A novel bilayer film material composed of polyaniline and poly(methylene blue)

Materials Letters 59 (2005) 3913 – 3916 www.elsevier.com/locate/matlet

A novel bilayer film material composed of polyaniline and poly(methylene blue) Xiang Li b, Ming Zhong b, Cheng Sun b,*, Yongming Luo a b

a Institute of Soil Science, Chinese Academy of Science, Nanjing 210008, PR China State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210093, PR China

Received 28 April 2005; accepted 14 July 2005 Available online 2 August 2005

Abstract Electrically conducting polyaniline(PAn)/poly(methylene blue)(PMB) bilayer film was prepared by electrochemical polymerization of methylene blue on the PAn-coated Pt working electrode. The bilayer film electrode exhibits higher electrocatalytic activity towards oxidation of methanol than PAn electrode and PMB electrode in neutral and alkaline solutions. FTIR spectroscopy reveals PAn-unit is a main unit in the structure of the bilayer film. In addition, the PAn/PMB bilayer film has also been characterized by scanning electron microscopy (SEM), and X-ray photoelectron spectra (XPS), respectively. D 2005 Elsevier B.V. All rights reserved. Keywords: Bilayer film; Polyaniline; Poly(methylene blue); Electrocatalytic behavior; Methanol

1. Introduction Recently, the fabrication of conducting polymer modified electrodes has attracted considerable interest because of their importance in the electrocatalytic field [1– 5]. Polyanilines have won particular attention for their high conductivity, good redox reversibility and excellent stability [6– 8]. These properties make them very useful in the preparation of modified electrodes and bring about favorable catalytic properties such as the oxidation of methanol. However, the energy density and catalytic activity of polyaniline are limited by pH value, while polyaniline (PAn) has little electrochemical activity at pH > 4. So, how to design new kinds of PAn and successfully immobilize them on electrode surfaces, while maintaining and enhancing their beneficial properties in alkaline solutions, is fascinating to chemists. The bilayer film technology has recently been reported to improve the properties of modified electrode through layer-

* Corresponding author. Tel.: +86 25 83593109; fax: +86 25 83707304. E-mail address: [email protected] (X. Li). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.07.033

by-layer self-assembly, based on many different inorganic and organic materials [9 –14]. This technique is shown to be a rapid and easy way to obtain ordered molecular assemblies with precise control of layer composition and thickness. Lupu et al. recently reported the preparation of two ultrathin multilayer films composed by prussian blue and conducting polymer [15]. The results display that the bilayer film possesses more excellent character than the relevant single layer film. This gives us a clue that methylene blue (MB) could electropolymerize on the PAn electrode and form a bilayer film with new characteristics. Surprisingly, there is no report in the literature on the bilayer films based on two different organic conducting polymers. For this, it will be of critical importance to successfully incorporate the functional properties of polyaniline-based bilayer films into the ultrathin material field. Furthermore, this new material is also hoped to be easily obtained and possesses high electrocatalytic activity. The present report focuses mainly on the design and properties of a new PAn/PMB material. The electrocatalytic behavior of PAn/PMB modified electrode towards oxidation of methanol was studied in detail in neutral and alkaline

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X. Li et al. / Materials Letters 59 (2005) 3913 – 3916

-0.03 0.00

i/mA

2.2. Preparation of the PAn/PMB bilayer film

3 2 1

0.03

3 0.06

2

0.09

1 -0.6

-0.3

0.0

0.3

0.6

E / V ( vs. SCE ) Fig. 1. Cyclic voltammograms at PAn/PMB film modified electrode in (1): 0.1 M phosphate buffer solution (pH 7.0), (2): (1) + 1 mM methanol and (3): (1) + 2 mM methanol. Scan rate: 50 mV s 1.

medium. In addition, the relative characterizations by SEM, FTIR and XPS have also been carried out.

The fabrication of the PAn/PMB bilayer film is described as follows. The Pt electrodes were all cleaned by chromic acid lotion. Aniline was deposited onto the cleaned electrode by cyclic voltammetry at 0.2¨+1.0 V with 50 mV s 1, from an aqueous solution containing 0.2 M aniline and 0.5 M H2SO4. The cycling number is three. After deposition, the modified electrodes were rinsed with 0.1M PBS solution (pH 7.0) and then immersed into a solution containing 1 mM methylene blue and 0.1 M PBS (pH 7.0), where the electrode potential was cycled at 0.4¨+ 1.3 V with 50 mV s 1 for 10 times, until a PAn/PMB bilayer film was obtained. In reverse, it results in a PMB/PAn bilayer film. In addition, PAn was prepared in the solution consisting of 0.2 M aniline and 0.5 M H2SO4 at 0.2¨1.0 V, while PMB was obtained in 0.1 M PBS (pH 7.0) containing 1mM MB with the potential between 0.4 and 1.3 V.

