- Email: [email protected]
The Gas Sensing Mechanism of the Low-Dimension Carbon Composites with Metal Oxide Quantum Dots
Available online at www.sciencedirect.com
Physics Procedia 32 (2012) 31 – 38
18th International Vacuum Congress (IVC-18)
The gas sensing mechanism of the low-dimension carbon composites with metal oxide quantum dots Hui Maa, Weiman Zhoua, Wu Yuana, Zheng Jieb, Hongzhong Liub, Xin. Lia a
Department of Microelectronics, Xi’an Jiaotong University, Xi’an 710049, China bKey Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710049, China
Abstract In this paper, we obtained three kinds of composite materials which were composed of metal oxides (ZnO, SnO2 and TiO2) and CNTs through catalytic pyrolysis method. Then we carried out the surface morphology, field emission and gas sensitivity properties test for them, and summarized the composite ways of metal oxides/CNTs by comparing three composite properties such as the changes in field emission and gas sensing properties, so that we might explore a set of preparation methods and processes of high performance gas sensors. At the same time, the study of field emission can also provide some improved methods to the traditional display technology. © 2012Published Publishedbyby Elsevier © 2010 Elsevier B.V.B.V. Selection and/or peer review under responsibility of Chinese Vacuum Society (CVS). PACS: 81.16.Hc; 81.05. –t; 82.80.-d
Key words: CNT; Metal oxide semiconductor; Composite materials; Gas sensitivity; Field emission
1. Introduction In 1991, CNT was first discovered by Dr Iijima (S. Iijima) who worked in the electronics company of NEC in Japanese [1]. CNT is an ideal one-dimensional material with excellent properties of large specific surface area, small scale, and showing conductivity of metal or semiconductor at different diameters and chirality [4], as well as a nanoscale material with some quantum effects, which determines the applications of CNT in field emission cathode materials and gas sensors. CNT is an ideal field emission cathode material because of its advantages of small curvature radius, large aspect ratio, high conductivity and excellent mechanical strength and chemical stability. In addition, large specific surface area makes CNT a strong adsorption capacity to some gas molecules. Accompanied with the electrical properties of semiconductor, the interactions between CNT and adsorbed gas molecules change the CNT’s Fermi level, which leads to change in material resistivity. Thus, measuring the resistivity change can be used to detect certain gases, and the change degree of resistance is directly related to the gas content. Furthermore, detecting gas by CNT is better than conventional gas sensing materials because of high sensitivity, fast response, small size and working at room temperature. However, due to the relatively stable of CNT’s electronic structure, only some strong oxidizing and reducing gas, such as O2 [5], NO2 [6], NH3 and SO2, can be adsorbed, and others are
Xin. Li.Tel: +86-029-8266-3041-601;fax:+86-029-8266-3426 E-mail address: [email protected]
1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer review under responsibility of Chinese Vacuum Society (CVS). doi:10.1016/j.phpro.2012.03.514
32
Hui Ma et al. / Physics Procedia 32 (2012) 31 – 38
not. Experimental and theoretical studies have shown that modifying CNT can improve the gas sensitivity, detection sensitivity and selectivity of gas sensors. Some modification methods of CNT are organic surface modification, oxidization, doping (boron, nitrogen, aluminum, etc.) and mechanical deformation [8]. Metal oxide semiconductor sensors are sensors based on resistive metal oxide materials, including WO3, SnO2, ZnO, In2O3 and TiO2 [9]. Its mechanism of gas sensing properties is that the resistance of metal oxides will change when a series of reactions happen between measured gas and metal oxides at a certain temperature. However, the traditional metal oxide gas sensors have a wide range of gas detection and good selectivity to different gases, but their detection sensitivity and response rate are low. A new type of gas sensor using composite material of metal oxide and CNTs will be invented with both advantages of the two materials, taking advantage of small scale of CNT