Ultrasensitive room temperature WO3/MXene heterostructures based ppb level NO2 sensor

Rising industrial revolution has led to the emission of various toxic gases such as ammonia (NH3), carbon dioxide (CO2), nitrogen dioxide (NO2) hydrogen sulfide (H2S), and various volatile organic compounds (VOCs) and have become a rising concern in context of environmental, and human safety. Among all hazardous gases, NO2 is most lethal, as even 10 ppb exposure can cause severe effect on immune system, lung inflammation, asthma, and other respiratory problems, as well as acid rain, and depletion of ozone layer [1, 2]. As a result high-performance NO2 selective sensors, are required to minuscule amounts within a sub-ppb concentration. Metal oxides, particularly tungsten trioxide (WO3), have received a lot of interest in the NO2 sensing field. Furthermore, the novel two-dimensional MXenes have garnered a lot of attention due to outstanding properties such as work function tunability and highly conductive nature to improve gas responsivity [3-5]. Based on these findings, the synergistic interaction of MXene with WO3 would be advantageous for generating strong NO2 sensing response at room temperature under sub-ppb concentrations. In the present work, we have synthesized WO3/Ti3C2 heterostructures using facile hydrothermal method (Fig. 1 A) and characterized through field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD) studies to investigate their structural and morphological properties. FESEM indicates the uniform distribution of WO3 nanoparticles over and in between the MXene sheets as well as XRD depicts the successful formation of their heterostructures. Further, WO3/MXene heterostructures have been brush painted over polyamide sheet having gold electrodes for the fabrication of sensing device. The fabricated sensor has been electrically tested at room temperature towards ethanol, methanol, acetone, ammonia, isopropanol, and NO2. It has been observed that WO3/MXene sensor showed high selectivity towards NO2 and high response of 27% at 60 ppb, attributable to the presence of large number of active sites provided by the scattered WO3 nanoparticles over MXene sheets. Also, it exhibited fast response and recovery time as compared to pristine WO3 based sensor owing to the highly conductive path provided by Ti3C2 resulting in efficient charge transfer processes. Moreover, WO3/MXene can detect NO2 gas down to ppb level at room temperature with high repeatability and stability. Fig. 1 B shows dynamic response–recovery curves as a function of time for WO3/MXene at different concentrations of NO2 gas (15–60 ppb). It demonstrates an increase in the resistance in NO2 atmosphere which returns to its baseline resistance on exposure to air, revealing the high reversibility of WO3/MXene sensor. The sensing parameters like response, response time (tres) and recovery time (trec) of WO3/MXene at different concentrations of NO2 are summarised in table 1. The WO3/MXene sensors have the potential to be used in industrial or wearable applications due to the ease of fabrication procedure, low-cost, flexible substrates, and high sensing capability. Fig.1. A. Experimental setup B. Resistance–time curves for WO3/MXene sensor obtained on exposure to different NO2 concentrations.   Table 1. Sensing parameters of WO3/MXene with varied NO2 concentrations at room temperature. NO2 concentration (ppb) Response (%) tres (s) trec (s) 15 7.3 211.6 24.1 30 13 183.8 22.0 60 27 162.0 19.8