FEATURE ARTICLE www.rsc.org/materials | Journal of Materials Chemistry
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Characterising chemical functionality on carbon surfaces
Gregory G. Wildgoose,* Poobalasingam Abiman and Richard G. Compton*
Received 25th November 2008, Accepted 13th February 2009
First published as an Advance Article on the web 17th March 2009
DOI: 10.1039/b821027f
This feature article introduces the reader to the surface chemistry and structure of graphitic carbon
materials, including carbon nanotubes. Recent work involving the development of dual labels that
allow us to selectively and quantitatively label carboxyl and general carbonyl groups (such as quinones,
ketones and aldehydes) and to distinguish between ortho- and para-quinone groups is reviewed. In
addition, the mechanisms of covalent, chemical derivatisation of these surfaces and the reactive sites
towards attack by radical and cationic intermediates are discussed, as well as the interesting effects on
the pKa values of organic molecules that attachment to a carbon surface can induce. When combined,
the methods described herein allow one to differentiate and explore the chemical functionality and
reactive sites on graphitic carbon surfaces.
1. Introduction
Carbon-based materials are widely used in many important
scientific and technological areas. Carbon is an attractive mate-
rial to use for these many varied applications because, whilst the
bulk of the material is reasonably chemically inert, the intrinsic
presence of reactive surface defects can be intelligently exploited.
For example, the surface chemistry of carbon materials enables
the facile chemical modification of the carbon surface to impart
desired properties and to tailor the surface chemistry to suit
a particular application. In addition, the presence of certain
surface defects is essential for rapid electron transfer reactions—
Gregory G: Wildgoose
Gregory G. Wildgoose
completed his MChem and
DPhil in 2003 and 2006 respec-
tively at Oxford University. In
2006, he was elected as a Junior
Research Fellow at St. John’s
College, Oxford. He has pub-
lished over 75 papers and is
a referee for several interna-
tional journals. He has broad
interests in electrochemistry and
electroanalysis in conjunction
with other surface analysis
techniques. Currently he is
focussed on studying the under-
lying physicochemical processes occurring at carbon-based elec-
trodes and at nanoscale materials such as carbon nanotubes and
inorganic nanomaterials.
Department of Chemistry, Physical and Theoretical ChemistryLaboratory, University of Oxford, South Parks Road, Oxford, OX13QZ, United Kingdom. E-mail: [email protected];[email protected]; Fax: +44 (0)1865 275410; Tel: +44(0)1865 275413; +44 (0)1865 275406
This journal is ª The Royal Society of Chemistry 2009
vital in the field of electrochemistry where the use of carbon
materials is now widespread.
This feature article provides an overview of some of the
insights into the surface chemistry and reactivity of predomi-
nantly graphitic carbon surfaces recently gained within our
laboratory and elsewhere. It aims to provide the reader with
a qualitative description of the range of reactive sites and surface
functional groups present on carbon materials, and seeks to
provide a ‘‘road map’’ of surface analysis methods and chemical
reagents to selectively and quantitatively identify their presence.
As our area of interest lies firmly in the field of electrochemistry,
we make no apology for emphasising the electrochemical impli-
cations within this work. However, readers from other fields
should not be discouraged, as much of the material presented
herein will be of value in a wide range of other applications,
particularly to those readers with a general interest in organic or
inorganic synthesis involving chemical modification on carbon
surfaces. Furthermore, these modified carbon materials are of
Poobalasingam Abiman
Poobalasingam Abiman
completed his undergraduate
degree in chemistry at the
University of Jaffna, Sri Lanka.
He is currently reading for
a DPhil in Chemistry, at the
University of Oxford within the
Compton group. His current
research focuses on the surface
modification of graphite powder,
glassy carbon microspheres and
carbon nanotubes with a range
of organic molecules, biological
molecules and metal nano-
particles. These modified
graphitic powders are used as biological sensors, pH sensors and in
some applications such as water purification. He has published 9
papers in these areas to date.
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increasing interest in other important relevant fields such as
catalysis, synthesis of so-called ‘‘designer interfaces’’, advanced
materials, energy storage, batteries, and environmental moni-
toring and remediation.
Scheme 1 Schematic representations of the structures of a) a graphite
crystal, b) a MWCNT, c) glassy carbon together with d) a schematic
cross-section through three different morphologies of MWCNT showing
their different internal structures and finally e) a representation of the
various oxygen-containing functionalities present on graphitic carbon
surfaces, adapted from reference 1. Reproduced by permission of the
Royal Society of Chemistry (RSC) on behalf of the Centre Nationale de
la Recherche Scientifique (CNRS), from reference 61.
2. A brief introduction to graphitic carbon structuresand the structure of the edge-plane defect sites
The structures of graphitic carbon materials, such as graphite
itself, glassy carbon (GC), carbon blacks, and, carbon nanotubes
(CNTs)—the latter of which has received enormous attention in
recent years—are well known and will only be briefly discussed
here. McCreery1 has attempted a detailed review of the general
structural properties of carbon materials, although certain
aspects of that review, particularly concerning the active sites of
electron transfer on graphite surfaces, have since been super-
seded.2
The basic structural units of any graphitic material are fused
hexagonal rings of sp2 hybridised carbon atoms. In graphite itself
these fused rings form planar sheets in a stacked arrangement
(Scheme 1), the degree of crystallographic ordering of which can
vary depending on the quality of the material. In CNTs, these
graphene sheets can be thought of as being seamlessly ‘‘rolled up’’
to form tubes with diameters in the nanometre range and lengths
of the order of a few microns. The tubes can either be single-
walled (SWCNTs) or multiwalled (MWCNTs), consisting of
several concentric graphene tubes fitted one inside the other.
There are also varying morphologies of MWCNTs that can
be formed such as ‘‘hollow tube’’ (h-MWCNTs), ‘‘bamboo-like’’
(b-MWCNTs) and ‘‘herringbone’’, shown in Scheme 1.
