Chapter 12 Graphene-based transparent
thin films and nanocomposites for
energy storage
12.1 Preparation of transparent conductive thin films
12.2 Large area flexible transparent conductive thin films
12.3 Flexible transparent conductive thin films for electrode
12.4 Graphene-based nanocomposites for energy storage
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12.1 Preparation of transparent conductive thin films[12-1]
Although carbon films possess high hardness, conductive carbon coatings do
not provide sufficient optical transparency and often have poor adhesion to
substrates. Either additional metal interlayers, periodic multilayers of carbon, or
pretreatment of substrates with ion implantation is required to promote adhesion
of the carbon films to the substrates. Moreover, the metal oxide coatings are
susceptible to ion diffusion from the metal oxide films into the substrates, which
can be unfavorable for long term device performance.
Attempts to exfoliate graphene-based sheets by intercalation of graphite with
potassium metal have been discussed.
GO prepared via the Hummers method was exfoliated in a water/ethanol
mixture to produce a stable suspension of individual graphene oxide sheets.
Addition of TMOS into this dispersion yielded graphene oxide-containing sols that
can be stored at room temperature for several days (weeks for the sols with high
weight percentage of graphene oxide (11 wt %)). Thin composite films can be
then prepared from these sols by spin-coating onto hydrophilic substrates of
either borosilicate glass or SiOx/silicon. Solvent evaporation leads to quick
gelation, and the resulting composite films
2
TMOS: Tetramethyl orthosilicate is the chemical compound with the formula
Si(OCH3)4. This molecule consists of four methyl groups.
Si(OCH3)4 + 2 H2O → SiO2 + 4 CH3OH
TEOS: Tetraethyl orthosilicate (Less toxic as compared with TMOS)
Prior to spin-coating, substrate surfaces were treated with an oxygen plasma
in order to increase their hydrophilicity. This step helps improve the adhesion of
the sol during spin-coating deposition.
During the spin-coating process, composite sols experience a combination
of in-plane centrifugal and counter-balancing viscous forces on the rotating
stage, and we suggest this leads to stretching and flattening of graphene oxide
sheets that might be to some degree crumpled/wrinkled in the colloidal
suspension.
3
During spinning, the thickness of the films increased slightly with increasing
graphene oxide concentration (Figure 2d). In all cases, the thickness of the films
decreased after the high-temperature curing step, indicating densification and
consolidation of the matrix.
TEM images of the cross sections of the sol-gel
derived composite films with (a) 0 wt %, (b) 11 wt
% of graphene oxide before the H.T.curing, and (c)
11 wt % of graphene oxide after the H.T. curing.
The layers are, from the bottom, the glass
substrate, composite film, Pt layer, and carbon
layer. (d) Film thickness of the same samples
after curing obtained from both TEM and XRR
and the surface roughness obtained from XRR4
XRR：X-ray reflectivity (thickness) before and after curing.
Peak
appear
I: The non-oxygenated C at 284.8 eV,
II. The carbon in C-O at 286.2 eV,
III. The carbonyl carbon
(C=O) at 287.9 eV
IV. The carboxylate carbon
(O-C=O) at 289.0 eV.
Pure
silica
Peak
disappear
The C 1s XPS spectra of the hydrazine-treated
film shows the presence of the same
functionalities (Figure 3b,c) but with much smaller
contribution of the oxygenated carbons (27.6% vs
81.8%),
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The lowest measured conductivity
(resistance <1 GΩ) could be observed
at loadings as low as 3.9 wt % of
graphene oxide. As a control, pure
silica film that were exposed to hydrazine
and cured showed no conductivity.
The chemical reduction step appears to be essential for converting insulating
graphene oxide sheets into conductive graphene-like sheets and inducing
electrical conductivity in composite samples.
Films subjected to curing without chemical reduction were simply nonconductive.
