The reduction of graphene oxide with hydrazine: elucidating its reductive capability based on a reaction-model approach

Article abstract of DOI:10.1039/c5cc08170j

We have performed an experimental investigation on the effects of hydrazine treatment on graphene oxide via a reaction-model approach. Hydrazine was reacted with small conjugated aromatic compounds containing various oxygen functional groups to mimic the structure of graphene oxide. The hydroxyl and carboxylic groups were not readily removed while carbonyl groups reacted with hydrazine to form the corresponding hydrazone complexes. In the presence of adjacent hydroxyl groups, carboxyl groups underwent thermal decarboxylation.

Full text of DOI:10.1039/c5cc08170j

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The reduction of graphene oxide with hydrazine:
elucidating its reductive capability based on a
reaction-model approach†
Cite this:DOI: 10.1039/c5cc08170j
Received 30th September 2015,
Accepted 27th October 2015
Chun Kiang Chua and Martin Pumera*
DOI: 10.1039/c5cc08170j
www.rsc.org/chemcomm
We have performed an experimental investigation on the effects
of hydrazine treatment on graphene oxide via a reaction-model
approach. Hydrazine was reacted with small conjugated aromatic
compounds containing various oxygen functional groups to mimic
the structure of graphene oxide. The hydroxyl and carboxylic groups
were not readily removed while carbonyl groups reacted with hydra-
zine to form the corresponding hydrazone complexes. In the presence
of adjacent hydroxyl groups, carboxyl groups underwent thermal
decarboxylation.
It has been a decade since graphene was successfully isolated.1
Graphene promises the development of state-of-the-art devices,
ranging from electronics,2 sensors,3 and healthcare devices4 to
energy storage and conversion devices,5 due to its exceptional
Fig. 1 Oxygen functional groups found on graphene oxide, including
epoxide, hydroxyl, carbonyl and carboxyl groups.
structural, physical and electronic properties.6 In order to achieve be removed upon base wash, chemical reduction or ultrasoni-
this, numerous synthesis strategies have been highlighted over cation treatment.10 Nevertheless, graphite oxide is typically
the past years to produce bulk quantity of high quality graphene. ultrasonicated and reduced to yield graphene sheets.8b One of
These strategies include top-down and bottom-up methods. the most common reducing agents applied for the reduction of
The bottom-up method produces high quality graphene sheets graphene oxide is hydrazine.11 In contrast to how hydrazine is
but suffers from high cost and low yield. On the other hand, the applied as a reducing agent in the context of graphene oxide,
top-down method realises the possibility of bulk production of hydrazine is typically applied in organic chemistry only as a
graphene but the integrity of the sp2 carbon network is often reducing agent for the Wolff–Kishner reaction (conversion of
compromised.
ketone to alkane) and as a nucleophile for most other types of
In a typical top-down approach based on chemical reaction, reactions.12 It is, as such, difficult to establish a direct link
graphite is first oxidised with strong acids and oxidants to produce between the functions of hydrazine in organic chemistry and in
graphite oxide, an intermediate which consists of oxygen-containing the chemistry of graphene oxide. This has thus encouraged the
groups, including hydroxyl, epoxide, carbonyl and carboxyl groups.7 development of experimental and computational investigations
Several models of graphite oxide have been proposed, with the Lerf– on the working mechanisms of hydrazine on graphene oxide. Ruoff
Klinowski model (Fig. 1) being the most frequently represented and co-workers have proven experimentally with the usage of solid
in the literature.8 A recent study also highlighted graphite oxide state NMR and XPS analyses of 13C- and 15N-labelled hydrazine-
as a bi-component material consisting of relatively unoxidised reduced graphene that five-membered pyrazole or pyrazoline
sheets and highly oxidized small aromatics,9 where the latter can rings are formed at the edges of the graphene sheets.13 On the
other hand, density functional theory was applied to investigate
the proposed mechanisms for the reduction of the epoxide
Division of Chemistry and Biological Chemistry, School of Physical and
group with hydrazine.14 Nagase and co-workers further inves-
Mathematical Sciences, Nanyang Technological University, Singapore 637371,
tigated the mechanisms and effects of hydrazine (25 1C) and
Singapore. E-mail: pumera.research@gmail.com
thermal annealing (700–1200 1C) treatments on various small
† Electronic supplementary information (ESI) available: Experimental details and
NMR spectra. See DOI: 10.1039/c5cc08170j
fragments of graphene sheets containing hydroxyl, epoxide,
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carbonyl and carboxylic groups.14b The computational study
showed that hydrazine treatment at room temperature does not
spontaneously remove the hydroxyl, epoxide, carbonyl and
carboxylic groups present at the edge plane of graphene sheets.
