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nano-materials and their application in acetone sensing properties
Accepted Manuscript Template-free hydrothermal synthesis of ZnO micro/nano-materials and their application in acetone sensing properties Dongmin An, Xiaoqiang Tong, Jincheng Liu, Qiong Wang, Qingjun Zhou, Juan Dong, Yan Li PII: DOI: Reference:
S0749-6036(14)00403-0 http://dx.doi.org/10.1016/j.spmi.2014.10.033 YSPMI 3472
To appear in:
Superlattices and Microstructures
Received Date: Revised Date: Accepted Date:
12 September 2014 22 October 2014 26 October 2014
Please cite this article as: D. An, X. Tong, J. Liu, Q. Wang, Q. Zhou, J. Dong, Y. Li, Template-free hydrothermal synthesis of ZnO micro/nano-materials and their application in acetone sensing properties, Superlattices and Microstructures (2014), doi: http://dx.doi.org/10.1016/j.spmi.2014.10.033
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Template-free hydrothermal synthesis of ZnO micro/nano-materials and their application in acetone sensing properties Dongmin An*, Xiaoqiang Tong, Jincheng Liu, Qiong Wang, Qingjun Zhou, Juan Dong, Yan Li
College of Science, Civil Aviation University of China, Tianjin, 300300, P. R. China
*Corresponding author. Tel.: +86-22-2409 2624; fax: +86-22-2409 2514
E-mail: [email protected]
Abstract ZnO micro/nano-materials with sheet and rod morphologies were hydrothermally synthesized with template-free. In synthesis, NaOH was used as the precipitant. The mass ratio of NaOH to zinc salt was the key factor in control the morphology of ZnO product, and synthesized ZnO exhibited a hexagonal wurtzite structure. The gas-sensing properties of the sensors based on the ZnO nanosheets and rods were systematically investigated. At 220 °C, the sensitivity of ZnO nanosheet to 100 ppm acetone was up to 24.9, and the response and recovery times were about 4.9 and 3.1 s, respectively, which was superior to the ZnO rods. The results indicated that ZnO products were potential candidates for acetone sensors, and ZnO nanosheets had a more excellent gas performance than ZnO rods (21.21, 6.0 and 3.2 s). Moreover, the formation and gas-sensing mechanisms of different morphologies of ZnO products were discussed. Keywords: ZnO micro/nano-materials; Template-free; Acetone sensing 1. Introduction 1
Zinc oxide (ZnO), which is a wide band-gap oxide semiconductor, has attracted great attention of scientists for years because of its comprehensive technological applications such as gas-sensing [1-5], catalysis [6,7], optoelectronics [8,9], and biomedicine [10]. Recently, more and more attentions have been paid on the utilization of the gas sensing property of ZnO materials to detect some nocuous gases [3-5]. In the point of ZnO sensor application, some gas-sensing performances will be considered, such as the operating temperature, sensitivity, selectivity and long-time stability of the sensor. The influencing factors of gas sensor properties are some, i.e. the morphology, the structure, the specific surface area of ZnO material, and so on. Therefore, kinds of methods have been developed to improve the gas-sensing properties and decrease the operating temperature of sensor. Many related papers reveal that morphology has a significant influence on the gas-sensing properties of nanomaterials [11,12]. ZnO is rich in morphology, such as the one-dimensional (1D) nanowires [13], nanorods [14], or nanobelts [15], two-dimensional (2D) nanoplates [12,16], are reported used as a gas sensor. Moreover, sheet-based 3D structure ZnO or large thick sheet ZnO was also applied as gas sensors in some literatures [17-19]. It is well known that ZnO is intrinsically and readily inclined to grow into 1D morphology in a liquid medium with its unique hexagonal crystal structure. For elongated ZnO material has both polar and nonpolar faces if it is grown from solution based methods [20], and the ZnO nuclei normally tend to aggregate along the polar face direction leading to form a 1D nanostructure (axial growth) [21]. However, the axial growth could be suppressed and then tabular nanostructures like the plate-or sheet-shape 2
ZnO could be obtained (equatorial growth), if the polar faces are passivated by some growth modifiers [21]. Therefore, researchers can selectively prepare ZnO crystals with different orientations or morphologies for exploring novel properties through choosing a proper growth modifier. In our study, using NaOH as precipitant and growth inducing agent, we developed a simple template-free route to synthesize ZnO with sheet and rod morphologies separately. Based on as-synthesized ZnO materials, the performance in acetone-sensing was investigated in detail and the results exhibited the sensors had a low operating temperature and high sensitivity to acetone gas. 2. Materials and method 2.1. Materials The analytical grade Zn(CH3COO)2·2H2O, Zn(NO3)2·6H2O, absolute ethanol (99.5%) and NaOH were used in our study, which were all supplied by Beihua Fine Chemicals (Beijing, China) and used as received without further purification. 2.2. Synthesis 1.5 g of Zn(CH3COO)2·2H2O was dissolved into 40 mL distilled water, then NaOH solution with different concentrations was dropped into the zinc acetate solution with vigorous magnetic stirring at room temperature, and the mass ratios of NaOH to Zn(CH3COO)2 were 1:2, 1:1, 2:1 and 4:1, respectively. When the NaOH solution was dropped completely, the mixed solution was continued stirred for 30 min at room temperature, and then was removed to the Teflon autoclave at 120oC for 12 h, afterward, the obtained slurry was filtrated, washed several times with water and absolute ethanol, 3
