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Selective-detection NO at room temperature on porous ZnO nanostructure by solid-state synthesis method
Journal of Colloid and Interface Science 556 (2019) 640–649
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Regular Article
Selective-detection NO at room temperature on porous ZnO nanostructure by solid-state synthesis method Ning Huang a, Yunhao Cheng b, Huiyu Li a, Li Zhao a, Zhongyu He a, Chun Zhao c, Fengmin Liu c,⇑, Lan Ding a,⇑ a b c
College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China Armed Police Unit, 088 Shuguang Street, Bayannaoer 015000, China State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 8 April 2019 Revised 4 July 2019 Accepted 6 July 2019 Available online 8 July 2019 Keywords: Solid-state synthesis Room temperature ZnO Gas sensors Nitric oxide
a b s t r a c t High sensitivity and selectivity detection of NO at room temperature has always been full of challenges. In this work, a kind of porous ZnO with coralline-like nanostructure was prepared by a rapid and simple solid-state synthesis strategy, using zinc acetate and oxalic acid as precursors. Structural analysis and morphological investigations of the ZnO powder showed that it has a large specific surface area (32.75 m2 g1) and many nanometer-sized channels between ZnO nanoparticles. This is beneficial to the adsorption and desorption of NO, which is an important reason for the selective detection of NO by the ZnO powder at room temperature. So based on the ZnO powder, a gas sensor was fabricated and its gas-sensing properties were investigated. It exhibited outstanding response (23.59) and fast response time (331 s) to 40 ppm of NO at room temperature (21 ± 2 °C). As the relative humidity study changed from 17% to 80% at 10 ppm of NO, the sensitivity of the sensor changed little, only decreased from 1.43 to 1.12. The stability study was also carried out. Under the concentration of 5 ppm of NO, the relative standard deviation was 0.33% within 8 days, which indicates that the obtained sensor is suitable for practical application. Ó 2019 Published by Elsevier Inc.
1. Introduction
⇑ Corresponding authors. E-mail addresses: [email protected] (F. Liu), [email protected] (L. Ding). https://doi.org/10.1016/j.jcis.2019.07.013 0021-9797/Ó 2019 Published by Elsevier Inc.
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
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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 [2–4], In2O3 [5,6], WO3 [7–10], 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 [11–13]. 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 [18–20]. 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 abovementioned 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.
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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. 2. Experimental 2.1. Chemicals All chemicals used were of analytical grade and without further purification. Zinc acetate dihydrate (Zn(CH3COO)22H2O) was obtained from Tianjin Yongsheng Fine Chemical Co. Ltd. (Tianjin, China), oxalic acid dihydrate (H2C2O42H2O) 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. 2.2. Solid-state synthesis of coralline-like porous ZnO powder In a representative solid-state synthesis process, 0.005 mol of (ZnCH3COO)22H2O and 0.005 mol of H2C2O42H2O were mixed together in a mortar without adding any other solvent. The mixture was ground for 20–25 min until a white slurry solid formed. The slurry solid was dried at 70 °C for 4 h in a drying oven, and then calcined at 450 °C in a tube furnace. The obtained ZnO powder was stored in a transparent glass bottle filled with nitrogen. 2.3. Characterization The structure and crystallinity of the ZnO powder were characterized by a XRD-6100 spectrometer (Shimadzu, Japan) with Cu Ka1 radiation (k = 0.15406 nm) operating at 40 kV in between 20° and 80°. Surface morphology of the ZnO powder was determined by scanning electron microscopy (SEM, JEOL, Japan) and transmission electron microscope (TEM, JEM-2100, JEOL, Japan), respectively. For the TEM measurement, the ZnO powder was dispersed in absolute ethanol with the help of ultrasound. Then several drops of the mixture were poured on a copper grid for the TEM measurements. Thermogravimetry (TG) analysis was performed by TGA Q500 Thermal Analyzer Instrument (TA, USA), heating rate of 10 °C/min from room temperature to 800 °C in air flow (100 cm3/min). The chemical bonding states of the ZnO was studied by XPS (Thermo Electron, USA). N2 adsorption was performed at 77 K and the specific surface area was analyzed with a Micromeritics Autochem II 2920 analyzer (Micromeritics, USA). Barrett-Joyner-Halenda (BJH) method was used to calculate the specific surface area and the pore size distribution. 2.4. Sensor fabrication Firstly, 0.01 g of the ZnO powder was dispersed into 200 lL of absolute ethanol and fully ground to form a uniform mixture. In order to obtain a gas sensor on the ZnO, 5 lL of the mixture was dropped on a gold interdigital electrode (10 5 mm) for 6 times by a pipette gun. After air-drying at room temperature, the gas sensor was calcined at 300 °C for 2 h (go through N2 for an hour and then close it). Finally, the gas sensor was placed in a transparent glass bottle filled with nitrogen to avoid other gas contamination before use.
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Fig. 1. Schematic diagram of the preparation of the gas sensor and determination of gases.
2.5. Gas sensing measurements In order to calculate the response of the gas sensor to target gases, a digital multimeter (8864A-Fluke) static-test sensing system was coupled to a computer to detect changes in the resistance value of the gas sensor in the presence of air and target gases. The preparation of the gas sensor and determination of gases are shown in Fig. 1. The gases in the study included NO, NO2, SO2, H2S, which were commercially procured in the form of canisters (concentrations of SO2 and NO2 are 500 ppm, concentrations of NO and H2S are 10,000 ppm). During the measurement, each test chamber was vacuumed before injection of the target gas, the chamber volume is 1000 cm3. Different concentrations of each gas in the chamber were acquired with the help of a syringe by injecting a certain amount of each gas accompanying with clean air as a gas distribution. All experiments were performed at room temperature and the relative humidity (RH) was 17%. Recovery of the gas sensor requires introducing clean air into the chamber. The response of the gas sensor was defined by the following formula:
Response ¼ Ra =Rg ðRa > Rg Þ or Response ¼ Rg =Ra ðRg > Ra Þ
ð1Þ
where Ra is the resistance of the gas sensor in air and Rg is that in the target gas. 3. Results and discussion 3.1. 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)22H2O and H2C2O42H2O, which was dried at 70 °C for 4 h, was studied by TGA (Fig. 2). In Fig. 2, the weight loss at
Fig. 2. TGA measurements of precursors ZnC2O42H2O.
