Spontaneous reduction and dispersion of graphene nano-platelets with in situ synthesized hydrazine assisted by hexamethyldisilazane

Article abstract of DOI:10.1039/c2jm34691e

The environmentally friendly reduction and a solution processability of chemically modified graphene nanosheets are most important for their applications. Here we report for the first time that in situ synthesis of hydrazine and spontaneous reduction of g

Full text of DOI:10.1039/c2jm34691e

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PAPER
Spontaneous reduction and dispersion of graphene nano-platelets with in situ
synthesized hydrazine assisted by hexamethyldisilazane†
a
a
a
b
a
a
Joong Tark Han,* Jeong In Jang, Bo Hwa Jeong, Beom Joon Kim, Seung Yol Jeong, Hee Jin Jeong,
b
b
Jeong Ho Cho and Geon-Woong Lee*
Received 17th July 2012, Accepted 16th August 2012
DOI: 10.1039/c2jm34691e
The environmentally friendly reduction and a solution processability of chemically modified graphene
nanosheets are most important for their applications. Here we report for the first time that in situ
synthesis of hydrazine and spontaneous reduction of graphene oxide (GO) in solution can be achieved
just by addition of hexamethyldisilazane (HMDS) and hydrogen peroxide into the GO suspension and
followed by step-wise heating. We also demonstrated that GO functionalized with carbonyl groups can
be used as an alternative to ketone molecules as a catalyst in a typical reaction cycle for hydrazine
synthesis by identifying intermediate azine structures. The successful synthesis of hydrazine and
ꢀ
spontaneous reduction of GO at 100 C was confirmed by measuring X-ray photoelectron spectra and
Fourier transform infrared spectra of RGO powder and UV absorbance spectra of RGO solution.
Moreover, trimethylsilanol molecules from HMDS contributed to a stable colloidal RGO suspension
even in ethanol. In situ synthesis of hydrazine molecules in GO suspension provides a new
environmentally friendly process for application of chemically modified graphene in electronics and
composites.
Introduction
synthesis of hydrazine in a GO suspension can be performed in a
typical reaction cycle involving ketone molecules as a catalyst
11
Chemically modified graphene, which is produced through the
oxidation of highly oriented pyrolytic graphite, is a typical two-
dimensional (2D) particle attracting great interests in colloidal
and ammonia and hydrogen peroxide as reagents. Importantly,
GO functionalized with carbonyl groups could be used as an
alternative to ketone molecules.
1–9
sciences. Oxidized graphene nano-platelets may be reduced by
chemical or thermal treatment to recover their electrical prop-
erties. The development of environmentally friendly methods for
the efficient reduction and solution processing of graphene nano-
platelets is central to the future use of such materials in elec-
Applications of GO are further complicated by the difficulties
associated with suspending reduced graphene oxide (RGO) in
polar solvents. Two-dimensional RGO irreversibly stacks and
agglomerates via p–p interactions. A variety of routes to RGO
colloidal stabilization have allowed RGO to be suspended in
12–15
1,4,7
tronics and composites.
Hydrazine has been used for the
polar solvents.
Alcohol-based formulations of graphene are
chemical reduction of graphene oxide (GO), however, hydrazine-
based methods are undesirable due to the toxicity of hydrazine to
essential for graphene processing and the fabrication of related
14
devices if the use of harsh organic solvents is not possible. In
10
human health and to the environment. Therefore, we focused
on the in situ synthesis of hydrazine molecules in a GO suspen-
sion and their immediate use for the reduction of GO in solution.
This approach precludes the risks of hydrazine exposure to
human body and the environment. We suggest that in situ
these contexts, hexamethyldisilazane (HMDS) may be used as a
source of ammonia molecules for hydrazine synthesis and RGO
dispersal because HMDS can be easily hydrolyzed into trime-
thylsilanol and ammonia in the presence of water (Scheme 1).
The trimethylsilanols generated from hydrolyzed HMDS act as
an RGO dispersant in organic solvents (Scheme 1(ii)).
