Redox chemistry between graphene oxide and mercaptan

Article abstract of DOI:10.1039/c2jm33467d

We report here redox reactions between graphene oxide (GO) and mercaptans, which reduces GO to reduced graphene oxide (RGO) and oxidizes mercaptans into disulfides. The reduction processes of GO using various mercaptans as the reducing agents are investigated through XPS, TGA, FT-IR, Raman and EA analysis. The degree of reduction of RGO depends on molecular structure of mercaptans and is controlled by the reaction time. The redox reaction is also employed to oxidize mercaptans into disulfides in medium to high yields under moderate conditions. The mechanism of the redox reaction may involve nucleophilic ring opening of oxirane on GO by alkylthio moiety, followed by addition of another alkylthio group, leaving the resulting disulfide. The reduction of the hydroxy group could be more complex, involving both radical and anionic processes.

Full text of DOI:10.1039/c2jm33467d

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PAPER
Redox chemistry between graphene oxide and mercaptan†
ab
a
a
a
a
a
Deqin Fan, Chao Zhang, Junpo He,* Rong Hua, Yang Zhang and Yuliang Yang
Received 30th May 2012, Accepted 16th July 2012
DOI: 10.1039/c2jm33467d
We report here redox reactions between graphene oxide (GO) and mercaptans, which reduces GO to
reduced graphene oxide (RGO) and oxidizes mercaptans into disulfides. The reduction processes of GO
using various mercaptans as the reducing agents are investigated through XPS, TGA, FT-IR, Raman
and EA analysis. The degree of reduction of RGO depends on molecular structure of mercaptans and is
controlled by the reaction time. The redox reaction is also employed to oxidize mercaptans into
disulfides in medium to high yields under moderate conditions. The mechanism of the redox reaction
may involve nucleophilic ring opening of oxirane on GO by alkylthio moiety, followed by addition of
another alkylthio group, leaving the resulting disulfide. The reduction of the hydroxy group could be
more complex, involving both radical and anionic processes.
Introduction
also observed in the oxidation of sulfur dioxide and amines by
48–52
molecular oxygen.
Mercaptans or thiols are essential chemicals in industry and
53,54
There is increasing interest in the chemistry of graphene and
related materials, such as graphene oxide (GO) and reduced
widely occurred in biosystem.
Oxidation processes of
1
–4
graphene oxide (RGO).
GO is the exfoliated product of
mercaptans are of great importance in the petroleum refinery due
55,56
oxidation reaction of graphite under harsh conditions, for
to its hazardousness to the environment.
Mercaptans are
5–7
example, H
2
SO /HNO
4
3
/KMnO
4
.
Oxygen-containing func-
usually oxidized by reagents such as hydrogen peroxide, iron
53,54
tional groups such epoxy, hydroxyl and carbonyl groups are
located either on the basal plane or on the edge of the single
atom-thin sheet. Reduction of GO may serve as a promising way
to mass produce graphene-related materials due to the significant
dispersibility of GO in water. GO can be transformed into RGO
chloride, oxygen and iodine.
The widely used Merox process
involves catalytic oxidation of mercaptans into disulfides with air
55
by metal phthalocyanines together with caustic soda. Treating
the wasted alkaline and metal residues may bring environmental
and economic issue. Improved processes using solid phase cata-
2–4,8,9
10–12
13
by chemical,
thermal,
and electrochemical reductions.
57,58
59,60
lysts,
biotechnology of desulfurization
and ultrasound-
The chemical reduction has been achieved by a number of
inorganic and organic reducing agents. Inorganic reductive salts
61
assisted oxidation have been developed. In biological system,
the oxidation of thiol groups of cysteine residue to disulfide bond
of cystine unit plays an important role in formation of protein
14–16
17,18
such as sodium boron hydrogen,
sodium bisulfite
and
19,20
metal iron or aluminium
base and acid, such as sodium hydroxide and hydroiodic acid,
also lead to efficient reduction of GO. Widely used reducing
show very high reductivity. Strong
54
tertiary and quaternary structures.
