Symposium EE2 : Advancements in Solar Fuels Generation---Materials, Devices and Systems

The emergence of hydrogen and fuel cell technologies in transportation and stationary power sectors offers the world important and potentially transformative environmental and energy security benefits. In recent years, research supported by the US Department of Energy’s (DOE) Fuel Cell Technologies Office has contributed substantially to the development of these technologies. Enhanced performance and reduced cost in automotive fuel cells are important examples of achievement. The research investments are clearly paying off, as commercial fuel-cell electric vehicles (FCEVs) are being rolled out by major car manufacturers today. With increasing market penetration of FCEVs, enabling technologies for the affordable and widespread production, storage and delivery of renewable hydrogen are becoming increasingly important. Long term commercial viability of hydrogen and fuel cells in the commercial marketplace will rely on continued materials research on several important fronts. Examples include the discovery and development of: (1) non-platinum-group-metal catalysts for next-generation fuel cells; (2) durable, high-performance photocatalytic materials systems for direct solar water splitting; (3) advanced materials-based systems for low-pressure, high-volumetric-density hydrogen storage; and (4) low-cost, hydrogen-compatible pipeline materials for hydrogen delivery and distribution. Research innovations in macro-, meso- and nano-scale materials are all needed for pushing forward the state-of-the-art in these areas. New approaches in accelerated materials development facilitated by a national energy materials network of advanced scientific resources in theory, computation and experimentation are being adopted at DOE. Application of these approaches to address the key materials challenges in hydrogen and fuel cell technologies are discussed.

A practical method to use sunlight to generate chemical fuels could dramatically change the landscape of global energy generation and storage. One specific implementation of this concept is an integrated photoelectrochemical (PEC) device that operates without added electrical bias with sunlight as the only energy input, producing either hydrogen by water splitting or carbon-based fuels via carbon dioxide reduction. Experimental demonstrations of such systems date back to the 1970s, and, since that time, there have been improvements in both the functional understanding and performance of the devices.
There are a number of design considerations arise when considering the full solar to fuel conversion system, both for water splitting and carbon dioxide reduction. These include the type and number of semiconductor junctions used to generate the requisite photovoltage, the use of co-catalysts, the electrolyte conditions, a product separation method such as a membrane, and the geometric layout and size of components.
To date, all approaches which have a solar to hydrogen (STH) conversion efficiency of >5% employ either 2 or more semiconductor junctions arranged in a voltage-additive tandem configuration. The degree of integration varies widely, from fully integrated “artificial leaves” to systems with decoupled photovoltaics and catalysts. A small subset of devices employ a mechanism, such as a proton or anion conducting membrane, to separate products and yield a pure H₂ stream. Reported STH conversion efficiencies range from <1% to over 20%. However, there are very few reports of long term operational stability, which is a clear prerequisite for commercial use [1].
Electrochemical reduction of CO₂ to fuel molecules such as methanol and ethanol could form the basis for the production of renewable and sustainable transportation fuels, replacing the fossil fuels which are used today. There has been substantial recent progress in solar driven PEC CO₂ reduction, with a number of reports of energy conversion efficiencies of over 1%. Product separation and the selective production of liquid fuels such as methanol remain as outstanding challenges for this technology.

This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.

(1) Ager, J. W.; Shaner, M. R.; Walczak, K. A.; Sharp, I. D.; Ardo, S. Energy Environ. Sci. 2015, 8 (10), 2811–2824

Photo-electrochemical electrolysis of water provides a direct pathway for the conversion of solar energy into an energy-dense, storable and transportable fuel. The practical implementation of photo-electrochemical approaches requires focusing simultaneously on four key developmental concerns: device and system i) efficiency, ii) durability and reliability, iii) environmental sustainability, and iv) scalability with economic viability.
In this talk, I will describe modeling efforts used in our group to provide a coupled techno-economic and life cycle assessment of photoelectrochemical hydrogen generation. This coupled modeling framework allows for the simultaneous prediction of solar-to-fuel efficiency, hydrogen cost, return on energy investment, and operational time. Based on a variety of characteristic PEC device types, incorporating irradiation concentration, current concentration and dilation, and various material and component combination, I will show how this tool can help us to provide holistic guidelines for practical and competitive PEC hydrogen generation approaches.
I will show how the coupled techno-economic-ecologic approach provides quantitative and holistic design guidelines for photo-electrochemical devices and can support decision-making for the implementation of large-scale, sustainable and competitive solar hydrogen production.
Subsequently, detailed multi-physics and multi-scale PEC reactor design models will be discussed which allow us to provide more detailed design guidelines for the reactor and its components and to provide input to required dedicated research efforts for the incorporated materials and components.

Most artificial photosynthetic systems mimic nature’s properties of light absorption, electronic charge separation, and electronic charge collection, where ultimately electrons make and break chemical bonds. Instead, we are motivated by nature’s ability to transduce photon energy into proton gradients whose electrochemical potential also drives chemical-bond formation.

In my presentation I will report on my research group’s recent success in demonstrating ionic power generation through solar light harvesting. Visible light was used to drive endergonic excited-state proton transfer from a photoacid-functionalized Nafion membrane. Photoacid molecules convert the energy in light into a change in the chemical potential of protons via a weakening of protic functional groups on the photoacid, i.e. a drop in its pK_a. The applicability and practicality of this material as a membrane in solar fuels devices and as a separate ionic photoelectrochemical device will also be discussed.

Specifically, we characterized several bipolar membranes to more clearly understand their electrochemical behavior under conditions relevant to solar fuels devices. We identified a condition where the energy required to electrolyze water was seemingly less than 1.23 V, which we showed was due to transport of ions other than protons and hydroxide ions. Then, we covalently linked a new derivative of hydroxypyrene photoacid (pyranine) to Nafion sulfonyl fluoride precursor membranes and verified their binding spectroscopically. After sandwiching this membrane between an anion-exchange membrane and a cation-exchange membrane to form a three-layer bipolar membrane composite structure, we demonstrated photovoltaic action during visible-light excitation using four-electrode photoelectrochemical membrane-measurement techniques. This composite material mimicked that of a solid-state semiconductor pin-junction with selective collection of positive majority carriers (i.e. protons) at the cation-exchange membrane contact and selective collection of negative majority carriers (hydroxide ions) at the anion-exchange membrane contact. As most of these materials and techniques are new to the solar fuels community, we plan to carefully present the results of our studies and provide ample explanation of the techniques. Collectively, this body of work represents several new directions in solar fuels research and development that are being pioneered by my research group.

The latest advances in materials design for solar hydrogen generation from water splitting will be presented. This will include a new strategy to combine and integrate molecular co-catalysts with inorganic semiconductors in highly ordered arrays as well as the effects of size, confinement and interfacial engineering onto the electronic structure of heteronanostructures for low cost solar water splitting.

High-efficiency solar-to-fuel energy conversion can be achieved using a photoelectrochemical (PEC) device consisting of an n-type photoanode in tandem with a p-type photocathode. However, the development of stable and inexpensive photoelectrodes are needed to make PEC devices economically competitive with traditional PV + electrolysis. In this presentation our laboratory’s progress in the development of economically-prepared, high performance photoelectrodes will be discussed along with the application toward overall PEC water splitting tandem cells. Specifically, how the use of scalable solution-processing techniques (e.g. colloidal processing of nanoparticles or sol-gels) leads to limitations in charge transport and charge transfer in the resulting thin-film photoelectrode will be examined. Strategies to overcome these limitations using chemical innovations such as using charge extraction buffer layers, catalysts, annealing/doping and nanoparticle self-assembly will be additionally presented. Materials of interest are delafossite CuFeO₂, CIGS, 2D-layered WSe₂, and semiconducting carbon-based materials.

The field of photoelectrochemical energy conversion has recently bifurcated into two approaches: stabilizing existing materials known to have high photovoltaic efficiencies with thin carrier conductive films and discovering and improving materials that are themselves stable under illumination in electrolyte solutions. In this talk I will review the progress in the search for new oxide materials for solar water splitting highlighting the different approaches and some promising new materials. I will discuss some progress on a new iron chromium aluminum material that shows some promise but may still have a fatal flaw. I propose a coordinated international effort to identify new oxide semiconducting materials, prepare them with different methods and in different forms and measure their important properties (band gap, carrier lifetimes, carrier mobilities…) with appropriate techniques to quickly determine if the new material has a fatal flaw like with iron oxide. If so then the field must agree to “fail quickly” and move on to other materials.

Solar photoelectrochemical generation of fuel is a promising energy technology, yet the lack of an efficient, robust photoanode remains a primary materials challenge in the development and deployment of solar fuels generators. Metal oxides comprise the most promising class of photoanode materials, but no known material meets the demanding requirements of low band gap energy, photoelectrocatalysis of the oxygen evolution reaction, and stability under highly oxidizing conditions. To identify new photoelectroactive materials, we have developed high throughput experimental methods for combinatorial materials synthesis, high-throughput photoelectrochemistry and high throughput optical spectroscopy. We have deployed this experimental pipeline in conjunction with a theory-experiment feedback loop, providing rapid discovery of new solar fuels photoanodes and unprecedented datasets that relate experimental and computational results. This effort has substantially expanded the set of known OER photoelectrocatalysts with band gap below 2 eV and revealed important structure-property relationships that pave the way for continued development of solar fuels materials.

Despite the promising developments in metal oxide photoelectrodes for water splitting in the past 5-10 years, concerns remain that the poor intrinsic properties of metal oxides (e.g., low carrier mobility due to polaron transport) will never be overcome. However, recent progress on BiVO₄ photoelectrodes paints a more optimistic picture. The performance of BiVO₄ is limited by the poor carrier transport and inefficient charge injection into the electrolyte. Based on these limitations, multiple strategies—surface modification with co-catalyst deposition, dopant introduction, and/or nanostructuring—have been effectively employed. In a very recent report, Pihosh et al. combined these strategies and showed a photocurrent exceeding 6.7 mA/cm² with their Co-Pi catalyzed guest-host BiVO₄/WO₃ nanorods [1]. This means that ~90% of the theoretical maximum photocurrent for BiVO₄ (7.5 mA/cm²) has been accomplished. This shows that despite the undoubtedly poor semiconducting properties of metal oxides compared to III-V materials or even silicon, highly-efficient photoelectrodes can be obtained by applying appropriate strategies.
To achieve efficiencies above 10%, metal oxides with smaller bandgaps have to be explored. Various calculations have indicated that a material with a bandgap of ~1.8 eV is ideal to be combined with a 1.2 eV bandgap material in a tandem configuration. Here, complex (ternary, quaternary) metal oxides offer plethora of possibilities. CuBi₂O₄, a p-type semiconductor with a bandgap of 1.8 eV, is a potential candidate. We have synthesized thin film CuBi₂O₄ photocathodes by drop-casting, and have systematically investigated its photoelectrochemical properties. Mott-Schottky analysis reveals the flat-band potential to be ~1.3 V vs RHE and a photocurrent onset potential of 1 V vs RHE, which is exceptionally positive. Microwave conductivity measurements reveal a carrier mobility in the range of 10^-2 cm²/Vs and a carrier lifetime of ~30 ns, comparable to values reported for BiVO₄. AM1.5 photocurrent densities of above 1 mA/cm² at 0.5 V vs. RHE are achieved, which is the highest value yet reported for this material.
Finally, we attempt to shed light on the charge carrier dynamics at the semiconductor-electrolyte interface, as inefficient charge injection is an important loss mechanism for many oxides. Using intensity-modulated photocurrent spectroscopy, we find that the photocurrent of BiVO₄ is mainly limited by surface recombination. Deposition of cobalt phosphate strongly suppresses surface recombination, but does not enhance the rate constant for charge injection into the electrolyte. The latter observation suggests that BiVO₄ itself is actually a quite good oxidation catalyst; indeed, modification of its surface with RuO_x, a well known (non-passivating) oxygen evolution catalyst, does not improve the performance. The implications of this surprising result will be discussed.

[1] Pihosh et al., Sci. Rep. 5 (2015) 11141

Hydrogen has been recognized as one of the most promising energy carriers for the future because it can generate enormous energy by clean combustion chemistry without any greenhouse gas emissions. Water splitting under visible light irradiation is an ideal route to cost-effective, large-scale, and sustainable hydrogen production. But it is challenging, because it requires a rare photocatalyst that carries a combination of suitable band-gap energy, appropriate band positions, and photochemical stability. To create this rare photocatalyst, we engineered the band edges of BiVO₄ by simultaneously substituting In³⁺ for Bi³⁺ and Mo⁶⁺ for V⁵⁺ in the host lattice of monoclinic BiVO₄, which caused partial phase transition from pure monoclinic BiVO₄ to a mixture of monoclinic and tetragonal BiVO₄.

Through the phase transition-induced band edge engineering via dual doping with In³⁺ and Mo⁶⁺, we developed a novel ‘greenish’ BiVO₄ (Bi_1-xIn_xV_1-xMo_xO₄). This new greenish BiVO₄ has a slightly larger band-gap energy (2.5 eV) than usual ‘yellow’ monoclinic BiVO₄ (2.4 eV), as supported by the unique color change to green, and a higher (more negative) conduction band than H⁺/H₂ potential (0 V_RHE at pH 7, RHE: reversible hydrogen electrode). Hence, it can split water into H₂ and O₂ under visible light irradiation without using any sacrificial reagents (e.g. CH₃OH or AgNO₃). This outcome represents the first example of a pure water-splitting photocatalyst responding to visble light without any noble-metal cocatalyst. Furthermore, greenish BiVO₄ shows the siginficantly increased activity of photocatalytic water oxidation by a factor of about 214 compared to commercial BiVO₄ (Alfa Aesar, 99.9%).

We also elucidated the physical origin of the augmented photo-response behaviors of greenish BiVO₄ through density functional theory (DFT) calculation of electronic structure as well as a variety of physical and electrochemical characterizations. Briefly, the In³⁺/Mo⁶⁺-dopant formation is more promoted within tetragonal BiVO₄ rather than monoclinic BiVO₄. This physicochemical tendency triggers the partial phase transition from pure monoclinic BiVO₄ to a mixture of monoclinic and tetragonal BiVO₄, which sequentially leads to unit-cell volume growth, compressive lattice-strain increase, conduction-band edge uplift, and band-gap widening. This In³⁺/Mo⁶⁺ doping-induced domino effect from phase transition to band edge engineering enables a previously unidentified mechanism to create a noble metal-free photocatalyst, achieving overall water splitting under visible-light irradiation. Therefore, the findings from this research possess great potential to realize cost-effective, large-scale, and sustainable hydrogen production from water.

Hematite (α-Fe₂O₃) is one of the best candidates for efficient conversion of solar energy to hydrogen via photo-electrochemical water splitting because of the unique combination of visible light absorption, stability in aqueous solutions, low cost and abundance. Attempts to identify and remove the loss mechanisms in α-Fe₂O₃ photoanodes are often obstructed by the complicated dependence of water photo-oxidation on physical properties, microstructure and operation conditions of the photoelectrode. Due to the limitations of conventional measurements such as linear scanning voltammetry and chronoamperometry, more detailed characterization methods capable of resolving the different processes are necessary to improve the understanding of the overall reaction. In this work, an operando diagnosis method of the water photo-oxidation process using α-Fe₂O₃ photoanodes is presented. By characterizing the hematite photoanode in a variety of electrolytes including H₂O₂ (hole scavenger) and Ferrocyanide- Ferricyanide redox couple, we identified and quantified photocurrent losses caused by surface charging effect and exposed substrate (shunts).

Acknowledgment: The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. [617516].

Hematite is a leading candidate for use as a photoanode material in solar powered water splitting due to its stability in aqueous solution, 2.1 eV band gap, vast abundance, and low cost. However, the short diffusion length (2 – 20 nm) of photogenerated minority carriers in hematite results in massive bulk recombination. Elimination of grain boundaries and reduction of defects by growth of heteroepitaxial films has potential to significantly suppress the deleterious effects of charge carrier recombination and increase water splitting efficiency. We present the growth of heteroepitaxial multilayer Pt(111)/Fe₂O₃(0001) films deposited on sapphire c-plane (0001) substrates by RF magnetron sputtering and pulsed laser deposition, respectively. We show that this is a model system for systematic investigations into hematite photoanode properties by a collection of materials, optical, and electrochemical characterization techniques.

The deposited films were highly crystalline, displaying an in-plane mosaic spread of less than 1 degree and a homogenous surface morphology with roughness of ~3 A, allowing for systematic investigation of hematite properties without changes to the microstructure. Ellipsometry and UV-Vis spectroscopy measurements were shown to be in excellent agreement with modelling, demonstrating that the optics of the system including absorption in the hematite layer are well described.

For polycrystalline hematite photoanodes deposited on platinum, full characterization of the system is hampered by the inability to make measurements in alkaline electrolyte containing 0.5 M hydrogen peroxide (H2O2) due to spontaneous decomposition of H2O2 by the exposed platinum. The pin-hole free high quality of the heteroepitaxial films is demonstrated by the ability to make stable and reproducible measurements in H2O2 containing electrolyte allowing for accurate extraction of charge separation and injection efficiency. The combination of excellent crystalline quality in addition to the well characterized optics and electrochemical properties of the heteroepitaxial hematite photoanodes demonstrate that Al₂O₃(0001)/Pt(111)/Fe₂O₃(0001) is a powerful model system for systematic investigation into solar water splitting photoanodes.

The solar driven photoelectrochemical (PEC) water splitting is a promising rout to directly store solar energy into chemical bonds. Recently, a new class of semiconductors, e.g. tantalum nitride (Ta₃N₅), emerged as a promising candidate for PEC water splitting. Most of the studies on tantalum nitride (Ta₃N₅) share a similar synthetic route, beginning with the oxidation of Ta(0) to Ta(IV), followed by ammonolysis at elevated temperatures ( > 850 °C). Despite of the simplicity that comes with this method, there are multiple negative consequences on the PEC performance. The high temperature ammonolysis limits the selection of TCO materials; therefore Ta₃N₅ is commonly prepared on Ta foil which excludes the applicability of the Ta₃N₅ as a photoanode in tandem PEC cells. It has been also reported that upon oxidation/ammonolysis some resistive nitrogen poor interfaces, between Ta₃N₅ and Ta, are formed which suppress the electron collection efficiency at Ta₃N₅/Ta junction. Further, high temperature ammonolysis makes it hard to control the morphology, interfaces and inherent properties of this semiconductor.
In order to overcome these issues, we synthesized Ta-doped TiO₂ (TTO) via atomic layer depositions (ALD) which we found to be a stable TCO in reducing atmospheres. In addition, to circumvent the high temperature ammonolysis, ALD was also used to directly deposit thin films of Ta₃N₅ on TTO. While initial as-deposited films are primarily amorphous TaO_xN_y, these films can be nitridized to Ta₃N₅ at far more moderate conditions compared to previous reports and ALD–deposited tantalum oxide analogues. The photoelectrochemical properties of the Ta₃N₅ films deposited on TTO were investigated and the PEC water oxidation performance was analyzed. The excellent material control reported here allowed for a detailed material structure – function relationship to be determined and a path to improved performance elucidated.

Photoelectrochemical splitting of water to produce hydrogen is a viable approach to transform sunlight into chemical energy, which has triggered a quest for suitable photocatalysts. Transition metal oxides such as α-Fe₂O₃ and TiO₂ are favorable candidates and provide intrinsic advantages in terms of high photo-stability and sufficient mobility of charge carriers, besides their earth-abundance. Despite these advantages, no commercially viable material exists that is able to maintain the proposed minimum 10 % requirement for solar energy to hydrogen fuel efficiency (STH). In the search for strategies enabling an enhancement of the photo electrochemical properties of metal oxide, various methods such as doping of metal oxide, co-catalyst or mutlilayering were conducted.
In this study, we have focused on Interfacial modification of α-metal oxide multilayer photoanodes deposited by plasma enhanced chemical vapor deposition (PE-CVD). Different mechanisms such as patterning of multilayering with different structure (bar structure or line structure) or graphene supporting were examined in this study. The α- bilayer electrode exhibited enhanced PEC responses in terms of a lower onset potential and a higher photocurrent density when compared to the single layer α-Fe₂O₃ electrode. This enhancement was observed to be due to synergistic light absorption with the bilayer electrode, although charge carrier recombination occurred faster due to interfacial defect states. The incorporation of a graphene layer between the α-Fe₂O₃/TiO₂ double layer and the FTO substrate resulted in a doubling of the photocurrent, but lead to a loss of the synergistic effect between the two active metal oxide layers. However, depositing the graphene between the two metal oxide layers resulted in an even higher photocurrent, while retaining the enhanced onset potential of the double layer electrode. This enhancement was observed to be due to either the passivation of the oxide defect states or enhancement of the charge transfer between the two oxide layers.

