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Platinum-Graphene Nanocomposites

as electrocatalysts in PEM Fuel Cells

Submitted by

T.V. Sridharan

(USN No. 1RV12ME108)

Under the guidance of

Professor Manoj Neergat

Department of Energy Sciences and Engineering

Indian Institute of Technology, Bombay

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CERTIFICATE

Certified that the summer internship project report “Platinum-Graphene

nanocomposite as electrocatalysts in PEM Fuel Cells” is the bonafide work of

T.V. Sridharan, USN No:1RV12ME108, 2rd year B.Tech in Mechanical

Engineering of R.V. College of Engineering, Bangalore carried out under my

supervision during 2.06.2014 to 28.07.2014.

Place: IITB, Powai Signature of the Supervisor

Date: 28-7-2014 Name of Supervisor-

Prof. Manoj Neergat

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Acknowledgement

With great pleasure, I would like to express my gratitude to my guide, Professor

Manoj Neergat for his ideas, suggestions and encouragement during the course of

the project. I would like to thank his PhD scholars Ramesh Singh, Naresh Nalajala,

Rahul R., Wasim Feroze, Tathaghat Kar, Devi Ruttala Varaprasad, Bapi Bera and

Arup Chakraborty for the academic and technical support they provided during the

project.

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CONTENTS

Title Page 1

Certificate by the Supervisor 2

Acknowledgement 3

CONTENTS

1. Abstract 5

2. Introduction 6

3. Literature Review 8

4. Experimental

4.1 Materials 11

4.2 Synthesis of Pt/rGO 11

4.3 Synthesis of Pt/C 12

4.4 Physical and Electrochemical Characterization 12

4.5 Electrode preparation 12

5. Results and Discussion

5.1 Physical Characterization

5.1.1 TEM Analysis 13

5.1.2 XRD Analysis 15

5.2. Electrochemical Characterization

5.2.1 Cyclic Voltammetry 17

5.2.2 Oxygen Reduction Reaction 20

6. Conclusion 22

7. References 23

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1. Abstract

The use of graphene as a support material for a dispersion of platinum nanoparticles was

explored as an alternative to the conventionally used carbon black. The Pt/rGO nanocomposite

was synthesized using a one pot modified polyol method. Platinum nanoparticles were deposited

onto graphene sheets by means of borohydride reduction of H2PtCl6 in a graphene oxide (GO)

suspension. Electrochemical experiments suggested a considerably higher effective catalytic

surface area for the Pt/rGO composite compared to Pt/C of similar metal loading. The activity of

the oxygen reduction reaction (ORR) however did not show any significant improvements. The

results indicated the need for future research efforts for graphene to vie for replacement of

carbon black as an effective and economical catalyst support.

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2. Introduction

Polymer Electrolyte Membrane fuel cells (PEMFCs) are regarded as promising energy sources

for mobile electronic applications due to their high efficiencies and low operating temperatures.

Their performance and cost are essentially governed by the nature of the electrocatalysts used. It

has long been acknowledged that platinum nanoparticles show superior performance in the

catalysis of the oxygen reduction reaction [1]. However, the commercial success of PEMFCs has

been greatly hindered by high cost of Pt and its ineffective utilization.

One of the prime objectives of PEMFC research is the reduction of precious metal loading on the

electrode without compromising the efficiency of the fuel cell. A great deal of work has focused

on supported metal catalysts which show higher activity and stability compared to unsupported

bulk metal catalysts [2]. A dispersion of the catalyst on support material increases the catalytic

surface area, thereby increasing the utilization efficiency of precious metal. Support materials are

characterized by their surface area, porosity, electrical conductivity, electrochemical stability and

have a strong influence on the performance and durability of catalysts.

Carbon black, because of its high surface area and low cost, has been extensively used as a

support material in PEMFCs [3]. However, carbon blacks are impaired by problems such as, (i)

the presence of organic impurities (ii) entrapment of catalyst nanoparticles in the deep

micropores making them inaccessible to reactants thus leading to reduced catalytic activity [2].

