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ZnO nanorod gas sensor for NO2 detection
Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 528–532
Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
ZnO nanorod gas sensor for NO2 detection Fang-Tso Liu, Shiang-Fu Gao, Shao-Kai Pei, Shih-Cheng Tseng, Chin-Hsin J. Liu * Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 31 October 2008 Received in revised form 22 March 2009 Accepted 23 March 2009
Vertically aligned ZnO nanorod arrays are prepared by a hydrothermal method with zinc acetate and hexamethylenetetramine, and used for NO2 gas sensing. The NO2 sensor based on nanorod arrays shows a higher sensitivity and lower operating temperature as compared to the sensor based on the ZnO film prepared by ultrasonic spray pyrolysis. The enhanced sensitivity is attributed to the higher aspect ratio of the nanorod structure. The response of the ZnO nanorod sensor is linearly proportional to the NO2 concentration in the 0.2–5.0 ppm range, and the sensitivity of the sensor increases with the length of the ZnO nanorod. ß 2009 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Zinc oxide Nanorod Gas sensor NO2
1. Introduction Zinc oxide is an intrinsically n-type semiconductor of wurtzite structure with a wide direct band gap of 3.37 eV and a large exciton ¨ zgu¨r et al., 2005) that finds many binding energy of 60 meV (O applications in transparent conductors (Lee et al., 2003), lightemitting diodes (Ye et al., 2007), solar cells (Olson et al., 2006; Peiro et al., 2006) and gas sensors. ZnO sensors for CO, H2, NH3, O3, NO2 and volatile organic compounds such as ethanol, acetone, toluene, have been reported (Agarwal and Speyr, 1998; Liao et al., 2007; Sadek et al., 2007). Recently, attention has been focused on the studies of nanostructure, i.e. nanowire, nanorod, or nanotube, of ZnO and its influence on the gas sensing performance (Wan et al., 2004). Many approaches have been demonstrated for the synthesis of one-dimensional ZnO nanostructures, including chemical vapor deposition (Wu and Liu, 2002), metal organic chemical vapor deposition (Jiang et al., 2007), carbothermal reduction (Park et al., 2005), vacuum evaporation (Jin et al., 1999) and hydrothermal growth (Fang et al., 2005). Among them, the hydrothermal method provides a convenient and low cost route for the construction of well-ordered ZnO nanorod arrays. NO2 sensor based on the sprayed ZnO thin film has been reported, and the enhanced sensitivity was attributed to the increase of surface area by doping (Ferro et al., 2005). ZnO nanowire sensor has been shown to be sensitive to various gases, and the sensitivity was found to depend on the nanowire diameter
* Corresponding author. Tel.: +886 2 27376647. E-mail address: [email protected] (C.J. Liu).
(Fan and Lu, 2006). The size dependency of gas sensitivity of ZnO nanorods was investigated, and the enhanced sensitivity of the thinner nanorods was attributed to the increase of oxygen vacancies and larger effective surface areas (Liao et al., 2007). Quasi-one-dimensional metal oxide materials for gas sensing have been reviewed recently (Lu et al., 2006). In this paper, we report a NO2 gas sensor based on ZnO nanorods prepared by the hydrothermal method. The effects of the nanorod and its length on the gas sensitivity of ZnO sensor will be discussed. 2. Experimental All chemicals were of analytical reagent grade. Substrates were 12 mm 12 mm alumina plates with sputtered platinum interdigitated electrodes. Thickness of the electrodes was 3 nm and both the linewidth and the spacing 200 mm. For thin film samples, the ZnO was deposited by the ultrasonic spray pyrolysis process. 0.05 M zinc acetate in methanol was used as the precursor solution. The solution was nebulized by a 2.4 MHz PZT ultrasonic generator. The resultant mist was carried into the Pyrex reactor by N2 gas at a flow rate of 6 sccm, and was pyrolyzed at the surface of the substrate maintained at 350 8C. The deposition time was 5–15 min. For nanorod samples, a seed layer of ZnO was deposited by ultrasonic spray pyrolysis, followed by the hydrothermal growth of the ZnO nanorods. The aqueous hydrothermal solution was prepared by mixing equal molar ratio of zinc acetate dihydrate (Acros organics 99%) and hexamethylenetetramine (HMTA, Acros organics 99%); both concentrations were kept at 0.02 M. The hydrothermal growth reaction was carried out in a Teflon-lined
1876-1070/$ – see front matter ß 2009 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jtice.2009.03.008
F.-T. Liu et al. / Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 528–532
stainless container. The substrate was put in the solution with the seeded face down, and the container was sealed and kept at 110 8C unless specified. The sample was removed after 3 h, rinsed with deionized water, and was dried in an oven at 50 8C. The morphology and size distribution of the nanorods were characterized using a JEOL JSM-6500F field-emission scanning electron microscope (FE-SEM) operated at 15 kV. The crystal structure of the nanorods was investigated by transmission electron microscopy and electron diffraction using a JEOL 2000 FX 11 TEM operated at 200 keV. X-ray diffraction analysis was performed with a Rigaku D/Max-Pc diffractometer with Cu Ka radiation. Two gold wires were then connected to the platinum interdigitated electrodes with conductive silver paste to construct the resultant sensor.
