Optimisation of Electrolytic Solvents for Simultaneous Electrochemical

Exfoliation and Functionalisation of Graphene with Metal Nanostructures

Andinet Ejigu*a,c, Benjamin Millera, Ian A. Kinlochb,c and Robert A. W. Dryfe*a,c

aSchool of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UKbSchool of Materials, University of Manchester, Oxford Road, Manchester M13 9PL, UK

cNational Graphene Institute, University of Manchester, Oxford Road, Manchester, M13 9PL, UK

Andinet Ejigu (andinet.aynalem@manchester.ac.uk)

Robert Dryfe (Robert.dryfe@manchester.ac.uk)

ABSTRACT: The development of a simple, simultaneous electrochemical exfoliation and

functionalisation of graphene with metal nanostructures in a one-pot, single step process is

reported. This approach is useful in terms of the reduction in processing time and cost, as

well as aiding the control of the aggregation of graphene sheets. This first part of this work

compares the efficiency of electrochemical graphite exfoliation in dimethyl sulfoxide

(DMSO), N-methylpyrrolidone (NMP) and in a mixture of dimethyl carbonate (DMC) and

ethylene carbonate (EC) in an electrolyte consisting of LiClO4 and tetraethylammonium

tetrafluoroborate. In the second part, the best performing electrolytic solvent was used for in-

situ functionalisation of graphene sheets with gold or cobalt nanostructures. The formation of

solid layer electrolyte interface in the DMC/EC system is believed to stabilise the graphite

from premature exfoliation and allowed the ions to intercalate efficiently to produce a

relatively high yield of monolayer graphene sheets. By contrast, the electrochemical

exfoliation of graphite in the other two solvents (DMSO and NMP) produced lower yields of

few layer graphene. In particular, the co-intercalation of DMSO fragments the electrode by

its decomposition by-products (sulfur/carbon oxides) before sufficient cation intercalation

occurs. The simulations electrochemical exfoliation and functionalisation of graphene at a

single applied potential in the presence of Au salt in DMC/EC solution resulted in the

functionalisation of graphene sheets with a variety of high surface area Au nanowhiskers,

nanodendrites, nanowires and lamellar nanoparticles. Alternatively, the use of Co(II) salt in

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the exfoliation solution resulted in the co-deposition of uniformly grown Co nanoparticles on

graphene sheets. The metal-functionalised graphene sheets showed high catalytic activity

and stability when used as an electrocatalyst for hydrogen evolution reactions. This process

could be extended to other metal salts, or mixtures of metal salts, to form graphene-metal

alloy composites for use in various applications.

1. Introduction

The name “graphene” was coined by Boehm et al. in 1986 to describe a flat monolayer of

carbon atoms that are arranged into a two-dimensional honeycomb lattice as the building

block of graphitic materials.[1] Although graphene has theoretically been investigated for

over 60 years under the guise of ‘2D graphite’, its initial isolation was achieved at the

University of Manchester in 2004 by A. K. Geim and co-workers.[2, 3] Graphene has

received a huge amount of research interest from academic and industrial researchers since

then due to its outstanding electronic, thermal, optical and mechanical properties.[3]

However, the method by which graphene was initially produced (mechanical exfoliation)

does not lend itself to mass production. Consequently, a great deal of attention has been paid

to developing cheap, simple and scalable production methods. There are a number of

approaches in the literature, but most can be categorised into one of the following: supported

growth of graphene on a solid substrate by chemical vapour deposition (CVD) or epitaxial

growth using bottom-up synthesis from a carbon precursor,[4, 5] or graphene production by

top-down approaches such as exfoliation of graphite (solution, chemical, mechanical and

electrochemical exfoliation).[6]

The production of “few layer graphene” using electrochemical exfoliation has been

considered attractive in terms of scalability, processability and affordability.[7]

Electrochemical exfoliation is the process of inserting a foreign species (cations or anions),

the intercalant, into the interstitial spaces between sheets of the graphite working electrode by

2

applying a bias potential/current in an electrolyte consisting of the intercalant ions and

solvent. When the crystallographic diameter of the intercalant is larger than the interlayer

distance of graphite, the intercalant dilates the interlayer spacing, which overcomes the van

der Waal forces to cause exfoliation of the material. Electrochemical exfoliation can be

divided into two processes: anodic (oxidative) or cathodic (reductive), based on the polarity

of applied potential or current. The latter technique is often performed using organic solvents

that have wider electrochemical windows than water. The reductive process produces high

quality non-oxidised graphene when compared to the oxidative process, but the efficiency

and yield is often inferior to the anodic exfoliation in aqueous solution.[7]

The crystallographic diameter of cations and their intercalation density into graphite

plays a major role when using them as an intercalant during reductive electrochemical

exfoliation of graphite. Simple cations like Li+ give a high intercalation density: for graphite

one Li+ is inserted for every six carbon atoms.[8] The ionic size is smaller than the interlayer

spacing of graphite (the diameter of Li+ is 0.118 nm [9] compared to the interlayer distance of

graphite, 0.335 nm) and hence the intercalation of Li+ does not cause exfoliation.[10]

Tetraethylammonium cations TEA+, for example, have a crystallographic diameter of 0.67

nm, about twice the size of the interlayer distance in graphite, and its electrochemical

intercalation into graphite results in exfoliation.[11] However, the intercalation density of

[TEA]+ is much lower than that of Li+ as for every 39 C atoms only one cation of TEA+

intercalates, compared to a 6:1 ratio with Li+ [11], which then significantly impacts the yield

of graphene production.