The present study describes, for the first time, the in situ
synthesis of hydrazine in the presence of GO and ammonia
generated during the hydrolysis of HMDS with hydrogen
peroxide. We demonstrated the successful reduction of GO by
the in situ synthesized hydrazine in a GO suspension by
measuring the X-ray photoelectron spectra (XPS) and Fourier
transform infrared (FTIR) spectra of the RGO powder and the
UV absorbance spectra of the RGO suspension. Importantly, the
a
Nano Carbon Materials Research Group, Korea Electrotechnology
Research Institute, 70 Boolmosangil, Changwon, Korea, 641-120. E-mail:
jthan@keri.re.kr; gwleephd@keri.re.kr; Fax: +82-55-280-1590; Tel:
+
b
82-22-280-1677
SKKU, Advanced Institute of Nanotechnology and Center for Human
Interface Nano Technology (HINT), Department of Chemical
Engineering, Sungkyunkwan University, 2066 Seobu-ro, Suwon, Korea,
4
40-746
† Electronic supplementary information (ESI) available: Additional
figures. See DOI: 10.1039/c2jm34691e
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ꢀ
hydrogen peroxide and stirring at 50 C for 10 h. The solution
ꢀ
was then heated at 100 C to reduce GO nano-platelets for 16 h.
Resulting reduced graphene oxide (RGO) suspension was
precipitated and redispersed in organic solvents by sonication at
1
20 W for 5 min.
GO and RGO films were fabricated at room temperature using
an automatic spray coater (NCS Co., NCS400) with a nozzle
1
.2 mm in diameter. The GO films were then reduced by heating
ꢀ
under hydrazine fumes at 100 C for 2 h to fabricate RGO films
as a control sample. Source and drain electrodes were then
deposited by evaporating Au through a shadow mask. The
channel length and width of a typical device were 100 mm and 800
mm, respectively. The transistor current–voltage characteristics
were measured using Keithley 2400 and 236 source/measure units
under vacuum (10–5 Torr).
The morphologies of the GO and RGO nanosheets were
imaged by AFM (DI Multi-modes), field emission scanning
electron microscopy (FE-SEM, HITACHI S4800), and field
emission transmission electron microscopy (FE-TEM,
JEM2100F). The absorbance of the GO suspension during
reduction was measured by UV-vis spectroscopy (Varian, Cary
Scheme 1 Roles of hexamethyldisilazane (HMDS): (i) ammonia source
for GO-assisted production of hydrazine upon the addition of hydrogen
peroxide and (ii) RGO dispersion agent in ethanol, via hydrophobic
interactions.
5
000). The structural characteristics of the RGO sheets were
investigated by a confocal Raman spectrometer (NTEGRA
SPECTRA, NT-MDT) with an excitation wavelength of 532 nm.
To confirm the change in the carbon to oxygen atomic ratio in
the functional groups of the films after reduction by hydrazine
molecules synthesized in situ, XPS analysis was conducted using
a Multilab2000 (Thermo VG Scientific Inc.) spectrometer with
monochromatized Al K X-ray radiation as the X-ray excitation
source. FT-IR spectroscopy (JASCO, 4200UP) analysis was
performed to assign the functional group of GO and RGO. The
electrical conductivity was measured by the four-point probe
method (Keithley 2000 multimeter) at room temperature.
trimethylsilanol hydrolyzed from the HMDS stabilized the
colloidal RGO suspension via hydrophobic interactions, even in
ethanol. This approach provides an environmentally friendly
method for reducing GO via the in situ synthesis of hydrazine
molecules in a GO suspension and the stable dispersion of RGO
in low boiling point relatively mild solvents.
Experimental
Graphene oxide was prepared from natural graphite (Alfa Aesar,
9
9.999% purity, À200 mesh) by a modified Hummers method.
Briefly, 20 g graphite and 460 mL H SO were mixed in a flask.
Then, 60 g KMnO were slowly added for 1 h. Stirring was
2
4
4
Results and discussion
continued for 2 h in an ice-water bath. After the mixture was
stirred vigorously for 18 h at room temperature, 920 mL deion-
ized water was added, and the solution was stirred for 10 min in
Hydrazine has been widely used for the production of RGO-based
10
nano-platelets with excellent physical properties. Hydrazine is
2 2
an ice-water bath. Fifty milliliters of H O (30 wt% aqueous
formed in a multitude of chemical reactions. In a typical process
11
solution) was then added, and the mixture was stirred for 2 h at
room temperature. The resulting mixture was precipitated and
filtered to obtain the graphite oxide powder. The graphite
involving acetone, ammonia, and hydrogen peroxide, acetone is
treated with ammonia to give an imine, which is then oxidized
with hydrogen peroxide to yield 3,3-dimethyloxaziridine.
