Symposium EN18 : Multiscale Designing and Constructing Photocatalytic Materials for Solar Fuels

A solar-driven vanadium redox cell, consisting of a carbon cloth cathode in 2.0 M H₂SO₄(aq) with 0.36 M V₂(SO₄)₃ (pH = 14.21) for vanadium reduction, a Ni mesh anode in 2.5 M KOH(aq) (pH = -0.16) for oxygen evolution reaction (OER) and a bipolar membrane that sustains the pH differentials between the cathode and anode chamber, was constructed to decouple the hydrogen evolution reaction (HER) from OER in space and time. Highly selective reduction of V³⁺ with a Faradaic efficiency that exceeded 99.8% was observed at the carbon cloth cathode in the presence of a high concentration of protons for HER at different cathodic potentials and at a range of charging depths. The produced V²⁺ species in the cathode chamber was then passed through a MoC_x-based catalyst to produce hydrogen and to regenerate V³⁺ for subsequent reduction, with an average hydrogen generation efficiency of 85% at different depths of charging. Coupled to a solar tracker, the solar-driven vanadium redox cell was charged outdoors under real-world illumination during the day and discharged at night to produce hydrogen with a diurnal-averaged solar-to-hydrogen conversion efficiency of 5.8%.

Conjugated polymer nanostructures (CPNs) emerge as a new class of photocatalysts for organic pollutant degradation under UV and visible light. Polypyrrole (PPy), as a conjugated polymer, exhibits a wide range of applications. We present here the first illustration of employing pure PPy nanostructures as a very efficient photocatalyst for the depollution of water. PPy was synthesized in soft template by chemical polymerization (PPy-c), obtained by radiolysis (PPy-γ), and synthesized without template via chemical method (PPy-b) as bulk. Among these three samples, PPy-c shows the best photocatalytic performance under UV light, while PPy-γ exhibits the highest activity for phenol degradation under visible light. These samples have been characterized by different techniques: SEM, TEM, NanoIR, FTIR, UV-Vis spectroscopy. We modified PPy nanostructures with co-catalysts based on Pt and Ni nanoparticles for H₂ production. The modified PPy nanostructures give also promising results for hydrogen generation under ultraviolet light. The effect of the nature of the metal precursors and their concentrations were studied.

Photocatalytic H₂ evolution using particulate semiconductors is a potentially scalable and economically feasible technology to utilize solar energy. A wide absorption range, long-term stability, high charge-separation efficiency and strong redox ability are the key features for ideal H₂ evolution photocatalyst. Composited photocatalytic systems are more favorable to improve the charge separation, and our work has been focused on the C₃N₄-W₁₈O₄₉ composite for visible-light-responsive H₂ evolution semiconductor and the latter is a good visible-light-responsive oxidative semiconductor [1-6].
We tuned the morphology of W₁₈O₄₉ by the structure-directing role of solvent to improve the activity and stability [1-3]. Urchin- and nanowire-like W₁₈O₄₉ are prepared by using ethanol and n-propanol as solvent, and interestingly, when acetic acid is used as solvent and structure-directing agent, uniform porous W₁₈O₄₉ with hollow architecture is synthesized. The spheres exhibit enhanced light harvesting, high surface area and adsorption capability, and thus are most active in photocatalysis compared with other two morphologies.
We fabricated g-C₃N₄ with simultaneous novel porous network and controllable O-doping[4,5]. First melamine was pre-treated with H₂O₂ or hydrothermal treatment to form hydrogen bond-induced supramolecular aggregates, then g-C₃N₄ was obtained upon calcinations. O doping preferentially occurs on two-coordinated N position, and the porous network and O-doping synergistically promote the light harvesting and charge separation. So g-C₃N₄ synthesized from H₂O₂-treated and hydrothermal melamine shows 6.1 and 11.3 times HER activity than bulk g-C₃N₄.
We further demonstrated that C₃N₄-W₁₈O₄₉, the type-II composite, can be switched to direct Z-scheme via modulating the interfacial band bending [6]. Adsorption of triethanolamine (TEOA) on C₃N₄ surface significantly uplifts its Femi level, inverses the continuous interfacial band bending to interrupted one, and thus switches the composite from type-II to Z-scheme, without the assistance of any electron shuttles. The Z-scheme C₃N₄/W₁₈O₄₉ composites exhibit superior H₂ production compared with pure C₃N₄, and reaction rate of 8597 μmol h^-1 g^-1 (AQY of 39.1% at 420 nm) with Pt as cocatalyst and TEOA as hole scavenger [6].
References
1. Z.-F. Huang, J. J. Song, L. Pan, X. W. Zhang, L. Wang, J.-J. Zou, Adv. Mater. 27 (2015) 5309-5327.
2. Z.-F. Huang, J. J. Song, L. Pan, F. Lv, Q. Wang, J.-J. Zou, X.W. Zhang, L. Wang, Chem. Commun. 50 (2014) 10959-10962.
3. Z.-F. Huang, J.-J. Zou, L. Pan, S. Wang, X. W. Zhang, L. Wang, Appl. Catal. B: Environ. 147 (2014) 167-174.
4. Z.-F. Huang, J. J. Song, L. Pan, Z. Wang, X. Q. Zhang, J.-J. Zou, W. B. Mi, X. W. Zhang, L. Wang, Nano Energy 12 (2015) 646-656.
5. J.-W. Zhang, S. Gong, N. Mahmood, L. Pan, X. W. Zhang, J.-J. Zou, Appl. Catal. B: Environ. 221 (2018) 9-16.
6. Z.-F. Huang, J. J. Song, X. Wang, L. Pan, K. Li, X. W. Zhang, L. Wang, J.-J. Zou, Nano Energy (40) 2017 308-316.

Developing earth-abundant and efficient electrocatalysts for photoelectrochemical water splitting is critical to realize high performance solar-to-hydrogen energy conversion process. Herein, we report a novel electrocatalyst of colloidal cobalt phosphide nanocrystals (CoP₂ NCs) by a modified “hot injection” method. Benefiting from the high phosphide content, size-uniformity, and appreciable electronic conductivity, the CoP₂ NCs exhibit superior Pt-like hydrogen evolution reaction (HER) electrocatalytic activity in acidic solution with a small overpotential of 39 mV to achieve -10 mA cm^-2, and also demonstrates long-term stability with negligible activity degradation within 36 h at an overpotential of 100 mV. Theoretical DFT calculation results reveal that the free energy of hydrogen adsorption on highly exposed P-rich (211) surface of CoP₂ is close to zero, leading to the desirable balance between hydrogen adsorption and desorption for Pt-like HER behavior. Further integration of the colloidal CoP₂ NCs with an atomic layer deposition (ALD) protective planar p-Si electrode to form a hybrid p-Si/20 nm AZO/10 nm TiO₂/CoP₂ photocathode. Under simulated solar illumination (AM-1.5G, 100 mW/cm²), an onset potential of photocurrent is observed at as positive as 0.48 V vs. RHE on this hybrid photocathode. Notably, the as-deposited p-Si/20 nm AZO/10 nm TiO₂/CoP₂ photocathode shows a remarkable photocurrent density of -16.7 mA cm^-2 at the reversible hydrogen potential (0 V vs. RHE), much higher than that of p-Si/20 nm AZO/10 nm TiO₂ without cocatalyst decoration (-5.62 mA cm^-2), and also exhibits excellent durability in acidic solution. The high performance and stability are ascribed to the intimate junction between p-Si and n-type AZO for fast interfacial electron transfer and high-photovoltage output, good corrosion-resistance of pinhole-free ultrathin TiO₂ protective layer, and favorable Fermi level position and fast HER kinetics of CoP₂ cocatalyst.

We have prepared the first crystalline and 3D periodically ordered mesoporous quaternary semiconductor photocatalyst in an evaporation-induced self-assembly assisted soft-templating process. Using lab synthesized triblock-terpolymer poly(isoprene-b-styrene-b-ethylene oxide) (ISO)^[1] a highly ordered 3D interconnected alternating gyroid pore morphology was achieved exhibiting near and long-range order, as evidenced by small angle X-ray scattering, physisorption, and electron microscopy, with mesopores larger than 40 ±10 nm and a surface area of 37 m² g^-1. Moreover, we revealed the formation process on the phase-pure construction of the material’s pore-walls with its high crystallinity, which proceeds along a highly stable W⁵⁺ compound, by both in-situ and ex-situ analyses, including X-ray powder diffraction, infrared spectroscopy, and electron paramagnetic resonance. The resulting photocatalyst CsTaWO₆ with its optimum balance between surface area and ordered mesoporosity ultimately shows superior hydrogen evolution rates over its non-ordered reference in photocatalytic hydrogen production. This work will help to pioneer new self-assembly preparation pathways towards multi-element multifunctional compounds for different applications, including improved battery and sensor electrode materials.

References
[1] M. Stefik, S. Wang, R. Hovden, H. Sai, M. W. Tate, D. Muller, U. Steiner, S. M. Gruner, U. Wiesner, J. Mater. Chem. 2012, 22 (3), 1078–1087.

Perovskite compounds stand for promising functional materials for a wealth of important applications such as photocatalysis, superconductors, optoelectronics, dielectrics, etc. This is largely due to the fact that perovskite crystal structures are extremely flexible with regard to cationic or anionic replacements and are also highly diverse in atomic arrangements. These properties are particularly useful for photocatalytic solar fuel productions which involves a number of critical processes such as light absorption, charge separation, charge transferring etc. Efficient solar fuel generations therefore hinges on the optimization of these processes which needs advanced materials engineering techniques. We have investigated several perovskite oxides/oxynitrides and their derivatives for photocatalytic water splitting via structural and compositional modifications [1-7]. Several importantly properties of perovskites such as optical absorption, charge transportation and defects levels etc. can be controlled which all link to the photocatalytic performance.

References
[1] F.F. Wu, G. Liu, X.X. Xu, J. Catal. 346 (2017) 10-20.
[2] L.W. Lu, M.L. Lv, D. Wang, G. Liu, X.X. Xu, Appl. Catal. B-Environ. 200 (2017) 412-419.
[3] X.Q. Sun, X.X. Xu, Appl. Catal. B-Environ. 210 (2017) 149-159.
[4] H.M. Chen, X.X. Xu, Appl. Catal. B-Environ. 206 (2017) 35-43.
[5] L. Jiang, S. Ni, G. Liu, X.X. Xu, Appl. Catal. B-Environ. 217 (2017) 342-352.
[6] M.L. Lv, Y.W. Wang, L.W. Lu, R.N. Wang, S. Ni, G. Liu, X.X. Xu, Phys. Chem. Chem. Phys. 18 (2016) 21491-21499.
[7] X.Q. Sun, Y.H. Xie, F.F. Wu, H.M. Chen, M.L. Lv, S. Ni, G. Liu, X.X. Xu, Inorg. Chem. 54 (2015) 7445-7453.

The exploration of new materials and/or structures as efficient electrodes for industry-level photoelectrochemical (PEC) water splitting into usable H₂ fuel has attracted intense interest from the whole world. As an advanced intelligent tactic, assembling nano-building blocks into desirable structures with new hierarchy and scaling-up laws has been explored to achieve high performance and functionality. In terms of methodology, a seeding method in solution phase still confronts some engineering obstacles notwithstanding it being regarded as a major avenue to fabricate heterogeneous nanostructures of semiconductors. To elevate the spatial occupancy of one-dimensional ZnO nanostructures and overcome the limitations of multistep seeding method currently widely used, we have developed a facile procedure with high yield to fabricate “caterpillar-like” ZnO nanostructured network (CZN) for photoelectrochemical applications. Moreover, by fine-tuning the synthesis procedure and manipulating their growth process, the dependence of their photoelectrochemical properties on geometry factors of the unique CZN consisting of branched ZnO nanowires onto ZnO nanofibers with tunable surface-to-volume ratio and roughness factor has been investigated. They offer mechanically and electrically robust interconnected networks with open micrometer-scale structures and short hole diffusion length. The preferential light-material interaction and charge separation to maximize the photo-to-hydrogen conversion efficiency were further studied. When used as photoanode, our CZN not only favors sunlight harvesting with multireflection ability, but also suppresses the recombination of photogenerated charge. Compared to the literature results, our CZN photoanodes with ZnO nanobranches of ~2.2 μm in length and ~25 nm in diameter exhibited the highest photocurrent density of 0.72 mA/cm² at +1.2 V (versus Ag/AgCl) and conversion efficiency of 0.209% at +0.91 V (versus RHE) without being decorated with noble metal cocatalysts or nonmetallic/metallic dopants due to their favorable structural features. Overall, our procedure to obtain the desirable CZN provides great opportunities for facile and efficient fabrication of model photoelectrochemical anodes and would be applied to other materials for sustainable chemistry and engineering applications.

Iron oxide (α-Fe₂O₃, hematite) is a promising material for use as a photoanode in photoelectrochemical cells for solar water splitting due to its long term stability under operating conditions, cost, abundance, and visible light absorption capabilities. However, state of the art photoanodes still fall significantly short of the theoretical efficiency. This poor performance is generally attributed to short lifetime of photogenerated minority carriers, i.e. holes, primarily measured by optical pump-probe methods, and also to a reported small diffusion length, resulting in significant bulk recombination. Despite much research into hematite photoanodes, an accurate model describing the device physics and charge transport of hematite photoanodes is still lacking. The most widely used model to describe photoanode behavior is the Gartner model, which describes the photocurrent as a sum of all carriers photogenerated within the depletion region plus those generated in the bulk which are able to diffuse to the depletion region. It is widely accepted that hematite photoanodes display short depletion and diffusion lengths of only several nm, therefore most of the efforts to improve hematite performance have centered on nanostructured porous layers. In this talk, we show that, in seeming contradiction to one or more of the preceding assumptions (i.e., Gartner model, diffusion length, or depletion region width), holes generated at least 700 nm away from the surface in a thick Ti-doped hematite planar photoanode are able to reach the surface and contribute to the photocurrent. Furthermore, we show that photogeneration of holes closer to the surface does not necessarily result in higher probability of charge carrier extraction and that the wavelength dependence of the generation plays a significant role in the ability to extract charges.

Spatial heterogeneous is one of the important features for thermal catalysis on heterogeneous catalysts. These local environments strongly affect the macroscopic properties. In artificial photocatalysis, the situation becomes even more challenge and complex: most of the elementary surface reactions of photocatalysis involve charge transfer processes. The spatial distribution of photogenerated charge carriers becomes a new and important factor to determine the overall performance of photocatalysis.