Furthermore, carbon supports and catalytic metals have been reported to degrade under

prevailing conditions of temperature, humidity and potential at the anode. Corrosion of the

support exacerbates the agglomeration and detachment of the catalyst from support materials [4].

Recently, various carbon based nanostructured support materials have come under investigation.

Graphene, a monolayer of graphite composed of carbon atoms in a honeycomb arrangement has

attracted the attention of a multitude of researchers [5]. This two-dimensional (2-D) material has

a theoretical surface area (2630 m2 g-1) and high conductivity (103–104 S m-1) that its use as a

catalyst support [5][6].

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The conductive support facilitates efficient collection and transfer of electrons to the electrode

surface. The large specific surface area coupled with its excellent thermal, electronic and

mechanical properties, make graphene a potential alternate substrate for the deposition of

inorganic nanoparticles to produce highly dispersed composites.

In this study, the effective surface area and activity of Pt/C was compared with Pt/G composite

of the same loading of platinum. Further, the catalytic performance of Pt/G nanocomposites with

different weight compositions of platinum and graphene was studied by means of their cyclic

voltammograms and oxygen reduction reaction (ORR) polarization curves. Pt/rGO catalyst was

prepared by simultaneous reduction of H2PtCl6.6H2O and graphene oxide using a modified

polyol method [7]. TEM and XRD were employed to characterize morphology and surface

composition of the samples. The role of graphene as an effective catalyst support was

investigated.

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3. Literature Review

The unique structural, physical and chemical properties of graphene draw attention to possibility

of the preparation of novel composite materials with superior catalytic properties. Although

single layer graphene catalytic supports have not been reported, promising results from few

layered graphene stacks are encouraging increased research efforts in this direction. Thus it is

important to discuss the properties of graphene which are of relevance to its application as a

catalyst support and its influence on the activity oxygen reduction reaction in PEMFCs.

Graphene has a hexagonal arrangement of carbon atoms in a 2 D plane forming a honeycomb

lattice (Figure 1). The planar sheet structure provides a very high surface area for the attaching

catalyst nanoparticles. The large surface area is an advantage lost when graphene sheets

irreversibly agglomerate. The extraordinary properties of graphene are associated with individual

layers.

Figure 1. Graphene monolayer with a honeycomb structure [20]

The carbon atoms in graphene are said to be sp2 hybridized. The bonds provide a strong

hexagonal backbone, and the out-of plane ∏ bonds are responsible for interaction between

different graphene layers. The lone pairs of ∏ electrons are delocalized and facilitate conduction

of charge through the plane normal to the c-axis of graphite (electron conduction along c-axis is

much lower).

Graphene is considered a zero-band gap semiconductor since the valence and conduction bands

overlap. The electronic properties of graphene vary with the thickness, or the number of stacked

layers. At one atom thickness, graphene is transparent suggesting its application in photocatalytic

reactions.

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Among the several exciting properties of graphene, the one that has attracted considerable

attention is its adsorption. Transition metals, and specifically platinum and palladium have been

shown to have remarkable catalytic properties. The adsorption mechanism and interaction

between metal atoms and the graphene support has become vital to fabricate graphene-based

composite catalyst materials. Several research groups have reported the performance of

Pt/Graphene composite as effective electrocatalysts.

Soin et al. synthesized Pt/Graphene nanoflakes electrode for the Methanol Oxidation Reaction.

Microwave plasma assisted chemical vapour deposition technique was used to grow the

vertically aligned graphene nanoflakes. Raman spectroscopy confirmed the characteristics of

highly crystallized few layered graphene. Pt nanoparticles were sputtered onto the graphene

nanoflakes. Cyclic Votammetry curves demonstrated fast electron transfer (ET) kinetics for the

Pt/Graphene electrodes. The rapid electron transfer kinetics was attributed to the highly

graphitized edge structure of the nanoflakes [8].

