Effect of Redox Cocatalysts Location on Photocatalytic Overall Water Splitting over Cubic NaTaO3 Semiconductor Crystals Exposed with Equivalent Facets

In the semiconductor photocatalyst system for overall water splitting, cocatalysts play crucial roles because they provide not only redox active sites but also charge separation function for photogenerated electrons and holes. In this work, we have investigated the cubic structured NaTaO3 with six equivalent {001} facets to address the following two important questions: Can charge separation occur among the equivalent facets? How can photogenerated charges be separated on the equivalent surface for photocatalytic reactions? Charge location probe experiments by photodepsotion of noble metals and metal oxides show that no spatial charge separation occurs among the six equivalent facets of NaTaO3. However, observation of efficient overall water-splitting reaction upon loading of well-known cocatalyst NiO on the NaTaO3 clearly demonstrates that photogenerated electrons and holes could still be well-separated. In-situ formation of Ni and NiO cocatalysts during the water-splitting process was revealed by X-ray photoelectron spectroscopy and synchrotron X-ray absorption spectroscopy, confirming the role of dual cocatalysts Ni/NiO, where nickel serves as an electron trap (catalytic sites for proton reduction) and NiO serves as a hole trap (catalytic sites for water oxidation). Such vicinal charge separation by dual cocatalysts leads to efficient overall water splitting.