**3.1. Apparatus**

Morphological characterization was carried out by using SEM Hitachi S-5500. The X-ray diffraction patterns were obtained from a Bruker D2 Phaser. The FTIR infrared analysis was measured with Platinum-ATR Alpha Bruker, and the ultraviolet visible spectra were obtained from Genesys 105 UV-Vis Thermo Scientific. Scanning electron microscope (SEM) was a LEO Synthesis and Characterization of Reduced Graphene Oxide/Polyaniline/Au Nanoparticles… http://dx.doi.org/10.5772/intechopen.77385 79

**Figure 2.** (a) GO sheets separation and (b) elemental analysis.

model operating at 15 kV, at 1, 5, 10, and 15 kX magnifications. Raman spectroscopy was a WITec alpha 300 AR, laser source: 532 nm (green), power: 15.6 mW, optical objective: 100×, integration time 5 s, eight accumulations.

### **3.2. Elemental analysis of the GO**

in 100 ml HCl 2 M separately, and put into an ice bath. The two solutions were mixed rapidly at 20°C temperature and put in ultrasonic bath for 3 h. The obtained green mixture was filtered and washed with ethanol and de-ionized water. The final product was put into a vacuum oven

The polyaniline/graphene oxide (GO/PANI) composite was prepared by the same polymerization method with the presence of graphene oxide. In this case, the aniline was fixed at 40% wt. with 60% wt. graphene oxide in the acidic solution of HCl 2 M, according to the method

1 ml of 38.8 mM sodium citrate to the vortex of the solution was added and resulted in a color change from yellow to burgundy. The heating at the same temperature and stirring was continued for an additional 15 min, and the resulting colloidal particles solution was stored at 4°C.

To prepare the working electrode samples, GO, PANI or GO/PANI, and PTFE were mixed (90:10, w/w) and dispersed in ethanol. For the system GO/PANI, AuNp, and PTFE, they were mixed (72:20:8%), respectively, and were also dispersed by sonication in ethanol. Carbon

The electrochemical experiments were performed in a three electrode cell arrangement. A graphite rod was used as a counter electrode, and the potentials were measured with respect to a Ag/AgCl standard electrode (saturated with KCl). The electrochemical impedance measurements were carried out by applying an AC voltage of 10 mV amplitude in the 10 kHz–0.01 Hz

at different scan rates of 2–100 mV/s in a voltage range of −0.4 to 1.2 V. Electrochemical Gill

To realize the characterization of the synthesized hybrid materials, they were analyzed with

Morphological characterization was carried out by using SEM Hitachi S-5500. The X-ray diffraction patterns were obtained from a Bruker D2 Phaser. The FTIR infrared analysis was measured with Platinum-ATR Alpha Bruker, and the ultraviolet visible spectra were obtained from Genesys 105 UV-Vis Thermo Scientific. Scanning electron microscope (SEM) was a LEO

frequency range. Cyclic voltammetry measurements were carried out in 1 M H<sup>2</sup>

AC Instruments analyzer ACM serial 1039 were used throughout the experiments.

brought to boil with vigorous stirring, rapid addition of

area) were coated with the mixture and

SO<sup>4</sup>

solution

at 60°C for 4 h.

reported and mentioned before [11].

78 Graphene Oxide - Applications and Opportunities

In a flask with 10 ml of 1 mM HAuCl<sup>4</sup>

dried at room temperature for 12 h.

**2.4. Synthesis of Au nanoparticles (AuNp)**

**2.5. Electrodes and electrochemical measurement**

cloth, stainless steel, and copper electrode (1 cm2

**3. Characterization of the hybrid materials**

the relevant methods and specified instruments.

**3.1. Apparatus**

As was mentioned in the methodology description, to obtain GO sheets, graphite was previously oxidized by a modified Hummer's method [11] consisting in the chemical oxidation of the structure through the use of concentrated sulfuric acid, potassium permanganate, and sodium nitrate. After oxidation, this was followed by ultrasonic bath, to break Van der Waals forces to separate the sheets and to obtain GO (**Figure 2a**). The incorporation of oxygen into the graphite crystalline network was corroborated, determining the carbon/oxygen ratio through SEM elemental X-ray analysis (**Figure 2b**).