Ali Grinou et a.l used an aniline stabilized Pt/rGO composite for electrochemical studies. The

nanocomposite was reduced by ethylene glycol solution and aniline stabilized the Pt

nanoparticles, without altering the reduced graphene oxide structure. A marked enhancement of

the electrical conductivity of the composite prepared using the aniline stabilizer was reported and

was attributed to the morphological structure, small particle size, uniform dispersion in large

quantities of Pt NPs and good interfacial interaction between the Pt NPs and rGO hybrid. [9]

Jafri et a.l synthesized Pt nanoparticles supported on nitrogen doped graphene by thermal

exfoliation of graphite oxide. This was performed by dispersing the Pt nanoparticles on the

support using the sodium borohydride reduction process. Electrochemical characterization using

Pt/N-G and Pt/G as the ORR catalyst showed a maximum power density of 440mWcm−2 and

390mWcm−2, respectively. The improved performance of Pt/N-G was attributed to the

incorporation of nitrogen in the C-backbone leading to increase in the conductivity of

neighbouring C atoms [10].

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The fast electron transport mechanism ascribed to the presence of graphene facilitates and speeds

up the Oxygen Reduction Reaction in fuel cells. Min Ho Seo et al explored the use of graphene

supported electrocatalysts for the oxygen reduction reaction (ORR) in alkaline medium. Both Pd

and Pt nanoparticles with a mean diameter of 1.8 nm were dispersed on graphene sheets (GNSs)

through chemical synthesis at a metal loading of 60 wt%. The ORR activity of these catalysts

was investigated in a 0.1 M NaOH solution and was reported to show significantly high activity

for ORR [11].

Several synthesis procedures have been adopted to prepare Pt-Graphene composites. Sequential

reduction involves the separate reduction of graphene from graphene oxide and platinum from its

precursor salt and the subsequent preparation of the composite. An alternate reduction method

involves the simultaneous reduction of both metal nanoparticles and the graphene oxide

[12][13][14]. In the microwave assisted synthesis methods, irradiation helps heating of the

reaction mixture uniformly and rapidly, allowing large-scale and efficient production of

graphene–metal composites [15].

Other techniques for metal nanoparticle decoration on the graphitic nanostructure include

electro-deposition [16], photochemical [17], and solventless bulk synthesis [18]. Although these

methods present some advantages over solution-based techniques, they are expensive and energy

consuming.

In this study, a single step modified polyol method has been employed to disperse Pt

nanoparticles on reduced graphene oxide using H2PtCl6 as a Pt precursor, and ethylene glycol

and NaBH4 as a reducing agent for Pt precursor and graphene oxide, respectively. This simpler

one-pot synthesis approach has been adopted to obtain a uniform dispersion of platinum

nanoparticles on the graphene support [7].

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4. Experimental Section

4.1 Materials. Graphite, H2PtCl6, NaBH4 and were obtained from Alfa-Aeser while KMnO4,

NaNO3, C2H6O2, H2O2 and were obtained from Merck. Nafion and N2H4-H2O was obtained from

Sigma Aldrich.

Graphite oxide (GO) was prepared by a modified Hummer’s method (Figure 2) [19]. In brief,

this method involves vigorous stirring of a mixture of graphite powder and sodium nitrate with

concentrated sulfuric acid followed by oxidation by potassium chlorate. The resulting solution is

then washed with deionized water, subject to several cycles of centrifugation and dried to obtain

graphite oxide flakes. This is stored and dispersed in solvents as needed.