Glassy carbon consists of interwoven ribbons of graphite,
whilst boron doped diamond (BDD) consists of an sp3 hybri-
dised, tetrahedral-coordination lattice of carbon atoms, with
approximately one carbon atom in every thousand replaced by
boron. This p-doping imparts electrical semi-conductivity to the
otherwise insulating diamond-like lattice structure.
The chemistry of graphitic carbon materials is predominantly
influenced by the presence of defects within the materials’
structure. Indeed, without these defects the chemistry of carbon
materials would be rather limited! For example, it has been
shown for the case of CNTs that the presence of pentagonal or
heptagonal carbon ring defects are sites of chemical reactivity
Richard G: Compton
Richard G. Compton is
a Professor of Chemistry at
Oxford University and Tutor in
Chemistry at St. John’s College.
He is Editor-in-Chief of the
journal Electrochemistry
Communications and has pub-
lished over 900 papers and
reviews in refereed journals. He
has broad interests in electro-
chemistry and electroanalysis.
4876 | J. Mater. Chem., 2009, 19, 4875–4886
and induce structural changes in the CNTs structure such as
kinks, bends and even splitting of the nanotube into ‘‘nano-
horns’’.3–11 However, the major type of surface defect occurs at
the termini of the graphene sheets within the graphitic carbon
structure, so-called ‘‘edge-plane’’ defects. This feature article will
focus on these reactive surface sites.
Consider a perfect crystal of graphite, as shown in Scheme 1.
We can identify two crystallographic planes on this surface. One
crystal plane, containing all the carbon atoms of a single sheet of
graphene, is called the basal-plane. We can also identify a plane
perpendicular to this, called the edge-plane, which encompasses
the termini of the graphene sheets.2 The electronic structure of
graphite can be understood in terms of the band theory of solids,
where an infinite sheet of graphite produces a series of closely
spaced HOMOs and closely spaced LUMOs, which effectively
overlap to form the valence and conduction bands respectively.
These bands are separated by a sufficiently small energy gap to
impart semi-metallic conductivity to the material. This conduc-
tivity is anisotropic, being greater along the sheet of graphite,
i.e. in the direction of the basal-plane, than in the direction
perpendicular to the basal-plane (due to the requirement of
electrons to hop between graphene sheets in the latter case). In
reality, the sheets of graphite are finite and must terminate at an
edge-plane site. This abrupt termination in the band structure,
which occurs at the edge-plane termini of the graphene sheet,
causes the energy of the electrons in the valence band to rise
sharply. Thus the edge-plane defect sites are high-energy defects,
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making them the sites at which electron transfer takes place,2 and
hence making these defect sites much more chemically reactive
than the relatively inert basal-plane regions. The role of edge-
plane sites as the dominant sites for electron transfer, and hence
the impact of this on our understanding of the electrochemistry of
graphitic carbon materials, particularly highly ordered pyrolytic
graphite, MWCNTs and most recently SWCNTs,12 has been
developed by the authors elsewhere2 and will not be repeated here.
Similar analogous edge-plane-like defects can be identified
on CNTs, occurring at the termini of the nanotubes and at hole
defects in the side-wall structure of the nanotubes. Recently
Iijima and co-workers have produced remarkable video TEM
images of the mobility of individual carbon atoms at the
terminus of a carbon nanotube, indicating that these are indeed
high energy, structurally reactive sites. In GC, where the material
is less crystallographically defined and takes on some amorphous
structural character, the situation is less well understood; but
edge-plane-like defects can still be envisaged to occur along the
edges of the graphitic ribbons that form the structure of glassy
carbon.
Being highly reactive chemical sites, the edge-plane defects can
react with atmospheric oxygen and/or moisture resulting in these
defect sites usually being decorated with a variety of surface oxo
groups, most notably quinonyl, hydroxyl and carboxylic acid
functional groups. The reaction of oxygen at edge-plane defects,
and the thermodynamic and kinetic parameters affecting the
formation of, for example, quinones and carboxyl groups, have
been studied theoretically and experimentally by a number of
researchers,13–23 and this area has been reviewed comprehensively
by Zhu et al.24 Typically, the number of synthetically useful
surface functional groups, particularly carboxyl groups, is too
low to be of value, and so a number of techniques to introduce
carboxyl groups onto carbon surfaces have been developed. Of
these, perhaps the most commonly used is to simply stir the
CNTs in mixtures of strong, oxidising acids, such as mixtures of
H2SO4 + HNO3.25,26 The effect of various parameters in this type
of method on the number and relative distribution of carboxyl
groups on carbon and CNT surfaces has been studied in some
detail by several workers, notably Hu et al.27–30 An interesting
study from the group of Mao et al., worthy of mention here,
demonstrates that the inherent redox properties of surface
functional groups on SWCNTs can be used to enhance electro-
catalytic detection of certain molecules such as thiols.31 To do
this the authors immobilised ortho-quinone derivatives onto
SWCNTs as a model for how these surface groups can interact in
a favourable fashion for these types of sensing applications.
Edge-plane defects are inherently chaotic in their structure,
with some sp3 hybridised carbon rings and chains and related oxo
groups such as ethers, carbonyl, cyclic esters and lactone-like
groups also formed. A schematic of an edge-plane defect site
showing some of the representative surface oxo groups that can
be envisaged to decorate these defects is given in Scheme 1.
