- Email: [email protected]
TiO2 microspheres prepared by a template-free method
Synthetic Metals 151 (2005) 1–5
Polyaniline/TiO2 microspheres prepared by a template-free method Lijuan Zhang a , Meixiang Wan a,∗ , Yen Wei b a b
Organic Solid Laboratory, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China The Center for Advanced Polymers and Materials Chemistry, Department of Chemistry, Drexel University, Philadelphia, PA 19104, USA Received 2 September 2004; received in revised form 6 December 2004; accepted 19 December 2004 Available online 13 June 2005
Abstract Polyaniline/TiO2 microspheres with 2.5–3.6 m in average diameter were synthesized by a template-free method in the presence of salicylic acid (SA) as the dopant. It was found that the morphology, conductivity and hydrophilicity of the PANI-SA/TiO2 microspheres were affected by the content of TiO2 nanoparticles. The micelles composed of SA anions and anilinium cations containing TiO2 were proposed to interpret the formation mechanism of the self-assembled composite microspheres. © 2005 Elsevier B.V. All rights reserved. Keywords: Polyaniline/TiO2 microspheres; Template-free method
1. Introduction Much researches dealing with the construction of polymer microspheres and microcapsules for a wide number of potential applications have been carried out over the past decade [1]. Especially, microspheres with hollow interiors have drawn a number of interests due to their applications including stationary phases for separation science, biomedical devices, coating additives, controlled release reservoirs and as small containers for micro-encapsulation [2]. Recently, a great deal of attention has been directed toward the functionality of these hollow microspheres. Various approaches have been designed to prepare the composite microspheres to obtain the required properties and promising applications in many fields, including optics, electronics, mechanics, membranes, protective coatings, catalysis, sensors, biology, and others [3]. Among these composites, many efforts have been focused on the combination of organic conjugated polymers and inorganic nano-crystals in order to yield new functional materials that may combine the advantages of each component [4].
∗
Corresponding author. Fax: +86 10 6255 9373. E-mail address: [email protected] (M. Wan).
0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.12.021
Conducting polymer as a typical organic conjugated polymer has received great attention because of their excellent electronic properties with conductivity covering the whole range from insulator to metal while retaining lightweight, mechanical properties and processing advantages of polymers [5]. Among these conducting polymers, polyaniline (PANI) is one of the most promising conducting polymers because of its low cost, easy preparation, controllable unique properties by oxidation and protonation state, excellent environmental stability and potential application in electronic devices [6]. Regarding inorganic materials, crystalline titanium dioxide (TiO2 ) nanoparticles are non-toxicity, chemical inertness, and low costs that lead to numerous promising applications in the fields of electronics and photonics [7]. Recently, we reported that hollow microspheres of PANI doped with salicylic acid (SA) could be prepared by a simple template-free method [8]. Moreover, we found that the method is essentially a self-assembly process because of micelles formed by SA and aniline acting as a template in the formation of those microspheres. This result promises us to examine whether we can use the self-assembly process to prepare the PANI-SA/TiO2 composite microspheres and to bring about new properties of PANI-SA microspheres. In this article, composite microspheres of PANI-SA/TiO2 with 2.5–3.6 m in average diameter were successfully
2
L. Zhang et al. / Synthetic Metals 151 (2005) 1–5
prepared by a template-free method in the presence of TiO2 nanoparticles (∼15 nm in diameter). Herein, the influence of the TiO2 concentration on the morphology, size, molecular structure, conductivity and hydrophilicity of the PANI-SA/TiO2 composite microspheres was investigated. The formation mechanism of the composite microspheres is also discussed.
2. Experimental 2.1. Materials Aniline monomer (Beijing Chem. Co.) was distilled under reduced pressure. Ammonium per-sulfate ((NH4 )2 S2 O8 , APS, Beijing Yili Chem. Co.), salicylic acid (SA) and other reagent were purchased from Beijing Chem. Co. and used as received without further treated. TiO2 nanoparticles were also used as received. 2.2. Polymerization A certain molar ratio of aniline and SA represented by [SA]/[An] were dissolved in de-ionized water with magnetic stirring at room temperature for 30 min. Then, TiO2 was added into the mixture and also stirred for 30 min. Finally, the stirring was stopped and followed by adding aqueous solution of APS and the reaction mixture was kept for 12 h. The resulting precipitate was washed with water, methanol and ether several times, respectively. Finally, the product was dried in vacuum at room temperature for 24 h. In all experiment, the molar ratio of aniline to SA (represented by [An]/[SA]) and to APS (represented by [An]/[APS]) for either PANISA microspheres or PANI-SA/TiO2 composite microspheres was 1:1 and 1:1, respectively. But the concentration of TiO2 nanoparticles was changed to understand the effect of the TiO2 nanoparticles on the morphology, structure, electrical properties and hydrophilicity of the resulting PANI-SA/TiO2 composite microspheres. 2.3. Characterization The morphologies of PANI-SA and PANI-SA/TiO2 were investigated with a JEOL-JSM-6700F field emission scanning electron microscope (SEM) and a JEOL JEM-2010 transmission electron microscopy (TEM), respectively. The samples for SEM measurements were mounted on aluminum stud using adhesive graphite tape and sputter-coated gold before analysis, while for TEM were dispersed on micro-grids copper coated with carbon support film. When the TEM was investigated, the synchronous energy depressive X-ray analysis was recorded with a LINK ISIS300 instrument. Raman spectra of PANI-SA and PANI-SA/TiO2 microspheres were carried out on a Renishaw Microscope (RM-2000) with 785 nm laser served as an excitation wavelength. The X-ray scatting patterns for PANI-SA
and PANI-SA/TiO2 microspheres were measured on an Xray diffraction instrument (Micscience M-18XHF with a Cu K␣ radiation). The electrical conductivity of PANI-SA and PANI-SA/TiO2 compressed pellets at room temperature were measured by a standard four-probe method, using a Keithley 196 System DMM Digital Multimeter and an Advantest R1642 programmable dc voltage/current generator as the current source. A Contact Angle System OCA Dataphysics DCAT 11 was used to measure the water contact angle of PANI-SA/TiO2 and PANI-SA films deposited on the glass substrate. The coated films on the glass substrate were polymerization by dipping the glass substrate into the reaction solution after adding oxidant (i.e. APS).
