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Original citation:
Guell, Aleix G., Ebejer, Neil, Snowden, Michael E., Macpherson, Julie V. and Unwin,
Patrick R.. (2012) Structural correlations in heterogeneous electron transfer at
monolayer and multilayer graphene electrodes. Journal of the American Chemical
Society, Vol.134 (No.17). pp. 7258-7261. ISSN 0002-7863
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Structural Correlations in Heterogeneous Electron Transfer at
Monolayer and Multilayer Graphene Electrodes
Aleix G. Güell, Neil Ebejer, Michael E. Snowden, Julie V. Macpherson and Patrick R. Unwin*
Department of Chemistry, University of Warwick, Coventry, CV4 7AL, United Kingdom
Graphene, Electrochemistry, Imaging
Supporting Information Placeholder
ABSTRACT: As a new form of carbon, graphene is attracting
intense interest as an electrode material with widespread ap-
plications. In the present study, the heterogeneous electron
transfer (ET) activity of graphene is investigated using scan-
ning electrochemical cell microscopy (SECCM), which allows
electrochemical currents to be mapped at high spatial resolu-
tion across a surface for correlation with the corresponding
structure and properties of the graphene surface. We establish
that the rate of heterogeneous ET at graphene increases sys-
tematically with the number of graphene layers, and show that
the stacking in multilayers also has a subtle influence on ET
kinetics.
Graphene-based materials are having a huge impact in elec-
trochemistry and electrochemical technologies, with promising
applications in areas such as supercapacitors,
1
batteries,
2
elec-
trocatalytic supports,
3
sensors for electroanalysis
4
and trans-
parent electrodes.
5
These important technologies typically use
graphene produced by chemical vapor deposition (CVD)
6
and
other scalable methods, yet important fundamentals questions
concerning heterogeneous electron transfer (ET) at such mate-
rials –intrinsic to many of these applications- remain to be
addressed. Electrical measurements have revealed that the
electron mobility
7
and the electronic band structure
8
are sensi-
tive to the number of graphene layers and their stacking order,
with implications for electrochemistry. In this communication,
we thus seek to elucidate how both the number of graphene
layers and arrangement of the layers influence heterogeneous
ET kinetics.
Graphene grown by CVD on nickel substrates
9
(see Sup-
porting Information section 1) was optimal for the present
study because it presents a heterogeneous continuous layer of
microsized multilayered flakes, which can be addressed with
high resolution scanning electrochemical cell microscopy
(SECCM).
10-13
Thus, on one sample it is possible to make
thousands of individual electrochemical (EC) measurements at
different locations and relate these to the corresponding gra-
phene structure. This provides datasets on a scale that would
be unfeasible with conventional photolithographic techniques
of the type employed in recent EC studies of exfoliated gra-
phene.
14-16
In order to study the unambiguous electrochemical
response of graphene without any interference from a conduc-
tive substrate, CVD graphene layers were transferred to a sili-
con substrate with a 300 nm thermal grown oxide layer. This
substrate allowed optical visualization and identification of the
morphological film features characteristic of graphene,
17,18
for
direct correlation with the local electrochemistry. Importantly,
the approach described herein makes possible the study of
graphene surfaces with minimal intrusion and avoids the need
for any post-processing lithographic step, which may result in
unavoidable damage and possible interference of residues.
19
Ferrocene-derivatives have proven particularly suitable for
the study of the ET activity of sp
2
carbon allotropes, such as
carbon nanotubes,
15,19,20
and so we consider the one-electron
oxidation of (ferrocenylmethyl) trymethylammonium (FcT-
MA
+/2+
) as an exemplar outer-sphere redox couple. The dual
channel theta pipet
21
(1 µm diameter) of the SECCM was
filled with aqueous electrolyte solution containing 2 mM
FcTMA
+
(as the hexafluorophosphate salt) and 30 mM KCl
supporting electrolyte together with silver-silver chloride quasi
counter reference electrodes (QCREs) to serve as both a con-
ductance cell and voltammetric cell, with the graphene as the
working electrode (WE) (Figure 1a). A linear sweep voltam-
mogram (LSV) obtained with the SECCM setup (Figure 1b)
demonstrates the electrochemical activity of graphene, with a
sigmoidal wave for the oxidation of FcTMA
+
which rises with
increasing potential to a clear transported-limited current ca.
