Photocatalytic overall water splitting promises to enable a sustainable large-scale hydrogen-based energy system using solar light, and great attention has been paid to the development of photocatalysts. It is necessary to develop photocatalysts that function under visible light to utilize sunlight efficiently. We have proposed non-oxide materials as candidates for visible-light-driven photocatalysts for overall water splitting, and this manuscript presents our recent research in photocatalyst development. Some oxynitride photocatalysts, modified with appropriate cocatalysts, showed performance for overall water splitting under visible light irradiation. The modification with cocatalysts drastically improved the efficiency of photocatalytic reactions, indicating the importance of controlling the surface active sites. Two-step excitation systems, known as Z-schemes, can significantly expand the range of light available for water splitting to longer wavelengths.

The worldwide demand for solar grade silicon reached an all time high between 2007 and 2008. Although growth in the solar industry is slowing due to the current economic downturn, demand is expected to rebound in 2011 based on current cost models. However, demand will increase even more than currently anticipated if costs are reduced. This situation creates an opportunity for new and innovative approaches to the production of photovoltaic grade silicon, especially methods which can demonstrate cost reductions over currently utilized processes.

Copper oxide films have been shown to be a promising electrode material for the direct production of hydrogen by the photoelectrochemical (PEC) decomposition of water. In this paper, we present our work in developing a hot-wall tubular CVD reactor for copper oxide film deposition using a solid-source copper precursor and oxygen. Initial deposition results have shown that the reactor can reliably deposit solid polycrystalline cuprous oxide films and porous cupric oxide films, both being promising PEC materials. Unusual spatial patters observed in the film composition prompted the development of a physically based mathematical model of the CVD process. Initial results of this modeling work and its role in understanding and optimizing the deposition process will be presented.

The use of pulsed laser deposition as a fabrication technique for metal oxide semiconductor photoelectrodes for solar-driven production of hydrogen from aqueous solutions is examined. The physical mechanisms of pulsed laser-material interactions facilitate the deposition of a wide variety of semiconductor materials quickly and controllably. Films prepared by this technique have proven to possess desirable characteristics for many applications, including highly sensitive electronic and optical devices. However, pulsed laser deposition of materials for photoelectrode films is relatively unexplored. Effectively utilizing this technique as a research tool for photoelectrode fabrication involves exploiting the physical phenomena associated with laser-material interactions and the characteristic ablation plume. Through control of process parameters one can engineer and study the composition and structural properties of photoelectrodes simultaneously, which is known to be required for high solar-to-hydrogen conversion efficiencies. Characteristics of photoanodes deposited by pulsed laser deposition are presented when illustrative of the fabrication technique discussed.

Titanium dioxide is a promising candidate for high-performance photocatalysts. Defect engineering may be applied for the modification of its properties, including the functional-related properties, in a controlled manner in order to achieve the desired/optimized performance. The present work reports the application of defect engineering for the modification of semiconducting properties of undoped TiO₂. The defect disorder is considered in terms of the predominant defect reactions. The related equilibrium constants are used to derive the defect disorder diagram for undoped TiO₂ in equilibrium (1273 K) in the gas phase of controlled oxygen activity (10−13 Pa < p(O2) < 105 Pa). The obtaind data on the concentration of electronic charge carriers have been used for the determination of the effect of p(O₂) on the change of Fermi energy within the band gap. The determined diagram may be applied for the selection of processing conditions of undoped TiO₂ with controlled semiconducting properties and the ability to donate or accept electrons.

Carbon-doped In₂O₃ and carbon-doped WO₃ films were produced using a spray pyrolysis methodology with octanoic acid as the carbon dopant source. C-doped and undoped In₂O₃ films showed a cubic polycrystalline In₂O₃ structure, and C-doped and undoped WO₃ films displayed a monoclinic polycrystalline WO₃ structure. C-doped In₂O₃ and WO₃, compared to their corresponding undoped materials, showed increased absorption in the 350-550 nm range with a red shift in the band gap transition. The presence of carbonate-type species in these C-doped samples was confirmed by XPS. The photoelectrochemical activity was evaluated under near UV-visible light and visible light only irradiation conditions. Under the same irradiation conditions, C-doped In₂O₃ and C-doped WO₃ electrodes produced greater photocurrent densities than their corresponding undoped electrodes. The C-doped In₂O₃ electrode exhibited photocurrent densities up to 1 mA/cm², with 40% from visible light irradiation, and the C-doped WO₃ electrode showed photocurrent densities up to 1.3 mA/cm², with 50% from visible light irradiation. These results indicate the potential for further development of In₂O₃ and WO₃ photocatalysts by simple wet chemical methods, and provide useful information towards understanding the structure and enhanced photoelectrochemical properties of these materials.

Ag deposited on TiO₂(110) forms nanoclusters ~5 nm across and 2 nm in height, shown by STM. These nanoclusters exhibit a plasmon loss at 3.8 eV as determined by EELS yet the substrate Fuchs-Kliewer phonon modes remain, indicating that the exposed TiO₂ is not perturbed by the Ag clusters. Titania is grown on top of these clusters by evaporation of Ti and subsequent oxidation and both EELS and optical measurements show that new excitations are produced in the 1.5-2 eV range, a much better match to the solar spectrum than the 3.8 eV Ag plasmon. AFM measurements indicate that the Ag clusters retain their morphology upon titania coating.