low temperature (below 200 °C) was 20.5 wt%, mainly due to the partial removal of hydration water of ZnC2O42H2O and production of ZnC2O4. To verify this conclusion, we used a tube furnace to simulate the heating condition of TGA (10 °C/min) to obtain the product at 200 °C in a N2 atmosphere. From the results of IR and XRD (Fig. 3), it can be seen that the obtained product is ZnC2O4. The weight loss between 200 °C and 420 °C indicates a fast decomposition process of ZnC2O4 into ZnO and CO2 (Fig. 2). Thus, in order to ensure complete decomposition of ZnC2O4, 450 °C was selected as the calcining temperature of ZnC2O4 to prepare the ZnO power. The total weight loss was about 42.11 wt%, which is basically consistent with the theoretical weight loss of ZnC2O42H2O (42.85 wt%). The ZnO powder on different proportions of zinc acetate and oxalic acid (mole ratio of 1:3, 1:2, 1:1 and 2:1) were prepared
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and their gas responses were subsequently studied, the results are shown in Fig. 4. Except for the ZnO powder prepared at the mole ratio of 1:1, the other ZnO powder prepared at mole ratios of 1:3, 1:2 and 2:1, respectively, almost showed very low sensitivity to NO. Therefore, the mole ratio of 1:1 of oxalic acid and zinc acetate,
Fig. 3. IR and XRD of ZnC2O4 at 200 °C.
Fig. 4. Effect of mole ratio of zinc acetate and oxalic acid on response of 50 ppm NO.
Fig. 5. XRD spectra of the coralline-like porous ZnO at 450 °C.
Fig. 6. (a) SEM images, (b) TEM images, (c) HRTEM image of the coralline-like porous ZnO, (d) SEM images after the final calcination step.
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drying at 70 °C for 4 h, calcining at 450 °C for 2 h were chosen for optimal preparation conditions of the ZnO powder in this study. 3.2. Characterization The structural characteristics of the ZnO powder were observed by XRD (Fig. 5). The XRD pattern of the ZnO powder clearly shows several peaks originating from (0 0 2), (1 0 0), (1 0 2), (1 1 0) and (1 0 1) planes of ZnO, which well indexed to the hexagonal wurtzite structure of ZnO (JCPDS No. 80-0075). The intensity of these peaks is very high, which indicates a high crystallinity of the ZnO powder [25]. The morphology and structure of the ZnO powder were further investigated by SEM and TEM. The SEM image in Fig. 6(a) showed that the ZnO powder was coral-like and consisted of ZnO nanoparticles, which had a large surface area volume ratio and roughness. The diameters of the ZnO nanoparticles were mainly distributed in the range of 15 to 45 nm by counting the sizes of 200 particles. The TEM image (Fig. 6(b)) showed that the ZnO powder had many mesoporous and nanometer-sized channels between the ZnO nanoparticles, which were conducive to gas adsorption and desorption in the process of gas sensing. The spacing of the adjacent lattice fringes in Fig. 6(c) is about 0.256 nm, which is equivalent to that of wurtzite ZnO (0 0 2). Fig. 6(d) showed the surface state of the sensor after the final calcination step. The results of SEM images confirmed that the size and morphology of the ZnO nanoparticles on the surface of the sensor were uniform and not different from that of the ZnO powder used to prepare the sensor. Nitrogen adsorption/desorption isotherms and BJH pore diameter measurements of the ZnO powder were shown in Fig. 7. The N2 isotherm could be classified as type IV, and a significant hysteresis loop is observed in the range of 0.8–1.0 p/p0, indicating the presence of mesopores (20–50 nm) in the ZnO powder. The pore size distribution of the ZnO powder exhibited a strong peak centered at 38.25 nm, which indicates that the pore size distribution was uniform in the ZnO powder. The BET surface area of the ZnO powder was 32.75 m2 g1, which was significantly higher than that of the commercial (16.52 m2 g1) and the nanoflower-like ZnO powders (20.56 m2 g1) [19]. The XPS spectrum showed in Fig. 8(a), only Zn, O and C elements were observed, and no other elements were found. The C 1s peak was attributed to hydrocarbons from the XPS instrument [26]. Fig. 8(b) shows that there are two strong peaks at 1021.3 eV and 1044.3 eV, corresponding to Zn 2p3/2 and Zn 2p1/2, respectively. The difference of binding energy between Zn 2p3/2 peak and Zn 2p1/2 peak is 23.00 eV, which indicates that zinc is in the
Fig. 7. Nitrogen adsorption/desorption isotherms and BJH pore size distribution of the coralline-like porous ZnO.
state of +2 oxidation [27,28]. From Fig. 8(c), it can be seen that the shape of O 1s peak of the ZnO powder is asymmetric and can be deconvoluted into three Gauss peaks, with the centers of 530 eV, 531 eV and 532 eV, respectively [29,30]. The peak at 530 eV can be indexed to O2 in the wurtzite structure of ZnO [31]. The peak at 531 eV represents chemisorbed oxygen species under oxygen deficiency conditions [32]. The peak located at 532 eV belongs to the hydroxy species (OH) or chemisorbed and dissociated oxygen species [33–35]. The presence of various oxy 2 gen (O (ads)) endows the ZnO nanomaterial 2 (ads), O (ads), and O with a large number of active sites, which makes them more sensitive to NO at room temperature. 3.3. Gas sensing performance study The responses of the gas sensor to 50 ppm of NO2, NO, SO2 and H2S at room temperature are showed in Fig. 9(a). When the reduc-
Fig. 8. (a) XPS spectrum of the coralline-like porous ZnO, (b) Zn 2p XPS spectra, and (c) High resolution O1s XPS spectra.
