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Selective detection of acetone and gasoline by temperature modulation in zinc oxide nanosheets sensors
Solid State Ionics 192 (2011) 688–692
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Selective detection of acetone and gasoline by temperature modulation in zinc oxide nanosheets sensors Huiqing Fan ⁎, Xiaohua Jia State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China
a r t i c l e
i n f o
Article history: Received 21 August 2009 Received in revised form 12 April 2010 Accepted 28 May 2010 Available online 2 July 2010 Keywords: Hydrothermal synthesis ZnO nanosheets Gas sensor Selective detection
a b s t r a c t Two-dimensional ZnO nanosheets with dimensions of several microns in length and tens of nanometers in thickness were synthesized via a simple mixed hydrothermal synthesis in the presence of cetyltrimethyl ammonium bromide (CTAB) and 1, 2-propanediol. The ZnO nanosheets are characterized by using X-ray powder diffraction, transmission electron microscopy, and field emission scanning electron microscopy. The formation mechanism and effect of CTAB on the morphology of ZnO nanosheets have also been discussed. Furthermore, gas sensing properties were measured between 180 and 360 °C by a static test system, which confirm that the as-synthesized ZnO nanosheets are of good selectivity and response to actone at low temperature and to gasoline at high temperature, and could be an ideal temperature selecting gas sensor for the detection of both acetone and gasoline. © 2010 Elsevier B.V. All rights reserved.
1. Introduction ZnO nanostructures are interesting to study not only because of the recent demonstrations of unique physical properties, but also because a wide variety of morphologies have been prepared. The size and morphology of ZnO nano-particles have great influences on their performances. Because the properties of nano-materials depend on their size and shape, new synthetic strategies in which the size and shape of nanostructures can be easily tailored are important. Some of the ZnO nanostructures exhibit nanowire, nanorod, nanoribbon, nanoplate, nanotube, tetrapod, cage-like and flower-like structures [1–9]. Among the various ZnO nanostructures, relatively few studies on the properties of two-dimensional ZnO nanosheets have been reported up to now. ZnO nanosheets have shown superior properties in nanoscale optoelectronics, solar cell electrode, catalysis and sensor devices [10–12]. The ZnO nanosheets have been prepared by various methods, such as, thermal oxidation of zinc powders, carbon-thermal redox of ZnO powders [13,14] and chemical vapor deposition [15]. These methods need high temperature and are also limited by their low yields. However, the sovolthermal route is an important and simple low-temperature method for wet chemistry, and has been employed to fabricate ZnO nanopowders. Gasoline is used as a fuel for automobiles. Normal human breath contains hundreds of volatile organic compounds (VOCs) in very low concentrations ranging from part-per-trillion to part-per-billion levels [16]. Some VOCs have been identified as biomarkers of specific
⁎ Corresponding author. Tel.: + 86 29 88494463; fax: + 86 29 88492642. E-mail addresses: [email protected] (H. Fan), [email protected] (X. Jia). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.05.058
diseases. For instance, acetone in human breath gas has been established as a biomarker for type-1diabetes (T1D). Hence, it will be very lucrative if a sensor can be developed to detect several gases. For a long time, metal oxide semiconductors gas sensors have played an important role in environment monitoring and chemical process controlling. Among the various solid-state sensors, zinc oxide (ZnO) is an interesting chemically and thermally stable n-type semiconductor with large exciton binding energy, large bandgap energy, and high sensitivity to toxic and combustible gases [17]. To date, various types of ZnO-based gas sensors, such as thick films [18], nanoparticles [19– 23], have been demonstrated. Here, we report an interesting observation for ZnO gas sensors, where the same sensors can selectively detect acetone and gasoline through temperature modulation. 2. Experimental ZnO nanosheets were synthesized by a simple mixed hydrothermal method. All reagents were 99.9% purity or better and purchased from Shanghai Chemical Reagent Co. and used without further purification. First, 0.004 mol zinc nitrate (Zn(NO3)2·6H2O) and 0.002 mol cetyltrimethyl ammonium bromide (CTAB, C19H42BrN) were dissolved in 46 mL 1, 2-propanediol and distilled water with the ratio of 1:1 under constant stirring. Then 0.002 mol urea was introduced into the abovementioned solution. After 10 min stirring, the mixed solution was transferred into a Teflon-lined stainless steel autoclave 56 mL in volume and 80% filled. Hydrothermal growth was carried out at 180 °C for 24 h. White precipitates were collected, filtered and washed with distilled water and ethanol several times to remove impurities. Finally, the precipitates were dried at 60 °C for 10 h.