Phase-pure Cu₂O thin films ~ 1.4 μm were obtained on Au-coated SnO₂:F (FTO) substrates via charge-controlled potentiostatic electrodeposition. The growth of preferentially (111) textured film was evinced XRD and Raman analysis. SEM images revealed a densely packed morphology of Cu₂O under this deposition scenario and an average growth rate of 17 Å/s. For the PEC measurements various loadings of the Pt co-catalyst was obtained by controlling the charge, during electrodeposition from a Chloroplatinic acid (H₂PtCl₆) bath. The optimum amount of co-catalyst was found to be that corresponding to a total deposition charge of 0.03 C/cm². The PEC device with the 1.4 μm thick film of Cu₂O loaded with Pt corresponding to a charge of 0.03 C/cm² resulted in generation of 1.5 mA/cm² photocurrent at 0.0 V (RHE) under 100 mW/cm² (AM 1.5G) in an electrolyte of pH 6.7. However, the photo-degradation of the Cu₂O was observed from light chopping chrono-amperometric experiement without use of any stable protective layer.

Photoelectrochemical water splitting has attracted significant attention as methods for generating carbon free fuels from solar energy. Various p-type semiconductors have been reported to evolve hydrogen from water, but stable responsive photocathodes are still very rare. Recently, Cu-delafossite (CuMO₂, M = Fe, Ga) thin films are reported as a promising low-cost p-type photocathode owing to their stability and high conductivity and tunable band gap by varying the Fe/Ga ratio (1.5 – 2.5 eV). Despite the advantages of Cu-delafossite materials, relatively short diffusion length of photo-generated carrier is a major impediment for achieving efficient photoelectrode. In general, ordered nanostructure can enhance the properties of low diffusion length materials by reducing transport distance of photocarrier, but complex synthetic route and high annealing temperature of multinary oxide tends to hinder achieving ordered nanostructure. Here, we suggest synthetic route for composition tunable p-type CuMO₂ inverse opal structure from silica-templated core-shell method. Precursor solution containing Cu, Fe and Ga ions was prepared, followed by surface reaction on silica to fabricate silica@CuMO₂ core-shell precursors. The silica@CuMO₂ core-shell precursors were places in a furnace and calcinated at 700 ^oC. The silica@CuMO₂ microspheres were submerged in a 10% HF solution to prepare hollow microspheres. Then, 3-dimensional inverse opal structures were fabricated by rubbing process with hollow CuMO₂. Morphological and structural characterizations were performed, and the photoelectrochemical, compositional and optical properties of CuMO₂ inverse opal structures were discussed depending upon the ratio of the Fe/Ga. This methodology can be a potential common low cost route to prepare various oxide inverse opal structures with a variety of compositions and size. Furthermore, this work is the first report on nanostructured Cu-delafossite photocathode.

Graphitic carbon nitride has emerged as a promising metal-free photocatalyst for solar energy conversion. Despite many merits including easy-availability, good stability, visible light responsive and environmentally friendly, its application is largely hindered by the high bulk recombination probability of photogenerated charge carriers and narrow visible light responsive range only to around 470 nm. To improve solar energy conversion efficiency, tailoring its microstructure at the atomic level is highly necessary. Graphitic carbon has a unique atomic structure with the coexistence of covalent bonds, hydrogen bonds and van der Waals forces, making it versatile to tailor its microstructure by controlling bonding structure. In this talk, the recent progress in forming amorphous graphitic carbon nitride by destroying weak hydrogen bonds and van der Waals forces will be introduced. It was found that amorphization can greatly narrow the bandgap of carbon nitride from 2.82 eV to 1.90 eV due to the formation of abundant band tails close to conduction and valence bands. Moreover, the radiative recombination in the amorphous carbon nitride is completely quenched. As a consequence, the photocatalytic hydrogen generation of carbon nitride is improved by one order of magnitude. Further study shows that selective destroying hydrogen bonds leads to the maximized photocatalytic hydrogen evolution. These results demonstrate the great potential of tailoring bonding structure of graphitic carbon nitride in improving photocatalytic activity.

References
[1] X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 2009, 8, 76 .
[2] Y. Y. Kang, Y. Q. Yang, L. C. Yin, G. Liu, H. M. Cheng, Adv. Mater. 2015, 27, 4572.
[3] P. Niu, L. C. Yin, Y. Q. Yang, G. Liu, H. M. Cheng, Adv. Mater. 2014, 26, 8046.
[4] P. Niu, Y. Q. Yang, J. C. Yu, G. Liu, H. M. Cheng, Chem. Commun., 2014, 50, 10837.
[5] Y. Y. Kang, Y. Q. Yang, L. C. Yin, G. Liu, H. M. Cheng, Submitted.

Materials development is needed to advance technologies for sustainable H₂ production, including solar photoelectrochemical (PEC) water-splitting. This talk will address challenges involving catalyst development and their integration onto semiconductors including Si, GaInP₂, Ta₃N₅, and BiVO₄ to improve performance of the resulting photocathodes and photoanodes. Approaches to deliberately structure the materials at either the nano- or micron-scale have led to improved efficiency and/or stability.

Development of a solar water splitting device requires design of a low-cost, efficient, and non-noble metal compound as alternative to noble metals. Here we showed that CoSe₂ and CoS₂ can function as co-catalyst in phototoelectrochemical hydrogen production. We designed a heterostructure of p-Si and marcasite-type CoSe₂ and pyrite-type CoS₂ for solar-driven hydrogen production. CoSe₂ and CoS₂ successively coupled with p-Si can act as a superior photocathode in solar-driven water splitting reaction. Photocurrents up to 9 mA/cm2 were achieved at 0 V vs. reversible hydrogen electrode. Electrochemical impedance spectroscopy showed that the high photocurrents can be attributed to low charge transfer resistance between the Si and CoSe₂ and CoS₂ interfaces and that between the CoSe₂ and CoS₂ and electrolyte interfaces. Our results suggest that this CoSe₂ and CoS₂ is a promising alternative co-catalyst for hydrogen evolution.

Metal particle co-catalysts such as Pt can be coupled to light harvesting semiconductors to facilitate charge separation and provide active surface reduction sites. On the Pt/TiO₂system, photogenerated electrons are transferred to the metal while the holes remain in the TiO₂ valence band thus suppressing electron-hole pair recombination. Under ultraviolet illumination, this material gives significant H₂ production during photocatalytical water splitting [1]. However, we find that the H₂ production rate drops by nearly a factor of 2 over a period of 10 hours during water splitting in pure water. Pt particle size evolution was carefully characterized using high-angle annular-dark field scanning transmission electron microscopy (HAADF-STEM). The average particle size increased from 1.7nm to 2.1 nm and the resulting decrease of surface area correlates with the activity drop. The Pt²⁺ concentration in different periods of reaction was determined to be negligible by inductively coupled plasma mass spectrometry (ICP-MS). The deactivation mechanism is believed to arise from a photo-electro-chemical form of Ostwald ripening. Under reaction conditions, small Pt particles, which are subject to more negative electrical potential because of the Gibbs-Thompson effect, prefer to oxidize to Pt²⁺ into H₂O solution, which then re-deposit onto large Pt particles [2]. Small and large Pt particles behave as the cathode and anode connected by conducting TiO₂ under irradiation. The charge transfers is very slow in dark when TiO₂ is much less conductive and thus the ripening rate is much slower. In-situ TEM characterization of the same materials when exposed to H₂O vapor and light showed no photo-induced sintering of Pt which supports the interpretation that the sintering happens through Ostwald ripening by of Pt ion transport through liquid H₂O.
Reference
[1]. Tabata, S.; Nishida, N.; Masaki Y.; Tabata, K. Catalysis Letters 1995, 34, 245.
[2]. Plieth. W. J. Jounal of Physical Chemistry 1982, 86, 3166-3170
[3]. The support from US Department of Energy (DE-SC0004954) and the use of ETEM at John M. Cowley Center for HR Microscopy at Arizona State University is gratefully acknowledged.

Photoactive and counter electrodes are the most important parts of a photoelectrochemical (PEC) solar cell. In a PEC cell having n-type semiconductor as photoactive electrode, the hydrogen generation takes place on the counter electrode, which is the platinum in most case. Although platinum is very expensive, it is the state-of-art material as counter electrode due to its stability and very high catalytic activity. In this study, we fabricated the nanostructured platinum counter electrode and compared its PEC performance with platinum sheet electrodes. We used zinc oxide (ZnO) flat and nanowire arrays deposited on indium tin oxide coated glass as the photoactive electrode. A flexible thin film platinum electrode with nanoflower morphology has been fabricated via alloying/dealloying technique. First platinum and aluminum were cosputtered on the Teflon^TM. Then aluminum in the alloys was selectively removed by dealloying in hydrochloric acid and NaBH₄ solutions, successively. After dealloying process highly porous (120 m²g^-1) nanoflower structured platinum electrodes were obtained. This method provides extremely cost effective production of the platinum electrodes with very high surface area. More specifically, we only used 44 μg of platinum per centimeter square. Besides, the usage of Teflon^TM as the substrate provided considerable amount of flexibility, which could be an advantage for the very large area applications. PEC measurements indicated that short circuit current density (J_sc) of the ZnO flat electrodes increased from 2.2 to 7.2 μA cm^-2 by using nanostructured platinum counter electrode. Moreover open cell potential (E_aoc) of the ZnO flat electrodes increased 1.3 times and reached to 70 mV when nanostructured platinum was used as counter electrode. More drastic change in J_sc and E_aoc was observed when we used ZnO nanowire array as the photoactive electrode. The J_sc and E_aoc of the ZnO nanowire- platinum sheet electrode PEC system were 110 μA cm^-2 and 525 mV, respectively. The maximum E_aoc of 530 mV was obtained for the ZnO nanowire- nanostructured platinum electrode PEC system. Solar-to-hydrogen (STH) conversion efficiency was also calculated for both sheet and nanostructured platinum electrodes. STH of the ZnO-NW/sheet and nanostructured platinum electrodes was 0.75 and 1.07 %, respectively. The maximum STH efficiency, which was 1.29%, was observed for the ZnO-flat/nanostuructured platinum electrodes. These efficiency values were very promising to utilize the nanostructured platinum films as counter electrode in PEC solar cells.

The oxygen-evolution reaction (OER) is one of the main kinetics bottlenecks that limit the efficiency of direct solar water splitting devices. Ruthenium dioxide (RuO₂) is one of the most active catalysts for OER, as measured by its relatively low overpotential. The high cost of RuO₂ precursors motivated us to develop synthetic methods for practical electrodes that incorporate low mass quantities of the expensive oxide in nanoparticulate froms. Recently, an NRL-patented solution-based synthetic protocol was developed to deposit conformal, ultrathin films of RuO₂ on technologically relevant electrode architectures. The solution-deposited RuO₂ forms a contiguous film of self-wired 2–3 nm particles, designated “nanoskins,” which exhibit an unprecedented combination of high carrier concentration (n), low mobility (μ), and broadband transparency not seen in bulk morphologies of RuO₂.

Here we present water oxidation at RuO₂ nanoskins deposited onto planar and three-dimensional (3D) substrates (i.e., SiO₂ fiber papers) and benchmark their performance for electrocatalytic OER activity and stability under device-relevant conditions. On planar conducting supports, we characterize how the specific activity for OER at RuO₂ nanoskins changes as a function of crystallinity, conductivity, and film thickness. We have demonstrated that the current density and overpotential for OER at RuO₂ supported on SiO₂ fiber papers is significantly improved compared to those on planar supports, thanks to the amplified surface area and mass loading of the 3D-expressed active material, despite the lack of conductivity provided by the support. Additionally, we have deposited RuO₂ on SiO₂ papers in which the silica fibers are pre-coated with a conductive graphitic-like carbon shell prior to RuO₂ deposition using an NRL-patented protocol. These composite electrodes have current densities and overpotentials comparable to state-of-the-art water oxidation catalysts in acid electrolyte while only incorporating micrograms per square of the active material.

IrO₂ is one of the most efficient electrocatalysts for the oxygen evolution reaction (OER) and water splitting process for the efficient solar energy into fuel (H₂) generation. In solar fuels, IrO₂ is used in the form of supported nanoparticles and its catalytic activity strongly depends on the size and shape of nanoparticles. Atomistic modeling of IrO₂ nanoparticles with different sizes and shapes can enable fundamental understanding of catalytic processes govern at nanoparticle surfaces. Here, we used density functional theory (DFT) and variable charge force field calculations to study catalytic activity changes at the surfaces, edges and corners on the nanoparticle. We used O adsorption energy as a descriptor for the catalytic activity for water splitting reaction and determined the activity changes with respect to atomic coordination and charge transfer at different active sites on the nanoparticle. Calculations with the first variable charge force field for IrO₂ [1], trained on DFT data using genetic algorithm optimization, enable investigation of thermodynamic stability of relatively large (1-4 nm) IrO₂ nanoparticles with different shapes. We revealed the effect of nanoparticle shape and size on the catalytic activity in terms of surface coordination changes, charge transfer and structural stability. Our results will shed light on the design and development of stable nanoscale IrO₂ nanoparticle electrocatalytsts that efficiently utilize solar energy for water splitting reaction.

[1] F. G. Sen, M. J. Davis, S. Gray, S. Sankaranarayanan, and M. Chan, “Towards accurate prediction of catalytic activity in IrO₂ nanoclusters via first principles-based variable charge force field,” Journal of Materials Chemistry A 3, 18970 (2015).

ACKNOWLEDGEMENT: Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The submitted abstract has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

The design of optimal absorbers for generating hydrogen from photoelectrochemical (PEC) water splitting requires control of both the band gap for good absorption and the band offsets with a suitable heterojunction partner to facilitate the desired charge transfer. The chalcopyrite material class, typically identified by its most popular photovoltaic alloy CuInGaSe₂, is known to exhibit a wide range of band gaps and provides exceptionally good candidates for PEC water splitting. Models for maximizing the solar-to-hydrogen (STH) efficiency in dual-absorber hybrid photoelectodes have predicted an optimal band gap of ~1.8 eV for the larger absorber, a value that cannot be obtained with conventional selenide-based chalcopyrites (e.g. CuInGaSe₂). Using hybrid functional calculations combined with cluster-expansion approaches we investigate the stability and electronic structure of a range of other alloys within the group I-III-VI₂ system (I=Ag,Cu ; III=In,Ga,Al ; VI=S,Se) to identify desirable compositions for optimal absorbers. Specifically, our analysis highlights alloys that are most favorable for water splitting in the context of their band gaps and band edge positions and identifies conditions for which the alloy should be stable relative to the parent compounds. We additionally make suggestions for n-type partner layers best matched to each absorber candidate for realizing high-efficiency STH conversion.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and funded by the Department of Energy office of Energy Efficiency & Renewable Energy (EERE).

The current consensus within the photoelectrochemical (PEC) water splitting community is that the development of a semiconductor of bandgap 1.7 – 2.1 eV that is both highly efficient and stable is necessary for an economically viable water splitting device to produce clean and renewable H₂. Copper chalcopyrites are excellent candidates for this purpose, and many other photo-active applications, as they output current densities close to their theoretical maximum, tend to be relatively durable compared to other chalcogenides, and can theoretically be engineered to have a bandgap from 1.0 to 3.0 eV. However, this material class covers a wide range of elements, many of which have yet to be explored.

In this work, we have focused on the CuGa(S,Se)₂ (CGSSe) alloy, which has been theoretically shown to have a bandgap within the ideal energy range. To the best of our knowledge, there is no research thus far that examines the application of this material to water splitting. With our process, a CuGaSe2 (CGSe) material is first co-evaporated onto a conductive substrate (FTO or Mo). Then, the CGSe thin film material is annealed with sulfur to form CGSSe. With this approach, we have been able to successfully synthesize single-phase CGSSe films of a bandgap anywhere between 1.7 and 2.4 eV. Adjusting the mass of sulfur used during the annealing process easily controlled the bandgap. Preliminary three-electrode photoelectrochemical measurements under AM1.5 illumination of these devices exhibit current densities exceeding 10 mA/cm² (hypothetical STH efficiency of about 12%) along with an anodic shifting of the photocurrent-onset voltage by approximately 0.3 V, relative to that of CuGaSe₂. A further discussion of the interesting facets of the synthesis process as well as the experimental results will be presented.

A copper indium gallium selenide (CIGS) has been considered as a good photoelectrode material due to its tunable bandgap of 1.0 ~ 2.4 eV by controlling the component ratio. In addition to its capability of visible light absorption, high conduction band position is advantageous for photoelectrochemical (PEC) hydrogen generation from water. However, expensive vacuum based processes such as co-evaporation or sputtering techniques have been generally used for CIGS thin film fabrication. In this study, we demonstrate that cost effective solution based printing method to prepare a CIGS photocathode on a fluorine-doped tin oxide (FTO), spin casting of metal precursor solution followed by sequential calcination and selenization. The PEC properties of the CIGS thin films were characterized for solar to hydrogen generation application. 20 mA/cm² of photocurrent was achieved with only 600 nm thin layer of CIGS on the FTO substrate. The photocurrent and transparency of the CIGS photocathodes were easily controlled by changing its thickness from 200 nm to 1000 nm. Additionally, detail characterization of CIGS semiconductor/electrolyte junction was carried out with/without protective layers to resolve the degradation of photocurrent issue and to study the effect of the protective layer types. The photocurrent and its onset potential were improved with decoration of co-catalysts for hydrogen evolution reaction. This active and transparent photocathode material can be useful to design photodiode, tandem cell, or other multi-junction water splitting cell.

A dual absorber (D4-type) photoelectrochemical (PEC) tandem cell, composed of a series-connected n-type photoanode and a p-type photocathode, has been considered as a promising target device for the inexpensive conversion of solar energy directly into chemical fuels through water splitting. Although numerous low-cost semiconductor materials and fabrication method have been investigated for n-type photoanode, efficient photocathode with both low-cost constituents and processing technique is still waited to be developed. Recently, Cu₂ZnSnS₄ (CZTS) has attracted intense attention as a low-cost light absorbing material for photo-energy conversion devices, such as photovoltaic cells and PEC water splitting cells. Most of the efficient photocathodes based on CZTS material, however, rely on expensive vacuum deposition. Here, we present a facile route to fabricate a CZTS thin film using low-cost solution processing and enhanced PEC properties of our fully low-cost PEC devices. Non-toxic alcohol based CZTS inks were employed to prepare thin-film photocathodes that served as a model system to interrogate the effect of different surface treatments, viz. n-type over-layer and co-catalyst. With the surface treatment, photoelectrochemical property of CZTS thin film was improved by enhanced charge transfer kinetics and shifting of the flat-band potential, which are analyzed by chronoamperometric measurement and Mott-schottky plot. We believe our approach for the fabrication of photocathode, reported here, will be the first step in realizing the development of efficient and fully low-cost photocathode for water splitting.

(Oxy) nitrides and (oxy) chalcogenides have been regarded as promising material groups for sunlight driven photocatalytic and photoelectrochemical (PEC) water splitting because of its attractive properties. BaTaO₂N (BTON) can absorb the light with wavelength of <660 nm and has a preferable band structure for water splitting. BTON photoanode prepared by particle transfer method shows relatively large photocurrent and stable water oxidation. PEC cell composed of BTON photoanode and La₅Ti₂Cu_1−xAg_xS₅O₇ (LTCA) photocathode with absorption edge of about 700 nm drove spontaneous water splitting under the light. Effects of surface modifications and electrolytes on the properties of PEC were investigated in detail and it was found that both anions and cations in the electrolyte are critical on the properties of the PEC cell, and the surface modifications can relax the requirements for electrolytes
Cu(In, Ga)Se₂ (CIGS) is one of the promising candidate of photocathodes for hydrogen evolution from water under sunlight because of large photo absorption coefficients, p-type conductivity, usability in polycrystalline state, and its long absorption edge. To achieve efficient water splitting using the photocathode, the too shallow potential of valence band maximum (VBM) of CIGS need to be deepened. We found that formation of solid solution of CIGS with ZnSe is one of the solutions to obtain enough deep VBM potential to compose PEC cell for solar hydrogen production from water under sunlight. We will discuss in detail about the PEC properties of the solid solution between CIGS and ZnSe in the presentation.