529
For gas sensing measurements, the sensor was put in a Pyrex glass test chamber with a volume of 34.4 mL, which was kept in a small oven maintained at a constant temperature with a PID temperature controller (Eurotherm model 808). NO2 gas diluted with high purity N2 was passed through the test chamber at a flow rate of 500 mL/min, controlled by a Sierra mass flow controller (Model 840). The dc conductivity of the ZnO sensor was measured using a Keithley 236 multimeter with a source
Fig. 2. X-ray diffraction patterns of (a) sprayed ZnO film and (b) ZnO nanorod arrays. ZnO deposition conditions are the same as that in Fig. 1.
Fig. 1. SEM micrographs of (a) sprayed ZnO film, top view; (b) ZnO nanorod arrays, top view; (c) ZnO nanorod arrays, side view. Spraying parameters: substrate temperature at 350 8C, spraying time 5 min; hydrothermal growth conditions: precursor concentrations are 0.02 M for both zinc acetate and HMTA, growth temperature 110 8C, growth time 3 h.
Fig. 3. (a) TEM image of a single ZnO nanorod with the corresponding electron diffraction pattern (inset) and (b) HRTEM image of the ZnO nanorod.
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F.-T. Liu et al. / Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 528–532
voltage of 10 V. The dc current was converted into resistance of the sensor. For n-type semiconductor, the resistance of ZnO increases with NO2, and the relative response was defined as R.S. = (R Ro)/Ro, where Ro is the resistance in carrier gas, R is the resistance in NO2–carrier gas mixture. An IBM microcomputer was used to record the time profile of gas response to adsorption and desorption of NO2. 3. Results and discussion Fig. 1 shows FE-SEM images of two ZnO samples. The spray pyrolyzed ZnO sample exhibits a relatively uniform film consisting of nanoparticles covering the alumina substrate surface, as shown in Fig. 1(a). And the hydrothermally grown ZnO sample exhibits nanorod arrays ca. 30 nm in diameter and 1.78 mm in length as shown in Fig. 1(b) and (c). The bundling of flexible ZnO nanorods at the tips is shown in Fig. 1(b). Wang et al. (2005) observed sudden bundling of ZnO nanorods with
gold tips during SEM measurement, and attributed this phenomenon to attraction between the accumulation charges near the Au/ZnO junctions of nearby nanorods. However, Liu et al. (2008) found that ZnO nanorods without gold tips also bundle together, and can be accounted for by the electrostatic interactions between Zn- and O-terminated (0 0 0 1) polar surfaces. Sun et al. (2006) attributed it to the surface tension forces during the air-drying. Fig. 2(a) and (b) shows the XRD patterns of the ZnO film and the ZnO nanorod array, respectively. The diffraction peaks in Fig. 2(b) can be indexed to a hexagonal wurtzite structure with lattice constants of a = 0.324 nm and c = 0.520 nm, in good agreement ¨ zgu¨r et al., 2005). The significantly higher with the literature (O intensity of the (0 0 2) diffraction peak indicates that the hydrothermally grown ZnO rods are preferentially oriented in the c-axis direction. The sprayed ZnO film also shows a small (0 0 2) peak, but not visible in Fig. 2(a) when displayed under the same scale with the nanorod sample.
Fig. 4. SEM micrographs of ZnO nanorod samples grown at three different temperatures: (a) 90 8C, top view; (b) 90 8C, side view; (c) 100 8C, top view; (d) 100 8C, side view; (e) 110 8C, top view; (f) 110 8C, side view. Other growth conditions the same as that in Fig. 1.
