Synthesis of Graphene Oxide by Oxidation of Graphite with Ferrate(VI) Compounds: Myth or Reality?

Article abstract of DOI:10.1002/anie.201603496

It is well established that graphene oxide can be prepared by the oxidation of graphite using permanganate or chlorate in an acidic environment. Recently, however, the synthesis of graphene oxide using potassium ferrate(VI) ions has been reported. Herein, we critically replicate and evaluate this new ferrate(VI) oxidation method. In addition, we test the use of potassium ferrate(VI) for the synthesis of graphene oxide under various experimental routes. The synthesized materials are analyzed by a number of analytical methods in order to confirm or disprove the possibility of synthesizing graphene oxide by the ferrate(VI) oxidation route. Our results confirm the unsuitability of using ferrate(VI) for the oxidation of graphite on graphene oxide because of its high instability in an acidic environment and low oxidation power in neutral and alkaline environments.

Full text of DOI:10.1002/anie.201603496

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201603496
German Edition: DOI: 10.1002/ange.201603496
Graphene Oxide
Synthesis of Graphene Oxide by Oxidation of Graphite with
Ferrate(VI) Compounds: Myth or Reality?
Zden eˇ k Sofer,* Jan Luxa, Ond rˇ ej Jankovsk y´ , David Sedmidubsk y´ , Tomꢀ sˇ Bystro nˇ , and
Martin Pumera*
[
5]
Abstract: It is well established that graphene oxide can be
prepared by the oxidation of graphite using permanganate or
chlorate in an acidic environment. Recently, however, the
synthesis of graphene oxide using potassium ferrate(VI) ions
has been reported. Herein, we critically replicate and evaluate
this new ferrate(VI) oxidation method. In addition, we test the
use of potassium ferrate(VI) for the synthesis of graphene
oxide under various experimental routes. The synthesized
materials are analyzed by a number of analytical methods in
order to confirm or disprove the possibility of synthesizing
graphene oxide by the ferrate(VI) oxidation route. Our results
confirm the unsuitability of using ferrate(VI) for the oxidation
of graphite on graphene oxide because of its high instability in
an acidic environment and low oxidation power in neutral and
alkaline environments.
composition of oxygen functionalities. The broad applica-
[6]
tion potential of graphene oxide has been widely reported.
Importantly, graphene oxide is a starting material for chemi-
[
7]
cally modified graphenes.
Very recently, a report appeared about a powerful new
oxidation agent—potassium ferrate(VI)—suitable for the
[8]
synthesis of graphite/graphene oxide. Because there is
substantial uncertainty in the literature thermodynamic data
regarding ferrate(VI) stability, we herein give rather con-
servative estimates of its redox potentials. Ferrate(VI) redox
potentials are strongly dependent on pH, giving a redox
potential of at least around 1.8 V in an acidic environment
(pH 0.0), at least around 0.9 Vat pH 7.0, and a relatively weak
redox potential in an alkaline environment (at least ca. 0.1 V
at pH 14.0). Nevertheless, the oxidation ability of ferrate(VI)
anion has been manifested by oxidizing water across a wide
range of available pH values. Thus, compared with the
previously used permanganates and chlorates, the high redox
potential of potassium ferrate(VI) provides an opportunity to
oxidize carbon in graphite to graphene oxide. In combination
with the low toxicity of iron ions, ferrate(VI) anion is
a “green” oxidation agent renowned for its high oxidizing
ability, especially in an acidic environment, where its redox
potential is the highest.