For photoelectrochemical (PEC) water splitting, cadmium selenide (CdSe) thin films (TFs) provide a narrow band gap and high photoconversion efficiencies, leading to enhanced sun-light to hydrogen conversion efficiencies. In this work, CdSe TFs, serving as the photoanode, were decorated with zirconium selenide (ZrSe) to form a heterojunction to improve the separation and transport of photo-induced electron−hole pairs. CdSe TFs and ZrSe were prepared with a simple and economical electrodeposition method. The PEC performances of the pristine and decorated CdSe TFs were evaluated by the photocurrent density under irradiation of an AM1.5G solar simulator at an intensity of 100 mW/cm². Pristine CdSe TF electrodes achieved a maximum photocurrent density of 2.47 mA/cm² at 1.23 V vs. RHE. The optimized ZrSe decorated CdSe TF electrodes achieved a maximum photocurrent density of 3.19 mA/cm². The ZrSe decorated CdSe TF electrodes exhibited a 30% improvement in photocurrent density over that of the pristine CdSe TF electrode.

Nanophotocatalysis is one of the potentially efficient ways of solar energy conversion. Translation of global power demand into novel emerging technologies could predictably lead to the cost of solar power drop below retail electricity in the next few years. Solar energy can be converted directly into a clean hydrogen H₂ chemical fuel via photocatalytic or photoelectrochemical water splitting reactions. In the natural world conversion of sunlight to chemical energy is carried by phototrophs, organisms that capable of capturing and converting sunlight photons to energy-storage organic molecules. Biological energy transformation is accomplished via two evolutionary-independent schemes by direct translocation of protons, using membrane proton pumps rhodopsins, or electrons, using photosynthetic reaction centers, across a membrane.
Not only do natural solar energy conversion systems serve as an inspiration, but they also provide functional biological structures as backbones for development of advanced hybrid materials for photoenergy conversion. We have been successfully utilized proton pump protein bacteriorhodopsin (bR) and its “purple membrane” complex (PM) from salt loving microorganisms Archaea as a building block in the bio-assisted photocatalytic systems for visible light-driven hydrogen production. In this nano-assembly, bR serves as a visible-light harvester on Pt/TiO₂ photocatalyst and also contributes into optimizing of the protons-Pt catalyst interface and therefore enhancing reduction of protons to hydrogen. The turnover rate on a per μmole basis of the active properly folded protein (as determined spectrophotometrically) of the hybrid photocatalyst was found to be 207 μmole of H₂ (μmole protein)⁻¹ h^–1 under monochromatic green light and 5275 mol of H₂ (μmole protein)⁻¹ h^–1 under white light illumination. Further introduction of rGO as an additional module that, along with the natural light-capturing membrane complex bR boosts performance of the photocatalyst under the visible light. Besides, rGO provides a nanoscaffold for seamless interface between biological molecules, semiconductor particles, and platinum cocatalyst. One of the most attractive features of our approach is that all biological and inorganic materials can readily self-assemble without additional chemical coupling steps to form a stable and functional hierarchical photocatalytic system. The rational engineering of the nano-bio catalyst via introduction of rGO results in boosting photocatalytic hydrogen production rates up to 11240 mol of H₂ (μmole protein)⁻¹ h^–1 under ambient conditions and remarkable reduction in the platinum cocatalyst content by 25%. Evolutionary evolved structural and functional elegance of natural purple “solar panels”, their robustness and low cost make them a great candidate for practical application in environmentally-friendly devices that provide energy from only infinite sources, salt water and sunlight.

This work addresses the critical variables limiting the performance of traditional TiO₂-based materials for solar-driven heterogeneous photocatalysis for solar fuels production, namely: (1) inefficient absorption of visible light; (2) short lifetimes of photogenerated electron-hole pairs; and; (3) lack of selectivity for fuels-generating reactions of interest. Our materials platform consists of high-surface area, mesoporous, titania aerogels which are modified with metal nanoparticles. To address poor sunlight utilization, we incorporate metal nanoparticles with local surface plasmon resonances that overlap the solar spectrum, and initiate photochemistry with visible photons. In order to extend electron-hole lifetimes, we synthetically modify particle-particle junctions within the networked oxide aerogels in order to enhance charge mobility. We address the selectivity of fuels-relevant chemical reactions by incorporating catalytic metal nanoparticles in order to improve the yield of the desired products (i.e. hydrogen). We explore how the interfacial arrangement between the metal nanoparticle and the mosporoous oxide support affect both plasmonic sensitization efficiency and catalytic activity.

Realization of photolytic devices generating renewable fuels is challenged by a number of requirements, one of which is efficient channeling of photogenerated electrons and holes to redox reactions at the interfaces. The present work investigates sol-gel synthesized vanadium oxyhydrate (V₃O₇●H₂O) nanowires decorated with Au nanoparticles. During the nanoparticle reduction on the nanowires, the vanadia oxidizes to V₂O₅●H₂O with optical band gap changing from 2.3 eV (indirect) to 2.7 eV (direct). Reproducible conversion and external quantum efficiencies of 5.3% and 11.3% have been recorded by gas chromatography, respectively, for the first hour of photolysis under 470 nm excitation (8 mW/cm²). H₂:O₂ ratio is reproducibly measured as 2.0 ± 0 1, suggesting true water splitting. Interestingly, under normal conditions our nanoconjugates are anticipated not to reduce hydrogen, as the conduction band edge of V₂O₅●H₂O is too deep, that is, 5.0 eV below vacuum level when measured by UV photoelectron spectroscopy (UPS). In other words, photogenerated electrons are expected to fall short in energy by 0.5 eV to reduce hydrogen. Therefore, to explain the observed hydrogen reduction, we have hypothesized the vanadia electron energy levels are lifted up by some negative surface charge. With the objective of validating this hypothesis, we performed cyclic current-voltage measurements on the aqueous suspensions of V₃O₇●H₂O nanowires and V₂O₅●H₂O nanowires conjugated with Au nanoparticles. The derived conduction and valence band edge energies are not only consistent with the optical band gaps, but also validate the hypothesized energy increase by 2.0 and 1.4 eV (relative to UPS-measured values), respectively. The negative surface charge is also corroborated by the zeta potential, which is measured as −58 mV. Based on the measured pH of 2.5, we attribute the negative surface charge to Lewis acid nature of the nanowires, establishing dative bonding with OH⁻. The present work establishes the importance of surface charge in photoelectrochemical reactions, where it can be instrumental and enabling in photolytic fuel production.

The natural photosynthetic process has fascinated chemists for long due to its high specificity in solar energy conversion to sugar. However, given the structural and functional complexity, it is a challenge to mimic this natural process. Nonetheless, efforts to develop efficient photosynthesis mimetic systems have been going on since 1970s. In recent years, these efforts have intensified due to increasing emphasis on the development of carbon-free or carbon-neutral systems/technologies for production of solar fuel/chemicals. Utilizing the natural photosynthesis as blueprint, a number of covalent, and non-covalent donor-acceptor conjugate dyes have been studied as systems for CO₂ fixation. Although capable of efficient photoinduced intra- and intermolecular electron transfer (ET), they suffer from poor conversion efficiency and lack photostability. For enhanced efficiency and photostability, a variety of photocatalytic materials, such as inorganic frameworks and metal complexes have been developed and evaluated. However, their direct utilization remains limited due to one or more reasons, which include, low NAD(P)H regeneration, poor selectivity, limited photostability and inability to work in visible light. This has led to emergence of the coupling of a suitable visible light active photocatalyst to an enzyme as an exciting avenue of research in this area.
In this regard, we developed the photocatalyst/enzyme integrated solar chemical factory platform system that exemplified solar energy in synthesis of solar fuel & solar chemicals. Generating NAD(P)H in non-enzymatic light-driven process and coupling it to the enzymatic dark reaction catalysis for the tailor-maid solar chemical synthesis via photobiocatalysis. The present work demonstrates successfully a new and potentially promising solar solar chemical factory platform system for the ultimate goal of utilization of solar energy in fuel & fine chemical synthesis.

A clean and sustainable energy source is a basic requirement for addressing the current increase in global energy demand and environmental issues. Semiconductor-based photocatalysis has received tremendous attention in the last few decades because of its potential for solving current energy and environmental problems. In a semiconductor photocatalytic system, photo–induced electron-hole pairs are produced when a photocatalyst is irradiated by light with frequencies larger than that of its band gap (hv> E_g). The photo-generated charge carriers can either recombine with no activity, or migrate to the surface of the semiconductor, where they can be involved in redox processes. The photocatalytic efficiency depends on the number of charge carriers taking part in the redox reactions and on the life time of the electron-hole pairs generated by the photoexcitation [1]. High recombination rate of charge carriers and limited efficiency under visible light irradiation are the two limiting factors in the development of efficient semiconductor-based photocatalysts. To overcome these drawbacks, a number of chemical and design strategies have been developed [2]. Among these strategies, the design and formation of composites using two or more semiconductor catalysts is a promising approach [3, 4]. Here, the complete study of composites of Bi₂O₃ and tantalum based compounds (Ta₂O₅/ or TaON/ or Ta₃N₅), composite of Bi₂O₃ and WO₃ and composite of g-C₃N₄/Sr₂Nb₂O₇ is reported and discussed. We used these three systems to demonstrate that the design and preparation of composites with proper band gaps and relative band positions can facilitate charge separation/migration and decrease the charge recombination probability, thus enhancing the photocatalytic efficiency in visible light [5-8]. On the basis of observed activity, band positions calculations, and photoluminescence data, a mechanism for the enhanced photocatalytic activity for the heterostructured composite is proposed and discussed.


References
1. R. Abe, J. Photochem. Photobiol. C, 2010, 11, 179-209.
2. M. Ni, M. K. H. Leung, D. Y. C. Leung and K. Sumathy, Ren. Sus. Energy Rev., 2007, 11, 401-425.
3. H. wang, L. zhang, Z. Chen, J. Hu, S. Li, Z. Wang, J. Liu and X. Wang, Chem. Soc. Rev., 2014, 43, 5234-5244.
4. R. Marschall, Adv. Funct. Mater., 2014, 24, 2421-2440.
5. S. P. Adhikari, L. Zhang, M. Gross and A. Lachgar, MRS Proceedings, 1738, mrsf14-1738-v05-52 DOI: 10.1557/opl.2015.194.
6. S. P. Adhikari, Z. D. Hood, K. L. More, I. Ivanov, L. Zhang, M. Gross and A. Lachgar, RSC Adv., 2015, 5, 54998–55005.
7. S. P Adhikari, H. Dean, Z. Hood, R. Peng, K. L. More, I Ivanov, Z. Wu, A. Lachgar, RSC Adv., Accepted
8. S. P Adhikari et. al., CN/SNO paper, ChemSusChem, (Submitted)

Overall water splitting is a huge challenge for the semiconductor photocatalysts. Herein, we investigated the high effective photocatalytic overall water stoichiometrically splitting into H₂ and O₂ activity using the SnO_x-NiGa₂O₄ (SNG) composites photocatalysts. Because of the effective charge separation and transfer in SnO_x-NiGa₂O₄ composites, the photocatalytic activity of composites photocatalysts (y=80) can reach up to more than one order of magnitude greater than that of NiGa₂O₄ (NGO) or SnO_x alone respectively. In addition, under visible light irradiation the photocatalysts also displayed well both photocatalytic hydrogen evolution and the fluorescence intensity experiments of 2-hydroxy terephthalic acid. More importantly, we further elucidated the essential band gap relation between the SnO_x and NiGa₂O₄ in the heterostructure, and a deep understanding of the charge separation mechanism based on the band alignment in such system was provided. Our study demonstrates great potential of the SnO_x-NiGa₂O₄ composites to be an attractive photocatalysts for the overall water splitting or pollution degradation under visible light irradiation.

A variety of electrocatalysts/photocatalysts (ranging from semiconducting to supramolecular, viz., metal oxides, nitrides, phosphides, sulphides; Pt- and ruthenium complexes) are being explored for solar water splitting, but their overall eficiency is limited. Such limitation of the efficiency is mainly due to the poor control over the recombinations of photogenerated charge carriers (excitons). To minimize the recombination losses, the “bulk-heterojunction” strategy in which a mixture of donor (viz. regioregular poly (3-hexylthiophene) (P3HT)) and acceptors (viz. (6, 6) phenyl C61 butyric acid methyl ester (PCBM)) are used to increase donor-acceptor interface and enhance exciton dissociation is being explored. With this motivation, we have synthesized P3HT-coupled CdS heterostructured by an inexpensive chemical bath deposition approach followed by drop casting. Structural characterization of CdS thin film undertaken using XRD and SEM and TEM suggests the formation of hexagonal phase and highly porous thin film, respectively. The shift of Π*(C=C) and σ* (C-C) peaks toward lower energy losses and prominent presence of σ* (C-H) in the case of P3HT-CdS observed in electron energy loss spectrum implies the formation of heterostructured P3HT-CdS. The current density recorded under illumination for the 0.2 wt % P3HT-CdS photoelectrode is 3 times higher than that of unmodified CdS and other loading concentration of P3HT coupled CdS photoelectrodes. The solar hydrogen generation studies using optimized photocatalyst (0.2 wt % P3HT-CdS) show drastic enhancement in the hydrogen generation rate i.e. 4108 µmol h^-1g^-1. The improvement in the photocatalytic activity of 0.2 wt% P3HT-CdS photocatalyst is ascribed to improved charge separation lead by the unison of shorter life time (τ₁ = 0.25 ns) of excitons, higher degree of band bending and increased donor density as revealed by transient photoluminescence studies. The detailed results on structural, optical, photoelectrochemical characterization and charge carrier dynamics at the interface of the P3HT-CdS photocatalysts will be presented.

Structuring semiconductor materials on the nanoscale to improve photocatalytic activity has gained increasing attention in recent years.
One strategy is the syntheses and in-situ formation of semiconductor multiphase or multicomponent heterojunctions to reduce charge carrier recombination via vectorial charge transfer in photocatalytic reactions. To enable efficient interfacial contact in semiconductor composites, several strategies will be presented to prepare such composite systems. For example, a three-component composite consisting of Ba₅Ta₄O₁₅ / Ba₃Ta₅O₁₅ / BaTa₂O₆, shows superior overall water splitting without any co-catalyst over Rh-modified Ba₅Ta₄O₁₅.
Nanofibers can be of advantage compared to nanoparticulate systems, exhibiting a high aspect ratio and surface area for good photocatalytic activity, however being usually thick enough to avoid band gap increase due to quantum confinement. We have prepared nanofibers of the complex mixed-oxide photocatalysts Ba₅Ta₄O₁₅, Ba₅Ta₂Nb₂O₁₅ and Ba₅Nb₄O₁₅ via electrospinning. The formation mechanism of the nanofibers will be presented in detail. Superior photocatalytic activity for hydrogen generation and overall water splitting was found compared to simple powder samples, the latter upon photodeposition of a Rh-Cr₂O₃ co-catalyst system.
Enabling mesoporosity in oxide photocatalysts is a third strategy to improve photocatalytic activity, by increasing the active surface area. However, there is no example for a mesoporous quaternary oxide photocatalyst. A soft-templating approach will be presented for highly crystalline mesoporous CsTaWO₆ and its photocatalytic performance will be presented, strongly depending on accessible pore diameter.

Photoelectrochemical (PEC) solar cells enable the conversion of sunlight into the chemical energy by means of solar water splitting. Low conversion efficiency and high manufacturing cost limit the application of PEC systems. In this work, we aimed to increase the efficiency of the zinc oxide (ZnO) nanowire (nw) photoelectrodes by copper indium sulfide (CIS) sensitization. We used very cost effective manufacturing techniques to build ZnO-CIS electrodes. First chemical bath deposition has been used to fabricate ZnO nw arrays on indium tin oxide (ITO) coated glass. Then spray pyrolysis (SP) has been used to sensitize the ZnO nw. Metal oxides such as TiO₂, ZnO and WO₃ with various morphologies have been widely studied for their suitability in water splitting. However, these large band gap metal oxides face with the limited light absorption. Utilization of lower band gap inorganic semiconductors in PEC water splitting is an ideal approach to harvest more visible light. Chalcopyrite semiconductors, like CIS, are very promising as a sensitizer for PEC water splitting due to their optimum band gap for sunlight absorption (e.g. 1.5 eV for CIS). Various methods, such as successive ionic layer and electrochemical deposition have been used to sensitize the metal oxide nanostructures in literature. These methods suffer from usage of large amount of chemical solution, which is especially important when the rare metals like indium are used. On the other hand, SP used in our study uses very little amount of solution to build thin films. We used approximately 0.75 ml of precursor solution per cm² to build 1 micrometer thick CIS films, which is the lowest solution amount reported in literature. Also this method is very proper to build very large area electrodes. In order to determine the effect of the CIS sensitization on PEC performance, we performed various cycle numbers, 2, 5 and 24. In other words, increased cycle number resulted thicker films. Also we compared the PEC performance of flat and nw ZnO electrodes. As expected nw formation enhanced the open circuit potential (Eaoc) and short circuit current (Jsc). For the non-sensitized electrodes, nw formation improved solar-to-hydrogen (STH) conversion efficiency more than 6 times compared to the flat electrodes at 0 bias voltage. This is most probably due to the drastic increase on surface area of ZnO and efficient charge transport on single crystalline nanoarrays. Furthermore, CIS sensitization provided a significant gain in photocurrent compare to the bare flat ZnO and ZnO-nw electrodes. The Jsc and STH efficiency of the ZnO-nw/CIS structures improved 9 and 45 times, respectively. The maximum efficiency of 4.98% has been obtained for 5 cycle sprayed ZnO-CIS electrodes. Increasing the thickness of the CIS on the ZnO-nw over 5 cycle caused decrease in STH efficiency most probably due to the loss in 1D nanostructure. This means sensitization is more efficient than thin film formation on ZnO-nw.

Water splitting for hydrogen generation, either using electricity or sunlight as a driving force, is considered as a promising approach for the demanding energy requirements. Catalysts play a key role in water splitting reactions to facilitate efficient gas evolution. One of the biggest challenges is to develop active and stable catalysts for hydrogen evolution reaction (HER) in alkaline solutions using earth-abundant materials. Here we present the synthesis of a novel cost-effective catalyst layer for efficient electrocatalytic or photoelectrochemical (PEC) water reduction in basic solutions. The catalyst film provides a high catalytic activity with robustness for the HER in 1 M KOH electrolyte with a low overpotential to achieve 10 mA/cm² current density. The robust catalyst layer significantly improves PEC water reduction of p-Si photocathodes in 1 M KOH solution resulting in a high positive onset potential and a high photocurrent at zero potential. On the other hand, a novel passivated GaP solar cell is fabricated for solar water oxidation in 1 M KOH solution offering a high photoanodic performance. Having developed efficient photocathodes and photoanodes, we present a tandem PEC cell for overall solar water splitting in 1 M KOH with a high solar-to-hydrogen (STH) efficiency. The achieved results in this study reveal the potential of earth-abundant catalysts for hydrogen production which may pave the way for designing efficient full PEC systems for overall solar water splitting.

(In,Ga)N alloys are promising materials for water splitting due to their large absorption coefficient and the possibility to tune the bandgap energy across the entire solar spectrum. Besides, their conduction and valence band edges can straddle the H⁺/H₂ and O₂/H₂O redox potentials. However, it is difficult to fabricate thick (In,Ga)N layers with high structural perfection due to the lack of lattice-matched substrates. In contrast, in nanowires (NWs), dislocations formed at the NW/substrate interface do not propagate along the NW axis. Therefore, (In,Ga)N NWs are attractive for solar energy harvesting. Meanwhile, photocorrosion is problematic for n-type (In,Ga)N NWs.¹ Stable photocurrent (PC) for several hours with n-(In,Ga)N NWs was reported for HBr as electrolyte,² but in this case the toxic gas Br₂ can evolve instead of O₂.³ Here we propose the use of H₂O₂, which works as hole scavenger, in order to prevent corrosion and to investigate the maximal performance of (In,Ga)N NWs as photoanodes. In-situ electrochemical mass spectroscopy during cyclic voltammetry revealed stable O₂ evolution, thus verifying the prevention of photocorrosion. (In,Ga)N NWs with an average In content of 16% as determined by x-ray diffractometry showed a PC of ~3 mA/cm² at 1.2 V vs RHE. Strikingly, this is the maximum PC expected from the bandgap of In_0.16Ga_0.84N. Moreover, high incident-photon-to-current conversion efficiencies (IPCE) of 20-60% were measured for these NWs in the range of 2.4-3.6 eV. In addition, onset of the IPCE occurs at much smaller energy (1.8 eV) than expected from the average In content (2.8 eV). We also investigated a sample with 28% In content. In this case, onset of the IPCE is actually at higher energy (2.1 eV) than for 16% In but still well below the bandgap (2.4 eV). However, to incorporate more In the second sample was grown at lower temperature, resulting in pronounced lateral growth and coalescence of NWs. We explain our results as follows. Fermi level pinning at the NW sidewalls leads to lateral surface band bending. In addition, the electronic potential is modified by the inevitable compositional fluctuations in (In,Ga)N. The combination of these two effects gives rise to a radial Stark effect that enables spatially indirect transitions at much lower energy than the bandgap.⁴ Thus, light is absorbed in a broader spectral range than normally possible, which in turn leads to the observed large PC. In the sample with higher In content, the morphology resembles a facetted film, and the radial Stark effect and its beneficial consequences are less eminent. In conclusion, our study demonstrates an unexpected benefit of (In,Ga)N NWs for solar energy harvesting: Much less In is needed in NWs than in planar films to absorb light of low energy.
References:
¹J. Kamimura et al., J. Am. Chem. Soc. 135, 10242 (2013).
²B. Alotaibi et al., Nano Lett. 13, 4356 (2013).
³M. Finken et al., Phys. Stat. Sol. B 252, 895 (2015).
⁴J. Lähnemann et al., submitted.

