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 degrees 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. re 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, O2-, into low temperature types, O-2(-) 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.