It has been demonstrated that graphene is a promising support material in electrocatalytic

reactions due to its high thermal and electrical conductivity, high surface area and chemical

inertness.[12] In addition, a synergetic interaction between graphene and an active material

(metal or metal oxide nanostructure) often occurs, i.e. there is higher activity and durability

3

when compared to the individual components due to the formation of a dual active site at the

electrocatalyst-support interface.[13, 14] The majority of the work reported to date however

involves multi-step synthesis whereby graphene or a graphene derivative (mostly graphene

oxide) is produced first, then decorated by metal nanostructures in separate subsequent steps.

The overall aim of this work was therefore to simultaneously electrochemically exfoliate

graphite and functionalise with metal (gold and cobalt) nanostructures in a single-pot process

in an optimised electrolytic solvent analogous to the method reported from this laboratory

recently for bulk simultaneous electrochemical exfoliation and functionalisation of graphene

with diazonium compounds.[15] To this end the efficiency of reductive electrochemical

exfoliation of graphite in dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP) and a

mixture of dimethyl carbonate (DMC) and ethylene carbonate (EC) in 1:1 v/v in an

electrolyte consisting of LiClO4 and TEA BF4 were systematically studied. The best

performing electrolyte was selected for metal nanostructure simultaneous exfoliation and

functionalisation. A mixture of Li+ and TEA+ was used, where the insertion of Li+ helps to

achieve good intercalation density with some degree of graphite expansion while TEA+ aids

the expansion and exfoliation of graphite. As we will describe, the electrochemical

exfoliation of graphite in DMC/EC mixture produced high yield and thinner graphene flakes

when compared to graphite exfoliation in DMSO or NMP. Simultaneous electrochemical

exfoliation and functionalisation of graphene with Au or Co nanostructures was then carried

out in the presence of the respective metal salts in the exfoliation solution, by applying a

single cathodic potential of −4.0 V vs Ag wire. The functionalisation of graphene with gold

produced a variety of nano-architectures including nano-whiskers, high surface area three

dimensional nano-dendrites, which consisted of nanowires with a diameter of 50–250 nm and

lamellar nanoparticles. In contrast, the use of Co(II) salt in the exfoliation solution resulted

4

in the co-deposition of amorphous Co nanostructures on the graphene sheets. Finally, the use

of these metal functionalised graphenes for the hydrogen evolution reaction is discussed.

2. Experimental methods

2.1 Materials and Reagents

Lithium perchlorate (>98%), cobalt (II) nitrate hexahydrate (98%), sodium tetrachloroaurate

(III) dihydrate (99 %), anhydrous EC (>99%), anhydrous DMC (>99%) and anhydrous

DMSO (99.9%) were obtained from SigmaAldrich. Tetraethylammonium tetrafluroborate

(TEABF4) (99%) and NMP (>99%) was obtained from Alfa Aesar. All electrochemical

measurements were performed using an Autolab potentiostat model (PGSTAT302N,

Metrohm Autolab, The Netherlands). Graphite foil (>99%) was obtained from Gee Graphite

Ltd (UK). Omnipore membrane filters made of poly(tetrafluoroethylene) (JVWP01300) were

used, pore size of 0.1 μm. Ultra-pure water (18.2 MΩ cm resistivity) was obtained from a

Milli-Q water purification system

2.2 Electrochemical Exfoliation and Functionalization with metal nanostructures

A graphite foil tape (4 × 2.5 × 0.2 cm) working electrode (pre-expanded by immersing in

liquid nitrogen for 30 s followed by transferring into absolute ethanol), a Pt mesh counter

electrode, and an Ag wire quasi-reference electrode were used for the electrochemical

measurements. Prior to performing electrochemical measurements, N2 gas was bubbled into

the electrolyte for 30 min, and during electrochemical measurements a N2 atmosphere was

maintained above the electrolyte. The electrolyte was prepared by dissolving 0.1 M LiClO4

and 0.1 M TEABF4 in the organic solvents (DMSO, NMP or in a mixture 1:1 v/v of DMC

and EC). Electrochemical exfoliation of graphite using each electrolyte was carried out for 4

h by applying a potential of −4.0 V vs Ag wire.

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Simultaneous electrochemical exfoliation and functionalization of graphene with either Au

or Co nanostructures was obtained as follows: for functionalisation with Au, 15 mM

NaAuCl4 was dissolved in an electrolyte containing 0.1 M LiClO4 and 0.1 M TEABF4 in

DMC/EC and the exfoliation and functionalisation was performed using chronoamperometry

by applying a potential of −4.0 V vs Ag wire for 12 h under a N2 atmosphere. Similarly, for

functionalisation with Co, 15 mM Co(NO3)2 in 0.1 M LiClO4 and 0.1 M TEABF4 in DMC/EC

was used, and the electrolysis was performed at −4.0 V for 12 h under N 2. The exfoliated

product was then washed with an excess water, hexane, acetone and NMP and re-dispersed in

NMP by sonicating for 30 min. The resulting mixture was centrifuged at 4000 rpm for 30

min, and the supernatant was extracted using a pipette without disturbing the residue for

analysis.

