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Nano Res
1
Multifunctional organically modified graphene with super-hydrophobicity
Huawen Hu1, Chan C. K. Allan1, Jianhua Li1, Yeeyee Kong1, Xiaowen Wang1, John H. Xin1 (), and
Hong Hu1 () Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0408-0
http://www.thenanoresearch.com on January 4 2014
© Tsinghua University Press 2014
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Nano Research
DOI 10.1007/s12274-014-0408-0
1
TABLE OF CONTENTS (TOC)
Multifunctional organically modified graphene with
super-hydrophobicity
Huawen Hu, Chan C.K. Allan, Jianhua Li, Yeeyee
Kong, Xiaowen Wang, John H. Xin,* Hong Hu*
The Hong Kong Polytechnic University, Hong Kong,
China
Page Numbers. The font is
ArialMT 16 (automatically
inserted by the publisher)
A multifunctional organically modified graphene with super-hydrophobicity
has been synthesized by a novel one-step organic modification of a
low-temperature thermally functionalized graphene. Unique structural
topology is found to exist in the as-prepared low-temperature thermally
functionalized graphene, along with a portion of reactive oxygen
functionalities preserved (see Figure), which facilitates the subsequently
highly effective fabrication of an organically modified graphene derivative
with multifunctional applications in liquid marbles and polymer
nanocomposites.
2
Multifunctional organically modified graphene with
super-hydrophobicity
Huawen Hu1, Chan C.K. Allan1, Jianhua Li1, Yeeyee Kong1, Xiaowen Wang1, John H. Xin1 (), and Hong Hu1
()
1 Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hong Kong 999077, China
Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher) © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011
ABSTRACT In order to bring graphene materials much closer to real word applications, it is imperative to have simple,
efficient and eco-friendly ways to produce processable graphene derivatives. In this study herein, a hydrophilic
low-temperature thermally functionalized graphene and its super-hydrophobic organically modified graphene
derivative were fabricated. A unique structural topology and a part of oxygen functionalities were found to
exist on the thermally functionalized graphene surfaces, which facilitated the subsequently highly effective
organic modification reaction and led to the super-hydrophobic organically modified graphene with
multifunctional applications in liquid marbles and polymer nanocomposites. The organic modification reaction
could also restore the graphenic conjugation structure of the thermally functionalized graphene, particularly
for the organic modifier having longer alkyl chains, confirmed by various characterization techniques such as
electrical conductivity measurement, ultraviolet/visible spectroscopy and selected area electron diffraction. The
free-standing soft liquid marble was fabricated by wrapping a water droplet with the super-hydrophobic
organically modified graphene, which showed a potential value in micro-reactors. As for the polymer
nanocomposites, a strong interfacial adhesion was believed to exist between an organic polymer matrix and the
modified graphene because of the organophilic coating formed on the graphene base, which resulted in large
improvements in the thermal and mechanical properties of the polymer nanocomposites with the modified
graphene even at a very low loading level. A new avenue was therefore opened up for large-scale production of
processible graphene derivatives with various practicable applications.
KEYWORDS low-temperature thermally functionalized graphene, organic modification, organically modified graphene,
liquid marbles, polymer nanocomposites
Nano Res DOI (automatically inserted by the publisher) Research Article
———————————— Address correspondence to J. H. Xin, [email protected]; H. Hu, [email protected]
3
1 Introduction
Graphene, a two-dimensional carbon honeycomb
nanostructure, has attracted substantial attention in
various areas owning to its extraordinary
mechanical [1], thermal [2] and electrical [3]
properties. Unfortunately, the cost of graphene, its
availability and the challenges that remain to
achieve good dispersion pose significant obstacles
to realization of these superior properties. Various
methods have thus been explored to produce
graphene effectively, among which a well-known
and cost-effective graphite oxide (GO) exfoliation
has been considered the most promising approach
to mass-scale production of graphene materials.
Starting from naturally abundant graphite, GO can
be first prepared based on Staudenmaier [4], Brodie
[5], or Hummers and Offeman [6] oxidation method.
Reduced graphene derivative can be subsequently
prepared via various reduction strategies [7-9].
However, reduction of GO usually cannot result in
single-layer graphene because of the irreversible
agglomeration and restacking of the reduced
graphene which eventually lead to the graphene
precipitate. This is caused by the strong interplanar
π-π stacking and van der Waals interactions
between reduced graphene [10,11]. Many
modification approaches have thus been developed
for fabrication of soluble graphene such as
noncovalent modification through π-π interactions
[12], covalent sulfonation modification [13], and
chemical reduction under controlled conditions [14,
15].
Alternatively, the disturbing problem of
graphene agglomeration has been addressed first
through a low-temperature thermal reduction of
GO in the present study. This technique leads not
only to a thermally reduced and functionalized
graphene (TrG) with a buckled, folded and
wrinkled surface topology, as reported similarly in
the Ref. [9,16], which can prevent re-graphitization
of the graphene sheets by inhibiting layering of one
reduced graphene sheet onto another [9,17], but to a
portion of oxygen functionalities remained, which
can impart hydrophilicity and reactivity to the
resulting TrG. In addition, compared with the
chemical reduction, the technique is more
environmentally friendly because of the absence of
any harmful chemical reducer such as
commonly-used hydrazine and its derivatives. On
another aspect, the present strategy of thermal
reduction and functionalization of GO involves the
heating temperature of 400 oC which is much lower
as compared to the conventional thermal reduction
approach involving the temperature of more than
1000 oC [9,18-21]. This thereby indicates that the
present technique is of much lower energy
consumption.
Furthermore, by considering the fact that the
thermal exfoliation of GO is currently used for
industrial production of functionalized graphene
[22], organic modification of thermally
functionalized graphene accordingly shows a
significance for large-scale production of
organophilic graphene derivatives. Although
organically modified graphene can be prepared
starting from GO which has abundant reactive
oxygen functionalities on its surfaces [23-29], many
in-plane properties of GO is heavily impaired
during its harsh preparation process. These
sacrificed properties of the prepared organically
modified graphene oxide should thus be further
restored by reduction processing, e.g., using
chemical reducer such as hydrazine, sodium
borohydride and hydroquinone to chemically
convert the organically modified graphene oxide to
graphene, which thereby violates the environment
safety due to the harmfulness of these chemicals.
Fortunately, this problem can be avoided using the
present approach of organic modification of TrG.
