This PDF file contains the front matter associated with SPIE Proceedings Volume 7770, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

Porous films of TiO₂ and TiO₂/WO₃ were deposited onto transparent electrodes from aqueous suspensions with polyethylene glycol, TiO2 particles and different amounts of tungistic acid. After annealing, crystalline samples were obtained. The band gap energy, approximately 3.1 eV for TiO₂, decreased from 2.9 to 2.7 eV for varying W/Ti molar ratios from 3 to 12 %. The electrochemical properties were investigated in Na₂SO₄ aqueous solution; for the TiO₂ electrode, the open circuit potential changed from 0.18 V in the dark to -0.25 V under irradiation from a solar simulator. For hybrid TiO₂/WO₃ electrodes, the VOC values were almost independent of the WO₃ content and corresponded to 0.3 V in the dark and -0.1 V under irradiation; however, photocurrent and interfacial capacitance increased with a higher WO₃ concentration. The electrodes were then used as photocatalysts for 17-α-etinylestradiol removal from water, and the mixed TiO₂/WO₃ exhibited better performance for photocatalytic oxidation of estradiol than TiO₂. Adding WO₃ enhances the visible light harvesting and minimizes the charge recombination resulting in higher efficiency for solar energy conversion.

We employ x-ray photoelectron spectroscopy (XPS), reflection high-energy electron diffraction (RHEED) and nuclear reaction analysis (NRA) to characterize the concentration-dependent structural properties of nitrogen doping into rutile TiO2. High quality N-doped TiO₂ were prepared on rutile single crystal TiO₂(110) substrates using plasma-assisted molecular beam epitaxy with an electron cyclotron resonance (ECR) plasma and Ti effusive sources. Films with N dopant concentrations at or below 2 at.% exhibited predominately substitutional doping based on NRA data, whereas films with concentrations above this limit resulted in little or no substitutional N and surfaces rich in Ti3⁺ . The binding energy of the N 1s feature in XPS did not readily distinguish between these two extremes in N-doping, rendering features within 0.4 eV of each other and similar peak profiles. Although widely used to characterize the state of N in anion-doped TiO₂ materials, we find that XPS is unsuitable for this task.

ZnO_1-xSe_x films have been prepared through pulsed laser deposition as a step toward stable films with a band gap appropriate for water splitting. The films show a clear red shift in absorption with increasing Se content and a shift in the flat band voltage toward spontaneity. Due to the films' electron affinities, there exists a natural tunnel junction between these n- ZnO_1-xSe_x films when grown on the p-side of a Si diode. The overall performance, emphasized by flat band potential measurements, can be improved by growing films on Si p-n diodes.

We perform extensive density-functional theory total-energy calculations and ab-initio molecular dynamics simulations to evaluate the stability and reactivity of surface oxides and hydroxides of InP(001) for photoelectrochemical water cleavage. In order to achieve maximal accuracy, our simulations include the full interface between the semiconductor surface and liquid water. Certain oxide contaminants are found to have a dramatic impact on the surface reactivity, pointing to the importance of surface oxide and hydroxide intermediates in facilitating the water-dissociation component of the hydrogen evolution process. Our results are used to relate the chemical activity of the surface towards water dissociation to the oxygen bond topology. The importance of the liquid hydrogen-bond network near the interface is discussed, particularly in relation to the generation of local configurations favorable for dissociative water adsorption on InP(001).