2.3 Characterization of the Exfoliated Product.

Raman spectra were obtained using a Renishaw inVia microscope with a 532 nm excitation

laser operated at a power of 3.32 mW with a grating of 1800 lines/mm and 100× objective.

The samples for Raman measurement were prepared by drop coating the dispersion onto a

Si/SiO2 wafer and then dried on a hot plate at 200 °C to evaporate the solvent. For AFM

analysis, the graphene dispersion was spray-coated onto a Si/SiO2 substrate which was dried

in a vacuum oven at 80 °C. SEM analysis was carried out using an FEI Quanta 650 FEG

environmental scanning electron microscope. The concentration of the graphene dispersion

was measured with UV−vis spectroscopy using a model DH-2000-BAL (Ocean Optics). X-

ray photoelectron spectroscopy (XPS) was performed using a Kratos Axis Ultra DLD

spectrometer with a monochromated Al Kα X-ray source (E = 1486.6 eV, 10 mA emission).

X-ray Diffraction (XRD) was performed on a Philips X’pert PRO diffractometer with Cu Kα

radiation (λ = 0.154 nm) operating at 40 kV and 30 mA.

6

2.4 Hydrogen evolution reaction

A three-electrode cell consisting of a 5 mm-diameter glassy carbon (GC) rotating disk

working electrode, a saturated calomel reference electrode, and a Pt mesh counter electrode

(area of 5 cm2) was used for hydrogen evolution reaction measurements. The GC working

electrode was polished with aqueous 0.3 mm alumina (Buehler, Lake Bluff, IL) slurries on

felt polishing pads and rinsed with deionized water prior to use. The desired electrocatalyst

ink was prepared by sonicating a mixture of 7 mg of either gold functionalised graphene or

cobalt functionalised graphene in 1 mL of N,N’-dimethylformamide and 50 µL of Nafion®

(5 %, Sigma-Aldrich) for 30 min. The GC electrode had 10 μL of the above solution drop

cast onto the surface, which was then dried at room temperature in air. Polarization curves

were obtained while rotating the GC electrode at 1600 rpm at 10 mV s−1 using deoxygenated

1.0 M H2SO4 (aq) under N2 atmosphere. Electrochemical impedance spectroscopy (EIS) was

obtained at an oscillation amplitude of 10 mV within a frequency range of 100 kHz to 0.1 Hz

at an applied potential of −0.3 V. The electrochemical active surface area of graphene

modified with Au was determined by integrating the charge passed during stripping of an

adsorbed oxide layer during CV in 0.1 M H2SO4 (Figure S7) assuming a stripping charge

density of 386 μC cm-2.[16] Based on this, the active surface area was 0.85 cm2 and the

corresponding roughness factors was 4.3.

3. Results and Discussion

To examine at which potential Li+ and TEA+ intercalate into graphite, cyclic voltammetry

was performed using a graphite working electrode. Figure 1 shows the electrochemical

7

behaviour of Li+ and TEA+ in the DMC:EC mixture at the graphite electrode. When 0.1 M

LiClO4 was used (black colour line), the insertion of Li+ into graphite started at −2.0 V and

reached a diffusion limited region around −3.5 V vs Ag wire. The removal of the inserted Li+

was observed at a peak potential of −0.75 V thus the corresponding Li+ insertion/de-insertion

peak-to-peak separation was 3.75 V. It has been shown that the immersion of

electrochemically or chemically Li-intercalated MoS2 in water spontaneously exfoliates it

into MoS2 nanosheets.[17, 18] It is assumed that the pressure of the evolving H2 gas due to

Figure 1. Cyclic voltammograms recorded at a graphite working electrode in 0.1 M LiClO4

(black colour), 0.1 M [TEA][BF4] (blue colour) and a mixture of 0.1 M LiClO4 and 0.1 M [TEA][BF4] (red colour) in EC:DMC (1:1) under a N2 atmosphere. The potential was swept between 1.0 and −5.0 V at 50 mV s-1

reaction between water and Li overcomes the van der Waals forces that hold the MoS2

nanosheets together. In this study, immersion of electrochemically Li-intercalated graphite

(from 0.1 M LiClO4/DMC/EC for 4 h electrolysis) into water was attempted, to see if the

reaction between Li and water causes the exfoliation observed in the MoS2 case. However,

8

graphite exfoliation was not observed, despite significant H2 gas being evolved at the surface

of the electrode. This observation suggests that the evolving H2 gas in the interstitial spaces

of graphite sheets was not enough to drive the graphene sheets apart. The fact that

exfoliation occurs in Li-intercalated MoS2 on the other hand mean that MoS2 is participating

in the reaction presumably via catalysis of the hydrogen evolution reaction. This

demonstrates the necessity of including a cation with a crystallographic diameter that is larger

than the interlayer distance of graphite. We used a mixture of a large organic cation, TEA+,

and a smaller size, high intercalation density ion, namely Li+.