#### **3.3. FTIR spectroscopy**

FTIR spectroscopy was used to elucidate the covalent grafting and to confirm the change in functional groups during each step. **Figure 3** represents the FTIR spectra of GO, PANI, GO/PANI, and GO/PANI/AuNp. The GO shows absorption bands at 3200 and 1734 cm−1, which correspond to O─H, C═O in COOH [12]. It can be also observed that there are bands around 1605 and 1376, which are due to the intercalated water and deformation vibrations of C─O in C─OH and C─O─C functional groups [13].

For pure PANI prominent attributed absorption peaks are seen at 1630 and 1394 cm−1, belonging to C═C stretching deformation of quinoid and C═N stretching of secondary aromatic amine, revealing the presence of emeraldine salt state in PANI [14]. The bands at 1184 and 805 cm−1 correspond to ─C─N stretching vibration and out of plane bending vibrations of C─H in the benzene ring. Around 3281 cm−1, it was observed an absorption band for N─H stretching of the amine group.

The FTIR spectrum of the GO/PANI composite was identical to that of PANI, which confirmed that the GO surface was wrapped by PANI [14]. There is no peak observed at 3200 and 1734 cm−1 (─OH and C═O vibrations, respectively), indicating the reduction of GO took

**Figure 3.** FT-IR spectrum of GO, PANI, and the composites GO/PANI and GO/PANI/AuNp.

place due to the polymerization of aniline, in addition to the confirmation that PANI has been covalently grafted onto the surface of the GO sheets. The FTIR bands were displaced, indicating a covalent bond and displacement due to the molecule arrangement. Functional groups responsible for stabilization of gold nanoparticles appeared in the band at 3300 cm−1 corresponding to N─H vibrations [15].

**Figure 4** exhibits the XRD crystallographic pattern of graphite, and the basal reflection (002) peak at 2θ = 25° indicates a d-spacing of 0.35 nm based on Bragg's equation. After chemical oxidation, the peaks shifted to a lower angle reflection plane (001) at 2θ =10.5°,

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**Figure 4.** X-ray diffraction (XRD) of graphite, GO, PANI, GO/PANI, and GO/PANI/AuNp.

This widening of the d-spacing can be attributed to the intercalation of water molecules and generation of oxygenated functional such as epoxy, hydroxyl, and carboxyl groups between the inter-layering of the graphite sheets during severe oxidation [9], the small peak observed at 2θ = 42.5° associated with (100) plane of graphite, and indicates that small amounts of graphite phases are still present [10]. The peaks for emeraldine form PANI were exhibited at 2θ = 25, 21, 15, and 9° corresponding to (200), (020), (011), (001) reflections for PANI,

which indicates a d-spacing of 0.84 nm.

respectively [16].

#### **3.4. X-ray diffraction**

Understating the morphological as well as structural changes of the products obtained, XRD studies on graphite, GO, PANI, GO/PANI, and GO/PANI/AuNp were analyzed. Synthesis and Characterization of Reduced Graphene Oxide/Polyaniline/Au Nanoparticles… http://dx.doi.org/10.5772/intechopen.77385 81

**Figure 4.** X-ray diffraction (XRD) of graphite, GO, PANI, GO/PANI, and GO/PANI/AuNp.

place due to the polymerization of aniline, in addition to the confirmation that PANI has been covalently grafted onto the surface of the GO sheets. The FTIR bands were displaced, indicating a covalent bond and displacement due to the molecule arrangement. Functional groups responsible for stabilization of gold nanoparticles appeared in the band at 3300 cm−1

**Figure 3.** FT-IR spectrum of GO, PANI, and the composites GO/PANI and GO/PANI/AuNp.

Understating the morphological as well as structural changes of the products obtained, XRD studies on graphite, GO, PANI, GO/PANI, and GO/PANI/AuNp were analyzed.

corresponding to N─H vibrations [15].

80 Graphene Oxide - Applications and Opportunities

**3.4. X-ray diffraction**

**Figure 4** exhibits the XRD crystallographic pattern of graphite, and the basal reflection (002) peak at 2θ = 25° indicates a d-spacing of 0.35 nm based on Bragg's equation. After chemical oxidation, the peaks shifted to a lower angle reflection plane (001) at 2θ =10.5°, which indicates a d-spacing of 0.84 nm.

This widening of the d-spacing can be attributed to the intercalation of water molecules and generation of oxygenated functional such as epoxy, hydroxyl, and carboxyl groups between the inter-layering of the graphite sheets during severe oxidation [9], the small peak observed at 2θ = 42.5° associated with (100) plane of graphite, and indicates that small amounts of graphite phases are still present [10]. The peaks for emeraldine form PANI were exhibited at 2θ = 25, 21, 15, and 9° corresponding to (200), (020), (011), (001) reflections for PANI, respectively [16].