[
9]
G
raphene oxide is the most common starting material for
[
1]
the synthesis of graphene. The synthesis of graphene oxide
was first reported in the mid-nineteenth century by Brodie,
who used potassium chlorate in combination with fuming
[2]
nitric acid (> 90 wt% HNO ). The initial methods were
3
generally based on the oxidation of graphite using potassium
chlorate in an environment of concentrated nitric and sulfuric
acids. Subsequently, methods were developed that used
mixtures of fuming nitric acid and concentrated sulfuric
acid, as well as mixtures of concentrated nitric (68 wt%
Conversely, it must be noted that the oxidation ability of
ferrate(VI) has also been linked to its instability, particularly
in acidic environments. In practical terms, ferrate(VI) stabil-
ity at room temperature is limited to days in a strongly
alkaline environment, to hours in a neutral environment, and
[3]
HNO ) and sulfuric acids. More recently developed meth-
3
ods are based on the procedure first reported by Hummers, in
which concentrated sulfuric acid is mixed with in situ formed
[
4]
[10]
manganese(VII) oxide. Hummersꢀ procedure has been
further modified in recent years to improve safety, scalability,
and the degree of graphite oxidation, as well as to tune the
to seconds in strongly acidic aqueous solutions. Its stability
is influenced by the various metal ions, such as Ni and Co ,
2
+
2+
[
11]
that catalyze its decomposition. Interestingly, the decom-
position process is also catalyzed by iron oxides/hydroxides,
which are the products of ferrate(VI) decomposition in an
aqueous environment. Thus, ferrate(VI) decomposition is an
autocatalytic process. Ferrate(VI) ions have been applied in
[*] Prof. Dr. M. Pumera
Division of Chemistry and Biological Chemistry
School of Physical and Mathematical Sciences
Nanyang Technological University
Nanyang Link 21, 637371 Singapore (Singapore)
E-mail: pumera@ntu.edu.sg
[
12]
various fields of organic synthesis, including water treat-
[
13]
[14]
ment,
tion.
energy storage,
and warfare agents decomposi-
[
15]
Prof. Dr. Z. Sofer, J. Luxa, Dr. O. Jankovsk y´ , Prof. Dr. D. Sedmidubsk y´
Department of Inorganic Chemistry
University of Chemistry and Technology Prague
Technickꢀ 5, 166 28 Prague 6 (Czech Republic)
E-mail: zdenek.sofer@vscht.cz
In their paper, Peng et al. claimed the highly efficient and
rapid oxidation of graphite by K FeO in a sulfuric acid
2
4
[8]
environment. Herein, we assess the premises that underlie
their use of potassium ferrate(VI) for the synthesis of
graphene oxide. We investigate the degree to which the
oxidation of graphite to graphite/graphene oxide by fer-
rate(VI) proceeds under various experimental conditions,
including replicating theirs. For all conditions, we perform
separate sets of experiments, using either laboratory-pre-
pared or commercially supplied K FeO . The subsequent
Dr. T. Bystronˇ
Department of Inorganic Technology
University of Chemistry and Technology Prague
Technickꢀ 5, 166 28 Prague 6 (Czech Republic)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201603496.
2
4
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detailed characterization of the products obtained from all of
these experiments enable us to determine whether or not the
reported method is suitable for the oxidation of graphite to
graphite/graphene oxide.
oxidation of graphite was observed (Figure 1B). The reaction
mixture after dilution with water and the addition of hydro-
gen peroxide is shown in Figure 1C. The obtained material,
shown in Figure 1D, was highly hydrophobic, which was
caused by a low degree of oxidation. There were no obvious
changes with respect to the starting graphite.
The thermodynamic instability of potassium ferrate(VI)
makes its use in synthesis complicated and expensive, and has
[
16]
limited its application to laboratory scale. Consequently, it
is not widely commercially available and, thus, we first
synthesized potassium ferrate(VI) according to the standard
In addition, other experiments were performed using
K FeO4 commercially obtained from BOC Sciences and
2
Chemical Point UG. Although both commercial samples
were declared to have a purity of over 90 wt%, detailed
analysis revealed that the concentration of K FeO in these
[
17]
procedure.
Our synthesis produced K FeO4 of 76 wt%
2
purity (for its detailed characterization, see Supporting
Information, Figures S1 and S2, and Table S1). Our labora-
tory-prepared K FeO was used in the replication of Peng
2
4
products was only about 20 wt%. They were also shown to
contain significant amounts of impurities, including KNO3
2
4
et al.ꢀs procedure for the synthesis of graphene oxide under
acidic conditions; the resulting sample was termed G-ORG.