The intercalation of TEA+ (blue line), in 0.1 M TEABF4, started at a similar potential as

for the Li+ solution, but the current measured was three times lower than the one obtained

using Li+. This could be due to its lower intercalation density as well as its associated lower

diffusion coefficient when compared to the smaller crystallographic size Li+. Moreover, the

formation of a solid electrolyte interface (SEI) due to the decomposition of the electrolyte

could also affect the insertion of TEA+. While the exact structure of the SEI is still under

debate, a generic model that has been proposed suggests that it comprises of a dense

inorganic layer (such as LiF or Li2CO3) directly on the graphite surrounded by porous

metastable, partially soluble organic compounds.[19, 20] It is also postulated that the organic

layer is permeable to all ions in solution while the inorganic layer is only permeable to Li

cations.[21] In this case, the insertion of TEA+ might have occurred through the porous

organic component while the inorganic portion might be impermeable to TEA+.

Nevertheless, the anodic peak seen at 0.1 V was related to the reduction process observed

between −2.0 and −4.0 V, which confirms the intercalation of TEA+. The stability of the CV

response upon multiple cycling (0.1 M TEABF4) also suggests that the insertion and de-

insertion of TEA+ was not affected by the continual formation of the SEI layer on cycling.

Furthermore, the de-intercalation of TEA+ was shifted positive by 0.8 V when compared to

9

Li+, which indicates that the former cation is more difficult to remove from graphite. The CV

response obtained using a mixture of 0.1 M LiClO4 and 0.1 M TEABF4 showed a broad

intercalation peak with enhanced current at −4.0 V, if compared to that in 0.1 M TEABF4,

with a de-intercalation peak at −0.25 V. We should also note that the optimum LiClO4

concentration was 0.1 M, as increasing above this value resulted in the electroplating of Li

rather than the continual intercalation of Li+ into the graphite galleries. Based on the CV

data, −4.0 V vs Ag wire was chosen as the optimal potential for chronoamperometric studies

of the intercalation of Li+ and TEA+: electrolysis was performed for 4h. Similar behaviour

was noted when DMSO or NMP (see Figure S1) were used: in each case −4.0 V was used as

the exfoliation potential.

The electrode expanded significantly without much exfoliation during the 4 h of

electrolysis in DMC/EC solvents whereas an instant electrode exfoliation/disintegration

occurred in DMSO. The same area electrode completely disintegrated within less than 20

minutes of electrolysis time in the DMSO electrolyte. This shows that DMC/EC can allow

efficient ion intercalation while preventing solvent co-intercalation, which we attribute to the

formation of the SEI. In doing so, it prevents the evolution of gas from solvent

decomposition which aggressively disintegrates the graphite electrode while allowing the

maximum level of ion intercalation staging to be achieved. In contrast, efficient ion

intercalation into graphite in the DMSO solvent system is problematic as the co-intercalation

of DMSO causes fragmentation of the electrode before sufficient cation intercalation occurs.

DMSO typically forms solvation shells[22] when mixed with Li+ or TEA+, and during

electrolysis the cations are intercalated along with DMSO into graphite. The co-intercalation

of DMSO then affects the yield of graphene in terms of reducing the ion intercalation density

as well as fragmentation of the graphite by the strain introduced by its gaseous decomposition

by-products such as sulfur or carbon oxides. This gives the misleading conclusion that

10

graphite exfoliation occurred via ion intercalation, while in reality most of the exfoliated

product was the result of graphite disintegration caused by DMSO co-intercalation. Early

research work on Li-ion batteries using DMSO as a solvent also reported fragmentation of

graphite electrode due to its co-intercalation.[23]

The expanded and exfoliated samples obtained using each solvent were washed with

acetone, water and NMP and re-dispersed in NMP with the aid of sonication for 30 min to

assist some of the expanded materials to exfoliate. The dispersed samples were then

centrifuged at 4000 rpm for 30 min and the supernatant was taken for analysis. The

dispersion concentration obtained from the exfoliation of graphite in DMC/EC (0.04 mg mL-

1) is about four times higher than the dispersion concentration obtained using DMSO or NMP

(0.01 mg mL-1) indicating that DMC/EC produces a high yield of graphene when compared

to the other two solvents. In NMP, the expansion of the electrode was found to be very slow

and the majority of the product analysed by Raman spectroscopy was found to be graphite-

like in nature and the graphene yield was similar to that of DMSO (Figure 2A), and this

observation is consistent with the previous report.[10]

The Raman samples were prepared by drop coating the dilute dispersion onto a Si/SiO 2

substrate by drying at 200 °C to ensure complete evaporation of the NMP. The graphite foil

starting material only showed two major peaks: the G-band at ca. 1570 cm-1 and the 2D-band

at ca. 2709 cm-1 (Figure 2B). The absence of the D-band indicates that the graphite foil was

defect free. However, after electrochemical exfoliation each sample showed a substantial D-

band at ca. 1332 cm-1: this band is typically associated with defects caused by edges in the

sp2-hybridised carbon network or by creation of sp3 hybridization through covalent chemistry.