After the polymerization, it is observed the reduction of graphene oxide by the interaction with PANI demonstrated with the decrease of (001) reflection plane angle (2θ = 10.5°). The XRD pattern of GO/PANI presents crystalline peaks similar to those of PANI. The peak around 2θ = 26° is correlated to the interlayer space between the graphene sheets, which overlap with the diffractions from PANI [16]. The AuNp peaks were observed at 2θ = 32, 38.4, and 44.6°, corresponding to (100), (111), and (200) planes of the face centered cubic crystal, respectively.

### **3.5. Ultraviolet-vis**

The AuNps were synthesized by Turkevich's method (1951) [17] in which reduction results were observed with the color change from pale yellow to burgundy. The color of AuNp is dependent upon the size and shape of the nanoparticles formed, which is correspondingly associated with the surface plasmon resonance due to collective oscillations of six electrons in the conduction band of AuNp, and this is the resonance frequency of the incident electromagnetic radiation [17, 18]. In **Figure 5a**, the burgundy particle dispersion attributed to circular shape particles with less than 40 nm in size can be observed, which agreed with circular shape with diameters 10-40 nm for AuNp, obtained by the citrate method [19].

The maximum absorption was observed at 520 nm (as seen in **Figure 5b**) corresponding to the plasmon collective oscillation of gold [20], indicating that the nanoparticles are evenly dispersed in the aqueous solvent. When the incident light wave frequency resonates with the electron coherent movement from the conduction band, it produces a strong absorption, which is the origin of the observed colloidal color [21]. For small metallic nanoparticles (less than 20 nm in diameter), the absorption spectrum only depends on the dipole oscillation, being the reason for the color change from the Au salt reaction (pale yellow) with the sodium citrate, forming gold nanoparticles (burgundy color), in which the resonant maximum absorption band of plasmon surface (520 nm) observed is AuNp characteristic nuclei response.

**3.6. High-resolution scanning electron microscopy**

**Figure 6.** HRSEM micrographs of (a–c) graphene oxide and (d) gold nanoparticles.

**3.7. Raman spectroscopy**

electric and magnetic properties.

High-resolution scanning electron microscopy (HRSEM) images were used to study size, shape, and dispersion about the synthesized nanomaterials. **Figure 6(a–c)** presents SEM micrographs at different magnifications showing views of the GO obtained by Hummer's modified method, graphene oxide sheets morphology are clearly seen, showing wrinkled and folded regions and the transparent property associated with them. Surface properties are associated with the shape and size of AuNp (**Figure 6d**) presenting sphere particles with an average size of 10–40 nm in diameter.

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Raman spectroscopy is one of the most important techniques to detect and distinguish the graphene properties. This technique provides valuable information regarding the number of sheet layers, edges, and defects. This evaluation is very important because structural defects transform the graphene in such a way that intrinsic defects in the band structure modify its

**Figure 5.** UV-Vis of gold nanoparticles (a) nanoparticles dispersion and (b) the maximum absorption observed at 520 nm.

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**Figure 6.** HRSEM micrographs of (a–c) graphene oxide and (d) gold nanoparticles.

#### **3.6. High-resolution scanning electron microscopy**

High-resolution scanning electron microscopy (HRSEM) images were used to study size, shape, and dispersion about the synthesized nanomaterials. **Figure 6(a–c)** presents SEM micrographs at different magnifications showing views of the GO obtained by Hummer's modified method, graphene oxide sheets morphology are clearly seen, showing wrinkled and folded regions and the transparent property associated with them. Surface properties are associated with the shape and size of AuNp (**Figure 6d**) presenting sphere particles with an average size of 10–40 nm in diameter.

#### **3.7. Raman spectroscopy**

After the polymerization, it is observed the reduction of graphene oxide by the interaction with PANI demonstrated with the decrease of (001) reflection plane angle (2θ = 10.5°). The XRD pattern of GO/PANI presents crystalline peaks similar to those of PANI. The peak around 2θ = 26° is correlated to the interlayer space between the graphene sheets, which overlap with the diffractions from PANI [16]. The AuNp peaks were observed at 2θ = 32, 38.4, and 44.6°, corresponding to (100), (111), and (200) planes of the face centered cubic crystal, respectively.