Herein, making use of the newly developed photoelectrical imaging technique, the distribution of photogenerated charges on the surface and across the interface of semiconductor photocatalyst is revealed. It is found that built-in electric field plays quite important roles in the spatially separated charges. Several exemplifications will be demonstrated: the anisotropic space charge region in the different facets of single crystal semiconductor^1,2, the inverted built-in electric fields in semiconductor photocatalyst aligned by cocatalyst³, and the effectively formed built-in electric field across the nano scale interface of anatase and rutile nanoparticles⁴. Moreover, the charge separation affected by the external fields such as asymmetric illumination and local enhanced electromagnetic fields will be also discussed. These results give deep insights into the nature of photogenerated charge separation in a single semiconductor photocatalyst particle and provide the scientific basis for enhancing the performance of solar chemical-conversion devices.


References:

1. R.G. Li, F.X. Zhang, D.E. Wang, J.X. Yang ,C. Li., et al. Nat. Comm. 4 (2013), 1432;
2. J. Zhu, F.T. Fan, R.T. Chen, H.Y. An. Z.C. Feng, C. Li., Angew. Chem. 54(2015), 9111;
3. J. Zhu, S. Pang, T. Dittrich, F.T. Fan, C. Li. et al. Nano Lett. (2017), ASAP;
4. Y. Gao. J. Zhu, F.T. Fan, C. Li. et al. J. Phys. Chem. Lett. ( 2017), 8, 1419.

Unlike thermally driven catalytic reactions by metals, the reaction rates in photo-catalysis do not scale with neither the amount of metals nor with their size. Because of the complexity of multi-component photo-catalysts in powder forms, this phenomenon that has been routinely observed for over three decades, has so far no fundamental explanations. In order to probe into this, hydrogen production rates from ethanol over Au clusters with different sizes deposited on TiO₂(110) rutile single crystal, were studied by scanning tunneling microscopy (STM) and online mass spectrometry. A non-linear increase of the rate of hydrogen with increasing surface coverage of gold was observed. While Au particles with sizes ranging from 4 to 8 Å, marginally affected the reaction rate, the inter-particle distance was found to be crucial. Increasing the separation distance resulted in increasing the normalized reaction rate. These results are explained in terms of competition between particles for excited electrons to reduce hydrogen ions of surface hydroxyls to molecular hydrogen. The reason for nonlinearity is postulated to be due to two considerably different time scale, the picosecond scale (associated with Debye length) of charge transfer at the interface Au/TiO₂ and the much slower time scale of electron transfer in chemical reactions.

Semiconductor photoelectrochemistry (PEC) has great potentials in supplying the worldwide energy demands via production of hydrogen/hydrocarbon fuels using sunlight. However, semiconductor materials are prone to (photo)corrosion reactions in contact with strong aqueous acidic and alkaline electrolytes and display limited stability when flowing currents through the electrodes under light illumination. Among them, technologically-important III-V semiconductors (InP, GaAs, GaP etc.) with superior photovoltaic performances suffer from such surface corrosion processes. Herein, we systematically probe the surface corrosion chemistry of these materials occurring at the semiconductor/electrolyte interfaces in strong acidic and alkaline media. Various dark/light and open-circuit/applied-bias conditions are studied and compared to fully reveal the chemical, electrochemical and photoelectrochemical stability of these III-V semiconductors. Experimental techniques including inductively coupled plasma mass spectroscopy (ICP-MS), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and atomic-force microscopy (AFM) are employed to probe both the physical and chemical changes after their surface corrosion transformations. Combined with considerations of thermodynamic pourbaix diagrams, comprehensive understandings of their diverse (photo)corrosion behaviors are gained over different potential regions. Eventually, protection strategies are rationally devised to inhibit rapid materials corrosion and enable these semiconductors for sustainable solar fuel production.

In semiconductor photoelectrochemistry, surface states behave as both recombination centers and the site of charge transfer to a redox system [1, 2], and their characterization is vital. Impedance spectroscopy is often employed for this purpose but it can modify the surface due to reaction current. We therefore try to characterize surface states by observing open-circuit potential (OCP) as a function of irradiation light intensity. This method, which we call as illuminative OCP spectroscopy, will provides us the information on the density of surface states (DoSS) with the minimum impact of reaction current.
As a light source, He-Cd laser (325 nm, within an absorption band of GaN) was employed. In contrast to our previous study with Xe-lamp illumination [3], light intensity absorbed by GaN was controlled precisely in a wide range from 1 μW/cm² to 175 mW/cm². The OCP of n-GaN photoelectrode, corresponding to electron fermi level, almost reached the flat-band potential (FBP) at a light intensity of 100 mW/cm², a similar behavior as reported in the literature [4].
The precise correlation between light intensity and OCP clarified a tendency that OCP is bound to a specific value at low light intensity range. The energy to which OCP was bound would correspond to the energy peak of DoSS. In order to confirm this hypothesis, we tried to increase DoSS intentionally by the exposure to Ar plasma, and characterized DoSS for “damaged” and “non-damaged” epitaxial n-GaN photoelectrodes by impedance spectroscopy, in order to explain the energy level at which OCP was bound.
The OCP of “non-damaged” n-GaN electrode was bound at -0.8 V v.s. Ag/AgCl under the light intensity range from 10^-3 to 10^-2 mW/cm². After impedance measurement, this value moved to -0.5 V v.s. Ag/AgCl, indicating that even a small reaction current during impedance measurement modifies the surface of GaN. Conversely, the proposed method of illuminative OCP spectroscopy can detect such a subtle change in the surface states. For the “damaged” n-GaN electrode, OCP stayed at -0.2 V v.s. Ag/AgCl in a much wider intensity range from 10^-3 to 10 mW/cm² than the case of “non-damaged” GaN, suggesting that DoSS in the damaged GaN is larger than the non-damaged GaN. For both samples, the energy values to which OCP was bound agreed with the peak energy of DoSS obtained by impedance spectroscopy.
In summary, it is probable that illuminative OCP spectroscopy provides information corresponding to DoSS with the minimum surface damage during measurement. Combining it with impedance analysis, which is more established though destructive, we can analyze DoSS for a variety of semiconductor photoelectrodes in a more comprehensive manner.
[1] Schäfer et al., JPC C 2012, 116, 22281
[2] Winnerl et al., JAP 2017, 122, 045302
[3] Imazeki et al., ECST 2017, 77(4), 25
[4] Deutsch et al., JPC B 2006, 110, 25297

Silver nanoparticle-silver halides (Ag-AgX, X = Cl, Br, I) have been attracted much attention as a new type of visible-light photocatalyst for solar-to-chemical transformations. Improving the activity of the “plasmonic photocatalysts” requires the compatibility of local electric field enhancement (LEFE) and effective absorption of the sunlight. Ag nanoparticle (NP) possesses strong LEFE effect, while the localized surface plasmon resonance (LSPR) peak located near the blue edge of the visible region. On the other hand, the absorption of Au NP well matches with the solar spectrum, but the LEFE is much smaller than that of Ag NP. Au_x-Ag_1-x alloy NP varies the optical property between those of Ag and Au NPs depending on the composition x, and thus, Au_x-Ag_1-x alloy NP-incorporated AgBr (Au_x-Ag_1-x@AgBr) can be a promising plasmonic photocatalyst. At the first step, a successive ionic layer adsorption and reaction (SILAR) method forms gold ion-doped AgBr NPs on mesoporous TiO₂ film. At the second step, UV-light irradiation to the sample in methanol yields Au_x-Ag_1-x alloy particles having diameter of ~5 nm in the interior of AgBr with crystallite size of ~50 nm. The LSPR peak position can be tuned in the range between 500 nm to 600 nm by the alloy composition. Au_x-Ag_1-x@AgBr/mp-TiO₂ exhibits a high level of photocatalytic activity under visible-light illumination.

We combined two different approaches for improving activity of TiO₂ photocatalyst; band gap narrowing by hetero-valent dopant and improving electron-hole separation rate by band offset at the anatase-rutile boundary. Polymorphic TiO₂ particles are usually synthesized by heating, and particles inevitably grow. Hetero-valent elements, such as W, doping reduces the band gap of TiO₂, and improving photon absorption efficiency. However, it also make space charge layer near the surface thinner which can reduce electron-hole separation rate. Therefore, it is difficult task to get synergetic effects of band gap narrower and high carrier separation rate of polymorphic particles. We achieved this by co-doping of tungsten, a band gap narrower, and tin, a promoter of anatase-to-rutile transformation (ART). By W-Sn co-doping, we synthesized ~ 10 nm sized nanoparticles of anatase-rutile mixed phase. The photo-catalytic activity was compared with solely doped TiO₂ and commercially available P25. We also reduced temperature by 50 K, which can save large amount of energy.

TiO2 nanomaterials have been widely studied for various photocatalytic and photoelectrochemical reactions in environmental pollution removal and renewable energy production (hydrogen production, CO2 reduction, fuel production, etc.). Their performances in those areas largely depend on the interaction with the electromagnetic radiation. Therefore, revealing those interactions is very important to many applications and may also open new applications. In this work, we will present our effort in studying the absorption of such electromagetic radition across various energy ranges with TiO2 nanomaterials and modified TiO2 nanomaterials. Mechanisms of such absorption are proposed and discussed, along with possible applications.

High energy conversion efficiency for solar fuel generation through photocatalytic water splitting necessitates visible light absorbing, high quantum efficiency materials. Historically, TiO₂ has become a widely studied ‘model’ system for this application at the expense of visible light absorption. In 2009, photocatalytic degradation of methylene blue under visible light using TiO₂-supported CeO₂ was demonstrated and attributed to a ‘coupled semiconductor’ mechanism¹. Here, the supported CeO₂ absorbs visible light photons (due Ce³⁺ at the grain boundaries) and transfers photoexcited electrons to TiO₂ due to its more negative conduction band minimum. More recent experimental evidence showing Ce³⁺ enrichment at the CeO₂-TiO₂ interface suggests a mixed-metal-oxide (MMO) mechanism wherein partially occupied Ce-4f levels introduce a donor state into TiO₂’s bandgap, effectively reducing the bandgap energy². However, structure-activity relationships regarding the impact of increasing Ce³⁺ concentration on O₂/H₂ evolution rates remain inconsistent, possibly due to the inability to distinguish Ce³⁺ at the interface vs. in the bulk of CeO₂ particles^2,3.

Using monochromated electron energy-loss spectroscopy (EELS) coupled to annular dark field scanning transmission electron microscopy (ADF-STEM), we aim to directly characterize the electronic structure of the CeO₂-TiO₂ MMO interface and correlate these properties to photocatalytic performance. For example, similar to previous work by our research group looking at Pr-doped CeO₂, a joint density of states approach could be applied to valence EELS data to deduce the energy position and width of bandgap states⁴. By applying this technique to valence EELS at the CeO₂-TiO₂ interface, we may be able to elucidate the electronic structure of this MMO and correlate it to Ce³⁺ concentration providing direct evidence of this mechanism. To this end, we have synthesized ‘model’ CeO₂-TiO₂ nanoparticles with 6 wt.% Ce-loading. The relatively high loading, well-defined shape, and small size of the TiO₂ support (<30 nm) should provide clean interfaces suitable for STEM-EELS at 60-kV.

To assess the photocatalytic performance, we plan to use a continuously-stirred photoreactor with recirculated Ar as a carrier gas so that the small nominal H₂/O₂ produced (due to the small amount of MMO interface per weight of sample) can be sampled every 2.5 minutes with a gas chromatograph. Fast sampling of produced gas will reveal transient photocatalytic behavior, if any, present in this system.

[1] G. Magesh et al. Indian J. Chem. 2009, 48A, 480-88. [2] S. Kundu et al. J. Phys. Chem. C 2012, 116, 14062-70. [3] S. Luo et al. J. Phys. Chem. C 2015, 119, 2669-79. [4] W.J. Bowman et al. Ultramicroscopy 2016, 167, 5-10. [5] We gratefully acknowledge support of DOE grant DE-SC0004954, ASU’s John M. Cowley Center for High Resolution Electron Microscopy and ASU’s Center for Solid State Science.

Photoelectrochemical (PEC) solar light-induced water splitting has received significant attention for CO₂-free hydrogen production. Recently, oxynitride semiconductors, such as LaTiO₂N and ATaO₂N (A = Ca, Sr, and Ba) are extensively studied as promising candidates for PEC water splitting under the visible light due to their chemical stability and appropriate band structure. Among these oxynitrides, BaTaO₂N has a smaller absorption edge (~1.9 eV) and thus high-efficiency is expected. In order to suppress the grain boundaries and defects deactivating photocarriers, single crystalline electrode is better than ceramic- or particle-based electrode. However, there are only few reports on the synthesis of BaTaO₂N thin films due to lack of commercially available substrate with good lattice matching. In this study, we fabricated BaTaO₂N epitaxial thin films by nitrogen plasma assisted pulsed laser deposition (NPA-PLD) on SrTiO₃ (STO) substrates (–5.4% mismatch), and improved their crystallinity by inserting a lattice-matched BaSnO₃ (+0.1% mismatch) as a buffer layer.

BaTaO₂N epitaxial thin films were grown on STO (100) and (110) substrates with atomically flat surfaces at various substrate temperature (T_s). A BaTaO_x target was ablated under supply of nitrogen gas activated by an RF plasma source. XRD measurements revealed that the films grown on STO (110) at T_s ≥550 °C showed diffraction peaks from (110)-oriented perovskite BaTaO₂N. However, the films grown at T_s ≥650 °C included impurity phases, whereas the films grown at T_s <600 °C showed weaker diffraction intensity as T_s decreased. The BaTaO₂N films grown on STO (100) showed lower crystallinity than those on STO (110) substrates. The BaTaO₂N film fabricated on STO (110) at 600 °C showed clear yellow color indicating almost stoichiometric chemical composition. On the other hand, its surface roughness was relatively large; root mean square (RMS) of ~5 nm.

To improve the crystallinity and surface roughness, double buffer layers of BaSnO₃ (~90 nm)/Sr_0.5Ba_0.5SnO₃ (~10 nm) were epitaxially grown on STO (110) before the deposition of BaTaO₂N. XRD pattern of the BaTaO₂N thin film fabricated on the buffer layers indicated epitaxial growth of a (110)-oriented perovskite BaTaO₂N thin film with much better crystallinity than the films directly grown on STO (110) substrate. The surface roughness of the BaTaO₂N film was also improved to RMS of ~1 nm. The band gap of the film was determined as 2.01 eV from a Tauc plot of (αhν)^1/2 vs hν, which shows a good agreement with the previous report on thin films (2 eV).

