Abstract
Titanium dioxide has been the benchmark semiconductor in photocatalysis for more than 40 years. Full water splitting, that is, decomposing water into H2 and O2 in stoichiometric amounts and with an acceptable activity, still remains a challenge, even when TiO2‐based photocatalysts are used in combination with noble‐metal co‐catalysts. The bottleneck of anatase‐type TiO2 remains the water oxidation, that is, the hole transfer reaction from pristine anatase to the aqueous environment. In this work, we report that “grey” (defect engineered) anatase can provide a drastically enhanced lifetime of photogenerated holes, which, in turn, enables an efficient oxidation reaction of water to peroxide via a two‐electron pathway. As a result, a Ni@grey anatase TiO2 catalyst can be constructed with an impressive performance in terms of photocatalytic splitting of neutral water into H2 and a stoichiometric amount of H2O2 without the need of any noble metals or sacrificial agents. The finding of long hole lifetimes in grey anatase opens up a wide spectrum of further photocatalytic applications of this material.
Highlights
Titanium dioxide has been the benchmark semiconductor in photocatalysis for more than 40 years
We investigated the water oxidation products
The open-circuit potential (OCP) potential of the electrode under illumination was measured to be at 0.45 V RHE, i. e. under operating conditions the electrode is in potential range where Ni(metal) can effectively catalyze H2 evolution, see Overall, the present results show clearly that water splitting by the Ni@grey anatase TiO2 catalyst proceeds via the twocharge pathway to form H2 and H2O2 rather than the fourcharge pathway to afford H2 and O2
Summary
A strong PL signal, which is centered at around 520–530 nm, is observed for grey TiO2, while this PL is weak in plain anatase TiO2 The origin of this PL peak in anatase has been investigated in depth by Pallotti et al.[48] – t can be attributed to a recombination of conduction band electrons with deep-trapped holes that lie approx. Ni@plain anatase or Ni@black anatase no appreciable photocatalytic formation of H2O2 over extended times of illumination could be observed, the unique characteristics of the catalyst introduced here must be ascribed to grey titania and its optimum hole-trapping ability and the provision of sufficiently long life-times of trapped holes to enable reaction with water to H2O2.
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