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SiO2 composite film
Journal of Hazardous Materials 285 (2015) 368–374
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Surface acoustic wave ammonia sensor based on ZnO/SiO2 composite film Shuang-Yue Wang a , Jin-Yi Ma b , Zhi-Jie Li a , H.Q. Su c , N.R. Alkurd c , Wei-Lie Zhou c , Lu Wang b , Bo Du b , Yong-Liang Tang a , Dong-Yi Ao a , Shou-Chao Zhang a , Q.K. Yu d , Xiao-Tao Zu a,∗ a Institute of Fundamental and Frontier Sciences and School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu 610054, PR China b Sichuan Institute of Piezoelectric and Acousto-Optic Technology, Chongqing 400060, PR China c Advanced Materials Research Institute, University of New Orleans, LA 70148, USA d Ingram School of Engineering, and Materials Science, Engineering and Commercialization Program, Texas State University, San Marcos, TX 78666, USA
h i g h l i g h t s • • • •
A surface acoustic wave ammonia sensor based on ZnO/SiO2 composite film. The sensor demonstrates good sensitivity and selectivity to NH3 . The dangling Si bands on the surface of the film enhance the sensitivity to NH3 . The reactions on the surface of the film improve the sensor’s selectivity to NH3 .
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Article history: Received 17 August 2014 Received in revised form 4 November 2014 Accepted 8 December 2014 Available online 10 December 2014 Keywords: NH3 gas sensor SAW ZnO SiO2 Composite film
a b s t r a c t A surface acoustic wave (SAW) resonator with ZnO/SiO2 (ZS) composite film was used as an ammonia sensor in this study. ZS composite films were deposited on the surface of SAW devices using the sol–gel method, and were characterized using SEM, AFM, and XRD. The performance of the sensors under ammonia gas was optimized by adjusting the molar ratio of ZnO:SiO2 to 1:1, 1:2 and 1:3, and the sensor with the ratio of ZnO to SiO2 equaling to 1:2 was found to have the best performance. The response of sensor was 1.132 kHz under 10 ppm NH3, which was much higher than that of the sensor based on a pristine ZnO film. Moreover, the sensor has good selectivity, reversibility and stability at room temperature. These can be attributed to the enhanced absorption of ammonia and unique surface reaction on composite films due to the existence of silica. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Ammonia (NH3 ) is highly toxic, volatile chemical gas and extensively utilized in many chemical industries, food processing, and medical diagnosis. A leak in these systems can result the health hazards. Detection of NH3 in trace amounts is important in terms of environmental protection and human health, as well as industry production. In recent years, numerous materials including
∗ Corresponding author. Tel.: +86 13679012605. E-mail address: [email protected] (X.-T. Zu). http://dx.doi.org/10.1016/j.jhazmat.2014.12.014 0304-3894/© 2014 Elsevier B.V. All rights reserved.
semiconducting oxide materials, such as ZnO [1,2], ZrO2 [3], CdO [4] and TiO2 [5], organic compounds [6,7] and novel materials such as carbon nanotubes [8] and graphene [9,10] are exploited for ammonia detection due to their superior ammonia gas sensing properties. Composite system based sensors have been demonstrated good sensitivity and selectivity to a particular gas [11,12] by varying the type and composition of the components of the composite such as metal oxide-metal oxide [1,13], novel materials (carbon nanotube and graphene)- metal oxide [9,14], polymer- metal oxide [8,15] etc. For ammonia gas detection, many efforts have been made to pursue both high response and good selectivity. Sharma et al. [16] reported a MWCNTs/alumina composite-based sensor. The sensor
S.-Y. Wang et al. / Journal of Hazardous Materials 285 (2015) 368–374
exhibited good sensitivity with the detection level up to 6 ppm, but the response time is up to 10 min, moreover, the sensor didn’t show a good selectivity. Pawar et al. [17] reported a nanostructured polyaniline–titanium dioxide gas sensor for ammonia recognition. The sensor showed a good selectivity to ammonia, but the detection level was as low as 20 ppm ammonia. Other attempts have been also reported, however, the high cost [18], complex preparation [19], or the poor response, and selectivity make them unsatisfactory. In this work, we present composite films of SiO2 and ZnO for ammonia gas sensing, showing high response as well as good selectivity. To our knowledge, the composite of SiO2 and ZnO for ammonia gas sensing has not been reported. ZnO, as a semiconductor material with a wide band gap of 3.4 eV, is gaining considerable attention due to its high mobility of conduction electrons, excellent piezoelectric properties and good chemical and thermal stability under operating conditions. SiO2 film is porous [20,21], and this characteristic can increase the surface area for better adsorption of the target gas. Moreover, it has intensive surface-active property [22–24], which was found to enhance the surface reaction of ammonia molecules. This mechanism will be explained in this work. Among different kinds of ammonia gas sensors, such as semiconductor sensors [25,26], electrochemical sensors [27], QCM sensors [28,29] and SAW sensors [30,31], SAW sensors have superior performance due to high sensitivity, high stability, high accuracy and low cost. Generally, a surface acoustic wave (SAW) has a high acoustic energy density on the top-surface of piezoelectric substrate within one or two wavelengths. Thus, it is very sensitive to the surface perturbations, for example, a slight change in conductivity. A change of the conductivity can change the velocity of a SAW device (shown in Fig. 1). As a result, the central frequency of the SAW device changes. The relationship between the change of central frequency or frequency shift (f) versus the sheet conductivity ( s ) of the film can be described as [32]: f v K2 s2 = ≈ v0 2 s2 + v2 Cs2 f0 0
(1)
where K2 is an electromechanical coefficient, v0 is the unperturbed SAW velocity, Cs = 0 + p , is the sum of permittivity of the region above the film and the substrate, and s is the sheet conductivity of the sensing film. The frequency shift can reflect the sensitivity of a SAW gas sensor. In the present study, a surface acoustic wave (SAW) ammonia sensor based on ZnO/SiO2 composite film was investigated. The sensor was fabricated and tested under different concentrations of ammonia gas at room temperature, and the sensing mechanism was also discussed.