F.-T. Liu et al. / Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 528–532
Fig. 3(a) shows a TEM image of a typical ZnO nanorod with a diameter of 33 nm. The corresponding electron diffraction pattern shown in the inset indicates that it is single crystal hexagonal wurtzite ZnO with a [0 0 1] nanorod growth direction. Fig. 3(b) shows the HRTEM image of the nanorod. The observed fringe spacing along the nanorod axis is 0.52 nm, which agrees well with the interplanar spacing of the (0 0 1) planes of the hexagonal ZnO. We have carried out a series of experiments on the growth kinetics of the ZnO nanorods. The results show that the growth condition, i.e. precursor concentration, growth temperature, and substrate type, will affect the characteristics of the nanorods such as length, diameter, and density. Details of that study will be reported elsewhere. Fig. 4 shows FE-SEM images of ZnO nanorod samples grown at three different temperatures. While the diameter of the nanorod remains relatively constant at ca. 30 nm with the precursor concentration kept at 0.02 M, the length increases as the growth temperature increases. The lengths are L = 1.78 mm, 1.43 mm, and 1.00 mm for T = 90 8C, 100 8C, and 110 8C samples, respectively, as measured from the side-view micrographs. The top-view micrographs in Fig. 4 also show that the degree of bundling increases as length of the nanorod increases. Before the sensing experiments, an I–V curve was plotted between the two platinum interdigitated electrodes. The I–V dependency was found to be linear between 10 V and +10 V that indicates the ZnO/Pt contacts are ohmic. The current at 10 V was taken as the sensing current, and the slope of the I–V curve as the sensor resistance, which varies during the sensing process. Fig. 5 shows the response curves of two ZnO sensors operating at 250 8C. Since ZnO is an n-type semiconductor, the oxidizing NO2 molecules adsorbed on the oxide surface may capture electrons from the conduction band and form NO2 (Sadek et al., 2007), as indicated by Eq. (1): NO2ðadsÞ þ e ! NO2ðadsÞ
(1)
A depletion layer is formed on the surface of ZnO, resulting in the increase in resistance. For the sprayed ZnO film sample, the sensor resistance is 4.2 MV under nitrogen carrier gas, and is increased to a steady-state value of 185 MV upon exposure to 5 ppm of NO2 in nitrogen. For the ZnO nanorod sample, the background resistance is 0.050 MV, and the steady-state resistance is 10 MV in 5 ppm of NO2 in nitrogen. These resistances are about two orders of magnitude lower than that of the ZnO film sample, possibly due to
Fig. 5. Response curves of the ZnO sensor based on (a) sprayed ZnO film and (b) ZnO nanorod arrays. ZnO deposition conditions are the same as that in Fig. 1. The testing gas is 5 ppm NO2 in nitrogen carrier gas. The sensor operating temperature is 250 8C.
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Fig. 6. Relative response as a function of operating temperature for (a) sprayed ZnO film sensor and (b) ZnO nanorod sensor. ZnO deposition conditions are the same as that in Fig. 1. The testing gas is 5 ppm NO2 in nitrogen carrier gas.
the higher defect concentration of the hydrothermally grown nanorod sample as compared to that of the spray pyrolyzed film sample. The relative responses calculated from Fig. 5 are R.S. = 43 for the sprayed ZnO film sample, and R.S. = 199 for the ZnO nanorod sample. The fact that the depleted region due to the adsorption of NO2 occupies a much larger volume fraction in the ZnO nanorod sample than in the ZnO film sample, may account for the higher sensitivity of the nanorod sample. Fig. 6 shows that the relative response of the sensor depends on the operating temperature and exhibits a maximum for both ZnO film and ZnO nanorod samples. In the temperature range prior to the maximum, the NO2 adsorption-electron capture process dominates, and the steady-state resistance increases as the temperature increases. In the temperature range past the maximum, the desorption process dominates, and the steadystate resistance decreases as the temperature increases. For the film sample, the maximum is reached at 290 8C, and for the nanorod sample at 250 8C. The lower optimum operating temperature and the higher relative response of the ZnO nanorod sensor could be interpreted by the high surface to volume ratio of the nanorods and the resultant faster adsorption and desorption kinetics (Dogo et al., 1992). Fig. 7 shows the resistance of a ZnO nanorod sensor in response to repetitive adsorption–desorption cycles. The stability and reproducibility of the NO2 sensor is demonstrated.
Fig. 7. Repetitive response curves of the ZnO nanorod sensor operated at 250 8C. The testing gas is 5 ppm NO2 in nitrogen carrier gas.
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F.-T. Liu et al. / Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 528–532
Acknowledgement Financial support from National Science Council (NSC 97-2221E011-074) is gratefully acknowledged. References
Fig. 8. Relative response as a function of NO2 concentration (in ppm) for ZnO nanorods grown at three temperatures: (&) 90 8C, (*) 100 8C, and (*) 110 8C.
