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ZnO microspheres for efficient sulfur dioxide detection
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One-pot hydrothermal synthesis of nano-sheet assembled NiO/ZnO microspheres for efficient sulfur dioxide detection Wenxu Sia, Wenjing Dua, Fenglong Wanga, Lili Wua, Jiurong Liua,∗, Wei Liub, Ping Cuic, Xiaomei Zhangc a Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education) and School of Materials Science and Engineering, Shandong University, Jinan, Shandong, 250061, People's Republic of China b State Key Laboratory of Crystal Materials, Shandong University, Shandong, 250100, China c School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Zinc oxide Nickel oxide Sulfur dioxide Gas sensing Heterojunction
The nano-sheet assembled NiO/ZnO microspheres with a diameter of ca. 2 μm have been synthesized via facile hydrothermal method followed by a thermal treatment process. The gas-sensing measurement results show that the sensor using nano-sheet assembled NiO/ZnO microspheres exhibits improved response to sulfur dioxide gas compared with the pure ZnO microspheres sensor. In particular, the sensor with a Ni/Zn molar ratio of 0.5 (0.5 mol% NiO/ZnO) shows high response value (S = 107) and superior selectivity to 10 ppm sulfur dioxide at a low operating temperature of 160 °C and the detection limit of the NiO/ZnO sensor toward sulfur dioxide is down to 1 ppm (S = 6). The enhanced sensing performance is attributed to the formation of p-n heterojunctions on the interface, the catalytic function of NiO and the three-dimensional microspheres composed of the lamellar structure with a large specific surface area. The three-dimensional microspheres provide more active sites for gas adsorption, and the interlayer gap facilitates the diffusion of gas in three-dimensional structure, resulting in high sensitivity and superior selectivity. This work provides a simple method for the synthesis of NiO/ZnO heterojunction microspheres with superior gas-sensing performance, which can be used as a potential material for sulfur dioxide detection.
1. Introduction The emission of toxic and flammable gases from urban industry and fossil energy combustion has dramatically increased [1–3]. Sulfur dioxide, one of the most common atmospheric pollutants, is the main cause for acid rain, threatening environment and human health [4,5]. When people are exposed to 500 ppm of sulfur dioxide, symptoms of respiratory diseases, pulmonary edema and even suffocation may occur [6,7]. Therefore, it is highly desirable to develop efficient sensors that can quickly and efficiently detect sulfur dioxide in the atmosphere so as to provide timely security alarms [8,9]. In recent years, the research of gas-sensing materials for detecting sulfur dioxide are mainly focused on metal oxide semiconductors, including ZnO [10–14], SnO2 [15–17], TiO2 [18,19], and WO3 [20,21]. Among them, ZnO is considered as a stable semiconductor material for sulfur dioxide detection because of its large band gap, low cost and high compatibility for microelectronic process [22,23], which has attracted increasing attention [24–26]. However, ZnO sensor has shortcoming for
∗
sulfur dioxide detection such as low response values, high working temperatures and poor selectivity. It is well known that the gas-sensing performance of pure metal oxide can be improved by heteroatom doping, surface modification and hybridization with other components [27–33]. Among these methods, the fabrication of composite material is an effective approach to enhance sensitivity and selectivity [34]. For instance, P. Tyagi et al. fabricated NiO dotted cluster/SnO2 thin films showing enhanced sensing performances to sulfur dioxide, including the high response (S = 56) and low operating temperature (160 °C) compared with the pristine SnO2 or NiO [35]. Y. Liu et al. reported that the Ru/Al2O3/ZnO material exhibited response (S = 1.25) to 25 ppm sulfur dioxide [4]. In addition, according to previous reports, NiO shows a catalytic overflow effect for sulfur dioxide. When the sulfur dioxide gas is contact with NiO, sulfur dioxide gas molecules can convert into unstable SO3 molecules, which can easily react with the chemisorbed oxygen on the surface of the composite and release electrons to the conduction band of the sensing material, reducing the potential barrier in the depletion layer and improving the response value of the
Corresponding author. E-mail address: [email protected] (J. Liu).
https://doi.org/10.1016/j.ceramint.2019.11.222 Received 28 October 2019; Received in revised form 16 November 2019; Accepted 25 November 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Wenxu Si, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.222
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sensor [36]. In the present work, the nano-sheet assembled NiO/ZnO heterojunction microspheres has been prepared through one-step facile hydrothermal route. Texture characteristic show that the NiO/ZnO microspheres with lamellar structure exhibit porous nature with high specific surface of 20.9 m2 g−1 and big pore size of 25 nm, which will provide more active sites and diffusion channels for oxygen and tested gas molecules. The sulfur dioxide sensing performance of the pristine ZnO and NiO/ZnO composites was systematically investigated. The results demonstrate that the NiO/ZnO microspheres exhibit high response value, superior selectivity, and low operating temperature to sulfur dioxide compared with the pristine ZnO. The sensing mechanism of improved gas-sensing performance was attributed to the heterojunction formed by the introduction of nickel oxide, the catalysis of NiO nanoparticles, and the microspheres structure composed of nano-sheets [37]. Therefore, the synthesized NiO/ZnO microspheres via one-step hydrothermal method can be used as a promising candidate for the development of sensor with superior gas-sensing properties for sulfur dioxide detection.
calcined at 400 °C to decompose the organic binder and form good connecting between sensitive material and flat electrode. After that, the flat electrode and the four legged pedestal were soldered and the fabricated sensor was measured after aging at 5 V heating voltage for one week. The aging process can enhance the stability and reliability of the sensor. The gas-sensing performance was measured via WS-30A gas-sensitive instrument under an experiment condition (13–20 %RH, 25 °C). The gas-sensing performance of sensing materials mainly comprises sensibility, response-recovery time, selectivity, and stability. Sensibility is calculated by formula S]Ra/Rg, where Ra and Rg represent the resistance value of the sensor in air and detected gas, respectively. Response-recovery time is defined as the time required for the gas sensor to reach 90% of the steady-state resistance changes in tested gas and air, respectively.
