17-11-2012, 04:03 PM
One-step synthesis of graphene/SnO2 nanocomposites and its application in electrochemical supercapacitors
[attachment=40038]
Abstract
A one-step method was developed to fabricate conductive graphene/SnO2 (GS) nanocomposites
in acidic solution. Graphite oxides were reduced by SnCl2 to graphene sheets in the presence of
HCl and urea. The reducing process was accompanied by generation of SnO2 nanoparticles.
The structure and composition of GS nanocomposites were confirmed by means of transmission
electron microscopy, x-ray photoelectron and Raman spectroscopy. Moreover, the
ultracapacitor characteristics of GS nanocomposites were studied by cyclic
voltammograms (CVs) and electrical impedance spectroscopy (EIS). The CVs of GS
nanocomposites are nearly rectangular in shape and the specific capacitance degrades slightly as
the voltage scan rate is increased. The EIS of GS nanocomposites presents a phase angle close
to π/2 at low frequency, indicating a good capacitive behavior. In addition, the GS
nanocomposites could be promisingly applied in many fields such as nanoelectronics,
ultracapacitors, sensors, nanocomposites, batteries and gas storage.
Introduction
Interest in understanding properties of graphene has led to
many theoretical and experimental efforts worldwide, due to its
prominent properties and broad potential applications in many
fields [1]. However, graphene sheets, unless well separated
from each other, tend to form irreversible agglomerates or even
restack to form graphite through van der Waals interactions.
Currently, some chemical [2–5] and physical [6, 7] methods
have been used to produce freestanding graphene sheets.
However, all previous graphene sheets were reduced by
alkaline reagents in aqueous solution and only dispersed well
in alkaline solution. Aggregation would occur with any change
of the condition of the solution, such as addition of salts or
acids [2, 8–10]. In contrast, many other materials are unstable
in an alkaline environment. This restricts the synthesis of many
hybrid graphene materials and application fields of graphene
sheets.
Fabrication of GS, CCG and GO films
The glassy carbon (GC, 3 mm in diameter) electrodes were
polished subsequently with 1.0, 0.3 and 0.05 μm alumina
slurry, and then sonicated in water for several times. To prepare
GO, CCG and GS-modified GC electrodes, an aliquot of 3 μl
of 2.1mgml−1 GO, CCG and GS aqueous solution was coated
on the clean GC electrode with a microsyringe, respectively.
Then they were dried in air before use.
Instruments and measurements
TEM image was taken with a JEOL 2000 transmission
electron microscope operating at 200 kV. XPS analysis was
carried out on an ESCALAB MK II x-ray photoelectron
spectrometer. Raman spectra were collected using a Renishaw
2000 system with an argon ion laser (514.5 nm) and chargecoupled
device detector. CVs were done with a CHI 660
electrochemical workstation (CHI, USA) with a conventional
three-electrode electrochemical cell. EIS was performed using
a Solartron 1255B Frequency Response Analyzer (Solartron
Inc., UK) with a three-electrode system too. Capacitance
values were calculated for the CV curves by dividing the
current by the voltage scan rate, C = I /(dV/dt). The
specific capacitance reported is the capacitance for the carbon
material of one electrode (specific capacitance = capacitance
of single electrode/weight material of single electrode). The
square resistance was measured under ambient conditions by a
standard four-probe method. A Keithley 2400 source meter and
a Keithley 2000 multimeter (Keithley Instruments, Cleveland,
OH) were connected to the sample. Four electrode contacts
with an inter-electrode spacing of 1.0 mm were formed on a
3 mm thick sample with 12 mm diameter. The sample was
made by the squash method at 40 MPa pressure.
Results and discussion
Figures 1(A) and (B) display the TEM images of GS
nanocomposites at low and high magnification. It is evident
that almost all graphene sheets are separated from each other
and coupled by SnO2 nanoparticles. On comparison with
the TEM image of CCG (figure 1©), the surface of GS
nanocomposites is much rougher than that of CCG, which
might be attributed to the growth of SnO2 nanoparticles
on graphene sheets.
