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Nano Res
1
Three-Dimensional Porous Graphene Sponges
Assembled with the Combination of Surfactant and
Freeze-drying
Rujing Zhang1, Yachang Cao1, Peixu Li2, Xiaobei Zang1, Pengzhan Sun1, Kunlin Wang1, Minlin Zhong1,
Jinquan Wei1, Dehai Wu2, Feiyu Kang1,3, Hongwei Zhu1,3,4 ()
Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-014-0508-x
http://www.thenanoresearch.com on June 5, 2014
© Tsinghua University Press 2014
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Nano Research
DOI 10.1007/s12274-014-0508-x
Three-Dimensional Porous Graphene Sponges
Assembled with the Combination of Surfactant
and Freeze-drying
Rujing Zhang, Yachang Cao, Peixu Li, Xiaobei Zang,
Pengzhan Sun, Kunlin Wang, Minlin Zhong, Jinquan
Wei, Dehai Wu, Feiyu Kang, Hongwei Zhu*
Tsinghua University, China
Three-dimensional hierarchical porous graphene sponges are
prepared with the combination of surfactant and freeze-drying.
Three-Dimensional Porous Graphene Sponges
Assembled with the Combination of Surfactant and
Freeze-drying
Rujing Zhang1, Yachang Cao1, Peixu Li2, Xiaobei Zang1, Pengzhan Sun1, Kunlin Wang1, Minlin Zhong1,
Jinquan Wei1, Dehai Wu2, Feiyu Kang1,3, Hongwei Zhu1,3,4 ()
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 2014
KEYWORDS
graphene sponge,
hierarchical,
freezing media,
porous, foams
ABSTRACT
With the combination of surfactant and freeze-drying, we have developed two
kinds of graphene spongy structures. On the one hand, using foams of soap
bubbles as templates, three-dimensional porous graphene sponges with rich
hierarchical pores are synthesized. Pores of the material contain three levels of
length scales, including millimeter, micrometer and nanometer. The structure
could be tuned by changing the freezing media, adjusting the stirring rate or
adding functional additives. On the other hand, by directly freeze-drying of
graphene oxide/surfactant suspension, porous framework with directional
alignment pores is prepared. The surfactant gives a better dispersion of
graphene oxide sheets, making a high specific surface area. Both of the obtained
materials exhibit excellent absorption capacity and good compression
performance, providing a broad range of possible applications, such as
absorbents, storage media, carriers, etc.
1 Introduction
Nanomaterials have shown a booming development
in recent decades due to their excellent properties.
Assembling nanomaterials into macroscopic
structures while keeping the unique properties of
nanoscale building blocks is of great significance to
advance the practical applications. Since the
discovery of graphene, it has been extensively
Nano Research
DOI (automatically inserted by the publisher)
Research Article
Address correspondence to H. W. Zhu, hongweizhu@tsinghua.edu.cn
| www.editorialmanager.com/nare/default.asp
2 Nano Res.
investigated as a typical carbon nanostructure. As the
precursor of graphene, graphene oxide (GO) can be
obtained easily by oxidizing expandable graphite
powders [1]. The introduced oxygen-containing
function groups can overcome hydrophobicity of
pristine graphene, thus making GO an attractive
candidate for the bottom-up assembly of graphene
into macroscale structures. Up to present, many
studies have been conducted on one-dimensional (1D)
fibers [2-4] or tubes [5], two-dimensional (2D) layered
papers [6] and woven fabrics [7], three-dimensional
(3D) porous structures [8-10], etc.
Given their outstanding properties, such as high
specific surface area and low density, 3D
graphene-based materials show great potential in the
application of supercapacitors, absorbent, energy
conversion and storage [11]. Synthesis methods of
some other ultra-light, three-dimensional porous
aerogels have model significance on the preparation
of 3D graphene-based materials. For example, carbon
nanofiber aerogels were prepared by
template-directed hydrothermal carbonization
process [12] and pyrolyzation of related precursors
[13]. Recent developments in the shapes, linkage
geometries and components of metal-organic
frameworks [14] and mesoporous materials [15] also
prompted great guiding significance. Traditionally,
template-direct deposition and self-assembly of GO
sheets are the two most studied methodologies to
prepare 3D graphene-based materials.
