Preparation and cyclic voltammetry studies on nickel-nanoclusters containing polyaniline composites having layer-by-layer structures

Electrochimica Acta 51 (2005) 984–990

Preparation and cyclic voltammetry studies on nickel-nanoclusters containing polyaniline composites having layer-by-layer structures Tran Trung a,∗ , Tran Huu Trung a , Chang-Sik Ha b a

Department of Electrochemical Technology and Metal Protection, Faculty of Chemical Technology, Hanoi University of Technology, Hanoi 10-000, Viet Nam b Department of Polymer Science and Engineering, Pusan National University, Pusan 609-735, South Korea Received 17 October 2004; received in revised form 1 March 2005; accepted 15 April 2005 Available online 22 August 2005

Abstract We suggest a novel route for the preparation of organic conductive polymer composites that are doped by the programmable controlled dispersion of nickel-nanoclusters into a polymer matrix having structure of layer-by-layer. The layered structures of polyaniline composites containing nickel-nanoclusters (PANI-Ni) were prepared electrochemically by a two-pot process in 0.1 M H2 SO4 and 0.5 M NiSO4 . We discuss on what is intrinsic nature of the mutual influences of the PANI chains and nickel-nanoclusters within a PANI-Ni film, on the change in structural morphology, and on the broadening and shifts of anodic waves to higher potentials during cyclic voltammetry. Also the role of nickel-nanoclusters as a source supplying protons to promote the protonation to form polaron and bipolaron charge carriers of PANI was suggested. © 2005 Elsevier Ltd. All rights reserved. Keywords: Polyaniline; Nickel-nanoclusters; Layer-by-layer structure; Conductive polymer composite; Cyclic voltammetry; SEM

1. Introduction Electrochemically incorporation of micron- or submicronsized particles of metal [1–4] and transition metal oxides [5–9] into the whole of polymer matrix of conjugated polymers (polypyrrole, polythiophene, and polyaniline) created metal/polymer composites having increased conductivity and special catalytic properties. In some cases, the particles of transition metal oxides, MnO2 and LiMn2 O4 , can be oxidized during electropolymerization, in which the manganese atoms are transformed into the higher Mn+6 oxidative state. Consequently, a strong hybrid is formed between the unfilled d orbitals of the manganese atom and the ␲ electrons of the polymer chains and/or the lone pair of electrons of the nitrogen atoms [5]. For all the mentioned above, the particles were prepared before they could be used as dopants for incorporation into polymer film. On other hand, the incorporation ∗

Corresponding author. Tel.: +84 4 8680122 E-mail address: tr [email protected] (T. Trung).

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of metal particles into the conducting polymers was carried out by directly reducing metal species, i.e., metal cations [10–13], or metal anion complexes [14,15]. The standard redox potentials (Pt, Pd, Ag in Refs. [10–12]), however, are enough positive (polymer layer is conductive at the deposition potentials of metals), or at least the potential window of electrodeposition overlapping with the conducting-to-insulating transition region, for example in the case of electrodepostion of copper in PANI layer [13]. In contrast to such the depositions, the electrodeposition of nanoclusters of metals (e.g., nickel and iron, which have standard redox potentials that are very negative versus the Ag/AgCl electrode) into organic conductive polymers is problematic. This challenge is due to the large differences between the redox potentials of such metals, which are significantly more negative than to the potentials, usually ranging from 0.2 to 1.2 V versus the Ag/AgCl electrode, used to electropolymerize aniline, pyrrole, and thiophene monomers. In this study, we attempted to find a route, using an electrochemical approach, by which we can overcome this

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Fig. 1. Schematic representations of the layer-by-layer structure of a polymer composite film (A) and a polymer composite film in which the particles are dispersed throughout the whole of the polymer matrix (B), and several aspects of the interconnection of metal-nanoclusters and polymer fibrils (C).

problem and control the distribution of particulates in the whole of polymer matrix or in alternately layered structured polymer films to prepare polyaniline composite films containing nickel-nanoclusters in layer-by-layer structures (Fig. 1). Also, the mutual influences of nickel-nanoclusters and PANI chains/fibrils during electrochemical process were investigated by cyclic voltammetry studies.

