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

Abstract

The development of high-performance NOx 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 NOx 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 NOx 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 NOx sensing performance (response of 12.9 at 50 ppm, 200 degrees 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.