ZnO Nanowires with Increasing Aspect Ratios for Room-Temperature NO₂ Gas Sensing

The oriented design and controllable preparation of sensing materials are extremely important and meaningful attributes that not only contribute to the realization of desirable gas-sensing properties but also facilitate the validation of the sensing mechanism. Because of the spatial effect derived from micelle assemblies formed by surfactant molecules under certain circumstances, radial growth of ZnO crystals is successfully suppressed in this work, which leads to the formation of ZnO nanowires with a larger aspect ratio. Furthermore, anionic surfactants of the same series but with longer carbon chains were utilized to create a greater spatial effect, which leads to more serious radial inhibition and, eventually, ever-increasing aspect ratios for the products. Except for a change in morphology, multiple characterization methods, such as XRD, FT-IR, and XPS, confirm the in situ doping/functionalization of surfactant molecules into ZnO crystals during the preparation process, which, on one hand, provide important evidence for the analysis of the product formation mechanism, and, on the other hand, serve as active sites for light-enhanced sensing. In subsequent gas-sensing tests, all surfactant–induced ZnO samples were applied as sensing materials to detect NO2 at room temperature (RT, 25 °C) in the dark, which show an increased response with increasing aspect ratio. In particular, at RT, the response toward 10 ppm NO2 of C16–ZnO with the highest aspect ratio reaches 271% in the dark, which is 24.64 times higher than that of pure ZnO without surfactant-assisted synthesis. Under UV irradiation, the response of C16–ZnO toward 10 ppm NO2 is further enhanced to 602% at RT, and this magnitude (2.22 times) of response improvement compared with that in the dark is maximal among all surfactant–induced ZnO samples. Moreover, the response/recovery time of C16–ZnO toward 10 ppm NO2 at RT is found to be drastically decreased from 393/953 s in the dark to 58/91 s under UV irradiation.