3. Results and discussion The scanning electron microscopy (SEM) and the scanning transmission (TEM) as well as the electron diffraction (ED) of the PANI-SA hollow microspheres and PANISA/TiO2 microspheres are shown in Fig. 1. The outer diameter of the composite microspheres ranges from 2.5 to 3.6 m. As one can see, the surface of PANI-SA hollow microspheres is smooth (Fig. 1a); however, the surface of PANI-SA/TiO2 microspheres is more rough and covered with granules (Fig. 1b). TEM image of PANI-SA/TiO2 (Fig. 1d) proved that TiO2 nanoparticles exist in the PANISA/TiO2 microspheres. The ED pattern of PANI-SA/TiO2 further proved that the microspheres are composed of polycrystalline TiO2 and amorphous PANI-SA. In particular, synchronous energy depressive X-ray spectrum measurements further revealed that the titanium element exists in the composite microspheres while which is absent in the PANISA microspheres. In order to further prove the existence of TiO2 nanoparticles in the microspheres, Raman spectra of PANI-SA, PANI-SA/TiO2 and TiO2 nanoparticles were measured with an excitation wavelength λL = 785 nm (Fig. 2). All typical characteristic bands of PANI were observed in both the Raman spectra of PANI-SA and PANI-SA/TiO2 . For instance, the bands at 1590 and 1369 cm−1 assigned to the C C stretching vibration of the benzenoid and quinoid rings were observed [9]. Moreover, the bands at 1505, 1339, and 1165 cm−1 corresponding to the N H stretching mode, the C N stretching of the cation radical and the C H bending vibration were also observed, respectively [10]. Especially, the bands at 807 and 415 cm−1 related to the C H deformation, the band at 578 cm−1 attributed to the amine deformation as well as the band at 514 cm−1 ascribed to the C N C torsion were observed too [11]. The Raman spectrum of TiO2 nanoparticles showed three characteristic modes at 395, 513 and 638 cm−1 . The modes at 395 and 513 cm−1 are assigned as B1 g and A1 g mode of anatase phase, while the mode at 638 cm−1 is assigned Eg phonon of the anatase structure, respectively [12]. All modes of TiO2 nanoparticles described above can be observed in the Raman spectrum of PANI-SA/TiO2 composite microspheres.
L. Zhang et al. / Synthetic Metals 151 (2005) 1–5
3
Fig. 1. SEM images (a and b), TEM (c and d) images and electron diffraction patterns (e and f) of PANI-SA/TiO2 microspheres synthesized under different concentration of TiO2 : (a, c and e) 0 M and (b, d and f) 0.04 M, synthetic conditions: [An] = 0.2 M, [An]/[SA] = 1:1, [An]/[APS] = 1:1.
Although the frontal two modes at 415 and 514 cm−1 are coincided with the ring deformation bands of PANI-SA, a new peak centered at 638 cm−1 , which is ascribed to the typical mode of TiO2 , was observed in the Raman spectrum of PANI-SA/TiO2 , while which is absent in PANI-SA. X-ray
diffraction of PANI-SA/TiO2 , PANI-SA and TiO2 nanoparticles was also measured to further prove the existence of TiO2 nanoparticles in the PANI-SA/TiO2 composite microspheres. As shown in Fig. 3, TiO2 nanoparticles are crystal, and the
Fig. 2. Raman spectra of (a) PANI-SA, (b) PANI-SA/TiO2 ([TiO2 ] = 0.04 M) and (c) TiO2 nanoparticles, excited with λL = 785 nm laser line, synthetic conditions: [An] = 0.2 M, [An]/[SA] = 1:1, [An]/[APS] = 1:1.
Fig. 3. X-ray scattering patterns of (a) PANI-SA, (b) PANI-SA/TiO2 ([TiO2 ] = 0.04 M) and (c) TiO2 nanoparticles, [An] = 0.2 M, [An]/[SA] = 1:1, [An]/[APS] = 1:1.
