Abstract
Titanium dioxide (TiO2) is often employed as a light absorber, electron-transporting material and catalyst in different energy and environmental applications. Heat treatment in a hydrogen atmosphere generates black TiO2 (b-TiO2), allowing better absorption of visible light, which placed this material in the forefront of research. At the same time, hydrogen treatment also introduces trap states, and the question of whether these states are beneficial or harmful is rather controversial and depends strongly on the application. We employed combined surface science and in situ electrochemical methods to scrutinize the effect of these states on the photoelectrochemical (PEC), electrocatalytic (EC), and charge storage properties of b-TiO2. Lower photocurrents were recorded with the increasing number of defect sites, but the EC and charge storage properties improved. We also found that the PEC properties can be enhanced by trap state passivation through Li+ ion intercalation in a two-step process. This passivation can only be achieved by utilizing small size cations in the electrolyte (Li+) but not with bulky ones (Bu4N+). The presented insights will help to resolve some of the controversies in the literature and also provide rational trap state engineering strategies.
Highlights
Photoelectrochemical (PEC) methods hold the promise to produce valuable chemical products by combining the functions of solar cells and electrolyzers.[1]
The photogenerated holes are responsible for the PEC reaction, not the electrons that are present on the trap states
The best photocatalytic activity was obtained for white TiO2 (w-TiO2)
Summary
Photoelectrochemical (PEC) methods hold the promise to produce valuable chemical products by combining the functions of solar cells and electrolyzers.[1]. A massive interest was devoted to hydrogen treatment induced modification of TiO2, which produces different defect states within the bandgap.[11,12] This process results in a drastic color change, from which the name black TiO2 (b-TiO2) originates.[13] The black color is the result of increased light absorption in the visible and near-infrared regime. The improved PEC performance of b-TiO2 was explained by the interplay of five effects: (i) increased light absorption, (ii) improved charge carrier separation, (iii) decreased resistance of charge transport in the bulk, (iv) enhanced charge transfer at the semiconductor/ electrolyte interface, and (v) suppressed recombination.[19] An interesting observation was that the photocurrent enhancement is mainly caused by the improved photoactivity in the UV region.[18,20] The narrowing of the bandgap did not automatically yield higher incident photon to charge carrier conversion (IPCE) values in the visible region. The photocurrent saturation, was achieved closer to the flatband potential, compared to its untreated counterpart, which indicates the better catalytic (i.e., charge transfer)
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