Formation of larger-area graphene from small GO sheets in the presence of basic divalent sulfide species and its use in biomass conversion

Article abstract of DOI:10.1039/c5ra24287h

Basic divalent sulfide species exhibit unique chemical activity in that they react with organic matter and function as a redox reagent. Herein, graphene oxide (GO) was modified by basic divalent sulfide species, and the modified materials were characterized by FT-IR spectra, XRD, UV-Vis, SEM, HRTEM, XPS, TG-MS analysis, Raman spectroscopy and electrical conductivity measurements. The results indicated that basic divalent sulfide species removed the abundant oxygen-containing groups from the basal plane of GO, in which the π-conjugated structure was largely restored. The basic divalent sulfide species also react with marginal oxygen-containing groups and were doped into GO to form large-area π-conjugated structures, resulting in the high electrical conductivity of sulfur-modified GO (SGO). TG-MS analysis showed that the basic divalent sulfide in SGO can be removed by heating, which provides a new method for preparing larger-area graphene sheets from GO with a smaller average size. In addition, SGO prepared without annealing displayed better performance in converting fructose, glucose, cellobiose and cellulose to 5-hydroxymethylfurfural (HMF) because it retained some acidic marginal oxygen-containing groups and basic divalent sulfide species.

Full text of DOI:10.1039/c5ra24287h

View Article Online
View Journal
RSC Advances
This article can be cited before page numbers have been issued, to do this please use: X. Wang, Z. Li, K.
Su and B. Cheng, RSC Adv., 2016, DOI: 10.1039/C5RA24287H.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. This Accepted Manuscript will be replaced by the edited,
formatted and paginated article as soon as this is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/advances

Page 1 of 26
RSC Advances
View Article Online
DOI: 10.1039/C5RA24287H
The larger area graphene formation from the small GO
sheet in presence of basic divalent sulfide species and its
usage in biomass conversion
Xuyan Wang1, Kunmei Su1, Zhenhuan Li*2, Bowen Cheng*2
1State Key Laboratory of Separation Membranes and Membrane Processes, School
of environment Science and Chemisty Engineering, Tianjin Polytechnic University,
Tianjin, China
2School of Materials Science and Engineering, Tianjin Polytechnic University,
300387, Tianjin, China
1

RSC Advances
Page 2 of 26
View Article Online
DOI: 10.1039/C5RA24287H
Abstract: The basic divalent sulfide exhibits a unique chemical activity both in its
ability to react with organic matter and as a redox reagent. Herein, graphene oxide
(GO) was modified by basic divalent sulfide species, and the modified materials were
characterized by FTꢀIR spectra, XRD, UVꢀVis, SEM, HRTEM, XPS, TGꢀMS analysis,
Raman spectroscopy and electrical conductivity measurement. The characterized
results indicated that basic divalent sulfide could well remove the abundant
oxygenꢀcontaining groups on the basalꢀplane of GO, in which the πꢀconjugated
structure was largely restored. However, the basic divalent sulfide species can also
react with marginal oxygenꢀcontaining groups and be doped into GO to form
largeꢀarea πꢀconjugated structure to result in the high electrical conductivity of sulfur
modified GO (SGO). TGꢀMS analysis showed that the basic divalent sulfide in SGO
can be well removed under heating conditions, which provides a new method to
realize the larger area graphene sheet preparations from GO of smaller average size.
In addition, the prepared SGO without annealing displayed a better performance in
fructose, glucose, cellobiose and cellulose conversion to 5ꢀhydroxymethylfurfural
(HMF) because it remained some acidic marginal oxygenꢀcontaining groups and basic
divalent sulfide species.
Keywords: Graphene oxide, the larger area graphene formation, S incorporation,
Basic divalent sulfide, 5ꢀhydroxymethylfurfural
2

Page 3 of 26
RSC Advances
View Article Online
DOI: 10.1039/C5RA24287H
Introduction
Graphene, a single layer of sp2ꢀbonded carbon atoms arranged in a honeycomb structure,1
has attracted much attention for its unique electronic properties, excellent mechanical
properties, and superior thermal properties.2 To date, the approaches to prepare graphene
contained mechanical exfoliation of bulk graphite,3 epitaxial growth,4 chemical vapor
deposition5 and chemical reduction of GO 6 et al. Among them, the chemical reduction of GO is
the most efficient method for lowꢀcost and largeꢀscale production of graphene. In the chemical
reduction process, GO, known as a material rich in oxygen containing groups such as hydroxyl,
carbonyl and epoxy group, was produced by the wellꢀknown Hummers method,7 and the
reduced graphene oxide was obtained by the chemical reduction of GO using a suitable
8
reducing agent. As a sort of efficient reducing agents, hydrazine, NaBH4,9 ascorbic acid,10
12
13, 14
HI,11 NaHSO3, Na2S2O4
et al had been used to reduce GO, and among them hydrazine
and NaBH4 were effective in removing oxygen of GO. However, when N2H4 was used as
reducing agent, N was doped in the asꢀprepared graphene9. Although NaBH4 showed excellent
ability in GO reduction, the hydrolysis property made it difficult to get a stable NaBH4 aqueous
solution, resulting in low efficiency in reduction15.
Since the bond energy of CꢀS is weaker than that of CꢀN, using sulfurꢀcontaining
compounds as the reducing agent may allow GO deoxygenation more easily.16 Therefore
sulfurꢀcontaining compounds, such as NaHSO3, Na2S2O3 and SO2,17 have been used as
reducing agents to reduce GO. It is wellꢀknown that the basic divalent sulfide species are
efficient reducing agents and can react with functionalized organic compounds to form organic
sulfur compound. Although the reductive reaction induced by sulfurꢀcontaining compounds
3