In an idealised structure of the edge-plane, the arrangement of
fused hexagonal rings can terminate in two principle ways,
forming either a ‘‘zigzag’’ arrangement or an ‘‘armchair’’ struc-
ture, and indeed these terms are often used to characterise the
morphology (i.e. the way in which the graphite sheet is ‘‘rolled
up’’) of SWCNTs, which in turn influences their electronic
properties. Oxygen can react at either type of ring terminus to
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produce ortho- and para-quinone structures, again shown sche-
matically in Scheme 1. The identification and selective discrimi-
nation between ortho- and para-quinones on carbon surfaces is
discussed later in section 4.2. First, we will briefly introduce some
of the earlier physical methods and experimental evidence for the
presence of this wide variety of surface oxo groups on carbon
surfaces.
3. Physical characterisation methods used to identifysurface oxo groups on carbon surfaces
There are several physical methods of investigating the surface
functionality on carbon that have been applied in the literature,
including thermal desorption methods (which provide little
information on the differing chemical identity of surface oxo
groups),32 TOF-SIMS,33 electrochemistry,18,25,26,34–41 and various
spectroscopic techniques. Of the latter, X-ray photoelectron
spectroscopy (XPS)42–47 and vibrational spectroscopic
methods48,49 (infra-red and Raman spectroscopy) are the most
common. The interested reader is directed to a review of different
surface characterisation techniques used on carbon surfaces
written by McGuire et al.50 Vibrational spectroscopy is capable
of providing qualitative data indicating the presence or absence
of certain kinds of surface oxo groups by, for example, identi-
fying an O–H stretch and a C]O stretch associated with the
presence of carboxyl groups. However, quantitative analysis of
the relevant amounts of each type of functional group is more
difficult, particularly where, for example, O–H stretches from
different functional groups such as hydroxyl, phenol or carboxyl
groups overlap to any extent. Furthermore, whilst Raman
spectroscopy of solid or powdered samples is relatively facile, IR
analysis of solid carbon samples is, from a practical standpoint,
more difficult than for a molecule in solution. The opacity of
carbon samples results in a decrease in the quality and spectral
detail that can be obtained, and can be a hindrance in quanti-
tative interpretation of the resulting spectra. Despite this,
McCreery and co-workers have carried out an extensive series of
studies coupling Raman and IR spectroscopic methods with
electrochemical identification of surface oxo groups, to which
the interested reader is directed.1,20,48
XPS analysis of surface oxo groups is usually undertaken in
one of two ways. Either the C1s spectral line can be examined
and deconvoluted into contributions from C–C bonds and C–O
bonds, or the O1s spectral line can be deconvoluted into contri-
butions from each type of surface oxo group e.g. O1s emission
from oxygen atoms in quinone, hydroxyl and carboxyl chemical
environments.42–47 The main drawback of this technique is
that the chemical shifts in the binding energies of atoms in
different chemical environments are rather small, often less than
2–3 eV,51–54 which is at the limit of the typical binding energy
resolution of the spectrometer. Thus, deconvolution of a spectral
line requires prior knowledge of what functional groups one
expects to find on the carbon surface. The deconvolution process
itself is therefore only semi-quantitative in that any number of
pre-determined peaks can be fitted to the experimentally deter-
mined C1s or O1s spectral line, to produce a good fit to the
experimental data. This in turn introduces a large degree of
uncertainty in the assignment of the relative amount of each
chemical species on the sample surface.
J. Mater. Chem., 2009, 19, 4875–4886 | 4877
Scheme 2 The modification of surface quinonyl groups decorating an
edge-plane-like defect site on a graphitic surface with 2,4-DNPH.
Reproduced by permission of the Royal Society of Chemistry (RSC) on
behalf of the Centre Nationale de la Recherche Scientifique (CNRS),
from reference 61.
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One method of overcoming this difficulty is to introduce
a ‘‘label’’—a molecule that can chemically interact with a target
surface group, say for example a carboxylic acid. The label
contains one or more atoms that are not normally present in the
carbon sample, and therefore the presence of this new spectral
peak can be directly correlated to the number of target functional
groups present on the sample surface. Several studies, notably
by Jones et al., have introduced a variety of labels, typically
halogenated compounds such as chloroform (for labelling of
COOH groups), and trifluoroacetic acid (for labelling of –OH
and –COOH groups), as well as the introduction of Ba2+ labels
for carboxylic group quantification.33,55 Collier et al.56 have also
introduced a titanium containing complex, titanium diisoprop-
oxide bis(2,4-pentanedionate) to quantify the number of surface
hydroxyl groups on glassy carbon samples. They found that
these groups typically constitute 1–10 mole percentage of the
carbon surface.
Aside from the problems of spectral deconvolution, the other
main drawback of using stand-alone XPS surface analysis tech-
niques to explore the chemical functionality on these types of
surfaces is the relative lack of sensitivity of the technique. An
alternative method worthy of special mention is the work of Feng
et al.57 who have developed an ultra-sensitive fluorescent label-
ling technique capable of quantifying the relative amounts of
surface hydroxyl, carboxyl and aldehyde groups on carbon
fibres, down to levels of 1011–1012 groups cm�2, which were
otherwise undetectable using XPS or FTIR techniques. Three
different chromophores were applied, namely 1-naph-
thalenethanol, 1-pyrenemethylamine, and triphenylmethyl-
chloride, for the detection of carboxyl, aldehyde and surface
hydroxyl groups respectively.57
In our work, discussed in the following sections, this problem
is overcome by introducing ‘‘dual-labelling’’, i.e. using labels that
can clearly be distinguished in XPS via identification of certain
heteroatoms within the label molecule, but which are also elec-
trochemically active compounds or adducts. We will begin
by examining ways of identifying the presence of quinones on
graphitic surfaces.
