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

Chromium oxide (Cr 2 O 3 ) 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, BaLa 4 Ti 4 O 15, and Al:SrTiO 3 particles as a function of the annealing process. The oxidation state of the Cr-oxide layer as deposited is found to be Cr 2 O 3 on the surface of P25 and Al:SrTiO 3 particles and Cr(OH) 3 on BaLa 4 Ti 4 O 15 . After annealing at 600 °C, for P25 (a mixture of rutile and anatase TiO 2 ), the Cr 2 O 3 layer diffuses into the anatase phase but remains at the surface of the rutile phase. For BaLa 4 Ti 4 O 15, Cr(OH) 3 converts to Cr 2 O 3 upon annealing and diffuses slightly into the particles. However, for Al:SrTiO 3, the Cr 2 O 3 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 Cr 2 O 3 on the P25, BaLa 4 Ti 4 O 15, and Al:SrTiO 3 particles is reduced to metallic Cr after annealing. The effect of Cr 2 O 3 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 Cr 2 O 3 for photocatalytic water splitting are discussed.