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Hydrothermal synthesis of Ag modified ZnO nanorods and their enhanced ethanol-sensing properties
Materials Science in Semiconductor Processing 75 (2018) 327–333
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Hydrothermal synthesis of Ag modified ZnO nanorods and their enhanced ethanol-sensing properties
T
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Ying Weia, Xiaodong Wanga, , Guiyun Yib, Lixing Zhoua, Jianliang Caob, Guang Suna, Zehua Chenb, Hari Balaa, Zhanying Zhanga a b
School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454000 Henan, China College of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo 454000 Henan, China
A R T I C L E I N F O
A B S T R A C T
Keywords: ZnO Composites Chemical preparation Sensors
Silver (Ag) nanoparticles decorated zinc oxide (ZnO) nanorods were synthesized by a simple hydrothermal treatment for enhancing gas-sensing performance toward ethanol. The X-ray diffraction (XRD) and energy dispersive X-ray spectroscopic (EDS) results indicate the presence of Ag nanoparticles. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results reveal that Ag nanoparticles are tightly anchored on the surface of ZnO nanorods. The UV–vis diffuse reflectance spectroscopy (UV–vis DRS) results confirm that the band gap energy is decreased due to the decoration of Ag. The gas sensing performance of assynthesized materials was investigated at different working temperatures and toward various ethanol concentrations. The gas-sensing results exhibit that the 1 wt% Ag/ZnO sensor shows the highest sensitivity of 389.6 toward 800 ppm ethanol and fast response among all the sensors. The improved response of Ag/ZnO sensors is ascribed to the catalysis and spillover effect of Ag. This result makes Ag/ZnO sensor a very promising gas sensing material for gas detection.
1. Introduction Gas sensors have attracted much attention for detecting explosive and toxic gases in our environment. Metal oxide semiconductors having irreplaceable advantages of low cost, small size, high sensitivity and facile integration are widely used for detection of gases [1,2]. Amongst the large number of metal oxides, zinc oxide (ZnO), an n-type semiconductor oxide, has been reported as a preferable candidate for gas sensing applications due to its wide band gap (3.37 eV), high electron mobility, ideal chemical and thermal stability [3–6]. However, the further applications of metal oxide based sensors are limited by the high working temperature, low sensitivity and poor selectivity. In order to overcome these existing shortages and to improve sensing characteristics, further work should be performed [7,8]. ZnO nanostructures have been prepared at different morphologies, such as nanosheets, nanoflowers, microwires, nanotubes and nanocones. Similar to the sensing behavior of other metal oxides, different nanostructures exhibit diverse sensing characteristics [9,10]. In view of relatively low response and poor selectivity of pure ZnO based gas sensor, there exists urgent need to improve its sensing characteristics [11]. Hence, some studies have been reported on the improvement of gas sensing performance of ZnO. For example, Drobek [12] et al. reported the preparation of ZIF-8
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molecular sieve membrane encapsulated ZnO nanowire (ZnO@ZIF-8 NWs), which shows the better selectivity at 300 °C for H2 than ZnO nanowires sensor. Viter [13] et al. reported the fabrication 1D ZnO/ polyacrylonitrile (PAN) nanostructures by the combination of electrospinning and atomic layer deposition (ALD) for optical sensor detection. Recently, the addition of noble metal to semiconducting oxide has been applied as a significant and effective technique for the enhancement of gas sensing performance [1,14]. It has been reported that the introduction of noble metal particles on metal oxide surface is identified as an efficient strategy to enhance gas sensitivity, which can influence the material's electronic and chemical distribution beneficial to the adsorption of oxygen species [15]. Ag nanoparticles have been reported to be a helpful catalyst to accelerate the reaction between oxygen species and target gas and greatly enhance their gas-sensing properties [16]. Meng et al. synthesized Ag-decorated ZnO nanosheets via a solvent reduction method, and the as-prepared products show a higher sensitivity toward ethanol with a detection limit of 1 ppb [17]. Cui et al. prepared Ag-ZnO nanorods through a two-step process and the Ag-ZnO sensors show dramatically high response to HCHO in UV light photoelectric at RT [18]. Chen et al. prepared Agdoped ZnO nanostructures by the hydrothermal process, and the 1 wt% Ag-doped ZnO nanostructures based sensor displays better gas-sensing
Corresponding author. E-mail address: [email protected] (X. Wang).
https://doi.org/10.1016/j.mssp.2017.11.007 Received 4 August 2017; Received in revised form 14 September 2017; Accepted 4 November 2017 Available online 22 November 2017 1369-8001/ © 2017 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 75 (2018) 327–333
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property to 10 ppm ethanol at 260 °C [19]. Nonetheless, it is difficult to control the size and distribution of Ag particles on the surface of ZnO nanostructures. Thus, it is still necessary for researchers to find an effective method to synthesize ZnO/Ag composite with high sensitivity. Herein, we synthesized pure and Ag nanoparticles decorated ZnO nanorods by the facile hydrothermal route, and fabricated an ethanol sensor by coating the sensing material onto the ceramic substrate. The gas-sensing properties of all products to ethanol were investigated at different temperatures. As a result, the sensing response is improved markedly by the decoration of Ag on ZnO nanorods. Especially, the sensor of 1 wt% Ag nanoparticles decorated ZnO nanostructures exhibits a high sensitivity and selectivity toward ethanol. Finally, the enhanced gas sensing mechanism of Ag/ZnO nanorods was discussed.
2. Experimental 2.1. Materials
Fig. 2. The XRD patterns of pure ZnO and Ag/ZnO nanorods.
All chemicals were the analytical-grade reagents and used without further treatment. Zinc nitrate, silver nitrate and sodium hydroxide were purchased from Sinopharm Chemical Reagent Co. Ltd. (China), distilled water was made in our laboratory.
