Influence of chemical oxidation upon the electro-catalytic properties of graphene–gold nanoparticle composite

Electrochimica Acta 91 (2013) 137–143

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Influence of chemical oxidation upon the electro-catalytic properties of graphene–gold nanoparticle composite Maria Coros a , Alexandru R. Biris a , Florina Pogacean a , Lucian Barbu Tudoran b , Camelia Neamtu a , Fumiya Watanabe c , Alexandru S. Biris c , Stela Pruneanu a,∗ a

National Institute for Research and Development of Isotopic and Molecular Technologies, Donath Street, No. 65-103, RO-400293, Cluj-Napoca, Romania Babes-Bolyai University, Electron Microscopy Center, Mihail Kogalniceanu Street No. 1, RO-400006, Cluj-Napoca, Romania c Center for Integrative Nanotechnology Sciences, University of Arkansas at Little Rock, 2801 S. University Ave., Little Rock, AR 72204, USA b

a r t i c l e

i n f o

Article history: Received 8 November 2012 Received in revised form 14 December 2012 Accepted 17 December 2012 Available online 5 January 2013 Keywords: Graphene-modified electrode Gold nanoparticles Guanine detection Tafel plots

a b s t r a c t A crystalline composite nanostructure based on graphene and gold nanoparticles (denoted as Gr–Au) was synthesized in a single-step process by Radio Frequency catalytic Chemical Vapour Deposition (RF-cCVD) over an Aux /MgO catalytic system (where x = 3 wt.% and represents the amount of metal loaded in the catalyst). After preparation, the composite was chemically oxidized with a mixture of sulfuric/nitric acid (3:1 vol.), being subsequently denoted as Gr–Au OH. No massive exfoliation of graphene layers occurred during oxidation, so the degree of crystallinity was preserved, as proved by X-ray powder diffraction (XRD) measurements. For both samples, the crystalline domain had a mean value of approximately 2.25 nm, corresponding to about 6 graphitic layers. The electro-catalytic properties of Gr–Au and Gr–Au OH composites were tested by modifying two gold electrodes with the same amount of each material (denoted Au/Gr–Au and Au/Gr–Au OH, respectively) and subsequently employed for the electrochemical analysis of guanine. A significant decrease in the electrochemical oxidation potential of guanine (∼100 mV) was obtained in both cases. Tafel analysis proved that the modified electrodes have a large value for the exchange current density (approximately 10−7 A) which is one order of magnitude larger than that corresponding to a bare gold electrode. Consequently, these composite materials greatly enhance the transfer of electrons from solution to electrode. However, the major drawback of the Gr–Au OH nanostructure is the large capacitive current induced by the oxygen functional groups and observed in all cyclic voltammetric measurements, which considerably diminishes the sensitivity of the modified electrode. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Guanine, one of the four bases in nucleic acids, is a fundamental compound in biological systems and participates in many biochemical processes. Purine bases in DNA have been detected and determined by many methods: electrophoresis coupled with electrochemical determination [1], high performance liquid [2] and micellar electrokinetic chromatography with indirect laser-induced fluorescence detection [3] isotope dilution mass spectrometry [4] and chemiluminescence [5]. These approaches are highly selective and sensitive; however, they are also expensive and time-consuming. The electrochemical detection of guanine has proven to be a simple and swift method with high selectivity and sensitivity [6–9]. In recent years, several electrodes have been used to detect

∗ Corresponding author. E-mail address: [email protected] (S. Pruneanu). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.12.122

guanine, such as highly ordered, basal and edge plane-pyrolytic graphite, glassy carbon, and carbon nanotubes [9]. The electrochemical detection of guanine has also been performed on mercury [10], gold [11], indium–tin oxide [6] and polymer-coated electrodes [12]. More recently, guanine sensing has been extended to the case of graphene-modified electrodes [7,13]. Graphene is a most attractive nanomaterial possessing excellent electrical, mechanical, thermal, and optical properties. Graphene is a zero-band gap semiconductor and its crystal lattice comprises a two-dimensional sheet of sp2 -bonded carbon atoms [14,15]. Given its high specific surface area, porosity, and elasticity, as well as its chemical stability, graphene has been used to prepare a new generation of electrodes for electrochemical studies [16–19]. The potential for the use of graphene in several areas is limited by its hydrophobicity; nonetheless, this disadvantage can be overcome by surface functionalization [20]. For guanine, electrochemical oxidation at graphite electrodes is controlled by two factors: the density of the basal plane sites, required for absorption, and the density of the edge plane sites,

