This PDF file contains the front matter associated with SPIE Proceedings Volume 8109, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

Efficient solar-driven catalytic water oxidation is one of the main challenges in solar-to-fuel conversion. In this proceeding, we investigate three approaches for constructing electron acceptor -sensitizer - catalysts systems for photocatalytic water oxidation and our current understanding of the relevant fundamental processes involved. We demonstrated that an all-inorganic molecular water oxidation catalyst (WOC), [{Ru₄O₄(OH)₂(H2O)₄}(γ-SiW10O36)2]10- (or Ru₄POM), catalyzed a homogenous O₂ evolution system with 27% quantum efficiency in homogeneous solution in the presence of sensitizer (Ru(bpy)3) and sacrificial electron donor.¹ This suggests the feasibility of a heterogeneous photoelectrochemical system in which the photoanode integrates all three components: electron acceptor, photosensitizer, and WOC. We prepare a photocatalytic electrode based on Ru₄POM and a dye-sensitized nanoporous TiO₂ film for efficient light-harvesting and charge separation. Ultrafast spectroscopic studies of this triadic nanocomposite indicate efficient charge separation from the excited sensitizer to TiO₂ and efficient regeneration of the ground state of the dye. The latter can be attributed to Ru₄POM oxidation by the photogenerated dye cation and has a yield of > 80% within 1 ns.

Transient absorption of visible light responsive powder photocatalysts, solid solution of GaN and ZnO (denoted as GaN:ZnO), and other related materials were measured by using femtosecond diffuse reflectance spectroscopy in order to evaluate the nature of photogenerated electrons and holes through spectral information in visible and near-infrared region as well as kinetics from 100 fs to 500 ps. The GaN:ZnO is known to be one of successful photocatalysts which are able to split water into oxygen and hydrogen molecules under visible-light irradiation. Since photoexcitation generates electrons and holes in the conduction and valence bands, respectively, it is important to understand their trapping and recombination processes in details. Generally efficient and quick trapping and slow recombination of them are required to increase the chance of charge transfer of them to protons and water molecules. We have elucidated that the charge trapping was within time resolution (< 1 ps) and recombination time was about 100 ps for 26% of carriers and much longer for 74%, clearly indicating that most of photogenerated carriers have long lifetime. Other photocatalysts with lower photocatalytic activity showed shorter lifetimes. These results indicated that the long carrier lifetime in GaN:ZnO is one of the reasons for the efficient reactivity.

In this paper, we present the band gap reduction effect of sulfur doping on TiO₂ in anatase phase. This study is based on Density Functional Theory (DFT). For these calculations, several types of supercells consisting of 48 atoms in anatase phase are used to study the optoelectronic properties and band gap energy of sulfur-doped TiO₂. The band gap reduction effect of sulfur doping as a function of concentration is also studied here. The most stable substitution site for sulfur is predicted based on theoretical calculations. Based on the previous experimental results and the recent theoretical calculations in this paper, it is proven that sulfur doping is a promising approach for band gap reduction of TiO₂ for a wide variety of energy-based applications.

We analyze the origin of the large (about 128 nm) difference in the maximum of the visible absorption spectrum of dyes 2-Cyano-3-[5'-(4''-(N,N-dimethylamino) phenyl) thiophen-2'-yl] acrylic acid and Cyano-[5-(4-(N,Ndimethyl-amino) benzylidene)-5H-thiophen -2-ylidene]-acetic acid which differ by the position of the methine unit that was observed in an acetonitrile solution. We perform an ab initio analysis of possible factors such as (non-)planarity of the molecule, isomerization, and solvent effects as well as of the influence of computational parameters. Ground state calculations failed to account for the difference in transition energies, but excited state optimization of deprotonated dyes in solution resulted in values comparable to the experiment. We conclude that the most likely explanation for the difference is different stabilization of the LUMO by the polar solvent.

New multi-cobalt-containing polyoxometalates (POMs) are soluble, fast and tunable water oxidation catalysts (WOCs). We report additional studies of [Co₄(H₂O)₂(PW₉O₃₄)₂]^10- (1), a very fast, soluble and oxidative stable WOC: new kinetics data further indicate that Co²⁺ is not kinetically important in water oxidations catalyzed by 1. Second, we report a new WOC, [{Co₄(ì-OH)(H₂O)₃}(Si₂W₁₉O₇₀)]^11- that coexists in a 1:1 ratio in the solid state (2a and 2b), and while it is oxidatively stable, it is not hydrolytically stable, rearranging to [Co(H₂O)SiW₁₁O₃₉]^6- in aqueous solution. All these studies provide insights relating structural, electronic and other features of these WOCs to their reactivity and stability.