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Fig. 9. (a) Selectivity study of the sensor for various gases, (b) Plot of resistance as a function time to 5, 10, 20, 40, 80 and 100 ppm NO with 17% RH at room temperature. (c) Variation of the response of the sensor to various concentration of NO, and (d) A plot of for a response and recovery times of the sensor exposure to 40 ppm NO at 17% RH.
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ing gas (SO2 or H2S) was introduced, the resistance of the sensor decreased. When the oxidizing gas (NO or NO2) was introduced, the resistance of the sensor increased. Fig. 9(a) clearly shows that the gas sensor has the highest response to NO compared to other test gases. Selectivity coefficient (K) of NO to other gases is calculated according to the following formula and the results show in Table 1.
K ¼ Sa =Sb
ð2Þ
where Sa and Sb represent the response to NO and other gases, respectively. Table 1 reveals the response of the gas sensor to NO is 15.7–22.9 times that of other gases with the same concentration, which indicates that the gas sensor is very suitable for the selective detection of NO at room temperature. The resistances of the gas sensor as a function of time for 5, 10, 20, 40, 80, and 100 ppm NO are shown in Fig. 9(b) and the sensitivity values are found to 1.11, 1.43, 8.07, 23.59, 25.01, 27.58, respectively. Fig. 9(c) displays the response behavior of the gas sensor from 5 ppm to 100 ppm of NO, which shows that the response of the sensor increases as the NO concentration increases. The low concentration of NO would tend to result in a lower surface reaction, which results from the low surface coverage of NO gas molecules on the gas sensor, thus causing a lower response. However, the higher the concentration of NO, the more NO covered the surface of the gas sensor and the more the reaction, so the response of the sensor is improved. Amazingly, the sensor for 5 ppm of NO has a remarkable response value of 111%, demonstrating that the sensor has the potential to detect low concentration of NO at room temperature. Fig. 9(d) is a plot of resistance as a function of time for a response time and recovery time of the sensor at 40 ppm NO. The response time (331 s) and recovery time (1258 s) for NO at 40 ppm were satisfactory. The response repeatability study of the sensor at 20 ppm NO is shown in Fig. 10(a). The relative standard deviation of the three consecutive response curves is 2.98%, indicating good repeatability. The sensing performances at different concentrations of NO are shown in Table 2 and the sensitivity, response and recovery time of the sensor increased accordingly with the increased concentration of NO. There have been only a few reports for the detection of NO based on metal oxide at room temperature as revealed in Table 3. RH is also a critical factor affecting the response of the sensors at room temperature (21 ± 2 °C), different RH (17%, 23%, 40%, 65% and 80%) were got by controlling the humidity chamber (Shanghai ESPC Environment Equipment Corporation, China). The sensitivities of the gas sensor to 10 ppm NO as a function of RH are shown in Fig. 10(b). As the humidity increased from 17% to 80%, the sensor sensitivity decreased from 1.43 to 1.12. We suspect that the probably reason is that H2O molecules occupy the surface position of oxygen, and the density of adsorbed oxygen decreases, resulting in decrease in the sensor response [36]. However, in the normal RH range of people’s living environment (40–65%), the sensor sensitivity slightly changed (from 1.46 to 1.20), indicating that the sensor is suitable for practical application. The stability of the sensor was measured at 5 ppm of NO at room temperature and 17% and the RH in Fig. 10(c) shows that the sensor has good stability in 8 days and the relative standard deviation was 0.33%. Table 1 Selectivity coefficient (K) of the coralline-like porous ZnO. Gases
H2S
SO2
NO2
Selectivity coefficient (K)
22.1
15.7
22.9
Fig. 10. (a) Plot of response repeatability of the sensor to 20 ppm NO with 17% RH at room temperature. In order to see the three repetitions more clearly, we chose vertical layout, (b) Sensor response to 10 ppm NO gas in the presence of different relative humidifies (17%, 23%, 40%, 65% and 80%) at room temperature, and (c) Stability study of the sensor towards 5 ppm NO with 17% RH once per day, for 8 days.
3.4. Discussion of gas sensing mechanism ZnO is typical n-type semiconductor material and its gas sensing mechanism is mainly based on a change in the electrical resistance involving target gas molecules adsorption-desorption in the surface of the gas sensor. The suggestive NO gas sensing mechanism based on the energy band diagram was demonstrated in Fig. 11.
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N. Huang et al. / Journal of Colloid and Interface Science 556 (2019) 640–649 Table 2 Comparison of different sensing parameters of the sensor at different concentrations of NO. Sample
Concentration of NO gas (ppm)
Sensitivity
Response time (s)
Recovery time (s)
ZnO
5 10 20 40 80 100
1.11 1.43 8.07 23.59 25.01 27.58
195 219 257 331 268 471
270 862 959 1285 1895 1876
Table 3 A summary of metal oxide based gas sensor to detect NO performance at room temperature. Sensing element
Gas concentration (ppm)
Response
Response time
Recovery time
Method of preparation
Refs.
