Dual-mode gas sensor for ultrasensitive and highly selective detection of xylene and toluene using Nb-doped NiO hollow spheres

Abstract

A single gas sensor with dual functionality for ultrasensitive and highly selective detection of p-xylene and toluene was designed using NiO hollow spheres doped with Nb. The pure and Nb-doped NiO hollow spheres were prepared by one-pot ultrasonic spray pyrolysis and subsequent heat treatment at 500 °C for 2 h. The Nb-doped NiO hollow spheres ([Nb]/[Ni] = 0.1) showed an ultrahigh response to 5 ppm of p-xylene (resistance ratio = 1752) and toluene (resistance ratio = 607), with negligible cross-responses to 5 ppm ethanol, benzene, carbon monoxide, and formaldehyde. In contrast, pure NiO hollow spheres showed negligibly low responses to 5 ppm of all analyte gases. In addition, the Nb-doped NiO hollow spheres exhibited dual sensing characteristics for selectively detecting p-xylene and toluene at 350 °C and 400 °C, respectively. The significant improvement of the response and selectivity for p-xylene and toluene can be explained by the high gas accessibility of hollow spheres, the Nb-doping-induced decrease in the charge carrier concentration, and the catalytic promotion of gas reforming reaction of less reactive xylene and toluene into more active species. The dual function of selectively detecting p-xylene and toluene in Nb-doped NiO hollow spheres is explained by the competition between oxidative filtering and gas reforming reaction depending on the operation temperature and sensing film thickness. The Nb-doped NiO hollow spheres can be used to design a single gas sensor with dual selectivity of xylene and toluene for reliable monitoring of the indoor air quality.