Chemical synthesis through oxidation of graphite[9-9]
I-4
(I) The Hummers Method
1.Natural graphite flake (325 mesh) was mixed with H2SO4.
2.Keep stirring in an ice-water bath.
3.Addition of KMnO4 and keep stirring at room temperature.
4.Pour DI water and H2O2.
5.Placed over night.
6.Diluted using centrifugation until neutral (pH 7).
(II) Reduction*
1.Dilute the concentrated graphite oxide solution to 500 mL
2.Addition of reducing agent (NaBH4) in GO mixture**.
3.Reaction (Keep heating at >120 ℃ and stirring for over 5 days.)
II-2
I-5
(III) Post-treatment
1.Filtered and washed with DI water until the filtration approached neutral
2.Dry the product and then grind it.
Advantages: Large scale production of few layer graphene sheets
Drawbacks: Defects on graphene sheets are inevitable
III-2
1
Stankovich et al. proposed the following mechanism for reduction of graphene
oxide using hydrazine. Reduction of graphene oxide restores electrical conductivity.
However, significant oxygen content remains: C/O~ 10/1.
*Reduction method
Thermal or chemical reduction have been used to convert insulating GOs to
conducting graphene-like layers. Thermal reduction has been highly effective
in producing graphene-like films with a C:O ratio of up to 9 and minimal defect
formation ※.
Chemical reduction is very simple, but it usually generates graphene-like
film exhibiting a relatively low C:O ratio and a considerable amount of
residual functional groups, resulting in a highly resistive film.
An alternative chemical reduction is dehydration of the hydroxyl groups on
graphene oxide in water at high pressure and temperature, 120-200 C.
Aluminum powder appears to catalyze this process in an acidic condition.(9-1)
2
2
**Adoption of NaBH4 and N2H4:
When N2H4 was used, nitrogen atoms behaved as donors compensating
p-type hole carriers in reduced graphite oxide.
In the case of NaBH4 reduction, the interlayer distance is first slightly
expanded by the formation of intermediate boron oxide complexes and then
contracted by the gradual removal of carbonyl and hydroxyl groups along with
the boron oxide complexes
The sheet resistance of graphite oxide film reduced using sodium boro hydride
(NaBH4) is much lower than that of films reduced using hydrazine (N2H4).
3
Processes of oxidation and reduction for graphene
(B)
(C)
(A) Graphite (3.38 A)
↓
(B) Graphite oxide (GO, 8.27 A)
↓
(C) Reduced GO (15 mM, 9.31 A)
(D)
(E)
(A)
↓
(D) Reduced bGO (50 mM, 9.72 A)
↓
(E) Reduced GO (150 mM, 3.73 A) : most
of the functional groups were removed,
4
9.6 Thermal exfoliation and reduction [9-1]
Thermally reduced graphene oxide (TRG) can be produced by rapid heating of
dry GO under inert gas and high temperature. Heating GO in an inert environment
at 1000 C for 30 s leads to reduction and exfoliation of GO, producing TRG sheets.
Exfoliation takes place when the pressure generated by the gas (CO2) evolved
due to the decomposition of the epoxy and hydroxyl sites of GO exceeds van der
waals forces holding the graphene oxide sheets together. About 30% weight loss is
associated with the decomposition of the oxygen groups and evaporation of water.
The exfoliation leads to volume expansion of 100-300 times producing very low-bulk
density TRG sheets (Figure 5d)
Because of the
structural defects
caused by the
loss of CO2, these
sheets are
highly wrinkled as
shown in Figure
5e.
Fig. 5 e
Fig. 5 d
5
80% of the TRG sheets are single layers with an average size of about 500 nm
independent of the starting GO size. The advantage of the thermal reduction
methods is the ability to produce chemically modified graphene sheets without the
need for dispersion in a solvent.
TRG has C/O ratio of about 10/1 compared to 2/1 for GO. This ratio has been
increased up to 660/1 through heat treatment at higher temperature (1500 C)
or for longer time. TRG sheets have high surface area, 1700 m2/g, as measured in
methylene blue and can be well dispersed in organic solvents such as
N,N-dimethylformamide (DMF) and tetrahydrofuran (THF).
6
9.7 Electrolytic exfoliation [9-2]
DC bias
Graphite rod
Facility:
1.Power supply
Materials:
Graphite rod
Electrolyte
Primary advantages and drawbacks
A: Simple
D: Dispersions are difficulty to remove
1.High purity graphite rods
2.Poly(sodium-4-styrenesulfonate)(PSS)
3.De-ionized (DI) water
Processes:
1.Apply constant potential of 5V
2.Dispersion subjected to centrifuge
3.Wash with DI water and ethanol
4.Dried to make powder
5.Vacuum filtration using AAO to obtain
graphene paper
Mechanism:
When PSS dissolved in water, it will
dissociate into Na+ cations and polystyrenesulfonate anions. During the electrolysis
process, polystyrenesulfonate anions were
forced to move to the positive graphite
electrode under electric force and interact
with graphite, leading to the electrolysis7
exfoliation of the graphite rod.