Synthesizing fuels or organic substances from sunlight, CO₂ and H₂O is an ideal reaction to help alleviate global warming and fossil fuel shortages. We have proposed the novel concept of selective CO₂ reduction using a hybrid photocatalyst comprising of semiconductor (SC) photosensitizer coated with a metal complex electrocatalyst (MCE).[1,2] The SC/MCE hybrid photocatalyst combining zinc-doped indium phosphide (InP) with a ruthenium complex polymer ([Ru{4,4’-di(1H-pyrrolyl-3-propylcarbonate)2,2’-bipyridine}(CO)₂]_n, RuCP) electrocatalyst can reduce CO₂ to formate with very high selectively under visible light irradiation in water.[3] Hence, by wiring the InP/[RuCP] photocathode for CO₂ reduction with a TiO₂ photoanode for H₂O oxidation in two compartment cell separated with proton exchange membrane, solar formate production by the Z-scheme reaction utilizing only sunlight, CO₂ and H₂O was successfully realized without an external electrical bias application in previous study.[4] However, the conversion efficiency for solar energy to chemical energy (formic acid) was 0.04% over the TiO₂//InP/[RuCP] system. Technical advantage of the semiconductor/complex system is the fact that they are applicable to lots of SCs and MCEs when appropriate materials are selected to certify uphill electron transfer in a total system. As an example, in the case that the TiO₂ photoanode was replaced with reduced SrTiO₃(SrTiO_3-x), the conversion efficiency from solar energy to chemical energy was improved from 0.04 % to 0.14% using SrTiO_3-x //InP/[RuCP] system. [5]
Recently, we have succeeded in further improvement of the solar-to-chemical conversion efficiency by replacing the SC to an amorphous Si-Ge junction and introducing carbon layer as a support for the Ru-complex catalyst. A monolithic tablet-shaped [RuCP]/carbon/SiGe-jn/IrO_x device was constructed for the artificial photosynthesis in a single-compartment reactor. By immersing the device in an aqueous phosphate buffer solution saturated with CO₂ (pH 6.), a large amount of O₂ bubbles were generated over IrO_x side under simulated solar irradiation (100mW/cm², AM1.5). Simultaneously, formic acid was produced linearly during 6h irradiation with a Faradaic efficiency of 94%. Stoichiometric reaction of photogenerated holes and electrons was also confirmed in a closed circulation system. The solar conversion efficiency for formate generation was calculated to be 4.6% as a Gibbs free energy change, which exceeded those of C3 crops.[6]

References
[1] S. Sato, et al. Angew. Chem. Int. Ed. 2010, 49, 5101.
[2] T. M. Suzuki, et al., Chem. Commun. 2011, 47, 8673
[3] T. Arai, et al. Chem. Commun. 2010, 46, 6944.
[4] S. Sato et al., J. Am. Chem. Soc., 2011, 133, 15240.
[5] T. Arai et al., Energy Environ. Sci., 2013, 6, 1274.
[6] T. Arai et al., Energy Environ. Sci., 2015, 8, 1998.

The development of photocatalysts to convert CO₂ into useful organic chemicals under irradiation with sunlight is increasingly important in respect of the fossil fuel shortage and global warming problem. The photocatalytic systems are classified in two types, photoelectrochemical and powdered photocatalytic systems. Although the construction of powdered photocatalytic system is challenging, the system is a particularly attractive as a practical cost-effective system to utilize sunlight as a renewable energy source [1]. In this regard, we have demonstrated visible light-induced selective reduction of CO₂ to HCOOH using a p-type semiconductor particles (N-doped Ta₂O₅) linked with a metal-complex electrocatalyst in acetonitrile (MeCN)/triethanolamine (TEOA) with quantum efficiencies of 1.9% and more [2, 3].
Not only oxide semiconductors but also sulfides which absorb visible light can be a candidate for CO₂ photoreduction by the hybrid system. Hence n-type metal sulfide, Ni doped ZnS (ZnS:Ni, Ni 0.2 mol%) [4] was employed as a semiconductor. Hybrid photocatalysts consisting of a Ru-complex and ZnS:Ni anchored by organic groups were prepared by an adsorption method. Photocatalytic CO₂ reduction was performed in MeCN/TEOA solution under visible light (≥410 nm) irradiation. Unmodified ZnS:Ni showed H₂ evolution while not CO₂ photoreduction. In contrast, modification of various Ru-complexes greatly enhanced generation of HCOOH and CO. The photocatalytic activities were dependent on the Ru-complex, where neutrally-charged complex with phosphonate anchors at bipyridine ligand showed highest turnover number of HCOOH for the Ru-complex was 93 after 16 h irradiation. We confirmed that the HCOOH was produced from CO₂ dissolved in the solutions by isotope tracer analyses involving ¹³CO₂.
In conclusion, we demonstrate that the semiconductor/complex hybrid system for photocatalytic CO₂ reduction is applicable for not only p-type but also n-type semiconductors and that n-type sulfide can also be applied to future construction of a powdered Z-scheme system which utilizes water as an electron donor.
References
[1] T. F. Jaromillo, et al., Energy Environ. Sci., 6, 1083 (2013). [2] S. Sato, et al., Angew. Chem. Int. Ed., 49, 5101 (2010). [3] T. M. Suzuki, et al., Chem. Commun., 47, 8673 (2011). [4] A. Kudo, et al., Chem. Commun., 1371 (2000).

Efficient sunlight-driven CO₂ reduction using anodized Ag cathode

Li Qin Zhou, Chen Ling, Hongfei Jia
Materials Research Department, Toyota Research Institute of North America, 1555 Woodridge Ave., Ann Arbor, MI 48105, USA.

Replacing fossil fuels with sustainable energy sources is the key to meet the current and future energy demands of the world. Sunlight is an abundant clean energy source. The intermittent nature of available sunlight necessitates its conversion into more convenient forms of energy. Although photovoltaic devices can efficiently convert solar energy into electricity, electricity storage is less convenient. In contrast, storing energy in chemical bonds of fuel molecules is an attractive option, which has become one of the hot topics in the energy research. The concept of using sunlight to generate chemical fuel, known as “artificial photosynthesis”, has been firstly directed towards sunlight-driven water splitting to obtain H₂ fuel. Great progress has been achieved on solar hydrogen generation in the past years and solar-to-hydrogen energy conversion efficiency exceeding 10% has been demonstrated.¹ Although H₂ is an important fuel and chemical feedstock, carbon-based fuel is another prospective alternative. Conversion of carbon dioxide and water into chemical fuel using sunlight not only offers an efficient path for a sustainable energy source, but also holds the promise of closing the anthropogenic carbon cycle. However, photoelectrochemical reduction of CO₂, a true mimic of natural photosynthesis, is rather primitive compared with hydrogen technology and are drawing significantly more attention in the research community.²
In an attempt to advance the carbon-based photosynthesis, this work demonstrates a photovoltaic-driven catalytic electrolysis to efficiently reduce CO₂ to carbon monoxide using water as the electron source. The system employed two series-connected triple-junction solar cells (GaInP₂/GaAs/Ge) and high-performance catalyst electrodes. Stable solar-to-CO conversion efficiency exceeding 10% was achieved. To a great extent, this superior performance was attributed to the highly active and selective anodized Ag cathode that the authors recently developed.³ This study further validated the stable operation of the anodized Ag as a suitable catalyst for CO₂ reduction.

1. J. W. Ager, M. R. Shaner, K. A. Walczak, I. D. Sharpae and S. Ardof, Energy Environ. Sci., 2015, 8, 2811.
2. J. Ronge, T. Bosserez, D. Martel, C. Nervi, L. Boarino, F. Taulelle, G. Decher, S. Bordigac and J. A. Martens, Chem. Soc. Rev., 2014, 43, 7963.
3. L. Q. Zhou, C. Ling, M. Jones and H. Jia, ChemComm., 2015, DOI: 10.1039/C5CC06752A.

Electrochemical CO₂ reduction has attracted intense interests for as a viable route to converting CO₂ to value-added products such as CO. Silver is one of the promising CO₂ reduction catalysts because of its high CO₂ reduction selectivity to CO toward hydrogen evolution reaction, both are competitive reaction in an aqueous solution. The main gas products from silver are H₂ and CO, known as syngas, and its ratio is important to make hydrocarbons through Fischer-Tropsch process. For instance, the optimal ratio of H₂/CO to produce methanol is known for 2. The problem to make syngas by electrochemical CO₂ reduction is that the ratio of H₂/CO can be easily affected by fluctuation of applied potentials. To get fixed quality of syngas from unstable potential sources like solar and wind power, constant H₂/CO ratio should be maintained for wide potential ranges during electrochemical CO₂ reduction. Here, we present silver nanowires (AgNWs)-based electrocatalysts for CO₂ reduction reaction in an aqueous solution to relieve dramatic CO production change depending on applied potential fluctuation. The AgNWs with diameter approximately 70 nm and length of approximately 20 ~ 30 mm are prepared by spraying the AgNWs solution on a carbon paper. The various amounts of the AgNWs were collected on carbon paper by controlling spraying time. The collected AgNWs have the sheet resistance from 0.5 to 1.4 ohm/sq, depending on the amount of AgNWs loading. In order to facilitate contact between AgNWs and carbon paper, the samples were annealed at 100 ^oC. Electrochemical CO₂ reduction reaction was conducted in CO₂-saturated 0.1 M KHCO₃ solution. The AgNWs on carbon papers show relatively constant CO selectivity compared to bare Ag foils over wide ranges of applied potentials. For instance, AgNWs with sheet resistance of 1.4 ohm/sq shows 13% to 37% of Faraday efficiency (F.E.) for CO from -0.78 V (vs. RHE) to -1.18 V while Ag foil shows 13% to 86% of F.E. for CO. Also, from -1.18 V to -1.58 V, F.E. of CO of the AgNWs with sheet resistance of 1.4 ohm/sq increases only from 37% to 53% while that of the Ag foil fluctuates from 60% to 94%. AgNWs on carbon papers show relatively stable H₂/CO gas production ratio for wide potential range compared to the pure Ag foil. Detailed CO₂ reduction activity of the AgNWs on a carbon paper as a function of applied potential as a function of nanowire loading will be presented.

In this work, a template free synthesis of 3d mesoporous alpha fe2o3 has been approached. The as- synthesized materials composes additional qualities of being layered and interbraided nanostructural network. The material has been well characterized by XRD, SEM, TEM, BET adsorption studies, DRS and raman. The as-synthesized alpha Fe₂O₃ nanostructural network possess high surface area of 53 m²/g and has about 60% more light harvesting properties as compared to commercially available alpha Fe₂O₃. In addition, the 3d mesoporous nanostructural network may also contribute to enhanced multilight reflection and scattering, leading to more visible light harvesting as confirmed by diffuse reflectance spectra. Furthermore, the interbraided nanostructural network facile fast path for electron transfer aiding the photo driven conversion. In the present state of the art of photocatalysts the as-synthesized alpha Fe₂O₃ has shown substantial enhancement in the photocatalytic conversion of CO₂ to Methane at ambient conditions as compared with the commercially available alpha Fe₂O₃.

Photoelectrochemical (PEC) reduction of carbon dioxide (CO₂) with semiconductor electrodes can produce energy source (CO, HCOOH, CH₄). Although it is difficult to drive the CO₂ reduction due to the high overpotential, it has been intensively studied to overcome the problem. 3C (cubic-type) silicon carbide (SiC) has been studied as photo-electrodes because 3C-SiC has a proper band gap of 2.2 eV for solar-light absorption [1, 2]. We have reported n-type 3C-SiC anodes can drive CO₂ reduction [3]. However, the n-type anodes suffered from the surface corrosion during the photo-reaction [2]. In this study, we demonstrate that p-type 3C-SiC photocathodes can reduce CO₂ with long-time stability.
We used a 30-μm-thick p-type 3C-SiC epitaxial layer deposited on a p-type 4H-SiC substrate with a 0.7° off-axis from (0001) face. For the PEC experiments, ohmic contact was fabricated on the backside of the sample [4]. The electrolyte was 0.1 M KHCO₃ saturated CO₂ in which photoelectrode, Pt, and Ag/AgCl (in saturated KCl) were immersed as the working (cathode), counter (anode), and reference electrode, respectively. A light source was a 500 W Xenon lamp with a power density of 100 mW/cm². Before the photo-response, CO₂ was introduced by gas bubbling. The CO₂ reduction was performed with applying -1 V vs. Ag/AgCl to the p-type 3C-SiC and the produced gases were analyzed by gas chromatography.
During the PEC reaction, the photo-current remained constant (∼1.5 mA/cm²) for more than 4 hours , indicating that the p-type 3C-SiC has good stability without chemical corrosion. It is worth noting that the reaction by 3C-SiC is mainly driven with visible light due to its band-gap. The product gases from the cathode consisted of H₂ and CO. We found that the selectivity of the product gas was H₂ : CO = 90 : 10. To prove the origin of the CO gas was the CO₂ reduction, we performed experiments with and without CO₂ injection into electrolyte and photo irradiation. It was found that the CO gas was produced only under the condition with both the CO₂ injection and photo irradiation, which is an unambiguous proof that the 3C-SiC photo-electrode can drive the CO₂ reduction. The selectivity of CO is still low, and most of photo-current is used by water splitting due to higher overpotential of the CO production than H₂ production or preferable adsorption of proton on the electrode surface compared with CO₂. One of the approaches to improve the selectivity of the CO₂ reduction is utilization of metal co-catalysts [5] on the p-type 3C-SiC.

[1] M. Kato et al., Int. J. Hydrogen Energy 39, 4845(2014). [2] J. T. Song et al., Jpn. J. Appl. Phys. 53 05FZ04(2014). [3] J. T. Song et al., Jpn. J. Appl. Phys. 54 04DR05(2015). [4] N. Ichikawa al., Appl. Phys. Express 8, 091301(2015). [5] Y. Hori et al., Electrochem. Acta, 39, 1833(1994).

Solar energy can be converted and stored in chemicals such as fuels with an artificial photosynthesis system using CO ₂ and water as feed-stock molecules. To achieve an efficient artificial photosynthesis system, an integrated device composed of photovoltaic cells and catalytic (photo)electrodes has been suggested. Recently, we developed a stand-alone monolithic solar-fuel device in which CIGS photovoltaic cell modules was incorporated with Au and Co ₃O ₄ catalytic electrodes to convert CO ₂ and H ₂O to CO and O ₂, respectively, and its solar to CO conversion efficiency was achieved over 4 %. ¹ Characterization of the solar-to-fuel device proves that development of efficient electro-catalysts is crucial to improve the final conversion efficiency. Especially, electrochemical reduction of CO ₂ in an aqueous electrolyte is challenging due to a requirement of high overpotential, competitive hydrogen evolution reaction, and poor product selectivity. Here, we demonstrate selective and efficient electrocatalysts for CO ₂ reduction with monodispersed Ag nanoparticles. These electrocatalysts were prepared by a direct one-pot method using a cysteamine anchoring agent which immobilizes particles on the carbon support. 5 nm Ag/C exhibits the highest CO ₂ reduction activity with low overpotential, and high mass and specific activity compared to 3 and 10 nm Ag/C, and Ag foil electrodes. 4.8 times enhanced Faradaic efficiency was achieved with 5 nm Ag/C compared to Ag foil. X-ray photoelectron spectroscopy analysis and density functional theory (DFT) calculation suggest that the specific interaction between Ag nanoparticle surface and the cysteamine anchoring agents contributes to the improved catalytic activities through a selectively higher affinity between the reaction intermediates. In addition, electrochemically prepared Ag nanostructure electrodes also showed that the subtle modulation of the Ag surface related to the oxygen species may cause the enhanced catalytic activity and durability.

Reference

1. H. S. Jeon et al. J. Mater. Chem. A. 2015, 3, 5835.

The interface of a semiconductor with the aqueous electrolyte and catalysts is crucial for its performance in direct solar water splitting. III-V semiconductors currently enable highest solar-to-hydrogen efficiencies for both direct water splitting and configurations using photovoltaics coupled to dark electrolysis due to their tunable optoelectronic properties [1,2]. Their surface is, however, not intrinsically stable against (photo)electrochemical corrosion, which also impedes the charge-transfer efficiency over the solid—liquid interface.

Based on surface chemistry considerations derived from model experiments in ultra-high vacuum [3], we developed an in situ surface-functionalisation processing sequence for a Ga_xIn_1-xP/Ga_xIn_1-xAs tandem absorber. Photoelectrochemical benchmarking combined with photoemission show that chemical and electronic passivation can be controlled by photoelectrochemical surface modifications on a sub-nanometre scale, involving the formation of phosphate species. Here, we will present the performance-critical steps of the in situ functionalisation in context with insights from photoelectron spectroscopy.

[1] M.M. May et al. Nat. Comm. 6 (2015) 8286.
[2] A. Nakamura et al. Appl. Phys. Express 8 (2015) 107101, .
[3] M.M. May, H.-J. Lewerenz, and T. Hannappel. J. Phys. Chem. C. 118 (2014) 19032-19041.

While photoelectrochemical (PEC) cells based on III-V semiconductor photocathodes have been demonstrated to achieve highly efficient solar-to-hydrogen conversion, photocorrosion of the electrodes in electrolyte solution remains a significant challenge. In order to improve durability while retaining high solar conversion efficiency of these devices, fundamental knowledge of chemical processes at the electrode-electrolyte interface, and their correlation with electronic properties is essential. One such process is surface oxidation which occurs natively and is thought to be connected to device stability and performance.

In this talk, we investigate oxidation processes of two representative III-V semiconductor electrodes, i.e. GaP and InP, at the interface with water using a combination of first-principles molecular dynamics simulations and advanced electronic structrure methods beyond density functional theory (DFT). Using this approach, we analyze the correlation between the surface structure and electronic properties of the interfaces, with focus on chemical composition and local topology. We demonstrate how subtle features in the surface mophology can significantly affect the band edge positions of the semiconductors due to the change in the surface polarization. Implications for the efficiency and stability of the III-V semiconductor photoelectrodes will be discussed.

This work was supported by the U.S. Department of Energy at the Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Greater solar-to-hydrogen (STH) conversion efficiencies are achieved with higher photocurrent densities made possible by lower band gap absorbers. The inherent current-for-voltage tradeoff is bound by the constraint of maintaining sufficient voltage for spontaneous water splitting. Thus, maximum STH efficiency is achievable only when voltage is optimized. We investigate the effects of doping density and profile in epitaxially-grown GaInP₂ photocathode test structures by characterizing their photoelectrochemical performance and spectral response. We show that cathodic photocurrent onset potential can be enhanced by as much as few hundred mV over traditional p-GaInP₂ that has uniform doping. Band bending calculations and comparisons to solid-state analogues will be discussed to help understand the enhancement.

