Electrochemical Reduction of Graphene Oxide and Its in Situ

7/30/2019 Electrochemical Reduction of Graphene Oxide and Its in Situ

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This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys.

Cite this: DOI: 10.1039/c2cp42253k

Electrochemical reduction of graphene oxide and its in situ

spectroelectrochemical characterization

Antti Viinikanoja,*a Zhijuan Wang,b Jussi Kauppilaac and Carita Kvarnstro m*a

Received 3rd July 2012, Accepted 20th August 2012

DOI: 10.1039/c2cp42253k

The electrochemical properties of self-assembled films of graphene oxide (GO) on

mercaptoethylamine (MEA) modified rough Au-surfaces were studied. The film deposition

process on MEA primed gold was followed by surface plasmon resonance measurements and the

film morphology on 3-aminopropyltriethoxysilane primed Si(100)-surface was studied by atomic

force microscopy. The deposited few layer thick GO films on gold were electrochemically reduced

by cyclic voltammetry simultaneously as the structural changes in the film were recorded byin situ vibrational spectroscopies. In situ surface enhanced infrared spectroscopy results indicate

that the effect of the applied potential on the GO structure could be divided into two parts where

the changes occurring at moderate negative potentials are mainly related to changes in the double

layer at the filmelectrolyte interface and to hydrogen bonding of intercalated water between the

GO sheets. At potentials more negative than 0.8 V vs. Ag/AgCl the reduction of GO starts to

take place with concomitant conversion of the different functional groups of the film.

Introduction

A pristine graphene sheet possesses high electron conductivity1

and fast heterogeneous electron transfer rate at the graphene

sheet edges,2 properties that are crucial for application in

electrochemical devices in energy storage or production or inelectrochemical sensors of different kinds.35 Since the revival

of the investigation of graphene in 20046 chemical exfoliation

of bulk graphite by strong oxidants or strong acid intercalation

has been developed with the expectation of obtaining graphene

like materials on a large scale and at low cost.3,4 The harsh

treatment of graphite in the production of bulk graphite or

graphene oxide (GO) excludes or depresses the properties found

in pristine graphene sheets. As a result, the graphite oxide or

GO is a highly hygroscopic, electrically insulating material,

and its stoichiometry is shown to depend on the method of

preparation and on the kind of graphite used.7 Subsequently,

the chemically oxidized graphite must be reduced by chemical,

thermal, or electrochemical means to retain at least some of

the graphene structure.8,9

The electrochemical reduction was introduced as a fast

and clean way (i.e. no contamination from reducing agents)

for producing a partly recovered sp2 lattice.10 Especially in

manufacturing flexible reduced graphene oxide (rGO) films

with complex patterns of a specific thickness electrochemical

reduction has shown its potential.11 Graphite or GO can be

electrodeposited and simultaneously reduced as thin conduct-

ing films from buffered aqueous solutions in a wide pH

range12,13 or from ionic liquids.14 The ionic surface groups

on the GO sheets enable a thin film preparation by the

electrostatic layer-by-layer self-assembly technique.15 The

method of fabrication and the way of reduction of the GO

film influence the final rGO product and its electrochemistry.

In comparison to thermally reduced graphene oxide that

contains a large amount of structural defects the chemically or

electrochemically reduced GO contains a higher amount of

oxygen containing groups.16 Electrochemical reduction has

been reported to only partially remove the oxygen containing

functionalities leaving groups mainly in the basal plane due to

a faster electron transfer rate around the edges of the GO

sheets.17 While electrochemistry alone cannot give details

about structural changes in thin films it is often applied in

combination with a spectroscopic technique.18,19 We have

previously used surface sensitive IR techniques to follow the

formation of self-assembled mono- and multilayers to probe

the structure, organization, chemical composition, and inter-

actions within these thin films.20,21 Previously, the thermal

reduction of single and multilayer GO was studied by these IR

techniques after the reduction process and the effect of inter-

calated water between the GO sheets on the final reduced

a Turku University Centre of Materials and Surfaces (MatSurf),Laboratory of Materials Chemistry and Chemical Analysis,University of Turku, Vatselankatu 2, 20014 Turku, Finland.E-mail: [email protected], [email protected];Fax: +358-2-333 6700; Tel: +358-2-333 6714, +358-2-333 6729

b School of Materials Science and Engineering, NanyangTechnological University, 50 Nanyang Avenue, Singapore 639798,Singapore

c The Finnish National Graduate School in Nanoscience(NGS-NANO), Nanoscience Center, P. O. Box 35, 40014,University of Jyvaskyla, Finland