Efficient photo-electrochemical (PEC) splitting of water to hydrogen usually requires photoelectrodes to have certain electronic properties. Unfortunately, at present available semiconductors do not meet all these criteria. So, a thorough understanding of band-engineering for mixed alloys is necessary to successfully design these photoelectrodes. Among the semiconductors, transition metal oxides are of particular interest due to their low cost and relatively high stability in aqueous media. Here, we will present a theoretical study of delafossite-alloys for PEC photo-electrodes. Previous studies have indicated that the group IIIA delafossite family (CuMO₂, M = Al, Ga, In) do not exhibit direct band gaps. Their fundamental band gaps are significantly smaller than their reported optical band gaps. On the other hand group IIIB delafossite family (CuMO₂, M = Sc, Y, La) in general show direct band gaps and, except for CuLaO2, band gaps are above 3.00 eV. However, both of these two families exhibit p-type conductivity. We will show that by appropriate alloying of these two delafossite-families we can tune their band gaps and other opto-electronic properties. These types of alloying are desirable, as these introduce no localized impurity states in the band gap due to isovalent alloying. Also, the electronic effective masses can be lowered by selective doping of main group elements. Finally, it will be discussed that, lowering the symmetry constraints of these alloys would enhance their optical absorption properties. We'll also discuss that alloying with other 3d metal elements may decrease the band gap, but would increase the effective masses of the photo-electrons.

Hematite is a promising material for solar energy conversion via photo-electrochemical water splitting. However, the precise control of substitutional doping and nanometer feature size is important for high photon harvesting efficiency. Doped and nanostructured hematite electrodes can be prepared by a simple solution-based colloidal approach however, a high temperature (800°C) annealing is required to activate the dopant atoms. This high temperature annealing step also increases the particle size above the dimension necessary for high photon harvesting efficiencies. Here we investigate a strategy to control the two kinetic processes occurring during sintering (particle size increase and dopant diffusion/activation) by incorporating Ti dopant directly into the colloid solution and reducing the annealing time. We find that this strategy leads to porous, high-surface area hematite electrodes giving a solar photocurrent density of 1.1 mA cm^-2 at 1.23 V vs. the reversible hydrogen electrode (RHE) under standard testing conditions where only 0.56 mA cm^-2 was observed at 1.23 V vs. RHE with our previous work. In addition, scanning electron micrographs examining the morphology of the electrodes suggests that our kinetic strategy is indeed effective and that further optimization may result in higher photocurrents.

As many water splitting photocatalysts only evolve hydrogen under irradiation due to complications with high energy water oxidation steps, a three component nano-catalyst was designed by combining sites for water reduction and oxidation with solar-charge-supplying semiconductor. The semiconductor framework was derived from K₄Nb₆O₁₇, a known UV-light photocatalyst with a band gap of 3.5 eV. Following ion exchange and exfoliation with terabutyalammonium hydroxide, the layered material separates into nanosheets that coil into 1 ± 0.5 um long and 10 ± 5 nm wide nanoscrolls to redue surface energy. PT (reduction sites) and IrOx (oxidation sites) were photochecmically deposited on the surface of the nanoscrolls to produce two-and three-component nanostructures. Upon irradiation with UV-light, H₂ was evolved from aqueous methanol and pure water, substoichiometric O₂ from aqueous AgNo₃. The band structures of each catalyst and reason for lack of O₂ evolution from pure water was evaluated with cyclic voltammetry and photoelectrochemistry.

We synthesized molybdenum disulfide (MoS₂) nanostructures and investigated their electrochemical activity for driving the hydrogen evolution activity as well as their photoelectrochemical activity for the water splitting reaction. MoS₂ nanoparticles were made using a reverse micelle encapsulation method and exhibit quantum confinement of the indirect band gap up to 1.8 eV. A MoS₂ double-gyroid bi-continuous structure was made using an evaporation induced self assembly method. Both nanostructures exhibit improved activity for the hydrogen evolution reaction versus bulk MoS₂. Photoelectrochemical activity was also observed in both nanostructures.

This paper summarises some recent results of applying in-situ EPR spectroscopy at low temperature to observe directly the creation and trapping of conduction band electrons in rutile, and indirectly in hematite thin films via observation of a photo-induced superparamagnetic resonance signal.

