Fourier transforms infrared study on the role of nickel-nanoclusters incorporated in polyaniline films having structure of layer by layer

Journal of Chemistry, Vol. 44 (1), P. 128 - 132, 2006<br /> <br /> <br /> Fourier transforms infrared study on the role of<br /> nickel-nanoclusters incorporated in polyaniline<br /> films having structure of layer by layer<br /> Received 26 July 2005<br /> Tran Trung<br /> Department of Electrochemistry, Faculty of Chemical Technology, HUT<br /> <br /> <br /> summary<br /> Fourier transform infrared (FTIR) studies showed the changes in a relative intensity of<br /> stretching vibrations of the PANI and PANI-Ni films, in a band of 1480 and 1590 cm-1, as<br /> well as those in a band of 1200 and 1350 cm-1, and evidenced the conversion of quinoid form<br /> of PANI to bipolaron structure, and finally to the more delocalized polaronic sites during<br /> protonation. This also shows that nickel-nanoclusters play a role as a source supplying<br /> protons to promote the protonation, giving a change in population of quinoid and aromatic<br /> forms of PANI, and increasing a number of NH bond. By the promoted protonation, the<br /> pernigraniline base and emeraldine base that were formed during electrosynthesis will be<br /> transformed into emeraldine salt forms having NH2+ groups breaking the coupling between<br /> the P-electrons of nitrogen, and making new positive sites. This may make advantages for the<br /> shuttling of electrons between polaronic sites consisting of C-N+ and C-N+•. Two processes,<br /> on which the conversion of quinoid-to-aromatic form of PANI can be done, are proposed as<br /> follows: i- during the one-step process the conversion occurs simultaneously in the coupling<br /> several segments of the same kind of PANI; ii- meanwhile the two-step process, the<br /> conversion and the coupling occur alternatively.<br /> Key words: FTIR, PANI, Ni-nanoclusters, conversion, protonation.<br /> <br /> I - Introduction source to promote an internal molecular electron<br /> transfer and also as a bridging center for<br /> So far, polyaniline (PANI) is still one of the shuttling electronic charge between polymer<br /> most widely studied organic conducting chains via a strong hybrid of the unfilled-d<br /> polymers, because an important concern of orbitals of transition metal oxide and the lone<br /> practical applications of PANI is the strongly pair of electrons of heteroatoms in polymer<br /> dependence of its conductivity on the synthesis chains, such as nitrogen atoms [6], or via<br /> approach and its parameters, nature of electrostatic interaction between doping<br /> electrolyte and of dopants, etc. This gives a molecular anions and potonated imine centers of<br /> number of various approaches to prepare a lot of PANI [7].<br /> kind of polyaniline composites in various The deep insights into electrode processes<br /> morphology and structures [1 - 5]. Especially, may be laid on IR-based techniques that have<br /> nanoparticles that are incorporated in been considered as useful tools in order to<br /> conducting polymer matrix are known as a investigate any electrochemical system. Since<br /> <br /> 128<br /> 1981, IR spectroscopy had been used to study containing polyaniline films (PANI-Ni) having<br /> the potential-dependent population of the structure of layer by layer were electrodeposited<br /> species in the double layer formed at a Pt on a platinum sheet under conditions that are<br /> electrode immersed in CH3CN/0.1 M described in elsewhere [2]. For FTIR<br /> LiClO4/(n-butyl)4NBF4 [8, 9]. Now there is a lot measurements, both the PANI and PANI-Ni<br /> of works [10 - 13], in which IR spectroscopy films were washed in acetone for several times,<br /> has been offering a major contribution yielding then scrapped from the working electrode and<br /> information on species at or near an electrode, heated at 45oC for 1 hour, under nitrogen<br /> such as the determination of molecular atmosphere. A thermonicolet NEXUS 870<br /> symmetries; the identification of functional spectrometer was used to record the FTIR<br /> groups, or compound identification; the nature spectra of both PANI and PANI-Ni.<br /> of chemical bond. FTIR study also offers further<br /> information of intramolecular forces acting III - Results and discussion<br /> between atoms of molecule; the intermolecular<br /> forces. The obtained FTIR spectra of PANI (Fig. 1)<br /> In previous work [2] the effects of nickel and PANI-Ni (Fig. 2) films, respectively show<br /> nanoclusters incorporated in PANI matrix on the stretching vibrations at the 1581 and 1486<br /> protonation and formation polaron lattice, as cm-1 bands and at the 1570 and 1481 cm-1 bands,<br /> well as on the conversion of quinoid rings to that are characteristics of non-symmetric C6 ring<br /> aromatics were investigated by XRD, SEM and stretching modes of quinoid and aromatic forms<br /> cyclic voltammetry. Here, a deeper insight into of PANI [14 - 16]. The relative intensities of<br /> the role of such nickel nanoclusters may be these bands (1486/1581 and 1481/1570) point<br /> given by FTIR studies on the polyaniline film towards the oxidative state of the polymer,<br /> having structure of layer by layer, via the which was resulting from the conversion of<br /> change in population of charge carriers of aromatic rings to quinoid during electro-<br /> PANI. polymerization. These relative intensities also<br /> involve in the protonation, on which the<br /> pernigraniline base and emeraldine base of<br /> PANI will be transformed to emeraldine salt<br /> forms [2]. The influences of nickel-nanoclusters<br /> on these transformations will be discussed<br /> further as follows.