A molecular approach to understand the photocatalytic degradation of small organic molecules adsorbed from the gas phase on anatase, rutile and doped TiO₂ nanoparticles is presented. Using in situ Fourier transform infrared (FTIR) spectroscopy and mass spectrometry the rate determining steps for the photocatalytic degradation of formic acid, acetone and propane are unraveled. Key intermediates are identified and correlated to structural properties of the TiO₂ nanoparticles. Specifically, stable bridging bidentate carboxylate (R-CO₂) and (bi)carbonate species forms preferentially on rutile particles, and are proposed to inhibit the total photodegradation efficiency. In particular, the concentration of R-CO₂ is found to decrease with increasing size of the anatase particles, and may at least partly explain why Degussa P25 is a good photocatalyst. Means to avoid R-CO₂ site-blocking is discussed. Improved solar light efficiencies are difficulty to achieve in cation doped TiO₂ despite higher visible light absorption and stronger adsorbate-surface interactions.

The stress on TiO₂ (110) and (100) surfaces with four types of adsorbent: (i) molecularly adsorbed water, (ii) dissociatively adsorbed water, (iii) dissociatively adsorbed water at an oxygen vacancy, and (iv) adsorbed hydrogen was investigated in the framework of density functional theory using a slab model. The calculations were intended to rationalize the change in dynamic hardness and the effect of artificially introduced stress that occurs in experimentally photoinduced hydrophilicity.

How do we learn about chemisorption and physisorption of hydrides and the kinetics of hydrogen adsorption and desorption? These are profound challenges with us for decades. Soft-x-ray spectroscopy will be will be a unique tool to study the electronic properties of fundamental materials, nanoporous, and complex hydrides and in-situ study the kinetics of hydrogen adsorption and desorption. To facilitate the search for most efficient hydrogen-generation and - storage compounds, a fundamental understanding of the electronic properties is essential. Hydrogen strongly affects the electronic and structural properties of many materials. The electronic structure ultimately determines the properties of matter. Photon-in/photon-out soft-x-ray spectroscopy has been the subject to a revived interest owing to the new generation synchrotron facilities and high performance beamline and instruments. Soft-x-ray absorption spectroscopy (XAS) probes the local unoccupied electronic structure, soft-x-ray emission spectroscopy (XES) probes the local occupied electronic structure, and resonant inelastic soft-x-ray scattering (RIXS) probes the intrinsic low-energy excitations, such as charge transfer, proton energy transfer etc. A number of examples, including some recent experimental findings, then illustrate the potential of XAS and XES applications in hydrogen energy sciences.

The behavior of water molecules on the (100) surface of BiVO₄ has been investigated using first-principles molecular dynamics in view of the crucial role in photo catalytic activities under visible light irradiation. The simulations show that H₂O molecules are adsorbed in a non-dissociated molecular form on the fivefold coordinated Bi site. The adsorption energy was estimated to be ~0.58 eV/molecule onto the Bi-exposing surface at 300 K. The band gap of the system shrinks slightly (by ~0.2 eV) upon water adsorption.

Molecule-based CVD is applied for the development of 1D semiconducting nanowires. By virtue of the chemical design of the metal-organic precursors, it is possible to achieve the required supersaturation ratio of phase-constituting elements in the gas phase, which allows to grow anisotropic structures with precisely controlled dimension and composition. [Ge(C₅H₅)₂] with labile Ge-C bonds was thermolysed at 300 °C to grow single crystalline Ge nanowires (NWs). For tin oxide nanostructures, [Sn(OBu^t)₄] with relatively strong and preformed Sn-O bonds was employed to synthesize anisotropic rutile phase. Determination of I-V characteristics of Ge NWs in different environments indicate surface passivation, possibly through hydrogen. Radial dimension of SnO₂ NWs was varied in the range 30-1000 nm by choosing appropriate size of catalyst particles. Photo-conductance studies on different NW samples revealed a significant 'blue shift' with shrinking wire diameters. Tin oxide nanowires were coated with vanadium oxide by CVD of [VO(OPrⁱ)₃] on as-grown tin oxide nanowires. Composite SnO₂/VO_x 1D nanostructures showed a shift to higher wavelength in photo-response peak, when compared to pure SnO₂ NWs. We also demonstrate the integration of single NW on pre-patterned electrodes for evaluating sensing and electrical properties on individual nanoobjects.