On the other hand, a different and unique structure
is introduced to the organically modified graphene
by the initial low-temperature thermal
functionalization of GO as compared to the main
4
chemical reduction way [7,30], which might
facilitate the subsequent applications, e.g., the bent
structure formed can be favorable for wrapping
water droplet and yielding liquid marble. Moreover,
compared with some other reported approaches for
preparation of organically modified graphene, such
as ball-milling intercalation and hydrothermal
reduction and modification [31,32] which involves
complex preparation procedures with high-energy
consumption and long processing time, the
developed method herein is a highly promising
alternative for saving energy and enhancing
efficiency.
In the present study, low-cost alkylamines with
different alkyl chain lengths, namely dodecylamine
(DA) and octadecylamine (OA), were adopted for
the organic modification of TrG. As expected, both
DA- and OA-modified graphene sheets denoted as
DA-G and OA-G, respectively, showed
super-hydrophobicity. The free-standing soft liquid
marbles were thus fabricated by wrapping water
droplet with the modified graphene powder. These
as-made liquid marbles behaved like a solid and
showed dramatically reduced adhesion to a solid
surface (the liquid marbles could move around
freely on the solid surface using the gravitational
field), which thereby exhibits great potential for
microfluidic applications [33-35]. In addition,
overall higher organic modification efficiency could
be found in OA-G as compared to that in DA-G as
far as hydrophobicity was concerned. Furthermore,
given the expected strong interfacial adhesion
between the alkylamines grafted on the modified
graphene surfaces and organic polymer matrix, an
investigation of doping effect of the organically
modified graphene on a poly
(styrene-co-acrylonitrile) matrix was conducted in
the present study. The results demonstrate that
incorporation of the modified graphene with a very
low concentration can largely increase the glass
transition and decomposition temperatures of the
polymer matrix, particularly for the nanocomposite
system with OA-G, together with large
improvement in the mechanical properties
including Young’s modulus and tensile strength.
In summary, the developed environmentally
friendly efficient strategies for preparation of
low-temperature thermally functionalized graphene
and its organically modified graphene derivatives
has opened up new avenues to accelerate industrial
production and applications of graphene materials.
2 Results and discussion
The primary preparation procedure is presented in
Scheme 1, with proposed structure models of
graphene oxide, TrG and organically modified
graphene displayed. The structure features are also
labeled in the schematic diagrams (using dotted lines
in red for specific indication of the structure details
in the models). Besides, the SEM images showing the
surface morphologies of GO, TrG and OA-G are
presented beside the corresponding proposed
structures (the typical shape and morphology have
been highlighted by a transparent shade with color).
A feature of smooth and translucent layer with a
huge surface area can be indexed to GO from its
SEM image. As for TrG, the wrinkles, buckles and
folds are generated after the low-temperature
thermal functionalization of GO, with an overall bent
structure, as a result of the structural defect formed
during decomposition of thermally unstable
functional groups and release of gas such as carbon
dioxide. Concerning OA-G, a feature of fluffy and
smooth surfaces in a stack state can be seen, with a
similarly bent structure to that of TrG (but the
bending degree is smaller due to restoring
conjugation structure and removing defects). Note
that the surface wrinkles of TrG can no longer be
observed which can be due to the organic moieties
attached on the TrG surfaces which thus enshroud
these wrinkles. The schematic diagrams depicted for
showing the applications for the present system are
also presented. The modified graphene derivatives
show multifunctional applications in liquid marbles
5
(microreactor) and polymer nanocomposites based
on the success of the interfacial modification.
The chemical structure and basic solution
property of GO and TrG are revealed in Fig. 1. The
photo images showing their water dispersibility and
XPS spectra confirming a part of functional groups
retained on TrG are shown in Fig. 1 a-g. The yellow
brown color of GO dispersion turns to black after the
low-temperature thermal functionalization, implying
the restored sp2-hybridized carbon network (Fig. 1 a).
In addition, TrG shows good water dispersibility,
indicative of the sufficient functional groups left
behind which thus render TrG hydrophilic by their
hydrophilicity and electrostatic repulsion. A
schematic illustration is also shown in Fig. 1 a in
order to unravel the microscopic water dispersion of
TrG. Also, a comparison study was conducted for
comparing the water dispersibilities among TrG,
extremely high-temperature thermally reduced
graphene @1000 oC, and chemically reduced
graphene using hydrazine hydrate as the reductant,
with the result presented in the Electronic
Supplementary Material (ESM) (Fig. S1). It can be
found that TrG shows much better water
dispersibility as compared to its counterparts. This
result can be attributed to the portion of oxygen
functionalities remained on the TrG surfaces which
help to solubilize TrG by hydrophilicity and
electrostatic repulsion interactions. In contrast, the
functional groups on both the control samples,
namely hydrazine-reduced graphene and
high-temperature thermally reduced graphene, are
almost entirely removed, which is evidenced by the
FTIR spectra as shown in Fig. S2 (see detailed
description in ESM).
Concerning the XPS survey spectra of GO and
TrG as shown in Fig. 1 b and e, respectively, the C 1s
peak intensity relative to that of O 1s peak is greatly
increased for TrG (C/O ≈ 2.07) as compared to GO
(C/O ≈ 0.65), which clearly indicates that TrG
contains less oxygen-rich functionalities than GO,
but with a moderate quantity remained. In addition,
high-resolution XPS C 1s and O 1s core-level spectra
of GO (Fig. 1 c,d) and TrG (Fig. 1 f,g) further clarify
the details of chemical bonds in GO and TrG. In the
XPS C 1s spectra, there exist four main deconvoluted
peaks for GO, which are centered at 284.9 (C=C/C-C),
286.7 (C-O), 287.7 (C=O) and 288.8 eV (O=C-OH),
whereas the carbonyl and C-O peaks are largely
lowered for TrG. Nevertheless, a comparative
quantity of carboxyl (O=C-OH) and C-O groups exist
on TrG planes. The XPS O 1s spectra also reflect that
the O component of TrG primarily comes from
C-OH (533.2 eV) and O=C-OH (531.9 eV), while GO
mainly contains C-OH (hydroxyl), C-O-C (epoxy)
and O=C-OH (carboxyl). These results of XPS spectra
are in good agreement with other reports relating to
GO and graphene derivatives such as
edge-carboxylated graphene [36,37]. In addition to
the portion of functional groups retained, the
partially recovered in-plane electronic conjugation
structure of TrG can be revealed by the large increase
of the electrical conductivity (from about 10-5 S m-1 of
the electrically insulating GO to approximately 0.9 S
m-1 of electrically conducting TrG), as detailed in
ESM (Fig. S3).
