Materials Chemistry and Physics 77 (2002) 826–830 Spectroscopic characterization of chromium in sol–gel derived silica Vinoy Thomas, Gin Jose, Gijo Jose, P.I. Paulose, N.V. Unnikrishnan ∗ School of Pure and Applied Physics, Mahatma Gandhi University, Priyadarsini Hills P.O., Kottayam 686 560, Kerala, India Received 11 September 2001; received in revised form 18 March 2002; accepted 26 March 2002 Abstract 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 Cr 3+ ions and no Cr 6+ (∼345 nm) ion was observed. Electron spin resonance studies showed that concentration of Cr 5+ 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. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Sol–gel; Optical materials; FTIR spectroscopy 1. Introduction Sol–gel methods are extensively used for preparation of optical materials by incorporating different cations or molecules in a silica host [1,2]. First row transition metal ions [3,4] and rare-earth ions [5,6] have been utilised for this purpose. The chromium ion is likely to play a central role in the development of new solid state laser materials because of its favourable absorption and emission properties [7]. In recent years study of the optical properties of crystals and glasses containing trivalent chromium has received a new impulse connected both to the exploitation of these materials as active media for infrared tunable laser, luminescent solar concentrators and to the fundamental understanding of the interaction between impurity and host lattice [8–10]. Doped glasses obtained via thermal densification of the correspond- ing gels show similar properties with that of the melt route glasses [11]. Thermal treatment of the doped gels changes the local environment of the metal ions due to various rea- sons resulting in fluorescence spectra. Though this behavior is common for most of the modifier ions doped gels studied, a detailed study of the incorporation of Cr 3+ ions in sol–gel silica host is yet to be well established. Since the optical properties of the doped silica systems depend on the local structure and bonding of the dopant cations [11], a detailed understanding of the structural evolution during the gel to ∗ Corresponding author. Tel.: +91-481-597923; fax: +91-481-597731. E-mail address: spapf@sancharnet.in (N.V. Unnikrishnan). glass conversion of Cr 3+ doped silica gels is very important to fabricate the material for optical applications. This short communication focuses on the spectroscopic properties of the gel derived monolithic Cr 3+ doped silica glasses. 2. Experimental Silica sols containing 0.1, 0.2 and 0.5 equivalent mol% Cr 2 O 3 were prepared from tetraethylorthosilicate (TEOS) (Fluka purum grade), Cr(NO 3 ) 3 ·9H 2 O (Merck, India), doubly distilled deionised water and high purity HNO 3 and NH 4 OH. The desired amount of Cr(NO 3 ) 3 ·9H 2 O dis- solved in deionised water in the presence of HNO 3 was poured in TEOS under stirring at room temperature. The TEOS/H 2 O/HNO 3 molar ratio was 1:14:0.01. The pH of the sols was adjusted to a value of 3 by adding NH 4 OH. The sols were cast in petridishes. The gels were aged for 1 week at room temperature. Transparent crack-free dried mono- lithic gels were obtained after drying for 14 days in air oven at 60 ± 2 ◦ C. The gels were heated at different temperatures ranging from 200 to 900 ◦ C in a programmable furnace with the rate of 3 ◦ C/h. (The gel with 0.5 mol% became opaque on heat treatment and was not taken for further studies). Trans- parent crack free and bubble free gels (diameter 30–35 mm, thickness 2–2.5 mm) were reproducibly obtained. Optical spectra of the gels and heat treated samples were recorded with UV–VIS (Shimadzu-UVPC2401) and spectrophotoflu- orimeter (Shimadzu-RFPC 5301). Thermogravimetric curve 0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0254-0584(02)00154-2