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required for oxidation on the electrode surface [9]. It has been demonstrated that graphene is anisotropic with respect to electron transfer: its edges resemble that of edge-plane graphite with fast electron transfer, while its sides resemble basal plane graphite with slow electron transfer [21]. In this work, we provide evidence for the first time that the electro-catalytic properties of graphene–gold nanoparticle composites largely depend on the amount of oxygen functional groups attached to their surface. Thermogravimetric (TGA) and Fourier Transform Infrared (FTIR) studies were employed to determine the thermal stability of Gr–Au and Gr–Au OH samples, respectively, to identify the oxygen functional groups attached to graphene, such as COOH, C OH, or C O groups. The Gr–Au composite was very stable, compared with the oxidized compound. There was no significant mass loss (∼1.7%) when this material was heated up to 1000 ◦ C, indicating that extremely small amounts of oxygen groups are present on graphene sheets. Moreover, the electrochemical oxidation of guanine at bare gold and graphene-modified electrodes was studied in detail by linear sweep voltammetry (LCV) and cyclic voltammetry (CV). The differences in the kinetics of oxidation were interpreted based on Tafel equations. 2. Experimental All chemicals were of analytical grade and used directly without any further purification. Guanine was purchased from Alfa Aesar (SUA-Germany). A 0.2 M acetate buffer pH 5 was prepared from 0.2 M acid acetic and 0.2 M sodium acetate by mixing the appropriate volumes. A stock solution of 10−4 M guanine was prepared in this buffer and then diluted to lower concentrations down to 10−6 M. N,N-dimethyl formamide (DMF) was purchased from Fluka-Germany and used for the dispersion of the Gr–Au and Gr–Au OH composites.

Transform Infrared (FTIR) Spectroscopy, and Thermogravimetric Analysis (TGA). TEM/HRTEM analysis (JEOL-JEM1010 and JEOL-JEM2100F) was performed by dispersing Gr–Au or Gr–Au OH composites in water and then drop-cast on a copper grid. X-ray powder diffraction data were collected in the 2 = 5–85◦ , with a Bruker D8 Advance diffractometer using Cu K␣1 radia˚ In order to increase the resolution, a Ge (1 1 1) tion ( = 1.5406 A). monochromator in the incident beam was used to eliminate the K␣2 radiation. Fourier Transform Infrared analysis was performed with a JASCO 6100 FTIR Spectrometer in the 4000–500 cm−1 spectral domain with a resolution of 4 cm−1 , using the KBr pellet technique. Thermogravimetric analysis was performed using a SDT Q600 (TA Instruments, USA) analyzer. The temperature calibration was made using the Curie temperatures of alumel and chromel standards. The measurements were carried out under a high-purity argon atmosphere (flow rate 100 ml/min), from ambient temperature to 1200 ◦ C, at a heating rate of 10 ◦ C/min. To ensure an inert gas environment for the sample, argon was purged for 90 min at 100 ml/min before starting the heating process. The samples were loaded in standard 90 ␮l alumina cups. Al2 O3 powder was used as reference. The mass of the sample and reference was the same, about 5 mg. Electrochemical measurements (Linear Sweep Voltammetry and Cyclic Voltammetry) were performed using an Autolab 302N Potentiostat/Galvanostat (Eco Chemie-Netherlands) connected to a three-electrode cell. The electrochemical data acquisition was conducted using NOVA1.8 software. A conventional three-electrode cell was employed for all electrochemical measurements, containing Ag/AgCl (KCl sat) as the reference electrode, a gold electrode as the working electrode (surface area 0.07 cm2 ), and a large area Pt electrode (approx. 2 cm2 ) as counter. A Mettler Toledo pH/ion Meter (Switzerland) was used for pH measurements.