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Phys. Chem. Chem. Phys. This journal is c the Owner Societies 2012

structure was discussed. The intercalated water was found to

affect the defect formation and carbonyl formation in multi-

layered graphene oxide films.22,23 By combining infrared

absorption spectroscopy and electrochemistry, the evolution

of oxygen containing defects can be followed in situ during the

electrochemical reduction of GO.

In this work in situ Surface Enhanced Infrared Spectroscopy

(SEIRAS)24 was used with quasi-single crystalline metal films

deposited on high refractive materials in attenuated total

reflection (ATR) geometry together with cyclic voltammetry.

SEIRAS enhances the signal-to-noise sensitivity by a factor of

1050 higher than that of IR reflection absorption spectro-

scopy which makes it a superior technique in studying reaction

processes on surfaces. We focus on the electrochemical

reduction in the aqueous media of a few layers of GO

deposited by self-assembly on the roughened gold plated

reflection element. Using the ATR geometry i.e. entering the

film from the backside through the substrate we can minimize

the interference from electrolyte contribution to the spectra

and get the main signal from intercalated water and the

potential dependent change in functional groups in the GO.

In situ surface enhanced Raman spectroscopy (SERS) and

atomic force microscopy (AFM) were further applied for rGO

characterization.

Experimental methods

Chemicals

3-Aminopropyltriethoxysilane (Fluka AG, 96%) (APTES), and

2-mercaptoethylamine hydrochloride (Aldrich, 98%) (MEA) were

used for the preparation of self-assembled monolayers. KCl

(FF-Chemicals, 99.5%), and NaF (J. T. Baker, Baker analyzed)

were used with Millipore water as electrolytes in electrochemical

and spectroelectrochemical measurements. The chemical reduction

of GO was done with hydrazine monohydrate (Aldrich, 98%).

HF (J. T. Baker, 48%), NH4F (Merck, p.a.), potassium tetra-

chloroaurate (Aldrich, 98%), sodium sulfite Na2SO3 (Merck, p.a.),

sodium thiosulfate Na2S2O35 H2O (FF-Chemicals, p.a.), and

NH4Cl (Acros Organics, 99%) were used in the gold deposition

process. 1,1,2-Trichloroethylene (Merck, 99%), MeOH, EtOH

were of analytical grade or higher and were used as received for

the cleaning of the Si-hemicylinder.

Synthesis of GO

Natural graphite (SP-1, Bay Carbon) was used for synthesizing

the graphite oxide by the modified Hummers method and was

further dispersed in water to form GO as previously reported.25

In brief, graphite flakes (0.3 g) were chemically oxidized using a

mixture of concentrated H2SO4 (2.4 mL), K2S2O8 (0.5 g), and

P2O5 (0.5 g) and kept at 80 1C for 4.5 h. This preoxidised

product was washed with distilled water to remove all traces of

acid and dried. The dried product was dropped into cold (0 1C)

concentrated H2SO4 (12 mL). Then, KMnO4 (1.5 g) was slowly

added and the temperature of the mixture was kept below

20 1C. After the addition, the solution temperature was kept

at 35 1C for 2 h and was diluted with distilled water (25 mL).

The mixture was stirred for 2 h; an additional 70 mL of H2O

was added to the solution. Shortly after the dilution 2 mL of

30% H2O2 was added to the mixture resulting in a bright yellow

coloured solution. The resulting mixture was washed with HCl

and distilled water after which graphite oxide was obtained.