Ag@plate-WO3 Pt/In2O3-WO3 ZnO nanobelts ZnO nanostructures Flower-like ZnO Haemin-functionalized Al-doped ZnO ZnO nanowires Vertically grown ZnO nanorods Coralline-like porous ZnO
5 0.025 10 40 1000 30 10 0.1 40
1.59 15.2 1.7 500% 20.1 30% 46% 7.4 23.59
600 s 41.6 min 135 s 30 s 22.1 min A few seconds. 600 s within 30 s 331 s
600 s 19.17 min 130 s 1 min 51.7 min 100 s A few seconds. Within 30 s 1285 s
Photo-induced method Impregnation method Carbothermal Carbon thermal reduction Template-free aqueous solution Sol-gel Chemical vapor deposition Hydrothermal method Solid-state synthesis method
[9] [10] [2] [22] [23] [24]
Small particle size, large surface area, defects such as oxygen vacancies are important factors in improving the performance of pure ZnO gas sensing [13]. The ZnO prepared in this work has coralline-like porous nanostructure, large specific surface area and mesopores with uniform pore size distribution, which enables this gas sensitive material to have more adsorption sites, faster adsorption and desorption time for NO. When NO appears, NO molecules take off electrons of the surface of ZnO, resulting in the increase of the resistance of sensor. The response of the sensor for NO is obviously greater than that of H2S, SO2 and NO2. Usually, ZnO has a poor response to H2S and SO2 at room temperature. The main reason is that the H2S and SO2 are mainly oxidized with O and O2– on the surface of the ZnO while the two oxygen species are present at above 100 °C [37–43]. The poor response of NO2 is due to nanometer-sized channels between ZnO nanoparticles, which are formed by the release of carbon dioxide during the solid phase decomposition of zinc oxalate into ZnO and CO2 (Fig. 6a and
[25] In this work
b). It should be noted that NO and CO2 are linear molecules with similar structures, so NO is more likely to diffuse into the channels, ZnO has high response sensitivity and fast response time to NO. In contrast, NO2 molecules are more difficult to diffuse into these channels due to their V-type structure, resulting in low response sensitivity. This can also be proved by the fact that the response of the gas sensor no longer increases significantly with the increase of the concentration of NO at higher concentrations (see Fig. 9c). Since NO molecule easily forms a dimer at a low temperature, the volume is similar to the size of NO2 molecules. Based on the above experimental results, it is speculated that the reaction paths between NO and the ZnO are as follows:
O2 (g) + e ! O2 (ads)
ð3Þ
NO + e ! NO (ads)
ð4Þ
Fig. 11. Schematic of sensing mechanism of the coralline-like porous ZnO.
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2NO (ads) ! N2 (gas) + 2O (ads)
ð5Þ
NO + O2 (ads) + e ! NO2 (ads) + O (ads)
ð6Þ
4. 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 solidphase synthesis method. Zinc acetate and oxalic acid as precursors were not only non-toxic and environmentally friendly, but also avoided the release of harmful gases in the preparation process [35,45]. Meanwhile, when the precursors decomposed, a large amount of CO2 released, which not only endowed the ZnO powder with a large number of mesoporous and large specific surface area, but also formed many nanometer-sized channels between the ZnO nanoparticles. The coralline-like ZnO has a specific surface area of 32.75 m2 g1, which is about 98.2% higher than that of commercial ZnO (16.52 m2 g1) [20]. The response of the coralline-like ZnO to 100 ppm of NO at room temperature was 27.58, which was much higher than 6.9 of the commercial ZnO at same concentration. The response of ZnO nanobelts to 40 ppm of NO was 5 [21] and that of the coralline-like ZnO was 23.59. The results implied that the mesoporous and nanometer-sized channels between ZnO nanoparticles make the ZnO more sensitive and selective to detect NO at room temperature. The developed sensor is expected to be a candidate in the field of environmental monitoring in the future. 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). References [1] L. Kreuzer, C. Patel, Nitric oxide air pollution: detection by optoacoustic spectroscopy, Science 173 (1971) 182–187. [2] M. Kaur, S. Kailasaganapathi, N. Ramgir, N. Datta, S. Kumar, A.K. Debnath, D.K. Aswal, S.K. Gupta, Gas dependent sensing mechanism in ZnO nanobelt sensor, Appl. Surf. Sci. 394 (2017) 258–266. [3] Z. Wen, L. Zhu, Z. Zhang, Z. Ye, Fabrication of gas sensor based on mesoporous rhombus-shaped ZnO rod arrays, Sens. Actuators B 208 (2015) 112–121. [4] C.Y. Lin, J.G. Chen, W.Y. Feng, C.W. Lin, J.W. Huang, J.J. Tunney, K.C. Ho, Using a TiO2/ZnO double-layer film for improving the sensing performance of ZnO based NO gas sensor, Sens. Actuators B 157 (2011) 361–367. [5] M. Ali, Ch.Y. Wang, C.C. Rohlig, V. Cimalla, Th. Stauden, O. Ambacher, NOx sensing properties of In2O3 thin films grown by MOCVD, Sens. Actuators B 129 (2008) 467–472. [6] Ch.Y. Wang, M. Ali, Th. Kups, C.C. Rohlig, V. Cimalla, Th. Stauden, O. Ambacher, NOx sensing properties of In2O3 nanoparticles prepared by metal organic chemical vapor deposition, Sens. Actuators B 130 (2008) 589–593. [7] P.I. Gouma, K. Kalyanasundaram, A selective nanosensing probe for nitric oxide, Appl. Phys. Lett. 93 (2008) 244102. [8] A.A. Tomchenko, V.V. Khatko, I.L. Emelianov, WO3 thick-film gas sensors, Sens. Actuators B 46 (1998) 8–14. [9] D.I. Chen, L. Yin, L.F. Ge, B.B. Fan, R. Zhang, J. Sun, G.S. Shao, Low-temperature and highly selective NO-sensing performance of WO3 nanoplates decorated with silver nanoparticles, Sens. Actuators B 185 (2013) 445–455. [10] B.Y. Chang, C.Y. Wang, H.F. Lai, R.J. Wu, M. Chavali, Evaluation of Pt/In2O3-WO3 nano powder ultra-trace level NO gas sensor, J. Taiwan Inst. Chem. E 45 (2014) 1056–1064. [11] K. Wetchakun, T. Samerjai, N. Tamaekong, C. Liewhiran, C. Siriwong, V. Kruefu, A. Wisitsoraat, A. Tuantranont, S. Phanichphant, Semiconducting metal oxides as sensors for environmentally hazardous gases, Sens. Actuators B 160 (2011) 580–591.