2. Experimental
The XRD patterns of as-prepared ZnO and NiO/ZnO microspheres are shown in Fig. 2. All diffraction peaks are clear and sharp, in agreement with the standard card JCPDS No. 36–1451, and no other impurity peak is found. The XRD peaks at 31.7°, 34.4°, 36.3°, 47.5°, 56.6°, 62.9° and 67.9° matched well with the (100), (002), (101), (102), (110), (103) and (112) crystal planes, indicating that pure ZnO has been obtained. However, for NiO/ZnO samples, no significant NiO peaks are observed in the sample (NZ0.25, NZ0.5, NZ1), which may be attributed to low content of NiO. The nitrogen adsorption-desorption measurement was performed to investigate the specific surface area and pore size of as-prepared ZnO and NZ0.5 microspheres. As shown in Fig. 3, the isotherms of ZnO and NZ0.5 microspheres show the typical type IV with typical H3 hysteresis loops, indicating the characteristic of macroporous structures in the ZnO and NZ0.5 microspheres. The pore size distribution curves (inset of Fig. 3) display that the average size of pores in samples is 25 nm. The Brunauer-Emmett-Teller (BET) surface areas of ZnO and NZ0.5 microspheres are calculated to be 20.9 m2 g−1 and 23.9 m2 g−1. There is no significant difference in specific surface area and pore size distribution, which illustrates that the introduction of optimal NiO has no change on the structure of pure ZnO. The elemental composition and chemical valence state of 10 mol.% NiO/ZnO surface were further illuminated by XPS. As shown in Fig. 4a, the survey-scan spectrum displays that the peaks can be ascribed to the elements of Zn, O, and Ni. Two strong peaks are discovered at 1021.4 and 1044.5 eV, which are corresponding to the position of Zn 2p3/2 and Zn 2p1/2, respectively (Fig. 4b) [38,39]. The high resolution spectrum of the Ni 2p is displayed in Fig. 4c. The Ni 2p peaks located at 855.1 eV and 860.7 eV can be ascribed to Ni 2p3/2, and peaks with the binding energy of 872.5 eV and 878.4 eV can be assigned to Ni 2p1/2. Therefore, the XPS spectrum of Ni 2p demonstrates the existence of Ni2+ in the product [40]. Fig. 4d exhibits the O 1s high resolution XPS spectrum. There is one symmetrical peak centered at 531.4 eV, which can be ascribed to the metal-oxygen bonds [8]. Therefore, the results of XPS demonstrate that the NiO/ZnO nanocomposites have been successfully synthesized. Porous pure ZnO and NiO/ZnO microspheres had been prepared via one-step hydrothermal synthesis method and the synthesis of NiO/ZnO microsphere was illustrated in Fig. 5. In the hydrothermal reaction, both Ni and Zn hydroxides were first formed. With the reaction continuing, the aggregated Ni and Zn hydroxides could grow along with the directions of 2D. Thermodynamically, the surface energy of individual nanosheet is quite high, so they tend to integrate self-assembly arrangements to reduce surface free energy. Therefore, the NiO/ZnO microspheres assembled by nano-sheet was obtained. Fig. 6a and b reveal that the precursor shows dispersed microsphere assembled by
3. Results and discussion 3.1. The characterization of NiO/ZnO microspheres
2.1. Synthesis of NiO/ZnO microspheres The NiO/ZnO microspheres were synthesized via a hydrothermal route. In brief, 0.5 g PVP, 1 mmol zinc acetate, 2.5 mmol glycine, various amounts of nickel acetate and 0.0015 mmol sodium sulfate were added to 30 mL deionized water. The mole percentages of nickle to zinc in the samples were 0.25 mol.%, 0.5 mol.%, and 1 mol.%, respectively. The as-prepared compound was stirred magnetically at room temperature for 20 min to form a uniform and transparent solution. Afterwards, 0.002 mol ammonium carbonate was dissolved in 20 mL deionized water and ammonium carbonate solution was dropped into the above mixture solution. Subsequently, the mixture was moved to 100 mL teflon-lined stainless autoclave and heated at 180 °C for 2 h. The white precipitate was obtained by centrifugation and was rinsed with deionized water and ethanol several times, and dried at 60 °C overnight in the oven. The ultimate ZnO microspheres and ZnO/NiO heterojunction products can been obtained through the precursor annealing at 500 °C for 2 h in air. According to the content of nickel, these products were named NZ0.25 (0.25 mol.% NiO/ZnO), NZ0.5 (0.5 mol. % NiO/ZnO), NZ1 (1 mol.% NiO/ZnO), respectively. Pure ZnO microspheres were synthesized based on the above method without Ni(CH3COO)2. 2.2. Characterizations The crystalline structure of as-prepared product was characterized by X-ray diffraction (XRD). The morphology and structure characteristics of samples were observed by a field emission scanning electron microscope (FE-SEM SU-70). The specific surface areas were determined through Brunauer-Emmelt-Teller (BET) model and the pore size distributions were calculated by the Barrett-Joyner-Halenda (BJH) method. The chemical composition and valence state of elements of products were analyzed by X-ray photoelectron spectra. 2.3. Fabrication and measurement of gas sensor The fabrication of gas sensor was carried out through a facile coating method and the manufacturing process is described as follows. 2.8925 g ethyl cellulose dispersed in 50 mL terpineol under magnetic stirring at 80 °C for 2 h to form transparent adhesive. The previously prepared materials were mixed with the adhesive together to form a paste, which was coated on the alumina planar electrode with two Ni–Cr lines for heating and two Pt lines for measurement (Fig. 1). Subsequently, the planar electrode with coating sensing material was 2
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Fig. 1. Schematic diagram for the alumina planar electrode.
3.2. Gas sensing performance of NiO/ZnO microspheres The working principle of metal oxide semiconductor (MOS) is based on the variation on the resistance of MOS caused by the adsorption/ desorption of objective gas on the surface of materials. As a typical ntype semiconductor, ZnO has been attracted increasing attention in the field of reducing gas detection. When the sensing material is exposed to air, the oxygen molecules can adsorb on the surface of the ZnO and form ionic oxygen species such as O2−, O−, O2− by grabbing electrons from the conduction band of zinc oxide at different temperature ranges of below 100 °C, between 100 °C and 300 °C, and above 300 °C, respectively (Equations (2)–(4)). The transportation of electrons from the conduction band of ZnO to chemisorbed oxygen results in a decrease in the number of free-electrons and an increase in resistance of ZnO. The kinematics of adsorption are explicated by the following formula:
Fig. 2. XRD patterns of pure ZnO and NiO/ZnO porous microspheres.
thin nano-sheets and the diameter size is about 2 μm. After the calcination of the precursor, the obtained ZnO remain the sphere-shaped structure, illustrating that the morphology of the sample was not destroyed during calcination (Fig. 6c and d). Fig. 6e and f shows the SEM images of the NZ0.5 composite and no significant difference is found compared with pure ZnO, indicating that the NZ0.5 well inherits the microsphere architecture of the pristine one.
O2 (gas) ⇔ O2 (ads)
(1)
O2 (ads) + e− ⇔ O−2 (ads)
(2)
O−2 (ads) + e− ⇔ 2O− (ads)
(3)
O− (ads)
(4)
+
e−
⇔
O2 − (ads)
When the sulfur dioxide gas is injected, the SO2 molecule will react with the O− on the surface of the ZnO to form SO3 and release electrons into conducting band of ZnO, resulting in a reduction in the resistance (Rg) of the sensor. When the SO2 gas is removed from the surface of ZnO, the unstable SO3 molecules capture electrons from the conducting band of the ZnO and decompose into SO2 and O−, and the resistance of ZnO will restore to the initial resistance (Ra) in air. The sensitivity of the NiO/ZnO sensor was studied by analyzing the
Fig. 3. Nitrogen adsorption-desorption isotherms and pore size distribution of (a) ZnO and (b) 0.5 mol.% NiO/ZnO microspheres. 3
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Fig. 4. XPS spectra of 10 mol.% NiO/ZnO: (a) survey, (b) Zn 2p, (c) Ni 2p, and (d) O 1s.
Fig. 5. Schematic illustration of the growth mechanism of the flowerlike NiO–ZnO composite. 4
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Table 1 The response (Ra/Rg) value of ZnO, NZ0.25, NZ0.5, NZ1 sensors to 10 ppm SO2. Temperature (°C)
ZnO(Ra/Rg)
NZ0.25(Ra/Rg)
NZ0.5(Ra/Rg)
NZ1(Ra/Rg)
100 160 200 240
4.5 15 4.68 3.23
5.33 46.15 25 15.34
23.5 107 31.25 10.42
17.9 34.48 33 15.12
the conduction band of ZnO, which causes the depletion region of the ZnO semiconductor increasing. In addition, the catalytic function of NiO facilitates the adsorption of oxygen on the surface of ZnO to form O−, O2−, which reduces the number of free electrons in the conduction band of ZnO. Therefore, the high resistance of the NZ0.5 sensor is obtained. When the sensors are exposed on 10 ppm SO2, the resistances of the ZnO and NZ0.5 sensors are lower than that in the air (Fig. 7c). Typically, the resistance of pure ZnO reduces from 100 to 22 at 100 °C, from 52.5 to 3.5 at 160 °C, from 11 to 2.35 at 200 °C, and from 8.2 to 2.54 at 240 °C, and the relevant response values are 4.5, 15, 4.7 and 3.2, respectively. For the NZ0.5 sensor, the resistance changes from 400 to 17 at 100 °C, from 140 to 1.3 at 160 °C, from 100 to 3.2 at 200 °C, and from 25 to 2.4 at 240 °C, and the response values are 23.5, 107.7, 31.3 and 10.4, respectively. The experimental results clearly demonstrate that the resistance variation of the NZ0.5 sensor in air and sulfur dioxide gas is larger than that of the pure ZnO sensor. Hence, a much improved response on NZ0.5 can be observed. Fig. 8a displays the response curves of the NZ0.5 sensor to SO2 gas with different concentrations at the working temperature of 160 °C. The NZ0.5 sensor shows good response and recovery curves to SO2 with the concentration ranging from 1 to 50 ppm, indicating the sensor can be used to detect ppm-level SO2 gas. In addition, with the injected concentration of sulfur dioxide increasing, the response values of the NZ0.5 sensor increase from 6 to 172. In addition, the response-recovery time of the sensor tends to reduce with the concentration of SO2 gas increasing and reaches a minimum value (701/237s). In order to investigate the relationship between the gas concentration and the response of the sensor, the logarithm of the response value (Lg S) can be described as a function of the logarithm of SO2 concentration (Lg C) by a linear curve. As shown in Fig. 8b, a well fitted linear relationship between gas concentration and the response of the sensor is obtained. The repeatability and stability are both important factors to evaluate the gas-sensing performance. Fig. 8c shows the repeatability of the NiO/ZnO sensor to 10 ppm SO2 at 160 °C. As shown in the curves, the response values of the three response-recovery cycles are maintained at 110. Besides, the identical response-recovery time (701-237 s) of asprepared NiO/ZnO microspheres sensor towards 10 ppm SO2 gas are observed. The results can be concluded that the NiO/ZnO sensor has an superb repeatability for SO2 gas sensing. The long response-recovery time may be caused by uneven pore size distribution, which increases the diffusion resistance in the pores, reduces the gas transport
Fig. 6. FE-SEM image of (a, b) precursor, (c, d) pure ZnO microspheres, and (e, f) NZ0.5 composite at different magnification.
