Enhanced photoelectrochemical performance for hydrogen generation via introducing Ti3+ and oxygen vacancies into TiO2 nanorod arrays

Abstract

Rutile TiO2 nanorod arrays were synthesized by a hydrothermal process, followed by NaBH4 treatment to obtain reduced TiO2 nanorods to be used as a photoanode for enhancement of photoelectrochemical (PEC) hydrogen generation. The NaBH4 treatment introduced Ti3+ and oxygen vacancy states in the TiO2 lattice, which increased the light absorption range and electrical conductivity of the photocatalyst, limiting the charge carrier recombination and thus enhancing the PEC properties performance of hydrogen generation. X-ray photoelectron spectra (XPS) and electron paramagnetic resonance (EPR) analysis confirmed the presence of Ti3+ and oxygen vacancies in the TiO2 nanorod after reducing. The optimal reduced TiO2 nanorod has been achieved after reaction with 2 M NaBH4 solution for 3 h, followed by HCl solution washing which improved the PEC performance. At 0.23 V vs Ag/AgCl (1.23 V vs RHE), the reduced TiO2 nanorod generated a photocurrent density of 4.03 mA/cm2 which is two times higher than the pristine TiO2 nanorod. Semiconductor/electrolyte interfacial charge transfer was investigated by electrochemical impedance spectra (EIS) using the Nyquist plots. So the reduced TiO2 nanorod presented a smaller resistance than that of TiO2 nanorod, thus oxygen vacancies enhanced the electrical conductivity in the reduced TiO2 nanorod. It is worth noticing that the Mott–Schottky plots revealed an increase in the donor density after NaBH4 treatment, while the reduced TiO2 remained n-type as the rutile TiO2.