68 pA. The waveshape is indicative of essentially reversible
electron transfer (difference in the potentials at ¾ and ¼ of the
limiting current, E
¾
-E
¼
= 57 mV). The wave highlights five
different potentials at which the local electrochemical activity
of CVD graphene was mapped by SECCM within the same
area, yielding EC current maps (3 of which are presented in
Figure 1d and the others in Supporting Information section 2).
These data show clearly that, at all potentials, the redox reac-
tion occurs across the entire surface, but with significant het-
erogeneity in the current values. Simultaneously with the sur-
face EC current, SECCM also acquires three complementary
maps: z piezo displacement (related to the substrate topogra-
phy), the ion conductance current between the QCREs in the
barrels, and the AC component of the migration current (used
as the set-point to control tip-to-sample separation).
10-13
Those
maps (provided in the Supporting Information section 2) con-
firm the stability of the electrolyte drop size (electrolyte con-
tact area of the order of the pipet size,
12,21
here as 550 nm ra-
dius) and tip-to-sample separation (found to be 180 nm, see
Supporting Information section 3, and ref. 12). Thus, the
changes in surface EC current can be assigned unequivocally
to differences in EC activity of the material and not to any
changes in wettability. This is further evident by comparing
the 5 EC maps (in Figure 1 and Supporting Information sec-
tion 2) in which it is evident that the most active and inactive
areas are in the same location in each map.
A finite element model
12
was developed to analyze the EC
maps (Supporting Information section 3) and extract and as-
sign standard heterogeneous ET rate constants at each mi-
cron-scale pixel of the images. For each pixel, we assumed
reasonably the
Figure 1: SECCM. (a) Schematic representation of the EC imaging setup. The graphene lies on a Si/SiO
2
substrate and is connected as the
working electrode via an evaporated Cr/Au band. A SECCM probe is employed as a local and mobile EC cell for electrochemical imaging.
(b) LSV for the oxidation of 2mM FcTMA
+
(30 mM KCl) acquired with a SECCM setup on a graphene surface, at 100 mV s
-1
, with a ≈1
m diameter pipet. (c) Optical microscope image of the CVD graphene area mapped by SECCM, showing the heterogeneity of the surface
and the presence of multiple-layer graphene flakes. (d) Set of three EC maps of the area shown in c) acquired by SECCM at three different
substrate electrode potentials (E – E
o
) indicated in the LSV in b) with labels E
1
, E
2
and E
3
. All images are at the same scale as c). The ar-
row-circle in part c) and d) indicates a small area where the silicon oxide was exposed and measured currents in this area are below the
lower limit on the scale bar. This area was used to calibrate the number of graphene layers (Supporting Information Section 5).
Butler-Volmer model for ET
22
and a uniformly active surface
given the tiny area investigated. Electrochemical kinetic anal-
yses are relatively insensitive to the value of the transfer coef-
ficient for α = 0.5 ± 0.2 (ref 23) and so we chose α = 0.5, giv-
en the large self exchange ET rate constant for ferrocene and
its derivatives.
24
Comparison between the observed heterogeneity in EC ac-
tivity of CVD graphene and the corresponding topography,
revealed by optical microscopy or atomic force microscopy
(AFM) (Supporting Information section 4) shows a clear cor-
relation between electrochemical activity and the number of
graphene layers. Qualitatively, there is close correspondence
between dark regions (multilayers) in Figure 1c and high EC
currents (Figure 1d and Supporting Information section 2).
In order to examine this relationship in more detail, EC cur-
rent maps and the optical image were correlated quantitatively.
