No abstract

Near edge X-ray absorption fine structure (NEXAFS) spectroscopy is used to precisely probe the alignment, uniformity in crystal growth direction, and electronic structure of single-crystalline V2O5 nanowire arrays prepared by a cobalt-catalyzed vapor transport process. The dipole selection rules operational for core-level electron spectroscopy enable angle-dependent NEXAFS spectroscopy to be used as a sensitive probe of the anisotropy of these systems and provides detailed insight into bond orientation and the symmetry of the frontier orbital states. The experimental spectra are matched to previous theoretical predictions and allow experimental verification of features such as the origin of the split-off conduction band responsible for the semiconducting properties of V2O5 and the strongly anisotropic nature of vanadyl−oxygen-derived (VO) states thought to be involved in catalysis. The strong anisotropy observed across thousands of nanowires in the NEXAFS measurements clearly demonstrates the uniformity of crystal growth direction in these nanowire arrays.

WO 3 is a promising candidate for a photoanode material in an acidic electrolyte, in which it is more stable than most metal oxides, but kinetic limitations combined with the large driving force available in the WO 3 valence band for water oxidation make competing reactions such as the oxidation of the acid counterion a more favorable reaction. The incorporation of an oxygen evolving catalyst (OEC) on the WO 3 surface can improve the kinetics for water oxidation and increase the branching ratio for O 2 production. Ir-based OECs were attached to WO 3 photoanodes by a variety of methods including sintering from metal salts, sputtering, drop-casting of particles, and electrodeposition to analyze how attachment strategies can affect photoelectrochemical oxygen production at WO 3 photoanodes in 1 M H 2 SO 4 . High surface coverage of catalyst on the semiconductor was necessary to ensure that most minority-carrier holes contributed to water oxidation through an active catalyst site rather than a sidereaction through the WO 3 /electrolyte interface. Sputtering of IrO 2 layers on WO 3 did not detrimentally affect the energy-conversion behavior of the photoanode and improved the O 2 yield at 1.2 V vs. RHE from B0% for bare WO 3 to 50-70% for a thin, optically transparent catalyst layer to nearly 100% for thick, opaque catalyst layers. Measurements with a fast one-electron redox couple indicated ohmic behavior at the IrO 2 /WO 3 junction, which provided a shunt pathway for electrocatalytic IrO 2 behavior with the WO 3 photoanode under reverse bias. Although other OECs were tested, only IrO 2 displayed extended stability under the anodic operating conditions in acid as determined by XPS.