21,22
In this work we report the redox chemistry between GO and
mercaptans under mild conditions. This chemistry can be used
not only to prepare RGO with controlled reduction degree, but
also to oxidize the odorous mercaptans into disulfides in good to
excellent yield. The mechanism of the redox chemistry is also
discussed.
9,23–27
agents in conventional organic synthesis, such as hydrazine,
29,30
28
hydroxyamine, urea,
32,33
aniline, amines,
as well as weak nucleophiles such as
31
34
35
36
ammonia, alcohols, and phenols, have
37
been used to reduce GO. Biochemicals, such as glucose,
39–41
heparin, ascorbic acid,
38
42
lysozyme, bovine serum albumin,
43
44
and bacterial provide green methods to reduce GO into RGO in
highly stable aqueous solution. On the other hand, GO has been
used as oxidant in oxidation of alcohols or alkynes into alde-
Experimental
Chemicals and materials
45–47
hydes or ketones in good yields.
Catalytic activity of GO was
Expandable graphite was provided by Yingtai Co. Ltd., China.
Tetrahydrofuran (THF, 99.0%), dimethyl formamide (DMF,
99.0%), barium chloride dihydrate (99.5%), hydrazine hydrate
solution (50%), NaNO3 (99.0%) and KMnO4 (99.5%) were
purchased from Shanghai Sinopharm Reagent Co. Ltd and used
as received. Ethyl alcohol (Shanghai Zhengxin Chemical
Company, 99.7%), hydrogen peroxide (Jiangsu Tongshen
a
The State Key Laboratory of Molecular Engineering of Polymer,
Department of Macromolecular Science, Fudan University, Shanghai,
00433, China. E-mail: jphe@fudan.edu.cn; Tel: +86-21-65643509
2
b
Department of Materials Science, Fudan University, Shanghai, 200433,
China
†
1
Electronic supplementary information (ESI) available. See DOI:
0.1039/c2jm33467d
1
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1
Dibenzothiazole disulfide. The product is a white crystal. H
Chemical Reagent Co. Ltd, 30%), n-dodecyl mercaptan (DM,
8%, Aladdin), amyl mercaptan (AM, 98%, Aladdin), propyl
9
NMR (CDCl ): d (ppm) ¼ 7.36–7.95 (m, 8H, Ar–H).
3
mercaptan (PM, 98%, Aladdin), hydroxyethyl mercaptan (HEM,
biotechnology grade, Solon Ind. Pkwy), 2-mercapto benzothia-
zoethiole (MBT, 98%, Aladdin), benzyl mercaptan (BM, AR,
Jiangsu Yonghua Chemical), L-cysteine (CysT, 98%, Aladdin),
Dipentyl disulfide. GC-MS (EI): m/z ¼ 206.
Deodorization of model petroleum by GO
5
-mercapto-1-methytetrazole (MMT, 98%, Aladdin) and
concentrated sulfuric acid (Taicang Zhitang Chemical Co. Ltd,
0
Model petroleum was made by addition of n-dodecyl mercaptan
ꢁ1
9
5–98%) were used as received. 2,2 -Azobis(isobutyronitrile)
into n-heptane to form 5 mg mL stock solution. Typically, GO
(
AIBN, Shanghai 4th Factory of Chemicals, 99%) was re-crys-
(125 mg, prepared by freeze-drying) or active carbon was mixed
with 20 mL model petroleum stock solution in a round flask
tallized from methanol.
(
50 mL). The mixture was sonicated for 5 min and degassed
through 3 cycles of freezing–pumping–thawing processes before
ꢀ
Preparation of aqueous solution of graphene oxide (GO)
thermoset at 80 C for 18 h. Samples were drawn at pre-
Graphene oxide was synthesized by modified Hummers method
determined time. After the reaction, the mixture was cooled
down to room temperature. GC-MS were used to determine the
relative ratio of mercaptans to disulfides.
