Impact of Different Ratios of Fluorine, Oxygen, and Hydroxyl Surface Terminations on Ti₃C₂T͓x MXene as Ammonia Sensor: A First-Principles Study

Abstract

A first-principles study was successfully employed to investigate the impact of different ratios of functional groups such as fluorine (-F), oxygen (-O), and hydroxyl (-OH) on ammonia (NH3) sensing of titanium carbide Mxene. Density functional theory (DFT) calculations were performed for studying the adsorption energy (Eads) and charge transfer (CT) between different gases (NH3, CO 2 , NO, H2S and SO 2 ) and TbC2T x material with a high ratio of fluorine surface functional groups, TbC2(OH)o.44Fo.ssO0.66. DFT calculations showed more sensitivity to NH3, with the highest CT (0.098 e) and the lowest Eads (-0.36 eV) among the mentioned gases. The adsorption of NH3 on TbC2T x MXene with a high and low ratios of fluorine surface functional groups, TbC2(OH)o.44Fo.ssOO.66 (Substrate 1) and TbC2(OH)o.66Fo.2201.11 (Substrate 2) respectively, resulted in adsorption energies of -0.36 eV and -0.49 eV, revealing a stronger adsorption of NH3 on Substrate 2 with low ratios of fluorine. In addition, the isosurfaces representation of charge difference illustrated that fluorine atoms have smaller charge transfer than oxygen atoms when interacting with NH3 molecules. The Bader charge difference for the closest oxygen and fluorine atoms to NH3 molecule showed that oxygen atoms have 60% to 180% larger Bader charge difference, when compared to fluorine atoms, supporting that TbC2T x sensor with a lower ratio of fluorine surface termination has a stronger interaction with NH3 gas molecules.