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 ∼112 m2 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 VSCE) is more cathodic than the water reduction level, thus allowing a spontaneous H2 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 cm3 H2 (g catalyst)−1 mn−1) occurs for 10% NiO at pH ∼13.1 with a light-to-chemical energy yield of 0.19% in presence of SO 3 2 − as hole scavenger. On the other hand, the decomposition of S 2 O 3 2 − 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: ( S 2 O 3 2 − + H 2 O + 2 OH − → 2 SO 3 2 − + 2 H 2 , ΔGf° = 101.04 kJ mol−1).