RSC Advances
Page 4 of 26
View Article Online
DOI: 10.1039/C5RA24287H
was expected to have little influence on the marginal carboxyl and hydroxyl groups owing to
the highꢀenergy barrier,16 the basic divalent sulfide species could open up epoxy groups to
form hydroxyls.18 These characteristics might facilitate a complex network of chemical
reactions during GO modification by basic divalent sulfide, including the reductive
deoxygenation reaction on the basal planes of pristine GO and the sulfur incorporation by the
nucleophilic reaction between basic divalent sulfide species and peripheral oxygen containing
groups (carbonyl and hydroxyl) of GO.
In this paper, GO was modified by basic divalent sulfide species, and it was found that the
basic divalent sulfide could remove the abundant hydroxyl and epoxide functionalities on
basalꢀplane of GO, and sulfur can also be incorporated into SGO to form the largeꢀarea
graphene sheets through GO sheet connection. According to the anthraquinone reduction by
basic divalent sulfide species,19 the mechanism of GO deoxygenation and sulfur incorporation
were provided. In addition, SGO was used to catalyze biomass conversion into HMF, which
provided some more information about sulfur incorporation.
2. Experiment section
2.1. Material preparation
GO was prepared from natural graphite powder (Purchased from Tianjin Kermel
Chemical Reagent Co., Ltd) by the Hummers method.7 In a typical GO modification
procedure, 1g GO and 500 mL water were loaded in a 1000 mL round bottom flask, and this
homogeneous mixture was sonicated using a KQ3200 ultrasonic bath cleaner (150 W) for 4h.
After that 15 g K2S or Na2S (94.7%, Yili Company) and 2 g NaOH (analytical grade,
Sinopharm Chemical Reagent Co.,Ltd) were added gradually (1 g/min) to above suspension
4

Page 5 of 26
RSC Advances
View Article Online
DOI: 10.1039/C5RA24287H
at 98 °C. The color of mixture turned from pale yellow (or brown) to black after 24 h, and
SGO precipitation appeared. SGO was obtained by filtration, and it was washed using
deionized water until the PH of eluate reached 7. SGO was dried at 60 °C for 24 h under the
vacuum condition.
GO modification by N2H4 was also carried out under the similar conditions (N2H4
Tianjin Beifang Tianyi Chemical Reagent Company, 80%), namely 3 mL N2H4 (80%, Tianjin
Beifang Tianyi Chemical Reagent Company) and 2 g NaOH (analytical grade, Sinopharm
Chemical Reagent Co., Ltd) were added to the above GO suspension. The mixture was heated
to 98 °C with the assistance of vigorous stirring (300 r/min). After 24 h, the color of the
mixture turned from pale yellow (or brown) to black, and the RGO precipitation appeared.
After filtration, the precipitation was washed using deionized water until the PH of the eluate
reached 7. The product was dried at 60 °C for 24 h under the vacuum condition. The undoped
TRGO was obtained by thermal reduction method at 800 oC in presence of Ar atmosphere.
2.2. Biomass conversion process
The catalytic experiments were performed in a 20 ml flask. In a typical experiment, 0.9 g
glucose and 0.03 g catalyst were added into 5 ml [EMIM]Br. After reaction system had been
purged with nitrogen, the reaction mixture was heated in an oilꢀbath at 140 oC for 4 h. After the
desired reaction time elapsed, reaction mixture was cooled to room temperature immediately.
2.3. Characterization
HRTEM images were obtained from a JEMꢀ2100 transmission electron microscope
(JEOL, Japan) and SEM images from a Hitachi 4800S electron microscope (Japan). Raman
spectra were obtained using a 514.5 nm argon ion laser with a high resolution via Raman
5

RSC Advances
Page 6 of 26
View Article Online
DOI: 10.1039/C5RA24287H
Microscope (RENISHAW, UK). UVꢀVis absorption spectra were recorded on a TUꢀ1901
UVꢀVis spectrophotometer (ThermoFisher). FTꢀIR spectra of the samples were recorded on a
TENSOR 37 (BRUKER Corporation, Germany) using KBrꢀdisk method. Thermal properties
were characterized by TG (STA409 PC thermogravimetry, NETZSCH, Germany) connected
to a Mass Spectrometry analyzer. This measurement was carried out at a heating rate of
5 °C/min in N2 atmosphere in the temperature range from 25 °C to 800 °C. The crystal
structure was characterized by Xꢀray diffraction (XRD, Elmer PHIꢀ5600) with Mg Kα
radiation. Xꢀray photoelectron spectroscopy (XPS, ThermoFisher Kꢀalpha) characterized the
C/O ratio and the change of functional groups. XPS peak deconvolution was carried out using
XPS peak 4.1 software in which a Shirley background was assumed. Electrical conductivity
was measured by a fourꢀprobe method and the corresponding volume conductivity (σ, S/cm)
was calculated using the formula: σ= 1/ρ, where ρ (unit: ꢁ·cm) refers to the electrical
resistivity. N2 adsorptionꢀdesorption isotherms were obtained on a AutosorbꢀiQ analyzer at
o
ꢀ196 C. Prior to the adsorptionꢀdesorption measurements, all samples were degassed at 150
oC overnight under a vacuum condition.
HMF yield was analyzed by the high performance liquid chromatograph (a 996
photodiode array detector and Waters1525 equipped with UV). HMF yield was quantified
with a Kromasil C18 (250×4.6mm, 5um) reverse phase column, using a mobile phase
CH3OHꢀH2O (80:20) at a flow rate of 1.0mL/min, and HMF yield was based on hexose
loading.
moles of HMF
5ꢀ HMF yield(mol%) =
×100%
moles of hexose
3. Results and discussion
6

