Preparation and characterization of nanocrystalline CdS doped monolithic and transparent TEOS silica xerogels using sol-gel method

Nanocrystalline cadmium sulfide (CdS) doped silica xerogels were prepared by hydrolysis and polycondensation of ethanol diluted tetraethoxysilane (TEOS) in the presence of HCl and NH4OH medium using cadmium acetate [Cd(CH3COO)2] and thiourea [SC(NH2)(2)] as the reactants in the silica sol followed by drying and slow heating of the resulting gel to 300 degrees C at atmospheric pressure. Monolithic and transparent CdS doped silica xerogels were obtained with HCl/TEOS and NH4OH/TEOS molar ratios of 0.1 and 0.0027 respectively. By varying the molar ratios of Cd/Si from 0.0005 to 0.08, the colour of the xerogels changed from light yellow to brown indicating a change in the size of the CdS nanocrystallites. The CdS doped xerogels were characterized using UV-visible spectroscopy and X-ray diffraction (XRD) techniques. In the optical absorption spectra, the absorption edge exhibited a blue shift compared with that of the bulk CdS crystal. The absorption threshold peaks were measured from the UV-visible spectra for different Cd/Si molar ratios. It was found that with increasing Cd/Si; molar ratio the absorption threshold peak shifted towards higher wavelengths. The crystallite sizes were calculated from XRD patterns and using tight binding calculation method. The XRD patterns revealed that the CdS nanocrystallites structure was hexagonal wurtzite. It was found that the crystallite sizes varied from 1 to 5 nm depending on the Cd/Si molar ratios (more the ratio, bigger the sizes). Increase of alcogel ageing resulted in the increase of CdS nanocrystallite sizes.