- Email: [email protected]
Polyaniline-coated halloysite nanotubes via in-situ chemical polymerization
Applied Surface Science 255 (2008) 2091–2097
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Polyaniline-coated halloysite nanotubes via in-situ chemical polymerization Long Zhang a, Tingmei Wang b, Peng Liu a,* a
State Key Laboratory of Applied Organic Chemistry and Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China b State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 May 2008 Received in revised form 28 June 2008 Accepted 28 June 2008 Available online 5 July 2008
Polyaniline coated halloysite nanotubes (PANI/HNTs) were prepared by the in-situ soapless emulsion polymerization of the anilinium chloride adsorbed halloysite nanotubes (HNTs), obtained by the dispersion of HNTs in acidic aqueous solution of aniline with magnetic stirring and ultrasonic irradiation, by using ammonium persulfate (APS) as oxidant. The effect of the acidities of the polymerizing media on the crystal structure of the nanotubes was investigated with X-ray diffraction (XRD) technique. The surface conducting coatings of the hybrids were characterized with X-ray photoelectron spectroscopy (XPS). The morphological analyses showed that the polyaniline coated halloysite nanotubes via the insitu chemical oxidation polymerization with ultrasonic irradiation had the better well-defined structures, by the transmission electron microscopy (TEM). The conductivities of the PANI/HNTs hybrids increased with the increasing of the amounts of HCl dopant added in the emulsion polymerization. ß 2008 Elsevier B.V. All rights reserved.
PACS: 61.46.+w 72.80.Tm Keywords: Polyaniline Halloysite nanotube Coating Nanocomposites
1. Introduction Polyaniline (PANI) has been known as one of the most technologically important conducting polymers because of its high electrical conductivity, easy producibility and environmental stability [1–3]. Polyaniline usually combined with other inorganic components to form nanocomposites in order to improve physical, mechanical, and electrical properties such as enhanced solubility, conductivity, magnetic and optoelectronic properties, etc. Recently, encapsulation of inorganic nanomaterials inside the shell of polyaniline has become the most popular and interesting aspect of nanocomposite synthesis. To date, a number of methods have been described for generating the polyaniline coated inorganic nanomaterials such as nanoparticles [4–10], nanotubes [11–14], nanobelts [15] and silicate clays [16] and so on by the insitu oxidative polymerization, gamma radiation-induced chemical polymerization or electropolymerization. These materials differ from the pure polymers in some of the physical and chemical properties and at the same time differ from each other also [17]. The halloysite (formula: Al2Si2O5(OH)42H2O, 1:1 layer aluminosilicate), a super-fine clay material, often occurs as an
ultramicroscopic hollow tubule with a multi-layer wall in nature, mined from natural deposits in countries such as China, America, Brazil, France and so on. It possesses a regular nanotubular morphology, bulk structure and rich mesopores and nanopores [18]. Recently, it was used as adsorbents [19,20], nanocomposites [21–26] and nanotemplates or nanoscale reaction vessels instead of carbon nanotubes or boron nitride nanotubes [27–29]. Luca and Thomson reported the intercalation and polymerization of aniline within halloysite nanotube by exposure of the Cu(II)-doped halloysite film to aniline vapor [30]. In the work, the polyaniline was polymerized within the nanotubes. In the present work, we reported the coating of polyaniline onto the surfaces of the halloysite nanotubes via the in-situ soapless emulsion polymerization after the anilinium chloride was adsorbed onto the halloysite nanotubes. The effect of the acidity of the polymerizing media on the crystal structure of the halloysite nanotubes and the adsorption conditions of anilinium chloride on the morphologies of the PANI/HNTs hybrids were emphasized. 2. Experimental 2.1. Materials and reagents
* Corresponding author. Tel.: +86 931 8912516; fax: +86 931 8912582. E-mail address: [email protected] (P. Liu). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.06.187
Halloysite mineral was obtained from Xuyong county in Sichuan province of China. Aniline (analytical grade reagent,
L. Zhang et al. / Applied Surface Science 255 (2008) 2091–2097
2092 Table 1 Polymerizing conditions with ultrasonic irradiation Samples PANI/HNTs PANI/HNTs PANI/HNTs PANI/HNTs PANI/HNTs PANI/HNTs a
1U 2U 3U 4U 5U 6U
Conc. hydrochloric acid (mL)
Ratios between PANI and HNT in PANI/HNTsa
Conductivity (S/cm)
1.50 3.00 4.50 6.00 7.50 9.00
0.35 0.39 0.41 0.44 0.34 0.30
6.5 105 2.6 104 7.2 104 9.9 104 4.1 103 1.2 103
Calculated from the element analysis.
