Preparation of hybrid films of aluminosilicate nanofiber and conjugated polymer

Synthetic Metals 159 (2009) 885–888

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Preparation of hybrid films of aluminosilicate nanofiber and conjugated polymer Nattha Jiravanichanun a , Kazuya Yamamoto a , Atsushi Irie b , Hideyuki Otsuka a,b,∗ , Atsushi Takahara a,b,∗∗ a b

Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

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Article history: Received 13 November 2008 Received in revised form 24 January 2009 Accepted 28 January 2009 Available online 28 February 2009 Keywords: Conjugated polymers Organic/inorganic hybrids Nanofiber Imogolite Layer-by-layer

a b s t r a c t We have demonstrated the preparation of hybrid films of aluminosilicate nanofiber (imogolite) and watersoluble poly(p-phenylene) (WS-PPP), which has sulfonate groups. The imogolite/WS-PPP hybrid gel could be prepared by mixing a solution of these two materials and subsequent centrifugation. The aluminol (Al OH) groups on the surface of imogolite would be protonated under acidic conditions to afford Al OH2 + groups that can interact with sulfonate groups (SO3 − ) of WS-PPP. Based on this ionic interaction, a layer-by-layer (LBL) assembly was applied to fabricate the hybrid films of imogolite nanofibers and WS-PPP. The deposited amounts of WS-PPP and imogolite were measured by quartz crystal microbalance (QCM) measurements and scanning electron microscopy (SEM) observation. Atomic force microscopy (AFM) observation revealed that imogolite nanofibers were well networked in the LBL hybrid film. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Imogolite is one of the clay minerals contained in volcanic ash soils [1]. It is a hydrous aluminosilicate and has a fibrous diameter of approximately 2 nm (Fig. 1). The chemical structure demonstrates that the aluminol (Al OH) groups are exposed on the outer surface of the fiber structure, and these groups can be protonated under acidic conditions to afford Al OH2 + groups. The surface charge therefore depends on the pH of the solution, with a positive charge being maintained under low pH conditions. Imogolite can be regarded as one of the positively charged polymers under acidic conditions. In addition, imogolite can be synthesized chemically from tetraethoxysilane and aluminum chloride [2,3], and it is a promising candidate for inorganic nanomaterials. The authors in the present study focused on imogolite not only due to its high aspect ratio and positively charged surface but also due to its optical transparency. The hybrid materials of imogolite and some functional polymers with important optical properties can be expected to show innovative functionality. In contrast, carbon nanotubes, a representative nanofiber, will waste the optical properties of the hybrid material due

∗ Corresponding author at: Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan. Tel.: +81 92 802 2515; fax: +81 92 802 2518. ∗∗ Corresponding author at: Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan. E-mail addresses: [email protected] (H. Otsuka), [email protected] (A. Takahara). 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.01.047

to their strong absorbance. In this study, imogolite nanofibers were used for assembly with conjugated polymers by ionic interactions. As a negatively charged conjugated polyelectrolyte, we employed poly[disodium 2,5-bis(3-sulfonatopropoxy)-1,4phenylene-alt-1,4-phenylene] (WS-PPP) [4], which is water-soluble and has a conjugated molecular structure. In this paper, the hybrid gel composed of WS-PPP and imogolite was prepared by a simple mixing method of the each acidic solutions and the obtained gel was characterized by IR spectroscopy. We then prepared ultrathin multilayer hybrid films by applying the layer-by-layer (LBL) self-assembly method [5]. The film-growth mechanism was investigated by quartz crystal microbalance (QCM). The film thickness and surface topology were observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively.

2. Experimental 2.1. Materials Imogolite was synthesized by the previously reported method [3]. Concentrated sulfuric acid, aqueous hydrogen peroxide solution, 3-mercaptopropyl-trimethylsilane (MTS), toluene, sodium 3-mercapto-1-propanesulfonate (MPS), sodium acetate, acetic acid, WS-PPP, and poly(4-hydroxystyrene) (PHS) were used as received. Acetate buffer pH 4 was prepared for a solvent of imogolite. Water in this experiment was purified with the Nanopure Water system,  > 18 M cm (Millipore, Inc.). Silicon wafer and quartz plate were used as substrates.

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Fig. 1. Schematic representation of the structure of imogolite and chemical structure of WS-PPP.