The developments of water-splitting systems that can efficiently use visible light have been a major challenge for many years in order to realize efficient conversion of solar light. We have developed a new type of photocatalysis system that can split water into H₂ and O₂ under visible light irradiation, which was inspired by the two-step photoexcitation (Zscheme) mechanism of natural photosynthesis in green plants. In this system, the water splitting reaction is broken up into two stages: one for H₂ evolution and the other for O₂ evolution; these are combined by using a shuttle redox couple (Red/Ox) in the solution. The introduction of a Z-scheme mechanism reduces the energy required to drive each photocatalysis process, extending the usable wavelengths significantly (~460 nm for H₂ evolution and ~600 nm for O₂evolution) from that in conventional water splitting systems (~460 nm) based on one-step photoexcitation in single semiconductor material.

The design and assembly of redox active polyoxometalate clusters, and their potential as water oxidation catalysts is discussed. The activity of the clusters is placed into context with comparisons to other systems and routes to the design and solution control of the assembly of novel polyoxometalate clusters with the correct characteristics for catalysis is also presented. Building on these features, a potential new polyoxometalate-based device architecture is presented that combines redox active polyoxometalate clusters, using systems that have been shown to be good water oxidation catalysts or structural models for photosystem II, with large cationic dyes to produce microtubular architectures that can be deposited on transparent substrates. The combination of a range of highly redox and catalytically active polyoxometalates with a range of possible cationic dye candidates allows the development of modular device architectures that can be screened and developed as potential new solar fuel cells.

Recently, preliminary experimental results of solar-to-hydrogen generation by wafer level InGaN nanowires were reported [Z. Mi et al. Nano Lett., 2011, 11 (6), pp 2353-2357]. In the present paper we report a theoretical investigation on the dissociation process of water molecules on wurtzite GaN (100) surface (M-Plane) using the density functional theory (DFT). We calculated the structure and energetic of the water adsorption, reaction barrier energies and pathway for water dissociation. The results suggest that the absorption of H2O is more favorable near Gallium atoms than near Nitrogen atoms and we determined the likely binding sites of water molecules on GaN (100) surface. We also analyzed a model for hydrogen evolution reaction on GaN (100) that involves three steps, where a water molecule first dissociates into a hydrogen atom plus the OH group, followed by the dissociation of the hydroxyl group, and finally the two hydrogen atoms recombine to form molecular hydrogen. For these reactions, the atomic positions and the reaction barriers were determined.

In this work we present an overview of the photo-reaction of ethanol over the surface of TiO2 (110) single crystal under photo-excitation and compare it to that over Au/TiO₂ nano-particle. Over rutile TiO₂(110) surface ethanol is present mainly in ethoxide (CH3CH2O(a)) form at 300K as evidenced by the presence of XPS C1s peak at 286.5 eV due to the -CH2-O(a) function; (a) for adsorbed. DFT computation of the same system indicated that the surface coverage is 50% or less in line with previous experimental results [1]. Exposing a pre-dosed surface to UV light in presence of oxygen resulted in the formation of acetaldehyde (CH₃CHO(g); (g) for gas phase) with the extent of reaction depending on the square root of the O₂ pressure in the 10^-10 - 10^-6torr range. Over the Au/TiO2 powder system we have focused the attention on the production of hydrogen as the oxidation of ethanol of ethanol to acetaldehyde has been previously studied [2]. The reaction is found to be sensitive to the polymorph nature of TiO₂ with anatase showing two orders of magnitudes higher activity than rutile. We have also addressed the TiO₂ particle size effect on the reaction and found that the TiO₂ particles, in the 150 to 10 nm range, to have the same reactivity.