response of the sample to 10 ppm SO2 in different measurement temperatures. Fig. 7a shows the response of ZnO, NZ0.25, NZ0.5, NZ1 to 10 ppm SO2 at different working temperatures of 100 °C, 160 °C, 200 °C and 240 °C. The response of NiO/ZnO and pure ZnO sensors show an ascending tendency from 100 °C to 160 °C. However, when the working temperature exceeds 160 °C, the declined response is observed. It indicates that the sensors exhibit the highest response value to 10 ppm SO2 at the optimized operating temperature of 160 °C. Table 1 shows the response values of ZnO, NZ0.25, NZ0.5, and NZ1 at 160 °C for 10 ppm SO2, which are 15, 46.15, 107, and 34.48 respectively. It reveals that the NiO/ZnO sensor with the NiO concentration of 0.5 mol.% shows the highest response value comparing with ZnO, NZ0.25 and NZ1 sensors at 160 °C. Furthermore, the highest response value of NiO/ ZnO is almost seven times than that of pure ZnO at 160 °C, illustrating that the introduction of NiO significantly enhances the sensitivity of ZnO microspheres to sulfur dioxide. In order to further illustrate the improvement of the sensor response value of NiO/ZnO, the resistance values of the ZnO and NZ0.5 sensor in air and SO2 gas at different operation temperatures were measured. As shown in Fig. 7b, the resistance of two sensors in air maintains a continuous decrease as the temperature increases. Moreover, the NZ0.5 sensor displays the higher resistance than the ZnO sensor. It can be attributed to the formation of p-n heterojunction at the interface of pNiO and n-ZnO caused by the introduction of NiO. The formation of p-n heterojunction results in a decrease in the concentration of electrons in
Fig. 7. (a) The response of ZnO, NZ0.25, NZ0.5 and NZ1 to 10 ppm SO2 at different operating temperatures. The resistance of ZnO and NZ0.5 sensor in air (b) and (c) 10 ppm SO2 gas at different operating temperature. 5
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Fig. 8. (a) The dynamic response curves of the NZ0.5 sensor to different concentrations (ppm) of sulfur dioxide gas at optimal operating temperature, (b) the fitting curve between the logarithm of the response value (Lg S) and the logarithm of SO2 concentration (Lg C), (c) repeatability and (d) stability measurements of the NiO/ ZnO sensor to 10 ppm SO2 at 160 °C, response of NZ0.5 and pure ZnO microspheres to various gases with different concentrations (e) 10 ppm and (f) 50 ppm.
SO2 at 85 °C; D. Zhang et al. [42] reported TiO2/rGO film with a low response of 1.12 at 25 °C, which is lower than NiO–ZnO microspheres (107) to 10 ppm SO2 at 160 °C in our work. The catalytic effect of NiO on sulfur dioxide is an important reason for the improvement of gas sensitivity. Table 2 shows that the gas-sensing material with NiO shows high response value. For example, Q. Zhou et al. reported NiO–ZnO nanodisks presented the higher response value (16.25) than Ru/Al2O3/ ZnO (1.25) to 20 ppm sulfur dioxide [4,5]. The SnO2/NiO thin film displays the higher response value (56) than rGO-SnO2 (22) to 500 ppm sulfur dioxide. Meanwhile, the layer-by-layer self-assembly structure is an important factor to improve the gas-sensing performance [42,43]. The layer gap facilitates the diffusion of gas in the three-dimensional structure, the larger specific surface provides a site for gas adsorption. Therefore, the self-assembled NiO/ZnO microspheres exhibited the highest response towards 10 ppm SO2 at a low temperature of 160 °C in comparison with the previously reported sensors.
efficiency, and affects the gas response/recovery time [10,41]. An endurance response value was measured to investigate the stability of the NiO/ZnO sensor. Fig. 8d exhibits the test result for seven days, and the response values to 10 ppm SO2 at 160 °C maintain at about 100. It indicates that the NiO/ZnO sensor has an admirable stability for SO2 gas sensing. The response values of NZ0.5 and pure ZnO microspheres to various gases were recorded. As shown in Fig. 8e, the pure ZnO sensor exhibits low response values of 15, 1, 2.5, 5.4 and 1.2–10 ppm SO2, NO2, CH4, H2 and NH3 gases, respectively. After the introduction of NiO, the improved response value (107) to 10 ppm SO2 is observed. For other gases such as NO2, CH4, H2 and NH3 gas with the concentration of 10 ppm, the NZ0.5 sensor exhibits negative response values. Although the concentration of tested gases increases to 50 ppm, the higher response value (172) of the NZ0.5 sensor to 50 ppm SO2 is found comparison with that at NO2, CH4, H2 and NH3 gases (Fig. 8f). Therefore, the NZ0.5 sensor exhibits superior selectivity. Table 2 exhibits the proposed SO2 sensor performance compared to previous work. The investigation results show that the material with high working temperature has a high response to sulfur dioxide, but the material with low working temperature has lower response value, for example, X. Ma et al. [6] reported SnO2 nanowires showing the response value of 13.7 to 10 ppm
3.3. Sensing mechanism of NiO/ZnO microspheres The excellent gas-sensing performance of the NZ0.5 sensor to sulfur dioxide is attributed to the formation of p-n heterojunction, the catalytic effect of NiO and the layer-by-layer self-assembly structure. Nickel oxide can react with sulfur dioxide to form unstable sulfur trioxide. Structurally, the porous structure of the NiO/ZnO microspheres provides more channels for gas diffusion, which favors the improvement of the gas-sensing performance of the NZ0.5 sensor. As a n-type semiconductor, the sensing behavior of the ZnO in air and SO2 gas is shown in Fig. 9a and b. In air, the adsorbed oxygen captures electrons from the semiconductor to form surface oxygen anions. Therefore, an electronic depletion layer in the outer layer of the semiconductor is formed. When sulfur dioxide is supplied, the SO2 gas molecules replace the site of chemisorbed oxygen ion to further form complex SO3−. And the unstable SO3− can convert to SO3 meanwhile release electrons into the conduction band of zinc oxide [48], resulting the electronic depletion layer reducing. The sensing reaction process can be depicted by the following equation (5):
Table 2 Comparison of gas sensing characteristic of sensing material to SO2 gas. Sensing material
Concentration (ppm)
SO2 response
Temperature (°C)
Ref.