2.4. Measurement of gas sensing performance The gas sensing properties of ZnO and Ag decorated Ag/ZnO nanorods were investigated by the sensor measuring system of CGS-4TPS (Beijing Eklite Tech Co., Ltd, China). For fabricating sensors, the synthesized products were mixed with ethanol to prepare a paste that painted onto the ceramic substrate (7 mm width, 13.4 mm length). After aged at 60 °C for 8 h, the fabricated sensors were fixed on the temperature controller plate. In the process of measurement, the fabricated sensors were adjusted to a suitable temperature by the temperature controller. After the resistances of sensors reach a stable state, a certain amount of ethanol was injected into test chamber to get the response curve. After the response reaches a steady value, the ceramic substrate was in air again by releasing the test gas. The gas response is estimated as Ra/Rg ratio, where Ra is the electrical resistance in air and Rg is the electrical resistance in test gas, respectively.
2.2. Synthesis of ZnO and Ag decorated ZnO (Ag/ZnO) nanorods The samples were prepared through the hydrothermal route, and the synthesis illustration of Ag/ZnO nanorods is displayed in Fig. 1. The preparation process is briefly described as follows: The Zn(NO3)2·6H2O (4.16 g) was dissolved into 320 mL of DI water with stirring for 30 min. Subsequently, 40 mL of NaOH solution (4 mol/L) and certain amount of AgNO3 (0 wt%, 0.5 wt%, 1.0 wt% and 2.0 wt%) was added into the above mixture solution under continuous stirring for 10 min to obtain the reaction solution system. The uniform solution was transferred into the Teflon-lined autoclave and heated at 180 °C for 12 h under autogenous pressure. Then, the final product was centrifuged and washed with deionized water for several times, and dried in oven at 65 °C. At last, the prepared samples were calcined at 400 °C for 2 h in the muffle furnace.
3. Results and discussions 3.1. Crystal structure and morphology Fig. 2 shows the XRD patterns of ZnO and Ag/ZnO nanorods. The diffraction peaks at 31.92°, 34.58°, 36.42°, 47.71°, 56.74°, 63.02°, 68.08°, and 69.24° can be unambiguously indexed to (100), (002), (101), (102), (110), (103), (112) and (201) planes of ZnO with hexagonal phase (JCPDS Card No.36-1451), respectively. The sharp diffraction peaks indicate that the ZnO materials are highly crystalline. As expected, the Ag/ZnO nanorods show obvious characteristic diffraction peaks at 38.20° 44.38° and 64.54° which can be corresponding to (111), (200) and (220) planes of cubic phase Ag (JCPDS Card No.04-0783), respectively. The Ag peak intensity becomes stronger with the increase of Ag concentration, which indicates the presence of Ag in obtained composites. The morphological investigation of resulting samples was performed by SEM, TEM and HRTEM. Fig. 3(a) reveals that pure ZnO has a radial structure assembled with nanorods. The average width and length of the ZnO nanorods is 200 and 900 nm, respectively. Fig. 3(b) shows the SEM images of Ag/ZnO nanorods. We can see clearly that the Ag nanoparticles with diameters ranging from 50 to 100 nm are firmly deposited on the surface of ZnO nanorods, and these Ag particles have a slight agglomeration. Fig. 3(c) displays the EDS spectrum of Ag/ZnO nanorods, indicating the existence of Ag element. According to XRD and EDS analyses, it can be concluded that Ag exists as a pure metal phase in Ag/ZnO composite. To further verify the above inference, the TEM analysis was performed. Fig. 4(a) further reveals the radial structure of pure ZnO
2.3. Characterizations of the samples The crystalline phase structure of the as-synthesized products was characterized via XRD using a Bruker-AXS diffractometer (D8 advance) with Cu Kα radiation. The morphologies of samples were investigated using FESEM (Quanta 250 FEG) and TEM (JEOL JEM-2100). Elemental composition of powders was studied by EDS that was connect to FESEM (Quanta 250 FEG). UV–Vis diffuse reflectance spectra (UV–Vis DRS) was performed for products by a UV–Vis spectrometer (Beijing Puxi GI Co., Ltd, TU-1900).
Fig. 1. The synthesis illustration of Ag/ZnO nanorods.
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Fig. 3. SEM images of (a) ZnO and (b) Ag/ZnO nanorods, (c) EDS spectrum of Ag/ZnO nanorods.
nanorods. Fig. 4(b, c) presents the TEM images of Ag/ZnO nanorods. As we can observe obviously, the Ag nanoparticles are attached tightly to the surface of ZnO. Fig. 4(d) presents the HRTEM image of Ag/ZnO nanorods, which suggests that the lattice fringes in Ag nanoparticles with a lattice spacing of 0.201 nm can be consistent with that of (200) crystal plane of cubic Ag. In addition, the lattice fringes of 0.262 nm can be assigned to (002) crystal plane of hexagonal ZnO. The HRTEM image analysis is done on a basis of corresponding XRD data, which reveals that Ag particles are tightly anchored onto ZnO. 3.2. UV–Vis DRS spectra UV–Vis diffuse reflectance spectra of ZnO and Ag/ZnO nanorods are displayed in Fig. 5. As we can see that ZnO and Ag/ZnO show a strong optical absorption below 390 nm in the ultraviolet light region, which is ascribed to the ZnO band gap [20]. Compared to ZnO, the Ag/ZnO nanorods displays a red shift, and the red shift is observed due to the strong interfacial coupling between ZnO and Ag, indicating that the band gap energy of Ag/ ZnO is lower than that of ZnO [21]. The optical band gap values of ZnO and Ag/ZnO samples were estimated by the (αhν)2 versus photon energy (hν) plot, as displayed in inset of Fig. 5. The band gaps of ZnO and Ag/ZnO were estimated to be 3.157 and 3.112 eV, respectively, which confirms that the band gap is narrowed by introducing Ag onto ZnO [22].