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Fig. 2. The schematic diagram of SAW device.
2. Experimental 2.1. Design and fabrication of the SAW device SAW devices based on piezoelectric ST-cut quartz substrate were used to fabricate the ammonia gas sensors, and the schematic diagram is shown in Fig. 2. The central frequency of the SAW device was 198.8 MHz and insertion loss is −15.5 dB. The SAW device consisted of input and output IDTs both with 30 pairs of fingers, and the width of each finger was 4 m, and the aperture of the IDTs was 3 mm. When the power was added to both sides of IDTs, the ST-cut quartz substrate was excited and produced surface acoustic wave due to the piezoelectric effect, spreading along the surface with a wavelength of 16 m. 2.2. Preparation of ZnO/SiO2 composite films on SAW devices ZnO/SiO2 (ZS) composite films were prepared by the sol-gel spin coating method. Zinc acetate dihydrate (Zn (CH3 COO)2 ·2H2 O) was dissolved in a mixture solution of 2-methoxyethanol (C3 H8 O2 , as a solvent) and ethanolamine (C2 H7 NO, EA, as a stabilizer) at room temperature. The molar ratio of EA to Zn(CH3 COO)2 2H2 O was 1.0, and the concentration of zinc acetate was 0.3 M. Then the resulting solution was stirred at 60 ◦ C for an hour to form a homogenous ZnO sol. The Stöber method was adopted to prepare silica sol [33]. The ethanol (analytic pure), TEOS (high purity), deionized water and ammonia (analytic pure liquid, 25 wt%) were successively added into a bunsen flask with a molar ratio of 1:3.25:37:0.17. The mixed solution was stirred at 30 ◦ C for 2 h and aged for 7 h. The concentration of the obtained silica sol was 0.5 M. Finally, the ZnO sol and silica sol were added simultaneously into a beaker at the molar ratio of 1:1, 1:2 and 1:3, expressed as R = 1:1, R = 1:2, and R = 1:3, respectively, and the mixed sols were stirred for another 30 min. Then the samples were aged for 24 h. For the preparation of composite films, the mixed sols were spin-coated onto the SAW devices at a speed of 6000 r/min for 30 s. Then, the as-coated devices were immediately pre-hearted at 300 ◦ C for 10 min. After repeating the coating and pre-heating procedures three times, the SAW devices were calcined in air at 480 ◦ C for 2 h to form the ZS composite films. 3. Results and discussion 3.1. Structural properties of ZS composite films
Fig. 1. The curve of velocity changes versus acoustoelectric parameter = s /v0 Cs .
XRD patterns of pristine ZnO powders and ZS (R = 1:1, R = 1:2 and 1:3) powders are shown in Fig. 3. XRD measurement of these powders corresponded to the different ZnO characteristic planes of (1 0 1), (1 0 0), (0 0 2), (1 0 2), (1 1 0), (1 0 3) and (1 1 2). The results proved ZnO existed as polycrystalline wurtzite crystal structure in these powders. As shown in Fig. 3(a–d), with the increasing of the SiO2 content, the intensity of characteristic peaks of ZnO became weaker. The crystallized-grain sizes of different powders were calculated. The sizes of pristine ZnO grain, ZS (R = 1:1) grain, ZS (R = 1:2) and ZS (R = 1:3) were 67.8 nm, 36.3 nm, 21.2 nm and
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was dense in Fig. 4(a), which consisted of uniform ZnO particles of average 15 nm. Fig. 4(b) and (c) presents the morphology of R = 1:1 and R = 1:2 ZS films respectively. As is shown, the ZS film (R = 1:1) was loose and some parts appeared surface undulation, which can provide a large surface area for an enhanced sensing response. As for the ZS film (R = 1:2), the surface became porous, surficial fluctuation became intense, and the overlapping and coalescence of the film became remarkable. Fig. 5 shows AFM images of pristine ZnO film and ZS(R = 1:1 and 1:2) composite films. From Fig. 5(a), it can be seen the pristine ZnO film was relatively flat. The ZS composite film surface (R = 1:1) raised partly as shown in Fig. 4(b), and the surface of the ZS composite film (R = 1:2) (shown in Fig. 5(c) became significantly more wrinkled and wavy. The surface roughness of pure ZnO film was 2.41 nm (RMS value expressed) and the composite films had a lager surface roughness than pure ZnO film as the surface roughness was 4.61 nm (R = 1:1) and 6.28 nm (R = 1:2). The larger surface roughness made larger surface area exposed to gas surroundings due to the wrinkled and wavy morphology. The larger surface area could adsorb more gas molecules when the target gas flowed over the surface.
