Cite this Article
Hu Fang, Li Wenhai, Zhang Ji, Meng Wei. Effect of Graphene Oxide as a Dopant on the Electrochemical Performance of Graphene Oxide/Polyaniline Composite. Journal of Materials Science & Technology, 2014, 30(4): 321-327  
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Copyright©2014, Editorial Board of Journal of Materials Science & Technology
Copyright 2014, Editorial Office of Journal of Materials Science & Technology
Effect of Graphene Oxide as a Dopant on the Electrochemical Performance of Graphene Oxide/Polyaniline Composite
Hu Fang, Li Wenhai, Zhang Ji, Meng Wei*
Department of Physical Chemistry, China Pharmaceutical University, Nanjing 211198, China
Corresponding author. Ph.D.; Tel.: +86 25 86185151.
Abstract

A method for preparing a graphene oxide/polyaniline (GO/PANI) composite electrode was developed to investigate the effect of GO doped in PANI. PANI was first prepared by the polymerisation of aniline and then dedoped by NH4OH to form emeraldine base (EB). The dedoped PANI and as-prepared GO were dissolved in N-methyl-2-pyrrolidone (NMP) to generate a homogeneous dispersion. The GO/PANI composites were redoped in HCl before use as electrode materials. These composites were characterised by Raman spectroscopy, X-ray diffraction, UV–vis adsorption spectroscopy, scanning electron microscopy, atomic force microscopy and electrochemical measurements. The GO/PANI composite electrode (containing 2.5% GO) has an initial gravimetric capacitance of 896 F g-1 at a scan rate of 5 mV s-1 and a retention life of 51% after 500 cycles, which is an improvement over that of pure PANI (23%). The results show that the synergy of GO and PANI attributes to the good electrochemical performance of the GO/PANI composite electrode.

Keyword: Electrical properties; Carbon film; Capacitance; Materials synthesis
1. Introduction

Since their discovery in 1960, intrinsically conducting polymers (ICPs) have attracted enormous interest due to their potential for use in many areas, including as actuators [1], electrochromic materials [2], films [3] and [4], supercapacitors [5], biosensors [6], electromagnetic shields and optical materials [7], [8] and [9]. Among the ICPs, polyacetylene (PA), polypyrrole (PPy), polythiophene (PT), polyparaphenylene vinylene (PPV) and polyaniline (PANI), have been investigated. However, because of their low conductivity compared with metal, poor solubility in solvents, infusibility and low environmental stability, few large-scale applications have been performed. Of these polymers, PANI is the most promising because of its low cost, ease of synthesis, good stability [10], high capacitive characteristics and fine-tuned properties [11], [12] and [13]. In particular, advancements in doping technology yielding a suitable dopant and nanostructured forms of PANI or its composites [7], [14] and [15] would accelerate progress in this field. Recently, PANI has been widely studied as an electrode material with better performance for supercapacitors.

Graphene and graphene oxide (GO) are two-dimensional carbon materials that have attracted great interest due to their high surface area, extraordinary electrochemical and mechanical properties. These advantages have led to various applications of graphene/PANI composites or GO/PANI composites [16] and [17]. These composites show enhanced electrochemical characteristics for energy storage devices. GO, which exhibits many oxygen functional groups on its basal planes and edges, is highly dispersible in water and thus a promising material for composites with PANI. In fact, the fabrication of many graphene/PANI composites has been performed by the reduction of GO/PANI composites. Most works employed in situ polymerisation approaches using aniline in the presence of GO, yielding high-performance electrodes [18] and [19]. In such a strategy, GO plays a dual role. On one hand, it acts as an efficient template for the nucleation and polymerisation of aniline [20], which changes aniline's polymerisation characteristics, such as its morphology, molecular weight and the molar fraction of oxidised units in the PANI structure [21]. On the other hand, GO also acts as a dopant, affecting the electrochemical properties and conformations of the molecular chain. Both of these aspects affect electrochemical performance; however, how to understand the distinction between the two roles remains unclear. Regarding this problem, to our best knowledge, few studies have been reported [22] and further studies are required.

