REACTIONS OF OXYGEN WITH ZNO-1010-SURFACES

Investigations are reported on the reaction of oxygen with electrostatic neutral ZnO-1010-surfaces studied by means of AES, LEED, EPR, thermal desorption spectroscopy, and isotopic exchange as well as changes in the surface conductivity and work function. Geometric and electronic structures of the surface are discussed. Reversible physisorption data are determined in the range of 120-250 K ( theta less than 10** minus **1 for P less than 10** minus **4 Pa), from which adsorption isotherms and isosteric heats of adsorption are derived. Chemisorption is found in the range of 300-650 K ( theta less than 2. 5 multiplied by 10** minus **4) on crystals with negligible amount of point defects at the surface. The energy of chemisorption and reaction rates can be described quantitatively by electron transfer models. Desorption rates for zinc and oxygen are derived from measurements of instationary crystal sublimation, the latter being strongly dependent on thermodynamically stable concentrations of oxygen vacancies at the surface, which can be determined quantitatively and removed during reactions with oxygen at lower temperatures (T less than equivalent to 700 K). Under these conditions a strong influence of oxygen vacancies on sticking coefficients and rates of electron transfer during oxygen exposure is found. The separation of chemisorption processes and reactions with point defects during oxygen interaction enables a deeper understanding of elementary steps during catalysis on ZnO.