Zinc oxide point defects

One feature of oxides is drat, like all substances, they contain point defects which are most usually found on the cation lattice as interstitial ions, vacancies or ions with a higher charge than dre bulk of the cations, refened to as positive holes because their effect of oxygen partial pressure on dre electrical conductivity is dre opposite of that on free electron conductivity. The interstitial ions are usually considered to have a lower valency than the normal lattice ions, e.g. Zn+ interstitial ions in the zinc oxide ZnO structure. 

Salt solutions When a zinc sheet is immersed in a solution of a salt, such as potassium chloride or potassium sulphate, corrosion usually starts at a number of points on the surface of the metal, probably where there are defects or impurities present. From these it spreads downwards in streams, if the plate is vertical. Corrosion will start at a scratch or abrasion made on the surface but it is observed that it does not necessarily occur at all such places. In the case of potassium chloride (or sodium chloride) the corrosion spreads downwards and outwards to cover a parabolic area. Evans explains this in terms of the dissolution of the protective layer of zinc oxide by zinc chloride to form a basic zinc chloride which remains in solution. 

Zinc oxide is normally an w-type semiconductor with a narrow stoichiometry range. For many years it was believed that this electronic behavior was due to the presence of Zn (Zn+) interstitials, but it is now apparent that the defect structure of this simple oxide is more complicated. The main point defects that can be considered to exist are vacancies, V0 and VZn, interstitials, Oj and Zn, and antisite defects, 0Zn and Zno-Each of these can show various charge states and can occupy several different... 

At low concentrations, defect clusters can be arranged at random, mimicking point defects but on a larger scale. This seems to be the case in zinc oxide, ZnO, doped with phosphorus, P. The favored defects appear to be phosphorus substituted for Zn, P n, and vacancies on zinc sites, Vzn. These defects are not isolated but preferentially form clusters consisting of (Pzn + 2V n). 

The conductivity measurements show that equilibrium (1) sets in rapidly at temperatures as low as 500°C. Since the melting point of zinc oxide is about 2100°C. and accordingly its Tamman temperature about 900°C., the process under consideration cannot possibly involve the bulk of the crystal because defects could not diffuse rapidly enough through the lattice at such low temperatures. Except at very high temperatures, the defect equilibrium is realized only at the surface of the crystal, that is, in a layer of a few unit cells thickness. 

Bowker et al.[12] have concluded that adsorption occurs only on [0001] face of zinc oxide and hence represents the active face. Also surface defects are reported to be present on the two polar surfaces[13,14], Akhter et al.[7] have shown that alcohols interact most strongly with zinc polar surface wherein the surface dipoles are pointing outwards. Berlowitz and Kung[6] have further shown that the decomposition rates of propan-2-ol were 3-5 times higher on zinc polar surface than 0-polar surface. Watos et al.[15] have concluded that charge of a surface zinc ion decreases as [0001] > [1010] > [0001] plane, making the latter plane most metallic. 

In our discussion of point defects in nonstoichiometric ZnO, we noted that excess zinc is present as interstitials the oxide is metal rich. 

---