Origin of the visible light absorption of Co2+ and NH4+ co‐doped hydrogen titanate nanotube thin films

Abstract

AbstractHydrogen titanate nanotube (HTNT) thin films were synthesized by hydrothermal method and then Co2+ and NH4+ co‐doped hydrogen titanate nanotube (Co, N‐HTNT) thin films were prepared by ion‐exchange method. The Co, N‐HTNT thin films exhibit strong absorption in the visible light range compared with the HTNT and NH4+ doped hydrogen titanate nanotube (N‐HTNT) thin films. The first‐principles calculations reveal that NH4+ doping has no effect on the visible light absorption of HTNTs. The red shift of Co, N‐HTNTs is only due to the mixture of the Co 3d and O 2p states in the top of the valence band, which results in the band gap narrowing. Relative to HTNTs and N‐HTNTs, both the valence band maximum (VBM) and conduction band minimum (CBM) of Co, N‐HTNTs shift to lower potential based on the valence band XPS spectra. Furthermore, the up‐shift of the VBM is much larger than that of the conduction band, which can result in band gap reduction explaining the origin of the visible light absorption of Co, N‐HTNTs.