PHOTOLUMINESCENCE FROM ZINC-OXIDE POWDER TO PROBE ADSORPTION AND REACTION OF O2, CO, H-2, HCOOH, AND CH3OH

Two emissions were observed at room temperature from zinc oxide powder excited with photons of higher energy than its band gap (3.2 eV). The sharp UV emission at 383 nm corresponded to band gap emission, and a broad visible (green) emission at ca. 500 nm corresponded to sub-band gap emission. Adsorption of O2, H-2, CO, methanol, and carboxylic acids (formic acid and acetic acid) changed the intensities of both emissions. At room temperature, O2, H-2, and CO exposure decreased the emission intensities; after evacuation at 10(-6) Torr the effect of O2 adsorption was only partially reversible, while those of H-2 and CO were virtually completely reversible. In contrast, adsorption of methanol and formic acid (and acetic acid) caused an irreversible increase of both emissions. Surface formate species were formed by dissociative adsorption of formic acid as well as by oxidation of methanol, as shown by temperature-programmed desorption and FT-IR. The decomposition of formate intermediates further increased the intensity of both emissions. Moreover, the increase of the visible emission intensity was more pronounced in the case of the decomposition of formates produced by methanol oxidation than of those resulting from formic acid dissociation. The increase of the visible emission intensity upon formate decomposition may be related to the creation of oxygen vacancies in the course of this reaction. These vacancies act as acceptor centers for deexcited electrons from the conduction band. This study demonstrates that photoluminescence can be used to probe adsorption, both reversible and irreversible, and reaction phenomena on the surface of zinc oxide.