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Zinc oxide nanorod and nanowire for humidity sensor
Applied Surface Science 242 (2005) 212–217 www.elsevier.com/locate/apsusc
Zinc oxide nanorod and nanowire for humidity sensor Yongsheng Zhanga,c, Ke Yua,b, Desheng Jiangd, Ziqiang Zhua,*, Haoran Genge, Laiqiang Luoa a
Department of Electric Engineering, East China Normal University, 3663 Zhongshan North Road, Shanghai 200062, PR China b School of Physics and Microelectronics, Shandong University, Jinan 250061, PR China c Department of Computer Science and Technology, Luoyang Technology College, Luoyang 471003, PR China d Fiber Optical Sensing Technology Research Center, Wuhan University of Technology, Wuhan 430070, PR China e School of Material Science and Engineering of Jinan University, Jinan 250022, PR China Received in revised form 16 August 2004; accepted 16 August 2004 Available online 25 September 2004
Abstract ZnO nanorod and nanowire films were fabricated on the Si substrates with comb type Pt electrodes by the vapor-phase transport method, and their humidity sensitive characteristics have been investigated. These nanomaterial films show highhumidity sensitivity, good long-term stability and fast response time. It was found that the resistance of the films decreases with increasing relative humidity (RH). At room temperature (RT), resistance changes of more than four and two orders of magnitude were observed when ZnO nanowire and nanorod devices were exposed, respectively, to a moisture pulse of 97% relative humidity. It appears that the ZnO nanomaterial films can be used as efficient humidity sensors. # 2004 Elsevier B.V. All rights reserved. PACS: 81.07.Bc; 81.10.Bk; 82.47.Rs Keywords: Zinc oxide nanomaterial; Vapor-phase transport; Humidity sensor
1. Introduction The humidity control is essential for various fields of industry as well as human life. There is a substantial * Corresponding author. Tel.: +86 21 62221912; fax: +86 21 62233780. E-mail address: [email protected] (Y. Zhang), [email protected] (Z. Zhu).
interest in the development of humidity sensors for application in monitoring relative humidity (RH) in moisture-sensitive environment (such as glove boxes and clean rooms), detection of trace moisture in many types of pure gases for semiconductor manufacturing and packaging, cryogenic process, medical and food science application, and so on. Several transduction techniques have been explored, for example, changes in the capacitance and resistance of polymer and
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.08.013
Y. Zhang et al. / Applied Surface Science 242 (2005) 212–217
ceramic films, in the oscillation frequency of thin piezoelectric quartz plates and in the luminescence of microporous silicon, thin films are being used to measure humidity levels [1,2]. The desirable characteristic of humidity sensors are high sensitivity, chemical and thermal stability, reproducibility, lowoperation temperature, low cost and long life. So far, however, there has been no optimum material that could fulfill all those requirements simultaneously [3–6]. ZnO is a versatile II–VI semiconductor with numerous applications ranging from optoelectronics to chemical sensors because of its distinctive optical, electronic and chemical properties. It is well known that dimension or the surface-to-volume ratio has great influence to the material performance. In recent years, one-dimensional (1D) ZnO nanostructure, such as nanowire, nanorods, nanobelts and nanotetrapod, have attracted much attention [7–11]. In this work, ZnO nanorod and nanowire films were fabricated, respectively, on the Si substrate with a grid of pre-deposited Pt electrodes by the vapor-phase transport process, and their humidity sensitivity characteristics have been investigated. It is experimentally demonstrated that ZnO nanorod and nanowire films show a promising application for humidity sensors.
2. Experimental Silicon (1 0 0) substrate was cleaned in a sonicating bath of acetone for about 1.5 h. A SiO2 insulating layer with thickness of about 200 nm was formed on the surface of Si substrate by thermal oxidizing. A comb type Pt electrode (Fig. 1) was deposited on the
Fig. 1. Scheme of a ZnO nanomaterial humidity sensor.