The highest efficiency 24.4% for solar-to-hydrogen (STH) energy conversion was obtained under natural sunlight by connecting concentrator photovoltaic (CPV) modules and polymer-electrolyte electrochemical (EC) cells. The CPV modules contained InGaP/GaAs/Ge 3-junction cells at the focal spot of sunlight and were mounted on a sun tracker. The power generation efficiency was approximately 31% under the operating conditions. The DC connection between the CPV modules and the EC cells with the almost optimized number of elements in series made the operation point of the CPV modules to be quite close to the power maximum point.
The STH efficiency is the product among 1) the power conversion efficiency of CPV modules at the power maximum point, 2) the ratio of the operation power (the operation voltage × the operation current) to the maximum power of the PV module, 3) the ratio of the redox potential of H₂ evolution (1.23 V) to the operation point voltage, and 4) Faraday efficiency. Under the best condition, the efficiency values were 1) 0.31, 2) 1.0, 3) 0.81 and 4) 0.98, leading to 0.24 as an STH efficiency. The values were quite stable and reproducible during the operation for hours, provided that the direct normal irradiance of sunlight was stable. These values of elementary efficiencies clarify that the bottleneck of STH efficiency exists primarily in the CPV modules, and secondary in the overpotential of the EC cells. In other words, only the materials/structures for high-efficiency photovoltaic operation can allow us to obtain high STH efficiency, which is true not only for PV-electrolysis approach such as the one in this work but also for photochemical or photoelectrochemical approaches in which semiconductor light absorbers exist in an electrolyte.
The results emphasized the general requirements for obtaining the maximum STH efficiency: 1) high-quality semiconductor materials and the optimized device structure for maximizing photovoltaic efficiency, 2) the best electrochemical catalyst and fast-enough mass transport of H⁺ or OH^- ions between the cathode and the anode for the minimum over-potential of water electrolysis, and 3) the best matching between the power-maximum voltage of the photovoltaic element and the operation voltage between the cathode and the anode. The third requirement can be satisfied more easily by PV-electrolysis approach but it is accompanied by the problem of power transmission loss. The design optimization in a system level will be a key to the real implementation of solar-powered hydrogen production.

A stable, high-performance photo(electro)chemical device for converting solar energy to energy dense fuels such as hydrogen would offer a promising path to address renewable energy intermittency.¹ While solar cells based on III-V semiconductors are the most efficient photovoltaic devices to date, their chemical instability under the typically strongly oxidizing or reducing photoelectrochemical (PEC) conditions limits their practical application. Protection of traditional semiconductors for use in photoelectrochemistry has thus been an active pursuit since early demonstrations of transient solar-to-hydrogen efficiencies above 10% using III-V solar cells,² with significant recent progress using amorphous^3,4 or crystalline oxides^5–7 and chalcogenides.^8,9 Still, such schemes typically require an additional, expensive catalytic layer.

In this work, we present a stable, catalyst-free oxide-on-III-V device that converts up to 50% of incident photons to hydrogen in neutral pH at the thermodynamic hydrogen evolution potential (0V vs. the reversible hydrogen electrode, RHE). All photocurrent is successfully converted to hydrogen with 100% Faradaic efficiency and an onset potential of ~0.3V RHE. We demonstrate that in the crystalline oxide on III-V platform, gains in photovoltaic performance of the underlying solar cell translate 1:1 to improved solar-to-hydrogen efficiency. By analyzing where potential drops remain within the device, we develop a roadmap for engineering the band structure of the crystalline oxide-on-III-V to capture large gains in solar-to-hydrogen efficiency. With proof-of-concept in hand, we demonstrate that integrating functional oxide surface layers and traditional semiconductor photovoltaics via low-loss interfaces provides a powerful and tunable platform for developing photoelectrochemical systems for solar fuels.

¹ D.G. Nocera and N.S. Lewis, Proc. Natl. Acad. Sci. 103, 15729 (2006).
² O. Khaselev and J.A. Turner, Science 280, 425 (1998).
³ E. Verlage, S. Hu, R. Liu, R.J.R. Jones, K. Sun, C. Xiang, N.S. Lewis, and H.A. Atwater, Energy Environ. Sci. doi:10.1039/C5EE01786F (2015).
⁴ S. Hu, M.R. Shaner, J. a Beardslee, M. Lichterman, B.S. Brunschwig, and N.S. Lewis, Science 344, 1005 (2014).
⁵ B. Seger, T. Pedersen, A.B. Laursen, P.C.K. Vesborg, O. Hansen, and I. Chorkendorff, J. Am. Chem. Soc. 135, 1057 (2013).
⁶ M. Malizia, B. Seger, I. Chorkendorff, and P.C.K. Vesborg, J. Mater. Chem. A 2, 6847 (2014).
⁷ L. Ji, M.D. McDaniel, S. Wang, A.B. Posadas, X. Li, H. Huang, J.C. Lee, A. A. Demkov, A.J. Bard, J.G. Ekerdt, and E.T. Yu, Nat. Nanotechnol. 10, 84 (2014).
⁸ A.B. Laursen, T. Pedersen, P. Malacrida, B. Seger, O. Hansen, P.C.K. Vesborg, and I. Chorkendorff, Phys. Chem. Chem. Phys. 15, 20000 (2013).
⁹ J.D. Benck, S.C. Lee, K.D. Fong, J. Kibsgaard, R. Sinclair, and T.F. Jaramillo, Adv. Energy Mater. 4, 1400739 (2014).

Charge transfer at photoelectrode interfaces is the primary event leading to energy conversion in photoelectrochemical systems. The charge-transfer event is often driven by a built-in field and produces a local transient electric field. Here, we develop transient photoreflectance (TPR) as a time-resolved spectroscopic probe that can directly monitor transient electric fields at semiconductor interfaces. We employ TPR to study p-type gallium-indium phosphide (GaInP₂), a well-known photocathode for hydrogen evolution. We monitored the formation and decay of the transient electric field that forms upon photoexcitation within bare GaInP₂, GaInP₂/platinum (Pt) and GaInP₂/amorphous titania (TiO₂) interfaces. We find that a built-in field at the GaInP₂/TiO₂ interface drives charge separation, similar to the field effects in the GaInP₂/Pt photoelectrode, but that charge recombination is hindered compared to GaInP₂/Pt due to the p-n nature of this interface compared to the Schottky nature of the GaInP₂/Pt interface. Our results provide a wealth of information about the role of amorphous TiO₂ in the photoconversion process and more generally reveal a new technique in our arsenal of ways to explore fundamental charge transfer processes at semiconductor interfaces.

Currently the energy conversion efficiency of photocatalysts, photoelectrochemical cells and photovoltaics based on wide band-gap semiconductors are limited by the spectral range of light absorption. In metal-semiconductor heterojunctions, plasmonic metals can serve as the photosensitizers to extend the light absorption range of the solar energy devices up to the near-infrared light region, which is enabled by the plasmon-induced resonant energy transfer (PIRET) and/or the plasmonic hot electron injection processes. This presentation deals with the concept, principle and applications of plasmonic metal photosensitizers

Efficiently harvesting visible and near infrared photons represents an attractive approach to improve the efficiency of photocatalysis and solar-to-fuel conversion. Plasmonic nanostructures have recently been explored for enhancing solar energy harvesting in the visible and near infrared regimes via several mechanisms, such as hot electron transfer and energy transfer. On the other hand, upconverting, doped lanthanide fluoride particles can convert longer wavelength radiation (near infrared) to shorter wavelength emission (ultraviolet and/or visible) and represent another interesting alternative for solar energy harvesting. In this talk, I will present some of our recent developments on nanohybrid photocatalysts involving plasmonic nanostructures (based on Au or Ag) and/or upconverting nanostructures and their promising applications in organic pollutant degradation and solar water splitting. Relevant hot electron transfer and energy transfer mechanisms in specific hybrid structures will also be discussed.

Hematite (α-Fe₂O₃) is a promising photoanode material for harvesting solar energy by splitting water into hydrogen and oxygen. It has a favorable bandgap energy (2.1 eV), good catalytic activity for water oxidation, low cost, is chemically stable in alkaline solutions and environmentally friendly. However, its water splitting efficiency is limited by poor charge carrier mobility and short life time of photogenerated charge carriers resulting in a short charge collection length of only 2-20 nm,^[1] much short than the extinction length of visible light in hematite (~1 µm). Our approach to overcome this problem uses thin (~20-30 nm thick) hematite films on specular back reflector substrates that give rise to resonant light trapping in the hematite film.^[2] This approach gives rise to new challenges, creating the need for an adapted fabrication sequence and deposition conditions. The hematite is typically deposited at high temperatures (> 500 °C) in oxygen atmosphere, conditions that give rise to oxidation of reactive metallic back reflectors such as Al and tarnishing of noble ones such as Ag. This contribution reports on a novel photoanode fabrication method employing film transfer and flip-over process, avoiding the degradation of the metallic back reflector. The presented method gives rise to new challenges, especially in the mechanical design of the stack, but it also opens up new opportunities such as the possibility to transfer the hematite photoanodes not only to rigid substrate but also to flexible ones. Our recent efforts and achievements in these directions will be reported in this contribution.

References
[1] J. H. Kennedy, K. W. Frese, J. Electrochem. Soc. 1978, 125, 709.
[2] H. Dotan, O. Kfir, E. Sharlin, O. Blank, M. Gross, I. Dumchin, G. Ankonina, A. Rothschild, Nat. Mater. 2013, 12, 158.

Acknowledgements: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 656132, the European Union's Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. [617516] and European Commission’s Framework Project 7 cofunded by Europe’s Fuel Cell and Hydrogen Joint Undertaking (FCH JU) under PECDEMO Grant 621252.

I will describe our recent results on employing a multi-layer photoelectrode architecture to enhance light absorption within small plasmonic particles and monolayer two-dimensional materials for solar water splitting [1,2]. Strong light-matter interactions with 10 or 30 nm plasmonic gold nanoparticles in these optical cavities result in efficient plasmon excitation and nonradiative decay into hot electrons. Our architecture circumvents the need for a Schottky barrier for charge generation. Instead, a wide band gap dielectric spacer layer, adjacent to the gold nanoparticles, serves as a blocking layer for the hot electrons that are therefore forced to be injected into the adsorbed molecules at the gold nanoparticle/ water interface. The observed photocurrents closely match the plasmon spectrum of our architecture. By contrast, negligible photocurrents are observed at short wavelengths where electrons are excited from the d-band and their final energy is too low to drive the hydrogen evolution reaction. The measured internal quantum efficiencies for 10 and 30 nm plasmonic gold nanoparticles are identical, suggesting that hot electron generation and injection efficiencies are similar and quantum size effects do not play a significant role for our structures.

[1] Direct Plasmon-Driven Photoelectrocatalysis, Hossein Robatjazi, Shah Mohammad Bahauddin, Chloe Doiron, and Isabell Thomann, Nano Letters, 2015, 15 (9), p 6155

[2] Photon management strategies for monolayer MoS₂, Shah M. Bahauddin, Hossein Robatjazi, Isabell Thomann, 2015, submitted

Efficient dye-sensitized photocathodes offer new opportunities for converting sunlight into storable energy cheaply and sustainably. We are developing dye-sensitized NiO cathodes for the photo-reduction of carbon dioxide or water to high energy products (solar fuels) using the lessons we have learnt from solar cells. Despite the infancy and complexity of this research area, we have brought about a number of exciting developments which have improved our understanding of the system. This talk will include recent work to benchmark the properties of NiO, methods to improve dyes for the p-type system and considerations in catalyst design. A major issue is the rapid charge recombination at the dye/NiO interface. Highlights from recent work examining charge-transfer between NiO and our photosensitizers using transient absorption spectroscopy, time-resolved infrared spectroscopy and resonance Raman spectroscopy will be presented.

Structured interfaces offer many advantages for photocatalytic applications. A relevant example is hydrogen evolution from water splitting at a photoactive semiconductor-liquid junction where the interface is decorated with nanocrystal catalysts to aid electron transfer. Reducing the size of catalysts allows for low mass loading of expensive materials, minimizes light reflection from the interface, and maintains the energetics of the semiconductor-liquid junction. Structuring can also impair reactions occurring at these interfaces by introducing mass transport overpotentials to the kinetics of the reaction. For reactions that evolve gas, the active surface area can be blocked by bubbles on discrete catalytic sites, possibly halting the reaction entirely. Persistent bubble residence on electrocatalysts has a negative effect on gas evolving reactions at a structured interface. Here, we explore these processes by measuring the dynamics of bubbles evolved from structured electrocatalysts at a semiconductor-electrolyte interface. We describe the use of transmission x-ray phase contrast microscopy to image gas-evolving reactions as a method to directly measure the effects of adhesion, catalyst structure, and buoyancy on electrolytically evolved bubbles. From these measurements, we develop a model for bubble evolution and transport that considers coalescence on neighboring sites, surface interactions, and the non-equilibrium shape dynamics of bubbles. This model can be used to identify favorable catalyst motifs that promote bubble clearance and mitigate their influence on reaction kinetics for water splitting applications.

Metal oxide semiconductors are promising electrode materials in photoelectrochemical (PEC) water splitting devices due to their high chemical stability and flexibility to manipulate physical properties [1]. In addition to developing their bulk properties (band gap, electrical conductivity, etc.), tailoring the oxide/electrolyte interface band edge alignment (BEA) can greatly impact the efficiency of these devices. The BEA not only defines the thermodynamic feasibility of the oxide to evolve hydrogen or oxygen, but also determines the spatial separation of photo-excited carriers within the oxide semiconductor [2]. However, independent control of the BEA from bulk properties over a wide range has remained a challenge, because it is generally fixed by the oxide electron affinity [3]. One potential way to overcome this limitation is to embed electrostatic dipole layers at the subsurface of the oxide/electrolyte interface, to create a large potential drop across atomic distances, as demonstrated recently for solid-state oxide interfaces [4].

Using atomic-scale thin film fabrication techniques, we explore this concept for (001)-oriented Nb-doped SrTiO₃ (Nb:SrTiO₃) single crystal and evaluated its BEA in basic aqueous solution. By forming ~1 nm epitaxial LaAlO₃ dipole layers capped by 2 nm SrTiO₃, we succeeded in varying the flat band potential over 1.3 V, significantly exceeding previous modulation of BEA in any semiconductor/aqueous electrolyte system. Detailed PEC characterization of these engineered photoelectrodes will be discussed in the presentation.

[1] A. Kudo, Y. Miseki, Chem. Soc. Rev. 38, 253 (2009).
[2] M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori, N. S. Lewis, Chem. Rev. 110, 6446 (2010).
[3] M. A. Butler and D. S. Ginley, J. Electrochem. Soc. 125, 228 (1978).
[4] T. Yajima, M. Minohara, C. Bell, H. Kumigashira, M. Oshima, H. Y. Hwang, and Y. Hikita, Nano Lett. 15, 1622 (2015).

Metal oxide protection layers for photoanodes may enable the development of large-scale solar fuel and solar chemical synthesis. ALD-TiO₂ remains the most widely used material because of its excellent stability under water oxidation conditions and potential for high electrical conductivity both as an ultrathin film and with thicknesses exceeding 100 nm [1-3]. However, devices fabricated with the most conductive ALD-TiO₂ films exhibit poor photovoltages of ~ 400 mV and less [3] while previous devices with higher voltage required ultrathin protection layers for efficient operation [1]. To achieve overall water splitting efficiencies over 20%, photovoltages over 600 mV with sufficiently thick TiO₂ for long-term protection are required for silicon to be a viable bottom cell in a tandem architecture.

In this work, a novel photovoltage loss is observed associated with an insulator-induced charge extraction barrier. By eliminating it, we are able to achieve photovoltages as high as 630 mV with thicker protection layers, the maximum reported to date for single-junction water-splitting silicon cells. The loss mechanism is systematically probed in MIS Schottky junction cells compared to buried junction p⁺n cells, revealing the need to maintain a characteristic hole density at the semiconductor/insulator interface. A capacitor model that predicts this loss is developed, and is related to the dielectric properties of the protective oxide, achieving excellent agreement with the data. From these findings, we extract design principles to maximize photovoltage by understanding the interplay between built-in field, extraction barriers, and interface quality [4].

Building on this work, we investigate strategies of controlling conductivity in ALD-TiO₂ to optimize fill factor without sacrificing voltage and accomplish voltages of over 600 mV with both the nSi MIS and p+n Si buried junction cell for water splitting devices uncovering new and more reliable pathways to high conductivity and overall high efficiency.

[1] Y.W. Chen, et al. Nature Mat. 2011, 10, 539-544.
[2] A. G. Scheuermann, et al. Energy Environ. Sci. 2013, 6, 2487–2496.
[3] S. Hu, et al. Science 2014, 344, 1005−1009.
[4] A.G. Scheuermann, et al. Accepted Nature Mat. 2015.

Metal-oxide protection layers for otherwise unstable photoanode materials represent a significant advancement toward large scale solar fuel synthesis. In particular, TiO₂ films grown by atomic layer deposition (ALD) have been widely successful for protecting silicon in metal-insulator-semiconductor (MIS) photoanodes. These TiO₂ films display a range of conductivities that have proven difficult to control and reproduce. Recent work, however, has shown that there exists a fundamental trade-off between the TiO₂ conductivity and the photovoltage.[i] Protective TiO₂ coatings that are only moderately conductive suffer from a hole extraction barrier at the Si/oxide interface that can decrease the photovoltage. As the TiO₂ is grown thicker, the effective photovoltage can even become negative when referencing the behavior of illuminated nSi photoanodes to that of p+Si anodes tested in the dark. Alternatively, TiO₂ coatings that are more highly conductive screen the effect of the high work-function catalyst, yielding photovoltages as low as 150-300 mV.[ii]

We have overcome this fundamental tradeoff by explicitly doping the TiO₂ with RuO₂ in an all-ALD process to create a hole-conductive metal-oxide protection layer with a high work function. These two materials are grown simultaneously in our ALD reactor using tetrakis(dimethylamido)titanium (TDMAT) / H₂O for TiO₂ growth [iii] and bis(2,4-dimethylpentadienyl)ruthenium(II) (Ru(DMPD)₂) / O₂ for RuO₂ growth.[iv] By changing the relative number of ALD cycles for each precursor, we can achieve precise control of the Ru content within the films. Using TiO₂ doped with 10-30% Ru, we have achieved average photovoltages of 500 mV for as-deposited nSi photoanodes, a significant improvement over devices with similarly conductive but un-doped TiO_2. Hole conduction through the films shows no significant ohmic loss as film thickness is increased from 3 to 24 nm. Thus, Ru-doped TiO₂ provides MIS photoanodes with both sufficient conductivity and high photovoltage. Finally, we have demonstrated that the work function is set by this Ru-doped TiO₂ layer, not by the overlying metal catalyst. This work illustrates the significant opportunity presented by ALD to tune the work function of corrosion resistant Schottky contacts in photoelectrochemical cells.


[i] Scheuermann A. et al, Nature Materials, accepted October 2015

[ii] McDowell, M.T. et al, N.S. ACS Appl. Mater. Interfaces 7, 15189-15199 (2015).

[iii] Scheuermann A. et al, ECS Transactions, 58 (10) 75-86 (2013)

[iv] Methaapanon, R. et al, J. Mater. Chem., 22, 25154 (2012)

The rising societal and environmental costs of fossil fuels have driven a resurgence of intense research interest into artificial photosynthesis. However, despite the rapid pace of progress leading to more efficient means of generating oxygen and hydrogen from water, the fundamental mechanisms are not well understood. In this work we attempt to address the non-equilibrium phenomena driving water splitting at semiconductor liquid interfaces within the context of a mesoscopic drift-diffusion theoretical model. It is shown that semiconductor carrier concentrations under active light illumination can differ significantly from those under dark current conditions, with competing recombination and charge transfer mechanisms playing a key role in solar fuel generation rates. In general, the results of this work are intended to drive the development of comprehensive engineering tools for the optimization of photocatalytic performance.

Direct conversion of solar energy to hydrogen is one of the most possible alternatives for fossil fuel. It is due to the high energy density of hydrogen, as well as its environmental friendliness by producing water as byproduct after combustion. Instead of producing hydrogen from water, obtaining through splitting of halogen acid(HX) could also produce concomitant X₂/X₃^-(X=F,Cl,Br,I) generation. Especially, produced dihalogen(X₂) is important chemical for shale gas reforming or water purification. However, due to the extremely acidic nature of HX, only few rare metals or semiconducting materials could work as stable solar splitting.
Methylammonium lead iodide(MPI) material has superior optoelectric property; large absorption coefficient throughout the solar light wavelength spectrum, small exciton binding energy, and balanced long-range charge diffusion length. However, MPI material can be easily degraded into methylammonium iodide(MAI) and lead iodide by absorbing moisture on air. Since the lead iodide cannot dissolve in water, MPI is commonly regarded as unstable materials under aqueous condition. Under hydroiodic acid condition, in contrast, it can make lead iodide dissolved into solution, lead MPI to be precipitated as powder form after saturating the hydroiodic acid solution with lead iodide and proper amount of MAI. The precipitated powder is purely MPI powder, confirmed by XRD analysis. The size of precipitated MPI has bulk powder form, size variation from sub 1um to over 30um. We also found the solubility of MPI, and dependency of solubility on lead iodide under hydroiodic acid solution.
Through UPS and absorbance study of precipitated MPI, band gap of MPI precipitated form has proper value for splitting under visible light condition of hydroiodic acid. Furthermore, by absorbing visible light of solar spectrum, MPI precipitate can produce hydrogen as well as triiodide from hydroiodic acid of solution. Without metal based cocatalyst such as platinum or gold, the hydrogen evolution is detected through gas chromatography. Despite the several times of evacuation of MPI precipitates under MAI/PbI₂ saturated system, stable hydrogen evolution could be verified.
In this study, we discovered H₂ evolution by MPI precipitates produced under MPI saturated solution. By utilizing visible light of solar spectrum, our system showed the ability to evolve hydrogen as well as triiodide. This system provides new methodology of photocatalytic hydrogen evolution reaction under hydroiodic acid condition without severe degradation of photocatalyst.