2.1. Preparation of Gr–Au and Gr–Au OH composites The Gr–Au nanostructures were synthesized by Radio Frequency catalytic Chemical Vapor Deposition using an Aux /MgO catalytic system (where x was set equal to 3 wt.% and represents the metal concentration within the catalyst) and methane as carbon source (120 mL/min). The synthesis temperature was 1000 ◦ C while the synthesis time was 60 min, as previously described in detail in Ref. [22]. The oxidized composite was prepared from Gr–Au nanostructure, after oxidation in a mixture of strong acids, and is denoted as Gr–Au OH. Briefly, 25 mg of Gr–Au composite were added to 1.5 ml solution of concentrated H2 SO4 and HNO3 acids (3:1 vol.). The suspension was mixed by ultrasound for 1 h at 40 ◦ C. Next, the product was centrifuged, washed with plenty of double-distilled water until neutral pH, and then dried at 40 ◦ C in an oven. 2.2. Gold electrode modified with Gr–Au or Gr–Au OH nanostructures (Au/Gr–Au respectively Au/Gr–Au OH) For electrode modification, 1 mg of Gr–Au or Gr–Au OH composite was dispersed in 2 ml DMF. Next, the mixture was sonicated with a cup-horn sonicator (Sonics, Vibra-Cell VC 505, 500 W, 20 kHz) in a water bath for 3 min at 30% amplitude, to give a black suspension. The cleaned gold electrode was modified by dropcasting 20 ␮L of the resulting suspension and then dried in air. 2.3. Apparatus The morphological and structural characteristics of Gr–Au and Gr–Au OH composites were investigated by Transmission Electron Microscopy (TEM/HRTEM), X-ray Powder Diffraction (XRD), Fourier

3. Results and discussion 3.1. Morphological and structural characterization of Gr–Au and Gr–Au OH composites The morphological characteristics of the Gr–Au and Gr–Au OH samples were investigated by TEM/HRTEM techniques. In Fig. 1a and b, we can see representative TEM images of the Gr–Au sample at low and high magnification, showing the characteristics of fewlayer graphene with gold nanoparticles (seen as black points). The presence of gold nanoparticles is also confirmed by STEM analysis (data not shown) [22]. The graphitic sheets have irregular shapes with a wide range of dimensions, from tens to several hundreds of square nanometers. The sheets are transparent, with homogeneous central areas quite smooth but with edges that tend to fold and roll. Wrinkles and corrugations are part of the intrinsic nature of graphene, which becomes thermodynamically stable via bending. The wrinkles and the number of layers within the graphene sheets can be better seen in Fig. 1c, along with a gold nanoparticle of about 15 nm. Many gold nanoparticles have spherical shape, varying from 3 to 37 nm (most are between 3 and 25 nm – see the histogram in Fig. 1d). Close analysis of several HRTEM images reveal the edges of graphene layers on top of metallic nanoparticles, so we hypothesize that the gold nanoparticles are covered by graphitic layers. Consequently, the sp2 hybridization of carbon atoms and the intrinsic properties of graphene sheets (e.g. high conductivity) were not disrupted by the attached gold nanoparticles. This observation is additionally supported by the fact that, after chemical oxidation of the Gr–Au sample, the size distribution

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Fig. 1. Gr–Au sample: (a,b) low and high magnification TEM images; (c) HRTEM image of graphene and gold nanoparticle; (d) histogram showing the size distribution of gold nanoparticles. Scale bar: (a) 200 nm; (b) 50 nm; (c) 2 nm.