Over the past decades, the photocatalytic materials that directly convert solar light into chemical energy had been extensively explored for energy and environmental applications. To improve the photocatalytic activity, not only the surface area but also the crystalline morphology should be controlled. The facets exposed on photocatalyst surface could affect the photocatalytic performance through various working mechanisms. Among the top-down approaches, the directional chemical etching is widely used to engineer the surface facets of photocatalytic semiconductors due to its low cost and simplicity. The featured surface morphologies are crucially affected by various processing parameters in directional chemical etching, such as the crystal orientation of the surface, the composition of the etchant, the etchant concentration, and temperature. To well understand and give insights into the growth mechanism of faceted morphologies by directional chemical etching, in this study a kinetic model was established to simulate the process of anisotropic chemical etching. By tuning the numerical parameters, such the etching rate, temperature, and the crystalline orientation, in the simulations, the effects of the relevant experimental parameters on the formation and evolution of faceted morphologies can be realized. Affecting by the anisotropy of the crystalline structures and etching rate, various featured surface morphologies, including the cusp-like hillock and nano-pyramid, were numerically reconstructed in accordance with the etching experiments. This numerical investigation has improved the practical knowledge for directional chemical etching to enhance the manufacturing process for photocatalytic materials.

Semiconductors involved in photocatalysis and water splitting often exhibit a bandgap in the UV, which represents only a few percents of the solar spectrum. This drastically limits photogeneration by direct interband absorption inside a semiconductor for solar driven photocatalysis.
In this frame, photogeneration of hot electrons in a metallic structure and their injection into an adjacent semiconductor is a crucial mechanism since it allows generating electrons in the conduction band of the semiconductor with incident photon energies larger than the energy barrier between the two media, however, in general much smaller than the bandgap of the semiconductor. This leads to an increase of the spectral range of photon energies participating to photocatalysis, e.g., a substantial part of the solar spectrum can be used.
The theoretical efficiency of the hot electron injection process can be described by Fowler’s law [1] that quantifies the percentage of hot electrons that carry kinetic energy from their velocity component normal to the surface larger than the energy barrier at the metal-semiconductor interface. This assumes a homogeneous distribution of initial energy of hot electrons and isotropic initial propagation directions. Using this model, the injection efficiency as a function of the incident photon energy is a square law and doesn’t exceed a few percents for a gold-titania system.
Some studies reported injection efficiencies that follow Fowler’s law [2] [3], but other studies also reported values up to 50%, and independent of the incident photon energy [4] [5] [6]. In this work, we quantify the limits of the hot electron injection efficiency by analyzing the ballistic transport of hot electrons in metallic nanostructures, and we compare these limits with the values reported in the studies mentioned previously. For a gold-titania system, we show that the size and shape of the gold nanostructure can increase the injection efficiency beyond what is possible when the planar interface between two half spaces of metal and semiconductor is considered. Additional effects that modify the propagation direction of the electrons can also increase the efficiency, such as electron-phonon scattering events and diffuse reflections at the interface. This is due to the fact that modifications of the propagation direction increase the probability that the propagation direction matches with the escape cone at the interface.
Nevertheless, the maximum efficiencies calculated are still far from those reported in [4] [5] [6], which suggest that additional effects, such as surface effects [5] [7], are predominant.
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[3]Y. K. Lee et al., Nano Lett. 11, 4251–4255 (2011).
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[6]D. Ratchford et al. , Nano Lett. 17 (10) , 6047-6055 (2017)
[7]C. Boeritger et al., ACS Nano 10, 6108-6115 (2016)

Titanium dioxide (TiO₂), one of semiconductor photocatalysts, has been widely used in the environmental remediation. However, due to its easy recombination of the electron–hole pair, the improvement to enhance TiO₂ photocatalytic activity has been applied, such as coating with graphene oxide (GO). Here, a simple hydrolysis method was used to synthesize titanium dioxide/reduced graphene oxide (TiO₂/rGO) hybrid. The hybrid is first synthesized by hydrolysis of titanium isopropoxide, and then annealed at 400 °C under flowing forming gas. The effects of various GO contents during the synthesis are investigated and the mass contents (8%, 15%, and 21%) of rGO in the hybrids are determined by thermogravimetric analysis. To compare with the TiO₂ nanoparticles, the structures and morphologies of TiO₂/rGO hybrids are characterized using XRD, TEM and SEM. The photocatalytic degradation of rhodamine B are studied under the illumination of solar simulation. All synthesized hybrids exhibit the superior photocatalytic activities than the bare TiO₂, which shows rGO can contribute to reduce the electron-hole pair recombination. This method also provides a simple way to obtain graphene-based semiconductor composites as photocatalysts.

A photoanode exhibiting high water-splitting efficiency at low bias potential is essential for stand-alone water-splitting devices through a tandem system combined with a photovoltaic device. However, many previous studies employing a typical BiVO₄/WO₃ heterojunctions focused on water oxidation at the maximum thermodynamic water splitting potential, 1.23 V vs. the reversible hydrogen electrode (V_RHE). Here, we suggest a strategy for high water oxidation efficiency at low potential using 3D BiVO₄/ZnO heterojunction photoanodes. The BiVO₄/ZnO heterojunction exhibits a lower onset potential compared to the commonly used WO₃ heterojunction. Due to the 3D ordered structure, the BiVO₄/ZnO achieves enhanced light harvesting efficiency and improve charge separation efficiency at low bias potential by ZnO heterojunction. As a result, the BiVO₄/ZnO photoanode exhibits a water-splitting photocurrent density of 3.3 ± 0.2 mA /cm² is obtained at 0.6 V_RHE under 1 sun illumination.

The development of solar fuels has been limited by the low conversion efficiency and lack of product selectivity of the current available photocatalysts. Recently, defective electro-catalysts have been successfully demonstrated to enhance hydrogen evolution reaction (HER) performance in liquid-phase reaction. However, gas-phase reaction for CO₂ conversion has yet to achieve the same status. Here, we present defect engineering in 2D materials with multiscale active sites as a promising photocatalysts for CO₂ reduction reaction (CO₂RR). Specifically, two types of defects have been investigated.
For the first type of zero-dimensional defect in photocatalyst, continuous MoS₂ thin films (MoS₂ TFs) with typical layer thickness of 5~10 nm and large area of 2x2 cm² were directly synthesized on SiO₂/Si substrates by chemical vapour phase reaction of MoO₃ and S powders. The MoS₂ TFs exhibited Raman peak frequency difference between A_1g and E_2g modes (Δ) of ~24 cm^-1. Subsequently, the films were post-treated with hydrogen plasma in order to create around 20% sulfur vacancies (S-V) with the resultant stoichiometry ratio of Mo/S confirmed by X-ray photoelectron spectroscopy. The MoS₂ TFs with S-V exhibited enhanced CO₂RR performance compared with pristine MoS₂ TFs and showed selective photoreduction of CO₂ to multi-hydrocarbon products such as acetaldehyde, acetone, methanol and ethanol.
For the second type of two-dimensional defect in photocatalyst, discontinuous MoS₂ TFs with different layer thickness of 1~5 nm were directly synthesized on multilayer graphene/SiO₂/Si substrates by chemical vapor deposition. The MoS₂ TFs grown on graphene-template exhibited different value of Δ (23~26 cm^-1), and also exhibited significantly enhanced CO₂RR performance compared with only pristine MoS₂ TFs. Plausible mechanisms of enhanced CO₂RR activity by defect engineering in 2D materials were further investigated by in situ near ambient pressure X-ray photoelectron spectroscopy experiments to reveal the essential steps for activating CO₂ on a MoS₂ TFs surface, and will be presented in the meeting.
This defective photocatalysts can serves as an initial platform to understand the basic working principle of active site from atomic to nano scale and open new opportunity to develop higher-efficiency photocatalysts for solar fuels application.

Photoelectrochemical water splitting cell enables the conversion of solar energy into hydrogen fuel from splitting water. Although promising, the efficiency of the overall photoelectrochemical cell is often limited by water oxygen evolution reaction (OER) in the anode, which is kinetically sluggish.¹ While finding more efficient and cheap (photo-)catalyst for OER is an active research area, some researchers have proposed the idea of substituting OER with oxidation of organic substances that derived from byproducts of biomass processing, e.g., ethanol, glycerol, glucose, etc. which is kinetically more favorable compared to OER. ^1–4 Moreover, the oxidation of organic substances are shown to produce value-added products, e.g.: oxidation of ethanol can produce acetate and hydrogen, which are more valuable compared to ethanol.²

Hematite (α-Fe₂O₃) as a popular photoanode for solar water splitting has already been demonstrated for the oxidation of organic substances with higher hydrogen yield compared to the system without the addition of organic substances.⁵ Other metal oxide semiconductors such as TiO₂ and WO₃ have also been investigated in similar works.^6,7 In this work, we would like to explored the possibilities of using earth-abundant bimetallic catalyst-hematite heterostructured photoanode (i.e., FeNiP/hematite) in a biomass oxidation configured with hydrogen reduction from water splitting cell in the quest to reduce the onset potential for photocurrent generated from photoanode and thus improve photoelectrochemical cell performance.

Reference
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Recently, our group demonstrated optical excitation at their localized surface plasmon resonance (LSPR) frequency and induced local enhancement of electromagnetic (EM) fields near the nanoparticles and bottom Au layer surface that can be markedly boosted using metal-dielectric hybrid-structure geometry. In the current study, we show that by engineering a photoanode which includes an array of pyramidal gold layer with a thin, thickness-varianced TiO₂ layer by oblique angle deposition technique on the surface in contact with gold nanoparticles, which works as a TiO₂ plasmonic platform for wide light absorption and their water splitting Applications.

Controlling matter and information across the nano-, meso-, and macro-scales is a challenge for science and the imagination. In this presentation, we highlight recent advances in our research efforts to develop synthetic methodologies for constructing an integrated photocathode for light activating chemical transformations that include capturing, converting, and storing solar energy as fuel. A recent example involves development of a direct one-step method to chemically graft porphyrin catalysts that chemically transform water to hydrogen as well as carbon dioxide to carbon monoxide onto a visible-light-absorbing gallium phosphide (GaP) semiconductor. The porphyrin complexes are prepared using a synthetic strategy that yields a tetrapyrrole macrocycle with a pendant 4-vinylphenyl attachment group. This structural modification allows use of the UV-induced immobilization chemistry of olefins to attach intact metalloporphyrin complexes to the semiconductor surface. Solar hydrogen production is demonstrated via photoelectrochemical testing in pH neutral aqueous solutions under simulated solar illumination. Key features of the constructs presented here include use of metalloporphyrins with built-in chemical sites for direct grafting to a GaP semiconductor, creating novel hybrid photoactive assemblies capable of converting photonic energy to fuel.

The abundance of solar energy makes it the premier alternative to fossil fuels as an energy source. Multiple approaches are taken for solar harvesting and storage, one being mimicking photosynthesis for water splitting. This has three basic parts: light collection, energy tunneling to the catalytic core, followed by electron transfer to the photocatalyst to convert water to hydrogen and oxygen. Facile molecular designs for donor-acceptor (D-A) dyads is critical for light absorption and energy transfer in photocatalytic systems. Forming dyads, and higher order assemblies often involves complicated synthetic schemes. This assembly was achieved in a simple, high fidelity process, through the coordination of chiral bisoxazolines (BOX) to zinc metal centers. Previous reports have demonstrated chiral discrimination between enantiomers of the BOX ligands upon formation of zinc complexes, with the heterochiral complex being the sole orientation in a racemic mixture. The meso-position of two enantiomers of BOX complexes was appended with chromophores to study exciton coupling and electron transfer between them in this self-assembled dyad. Exciton coupling was measured through uv-vis absorption, fluorescence spectroscopy, quantum yield and lifetime studies.

High efficiency, high stability and easy recovery are three key factors for practical photocatalysts. Z-scheme heterostructure is one of the most promising photocatalytic systems to meet all above requirements. However, efficient Z-scheme visible-light-driven photocatalysts are still few and difficult to implement at present. Searching for appropriate p-type semiconductors with narrow band gap and high energy level of CB minimum (CBM) is essential for constructing highly efficient Z-schematic photocatalytic systems.
As two p-type semiconductors, Cu₂O and ZnFe₂O₄ have narrow direct band gap with much negative CBM. Meanwhile, AgBr is an n-tpye semiconductor with the band gap of 2.6 eV, which has been extensively studied in photocatalysis and exhibited the outstanding photocatalytic activity. In this work, firstly, a series of Cu₂O/Cu/AgBr/Ag photocatalysts were synthesized by a redox procedure followed by photo-assisted deposition. It is found that Cu nanoparticles (NPs) could be controllable to grow between Cu₂O and AgBr. If without Cu NPs, only a low photodegradation rate on methyl orange (MO) (~51% in 50 min) was observed compared with the high MO photodegradation rate (~98% in 50 min) in the presence of Cu NPs. Furthermore, introducing Cu NPs assists in accelerating excited carriers transfer at the interface between Cu₂O and AgBr, proved by the photoluminescence spectra, photocurrent and electrochemical impedance spectra, which thus helps to increase the stability of the photocatalyst. Secondly, the composite photocatalysts ZnFe₂O₄/AgBr/Ag were prepared through a two-step method. Under visible-light irradiation, a high photodegradation rate of ~92% on methyl orange was observed within 30 min, which is much better than that of individual ZnFe₂O₄ or AgBr/Ag. The stability was also greatly improved compared with AgBr/Ag. Besides, ZnFe₂O₄/AgBr/Ag is ferromagnetic and can be recovered by magnet. Moreover, photocatalytic activity can further be improved under an external magnetic field because of the easier carrier transfer induced by magnetic field. The increased photocatalytic activity and stability of these two composite photocatalysts can be attributed to the suitable band alignment between Cu₂O or ZnFe₂O₄ and AgBr, which is known as the Z-scheme mechanism confirmed by the detection of active species and electrochemical impedance spectroscopy. These results demonstrate that Z-scheme photocatalysts of Cu₂O/Cu/AgBr/Ag and ZnFe₂O₄/AgBr/Ag are potential visible-light driven photocatalysts for pollutants degradation.

The field of semiconductor photoelectrochemistry and photocatalys is now more than 40 years old. Yet, the complexity of the interfacial charge transfer in nanostructured semiconductor systems continues to intrigue material chemists, spectroscopists and surface scientists. Although the primary process following the bandgap excitation of semiconductor nanoparticle is the electron-hole separation, the competing charge transfer processes at the semiconductor interface dictate the overall performance of semiconductor photocatalysts. Bipolar membranes provide interesting opportunities to build photocatalyst assemblies and separate oxidation and reduction reactions. Kinetic and mechanistic aspects of electron transfer processes within such membranes and their implications in water splitting reaction will be discussed.