The obtained graphite oxide was dispersed in water at a

certain concentration and subsequently sonicated to obtain

GO. UV/Vis (GO): 231(pp*), B300 nm (np*); rGO,

reduced with hydrazine: 270 (pp*), >300 nm (increase in

absorbance).26,27 IR (graphite oxide): 3615 (sh, COH), 3410

(COH), 3210 (H2O), 1740 (CQO), 1620 (CQC, H2O), 1049

(CO), 14001300 (OH, COH), 12001000 (ArOH), 986

(sh, COC), 848 (COOC) cm1.28

Preparation of the substrates

An Au-minielectrode (CHI101, 2 mm dia., CH Instruments,

Inc.) was mechanically polished with an EtOH wetted polishing

cloth (Struers, DP-Nap) using diamond paste of gradually

finer grades (Struers, 3, 1, and 0.25 mm) with careful wash by

EtOH in between. The mechanically polished electrodes were

electrochemically cleaned in 0.5 M H2SO4 (Aldrich, Suprapur)

using cyclic voltammetry recording 50 potential cycles between

0.0 and 1.2 V at 100 mV s

1 vs. a Hg/HgSO4 referenceelectrode. In SERS measurements, the Au-minielectrode was

additionally roughened electrochemically in 0.1 M KCl before

the electrochemical cleaning step based on the previously

reported procedure.29 Microscope glass slides and Si(100)

wafers (Okmetic, Finland) for polarization modulation infrared

reflection-absorption spectroscopy (PM-IRRAS) and AFM

measurements, respectively, were cleaned in fresh piranha

solution (concentrated H2SO4/30% H2O2 (3 : 1)), rinsed

thoroughly with Millipore water and dried. A gold film

(ca. 100 nm) was evaporated on microscope slides using the

Edwards E306A coating system. The microscope slides were

previously covered by an evaporated chromium layer (ca. 5 nm)

for improving the adhesion of Au. The reflecting plane of thesilicon hemicylinder (Harrick Scientific) was cleaned by

soaking in hot 1,1,2-trichloroethylene, acetone, and MeOH for

10 min in each, thereafter etching by 1 min treatment in argon

saturated 10% HF (aq.). The oxide layer on the surface was

removed by soaking in argon saturated 40% NH4F (aq.) for

6 min.30 A thin gold film was deposited chemically on

the cleaned silicon hemicylinder according to a previous

report by H. Miyake et al.31 In brief, the reflecting plane of

the silicon hemicylinder was put into contact with a mixture of

2% HF and a plating solution of 0.015 M KAuCl 4 + 0.15 M

Na2SO3 + 0.05 M Na2S2O35 H2O + 0.05 M NH4Cl

(aq.) (2 : 1 v/v) for 15 minutes at room temperature.

The hemicylinder was rinsed with Millipore water to finishthe deposition.

Deposition of the GO film

Positive charge on substrates, required for GO adsorption,

was accomplished by immersing cleaned silicon or gold

substrates in 1% (v/v) APTES in EtOH for 30 min32 or in

1 mM MEA in H2O for one hour, respectively. Self-assembled

thin films were deposited on the primed substrates from

dilute aqueous GO solutions. For this purpose solid graphite

oxide was dispersed in Millipore water (0.51 mg mL1)

with the aid of mild ultrasound treatment for 30 min.

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Additionally, the resulting yellow suspension was centrifuged

at 4500 rpm (3939g) for 30 min. The pH of the supernatant was

set at 2.5 with H2SO4 and used for the thin film deposition. The

adsorption time of 240 min and pH 2.5 of the GO solution were

based on a previously reported procedure and on surface

plasmon resonance (SPR) measurements (vide infra).