[attachment=40038]
Abstract
A one-step method was developed to fabricate conductive graphene/SnO2 (GS) nanocomposites
in acidic solution. Graphite oxides were reduced by SnCl2 to graphene sheets in the presence of
HCl and urea. The reducing process was accompanied by generation of SnO2 nanoparticles.
The structure and composition of GS nanocomposites were confirmed by means of transmission
electron microscopy, x-ray photoelectron and Raman spectroscopy. Moreover, the
ultracapacitor characteristics of GS nanocomposites were studied by cyclic
voltammograms (CVs) and electrical impedance spectroscopy (EIS). The CVs of GS
nanocomposites are nearly rectangular in shape and the specific capacitance degrades slightly as
the voltage scan rate is increased. The EIS of GS nanocomposites presents a phase angle close
to π/2 at low frequency, indicating a good capacitive behavior. In addition, the GS
nanocomposites could be promisingly applied in many fields such as nanoelectronics,
ultracapacitors, sensors, nanocomposites, batteries and gas storage.
Introduction
Interest in understanding properties of graphene has led to
many theoretical and experimental efforts worldwide, due to its
prominent properties and broad potential applications in many
fields [1]. However, graphene sheets, unless well separated
from each other, tend to form irreversible agglomerates or even
restack to form graphite through van der Waals interactions.
Currently, some chemical [2–5] and physical [6, 7] methods
have been used to produce freestanding graphene sheets.
However, all previous graphene sheets were reduced by
alkaline reagents in aqueous solution and only dispersed well
in alkaline solution. Aggregation would occur with any change
of the condition of the solution, such as addition of salts or
acids [2, 8–10]. In contrast, many other materials are unstable
in an alkaline environment. This restricts the synthesis of many
hybrid graphene materials and application fields of graphene
sheets.
Fabrication of GS, CCG and GO films
The glassy carbon (GC, 3 mm in diameter) electrodes were
polished subsequently with 1.0, 0.3 and 0.05 μm alumina
slurry, and then sonicated in water for several times. To prepare
GO, CCG and GS-modified GC electrodes, an aliquot of 3 μl
of 2.1mgml−1 GO, CCG and GS aqueous solution was coated
on the clean GC electrode with a microsyringe, respectively.
Then they were dried in air before use.
Instruments and measurements
TEM image was taken with a JEOL 2000 transmission
electron microscope operating at 200 kV. XPS analysis was
carried out on an ESCALAB MK II x-ray photoelectron
spectrometer. Raman spectra were collected using a Renishaw
2000 system with an argon ion laser (514.5 nm) and chargecoupled
device detector. CVs were done with a CHI 660
electrochemical workstation (CHI, USA) with a conventional
three-electrode electrochemical cell. EIS was performed using
a Solartron 1255B Frequency Response Analyzer (Solartron
Inc., UK) with a three-electrode system too. Capacitance
values were calculated for the CV curves by dividing the
current by the voltage scan rate, C = I /(dV/dt). The
specific capacitance reported is the capacitance for the carbon
material of one electrode (specific capacitance = capacitance
of single electrode/weight material of single electrode). The
square resistance was measured under ambient conditions by a
standard four-probe method. A Keithley 2400 source meter and
a Keithley 2000 multimeter (Keithley Instruments, Cleveland,
OH) were connected to the sample. Four electrode contacts
with an inter-electrode spacing of 1.0 mm were formed on a
3 mm thick sample with 12 mm diameter. The sample was
made by the squash method at 40 MPa pressure.
Results and discussion
Figures 1(A) and (B) display the TEM images of GS
nanocomposites at low and high magnification. It is evident
that almost all graphene sheets are separated from each other
and coupled by SnO2 nanoparticles. On comparison with
the TEM image of CCG (figure 1©), the surface of GS
nanocomposites is much rougher than that of CCG, which
might be attributed to the growth of SnO2 nanoparticles
on graphene sheets.