This study was designed to examine the possible photosensitization effect of zinc oxide (ZnO) nanowires (NWs) by Au nanoparticles (AuNPs) by directly monitoring the charge carrier lifetime in AuNP-decorated ZnO NWs. ZnO-Au nanocomposite structures showed reduced photocurrent compared to pristine ZnO NWs due to the combined effect of ZnO etching during the AuNPs growth and competitive absorption/scattering effects from AuNPs of incident UV photons. Ultrafast transient pump-probe spectroscopy was utilized to characterize the charge carrier dynamics. The bleach recovery of ZnO indicates electron-hole recombination on the 150 ps time scale attributed to shallow donor recombination. The AuNP-decorated ZnO NWs exhibit a fast decay of 3 ps in addition to the decays observed for ZnO NWs. This fast decay is similar to the hot electron relaxation lifetime observed for AuNPs in solution. Overall, the dynamics features for AuNP-decorated ZnO NWs appear as a simple sum of those from AuNPs and ZnO NWs alone. There is no evidence of photosensitization of the ZnO NWs by AuNPs investigated in this study.

Cadmium sulfide (CdS) quantum dots with a series of electron donor and acceptors were employed to investigate thermodynamic and kinetic influences on photo-induced electron transfer reactions at CdS surface. Although the potential energy levels of all electron donor and acceptors are located within the QD band gap, the QD photoluminescence (PL) behavior is dependent upon the type of quencher. PL decreased into half, when one methyl viologen or benzyl viologen molecule per one QD was added into the QD solution, implying that the molecule attaches to the QD surface. In contrast, the dynamic quenching behavior was observed when thionine or o-tolidine was employed as a quencher. PL quenching efficiency decreased, when the distance between the QD surface and the quencher was increased by capping the QD with butylamine. Therefore, the PL quenching is mainly controlled kinetically rather than thermodynamically.

studied primary photocatalytic reactions in TiO2 nanoparticle films by using time-resolved laser spectroscopy. In this report, we assign absorption spectra due to active species in TiO2. We also observed the reaction of holes in TiO2 with methanol. Finally, we detected mobile electrons in a strong light-scattering sample by means of time-resolved microwave conductivity.k

The production of fuels directly or indirectly from sunlight represents one of the major challenges to the development of a sustainable energy system. Hydrogen is the simplest fuel to produce and while platinum and other noble metals are efficient catalysts for photoelectrochemical hydrogen evolution, earth-abundant alternatives are needed for largescale use. We show that bio-inspired molecular clusters based on molybdenum sulfides and tungsten sulfides mimic nature's enzymes for hydrogen evolution, molybdenum sulfides evolve hydrogen at a slightly higher overpotential than platinum when deposited on various supports. It will be demonstrated how this overpotential can be eliminated by depositing the same type of hydrogen evolution catalyst on p-type Si which can harvest the red part of the solar spectrum. Such a system could constitute the cathode part of a tandem dream device where the red part of the spectrum is utilized for hydrogen evolution while the blue part is reserved for the more difficult oxygen evolution. The samples have been illuminated with a simulated red part of the solar spectrum i.e. long wavelength (ë > 620 nm) part of simulated AM 1.5G radiation. The current densities at the reversible potential match the requirement of a photoelectrochemical hydrogen production system with a solar-to-hydrogen efficiency in excess of 10%. The experimental observations are supported by DFT calculations of the Mo₃S₄ cluster adsorbed on the hydrogen-terminated silicon surface providing insights into the nature of the active site.

Photocatalytic water splitting to produce H₂ and O₂ with semiconductor photocatalysts provides an attractive solution to global energy and environmental problems. The development of photocatalysts with high efficiency, availability, and stability under wide solar spectrum is paramount for the practical application of this technology. Nitrogen doping and preparation of materials with desirable crystal structure and morphology are two important strategies of fine-tuning the properties of semiconductor photocatalysts. In the present work, by synchronizing the two strategies, photocatalysts with typical structures were doped with nitrogen with the aim of realizing efficient water splitting under wide solar spectrum. After nitrogen doping, the absorption of the as-obtained N-doped photocatalysts was extended from the UV to the visible region. The doped photocatalysts exhibited not only increased visible light absorbance but enhanced photocatalytic hydrogen or oxygen production under light irradiation, in comparison to that of undoped parent compound. DFT calculations indicated that the top of the valence band is composed of N2p states mixed with pre-existing O2p states, which moved the valence band maximum (VBM) upwards, as a result, decreasing the band gap of the parent oxide photocatalysts tremendously. The unique structures of the pristine materials were found to facilitate the homogeneous of nitrogen nitrogen in the whole materials by offering excellent pathways for nitrogen doping process. This work highlighted the importance of crystal structures on the doping of nitrogen, paving a new way for developing novel functional photocatalytic materials.