Fig. 5. The UV–visible diffuse reflectance spectra and band gap (inset) of ZnO and Ag/ ZnO nanorods.
Fig. 4. TEM images of (a) ZnO and (b, c) Ag/ZnO nanorods, HRTEM images of (d) Ag/ZnO nanorods.
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samples increase rapidly by injecting ethanol into the chamber and recover quickly to the baseline after introducing clean air. Obviously, the Ag concentration in ZnO nanorods strongly influences the responses of sensors. By comparing the response of the sensors, it can be found that the 1 wt% Ag/ZnO sensor shows the maximum sensitivity and fast response among all the sensors. Therefore, the 1 wt% Ag/ZnO sensor was selected to accomplish the subsequent research for ethanol sensing performance. The enhancement in gas response of 1 wt% Ag/ZnO sensor is attributed to appropriate Ag concentration which increases the chemical adsorption and promotes the reaction on the ZnO surface. The Ag nanoparticles are used as a catalytic agent to strengthen the reaction between Ag/ZnO and ethanol, and create additional active sites on the Ag/ZnO surface [15,17]. The transient response of 1 wt% Ag/ZnO sensor toward 100 ppm ethanol at 360 °C is shown in Fig. 7(b). Apparently, the response increases sharply via exposing the Ag/ZnO sensor to ethanol and recovers reach a saturation state. The response and recovery time of Ag/ZnO sensor to 100 ppm ethanol at 360 °C is about 50 s and 28 s, respectively. It is shown that Ag/ZnO sensor possesses a relatively short response and recovery time, which possibly ascribes to the spillover effect of Ag [23]. The relationship of sensor response to ethanol concentration for the fabricated sensor devices at 360 °C is shown in Fig. 8. We can see clearly that the responses of pure ZnO and the 1 wt% Ag/ZnO sensor increase with the ethanol concentration in the range of 50–800 ppm. Initially, within 50–500 ppm gas concentration the increase of the responses to ethanol is faster, while over 500 ppm the increase tends to saturation. The 1 wt% Ag/ZnO sample exhibits a higher response than pure ZnO, and its response is 21.5, 36.5, 81.7, 122.5, 332.8 and 389.6 to 50, 100, 200, 300, 500 and 800 ppm of ethanol, respectively, which indicates that the appropriate decoration of Ag can enhance the gas-sensing mechanism. This result might be attributed to the catalysis of Ag, which can accelerate the reactions between reducing gas and ZnO nanorods [10,24]. Fig. 9 shows the transient response-recovery curves of the ZnO and 1 wt% Ag/ZnO based sensors at 360 °C to different ethanol concentrations. It is observed that the responses of the ZnO and 1 wt% Ag/ZnO based sensors raise quickly with the increasing of ethanol concentration, and the 1 wt% Ag/ZnO nanorods indicates a higher sensitivity than pure ZnO to the same concentration of ethanol. When releasing the ethanol, the response can fully recovered to its primal value, indicating that the sensor is recoverable. The selectivity to test gas is a key factor of gas sensors. Thus, to measure the selectivity of Ag/ZnO nanorods, the responses of the 1 wt% Ag/ZnO based sensor to 100 ppm various tested gases namely acetone, toluene, formaldehyde and ethanol have been investigated at 360 °C as shown in Fig. 10. Evidently, 1 wt% Ag/ZnO based sensor shows
Fig. 6. Response of the sensors towards 300 ppm ethanol gas at different temperature.
3.3. Gas sensing properties The operating temperature plays an important factor in the sensitivity of gas-sensing materials. Therefore, to investigate the gas sensing properties of products, determining the optimum working temperature of a sensor is important. The responses of pure ZnO nanorods and Ag/ ZnO based sensors toward 300 ppm ethanol were investigated at different elevated temperature within a range of 300–400 °C. Fig. 6 displays the responses of the sensors toward 300 ppm ethanol at various working temperatures. For the ZnO, the response increases with the increase of temperature until gets its maximum at 400 °C. However, the responses of Ag/ZnO nanorods based sensors increase firstly with the increase of operating temperature till 360 °C and then gradually decrease. It is found that the optimum operating temperatures of Ag/ZnO based sensors are lower than the pure ZnO sensor. Meanwhile, we can notice that the responses of pure ZnO nanorods, 0.5 wt% Ag/ZnO nanorods, 1 wt% Ag/ZnO nanorods and 2 wt% Ag/ZnO nanorods are 88.95, 49.01, 122.47, and 87.53 toward 300 ppm ethanol under optimal working temperature, respectively. Additionally, the 1 wt% Ag/ ZnO sensor shows the highest response among all sensors. This result indicates that the 1 wt% Ag/ZnO sensor is very helpful to detect ethanol. To demonstrate the sensing behaviors of Ag/ZnO nanorods, all sensor samples were investigated to make a comparison. Fig. 7(a) displays the sensing transients of all gas sensors towards 300 ppm concentration of ethanol at 360 °C. It is observed that the responses of all
Fig. 7. (a) The sensing transients of all sensors towards 300 ppm ethanol gas at 360 °C, (b) The transient response of the 1 wt% Ag/ZnO sensor toward 100 ppm ethanol at 360 °C.