In this work, PANI was first synthesised by chemical polymerisation and then converted into emeraldine base (EB) after dedoping by 1 mol/L NH4OH for 24 h. The EB and as-prepared GO were dissolved in N-methyl-2-pyrrolidone (NMP) to generate a homogeneous dispersion, which was attributed to good solubility of EB and the high degree of dispersion of GO in NMP. In this process, GO acted only as a dopant. The interaction between GO and PANI was investigated. The introduction of GO into PANI was found to enhance the electrochemical performance of PANI; thus, this composite was a promising electrode material.

2. Experimental
2.1. Materials

High-purity (99.5%) natural flake graphite (G) was purchased from Alpha. Other chemicals (analytical-grade reagents) were provided by Sinopharm Chemical Reagent Co., Ltd. and used without purification.

2.2. Preparation of GO

Based on the method of Hummers and Offeman [23], a modified method was proposed for preparing a high-quality GO monolayer from G. The modified procedure involved an additional G preoxidation step. G (2 g) was pre-oxidized with a mixture of 20 mL of concentrated H2SO4, 5 g of K2S2O8 and 5 g of P2O5 at 80 °C for 6 h under stirring and then carefully diluted with distilled water, filtered, and washed until the pH of the rinse water was neutral. The product was dried in air at ambient temperature overnight. The peroxidised G powder (2 g) was added to concentrated H2SO4 (0 °C, 80 mL). KMnO4 (10 g) was then added slowly with stirring and cooling such that the temperature of the mixture remained below 10 °C with the aid of an ice bath. The mixture was then stirred at 35 °C for 3 h, after which distilled water (150 mL) was added and the temperature was allowed to rise to approximately 98 °C. After 15 min, the reaction was terminated by the addition of a large amount of distilled water (350 mL) and 30% H2O2 solution (10 mL). The GO dispersion was centrifuged and then washed in succession with 300 mL of 5% HCl and distilled water. A 2% GO dispersion was prepared and subjected to dialysis for a week to completely remove metal ions and acids. Finally, solid GO was obtained after centrifugation and drying in a vacuum oven at 40 °C for 24 h.

2.3. Preparation of PANI

A typical preparation route is as follows. Aniline (5 mL) and hydrochloric acid (15 mL, 37%) were added to 190 mL of water with stirring. The dispersion was kept in an ice bath and the temperature was controlled between 2 and 4 °C. Chemical polymerisation was then performed by the slow addition of 20 mL of ammonium persulfate (APS) solution (molar ratio: APS/aniline = 1:1). The aniline polymerisation was appreciable as the dispersion turned green. 2 h later, the product (emeraldine salt) was filtered and washed with water and acetone. The as-prepared PANI was then dedoped by immersion in 1 mol/L NH4OH and stirring for 24 h. EB was obtained by filtering, washing with distilled water and then drying in a vacuum oven at 60 °C for 24 h [24].

2.4. Preparation of electrodes

EB was dissolved in NMP to form an 82 mg/mL solution. GO was added to another NMP and ultrasonicated (250 W, 220 V) for 1 h to obtain an exfoliated yellow-brown GO suspension with a concentration of 1.13 mg/mL [25]. These two solutions were then mixed in different ratios, and the dedoped GO/PANI composite dispersions were obtained by an additional 0.5 h of ultrasonication. The GO/PANI composite electrodes were prepared as follows. A stainless-steel mesh was cleaned in 1 mol/L H2SO4 electrolyte by cyclic voltammetry (CV) with the potential range of -0.2 V and 0.8 V and used for preparing the working electrode [5]. It was first coated with the dedoped GO/PANI composite dispersion and then dried at 60 °C under vacuum to assure good electrical contact. The repeated coating and drying process ensures a high-quality product. The dedoped GO/PANI composite electrode was redoped with 1 mol/L HCl for 1 h. The working electrode has an area of approximately 2 cm2. The pure PANI electrode was prepared in the absence of GO via a similar procedure. The different GO/PANI composites were denoted as GO/PANI composite A, B, C and D, which contain 1.2%, 2%, 2.5% and 3% GO, respectively.