and good selectivity of metal oxide. Moreover, combining metal oxide with CNTs will make metal oxide particles in nano-scale so as to increase the contact area between measured gas and metal oxide, which will improve the gas sensing properties according to the metal oxide gas sensing principle that the smaller material particle size is, the larger the contact area with the measured gas is, and the more obvious gas sensing properties will be. Thus this kind of gas sensing material will have both advantages of metal oxide and CNTs as good selectivity, fast response and working at room temperature. Gas sensors using nano-materials have advantages that conventional sensors can not be replaced as follows: (1) high sensitivity, due to small particle size and large specific surface area of nano-solid materials, which enlarge the interface with gas so as to provide a large number of channels for gas; (2) Room temperature testing. 2. Experiment Three kinds of metal oxides (ZnO, SnO2 and TiO2) were selected to be combined with CNTs in this experiment. The samples’ making process referring to the patent of growth and purification of CNTs by catalytic pyrolysis method [7] was as follows: Initially, thick film of metal oxide nano-particles was printed on substrate by screen printing. Then, CNTs grew on the thick film by catalytic pyrolysis method, and the composite process of metal oxides and CNTs would also be completed at the same time. After the preparation of samples, tests of the composite materials were done, including surface morphology by SEM, field emission at vacuum condition and gas sensitivity properties by I-V. There were two kinds of substrates used in this experiment. One kind was low resistivity silicon (ij = 100mm, Ptype, 480ȝm in thickness, ȡ = 10-2-10-3ȍ • cm). The other kind was a Pt interdigital electrode on SiO2/Si shown in figure 1.
Figure 1 photo of interdigital electrode
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Hui Ma et al. / Physics Procedia 32 (2012) 31 – 38
Figure 2 shows the testing principle of field emission properties. DC voltage was added between two plates while cathode plate was the sample, and anode plate was conductive glass coated with phosphor. Then the part of anode plate coated with phosphor would glow caused by electrons emitted from the sample surface bombarding phosphor when applied voltage reached the threshold voltage under vacuum condition. The ampere meter could show the discharge current at the same time. Finally, field emission properties were studied by comparing threshold voltages and discharge currents of different samples.
Figure 2 the testing principle of field emission properties
3. Results and discussion The corresponding relationship between sample number and sample kinds are shown in Table 1. Table 1 the corresponding relationship between sample number and sample kinds Samples
ZnO/MWCNT
SnO2/MWCNT
TiO2/MWCNT
ZnO
MWCNT
Number
1
2
3
4
5
3.1. SEM analysis of the samples’ surface morphology
Figure 3 SEM image of ZnO / MWCNT composite material
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Hui Ma et al. / Physics Procedia 32 (2012) 31 – 38
The surface morphology of composite material that CNTs grew on ZnO thick film shown in Figure 3 is that there are many tubes of different orientations with a lot of tiny white particles on their side wall. The tubes woven into fibrous network are CNTs, and the white nano-particles are ZnO dot.
Figure 4 SEM image of SnO2/ MWCNT composite material (CNTs grew on the edge of SnO2)
Figure 5 SEM image of SnO2 / MWCNT composite material(CNTs grew on evenly distributed part of SnO2)
The surface morphology of composite material that CNTs grew on SnO2 thick film shown in Figures 4-5 is that: On the edge of SnO2 thick film, CNTs were similar to those on the edge of ZnO thick film. However, there are a few two-dimensional plane carbon nano-walls nearby the big SnO2 particles. On the evenly distributed part of SnO2 thick film, the size of SnO2 particles is similar, and most of the composite structures are carbon nano-walls while there are both carbon nano-walls and CNTs that carbon nano-walls appear nearby the big SnO2 particles, and CNTs and small SnO2 particles are distributed between them on unevenly distributed part of SnO2.
