p–p Heterojunction Sensors of p‑Cu₃Mo₂O₉ Micro/Nanorods Vertically Grown on p‑CuO Layers for Room-Temperature Ultrasensitive and Fast Recoverable Detection of NO₂

High sensitivity, low limit of detection (LOD), and short response and recovery times at room temperature (RT) are critical for gas sensors. For NO₂, different binary metal oxide-based sensors were developed to achieve superior performance at elevated temperatures instead of RT. Herein, we report on CuO@CuO and Cu₃Mo₂O₉@CuO sensors with CuO and Cu₃Mo₂O₉ micro/nanorods vertically aligned on the CuO layers, which were directly fabricated using a facile, low-cost, and catalyst-free chemical vapor deposition (CVD) technique. Their sensing performance tests revealed that the Cu₃Mo₂O₉@CuO p–p heterojunction sensors exhibited a high response of 160% to 5 ppm NO₂, an excellent sensitivity of 50% ppm^–1, a low LOD of 2.30 ppb, a short response time of 49 s, and a rapid recovery of 241 s at RT, obviously better than those for CuO@CuO sensors. The superior performance of Cu₃Mo₂O₉@CuO sensors could be attributed to the Schottky heterojunction formed between p-Cu₃Mo₂O₉ micro/nanorods and p-CuO films, the catalytic effect, and the anisotropic nature of Cu₃Mo₂O₉ micro/nanorods. This study not only provides a simple, low-cost, and batchable fabrication method of homo/heterojunction sensors with micro/nanorods vertically aligned on films but also opens an avenue for sensor design by tuning the Schottky barrier height to enhance RT performance.