Nickel-Nanoclusters-Containing Polyaniline Composites Having Structure Layer-by-Layer Part I - the Development of Concept and Structural Morphology

236 Journal of Chemistry, Vol. 42 (2), P. 236 - 240, 2004 Nickel-Nanoclusters-Containing Polyaniline Composites Having Structure Layer-by-Layer Part I - the Development of Concept and Structural Morphology Received 17-4-2003 Tran Trung, Tran Huu Trung Department of Electrochemical Technology and Metal Protection, Faculty of Chemical Technology, Hanoi University of Technology Abstract The development of concept in structural morphology lets description the location of nickel nanoclusters in conductive organic polymers, which may be a new kind of structure in conductive organic polymer composites family. As a new route, such the development was supported by electrochemical two-pot process, on which polianiline containing nanoclusters of nickel incorporated in layer by layer were prepared, in H2SO4 0.1 M solutions. The presence of nickel nanoclusters in polymer matrix and the changes in morphology were confirmed by SEM/EDS. I – Develo`pment of concept Electropolymerization, an interesting method, has been widely used to prepare conjugated polymers, such as polypyrrole, polythiophen, polyaniline [1 - 3]. Such all conjugated polymers with spatially extended
-bonding system found exhibit unique physical and electrochemical properties unobtainable for conventional polymers. And it is very well known that depending on the potential value at the end of the electrosynthesis, the polymers have been in various the oxidative states, therefore leading to the different physical and electrochemical properties. To improve physical and electrochemical properties, conjugated polymers have been modified by incorporation of small size ions, Cl-, Br-, BF4 -, NO3 -, ClO4 -, SO2 2- etc. that can be diffusion into and diffusion out polymer matrix [1 - 3], or by incorporation of immobilized organic anions [4 - 6]. On the other way, very small size particles, in micron or submicron scale, of transition metal oxides including MnO2, LiMn2O4, WO3, RuO2 etc. and inorganic polyanions used as dopants for doping conjugated polymers [1, 7 - 10]. In some cases, microparticles and submicro-particles such as MnO2 and LiMn2O4 can be oxidized during electrosynthesis, and manganese atoms in low oxidative states will transform into high oxidative state Mn+6. In consequences, a strongly hybrid between d-unfilled orbitals of manganese atom and
-electron of polymer chains or/and lone-pair electrons of nitrogen was formed [1]. It is unobtainable and quite different from chemically way used to prepare such organic conductive polymer composites. Metal nanopar- ticles, such as Pt and Pd nanoparticles [11, 12], and metal complexes [13] have been already 237 electrochemically incorporated into polyaniline (PANI). PANI Films A B PANI layer Nickel-enriched PANI layer Aspects of the interconnection of metal nanoclusters and polymer fibrils Nanoclusters of nickel Polymer fibrils Figure 1: Schematic representation of the layer-by layer structured polymer composite film (A) and polymer composite film in which particles dispersed in to the whole of the polymer matrix (B), and of several aspects of the interconnection of metal nanoclusters and polymer fibrils In all cases mentioned above such dopants were electrochemically homogeneously disper- sed into the whole of conjugated polymer film (Fig. 1b). In this work, a new concept in structural morphology, the metal-nanoclusters- containing conductive organic polymers having structure layer-by-layer was suggested and developed. The success of preparation of nickel- nanoclusters-containing polyaniline is obviously evidence to support the development. The location of nickel nanocluster, as bridging nanocluster between polymers fibrils, in polymer matrix was also suggested (Fig. 2). Another striking point in such the preparation is that different from the electrochemically dispersion of Pd and Pt microparticles into PANI film, the dispersion of nanoclusters of metals, for