Material dependent and temperature driven adsorption switching (p- to n- type) using CNT/ZnO composite-based chemiresistive methanol gas sensor

Abstract

• Correlations of two simultaneous as well as significant observations coming out from a single composite (CNT/ZnO) for the detection of VOCs. • A unique adsorption switching followed by p- to n- transition is observed in VOC detection. • The high reproducible sensors are selective towards methanol (R∼73 ± 3 %) and 8-fold enhancement in response compared to ethanol. • Composite sensor working looks like a full wave rectification process. The present study correlates two simultaneous as well as significant observations coming out from a single sensing prototype concerning the detection of volatile organic compounds (VOCs) by a carbonaceous material based sensor. We have developed a composite based chemiresistive sensor utilizing two different components (carbon nanotube (CNT) and zinc oxide (ZnO)). This is reflected in a unique adsorption switching phenomena followed by a ‘ p - to n -’ type transition characteristics above a certain operating temperature (150 °C) in the VOC detection process. Noticeably, by the virtue of adsorption switching, the CNT/ZnO composite is able to operate as a dual mode sensor, in which CNT dominates in low temperature region (≤ 150 °C) and ZnO at high temperature region (>150 °C) with a realistic detection ability. The highly reproducible sensors (29 prototypes) are selective towards methanol (Response, R ∼ 73 ± 3 %) and shows 8-fold enhancement in response value compared to neighbouring VOC i.e., ethanol at an operating temperature of 150 °C with a very low bias voltage of 10 mV. Finally, the adsorption switching phenomena (physisorption to chemisorption) has been explained by Fourier Transform Infrared Spectroscopy (FTIR) study and activation energy values along with ‘ p - to n -’ type transition is compared qualitatively with a typical full wave rectification process.