Titanium dioxide (TiO2) as a benchmark photocatalyst has been attracting attention due to its photocatalytic activity combined with photochemical stability. In particular, TiO2 with anatase polymorph holds promise for driving reduction reactions, such as proton reduction to evolve H2 via photocatalysis. In this study, anatase TiO2 is loaded with CoS2 through the hydrothermal route to form a CoS2@TiO2 photocatalyst system. X-ray absorption near edge structure confirms the +2-oxidation state of the Co cation, while extended x-ray absorption fine structure shows that each Co2+ cation is primarily coordinated to six S− anions forming a CoS2-like species. A small fraction of the Co2+ species is also coordinated to O2− anions forming Co x O y species and substitutionally resides at the Ti4+-sites. Further investigations with steady-state IR absorption induced by UV-light and time-resolved microwave conductivity suggest an efficient electron transfer from the conduction band of TiO2 to the surface-loaded CoS2 which acts as a metallic material with no bandgap. The CoS2 shallowly traps electrons at the host surface and facilitates proton reduction. An appreciably enhanced H2 evolution rate (8 times) is recognised upon the CoS2 loading. The CoS2 is here proposed to function as a proton reduction cocatalyst, which can potentially be an alternative to noble metals.

The reduction of anatase TiO2 with NaBH4 under argon atmosphere at a high temperature resulted in a longer electron lifetime and a larger electron population. The reduced gray anatase sample with disorder layer showed a higher evolution rate of H2 (130.2 μmol h−1 g−1) compared to pristine TiO2 (24.1 μmol h−1 g−1) in the presence of Pt co-catalyst in an aqueous glucose solution under exposure to ultraviolet light (λ ⩽ 400 nm). Ti3+ and oxygen vacancy defects were proposed to exist in the reduced TiO2. A continuum tail forms above the valence band edge top as a result of these two defects, which contribute to the lattice disorder. This is presumably also the case with the conduction band, which has a continuum tail composed of mid-gap states as a result of the defects. The Ti3+ and oxygen vacancy defects operate as shallow traps for photoexcited electrons, thereby preventing recombination. Since the defects are primarily located at the surface, i.e. in the disorder layer, the photoexcited electrons in shallow traps hence become readily available for the reduction of H3O+ into H2. The prolonged electron lifetime increases the photoexcited electron population in the reduced TiO2, resulting in enhanced water reduction activity.