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Table 1 Comparison of ethanol-sensing performances of the present work with previous studies. Sensing material
Concentration
Operating temperature
Response (Ra/Rg)
References
AgFeO2 nanoparticles flower-like PdOZnO NiO/ZnO nanoplates Al-doped ZnO
100 ppm
180 °C
30.2
100 ppm
320 °C
35.4
100 ppm
400 °C
2.88
1000 ppm
290 °C
108
Ag/ZnO nanorods
100 ppm
360 °C
36.52
Wang et al. [25] Lou et al. [26] Deng et al. [27] Yang et al. [28] This paper
Fig. 8. Response of the sensors based on pure ZnO and the 1 wt% Ag/ZnO towards different concentration of ethanol gas at 360 °C.
Fig. 11. The stability of the 1 wt% Ag/ZnO exposed to 50, 100 and 200 ppm ethanol.
Ag/ZnO sensor shows relative higher response to ethanol. Hence, the 1 wt% Ag/ZnO sample is a hopeful candidate for efficient and highly sensitive in ethanol detection. The stability is also an important parameter of gas sensors. To investigate the stability of the 1 wt% Ag/ZnO sensor, the responses to various concentrations of ethanol gas were tested for 15 days. Fig. 11 displays the stability of the 1 wt% Ag/ZnO sensor to 50, 100, and 200 ppm of ethanol gas. The responses were measured for over 15 days with 3 days as interval at 360 °C. It is observed that all the responses of 1 wt% Ag/ZnO sensor to 50, 100, and 200 ppm of ethanol gas show little variation within 30 days, which suggests that the 1 wt% Ag/ZnO sensor has a satisfying stability to ethanol.
Fig. 9. The transient response-recovery curves of the sensors based on pure ZnO and the 1 wt% Ag/ZnO towards different concentration of ethanol gas at 360 °C.
3.4. Ethanol sensing mechanism ZnO as a typical n-type semiconductor, its gas sensing mechanism is considered to be a surface-controlled type [7,29]. There will be some holes on ZnO surface due to the particular nature of semiconductor, and these holes can absorb the oxygen molecules from the air. Then the adsorbed oxygen forms O2-, O- and O2- through trapping electrons from the conductive band of ZnO, resulting in a depletion layer on the surface [30–32]. For this reason, the carrier concentration and electrical conductivity of ZnO are decreased. However, once the ZnO exposes to the reducing gas (ethanol), there will be a reaction between ethanol and adsorbed oxygen species on ZnO surface, liberating electrons back to the conductive band, causing a decreased resistance [33,34]. The enhanced sensing performance of Ag/ZnO sensor is predominantly ascribe to catalysis of Ag nanoparticles [35,36]. Fig. 12 represents the schematic diagram sensing mechanism for Ag/ZnO nanorods. The introduction of Ag will provide more adsorption sites to facilitate the interaction between ethanol and Ag/ZnO nanorods through the spillover effect [23,37]. The addition of Ag nanoparticles can make more electrons be extracted and the thickness of electron
Fig. 10. Response of 1 wt% Ag/ZnO based sensor towards 100 ppm various test gases at 360 °C.
responses to all the examined gases, especially ethanol and acetone. Moreover, the 1 wt% Ag/ZnO based sensor displays a relative higher response of 36.52 to 100 ppm ethanol, which indicates that it has exceptional selectivity toward ethanol. Meanwhile, for better understanding the ethanol-sensing properties of the 1 wt% Ag/ZnO sensor, the response of the sensor was compared with previous work. Table 1 presents the comparison of ethanol-sensing performances of the present work with previous studies [25–28]. We can clearly see that the 1 wt% 331
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Fig. 12. Schematic diagram sensing mechanism for Ag/ZnO nanorods.
depletion layer will be increased. Compared with the pure ZnO, more reactive sites were generated in the Ag/ZnO nanorods, which can increase adsorption and enhance dissociation of adsorbed oxygen molecules on Ag surface into oxygen ion species, and these oxygen ion species will spill over to the surface of ZnO [38,39]. When the Ag/ZnO sensors are exposed toward ethanol, the ethanol molecules on the Ag surface will react with more adsorbed oxygen ions, and more electrons will be released into the conduction band, which leads to a thicker electron depletion layer and a larger variation in resistivity of Ag/ZnO nanorods [40]. The resistance variation of Ag/ZnO in target gas is much larger than that of pure ZnO, which causes the remarkable enhancement of the sensor response. Actually, Ag nanoparticles can catalyze the dissociation of oxygen molecules and ethanol molecules on its surface, which accelerates the redox reaction during the sensor working. Therefore, the Ag/ZnO sensor exhibits a high response and recovery rate [18,41].
