A single response to reducing gases by NiO-TiO2 heterojunction nanocrystals

Abstract

Utilizing p-n composite material to fabricate metal-oxide semiconductors (MOSs) gas sensors is a promising strategy to achieve ultrahigh selectivity detection of reducing gases, such as H2 and CO. This work utilized a facile doping method to synthesize p-NiO/n-xTiO2 nanoparticles with p-n heterojunction. The p-NiO/n-1.0TiO2 nanoparticles annealed at 550 °C exhibited ultrahigh selectivity with an n-type gas-sensitive response to H2 at 300 °C, eliminating the cross-sensitivity phenomenon. This gas selectivity to H2 and CO highly depends on the Ni:Ti molar ratios and operating temperature. Under repeated ventilation conditions, the response values of p-NiO/n-1.0TiO2 sensor to H2 and CO remained unchanged, confirming its excellent stability. Multiple gas experiments show that the p-NiO/n-1.0TiO2 sensor can be utilized to selectively detect any of H2, CH4, NO2, and SO2 in the presence of CO. Furthermore, the study of H2 and CO adsorption behavior on the NiO-TiO2 revealed that H2 is more easily adsorbed on the NiO-TiO2 heterojunction surface than CO. Introduction of H2 caused electron transfer between the gas molecule and surface atoms of the heterojunction, while the electronic structure of the system remained unchanged after CO adsorption. This study provides an effective method to improve the selectivity of MOS-based gas sensors.