2. Experimental 3. Results and discussion

2.1. Reagents and apparatus

-0.06

C c a A

-0.03

i/mA

Aniline, methylene blue and other chemicals used in this work were obtained at the reagent grade. Doubly distilled water was used for all experiments; 0.1 M phosphate buffer solutions (PBS) at various pH values were prepared by mixing the stock solutions of NaH2PO4 and Na2HPO4 and then adjusting the pH with 0.1 M NaOH and H3PO4. The deposited films were analyzed using various characterization techniques. The surface morphology of the films was observed using a model X650 scanning electron microscopy (SEM) (Hitachi, Japan). The atomic composition and chemical binding states of the films were identified using X-ray photoelectron spectroscopy (XPS), which were carried by ESCALab MK2 spectrometer with an Mg Ka X-ray source (1253.6 eV). All binding energies were referenced to C1s neutral carbon peak at 284.6 eV. The film structures were determined by a Nicolet NEXUS 870 Fourier transform-infrared (FTIR) attenuated total reflection (ATR) spectrometer. Electrochemical measurements were made using a CHI 650A electrochemical workstation (CH Instrument Corp. USA) connected to a computer. A conventional three-electrode system was used. The working electrode was a polymer film modified Pt electrode, whose area was 5  5 mm2. A saturated calomel reference electrode (SCE) was used as the reference electrode and Pt foil as a counter electrode. The Pt was polished with Al2O3 paste, washed with distilled water then ultrasonicated in deionized water and acetone successively. A pHs-2C model acidimeter was used for pH measurement (made in Shanghai, China). The temperature for the electrochemical experiments was controlled at 25 -C.

In order to evaluate its eletrocatalytic activity, the PAn/PMB bilayer film was firstly fabricated with a modified electrode and immerged in 0.1 M PBS buffer (pH 7.0) with and without methanol. The correlative cyclic voltammograms were shown in Fig. 1. Upon addition of 1 mM methanol to the buffer, the anodic peak current at 0.25 V increased and the cathodic peak current decreased dramatically, indicating a typical electrocatalytic oxidation process of methanol. The increase in anodic current might be due to the fact that methanol in solution diffused to the electrode surface and reacted with the oxidized state of the polymer and then the reduced state polymer was formed, which led to a current increase in the anodic region. Furthermore, the oxidation peak current increased with the increasing methanol concentration. These phenomenons were not observed at PAn, PMB, PMB/PAn electrode in the above solution (pH 7.0) (Fig. 2). This shows the synthetical PAn/PMB is a novel material that can be used in neutral and alkaline medium. In addition, the stability of the composite film electrode was examined by measuring the decrease in voltammetric currents of the composite film electrode during

B b

0.00

Bb

0.03 0.06

C c

A

0.09

a

-0.6

-0.3

0.0

0.3

0.6

E / V (vs. SCE ) Fig. 2. Cyclic voltammograms of PMB/PAn, PAn, PMB film modified electrode in 0.1 M phosphate buffer solution with (a, b, c) and without (A, B, C) 1 mM methanol (pH 7.0). Scan rate: 50mV s 1.

X. Li et al. / Materials Letters 59 (2005) 3913 – 3916

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Fig. 3. Scanning electron microphotographs of PMB (A), PAn (B) and PAn/PMB (C).

potential cycling. For example, the film electrode was subjected in 0.1 M PBS solution (pH 7.0) to 100 potential cycles in the potential range of 0.6 to 0.6 V at 50 mV s 1, a decrease in the peak’s current of less than 10% was noticed. After soaking the composite film modified electrode in 0.1 M PBS solution (pH 7.0) for 10 days, almost no change of the electrochemical response was observed. It is well known that the electrocatalytic activity of an electrode is affected by its morphology [16]. To investigate their images, the reasons of their different electrocatalytic activities might be found. Before PMB deposition, the PAn surface is rough and caky (see Fig. 3B). After deposition of followed PMB, the yielded bilayer film consists of a tight net structure and a few cavities (Fig. 3C). In addition, the SEM image of the PMB shows an almost uniform and smooth glaze (Fig. 3A). It is clear that the morphology of the bilayer film is different from those of PAn and PMB. So to a certain extent, this also indicates that bilayer film electrode possesses preferable electrocatalytic activities to PAn and PMB. The results of SEM show that it is consistent with the above electrocatalytic research. FTIR spectroscopy is used to characterize the film structures. Fig. 4 shows the IR spectra of PAn, PMB and PAn/PMB bilayer film. There were six important absorption bands present in polyaniline spectrum (Curve 1). In 3000 – 3500 cm 1, this is the region of the N – H stretching vibrations. The band at 1587 cm 1 represented the C_N stretching of the quinoid ring. The band at 1499 cm 1 indicated the stretching of the benzenoid ring. The C – N stretching of the benzenoid ring appeared as the 1303 cm 1. The band at 1137 cm 1 reflected the in-plane C – H bending motion of the