Semiconducting metal oxide nanowires represent a class of novel materials that are of superior properties to naoparticles currently used in dye sensitized solar cell and polymer hybrid solar cells. The quasi one-dimensional nanostructure and surface states of nanowires improve carrier mobility and charge transfer through interface interactions of theses nanocomposite materials. Raman spectroscopy, especially resonant Raman spectroscopy, is used to correlate nanomaterial synthesis condition to the structural, optical and electric transport properties that are important to photocatalysis, exciton transport and recombination and hydrogen storage mechanism. For example, highly orientated ZnO nanowires studied with Raman and photoluminescence spectroscopy demonstrated the high efficiency of the phonon and electron coupling. These results are compared with that of other ZnO forms such as thin film, polycrystalline powder and solid. The Raman bandwidths and shifts of nanowires revealed the phonon confinement in the quasi one-dimensional nanostructures, which is further demonstrated with In₂O₃ nanowires at 5, 10, 20, 30 nm in diameters. Room temperature photoluminescence results also show band gap shifts with nanowire dimensions. Nanowire sizes, defects and strains, controlled by synthesis conditions, have shown to determine band structure and optical phonon properties. We also discuss characterization and synthesis of carbon nanotube based composite materials including polymer electropolymerization and infiltration. Combining significantly enhanced mechanical compressive strength and excellent electric conductivity, these composite materials offer potentials to fuel cell anode materials as multifunctional hydrogen storage media.

We report the rapid high-yield generation of inorganic fullerene-like cesium oxide (IF-Cs₂O) nanoparticles, activated by highly concentrated sunlight. The solar process represents an alternative to the only reported method for synthesizing IF-Cs₂O nanostructures: laser ablation. IF-Cs₂O formed at solar irradiation greater-than or equal to 6W, confirmed by high resolution transmission electron microscopy. These closed-cage Cs₂O nanostructures are stable under electron microscope conditions, and also when exposed temporarily to air - of significance for their use in a variety of photonic devices.

The present work considers the application of defect chemistry for engineering of semiconducting properties of metal oxides in general and TiO₂ in particular. The performance-related functional properties of TiO₂-based photoelectrode for hydrogen generation through water splitting using solar energy (solar-hydrogen) are considered in terms of (i) electronic structure, (ii) charge transport, (iii) near-surface charge distribution and the related electric fields, and (iv) defect disorder of the outermost surface layer. The present work considers the modification of these functional properties for TiO₂ through the imposition of controlled defect disorder. The defect disorder is considered in terms of defect equilibria and the defect diagram describing the effect of oxygen activity on the concentration of both ionic and electronic defects.

The paper describes methodologies for deposition of nanostructured films in single step processes using flame aerosol reactors. An understanding of the process parameters such as precursor feed rate, temperature histories and residence times that control resultant film parameters such as thickness, crystallinity and morphology are developed. Control of temperature profiles allow control of sintering rates to produce desired nanostructured thin films. These films are then tested for photocurrent generation under uv light illumination - and overall conversion efficiencies of around 5 % are readily obtained. Results of the study indicate that conditions could be optimized to improve water splitting efficiencies.