The effect of organic modification of TrG is
illustrated in Fig. 2. The UV/vis characterization
results of the ethanol dispersions of GO, TrG, DA-G
and OA-G are shown in Fig. 2 a. As a well-known
absorption feature for GO, the band at ~227 nm can
be clearly observed which corresponds to the π→π*
transitions of aromatic C-C bonds [38] and can be
bathochromically shifted by conjugation [39-41]. The
typical absorption band was red-shifted to ~254 nm
as for TrG due to the increased conjugation, in line
with the electrical conductivity results. A further red
shift to ~259 nm can be seen for DA-G, indicating the
restoration of the graphenic plane by organic
modification with DA. In contrast, modification with
OA leads to a larger extent of red shift, up to ~261
nm, which implies that a stronger chemical
modification reaction has been achieved between OA
and TrG. As expected, these restorations of
6
graphenic conjugation structure contribute to the
further increase of the electrical conductivities of
DA-G and OA-G relative to that of TrG, with around
6- and 107-fold increase, respectively (see detailed
description in ESM, Fig. S3).
As shown in Fig. 2 b, a water droplet penetrates
and disturbs the compressed plate made of GO
powder once it contacts the plate, thus confirming
the abundant hydrophilic functional groups on the
GO planes, while the plate made of TrG shows an
improved hydrophobicity to a certain extent with a
water contact angle of about 105.6o (the water
droplet can still partially infiltrate into the plate) due
to the thermal removal of a part of functional groups
and reduction of hydrophilicity to a certain extent.
DA-G presents a clear hydrophobic nature, with a
contact angle of about 147.5o, resulting from the
hydrophobic alkyl chains grafted on the TrG surfaces.
As for OA-G, a further enhanced hydrophobicity
(with a contact angle of approximately 154.7o) is
achieved. Fig. 2 c illustrates the digital images
showing the as-constructed free-standing liquid
marble (with a radius of around 1 mm) on a glass
slide which would otherwise be wetted by water.
Owing to the presence of the modified graphene
sheets at the liquid-air interface, the wetting between
the water and glass is suppressed, and the formed
liquid marbles can easily roll off the glass slide
surface. Therefore, the liquid marble fabrication can
solve the problem of driving small amounts of liquid
on solid (or even liquid) substrates, and pave a way
for potential microfluidic applications. Fig. 2 d
presents the digital images which show the solution
properties of TrG before and after the organic
modification. The bottom and upper liquid layers
correspond to deionized (DI) water and toluene,
respectively. It can be seen that the surface property
of TrG transforms from hydrophilicity to
hydrophobicity and organophilicity after organic
modification of TrG with DA or OA, evidenced by
the homogenous dispersion in toluene. In addition,
digital images showing the dispersing stabilities of
DA-G and OA-G in toluene are presented in ESM
(Fig. S4). The result demonstrates an overall higher
stability of OA-G than that of DA-G, thereby
suggesting higher modification efficiency of OA-G
by considering the hydrophobicity and
organophilicity.
Figure 3 further depicts the microstructure
details of various samples. XRD pattern for the
pristine graphite presents a typical sharp band
centered at 2θ ≈ 26.4o which is assigned to the
graphitic 002 crystal diffraction. This typical band
disappears for GO, with concomitant emergence of
a new peak located at 2θ ≈ 10.7o which is ascribed to
the 001 plane reflection of GO (Fig. 3 a). With
respect to TrG, no obvious diffraction features can
be observed, with only a small broad diffraction
band centered at 2θ ≈ 25.3o which is only visible
under magnification, thus indicating the
disorderedly stacked TrG [42,43]. Note that the
XRD patterns of neat DA and OA present many
sharp diffraction peaks, which suggests their
well-defined crystal structures. In contrast, the XRD
patterns of DA-G or OA-G present a broad peak
over 2θ = 15-30o with much weaker intensity, which
indicates, most probably, that the alkylamine
moieties, unlike their pristine alkylamine
counterparts, show a largely disordered structure
with a rather low crystallinity. Moreover, the
magnified XRD patterns of both DA-G and OA-G
exhibit small peaks at 2θ ≈ 26.4o, probably implying
the restoration of in-plane sp2-hybridized carbon
structure of the modified graphene sheets and
increased π-π interactions, which is in line with the
results of electrical conductivity and UV/vis spectra.
Besides, an additional small protuberance at 2θ ≈
21.4o can be found for OA-G, likely resulting from a
trace amount of crystalline structure formed by the
ordered alignment of OA molecular chains on the
TrG surfaces. By contrast, such protuberance cannot
be found for DA-G, attributable to the lower
quantity of DA molecules grafted on the TrG
surfaces which is not enough for forming a
7
detectable crystalline structure by XRD test. FTIR
spectra are shown in Fig. 3 b. As for TrG, absorption
bands centered at ~1724 and 1567 cm-1, attributed to
C=O of carboxyl and C=C stretching vibrations,
respectively, can be indexed to the carboxyl groups
and sp2-hybridized carbon structure, which is in
line with the results of XPS spectra. After the
chemical reaction of TrG with organic modifiers DA
and OA, the band at ~1724 cm-1 becomes much
weaker and disappears completely for DA-G and
OA-G, respectively, together with the emergence of
a new band at ~1658 cm-1 which can be assigned to
C=O stretch mode of amide carbonyl for both DA-G
and OA-G. Besides, the bands at 2918-2922 and
~2851 cm-1 associated with C–H stretches of alkyl
chain, and 1564-1571 and 1460-1461 cm-1
corresponding to N–H bends and C–N stretches,
respectively, can be observed for both DA-G and
OA-G [26]. These results indicate the effective
chemical reaction between the reactive functional
groups of alkylamine and TrG, and the larger
reaction extent is found for OA-G.
There exist typical D and G bands in the
Raman spectra of graphene-based specimens. While
the D band is indexed to structural defects,
amorphous carbon, or edges that can break the
symmetry and selection rule, the G band is
associated with the first-order scattering of the E2g
mode observed for sp2-hybridized carbon domains.
The variation of the integrated intensity ratio of the
D and G bands, namely ID/IG, in the Raman spectra
of GO during thermal reduction and subsequent
organic modification can evidence the change of the
electronic conjugation state [44]. Figure 3 c presents
the typical Raman spectra of various specimens and
a comparison plot of ID/IG ratios versus these
specimens. The obvious D and G bands in the
Raman spectra of GO, TrG, DA-G and OA-G are
centered at approximately 1351 and 1575 cm-1,
respectively. It can be seen that the ID/IG ratio of TrG
(~1.65) is obviously larger than that of GO (~0.92),
which probably attributed to the fact that new
graphenic sp2 domains have been generated, with
smaller size but larger quantities as compared to
that existing in GO [7]. The lattice defects and
preserved functional groups existing in TrG may
also result in the ID/IG ratio increase [17]. Further
slight increases of the ID/IG ratios can be found for
DA-G and OA-G as compared to that for TrG,
probably owning to the structural distortions
induced by the bulky alkyl chains of DA and OA
[35]. Note that the ID/IG ratio is slightly larger for
OA-G than that for DA-G, which is an indication of
the higher disturbance effect of OA on TrG. This is
in agreement with the results obtained from UV/vis,
FTIR and XRD tests.
