Nature of Conduction Band Tailing in Hydrogenated Titanium Dioxide for Photocatalytic Hydrogen Evolution

The involvement of oxygen vacancies in the electronic and structural properties and photocatalytic activity of hydrogenated TiO2 (H-TiO2) for water splitting are currently under debate. Herein, the process for the formation of oxygen vacancies in H-TiO2 is illustrated for the first time to verify the role of these vacancies on the surface and in the bulk of H-TiO2 by using surface-sensitive soft X-ray techniques and bulk-information-included hard X-ray techniques for the O K-edges and Ti K-edges, respectively. We find that oxygen vacancies on the surface and in the bulk contribute differently to tailing of the conduction band of H-TiO2. By controlling the degree of hydrogenation to restrict tailing of the conduction band to just above the H+/H-2 redox potential, the most favorable disordered structure for photocatalytic H-2 evolution is developed. This work promotes a better understanding of the possible negative consequence of hydrogenated semiconductors in H-2 evolution apart from the positive effect on visible-light response.