<br /> <br /> <br /> <br /> <br /> Figure 1: FTIR spectrum of polyaniline that was<br /> electrosynthesized in aqueous H2SO4 (pH = 3)<br /> solution containing 0.1 M aniline monomer,<br /> under cyclic voltammetry, in the potential range<br /> of 0.2 and 0.8V vs. Ag/AgCl, at a potential<br /> scanning rate of 100 mV.s-1<br /> Figure 2: FTIR spectrum of polyaniline<br /> II - Experimental containing nickel-nanoclusters was electro-<br /> synthesized via a two-pot process, the detailed<br /> Polyaniline films and nickel-nanoclusters- procedures for preparation is shown in [2]<br /> <br /> 129<br />  Like an ideal polyaniline, a relative intensity charges). Also due to the formation of NH2+<br /> (1486/1581) of the bands at 1486 and 1581 cm-1 groups breaking the coupling between the<br /> in the FTIR spectrum of the PANI film (Fig. 1) P-electrons of nitrogen, new positive sites<br /> is slightly less than unity (about 0.96). resulted in. This may make advantages for the<br /> Therefore it can be said that the obtained PANI shuttling of odd electrons between polaronic<br /> is mostly in the emeraldine form. However, due sites (Fig. 3), and consequently a polaron lattice<br /> to the presence of nickel-nanoclusters in the of PANI was composed.<br /> PANI-Ni film, there is a noticeable shift of such<br /> the stretching vibrations to the lower wave-<br /> numbers, and centered at the 1481 and 1570 cm-<br /> 1<br /> . As seen in Fig. 2, a relative intensity<br /> (1481/1570) is of 1.14 significantly larger than<br /> unity. This evidences that a part of the quinoid<br /> rings was converted into aromatic rings.<br /> Consequently, the number of aromatic ring<br /> increased, meanwhile the number of quinoid<br /> ring decreased. This change in population is<br /> involved in transformations of the emeraldine<br /> and pernigraniline base to emeraldine salts that<br /> are coupled with protonation. Obviously, the<br /> protonation requires protons. So, where protons<br /> come from? In previous work [2], it was<br /> revealed that nickel-nanoclusters play a role as a<br /> local source supplying protons for such the<br /> conversion in PANI-Ni film. Protonation that<br /> promoted by the presence of nickel-nanoclusters<br /> in PANI-Ni film not just gives a change in Figure 3: The schematically illustration shows a<br /> population of quinoid and aromatic forms, but role of nickel-nanoclusters as a source supplying<br /> also increase a number of NH bond in PANI protons for protonation, on which the quinoid<br /> matrix. Indeed, except for the absorption forms of PANI are concerted to aromatics<br /> intensities are significantly greater due to the<br /> presence of nickel-nanoclusters, the appearance Beside this, the C-N stretching vibrations of<br /> of a wider band centered at 3213 cm-1 (Fig. 2) secondary aromatic amines have already been<br /> instead of two separately bands centered at the observed in the region of about 1200 and 1350<br /> 3233 and 3431 cm-1 (Fig. 1) evidenced a higher cm-1 (Figs. 1-2). In this region of frequency, the<br /> concentration of NH group in the PANI-Ni film. bands at 1306 cm-1 and 1249 cm-1 are<br /> This mention, as well as the stretching vibration respectively attributed to C-N+ stretching of<br /> band at 2930 cm-1 (Fig. 2) that presents NH2+ in secondary amines [16, 20, 23] and C-N+•<br /> C6H4 NH2+ C6H4 [18, 19], both revealed a stretching vibrations [21 ÷ 23], that were<br /> greater degree of oxidation and hence greater strengthened during the protonation of PANI<br /> amount of emeraldine salts. Also, the red shift chains. The later is associated with the<br /> of the wider band 3213 cm-1 from 20 to about formation of a polaron lattice from the<br /> 210 cm-1 that observed in FTIR spectrum of protonated bipolaron structure of PANI forms<br /> PANI-Ni film may be associated with the higher (Fig. 3), corresponds to the vibration of<br /> degree of electron delocalization [16, 17]. This polaronic sites and indicates the presence of<br /> result reveals that the intrinsic oxidative state of more delocalized polarons. Whilst, the first one<br /> the PANI is associated with the electronic is related to a transformation, due to the<br /> absorption of protonated forms of PANI by free pronation, of secondary amines from quinoid<br /> carriers (unpaired electrons and positive form to bipolaron structure, and associated with<br /> <br /> <br /> 130<br /> the more localized polaronic sites. Interestingly, directly evidences for the conversion of quinoid<br /> a noticeable change in the relative intensity rings into aromatics.