4. XPS and electrochemical ‘‘dual-labelling’’ ofsurface carbonyl and quinone groups on graphiteand CNTs
4.1 Labelling the total number of carbonyl groups with 2,4-
dinitrophenylhydrazine
When Elliot and Murray first coined the term ‘‘chemically
modified electrode’’ in their seminal 1976 paper,37 one of the
reactions they exploited was the labelling of surface carbonyl
groups on a graphite rod and a GC electrode with a classical
reagent commonly used in organic chemistry to identify the
presence of these groups, namely 2,4-dinitrophenylhydrazine
(2,4-DNPH). The derivatisation conditions and mechanism are
shown in Scheme 2. Only the non-aqueous voltammetry of the
resulting 2,4-DNPH adduct (shown in Scheme 2) was reported
and that was described as being ‘‘poorly resolved’’.37 XPS
characterisation was also performed to identify that the nitro
groups within the 2,4-DNPH adduct underwent reduction ‘‘at
least in part’’. Unwittingly, Elliot and Murray had introduced
4878 | J. Mater. Chem., 2009, 19, 4875–4886
one of the first ‘‘dual-labels’’ capable of quantifying the number
of surface carbonyl groups using both XPS and electrochem-
istry, but at the time, this was not the focus of their efforts, and
no quantification of the surface groups was performed. Later
McCreery et al. also used 2,4-DNPH to label surface quinone/
carbonyl groups and studied the electrochemical response and
the structure of the resulting adduct using Raman spectros-
copy,48 which indicated that the adduct forms as the predomi-
nantly azo-linked tautomer shown in Scheme 2. This time,
a quantitative analysis of the surface coverage of ‘‘quinone’’
groups on a GC surface was performed via Raman spectroscopy,
and using the 2,4-DNPH as a single-mode label—the quantita-
tive redox quantification using voltammetric techniques was
not combined. However, this method does assume that the only
carbonyl groups present on the surface that the 2,4-DNPH could
react with were in the form of quinones—vide infra. The Raman
labelling method was successful though, in that around 1.2% of
all carbon atoms on the surface were found to have a quinone
group attached.48 This is in agreement with earlier XPS labelling
(1.4–2.8 atomic percent) methods,58 and not too dissimilar to
methods involving simple deconvolution of C1s spectra (ca. 5
atomic percent),48 again demonstrating that the latter method is
less than ideal.
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The difficulty in combining the redox response of the 2,4-
DNPH label with either the XPS or Raman spectroscopic data
appears to be that the complex mechanism of the electrochemical
reduction of the adduct was not fully understood, hindering
quantitative voltammetric analysis of the number of surface
groups present. Fortuitously we have recently elucidated
a mechanism for the electrochemical reduction in aqueous media
of an azo dye, Fast Black K (FBK, 2,5-dimethoxy-4-[(4-nitro-
phenyl)azo]benzenediazonium chloride), covalently attached to
carbon surfaces,59,60 which is structurally very similar to the
structure of the quinone-2,4-DNPH adduct.
The voltammetric reduction mechanism of FBK involves the
simultaneous reduction of a nitro group together with reduction
and subsequent cleavage of the azo linkage in the molecule,
releasing a p-phenylenediamine fragment into solution and
leaving a dimethoxyaniline fragment bound to the carbon
surface.59,60 By chemically modifying graphite powder and
MWCNTs with 2,4-DNPH, and using our insights gleaned from
the FBK studies, together with voltammetric comparisons with
model compounds, we were able to elucidate the mechanism of
reduction of the carbonyl/quinone-2,4-DNPH adduct in aqueous
media. This involves the four-electron, four-proton reduction of
one nitro group in the 2,4-DNPH adduct and simultaneously
the two-electron, two-proton reduction and cleavage of the azo
linkage formed when 2,4-DNPH forms an adduct with a surface
C]O group.61 In total, the overall reduction process involves six
electrons and six protons. This again results in the potentios-
tatically controlled chemical release of a fragment of the adduct
into solution and leaves an aryl nitroso fragment covalently
bound to the carbon surface, as shown in Scheme 3.61 We were
Scheme 3 The proposed mechanism for the reduction of 2,4-DNPH
modified graphitic surfaces in aqueous media showing the cleavage of
fragment A and the formation of a new surface bound arylnitroso species.
Reproduced by permission of the Royal Society of Chemistry (RSC) on
behalf of the Centre Nationale de la Recherche Scientifique (CNRS),
from reference 61.
This journal is ª The Royal Society of Chemistry 2009
also able to propose a tentative mechanism explaining the non-
aqueous voltammetry of the 2,4-DNPH adduct,61 which will not
be discussed here.
Thus, we now have a dual XPS label, whereby a sample of
graphitic carbon can be chemically modified with 2,4-DNPH
resulting in two distinct N1s signals in the XPS spectrum, one
from the nitro groups and one from the azo group, and which can
be used for quantitative voltammetric analysis. The drawback of
using the 2,4-DNPH system, as alluded to above, is that it only
tells us the total number of carbonyl groups on the surface. It can
not distinguish between quinone groups, ketones and aldehydes.
The next section details how quinones can be selectively labelled
and quantified and also how to distinguish between ortho- and
para-quinone groups.
4.2 Selective dual labelling of quinone groups and
differentiation between ortho- and para-quinones
It is clear from the above discussion that numerous studies have
been performed to identify quinone groups on carbon surfaces.
However, to the best of our knowledge only one study prior to
our own work has addressed the issue of differentiating between
ortho- and para-quinone groups. Schreurs et al. examined the
surface of a GC electrode which had undergone pre-treatment
with radio frequency plasma etching to introduce surface oxo
groups, and found that the majority of surface quinone groups
were ortho-quinones.62 They achieved this by selectively labelling
the ortho-quinone groups with 1,2-phenylenediamine, forming
redox active benzophenazines.62
Armstrong et al. have also reported the electrochemical
modification of carbon surfaces via the electroreduction of an
inorganic hexamminechromium(III) complex, which selectively
binds to ortho-quinones on the carbon surface in preference
to para-quinones via the chelate effect.63–66 In this case the focus
of their work was not to examine the presence of these surface
quinone groups, but this approach does provide us with yet
another dual-labelling technique to quantify these groups.
