Spectroscopic characterization of chromium in sol–gel derived silica

Trivalent chromium was introduced into silica gel monoliths, prepared via sols obtained from tetraethylorthosilicate and metal salts at constant pH (pH=3). Gels were heat-treated up to 900 °C. The optical properties of the silica glass and its in-process gel, including the ultraviolet–visible (UV–VIS) absorption spectra, Fourier transform infrared (FTIR) spectra, fluorescence emission spectra were measured and analyzed. The UV visible spectra of the gels dried at 60 °C indicated only the presence of Cr3+ ions and no Cr6+ (∼345 nm) ion was observed. Electron spin resonance studies showed that concentration of Cr5+ increased with increasing heat treatment temperatures. The FTIR spectra showed that high temperature heat treatment can greatly reduce the amount of OH groups and organic residues in the silica. It was also observed that a high OH content contributes to the fluorescence inefficiency of the gels as well as porous glass.