Given the linear increase of green component contrast with the
number of graphene layers,
9,17,18
and with further confirmation
from micro-Raman spectroscopy (vide infra), the full range of
light contrast was segmented into 8 different bins assigned to a
defined number of graphene layers (see Supporting Infor-
mation section 5).
Figure 2a shows the local EC current at potential E
2
versus
the number of graphene layers. Similar correlations at poten-
tials E
1
and E
3
are provided in Supporting Information section
2. From this plot, it is clear that single layer graphene exhibits
the lowest EC activity, and that the activity increases systemat-
ically with the number of layers, to a situation where the flakes
are so active that the ET process becomes essentially reversi-
ble
13
within experimental error (see Supporting Information,
Figure S9).
EC current distributions were analyzed to obtain the corre-
sponding ET standard rate constants (k
0
) for potentials E
1
, E
2
and E
3
(full details in ref 12 and Supporting Information sec-
tion 3). Figure 2b reveals that the ET kinetics evolves with the
number of layers towards faster ET and a broader range of k
0
(and current magnitudes) from monolayer to multilayer gra-
phene. This is found consistently at all three potentials. Alt-
hough there will be some cross contribution of different flakes
at some single point measurements (where the tip is at the
boundary between flakes), the different stacking order within
the graphene multilayers could also play a role in the broad-
ness of ET kinetics, seen for bilayer, trilayer and thicker
flakes, especially for epitaxial of CVD multilayer graphene,
where non-Bernal or AB stacking order is very common.
25
Raman spectroscopy was employed to determine both the
stacking order and the corresponding number of layers on
different graphene flakes for correlation with EC (Figure 3).
Figure 3a shows a zoom of the optical image and the associat-
ed SECCM map. We differentiate four different graphene
flakes labeled A1, A2, A3 and A4, being categorized as mono-
layer, bilayer, trilayer and multilayer graphene, respectively
(vide infra). The Raman spectra of those areas (Figure 3c)
present the three characteristic graphene D, G and 2D peaks.
26
For the A1 and A2 areas, the 2D bands are slightly more in-
tense than the G peak, and the FWHM of the 2D peaks are
around 35-40 cm
-1
, hallmarks of single layer CVD
graphene.
9,27
However, the upshift of about 10 cm
-1
(ref 28,29)
for the 2D peak (Figure 3d), in addition to light contrast values
of 0.15 (Supporting information section 5), indicate that the
A2 region actually corresponds to a non-AB stacking bilayer.
The lack of AB stacking (Figure 3d) reduces electronic cou-
pling between the graphene layers, so that bilayer graphene in
this configuration has electronic properties similar to that of
monolayer graphene.
30-32
This evidently impacts directly the
EC activity: current values for the A2 spot are very similar to
the A1 region (Figure 3b and
Figure 2: (a) Pixel-by-pixel correlation between the EC current map at potential E
2
and the number of graphene layers. (b) Histograms of
the EC current and standard rate constant, k
0
, for each defined number of CVD graphene layers, for potentials E
1
, E
2
and E
3
(from left to
right). The dashed line in a) and the blue area in b) denotes the conditions where the ET process becomes entirely reversible.
Supporting Information section 6), which corresponds to a
single layer. It is accepted that the electronic structure and
density of states play a key role in heterogeneous ET rates,
22,33
and these results show that different graphene layers (mono-
layer and bilayer), with closely similar band structures, behave
analogously in terms of electrochemistry. This result also
allows us to rule out a strong influence of charge carrier mo-
bility to the electrochemical activity measured. An increase of
mobility is expected for a non-AB stacking bilayer, compared
to monolayer graphene, since the substrate effect is, to some
extent, screened by the additional graphene layer beneath the
top layer in the case of bilayer graphene
34
but this does not
enhance ET kinetics compared to the intrinsic activity of mon-
olayer graphene.
The areas A3 and A4 are assigned to trilayer and multilayer
(>trilayer), respectively, based on the much broader 2D peak
(Figure 3e) and the intensity and peak position of the G peak
(Figure 3c). For these domains, an increase of EC activity is
observed with the number of layers (Figure 3b), consistent
with the evolution of the density of electronic states through
single layer, AB-bilayer and trilayer graphene.
