A density functional theory study on the formaldehyde detection mechanism by Pd-decorated ZnO nanotube

Experimental works have shown that decoration of noble metals on the metal oxides significantly increases their sensitivity toward different gases. In this work, density functional theory calculations were used to explore the effect of Pd-decoration on the sensitivity of a ZnO nanotube toward formaldehyde (HCHO) gas. We showed that the pristine ZnO nanotube physically adsorbs the HCHO gas with adsorption energy of -8.0 kcal/mol with no impact on the conductivity of ZnO. The response of pristine ZnO nanotube to HCHO gas was calculated to be 0.04, which it was increased to 102.55 by the Pd-decoration. The Pd atom catalyzes the following reaction: O2 + HCHO -> CO2 + H2O by facilitating the O-O bond dissociation and passing through an energy barrier of 5.1 kcal/mol at 298 K. This reaction is the origin of the resistivity change of Pd-ZnO nanotube because of the electron donation and back-donation processes between the reactants and the ZnO surface. Also, a short recovery time of 12 ms was predicted for Pd-ZnO nanotube-based sensor. The results indicate that the Pd-ZnO nanotube is a promising material for application in the HCHO gas sensors.