The concentration of point defects in the surface layer of binary oxides

Point defects within the surface layer of oxides affect reactions on the surfaces hence heterogeneous catalysis on those oxides as well as surface electrical conduction. The oxides discussed here are binary ones and the point... [ view full abstract ]

Point defects within the surface layer of oxides affect reactions on the surfaces hence heterogeneous catalysis on those oxides as well as surface electrical conduction. The oxides discussed here are binary ones and the point defects are oxygen vacancies, electrons and holes. The dependence of the defect concentrations on oxygen partial pressure, P(O₂), and possible acceptor doping concentration, A_b, is evaluated for the first time. Further, with one calibration point the absolute defect concentration under any P(O₂) and A_b is determined. A relation between the dependence of the point defects in the surface layer and that of the corresponding defects deep in the neutral bulk on P(O₂) and A_b, is presented. The relation is controlled by two key parameters: whether the electronic states in the surface layer exhibit Fermi level pinning (FLP) and whether a dense chemisorbed layer exists. In the case there is no FLP and the chemisorbed layer is dilute the dependence of the defect concentrations in the surface layer on P(O₂) and A_b is the same as that of the defect concentration deep in the neutral bulk, the difference being only in a proportionality constant. This situation is quite common for oxides as in most no FLP prevails while at elevated temperatures chemisorbed layers are dilute. For acceptor doped oxides chemisorption is also low at low temperature. Different relations are derived under other conditions. In particular when FLP prevails, the oxygen vacancy concentration in the surface layer follows only one dependence, P(O₂)^-1/2A_b⁰, irrespective of the dependence of the bulk vacancy concentration on P(O₂) and A_b and irrespective of chemisorption.