Page 7 of 26
RSC Advances
View Article Online
DOI: 10.1039/C5RA24287H
3.1 Graphene characterization
The modification of GO in the presence of basic divalent sulfide can be easily observed by
the color change of mixture, from brown to black. After modification, the black SGO
precipitated out from the solution. Fig 1a showed the FTꢀIR spectra of GO and SGO. The
characteristic peaks of GO contains carbonyl C=O (1735 cmꢀ1), hydroxyl OꢀH (1380 cmꢀ1),
carboxy CꢀO (1230 cmꢀ1) and epoxy CꢀO (1065 cmꢀ1).20 The broad peak at 3500 cmꢀ1 was
assigned to the vibrations of hydroxyl groups. After GO modification by N2H4 (NGO), two new
peaks appeared at 3418 cmꢀ1 and 3126 cmꢀ1, indicating the doping of N. As for the CꢀH
stretching vibration at 2800ꢀ3000 cmꢀ1, those peaks of SGO were much weaker than that of
NGO, which confirmed the efficient restoration of the conjugated structure in presence of basic
divalent sulfide species. In addition, it could be observed that a weak visible absorption at 410
nm for SGO, which suggested sulfide incorporation.
1634
410
a
b
GO
SGO
NGO
SGO
NGO
Graphite
GO
1400
1733
5
10 15 20 25 30 35 40
3500
3000
1500 1000 500
-1
2heta(Degree)
Wavenumber (cm )
7

RSC Advances
Page 8 of 26
View Article Online
DOI: 10.1039/C5RA24287H
c
233 nm
NGO
GO
264 nm
SGO
350
200
250
300
400
Wavelength (nm)
Fig. 1 Characterization of GO, SGO and NGO: (a) FTIR spectra; (b) XRD patterns of
graphite, GO, SGO and NGO. (c) UVꢀVis absorption spectra.
Fig.1 b showed the XRD patterns of GO, graphite, SGO and NGO. The diffraction peak
at 2θ = 12.5° confirmed the relatively complete exfoliation of graphite.21 The interlayer
distance obtained from the (002) peak of graphite was 3.46 Å (2θ = 25.70°), which was
expanded to 7.10 Å (2θ = 12.45°) after oxidation by the Hummers method, indicating the
formation of oxygenꢀcontaining groups such as hydroxyl, epoxy and carboxyl groups.22 After
GO modification by N2H4 and basic divalent sulfide, the intensive peak of GO at 2θ = 12.45°
disappeared, and the new broadened peaks at 2θ = 24.3° and 2θ = 25.1° appeared. The
interlayer distance of NGO and SGO returned back to 3.69 and 3.55 Å, respectively, which
implied the removal of the most oxygen functional groups and the disorder agglomeration of
planar sheet by πꢀπ stacking interactions.
Fig. 1c showed a maximum UVꢀVis absorption peak at 233 nm in GO, which was mainly
due to the πꢀπ* transition of aromatic CꢀC bonds in GO.23 After GO modification, the
8

Page 9 of 26
RSC Advances
View Article Online
DOI: 10.1039/C5RA24287H
absorption peak shifted to the red region, reaching a value of 264 nm. Those results indicated
that the extended πꢀπ conjugated structure of GO had been significantly destroyed during
oxidization. However, as the deoxygenation took place, the extensive conjugated sp2ꢀcarbon
network has been rebuilt in the modifying process.24 By the comparison of NGO with SGO, it
was found that NGO showed obvious visible light absorption extending up to 400 nm, while
almost no visible absorption can be seen at 400 nm for SGO, which suggested sulfide was
incorporated into SGO.
Fig. 2. HRTEM image of NGO (a and c) and SGO (b and d).
Fig. 2 showed the HRTEM images of NGO and SGO. HRTEM images revealed the
layered structure of the reduced material. As shown in Fig 2a, GO modification by N2H4 was
observed with more wrinkled structures, and these small sheets were randomly aggregated
9

