Photocatalysis and Hydrogen Evolution of Al- and Zn-Doped TiO Nanotubes Fabricated by Atomic Layer Deposition.

Highly homogeneous Al- and Zn-doped TiO nanotubes were fabricated by atomic layer deposition (ALD) via nanolaminated stacks of binary layers of TiO/AlO and TiO/ZnO, respectively. The bilayers were alternately deposited on the polycarbonate (PC) membrane template by ALD with various cyclic sequences. The nanotubes in a length of 20 μm and a diameter of 220 nm were obtained after removal of the PC membrane by annealing at 450 °C. The effects of doping composition on the photocatalytic and photoelectrochemical (PEC) activities were investigated. Increasing the Al doping reduced the photocatalytic activity of TiO due to formation of charge recombination sites and reduction of hydroxide radicals. In contrast, there was an optimal range of Zn doping to get enhanced photocatalytic activity and higher PEC efficiency. With a doping ratio of 0.01, the hydrogen production rate from water splitting was 6 times higher than that of commercial P25 TiO. The energy-band diagram of Zn-doped TiO determined by ultraviolet photoelectron spectroscopy revealed shift up of the Fermi level to provide more electrons to the conduction band. The photoinduced trapped electrons and holes were detected in Zn-doped TiO by in situ electron paramagnetic resonance spectroscopy, which revealed that Ti sites on the surface and surface oxygen vacancies played a key role in promoting the photocatalytic process.