Piezoelectric effect induced by piezoelectric polarization has been widely applied in nano-generators, piezoelectric field effect transistors, flexible piezoelectric sensors and so on. Recently, the application of piezoelectric effect in environmental purification and hydrogen production has also attracted the attention of researchers. At present, the research of piezoelectric catalysis is mainly focused on one-dimensional piezoelectric / ferroelectric materials, such as BaTiO₃ one-dimensional structure, PZT fiber, single layer or few layers self-assembly sheet structure of MoS₂. For example, the dendritic structure of BaTiO₃ achieves the degradation of azo dye and hydrogen production. One or two dimensional material bent easily by mechanical force, thus generating opposite polarity charge (piezoelectric potential) on the two opposite sides. Thus, the formed piezoelectric field can induce charge migration. In photocatalytic process, charge separation is an important factor affecting photocatalytic efficiency. The separation of photogenerated charge driven by piezoelectric potential is a very effective way to improve photocatalytic efficiency. Through the combination of piezoelectric and photocatalytic properties, the efficient separation of electrons and holes was demonstrated. This provides a new idea for the design and fabrication of new environmental purification materials.

The possibility of incident sunlight passively splitting water to constituent hydrogen and oxygen, through the designed placement of the semiconductor conduction band (CB) and the valence band (VB) with respect to the H₂O/H₂ reduction potential and the H₂O/O₂ oxidation potential, respectively, is of immense scientific and technological interest.

The role of non-stoichiometry in a hierarchically structured WO_3-x electrode, constituted from nanoscale fuzziness as well as microscale wire morphology, on the photoelectrochemical response is investigated. Through x-ray photoelectron spectroscopy (XPS) studies, the relative amounts of the various oxidation states of the constituent W are probed with respect to the observed response. It is concluded that an intermediate/optimal number of vacancies, yielding a W⁶⁺/ (W⁵⁺ + W⁴⁺) ratio of around 2, would be beneficial for increasing the photocurrent. It is posited that defect engineering combined with optimized band structure modulation could be used for enhanced photocurrent density as well as electrode stability. The work would help considerably elucidate the role of defects as well as charge carriers for oxygen evolution reaction (OER) efficiency increase.

Given that OER proceeds through the interaction of hole charge carriers (h⁺) with water and that the presence of h⁺ implies surface instability, we indicate that a charge compensating mechanism through electrons from oxygen vacancies may be beneficial. It is hoped that the consideration of such defect engineering aspects, perhaps at the atomic level, would be helpful in designing higher efficiency electrodes for water oxidation.

Figure 1: Schematic diagram of separation and transfer of photo-generated carriers in the CN/SNON-700 heterojunction under visible light irradiation.

Semiconductor-based photocatalysis has received tremendous attention in the last few decades because of its potential for solving current energy and environmental issues. In a semiconductor photocatalytic system, photoinduced electron-hole pairs are produced when a photocatalyst is irradiated by light with frequencies larger than that of its band gap [1]. Semiconductor-based heterojunctions have been shown to overcome the drawbacks of low photocatalytic efficiency that result from electron−hole recombination and narrow photo-response range [3, 4] Different types of visible-light-active heterojunctions made by two different semiconductors will be presented [5-7]. With the example of our recent study based on the heterojunction of g-C₃N₄ and nitrogen-doped Sr₂Nb₂O₇, the importance of design and preparation of heterojunctions to facilitate charge separation/migration for enhanced photocatalytic activity will be discussed.

Nanoparticles of very small size (below 10 nm) of TiO₂ material are nowadays the functional building blocks for many applications in photocatalysis and photovoltaics. [1] In order to improve the photocatalytic performances of this nanomaterial, it is of great importance to understand how the size confinement, the morphology and the interaction with water influence charge carriers separation and migration to the surface.
In the first part of the talk, we present a hybrid density functional theory (DFT) investigation [2,3] of the size and shape effects on the life path of energy (excitons) and charge (electrons and holes) carriers in real-size anatase TiO₂ nanoparticle models with different size (2-3 nm) and shape (faceted vs spherical). We focus our attention on the exciton/charge carriers formation, separation, recombination, self-trapping processes, which are analyzed in terms of structural distortions, energy gain or cost, charge localization/delocalization and electronic transitions of the trapped charges. The migration of photoinduced charges from the bulk towards the surface is always computed to be a downhill process, although differences are observed for spherical vs faceted nanoparticles because of the higher disorder and larger diversity of surface sites. The computational models are corroborated by an extensive comparison with available experimental data from photoluminescence measurements, electron paramagnetic resonance and transient absorption spectroscopies.
In the last part of the talk, we briefly discuss a recent combined experimental and theoretical study [4] about the effect of a water environment on the hole trapping mechanism for TiO₂ nanoparticles with different morphologies. Comparing the results from steady and transient infrared spectroscopy with DFT calculations, we clarify why water enhances hole trapping at the surface of spherical TiO₂ nanoparticles, but not of well-faceted ones.
The project has received funding from the European Research Council (ERC) under the European Union's HORIZON2020 research and innovation programme (ERC Grant Agreement No [647020]) and from the network QM-FORMa: Designing New Materials with Quantum Mechanics.

[1] Sang, L.; Zhao, Y.; Burda, C. Chem. Rev. 2014, 114, 9283–9318.
[2] Fazio, G.; Ferrighi, L.; Di Valentin, C. Nano Energy 2016, 27, 673 – 689.
[3] Selli, D.; Fazio, G.; Di Valentin, C. J. Chem. Phys. 2017, 147, 164701.
[4] Shirai, K.; Fazio, G.; Sugimoto, T.; Selli, D.; Ferraro, L.; Watanabe, K.; Haruta, M.; Ohtani, B.; Kurata, H.; Di Valentin, C.; Matsumoto, Y., J. Am. Chem. Soc. 2017, under review.

Graphitic carbon nitrides (g-CN_xH_y’s) demonstrate immense potential for efficient photocatalytic hydrogen generation, attributed to their high surface area and ability to absorb visible light¹. Due to kinetic limitations afforded during synthesis, involving calcination of N-rich precursors, g-CN_xH_y’s contain a range of residual hydrogen, perturbing structural condensation². Efforts to improve the efficiency of g-CN_xH_y’s have focused on reducing H-content to give higher hydrogen evolution rates (HER’s), suggesting that amine (N-H_x) defects slow down charge transfer². However, the undesirable need of Pt remains as g-CN_xH_y must be functionalized with ~2-3 wt.% Pt to achieve appreciable HER’s. Recently, g-CN_xH_y’s with supported single Pt atoms and very low loadings (<0.2 wt.%) have been shown to give improved HER’s on a per Pt-atom basis³. Yet, low-loading, highly-dispersed Pt co-catalysts still cannot surpass the high HER’s of the traditional photodeposition (PD), high-loading routes.

By combining annular dark field scanning transmission electron microscopy (ADF-STEM) and photoreaction data, we systematically determine the effect of co-catalyst dispersion and support structure on the photocatalytic performance of Pt/g-CN_xH_y’s. To capture a range of support structural disorder, three g-CN_xH_y samples are selected based on the broadening of the (002)-peak observed in powder x-ray diffraction. Here, g-CN:1, g-CN:2, and g-CN:3 refer to g-CN_xH_y’s with average domain sizes of 9.9, 4.5, and 3.2 nm, respectively, determined by Scherrer analysis. When loaded with 2 wt.% Pt via PD, the HER of g-CN:2 is ~2x higher than g-CN:1 and g-CN:3. ADF-STEM reveals that g-CN:2 also possesses the highest Pt dispersion quantified by a specific surface area (SSA) of 27.6 m²/g. By normalizing the HER’s by the number of exposed surface Pt atoms (based on measured SSA’s) the TOF of each support can be derived and are found to correlate with the average domain size.

With a TOF of 182/hr, g-CN:1 is the most active photocatalyst but suffers a low Pt dispersion. Based on this knowledge, we can predict the combination of support and Pt dispersion/loading to give desired HER’s, resulting in rational design strategies for reducing Pt consumption while maintaining high energy conversion efficiency. HER’s in excess of 5000 μmol/g/hr are predicted for 0.5 wt.% Pt on g-CN:1 with single atom stabilization. Current efforts confirm that the Pt particle size can be dramatically reduced on g-CN:1 by employing NaBH₄-assisted chemical deposition⁴, giving a 60% improvement in HER for a 4x reduction in Pt loading.

[1] X. Wang et al. Nat. Mater. 2009, 8, 76-80. [2] D.J. Martin et al. Ange. Chem. Int. Ed. 2014, 53, 9240-45. [3] X. Li et al. Adv. Mater. 2016, 28, 2427-31. [4] Z. Chen et al. Adv. Funct. Mater. 2017, 1605785. [5] We gratefully acknowledge the support of DOE grant DE-SC0004954, ASU’s John M. Cowley Center for High Resolution Electron Microscopy and ASU’s Center for Solid State Science.

Photocatalysis has attracted more and more attention on the sustainable energy and environmental remedy. However, the light adsorption is still a critical issue in this research field. In this presentation we demonstrate two strategies for constructing the photocatalyst with visible light response. One is the using mixed transition metal element at B site in ABO3 type oxides. We employed Nb and Fe replaced the Ti in CaTiO3 nanocarystals. In the band diagram of CaTiO3, the conduction band (CB) is origniated from the d orbital of Ti and valence band (VB) is contributed from the p orbital of O. When the Ti was replaced with Nb and Fe, the d orbital of Fe results in a lower position of CB, which further narrow the band gap. Another example is conducting polymer polythiophene. As well known, polythiophene is a typical light adsorption materials in solar cells. Here we demeonstrate it also can be a stable photocatalyst for H2 production.
[1] G. Zhang, Shaorui Sun, Wenshuai Jiang, Xiang Miao, Zhao Zhao, Xiaoyan Zhang, Dan Qu, Duoying Zhang, Dabing Li, and Z. Sun. A Novel Perovskite SrTiO₃-Ba₂FeNbO₆ Solid Solution for Visible Light Photocatalytic Hydrogen Production Adv. Energy Mater.: 2016, 6, 1600932
[2] Xupeng Zong, Xiang Miao, Shixin Hua, Li An, Xiang Gao, Wenshuai Jiang, Dan Qu, Zhijun Zhou, Xingyuan Liu, Z. Sun Structure defects assisted photocatalytic H2 production for polythiophene nanofibers Applied Catalysis B: Environmental 2017, 211, 98–105

Visible-light responsive photocatalysts can directly harvest energy from solar light offering a desirable way to solve energy and environment issues.
We have shown that conjugated polymers (in particular Polydiphenylbutadiyne, (PDPB) Poly(3,4-ethylenedioxythiophene (PEDOT) Poly(3-hexylthiophene) nanostructures, (P3HT)) and poly(pyrrole) (PPy) emerge as a new class of photocatalysts very active under visible light without the assistance of sacrificial reagents or precious metal co-catalysts.^1,2,3 These polymer nanostructures are synthesized in soft templates provided by hexagonal mesophases. These stable and cheap polymer nanofibers are easy to process and can be reused without appreciable loss of activity.
Addition of scavengers and mechanistic studies show that superoxide radical is the main radical responsible for degradation of phenol taken as a model pollutant. P3HT nanostructures can easily be deposited on flat supports such as quartz for photocatalytic applications avoiding a separation step by centrifugation. The photocatalytic activity of these P3HT nanostructures is highly enhanced when they are supported on a solid surface opening new perspectives in photocatalytic reactors and self-cleaning surfaces.³
PDPB nanostructured conjugated polymers when dispersed in water, and in the absence of sacrificial agents or co-catalysts can perform photocatalytic water oxidation under visible light excitation.
Our results demonstrate that conducting polymer nanostructures offer the perspective of development of a new generation of efficient and cheap visible light driven photocatalysts for environmental protection. These polymer nanostructures can also find applications in self-cleaning surfaces and water splitting.


References:
1- Ghosh, S.; Kouamé, N.A.; Ramos, L.; Remita, S.; Dazzi, A.; Deniset-Besseau, A..Beaunier, P.;Goubard, F.; Aubert, P.-H.; Remita, H. Nature Materials (2015) 14, 505 – 511.
2- Ghosh, S.; Kouame, N.A.; Remita, S.; Ramos, L.; Goubard, F.; Aubert, P.-H.; Dazzi, A.; Deniset-Besseau, A.; Remita, H. Sci. Rep. 5 (2015) 18002.
Floresyona, D. ; Goubard, F. ; Aubert, P.-H. ; Lampre, I. ; Mathurin, J.;Dazzi, A.; Ghosh, S.; Beaunier, P.; Brisset,F.; Remita, S.; Ramosand Remita, H. Appl. Catal., B Environmental 209 (2017) 23-32.

We report latest results using a dual III-V semiconductor based tandem solar photoelectrochemical cell with record solar-to-fuel efficiency for hydrogen generation. Unassisted water splitting was carried out with a tandem photocathode device featuring a cathode surface with a highly transparent catalyst Rh layer supported on an anatase TiO₂, which in turn coats a PO₄-terminated AlInP photoelectrode window layer of the tandem photoelectrode in order to achieve optimal band alignment. To maximize the photocurrent density, we have also employed several light management strategies that enable increased transparency of the metal catalyst ensembles. Two pathways have been pursued: i) an optically transparent but dense Rh metal nanoparticle layer, and ii) a general approach to design of high-activity effectively transparent catalysts that employ opaque catalyst materials, in which a tailored mesophotonic dielectric cone structure is used as a light coupler to efficiently guide incident light into the light absorber. We have observed solar-to-hydrogen conversion efficiencies as high as 19.3% at low pH, but saw a time-dependent loss of device performance related to loss of catalyst activity. The origin of the activity losses was analyzed using surface analysis methods (XPS, UPS, AFM) and strategies for improvement are addressed. We also have demonstrated hydrogen generation with an 18.5% STH efficiency operating with a tandem photoelectrode with improved stability at neutral pH. This development also opens up a new route for other photoelectrochemical applications, as for example, aqueous CO₂ reduction is best performed near pH 7. The catalyst integration and device design criteria for direct photoelectrochemical CO₂ reduction for production of higher value solar fuels will also be discussed.

Introduction of atom vacancies and heteroatom dopants have been widely used to improve their visible-light responding photocatalytic activities [1, 2]. However, it always results in limited visible-light absorption increase, like an extra absorption shoulder or up-shifted absorption tail, coming from the accompanying donor electrons filling the donor states beneath the conduction band or the narrow newly-formed energy level above the conduction band. To improve the visible-light absorption further, we developed a methodology of introducing hydrogen-mediated anion vacancies[3]. Taking TiO2 as an example, theoretical simulations and experimental results showed that the introduced atomic H-mediated oxygen vacancies (OVH) could induce a sub-valence band of certain width within the bandgap of TiO2, and bring about an unusual strong absorption band ranging across the full spectrum of visible light, with the bandgap of TiO2 decreasing from 3.2 eV to 1.98 eV. Compared with conventional blue-colored oxygen-vacancy-containing TiO2, OVH-TiO2 showed much higher photoelectrochemical current. This successful trial in TiO2 sheds new light on the modification of solar-conversion materials.

