Thermal Stability of Structurally Controlled Lamellae and Hexagonal Mesoporous Silicate Thin Films
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decomposition of the surfactant in the composite film will inevitably affect the intensity of these two bands. In this communication, we report about a simple process to synthesize both lamellar and one-dimensional hexagonal (1-dH) silica-surfactant composite films on ordinary glass slide by spin coating process. Thermal decomposition of the surfactant from the composite films has been studied for the formation of hexagonal mesoporous thin films. EXPERIMENTAL Silicasols with different SiO 2 :C16 TMACI ratios were prepared in the following way. The required amount of TEOS was mixedwith 1-propanol. The TEOS was hydrolysed via the addition of previously mixed HC1 and water (1/3 of total volume). The solution was stirred for 60 min. The required amount of 2-butanol was added and stirred for another 30 min. The rest amount of the water (2/3 volume of total) was used for the dissolution of the surfactant. The surfactant-water mixture was then added slowly under stirring into the previously prepared sol. The sol was then kept under stirring for another 60 mins and finally used for the deposition of films on glass by spin coatings (2000 rpm). The silica concentration in the sol was maintained at 6 wt% equivalent without consideringthe weight of C 16 TMACl in the calculation. The volume ratio of TEOS: 1-propanol: 2-butanol: H 2 0: HCl was 1: 2.18 : 1.09 : 0.84 : 0.004. The mole ratio of SiO 2 :C 16 TMACl was maintained at 0.25 and 0.093. The coated slides were heated at different temperatures (e.g. 100 0 C, 150 0 C, 200 0 C, 250 0 C, 300 0 C, 3 50 0 C and 400 0 C) in air using 60 min soaking time for each step in a cumulative heating procedure. Both as-prepared samples and the samples after heat-treatment at each step were subjected to Low angle X-ray diffraction (LAXRD) studies at 40 kW and 40 mA ( RIGAKU). For the FTIR studies of the films, silicon wafers (100) were used as substrate instead of glass slides. ETIR spectra (400 - 4000 cm- 1 ) of the heat-treated films were recorded in N 2 atmosphere with 4 cm- 1 resolution in a Digilab spectrometer ( model: FTS 60). RESULTS AND DISCUSSION Figures 1 and 2 show LAXRD patterns of heat-treated thin films. As-deposited films showed similar low angle X-ray diffraction patterns. The XRD results for the high and low surfactant compositions show a very intense peak with d values at 35.73 A and 37.24 A, respectively. This intense peak is associated with a weak second order reflection (not shown in the Figs.) for both the compositions. The absence of any XRD peak for high surfactant containing films on heat-treatment at 400 0 C suggests the formation of a lamellar phase silica surfactant nanocomposites in the as-deposited thin films [3]. The removal of surfactant may have helped to collapse the lamellarstructure to an amorphous one. On the other hand, the XRD two peaks remained for low surfactant containing samples even after heating at 400C. Yang et al [2] and later on Lu et al [3] have found the similar observation and concluded the formation 236
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