direct electro-deposition of graphene from aqueous suspensions

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This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys.,2011, 13, 91879193 9187

Cite this:Phys. Chem. Chem. Phys., 2011, 13, 91879193

Direct electro-deposition of graphene from aqueous suspensionsw

Matthias Hilder,*a Bjorn Winther-Jensen,b Dan Li,b Maria Forsythc and

Douglas R. MacFarlanea

Received 20th January 2011, Accepted 4th March 2011

DOI: 10.1039/c1cp20173e

We describe the direct electro-chemical reduction of graphene oxide to graphene from aqueous

suspension by applying reduction voltages exceeding 1.0 to 1.2 V. The conductivity of the

deposition medium is of crucial importance and only values between 425 mS cm1 result in

deposition. Above 25 mS cm1 the suspension de-stabilises while conductivities below 4 mS cm1

do not show a measurable deposition rate. Furthermore, we show that deposition can be

carried out over a wide pH region ranging from 1.5 to 12.5. The electro-deposition process is

characterised in terms of electro-chemical methods including cyclic voltammetry, quartz crystal

microbalance, impedance spectroscopy, constant amperometry and potentiometric titrations,

while the deposits are analysed via Raman spectroscopy, infra-red spectroscopy, X-ray

photoelectron spectroscopy and X-ray diffractometry. The determined oxygen contents are similar

to those of chemically reduced graphene oxide, and the conductivity of the deposits was found to

be B20 S cm1.

1. Introduction

Although graphene was only first isolated in the early 60s,1 the

discovery of Geim et al. in 2004 sparked great interest of this

new member of the carbon family,2 which since then has

become one of the most intensively studied materials in recent

times.35 Graphene is a monolayer of aromatic sp2 hybridised

carbon which forms 2D crystals6,7 and is the building block of

other carbon species. It can, for example, be rolled up into 1D

carbon nanotubes, scrunched into 0D fullerenes or stacked

to form 3D graphite.3,8,9 Its thinness, mechanical strength,

transparency and conductive properties make it a popular

research subject for electro-chemical applications including

sensors,9 electrodes,1012 transistors,6 solar cells13,14 and fuel

cells1517 to name only a few. Graphene is thought to be a

promising candidate to replace the transparent conductor

indium tin oxide since thin layers are both transparent and

conductive. The lack of functional groups makes it chemically

compatible for most composite applications. Additionally,

functionality can be introduced chemically, thus adding specific

reactivity to it. Graphene is also one of the few materials which

can be produced in the form of single atomic sheets,18 which

makes it very useful for fundamental research.

Although graphene was predicted not to exist for thermo-

dynamic reasons19 in 2004 Geim et al. produced single atomic

layers of honeycomb carbon by mechanical exfoliation of

graphite.2 This method involves peeling off the surface of a

piece of graphite, one layer at a time, using adhesive tape.

Despite the laborious process, the sheets obtained are of very

good quality. Since then other methods have been reported

including thermal desorption of Si from SiC,20,21 chemical

vapour deposition involving hydrocarbons,22,23 unzipping of

carbon nanotubes,24,25 following protocols of organic

synthetic ring condensations,26,27 electron beam irradiation

of polymers,28 arc discharge involving graphite,29 reacting

ethanol with sodium followed by pyrolysis,30 exfoliation of

graphene in organic solvents,31 heat treatment of nano

diamond32 and solid state reactions.33 All of these approaches

have their own advantages and disadvantages, however one of

the simplest is the reduction of graphene oxide.3439 Graphene

oxide is a highly oxidised form of graphene,4042 which

contains polar organic groups including carboxylic acid,

phenol, aldehyde, and epoxide functional groups.43,44 These

make graphene oxide highly hydrophilic so it can easily be

dispersed in aqueous suspensions.