In view of the fact that the regular alignment of
the organic alkylamines grafted on the TrG surfaces
might give rise to the crystallization signature
which can be detected by DSC test. Crystallization
behaviors were thus investigated, with the results
presented in Fig. 4 a-d. In the cooling step of the
DSC curves, the sharp exothermic peaks assigned to
the crystallization temperatures can be clearly
observed for the pure organic modifiers DA and
OA (Fig. 4 a,c). In addition, the corresponding sharp
endothermic peaks, associated with the crystal
melting points, can be seen in the reverse second
heating step (Fig. 4 b,d). These results suggest that
the pure DA and OA have well-defined crystal
structures. After grafting of the organic modifiers
onto the TrG surfaces, the crystallization signals
become much weaker. As for OA-G, broadened and
left-shifted exothermic and endothermic peaks can
be carefully observed in the cooling and second
heating steps, respectively, while no typical
crystallization feature can be detected from the DSC
curves for DA-G. These results imply that the
well-ordered structures of the pristine organic
modifiers are heavily disturbed by TrG. In addition,
from the TGA curves shown in Fig. 4 e, the amount
of DA and OA grafted on the graphene surfaces are
calculated as ~11 and 36 wt.%, respectively. The
much higher amount of OA grafted enables the
8
emergence of the related DSC crystallization signals
for OA-G resulting from a certain extent of
molecule alignment and formation of ordered
crystalline structure, whereas no signals can be
found for DA-G, which is in good agreement with
the XRD results.
As shown in the AFM image of graphen oxide,
along with the height profile (Fig. 5 a), its average
thickness calculated is approximately 1.03 nm,
which indicates the single-layer graphene oxide
prepared, as similarly reported elsewhere [45].
Likewise, the average thickness of TrG is calculated
as approximately 1.11 nm (Fig. 5 b), similarly to the
reported thickness value of single-layer graphene
sheets (approximately 1 nm) [7,46,47]. This is also
an indication of the well-exfoliated TrG. Moreover,
the apparently larger average thicknesses of DA-G
and OA-G (corresponding to approximately 2.35
and 3.69 nm, respectively) than that of TrG can be
owning to the incorporation of DA and OA on both
sides of the graphene sheets (Fig. 5 c,d). The higher
amount of OA grafted on the TrG surfaces results in
the larger average thickness of OA-G than that of
DA-G.
Figure 6 reveals the TEM morphologies of
graphene oxide, TrG, DA-G and OA-G, as shown in
Fig. 6 a, b, c and d, respectively, with the insets
displaying the corresponding magnified images. As
for the TEM image of graphene oxide (Fig. 6 a), a
huge and transparent layer, featuring folded basal
plane and rolled edges which is intrinsic to
graphene for gaining thermodynamic stability [48],
can be observed. After thermal treatment, increased
wrinkles, folds and buckles are generated on the
TrG surface (Fig. 6 b), which can be due to the
decomposition of functional groups and formation
of new structural defect. This is in good agreement
with SEM surface morphologies. Upon modification
of TrG with DA, a fluffy bulk material on the
graphene surface can be carefully observed which
can be indexed to the DA molecules irregularly
arranged on the graphene surface, especially at the
wrinkles and edges which are more active because
of the lattice defect (Fig. 6 c). It should be noted that
the grafted DA is inhomogeneously covered on the
graphene, with an obvious profile difference from
the TrG surface (inset of Fig. 6 c), whereas such
profile difference cannot be observed in the TEM
image of OA-G which shows a much more uniform
coating (inset of Fig. 6 d), possibly suggesting the
higher compatibility between the modifier OA and
TrG.
Moreover, the selected area electron diffraction
(SAED) patterns of graphene oxide, TrG, DA-G and
OA-G are presented in Fig. 6 e, f, g and h,
respectively. From the SAED pattern of graphene
oxide, it can be seen that the intensity of the
0011 -type reflection is higher than that of the
2011 -type reflection, which indicates that the
graphene oxide prepared is a monolayer and/or
disordered stacking of monolayers. This is in
accordance with the previous analysis of the
samples that may consist of randomly oriented and
oxidized carbon monolayers, which lacks any
oxygen superlattice ordering [49,50]. As for the
SAED pattern of TrG, the diffraction intensity is
largely decreased, with almost no diffraction spots
remained, thereby indicating the loss of long-range
ordering between the graphene sheets [18].
Concerning DA-G, diffraction rings take the main
role, with vaguely exhibited diffraction spots,
possibly attributed to the limited reaction effect of
DA on the graphenic in-plane structure of TrG. By
contrast, evolution of much clearer regular
diffraction spots can be observed for OA-G, which
implies the larger extent of chemical reaction
between OA and TrG and higher-level restoration
of the destructed and disordered structure of TrG.
This is in close agreement with the results of
electrical conductivities, UV/vis spectra and XRD
patterns.
9
Based on the various characterization results,
the overall higher efficiency in modification of TrG
with OA (as compared to that with DA) is likely
related to the regular molecule alignment of OA
and formation of crystalline structure. As
demonstrated in DSC and XRD sections, OA-G
shows the crystallization behavior of the OA
molecules grafted on OA-G surfaces; in contrast,
there exists no signal showing the similar behavior
of the DA molecules grafted on DA-G surfaces. This
result might suggest that OA molecules with longer
alkyl chains have higher tendency to be oriented
and aligned, leading to the easier formation of the
crystalline structure. This can also be reflected from
the TEM images which show the irregularly
arranged DA molecules grafted on the graphene
surface. On the other hand, the steric hindrance
effect of OA is larger than that of DA on the organic
modification reaction with TrG due to the
longer/bulkier alkyl chains of OA, which probably
leads to the slower reaction between TrG and OA
molecules. Nevertheless, this slower reaction might
be more favorable for the regular arrangement of
the OA molecules and formation of crystalline
structure by considering that the slower reaction
can provide more time for the OA molecules to be
regularly arranged, as well as to sufficiently attack
the reactive centers on TrG surfaces. In contrast, DA
molecules can initially consume the reactive centers
on TrG more quickly, indicating that a larger bunch
of DA molecules can attack the reactive centers at
the same time. After being grafted on the graphene
surfaces, these DA molecules probably impede
further reaction between the free DA molecules and
reactive centers remained on the graphene surfaces
because of the extremely large steric hindrance
formed by the grafted layer of DA molecules which
may even lead to inaccessibility of the reactive
centers remained. Moreover, owning to the lack of
time for their regular arrangement, the DA
molecules grafted are likely in a disordered state,
which suggests that no crystalline structure can be
formed. Besides, the limited quantities of DA
molecules grafted are even not enough for their
arrangement into a detectable crystalline structure.