Figure 2. Schematic diagram of Hummer’s method [21]

4.2 Synthesis of Pt/graphene nanocomposite.50mg of graphite oxide obtained from the

modified Hummer’s method was dispersed in ethylene glycol (1mg/ml). It was then

ultrasonicated for one hour to exfoliate the graphite oxide to graphene oxide. A homogeneous

graphene oxide slurry was obtained. Subsequently, 120mg of H2PtCl6.6H2O was dissolved in the

graphene oxide slurry and sonicated. The pH of the resultant solution was adjusted to 10 by the

addition 100mg of NaOH and stirring vigorously. The solution was transferred to a round bottom

flask and the temperature was increased up to 120˚C. When the reaction temperature reached

120˚C, 2ml of NaBH4 (20mg/ml) was added dropwise and refluxed at 120˚C for 1 hour. After

complete reduction of H2PtCl6.6H2O to platinum, the solution is cooled and neutralized using 0.1

M HCl (aq) so as to obtain a pH of 7, washed and centrifuged 3 times with water.

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The residue was suspended in 20ml of ethanol and left to dry in the oven at 80˚C for 12 hours.

After drying, the catalyst was ground to a fine powder and used for characterization.

4.3 Synthesis of Pt/carbon nanocomposite. The Pt/C nanocomposite was synthesized through

the sulfito complex route followed by reduction using sodium borohydride. 156.10mg of the

platinum precursor, Na6Pt(SO3)4.6 H2O was dissolved in 25ml of 0.5M H2SO4. The platinum

sulfito complex was added to 200ml of water and the temperature of the mixture was raised to

80˚C. Separately, 40mg of carbon black was dispersed in 20ml of H2O and sonicated for 10

minutes. This was further added to the above platinum solution and stirred continuously. Next,

25ml of H2O2 (30%) was added dropwise for 1½ hours. 25ml of NaBH4 (1mg/ml) was then

dropped into the solution over half an hour to reduce the platinum. The resultant solution was

cooled, washed and centrifuged thrice. The residue was dispersed in 20ml of ethanol and dried in

the oven at 80°C for 12 hours. After drying, the catalyst was ground to a fine powder and used

for characterization.

4.4 Physical and Electrochemical Characterization. The XRD was conducted on a Rigaku

SmartLab® X-ray diffractometer using Cu K radiation (= 0.15406 nm). The HRTEM images

were recorded with JEOL JEM 2100 Field emission electron microscope.

Electrochemical measurements were performed in a three-electrode electrochemical cell using

WaveDriver 20 Bipotentiostat/Galvanostat system from Pine Research Instrumentation, USA.

Platinum served as the counter electrode while Ag/AgCl system was used as the reference

electrode.

4.5 Electrode Preparation: Catalyst ink was prepared by dispersing catalyst powder (5.0 mg)

ultrasonically in 5.0 mL distilled water to form a homogeneous black suspension. Then, 7μL of

Nafion was dropped into the dispersion followed by 10ml of isopropyl alcohol and sonicated for

half an hour. A volume of 56μL of this solution was drop cast onto a clean glassy carbon

electrode (0.196mm2 area) and used as the working electrode. For cyclic voltammetry

measurements, the working electrode was immersed in 0.1 M H2SO4 saturated with highly

purified argon and scanned between -200mV to 800mV. Before the conduction of oxygen

reduction experiment, the solution was purged with 99.9995% O2 for about 10 min. The ORR

was carried out in the same potential range as the cyclic voltammogram at the rate of 20mVs-1.

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5. Results and Discussions

5.1 Physical Characterization

5.1.1 TEM Analysis

The surface morphology and dispersion of the platinum were determined from TEM analysis.

For the analysis, a drop of colloidal sample was dispersed on a lacey-carbon grid and dried in air.

The TEM images revealed the formation of platinum nanoparticles. Figure 3(a) displays the

reduced graphene oxide sheets on the lacey grid supporting a dispersion of platinum particles.

The folds depict an overlap of few graphene layers at the boundary.

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Figure 3. (a) TEM image of reduced graphene oxide and platinum nanoparticles. (b) Uniform

dispersion of platinum nanoparticles (c) HRTEM of higher magnification (d) selected area

electron diffraction (SAED) pattern of the Pt/rGO composite.

