Highly Sensitive and Fast Optoelectronic Room-Temperature NO₂ Gas Sensor Based on ZnO Nanorod-Assembled Macro-/Mesoporous Film

UV illumination has been demonstrated as an effective approach to realize high-performance gas sensors operating at room temperature. Here, a macro-/mesoporous film structure design was presented for optoelectronic NO2 gas sensors exhibiting high response, fast sensing, full reversibility, and a sub-ppb detection limit. We developed a rapid (≤120 s), microwave-assisted, precursor self-sacrificing templated synthesis of macro-/mesoporous ZnO films composed of hierarchically networked nanorods. Compared to dense nanorod film, the device based on macro-/mesoporous ZnO films exhibited significantly improved optoelectronic and gas sensing performances at the ppb level of NO2 under UV irradiation. The optimal sensor responses to 2.5–1000 ppb NO2 range from 6.6% to 2900% at room temperature, and the response and recovery time was 19 and 32 s, respectively (to 400 ppb NO2). Furthermore, the sensor showed full reversibility, a sub-ppb detection limit (0.2 ppb), and high selectivity to NO2. The excellent optoelectronic property and UV-activated NO2 sensing performance were due to the hierarchically macro-/mesoporous structure which facilitated the efficient penetration and scattering of UV light within the film, as well as the enhanced diffusion and adsorption of gas molecules. These findings demonstrate a high-performance sensing film structure as well as a low-cost fabrication technology, which could also be employed to other semiconductors for optoelectronic devices.