Growth-Temperature Dependent Unpassivated Oxygen Bonds Determine the Gas Sensing Abilities of Chemical Vapor Deposition-Grown CuO Thin Films

CuO is a multifunctional metal oxide excellent for chemiresistive gas sensors. In this work, we report CuO-based NO₂ sensors fabricated via chemical vapor deposition (CVD). CVD allows great control on composition, stoichiometry, impurity, roughness, and grain size of films. This endows sensors with high selectivity, responsivity, sensitivity, and repeatability, low hysteresis, and quick recovery. All these are achieved without the need of expensive and unscalable nanostructures, or heterojunctions, with a technologically mature CVD. Films deposited at very low temperatures (≤350 °C) are sensitive but slow due to traps and small grains. Films deposited at high temperatures (≥550 °C) are not hysteretic but suffer from low sensitivity and slow response due to lack of surface states. Films deposited at optimum temperatures (350–450 °C) combine the best aspects of both regimes to yield NO₂ sensors with a response of 300 % at 5 ppm, sensitivity limit of 300 ppb, hysteresis of <20%, repeatable performance, and recovery time of ∼1 min. The work demonstrates that CVD might be a more effective way to deposit oxide films for gas sensors.