Formation of CdS Nanoparticles within Modified MCM-41 and SBA-15

Abstract

Cadmium sulfide nanoparticles were formed within both modified mesoporous aluminosilicate MCM-41 and siliceous SBA-15. Syntheses involved the formation of a monolayer of silylation reagent, (3-mercaptopropyl)trimethoxysilane, on the surfaces of both MCM-41 and SBA-15, which served to preposition the sulfide source within the mesoporous channels. A source of Cd ions was then added, leading to reaction with the prepositioned sulfide and with added H2S to result in occluded CdS nanoparticles. The average diameters of nanoparticles of CdS formed within the mesochannels of MCM-41 and SBA-15, as characterized by XRD and UV−vis diffuse reflectance, were ca. 2.1 and 3.4 nm, respectively. The thicker wall and larger pore size of SBA-15 compared to MCM-41, represents a more stable matrix for forming the occluded nanoparticle, as indicated by the stability of the XRD patterns for the nanocomposite utilizing SBA-15 versus that when MCM-41 was used. Also, from Raman spectra for the CdS/MCM-41 system, the relative intensities of the fundamental band and the overtone band were found to differ for the bulk and nanoparticle samples. We discussed this observation in terms of electronic anharmonicity. Furthermore, blue shifts for both fundamental and overtone Raman bands were observed when compared to bulk CdS: 303 vs 299 cm-1 and 605 vs 599 cm-1, respectively. We rationalized this observation in terms of CdS lattice compression and/or sulfide vacancies, particularly at the interface between the nanoparticle and the mesoporous cage matrix. We further suggest that the stoichiometry for the nanoparticle is more appropriately CdS1-x. Lastly, photoluminescence shifts were rationalized on the basis of nanoparticle size, incident excitation intensity, as well as temperature.