Critical Role of the NiO–Ni(OH)₂ Species Loaded Onto TiO₂ Photocatalyst in the Enhancement of H₂ Evolution Activity

Abstract

One of the bottlenecks in photocatalysis is to obtain reasonably cheap co-catalysts beneficially contributing to efficient photophysical and photochemical processess. Herein, we prepare a co-catalyst system consisting of Ni species embedded in anatase TiO2. We focus on clarifying its role in regulating electron behavior, and hence, photocatalytic activity. Based on the transmission electron microscopy, X-ray absorption near-edge structure, and X-ray absorption fine structure, the surface-loaded Ni species are found to exist as crystalline NiO, together with a small fraction of amorphous Ni(OH)2. Using a combination of transient microwave conductivity and static infrared absorption spectroscopy, we show that electron transfer to the Ni species instead of electron trapping by structural defects, such as oxygen vacancies or reduced titanium cations, is key mechanism responsible for the prolonged lifetime of photoexcited electrons. The prolonged electron lifetime due to an efficient electron extractionby the Ni species are behind the increased H2 evolution activity. We demonstrate that the NiO-Ni(OH)2 co-catalyst system, which works as a H2 evolution site, is auspicious to make photoexcited electrons abundantly available on the surface by localizing them, and thus, readily accessible for the desired surface reactions.