References
[1] Zuo, F., Wang, L., Wu, T., Zhang, Z., Borchardt, D., Feng, P. Self-Doped Ti3+ Enhanced Photocatalyst for Hydrogen Production under Visible Light. Journal of the American Chemical Society, 132, 11856-11857 (2010).
[2] Asahi, R., Morikawa, T., Ohwaki, T., Aoki, K., Taga, Y. Visible-light photocatalysis in nitrogen-doped titanium oxides. Science, 293, 269-271(2001).
[3] Yang, Y. Q., Yin, L.-C., Gong, Y., Niu, P., Wang, J. Q., Gu, L., Liu, G., Wang, L. Z., Cheng, H.-M., An Unusual Strong Visible-light Absorption Band in Anatase TiO2 Photocatalyst induced by Atomic Hydrogen-occupied Oxygen Vacancies. Advanced Materials, accepted.

High-pressure hydrogenation of anatase TiO₂ has emerged as one of the most effective procedures for extending the photo-absorption of TiO₂ to the visible while maintaining its excellent photocatalytic properties. The resulting black TiO₂ nanoparticles are known to have a crystalline core covered by a reduced, disordered TiO_2-x shell. However, the detailed atomic scale structure of the black TiO₂ nanoparticles and the mechanism of their superior activity remain largely unknown. We have studied the evolution of low-index anatase surfaces and a spherical anatase nanoparticle (NP) in high temperature, high pressure H₂ atmosphere using molecular dynamics simulations based on reactive force field combined with first principles density functional calculations. Our results reveal the main reduction pathways of anatase, and the associated changes in the structural and electronic properties.

The generation of hydrogen from water and sunlight through photoelectrochemical cells (PECs) offers a promising approach for producing scalable and sustainable carbon-free energy. However, the design of high-performance PECs requires a detailed understanding of physicochemical properties of the interface between semiconductor photoelectrode materials and liquid water, which is often difficult to probe under operating conditions. In this presentation, we discuss how high-level first-principles simulations can be integrated with advanced experimental characterization techniques to unravel the key chemical and electronic properties of solid/liquid electrochemical interfaces. Specific discussion focuses on the combination of X-ray photoelectron spectroscopy (XPS) simulations and near-ambient-pressure XPS experiments for the identification of solid/liquid interfacial chemical composition and speciation. We also show how first-principles molecular dynamics simulations can be coupled to many-body perturbation theory to directly probe the link between interfacial electronic properties and local chemistry, which is necessary for devising meaningful strategies to improve PEC performance and durability. The integrated computational-experimental approach is demonstrated to understand the surface chemistry of GaP and InP photoelectrodes in contact with water.

Photoelectrochemical water-splitting cells are typically composed of at least one semiconductor photoelectrode which is prone to deleterious degradation and/or oxidation. Various surface modifications are known for stabilizing semiconductor photoelectrodes, yet these stabilization techniques are often accompanied by a decrease in photoelectrode performance creating a roadblock for further improvements. In this work, we present a multiscale computational tool combining density functional theory (DFT) and finite-element device simulations. This integrated computational approach can be utilized to provide insight into charge transport across modified photoelectrodes and subsequently design functionalized photoelectrode for high efficiency photoelectrochemical water-splitting. To demonstrate the approach, we present how the tool has been used to analyze the performance of Si(111) photoelectrodes functionalized with mixed organic monolayers. DFT is used to calculate the changes in the electronic properties of the silicon induced by the organic functionalization. Then, the DFT results are used as input parameters for the finite-element device simulations. The device simulations are used to predict the efficiency of the operating photoelectrode based off calculations of the charge transfer behavior. This computational approach provides an inexpensive multiscale methodology that combines the angstom-scale results obtained using DFT with the micron-to-nanometer scale capability of device modeling.

Electrocatalysts play a prominent role in the renewable energy conversion and storage applications, including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and carbon dioxide reduction (CO₂RR). Here we will briefly introduce our recent work in developing some of these electrocatalysts [1-5], together with theoretical calculations for rational structure designs and reaction mechanism understandings. Several representative examples using earth-abundant metal (hydro)oxides and carbons include: (1) hybrid hydroxide nanowire-nanoflake assembly for bifunctional HER/OER, (2) metal oxide@carbon superlattices for bifunctional HER/OER, (3) mesoporous oxide@carbon for bifunctional ORR/OER, and (4) tuning of nitrogen doping types in carbon nanostructures for CO₂RR. Attributed to their high electrochemically active surface area, fast charge transport, efficient mass transfer and gas release, these nanostructured electrocatalysts enable much enhanced activity, such as reduced overpotentials, high current densities and long stability. In addition, we also demonstrate that by reversing the catalyst design concepts, new battery electrodes with substantially enhanced energy storage density and power density can also be realized [6].

References
(1) Li, J.; Wang, Y.; Zhou, T.; Zhang, H.; Sun, X.; Tang, J.; Zhang, L.; Yang, Z.; Zheng, G. F. J. Am. Chem. Soc. 2015, 137, 14305-14312.
(2) Cha, M.; Da, P.; Wang, J.; Wang, W.; Chen, Z.; Xiu, F.; Zheng, G. F.; Wang, Z. S. J. Am. Chem. Soc. 2016, 138, 8581-8587.
(3) Wang, Y.; Chen, L.; Yu, X.; Wang, Y. G.; Zheng, G. F. Adv. Energy Mater. 2017, 7, 1601390.
(4) Kuang, M.; Wang, Q.; Han, P.; Zheng, G. F. Adv. Energy Mater. 2017, 7, 1700193.
(5) Kuang M.; Han, P.; Qang, Q.; Li, J.; Zheng, G. F. Adv. Func. Mater. 2016, 26, 8555-8561.
(6) Wang, Y.; Cui, X.; Zhang, Y.; Zhang, L.; Gong, X. G.; Zheng, G. F. Adv. Mater. 2016, 28, 7626-7632.

In the semiconductor and/or metal-based heteronanostructures (HNSs) for solar energy conversion, “nanoscale band engineering” taking the size quantization and interfacial design into consideration is essential for achieving the long-range charge transfer to enhance the efficiency because the effective space charge layer is usually not formed. HNSs consisting of metal chalcogenide quantum dot and metal oxide semiconductor (MX QD/MO, X = S, Se, Te) represented by QD/TiO₂ are highly promising as the photoelectrode for not only solar cells but also photoelectrochemical cells for hydrogen generation from water. In this case, rational hybridization of the QDs (MX-MX, MX-plasmonic metal, plasmonic metal-metal) with atomically commensurate junctions between the components can improve the cell performances by tailoring the optical and electrical properties. This talk focuses on the in-situ synthesis of hybrid QDs/MO with epitaxial (EPI) junctions on the TiO₂ (or ZnO) scaffold surface, and the effects of the interfacial bond and state of junctions between the components on the photoinduced charge generation and separation, and the cell performance for solar-to-hydrogen conversion. Firstly, the basic structure of the QD-sensitized photoelectrochemical (QD-SPEC) cell and the design of the QD/TiO₂ photoanode are presented. Secondly, the mechanism on the electron injection from QD to TiO₂ is dealt with. Finally, the synthesis of TiO₂ coupled with hybrid QDs with EPI junctions, and the state-of-the-art QD-SPEC cells using the hybrid QD/TiO₂ as the photoelectrode are described.

To successfully scale photo-electrochemical water-splitting technologies from bench to demonstration size requires addressing predictable and unpredictable complications. This talk will identify the challenges and describe solutions for successful scaling of the absorber area of inverted metamorphic multijunction III-V cells[1] from ~0.15 cm² up to 16 cm² and incorporating them in a photoreactor capable of generating 3 standard liters of hydrogen in 8 hours under natural sunlight. Despite using Comsol multiphysics to model our photoreactor and identify suitable specifications for a prototype, several practical issues that were uncovered during testing led to multiple iterations between the initial and final photoreactor design. Several bottlenecks that ranged from counter electrode composition and orientation to bubble removal needed redress in order to meet our performance targets. Ultimately, the demonstration scale system was able to generate nearly twice the target volume of hydrogen in an 8-hour outdoor trial.

[1] NATURE ENERGY 2, 17028 (2017)

Abstract:
The design of ultra-thin, highly absorbing metallic nanostructures has become increasingly important for a variety of applications ranging from photo-detection to photo-catalysis. Indeed, it has been demonstrated that, despite their short mean-free paths (10-20nm), hot-carriers generated in a metal can be transferred to either an adjacent semiconductor or an adsorbed molecule leading to detectable photo-currents and measurable reaction products, respectively. Furthermore, localized heat generation can be beneficial for photothermal processes. Typically, plasmonic nanostructures are utilized for achieving broadband or narrowband, efficient light absorption at the small dimensions required for the efficient transfer of plasmonic hot-carriers or heat generation. However, relying on nano-patterned designs challenges the scalability of these structures for practical applications, in particular for solar-energy conversion devices.
Here, we study light absorption in ultrathin (10-15 nm thick) planar metal films. Our absorbers are comprised of a low-loss silver backreflector, a subwavelength transparent dielectric (such as SiO2) or wide-bandgap semiconductor (i.e NiO or TiO2), typically 50-250nm thick, and an ultrathin (5-30 nm) metallic active absorbing layer. As a thin absorbing metal layer we consider several different materials, including Au, Ti, Cu, Ni, Pt and Pd.
First, we demonstrate theoretically and experimentally that both broadband and tunable narrowband, near-unity light absorption is possible in these unpatterned, ultrathin metallic systems which could be easily upscaled. Next, we test our absorbers as photoelectrodes. We measure the photoresponse of our ultra-thin Pt broadband absorber in a 0.1M sulfuric acid solution under cathodic potentials and perform the hydrogen evolution reaction (HER). For the case of Pt-on-insulator, the electrode exhibit a response time of approx. 1s, consistent with fast-heating. Instead, for the case of Pt-on-semiconductor we observe a fast response (~1uA/cm² of photocurrent at 4 Suns illumination intensity) which can be attributed to charge injection. We also study CO₂ photoelectrochemical reduction (50mM K₂CO3 solution saturated with CO₂, V=-0.8 to -1.2V vs RHE) using a series of Cu narrowband absorbers with absorption peak tuned from 350nm to 750nm. Tuning the absorption peak, and hence the energy of the absorbed photons, enable us to control the energy distribution of the photo-excited hot-electrons. We report results of product analysis for our series of Cu absorbers and evaluate the impact of light energy on the selectivity of our electrodes.
Our work opens new pathway for efficient light collection and enables a simple platform for photon assisted hot carrier generation or localized heating.

Silicon based photocathodes with various co-catalysts have been studied extensively for solar fuels production, especially for hydrogen evolution reaction of photoelectrochemical (PEC) water splitting.1 In contrast, Si photocathodes which perform CO₂ reduction (CO₂R) are less well investigated.2 Moreover, in most prior studies, Si photocathodes produce two electron reduction products such as carbon monoxide and formate.
A strategy to produce C₂ and C₃ products such as ethylene, ethanol, and propanol will described. The choice of the CO₂ reduction catalyst is crucial as Si is not able to selectively convert CO₂ in to C₂-C₃ products by itself. Also, the light absorption properties and effects on surface recombination need to be considered, as well as the selectivity for CO₂R over hydrogen evolution.
To permit the use of absorbing metallic catalysts, we employed a back illumination geometry using a both side-textured n-type Si absorber and a p⁺⁺ implanted hole back contact (illumination side) and n++ electron front contact (electrolyte side). Front surface passivation, electron collection, and stability under aqueous PEC CO₂R conditions are achieved via atomic layer deposition of TiO₂. A CuAg nanococtus catalyst was deposited in two-step methods; i) a 100 nm layer of Ag was deposited via ebeam and ii) copper was electrodeposited at high current to generate nanocactus morphology.
Under 1-sun back illumination, the onset potential for production of hydrocarbon products is shifts to 0.5-0.6 V vs. RHE, which represents a cathodic shift of ~500 mV due to the Si PV. The device has excellent stability (6 hrs in 0.1 M CsHCO₃) and over 60% faradic efficiency for hydrocarbons and oxygenates. This nanocactus bimetallic catalyst grown on planar and textured silicon and compared for CO₂ reduction. Textured silicon shows higher photocurrent than planar silicon at a fixed potential due to high surface area but there was no change observed in products distribution.

[1] J. Oh, T. G. Deutsch, H.-C. Yuan, H. M. Branz, Energy & Environmental Science 2011, 4, 1690; Y. Hou, B. L. Abrams, P. C. K. Vesborg, M. E. Björketun, K. Herbst, L. Bech, A. M. Setti, C. D. Damsgaard, T. Pedersen, O. Hansen, J. Rossmeisl, S. Dahl, J. K. Nørskov, I. Chorkendorff, Nat Mater 2011, 10, 434; S. W. Boettcher, J. M. Spurgeon, M. C. Putnam, E. L. Warren, D. B. Turner-Evans, M. D. Kelzenberg, J. R. Maiolo, H. A. Atwater, N. S. Lewis, Science 2010, 327, 185.
[2] K. D. Yang, Y. Ha, U. Sim, J. An, C. W. Lee, K. Jin, Y. Kim, J. Park, J. S. Hong, J. H. Lee, H.-E. Lee, H.-Y. Jeong, H. Kim, K. T. Nam, Advanced Functional Materials 2016, 26, 233; E. Torralba-Peñalver, Y. Luo, J.-D. Compain, S. Chardon-Noblat, B. Fabre, ACS Catalysis 2015, 5, 6138; J. T. Song, H. Ryoo, M. Cho, J. Kim, J.-G. Kim, S.-Y. Chung, J. Oh, Advanced Energy Materials 2017, 7, n/a.

Surface electronic density of states can be adjusted via surface structure such as surface vacancies, surface hybrid and ligands. The density of states (DOS) of the photocatalysts valence band (VB), can be regulated and controlled by the structure, number and the kind of surface oxygen vacancies, which can be contributed to the narrowed the band gap and the broaden the VB, thus promoting the separation efficiency of photoinduced electron-hole pairs, and improving the photocatalytic activity. In addition, the theory calculation indicates that the influence of surface oxygen-defect states on the band structure, the DOS of the VB, photoabsorption performance and oxidation-reduction potential for photocatalysts. A variety of technologies were utilized to reveal the physical structure of surface defects and bulk defects, and to display the external relations of defect states with the photodegradation process and the photocurrent, and to imply the inherent law of defect states with the separation and transfer of the photoinduced electrons. This work discovers the promotion mechanism of surface oxygen vacancies on the separation of photoinduced electron-hole pairs, thus new high activity photocatalysts were prepared successfully. In a word, it is beneficial to further understand the relationship of surface oxygen defects with the enhanced separation and transfer of photoinduced electrons, and it has a promoting role in the development of physics and chemistry. The photocatalytic performances of ZnO, BiPO₄ and Bi₂WO₆ photocatalysts have been enhanced greatly via surface oxygen-vacancy.
The surface hybridization on photocatalyst by using the rapid electron and hole transpoting property of delocalized conjugated π materials, on a purpose of enhancing the transportation and the separation of the photocarriers. Therefore, the photocatalytic activity of semiconductor would be increased. The synergic effect between conjugated π materials and photocatalysts such as TiO₂,ZnO, Bi₂WO₆,BiPO₄ etc were elucidated. In photocatalysis, the holes in the valence band of ZnO and Bi₂WO₆ could directly transfer to the HOMO orbital of C₃N₄, making charge separation more efficient and leading to an enhanced photocatalytic activity. The photocorrosion of ZnO was caused by photogenerated holes, the rapid transportion of holes from ZnO to C₃N₄ could effectively suppress the photocorrosion of ZnO. Under visible light irradiation, the excited electron from HOMO to LUMO orbital of C₃N₄ could directly inject into the conduction band (CB) of ZnO, making C₃N₄/ZnO present a dramatic visible light photocatalytic activity.

