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Influence of Sn doping on ZnO sensing properties for ethanol and acetone
Materials Science and Engineering C 32 (2012) 817–821
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Influence of Sn doping on ZnO sensing properties for ethanol and acetone Xueda Li 1, Yongqin Chang ⁎, Yi Long School of Materials Science and Engineering, University of Science and Technology Beijing, PR China
a r t i c l e
i n f o
Article history: Received 14 October 2010 Received in revised form 26 August 2011 Accepted 30 January 2012 Available online 6 February 2012 Keywords: Sn doped ZnO Gas sensing property Ethanol Acetone
a b s t r a c t Nanostructured pure ZnO (PZO) and Sn doped ZnO (SZO) were deposited on glass templates by chemical deposition method. Before the deposition, electrically conductive silver paste was pasted at the two ends of the glass templates as electrodes. The gas sensing properties of the nanostructured PZO and SZO were systematically investigated by our home-made system, and the results show that Sn doping decreases the response to tested gases. However, it remarkably shortens the response time and recovery time of the sensor. Both PZO and SZO show higher response to acetone than to ethanol, while the response time and recovery time of PZO and SZO to ethanol are shorter than those to acetone, which indicates that the selectivity of ZnO nanostructures to ethanol and acetone are different. © 2012 Elsevier B.V. All rights reserved.
1. Introduction As a functional semiconductor, ZnO has been widely studied for various practical applications, including field-effect transistor [1], optical device [2], dye-sensitized solar cell [3], solid-state gas sensor [4,5], and so on. Solid-state gas sensor is one of its important applications. It is well known that ZnO is n-type semiconductor [6] and the responsible donors are usually identified as O vacancy VO, Zn interstitial ZnI, or complex defects [7]. When a ZnO gas sensor is exposed to reductive gas, like ethanol, its molecules are adsorbed on the surface of zinc oxide and released electrons induce the decrease of the resistance [8]. The sensing properties of ZnO suffer from several problems, such as high operation temperature, poor sensitivity, long response and recovery time and so on [9]. Doping with different elements such as Al, Fe, Sn, Ti, Cu, In, Nd [9–16] is one of important solutions. The noble elements act as catalyst that can improve the adsorption of gas [17]. SnO2 and ZnO are both widely used as gas sensors because of their good sensing property and cost-effective synthesis route [18]. Sn doped nanostructured ZnO [14] may have better gas sensing properties as the combination of the two elements. Most research was focused on ZnO thin film, but recently individual ZnO nanowire attracted more interests [19]. Many kinds of oxidative or reductive gases have been researched, such as NO2 [15], formaldehyde [11], ethanol [13], acetone [20], CO [12], hydrogen [5], and so on. Detection
of ethanol and acetone is of great importance to the safety of petrochemical industry. In this work, the gas sensing properties of PZO and SZO respectively to ethanol and acetone are reported. The response (S), response time and recovery time were systematically studied at different temperatures and different concentrations of target gases in this literature. 2. Experimental 2.1. Syntheses The mixed powders of Zn and Sn (weight ratio 5:1) carried by ceramic boat was kept in the middle of tubular furnace. Argon gas was continuously ventilated in the quartz tube with a flow rate of 200 sccm. The furnace was heated at the rate of 12.5 °C up to 575 °C and kept at this temperature for 20 min. During the growth process, the flow rate of argon gas was changed to 100 sccm, and controlled flow rate of oxygen (6 sccm) was introduced. Once the growth process ended, the Al2O3 boat was taken out immediately and then cooled down in the air in case the nanostructure grew bigger. The PZO nanostructures were prepared in the similar way using pure Zn powder as source material. 2.2. Characterization
⁎ Corresponding author at: School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China. Tel.: + 86 10 62334958; fax: + 86 10 62334807. E-mail addresses: [email protected] (X. Li), [email protected] (Y. Chang), [email protected] (Y. Long). 1 Present Address: PO Box 211, 30 Xueyuan Road, Haidian District, Beijing 100083 PR China. 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2012.01.032
Scanning electron microscope (SEM) images were taken on Cambridge S-360, and the morphology of PZO and SZO was shown in Fig. 1(a) and (b) respectively. The PZO film is composed of two layers. The upper layer is composed of flowerlike particles, while the lower layer is composed of nanorods. Fig. 1(b) shows that the SZO film is composed of vertical aligned nanorods.