Fig. 8 shows the relative response of the sensor is linearly proportional to the NO2 concentration in 0.2–5.0 ppm range. It also shows that the sensor sensitivity increases with the length of the ZnO nanorod. The sensitivities can be calculated from the slopes as 40.9 ppm1, 31.8 ppm1, and 11.2 ppm1 for the L = 1.78 mm, 1.43 mm, and 1.00 mm samples, respectively. The higher sensitivity for the longer nanorod may be explained by the larger aspect ratio of the nanorod, and the larger fraction of the nanorod is depleted upon adsorption of NO2. However, the effect of the nanorod bundling on the gas sensitivity is not clear yet. Further studies are needed to control the bundling of ZnO nanorods and to clarify the effects of length and diameter on their gas sensitivity. 4. Conclusion In conclusion, ZnO nanorod arrays synthesized by the hydrothermal method align vertically along the c-axis. The NO2 sensor based on the ZnO nanorods shows higher sensitivity and lower operating temperature as compared to that based on the sprayed ZnO thin film without the nanorod structure. The relative response of the sensor is linearly proportional to the NO2 concentration in the 0.2–5.0 ppm range. And the sensitivity of the sensor increases with the length of the ZnO nanorod. The sensitivities are 40.9 ppm1, 31.8 ppm1, and 11.2 ppm1 for the L = 1.78 mm, 1.43 mm, and 1.00 mm ZnO nanorod arrays, respectively. The higher sensitivity for the longer nanorod can be explained by the larger aspect ratio. Further research is needed to understand the effect of the nanorod bundling on the gas sensing behavior of the ZnO sensor.
Agarwal, G. and R. F. Speyr, ‘‘Current Change Method of Reducing Gas Sensing Using ZnO Varistors,’’ J. Electrochem. Soc., 145, 2920 (1998). Dogo, S., J. P. Germain, C. Maleysson, and A. Pauly, ‘‘Interaction of NO2 with Copper Phthalocyanine Thin Films. II. Application to Gas Sensing,’’ Thin Solid Films, 219, 251 (1992). Fan, Z. and J. G. Lu, ‘‘Chemical Sensing with ZnO Nanowire,’’ IEEE Trans. Nanotechnol., 5, 393 (2006). Fang, Y., X. Wen, S. Yang, Q. Pang, L. Ding, J. Wang, and W. Ge, ‘‘Hydrothermal Synthesis and Optical Properties of ZnO Nanostructured Films Directly Grown from/on Zinc Substrates,’’ J. Sol–Gel Sci. Technol., 36, 227 (2005). Ferro, R., J. A. Rodriguez1, and P. Bertrand, ‘‘Development and Characterization of a Sprayed ZnO Thin Film-based NO2 Sensor,’’ Phys. Status Solidi C, 2, 3754 (2005). Jiang, F., C. Zheng, L. Wang, W. Fang, Y. Pu, and J. Dai, ‘‘The Growth and Properties of ZnO Film on Si(1 1 1) Substrate with an AlN Buffer by AP-MOCVD,’’ J. Lumin., 122123, 162 (2007). Jin, M., J. Feng, D. H. Zhang, H. L. Ma, and S. Y. Li, ‘‘Optical and Electronic Properties of Transparent Conducting ZnO and ZnO:Al Films Prepared by Evaporating Method,’’ Thin Solid Films, 357, 98 (1999). Lee, J. H., K. H. Ko, and B. O. Park, ‘‘Electrical and Optical Properties of ZnO Transparent Conducting Films by the Sol–Gel Method,’’ J. Cryst. Growth, 247, 119 (2003). Liao, L., H. B. Lu, J. C. Li, H. He, D. F. Wang, D. J. Fu, C. Liu, and W. F. Zhang, ‘‘Size Dependence of Gas Sensitivity of ZnO Nanorods,’’ J. Phys. Chem. C, 111, 1900 (2007). Liu, J., S. Lee, K. Lee, Y. H. Ahn, J. Y. Park, and K. H. Koh, ‘‘Bending and Bundling of Metalfree Vertically Aligned ZnO Nanowires Due to Electrostatic Interaction,’’ Nanotechnology, 19, 185607 (2008). Lu, J. G., P. Chang, and Z. Fan, ‘‘Quasi-One-Dimensional Metal Oxide Materials— Synthesis Properties and Applications,’’ Mater. Sci. Eng. R, 52, 49 (2006). Olson, D. C., J. Piris, R. T. Collins, S. E. Shaheen, and D. S. Ginley, ‘‘Hybrid Photovoltaic Devices of Polymer and ZnO Nanofiber Composites,’’ Thin Solid Films, 496, 26 (2006). ¨ zgu¨r, , Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dog˘an, V. Avrutin, S. J. Cho, and H. O Morkocd, ‘‘A Comprehensive Review of ZnO Materials and Devices,’’ J. Appl. Phys., 98, 041301 (2005). Park, J. H., Y. J. Choi, and J. G. Park, ‘‘Synthesis of ZnO Nanowires and Nanosheets by an O2-assisted Carbothermal Reduction Process,’’ J. Cryst. Growth, 280, 161 (2005). Peiro, A. M., P. Ravirajan, K. Govender, D. S. Boyle, P. O’Brien, D. C. Bradley, J. Nelson, and J. R. Durrant, ‘‘Hybrid Polymer/Metal Oxide Solar Cells Based on ZnO Columnar Structures,’’ J. Mater. Chem., 16, 2088 (2006). Sadek, A. Z., S. Choopun, W. Wlodarski, S. J. Ippolito, and K. Kalantar-zadeh, ‘‘Characterization of ZnO Nanobelt-based Gas Sensor for H2, NO2, and Hydrocarbon Sensing,’’ IEEE Sensor J., 7, 919 (2007). Sun, Y., N. G. Ndifor-Angwafor, D. J. Riley, and M. N. R. Ashfold, ‘‘Synthesis and Photoluminescence of Ultra-thin ZnO Nanowire/Nanotube Arrays Formed by Hydrothermal Growth,’’ Chem. Phys. Lett., 431, 352 (2006). Wan, Q., Q. H. Li, Y. J. Chen, T. H. Wang, X. L. He, J. P. Li, and C. L. Lin, ‘‘Fabrication and Ethanol Sensing Characteristics of ZnO Nanowire Gas Sensors,’’ Appl. Phys. Lett., 84, 3654 (2004). Wang, X., C. J. Summers, and Z. L. Wang, ‘‘Self-Attraction among Aligned Au/ZnO Nanorods under Electron Beam,’’ Appl. Phys. Lett., 86, 013111 (2005). Wu, J. J. and S. C. Liu, ‘‘Catalyst-Free Growth and Characterization of ZnO Nanorods,’’ J. Phys. Chem. B, 106, 9546 (2002). Ye, Z. Z., J. G. Lu, Y. Z. Zhang, Y. J. Zeng, L. L. Chen, F. Zhuge, G. D. Yuan, H. P. He, L. P. Zhu, J. Y. Huang, and B. H. Zhao, ‘‘ZnO Light-Emitting Diodes Fabricated on Si Substrates with Homobuffer Layers,’’ Appl. Phys. Lett., 91, 113503 (2007).
Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
ZnO nanorod gas sensor for NO2 detection Fang-Tso Liu, Shiang-Fu Gao, Shao-Kai Pei, Shih-Cheng Tseng, Chin-Hsin J. Liu * Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 31 October 2008 Received in revised form 22 March 2009 Accepted 23 March 2009
Vertically aligned ZnO nanorod arrays are prepared by a hydrothermal method with zinc acetate and hexamethylenetetramine, and used for NO2 gas sensing. The NO2 sensor based on nanorod arrays shows a higher sensitivity and lower operating temperature as compared to the sensor based on the ZnO film prepared by ultrasonic spray pyrolysis. The enhanced sensitivity is attributed to the higher aspect ratio of the nanorod structure. The response of the ZnO nanorod sensor is linearly proportional to the NO2 concentration in the 0.2–5.0 ppm range, and the sensitivity of the sensor increases with the length of the ZnO nanorod. ß 2009 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Zinc oxide Nanorod Gas sensor NO2
1. Introduction Zinc oxide is an intrinsically n-type semiconductor of wurtzite structure with a wide direct band gap of 3.37 eV and a large exciton ¨ zgu¨r et al., 2005) that finds many binding energy of 60 meV (O applications in transparent conductors (Lee et al., 2003), lightemitting diodes (Ye et al., 2007), solar cells (Olson et al., 2006; Peiro et al., 2006) and gas sensors. ZnO sensors for CO, H2, NH3, O3, NO2 and volatile organic compounds such as ethanol, acetone, toluene, have been reported (Agarwal and Speyr, 1998; Liao et al., 2007; Sadek et al., 2007). Recently, attention has been focused on the studies of nanostructure, i.e. nanowire, nanorod, or nanotube, of ZnO and its influence on the gas sensing performance (Wan et al., 2004). Many approaches have been demonstrated for the synthesis of one-dimensional ZnO nanostructures, including chemical vapor deposition (Wu and Liu, 2002), metal organic chemical vapor deposition (Jiang et al., 2007), carbothermal reduction (Park et al., 2005), vacuum evaporation (Jin et al., 1999) and hydrothermal growth (Fang et al., 2005). Among them, the hydrothermal method provides a convenient and low cost route for the construction of well-ordered ZnO nanorod arrays. NO2 sensor based on the sprayed ZnO thin film has been reported, and the enhanced sensitivity was attributed to the increase of surface area by doping (Ferro et al., 2005). ZnO nanowire sensor has been shown to be sensitive to various gases, and the sensitivity was found to depend on the nanowire diameter
* Corresponding author. Tel.: +886 2 27376647. E-mail address: [email protected] (C.J. Liu).
