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, john.xin@polyu.edu.hk; H. Hu, hu.hong@polyu.edu.hk

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-***-****-*

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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.