The AuNps were synthesized by Turkevich's method (1951) [17] in which reduction results were observed with the color change from pale yellow to burgundy. The color of AuNp is dependent upon the size and shape of the nanoparticles formed, which is correspondingly associated with the surface plasmon resonance due to collective oscillations of six electrons in the conduction band of AuNp, and this is the resonance frequency of the incident electromagnetic radiation [17, 18]. In **Figure 5a**, the burgundy particle dispersion attributed to circular shape particles with less than 40 nm in size can be observed, which agreed with circular shape

The maximum absorption was observed at 520 nm (as seen in **Figure 5b**) corresponding to the plasmon collective oscillation of gold [20], indicating that the nanoparticles are evenly dispersed in the aqueous solvent. When the incident light wave frequency resonates with the electron coherent movement from the conduction band, it produces a strong absorption, which is the origin of the observed colloidal color [21]. For small metallic nanoparticles (less than 20 nm in diameter), the absorption spectrum only depends on the dipole oscillation, being the reason for the color change from the Au salt reaction (pale yellow) with the sodium citrate, forming gold nanoparticles (burgundy color), in which the resonant maximum absorption band of plasmon surface (520 nm) observed is AuNp characteristic nuclei

**Figure 5.** UV-Vis of gold nanoparticles (a) nanoparticles dispersion and (b) the maximum absorption observed at 520 nm.

with diameters 10-40 nm for AuNp, obtained by the citrate method [19].

**3.5. Ultraviolet-vis**

82 Graphene Oxide - Applications and Opportunities

response.

Raman spectroscopy is one of the most important techniques to detect and distinguish the graphene properties. This technique provides valuable information regarding the number of sheet layers, edges, and defects. This evaluation is very important because structural defects transform the graphene in such a way that intrinsic defects in the band structure modify its electric and magnetic properties.

hybrid materials show very small diameter semicircle demonstrating the good electrical conductivity of the three-element composite, with a semi-straight line at mid to low frequencies, associated to capacitive behavior (mass transfer control) and lower charge-transfer resistance. The Nyquist and Bode plots obtained are presented in **Figure 8a** and **b**, respectively, and for comparison purposes, the carbon cloth presents the highest overall or total impedance

**Figure 8.** Impedance plots (a) Nyquist and (b) Bode for carbon cloth material support, and each synthesized material;

intermediate frequencies corresponding to double layer capacitance. For the PANI system, the impedance values diminished in all the frequency bandwidth considered around 45

The GO/PANI/AuNp hybrid exhibits the semicircle smaller than GO/PANI and PANI, suggesting better conductivity and lower charge transfer resistance. A straight sloping line in the lower frequency represents the diffusion resistance (Warburg impedance, W), which reflects the diffusion or mass transfer of redox species in the electrolyte, and a steeper line usually

Based on these electrochemical analyses, the enhanced capacitive behavior was due to the synergistic effect between graphene oxide and PANI, besides the high conductivity of the AuNp. In addition, the small nanometer size can exhibit enhanced electrode/electrolyte interface areas, providing high electro-active regions and short diffusion lengths. This is also true for the GO/PANI and GO/PANI/AuNp hybrid materials showing a further decrease of

, presenting low impedance behavior all around [22]. Bode impedance diagrams (**Figure 8**) demonstrate that the synthesized materials PANI, GO/ PANI, and GO/PANI/AuNp present good conductivity properties with low impedance values. Also, the capacitive behavior was observed with the carbon cloth (blank) and the GO material. This analysis reveals that the good electrical conductivity and ion diffusion behavior resulted in the electrochemical performance of GO/PANI/AuNp of the three element hybrid

, reflecting the conductive properties of the polymer material.

at the low frequency limit (0.01 Hz). The GO system shows a lower

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, both showing an inverse frequency behavior at

value above 1 kohm·cm<sup>2</sup>

GO, PANI, GO/PANI, GO/PANI/NpAu.

indicates faster ion diffusion.

around 10 ohms·cm2

material [23].

ohms·cm2

impedance value around 4500 kohms·cm<sup>2</sup>

**Figure 7.** Raman spectra of graphite, GO, and GO/PANI composite.

**Figure 7** shows the Raman spectra of graphite, GO, and GO/PANI composite. Graphite 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 *sp*<sup>2</sup> -hybrided carbon. When GO is intercalated and oxidized, the band shifts to a higher wavenumber (1589 cm−1) and widens as a result of a loss of interaction between the adjacent layers. The band of GO/PANI at 1358 cm−1 is more intense than that of GO due to the intercalation of oxygen-containing functional groups with covalent bonding in the GO layer [13, 14].