4
L. Zhang et al. / Synthetic Metals 151 (2005) 1–5
positions of all sharp peaks are in agreement with the results reported by Tang et al. [13]. PANI-SA microspheres are amorphous, on the other hand, only one broad peaks centered at 2θ = 21◦ were observed, which are ascribed to the periodicity parallel to the polymer chain [14]. The X-ray scattering pattern of PANI-SA/TiO2 composite microspheres consists of one broad peak at 2θ = 21◦ , which is belong to PANI-SA, and all sharp peaks of TiO2 nanoparticles, proving the existence of TiO2 nanoparticles in the composite microspheres. Although the above-mentioned results proved the existence of the TiO2 nanoparticles in the PANI-SA microspheres, we cannot confirm where TiO2 nanoparticles exist? Since the TiO2 nanoparticles are hydrophobic, we thought the measurement of water contact angle maybe provide some information about the position of TiO2 nanoparticles in the composite microspheres. As a result, we measured the water contact angle of PANI-SA/TiO2 films deposited on the glass substrate and compared with PANI-SA samples. It was found that the water contact angle of PANI-SA/TiO2 microspheres was increased from 41.2◦ to 57.5◦ when the content of TiO2 nanoparticles changed from 0 to 0.2 M (Fig. 4), indicating the influence of the content of TiO2 nanoparticles on the hydrophilicity of the composite microspheres is weak. On the other hand, our previous results showed that the water contact angle of PANI--NSA/TiO2 nanotubes prepared by a template-free method increased from 53.5◦ to 98.5◦ as the concentration of TiO2 changed from 0 to 0.08 M [15]. The above results showed that the PANI-SA/TiO2 microspheres is more hydrophilic than that of the PANI--NSA/TiO2 nanotubes although the content of the TiO2 nanoparticles in PANI-SA/TiO2 microspheres is higher than that of PANI-NSA/TiO2 nanotubes, indicating most TiO2 nanoparticles are filled in the hollow interiors of PANI-SA microspheres. According to our previous studies [8], it has been demonstrated that the spherical micelles formed by SA and anilinium cations act as templates in the formation of PANI-SA
Fig. 5. Effect of the TiO2 concentration on the room-temperature conductivity of PANI-SA/TiO2 microspheres, synthetic conditions: [An] = 0.2 M, [An]/[SA] = 1:1, [An]/[APS] = 1:1.
hollow microspheres and the hydrogen bond between OH group of SA and amine group of polymer chain is a driving force to form self-assembled microspheres. As described in the experiment, the TiO2 nanoparticles dispersed into SA/An solution before polymerization. Therefore the micelles in this study have a “core-shell” structure, where TiO2 nanoparticles are assigned as a “core” of the micelles due to its hydrophobic feature, while SA and anilinium cations as a “shell” of the micelles due to hydrophilicity of SA dopant ( COOH group attached on the benzene ring). On the basis of our previous micelle mechanism [8] and the above described, it is reasonable to understand that these micelles containing TiO2 act as templates in the formation of PANI-SA/TiO2 microspheres. In general, it is expected that polymerization only took place at the interface of micelle/water due to the hydrophilicity of APS as an oxidant [16] and the growth of the microspheres are controlled by accretion process [17] The above-mechanism suggests that PANI-SA is co-structured with TiO2 nanoparticles to form composite microspheres through a self-assembly process. It was found that the conductivity of PANI-SA/TiO2 composites strongly depended on the content of TiO2 as shown in Fig. 5. When the concentration of TiO2 is lower than 0.04 M, the conductivity increases with the increase of the concentration of TiO2 , the maximum conductivity at room temperature is about 10−2 S/cm. However, it decreases when the TiO2 concentration is higher than 0.06 M. This is consistent with the results of PANI-DBSA/TiO2 reported by Su and Kuramoto [18] and PANI--NSA/TiO2 nanotubes reported by us [15].
4. Conclusions Fig. 4. Effect of the concentration of TiO2 on the water contact angle of the PANI-SA/TiO2 films deposited on the glass substrate, synthetic conditions: [An] = 0.2 M, [An]/[SA] = 1:1, [An]/[APS] = 1:1.
PANI-SA microspheres containing TiO2 nanoparticles were prepared by a template-free method that opens a sample and cheap route to prepare composite microspheres of
L. Zhang et al. / Synthetic Metals 151 (2005) 1–5
conducting polymers with inorganic nanoparticles. It was found that changing the content of TiO2 can adjust the morphology, size, electrical property and hydrophilicity of the composite microspheres.
Acknowledgments
[4]
[5]
This project was supported by the National Natural Science Foundation of China (No. 50133010, and 29974037) and Grand of Over-sea Outer-standing Scientists of Chinese Academy of Sciences. TiO2 nanoparticles were kindly supplied by Professor Jincai Zhao, Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences.