[24] The intensity ratio of the D-band to the G-band (iD/iG) for the graphene sample obtained

in DMC/EC, DMSO and NMP was 1.4, 0.65 and 0.3 respectively. This showed that the

graphene sample obtained in DMC/EC was the most defective product. As shown in Figure

11

2C, statistical analysis of over 200 flakes obtained in DMC/EC shows that the lateral flake

size varies between 100 nm and 500 nm, whilst the majority of the flakes are 200 nm in

diameter. This indicates that most of the flakes are much smaller than the spot diameter of

the Raman laser ( 800 nm) and the high intensity of the D-band is partly due to

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Figure 2. (A) UV-Vis spectra of graphene dispersions in NMP obtained after electrochemical exfoliation of graphite in an electrolyte containing 0.1 M LiClO4 and 0.1 M TEABF4 in DMC/EC, DMSO and NMP. The inset shows the dispersions of graphene materials in NMP obtained from prior electrolysis in the solvents indicated. (B) Raman spectra of electrochemically exfoliated graphene for the indicated electrolytes: the samples for Raman analysis were prepared by drop-coating the dispersion of graphene on to Si/SiO2 wafer and dried on a hot plate at 200 °C to evaporate the NMP and (C) AFM of graphene flakes that were obtained by electrochemical exfoliation of graphite foil at −4.0 V vs Ag wire in 0.1 M LiClO4 and 0.1 M TEABF4 in DMC/EC dilute dispersion deposited on Si/SiO2. The inset in (C) shows height profile for the selected region.

the response from the edge. The functionalisation of the graphene flakes with SEI could also

contribute to the D-band: see the XPS section for a further discussion.

A large change in the shape, intensity and position of the 2D peak can be seen when

comparing the exfoliated samples to bulk graphite. The 2D band of graphene sample

obtained in DMC/EC, DMSO and NMP was down shifted by 40 cm -1, 24 cm-1 and 21 cm-1,

respectively. Furthermore, the intensity ratio of the 2D-band to the G-band (i2D/iG) for

graphene sample obtained in DMC/EC was 1.7 while for graphene samples obtained in NMP

or DMSO it was below 0.4. These observations indicate that electrochemical exfoliation of

graphite in DMC/EC can produce monolayer graphene while exfoliations in DMSO or NMP

produce few layer graphene (≤ 5) since monolayer CVD grown graphene typically shows an

i2D/iG of 1.8.[23] The formation of monolayer graphene can also be confirmed when

analysing the flake thickness by AFM. As shown in inset of Figure 2C, the thickness of most

of the flakes was 0.8 nm, which is similar to the thickness of monolayer graphene flakes on a

Si/SiO2 substrate.[25] Analysis of Raman spectroscopy and AFM data demonstrates that the

formation of the SEI in DMC/EC electrolyte stabilised the graphite from premature

exfoliation and allowed the ions to intercalate to high staging to produce high yield

monolayer graphene flakes.

The electrochemical exfoliation of graphite in DMC/EC produced the highest yield of

graphene dispersion when compared to DMSO or NMP as such; DMC/EC was thus

13

employed for simultaneous exfoliation and functionalisation of graphite with Co or Au

nanostructures. A single applied potential of −4.0 V vs Ag wire was used for simultaneous

co-deposition of metals and exfoliation of graphite. The reduction potential of these metals

are typically much lower than the intercalation potential of Li+ or TEA+. For example, the

electrodeposition of Au on graphite surface was observed at −0.4 V vs Ag wire as shown in

Figure S2. The CV also showed that the pre-electrodeposition of Au on graphite surface did

14

Figure 3. UV−vis spectra of the electrolyte recorded after applying −4.0 V vs Ag wire to the graphite working electrode at the electrolysis times indicated. The initial electrolyte consisted of (A) 15 mM Co(NO3)2 in 0.1 M LiClO4 and 0.1 M TEABF4 in DMC/EC and (B) 15 mM of NaAuCl4 in 0.1 M LiClO4 and 0.1 M TEABF4 in DMC/EC. The inset pictures show the colour change of the electrolyte at indicated electrolysis times.

not inhibit the intercalation of Li+ and TEA+ indicating the possibility of performing

simultaneous exfoliation and functionalisation of graphene with metal nanostructures.

The reaction progress of simultaneous metal co-deposition during electrochemical

exfoliation of graphite was examined using UV-visible absorption spectroscopy. During the

chronoamperometric measurement, an aliquot of the electrolyte solution was taken at certain

period of times and analysed via its absorbance. Figure 3A shows the reaction progress of 15

mM Co(NO3)2 in a solution consisting of 0.1 M LiClO4 and 0.1 M TEABF4 in DMC/EC as a

function of electrolysis time. A broad absorption maximum between 520 and 530 nm was

seen due to the electronic transition from Co (II).[26] The intensity of the peak continuously

decreased as electrolysis time (t) increased from t = 0 to t = 2 hr indicating that Co was being

consumed from the electrolytic solution (see the colour change of Co2+ with increasing

electrolysis time in inset of Figure 3A). Even after 3 hr of electrolysis, however, the Co2+ was

not completely depleted from the electrolytic solution. Accordingly electrolysis was

conducted overnight (12 hr) to ensure its complete reaction as well as to attain efficient ion

intercalation (at t = 12 hr all the Co2+ reacted as shown in Figure 3A). Similar behaviour was

also observed for electrodeposition of Au when examining the UV-vis spectra of 15 mM of

NaAuCl4 in 0.1 M LiClO4 and 0.1 M TEABF4 in DMC/EC (Figure 3B). The presence of

AuCl4 ions was confirmed via a strong absorption maximum at 322 nm and this peak is

related to ligand-metal charge transfer.[27] The intensity of the peak decayed by a factor of

approximately ten as electrolysis time was increased from t = 0 to t = 3 hr due to the

continual deposition of Au on graphite surface. Moreover, the broad peak was gradually split

at =362 nm and =313 nm when electrolysis time increased above 2 hr. The expanded and

15

functionalised graphite electrode was then washed with acetone, water and NMP and re-

dispersed in NMP by a 30 min period of sonication.

