Chemical synthesis through oxidation of graphite[9-9]

1
Chemical synthesis through oxidation of
graphite9-9
I-4

• (I) The Hummers Method
• Natural graphite flake (325 mesh) was mixed with
  H2SO4.
• Keep stirring in an ice-water bath.
• Addition of KMnO4 and keep stirring at room
  temperature.
• Pour DI water and H2O2.
• Placed over night.
• Diluted using centrifugation until neutral (pH
  7).

II-2
(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 gt120 ? and stirring
for over 5 days.) (III) Post-treatment 1.Filtered
and washed with DI water until the filtration
approached neutral 2.Dry the product and then
grind it.
I-5
Advantages Large scale production of few layer
graphene sheets
III-2
Drawbacks Defects on graphene sheets are
inevitable
1
2
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 CO 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 CO 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
3
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).
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4
Processes of oxidation and reduction for graphene
4
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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.
5
Fig. 5 e
Fig. 5 d
6
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
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9.7 Electrolytic exfoliation 9-2
Facility
1.Power supply
Materials
1.High purity graphite rods 2.Poly(sodium-4-styren
esulfonate)(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
polystyrene- sulfonate 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 electrolysis
exfoliation of the graphite rod.
Primary advantages and drawbacks
A Simple
D Dispersions are difficulty to remove
8
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
9.8.1 X-ray diffraction (9-14, 9-16)
XRD spectrum shows that the interlayer
distance of the r-GO film is decreased to 3.57A
(2h 24.4) from 8.10A (2h 10.9) for the
original GO film.(9-14)
10
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
0.253 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 (CO), epoxy
(COC), and hydroxyl (COH) bonds in GO and RGO
(9-16).
10
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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 CO (alkoxy),
phenolic OH, CO (epoxy), COH bending, CO,
aromatic CC, PhCO, deformed CC, COH,
respectively. The deformed CC stretching
vibration at 1566 cm-1 is due to the presence of
the neighboring epoxy groups. The peaks
(10601290 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 CO peak at
1720 cm-1 in Fig. 2b disappears when the reducing
temperature is above 55 C, while the PhCO and
COH peaks in Fig. 2b have no obvious changes at
different reducing temperatures. In addition, the
CO peak in Fig. 2c completely disappears until
the reducing time reaches 3 h. The deformed CC
peak at 1566 cm-1 in Fig. 2c decreases with
increasing the reducing time and the aromatic CC
at 1640 cm1 simultaneously increases, indicating
that the GO is gradually reduced into graphene.
(9-16)
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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(???)
Vibrated state(???)
Ground state(??)
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)
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In Wangs work9-3, 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 S3Second-order peak due to D-G combination
Wang JACS 2009 9910
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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.
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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.
AFM images of spray deposited graphene flakes.
(9-2)
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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)
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9.8.5 FE-SEM, TEM and HRTEM9-6
(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)
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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
23 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)
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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.
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C-O, hydroxyl and epoxy (286.5 eV) CO, carbonxy
(288.3 eV) CC/C-C (284.6 eV) O-CO, 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
oxygen- related functional groups.
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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.
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Conductivity relies on CO ratio as a
function of the molar concentration of NaBH4. The
behavior of the conductivity and CO 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.
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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 15581565
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