Photoelectrochemical water splitting into H₂ and O₂ was investigated using TiO₂ based photoelectrodes. First, influence of photoelectorde structure on water splitting was studied through photocurrent observation. Solar energy conversion efficiency to H₂ (STH) of mesoporous TiO₂ photoelectode, composed of anatase TiO₂ particles of 20nm in diameter, with 10μ thickness on FTO glass was 0.32% under 0.4V vs RHE, producing 0.39mA/cm². The quantum efficiency of water splitting at 360nm was 27%. Then, visible light absorbing mesoporous N-doped and S-doped anatase TiO₂ photoelectrodes were studied. Visible light absorbing properties of these photoelectrodes were dramatically decreased with increasing calcination temperature to 550°C. However, photocurrent such as 1μA/cm² was observed under 0.94V vs RHE and visible light irradiation using 300W-Xe lamp with 410nm cut off filter. Overall photocurrent of N-doped and S-doped TiO₂ photoelectrode was about 1/5 to 1/10 of that of non-doped TiO₂ photoelectrodes. Finally, solar hydrogen production by a tandem cell, composed of a mesoporous TiO₂ based photoelectrode, a Pt wire electrode and a Black dye-sensitized solar cell, was studied. STH of a non-doped TiO₂ photoelectrode system was 0.53% but STH of a S-doped TiO₂ photoelectrode system was 0.15%, which was 1/3 lower than that of a non-doped TiO₂ photoelectrodes.

Metal sulfide (CdS or PbS) quantum dots were synthesized in nanoporous TiO₂ films for applications in solar energy conversion devices. Sandwich type regenerative solar cells, based on the quantum dots sensitized TiO₂ film, exhibit a high IPCE over visible wavelengths by optimizing the polysulfide electrolyte composition. The CdS QD shows a higher IPCE, compared to PbS, related to an increased light harvesting efficiency when the number and size of the QDs intensified. In contrast, QD size dependence on the IPCE was observed for the PbS, likely resulting from the QD size dependence on a conduction band edge potential (associated with quantum size effect) relative to the TiO₂ conduction band edge, or the kinetic competition between the hot electron injection and the electron relaxation in the PbS conduction band. We also propose that an I₃ ^-/I^- redox electrolyte, with NaSCN addition, can be employed to enhance the solar cell performance. SCN^- ions may attach to the QD surface forming a shell type structure to prevent the photocorrosion reaction, and act as an intermediate electronic state to induce the sequential step electron transfer reactions for the QD re-reduction.

Anodization of Ti in acidified fluoride solution resulted in a vertically oriented and an ordered nanotubular titanium oxide surface. Annealing of the TiO2 nanotubular arrays in a carbonaceous or nitrogen containing atmosphere presumably resulted in band-gap states, which enhanced the photo-activity. Composite electrode of nanotubular TiO2 + carbon doping resulted in a photocurrent density of more than 2.75 mA/cm2 at 0.2 V(Ag/AgCl) under simulated solar light illumination. The enhanced photo-activity of the carbon-modified nanotubular TiO2 is highly reproducible and sustainable for longer duration. The charge carrier densities, calculated based on the Mott-Schottky analyses, were in the range of 1-3 x 1019 cm-3 for both the carbon modified and the nitrogen-annealed nanotubular TiO2 samples. The asanodized and oxygen-annealed samples showed a charge carrier density of 5 x 1017 and 1.2 x1015 cm-3 respectively. In this study, the measured photo current density was not directly related to the charge carrier densities of the nanotubes. Presence of different phases, such as amorphous, anatase and rutile, influenced the photo activity more than the charge carrier density.

About 3 μm thick tungsten trioxide film electrodes consisting of partly sintered, 40-80 nm in diameter, particles deposited on conducting glass substrates exhibit high photon-to-current conversion efficiencies for the photooxidation of water, exceeding 70% at 400 nm. This is facilitated by a ca. 40% film porosity resulting in high contact area with the electrolyte. It is shown that the activity of the WO₃ electrodes towards photooxidation of water is enhanced by addition of even small amounts of halide (Cl^-, Br^-) ions to the acidic electrolyte. Photoelectrolysis experiments performed either in acidic electrolytes containing chloride or bromide anions or in a 0.5 M NaCl solution, under simulated 1.5 AM solar illumination, demonstrated long term stability of the photocurrents. Oxygen remains the main product of the photoanodic reaction even in a 0.5 M NaCl solution, a composition close to the sea water, with chlorine accounting for ca. 20% of current efficiency.