RSC Advances
Page 10 of 26
View Article Online
DOI: 10.1039/C5RA24287H
and closely associated with each other in a low magnification. The corrugations are attributed
to the disruption of the planar sp2 carbon sheets and the existence of some residual
sp3ꢀhybridized carbon on graphene sheets. As shown in Fig 2b, SGO showed a transparent,
bulk planar sheet and wrinkled paperꢀlike structure, which indicated that SGO exhibited a
better and larger sheet area than NGO. Congruent with XRD results, the edgeꢀon HRTEM
image of NGO (Fig 2c) showed an interꢀgraphene spacing of 0.369 nm, while the
interꢀgraphene spacing of SGO declined to 0.355 nm. Further more, the HRTEM images of
NGO from an edgeꢀon view also showed graphene sheets arranged in a disordered structure.
However, there existed the ordered stacking of larger sheet in SGO, which indicated the well
restored conjugated structure of SGO and the sulfur incorporation among GO sheets into the
larger area graphene. To prove the existed connection between small GO sheets after the
sulfide reduction, TEM images of SGO, GO and NGO with low magnification were provided
in Fig. 3, which further proved that the size of SGO is apparently larger than that of GO and
NGO. From those low magnification TEM images, we can conclude that some small pieces
of GO were connected into larger pieces of SGO during GO reduction by basic divalent
sulfide species.
SGO
GO
NGO
500 nm
500 nm
500 nm
Fig. 3. the low magnification TEM image of SGO, GO and NGO
10

Page 11 of 26
RSC Advances
View Article Online
DOI: 10.1039/C5RA24287H
GO, NGO and SGO were characterized by XPS, and the characterized results were
shown in Fig 4. C1s XPS spectra of GO were deconvoluted into four peaks according to the
carbon atoms in different chemical states. GO contains the nonꢀoxygenated C (CꢀC/C=C,
284.8 eV), hydroxyl or epoxy C (CꢀO 285.8 eV), carbonyl C (C=O, 287.6 eV) and the
carboxyl C (ꢀOꢀC=O, 288.8 eV).25, 26 After the chemical modification by basic divalent
sulfide, the oxygen species of CꢀO (hydroxyl and epoxyl), C=O (carbonyl) and O=CꢀO
(carboxyl) reduced significantly, which indicated an efficient deoxidization. The remaining
specie was sp2 C=C, while SP3 CꢀC (285.1eV) and C=O species were completely removed. In
addition, a trace of the marginal O=CꢀO (288.8eV) and CꢀOH (285.8 eV) existed after
modification, and some CꢀS (285.3 eV) also exited in SGO. As shown in Fig 3d, when GO
was modified by N2H4, a new intensive SP3 CꢀC (285.1eV) peak existed, which conformed
the existence of saturated CꢀH in NGO. Be similar with SGO, some O=CꢀO also existed after
N2H4 reduction, but a new peak at 286.3 eV appeared due to C=N formation. Those results
were consistent with FTꢀIR characterization. After GO modification by basic divalent sulfide,
C/O ratio increased from 1.95 to 4.98 which was comparable to GO reduced by NaBH4,27
indicating an efficient deoxidization. N1s XPS spectra and S2p XPS spectra of GO, NGO and
SGO were listed in Fig.4e and Fig.4f, and those characterized results also conformed the
formation of CꢀN bonds in NGO (N1s peak appearance at 400.0 eV) and the existence of CꢀS
bonds in SGO (S2p peak at 164.4 eV). Because S was incorporated at low temperature (98
oC), some CꢀS peaks around 285.3 eV exited and the S peak at 164.4 eV appeared, which is
attributed to the S existence inside the lattice structure of the graphene and it is covalently
bonded to the graphene in the form of thiophenic, disulfide and thioether species.28 In
11

RSC Advances
Page 12 of 26
View Article Online
DOI: 10.1039/C5RA24287H
addition, a weak S2p peak at 169.1 eV was also detected in GO and SGO, which was
attributed to a trace of SO3H existence. Those results also indicated GO can be sulfonated
during the exfoliation of graphite, but sulfonic acid group can be eliminated by N2H4
reduction
a
b
GO
285.8
SGO
287.6
NGO
GO
288.8
284.8
280 282 284 286 288 290 292 294
Binding Energy (eV)
280
282
284
286
288
290
Binding Energy (eV)
284.5
c
SGO
d
NGO
285.1
285.8
286.3
288.8
288.8
284.5
280
282
284
286
288
290
280 282 284 286 288 290 292
Binding Energy(eV)
Binding Energy (eV)
164.4
e
f
SGO
GO
NGO
GO
SGO
NGO
169.1
400
200 190 180 170 160 150 140
Binding Energy (eV)
480 460 440 420 400 380 360 340 320
Binding Energy (eV)
Fig 4. (a) is the C1s spectra of GO, SGO and NGO; (bꢀd) are the deconvoluted C1s spectra of
the ones of each samples. (e) N1s Xꢀray photoelectron spectra of GO, NGO and SGO; (f) S2p
12

