p-/n-type conduction in activated carbon NO2 sensors with enhanced low-concentration sensitivity at room temperature

Abstract

Gas sensors play a vital role in monitoring air quality across a wide range of applications, including industry, transport and healthcare. The present study investigates the nitrogen dioxide (NO₂) sensing capability of four commercial activated carbons (ACs), including two coal-based (MSC 30 and CW 30) and two bio-based (A supra and PK1-3) ACs at room temperature (25 °C). The ACs exhibited distinct textural properties, with specific surface areas ranging from 916 to 2233 m²g^-1. Key sensing parameters including responses, R and R_ci (%), response time, recovery time, sensitivity, linearity, repeatability, reversibility and stability were examined under controlled NO₂ exposures. The fabricated sensors were exposed to continuous cycles of varying NO₂ concentrations, from 1 to 10 ppm, in 1 ppm increments, and from 1 to 20 ppm in 5 ppm increments. The sensors exhibited p- or n-type conduction behavior, depending on the AC, confirmed by Mott-Schottky measurements. Reversible sensing was governed by weak physical interactions (physisorption) of NO₂ gas on the sensor surface and charge transport via charge hopping. These findings offer valuable guidance for selecting appropriate materials in the development of high-performance, room temperature AC-based NO₂ sensors, which are an essential component of effective environmental monitoring.