H. Fan, X. Jia / Solid State Ionics 192 (2011) 688–692
The phase structure and phase purity of the as-synthesized powders were examined by X-ray diffraction (XRD; X'pert, Philips, Holland) with Cu-Kα radiation (λ = 1.5406 Å) at 40 kV, 30 mA over the 2θ range 20– 90°. The morphology of the obtained samples was also investigated using field emission scanning electronic microscopy (FE-SEM; JSM6701F, JEOL, Japan) with an energy dispersion spectrum (EDS; FeatureMax, Oxford Instruments, UK). The specific surface area of ZnO nanosheets was measured by using nitrogen absorption through a gas adsorption analyzer (NOVA 2000E, Quantachrome Instruments, USA). After structural characterization, the gas sensing properties of ZnO nanosheets were also studied at different heating temperatures. The obtained ZnO nanosheets were mixed and ground with adhesive in an agate mortar to form a gas sensing paste. The paste used as sensitive body was coated on an alumina tube with a diameter of 1 mm and length of 4 mm, which is positioned with a pair of Au electrodes and four Pt wires on both ends of the tube. A Ni–Cr alloy crossing alumina tube was used as a heating resistor which ensured both substrate heating and temperature controlling. Each element was sintered at 600 °C for 1 h in air. The side-heated type the sensitive body based on ZnO nanosheets was formed as shown in Fig. 1a. In order to improve their stability and repeatability, the gas sensors were aged at 300 °C for 240 h in air. Gas sensing tests were performed on a static state gas sensing measurement system (HW-30A, Hanwei Electronics, China) at a relative humidity of 20–30%. Target gas including acetone, ammonia, ethanol, gasoline (93% octane) and toluene in the concentration of 50 ppm was introduced into the testing chamber in volume of 15 L on HW-30A by a microsyringe. The schematic circuit is shown in Fig. 1b. Gas response (S) is defined as the ratio of Rair/Rgas, where Rair and Rgas are the resistance values measured in
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air and reducing atmosphere, respectively. In the experiment, gas sensor was put into this static testing system by injecting gas within 1 s and releasing it within 1 s, so the working condition is almost the same as that of the real application for gas sensor and the critical gas flow rate was not considered any more. The response time is expected as the time required for the variation in conductance of the sensor to reach 90% of the equilibrium value after injection of a testing gas, and the recovery time is the time necessary for the sensor to return to 10% above the original conductance in air after releasing the test gas. 3. Results and discussion To obtain structure and chemical composition evidence of ZnO nanosheets, X-ray powder diffraction was performed immediately after sample preparation. As shown in Fig. 2, all the reflection peaks of the products can be well indexed to pure hexagonal phase ZnO, which are in good agreement with the literature values (JCPDS card number 79-2205). No diffraction peaks from any other impurities are detected. The obtained particles composed of high crystalline ZnO with wurtzite crystal structure. The wurtzite structure of ZnO belongs to the P63mc (No. 186) space group and has the hexagonal symmetry. The overall morphology of as-grown ZnO nanosheets was examined by field emission scanning electron microscopy. The low-magnification and high-magnification SEM images of ZnO nanosheets are shown in Fig. 3a and b. It can be seen from the Fig. 3a that the ZnO nanosheets formed a nanoflower. The thickness of the nanosheets is in the range of 10–20 nm, the width-to-thickness ratios of the nanosheets almost reach to one thousand. Furthermore, the EDS method was utilized to identify
Fig. 1. (a) Scheme of the gas sensor made by ZnO nanosheets (The insert figure gives a photograph of ZnO nanosheets gas sensor), (b) the working principle of HW-30A gas sensing measurement system (V: heating voltage, Vc: circuit voltage, R: load resistor).