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4. Conclusion Undecorated and Ag decorated ZnO nanorods were prepared through the hydrothermal treatment, and the characterization displays that Ag nanoparticles are tightly anchored onto ZnO. The Ag content is modulated through adjusting the concentration of Ag precursor. The gas-sensing performance of ZnO and Ag decorated ZnO nanorods was studied and the results demonstrate that the 1 wt% Ag/ZnO sensor shows the highest sensitivity and fast response among all the sensors. The transient response of 1 wt% Ag/ZnO sensor was investigated at 360 °C with the ethanol concentration in the range of 50–800 ppm, and the response can fully recover to its starting value when the ethanol is released, indicating a good recovery of 1 wt% Ag/ZnO sensor. The enhanced sensing performance of Ag/ZnO is ascribed to the catalysis and spillover effect of Ag, which makes Ag/ZnO nanorods a very promising gas sensing material for the detection of ethanol. Acknowledgements This work is supported by the National Natural Science Foundation of China (51404097, 61474038, U1404613) and Key Scientific Research Projects of Henan Higher Education (16A150009). References [1] N. Tamaekong, C. Liewhiran, A. Wisitsoraat, S. Phanichphant, Acetylene sensor based on Pt/ZnO thick films as prepared by flame spray pyrolysis, Sens. Actuators B: Chem. 152 (2) (2011) 155–161. [2] C. Liu, B.Q. Wang, T. Liu, P. Sun, Y. Gao, F.M. Liu, G.Y. Lu, Facile synthesis and gas sensing properties of the flower-like NiO-decorated ZnO microstructures, Sens. Actuators B: Chem. 235 (2016) 294–301. [3] A.J. Kulandaisamy, J.R. Reddy, P. Srinivasan, K.J. Babu, G.K. Mani, P. Shankar, J.B.B. Rayappan, Room temperature ammonia sensing properties of ZnO thin films grown by spray pyrolysis: effect of Mg doping, J. Alloy. Compd. 688 (2016) 422–429. [4] Z.H. Ibupoto, N. Jamal, K. Khun, X. Liu, M. Willander, A potentiometric immunosensor based on silver nanoparticles decorated ZnO nanotubes, for the selective detection of d-dimer, Sens. Actuators B: Chem. 182 (3) (2013) 104–111. [5] Z.H. Ibupoto, K. Khun, M. Eriksson, M. Alsalhi, M. Atif, A. Ansari, M. Willander, Hydrothermal growth of vertically aligned ZnO nanorods using a biocomposite seed layer of ZnO nanoparticles, Materials 6 (8) (2013) 3584–3597.
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Hydrothermal synthesis of Ag modified ZnO nanorods and their enhanced ethanol-sensing properties
T
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Ying Weia, Xiaodong Wanga, , Guiyun Yib, Lixing Zhoua, Jianliang Caob, Guang Suna, Zehua Chenb, Hari Balaa, Zhanying Zhanga a b
School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454000 Henan, China College of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo 454000 Henan, China
A R T I C L E I N F O
A B S T R A C T
Keywords: ZnO Composites Chemical preparation Sensors
Silver (Ag) nanoparticles decorated zinc oxide (ZnO) nanorods were synthesized by a simple hydrothermal treatment for enhancing gas-sensing performance toward ethanol. The X-ray diffraction (XRD) and energy dispersive X-ray spectroscopic (EDS) results indicate the presence of Ag nanoparticles. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results reveal that Ag nanoparticles are tightly anchored on the surface of ZnO nanorods. The UV–vis diffuse reflectance spectroscopy (UV–vis DRS) results confirm that the band gap energy is decreased due to the decoration of Ag. The gas sensing performance of assynthesized materials was investigated at different working temperatures and toward various ethanol concentrations. The gas-sensing results exhibit that the 1 wt% Ag/ZnO sensor shows the highest sensitivity of 389.6 toward 800 ppm ethanol and fast response among all the sensors. The improved response of Ag/ZnO sensors is ascribed to the catalysis and spillover effect of Ag. This result makes Ag/ZnO sensor a very promising gas sensing material for gas detection.
1. Introduction Gas sensors have attracted much attention for detecting explosive and toxic gases in our environment. Metal oxide semiconductors having irreplaceable advantages of low cost, small size, high sensitivity and facile integration are widely used for detection of gases [1,2]. Amongst the large number of metal oxides, zinc oxide (ZnO), an n-type semiconductor oxide, has been reported as a preferable candidate for gas sensing applications due to its wide band gap (3.37 eV), high electron mobility, ideal chemical and thermal stability [3–6]. However, the further applications of metal oxide based sensors are limited by the high working temperature, low sensitivity and poor selectivity. In order to overcome these existing shortages and to improve sensing characteristics, further work should be performed [7,8]. ZnO nanostructures have been prepared at different morphologies, such as nanosheets, nanoflowers, microwires, nanotubes and nanocones. Similar to the sensing behavior of other metal oxides, different nanostructures exhibit diverse sensing characteristics [9,10]. In view of relatively low response and poor selectivity of pure ZnO based gas sensor, there exists urgent need to improve its sensing characteristics [11]. Hence, some studies have been reported on the improvement of gas sensing performance of ZnO. For example, Drobek [12] et al. reported the preparation of ZIF-8
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molecular sieve membrane encapsulated ZnO nanowire (ZnO@ZIF-8 NWs), which shows the better selectivity at 300 °C for H2 than ZnO nanowires sensor. Viter [13] et al. reported the fabrication 1D ZnO/ polyacrylonitrile (PAN) nanostructures by the combination of electrospinning and atomic layer deposition (ALD) for optical sensor detection. Recently, the addition of noble metal to semiconducting oxide has been applied as a significant and effective technique for the enhancement of gas sensing performance [1,14]. It has been reported that the introduction of noble metal particles on metal oxide surface is identified as an efficient strategy to enhance gas sensitivity, which can influence the material's electronic and chemical distribution beneficial to the adsorption of oxygen species [15]. Ag nanoparticles have been reported to be a helpful catalyst to accelerate the reaction between oxygen species and target gas and greatly enhance their gas-sensing properties [16]. Meng et al. synthesized Ag-decorated ZnO nanosheets via a solvent reduction method, and the as-prepared products show a higher sensitivity toward ethanol with a detection limit of 1 ppb [17]. Cui et al. prepared Ag-ZnO nanorods through a two-step process and the Ag-ZnO sensors show dramatically high response to HCHO in UV light photoelectric at RT [18]. Chen et al. prepared Agdoped ZnO nanostructures by the hydrothermal process, and the 1 wt% Ag-doped ZnO nanostructures based sensor displays better gas-sensing
Corresponding author. E-mail address: [email protected] (X. Wang).
https://doi.org/10.1016/j.mssp.2017.11.007 Received 4 August 2017; Received in revised form 14 September 2017; Accepted 4 November 2017 Available online 22 November 2017 1369-8001/ © 2017 Elsevier Ltd. All rights reserved.
