Distinguishing surface and bulk electron charge carriers for ZnO powders

A two-charge-carrier model that assumes coexisting high-mobility, low-concentration bulk, semiconduction electron charge carriers and low-mobility, variable-concentration surface-trapped-electron charge carriers is used to explain measured electrical and chemisorption properties of ZnO powders. This model resolves two serious quantitative issues not explained by the single-charge-carrier-type model used for our previously reported studies. Not explainable using a single-charge-carrier model are (1) wide variations in measured electron mobility values due to variations in surface treatments and (2) calculated electron number densities too low to match measured electron-transfer chemisorption results. Our two-charge-carrier model for ZnO powders assigns high-mobility bulk electrons to n-type ZnO semiconduction and low-mobility surface electrons to (V-o)(2-) and (V-o(t))(-) Surface oxide ion electron trapping vacancies. This model results in a high variation in surface electron number density due to surface treatments, while the mobilities for both the bulk and surface charge carriers remain constant. The model also calculates much higher surface electron number densities that better match charge-transfer chemisorption results. The two-charge-carrier model is expected to have significant importance in explaining chemisorption and catalysis on ZnO and other similar powder oxides. In particular, the two-charge-carrier model can yield two to three orders of magnitude higher calculated concentrations of surface electrons than for the single-charge-carrier model for any powder with coexisting high-mobility, semiconduction, bulk, charge carriers and variable concentrations of low-mobility, surface charge carriers. (C) 2000 Published by Elsevier Science B.V.