To this end we explored both the hexamminechromium(III)
label, and also a range of phenylenediamine derivatives
for quantitative dual-labelling of ortho-quinone groups on
graphite, glassy carbon and CNT surfaces.67 Phenylenediamine
compounds with the amino groups in both the 1,2-positions and
the 1,4-positions as well as with the nitro derivatives of these
(which provide us with further redox chemistry and nitrogen
chemical environments for electrochemical and XPS character-
isation respectively) were studied, as shown in Fig. 1. The phe-
nylenediamine derivatives are capable of reacting with both
ortho- and para-quinone groups to form imine-like adducts.
However in the case of ortho-quinones reacting with ortho-phe-
nylenediamines the product is a benzophenazine product, 8,
which has an electrochemical signal distinct from the imine-like
product formed with para-quinones such as the structure 9
shown in Fig. 1. Initially we demonstrated that in the case of the
phenylenediamine labels, only the 1,2-phenylenediamine deriv-
atives 1 and 2, could selectively label ortho-quinones, by reacting
them with anthraquinone, 7, and 9,10-phenanthrequinone, 6, as
model para- and ortho-quinone substrates. The resulting adducts,
8, and 9, were characterised voltammetrically and using 1H-
NMR. The voltammetric response of the benzo[a,c]phenazine
J. Mater. Chem., 2009, 19, 4875–4886 | 4879
Fig. 1 The structures of the various organic phenylamine derivatives,
1–5, used to label quinone groups together with the structures of the two
model quinone compounds 9,10-phenanthraquinone 6 and 9,10-anthra-
quinone 7 and the corresponding adducts, 8 and 9 respectively, formed by
their reaction with 1. Also shown is the reaction mechanism for the
labeling of ortho-hydroquinone groups with the inorganic [Cr(NH3)6]2+/3+
complex. Reproduced with permission from reference 67. Copyright
American Chemical Society 2007.
Fig. 2 The cyclic voltammetric response of a) graphite powder and b)
GC powder labeled with 1 in 0.1 M KCl. Reproduced with permission
from reference 67. Copyright American Chemical Society 2007.
Table 1 The surface coverage of ortho-quinones on graphite and GCpowders determined by labeling with 1 and the surface coverage of para-quinones (unlabelled) determined from the corresponding voltammetricpeak (system III). Data reproduced with permission from reference 67.Copyright American Chemical Society 2007
Material
Surface coverageof ortho-quinones/mol cm�2
Surface coverageof para-quinones/mol cm�2
Graphite powder 1.3 � 10�10 3.4 � 10�11
GC powder 5.0 � 10�12 1.7 � 10�11
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adduct, 8, is indeed distinct from the adduct 9 formed by reaction
of either 1 or 2 with para-quinones such as 6. Thus, our labels are
selective only for ortho-quinones as desired. The voltammetric
responses of graphite and glassy carbon powders labelled with 1
are shown in Fig. 2. The peaks labelled as Ia/Ic correspond to the
voltammetry of the benzophenazine adduct, whilst IIa/IIc are due
to impurities in the graphite. A further poorly resolved quasi-
reversible redox process, labelled III, can be observed at ca. 0.1 V
vs. SCE. This is commonly observed in the voltammetry of
graphitic electrodes (frequently but erroneously referred to in
the literature as a ‘‘pseudo-capacitance’’) and is attributed to the
quasi-reversible behaviour of the surface quinone/hydroquinone
redox couple.68–72 Assuming that the reaction of 1 with surface
ortho-quinones is quantitative, the voltammetry of III can be
tentatively assigned to any remaining para-quinones on the
carbon surface. Thus by measuring the peak areas under I and
III, we can obtain a quantitative distribution of both types of
surface quinone, as shown in Table 1.
Interestingly, the labelling of GC powder reveals that only
a small number of surface quinones are in the ortho from, and
that the surface quinone groups in this sample are predominantly
para-quinones. This is in contrast to the work of Schreurs et al.,62
but one must remember that the nature of the surface can vary
from sample to sample and also depends on the method of pre-
treatment, such as the radio plasma etching used by Schreurs.
4880 | J. Mater. Chem., 2009, 19, 4875–4886
XPS analysis was also performed on the graphite and GC
powder samples after labelling with either 1 or 2. Only emission
from the C1s, O1s and N1s levels was observed in the survey
spectrum shown for the case of graphite powder labelled with 1 in
Fig. 3. Careful analysis of these spectral peaks reveals that the
atomic percentages are distributed according to Table 2. By
comparing the stoichiometry of the labelling molecule and the
ratio of N to O atoms it is possible to attribute the percentage
contribution of ortho-quinone groups to the total O1s signal, and
hence to further quantify their distribution (vide infra).
Similarly, labelling of graphite, GC, b-MWCNTs, h-
MWCNTs and SWCNTs via the electrochemical reduction of the
hexamminechromium(III) complex produced voltammetry cor-
responding to the adduct of this inorganic label with surface
quinones shown in Fig. 4. Again it is possible to calculate the
This journal is ª The Royal Society of Chemistry 2009
Table 2 The percentage elemental surface composition of graphitepowder labeled with 1 and 2 and the percentage of the total oxygen-containing groups attributed to ortho-quinones. Data reproduced withpermission from reference 67. Copyright American Chemical Society2007
Labelingcompound % C � 0.1% % O � 0.1% % N � 0.1%
Percentage oftotal oxygenattributed toortho-quinones
1 95.8 3.3 0.9 272 93.3 5.5 1.2 22
Table 3 The surface coverage of quinone groups on each carbonaceousmaterial studied as determined using voltammetric labeling with thehexamminechromium(III) complex. Note that eppg and bppg refer toedge-plane and basal-plane pyrolytic graphite electrodes respectively.Data reproduced with permission from reference 67. Copyright Amer-ican Chemical Society 2007
MaterialSurface coverage of ortho-quinones/mol cm�2
eppg 1.7 � 10�10
bppg 2.0 � 10�12
GC —b-MWCNTs 3.2 � 10�12
h-MWCNTs 2.5 � 10�12
SWCNTs 5.1 � 10�13
Fig. 3 The wide survey XPS spectrum (0–1200 eV) of graphite powder
labeled with 1. Reproduced with permission from reference 67. Copyright
American Chemical Society 2007.