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X. Li et al. / Materials Science and Engineering C 32 (2012) 817–821
Fig. 1. Typical SEM image of pure ZnO (a) and Sn doped ZnO (b).
X-ray diffraction (XRD) analysis was performed using DMAX-RB X diffractometer with Cu Kα radiation (λ = 1.5418 Å), and the results were shown in Fig. 2. Both PZO and SZO XRD patterns are well matched with wurtzite structure (JCPDS 36-1451). Two weak peaks (marked with *) corresponding to SiO2 glass template were also observed. These two weak peaks may be caused by small SiO2 crystal grains in the glass template. In the diffraction pattern of SZO, no other characteristic peaks from impurities were detected. The strongest peak of both PZO and SZO which is indexed as (002) shows the preferential growth of the nanostructure along c-axis, which is in accordance with the morphology of Fig. 1a) and b. The pattern of SZO shifts to the left with the angle of about 0.11° compared with PZO. According to Bragg Equation [21], it reveals that the interplanar distance of SZO increased. The lattice constants of PZO calculated from the XRD pattern are a = 3.256 Å, c = 5.223 Å (with the uncertainty δd = 0.002 Å), while the lattice constants of SZO are a = 3.262 Å, c = 5.226 Å (with the uncertainty δd = 0.002 Å). Compared with the lattice parameters of pure ZnO, a = 3.251 Å, c = 5.208 Å (JCPDS 361451), there is a little increasement of the lattice parameter of SZO, which indicates that Sn dopes into the lattice of ZnO instead of forming impurity phase.
electrodes. Fig. 3(a) shows the schematic of the gas sensor device. I–V curve of the device was measured and the results were shown in Fig. 3(b). The curve is almost linear, which reveals that the contact between ZnO and Ag paste is Ohmic contact. Compared with the resistance of the ZnO film, the contact resistance is so small that it can hardly influence the measurement of the gas sensing property. Therefore, the contact resistance can be omitted. Fig. 4 is the schematic diagram of the gas sensing system, which includes a sealed test chamber, heating equipment (a furnace tube) and recording equipment. Fixed quantity of liquid ethanol or acetone is injected into the sealed test chamber by a syringe to obtain designed concentration. The calculating method of concentration is described as follows (taking ethanol as an example), V ethanol;gas ¼ C V s
ð1Þ
where Vethanol, gas is the volume of gaseous state ethanol, C (ppm) is the concentration of the tested gas, Vs is the volume of the test chamber.with PV = nRT [22], nethanol ¼
2.3. Sensor fabrication and sensing system
PV ethanol;gas PCV s ¼ RT RT
ð2Þ
where V = Mn/ρ [22], this gives Conductive silver paste was pasted at the two ends of glass template which is 1 cm long and 0.5 cm wide as electrodes. The distance between the two electrodes was about 6–8 mm. In order to obtain good mechanical and electrical property, the silver paste was dried in the air for 12 h, and then was treated at 150 °C for 2 h. The nanostructured materials were deposited on these templates with
V inject ¼
Mnethanol MPV s C ¼ ⋅ ρ Rρ T
ð3Þ
M (g/mol) is the molecular weight of the liquid, ρ (g/ml) is the density of the liquid, and T (K) is the average temperature of the test chamber. In our work, the values of M, P, Vs, R, ρ are 46 g/mol, 101325 Pa, 8.68 L, 8.31441 J/(mol·K), 0.816 g/cm 3, respectively, this yields V inject;ethanol ¼ 5:96⋅
C T
ð4Þ
For acetone, the values of M, P, Vs, R, ρ are 46 g/mol, 101325 Pa, 8.68 L, 8.31441 J/(mol·K), 0.788 g/cm 3, respectively. With Eq. (3) we obtain V inject;acetone ¼ 7:94⋅
C T
ð5Þ
The designed concentration for ethanol and acetone can be calculated from Eqs. (4) and (5) with the quantity of the liquid. 2.4. Measurements of gas sensing properties
Fig. 2. XRD pattern of pure ZnO and Sn doped ZnO (PDF card number for ZnO: 36-1451; for SiO2: 50-1432).
