Evolution of the oxidation states at the WO3 thin film surface during annealing in gases

The main objective of the present study was identification of the electronic states associated with W ions and defects (oxygen vacancies), which determine the surface stoichiometry of WO3 thin films and consequently their reactivity. The electronic surface structure of WO3 thin films was deduced from electron energy loss spectroscopy (EELS) analyses. We have investigated the changes of the electronic structure associated with thermal treatments under reducing and oxidizing atmospheres. We analyzed 200 spectra obtained on 50 samples and we present here the main tendencies of surface evolution. We identified three kinds of surface electronic structure, which correspond to decrease in oxidation level of the surface. They were attributed to clean, slightly reduced reconstructed and WO2 terminated surfaces, respectively.