Abstract:
The paper describes studies of the dissolution rate of NiO (a p-type semiconductor) in nitric acid solution at 60°C as a function of prior annealing temperature in air. The dissolution rate per unit surface area decreased markedly on increasing the annealing temperature from 500 to 1450°C. This effect is not due to gross structural change or to major changes in dislocation density. The higher dissolution rates (i.e., > 10–12 mol cm–2 s–1) of oxide annealed at temperatures below 700°C is due to an excess concentration over thermodynamic equilibrium of point defects (nickel vacancies), introduced during decomposition of the hydroxide, and maintained as a consequence of limited diffusion. For annealing temperatures above 900°C, the defect concentration is roughly equal in all samples because of rapid equilibration in polycrystalline samples during cooling. The decreasing rates (i.e., < 4 × 10–13 mol cm–2 s–1) may be due to (i) limited conduction of charge due to changes in the space-charge region of the semiconductor and/or (ii) reduction of the density of kink sites on the surface of the more perfect crystallites.