3.2 Characterisation of the exfoliated functionalised products

Figure 4 shows the SEM image of Au and Co functionalised graphene. The electrodeposition

of gold at such high overpotential (−4.0 V vs Ag wire) with respect to the equilibrium

potential for Au deposition on graphene produced a variety of nano-architectures including

nano-whiskers, high surface area three dimensional nano-dendrites and lamellar nanoparticles

(Figure 4A-4C). The majority of the gold nano-dendrites grew perpendicular to the graphene

surface and this dendritic structure consists of nanowires with diameters in the range 50 nm–

150 nm. Close examination of Figure 4A also show that the deposits consist of many sharp

tips with high surface area nano-sized junctions. The electrodeposition also produced a long

nanowire of up to 4 µm with a diameter that varies between 50-250 nm (Figure 4B).

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Figure 4. (A-C) SEM images of G-Au that were obtained by electrochemical exfoliation of graphite foil at −4.0 V vs Ag wire in 15 mM NaAuCl4 in 0.1 M LiClO4 and 0.1 M TEABF4 in DMC/EC and (D) G-Co from 15 mM of Co(NO3)2 in 0.1 M LiClO4 and 0.1 M TEABF4 in DMC/EC.

TEM was also used to show the deposition of Au nanostructures in the form of lamellar

nanoparticles (with lateral sizes between 40-100 nm, Figure 5A) and some rod-like structures

(Figure S3) on graphene sheets. The high resolution TEM image (Figure 5B) shows the

typical twinning structure of Au nanostructures with five-fold symmetry.[28]. Powder X-ray

diffraction (XRD) data shows patterns that are associated with graphitic and gold materials

demonstrating that the Au deposits are highly crystalline (Figure 5C). The XRD pattern of

Au is consistent with that expected for face centred cubic structure (Joint Committee for

Powder Diffraction Standards (JCPDS) File Card No. 00-004-0784). Several synthetic routes

to dendritic gold nanostructures have been reported previously including electrochemical,

hydrothermal and aqueous/organic interfacial deposition.[29-32]. On the other hand, the use

of Co(II) salt resulted in the co-deposition of uniformly grown Co nanoparticles on graphene

sheets as shown in the SEM (Figure 4D) and TEM images ( Figure 5D and 5E), and the

average diameter of Co particles was 4 nm. High-resolution TEM image (Figure S4) and

powder X-ray diffraction data showed the deposits were an amorphous structure (Figure

S6B). Furthermore, the selected area electron diffraction pattern presented in Figure 5F

showed the response expected for graphene and an amorphous cobalt structure.

17

Figure 5. TEM images (A and B) and XRD of G-Au that were obtained by electrochemical exfoliation of graphite foil at −4.0 V vs Ag wire in 15 mM NaAuCl4 in 0.1 M LiClO4 and 0.1 M TEABF4 in DMC/EC. The scale bar in the inset of Figure 5A is 20 nm. TEM images (D and F) and selected area diffraction pattern (F) of G-Co that were obtained by electrochemical exfoliation of graphite foil at −4.0 V vs Ag wire in 15 mM Co(NO3)2 in 0.1 M LiClO4 and 0.1 M TEABF4 in DMC/EC.

XPS was used to confirm the functionalisation of the exfoliated product with the desired

metal nanostructures. Figure 6A shows the survey scans obtained for a graphite sample that

was exfoliated in the presence of Co2+ (G-Co) or Au3+ (G-Au) salt as well as graphene

obtained in the absence of those salt (control). The control sample consists of peaks due to C

1s (285 eV), O 1s (533 eV) and F 1s (685 eV) with corresponding atomic concentrations of

95 %, 3 % and 2 % respectively. The graphite foil starting material was also analysed and

found to consist of 98.6 % carbon and 1.4% oxygen, indicating an increase in oxygen content

of 1.6 % as a result of the exfoliation process. The increase in oxygen content could be due

to the functionalisation of graphene with SEI layers or contamination of the sample during

sample transfer to XPS chamber. The reduction of electrolyte solvent and salt reduction

18

during electrolysis produces SEI layers which consist of a mixture of polymers, Li2CO3, Li2O

and LiF (in fluorine containing salt and in our case is TEABF4).[20] The presence of LiF can

easily be distinguished using XPS analysis of the F1s spectra since the sensitivity of Li 1s is

not so strong. Each of the samples analysed contained F 1s, which suggests that the graphene

samples were functionalised with SEI. The extent of graphene functionalisation with SEI

was found to be dependent on the electrolyte compositions: the atomic concentration of F was

about 1.5 % in the control and 3 % in G-Co but dramatically increased to 14.5 % for G-Au.