The as-prepared gas sensor devices were put into the gas sensing system, and tested at different concentrations (400 ppm, 800 ppm, 1600 ppm, 2400 ppm, 4000 ppm) of ethanol or acetone and tested
X. Li et al. / Materials Science and Engineering C 32 (2012) 817–821
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Fig. 3. (a) Schematic of the gas sensor element, (b) I-V curve of the gas sensor device.
Fig. 4. Schematic of gas sensing system.
at 225 °C, 250 °C, 275 °C, 300 °C in each kind of target gas respectively, so 16 response curves of the sensor elements were recorded. Two typical curves of them were shown in Fig. 5(a) and (b).The resistance of the elements was measured by the multimeter (Fluke 8846A) and recorded by the computer. The value of the resistance was recorded at intervals of 2 s. 3. Results and discussion 3.1. Response S (%) The gas response is given by S (%) defined as below [8]:
S¼
Ra 100% Rg
ð6Þ
where Ra is the resistance in the air and Rg is the resistance in the tested gas. Fig. 6 shows the comparison of the response of PZO and SZO at the atmosphere of ethanol and acetone, respectively. PZO shows higher response value than SZO whenever at the atmosphere of ethanol or acetone, which reveals that the gas sensing property of PZO is better than that of SZO. The average response of PZO is 2.2 times of SZO. That is to say, Sn doping decreases the sensing property of ZnO, which agree with the report of reference [14]. The probable reason is that Sn belongs to IVA subgroup, there are four outer-shell electrons, Sn acts as electron donator which increases the amount of free electrons, and more free electrons create more oxygen vacancies, which makes the resistance of ZnO nanostructure decrease but also causes the depletion layer thinner [7]. Therefore, the relative change of the resistance of ZnO nanostructure decreases. That is to say, S (Ra/Rg) decreases.
Fig. 5. Resistance depends on time for (a) Sn doped ZnO sensor in acetone atmosphere at 300 °C; (b) Pure ZnO sensor in ethanol atmosphere at 300 °C.
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Fig. 6. Response of pure ZnO and Sn doped ZnO for ethanol (a) and acetone (b) at different temperature (X axis: temperature (°C); Y axis: response of pure ZnO and Sn doped ZnO (%)).
Fig. 6 also shows that: (1) To acetone, the S (%) of both PZO and SZO is the largest at 300 °C and S300 °C > S275 °C > S250 °C > S225 °C; (2) To ethanol, The S (%) of both PZO and SZO is the largest at 275 °C and S275 °C > S300 °C > S250 °C > S225 °C. The results show that the best testing temperature for acetone and ethanol is 300 °C and 275 °C, respectively. The response S (%) of PZO or SZO increases with the concentration of ethanol or acetone. Fig. 7 is the S-concentration curve of PZO at the atmosphere of acetone at 300 °C. It is all most linear. This is a good property that can be used for detecting different concentrations of gases.
Fig. 8. Average response of pure ZnO and Sn doped ZnO to acetone and ethanol (X axis: temperature (°C); Y axis: response of pure ZnO and Sn doped ZnO (%)).
Fig. 7. Gas response versus concentration curve (acetone for sample Sn-doped ZnO at 300 °C).
Fig. 9. Schematic of response curve.
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Fig. 10. (a) Response time and (b) Recovery time of pure ZnO and Sn doped ZnO.