213
substrate. The substrate was coated by an Au thin film with thickness of about 5 nm using a JS-450 cold sputter unit. An equal amount of ZnO powder (purity 99.99%) and graphite powder (purity 99.9%) were mixed and loaded in a 15 cm long quartz boat. The substrate with gold thin film was put 5 cm downstream of the starting materials. The assembly was then placed in the middle of a quartz tube in a horizontal furnace. The furnace temperature was increased to 450–600 8C at a flow rate of 150 sccm Ar (99.9%), and O2 was then added at a flow rate of 15 sccm for a further 1 h. After the system had cooled to room temperature (RT), light or dark gray material film was found on the surface of the substrate. Scanning electron microscopy (SEM, JEOL-JSM6700F), X-ray diffraction (XRD, D/max 2550 V) and high-resolution transmission electron microscopy (HRTEM) (Hitachi H-9000, NAR 300 kV) were used to characterize the morphology and crystal structure of the products, respectively. The electrical properties of the materials were investigated using a LF impedance analyzer and electrometer. The layout of the sensor is shown in Fig. 1. The controlled humidity environments were achieved using anhydrous P2O5 and saturated aqueous solutions of LiCl, MgCl2, NaBr, NaCl, KCl and K2SO4 in a closed glass vessel at an ambient temperature of 25 8C, which yielded approximately 12.0, 33.2, 57.6, 75.8, 84.3 and 96.7% relative humidity, respectively. These RH levels were independently monitored by using a standard hygrometer.
3. Results and discussion Fig. 2(a) and (b) show the XRD patterns of products grown on the substrates under 450 and 600 8C, respectively. The strong diffraction peaks can be indexed as those from the known highly crystallized wurtzite-structure ZnO with lattice constants of a = 0.325 nm and c = 0.521 nm. Fig. 3(a) and (b) show a top-view SEM images of high-density ZnO nanorods and nanowires grown on the substrates under 450 and 600 8C, respectively. As shown in Fig. 3(a), a lot of short ZnO multipod nanorods with a mean diameter of about 300 nm and an average length of about 1.0 mm were obtained at low temperature (450 8C). When the temperature went higher (600 8C), ZnO nanowires
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Fig. 2. XRD powder patterns of the synthesized samples: (a) ZnO nanorods; (b) ZnO nanowires.
with a mean diameter of about 40 nm and a length in the range of 2–6 mm were observed in Fig. 3(b). Hexagon and planes of a single nanowire can be clearly identified, as shown by the inset SEM image in Fig. 3(b), providing strong evidence that it grows in the direction of (0 0 0 1). Fig. 4 shows a HRTEM image of the ZnO nanowire. Clear fringes, indicating a single crystalline wurtzite structure, can be observed. The amorphous layer of graphite on the nanowire surface is not found, proving that ZnO nanowires are pure. The growth process of the ZnO nanorods and nanowires can be interpreted by means of the vapor– liquid–solid mechanism [12,13]. First, ZnO powder in the source is reduced to Zn and its suboxide (ZnOx, x < 1) of low-melting point by carbon and carbon monoxide. The Zn and ZnOx vapor are then transferred by the processing gas (Ar) to the lowtemperature region to be condensed into liquid
Fig. 3. SEM images of the synthesized samples: (a) ZnO nanorods; (b) ZnO nanowires.
nanodroplets, which recombine with oxygen to form nano-ZnO as nuclei on the substrate with Au catalyst and further grow into the nanorods and nanowires along the (0 0 0 1) direction. At lower reaction temperature (450 8C), ZnO powders of starting materials could be reduced deficiently and the size of Au catalyst droplets is larger, thus the shorter and thicker ZnO nanorods with faulty crystal structure were formed. When reaction temperature is higher (600 8C), ZnO powder could be reduced sufficiently, the size of Au catalyst droplets is smaller; the longer and thinner ZnO nanowires with perfect crystal structure were obtained.
Y. Zhang et al. / Applied Surface Science 242 (2005) 212–217
Fig. 4. HRTEM image of a ZnO nanowire.