Figure 6 SEM image of TiO2 / MWCNT composite material
The surface morphology of composite material that CNTs grew on TiO2 thick film shown in Figure 6 is that: On the edge of TiO2 thick film, CNTs also appear as fibrous network with some TiO2 particles in it. On the evenly distributed part of TiO2 thick film, TiO2 particles are embedded in CNTs so that the diameters of CNTs are larger than normal while there are great differences in the diameters of CNTs on unevenly distributed part of TiO2 thick
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Hui Ma et al. / Physics Procedia 32 (2012) 31 – 38
film. Therefore, the combining of TiO2 and CNTs is that TiO2 particles are embedded in CNTs, and different sizes of TiO2 particles will get different diameters of CNTs. Microstructure comparisons of three kinds of CNTs / metal oxide composite materials are as follows: (1) There are significant differences in surface morphologies of three CNTs / metal oxide composite materials: In CNTs / ZnO composite materials, ZnO particles are distributed on the ektexines of CNTs. In CNTs / SnO2 composite materials, SnO2 particles are centers of growth of carbon nano-walls. In CNTs / TiO2 composite materials, TiO2 particles are embedded in CNTs, and different sizes of TiO2 particles will get different diameters of CNTs. (2) Metal oxide particles diffuse to the edge in the process of growth, because metal oxide particles can be found at the edge of sample where there is no film. However, the particles’ size is smaller, and the CNTs are finer there. (3) The size of metal oxide particles will affect the shape of CNTs. So to get uniform and reliable composite CNTs requires accurate control on particles size.
3.2. Field emission phenomena and data analysis Field emission I-V curves were done with three samples of ZnO / MWCNT, SnO2/MWCNT and TiO2/MWCNT as the cathode, and the I-V curves are shown in Figure 7. From the I-V curves, it can be seen that the threshold voltage of SnO2 / MWCNT sample is higher than others, and its discharge current intensity increases slowly while ZnO / MWCNT sample is similar to TiO2/MWCNT sample, caused by the different surface morphologies of three composite materials that ZnO/MWCNT and TiO2/MWCNT present CNTs structure, but SnO2/MWCNT presents carbon nano-walls structure. In addition, the comparisons of threshold voltages and emission currents at 500V voltage listed at Tables 2-3 indicate that the threshold voltage of ZnO / MWCNT sample is the minimum, and discharge current of TiO2/MWCNT sample is the maximum at the same operating voltage as 500V. ZnO/MWCNT SnO2/MWCNT TiO2/MWCNT
500
Current (uA)
400
300
200
100
0 0
50 100 150 200 250 300 350 400 450 500 550 600 650 700 750
Voltage (V)
Figure 7 field emission IV character of three metal oxide/MWCNT composite materials
Table 2˖Comparison of threshold voltages Samples
ZnO/MWCNT
SnO2/MWCNT
TiO2/MWCNT
Turn-on voltage˄V˅
200
340
230
Table 3: Comparison of currents in the same voltage˄500V˅ Samples
ZnO/MWCNT
SnO2/MWCNT
TiO2/MWCNT
Current˄ȝA˅
130
20
140
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Hui Ma et al. / Physics Procedia 32 (2012) 31 – 38
Field emission images of three composite materials are shown as Figures 8. These images show the vacuum field emission at a high voltage without ampere meter. They suggest that there are more bright spots at the edge of composite materials with the reason that, the composite CNTs at the edge was deformed, even turned up, when the sample was padded with the insulation layer, which leads to the phosphor light earlier for the plate spacing of anode and cathode is shortened. It also can be seen, from Figure 8, that field emission property of SnO2/MWCNT composite material is poorer than other two.
