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
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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.
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[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
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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.
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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.