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Regular Article
Selective-detection NO at room temperature on porous ZnO nanostructure by solid-state synthesis method Ning Huang a, Yunhao Cheng b, Huiyu Li a, Li Zhao a, Zhongyu He a, Chun Zhao c, Fengmin Liu c,⇑, Lan Ding a,⇑ a b c
College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China Armed Police Unit, 088 Shuguang Street, Bayannaoer 015000, China State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 8 April 2019 Revised 4 July 2019 Accepted 6 July 2019 Available online 8 July 2019 Keywords: Solid-state synthesis Room temperature ZnO Gas sensors Nitric oxide
a b s t r a c t High sensitivity and selectivity detection of NO at room temperature has always been full of challenges. In this work, a kind of porous ZnO with coralline-like nanostructure was prepared by a rapid and simple solid-state synthesis strategy, using zinc acetate and oxalic acid as precursors. Structural analysis and morphological investigations of the ZnO powder showed that it has a large specific surface area (32.75 m2 g1) and many nanometer-sized channels between ZnO nanoparticles. This is beneficial to the adsorption and desorption of NO, which is an important reason for the selective detection of NO by the ZnO powder at room temperature. So based on the ZnO powder, a gas sensor was fabricated and its gas-sensing properties were investigated. It exhibited outstanding response (23.59) and fast response time (331 s) to 40 ppm of NO at room temperature (21 ± 2 °C). As the relative humidity study changed from 17% to 80% at 10 ppm of NO, the sensitivity of the sensor changed little, only decreased from 1.43 to 1.12. The stability study was also carried out. Under the concentration of 5 ppm of NO, the relative standard deviation was 0.33% within 8 days, which indicates that the obtained sensor is suitable for practical application. Ó 2019 Published by Elsevier Inc.
1. Introduction
⇑ Corresponding authors. E-mail addresses: [email protected] (F. Liu), [email protected] (L. Ding). https://doi.org/10.1016/j.jcis.2019.07.013 0021-9797/Ó 2019 Published by Elsevier Inc.
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
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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 [2–4], In2O3 [5,6], WO3 [7–10], 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 [11–13]. 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 [18–20]. 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 abovementioned 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.
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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. 2. Experimental 2.1. Chemicals All chemicals used were of analytical grade and without further purification. Zinc acetate dihydrate (Zn(CH3COO)22H2O) was obtained from Tianjin Yongsheng Fine Chemical Co. Ltd. (Tianjin, China), oxalic acid dihydrate (H2C2O42H2O) 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. 2.2. Solid-state synthesis of coralline-like porous ZnO powder In a representative solid-state synthesis process, 0.005 mol of (ZnCH3COO)22H2O and 0.005 mol of H2C2O42H2O were mixed together in a mortar without adding any other solvent. The mixture was ground for 20–25 min until a white slurry solid formed. The slurry solid was dried at 70 °C for 4 h in a drying oven, and then calcined at 450 °C in a tube furnace. The obtained ZnO powder was stored in a transparent glass bottle filled with nitrogen. 2.3. Characterization The structure and crystallinity of the ZnO powder were characterized by a XRD-6100 spectrometer (Shimadzu, Japan) with Cu Ka1 radiation (k = 0.15406 nm) operating at 40 kV in between 20° and 80°. Surface morphology of the ZnO powder was determined by scanning electron microscopy (SEM, JEOL, Japan) and transmission electron microscope (TEM, JEM-2100, JEOL, Japan), respectively. For the TEM measurement, the ZnO powder was dispersed in absolute ethanol with the help of ultrasound. Then several drops of the mixture were poured on a copper grid for the TEM measurements. Thermogravimetry (TG) analysis was performed by TGA Q500 Thermal Analyzer Instrument (TA, USA), heating rate of 10 °C/min from room temperature to 800 °C in air flow (100 cm3/min). The chemical bonding states of the ZnO was studied by XPS (Thermo Electron, USA). N2 adsorption was performed at 77 K and the specific surface area was analyzed with a Micromeritics Autochem II 2920 analyzer (Micromeritics, USA). Barrett-Joyner-Halenda (BJH) method was used to calculate the specific surface area and the pore size distribution. 2.4. Sensor fabrication Firstly, 0.01 g of the ZnO powder was dispersed into 200 lL of absolute ethanol and fully ground to form a uniform mixture. In order to obtain a gas sensor on the ZnO, 5 lL of the mixture was dropped on a gold interdigital electrode (10 5 mm) for 6 times by a pipette gun. After air-drying at room temperature, the gas sensor was calcined at 300 °C for 2 h (go through N2 for an hour and then close it). Finally, the gas sensor was placed in a transparent glass bottle filled with nitrogen to avoid other gas contamination before use.
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Fig. 1. Schematic diagram of the preparation of the gas sensor and determination of gases.
2.5. Gas sensing measurements In order to calculate the response of the gas sensor to target gases, a digital multimeter (8864A-Fluke) static-test sensing system was coupled to a computer to detect changes in the resistance value of the gas sensor in the presence of air and target gases. The preparation of the gas sensor and determination of gases are shown in Fig. 1. The gases in the study included NO, NO2, SO2, H2S, which were commercially procured in the form of canisters (concentrations of SO2 and NO2 are 500 ppm, concentrations of NO and H2S are 10,000 ppm). During the measurement, each test chamber was vacuumed before injection of the target gas, the chamber volume is 1000 cm3. Different concentrations of each gas in the chamber were acquired with the help of a syringe by injecting a certain amount of each gas accompanying with clean air as a gas distribution. All experiments were performed at room temperature and the relative humidity (RH) was 17%. Recovery of the gas sensor requires introducing clean air into the chamber. The response of the gas sensor was defined by the following formula:
Response ¼ Ra =Rg ðRa > Rg Þ or Response ¼ Rg =Ra ðRg > Ra Þ
ð1Þ
where Ra is the resistance of the gas sensor in air and Rg is that in the target gas. 3. Results and discussion 3.1. 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)22H2O and H2C2O42H2O, which was dried at 70 °C for 4 h, was studied by TGA (Fig. 2). In Fig. 2, the weight loss at
Fig. 2. TGA measurements of precursors ZnC2O42H2O.