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H. Fan, X. Jia / Solid State Ionics 192 (2011) 688–692
Fig. 2. XRD patterns of the ZnO nanosheets prepared by the mild mixed hydrothermal at 180 °C for 24 h, All peaks can be indexed to hexagonal ZnO with a wurtzite structure (JCPDS card number 79-2205).
the chemical composition of our products. It was observed that the asprepared sample mainly contained Zn and O elements (Fig. 3c) in a Zn/O atomic ratio of approximately 1:0.74, suggesting an oxygen deficiency in the sample. The signal of Cu originates from the copper substrate, further confirming the formation of ZnO. The surface structural characteristics of the ZnO nanosheets were also analyzed by nitrogen adsorption and desorption isotherm techniques, the Brunauer– Emmett–Teller (BET) analysis showed that the surface areas of the ZnO nanosheets were 38.3 m2 g− 1. On the basis of the above-mentioned experimental results, a possible growth mechanism is proposed as following. The surfactant CTAB plays a key role in controlling the nucleation and growth of the samples. In the presence of CTAB, the surface tension of solution is educed to the existence of surfactant, which lowers the energy needed for the formation of a new phase. In this experimental, the urea is decomposed into CO2 and NH3 during heating in the autoclave [24,25]. The ammonia is hydrolyzed and complexes with the metal ions in solution to form Zn(OH)2 in the early reaction stages. The growth mechanism is believed to be similar to a mechanism proposed by Wang et al. [26], the reaction products are Zn(OH)2 in this reference because of the low reaction temperature. In our experimental, the Zn(OH)2 particle was decomposed into ZnO with the reaction going on under a relatively high reaction temperature. An interesting difference in the acetone and gasoline sensing behavior of ZnO nanosheets gas sensor was found in the Fig. 4. The gas response of ZnO gas sensor towards gasoline decreases drastically with increasing of the heating temperature. However, to acetone the gas response increases with increasing of the heating temperature. As a result, the same ZnO gas sensor can detect both acetone and gasoline at different heating temperatures. Such behavior can be understood by considering the role of the kind of adsorption oxygen and the characteristic of gasoline and acetone, the oxygen adsorption depends on the particle size, large specific area of the material, and the operating temperature of the sensor [26]. With increasing the temperature in ambience, the state of oxygen adsorbed on the surface of the ZnO nanosheets material undergoes the following reactions [27,28]: O2gas ↔O2ads −
ð1Þ −
O2ads + e ↔O2ads −
−
−
O2ads + e ↔2Oads −
−
2−
Oads + e ↔Oads :
ð2Þ ð3Þ ð4Þ
Fig. 3. Typical FE-SEM morphologies of the ZnO nanosheets prepared by hydrothermal at 180 °C for 24 h, (a) low-magnification image revealing large quantities of ZnO nanosheets formed a nanoflower. (b) high-magnification image of ZnO nanosheets. (c) EDS result of the ZnO nanosheets.
The oxygen species capture electrons from the material, leading to an increase in hole concentration and a decrease in electron concentration. When the gasoline was injected in the test chamber, the alkene in the gasoline was adsorbed on the surface of the gas sensing materials, and then reacted with the oxygen adsorbed on the surface of the ZnO nanosheets. The electrons trapped by the adsorptive states will be released, leading to a decrease in sensor resistance. So, the ZnO nanosheets sensor is sensitive to gasoline. The same mechanism could be used to explain the gas sensing properties of acetone when the heating temperature was 360 °C. Long-term
H. Fan, X. Jia / Solid State Ionics 192 (2011) 688–692
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Fig. 4. The influence of working temperature on the gas response of ZnO nanosheets gas sensor.
Fig. 6. The response and recovery curves of the ZnO nanosheets gas sensor for 50 ppm acetone at different heating temperatures.