To reduce the non-conductive graphene oxide (GO) to

graphene, various methods have been established including

thermal reduction,45,46 flash reduction,47 enzymatic reduction48

or heating in reducing atmospheres (e.g. ammonia, hydrazine,

hydrogen).49,50 The most common approach however, is

reduction using hydrazine or dimethylhydrazine from an

aqueous suspension.49,5156 Although hydrazine and its

derivatives are highly toxic, carcinogenic and explosive, it is

one of the few reducing reagents compatible with water.

a Monash University, School of Chemistry, Clayton, Victoria, 3800,Australia. E-mail: [email protected];Tel: +61 3 990 20323

b Monash University, Materials Science, Clayton, Victoria, 3800,Australia

c Deakin University, 221 Burwood Highway, Burwood, Victoria 3125,Australiaw Electronic supplementary information (ESI) available. See DOI:10.1039/c1cp20173e

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9188 Phys. Chem. Chem. Phys.,2011, 13, 91879193 This journal is c the Owner Societies 2011

More traditional reducing agents including NaN3, NaBH4,

NaH2,etc.can be used in organic media,5759 however in order

to suspend GO in organic solvents it has to be chemically

modified.

Despite the vast amount of information published in recent

years, there has surprisingly been little progress in making

graphene electro-chemically. This is potentially the simplest

and least expensive method, and it is suitable for mass

production, even for large area applications. In principle,

highly controllable, conformal films can be formed without

the need for volatile solvents or reducing agents. The few

publications on electro-chemically reduced graphene involve

either pre-applying GO dispersions directly onto electrodes

followed by reduction in an electrolyte,6063 functionalising the

electrodes in order to add templating properties to GO 64,65 or

chemically functionalising GO and carrying out the deposition

process from organic solvents.66 Electro-depositing from

aqueous suspension rather than post-reducing a pre-

applied coating of graphene oxide electrochemically would

simplify the process and make graphene materials more readily

accessible for new applications and devices. Here we describe

for the first time, the direct electro-chemical reduction of

graphene oxide from an aqueous suspension to form thin

films on the electrode surface, through the control of the ionic

conductivity of the electrolyte mixture. We also discuss the

completeness of the reduction process and compare it to

graphene obtained from reducing GO pre-applied to the

electrode. The products are characterised via infra-red

spectroscopy, Raman spectroscopy, X-ray diffraction, X-ray

photoelectron spectroscopy, impedance spectroscopy and

surface charge titration.

2. ExperimentalGraphene oxide

Graphite oxide was synthesized from natural graphite

(SP-1, Bay Carbon) by a modified Hummers method as

originally presented by Kovtyukhova and colleagues.42,67

As-synthesized graphite oxide was then purified by dialysis

and then subjected to ultra-sonication protocols which are

described in detail elsewhere.39

Reagents

Reagents were either used to act as electrolytes or to adjust the

pH and were sodium chloride (Merck, p.a.), potassium

chloride (Merck, p.a.), sodium hydroxide (Merck, p.a.) andhydrochloric acid (UNIVAR, analytical reagent).

Electro-chemical techniques

All depositions and electro-chemical characterisations were

carried out using a multi-channel potentiostat and controlled

using VMP2 EC-lab 9.56 software. CVs were recorded in the

range of1.6 V to +1.0 V using a scan speed of 0.1 V min1.

The CV as well as the deposition experiments were carried out

using a three electrode setup including a calomel reference

electrode, a titanium mesh counter electrode and a gold mylar

or glassy carbon working electrode. The solution was stirred

with a magnetic stirrer and nitrogen was bubbled through the

medium. A pre-reduction step (5 min, 0.8 V) was introduced

to oxidise the dissolved oxygen.

Constant potential deposition was carried out applying

voltages of 1.0 to 1.4 V. An aqueous GO suspension

(0.5 mg mL1) was mixed 1 : 1 with the NaCl electrolyte

(0.25 M for the pH experiments and varying concentrations

to determine the conductivity thresholds). The pH was

adjusted to values described in the Experimental section using

diluted solutions of sodium hydroxide or hydrochloric acid. The

depositions were carried out for 15 min. Black spongy deposits

with high water contents were obtained. Upon drying at room

temperature, a grey shiny conductive film was formed. For the

XPS study, these films were immersed into a GO-free NaCl0.25Melectrolyte for post-reduction (1.0 to 1.4 V). GO suspension

was also cast straight onto the working electrode and post-

treated under the same conditions as described above.

