Atomic Level In-situ Characterization of NiO-TiO2 Photocatalysts under Light Irradiation in Water Vapor

Photocatalysts are important for environmental cleanup of undesirable organic compounds and have potential applications for solar fuel generation either through water splitting or CO2 reduction [1]. It is now recognized that atomic level in-situ observations of catalytic materials are critical for understanding the structure-reactivity in catalysts. For photocatalysts, this requires that the system be observed not only in the presence of reactant and product species but also during in-situ light illumination. NiO loaded semiconductor photocatalyst with Ni first reduced and then partially re-oxidized at the surface has been reported to have good photocatalytic properties by forming a metallic Ni Ohmic contact between NiO and the semiconductors. NiO-Ni-SrTiO3 performs better than NiO/SrTiO3 while Ni/SrTiO3 does not have high photocatalytic activity [2]. TiO2 is a promising photocatalyst which has attracted intense research interest for decades since photo-decomposition of water by TiO2 was discovered [3]. The TiO2 photocatalysts are either anatase or rutile which has been well known. Herein we use TiO2 as a model material to develop in situ photocatalytic experimental methodology and explore structure changes of NiO/semiconductor photocatalysts. In-stiu heat treatment in H2 or O2 is applied to prepare initially Ni/TiO2, NiO/TiO2 or NiO-Ni-TiO2 materials in an environmental transmission electron microscope (ETEM). Then, without exposure to air, analysis can be performed in the same modified ETEM under in situ conditions in the presence of light and reactants to explore oxidation/reduction or interface changes under photocatalytic reactions. Insights from these experiments can help in the design of photocatalysts with better performance and stability.