This PDF file contains the front matter associated with SPIE Proceedings Volume 8822 including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Titanium dioxide is a versatile material with ubiquitous applications, many of which are critically linked to either light absorption or transparency in the visible spectral range in addition to electrical conductivity. Doping is a well-known way to influence those properties in order to bring them into a desired range. Working towards a comprehensive understanding of the electronic and optical properties of TiO₂ (as well as of the link between them) we review and summarize electronicstructure results that we obtained using cutting-edge theoretical spectroscopy techniques. We focus on the formation of electron and hole polarons and we elucidate the influence of doping on the optical properties of TiO₂. In addition, we present new results for the reflectivity of pure TiO₂.

The effect of metal ion doping on electronic band structure and charge carriers' effective mass of Ta2O5 were studied using hybrid functionals in density functional theory. PBE0 hybrid functional proved to be very efficient in predicting the band structure with less than 5% error compared to experimental data. The bandgap decreases monotonically as the percentage of the dopant increases. Furthermore, the indirect bandgap behavior of Ta2O5 was found to initially increase with doping before it decreases again to its original value of pristine Ta₂O₅. We found that high doping content or even mixing with another metal is required in order to modify the band structure of Ta₂O₅. The effect of doping on the crystal structure was also studied. XRD measurements show that the crystal lattice tends to expand upon doping with metals with larger atomic radius than Ta and this effect is more pronounced as the dopant content increases.

This contribution shows the potential of cold gas spraying for the production of photoelectrodes employing photoelectrocatalysts for the water oxidation reaction. Conventional methods of coating usually employ sol-gel methods and calcination to obtain a good binding of the coating to the substrate. In cold gas spraying, particles are accelerated to high velocities by a pressurized gas. Nitrogen is used as process gas, preheated and then expanded in a De Laval type nozzle. On impact with the substrate the particles deform, break up and build an efficient interface to the back contact (as revealed, for example, by scanning electron microscopy). Cold gas spraying is a method for the direct bonding of particles to a substrate and does not require additives that have to be removed e.g. by a calcination step. Thereby it allows the direct fabrication of a working electrode ensemble. In our initial experiments, the state-of-the-art photocatalyst titanium dioxide (TiO2) was explored. The cold-gas-sprayed coatings revealed significantly higher activities for the oxygen evolution reaction (OER), as compared to films derived from wet-chemical processes. Due to the demand for photocatalysts with band gap suitable for visible light absorption, this approach was extended to the promising catalyst material hematite. In correlation with photoelectrochemical measurements, the operating parameters of the cold gas spray process are discussed in terms of their influence on the photocatalytic properties of the semiconductor.

We use theoretical simulations to study how oxidative and humid environments affect the chemical composition, shape and structure of ceria nanoparticles. Based on our calculations, we predict that small stoichiometric ceria nanoparticles will have a very limited stability range when exposed to these environments. Instead, we find that reduced ceria nanoparticles are stabilized without changing their inherent shape through the adsorption of oxygen molecules in the form of superoxo species and water in the form of hydroxo species. Based on our results, we propose a redox-cycle for meta-stable ceria nanoparticles without the formation of explicit oxygen vacancies, which is important for understanding the low-temperature oxygen chemistry of ceria at the nanoscale.

In the present paper, we report our efforts on the development of metal tungstate alloys for efficient and economical photoelectrochemical water splitting. As suggested by density functional theory (DFT), the addition of copper to the host tungsten trioxide improves the visible light absorption. Past studies at the Hawaii Natural Energy Institute have demonstrated that water splitting with co-sputtered and spray-deposited CuWO4 with 2.2 eV band gap was feasible, although the efficiency of the process was severely limited by charge carrier recombination. Density functional theory calculation showed that CuWO4 contains unfilled mid-gap states and high electron effective mass. To improve transport properties of CuWO4, we hypothesized that copper tungstate (CuWO4) hollow nanospheres could improve holes transfer to the electrolyte and reduce recombination, improving the water splitting efficiency. Nanospheres were synthesized by sonochemical technique in which the precursors used were copper acetate, ammonium meta-tungstate and thiourea (used as a fuel to complete the reaction). All chemicals undergo a high-energy sonication by using ethylene glycol as a solvent. Preliminary linear scan voltammetry (LSV) performed for annealed CuWO4 under front side and back side simulated AM-1.5 illumination demonstrated that the CuWO4 hollow nanospheres were photoactive. Subsequent scanning (SEM) and transmission (TEM) electron microscopy studies revealed the clear formation of nano sized hollow spherical shaped CuWO4 particles. X-ray diffraction analysis showed a clear formation of triclinic CuWO4 structure during the sonochemical process.