3
in which the expandable graphite was treated with NaNO ,
62–64
H
2
SO
4
and KMnO
4
.
After purification, graphene oxide in
aqueous stock solution was obtained. The content of graphene
oxide was measured three times by the gravimetric method to be
Characterization
2
.62% by weight. For the characterization of the GO refer to a
62
previous publication.
Transmission electron microscopy (TEM) images were recorded
on a JEOL JEM2010 electron microscope operating at 200 kV.
The specimens for TEM observations were prepared by placing
one drop of the sample dispersion in alcohol on a copper grid
coated with carbon. Scanning electron microscopy (SEM)
images were recorded on a FE-SEM S-4800. The samples were
loaded on electrically conductive adhesives without a sputtering
film. Raman measurements were carried out on a Jobin Yvon
LabRam-1B instrument using excitation laser with wavelength
632.8 nm. TGA was conducted on a NETZSCH TG-209
Preparation of reduced graphene oxides (RGOs) using various
mercaptans as the reductants
Typically, GO aqueous solution (1.0 g, 2.62 wt%) was mixed with
1
0 mL DMF in a round flask (25 mL) which was then put into a
sonication bath for 1 h. Mercaptan (13.3 mmol) (Cautious:
mercaptans are highly odorous!) was charged into the flask. The
mixture was degassed through 3 cycles of freezing–pumping–
ꢀ
ꢁ1
thawing processes. The flask was thermoset at 80 C for 11 h.
instrument under nitrogen atmosphere (20 mL min ).
ꢀ ꢀ ꢀ
Samples were drawn at a predetermined time. After the reaction,
the mixture was cooled down to room temperature, diluted with
Temperature was elevated from 30 C to 650 C at a rate of 1 C
ꢁ
1
min . The photos of the samples were taken with a digital
camera (Canon IXUS 860). Attenuated total reflection FT-IR
Spectroscopy was performed using a NEXUS-470 FT-IR spec-
trometer with the solid samples on a ZnSe crystal. Elemental
analysis (EA) was conducted on a ELEMENTAR, VARIO EL
2
0 mL DMF and filtered with 450 nm cellulose filter. The filter
cake was washed with 50 mL DMF for at least 3 times until no
mercaptans can be detected in filtrate. Final wash with chloro-
form removed the residue DMF solvent. The filtered sample was
dried at room temperature under vacuum. A black powder was
obtained. For comparison, a control experiment in the absence
of mercaptan under otherwise identical condition, e.g., GO in
1
III. H NMR measurements were carried out on a Bruker
3 6
(500 MHz) NMR instrument using CDCl or DMSO-d as the
solvent and tetramethylsilane (TMS) as the reference. Gas
chromatography-mass spectrometry (GC-MS) was performed
on an Agilent Technologies 6890N system, equipped with a
mass-selective detector using electron impact ionization. Analy-
tes were separated by a HP-5MS capillary column of 30 m
(0.25 i.d. and 0.25 mm film thickness), which was inserted directly
into the ion source of the MS system. Helium (99.999%) was the
ꢀ
water heated at 80 C, was performed.
Synthesis of disulfides by GO oxidation of mercaptans
The reaction process was nearly identical to RGO preparation
process except the mass ratio of mercaptans to GO was 1 : 0.75.
At the end of the reaction, the solid was separated by centrifu-
gation. The supernatant was collected and condensed by rotation
evaporation. The residue was purified by recrystallization (for
DM and MBT), or by column chromatography (for BM).