2.5. Characterisation of materials

GO, PANI and their composites were characterized by Fourier transform infrared (FT-IR) spectroscopy, UV–vis spectroscopy, atomic force microscopy (AFM), Raman spectroscopy, scanning electron microscope (SEM) and X-ray powder diffraction (XRD). FT-IR spectra were recorded with an FT-IR spectrophotometer (SHIMADZU FTIR-8400S) using KBr pellets. SEM images were obtained using the Hitachi S-4800. UV–vis absorption spectra were collected on a TU-1810 UV–vis spectrophotometer (Beijing Purkinje General Instrument, China) with standard 1-cm-optical-path quartz cells. Raman spectra were measured on a Renishaw Micro-Raman spectroscopy system. The excitation line, 514 nm, was provided by an argon ion laser. XRD was performed on a Bruker D8 Advanced X-ray diffractometer with Cu Kα radiation. AFM images were obtained using an SPA300HV (Seiko Instruments Inc.) instrument in tapping mode. AFM samples were prepared by drop-casting GO dispersions onto freshly cleaved mica substrates and drying in air. All electrochemical experiments were performed using a three-electrode setup with a CHI660C workstation, in which platinum was used as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. The electrolyte was 1 mol/L H2SO4. The electric conductivity measurements of compressed powder pellets of samples were performed using a four-probe method (SDY-4, China).

3. Results and Discussion

Fig. 1 shows typical XRD patterns for G, GO, PANI and the representative GO/PANI composite C, which has high electrochemical performance and will be discussed later. No characteristic G peaks are detected in GO, whereas a peak is observed for GO at 10.72° (2 θ), corresponding to the (001) reflection of GO. This result indicates that a highly oxidised GO product has been synthesised [26]. As shown in Fig. 1, an intense peak around 2 θ = 25° and some weak reflections from PANI indicate some crystalline order in PANI. The relative intense peak of GO (2 θ = 10.72°) disappears in GO/PANI composite, which indicates that low-content GO is exfoliated and loosely dispersed in GO/PANI composite. XRD patterns of GO/PANI composite and PANI reveal their different crystalline forms. There is a new broad peak at 15°–30° existing in the XRD pattern of the GO/PANI composite, which is ascribed to the reflection of PANI. Therefore, the XRD results confirm the formation of GO/PANI composite but not a mixture of GO and PANI [27].

Fig. 1. XRD patterns of G, GO, PANI and GO/PANI composite C.

The FT-IR spectrum of GO ( Fig. 2(a)) dispersed in KBr shows the introduction of oxygen-containing functional groups on the basal planes and the edges. Bands at 3404 cm-1 and 1624 cm-1 are attributed to the O–H stretching vibration of C–OH and O–H deformation, respectively. The peaks at 1719, 1384 and 1114 cm-1 are assigned to the stretching vibrations of C=O and C–O in COH and C–O in COC [28], [29] and [30], respectively. This spectrum is quite different from that of G, which contains few oxygen-containing functional groups. A peak at 1114 cm-1 (stretching vibration of C–O in COC) is present for GO but not for G, indicating that oxygen-containing functional groups exist not only on the edges but also on the basal planes of GO [31]. Fig. 2(b) represents the FT-IR spectra of PANI and GO/PANI composite C. The peaks at 1559, 1473, 1296, 1126 cm-1 correspond to C=C stretching deformation in quinoid ring, C=C stretching deformation in benzenoid ring, C–N stretching of secondary aromatic amine and C=N stretching (–N=quinoid=N–), respectively. The broad peak at 3421 cm-1 is due to the N–H stretching of PANI. All character bands of PANI chains are observed in GO/PANI composite. However, compared with the bands of PANI, most bands of GO/PANI composite are clearly red-shifted. The spectral red-shift phenomena reveal the π-stacking and hydrogen bonding between GO nanosheets and the PANI backbone [16] and [22]. In addition, the peak at 3407 cm-1 in GO/PANI composite becomes intense, which is ascribed to the overlap of O–H stretching vibration of GO and N–H stretching of PANI.

Fig. 2. FT-IR spectra of G, Go (a), PANI and GO/PANI composite C (b).