To determine the conductivity of the deposit, a glass slide

was plasma cleaned and sputtered with gold (SPI Module

Sputter Coater) leaving a 1 mm gap. Onto this glass strip

graphene oxide was cast connecting the two gold-coated areas.

The GO was electro-chemically reduced (1.2 V, NaCl0.25M).

After drying the thickness was determined using a VEECO

Dektak150 to be B1.0 mm. Then the two gold areas were

electrically connected and the resistivity was measured by

impedance methods (1 Hz to 1 MHz, 3 measurements per

frequency). The surface conductivity was calculated from the

resistivity, the area and the thickness.

A QCM200 Quartz Crystal Microbalance combined with a

QCM25 5 MHz Crystal Oscillator was used for the QCM

experiments. The QCM was used in the same manner as like

the working electrode.

Conductivities of the GO/NaCl suspensions were also

determined by impedance methods (1 Hz to 5 MHz,

3 measurements per frequency). A two platinum electrode

setup was used and the results were calibrated against a KCl

standard solution of known concentration and conductivity in

order to determine the cell constant.

The deposition rates were determined by measuring the

weight of the deposit after 1 h. Using Faradayss law the

molecular weight was calculated from the time, the weight,

the current and the faraday constant assuming a one electro-

reduction process.

IR

IR samples were recorded using a Perkin Elmer Spectrum

RX1 FT-IR spectrometer. KBr pellets were made by grindingthe samples with dried potassium bromide (Merck, p.a.) and

applying the equivalent of 10 tons pressure for 15 min while

applying vacuum. The samples were then spectroscopically

characterised in the region of 500 to 4000 cm1 (16 scans, 4 cm1).

The background was manually removed and the intensity

normalised against the intensity of the CQC stretching

vibration.

XRD

Powder X-ray diffractograms were collected using a Philips

PW1130 equipped with a Cu cathode ray tube PW2213/20

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(60 kV, 1500 W) which generated CuKa

radiation. A 2yangle

range of 21 to 501 was measured (step width = 0.021, scan

speed of 11per minute).

XPS

XPS analysis was performed using an AXIS-HSi spectrometer

(Kratos Analytical Inc., Manchester, UK) with a monochro-

mated Al Kasource at a power of 144 W (12 kV

12 mA), ahemispherical analyser operating in the fixed analyser trans-

mission mode and the standard aperture (1 mm 0.5 mm).

The total pressure in the main vacuum chamber during

analysis was of the order of 108 mbar. A magnetic immersion

lens was used for charge compensation (typically 1 to 4 eV).

The area of analysis is estimated to be 0.35 mm2. A pass energy

of 320 eV was chosen for survey spectra (range 5 to 1100 eV)

while 40 eV was used for high resolution C1s spectra (292277 eV).

The data were analysed using CasaXPS software. The spectra

were corrected using the Shirley approach and the sensitivity

factors supplied by the manufacturer were used for identification.

The position was calibrated against the C1s peak of graphene

which is expected to occur at 284.4 eV.

Raman

A Renishaw Invia Raman Confocal Microscope was used to

study the deposits. The excitation source was a 632.8 nm

HeNe laser system. The excitation energy was set to 50%

and a scan rate of 10 cm1 s1 was chosen. The signal position

was calibrated against silicone.

Micrographs

A Nikon Eclipse ME600 was used to record micrographs of

the films.

3. Results and discussion

Electro-chemical characterisation of the electro-deposition

process and the deposits

Successful electro-reduction of graphene oxide (GO) to

graphene from aqueous suspension was observed if a reduction

potential of1.2 V was applied to a neutral GO suspension

(0.5 mg mL1) which was mixed 1 : 1 with a sodium chloride

solution (NaCl = 0.25 M). Fig. 1 shows the cyclic voltammogram

(CV) of the GO dispersion compared to the NaCl electrolyte

alone. The additional reduction current in the region of1.0 V

to 1.2 V indicates the reduction of GO.