and then dried at 100 °C for 12 h to obtain the final product of ZnO, which was designated as ZnO-A. The products obtained with different mass ratios of 1:2, 1:1, 2:1 and 4:1 were designated as ZnO-A1, ZnO-A2, ZnO-A3 and ZnO-A4, respectively. The zinc salt of Zn(NO3)2 was also used to synthesize ZnO product in accord with the above steps, and products prepared with different ratios were designated as ZnO-N1, ZnO-N2, Zn-N3 and Zn-N4, respectively. 2.3. Sensor fabrication and test The prepared ZnO micro/nano-material and distilled water were mixed to form a slurry, which was used to fabricate the gas sensor by coating the slurry onto a ceramic tube (length 4 mm, diameter 1 mm) to make a thin film structure. Two Au electrodes circled the both sides of tube and four Pt wires were stretched on both ends of the tube. A Ni-Cr alloy coil as a heater went through the ceramic tube to control the operating temperature. Target gas acetone was first diluted in the air and then a calculated amount of test gas was introduced by a microsyringe into the testing chamber on Chemical Gas Sensor-8 (CGS-8, Ailite Technology Co., Ltd., Beijing, China) at a relative humidity of 20~30%. The sensor sensitivity signal S, was defined as the ratio Ra/Rg, where Ra and Rg were the electrical resistances of the sensor in air and in test gas, respectively. The scheme of the sensor is shown in Fig.1. 2.4. Characterizations A scanning electron microscopy (SEM) with a field-emission-scanning electron microscope (LEO1530VP) was used to observe the morphologies of ZnO products. A DX-2000 X-ray diffractometer, using Cu Kα (λ=0.1542 nm) radiation, was used to 4
characterize the crystalline phase of the ZnO samples. The specific surface area was measured by N2 adsorption-desorption analysis on an ASAP 2020 instrument. The photoluminescence (PL) spectra of ZnO particles were recorded by an F-7000 luminescence spectrophotometer (Hitachi High-Technologies Corporation, Tokyo, Japan, Xe lamp). 3.1 Results and Discussion 3.1 The properties of as-prepared ZnO product Fig.2 shows the XRD patterns of the as-prepared ZnO-A samples. All the diffraction peaks of the prepared ZnO-A products can be well indexed to the standard card of hexagonal wurtzite ZnO structures (JCPDS No.01-1136-1) as shown in Fig.2. No other characteristic diffraction peaks or impurities were detected, which implies that pure ZnO was formed. The representative SEM images of ZnO product are shown in Fig.3. As can be seen, the ZnO product with sheet shape (Fig.3a and b; Fig.3c and d) was obtained when the mass ratios of NaOH to Zn(CH3COO)2 were 1:2 and 1:1. The thickness of the sheet was about 30 nm and the ZnO sheet was made up of small nanoparticles with well dispersion and close packed, which were clearly visible in Fig. 3b and Fig.3d. ZnO rod (see Fig.3e) was obtained when the mass ratio of NaOH to Zn(CH3COO)2 reached 2:1. The ZnO rod was hexagonal structure with smooth surface. Moreover, most of the hexagonal rod had a pointed end, and the surface of ZnO rod was very smooth which can be seen in Fig.3f. The ZnO hexagonal rod with a diameter of 1~2 μm and a length of 10 μm in Fig.3g was obtained when the mass ratio of NaOH to Zn(CH3COO)2 was 5
continued increased to 4:1. The surface of these rod was not smooth, and was consisted of small particles about 20~30 nm and some pores, as can be clearly seen in Fig.3h. The reason may be due to the etching of superfluous NaOH of the reaction system on the surface of ZnO rod. Fig.4 shows the PL spectrum curves of the ZnO samples. As we know that the photoluminescent property of ZnO is sensitive to its defects [22, 23]. In Fig.4, both the UV emission and the yellow emission can be clearly observed, in which the UV emission was due to the near band-edge transition, namely, the recombination of free excitions through an excition-excition collision process. The curve a in Fig.4 was the PL spectrum of ZnO-A1, in which the UV emission was slightly stronger than other samples. The peak around 545 nm was due to the yellow emission that was commonly referred to as deep-level and trap state emission, which had been associated to the oxygen excess [24]. The curve of Fig.4b shows a relative weak UV emission and yellow emission compared with the Fig.4a. The curve c in Fig.4 is the PL spectrum of ZnO rod, which showed a strong yellow emission but a weak UV emission compared with the ZnO sheet product, and the same result could be seen from the PL spectrum of the as-prepared ZnO-A4 product. The results of the PL spectrum indicated that the intensity ratios of the UV emission to the yellow emission varied with the morphologies change of ZnO product and the same result was also reported by Li et al.[25]. Semiconductor metal oxides are promising materials for gas sensor, especially for all kinds of morphologies of ZnO. Many studies have proven that these special morphology structures could significantly enhance the sensor performance. In our case, the ZnO 6