example nickel and iron (their standard redox potentials are very negative vs. Ag/AgCl electrode), into organic conductive polymers has met a problem. It is due to the big difference between the redox potential of such a metal and the potentials, usually ranging from 0.2 V to 1.2 V vs. Ag/AgCl electrode, used to electro- polymerize of aniline, pyrrole and thiophen monomers by potential sweep. Oppositely, the electro-deposition of metal ions conducted at potentials very negative to Ag/AgCl electrode. This study attempt to find a route, by using electrochemical approach, on which we can overcome such problems and can control the distribution of particulates in the whole of polymer matrix or in an alternatively layer structured polymer film, as shown in figure 1. In such composites there exist several aspects of interconnect of nanoclusters of metal and polymer fibrils (Fig. 1c). II - Experimental Polyaniline composites containing nanoclus- ters of nickel were electrochemically prepared by two-pot process and structured layer-by-layer. The controlled electropolymerization system for preparation of aniline or the controlled electro- deposition system for incorporating nanoclusters of nickel into PANI film were composed of a potentiostat, the EG&E Priceton Applied Research model 362 with program Ecuniv-HH5, connected with a standard three electrodes cell containing an aqueous solution of 0.1 M aniline monomer or of 0.5 M nickel sulfate, respectively. The potential applied on the PANI composite films, which was electrodeposited onto platinum electrode (S = 1 cm2), was vs. Ag/AgCl reference electrode for all electrochemical measurements 238 and a platinum sheet was serving as auxiliary electrode. All chemicals used are in AR grade and supplied by Merck. To deoxygenate doubly distilled water, for preparation of solutions, and the electrolyte solution, nitrogen gas was bubbled before and during experiments. The presence of nickel in the obtained PANI composite films was confirmed by energy dispersion X-ray spectroscopy (EDXS, the incident angle kept constantly at 35o) equipped with a scanning electron microscope (SEM) model JEOL JSM-5410LV, which was used to investigate the surface morphology of the films. Figure 2: Schematic representation of the structures involving in the generation of principle forms and charge carriers of PANI as well as of illustration of the role of nanoclusters of nickel III - Structural morphology of nickel nanoclusters containing polyaniline As the known well, the PANI film electro- oxidized during potential sweep in voltammetry can be existed in a variety of form, which differs in their oxidative level. Principle neutral forms of PANI were consisting of the most reduced form commonly called leucoemeraldine, the fully oxidized form termed pernigraniline, and the half-oxidized form, emeraldine. The oxidation state of a PANI film onto the working electrode immersed in aqueous acidified solution depends on the applied potential and the presence of dopants (Fig. 3). The presence of nickel nanoclusters incorporated into PANI matrix leads to the changes in the density of 239 charge carriers, consequently leading the shift of anodic waves of cyclic voltammetry, as shown in figure 3. Like cyclic voltammograms in aqueous acidified solution, the cyclic voltammograms of PANI film conducted in 0.1 M H2SO4 solution, at potential scanning rate of 50 mV.s-1 consists of two main peaks of oxidation (Fig. 3d). The first one maximum at  0.2 V vs. Ag/AgCl reference electrode corresponds to the oxidization of leuco- emeraldine to emeraldine, and the second one maximum at a higher potential of  0.7 V attribu- ted to the oxidization of emeraldine to pernigra- niline. Except for just mentioned there also exist an obtuse peak (a shoulder) in the middle. The obtuse peak formed in combination of middle peaks in the cyclic voltammetry of PANI reported in [14]. It signifies that there is coexistence of reactions between nitronium aniline cation (C4H5NH +) and the nitronium in PANI matrix (C6H4N+), and of