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: gregory.wildgoose@chem.ox.ac.uk;richard.compton@chem.ox.ac.uk; 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.

J. Mater. Chem., 2009, 19, 4875–4886 | 4875

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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

This journal is ª The Royal Society of Chemistry 2009

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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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