Sensors and Actuators B 134 (2008) 62–65 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb La 0.5 Sr 0.5 CoO 3−ı nanotubes sensor for room temperature detection of ammonia Wei Liu a,b,c , Sheng Wang a , Yu Chen a , Guojia Fang a,b,c , Meiya Li a,b,c , Xing-zhong Zhao a,b,c,∗ a Department of Physics, Wuhan University, Wuhan 430072, People’s Republic of China b Key Laboratory of Acoustic and Photonic Materials and Devices of Ministry of Education, Wuhan University, Wuhan 430072, People’s Republic of China c Center of Nanoscience and Nanotechnology, Wuhan University, Wuhan 430072, People’s Republic of China article info Article history: Received 26 November 2007 Received in revised form 13 April 2008 Accepted 14 April 2008 Available online 30 April 2008 Keywords: LSCO nanotubes Gas sensors Fast response abstract La 0.5 Sr 0.5 CoO 3−ı (LSCO) nanotubes were synthesized by using a porous anodic aluminum oxide (AAO) template from a sol–gel solution.Based on theachievement of synthesis ofLSCO nanotubes, ananotube gas sensor was fabricated with microelectromechanical system technology andits NH 3 sensing characteristics were investigated. Capacitanceof LSCO nanotubes was changed by two orders of magnitude withinseveral seconds of exposure to NH 3 molecules at room temperature. The detection limit of the LSCO nanotube sensor was several ppm,and the typicalresponse and recovery timeof the sensorat room temperature was only several seconds. Our results demonstrate the potential application of LSCO nanotubes for fabricating a highly sensitive and fast response gas sensor. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Recently, one-dimensional (1-D) nanostructures, including semiconducting carbon nanotubes [1,2], functionalized carbon nanotubes [3,4], Si nanowires [5,6], ZnO nanowires [7],V 2 O 5 nanowires [8],In 2 O 3 nanowires [9,10],WO 3 nanowires [11], SnO 2 nanowires [12], and metal nanowires [13,14], have been demon- strated as effective ultrasensitive chemical and biological sensors because of their high surface-to-volume ratio and their unique electrical properties. These features may enable a sensitivity high enough to charged analytes so that single molecule detection becomes possible [15]. In addition, the direct conversion that from chemical information into electrical signal can take advantage of existing low-power microelectronic technology and lead to minia- turized sensor devices. The recovery time is a very important parameter for gas sensors. Most of the nanotube based gas sensors have slow recovery time ranging from several minutes to several hours [1,11,12,16], which limits the practical application of these sensors. Some assistant methods, such as UV irradiation and high voltage pulse [11,16,17] are used to decrease the recovery time, but these assistant methods also make the sensors inconvenient to use. In this letter we report a fast recover gas sensor based on LSCO nanotubes. Our devices exhibit a large response to NH 3 at room temperature. Moreover, ∗ Corresponding author at: Department of Physics, Wuhan University, Wuhan 430072, People’s Republic of China. E-mail address: wliu.whu@gmail.com (X z. Zhao). Response time as short as several seconds has also been achieved, which is far better than the results previously obtained [1,11,12,16]. 2. Experiment methods 2.1. AAO membrane preparation High-purity aluminum sheets (99.999%, 20 mm × 10 mm) were used in this experiment. Prior to anodization, the metal surfaces were degreased, etched in an alkaline solution, rinsed in distilled water, and electropolished to achieve a smooth surface. It was nec- essary to immerse the samples in a concentrated acid or alkaline solution for several minutes to remove the oxide layer formed during the electropolishing process. All samples were rinsed in dis- tilled water again and then transferred to a nitrogen environment. The resultant clean aluminum samples were anodized at constant potential in 0.3 M oxalic acid (C 2 H 2 O 4 ) (40 V, 4 ◦ C, Pt sheet as a counter electrode). The anode was then immersed in an aqueous solution of 0.6 M H 3 PO 4 and 0.15 M H 2 CrO 4 at 60 ◦ C for 10 h to remove the alumina layer. Subsequently, the Al sheet was rean- odized for 20 h under the same condition again and became an AAO template with highly ordered nanoporous arrays. 