Fig. 4 The overlaid cyclic voltammograms recorded in 0.1 M KCl
of native SWCNTs and SWCNTs after labeling with hex-
aamminechromium(III) (first five cycles). The SWCNTs were abrasively
immobilized onto a bppg electrode. Scan rate 100 mVs�1. Reproduced
with permission from reference 67. Copyright American Chemical
Society 2007.
Table 4 The elemental surface composition of an eppg, bppg and GCelectrode labeled with the hexamminechromium(III) relative to the C1s
peak (as 100%) and after correction to take into account the emissionfrom the PTFE surrounding mantle (correlated to the F1s peak). Alsoshown is the percentage of the total O1s peak attributed to ortho-quinonegroups. Note that eppg and bppg refer to edge-plane and basal-planepyrolytic graphite electrodes respectively. Data reproduced withpermission from reference 67. Copyright American Chemical Society2007
Electrodematerial % Cr � 0.05% % O � 0.05% % N � 0.05%
% of the totalO1s peakattributed toortho-quinones
eppg 3.54 36.33 2.33 19.5bppg 1.62 48.51 3.34 13.8GC 0.61 21.11 1.92 5.8
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surface coverage of these groups by measuring the area under the
voltammetric peaks. Doing so reveals the surface coverage of
ortho-quinones to be that shown in Table 3 for these particular
samples. Again, the GC powder possessed a surprisingly low
surface coverage, indeed too low to quantify with any acceptable
This journal is ª The Royal Society of Chemistry 2009
degree of certainty, of ortho-quinones in agreement with the
labelling with compounds 1 and 2.
XPS characterisation of the hexaaminechromium(III) labelled
carbon surfaces produced distinct spectral features correspond-
ing to Cr and N atoms from the label, in addition to the usual C1s
and O1s emissions. Using both the Cr2p and N1s spectral peaks it
is again possible to attribute the percentage contribution to
the total O1s signal arising from ortho-quinone surface groups
as shown in Table 4.
Thus we have developed a series of dual electrochemical and
XPS labelling methods that can be combined to allow one to
quantify the presence and distribution of all carbonyl groups, and
to selectively distinguish, quantitatively, between ortho- and para-
quinone groups on graphitic carbon surfaces. In the next section,
we will complete our study of the common surface oxo groups by
the development of dual-labels for surface carboxyl groups.
5. XPS and voltammetric labelling of carboxylgroups using 4-nitrophenol
Using a similar dual-labelling approach as outlined above for the
determination of quinones, we have successfully exploited the
covalent modification of carbon surfaces with 4-nitrophenol (4-
NP) to quantify surface carboxyl groups.73 The general reaction
scheme is shown in Scheme 4 and is applicable to graphite, CNT
and GC surfaces. The electrochemical reduction of the nitro
group provides a distinct four-electron, four-proton irreversible
reduction peak upon first scanning in a reductive direction to
form the corresponding arylhydroxylamine. This in turn gives
J. Mater. Chem., 2009, 19, 4875–4886 | 4881
Scheme 4 The modification of CNTs with 4-NP via the conversion of surface carboxyl groups to the corresponding acyl chlorides. Reproduced by
permission of the Royal Society of Chemistry (RSC) from reference 73.
Table 6 The relative values of the surface coverage of carboxyl andquinonyl groups on the surface of 4-NP labelled b-MWCNTs andh-MWCNTs (expressed as a percentage) determined by deconvolutingthe voltammetric peaks of system II and the surface quinonyl groups (seetext). Also shown is the percentage increase of each functional group afteroxidation compared to the native state. Data reproduced by permissionof the Royal Society of Chemistry (RSC) from reference 73
Material% Carboxyl�1%
% Quinonyl�1%
% change incoverageof each group afteroxidation cf. nativeMWCNTs
� %Carboxyl
� %Quinonyl
b-MWCNTs 76 24b-MWCNTs (oxidised) 37 63 +590 +3200h-MWCNTs 47 53h-MWCNTs (oxidised) 64 36 +940 +460
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rise to yet another distinct quasi-reversible system at slightly
more positive potentials than the initial reduction wave (the exact
peak potentials depend on the solution pH) corresponding to the
redox behaviour of the arylhydroxylamine/arylnitroso system.
The N1s signal from the nitro group is also distinct in the XPS
spectrum.
