SURFACE-CHEMISTRY AND ELECTRONIC EFFECTS OF O-2, NO AND NO/O2 ON SNO2

SnO2 samples submitted to thermal treatments in vacuo and O2 show strong modification both in shape and position of the valence band-conduction band (VB-CB) absorption edge, its stoichiometry being seriously perturbed. Interaction with pure NO at room temperature (RT) originates a set of surface reactions: a reversible one related to a reversible release of electrons to the solid and others giving as products NO(delta-), NO2- and N2O, this last being related to the perturbation of defects responsible for the tailing observed in the fundamental transition band edge. At 773 K in pure NO an electron release mainly occurs, which increases the electron density in the CB, already populated by thermal excitation. Admission of NO/O2 mixtures gives rise at RT to NO(delta+) and NO3- species, without any detectable electron release. At 773 K the mixture is able to demonstrate a contrast to the pure thermal effect, decreasing the free electron density in the CB.