Photosensitised oxidation of water by CdS-based suspensions
Dispersions of CdS powder with or without metal or metal oxide deposits, such as Pt, Au, Ag, Rh, Rh2O3 and RuO2, have been used to sensitise the oxidation of water by PtCl2–6. The most active of the CdS-based photosensitisers was found to be one in which Pt was deposited by precipitation of a Pt colloid onto the surface of a CdS sample which had been annealed in air for 3 h prior to platinisation. A study was made to determine the optimum conditions for O2 evolution. The initial rate of O2 evolution was found to depend upon a number of factors including: pH, [CdS/Pt], [PtCl2–6], [O2], Pt loading and CdS annealing temperature and environment. Although a number of different electron acceptors were tried, including PtCl2–6, PtCl2–4, Pt(OH)2–6, H2AuO–3, S2O2–8, Co(NH3)5Cl2+ and Fe(CN)3–6, O2 photogeneration was observed with only PtCl2–6 and Fe(CN)3–6. A number of other materials were tested as photocatalysts for the oxidation of water by PtCl2–6, including TiO2, TiO2/Pt, TiO2/Rh2O3, CdO, CdO/Pt, HgS/Pt, SiO2/Rh2O3 and Al2O3/Pt; however, only the TiO2-based materials showed any activity. The results of this work are discussed with respect to the current controversy over the mechanism for O2 evolution.