Highly Sensitive NO₂ Gas Sensors Based on ZnO-Coated Black Silicon Nanostructures

We report highly sensitive NO₂ gas sensors based on ZnO thin films prepared via a sol-gel method and deposited onto nanostructured black silicon (b-Si). The b-Si layers, fabricated using maskless reactive ion etching, consist of densely packed silicon nanoneedles with an average height of ~810 nm, a base diameter of ~160 nm, and a characteristic periodicity of ~190 nm. Owing to this highly developed surface morphology, the effective surface area of the b-Si layer is estimated to be approximately one order of magnitude higher than that of planar silicon, thereby enhancing gas adsorption and charge-transfer processes in the ZnO film. ZnO/b-Si/Si sensors exhibit a response of 448% at 25 ppm NO₂ at an optimal operating temperature of 200 °C, which is approximately 1.5 times higher than that of planar ZnO/Si sensors at the same concentration and temperature. Notably, a comparable response (~300%) is achieved at a reduced temperature of 140 °C, indicating the potential for low-power operation. The sensing mechanism is governed primarily by the ZnO layer, while b-Si serves as a morphological scaffold, increasing the effective surface area. These results demonstrate that ZnO-coated b-Si nanostructures represent a promising platform for high-performance NO₂ sensing and offer strong potential for integration with silicon-based microelectronic technologies.