We report the use of amorphous silicon (a-Si) tandem junctions as part of an integral "hybrid" photoelectrochemical (PEC) cell to produce hydrogen directly from water using sunlight. The device configuration consists of stainless steel (SS)/ni₂pni₁p/ZnO/WO₃. When the device is immersed in an electrolyte and illuminated, O₂ is evolved at the WO₃/electrolyte interface and H₂ is produced at the counter electrode. A voltage >1.23V is required to split water; typically 1.6-1.8V are needed, taking account of losses in a practical water-splitting system. We use a-Si tandem cells, deposited by plasma-enhanced chemical vapor deposition, to supply this voltage. Current matching in the two a-Si subcells is achieved by altering the thicknesses of the two layers (i₁ and i₂) while keeping their band gaps at ~1.75eV, which results in a device with an open circuit voltage >1.6V, short circuit current density (J_sc) >6mA/cm² (on SS substrates), and a fill factor >0.6. Deposition on a textured SnO₂ coated glass has resulted in J_sc >9mA/cm². Photoactive WO₃ films, deposited using the RF sputtering technique, have achieved photocurrents >3mA/cm² at 1.6V vs. saturated calomel electrode (SCE). The PEC device operates at the point at which the WO₃ photocurrent IV curve and the a-Si (filtered by WO₃) light IV curve cross, leading to operating currents of 2.5mA/cm² and solar-to-hydrogen (STH) conversion efficiency of >3%.

An H₂WO_4(aq)-based sol processing route has been developed to allow the ink jet printing of photocatalytically active WO₃ films. The effect of different heat treatment atmospheres and the addition of triethanolamine upon the structure, composition, optical properties and IPCE response of films printed upon conducting glass substrates (ITO) have been studied using x-ray diffraction, Raman microscopy, UV-visible spectroscopy and photocurrent spectroscopy. It has been discovered that heat treatment under a nitrogen atmosphere inhibits formation of a well defined crystal structure but may extend the tail of the IPCE response curve into the visible range as far as 700 nm. Likewise, the presence of triethanolamine in the precursor sol tends to disrupt the WO₃ crystallization process leading to the formation of amorphous material and residual organic material in the heat treated film. However, UV-visible spectroscopy of these films indicates optical absorption similar to that of crystalline WO3 except with increased absorption in the visible region from 350 nm to 600 nm. These observations are supported by ab initio calculations predicting that the incorporation of nitrogen into the monoclinic WO₃ lattice leads to band gap narrowing and the introduction of mid-gap states.

Ceramic semiconductor photoelectrodes made of the Fe₂O₃-Nb₂O₅ solid solutions were synthesized. The spectral and capacitance-voltage characteristics of the photoelectrodes were determined, and the dynamic polarization with chopped light was investigated. The anodic photocurrent onset potential, the flat band potential and the shallow and deep donor density of these materials were determined. The threshold photon energies corresponding to the inter-band optical transitions near the edge of the fundamental absorption of the semiconductor photoelectrode were calculated. Analysis of the frequency dispersion of the real and imaginary parts of the complex impedance of photoelectrochemical cell was carried out. On the basis of this analysis, equivalent circuits describing the structure of the double electrical layer of the semiconductor - electrolyte interface were proposed and their parameters were calculated. The main limiting steps of the electrode processes, which determine the electrode polarization and current, are determined.

