Photocatalytic hydrogen evolution by co-catalyst-free TiO₂/C bulk heterostructures synthesized under mild conditions

Hydrogen production by photocatalytic water splitting is one of the most promising sustainable routes to store solar energy in the form of chemical bonds. To obtain significant H₂ evolution rates (HERs) a variety of defective TiO₂ catalysts were synthesized by means of procedures generally requiring highly energy-consuming treatments, e.g. hydrogenation. Even if a complete understanding of the relationship between defects, electronic structure and catalytic active sites is far from being achieved, the band gap narrowing and Ti³⁺-self-doping have been considered essential to date. In most reports a metal co-catalyst (commonly Pt) and a sacrificial electron donor (such as methanol) are used to improve HERs. Here we report the synthesis of TiO₂/C bulk heterostructures, obtained from a hybrid TiO₂-based gel by simple heat treatments at 400 °C under different atmospheres. The electronic structure and properties of the grey or black gel-derived powders are deeply inspected by a combination of classical and less conventional techniques, in order to identify the origin of their photoresponsivity. The defective sites of these heterostructures, namely oxygen vacancies, graphitic carbon and unpaired electrons localized on the C matrix, result in a remarkable visible light activity in spite of the lack of band gap narrowing or Ti³⁺-self doping. The materials provide HER values ranging from about 0.15 to 0.40 mmol h^-1 g_cat ^-1, under both UV- and visible-light irradiation, employing glycerol as sacrificial agent and without any co-catalyst.