ZnSe/SnO2 Heterostructure-Based Sensor for Ultralow Concentration of NO2 Detection

Abstract

Introduction Nitrogen dioxide is a hazardous gas species, which commonly causes the air pollution, acid rain, photochemical smog and is known to be harmful to human health.[1,2] According to World Health Organization (WHO) reports, approximately two million premature deaths of humans per year around the world are due to the influence of NO2 contaminants in the atmosphere.[3] Therefore, it is imperative to develop a reliable sensor with high sensitivity and selectivity for NO2 detection. In this aspect, ZnSe-core/SnO2-shell hybrid microspheres based sensor has been synthesized for ppb-level NO2 detection. The as-fabricated sensor exhibits a response of 1.21 to ultralow concentration of NO2 detection (75 ppb) at 160 °C. It has been demonstrated that the sensor presents an excellent reproducibility, long-term stability and selectivity. Experiment ZnSe microspheres were synthesized through the hydrothermal reaction. In a typical procedure, 4.5 mmol ZnCl2 and 3 mmol SeO2 were added to a mixed solution of triethylenetetramine (TETA, C6H18N4) and deionized water (DIW). The mixture was transferred into a 50 ml Teflon-lined autoclave and heated at 180 °C for 10 h. The as-prepared ZnSe microspheres were dispersed in an aqueous solution of tin chloride, and control the Zn source to Sn source in different mole ratios. The mixture was then maintained at 130 °C for 4 h. The obtained precipitates were washed by deionized water and absolute alcohol until the absence of chloride ion. Results and conclusions Figure 1a-d show the low-magnification SEM images of as-fabricated samples. The insets at the left-top corners are the enlarged images of the corresponding selected areas. Obviously, the pristine ZnSe sample exhibits a microspherical morphology with a particle size in the range of 4 μm ~ 6 μm. As the amount of SnO2 coated on the ZnSe surface increases, the spherical surface of the sample became rougher and coarser. However, with further increasing the content of SnO2 to Zn/Sn-0.5, the surface of the sample instead became smoother. Figure 1e shows the HRTEM of the Zn/Sn-1 sample. The fringe interval of 0.33 nm can be attributed to the (111) crystal planes of ZnSe, whereas the fringe intervals of 0.34 nm and 0.27 nm are consistent with the (110) and (101) crystal planes of SnO2. Figure 1f reveals the selected area electron diffraction pattern of the Zn/Sn-1 sample. The ring-shape patterns are assigned to the (110), (101), (200) and (211) crystal planes of SnO2. Two spots represent the (111) and (220) crystal planes of ZnSe, which confirms that both ZnSe and SnO2 are present in the hybrid Zn/Sn-1 sample. The elemental distributions on the surface of the Zn/Sn-1 sample are recorded in Figure 1h-l. It can be observed that Zn, Se, Sn and O elements are uniformly and continuously distributed along the shape of the core-shell microsphere structure, indicating the homogeneous distribution of ZnSe and SnO2 in the sphere-shape matrix. Figure 2a shows a comparison of the responses of the mesoporous ZnSe/SnO2 heterostructure based sensors with different concentrations of SnO2 shells to 2.4 ppm NO2 at different operating temperatures from 140 °C to 180 °C. Among these samples, the Zn/Sn-1 heterostructure based sensor achieves the highest response approximately 6.94 at the optimum operating temperature of 160 °C, which is higher than that of Zn/Sn-0.5 (~4.55) and SnO2 (~2.86) based sensors. Figure 2b reveals the selectivity of the Zn/Sn-1 based sensor for several possible interferents with a concentration of 100 ppm such as SO2, CO2, CO, methanol, ethanol, acetone, benzene, methylbenzene, xylene, ammonia and formaldehyde, respectively. The sensor represents the highest response to 2.4 ppm NO2 than others, which indicates an excellent selectivity.