We report the methodology and fabrication of α-Fe₂O₃ nanostructured photoelectrodes for water splitting applications. Thin films of α-Fe₂O₃ (hematite) were deposited onto nanostructured substrates (ZnO nanowires and TiO2 nanotubes) using filtered arc deposition (FAD). It is proposed that such nanostructured electrodes can overcome the poor absorption and high charge carrier recombination of planar α-Fe₂O₃ films used for water splitting. Results of the characterization and optimization of the α-Fe₂O₃ films and the nanostructured substrates are presented. The filtered arc deposition technique is shown to produce high purity α-Fe₂O₃ films. Results of preliminary studies of silicon doping of the hematite films are presented. The filtered arc deposition technique is shown to be suitable for coating highly structured substrates.

Synchrotron-based spectroscopic investigations of 1D nanomaterials consisting of designed oriented nanorod-arrays of hematite grown by aqueous chemical growth reveal significant differences in the electronic structure and bandgap compared to bulk samples. Resonant inelastic x-ray scattering (RIXS) study of α-Fe₂O₃ crystalline nanorod bundle arrays at the Fe L-edge is reported. The low energy excitations, namely d-d and charge-transfer excitations, are identified in the region from 1 to 5 eV. The 1-eV and 1.6-eV energy-loss features are weak transitions from multiple excitations. The 2.5-eV excitation which corresponds to the bandgap transition appears significantly larger than the typical 1.9-2.2-eV-bandgap of single-crystal or polycrystalline hematite samples, revealing a one-dimensional (1D) quantum confinement effect in the bundled ultrafine nanorod-arrays. Such conclusion strongly suggest that bandgap and band edge position criteria for direct photo-oxidation of water by solar irradiation without an applied bias are therefore satisfied for such purpose-built nanomaterials. The outcome of such a result is of great importance for the solar production of hydrogen, an environmental friendly energy source carrier for the future. Indeed, the generation of hydrogen by visible light irradiation with an environmental friendly and economical photoactive material would thus advance a step closer to reality.

The short diffusion length of photo-excited charge carriers in Fe₂O₃ is one of the factors limiting the water splitting efficiency of iron oxide based materials. To overcome this problem we are engineering transparent arrays of nanowires to act as conducting substrates for the Fe₂O₃. To help understand the charge transport characteristics of the Fe₂O₃ component we report transient photocurrent measurements performed on an absorbing thin film of Fe₂O₃ deposited by filtered arc deposition on conducting glass with a semi-transparent silver Schottky top contact. Ultraviolet laser pulses were used to generate charge carriers near the surface and the resulting current transients were measured. A simulation of this charge transport has also been developed. The sign of the observed transients was independent of applied bias, consistent with a fully depleted film. The measurements also suggest that recombination may play a significant factor in determining the transient shape. Further investigation is required to confidently predict mobilities.

Oxynitrides are presented as effective non-oxide photocatalysts for overall water splitting. RuO₂-loaded germanium nitride (β-Ge₃N₄) is shown to achieve stoichiometric decomposition of H₂O into H₂ and O₂ under ultraviolet irradiation (λ > 200 nm). A novel solid solution of GaN and ZnO, (Ga_1-xZn_x)(N_1-xO_x), with a band gap of 2.4-2.8 eV (depending on composition) achieves overall water splitting under visible light (λ > 400 nm) when loaded with an appropriate cocatalyst. The narrower band gap of the solid solution originates from the bonding between Zn and N atoms at the top of the valence band. The photocatalytic activity of (Ga_1-xZnx)(N_1-xO_x) for overall water splitting is dependent strongly on both the cocatalyst and the crystallinity and composition of the material. The quantum efficiency of (Ga_1-xZnx)(N_1-xO_x) with Rh and Cr mixed-oxide (Rh_2-yCr_yO₃) nanoparticles reaches 2-3 % at 420-440 nm, which is the highest reported efficiency for overall water splitting in the visible-light region.