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property to 10 ppm ethanol at 260 °C [19]. Nonetheless, it is difficult to control the size and distribution of Ag particles on the surface of ZnO nanostructures. Thus, it is still necessary for researchers to find an effective method to synthesize ZnO/Ag composite with high sensitivity. Herein, we synthesized pure and Ag nanoparticles decorated ZnO nanorods by the facile hydrothermal route, and fabricated an ethanol sensor by coating the sensing material onto the ceramic substrate. The gas-sensing properties of all products to ethanol were investigated at different temperatures. As a result, the sensing response is improved markedly by the decoration of Ag on ZnO nanorods. Especially, the sensor of 1 wt% Ag nanoparticles decorated ZnO nanostructures exhibits a high sensitivity and selectivity toward ethanol. Finally, the enhanced gas sensing mechanism of Ag/ZnO nanorods was discussed.
2. Experimental 2.1. Materials
Fig. 2. The XRD patterns of pure ZnO and Ag/ZnO nanorods.
All chemicals were the analytical-grade reagents and used without further treatment. Zinc nitrate, silver nitrate and sodium hydroxide were purchased from Sinopharm Chemical Reagent Co. Ltd. (China), distilled water was made in our laboratory.
2.4. Measurement of gas sensing performance The gas sensing properties of ZnO and Ag decorated Ag/ZnO nanorods were investigated by the sensor measuring system of CGS-4TPS (Beijing Eklite Tech Co., Ltd, China). For fabricating sensors, the synthesized products were mixed with ethanol to prepare a paste that painted onto the ceramic substrate (7 mm width, 13.4 mm length). After aged at 60 °C for 8 h, the fabricated sensors were fixed on the temperature controller plate. In the process of measurement, the fabricated sensors were adjusted to a suitable temperature by the temperature controller. After the resistances of sensors reach a stable state, a certain amount of ethanol was injected into test chamber to get the response curve. After the response reaches a steady value, the ceramic substrate was in air again by releasing the test gas. The gas response is estimated as Ra/Rg ratio, where Ra is the electrical resistance in air and Rg is the electrical resistance in test gas, respectively.
2.2. Synthesis of ZnO and Ag decorated ZnO (Ag/ZnO) nanorods The samples were prepared through the hydrothermal route, and the synthesis illustration of Ag/ZnO nanorods is displayed in Fig. 1. The preparation process is briefly described as follows: The Zn(NO3)2·6H2O (4.16 g) was dissolved into 320 mL of DI water with stirring for 30 min. Subsequently, 40 mL of NaOH solution (4 mol/L) and certain amount of AgNO3 (0 wt%, 0.5 wt%, 1.0 wt% and 2.0 wt%) was added into the above mixture solution under continuous stirring for 10 min to obtain the reaction solution system. The uniform solution was transferred into the Teflon-lined autoclave and heated at 180 °C for 12 h under autogenous pressure. Then, the final product was centrifuged and washed with deionized water for several times, and dried in oven at 65 °C. At last, the prepared samples were calcined at 400 °C for 2 h in the muffle furnace.
3. Results and discussions 3.1. Crystal structure and morphology Fig. 2 shows the XRD patterns of ZnO and Ag/ZnO nanorods. The diffraction peaks at 31.92°, 34.58°, 36.42°, 47.71°, 56.74°, 63.02°, 68.08°, and 69.24° can be unambiguously indexed to (100), (002), (101), (102), (110), (103), (112) and (201) planes of ZnO with hexagonal phase (JCPDS Card No.36-1451), respectively. The sharp diffraction peaks indicate that the ZnO materials are highly crystalline. As expected, the Ag/ZnO nanorods show obvious characteristic diffraction peaks at 38.20° 44.38° and 64.54° which can be corresponding to (111), (200) and (220) planes of cubic phase Ag (JCPDS Card No.04-0783), respectively. The Ag peak intensity becomes stronger with the increase of Ag concentration, which indicates the presence of Ag in obtained composites. The morphological investigation of resulting samples was performed by SEM, TEM and HRTEM. Fig. 3(a) reveals that pure ZnO has a radial structure assembled with nanorods. The average width and length of the ZnO nanorods is 200 and 900 nm, respectively. Fig. 3(b) shows the SEM images of Ag/ZnO nanorods. We can see clearly that the Ag nanoparticles with diameters ranging from 50 to 100 nm are firmly deposited on the surface of ZnO nanorods, and these Ag particles have a slight agglomeration. Fig. 3(c) displays the EDS spectrum of Ag/ZnO nanorods, indicating the existence of Ag element. According to XRD and EDS analyses, it can be concluded that Ag exists as a pure metal phase in Ag/ZnO composite. To further verify the above inference, the TEM analysis was performed. Fig. 4(a) further reveals the radial structure of pure ZnO
2.3. Characterizations of the samples The crystalline phase structure of the as-synthesized products was characterized via XRD using a Bruker-AXS diffractometer (D8 advance) with Cu Kα radiation. The morphologies of samples were investigated using FESEM (Quanta 250 FEG) and TEM (JEOL JEM-2100). Elemental composition of powders was studied by EDS that was connect to FESEM (Quanta 250 FEG). UV–Vis diffuse reflectance spectra (UV–Vis DRS) was performed for products by a UV–Vis spectrometer (Beijing Puxi GI Co., Ltd, TU-1900).
Fig. 1. The synthesis illustration of Ag/ZnO nanorods.