stability of the gas sensor was also measured by repeating the sensing measurement 8 times within 2 months, and no appreciable variations were detected during the test. Therefore, this ZnO gas sensor has an attractive stability for future commercial application. In order to investigate the selectivity of the sensors, our studies were extended to acetone, ammonia, ethanol, gasoline and toluene. The results under the heating temperature of 180 and 360 °C are shown in Fig. 5, which shows that the response to gasoline is excellently higher than those to other gases when the temperature at 180 °C, the response magnitude was about 30, while that to other gases was no greater than 5. It is obvious that the selectivity of the sensor to gasoline against all other gases is exceeding almost by 6 times. While the response to acetone can reaches 31 when the heating temperature is at 360 °C, it is apparent that acetone lead to significantly stronger responses compared to other gases, even if the response is only 1 at a low heating temperature, in other words, there almost no reaction to the acetone at low heating temperature. These results clearly demonstrate the high selectivity of ZnO nanosheets gas sensor for acetone and gasoline at different heating temperature, which is much different from that of gas sensors made by ZnO nanopowders [23]. Fig. 6 shows typical response and recovery curves for 50 ppm acetone at different heating temperatures. It is clear that both the response and recovery time improve with increasing operating temperature. At low heating temperature the response time for the sensor element lasted for several minutes, and the gas sensor had no apparent response to acetone when the heating temperature was
180 °C, 200 °C and 240 °C, and the curves almost not recovery when the gas sensors were exposed to air. The sensor signal at a high heating temperature was obviously improved, The 90% response time of the sensor at 300 °C was nearly 13 s and the recovery time of it was less than 30 s. The response time and recovery time of the sensor was more rapid at a higher heating temperature, which were less than 2 s and 15 s, respectively. It reveals that the response and recovery properties of ZnO nanosheets gas sensor are quite satisfying. 4. Conclusions ZnO nanosheets have been synthesized successfully by using a simple mixed hydrothermal method. The thickness of the ZnO nanosheets is in the range of 10–20 nm, and the width-to-thickness ratios of the ZnO sheets almost reach up to one thousand. The formation mechanism and effect of CTAB on the morphology of ZnO nanosheets have also been discussed. It has been found that the gas sensor made by these ZnO nanosheets can detect gasoline at 180 °C, and the same sensor can selectively detect acetone at 360 °C interestingly. Acknowledgements This work has been supported by the National Nature Science Foundation (50672075), the NCET and 111 Program (B08040) of MOE, and Xi'an Science & Technology Foundation (CXY08006, XAAM-200905, and XA-AM-200906), and the Fundamental Research Foundation (NPU-FFR-200703) and Doctorate Foundation (CX200804) of NPU, and the SKLSP Research Fund (40-QZ-2009) of China. References
Fig. 5. Sensitivity and selectivity of the gas sensor based on ZnO nanosheets to different tested gases.
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Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Selective detection of acetone and gasoline by temperature modulation in zinc oxide nanosheets sensors Huiqing Fan ⁎, Xiaohua Jia State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China
a r t i c l e
i n f o
Article history: Received 21 August 2009 Received in revised form 12 April 2010 Accepted 28 May 2010 Available online 2 July 2010 Keywords: Hydrothermal synthesis ZnO nanosheets Gas sensor Selective detection
a b s t r a c t Two-dimensional ZnO nanosheets with dimensions of several microns in length and tens of nanometers in thickness were synthesized via a simple mixed hydrothermal synthesis in the presence of cetyltrimethyl ammonium bromide (CTAB) and 1, 2-propanediol. The ZnO nanosheets are characterized by using X-ray powder diffraction, transmission electron microscopy, and field emission scanning electron microscopy. The formation mechanism and effect of CTAB on the morphology of ZnO nanosheets have also been discussed. Furthermore, gas sensing properties were measured between 180 and 360 °C by a static test system, which confirm that the as-synthesized ZnO nanosheets are of good selectivity and response to actone at low temperature and to gasoline at high temperature, and could be an ideal temperature selecting gas sensor for the detection of both acetone and gasoline. © 2010 Elsevier B.V. All rights reserved.