ZnO nano/microstructures offer the opportunity to design new types of photoelectrochemical devices. Arrays of single crystal ZnO nanowires present very interesting properties to enhance the performance in these devices. A systematic study of the deposition of single crystal ZnO nanowire arrays from the oxygen electroreduction method is reported in order to gain a further insight into the nanowire growth mechanisms and to develop an efficient electrochemical method which allows tailoring the nanowire dimensions. The influence of deposition parameters such as zinc precursor and supporting electrolyte concentrations on the formation of a polycrystalline compact ZnO layer or a ZnO nanowire array, as well as on the dimensions of the single crystal nanowires is analyzed. The effect of the polycrystalline compact ZnO buffer layer on the nanowire nucleation process and therefore on the nanowire diameter and density is also discussed. The results show that electrodeposition is a versatile and cost-effective technique which allows growing ZnO single crystal nanowire arrays with tailored dimensions. The structural and optical properties of electrodeposited nanowire arrays are discussed. ZnO nanowires can be sensitized by the coating of a thin layer of CdSe. The ZnO/CdSe photoanode exhibits excellent photoelectrochemical properties and external quantum efficiency larger than 70 % are observed in ferri/ferrocyanide solutions.

We propose core-shell nanorods such as InP-CdS and InP-ZnTe to be photoelectrodes for efficient photoelectrochemical hydrogen production. Based on our systematic study using strain-dependent k.p theory, we find that in these heterostructures both energies and wave-function distributions of electrons and holes can be favorably tailored to a considerable extent by exploiting the interplay between quantum confinement and strain. Consequently, these core-shell nanorods with proper dimensions (height, core radius, and shell thickness) may simultaneously satisfy all criteria for effective photoelectrodes in solar-based hydrogen production.

We have studied solar water splitting with a composite semiconductor electrode, composed of an n-i-p junction amorphous silicon (a-Si, E_g≈ 1.7 eV) layer, an indium tin oxide (ITO) layer, and a tungsten trioxide (WO₃, E_g 2.8 eV) particulate layer. The n-i-p a-Si layer, which had more accurately a structure of n-type microcrystalline ( c) 3C-SiC:H (25 nm)/i-type a-Si:H (400 nm)/p-type a-SiC_x:H (25 nm), was prepared on a TiO₂-covered F-doped SnO₂ (FTO)/glass plate by a Hot-Wire CVD method. The ITO layer (100 nm thick) was deposited on the p-type a-Si by the DC magnetron sputtering method, and the WO3 particulate layer was formed by a doctor-blade method, using a colloidal solution of commercial WO₃ powder of 10-30 nm in diameter. The composite electrode thus prepared was finally heat-treated at 300°C for 1 h. The anodic (water oxidation) photocurrent for the composite electrode in 0.1 M Na₂SO₄ yielded an IPCE (incident photon to current efficiency) of about 6 % at 400 nm and was stable for more than 24 h. Besides, the onset potential lay a little (by about 0.05 V) more negative than the equilibrium hydrogen evolution potential, indicating a possibility of solar water splitting with no external bias. A preliminary result for the water photooxidation with an "n- GaP/p-Si/Pt dot" electrode is also reported briefly.

The conversion of light energy into chemical energy is a focus of much research. Solar energy is of sufficient energy to drive water splitting to generate hydrogen and oxygen. The splitting of water involves multi-electron reactions and the breaking and formation of chemical bonds. Light driven water splitting has therefore proven elusive. Supramolecular complexes that contain ruthenium or osmium polyazine units can efficiently absorb visible light and generate charge transfer excited states. While many supramolecular complexes can absorb solar light efficiently, few are able to convert this energy into chemical energy via the conversion of a readily available chemical feedstock into a fuel. One process that is proposed as applicable for light to energy conversion is photoinitiated electron collection. Photoinitiated electron collection is a multi-step process whereby light energy is used to collect reducing equivalents. The collection of reducing equivalents is an essential step in the use of light energy to drive multi-electron reactions such as water splitting. The development of mixed-metal complexes as photoinitiated electron collectors is described, including the factors impacting device function. The use of Rh based electron collectors allows for the reducing equivalents generated by photoinitiated electron collection to be transferred to substrates, such as the reduction of water to produce hydrogen.

