ELEMENTARY SURFACE-REACTIONS IN THE PREPARATION OF VANADIUM-OXIDE OVERLAYERS ON SILICA BY CHEMICAL VAPOR-DEPOSITION

Elementary processes of the formation of vanadium oxide overlayers by the adsorption-decomposition of VO(OC2H5)3 vapor on high-surface-area SiO2 were studied by IR (infrared) spectroscopy, TPD (temperature-programmed decomposition), and the stoichiometry of the surface reactions. In this study the adsorption-decompostion process will be called a CVD (chemical vapor deposition) cycle. The structure of the vanadium oxide prepared by repeated CVD cycles was characterized by XRD (X-ray diffraction) and XPS (X-ray photoelectron spectroscopy). After the introduction of VO(OC2H5)3 onto the SiO2 at 423 K, the IR peak due to the surface Si-OH groups disappeared, and the number of ethanol molecules evolved agreed with that of the surface Si-OH, indicating that all the Si-OH groups reacted with VO(OC2H5)3. On the basis of the stoichiometry of the gas-phase products and the vanadium atoms, it was confirmed that two surface species, Si-O-VO(OC2H5)2 (1) and (Si-O)2-VO(OC2H5) (2) were formed, the fraction of species 2 being 0.72-0.82 and 0.57-0.59 for SiO2 pretreated at 523 and 773 K, respectively. IR and TPD revealed that these species decomposed upon heat treatment to form vanadium oxide through the formation of V-OH and ethylene. The XPS peak ratios of V to Si as well as the XRD data showed that repeated CVD cycles gave highly dispersed vanadium oxides on SiO2 as compared with that prepared by an impregnation method, especially at high loading levels.