Figure 8 Field emission images ˄a˅ZnO/MWCNT (b) SnO2/MWCNT (c) TiO2/MWCNT
3.3. Gas sensitivity experiment The curves of current changes (BI) with time (t) under the condition that ethanol gas flows across the surfaces of samples are shown in Figure.9. Figures 9a-9b suggest that current decreases when ethanol gas passes across the surfaces of samples, that is, the number of carriers between two electrodes is reduced after adsorbing ethanol gas, which leads to the increase of resistivity in macroscopic view. There are some results by comparing all the samples as follows: (1) The current of pure metal oxide film is small. It fluctuates with the process of test due to some uncertain conditions, and it is saturated in a short time. Moreover, the measuring range of gas concentration of the pure metal oxide film is small too. (2) The current of CNTs decreases significantly with time, and the sensitivity of CNTs are higher than metal oxide. (3) The current change rates of different samples under the condition of 3.0v are calculated: ȡ1 = 2.55682 × 10-1%, ȡ2 = 55.7417%. Thus, the samples of higher current change rate are metal oxide MWCNT composite film samples, and the lower are pure CNTs samples. In addition, the current of pure metal oxide film samples is low and easy to be interfered with for their bad electron conductivity whereas composite material samples of metal oxide and CNTs are higher than pure CNTs in current change rate. BI
BI 0.012296
0.003168 0.012294 0.012292
0.003166
BI (A)
BI (A)
0.012290 0.012288
0.003164
0.012286
0.003162
0.012284 0.012282
0.003160 0.012280 0
2
4
t (min)
(a)
6
8
10
0
2
4
6
8
t (min)
(b)
Figure 9 the BI-t curve of the different samples. (a) MWCNT sample, (b) SnO2/MWCNT sample
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Hui Ma et al. / Physics Procedia 32 (2012) 31 – 38
37
4. Conclusions This paper presents the preparation of composite materials, the test & analysis of composite materials, and some satisfactory results. We prepared three kinds of CNTs/metal oxide composite materials by catalytic pyrolysis method. Then, we tested these composite materials, including surface morphology by SEM, field emission at vacuum condition and gas sensitivity properties by I-V, and analyzed the experimental results. Finally, we draw some conclusions as follows: (1) ZnO particles are distributed on the side wall of CNTs, SnO2 particles are attached to carbon nano-walls, and TiO2 particles are embedded in CNTs. (2)Field emission properties of composite materials are affected by composite micro-morphology, the field emission property of CNTs/SnO2 composite material is worse than other two composite materials. (3) It is verified that the metal-oxides show semiconductor IV characteristics in the test of gas sensing properties. All the samples electrical conductivity decreases when ethanol gas passes through, the electrical conductivity of metal oxide thick film increases after combined with CNTs, but the current change rate decreases at gas test. Nevertheless, gas sensing properties of composite materials are accord with expectations. Acknowledgements This paper was supported by National Natural Science Foundation of China (No. 729092923040, 60801022ˈ 60476037, 50975226, 60976058HZ), National Basic Research Program of China (No. 2009CB724202), New Century Excellent Talents (NCET-08-0447), and the Fundamental Research Funds for the Central Universities. References [1] Iijima S.Helical microtubules of graphitic carbon[J].1991. [2] Y.H.Yuan and Z.J.Liu, Z.W. Li.The mechanism of improving the CNTs gas sensors by doping[R].2004 [3] PHILIP G.COLLINS, KEITH B, et al. Extreme oxygen sensitivity of electronic properties of carbon nanotubes[R]. 2000. [4] J. Kong, N. R.Franklin, et al. Nanotube molecular wires as chemical sensors [J].2000. [5] C.C.Zhu and X.Li. W.H. Liu. H.J.Liu. The CNT growth process by single temperature zone resistance furnace pyrolysis method and purification process. 200510096426 [P].2006. [6] X..Y Wen. The modifying CNT gas sensors [J].2008. [7] X.J.Liu and X.Q.Tan. S.Huan. The metal oxide gas sensors [J].2007.
