All-Transparent NO₂ Gas Sensors Based on Freestanding Al-Doped ZnO Nanofibers

Transparent optoelectronics can enable a new class of applications such as transparent displays, smart windows, and invisible sensors. Here, we demonstrate all-transparent NO2 gas sensors based on aluminum-doped zinc oxide (AZO) freestanding hollow nanofibers. Freestanding AZO nanofibers are fabricated by sputtering AZO on template polyvinylpyrrolidone (PVP) nanofibers, which are electrospun on a glass frame with indium zinc oxide (IZO) transparent electrodes, followed by a heat treatment to remove the PVP template nanofibers. Not only the gas-sensing active material but also other components such as the substrate and electrodes are all transparent in the visible region. The optical transparency of the nanofibers is controlled by changing the AZO nanofibers density without compromising the sensitivity. The gas-sensing measurements of the transparent sensor depict n-type response behavior with full recovery even at low NO2 concentrations (0.5 ppm). The high sensitivity of the transparent sensors is attributed to the higher surface area of the hollow nanofibers and the high impact frequency of trapped NO2 gas inside the hollow compared to solid counterpart nanofibers. The unique combination of transparency and high sensitivity can potentially have applications in advanced sensor systems that can be attached to windows integrated with the Internet of Things.