This PDF file contains the front matter associated with SPIE Proceedings Volume 7044, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

We report the synthesis and characterization of nanostructured semiconductors such as CdS, CuInSe₂ (CIS) and TiO₂ for photovoltaic cells and photoelectrochemical cells for hydrogen production. The CdS was prepared by chemical deposition, CuInSe₂ by electrodeposition and chemical method and TiO₂ by sol-gel method. All the three semiconductors were prepared in the thin film and powder form. The CdS was synthesized as wide band gap n-type material in the nanostructured form. The p-CdS was prepared also in the nanostructured form with Cu doping. P-type CuInSe₂ films and powders were synthesized in the nanostructured form. TiO₂ was always formed in the nanostructured and n-type form. The films and powders were characterized by x-ray diffraction, atomic force microscopy, and opto-electronic methods. All the semiconductors were formed in the nanostructured form with different band gaps depending on the particle size and post-deposition treatments.

New water splitting solid solution photocatalysts with the composition of Gd_1-xBi_xVO₄ (x = 0, 0.3, 0.5, 0.7, 0.8, 0.9, 0.95, 1.0) were synthesized by a solid-state reaction. Gd_0.3Bi_0.7VO₄ was found as novel photocatalyst with both O₂ evolution from aqueous solution of sacrificial reagent AgNO₃ under visible-light irradiation (λ > 420nm) and H₂ evolution from aqueous solution of sacrificial reagent CH₃OH under near visible-light irradiation (λ > 380nm). The obtained solid solutions such as GdVO₄, Gd_0.7Bi_0.3VO₄, Gd_0.5Bi_0.5VO₄, and Gd_0.3Bi_0.7VO₄ crystallized in zircon-tetragonal crystal structures, while Gd_0.05Bi_0.95VO₄ and BiVO₄ crystallized in scheelite-monoclinic structures. The diffuse reflectance spectra of the solid solutions shift monotonically to a long wavelength as the ratio of Bi ions to Gd ions increases in the solid solution. The structure and water splitting activity were discussed in relation to the solid solution compositions and photophysical properties. Furthermore, new thin film photoelectrodes of Gd_0.7Bi_0.3VO₄ and BiVO₄ for solar hydrogen production were prepared by metal organic decomposition (MOD) method and polymerized complex (PC) method. The photoelectrodes were characterized by using Grazing Incidence X-ray Analysis (GIXA), SEM, cyclic voltammetry (CV) and IPCE measurement. Finally, solar energy conversion efficiency for water splitting (STH efficiency) was measured. Best STH efficiencies of BiVO₄ and Gd_0.3Bi_0.7VO₄ thin film photoelectrodes were 0.05% at the applied potential of 0.9 V and 0.025% at the applied potential of 0.5 V vs NHE, respectively.

Photoelectrochemical (PEC) water splitting at a semiconductor-electrolyte interface using sunlight is of considerable interest as it offers a clean approach to hydrogen production. PEC cells require semiconductor photoelectrode materials fulfilling a number of important requirements, such as band-edge alignment, corrosion resistance to electrolyte, and adequate current generation. We report the development of RF-PECVD-deposited hydrogenated amorphous silicon carbide (a-SiC:H) photoelectrodes with improved durability, which, when combined with a-Si:H tandem photovoltaic devices, should produce hydrogen directly from water under sunlight. The a-SiC:H is commonly grown with a bandgap in excess of 2.0 eV and completes the PEC device by providing contact with the electrolyte, proper band-edge alignment, and acts as a buffer for the a-Si:H tandem structure. Effects of the pH of electrolyte, type of substrates, and a platinum nanoparticle coating on the durability of a-SiC photoelectrodes will be presented. From these studies we surmise that corrosion or damage mechanism occurring on a-SiC:H layer could be divided into different aspects of physical and chemical. From the physical point of view, defects associated with spikes in textured TCO substrates, roughness of stainless steel, or other sources of pinholes may initiate delamination as confirmed by SEM (Scanning Electron Microscopy) and EDS (Energy-Dispersive X-ray Spectroscopy) studies. Chemically, the production of hydrogen involves reactions that may etch the electrode, especially when physical defects are involved. We observe that reducing the acidity of the electrolyte (increasing the pH from 0 to 2) significantly reduces corrosion while the useful photocurrent output of the a-SiC:H p/i structure is unaffected.