215
sensitivity factor Sf = R12%/R97% = 5442, where R12% and R97% are the dc resistances at 12 and 97% RH, respectively. For ZnO nanorod sample, the linearity of the resistance versus RH and sensitivity are relatively poor over a wide humidity range, and Sf = R12%/R97% = 183. The resistance variations with time for two samples are shown in Fig. 6. The measurements were repeated at 25 8C, every 5 days for 1 month. Slight variation in resistance is observed over time. The resistance variation is less than 3% RH at each humidity region, for 1 month, resistance that the ZnO nanomaterial films are relatively stable to the exposure to water in air. Compared with nanorods, the linearity and sensitivity of resistance versus RH for ZnO nanowires are better; it is mainly due to the homogenous
Concerning the resistive-type sensor, I–V characteristics measured in the RH range of 12–97% indicate an almost ohmic behavior, and evidence a resistance strongly dependent on RH. The results of resistance measurement as a function of RH at a fixed ambient temperature of 25 8C are presented in Fig. 5. It can be seen that the resistance of the ZnO nanomaterial films decreases with increasing relative humidity, and the resistance of ZnO nanowire sample changes linearly with RH for approximately four orders of magnitude (108–104) on a semilogarithmic scale over the range of 12–97% RH, showing very high sensitivity and good linearity. The
Fig. 5. Relative humidity vs. dc resistance plots at 25 8C for ZnO nanorods and nanowires.
Fig. 6. Resistance variations with time for the samples at various RH levels: (a) ZnO nanorods; (b) ZnO nanowires.
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Y. Zhang et al. / Applied Surface Science 242 (2005) 212–217
field, which promotes water dissociation [15]. The dissociation provides protons as charge carriers of the hopping transport. At high humidity, liquid water condensed on the nanorods and nanowires, and electrolytic conduction between nanorods and nanowires takes place along with protonic transport.
4. Conclusions
Fig. 7. Electrical responses of the ZnO nanorods and nanowires to a pulse of 97% relative humidity.
morphology and size of ZnO nanowires and the higher specific surface area. Thus, the ZnO nanowires absorb moisture easily and uniformly. It was experimentally demonstrated that the ZnO nanomaterials present fast response to the humidity pulse at RT. As shown in Fig. 7, the current increases abruptly in 3 s when the sensors were exposed to the moist air of 97% RH. For nanorods and nanowires, the maximum currents are about 1.7 and 1.9 mA, respectively. About 10 s later, ZnO nanomaterial sensors were placed in air ambient again. The current decreases to about 80 mA in 20 s for nanorods, and 50 mA in 30 s for nanowires, while a relatively long current tail was also observed. If devices were operated in a ventilated ambient or at a relatively high temperature, the recovery characteristic would be improved farther. Here, we give some discussions about the humidity sensing mechanism of the ZnO nanomaterials. Waterrelated conduction in ceramic and porous materials is known to mainly occur as a surface mechanism [14]. We believe that the large increase in conductivity with increased RH of ZnO nanomaterials is also related to the adsorption of water molecules. There is space between the ZnO nanorods or nanowires as their growth is irregular, and the films consisting of ZnO nanorods or nanowires are similar to porous structure materials, which have higher specific surface area, and the films absorb moisture easily. At low humidity, tips and defects of the nanorods and nanowires present a high local charge density and a strong electrostatic
ZnO nanorods and nanowires were synthesized on the Si substrates with a comb type pre-deposited Pt electrode by the vapor-phase transport process. These nanomaterial films show high-humidity sensitivity, good long-term stability and fast response time. The resistance of the films decreases with increasing relative humidity. At RT, the resistance changes of more than four and two orders of magnitude were observed when ZnO nanowire and nanorod devices were exposed, respectively, to a moisture pulse of 97% relative humidity. Therefore, the ZnO nanorods and nanowires show a potential application for humidity sensors. Acknowledgements The authors acknowledge the financial support from the NSF of China (No.10374027), and the PSF of Shandong Province, China (No. Y2001 G02).
