IEEE SENSORS JOURNAL, VOL. 12, NO. 6, JUNE 2012
CuO Nanowire-Based Humidity Sensor Sheng-Bo Wang, Chih-Hung Hsiao, Shoou-Jinn Chang, Senior Member, IEEE, Kin-Tak Lam, Kuo-Hsun Wen, Sheng-Joue Young, Shang-Chao Hung, and Bohr-Ran Huang
Abstract— The authors report the growth of CuO nanowires on an oxidized Cu wire and the fabrication of a CuO nanowire humidity sensor. It was found that we could transform a Cu wire into CuO/Cu2 O/Cu core-shell tri-layers covered with high density CuO nanowires by thermal annealing. It was also found that steady state currents of the sensor were about 2.44, 2.32, 2.23, and 2.15 µA, respectively, when measured with 20, 40, 60, and 80% relative humidity. Furthermore, it was found that sensing property of the fabricated device was stable and reproducible. Index Terms— CuO nanowire, humidity sensor.
I. I NTRODUCTION
UMIDITY sensing is an important issue in our daily life. Humidity sensors can monitor the environmental moisture for human comfort. Humidity sensors can also be used in automotive, medical, construction, semiconductor, meteorological and food processing industries [1-4]. Conventional methods to determine relative humidity (RH) are to measure changes in oscillation frequency of thin piezoelectric quartz plates or changes in the luminescence of micro-porous thin films. It is also possible to determine RH by measuring the changes in the capacitance or the resistance of moisture sensitive materials such as polymer and ceramic films. For these humidity sensors, the changes in capacitance or resistance are originated from the chemical reaction between water vapor molecular and the sample surface. With large surface area, Manuscript received May 3, 2011; revised November 24, 2011; accepted December 9, 2011. Date of publication December 16, 2011; date of current version April 25, 2012. This work was supported in part by the Center for Frontier Materials and Micro/Nano Science and Technology, the Advanced Optoelectronic Technology Center, the National Cheng Kung University under projects from the Ministry of Education, Taiwan, the Ministry of Economic Affairs under Grant NSC 98-EC-17-A-09020769 and Grant NSC 98-2221E158-006, and the Center for Condensed Matter Sciences, National Taiwan University. The Associate Editor coordinating the review of this paper and approving it for publication was Prof. Michiel J. Vellekoop. S. B. Wang, C. H. Hsiao, and S. J. Chang are with the Department of Electrical Engineering, Institute of Microelectronics, Center for Micro/Nano Science and Technology, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 70101, Taiwan (e-mail: [email protected]
; [email protected]
; [email protected]
). K. T. Lam and K. H. Wen are with the College of Physics and Information Engineering, Fuzhou University, Fujien 350108, China (e-mail: [email protected]
; [email protected]
). S. J. Young is with the Department of Electronic Engineering, National Formosa University, Yunlin 632, Taiwan (e-mail: [email protected]
). S. C. Hung is with the Department of Information Technology and Communication, Shih Chien University, Kaohsiung 845, Taiwan (e-mail: [email protected]
). B. R. Huang is with the Department of Electronic Engineering, Graduate Institute of Electro-Optical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan (e-mail: [email protected]
). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2011.2180375
nano-structured materials should provide a large response as compared to bulk and thin film materials for these applications. Indeed, humidity sensors based on nanowires [5, 6], nanorods [7, 8], nanotubes , nano-composites  have all been demonstrated. Among the nano-structured materials, one dimensional (1D) metal oxides have attracted much attention due to their potential applications in nano-scaled electronic and photonic devices. For example, 1D ZnO nanowires have been extensive studied. ZnO is an n-type semiconductor with large bandgap energy of 3.37 eV at room temperature [11, 12]. It has also been shown that ZnO nanowires could be used for force sensors , ultraviolet (UV) photo sensors , gas sensors  and humidity sensors . In