of gold nanoparticles did not significantly change (see next, the characterization of Gr–Au OH sample). After chemical oxidation of the Gr–Au sample (Fig. 2a,b) with a mixture of sulfuric/nitric acid, we observed that the sample preserved its morphological characteristics and that gold nanoparticles were also present on the graphene sheets. Most important, the crystalline structure of graphene was preserved, as demonstrated by XRD study. The mild oxidative treatment we employed led to the formation of a small amount of oxygen functional groups (carboxyl, carbonyl, epoxy) on graphene sheets and did not exfoliate the layers of graphene. No significant change in nanoparticle size was noticed, and this may be attributed to the fact that they were not directly exposed to the acidic solution, being covered by graphitic layers (see the histogram in Fig. 2c.) Fig. 3 shows the XRD patterns of Gr–Au and Gr–Au OH samples, which indicate several distinct diffraction peaks, both for graphene and gold. The dominant diffraction peak from graphene is centered at 25.5◦ and corresponds to reflection from the (0 0 2) plane [23]. The peak is sharp, which reveals a good ordering of graphene layers along the stacking direction, with an interlayer spacing of 0.35 nm. The mean value of the crystalline domain, perpendicular to the Gr(0 0 2) crystallographic plane was calculated using the Scherrer equation [24] both for the Gr–Au and the Gr–Au OH samples. This value is approximately 2.2 nm and corresponds to about 6 graphitic layers, being in excellent agreement with the number of layers observed in several HRTEM images. The gold crystallite size was also determined with the Scherrer formula, and the obtained value (24 nm) was in good correlation with the size

determined from TEM images (see the histograms presented in Figs. 1d and 2c). The XRD measurements clearly proved that the crystalline structure of the Gr–Au OH sample was not destroyed by the oxidative treatment, and, most probably, the oxygen functional groups were attached on the edges and outer basal planes of the graphene sheets. No massive exfoliation of graphene layers occurred. FTIR study was next employed to identify the oxygen functional groups attached to the graphene sheets (Fig. 4). The FTIR spectrum of the Gr–Au OH sample shows the presence of the carboxylic group: a medium band at 1716 cm−1 which is attributed to the C O stretch of the COOH group, a strong band at 1174 cm−1 attributed to C O stretching vibrations, and a weak shoulder at 2853 cm−1 attributed to the hydroxyl stretching vibrations of the C OH groups [25]. Bands at 2922 and 2318 cm−1 represent the stretching of the CH2 group. The broad band at 3100 to 3500 cm−1 , assigned to the O H stretch from absorbed H2 O, also appeared in the pure Gr–Au spectrum. The presence of the absorbed water molecules supports the fact that graphene is a highly absorptive material [26]. The oxygen functional groups were also investigated by TGA. In order to avoid the pyrolysis of carbon atoms at temperatures higher than 650 ◦ C, TGA measurements were carried out under Ar flow. For the Gr–Au OH sample, we noticed a first weight loss due to physisorbed water in the region up to 100 ◦ C. In agreement with previous reports [27,28] the primary loss occurred in the 150–400 ◦ C range (∼9.7%) and may be the result of the decomposition of carboxylic, anhydride, and lactone functional groups in the material [29]. Next, a steady mass loss (∼7.6%) was observed in

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Fig. 2. (a,b) TEM images of Gr–Au OH sample (scale bar 200 nm); (c) histogram showing the size distribution of gold nanoparticles.

the 400–1000◦ C temperature range, which may have resulted from the removal of more stable phenol, carbonyl, and quinone groups [30,31] (Fig. 5). As expected, Gr–Au was more stable and no significant mass loss (only 1.7%) was detected when this material was heated up to 1000 ◦ C. This clearly demonstrates that the sample contained extremely small amounts of labile oxygen functionalities on its surface. Due to its high hydrophobicity, the amount of physisorbed water was considerably smaller.

3.2. Electrochemical investigation of guanine oxidation using gold electrodes modified with Gr–Au or Gr–Au OH composite Although the Gr–Au and Gr–Au OH samples have very similar crystalline structures, the presence of a small amount of oxygen functional groups has a strong effect on the electro-catalytic properties of graphene-modified electrodes. The electrochemical oxidation of guanine (10−4 M in acetate buffer, pH5) was next considered using gold electrodes modified with the same amount

900

Intensity (a.u.)