Given that G-Au showed high surface area nanowhiskers when compared to G-Co, it could

be that the growth of the SEI layers was more favoured on the Au modified surface than on

the Co modified carbon surface. The functionalisation of graphene with SEI layers may

explain the high iD/iG observed when analysing by Raman spectroscopy.

The presence of Au (4f or 3d) in G-Au and Co2p in G-Co confirms the functionalisation

of the graphene product with Au or Co metals. The atomic concentrations of gold and

oxygen in G-Au samples were 2.2 % and 2.9 % respectively. This shows that the presence of

Au3+ salt in the exfoliation solution did not affect the oxygen content when compared to the

control sample. In contrast, the oxygen content of G-Co sample increased considerably to

13.5 %. Close examination of the high resolution C1s of G-Co shows an identical response

to that of either G-Au or control sample indicating that the graphene sample was not oxidised

during electrochemical exfoliation in the presence of Co2+ (Figure 6B). The high oxygen

content in G-Co sample could arise from oxidation of metallic Co when exposed to air at

open circuit potential, and this is consistent with previous report where Co deposited on

reduced graphene oxide undergoes a substantial oxidation in air.[33] The bulk chemical

composition of the functionalised graphene sample also analysed by EDX and Figure S5

confirms the functionalisation of the graphene samples with either Au or Co nanostructures.

19

Raman spectroscopy data shows the formation of few layer functionalised graphene with the

metal nanostructures (Figure S6A).

Figure 6 (A) Wide-scan XP spectrum of control, G-Au, and G-Co. (B) High-resolution XP spectrum of G-Au, G-Co and control in the C1s region and (C) high-resolution XP spectrum of G-Co in the Co2p region and (D) high-resolution XP spectrum of G-Au in the Au4f region. All peak positions were charge-corrected by setting the binding energy of the C 1s signal to 285 eV.

3.2 Hydrogen Evolution Reaction

Figure 7A shows the rotating disk (RDE) polarisation curve obtained at 1600 RPM at Pt, G-

Au and G-Co electrocatalysts in deoxygenated 1.0 M H2SO4 (aq). Current densities were

normalized to the geometric area of each electrode and are quoted vs. the reversible hydrogen

electrode (RHE). As expected, Pt exhibited high HER activity with negligible overpotential,

20

while non-functionalised graphene (control) showed poor electrocatalytic activity

characterised by the low magnitude of cathodic current and high onset potential (−0.4 V).

But after modification of the GC electrode with G-Au or G-Co ink, the onset potential of

HER shifted to −0.05 V for G-Au and −0.2 V for G-Co. The overpotential required to sustain

a current density of 10 mA cm-2 was 0.13 V, and for 50 mA cm-2 was 0.27 V on G-Au; at G-

Co the same current densities were achieved at 0.31 V and 0.42 V, respectively.

The high activity of G-Au over G-Co can also be seen when examining their

electrochemical impedance spectroscopy (EIS) data. The EIS was obtained at an

overpotential of −0.3 V vs RHE at each electrocatalyst (Figure 7B). The charge transfer

resistance (RCT) of HER at G-Au was 1.4 cm2, which was an order of magnitude lower than

the RCT obtained at G-Co (15 cm2). The high activity of G-Au over G-Co is partly due to

the differing surface area as the Au (2.2 % atomic) loading over graphene was twice as large

as the Co (1 %) loading. The effective deposition of Au with various morphologies is most

likely due to the large thermodynamic driving force for Au deposition when compared to Co

deposition.[9, 34] Moreover, the majority of the Co on graphene has already undergone

aerial oxidation (as shown by XPS) which would also impact the intrinsic catalytic activity

towards HER.

The catalytic efficiency of G-Au may emanate from its high electrochemical surface area

(0.85 cm2) in combination with the synergistic effect between Au and graphene. The

synergetic interaction between Au and graphene may decrease the overall energy barrier for

the adsorption of H+ by inducing a change in the electronic density of states around the

carbon as well as the metal, and this may generate more catalytically active sites on the Au-

carbon surface. Such a synergistic effect was also reported for Au and graphene for

electrocatalytic CO2 reduction,[35] for hydrogen evolution,[36] for decomposition of

H2O2[37] and as effective selective detection of ascorbic acid.[38]

21

Figure 7 RDE polarization curve recorded at 1600 rpm in deoxygenated 1.0 M H2SO4 (aq) at indicated electrodes between 0.0 V and −0.6 V at 10 mV s-1. (B) Nyquist plots obtained in deoxygenated 1.0 M H2SO4 (aq) at G-Co and G-Au electrodes in a three-electrode cell. The measurements were carried out at oscillation amplitude of 5 mV in the frequency range of 100 mHz to 100 kHz, at an applied potential of −0.3 V vs. the RHE. (C) Tafel plot (log of current versus overpotential) generated from the polarisation curve shown in (A) and (D) RDE polarization curve recorded at 1600 rpm in deoxygenated 1.0 M H2SO4 at G-Au electrode before and after 5000 cycle between 0.0 V to −0.6 V at 300 mV s-1

A wide range of non-Pt HER electrocatalysts have been reported in recent years, including

MoS2,[17, 39] Mo2C,[40] Ni2P,[41] and Ni-Mo-N nanosheets [42]: typically these catalysts

showed an overpotential of 0.13 V to 0.24 V at reductive current densities of 10−20 mA cm-2.