Ammonolysis then produces the hydrazone, which reacts with one
more equivalent of acetone. The resulting azine is hydrolyzed to
produce hydrazine and regenerate two molecules of acetone. In
this study, GO nano-platelets functionalized with ketone groups
were utilized as an alternative ketone molecule for the in situ
production of hydrazine and the spontaneous reduction of GO in
solution. As shown in Scheme 1, one ammonia molecule is
generated by the hydrolysis of HMDS in water. Thus, two HMDS
molecules and one hydrogen peroxide molecule were needed to
synthesize one hydrazine molecule in the presence of GO nano-
platelets (Fig. S1 in the ESI†).
À1
powder was dispersed in water by the centrifugation of a 10 g L
aqueous GO suspension under magnetic stirring, and the highly
functionalized graphite oxide with carboxylic acid groups was
removed by centrifugating. Purified graphite oxide was then
À1
exfoliated into graphene oxide (GO) nano-platelets (1 g L ) in
deionized water–dimethylformamide (DMF) (1/9 v/v) by bath
sonication for 1 h.
For the reduction of GO in solution, hydrazine was in situ
synthesized in a GO suspension by mimicking a typical reaction
cycle involving GO (alternative ketone molecules as a catalyst)
and ammonia and hydrogen peroxide as reagents. Hexame-
thyldisilazane (HMDS) molecules which can be easily hydro-
lyzed into ammonia and trimethylsilanol in the presence of water
molecules were used as a source of ammonia molecules. First,
GO nano-platelets were prepared by exfoliation of graphite
oxide produced by the modified Hummer’s method using pure
graphite (with an average size of 70 mm from the natural graphite
powder). A homogeneous colloidal suspension of GO nano-
platelets was prepared in water–dimethylformamide (DMF)
ꢀ
HMDS was added into the GO suspension at 25 C and stirred
for 30 min for hydrolysis of HMDS moleculesto produce
ammonia and trimethylsilanols, followed by addition of
À1
(1/9 v/v) mixture solvent at a concentration of 500 mg L by
2
0478 | J. Mater. Chem., 2012, 22, 20477–20481
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bath sonication of a graphite oxide solution for 30 min (Fig. 1a).
The average size of the purified GO exceeded one micrometer
overlap with the C]N stretching peak of the azine. The XPS
data in Fig. 2b and c confirmed that the azine and oxaziridine
structures formed at low temperatures. The C1s peak at 287.87
eV arose from the C–N bond, and the N1s peaks at 399.33 and
401.7 eV indicated the formation C]N bonds in the azine and
N–O bonds in the oxaziridine. The N1s peak (399.33) from the
azine (Fig. 2c) differed from the peak (399.6 eV) corresponding
(
Fig. 1b and S2†).
Importantly, we attempted to reduce the GO nano-platelets
with hydrazine synthesized in situ in a GO suspension in the
presence of ammonia and hydrogen peroxide without adding
other reduction agents. A quantity of HMDS (98 wt%) was
À1
16
added to the 500 mg L GO suspension in a water–DMF
ꢀ
to hydrazone, formed by the addition of hydrazine (Fig. 2d). As
mixture (50 mL). The suspension was then stirred at 50 C for 30
shown in Fig. 1a and the SEM image (Fig. 1c), the GO nano-
platelets underwent severe aggregation at low temperatures, even
though the GO nano-platelets were not reduced. Moreover, the
min to hydrolyze the HMDS, producing ammonia and trime-
thylsilanol. The amount of HMDS added to the GO suspension
was varied between 4 moles per 100 mg GO and 60 moles per
1
00 mg GO by considering the number of hydrazine molecules
that could be synthesized in situ in the GO suspension. Subse-
quently, a stoichiometric amount of hydrogen peroxide was
added to the GO–HMDS mixture solution, followed by stirring
ꢀ
at 50 C for 10 h. A quantity of the GO–HMDS–H
2
O
2
mixture
solution was characterized to investigate the intermediates
oxaziridine, hydrazone, and azine structures, Fig. S1†) formed
(
during the in situ synthesis of hydrazine using FTIR spectroscopy
and XPS. Fig. 2a shows the FTIR spectra of a GO–HMDS–
ꢀ
H O sample heated at 50 C, followed by heat-treatment at 100
2
2
ꢀ
ꢀ
C. After heating at 50 C, the carbonyl groups in the GO dis-
appeared, and an N–O stretching peak from the oxaziridine
structure appeared. The C]C stretching peak was broad due to
Fig. 2 Characterization of GO and RGO. (a) FTIR spectra of (1) GO,
ꢀ
the GO–HMDS–H
ꢀ
2
O
2
