Charge transport mechanism in HfO2-modified ZnO composite for NOx gas sensing applications

Abstract

The development of high-performance NO_x gas sensors and multifunctional dielectric materials requires a clear understanding of defect-mediated charge transport in oxide ceramics. Although ZnO-based composites have been widely investigated, a systematic correlation between grain boundary dominated electrical transport and NO_x gas sensing performance has not been widely explored. In this work, ZnO–HfO₂ (HZO) composites with varying HfO₂ content (0–10%) were synthesized via sol–gel route to tailor their structural, optical, dielectric, electrical, and NO_x gas sensing characteristics. Structural and microstructural analyses confirmed the coexistence of ZnO and HfO₂ phases, accompanied by lattice strain, grain refinement, and modified grain boundary density upon HfO₂ compositing. Optical studies revealed modulation of the band gap due to defect-induced electronic states. Dielectric and impedance spectroscopy demonstrated thermally activated, non-Debye relaxation behavior dominated by grain and grain boundary effects, with minimum grain boundary resistance in ZnO–4 wt% HfO₂ (HZO 4) composite. The optimized composite exhibited superior NO_x sensing performance (response of 12.9 at 50 ppm, 200 °C), attributed to enhanced oxygen vacancy concentration and efficient charge carrier modulation. This study establishes a direct structure–transport–sensing mechanism and provides a defect-engineering strategy for ZnO-based gas sensors.