Enhanced High-Temperature (600 °C) NO₂ Response of ZnFe₂O₄ Nanoparticle-Based Exhaust Gas Sensors

Fabrication of gas sensors to monitor toxic exhaust gases at high working temperatures is a challenging task due to the low sensitivity and narrow long-term stability of the devices under harsh conditions. Herein, the fabrication of a chemiresistor-type gas sensor is reported for the detection of NO₂ gas at 600 °C. The sensing element consists of ZnFe₂O₄ nanoparticles prepared via a high-energy ball milling and annealed at different temperatures (600-1000 °C). The effects of annealing temperature on the crystal structure, morphology, and gas sensing properties of ZnFe₂O₄ nanoparticles are studied. A mixed spinel structure of ZnFe₂O₄ nanoparticles with a lattice parameter of 8.445 Å is revealed by X-ray diffraction analysis. The crystallite size and X-ray density of ZnFe₂O₄ nanoparticles increase with the annealing temperature, whereas the lattice parameter and volume are considerably reduced indicating lattice distortion and defects such as oxygen vacancies. ZnFe₂O₄ nanoparticles annealed at 1000 °C exhibit the highest sensitivity (0.13% ppm^-1), sharp response (τ = 195 s), recovery (τ = 17 s), and linear response to 100-400 ppm NO₂ gas. The annealing temperature and oxygen vacancies play a major role in determining the sensitivity of devices. The plausible sensing mechanism is discussed. ZnFe₂O₄ nanoparticles show great potential for high-temperature exhaust gas sensing applications.