1. Introduction ZnO nanostructures are interesting to study not only because of the recent demonstrations of unique physical properties, but also because a wide variety of morphologies have been prepared. The size and morphology of ZnO nano-particles have great influences on their performances. Because the properties of nano-materials depend on their size and shape, new synthetic strategies in which the size and shape of nanostructures can be easily tailored are important. Some of the ZnO nanostructures exhibit nanowire, nanorod, nanoribbon, nanoplate, nanotube, tetrapod, cage-like and flower-like structures [1–9]. Among the various ZnO nanostructures, relatively few studies on the properties of two-dimensional ZnO nanosheets have been reported up to now. ZnO nanosheets have shown superior properties in nanoscale optoelectronics, solar cell electrode, catalysis and sensor devices [10–12]. The ZnO nanosheets have been prepared by various methods, such as, thermal oxidation of zinc powders, carbon-thermal redox of ZnO powders [13,14] and chemical vapor deposition [15]. These methods need high temperature and are also limited by their low yields. However, the sovolthermal route is an important and simple low-temperature method for wet chemistry, and has been employed to fabricate ZnO nanopowders. Gasoline is used as a fuel for automobiles. Normal human breath contains hundreds of volatile organic compounds (VOCs) in very low concentrations ranging from part-per-trillion to part-per-billion levels [16]. Some VOCs have been identified as biomarkers of specific
⁎ Corresponding author. Tel.: + 86 29 88494463; fax: + 86 29 88492642. E-mail addresses: [email protected] (H. Fan), [email protected] (X. Jia). 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.05.058
diseases. For instance, acetone in human breath gas has been established as a biomarker for type-1diabetes (T1D). Hence, it will be very lucrative if a sensor can be developed to detect several gases. For a long time, metal oxide semiconductors gas sensors have played an important role in environment monitoring and chemical process controlling. Among the various solid-state sensors, zinc oxide (ZnO) is an interesting chemically and thermally stable n-type semiconductor with large exciton binding energy, large bandgap energy, and high sensitivity to toxic and combustible gases [17]. To date, various types of ZnO-based gas sensors, such as thick films [18], nanoparticles [19– 23], have been demonstrated. Here, we report an interesting observation for ZnO gas sensors, where the same sensors can selectively detect acetone and gasoline through temperature modulation. 2. Experimental ZnO nanosheets were synthesized by a simple mixed hydrothermal method. All reagents were 99.9% purity or better and purchased from Shanghai Chemical Reagent Co. and used without further purification. First, 0.004 mol zinc nitrate (Zn(NO3)2·6H2O) and 0.002 mol cetyltrimethyl ammonium bromide (CTAB, C19H42BrN) were dissolved in 46 mL 1, 2-propanediol and distilled water with the ratio of 1:1 under constant stirring. Then 0.002 mol urea was introduced into the abovementioned solution. After 10 min stirring, the mixed solution was transferred into a Teflon-lined stainless steel autoclave 56 mL in volume and 80% filled. Hydrothermal growth was carried out at 180 °C for 24 h. White precipitates were collected, filtered and washed with distilled water and ethanol several times to remove impurities. Finally, the precipitates were dried at 60 °C for 10 h.
H. Fan, X. Jia / Solid State Ionics 192 (2011) 688–692
The phase structure and phase purity of the as-synthesized powders were examined by X-ray diffraction (XRD; X'pert, Philips, Holland) with Cu-Kα radiation (λ = 1.5406 Å) at 40 kV, 30 mA over the 2θ range 20– 90°. The morphology of the obtained samples was also investigated using field emission scanning electronic microscopy (FE-SEM; JSM6701F, JEOL, Japan) with an energy dispersion spectrum (EDS; FeatureMax, Oxford Instruments, UK). The specific surface area of ZnO nanosheets was measured by using nitrogen absorption through a gas adsorption analyzer (NOVA 2000E, Quantachrome Instruments, USA). After structural characterization, the gas sensing properties of ZnO nanosheets were also studied at different heating temperatures. The obtained ZnO nanosheets were mixed and ground with adhesive in an agate mortar to form a gas sensing paste. The paste used as sensitive body was coated on an alumina tube with a diameter of 1 mm and length of 4 mm, which is positioned with a pair of Au electrodes and four Pt wires on both ends of the tube. A Ni–Cr alloy crossing alumina tube was used as a heating resistor which ensured both substrate heating and temperature controlling. Each element was sintered at 600 °C for 1 h in air. The side-heated type the sensitive body based on ZnO nanosheets was formed as shown in Fig. 1a. In order to improve their stability and repeatability, the gas sensors were aged at 300 °C for 240 h in air. Gas sensing tests were performed on a static state gas sensing measurement system (HW-30A, Hanwei Electronics, China) at a relative humidity of 20–30%. Target gas including acetone, ammonia, ethanol, gasoline (93% octane) and toluene in the concentration of 50 ppm was introduced into the testing chamber in volume of 15 L on HW-30A by a microsyringe. The schematic circuit is shown in Fig. 1b. Gas response (S) is defined as the ratio of Rair/Rgas, where Rair and Rgas are the resistance values measured in