We also used our K FeO for graphite oxidation by ferrate-
and KClO , both of which are strong oxidants commonly used
3
in permanganate- and chlorate-based methods for the syn-
[
3,4]
thesis of graphene oxide.
For a detailed chemical analysis
2
4
(
(
VI) in environments of sulfuric acid (sample G-0), of water
sample G-7), or of 1m potassium hydroxide (sample G-14);
of the commercially-obtained K FeO , see Supporting Infor-
2
4
mation and Figure S2 and Table S1. Using the commercially
supplied K FeO of low purity, we performed four further
in other words, under acidic, neutral, or alkaline conditions as
reflected by the pH value in the sample name. The main steps
of graphite oxidation by sulfuric acid and the final product are
shown in Figure 1. The reaction of K FeO with sulfuric acid
2
4
experiments similar to Peng et al.ꢀs method (full details of the
syntheses and all characterizations can be found in the
Experimental Section in the Supporting Information). The
presence of KClO and KNO led to the formation of a large
2
4
3
3
amount of nitrous oxide during the oxidation procedure and
to a slightly higher degree of graphite oxidation. However, the
formation of graphene oxide was not observed in any of these
experiments, not even when three-times the standard amount
[
8]
of K FeO4 was used for graphite oxidation. A control
2
experiment performed with a purified commercial K FeO
2
4
(
free of KClO , KNO , KCl, and K CO ; 91 wt% K FeO ) led
3 3 2 3 2 4
only to minimal oxidation of graphite and was comparable to
the results obtained with K FeO synthesized in laboratory.
2
4
Images of the oxidation reaction mixtures are shown in the
Supporting Information (Figure S3).
SEM was used to compare the morphology of the samples
prepared from our laboratory K FeO . It revealed no
2
4
significant differences to the graphite (Figure S4), thereby
indicating a negligible effect of graphite oxidation by ferrate-
(VI) ions.
To obtain more information about the chemistry of the
obtained materials, X-ray photoelectron spectroscopy (XPS)
analysis was performed. The spectra for the samples prepared
from our laboratory K FeO showed a low concentration of
2
4
oxygen in the range of 1–5 at% (Figure 2). The lowest
concentration of oxygen was observed in G-ORG, the
sample prepared according to the Peng et al.ꢀs procedure,
Figure 1. Image of reaction mixture after first addition of K FeO to
2
4
[8]
sulfuric acid (A). Reaction mixture after one hour (B) and after dilution
with water and the addition of hydrogen peroxide (C) indicate the
presence of weakly oxidized graphite with hydrophobic properties.
Final oxidized graphite (D).
in which ferrate(VI) was added to sulfuric acid before the
addition of graphite. Our measured C/O ratio was 56.1
compared to their reported C/O ratio of 2.2; the C/O ratio
measured of the starting graphite was 89.9. The decomposi-
tion of ferrate(VI) was so fast that it did not allow for the
further oxidation of subsequently added graphite. In the case
of the three other samples, the concentration of oxygen was
only slightly higher, yielding a C/O ratio of about 22 for the
samples oxidized in sulfuric acid and neutral environments
and of about 28 for the graphite oxidized in an alkaline
environment. Collectively, these results show that graphite
cannot be oxidized by ferrate(VI) ions because of their
insufficient stability in an acidic environment and relatively
was extremely vigorous (to prevent an explosive reaction with
sulfuric acid, K FeO4 must be completely free of KOH).
2
K FeO4 underwent immediate decomposition, which was
2
characterized by the vigorous evolution of oxygen and the
disappearance of the ferrate(VI) ionꢀs normal red color
within a few minutes. The reaction mixture after the first
addition of K FeO is shown in Figure 1A. After one hour,
2
4
the slight evolution of oxygen was still evident, but no visible
2
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graphite oxidized by the commercially supplied K FeO are
2
4
shown in the Supporting Information (Figure S5), together
with the chemical analysis results obtained from these spectra
(Table S2). No higher degree of oxidation was observed in the
high-resolution XPS spectra, in which the absence of ketone
and/or carboxylic acid functionalities was clearly seen (Fig-
ure S5).
