Precise 3D structure determination of Cu single atoms on α-Al2O3(0001) surface by polarization-dependent total reflection fluorescence X-ray absorption fine structure and first-principles calculation

Abstract

The valence state and three-dimensional (3D) structure of vacuum-deposited Cu species on an Al-terminated (1×1) α-Al₂O₃(0001) surface have been studied using polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS) and density functional theory (DFT) calculations. The Cu species were atomically dispersed on the α-Al₂O₃(0001) surface in its low-coverage region (0.07 ML on Cu/O basis) with a Cu-O distance of 0.194±0.003 nm and present in the form of Cu(I). The adsorption site for the Cu single atoms was determined to be a three-fold hollow oxygen site corresponding to the imaginary Al position in the next Al layer over the α-Al₂O₃(0001) surface, accompanied with an upward relaxation for the nearest neighbor oxygen atoms due to expansion of the oxygen-oxygen bond distance. A similar local structure and surface relaxation mechanism have been reported in our previous PTRF-EXAFS study on Ni single atoms deposited on the α-Al₂O₃(0001) surface (Chem. Phys. Lett., 2004, 384, 134–138), indicating important implications for stabilizing single metal atoms on the Al₂O₃ surface.