Regular ArticleSelective-detection NO at room temperature on porous ZnO nanostructure by solid-state synthesis method
Graphical abstract
Introduction
Nitric oxide (NO) is a harmful gas, which is produced in the form of by-products of industrial production and combustion. NO is not only one of the important sources of acid rain, but also destroys the olfactory oxygen layer [1]. Moreover, NO can also react with water and oxygen in mucosa, skin and eyes, finally turn into irritant substances, such as nitric acid and nitrite, which do harm to human health. Thus, it is essential to develop a simple, low-cost NO sensor for environmental monitoring with high sensitivity and selectivity at room temperature.
Resistance gas sensors based on metal oxides are widely concerned due to their high sensitivity, fast response, high compatibility and low power consumption. Many metal oxides, such as ZnO [2], [3], [4], In2O3 [5], [6], WO3 [7], [8], [9], [10], are widely applied to prepare NO gas sensors. However, these materials only response well to NO at high temperature (≥200 °C), which hinders their practical applications. In order to solve this problem, methods such as doping, surface modification and regulating material morphology [3], [23] are widely used to improve their sensing performance and reduce the response temperature [11], [12], [13]. Chen et al. [9] prepared a gas sensor with sensitive response to NO at lower temperature based on Ag nanoparticles-modified WO3 nanoplates. They found that the doping amount of Ag nanoparticles had an important effect on the sensitivity of NO at 50 °C. By introducing Pt into In2O3-WO3, Chang et al. [10] developed a gas sensor based on 0.25% Pt/In2O3-WO3 (4:1). Compared with the reported sensors based on WO3 [14] or In2O3 [15], which were operated at 200 °C and 500 °C, respectively, this sensor could not only realize the detection of NO at room temperature, but also significantly improve the sensitivity.
ZnO is also often used as a gas sensitive material for preparation NO gas sensors [16], [17]. ZnO is a typical n-type semiconducting metal oxide with low-cost, environment-friendly, large exciton binding energy, a wide band gap of 3.37 eV and high electron mobility at room temperature [18], [19], [20]. ZnO nanobelts are obtained by calcining the mixture of ZnO and graphite (weight ratio of 3:1) at 1050 °C in a tube furnace. The sensor based on the ZnO nanobelts has high sensitivity and the response is 1.7 and 6.5 for 10 ppm and 50 ppm of NO at 28 °C, respectively. But at same temperature, the response of 10 ppm H2S is 8, which is better than that of NO [2]. Kaur et al. [21] prepared bare ZnO nanostructures by carbon thermal reduction method. The response of the sensor to 40 ppm of NO was 500% at room temperature. The response and recovery times were about 30 s and 1 min, respectively. Unfortunately, this sensor could respond to many gases at the same time, such as CO, H2S, NH3, NO and ethanol etc. Wu et al. [22] prepared a NO gas sensor based on flower-like nanoscale ZnO powder. It has higher responsiveness to NO than the sensor based on commercial ZnO powder. At room temperature, the responsiveness to 1000 ppm of NO are 20.1 and 5.6, respectively. The gas sensor on haemin-functionalized Al-doped porous ZnO was also used to detect NO at room temperature [23]. The response of sensor (30%) was better than that on the haemin-functionalized ZnO without Al-doping (27%) and bare ZnO (5.8%). The research results show that the specific surface area and pore permeability of ZnO can be improved by doping Al, thus the sensitivity is improved and the operating temperature is reduced. Besides, the selectivity of the sensor was also enhanced by doping haemin. The vertically grown ZnO nanorods were prepared by heating ZnCl2 and NH4OH at 95 °C in a sealed autoclave. Ag was uniformly sprayed onto the surface of ZnO nanorods. The sensor on the Ag-modified ZnO nanorods is highly sensitive to NO, and has a sensitivity of up to 7.4% to 100 ppb of NO at room temperature [24]. Although the above-mentioned sensors can significantly improve the response sensitivity and reduce operating temperature, there are still some problems, such as complex preparation of sensitive materials, unsatisfactory selectivity of sensors and so on. Therefore, it is still necessary to develop a simple and rapid preparation method for selective detection of NO at room temperature.
In this study, a fast and simple solid-state method was developed to obtain a coralline-like porous ZnO nanostructure powder. A gas sensor based the ZnO powder was prepared to selectively detect NO at room temperature. The reactant precursors are environmental-friendly and non-toxic zinc acetate, which is better than commonly used zinc nitrate to avoid the generation of harmful nitrogen dioxide gas during calcination. The properties of the sensor were systematically studied. The experimental results show that the sensor has good sensitivity and selectivity to NO at room temperature.
Section snippets
Chemicals
All chemicals used were of analytical grade and without further purification. Zinc acetate dihydrate (Zn(CH3COO)2·2H2O) was obtained from Tianjin Yongsheng Fine Chemical Co. Ltd. (Tianjin, China), oxalic acid dihydrate (H2C2O4·2H2O) was purchased from Tianjin Guangfu Institute of Fine Chemicals (Tianjin, China) and ethanol (C2H5OH) was purchased from Beijing Chemical Factory (Beijing, China). Distilled water was used throughout the experiments.
Solid-state synthesis of coralline-like porous ZnO powder
In a representative solid-state synthesis process,
Optimization of the synthesis conditions of the coralline-like porous ZnO powder
Preliminary experiments show that the calcination temperature and the ratio of reaction precursors have a great influence on the response of NO. In order to obtain the optimal calcination temperature, the thermal decomposition process of the mixture of (ZnCH3COO)2·2H2O and H2C2O4·2H2O, which was dried at 70 °C for 4 h, was studied by TGA (Fig. 2). In Fig. 2, the weight loss at low temperature (below 200 °C) was 20.5 wt%, mainly due to the partial removal of hydration water of ZnC2O4·2H2O and
Conclusions
Based on previous reports, the sensing performances could be improved by changing the particle size, specific surface area and oxygen deficiency of metal oxide [3], [13], [44]. This work has proved that the sensitive and selective detection of NO could be achieved at room temperature by adjusting the morphology of ZnO. Compared with the reported methods [23], [31], [41], we prepared a kind of coralline-like ZnO powder by a simple and low-cost solid-phase synthesis method. Zinc acetate and
Acknowledgements
This work was supported by Development Program of the Ministry of Science and Technology of Jilin Province, China (No. 20180201011GX) and The National Natural Science Foundation of China (NSFC No. 61474057).
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