Black TiO₂ nanomaterials have recently emerged as promising candidate for efficient solar-driven photocatalytic hydrogen production. Despite the great efforts to synthesize highly reduced TiO₂, it is apparent that intermediate degree of reduction (namely grey titania) brings about the formation of unique defective catalytic sites enabling co-catalyst free hydrogen generation. A precise understanding on the structural and electronic nature of this catalytically active site is still elusive, as well as the fundamental structure-activity relationships that govern formation of crystal defects, increased light absorption, charge separation and photocatalytic activity. In this perspective we discuss basic concepts that underlies an effective design of reduced TiO₂ photocatalysts for hydrogen production such as (i) defects formation in reduced TiO₂, (ii) analysis of structure deformation and presence of unpaired electrons through electron paramagnetic resonance spectroscopy, (iii) insights from surface science on electronic singularities due to defects, and (iv) photocatalysis with reduced TiO₂ classified as Pt-modified and co-catalyst-free photocatalytic hydrogen generation. Finally, future directions to improve the performance of reduced TiO₂ photocatalysts are outlined.

Noble metal dewetting on self-organized TiO ₂ nanotubes – nanoscopic design of photocatalysts towards green H ₂ generation.