low temperature (below 200 °C) was 20.5 wt%, mainly due to the partial removal of hydration water of ZnC2O42H2O and production of ZnC2O4. To verify this conclusion, we used a tube furnace to simulate the heating condition of TGA (10 °C/min) to obtain the product at 200 °C in a N2 atmosphere. From the results of IR and XRD (Fig. 3), it can be seen that the obtained product is ZnC2O4. The weight loss between 200 °C and 420 °C indicates a fast decomposition process of ZnC2O4 into ZnO and CO2 (Fig. 2). Thus, in order to ensure complete decomposition of ZnC2O4, 450 °C was selected as the calcining temperature of ZnC2O4 to prepare the ZnO power. The total weight loss was about 42.11 wt%, which is basically consistent with the theoretical weight loss of ZnC2O42H2O (42.85 wt%). The ZnO powder on different proportions of zinc acetate and oxalic acid (mole ratio of 1:3, 1:2, 1:1 and 2:1) were prepared
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and their gas responses were subsequently studied, the results are shown in Fig. 4. Except for the ZnO powder prepared at the mole ratio of 1:1, the other ZnO powder prepared at mole ratios of 1:3, 1:2 and 2:1, respectively, almost showed very low sensitivity to NO. Therefore, the mole ratio of 1:1 of oxalic acid and zinc acetate,
Fig. 3. IR and XRD of ZnC2O4 at 200 °C.
Fig. 4. Effect of mole ratio of zinc acetate and oxalic acid on response of 50 ppm NO.
Fig. 5. XRD spectra of the coralline-like porous ZnO at 450 °C.
Fig. 6. (a) SEM images, (b) TEM images, (c) HRTEM image of the coralline-like porous ZnO, (d) SEM images after the final calcination step.
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drying at 70 °C for 4 h, calcining at 450 °C for 2 h were chosen for optimal preparation conditions of the ZnO powder in this study. 3.2. Characterization The structural characteristics of the ZnO powder were observed by XRD (Fig. 5). The XRD pattern of the ZnO powder clearly shows several peaks originating from (0 0 2), (1 0 0), (1 0 2), (1 1 0) and (1 0 1) planes of ZnO, which well indexed to the hexagonal wurtzite structure of ZnO (JCPDS No. 80-0075). The intensity of these peaks is very high, which indicates a high crystallinity of the ZnO powder [25]. The morphology and structure of the ZnO powder were further investigated by SEM and TEM. The SEM image in Fig. 6(a) showed that the ZnO powder was coral-like and consisted of ZnO nanoparticles, which had a large surface area volume ratio and roughness. The diameters of the ZnO nanoparticles were mainly distributed in the range of 15 to 45 nm by counting the sizes of 200 particles. The TEM image (Fig. 6(b)) showed that the ZnO powder had many mesoporous and nanometer-sized channels between the ZnO nanoparticles, which were conducive to gas adsorption and desorption in the process of gas sensing. The spacing of the adjacent lattice fringes in Fig. 6(c) is about 0.256 nm, which is equivalent to that of wurtzite ZnO (0 0 2). Fig. 6(d) showed the surface state of the sensor after the final calcination step. The results of SEM images confirmed that the size and morphology of the ZnO nanoparticles on the surface of the sensor were uniform and not different from that of the ZnO powder used to prepare the sensor. Nitrogen adsorption/desorption isotherms and BJH pore diameter measurements of the ZnO powder were shown in Fig. 7. The N2 isotherm could be classified as type IV, and a significant hysteresis loop is observed in the range of 0.8–1.0 p/p0, indicating the presence of mesopores (20–50 nm) in the ZnO powder. The pore size distribution of the ZnO powder exhibited a strong peak centered at 38.25 nm, which indicates that the pore size distribution was uniform in the ZnO powder. The BET surface area of the ZnO powder was 32.75 m2 g1, which was significantly higher than that of the commercial (16.52 m2 g1) and the nanoflower-like ZnO powders (20.56 m2 g1) [19]. The XPS spectrum showed in Fig. 8(a), only Zn, O and C elements were observed, and no other elements were found. The C 1s peak was attributed to hydrocarbons from the XPS instrument [26]. Fig. 8(b) shows that there are two strong peaks at 1021.3 eV and 1044.3 eV, corresponding to Zn 2p3/2 and Zn 2p1/2, respectively. The difference of binding energy between Zn 2p3/2 peak and Zn 2p1/2 peak is 23.00 eV, which indicates that zinc is in the
Fig. 7. Nitrogen adsorption/desorption isotherms and BJH pore size distribution of the coralline-like porous ZnO.
state of +2 oxidation [27,28]. From Fig. 8(c), it can be seen that the shape of O 1s peak of the ZnO powder is asymmetric and can be deconvoluted into three Gauss peaks, with the centers of 530 eV, 531 eV and 532 eV, respectively [29,30]. The peak at 530 eV can be indexed to O2 in the wurtzite structure of ZnO [31]. The peak at 531 eV represents chemisorbed oxygen species under oxygen deficiency conditions [32]. The peak located at 532 eV belongs to the hydroxy species (OH) or chemisorbed and dissociated oxygen species [33–35]. The presence of various oxy 2 gen (O (ads)) endows the ZnO nanomaterial 2 (ads), O (ads), and O with a large number of active sites, which makes them more sensitive to NO at room temperature. 3.3. Gas sensing performance study The responses of the gas sensor to 50 ppm of NO2, NO, SO2 and H2S at room temperature are showed in Fig. 9(a). When the reduc-
Fig. 8. (a) XPS spectrum of the coralline-like porous ZnO, (b) Zn 2p XPS spectra, and (c) High resolution O1s XPS spectra.