References [1] Z.M. Rittersma, Sens. Actuators A 96 (2002) 196. [2] G. Harsanyi, Mater. Chem. Phys. 43 (1996) 199. [3] C.W. Lee, H.W. Rhee, M.S. Gong, Sens. Actuators B 73 (2001) 124. [4] M.S. Gong, M.H. Lee, H.W. Rhee, Sens. Actuators B 73 (2001) 185. [5] M.R. Yang, K.S. Chen, Sens. Actuators B 49 (1998) 240. [6] M. Matsuguichi, Y. Sadaoka, Y. Nuwa, M. Shinmoto, Y. Sakai, T. Kuroiwa, J. Electrochem. Soc. 141 (3) (1994) 614. [7] Y. Li, G.W. Meng, L.D. Zhangm, F. Phillipp, Appl. Phys. Lett. 76 (2000) 2011. [8] B.D. Yao, Y.F. Chan, N. Wang, Appl. Phys. Lett. 81 (2002) 757. [9] D. Banerjee, J.Y. Lao, D.Z. Wang, J.Y. Huang, Z.F. Ren, D. Steeves, B. Kimball, M. Sennett, Appl. Phys. Lett. 83 (2003) 2061. [10] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947.
Y. Zhang et al. / Applied Surface Science 242 (2005) 212–217 [11] Q. Wan, K. Yu, T.H. Wang, C.L. Lin, Appl. Phys. Lett. 83 (2003) 2253. [12] P. Yang, C.M. Lieber, J. Mater. Res. 12 (1997) 2981. [13] B.D. Yao, Y.F. Chan, N. Wang, Appl. Phys. Lett. 81 (2002) 757.
217
[14] M.B. Kulwicki, J. Am. Ceram. Soc. 74 (1991) 697. [15] R. Schaub, P. Thostrup, N. Lopez, E. Laegsgaard, I. Stensgaard, J.K. Norskov, F. Besenbacher, Phys. Rev. Lett. 87 (2001) 266104.
Zinc oxide nanorod and nanowire for humidity sensor Yongsheng Zhanga,c, Ke Yua,b, Desheng Jiangd, Ziqiang Zhua,*, Haoran Genge, Laiqiang Luoa a
Department of Electric Engineering, East China Normal University, 3663 Zhongshan North Road, Shanghai 200062, PR China b School of Physics and Microelectronics, Shandong University, Jinan 250061, PR China c Department of Computer Science and Technology, Luoyang Technology College, Luoyang 471003, PR China d Fiber Optical Sensing Technology Research Center, Wuhan University of Technology, Wuhan 430070, PR China e School of Material Science and Engineering of Jinan University, Jinan 250022, PR China Received in revised form 16 August 2004; accepted 16 August 2004 Available online 25 September 2004
Abstract ZnO nanorod and nanowire films were fabricated on the Si substrates with comb type Pt electrodes by the vapor-phase transport method, and their humidity sensitive characteristics have been investigated. These nanomaterial films show highhumidity sensitivity, good long-term stability and fast response time. It was found that the resistance of the films decreases with increasing relative humidity (RH). At room temperature (RT), resistance changes of more than four and two orders of magnitude were observed when ZnO nanowire and nanorod devices were exposed, respectively, to a moisture pulse of 97% relative humidity. It appears that the ZnO nanomaterial films can be used as efficient humidity sensors. # 2004 Elsevier B.V. All rights reserved. PACS: 81.07.Bc; 81.10.Bk; 82.47.Rs Keywords: Zinc oxide nanomaterial; Vapor-phase transport; Humidity sensor
1. Introduction The humidity control is essential for various fields of industry as well as human life. There is a substantial * Corresponding author. Tel.: +86 21 62221912; fax: +86 21 62233780. E-mail address: [email protected] (Y. Zhang), [email protected] (Z. Zhu).