TiO₂ and TiO₂/WO₃ porous films were deposited onto transparent conducting glass electrodes, resulting in uniform films consisted of agglomerated particles with average diameters ranging from 50 to 200 nm; Ti, O and W atoms were homogeneously distributed at the surface of hybrid film. Comparable electrochemical properties were observed in the dark, with small capacitive currents and similar potentials for O₂ and H₂ evolution reactions in aqueous solution. Under polychromatic irradiation, the hybrid film electrode, molar ratio WO₃/TiO₂ = 12 %, reveled higher photocurrent and photocatalytic activity for oxidation of phenol and 17-α-ethinylestradiol. The visible light harvesting ability of hybrid film, with band gap energy estimated as 2.3 eV, and the relative position of conduction and valence band edges that inhibits charge recombination, should improve its photocatalytic activity for organic pollutant removal.

Fe-TiO₂ particles were synthesized by sol-gel process from hydrolysis of titanium tetra-isopropoxide with nitric acid and ferric nitrate aqueous solutions (relative Fe:Ti molar ratio ranging from 1 to 6 at %) followed by hydrothermal treatment. Thin films were deposited onto conducting glass electrodes from a suspension with polyethylene glycol and heating at 450 °C for 30 min, which resulted in 1.5 μm thick transparent porous films. Crystalline samples, 93 % anatase and 7 % brookite, were obtained. Increasing the iron amount, the crystallite size estimated from XRD patterns ranged from 18 to 11 nm and the color varied from slightly yellow to brown. The optical properties have also changed; the absorption edge shifted towards longer wavelengths, with band gap energy decreasing from 3.0 to 2.7 eV. The films exhibited photocatalytic activity for phenol degradation that indicates promising applications in solar energy conversion.

Inorganic semiconductors are promising materials for driving photoelectrochemical water-splitting reactions. However, there is not a single semiconductor material that can sustain the unassisted splitting of water into H₂ and O₂. Instead, we are developing a three part cell design where individual catalysts for water reduction and oxidation will be attached to the ends of a membrane. The job of splitting water is therefore divided into separate reduction and oxidation reactions, and each catalyst can be optimized independently for a single reaction. Silicon might be suitable to drive the water reduction. Inexpensive highly ordered Si wire arrays were grown on a single crystal wafer and transferred into a transparent, flexible polymer matrix. In this array, light would be absorbed along the longer axial dimension while the resulting electrons or holes would be collected along the much shorter radial dimension in a massively parallel array resembling carpet fibers on a microscale, hence the term "solar carpet". Tungsten oxide is a good candidate to drive the water oxidation. Self-organized porous tungsten oxide was successfully synthesized on the tungsten foil by anodization. This sponge-like structure absorbs light efficiently due to its high surface area; hence we called it "solar sponge".

We report a novel method to fabricating single crystal and highly oriented 1-D Silicon micropillars and nanowires and then transferring them to coat a target surface of any topology using an innovative harvest/lift-off process. This method enables highly crystalline micro- and nano- pillars of different materials with diverse bandgaps and physical properties to be fabricated on appropriate mother substrates and transferred to form multilayered 3D stacks for multifunctional devices. This approach not only ensures the incorporation of any kind of material (with the best device characteristics) on a single substrate facilitating substrate-free device fabrications on any topology, but also allows the repeated use of a mother substrate for continual production of new devices. This capability of fabricating substrate-less devices will offer a universal platform for material integration and allow solar active devices to be coated on various surface topologies that would be suitable for solar hydrogen generation.

Most transport fuels are derived from fossil fuels, generate greenhouse gases, and consume significant amounts of water in the extraction, purification, and/or burning processes. The generation of hydrogen using solar energy to split water, ideally from abundant water sources such as sea water or other non-potable sources, could potentially provide an unlimited, clean fuel for the future. Solar, electrochemical water splitting typically combines a photoanode at which water oxidation occurs, with a cathode for proton reduction to hydrogen. In recent work, we have found that a bioinspired tetra-manganese cluster catalyzes water oxidation at relatively low overpotentials (0.38 V) when doped into a Nafion proton conduction membrane deposited on a suitable electrode surface, and illuminated with visible light. We report here that this assembly is active in aqueous and organic electrolyte solutions containing a range of different salts in varying concentrations. Similar photocurrents were obtained using electrolytes containing 0.0 - 0.5 M sodium sulfate, sodium perchlorate or sodium chloride. A slight decline in photocurrent was observed for sodium perchlorate but only at and above 5.0 M concentration. In acetonitrile and acetone solutions containing 10% water, increasing the electrolyte concentration was found to result in leaching of the catalytic species from the membrane and a decrease in photocurrent. Leaching was not observed when the system was tested in an ionic liquid containing water, however, a lower photocurrent was generated than observed in aqueous electrolyte. We conclude that immersion of the membrane in an aqueous solution containing an electrolyte concentration of 0.05 - 0.5M represent good conditions for operation for the cubium/Nafion catalytic system.

An apparatus is described for housing artificial photosynthesis processes. The apparatus is solar powered and employs two separate compartments for the respective oxidation and reduction reactions. A proton exchange membrane (PEM) partitions the two compartments and enables proton conduction therebetween. Faradic losses due to proton currents are minimized by use of a novel geometry. A zigzag design for the solar cell/electrode/PEM partition between the two compartments introduces large fringe fields which help drive proton current from one compartment to the other and reduce faradic losses by shortening the average proton conduction path. Facilitating proton current also improves the pH gradient and enhances water splitting reaction rates. The zigzag design also improves capture of solar flux by shading the PEM under the solar cells.