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Conference Title˖18th International Vacuum Congress (IVC-18) The title of paper: The gas sensing mechanism of the low-dimension carbon composites with metal oxide quantum dots Author: Hui Ma, Weiman Zhou, Wu Yuan, Zheng Jie, Hongzhong Liu, Xin. Li Email: [email protected], [email protected] First affiliation: Department of Microelectronics, Xi’an Jiaotong University, Xi’an 710049, China Second affiliation: Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710049, China
Abstract In this paper, we obtained three kinds of composite materials which were composed of metal oxides (ZnO, SnO2 and TiO2) and CNTs through catalytic pyrolysis method. Then we carried out the surface morphology, field emission and gas sensitivity properties test for them, and summarized the composite ways of metal oxides/CNTs by comparing three composite properties such as the changes in field emission and gas sensing properties, so that we might explore a set of preparation methods and processes of high performance gas sensors. At the same time, the study of field emission can also provide some improved methods to the traditional display technology. © 2012 Elsevier B.V. Selection and/or peer review under responsibility of Chinese Vacuum Society (CVS). © 2010Published Publishedbyby Elsevier B.V. PACS: 81.16.Hc; 81.05. –t; 82.80.-d
Key words: CNT; Metal oxide semiconductor; Composite materials; Gas sensitivity; Field emission
Physics Procedia 32 (2012) 31 – 38
18th International Vacuum Congress (IVC-18)
The gas sensing mechanism of the low-dimension carbon composites with metal oxide quantum dots Hui Maa, Weiman Zhoua, Wu Yuana, Zheng Jieb, Hongzhong Liub, Xin. Lia a
Department of Microelectronics, Xi’an Jiaotong University, Xi’an 710049, China bKey Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710049, China
Abstract In this paper, we obtained three kinds of composite materials which were composed of metal oxides (ZnO, SnO2 and TiO2) and CNTs through catalytic pyrolysis method. Then we carried out the surface morphology, field emission and gas sensitivity properties test for them, and summarized the composite ways of metal oxides/CNTs by comparing three composite properties such as the changes in field emission and gas sensing properties, so that we might explore a set of preparation methods and processes of high performance gas sensors. At the same time, the study of field emission can also provide some improved methods to the traditional display technology. © 2012Published Publishedbyby Elsevier © 2010 Elsevier B.V.B.V. Selection and/or peer review under responsibility of Chinese Vacuum Society (CVS). PACS: 81.16.Hc; 81.05. –t; 82.80.-d
Key words: CNT; Metal oxide semiconductor; Composite materials; Gas sensitivity; Field emission
1. Introduction In 1991, CNT was first discovered by Dr Iijima (S. Iijima) who worked in the electronics company of NEC in Japanese [1]. CNT is an ideal one-dimensional material with excellent properties of large specific surface area, small scale, and showing conductivity of metal or semiconductor at different diameters and chirality [4], as well as a nanoscale material with some quantum effects, which determines the applications of CNT in field emission cathode materials and gas sensors. CNT is an ideal field emission cathode material because of its advantages of small curvature radius, large aspect ratio, high conductivity and excellent mechanical strength and chemical stability. In addition, large specific surface area makes CNT a strong adsorption capacity to some gas molecules. Accompanied with the electrical properties of semiconductor, the interactions between CNT and adsorbed gas molecules change the CNT’s Fermi level, which leads to change in material resistivity. Thus, measuring the resistivity change can be used to detect certain gases, and the change degree of resistance is directly related to the gas content. Furthermore, detecting gas by CNT is better than conventional gas sensing materials because of high sensitivity, fast response, small size and working at room temperature. However, due to the relatively stable of CNT’s electronic structure, only some strong oxidizing and reducing gas, such as O2 [5], NO2 [6], NH3 and SO2, can be adsorbed, and others are
Xin. Li.Tel: +86-029-8266-3041-601;fax:+86-029-8266-3426 E-mail address: [email protected]
1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer review under responsibility of Chinese Vacuum Society (CVS). doi:10.1016/j.phpro.2012.03.514
32
Hui Ma et al. / Physics Procedia 32 (2012) 31 – 38
