In this work, anodized TiO2 nanotubes were modified via hydrogen or ammonia gas treatments to generate various substoichiometric phases, characterized by reduced bandgap and higher conductivity. In particular, annealing in partial hydrogen pressure (5 at%) leads to a gradual phase transformation of anatase to rutile and various Magneli phases, yielding an enhanced photoelectrochemical response up to 2.2-fold at 1.23VRHE. Annealing in ammonia and argon flow on the other hand results in oxygen loss with barely detectable N incorporation, resulting in a 4-5 fold photocurrent enhancement at 1.23VRHE with a significant cathodic shift (~100 mV) of the onset potential. The fraction of the photogenerated charges utilized for water splitting was estimated by comparing photoelectrochemical performance with or without hole scavenger species.3 All modified samples displayed higher water oxidation efficiency (above 80%) compared with unmodified TiO2(~65%). In order to evaluate the surface disorder introduced by the modification, electrochemical impedance spectroscopy (EIS) was carried out, showing higher charge carrier density. Overall, this work demonstrates that hydrogen or ammonia annealing treatments enhance significantly photoelectrochemical performance.
1. L. Tsui and G. Zangari, J. Electrochem. Soc. , 161, D3066–D3077 (2014)
2. X. Chen, L. Liu, and F. Huang, Chem. Soc. Rev., 44, 1861–1885 (2015)
3. L. Tsui, Y. Xu, D. Dawidowski, D. Cafiso, and G. Zangari, J. Mater. Chem. A, 4, 19070–19077 (2016).