3.2. Ammonia gas sensing performance
Fig. 3. XRD patterns of ZnO/SiO2 powders at different ratio.
17.1 nm respectively, which indicated the degree of crystallinity became poorer with the increasing of proportion of SiO2 . The SEM images of pristine ZnO film and ZS films (R = 1:1 and 1:2) are shown in Fig. 4. It can be clearly seen the pristine ZnO film
The responses of ammonia sensors with different ZS composite films under 50 ppm NH3 are given in Fig. 6. It shows that ZS composite films exhibited better sensing performance than the pristine ZnO film. The ZS composite film with R = 1:2 exhibited the maximum response with a frequency shift of 3.84 kHz. The recovery process of ZS composite film with R = 1:3 was slower than that of other films. Fig. 7 shows the dynamic responses of the sensors with pristine ZnO film and ZS composite film (R = 1:2) to a sequence
Fig. 4. SEM images of (a) pristine ZnO film (b) ZS film (R = 1:1) and (c) ZS film (R = 1:2).
Fig. 5. AFM images of (a) pristine ZnO film, (b) ZS film (R = 1:1) and (c) ZS film (R = 1:2).
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Fig. 6. The sensor responses with different ratio between ZnO and SiO2 under 50 ppm ammonia.
Fig. 7. The dynamic sensor responses under different concentrations of ammonia/synthetic air mixture at room temperature.
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of NH3 gas concentration of 10 ppm, 20 ppm, 50 ppm, 100 ppm, and 200 ppm in synthetic air at room temperature. The responses of the sensor with pristine ZnO film were 98 Hz, 235 Hz, 720 Hz, 1.52 kHz, and 2.86 kHz respectively and the sensor’s response with ZS film were 1.132 kHz, 1.95 kHz, 3.84 kHz, 5.15 kHz, and 8.276 kHz, respectively. The responses of the sensor with composite film were remarkable and virtual one order of magnitude lager than that of the sensor with pristine ZnO film at low concentration. The response and recovery times of the sensor with the optimum composite film (R = 1:2) and pristine ZnO film under different concentrations of ammonia are shown in Fig. 8. From Fig. 8(a), it can be seen that the sensor with composite film had a similar response tendency with pristine ZnO film. Considering the high response of the sensor with composite film, the response time is not too long, so the SAW sensor is suitable for the efficient selective detection of ammonia. The recovery time of the sensor with composite film (Fig. 8(b)) exhibited an abnormal tendency as the maximum recovery time appeared under the ammonia concentration of 50 ppm. This might be related to the complex desorption process. To investigate the selectivity of the sensor with optimum ZS film (R = 1:2), four different type non-target gases including reducing gases C2 H6 O, CO, and H2 , and oxidizing gas NO2 were tested under the same concentration of 100 ppm. The results are shown in Fig. 9. The frequency shifts under CO, NO2 , C2 H6 O, and H2 gases remained almost constant. Compared with the frequency shift of 5.27 kHz under NH3 gas, the responses under non-target gases were weak and negligible, suggesting that the sensor had an excellent selectivity to ammonia. The selectivity of the sensor with pristine ZnO film was also tested under the same conditions in contrast to the ZS film, and the results are shown Fig. 10. As can be seen that the sensor showed a much poorer selectivity as the sensor was sensitive to multiple gases tested. In fact, the composite of ZnO and SiO2 significantly improved the selectivity to ammonia, which can be due to the existence of silica surface. The mechanism would be elaborated in the part of the sensing mechanism. The sensor was exposed to 100 ppm ammonia/synthetic air mixture at room temperature for five cycles to investigate the reversibility and stability of the optimum composite film based ammonia sensor. The results are shown in Fig. 11. The series of responses were 5.14 kHz, 5.21 kHz, 5.30 kHz, 5.24 kHz and 5.18 kHz respectively. The fluctuation of frequency shift was less than 5%, revealing that the sensor had good stability.
Fig. 8. The response (a) and recovery times (b) of sensors under different ammonia concentrations.