ꢁ
1
carrier gas maintained at a flow rate of 1.0 mL min . The
ꢀ
injector was kept at 280 C, and samples were split injected
(1 mm, split ratio 100 : 1). The column oven temperature was
ꢁ1
ꢀ
ꢀ
programmed to start at 80 C for 2 min, ramp at 10 C min to
ꢀ ꢀ
2
80 C, and held at 280 C for 5 min. EI spectra were scanned
Didodecyl disulfide. The product is a white needle-like crystal.
between 50 and 550 Da in the full-scan acquisition mode. The
ꢀ
1
H NMR (CDCl ): d (ppm) ¼ 0.88 (t, 6H, CH –CH ), 1.26–1.38
transfer line and the EI source temperature were both 230 C.
3
3
2
(
m, 18H, CH –(CH ) –CH ), 1.67 (m, 4H, –CH –CH –CH –S),
2
The electron multiplier voltage was set to 70 eV. XPS experi-
ments were carried out on a RBD upgraded PHI-5000C ESCA
system (Perkin Elmer) with Mg Ka radiation (hn ¼ 1253.6 eV) or
Al Ka radiation (hn ¼ 1486.6 eV). In general, the X-ray anode
was run at 250 W and the high voltage was kept at 14.0 kV with a
3
2 9
2
2
2
2
.68 (t, 4H, –CH –S). GC-MS (EI): m/z ¼ 402.
2
1
Dibenzyl disulfide. The product is a white crystal. H NMR
–Ph), 7.26–7.36 (m, 10H,
(
DMSO): d (ppm) ¼ 3.74 (s, 4H, –CH
2
ꢀ
Ph). GC-MS (EI): m/z ¼ 246.
detection angle at 54 . The pass energy was fixed at 23.5, 46.95 or
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9
3.90 eV to ensure sufficient resolution and sensitivity. The base
ꢁ
8
pressure of the analyzer chamber was about 5 ꢂ 10
Pa.
Samples were directly pressed to a self-supported disk (10 ꢂ
1
0 mm) and mounted on a sample holder then transferred into
the analyzer chamber. The whole spectra (0–1100 eV) and the
narrow spectra of all the elements with much high resolution
were both recorded by using RBD 147 interface (RBD Enter-
prises, USA) through AugerScan 3.21 software. Binding energies
were calibrated by using the containment carbon (C1s ¼
2
84.6 eV). Data analysis was carried out by using the RBD
AugerScan 3.21 software provided by RBD Enterprises or
XPSPeak4.1.
Fig. 1 Photos of reaction mixtures between GO and mercaptans at 80
ꢀC. (A) and (B) are reaction mixture using n-dodecyl mercaptan, while
Results and discussion
(
C) and (D) are those using hydroxyethyl mercaptan as the reducing
Reduction of GO by the mercaptans
agent. (A) and (C) are product mixture at 660 min of both systems; (B)
and (D) are DMF-diluted GO dispersion and reaction mixture at
different reaction time. The purpose of dilution is for more clear obser-
vation of the change in colour and dispersion stability of the reaction
mixture.
The redox reactions are carried out using GO, prepared by
Hummers’ method, and a number of mercaptans listed in
2
Scheme 1 as the reducing agents in DMF/H O (v/v: 10/1) at
ꢀ
8
0
C. The reduced products, RGOs, are obtained as black
powder after filtration and washing with DMF and chloroform.
Fig. 1 shows an example using n-dodecyl mercaptan (A) and
hydroxyethyl mercaptan (C) as the reducing agents. In the former
system, the resulting RGO dispersion is stable for over one
month, whereas in the latter the product immediately precipi-
tates. Dilution by DMF facilitates the observation of the change
in colour during the reaction. For both systems the brown colour
of GO turns to black as the reaction proceeds. The black colour is
due to the recovery of the conjugated plane. The reaction mixture
of GO and hydroxyethyl mercaptan (B) turns to black at an
earlier time than that using n-dodecyl mercaptan (D), implying a
faster reaction rate for the former. After dilution, samples of
longer reaction time give precipitates of RGO for both systems.
