Reduction and Diffusion of Cr-Oxide Layers into P25, BaLa₄Ti₄O₁₅, and Al:SrTiO₃ Particles upon High-Temperature Annealing

Chromium oxide (Cr₂O₃) is a beneficial metal oxide used to prevent the backward reaction in photocatalytic water splitting. The present work investigates the stability, oxidation state, and the bulk and surface electronic structure of Cr-oxide photodeposited onto P25, BaLa₄Ti₄O₁₅, and Al:SrTiO₃ particles as a function of the annealing process. The oxidation state of the Cr-oxide layer as deposited is found to be Cr₂O₃ on the surface of P25 and Al:SrTiO₃ particles and Cr(OH)₃ on BaLa₄Ti₄O₁₅. After annealing at 600 °C, for P25 (a mixture of rutile and anatase TiO₂), the Cr₂O₃ layer diffuses into the anatase phase but remains at the surface of the rutile phase. For BaLa₄Ti₄O₁₅, Cr(OH)₃ converts to Cr₂O₃ upon annealing and diffuses slightly into the particles. However, for Al:SrTiO₃, the Cr₂O₃ remains stable at the surface of the particles. The diffusion here is due to the strong metal-support interaction effect. In addition, some of the Cr₂O₃ on the P25, BaLa₄Ti₄O₁₅, and Al:SrTiO₃ particles is reduced to metallic Cr after annealing. The effect of Cr₂O₃ formation and diffusion into the bulk on the surface and bulk band gaps is investigated with electronic spectroscopy, electron diffraction, DRS, and high-resolution imaging. The implications of the stability and diffusion of Cr₂O₃ for photocatalytic water splitting are discussed.