Using this dual label, we were able to quantitatively compare
the number of surface groups on two different morphologies of
MWCNTs, namely b-MWCNTs and h-MWCNTs. We were also
able to study the effect of acid oxidation (mentioned earlier as
one of the most common methods of introducing surface
carboxyl groups onto graphitic carbon) on the relative numbers
of carboxyl groups and other surface oxo groups.73 Table 5
details the surface coverage of carboxyl groups on the
b-MWCNTs and h-MWCNTs before and after oxidation,
determined voltammetrically using 4-NP labelling.73 Prior to acid
pre-treatment, the b-MWCNTs possess approximately three
times as many surface carboxyl groups as the h-MWCNTs. This
is consistent with the morphology of the b-MWCNTs resulting in
a greater number of edge-plane defect sites, where the carboxyl
groups are located, than the h-MWCNTs. After acid pre-treat-
ment we see an overall increase in the number of surface carboxyl
groups in both cases, but this increase is around five times greater
for h-MWCNT than b-MWCNTs. This is due to the acid
pre-treatment damaging the otherwise pristine structure of the
h-MWCNTs, introducing holes and edge-plane defect sites in the
side-walls of the h-MWCNTs and decorating these with carboxyl
groups, so that after the acid pre-treatment step the surface
coverage of carboxyl groups is similar for both morphologies.73
Table 5 The surface coverage, G/mol cm�2 of carboxyl groups on thesurface of b-MWCNTs and h-MWCNTs, before and after oxidative pre-treatment, determined using the reduction peak area of system I afterlabelling of the carboxyl groups with 4-NP. Data reproduced bypermission of the Royal Society of Chemistry (RSC) from reference 73
MaterialG/mol cm�2
before oxidationG/mol cm�2
after oxidation
4-NP-b-MWCNTs 6.7 � 0.5 � 10�12 1.3 � 0.2 � 10�11
4-NP-h-MWCNTs 2.3 � 0.5 � 10�12 2.4 � 0.2 � 10�11
4882 | J. Mater. Chem., 2009, 19, 4875–4886
Table 6 details the results of XPS analysis of the same samples
before and after oxidation. Again the trends described above are
also observed using XPS labelling with 4-NP. Interestingly, while
oxidative acid pre-treatment does result in an increase in
carboxyl groups in both cases, the XPS analysis shows that, in
the case of the b-MWCNT sample, there is a much greater
increase in the atomic percentage of the other surface oxo
groups, presumably quinones, introduced during the oxidation
step. Whilst it is often reported in the literature that carboxyl
groups are introduced using these kinds of acid pre-treatment,
the introduction of other functional groups is often overlooked.
We have now introduced four different dual-labels for vol-
tammetric and XPS analysis of surface oxo groups. The distri-
bution of carboxyl, ortho- and para-quinones, carbonyl and
‘‘other’’ groups can now be determined by separately labelling
aliquots of the same powdered sample with each as shown in the
flow chart in Fig. 5. Doing this provides us with the quantitative
distributions of each type of surface oxo group, and the relative
amount of oxygen groups in other forms (lactones, ethers, etc.)
can then be calculated by subtraction. The presence of various
surface oxo groups can also have profound influences on the
rate of electron transfer at these surfaces,74 affecting both their
This journal is ª The Royal Society of Chemistry 2009
Fig. 5 The road map demonstrating how the various dual-labelling techniques can be combined to quantitatively determine the distribution of different
surface oxo groups on graphitic carbon samples.
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chemical reactivity and their potential for use in electrochemical
sensing devices. Having concentrated so far on surface oxo group
chemistry, in the final section we briefly review some of our
recent findings on the reactive sites on graphitic carbon surfaces
towards radical and cationic attack.
6. Reactive sites towards radical and cationic specieson carbon surfaces
6.1 Mechanisms of covalent derivatisation of carbon surfaces
using diazonium salts and the different reactive sites towards aryl
radical and aryl cation intermediates
The modification of graphitic surfaces using aryldiazonium salts
is of great interest. A variety of methods have been used to
generate reactive intermediates from the aryldiazonium salts
such as electrochemical reduction,75,76 thermolysis,77–79 photol-
ysis80 and chemical reduction.81–83 The resulting reactive inter-
mediates formed, such as aryl radical or cationic species, can then
react with carbon/CNT surfaces resulting in covalent bond
formation. We have demonstrated a method for the derivatisa-
tion of carbon/CNT surfaces chemically, in bulk quantities, using
aryldiazonium salts both in the presence of hypophosphorous
acid reducing agent and simply in water (ie. with no hypophos-
phorous acid reducing agent present).84,85
We have investigated the mechanisms involved in the deriva-
tisation of carbon powder, in both the presence and the absence
of the hypophosphorous acid reducing agent. In the presence
of a mild reducing agent such as hypophosphorous acid the
diazonium species can be reduced in a one-electron step to form
an aryl radical intermediate. It should be noted that, even in the
presence of a mild reducing agent such as hypophosphorous acid,
the diazonium salt can still undergo a competing mechanistic
This journal is ª The Royal Society of Chemistry 2009
pathway involving the spontaneous loss of dinitrogen to form an
aryl radical cation intermediate (see below). Both pathways
are driven by the thermodynamically favourable liberation of
dinitrogen gas. The aryl radicals can form a carbon-carbon bond
with the H-terminated regions of the graphitic carbon surface
(again at edge-plane defect sites)86 via a hydrogen abstraction
mechanism as shown in Scheme 5a.87,88 Alternatively, the aryl
radicals may react directly with free radical sites.
In the absence of any reducing agent the most likely decom-
position pathway for an aryldiazonium salt is again the loss of
dinitrogen resulting in an aryl cation intermediate being formed.
The aryl cations can then undergo a reaction with the graphite
surface via electrophilic attack and subsequent loss of H+, as
shown in Scheme 5b.88–90 In addition to this pathway, the aryl
cation intermediate can also react with surface carboxylate
groups to form an arylester linkage, as shown in Scheme 5c.91
The mechanistic pathways involving carbon-carbon bond
formation via either the radical or cationic intermediate result in
the formation of the same product, whilst the formation of an
aryl ester linkage is structurally and chemically different. Using
cyclic voltammetry we were able to use slight differences in the
peak potentials of the C–C bonded aryl species and the arylester
bonded species to understand the reaction mechanisms involved
in their derivatisation. The covalent attachment of the modifier
to the carbon/CNT surface via radical or cationic (via loss of H+)
intermediates is reduced at the same peak potentials, whilst the
attachment through ester linkage alters the peak potentials,
shifting them to more negative values.84,85
6.2 pKa effects on chemically modified carbon surfaces
The pKa values of surface-immobilized molecules often differ
from those in bulk solution. This effect has been observed for
J. Mater. Chem., 2009, 19, 4875–4886 | 4883
Scheme 5 Mechanistic illustration for the derivatisation of NP-carbon through a) radical, b) and c) cationic intermediates. Reproduced from reference
88 with permission. Copyright John Wiley & Sons Ltd., 2008.