Figure 3(b) shows a uniform, well ordered distribution of platinum nanoparticles on the surface

of the graphene sheets. The high percentage of metal loading is clearly observed. TEM images of

relatively higher magnification from figure 3(c) enabled a rough estimation of the particle size.

The average size of the particles was expected to be ~4-7 nm. The selected area electron

diffraction (SAED) pattern (Figure 3(d)) shows bright rings due to the presence of platinum.

The sharp hexagonal spot patterns correspond to the presence of the graphene sheets.

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5.1.2 XRD Analysis

Figure 4 depicts the XRD patterns obtained from the characterization of reduced graphene oxide

(rGO), 50 wt% Pt/C and 50 wt% Pt/rGO composite catalyst. A sharp peak is observed at

26.6°from the XRD pattern of the rGO. With the reduction of graphite oxide, this peak is

expected to be close to that of graphite structure as seen. Both Pt/C and Pt/rGO displayed

diffraction peaks at 2 theta angles of 39.8°, 46.3°, 67.6° and 81.4° can be indexed to the (1 1 1),

(2 0 0), (2 2 0) and (3 1 1) planes of the face-centered cubic (FCC) Pt crystal (JCPDS card NO.

04-0802.)

Figure 4. X-ray diffraction pattern of rGO (blue), Pt/rGO (red), Pt/C

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From the XRD, particle size of the platinum nanoparticles dispersed on the graphene support was

estimated using Scherrer’s equation (Equation1).

Equation1. = k/ cos

is the particle Size

k is a dimensionless shape factor , typical taken as 0.94, but varies with the actual shape

of the crystallite;

λ is the wavelength of the X-ray = 0.154 nm for Cu k radiation;

β is the line broadening at half the maximum intensity (FWHM), after.

θ is the Bragg angle.

The plane (2 2 0) was used for calculation at a 2θ angle of = 67.23°. The FWHM, β , was found

to be 2.30576°. Upon calculation using equation1, = 4.32 nm. The size of Pt particles indicated

a good dispersion on the graphene support.

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5.2 Electrochemical Characterization

5.2.1 Cyclic Voltammetry

To investigate the utilization and electrochemical activity of the Pt/rGO composite catalyst in

comparison to the Pt/C catalyst, a cyclic voltammogram was performed in 0.1 M H2SO4

saturated with highly purified argon at a sweep rate of 20mVs-1. As the potential was increased

in the forward scan, hydrogen desorption peaks were observed in the potential window -0.2 to

0.05V (Figure 5). The potential range 0.05 to 0.5V corresponds to the charge of the double

layer by the oxygenated groups on the carbon/graphene support surface. The oxide formation

region is between 0.55-0.8V to form platinum oxides. In the reverse scan, oxygen evolution from

the platinum surface results in a reduction peak at 0.51V as Pt-O reduces to platinum metal. As

lower potentials from 0.05 to -0.2V on the reverse scan, peaks corresponding to the

adsorption of hydrogen on the surface of platinum are observed. The potential range for

hydrogen adsorption/ desorption processes comprises the hydrogen underpotential deposition

(HUPD) region.

Figure 5. Cyclic voltammograms of Pt/rGO (black) and Pt/C recorded at room temperature in an

argon saturated solution of 0.1M H2SO4

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A clear distinction was observed in the voltammograms of Pt/C and Pt/rGO. The double layer

current in the potential regime 0.05-0.5V in the case of the Pt/rGO composite is significantly

higher as when compared with conventional Pt/C (with equal Pt loading). Graphene is known to

display a high interfacial capacitance partly due to its large specific surface area. The enhanced

double layer consequently renders a higher HUPD current.

Electrochemically active Surface Area: The electrochemically active surface area (ECSA) is

an important parameter that provides information about the number of available active sites. It

accounts not only for the catalytic surface area available for charge transfer but also the access of

a conductive path for electron transfer between the catalyst and the electrode surface. Hydrogen

adsorption/desorption region in an electrochemical system is commonly used to evaluate the

ECSA. The area under the curve is a measure of the hydrogen desorbed, which provides an

estimate of the ECSA.