Xi’an Reagent Co., Xi’an, China) was freshly distilled under pressure before use. Concentrated hydrochloric acid, ammonium persulfate (APS) were analytical grade reagents received from Tianjin Chemical Co., Tianjin, China and used without further purification as received. Distilled water was used throughout. 2.2. Soapless emulsion polymerization The anilinium chloride adsorbed halloysite nanotubes were prepared by the followed process before the soapless emulsion polymerization: halloysite nanotubes (3.00 g), aniline (1.50 mL), and certain amounts of HCl (Table 1) were mixed into 450 mL water. The mixtures were stirred with magnetic stirrer or ultrasonic irradiated for 30 min. Then 100 mL of the acidic aqueous solution of APS (containing APS 6.80 g and conc. hydrochloric acid 1.00 mL) was added dropwise into the colloidal mixtures within 30 min with magnetic stirring in an ice-water bath. And the mixture was stirred for 12 h with magnetic stirring. After the polymerization, the mixtures were centrifugated and the dark green powders were obtained. The products were washed by water for several times until neutral and dried under vacuum at 40 8C overnight. 2.3. Characterization and analysis Elemental analysis (EA) of C, N and H was performed on Elementar vario EL instrument. Bruker IFS 66 v/s infrared spectrometer was used for the Fourier transform infrared (FTIR) spectroscopy analysis. The X-ray diffraction (XRD) patterns were recorded in the range of 2u = 10–808 by step scanning with a Shimadzu XRD-6000 X-ray diffractometer (Shimadzu Corp., Kyoto, Japan). Nickel-filter Cu Ka radiation (l = 0.15418 nm) was used with a generator voltage of 40 kV and a current of 30 mA. X-ray photoelectron spectroscopy (XPS) was accomplished using a PHI-5702 multi-functional X-ray photoelectron spectrometer (Physical Electronics Inc., Chanhassen, MN, USA) with pass energy of 29.35 eV and an Mg Ka line excitation source. The binding energy of C 1s (284.6 eV) was used as a reference. The morphologies of the halloysite nanotubes were characterized with a JEM-1200 EX/S transmission electron microscope (TEM) (JEOL, Tokyo, Japan). The powders were dispersed in water in an ultrasonic bath for 5 min, and then deposited on a copper grid covered with a perforated carbon film. The electrical conductivities of the PANI/HNTs hybrids prepared with different HCl concentrations were measured using SDY-4 Four-Point Probe Meter (Guangzhou Institute of Semiconductor Materials, Guangzhou, China) at ambient
temperature employing the method on a pressed pellet according to the formula [31]: 1 V D W F W Fsp s ¼ ¼ F S S r I where s referred to electrical conductivity, V the voltage, I the current, D the diameter of the pallets, W the thickness of the pallets, S the average space between of the probes, F(D/S) the amendatory coefficient of the diameter of the pallets, F(W/ S) the amendatory coefficient of the thickness of the pallets and Fsp was the amendatory coefficient of the space between of the probes. The pellets were obtained by subjecting the powder sample to a pressure of 30 MPa. The reproducibility of the result was checked by measuring the resistance three times for each pellet. 3. Results and discussion In the proposed method, aniline reacted with HCl and formed the anilinium chloride salts when it was added to the HCl solutions. The anilinium cations were absorbed to the surfaces of the halloysite nanotubes (HNTs) because of the negative centers introduced by the reaction of HNTs with hydrochloric acid [16]. Thus, the surfaces of the HNTs changed more hydrophobic so that they have more affinity for the aniline monomers, facilitating the adsorption. So the anilinium chloride salts acted as both the monomer and the surface modifier in the oxidative polymerization. When the oxidant added, the polymerization occurred and the halloysite nanotubes were coated by PANI.
Fig. 1. XRD patterns of the HNTs and PANI/HNTs hybrids with ultrasonic irradiation.
L. Zhang et al. / Applied Surface Science 255 (2008) 2091–2097
Furthermore, the more acid dopant added might increase the electrical conductivities of the PANI/HNTs hybrids. So the polymerizing media with high acidity was desired in the work. However, the acid might destroy the crystal structure of the HNTs. So the effect of the amounts of hydrochloric acid added on the crystal structure of the halloysite nanotubes was studied with XRD. The results were given in Fig. 1. The XRD peak at about 138 disappeared when the amount of conc. hydrochloric acid added reached 9.00 mL. It indicated that the crystal structure of the halloysite nanotubes was destroyed. So the maximum amount of hydrochloric acid added in the polymerizing media was selected as 9.00 mL. The polyaniline coated halloysite nanotubes hybrids were prepared via the in-situ chemical polymerization of the anilinium chloride adsorbed halloysite nanotubes. The adsorption condition of the anilinium chloride molecules onto the surfaces of the halloysite nanotubes might affect the structure of the hybrids obtained. So the adsorption of the anilinium chloride molecules
2093
onto the surfaces of the halloysite nanotubes was achieved with stirring or ultrasonic irradiation. The morphologies of the products with stirring (PANI/HNTs 1–6S) and ultrasonic irradiation (PANI/ HNTs 1-6U) were analyzed using TEM technique and the TEM images were given in Figs. 2 and 3, respectively. Compared with that of the raw halloysite nanotubes in Fig. 2a, direct evidence for the formation of the polyaniline layers on the surfaces of the halloysite nanotubes is provided in the TEM images. It is clear from the images that the PANI/HNTs 1-6U, from the adsorption with ultrasonic irradiation, had the better results. The coated polyaniline layers were more symmetrical and the PANI/HNTs 1-6U hybrids had the better dispersibilities in water than those from the adsorption with stirring (PANI/HNTs 1–6S), which were conglomeration. Furthermore, the diameter of the PANI/HNTs 1-6U hybrids increased from 30 nm of the raw HNTs to about 60 nm. And the inner hollow cavity of the PANI/HNTs 1-6U hybrids remained about 10 nm as the raw HNTs. It indicated that the polyaniline layer with the thickness of about 15 nm was coated only onto the
Fig. 2. TEM images of the PANI/HNTs with stirring. (a) HNTs; (b) PANI/HNTs 1S; (c) PANI/HNTs 2S; (d) PANI/HNTs 3S; (e) PANI/HNTs 4S; (f) PANI/HNTs 5S; (g) PANI/HNTs 6S.