7
These more
detailed analyses (Figure 3b and Supporting Information sec-
tion 6) confirm the trend (vide supra) between EC current and
light contrast in the optical image (interpreted as the number
of graphene layers).
Complementary experiments were carried out to eliminate
other possible causes for the observed changes in EC activity
with the number of graphene layers. An exhaustive analysis of
surface roughness was performed over the sample with AFM
(Supporting Information section 4) to discard the possibility
that the observed increase of EC activity was due to a change
in the roughness of the surface with the number of layers. The
presence of wrinkles is unavoidable for synthetic graphene
and
Figure 3: (a) Optical image of CVD graphene with 4 different flakes labeled A1, A2, A3 and A4, and corresponding SECCM data. Scale
bar is 5 m. (b) Histograms of the EC current in each designated flake at potential E
2
. (c) Raman spectra acquired with an excitation wave-
length of 633 nm and spot size of 500 nm at each graphene flake. The three characteristic Raman peaks for graphene are labelled as D, G
and 2D. (d) Raman 2D peak for regions A1 (red line) and A2 (blue line) plotted together highlighting the ≈10 cm
-1
Raman upshift charac-
teristic for a non-AB stacking bilayers (blue line). Schematic of Bernal (AB-stacking) for a bilayer of graphene. The basic structure of
graphene is defined with two atoms in the unit cell, denoted A (red dot) and B (blue dot). For an AB stacking bilayer, the A atom of the
top layer lies directly over the B atom of the bottom layer. (e) The Raman 2D peak for areas A1 (red line), A3 (green line) and A4 (orange
line)
they are responsible for local changes in the electronic struc-
ture,
35
but were essentially uniform (as evidenced by AFM in
Supporting Information) over the entire surface area and inde-
pendent of the number of layers and flakes. The Raman D
peak at 1350 cm
-1
is usually used to determine the density of
defects on graphene,
26,27
either as the peak intensity itself, or
with the ratio of D and G peaks (I
D
/I
G
). In all spectra obtained,
the D peak intensity was essentially constant for all flakes
studied and independent of the number of layers. Indeed, if the
I
D
/I
G
ratios are compared, the multilayered flakes have the
lowest density of defects, yet have higher activity. It is further
well known
36,37
that edges accumulate a higher density of de-
fects, but it is clear that we see no increase of EC activity
along the edges of either the flakes or at the (step-edge)
boundary between flakes, at the spatial resolution of the inves-
tigation.
In conclusion, we have demonstrated how the ET activity of
a complex graphene material can be elucidated, analyzed and
correlated with intrinsic structural properties using high reso-
lution SECCM in tandem with Raman microscopy, optical
microscopy and AFM. The unprecedented insights on the
structural controls of ET are of fundamental value, and pro-
vide a rational basis for the design and use of graphene in
electrochemical technologies. The SECCM methodology de-
scribed is general and we expect it will find increasing use for
structure – function imaging of surface and interfacial pro-
cesses.
ASSOCIATED CONTENT
Supporting Information. Full experimental details of graphene
synthesis, SECCM operation and complementary maps, AFM
images, light contrast calibration and FEM simulations. This ma-
terial is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
p.r.unwin@warwick.ac.uk
ACKNOWLEDGMENT
This project was supported by the European Research Council
through project ERC-2009-AdG 247143-QUANTIF and a Marie
Curie IntraEuropean Fellowship (236885) (A.G.G). Funding from
the EPSRC (EP/H023909/1) and UK National Physical Laborato-
ry it is also acknowledged. Equipment used in this research was
obtained through Science City (AM2), with support from Ad-
vantage West Midlands and part funded by the European regional
Development Fund. The authors thank Mr. Kim McKelvey for
assistance in data analysis and Mr. Tom Miller and Mr. Anatolii
Kuharuk for contributions to CVD graphene synthesis.
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