The visible light absorption property of Cu-doped hydrogen titanate nanotube thin films: An experimental and theoretical study

Hydrogen titanate nanotube thin films were synthesized by alkaline hydrothermal method using titanium plates as starting material and then doped with Cu through ion-exchange method. The experimental results show that Cu ions incorporate into the walls of nanotubes and exist as Cu2+. Cu doping leads to a red shift of the optical absorption edge from the ultraviolet region to the visible light region. First-principles calculation results reveal that Cu doping substantially reduces the band gap of hydrogen titanate nanotubes due to the presence of the impurity energy band of Cu 3d in intrinsic band gap. The valence and conduction band positions of Cu-doped hydrogen titanate nanotubes were determined by valence band spectra. The valence band is shifted to lower potential while the conduction band is shifted to higher potential after Cu doping, explaining the origin of the band gap narrowing and red shift.