Reduction and Diffusion of Cr-Oxide Layers into P25, BaLaTiO, and Al:SrTiO Particles upon High-Temperature Annealing.

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