interest in the development of humidity sensors for application in monitoring relative humidity (RH) in moisture-sensitive environment (such as glove boxes and clean rooms), detection of trace moisture in many types of pure gases for semiconductor manufacturing and packaging, cryogenic process, medical and food science application, and so on. Several transduction techniques have been explored, for example, changes in the capacitance and resistance of polymer and
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.08.013
Y. Zhang et al. / Applied Surface Science 242 (2005) 212–217
ceramic films, in the oscillation frequency of thin piezoelectric quartz plates and in the luminescence of microporous silicon, thin films are being used to measure humidity levels [1,2]. The desirable characteristic of humidity sensors are high sensitivity, chemical and thermal stability, reproducibility, lowoperation temperature, low cost and long life. So far, however, there has been no optimum material that could fulfill all those requirements simultaneously [3–6]. ZnO is a versatile II–VI semiconductor with numerous applications ranging from optoelectronics to chemical sensors because of its distinctive optical, electronic and chemical properties. It is well known that dimension or the surface-to-volume ratio has great influence to the material performance. In recent years, one-dimensional (1D) ZnO nanostructure, such as nanowire, nanorods, nanobelts and nanotetrapod, have attracted much attention [7–11]. In this work, ZnO nanorod and nanowire films were fabricated, respectively, on the Si substrate with a grid of pre-deposited Pt electrodes by the vapor-phase transport process, and their humidity sensitivity characteristics have been investigated. It is experimentally demonstrated that ZnO nanorod and nanowire films show a promising application for humidity sensors.
2. Experimental Silicon (1 0 0) substrate was cleaned in a sonicating bath of acetone for about 1.5 h. A SiO2 insulating layer with thickness of about 200 nm was formed on the surface of Si substrate by thermal oxidizing. A comb type Pt electrode (Fig. 1) was deposited on the
Fig. 1. Scheme of a ZnO nanomaterial humidity sensor.
213
substrate. The substrate was coated by an Au thin film with thickness of about 5 nm using a JS-450 cold sputter unit. An equal amount of ZnO powder (purity 99.99%) and graphite powder (purity 99.9%) were mixed and loaded in a 15 cm long quartz boat. The substrate with gold thin film was put 5 cm downstream of the starting materials. The assembly was then placed in the middle of a quartz tube in a horizontal furnace. The furnace temperature was increased to 450–600 8C at a flow rate of 150 sccm Ar (99.9%), and O2 was then added at a flow rate of 15 sccm for a further 1 h. After the system had cooled to room temperature (RT), light or dark gray material film was found on the surface of the substrate. Scanning electron microscopy (SEM, JEOL-JSM6700F), X-ray diffraction (XRD, D/max 2550 V) and high-resolution transmission electron microscopy (HRTEM) (Hitachi H-9000, NAR 300 kV) were used to characterize the morphology and crystal structure of the products, respectively. The electrical properties of the materials were investigated using a LF impedance analyzer and electrometer. The layout of the sensor is shown in Fig. 1. The controlled humidity environments were achieved using anhydrous P2O5 and saturated aqueous solutions of LiCl, MgCl2, NaBr, NaCl, KCl and K2SO4 in a closed glass vessel at an ambient temperature of 25 8C, which yielded approximately 12.0, 33.2, 57.6, 75.8, 84.3 and 96.7% relative humidity, respectively. These RH levels were independently monitored by using a standard hygrometer.
3. Results and discussion Fig. 2(a) and (b) show the XRD patterns of products grown on the substrates under 450 and 600 8C, respectively. The strong diffraction peaks can be indexed as those from the known highly crystallized wurtzite-structure ZnO with lattice constants of a = 0.325 nm and c = 0.521 nm. Fig. 3(a) and (b) show a top-view SEM images of high-density ZnO nanorods and nanowires grown on the substrates under 450 and 600 8C, respectively. As shown in Fig. 3(a), a lot of short ZnO multipod nanorods with a mean diameter of about 300 nm and an average length of about 1.0 mm were obtained at low temperature (450 8C). When the temperature went higher (600 8C), ZnO nanowires
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Y. Zhang et al. / Applied Surface Science 242 (2005) 212–217
Fig. 2. XRD powder patterns of the synthesized samples: (a) ZnO nanorods; (b) ZnO nanowires.