Page 13 of 26
RSC Advances
View Article Online
DOI: 10.1039/C5RA24287H
Xꢀray photoelectron spectra of GO, NGO and SGO.
As shown in Fig. 5, Fig.5a displayed the TGA curves of GO, NGO and SGO, and Fig.5
b~d showed the mass intensity profiles of different thermal decomposition product from GO,
BGO and SGO. TGA curves of GO, NGO and SGO displayed a small weight loss around
100 °C, mainly due to the adsorbed water. The major mass loss of GO took place at around
200 °C, and the mass loss was ascribed to the gaseous escape of H2O and CO2, indicating the
decomposition of oxygenꢀcontaining groups that were thermally unstable. The minor mass loss
of GO took place at 500ꢀ800 °C, which was mainly attributed to H2O escape, while only a trace
of CO2 gas was detected at around 600 °C. In contrast with GO, NGO and SGO exhibited a less
weight loss in the temperature range from room temperature to 800 °C, and the mass loss at
around 200 °C was small, indicating the efficient removal of oxygen functional groups after
modification. As for NGO, the mass loss was attributed to the removal of absorbed H2O,
produce of NH3 from the decomposition of CꢀN species and escape of CO2 from residual
oxygen functional groups (see Fig 3c). The release of CO2 inevitably left behind vacancies and
topological defects on the graphene sheets, and single carbon vacancies could coalesce into
double vacancy and result in topological defects lined up along the wrinkle. It was interesting
to note here that the thermal stability of NGO was a little better than that of SGO, which was
mainly attributed to S elimination from SGO (see Fig 3d). Oxygen, nitrogen and carbon were
released as relatively nonꢀactive lowꢀmolecularꢀweight compounds (e.g. CO2, H2O, NH3), but
sulfur often was released as highly active species, such as H2S, S element and polysulfide ions.
Herein, S element was detected as the main escaping specie during SGO thermal cleavage
13

RSC Advances
Page 14 of 26
View Article Online
DOI: 10.1039/C5RA24287H
process at 167, 267, 493, 756 and 900 °C.
GO
100
90
80
70
60
50
40
30
a
b
CO2
GO
RGO-N H
2 4
RGO-Na S
100 200 300 400 500 600 700 800 900
o
Temperature ( C)
CO
2
2
H O
2
100 200 300 400 500 600 700 800
o
100 200 300 400 500 600 700 800 900
Temperature
( C)
o
( C)
Temperature
H O
2
SGO
H O
2
c
NGO
d
167
S
267
493
756
CO
2
CO
SO
2
NH
H S
2
3
2
N H
2 4
Phenthiol
100 200 300 400 500 600 700 800
(o
100 200 300 400 500 600 700 800 900
(o
Temperature
C)
Temperature
C)
Fig 5. TG and TGꢀMS curves of GO, NGO and SGO: (a) TG curves of GO, NGO and SGO;
(b) Mass intensity profiles of H2O and CO2 from GO TGꢀMS curves; (c) Mass intensity
profiles of H2O, CO2, NH3 and N2H4 from NGO TGꢀMS curves; (d) Mass intensity profiles
of H2O, CO2, S, SO2 and phenthiol from SGO TGꢀMS curves. The inset of Fig 4b is the Mass
intensity profiles of CO2, which can be viewed as the amplification of the black line in Fig 4b.
We use the inset to confirm the trace of CO2 escape around 600 °C.
The significant changes of GO structure during the chemical modification processing were
recorded in the Raman spectrum in Fig 6. The spectrum of GO displayed two peaks, that was D
band at around 1364 cmꢀ1 and G band at around 1592 cmꢀ1. The ID/IG ratio was often used to
14

Page 15 of 26
RSC Advances
View Article Online
DOI: 10.1039/C5RA24287H
estimate the defect of the sample and the inꢀplane crystallite sizes in disordered carbon
materials.29, 30 ID/IG ratio of GO was 0.76, which increased to 0.90 of SGO and 0.96 of NGO,
indicating the modification in size of the inꢀplane sp2 domains. As for SGO and NGO, the G
band moved to 1584 cmꢀ1, which was close to the value of the pristine graphite, confirming the
reduction of GO during the chemical modification. Those changes suggested more sp2 domain
formation and the decrease in the average size of the sp2 domains upon modification of the
exfoliated GO.6,31 Most importantly, the ID/IG ratio of GO was slightly lower than that of
modified GO, which was possibly related to the ordered stacking of GO layers by the numerous
hydrogen bonds. The above conclusion was consistent with the earlier reports that the reduction
of GO increased the number of aromatic domains of smaller average size in graphene and led to
an increase of the ID/IG ratio.6 However, ID/IG ratio of SGO was slightly lower than that of NGO,
indicating the aromatic domains of smaller average size in NGO while the largeꢀarea
πꢀconjugated structure formation through GO sheets connection by basic divalent sulfide
species.
Besides the D and G bands, both GO and modified GO contains 2D peak and D+G peak at
2710 cmꢀ1 and 2926 cmꢀ1 respectively. The 2D band is a characteristic band for defectꢀfree
graphitic structures, whereas the D+G band is induced by structural disorder.32 As for GO,
NGO and SGO, the I2D/ID+G ratios were 0.78, 0.79 and 0.91, respectively, strongly suggesting
the restoration of sp2 carbon in modified GO.33 The I2D/ID+G ratio of SGO was higher than that
of NGO, indicating the less defection formation in SGO.
15