(Fan and Lu, 2006). The size dependency of gas sensitivity of ZnO nanorods was investigated, and the enhanced sensitivity of the thinner nanorods was attributed to the increase of oxygen vacancies and larger effective surface areas (Liao et al., 2007). Quasi-one-dimensional metal oxide materials for gas sensing have been reviewed recently (Lu et al., 2006). In this paper, we report a NO2 gas sensor based on ZnO nanorods prepared by the hydrothermal method. The effects of the nanorod and its length on the gas sensitivity of ZnO sensor will be discussed. 2. Experimental All chemicals were of analytical reagent grade. Substrates were 12 mm 12 mm alumina plates with sputtered platinum interdigitated electrodes. Thickness of the electrodes was 3 nm and both the linewidth and the spacing 200 mm. For thin film samples, the ZnO was deposited by the ultrasonic spray pyrolysis process. 0.05 M zinc acetate in methanol was used as the precursor solution. The solution was nebulized by a 2.4 MHz PZT ultrasonic generator. The resultant mist was carried into the Pyrex reactor by N2 gas at a flow rate of 6 sccm, and was pyrolyzed at the surface of the substrate maintained at 350 8C. The deposition time was 5–15 min. For nanorod samples, a seed layer of ZnO was deposited by ultrasonic spray pyrolysis, followed by the hydrothermal growth of the ZnO nanorods. The aqueous hydrothermal solution was prepared by mixing equal molar ratio of zinc acetate dihydrate (Acros organics 99%) and hexamethylenetetramine (HMTA, Acros organics 99%); both concentrations were kept at 0.02 M. The hydrothermal growth reaction was carried out in a Teflon-lined
1876-1070/$ – see front matter ß 2009 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jtice.2009.03.008
F.-T. Liu et al. / Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 528–532
stainless container. The substrate was put in the solution with the seeded face down, and the container was sealed and kept at 110 8C unless specified. The sample was removed after 3 h, rinsed with deionized water, and was dried in an oven at 50 8C. The morphology and size distribution of the nanorods were characterized using a JEOL JSM-6500F field-emission scanning electron microscope (FE-SEM) operated at 15 kV. The crystal structure of the nanorods was investigated by transmission electron microscopy and electron diffraction using a JEOL 2000 FX 11 TEM operated at 200 keV. X-ray diffraction analysis was performed with a Rigaku D/Max-Pc diffractometer with Cu Ka radiation. Two gold wires were then connected to the platinum interdigitated electrodes with conductive silver paste to construct the resultant sensor.
529
For gas sensing measurements, the sensor was put in a Pyrex glass test chamber with a volume of 34.4 mL, which was kept in a small oven maintained at a constant temperature with a PID temperature controller (Eurotherm model 808). NO2 gas diluted with high purity N2 was passed through the test chamber at a flow rate of 500 mL/min, controlled by a Sierra mass flow controller (Model 840). The dc conductivity of the ZnO sensor was measured using a Keithley 236 multimeter with a source
Fig. 2. X-ray diffraction patterns of (a) sprayed ZnO film and (b) ZnO nanorod arrays. ZnO deposition conditions are the same as that in Fig. 1.
Fig. 1. SEM micrographs of (a) sprayed ZnO film, top view; (b) ZnO nanorod arrays, top view; (c) ZnO nanorod arrays, side view. Spraying parameters: substrate temperature at 350 8C, spraying time 5 min; hydrothermal growth conditions: precursor concentrations are 0.02 M for both zinc acetate and HMTA, growth temperature 110 8C, growth time 3 h.