Gr(002)

Au(111)

Au(200)

600

Au(220) Au(311)

Gr-Au

300

Gr(100)

Gr-Au-OH

0 20

40

60

80

2θ (degree) Fig. 3. XRD patterns of the Gr–Au (red) and Gr–Au OH (blue) samples showing several distinct diffraction peaks, both for graphene and gold. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Fig. 4. FTIR spectra of Gr–Au (red) and Gr–Au OH (blue) samples, in KBr pellets. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

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recordings with the Au/Gr–Au and Au/Gr–Au OH electrodes. In the first instance, the peak potentials (0.83 respectively 0.84 V) and the peak heights (1.06 × 10−6 and 1.02 × 10−6 A, respectively) have very close values. However, the capacitive current (Icap ) is about four times larger for the Au/Gr–Au OH electrode, in comparison with the Au/Gr–Au. This current is always present at the solution/electrode interface due to the electrical double layer and is not influenced by the movement of ions in solution. A plausible explanation for the large Icap may be related to the fact that the charges associated with oxygen functional groups increase the double-layer capacitance (Cdl ). As is well known, Icap is proportional with the double-layer capacitance (Cdl ) and the scanning rate (dE/dt) according to the following equation [32]:

1.7%

Gr-Au 9.7%

Weight (%)

95

90 7.6%

Gr-Au-OH 85

80 0

200

400

600

800

1000

1200 Icap = Cdl

o

Temperature ( C) Fig. 5. Thermogravimetric curves for Gr–Au (red) and Gr–Au OH (blue) samples. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

(20 ␮L) of Gr–Au or Gr–Au OH composite. Typical LCV measurements are displayed in Fig. 6a, in comparison with results from a bare gold electrode. In the case of graphene-modified electrodes, a clear peak due to guanine oxidation appears at around 0.83 V, while, with bare gold, a broad wave is seen at approximately 0.93 V. We noticed similarities, as well as differences, between the

-5

1.8x10

10-4 M Guanine

-5

a.

1.5x10

0.84 V

I (A)

I = I0

Au/Gr-Au-OH

-6

9.0x10

-6

6.0x10

0.83 V

-6

Au/Gr-Au

3.0x10

0.93 V

Au

0.0 -6

-3.0x10

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

E (V) vs Ag/AgCl

-5

2.5x10

3 x 10-5 M Guanine

b.

-5

dE dt

(1)

This large capacitive current has a strong effect on the electrocatalytic properties of the Au/Gr–Au OH electrode, when lower concentrations of guanine are analyzed (e.g., 3 × 10−5 M; see Fig. 6b). Careful inspection of this figure reveals that no signal for guanine oxidation was recorded from the Au/Gr–Au OH electrode. In contrast, the Au/Gr–Au electrode exhibited a clear peak at 0.83 V, having a height of 9.3 × 10−7 A. Next, we turned our attention to the kinetics of interfacial charge transfer, which provides more information about the origin of the enhanced electrochemical oxidation of guanine on graphenemodified electrodes. The magnitude of the current passing through the electrode/solution interface at any potential is described by the Butler–Volmer equation (Eq. (2)), which shows the variation of current density (I) as a function of over-potential (), exchange current density (I0 ), and transfer coefficient (˛) [32]:

-5

1.2x10

141



exp

 (1 − ˛)nF  RT

 ˛nF 

 − exp −

RT



(2)

where n is the number of electrons transferred during the redox process, F is the Faraday constant (96,485 C mol−1 ), R is the gas constant (8.314 J K−1 mol−1 ), and T is the temperature (K). The Butler–Volmer equation is valid over the full potential range, but simpler equations can be derived over more restricted ranges of potential. At either positive or negative over-potentials (generally larger than 0.05 V), the over-potential can be considered as logarithmically dependent on the current density. The usual plots of over-potential versus log current density are known as Tafel plots (Eqs. (3) and (4)) and can be used to determine the exchange current density (where IC and IA are the cathodic and anodic current density, respectively). I0 is an extensive parameter and is influenced by kinetics or the speed of the reaction (a large I0 characterizes a surface that allows a swift transfer of electrons).