RSC Advances
Page 16 of 26
View Article Online
DOI: 10.1039/C5RA24287H
G band
D band
2D
D+G
1592
1364
GO
NGO
1352
1584
1500
SGO
2926
2710
1200
2400
2700
3000
-1
)
Wavenumber cm
(
Fig. 6 Raman spectra of GO, NGO and SGO
The increase in the electrical conductivity of the graphene materials often signifies the
efficiency of a particular reduction method, therefore the structure difference of SGO and
NGO may explain the difference in the electrical conductivity. The electrical conductivity
was measured using a fourꢀprobe method and was calculated using the formula: σ= 1/ρ,
where ρ (unit: ꢁ·cm) referred to the electrical resistivity that can be obtained by the test
directly. After GO reduction by basic divalent sulfide, SGO reached a relatively higher
electrical conductivity of 7690 S/m, however, NGO showed some lower conductivity at
6578.9 S/m, probably due to the presence of some saturated carbon CꢀH moiety and the
smaller average size of aromatic domains. In other words, GO can undergo more effective
deoxygenation and the largerꢀarea πꢀconjugated structure in presence of basic divalent
sulfide.
3.2 Graphene oxidation modification mechanism
16

Page 17 of 26
RSC Advances
View Article Online
DOI: 10.1039/C5RA24287H
The mechanism for the chemical modification of GO remained obscure, but it had been
explained by SN2 nucleophilic reactions8, 12. The basalꢀplane of GO is highly populated with 1,
2ꢀepoxides and hydroxyl groups while the edgeꢀplane mainly consists of carboxyl and carbonyl
groups. Those oxygen containing groups account for the structural defects that lead graphene
oxide to deviate from the state of pristine graphene. Pablo A. Denis and C. Pereyra Huelmo28
gave us a better understand of S incorporation mechanism under high temperature condition,
however, inspired by the reduction of anthraquinone and the role of sulfur in the
transformations of sedimentary organic matter,34 a possible mechanism of GO modification by
basic divalent sulfide species at low temperature (<100 oC) was provided. Basic divalent sulfide
species is often utilized in synthetic chemistry for electrophilic addition and nucleophilic
substitution reactions. On one hand, acting as the reducing agent basic divalent sulfide played a
role on removing the oxygen containing groups. On the other hand, basic divalent sulfide
species are nucleophilic in nature and react with GO via SN1 or SN2 pathway. The proposed
process was listed in Scheme 1, through which the conjugated structure was restored. Although
the reductive reaction induced by basic divalent sulfide species was expected to have little
influence on the elimination of marginal oxygenꢀcontaining groups owing to the highꢀenergy
barrier16, Basic divalent sulfide species were thought to be doped in GO (process was listed in
Scheme 2). Sulfide might act as the medium to connect the adjacent GO sheets, therefore large
area graphene sheets formed. At the same time, a part of the structural defects were repaired by
the basic divalent sulfide modification, thus enhancing the electrical conductivity. The dope
mechanism was speculated in Scheme 3.
17

RSC Advances
Page 18 of 26
View Article Online
DOI: 10.1039/C5RA24287H
SH
H2O
OH
O
O
HO
OH
S2-
S2-
O
OH
H2O
SH
Scheme 1: Possible deoxygenation mechanism by the basic divalent sulfide
S2-
R1
R2
R1
R2
R1
R
R1
R2
S
S
R1
R2
O
or
O
S
H2O
2 H
H
O
O
R2
R1
R1
O
R2
S2-
R2
R2
or
R1
R2
H2O
R1 S S R1
S
O
Scheme 2: The basic divalent sulfide incorporation process
18

Page 19 of 26
RSC Advances
View Article Online
DOI: 10.1039/C5RA24287H
COOH
O
O
O
O
HO
OH
O
O
OH
OH
Small GO
OH
O
O
HO
O
COOH
O
H2O
Na2S
Oxygen-containing groups on basal-plane
of GO were removed by reduction
COOH
Large SGO
HO
S
S
Sulfur was incorporated through
S
the reaction between Na S and
HO
2
carbonyl groups in edge-plane
OH
COOH
S
S
S
S
OH
HO
Annealing
COOH
HO
Large graphene
Scheme 3: The larger area graphene formation process from the small GO sheet
3.3 Biomass conversion into HMF over SGO without annealing
Recently the selective conversion of fructose and glucose (or cellulose) into HMF has
attracted significant interest because HMF and its derivatives are potential substitutes for
petroleumꢀbased chemicals. The conversion of glucose into HMF is generally achieved by two
19

RSC Advances
Page 20 of 26
View Article Online
DOI: 10.1039/C5RA24287H
steps: glucose is first isomerized to fructose using catalysts based on a Lewis acid or a base,
followed by fructose dehydration to yield HMF under acidic conditions.35 CrCl2 exhibited
excellent performance for the isomerization of glucose into fructose and further dehydration of
the generated fructose in an ionic liquid solvent, and HMF yield was in the range of 60–70%.36
However, the homogeneous chromium based catalysts have serious environmental and
operational problems for the largeꢀscale production of HMF from glucose.
Table 1 HMF yields from carbohydrates over different catalyst
HMF yield /Selectivity (mol%)
Surface area
Catalysts
Fructose
Glucose
Cellobiose
Cellulose
(m2 g−1)
(140 oC/4h) (140 oC/4h) (140 oC/4h) (180 oC/2h)
SGO
NGO
GO
360
465
867
760
73.4
45.4
76.9
49.8
56.9
1.56
77.8
36.7
51.1
2.10
66.2
30.2
49.4
8.29
84.6
TRGO
68.93
Conditions: 0.9g carbohydrate, 0.03g catalyst, 5mL 1ꢀethylꢀ3ꢀmethylimidazolium sodium
bromide ([EMIM] Br) and 4h.
It is well know that the introduction of heteroatoms could increases reactivity of
graphene, and this effect was attributed to the strong charge redistribution induced by the
dopants28. Herein, the S incorporation in graphene might act as reactive sites to promote the
transform from glucose to fructose. SGO, NGO and GO were used to catalyze fructose,
20