Instrumentation

The UV-vis spectra of the GO and chemically reduced GO

solutions were recorded using a Hewlett-Packard 8453 spec-

trophotometer. The self-assembly process of GO from dilute

aqueous solution was followed by a Texas Instruments Spreeta

integrated surface plasmon resonance affinity sensor.33 The

electrochemical reduction of the thin GO films on gold

electrodes was performed by cyclic voltammetry in deaerated

0.1 M NaF aqueous solutions at a scan rate of 10 mV s1 using

a standard calomel reference electrode (+43 mV vs. Ag/AgCl/

3.5 M KCl reference system) and a platinum wire as an

auxiliary electrode. PM-IRRAS differential reflectance spectra

and SERS spectra of the thin self-assembled films on primed

gold were measured using a dry-air-purged Nexus 870 FTIR

spectrometer (Nicolet) equipped with a Tabletop Optical

Module (TOMt) for Nicolett FT-IR spectrometers for

polarization modulation experiments and FT-Raman

accessory. For each PM-IRRAS spectrum, 4096 scans at

4 cm1 spectral resolution were collected. SEIRAS spectra

were measured using the Kretschmann attenuated total reflec-

tion (ATR) configuration24 in the same TOM configuration as

the PM-IRRAS spectra without the polarization modulation

of the IR-signal. The p-polarized IR-beam passed at an angle

of incidence of 701 relative to the surface normal through a

silicon hemicylinder mounted on a homemade spectroelectro-

chemical cell. The total reflected beam from the thin film was

detected using an HgCdTe detector. All IR spectra shown are

differential spectra where the previous spectrum has been used

as reference for the following one in the in situ recorded

spectral array. SERS spectra were obtained from the thin

films deposited on roughened and primed gold electrodes

placed in a homemade spectroelectrochemical cell equipped

with a glass window using a Nd:YAG laser at 1064 nm in 1801

backscattering geometry and a germanium detector. For each

SEIRAS and SERS spectrum, 1024 scans were collected

at constant potential using 4 cm1 spectral resolution. An

Ag/AgCl-minireference-electrode (Cypress Systems, Inc., leak-

free) and a platinum wire served as reference and counter

electrodes. The potentiostat was an EG & G 283 (Princeton

Applied Research). Argon purged 0.1 M NaF aqueous

solutions were used as an electrolyte. The surface morphology

of MEA/GO thin films on APTES modified silicon was

investigated using a Veeco diCaliber AFM instrument (Veeco

Instruments Inc.) under ambient conditions in the tapping

mode. Rectangular silicon cantilevers (TESPA) of 125 mm

length having a resonance frequency of approximately

320 kHz were used. The tip height was 1015 mm and the

nominal radius of curvature 8 nm. The raw data obtained were

processed by flattening and plane fitting with the SPMLab-

Analysis 7.00 (Veeco Instruments, Inc.) and Gwyddion

2.19 data visualization and analysis software.

Results and discussion

The adsorption of GO on a MEA primed gold surface was

followed by SPR measurements in order to optimize the condi-

tions for the self-assembly process, namely the adsorption time,

concentration and pH of the GO deposition solution. The pKavalue of the NH2-group in a MEA monomer is close to 7.6,

3436

suggesting that the amine group is mainly protonated during the

GO adsorption process at pH 6 and 2.5 making the surfacepositively charged. Attracted by the opposite charge, GO sheets

adsorb on top of the MEA layer attached to the gold layer. This

self-assembly process is terminated when enough of negatively

charged GO platelets are adsorbed on the surface to reverse

the surface charge. The change in pH from 6 to 2.5 has an

effect on this equilibrium. At pH 2.5, the protonation degree of

the NH2-groups of MEA is higher increasing the positive

surface charge. On the other hand, the carboxylic acid groups

responsible for the negative charge of the GO sheets (surface

charge density is 0.4 units per 100 A 2)15 are mostly protonated

at this pH, which lowers the negative surface charge density

of the platelets. As a result, more GO sheets are required

to reverse the surface charge at lower pH which is seen asa greater change in the refractive index of the SPR curves at

pH 2.5 in Fig. 1.

Additionally, at lower concentrations (but the same pH) less

material is adsorbed on the electrode at the same time indicating a

diffusion limited mass transport process. SPR data revealed that

the growth of the GO film levels-off after ca. 80 min (ca. 15 min at

lower concentration). A small drift in the SPR signal after the

plateau is comparable to that observed in experiments done

with pure solvent and is related to instrumental and other

e.g. temperature and diffusion effects. Based on this and

previously reported results an adsorption time of 240 min,

cGO = 0.51 mg mL1 and a pH of 2.5 was chosen.34

Fig. 2a and b show the AFM images of a few layers thickfilm of GO on an APTES modified silicon surface after short

(30 min) and long (240 minutes) deposition times. The line in

the figures indicates the position from where the height profiles

below the images are obtained. After 30 minutes, the surface is

unevenly covered with relatively thin particles whose heights

are below 4.0 nm and a few bigger particles with height rising

up to 14.4 nm. After 240 minutes, the surface is more evenly

covered with GO particles whoseheights are below 3.4 nm although

Fig. 1 SPR curves obtained from a MEA modified gold surface upon

interaction with aqueous 0.01 mg mL1 GO suspension at pH 6.0 (&),

pH 2.5 (B), and with 1 mg mL1 GO suspension at pH 2.5 (J).

