2D Molybdenum Carbide MXenes for Enhanced Selective Detection of Humidity in Air.

2D transition metal carbides and nitrides (MXenes) open up novel opportunities in gas sensing with high sensitivity at room temperature. Herein, 2D Mo CT flakes with high aspect ratio are successfully synthesized. The chemiresistive effect in a sub-µm MXene multilayer for different organic vapors and humidity at 10 -10  ppm in dry air is studied. Reasonably, the low-noise resistance signal allows the detection of H O down to 10 ppm. Moreover, humidity suppresses the response of Mo CT to organic analytes due to the blocking of adsorption active sites. By measuring the impedance of MXene layers as a function of ac frequency in the 10 -10  Hz range, it is shown that operation principle of the sensor is dominated by resistance change rather than capacitance variations. The sensor transfer function allows to conclude that the Mo CT chemiresistance is mainly originating from electron transport through interflake potential barriers with heights up to 0.2 eV. Density functional theory calculations, elucidating the Mo C surface interaction with organic analytes and H O, explain the experimental data as an energy shift of the density of states under the analyte's adsorption which induces increasing electrical resistance.