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3.3. The sensing mechanism For pristine ZnO film, the sensing mechanism is based on the reactions that occurred at the sensor surface between ammonia gas and pristine ZnO film. According to Wang et al. [9] revelation that in an air environment, oxygen molecules absorbed onto the surface of ZnO films to form O2− , O− , O− 2 ions depending on the temperature by extracting electrons from the conduction band. This phenomenon would result in the formation of a depletion region at the surface. When ZnO film was exposed to ammonia gas, some ammonia molecules would become adsorbed to the surface of film, and they would get oxidized to NO, N2 , and H2 O due to the reducibility of ammonia gas. The mechanism was as follows [34] NH3 + 5O− (abs) → 2NO + 3H2 O + 5e− −
2NH3 +3O (abs) → N2 + 3H2 O + 3e
Fig. 9. The sensor responses with optimum ZS film to different gases under the same concentration of 100 ppm.
Fig. 10. The sensor responses with pristine ZnO film to different gases under the same concentration of 100 ppm.
(2) (3)
This progress would consume chemisorbed oxygen from the film surface and release electrons into the conduction band, resulting in a reduction of the surface depletion region, and then the film conductivity would increase, but the poor sensitivity of ZnO films may be due to the ZnO films are too conductive and tend to lie to the right of the sensitivity maximum in Fig. 1, so the increasing of the film conductivity could not result in the increasing of the sensitivity. The Reactions showed in (2) and (3) are also applicable for ZS films, moreover, the absolute value of ZS films’ conductivity can be tend to the right of the sensitivity maximum in Fig. 1 by adjusting the molar ratio of ZnO:SiO2 as 1:1, 1:2 and 1:3, so the increasing of the film conductivity could result in the enhanced sensitivity. Additionally, according to Sneh and George [35] and Leeuw et al. [36] reports, at atmospheric pressure and at temperatures below 150 ◦ C, a SiO2 surface was fully terminated by hydroxyl groups. It might be thought there were a mass of dangling Si bands on the surface, which leaded to the effective chemical absorption of hydroxyl groups. Once the SiO2 surface was fully hydroxylated, molecular water absorbed to OH− groups [37]. Ammonia gas has a large solubility in water (volume ratio of ammonia and water is 700:1). Thus, when the gas flowed over the film surface, ammonia molecules were captured by water on the surface, and became oxidized. This process would enhance the release of electrons into the conduction band and increase the film conductivity. Moreover, ammonia molecules could react with water molecules on the surface of SiO2 and initiate the process of proton conductivity via NH4 + at room temperature. The schematic diagram of the mechanism is shown in Fig. 12. The following reaction was expected to take place on the surface of SiO2 NH3(g) + H2 O(surface) NH4 + + OH− +
Fig. 11. The sensor responses under 100 ppm ammonia gas for five cycles.
−
(4)
The NH4 constituted proton conductivity, leading to a crucial increase in surface conductivity. The phenomenon might be also the reason for the good selectivity of this sensor. When ammonia gas was off, the Reaction (4) would antidromicly proceed. Then the conductivity would decrease and be back to its initial state, explaining the recovery of the sensor. We think the mechanism occurred on the SiO2 surface can also explain the selectivity of the sensor. To verify the key role of the SiO2 surface in the enhancing the sensor’s response to ammonia, the sensor with pristine SiO2 film was exposed to 50 ppm ammonia gas, and the results are shown in Fig. 13. We can observe the frequency shift was about 1 kHz. The hydroxyl groups and the unique reaction on the silica surface mentioned above were expected to be responsible for the response, however, the response was relatively small and unstable, and the recovery process was slow. Compared with ZS film, the composite of ZnO and SiO2 not only enhanced the response but also improved the recovery process.
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the National Natural Science Foundation of China and the China Academy of Engineering Physics(U1330108) and the National Natural Science Foundation of China(No. 11304032). References
Fig. 12. Schematic diagram of the reaction between NH3 and H2 O molecules.
Fig. 13. The response of the sensor with pristine SiO2 film under 50 ppm ammonia gas.