reaction between PANI chains itself, through the substitution of a nitronium cation in another PANI chain. On the other words, the transform from emeraldine to pernigraniline occurred simultaneously with the formation of charge carriers of PANI consisting of polaron and bipolaron forms delocalized on PANI chains. By such cross-linking reactions during electropolymerization of aniline, the branched PANI chains were performed and twisted together to form the branched PANI fibrils as observed by SEM studies (Fig. 4). As seen there is quite difference in structural surface morphology between PANI film and PANI film containing nanoclusters of nickel (PANI-Ni). PANI-Ni film seems just structured of a number of the coral-like branched polymer matrix consisting of twisted polymer fibrils. Meanwhile, PANI film shows its structural surface morphology like a “fishing-net” with unit cells covered by slab of PANI. Especially, the PANI polymers in coil shape, present in PANI film, however seem disappeared in PANI-Ni film. It may be consisted with the presence of nanoclusters of nickel in PANI-Ni film. The mentioned changes show the change in popula- tion of charge carriers in PANI-Ni film in comparing with those in PANI film. It is a main reason to cause the broadening and shift of the first anodic wave in cyclic voltammogram, and the second peak seems shifted to the higher potential, which is out of the studying potential range (Fig. 3 a - c). Further studies in order to elucidate the influence of nickel in PANI-Ni film are going to show in the next paper. Figure 3: Multi-cyclic voltammograms of PANI-Ni films consisting of a layer of nanoclusters of nickel (a), of two layer of nanoclusters of nickel (b) and three (c), and of no layer of nanoclusters of nickel (d) 240 A B Figure 4: SEM photographs of PANI films in magnification of 10,000 (A), and of PANI-Ni film in magnification of 10,000 (B) IV - ConclusionS A new concept in structural morphology was suggested and developed for conductive organic polymer composites containing nano- clusters of metals and nanoparticles. The success of the preparation of PANI-Ni films having structure layer-by-layer, by using electrochemical two-pot process was obviously evidence to support the mentioned develop- ment. The presence of nanoclusters of nickel in PANI-Ni films is main reason to cause the changes in the density of charge carriers and the broadening, as well as the shift of the first anodic waves. Followed that the flow of electrons delivered through metal substrate electrode increases significantly (Fig. 3). The further works on such the composite polymers will focus on the mechanism of processes coexisted during electrooxidation and on possible applications of those. References 1. T. Tran and V. T. Nguyen. Functional Materials (Proceedings of EUROMAT'99), Ed. by VCH-Wiley, P. 309 (2000). 2. V. Aboutanos, J. N. Barisci, L. A. P. Kane- Maguire, and G. G. Wallace. Synth. Met., Vol. 106, P. 89 (1999). 3. K. Teshima, K. Yamada, N. Kobayashi, and R. M. Hirohashi. J. Electroanal. Chem., Vol. 426, P. 97 (1997). 4. M. Zhou, J. J. Xu, H. Y. Chen, and H. Q. Fang. Electroanalysis, Vol. 9, P. 1185 (1997). 5. N. Oyama, J. M. Pope, T. Tatsuma, O. Hatazaki, F. Matsumoto, Q. J. Chi, S. C. Paulson, and M. Iwaku. Macromol. Symp., Vol. 13, P. 103 (1998). 6. J. Huang and M. Wan. Solid State Communi- cation, Vol. 108, P. 255 (1998). 7. N. Endo, Y. Miho, and K. Ogura. J. Mol. Catal. A: Chem., Vol. 127, P. 49 (1997). 8. K. Pielichowski and M. Hasik. Synth. Met., Vol. 89, P. 1999 (1997). 9. P. Wang and Y. F. Li. J. Electroanal. Chem., Vol. 408, P. 77 (1996). 10. M. Barth, M. Lapkowski, W. Turek, J. Muszynski, and S. Lefrant. Synth. Met., Vol. 84, P. 111 (1997). 11. C. H. Yang and T. C. Wen. Electrochim. Acta, Vol. 44, P. 207 (1998). 12. H. Kim and W. Chang. Synth. Met. Vol. 101, P. 150 (1999). 13. C. Coutanceau, P. Crouigneau, J. M. LÐger, and C. Lamy. J. Electroanal. Chem., Vol. 379, P. 389 (1994). 14. E. M. GeniÌs, M. Lapkowski, and J. E. Penneau. J. Electroanal. Chem., Vol. 249, P. 97 (1988).