Does carbon doping of TiO2 allow water splitting in visible light? Comments on “Nanotube enhanced photoresponse of carbon modified (CM)-n-TiO2 for efficient water splitting”

Reports of water splitting by carbon-doped titanium dioxide (TiO2) photoelectrodes under visible illumination are critically examined. Xu et al. [Sol. Energy Mater. Sol. Cells 91 (2007) 938] recently reported significant incident photon conversion efficiencies (IPCEs) at visible wavelengths for carbon-doped TiO2 in thin film and nanotube form. Evidence is given here that these results were due to an artefact in the measurements. Further, it is pointed out that the mechanism proposed for water splitting under visible illumination is unphysical, and the photocurrents presented are shown to be grossly inconsistent with the IPCE data. Other workers have also measured non-zero IPCEs at visible wavelengths for carbon-doped TiO2, but have not presented this as evidence of water splitting. In other cases, carbon doping was performed in a reducing atmosphere, and measured visible activity is most likely a result of oxygen vacancies. It is concluded that there is no convincing evidence in the literature of water splitting under visible light in carbon-doped TiO2.