For revealing more clearly the difference in the
reaction mechanisms of the two reaction systems,
i.e., DA-TrG and OA-TrG, schematic structural
models of the resulting modified graphenes, DA-G
and OA-G, are created and presented in Scheme 2.
Since the strong interfacial adhesion between
the modified graphene and organic polymer matrix
can be expected based on the high compatibility of
the organic alkylamines grafted and an organic
polymer matrix, the superior in-plane properties of
the modified graphene can be transferred
effectively to the polymer matrix, leading to the
high-performance polymer nanocomposites. Note
that, owning to the organophilicity of the
organically modified graphene, the nanocomposite
materials could be fabricated easily through a
simple solution route. The characterization results
and detailed descriptions of the as-prepared poly
(styrene-co-acrylonitrile)-based nanocomposites
with DA-G and OA-G are presented in ESM (Fig.
S5-S8). Typically, the glass transition and
degradation temperatures of the neat polymer have
been shifted by ~16 and 11 oC, respectively, after
incorporation of only 0.5 wt.% of OA-G, in addition
to the ~30% and 11% increases in Young’s modulus
and tensile strength, respectively (with only a
slightly decease in elongation). Therefore, the
developed organically modified graphene sheets
show a great value in the area of polymer
nanocomposites. The present work has also opened
up a general route to mass-scale fabrication of other
multifunctional graphene derivatives and
high-performance graphene-based composite
materials starting from TrG or the organically
modified graphene for various functional and
multifunctional applications.
3 Conclusions
10
A hydrophilic functionalized graphene was
prepared by a low-temperature thermal reduction
and functionalization technique. This kind of
graphene with a part of reactive oxygen
functionalities and a unique structural topology
was subsequently reacted with alkylamines, leading
to an organically modified graphene with
super-hydrophobicity. The organic modification
was found to induce a further restoration of the
sp2-hydridized carbon network of the thermally
functionalized graphene, and the organic modifier
having larger alkyl chain length could enable an
even higher efficiency in hydrophobicity and
organophilicity. The proposed mechanism and
schematic structural models were presented for
explaining the higher efficiency in organic
modification of TrG with OA than that with DA.
With these organically modified graphene, the
liquid marbles and polymer nanocomposites were
effectively fabricated, which thus exhibit the
multifunctionality of the modified graphene. The
developed low-temperature thermal
functionalization and highly efficient organic
modification will pave the way for industrial scale
production of processible graphene derivatives
with multifunctional properties, thus facilitating
various practicable applications.
4 Experimental
4.1 Materials
Dodecylamine and octadecylamine (analytical
reagents) were purchased from Sigma-Adrich, and
used without further purification. Poly
(styrene-co-acrylonitrile) pellets (average Mw
~165,000 by GPC, styrene 75 wt.%),
dicyclohexylcarbodiimide (DCC), and graphite fine
powder were obtained from Tianheng Technology
Co. Ltd. (Hong Kong, China). Hydrazine hydrate (60
wt.%) was supplied by Oriental Chem. & Lab.
Supplies Ltd. (Hong Kong, China). All other
chemicals were purchased from Sigma-Adrich and
used as received.
4.2 Synthesis of chemically reduced graphene
GO was first prepared according to the modified
Hummers method, as reported elsewhere [6, 51]. As
a control experiment, chemical reduction of GO was
conducted first in the present study.
Commonly-used hydrazine hydrate was adopted as
the chemical reducer to convert graphene oxide to
chemically reduced graphene. The experiment
procedure is given as follows. GO powder (25 mg)
was dispersed in DI water by ultrasonication for 30
min. Hydrazine hydrate (2 mL, 60 wt.%) was then
added into the GO dispersion, followed by
magnetically stirring at 90 oC under a water-cooled
condenser. The reaction was continued for 12 h, and
the final mixture was vacuum-filtered and washed
with water and methanol before vacuum-dried. The
resulting powder sample was stored in a desiccator
before use.
4.3 Preparation of TrG and high-temperature
thermally reduced graphene
A given amount of the as-synthesized GO was
placed in a ceramic container, followed by insertion
of the container into a muffle furnace which was
preheated to 400 oC and saturated with nitrogen
atmosphere beforehand. Upon holding at the
temperature for 30 s, the furnace was cooled down to
room temperature. The ceramic container was then
withdrawn, and the functionalized graphene sheets,
namely TrG, were collected for the subsequent
organic modification experiment. For comparison,
the high-temperature heat treatment of GO at 1000
oC was also conducted using the same thermal
treatment procedure, except that the preheated
temperature the muffle furnace was raised to 1000
oC.
4.4 One-step organic modification of TrG
11
Typically, TrG (100 mg) and organic modifier DA
(3.0 g) were dispersed into tetrahydrofuran (THF, 50
mL) by ultrasonication (150 W) at room temperature
for 1 h. After that, the homogeneous mixture of TrG
and the modifier in THF was refluxed using DCC
(100 mg) as the catalyst under magnetic stirring for
12 h. Once the refluxing reaction was completed, the
resulting product was vacuum-filtered through a
0.22 μm PVDF membrane. The filter cake was
redispersed into 80 mL of THF and poured into
methanol (200 mL) to precipitate the modified
graphene sheets, which was repeated by five times to
remove unreacted free and weakly adsorbed DA or
OA particles. The final filter cake was vacuum-dried
at 50 oC for 24 h. Similarly, the OA-modified
graphene was prepared by replacing the starting DA
with OA having the same weight loading.
4.5 Constructing Liquid Marbles
A syringe loaded with DI water was squeezed to
form a 3-4 μL water droplet on the tip of the syringe.
The water droplet was then let to come into contact
with the surfaces of loose organically modified
graphene powder supported by a glass slide. By
carefully moving the glass slide back and forth, the
modified graphene powder could spontaneously
wrap the water droplet. After the whole surface of
the water droplet was homogeneously wrapt by the
modified graphene particles, it was detached from
the tip of the syringe, and the free-standing liquid
marble with the modified graphene sheets was
yielded.
