The Role of Lattice Defects on the Optical Properties of TiO 2 Nanotube Arrays for Synergistic Water Splitting.

In this study, we report a facile one-step chemical method to synthesize reduced titanium dioxide (TiO 2 ) nanotube arrays (NTAs) with point defects. Treatment with NaBH 4 introduces oxygen vacancies (OVs) in the TiO 2 lattice. Chemical analysis and optical studies indicate that the OV density can be significantly increased by changing reduction time treatment, leading to higher optical transmission of the TiO 2 NTAs and retarded carrier recombination in the photoelectrochemical process. A cathodoluminescence (CL) study of reduced TiO 2 (TiO 2- x ) NTAs revealed that OVs contribute significantly to the emission bands in the visible range. It was found that the TiO 2 NTAs reduced for a longer duration exhibited a higher concentration of OVs. A typical CL spectrum of TiO 2 was deconvoluted to four Gaussian components, assigned to F, F + , and Ti 3+ centers. X-ray photoelectron spectroscopy measurements were used to support the change in the surface chemical bonding and electronic valence band position in TiO 2 . Electron paramagnetic resonance spectra confirmed the presence of OVs in the TiO 2- x sample. The prepared TiO 2- x NTAs show an enhanced photocurrent for water splitting due to pronounced light absorption in the visible region, enhanced electrical conductivity, and improved charge transportation.