not. Experimental and theoretical studies have shown that modifying CNT can improve the gas sensitivity, detection sensitivity and selectivity of gas sensors. Some modification methods of CNT are organic surface modification, oxidization, doping (boron, nitrogen, aluminum, etc.) and mechanical deformation [8]. Metal oxide semiconductor sensors are sensors based on resistive metal oxide materials, including WO3, SnO2, ZnO, In2O3 and TiO2 [9]. Its mechanism of gas sensing properties is that the resistance of metal oxides will change when a series of reactions happen between measured gas and metal oxides at a certain temperature. However, the traditional metal oxide gas sensors have a wide range of gas detection and good selectivity to different gases, but their detection sensitivity and response rate are low. A new type of gas sensor using composite material of metal oxide and CNTs will be invented with both advantages of the two materials, taking advantage of small scale of CNT and good selectivity of metal oxide. Moreover, combining metal oxide with CNTs will make metal oxide particles in nano-scale so as to increase the contact area between measured gas and metal oxide, which will improve the gas sensing properties according to the metal oxide gas sensing principle that the smaller material particle size is, the larger the contact area with the measured gas is, and the more obvious gas sensing properties will be. Thus this kind of gas sensing material will have both advantages of metal oxide and CNTs as good selectivity, fast response and working at room temperature. Gas sensors using nano-materials have advantages that conventional sensors can not be replaced as follows: (1) high sensitivity, due to small particle size and large specific surface area of nano-solid materials, which enlarge the interface with gas so as to provide a large number of channels for gas; (2) Room temperature testing. 2. Experiment Three kinds of metal oxides (ZnO, SnO2 and TiO2) were selected to be combined with CNTs in this experiment. The samples’ making process referring to the patent of growth and purification of CNTs by catalytic pyrolysis method [7] was as follows: Initially, thick film of metal oxide nano-particles was printed on substrate by screen printing. Then, CNTs grew on the thick film by catalytic pyrolysis method, and the composite process of metal oxides and CNTs would also be completed at the same time. After the preparation of samples, tests of the composite materials were done, including surface morphology by SEM, field emission at vacuum condition and gas sensitivity properties by I-V. There were two kinds of substrates used in this experiment. One kind was low resistivity silicon (ij = 100mm, Ptype, 480ȝm in thickness, ȡ = 10-2-10-3ȍ • cm). The other kind was a Pt interdigital electrode on SiO2/Si shown in figure 1.
Figure 1 photo of interdigital electrode
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Hui Ma et al. / Physics Procedia 32 (2012) 31 – 38
Figure 2 shows the testing principle of field emission properties. DC voltage was added between two plates while cathode plate was the sample, and anode plate was conductive glass coated with phosphor. Then the part of anode plate coated with phosphor would glow caused by electrons emitted from the sample surface bombarding phosphor when applied voltage reached the threshold voltage under vacuum condition. The ampere meter could show the discharge current at the same time. Finally, field emission properties were studied by comparing threshold voltages and discharge currents of different samples.
Figure 2 the testing principle of field emission properties
3. Results and discussion The corresponding relationship between sample number and sample kinds are shown in Table 1. Table 1 the corresponding relationship between sample number and sample kinds Samples
ZnO/MWCNT
SnO2/MWCNT
TiO2/MWCNT
ZnO
MWCNT
Number
1
2
3
4
5
3.1. SEM analysis of the samples’ surface morphology
Figure 3 SEM image of ZnO / MWCNT composite material
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Hui Ma et al. / Physics Procedia 32 (2012) 31 – 38
The surface morphology of composite material that CNTs grew on ZnO thick film shown in Figure 3 is that there are many tubes of different orientations with a lot of tiny white particles on their side wall. The tubes woven into fibrous network are CNTs, and the white nano-particles are ZnO dot.
Figure 4 SEM image of SnO2/ MWCNT composite material (CNTs grew on the edge of SnO2)
Figure 5 SEM image of SnO2 / MWCNT composite material(CNTs grew on evenly distributed part of SnO2)
The surface morphology of composite material that CNTs grew on SnO2 thick film shown in Figures 4-5 is that: On the edge of SnO2 thick film, CNTs were similar to those on the edge of ZnO thick film. However, there are a few two-dimensional plane carbon nano-walls nearby the big SnO2 particles. On the evenly distributed part of SnO2 thick film, the size of SnO2 particles is similar, and most of the composite structures are carbon nano-walls while there are both carbon nano-walls and CNTs that carbon nano-walls appear nearby the big SnO2 particles, and CNTs and small SnO2 particles are distributed between them on unevenly distributed part of SnO2.
