Effect of nanostructured graphene oxide on electrochemical activity of its composite with polyaniline titanium dioxide

Most chemicals used in this study were provided by Merck (Germany). Aniline (Anil) was fresh distilled under vacuum before use. The titanium electrodes as electrode substrate were polished by sandpaper with 180 grits and then pretreated following the procedure in [6] before use. The PANi-TiO₂-GO nanocomposites were prepared by chemical method using TiO₂ in sol gel form (from the Institute of Applied Physics, VAST, Vietnam), GO in powder form (from the Institute of Chemistry, VAST, Vietnam) and ammoniumpersulfate (molar ratio to Anil was equal 1) as an oxidation agent under stirring at a temperature of about 0 °C–5 °C. GO content was varied from 0 to 10% compared with Anil, while the mass ratio of TiO₂ to Anil was equal 1/6. The pastes of those composites using chitosan solution in acetic acid (1%) as a binder were performanced on pretreated titanium substrate and dried at a temperature of 120 °C for 2 h. Brewery wastewater (chemical oxygen demand (COD) was 3555 mg l⁻¹) was chosen as a substrate electrolyte used for electrochemical measurements.

The structure of materials was carried out by infrared spectra on IMPACT 410-Nicolet unit. The morphology of the material was examined by scanning electron microscopy (SEM) on an equipment FE-SEM Hitachi S-4800 (Japan) and transmission electron microscopy (TEM) on a Jeol 200CX (Japan). The x-ray diffraction of the samples was obtained by an x-ray diffractometer D8-Advance Bruker (Germany). The electrical conductivity measurement by cyclic voltammetry (CV) and electrochemical impedance spectroscopy analysis as well as dynamic current polarization measurement were carried out on the electrochemical workstation unit IM6 (Zahner-Elecktrik, Germany).

The conductivity of composites was determined through CV diagrams in figure 1 and table 1. The higher slope has the CV line, the higher electrical conductivity obtains the material [7]. The conductivity was calculated by the following equation

$$\begin{eqnarray}&&\delta =d\displaystyle \frac{{\rm{\Delta }}I}{{\rm{\Delta }}E},\end{eqnarray} \tag{ 1 }$$

where δ is electrical conductivity (mS cm⁻¹), ΔE is the potential difference (mV), ΔI is the responsive current difference (mA) and d is the thickness of the sample (cm).

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The results showed an increase of electrical conductivity when GO was used for fabricating composite, among them the highest value reached in the case of GO/Anil ratio of 1/12 (127.02 mS cm⁻¹).

Figure 2(a) illustrates Nyquist plots simulated following electrical equivalent circuits on figures 2(b) and (c) where the symbols are measured points and the solid lines are fitting ones. Two schemas were found where the first one with six elements belonged to composites with the presence of GO (figure 2(b)) and the other one with seven elements belonged to those without GO (figure 2(c)). The obtained data given in table 2 showed an important effect of GO not only on both capacitance (C_f) and resistance (R_f) of the layer but also on the electrochemical process due to an appearance of adsorption capacitance (C_ad) and Warburg diffusion (W). In fact, GO-composites have very small values of R_f and C_f in comparison with that non-containing GO at which the electrochemical reactions may be prevented owing to the appearance of R_ad and very small constant phase element (CPE) (0.3 nF). However, it was found the highest R_f (326.1 Ω) and W (118.5 Ω s^−1/2) values for composite with GO/Anil ratio of 1 to 12.

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The diffusion coefficient D can be calculated by the following equation [8]

$$\begin{eqnarray}&&D=\displaystyle \frac{\nu {R}^{2}{T}^{2}}{{n}^{4}{F}^{4}{A}^{2}{C}^{2}{\sigma }^{2}},\end{eqnarray} \tag{ 2 }$$

where ν is the reaction order, R is the Boltzman gas constant, T is the absolute temperature, n is the exchange electron number in the charge transfer process, F is the Faraday constant, A is the electrode surface area, C is the oxidant/reductant concentration on the electrode material (C = 1); σ is the Warburg constant obtained by fitting.

It suggests that both reaction order (ν) and exchange electron number (n) equal 1 because bateria in brewery wastewater as a substrate in MFC are reactants taking part in the charge transfer process. The values of the calculated diffusion coefficient are ranged 10⁻¹⁶ cm² s⁻¹, among them the diffusion process in the case of GO/Anil equal 1 to 14 used for the fabricating composite was faster than the other owing to the biggest one (9.37 × 10⁻¹⁶ cm² s⁻¹) caused by the smallest R_f (156.3 Ω).

According to [9], the current dynamic polarization can be used for the evaluation of the performance and electrocatalytic activity with microbial fermentation products. In figure 3 the galvanodynamically recorded polarization curves of composites are shown and compared in terms of their current density at 0.4 V versus Ag/AgCl. As can be seen, the composite with GO/Anil ratio of 1 to 14 exhibits a significantly better performance, yielding a current density of 280 μA cm⁻², compared to only 170–200 μA cm⁻² observed with other rest composites.

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The results given in figure 4 and table 3 explained that GO existed in composite due to –OH stretching mode at 3500 and 3362 cm⁻¹, 3057 cm⁻¹ (C–H), 1718 and 1654 cm⁻¹ (C=O), 1558 and 1517 cm⁻¹ (C=C), 1227 cm⁻¹ (C–O) [10–12]. Figure 5 shows some signals belonging to GO such as at 3500 cm⁻¹ (–OH), 1560 cm⁻¹ (C=C) and 1294 cm⁻¹ (C–O). It was also found that PANi existed in composite owing to vibration signals of benzoid and quinoid rings at 1560 and 1485 cm⁻¹, respectively [13]. Some other signals were found at 3542 cm⁻¹ assigning N–H stretching mode, 2833 cm⁻¹ (C-H), 1438 cm⁻¹ (−N=quinoid=N−), 1127 cm⁻¹ (C–N⁺).

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The data given in figure 6 provided separately the spectra of PANi (curve (a)) and TiO₂ (curve (b)) with which the spectrum belonging to the composite could be compared. It was found on the spectrum of curve (c) the peak at 2θ degree of about 27 presented for amorphous PANi and the other peaks belonging to anatase TiO₂ at those of 38°, 48°, 55° and 63°.

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The SEM images on figure 7 show that TiO₂, which was provided in sol-gel form (50 g l⁻¹), existed in grain with a size approximately smaller than 20 nm and GO in the typical layered wrinkle structure. Compared with those images, the GO-composite existed in a layered structure of GO on which PANi was deposited to form the fibre bundles.

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The TEM images in figure 8 evidenced convincingly that among three clearly different colours, the light one belongs to PANi enclosing the big dark one which belongs to graphene oxide and the other small dark one belongs to TiO₂. All of them had a size in the nanorange. The gained results from SEM and TEM analysis explained that nanostructured PANi-TiO₂-GO composites were successfully prepared by chemical method.

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