Investigation of Ligand-Stabilized Gold Clusters on Defect-Rich Titania

Chemically synthesized atomically precise gold clusters stabilized by triphenylphosphine ligands [Au₉(PPh₃)₈]­(NO₃)₃] were deposited onto the surface of titania fabricated via atomic layer deposition. The titania surface was pretreated by heating and sputtering. After deposition of the clusters onto pretreated titania, the samples were heated at 200 °C for 20 min under ultrahigh vacuum and subsequently investigated using metastable-induced electron spectroscopy to study the electronic structure of the outermost layer of the sample and X-ray photoelectron spectroscopy to determine the chemical composition of the surface of the sample. The former study revealed that two reference spectra are needed to explain the electronic structure of the sample. One reference spectrum is related to the titania substrate, while the second spectrum is related to the presence of the Au cluster cores and the ligands removed from the cluster cores. The latter study found that the Au 4f peak is shifted to lower binding energy and the P 2p peak to higher binding energy after heating. These are interpreted in the light of ligand removal and size evolution of Au particles upon heating of the clusters on titania. The important outcome of the present work is that defects introduced at the ALD titania surface via sputtering and heating strongly reduce the agglomeration of the Au clusters adsorbed to the surface

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