Coverage-dependent adsorption thermodynamics of oxygen on ZnO(0001)

The equilibrium behavior of oxygen adsorbed on metal oxide surfaces such as ZnO(0001) often affects their behavior in applications such as gas sensing. The present work attempts to bridge this distinct gap to applications from an alternate perspective by employing the optical technique of photoreflectance (PR) to measure the enthalpy ΔHads and entropy ΔSads of oxygen adsorption as a function of coverage on Zn-terminated ZnO(0001). The large and strongly coverage-dependent parameters, stemming from a multiplicity of sites, chemical species and reconstructions, lead to a nonmonotonic coverage variation in both ΔHads and ΔSads. The entropic contribution to the free energy is comparable to the enthalpic contribution even near room temperature, so that temperature effects on oxygen adsorption may be uncommonly large.