recently years, CuO has also attracted much attention. CuO is a p-type semiconductor material with small bandgap energy of only 1.2 eV at room temperature. It has been shown that CuO can be used in high temperature high temperature superconductors , gas sensor [18, 19], magnetic storage media , catalysis [21, 22] and field emitters . Very recently, Li et al. reported the growth of CuO nanowires by thermal oxidation of Cu foils . By suspending the CuO nanowires in isopropyl alcohol and depositing them onto oxidized Si substrates, they also fabricated individual CuO nanowire field effect transistors. They also observed a strong interaction between H2 O and CuO surface which lead to rapid decrease in the conductance of an individual CuO nanowire . Such a result suggests that conductance of CuO nanowires is extremely sensitive to RH. Compared with other humidity sensors, the CuO nanowire sensors should be cost effective and easy to fabricate. In this study, we report the growth of CuO nanowires on an oxidized Cu wire and the fabrication of a CuO nanowire humidity sensor. Physical and electro-optical properties of the fabricated device will also be discussed. II. E XPERIMENTS Prior to the growth of CuO nanowires, a 1-cm-long 99.999% pure Cu wire with a diameter of 0.1 mm was first rinsed in HCl and vibrated ultrasonically for 60 sec. The chemically cleaned Cu wire was subsequently rinsed with de-ionized water and then dried by N2 flow. To grow the nanowires, we inserted the Cu wire into a quartz tube in a conventional horizontal furnace under atmospheric pressure at 500 °C in air for 2 hours. After the growth, surface morphology of the sample was characterized by a Hitachi S-4700I field-emission scanning electron microscope (FESEM) operated at 15kV. Crystallographic and structural properties of the thermally treated sample were characterized by a Siemens D5000 X-ray Diffraction (XRD) system and a Philips FEI TECNAI G2 high resolution transmission electron microscopy (HRTEM) operated at 200kV.
1530–437X/$26.00 © 2011 IEEE
WANG et al.: CuO NANOWIRE-BASED HUMIDITY SENSOR
Flow control Dry air
Chamber Fig. 1. Schematic diagram of the system used to evaluate the sensor performances.
Wet air Flow control
Fig. 2. (a) Top-view. (b) Cross-sectional FESEM images of the thermally treated sample.
III. R ESULTS AND D ISCUSSION Figure 2(a) shows top-view FESEM image of the thermally treated sample. After 500 °C thermal treatment in air for 2 hours, it can be seen that surface of the wire was covered with high-density, well-aligned nanowires. Inset in figure 2(b) shows cross-sectional FESEM image while figure 2(b) shows enlarged cross-sectional FESEM image of the same sample. As shown in these cross-sectional FESEM images, it was found that the original Cu wire was oxidized into a tri-layer coreshell wire covered with numerous nanowires. It was also found that these nanowires were reasonably uniform with an average length of 2-5 µm and an average diameter of 50 nm. Figure 3 shows XRD spectrum measured from the thermally treated sample. It was found that Cu2 O and CuO related peaks were both observed in the spectrum and could be well identified by JCPDS (No. 45-0937, 05-0667). During thermal treatment, surface of the original Cu wire was first oxidized into a Cu2 O layer (i.e., Cu + O2 → Cu2 O) and
XRD spectrum of the thermally treated sample.
For the fabrication of humidity sensor, we placed the thermally treated sample on a glass substrate and used silver glue to form solid contact electrodes on both ends of the wire. The sample was subsequently placed in a sealed chamber. Current-voltage (I-V) characteristics of the sample were then measured from the two electrodes by a Keithley 4200-SCS semiconductor characterization system. Figure 1 show schematic diagram of the system used to evaluate the sensor performances. As shown in figure 1, we used the dry air through pure water to control the humidity level in the test chamber by adjusting the flow rates of dry air and wet air at room temperature. The humidity level inside the chamber was also monitored by a standard hygrometer during these measurements.