4.6 Fabrication of polymer nanocomposites
Briefly, 2.0 g of the poly (styrene-co-acrylonitrile)
pellets were dissolved into DMF (60 mL) under
magnetic stirring at 80 oC. The homogeneous
dispersion of DA-G or OA-G (10.0 mg) in DMF (20
ml) was obtained by ultrasonication (150 W) for 40
min at room temperature. The polymer solution and
modified graphene dispersion were then
compounded by ultrasonic treatment for additional
40 min, followed by casting into a preheated
PTFE-lined mold and dried at 90 oC for 12 h. The
nanocomposite films incorporated with 0.5 wt.% of
DA-G and OA-G were then obtained. The pure
polymer and nanocomposites with 1.0 wt.% of the
modified graphene sheets were similarly prepared.
4.7 Characterization
The X-ray photoelectron spectroscopy (XPS)
measurement was performed using a Sengyang
SKL-12 electron spectrometer equipped with a VG
CLAM 4 MCD electron energy analyzer. The
Ultraviolet/visible (UV/vis) spectra were recorded on
a Lambda 18 UV/VIS Spectrometer. The Fourier
transformed infrared (FTIR) spectra were collected
by a FTIR spectrometer (Perkin Elmer System 2000)
in KBr mode. The powder X-ray diffraction (XRD)
patterns were recorded on a Bruker D8 Advance
X-ray diffractometer (Bruker AXS, Karlsruhe,
Germany). The Raman spectra were excited with a
laser of 488 nm and recorded on solid powder
samples in a Lab-RAM HR800 spectrometer. The
thermal behaviors of the organically modified
graphene powders and the polymer-based
nanocomposites were measured by a differential
scanning calorimeter (DSC, Perkin Elmer DSC-7)
according to a heating-cooling-heating procedure at
a rate of 10 oC/min or -10 oC/min. The first heating
process was to eliminate the thermal history and the
second cooling and third heating processes were
adopted for analyzing crystallization, and melting
and glass transition behaviors, respectively. The
thermogravimetric analysis (TGA) was conducted on
a Mettler Toledo TGA/SDTA851 under N2
atmosphere at a heating rate of 10 oC/min. The
surface morphologies were observed by a
field-emission scanning electron microscopy
(FE-SEM, JEOL JSM-6335F). In order to determine
the thickness of the graphene oxide, thermally
functionalized graphene and organically modified
graphene, the atomic force microscopy (AFM) was
performed on a Nanoscope Multimode IIIa scanning
12
probe microscopy system in a tapping mode. The
specimens used for AFM test were prepared by
dropping diluted ethanol dispersion of the graphene
sample onto a silicon wafer, followed by drying
treatment at room temperature. The transmission
electron microscope (TEM) images and selected area
electron diffraction (SAED) patterns were obtained
by a Jeol JEM-2011 TEM facility at an acceleration
voltage of 100 kV. The specimens for the TEM
observation were similarly prepared as mentioned in
the AFM test. The water contact angle measurement
was performed using a contact-angle meter (Tantec,
Schaumburg, IL). For electrical conductivity
measurement, the compacts of GO, TrG, DA-G and
OA-G were fabricated by pressing the powder
samples with a Specac Atlas manual hydraulic press
under the pressure of 40 MPa. The volume
conductivities of the as-fabricated compacts with the
average diameter of 13 mm were measured on a
4-point probes resistivity measurement system at
room temperature. The tensile tests for the polymer
nanocomposites were performed on a universal
tensile machine (Instron 5566) at an extension rate of
5 mm/min at room temperature. For each sample,
five specimens were tested and the average of the
testing values was plotted.
Acknowledgements
The authors acknowledge the funding from RGC of
the Hong Kong SAR Government (PolyU 5316/10E).
Electronic Supplementary Material: Water
dispersibilities of TrG, chemically reduced graphene
using hydrazine hydrate as the reducing reagent,
and high-temperature thermally reduced graphene
@1000 oC, FTIR spectra and electrical conductivity
measurement results of various kinds of graphene
powders, dispersing stabilities of TrG and its
organically modified derivatives in toluene,
characterization results of the structure and
properties of the polymer nanocomposites filled with
the organophilic modified graphene are available in
the online version of this article at
http://dx.doi.org/10.1007/s12274-***-****-*
(automatically inserted by the publisher). References [1] Geim, A. K. Graphene: status and prospects. Science 2009,
324, 1530-1534.
[2] Zhang, Y.; Tan, Y. W.; Stormer, H. L.; Kim, P.
Experimental observation of the quantum Hall effect and
Berry's phase in graphene. Nature 2005, 438, 201-204.
[3] Loh, K. P.; Bao, Q.; Ang, P. K.; Yang, J. The chemistry of
graphene.J. Mater. Chem. 2010, 20, 2277-2289.
[4] Staudenmaier, L. Method of preparation of graphite-acid.
Ber. Dtsch. Chem. Ges. 1898, 31, 1481-1499.
[5] Brodie, B. C. Sur le poids atomique du graphite. Ann.
Chim. Phys. 1860, 59, 466-472.
[6] Hummers Jr, W. S.; Offeman, R. E. Preparation of
graphitic oxide. J. Am. Chem. Soc. 1958, 80, 1339.
[7] Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K.
A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff,
R. S. Synthesis of graphene-based nanosheets via chemical
reduction of exfoliated graphite oxide. Carbon 2007, 45,
1558-1565.
[8] Ding, Y.; Zhang, P.; Zhuo, Q.; Ren, H.; Yang, Z.; Jiang, Y.
A green approach to the synthesis of reduced graphene
oxide nanosheets under UV irradiation. Nanotechnology
2011, 22, 215601.
[9] Schniepp, H. C.; Li, J. L.; McAllister, M. J.; Sai, H.;
Herrera-Alonso, M.; Adamson, D. H.; Prud'homme, R. K.;
Car, R.; Saville, D. A.; Aksay, I. A. Functionalized single
graphene sheets derived from splitting graphite oxide. J.
Phys. Chem. B 2006, 110, 8535-8539.
[10] Deng, S.; Lei, J.; Cheng, L.; Zhang, Y.; Ju, H. Amplified
electrochemiluminescence of quantum dots by
electrochemically reduced graphene oxide for
nanobiosensing of acetylcholine. Biosens. Bioelectron.
2011, 26, 4552-4558.
[11] Yang, X.; Zhu, J.; Qiu, L.; Li, D. Bioinspired Effective
Prevention of Restacking in Multilayered Graphene Films:
Towards the Next Generation of High-Performance
Supercapacitors. Adv. Mater. 2011, 23, 2833-2838.
