Origin of the visible light absorption of Co2+ and NHR4+ co-doped hydrogen titanate nanotube thin films

Hydrogen 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.