Kundu et al.[43] reported a current density of 10 mA cm-2 at an overpotential of 0.185 V in

0.5 M H2SO4 at gold aerogel supported on thin carbon nitride sheets and Siddhardha et al.[44]

reported the same current density at approximately 0.4 V at Au-graphene composite.

22

There are three possible elementary steps by which the HER may occur, namely the

Volmer, the Heyrovsky or the Tafel step.[45] The initial process is the Volmer step

(Equation 1) whereby a H+ from solution adsorbs onto a vacant catalyst surface site () to

form Had. The Heyrovsky process (Equation 2) is a chemical-electrochemical step where Had

reacts with a H+ to produce H2 and the Tafel step is a chemical step where two Had diffuse

into close proximity of one another and react (Equation 3). The combination of the first and

third steps is called the Tafel–Volmer mechanism, and the combination of the first and

second steps is known as the Heyrovsky–Volmer mechanism.

H+ + e + → Had (1)

Had + H++ e → H2 + (2)

Had + Had → H2 + 2 (3)

The mechanism by which this reaction proceeds therefore dictates the rate determining step,

which can be identified by analysis of the Tafel slope. A Tafel slope of 108 mV decade -1 at

G-Au and 142 mV decade-1 at G-Co were obtained (Figure 7C). This suggests that the

Volmer step is the rate demining step at each electrocatalyst. Au is known to bind H+ poorly

(in the so-called “volcano plot” analysis)[45] and therefore it is not surprising that the

adsorption of the H atom is the rate determining step. We note that the Tafel slope measured

using the data obtained using G-Co electrocatalysts are higher than the theoretical value

expected for a rate-determining first electron transfer (118 mV decade-1). This deviation has

been observed previously at base metal electrocatalysts and was attributed to the formation of

an oxide layer which may impede the rate of electron transfer during electrocatalysis of the

HER.[46, 47] This is in agreement with XPS analysis which showed the oxidation of Co.

Finally, the durability of G-Au was assessed by cycling the potential of the electrode between

0.0 V and -0.6 V versus RHE for 5000 cycles at 0.3 V s -1. Figure 7D shows the RDE

23

polarisation curve of G-Au before and after performing an accelerated stability test, and the

data shows that G-Au is stable after 5000 continuous cycles. The electrodeposition process

produces strong adhesion between graphene and gold which aids against aggregation or

dissolution of Au from the graphene support.

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4 Conclusions

A novel, facile, single-stage simultaneous electrochemical exfoliation and decoration of

graphene with metal nanostructures process was demonstrated. Combination of the high

intercalation density cation (Li+) and large crystallographic diameter cation (TEA+) with a 1:1

v/v mixture of DMC/EC was found to be the optimum electrolyte for electrochemical

exfoliation and functionalisation of graphene. On one hand, the formation of the solid layer

electrolyte interface in DMC/EC system stabilised the graphite from premature exfoliation

and allowed the ions to intercalate efficiently to produce a relatively high yield of thinner

graphene flakes unlike in dimethyl sulfoxide-based electrolyte. On the other hand, the solid

layer electrolyte interface functionalised the graphene, as evidenced by analysis of Raman

and X-ray photoelectron spectroscopic data. Further work should concentrate on identifying

the impact of this functionalisation on the physical and chemical properties of the graphene.

The functionalisation of graphene with gold produced a variety of nanoarchitectures

including nanowhiskers, high surface area three dimensional nanodendrites, which consisted

of nanowires with a diameter of 50–250 nm and lamellar nanoparticles whilst the use of

cobalt salt in the exfoliation solution produced an amorphous nanoparticle nanostructure.

These metal-graphene composites showed high catalytic activity and stability when used as

electrocatalysts for hydrogen evolution reactions. The extension of this approach to other

base metal salts, or mixtures of metal salts to form graphene-metal alloy composites, could

further enhance the catalytic activity of HER and future work should focus on this. The

possibility of obtaining bulk, functionalised graphene with gold may also open up further

chemistry for tailored applications, e.g. in imaging of biological samples. Furthermore,

future work may investigate on the use of Cs+ (which has similar size to the interlayer spacing

of graphite and high intercalation density to graphite) in combination with DMC/EC solvent

which could even simplify the reaction setup and might produce a high yield of graphene.

25

ACKNOWLEDGMENTS

We would like to thank the European Union Seventh Framework Programme for funding,

under grant agreement no. 604391 Graphene Flagship, and EPSRC (UK) for further financial

support (Grant refs EP/K016954/1, EP/I023879/1). B.M. thanks the B.J. Bennie Ben

Foundation for financial support. The authors also thank Dr. Alok Mani Tripathi for his

assistance with TEM.

SUPPORTING INFORMATION

Supporting information is available from ****

Corresponding authors:

Andinet Ejigu (andinet.aynalem@manchester.ac.uk)

Robert Dryfe (robert.dryfe@manchester.ac.uk)

DATA AVAILABILITY STATEMENT

Original data files are available on reasonable request from the authors.

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