6. Conclusions A surface acoustic wave NH3 gas sensor based on ZS composite film has been prepared by an aqueous sol–gel technique. The ZS composite films have a larger surface area and rougher morphology than pristine ZnO film due to the wrinkled structure. The sensor with ZS film (R = 1:2) shows the best sensing properties and its maximum frequency shift is 1.132 kHz under 10 ppm ammonia gas. Moreover, the sensor also has good selectivity, stability and reversibility. The absolute value of ZS films conductivity can be tend to the right of the sensitivity maximum by adjusting the molar ratio of ZnO and SiO2 . Additionally, the existence of water molecules on the surface of SiO2 particles may contribute to the large response as it can absorb abundant ammonia molecules and constitutes proton conductivity via NH4 + by reacting with ammonia molecules. The results above suggest the possibility of utilizing the ZnO/SiO2 composite film sensor for the efficient selective detection of ammonia. Acknowledgements This work was supported by theFundamental Research Funds for the central Universities(ZYGX2012J047), the Joint Fund of
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Surface acoustic wave ammonia sensor based on ZnO/SiO2 composite film Shuang-Yue Wang a , Jin-Yi Ma b , Zhi-Jie Li a , H.Q. Su c , N.R. Alkurd c , Wei-Lie Zhou c , Lu Wang b , Bo Du b , Yong-Liang Tang a , Dong-Yi Ao a , Shou-Chao Zhang a , Q.K. Yu d , Xiao-Tao Zu a,∗ a Institute of Fundamental and Frontier Sciences and School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu 610054, PR China b Sichuan Institute of Piezoelectric and Acousto-Optic Technology, Chongqing 400060, PR China c Advanced Materials Research Institute, University of New Orleans, LA 70148, USA d Ingram School of Engineering, and Materials Science, Engineering and Commercialization Program, Texas State University, San Marcos, TX 78666, USA
h i g h l i g h t s • • • •
A surface acoustic wave ammonia sensor based on ZnO/SiO2 composite film. The sensor demonstrates good sensitivity and selectivity to NH3 . The dangling Si bands on the surface of the film enhance the sensitivity to NH3 . The reactions on the surface of the film improve the sensor’s selectivity to NH3 .
a r t i c l e
i n f o
Article history: Received 17 August 2014 Received in revised form 4 November 2014 Accepted 8 December 2014 Available online 10 December 2014 Keywords: NH3 gas sensor SAW ZnO SiO2 Composite film
a b s t r a c t A surface acoustic wave (SAW) resonator with ZnO/SiO2 (ZS) composite film was used as an ammonia sensor in this study. ZS composite films were deposited on the surface of SAW devices using the sol–gel method, and were characterized using SEM, AFM, and XRD. The performance of the sensors under ammonia gas was optimized by adjusting the molar ratio of ZnO:SiO2 to 1:1, 1:2 and 1:3, and the sensor with the ratio of ZnO to SiO2 equaling to 1:2 was found to have the best performance. The response of sensor was 1.132 kHz under 10 ppm NH3, which was much higher than that of the sensor based on a pristine ZnO film. Moreover, the sensor has good selectivity, reversibility and stability at room temperature. These can be attributed to the enhanced absorption of ammonia and unique surface reaction on composite films due to the existence of silica. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Ammonia (NH3 ) is highly toxic, volatile chemical gas and extensively utilized in many chemical industries, food processing, and medical diagnosis. A leak in these systems can result the health hazards. Detection of NH3 in trace amounts is important in terms of environmental protection and human health, as well as industry production. In recent years, numerous materials including
∗ Corresponding author. Tel.: +86 13679012605. E-mail address: [email protected] (X.-T. Zu). http://dx.doi.org/10.1016/j.jhazmat.2014.12.014 0304-3894/© 2014 Elsevier B.V. All rights reserved.
semiconducting oxide materials, such as ZnO [1,2], ZrO2 [3], CdO [4] and TiO2 [5], organic compounds [6,7] and novel materials such as carbon nanotubes [8] and graphene [9,10] are exploited for ammonia detection due to their superior ammonia gas sensing properties. Composite system based sensors have been demonstrated good sensitivity and selectivity to a particular gas [11,12] by varying the type and composition of the components of the composite such as metal oxide-metal oxide [1,13], novel materials (carbon nanotube and graphene)- metal oxide [9,14], polymer- metal oxide [8,15] etc. For ammonia gas detection, many efforts have been made to pursue both high response and good selectivity. Sharma et al. [16] reported a MWCNTs/alumina composite-based sensor. The sensor
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exhibited good sensitivity with the detection level up to 6 ppm, but the response time is up to 10 min, moreover, the sensor didn’t show a good selectivity. Pawar et al. [17] reported a nanostructured polyaniline–titanium dioxide gas sensor for ammonia recognition. The sensor showed a good selectivity to ammonia, but the detection level was as low as 20 ppm ammonia. Other attempts have been also reported, however, the high cost [18], complex preparation [19], or the poor response, and selectivity make them unsatisfactory. In this work, we present composite films of SiO2 and ZnO for ammonia gas sensing, showing high response as well as good selectivity. To our knowledge, the composite of SiO2 and ZnO for ammonia gas sensing has not been reported. ZnO, as a semiconductor material with a wide band gap of 3.4 eV, is gaining considerable attention due to its high mobility of conduction electrons, excellent piezoelectric properties and good chemical and thermal stability under operating conditions. SiO2 film is porous [20,21], and this characteristic can increase the surface area for better adsorption of the target gas. Moreover, it has intensive surface-active property [22–24], which was found to enhance the surface reaction of ammonia molecules. This mechanism will be explained in this work. Among different kinds of ammonia gas sensors, such as semiconductor sensors [25,26], electrochemical sensors [27], QCM sensors [28,29] and SAW sensors [30,31], SAW sensors have superior performance due to high sensitivity, high stability, high accuracy and low cost. Generally, a surface acoustic wave (SAW) has a high acoustic energy density on the top-surface of piezoelectric substrate within one or two wavelengths. Thus, it is very sensitive to the surface perturbations, for example, a slight change in conductivity. A change of the conductivity can change the velocity of a SAW device (shown in Fig. 1). As a result, the central frequency of the SAW device changes. The relationship between the change of central frequency or frequency shift (f) versus the sheet conductivity ( s ) of the film can be described as [32]: f v K2 s2 = ≈ v0 2 s2 + v2 Cs2 f0 0
(1)
where K2 is an electromechanical coefficient, v0 is the unperturbed SAW velocity, Cs = 0 + p , is the sum of permittivity of the region above the film and the substrate, and s is the sheet conductivity of the sensing film. The frequency shift can reflect the sensitivity of a SAW gas sensor. In the present study, a surface acoustic wave (SAW) ammonia sensor based on ZnO/SiO2 composite film was investigated. The sensor was fabricated and tested under different concentrations of ammonia gas at room temperature, and the sensing mechanism was also discussed.
