Nanocomposites, including nanoparticles of semiconductor metal oxide (MO) and reduced graphene oxide (rGO), are of exceptional interest for light-activated gas sensors functioning without thermal heating. In this paper, we discuss the sensor properties of electrospun ZnO nanofibers and ZnO/rGO composites. The materials were characterized by transmission and scanning electron microscopy (TEM, SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and IR spectroscopy (FTIR). The sensor characteristics were studied when detecting reducing gases CO, H2, and NH3 and oxidizing gases NO and NO2 at 25–150 °C in dark conditions and under UV illumination. ZnO nanofibers and ZnO/rGO composites have no sensitivity when detecting CO, NH3, and H2 either in dark conditions or under UV illumination. At T = 25 °C, UV illumination is a necessary condition for the appearance of a sensor response when detecting both NO and NO2. The increased sensitivity of ZnO/rGO composites when detecting nitrogen oxides at T = 25 °C is discussed in terms of the heterojunction formation. Observed at T = 150 °C, opposite trends in the UV illumination influence on the sensor response toward NO and NO2 are due to the participation of chemisorbed oxygen in the processes responsible for the sensor response formation when detecting NO.

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