DFT study of the effect of platinum on the H2 gas sensing performance of ZnO nanotube: Explaining the experimental observations

An experimental report displays that the Pt-decoration meaningfully increases the ZnO nanostructure sensitivity toward H-2 gas. Here, DFT calculations are employed to explain the origin of Pt effect on the sensitivity of a ZnO nanotube (ZnONT) to H-2. An H-2 molecule weakly interacts with the ZnONT with adsorption energy of -2.1 kcal/mol) and cannot be detected. The Pt atom locates above the center of a hexagonal ring of ZnONT, releasing an energy of -39.7 kcal/mol. We showed that there is a relation between the experimental change of the sensor sensitivity and the change of the HOMO-LUMO gap of the ZnONT. Based on our results, the Pt metal first catalyzes the dissociation of O-2 gas of air on the ZnONT which causes a charge transfer from the tube to the O atoms, forming O- anions and decreasing the gap. Then the H-2 reacts with the pre-chemisorbed O ions and generates H2O molecules, releasing some of the trapped electrons back into the Pt-ZnONT and increasing the gap. This process needs to overcome an energy barrier of 18.9 kcal/mol, which can be accomplished at room temperature. After the H-2 adsorption and H2O production, the gap of Pt-ZnONT somewhat back toward its initial value because of an electron back-donation. The sensing mechanism is based on the gap change, which is related to the change of electrical conductivity. We predicted that by Pt-decoration, the sensor response increases from 1.12 to 11524 in agreement with experiment. The calculated recovery time is 13.5 ms for ZnONT sensor. (C) 2020 Elsevier B.V. All rights reserved.