products with different morphologies were used as gas sensor to detect acetone gas. Fig.5 shows the sensitivity of the ZnO gas sensors as a function of temperature exposed to 100 ppm acetone testing gas. All the sensors showed unsteadily response to acetone at the working temperature below 140 oC, while the working temperature was up to 140 o
C, all the sensors began to show response to acetone gas. The sensitivity remarkably
increased along with the temperature up to 220 oC, and reached the maximum value at 220oC. However, the sensitivity was suddenly dropped with further increment in temperature. As the curve a and b shown in Fig.5, the sensitivity of ZnO-A1 and ZnO-A2 sheet product to acetone gas were slowly increased with the increment of temperature below 180 oC, but the sensitivity was with a sharp increment when the temperature beyond 180 oC, and reached the maximum value of 24.9 and 21.21 at 220 o
C, respectively. The optimum operating temperature was much lower to the results of
ZnO, SnO2 and ZnSnO3 sensors to acetone gas reported in the literatures [26-29], in which the optimum operating temperature of these sensors was 300 oC, and some were even above 300 oC. For the rod-shape samples of ZnO-A3 and Zn-A4, the sensitivity to acetone gas was slowly climbed when the temperature below 200 oC as the Fig.5c and Fig.5d shown, and reached the maximum value of 16.42 and 14.89 at 220 oC, respectively, which was in the shade compared with the sensitivity of the sheet ZnO product. Different acetone gas concentrations were tested at an operating temperature of 220 o
C in the sequence of 10, 50, 100, 200, 300, 400, 500 and 600 ppm as Fig.6 shown. As
we can see, it was obvious that the sensitivity of all ZnO gas sensors increased with the 7
concentration of acetone gas increasing. The ZnO sensors showed considerable sensitivity to acetone gas even at a low concentration of 10 ppm. For the rod-like ZnO gas sensors, the sensitivities of ZnO-A3 and ZnO-A4 (Fig.6c and d) to acetone gas were stable and slowly increased, but for the sheet-like ZnO gas sensors (Fig.6a and b), the sensitivities to low concentration of acetone gas quickly increased and slowly increased to high concentration (above 500 ppm) of acetone gas. Therefore, the ZnO sheet products were more suitable to detect low concentration of acetone gas. The response-recovery curves of ZnO-A3 sensor to different concentrations of acetone gas at the working temperature of 220 oC is shown in Fig.7. It can be seen that the ZnO rod sensor presented sensitive and reversible responses to acetone gas with different concentrations. The response ascended with the increased of the concentration of acetone gas. It can also be noted from Table 1 that all the ZnO products did not have a large surface area, but all the ZnO sensors still showed a short response and recovery time to acetone gas with several seconds, which indicated that the ZnO product was suitable to detect acetone gas. It’s well known that ZnO is a typical n-type semiconductor and the sensing mechanism is dominated by the surface process. In an air atmosphere, oxygen molecules are firstly absorbed on the surface of ZnO and capture free electrons to produce ionic species such as O-, O2- or O2-. When ZnO materials are exposed to acetone gas at higher temperature, the acetone gas molecules react with these ionic species, which leads to more free electrons liberated and an increase in the resistance of the ZnO semiconductor. The reaction kinematics is as follow: 8
C3H6O + 8O- (ads) → 3H2O + 3CO2 + 8eTherefore, the contact area between the sensor and the tested gas plays an important role in the surface absorption of gas and subsequent reaction of oxygen species with the tested gas. In our case, compared with the ZnO rod, the ZnO sheet had a relative high surface area (Table 1), leading to more surface area to absorb acetone gas and high gas sensitivity. 3.2. Growth mechanism for the formation of ZnO rods and sheets The growth habit of ZnO crystal in liquid medium is mainly determined by its intrinsic structure, but some external conditions, such as pH value of the solution, the concentration of Zn(OH)42− ions, and the addition of growth modifier, can also affect the growth of ZnO crystal. In order to study the growth mechanism of hexagonal ZnO rod and sheet in our work, the zinc salt of Zn(NO3)2 was also used to synthesize ZnO product while other conditions were kept unchanged. As Fig.8 shown, the morphologies of ZnO-N products were also changed from sheet (Fig.8a and b) to rod structure (Fig.8c and d) with the same ratio change of OH- to Zn2+, which implied that the morphology change in our case was only relevant to the amount of OH- in the solution. The growth process of ZnO crystallites is generally accepted via the following mechanism [30]: Zn2+ + 2OH−→ Zn(OH)2↓ (1) Zn(OH)2→ZnO + H2O (2) Zn(OH)2+ 2OH−→ Zn(OH)42− (3) Zn(OH)42−→ ZnO + H2O + 2OH− (4) Based on the above experimental results, a possible formation mechanism of the ZnO 9