with a mean diameter of about 40 nm and a length in the range of 2–6 mm were observed in Fig. 3(b). Hexagon and planes of a single nanowire can be clearly identified, as shown by the inset SEM image in Fig. 3(b), providing strong evidence that it grows in the direction of (0 0 0 1). Fig. 4 shows a HRTEM image of the ZnO nanowire. Clear fringes, indicating a single crystalline wurtzite structure, can be observed. The amorphous layer of graphite on the nanowire surface is not found, proving that ZnO nanowires are pure. The growth process of the ZnO nanorods and nanowires can be interpreted by means of the vapor– liquid–solid mechanism [12,13]. First, ZnO powder in the source is reduced to Zn and its suboxide (ZnOx, x < 1) of low-melting point by carbon and carbon monoxide. The Zn and ZnOx vapor are then transferred by the processing gas (Ar) to the lowtemperature region to be condensed into liquid
Fig. 3. SEM images of the synthesized samples: (a) ZnO nanorods; (b) ZnO nanowires.
nanodroplets, which recombine with oxygen to form nano-ZnO as nuclei on the substrate with Au catalyst and further grow into the nanorods and nanowires along the (0 0 0 1) direction. At lower reaction temperature (450 8C), ZnO powders of starting materials could be reduced deficiently and the size of Au catalyst droplets is larger, thus the shorter and thicker ZnO nanorods with faulty crystal structure were formed. When reaction temperature is higher (600 8C), ZnO powder could be reduced sufficiently, the size of Au catalyst droplets is smaller; the longer and thinner ZnO nanowires with perfect crystal structure were obtained.
Y. Zhang et al. / Applied Surface Science 242 (2005) 212–217
Fig. 4. HRTEM image of a ZnO nanowire.
215
sensitivity factor Sf = R12%/R97% = 5442, where R12% and R97% are the dc resistances at 12 and 97% RH, respectively. For ZnO nanorod sample, the linearity of the resistance versus RH and sensitivity are relatively poor over a wide humidity range, and Sf = R12%/R97% = 183. The resistance variations with time for two samples are shown in Fig. 6. The measurements were repeated at 25 8C, every 5 days for 1 month. Slight variation in resistance is observed over time. The resistance variation is less than 3% RH at each humidity region, for 1 month, resistance that the ZnO nanomaterial films are relatively stable to the exposure to water in air. Compared with nanorods, the linearity and sensitivity of resistance versus RH for ZnO nanowires are better; it is mainly due to the homogenous
Concerning the resistive-type sensor, I–V characteristics measured in the RH range of 12–97% indicate an almost ohmic behavior, and evidence a resistance strongly dependent on RH. The results of resistance measurement as a function of RH at a fixed ambient temperature of 25 8C are presented in Fig. 5. It can be seen that the resistance of the ZnO nanomaterial films decreases with increasing relative humidity, and the resistance of ZnO nanowire sample changes linearly with RH for approximately four orders of magnitude (108–104) on a semilogarithmic scale over the range of 12–97% RH, showing very high sensitivity and good linearity. The
Fig. 5. Relative humidity vs. dc resistance plots at 25 8C for ZnO nanorods and nanowires.
Fig. 6. Resistance variations with time for the samples at various RH levels: (a) ZnO nanorods; (b) ZnO nanowires.
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Y. Zhang et al. / Applied Surface Science 242 (2005) 212–217
field, which promotes water dissociation [15]. The dissociation provides protons as charge carriers of the hopping transport. At high humidity, liquid water condensed on the nanorods and nanowires, and electrolytic conduction between nanorods and nanowires takes place along with protonic transport.