2094
L. Zhang et al. / Applied Surface Science 255 (2008) 2091–2097
Fig. 2. (Continued ).
outer surfaces of the HNTs. The interaction between the polyaniline layer and the nanotubes was quite strong because thorough washing did not remove the coated layers from the surfaces. It was found that the length of the PANI/HNTs 6U hybrids was shorter than the PANI/HNTs 1-5U hybrids. It might be resulted from the cutting of the HNTs in the adsorption media with the higher acidity, although there was no evidence that its crystal structure was destroyed. The XPS technique was used for the surface analysis of the PANI/HNT 6U hybrids and the XPS survey and the main surface compositional data were given in Fig. 4 and Table 2, respectively. The raw halloysite nanotube is composed of silicon, aluminum and oxygen. After the soapless emulsion polymerization, the surface element contents of C and N of the products increased to more than 70 and 5% and those of Si and Al decreased to less than 3%, respectively. It showed the polyaniline layer had been coated
onto the surfaces of the halloysite nanotubes successfully. Moreover, Cl in the dopant HCl and S in the remained SO42 were also found in the XPS survey of these PANI/HNTs 6U hybrids. In the PANI/HNTs 1-6U hybrids, the ratios between the polyaniline and the halloysite nanotubes (HNTs) were given in Table 2, calculated from the elemental analysis results. It could be found that the content of organic polymer in the hybrids reached the maximum with 6.00 mL hydrochloric acid added in the polymerizing media (PANI/HNT 4U). The weight ratio reached 0.44. It indicated that the almost all the aniline added had been polymerized and the conversion was close to 100% in the polymerizing medium. The electrical conductivities of the PANI/HNTs 1-6U hybrids were also given in Table 2. It could be concluded that the electrical conductivities of the PANI/HNTs 16U hybrids increased with the increasing of the acidities of the
L. Zhang et al. / Applied Surface Science 255 (2008) 2091–2097 Table 2 The surface compositional data from XPS analysis Samples
PANI/HNTs PANI/HNTs PANI/HNTs PANI/HNTs PANI/HNTs PANI/HNTs
Surface element contents (at.%)
1U 2U 3U 4U 5U 6U
C 1s
N 1s
Si 2p
Al 2p
O 1s
74 76 74 80 77 77
5.2 5.3 8.1 6.4 3.7 8.7
3.3 1.6 2.1 0 2.1 1.5
1.8 2.4 1.0 0 0.5 1.9
16 14 15 14 17 11
2095
polymerizing media. As for the PANI/HNTs 6U hybrid, the electrical conductivity decreased than that of the PANI/HNTs 5U hybrid. It could be explained with the two facts followed: one is the lower content of the conducting polymer in the PANI/HNTs 6U hybrid, and the other is the destroyed structure given in Fig. 3f. The electrical conductivities of the PANI/HNTs 1-6U hybrids were lower due to the deprotonation in the treatment procedures of the samples by washing and drying. In the FT-IR spectra of the PANI/HNTs 1-6U hybrids (Fig. 5), the absorbance at about 1140 cm1 of the plane bending vibration of C–H (modes of N=Q=N, Q=N+H–B and B–N+H–B), which is formed during protonation and considered to be a measure of the degree of electron delocalization in PANI chains and thus is a characteristic peak related to PANI conductivity [32,33], could not be found. It was also proved by the small signals of Cl atoms in the XPS surveys (Fig. 4).
Fig. 3. TEM images of the PANI/HNTs with ultrasonic irradiation: (a) PANI/HNTs 1U; (b) PANI/HNTs 2U; (c) PANI/HNTs 3U; (d) PANI/HNTs 4U; (e) PANI/HNTs 5U; (f) PANI/ HNTs 6U.
L. Zhang et al. / Applied Surface Science 255 (2008) 2091–2097
2096
Fig. 3. (Continued ).
