Enhancement of photoelectrochemical performance for nitrogen-doped titanium dioxide-based thin films produced by a facile air-based sputtering deposition

Abstract

This work explores the use of air as a reactive gas in the sputtering deposition of N-doped TiO2-based bilayer, gradient, and multilayer thin films for photoelectrochemical evaluations. By merely adjusting the air/Ar flow ratio, crystalline phases, nitrogen doping contents, electrical properties, optical bandgaps, and valence band maximum (VBM) positions of N–TiO2 films were tailored. The N–TiO2/TiN(O) bilayer on the glass substrates exhibited a peak photocurrent density of 776 μA cm−2 at an air/Ar ratio of 1.60, which may be due to suitable bandgap and carrier mobility of the TiO2 overlayer as the bandgap was affected by the nitrogen content and rutile fraction in the TiO2 overlayer. To further improve the performance of N–TiO2, stepwise and sequential gradient structures with gradient bandgaps and nitrogen contents were produced, which could reach photocurrent densities of 805 ± 4 and 837 ± 3 μA cm−2, respectively. The gradient structures lead to enhanced light absorption efficiency and reduce carrier recombination. The N–TiO2/TiN(O) multilayer structures were also designed and revealed the highest photocurrent density of 865 ± 7 μA cm−2. Such evident enhancement is mainly due to the thin conductive while translucent TiN(O) layers between the photoelectrochemical-active N–TiO2 layers provide multiple low-impedance channels in enhancing carrier transport.