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some regions with higher height profiles are visible as well.

Both profiles indicate films that are thicker than a single

GO layer. The widths of the largest particles are about

150300 nm. The thickness of a single graphene sheet is

approx. 0.8 nm26,37 and a GO sheet that contains oxygen-rich

defects has a thickness of ca. 1.2 nm.32,3841 According to this

the highest spot within a smooth region in our sample would

consist of three layers of GO sheets if one considered a sheet

thickness of 1.21.3 nm that is in agreement with the above

and with the step height of 1.01.3 nm that can be observed in

some GO sheets. Generally, the film is up to ca. 4 nm thick

with occasional multilayer GO structures. Importantly, the

AFM results show a film that consists mainly of a few layer

GO sheets and occasional multilayer GO particles. It has been

reported that intercalated water has a significant impact on the

course of reduction processes in similar kinds of films,22 which

is also confirmed by SEIRAS results (vide infra).

Vibrational spectral characterization

The PM-IRRAS results of the MEA-primed gold substrate in

Fig. 3 show that the asymmetric and symmetric methylene

stretching vibrations belonging to a short ethylene chain in the

monolayer appear at 2925 and 2856 cm1, respectively. Upon

the adsorption of a GO layer the relative intensity of the

aforementioned bands increases probably due to the orienta-

tion effect of the monolayer. The bulk spectrum of graphite

oxide (data not shown) did not reveal any absorption bands

at these wavenumbers. An additional broad band around

ca. 3300 cm1 is assigned to OH stretching vibrations related

to adsorbed water molecules and structural OH groups. In

the lower wavenumber range the spectrum of the MEA

monolayer is featureless whereas after GO adsorption several

peaks appear below 1800 cm1. The vibration at 1739 cm1

can be related to CQO stretching from carboxylic acid groups.

Vibrations belonging to the carboxylate group are seen at

1541 and 1465 cm1. The relative intensity of the CQO

absorbance (vs. bulk) has decreased too indicating the ioniza-

tion of the acidic groups. This is expected because some of the

surface groups are to be negatively charged in order to enable

a successful adsorption of the GO sheets on the positively

charged electrode surface. The band around 1662 cm1 is

usually assigned to either the CQC vibrations of the basal

plane of graphitic regions or oxygen containing carbon species

other than carboxylic acids.42,43 Additionally, the possibility

for traces of water remaining in the film structure, giving rise

to absorptions from HOH bending (ca. 1620 cm1), has to

be taken into account. Hydroxyl or ether (CO) related

vibrations contribute to the weak peak at 1087 cm1.28

In situ spectroelectrochemical characterization of the GO

reduction

Surface enhanced Raman spectroscopy. Raman spectroscopyrepresents one frequently used technique for the characteriza-

tion of carbon-based materials and graphene structures in

particular.44,45 The Raman spectra of GO at different potentials

are shown in Fig. 4a. All Raman spectra exhibit two prominent

features, the G-band (B1599 cm1), corresponding to the sp2

lattice, and the D-band (B1305 cm1), which is related to the

defects in the sp2 lattice. For a better comparison, all signal

intensities are normalized to the G-band (1599 cm1) intensity.

From the presented Raman spectra, it is clear that the intensity

of the D-band, which is related to disorder structures in the sp2

lattice, is high in graphene oxide (D/G ratio of 1.41 in Fig. 4b)

as it exhibits the large abundance of oxygenated functionalities

that disrupt the planar sp2 structure. In Fig. 4b, the electro-chemical reduction of GO products exhibits D/G ratios between

1.59 (at 0.4 V) and 1.29 (at 1.0 V), suggesting that the order

of graphene is partly recovered during the electrochemical

Fig. 2 The AFM images of Si/APTES/GO (a) after 30, and

(b) 240 minutes adsorption time from 0.5 mg mL1 aqueous solution.

A line in an AFM image indicates the position of the corresponding

height profile.

Fig. 3 The PM-IRRAS spectrum of Au/MEA (- - -) and Au/MEA/

GO (___) before the reduction.

