Processing and characterization of PbS nanocrystallites in TMOS silica xerogels

Nanocrystalline PbS-doped tetramethoxysilane (TMOS) silica xerogels are prepared by reacting lead nitrate with thiourea, followed by hydrolysis and polycondensation of TMOS in the presence of an ammonium hydroxide (NH4OH) catalyst. The effect of NH4OHTMOS molar ratio on PbS crystallite size is studied by keeping the PbSTMOS molar ratio constant at 0.006. The PbSTMOS molar ratio is varied from 0.0008 to 0.08 and its effect on the optical properties of the PbS-doped xerogels is studied. The color of the xerogels varied form yellow to black by varying the PbSTMOS molar ratio from 0.0008 to 0.08, respectively. The threshold absorption peak shifted from higher to lower wavelength with decreasing PbSTMOS molar ratio from 0.08 to 0.0008, due to a size quantization effect. X-ray diffraction (XRD) measurements show that the crystallite size of PbS varied from 2 to 15 nm by varying the PbSTMOS molar ratio from 0.0008 to 0.08, respectively. The effect of temperature on PbS crystallite size is also studied. It is found that with increasing temperature from 100 to 300 °C, crystallite size increased from 3 to 14 nm. The XRD measurements show that the PbS crystallites in TMOS xerogels have cubic rock salt structure.