Gas Sensing Properties Comparison between SnO2 and Highly Antimony-Doped SnO2 Materials

Abstract

Introduction Gas sensors are widely applied to monitor combustible and harmful gases which can be detrimental to safety, health and environment. SnO2, a wide gap semiconductor, is the most promising gas sensor material because of high chemical stability and low fabrication cost[1].While due to the drawbacks of SnO2 working as gas sensing material, such as low selectivity and high working temperature, doping by noble or transition metals is a particularly efficient way to improve its sensing properties[2]. SnO2 doped by antimony (SnO2/Sb) is a typical n-type oxide material, which has high electrical conductivity and thermal stability. Materials and Method The impacts of Sb doping (10 wt.% doping content) on morphology, nanostructure of commercial SnO2 were investigated by scanning electron microscopy (SEM) and x-ray diffraction (XRD) technique. The samples were added into a composed solution mixed with glycol ether as wetting agent and an acrylic resin, and then screen printed onto alumina substrates, where Au was applied as interdigitated electrodes on front-side, and the back-side was equipped with a heater to provide with working temperature. In the last, the screen-printed films were treated at 180 °C in a muffle oven for 12 h in air to obtain the thermal stabilization[3]. The response values (R) were obtained by the equation (1), hence Ra is the resistance while exposed to the dry air, and Rg is the resistance while exposed to target gas, which were measured by home-made gas sensing system. The samples under different temperatures were named as SnO2-x and SnO2/Sb-x (x=300°C, 350°C, 400°C and 450°C). The gas sensing properties dependence of samples on different temperatures under acetone gas condition was clarified, and the relationship between doping and selectivity of samples under different target gases at 350°C were demonstrated. The results about high antimony doping content at different working temperatures and target gas situations were investigated in this work. Results and Conclusions The SEM reveled that antimony doping SnO2 contributed to harshen the surface of SnO2, which is beneficial to increase the specific surface area and the adsorption efficiency to target gases, as shown in Fig. 1.[4]. XRD characterization shows that no other phases such as SnO and Sn3O4 were detected, which means the high purity of the SnO2[5]. Meanwhile, Sb doping did not change the tetragonal structure of SnO2, but it distinctly affected the preferred (110), (101) and (211) orientation growth, which can be detected in Fig.2. Furthermore, the gas sensing properties between pure SnO2 and SnO2/Sb were compared at different working temperatures and under different target gases environment. The measurement results in Fig.3 (a) and (b) demonstrated that the response of SnO2 and SnO2/Sb reached highest value at 350°C. While SnO2 and SnO2/Sb showed abnormal trend at 450°C that is conducive to oxygen atoms insertion into the crystal structure[6]. Specifically, SnO2/Sb showed p-type shape response at 450°C, maybe it is because antimony doping decreased the grain size of SnO2 so that the whole grains were fully depleted. In the beginning of reaction with target gas, the concentration of O2 on the surface of SnO2/Sb decreased and the conductance increased. Whilst, when the depletion zone was smaller than the grain size, the conductance decreased a lot. When stopping injection of target gases, the trend processed reversely [6]. Otherwise, this p-type shape response of SnO2/Sb can only be observed under acetone environment at 450°C, which can be used to distinguish acetone among other gases. From Fig.4 (a) and (b), it can be found that Sb doping facilitated SnO2 the highest response to acetone among H2S, ethylene, ethanol, and CO environments, which means antimony doping can improve the selectivity of SnO2.