Microstructural Evolution and Order-Disorder Transitions in Mesoporous Silica Films Studied by FTIR Spectroscopy
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Microstructural Evolution and Order-Disorder Transitions in Mesoporous Silica Films Studied by FTIR Spectroscopy Plinio Innocenzi, Paolo Falcaro Dipartimento di Ingegneria Meccanica, settore materiali, Università di Padova, via Marzolo 9, 35131 Padova, Italy David Grosso and Florence Babonneau Chimie de la Matière Condensée, Université Paris 6 - T54 -E5, 4 place Jussieu, 75252 Paris Cedex 05 - France Abstract Silica mesoporous thin films have been synthesised with a self-assembling process employing cetyltrimethylammonium bromide as the organic template and tetraethyl orthosilicate as the silica source. Mesoporous films with Pm3n cubic phase phases have been obtained and the films have been thermally treated in air with a progressive heating schedule from asdeposited up to 1000°C. The evolution of the microstructure has been studied with transmission Fourier transformed infrared (FTIR) spectroscopy. FTIR spectra of the as-deposited films have shown the presence of cyclic species, which at temperatures larger than 350°C have been no more observed. In the 1000-1300 cm-1 region several overlapped absorption bands have been detected. In particular, the pair LO3-TO3, the cyclic species absorption bands and the pair LO4-TO4 have been resolved. These last bands, in particular, are associated with disorder-order transitions in the silica microstructure. These disorder-induced optical modes are due to the large interface area and related to bond strains. The evolution of the bands in the 1000-1300 cm-1 region has been followed with the Berreman configuration, performing the transmission FTIR analysis at 45° with respect to the normal incidence angle. The LO3 band, which in silica sol-gel films is indicative of the network condensation and is activated by scattering of the light in the pores, was resolved as a single sharp band from 250°C. 1. Introduction Infrared (IR) spectroscopy has been widely applied in the last years to characterize the different stages of sol-gel process from alkoxide precursors to gels and glasses [1,2,3,4,5,6,7]. IR spectra have been correlated to microstructure and composition of different types of sol-gel materials, especially silica glasses, which have been the subject of extensive specific investigations. Detailed studies by IR spectroscopy have been reported on the different stages of sol-gel processing of silica glasses, in particular on the oligomeric species formed during the hydrolysis-polycondensation of TEOS [1], on evolution of TEOS to silica gels and glass [2], on silica xerogels from TEOS [3,4,5] and on structural evolution of silica gels in the early stages of conversion to glasses [6,7]. More recently new silica materials with a mesostructured organization have been synthesized via surfactant templating supramolecular chemistry. Mesoporous materials have been obtained as powders for a large variety of oxides, whilst thin oxide films have been more difficult to fabricate, because of the need for a strict control of the experimental parameters during the evaporation-induced self-assembl
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