The Mystery of Black TiO2: Insights from Combined Surface Science and In Situ Electrochemical Methods

Titanium dioxide(TiO2) is often employed as a lightabsorber, electron-transporting material and catalyst in differentenergy and environmental applications. Heat treatment in a hydrogenatmosphere generates black TiO2 (b-TiO2), allowingbetter absorption of visible light, which placed this material inthe forefront of research. At the same time, hydrogen treatment alsointroduces trap states, and the question of whether these states arebeneficial or harmful is rather controversial and depends stronglyon the application. We employed combined surface science and in situelectrochemical methods to scrutinize the effect of these states onthe photoelectrochemical (PEC), electrocatalytic (EC), and chargestorage properties of b-TiO2. Lower photocurrents wererecorded with the increasing number of defect sites, but the EC andcharge storage properties improved. We also found that the PEC propertiescan be enhanced by trap state passivation through Li+ ionintercalation in a two-step process. This passivation can only beachieved by utilizing small size cations in the electrolyte (Li+) but not with bulky ones (Bu4N+). Thepresented insights will help to resolve some of the controversiesin the literature and also provide rational trap state engineeringstrategies.