Much effort has been conducted to develop the solar water splitting system which can directly convert solar energy into storable and transportable chemical fuel, hydrogen. However, two main issues of poor stability and low solar-to-hydrogen conversion power have been limited to realize commercially viable water-splitting systems. In particular, a tradeoff relation between photovoltage and photocurrent in solar water splitting system leads to inherent constraints to increase solar-to-hydrogen conversion power.
To overcome this drawback, we have been proposed a novel concept of the hybrid system, which is consist of photoelectrochemical (PEC) and thermoelectric (TE) devices, utilizing a synergistically harnessing photon and waste heat energy, respectively. The PEC-TE hybrid system, of which PEC system are electrically connected with thermoelectric device in series, could provide an additional output voltage from a thermal gradient in TE to overcome the potential barrier required for water splitting reaction while the overall current is determined by the high photocurrent level of Si photocathode in the PEC cell. This feature in our hybrid system can decouple the tradeoff relation between photovoltage and photocurrent that can greatly improve solar-to-hydrogen conversion power. HfOx/SiOx stack layer was grown on Si photocathode to guarantee sustainable long-term water splitting with efficient charge transfer. High atomic mass of hafnium in HfOx contributed significantly to protect a silicon surface from oxidizing in acid solution. In addition, a small conduction-band offset between HfOx and Si lowered a barrier for electron tunneling, leading to a lower overpotential to drvie water splitting reaction.

Hematite (α-Fe₂O₃) is a promising photoanode in solar photoelectrochemical (PEC) water splitting with a theoretical solar-to-fuel conversion of 14-17%, which corresponds to a photocurrent of 11–14 mA cm⁻². [1-3] However, α-Fe₂O₃ performances are limited by intrinsic properties such as low conductivity and short holes diffusion length (only few nanometers). Among the methods used for improving its photocurrent, Si-doping has driven intense research during recent years delivering Si-doped α-Fe₂O₃ photoanodes with record efficiency around 3-4 mA/cm². [1] Besides the increased conductivity, the understanding of the effects produced by the introduction of tetravalent dopants such as Si⁴⁺ (or Ti⁴⁺) on structural and electrochemical properties remains elusive. [4,5]

In this contribution, we prepared α-Fe₂O₃ nanostructures through the solvothermal reaction of Fe (II) acetate in ethanol at 150°C for 15 h. Si-doped α-Fe₂O₃ samples were synthesized by adding a proper amount (1-5-10-15-20 mol%) of tetramethoxysilane as Si source. The resulting materials were all goethite (α-FeOOH) 100% in phase composition and after a thermal treatment at 550°C for 1h they turned into pure α-Fe₂O₃. SEM and TEM analysis revealed that pure α-Fe₂O₃ crystallized in hollow spheres morphology. Interestingly, the introduction of Si induced the morphology transition to Si-α-Fe₂O₃ with acicular shape. Raman, synchrotron radiation powder diffraction, XPS and EDX/STEM measurements were employed to detect the structural changes due to the Si inclusion in the α-Fe₂O₃ lattice. As the amount of Si in the α-Fe₂O₃ nanostructures increased, the atomic % of oxygen and Fe²⁺ augmented. This pointed out to a doping mechanism where the additional charge introduced by the substitution of Fe³⁺ with Si⁴⁺ was compensated both by iron valence reduction and interstitial oxygen. The variation of the atomic composition of α-Fe₂O₃ structure was reflected by increased structural disorder as detected by Raman spectroscopy and trend of lattice atomic distances as resulting from synchrotron radiation powder diffraction.

Finally, α-Fe₂O₃ powders were deposited on FTO conductive glass through electrophorethic deposition and tested in a traditional three-electrodes PEC cell under AM1.5G illumination. Voltammetric analysis were performed observing an optimum of 1% Si-doping. Through impedance measurements the charge transfer resistance and donor density were extracted and correlated to Si-content, structure and morphology of α-Fe₂O₃ nanostructures.



[1] S. C.Warren et al. Nat. Mater. 2013, 12, 842–849. [2] M. Marelli et al. Appl. Mater. Interfaces 2014, 6, 11997–12004. [3] F. Malara et al. ACS Catal. 2015, 5, 5292−5300. [4] A. Kay et al. J. Am. Chem. Soc. 2006, 128, 15714−15721. [5] D. Monllor-Satoca et al. Energy Environ. Sci. 2015, DOI: 10.1039/C5EE01679G.

Amorphous TiO2 deposited by atomic layer deposition (ALD) has been recently used as a means to protect photoanodes from harsh operating conditions. The use of this protection layer has enabled the use of semiconductors (e.g. GaAs, GaP, CdTe) that were previously not usable due to their stability. The TiO2’s intrinsic stability makes it an ideal candidate for as a protecting layer. However, the band alignment of TiO2 should make it such that photogenerated holes are not able to pass though the TiO2, but that's not the case as photogenerated holes are able to. It has been suggested that mid-gap states are the source of the TiO2’s ability to allow photogenerated holes to traverse through the dielectric; however, these mid-gap states and their relation to the transport properties of this amorphous film have not been fully investigated. Thus, in this work we grow the amorphous TiO2 film on p+ and n+ Si, demonstrate the role of the mid-gap states towards conduction, the relation of these mid-gap states to the top contact as well as adventitious integration methods to enable the TiO2 for water oxidation in acidic environments.

Despite its potential economic advantages, integrated solar-driven water splitting, different from the discrete design, has suffered from severe degradation under operation conditions either in strong acid or alkaline electrolyte. Since the very first demonstration of a high-efficiency III-V material based photovoltaic-biased photoelectrosynthetic cell by John Tuner in 1998, there have been tremendous efforts in understanding the photocorrosion and photopassivation processes of materials especially those based on traditional photovoltaic semiconductors, as well as on developing protective coatings to enable the unstable semiconductors for integrated solar water splitting. Outstanding performances of these protective strategies have been mainly achieved on crystalline Si, for example, using an ultrathin catalytic metal film, an ultrathin tunneling oxide (SiO_x+TiO_x, or SiO_x+CoO_x), a thick TiO₂ film, and a multifunctional NiO_x film. Despite all these successes, the lifetime of unassisted high-efficiency integrated devices is still limited. In this work, we report our effort in developing efficient, unassisted, and integrated solar water-splitting cell with large active surface area and benchmarking stability operating at a constant pH gradient.

In the past 5 years, BiVO₄ has emerged as one of the most promising photoanode materials for solar water splitting. One of the main limitations of BiVO₄ is its poor carrier transport, as reflected in the low carrier mobility value of ~10^-2 cm²/Vs [1]. To compensate this, many researchers have introduced donor-type dopants to increase the carrier concentration. Tungsten (W) and molybdenum (Mo) are among the most effective dopants; record AM1.5 photocurrents have been reported with W- or Mo-doped BiVO₄ photoanodes. While efforts on donor doping of BiVO₄ are numerous, there is practically no report on an acceptor-doped BiVO₄ photoelectrode. This may be simply caused by the less obvious motivation for such modification, but a p-type BiVO₄ could open up further material and/or device development possibilities. It offers more control of the material properties; p-n or p-i-n homojunctions based on BiVO₄ can be fabricated. The higher internal electric field is expected to improve the carrier separation within the BiVO₄ film.
We have synthesized a thin film of Ca-doped BiVO₄ photoanode by spray pyrolysis. A DFT calculation has shown that introducing Ca atoms on Bi sites lead to the formation of very shallow acceptors [2]. Upon doping up to 1 at% Ca, only monoclinic BiVO₄ phase were detected with X-ray diffraction, and no noticeable change in the optical and morphological properties was observed. The change in open circuit potential (OCP) upon illumination, however, shows an interesting behavior. In an undoped BiVO₄, OCP shifts to a more negative potential upon illumination, which indicates the flattening of a downward band bending. For the case of the Ca-doped BiVO₄, the exact opposite is observed, i.e., OCP shifts to a more positive potential upon illumination. This result suggests that an upward band bending, which is typical of a p-type semiconductor-electrolyte junction, is present in the Ca-doped BiVO₄ film. To fully understand this interesting observation, we have performed high kinetic energy (2-6 keV) X-ray and ultraviolet photoelectron spectroscopy measurements on the undoped and Ca-doped BiVO₄. The detailed analysis of these measurements will be presented. Finally, we have fabricated a homojunction consisting of undoped and Ca-doped BiVO₄, and an improvement of up to a factor of 2 in the carrier separation efficiency has been successfully achieved.
[1] Abdi et al., J Phys. Chem. Lett. 4 (2013) 2752
[2] Yin et al., Phys. Rev. B 83 (2011) 155102

Semiconductor based heterojunctions have been shown to be effective photocatalytic materials to overcome the drawbacks of low photocatalytic efficiency that results from electron−hole recombination and narrow photo-response range.^1,2 A novel visible-light-driven g-C₃N₄/Sr₂Nb₂O₇ nanocomposite photocatalysts were fabricated by direct heating melamine and hydrothermally synthesized strontium niobate (Sr₂Nb₂O₇) nanoparticles.³ The composites were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), UV-vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) surface area measurements, Fourier transform infrared spectroscopy (FT-IR), photoluminescence spectra (PL), and electrochemical impedance spectroscopy (EIS) to better understand its structure, composition, morphology, and optical properties. The photocatalytic performance was evaluated for hydrogen generation from water under visible light illumination. The results show that as-prepared composite photocatalysts extend light absorption range and enhances the life time of photogenerated charge-carriers resulting in enhanced photocatalytic activity compared to pristine g-C₃N₄. The plausible mechanism ) for the enhanced photocatalytic activity for the heterostructured composites is proposed based on observed activity and band positions calculations.
References
1. R Marschall, Adv. Func. Mater., 2014, 24, 2421-2440
2. H. wang, L. zhang, Z. Chen, J. Hu, S. Li, Z. Wang, J. Liu and X. Wang, Chem. Soc. Rev., 2014, 43, 5234-5244.
3. S. P Adhikari et.al., ChemSusChem, (Submitted)

For the first time, vertically grown ZnSe nanowires were successfully synthesized on a conducting substrate by a low-temperature solution method. Bi catalysts were used for the solution-liquid-solid (SLS) growth of ZnSe nanowires. They were highly crystalline and in zinc blende structure. Their average diameter and length were ~30 nm and ~1 mm, respectively. There exist only a few papers reporting the application of ZnSe for water splitting. The role of ZnSe (~2.7 eV) was only sensitizing TiO₂ having wide band gap (~3.2 eV) under visible light illumination. In this study we demonstrate the combination of n-ZnSe and n-CdS as a core/shell heterostructure for the construction of effective Z-scheme water splitting. Both ZnSe and CdS (energy band gap of ~2.5 eV) can be activated under visible light. At the thermal equilibrium, to meet the Fermi energy level the energy bands of ZnSe shift down to positive potential and those of CdS shift up to the negative potential. The excited electrons in ZnSe can move to a Pt electrode and reduce hydrogen ions to evolve hydrogen gas, while the excited electrons in CdS can combine with holes in ZnSe at the ZnSe/CdS interface. The holes in CdS can oxidize water to evolve oxygen gas. The excited electrons from ZnSe/CdS core/shell nanowires could transfer to the Pt electrode through FTO substrates and the photocurrent density was measured to be ~2.9 mA/cm² at 0 bias voltage (vs. Ag/AgCl) under visible light irradiation. This photocurrent value is an almost two times higher value than that from only n-ZnSe nanowire anodes. This increased photocurrent density could originate from the effective electron and hole separation of ZnSe by the formation of Z-scheme and increased reduction and oxidation potentials by energy band aligning between ZnSe and CdS. Here, we report that the first successful growth of ZnSe nanowires on a glass substrate by a solution method. Large-area nanowire photoanodes were achieved using the low-temperature SLS growth and the Z-scheme band alignment of n-ZnSe/n-CdS induced significant improvement of photoelectrochemical performance.

We have developed a tandem photoelectrochemical cell which is composed of WO₃/dye-sensitized TiO₂ (WO₃/dye) dual absorber photoanode and copper oxide (Cu₂O/CuO) wire array cathode. This tandem cell is demonstrated as a stand-alone and durable device for CO₂ photoelectrochemical reduction. The Cu₂O/CuO wire arrays exhibits a superior electrocatalytic activity of CO₂ reduction compared to metallic Cu (Cu⁰). Upon irradiation of a simulated light (AM 1.5; 100 mW/cm²), the single absorber system (WO₃ and Cu₂O/CuO couples) shows low open circuit potential at -0.2 V (0.1 M potassium bicarbonate; pH 6.8; carbon dioxide-purged). In the dual absorber system, the long wavelength (l > 450 nm) passed through the semi-transparent WO₃ film is absorbed by dye. The tandem PEC cell shows open circuit potential gain of ~0.7 V, which can drive the CO₂ conversion without any external bias. The primary CO₂ conversion product is CO with energy efficiency ~3.5 % while H₂ and low amount of formic acid are obtained with the energy efficiencies of ~0.9 % and ~0.35 % in 5 hours, respectively. The significant low yield of formic acid is attributed to the limited availability of proton/electron pairs (H*) at less negative potentials leading to predominant CO formation, as well as at high negative potentials due to predominant H₂ production. Neither CO₂ conversion products nor H₂ are not found in the single absorber system.

Solar-to-fuels production has attracted substantial attention due to a need for developing sustainable future energy as well as chemical resources. In this study, we successfully demonstrate a highly efficient solar-to-fuel conversion device using CO₂ and water as feedstocks. Cu(In_xGa_1-x)(S_ySe_1-y)₂ (CIGS) thin film module technology was applied to provide the sufficient power to split water and CO₂. Unlike the commercialized CIGS thin film solar cell technology we adapted a low cost solution based method for fabricating the CIGS absorber film which is expected to be more beneficial in terms of cost, throughput, and scale-up. On reverse side of this CIGS module, Co₃O₄ electrocatalytic film for water oxidation was prepared based on a low temperature coating method in order to not damage the performance of the pre-made CIGS module part. Nanostructured gold was applied as the electrocatalyst for selective reduction of CO₂ to CO, since it requires relatively low overpotential with a high faradaic efficiency for CO₂ reduction in aqueous media. Each component was successfully incorporated into a solar-to-fuel conversion device, with only ~5% coupling efficiency loss. Remarkably, the actual solar-to-fuel conversion device has 4.23% solar-to-CO chemical conversion efficiency, which is comparable to that of photosynthesis in nature.

Semiconducting metal-oxides, such as TiO₂, Fe₂O₃, WO₃ and SrTiO_3, are leading candidates for hydrogen production through solar water splitting. One of the most promising materials for the oxygen evolution reaction (OER) is iron oxide (α-Fe₂O₃, hematite), which is stable in alkaline solutions, has a favourable bandgap energy of 2.1 eV (λ ≈ 600 nm), is a good catalyst for water oxidation, and is cheap and abundant. However, hematite also possesses two major drawbacks: the mobility of the charge carriers in α-Fe₂O₃ is low (~0.1cm²V^-1s^-1) and the lifetime of minority charge carriers is very short (~100 ps). Thus leads to short (~ 2-5 nm) diffusion length of photogenerated minority carriers (holes) and high bulk recombination. Consequently, hematite photoanodes photocurrent and efficiency are significantly lower than their theoretical limit.
Possible approach to reduce bulk recombination is by tailoring built-in electrical fields, besides the one at the photoanode/electrolyte interface. In this work, we explore this approach by several doping schemes aiming to supress the bulk recombination by generating internal electric fields. Specifically, we investigate p-n and p-i-n structures similar to conventional photovoltaic devices rather than homogenously doped layers as in conventional α-Fe₂O₃ photoanodes.
Ultrathin films of 1% Zn-doped (acceptor, p), undoped (i), and 1% Ti-doped (donor, n) α-Fe₂O₃ were deposited on FTO-coated glass substrates by pulsed laser deposition (PLD). The use of thin films reduces recombination in the photoanode and provides a high degree of structural and compositional control. SnO₂ thin film buffer layer between the substrate and the α -Fe₂O₃ films was used in order to suppress backward current that notoriously reduces the water photo-oxidation current in ultrathin film photoanodes. We observe enhancement in the plateau photocurrent by almost 20% and cathodic shift of the onset potential by 150 mV in p-n and p-i-n structures compared to their homogenously doped counterparts. These results are encouraging for the development of highly efficient α-Fe₂O₃ photoanodes for water splitting.

Harvesting solar energy to produce hydrogen from water on a large scale is an ultimate goal for sustainable energy. Plasmon-assisted photoelectrocatalysis has recently come into focus as a promising technology. It disperses noble metal nanoparticles into semiconductor catalysts, which possesses distinguish features with a Schottky junction and localized surface plasmonic resonance. The former is of benefit to charge separation and transfer whereas the latter contributes to the strong absorption of visible light and the excitation of active charge carriers. This paper aims to provide a systematic study of physical mechanisms of plasmonic photoelectrodes (PPE) as well as to rationalize experimental observations. We identify popular material systems of PPEs that have shown excellent performance on water splitting and elucidate their key features. Four common PPE materials configurations are generalized as nanostructured, embedded, buried and isolated structures respectively according to their spatial integration. The noble metal nanostructures and spatial integration have important effect on the water-splitting performance. Meanwhile, plasmonic resonances in metallic nanostructures and/or multilayer interference effects can be engineered to enhance light absorption, hot-electron injection or plasmon-induced resonance energy transfer. This work opens up a new door to photoelectrocatalysis by leveraging the advantages of plasmonics, enabling a novel route toward high efficiency solar hydrogen.

Hydrogen is regarded as one of clean energy sources, which tackles the current energy crisis and environmental issues. As a process for the utilization of the solar energy to generate hydrogen, photoelectrochemical (PEC) water splitting has received extensive research interests. Si has been widely used in the field of solar energy harvesting. In practice, the PEC performance of Si as a photoelectrode material degrades owing to its higher valence band energy than that of the water oxidation energy level and poor chemical stability in aqueous solution. Zinc indium sulfide (ZnIn2S4), a ternary chalcogenide semiconductor with excellent physical and chemical properties, its energy band levels are suitable to achieve PEC water splitting with a considerable chemical stability. Considering the advantages of Si and ZnIn2S4, it would be insightful to combine these two materials together to obtain a high performance of PEC efficiency.
In this work, we present a novel multi-junction by compositing ZnIn2S4 nanosheets on TiO2 film coated Si nnaowire arrays.[1] The Si nanowires were synthesized by metal-assisted chemical etching a Si wafer; then a TiO2 protective thin layer was deposited on the Si nanowires using an Atomic Layer Deposition (ALD) system; Finally, ZnIn2S4 nanosheets were grown on the TiO2/Si nanowires by a simple hydrothermal reaction. This junction exhibited a superior photoelectrochemical performance with a maximum photoconversion efficiency of 0.51%, which is 795 and 64 times higher than that of the bare Si wafer and nanowires, respectively. The large enhancement was attributed to the effective electron-hole separation and fast excited carrier transport within the multi-junctions due to their favorable energy band alignments with water redox potentials, and enlarged contact area for facilitating the electron transfer at the multi-junction/ electrolyte interface.

Reference:
[1] Qiong Liu, Liang Li, et al. Nano Res., 2015, DOI: 10.1007/s12274-015-0852-5.