SnO2 nanowires Ru/Al2O3/ZnO SnO2/NiO SnO2 dodecahedro rGO-SnO2 TiO2/rGO V-doped SnO2 NiO–ZnO nanodisks NiO–ZnO microspheres
10 25 500 20
13.7 1.25 56 10.4
85 180 180 200
[6] [4] [35] [7]
500 5 100 20
22 1.12 3.3 16.25
60 25 350 240
[46] [42] [47] [5]
10
107
160
This work
SO2 + O− → SO−3 → SO3 + e−
(5)
Fig. 9c and d shows the band structure of the NiO/ZnO 6
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Fig. 9. The energy band structure diagram for ZnO (a) in air and (b) in sulfur dioxide; for NiO and ZnO (c) and p-NiO/n-ZnO heterojunctions (d) in vacuum; for pNiO/n-ZnO heterojunctions (e) in air and (f) in sulfur dioxide.
microspheres are exposed to air, a hole accumulation layer and an electronic depletion layer form in the surface region of NiO and ZnO nanoparticles, leading to the formation of a high potential barrier ΔEc at p-n heterojunction (Fig. 9e). Thus, the electrons transport channel is inhibited, the effective cross-sectional area of charge conduction is reduced, and the resistance (Ra) of the NZ0.5 sensor is increased. For MOS composites, the catalytic function of metal oxide nanoparticles is an important factor to improve the gas-sensing performance of the material. Fig. 9f reveals the response mechanism of NZ0.5 microspheres to SO2. When the NiO/ZnO sensor is exposed to SO2, the SO2
semiconductor in vacuum. After the modification of NiO nanoparticles, two metal oxide semiconductor are in intimate contact. Due to the difference in the Fermi levels of n-type ZnO and p-type NiO, the electrons in the higher energy level will transfer to the lower energy level. Therefore, when the n-type ZnO and p-type NiO are contacted with each other, electrons transfer from n-ZnO to p-NiO, meanwhile holes shift in the opposite direction until the Fermi energy level are in equilibrium. As a result, a p-n heterojunction structure is formed at the interface of n-ZnO and p-NiO, the energy band of the interface bends and the energy barrier (Ec) appears (Fig. 9d). When the NiO/ZnO 7
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gas molecules can react with the NiO to form the unstable SO3 due to the catalytic function of NiO nanoparticles, which can be depicted as equation (6). Subsequently, the as-obtained unstable SO3 molecules further combine with the chemisorbed oxygen (O−) on the surface of ZnO and convert into SO2 and O2 (equation (7)). Simultaneously, the electrons transfer from the chemisorbed oxygen to the conduction band of the ZnO and NiO. Therefore, the hole accumulation layer and electronic depletion layer decreasing in the surface region of NiO and ZnO nanoparticles are obtained, and a reduced broken band (ΔEc’) appears on the conduction band (Fig. 9f), resulting in the resistance of the material decreasing. Besides, the spilled SO2 gas that is the by-product of formula (equation (7)) further react with ZnO and release more electrons, resulting in a further decrease in the resistance (Rg) of the sensor, improving the response of the material [35].
NiO + 4SO2 → NiS + 3SO3
(6)
SO3 + O− → SO2 + O2 + e−
(7)
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Shen, S. Zhao, G. Li, Y. Yin, R. Lu, S. Gao, C. Han, D. Wei, NO2 sensing properties of WO3 porous films with honeycomb structure, J. Alloy. Comp. 789 (2019) 129–138. [21] X. Wang, F. Chen, M. Yang, L. Guo, N. Xie, X. Kou, Y. Song, Q. Wang, Y. Sun, G. Lu, Dispersed WO3 nanoparticles with porous nanostructure for ultrafast toluene sensing, Sens. Actuators B Chem. 289 (2019) 195–206. [22] R. Yoo, A.T. Güntner, Y. Park, H.J. Rim, H.-S. Lee, W. Lee, Sensing of acetone by Aldoped ZnO, Sens. Actuators B Chem. 283 (2019) 107–115. [23] G. Ren, Z. Li, W. Yang, M. Faheem, J. Xing, X. Zou, Q. Pan, G. Zhu, Y. Du, ZnO@ZIF8 core-shell microspheres for improved ethanol gas sensing, Sens. Actuators B Chem. 284 (2019) 421–427. [24] Y. Gong, X. Wu, X. Zhou, X. Li, N. Han, Y. Chen, High acetone sensitive and reversible P- to N-type switching NO2 sensing properties of Pt@Ga-ZnO core-shell nanoparticles, Sens. Actuators B Chem. 289 (2019) 114–123. [25] X. Liu, J. Zhang, T. Yang, L. Wang, Y. Kang, S. Wang, S. Wu, Self-assembled hierarchical flowerlike ZnO architectures and their gas-sensing properties, Powder Technol. 217 (2012) 238–244. [26] J. Han, T.-y. Wang, T.-t. Li, H. Yu, Y. Yang, X.-t. Dong, Enhanced NOx gas sensing properties of ordered mesoporous WO3/ZnO prepared by electroless plating, Adv. Mater. Interfaces 5 (2018). [27] C. Han, X. Li, C. Shao, X. Li, J. Ma, X. Zhang, Y. Liu, Composition-controllable pCuO/n-ZnO hollow nanofibers for high-performance H2S detection, Sens. Actuators B Chem. 285 (2019) 495–503. [28] D. Fu, C. Zhu, X. Zhang, C. Li, Y. Chen, Two-dimensional net-like SnO2/ZnO heteronanostructures for high-performance H2S gas sensor, J. Mater. Chem. 4 (2016) 1390–1398. [29] J. Hao, D. Zhang, Q. Sun, S. Zheng, J. Sun, Y. Wang, Hierarchical SnS2/SnO2 nanoheterojunctions with increased active-sites and charge transfer for ultrasensitive NO2 detection, Nanoscale 10 (2018) 7210–7217. [30] X. Wang, M. 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4. Conclusions In summary, the ZnO and NiO/ZnO microspheres have been synthesized by a one-step hydrothermal method for the detection of toxic sulfur dioxide. Texture characterizations exhibit that the NiO/ZnO microsphere with a diameter of ca. 2 μm show porous structure with specific surface of 20.9 m2 g−1 and big pore size of 25 nm. By taking the advantages of the catalytic functions of nickel oxide (chemical effect), the formation of semiconductor heterojunctions (electronic effect) and the large specific surface area and porous structure (geometric effect) of NiO/ZnO composites, the gas-sensing performance of the sensor to sulfur dioxide significantly enhanced compared with previously reported pure ZnO sensors and other-based sensors. This work indicates that the NiO/ZnO microspheres are promising candidates for fabrication of high-performance SO2 sensors. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (No.51572157), the Science and Technology Development Plan (2014GGX102004), the Qilu Young Scholar Scheme of Shandong University (No.31370088963043) and the Fundamental Research Funds for the Central Universities (2018JC036, 2018JC046). References [1] C. Ma, X. Hao, X. Yang, X. Liang, F. Liu, T. Liu, C. Yang, H. Zhu, G. Lu, Sub-ppb SO2 gas sensor based on NASICON and LaxSm1-xFeO3 sensing electrode, Sens. Actuators B Chem. 256 (2018) 648–655. [2] J. Cao, C. Qin, Y. Wang, B. Zhang, Y. Gong, H. Zhang, G. Sun, H. Bala, Z. Zhang, Calcination method synthesis of SnO2/g-C3N4 composites for a high-performance ethanol gas sensing application, Nanomaterials 7 (2017). [3] C. Wang, L.-J. Wang, L. Zhang, R. Xi, H. Huang, S.-H. Zhang, G.-B. Pan, Electrodeposition of ZnO nanorods onto GaN towards enhanced H2S sensing, J. Alloy. Comp. 790 (2019) 363–369. [4] Y. Liu, X. Xu, Y. Chen, Y. Zhang, X. Gao, P. Xu, X. Li, J. Fang, W. Wen, An integrated micro-chip with Ru/Al2O3/ZnO as sensing material for SO2 detection, Sens. Actuators B Chem. 262 (2018) 26–34. [5] Q. Zhou, W. Zeng, W. Chen, L. Xu, R. Kumar, A. Umar, High sensitive and lowconcentration sulfur dioxide (SO2) gas sensor application of heterostructure NiOZnO nanodisks, Sens. Actuators B Chem. 298 (2019). [6] X. Zhong, Y. Shen, S. Zhao, X. Chen, C. Han, D. Wei, P. Fang, D. Meng, SO2 sensing properties of SnO2 nanowires grown on a novel diatomite-based porous substrate, Ceram. Int. 45 (2019) 2556–2565.