6 4 Energy (keV) (b)
Fig. 4. (a) HRTEM image taken from one single CuO nanowire. Inset shows SAED image of the CuO nanowire. (b) EDX spectrum of our CuO nanowires.
then further oxidized into CuO (i.e., Cu2 O + O2 → CuO) . Very recently, Park et al. demonstrated the growth of CuO nanowires by furnace annealing a Cu thin film in air . We believe similar mechanism should also occur as we continuously oxidized the Cu wire in air. As a result, we observed the tri-layer core-shell wire covered with numerous nanowires shown in figure 2(b). Figure 4(a) shows HRTEM image taken from a randomly picked nanowire of the sample. It can be seen clearly that the nanowire is well-oriented single crystal with no observable defects in this region. The 0.265 nm lattice spacing observed from this image is equivalent to that between the (−2 0 2) planes of the bulk CuO crystal. Inset in figure 4(a) shows selected area electron diffraction (SAED) image of the same CuO nanowire. It can be seen from the SAED pattern that pairs of reciprocal lattice peaks existed along two different lattice directions. Such a pattern suggests that the twin plane exists in our CuO nanowire. Such an observation results suggest that our nanowires were dual-phased monoclinic CuO . Figure 4(b) shows energy dispersive x-ray (EDX) spectrum taken from the same nanowire. The small amount of nickel and carbon signal shown in the spectrum should originate from TEM nickel grids and the carbon adhesion film. From the EDX spectrum, it was found that the nanowire contains
IEEE SENSORS JOURNAL, VOL. 12, NO. 6, JUNE 2012
2.4 40 %
Current (×10−6 A)
20 % Current(×10−6 A)
CuO NW CuO film Cu2O Cu
2.2 80 % 0
500 1000 Time (s)
80 % 2.3 Fig. 5.
Schematic diagram of the fabricated humidity sensor.
4Cu + O2 → 2Cu2 O 2Cu2 O + O2 → 4CuO
During high temperature treatment, Cu atoms were oxidized through a series of reactions. Firstly, the Cu atoms could migrate to the face of the wire and react with oxygen in the air to form a Cu2 O layer. Secondly, surface of the Cu2 O layer could be further oxidized into CuO to form the CuO/Cu2 O/Cu core-shell tri-layer structure. Finally, selfassembled CuO nanowires could thus be grown directly on the wire surface . Since these CuO nanowires were grown by thermal treatment process in air, the geometry (i.e., density, length and diameter) of these CuO nanowires should depend strongly on growth temperature and growth time. Sensing properties of the fabricated CuO nanowires should also depend on these parameters. Figure 5 shows schematic diagram of the fabricated humidity sensor. Upon thermal oxidation treatment, surface of the Cu wire will be oxidized to form the CuO/Cu2 O/Cu coreshell tri-layers. It should be noted that the silver glue was only pasted on the CuO surface layer as the electrodes. Thus, electrical short circuit will not occur through the Cu core layer. To evaluate humidity sensing properties of the fabricated devices, we measured the time-current characteristics of the sensor under various RHs. Figure 6(a) shows measured current as a function of time as we changed RH in the sealed chamber. It can be seen that measured current decreased monotonically with the increase of RH. Figure 6(b) plots measured current as a function of RH in the test chamber. As shown in figure 6(b), it was found that steady state currents were about 2.44, 2.32, 2.23 and 2.15 µA, respectively, when measured with 20%, 40%, 60% and 80% RH. It is known that water vapor will be adsorbed on the surface of metal oxide nanowires. For example, it has been shown that water vapor reacts reversibly with ZnO lattice and release free electrons in the nanowires. As a result, the n-type ZnO nanowires should become more conductive due to increased number of free electrons . Similar situation should also occur for the CuO nanowires. Using simulation tools, Li also found that H2 O molecule can
1000 1500 Time (s) (a)
Experiment data Linear fir
2.40 Current(×10−6 A)
49% copper and 51% oxygen. These values again indicate that the nanowires prepared in this study were indeed CuO nanowires. The growth mechanism of our CuO/Cu2 O/Cu core-shell tri-layer structure was formed through a two-step oxidation process which could be described as :
2.35 2.30 2.25 2.20 2.15 20
40 50 60 Relative humidity % (b)
Fig. 6. (a) Measured current as a function of time and inset the reproducible sensing response. (b) Measured current as a function of RH.