mixture treated (2) at 50 C for 10 h, subsequently
ꢀ
Fig. 1 Reduction and dispersion of chemically modified graphene nano-
heated at 50 C and 100 C (3) for 90 min and (4) 16 h, and (5) RGO
ꢀ
platelets. (a) Route (i): sequential addition of HMDS and hydrogen
ꢀ
reduced by hydrazine at 100 C for 16 h. (b and c) C1s and N1s XPS
ꢀ
peroxide to a GO suspension at 50 C, followed by stirring for 10 h and
ꢀ
spectra of GO treated at 50 C for 10 h, respectively. (d) N1s spectrum of
further heating at 100 C for 16 h. The resulting aggregated RGO
the hydrazine fume-reduced graphene oxide. (e) The UV absorbance
ꢀ
precipitate was redispersed in ethanol. Route (ii): direct reduction with
hydrazine monohydrate without any dispersant. (b and c) SEM images of
peak positions of the GO solution shifted upon step-wise heating at 50 C
ꢀ
ꢀ
and 100 C (solid red), relative to the one-step heating at 100 C. Star
well-exfoliated GO nano-platelets and aggregated GO–azine on SiO
2
,
2 4
symbols indicate the RGO suspension reduced by the addition of N H at
ꢀ
respectively. (d) TEM image of RGO nano-platelets well-dispersed with
100 C. (f) C1s XPS spectra of GO, partially reduced GO (RGO1), and
trimethylsilanol, showing the wrinkled structures produced via the route
2 4
fully reduced GO (RGO2) by the in situ synthesized N H . The numbers
(i) method.
shown in (e) correspond to those in (f).
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solution color did not change from dark brown. These results
clearly supported that the interconnected azine structure of GO
formed mainly without the reduction of the GO nano-platelets.
The intermediated GO was denoted as GO–azine. The GO–
shows the XPS spectra of the GO and RGO films reduced by the
HMDS–H O mixture without other reduction agents. The
2
2
addition of 4 moles HMDS and 2 moles H O per 100 mg GO
2
2
2 4
(the stoichiometric yield of N H is 2 moles under these condi-
HMDS–H
ꢀ
2
O
2
mixture solution was subsequently heated to 100
tions) reduced the number of C–O bonds in the epoxides and
hydroxyl groups, whereas carbonyl groups and C–O bonds
C because the reduction of GO by hydrazine can usually be
ꢀ
performed at an elevated temperature of 100 C. The dark brown
2 2
remained. A ten-fold increase in the HMDS and H O concen-
GO–azine suspension turned black within 30 min, indicating the
production of hydrazine and the spontaneous reduction of GO.
trations dramatically reduced the remaining C–O and C]O
bonds. In addition, the D/G ratio decrease and the 2D peak
increase in the Raman spectra of GO and RGO films (Fig. S5†)
demonstrate the successful reduction of GO by the in situ
ꢀ
Heating at 100 C for 90 min resulted in a decrease in the N–O
stretching peak in the FTIR spectra (Fig. 2a). The peak dis-
appeared after sufficient reduction of GO, and a C]C stretching
peak gradually sharpened after 16 h. Importantly, the N–H
stretching peak corresponding to the hydrazone groups (–C]N–
17
synthesized hydrazine. The I–V plot shown in Fig. 3a also
shows that electrically insulating GO was successfully reduced,
À1
yielding a conductivity of 5000 S m for a 5 mm RGO film
NH
2
) attached to the RGO nanosheets upon the in situ synthesis
ꢀ
(Fig. S6†). These results clearly demonstrated that, as suggested,
the hydrazine molecules were successfully synthesized in situ, and
the GO nano-platelets were reduced in solution.
of hydrazine molecules appeared after heating at 100
C
(
Fig. 2d). As a control experiment, we heated the GO suspension
ꢀ
at 100 C for 10 h without HMDS in the presence or absence of
and in the presence of HMDS. The GO nano-platelets were
The survey XPS spectral scans (Fig. S4b†) show that trime-
thylsilanol groups were present in the RGO film prepared on an
Au-coated Si wafer, as indicated by the Si 2p peak. The effects of
trimethylsilanol in the RGO film were investigated by measuring
the electrical transport characteristics of the RGO–HMDS films
by preparing graphene field-effect transistors (FETs) on heavily
doped Si substrates. As a control sample, the FET characteristics
of the RGO film prepared by reducing the GO film using
hydrazine fumes were obtained. Fig. 3b and c show the transfer
2 2
H O
not reduced in this solution; therefore, we believe the hydrazine
molecules appeared to be synthesized in situ in the presence of
GO, HMDS, and H O in solution.
