Electronic structure of chromia aerogels from soft x-ray absorption spectroscopy

The electronic structure of ultralow density nanoporous chromia monoliths (aerogels) is studied by soft x-ray absorption near-edge structure (XANES) spectroscopy. These aerogels are prepared by the epoxide sol-gel method. High-resolution O K edge and Cr L2,3 edge XANES spectra of aerogels (before and after thermal annealing at 550°C) are compared with spectra of microcrystalline α-Cr2O3 and orthorhombic CrO3 powders, α-Cr2O3 (0001) and (11¯02) single crystals, and an unrelaxed amorphous phase of full-density Cr2O3. Spectra are interpreted based on the results of crystal-field multiplet calculations (for Cr L2,3 edge) and ab initio spin-polarized density functional theory calculations with and without a Hubbard-type on-site Coulomb repulsion (for O K edge). Results show that the electronic structure of as-prepared aerogels, with a large fraction of undercoordinated surface atoms, is significantly different from that of bulk α-Cr2O3, CrO3, or amorphous Cr2O3. Thermal treatment transforms the as-prepared amorphous aerogel into α-Cr2O3 nanopowder.