[6]
References
[7]
[1] (a) L. Dalhne, S. Leporatti, E. Donath, H. M¨ohwald, J. Am. Chem. Soc. 123 (2001) 5431; (b) K. Kamata, Y. Lu, Y. Xia, J. Am. Chem. Soc. 125 (2003) 2384; (c) T.K. Mandal, M.S. Fleming, D.R. Walt, Chem. Mater. 12 (2000) 3481; (d) Z. Chang, G. Liu, Y. Tian, Z. Zhang, Mater. Lett. 58 (2004) 522; (e) S.M. Klein, V.N. Manoharan, D.J. Pine, F.F. Lange, Colloid Polym. Sci. 282 (2003) 7; (f) S.M. Marinakos, D.A. Shultz, D.L. Feldheim, Adv. Mater. 11 (1999) 34; (g) I. Pastoriza-Santos, B. Sch¨oler, F. Caruso, Adv. Funct. Mater. 11 (2001) 122. [2] (a) F. Caruso, R.A. Caruso, H. M¨ohwald, Science 282 (1998) 1111; (b) Y. Xia, B. Gates, Y. Yin, Y. Lu, Adv. Mater. 12 (2000) 693; (c) K.B. Thurmond II, T. Kowalewski, K.L. Wooley, J. Am. Chem. Soc. 119 (1997) 6656; (d) R. Gref, Y. Minamitake, M.T. Peracchia, V. Trubetskoy, V. Torchilin, R. Langer, Science 263 (1994) 1600; (e) I. Gill, A. Ballesteros, J. Am. Chem. Soc. 120 (1998) 8587; (f) F. Caruso, M. Spasova, V.S. Maceira, M.L. Marz´an, Adv. Mater. 13 (2001) 1090; (g) S.M. Marinakos, J.P. Novak, L.C. Brousseau III, A.B. House, E.M. Edeki, J.C. Feldhaus, D.L. Feldheim, J. Am. Chem. Soc. 121 (1999) 8518; (h) Z. Wei, M. Wan, T. Lin, L. Dai, Adv. Mater. 15 (2003) 136. [3] (a) J.P. Spatz, T. Herzog, S. M¨omer, P. Ziemann, M. M¨oller, Adv. Mater. 11 (1999) 149; (b) G. Sukhorukov, L. Dahne, J. Hartmann, E. Donath, H. M¨ohwald, Adv. Mater. 12 (2000) 112;
[8] [9]
[10] [11] [12]
[13] [14]
[15] [16]
[17] [18]
5
(c) M. Antonietti, C. Goltner, Angew. Chem. Int. Ed. 36 (1997) 910; (d) A. Bigi, E. Boanini, D. Walsh, S. Mann, Angew. Chem. Int. Ed. 41 (2002) 2163; (e) C. Bai, Y. Fang, Y. Zhang, B. Chen, Langmuir 20 (2004) 263. (a) H. Yang, P.H. Holloway, J. Phys. Chem. B 107 (2003) 9705; (b) N.J. Torrence, C.M. Moore, S.D. Minteer, Langmuir 18 (2002) 6254; (c) F. Caruso, Adv. Mater. 13 (2001) 11. (a) T.A. Skotheim, R.L. Elsenbaumer, J.R. Reynolds (Eds.), Handbook of Conducting Polymers, Marcel Dekker, Inc., New York, 1998; (b) C.R. Martin, Science 266 (1994) 1961; (c) A.J. Heeger, Chin. J. Polym. Sci. 19 (2001) 545; (d) A.J. Heeger, J. Phys. Chem. B 105 (2001) 8475. (a) A.G. MacDiarmid, J.C. Chiang, W.S. Huang, B.D. Humphrey, N.L.D. Somasiri, Mol. Cryst. Liq. Cryst. 125 (1985) 309; (b) A.G. MacDiarmid, J.C. Chiang, A.F. Richter, A.J. Epstein, Synth. Met. 18 (1987) 285; (c) Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 48 (1992) 91; (d) J. Huang, S. Virji, B.H. Weiller, R.B. Kaner, J. Am. Chem. Soc. 125 (2003) 314. (a) P.A. van Hal, M.P.T. Christiaans, M.M. Wienk, J.M. Kroon, R.A.J. Janssen, J. Phys. Chem. B 103 (1999) 4352; (b) C. Longo, A.F. Nogueira, M.D. Paoli, H. Cachet, J. Phys. Chem. B 106 (2002) 5925; (c) A.L. Linsebigler, G. Lu, J.T. Yates, Chem. Rev. 95 (1995) 735; (d) Y.V. Zubavichus, Yu.L. Slovokhotov, M.K. Nazeeruddin, S.M. Zakeeruddin, M. Gratzel, V. Shklover, Chem. Mater. 14 (2002) 3556. L. Zhang, M. Wan, Adv. Funct. Mater. 13 (2003) 815. (a) G. Louarn, M. Lapkoaski, S. Quillard, A. Pron, J.P. Buisson, S. Lefrant, J. Phys. Chem. 100 (1996) 6998; (b) M. Bernard, A.H. Goff, Synth. Met. 85 (1997) 1145; (c) M.I. Boyer, S. Quillard, E. Rebourt, G. Louarn, J.P. Buisson, A. Monkman, S. Lefrant, J. Phys. Chem. B 102 (1998) 7382. S. Quillard, G. Louarn, S. Lefrant, A.G. MacDiarmid, Phys. Rev. B 50 (1994) 12496. M. Cochet, G. Louarn, S. Quillard, J.P. Buisson, S. Lefrant, J. Raman Spectrosc. 31 (2000) 1041. (a) C.A. Melendres, A. Narayanasamy, V.A. Maroni, R.W. Siegel, J. Mater. Res. 4 (1989) 1246; (b) M. Zhang, Z. Yin, Q. Chen, Ferroelectrics 168 (1995) 131. H. Tang, K. Prasad, R. Sanjin`es, P.E. Schmid, F. L´evy, J. Appl. Phys. 75 (1994) 2042. (a) Y. Yang, M. Wan, J. Mater. Chem. 12 (2002) 897; (b) C.R. Martin, Chem. Mater. 8 (1996) 1739; (c) J.P. Pouget, M.E. J´ozefowicz, A.J. Epstein, X. Tang, A.G. MacDiarmid, Macromolecules 24 (1991) 779. L. Zhang, M. Wan, J. Phys. Chem. B 107 (2003) 6748. (a) J.H. Fuhrhop, W. Helfrich, Chem. Rev. 93 (1993) 1565; (b) Z. Wei, Z. Zhang, M. Wan, Langmuir 18 (2002) 917; (c) Z. Zhang, Z. Wei, M. Wan, Macromolecules 35 (2002) 5937. B.J. Kim, S.G. Oh, M.G. Han, S.S. Im, Langmuir 16 (2000) 5841. S. Su, N. Kuramoto, Synth. Met. 114 (2000) 147.