In our research, both PZO and SZO show better sensing property to acetone than to ethanol under the same condition (the same temperature and the same concentration). The average response to acetone is 1.2 times of ethanol (Fig. 8), which demonstrates that ZnO nanostructure shows better selectivity to acetone than to ethanol. It also consists with the results of reference [14]. 3.2. Response time and recovery time The response or recovery time is defined as the time taken for the change of the resistance of sensor reach 90% of its total changed value (Ra–Rg) [7]. Fig. 9 is the schematic of one response curve. The research shows, (1) The response time of SZO is shorter than PZO except at 300 °C. Under other conditions, the average response time of SZO to ethanol is shortened from 69.3 s to 36.7 s, which is shortened by 47%, and to acetone from 133.5 s to 79.2 s, which is shortened by 41%. (2) The recovery time of SZO is shorter than PZO under all conditions. The average recovery time of SZO to ethanol is shortened from 175.1 s to 92.2 s which is shortened by 47%, and to acetone from 244.3 s to 97.5 s which is shortened by 60%. Fig. 10 shows the average response time and recovery time of PZO and SZO respectively to ethanol and acetone. The above results show that Sn doping greatly shortens the response time and recovery time of ZnO nanostructures. It is probably because Sn atoms doped in ZnO lattice are more active than Zn atoms. The adsorption and desorption of Sn atoms with O2/ tested gases are more quickly than that of Zn atoms. Also, Sn doping introduces some defects in the nanostructure, and these defects make the nanostructure unstable and more possible to react with O2 or reducing gas molecules. Therefore, both the response and recovery time of SZO are shorter than those of PZO. Fig. 10 also shows that the response and recovery time of both PZO and SZO to ethanol are shorter than to acetone. The results show that the average response time to ethanol is 55.3 s while to acetone 110.2 s, the average recovery time to ethanol is 139.6 s while to acetone 181.4 s. This phenomenon reveals that the selectivity of ZnO structures to ethanol and acetone is significantly different. 4. Conclusion In this work, the gas sensing properties of pure ZnO and Sn-doped ZnO thin film to ethanol and acetone were systematically tested in our home-made testing system. The response, response time and recovery time were tested and calculated. The results show that Sn dop-
ing makes the response of ZnO decrease, and the average response of PZO is 2.2 times of SZO. But it notably shortens the response time and recovery time, and Sn doping shortens the response time and recovery time by 40%–60%. Either PZO or SZO shows better response to acetone than ethanol, and the average response to acetone is 1.2 times of ethanol. But the response time and recovery time of PZO and SZO to ethanol are shorter than acetone (the average response time to ethanol is 55.3 s while to acetone 110.2 s; the average recovery time to ethanol is 139.6 s while to acetone 181.4 s), which shows that ZnO has different selectivity to ethanol and acetone.
Acknowledgements This project is financially supported by the National Natural Science Foundation of China (No. 50502005), Beijing Natural Science Foundation (No. 1062008, 1092014) and Metallurgy Foundation of University of Science and Technology Beijing. One of the authors (Y. Q. Chang) is supported by Program for New Century Excellent Talents in University (No.NCET-07-0065) and Beijing Novel Program.
