DOE OSTI2011
The efficient conversion of solar photons into solar electricity and solar fuels is one of the most important scientific challenges of this century owing to dwindling fossil fuel reserves and the need for clean energy. While research in the direct conversion of solar energy to electricity in the areas of low-cost photovoltaic (PV) systems based on all-inorganic semiconductors, dye-sensitized solar cells, organic, and molecular PV is more technically advanced than its direct conversion to fuels, electricity may not be the ultimate primary solar energy conversion choice owing to the intermittence of solar radiation, the considerable energy loss during transmission, the availability of cost-effective storage media for electricity, and the continuing need for liquid transportation fuels. On the other hand, the direct conversion of solar photons to fuels such as H{sub 2}, CO, alcohols, and hydrocarbons using H{sub 2}O and CO{sub 2} as feedstocks offers a solution for the storage and distribution of solar energy in the form of stable chemical bonds that can be activated to provide energy at arbitrary times and locations. The latter approach to photocatalysis is generally called artificial photosynthesis, and has received renewed interest over the past five or so years. While 'photocatalysis' has not traditionally been restricted to the generation of 'solar fuels,' and has included the production of other useful chemicals, polymerization, and environmental remediation applications, the recent upsurge of interest has been driven mostly by renewable energy issues. It was the pioneering work on photo-electrochemical splitting of water to H{sub 2} and O{sub 2} by n-type TiO{sub 2} using ultraviolet light, by Fujishima and Honda in 1972, that ushered in the area of research that has come to be known as 'solar fuels,' and that has led to the terms 'photocatalysis' and 'solar fuels' becoming almost synonymous. This special issue of ChemSusChem is devoted to providing a current perspective on the field of photocatalysis. It contains invited papers from leading researchers in a wide range of important aspects of the field that address materials, photophysical, photochemical, and electrocatalysis issues. The area remains primarily the domain of basic research studies because progress toward the promise offered by the early work has (at least until recently) been slow, despite its significance having become increasingly recognized. The present collection of papers deals with new semiconductor photocatalysts, molecular catalysts for hydrogen production and water oxidation, dye-sensitized photoelectrochemical cells, and electrochemical CO{sub 2} reduction. Overall photochemical water splitting without any applied bias potential is achieved in several systems, especially under UV irradiation. Further advances are also achieved in a few semiconductor systems, such as GaZn oxynitrides or two-step (so-called 'Z-scheme') systems to produce H{sub 2} and O{sub 2} without any sacrificial reagent under visible irradiation. When band gaps of semiconductors are narrowed to absorb more visible light for greater efficiency, or when band positions are not suitable for carrying out one-electron redox processes, multielectron catalysts are required to promote proton-coupled electron transfer reactions in producing solar fuels. In homogeneous photocatalysis systems, sacrificial reagents are typically used to investigate the catalytic activity, detailed kinetics, and mechanisms of a half reaction. Photoelectrolysis systems with immobilized catalysts (metals, metal oxides, or molecular catalysts) on electrodes can separate oxidized products, such as O{sub 2}, and reduced products, such as H{sub 2}, CO, CH{sub 3}OH, and others, by means of proton- or hydroxide-conducting membranes. The following paragraphs briefly summarize these contributions. In the area of UV-driven water splitting, Townsend et al. prepared Pt-and/or IrO{sub x}-coated niobate (Nb{sub 6}O{sub 17}{sup 4-}) nanoscrolls and tested photochemical water reduction with methanol as a sacrificial reagent, and water oxidation with AgNO{sub 3} as a sacrificial reagent. In this work, factors for improving the limited photocatalytic activity of the system are explored. Nishiyama et al. investigated factors controlling the photocatalytic water splitting activity, such as preparation temperature and crystallinity of the semiconductor materials, the amount of co-catalyst loading, and the degree of dispersion of the co-catalyst with RuO{sub 2}-loaded niobium and tantalum bronzes, M{sub 8}P{sub 4}O{sub 32}{sup 4-} (M=Nb, Ta). Their DFT calculations demonstrate that severe distortion of NbO{sub 6} octahedra plays an important role in photocatalytic water splitting.