Instead, the epoxide and hydroxyl groups present in the basal
plane are removed favourably.
Table 1 Reactivity of compounds 1 to 8 in the presence of hydrazine
(100 1C for 24 hours) and in the absence of hydrazine as a control case
(100 1C for 24 hours)
Materials
Reaction with hydrazine
Control case
No reaction
No reaction
In this work, the effects of hydrazine as a reducing agent, in the
context of graphene oxide reduction, are investigated experimentally
on small conjugated aromatic compounds. While graphene oxide
should be considered as a highly conjugated macromolecule and its
chemistry, as such, should be treated differently, it is nevertheless
worthwhile to examine the possible chemical transformations
occurring in simple organic molecules upon treatment with
hydrazine. In view of this, small conjugated aromatic com-
pounds containing specific oxygen functional groups (i.e., hydroxyl,
carbonyl, carboxylic, methoxy) were treated with hydrazine
and analysed for possible functional group transformations.
These include 2-naphthol (1), 1,2,3,4-tetrahydro-1-naphthol (2),
2-methoxynaphthalene (3), 1-tetralone (4), 2-acetonapthone (5),
2-naphthoic acid (6), 2-hydroxy-1-naphthaldehyde (7) and 2-hydroxy-
1-naphthoic acid (8) (see Table 1). Control experiments which
excluded the addition of hydrazine were also performed simulta-
neously in order to determine the effects of thermal treatment on
compounds 1–8. This work provides a basis for future investigations
on more complex conjugated molecules as models to understand
the reductive effects of hydrazine on graphene oxide.
In a typical reaction of graphene oxide with hydrazine, hydrazine
was added into a dispersion of graphene oxide in water and
subsequently heated to 100 1C for 24 hours. However, in order to
ensure the complete dissolution of the conjugated aromatic com-
pounds applied as model molecules in this study, the procedure
was modified by changing the dispersion medium from water to a
mixture of DMF and water (9 : 1). This new procedure was applied
for the reduction of graphene oxide, whereby graphene oxide was
observed to agglomerate into clusters over the course of the reaction,
which is consistent with typical observations.
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
Further X-ray photoelectron spectroscopy (XPS) analyses indi-
cated a well-reduced graphene sample (Fig. 2). The survey scan
showed the peaks of C1s, N1s and O1s in the ratio of 83: 12: 5.
The derived C/O ratio of 6.9 was typical for hydrazine-reduced
graphene oxide.15 Moreover, the C1s high-resolution core-level
spectrum showed a sharp peak at 284.5 eV (CQC bond) with
extended tailing into the region of higher binding energy. The
presence of C–C, C–O, CQO, O–CQO and p–p* peaks was
obvious as well. Since the revised protocol provided a good graphene oxide were expected to dissociate or migrate to the edges
extent of reduction on the graphene oxide, compounds 1–8, of the aromatic domains, which thereafter would dissociate off
which were mainly naphthalene-based, were subjected to hydra- upon thermal treatment (700–1200 1C).14b In this study, both
zine treatment at 100 1C for 24 hours in a DMF/water medium. compounds 1 and 2 represented hydroxyl groups at the edges
Alongside that, control experiments performed in the absence of of graphene oxide sheets, with compound 1 being positioned
hydrazine were conducted to determine the effects of thermal in an aromatic domain while compound 2 being situated in a
treatment on the compounds. A summary of hydrazine and the non-aromatic domain. When compound 1 was reacted with
thermal effects on compounds 1–8 is listed in Table 1.