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Fig. 2. The schematic diagram of SAW device.
2. Experimental 2.1. Design and fabrication of the SAW device SAW devices based on piezoelectric ST-cut quartz substrate were used to fabricate the ammonia gas sensors, and the schematic diagram is shown in Fig. 2. The central frequency of the SAW device was 198.8 MHz and insertion loss is −15.5 dB. The SAW device consisted of input and output IDTs both with 30 pairs of fingers, and the width of each finger was 4 m, and the aperture of the IDTs was 3 mm. When the power was added to both sides of IDTs, the ST-cut quartz substrate was excited and produced surface acoustic wave due to the piezoelectric effect, spreading along the surface with a wavelength of 16 m. 2.2. Preparation of ZnO/SiO2 composite films on SAW devices ZnO/SiO2 (ZS) composite films were prepared by the sol-gel spin coating method. Zinc acetate dihydrate (Zn (CH3 COO)2 ·2H2 O) was dissolved in a mixture solution of 2-methoxyethanol (C3 H8 O2 , as a solvent) and ethanolamine (C2 H7 NO, EA, as a stabilizer) at room temperature. The molar ratio of EA to Zn(CH3 COO)2 2H2 O was 1.0, and the concentration of zinc acetate was 0.3 M. Then the resulting solution was stirred at 60 ◦ C for an hour to form a homogenous ZnO sol. The Stöber method was adopted to prepare silica sol [33]. The ethanol (analytic pure), TEOS (high purity), deionized water and ammonia (analytic pure liquid, 25 wt%) were successively added into a bunsen flask with a molar ratio of 1:3.25:37:0.17. The mixed solution was stirred at 30 ◦ C for 2 h and aged for 7 h. The concentration of the obtained silica sol was 0.5 M. Finally, the ZnO sol and silica sol were added simultaneously into a beaker at the molar ratio of 1:1, 1:2 and 1:3, expressed as R = 1:1, R = 1:2, and R = 1:3, respectively, and the mixed sols were stirred for another 30 min. Then the samples were aged for 24 h. For the preparation of composite films, the mixed sols were spin-coated onto the SAW devices at a speed of 6000 r/min for 30 s. Then, the as-coated devices were immediately pre-hearted at 300 ◦ C for 10 min. After repeating the coating and pre-heating procedures three times, the SAW devices were calcined in air at 480 ◦ C for 2 h to form the ZS composite films. 3. Results and discussion 3.1. Structural properties of ZS composite films
Fig. 1. The curve of velocity changes versus acoustoelectric parameter = s /v0 Cs .
XRD patterns of pristine ZnO powders and ZS (R = 1:1, R = 1:2 and 1:3) powders are shown in Fig. 3. XRD measurement of these powders corresponded to the different ZnO characteristic planes of (1 0 1), (1 0 0), (0 0 2), (1 0 2), (1 1 0), (1 0 3) and (1 1 2). The results proved ZnO existed as polycrystalline wurtzite crystal structure in these powders. As shown in Fig. 3(a–d), with the increasing of the SiO2 content, the intensity of characteristic peaks of ZnO became weaker. The crystallized-grain sizes of different powders were calculated. The sizes of pristine ZnO grain, ZS (R = 1:1) grain, ZS (R = 1:2) and ZS (R = 1:3) were 67.8 nm, 36.3 nm, 21.2 nm and
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was dense in Fig. 4(a), which consisted of uniform ZnO particles of average 15 nm. Fig. 4(b) and (c) presents the morphology of R = 1:1 and R = 1:2 ZS films respectively. As is shown, the ZS film (R = 1:1) was loose and some parts appeared surface undulation, which can provide a large surface area for an enhanced sensing response. As for the ZS film (R = 1:2), the surface became porous, surficial fluctuation became intense, and the overlapping and coalescence of the film became remarkable. Fig. 5 shows AFM images of pristine ZnO film and ZS(R = 1:1 and 1:2) composite films. From Fig. 5(a), it can be seen the pristine ZnO film was relatively flat. The ZS composite film surface (R = 1:1) raised partly as shown in Fig. 4(b), and the surface of the ZS composite film (R = 1:2) (shown in Fig. 5(c) became significantly more wrinkled and wavy. The surface roughness of pure ZnO film was 2.41 nm (RMS value expressed) and the composite films had a lager surface roughness than pure ZnO film as the surface roughness was 4.61 nm (R = 1:1) and 6.28 nm (R = 1:2). The larger surface roughness made larger surface area exposed to gas surroundings due to the wrinkled and wavy morphology. The larger surface area could adsorb more gas molecules when the target gas flowed over the surface.