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a wide number of relevant systems, for example, phenyl
carboxylic acids on glassy carbon electrodes,92,93 azo dyes,94 and
self assembled monolayers of thiols95–98 and thiotic acids99 on
gold electrode surfaces. We have reported that the pKa values of
1-anthraquinone groups on the surface of graphite powder and
CNTs exhibit large changes in their pKa values compared to
those of free anthraquinone in solution, and that the shifts are >3
pKa units, one of the largest pKa shifts yet reported.86 In addition
we have also chemically modified graphite and glassy carbon
surfaces with benzoic acid groups, and have compared the
changes in pKa values of benzoic acid from its solution value.
Interestingly, the pKa value of benzoic acid covalently attached
to graphite surfaces is shifted to higher values by more than 2 pKa
units; in contrast, the pKa of benzoic acid on glassy carbon
surfaces is shifted by just over 1 pKa unit to lower values than
that of benzoic acid in solution.81 We have also reported that the
pKa values of bis(3-aminopropyl)-terminated polyethylene glycol
(jeffamine) on graphite surfaces shifted by more than 2 pKa units
from its solution value.100
The reasons for the shifts in pKa are not fully understood,
but have been attributed to a variety of factors. These include
such considerations as whether the interface is charged or not,
the nature of the surface such as its hydrophobicity, lateral
4884 | J. Mater. Chem., 2009, 19, 4875–4886
interactions between adsorbed species e.g. hydrogen bonding,
electronic interactions between surface bound species and the
bulk solid electrode and differing molecular environments, such
as solvation shell, solvent accessibility, etc.) caused by the
structure and location at defect sites on the carbon surface.86
Some insight into the thermodynamic parameters controlling
the shift in pKa has been given in a recent study comparing the
pKa of bis(3-aminopropyl)-terminated polyethylene glycol (jeff-
amine) in solution (pKa ¼ 9.7) with that when the jeffamine is
attached to a carbon surface (pKa ¼ 7.1).101 Jeffamine is a weak
base that reacts with an acid according to equation (1):
RNH2 + HA 4 RNH3+ + A� (1)
The pKa of jeffamine can then be determined from a plot of pH
vs. the log term in equation (2) provided the other quantities are
known.
pH ¼ pKa þ log
�RNH2
��RNHþ3
�!
(2)
The value of Ka can then be related to the change in entropy,
enthalpy and Gibbs energy for dissociation of the protonated
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jeffamine via the integral form of the van’t Hoff isochore
(Eq. (3a)) and standard thermodynamic relationships given by
Eq. (3b–c).
lnKa ¼ �
DHoa
RT
!þ const (3a)
DGo ¼ �RTlnKa (3b)
DGo ¼ DHo � TDSo (3c)
Analysis of the thermodynamic parameters controlling the
ionization of jeffamine in solution and attached to a carbon
surface revealed that the enthalpy changes for each case were
negligibly small (0.9 and 0.7 kJmol�1 for jeffamine in solution
and on a carbon surface respectively). This suggests that lateral
interactions or hydrogen bond formation were not responsible
for the observed pKa shift. However, significant differences in
the entropy of ionization between jeffamine in free solution and
attached to the carbon surface were observed.101 This suggests
that the principle reason for the shift in pKa was due to differ-
ences in solvent ordering/disordering at the interface between the
carbon substrate and the solution.
The changes in magnitude and direction of shifts in the
observed pKa of the same molecules attached to different carbon
surfaces have to be considered very carefully when one is plan-
ning or attempting a synthesis to produce chemically modified
carbon materials. However, these shifts can also be used to our
advantage. For example, one important application of this pKa
effect on chemically modified carbon surfaces is the recent
development of reagentless, calibration free, solid-state pH
sensing electrodes based on modified CNT and graphite mate-
rials.102–107 Although the pKa values for some of the pH sensitive
species used, for instance 9,10-anthraquinone (AQ), are around
8–10 in solution, the chemically modified carbon pH sensors
(e.g. anthraquinone modified carbon nanotubes, AQ-MWCNTs)
can exhibit peak potentials that vary in a linear, Nernstian
fashion from pH 1 to beyond pH 14!86,102,105,107
7. Conclusions and outlook
In summary, we have briefly introduced physical methods of
characterising the surface chemistry and functional groups
present on carbon surfaces, and shown how we can extend these
to develop chemical methods using dual-labels that provide both
distinct XPS and voltammetric signals. Several different labels
have been introduced to explore total C]O groups, selectively
distinguish between different surface quinone groups, and to
label carboxyl groups. This has provided us with a unique insight
into carbon surface chemistry, the effects of chemical oxidation,
and the role that these groups can play in affecting electron
transfer rates at these surfaces.
We have also explored radical and cationic mechanisms of
derivatising carbon surfaces, and identified plausible reaction
pathways that explain the observed voltammetry. Large changes
in the pKa values of certain organic molecules attached to carbon
surfaces can be observed, and reasons for these changes have
been explored, and exploited. It is important for synthetic
chemists and materials chemists working in the area of modified
This journal is ª The Royal Society of Chemistry 2009
carbon materials to bear in mind that the changes in surface pKa
effects can have a profound impact on any proposed synthetic
strategy, but these changes can also be exploited beneficially.
The surface chemistry of graphitic materials is rich. We
envisage that this area will see yet more development over the
coming years and will enable materials chemists and analytical
chemists alike to develop new and interesting materials and
systems to meet the challenges facing us at the start of the twenty-
first century.
Acknowledgements
GGW thanks St. John’s College, Oxford, for a Junior Research
Fellowship.
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