sheet and rod was proposed. In our experiments, when the mass ratio of NaOH to Zn(Ac)2 was lower than 1:1, the OH- was a little superfluous according to the stoichiometric ratio of Zn(OH)2, so the main product of Zn(OH)2 and small quantity of growth units ([Zn(OH)4]2−) were obtained after the mixed of NaOH and Zn(Ac)2 in the solution, as was approved by the experimental phenomenon occurred in the process of reaction for that white slurry was obtained when the NaOH solution was dropped completely. When the obtained white slurry was removed to Teflon autoclave, the main reaction occurred were Eq. (2). The ZnO crystal was hexagonal, and the anion in crystal was tetrahedron coordination [Zn-O4]6. The angle of ZnO4 was connected, and a three-fold symmetry axis was parallel to the c axis; a side surface was parallel to the cathode surface, the angle of which was corresponded to the negative side point. The ZnO4 tetrahedron was presented in layered distribution along the c-axis. When the dehydrated reaction occurred, ZnO sheet was formed. When the mass ratio of NaOH to Zn(Ac)2 was above 1:1, because OH− was abundant in the mixed aqueous solution, freshly formed Zn(OH)2 precipitation (Eq. (1)) could be dissolved immediately by reacting with superfluous OH− ions and a transparent Zn(OH)42− solution was obtained (Eq. (3)), as could also been approved by the experimental phenomenon for that white slurry was firstly obtained when the NaOH solution was partly dropped, while the white precipitate gradually disappeared with the following dropping of NaOH. Ultimately a transparent Zn(OH)42− solution was obtained before removed to Teflon autoclave. In the hydrothermal process, the Zn(OH)42− growth units combined with each other and dehydrated into ZnO nuclei simultaneously. And then these ZnO nuclei exhibited fast 10
growth orientation [0001]. Finally, ZnO rods were formed. 4. Conclusions In summary, a template-free hydrothermal method was demonstrated for the synthesis of ZnO micro/nano-structures and a proper mechanism was proposed, which implied that the morphology of ZnO product depended on the mass ratio of NaOH to zinc salts. The as-synthesized ZnO nanosheet or rod products were all hexagonal wurtzite structures through XRD analysis. The intensity of PL emission peak of UV and visible light was varied with the morphology change of ZnO product. Sensors properties based on the sheet and rod samples were investigated. The sensitivities of all the ZnO gas sensors increased with the increment of the gas concentrations of acetone. The results of systematical gas sensing studies demonstrated that the sensor based on ZnO sheet or rod micro/nano-materials had a low operating temperature, high sensitivity, short response and recovery time to acetone gas. The sensitivity of ZnO nanosheet to 100 ppm acetone gas was up to 24.9 at 220 °C, and the response and recovery times were about 4.9 and 3.1 s, respectively, which was superior to the ZnO rod sensor. It demonstrated that ZnO sheet was more suitable for use as gas-sensing material and the gas-sensing mechanism was discussed in detail. Acknowledgements This research was mainly supported by the Fundamental Research Funds for the Central Universities (ZXH2012K008, 3122013c014 and 3122013k007), the Significant Pre-research Funds of Civil Aviation University of China (No. 3122013P001), and Students innovation training project (IECAUC14060). Here, we also gave thanks to the 11
Applied Basic and Cutting-edge Research Programs of Science and Technology Foundation of Tianjin (No. 13JCQNJC07100). References [1] C. Soci, A. Zhang, B. Xiang, S.A. Dayeh, D.P.R. Aplin, J. Park, X.Y. Bao, Y.H. Lo, D. Wang, ZnO nanowire UV photodetectors with high internal gain, Nano Lett. 7(2007)1003-1009. [2] C.S. Lao, M.C. Park, Q. Kuang, Y.L. Deng, A.K. Sood, D.L. Polla, Z.L. Wang, Giant
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Figure captions Fig.1 The scheme of the prepared ZnO-based sensor. Fig.2 The XRD of ZnO-A1, ZnO-A2, ZnO-A3, ZnO-A4 and hexagonal wurtzite ZnO structures (JCPDS No.01-1136-1), respectively. Fig.3 The SEM images of ZnO-A1(a and b), ZnO-A2 (c and d), ZnO-A3 (e and f) and ZnO-A4 (g and h), respectively. Fig.4 The PL spectrum of the ZnO samples: ZnO-A1(a), ZnO-A2 (b), ZnO-A3 (c) and ZnO-A4 (d), respectively. Fig.5 Response of the ZnO product sensors to 100 ppm acetone at different working temperatures: curves a, b, c and d represented ZnO-A1, ZnO-A2, ZnO-A3 and ZnO-A4, respectively. Fig.6 Response of the ZnO product sensors to different concentrations of acetone at 220oC: curves a, b, c and d represented ZnO-A1, ZnO-A2, ZnO-A3 and ZnO-A4, respectively. Fig.7 Response-recovery curves of the ZnO-A3 rod sensor to different acetone concentrations at 220oC. Fig.8 The SEM images of ZnO-N1(a), ZnO-N2 (b), ZnO-N3 (c) and ZnO-N4 (d), respectively.
Table 1 The BET, response and recovery times of ZnO-A1, ZnO-A2, ZnO-A3 and ZnO-A4 to 100 ppm acetone gas, respectively.
16
Fig.1
17
Fig.2
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Fig.3
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Fig.4
21
Fig.5
a b c d
25
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0 140
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Highlights 1. Sheet and rod ZnO micro/nano-materials were synthesized with a template-free
hydrothermal method. 2. The morphology of ZnO was depended on the weight ratio of NaOH to zinc salt. 3. The formation mechanism of different morphologies of ZnO products was discussed. 4. The ZnO sensors exhibited a high recovery and fast response to acetone gas. 5. The ZnO sensors exhibited a relative low operating temperature and high sensitivity
to acetone gas.