quinoid ring and the band at 820 cm 1 was identified with the outof-plane bending of C – H bond in the aromatic ring. The peaks identified are consistent with previously published data [17]. PMB exhibits some different behaviours (Curve 3). There is the N – H stretching vibrations at 3000¨3500 cm 1. The peak at 1658 cm 1 is to be explained as phenyl ring stretching vibration, or a mixture of vibrations (t C_C and t C_N). The peak at 1264, 1103 and 1045 cm 1 is attributed to the bending vibration of N – C of phenothiazine ring and N – CH3 [18], respectively. The bands at 962 and 889 cm 1 were identified with the bending of C – H bond in the phenothiazine ring. Compared with PAn and PMB spectrum (Curve 1, 3), it can be found that the FTIR spectra of PAn/PMB bilayer film (Curve 2) is not a simple superposition of each individual polymer, while giving a similar PAn spectrum. This shows that PAn-unit is a main unit in the structure of the bilayer film. Furthermore, it also indicates that electrochemical properties of bilayer film are mainly attributed to polyaniline, but the bilayer has better electrochemical reversibility and catalysis in neutral solution than those of polyaniline. This is mainly caused by PMB on PAn, which plays an important role in the charge transfer between methanol and polyaniline. In addition, the deposition of composite film was also evident from the observed changes in growth rate, redox potentials, and transport mechanisms of deposited films in contrast to PAn and PMB. To relate the results of SEM, they all show that the bilayer is not a mixture of homopolymers. To identify the elemental composition of the multilayer film, the XPS of the film was measured (Fig. 5). Although the XPS measuremet gives only semiquantitative elemental composition, the presence of C, O, N, P and S elements in the film is confirmed. C1s

1 2

3

3600

3000

2400

1800

1200

600

/ cm-1 Fig. 4. FTIR spectra of PAn (curve 1), PAn/PMB (curve 2) and PMB (curve 3).

Relative intensity (c/s)

% Transmittance

60 k 50 k

O1s N1s

40 k 30 k 20 k P2p S2p

10 k 0

0

150

300

450

600

750

Binding energy (eV) Fig. 5. XPS spectrum of PAn/PMB film.

900

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X. Li et al. / Materials Letters 59 (2005) 3913 – 3916

Table 1 The bending energy of S2p elements in PAn, PMB, PAn/PMB

Acknowledgements

Films

Binding energy (eV) (determined)

Binding energy (eV) (corrected)

Atom percentage (%) at.%

Area

PAn PMB PAn – PMB

172.65 170.35 168.40

167.95 163.5 168.05

0.11 4.38 1.22

46 1745 443

The authors are grateful for the assistance from Laboratory of Material Cycling in Pedosphere, Institute of Soil Science, Chinese Academy of Science.

References The atom percentage concentration (at.%) of S element in the bilayer film is 1.22%, in which S is from PMB (S%: 4.38%) (Table 1). Furthermore, the presence of a single S2p peak in the XPS measurement reflects that the valence state of the S atoms from PMB does not change after bilayer deposition; this indicates that only a simple electrostatic interaction exists between the layers [14].

4. Conclusions In the present work, it has firstly presented a promising and easy way for the preparation of a novel film through a two-step method. The resulting material has been characterized by SEM image, FTIR spectroscopy and XPS method. Electrochemical research reveals that the PAn/ PMB film has good electrocatalytic properties on the electrooxidation of methanol in neutral and alkaline medium. The convenience of this method compared to the copolymerization process [19,20] is that it is easy to control the amount of each polymer, thus obtaining the desired properties. However, the interacting mechanism of each polymer is still unclear. More extensive work on PAn-radical composites and their electrocatalytic property is under study in our laboratory. Also, application and optimization of these modified electrodes for fuel cell electrocatalysis will be the focus of our interest in the future investigations.

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