2.2. Hybrid gel preparation Aqueous solutions of imogolite (1 mM, pH 4.0) and WS-PPP (1 mM, pH 4.0) were mixed at room temperature. The mixture was centrifuged at 6000 rpm for 10 min to afford a hybrid gel. In order to characterize the gel, its IR spectrum was measured after drying. 2.3. Substrate preparation Substrates were cleaned in a piranha solution before the chemical vapor adsorption (CVA) process of MTS [6]. The substrates were then irradiated by UV-ray (wavelength 365 nm) for over 10 h in air to transform terminal mercapto groups to sulfonic acid groups [6]. Negatively charged substrates with sulfonic acid groups were immersed in the imogolite solution (1 mM) and the WS-PPP solution (1 mM) alternatively. The immersion time was 20 and 10 min, respectively, followed by 2 min of immersion in water after each dipping step. Substrates were dried in air after each rinsing step. 2.4. Characterization IR spectroscopic measurement was carried out with a Spectrum One (PerkinElmer Japan Co., Ltd.). The intermittent contact-mode AFM observations were carried out using an SPA400 AFM head with a SPI4000 Probe Station (SII Nano Technology, Inc.). The crosssectional SEM images of the multilayer films were performed using an S-4300SE (Hitachi Co., Ltd.). UV absorption spectrum of the hybrid film was determined on a Shimadzu UV-3600 UV–VIS–NIR spectrophotometer. Fluorescence (FL) spectrum was analyzed on a Jasco FP-6600 spectrofluorometer by exiting at 340 nm. A 9-MHz resonator was manufactured by an USI system, Japan. The frequency was monitored by an Agilent universal frequency

Fig. 3. IR spectra of (a) dried imogolite/WS-PPP gel; (b) WS-PPP; (c) synthetic imogolite.

counter (model 53131A), and was recorded by a personal computer. A crystal was coated on both sides with gold electrodes 4.4 mm in diameter. Gold-coated QCM crystal was modified with MPS [7] for the LBL assembly process. 3. Results and discussion 3.1. The hybrid gel structure and interaction In order to confirm the interaction between WS-PPP and imogolite nanofibers, acidic aqueous solutions of WS-PPP and imogolite were mixed. The viscosity of the imogolite solution was immediately enhanced after adding the WS-PPP solution. As we expected, the mixture of imogolite and WS-PPP formed a gel product after centrifugation, even showing a clear solution before mixing. Under the irradiation of UV light (ex 365 nm), the imogolite/WS-PPP hybrid gel was fluorescent blue in color and could be clearly seen in the dark (Fig. 2). The hybrid gel was freeze-dried to characterize its structure by IR measurement. IR spectra of the hybrid gel, synthetic imogolite, and WS-PPP are shown in Fig. 3. The spectrum of imogolite showed two sharp absorption bands at 930 and 950 cm−1 corresponding to the Si O Al stretching vibration and a large absorption at 3440 cm−1 corresponding to the OH stretching vibration. WS-PPP showed two strong absorption bands at 1039 and 1204 cm−1 corresponding to the S O stretching of sulfonate groups. The aromatic C C stretching of a phenylene ring was found at 1485 and 1643 cm−1 . Aromatic and aliphatic C H stretchings were found at 2951 and 2869 cm−1 , respectively. The hybrid gel spectrum

Fig. 2. Photographic images of the imogolite/WS-PPP hybrid gel luminescent under 365 nm UV irradiation (a) in light; (b) in dark.

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Fig. 4. Increase in the frequency shift of imogolite and WS-PPP in the LBL film. The odd and even layers represent imogolite and WS-PPP, respectively. Fig. 5. SEM cross-section image of the LBL film with a thickness of 103 nm.

showed both characteristic peaks of WS-PPP and imogolite. These results suggested that WS-PPP had a strong ionic interaction with imogolite nanofibers. 3.2. Multilayer film structure and thickness In the case of the LBL assembly, the substrates for the preparation of hybrid films required some interactions with the film component. The substrates were treated with MTS to prepare an organosilane monolayer with mercapto groups, and subsequent oxidation was carried out to convert the mercapto group into sulfonic acid groups, which can form electrostatic interactions with the surface of imogolite. LBL films were successfully prepared on silicon wafer substrates. We were able to characterize the internal structure of the films by investigating the deposition amount in each layer. The mechanism for film growth in each layer was investigated by QCM measurement. The adsorption measurements were carried out after deposition of each layer of imogolite and WS-PPP. Fig. 4 shows the dependence of frequency shift on the assembly steps of the 10 bilayers LBL film. The frequency decrements indicated regular film growth at a nanometer scale during consecutive deposition cycles of imogolite and WS-PPP in the LBL films. The odd and even layers represent the adsorption steps of imogolite and WS-PPP layers, respectively. The results indicated that imogolite and WS-PPP were successfully assembled by the LBL assembly method. The average deposited amounts per bilayer of imogolite and WS-PPP were found to be 326 ± 121 and 35 ± 22 ng, respectively. The large frequency change is probably due to the aggregation of imogolite nanofiber adsorption. We tried to investigate the bilayer structures in nano-scale of WS-PPP and imogolite nanofibers in hybrid films. The LBL hybrid film was cut to afford cross-sections, and they were observed by SEM. The cross-section image of the LBL film showed a thick-

