Enhanced solar-driven photoelectrochemical water splitting of H/N co-doped TiO2: Role of defect states in a band gap reduction and promotion of charge transfer

Photoelectrodes based on reduced TiO2 are being extensively studied in the search for photocatalytic materials that can perform solar-driven water splitting and renewable hydrogen production. To date, substantial efforts have been devoted to understanding the microscopic mechanisms that underlie their photoelectrochemical (PEC) behavior, but this is a critical missing input to the Ti 3d carriers that govern the PEC activity. In this study the single and co-doping (N, H) in anatase TiO2 are prepared for PEC hydrogen production. The codoped photocathode delivers much better PEC performance from the enhanced solar light response and a fast electron pathway. Soft x-ray spectroscopy including x-ray absorption (XAS) and photoelectron spectroscopy (XPS) has been used to investigate the location and occupation of the defect levels from doping, through which the relevant engineered electronic properties in doped TiO2 are determined. It is shown that co-doping effectively produce the substitutional nitrogen in comparison with single doping. The Ti3+ defect states promoted by doping introduces localized Ti 3d-derived states below Fermi level (EF), resulting in a large reduction (approximately 1 eV) in the band gap of co-doped TiO2. The progressive production of oxygen vacancies eventually causes a surface band upward-bending induced electron transfer mechanism critical for charge separation. The present insights into defect chemistry and its determination to the electronic structure of co-doped TiO2 provide important guidance for using TiO2 in electrocatalysis and optoelectronics.