Fig. 3 shows the Raman spectra of G, GO, dedoped PANI and dedoped GO/PANI composite C. G exhibits two peaks: a D band at 1363 cm-1 corresponding to defects or edge areas and a G band at 1577 cm-1 related to the vibration of sp2-hybrided carbon. When G is intercalated and oxidised, the G band shifts to a higher wavenumber (1589 cm-1) and widens as a result of a loss of interaction between the adjacent layers. The D band of GO at 1358 cm-1 is more intense than that of G due to the intercalation of oxygen-containing functional groups with covalent bonding in the GO layer [29] and [32].

Fig. 3. Raman spectra of G, GO, dedoped PANI and dedoped GO/PANI composite C.

To confirm the complete exfoliation of GO in NMP, AFM images of the GO sheets deposited from GO dispersion onto mica substrates were recorded, as shown in Fig. 4. Irregularly shaped sheets of uniform thickness are observed. As illustrated in Fig. 4(b) for the sheet marked by the green line, the height is typically between 1.2 nm and 1.7 nm relative to the background. This thickness is greater than that of single-layer graphene (0.34 nm) and is due to oxygen-containing functional groups on the edges and basal planes of GO. Previous AFM studies indicate that the thicknesses of single-layer and double-layer GO are between 1 nm and 1.4 nm and at least 2 nm [28], respectively, when GO is exfoliated in water, which reveals the presence of single-layer GO sheets in NMP. In addition, some corrugated sheets and sheets with turned-in edges are observed, with thicknesses in the range of 1.9–2.4 nm.

Fig. 4. (a) AFM images of GO sheets deposited from a dispersion, (b) height profile through the line shown in (a).

Fig. 5 shows the UV–vis spectra recorded in NMP. The dedoped PANI sample has a sharp, intense peak at 331 nm and a broad peak at 638 nm, which is characteristic of dedoped insulating PANI [33]. The peak at 331 nm (B peak) is assigned to the π–π transition associated with the π electrons in the benzene rings. The peak at 638 nm (Q peak) is attributed to the excitation of an electron from the highest occupied molecular orbital (HOMO) of the benzenoid rings to the lowest unoccupied molecular orbital (LUMO) quinoid rings. Due to the strong absorption of NMP at lower wavelengths, the absorption peak of GO is shielded and not observed, which is consistent with another report [25]. However, when GO was added to PANI in NMP, the B peak is almost unchanged and the Q peak red-shifts by 4 nm. Remarkably, the absorbance of GO increases with the decrease in wavelength and its addition would prevent the red shift of the Q peak. Thus, it can be deduced that the red shift of the Q peak should be greater than 4 nm. Generally speaking, the B peak is mainly a function of intra-chain interactions. Therefore, the surroundings have less influence on its intensity and wavelength, but the Q peak is strongly affected by solubility, the use of additives and concentration. This peak reflects not only intra-chain but also inter-chain interactions [34]. The clear shift of the Q peak from 638 nm to 642 nm indicates interaction between GO and PANI. However, if the absorbance of GO at 642 nm in the GO/PANI/NMP solution is considered, the virtual absorbance of PANI at 642 nm in the GO/PANI/NMP solution is 0.26, which is lower than that in the PANI/NMP solution. All of these results indicate that GO dopes on the quinoid ring of PANI to form a conductive GO/PANI composite [35]. Possible interactions include π–π stacking, electrostatic interaction and hydrogen bonding (–OHċN– or –OċHN–) [18] and [36]. The doping is also confirmed by the Raman spectra (see Fig. 3). The spectrum of pure dedoped PANI shows peaks at 1167 cm-1, 1498 cm-1 and 1588 cm-1, corresponding to in-plane C–H bending, C=N stretching and C–C stretching in benzenoid, respectively [18], [36] and [37]. In dedoped GO/PANI composite C, the intensity of the peak for C=N stretching decreases and a peak for C–N++. (radical cation) stretching at 1343 cm-1 emerges. These phenomena were also observed by Tao et al. [38] when EB was doped with HCl, indicating that GO doping occurs and forms a conductive GO/PANI composite. All of these results reveal that PANI is in a doped state because of the carboxyl group on the GO surface and the proton doping mechanism of PANI [38].