There is increasing interest in photochemical schemes for converting CO₂ into a useful product as a means of mitigating atmospheric levels of this gas. Although photoelectrochemical schemes have been considered for this application, typically very high overpotentials are observed, and thus, semiconductor-electrolyte interfaces have not been observed to actually convert light energy to chemical energy in the aqueous CO₂ redox system. We report here on a catalytic system that efficiently converts CO₂ to methanol and other alcohols. The system couples a III-V p-type semiconductor electrode with a pyridinium catalyst. The conversion of CO₂ to alcohols can be driven solely with light to yield faradaic efficiencies approaching 100% at potentials well below the thermodynamic potential. Mechanistic studies on the formation of methanol indicate that the observed six-electron reduction occurs via a series of one electron reductions mediated by pyridinium.

The properties of thin film InVO4 photoanodes for water splitting have been studied. Compact films of InVO4 were prepared by spray pyrolysis and are found to be stable between pH 3 - 11. Although the indirect bandgap is 3.2 eV, a modest amount of visible light absorption is observed. The origin of this absorption is attributed to the presence of deep donor states at ~0.7 eV below the conduction band. Shallow donors are absent in this material, in contrast to what is normally observed for metal oxides. The deep donor model explains the much stronger visible light absorption of powders compared to thin films, and is supported by photoluminescence data. The origin of the deep donors is attributed to deviations in the In:V ratio, and the corresponding defect-chemical reactions will be discussed.

For more than a decade, the Hawaii Natural Energy Institute has conducted research on photoelectrochemical (PEC) technologies and achieved major milestones, including the fabrication of high-performance photoactive thin film materials and the development of innovative device integrations (hybrid-photo-electrode). In this paper, we focus our discussion on tungsten oxide-based materials, one of our two principal topics of research in this field. After a description of pure WO₃ physical, chemical and energetic properties we present our latest results on tungsten oxide PEC properties improvement. In our general approach, each component of the PEC electrode is addressed, from the absorber (bulk) to the surface energetics (near-surface) and catalysis (surface). Recently, progresses have been made on surface treatment for catalytic purposes as well as on PEC materials integration. In the case of catalytic treatment, our studies show that reactive sputtering technique is suitable to form high quality RuO₂ thin films and nanoparticles. Tests conducted on RuO2 thin films pointed out an oxygen evolution reaction potential as low as 0.2 V. When used as an anode in 2- electrode configuration, RuO2 thin films lead to a photocurrent onset potential reduction as low as 500 mV for p-type PEC materials (CGSe₂ and a-SiC, so far tested) when compared to platinum. In the case of RuO2 nanoparticles, a photocurrent density increase of approx. 20% was observed on treated tungsten oxide films. Finally, we present a new integration scheme to increase photocurrent density using highly textured substrates (HTS). In our approach, HTS were obtained by anisotropic etching of [100] silicon substrates in KOH solution. Initial results indicated a very good coverage of WO₃ onto the silicon pyramids and a photocurrent doubling is observed when compared to WO3 deposited on flat silicon substrates.

Photoelectrochemical (PEC) water dissociation into hydrogen and oxygen at a semiconductor-liquid interface offers an environmentally benign approach to hydrogen production. We have developed an integrated PEC device using hydrogenated amorphous silicon carbide (a-SiC or a-SiC:H) material as photoelectrode in conjunction with an amorphous silicon (a-Si) tandem photovoltaic device. Such a "hybrid PV/a-SiC" PEC cell produces photocurrent of about 1.3 mA/cm² in a short-circuit configuration and is durable in a pH2 electrolyte. On the other hand, the aforementioned structure finished with ITO contacts and measured as a solid-state device features a current density of 5 mA/cm², indicating a potential solar-to-hydrogen (STH) conversion efficiency of about 6% in the hybrid PV/a-SiC PEC cell. The much lower photocurrent measured in the hybrid PEC cell suggests that there exists an interfacial barrier between the a-SiC and electrolyte, which hinders the photocurrent extraction. In order to mitigate against the interfacial barrier and hence improve the photo-generated charge carrier transport through the a-SiC/electrolyte interface, we have explored several surface modification techniques, namely the use of metallic nano-particles (such as platinum or palladium) and the growth of an additional thin layer (a-SiNx, carbon-rich a-SiC, a-SiF, etc.) on the top of a-SiC by PECVD. In the latter case, it is observed that the addition of a thin PECVD-fabricated layer does not significantly improve the photocurrent, presumably due to a poor band alignment at the a-SiC/electrolyte interface. The use of lower work function nanoparticles like titanium has led to promising results in terms of photocurrent enhancement and an a nodic shift in the onset potential.

