TiO₂ nanotube arrays (NTs) made by electrochemical anodization exhibit significant potential as a photoanode in low cost, sustainable photoelectrochemical cells for solar water oxidation.¹ The wide bandgap (3.2eV, λ < 387 nm) of these materials, however, limits the photocurrent, and the low mean free path of charge carriers further limits performance. Formation of substoichiometric phases or generation of suitable defects have been proven to enhance photoresponse.² Thermal treatment of TiO₂ under reducing atmosphere, for example, generates high-conductivity, oxygen deficient TiO_2-xmaterials, including the Magneli phases. Similarly, nitrogen doping may decreases the band gap, enabling increased light absorption.

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 1.23V_RHE. 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 1.23V_RHE 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.

References:

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).

Figure 1. Photochemical response of hydrogen reduced TiO₂ (a) and nitrogen modified TiO₂ (b).