Figure 6 SEM image of TiO2 / MWCNT composite material
The surface morphology of composite material that CNTs grew on TiO2 thick film shown in Figure 6 is that: On the edge of TiO2 thick film, CNTs also appear as fibrous network with some TiO2 particles in it. On the evenly distributed part of TiO2 thick film, TiO2 particles are embedded in CNTs so that the diameters of CNTs are larger than normal while there are great differences in the diameters of CNTs on unevenly distributed part of TiO2 thick
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Hui Ma et al. / Physics Procedia 32 (2012) 31 – 38
film. Therefore, the combining of TiO2 and CNTs is that TiO2 particles are embedded in CNTs, and different sizes of TiO2 particles will get different diameters of CNTs. Microstructure comparisons of three kinds of CNTs / metal oxide composite materials are as follows: (1) There are significant differences in surface morphologies of three CNTs / metal oxide composite materials: In CNTs / ZnO composite materials, ZnO particles are distributed on the ektexines of CNTs. In CNTs / SnO2 composite materials, SnO2 particles are centers of growth of carbon nano-walls. In CNTs / TiO2 composite materials, TiO2 particles are embedded in CNTs, and different sizes of TiO2 particles will get different diameters of CNTs. (2) Metal oxide particles diffuse to the edge in the process of growth, because metal oxide particles can be found at the edge of sample where there is no film. However, the particles’ size is smaller, and the CNTs are finer there. (3) The size of metal oxide particles will affect the shape of CNTs. So to get uniform and reliable composite CNTs requires accurate control on particles size.
3.2. Field emission phenomena and data analysis Field emission I-V curves were done with three samples of ZnO / MWCNT, SnO2/MWCNT and TiO2/MWCNT as the cathode, and the I-V curves are shown in Figure 7. From the I-V curves, it can be seen that the threshold voltage of SnO2 / MWCNT sample is higher than others, and its discharge current intensity increases slowly while ZnO / MWCNT sample is similar to TiO2/MWCNT sample, caused by the different surface morphologies of three composite materials that ZnO/MWCNT and TiO2/MWCNT present CNTs structure, but SnO2/MWCNT presents carbon nano-walls structure. In addition, the comparisons of threshold voltages and emission currents at 500V voltage listed at Tables 2-3 indicate that the threshold voltage of ZnO / MWCNT sample is the minimum, and discharge current of TiO2/MWCNT sample is the maximum at the same operating voltage as 500V. ZnO/MWCNT SnO2/MWCNT TiO2/MWCNT
500
Current (uA)
400
300
200
100
0 0
50 100 150 200 250 300 350 400 450 500 550 600 650 700 750
Voltage (V)
Figure 7 field emission IV character of three metal oxide/MWCNT composite materials
Table 2˖Comparison of threshold voltages Samples
ZnO/MWCNT
SnO2/MWCNT
TiO2/MWCNT
Turn-on voltage˄V˅
200
340
230
Table 3: Comparison of currents in the same voltage˄500V˅ Samples
ZnO/MWCNT
SnO2/MWCNT
TiO2/MWCNT
Current˄ȝA˅
130
20
140
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Hui Ma et al. / Physics Procedia 32 (2012) 31 – 38
Field emission images of three composite materials are shown as Figures 8. These images show the vacuum field emission at a high voltage without ampere meter. They suggest that there are more bright spots at the edge of composite materials with the reason that, the composite CNTs at the edge was deformed, even turned up, when the sample was padded with the insulation layer, which leads to the phosphor light earlier for the plate spacing of anode and cathode is shortened. It also can be seen, from Figure 8, that field emission property of SnO2/MWCNT composite material is poorer than other two.