tightly bond on the CuO(111) surface and each H2 O molecule could transfer 0.02 extra electrons to the CuO nanowire . This will result in a decrease in free hole concentration in the p-type CuO nanowires. More specifically, the H2 O molecules adsorbed on the CnO nanowire surface could capture the majority charge carriers (hole) and reduce hole concentration. Such chemical reaction could be described as : 1 1 H2 O + h + ↔ O2 + H + 2 4
This will result in an increase in resistivity of the nanowires. As a result, we can thus use the fabricated device for humidity sensing. As a result, we observed a monotonic decrease in current as we increased the RH. Inset in figure 6(a) shows dynamic response measured from the fabricated humidity sensor. During this measurement, the RH level in the testing environment was switched between 60% and 80%. As we increased the RH from 60% to 80% and then decreased back to 60%, it was found that measured current decreased and then increased to its original value. Such a result indicates that sensing property of the fabricated device was stable and reproducible. From this transit response measurement, it was found that rise-time and fall-time of the fabricated sensor were both around 120 sec. Such a value also agrees with those observed from the CuO nanowire-based humidity sensor fabricated from two-dimensional CuO film . However, similar to other semiconductor nanowire materials (such as Si, ZnO, GaN, SnO2 …. etc.), the fabricated CuO nanowire sensor
WANG et al.: CuO NANOWIRE-BASED HUMIDITY SENSOR
also responses to reducing gases, such as CO, NO and alcohol. Very recently, He et al. reported the fabrication of Si nanowire sensors with high specificity and high-selectivity . By measuring the changes in impedance under different frequencies, they successfully identified simultaneously water (H2 O) and ethanol (EtOH), respectively. We should also be able to solve the cross-sensitivity issue of our CuO nanowire humidity sensors using the same method. Such experiments are under preparation and the results will be reported later. IV. C ONCLUSION In summary, we report the growth of CuO nanowires on an oxidized Cu wire and the fabrication of a CuO nanowire humidity sensor. It was found that we could transform a Cu wire into CuO/Cu2 O/Cu core-shell tri-layers covered with high density CuO nanowires by thermal annealing. It was also found that steady state currents of the sensor were about 2.44, 2.32, 2.23 and 2.15 µA, respectively, when measured with 20%, 40%, 60% and 80% RH. Furthermore, it was found that sensing property of the fabricated device was stable and reproducible. ACKNOWLEDGMENT The authors would like to thank the Light Emitting Diode Lighting Research Center of National Cheng Kung University, Tainan, Taiwan, for the assistance in device characterization. R EFERENCES  L. T. Chen, C. Y. Lee, and W. H. Cheng, “MEMS-based humidity sensor with integrated temperature compensation mechanism,” Sensors Actuat. A: Phys., vol. 147, no. 2, pp. 522–528, Oct. 2008.  T. L. Yeo, T. Sun, and K. T. V. Grattan, “Fibre-optic sensor technologies for humidity and moisture measurement,” Sensors Actuat. A: Phys., vol. 144, no. 2, pp. 280–295, Jun. 2008.  P. G. Su and C. P. Wang, “Flexible humidity sensor based on TiO2 nanoparticles-polypyrrole-poly-[3-(methacrylamino)propyl] trimethyl ammonium chloride composite materials,” Sensors Actuat. B: Chem., vol. 129, no. 2, pp. 538–543, Feb. 2008.  A. Vijayan, M. Fuke, R. Hawaldar, M. Kulkarni, D. Amalnerkar, and R. C. Aiyer, “Optical fibre based humidity sensor using Co-polyaniline clad,” Sensors Actuat. B: Chem., vol. 129, no. 1, pp. 106–112, Jan. 2008.  B. Tao, J. Zhang, F. Miao, H. Li, L. Wan, and Y. Wang, “Capacitive humidity sensors based on Ni/SiNWs nanocomposites,” Sensors Actuat. B: Chem., vol. 136, no. 1, pp. 144–150, Feb. 2009.  H. Li, J. Zhang, B. Tao, L. Wan, and W. Gong, “Investigation of capacitive humidity sensing behavior of silicon nanowires,” Phys. E: Low-Dimensional Syst. Nanostr., vol. 41, no. 4, pp. 600–604, Feb. 2009.  L. Gu, K. B. Zheng, Y. Zhou, J. Li, X. L. Mo, G. R. Patzke, and G. R. Chen, “Humidity sensors based on ZnO/TiO2 core-shell nanorod arrays with enhanced sensitivity,” Sensors Actuat. B: Chem., vol. 159, no. 1, pp. 1–7, Nov. 2011.  Y. Zhang, K. Yu, D. Jiang, Z. Zhu, H. Geng, and L. Luo, “Zinc oxide nanorod and nanowire for humidity sensor,” Appl. Surface Sci., vol. 242, nos. 1–2, pp. 212–217, Mar. 2005.  J.-R. Huang, M.-Q. Li, Z.-Y. Huang, and J.-H. Liu, “A novel conductive humidity sensor based on field ionization from carbon nanotubes,” Sensors Actuat. A: Phys., vol. 133, no. 2, pp. 467–471, Feb. 2007.  Y. Pimtong-Ngam, S. Jiemsirilers, and S. Supothina, “Preparation of tungsten oxide-tin oxide nanocomposites and their ethylene sensing characteristics,” Sensors Actuat. A: Phys., vol. 139, nos. 1–2, pp. 7–11, Sep. 2007.  K. J. Chen, F. Y. Hung, S. J. Chang, and S. J. Young, “Optoelectronic characteristics of UV photodetector based on ZnO nanowire thin films,” J. Alloys Compoun., vol. 479, nos. 1–2, pp. 674–677, Jun. 2009.