Template-direct synthesis leads to the inherited
structures of 3D scaffold templates, including
chemical vapor deposition (CVD) [16], ice templating
[17, 18], or chemical conversion of amorphous porous
carbon [19]. Self-assembly strategies, such as
convenient one-step hydrothermal method [20,
21],chemical reduction-induced method [22], and
metal ion induced process [23], make graphene
sheets interconnect with each other via hydrogen
bonding, π-π stacking or electrostatic interactions.
Other main approaches, including simple centrifugal
evaporation process [24], leavening strategy [25],
electrochemical reduction method [26-28], and freeze
casting [29, 30] were also widely investigated.
However, the restacking and aggregation of
graphene sheets during assembly remains a problem
in liquid phase methods.
Hierarchical structures could offer a huge increase
in specific surface area due to the large range of
pores, having great potentials in energy storage,
energy conversion, drug delivery and catalytic
applications [31]. Early studies were mainly
concentrated on carbon-based materials [32, 33].
Recently, hierarchical porous graphene-based
materials have been prepared [34-37]. However, most
3D porous graphene structures only have small pores
ranging from a few nanometers to dozens of microns
in diameter. Few studies have been conducted on
structures with macroscopic and visible pores [38, 39].
Additionally, preparing highly ordered
graphene-based materials is still very challenging.
Here we present two kinds of graphene sponges
prepared with the combination of surfactant and
freeze-drying. To provide a supplementary of the
existing porous graphene-based 3D structures,
graphene sponges with hierarchical ordered pores
were synthesized using foams of soap bubbles as
templates. The typical fast-frozen architectures have
three levels of pore scales, ranging from nanometer
to millimeter. By tuning the stirring rate, freezing rate
or adding additives, the obtained pores display
significantly different features. Graphene sponge
prepared by directly freeze-drying of GO/surfactant
solution shows higher specific surface area due to the
relatively good dispersion of GO sheets. Excellent
absorption capability makes the materials attractive
in many situations. The method is facile and
environment-friendly, and the surfactant can be
removed by high-temperature treatment with
subsequent washing process, making the method
suitable for the synthesis of many other porous
materials.
2 Results and Discussion
2.1 Synthesis and Characterizations of Graphene
Sponges
Figure 1a illustrates the synthesis process of
graphene sponge. Commercial detergent was added
into deionized water. Then the mixture was stirred to
obtain detergent bubbles and solution, as shown in
Figure 1b. The inset shows GO dispersion with a
high concentration of 10 mg mL-1.
For the upper part, by adding GO suspension into
the cluster of detergent bubbles, with subsequent
uniform stirring, vacuum freeze-drying and thermal
annealing, thermal-reduced GO-bubble liquid
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research
3 Nano Res.
nitrogen-frozen sponge (trGO-B-LN sponge) was
prepared. The detergent bubbles are light-weight and
non-flowing (Figure 1c), acting as a special template
of the sponge. After adding GO suspension into the
bubble-clusters and evenly stirring, “wet GO
sponge” was prepared, maintaining the
morphologies of detergent bubbles (the inset of
Figure 1c). Water was removed in the vacuum
freeze-drying and removal of oxygen-containing
groups occurs in the thermal-annealing process. The
finally obtained trGO-B-LN sponge on a green
bristle-grass is shown in Figure 1d, exhibiting the
light weight (also shown in Movie S1). The density of
trGO sponge varies from about 2 to 8 mg cm-3 via
controlling the volume ratio of graphene dispersion
and raw detergent bubbles.
Freezing
Freezing
DI
water
Detergent
GO suspension
Stirring
Thermal-annealing
trGO-B sponge
GO suspension
GO
Bubbles
Freeze-drying
mixed
solution
Bubbles
Thermal-annealing
trGO-S sponge
Freeze-drying
Bubbles
Upper part
Lower partSonication
Stirring
Detergent solution GO/detergent mixed solution
(a)
(b) (c) (d)
(e) (f)
Figure 1. (a) Fabrication illustration of the two kinds of trGO
sponge. (b) The seperation of upper bubbles and lower solution
after stirring. The inset shows GO dispersion of 10 mg mL-1. (c) A
cluster of detergent bubbles. The inset reveals a breaker of GO
bubbles. (d) A ultra-light weight trGO-B-250-LN sponge on a
green bristle-grass (apparent density=2.1 mg cm-3). (e)
GO/detergent solution mixed by sonication. (f) A ultra-light weight
trGO-S sponge on a mass of catkin.
