CdS-silica xerogel nanocomposites: Processing-induced textural changes
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CdS-silica xerogel composites were prepared from tetramethoxysilane, acidic water, and formamide mixtures homogenized by high power ultrasounds. Different concentrations of Cd(NO 3 ) 2 were added. CdS semiconductor nanoparticles were precipitated by H 2 S gas diffusion through the sonogel porous structure. Composite mechanical properties were enhanced by an impregnation process by the gel soaking in the same sol as-prepared. Textural parameters evolution is compared to the salt content in order to find the best performance to CdS nanocrystal growth. The impregnation process allows a better composite protection.
I. INTRODUCTION From the first work of Jain and Lind,1 semiconductor particles in dielectric matrix composites have attracted much interest for optical communications and optical signal processing because of their large optical nonlinearity and fast response time.2'3 It is well established that when the diameter of the semiconductor crystallite is of the same magnitude order of the exciton Bohr diameter (typically 2 - 8 nm for II-VI semiconductors) the semiconductor nanocrystal electronic properties do not behave in the same manner as in bulk material.4 This size effect transforms the continuous valence and conduction bands of the bulk crystal into a series of discrete states since electron and hole wave functions are confined. Efros and Efros5 showed that the effective band gap between the top of the valence subband and the bottom of the conduction subband is a function of microcrystallite size. It has been suggested that such a three-dimensional confinement of electron-hole pair (related with the volume normalized oscillator strength) combined with a narrow microcrystallite size distribution (wavelength selectivity) are responsible for the increase of the third-order nonlinear susceptibility.6 The so-called quantum size effect can be observed as a blue shift in the optical band gap or exciton energy.7 It has been shown how the sol-gel route allows one to produce high porosity silica monolithic pieces by a soft and wet chemical procedure8 by using ultrasound as a reaction promoter and formamide as an additive for gel-drying control. Recently,9 we have reported the processing of CdS-silica xerogel composites, showing the aforementioned blue shift which abides by the Efros and Efros model.5 In this processing a cadmium salt is added to a silica sol and after gelation, CdS nanoparticles precipitate when H 2 S gas is diffused through the porous network. On the basis of cation effect in the gelation J. Mater. Res., Vol. 9, No. 11, Nov 1994 http://journals.cambridge.org
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process, a relation between estimated sample porosity, from nitrogen adsorption, and particle size was found. In a different way, once the CdS microcrystallites are grown, a certain amount of pores remain, weakening the mechanical durability of the preforms. To overcome this problem, the composites were reinforced by impregnation with a silica sol10 with the aim of filling the pores by gelation of the flowing sol inside
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