26
S0749-6036(14)00403-0 http://dx.doi.org/10.1016/j.spmi.2014.10.033 YSPMI 3472
To appear in:
Superlattices and Microstructures
Received Date: Revised Date: Accepted Date:
12 September 2014 22 October 2014 26 October 2014
Please cite this article as: D. An, X. Tong, J. Liu, Q. Wang, Q. Zhou, J. Dong, Y. Li, Template-free hydrothermal synthesis of ZnO micro/nano-materials and their application in acetone sensing properties, Superlattices and Microstructures (2014), doi: http://dx.doi.org/10.1016/j.spmi.2014.10.033
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Template-free hydrothermal synthesis of ZnO micro/nano-materials and their application in acetone sensing properties Dongmin An*, Xiaoqiang Tong, Jincheng Liu, Qiong Wang, Qingjun Zhou, Juan Dong, Yan Li
College of Science, Civil Aviation University of China, Tianjin, 300300, P. R. China
*Corresponding author. Tel.: +86-22-2409 2624; fax: +86-22-2409 2514
E-mail: [email protected]
Abstract ZnO micro/nano-materials with sheet and rod morphologies were hydrothermally synthesized with template-free. In synthesis, NaOH was used as the precipitant. The mass ratio of NaOH to zinc salt was the key factor in control the morphology of ZnO product, and synthesized ZnO exhibited a hexagonal wurtzite structure. The gas-sensing properties of the sensors based on the ZnO nanosheets and rods were systematically investigated. At 220 °C, the sensitivity of ZnO nanosheet to 100 ppm acetone was up to 24.9, and the response and recovery times were about 4.9 and 3.1 s, respectively, which was superior to the ZnO rods. The results indicated that ZnO products were potential candidates for acetone sensors, and ZnO nanosheets had a more excellent gas performance than ZnO rods (21.21, 6.0 and 3.2 s). Moreover, the formation and gas-sensing mechanisms of different morphologies of ZnO products were discussed. Keywords: ZnO micro/nano-materials; Template-free; Acetone sensing 1. Introduction 1
Zinc oxide (ZnO), which is a wide band-gap oxide semiconductor, has attracted great attention of scientists for years because of its comprehensive technological applications such as gas-sensing [1-5], catalysis [6,7], optoelectronics [8,9], and biomedicine [10]. Recently, more and more attentions have been paid on the utilization of the gas sensing property of ZnO materials to detect some nocuous gases [3-5]. In the point of ZnO sensor application, some gas-sensing performances will be considered, such as the operating temperature, sensitivity, selectivity and long-time stability of the sensor. The influencing factors of gas sensor properties are some, i.e. the morphology, the structure, the specific surface area of ZnO material, and so on. Therefore, kinds of methods have been developed to improve the gas-sensing properties and decrease the operating temperature of sensor. Many related papers reveal that morphology has a significant influence on the gas-sensing properties of nanomaterials [11,12]. ZnO is rich in morphology, such as the one-dimensional (1D) nanowires [13], nanorods [14], or nanobelts [15], two-dimensional (2D) nanoplates [12,16], are reported used as a gas sensor. Moreover, sheet-based 3D structure ZnO or large thick sheet ZnO was also applied as gas sensors in some literatures [17-19]. It is well known that ZnO is intrinsically and readily inclined to grow into 1D morphology in a liquid medium with its unique hexagonal crystal structure. For elongated ZnO material has both polar and nonpolar faces if it is grown from solution based methods [20], and the ZnO nuclei normally tend to aggregate along the polar face direction leading to form a 1D nanostructure (axial growth) [21]. However, the axial growth could be suppressed and then tabular nanostructures like the plate-or sheet-shape 2
ZnO could be obtained (equatorial growth), if the polar faces are passivated by some growth modifiers [21]. Therefore, researchers can selectively prepare ZnO crystals with different orientations or morphologies for exploring novel properties through choosing a proper growth modifier. In our study, using NaOH as precipitant and growth inducing agent, we developed a simple template-free route to synthesize ZnO with sheet and rod morphologies separately. Based on as-synthesized ZnO materials, the performance in acetone-sensing was investigated in detail and the results exhibited the sensors had a low operating temperature and high sensitivity to acetone gas. 2. Materials and method 2.1. Materials The analytical grade Zn(CH3COO)2·2H2O, Zn(NO3)2·6H2O, absolute ethanol (99.5%) and NaOH were used in our study, which were all supplied by Beihua Fine Chemicals (Beijing, China) and used as received without further purification. 2.2. Synthesis 1.5 g of Zn(CH3COO)2·2H2O was dissolved into 40 mL distilled water, then NaOH solution with different concentrations was dropped into the zinc acetate solution with vigorous magnetic stirring at room temperature, and the mass ratios of NaOH to Zn(CH3COO)2 were 1:2, 1:1, 2:1 and 4:1, respectively. When the NaOH solution was dropped completely, the mixed solution was continued stirred for 30 min at room temperature, and then was removed to the Teflon autoclave at 120oC for 12 h, afterward, the obtained slurry was filtrated, washed several times with water and absolute ethanol, 3