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Fig. 9. (a) Selectivity study of the sensor for various gases, (b) Plot of resistance as a function time to 5, 10, 20, 40, 80 and 100 ppm NO with 17% RH at room temperature. (c) Variation of the response of the sensor to various concentration of NO, and (d) A plot of for a response and recovery times of the sensor exposure to 40 ppm NO at 17% RH.
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ing gas (SO2 or H2S) was introduced, the resistance of the sensor decreased. When the oxidizing gas (NO or NO2) was introduced, the resistance of the sensor increased. Fig. 9(a) clearly shows that the gas sensor has the highest response to NO compared to other test gases. Selectivity coefficient (K) of NO to other gases is calculated according to the following formula and the results show in Table 1.
K ¼ Sa =Sb
ð2Þ
where Sa and Sb represent the response to NO and other gases, respectively. Table 1 reveals the response of the gas sensor to NO is 15.7–22.9 times that of other gases with the same concentration, which indicates that the gas sensor is very suitable for the selective detection of NO at room temperature. The resistances of the gas sensor as a function of time for 5, 10, 20, 40, 80, and 100 ppm NO are shown in Fig. 9(b) and the sensitivity values are found to 1.11, 1.43, 8.07, 23.59, 25.01, 27.58, respectively. Fig. 9(c) displays the response behavior of the gas sensor from 5 ppm to 100 ppm of NO, which shows that the response of the sensor increases as the NO concentration increases. The low concentration of NO would tend to result in a lower surface reaction, which results from the low surface coverage of NO gas molecules on the gas sensor, thus causing a lower response. However, the higher the concentration of NO, the more NO covered the surface of the gas sensor and the more the reaction, so the response of the sensor is improved. Amazingly, the sensor for 5 ppm of NO has a remarkable response value of 111%, demonstrating that the sensor has the potential to detect low concentration of NO at room temperature. Fig. 9(d) is a plot of resistance as a function of time for a response time and recovery time of the sensor at 40 ppm NO. The response time (331 s) and recovery time (1258 s) for NO at 40 ppm were satisfactory. The response repeatability study of the sensor at 20 ppm NO is shown in Fig. 10(a). The relative standard deviation of the three consecutive response curves is 2.98%, indicating good repeatability. The sensing performances at different concentrations of NO are shown in Table 2 and the sensitivity, response and recovery time of the sensor increased accordingly with the increased concentration of NO. There have been only a few reports for the detection of NO based on metal oxide at room temperature as revealed in Table 3. RH is also a critical factor affecting the response of the sensors at room temperature (21 ± 2 °C), different RH (17%, 23%, 40%, 65% and 80%) were got by controlling the humidity chamber (Shanghai ESPC Environment Equipment Corporation, China). The sensitivities of the gas sensor to 10 ppm NO as a function of RH are shown in Fig. 10(b). As the humidity increased from 17% to 80%, the sensor sensitivity decreased from 1.43 to 1.12. We suspect that the probably reason is that H2O molecules occupy the surface position of oxygen, and the density of adsorbed oxygen decreases, resulting in decrease in the sensor response [36]. However, in the normal RH range of people’s living environment (40–65%), the sensor sensitivity slightly changed (from 1.46 to 1.20), indicating that the sensor is suitable for practical application. The stability of the sensor was measured at 5 ppm of NO at room temperature and 17% and the RH in Fig. 10(c) shows that the sensor has good stability in 8 days and the relative standard deviation was 0.33%. Table 1 Selectivity coefficient (K) of the coralline-like porous ZnO. Gases
H2S
SO2
NO2
Selectivity coefficient (K)
22.1
15.7
22.9
Fig. 10. (a) Plot of response repeatability of the sensor to 20 ppm NO with 17% RH at room temperature. In order to see the three repetitions more clearly, we chose vertical layout, (b) Sensor response to 10 ppm NO gas in the presence of different relative humidifies (17%, 23%, 40%, 65% and 80%) at room temperature, and (c) Stability study of the sensor towards 5 ppm NO with 17% RH once per day, for 8 days.
3.4. Discussion of gas sensing mechanism ZnO is typical n-type semiconductor material and its gas sensing mechanism is mainly based on a change in the electrical resistance involving target gas molecules adsorption-desorption in the surface of the gas sensor. The suggestive NO gas sensing mechanism based on the energy band diagram was demonstrated in Fig. 11.
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N. Huang et al. / Journal of Colloid and Interface Science 556 (2019) 640–649 Table 2 Comparison of different sensing parameters of the sensor at different concentrations of NO. Sample
Concentration of NO gas (ppm)
Sensitivity
Response time (s)
Recovery time (s)
ZnO
5 10 20 40 80 100
1.11 1.43 8.07 23.59 25.01 27.58
195 219 257 331 268 471
270 862 959 1285 1895 1876
Table 3 A summary of metal oxide based gas sensor to detect NO performance at room temperature. Sensing element
Gas concentration (ppm)
Response
Response time
Recovery time
Method of preparation
Refs.
