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Characterization of nanostructured PbO2–PANi composite materials synthesized by combining electrochemical and chemical methods

Nanostructured PbO2–PANi composite materials were prepared by combining electrochemical and chemical methods. Firstly, PbO2 was deposited on a stainless steel substrate by pulsed current method and then obtained PbO2 electrode was immersed into acidic aniline solution to form nanostructured PbO2–PANi composites. The synthesized samples were characterized by infrared (IR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and x-ray diffraction (XRD). The electrocatalytic oxidation of methanol on those composites was evaluated by potentiodynamic polarization from 1.4 to 2.2 V versus Ag/AgCl/saturated KCl electrode. The adsorption of N–H group as well as the presence of benzoid and quinoid ring vibrations on IR-spectrum asserts that PANi coexisted with β-PbO2 which is evidenced by x-ray analysis. With increasing immersion times of the PbO2 electrode in the acidic aniline solution the electrocatalytic performance of the obtained PbO2–PANi composites for methanol oxidation was improved due to the formation of less closely knitted nano-sized PANi fibers, which was confirmed by surface morphology analysis.


Introduction
It is known that PbO 2 material has good electrical conductivity that is similar to metal and PbO 2 is an excellent electrical catalyst and catalyst carrier [1,2]. Polyaniline (PANi) is non-toxic, inexpensive and a stable conducting polymer, which can be synthesized by simple chemical and electrochemical routes [3][4][5]. Currently, it is hybridized with oxides of some metals like titanium, lead and manganese to form composites as materials for energy storage and conversion as well as electrocatalysis [6][7][8][9]. Among them, there is a lack of publications about PbO 2 -PANi which may be obtained by combining chemical and electrochemical methods resulting in a better catalyst. Hence, it is of significance in this research to prepare PbO 2 -PANi catalyst by this method and investigate its electrocatalytic performance for oxidation of a small organic molecule such as methanol.

Material and methods
Analytical grade nitric acid, Cu(NO 3 ) 2 , Pb(NO 3 ) 2 , ethylenglycol (Merck) and methanol were used without any purification. Aniline (Merck) was freshly distilled under vacuum condition at about 120 • C before use. The pulsed current method was applied to electrosynthesize PbO 2 (the height of pulse: 30 mA cm −2 , the width of pulse: 3 s, Q = 9 A s cm −2 ). For each time, the obtained PbO 2 electrode was immersed 60 s into acidic aniline solution (0.1 M),  washed by distilled water, immersed in acetone to remove the excess of aniline and after that dried for half an hour at room temperature. This procedure was used for preparing samples of two kinds. The first one was immersed twice and the second one five times.

Detection method
The structure of the film layer was carried out by infrared spectrum on IMPACT 410-Nicolet unit. The surface morphology of coatings was examined by SEM on an FE-SEM Hitachi S-4800 (Japan) and TEM on a Jeol 200CX (Japan). The analysis of crystaline structure of those layers was performed by using an x-ray diffractometer D5000, Siemens (Germany). The electrocatalytic oxidation of methanol was measured by electrochemical workstation unit IM6 (Zahner-Elektrik, Germany).

SEM images
The image in figure 1(a) showed that non-uniform tetragonal β-PbO 2 crystals were formed on the surface of stainless steel substrate under pulsed current method. However, its surface morphology was changed completely after immersing into    aniline solution (figures 1(b) and (c)). It can be explained that aniline was chemically polymerized on the surface of an electrode owing to the presence of PbO 2 layer as an oxidation agent by the following reaction [10]: Aniline has converted to anilinume cation radical which can begin the polymerization reaction leading to PANi product, while Pb 2+ in the PANi lattice can be solved because we used 0.1 M HNO 3 as an electrolyte. The surface of PANi-PbO 2 composite electrode in case (b) was more closely knitted by PANi fibers than that in case (c). Reasonably, this can be explained as follows: the more time the electrode was immersed in the above solution, the less aniline was oxydized on its surface because a conductive prelayer like a barrier reduces the following polymerization process of aniline resulting in a spongy surface.

TEM images
The TEM images in figure 2 evidence convincingly that among two clearly different colors, the light one belongs to PANi enclosing the dark one belongs to PbO 2 . Both of them had size in the nano range. The gained results from SEM and TEM analysis explained that nanostructured PbO 2 -PANi composites were successfully prepared by combining electrochemical and chemical methods.

X-ray diffraction
X-ray diffraction (XRD) was used to determine the structure of the electrode before and after immersion into acidic monomer solution following the above-mentioned procedures, as shown in figure 3. For all cases the first peak was found to be located at 2θ degree of 30 • and the second peak strongly oriented at 2θ of over 62 • indicated β-PbO 2 as reported in [11]. This explains that β-PbO 2 existed in our prepared composites. This is evidence to prove that only a part of the surface of PbO 2 layer was reduced by aniline to Pb 2+ which might be moving into electrolyte, and the rest of it remains in composite matrix.  [12,13] which showed that PANi co-existed in composite matrix in emeraldine salt form (PANi + NO − 3 ) because of using HNO 3 as an electrolyte.

Electrocatalytic oxidation of methanol
The electro-oxidation of methanol on the surface of anodic PbO 2 -PANi composites can occur by the following reaction: From data given in figure 5, the electro-oxidation current i of methanol can be calculated by equation where i is corresponding current measured in acidic methanol solution and i base line is that measured in acidic solution without methanol (base line). The results given in figure 6 showed that only one oxidation peak ( i p ) of methanol in the potential range of 2.05-2.10 V (versus Ag/AgCl/saturated KCl electrode) was found. Additionally, the height of i p linearly increased with increasing methanol concentration in solution, among which the composite prepared by increasing immersion times into acidic aniline solution could electrocatalyse better for oxidation of methanol because its i p shift is higher (green line in figure 7). The i p values in this research were very high (up to 30 mA cm −2 ) in comparison with those reported in our previous report (less than 8 mA cm −2 ), which resulted from only the electrochemical pulsed current method [14]. It can be said that methanol oxidation could be positively catalysed on the surface of PANi-PbO 2 composites prepared by combining chemical and electrochemical methods.

Conclusions
From the above results we conclude that nanostructured PbO 2 -PANi composite prepared by combining electrochemical and chemical methods improved its electrocatalytic ability for methanol oxidation in comparison with that prepared by the pulsed current method (only by the electrochemical way).
By increasing immersion time of PbO 2 electrode in acidic aniline solution, the surface morphology was obtained with less closely knitted PANi fibers resulting in nanostructured PbO 2 -PANi composite on which electrocatalytic ability for methanol oxidation improved positively.