PbS nanoparticles were synthesized in zeolite Na-X by means of Pb2+ exchange followed by sulfidation produced by reaction with thiourea in solution at several temperatures. Three samples were prepared at sulfidation temperatures of 65, 75 and 95ºC. The crystalline structure of system PbS-Na-X was studied by x-ray diffraction measurements. The average size of the nanoparticles and their crystalline structure were determined from transmission electron microscopy and electron diffraction patterns, respectively. The optical properties of the samples were studied by diffuse reflectance spectroscopy. The results show that spherical PbS nanoparticles are formed not inside the zeolite cages but outside, embedded in the zeolite matrix. The PbS nanoparticles have an average size of about 4 nm with a cubic crystalline structure. The absorption spectra of the samples display exciton absorption bands at much higher energy than the fundamental absorption edge of bulk PbS. This result is a consequence of strong quantum confinement effects produced by the reduced size of the PbS nanoparticles as compared to the exciton Bohr diameter of bulk PbS.
Keywords

Zeolites, Porous, Nanostructures, Semiconductors, Characterization.
Introduction

The study of semiconductor nanoparticles embedded in several types of matrix is currently a very active research area. Small particles, with size in the range of 1-20 nm, have physicochemical properties quite different from those observed in the bulk material [1]. A great variety of semiconductors nanoparticles have been synthesized in different matrix such as polymers, glasses and zeolites with the main purpose to modify their properties by controlling the particle size. Thus, the interest on these systems has been driven because the possibility to tailor the properties of a material for specific technological applications in the fields of non-linear optics, photovoltaic conversion, catalysis, optoelectronics, etc. Zeolites are nanoporous crystalline materials composed by Si, Al and O atoms, which constitute a framework structure of cages interconnected through channels with molecular dimensions. There are different techniques to incorporate compound semiconductors into the well defined and ordered zeolite cages. The cages and channels of the zeolite matrix, both of nanometric dimensions, are very appropriate sites to self-assemble and stabilize semiconductor clusters. Several semiconductor-zeolite systems have been studied and reported in literature [2-6].

The modifications of the properties of a semiconductor material are much stronger when its dimensions are comparable to the exciton Bohr diameter. PbS is a IV-VI compound semiconductor with narrow energy band gap, 0.41 eV at 300 K, high dielectric constant, 17.3 and small electron effective mass, <0.1 m* [7]. The values of the last two parameters yield to a large exciton Bohr diameter, about 18 nm [8-10], for this semiconductor material. Therefore, PbS is an adequate material to study particle size effects because it is not very difficult to synthesize PbS nanoparticles smaller than 18 nm. In fact, the strong confinement regime in which the particle size is much less than the exciton Bohr radius, has been achieved in systems with PbS nanoparticles having a size between 1-4 nm [11-15]. In this work we have synthesized PbS nanoparticles with an average size of 4 nm in zeolite Na-X, by means of ionic exchange processes in alkaline aqueous solutions. We report here the optical and structural properties of the system PbS-Na-X zeolite.