and then dried at 100 °C for 12 h to obtain the final product of ZnO, which was designated as ZnO-A. The products obtained with different mass ratios of 1:2, 1:1, 2:1 and 4:1 were designated as ZnO-A1, ZnO-A2, ZnO-A3 and ZnO-A4, respectively. The zinc salt of Zn(NO3)2 was also used to synthesize ZnO product in accord with the above steps, and products prepared with different ratios were designated as ZnO-N1, ZnO-N2, Zn-N3 and Zn-N4, respectively. 2.3. Sensor fabrication and test The prepared ZnO micro/nano-material and distilled water were mixed to form a slurry, which was used to fabricate the gas sensor by coating the slurry onto a ceramic tube (length 4 mm, diameter 1 mm) to make a thin film structure. Two Au electrodes circled the both sides of tube and four Pt wires were stretched on both ends of the tube. A Ni-Cr alloy coil as a heater went through the ceramic tube to control the operating temperature. Target gas acetone was first diluted in the air and then a calculated amount of test gas was introduced by a microsyringe into the testing chamber on Chemical Gas Sensor-8 (CGS-8, Ailite Technology Co., Ltd., Beijing, China) at a relative humidity of 20~30%. The sensor sensitivity signal S, was defined as the ratio Ra/Rg, where Ra and Rg were the electrical resistances of the sensor in air and in test gas, respectively. The scheme of the sensor is shown in Fig.1. 2.4. Characterizations A scanning electron microscopy (SEM) with a field-emission-scanning electron microscope (LEO1530VP) was used to observe the morphologies of ZnO products. A DX-2000 X-ray diffractometer, using Cu Kα (λ=0.1542 nm) radiation, was used to 4
characterize the crystalline phase of the ZnO samples. The specific surface area was measured by N2 adsorption-desorption analysis on an ASAP 2020 instrument. The photoluminescence (PL) spectra of ZnO particles were recorded by an F-7000 luminescence spectrophotometer (Hitachi High-Technologies Corporation, Tokyo, Japan, Xe lamp). 3.1 Results and Discussion 3.1 The properties of as-prepared ZnO product Fig.2 shows the XRD patterns of the as-prepared ZnO-A samples. All the diffraction peaks of the prepared ZnO-A products can be well indexed to the standard card of hexagonal wurtzite ZnO structures (JCPDS No.01-1136-1) as shown in Fig.2. No other characteristic diffraction peaks or impurities were detected, which implies that pure ZnO was formed. The representative SEM images of ZnO product are shown in Fig.3. As can be seen, the ZnO product with sheet shape (Fig.3a and b; Fig.3c and d) was obtained when the mass ratios of NaOH to Zn(CH3COO)2 were 1:2 and 1:1. The thickness of the sheet was about 30 nm and the ZnO sheet was made up of small nanoparticles with well dispersion and close packed, which were clearly visible in Fig. 3b and Fig.3d. ZnO rod (see Fig.3e) was obtained when the mass ratio of NaOH to Zn(CH3COO)2 reached 2:1. The ZnO rod was hexagonal structure with smooth surface. Moreover, most of the hexagonal rod had a pointed end, and the surface of ZnO rod was very smooth which can be seen in Fig.3f. The ZnO hexagonal rod with a diameter of 1~2 μm and a length of 10 μm in Fig.3g was obtained when the mass ratio of NaOH to Zn(CH3COO)2 was 5
continued increased to 4:1. The surface of these rod was not smooth, and was consisted of small particles about 20~30 nm and some pores, as can be clearly seen in Fig.3h. The reason may be due to the etching of superfluous NaOH of the reaction system on the surface of ZnO rod. Fig.4 shows the PL spectrum curves of the ZnO samples. As we know that the photoluminescent property of ZnO is sensitive to its defects [22, 23]. In Fig.4, both the UV emission and the yellow emission can be clearly observed, in which the UV emission was due to the near band-edge transition, namely, the recombination of free excitions through an excition-excition collision process. The curve a in Fig.4 was the PL spectrum of ZnO-A1, in which the UV emission was slightly stronger than other samples. The peak around 545 nm was due to the yellow emission that was commonly referred to as deep-level and trap state emission, which had been associated to the oxygen excess [24]. The curve of Fig.4b shows a relative weak UV emission and yellow emission compared with the Fig.4a. The curve c in Fig.4 is the PL spectrum of ZnO rod, which showed a strong yellow emission but a weak UV emission compared with the ZnO sheet product, and the same result could be seen from the PL spectrum of the as-prepared ZnO-A4 product. The results of the PL spectrum indicated that the intensity ratios of the UV emission to the yellow emission varied with the morphologies change of ZnO product and the same result was also reported by Li et al.[25]. Semiconductor metal oxides are promising materials for gas sensor, especially for all kinds of morphologies of ZnO. Many studies have proven that these special morphology structures could significantly enhance the sensor performance. In our case, the ZnO 6
