Facet-Dependent Gas Adsorption Selectivity on ZnO: A DFT Study

Abstract

Semiconductor-based gas sensors are of great interest in both industrial and research settings, but poor selectivity has hindered their further development. Current efforts including doping, surface modifications and facet controlling have been proved effective. However, the “methods-selectivity” correlation is ambiguous because of uncontrollable defects and surface states during the experiments. Here, as a case study, using a DFT method, we studied the adsorption features of commonly tested gases—CH2O, H2, C2H5OH, CH3COCH3, and NH3—on facets of ZnO(0001¯), ZnO(101¯0) and ZnO(101¯1). The adsorption energies and charge transfers were calculated, and adsorption selectivity was analyzed. The results show ZnO(0001¯) has obvious CH2O adsorption selectivity; ZnO(101¯0) has a slight selectivity to C2H5OH and NH3; and ZnO(101¯1) has a slight selectivity to H2, which agrees with the experimental results. The mechanism of the selective adsorption features was studied in terms of polarity, geometric matching and electronic structure matching. The results show the adsorption selectivity is attributed to a joint effort of electronic structure matching and geometric matching: the former allows for specific gas/slab interactions, the latter decides the strength of the interactions. As the sensing mechanism is probably dominated by gas–lattice interactions, this work is envisioned to be helpful in designing new sensing material with high selectivity.