A control experiment of heating GO in water in the absence of
ꢁ
1
) at ca. 1021 cm , C–O stretching of
vibration of epoxy (n
C–O–C
ꢁ1
alkoxy (nC–O) at 1057 cm , C]O stretching of carbonyl (nC]O
at 1727 cm
)
ꢁ1
and C]C of graphitic domains (nC]C) at
ꢁ
1
1640 cm , respectively.
15,65,66
Carboxylic groups show typical
ꢁ1
ꢁ
1
absorption at about 3200 cm (coupled nO–H) and 1399 cm
ꢁ
1
(nO–H). The absorption at about 3400 cm may results from the
stretching mode of absorbed water. The intensities of these peaks
become weak gradually along with the reaction, corresponding
27,37,66
to stepwise removal of oxygen-containing moieties.
Raman spectra of GO and RGO reduced by n-dodecyl
mercaptan are presented in Fig. 3 (for other reaction systems
ꢁ
1
refer to Fig. S3†). The G band at about 1590 cm results from
the in-phase vibration of graphite lattice while the D band at
ꢁ1
ꢀ
67ꢁ69
mercaptan shows limited reduction of GO at 80 C. Although the
1330 cm is caused by graphite edges.
ꢁ
G band shifts slightly
1
ꢁ1
reaction mixture turns black in colour, no precipitation is
observed for samples at different reaction time before and after
dilution with DMF (Fig. S1†). In addition, TGA shows that the
from 1590 cm for GO to 1583 cm for samples reduced for
300 and 600 min, which is more close to that of the graphite. The
D G
relative intensity of D band to G band, I /I , increases from 1.21
ꢀ
weight loss from 100 to 500 C of GO samples due to oxygen-
to 1.71 along with reaction time. This indicates the formation of
small sized in-plane domains of graphite after mercaptan
reduction, consistent to the results using other reducing
containing functionality is only slightly less than that of pristine
GO (Fig. S2†). These results indicate that the reduction by water
is insignificant in comparison with that by mercaptans.
9,25,70–74
agents.
The small sized graphite domains are a conse-
FT-IR spectra of GO and RGO at different reaction time are
shown Fig. 2. The plots are up shifted for comparison. The
spectrum of GO shows absorption of asymmetry stretching
quence of the presence of residual functional groups and some
Scheme 1 Schematic drawing illustrating the process of GO-mercaptan
Fig. 2 FT-IR spectra of GO and RGO samples at different reaction
redox chemistry.
time.
1
8566 | J. Mater. Chem., 2012, 22, 18564–18571
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Fig. 3 Raman spectrum of GO and RGO with reaction time.
unrecoverable defects resulted from the harsh oxidation. These
scattered functionalities and defects will serve as the border of the
restored graphite domains, therefore, limit the size but increase
4,9,75
the number of the reduced domains.
The reduction degree of GO by the n-dodecyl mercaptan is
also analyzed by XPS measurements. Fig. 4A shows the full
spectrum of GO and RGO samples along with the reaction time.
The atomic ratios of carbon to oxygen (C/O) are calculated
according to the peak areas. The C/O values increase from 2.3 of
GO to 9.8 with increase in reaction time. Fig. 4B are the C1s peak
spectra and the fitting curves of GO and reduced GO samples.
The C1s peaks are deconvoluted in terms of four main compo-
nents: graphitic C, epoxy and hydroxyl groups C–O–C/C–O,
9,19,21,29
carbonyl groups C]O and carboxyl groups –C(]O)O–.
The C1s spectrum of GO shows the corresponding peak centered
at around 284.9, 286.7, 287.8 and 288.9 eV, respectively. The
intensity of the oxygen species decreases gradually, while that of
14,30,32,34
the graphitic carbon increases due to the reduction.
Correlating to the C1s changes of RGO samples, the O1s
peaks of RGO at around 534.7, 533.6, 532.7, 531.8 and 530.9 eV
(
Fig. 4C) are assigned to contribution from –OH, O–C, O]C,
O–C(O) and quinone, respectively.