[12] Xu, Y.; Bai, H.; Lu, G.; Li, C.; Shi, G. Flexible Graphene
Films via the Filtration of Water-Soluble Noncovalent
13
Functionalized Graphene Sheets. J. Am. Chem. Soc. 2008,
130, 5856-5857.
[13] Si, Y.; Samulski, E. T.; Synthesis of water soluble
graphene. Nano Lett. 2008, 8, 1679-1682.
[14] Xu, Y.; Sheng, K.; Li, C.; Shi G. Highly conductive
chemically converted graphene prepared from mildly
oxidized graphene oxide. J. Mater. Chem. 2011, 21,
7376-7380.
[15] Li, D.; Müller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G.
G. Processable aqueous dispersions of graphene
nanosheets. Nat. Nanotechnol. 2008, 3, 101-105.
[16] Zhu, Y.; Cai, W.; Piner, R. D.; Velamakanni, A.; Ruoff, R.
S. Transparent self-assembled films of reduced graphene
oxide platelets. Appl. Phys. Lett. 2009, 95,
103104-103104-3.
[17] Kudin, K. N.; Ozbas, B.; Schniepp, H. C.; Prud'Homme,
R. K.; Aksay, I. A.; Car, R. Raman spectra of graphite
oxide and functionalized graphene sheets. Nano Lett. 2008,
8, 36-41.
[18] McAllister, M. J.; Li, J.-L.; Adamson, D. H.; Schniepp,
H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius,
D. L.; Car, R.; Prud'homme, R. K. Single sheet
functionalized graphene by oxidation and thermal
expansion of graphite. Chem. Mater. 2007, 19, 4396-4404.
[19] Jing, X.; Qiu, Z. Effect of Low Thermally Reduced
Graphene Loadings on the Crystallization Kinetics and
Morphology of Biodegradable Poly (3-hydroxybutyrate).
Ind. Eng. Chem. Res. 2012, 51, 13686-13691.
[20] Tung, N. T.; Van Khai, T.; Jeon, M.; Lee, Y. J.; Chung, H.;
Bang, J.-H.; Sohn, D. Preparation and characterization of
nanocomposite based on polyaniline and graphene
nanosheets. Macromol. Res. 2011, 19, 203-208.
[21] Yan, D.; Zhang, H.-B.; Jia, Y.; Hu, J.; Qi, X.-Y.; Zhang,
Z.; Yu, Z.-Z. Improved electrical conductivity of
polyamide 12/graphene nanocomposites with maleated
polyethylene-octene rubber prepared by melt
compounding. ACS Appl. Mater. Interfaces 2012, 4,
4740-4745.
[22] Roy-Mayhew, J. D.; Bozym, D. J.; Punckt, C.; Aksay, I.
A. Functionalized graphene as a catalytic counter electrode
in dye-sensitized solar cells. ACS Nano 2010, 4,
6203-6211.
[23] Wang, G.; Shen, X.; Wang, B.; Yao, J.; Park, J. Synthesis
and characterisation of hydrophilic and organophilic
graphene nanosheets. Carbon 2009, 47, 1359-1364.
[24] Park, S.; An, J.; Jung, I.; Piner, R. D.; An, S. J.; Li, X.;
Velamakanni, A.; Ruoff, R. S. Colloidal suspensions of
highly reduced graphene oxide in a wide variety of organic
solvents. Nano Lett. 2009, 9, 1593-1597.
[25] Hu, H.; Wang, X.; Wang, J.; Liu, F.; Zhang, M.; Xu, C.;
Microwave-assisted covalent modification of graphene
nanosheets with chitosan and its electrorheological
characteristics. Appl. Surf. Sci. 2011, 257, 2637-2642.
[26] Cao, Y.; Feng, J.; Wu, P. Alkyl-functionalized graphene
nanosheets with improved lipophilicity. Carbon 2010, 48,
1683-1685.
[27] Lin, Z.; Liu, Y.; Wong, C.-p. Facile fabrication of
superhydrophobic octadecylamine-functionalized graphite
oxide film. Langmuir 2010, 26, 16110-16114.
[28] Zhang, S. P.; Song, H. O. Supramolecular graphene
oxide-alkylamine hybrid materials: variation of
dispersibility and improvement of thermal stability. New. J.
Chem. 2012, 36, 1733-1738.
[29] Shanmugharaj, A.; Yoon, J.; Yang, W.; Ryu, S. H.
Synthesis, characterization and surface wettability
properties of amine functionalized graphene oxide films
with varying amine chain lengths. J. Colloid Interface Sci.
2013, 401, 148-154.
[30] Li, W.; Tang, X. Z.; Zhang, H. B.; Jiang, Z. G.; Yu, Z. Z.;
Du, X. S.; Mai, Y. W. Simultaneous surface
functionalization and reduction of graphene oxide with
octadecylamine for electrically conductive polystyrene
composites. Carbon 2011, 49, 4724-4730.
[31] Nethravathi, C.; Rajamathi, M. Chemically modified
graphene sheets produced by the solvothermal reduction of
colloidal dispersions of graphite oxide. Carbon 2008, 46,
1994-1998.
[32] Matsuo, Y.; Higashika, S.; Kimura, K.; Miyamoto, Y.;
Fukutsuka, T.; Sugie, Y. Synthesis of
polyaniline-intercalated layered materials via exchange
reaction. J. Mater. Chem. 2002, 12, 1592-1596.
[33] Aussillous, P.; Quéré, D. Liquid marbles. Nature 2001,
411, 924-927.
[34] Aussillous, P.; Quéré, D. Properties of liquid marbles.
14
Proc. Math. Phys. Eng. Sci. 2006, 462, 973-999.
[35] Xue, Y.; Liu, Y.; Lu, F.; Qu, J.; Chen, H.; Dai, L.
Functionalization of Graphene Oxide with Polyhedral
Oligomeric Silsesquioxane (POSS) for Multifunctional
Applications. J. Phys. Chem. Lett. 2012, 3, 1607-1612.
[36] Jeon, I.-Y.; Shin, Y.-R.; Sohn, G.-J.; Choi, H.-J.; Bae,
S.-Y.; Mahmood, J.; Jung, S.-M.; Seo, J.-M.; Kim, M.-J.;
Chang, D. W. Edge-carboxylated graphene nanosheets via
ball milling. Proc. Natl. Acad. Sci. U.S.A. 2012, 109,
5588-5593.
[37] Chen, C.; Long, M.; Xia, M.; Zhang, C.; Cai, W.
Reduction of graphene oxide by an in-situ
photoelectrochemical method in a dye-sensitized solar cell
assembly. Nanoscale Res. Lett. 2012, 7, 1-5.
