X-ray photoelectron/X-ray excited auger electron spectroscopic study of highly dispersed semiconductor CdS and CdO species in zeolites

Abstract

The identification and study of the electronic properties and location of CdS_x(x < 1) and CdO particles in faujasites have been performed using X-ray photoelectron spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES) and fluorescence spectroscopy. These data were compared with results obtained by transmission electron microscopy (TEM), UV–VIS reflectance spectroscopy and adsorption measurements reported earlier for the same samples. Depending on the preparation procedure, CdS aggregates have different dispersions and can be located either inside the zeolite structure or on the external surface. Small CdS particles (1.5–2 nm) exhibit both XPS and fluorescence band shifts indicating a semiconductor size quantization (Q-size) effect. In addition, the non-stoichiometry of these species was also displayed in the fluorescence spectra. Larger CdS particles exhibit electronic and relaxation (polarization) properties rather close to those of bulk CdS. Strong surface enrichment with these particles was also observed. The CdO aggregates are formed during CdY treatment with NaOH and calcination. The identification of CdO is based on an abnormally low binding energy of Cd, which is characteristic for cadmium oxide only.