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Fig. 3. SEM images of (a) ZnO and (b) Ag/ZnO nanorods, (c) EDS spectrum of Ag/ZnO nanorods.
nanorods. Fig. 4(b, c) presents the TEM images of Ag/ZnO nanorods. As we can observe obviously, the Ag nanoparticles are attached tightly to the surface of ZnO. Fig. 4(d) presents the HRTEM image of Ag/ZnO nanorods, which suggests that the lattice fringes in Ag nanoparticles with a lattice spacing of 0.201 nm can be consistent with that of (200) crystal plane of cubic Ag. In addition, the lattice fringes of 0.262 nm can be assigned to (002) crystal plane of hexagonal ZnO. The HRTEM image analysis is done on a basis of corresponding XRD data, which reveals that Ag particles are tightly anchored onto ZnO. 3.2. UV–Vis DRS spectra UV–Vis diffuse reflectance spectra of ZnO and Ag/ZnO nanorods are displayed in Fig. 5. As we can see that ZnO and Ag/ZnO show a strong optical absorption below 390 nm in the ultraviolet light region, which is ascribed to the ZnO band gap [20]. Compared to ZnO, the Ag/ZnO nanorods displays a red shift, and the red shift is observed due to the strong interfacial coupling between ZnO and Ag, indicating that the band gap energy of Ag/ ZnO is lower than that of ZnO [21]. The optical band gap values of ZnO and Ag/ZnO samples were estimated by the (αhν)2 versus photon energy (hν) plot, as displayed in inset of Fig. 5. The band gaps of ZnO and Ag/ZnO were estimated to be 3.157 and 3.112 eV, respectively, which confirms that the band gap is narrowed by introducing Ag onto ZnO [22].
Fig. 5. The UV–visible diffuse reflectance spectra and band gap (inset) of ZnO and Ag/ ZnO nanorods.
Fig. 4. TEM images of (a) ZnO and (b, c) Ag/ZnO nanorods, HRTEM images of (d) Ag/ZnO nanorods.
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samples increase rapidly by injecting ethanol into the chamber and recover quickly to the baseline after introducing clean air. Obviously, the Ag concentration in ZnO nanorods strongly influences the responses of sensors. By comparing the response of the sensors, it can be found that the 1 wt% Ag/ZnO sensor shows the maximum sensitivity and fast response among all the sensors. Therefore, the 1 wt% Ag/ZnO sensor was selected to accomplish the subsequent research for ethanol sensing performance. The enhancement in gas response of 1 wt% Ag/ZnO sensor is attributed to appropriate Ag concentration which increases the chemical adsorption and promotes the reaction on the ZnO surface. The Ag nanoparticles are used as a catalytic agent to strengthen the reaction between Ag/ZnO and ethanol, and create additional active sites on the Ag/ZnO surface [15,17]. The transient response of 1 wt% Ag/ZnO sensor toward 100 ppm ethanol at 360 °C is shown in Fig. 7(b). Apparently, the response increases sharply via exposing the Ag/ZnO sensor to ethanol and recovers reach a saturation state. The response and recovery time of Ag/ZnO sensor to 100 ppm ethanol at 360 °C is about 50 s and 28 s, respectively. It is shown that Ag/ZnO sensor possesses a relatively short response and recovery time, which possibly ascribes to the spillover effect of Ag [23]. The relationship of sensor response to ethanol concentration for the fabricated sensor devices at 360 °C is shown in Fig. 8. We can see clearly that the responses of pure ZnO and the 1 wt% Ag/ZnO sensor increase with the ethanol concentration in the range of 50–800 ppm. Initially, within 50–500 ppm gas concentration the increase of the responses to ethanol is faster, while over 500 ppm the increase tends to saturation. The 1 wt% Ag/ZnO sample exhibits a higher response than pure ZnO, and its response is 21.5, 36.5, 81.7, 122.5, 332.8 and 389.6 to 50, 100, 200, 300, 500 and 800 ppm of ethanol, respectively, which indicates that the appropriate decoration of Ag can enhance the gas-sensing mechanism. This result might be attributed to the catalysis of Ag, which can accelerate the reactions between reducing gas and ZnO nanorods [10,24]. Fig. 9 shows the transient response-recovery curves of the ZnO and 1 wt% Ag/ZnO based sensors at 360 °C to different ethanol concentrations. It is observed that the responses of the ZnO and 1 wt% Ag/ZnO based sensors raise quickly with the increasing of ethanol concentration, and the 1 wt% Ag/ZnO nanorods indicates a higher sensitivity than pure ZnO to the same concentration of ethanol. When releasing the ethanol, the response can fully recovered to its primal value, indicating that the sensor is recoverable. The selectivity to test gas is a key factor of gas sensors. Thus, to measure the selectivity of Ag/ZnO nanorods, the responses of the 1 wt% Ag/ZnO based sensor to 100 ppm various tested gases namely acetone, toluene, formaldehyde and ethanol have been investigated at 360 °C as shown in Fig. 10. Evidently, 1 wt% Ag/ZnO based sensor shows
Fig. 6. Response of the sensors towards 300 ppm ethanol gas at different temperature.
