Abstract:
The effect of decreasing dissolution rate of nickel oxide per unit surface area in acid solution with increasing prior annealing temperature (700–1450°C) has been shown to occur over a wide range of pH. The linear dependence of log (rate) on pH may be explained on a model of non-oxidative dissolution in which the pH variation changes the overpotential at the surface. Electron microscopy shows a different mode of attack at pH < 0. The presence in solution of a strong oxidizing ion such as cobaltic causes a very large increase in dissolution rate (> 200 fold) for all prior annealing temperatures but the oxide annealed at 1450°C is still the slowest to dissolve in the presence of cobaltic ions. This is believed to be because it has the lowest kink site density, the role of the Co3+ being hole injection into the p-type semiconductor at kink sites. The general conclusion of the work is that the supply of the majority carriers (the holes) may be rate limiting in the dissolution process.
