Photocatalytic hydrogen production over NiO modified silica under visible light irradiation

The novel hetero-system xNiO/SiO2 (x = 0, 4 and 10%), prepared by impregnation/calcination, is assessed photoelectrochemically for the hydrogen evolution upon visible light. The X ray diffraction show mixed phases with a good crystallinity of NiO, spread uniformly over amorphous silica. The BET measurements give a specific surface area of similar to 112 m(2) g(-1) for loaded compounds. The optical properties show strong light absorption of wavelengths shorter than 860 nm. The Mott-Schottky characteristic (C-2-V) of NiO exhibits a positive slope, characteristic of n-type conductivity. The flat band potential (-0.93 V-SCE) is more cathodic than the water reduction level, thus allowing a spontaneous H-2 evolution. The electrochemical impedance spectroscopy reveals the predominance of the diffusion of finite length type and the Nyquist response is decomposed on R-C parallel circuit. NiO plays a key role as sensitizer and the photoactivity increases with increasing its loading. The best performance (0.2 cm(3) H-2 (g catalyst)(-1) mn(-1)) occurs for 10% NiO at pH similar to 13.1 with a light-to-chemical energy yield of 0.19% in presence of SO32- as hole scavenger. On the other hand, the decomposition of S2O32- is an energy storing process that could be a source of hydrogen from the solar light and the hetero-system is evaluated for the chemical energy storage through the photosynthetic reaction: (S2O32- + H2O + 2OH(-) -> 2H(2), Delta G(f)degrees = 101.04 kJ mol(-1)). Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.