Fig. 4 (a) The normalized Raman spectra of the Au/MEA/GO film

as a function of applied potential. The spectra are shifted for clarity.

(b) The D/G-intensity ratio as a function of applied potential. (c) The

absolute Raman spectra of the Au/MEA/GO film at 0.0 V before (- - -)

and after (___) the reduction. (d) Cyclic voltammogram of Au/MEA/

GO in 0.1 M NaF, a scan rate of 10 mV s1. The arrow indicates the

direction of the potential sweep.

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reduction that is consistent with previous reports.46,47 Based

on Raman spectra, it is possible to calculate the average

crystallite size (La) from the observed D/G ratio.48 The D/G

ratio is inversely proportional to the crystallite size and for

graphene oxide and electrochemically reduced GO the sizes

16 nm and 15 nm, respectively, have been reported.16 Similarly,

an increase in the D/G ratio up to 0.4 V indicates a degrada-

tion of the crystallite size after which the size of the graphitic

regions grows slightly at higher negative potentials as indicated

by the decrease in the D/G ratio. The G band position was

shifted by 8 cm1 from 1599 to 1591 cm1 with increasing

applied potential. This shift in the G band is in line with the

reduction of GO.49 The D band also shows a similar variation

in its position from 1305 to 1295 cm1 in Fig. 4c. This is in

consistence with the data reported by Ramesha and Sampath

from studies in the identical potential range.34

Surface enhanced IR-reflection absorption spectroscopy. The

reduction of GO to rGO in aqueous solutions was followed

spectroelectrochemically during stepwise increase in applied

potential towards more negative values between 0 and 1.0 V.

Assuming that the spectral treatment and the use of an

internal reflection set-up compensate the contribution from

the environment and the electrolyte in the cell upward extending

peaks show the gain and downward extending peaks the loss in

the structure of the film during electrochemical reduction shown

in Fig. 5a. The dashed line in Fig. 5a shows the total change in

structure induced during the potential cycle. The vibrations

related to structural OH groups in GO are overlapped with the

vibrations of intercalated and/or free water molecules. A lot of

efforts have been made to distinguish between the COH modes

originating from a wide range of functional groups on the GO

sheets all overlapping in the range of 36003200 cm1.23,50,51

Within the band above 3200 cm1 at least four main vibrations

at around 3200 (overtones of scissor vibrations of adsorbed

water molecules), 3370 (closely neighbouring hydroxyl groups),

3500 (carboxyl groups of GO), and 3625 cm1 (COH stretch

associated with five-membered-ring lactols and hydroxyl groups

of GO from both the basal plane and from the sheet edges) can

be distinguished.23,50 The plot in Fig. 5b shows the potential

induced change in absorbance of the aforementioned bands at

specific potential related to the previous applied potential. The

potential induced change in absorbance can be divided into two

sections, the first covering the potential range from 0 to 0.8 V

where only a minor increase in the intensity of all the vibration

modes takes place. Within this potential range mainly changes

in the double layer at the interface between the GO layer and

the electrolyte are detected together with the enhancement of the

absorbance due to hydrogen bonding interactions (hydrogen

bridges) with COH and COOH groups on the GO sheets.

These modes are known to be more enhanced in a multilayer

film due to the contribution of intercalated water.22,28,50,52,53The

multilayer nature of the investigated GO films is also supported

by the AFM results in Fig. 2. The reduction of GO involving the

conversion of the functional groups starts to take place at

potentials more negative than 0.8 V. This is seen as an abrupt

change observed above 3200 cm1 where both the width and

the intensity of the absorption bands strongly increase. This

coincides well with the significant reduction current increase

observed in the cyclic voltammogram of the GO film at around

0.8 V vs. Ag/AgCl in Fig. 4d. At 1.0 V finally, a part of the

broad band develops a broad negative band with its centre at

3650 cm1 while the rest of the band remains positive.

