Structure and properties of lead and lead sulfide nanoparticles in natural zeolite

We have synthesized lead and lead sulfide nanoparticles embedded in a natural zeolite (clinoptilolite) matrix by a simple hydrothermal process. The process steps involve the partial removing of the natural cations in clinoptilolite, the ion-exchange process to enclose Pb ions and nanoparticles and finally a sulfuration process at different temperatures to obtain lead sulfide phases in the zeolite matrix. The samples were studied by X-ray diffraction, diffuse reflectance spectroscopy, energy dispersive X-ray spectroscopy, X-ray photon spectroscopy and transmission electron microscopy. The experimental results show the inclusion of three Pb species with different valence states after the Pb ion-exchange step, namely Pb²⁺, Pb⁴⁺, and Pb⁰. At the end of the process, two simultaneous lead sulfide crystalline phases, PbS (Galena) and PbS₂ (tetragonal) were synthesized in the clinoptilolite matrix. The optical absorption spectra of the samples show the exciton absorption peaks typical of colloidal PbS nanoparticles. The average size of the PbS nanoparticles was about 10 nm and their crystalline structure was determined from diffraction electron patterns. The high-pressure phase PbS₂ was also identified and its formation was attributed to the influence of the special conditions of clinoptilolite matrix as crystallization media to induce some selective nucleation process of this crystalline phase.