Our XPS analysis results are also supported by elemental
combustion analysis (see Table 1 for the samples prepared
Table 1: Content of C and O together with C/O ratios obtained from
elemental combustion analysis.
Sample
C [at%]
O [at%]
S [at%]
C/O
Graphite
G-ORG
G-0
G-7
G-14
98.9
98.1
95.7
95.6
96.6
1.1
1.9
4.3
4.4
3.4
0.0
0.0
0.2
0.1
0.1
89.9
51.6
22.3
21.7
28.4
from our laboratory K FeO and Table S3 for the samples
2
4
prepared from commercially supplied K FeO ). Only
2
4
a minute amount of oxygen and hydrogen was detected,
suggesting only slight surface oxidation. Slightly lower C/O
ratios were observed for the samples prepared using the
commercially obtained K FeO4 because of the impurities
2
present. In particular, both KNO and KClO have the ability
3
3
to oxidize graphite under strongly acidic conditions, such as
those used in the chlorate method of graphene oxide syn-
thesis. However, the C/O ratio was still about one order of
magnitude higher than for the typical composition of
graphene oxide. In addition, about 1 at% of sulfur was
observed in the samples oxidized by the commercially
available K FeO . For all samples, the obtained values were
2
4
Figure 2. XPS survey spectra and C 1s detail of graphite and oxidized
graphite.
close to the C/O ratios measured by XPS.
Raman spectroscopy is a very useful method for the
investigation of graphene oxide-based materials. Typically,
such spectra are dominated by two bands: 1) a D-band at
weak oxidation power in neutral or alkaline reaction mix-
tures. These results are also supported by the high-resolution
XPS spectra of the C 1s peak, in which carbon predominated
in the form of C=C bonds and only a negligible amount of
À1
3
1360 cm associated with the sp hybridized carbon atoms
and defects in the graphene structure, and 2) a G-band at
À1
2
1560 cm related to the vibrations of sp hybridized carbon
[
19]
oxygen functionalities was observed (Figure 2). In fact, all
C 1s spectra were almost identical. Such results are incon-
gruous with those obtained for graphene oxide prepared by
the classical permanganate- and chlorate-based methods, in
which the C/O ratio typically reaches values in the range of
atoms that form the graphene skeleton. In addition, a 2D-
band can also be often observed around 2700 cm .
À1 [19]
However, this is not what we found. The Raman spectra
were almost identical for all samples. The D-band for the
samples prepared from our laboratory K FeO4 (Figure 3)
2
[18]
2
–4.
For the samples prepared from commercially supplied
K FeO which contained high concentrations of KNO and
revealed extremely low intensities, confirming both the low
number of defects and insufficient oxidation of the starting
material. Only in the G-ORG sample, the D-band was slightly
visible owing to the surface oxidation of the graphite. The
Raman spectra for the samples prepared from commercially
2
4,
3
KClO XPS analysis showed a significant increase in the C/O
3
ratio, which reached 10–12. An even lower C/O ratio was
obtained when we carried out a larger scale experiment with
supplied and also purified K FeO4 (Figure S6) showed
2
6
0 g of commercial K FeO . These results show that the slight
a stronger, but still weak d-band that resulted from the
surface oxidation of the graphite; this again was due to the
oxidation ability of the impurities present in the commercial
products. However, even in this case, the D/G ratio remained
extremely low, indicating minimal oxidation of the graphite).
Collectively, these results are in good agreement with the low
2
4
oxidation of graphite using commercial potassium ferrate(VI)
is caused by the oxidation ability of the impurities present
within K FeO and not by its own oxidation ability. This was
also proved by performing experiments with purified K FeO
free of KNO and KClO3 impurities. XPS spectra of the
2
4
2
4
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Figure 3. Raman spectra of starting graphite and prepared materials.
Figure 5. XRD diffraction patterns of starting graphite and prepared
materials.