A comparative study was done on the photo electrochemical (PEC) characteristics of InGaN/GaN multi quantum well (MQW) structures grown coaxially and uniaxially on n-GaN nanowires for water splitting applications. A cost effective method of metal-organic chemical vapor deposition (MOCVD) was used for growing both kind of heterostructures. Uniaxial InGaN/GaN MQW layers were grown on the c-plane whereas coaxial InGaN/GaN MQW layers were grown on the {1-100} sidewalls of c-axis of n-GaN nanowire. For a better understanding of the material and structural properties of both coaxially and uniaxially grown InGaN/GaN MQW structures, high-resolution transmission electron microscopy (HR-TEM) studies were carried out. In order to record the PEC measurements, a three electrode system was used: sample was used as the working electrode, Pt mesh as counter electrode and Ag/AgCl (1 M KCl) as reference electrode. Incident light with a power density of 100 mW/cm² was used as a light source during water splitting experiment. PEC measurements were carried out in a homemade quartz chamber at room temperature. Before the measurement, deionized water was purged in to the setup using Ar for 40 minutes. Using a TCD GC system (Agilent-6890N), amount of the evolved H₂ was measured. Sampling was carried out using a vacuum syringe. High purity Ar (99.99999%) was used as a carrier gas. An incident photon-to-current efficiency (IPCE) of 12 % and 8 % was recorded for coaxial and uniaxial type MQW structures, respectively, at 365 nm incident wavelength. IPCE of uniaxial type MQW showed a linear decrease whereas the coaxial structure showed almost no change at 450 nm wavelength. This tendency of the IPCE value of coaxial heterostructure was attributed to its large active area compared to the uniaxial MQW structure as well as elimination of piezoelectric fields in coaxial InGaN/GaN MQW active region.

Transition metal oxide films prepared using a low temperature atomic-layer deposition were found to be effective surface passivation layers for n-type crystalline silicon (n-Si). Particularly, an ultrathin cobalt oxide (CoO_x) film provided both surface passivation and large band bending to lightly doped n-Si interfaces, which, in-turn led to a low surface recombination and a hole selective contact for efficient charge separation/collection. Together with a recently developed multifunctional NiO_x catalyst coating, the n-Si/SiO_x,RCA/CoO_x/NiO_x photoanode exhibited enhanced blue responses and comparable photoelectrochemical performances to its homogeneous p-n junction (np⁺-Si/NiO_x) based counterparts. Specifically, the n-Si/SiO_x,RCA/CoO_x/NiO_x photoanode showed photocurrent onset potentials of -240 mV relative to the equilibrium water oxidation potential, photocurrent densities at the equilibrium water oxidation potential of 28 mA cm^-2, and benchmarking solar-to-O₂(g) conversion figure-of-merits of 2.2 ± 0.1%. Moreover, the photoanode showed a benchmarking stability of continuous solar water oxidation for >1700 h in 1.0 M KOH(aq) under 100 mW cm^-2 of simulated solar illumination with ~100% Faradaic efficiency for O₂(g) evolution. Finally, this CoO_x layer from low temperature growth studied herein represents a passivation strategy to a crystalline n-Si photovoltaic cell that is effective and stable under continuous solar illumination and in strong oxidizing environment.

The promise of generating renewable energy from sunlight, carbon dioxide, and water can be realized through the discovery of efficient and robust photoelectrochemical (PEC) cell materials, specifically photoanodes for oxygen evolution reaction (OER). We develop a theory-experiment joint data-driven discovery approach for rapid identification of the most promising classes of transition metal oxide photoanodes using key material parameters, such as band gap energy, band positions, and stability under operational conditions. Capitalized on the Materials Project database, a broadly-applicable multiple-layer screening workflow is built using automated density functional theory (DFT) and hybrid functional calculations of bulk electronic, surface electronic and magnetic structures. Combined with combinatorial materials synthesis and photoelectrochemistry measurements, we successfully identified several earth-abundant complex oxides that have the desirable band gap energies at or below 2 eV and excellent photocatalytic stability in electrolyte solutions. These compounds include β–Mn₂V₂O₇ and four distinct CuO-V₂O₅ phases: α–Cu₂V₂O₇, β–Cu₂V₂O₇, γ–Cu₃V₂O₈, and Cu₁₁V₆O₂₆. Interestingly, these compounds share common structure motifs with a promising photoanode BiVO₄, which has a band gap of 2.4 eV. Utilizing the massive computed data, we observe a strong correlation between structure motif and the desirable electronic structure. The structure-property correlation provides us a new design rule for the understanding and discovery of novel light absorbers and photocatalysts in the future.

In recent years, considerable efforts have been made to improve the performance of photoactive nanostructured materials for water splitting applications. Herein, we report on the assembly and use of a bacteriorhodopsin (bR)/TiO₂ nanotube array hybrid electrode system. Photoanode materials composed of � 7mm long self-ordered and vertically oriented nanotube array of titanium dioxide films were fabricated via the anodization of Ti foil in formamide electrolytes containing NH₄F at room temperature followed by sensitization of the electrodes with bR. The stability of bR on the TiO₂ surface was found to depend on the pretreatment process of the TiO₂ films. Our results demonstrate the
opportunity to fabricate fairly stable bR/TiO₂ hybrid electrodes that can be used as photoanodes for photoelectrochemical water splitting. Under AM 1.5 illumination (100 mW/cm²), the hybrid electrodes achieved a photocurrent density of 0.65 mA/cm² which is a � 50% increase over that measured for pure TiO₂ nanotubes (0.43 mA/cm²) fabricated and tested under the same conditions. In the presence of a redox electrolyte, the photocurrent increased to 0.87 mA/cm². To the best of our knowledge, this is the first report on the use of bR/TiO₂ hybrid electrodes in photoelectrochemical water oxidation cells. We believe the proton pumping property of bR can be used in a variety of applications, especially those related to third generation photovoltaic cells.

Solar fuels have a tremendous potential to reduce the world’s dependence on the declining reserves of fossil fuels, therefore, not only providing a solution to environmental problems but also geopolitical ones. Various solar fuels, in particular hydrogen, are viewed as an alternative to replace fossil fuels. Sunlight of low power density and with time variance is transformed into compact and deployable fuels.

In contrast to conventional solar power systems, we propose an entirely novel approach to solar fuel generation: Instead of electrochemically driving chemical reactions as attempted by artificial photosynthesis, we thermally overcome the activation energy. In contrast to the conventional solarthermal approach, we avoid the heating of the entire reactor and, thus, the associated high-energy losses, by locally at nanoscale creating a thermally excited environment and by in-situ light absorption and catalytic reaction. The objective of this proposed project is to take advantage of localized surface plasmon resonance (LSPR) occurring on plasmonic nanostructures to thermally drive catalytic reactions that normally require elevated temperatures to overcome the reaction activation energy.

This experimental study demonstrates the basic feasibility of performing methanol steam reforming in a cold environment by plasmon-assisted catalysis. Polystyrene spheres of 430 nm diameter were coated with a 200-nm thick layer of Au as plasmonic nanostructures. Catalytic CuO/Al₂O₃/ZnO nanoparticles (15 nm diameter) were then deposited on top of the plasmonic substrate and immersed in liquid water-methanol. When solar irradiance of 1 sun (1000 W/m²) was focused with 13-56x concentration onto the sample, strong generation of gas bubbles was visible for hours. Using known reaction rate constants of the used catalyst and an Arrhenius-type reaction rate, apparent local temperatures on the catalyst of 200-400°C were demonstrated, while the bulk liquid and reactor remained at a temperature very close to ambient (below 30°C). The hydrogen production rate increased exponentially with solar intensity, reaching hydrogen generation up to 100 L/(min m²). The product gas consisted of approximately 25% CO₂ and 75% H₂, as expected for methanol steam reforming. Reference tests with identical plasmonic substrates (without catalyst) and with catalyst only (without plasmonic structures) did not show any gas bubble generation, indicating the effective interaction between catalytic and plasmonic structures. The linear relationship between gas production rate and intensity suggests a thermally driven process, in contrast to electron-driven photocatalysis.

Global environmental deterioration has become more serious year by year and thus scientific interests in the renewable energy as environmental technology and replacement of fossil fuels have grown exponentially. Photoelectrochemical (PEC) cell consisting of semiconductor photoelectrodes that can harvest light and use this energy directly to split water, also known as photoelectrolysis or solar water splitting, is a promising renewable energy technology to produce hydrogen for uses in the future hydrogen economy. A major advantage of PEC systems is that they involve relatively simple processes steps as compared to many other H₂ production systems. Until now, a number of materials including TiO₂, WO₃, Fe₂O₃, CuWO₄, and BiVO₄ were exploited as the photoelectrode. However, the PEC performance of these single absorber materials is limited due to their large charge recombinations in bulk, interface and surface, leading low charge separation/transport efficiencies. Recently, coupling of two materials, e.g., BiVO₄/WO₃, Fe₂O₃/WO₃ and CuWO₄/WO₃, to form a type II heterojunction has been demonstrated to be a viable means to improve the PEC performance by enhancing the charge separation and transport efficiencies. For instance, R. Saito et al., demonstrated 3.04 mA/cm² at 1.23V vs. RHE in a highly concentrated carbonate electrolyte by using BiVO₄/SnO₂/WO₃ multi-composite photoanode. More recently, P. Rao et al., synthesized WO₃/BiVO₄ core/shell nanowire photoanode by using a flame vapor deposition and sol-gel coating methods which achieves ~3.1 mA/cm² at 1.23V vs. RHE.

In this study, we have prepared a triple-layer heterojunction BiVO₄/WO₃/SnO₂ photoelectrode that shows a comparable PEC performance with previously reported best-performing nanostructured BiVO₄/WO₃ heterojunction photoelectrode via a facile solution method. Interestingly, we found that the incorporation of SnO₂ nanoparticles layer in between WO₃ and FTO largely promotes electron transport and thus minimizes interfacial recombination. The impact of the SnO₂ interfacial layer was investigated in detail by TEM, hall measurement and electrochemical impedance spectroscopy (EIS) techniques. In addition, our planar-structured triple-layer photoelectrode shows a relatively high transmittance due to its low thickness (~300 nm), which benefits to couple with a solar cell to form a tandem PEC device. The overall PEC performance, especially the photocurrent onset potential (V_onset), were further improved by a reactive-ion etching (RIE) surface etching and electrocatalysts (CoO_x and NiO_x) deposition. Finally, we have demonstrated bias-free PEC water splitting by coupling of our planar-structured triple-layer photoelectrode with a perovskite solar cell, i.e., tandem configuration.

While metal oxide semiconductors display improved stability under irradiation in aqueous solutions compared to other materials, photocorrosion processes can still hinder their long-term application in solar water splitting applications. In this talk, the effects of photocorrosion will be discussed in relation to the photocatalytic hydrogen generation rates obtained in (K,La)TiO₃ perovskite (KLTO) and in ZnO photoanodes. KLTO with hyperbranched nanostructured morphology was synthesized using a hydrothermal reaction followed by acid treatment to remove La(OH)₃ side products. Higher degrees of potassium loss were obtained in samples exposed to long acid etching times, which were correlated to H₂ production rates that were less stable over time. The proposed origin of the decrease in activity is due to a proton exchange reaction in the KLTO structure, which affects the electronic structure and conduction band energy. Additionally, our investigations on the effectiveness of plasma-enhanced atomic layer deposition (PEALD) as a surface protecting layer on ZnO thin films and single crystal photoanodes will be presented. Thin PEALD deposited films of SiO₂ and Al₂O₃ with different thicknesses were evaluated and characterized with atomic force microscopy, scanning electron microscopy, and transmission electron microscopy to understand the surface and interfacial structure of the films and the mechanism of photocorrosion.

Metal oxide n-type semiconductors have been extensively explored as photoanodes for solar water oxidation because of their flexible synthesis, chemical stability and relatively low cost. However, one of the approach to overcome their limitations connected with sluggish surface water oxidation kinetics, involves use of a catalyst for the photoelectrochemical oxygen evolution reaction (OER). Identification of a new molecular oxygen evolution catalysts active in highly corrosive media is not a trivial case. N-type mesoporous tungsten trioxide, WO₃, is one of few stable semiconductor materials able to photo-electrochemically split water under solar light irradiation but it’s operational acidic environment restricts use of any catalyst to the precious metal based OER. In an attempt to overcome this limitation, adding to the acidic electrolyte of Keggin-type compounds, e.g., silico-tungstic H₄SiW₁₂O₄₀ or phospho-molybdenic H₃PMo₁₂O₄₀ acid (POMs) species was investigated in function of their electrocatalytic capabilities to drive oxygen evolution. At the employed concentration levels, 10^-4 M, the POMs were transparent to the visible wavelengths and contributed to the substantial increase of anodic photocurrents generated onto WO₃ photoanode. The presence of this catalyst in the electrolyte makes it particularly efficient for use with nanoporous structure allowing deep penetration of the catalyst dissolved in the electrolyte. Moreover, moderate doping of WO₃ has been found to enhance visible blue light absorption of tungsten trioxide and, consequently, its photocurrent conversion efficiency. Combination of those two effects allowed achieving of record high water photooxidation currents attained under simulated solar AM 1.5G irradiation. These photocurrents reach a plateau at 1-1.1 V versus reversible hydrogen electrode what opens the prospect to attain a 5% solar to hydrogen (STH) efficiency in a tandem water splitting device employing a single junction photovoltaic cell combined with the photoelectrolyser. The enhancement of anodic photocurrents will be discussed in the frame of functionalization of Na(WO₃) with the POMs and correlated with the measurements of the amounts of oxygen evolved at the modified WO₃ photoanode.

One- to three-dimensional arrangements of semiconductors on the microscale or nanoscale have been achieved to control their surface properties as well as optical properties, accordingly. Here, we present the patterned WO₃ photoelectrode prepared by the electrodeposition of WO₃ on the photolithographically patterned indium tin oxide (ITO) substrate. WO₃ is deposited by filling holes of the patterned ITO where conductive ITO is exposed, and the WO₃ deposition is optimized by controlling a passed charge for the electrodeposition. The three-dimensional deposited WO₃ can improve the light absorption and it is proved by a finite-difference time-domain (FDTD) simulation that shows a strong resonant absorption peaks due to its patterned structure. In addition, it also improves a charge separation by increasing the surface area to accommodate the water oxidation and shortening the distance that holes need to travel to the electrolyte. As the result, the current density of 0.82 mA cm^-2 at 1.5 V_RHE has been achieved on a patterned-WO₃ electrode, which is about two times of that for a flat (planar)-WO₃ electrode. The improved charge separation in patterned-WO₃ electrode compared to flat-WO₃ electrode is also supported by the photoluminescence and the photodeposition of cocatalyst.

In this contribution we present our recent work on quasi-1D hyperbranched WO₃ nanostructures exhibiting stable photocurrent values of 1.7 mA/cm² at 0.75 V vs RHE, an unmet result as such low bias voltage.^[1] The quasi-1D hyperbranched WO₃ nanostructures are fabricated by pulsed laser deposition (PLD) exploiting self assembly from the gas phase. The deposited photoanodes resemble a forest composed of individual, high aspect-ratio, tree-like structures, assembled from crystalline monoclinic WO₃ nanoparticles. These tree-like structures represent an innovative compromise between nanorods/nanotubes (better electron transport) and the conventional isotropic nanoparticle photoanodes (high surface area). Moreover, the proposed morphology is capable of scattering and trapping light, improving its effective optical density. Optical characterization and IPCE measurements confirm enhanced absorption and photoactivity at wavelengths as high as 500 nm. The excellent performances at low biases are attributed to the low density of defects and efficient transport in the crystalline structure of the hyperbranched nanostructure, as suggested by electrochemical impedance spectroscopy (EIS). This ensemble of peculiar properties candidates these quasi-1D WO₃ nanostructures as a promising material for PEC-WS per se and as an efficient electrochromic electrodes.^[2] The low charge transfer and ion diffusion resistance of the hyperbranched nanostructures allows for fast intercalation of the Li ions inside the scaffold, thus achieving a high optical modulation in the visible range (ΔT > 60%) with exceptionally fast coloration kinetics (τ_c
^[1] M. Balandeh, A. Mezzetti, A. Tacca, S. Leonardi, G. Marra, G. Divitini, C. Ducati, L. Meda, F. Di Fonzo, J. Mater. Chem. A, 2015, 3, 6110

^[2] R. Giannuzzi, M. Balandeh, A. Mezzetti, L. Meda, P. Pattathil, G. Gigli, F. Di Fonzo, M. Manca, Adv. Opt. Mater., 2015

Experiments using well-defined model catalysts under controlled conditions and utilizing a range of spectroscopic techniques for characterization of surface and interface properties and the nature and reactivity of surface-bound species in a surface science approach can greatly advance understanding of the structure and reactivity of photoelectrocatalysts for solar fuels. We report on several of our recent studies, which include investigations of the effects of dopant incorporation on the structural and chemical properties of the α-Fe₂O₃(0001) surface for water oxidation catalysis, facet-dependent activity and stability of Co₃O₄ nanocrystals towards OER, and the interaction of water with GaP(110), a semiconductor that is known to enable selective CO₂ reduction to methanol in aqueous solutions of CO₂ and nitrogen-containing heteroaromatics. For water oxidation on α-Fe₂O₃, we found that Ni doping in thin films of model catalysts caused a new termination for the films and induced formation of more stable surface-bound OH groups. For the Co₃O₄ system, we used the well-defined morphologies of nanocubes and nanooctahedra to demonstrate that the (111) surfaces vastly out-perform the (100) surfaces for OER activity (overpotential and current density). Finally we have spectroscopically identified in situ the surface-bound species on GaP(110) associated with exposure to water using ambient pressure photoelectron spectroscopy (APPES). These observations on model systems afford further analysis and discussion of the role of surface-bound species in mechanisms for catalyzed water oxidation and CO₂ reduction.

The direct conversion of solar energy into fuels, H₂ in particular, at a simple and low cost semiconductor/water junction is still a challenge. Despite the theoretical simplicity of such a device, limitations in suitable semiconductor materials have hindered its development. Recently, photoelectrochemical hydrogen production through hybrid organic/inorganic interfaces emerged as an alternative route and H₂ evolving hybrid photocathodes architectures emerged with the demonstration of the first device. In this work, we report unprecedented hybrid organic-inorganic H₂ evolving photocathodes for the direct conversion of solar light into chemical energy. The relevance of our findings can be summarized in few key points: (i) photocurrents above 3 mA/cm² at 0 V vs the reversible hydrogen electrode (RHE); (ii) optimal process stability with 100% faradaic efficiency along the whole electrode’s lifetime; (iii) excellent energetics with onset potential as high as +0.67 V vs RHE; (iv) promising operational activity of several hours and (vi) by-design compatibility for implementation in a tandem architecture. Collectively, this set of features establishes the hybrid architecture we developed well ahead of existing reports on photoelectrodes employing organic semiconductors as photoactive material and suggests that organic photoelectrochemical systems are mature to step into the realm of state-of-the-art photocathodes based on inorganic semiconductors. In the discussion we also present an in-depth study of different architectures investigating of the role of various interfaces and enlightening their effects on the optoelectronic performances of the device. The photocatalityic activity and long-term stability of a simple, catalysed, bulk heterojunction is proven and the effect on hydrogen generation performances of properly engineered selective contacts is investigated separately. Introduction of an electron selective layer is found to increase the photocurrent response while the major effect is attributed to the introduction an hole blocking layer which shifts the onset potential towards positive voltages allowing operation in a electrical region compatible with a tandem photoanode and/or a PV cell. From the knowledge learnt, we anticipate that stable operation at photocurrents close to the maximum achievable with OPV, on the order of 10 mA/cm² at positive bias, are achievable. This work opens up the way to the exploration of the rich library of organic semiconductors developed for OPVs in photoelectrochemistry and to the realization of a new generation of large area, solution processed tandem water splitting devices for renewable and low cost direct conversion of solar energy into hydrogen.

Conversion and storage of solar energy in a usable form is a pressing issue in the world’s energy economy.^[1] Employing hydrogen generated from water as an energy carrier would be an excellent choice, although water splitting is a thermodynamically unfavored reaction.^[2] In particular, the four-step, proton-coupled electron transfer reaction of water oxidation is a “kinetic bottleneck”. This kinetic hindrance manifests as an overpotential in electrochemical water oxidation experiments. Various homo- and heterogeneous catalysts are capable of mediating this reaction, but they usually are based on the rather scarce elements Ruthenium and Iridium. Compounds of earth abundant first row transition metals have been shown to also mediate this reaction.^[3] For example, amorphous nickel-oxo species have been shown to be effective water oxidation catalysts.^[4]

Motivated by these results, a partially (9.1%) Ni-substituted ZnO (ZnO:Ni) was prepared in nanoparticulate form via solvolysis of heterobimetallic di-2-pyridylmethanediolate precursors.^[5] The obtained material was grafted onto FTO substrates and used as the working electrode in cyclovoltammometric and chronoamperometric experiments in an alkaline electrolyte (1 mol/l KOH). A self-activation of the initially almost inactive material was observed, accompanied by a partial loss of crystallinity and Zn content as well as a change of color from greyish to black.^[5] Remarkably, the activated material surpassed its references (e.g., anodized solvolytically prepared NiO particles, commercial NiO particles) in activity and stability.

