A sonochemical method for the selective synthesis of a-HgS and b-HgS nanoparticles

A novel sonochemical method for the selective synthesis of a-HgS (cinnabar) and b-HgS (metacinnabar) nanoparticles in aqueous solutions is reported in this paper. a-HgS and b-HgS nanoparticles have been selectively prepared by choosing sodium thiosulfate and thiourea as the sulfur source respectively. To study the crystalline structure, size, morphology and composition of the products, characterization techniques including X-ray powder diffraction, transmission electron microscopy, selected area electron diffraction, X-ray photoelectron spectroscopy and energy-dispersive X-ray analysis are employed. The optical properties of the nanoparticles are investigated by UV–visible absorption spectroscopic measurements. The direct band gap of the as-prepared a-HgS nanoparticles with an average size of 12 nm is calculated to be 2.8 eV according to the absorption spectrum. In the case of the b-HgS nanoparticles with an average size of 13 nm, a broad absorption peak is observed in theUV–visible absorption spectrum, which can be ascribed to the special surface state of this sample. Probable mechanisms for the sonochemical formation of a-HgS and b-HgS nanoparticles in aqueous solutions are presented. The optimum pH value of the stock solutions and the effect of sonication time on the particle size are also investigated. ! 2003 Elsevier B.V. All rights reserved.