Chemical reduction alone renders the uncured hydrazine-treated films sufficiently
conductive to be measured with our testing system but only at higher loading levels of
the graphene oxide filler. ((1.1±0.1) ×10-3 S/cm at 11 wt % to (7.0±0.7) ×10-5 S/cm at
9.1 wt %,
The combination of chemical reduction and high-temperature treatment improved
the overall conductivity of the samples. Presumably, the consolidation of the film
upon curing increases the density of the graphene-based sheets inside the matrix,
reducing the average intersheet distances (change of film thickness) and resulting6
in more pathways for electrical conduction.
However, even at the highest loading (11 wt %), the transmittance is consistently
high, ranging from 0.94 to 0.96 in the wavelength range of 380-1000 nm. The
transparency of the graphene oxide-silica composite is further reduced after
chemical reduction and curing (Figure 5b), primarily due to the “graphenization” of
the nanofiller, and the transparency at 650 nm drops at most by only 4% after
chemical reduction and curing.
11wt % graphene-silica composite spun-cast films 0.45 S/cm.
The bulk conductivity of ITO films reported in the literature is 1×104 S/cm.
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Preparation of transparent, conductive films by graphene nanosheet deposition
on hydrophilic or hydrophobic surfaces through control of the pH value [12-2]
GNS treated with N2H4 or NaBH4 precipitated as a hydrophobic surface, which is
insoluble in water and organic solvents, resulting in further processing difficulties.
The problem can be solved by treating the GNS with alkaline materials. Treated by
alkaline materials, the surface of the GNS is charged because the carboxylic acid
functional groups (–COOH) formed carboxylate anion (–COO), which may also
enable the formation of well-dispersed GNS colloids.
Use sodium borohydride (NaBH4) to reduce graphene oxide to graphene
nanosheets (GNS), which contain the carboxylic functional group that becomes
carboxyl (–COOH) or carboxylate anion (–COO)-type in the acid or alkaline
environment, respectively. The GNS with didodecyldimethylammonium bromide
(DDAB) particles becomes hydrophilic (A-GNS) or hydrophobic (B-GNS) through
control of the pH value, which can be dispersed efficiently in water or a water/THF
medium to deposit on the hydrophilic (poly(acrylic acid-acryl amide) PAA-AAM) or
hydrophobic (polystyrene) substrate for preparing the transparent, conductive film
(TCF) by spin coating.
-COOH
PAA-AAA
A-GNS
in acid
substrate
+ NaBH4
Natural
GNS
+ DDAB
graphite
(-COO)- in
Polystyrene 8
B-GNS
alkaline
substrate
By placing surfactant particles in the GO solution before reduction, the
reduced GO still remain exfoliated and disperse in an aqueous medium.
The GNS can also be dispersed in a polar organic solution. When the pH value of
the GNS solution is controlled to approximately 9, thecarboxylic acid (–COOH)
becomes a carboxylate anion (–COO), which is negatively charged. The cationic
surfactant is added into the solution, which may attract the negative charged
functional group on the GNS, and the hydrophobic chains of the surfactant can then
enable the GNS to disperse in organic solvent. However, the surfactant particles
remain on the surface of the GNS, and obstruct the transmission of electrons and
reduce electrical conductivity. To solve this problem, the surfactant particles can be
removed after HNO3 treatment.
When using spin coating to prepare TCFs, the surface of the substrate must
be treated by chemical materials to allow the surface, which becomes hydrophilic
or hydrophobic, to ‘‘catch’’ the solution of GNS.
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GNS with DDAB particles forms A-GNS or B-GNS through control of pH value,
enabling GNS disperse in water or water/THF medium to deposit on the
PAA-AAM or PS substrate for preparing the TCF
10
Before using NaBH4 to reduce GO,
NH4OH was used to treat the GO solution,
which sets the pH value of the GO solution
to approximately 9. The carboxylic functional
groups remain on the GO surface after
reduction. The carboxylic functional groups
with a pH value of approximately 9 supply
the negative charges (COO-) on the GNS
surface to be mixed with the cathode
surfactant (DDAB).
The hydrophilic heads of DDAB carry
positive charges, which can attract the
carboxylic functional groups, and the
hydrophobic tail of DDAB is then oriented
towards the water/THF phase.
