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

MXenes, a rapidly developing family of two-dimensional materials possessing tunable electronic properties and abundant surface functional groups, are promising gas-sensing materials. Ti2CTx, with a thinner unit cell thickness compared to its compositional analogue Ti3C2Tx 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 Ti2CTx (HF) nanosheets and metallic Ti2CTx (LiF/HCl) nanosheets were obtained, respectively, arising from their treatment-dependent surface functionalization. In addition, the resulting metallic nanosheets were partially oxidized into TiO2/Ti2CTx (LiF/HCl) hybrid, which exhibited superior sensitivity toward NH3 gas as compared with Ti2CTx (HF) and Ti2CTx (LiF/HCl). Detailed analysis suggests that a high concentration of surface oxygen containing species, such as -Ox, -(OH)x and Ti-O-Ti, is generally beneficial for NH3 sensing, and a relatively higher -Ox concentration allows rapid gas desorption and sensor recovery.