products with different morphologies were used as gas sensor to detect acetone gas. Fig.5 shows the sensitivity of the ZnO gas sensors as a function of temperature exposed to 100 ppm acetone testing gas. All the sensors showed unsteadily response to acetone at the working temperature below 140 oC, while the working temperature was up to 140 o
C, all the sensors began to show response to acetone gas. The sensitivity remarkably
increased along with the temperature up to 220 oC, and reached the maximum value at 220oC. However, the sensitivity was suddenly dropped with further increment in temperature. As the curve a and b shown in Fig.5, the sensitivity of ZnO-A1 and ZnO-A2 sheet product to acetone gas were slowly increased with the increment of temperature below 180 oC, but the sensitivity was with a sharp increment when the temperature beyond 180 oC, and reached the maximum value of 24.9 and 21.21 at 220 o
C, respectively. The optimum operating temperature was much lower to the results of
ZnO, SnO2 and ZnSnO3 sensors to acetone gas reported in the literatures [26-29], in which the optimum operating temperature of these sensors was 300 oC, and some were even above 300 oC. For the rod-shape samples of ZnO-A3 and Zn-A4, the sensitivity to acetone gas was slowly climbed when the temperature below 200 oC as the Fig.5c and Fig.5d shown, and reached the maximum value of 16.42 and 14.89 at 220 oC, respectively, which was in the shade compared with the sensitivity of the sheet ZnO product. Different acetone gas concentrations were tested at an operating temperature of 220 o
C in the sequence of 10, 50, 100, 200, 300, 400, 500 and 600 ppm as Fig.6 shown. As
we can see, it was obvious that the sensitivity of all ZnO gas sensors increased with the 7
concentration of acetone gas increasing. The ZnO sensors showed considerable sensitivity to acetone gas even at a low concentration of 10 ppm. For the rod-like ZnO gas sensors, the sensitivities of ZnO-A3 and ZnO-A4 (Fig.6c and d) to acetone gas were stable and slowly increased, but for the sheet-like ZnO gas sensors (Fig.6a and b), the sensitivities to low concentration of acetone gas quickly increased and slowly increased to high concentration (above 500 ppm) of acetone gas. Therefore, the ZnO sheet products were more suitable to detect low concentration of acetone gas. The response-recovery curves of ZnO-A3 sensor to different concentrations of acetone gas at the working temperature of 220 oC is shown in Fig.7. It can be seen that the ZnO rod sensor presented sensitive and reversible responses to acetone gas with different concentrations. The response ascended with the increased of the concentration of acetone gas. It can also be noted from Table 1 that all the ZnO products did not have a large surface area, but all the ZnO sensors still showed a short response and recovery time to acetone gas with several seconds, which indicated that the ZnO product was suitable to detect acetone gas. It’s well known that ZnO is a typical n-type semiconductor and the sensing mechanism is dominated by the surface process. In an air atmosphere, oxygen molecules are firstly absorbed on the surface of ZnO and capture free electrons to produce ionic species such as O-, O2- or O2-. When ZnO materials are exposed to acetone gas at higher temperature, the acetone gas molecules react with these ionic species, which leads to more free electrons liberated and an increase in the resistance of the ZnO semiconductor. The reaction kinematics is as follow: 8
C3H6O + 8O- (ads) → 3H2O + 3CO2 + 8eTherefore, the contact area between the sensor and the tested gas plays an important role in the surface absorption of gas and subsequent reaction of oxygen species with the tested gas. In our case, compared with the ZnO rod, the ZnO sheet had a relative high surface area (Table 1), leading to more surface area to absorb acetone gas and high gas sensitivity. 3.2. Growth mechanism for the formation of ZnO rods and sheets The growth habit of ZnO crystal in liquid medium is mainly determined by its intrinsic structure, but some external conditions, such as pH value of the solution, the concentration of Zn(OH)42− ions, and the addition of growth modifier, can also affect the growth of ZnO crystal. In order to study the growth mechanism of hexagonal ZnO rod and sheet in our work, the zinc salt of Zn(NO3)2 was also used to synthesize ZnO product while other conditions were kept unchanged. As Fig.8 shown, the morphologies of ZnO-N products were also changed from sheet (Fig.8a and b) to rod structure (Fig.8c and d) with the same ratio change of OH- to Zn2+, which implied that the morphology change in our case was only relevant to the amount of OH- in the solution. The growth process of ZnO crystallites is generally accepted via the following mechanism [30]: Zn2+ + 2OH−→ Zn(OH)2↓ (1) Zn(OH)2→ZnO + H2O (2) Zn(OH)2+ 2OH−→ Zn(OH)42− (3) Zn(OH)42−→ ZnO + H2O + 2OH− (4) Based on the above experimental results, a possible formation mechanism of the ZnO 9
