Surface chemistry and electronic effects of H 2(D 2) on pure SnO 2 and Cr-doped SnO 2

Abstract

The interaction of pure SnO 2 and Cr-doped SnO 2 with H 2(D 2) at r.t. has been studied by FT-IR spectroscopy, to provide information both on surface reactions and changes in the electronic properties. On pure SnO 2, H 2 (D 2) interaction ( p = 1.33 × 10 −3 Pa) causes the fast and complete loss of IR transmission, only partially restored by subsequent admission of O 2. No increase in OH vibrations appears; this is justified as follows: H 2 is dissociatively adsorbed in the interstitial symmetric sites between oxidic ions, both on the surface and in the SnO 2 sublayers, as proton. On Cr-doped SnO 2, pressures five orders of magnitude higher are necessary to see a sensible transmission loss. Simultaneously, vibrational peaks characteristic of CrH (CrD) and OH (OD) species appear, formed by H 2(D 2) dissociation on CrO pairs. The broad absorption responsible for the transmission loss is actually due to the superposition of two bands of different shape. One shows an asymmetrical shape, a sharp increase in the range 1000–2000 cm −1 (0.12–0.25 eV) and a slow drop in the high energy range. The shape and energy of the absorption edge are those expected for an electron transition from the second level of oxygen vacancies to the conduction band (CB). The other one shows a symmetrical shape, with a half width ∼3000 cm −1 and a maximum at 4750 cm −1 (≈0.6 eV). This band reveals the presence of donor levels at ∼0.6 eV from the bottom of the conduction band. The intensity ratio of the two bands is deeply affected by the presence of Cr, that preferentially destroys the first electronic transition, showing that its major effect is to decrease the number of oxygen vacancies.