Triethylamine gas sensor based on ZnO nanorods prepared by a simple solution route

Triethylamine gas sensor based on ZnO nanorods prepared by a simple solution route

Sensors and Actuators B 141 (2009) 85–88 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier...

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Sensors and Actuators B 141 (2009) 85–88

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Triethylamine gas sensor based on ZnO nanorods prepared by a simple solution route Yu-zhen Lv a,b,c , Cheng-rong Li a , Lin Guo b,∗ , Fo-chi Wang a , Yue Xu b,∗ , Xiang-feng Chu d a

Beijing Key Laboratory of High-Voltage & EMC, North China Electric Power University, Beijing, 102206, PR China School of Chemistry and Environment, Beijing University of Aeronautics and Astronautics, Xueyuan Road 37, Haidian District, Beijing, 100083, PR China Key Laboratory of Condition Monitoring and Control for Power Plant Equipment, Ministry of Education, North China Electric Power University, Beijing, 102206, PR China d School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou, 510275, PR China b c

a r t i c l e

i n f o

Article history: Received 18 December 2008 Received in revised form 13 May 2009 Accepted 20 June 2009 Available online 27 June 2009 Keywords: ZnO nanorods Solution route Triethylamine Gas sensor

a b s t r a c t Well-crystallized ZnO nanorods were prepared by a simple solution route using dodecyl benzene sulfonic acid sodium salt (DBS) as a modifying agent. The crystal structure and morphology of the as-grown product were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM) and highresolution transmission electron microscopy (HRTEM). It was found that the hexagonal ZnO nanorods can be controllably prepared by the modification of DBS, which have an average diameter of 75 nm and the length of 2–4 ␮m. ZnO sensor was fabricated from the ZnO nanorods and its gas-sensing properties were investigated. High selectivity and superior sensitivity of the ZnO sensor to dilute triethylamine (TEA) were observed at a low operating temperature of 150 ◦ C. The sensor is a promising candidate for practical detector for low-concentration TEA. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Metal oxide semiconductor sensors have attracted great attention for a long time due to their advantageous features, such as high sensitivity under ambient conditions, low cost, low power consumption, on-line operation, and high compatibility with microelectronic processing [1–3]. Among them, ZnO has been widely used to detect low-concentration gases, such as benzene, ethanol, nitrogen oxide, liquid petroleum gas and other species due to its range of conductance variability and response towards both oxidative and reductive gases [4–8]. It is well known that the gas-sensing property of semiconductor sensors is based on the surface reactivity of semiconducting oxides. Therefore, microstructure of ZnO, namely, surface state and morphology, is one of the most important factors for high sensitivity of ZnO sensors. In the past decade, ZnO films [9,10], nanoparticles [11], and nanorods [12] have been well synthesized to detect dilute toxic gases released from harvested fish and other seafood with aging, such as trimethylamine (TMA) and triethylamine (TEA). Ryu et al. [9] reported that ZnO film sensors doped with Al2 O3 , TiO2 and V2 O5 exhibited fairly high sensitivity and selectivity to TMA at 300 ◦ C and responded well to the deterioration of a mackerel during storage. Very recently, Zhang et al. [13] reported that SnO2 –ZnO nanocomposite sensor had high

∗ Corresponding author. E-mail address: [email protected] (L. Guo). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.06.033

and quick responses to TMA (1–500 ppm) at 190–330 ◦ C. However, ZnO nanomaterial sensors for detecting TEA are rarely reported. Table 1 summarizes the TMA and TEA concentration, operating temperature, and gas sensitivity of ZnO and other nanomaterial-based sensors. It is worth noted that to obtain TEA sensors of high sensitivity and selectivity at a low working temperature still remains a challenge. To the best of our knowledge, there is no report about 1D ZnO nanorods with high selectivity and sensitivity to less than 1 ppm TEA at a relatively low operating temperature up to now. In this work, ZnO nanorods were synthesized by a facile solution route and their gas-sensing properties were investigated. High selectivity and superior sensitivity of the ZnO sensors to TEA were observed and the response to 0.001 ppm TEA attained 6 at 150 ◦ C. 2. Experimental All the chemicals are of analytical grade and used as received without further purification. Briefly, 0.001 mol zinc acetate dihydrate [Zn(AC)2 ·2H2 O] and DBS with a ratio of 1:8.5 were introduced into a mixed solvent (50 mL) of xylene and ethylene glycol (E.G) under a constant stirring. After 30 min of stirring, a hydrazine monohydrate ethanol solution was added into the above solution at room temperature with the simultaneous and vigorous agitation, resulting in a white solution. The stirring process lasted for 1 h after finishing the dropping to ensure the completion of the reaction. The mixture was subsequently heated to the boiling point (140 ◦ C). After