4. Conclusion The conducting polyaniline was coated onto the natural nanotubes, halloysite nanotubes, by the chemical polymerization of the anilinium chloride adsorbed halloysite nanotubes via soapless emulsion polymerization. The anilinium chloride adsorbed halloysite nanotubes were prepared by the adsorption of aniline onto halloysite nanotubes in acidic media with magnetic stirring or ultrasonic irradiation, respectively. The results showed that the adsorption with ultrasonic irradiation is propitious to the core/shell structures. The effect of the acidity of the polymerizing media on the structures and the electrical conductivities of the PANI/HNTs hybrids were investigated. References
Fig. 4. XPS survey spectra.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
[11] [12] [13] [14] [15] [16] [17] [18] [19] Fig. 5. FT-IR spectra.
[20]
N. Gospodinona, L. Terlemezyan, Prog. Polym. Sci. 23 (1998) 1443. E.T. Kang, K.G. Neoh, K.L. Tan, Prog. Polym. Sci. 23 (1998) 277. Y.S. Neqi, P.V. Adhyapak, J. Macromol. Sci. Polym. Rev. 42 (2002) 35. F.Y. Chuang, S.M. Yang, J. Colloid Interface Sci. 320 (2008) 194. X.L. Luo, A.J. Killard, A. Morrin, M.R. Smyth, Electrochim. Acta 52 (2007) 1865. N. Asim, S. Radiman, M.A. Bin Yarmo, Mater. Lett. 62 (2008) 1044. O. Yavuz, M.K. Ram, M. Aldissi, P. Poddar, S. Hariharan, J. Mater. Chem. 15 (2005) 810. M.R. Karim, K.T. Lim, C.J. Lee, M.T.I. Bhuiyan, H.J. Kim, L.S. Park, M.S. Lee, J. Polym. Sci.: Polym. Chem. 45 (2007) 5741. K.H. Wu, Y.M. Shin, C.C. Yang, W.D. Ho, J.S. Hsu, J. Polym. Sci.: Polym. Chem. 44 (2006) 2657. A. Kassiba, W. Bednarski, A. Pud, N. Errien, M. Makowska-Janusik, L. Laskowski, M. Tabellout, S. Kodjikian, K. Fateyeva, N. Ogurtsov, Y. Noskov, J. Phys. Chem. C 111 (2007) 11544. T.M. Wu, Y.W. Lin, C.S. Liao, Carbon 43 (2005) 734. X.T. Zhang, Z. Lu, M.T. Wen, H.L. Liang, J. Zhang, Z.F. Liu, J. Phys. Chem. B 109 (2005) 1101. C.Y. Zhi, Y. Bando, C.C. Tang, S. Honda, K. Sato, H. Kuwahara, D. Golberg, Angew. Chem. Int. Ed. 44 (2005) 7929. M. Ginic-Marlena, J.G. Matisons, R. Cervini, G.P. Simon, P.M. Fredericks, Chem. Mater. 18 (2006) 6258. J. Xu, X.L. Li, J.F. Liu, X. Wang, Q. Peng, Y.D. Li, J. Polym. Sci.: Polym. Chem. 43 (2005) 2892. Y.S. Liu, P. Liu, Z.X. Su, Syn. Metal. 157 (2007) 585. R. Gangopadhyay, A. De, Chem. Mater. 12 (2000) 608. S.R. Levis, P.B. Deasy, Int. J. Pharm. 243 (2002) 125. M.T. Viseras, C. Aguzzi, P. Cerezo, C. Viseras, C. Valenzuela, Micropor. Mesopor. Mater. 108 (2007) 112. M.F. Zhao, P. Liu, Micropor. Mesopor. Mater. 112 (2008) 419.
L. Zhang et al. / Applied Surface Science 255 (2008) 2091–2097 [21] [22] [23] [24] [25]
M.F. Zhao, P. Liu, J. Macromol. Sci. B Phys. 46 (2007) 891. Y.P. Ye, H.B. Chen, J.S. Wu, L. Ye, Polymer 48 (2007) 6426. M.L. Du, B.C. Guo, D.M. Jia, Eur. Polym. J. 42 (2006) 1362. M.X. Liu, B.C. Guo, M.L. Du, D.M. Jia, Appl. Phys. A 88 (2007) 391. M.X. Liu, B.C. Guo, M.L. Du, X.J. Cai, D.M. Jia, Nanotechnology 18 (2007) 455703. [26] M.X. Liu, B.C. Guo, Q.L. Zou, M.L. Du, D.M. Jia, Nanotechnology 19 (2008) 205709. [27] S.J. Antill, Aust. J. Chem. 56 (2003) 723.