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IEEE SENSORS JOURNAL, VOL. 12, NO. 6, JUNE 2012
Sheng-Bo Wang was born in Taoyuan, Taiwan, on May 29, 1984. He received the B.S. degree from the Department of Electrical Engineering, National Yunlin University of Science and Technology (NYUST), Yunlin, Taiwan, and the M.S. degree from the Graduate School of Optoelectronics, NYUST, in 2006 and 2008, respectively. He is currently pursuing the Ph.D. degree with the Institute of Microelectronics, National Cheng Kung University, Tainan, Taiwan. His current research interests include optoelectronics devices of silicon based compound semiconductors and 1-D metal-oxide-semiconductors. Chih-Hung Hsiao was born in Taipei, Taiwan, on December 3, 1982. He received the M.S. degree from the Institute of Electronic Engineering, National Yunlin University of Science and Technology, Yunlin, Taiwan, and the Ph.D. degree from the Institute of Microelectronics, National Cheng Kung University (NCKU), Tainan, Taiwan, in 2007 and 2010, respectively. He was a Visiting Research Student with the Solid-State Devices Section of Materials and Electro-Optics Research Division, Chung-Shan Institute of Science and Technology, Taoyuan, Taiwan, from 2007 to 2008, and was a Visiting Research Student with Waseda University, Tokyo, Japan, from October 2009 to March 2010. Currently, he serves as a Post-Doctoral Researcher with the Institute of Microelectronics, NCKU. His current research interests include growth and characterization of II–V compound optoelectronics devices and 1-D semiconductor nanowires. Shoou-Jinn Chang (M’06–SM’10) was born in Taipei, Taiwan, on January 17, 1961. He received the B.S. degree from National Cheng Kung University (NCKU), Tainan, Taiwan, the M.S. degree from the State University of New York, Stony Brook, and the Ph.D. degree from the University of California, Los Angeles, in 1983, 1985, and 1989, respectively, all in electrical engineering. He was a Research Scientist with NTT Basic Research Laboratories, Tokyo, Japan, from 1989 to 1992. In 1992, he became an Associate Professor with the Department of Electrical Engineering, NCKU, and was promoted to Full Professor in 1998. Currently, he serves as the Deputy Director of the Center for Micro/Nano Science and Technology, the Deputy Director of the Advanced Optoelectronic Technology Center, and the Director of the Institute of Microelectronics, NCKU. He was a Royal Society Visiting Scholar with the University of Wales, Wales, U.K., from January 1999 to March 1999, a Visiting Scholar with the Research Center for Advanced Science and Technology, University of Tokyo, Tokyo, from July 1999 to February 2000, a Visiting Scholar with the Institute of Microstructural Science, National Research Council, Ottawa, ON, Canada, from August 2001 to September 2001, a Visiting Scholar with the Institute of Physics, Stuttgart University, Stuttgart, Germany, from August 2002 to September 2002, and a Visiting Scholar with the Faculty of Engineering, Waseda University, Tokyo, from July 2005 to September 2005. He is an Honorary Professor with the Changchun University of Science and Technology, Jilin, China. His current research interests include semiconductor physics, optoelectronic devices, and nanotechnology. Prof. Chang received the Outstanding Research Award from the National Science Council, Taiwan, in 2004. Kin-Tak Lam was born in Hong Kong, on February 23, 1965. He received the B.S. degree in mechanical engineering from National Cheng Kung University, Tainan, Taiwan, and the Ph.D. degree in mechanical engineering from National Sun Yat-Sen University, Kaohsiung, Taiwan, in 1987 and 1993, respectively. He is currently a Full Professor with the College of Physics and Information Engineering, Fuzhou University, Fujien, China.