Figure 8 Field emission images ˄a˅ZnO/MWCNT (b) SnO2/MWCNT (c) TiO2/MWCNT
3.3. Gas sensitivity experiment The curves of current changes (BI) with time (t) under the condition that ethanol gas flows across the surfaces of samples are shown in Figure.9. Figures 9a-9b suggest that current decreases when ethanol gas passes across the surfaces of samples, that is, the number of carriers between two electrodes is reduced after adsorbing ethanol gas, which leads to the increase of resistivity in macroscopic view. There are some results by comparing all the samples as follows: (1) The current of pure metal oxide film is small. It fluctuates with the process of test due to some uncertain conditions, and it is saturated in a short time. Moreover, the measuring range of gas concentration of the pure metal oxide film is small too. (2) The current of CNTs decreases significantly with time, and the sensitivity of CNTs are higher than metal oxide. (3) The current change rates of different samples under the condition of 3.0v are calculated: ȡ1 = 2.55682 × 10-1%, ȡ2 = 55.7417%. Thus, the samples of higher current change rate are metal oxide MWCNT composite film samples, and the lower are pure CNTs samples. In addition, the current of pure metal oxide film samples is low and easy to be interfered with for their bad electron conductivity whereas composite material samples of metal oxide and CNTs are higher than pure CNTs in current change rate. BI
BI 0.012296
0.003168 0.012294 0.012292
0.003166
BI (A)
BI (A)
0.012290 0.012288
0.003164
0.012286
0.003162
0.012284 0.012282
0.003160 0.012280 0
2
4
t (min)
(a)
6
8
10
0
2
4
6
8
t (min)
(b)
Figure 9 the BI-t curve of the different samples. (a) MWCNT sample, (b) SnO2/MWCNT sample
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4. Conclusions This paper presents the preparation of composite materials, the test & analysis of composite materials, and some satisfactory results. We prepared three kinds of CNTs/metal oxide composite materials by catalytic pyrolysis method. Then, we tested these composite materials, including surface morphology by SEM, field emission at vacuum condition and gas sensitivity properties by I-V, and analyzed the experimental results. Finally, we draw some conclusions as follows: (1) ZnO particles are distributed on the side wall of CNTs, SnO2 particles are attached to carbon nano-walls, and TiO2 particles are embedded in CNTs. (2)Field emission properties of composite materials are affected by composite micro-morphology, the field emission property of CNTs/SnO2 composite material is worse than other two composite materials. (3) It is verified that the metal-oxides show semiconductor IV characteristics in the test of gas sensing properties. All the samples electrical conductivity decreases when ethanol gas passes through, the electrical conductivity of metal oxide thick film increases after combined with CNTs, but the current change rate decreases at gas test. Nevertheless, gas sensing properties of composite materials are accord with expectations. Acknowledgements This paper was supported by National Natural Science Foundation of China (No. 729092923040, 60801022ˈ 60476037, 50975226, 60976058HZ), National Basic Research Program of China (No. 2009CB724202), New Century Excellent Talents (NCET-08-0447), and the Fundamental Research Funds for the Central Universities. References [1] Iijima S.Helical microtubules of graphitic carbon[J].1991. [2] Y.H.Yuan and Z.J.Liu, Z.W. Li.The mechanism of improving the CNTs gas sensors by doping[R].2004 [3] PHILIP G.COLLINS, KEITH B, et al. Extreme oxygen sensitivity of electronic properties of carbon nanotubes[R]. 2000. [4] J. Kong, N. R.Franklin, et al. Nanotube molecular wires as chemical sensors [J].2000. [5] C.C.Zhu and X.Li. W.H. Liu. H.J.Liu. The CNT growth process by single temperature zone resistance furnace pyrolysis method and purification process. 200510096426 [P].2006. [6] X..Y Wen. The modifying CNT gas sensors [J].2008. [7] X.J.Liu and X.Q.Tan. S.Huan. The metal oxide gas sensors [J].2007.
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Conference Title˖18th International Vacuum Congress (IVC-18) The title of paper: The gas sensing mechanism of the low-dimension carbon composites with metal oxide quantum dots Author: Hui Ma, Weiman Zhou, Wu Yuan, Zheng Jie, Hongzhong Liu, Xin. Li Email: [email protected], [email protected] First affiliation: Department of Microelectronics, Xi’an Jiaotong University, Xi’an 710049, China Second affiliation: Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710049, China
Abstract In this paper, we obtained three kinds of composite materials which were composed of metal oxides (ZnO, SnO2 and TiO2) and CNTs through catalytic pyrolysis method. Then we carried out the surface morphology, field emission and gas sensitivity properties test for them, and summarized the composite ways of metal oxides/CNTs by comparing three composite properties such as the changes in field emission and gas sensing properties, so that we might explore a set of preparation methods and processes of high performance gas sensors. At the same time, the study of field emission can also provide some improved methods to the traditional display technology. © 2012 Elsevier B.V. Selection and/or peer review under responsibility of Chinese Vacuum Society (CVS). © 2010Published Publishedbyby Elsevier B.V. PACS: 81.16.Hc; 81.05. –t; 82.80.-d
Key words: CNT; Metal oxide semiconductor; Composite materials; Gas sensitivity; Field emission