Photocatalysis with Reduced TiO2: From Black TiO2 to Cocatalyst-Free Hydrogen Production

Black TiO2 nanomaterials have recently emerged as promising candidates for solar-driven photocatalytic hydrogen production. Despite the great efforts to synthesize highly reduced TiO2, it is apparent that intermediate degree of reduction (namely, gray titania) brings about the formation of peculiar defective catalytic sites enabling cocatalyst-free hydrogen generation. A precise understanding of the structural and electronic nature of these catalytically active sites 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 Review, we discuss the basic concepts that underlie 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) the key differences between black and gray titania, that is, photocatalysts that require Pt-modification and cocatalyst-free photocatalytic hydrogen generation. Finally, future directions to improve the performance of reduced TiO2 photocatalysts are outlined.