Rare earth doped sol–gel materials as potential absorbance standards

Abstract

Our studies are aimed towards the preparation of absorbance materials for use in the UV and combined UV–VIS regions using sol–gel derived silica as the matrix. Our current research is directed towards understanding the extent of molecular interactions between the dopant phase (in this case metal salts) and the gel matrix and the effect of these interactions on the spectroscopic signature of all components in the gel–glass composite. The addition of cerium(III) to the sol–gel reagents affected the rate of drying of the gel–silicas suggesting a direct interaction between the gel matrix and the metal ions themselves. Confirmatory evidence was provided by low temperature 29Si solution NMR measurements during the early stages of gel formation where Q 0 and Q 1 species disappeared from the reaction medium more quickly in the presence of the dopant. The UV–VIS spectrum of dried sol–gel glasses containing Ce(III) showed that the as-prepared material contained principally the aqua-ion with nine-fold coordination. Evacuation of the cerium doped glass gave a spectrum commensurate with the metal ion having a lower coordination number and after heating the Ce(III) spectral response was no longer observed. It is suggested that oxidation of Ce(III) to Ce(IV) occurred during thermal treatment. When gel glasses were prepared with both cerium(III) and neodymium(III) the signal arising from the presence of the cerium ions was diminished. Moreover, after evacuation of the glasses and heat treatment (800°C), the Ce(III) spectrum could no longer be observed.