hydrazine, no obvious dehydroxylation was observed as only the
The possible events of hydrazine- and thermal-induced starting material was detected by NMR analysis. However, the
dehydroxylation were investigated with compounds 1 and 2. Based presence of naphthalene was occasionally observed at a very
on previous theoretical studies by Nagase and co-workers, the low proportion by NMR analysis over the span of several
hydroxyl groups attached to the inner aromatic domains of repetitions. Similarly, treatment of compound 2 with hydrazine
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We extrapolated the study to conjugated aromatic compounds
containing a mixture of oxygen functional groups, specifically
compounds 7 and 8, which consisted of carbonyl/hydroxyl and
carboxylic/hydroxyl functional groups, respectively. In the case of
compound 7, the corresponding hydrazone 7a was obtained as a
major product (84%) when treated with hydrazine. Thermal treat-
ment did not result in any functional group transformation. As for
compound 8, thermal treatment caused a decarboxylation process
toward the formation of 2-naphthol (8a, 80%), while no further
reactions were observed in the presence of hydrazine. Thermal
decarboxylation was however not observed in compound 6, which
suggested a possible hydroxyl group effect on this process.
In summary, the reactions of hydrazine with various conju-
gated aromatic compounds were investigated based on a reaction-
model approach to understand the reactivity of hydrazine on
graphene oxide. This study showed that hydroxyl and carboxylic
groups were not readily removed while carbonyl groups formed
the corresponding hydrazone complexes. These observations may
be anticipated from the organic chemistry point of view, but it is
nevertheless important to extend the investigations to more
complex conjugated aromatic compounds to further understand
the underlying effects of hydrazine treatment on graphene oxide.
Furthermore, the phenomenon of the thermal decarboxylation
of a carboxyl group containing an adjacent hydroxyl group
could provide useful insights into the observed precipitation
Fig. 2 X-ray photoelectron spectra of hydrazine-reduced graphene oxide
based on the (a) survey and (b) high-resolution C1s core-level analyses.
did not result in any dehydroxylation effect although slight traces of of graphene during the reduction of graphene oxide with
undetermined complexes were observed by NMR analysis in very hydrazine. This work highlights the importance of using small
small proportions. In an effort to study the possible demethoxyl- organic molecules as models to study the reactions occurring
ation or demethylation processes occurring in the aromatic on graphene oxide.
domains, compound 3 was treated with hydrazine but it remained
unreactive.
This work was supported by a Tier 2 grant (MOE2013-T2-1-056;
ARC 35/13) from the Ministry of Education, Singapore.
Subsequently, the decarbonylation process was examined
with compounds 4 and 5. Similar to dehydroxylation, Nagase
and co-workers have theoretically proven that carbonyl groups
were resistant to decarbonylation under both hydrazine (25 1C)
and thermal treatment (700–1200 1C).14b In our case, when
compound 4 was reacted with hydrazine at 100 1C for 24 hours,
the starting material was fully consumed to provide the corres-
ponding hydrazone 4a in qualitative yield. Interestingly, traces
of the corresponding azine complex were also detected, which
supported the possible formation of pyrazole from diketone
moieties.13 Moreover, hydrazone 4a was observed to react with trace
acetone to form an azine complex. As a matter of fact, while Nagase
and co-workers predicted the presence of hydrazino alcohol from
the reaction between the edge-terminated epoxide group and hydra-
zine,14b it may be useful to note that the hydrazino alcohol could
further react with typical organic solvents (e.g. acetone) applied for
washing hydrazine-reduced graphene materials. Further studies on
the reaction of 2-tetralone and hydrazine provided a mixture con-
sisting of mainly decomposed materials. As for compound 5 which
consisted of a free carbonyl group, the corresponding hydrazone 5a
was obtained in 85% yield. While decarboxylation was predicted
by Nagase and co-workers to occur slowly at room temperature14b
and even at 100–150 1C,11,16 decarboxylation was not observed
in this study when compound 6 was treated with hydrazine at
100 1C for 24 hours.
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