76,77
The atom ratios of the
functional groups have been summarized in Table S1.† The
intensity of both O–H and O–C group peak become weaker
along with the reaction time. The atom content for –OH
decreases from 9.5% to 3.3% and that for O–C group from 14.5%
to 1.2%. Whereas, the contents of O]C, O–C(O) and quinone
groups seem to be stable, indicating that mercaptans are unable
to reduce these relatively inert groups. It is noted that during the
initial two hours of reaction, the ratio of both O–H and O–C
group dropped rapidly, in good agreement with the results of EA
(
Fig. 6) and TGA (Fig. 5). It is also found that functional groups,
such as –OH and O–C moieties, still exist on RGO after 660 min
reduction.
The thermographs of GO and RGO samples taken at different
reduction time are shown in Fig. 5. The weight losses of RGOs
are in general less than that of pristine GO due to the removal of
oxygen-containing moieties by the reduction. The weight loss at
ꢀ
temperatures lower than 100 C is ascribed to absorbed water.
Water adsorbs very easily to the hydrophilic surface of GO. After
reduction, the surface becomes more hydrophobic. Therefore,
the weight loss of water is significantly reduced for samples taken
at the reaction time of 120, 300, and 660 min. The weight losses
Fig. 4 XPS full spectra of RGO samples with time (A), deconvoluted
XPS spectra of C1s (B) and O1s (C) regions of GO and RGO samples at
different reaction times.
ꢀ
within the range from 100 to 200 C are attributed to the
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The carbon contents determined by elemental analysis of
RGOs using various mercaptans after 480 min reaction are
summarized in Table 1. The weight percent of carbon increase
from 47.3% for pristine GO to the range of 65.6–80.1% for RGO
due to the removal of the oxygen in the redox reaction. The
structure of mercaptans affects the reduction degree. For
example, MBT and MMT give the lower carbon content than
alkyl mercaptans because the thiol group is in conjugation with
aromatic ring and thus have lower reducing reactivity. The
effects of hydrocarbon chains and their functionalities on the
redox reaction are complicated by the solubility of mercaptans
and the dispersibility of the products. Two typical examples are
compared in Fig. 6, which shows the variation of carbon content
along with the reaction time. HEM results in ꢃ75% carbon
content at the very initial period, forming instant precipitation of
RGO which impedes further reduction. DM reacts at a much
slower rate in early stage but the dispersion of the reaction
mixture is more stable and thus allows the reduction to proceed
progressively. The carbon contents of the two systems reach very
close values of approximately 80%. The faster reduction by HEM
over DM is ascribed to two factors: (1) the alkyl chain of HEM is
short, therefore the steric hindrance is small; (2) hydroxyl group
not only increases the solubility of HEM but also take part in the
reduction reaction. The more stable dispersion in the DM system
may be ascribed to the emulsification effect of DM on the
resulting RGO. Dilution with more solvent (DMF) may destroy
the stabilized dispersion, as shown in Fig. 1. Small amount of
grafted DM onto the surface of GO, as evidenced by TGA results
Fig. 5 Normalized TGA plots of GO and RGOs reduced by n-dodecyl
mercaptan at different reaction times.
(
Fig. 5), may also contribute to the stabilization effect. Other
reducing agents show nearly negligible weight loss within this
temperature range (Fig. S4†).