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[36] S.K. Hasan, M. Mobin, High temperature interaction of NiO with sodium sulphate in SO2 environment at 1100 and 1200 K, Port. Electrochim. Acta 29 (2011) 187–196. [37] P. Zheng, Y. Zhang, Z. Dai, Y. Zheng, K.N. Dinh, J. Yang, R. Dangol, X. Liu, Q. Yan, Constructing multifunctional heterostructure of Fe2O3@Ni3Se4 nanotubes, Small 14 (2018). [38] X. Chen, Y. Shen, X. Zhong, T. Li, S. Zhao, P. Zhou, C. Han, D. Wei, Y. Shen, Synthesis of ZnO nanowires/Au nanoparticles hybrid by a facile one-pot method and their enhanced NO2 sensing properties, J. Alloy. Comp. 783 (2019) 503–512. [39] X. Chen, Y. Shen, P. Zhou, X. Zhong, G. Li, C. Han, D. Wei, S. Li, Bimetallic Au/Pd nanoparticles decorated ZnO nanowires for NO2 detection, Sens. Actuators B Chem. 289 (2019) 160–168. [40] D. Meng, D. Liu, G. Wang, Y. Shen, X. San, M. Li, F. Meng, Low-temperature formaldehyde gas sensors based on NiO-SnO2 heterojunction microflowers assembled by thin porous nanosheets, Sens. Actuators B Chem. 273 (2018) 418–428. [41] H. Aoki, T. Asaka, M. Higuchi, Y. Azuma, K. Asaga, K. Katayama, Effect of pore structures on the humidity sensitivity of TiO2-based ceramic humidity sensors, Key
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
One-pot hydrothermal synthesis of nano-sheet assembled NiO/ZnO microspheres for efficient sulfur dioxide detection Wenxu Sia, Wenjing Dua, Fenglong Wanga, Lili Wua, Jiurong Liua,∗, Wei Liub, Ping Cuic, Xiaomei Zhangc a Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education) and School of Materials Science and Engineering, Shandong University, Jinan, Shandong, 250061, People's Republic of China b State Key Laboratory of Crystal Materials, Shandong University, Shandong, 250100, China c School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Zinc oxide Nickel oxide Sulfur dioxide Gas sensing Heterojunction
The nano-sheet assembled NiO/ZnO microspheres with a diameter of ca. 2 μm have been synthesized via facile hydrothermal method followed by a thermal treatment process. The gas-sensing measurement results show that the sensor using nano-sheet assembled NiO/ZnO microspheres exhibits improved response to sulfur dioxide gas compared with the pure ZnO microspheres sensor. In particular, the sensor with a Ni/Zn molar ratio of 0.5 (0.5 mol% NiO/ZnO) shows high response value (S = 107) and superior selectivity to 10 ppm sulfur dioxide at a low operating temperature of 160 °C and the detection limit of the NiO/ZnO sensor toward sulfur dioxide is down to 1 ppm (S = 6). The enhanced sensing performance is attributed to the formation of p-n heterojunctions on the interface, the catalytic function of NiO and the three-dimensional microspheres composed of the lamellar structure with a large specific surface area. The three-dimensional microspheres provide more active sites for gas adsorption, and the interlayer gap facilitates the diffusion of gas in three-dimensional structure, resulting in high sensitivity and superior selectivity. This work provides a simple method for the synthesis of NiO/ZnO heterojunction microspheres with superior gas-sensing performance, which can be used as a potential material for sulfur dioxide detection.
1. Introduction The emission of toxic and flammable gases from urban industry and fossil energy combustion has dramatically increased [1–3]. Sulfur dioxide, one of the most common atmospheric pollutants, is the main cause for acid rain, threatening environment and human health [4,5]. When people are exposed to 500 ppm of sulfur dioxide, symptoms of respiratory diseases, pulmonary edema and even suffocation may occur [6,7]. Therefore, it is highly desirable to develop efficient sensors that can quickly and efficiently detect sulfur dioxide in the atmosphere so as to provide timely security alarms [8,9]. In recent years, the research of gas-sensing materials for detecting sulfur dioxide are mainly focused on metal oxide semiconductors, including ZnO [10–14], SnO2 [15–17], TiO2 [18,19], and WO3 [20,21]. Among them, ZnO is considered as a stable semiconductor material for sulfur dioxide detection because of its large band gap, low cost and high compatibility for microelectronic process [22,23], which has attracted increasing attention [24–26]. However, ZnO sensor has shortcoming for
∗
sulfur dioxide detection such as low response values, high working temperatures and poor selectivity. It is well known that the gas-sensing performance of pure metal oxide can be improved by heteroatom doping, surface modification and hybridization with other components [27–33]. Among these methods, the fabrication of composite material is an effective approach to enhance sensitivity and selectivity [34]. For instance, P. Tyagi et al. fabricated NiO dotted cluster/SnO2 thin films showing enhanced sensing performances to sulfur dioxide, including the high response (S = 56) and low operating temperature (160 °C) compared with the pristine SnO2 or NiO [35]. Y. Liu et al. reported that the Ru/Al2O3/ZnO material exhibited response (S = 1.25) to 25 ppm sulfur dioxide [4]. In addition, according to previous reports, NiO shows a catalytic overflow effect for sulfur dioxide. When the sulfur dioxide gas is contact with NiO, sulfur dioxide gas molecules can convert into unstable SO3 molecules, which can easily react with the chemisorbed oxygen on the surface of the composite and release electrons to the conduction band of the sensing material, reducing the potential barrier in the depletion layer and improving the response value of the
Corresponding author. E-mail address: [email protected] (J. Liu).
https://doi.org/10.1016/j.ceramint.2019.11.222 Received 28 October 2019; Received in revised form 16 November 2019; Accepted 25 November 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Wenxu Si, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.222
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sensor [36]. In the present work, the nano-sheet assembled NiO/ZnO heterojunction microspheres has been prepared through one-step facile hydrothermal route. Texture characteristic show that the NiO/ZnO microspheres with lamellar structure exhibit porous nature with high specific surface of 20.9 m2 g−1 and big pore size of 25 nm, which will provide more active sites and diffusion channels for oxygen and tested gas molecules. The sulfur dioxide sensing performance of the pristine ZnO and NiO/ZnO composites was systematically investigated. The results demonstrate that the NiO/ZnO microspheres exhibit high response value, superior selectivity, and low operating temperature to sulfur dioxide compared with the pristine ZnO. The sensing mechanism of improved gas-sensing performance was attributed to the heterojunction formed by the introduction of nickel oxide, the catalysis of NiO nanoparticles, and the microspheres structure composed of nano-sheets [37]. Therefore, the synthesized NiO/ZnO microspheres via one-step hydrothermal method can be used as a promising candidate for the development of sensor with superior gas-sensing properties for sulfur dioxide detection.
calcined at 400 °C to decompose the organic binder and form good connecting between sensitive material and flat electrode. After that, the flat electrode and the four legged pedestal were soldered and the fabricated sensor was measured after aging at 5 V heating voltage for one week. The aging process can enhance the stability and reliability of the sensor. The gas-sensing performance was measured via WS-30A gas-sensitive instrument under an experiment condition (13–20 %RH, 25 °C). The gas-sensing performance of sensing materials mainly comprises sensibility, response-recovery time, selectivity, and stability. Sensibility is calculated by formula S]Ra/Rg, where Ra and Rg represent the resistance value of the sensor in air and detected gas, respectively. Response-recovery time is defined as the time required for the gas sensor to reach 90% of the steady-state resistance changes in tested gas and air, respectively.
