Structural evolution of titanium dioxide during reduction in high-pressure hydrogen

The excellent photocatalytic properties of titanium oxide (TiO₂) under ultraviolet light have long motivated the search for doping strategies capable of extending its photoactivity to the visible part of the spectrum. One approach is high-pressure and high-temperature hydrogenation, which results in reduced 'black TiO₂' nanoparticles with a crystalline core and a disordered shell that absorbs visible light. Here we elucidate the formation mechanism and structural features of black TiO₂ using first-principles-validated reactive force field molecular dynamics simulations of anatase TiO₂ surfaces and nanoparticles at high temperature and under high hydrogen pressures. Simulations reveal that surface oxygen vacancies created upon reaction of H₂ with surface oxygen atoms diffuse towards the bulk material but encounter a high barrier for subsurface migration on {001} facets of the nanoparticles, which initiates surface disordering. Besides confirming that the hydrogenated amorphous shell has a key role in the photoactivity of black TiO₂, our results provide insight into the properties of the disordered surface layers that are observed on regular anatase nanocrystals under photocatalytic water-splitting conditions.