1. J. Mater. Chem. A, 2014, 2, 1174.
2. Appl. Cata. B: Environ. 2013, 138, 26.
3. Adv. Funct. Mater.,2012, 22,1518
4. Eenergy & Eevironmental Science,2011,4(8), 2922

Carbon dots (CDs) are a versatile nanomaterial with attractive photoluminescent and photocatalytic properties. In contrast to inorganic semiconductors, they have a complex internal structure, considered to comprise sp²-hybridized aromatic domains embedded in an amorphous sp³–hybridized matrix.[1] The structure manifests itself in an intricate interplay between the constituent parts with multiple charge and energy transfer pathways determining the photocatalytic properties.[2]
Here, we investigate the structure of CDs formed by microwave and hydrothermal approaches from citric acid and diamines and show that the function of the CDs can be easily tuned through simple synthetic means.[3] In particular, we demonstrate that the mode of nitrogen inclusion within the aromatic domains controls the charge transfer and separation, and thereby the photocatalytic activity for water reduction. In this context, the merit of introduction of specific active sites for hydrogen production into the structure is also discussed. It is used to demonstrate how a critical understanding of the structure and optoelectronic properties of the CDs can lead to substantial improvements in photocatalytic efficiency.
[1] Bhattacharyya et al., Effect of nitrogen atom-positioning on the trade-off between emissive and photocatalytic properties of carbon dots, Nat. Commun. 2017, accepted, DOI: 10.1038/s41467-017-01463-x
[2] Fu et al., Carbon Dots: A Unique Fluorescent Cocktail of Polycyclic Aromatic Hydrocarbons, Nano Lett. 2015, 15, 6030-6035.
[3] Lau et al., Urea-modified carbon nitrides: Enhancing Photocatalytic Hydrogen Evolution by Rational Defect Engineering, Adv. En. Mater. 2017, 7, 1602251.

Photoelectrosynthetic assemblies provide an approach to capture, convert, and store solar energy as a fuel. However, the ability to effectively assemble the requisite components and control their properties remains an outstanding challenge. Our research group has recently developed a bioinspired synthetic methodology for interfacing polymeric surface coatings to (semi)conducting surfaces. The surface-grafted polymer chains provide: 1) a protective coating for the underpinning surface, 2) appropriate functional groups to direct, template, and assemble molecular components, including catalysts, and 3) a stabilizing three-dimensional environment for catalysts embedded within the polymers. Incorporation of rational synthetic design principles affords further control over the activity of the hybrid assemblies. The reported constructs thus set the stage for an improved understanding of the nano-, meso-, and macro-scale structure−function relationships governing the optoelectronic and catalytic properties of surface-immobilized catalyst−polymer architectures.

Electrical transport in amorphous titanium dioxide films deposited by atomic-layer deposition (ALD) and their heterostructures with p⁺-Si substrates have been characterized by AC conductivity, temperature-dependent DC conductivity, space-charge-limited current (SCLC) spectroscopy, electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS) and current density-voltage (J-V) characteristics. The comprehensive characterization reported herein, indicates a Ti³⁺ defect-mediated transport model: a hopping mechanism with a defect density of 10¹⁹ cm^-3, defect band edge ~0.6 eV below the conduction band and a free carrier concentration of 10¹⁶ cm^-3, which agrees with previous Hall measurements. Amorphous TiO₂ films that were fabricated using TiCl₄ as the ALD precursor exhibited less DC conductivity than films formed using tetrakis(dimethylamido)-titanium (TDMAT) as the precursor. The DC conductance increased proportionally to the peak height of the defect states in the valence band for TiCl₄-precursor TiO₂ films grown at 50°C, indicating that substantial room temperature conductivity is not intrinsic to amorphous TiO₂ but is dependent on the introduction of defect states during the ALD fabrication process.

Utilizing solar energy to fix CO₂ with water into chemical fuels and oxygen, a mimic process of photosynthesis in nature, is becoming increasingly important but still challenged by low selectivity and activity, especially in CO₂ electrocatalytic reduction. Here, we report transition-metal atoms coordinated in a graphene shell as active centers for aqueous CO₂ reduction to CO with high faradic efficiencies over 90% under significant currents up to ~60 mA/mg. We employed three-dimensional atom probe tomography to directly identify the single Ni atomic sites in graphene vacancies. Theoretical simulations suggest that compared with metallic Ni, the Ni atomic sites present different electronic structures that facilitate CO₂-to-CO conversion and suppress the competing hydrogen evolution reaction dramatically. Coupled with Li⁺-tuned Co₃O₄ oxygen evolution catalyst and powered by a triple-junction solar cell, our artificial photosynthesis system achieves a peak solar-to-CO efficiency of 12.7% by using earth-abundant transition-metal electrocatalysts in a pH-equal system.

Direct photochemical conversion of H₂O and CO₂ into H₂ and CO is a promising strategy to mitigate CO₂ emissions and simultaneously store solar energy in renewable fuels, but most of the currently known catalysts for this purpose are based on precious metals, require organic solvents or suffer from low stability and selectivity.

We study hybrid materials that combine the photophysical properties of semiconductor nanocrystals with the selectivity of well-defined molecular electrocatalysts. Engineering the particle surface is of paramount importance to achieve efficient charge transfer in such a system. By designing material-specific surface anchors, we can attach molecular catalysts to chalcogenide quantum dots (QDs) to drive H₂ evolution and CO₂ reduction with visible light in water. Direct comparison of different anchoring groups allows us to correlate the photocatalytic activity with the QD/catalyst interface.[1] We use transient absorption spectroscopy to follow the charge-transfer processes in hybrid photocatalysts.

We further develop novel strategies to control the competition between CO₂ reduction and H₂ evolution from aqueous QDs, by exploiting the often overlooked effects of capping ligands on their photocatalytic activity.[2] Modulating the capping ligand surface coverage not only allows us to control the H₂/CO selectivity, but it also reveals new reactivities such as tuneable formic acid decomposition and biomass photoreforming.[3-4]

References
[1] J. Amer. Chem. Soc., 2017, 139, 7217; [2] J. Mat. Chem. A, 2016, 4, 2856; [3] Angew. Chem. Int. Ed., 2015, 54, 9627; [4] Nat. Energy, 2017, 2, 201721.

Solar fuel production by semiconductor photoelectrochemical water splitting entails the convertion of water by solar radiation into a storable clean fuel. Various materials have been investigated for obtaining an efficient and stable photoelectrodes, including silicon, various oxides, and III-V compounds. Oxides are stable in aqueous evironments but suffer from the low efficiency due to their wide bandgaps. Silicon and III-V compundes have bandgaps well-suited to absorbing a large portion of the solar spectrum, but their stability in aqueous electrolyte are very poor. Thus, metal-oxide-semiconductor (MOS) architecture has been proposed for photoelectrochemical solar fuel production.

In a typical MOS photoelectrodes, oxide layer protects the semiconductor substrates from corrosion, while maintains the facile transport of photo-generated carriers from semiconductor to metal catalysts. Here is a trade-off between efficiency and stability. A thin oxide layer would allow facile electron/hole transport but the long-term stability will be problematic. A thick oxide layer provides high stability while the efficiency would be affected. So, in this work, to address this issue, several strategies were investigated to engineering the oxide layer to obtain an efficient photoelectrode with high stability.

Molybdenum disulfide (MoS₂) and its related layered transition-metal dichalcogenides (TMDs) have attracted much attention as potential electrocatalysts for converting carbon dioxide to fuels due to their lower price compared with precious metals and their prominent catalytic features. MoS₂ and MoSe₂ have recently been shown to perform as excellent electrocatalysts in ionic-liquid-based systems for the CO₂ reduction reaction (CO₂RR)^1,2. However, achieving selectivity in the CO₂RR is challenging due to the numerous possible chemical reaction pathways and their very similar reduction potentials, often leading to a multitude of CO₂RR products. Theoretical calculations³ indicate that the conduction band (CB) edge position of TMD materials can be tuned by adjusting the layer thickness (i.e. monolayer vs. bilayer), as well as by chemically alloying (e.g. MoSSe). The TMDs therefore offer a suitable material system for adjusting the CB edge of the catalyst relative to a given reduction potential for CO₂RR. Herein, we report the thickness-controllable, large-area (1 cm²) growth of Mo(S,Se)₂ thin-films synthesized via metal–organic chemical vapor deposition (MOCVD) as the catalysts for CO₂RR. As a first step, we evaluated bulk crystals of MoS₂, MoSSe, and MoSe₂ for electrochemical CO₂RR in aqueous K₂CO₃ solution (pH = 6.8) at -0.6 V vs. RHE. The results show that both MoSSe and MoSe₂ produce 4 times more CO and CH₄ than MoS₂. To further explore the effect of the CB edge position relative to the CO₂RR, we synthesized thickness-controllable, large-area Mo(S,Se)₂ electrodes integrated with degenerately-doped Si substrates via MOCVD. This process enables the incorporation of organic selenium vapor source during the MOCVD growth process to change the S/Se ratio in Mo(S,Se) alloys. The CO₂RR product analysis as a function of film composition and thickness will be discussed and compared to the relevant CO₂RR reduction potentials.


Reference
1. Asadi, M. et al. Robust carbon dioxide reduction on molybdenum disulphide edges. Nat. Commun. 5, 4470 (2014).
2. Asadi, M. et al. Nanostructured transition metal dichalcogenide electrocatalysts for CO₂ reduction in ionic liquid. Science 353, 467-469 (2016)
3. Kang, J. et al. Band offsets and heterostructures of two-dimensional semiconductors, Appl. Phys. Lett. 21, 012111 (2013)

Plasmonic-metal nanostructures exhibit broadly tunable optical properties coupled with catalytically active surfaces that offer unique opportunities for solar photocatalysis. Of particular interest is the resonant optical excitation of surface plasmons to produce energetic “hot” carriers at both metal-semiconductor and metal-electrolyte interfaces that can drive photochemical reactions. While examples of hot-electron-driven carrier collection and photoelectrochemical processes have been widely reported, little is known about the nature of plasmon-derived hot holes and their role in hot carrier photocatalysis. Here, we report the demonstration of a plasmon-driven photoelectrochemical CO₂ reduction process occurring in a Au/p-type GaN photocathode. Hot-hole collection is observed to occur by hole injection from gold (Au) nanoparticles into a p-type gallium nitride (p-GaN) semiconductor support. Despite an interfacial Schottky barrier to hole transport of more than 1 eV across the Au-GaN heterojunction, plasmonic Au/p-GaN photocathodes exhibit photoelectrochemical properties consistent with the injection of hot holes into GaN upon plasmon excitation of Au nanoparticles. The photocurrent spectral response of our plasmonic Au/p-GaN photocathode faithfully follows the surface plasmon resonant absorption spectrum of the Au nanoparticles and open-circuit voltage studies demonstrate the ability to sustain a plasmonic photovoltage of 20 mV across the Au-GaN heterojunction during plasmon excitation. For comparison, Au/p-NiO heterojunction were formed by Au nanoparticle deposition onto p-type nickel oxide (p-NiO) photocathodes, for which there is no Schottky barrier across the metal-semiconductor heterojunction. Significantly, there is little difference in device performance between these two distinct systems, supporting previous theoretical predictions about the distribution of hot holes deep below the Au Fermi level. The plasmonic Au/p-GaN photocathodes were further employed for plasmon-driven CO₂ reduction in aqueous electrolyte, illustrating the concept of plasmon-driven artificial photosynthesis. Taken together, our results offer experimental verification of optically-excited hot holes more than 1 eV below the Au Fermi level and demonstrate a photoelectrochemical platform for harvesting them to drive solar-to-fuel energy conversion.

Semiconducting materials hold the key for efficient photocatalytic and photoelectrochemical water splitting. In this talk, we will give a brief overview of our recent progresses in designing semiconductor metal oxides materials for photoelectrochemical energy conversion including photocatalytic solar fuel generation. In more details, we have been focusing the following a few aspects; 1) band-gap engineering of layered semiconductor compounds including layered titanate, tantalate and niobate-based metal oxide compounds for visible light phtocatalysis, and 2) two-dimensional nanosheets/nanoplates of TiO₂, Fe₂O₃, WO₃, BiVO₄ as building blocks for new photoelectrode design，and 3) the combination of a high performance photoelectrode BiVO₄ with perovskite solar cells can lead to unassisted solar driven water splitting process with solar-to-hydrogen conversion efficiency of >6.5%; The resultant material systems exhibited efficient visible light photocatalytic performance and improved power conversion efficiency in solar energy, which underpin important solar-energy conversion applications including solar fuel generation and simultaneous environmental application.

High efficiency artificial photosynthesis, that can convert solar energy directly into chemical fuels, is one of the key sustainable technologies to enable a carbon-free, storable and renewable source of energy. To date, however, success in finding abundant visible-light active photocatalyst has been very limited. In this context, we have investigated the photocatalytic and photoelectrochemical properties of InGaN nanowires. Compared to conventional metal-oxide and other semiconductor photocatalysts/photoelectrodes, the energy bandgap of InGaN can be tuned across nearly the entire solar spectrum. Moreover, InGaN is the only known semiconductor whose conduction and valence band edges can straddle water redox potentials under deep visible light irradiation. We have demonstrated that the quantum efficiency of solar-to-hydrogen conversion on InGaN nanowires can be enhanced by two orders of magnitude through precise tuning of the near-surface Fermi level. We have further demonstrated an integrated InGaN/Si nanowire photoelectrode system that can lead to significantly enhanced efficiency and stability in strong acidic solution. Moreover, we have demonstrated the reduction of CO₂ into methanol (CH₃OH) and syngas on InGaN nanowires utilizing sunlight.