Page 21 of 26
RSC Advances
View Article Online
DOI: 10.1039/C5RA24287H
glucose, cellobiose and cellulose conversion to HMF (Table 1). GO display a better
performance in fructose dehydration to HMF, and HMF yield over GO achieved 84.6%.
However, glucose, cellobiose and cellulose were difficult to be conversed into HMF over GO.
When SGO was used as catalysts in [EMIM]Br, 73.4% HMF yield was obtained from
fructose dehydration, but SGO also showed an excellent performance in HMF synthesis from
glucose, cellobiose and cellulose. However, HMF yields from fructose, glucose, cellobiose
and cellulose over NGO declined to 45.4%, 49.8%, 36.7% and 30.2%. The plausible
mechanisms of glucose conversion to HMF over SGO were proposed (Scheme 4). Namely,
glucose was isomerized into fructose over Lewis basic sulfide sites of SGO, while the
marginal COOH, phenolic hydroxyl group and a trace of SO3H played a key role in fructose
dehydration into HMF. Although TRGO have high surface area, it displayed no activity for
glucose conversion into HMF.
HO
HO
OH
Lewis basic
sulfide sites
O
O
O
Acidic marginal oxygen
containing groups
HO
O
HO
HO
HO
OH
isomerization
OH
OH
OH
Dehydration
Glucose
HMF
Fructose
Scheme 4: HMF formation process over SGO
4. Conclusion
The basic divalent sulfide is an excellent reducing agents for GO deoxygenation, and it
can be incorporated into GO. When GO was modified by basic divalent sulfide species, the
formation of saturated CꢀH was restrained and πꢀconjugated structure was more easily restored.
There existed some CꢀS bonds in SGO, and the existence of the basic divalent sulfide species
acted as the medium to connect adjacent GO sheets to form the largeꢀarea conjugated
21

RSC Advances
Page 22 of 26
View Article Online
DOI: 10.1039/C5RA24287H
sp2ꢀcarbon network, which made SGO to have a relatively higher electrical conductivity of
7690 S/m. However, NGO showed some lower conductivity at 6578.9 S/m, due to some
saturated CꢀH moiety, CꢀN specie and the smaller average size of aromatic domains in NGO.
When SGO was used as catalysts to catalyze biomass conversion in [EMIM]Br, 73.4% HMF
yield was obtained from fructose dehydration, and SGO showed an excellent performance in
HMF formation from glucose, cellobiose and cellulose. Those results further indicated that the
basic divalent sulfide was incorporated into SGO and the residual oxygenꢀcontaining groups
were the acidic marginal oxygenꢀcontaining groups. At the same time, a possible mechanism of
GO modification by basic divalent sulfide species was provided, that is the basic divalent
sulfide species, acting as the reducing agent, are capable of removing the abundant hydroxyl
and epoxide functionalities on basalꢀplane of GO to obtain the conjugated structure, while basic
divalent sulfide species can be incorporated to connect adjacent GO sheets.
Acknowledgements
The authors are grateful for the financial support of the National and Tianjin Natural
Science Foundation of China (nos. 21376177 and 12JCZDJC29800 and 15JCZDJC7000).
This work is also supported by China National Textile and Apparel Council (J201406) and
Science and technology innovation project of Shanxi Province
(2014101002).
References
1 M. Ishigami, J.H. Chen, W.G. Cullen, M.S. Fuhrer, E.D. Williams, Nano Lett. 2007, 7,1643.
22

