Room Temperature UV-Activated NO2 and NO Detection by ZnO/rGO Composites

Abstract

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.

Au- or Ag-Decorated ZnO-Rod/rGO Nanocomposite with Enhanced Room-Temperature NO2-Sensing Performance

“…When rGO is exposed to air at a moderate temperature, oxygen molecules chemisorbed on the surface form oxygen species (O 2 − , O − ) (Equations (3) and (4)), which simultaneously induce holes (h + ) and positive charge carriers, and remove some electrons from the valence band, converting graphene into a p-type metal with a Fermi level below the K-point. As a result of being exposed to NO 2 , molecules of NO 2 are chemisorbed into NO 2 − ions (Equation (5)), which further remove electrons from the valence band and increase the concentration of holes, causing the resistance to drop [ 40 ]. O 2 (g) + 2e − → 2O − (ads) O 2 (g) + e − → O 2 − (ads) NO 2 (g) + e − → NO 2 − (ads) NO 2 (ads) + O − (ads) → NO 3 − (ads) NO 2 (ads) + O 2 − (ads) → NO 2 − (ads) + O 2 (g) …”

Preparation of ZnO Thick Films Activated with UV-LED for Efficient H2S Gas Sensing

“…It exhibits transparency in the visible spectrum and high reflectivity in the infrared region [10]. These properties allow ZnO to be employed effectively in various electronic and optoelectronic applications such as solar cells, lightemitting diodes, piezoelectric devices, gas sensors (thermal-activated) [11] and gas sensors (UV-activated) [12], among others.…”

“…UV-LEDs are able to excite the surface of a chemiresistive sensor with low energy consumption, about 50 to 500 mW for optimum operating conditions. For this reason, the use of UV-LEDs is attractive for the development of highly sensitive gas sensors with low energy consumption [12].…”