Figure 4. FTIR spectra of starting graphite and prepared samples.
graphene oxide typically leads to an increase in interlayer
[
21]
spacing from 0.34 nm to around 0.6–0.8 nm. However, in
this case, none of the samples showed an increase in interlayer
distance compared with the starting graphite. Again, as with
the chemical analyses, this indicates a negligible degree of
oxidation. XRD was also performed for the graphite oxidized
by commercially supplied potassium ferrate(VI) including
degree of oxidation revealed by XPS and elemental combus-
tion analysis.
FTIR spectroscopy was also used to characterize the
prepared materials (see Figure 4 for the spectra of the
samples prepared from our laboratory K FeO ). In agreement
purified K FeO4 from BOC Sciences, but no significant
2
4
2
with the results obtained from all other analytical methods,
FTIR did not detect any functional groups on the surface of
the obtained materials. For all samples, only a weak absorp-
changes were visible on the X-ray diffractograms (Figure S8).
Thermal analysis was also performed in air atmosphere
(Figure S9). Typically, the exfoliation temperature of gra-
phene oxide is around 2008C and accompanied by a significant
reduction of mass and exothermic effects. However, none
of these effects was observed on our samples below 6008C.
This shows the absence of any thermally labile oxygen
functional groups in the oxidized graphite.
À1
tion band of the C=C bond can be seen at 1590 cm . Again,
[
6a]
no absorption bands of oxygen-containing functional groups
were observed on the FTIR spectra of the graphites oxidized
using the commercially-obtained samples as well as purified
commercial K FeO (Figure S7). Usually, -C=O, -OH, and
2
4
-
COOH groups are present even on chemically reduced
Generally, our careful analysis suggests that none of the
above-mentioned experimental conditions led to the forma-
tion of graphene oxide. In fact, using Peng et al.ꢀs procedure
only resulted in the formation of poorly oxidized graphite
with a very small amount of oxygen that resulted from the
[20]
graphenes.
XRD was used to evaluate the structure of the oxidized
graphite (see Figure 5 for the spectra of the samples prepared
from our laboratory K FeO ). The formation of graphite/
2
4
4
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Angewandte
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surface oxidation of the graphite. The higher oxidation of
graphite observed for the samples prepared using commer-
cially obtained K FeO originates from oxidizing impurities
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WARNING: The synthesis of potassium ferrate(VI) requires the
handling of gaseous chlorine, which is highly toxic. Concentrated
hydroxide solution is highly corrosive and protective equipment must
be used. The reaction of potassium ferrate(VI) with acid can be
extremely violent. Potassium ferrate(VI) is a strong oxidation agent
and mixing it with an organic compound should be avoided. High
purity potassium ferrate(VI) may spontaneously decompose with
a large evolution of heat and to ignite organic solvents used for
purification. For full experimental details, see the Supporting
Information.
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Acknowledgements
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ˇ
The project was supported by the Czech Science Foundation
(
(
GACR No. 15-09001S) and by specific university research
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10543.
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Keywords: ferrate(VI) · graphene · graphene oxide · oxidation
ˇ
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1
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[
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Received: April 10, 2016
M. Pumera, Carbon 2014, 77, 508 – 517; c) H. C. Schniepp, J.-L.
Revised: June 8, 2016
Li, M. J. McAllister, H. Sai, M. Herrera-Alonso, D. H. Adamson,
Published online: && &&, &&&&
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Angewandte
Communications
Chemie
Communications
Graphene Oxide
Ferrate-ing around: The oxidation of
graphite by ferrate(VI) is impossible
owing to its fast decomposition in an
acidic environment. Any oxidation occur-
ring when using commercial potassium
ferrate(VI) is caused by impurities.
Z. Sofer,* J. Luxa, O. Jankovsk y´ ,
D. Sedmidubsk y´ , T. Bystro nˇ ,
M. Pumera*
&&&& — &&&&
Synthesis of Graphene Oxide by
Oxidation of Graphite with Ferrate(VI)
Compounds: Myth or Reality?
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www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!