2097
[28] D.G. Shchukin, G.B. Sukhorukov, R.R. Price, Y.M. Lvov, Small 1 (2005) 510. [29] A.P. Wang, F.Y. Kang, Z.H. Huang, Z.C. Guo, X.Y. Chuan, Micropor. Mesopor. Mater. 108 (2008) 318. [30] V. Luca, S. Thomson, J. Mater. Chem. 10 (2000) 2121. [31] The manual of the SDY-4 Four-Point Probe Meter, Guangzhou Institute of Semiconductor Materials, Guangzhou, China. [32] E.T. Kang, K.G. Noeh, K.L. Tan, Prog. Polym. Sci. 23 (1998) 277. [33] D. Lee, K. Char, S.W. Lee, Y.W. Park, J. Mater. Chem. 13 (2003) 2942.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Polyaniline-coated halloysite nanotubes via in-situ chemical polymerization Long Zhang a, Tingmei Wang b, Peng Liu a,* a
State Key Laboratory of Applied Organic Chemistry and Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China b State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 May 2008 Received in revised form 28 June 2008 Accepted 28 June 2008 Available online 5 July 2008
Polyaniline coated halloysite nanotubes (PANI/HNTs) were prepared by the in-situ soapless emulsion polymerization of the anilinium chloride adsorbed halloysite nanotubes (HNTs), obtained by the dispersion of HNTs in acidic aqueous solution of aniline with magnetic stirring and ultrasonic irradiation, by using ammonium persulfate (APS) as oxidant. The effect of the acidities of the polymerizing media on the crystal structure of the nanotubes was investigated with X-ray diffraction (XRD) technique. The surface conducting coatings of the hybrids were characterized with X-ray photoelectron spectroscopy (XPS). The morphological analyses showed that the polyaniline coated halloysite nanotubes via the insitu chemical oxidation polymerization with ultrasonic irradiation had the better well-defined structures, by the transmission electron microscopy (TEM). The conductivities of the PANI/HNTs hybrids increased with the increasing of the amounts of HCl dopant added in the emulsion polymerization. ß 2008 Elsevier B.V. All rights reserved.
PACS: 61.46.+w 72.80.Tm Keywords: Polyaniline Halloysite nanotube Coating Nanocomposites
1. Introduction Polyaniline (PANI) has been known as one of the most technologically important conducting polymers because of its high electrical conductivity, easy producibility and environmental stability [1–3]. Polyaniline usually combined with other inorganic components to form nanocomposites in order to improve physical, mechanical, and electrical properties such as enhanced solubility, conductivity, magnetic and optoelectronic properties, etc. Recently, encapsulation of inorganic nanomaterials inside the shell of polyaniline has become the most popular and interesting aspect of nanocomposite synthesis. To date, a number of methods have been described for generating the polyaniline coated inorganic nanomaterials such as nanoparticles [4–10], nanotubes [11–14], nanobelts [15] and silicate clays [16] and so on by the insitu oxidative polymerization, gamma radiation-induced chemical polymerization or electropolymerization. These materials differ from the pure polymers in some of the physical and chemical properties and at the same time differ from each other also [17]. The halloysite (formula: Al2Si2O5(OH)42H2O, 1:1 layer aluminosilicate), a super-fine clay material, often occurs as an
ultramicroscopic hollow tubule with a multi-layer wall in nature, mined from natural deposits in countries such as China, America, Brazil, France and so on. It possesses a regular nanotubular morphology, bulk structure and rich mesopores and nanopores [18]. Recently, it was used as adsorbents [19,20], nanocomposites [21–26] and nanotemplates or nanoscale reaction vessels instead of carbon nanotubes or boron nitride nanotubes [27–29]. Luca and Thomson reported the intercalation and polymerization of aniline within halloysite nanotube by exposure of the Cu(II)-doped halloysite film to aniline vapor [30]. In the work, the polyaniline was polymerized within the nanotubes. In the present work, we reported the coating of polyaniline onto the surfaces of the halloysite nanotubes via the in-situ soapless emulsion polymerization after the anilinium chloride was adsorbed onto the halloysite nanotubes. The effect of the acidity of the polymerizing media on the crystal structure of the halloysite nanotubes and the adsorption conditions of anilinium chloride on the morphologies of the PANI/HNTs hybrids were emphasized. 2. Experimental 2.1. Materials and reagents
* Corresponding author. Tel.: +86 931 8912516; fax: +86 931 8912582. E-mail address: [email protected] (P. Liu). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.06.187
Halloysite mineral was obtained from Xuyong county in Sichuan province of China. Aniline (analytical grade reagent,
L. Zhang et al. / Applied Surface Science 255 (2008) 2091–2097
2092 Table 1 Polymerizing conditions with ultrasonic irradiation Samples PANI/HNTs PANI/HNTs PANI/HNTs PANI/HNTs PANI/HNTs PANI/HNTs a
1U 2U 3U 4U 5U 6U
Conc. hydrochloric acid (mL)
Ratios between PANI and HNT in PANI/HNTsa
Conductivity (S/cm)
1.50 3.00 4.50 6.00 7.50 9.00
0.35 0.39 0.41 0.44 0.34 0.30
6.5 105 2.6 104 7.2 104 9.9 104 4.1 103 1.2 103
Calculated from the element analysis.
