Change in the Oxidation State of the Adsorbed Oxygen Equilibrated at 25°C on ZnO Surface during Room Temperature Annealing after Rapid Quenching

The porous ZnO quenched from 500°C in air showed very low electrical resistance with non-ohmic current–voltage behavior. This characteristic was resulted from the surface energy barrier formed by negatively charged chemisorbed oxygen. The resistance and the non-ohmic exponent of the quenched specimen gradually decreased by holding the specimen at room temperature. Electron spin resonance measurements showed a change in the oxidation state of chemisorbed oxygen from its high temperature type, O^2-, into low temperature types, O₂^- and O^- while being kept at room temperature. From this viewpoint, it was judged that the decrease in the resistance and the non-ohmic exponent resulted from the release of the trapped electron by this change in the oxidation state. The resistance and the non-ohmic exponent of the sample doped with CoO were higher than those of the undoped sample, and their decrease for the CoO-doped sample while being kept at room temperature was slower than those for the undoped sample. The rate of change in the oxidation state was also slower in the CoO doped sample.