High-performance NO2-gas sensing of ultrasmall ZnFe2O4 nanoparticles based on surface charge transfer

Abstract

Monitoring the concentration of NO₂ could effectively protect the health of human beings and the environment from damage from NO₂ emissions and leakage. However, the high sensing performances (e.g., high response, fast response and recovery time, excellent selectivity, and low operating temperature) of the current NO₂-gas sensors still remain a considerable challenge. Herein, to address the above-stated issues, pure ultrasmall ZnFe₂O₄ nanoparticles, which have exhibited outstanding selectivity to NO₂ molecules, were successfully synthesized via a typical one-step hydrothermal synthetic procedure. We found that our ZnFe₂O₄-based sensor exhibited an ultrahigh response (R_gas/R_air = 247.7) and fast response time (T_res. = 6.5 s) and recovery time (T_rec. = 11 s) toward 10 ppm NO₂ at a low operating temperature of 125 °C, which are superior to the vast majority of NO₂ semiconducting sensors, and more importantly, a sensor based on spinel structure materials presenting such excellent sensing performances for NO₂ has barely been reported before. Ex situ photoluminescence characterization was applied to reveal the gas sensing mechanism based on charge transfer. Furthermore, to deeply investigate the experimental phenomena, the adsorption energy and charge transfer between gas molecules and the ZnFe₂O₄-based sensor were explored by density functional theory calculation and Bader charge analysis in this work. As a result, the theory calculations demonstrated that NO₂ molecules have a larger negative adsorption energy (−1.32 eV) and that 0.35 electrons transfer from Zn and Fe atoms of the ZnFe₂O₄ to NO₂ molecules during the process of NO₂ adsorption. Also, according to the results of our theoretical calculations, the existence of an oxygen vacancy can also enhance the adsorption energy and charge transfer between the surface of ZnFe₂O₄ and NO₂ molecules. Significantly, our results not only provide the results from fundamental studies elaborating the gas sensing mechanism based on charge transfer but may pave the way for the commercial application of spinel structure materials in gas-sensing fields.