References [1] X.D. Wang, J. Zhou, J.H. Song, J. Liu, N. Xu, Z.L. Wang, Nano Lett. 6 (2006) 2768–2772. [2] P.D. Yang, H.Q. Yan, S. Mao, et al., Adv. Funct. Mater. 12 (2002) 323–331. [3] M. Law, L.E. Greene, J.C. Johnson, et al., Nat. Mater. 4 (2005) 455–459. [4] S.C. Navale, S.W. Gosavi, I.S. Mulla, Talanta 75 (2008) 1315–1319. [5] O. Lupan, G.Y. Chai, L. Chow, Microelectron. J. 38 (2007) 1211–1216. [6] F. Porter, Zinc Oxide Handbook, CRC Press, Boca Raton, 2006. [7] M.-W. Ahn, K.-S. Park, J.-H. Heo, et al., Sens. Actuators B 138 (2009) 168–173. [8] N. Hongsith, E. Wongrat, T. Kerdcharoen, et al., Sens. Actuators B 144 (2010) 67–72. [9] Y. Cao, Weiyu Pan, Ying Zong, et al., Sens. Actuators B 138 (2009) 480–484. [10] Z. Yang, Y. Huang, G. Chen, et al., Sens. Actuators B 140 (2009) 549–556. [11] N. Han, Y.J. Tian, X.F. Wu, et al., Sens. Actuators B 138 (2009) 228–235. [12] H. Gong, J.Q. Hu, J.H. Wang, et al., Sens. Actuators B 115 (2006) 247–251. [13] F. Paraguay, et al., Thin Solid Films 373 (2000) 137–140. [14] S.C. Navale, I.S. Mulla, Mater. Sci. Eng. C 29 (2009) 1317–1320. [15] T. Sergiu, T.S. Shishiyanu, O.I. Lupan, Sens. Actuators B 107 (2005) 379–386. [16] P.P. Sahay, R.K. Nath, Sens. Actuators B 134 (2008) 654–659. [17] L.C. Tien, P.W. Sadik, D.P. Norton, et al., Appl. Phys. Lett. 87 (2005) 222106–222109. [18] L. Peng, T.F. Xie, Min Yang, et al., Sens. Actuators B 131 (2008) 660–664. [19] O. Lupan, V.V. Ursaki, G. Chai, et al., Sens. Actuators B 144 (2010) 56–66. [20] L. Wang, K. Kalyanasundaram, M. Stanacevic, et al., Sens. Lett. 8 (2010) 709–712. [21] V. Luzzati, International Tables for X-ray Crystallography, Kynoch Press, England, 1959. [22] Peter Atkins, Julio de Paula, Atkins’ Physical Chemistry, seventh edition Higher Education Press, 2006.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Influence of Sn doping on ZnO sensing properties for ethanol and acetone Xueda Li 1, Yongqin Chang ⁎, Yi Long School of Materials Science and Engineering, University of Science and Technology Beijing, PR China
a r t i c l e
i n f o
Article history: Received 14 October 2010 Received in revised form 26 August 2011 Accepted 30 January 2012 Available online 6 February 2012 Keywords: Sn doped ZnO Gas sensing property Ethanol Acetone
a b s t r a c t Nanostructured pure ZnO (PZO) and Sn doped ZnO (SZO) were deposited on glass templates by chemical deposition method. Before the deposition, electrically conductive silver paste was pasted at the two ends of the glass templates as electrodes. The gas sensing properties of the nanostructured PZO and SZO were systematically investigated by our home-made system, and the results show that Sn doping decreases the response to tested gases. However, it remarkably shortens the response time and recovery time of the sensor. Both PZO and SZO show higher response to acetone than to ethanol, while the response time and recovery time of PZO and SZO to ethanol are shorter than those to acetone, which indicates that the selectivity of ZnO nanostructures to ethanol and acetone are different. © 2012 Elsevier B.V. All rights reserved.
1. Introduction As a functional semiconductor, ZnO has been widely studied for various practical applications, including field-effect transistor [1], optical device [2], dye-sensitized solar cell [3], solid-state gas sensor [4,5], and so on. Solid-state gas sensor is one of its important applications. It is well known that ZnO is n-type semiconductor [6] and the responsible donors are usually identified as O vacancy VO, Zn interstitial ZnI, or complex defects [7]. When a ZnO gas sensor is exposed to reductive gas, like ethanol, its molecules are adsorbed on the surface of zinc oxide and released electrons induce the decrease of the resistance [8]. The sensing properties of ZnO suffer from several problems, such as high operation temperature, poor sensitivity, long response and recovery time and so on [9]. Doping with different elements such as Al, Fe, Sn, Ti, Cu, In, Nd [9–16] is one of important solutions. The noble elements act as catalyst that can improve the adsorption of gas [17]. SnO2 and ZnO are both widely used as gas sensors because of their good sensing property and cost-effective synthesis route [18]. Sn doped nanostructured ZnO [14] may have better gas sensing properties as the combination of the two elements. Most research was focused on ZnO thin film, but recently individual ZnO nanowire attracted more interests [19]. Many kinds of oxidative or reductive gases have been researched, such as NO2 [15], formaldehyde [11], ethanol [13], acetone [20], CO [12], hydrogen [5], and so on. Detection