689
air and reducing atmosphere, respectively. In the experiment, gas sensor was put into this static testing system by injecting gas within 1 s and releasing it within 1 s, so the working condition is almost the same as that of the real application for gas sensor and the critical gas flow rate was not considered any more. The response time is expected as the time required for the variation in conductance of the sensor to reach 90% of the equilibrium value after injection of a testing gas, and the recovery time is the time necessary for the sensor to return to 10% above the original conductance in air after releasing the test gas. 3. Results and discussion To obtain structure and chemical composition evidence of ZnO nanosheets, X-ray powder diffraction was performed immediately after sample preparation. As shown in Fig. 2, all the reflection peaks of the products can be well indexed to pure hexagonal phase ZnO, which are in good agreement with the literature values (JCPDS card number 79-2205). No diffraction peaks from any other impurities are detected. The obtained particles composed of high crystalline ZnO with wurtzite crystal structure. The wurtzite structure of ZnO belongs to the P63mc (No. 186) space group and has the hexagonal symmetry. The overall morphology of as-grown ZnO nanosheets was examined by field emission scanning electron microscopy. The low-magnification and high-magnification SEM images of ZnO nanosheets are shown in Fig. 3a and b. It can be seen from the Fig. 3a that the ZnO nanosheets formed a nanoflower. The thickness of the nanosheets is in the range of 10–20 nm, the width-to-thickness ratios of the nanosheets almost reach to one thousand. Furthermore, the EDS method was utilized to identify
Fig. 1. (a) Scheme of the gas sensor made by ZnO nanosheets (The insert figure gives a photograph of ZnO nanosheets gas sensor), (b) the working principle of HW-30A gas sensing measurement system (V: heating voltage, Vc: circuit voltage, R: load resistor).
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H. Fan, X. Jia / Solid State Ionics 192 (2011) 688–692
Fig. 2. XRD patterns of the ZnO nanosheets prepared by the mild mixed hydrothermal at 180 °C for 24 h, All peaks can be indexed to hexagonal ZnO with a wurtzite structure (JCPDS card number 79-2205).
the chemical composition of our products. It was observed that the asprepared sample mainly contained Zn and O elements (Fig. 3c) in a Zn/O atomic ratio of approximately 1:0.74, suggesting an oxygen deficiency in the sample. The signal of Cu originates from the copper substrate, further confirming the formation of ZnO. The surface structural characteristics of the ZnO nanosheets were also analyzed by nitrogen adsorption and desorption isotherm techniques, the Brunauer– Emmett–Teller (BET) analysis showed that the surface areas of the ZnO nanosheets were 38.3 m2 g− 1. On the basis of the above-mentioned experimental results, a possible growth mechanism is proposed as following. The surfactant CTAB plays a key role in controlling the nucleation and growth of the samples. In the presence of CTAB, the surface tension of solution is educed to the existence of surfactant, which lowers the energy needed for the formation of a new phase. In this experimental, the urea is decomposed into CO2 and NH3 during heating in the autoclave [24,25]. The ammonia is hydrolyzed and complexes with the metal ions in solution to form Zn(OH)2 in the early reaction stages. The growth mechanism is believed to be similar to a mechanism proposed by Wang et al. [26], the reaction products are Zn(OH)2 in this reference because of the low reaction temperature. In our experimental, the Zn(OH)2 particle was decomposed into ZnO with the reaction going on under a relatively high reaction temperature. An interesting difference in the acetone and gasoline sensing behavior of ZnO nanosheets gas sensor was found in the Fig. 4. The gas response of ZnO gas sensor towards gasoline decreases drastically with increasing of the heating temperature. However, to acetone the gas response increases with increasing of the heating temperature. As a result, the same ZnO gas sensor can detect both acetone and gasoline at different heating temperatures. Such behavior can be understood by considering the role of the kind of adsorption oxygen and the characteristic of gasoline and acetone, the oxygen adsorption depends on the particle size, large specific area of the material, and the operating temperature of the sensor [26]. With increasing the temperature in ambience, the state of oxygen adsorbed on the surface of the ZnO nanosheets material undergoes the following reactions [27,28]: O2gas ↔O2ads −
ð1Þ −
O2ads + e ↔O2ads −
−
−
O2ads + e ↔2Oads −
−
2−
Oads + e ↔Oads :
ð2Þ ð3Þ ð4Þ
Fig. 3. Typical FE-SEM morphologies of the ZnO nanosheets prepared by hydrothermal at 180 °C for 24 h, (a) low-magnification image revealing large quantities of ZnO nanosheets formed a nanoflower. (b) high-magnification image of ZnO nanosheets. (c) EDS result of the ZnO nanosheets.