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Table 1 TMA and TEA sensing properties of ZnO and other gas sensors with different operating temperatures and gas concentrations. Material

Gas concentration (ppm)

ZnO film [9] ZnO + Al2 O3 film [9] ZnO + Al2 O3 + TiO2 film [9] ZnO + Al2 O3 + TiO2 + V2 O5 film [9] ZnO film [10] ZnO + Al film [10] ZnO nanoparticle + PVP [11] ZnO nanorods [12] SnO2 + ZnO nanocompsite [13] SnO2 nanorods [15] NiFe2 O4 nanorods [16] CoFe2 O4 nano-crystallines [17]

160 (TMA) 160 (TMA) 160 (TMA) 160 (TMA) 400 (TMA) 400 (TMA) 0.4 (TMA) 10 (TMA) 100(TMA) 1.0 (TEA) 1.0 (TEA) 1.0 (TEA)

refluxing for 5 h, the resulting products were cooled down naturally and washed with distilled water and absolute ethanol for several times to remove the ions possibly remaining in the final products, and finally dried in the vacuum at 70 ◦ C. For comparison, we also fabricated ZnO nanorods with a diameter of 95 nm and measured its response to TEA at 150 ◦ C. The structure and morphology of the asgrown product were characterized by XRD (Rigaku D/max 2200 V, Cu K␣), SEM (Quanta 200F, Philips-FEI) and HRTEM (Tecnai F30, FEI). The paste prepared from a mixture of ZnO nanorods with a PVA solution was coated onto an Al2 O3 tube on which two gold leads had been installed at each end. Then it was heated in an electric furnace at 400 ◦ C for 2 h to get rid of the PVA solution. The Al2 O3 tube was about 8 mm in length, 2 mm in external diameter and 1.6 mm in internal diameter. A heater of Ni–Cr wire was inserted into the Al2 O3 tube to supply the operating temperature which could be controlled in the range of 100–500 ◦ C. The sketch of the sensor device has been given before [14]. The electrical resistance of a sensor was measured in air and in sample gases. The sensing response of the sensor was defined as the ratio of the electrical resistance in air (Ra ) to that in a sample gas (Rg ).

3. Results and discussion

Operating temperature (◦ C) 300 300 300 300 300 300 <250 170 330 350 175 190

Sensitivity (Ra /Rg ) 60 80 340 450 3.5 1.4 10000 12 200 3.0 7.0 2.0

Fig. 1. XRD pattern of the as-prepared product.

have the average diameter of 75 nm and length of 2–4 ␮m. Fig. 2(b) is a lattice-resolved HRTEM image taken from one side of the ZnO nanorod. The lattice spacing amounts to 0.26 nm which is in agreement with interplanar distance of (0 0 2). This result demonstrated the single-crystalline nature of the ZnO nanorods.

3.1. Crystal structure and morphology of the prepared product 3.2. Gas-sensing properties of the prepared product Fig. 1 shows XRD pattern of the as-prepared product. All the diffraction peaks of the product are well index to the standard hexagonal phase ZnO (JCPDS card No. 36-1451). No obvious characteristic diffraction peaks from other impurities were detected, showing the high purity of the products. The as-prepared ZnO are well crystallized according to the strong diffraction peaks. Morphology of the ZnO product is shown in Fig. 2. The sample consists of straight and uniform rods. The well-proportioned ZnO nanorods

The responses of ZnO sensor based on ZnO nanorods to 1000 ppm ethanol, benzene, toluene, acetone and TEA gases at the operating temperatures of 100–400 ◦ C are shown in Fig. 3. The ZnO sensor has high sensitivity to TEA and low responses to other gases. The sensing response of the ZnO sensor to 1000 ppm TEA is as high as 566 at 150 ◦ C. Unlike other sensors which usually detected dilute TEA at the high temperatures (190–350 ◦ C) [15–17], the effective

Fig. 2. (a) SEM image of the as-grown ZnO nanorods and (b) HRTEM image of an individual ZnO nanorod.

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Fig. 3. The sensing responses of the ZnO sensor to volatile gases as a function of operating temperature.

working temperature of our sensor is 150 ◦ C. These observations clearly indicate that our ZnO sensor exhibits high selectivity to TEA among ethanol, benzene, toluene and acetone at a low operating temperature. The sensing responses of ZnO sensor to TEA (150 ◦ C) are depicted in Fig. 4 as a function of gas concentration. The correlation line was approximately linear and the sensing responses of ZnO sensor to low-concentration TEA were considerable high. For example, the sensing responses of ZnO sensor to 0.1 and 1 ppm TEA were as high as 23 and 39. The sensitivities of our ZnO sensor to TEA were higher than those of sensors made from SnO2 nanorods, NiFe2 O4 nanorods and CoFe2 O4 nano-crystallines [15–17]. It should be noted that the sensing response of our sensor to 0.001 ppm TEA attained 6, which could be applied to detect low-concentration (0.001–1000 ppm) TEA. The response transients of the sensor of ZnO nanorods to 1 and 500 ppm TEA at 150 ◦ C are depicted in Fig. 5. It was noteworthy