ness of 103 nm (Fig. 5). Although it was difficult to observe the internal structure in terms of the topology of each layer by crosssectional SEM, we could estimate the thickness of the hybrid film. 3.3. AFM surface morphology Fig. 6 shows AFM surface images of imogolite/WS-PPP hybrid films on silicon wafer substrates of 1, 5, 10, and 15 bilayers. Imogolite nanofibers were deposited on the surface of the LBL films to form a well-networked structure. Surface coverage was not good in the single-layer film, but the coverage of nanofibers increased when the number of deposition bilayers increased. 3.4. Formation of free-standing film and optical properties The film-forming ability of the imogolite/WS-PPP hybrid film was tested by the prepared multilayered film on silicon wafer applied with PHS. PHS was employed as a sacrificial layer which can be dissolved in ethanol to detach from silicon wafer. The sample was then annealed at 100 ◦ C and immersed in ethanol for detachment. The preparation of WS-PPP film was also attempted on a PHS coated substrate by the same method. The WS-PPP showed very poor film-forming ability. In contrast, the hybrid imogolite/WS-PPP film formed a free-standing film after detaching from the substrate. The hybrid film was colorless and transparent. The hybridization with imogolite nanofibers enhanced film-formation due to the well-networked structure of imogolite nanofibers. In order to investigate the optical properties of imogolite/WSPPP hybrid film, the UV adsorption and fluorescence (FL) spectra of the multilayer imogolite/WS-PPP film on quartz substrate were measured. The UV adsorption spectrum showed max = 340 nm due

Fig. 6. AFM images of surface topology of 1, 5, 10, and 15 bilayer films prepared by LBL assembly.

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to the conjugated aromatic system on WS-PPP, and FL spectrum of the multilayer film shows maximum intensity at 425 nm. A WSPPP film without imogolite shows max of UV adsorption and em of FL spectra at 337 and 427 nm, respectively, which were not significantly different from those of the multilayer film. These results indicated that imogolite is optically transparent in the multilayer film. 4. Conclusions Sulfonate groups of WS-PPP formed a strong interaction with aluminol groups on the surface of imogolite nanofibers, as observed in the hybrid gel. Moreover, hybrid films of imogolite and WSPPP were successfully prepared by LBL assembly. The quantitative deposited amount was achieved by QCM measurement, and the film thickness was also observed by cross-sectional SEM. This hybrid material was able to form a free-standing film. Imogolite nanofibers were used as templates for conjugated polymer at the surface of nanofibers by ionic interaction. This is a simple and applicable method for preparing highly ordered hybrid films of nanofibers and conjugated polymers.

Acknowledgments This work was supported by a Grant-in-Aid for Science Research in a Priority Area “Super-Hierarchical Structures” (Grant No. 446) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. The present work was also supported by a Grant-in-Aid for the Global COE Program, “Science for Future Molecular Systems” from MEXT, Japan. We thank Prof. Shin-Ichiro Wada, Faculty of Agriculture, Kyushu University, for his helpful advice regarding imogolite synthesis. References [1] K. Yamamoto, H. Otsuka, A. Takahara, Polym. J. 39 (2007) 1. [2] S.-I. Wada, A. Eto, K. Wada, J. Soil Sci. 30 (1979) 347. [3] K. Yamamoto, H. Otsuka, S.-I. Wada, D. Sohn, A. Takahara, Soft Matter 1 (2005) 372. [4] S. Kim, J. Jackiw, E. Robinson, K.S. Schanze, J.R. Reynolds, Macromolecules 31 (1998) 964. [5] G. Decher, Science 277 (1997) 1232. [6] T. Koga, H. Otsuka, A. Takahara, Bull. Chem. Soc. Jpn. 78 (2005) 1691. [7] J. Hodak, R. Etchenique, E.J. Calco, Langmuir 13 (1997) 2708.