Chlorine gas reaction with ZnO wurtzoid nanocrystals as a function of temperature: a DFT study

In the present work, we applied density functional theory and temperature-dependent Gibbs free energy calculations to wurtzoid structures to explain the sensitivity of ZnO nanocrystals towards chlorine molecules. In agreement with experimental findings, our results revealed that chlorine sensing under ambient conditions is feasible. Higher temperatures increased the sensitivity of ZnO nanocrystals towards chlorine gas molecules. Peak calculated sensitivities were in the temperature ranges (167-220 °C), (447-578 °C) and (952-1159 °C), which is in good agreement with experimentally determined temperatures. According to the calculated Gibbs free energy, these three ranges correspond to the van der Waals attachment of Cl₂ molecules on Zn-polar sites, van der Waals attachment of Cl₂ molecules on O sites, and dissociation of Cl₂ molecules on ZnO nanocrystal surfaces, respectively. The removal of chlorine atoms from the surface of ZnO nanocrystals is difficult at low temperatures because of the high electron affinity of chlorine gas atoms, which results in a long recovery time and accumulation of chlorine atoms and molecules on the ZnO surface. Atomic charges and charge transfer are depicted using natural bond orbital analysis to explain the present mechanisms.