A quartz crystal microbalance (QCM) was also employed to

monitor the electro-reduction process. The inset in Fig. 1

shows the frequency plot as a function of the deposition time.

The frequency dip indicates graphene deposition. Although it

is, in principle, possible to determine the deposited mass from

QCM experiments, the spongy graphene deposits contain

entrapped water (up to 95%) and thus a meaningful direct

measure of mass proved not to be possible.

Experiments showed that adding a supporting electrolyte to

the GO suspension is essential for electro-deposition. An

excess of reagent (acids, bases or salts), however, results in

destabilisation of the suspended GO particles. Graphene oxide

contains anionic functional groups (e.g. phenolates and

carboxylates); partial dissociation of these groups results in

negative charges bound to the particle surface. In suspension

these surface charges are responsible for the formation of an

electro-chemical double layer which surrounds the particles.

This layer stabilises the suspended particles by physically

preventing them to agglomerate. The thickness of the double

layer decreases with increasing electrolyte concentration,

hence stabilisation can only be achieved in very dilute systems.

By adding NaCl solutions of varying concentrations to the GO

suspensions (0.5 mg mL1), the quality of the suspension was

evaluated visually and correlated to the overall conductivity of

the medium at pH 7, using impedance spectroscopy. The

threshold conductivity regarding the stability of the GO

suspension was found to be 25 mS cm1. Suspensions of

higher conductivity de-stabilise, forming lumpy agglomerates.

The same approach was adopted to determine the lower

conductivity threshold for electro-reduction of the GO. By

adding NaCl solutions of various concentrations to GO0.5mg mL1

suspension (pH 7) the threshold conductivity for electro-

deposition was determined to be 4 mS cm1 or higher if a

reduction potential of1.2 V was applied. Combining these

two findings means that there is an optimum conductivity

range between 4 and 25 mS cm1 in which (i) the GO

suspensions are stable and (ii) graphene can be electro-deposited

from aqueous suspensions. It should be mentioned that for

conductivities larger than 25 mS cm1 deposition is still

observed, but the non-uniform nature of the destabilised

suspension results in poor quality films. Fig. 2a summarises

these findings.

Another important parameter is the pH of the deposition

medium since it strongly influences the state of protonation/

deprotonation of functional groups on the GO surface and

therefore influences both the dispersion stability and the

electro-chemical properties. To study the pH range under

which GO can be electro-chemically reduced, the NaCl0.25Melectrolyte was added to GO0.5mg mL1 suspensions (1 : 1 by

volume) followed by pH adjustment with dilute HCl or NaOH

solutions. The addition of NaCl ensured that the suspension

remained within the optimum conductivity range for electro-

deposition (as established above). The CVs for GO/NaCl

Fig. 1 Cyclic voltammogram of (i) the NaCl electrolyte and (ii) the

GO/NaCl system (working electrode area = 1.5 cm 2, scan rate =

10 mV s1), GO starts to reduce at potentials below 1.0 V which is

before the NaCl electrolyte itself begins to reduce. The inset shows the

drop in frequency of an oscillating quartz microbalance electrode

indicating deposition of graphene oxide onto the oscillating quartz

crystal. Reference electrode = saturated calomel electrode.

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dispersions at different pH values are presented in the ESIw

(Fig. S2). The working electrode was visually inspected for the

presence of deposits after applying a reduction potential of

1.2 V. Indeed it is possible to deposit graphene over a wide

pH range starting from pH 1.5 to 12.5 (see Fig. 2b). Beyond

those pH ranges (pH o 1.5; pH > 12.5) the conductivity

exceeds 25 mS cm1 and the suspension destabilises due to

agglomerate formation. Besides compatibility issues at low pH

(applying a reduction potential of 1.2 V, the gold mylar

substrate disintegrated at pH 1.0) there are also side-reactions

competing with GO reduction. Not only does reduction of H+

compete with GO reduction electro-chemically, but the

formation of hydrogen bubbles also create a physical barrier

which hinders the GO particles to approach the working

electrode, thus interfering with the process. The CVs in the

basic region were similar to those from the neutral region thus

there are no interfering side-reactions and the limitation at

high pH is thought to be caused by compatibility issues. While

electro-deposition is observed at 1.0 V for basic and acidic

suspensions, graphene deposits at slightly higher potentials for

neutral suspensions (Fig. S3 in ESIw).