In this work, InGaN nanowires are grown on Si substrate by molecular beam epitaxy. The water splitting reaction takes place on the nonpolar surface (m-planes) of InGaN nanowire photocatalysts. With increasing Mg-dopant incorporation, the efficiency for solar-to-hydrogen conversion is enhanced by nearly two orders of magnitude. The internal quantum efficiency reaches ~75% with optimum Mg doping concentration. The significantly enhanced efficiency is directly related to the optimized surface electronic properties that lead to both efficient water oxidation and proton reduction. A solar-to-hydrogen conversion efficiency of 3.4% was demonstrated without any energy input other than sunlight. We have further demonstrated multi-band InGaN/GaN nanowire photoelectrodes monolithically integrated on a Si solar cell wafer. The tandem PEC device consists of a planar n⁺-p Si solar cell wafer and p-InGaN nanowire segments. The p-InGaN nanowire arrays are designed to absorb the ultraviolet and visible solar spectrum. The remaining photons with wavelengths up to 1.1 µm are absorbed by the underlying planar Si p-n junction. Such a monolithically integrated photocathode promises solar-to-hydrogen conversion efficiency >20%. With the use of such a photoelectrode, we have also demonstrated that syngas, a key feedstock to produce methanol and liquid fuels in industry, can be produced from a CO₂ and H₂O with a benchmark turnover number of 1330 and a desirable CO/H₂ ratio of 1:2. Work is currently in progress to achieve high efficiency syngas and methanol generation in an aqueous photoelectrochemical cell.

Photosystem I (PS I), the photosynthetic membrane protein, undergoes light activated (λ=680 nm) charge separation and unidirectional electron transfer with near-unity quantum efficiency. The robust photoelectrochemical (PEC) activities of PSI make it an ideal biomaterial for bio-hybrid photovoltaic and/or, optoelectronic devices.^1,2 But, the first step towards rational design of such devices requires systematic electrochemical characterizations of PSI assembly in tailored biotic-abiotic interfaces.³ In the past, such interfaces have been created using plasmonic metal nanostructures to tune optoelectronic properties of molecular fluorophores. Herein, we investigate plasmon-enhanced photocurrents from PSI assembled with plasmonic Ag and Au nanopatterned structures. Based on our recent works, we present the first-ever experimental verification of plasmon-induced photocurrent enhancements from PSI attached to Fischer patterns of Ag nano-pyramids (Ag-NP) whose resonance peaks are tuned to match the PSI absorption peaks at ~450 and ~680 nm. Detailed atomic force microscopy (AFM) characterizations reveal both the background Fischer patterns and the PSI immobilization on them. The plasmon enhanced photocurrents indicate enhancement factors of ~6.5 and ~5.8 as compared to PSI assembly on planar Ag substrates for nominal excitation wavelengths of 660 and 470 nm respectively.⁴ The comparable enhancement factors from both 470 nm and 660 nm excitations, in spite of a significantly weaker plasmon absorption peak at ~450 nm for the Ag-NP structures, can be explained by previously reported observations of excessive plasmon-induced fluorescence emission losses from PSI in red region of the excitation wavelengths. Based on these results, our on-going efforts are directed towards the systematic investigation of the effect of varying plasmon peak positions tuned with designer nanopatterns on the plasmon-enhanced photocurrents from PSI assembly on these structures. We aim to carry out systematic studies on the role of tailored Ag and Au nanopatterned substrates designed with E-beam lithography for specific peak plasmonic resonance peaks and the distance of separation between PSI to the plasmonic surfaces on plasmon-induced photocurrents from PSI complexes. Aforementioned work will shine light on the fundamental biophysics behind the alterations in excitation energy transfer mechanism among chlorophylls in PSI under plasmon-induced localized electric field.<!--![endif]---->
References:
(1) D.Mukherjee, M. May, B. Khomami; J. Coll. Interf. Sci., 2011, 358, 477..
(2) D. Mukherjee, M. May, M. Vaughn, B. D. Bruce, B. Khomami; Langmuir, 2010, 26, 16048.
(3) T. Bennett, H. Nirooman, R. Pamu, I. Ivanov, D. Mukherjee, B. Khomami; PCCP, 2016, 18, 8512.
(4) R. Pamu, B. Lawrie, R. Kalyanaraman, D. Mukherjee, B. Khomami;Nature Comm., 2016, Submitted.

Energy conversion such as CO₂ reduction to fuel and water splitting needs catalysts with high activity. Nanostructured materials are promising for future application in this area. Although there are a large number of related publications, the catalytic activity and stability for energy conversion is still far from application. So far, Cu and CuO_x materials have been widely applied as catalysts for electrochemical, photochemical and photoelectrochemical CO₂ conversion. Additionally, Cu as a good conductive material works well for electrode substrate. In order to take advantage of Cu and CuxO materials and overcome their problems, we have prepared nanostructured Cu and CuxO based materials for photochemical, electrochemical and photoelectrochemical CO₂ reduction to organic fuel.^[1-4] Besides, Cu nanowires have been used as the substrate to fabricate a highly efficient three-dimensional (3D) bulk catalysts of core-shell structure, in which thin NiFe layered double hydroxide (LDH) nanosheets are grown on the substrate cores supported on Cu foams, toward overall water splitting.^[5-6] The preliminary conclusion can be reached that Cu and CuxO materials are prospective in future practical application in energy conversion and the 3D core-shell electrocatalysts significantly advances the research towards large-scale practical water electrolysis.

References:
[1] Y. Li, W. Zhang, X. Shen, P. Peng, L. Xiong and Y. Yu, Chin. J. Catal. 2015, 36, 2229-2236.
[2] L. Yu, G. Li, X. Zhang, X. Ba, G. Shi, Y. Li, P.K. Wong, J. C. Yu, and Y. Yu, ACS Catal. 2016, 6, 6444-6454.
[3] X. Ba, L. Yan, S. Huang, X. Xia and Y. Yu, J. Phys. Chem. C, 2014, 118, 24467-24478.
[4] G. Shi, L. Yu, X. Ba, X. Zhang, J. Zhou and Y. Yu, Dalton Trans. 2017, 46: 10569-10577.
[5] Luo Yu, Haiqing Zhou, Jingying Sun, Fan Qin, Fang Yu, Jiming Bao, Ying Yu, Shuo Chen and Zhifeng Ren, Energy Environ. Sci. 2017, 10: 1820-1827.
[6] Luo Yu, Haiqing Zhou, Jingying Sun, Fan Qin, Dan Luo, Lixin Xie, Fang Yu, Jiming Bao, Yong Li, Ying Yu, Shuo Chen and Zhifeng Ren, Nano Energy. 2017, 41: 327-336.

Metal oxide semiconductors are promising materials in photocatalytic and photoelectrochemical (PEC) water-splitting devices due to their high chemical stability, and flexibility in manipulating their physicochemical properties [1]. In addition to developing their bulk properties (optical absorption, electrical conductivity, etc.), tailoring the flatband potential at the oxide/electrolyte interface is essential for the enhancement of the spatial separation efficiency of photo-excited carriers [2]. We recently demonstrated the modulation of the flatband potential over 1.3 V by embedding a perovskite atomic dipole layer consisting of (LaO)⁺ and (AlO₂)^- just at the sub-surface of a (001)-oriented SrTiO₃ (SrTiO₃) photoanode [3]. While this large modulation showed the potential of this approach, the direction of the shift was opposite to that needed for improving the PEC performance for an n-type photoanode.

Here, we present a 400 mV flatband potential shift at the SrTiO₃ (001)/electrolyte interface, successfully enhancing the photocurrent by ~10%. On a TiO₂-terminated Nb-doped SrTiO₃ (001) substrate, a negatively-charged oxide layer (AlO₂)^- and an ultrathin charge neutral SrTiO₃ layers were sequentially grown by pulsed laser deposition. By immersing the entire structure in solution, the positively-charged ions adsorb onto the SrTiO₃ surface to complete the dipole. This design strategy is based on our work on the solid-state analog, where we demonstrated a barrier height increase at oxide Schottky interfaces using a dipole created by a fixed oxide charge layer and induced electronic screening charge, instead of two oxide fixed charge layers [4]. The ability to independently manipulate the oxide/electrolyte interface energy levels greatly impacts the fundamental approach in developing heterostructures for photocatalytic and PEC applications. Details of the fabrication, PEC characterization, as well as spectroscopic results 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 et al., Chem. Rev. 110, 6446 (2010).
[3] Y. Hikita et al., Adv. Energy Mater. 6, 1502154 (2016).
[4] T. Yajima et al., Nature Commun. 6, 6759 (2015).

After a brief introduction on the principle of photoelectrochmical water splitting, we will present our recent results on water splitting photoelectrodes. Firstly, we show high performance Si-based water splitting photocathodes using transition metal chalcogenides (TMCs) such as MoS₂, WS₂, and MoP. It is emphasized that benign band bending and defect engineering of TMCs are crucial to enhance charge transfer processes between the light absorber and the catalytic layer and between the solid catalyst and the liquid electrolyte, respectively. Secondly, we show water oxidation performance of MnO/BiVO₄/WO₃/FTO photoanodes. It is revealed that charge transfer between MnO nanoparticles and the underlying BiVO₄ layer is strongly depending on the ligands of MnO nanoparticles, by which the position of the band edges changes considerably. At last, we present our study on the role of plasmonic Au nanoparticles on water splitting activities of various metal oxide photonanodes. Our results indicate that shape control of Au nanoparticles is important and the catalytic hole transfer should be accompanied for direct electron injection from the nanoparticles to the metal oxide. It is shown that plasmonic resonance energy transfer can be dominant over direct energy transfer in Au nanoparticles/BiVO₄ system.

TiO₂ nanomaterials have over the last 30 years attracted tremendous scientific and technological interest. Particularly various 1D and highly defined TiO2 morphologies were explored for the replacement of nanoparticle networks and were found in many cases far superior to nanoparticles or their assemblies. Nanotubes or wires can be grown by hydrothermal or template methods, or even more elegantly, by self-organizing anodic oxidation. The latter is not limited to TiO₂ but to a full range of other functional oxide structures on various metals and alloys can be formed. These advanced and doped morphologies can be grown on conductive substrates as ordered layers and therefore can be directly used as functional electrodes (e.g. photo-anodes). The presentation will focus on these highly ordered nanotube arrays of TiO₂ (and similar) and discuss most recent progress in synthesis, modification and applications towards photocatalytic and photoelectrochemical applications such as noble-metal-free H2 generation or the site-selective placement of active centers onto/into these tube layers.
Literature:
- P. Roy, S. Berger, P. Schmuki, Angew. Chem. Int. Ed. (2011), 2904
- K. Lee, A. Mazare, P.Schmuki, CHEMICAL REVIEWS, 114 (2014) 9385
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- N. Liu, P. Schmuki et.al. Angew. Chem. Int. Ed. 53 (2014) 14201
- X. Zhou, N.Liu, P.Schmuki ACS Catal., ,7 (2017) 3210

Here we report a new motif for highly catalytically active, large-area, stable nanoporous metal electrodes of Au and Cu with a nanofoam morphology, synthesized by dealloying of metal alloy thin films. This process yields 500 nm thick nanoporous Au electrodes with 5-15x larger electrochemical activity that than for comparable planar Au films, owing to increased surface area. The nanoporous Au (np-Au) structure is fabricated via electron beam co-deposition of a gold/silver (Au/Ag) alloy of tunable elemental composition (10/90 to 30/70) and variable thickness (0.1 μm to 1 μm), followed by a chemical etch of silver with nitric acid to yield a monolithic, np-Au structure. Depending on the etching temperature, we can alter the Au feature size from 10-25nm. The np-Au also possesses near-unity absorption throughout a broad portion of the visible spectrum from (400nm to 600 nm). Gas chromatography indicates H₂ and CO as reduction products of CO₂ for np-Au electrodes in a 50 mM K₂CO₃ buffer at pH 6.8 with similar CO:H₂ product ratio vs applied potential as that obtained for flat Au electrodes. Chronoamperometry measurements indicate stable np-Au electrode operation over periods > 6 hours at a potential of -1.34 V vs RHE and current of -10mA/cm². Nanoporous Cu (np-Cu) electrodes are synthesized in a similar way, but using a Cu/Al alloy and HCl as the etchant for selective Al removal. Both np-Cu and np-Au electrodes show a photocatalytic response marked by a small increase in current density of ~5μA/cm² upon irradiation with white light at a power of 300mW/cm². Experiments to understand the product selectivity for np-Au and np-Cu photocatalytic electrodes as a function of temperature, illumination wavelength, and incident light power will be discussed. In summary, nanoporous metallic electrodes exhibit high activity and stability, and are interesting candidates as cathodes for electrochemical and photoelectrochemical CO₂ reduction.

Transition metal dichalcogenides (TMDs) are efficient electrodes for photoelectrochemical solar energy conversion to electricity and chemical fuels. One approach to produce large-area thin film photoelectrodes is to exfoliate single or multiple TMD layers from high quality bulk samples. However, exfoliated TMD thin films exhibit lower photocurrent collection efficiencies than bulk electrodes. Here we use a single-nanoflake photoelectrochemical microscopy approach to determine how variations in nanoflake area, thickness, and surface structural features contribute to the lower photocurrent collection efficiency in n-MoSe2/I-,I3-/Pt solar cells. Photocurrent efficiency increases with nanoflake area, but is independent of nanoflake thickness over the range of 40-400 nm. Surprisingly, there is a small, "champion" population whose photocurrent efficiency values are nearly equivalent to or exceed the bulk crystal. This observation, which is hidden in ensemble-average measurements, shows that exfoliated nanoflakes can achieve similar photocurrent collection efficiencies to bulk crystals. There is a also a large, "spectator" population that is mostly responsible for the overall lower photocurrent efficiency compared to the bulk crystal. Photocurrent mapping showed that charge carrier recombination near perimeter edges is more significant than at interior steps. Our results highlight nanoflake properties that should be considered and optimized for high performance exfoliated TMD photoelectrodes.

Photoelectrochemical water splitting, the production of hydrogen and oxygen by solar irradiation of semiconductor materials in water, is an elegant method of sustainable energy production. In recent years, significant progress has been made in the development of materials for water splitting, especially metal oxide semiconductors. For example, bismuth vanadate (BiVO₄), an n-type photoanode material, has been optimized to operate at 90% of the theoretical maximum efficiency based on its bandgap energy of 2.4 eV.¹ To make further progress in photoelectrochemical water splitting, we must optimize new and emerging semiconductor materials with smaller bandgap energies in the range of 1.5 to 2.0 eV. One such material is copper bismuth oxide (CuBi₂O₄), a p-type semiconductor with a bandgap energy of 1.6 - 1.8 eV.^1,2 Recently, we identified the main challenges that must be overcome to develop CuBi₂O₄ as a photocathode material including its relatively poor charge carrier transport and photo-corrosion under the operating conditions for the hydrogen evolution reaction.³ To address these challenges we have explored several strategies such as new synthesis techniques to control the surface structure and morphology, modifications in the chemical composition to improve the electrical properties, and conformal deposition of protection layers to prevent photo-corrosion.^4,5 This has resulted in continuous improvements in the efficiency and stability. Presently, our CuBi₂O₄ photocathodes can evolve hydrogen at current densities on the order of 1 mA/cm² with Faradaic efficiencies above 90%.

