UV Effect on Indium Oxide Resistive Sensors

Metal oxide semiconductors have been the most intensely investigated materials in gas sensing area for the past several years due to their unique electric properties. Among them, indium oxide (In2O3) is an important n-type III-V semiconductor with a band gap of 3.6 eV, which has been widely used for the detection of both oxidizing (e.g. O3 and NOx) and reducing (e.g. CO, H2S and H2) gases [1]. As is well known, the response of gas sensors can be improved by increasing the surface area of the sensing element. Accordingly, gas sensors based on In2O3 nanowires (NWs) and nanotubes (NTs) have been extensively studied because of their improved properties in comparison with the traditional bulk materials, such as the lower operating temperature and better response in the detection of gas concentration at trace levels [2, 3]. Being a new, simple and flexible technique to synthesize NWs/NTs, electrospinning process has gained increased interest in gas sensing area. Furthermore, to overcome some problems during the functioning at room/low operating temperature such as the poor sensitivity and long recovery time, illuminating the sensor with UV radiation has been also recently recognized [4]. In this paper, electrospinning technique has been employed to synthesize PVP (polyvinyl pyrrolidone)-In2O3 composite fibers. The produced In2O3 fibers after annealing at 600°C were investigated as sensitive layer in resistive sensors for monitoring NO2 at low concentrations in air. In the specific, the step followed in this work are: (i) synthesis of In2O3/PVP composites; (ii) characterization of samples prepared by using TGA, XRD and SEM; (iii) investigation of their NO2 sensing properties; (iv) study of the effect of UV radiation on the response of sensors fabricated. By optimizing the spinning conditions and the subsequent annealing treatment, In2O3 fibers have been successfully obtained (see SEM micrographs in Fig. 1). The annealed samples have a high crystallization degree (Fig. 2). Their sensing behavior towards nitrogen dioxide has been then tested both in the dark and under UV light, utilizing a LED with a wavelengths of 350 nm. To analyze in detail the effect of UV radiation, sensing test have been performed by using three different procedures: without UV illumination, under continuous UV illumination and under pulsed UV illumination (provided only during the recovery time). It has been proved that the optimal characteristics (higher sensibility and very shorter recovery time) of the sensor occurs when the sample is irradiated with pulsed UV light (Fig. 3).