2. Experimental 2.1. Chemicals and apparatus Layer-by-layer-structured composites of polyaniline containing nickel-nanoclusters were prepared electrochemically by a two-pot process. The controlled electropolymerization system for the preparation of polyaniline and the controlled electrodeposition system for incorporating nickelnanoclusters into PANI film were composed of a potentiostat, a EG&E Princeton Applied Research model 362 incorporating the Ecuniv-HH5 program, connected to a standard three-electrode cell containing an aqueous solution of either 0.1 M aniline monomer or 0.5 M nickel sulfate, respectively. The potential applied to the PANI composite films, which were electrodeposited onto a platinum electrode (S = 1 cm2 ) serving as working electrode, was relative to an Ag/AgCl reference electrode for all electrochemical measurements; a platinum sheet serving as an auxiliary electrode. All chemicals used in our experiments were of AR grade and supplied by Merck. Doubly distilled water and the electrolyte solutions were deoxygenated by bubbling nitrogen gas before and during the experiments. The presence of nickel in the obtained PANI composite films was confirmed by X-ray diffraction measurements (the incident angle kept constantly at 1◦ ), by using a D5005 Diffractometer (Siemens, Germany). The surface morphology of the films was investigated by scanning electron microscopy (SEM; JEOL model JSM-

5410LV), coupled with an energy dispersion X-ray spectroscope (EDS). 2.2. Procedures used to prepare the PANI films having layer-by-layer structure Nickel-nanoclusters containing polyaniline composites having layer-by-layer structures were electrosynthesized by using two-pot approach. Firstly PANI layer were electrodeposited onto a working electrode in aqueous solution containing 0.1 M H2 SO4 and 0.1 M aniline monomer, by cyclic potential sweep from initial potential of +0.2 V to upper potential of +0.8 V versus Ag/AgCl, with scanning rate of 100 mV s−1 , and in two cycles. The electrodeposition of nickel-nanoclusters was then taken place on the PANIlayered electrode in second pot containing 0.5 M NiSO4 , also by potential sweep ranging from −0.4 to −0.7 V, with scanning rate of 100 mV s−1 for two cycles. Such the electrodepositions were repeated alternately to prepare covered nickel-nanoclusters containing polyaniline electrodes having structure shown in Fig. 1A, and having as-desired number of nickel-nanocluster layer.

3. Results and discussion It is well known that PANI films can be electrooxidized during a voltammetry potential sweep to exist in a variety of forms that differ by their oxidative levels. The principle neutral forms of PANI are the most-reduced form, commonly called leucoemeraldine, the fully oxidized form, pernigraniline, and the half-oxidized form, emeraldine. The oxidation state of a PANI film on the working electrode immersed in aqueous acidified solution depends on the applied potential (Fig. 2). As observed generally in cyclic voltammograms of PANI in aqueous acidified solution, the cyclic voltammograms of a

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Fig. 2. A typical multicyclic voltammogram of PANI film recorded in H2 SO4 solution at a potential scanning rate of 50 mV s−1 , which shows the formation of oxidative forms of PANI.

PANI film conducted in 0.1 M H2 SO4 at a potential scanning rate of 50 mV s−1 consists of two main oxidation peaks. The first maximum, at about 0.2 V versus Ag/AgCl (Fig. 2), corresponds to the oxidization of leucoemeraldine to emeraldine, and the second maximum at the higher potential of 0.7 V is attributed to the oxidization of emeraldine to pernigraniline. There also exists an obtuse peak (a shoulder) in the middle of these two peaks, as has been reported previously [16],

which signifies that reactions coexist between the aniline • nitronium radical (C6 H5 NH ), the aniline nitronium cation (C6 H5 NH+ ) and the nitronium radical cation (–C6 H4 N+ –) • or (–C6 H4 NH + –) in the PANI matrix and between PANI chains themselves, through the substitution of a nitronium cation in another PANI chain. Thus, Fig. 2 suggests that the transformation from emeraldine to pernigraniline occurs simultaneously with the formation of PANI charge carriers consisting of polaron (radical cation) and bipolaron (dication) forms delocalized on PANI chains. By such cross-linking reactions during electropolymerization of aniline, branched PANI chains are prepared and twisted together to form the branched PANI fibrils. Fig. 3 displays that there is a large difference between the surface morphology of the PANI film and the PANI film containing nickel-nanoclusters (PANI-Ni film). The PANINi film appears to exist as a number of coral-like branched polymer matrixes consisting of twisted polymer fibrils. In contrast, the PANI film displays a surface morphology akin to a “fishing net” with unit cells covered by slabs of PANI. Also there are several of the coil-like shaped PANI polymers, the slabs of compact PANI, presented within the PANI film (Fig. 3A). But they seem disappeared in the nickelnanoclusters containing PANI film. The changes in structural morphology, especially the disappearance of coil-shaped

Fig. 3. SEM images of (A) a PANI film and (B) a PANI-Ni film (magnification ×10,000), both the films electrodeposited on ITO electrode. The presence of nickel-nanoclusters in the PANI-Ni film was confirmed by EDS measurement.