Fig. 5. UV–vis absorption spectra of GO, dedoped PANI and dedoped GO/PANI composite C.

To further evaluate the quality of GO/PANI composite, SEM was utilized to analyze the morphologies of GO, PANI and their composites. The structure for GO agglomerate can be observed in Fig. 6(a), which is consistent with the report given by Ramesh et al. [29]. Fig. 6(b) depicts as-prepared PANI. The as-prepared PANI consists of dominant rod-like particles and some granular agglomerate. After dedoping, dissolving in NMP and subsequently redoping, the morphology of PANI as electrode material changes and shows some layered structure (see Fig. 6(c)). Though NMP works as plasticizer which can produce a film, the layered structure has low quality and some pores are also found at the surface of PANI electrode, which is the result of the evaporation of NMP. Fig. 6(c and d) reveals the difference in the microstructure or morphology between PANI electrode and GO/PANI composite electrode. GO/PANI composite C electrode shows good layered structure and few pores are found. This composite morphology differs from individual components. The changes in microstructure or morphology reveal that the introduction of GO plays an important role in the preparation of GO/PANI composite. The adsorption and intercalation of PANI on the surface of GO should be responsible for the composite morphology and further affect the electrochemical performances greatly. The result of conductivity measurement shows that the GO/PANI composite C has the conductivity of 4.3 S cm-1, which is higher than that of pure PANI (3.5 S cm-1). The enhanced conductivity of GO/PANI composite C is mainly attributed to its special microstructure and the π-stacking between PANI backbone and the GO sheets.

Fig. 6. SEM images of GO (a), as-prepared PANI (b), PANI (c) and GO/PANI composite C (d).

Cyclic voltammetry (CV) was performed in 1 mol/L H2SO4 electrolyte in the potential range of -0.2 V and 0.8 V. The gravimetric capacitance (the specific capacitance per gram) of electrodes can be calculated from the CV curves using the following equation [5], [37] and [39]:

CS=(∫IdV)/(ν·ΔV)

where Cs is the gravimetric capacitance (F g-1), I is the current density (A g-1), ν is the scan rate (V s-1) and Δ V is the potential interval (V).The experimental results indicate that GO/PANI composite C has the highest gravimetric capacitance and that the capacitances of GO/PANI composites A, B, C and D are 525, 561, 600 and 550 F g-1 at a scan rate of 20 mV s-1, respectively. In comparison, the capacitance of pure PANI is 475 F g-1. When the GO concentration in the composites exceeds 2.5%, the composites become porous and the adhesion ability of the products decreases, resulting in poor contact with the stainless-steel mesh and declining gravimetric capacitance. Fig. 7(a and b) shows the representative CVs of pure PANI and GO/PANI composite C at the scan rates ranging from 5 to 100 mV s-1. The CV curves of PANI and GO/PANI composite C from the 1st cycle to the 500th cycle at a scan rate of 20 mV s-1are shown in Fig. 8for the calculation of the retention life. Both pure PANI and GO/PANI composite C exhibit two pairs of redox peaks, which are characteristic of pseudocapacitance. One pair is attributed to the transition of PANI from a semiconducting state (leucoemeraldine) to its conductive form (polaronic emeraldine) (C1/A1) and the other pair is attributed to the transition from polaronic emeraldine to pernigraniline (C2/A2) (see inset in Fig. 7(a)) [36] and [37]. With increasing scan rate, the current density of both samples increases. It should be noted that the curved shape of pure PANI changes and a pair of peaks remains up to 20 mV s-1. However, the curved shape of GO/PANI composite C is retained up to 50 mV s-1; thus, GO/PANI composite C has better electrochemical stability as an electrode material. This behaviour is likely due to the introduction of GO, which has a large surface area and very large oxygen groups and thus enhances the conductivity and electrochemical activity of PANI.