The effect of tungsten doping and hydrogen annealing treatments on the photoelectrochemical (PEC) performance of bismuth vanadate (BiVO4) photoanodes for solar water splitting was studied. Thin films of BiVO4 were deposited on ITO-coated glass slides by ultrasonic spray pyrolysis of an aqueous solution containing bismuth nitrate and vanadium oxysulfate. Tungsten doping was achieved by adding either silicotungstic acid (STA) or ammonium metatungstate (AMT) in the aqueous precursor. The 1.7 μm – 2.2 μm thick films exhibited a highly porous microstructure. Undoped films that were reduced at 375 ºC in 3% H2 exhibited the largest photocurrent densities under 0.1 W cm^-2 AM1.5 illumination. This performance enhancement was believed to be due to the formation of oxygen vacancies, which are shallow electron donors, in the films. Films doped with 1% or 5% tungsten from either STA or AMT exhibited reduced photoelectrochemical performance and greater sample-to-sample performance variations. Powder X-ray diffraction data of the undoped films indicated that they were comprised primarily of the monoclinic scheelite phase while unidentified phases were also present. Scanning electron microscopy showed slightly different morphology characteristics for the Wdoped films. It is surmised that the addition of W in the deposition process promoted the morphology differences and the formation of different phases, thus reducing the PEC performance of the photoanode samples. Significant PEC performance variability was also observed among films deposited using the described process.

In this work, we investigate the initial interaction of water and oxygen with different surface reconstructions of GaP(100) applying photoelectron spectroscopy, low-energy electron diffraction, and reflection anisotropy spectroscopy. Surfaces were prepared by metal-organic vapour phase epitaxy, transferred to ultra-high vacuum, and exposed to oxygen or water vapour at room temperature. The (2 4) reconstructed, Ga-rich surface is more sensitive and reactive to adsorption, bearing a less ordered surface reconstruction upon exposure and indicating a mixture of dissociative and molecular water adsorption. The p(2 2)=c(4 2) P-rich surface, on the other hand, is less reactive, but shows a new surface symmetry after water adsorption. Correlating findings of photoelectron spectroscopy with reflection anisotropy spectroscopy could pave the way towards optical in-situ monitoring of electrochemical surface modifications with reflection anisotropy spectroscopy.

The paper revisits the mechanism of reversible hydrogen evolution and oxidation reactions (HER and HOR) based on detailed thermodynamic analysis and survey of recent literature data. An assumption about the participation of adsorbed hydrogen atoms as intermediates in these processes, which was made about half a century ago and cannot account for a number of recent experimental observations, is critically analyzed. We propose that adsorbed molecular ion (H+ 2 )ad acts as intermediate in HER and HOR reactions described by the overall reversible process as follows: 2H3O+ + 2e (H+2 )ad H2" + 2H2O. New insights into the surface electrochemical processes taking place on Pt electrode at E ERHE (RHE { reference hydrogen electrode) as well as electrocatalysis of HER at E < ERHE, i.e. under overvoltage conditions, are obtained. The presented concept explains quantitatively the HER and HOR phenomena observed experimentally.

Silicon nanowires are considered as a promising architecture for solar energy conversion systems. By metal assisted chemical etching of multi-crystalline upgraded metallurgical silicon (UMG-Si), large areas of silicon nanowires (SiNWs) with high quality can be produced on the mother substrates. These areas show a low reflectance comparable to black silicon. More interestingly, we find that various metal impurities inside UMG-Si are removed due to the etching through element analysis. A prototype cell was built to test the photoelectrochemical (PEC) properties of UMG-SiNWs for water splitting. The on-set potential for hydrogen evolution was much reduced, and the photocurrent density showed an increment of 35% in comparison with a ‘dirty’ UMG-Si wafer.