Photocatalytic hydrogen evolution over CuCrO_2

摘要

We have been studying the technical feasibility of a photochemical H_2 evolution based on a dispersion of CuCrO_2 powder in aqueous electrolytes containing various reducing agents (S~(2-), SO_3~(2-) and S_2O_3~(2-)). The title oxide combines a fair resistance to corrosion with an optimal band gap E_g of 1.32 eV. The intercalation of a small amount of oxygen should be accompanied by a partial oxidation of Cu~+ into Cu~(2+) implying a p-type semiconductivity. The S~(2-) oxidation inhibits the photocorrosion and the H_2 evolution increases parallel to polysulfides S_n~(2-) formation. Most of H_2 is produced when p-CuCrO_2 is connected to n-Cu_2O formed in situ. H_2 liberation proceeds mostly on CuCrO_2 while the oxidation of S~(2-) takes place over Cu_2O surface and the hetero system Cu_2O/CuCrO_2 is optimized with respect to some physical parameters. The photoactivity is dependent on preparation conditions and lowering the synthesis temperature through nitrate route leads to an increase in specific surface area S_(sp). The photoelectrochemical H_2 production is a mul-tistep process where the rate determining step is the arrival of electrons at the interface because of their low mobility. Prolonged irradiation ( > 80 min) leads to a pronounced decrease of the photoactivity; the tendency toward saturation is due to the undesired back reduction of polysulfides S_n~(2-) in a closed system and to their strong absorption in the visible region (λ_(max) = 520 nm).