Treatment-dependent surface chemistry and gas sensing behavior of the thinnest member of titanium carbide MXenes

Abstract

MXenes, a rapidly developing family of two-dimensional materials possessing tunable electronic properties and abundant surface functional groups, are promising gas-sensing materials. Ti₂CT_x, with a thinner unit cell thickness compared to its compositional analogue Ti₃C₂T_x and thus more profound surface-dependent properties, has been less explored over the past years. Herein, by etching the precursor Ti₂AlC with a concentrated HF or LiF/HCl mixture, semiconducting Ti₂CT_x (HF) nanosheets and metallic Ti₂CT_x (LiF/HCl) nanosheets were obtained, respectively, arising from their treatment-dependent surface functionalization. In addition, the resulting metallic nanosheets were partially oxidized into TiO₂/Ti₂CT_x (LiF/HCl) hybrid, which exhibited superior sensitivity toward NH₃ gas as compared with Ti₂CT_x (HF) and Ti₂CT_x (LiF/HCl). Detailed analysis suggests that a high concentration of surface oxygen containing species, such as -O_x, -(OH)_x and Ti-O-Ti, is generally beneficial for NH₃ sensing, and a relatively higher -O_x concentration allows rapid gas desorption and sensor recovery.