Highly sensitive work function type room temperature gas sensor based on Ti doped hBN monolayer for sensing CO2, CO, H2S, HF and NO. A DFT study

Abstract

The adsorptions of toxic gas molecules (CO₂, CO, H₂S, HF and NO) on pristine and Ti atom doped hexagonal boron nitride (hBN) monolayer are investigated by density functional theory. Weak physisorption of gas molecules on pristine hBN results in micro seconds recovery time, limiting the gas sensing ability of pristine hBN. However Ti atom doping significantly enhances the adsorption ability. Ti atom best fits to be doped at B vacancy in hBN with lowest formation energy (−3.241 eV). Structural analysis reveals that structures of gas molecules change after being chemisorbed to Ti doped hBN monolayer. Partial density of states analysis illustrates strong hybridization among Ti-3d, gas-2p and BN-2p orbitals, moreover Bader charge transfer indicates that gas molecules act as charge acceptors. Ti doped hBN monolayer undergoes transition from semiconductor to narrow band semiconductor with adsorption of CO₂, H₂S and NO, while with CO and HF adsorption it transforms into metal. The change of conductance of Ti doped hBN monolayer in response to adsorption of gas molecules reveals its high sensitivity, however it is not selective to HF and NO gases. The recovery times of gas molecules desorption from monolayer are too long at ambient condition however it can significantly be shortened by annealing at elevated temperature with UV exposure. Since recovery time for NO removal from monolayer is still very long at 500 K with UV exposure, Ti doped hBN monolayer is more suitable as a scavenger of NO gas rather than as a gas sensor. It is thus predicted that Ti doped hBN monolayer can be a work-function type CO₂, CO, H₂S and HF sensor and NO gas scavenger.