Morphological changes and covalent reactivity assessment of single-layer graphene oxides
under carboxylic group-targetted chemistry
Raymond L. D. Whitby,1, Alina Korobeinyk,1 Katya V. Glevatska2
1. Nanoscience and Nanotechnology Group, PABS, University of Brighton, Lewes Road, Brighton,
BN2 4GJ, United Kingdom
2. Chuiko Institute of Surface Chemistry, General Naumov Street 17, 03164 Kiev, Ukraine
Single-layer graphenes possess electronic and thermal conduction properties superior to those
for carbon nanotubes [1-3] and their use promises to transcend a number of fields from
electronics to polymer strengthening additives to heterogeneous catalysis. Therefore, assessing
their surface chemistry reactivity, in order to tune their properties, and resulting physical
changes of the individual layers is of paramount importance. Numerous synthetic strategies
have successfully resulted in single-layer graphenes, which often leads to extensive
incorporation of oxygen containing groups, around the periphery and across the graphene
surface [4], hence termed single-layer graphene oxide (SLGO).
In order to release their potential, chemical modification of their surface, which allows
for subsequent chemical reactions, is likely necessary to tailor (or integrate) SLGO for a
specific end purpose [5]. In particular, carboxylic groups have proven useful candidates for the
chemical modification of other nanocarbon systems [6] and therefore are an appropriate target
chemical group in order to understand the chemical and physicochemical nature of SLGO.
Herein, Boehm titration analysis [7-9] was conducted to directly assess the quantity of
carboxylic groups available in the SLGO system and their chemical reactivity, which has not
been reported before.
SLGO was obtained from a commercial supplier (CheapTubes Inc) and chemically

Corresponding author. Fax: +44 1273 642674. E-mail address: r.whitby@brighton.ac.uk (R.L.D. Whitby)
reacted according to procedures detailed in the Supplementary Information. The total number of
oxygen containing groups, which includes carboxylic, lactone and phenolic environments, was
7.521 (± 0.144) meq g-1, although likely to be slightly higher for the presence of carbonyl
groups that were undetected in the titrations, and the average sheet size was ca. 800 nm
according to AFM analysis conducted by the supplier. This SLGO sample appears similar in
nature and functionality of SLGO reported elsewhere [5]. Based on the calculation of reactive
oxygen-containing groups distributed solely at the periphery of SLGO would give a value in the
region of 0.1 meq g-1, which is substantially less than the value obtained and supports the notion
that the extensive oxygen-containing group functionality decorates the periphery and across the
surface of each sheet, in accordance with models described elsewhere [10].
Fig. 1. Pure single-layer graphene oxide suspension in methanol (a) retains its (near) planar
morphology, as observed in the TEM image (b, scale bar = 1 micron). However, once treated
with thionyl chloride, the chlorinated graphene oxide appears as a black powder that readily
falls out of solution (c) where the planar sheets have reduced in size and overlap (arrow A) or
have folded (arrow B) (d, scale bar = 5 microns).
Treatment with thionyl chloride precludes analysis of the intermediate stage, as rapid
hydrolysis converts the acyl chloride groups back to its carboxylic state, therefore carrying the
reaction through with an amine and analysis of the final amide is warranted. Covalent bonding
of n-butylamine to graphene-acyl chloride led to a 83 % reduction in the number of carboxylic
groups to 0.495 (± 0.021) meq g-1. It is noted that the initial brown coloration (Fig. 1a) of the
graphene oxide changes to black (Fig. 1c). TEM analysis showed that the SLGO sheets
appeared typically flat yet wrinkled (Fig. 1b), but on chlorination the graphene sheet size has
substantially reduced and in places appears to have folded (Fig. 1d arrow B), noted in the dark
contrast frequently exhibited by overlapping layers. However, separate layers causing this
overlap are often easy to differentiate from folding (Fig. 1d arrow A).
A separate thionyl chloride treated SLGO sample was subsequently regenerated with
hydrochloric acid and revealed that the number of carboxylic groups detected had dropped by 20 %
to 2.273 (± 0.018) meq g-1. It was not possible to re-suspend the final black product in water (or
solvents), despite the high number of carboxylic groups still present. It is apparent that the
chlorinating agent acting on a large aromatic single layer network that incorporates a high degree of
oxygen-containing group functionality leads to degradation of the structure and loss of acidic
groups. Taking the 20 % loss of functionality into consideration during the thionyl chloride reflux,
the reactivity of SLGO-carboxylic groups through an acyl chloride intermediate to the final nbutylamide was 78 %.
SLGO-acyl chloride was also reacted with aniline causing a depletion to 0.746 (± 0.075)
meq g-1 (67 % reactivity) of the number of carboxylic groups. The 11 % difference between this
SLGO-phenylamide and SLGO-n-butylamide is likely to reflect the small variation of subtle steric
hindrance effects of the available carboxylic group where linear aliphatic amines do react and
aromatic amines do not.