9.8 Characterization
9.8.1 X-ray diffraction
9.8.2 FTIR
9.8.3 Raman
9.8.4 AFM
9.8.5 FE-SEM, TEM and HRTEM
9.8.6 X-ray photoemission spectroscopy (XPS)
8
9.8.1 X-ray diffraction (9-14, 9-16)
(B)
(C)
(D)
(E)
(A)
(A) Graphite (3.38 A)
(B) Graphite oxide (GO, 8.27 A)
XRD spectrum shows
that the interlayer distance
(C) Reduced GO (15 mM, 9.31 A)
of the r-GO film is
decreased to 3.57A° (2h = (D) Reduced bGO (50 mM, 9.72 A)
24.4) from 8.10A° (2h =
(E) Reduced GO (150 mM, 3.73 A) :
10.9) for the original GO
most of the functional groups
9
film.(9-14)
were removed.
The d-spacing of graphite sharp feature peak (002) at
26.38 is 3.38A°. The GO feature peak at 9.26, whose
corresponding d-spacing is 9.55A°, disappears when
the reducing temperature increases above 80 C, which
indicates that most of the oxygen functional groups,
having marked effects on the d-spacing, have been
reduced above 80C (9-16).
The feature peaks (002) of the RGO reduced for 0.25–3 h
almost have no shift, which indicates that the content of
carbonyl and epoxy in GO or RGO is the main factor that
affects the d-spacing values of GO and RGO, combined
with the changes of the relative contents of carbonyl
(C=O), epoxy (C–O–C), and hydroxyl (C–OH) bonds in
GO and RGO (9-16).
10
9.8.2 FTIR
The peaks at 1060, 1186, 1226, 1290,1720, 1640, 1620, 1566, and 1393 cm-1
are assigned to the stretching vibration of C–O (alkoxy), phenolic OH, C–O (epoxy),
C–OH bending, C=O, aromatic C–C, Ph–CO, deformed C–C, C–OH, respectively.
The deformed C–C stretching vibration at 1566 cm-1 is due to the presence of the
neighboring epoxy groups. The peaks (1060–1290 cm-1) corresponding to oxygen
functional groups dramatically decrease with increasing the reducing temperatures
from 80 to 140 C in Fig. 2a and with increasing the reducing time from 0.25 to 3 h in
Fig. 2c. The C=O peak at 1720 cm-1 in Fig. 2b disappears when the reducing
temperature is above 55 C, while the Ph–CO and C–OH peaks in Fig. 2b have no
obvious changes at different reducing temperatures. In addition, the C=O peak in Fig.
2c completely disappears until the reducing time reaches 3 h. The deformed C–C
peak at 1566 cm-1 in Fig. 2c decreases with increasing the reducing time and the
aromatic C–C at 1640 cm1 simultaneously increases, indicating that the GO is 11
gradually reduced into graphene. (9-16)
9.8.3 Raman: Non-destructive technique to characterize graphite materials in
particular to determine the defects, ordered and disordered structure of graphene.
Excited state(激發態)
Vibrated state(振動態)
Ground state(基態)
Raman scattering or the Raman effect is the inelastic scattering of a photon.
When photons are scattered from an atom or molecule, most photons are
elastically scattered (Rayleigh scattering), such that the scattered photons have
the same kinetic energy (frequency and wavelength) as the incident photons.
However, a small fraction of the scattered photons (approximately 1 in 10 million)
are scattered by an excitation, with the scattered photons having a frequency
different from, and usually lower than, that of the incident photons. (Wikipedia)
12
In Wang’s work[9-3], they proposed a mild exfoliation-reintercalation expansion
method for forming high-quality GS with higher conductivity and a lower degree of
oxidation than GO. Here we present a 180 °C solvothermal reduction method for
our GS and GO. The solvothermal reduction is more effective than the earlier
reduction methods in lowering the oxygen and defect levels in GS, increasing the
graphene domains, and bringing the conductivity of GS close to that of pristine
graphene. The reduced GS possess the highest degree of pristinity among
chemically derived graphene.
D: Defect peak due to intervalley scattering
G: Graphene G peak
D’: Defect peak due to intervalley scattering
2D: Overtone of D peak
S3:Second-order peak due to D-G combination
Wang JACS 2009 9910
13
The D/G intensity
ratios increase from GO
to the RGO reduced at
100 C, decrease for the
RGO reduced from 100
to 140 C, and increase
again for the RGO
reduced from 140 to 150
C.
The first increase stage of the D/G intensity ratios is attributed to an
increase in the number of small crystalline graphene domains, the next
decrease is due to an increase of the average size of the crystalline graphene
domains with increasing the reducing temperatures, and the last increase of
the D/G intensity ratios at 150 C is resulted from the decrease of reducing
ability of NaBH4 with Anhydrous AlCl3 because of over high reducing
temperature.