Xi’an Reagent Co., Xi’an, China) was freshly distilled under pressure before use. Concentrated hydrochloric acid, ammonium persulfate (APS) were analytical grade reagents received from Tianjin Chemical Co., Tianjin, China and used without further purification as received. Distilled water was used throughout. 2.2. Soapless emulsion polymerization The anilinium chloride adsorbed halloysite nanotubes were prepared by the followed process before the soapless emulsion polymerization: halloysite nanotubes (3.00 g), aniline (1.50 mL), and certain amounts of HCl (Table 1) were mixed into 450 mL water. The mixtures were stirred with magnetic stirrer or ultrasonic irradiated for 30 min. Then 100 mL of the acidic aqueous solution of APS (containing APS 6.80 g and conc. hydrochloric acid 1.00 mL) was added dropwise into the colloidal mixtures within 30 min with magnetic stirring in an ice-water bath. And the mixture was stirred for 12 h with magnetic stirring. After the polymerization, the mixtures were centrifugated and the dark green powders were obtained. The products were washed by water for several times until neutral and dried under vacuum at 40 8C overnight. 2.3. Characterization and analysis Elemental analysis (EA) of C, N and H was performed on Elementar vario EL instrument. Bruker IFS 66 v/s infrared spectrometer was used for the Fourier transform infrared (FTIR) spectroscopy analysis. The X-ray diffraction (XRD) patterns were recorded in the range of 2u = 10–808 by step scanning with a Shimadzu XRD-6000 X-ray diffractometer (Shimadzu Corp., Kyoto, Japan). Nickel-filter Cu Ka radiation (l = 0.15418 nm) was used with a generator voltage of 40 kV and a current of 30 mA. X-ray photoelectron spectroscopy (XPS) was accomplished using a PHI-5702 multi-functional X-ray photoelectron spectrometer (Physical Electronics Inc., Chanhassen, MN, USA) with pass energy of 29.35 eV and an Mg Ka line excitation source. The binding energy of C 1s (284.6 eV) was used as a reference. The morphologies of the halloysite nanotubes were characterized with a JEM-1200 EX/S transmission electron microscope (TEM) (JEOL, Tokyo, Japan). The powders were dispersed in water in an ultrasonic bath for 5 min, and then deposited on a copper grid covered with a perforated carbon film. The electrical conductivities of the PANI/HNTs hybrids prepared with different HCl concentrations were measured using SDY-4 Four-Point Probe Meter (Guangzhou Institute of Semiconductor Materials, Guangzhou, China) at ambient
temperature employing the method on a pressed pellet according to the formula [31]: 1 V D W F W Fsp s ¼ ¼ F S S r I where s referred to electrical conductivity, V the voltage, I the current, D the diameter of the pallets, W the thickness of the pallets, S the average space between of the probes, F(D/S) the amendatory coefficient of the diameter of the pallets, F(W/ S) the amendatory coefficient of the thickness of the pallets and Fsp was the amendatory coefficient of the space between of the probes. The pellets were obtained by subjecting the powder sample to a pressure of 30 MPa. The reproducibility of the result was checked by measuring the resistance three times for each pellet. 3. Results and discussion In the proposed method, aniline reacted with HCl and formed the anilinium chloride salts when it was added to the HCl solutions. The anilinium cations were absorbed to the surfaces of the halloysite nanotubes (HNTs) because of the negative centers introduced by the reaction of HNTs with hydrochloric acid [16]. Thus, the surfaces of the HNTs changed more hydrophobic so that they have more affinity for the aniline monomers, facilitating the adsorption. So the anilinium chloride salts acted as both the monomer and the surface modifier in the oxidative polymerization. When the oxidant added, the polymerization occurred and the halloysite nanotubes were coated by PANI.
Fig. 1. XRD patterns of the HNTs and PANI/HNTs hybrids with ultrasonic irradiation.
L. Zhang et al. / Applied Surface Science 255 (2008) 2091–2097
Furthermore, the more acid dopant added might increase the electrical conductivities of the PANI/HNTs hybrids. So the polymerizing media with high acidity was desired in the work. However, the acid might destroy the crystal structure of the HNTs. So the effect of the amounts of hydrochloric acid added on the crystal structure of the halloysite nanotubes was studied with XRD. The results were given in Fig. 1. The XRD peak at about 138 disappeared when the amount of conc. hydrochloric acid added reached 9.00 mL. It indicated that the crystal structure of the halloysite nanotubes was destroyed. So the maximum amount of hydrochloric acid added in the polymerizing media was selected as 9.00 mL. The polyaniline coated halloysite nanotubes hybrids were prepared via the in-situ chemical polymerization of the anilinium chloride adsorbed halloysite nanotubes. The adsorption condition of the anilinium chloride molecules onto the surfaces of the halloysite nanotubes might affect the structure of the hybrids obtained. So the adsorption of the anilinium chloride molecules
2093
onto the surfaces of the halloysite nanotubes was achieved with stirring or ultrasonic irradiation. The morphologies of the products with stirring (PANI/HNTs 1–6S) and ultrasonic irradiation (PANI/ HNTs 1-6U) were analyzed using TEM technique and the TEM images were given in Figs. 2 and 3, respectively. Compared with that of the raw halloysite nanotubes in Fig. 2a, direct evidence for the formation of the polyaniline layers on the surfaces of the halloysite nanotubes is provided in the TEM images. It is clear from the images that the PANI/HNTs 1-6U, from the adsorption with ultrasonic irradiation, had the better results. The coated polyaniline layers were more symmetrical and the PANI/HNTs 1-6U hybrids had the better dispersibilities in water than those from the adsorption with stirring (PANI/HNTs 1–6S), which were conglomeration. Furthermore, the diameter of the PANI/HNTs 1-6U hybrids increased from 30 nm of the raw HNTs to about 60 nm. And the inner hollow cavity of the PANI/HNTs 1-6U hybrids remained about 10 nm as the raw HNTs. It indicated that the polyaniline layer with the thickness of about 15 nm was coated only onto the
Fig. 2. TEM images of the PANI/HNTs with stirring. (a) HNTs; (b) PANI/HNTs 1S; (c) PANI/HNTs 2S; (d) PANI/HNTs 3S; (e) PANI/HNTs 4S; (f) PANI/HNTs 5S; (g) PANI/HNTs 6S.