Ag@plate-WO3 Pt/In2O3-WO3 ZnO nanobelts ZnO nanostructures Flower-like ZnO Haemin-functionalized Al-doped ZnO ZnO nanowires Vertically grown ZnO nanorods Coralline-like porous ZnO
5 0.025 10 40 1000 30 10 0.1 40
1.59 15.2 1.7 500% 20.1 30% 46% 7.4 23.59
600 s 41.6 min 135 s 30 s 22.1 min A few seconds. 600 s within 30 s 331 s
600 s 19.17 min 130 s 1 min 51.7 min 100 s A few seconds. Within 30 s 1285 s
Photo-induced method Impregnation method Carbothermal Carbon thermal reduction Template-free aqueous solution Sol-gel Chemical vapor deposition Hydrothermal method Solid-state synthesis method
[9] [10] [2] [22] [23] [24]
Small particle size, large surface area, defects such as oxygen vacancies are important factors in improving the performance of pure ZnO gas sensing [13]. The ZnO prepared in this work has coralline-like porous nanostructure, large specific surface area and mesopores with uniform pore size distribution, which enables this gas sensitive material to have more adsorption sites, faster adsorption and desorption time for NO. When NO appears, NO molecules take off electrons of the surface of ZnO, resulting in the increase of the resistance of sensor. The response of the sensor for NO is obviously greater than that of H2S, SO2 and NO2. Usually, ZnO has a poor response to H2S and SO2 at room temperature. The main reason is that the H2S and SO2 are mainly oxidized with O and O2– on the surface of the ZnO while the two oxygen species are present at above 100 °C [37–43]. The poor response of NO2 is due to nanometer-sized channels between ZnO nanoparticles, which are formed by the release of carbon dioxide during the solid phase decomposition of zinc oxalate into ZnO and CO2 (Fig. 6a and
[25] In this work
b). It should be noted that NO and CO2 are linear molecules with similar structures, so NO is more likely to diffuse into the channels, ZnO has high response sensitivity and fast response time to NO. In contrast, NO2 molecules are more difficult to diffuse into these channels due to their V-type structure, resulting in low response sensitivity. This can also be proved by the fact that the response of the gas sensor no longer increases significantly with the increase of the concentration of NO at higher concentrations (see Fig. 9c). Since NO molecule easily forms a dimer at a low temperature, the volume is similar to the size of NO2 molecules. Based on the above experimental results, it is speculated that the reaction paths between NO and the ZnO are as follows:
O2 (g) + e ! O2 (ads)
ð3Þ
NO + e ! NO (ads)
ð4Þ
Fig. 11. Schematic of sensing mechanism of the coralline-like porous ZnO.
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N. Huang et al. / Journal of Colloid and Interface Science 556 (2019) 640–649
2NO (ads) ! N2 (gas) + 2O (ads)
ð5Þ
NO + O2 (ads) + e ! NO2 (ads) + O (ads)
ð6Þ
4. 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 solidphase synthesis method. Zinc acetate and oxalic acid as precursors were not only non-toxic and environmentally friendly, but also avoided the release of harmful gases in the preparation process [35,45]. Meanwhile, when the precursors decomposed, a large amount of CO2 released, which not only endowed the ZnO powder with a large number of mesoporous and large specific surface area, but also formed many nanometer-sized channels between the ZnO nanoparticles. The coralline-like ZnO has a specific surface area of 32.75 m2 g1, which is about 98.2% higher than that of commercial ZnO (16.52 m2 g1) [20]. The response of the coralline-like ZnO to 100 ppm of NO at room temperature was 27.58, which was much higher than 6.9 of the commercial ZnO at same concentration. The response of ZnO nanobelts to 40 ppm of NO was 5 [21] and that of the coralline-like ZnO was 23.59. The results implied that the mesoporous and nanometer-sized channels between ZnO nanoparticles make the ZnO more sensitive and selective to detect NO at room temperature. The developed sensor is expected to be a candidate in the field of environmental monitoring in the future. 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). References [1] L. Kreuzer, C. Patel, Nitric oxide air pollution: detection by optoacoustic spectroscopy, Science 173 (1971) 182–187. [2] M. Kaur, S. Kailasaganapathi, N. Ramgir, N. Datta, S. Kumar, A.K. Debnath, D.K. Aswal, S.K. Gupta, Gas dependent sensing mechanism in ZnO nanobelt sensor, Appl. Surf. Sci. 394 (2017) 258–266. [3] Z. Wen, L. Zhu, Z. Zhang, Z. Ye, Fabrication of gas sensor based on mesoporous rhombus-shaped ZnO rod arrays, Sens. Actuators B 208 (2015) 112–121. [4] C.Y. Lin, J.G. Chen, W.Y. Feng, C.W. Lin, J.W. Huang, J.J. Tunney, K.C. Ho, Using a TiO2/ZnO double-layer film for improving the sensing performance of ZnO based NO gas sensor, Sens. Actuators B 157 (2011) 361–367. [5] M. Ali, Ch.Y. Wang, C.C. Rohlig, V. Cimalla, Th. Stauden, O. Ambacher, NOx sensing properties of In2O3 thin films grown by MOCVD, Sens. Actuators B 129 (2008) 467–472. [6] Ch.Y. Wang, M. Ali, Th. Kups, C.C. Rohlig, V. Cimalla, Th. Stauden, O. Ambacher, NOx sensing properties of In2O3 nanoparticles prepared by metal organic chemical vapor deposition, Sens. Actuators B 130 (2008) 589–593. [7] P.I. Gouma, K. Kalyanasundaram, A selective nanosensing probe for nitric oxide, Appl. Phys. Lett. 93 (2008) 244102. [8] A.A. Tomchenko, V.V. Khatko, I.L. Emelianov, WO3 thick-film gas sensors, Sens. Actuators B 46 (1998) 8–14. [9] D.I. Chen, L. Yin, L.F. Ge, B.B. Fan, R. Zhang, J. Sun, G.S. Shao, Low-temperature and highly selective NO-sensing performance of WO3 nanoplates decorated with silver nanoparticles, Sens. Actuators B 185 (2013) 445–455. [10] B.Y. Chang, C.Y. Wang, H.F. Lai, R.J. Wu, M. Chavali, Evaluation of Pt/In2O3-WO3 nano powder ultra-trace level NO gas sensor, J. Taiwan Inst. Chem. E 45 (2014) 1056–1064. [11] K. Wetchakun, T. Samerjai, N. Tamaekong, C. Liewhiran, C. Siriwong, V. Kruefu, A. Wisitsoraat, A. Tuantranont, S. Phanichphant, Semiconducting metal oxides as sensors for environmentally hazardous gases, Sens. Actuators B 160 (2011) 580–591.
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