Ultraviolet–visible–near-infrared and EXAFS study of CoII coordination chemistry in post-doped silica sol–gel glasses

Abstract

UV–VIS–NIR and EXAFS spectroscopies have been used to probe the coordination chemistry of cobalt(II) in post-doped silica sol–gel glasses. Materials doped with cobalt(II) chloride, nitrate and acetate were prepared and the cobalt(II) coordination chemistry was studied in relation to post-doping treatments such as dehydration, rehydration and densification. Both octahedral and tetrahedral coordination geometries were observed for the hydrated and dehydrated gel glasses with pronounced anion coordination present in all dehydrated samples. The presence of salts in the gel-glass pore structure provided additional absorbing centres for water. The extent of associated water adsorption (hydrogen bonded to silanol groups and hydrogen bonded to other water molecules) in the pores varied in the order nitrate > chloride > acetate. When the sample was sintered cobalt was incorporated in the glass framework and anions and water were lost.