Refluxing SLGO in hydrazine [11] also generates a black material (similar to Fig. 1c).
However, titrations showed that the SLGO only lost 0.592 (± 0.088) meq g-1 (21 %) of its
carboxylic groups and is far from the desired pure form of single-layer graphene. Moreover, the
partly reduced graphene sheet is not solubilised in aqueous solution, so it appears that the surface
charge is likely to have changed where inter-sheet repulsion can no longer maintain a stable
aqueous suspension of the graphene system [12]. Further treatment of these reduced graphenes with
thionyl chloride and regenerating in hydrochloric acid removed an additional 1.322 (± 0.076) meq
g-1 (59 %) of carboxylic groups implying that the reduction process had actually structurally
weakened the graphene sheet, thus leading to a significant loss of functionality through the
subsequent chlorination step. Generation of the acyl chloride and conversion to the final nbutylamide lowered the number of carboxylic groups to 0.217 (± 0.043) meq g-1, giving an overall
reactivity of 76 %.
The incomplete conversion of all carboxylic groups during chlorination and amidation could
be due to a number of reasons. Two carboxylic (or acyl chloride) groups on the periphery could be
in close proximity generating steric hindrance; a small number of carboxylic groups detected were
actually part of peripheral acid anhydride groups that would lead to two COOH groups detected, but
the formation of only a single amide; carboxylic groups could be located within a lattice defect that
again generates steric hindrance for amine molecules, but not the titrant ions.
Fig. 2. Pure single-layer graphene oxide is stably suspended in water (a) where it retains (near)
planar morphology (b, scale bar = 200 nm). However, once EDAC was added graphene oxide
immediately agglomerated out of solution (c). It was observed under TEM that individual graphene
oxide sheets condensed into star-like clusters (d, scale bar = 100 nm).
Given the hydrophilic nature of graphene oxide, carbodiimide chemistry is an obvious
avenue of exploration, plus its room temperature conditions should maintain the structural integrity
of graphene. On addition of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDAC), graphene
oxide (Fig. 2a) separates rapidly from solution (Fig. 2c) and remains in an agglomerated state on
stirring with N-hydroxysuccinimide (NHS). Once purified, the intermediate SLGO-NHS exhibited
0.788 (± 0.173) meq g-1 loss of carboxylic groups, representing a 37 % conversion. As the
intermediate can react with monoamine molecules in a one-to-one ratio, the effectiveness of using
n-butylamine to monitor the reaction progress is negated as it would be difficult to distinguish the
intermediate from the end product using Boehm titration, a problem reported before [13]. Therefore,
glutamic acid was used as it contains two carboxylic groups; herein it is assumed that all
intermediate groups that have formed will also be capable of participating in further reactions. The
increase in number of carboxylic groups was measured at 0.087 (± 0.087) meq g-1 above the
intermediate compound giving only a 4 % reactivity of the intermediate, which is much lower than
that of the graphene-acyl chloride compound. Moreover, this value falls inside the error limits,
potentially indicating that no reactivity has occurred. However, through an aqueous chemistry route
the final compound retains its brown coloration (Fig. 2c) indicating that the structural integrity of
the graphene sheets was maintained, although the low degree of reactivity with glutamic acid does
not provide sufficient acidic groups for resolubilisation. TEM reveals that the initially planar
graphene sheets (Fig. 2b) have condensed into star-like clusters (Fig. 2d), which may imply intrasheet coupling. However, similar morphological changes of SLGO were also induced through
simple pH changes of its aqueous suspension. Therefore, an alternative explanation may be due to
the addition of the EDAC group, which incorporates tertiary amino groups and could induce van der
Waals attractive forces to operate within the sheet resulting in a collapsed morphology that provides
an energetic minimum [14], thus reducing the number of available carboxylic sites for amine
coupling.
Ultimately, the presence of a large number of peripheral and intra-sheet oxygen-containing
functional groups leads to weakening and reduction in the size of the SLGO structure through
chemical modification with thionyl chloride and hydrazine. This may have been overcome by
exploiting room-temperature carbodiimide chemistry, however, the graphene sheet was observed to
condense into itself, thus lowering the number of available reactive carboxylic groups and
preventing further reactions with amines. Therefore, traditional chemical treatment for
functionalisation of SLGO-carboxylic groups leads to potentially undesirable changes in sheet
morphology, which also affects its overall reactivity, and may restrict the usability of SLGO and its
reduced forms.
Acknowledgements
We thank the RCUK Academic Fellowship scheme and CheapTubes Inc. for useful discussions.
Supplementary Information
Experimental details
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