2.0x10

-5

I (A)

1.5x10

log(IA ) = log(I0 ) +

-5

1.0x10

0.83 V

-6

5.0x10

Au/Gr-Au Au

0.0 0.65

0.70

0.75

˛nF  2.3RT

(3)

(1 − ˛)nF  2.3RT

(4)

log(IC ) = log(−I0 ) −

Au/Gr-Au-OH

0.80

0.85

0.90

0.95

E (V) vs Ag/AgCl Fig. 6. Linear sweep voltammetry recorded in solution of 10−4 M guanine (a) or 3 × 10−5 M guanine (b), with bare gold (black) and graphene-modified electrodes: Au/Gr–Au (red); Au/Gr–Au OH (blue); scan rate 50 mV s−1 . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Fig. 7a shows the cyclic voltammograms recorded in solution of 10−4 M guanine (scan rate 5 mV s−1 ) with bare gold (black) and the graphene-modified electrodes, Au/Gr–Au (red) and Au/Gr–Au OH (blue), respectively. Similar to Fig. 6a, the capacitive current of the Au/Gr–Au OH electrode is considerably larger than that corresponding to the Au/Gr–Au electrode. Fig. 7b shows the corresponding Tafel plots obtained after analyzing the cyclic voltammograms from Fig. 7a. As expected, in the case of graphenemodified electrodes, the exchange current density (I0 ) is one order of magnitude larger than that of the bare gold electrode. Based on the fact that the I0 depends on the charge-transfer resistance (Rct ) at the electrode/solution interface (Rct = RT/nFI0 ) [32] we can

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4.5x10

a. Au/Gr-Au-OH

-6

-4 10 M Guanine

3.0x10

0.8 V -6

I (A)

1.5x10

0.79 V

Au/Gr-Au

0.0

Au

-6

-1.5x10

-6

with the same amount of each material and then testing for the electrochemical oxidation of guanine. In the case of graphenemodified electrodes, a clear peak due to guanine oxidation appeared at around 0.83 V, whereas, with bare gold, a broad wave was seen at around 0.93 V. However, the capacitive current (Icap ) was about four times larger for the Au/Gr–Au OH electrode, in comparison with the Au/Gr–Au. The primary reason for the large Icap may be related to the charges of oxygen functional groups which increase the double-layer capacitance (Cdl ). This large capacitive current was observed in all linear sweep or cyclic voltammetric measurements and considerably diminished the sensitivity of the modified electrode.

-3.0x10

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Acknowledgments

E (V) vs Ag/AgCl

-6

b.

10

Log(I)

-7

10

This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS-UEFISCDI, Project Number PN-II-ID-PCE-2011-3-0129. The editorial assistance of Dr. Marinelle Ringer is gratefully acknowledged.

Au/Gr-Au-OH Io= 1.5 x 10-7 A

References

Au/Gr-Au Io= 1.1 x 10-7 A Au Io = 0.91 x 10-8 A

-8

10

-9

10

-10

10

-0.2

-0.1

0.0 0.1 Over-potential (V)

0.2

Fig. 7. (a) Cyclic voltammograms recorded in solution of 10−4 M guanine with bare gold (black) and graphene-modified electrodes: Au/Gr–Au (red); Au/Gr–Au OH (blue); scan rate 5 mV s−1 . (b) Tafel plots obtained after analysis of cyclic voltammograms from (a): bare gold electrode (black); Au/Gr–Au (red); Au/Gr–Au OH (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

conclude that graphene greatly promotes the transfer of electrons from solution to electrode. Since both modified electrodes have very close values for I0 (Fig. 7b) we can conclude that, in the case of the Gr–Au OH composite, the presence of the small number of oxygen functional groups does not critically disrupt the sp2 network. Consequently, the intrinsic properties of graphene sheets (e.g. high conductivity) are preserved. However, the major drawback of the Gr–Au OH composite is the large capacitive current observed in all linear sweep or cyclic voltammetric measurements, which considerably diminishes the sensitivity of the modified electrode. 4. Conclusions In this work, we prepared graphene–gold nanoparticle composite by RF-cCVD technique over an Aux /MgO catalyst (where x = 3 wt.%). The composite material was mildly oxidized with a mixture of sulfuric/nitric acid (3:1) and next characterized by TEM/HRTEM, as well as by FTIR spectroscopy and thermogravimetric analysis. TGA and FTIR measurements gave us valuable information about the thermal stability of the Gr–Au and Gr–Au OH samples, respectively, as well as the oxygen functional groups attached to the graphene (COOH, C OH or C O). The electro-catalytic properties of the Gr–Au and Gr–Au OH composites were demonstrated by modifying two gold electrodes

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