2094
L. Zhang et al. / Applied Surface Science 255 (2008) 2091–2097
Fig. 2. (Continued ).
outer surfaces of the HNTs. The interaction between the polyaniline layer and the nanotubes was quite strong because thorough washing did not remove the coated layers from the surfaces. It was found that the length of the PANI/HNTs 6U hybrids was shorter than the PANI/HNTs 1-5U hybrids. It might be resulted from the cutting of the HNTs in the adsorption media with the higher acidity, although there was no evidence that its crystal structure was destroyed. The XPS technique was used for the surface analysis of the PANI/HNT 6U hybrids and the XPS survey and the main surface compositional data were given in Fig. 4 and Table 2, respectively. The raw halloysite nanotube is composed of silicon, aluminum and oxygen. After the soapless emulsion polymerization, the surface element contents of C and N of the products increased to more than 70 and 5% and those of Si and Al decreased to less than 3%, respectively. It showed the polyaniline layer had been coated
onto the surfaces of the halloysite nanotubes successfully. Moreover, Cl in the dopant HCl and S in the remained SO42 were also found in the XPS survey of these PANI/HNTs 6U hybrids. In the PANI/HNTs 1-6U hybrids, the ratios between the polyaniline and the halloysite nanotubes (HNTs) were given in Table 2, calculated from the elemental analysis results. It could be found that the content of organic polymer in the hybrids reached the maximum with 6.00 mL hydrochloric acid added in the polymerizing media (PANI/HNT 4U). The weight ratio reached 0.44. It indicated that the almost all the aniline added had been polymerized and the conversion was close to 100% in the polymerizing medium. The electrical conductivities of the PANI/HNTs 1-6U hybrids were also given in Table 2. It could be concluded that the electrical conductivities of the PANI/HNTs 16U hybrids increased with the increasing of the acidities of the
L. Zhang et al. / Applied Surface Science 255 (2008) 2091–2097 Table 2 The surface compositional data from XPS analysis Samples
PANI/HNTs PANI/HNTs PANI/HNTs PANI/HNTs PANI/HNTs PANI/HNTs
Surface element contents (at.%)
1U 2U 3U 4U 5U 6U
C 1s
N 1s
Si 2p
Al 2p
O 1s
74 76 74 80 77 77
5.2 5.3 8.1 6.4 3.7 8.7
3.3 1.6 2.1 0 2.1 1.5
1.8 2.4 1.0 0 0.5 1.9
16 14 15 14 17 11
2095
polymerizing media. As for the PANI/HNTs 6U hybrid, the electrical conductivity decreased than that of the PANI/HNTs 5U hybrid. It could be explained with the two facts followed: one is the lower content of the conducting polymer in the PANI/HNTs 6U hybrid, and the other is the destroyed structure given in Fig. 3f. The electrical conductivities of the PANI/HNTs 1-6U hybrids were lower due to the deprotonation in the treatment procedures of the samples by washing and drying. In the FT-IR spectra of the PANI/HNTs 1-6U hybrids (Fig. 5), the absorbance at about 1140 cm1 of the plane bending vibration of C–H (modes of N=Q=N, Q=N+H–B and B–N+H–B), which is formed during protonation and considered to be a measure of the degree of electron delocalization in PANI chains and thus is a characteristic peak related to PANI conductivity [32,33], could not be found. It was also proved by the small signals of Cl atoms in the XPS surveys (Fig. 4).
Fig. 3. TEM images of the PANI/HNTs with ultrasonic irradiation: (a) PANI/HNTs 1U; (b) PANI/HNTs 2U; (c) PANI/HNTs 3U; (d) PANI/HNTs 4U; (e) PANI/HNTs 5U; (f) PANI/ HNTs 6U.
L. Zhang et al. / Applied Surface Science 255 (2008) 2091–2097
2096
Fig. 3. (Continued ).
4. Conclusion The conducting polyaniline was coated onto the natural nanotubes, halloysite nanotubes, by the chemical polymerization of the anilinium chloride adsorbed halloysite nanotubes via soapless emulsion polymerization. The anilinium chloride adsorbed halloysite nanotubes were prepared by the adsorption of aniline onto halloysite nanotubes in acidic media with magnetic stirring or ultrasonic irradiation, respectively. The results showed that the adsorption with ultrasonic irradiation is propitious to the core/shell structures. The effect of the acidity of the polymerizing media on the structures and the electrical conductivities of the PANI/HNTs hybrids were investigated. References
Fig. 4. XPS survey spectra.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
[11] [12] [13] [14] [15] [16] [17] [18] [19] Fig. 5. FT-IR spectra.
