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

Graphene oxide (GO) significantly affects the electrochemical activity of its composite with polyanline titanium dioxide (TiO2). In this work various composites with different GO contents have been successfully synthesized by chemical method to compare not only their material properties but also electrochemical characteristics with each other. The results of an electrochemical impedance study showed that their electrochemical property has been improved due to the presence of GO in a composite matrix. The galvanodynamic polarization explained that among them the composite with GO/Ani ratio in the range of 1–14 exhibits a better performance compared to the other due to yielding a higher current desity (280 μA cm−2). The TEM and SEM images which presented the fibres of a composite bundle with the presence of PANi and TiO2 were examined by IR-spectra and x-ray diffraction, respectively.


Introduction
Currently, the function materials and their applications have attracted the most attention due to their outstanding properties concerning the bioelectrocatalitic degradation of organic molecules in organic wastes, especially in brewery wastewater. They are potential materials for fabricating fuel substrate for microbial fuel cell (MFC) due to the food-derived nature of the organic matter and the lack of high concentrations of inhibitory substances (for example, ammonia in animal wastewaters) [1]. The hybrid materials based on graphene oxide (GO) and polyaniline (PANi) are fabricated for different applications in a supercapacitor and biosensor because of their excellent electrochemical performances and biocompatibility [2,3].
Graphene oxide is chemically oxidized graphene with advantageous characteristics such as thermal and chemical stability, high mechanical strengh, large specific surface area, good electron conductivity and water solubility. Titanium dioxide (TiO 2 ) is one of the most studied semiconductors owing to excellent electronic properties, non-toxicity, low cost, physical and chemical stability and high reactivity, whose composites with GO [4] or PANi [5] are promising antibacterial materials.
In this work PANi-TiO 2 -GO composites were prepared by chemical method and performanced in paste form on titanium substrate. The effects of GO content in composites on their electrochemical properties were considered by electrochemical impedance measurement and dynamic current polarization one in brewery wastewater. Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Materials and methods
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-TiO 2 -GO nanocomposites were prepared by chemical method using TiO 2 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 TiO 2 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.

Detection method
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).

Electrical conductivity measurement
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 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).
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, GOcomposites 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.

Electrochemical impedance study
The diffusion coefficient D can be calculated by the following equation [8] 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 cm 2 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 cm 2 s −1 ) caused by the smallest R f (156.3 Ω).

Current dynamic polarization
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.

X-ray diffraction
The data given in figure 6 provided separately the spectra of PANi (curve (a)) and TiO 2 (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 TiO 2 at those of 38°, 48°, 55°and 63°.

SEM images
The SEM images on figure 7 show that TiO 2 , 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.

TEM images
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 TiO 2 . All of them had a size in the nanorange. The gained results from SEM and TEM analysis explained that nanostructured PANi-TiO 2 -GO composites were successfully prepared by chemical method.

Conclusion
The GO composite was succesfully synthesized by chemical method, which existed in a layered structure combined with fibre bundles in nano size resulting in very small electrochemical impedance in comparision with that without GO. It explained that the presence of GO improved the electrochemical property of the composite on which the adsorption