3.2. Ammonia gas sensing performance
Fig. 3. XRD patterns of ZnO/SiO2 powders at different ratio.
17.1 nm respectively, which indicated the degree of crystallinity became poorer with the increasing of proportion of SiO2 . The SEM images of pristine ZnO film and ZS films (R = 1:1 and 1:2) are shown in Fig. 4. It can be clearly seen the pristine ZnO film
The responses of ammonia sensors with different ZS composite films under 50 ppm NH3 are given in Fig. 6. It shows that ZS composite films exhibited better sensing performance than the pristine ZnO film. The ZS composite film with R = 1:2 exhibited the maximum response with a frequency shift of 3.84 kHz. The recovery process of ZS composite film with R = 1:3 was slower than that of other films. Fig. 7 shows the dynamic responses of the sensors with pristine ZnO film and ZS composite film (R = 1:2) to a sequence
Fig. 4. SEM images of (a) pristine ZnO film (b) ZS film (R = 1:1) and (c) ZS film (R = 1:2).
Fig. 5. AFM images of (a) pristine ZnO film, (b) ZS film (R = 1:1) and (c) ZS film (R = 1:2).
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Fig. 6. The sensor responses with different ratio between ZnO and SiO2 under 50 ppm ammonia.
Fig. 7. The dynamic sensor responses under different concentrations of ammonia/synthetic air mixture at room temperature.
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of NH3 gas concentration of 10 ppm, 20 ppm, 50 ppm, 100 ppm, and 200 ppm in synthetic air at room temperature. The responses of the sensor with pristine ZnO film were 98 Hz, 235 Hz, 720 Hz, 1.52 kHz, and 2.86 kHz respectively and the sensor’s response with ZS film were 1.132 kHz, 1.95 kHz, 3.84 kHz, 5.15 kHz, and 8.276 kHz, respectively. The responses of the sensor with composite film were remarkable and virtual one order of magnitude lager than that of the sensor with pristine ZnO film at low concentration. The response and recovery times of the sensor with the optimum composite film (R = 1:2) and pristine ZnO film under different concentrations of ammonia are shown in Fig. 8. From Fig. 8(a), it can be seen that the sensor with composite film had a similar response tendency with pristine ZnO film. Considering the high response of the sensor with composite film, the response time is not too long, so the SAW sensor is suitable for the efficient selective detection of ammonia. The recovery time of the sensor with composite film (Fig. 8(b)) exhibited an abnormal tendency as the maximum recovery time appeared under the ammonia concentration of 50 ppm. This might be related to the complex desorption process. To investigate the selectivity of the sensor with optimum ZS film (R = 1:2), four different type non-target gases including reducing gases C2 H6 O, CO, and H2 , and oxidizing gas NO2 were tested under the same concentration of 100 ppm. The results are shown in Fig. 9. The frequency shifts under CO, NO2 , C2 H6 O, and H2 gases remained almost constant. Compared with the frequency shift of 5.27 kHz under NH3 gas, the responses under non-target gases were weak and negligible, suggesting that the sensor had an excellent selectivity to ammonia. The selectivity of the sensor with pristine ZnO film was also tested under the same conditions in contrast to the ZS film, and the results are shown Fig. 10. As can be seen that the sensor showed a much poorer selectivity as the sensor was sensitive to multiple gases tested. In fact, the composite of ZnO and SiO2 significantly improved the selectivity to ammonia, which can be due to the existence of silica surface. The mechanism would be elaborated in the part of the sensing mechanism. The sensor was exposed to 100 ppm ammonia/synthetic air mixture at room temperature for five cycles to investigate the reversibility and stability of the optimum composite film based ammonia sensor. The results are shown in Fig. 11. The series of responses were 5.14 kHz, 5.21 kHz, 5.30 kHz, 5.24 kHz and 5.18 kHz respectively. The fluctuation of frequency shift was less than 5%, revealing that the sensor had good stability.
Fig. 8. The response (a) and recovery times (b) of sensors under different ammonia concentrations.