At pH 7, the deposition rate was determined gravimetrically

to be 1 mg h1 under the conditions used here. The average

current was 0.52 mA over this time and using Faradays law it

can be estimated that one electron is consumed for every 4

carbons deposited (or equivalently one two-electron reduction

for every 8 carbons deposited). This is in accord with the

degree of oxidation of GO reported.6769

The grey/black colour of the deposit suggests that graphene

oxide (brown) has been electro-chemically reduced to graphene

(black). To determine the conductivity of the deposited film,

impedance measurements were conducted on the samples over

a 1 mm gap between two gold electrodes. The resistance was

43 O and, with an average thickness of 1.0 mm as determined

by profilometry, the bulk conductivity was calculated to be

23 5 S c m1. The review comparing conductivities of

graphene materials by Park and Ruoff reports values ranging

from 2 to 7200 S cm1.70 The present conductivity is therefore

in reasonable accord with these results, though it is important

to note that the surface of the deposit is quite rough and that

an average thickness was used in the calculation. The

determined result is therefore probably an underestimate of

the conductivity of the bulk material.

Spectroscopic characterisation of the deposits

Raman spectroscopy has been widely used in the characterisation

of graphene and there are several reviews available.71,72 The

spectrum of graphene oxide and of the electro-deposited

sample is shown in Fig. 3. The Raman spectrum of graphene

usually consists of three main components, the D (A1g) band

which occurs between 1330 to 1360 cm1, the G band (E2g) at

around 1580 cm1 and the G 0 band (2nd order, two phonon

mode) at 2700 cm1. The D peak, which does not occur in

bulk graphite, indicates a decrease in symmetry due to the

defects on the edges. The intensity of the D and G peaks is

reversed once graphene oxide is reduced to graphene. The D

band reflects the amount of defects present and it has been

Fig. 2 (a) After adding NaCl (concentration gradient) to an aqueous

GO suspension (0.5 mg mL1) the pH was adjusted to 7. Then the

stability of the suspension was evaluated visually and deposition

experiments were carried out (1.2 V, Ti counter electrode, calomel

reference electrode) and the gold mylar working electrode was visually

examined for deposits. The conductivity of the suspensions was

determined by impedance measurements (1 Hz to 1 MHz,) at room

temperature (calibrated against a KCl standard solution). There is an

optimum conductivity range between 4 and 25 mS cm1. (b) Deposition

behaviour of GO (0.5 mg mL1) and NaCl (0.25 M) at different

pH values. The conductivity for all samples was in the range of

optimum conductivity regarding deposition and stability. Deposits

were obtained in the pH range from 1.5 to 12.5 (1.2 V, calomel

reference electrode, Ti counter electrode).

Fig. 3 Raman spectrum of graphene electro-deposited from neutral

suspension at 1.2 V vs. SCE. The intensity changes in the G and D

bands indicate the formation of graphene. The double peak of the G 0

band indicates the presence of bulk graphene (e.g. several layers). The

spectra were obtained by excitation of the sample with a HeNe laser

(632.8 nm, 50% intensity).

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observed by others that the defect concentration is increased in

graphene.63 The reversal is a good indication of the change

that occurred during electro-reduction. The ESIw shows the

diagrams for the deposits obtained at various pH values

(Fig. S4).