Polyaniline/TiO2 microspheres prepared by a template-free method Lijuan Zhang a , Meixiang Wan a,∗ , Yen Wei b a b
Organic Solid Laboratory, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China The Center for Advanced Polymers and Materials Chemistry, Department of Chemistry, Drexel University, Philadelphia, PA 19104, USA Received 2 September 2004; received in revised form 6 December 2004; accepted 19 December 2004 Available online 13 June 2005
Abstract Polyaniline/TiO2 microspheres with 2.5–3.6 m in average diameter were synthesized by a template-free method in the presence of salicylic acid (SA) as the dopant. It was found that the morphology, conductivity and hydrophilicity of the PANI-SA/TiO2 microspheres were affected by the content of TiO2 nanoparticles. The micelles composed of SA anions and anilinium cations containing TiO2 were proposed to interpret the formation mechanism of the self-assembled composite microspheres. © 2005 Elsevier B.V. All rights reserved. Keywords: Polyaniline/TiO2 microspheres; Template-free method
1. Introduction Much researches dealing with the construction of polymer microspheres and microcapsules for a wide number of potential applications have been carried out over the past decade [1]. Especially, microspheres with hollow interiors have drawn a number of interests due to their applications including stationary phases for separation science, biomedical devices, coating additives, controlled release reservoirs and as small containers for micro-encapsulation [2]. Recently, a great deal of attention has been directed toward the functionality of these hollow microspheres. Various approaches have been designed to prepare the composite microspheres to obtain the required properties and promising applications in many fields, including optics, electronics, mechanics, membranes, protective coatings, catalysis, sensors, biology, and others [3]. Among these composites, many efforts have been focused on the combination of organic conjugated polymers and inorganic nano-crystals in order to yield new functional materials that may combine the advantages of each component [4].
∗
Corresponding author. Fax: +86 10 6255 9373. E-mail address: [email protected] (M. Wan).
0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.12.021
Conducting polymer as a typical organic conjugated polymer has received great attention because of their excellent electronic properties with conductivity covering the whole range from insulator to metal while retaining lightweight, mechanical properties and processing advantages of polymers [5]. Among these conducting polymers, polyaniline (PANI) is one of the most promising conducting polymers because of its low cost, easy preparation, controllable unique properties by oxidation and protonation state, excellent environmental stability and potential application in electronic devices [6]. Regarding inorganic materials, crystalline titanium dioxide (TiO2 ) nanoparticles are non-toxicity, chemical inertness, and low costs that lead to numerous promising applications in the fields of electronics and photonics [7]. Recently, we reported that hollow microspheres of PANI doped with salicylic acid (SA) could be prepared by a simple template-free method [8]. Moreover, we found that the method is essentially a self-assembly process because of micelles formed by SA and aniline acting as a template in the formation of those microspheres. This result promises us to examine whether we can use the self-assembly process to prepare the PANI-SA/TiO2 composite microspheres and to bring about new properties of PANI-SA microspheres. In this article, composite microspheres of PANI-SA/TiO2 with 2.5–3.6 m in average diameter were successfully
2
L. Zhang et al. / Synthetic Metals 151 (2005) 1–5
prepared by a template-free method in the presence of TiO2 nanoparticles (∼15 nm in diameter). Herein, the influence of the TiO2 concentration on the morphology, size, molecular structure, conductivity and hydrophilicity of the PANI-SA/TiO2 composite microspheres was investigated. The formation mechanism of the composite microspheres is also discussed.
2. Experimental 2.1. Materials Aniline monomer (Beijing Chem. Co.) was distilled under reduced pressure. Ammonium per-sulfate ((NH4 )2 S2 O8 , APS, Beijing Yili Chem. Co.), salicylic acid (SA) and other reagent were purchased from Beijing Chem. Co. and used as received without further treated. TiO2 nanoparticles were also used as received. 2.2. Polymerization A certain molar ratio of aniline and SA represented by [SA]/[An] were dissolved in de-ionized water with magnetic stirring at room temperature for 30 min. Then, TiO2 was added into the mixture and also stirred for 30 min. Finally, the stirring was stopped and followed by adding aqueous solution of APS and the reaction mixture was kept for 12 h. The resulting precipitate was washed with water, methanol and ether several times, respectively. Finally, the product was dried in vacuum at room temperature for 24 h. In all experiment, the molar ratio of aniline to SA (represented by [An]/[SA]) and to APS (represented by [An]/[APS]) for either PANISA microspheres or PANI-SA/TiO2 composite microspheres was 1:1 and 1:1, respectively. But the concentration of TiO2 nanoparticles was changed to understand the effect of the TiO2 nanoparticles on the morphology, structure, electrical properties and hydrophilicity of the resulting PANI-SA/TiO2 composite microspheres. 2.3. Characterization The morphologies of PANI-SA and PANI-SA/TiO2 were investigated with a JEOL-JSM-6700F field emission scanning electron microscope (SEM) and a JEOL JEM-2010 transmission electron microscopy (TEM), respectively. The samples for SEM measurements were mounted on aluminum stud using adhesive graphite tape and sputter-coated gold before analysis, while for TEM were dispersed on micro-grids copper coated with carbon support film. When the TEM was investigated, the synchronous energy depressive X-ray analysis was recorded with a LINK ISIS300 instrument. Raman spectra of PANI-SA and PANI-SA/TiO2 microspheres were carried out on a Renishaw Microscope (RM-2000) with 785 nm laser served as an excitation wavelength. The X-ray scatting patterns for PANI-SA
and PANI-SA/TiO2 microspheres were measured on an Xray diffraction instrument (Micscience M-18XHF with a Cu K␣ radiation). The electrical conductivity of PANI-SA and PANI-SA/TiO2 compressed pellets at room temperature were measured by a standard four-probe method, using a Keithley 196 System DMM Digital Multimeter and an Advantest R1642 programmable dc voltage/current generator as the current source. A Contact Angle System OCA Dataphysics DCAT 11 was used to measure the water contact angle of PANI-SA/TiO2 and PANI-SA films deposited on the glass substrate. The coated films on the glass substrate were polymerization by dipping the glass substrate into the reaction solution after adding oxidant (i.e. APS).