This study describes a simple two-step approach to coat gold nanorods with a silica/titania shell. Gold nanorods with an aspect ratio of 2.5 (L=48±2 and d=19±1) are synthesized by a silver-seed mediated growth approach according to our previously reported procedure (Hunyadi Murph ACS Symposium Series, Volume 1064, Chapter 8, 2011, 127-163 and reference herein). Gold nanorods are grown on pre-formed gold nano-seeds in the presence of surfactant, cetyltrimethylammonium bromide (CTAB), and a small amount of silver ions. A bifunctional linker molecule which has a thiol group at one end and a silane group at the other is used to derivatize gold nanorods. The silane group is subsequently reacted with both sodium silicate and titanium isopropoxide to a silica/titania shell around the gold nanorods. By fine tuning the reaction conditions, the silica/titania shell thickness can be controlled from ~5 to ~40nm. The resulting nanomaterials are stable, amenable to scale up and can be isolated without core aggregation or decomposition. These new materials have been characterized by scanning electron microscopy, energy dispersive X-ray analysis, UV-Vis spectroscopy and dynamic light scattering analysis. Photocatalytic activity of Au-silica/titania nanomaterials under visible and UV illumination is measured via degradation of a model dye, methyl orange (MO) under visible and UV illumination. The results indicate a 3 fold improvement in the photocatalytic decomposition rate of MO under visible illumination vs. UV illumination.

Vertically aligned bundles of TiO₂ nanocrystals were fabricated by pulsed laser deposition (PLD) and tested as a photoanode material in dye sensitized solar cells (DSSC) using scanning electron microscopy (SEM), light absorption spectroscopy (UV-Vis), and incident photon-to-current efficiency (IPCE) experiments. An optimal background pressure of oxygen during deposition was discovered to produce a photoanode structure that simultaneously achieves high surface area and improves charge transport for enhanced photoelectrochemical performance. UV-Vis studies show that there is a 1.4x enhancement of surface area for PLD-TiO₂ photoanodes compared to the best sol-gel films. PLD-TiO₂ DSSC IPCE values are comparable to 3x thicker sol-gel films and nearly 92% APCE values have been observed for optimized structures.

Hematite is a potential candidate for hydrogen production by photoelectrochemical (PEC) decomposition of water due to its good bad gap and excellent chemical stability. However, its poor conductivity limits its PEC performance. Titanium has been predicted to be a good choice of dopant for improving the conductivity. Most of the Ti-doped hematite films are produced by solution based method. However, such procedure may introduce impurities. RF sputtering is a clean vacuum deposition technique, which is perfect for the synthesis of metal oxide. In this paper, we report our synthesis of Tidoped hematite thin films by RF magnetron co-sputtering of iron oxide and titanium targets at various conditions. Our work shows that the structure and morphology of iron oxide can be modified by controlling the doping concentration of titanium. Moreover, we confirmed that the PEC performance of Ti-doped iron oxide film is significantly better than the undoped one.

In this work we describe present experimental results for two related ternary phosphide materials, N-alloyed GaP and ZnGeP₂. These materials represent two potential mid-bandgap photoelectrode materials for artificial photosynthetic systems for solar energy conversion/storage. For photoelectrochemical cells designed to generate energyrich chemical fuels under illumination, candidate photoelectrode materials should demonstrate the capacity to sustain large photovoltages and photocurrent densities under solar insolation. The results in this work show that the optical properties of these two materials should enable the possibilities for light collection out past 600 nm. For N-alloyed GaP nanowire films, diffuse reflectance spectra show the increase of light absorption at sub-bandgap wavelengths with increasing NH₃(g) used during the annealing step. Corresponding photoelectrochemical data show that the quantum efficiency for light collection at sub-bandgap wavelengths does not follow the same monotonic trend. Separately, we report the first demonstration of ZnGeP₂ nanowire films. The as-prepared materials show reflectance responses consistent with a mid-bandgap material featuring a pseudo-direct bandgap.