2
2
This sequential process precipitated the RGO suspension
Fig. 1a). The precipitate was separated and could be redispersed
(
in various organic solvents such as DMF, dimethylsulfoxide, and
ethanol (EtOH) (Fig. S3†). The linear increase in the UV
absorbance of the RGO suspension at 550 nm as a function of
concentration even in EtOH indicated the stable dispersion. In
stark contrast, graphene nano-platelets reduced by the hydrazine
monohydrate without HMDS molecules (route (ii) in Fig. 1a)
precipitated as an irreversible agglomerate, making further pro-
cessing difficult, as shown in Fig. 1a.
D G 2 4
characteristics (I vs. V ) of graphene FETs based on the N H
fume-reduced RGO and RGO–HMDS films, respectively. A
characteristic V-shaped ambipolar behavior, which indicates
2 4
electron and hole conduction, was obtained from the N H fume-
reduced RGO film. Hysteresis was observed in the RGO FETs
without HMDS, even under vacuum, as shown in Fig. 3b. RGO
multilayer films and RGO–polymer composite films have previ-
ously been observed to display hysteresis, even under vacuum,
whereas single-layer RGO sheets display hysteresis-free back-
The reduction efficiency was investigated by monitoring the
UV absorbance peak position of the RGO suspension. The XPS
spectra of the films were also observed for varying concentrations
of HMDS and for various heating conditions. The UV absor-
bance peak position of the graphene solution shifted from 232 to
18
gate-dependent conductivity under vacuum. The residual
oxidative moieties and adsorbed water molecules in the RGO
multilayer films, both of which may become polarized under an
applied electric field, introduced a current hysteresis during the
2
73 nm upon an increase in the HMDS concentration (Fig. 2e
ꢀ
ꢀ
and S4a†). Importantly, step-wise heating at 50 C and 100 C
was more efficient than a one-step process involving heating at
ꢀ
G
V sweep due to charge carrier trapping. Importantly, the
1
00 C because at high temperatures, GO, which acts as a ketone
current hysteresis completely vanished in the RGO–HMDS film
(Fig. 3c). Moreover, the threshold voltage shifted toward posi-
tive values due to the interaction between RGO and the silanol
groups, even in the presence of hydrazone groups, which acted as
molecule in the hydrazine synthesis, can be spontaneously
reduced by the in situ synthesized hydrazine molecules. Fig. 2f
19
an n-type dopant. We suggest that the trimethylsilanol mole-
cules from the hydrolyzed HMDS present between the RGO
nano-platelets interacted with the remaining oxygen moieties via
2
hydrogen bonding, and the silanol groups interacted with the sp
carbons to withdraw electrons from the RGO after evaporation
of solvents.
Conclusions
We demonstrated that the in situ synthesis of hydrazine in a GO
suspension and the spontaneous reduction of GO nano-platelets
in solution could be achieved in a reaction involving ammonia
produced from hydrolyzed HMDS, hydrogen peroxide, and GO
nanosheets. Intermediate oxaziridine and azine structures
formation during the in situ hydrazine synthesis and the
Fig. 3 (a) I–V plot of the GO and RGO reduced by hydrazine synthe-
sized in situ in the presence of HMDS molecules. (b and c) Transistor
2 4
properties of the RGO films reduced by hydrazine fumes (N H fume–
2 2
RGO) and by the in situ synthesized hydrazine in the HMDS–H O
mixture solution (HMDS–RGO), respectively.
2
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spontaneous reduction of GO were supported by the FTIR and
XPS spectra at low temperatures. Moreover, trimethylsilanol
molecules from the hydrolyzed HMDS molecules successfully
acted as dispersants for the RGO in the organic solvents. HMDS
molecules have been usually used to modify the interfacial energy
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