[38] Villar-Rodil, S.; Paredes, J. I.; Martínez-Alonso, A.;
Tascón, J. M. D. Preparation of graphene dispersions and
graphene-polymer composites in organic media. J. Mater.
Chem. 2009, 19, 3591-3593.
[39] Hu, H. W.; Chen, G. H.; Fang, M.; Zhao, W. F.
Modification of graphite oxide nanoparticles prepared via
electrochemically oxidizing method. Synth. Met. 2009, 159,
1505-1507.
[40] Hu, H. W.; Xin, J. H.; Hu, H. Highly Efficient
Graphene-Based Ternary Composite Catalyst with
Polydopamine Layer and Copper Nanoparticles.
ChemPlusChem 2013, 78, 1483-1490.
[41] Rani, J.; Lim, J.; Oh, J.; Kim, J.-W.; Kim, D.; Lee, D.;
Shin, H. S.; Kim, J. H.; Jun, S. C. Substrate and Buffer
layer Effect on the Structural and Optical Properties of
Graphene Oxide Thin Films. RSC Adv. 2013, 3,
5926-5936.
[42] Mai, Y.; Wang, X.; Xiang, J.; Qiao, Y.; Zhang, D.; Gu, C.;
Tu, J. CuO/graphene composite as anode materials for
lithium-ion batteries. Electrochim. Acta 2011, 56,
2306-2311.
[43] Qi, Y.; Zhang, H.; Du, N.; Yang, D. Highly loaded
CoO/graphene nanocomposites as lithium-ion anodes with
superior reversible capacity. J. Mater. Chem. A 2013, 1,
2337-2342.
[44] Zhang, J.; Yang, H.; Shen, G.; Cheng, P.; Guo, S.
Reduction of graphene oxide vial-ascorbic acid. Chem.
Commun. 2010, 46, 1112-1114.
[45] Sun, X.; Liu, Z.; Welsher, K.; Robinson, J. T.; Goodwin,
A.; Zaric, S.; Dai. H. Nano-graphene oxide for cellular
imaging and drug delivery. Nano Res. 2008, 1, 203-212.
[46] Liu, N.; Pan, Z.; Fu, L.; Zhang, C.; Dai, B.; Liu, Z. The
origin of wrinkles on transferred graphene. Nano Res.
2011, 4, 996-1004.
[47] Zhang, H.; Xu, P.; Du, G.; Chen, Z.; Oh, K.; Pan, D.; Jiao,
Z. A facile one-step synthesis of TiO2/graphene
composites for photodegradation of methyl orange. Nano
Res. 2011, 4, 274-283.
[48] Shen, L.; Yuan, C.; Luo, H.; Zhang, X.; Yang, S.; Lu, X.
In situ synthesis of high-loading Li4Ti5O12–graphene
hybrid nanostructures for high rate lithium ion batteries.
Nanoscale 2011, 3, 572-574.
[49] Mattson, E.; Cui, S.; Schofield, M.; Lu, G.; Pu, H.;
Weinert, M.; Chen, J.; Hirschmugl, C.;
Gajdardziska-Josifovska, M. Real-Time Observations of
Structural Ordering in Graphene Oxide During Thermal
Reduction in Vacuum. Microsc. Microanal. 2011, 17,
1500-1501.
[50] Mattson, E.; Cui, S.; Mao, S.; Lu, G.; Chen, J.;
Hirschmugl, C.; Gajdardziska-Josifovska, M. Structure of
Graphene Oxide-Tin Oxide Hybrid Nanomaterials for Gas
Sensors. Microsc. Microanal. 2010, 16, 1708-1709.
[51] Kim, H.; Kim, S.; Park, Y.; Gwon, H.; Seo, D.; Kim, Y.;
Kang, K. SnO2/graphene composite with high lithium
storage capability for lithium rechargeable batteries. Nano
Res. 2010, 3, 813-821.
15
Scheme 1 Schematic illustration of the main content for the present study. The schematic structure models of graphene oxide, TrG and organic modified graphene with OA are presented, with the corresponding SEM images shown in their right sides. The typical surface morphologies are highlighted with transparent colored shade in the SEM images for clearly revealing the structure features of different samples, and the structure details are described and indicated by the red dotted lines. The multifunctional applications in liquid marbles and polymer nanocomposites for the present research system are also explained by the structural models with microscopic details.
16
Figure 1 Structure analysis of GO and TrG. (a) Digital images and schematic illustrations showing the water dispersions of GO and
TrG. XPS survey spectra of GO (b) and TrG (e), high-resolution XPS C 1s core-level spectra of GO (c) and TrG (f), and
high-resolution XPS O 1s core-level spectra of GO (d) and TrG (g).
Figure 2 Structure and surface property analysis of various graphene-based samples and applications of the organically modified
graphenes in liquid marbles. (a) UV/vis spectra of ethanol dispersions of GO, TrG, DA-G and OA-G. (b) Contact angle images of water
droplets on the compressed plates made of GO, TrG, DA-G and OA-G powders. (c) Digital images showing the as-constructed liquid
marbles by the means of wrapping water droplets with DA-G and OA-G powders. (d) Digital images revealing the solution properties of
TrG before and after organic modification.
(a)
(b) (c) (d)
(e) (f) (g)
17
Figure 3 Structure characterizations of various powder samples by X-ray diffraction, Fourier transform infrared spectroscopy and
Raman spectroscopy. (a) XRD patterns (left) along with the selectively magnified patterns (right). (b) FTIR spectra showing
comparisons among DA, TrG and DA-G (tope), and among OA, TrG and OA-G (bottom). (c) Raman spectra of GO, TrG, DA-G and
OA-G together with the comparison plot of ID/IG ratio versus sample code.
(a)
(c)
(b)
18
Figure 4 Thermal characterization for clarifying the crystallization behavior of the organic modifier grafted on the graphene
surfaces. DSC curves of the cooling (a,c) and second heating (b,d) processes, and TGA patterns (e) of various samples.
19
Figure 5 Thickness measurement for various graphene-based samples using atomic force microscope. AFM images along with the
height profiles of graphene oxide (a), TrG (b), DA-G (c) and OA-G (d).
Figure 6 Microstructure observation and electron diffraction analysis of various graphene-based samples. TEM images coupled
with SAED patterns of graphene oxide (a,e), TrG (b,f), DA-G (c,g) and OA-G (d,h). The insets of Figure 6 c and d present the
corresponding magnified TEM images.
(e) (f) (g) (h)
(a) (b) (c) (d)
20
Scheme 2 Schematic structural models created for explaining the reaction mechanisms of organic modification of TrG with the
modifiers OA and DA, as well as for illustrating the higher modification efficiency for the modifier OA than that for the modifier DA.