3. Results and discussion The scanning electron microscopy (SEM) and the scanning transmission (TEM) as well as the electron diffraction (ED) of the PANI-SA hollow microspheres and PANISA/TiO2 microspheres are shown in Fig. 1. The outer diameter of the composite microspheres ranges from 2.5 to 3.6 m. As one can see, the surface of PANI-SA hollow microspheres is smooth (Fig. 1a); however, the surface of PANI-SA/TiO2 microspheres is more rough and covered with granules (Fig. 1b). TEM image of PANI-SA/TiO2 (Fig. 1d) proved that TiO2 nanoparticles exist in the PANISA/TiO2 microspheres. The ED pattern of PANI-SA/TiO2 further proved that the microspheres are composed of polycrystalline TiO2 and amorphous PANI-SA. In particular, synchronous energy depressive X-ray spectrum measurements further revealed that the titanium element exists in the composite microspheres while which is absent in the PANISA microspheres. In order to further prove the existence of TiO2 nanoparticles in the microspheres, Raman spectra of PANI-SA, PANI-SA/TiO2 and TiO2 nanoparticles were measured with an excitation wavelength λL = 785 nm (Fig. 2). All typical characteristic bands of PANI were observed in both the Raman spectra of PANI-SA and PANI-SA/TiO2 . For instance, the bands at 1590 and 1369 cm−1 assigned to the C C stretching vibration of the benzenoid and quinoid rings were observed [9]. Moreover, the bands at 1505, 1339, and 1165 cm−1 corresponding to the N H stretching mode, the C N stretching of the cation radical and the C H bending vibration were also observed, respectively [10]. Especially, the bands at 807 and 415 cm−1 related to the C H deformation, the band at 578 cm−1 attributed to the amine deformation as well as the band at 514 cm−1 ascribed to the C N C torsion were observed too [11]. The Raman spectrum of TiO2 nanoparticles showed three characteristic modes at 395, 513 and 638 cm−1 . The modes at 395 and 513 cm−1 are assigned as B1 g and A1 g mode of anatase phase, while the mode at 638 cm−1 is assigned Eg phonon of the anatase structure, respectively [12]. All modes of TiO2 nanoparticles described above can be observed in the Raman spectrum of PANI-SA/TiO2 composite microspheres.
L. Zhang et al. / Synthetic Metals 151 (2005) 1–5
3
Fig. 1. SEM images (a and b), TEM (c and d) images and electron diffraction patterns (e and f) of PANI-SA/TiO2 microspheres synthesized under different concentration of TiO2 : (a, c and e) 0 M and (b, d and f) 0.04 M, synthetic conditions: [An] = 0.2 M, [An]/[SA] = 1:1, [An]/[APS] = 1:1.
Although the frontal two modes at 415 and 514 cm−1 are coincided with the ring deformation bands of PANI-SA, a new peak centered at 638 cm−1 , which is ascribed to the typical mode of TiO2 , was observed in the Raman spectrum of PANI-SA/TiO2 , while which is absent in PANI-SA. X-ray
diffraction of PANI-SA/TiO2 , PANI-SA and TiO2 nanoparticles was also measured to further prove the existence of TiO2 nanoparticles in the PANI-SA/TiO2 composite microspheres. As shown in Fig. 3, TiO2 nanoparticles are crystal, and the
Fig. 2. Raman spectra of (a) PANI-SA, (b) PANI-SA/TiO2 ([TiO2 ] = 0.04 M) and (c) TiO2 nanoparticles, excited with λL = 785 nm laser line, synthetic conditions: [An] = 0.2 M, [An]/[SA] = 1:1, [An]/[APS] = 1:1.
Fig. 3. X-ray scattering patterns of (a) PANI-SA, (b) PANI-SA/TiO2 ([TiO2 ] = 0.04 M) and (c) TiO2 nanoparticles, [An] = 0.2 M, [An]/[SA] = 1:1, [An]/[APS] = 1:1.