of ethanol and acetone is of great importance to the safety of petrochemical industry. In this work, the gas sensing properties of PZO and SZO respectively to ethanol and acetone are reported. The response (S), response time and recovery time were systematically studied at different temperatures and different concentrations of target gases in this literature. 2. Experimental 2.1. Syntheses The mixed powders of Zn and Sn (weight ratio 5:1) carried by ceramic boat was kept in the middle of tubular furnace. Argon gas was continuously ventilated in the quartz tube with a flow rate of 200 sccm. The furnace was heated at the rate of 12.5 °C up to 575 °C and kept at this temperature for 20 min. During the growth process, the flow rate of argon gas was changed to 100 sccm, and controlled flow rate of oxygen (6 sccm) was introduced. Once the growth process ended, the Al2O3 boat was taken out immediately and then cooled down in the air in case the nanostructure grew bigger. The PZO nanostructures were prepared in the similar way using pure Zn powder as source material. 2.2. Characterization
⁎ Corresponding author at: School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China. Tel.: + 86 10 62334958; fax: + 86 10 62334807. E-mail addresses: [email protected] (X. Li), [email protected] (Y. Chang), [email protected] (Y. Long). 1 Present Address: PO Box 211, 30 Xueyuan Road, Haidian District, Beijing 100083 PR China. 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2012.01.032
Scanning electron microscope (SEM) images were taken on Cambridge S-360, and the morphology of PZO and SZO was shown in Fig. 1(a) and (b) respectively. The PZO film is composed of two layers. The upper layer is composed of flowerlike particles, while the lower layer is composed of nanorods. Fig. 1(b) shows that the SZO film is composed of vertical aligned nanorods.
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X. Li et al. / Materials Science and Engineering C 32 (2012) 817–821
Fig. 1. Typical SEM image of pure ZnO (a) and Sn doped ZnO (b).
X-ray diffraction (XRD) analysis was performed using DMAX-RB X diffractometer with Cu Kα radiation (λ = 1.5418 Å), and the results were shown in Fig. 2. Both PZO and SZO XRD patterns are well matched with wurtzite structure (JCPDS 36-1451). Two weak peaks (marked with *) corresponding to SiO2 glass template were also observed. These two weak peaks may be caused by small SiO2 crystal grains in the glass template. In the diffraction pattern of SZO, no other characteristic peaks from impurities were detected. The strongest peak of both PZO and SZO which is indexed as (002) shows the preferential growth of the nanostructure along c-axis, which is in accordance with the morphology of Fig. 1a) and b. The pattern of SZO shifts to the left with the angle of about 0.11° compared with PZO. According to Bragg Equation [21], it reveals that the interplanar distance of SZO increased. The lattice constants of PZO calculated from the XRD pattern are a = 3.256 Å, c = 5.223 Å (with the uncertainty δd = 0.002 Å), while the lattice constants of SZO are a = 3.262 Å, c = 5.226 Å (with the uncertainty δd = 0.002 Å). Compared with the lattice parameters of pure ZnO, a = 3.251 Å, c = 5.208 Å (JCPDS 361451), there is a little increasement of the lattice parameter of SZO, which indicates that Sn dopes into the lattice of ZnO instead of forming impurity phase.
