XAS Study of Amorphous WO3 Formation from a Peroxo-Tungstate Solution
Résumé
Electrodeposition is a low cost method for WO3 thin film preparation. The starting dissolved tungsten precursor is a peroxo-compound which can be prepared by an equimolar mixing of sodium tungstate and hydrogen peroxide. The precursor preparation as well as the electrodeposition and precursor solution aging processes have been studied by X-ray absorption spectroscopy (XAS) at the W L3 edge. By extended X-ray absorption fine structure (EXAFS) analysis, the local structure of the first oxygen octahedral shell around the W absorber has been characterized both in the colloidal oxide formed after deposition solution aging and in electrodeposited films. The colloidal compound could be fitted with the same procedure as that successfully used with a monoclinic WO3 reference and involving two different W−O interatomic distances. The aging product is identified as amorphous WO3. If the spectra of as-electrodeposited films could not be fitted by the same procedure, the structural parameters of those cycled electrochemically in a lithium ion containing organic medium have been determined. If compared to crystallized WO3, the mean W−O distances in amorphous WO3 (films and colloids) are found significantly shorter, whereas the Debye−Waller factors are much larger. The condensation process has been investigated by analyzing the variation of the X-ray absorption near edge structure (XANES) spectra. We show that the white line height of the W L3 edge is a good indicator of the condensation state of the matter during the deposition. According to this parameter, the electrodeposited WO3 films would undergo a dramatic reorganization process during the first potential cyclings in the lithium ion containing organic medium. The same behavior is observed with electrochromic films deposited by sputtering, a more classical method.