Kuo-Hsun Wen received the B.S. degree from Feng Chia University, Taichung, Taiwan, the M.S. degree from the University of Manchester, Manchester, U.K., and the Ph.D. degree from the Swinburne University of Technology, Melbourne, Australia, in 1990, 1993, and 2003, respectively. He is currently a Full Professor with the College of Physics and Information Engineering, Fuzhou University, Fujien, China.
Sheng-Joue Young was born in Tainan, Taiwan, on June 22, 1981. He received the B.S. degree from the Department of Physics, National Changhua University of Education, Changhua, Taiwan, the M.S. degree from the Institute of Electro-Optical Science and Engineering, National Cheng Kung University (NCKU), Tainan, and the Ph.D. degree from the Institute of Microelectronics, NCKU, in 2003, 2005, and 2008, respectively. He is currently an Assistant Professor with the Department of Electronic Engineering, National Formosa University, Yunlin, Taiwan. His current research interests include optoelectronic devices and growth of semiconductor nanostructures.
Shang-Chao Hung was born in Taipei, Taiwan, in 1965. He received the M.S.E. degree in electrical engineering from the University of Alabama, Huntsville, in June 1992, and the Ph.D. degree from Institute of Microelectronics, National Cheng Kung University, Tainan, Taiwan, in 2006. He was with MATRA, Paris, France, as an Electrical Engineer responsible for constructing the first subway in Taiwan in 1992. He is now an Associate Professor with the Department of Information Technology and Communication, Shih Chen University, Kaoshuing, Taiwan. His current research interests include nano-structure design, fabrication, characterization, and optoelectronic devices.
Bohr-Ran Huang was born in Nantou, Taiwan, in 1961. He received the B.S. degree in electrophysics from National Chiao Tung University, Hsinchu, Taiwan, in 1983, and the M.S. and Ph.D. degrees in electrical engineering from Michigan State University, East Lansing, in 1986 and 1992, respectively. He joined the National Yunlin University of Science and Technology (NYUST), Yunlin, Taiwan, in 1992, as an Associate Professor of electronic engineering and became a Full Professor in 2000. He served as a Chairperson with the Department of Electronic Engineering and the Director of the Institute of Optoelectronics, NYUST, from 2002 to 2007. He joined the National Taiwan University of Science and Engineering (NTUST), Taipei, Taiwan, in 2007. Currently, he is a Professor with the Graduate Institute of Electro-Optical Engineering and the Department of Electronic Engineering, NTUST. His current research interests include synthesis of nanomaterials (including carbon nano tubes, silicon nanowires, ZnO, WO3, GaN, and nanodiamond), light emitting diodes, nano-imprint technology, and nano-optoelectronic devices. Prof. Huang is a member of the Chinese Institute of Electrical Engineering, the Chinese Society for Material Science, and the Council of Taiwan Association for Coatings and Thin Films Technology. He received the Teaching Professor Award from the Ministry of Education, Taiwan, in 2002 and the Excellent Research Award from NTUST in 2009.