sheet and rod was proposed. In our experiments, when the mass ratio of NaOH to Zn(Ac)2 was lower than 1:1, the OH- was a little superfluous according to the stoichiometric ratio of Zn(OH)2, so the main product of Zn(OH)2 and small quantity of growth units ([Zn(OH)4]2−) were obtained after the mixed of NaOH and Zn(Ac)2 in the solution, as was approved by the experimental phenomenon occurred in the process of reaction for that white slurry was obtained when the NaOH solution was dropped completely. When the obtained white slurry was removed to Teflon autoclave, the main reaction occurred were Eq. (2). The ZnO crystal was hexagonal, and the anion in crystal was tetrahedron coordination [Zn-O4]6. The angle of ZnO4 was connected, and a three-fold symmetry axis was parallel to the c axis; a side surface was parallel to the cathode surface, the angle of which was corresponded to the negative side point. The ZnO4 tetrahedron was presented in layered distribution along the c-axis. When the dehydrated reaction occurred, ZnO sheet was formed. When the mass ratio of NaOH to Zn(Ac)2 was above 1:1, because OH− was abundant in the mixed aqueous solution, freshly formed Zn(OH)2 precipitation (Eq. (1)) could be dissolved immediately by reacting with superfluous OH− ions and a transparent Zn(OH)42− solution was obtained (Eq. (3)), as could also been approved by the experimental phenomenon for that white slurry was firstly obtained when the NaOH solution was partly dropped, while the white precipitate gradually disappeared with the following dropping of NaOH. Ultimately a transparent Zn(OH)42− solution was obtained before removed to Teflon autoclave. In the hydrothermal process, the Zn(OH)42− growth units combined with each other and dehydrated into ZnO nuclei simultaneously. And then these ZnO nuclei exhibited fast 10
growth orientation [0001]. Finally, ZnO rods were formed. 4. Conclusions In summary, a template-free hydrothermal method was demonstrated for the synthesis of ZnO micro/nano-structures and a proper mechanism was proposed, which implied that the morphology of ZnO product depended on the mass ratio of NaOH to zinc salts. The as-synthesized ZnO nanosheet or rod products were all hexagonal wurtzite structures through XRD analysis. The intensity of PL emission peak of UV and visible light was varied with the morphology change of ZnO product. Sensors properties based on the sheet and rod samples were investigated. The sensitivities of all the ZnO gas sensors increased with the increment of the gas concentrations of acetone. The results of systematical gas sensing studies demonstrated that the sensor based on ZnO sheet or rod micro/nano-materials had a low operating temperature, high sensitivity, short response and recovery time to acetone gas. The sensitivity of ZnO nanosheet to 100 ppm acetone gas was up to 24.9 at 220 °C, and the response and recovery times were about 4.9 and 3.1 s, respectively, which was superior to the ZnO rod sensor. It demonstrated that ZnO sheet was more suitable for use as gas-sensing material and the gas-sensing mechanism was discussed in detail. Acknowledgements This research was mainly supported by the Fundamental Research Funds for the Central Universities (ZXH2012K008, 3122013c014 and 3122013k007), the Significant Pre-research Funds of Civil Aviation University of China (No. 3122013P001), and Students innovation training project (IECAUC14060). Here, we also gave thanks to the 11
Applied Basic and Cutting-edge Research Programs of Science and Technology Foundation of Tianjin (No. 13JCQNJC07100). References [1] C. Soci, A. Zhang, B. Xiang, S.A. Dayeh, D.P.R. Aplin, J. Park, X.Y. Bao, Y.H. Lo, D. Wang, ZnO nanowire UV photodetectors with high internal gain, Nano Lett. 7(2007)1003-1009. [2] C.S. Lao, M.C. Park, Q. Kuang, Y.L. Deng, A.K. Sood, D.L. Polla, Z.L. Wang, Giant
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Figure captions Fig.1 The scheme of the prepared ZnO-based sensor. Fig.2 The XRD of ZnO-A1, ZnO-A2, ZnO-A3, ZnO-A4 and hexagonal wurtzite ZnO structures (JCPDS No.01-1136-1), respectively. Fig.3 The SEM images of ZnO-A1(a and b), ZnO-A2 (c and d), ZnO-A3 (e and f) and ZnO-A4 (g and h), respectively. Fig.4 The PL spectrum of the ZnO samples: ZnO-A1(a), ZnO-A2 (b), ZnO-A3 (c) and ZnO-A4 (d), respectively. Fig.5 Response of the ZnO product sensors to 100 ppm acetone at different working temperatures: curves a, b, c and d represented ZnO-A1, ZnO-A2, ZnO-A3 and ZnO-A4, respectively. Fig.6 Response of the ZnO product sensors to different concentrations of acetone at 220oC: curves a, b, c and d represented ZnO-A1, ZnO-A2, ZnO-A3 and ZnO-A4, respectively. Fig.7 Response-recovery curves of the ZnO-A3 rod sensor to different acetone concentrations at 220oC. Fig.8 The SEM images of ZnO-N1(a), ZnO-N2 (b), ZnO-N3 (c) and ZnO-N4 (d), respectively.
Table 1 The BET, response and recovery times of ZnO-A1, ZnO-A2, ZnO-A3 and ZnO-A4 to 100 ppm acetone gas, respectively.
16
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Highlights 1. Sheet and rod ZnO micro/nano-materials were synthesized with a template-free
hydrothermal method. 2. The morphology of ZnO was depended on the weight ratio of NaOH to zinc salt. 3. The formation mechanism of different morphologies of ZnO products was discussed. 4. The ZnO sensors exhibited a high recovery and fast response to acetone gas. 5. The ZnO sensors exhibited a relative low operating temperature and high sensitivity
to acetone gas.
26