Fig. 7. CV curves for PANI (a) and GO/PANI composite C (b) at different scan rates of 5, 10, 20, 30, 50, 70 and 100 mV s-1 in 1 mol/l H2SO4 from -0.2 V to 0.8 V.

Fig. 8. CV curves for PANI (a) and GO/PANI composite C (b) from 1st cycle to 500th cycle at a rate of 20 mV s-1.

Fig. 9(a) shows the gravimetric capacitance of pure PANI and GO/PANI composite C at different scan rates. GO has a very low gravimetric capacitance (<2 F g-1), so it is ignored in calculating the gravimetric capacitance. In general, the gravimetric capacitance decreases as the scan rate increases. The gravimetric capacitances of pure PANI are 739, 605, 473, 408, 341, 293 and 223 F g-1, and those of GO/PANI composite C are 896, 733, 601, 513, 441, 387 and 342 F g-1 at 5, 10, 20, 30, 50, 70 and 100 mV s-1, respectively. The gravimetric capacitance of GO/PANI composite C is greater than that of pure PANI by approximately 20% at all scan rates, corresponding to a higher electrochemical capacitance. At the highest scan rate (100 mV s-1), the gravimetric capacitance of pure PANI is 30% of that at the lowest scan rate (5 mV s-1), whereas that of GO/PANI composite C remains at 38%. The decrease in the gravimetric capacitance of GO/PANI composite C is slower than that of pure PANI. The initial gravimetric capacitances of pure PANI and GO/PANI composite C are 739 F g-1 and 896 F g-1, respectively, at 5 mV s-1. The composite C electrode has a retention life of 51% after 500 circles, whereas that of pure PANI is 23% (see Fig. 9(b)). Interestingly, the gravimetric capacitance of GO/PANI composite C rapidly decreases for the 200 cycles, similarly to PANI, but then increases slowly for the next 300 cycles. This variation in the behaviour of the gravimetric capacitances is most likely due to the degradation of PANI and the reduction of GO. The former decreases gravimetric capacitance, whereas the latter increases gravimetric capacitance. As mentioned above, GO doping occurs, forming a conductive GO/PANI composite, because of the carboxyl groups on the GO surface and the proton doping mechanism of PANI. The synergetic effect between GO and PANI includes π–π stacking, electrostatic interaction and hydrogen bonding. Both the synergetic effect and the desirable mechanical properties of the composite due to the introduction of GO [40] are responsible for its good electrochemical performance. These factors greatly improve the gravimetric capacitance and retention life of the composite material. Though these values are not the best in the previously reported data [5] and [20], the results are acceptable. The difference is possibly due to the use of different methods to prepare the electrodes. To investigate the effect of GO as a dopant on the electrochemical performance of GO/PANI composites, polytetrafluoroethylene (PTFE), often used as a binder and acetylene, often used as a conductor, are both absent in the work electrodes. In addition, as mentioned above, for most in situ polymerisation GO plays an important role in changing aniline's polymerisation characteristics, such as its morphology, molecular weight and the molar fraction of oxidised units in the PANI structure, but it is used as a dopant in our experiment. So this finding can shed light on the effect of GO as a dopant.

Fig. 9. (a) Gravimetric capacitances of pure PANI and GO/PANI composite C at different scan rates, (b) their gravimetric capacitances as a function of number of cycles at a rate of 20 mV s-1.

4. Conclusion

We have proposed a method to prepare high-performance GO/PANI composite electrode materials. The use of NMP solution leads to a more homogeneous dispersion of dedoped PANI and GO and enables the study of the interaction between PANI and GO while excluding the effect of GO on the polymerisation of PANI. As a dopant, GO has high surface area and very large oxygen groups, which provides the GO/PANI composite electrode with good electrochemical performance. The pure PANI electrode and composite C electrode (containing 2.5% GO) have an initial gravimetric capacitance of 739 and 896 F g-1, respectively, at a scan rate of 5 mV s-1. The capacitance retention of the composite material is also enhanced. The high-performance of GO/PANI composites is attributed solely to the synergy between PANI and GO.

Acknowledgement

This work was supported by research funding from the School of Sciences of China Pharmaceutical University.

The authors have declared that no competing interests exist.

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