4
L. Zhang et al. / Synthetic Metals 151 (2005) 1–5
positions of all sharp peaks are in agreement with the results reported by Tang et al. [13]. PANI-SA microspheres are amorphous, on the other hand, only one broad peaks centered at 2θ = 21◦ were observed, which are ascribed to the periodicity parallel to the polymer chain [14]. The X-ray scattering pattern of PANI-SA/TiO2 composite microspheres consists of one broad peak at 2θ = 21◦ , which is belong to PANI-SA, and all sharp peaks of TiO2 nanoparticles, proving the existence of TiO2 nanoparticles in the composite microspheres. Although the above-mentioned results proved the existence of the TiO2 nanoparticles in the PANI-SA microspheres, we cannot confirm where TiO2 nanoparticles exist? Since the TiO2 nanoparticles are hydrophobic, we thought the measurement of water contact angle maybe provide some information about the position of TiO2 nanoparticles in the composite microspheres. As a result, we measured the water contact angle of PANI-SA/TiO2 films deposited on the glass substrate and compared with PANI-SA samples. It was found that the water contact angle of PANI-SA/TiO2 microspheres was increased from 41.2◦ to 57.5◦ when the content of TiO2 nanoparticles changed from 0 to 0.2 M (Fig. 4), indicating the influence of the content of TiO2 nanoparticles on the hydrophilicity of the composite microspheres is weak. On the other hand, our previous results showed that the water contact angle of PANI--NSA/TiO2 nanotubes prepared by a template-free method increased from 53.5◦ to 98.5◦ as the concentration of TiO2 changed from 0 to 0.08 M [15]. The above results showed that the PANI-SA/TiO2 microspheres is more hydrophilic than that of the PANI--NSA/TiO2 nanotubes although the content of the TiO2 nanoparticles in PANI-SA/TiO2 microspheres is higher than that of PANI-NSA/TiO2 nanotubes, indicating most TiO2 nanoparticles are filled in the hollow interiors of PANI-SA microspheres. According to our previous studies [8], it has been demonstrated that the spherical micelles formed by SA and anilinium cations act as templates in the formation of PANI-SA
Fig. 5. Effect of the TiO2 concentration on the room-temperature conductivity of PANI-SA/TiO2 microspheres, synthetic conditions: [An] = 0.2 M, [An]/[SA] = 1:1, [An]/[APS] = 1:1.
hollow microspheres and the hydrogen bond between OH group of SA and amine group of polymer chain is a driving force to form self-assembled microspheres. As described in the experiment, the TiO2 nanoparticles dispersed into SA/An solution before polymerization. Therefore the micelles in this study have a “core-shell” structure, where TiO2 nanoparticles are assigned as a “core” of the micelles due to its hydrophobic feature, while SA and anilinium cations as a “shell” of the micelles due to hydrophilicity of SA dopant ( COOH group attached on the benzene ring). On the basis of our previous micelle mechanism [8] and the above described, it is reasonable to understand that these micelles containing TiO2 act as templates in the formation of PANI-SA/TiO2 microspheres. In general, it is expected that polymerization only took place at the interface of micelle/water due to the hydrophilicity of APS as an oxidant [16] and the growth of the microspheres are controlled by accretion process [17] The above-mechanism suggests that PANI-SA is co-structured with TiO2 nanoparticles to form composite microspheres through a self-assembly process. It was found that the conductivity of PANI-SA/TiO2 composites strongly depended on the content of TiO2 as shown in Fig. 5. When the concentration of TiO2 is lower than 0.04 M, the conductivity increases with the increase of the concentration of TiO2 , the maximum conductivity at room temperature is about 10−2 S/cm. However, it decreases when the TiO2 concentration is higher than 0.06 M. This is consistent with the results of PANI-DBSA/TiO2 reported by Su and Kuramoto [18] and PANI--NSA/TiO2 nanotubes reported by us [15].
4. Conclusions Fig. 4. Effect of the concentration of TiO2 on the water contact angle of the PANI-SA/TiO2 films deposited on the glass substrate, synthetic conditions: [An] = 0.2 M, [An]/[SA] = 1:1, [An]/[APS] = 1:1.
PANI-SA microspheres containing TiO2 nanoparticles were prepared by a template-free method that opens a sample and cheap route to prepare composite microspheres of
L. Zhang et al. / Synthetic Metals 151 (2005) 1–5
conducting polymers with inorganic nanoparticles. It was found that changing the content of TiO2 can adjust the morphology, size, electrical property and hydrophilicity of the composite microspheres.
Acknowledgments
[4]
[5]
This project was supported by the National Natural Science Foundation of China (No. 50133010, and 29974037) and Grand of Over-sea Outer-standing Scientists of Chinese Academy of Sciences. TiO2 nanoparticles were kindly supplied by Professor Jincai Zhao, Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences.