electrodes. Fig. 3(a) shows the schematic of the gas sensor device. I–V curve of the device was measured and the results were shown in Fig. 3(b). The curve is almost linear, which reveals that the contact between ZnO and Ag paste is Ohmic contact. Compared with the resistance of the ZnO film, the contact resistance is so small that it can hardly influence the measurement of the gas sensing property. Therefore, the contact resistance can be omitted. Fig. 4 is the schematic diagram of the gas sensing system, which includes a sealed test chamber, heating equipment (a furnace tube) and recording equipment. Fixed quantity of liquid ethanol or acetone is injected into the sealed test chamber by a syringe to obtain designed concentration. The calculating method of concentration is described as follows (taking ethanol as an example), V ethanol;gas ¼ C V s
ð1Þ
where Vethanol, gas is the volume of gaseous state ethanol, C (ppm) is the concentration of the tested gas, Vs is the volume of the test chamber.with PV = nRT [22], nethanol ¼
2.3. Sensor fabrication and sensing system
PV ethanol;gas PCV s ¼ RT RT
ð2Þ
where V = Mn/ρ [22], this gives Conductive silver paste was pasted at the two ends of glass template which is 1 cm long and 0.5 cm wide as electrodes. The distance between the two electrodes was about 6–8 mm. In order to obtain good mechanical and electrical property, the silver paste was dried in the air for 12 h, and then was treated at 150 °C for 2 h. The nanostructured materials were deposited on these templates with
V inject ¼
Mnethanol MPV s C ¼ ⋅ ρ Rρ T
ð3Þ
M (g/mol) is the molecular weight of the liquid, ρ (g/ml) is the density of the liquid, and T (K) is the average temperature of the test chamber. In our work, the values of M, P, Vs, R, ρ are 46 g/mol, 101325 Pa, 8.68 L, 8.31441 J/(mol·K), 0.816 g/cm 3, respectively, this yields V inject;ethanol ¼ 5:96⋅
C T
ð4Þ
For acetone, the values of M, P, Vs, R, ρ are 46 g/mol, 101325 Pa, 8.68 L, 8.31441 J/(mol·K), 0.788 g/cm 3, respectively. With Eq. (3) we obtain V inject;acetone ¼ 7:94⋅
C T
ð5Þ
The designed concentration for ethanol and acetone can be calculated from Eqs. (4) and (5) with the quantity of the liquid. 2.4. Measurements of gas sensing properties
Fig. 2. XRD pattern of pure ZnO and Sn doped ZnO (PDF card number for ZnO: 36-1451; for SiO2: 50-1432).
The as-prepared gas sensor devices were put into the gas sensing system, and tested at different concentrations (400 ppm, 800 ppm, 1600 ppm, 2400 ppm, 4000 ppm) of ethanol or acetone and tested
X. Li et al. / Materials Science and Engineering C 32 (2012) 817–821
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Fig. 3. (a) Schematic of the gas sensor element, (b) I-V curve of the gas sensor device.
Fig. 4. Schematic of gas sensing system.
at 225 °C, 250 °C, 275 °C, 300 °C in each kind of target gas respectively, so 16 response curves of the sensor elements were recorded. Two typical curves of them were shown in Fig. 5(a) and (b).The resistance of the elements was measured by the multimeter (Fluke 8846A) and recorded by the computer. The value of the resistance was recorded at intervals of 2 s. 3. Results and discussion 3.1. Response S (%) The gas response is given by S (%) defined as below [8]:
S¼
Ra 100% Rg
ð6Þ
where Ra is the resistance in the air and Rg is the resistance in the tested gas. Fig. 6 shows the comparison of the response of PZO and SZO at the atmosphere of ethanol and acetone, respectively. PZO shows higher response value than SZO whenever at the atmosphere of ethanol or acetone, which reveals that the gas sensing property of PZO is better than that of SZO. The average response of PZO is 2.2 times of SZO. That is to say, Sn doping decreases the sensing property of ZnO, which agree with the report of reference [14]. The probable reason is that Sn belongs to IVA subgroup, there are four outer-shell electrons, Sn acts as electron donator which increases the amount of free electrons, and more free electrons create more oxygen vacancies, which makes the resistance of ZnO nanostructure decrease but also causes the depletion layer thinner [7]. Therefore, the relative change of the resistance of ZnO nanostructure decreases. That is to say, S (Ra/Rg) decreases.
Fig. 5. Resistance depends on time for (a) Sn doped ZnO sensor in acetone atmosphere at 300 °C; (b) Pure ZnO sensor in ethanol atmosphere at 300 °C.
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X. Li et al. / Materials Science and Engineering C 32 (2012) 817–821
Fig. 6. Response of pure ZnO and Sn doped ZnO for ethanol (a) and acetone (b) at different temperature (X axis: temperature (°C); Y axis: response of pure ZnO and Sn doped ZnO (%)).
