Study on electrical conductivity of single polyaniline microtube

Materials Letters 61 (2007) 2965 – 2968 www.elsevier.com/locate/matlet

Study on electrical conductivity of single polyaniline microtube Shanxin Xiong a , Qi Wang b,⁎, Yinghong Chen b a

b

Temasek Laboratories of Nanyang Technological University, 50 Nanyang Drive, Singapore 637553, Singapore State Key Lab. of Polymer Materials Engineering (Sichuan University), Polymer Research Institute of Sichuan University, Chengdu 610065, China Received 17 March 2006; accepted 19 October 2006 Available online 7 November 2006

Abstract The PANI microtubes with diameter of 200 nm were synthesized by the template synthesis technique. The electrical conductivity of individual PANI microtube was measured directly in the template channel using scanning probe microscope (SPM). The average conductivity of the microtube is 5.81 S/cm, which is higher than that of bulk PANI (1.75 S/cm). The higher average conductivity is due to the enhancement of electrical conductivity caused by the confined environment and ordered structure of the template channels. Moreover, most of the conductivities of the microtubes are in the range of 100 S/cm magnitude, which suggests that the SPM method possesses good reproducibility and feasibility for conductivity measurement of individual microtubes. © 2006 Elsevier B.V. All rights reserved. Keywords: Scanning probe microscope; Electrical properties; Microstructure

1. Introduction Since the conductive polyacetylene (PA) was reported in 1977 [1], the nanostructures of conductive polymers have attracted considerable interests [2–4]. Template synthesis [5–7] and self-assembly [8] are effective methods for the preparation of conductive polymer nanostructure. The one-dimensional conductive polymer (nanotube, nanowire) exhibits enhanced electrical conductivity [7]. Hence, it is essential to study the electrical property of an individual conductive polymer nanostructure. However, the tiny size and difficulty of manipulation are two main obstacles for this study. In recent years, various measurement methods including experimental techniques and new testing equipment have been developed and applied. Plating electrode [9], as an effective approach, was used to measure the electrical properties of individual CNT bundles or “ropes”. However, the random sedimentation of nanostructure caused the manipulation and measurement to be more complicated. An in-situ testing method was designed by K. Ramanathan et al. [10] and Yun et al. [11] to

⁎ Corresponding author. Tel.: +86 28 85405133; fax: +86 28 85402465. E-mail address: [email protected] (Q. Wang). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.10.049

study the I–V characteristics of single nanowire through the direct synthesis of conductive polymer wires by electrodeposition within channels between two electrodes on the surface of silicon wafers. Also, a manipulator-equipped scanning electron microscope [12] was applied to characterize the electrical property of the carbon nanocoils. As a powerful and unique tool, scanning tunneling microscope (STM) can be utilized to understand the surface structure and morphology at the nanometer scale. A. Hassanien et al. [13] obtained the I–V characteristic of aligned coaxial nanowires of PANI passivated carbon nanotubes with STM. The multi-tips STM technique [14] is also a utilitarian means to directly map the surface conductivity of the sample. Nevertheless, SPM was successfully applied for the studies of the electrical property of PPy nanotube [2]. For all the above mentioned work, the first step is to obtain a dispersed single nanostructure for plating the electrodes or other manipulations. Unfortunately, this stage is the key process, and also a difficult one. Are there any other methods that can work without this stage? Actually, the template synthesis method can provide well-dispersed and highly ordered nanostructure array. Herein, the PANI microtubes were prepared by the template synthesis method. An extra circuit uses the tip and sample stage of SPM as positive and

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Fig. 1. Schematic process for the measurement of the electrical property of PANI microtube.

negative electrodes, respectively. The electrical conductivity of individual PANI microtube was measured easily by moving the position of the SPM tip. 2. Experimental 2.1. Materials Analytic reagent grade aniline (C6H5NH2), ammonium peroxydisulphate ((NH4)2S2O8, APS) and hydrochloric acid (HCl) were employed in the experiment. Aniline was distilled under reduced pressure, and stored at low temperature prior to use. APS and HCl were used as received. Anodic Aluminum Oxide (AAO) with pore diameter of 200 nm was obtained from Whatman International Ltd.