2.2. LSCO sol preparation The LSCO sol were prepared from the starting materials of lanthanum acetate (La(CH 3 COO) 3 ·1.5H 2 O), strontium acetate (Sr(CH 3 COO) 2 ·0.5H 2 O) and cobalt acetate (Co(CH 3 COO) 2 ·4H 2 O). The starting materials were mixed at a molar ratio of La:Sr:Co 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.04.034 W. Liu et al. / Sensors and Actuators B 134 (2008) 62–65 63 Fig. 1. Apparatus for gas sensing test. =1− x:x:1 and dissolved in heated acetic acid and deionized water; acetylacetone (CH 3 COCH 2 COCH 3 ) was added to stabilize the solution at a the volume ratio of CH 3 COCH 2 COCH 3 /H 2 O =1:1). For- mamide (HCONH 2 ) was also added to the system at a volume ratio of HCONH 2 /H 2 O =1:3) in order to avoid cracking during heating. The concentration of solution was diluted to 0.3 M Co. 2.3. LSCO nanotubes preparation The alumina template membrane was dipped into the sol for a desired period of time and then removed, the excess sol on the membrane surface was wiped off using a laboratory tissue,followed by drying under vacuum at 100 ◦ C for 24 h. The membrane surface was carefully wiped again to remove salts crystallized on the sur- face and heated at 700 ◦ C for 4 h in open air, resulting in formation of arrays of LSCO nanotubes in the inside of the pores of the AAO template. 2.4. Characterization The morphologies of the LSCO nanowires were characterized by a scanning electron microscope (SEM, Sirion FEG, FEI). For the capacitance measurement, a pair of interdigitated elec- trodes (IDE) was fabricated using a conventional photolithographic method with a finger width of 8 ␮m and a gap size of 8 ␮m. The IDE fingers were made by sputtering 20-nm Ti and 40-nm Pt on a layer of silicon dioxide (SiO 2 ) thermally grown on top of a silicon wafer. The suspension of LSCO was strewn on the IDE fingers. 2.5. Measurement of sensing characteristics Gas-sensing experiments were carried out using a capacitance measurement system, as represented in Fig. 1. During the experi- ment, an LSCO nanotube gas sensor was placed in a sealed chamber. Diluted NH 3 in a carrier gas of air flowed through the sealed cham- ber while we are monitoring the capacitance and dielectric loss of the LSCO nanotubes. All measurements were operated at room temperature. The capacitance and dielectric loss responses during testing were monitored by a precision impedance analyzer (Agilent 4294a). 3. Results and discussions Fig. 2 illustrates the SEM images of LSCO nanotubes on micro- electrodes. It can be seen in Fig. 2, the diameter of the LSCO Fig. 2. SEM image of LSCO nanotubes on Au microelectrodes. nanotubes is about 50 nm, which is similar to the pore diameter of the template. These LSCO nanotubes are put over two Pt/Ti elec- trodes. The capacitive NH 3 sensing properties of LSCO nanotubes were measured at room temperature by placing the device in a testing chamber. Exposure to NH 3 molecules increased the capacitance of the sensor (Fig. 3). It has been found that there exists a dependency of the capacitance on the applied signal frequency. Clearly, the device’s response to NH 3 gas was more sensitive at lower frequency. A capacitance change of about three orders of magnitude had been achieved to 1000 ppm NH 3 at a frequency of 100 Hz. However, the noise at low frequency was not neglected. Therefore 10,000 Hz was chosen as the applied signal frequency to obtain enough sensitivity and negligible noise. The dielectric loss versus frequency in dif- ferent concentrations of NH 3 is depicted in Fig. 4. When the NH 3 concentration increased, the resonant frequency shifts from low to high frequency, which is corresponding to faster ion transport in high NH 3 concentration of NH 3 . Typical response curve obtained with different steps ofNH 3 con- centration variation at the 10,000 Hz frequency is reported in Fig. 5; the measurements were performed at room temperature. After 0.5% NH 3 was induced, the capacitance of the PAA sample increased by about three orders of magnitude (Fig. 5a). And then the NH 3 con- centration decreased by 20% in every step. The capacitance of the device decreased along with the decreasing NH 3 concentration. The capacitanceand dielectric loss variation versus NH 3 concen- tration was measured for the same device and the plots are shown Fig. 3. Capacitance of LSCO nanotubes versus frequency at different NH 3 concen- trations. 