The promise of efficient, economic and renewable H₂ photoproduction from water can potentially be met by green algae. These organisms are able to functionally link photosynthetic water oxidation to the catalytic recombination of protons and electrons to generate H₂ gas through the activity of the hydrogenase enzyme. Green algal hydrogenases contain a unique metallo-catalytic H-cluster that performs the reversible H₂ oxidation /evolution reactions. The H-cluster, located in the interior of the protein structure is irreversibly inactivated by O₂, the by-product of water oxidation. We developed an Escherichi coli expression system to produce [FeFe]-hydrogenases from different biological sources and demonstrated that clostridial [FeFe]-hydrogenases have higher tolerance to O₂ inactivation compared to their algal counterparts. We have been using computational simulations of gas diffusion within the Clostridium pasteurianum CpI hydrogenase to identify the pathways through which O₂ can reach its catalytic site. Subsequently, we modify the protein structure at specific sites along the O₂ pathways (identified by the computational simulations) by site-directed mutagenesis with the goal of generating recombinant enzymes with higher O₂ tolerance. In this paper, we review the computational simulation work and report on preliminary results obtained through this strategy.

Research efforts to develop efficient systems for H₂ production encompass a variety of biological and chemical approaches. For solar-driven H₂ production we are investigating an approach that integrates biological catalysts, the [FeFe] hydrogenases, with a photoelectrochemical cell as a novel bio-hybrid system. Structurally the [FeFe] hydrogenases consist of an iron-sulfur catalytic site that in some instances is electronically wired to accessory iron-sulfur clusters proposed to function in electron transfer. The inherent structural complexity of most examples of these enzymes is compensated by characteristics desired for bio-hybrid systems (i.e., low activation energy, high catalytic activity and solubility) with the benefit of utilizing abundant, less costly non-precious metals. Redesign and modification of [FeFe] hydrogenases is being undertaken to reduce complexity and to optimize structural properties for various integration strategies. The least complex examples of [FeFe] hydrogenase are found in the species of photosynthetic green algae and are being studied as design models for investigating the effects of structural minimization on substrate transfer, catalytic activity and oxygen sensitivity. Redesigning hydrogenases for effective use in bio-hybrid systems requires a detailed understanding of the relationship between structure and catalysis. To achieve better mechanistic understanding of [FeFe] hydrogenases both structural and dynamic models are being used to identify potential substrate transfer mechanisms which are tested in an experimental system. Here we report on recent progress of our investigations in the areas of [FeFe] hydrogenase overexpression, minimization and biochemical characterization.

Clean structures are fabricated in ultrahigh vacuum conditions by evaporation through a shadow mask, avoiding contamination by resist, chemicals or exposure to air. Moving the shadow mask with nanometer precision during the growth of the structures gives additional freedom in determining the lateral shape and the thickness profile. Different materials can easily be combined and chosen from a large range of metals, semiconductors or insulators. In-situ treated surfaces or, conversely, ex-situ pre-fabricated samples can be used as substrates.

We fabricated and characterized one- and two- dimensional nanoscale arrays of platinum for study of model catalysts. One-dimensional arrays of nanoscale facets were fabricated by annealing a high-index plan of platinum single crystals. The high-index plane forms rows of alternating two low-index facets, (111) and (100), widths of which are ~10 nanometers. Two-dimensional arrays were fabricated lithographically from the epitaxial films of platinum grown on SrTiO3 substrates. Electron beam lithography was used to create precisely registered square arrays of millions of identical platinum nanocrystals with ~30 nm in diameter.

Recent progress in nanotechnology has stimulated research in the area of advanced materials and resulted in the development of a new class of nano structured materials. These materials are finding uses in a wide range of applications including photoelectrochemical water splitting using Tandem Cells^TM. . This is a novel concept designed to generate hydrogen by splitting water under direct sunlight [1]. A Tandem Cell^TM consists of two photocells which are connected optically in series, each containing a nanostructured semiconductor photoelectrode (Fig. 1). The Tandem Cell^TM is a low cost alternative to solar water splitting configurations proposed by others and Hydrogen Solar is currently in the process of developing the technology.