Fig. 6 Carbon content of RGOs with reaction time for DM and HEM
RGO was obtained as black powder after purification by
repeated washing and drying. Microscopy of the powder was
performed using SEM and TEM. As shown in Fig. 7, crumpled
nanosheets and the wrinkled textures of the aggregated gra-
phene sheets were clearly observed by SEM (A and B) and TEM
(C and D), respectively. This is in contrast to the flat
measured by EA. The lines are drawn only for guidance.
decomposition of functional groups such as epoxy, hydroxyl,
carboxylic and carbonyl groups. The contents of these func-
tionalities decrease along with the reaction time, as indicated by
reduced, or even totally diminished, weight loss on TGA curves
within this range. This is evidence for gradual reduction, in good
agreement with the previous findings using other reducing
62
morphology of pristine GO observed by TEM. It seems that
the sheet-like structure of graphene-derived material has been
sustained during the reduction, but was coiled during the
process of aggregation.
18,22,32
agents.
The weight losses within the range from 200 to 400
ꢀ
C are attributed to the decomposition of residual carbonyl
groups and possibly grafted alkylthio chains. The weight losses
for GO and RGO sample reduced for 30 min are nearly finished
D G
Table 1 Carbon contents from EA and I /I values from Raman
spectroscopy of GO and RGO using various reducing agents after 480
ꢀ
ꢀ
min reaction at 80 C
at 300 C. For RGO at longer reaction time, further weight loss is
ꢀ
observed up to 400 C, which can be the result of small amount of
Reducing agent
HEM
PM
AM
DM
I
D
/I
G
C wt%
grafted mercaptans. The elimination of these side chains may
cause additional defects that lead to further weight loss at
1.78
1.73
1.65
1.67
1.66
1.59
1.87
1.78
1.68
1.61
1.21
80.1
78.2
79.1
78.9
74.0
75.6
75.4
65.6
68.9
—
a
ꢀ
temperatures above 590 C. This is evidenced by increasing
weight loss at highest temperature for samples reduced at longer
periods. It is difficult to determine the exact amount of graft
mercaptans by TGA because the weight loss of grafted
mercaptans may be complicated by other functionalities. The
atom content of sulfur in the final sample (660 min reduction) is
ca. 0.59% by XPS analysis (peak at 163.2 eV), corresponding to
MBT
MMT
CysT
b
HEM
DM
b
Hydrazine
None (GO)
7
8–80
47.4
ca. 8.6% grafted mercaptans by weight.
Thermal annealing at
00–500 C may serve as an efficient way to remove the grafted
mercaptans.
ꢀ
4
a
ꢀ
b
Reaction at 60 C. In the presence of AIBN (0.66 mmol).
1
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ꢀ
temperatures from 60 to 80 C. GO as a solid oxidant facilitates
the separation process by simple centrifugation or filtration after
reaction due to the heterogeneous nature of the reaction. Thus
the present redox chemistry can serve as a new method for
preparation of disulfides at mild conditions.
The driving force of the redox chemistry is the recovery of the
conjugated plane of graphene. The oxidative activity of GO is
moderate because disulfides are the dominant products and no
product of higher oxidative state, such as sulfonic acid, is
observed. Quantitative stoichiometry is difficult due to lack of
information of exact content of oxygen containing groups. The
situation is further complicated by the heterogeneity of the
reaction mixture. Either ionic or radical, or both, processes has
53,54
been proposed for the reduction of GO.
In the present work,
the primary step may be the ring-opening of epoxy group by an
attacking alkylthio anion (Scheme 2). The addition of another
alkylthio anion to the attached sulfur atom results in the
formation of an intermediate, which undergoes subsequent
N
elimination of disulfides. This S 2 process also leaves a negative
Fig. 7 SEM (A and B) and TEM (C and D) images of RGO materials
3
charge on the sp carbon of graphene. The charge is delocalized
to the hydroxyl on the neighboring carbon. The hydroxy anion is
obtained from redox reaction using DM as the reductant.
neutralized by a proton into H O.
2
Reaction mechanism on the hydroxyl group is of higher
uncertainty because we have no information about the valent
state of the neighbouring carbon. The hydroxyl group or
4,12,40,64,75
hydrogen has been proposed as the substituent,
which
may lead to different mechanisms. Alkylthio radicals may be
formed as the intermediates, which can recombine to give
disulfides. Evidence for the radical process is provided by the
effect of AIBN on the redox reaction. As shown in Table 2, the
carbon content in RGO samples, reduced by HEM and DM in
the presence of AIBN, is clearly less than in those without AIBN.
