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 subppb 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.