The C1s peak in the XPS spectrum
of (a) GO and (b) GNS.
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When GO was reduced to the GNS, the large interlayer spacing no longer
existed, and an intense steep peak of GO will shift back to the graphite type,
due to the vanishing of the functional groups at the surface.
GO
A-GNS
Fig. 3(b) demonstrates that the inclusion of
DDAB within the interlayer spaces may separate
the sheets, preventing the GNS from aggregating.
After reduction, the sharp peak of GO was no
longer present and does not shift back to the
graphite type, and either becomes weak or
disappears
B-GNS
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For the GO, the G-band becomes broad and up-shifts to 1594 cm1 because
the resonance at frequencies of the isolated double bonds is higher than that of
Graphene.
For few-layer GNS, the ID/IG ratio is
related to the in-plane crystallite size (La),
which is given as La =4.4 (IG/ ID). The
La value of GO can be obtained from
the Raman spectrum that is 5.06
The ID/IG ratio of B-GNS, which is 1.08
(La=4.07), is higher than that of GO due to
The presence of unrepaired defects that
remain after the removal of large number
of oxygenic functional groups.
La:
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The static contact angles of water
droplet on the (a) PAA-AAM,
(b) A-GNS, (c) HA-GNS, (d) PS,
(e) B-GNS and (f) HB-GNS film
surfaces.
Sheet surface resistance and (b) transmittance
varies with different cycles of spin coating.
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Transparent and conductive thin films of graphene/polyaniline nanocomposites
prepared through interfacial polymerization [12-3]
Excellent transparent and conductive thin films can be obtained using polyaniline
as matrix and graphene as filler prepared based on an interfacial polymerization.
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Polyaniline is especially attractive because it is relatively inexpensive, has three
distinct oxidation states with different colors and has an acid/base doping response.
This latter property makes polyaniline an attractive for acid/base chemical vapor
sensors. The different colors, charges and conformations of the multiple oxidation
states also make the material promising for applications such as actuators,
supercapacitors and electrochromics. (from wiki)
Attractive fields for current and potential utilization of polyaniline is in antistatics,
charge dissipation or electrostatic dispersive (ESD) coatings and blends, (EMI),
coatings, hole injection layers, transparent conductors, ITO replacements, actuators,
chemical vapor and solution based sensors, electrochromic coatings (for color
change windows, mirrors etc.), PEDOT-PSS replacements, toxic metal recovery,
catalysis, fuel cells and active electronic components such as for non-volatile
memory. (from wiki)
The majority of the reports on the graphene/polyaniline nanocomposites take
advantage of the in situ polymerization of aniline over a graphene (or graphite
oxide) dispersion as the synthetic approach.
The graphene prepared in their work is easily dispersed in toluene.
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Synthesis of the nanocomposites:
1. 0.63 mg of graphene were dispersed in 20 mL of a toluene solution of aniline.
Different graphene/aniline ratios have been used, by fixing the graphene
amount and varying the amount of aniline (2.5; 5; 10; 20 and 60 mL,
corresponding to graphene/aniline weight ratios of 1/4; 1/8; 1/16; 1/32
and1/100, respectively).
2. The resulting mixture was transferred to a 50 mL round-flask containing 20 mL
of a 1 mol L1 HCl aqueous solution in which a suitable amount of ammonium
persulfate was previously dissolved, according to the starting amount of
aniline.
The magnetic stirring was subsequently interrupted and immediately a
continuous, self-standing, homogeneous and transparent film was spontaneously
formed at the interface. The film was transferred from the water/oil interface and
deposited on glass substrates.
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The spectrum of the neat polyaniline presents the
following PANI-ES bands: a shoulder at
1640 cm-1 (cross-linked chains),
1623 cm-1 (nC–C of the benzene rings),
1580 cm-1 (nCQC of the quinoid rings),
1517 cm-1 (nCQNH+ of the quinoid protonated di-imine
units),
1485 cm-1(nCQN of the quinoid non-protonated diimine units),
1337 and 1318 cm-1 (nC–N+, characteristic band of the
polaron radical cation),
1255 cm-1 (nC–N of benzenoid and quinoid rings),
1186 (sh) and 1168 cm-1 (C–H bending of the
benzenoid and quinoid rings, respectively).