References
(1) Abdi, F. F.; Berglund, S. P. Journal of Physics D: Applied Physics 2017, 50, 193002.
(2) Berglund, S. P.; Lee, H. C.; Nunez, P. D.; Bard, A. J.; Mullins, C. B. Physical Chemistry Chemical Physics 2013, 15, 4554.
(3) Berglund, S. P.; Abdi, F. F.; Bogdanoff, P.; Chemseddine, A.; Friedrich, D.; van de Krol, R. Chemistry of Materials 2016, 28, 4231.
(4) Wang, F.; Chemseddine, A.; Abdi, F. F.; van de Krol, R.; Berglund, S. P. Journal of Materials Chemistry A 2017, 5, 12838.
(5) Wang, F.; Septina, W.; Chemseddine, A.; Abdi, F. F.; Friedrich, D.; Bogdanoff, P.; van de Krol, R.; Tilley, S. D.; Berglund, S. P. Journal of the American Chemical Society 2017, 139, 15094.

The choice of photoelectrodes used in solar water splitting depends on a few intricate factors such as suitable optical absorption range, efficient charge transport properties and long term stabilities in aqueous environments. As nature does not provide such efficient material, careful design strategies are required to realize an efficient photoelectrode. For example, wide bandgap metal oxides, which are usually stable in aqueous solution, are not efficient light absorber and possess poor electronic properties. On the other hand, low bandgap metal sulfides, which absorbs visible light efficiently with reasonable electrical properties, are usually not stable in aqueous solution. In this talk, I will show our latest efforts in altering the intrinsic properties of metal oxide photoanodes, such as iron oxide and metal vanadates. I will also show how cation substitution and surface modifications are used to improve the photocatalytic efficiency of metal sulfide photocathode. Unassisted water splitting using hematite photoanode stacked with a perovskite solar cell is also demonstrated.

The chemical literature does not strongly differentiate between photocatalytic (PC) and photosynthetic (PS) reactions (including artificial photosynthesis), even though these are different processes. Photocatalytic processes are thermodynamically downhill (DG<0) and are merely accelerated by a catalyst whereas photosynthetic processes are thermodynamically unfavorable (DG>0) and require photochemical energy input to occur. Here we apply this differentiation to formulate design criteria for PC and PS devices, with a special emphasis on solar water photoelectrolysis / solar hydrogen generation. As will be shown, under conditions of optimal light absorption, carrier lifetimes, and electrochemical rates, the performance of PCs is limited only by their surface area, while type 1 PS devices are limited by their carrier mobility and mass transport, and type 2 PS devices are limited by electrochemical charge-transfer selectivity. Strategies for the optimization of type 1 and 2 photosynthetic devices and photocatalysts are also discussed.

The light-activated plasmonic properties of metal nanodeposits loaded on an appropriate support can be harnessed so as to promote the activity and/or influence the selectivity of a thermal catalyst system. Nobel metals such as Au and Ag, which possess well-known visible-light-activated plasmons, can be coupled with other transition metals, such as nickel, to alter their catalytic activity. Alternately the transition metal plasmon can itself be used to promote catalytic performance although assistance by a promoter may be required. Here, using nickel as the catalytically active metal loaded on a metal oxide support, different strategies for integrating plasmonics into the system so as to promote the carbon dioxide methanation reaction are examined. In the first instance, the nickel is coupled with gold or silver on a ceria support and subject to laser illumination (l = 520nm) so as to capture the noble metal plasmon and improve catalyst performance. While the Au/Ag exhibits a positive plasmonic effect it is overshadowed by the poor activity of the noble metal toward carbon dioxide methanation. In the second instance, the plasmonic properties of metallic nickel loaded on a titania support are able to be harnessed for carbon dioxide methanation only when the support is promoted with lanthanum oxide. The lanthanum oxide ‘activates’ the carbon dioxide with the ensuing carbonate species then being susceptible to the nickel plasmon. Overall, the findings illustrate that while light-activated plasmonics can be useful for catalytic reactions such as methanation, successfully ultilising the phenomenon is not always straight-forward.
If time permits, the findings from recent research relating to using light pre-treatment to promote catalytic oxygen activation will be presented. Oxygen activation, via the oxygen reduction reaction ORR), is thought to be the bottle-neck process in fuel cell systems. Here, the use of light pre-treatment to accelerate the ORR in the form of either plasmonic enhancement or defect generation of the metal oxide support will be detailed.

First principles calculation has become a powerful tool for materials design in many fields [1,2]. However, theoretical design of photocatalysts and photoelectrodes has met with limited success, due to the lack of adequate modeling criteria. Under illumination, in the case of usual n-type semiconductor photoelectrode, owing to the space charge region formed in the vicinity of the semiconductor surface, photogenerated (excess) electrons drift to the bulk of the semiconductor whereas excess holes accumulate at the semiconductor surface. However, to the best of our knowledge, there are no computational studies that systematically examine the effect of “surface charging” on photocatalytic activities. This is presumably because, in the periodic boundary conditions, excess surface charge cannot be handled with the standard first-principles computational approaches in a straightforward manner.
In this work we study the effect of “surface charging” on the electronic and geometrical structures of the photoelectrode/electrolyte interface, with the aid of first-principles density functional theory (DFT) calculations. The effective screening medium (ESM) method [3], which can handle non-integer charge, is utilized in order to methodically vary the concentration of excess holes at the semiconductor surface. Well studied polar GaN(0001)/water interface is chosen as a model system. The bare surfaces, terminated with H atom and hydroxyl group, are considered. In addition, in order to account for the Helmholtz layer which is formed within the electrolyte, we adopt the recently developed ESM-reference interaction site method (RISM) [4], which can treat non-integer counter ions in the solvent.
The computational results show that the surface state originating from Ga dangling bonds is located roughly 1 eV below the conduction band minimum (CBM) and has a dispersion larger than 1 eV. The surface states originating from Ga dangling bonds diminished with increasing coverage of H atoms and hydroxyl groups, resulting in the decrease in the band bending and the strength of the Fermi level pinning induced by the surface states. Moreover, we find that the adsorption energy of H and hydroxyl group is strongly dependent not only on the adsorbate coverage, but also on the amount of excess charge at the surface. We will also report the effect of water molecule alignment and ion distribution on the electronic and geometrical structures of the interface. These results demonstrate that our surface-charge-sensitive modeling approach can provide critical insight into photocatalytic activities.

[1] S. Charkraborty et al., ACS Energy Lett. 2017, 2, 837
[2] E. I. Izgorodina et al., Chem. Rev. 2017, 117, 6696
[3] M. Otani and O. Sugino, Phys. Rev. B 2006, 73, 115407
[4] S. Nishihara and M. Otani, Phys. Rev. B 2017, 96, 115429

Water splitting to oxygen and hydrogen under sun light using semiconductor photoanodes has recently attracted lot of interest. A photoelectrochemical water splitting device employs in many cases metal-insulator-semiconductor (MIS) structure [1] with a top thin metallic film, which catalyses water oxidation reaction.
In our contribution we have examined the properties of Si photoanodes covered by RuO₂ thin films. Thin RuO₂ films are considered as a promising catalyst for oxygen evolution reactions. The RuO₂ thin films are transparent for sun light spectrum, exhibit high chemical stability, high work function and low resistivity. However, there are only few attempts in literature on application of RuO₂ films for water splitting.
Metal-insulator-silicon photoanode was prepared on silicon substrate. Ultrathin SiO₂ films were prepared by exposure of silicon to ozone treatment at 300 °C. Top RuO₂ layers were prepared by liquid injection chemical vapor deposition using [Ru(thd)₂(cod)] precursor dissolved in iso-octane at 300 °C [2].
The RuO₂ film exhibited low resistivity of 10^-4 Ωcm and optical transmittance up to 80% in the visible sun light region. RuO₂/SiO₂/Si photoanode exhibit high photovoltage of 0.5 V in the acid (1M H₂SO₄) and base (1M KOH) solutions. It was observed, that RuO2/SiO₂/Si photoanode can generate photocurrent density of 10 mA/cm² at thermodynamic water oxidation potential in the acid solution (E⁰_H2O/O2=1.23 V). These parameters are comparable to the state-of-art results [3] and indicate great potential of RuO₂ protected Si photoanodes for water splitting. Unfortunately, initial experiments revealed insufficient stability of the RuO₂/SiO₂/Si photoanode for long term operation. Further steps to enhance electrochemical corrosion resistance of the RuO₂/SiO₂/Si photoanode are discussed.


Acknowledgements
The authors acknowledge support of VEGA projects 1/0651/16 and 2/0136/18.

References
[1] T. Zhu, M. N. Chong, Nanoenergy 12 (2015) 12347.
[2] K. Fröhlich, V. Cambel, D. Machajdík, P. K. Baumann, J. Lindner, M. Schumacher, Mater. Science Semicond. Processing 5 (2003) 173.
[3] Y. W. Chen, J. D. Prange, A. Dühnen, Y. Park, M. Gunji, C. E. D. Chidsey and P. McIntyre, Nature Mater. 10 (2011) 539.

Artificial photosynthesis (AP), producing carbon based liquid fuel compounds by reducing CO₂ with water and sunlight, is regarded as one of the environment friendly methods on CO₂ reduction. Finding the efficient ways to solar energy utilization for CO₂ reduction has attracted great attention due to its potential possibilities, which can produce useful chemicals such as methane, formaldehyde, mehtanol and carbon monooxide from CO₂. Since the first concept of AP suggested by G. Ciamician at 1912, a lot of researchers tried to realize AP with chemical approach, electrochemical approach and photochemical approach. However, the major drawbacks of these technologies are low amount of reaction product and limited on specific few reduction products such as formic acid, formaldehyde and methane, derived from poor selectivity during CO₂ reduction reactions. Herein, we suggest the efficient approaches on increasing efficieny of AP reaction based on few parameters. By adjusting those key parameters, we successfully reached on 1.06 % of solar to fuel efficiency with over 90 % of CO₂ reduction product selectivity.

Photoelectrochemistry (PEC) is an attractive method to renewably produce H₂, useful both as a chemical stock for industrial processes, and a renewable replacement to fossil fuels. Chalcopyrite materials in particular, with a demonstrated success as photovoltaic absorbers, offer exceptional candidates for cheap and sustainable solar fuels production. For example, previous results from our group have already demonstrated high saturated photocurrent densities (>15 mA/cm²) and high Faradaic efficiency (> 85%) from single-junction CuGaSe₂ photocathodes.
However, single-junction chalcopyrite devices are not feasible for stand-alone water splitting, in which photovoltages over 1.6 V are required. This has motivated research into multi-junction PEC devices as an alternative. Several theoretical analyses of tandem PEC devices show that the bandgap of the top absorber material must be > 1.70 eV to reach over 15% solar-to-hydrogen efficiency.
Cu(In,Ga)S₂ (CIGS) in particular has a bandgap tunable from 1.5 to 2.4 eV, making it attractive as a top-absorber for tandem PEC devices. Previous efforts by our group have led to 2.0 eV CIGS generating promising saturated photocurrent density in excess of 5 mA/cm², synthesized by sulfurizing a Cu(In,Ga)Se₂ precursor.
In this communication, we present an improved route to synthesize higher quality CIGS through a facile sulfurization of co-evaporated Cu(In,Ga) alloys. These materials were assessed as both solid-state photovoltaic (PV) devices and as PEC photocathodes. Bare (etched PV) CIGS photocathodes generated photocurrent densities over 10 mA/cm² at saturation, under AM1.5G simulated illumination in 0.5M H₂SO₄. In a solid-state configuration (with CdS n-type buffer), QE analysis performed at 0 V bias revealed a conversion efficiency up to 50% in the visible, with integrated QE over AM1.5G equal to 8.6 mA/cm². The solid-state CIGS cells were further coated with Pt and tested as integrated PV electrolyzer devices. A 300 mV anodic onset potential shift was achieved compared to the bare CIGS, and IPCE measurement led to a photocurrent density close to that measured in air on the solid-state device (8.15 mA/cm²). The perspective gained from the combined PV and PEC performance will lead to a better understanding of CIGS as a photocatalytic material.

Rational design of soft-to-hard material interfaces offers new opportunities to control matter and energy across the nano- and meso-scales, thus providing a chemical strategy to tailor the structural and physical properties of surfaces with molecular level precision. In the context of energy transduction, interfacing molecular catalysts with solid-state substrates is a promising approach to developing hybrid materials for generating solar fuels. However, effective integration of the requisite components, while controlling their redox properties and stability, remains a major challenge. Taking inspiration from nature, where specific amino acid residues and soft-material coordination environments control the redox properties of metal centers in proteins during enzymatic catalysis, we show that thin-film polymer surface coatings provide a novel strategy for assembling human-engineered catalysts onto solid supports. This presentation describes recent results from our laboratory aimed at better understanding the electrochemical and optical properties of hydrogen production catalysts assembled onto polymer-modified electrode surfaces. The polymer immobilization method results in unique electronic and vibrational spectroscopic signals associated with the immobilized molecular species. In addition, the use of discrete polymer architectures, coupled with rational synthetic modifications to the catalyst’s ligand environment, affords control over the chemical stability and redox potentials of surface immobilized molecular complexes, spanning a ~250 mV range.

The use of doped TiO₂ as a photocatalyst in water splitting for hydrogen production continues to demonstrate significant potential in renewable energy technology devices. We characterised the behaviour, nature and spatial location of non-metal ion dopants in the TiO₂ matrix. To create a stable dopant distribution profile that can be probed in the 3D space, single crystal substrate forms of TiO₂ were used.
We investigated the diffusion of nitrogen, sulphur, carbon and boron dopants into TiO₂ (110), (100), (001) substrates using a novel high temperature method. The former three dopants were found to be in interstitial and substitutional sites with a concentration gradient into the bulk, as measured by the Thermoscientific (K-alpha & Theta probe) X-ray photoelectron spectroscopy.
We also report the growth of a TiBO₃ phase (at least 20nm thick) in multiple orientations, different to that of the TiO₂ (110) substrate. XRD has confirmed a film-like layer of TiBO₃ on the TiO₂ surface whilst Raman spectroscopy has shown potential uniform growth of TiBO₃ on the TiO₂ (100) and (001) surfaces but not on the TiO₂ (110) surface. Reciprocal space maps revealed the orientation of the TiBO₃ layer.
Whilst the ability to grow thick films of TiBO₃ is crucial for the advancement of facile film growth technology, fundamental insight into dopant diffusion gradients is significant for the investigation of structure-function relationship, essential for designing materials with optimised photoactivity.