64 W. Liu et al. / Sensors and Actuators B 134 (2008) 62–65 Fig. 4. Dielectric loss of LSCO nanotubes versus frequency at different NH 3 concen- trations. Fig. 5. (a) Capacitance and (b) dielectric loss responses to the stepwise decreases of the NH 3 concentration at a frequency of 10,000 Hz. in Fig. 6. From the plot we can see that the capacitance of the device increases along with the increasing NH 3 concentration. It is clearly evident that the LSCO nanotube sensor exhibits a greater sensitiv- ity towards NH 3 . The detection limit of the LSCO nanotube sensor is several ppm. The capacitance varied with the NH 3 concentration monotonically but nonlinearly, while the dielectric loss was non- monotonically related to the NH 3 concentration. When the NH 3 Fig. 6. The plots of (a) and (b) dielectric loss variation as a function of NH 3 concen- tration at a frequency of 10,000 Hz. Fig. 7. (a) Capacitance and (b) dielectric loss changes at 10,000 Hz upon exposure to NH 3 of 10–200 ppm. concentration is lower than 200 ppm, the dielectric loss increased with increasing NH 3 concentration. However, in the range from 200 to 1000 ppm, thedielectric loss decreased with increasing NH 3 con- centration. These results matched well with the data in Fig. 4.In Fig. 4, the points on the dashed line correspond to dielectric losses at 10,000 Hz. We can see that the dielectric loss peak is obtained to about 200 ppm of NH 3 , which corresponds to the peak in curve (b) in Fig. 6. To understand the sensitivity in low NH 3 concentration, the dynamics gas response of the LSCO nanotube sensor to low con- centrations of NH 3 is shown in Fig. 7. Curves (a) and (b) represent how thecapacitance and dielectricloss responses to NH 3 of 200, 50, 30, 20 and 10 ppm, respectively. The variation amplitudes at vari- ous NH 3 concentrations could be reflected clearly by the functionof device sensitivity. We define capacitance response (S C ) as the ratio S C =((C A − C G )/C A ) × 100%, where C A represents the capacitance in air and C G the capacitance in gas. The dielectric loss response (S D ) is defined as the ratio S D =((D A − D G )/D A ) × 100%, where D A rep- resents the dielectric loss in air and D G the dielectric loss in gas. We can see that the capacitance response is 126, 61, 33, 16 and 12%toNH 3 of 400, 100, 50, 30 and 20 ppm respectively, while the dielectric loss response is 337, 220, 168, 112 and 80%. The dielectric loss response is much higher than the capacitance response. These results are in good accordance with the data in Fig. 6. Response time is one of the most important parameters for all sensors. Generally, this property of a gas sensor mainly depends upon the response time at low gas concentrations. The room- temperature response and recovery time of the LSCO nanotube sensor at low NH 3 concentrations are presented in Fig. 7. The exper- imental data showed that about only several seconds was needed for the capacitance to reach 90% of the total variation values dur- ing both NH 3 adsorption and desorption processes, These results were far better than most of other 1-D nanostructrued materials [1 ,11,12,16]. The sensing mechanism of LSCO nanotubes to NH 3 was sug- gested to be related with the change of the overall dielectric constant or a surface reaction process. The capacitance and dielec- tric loss variation with NH 3 of LSCO nanotubes may have originated mainly from theNH 3 molecule adsorption on the walls of LSCO nan- otubes, replacement of the air in the voids of the nanopores by NH 3 vapors, and possible surface reaction. The fast response and recov- ery time may be due to the physical adsorption of NH 3 on the LSCO nanotube surface. W. Liu et al. / Sensors and Actuators B 134 (2008) 62–65 65 4. Conclusions La 0.5 Sr 0.5 CoO 3 (LSCO) nanotubes were synthesized using a porous anodic aluminum oxide (AAO) template from a sol–gel solution. The ammonia response of LSCO nanotubes was tested. The experiments showed that the LSCO sensor exhibited a large response to NH 3, quick response and recovery time, and reversibil- ity at room temperature. These excellent NH 3 sensing properties were attributed to the high surface-to-volume ratio of the 1-D nanostructured materials. Thus, it is strongly indicated that LSCO nanotubes are a promising practical NH 3 sensing material. Acknowledgement This work was supported by National Key Basic Research and Development Program (973 Project) of China (Grant No. 2006CB932305). References [1] E.S. Snow, F.K. Perkins, E.J. Houser, S.C. Badescu, T.L. Reinecke, Chemical detec- tion with a single-walled carbon nanotube capacitor, Science 307 (2005) 1942–1945. [2] J. Kong, N.R. Franklin, C.W. Zhou, M.G. Chapline, S. Peng, K.J. Cho, H.J. Dai, Nan- otube molecular wires as chemical sensors, Science 287 (200 0) 622–625. [3] K. Besteman, J.O. Lee, F.G.M. Wiertz, H.A. Heering, C. Dekker, Enzyme-coated carbon nanotubes as single-molecule biosensors, Nano Lett. 3 (2003) 727–730. [4] J. Kong, M.G. Chapline, H.J. Dai, Functionalized carbon nanotubes for molecular hydrogen sensors, Adv. Mater. 