Raman spectra of the films:
(a) neat polyaniline;
(b) GR/PANI-1/32;
(c) GR/PANI-1/16;
(d) GR/PANI-1/8;
(e) GR/PANI-1/4;
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(f) neat graphene.
Emeraldine PANI salt
(PANI-SE)
The emeraldine (n = m = 0.5) form of
polyaniline, often referred to as
emeraldine base (EB), is neutral, if
doped (protonated) it is called
emeraldine salt (ES).
The bands associated to the oxidized
portions of polyaniline chains (quinoid
rings and imine nitrogen, associated to
the bi-polaron carrier in PANI-ES) are
strongly modified by the presence of
graphene, which can be an indicative
that the interaction between the
graphene and the polymer occurs
through these segments:
(i) the band of polyaniline at 1168 cm-1 is
red shifted to 1160 cm-1 by the presence
of graphene. This shift has been usually
associated to an increase in the bipolaron segments on the polymer chain;
(ii) the bands at 1485 and 1517 cm-1 collapse to one single and broad band that is
red shifted according to the increase in the amount of graphene (1480, 1472, 1464
and 1464 cm-1 for samples PANI/GR 1/32, 1/16, 1/8 and 1/4, respectively);
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(iii) the band at 1255 cm1 is red shifted to 1218
cm-1.
The low-frequency region of the Raman spectra
(200–1000-1) is very sensitive to the structure and
conformation of the polyaniline chains. As showing in
Fig. 2, right, the polymer bands become broader and
less definite in the presence of graphene, showing
less uniformity in the polymer structure.
There are some indicatives that the polymer chain
becomes less planar (more coiled) in the presence
of graphene.
(1) The band at 294 cm-1 present in the neat polyaniline
is characteristic of a pseudo-orthorhombic crystalline
structure of the polymer, which is not observable in
the nanocomposites.
(2) The bands at 810, 835 and 873 cm-1 are attributed to out-of-plane C–H motions
and are very sensitive to the torsion angle between two aniline rings.
(3) All three modes are clearly observed in the neat PANI and almost disappear in
the samples containing graphene, corroborating the model in which the
graphene induces the occurrence of less-planar polyaniline chains.
(4) The intensity of the cross-linked band at 574 cm-1 decreases in the presence of
graphene (and the band at 1640 cm-1 disappears), which indicates that the
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cross-linking degree of the samples decreases in the presence of graphene.
The interpretation of the Raman data indicates that apparently the interaction
between the graphene and PANI-ES occurs through the oxidized segments of
the polymer, inducing a more bipolaronic and less planar structure of the PANI.
It is well known that to be used in optoelectronic devices transparent electrodes
must have a sheet resistance lower than 100 Ω sq-1 coupled with optical
transmittance at 550 nm of approximately 90%. The best sample (GR/PANI-1/32)
presented a sheet resistance of 60.6 Ω sq-1 and transmittance of 89%.
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References
12-1. Nano Letters, Watcharotone 2007 7 721888
12-2. J of Materials Chemistry, Tien 2012 22 2545
12-3. ChemComm, Dominggues 2011 47 2592
12-4. Applied Physics Letters, Kim 2011 98 091502
12-5. ACS nano, Zheng, 2011 5 7 6039
12-6. Nature Nanotechnology 2010 5 574
12-7. Applied Physics Letters, Kobayashi 2013 102 023112
12-8. Nano Letters, Wang 2008 8 323
12-9. Chemistry of Materials 2010 22 1392
12-10.Energy Environ. Sci., Pumera 2011 4 668-674
12-11.Nano Letters, Li 2009 9 12 4359
12-12.Nanoscale, Nguyen 2011
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