<br /> (1306/1249) from 1.8 in PANI film to 1.5 For further illustration of the significant<br /> (1307/1247) in PANI-Ni film was observed. conversion of quinoid rings of PANI to<br /> This evidenced that the populations of the aromatics, we can propose that there exist two<br /> polaron structure and the protonated bipolaron processes that are highly competitive to do<br /> increased. It means that a quinoid-to-aromatic occurrence. The first one is a two-step process,<br /> conversion is strengthened due to the presence on which a part of or a segment of quinoid<br /> of nickel nanoclusters. Such the change in converted into an aromatic (Step 1), and<br /> population also revealed in the change of the accompanied with the coupling of a remaining<br /> 1130 cm-1 strong and obtuse band (Fig. 1) that is part to another quinoid segment (Step 2, see Fig.<br /> characteristic of the stretching vibration of the 4a). The second is a one-step process, on which<br /> NH+ and NH+= structure, both performed the conversion of a quinoid segment into an<br /> by protonation [16, 24]. This band is considered aromatic occurs simultaneously in the coupling<br /> as a measure of the degree of delocalization of of performed aromatic segments and existing<br /> electron in PANI matrix and referred to as the aromatic segments (Fig. 4b). For both processes,<br /> electronic like absorption. With presence of the length of several segments of quinoid may<br /> nickel nanoclusters in PANI matrix, this band increase, but the number of quinoid segment<br /> was split into two bands at 1144 cm-1 and 1108 and also of quinoid rings must decrease. And of<br /> cm-1 (Fig. 2), which are characteristic of course, the number of aromatic ring and the<br /> Ar NH+ Q and Q NH+ = Q bonds (where Ar length of an aromatic segment increase. In the<br /> refers to the aromatic type rings and Q to the nature, the number of polaronic site<br /> quinoid type rings). The higher intensity of the significantly increase and a polaron lattice is<br /> band at 1144 cm-1 in comparison with the one of performed then in PANI-Ni.<br /> the 1108 cm-1 band is also considered as one of<br /> <br /> A Q A Q A Q aA Q A Q A A Q A Q A Q AaA Q A Q A<br /> Conversion of a part of any A part of a quinoid segment<br /> quinoid segment into aromatic converted into aromatic<br /> <br /> A Q A Q A Q A Q A Q A A Q A Q A Q A Q A Q A<br /> Conversion of a quinoid segment<br /> <br /> A Q A Q A Q AAA Q A A Q A Q AAA Q A Q A<br /> Coupling<br /> <br /> <br /> A Q Q AA Q AAA Q A<br /> (a) (b)<br /> A or a: notice of aromatic segment; Q : notice of quinoid segment.<br /> Figure 4: The schematically depiction of two proposed processes, on which the conversion<br /> of quinoid form of PANI to aromatics occurred; a two-step process (a) and a one-step process (b)<br /> <br /> IV - Conclusion frequency region. This may give a deeper<br /> insight into the role of nickel-nanoclusters as an<br /> The conversion of quinoid rings to additional source supplying protons for<br /> aromatics in PANI matrix due to the presence of protonation, on which the polaron lattice of<br /> nickel-nanoclusters has been studied by FTIR PANI was performed in a higher degree of<br /> spectroscopic measurements on PANI and electron delocalization.<br /> PNAI-Ni samples in a 400 ÷ 4000 cm-1 The protonation promoted by nickel-<br /> <br /> 131<br /> nanoclusters incorporated in PANI matrix was Materials, Vol. 13, P. 309, (Proceedings of<br /> also evidenced by the appearance of a wider EUROMAT'99), Ed. by K. Graissie, E.<br /> band centered at 3213 cm-1 (Fig. 2) instead of Tenckhoff, G. Wegner, J. Hau elt and H.<br /> two separately bands centered at the 3233 and Hanselka; Published by VCH-Wiley (2000).<br /> 3431 cm-1 (Fig. 1) and also the appearance of 7. V. P. Parkhutik, E. Matveeva. J. Phys.<br /> stretching vibration band at 2930 cm-1 that is Chem. B, 102,1549 (1998).<br /> characteristic of NH2+ in C6H4 NH2+ C6H4 8. T. Davidson, S. Pons, A. Bewick, P. P.<br /> segments. Shmidt. J. Electroanal. Chem., 125, 237<br /> The considerably change from 0.96 up to (1981).<br /> 1.14 in the relative intensity (1486/1581) and 9. S. Pons, T. Davidson, A. Bewick. J.<br /> (1481/1570) of the stretching bands Electroanal. Chem., 140, 211 (1982).<br /> characteristic the oxidative state of the polymer,<br /> 10. H. Neugerbauer, G. Nauer, A. Neckel, G.<br /> as well as the noticeable change in the relative<br /> Turrillon, F. Garnier, P. Lang. J. Phys.<br /> intensity (1306/1249) from 1.8 in PANI film to<br /> Chem., 88, 652 (1984).<br /> 1.5 (1307/1247) in PANI-Ni film are considered<br /> as the evidences for the significantly conversion. 11. H. Tabil, B. Humbert, D. Billaud,<br /> Spectrochim. 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