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

Analytical grade nitric acid, Cu(NO₃)₂, Pb(NO₃)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 PbO₂ (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 PbO₂ 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.

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).

The image in figure 1(a) showed that non-uniform tetragonal β-PbO₂ 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 PbO₂ layer as an oxidation agent by the following reaction [10]:

$$\begin{equation} {\rm{Pb}}^{4 + } + {\rm{2C}}_{\rm{6}} {\rm{H}}_{\rm{5}} {\rm{NH}}_{\rm{2}} \to {\rm{Pb}}^{2 + } + 2{\rm{C}}_{\rm{6}} {\rm{H}}_{\rm{5}} {\rm{NH}}_{\rm{2}} ^ {+.} . \end{equation} \tag{ 1 }$$

Aniline has converted to anilinume cation radical which can begin the polymerization reaction leading to PANi product, while Pb²⁺ in the PANi lattice can be solved because we used 0.1 M HNO₃ as an electrolyte. The surface of PANi–PbO₂ 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.

Zoom In Zoom Out Reset image size

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 PbO₂. Both of them had size in the nano range. The gained results from SEM and TEM analysis explained that nanostructured PbO₂–PANi composites were successfully prepared by combining electrochemical and chemical methods.

Zoom In Zoom Out Reset image size

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-PbO₂ as reported in [11]. This explains that β-PbO₂ existed in our prepared composites. This is evidence to prove that only a part of the surface of PbO₂ layer was reduced by aniline to Pb²⁺ which might be moving into electrolyte, and the rest of it remains in composite matrix.

Zoom In Zoom Out Reset image size

Figure 4 shows the spectrum of the PbO₂–PANi composite prepared by procedure 2 (immersion of PbO₂ into acidic aniline solution five times). The band from 3447 to 3206 cm⁻¹ is assigned to the N–H stretching mode, from 3043 to 2916 cm⁻¹ (aromatic C–H), from 1447 to 1353 cm⁻¹(–N = quinoid = N–). It exhibits clearly the presence of benzoid and quinoid ring vibrations at 1652 and 1533 cm⁻¹, respectively. The signals at 1182 and 1096 cm⁻¹ were determined due to the C–N⁺ group, at 905 and 825 cm⁻¹ confirmed due to the absorption of the N–H group, at 650 and 592 cm⁻¹ explaining the adsorption of NO₃⁻ ion. These vibration signals were similar to those reported in the literature [12, 13] which showed that PANi co-existed in composite matrix in emeraldine salt form (PANi⁺NO₃⁻) because of using HNO₃ as an electrolyte.

Zoom In Zoom Out Reset image size

The electro-oxidation of methanol on the surface of anodic PbO₂–PANi composites can occur by the following reaction:

$$\begin{equation} {\rm{CH}}_{\rm{3}} {\rm{OH}} + {\rm{H}}_{\rm{2}} {\rm{O}} \to {\rm{6H}}^ + + {\rm{6e}}^ - + {\rm{CO}}_2 . \end{equation} \tag{ 2 }$$

From data given in figure 5, the electro-oxidation current Δi of methanol can be calculated by equation

$$\begin{equation} \Delta i = i - i_{{\rm{base}}\,{\rm{line}}} , \end{equation} \tag{ 3 }$$

where i is corresponding current measured in acidic methanol solution and i_base line is that measured in acidic solution without methanol (base line).

Zoom In Zoom Out Reset image size

The results given in figure 6 showed that only one oxidation peak (Δi_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 Δi_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 Δi_p shift is higher (green line in figure 7). The Δi_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–PbO₂ composites prepared by combining chemical and electrochemical methods.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size