Impact of Defects on the Surface Chemistry of ZnO(0001̄)−O

Abstract

Scanning tunneling microscopy and core level photoelectron spectroscopy measurements have been used to investigate the morphology of ZnO(0001 macro)-O, and its reactivity with carbon monoxide and carbon dioxide, as a function of surface preparation. Real space images of the surface indicate that increasing the substrate anneal temperature during preparation significantly reduces the surface step density. Surface defect concentration is also monitored by employing formic acid as a chemical probe, which is shown to adsorb dissociatively (HCOOH --> [HCOO](-) + H(+)) only on zinc cations at step edges. Carbon 1s X-ray photoelectron spectra show that carbon monoxide and carbon dioxide both react to form surface carbonate species. Spectra, recorded both as a function of surface preparation and following coadsorption, demonstrate that the carbonate formed from either reactant molecule is located at oxygen vacancies at step edges, evidencing the significant role that defects can play in the surface chemistry of ZnO(0001 macro)-O.

Molecular scale investigations of the reactivity of magnetite with formic acid, pyridine, and carbon tetrachloride

“…This is consistent with results of an earlier XPS study of HCOOH adsorption on ZnO (0001) (Lindsay et al, 2002). In this case, the partial dissociation of HCOOH to HCOO À and H + was restricted to point defects on terraces and at step sites, both of which expose underlying Zn 2+ cations.…”

Oxidation behaviour of carbon monoxide at the photostimulated surface of ZnO nanowires

“…As discussed before, these vacancies are the active centers in the CO adsorption on ZnO and, thus, are also responsible for their sensing mechanism [26,27].…”

“…When CO and residual gases approach the surface, the CO or adsorbed oxygen molecules do not have enough thermal energy to recombine with oxygen vacancies at room temperature, which is reasonable due to the fact that even at high temperatures above 1000 K, the oxygen vacancies cannot be completely annihilated in an oxygen environment [27]. Thus, a possible mechanism is that the ionized CO molecules adsorb at the nanowire surface with the C-atoms weakly bonded to Zn ions at surface defects (oxygen vacancies) at room temperature [32], leading to a decrease of surface electron density.…”

“…According to photoelectron spectroscopy analysis [26,27], these defects are oxygen vacancies. This indicates that the defects observed in the former PL experiments are generated by the UV illumination.…”

Self-assembly of ZnO nanoparticles and subsequent formation of hollow microspheres

“…Due to its direct and wide band gap (3.37 eV) and large exciton binding energy (60 meV) [1], zinc oxide (ZnO) is a versatile semiconducting material with efficient excitonic emission at room temperature and unique acoustic [2], electronic [3], catalytic [4] and photocatalytic properties [5], which make it an appropriate material for various applications such as sensors [6], solar cells [7], varistors [1], electroluminescent [8] and optoelectric devices [9]. Among various morphologies which were reported previously for ZnO nanostructures (i.e.…”