3.3. Gas sensing properties The operating temperature plays an important factor in the sensitivity of gas-sensing materials. Therefore, to investigate the gas sensing properties of products, determining the optimum working temperature of a sensor is important. The responses of pure ZnO nanorods and Ag/ ZnO based sensors toward 300 ppm ethanol were investigated at different elevated temperature within a range of 300–400 °C. Fig. 6 displays the responses of the sensors toward 300 ppm ethanol at various working temperatures. For the ZnO, the response increases with the increase of temperature until gets its maximum at 400 °C. However, the responses of Ag/ZnO nanorods based sensors increase firstly with the increase of operating temperature till 360 °C and then gradually decrease. It is found that the optimum operating temperatures of Ag/ZnO based sensors are lower than the pure ZnO sensor. Meanwhile, we can notice that the responses of pure ZnO nanorods, 0.5 wt% Ag/ZnO nanorods, 1 wt% Ag/ZnO nanorods and 2 wt% Ag/ZnO nanorods are 88.95, 49.01, 122.47, and 87.53 toward 300 ppm ethanol under optimal working temperature, respectively. Additionally, the 1 wt% Ag/ ZnO sensor shows the highest response among all sensors. This result indicates that the 1 wt% Ag/ZnO sensor is very helpful to detect ethanol. To demonstrate the sensing behaviors of Ag/ZnO nanorods, all sensor samples were investigated to make a comparison. Fig. 7(a) displays the sensing transients of all gas sensors towards 300 ppm concentration of ethanol at 360 °C. It is observed that the responses of all
Fig. 7. (a) The sensing transients of all sensors towards 300 ppm ethanol gas at 360 °C, (b) The transient response of the 1 wt% Ag/ZnO sensor toward 100 ppm ethanol at 360 °C.
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Table 1 Comparison of ethanol-sensing performances of the present work with previous studies. Sensing material
Concentration
Operating temperature
Response (Ra/Rg)
References
AgFeO2 nanoparticles flower-like PdOZnO NiO/ZnO nanoplates Al-doped ZnO
100 ppm
180 °C
30.2
100 ppm
320 °C
35.4
100 ppm
400 °C
2.88
1000 ppm
290 °C
108
Ag/ZnO nanorods
100 ppm
360 °C
36.52
Wang et al. [25] Lou et al. [26] Deng et al. [27] Yang et al. [28] This paper
Fig. 8. Response of the sensors based on pure ZnO and the 1 wt% Ag/ZnO towards different concentration of ethanol gas at 360 °C.
Fig. 11. The stability of the 1 wt% Ag/ZnO exposed to 50, 100 and 200 ppm ethanol.
Ag/ZnO sensor shows relative higher response to ethanol. Hence, the 1 wt% Ag/ZnO sample is a hopeful candidate for efficient and highly sensitive in ethanol detection. The stability is also an important parameter of gas sensors. To investigate the stability of the 1 wt% Ag/ZnO sensor, the responses to various concentrations of ethanol gas were tested for 15 days. Fig. 11 displays the stability of the 1 wt% Ag/ZnO sensor to 50, 100, and 200 ppm of ethanol gas. The responses were measured for over 15 days with 3 days as interval at 360 °C. It is observed that all the responses of 1 wt% Ag/ZnO sensor to 50, 100, and 200 ppm of ethanol gas show little variation within 30 days, which suggests that the 1 wt% Ag/ZnO sensor has a satisfying stability to ethanol.
Fig. 9. The transient response-recovery curves of the sensors based on pure ZnO and the 1 wt% Ag/ZnO towards different concentration of ethanol gas at 360 °C.
3.4. Ethanol sensing mechanism ZnO as a typical n-type semiconductor, its gas sensing mechanism is considered to be a surface-controlled type [7,29]. There will be some holes on ZnO surface due to the particular nature of semiconductor, and these holes can absorb the oxygen molecules from the air. Then the adsorbed oxygen forms O2-, O- and O2- through trapping electrons from the conductive band of ZnO, resulting in a depletion layer on the surface [30–32]. For this reason, the carrier concentration and electrical conductivity of ZnO are decreased. However, once the ZnO exposes to the reducing gas (ethanol), there will be a reaction between ethanol and adsorbed oxygen species on ZnO surface, liberating electrons back to the conductive band, causing a decreased resistance [33,34]. The enhanced sensing performance of Ag/ZnO sensor is predominantly ascribe to catalysis of Ag nanoparticles [35,36]. Fig. 12 represents the schematic diagram sensing mechanism for Ag/ZnO nanorods. The introduction of Ag will provide more adsorption sites to facilitate the interaction between ethanol and Ag/ZnO nanorods through the spillover effect [23,37]. The addition of Ag nanoparticles can make more electrons be extracted and the thickness of electron
Fig. 10. Response of 1 wt% Ag/ZnO based sensor towards 100 ppm various test gases at 360 °C.
responses to all the examined gases, especially ethanol and acetone. Moreover, the 1 wt% Ag/ZnO based sensor displays a relative higher response of 36.52 to 100 ppm ethanol, which indicates that it has exceptional selectivity toward ethanol. Meanwhile, for better understanding the ethanol-sensing properties of the 1 wt% Ag/ZnO sensor, the response of the sensor was compared with previous work. Table 1 presents the comparison of ethanol-sensing performances of the present work with previous studies [25–28]. We can clearly see that the 1 wt% 331
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Fig. 12. Schematic diagram sensing mechanism for Ag/ZnO nanorods.
depletion layer will be increased. Compared with the pure ZnO, more reactive sites were generated in the Ag/ZnO nanorods, which can increase adsorption and enhance dissociation of adsorbed oxygen molecules on Ag surface into oxygen ion species, and these oxygen ion species will spill over to the surface of ZnO [38,39]. When the Ag/ZnO sensors are exposed toward ethanol, the ethanol molecules on the Ag surface will react with more adsorbed oxygen ions, and more electrons will be released into the conduction band, which leads to a thicker electron depletion layer and a larger variation in resistivity of Ag/ZnO nanorods [40]. The resistance variation of Ag/ZnO in target gas is much larger than that of pure ZnO, which causes the remarkable enhancement of the sensor response. Actually, Ag nanoparticles can catalyze the dissociation of oxygen molecules and ethanol molecules on its surface, which accelerates the redox reaction during the sensor working. Therefore, the Ag/ZnO sensor exhibits a high response and recovery rate [18,41].
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