Controlled growth of mixed-phase CuOx-ZnO (CZO) 1D nanorods for enhanced methanol detection at low operating...

Achieving high sensing performance at low operating temperatures remains a major challenge for metal oxide-based chemiresistive gas sensors, particularly those fabricated via physical vapour deposition (PVD) techniques. In this work, we demonstrate the controlled growth of mixed CuOx-ZnO (CZO) one-dimensional (1D) nanorods through post-oxidation of co-sputtered Cu-ZnO thin films. This approach enables the integration of complementary p-type and n-type semiconducting phases within a single nanostructured framework. Comprehensive structural and spectroscopic analyses confirm the coexistence of Cu2O, CuO, and ZnO phases. Notably, ZnO incorporation effectively tailors the oxidation kinetics of Cu by suppressing rapid CuO formation and stabilizing the intermediate Cu2O phase, resulting in diffusion-limited oxidation and controlled phase evolution. Consequently, dense and well-aligned mixed oxide nanorod networks with high surface activity are formed under optimized conditions. These Cu-rich CZO nanorod-based sensors exhibit a methanol sensing response of 57 % toward 50 ppm methanol at a significantly reduced operating temperature of 75 °C, along with a low detection limit of 200 ppb. This performance markedly surpasses that of pristine CuO sensors, which require higher operating temperatures (150 °C) and show comparatively lower response (~34 %). The enhanced sensing characteristics are attributed to the synergistic effects of high-aspect-ratio 1D morphology, increased defect density, and the formation of CuO-ZnO (CZO) p-n heterojunctions, which promote efficient charge separation, favourable band alignment, and enhanced modulation of the surface depletion layer during gas interaction. Additionally, the PVD-derived sensors demonstrate good reproducibility and long-term thermal stability, offering advantages over conventionally synthesized oxide materials. These results establish mixed-phase and heterojunction engineering via controlled oxidation as an effective strategy for developing high-performance, low-temperature VOC sensing platforms.