[6]
References
[7]
[1] (a) L. Dalhne, S. Leporatti, E. Donath, H. M¨ohwald, J. Am. Chem. Soc. 123 (2001) 5431; (b) K. Kamata, Y. Lu, Y. Xia, J. Am. Chem. Soc. 125 (2003) 2384; (c) T.K. Mandal, M.S. Fleming, D.R. Walt, Chem. Mater. 12 (2000) 3481; (d) Z. Chang, G. Liu, Y. Tian, Z. Zhang, Mater. Lett. 58 (2004) 522; (e) S.M. Klein, V.N. Manoharan, D.J. Pine, F.F. Lange, Colloid Polym. Sci. 282 (2003) 7; (f) S.M. Marinakos, D.A. Shultz, D.L. Feldheim, Adv. Mater. 11 (1999) 34; (g) I. Pastoriza-Santos, B. Sch¨oler, F. Caruso, Adv. Funct. Mater. 11 (2001) 122. [2] (a) F. Caruso, R.A. Caruso, H. M¨ohwald, Science 282 (1998) 1111; (b) Y. Xia, B. Gates, Y. Yin, Y. Lu, Adv. Mater. 12 (2000) 693; (c) K.B. Thurmond II, T. Kowalewski, K.L. Wooley, J. Am. Chem. Soc. 119 (1997) 6656; (d) R. Gref, Y. Minamitake, M.T. Peracchia, V. Trubetskoy, V. Torchilin, R. Langer, Science 263 (1994) 1600; (e) I. Gill, A. Ballesteros, J. Am. Chem. Soc. 120 (1998) 8587; (f) F. Caruso, M. Spasova, V.S. Maceira, M.L. Marz´an, Adv. Mater. 13 (2001) 1090; (g) S.M. Marinakos, J.P. Novak, L.C. Brousseau III, A.B. House, E.M. Edeki, J.C. Feldhaus, D.L. Feldheim, J. Am. Chem. Soc. 121 (1999) 8518; (h) Z. Wei, M. Wan, T. Lin, L. Dai, Adv. Mater. 15 (2003) 136. [3] (a) J.P. Spatz, T. Herzog, S. M¨omer, P. Ziemann, M. M¨oller, Adv. Mater. 11 (1999) 149; (b) G. Sukhorukov, L. Dahne, J. Hartmann, E. Donath, H. M¨ohwald, Adv. Mater. 12 (2000) 112;
[8] [9]
[10] [11] [12]
[13] [14]
[15] [16]
[17] [18]
5
(c) M. Antonietti, C. Goltner, Angew. Chem. Int. Ed. 36 (1997) 910; (d) A. Bigi, E. Boanini, D. Walsh, S. Mann, Angew. Chem. Int. Ed. 41 (2002) 2163; (e) C. Bai, Y. Fang, Y. Zhang, B. Chen, Langmuir 20 (2004) 263. (a) H. Yang, P.H. Holloway, J. Phys. Chem. B 107 (2003) 9705; (b) N.J. Torrence, C.M. Moore, S.D. Minteer, Langmuir 18 (2002) 6254; (c) F. Caruso, Adv. Mater. 13 (2001) 11. (a) T.A. Skotheim, R.L. Elsenbaumer, J.R. Reynolds (Eds.), Handbook of Conducting Polymers, Marcel Dekker, Inc., New York, 1998; (b) C.R. Martin, Science 266 (1994) 1961; (c) A.J. Heeger, Chin. J. Polym. Sci. 19 (2001) 545; (d) A.J. Heeger, J. Phys. Chem. B 105 (2001) 8475. (a) A.G. MacDiarmid, J.C. Chiang, W.S. Huang, B.D. Humphrey, N.L.D. Somasiri, Mol. Cryst. Liq. Cryst. 125 (1985) 309; (b) A.G. MacDiarmid, J.C. Chiang, A.F. Richter, A.J. Epstein, Synth. Met. 18 (1987) 285; (c) Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 48 (1992) 91; (d) J. Huang, S. Virji, B.H. Weiller, R.B. Kaner, J. Am. Chem. Soc. 125 (2003) 314. (a) P.A. van Hal, M.P.T. Christiaans, M.M. Wienk, J.M. Kroon, R.A.J. Janssen, J. Phys. Chem. B 103 (1999) 4352; (b) C. Longo, A.F. Nogueira, M.D. Paoli, H. Cachet, J. Phys. Chem. B 106 (2002) 5925; (c) A.L. Linsebigler, G. Lu, J.T. Yates, Chem. Rev. 95 (1995) 735; (d) Y.V. Zubavichus, Yu.L. Slovokhotov, M.K. Nazeeruddin, S.M. Zakeeruddin, M. Gratzel, V. Shklover, Chem. Mater. 14 (2002) 3556. L. Zhang, M. Wan, Adv. Funct. Mater. 13 (2003) 815. (a) G. Louarn, M. Lapkoaski, S. Quillard, A. Pron, J.P. Buisson, S. Lefrant, J. Phys. Chem. 100 (1996) 6998; (b) M. Bernard, A.H. Goff, Synth. Met. 85 (1997) 1145; (c) M.I. Boyer, S. Quillard, E. Rebourt, G. Louarn, J.P. Buisson, A. Monkman, S. Lefrant, J. Phys. Chem. B 102 (1998) 7382. S. Quillard, G. Louarn, S. Lefrant, A.G. MacDiarmid, Phys. Rev. B 50 (1994) 12496. M. Cochet, G. Louarn, S. Quillard, J.P. Buisson, S. Lefrant, J. Raman Spectrosc. 31 (2000) 1041. (a) C.A. Melendres, A. Narayanasamy, V.A. Maroni, R.W. Siegel, J. Mater. Res. 4 (1989) 1246; (b) M. Zhang, Z. Yin, Q. Chen, Ferroelectrics 168 (1995) 131. H. Tang, K. Prasad, R. Sanjin`es, P.E. Schmid, F. L´evy, J. Appl. Phys. 75 (1994) 2042. (a) Y. Yang, M. Wan, J. Mater. Chem. 12 (2002) 897; (b) C.R. Martin, Chem. Mater. 8 (1996) 1739; (c) J.P. Pouget, M.E. J´ozefowicz, A.J. Epstein, X. Tang, A.G. MacDiarmid, Macromolecules 24 (1991) 779. L. Zhang, M. Wan, J. Phys. Chem. B 107 (2003) 6748. (a) J.H. Fuhrhop, W. Helfrich, Chem. Rev. 93 (1993) 1565; (b) Z. Wei, Z. Zhang, M. Wan, Langmuir 18 (2002) 917; (c) Z. Zhang, Z. Wei, M. Wan, Macromolecules 35 (2002) 5937. B.J. Kim, S.G. Oh, M.G. Han, S.S. Im, Langmuir 16 (2000) 5841. S. Su, N. Kuramoto, Synth. Met. 114 (2000) 147.