Page 23 of 26
RSC Advances
View Article Online
DOI: 10.1039/C5RA24287H
2 S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, R.D.
Piner, S.T. Nguyen, R.S. Ruoff, Nature , 2006, 442, 282.
3 H.C. Schniepp, J. L. Li, M.J. McAllister, H. Sai, M. HerreraꢀAlonso, D.H. Adamson, R.K.
Prud’homme, R. Car, D.A. Saville, I.A. Aksay, J. Phys. Chem. B 2006, 110, 8535.
4 C. Berger, Z. M. Song, T. B. Li, X.B. Li, A. Y. Ogbazghi, R. Feng, Z.T. Dai, A.N.
Marchenkov, E.H. Conrad, P.N. First, W.A. De Heer, J. Phys. Chem. B 2004, 108, 19912.
5 K. S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.H. Ahn, P. Kim, J.Y. Choi,
B.H. Hong, Nature, 2009, 457, 706.
6 S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y.Y. Jia, Y. Wu,
S.T. Nguyen, R.S. Ruoff, Carbon 2007, 45, 1558.
7 W. Chen, L. Yan, P.R. Bangal, Carbon 2010, 48, 1146.
8 X.F. Gao, J. Jang, S. Nagase, J. Phys. Chem. C 2009, 114, 832.
9 H.J. Shin, K.K. Kim, A. Benayad, S.M. Yoon, H.K. Park, I.S. Jung, M.H. Jin, H.K. Jeong,
J.M. Kim, J.Y. Choi, Y.H. Lee, Adv. Funct. Mater. 2009, 19, 1987.
10 M.J. FernandezꢀMerino, L. Guardia, J.I. Paredes, S. VillarꢀRodil, P. SolisꢀFernandez, A.
MartinezꢀAlonso, J.M.D. Tascon, J. Phys. Chem. C 2010, 114, 6426.
11 P.F. Song, J.P. Zhao, J.H. Du, W.C. Ren, H.M. Cheng, Carbon. 2010, 48, 4466.
12 W.F. Chen, L.F. Yan, P.R. Bangal, J. Phys. Chem. C 2010, 114, 19885.
13 T.N. Zhou, F. Chen, K. Liu, H. Deng, Q. Zhang, J.W. Feng, Q. Fu, Nanotechnology. 2011,
22, 045704.
14 H.Y. Fu, X.L. Qu, W. Chen, D.Q. Zhu, Environ. Toxicol. Chem. 2014, 33, 2647.
15 W. Gao, L.B. Alemany, L.J. Ci, P. M. Ajayan, Nature Chem. 2009, 1, 403.
23

RSC Advances
Page 24 of 26
View Article Online
DOI: 10.1039/C5RA24287H
16 Y. Su, X. Gao, J. Zhao, Carbon 2014, 67, 146.
17 Y. Long, C. Zhang, X. Wang, J. Gao, W. Wang, Y. Liu, J. Phys. Chem. 2011, 21, 13934.
18 A. Lerf, H. He, M. Forster, J. Klinowski, J. Phys. Chem. B 1998, 102, 4477.
19 J.F. Tijero, M. Oliet, J.C. Burillo, F. Rodriguez, Ind. Eng. Chem. Res. 1991, 30, 1791.
20 S.J. Park, J. An, R.D. Piner, I. Jung, D.X. Yang, A. Velamakanni, S.T. Nguyen, R.S. Ruoff,
Chem. Mater. 2008, 20, 6592ꢀ6594.
21 Y.X. Xu, H. Bai, G. Lu, C. Li, G.Q. Shi, J. Am. Chem. Soc. 2008, 130, 5856.
22 O.C. Compton, S.T. Nguyen, Graphene Oxide, Small. 2010, 6, 711.
23 D.C. Luo, G. X. Zhang, J. F. Liu, X.M. Sun, J. Phys. Chem. C 2011, 115, 11327.
24 D. Li, M.B. Mueller, S. Gilje, R.B. Kaner, G.G. Wallace, Nat. Nanotechnol. 2008, 3, 101.
25 D. Briggs, G. Beamson, High Resolution XPS of Organic Polymers: Thescienta ESCA300
Database. John Wiley and Sons: New York, 1992.
26 R.J. Waltman, J. Pacansky, C.W. Bates Jr, Chem Mater. 1993, 5, 1799.
27 S. Zhang, Y.Y. Shao, H.G. Liao, M.H. Engelhard, G. Yin, Y.H. Lin, ACS Nano 2011, 5,
1785.
28. P.A. Denis, C. Pereyra Huelmo, Carbon 2015, 87, 106.
29 D.S. Knight, W.B. White, J. Mater. Res. 1989, 4, 385.
30 S. Park, J.H. An, R.D. Piner, I. Jung, D.X. Yang, A. Velamakanni, S.T. Nguyen, R.S. Ruoff,
Chem Mater. 2008, 20, 6592.
31 F. Tuinstra, J. L. Koenig, J. Chem. Phys. 1970, 53, 1126.
32 M. A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, L.G. Cancado, A. Jorio, R. Saito, Phys
Chem Chem Phys. 2007, 9, 1276.
24

Page 25 of 26
RSC Advances
View Article Online
DOI: 10.1039/C5RA24287H
33 M.S. Dresselhaus, A. Jorio, M. Hofmann, G. Dresselhaus, R. Saito, Nano Lett 2010, 10,
751.
34 Z. Aizenshtat, E.B. Krein, M. A. Vairavamurthy, T.P. Goldstein, Role of Sulfur in the
Transformations of Sedimentary Organic Matter: A Mechanistic Overview. ACS Symposium
Series, Geochemical Transformations of Sedimentary Sulfur, Chapter 2, 612(1995)16ꢀ37.
35 M.E. Zakrzewska, E. BogelꢀŁukasik, R. BogelꢀŁukasik, Chem. Rev. 2011, 111, 397–417.
36 H. Zhao, J.E. Holladay, H. Brown, Z.C. Zhang, Science 2007, 316, 1597.
25

RSC Advances
Page 26 of 26
View Article Online
DOI: 10.1039/C5RA24287H
COOH
O
O
O
HO
OH
O
O
O
OH
OH
Small GO
OH
O
O
HO
O
O
COOH
Biomass
HMF
S2-
COOH
HO
S
Large SGO
S
S
HO
OH
COOH
S
S
S
S
OH
HO
Annealing
COOH
HO
Large graphene