Infra-red spectra (IR) of GO and electro-deposited

graphene are presented in Fig. 4. As discussed previously,

graphene oxide contains various oxygen containing functional

groups including ether, epoxide, carboxylic acid, phenolate

and aldehyde groups. The IR indicates the presence of OH,

CH, CQC, CQO and CO groups in GO. The most

dramatic change on reduction occurs in the region of the

CO stretching vibration. After reduction, the signals in this

region are much less intense and less complex compared to the

GO starting material. The same is true for the CQO region.

Reduction thus seems to eliminate mainly CO groups (ethers,

epoxides, esters, carboxylic acids and alcohols/phenols). There

are, however, oxygen containing groups still present even after

reduction. These findings are in close agreement with the work

of Zangmeister.68

X-Ray diffractograms (XRD) were recorded for graphene

oxide and the electro-reduced deposits. While GO has a sharp

diffraction signal at 10.81 (d = 0.82 nm) which is associated

with stacking in the (0 0 2) direction, the signal disappears

upon electro-reduction (Fig. S5 in the ESIw). Although this is

of little diagnostic value, the change still indicates that the GO

precursor has been electro-chemically converted.

As discussed above the IR of the deposited graphene films

indicated the presence of oxidised carbon species. It was thus of

interest to quantify the degree of reduction. To do so, various

treated films were prepared and investigated by X-ray photo-

electron spectroscopy (XPS). Films were deposited by applying

1.2 V for 15 minutes to an aqueous suspension of graphene

oxide (0.5 mg mL1, pH 7.0). The films were then post-treated by

immersion in a GO-free solution of NaCl (0.25 M) and applying

a reduction potential of1.0 V,1.2 V or 1.4 V for 15 min. To

compare the quality of products electro-deposited from aqueous

suspension to previously described reductive techniques,6063

graphene oxide suspensions (15 mg mL1) were cast directly

onto the working electrode and post-treated in an identical

manner (15 min at 1.0, 1.2 and 1.4 V in 0.25 M NaCl).

Fig. 5 shows the C1s XPS spectra of the graphene oxide

(applied and post-treated) and graphene deposits from suspen-

sion (deposited and post-treated) and Fig. 6 shows an example

of a C1s XPS spectrum of graphene oxide and a deposited

graphene film, including curve fits for the various carbon

components. It is evident that graphene oxide shows strong

signals due to the presence of oxidised carbon species

(CO, CQO and OCQO). Upon electro-chemical reduction,

the intensity of these signals decreases. Table 1 shows the

Fig. 4 IR spectra of graphene oxide and graphene electro-deposited

at pH 7.0 at 1.2 V. Although oxygen containing groups are

still present, their concentration is significantly decreased. This is

particularly true for the presence of groups containing CO single

bonds.

Fig. 5 C1s XPS spectra of graphene electro-deposited from suspen-

sion (15 min, 1.2 V) and post-treated in NaCl (15 min, 1.2 V),

compared with graphene oxide applied and post-treated in NaCl

(15 min, 1.2 V). The amount of highly oxidised carbon species

decreases after reduction.

Fig. 6 C1s XPS spectrum including curve fits for the individual

components of (a) graphene oxide and (b) electro-deposited graphene.

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C : O ratios derived from the XPS survey spectra and from

the C1s spectra. This shows that the C : O ratios increase with

increasingly negative reducing potentials indicating a decrease

in the oxygen content. The trends from the survey spectra are

in agreement with the results obtained from the C1s spectra.

Differences result from the assumption that each oxygen atom

is bound to one carbon atom while in reality the oxygen might

be shared between two carbons (e.g. in ether COC bonds)

while other carbon atoms are bound to more than one

oxygen (e.g. CO2 ester or carboxylic acid bonds). The films

electro-deposited from suspension contain less oxygen

compared to those obtained by reducing pre-applied graphene

oxide. Table 2 monitors the change of various functional

groups during the reduction process. The most significant

change is observed in the concentration of CO groups

(phenol, ether) which significantly decreases as the reduction

process proceeds. Although it is possible to remove all oxygen

by for example heating it for 12 hours at 800 1C in a H2/Ar

stream (20% H2, 50 cm3 min1),73 our findings are in

agreement with reported C : O ratios ranging from 3.6 to

6.4 for chemically reduced graphene oxide employing

hydrazine.6769,73 The observation that the reduction process

mostly affects CO single bond groups is in agreement with the

IR results.