4. Conclusions
Fig. 7. Electrical responses of the ZnO nanorods and nanowires to a pulse of 97% relative humidity.
morphology and size of ZnO nanowires and the higher specific surface area. Thus, the ZnO nanowires absorb moisture easily and uniformly. It was experimentally demonstrated that the ZnO nanomaterials present fast response to the humidity pulse at RT. As shown in Fig. 7, the current increases abruptly in 3 s when the sensors were exposed to the moist air of 97% RH. For nanorods and nanowires, the maximum currents are about 1.7 and 1.9 mA, respectively. About 10 s later, ZnO nanomaterial sensors were placed in air ambient again. The current decreases to about 80 mA in 20 s for nanorods, and 50 mA in 30 s for nanowires, while a relatively long current tail was also observed. If devices were operated in a ventilated ambient or at a relatively high temperature, the recovery characteristic would be improved farther. Here, we give some discussions about the humidity sensing mechanism of the ZnO nanomaterials. Waterrelated conduction in ceramic and porous materials is known to mainly occur as a surface mechanism [14]. We believe that the large increase in conductivity with increased RH of ZnO nanomaterials is also related to the adsorption of water molecules. There is space between the ZnO nanorods or nanowires as their growth is irregular, and the films consisting of ZnO nanorods or nanowires are similar to porous structure materials, which have higher specific surface area, and the films absorb moisture easily. At low humidity, tips and defects of the nanorods and nanowires present a high local charge density and a strong electrostatic
ZnO nanorods and nanowires were synthesized on the Si substrates with a comb type pre-deposited Pt electrode by the vapor-phase transport process. These nanomaterial films show high-humidity sensitivity, good long-term stability and fast response time. The resistance of the films decreases with increasing relative humidity. At RT, the resistance changes of more than four and two orders of magnitude were observed when ZnO nanowire and nanorod devices were exposed, respectively, to a moisture pulse of 97% relative humidity. Therefore, the ZnO nanorods and nanowires show a potential application for humidity sensors. Acknowledgements The authors acknowledge the financial support from the NSF of China (No.10374027), and the PSF of Shandong Province, China (No. Y2001 G02).
References [1] Z.M. Rittersma, Sens. Actuators A 96 (2002) 196. [2] G. Harsanyi, Mater. Chem. Phys. 43 (1996) 199. [3] C.W. Lee, H.W. Rhee, M.S. Gong, Sens. Actuators B 73 (2001) 124. [4] M.S. Gong, M.H. Lee, H.W. Rhee, Sens. Actuators B 73 (2001) 185. [5] M.R. Yang, K.S. Chen, Sens. Actuators B 49 (1998) 240. [6] M. Matsuguichi, Y. Sadaoka, Y. Nuwa, M. Shinmoto, Y. Sakai, T. Kuroiwa, J. Electrochem. Soc. 141 (3) (1994) 614. [7] Y. Li, G.W. Meng, L.D. Zhangm, F. Phillipp, Appl. Phys. Lett. 76 (2000) 2011. [8] B.D. Yao, Y.F. Chan, N. Wang, Appl. Phys. Lett. 81 (2002) 757. [9] D. Banerjee, J.Y. Lao, D.Z. Wang, J.Y. Huang, Z.F. Ren, D. Steeves, B. Kimball, M. Sennett, Appl. Phys. Lett. 83 (2003) 2061. [10] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947.
Y. Zhang et al. / Applied Surface Science 242 (2005) 212–217 [11] Q. Wan, K. Yu, T.H. Wang, C.L. Lin, Appl. Phys. Lett. 83 (2003) 2253. [12] P. Yang, C.M. Lieber, J. Mater. Res. 12 (1997) 2981. [13] B.D. Yao, Y.F. Chan, N. Wang, Appl. Phys. Lett. 81 (2002) 757.
217
[14] M.B. Kulwicki, J. Am. Ceram. Soc. 74 (1991) 697. [15] R. Schaub, P. Thostrup, N. Lopez, E. Laegsgaard, I. Stensgaard, J.K. Norskov, F. Besenbacher, Phys. Rev. Lett. 87 (2001) 266104.