Chemical and electronic characterization of pure SnO2 and Cr-doped SnO2 pellets through their different response to NO.

The interaction of NO at room temperature (RT) with pure and Cr-doped SnO, self-supporting pellets, pretreated in dry oxygen at 673-873 K, has been studied by FT-IR spectroscopy. With both pure and doped pellets treated at 673 K, NO at pressures ~10 N m2 causes a reaction (NO = NO+ +e-) that is responsible for the increase of a broad electronic absorption, which is irreversible to evacuation at RT, but no peaks due to surface species appear. For pressures > 10 N m2, peaks due to nitrites, nitrosyls and N2O appear, without further increase of the electronic absorption, which actually becomes completely reversible at RT, while surface nitrites and nitrosyls are irreversible under evacuation. The formation of these species is responsible for the shift of the equilibrium involving electrons. The electronic absorption is due to the overlap of two bands: one at 0.1-0.18 eV, assigned to the electron transition from monoionized oxygen vacancies to the conduction band; the other, at 0.45-0.6 eV, assigned to a similar electronic transition involving oxygen divacancies. Thermal treatment at 873 K affects the electronic response of both the peilets. On Cr-doped pellets the electronic response is completely killed. SnO, pellets show the behaviour described for pretreatment at 673 K, except for the shape of the electronic absorption: the relativeintensities of the two overlapping bands at 0.1-0.18 and 0.45-0.6 eV bands change in favour of the latter.