14
9.8.4 AFM (9-2, 9-6)
Contact mode (contact between probe and surface)
Non-contact mode (van der Waals force, signal amplified)
Tapping mode (reduce distance between probe and
surface, enlarge amplitude, probe contact surface at the
valley of vibration)
A zoomed image of graphene flakes. Below the
image is a line scan taken horizontally through
the image as marked with a red line, from which
the height of a small graphene flake and a large
graphene flake were determined to be about 0.8
nm, indicating the monolayer graphene sheet.
15
AFM images of spray deposited graphene flakes. (9-2)
9.8.4 AFM (9-2, 9-6)
(a)-(c) OM image of Cu foil surface after being annealed at 990 C, 80 mbar for 20
min, and it can be observed that there are a lot of ‘‘polishing marks’’ on the Cu
surface even after the annealing at 990 C. (9-6)
(d)-(f)low pressure annealing could greatly enhance the uniformity of Cu surface
and decrease the number of the sharp structures, thereby making the Cu surface
smoother. (9-6)
16
9.8.5 FE-SEM, TEM and HRTEM[9-6]
(a) TEM image of CVD bilayer graphene
(area containing the purple ring) on Cu
grid. (b) Electron diffraction patterns
taken within the purple ring in (a).
(c) High resolution transmission electron
microscopy (HRTEM) image of bilayer
graphene. Red lines are along the
direction of the carbon lattice atoms,
which also indicates the existence of
different orientations. (9-6)
17
Fig. 3a shows a FESEM
image of the graphene
nanosheets. The morphology of
individual graphene sheets
resembles waves in a crumpled
silk veil. The graphene sheets
look transparent under the
electron microscope. (9-2)
Fig. 3b shows a low magnification
TEM image of graphene sheets.
Most of the graphene sheets are
stacked multilayers. These
graphene sheets are rippled and
entangled. (9-2)
Well diluted graphene dispersion was also prepared for TEM analysis. Fig. 3c shows a
TEM view of a few flat graph ne sheets in larger sizes (a few square micrometers), in
which about 2–3 layers of graphene overlap. Selected area electron diffraction (SAED)
was performed on the graphene sheets and the corresponding SAED pattern is shown
as the inset in Fig. 3c. (9-2)
18
9.8.6 X-ray photoelectron spectroscopy* (9-13)
X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique
that measures the elemental composition, empirical formula, chemical state and
electronic state of the elements that exist within a material. XPS spectra are obtained
by irradiating a material with a beam of X-rays while simultaneously measuring the
Kinetic energy and number of electrons that escape from the top 1 to 10 nm of the
material being analyzed. XPS requires ultra-high vacuum (UHV) conditions.*
19
C-O, hydroxyl and epoxy (~286.5 eV)
C=O, carbonxy (~288.3 eV)
C=C/C-C (284.6 eV)
O-C=O, carboxyl (~290.3 eV)
Boron oxide complexes were
visible in films treated with
15mM NaBH4
15 mM: carboxyl groups were
partially removed
50 mM: all of the carbonyl
groups were nearly removed
and CO bonds were reduced .
150 mM: No significant
reduction of the oxygenrelated functional groups. 20
The UV-vis spectra of GO and three RGO films showed the absorption peak of GO
around 230nm gradually red-shifting towards 260nm in films treated with higher
concentrations of NaBH4. The peaks of GO at 300 and 360nm evidently disappeared.
This indicated the formation of highly conjugated structure like that of graphite.
21
Conductivity relies on C:O ratio as a function of the molar concentration of
NaBH4. The behavior of the conductivity and C:O ratio were very similar to each
other the NaBH4 concentration.
Conductivity of the film was directly related
to the oxygen content. Complete removal of
residual oxygen content even when using
high NaBH4 concentrations was the key
factor in further reducing the sheet resistance.
The presence of oxygen atoms also affects
the electronic density of states, as shown in
Figure 4b.
22
* Wikipedia
9-1. Macromolecules, Kim 2010 43 6515 (also 10-5)
9-2. Carbon, Wang 2009 3242
9-3. JACS, Wang 2009 9910
9-4. Nature Nanotechnology 2009 4 30
9-5 Science 2004 306 666
9-6 Carbon, Wei Liu 2011
9-7 Nature, Kim 2009
9-8 Science, Berger 2006 312 1191
9-9 Carbon, Ma 2011 49 1550
9-10 NTT Technical Review, Hibino 2010 8 8 1
9-11 Progress in Materials Science, Singh 2011 56 8
9-12 Nature Materials, Emtsev 2009 8 203
9-13 PRL, Ferrari 2006 97 187401
9-14 Advanced Functional Materials, Shin 2009 19 1987
9-15 Carbon Pei 2010 4466
9-16 Carbon Li 2011 3024
9-17 Macromolecular Chemistry and Physics, Du 2012 213 1060 (also 10-6)
9-18 Carbon Stankovich 2007 45 1558–1565
23