Fig. 3. (a) TEM image of a single ZnO nanorod with the corresponding electron diffraction pattern (inset) and (b) HRTEM image of the ZnO nanorod.
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F.-T. Liu et al. / Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 528–532
voltage of 10 V. The dc current was converted into resistance of the sensor. For n-type semiconductor, the resistance of ZnO increases with NO2, and the relative response was defined as R.S. = (R Ro)/Ro, where Ro is the resistance in carrier gas, R is the resistance in NO2–carrier gas mixture. An IBM microcomputer was used to record the time profile of gas response to adsorption and desorption of NO2. 3. Results and discussion Fig. 1 shows FE-SEM images of two ZnO samples. The spray pyrolyzed ZnO sample exhibits a relatively uniform film consisting of nanoparticles covering the alumina substrate surface, as shown in Fig. 1(a). And the hydrothermally grown ZnO sample exhibits nanorod arrays ca. 30 nm in diameter and 1.78 mm in length as shown in Fig. 1(b) and (c). The bundling of flexible ZnO nanorods at the tips is shown in Fig. 1(b). Wang et al. (2005) observed sudden bundling of ZnO nanorods with
gold tips during SEM measurement, and attributed this phenomenon to attraction between the accumulation charges near the Au/ZnO junctions of nearby nanorods. However, Liu et al. (2008) found that ZnO nanorods without gold tips also bundle together, and can be accounted for by the electrostatic interactions between Zn- and O-terminated (0 0 0 1) polar surfaces. Sun et al. (2006) attributed it to the surface tension forces during the air-drying. Fig. 2(a) and (b) shows the XRD patterns of the ZnO film and the ZnO nanorod array, respectively. The diffraction peaks in Fig. 2(b) can be indexed to a hexagonal wurtzite structure with lattice constants of a = 0.324 nm and c = 0.520 nm, in good agreement ¨ zgu¨r et al., 2005). The significantly higher with the literature (O intensity of the (0 0 2) diffraction peak indicates that the hydrothermally grown ZnO rods are preferentially oriented in the c-axis direction. The sprayed ZnO film also shows a small (0 0 2) peak, but not visible in Fig. 2(a) when displayed under the same scale with the nanorod sample.
Fig. 4. SEM micrographs of ZnO nanorod samples grown at three different temperatures: (a) 90 8C, top view; (b) 90 8C, side view; (c) 100 8C, top view; (d) 100 8C, side view; (e) 110 8C, top view; (f) 110 8C, side view. Other growth conditions the same as that in Fig. 1.
F.-T. Liu et al. / Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 528–532
Fig. 3(a) shows a TEM image of a typical ZnO nanorod with a diameter of 33 nm. The corresponding electron diffraction pattern shown in the inset indicates that it is single crystal hexagonal wurtzite ZnO with a [0 0 1] nanorod growth direction. Fig. 3(b) shows the HRTEM image of the nanorod. The observed fringe spacing along the nanorod axis is 0.52 nm, which agrees well with the interplanar spacing of the (0 0 1) planes of the hexagonal ZnO. We have carried out a series of experiments on the growth kinetics of the ZnO nanorods. The results show that the growth condition, i.e. precursor concentration, growth temperature, and substrate type, will affect the characteristics of the nanorods such as length, diameter, and density. Details of that study will be reported elsewhere. Fig. 4 shows FE-SEM images of ZnO nanorod samples grown at three different temperatures. While the diameter of the nanorod remains relatively constant at ca. 30 nm with the precursor concentration kept at 0.02 M, the length increases as the growth temperature increases. The lengths are L = 1.78 mm, 1.43 mm, and 1.00 mm for T = 90 8C, 100 8C, and 110 8C samples, respectively, as measured from the side-view micrographs. The top-view micrographs in Fig. 4 also show that the degree of bundling increases as length of the nanorod increases. Before the sensing experiments, an I–V curve was plotted between the two platinum interdigitated electrodes. The I–V dependency was found to be linear between 10 V and +10 V that indicates the ZnO/Pt contacts are ohmic. The current at 10 V was taken as the sensing current, and the slope of the I–V curve as the sensor resistance, which varies during the sensing process. Fig. 5 shows the response curves of two ZnO sensors operating at 250 8C. Since ZnO is an n-type semiconductor, the oxidizing NO2 molecules adsorbed on the oxide surface may capture electrons from the conduction band and form NO2 (Sadek et al., 2007), as indicated by Eq. (1): NO2ðadsÞ þ e ! NO2ðadsÞ
(1)
A depletion layer is formed on the surface of ZnO, resulting in the increase in resistance. For the sprayed ZnO film sample, the sensor resistance is 4.2 MV under nitrogen carrier gas, and is increased to a steady-state value of 185 MV upon exposure to 5 ppm of NO2 in nitrogen. For the ZnO nanorod sample, the background resistance is 0.050 MV, and the steady-state resistance is 10 MV in 5 ppm of NO2 in nitrogen. These resistances are about two orders of magnitude lower than that of the ZnO film sample, possibly due to
Fig. 5. Response curves of the ZnO sensor based on (a) sprayed ZnO film and (b) ZnO nanorod arrays. ZnO deposition conditions are the same as that in Fig. 1. The testing gas is 5 ppm NO2 in nitrogen carrier gas. The sensor operating temperature is 250 8C.
