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

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 were 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 by an upward shift of the nearest neighbor oxygen atoms and expansion of the oxygen–oxygen 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 (K. Ijima et al., Chem. Phys. Lett., 2004, 384, 134–138), indicating important implications for stabilizing single metal atoms on the Al₂O₃ surface.