SHEC LABS - Solar Hydrogen Energy Corporation constructed a pilot-plant to demonstrate a Dry Fuel Reforming (DFR) system that is heated primarily by sunlight focusing-mirrors. The pilot-plant consists of: 1) a solar mirror array and solar concentrator and shutter system; and 2) two thermo-catalytic reactors to convert Methane, Carbon Dioxide, and Water into Hydrogen. Results from the pilot study show that solar Hydrogen generation is feasible and cost-competitive with traditional Hydrogen production. More than 95% of Hydrogen commercially produced today is by the Steam Methane Reformation (SMR) of natural gas, a process that liberates Carbon Dioxide to the atmosphere. The SMR process provides a net energy loss of 30 to 35% when converting from Methane to Hydrogen. Solar Hydrogen production provides a 14% net energy gain when converting Methane into Hydrogen since the energy used to drive the process is from the sun. The environmental benefits of generating Hydrogen using renewable energy include significant greenhouse gas and criteria air contaminant reductions.

The present paper considers the effect of segregation on the performance of photo-electrode materials for photo-electrochemical water splitting. This phenomenon, which alters the surface composition of a material during processing at elevated temperatures, has the capacity to dominate interfacial charge transfer between the photo-electrode and the electrolyte. As the present understanding of segregation in metal oxides is limited, this paper aims at addressing the need to collect empirical data which can be used for the development of novel materials. In the present investigation, Nb surface segregation was investigated at 1273 K under high and low oxygen activity using secondary ion mass spectrometry (SIMS). A calibration procedure was used to enable quantifiable data and Nb was observed to segregate strongly, especially at high oxygen activity. While this was attributed to the defect disorder, it remained unclear whether gas/solid equilibrium was achieved, and consequently whether the observed behaviour represents equilibrium segregation. Irrespectively, the observed behaviour clearly illustrates how the surface composition of a metal oxide can be altered through the control of segregation. This must be considered in the pursuit of high performance photo-electrode materials for water splitting under sunlight.

The semiconducting properties of TiO₂ single crystal and their changes during oxidation and reduction at elevated temperatures (1073 - 1323 K) under controlled oxygen activity (10^-9 - 10⁵ Pa) were monitored using measurements of electrical conductivity and thermoelectric power. The experimental data obtained in equilibrium led to a TiO₂ defect disorder model. According to this model, oxygen vacancies are the predominant defect species in TiO₂ across a wide range of oxygen activities. This work has discovered the diffusion of Ti vacancies, which are formed during prolonged oxidation at elevated temperatures and in a gas phase of high oxygen activity. Observations indicate that appreciable concentrations of Ti vacancies are formed on the TiO₂ surface and then are very slowly incorporated into the bulk. The obtained diffusion data has shown that in the commonly studied temperature range (1000-1400 K) the Ti vacancy concentration is quenched and can be considered as constant. Prolonged oxidation involves two kinetic regimes that are related to the transport of defects of different mobilities. The defect disorder model derived in this work may be beneficial for engineering TiO₂ for enhanced water splitting through the selection of optimal processing conditions, including temperature and oxygen activity.

In the investigation of alternative energy sources, specifically, solar hydrogen production from water, the ability to perform experiments with a consistent and reproducible light source is key to meaningful photochemistry. The design, construction, and evaluation of a series of LED array photolysis systems for high throughput photochemistry have been performed. Three array systems of increasing sophistication are evaluated using calorimetric measurements and potassium tris(oxalato)ferrate(II) chemical actinometry and compared with a traditional 1000 W Xe arc lamp source. The results are analyzed using descriptive statistics and analysis of variance (ANOVA). The third generation array is modular, and controllable in design. Furthermore, the third generation array system is shown to be comparable in both precision and photonic output to a 1000 W Xe arc lamp.