In order to understand the peculiarities of this material, the chemical and electronic structure of the ZnO:Ni layers was studied before and after activation by soft x-ray emission and absorption spectroscopy as well as hard x-ray photoelectron spectroscopy. While the electrolyte-induced changes in chemical structure are limited to the very surface of the NiO reference, for the ZnO:Ni materials we find evidence for a massive material loss and the conversion of the chemical environment initially dominated by ZnO into a material best described by a mixture of Ni²⁺, Ni³⁺, and a nickel hydroxide species. In our contribution, we will discuss why this material shows a catalytic activity and stability enhancement in comparison to pure anodized NiO standard.

[1] M. A. Pellow et al., Energy Environ. Sci. 2015, 8, 1938.
[2] H. Dau et al., ChemCatChem 2010, 2, 724.
[3] a) W. Hong et al., Energy Environ. Sci. 2015, 8, 1404; b) C.C.L. McCrory et al., J. Am. Chem. Soc. 2013, 135, 16977.
[4] a) A. Singh et al., Catal. Sci. Technol. 2013, 3, 1725; b) A. Singh et al., Energy Environ. Sci. 2013, 6, 579.
[5] a) J. Pfrommer et al., Angew. Chem. Int. Ed. 2014, 126, 528; b) J. Pfrommer et al., in preparation.

Conversion of solar irradiation into chemical fuels could meet our global demand for clean and renewable energy, but only if it can be accomplished at high efficiency and with abundant materials. These two demands are difficult to reconcile, as the best performing materials are usually rare or difficult to synthesize, while more abundant compounds are often plagued with shortcomings. Using photocathodes based on cuprous oxide (Cu₂O) as a case study, our efforts to address the deficiencies of this this Earth-abundant material will be detailed. Significant enhancement of photovoltage, photocurrent, and stability have been achieved through judicious design of charge-selective layers, nanostructuring of the absorber, and protection layer engineering, respectively. The role of each approach was elucidated by a variety of experimental techniques, including the unique demonstration of potential-dependent quantum efficiency measurements as a method capable of probing device characteristics including depletion depth and carrier diffusion length. A new benchmark in metal oxide-driven photoelectrolysis was achieved, enabling the demonstration of a tandem device capable of solar-to-hydrogen efficiencies exceeding 5%.

Photochemical charge generation, separation, and transport across particle interfaces are central to photoelectrochemical water splitting, a pathway to hydrogen from solar energy. Here we use surface photovoltage spectroscopy (SPS) to probe these processes in films made of BiVO4 and Rh:SrTiO3 particles on fluorine doped tin oxide (FTO) or p-doped silicon wafers. Particle films were made by drop-coating premade particle suspensions in water followed by thermal annealing. Photochemical charge separation in the films was measured as a function of layer thickness and illumination intensity, and in the presence of sacrificial electron donors. The formation of Tandem junctions is clearly observed and to result in a maximum photovoltage of up to -150 mV under 0.3 mW cm-2 illumination. Charge separation is limited by light absorption and by slow electron transport. The ability to measure the built-in potential in particle junctions promotes the development of particle-based systems for artificial photosynthesis.

The requirements of expansive platinum (Pt) based materials as electrocatalysts and the sluggish kinetics of oxygen reduction reaction (ORR) in a cathode have significantly impeded the commercialization of proton exchange membrane fuel cells. Much effort has been directly made to search for less expansive and high performance non-platinum and non-noble cathode catalysts. Chalcogenides are promising for the potential replacement of Pt based cathode catalysts because of their good electrocatalytic activity and high selectivity toward ORR in both acidic and basic media. This talk will briefly review the recent progress in research and development of chalcogenides as high ORR activity cathode catalysts for fuel cells applications. In addition, our research works in chalcogenides will also be introduced. Ruthenium selenides with Ru₈₅Se₁₅, cobalt selenides with CoSe₂ and iron selenides with FeSe₂ nanoparticles supported on commercial carbon blacks or multiwall carbon nanotubes were prepared by simple microwave assisted methods. The effects of preparation parameters such as solution pH, catalyst loading, molar ratio, pretreatment and type of carbon supports were systematically investigated. Correlations among compositions, microstructures and electrocatalytic activities are discussed.

Here, we present a reduced anodic Au thin film for enhanced electrochemical CO₂ reduction in an aqueous solution. The highly increasing dependence on fossil fuels as energy has been leading to global warming and depletion of resource. Electrochemical CO₂ reduction for converting CO₂ to chemical fuels is considered as a promising method to solve the problems. However, high overpotential is a major obstacle to reduce CO₂ due to unstable intermediate species. Furthermore, in case of aqueous CO₂ reduction, large applied potential required to overcome high overpotential instigates H₂ evolution while lowering selectivity for CO₂ reduction. In this vein, efficient catalysts are essential to succeed in CO₂ reduction. Au is one of novel catalysts for CO₂ reduction with the highest activity to produce CO in aqueous solution. In recent progresses, thick Au oxide layer formed on Au foil by applying periodic pulse potential was reported to show much lower overpotential than polycrystalline Au [1]. However, use of Au foils can result in high-cost for CO₂ reduction. Herein, we use 200 nm-thick Au films for low-cost and highly-efficient CO₂ reduction electrocatalyts. In order to lower overpotential, we investigated the effect of anodization on Au thin film deposited on Si substrate by e-beam evaporator. The surface anodization is performed in 0.1 M KHCO₃ solution under constant potential (2.3 V (vs RHE)) for various times (1, 10, 40 and 70 min). Before all evaluations of CO₂ reduction properties, AuO₂ formed on surface was removed by a reduction process. In result, proportional increase of CO₂ reduction selectivity is found from 1 to 40 min in potential range of -0.39 – -0.99 V. Anodized-Au for 40 min shows the greatest CO/H₂ of 17.5 at -0.45 V while one of bare Au is recorded to 0.4. In addition, anodization for 70 min results in similar selectivity with 40 min, but highly encourages CO production at lower potential, -0.35 V with Faradaic efficiency of 41.8 % while only H₂ is generated with bare Au. The adsorbed oxygen on Au surface should have a key role for enhancing selectivity for CO₂ reduction from XPS analysis. O1s peak position for all anodized-Au is slightly shifted to ~1 eV higher energy indicating the presence of adsorbed oxygen on the surface. The ratio of O1s peak in all components also grows in proportional to anodization time. It is similar tendency of CO₂ reduction selectivity change. From SEM observation, anodization induces aggregated Au surface and grain structure. Especially, grain boundary is clearly formed for anodized-Au for 70 min. That is, lowered overpotential for anodized-Au for 70 min might be originated from increase of active sites at grain boundary [2]. The improvement of CO₂ reduction activity for Au thin film opens the feasibility towards efficient device application such as photoelectrochemical CO₂ conversion.

References
1. Y. Chen et al., J. Am. Chem. Soc. 134, 19969 (2012)
2. X. Feng et al., J. Am. Chem. Soc. 137, 4606 (2015)

The progress of artificial photosynthesis technologies lies in the development of a low-cost and high-efficiency photocatalyst that can boost CO₂ reduction(CO₂RR) and water oxidation reaction as a potential way to convert fossil fuel into a sustainable alternative energy sources. In this study, a common organic semiconductor poly(3-hexylthiophene-2,5-diyl) (P3HT) hybrid with improved graphene oxide (iGO) acts as a co-photocatalyst under visible light irradiation in photosynthetic process. The high product yield and selectivity is demonstrated by using P3HT nanoparticles as co-catalysts. This is due to enhance visible light absorption of the catalyst from the conducting polymer. The average CO₂ reduction to chemical fuels formation rate of P3HT-iGO hybrid is more than 5-fold higher than the pristine iGO. The results show a potential and prospective application of the conjugated polymer sensitized on 2D carbon materials in solar fuel conversion and offer significant guidance to develop stable and efficient photocatalytic systems based on organic semiconductors.

Solar conversion to fuels or to electricity in semiconductors using the wide spectrum and quantum rich solar radiation field (far red-to-NIR light), which accounts for about 40% of solar energy, is a highly significant scientific and commercial mission. However, problems lie in limited NIR active photocatalysts, low efficiency, and restricted mechanisms. One main challenge towards NIR photocatalysis is the development of novel strategies for enhanced activity and new basic principles for NIR response. Biomimetics is a splendiferous strategy for scientific and technological problems. Mother Nature has already known to smartly use far red-to-NIR light via their evolved intelligent systems with superior components and elaborated architectures. In photosynthesis, a water-oxidizing cyanobacteria has evolved to use 700~740 nm light while bacterial reaction centers are able to utilize photons as ‘red’ as 1030 nm. Some pioneering endeavors discovered that certain biological systems (e.g. snake skins, butterfly wings) are able for NIR harvesting due to their unique micro/nanoarchitectures with underlying mechanisms. However, so far as we know, these NIR harvesting mechanisms in biology have never been applied for the design of new efficient NIR responsive systems.
Here we report the first demonstration of a new strategy, based on adopting nature’s NIR light responsive architectures, for an efficient far red-to-NIR plasmonic hybrid photocatalytic system. The system is constructed by controlled assembly of light-harvesting plasmonic nanoantennas (gold nanorods) on a typical photocatalytic unit having butterfly wings’ 3D architectures—a representative biological prototype. Experiments and finite-difference time-domain (FDTD) simulations demonstrate the structural effects on obvious far red-to-NIR photocatalysis enhancement (photocatalytic gaseous degradation and photocatalytic CO₂ reduction) which originates from the synergetic effects of (1) Enhancing far red-to-NIR light (700~1200 nm) harvesting ability, up to 25%. (2) Enhancing electric-field amplitude of localized surface plasmon (LSPs) to more than 3.5 times than that of the non-structured one, which promotes the rate of electron-hole pair formation, thus substantially reinforcing photocatalysis.
This research would potentially lead to an entirely new paradigm in harvesting NIR photons for practical use. We view this study to be the first step in the development of a new methodology for NIR photocatalysis via morphology control. A next step will be to tailor and engineer the NIR responsive of elaborated nanostructures with artificial nanofabrication methods such as electron beam lithography, photolithography, 3D printing technology, self-assembly and so forth. We anticipate that this bio-inspired strategy could be extended to other solar conversion systems such as solar cells, thermoelectric devices, photodetector and so forth.

Graphene is probably one of the best discoveries of the last decade which fascinated research communities globally due to its unmatched electronic, optical and mechanical properties . However, lack of an intrinsic band gap and limited amenability to chemical modification has sparked increasing interest in other 2D layered nanomaterials like MoS₂. This unlike graphene has a direct band gap of 1.9 eV, making it suitable candidate for optoelectronic and photocatalytic applications. The ease of doping and heterostructure formation with tunable and desired properties makes it an attractive alternative for graphene. Noble metal nanostructures with strong plasmonic behavior absorbs plasmonic energy to generate electron-hole pairs (EHPs). Such electron may take part in formation of H₂ from H⁺ ions as a cathodic reduction process. This is analogous to cathodic reduction of metallic substrate during electrolysis of water. When the plasmonic metallic nanostructures (such as Au, Ag, and Cu) are coupled to 2D-materials , the plasmon-excited hot electrons can be injected from the metal nanoparticles into the conduction band of the semiconductors by overcoming the Schottky barrier which is already demonstrated in Au-MoS₂ composite . This affects physical separation of the charge carriers and thereby enhances the life time of the photogenerated carriers, a prerequisite for better utilization in redox reactions. Semiconducting 2H-MoS2-Au composite can also be used for water splitting reaction but it is found that metallic single layer trigonal (1T-MoS₂) is even better than 2H-MoS₂-Au composite. The metallic 1T-MoS₂ phase can in principle replace the cathode with added benefit of possessing extremely high active surface area- a characteristic of 2D materials. However, 1T-MoS₂ is inherently unstable, plasmonic hot electrons from Au here plays a dual role of stabilizing the 1T-MoS₂ plasmon induced stress assisted conversion of 2H phase to 1T phase.

Thus by producing noble metal/alloy nanostructures-MoS₂ composite with locally distributed 1T-MoS₂ drastically enhance the HER properties superior to graphene. Microstructure and optical properties of the composite has been chacterized using SEM, HRTEM,, XRD, UV-Visible-NIR absorption spectroscopy and raman spectroscopy. photoelectrochemical properties has been characterized using current-voltage characteristics, and electrochemical impedance spectroscopy. Exchange current density and hydrogen evolution over voltage has been calculated from Tafel plot. Solar to hydrogen genaration efficiency has been calculated from gas chromatography experiment and compared with the efficiency calculated from the I-V characteristics.

Solar-driven production of hydrogen (H₂) from water splitting is considered as one of the promising ways to provide clean fuels.^1-3 There are two critical issues for high efficient photocatalytic solar energy conversion. One is the extension of light absorption into visible light region, and the other is the enhancement of quantum efficiency at each wavelength.⁴ In the former case, the light absorption band strongly depends on the intrinsic band gap of the photocatalyst materials. Regarding to the latter case, enhancement of quantum efficiency has been attempted by improving the methods of modification and synthesis. In this presetation, defects, like oxygen vacancies, have been demonstrated to enhance photocatalytic activity for TiO₂, SrTiO₃ and other oxides due to excellent charge separation efficiency within their whole absorption band.[1,2] besides the surface defect, bulk defect like Ti³⁺ doped TiO₂ can be treated as another kind of bulk defect also were invstigated.[3] Normally, these oxides only absorb UV light due to their large band gap. We also investigated the effect of defects on the photocatalytic activity of visible light photocatalyst, like sulfides, which have an extended visible light absorption from UV to visible light region (~ 550 nm).


References
(1) H. Tan, Z. Zhao, M. Niu, C. Mao, D. Cao, D. Cheng, P. Feng and Z. Sun. Nanoscale, 2014, 6, 10216.
(2) H. Tan, Z. Zhao, W. Zhu, E. N. Coker, B. Li, M. Zheng, W. Yu, H. Fan, and Z. Sun ACS Appl. Mater. Interfaces,: 2014 ,6 ,19184-19190.
(3) Z. Zhao, H. Tan, H. Zhao, Y. Lv, L. Zhou, Y. Song and Z. Sun. Chem. Commun., 2014, 50, 2755.

In the past decade, anodically formed self-organized TiO₂ nanotube layers attracted considerable scientific interest (see e.g. reviews [1,2]). This is mainly due to the fact that these layers combine the defined nanotubular geometry with a broad range of functional features inherent to TiO₂.
Here we report that TiO2 nanotube (NT) arrays, converted by a high pressure H2 treatment to anatase-like "black titania", show a high open-circuit photocatalytic hydrogen production rate without the presence of a cocatalyst. Tubes converted to black titania using classic reduction treatments (e.g., atmospheric pressure H2/Ar annealing) do not show this effect. The main difference caused by the high H2 pressure annealing is the resulting room-temperature stable, isolated Ti 3+ defect-structure created in the anatase nanotubes, as evident from electron spin resonance (ESR) investigations. This feature, absent for conventional reduction, seems thus to be responsible for activating intrinsic, cocatalytic centers that enable the observed high open-circuit hydrogen generation.

Solar-driven thermochemical production of chemical fuels using redox active oxides has emerged as an attractive means for storing solar energy for use on demand. In this process a reactive oxide is cyclically exposed at high temperatures to an inert gas, which induces partial reduction of the oxide, and to an oxidizing gas of either H₂O or CO₂, which reduces the oxide, releasing H₂ or CO. The oxidation (fuel production) step is typically carried out at a lower temperature than the reduction (oxygen release) step, constituting a temperature swing cycle, but the entire process may be carried out at close to isothermal conditions in a pressure swing cycle. Here we explore material behavior at the high temperatures (800 – 1500 °C) relevant to thermochemical fuel production. In particular, we show that typical candidate materials have such high bulk diffusivity and surface reaction rates, that global kinetic behavior is often limited, not by these parameters, but rather by the thermodynamic capacity of the reactant gas to change the oxidation state of the redox active material. We discuss the implications of this insight on strategies for material and reactor design.

Solar thermal fuels utilize molecules that undergo reversible photo-isomerization to convert solar energy into stored thermal energy [Kucharski, T. J., et al. Energy Environ. Sci. 4, 4449 (2011)]. Because solar thermal fuels produce no emissions and can store and convert energy within one material, they are an attractive option for a renewable energy source. However, it has remained a challenge to identify a suitable solar thermal fuel material that exhibits high energy density, high energy conversion efficiency, long energy storage lifetime, and can be produced at low cost. A recent proposal is a nanotemplate-photoisomer hybrid system, e.g. functionalized azobenzene, a well-known photoisomer molecule, attached to carbon nanostructure templates such as carbon nanotubes, graphene, pentacene or alkene chains. Such structures have been suggested and tested as candidate solar thermal fuel materials with high energy density and long storage time [Kolpak, A. M., et al. Nano Lett. 11, 3156–3162 (2011)., Kolpak, A. M., et al. J. Chem. Phys. 138, 034303 (2013)., Kucharski, T. J., et al. Nat. Chem. 6, 441–447 (2014)]. In this study, we aim to maximize the overlap of the absorption spectrum of the trans-isomer (stable state) with the solar spectrum while simultaneously minimizing that of the cis-isomer (metastable state) with the solar spectrum. The greater this difference of overlap, the higher the fraction of charged fuel in the photostationary state, where both forward and backward photoisomerization reactions are in a steady state. This yield of the cis-isomer in the photostationary state should be maximized in order to realize the maximum energy harvesting capacity of solar thermal fuels. We employ time-dependent density functional theory methods to analyze spectra of pyrene-azobenzene structures, and show that the template-azobenzene interaction greatly improves absorption of both trans- and cis-azobenzene. Further, by exploring a broad range of nanotemplate-azobenzene structures, we analyze and elucidate the role of both geometric confinement and electronic interactions on optical properties and use this understanding to identify candidate solar thermal fuels with improved yield upon charging.

The availability of renewable hydrogen fuel is critical to solve many of the world’s energy and environmental issues. Photocatalytic water splitting using sunlight is considered a promising route to clean and renewable hydrogen but the current efficiency of the process is still below what is needed for commercialization. TiO₂ remains the leading semiconducting material for water splitting with its good conversion efficiency and stability. Improving the light absorption and charge carrier separation in TiO₂ remains the biggest challenge. In this study, we investigate the photocatalytic water splitting activity of ultra-thin plasmonic gold films coated with anatase TiO₂ photocatalyst. A series of ultra-thin Au films with different thickness (2-20 nm) were deposited on a glass substrate and the growth and plasmonic property was studied using scanning electron microscopy (SEM) and UV-Vis absorption spectroscopy. At initial stages of film growth, isolated, 3D metal islands are formed and a continuous film is eventually formed around 10 nm. The localized surface plasmon resonance (LSPR) peaks was clearly visible for Au films with thickness < 12 nm. For films thicker than 12 nm due to delocalization of the free electrons the drude absorption is more significant which suppresses the LSPR. The photocatalytic activity of TiO₂ coated Au thin films was evaluated with 5 vol.% glycerol as sacrificial agent under UV and visible light. The H₂ evolution showed a dramatic increase from ~ 200 µmolg^-1min^-1 without Au to maximum activity for 8 nm Au films of ~ 850 µmolg^-1min^-1. Thicker films saw a drop in H₂ production which coincided with a suppression of LSPR. The water splitting activity was further evaluated in detail under UV light, using Pt thin films and also SiO₂ as an interlayer. All our results clearly indicate that the enhancement in photocatalytic activity was due to the LSPR of Au and the resulting electric field produced at the interface of Au and TiO₂. The mechanism was further supported using finite difference time domain simulations (FDTD) to study the electric field (EF) at the interface of TiO₂ and Au. With 2 nm Au film the EF enhancement is ~ 5 times at the surface of TiO₂ particle. Increasing the Au film thickness dramatically improves the EF enhancement with upto 19 times higher EF for 8 nm films and then starts dropping for thicker films. This matches the trend seen in the photocatalytic activity and LSPR measurements. This is probably the first time that such a direct correlation has been seen observed between the electric field enhancement and the photocatalytic activity.