Equation2 below is commonly employed to calculate the effective surface area.

Equation2. ECSA [cm2Pt/g Pt] = charge [Qh μC/cm2]/ (210 [ μC/cm2]*electrode loading

[gPt/cm2])

QH- average charge integrated from the voltammogram of the adsorbtion/desorbtion

hydrogen process on the CV curve (mC)

constant 210 shows the charge in theoretical calculation to oxidize a single hydrogen

layer adsorbed on bright platinum (mC)

mPt is the platinum loading on the surface sample (g cm−2)

The ESCA of the Pt/rGO composite was calculated to be 61.67 m2/g Pt while that of Pt/C was

35.51 m2/g Pt. This result indicates a smaller particle size and a far better utilization of Pt in the

Pt/rGO nanocomposites which is essential for improving the practical performance of the

PEMFCs.

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The following cyclic voltammogram, (Figure 6.) depicts the cyclic voltammogram of Pt/rGO

nanocomposite with different Pt loading. It was observed that graphene largely masks the Pt

features for loadings less than 50%. This is due to the high double layer capacitance of the

graphene support.

Figure6. Cyclic voltammograms of Pt/rGO with platinum loading of 20% (red) and 50% (blue)

recorded at room temperature in an argon saturated solution of 0.1M H2SO4

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5.2.2 Oxygen Reduction Reaction

The oxygen reduction reaction was carried out in the potential regime -200 to 800mV in an

oxygen purged solution of 0.1M H2SO4 to compare the activity of Pt/C and Pt/rGO catalyst.

From the ORR depicted in Figure 7, it is observed that the curves are comparable. In the case of

the Pt/rGO composite however, current decays more rapidly as the potential is increased between

-200 and 600mV. This can be ascribed to the presence of unreduced functional groups on the

graphene sheets that hinder the diffusion of oxygen to the surface of the electrocatalyst.

Figure7. ORR polarization curves of Pt/rGO and Pt/C catalyst recorded at room temperature

with a sweep rate of 20mVs-1 in O2-saturated 0.1 M H2SO4 solution.

At higher potentials in the oxide formation region, it is observed that the Pt/rGO current is

marginally higher.

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The effect of varying the platinum loading on the catalytic performance was also studied. It is

seen from Figure 8 that an increase in the metal loading from 20% to 50% resulted in an

increase in the limiting current and half wave potential.

Figure8. ORR polarization curves of Pt/rGO catalyst at metal loading of 20% (black) and 50%

(blue) recorded at room temperature at a rate of 20mVs-1 in O2-saturated 0.1 M H2SO4 solution.

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6. Conclusions

1. A single step modified polyol method was adopted to prepare Pt/rGO composite. This

strategy allowed for efficient synthesis of highly loaded Pt catalyst with small

nanoparticle size and uniform particle dispersion.

2. The platinum features were masked at metal loadings less than 50% on graphene

supports. This is due to the high double layer capacitance of graphene. The requirement

of high metal loadings is a potential area for future research efforts to realize the

application of Pt/rGO composite electrocatalysts.

3. Comparison of the effective surface area of Pt/rGO to that of Pt/C with similar metal

loading revealed the presence of increased number of active sites and higher utilization of

the platinum supported on graphene.

4. Comparison of the oxygen reduction reaction however did not show any significant

improvement in activity of the Pt/rGO composite. This was attributed to the presence of

oxygen moieties on the surface of partially reduced graphene hampering mass transport

of oxygen to the electrode.

The effort to utilize graphene as an alternative support material for platinum catalysts in

fuel cells showed both promise as well as the challenges involved in leveraging the

theoretical properties of high specific surface area, thermochemical stability and

conductivity of graphene that suggest its use as an excellent catalytic support. Further

work is necessary to develop strategies to improve the sluggish oxygen reduction kinetics

and reduce the precious metal loading for to make it an economical and viable alternative

to the presently used carbon supports.

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