[20]
N. Gospodinona, L. Terlemezyan, Prog. Polym. Sci. 23 (1998) 1443. E.T. Kang, K.G. Neoh, K.L. Tan, Prog. Polym. Sci. 23 (1998) 277. Y.S. Neqi, P.V. Adhyapak, J. Macromol. Sci. Polym. Rev. 42 (2002) 35. F.Y. Chuang, S.M. Yang, J. Colloid Interface Sci. 320 (2008) 194. X.L. Luo, A.J. Killard, A. Morrin, M.R. Smyth, Electrochim. Acta 52 (2007) 1865. N. Asim, S. Radiman, M.A. Bin Yarmo, Mater. Lett. 62 (2008) 1044. O. Yavuz, M.K. Ram, M. Aldissi, P. Poddar, S. Hariharan, J. Mater. Chem. 15 (2005) 810. M.R. Karim, K.T. Lim, C.J. Lee, M.T.I. Bhuiyan, H.J. Kim, L.S. Park, M.S. Lee, J. Polym. Sci.: Polym. Chem. 45 (2007) 5741. K.H. Wu, Y.M. Shin, C.C. Yang, W.D. Ho, J.S. Hsu, J. Polym. Sci.: Polym. Chem. 44 (2006) 2657. A. Kassiba, W. Bednarski, A. Pud, N. Errien, M. Makowska-Janusik, L. Laskowski, M. Tabellout, S. Kodjikian, K. Fateyeva, N. Ogurtsov, Y. Noskov, J. Phys. Chem. C 111 (2007) 11544. T.M. Wu, Y.W. Lin, C.S. Liao, Carbon 43 (2005) 734. X.T. Zhang, Z. Lu, M.T. Wen, H.L. Liang, J. Zhang, Z.F. Liu, J. Phys. Chem. B 109 (2005) 1101. C.Y. Zhi, Y. Bando, C.C. Tang, S. Honda, K. Sato, H. Kuwahara, D. Golberg, Angew. Chem. Int. Ed. 44 (2005) 7929. M. Ginic-Marlena, J.G. Matisons, R. Cervini, G.P. Simon, P.M. Fredericks, Chem. Mater. 18 (2006) 6258. J. Xu, X.L. Li, J.F. Liu, X. Wang, Q. Peng, Y.D. Li, J. Polym. Sci.: Polym. Chem. 43 (2005) 2892. Y.S. Liu, P. Liu, Z.X. Su, Syn. Metal. 157 (2007) 585. R. Gangopadhyay, A. De, Chem. Mater. 12 (2000) 608. S.R. Levis, P.B. Deasy, Int. J. Pharm. 243 (2002) 125. M.T. Viseras, C. Aguzzi, P. Cerezo, C. Viseras, C. Valenzuela, Micropor. Mesopor. Mater. 108 (2007) 112. M.F. Zhao, P. Liu, Micropor. Mesopor. Mater. 112 (2008) 419.
L. Zhang et al. / Applied Surface Science 255 (2008) 2091–2097 [21] [22] [23] [24] [25]
M.F. Zhao, P. Liu, J. Macromol. Sci. B Phys. 46 (2007) 891. Y.P. Ye, H.B. Chen, J.S. Wu, L. Ye, Polymer 48 (2007) 6426. M.L. Du, B.C. Guo, D.M. Jia, Eur. Polym. J. 42 (2006) 1362. M.X. Liu, B.C. Guo, M.L. Du, D.M. Jia, Appl. Phys. A 88 (2007) 391. M.X. Liu, B.C. Guo, M.L. Du, X.J. Cai, D.M. Jia, Nanotechnology 18 (2007) 455703. [26] M.X. Liu, B.C. Guo, Q.L. Zou, M.L. Du, D.M. Jia, Nanotechnology 19 (2008) 205709. [27] S.J. Antill, Aust. J. Chem. 56 (2003) 723.
2097
[28] D.G. Shchukin, G.B. Sukhorukov, R.R. Price, Y.M. Lvov, Small 1 (2005) 510. [29] A.P. Wang, F.Y. Kang, Z.H. Huang, Z.C. Guo, X.Y. Chuan, Micropor. Mesopor. Mater. 108 (2008) 318. [30] V. Luca, S. Thomson, J. Mater. Chem. 10 (2000) 2121. [31] The manual of the SDY-4 Four-Point Probe Meter, Guangzhou Institute of Semiconductor Materials, Guangzhou, China. [32] E.T. Kang, K.G. Noeh, K.L. Tan, Prog. Polym. Sci. 23 (1998) 277. [33] D. Lee, K. Char, S.W. Lee, Y.W. Park, J. Mater. Chem. 13 (2003) 2942.