2. Experimental
The XRD patterns of as-prepared ZnO and NiO/ZnO microspheres are shown in Fig. 2. All diffraction peaks are clear and sharp, in agreement with the standard card JCPDS No. 36–1451, and no other impurity peak is found. The XRD peaks at 31.7°, 34.4°, 36.3°, 47.5°, 56.6°, 62.9° and 67.9° matched well with the (100), (002), (101), (102), (110), (103) and (112) crystal planes, indicating that pure ZnO has been obtained. However, for NiO/ZnO samples, no significant NiO peaks are observed in the sample (NZ0.25, NZ0.5, NZ1), which may be attributed to low content of NiO. The nitrogen adsorption-desorption measurement was performed to investigate the specific surface area and pore size of as-prepared ZnO and NZ0.5 microspheres. As shown in Fig. 3, the isotherms of ZnO and NZ0.5 microspheres show the typical type IV with typical H3 hysteresis loops, indicating the characteristic of macroporous structures in the ZnO and NZ0.5 microspheres. The pore size distribution curves (inset of Fig. 3) display that the average size of pores in samples is 25 nm. The Brunauer-Emmett-Teller (BET) surface areas of ZnO and NZ0.5 microspheres are calculated to be 20.9 m2 g−1 and 23.9 m2 g−1. There is no significant difference in specific surface area and pore size distribution, which illustrates that the introduction of optimal NiO has no change on the structure of pure ZnO. The elemental composition and chemical valence state of 10 mol.% NiO/ZnO surface were further illuminated by XPS. As shown in Fig. 4a, the survey-scan spectrum displays that the peaks can be ascribed to the elements of Zn, O, and Ni. Two strong peaks are discovered at 1021.4 and 1044.5 eV, which are corresponding to the position of Zn 2p3/2 and Zn 2p1/2, respectively (Fig. 4b) [38,39]. The high resolution spectrum of the Ni 2p is displayed in Fig. 4c. The Ni 2p peaks located at 855.1 eV and 860.7 eV can be ascribed to Ni 2p3/2, and peaks with the binding energy of 872.5 eV and 878.4 eV can be assigned to Ni 2p1/2. Therefore, the XPS spectrum of Ni 2p demonstrates the existence of Ni2+ in the product [40]. Fig. 4d exhibits the O 1s high resolution XPS spectrum. There is one symmetrical peak centered at 531.4 eV, which can be ascribed to the metal-oxygen bonds [8]. Therefore, the results of XPS demonstrate that the NiO/ZnO nanocomposites have been successfully synthesized. Porous pure ZnO and NiO/ZnO microspheres had been prepared via one-step hydrothermal synthesis method and the synthesis of NiO/ZnO microsphere was illustrated in Fig. 5. In the hydrothermal reaction, both Ni and Zn hydroxides were first formed. With the reaction continuing, the aggregated Ni and Zn hydroxides could grow along with the directions of 2D. Thermodynamically, the surface energy of individual nanosheet is quite high, so they tend to integrate self-assembly arrangements to reduce surface free energy. Therefore, the NiO/ZnO microspheres assembled by nano-sheet was obtained. Fig. 6a and b reveal that the precursor shows dispersed microsphere assembled by
3. Results and discussion 3.1. The characterization of NiO/ZnO microspheres
2.1. Synthesis of NiO/ZnO microspheres The NiO/ZnO microspheres were synthesized via a hydrothermal route. In brief, 0.5 g PVP, 1 mmol zinc acetate, 2.5 mmol glycine, various amounts of nickel acetate and 0.0015 mmol sodium sulfate were added to 30 mL deionized water. The mole percentages of nickle to zinc in the samples were 0.25 mol.%, 0.5 mol.%, and 1 mol.%, respectively. The as-prepared compound was stirred magnetically at room temperature for 20 min to form a uniform and transparent solution. Afterwards, 0.002 mol ammonium carbonate was dissolved in 20 mL deionized water and ammonium carbonate solution was dropped into the above mixture solution. Subsequently, the mixture was moved to 100 mL teflon-lined stainless autoclave and heated at 180 °C for 2 h. The white precipitate was obtained by centrifugation and was rinsed with deionized water and ethanol several times, and dried at 60 °C overnight in the oven. The ultimate ZnO microspheres and ZnO/NiO heterojunction products can been obtained through the precursor annealing at 500 °C for 2 h in air. According to the content of nickel, these products were named NZ0.25 (0.25 mol.% NiO/ZnO), NZ0.5 (0.5 mol. % NiO/ZnO), NZ1 (1 mol.% NiO/ZnO), respectively. Pure ZnO microspheres were synthesized based on the above method without Ni(CH3COO)2. 2.2. Characterizations The crystalline structure of as-prepared product was characterized by X-ray diffraction (XRD). The morphology and structure characteristics of samples were observed by a field emission scanning electron microscope (FE-SEM SU-70). The specific surface areas were determined through Brunauer-Emmelt-Teller (BET) model and the pore size distributions were calculated by the Barrett-Joyner-Halenda (BJH) method. The chemical composition and valence state of elements of products were analyzed by X-ray photoelectron spectra. 2.3. Fabrication and measurement of gas sensor The fabrication of gas sensor was carried out through a facile coating method and the manufacturing process is described as follows. 2.8925 g ethyl cellulose dispersed in 50 mL terpineol under magnetic stirring at 80 °C for 2 h to form transparent adhesive. The previously prepared materials were mixed with the adhesive together to form a paste, which was coated on the alumina planar electrode with two Ni–Cr lines for heating and two Pt lines for measurement (Fig. 1). Subsequently, the planar electrode with coating sensing material was 2
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Fig. 1. Schematic diagram for the alumina planar electrode.
3.2. Gas sensing performance of NiO/ZnO microspheres The working principle of metal oxide semiconductor (MOS) is based on the variation on the resistance of MOS caused by the adsorption/ desorption of objective gas on the surface of materials. As a typical ntype semiconductor, ZnO has been attracted increasing attention in the field of reducing gas detection. When the sensing material is exposed to air, the oxygen molecules can adsorb on the surface of the ZnO and form ionic oxygen species such as O2−, O−, O2− by grabbing electrons from the conduction band of zinc oxide at different temperature ranges of below 100 °C, between 100 °C and 300 °C, and above 300 °C, respectively (Equations (2)–(4)). The transportation of electrons from the conduction band of ZnO to chemisorbed oxygen results in a decrease in the number of free-electrons and an increase in resistance of ZnO. The kinematics of adsorption are explicated by the following formula:
Fig. 2. XRD patterns of pure ZnO and NiO/ZnO porous microspheres.
thin nano-sheets and the diameter size is about 2 μm. After the calcination of the precursor, the obtained ZnO remain the sphere-shaped structure, illustrating that the morphology of the sample was not destroyed during calcination (Fig. 6c and d). Fig. 6e and f shows the SEM images of the NZ0.5 composite and no significant difference is found compared with pure ZnO, indicating that the NZ0.5 well inherits the microsphere architecture of the pristine one.
O2 (gas) ⇔ O2 (ads)
(1)
O2 (ads) + e− ⇔ O−2 (ads)
(2)
O−2 (ads) + e− ⇔ 2O− (ads)
(3)
O− (ads)
(4)
+
e−
⇔
O2 − (ads)
When the sulfur dioxide gas is injected, the SO2 molecule will react with the O− on the surface of the ZnO to form SO3 and release electrons into conducting band of ZnO, resulting in a reduction in the resistance (Rg) of the sensor. When the SO2 gas is removed from the surface of ZnO, the unstable SO3 molecules capture electrons from the conducting band of the ZnO and decompose into SO2 and O−, and the resistance of ZnO will restore to the initial resistance (Ra) in air. The sensitivity of the NiO/ZnO sensor was studied by analyzing the
Fig. 3. Nitrogen adsorption-desorption isotherms and pore size distribution of (a) ZnO and (b) 0.5 mol.% NiO/ZnO microspheres. 3
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Fig. 4. XPS spectra of 10 mol.% NiO/ZnO: (a) survey, (b) Zn 2p, (c) Ni 2p, and (d) O 1s.