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3.3. The sensing mechanism For pristine ZnO film, the sensing mechanism is based on the reactions that occurred at the sensor surface between ammonia gas and pristine ZnO film. According to Wang et al. [9] revelation that in an air environment, oxygen molecules absorbed onto the surface of ZnO films to form O2− , O− , O− 2 ions depending on the temperature by extracting electrons from the conduction band. This phenomenon would result in the formation of a depletion region at the surface. When ZnO film was exposed to ammonia gas, some ammonia molecules would become adsorbed to the surface of film, and they would get oxidized to NO, N2 , and H2 O due to the reducibility of ammonia gas. The mechanism was as follows [34] NH3 + 5O− (abs) → 2NO + 3H2 O + 5e− −
2NH3 +3O (abs) → N2 + 3H2 O + 3e
Fig. 9. The sensor responses with optimum ZS film to different gases under the same concentration of 100 ppm.
Fig. 10. The sensor responses with pristine ZnO film to different gases under the same concentration of 100 ppm.
(2) (3)
This progress would consume chemisorbed oxygen from the film surface and release electrons into the conduction band, resulting in a reduction of the surface depletion region, and then the film conductivity would increase, but the poor sensitivity of ZnO films may be due to the ZnO films are too conductive and tend to lie to the right of the sensitivity maximum in Fig. 1, so the increasing of the film conductivity could not result in the increasing of the sensitivity. The Reactions showed in (2) and (3) are also applicable for ZS films, moreover, the absolute value of ZS films’ conductivity can be tend to the right of the sensitivity maximum in Fig. 1 by adjusting the molar ratio of ZnO:SiO2 as 1:1, 1:2 and 1:3, so the increasing of the film conductivity could result in the enhanced sensitivity. Additionally, according to Sneh and George [35] and Leeuw et al. [36] reports, at atmospheric pressure and at temperatures below 150 ◦ C, a SiO2 surface was fully terminated by hydroxyl groups. It might be thought there were a mass of dangling Si bands on the surface, which leaded to the effective chemical absorption of hydroxyl groups. Once the SiO2 surface was fully hydroxylated, molecular water absorbed to OH− groups [37]. Ammonia gas has a large solubility in water (volume ratio of ammonia and water is 700:1). Thus, when the gas flowed over the film surface, ammonia molecules were captured by water on the surface, and became oxidized. This process would enhance the release of electrons into the conduction band and increase the film conductivity. Moreover, ammonia molecules could react with water molecules on the surface of SiO2 and initiate the process of proton conductivity via NH4 + at room temperature. The schematic diagram of the mechanism is shown in Fig. 12. The following reaction was expected to take place on the surface of SiO2 NH3(g) + H2 O(surface) NH4 + + OH− +
Fig. 11. The sensor responses under 100 ppm ammonia gas for five cycles.
−
(4)
The NH4 constituted proton conductivity, leading to a crucial increase in surface conductivity. The phenomenon might be also the reason for the good selectivity of this sensor. When ammonia gas was off, the Reaction (4) would antidromicly proceed. Then the conductivity would decrease and be back to its initial state, explaining the recovery of the sensor. We think the mechanism occurred on the SiO2 surface can also explain the selectivity of the sensor. To verify the key role of the SiO2 surface in the enhancing the sensor’s response to ammonia, the sensor with pristine SiO2 film was exposed to 50 ppm ammonia gas, and the results are shown in Fig. 13. We can observe the frequency shift was about 1 kHz. The hydroxyl groups and the unique reaction on the silica surface mentioned above were expected to be responsible for the response, however, the response was relatively small and unstable, and the recovery process was slow. Compared with ZS film, the composite of ZnO and SiO2 not only enhanced the response but also improved the recovery process.
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the National Natural Science Foundation of China and the China Academy of Engineering Physics(U1330108) and the National Natural Science Foundation of China(No. 11304032). References
Fig. 12. Schematic diagram of the reaction between NH3 and H2 O molecules.
Fig. 13. The response of the sensor with pristine SiO2 film under 50 ppm ammonia gas.
6. Conclusions A surface acoustic wave NH3 gas sensor based on ZS composite film has been prepared by an aqueous sol–gel technique. The ZS composite films have a larger surface area and rougher morphology than pristine ZnO film due to the wrinkled structure. The sensor with ZS film (R = 1:2) shows the best sensing properties and its maximum frequency shift is 1.132 kHz under 10 ppm ammonia gas. Moreover, the sensor also has good selectivity, stability and reversibility. The absolute value of ZS films conductivity can be tend to the right of the sensitivity maximum by adjusting the molar ratio of ZnO and SiO2 . Additionally, the existence of water molecules on the surface of SiO2 particles may contribute to the large response as it can absorb abundant ammonia molecules and constitutes proton conductivity via NH4 + by reacting with ammonia molecules. The results above suggest the possibility of utilizing the ZnO/SiO2 composite film sensor for the efficient selective detection of ammonia. Acknowledgements This work was supported by theFundamental Research Funds for the central Universities(ZYGX2012J047), the Joint Fund of
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