This is possibly due to quenching of the reactive radicals by the
AIBN-derived radical.
Deodorization of model petroleum using GO or active carbon
ꢁ1
(
at identical mass amount) is conducted with 5 mg mL n-
Scheme 2 Proposed mechanism for removal of epoxy group in the redox
ꢀ
dedocyl mercaptan in n-heptane solution at 80 C. GO prepared
by freeze-drying is yellowish with higher apparent volume than
that of active carbon (Fig. S9†). After reaction, the yellowish GO
became dark in colour in dispersion, indicating the reduction.
The chemical composition of the reaction mixtures are charac-
terized by GC-MS (Fig. S10†), from which the ratios of disulfides
to mercaptans are obtained. The ratio increases with reaction
time in both cases. After 18 hours, GO leads to much higher ratio
of disulfide to mercaptan (ꢃ4 times higher than that of active
carbon). The result clearly demonstrates that the efficiency of
GO in deodorization is much higher than active carbon.
reaction of GO and mercaptans.
Disulfide synthesis by the redox chemistry
In addition to the application in GO reduction, the redox
chemistry is employed to oxidize mercaptans into disulfides.
Mercaptans such as DM, BM, AM and MBT are used as model
compounds to prepare corresponding disulfides. The reactions
ꢀ
are conducted at 60 C in THF or DMF/water mixed solvents
under nitrogen. After the reaction, the odor of mercaptans is
significantly reduced. The products are obtained as solids in good
to excellent yields after isolation through either recrystallization
or column chromatography. These products were characterized
Table 2 Yields of disulfides synthesized by GO oxidation of mercaptans
ꢀ
1
by GC-MS and H NMR (Fig. S5–8†). Strong effects of the
with mass ratio of mercaptan to GO 1 : 0.75 at 60 C in THF
solvents on yield are observed. For example, the yields of
dimerization of DM are 46% and 70% in DMF/water and in
THF, respectively. This solvent effect arises from the solubility of
the starting mercaptans and the dispersibility of GO in the
certain solvents. Better solubility and dispersibility facilitate the
redox reaction.
Substrate
Product
Yield/%
DM
BM
AM
MBT
Didodecyl disulfide
Dibenzyl disulfide
Dipentyl disulfide
70
97
a
85
Dibenzothiazole disulfide.
56
a
Oxidation under mild conditions is very important in organic
synthesis. The present redox reactions are conducted at
Estimated by GC–MS results.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 18564–18571 | 18569

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2
2
2
6 V. C. Tung, M. J. Allen, Y. Yang and R. B. Kaner, Nat. Nanotechnol.,
Conclusions and perspective
2
009, 4, 25.
7 Y. X. Xu, H. Bai, G. W. Lu, C. Li and G. Q. Shi, J. Am. Chem. Soc.,
008, 130, 5856.
GO and mercaptans undergo a redox reaction under mild
conditions. The reaction reduces GO to RGO, and oxidize
mercaptans into disulfides. The reduction degree of GO can be
controlled by selecting suitable mercaptans under predetermined
time. The oxidation potential of GO is moderate so that the
product is disulfide and no organosulfur with high valency sulfur
atoms has been observed. The redox chemistry may be used in
industry for desulfurization in, for example, the sweetening of
petroleum or natural gas. Preliminary results in our laboratory
show that the deodorization efficiency of GO for hydrocarbons is
much higher than that of traditional active carbon. The chem-
istry may also provide fundamental insight in the toxicity of
graphene materials to bio-systems.
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Acknowledgements
This work is subsidized by the National Basic Research Program
of China (2011CB605701) and the National High Technology
Research and Development Program of China (2008AA032102).
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