Fig. 5. Schematic illustration of the growth mechanism of the flowerlike NiO–ZnO composite. 4
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Table 1 The response (Ra/Rg) value of ZnO, NZ0.25, NZ0.5, NZ1 sensors to 10 ppm SO2. Temperature (°C)
ZnO(Ra/Rg)
NZ0.25(Ra/Rg)
NZ0.5(Ra/Rg)
NZ1(Ra/Rg)
100 160 200 240
4.5 15 4.68 3.23
5.33 46.15 25 15.34
23.5 107 31.25 10.42
17.9 34.48 33 15.12
the conduction band of ZnO, which causes the depletion region of the ZnO semiconductor increasing. In addition, the catalytic function of NiO facilitates the adsorption of oxygen on the surface of ZnO to form O−, O2−, which reduces the number of free electrons in the conduction band of ZnO. Therefore, the high resistance of the NZ0.5 sensor is obtained. When the sensors are exposed on 10 ppm SO2, the resistances of the ZnO and NZ0.5 sensors are lower than that in the air (Fig. 7c). Typically, the resistance of pure ZnO reduces from 100 to 22 at 100 °C, from 52.5 to 3.5 at 160 °C, from 11 to 2.35 at 200 °C, and from 8.2 to 2.54 at 240 °C, and the relevant response values are 4.5, 15, 4.7 and 3.2, respectively. For the NZ0.5 sensor, the resistance changes from 400 to 17 at 100 °C, from 140 to 1.3 at 160 °C, from 100 to 3.2 at 200 °C, and from 25 to 2.4 at 240 °C, and the response values are 23.5, 107.7, 31.3 and 10.4, respectively. The experimental results clearly demonstrate that the resistance variation of the NZ0.5 sensor in air and sulfur dioxide gas is larger than that of the pure ZnO sensor. Hence, a much improved response on NZ0.5 can be observed. Fig. 8a displays the response curves of the NZ0.5 sensor to SO2 gas with different concentrations at the working temperature of 160 °C. The NZ0.5 sensor shows good response and recovery curves to SO2 with the concentration ranging from 1 to 50 ppm, indicating the sensor can be used to detect ppm-level SO2 gas. In addition, with the injected concentration of sulfur dioxide increasing, the response values of the NZ0.5 sensor increase from 6 to 172. In addition, the response-recovery time of the sensor tends to reduce with the concentration of SO2 gas increasing and reaches a minimum value (701/237s). In order to investigate the relationship between the gas concentration and the response of the sensor, the logarithm of the response value (Lg S) can be described as a function of the logarithm of SO2 concentration (Lg C) by a linear curve. As shown in Fig. 8b, a well fitted linear relationship between gas concentration and the response of the sensor is obtained. The repeatability and stability are both important factors to evaluate the gas-sensing performance. Fig. 8c shows the repeatability of the NiO/ZnO sensor to 10 ppm SO2 at 160 °C. As shown in the curves, the response values of the three response-recovery cycles are maintained at 110. Besides, the identical response-recovery time (701-237 s) of asprepared NiO/ZnO microspheres sensor towards 10 ppm SO2 gas are observed. The results can be concluded that the NiO/ZnO sensor has an superb repeatability for SO2 gas sensing. The long response-recovery time may be caused by uneven pore size distribution, which increases the diffusion resistance in the pores, reduces the gas transport
Fig. 6. FE-SEM image of (a, b) precursor, (c, d) pure ZnO microspheres, and (e, f) NZ0.5 composite at different magnification.
response of the sample to 10 ppm SO2 in different measurement temperatures. Fig. 7a shows the response of ZnO, NZ0.25, NZ0.5, NZ1 to 10 ppm SO2 at different working temperatures of 100 °C, 160 °C, 200 °C and 240 °C. The response of NiO/ZnO and pure ZnO sensors show an ascending tendency from 100 °C to 160 °C. However, when the working temperature exceeds 160 °C, the declined response is observed. It indicates that the sensors exhibit the highest response value to 10 ppm SO2 at the optimized operating temperature of 160 °C. Table 1 shows the response values of ZnO, NZ0.25, NZ0.5, and NZ1 at 160 °C for 10 ppm SO2, which are 15, 46.15, 107, and 34.48 respectively. It reveals that the NiO/ZnO sensor with the NiO concentration of 0.5 mol.% shows the highest response value comparing with ZnO, NZ0.25 and NZ1 sensors at 160 °C. Furthermore, the highest response value of NiO/ ZnO is almost seven times than that of pure ZnO at 160 °C, illustrating that the introduction of NiO significantly enhances the sensitivity of ZnO microspheres to sulfur dioxide. In order to further illustrate the improvement of the sensor response value of NiO/ZnO, the resistance values of the ZnO and NZ0.5 sensor in air and SO2 gas at different operation temperatures were measured. As shown in Fig. 7b, the resistance of two sensors in air maintains a continuous decrease as the temperature increases. Moreover, the NZ0.5 sensor displays the higher resistance than the ZnO sensor. It can be attributed to the formation of p-n heterojunction at the interface of pNiO and n-ZnO caused by the introduction of NiO. The formation of p-n heterojunction results in a decrease in the concentration of electrons in
Fig. 7. (a) The response of ZnO, NZ0.25, NZ0.5 and NZ1 to 10 ppm SO2 at different operating temperatures. The resistance of ZnO and NZ0.5 sensor in air (b) and (c) 10 ppm SO2 gas at different operating temperature. 5
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Fig. 8. (a) The dynamic response curves of the NZ0.5 sensor to different concentrations (ppm) of sulfur dioxide gas at optimal operating temperature, (b) the fitting curve between the logarithm of the response value (Lg S) and the logarithm of SO2 concentration (Lg C), (c) repeatability and (d) stability measurements of the NiO/ ZnO sensor to 10 ppm SO2 at 160 °C, response of NZ0.5 and pure ZnO microspheres to various gases with different concentrations (e) 10 ppm and (f) 50 ppm.
SO2 at 85 °C; D. Zhang et al. [42] reported TiO2/rGO film with a low response of 1.12 at 25 °C, which is lower than NiO–ZnO microspheres (107) to 10 ppm SO2 at 160 °C in our work. The catalytic effect of NiO on sulfur dioxide is an important reason for the improvement of gas sensitivity. Table 2 shows that the gas-sensing material with NiO shows high response value. For example, Q. Zhou et al. reported NiO–ZnO nanodisks presented the higher response value (16.25) than Ru/Al2O3/ ZnO (1.25) to 20 ppm sulfur dioxide [4,5]. The SnO2/NiO thin film displays the higher response value (56) than rGO-SnO2 (22) to 500 ppm sulfur dioxide. Meanwhile, the layer-by-layer self-assembly structure is an important factor to improve the gas-sensing performance [42,43]. The layer gap facilitates the diffusion of gas in the three-dimensional structure, the larger specific surface provides a site for gas adsorption. Therefore, the self-assembled NiO/ZnO microspheres exhibited the highest response towards 10 ppm SO2 at a low temperature of 160 °C in comparison with the previously reported sensors.