4. Conclusions

Graphene films have been successfully produced by electro-

chemical reduction of graphene oxide from aqueous

suspension. A critical parameter in the formation of good

quality films is the conductivity of the medium; the optimum

conductivity range for the system studied (GO = 0.5 mg mL1)

was found to be between 4 and 25 mS cm1. Below 4 mS cm1

no graphene can be electro-deposited and above 25 mS cm1

the GO suspension becomes unstable. Although these results

are dependent on the system (concentration, history of the GO

sample, electrolyte,etc.) the overall concept of electro-deposition

and suspension destabilisation still applies to all samples. The

electro-deposition can be carried out over the wide pH range

from 1.5 to 12.5. Beyond those pH limits the suspension

destabilises. While in the acidic region H+ reduction competes

with GO reduction, no interfering chemical processes are

occurring in the neutral and basic regions. Deposits were

obtained at 1.0 V for acidic and basic suspensions and at

1.2 V for neutral suspensions. The deposition current at a

given potential was approximately constant over time and the

deposition rate was gravimetrically determined at B1 mg h1

at 1.2 V. Thus film thickness control could easily be achieved

over a wide range viachoice of deposition time. The conductivity

of the films was of the order of 20 S cm1. While XRD and

Raman indicated that a chemical and structural change occurs

during the electro-chemical conversion, IR and XPS confirm

that the content of CO groups is significantly reduced upon

electro-deposition. The oxygen content is lower for samples

deposited straight from suspension compared to GO which

was cast onto electrodes followed by reduction in the electro-

lyte. The oxygen content decreases with increasingly negative

reduction potential.

Acknowledgements

We would like to thank the Australian Centre of Excellence

for Electromaterials Science for funding. DRM and BWJ are

grateful to the Australian Research Council for Fellowships.

We would also like to thank Dr Noel Clark (Commonwealth

Scientific and Industrial Research Organisation) for his input.

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Table 1 C : O ratios for various graphene electro-deposits derivedfrom XPS. First graphene was deposited from aqueous suspension(GO0.5mg mL1, 1.2 V, 15 min). After drying the samples were post-treated in NaCl0.25Mfor 15 min applying 1.0 V, 1.2 V or 1.4 V.This method was compared to GO which was cast onto the workingelectrode and post-treated using the same conditions. From the surveyspectra the C/O ratio was determined. The results from the surveyspectra were compared to the C/O ratios derived from the C1s spectra.Components occurring in the range of 284285 eV were assigned toreduced carbon species while components of higher binding energywere assigned to oxidised carbon species. The C/O ratios of the twoapproaches follow a similar trend

XPS survey C1s

Electro-deposited Cast Electro-deposited Cast

GO 3.3 3.3 2.5 2.5Initial deposition 5.1 NA 3.5 NA1.0 V 6.3 5.3 4.9 3.61.2 V 6.8 6.5 4.7 3.61.4 V 7.8 6.9 5.9 4.7

Table 2 Carbon components in the graphene films based on

components observed in the XPS C1s spectrum. The C1s spectra werecomposed of reduced carbon species (e.g. 284285.5 eV), carbonsingle bond species (286287 eV) and carbon double bond species(around 288 eV). Electro-reduction seems mostly to eliminate COcomponents

GO Initial deposition 1.0 V 1.2 V 1.4 V

Electro-depositedCreduced 1.00 1.00 1.00 1.00 1.00CO 0.57 0.30 0.15 0.16 0.16CQO 0.09 0.09 0.11 0.11 0.04CastCreduced 1.00 NA 1.00 1.00 1.00CO 0.57 NA 0.21 0.21 0.20CQO 0.09 NA 0.18 0.18 0.07

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direct electro-deposition of graphene from aqueous suspensions

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