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Fig. 5. The response and recovery times of the ZnO sensor to triethylamine gas.

that the response times and recovery times to 1 ppm and 500 ppm TEA were about 15 s. ZnO sensor belongs to the surface-resistance controlled type, i.e., using the change of surface resistance to detect gases [8–13]. Its gas-sensing properties are strongly based on the changes in conductance of ZnO. The adsorbed atmospheric oxygen extracts the conduction electrons from the surface region of ZnO nanorods, leaving positively charged donor ions behind. An electric field develops between the positively charged donor ions and the negatively charged oxygen ions such as O2− , O− or O2 − on the surface [18]. The more the oxygen ions are on the surface, the higher the potential barrier and therefore the higher the resistance [19]. As the concentration of TEA gas increases, the amount of O2− , O− or O2 − decreases due to the reaction with TEA molecules, resulting in a decrease in resistance. The oxygen absorbed on the surface significantly influences the conductance of ZnO sensor and the amount of absorbed oxygen depends on the morphology, particle size, specific surface area and active surface area of ZnO [5–13]. According to the above observations, the modification of DBS could effectively prevent ZnO nanorods from aggregating, and control the diameter to 75 nm, i.e., increasing the ratio of surface to volume. As a result, gas molecules are easy to be adsorbed on the surface of the ZnO sensor. Furthermore, well crystallization of our ZnO nanorods can increase the electron–hole transportation. This would be the reason for the fact that the responses of our ZnO sensor to low-concentration TEA (0.001–1000 ppm) at a low operating temperature were higher than that of the other nanomaterial sensors. For comparison, we also investigated the gas-sensing property of ZnO sensor based on ZnO nanorods with a diameter of 95 nm. Its response to 1000 ppm TEA at 150 ◦ C is 275, which is more than twice lower than that of ZnO sensors (75 nm). This result further proved that the decrease size of ZnO nanorods could increase the sensitivity of ZnO sensor. However, the reason for the good selectivity of ZnO sensor to TEA among the gases of ethanol, benzene, toluene and acetone is still not very clear. More details of the high selectivity of the ZnO nanorods to TEA need further investigation. 4. Conclusions

Fig. 4. The sensing responses of the ZnO sensor to triethylamine gas as a function of gas concentration.

In summary, we prepared well-crystallized ZnO nanorods by a facile solution route employing DBS as a modifying agent, and investigated their gas-sensing properties. The results demonstrated that the sensors based on ZnO nanorods are highly sensitive and selec-

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tive to low-concentration (0.001–1000 ppm) TEA among the gases of ethanol, benzene, toluene and acetone at the operating temperature of 150 ◦ C. The response of the ZnO sensor to 0.001 ppm TEA is as high as 6. Thus, the sensor based on ZnO nanorods is a promising candidate for practical detector for low-concentration TEA. Acknowledgments This project was supported by Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT0720) and National Natural Science Foundation of China (50725208) as well as State Key Project of Fundamental Research for Nanoscience and Nanotechnology (2006CB932301).

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Yu-zhen Lv received her PhD in materials science from Beijing University of Aeronautics and Astronautics in 2006. She is currently an associate professor at North China Electric Power University. Her research interests are in gas sensor, synthesis, characteristic and applications of nanodielectrics. Cheng-rong Li received the PhD degree in electrical engineering from Tsing Hua University in 1989. He joined the University of South Carolina as a Visiting Scholar in 1992. Presently, he is a professor in North China Electric Power University. His current fields of interests are gas sensors for GIS insulation diagnosis, nanodielectrics and partial discharge location in power transformer. Lin Guo received the PhD degree in materials science from Beijing Institute of Technology. He got Professor position in 2001 in School of Material Science and Engineering, Beijing University of Aeronautics and Astronautics. Now he is a professor in School of Chemistry and Environment, BUAA. His research interests lie in synthesis, optical, magnetic, catalytic as well as gas-sensing properties of nanostructured materials. Fo-chi Wang got his MS degree in high voltage and insulation from Harbin University of Science and Technology in 2005. He is currently a teacher in North China Electric Power University. His research interest lies in the synthesis of nanodielectrics and icephobic coating. Yue Xu recived his PhD degree in materials science from Harbin Institute of Technology in 2001. Now he is an associate professor in School of Chemistry and Environment, Beijing University of Aeronautics and Astronautics. Xiang-feng Chu received his PhD degree in materials science from China University of Science and Technology. He is an associate professor in School of Chemistry and Chemical Engineering, Sun Yat-sen University. His research interest lies in gas sensors of nanomaterials.