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Table 1 Comparison of the current peak and the amount of the delivered charge of the anodic waves of during cyclic voltammetry of both PANI and PANI-Ni films Wave

Ipeak Q

PANE

PANI/outer-Ni

PANI/inner-Ni

First

Second

Middle

Ratio

First

Second

Middle

Ratio

First

Second

Middle

Ratio

7.6 32.6

2.4 7.1

0.8 2.6

∼9.5:3:1 ∼12.5:2.7:1

7.2 37.4

4.5 14.9

1.6 3.8

∼4.5:3:1 ∼9.8:4:1

8.5 64.7

– –

0.7 1.2

∼12:1 ∼54:1

[Ipeak ] = mA cm−2 and [Q] = mC cm−2 .

PANI polymers (Fig. 3A and B), signify that there would be changes in population of charge carriers and neutral in the PANI-Ni film in comparing with those in PANI film. This point was also confirmed by the change in the amount of the delivered charge of the anodic waves corresponding to the transformations of PANI-oxidative forms (Figs. 2 and 5), and in their ratios (Table 1). To elucidate the influence of nickel-nanoclusters incorporated in the PANI-Ni film, the following experiments were carried out: a layer of nickel electrodeposited on platinum was dissolved in 0.1 M H2 SO4 under multicyclic voltammetry (Fig. 4) in order to observe electrochemical behavior of the dissolution and its difference in comparing with those of nickel-nanoclusters in PANI-Ni film. Fig. 4 displays that the dissolution of the electrodeposited nickel began at −0.12 V and maximum at about −0.01 V, was only observed during anodic scanning of the first cycle of the multi-cycle 0 voltammogram (ENi 2+ /Ni = −0.23 V versus NHE [17a], or 0 ENi2+ /Ni = −0.45 V versus Ag/AgCl electrode). The remaining cycles, which do not exhibit the peak for the dissolution of nickel, have almost the same shapes and resemble electrochemical behavior of the platinum electrode under the same conditions, however quite different from the electrochemical behavior of covered PANI film electrode and of covered PANI-Ni film electrode. The dissolution of nickel layer appears to be complete at about 0.2 V (Fig. 4). Meanwhile, the first anodic wave in the multicyclic voltammogram of PANI begins at about −0.05 V and shows a maximum at 0.2 V (Figs. 2 and 5a). This

Fig. 4. Cyclic voltammograms indicating the dissolution of the Ni layer deposited on a Pt electrode at a potential scanning rate of 50 mV s−1 in 0.1 M H2 SO4 . The anodic wave appears only during the first cycle.

