N-doped TiO₂ nanotubes coated with a thin TaO_xN_y layer for photoelectrochemical water splitting: Dual bulk and surface modification of photoanodes

TaON is a good photoanode material with a suitable band structure for water splitting as well as coupling with TiO₂ for efficient charge separation. However, the synthesis of TaON that requires high temperature nitridation (850 °C) limits the combination with other materials. In this work, we deposited a thin amorphous TaO_xN_y layer on N-doped TiO₂ nanotubes (N-TNTs) through low temperature nitridation (500 °C) and demonstrated its successful performance as an efficient photoanode for water-splitting. Since the preparation temperature is low, TaO_xN_y on N-TNTs has a unique amorphous structure with a smooth thin layer (5 nm). It is proposed that the thin amorphous TaO_xN_y layer plays dual roles: (i) surface sensitization and/or charge rectification at the heterojunction between the TaO_xN_y layer and N-TNTs, and (ii) passivation of N-TNT surface trap states to retard the charge recombination. TaO_xN_y layer-decorated N-TNTs as dual modified TNTs (N-doping in the bulk and TaO_xN_y overlayer deposition on the surface) have significantly improved both visible (ca. 3.6 times) and UV (ca. 1.8 times) activities for PEC water-splitting as well as the faradaic efficiency (ca. 1.4 times, η = 98%) for H₂ production. Making the amorphous TaO_xN_y layer crystalline at higher temperatures reduced the PEC activity of the hybrid photoanode, in contrast, which indicates that the amorphous TaO_xN_y layer deposition on N-TNTs through low temperature nitridation (500 °C) is optimized for the PEC activity. A range of spectroscopic and electrochemical techniques were systematically employed to investigate the properties of the PEC process.