The oxygen species capture electrons from the material, leading to an increase in hole concentration and a decrease in electron concentration. When the gasoline was injected in the test chamber, the alkene in the gasoline was adsorbed on the surface of the gas sensing materials, and then reacted with the oxygen adsorbed on the surface of the ZnO nanosheets. The electrons trapped by the adsorptive states will be released, leading to a decrease in sensor resistance. So, the ZnO nanosheets sensor is sensitive to gasoline. The same mechanism could be used to explain the gas sensing properties of acetone when the heating temperature was 360 °C. Long-term
H. Fan, X. Jia / Solid State Ionics 192 (2011) 688–692
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Fig. 4. The influence of working temperature on the gas response of ZnO nanosheets gas sensor.
Fig. 6. The response and recovery curves of the ZnO nanosheets gas sensor for 50 ppm acetone at different heating temperatures.
stability of the gas sensor was also measured by repeating the sensing measurement 8 times within 2 months, and no appreciable variations were detected during the test. Therefore, this ZnO gas sensor has an attractive stability for future commercial application. In order to investigate the selectivity of the sensors, our studies were extended to acetone, ammonia, ethanol, gasoline and toluene. The results under the heating temperature of 180 and 360 °C are shown in Fig. 5, which shows that the response to gasoline is excellently higher than those to other gases when the temperature at 180 °C, the response magnitude was about 30, while that to other gases was no greater than 5. It is obvious that the selectivity of the sensor to gasoline against all other gases is exceeding almost by 6 times. While the response to acetone can reaches 31 when the heating temperature is at 360 °C, it is apparent that acetone lead to significantly stronger responses compared to other gases, even if the response is only 1 at a low heating temperature, in other words, there almost no reaction to the acetone at low heating temperature. These results clearly demonstrate the high selectivity of ZnO nanosheets gas sensor for acetone and gasoline at different heating temperature, which is much different from that of gas sensors made by ZnO nanopowders [23]. Fig. 6 shows typical response and recovery curves for 50 ppm acetone at different heating temperatures. It is clear that both the response and recovery time improve with increasing operating temperature. At low heating temperature the response time for the sensor element lasted for several minutes, and the gas sensor had no apparent response to acetone when the heating temperature was
180 °C, 200 °C and 240 °C, and the curves almost not recovery when the gas sensors were exposed to air. The sensor signal at a high heating temperature was obviously improved, The 90% response time of the sensor at 300 °C was nearly 13 s and the recovery time of it was less than 30 s. The response time and recovery time of the sensor was more rapid at a higher heating temperature, which were less than 2 s and 15 s, respectively. It reveals that the response and recovery properties of ZnO nanosheets gas sensor are quite satisfying. 4. Conclusions ZnO nanosheets have been synthesized successfully by using a simple mixed hydrothermal method. The thickness of the ZnO nanosheets is in the range of 10–20 nm, and the width-to-thickness ratios of the ZnO sheets almost reach up to one thousand. The formation mechanism and effect of CTAB on the morphology of ZnO nanosheets have also been discussed. It has been found that the gas sensor made by these ZnO nanosheets can detect gasoline at 180 °C, and the same sensor can selectively detect acetone at 360 °C interestingly. Acknowledgements This work has been supported by the National Nature Science Foundation (50672075), the NCET and 111 Program (B08040) of MOE, and Xi'an Science & Technology Foundation (CXY08006, XAAM-200905, and XA-AM-200906), and the Fundamental Research Foundation (NPU-FFR-200703) and Doctorate Foundation (CX200804) of NPU, and the SKLSP Research Fund (40-QZ-2009) of China. References
Fig. 5. Sensitivity and selectivity of the gas sensor based on ZnO nanosheets to different tested gases.
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