observation suggests that in the potential range of −0.05 and 0.2 V, the nickel-nanoclusters incorporated in the PANI-Ni film would be completely dissolved if it were not prevented from another processes occurring in PANI-Ni film. However, it could not happen, even though nickel-nanoclusters located on the outermost layer of PANI-Ni film. Nickel-nanoclusters are still presented in PANI-Ni film as confirmed by XRD and EDS measurements. It signifies that the dissolution of nickelnanoclusters incorporated in the PANI-Ni film is influenced by the electrooxidation of the PANI chains. It is reasonable to suggest that this dissolution, to be exactly the electrooxidation, may begin when innermost layer of PANI (the PANI first layer) transforms from an electrically insulating state to an electrically conductive state. Obviously, the transformation of innermost layer of PANI begins at −0.05 V (Figs. 2 and 5). Thus, we suggest that the electrooxidation of nickel atoms (belonged to nickel-nanoclusters) begins at −0.05 V. At higher potentials resulting from potential sweep, the electrooxidation of PANI chains located in the PANI second layer (see Fig. 1A) results in a flow of electrons toward the electrode-first-layer of PANI interface and may decreases the electrooxidation of nickel-nanoclusters located between the PANI first layer and PANI second layer. By a mutual influence, the electrooxidation of the nickel-nanoclusters also produces electrons and protons (see Eqs. (2)–(4)), which may cause two processes having positive influences on conductivity of PANI matrix. The first one may promote the formation of resonance form of polaron of PANI. The second may also promote the protonation, leading to the increase in population of charge carriers of PANI (the polaron and bipolaron, see Fig. 6), as exhibiting via the increase in the amount of the charge delivered through electrode surface (Table 1). Obviously, the transformation of PANI chains from leucoemeraldine form to emeraldine form become more intensively than before due to the presence of nickel-nanoclusters (Figs. 5 and 7). In the multicyclic voltammogram of the PANINi film (Fig. 5b), the anodic current is significantly increased relative to that of the PANI film (Fig. 5a), such that the first anodic wave is shifted to the left. This shift also indicates that the electrooxidation of nickel-nanoclusters is almost complete at the potential of the maximum of the first anodic wave. In fact, the first recorded anodic wave is an integrated line and arisen from both the electrooxidation of PANI chains and the electrooxidation of nickel-nanoclusters. Thereby, at a potential of about 0.10 V, the slope of the first anodic wave begins to change considerably downward (Fig. 5b), suggesting that the electrooxidation of nickel-nanoclusters may be maximized at

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sition of nickel, the formation S2− anions may be occurred as follows: SO4 2− + 10H+ + 8e− → H2 S + 4H2 O (E0 = 0.30 versus NHE [17b])

(1)

As predicted, the obtained XRD data (not presented here) show the coexistence of NiO(H2 O), Nix Sy , and NiHz in the PANI-Ni films, which leads us to propose the oxidation of nickel-nanoclusters during electropolymerization:

Fig. 5. Multicyclic voltammograms conducted in 0.1 M H2 SO4 solutions at a potential scanning rate of 50 mV s−1 of (a) a PANI film, (b) a two-layer PANINi film comprising an inner PANI layer and an outer nickel-nanocluster layer, and (c) a three-layer PANI-Ni film comprising a layer of nickel-nanoclusters sandwiched between inner and outer layers of PANI.

this potential, but decreases intensively thereafter. Although the nickel-nanoclusters layer was covered by the PANI outer layer (Fig. 5c) the first anodic wave still begins at −0.05 V, but reaches a maximum at 0.27 V, which is considerably larger than potential value of 0.2 V the first anodic peak of PANI cyclic voltammogram reached (Fig. 5a). The anodic shift of the first anodic peak is due to that for reaching such the same oxidation level, it (Fig. 5c) must take the times longer than those for PANI-Ni film in the cases of Fig. 5a and b. Also Fig. 5b and c displayed that in both multicyclic voltammograms the intercepts ranged from −0.05 to 0.07 V were almost identical. It means that in the initial stages of electrooxidation the transformation from an electronically insulating state to an electronically conductive state in the innermost layer of PANI in both kinds of PANI-Ni films (having or lacking the outermost layer of PANI) is expected to be almost the same, before the electrooxidation of nickelnanoclusters begins. When the electrooxidation of nickelnanoclusters begins, both electrooxidations begin to become mutually influencing. Very interestingly, in each multicyclic voltammogram (Figs. 5b and c and 7a–c) we observed no change in shape over all cycles, whereas there is a significant difference in shape of the first cycle with respect to the latter ones in the multicyclic voltammogram obtained (Fig. 4) for the dissolution of the nickel layer deposited onto the Pt electrode, in which the anodic wave in the first cycle exhibits a maximum at −0.01 V and the dissolution of nickel layer into H2 SO4 solution disappears in the following cycles. The findings described above signify that the nickel-nanoclusters in the PANI-Ni films were oxidized into insoluble forms, but do not form discrete Ni2+ cations, which can diffuse in and out easily. Thus, we believe that either a special bonding occurs between nickel atoms and PANI chains or that the electrooxidation of nickel-nanoclusters is accompanied by chemical reactions taking place to form insoluble products, and that both processes occur. Indeed, during the electrodepo-

Ni + xH2 O → NiO(H2 O)x−1 + 2H+ + 2e−

(2)

2Ni + yH2 O → NiH2 + NiO(H2 O)y−1

(3)