3. Results and discussion 3.1. Template synthesis of PANI microtubes As expected, the nanochannels of the template confine the geometry shape of PANI during the polymerization process. The resultant material possesses similar shape and size as those of template channels. Fig. 2a shows the typical SEM image of PANI microtubes, which is the duplicate of the template channels. The diameter of the microtubes is close to the pore diameter (200 nm) while the length of the microtubes is close to the thickness (60 μm) of the template. PANI microtubes can form a uniform and ordered array. However, some microtubes are randomly distributed after the template is removed. As shown in the

2.2. Template synthesis of PANI microtubes PANI microtubes were prepared through in-situ polymerization of aniline in the template channels [15]. As a contrast, the aniline was polymerized simultaneously without the template under the same condition. The morphologies of the microtubes were observed through JEOL JSM-5600LV scanning electron microscope (SEM) and JEM-100CX transmission electron microscope (TEM). The template was partially and fully removed for SEM and TEM observations, respectively. 2.3. Measurement of electrical conductivity Four probe resistance and SPA400 SPM were used to measure the electrical conductivities of bulk PANI and PANI microtube, respectively. The schematic diagram of I–V measurement is shown in Fig. 1. Details of the experimental procedures are given as follows: Firstly, the surface of template was treated with 1% (w/w) NaOH solution to partially remove the template and expose the PANI microtubes from the template. Then, Aurum film was sputtered onto the lower surface of the template. Finally, SPM was used to test the I–V characteristics. Here, the SPM tip and Aurum film acted as positive and negative electrodes, respectively. In the entire circuit, the resistance of the Aurum film can be neglected compared to the microtube. Hence, the I–V characteristics of individual microtubes were obtained.

Fig. 2. Morphologies of PANI microtubes. (a) SEM image; (b) TEM image.

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Fig. 3. SPM analyses of PANI microtubes. (a) and (b) morphologies of PANI microtubes, and (c) I–V curves of PANI microtubes.

TEM image (Fig. 2b), the PANI microtube is straight and regular. The diameter of microtube is about 240 nm, which is bigger than the pore diameter of template channels owing to expansion of microtube [15]. The wall thickness of the microtubes is about 20 nm and the length of the tube is about several micrometers. The length of the microtubes is shorter than that of template channels due to breakage during the treatment process. The morphology observation established that the PANI microtubes were prepared successfully.

Panels a and b of Fig. 3 are the surface morphology and 3-D image of PANI microtubes array (top part), respectively. The particle-like matters (∼200 nm in diameter) in the images are the exposed PANI microtubes. Several big aggregations (N 200 nm in diameter) are attributed to the leftover template. Fig. 3c shows the I–V characteristics of ten pieces of PANI microtubes, as seen by the labeled Arabic numerals from 1 to 10. It is found that the I–V curves of PANI microtubes are not always linear. At the applied voltage of lower than 0.4 V for most of the microtubes, the I–V curves are linear, indicating

3.2. Electrical conductivity of individual PANI microtube The electrical conductivity of bulk PANI in doping status is 1.75 S/ cm. H. Yan et al. [16] found that the PPy nanotubes revealed extremely high anisotropy in the electrical conductivity. The ratios of the electrical conductivity parallel to the axis (σ) to that perpendicular to the axis in plane (σ′) and to that across the plane (σ″) are as high as 103 (σ/σ′) and 105 (σ/σ″), respectively. Hence, it is very interesting to study the electrical property of PANI microtube in its axis direction.

Table 1 Electrical conductivities of PANI microtubes Sample

1

2

3

4

Conductivity (S/cm)

4.44

11.29

6.9

3.11

Sample Conductivity (S/cm)

6 0.89

7 15.15

8 7.31

9 2.01

5 1.24 10 0

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the ohmic contact between the tip and microtubes [17]. The linear curve also showed the metallic behavior of PANI microtubes [13]. While for the applied voltage higher than 0.5 V, the I–V curves are nonlinear. The slopes of I–V curves decreased, indicating the increase of electrical resistance. The undoping of HCl at high temperature might cause this phenomenon. In the experiment that we carried out, the applied maximum current is 100 nA. Taking the diameter and pore wall into account, it corresponds to the current density of 500 A/cm2. At such high current density, the thermal effect (Joule heating) is very obvious, which leads to the volatilization of the doping agent HCl, as well as the decrease in electrical conductivity. Also, it should be noted that when the applied voltage is lower than 0.1 V there is no positive current in the entire circuit. We supposed that there is a threshold value in the circuit. As for the details, it is not known to us yet and will be carried out in a future work. On the other hand, the slopes of the I–V curves of different PANI microtubes are different. This suggests that the electrical conductivities are different. It results from the different sizes and shapes of the microtubes. This might be caused by different local chemical environments during the process of preparation as well. We assume that the tube wall of the PANI microtube is uniform and based equally from the TEM image. The conductivity of a single microtube can be evaluated according to the following relationship [3]: r¼