Fig. 6 also shows that: (1) To acetone, the S (%) of both PZO and SZO is the largest at 300 °C and S300 °C > S275 °C > S250 °C > S225 °C; (2) To ethanol, The S (%) of both PZO and SZO is the largest at 275 °C and S275 °C > S300 °C > S250 °C > S225 °C. The results show that the best testing temperature for acetone and ethanol is 300 °C and 275 °C, respectively. The response S (%) of PZO or SZO increases with the concentration of ethanol or acetone. Fig. 7 is the S-concentration curve of PZO at the atmosphere of acetone at 300 °C. It is all most linear. This is a good property that can be used for detecting different concentrations of gases.
Fig. 8. Average response of pure ZnO and Sn doped ZnO to acetone and ethanol (X axis: temperature (°C); Y axis: response of pure ZnO and Sn doped ZnO (%)).
Fig. 7. Gas response versus concentration curve (acetone for sample Sn-doped ZnO at 300 °C).
Fig. 9. Schematic of response curve.
X. Li et al. / Materials Science and Engineering C 32 (2012) 817–821
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Fig. 10. (a) Response time and (b) Recovery time of pure ZnO and Sn doped ZnO.
In our research, both PZO and SZO show better sensing property to acetone than to ethanol under the same condition (the same temperature and the same concentration). The average response to acetone is 1.2 times of ethanol (Fig. 8), which demonstrates that ZnO nanostructure shows better selectivity to acetone than to ethanol. It also consists with the results of reference [14]. 3.2. Response time and recovery time The response or recovery time is defined as the time taken for the change of the resistance of sensor reach 90% of its total changed value (Ra–Rg) [7]. Fig. 9 is the schematic of one response curve. The research shows, (1) The response time of SZO is shorter than PZO except at 300 °C. Under other conditions, the average response time of SZO to ethanol is shortened from 69.3 s to 36.7 s, which is shortened by 47%, and to acetone from 133.5 s to 79.2 s, which is shortened by 41%. (2) The recovery time of SZO is shorter than PZO under all conditions. The average recovery time of SZO to ethanol is shortened from 175.1 s to 92.2 s which is shortened by 47%, and to acetone from 244.3 s to 97.5 s which is shortened by 60%. Fig. 10 shows the average response time and recovery time of PZO and SZO respectively to ethanol and acetone. The above results show that Sn doping greatly shortens the response time and recovery time of ZnO nanostructures. It is probably because Sn atoms doped in ZnO lattice are more active than Zn atoms. The adsorption and desorption of Sn atoms with O2/ tested gases are more quickly than that of Zn atoms. Also, Sn doping introduces some defects in the nanostructure, and these defects make the nanostructure unstable and more possible to react with O2 or reducing gas molecules. Therefore, both the response and recovery time of SZO are shorter than those of PZO. Fig. 10 also shows that the response and recovery time of both PZO and SZO to ethanol are shorter than to acetone. The results show that the average response time to ethanol is 55.3 s while to acetone 110.2 s, the average recovery time to ethanol is 139.6 s while to acetone 181.4 s. This phenomenon reveals that the selectivity of ZnO structures to ethanol and acetone is significantly different. 4. Conclusion In this work, the gas sensing properties of pure ZnO and Sn-doped ZnO thin film to ethanol and acetone were systematically tested in our home-made testing system. The response, response time and recovery time were tested and calculated. The results show that Sn dop-
ing makes the response of ZnO decrease, and the average response of PZO is 2.2 times of SZO. But it notably shortens the response time and recovery time, and Sn doping shortens the response time and recovery time by 40%–60%. Either PZO or SZO shows better response to acetone than ethanol, and the average response to acetone is 1.2 times of ethanol. But the response time and recovery time of PZO and SZO to ethanol are shorter than acetone (the average response time to ethanol is 55.3 s while to acetone 110.2 s; the average recovery time to ethanol is 139.6 s while to acetone 181.4 s), which shows that ZnO has different selectivity to ethanol and acetone.
Acknowledgements This project is financially supported by the National Natural Science Foundation of China (No. 50502005), Beijing Natural Science Foundation (No. 1062008, 1092014) and Metallurgy Foundation of University of Science and Technology Beijing. One of the authors (Y. Q. Chang) is supported by Program for New Century Excellent Talents in University (No.NCET-07-0065) and Beijing Novel Program.
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