Visible light absorption of surface-modified Al2O3 powders: A comparative DFT and experimental study

Surface modification of Al2O3 powders, prepared using reproducible sol-gel synthetic route with small colorless organic molecules, induces charge transfer complex formation and the appearance of absorption in the visible spectral region. Comprehensive microstructural characterization involving transmission electron microscopy, X-ray diffraction analysis, and nitrogen adsorption desorption isotherms, revealed that gamma-crystalline alumina powders consist of mesoporous particles in the size range from 0.1 to 0.3 mu m, with specific surface area of 54.8 m(2)/g, and pore radius between 3 and 4 nm. The attachment of catecholate-type of ligands (catechol, caffeic acid, gallic acid, dopamine and 2,3-dihydroxy naphthalene), salicylate-type of ligands (salicylic acid and 5-amino salicylic acid), and ascorbic acid, to the surface such gamma-Al2O3 particles leads to the formation of colored powders and activates their absorption in visible-light spectral region. To the best of our knowledge, similar transformation of an insulator (Al2O3), with the band gap energy of 8.7 eV, into a semiconductor-like hybrid material with tunable optical properties has not been reported in the literature before. The density functional theory (DFT) calculations with periodic boundary conditions were performed in order to estimate the energy gaps of various inorganic/organic hybrids. The calculated values compare well with the experimental data. The good agreement between the calculated and experimentally determined band gaps was found, thus demonstrating predictive ability of the theory when proper model is used.