Theoretical Study on the Al-Doped ZnO Nanoclusters for CO Chemical Sensors

Experimental work has already demonstrated that Al-doped ZnO nanostructures exhibit a higher response than the pure ZnO sample to CO gas and can detect it at sub-ppm concentrations. In this work, using density functional theory calculations (at B3LYP, M06-L, and B97D levels), we studied the effect of Al-doping on the sensing properties of a ZnO nanocluster. We investigated several doping and adsorption possibilities. This study explains the electrical behavior that has been obtained from the ZnO nanostructures upon the CO adsorption. There is a relationship between the HOMO–LUMO energy gap (Eg) and the resistivity of the ZnO nanostructure. If a Zn atom of the ZnO nanocluster is replaced by an Al atom, a CO molecule can be adsorbed from its C-head on the doped site with ΔG of −5.0 kcal/mol at room temperature. In contrast to the pristine cluster, Al-doped ZnO cluster can detect CO molecules due to a significant decrease in the Eg and thereby in the resistivity. We also found that the Eg decreases by increasing the number of Al atom up to 4, and then it starts to increase by increasing the Al atoms with its trend analogous to the resistivity change in the experimental work.