Two-Dimensional CrCl3-Layered Trihalide Nanoflake Sensor for the Detection of Humidity, NO2, and H2

This study demonstrates that few-layer two-dimensional (2D) CrCl3 transition-metal trihalides (TMTHs; MX3, where M = Ti, V, Cr, Mo, Fe, Ru, and X = Cl, Br, or I) exhibit promising capabilities as chemoresistive sensors for humidity and NO2, H2, and NH3 gases, representing suitable 2D interfaces for gas-sensing applications. Liquid-phase-exfoliated 2D-CrCl3 flakes spin-coated over interdigital substrates exhibit higher chemical stability than CrI3 and VI3 with an excellent reproducible and long-term stable electrical response at an operating temperature (OT) of 100 °C in dry/wet air environments. Results show that with an increase/decrease in resistance, 2D-CrCl3 exhibits p-type responses to NO2 (400 ppb to 1 ppm) and H2O (10–80% relative humidity (RH) at 25 °C), H2 (10–250 ppm), and NH3 (10–250 ppm) oxidizing/reducing gases, respectively. The humidity cross-response of 2D-CrCl3 to NO2 reveals that an environment with 40% RH improves the relative responses and limits of detection of exfoliated CrCl3. The humidity- and gas-sensing response mechanism of 2D-CrCl3 sensor explains its reversal in resistance from an ionic (OT = 25 °C) to an electronic conduction regime (OT = 100 °C) under humid air conditions. These findings provide insights and possibilities for using 2D-CrCl3 TMTHs as humidity- and gas-sensing interfaces.