efficiency, and affects the gas response/recovery time [10,41]. An endurance response value was measured to investigate the stability of the NiO/ZnO sensor. Fig. 8d exhibits the test result for seven days, and the response values to 10 ppm SO2 at 160 °C maintain at about 100. It indicates that the NiO/ZnO sensor has an admirable stability for SO2 gas sensing. The response values of NZ0.5 and pure ZnO microspheres to various gases were recorded. As shown in Fig. 8e, the pure ZnO sensor exhibits low response values of 15, 1, 2.5, 5.4 and 1.2–10 ppm SO2, NO2, CH4, H2 and NH3 gases, respectively. After the introduction of NiO, the improved response value (107) to 10 ppm SO2 is observed. For other gases such as NO2, CH4, H2 and NH3 gas with the concentration of 10 ppm, the NZ0.5 sensor exhibits negative response values. Although the concentration of tested gases increases to 50 ppm, the higher response value (172) of the NZ0.5 sensor to 50 ppm SO2 is found comparison with that at NO2, CH4, H2 and NH3 gases (Fig. 8f). Therefore, the NZ0.5 sensor exhibits superior selectivity. Table 2 exhibits the proposed SO2 sensor performance compared to previous work. The investigation results show that the material with high working temperature has a high response to sulfur dioxide, but the material with low working temperature has lower response value, for example, X. Ma et al. [6] reported SnO2 nanowires showing the response value of 13.7 to 10 ppm
3.3. Sensing mechanism of NiO/ZnO microspheres The excellent gas-sensing performance of the NZ0.5 sensor to sulfur dioxide is attributed to the formation of p-n heterojunction, the catalytic effect of NiO and the layer-by-layer self-assembly structure. Nickel oxide can react with sulfur dioxide to form unstable sulfur trioxide. Structurally, the porous structure of the NiO/ZnO microspheres provides more channels for gas diffusion, which favors the improvement of the gas-sensing performance of the NZ0.5 sensor. As a n-type semiconductor, the sensing behavior of the ZnO in air and SO2 gas is shown in Fig. 9a and b. In air, the adsorbed oxygen captures electrons from the semiconductor to form surface oxygen anions. Therefore, an electronic depletion layer in the outer layer of the semiconductor is formed. When sulfur dioxide is supplied, the SO2 gas molecules replace the site of chemisorbed oxygen ion to further form complex SO3−. And the unstable SO3− can convert to SO3 meanwhile release electrons into the conduction band of zinc oxide [48], resulting the electronic depletion layer reducing. The sensing reaction process can be depicted by the following equation (5):
Table 2 Comparison of gas sensing characteristic of sensing material to SO2 gas. Sensing material
Concentration (ppm)
SO2 response
Temperature (°C)
Ref.
SnO2 nanowires Ru/Al2O3/ZnO SnO2/NiO SnO2 dodecahedro rGO-SnO2 TiO2/rGO V-doped SnO2 NiO–ZnO nanodisks NiO–ZnO microspheres
10 25 500 20
13.7 1.25 56 10.4
85 180 180 200
[6] [4] [35] [7]
500 5 100 20
22 1.12 3.3 16.25
60 25 350 240
[46] [42] [47] [5]
10
107
160
This work
SO2 + O− → SO−3 → SO3 + e−
(5)
Fig. 9c and d shows the band structure of the NiO/ZnO 6
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Fig. 9. The energy band structure diagram for ZnO (a) in air and (b) in sulfur dioxide; for NiO and ZnO (c) and p-NiO/n-ZnO heterojunctions (d) in vacuum; for pNiO/n-ZnO heterojunctions (e) in air and (f) in sulfur dioxide.
microspheres are exposed to air, a hole accumulation layer and an electronic depletion layer form in the surface region of NiO and ZnO nanoparticles, leading to the formation of a high potential barrier ΔEc at p-n heterojunction (Fig. 9e). Thus, the electrons transport channel is inhibited, the effective cross-sectional area of charge conduction is reduced, and the resistance (Ra) of the NZ0.5 sensor is increased. For MOS composites, the catalytic function of metal oxide nanoparticles is an important factor to improve the gas-sensing performance of the material. Fig. 9f reveals the response mechanism of NZ0.5 microspheres to SO2. When the NiO/ZnO sensor is exposed to SO2, the SO2
semiconductor in vacuum. After the modification of NiO nanoparticles, two metal oxide semiconductor are in intimate contact. Due to the difference in the Fermi levels of n-type ZnO and p-type NiO, the electrons in the higher energy level will transfer to the lower energy level. Therefore, when the n-type ZnO and p-type NiO are contacted with each other, electrons transfer from n-ZnO to p-NiO, meanwhile holes shift in the opposite direction until the Fermi energy level are in equilibrium. As a result, a p-n heterojunction structure is formed at the interface of n-ZnO and p-NiO, the energy band of the interface bends and the energy barrier (Ec) appears (Fig. 9d). When the NiO/ZnO 7
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gas molecules can react with the NiO to form the unstable SO3 due to the catalytic function of NiO nanoparticles, which can be depicted as equation (6). Subsequently, the as-obtained unstable SO3 molecules further combine with the chemisorbed oxygen (O−) on the surface of ZnO and convert into SO2 and O2 (equation (7)). Simultaneously, the electrons transfer from the chemisorbed oxygen to the conduction band of the ZnO and NiO. Therefore, the hole accumulation layer and electronic depletion layer decreasing in the surface region of NiO and ZnO nanoparticles are obtained, and a reduced broken band (ΔEc’) appears on the conduction band (Fig. 9f), resulting in the resistance of the material decreasing. Besides, the spilled SO2 gas that is the by-product of formula (equation (7)) further react with ZnO and release more electrons, resulting in a further decrease in the resistance (Rg) of the sensor, improving the response of the material [35].
NiO + 4SO2 → NiS + 3SO3
(6)
SO3 + O− → SO2 + O2 + e−
(7)
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4. Conclusions In summary, the ZnO and NiO/ZnO microspheres have been synthesized by a one-step hydrothermal method for the detection of toxic sulfur dioxide. Texture characterizations exhibit that the NiO/ZnO microsphere with a diameter of ca. 2 μm show porous structure with specific surface of 20.9 m2 g−1 and big pore size of 25 nm. By taking the advantages of the catalytic functions of nickel oxide (chemical effect), the formation of semiconductor heterojunctions (electronic effect) and the large specific surface area and porous structure (geometric effect) of NiO/ZnO composites, the gas-sensing performance of the sensor to sulfur dioxide significantly enhanced compared with previously reported pure ZnO sensors and other-based sensors. This work indicates that the NiO/ZnO microspheres are promising candidates for fabrication of high-performance SO2 sensors. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (No.51572157), the Science and Technology Development Plan (2014GGX102004), the Qilu Young Scholar Scheme of Shandong University (No.31370088963043) and the Fundamental Research Funds for the Central Universities (2018JC036, 2018JC046). References [1] C. Ma, X. Hao, X. Yang, X. Liang, F. Liu, T. Liu, C. Yang, H. Zhu, G. Lu, Sub-ppb SO2 gas sensor based on NASICON and LaxSm1-xFeO3 sensing electrode, Sens. Actuators B Chem. 256 (2018) 648–655. [2] J. Cao, C. Qin, Y. Wang, B. Zhang, Y. Gong, H. Zhang, G. Sun, H. Bala, Z. Zhang, Calcination method synthesis of SnO2/g-C3N4 composites for a high-performance ethanol gas sensing application, Nanomaterials 7 (2017). [3] C. Wang, L.-J. Wang, L. Zhang, R. Xi, H. Huang, S.-H. Zhang, G.-B. Pan, Electrodeposition of ZnO nanorods onto GaN towards enhanced H2S sensing, J. Alloy. Comp. 790 (2019) 363–369. [4] Y. Liu, X. Xu, Y. Chen, Y. Zhang, X. Gao, P. Xu, X. Li, J. Fang, W. Wen, An integrated micro-chip with Ru/Al2O3/ZnO as sensing material for SO2 detection, Sens. Actuators B Chem. 262 (2018) 26–34. [5] Q. Zhou, W. Zeng, W. Chen, L. Xu, R. Kumar, A. Umar, High sensitive and lowconcentration sulfur dioxide (SO2) gas sensor application of heterostructure NiOZnO nanodisks, Sens. Actuators B Chem. 298 (2019). [6] X. Zhong, Y. Shen, S. Zhao, X. Chen, C. Han, D. Wei, P. Fang, D. Meng, SO2 sensing properties of SnO2 nanowires grown on a novel diatomite-based porous substrate, Ceram. Int. 45 (2019) 2556–2565.
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