Adsorption of gas molecules of CH4, CO and H2O on the vanadium dioxide monolayer: Computational method and model.

Inspired by the recent use of two-dimensional nanomaterials as gas sensors, we used density functional theory calculations to study the adsorption of gas molecules (CH$_4$, CO and H$_2$O) on sandwich vanadium dioxide tablets. The results showed that of all these gases, only the CH$_4$ gas molecule was the electron acceptor with significant charge transfer on the VO$_2$ sheet. The adsorption energies of CH$_4$, CO and H$_2$O are -229.5 meV, -239.1 meV and -388.3 meV, respectively. We have also compared the adsorption energy of three different gas molecules on the VO$_2$ surface, our calculation results show that when the three kinds of gases are adsorbed on the VO$_2$ surface, the order of the surface adsorption energy is H$_2$O$>$ CO$>$ CH$_4$. It is also found that after adsorption of CH$_4$, CO and H$_2$O molecules, the electronic properties of VO$_2$ sheet changed obviously. However, due to the strong adsorption of H$_2$O molecule on VO$_2$ sheet, it is difficult to desorption, which hinders its application in gas molecular sensors. The optical properties of VO$_2$ sheet are further calculated. The absorption of CH$_4$, CO and H$_2$O molecules is introduced to red-shift the dielectric function of the thin film, which indicates that the optical properties of the thin film have changed significantly. According to the change of optical properties of VO$_2$ sheet before and after molecular adsorption, VO$_2$ can be used as a highly selective optical gas sensor for CH$_4$, CO and H$_2$O detection. These results provide a new approach for the potential application of VO$_2$ based optical gas sensors.