L Journal of Alloys and Compounds 300–301 (2000) 224–229 www.elsevier.com / locate / jallcom Rare earth doped sol–gel materials as potential absorbance standards * ´ Severine Aubonnet, Carole C. Perry Department of Chemistry and Physics, The Nottingham Trent University, Clifton Lane, Nottingham NG11 8NS, UK 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 29 temperature Si solution NMR measurements during the early stages of gel formation where Q and Q species disappeared from the 0 1 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 (8008C), the Ce(III) spectrum could no longer be observed.  2000 Published by Elsevier Science S.A. All rights reserved. 29 Keywords: Cerium(III); Neodymium(III); Sol–gel; Si NMR; UV–VIS spectroscopy 1. Introduction phosphors, scintillators, detectors, UV absorbers, emitters and activators due to their luminescence behaviour [5]. Glasses have been conventionally prepared by high Neodymium doped densified sol–gel materials are current- temperature methods [2] but the use of the sol–gel process ly being investigated for high power laser systems and as enables the preparation of porous or dense glasses with fiber optic amplifiers [6]. superior homogeneity, purity, and good optical qualities (high transmittance) at significantly lower temperature [3]. Sol–gel materials provide an excellent vehicle for the 2. Experimental incorporation of secondary phases including metal ions, organic molecules or macromolecules. These species may Solutions containing various dopants (Nd(NO ) ?6H O, 3 3 2 be doped into the gel-matrix as it is being formed (pre- CeCl ?7H O, Ce(NO ) ?6H O from Aldrich) were pre- 3 2 3 3 2 doping) [4] or incorporated after the glass has been pared in dry ethanol (distilled from 95% ethanol over prepared (post-doping). In this paper we present results magnesium) and distilled deionised water. obtained from gel–glass monoliths prepared using the The solutions were analysed by UV–VIS spectroscopy pre-doping method. The effect of the dopant on the using a Unicam UV2 UV–VIS spectrometer (scan speed 21 development of the sol–gel matrix was studied as well as 600 nm min , data interval 0.5 nm). the effect of the matrix on the dopant. Using the sol–gel process silica monoliths were pre- Our goal is to develop suitable materials for applications pared. Non-doped and doped silica gel glasses were made 31 31 as optical standards. Ce and Nd ions were used for the using tetraethylorthosilicate 98% (Aldrich) mixed with dry preparation of standards as they have useful spectroscopic ethanol (distilled from 95% ethanol over magnesium) and signatures in the UV and Visible regions, respectively. stirred gently for few seconds to obtain an homogeneous Cerium doped glasses also have potential application as solution. Then, if necessary, the dopant was added. Finally, 0.5 M HCl (prepared from 11.3 M HCl (Fisher)) was *Corresponding author. added at the following molar ratio: for normal sol–gels: 1 0925-8388 / 00 / $ – see front matter  2000 Published by Elsevier Science S.A. All rights reserved. PII: S0925-8388(99)00774-4