13 (2001) 1384–1386. [5] W.W. Chen, H. Yao, C.H. Tzang, J.J. Zhu, M.S. Yang, S.T. Lee, Silicon nanowires for high-sensitivity glucose detection, Appl. Phys. Lett. 88 (2006) 213104–213106. [6] Z. Li, Y. Chen, X. Li, T.I. Kamins, K. Nauka, R.S. Williams, Sequence-specific label- free DNA sensors based on silicon nanowires, Nano Lett. 4 (2004) 245–247. [7] Q. Wan, Q.H. Li, Y.J. Chen, T.H. Wang, X.L. He, J.P. Li, C.L. Lin, Fabrication and ethanol sensing characteristics of ZnO nanowire gas sensors, Appl. Phys. Lett. 84 (2004) 3654–3656. [8] H.Y. Yu, B.H. Kang, U.H. Pi, C.W. Park, S.Y. Choi, G.T. Kim, V 2 O 5 nanowire- based nanoelectronic devices for helium detection, Appl. Phys. Lett. 86 (2005) 253102–253104. [9] D.H. Zhang, Z.Q. Liu, C. Li, T. Tang, X.L. Liu, S. Han, B. Lei, C.W. Zhou, Detection of NO 2 down to ppb levels using individual and multiple In 2 O 3 nanowire devices, Nano Lett. 4 (2004) 1919–1924. [10] D.J. Zhang, C. Li, X.L. Liu, S. Han, T. Tang, C.W. Zhou, 2003 Doping depen- dent NH 3 sensing of indium oxide nanowires, Appl. Phys. Lett. 83 (2003) 1845–1847. [11] Y.S. Kim, S.C. Ha, K. Kim, H. Yang, S.Y. Choi, Y.T. Kim, J.T. Park, C.H. Lee, J. Choi, J. Paek, K. Lee, Room-temperature semiconductor gas sensor based on nonstoichiometric tungsten oxide nanorod film, Appl. Phys. Lett. 86 (2005) 213105–213107. [12] A. Kolmakov, Y.X. Zhang, G.S. Cheng, M. Moskovits, Detection of CO and O 2 using tin oxide nanowire sensors, Adv. Mater. 15 (2003) 997–1000. [13] F. Favier, E.C. Walter, M.P. Zach, T. Benter, R.M. Penner, Hydrogen sensors and switches from electrodeposited palladium mesowire arrays, Science 293 (2001) 2227–2231. [14] B.J. Murray, E.C. Walter, R.M. Penner, Amine vapor sensing with silver mesowires, Nano Lett. 4 (2004) 665–670. [15] A. Kolmakov, M. Moskovits, Chemical sensing and catalysis by one- dimensional metal-oxide nanostructures, Annu. Rev. Mater. Res. 34 (200 4 ) 151–180. [16] C. Li, D.H. Zhang, X.L. Liu, S. Han, T. Tang, J. Han, C.W. Zhou, In 2 O 3 nanowires as chemical sensors, Appl. Phys. Lett. 82 (2003) 1613–1615. [17] Z.Y. Fan, J.G. Lu, Gate-refreshable nanowire chemical sensors, Appl. Phys. Lett. 86 (2005) 123510–123512. Biographies Wei Liu received his MS degree in physics at Wuhan University in 2003 and presently is a graduate student for her PhD degree in physics at Wuhan University. Her field of interest is nanomaterials and Lab on a Chip. Sheng Wang received his MS degree in physics at Wuhan University in 2005 and presently is a graduate student for her PhD degree in physics at Wuhan University. His field of interest is nanomaterials and devices. Yu Chen received his BSdegree inphysics at Wuhan University in 2007 and presently is a graduate student for his MS degree in physics at Wuhan University. Meiya Li received his PhD in physics at Beijing University in China (1997) and presently is a professor in Department of Physics of Wuhan University. His current fields of interest are nanomaterials. Guojia Fang received his PhD in physics at Huazhong University of Science and Technology in China (2000) and presently is a professor in Department of Physics of Wuhan University. His current fields of interest are nanomaterials Xing-Zhong Zhao received his PhD in physics at University of Science and Technology of Beijing in China (1989) and presently is a professor in Depart- ment of Physics of Wuhan University. His current fields of interest are Lab on a Chip. . Chemical journal homepage: www.elsevier.com/locate/snb La 0.5 Sr 0.5 CoO 3−ı nanotubes sensor for room temperature detection of ammonia Wei Liu a,b,c , Sheng Wang a ,. heated at 700 ◦ C for 4 h in open air, resulting in formation of arrays of LSCO nanotubes in the inside of the pores of the AAO template. 2.4. Characterization The