The surface structures of ZnO surfaces and ZnO nanoparticles, with and without water, were studied with a reactive force field (FF) within the ReaxFF framework, and molecular dynamics (MD) simulations. The force field parameters were fitted to a training set of data points (energies, geometries, charges) derived from quantum-mechanical B3LYP calculations. The ReaxFF model predicts structures and reactions paths at a fraction of the computational cost of the quantum-mechanical calculations. Our simulations give the following results for the (10-10) surface. (i) The alternating H-bond pattern of Meyer et al. for one monolayer coverage is reproduced and maintained at higher temperatures. (ii) Coverages beyond one water monolayer enhances ZnO hydroxylation at the expense of ZnO hydration. (iii) This is achieved through an entirely new H-bond pattern mediated via the water molecules in the second layer above the ZnO surface. (iv) During a desorption process, the desorption rate slows significantly when two monolayers remain. Simulations of nanoparticles in water suggest that these conclusions are relevant also in the nano case.

Modification in the properties of material by swift heavy ion irradiation is an interesting area of research. It provides the researchers a new dimension of introducing desired changes to the behaviour of the material, which largely influence their PEC properties also. This communication describes a study on the swift heavy ion irradiation induced modification in the PEC properties of nanostructured hematite thin films. Thin films of nanostructured α-Fe₂O₃ prepared by spray-pyrolysis were irradiated with Si⁷⁺ ions at fluence ranging from 5x10¹² to 4x10¹³ ions/cm². After characterizing them for structural, morphological and optical parameters, photoelectrochemical response of the irradiated and unirradiated samples of hematite thin films in terms of photocurrent density as a function of different ions fluence, towards the solar splitting of water was studied. It was observed that ion fluence plays an important role in modifying the PEC properties of the material. Sample irradiated with 100 MeV Si⁷⁺ ions at fluence 2x10¹³ ions/cm² exhibited the best photocurrent density.

Overall water splitting to form hydrogen and oxygen on a heterogeneous photocatalyst using solar energy is an attractive process for large-scale hydrogen production. In recent years, numerous attempts have been made for the development of visible-light-responsive photocatalysts to efficiently utilize solar energy. In this article, recent research progress in the development of visible-light-driven photocatalysts is described, specifically focusing on our efforts made on the development of (oxy)nitride photocatalysts for overall water splitting.

In view of this, we investigated new visible light nanostructured semiconductor photocatalyst especially in the field of composite metal oxide photocatalyst for solar hydrogen production from H₂S. For this purpose, two methodologies using a quantum mechanical material design and the microscopic surface analysis on a nanometer scale are adopted. The catalysts are synthesized by our proprietary soft chemical approaches. Also, a demonstrative reaction system for the effective solar hydrogen production is presented.

The surface grating technologies enable to control the thermal radiation spectrum. We are applying this technique to promote the chemical reaction to produce hydrogen in the methane steam reforming process by spectrally resonant thermal radiation. The thermal radiation spectrum is adjusted to vibrational absorption bands of methane and water molecules near 3 μm by making a two-dimensional surface grating of period Λ=2.6 μm on the radiative surface. By matching the peak of thermal radiation to the absorption bands of gases, it is clearly observed that the hydrogen production is promoted five times as much as the case without spectrally resonant thermal radiation by the optical excitation of vibrational energy levels of molecules. From a series of experiments and analysis, it is suggested that radiative gas effectively excited the molecules up of high energy vibrational and rotational levels, and this lead to the high production rate of hydrogen in methane steam reforming process.