Ni + 2S2− + 4H2 O → NiS + SO4 2− + 8H+ + 10e− (4) Excepted for the formed insoluble products, the local concentration of protons increases by those arising from the electrooxidation, which promote the protonation of PANI chains to form charge carriers of PANI (i.e., the polarons and bipolarons), increasing the conductivity of PANI matrix, then. It means that nickel-nanoclusters play a role as source supplying protons for protonation (Fig. 6). More interestingly, nickel-nanoclusters also play as a location to keep water molecules out of reactions with PANI chains (these reactions can shorten the spatially extended ␲-bonding system). Fig. 6 illustrates schematically the structures in mutual influences involved in the processes of electrooxidation of the nickelnanoclusters, the generation of principle forms and charge carriers of PANI and the protonation. Thus, three mentioned reactions themselves lead to changes in the ratio of the relative intensities of both the anodic waves and the middle peak, as well as in the amount of charge delivered by each anodic wave. Of course, these processes are also related to the transformations of the PANI chains during electrochemical process. As displayed in Fig. 7a–c, the broadening and shift of anodic waves, and the shapes of the cyclic voltammogram change obviously as the number of layers of nickelnanoclusters in the PANI-Ni film increases, which is quite different from that observed in the normal cyclic voltammogram of PANI (Figs. 2 and 7d) in an aqueous acidified solution. The transformation of leucoemeraldine to emeraldine and the interconnection of emeraldine fibrils result in electrically conductive zones of the first PANI layer (i.e., the innermost layer of PANI or the first layer directly interconnected with the metal electrode), which are interconnected to the nickel-nanoclusters and to the PANI fibrils of the second layer. These processes continue until the PANI fibrils of the outermost PANI layer interconnect together to form a homogeneous electrically conductive zone. Thus, as the number of layers increases, including the PANI layers and layers of nickel-nanoclusters, the innermost layer of PANI reaches the maximum oxidative level first, followed by the next PANI layer, and so on. A striking point here

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Fig. 6. Schematic representation of the structures involved in the generation of principle forms and charge carriers of PANI and the protonation, as well as an illustration of the role of the nickel nanoclusters.

is that such leucoemeraldine-to-emeraldine transformation in the PANI first layer of the PANI-Ni films (Fig. 7a–c) is more intensively than one in PANI film (Fig. 7d). The edge of such the transformation shifted on the left obviously. It should be considered as a directly evidence for the mentioned promotions due to the presence of nickelnanoclusters. Indeed at potential smaller 0.2 V, corresponding to leucoemeraldine-to-emeraldine transformation, the number of electron (observed current) delivered through metallic substrate from PANI-Ni film (Figs. 7a–c and 5b and c) is considerably larger than one from PANI film (Figs. 7d and 5a). It signifies that during potential sweep from −0.2 to 0.2 V the presence of nickel-nanoclusters promoted intensively the leucoemeraldine-to-emeraldine transformation of PANI fibrils, which were of PANI first layer and directly interconnected to metallic substrate, even though of PANI fibrils in insulating. It also means that the propagation of conductive

Fig. 7. Multicyclic voltammograms of (a) PANI-Ni films consisting of one, (b) two, and (c) three layers of nickel-nanocluster, and (d) of PANI film (no layers of nickel nanoclusters).

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zones was induced intensively by the presence of nickelnanoclusters. 4. Conclusions The PANI-Ni films, structured in a layer-by-layer style, were prepared successfully in a two-pot process. This is a new route for the preparation of organic conductive polymers incorporating layers of nanoclusters of dopants in a polymer matrix. The presence of the nickel-nanoclusters in the PANI-Ni films is the main cause of the change in structural morphology of PANI matrix and the broadening and shifts of the first anodic wave, as well as arising new processes mutually influenced on the electrooxidation of PANI chains. In these mutual influences, the nickel-nanoclusters play a role as bridging for electron hoping of PANI chains/fibrils, and as a source supplying protons for protonation in order to form charge carriers of PANI, and electrons for promoting the delocalization of ␲-bonding electrons of polymer matrix, meaning promoting the formation of resonance forms of polaron of PANI (Fig. 6). Further work is required on these polymer composites to address on their possible applications and to determine the mechanisms of the processes that coexist during electrooxidation. Acknowledgements The work was supported by the state basic research project KHCB55-01-03 funded by Ministry of Science and Technology of Vietnam and the National Research Labora-

tory Program, the Center for Integrated Molecular Systems, POSTECH, Korea, and the BK21 Project.

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