I L d U kðro2 −ri2 Þ

Where I is the current, U is the applied voltage, L = 60 μm is the length of the microtube, ro = 100 nm and ri = 80 nm. ro and ri are the outer and inner radius of the microtube, respectively. The linear part was chosen to calculate the electrical conductivity. As listed in Table 1, the electrical conductivities of most microtubes are higher than bulk PANI. This attribution is due to the alignment of the PANI molecular chains, which is induced by the confined space of the template channels [7]. The maximum conductivity, minimum conductivity, and the average conductivity of single microtube are 15.15 S/cm, 0.89 S/cm, and 5.81 S/cm respectively. Although the average conductivity of individual microtubes is higher than bulk PANI, the value is still lower than those in other works [18,19] using similar synthesis methods. We proposed that the PANI microtubes were partially undoped by NaOH solution during the process of removing template. The conductivity of microtube (number 10) is zero. This is attributed to the rupture of the PANI microtube or poor contact between the SPM tip and microtube. Anyway, most of the PANI microtube conductivities are in the range of 100 S/cm magnitude, which suggests that the applied method in this paper possesses good reproducibility and feasibility.

4. Conclusion We have successfully measured the electrical conductivity of individual PANI microtubes synthesized by the template meth-

od. It testifies that SPM is feasible and an easy method for the direct measurement of the electrical conductivity of individual microstructures. The average conductivity of a single microtube is 5.81 S/cm, which is higher than bulk PANI. This result confirms that the synthesis in the confined space of the template channels can induce alignment of PANI molecular chains, as well as the enhancement of the electrical conductivity of the microtube. Acknowledgements We would like to show our appreciation to the financial support from the National Natural Science Foundation of China (20034010) and the National Basic Research Program of China (2003CB615600). References [1] C.K. Chiang, C.R. Fincher Jr., Y.W. Park, et al., Phys. Rev. Lett. 39 (1977) 1098. [2] J.G. Park, B. Kim, S.H. Lee, et al., Thin Solid Films 438–439 (2003) 118–122. [3] Y.Z. Long, L.J. Zhang, Y.J. Ma, et al., Macromol. Rapid Commun. 24 (2003) 938–942. [4] W.G. Li, M.X. Wan, Synth. Met. 92 (1998) 121. [5] C.R. Martin, Science 266 (1994) 1961–1966. [6] C.G. Wu, T. Bein, Science 264 (1994) 1757–1759. [7] Z.H. Cai, C.R. Martin, J. Am. Chem. Soc. 111 (1989) 4138–4139. [8] M.X. Wan, Z.X. Wei, Z.M. Zhang, et al., Synth. Met. 135 (2003) 175. [9] M. Bockrath, D.H. Cobden, R.E. Smalley, et al., Science 275 (1997) 1922–1925. [10] K. Ramanathan, M.A. Bangar, M. Yun, et al., Nano Lett. 4 (2004) 1237–1239. [11] M. Yun, N.V. Myung, R.P. Vasquez, Nano Lett. 4 (2004) 419–422. [12] T. Hayashida, L. Pan, Y. Nakayama, Phys., B Condens. Matter 323 (2002) 352–353. [13] A. Hassanien, M. Gao, M. Tokumoto, et al., Chem. Phys. Lett. 342 (2001) 479–484. [14] S.J. Hasegawa, Curr. Opin. Solid Mater. Sci. 4 (1999) 429–434. [15] S.X. Xiong, Q. Wang, H.SH. Xia, Mater. Res. Bull. 39 (2004) 1569–1580. [16] H. Yan, T. Ishida, N. Toshima, Chem. Lett. 8 (2001) 816–817. [17] K. Kaneto, M. Tsuruta, G. Sakai, Synth. Met. 103 (1999) 2543–2546. [18] M. Delvanx, J. Duchet, R.Y. Stavaux, et al., Synth. Met. 113 (2000) 275. [19] Y. Cao, J.J. Qin, P. Smith, Synth. Met. 69 (1995) 187.