Metal oxides are promising for solar energy conversion applications since they are less expensive and easier to produce than conventional single crystal silicon. In this paper we focused on CdSe and CdS quantum dot (QD) co-sensitized and nitrogen doped TiO₂ (N:TiO₂) nanocomposite photoanodes for photoelectrochemical (PEC) water splitting for hydrogen generation. PEC, UV-vis and scanning electron microscopy (SEM) characterizations of the nanostructured films were carried out. The QD co-sensitized and N-doped TiO₂ photoanodes exhibits increased photocurrent compared to cosensitized TiO₂ without N-doping This is tentatively attributed to enhanced hole transport by oxygen vacancy states that are increased upon nitrogen doping. The enhanced hole transport facilitates overall charge transfer and transport and thereby results in improved photocurrent.

Conversion of solar energy into hydrogen is one of the most promising renewable energy technologies. Photocatalytic production of hydrogen from water, H2S and organic wastes using semiconductors is one of the potential strategies for converting the sunlight energy into chemical energy. Korea government paid great attention to the hydrogen economy and launched the HERC (Hydrogen Energy R&D Center) for supporting the R&D topics on hydrogen related technologies. The key issue for realizing the commercial application of solar water splitting hydrogen production technique is to find an efficient, stable and low-cost photocatalyst. Our research groups have continuously investigated to find oxide and composite photocatalysts for photoelectrochemical cell with high efficiency using computational design and synthesis method. But, fundamental research on semiconductor doping for band gap shifting and surface chemistry modification is still required. Various reaction media containing sacrificial agents should be developed to match with high activity photocatalysts to further improve the system efficiency. Water containing organic/inorganic waste and sea water are particularly suggested in the consideration that all these water sources are the most available water on the earth to the final commercial application of photocatalytic water splitting technique.

The work presents the photo-catalytic reactions of ethanol over Au particles deposited on TiO2 anatase nano- (≤ 10 nm) and micro- (ca. 0.15 μm) particle catalysts. The Au particles are of uniform and similar dimension (mean particle size = ca. 5 nm and 7 nm on the microand nano-sized TiO2, respectively). XPS Au4f indicated that in both cases Au particles are present in their metallic state with no evidence of charge transfer to (or from) the semiconductor. Liquid slurry photoreaction indicated the production of hydrogen with a rate ≈ 2 L/kgCatal.min on 2 wt. % Au/TiO2 anatase nano-particles under UV photo irradiation of comparable intensity to solar radiation. While the reaction rate per unit mass was lower on the micro-sized Au/TiO2 it simply scaled up to an equivalent rate for the nano-sized Au/TiO2 catalyst when normalised by unit area.

Hetero-structured TiO₂ and WO₃ nanorod arrays have been fabricated by dynamic shadowing growth. The two-layer WO₃/TiO₂ nanorod arrays grown by consecutive deposition of two materials at oblique angle are found to greatly enhance the catalytic performance by the increased TiO2 surface area and the potential enhanced charge separation due to the WO₃-TiO₂ interface. Also, two important factors have been found to affect the observed photocatalytic enhancement: the crystal phase of each material, and the interfacial area between TiO₂ and WO₃. To further increasing the interfacial area between the TiO₂ and WO₃, a dynamic shadowing growth method is used to create a quazi-core-shell nanorod array. WO₃ nanorods are first grown perpendicularly on a bare substrate to serve as the "core", and a TiO₂ "shell" is then deposited. Such a core-shell structure maximizes the interfacial area between the two materials. The photocatalytic decay rate for these core-shell samples again shows further improvement over single layer TiO₂ thin films and multi-layer c-TiO₂/a-WO₃ films by 13 and 3 times respectively. Our results demonstrate that heterostructured WO3/TiO₂ nanorod arrays are promising photocatalyst for water decontamination and water splitting for hydrogen generation.