In the lower wavenumber region (Fig. 5a) the same potential

dependent increase in absorbance is also seen at 1640 cm1

where a rather broad band is growing. This is the range for the

asymmetric vibrational stretching of sp2-hybridized CQC,

non-oxidized or reconstructed graphitic domains.10,22 Within

this same range also scissor modes of water that are broadened

through hydrogen interaction with COH are visible. Additionally

also vibrations from edge carbonyl groups might be visible in

this region. An additional weak feature arising at 1444 cm1

can be assigned to the deformation vibrations of carboxyl and

COH groups (seen only in the spectrum measured upon

completion of the potential cycle in Fig. 5a).23 Simultaneously

with the abovementioned increasing vibrations, a negative band

from vanishing structures appears at around 1220 cm1. The

asymmetric shape of the peak at 1224 cm1 indicates that

the band consists of several oxygen related vibrations. This is

the region for epoxides COC but also for peroxides, ethers,

ketones, lactols and benzoquinones.22,28

The response from the intercalated water between the GO

sheets resulting in further overlapping in the high wavenumber

region in the IR spectra can be compared with the results

reported from water in polymer membranes.54 In a LB film of

a polymer, monomeric non-hydrogen-bonded water shows up

at 3616 cm1, hydrogen-bonded dimers and small hydrogen-

bonded clusters have their vibration modes at 3536 cm1 and

3424 cm1. A fourth mode at 3246 cm1 contains a wide

Fig. 5 (a) Differential SEIRA spectra of the Au/MEA/GO film in

0.1 M NaF solution; the reference and sample potentials were 0.0 and

0.2 (1), 0.2 and 0.4 (2), 0.4 and 0.6 (3), 0.6 and 0.8 (4),

0.8 and 1.0 V (5) vs. Ag/AgCl, respectively. Curve (6) shows the

difference in absorbance at 0.0 V vs. Ag/AgCl before and after the reduction

cycle. (b) The potential dependence of GO (3500 and 3625 cm1) and H2O

(3200 and 3370 cm1) related vibrational bands.

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distribution of hydrogen-stretch vibrations associated with

water chains. The effect of hydrogen bonding causing

enhancement of the IR intensity as well as the broadening of

the OH stretch band is as strongest in the lower wavenumber

region in the 36003200 cm1 band, i.e. 34003200 cm1.

The electrochemical reduction of epoxides, carboxylates,

ethers, etc. has been mostly studied in organic solvents and the

reduction potentials reported for these species lie close to each

other and are therefore hard to identify in a mixture.55,56 In

aqueous solution, the potential window is narrow and the order

of easiness of the electrochemical reductions of the different

functional groups in the GO layers might be due to the access

of electrons and the electrolyte to these sites. According to the

LerfKlinowski model7 the major components in the basal plane

of the GO sheets are epoxide and hydroxyl groups.57,58 The

epoxide group can be considered as the most easily reduced

functional group in the GO film.59 The reduction potential of

aromatic epoxide groups has been reported to occur around

0.75 to 1.5 V vs. SCE in aqueous solutions.6062 The electro-

chemical reduction of epoxides CHOCH leads mainly to

CHQCH, CH2CH2 or CH2CHOH slightly depending

on the medium and its pH where the reduction is made.55

Aromatic carboxylic acids are converted to the corresponding

carboxylate anion, aldehyde, alcohol, or hydrocarbon depending

on the pH and the number of electrons consumed upon electro-

chemical reduction.56,63,64 Moreover, in aqueous solutions the

formed aldehydes may exist predominantly as the hydrate. The

formed hydroxyl groups COH originating from the reduction

of the epoxides and carboxyl groups can explain the increase in

absorbance intensity at 36003640 cm1 at the beginning of the

reduction. The simultaneous reduction of intercalated water

contributes to the increase in OH species.

ConclusionsBased on the experimental results obtained from thin GO films

prepared by the self-assembly technique utilizing electrostatic inter-

actions an overview of the potential dependent electrochemical

reduction process could be presented. Due to the fact that the redox

reactions of the different oxygen containing functional groups lie

very close to each other a complete identification of each group by

potential was not possible by electrochemistry in aqueous solutions.

The in situ spectroelectrochemical measurements, however, give a

possibility to correlate the specific structural changes with the

reduction potential. The role of intercalated water in multilayer

GO samples is taken into account in the spectral interpretation.

Spectroelectrochemical studies in organic solvents are in progress.

Acknowledgements

Financial support from Academy of Finland (Grant No.

128535) and the Graduate School of Materials Research

(JK) is gratefully acknowledged.

Notes and references

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