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Fig. 6. Relative response as a function of operating temperature for (a) sprayed ZnO film sensor and (b) ZnO nanorod sensor. ZnO deposition conditions are the same as that in Fig. 1. The testing gas is 5 ppm NO2 in nitrogen carrier gas.
the higher defect concentration of the hydrothermally grown nanorod sample as compared to that of the spray pyrolyzed film sample. The relative responses calculated from Fig. 5 are R.S. = 43 for the sprayed ZnO film sample, and R.S. = 199 for the ZnO nanorod sample. The fact that the depleted region due to the adsorption of NO2 occupies a much larger volume fraction in the ZnO nanorod sample than in the ZnO film sample, may account for the higher sensitivity of the nanorod sample. Fig. 6 shows that the relative response of the sensor depends on the operating temperature and exhibits a maximum for both ZnO film and ZnO nanorod samples. In the temperature range prior to the maximum, the NO2 adsorption-electron capture process dominates, and the steady-state resistance increases as the temperature increases. In the temperature range past the maximum, the desorption process dominates, and the steadystate resistance decreases as the temperature increases. For the film sample, the maximum is reached at 290 8C, and for the nanorod sample at 250 8C. The lower optimum operating temperature and the higher relative response of the ZnO nanorod sensor could be interpreted by the high surface to volume ratio of the nanorods and the resultant faster adsorption and desorption kinetics (Dogo et al., 1992). Fig. 7 shows the resistance of a ZnO nanorod sensor in response to repetitive adsorption–desorption cycles. The stability and reproducibility of the NO2 sensor is demonstrated.
Fig. 7. Repetitive response curves of the ZnO nanorod sensor operated at 250 8C. The testing gas is 5 ppm NO2 in nitrogen carrier gas.
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F.-T. Liu et al. / Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 528–532
Acknowledgement Financial support from National Science Council (NSC 97-2221E011-074) is gratefully acknowledged. References
Fig. 8. Relative response as a function of NO2 concentration (in ppm) for ZnO nanorods grown at three temperatures: (&) 90 8C, (*) 100 8C, and (*) 110 8C.
Fig. 8 shows the relative response of the sensor is linearly proportional to the NO2 concentration in 0.2–5.0 ppm range. It also shows that the sensor sensitivity increases with the length of the ZnO nanorod. The sensitivities can be calculated from the slopes as 40.9 ppm1, 31.8 ppm1, and 11.2 ppm1 for the L = 1.78 mm, 1.43 mm, and 1.00 mm samples, respectively. The higher sensitivity for the longer nanorod may be explained by the larger aspect ratio of the nanorod, and the larger fraction of the nanorod is depleted upon adsorption of NO2. However, the effect of the nanorod bundling on the gas sensitivity is not clear yet. Further studies are needed to control the bundling of ZnO nanorods and to clarify the effects of length and diameter on their gas sensitivity. 4. Conclusion In conclusion, ZnO nanorod arrays synthesized by the hydrothermal method align vertically along the c-axis. The NO2 sensor based on the ZnO nanorods shows higher sensitivity and lower operating temperature as compared to that based on the sprayed ZnO thin film without the nanorod structure. The relative response of the sensor is linearly proportional to the NO2 concentration in the 0.2–5.0 ppm range. And the sensitivity of the sensor increases with the length of the ZnO nanorod. The sensitivities are 40.9 ppm1, 31.8 ppm1, and 11.2 ppm1 for the L = 1.78 mm, 1.43 mm, and 1.00 mm ZnO nanorod arrays, respectively. The higher sensitivity for the longer nanorod can be explained by the larger aspect ratio. Further research is needed to understand the effect of the nanorod bundling on the gas sensing behavior of the ZnO sensor.
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