There is an increasing interest in semiconductor/ electrolyte systems in connection with their application as phototelectrolytic energy conversion devices (e.g. hydrogen evolution). There are several requirements in order to produce hydrogen by photoelectrolysis using oxides metals and semiconductors. One of the most interesting semiconductor materials is the GaN which is a direct ban gap semiconductor. In this paper shows the formation of GaN prepared via electrodeposition, using ammonium nitrate at different concentration as type sources of nitrogen in order to growth a thin film. A standard three-electrode cell was used to prepare it using potenciostatic conditions. The average thickness of the samples was measured. The annealed films were characterized by electrochemical; photoelectrochemical, compositional, and morphologic methods in order to know its potential for water splitting.

For the first time, TiO₂ nanopowders were used as a modifier for the metal Al powders in the reaction with ordinary tap water to generate hydrogen at ambient temperature. The present study systematically investigated the effect of additives such as Al(OH)₃, AlO(OH), α-Al₂O₃, γ-Al₂O₃, SiO₂, CaO, Fe₂O₃, WO₃, and four different TiO₂ ceramic powders on the promotion of hydrogen generation in the reaction of metal Al powders and tap water. Effect of mechanical ball-mixing was also evaluated. It was found that AlO(OH), CaO, α-Al₂O₃and TiO2 (P90) powders were very effective to facilitate the hydrogen production from the reaction of metal Al powders and tap water under ambient condition. TiO2 (P90) nanopowders exhibited a slightly better effect than those of α-Al₂O₃in the facilitation of hydrogen generation from water splitting. The generation of hydrogen from the reaction of modified Al powders and water was found to be dependent on the sizes of metal Al powders, the modifiers, the size of the modifiers, weight ratio of metal Al powders to the modifiers, and ball-milling durations. Further studies are required to fully understand the contradictory behaviors to the previously known mechanism.

The development of low cost, scalable, renewable energy technologies is one of today's most pressing scientific challenges. We report on progress towards the development of a photoelectrochemical water-splitting system that will use sunlight and water as the inputs to produce renewable hydrogen with oxygen as a by-product. This system is based on the design principle of incorporating two separate, photosensitive inorganic semiconductor/liquid junctions to collectively generate the 1.7-1.9 V at open circuit needed to support both the oxidation of H₂O (or OH^-) and the reduction of H⁺ (or H₂O). Si microwire arrays are a promising photocathode material because the high aspect-ratio electrode architecture allows for the use of low cost, earth-abundant materials without sacrificing energy-conversion efficiency, due to the orthogonalization of light absorption and charge-carrier collection. Additionally, the high surfacearea design of the rod-based semiconductor array inherently lowers the flux of charge carriers over the rod array surface relative to the projected geometric surface of the photoelectrode, thus lowering the photocurrent density at the solid/liquid junction and thereby relaxing the demands on the activity (and cost) of any electrocatalysts. Arrays of Si microwires grown using the Vapor Liquid Solid (VLS) mechanism have been shown to have desirable electronic light absorption properties. We have demonstrated that these arrays can be coated with earth-abundant metallic catalysts and used for photoelectrochemical production of hydrogen. This development is a step towards the demonstration of a complete artificial photosynthetic system, composed of only inexpensive, earth-abundant materials, that is simultaneously efficient, durable, and scalable.