While for the lower part of detergent solution, GO
suspension was added into it. The mixture was
mixed by ultrasound instead of violent stirring to
avoid the formation of visible bubbles (Figure 1e),
with subsequent directly freeze-drying and
thermal-reduction. The finally obtained material is
called thermal-reduced GO-sonication, liquid
nitrogen-frozen sponge (trGO-S-LN sponge). The
low density make it stand on a mass of catkin, as
shown in Figure 1f.
Microstructures of the sponges were investigated
by scanning electron microscope (SEM). Typical
architectures of trGO-B-LN sponge was prepared by
the stirring rate of 250 r min-1 and frozen by liquid
nitrogen (trGO-B-250-LN sponge), as shown in
Figure 2a and Figure S1. Most of the pores are
quasi-round, approximately the same as detergent
bubbles. The macroscopic pores on the surface are
visible to the naked eyes, with diameters in the scale
of several hundred microns in average. GO-B sponge
shows ordered large pores with slot pores on the
walls (Figure S1a,b). Slot pores are hundreds of
microns in length and tens of microns in width.
Besides slot pores (shown as the yellow lines in
Figure S1c), some near-round pores with dozens of
microns in diameter can also be observed on the wall,
especially on the connection joint where ice crystals
of adjacent soap bubbles meet, as indicated by the
yellow cycles in Figure S1c and the high
magnification image in Figure S1d.
SEM images of trGO-S-LN sponge are displayed in
Figure 1g and h, revealing morphologies of the
surface and cross section, respectively. Flexible
graphene sheets overlap and coalesce with each other,
forming porous well-defined framework. The yellow
arrow in Figure 1h shows the direction of graphene
sheets arrangement, which is consistent with the
growth direction of ice crystals.
Transmission electron microscopy (TEM) were
used to observe structures in nanoscale (Figure S2).
As to trGO-B-250-LN sponge, Figure S2a shows that
some parts of the sponge were made up with
relatively clean graphene sheets, while some areas
had residual impurities resulting from the laundry
detergent after thermal annealing at 300 ℃(Figure
S2b,c). The diameter of pores ranged from dozens of
nanometers (Figure S2b) to several nanometers
(Figure S2c).
From the above, pores of trGO-B-250-LN sponge
prepared with liquid nitrogen as the freezing
medium can be divided into three levels of length
scale, namely large pores in millimeter scale which
directly inherit the morphology of bubbles, slot or
near-round pores on the walls in micrometer scale
resulting from ice crystals, and pores in nanometer
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4 Nano Res.
scale originating in the residua of the detergent. For
trGO-S-LN sponge, the distribution of pores is
relatively narrow, which come from ice crystals.
Fa
st
Slo
w
Fre
ezin
g s
pee
d
Stirring Rate
Slow Fast
Add
ing S
ug
ar
No
Ye
s
Ordered quasi-round
pores with slot pores
on the wall
Near-round pores with
irregular pores on the
wall
Disordered irregular
pores
Disordered pores with
no pores on the wall
Transparent graphene
sheets in overlap form
quasi-round pores with
filamentous assembled
graphene sheets
10 μm
(c)
20 μm
(f)
20 μm
(b)
20 μm
(a)
10 μm
(e)
20 μm
(d)
20 μm
(g)
10 μm
(h)
Figure 2. Microscopic structures of graphene sponge. (a-f):
trGO-B sponges prepared with different stirring rates and freezing
media display distinguished structures. (a) 250 r min-1, liquid
nitrogen. (b) 5000 r min-1, liquid nitrogen. (c) 20000 r min-1, liquid
nitrogen. (d) 250 r min-1, refrigerator. (e) Partial enlargement
image of (d). (f) 20000 r min-1, liquid nitrogen, adding sugar. (g-h):
SEM images of the surface and cross section of trGO-S-LN
sponge, respectively.
Raman spectrum of the prepared trGO-B-250-LN
sponge reveals the existence of D, G and 2D bands
(Figure S2d). The relatively high D band at 1350 cm-1
shows the existence of thermally induced defects and
amorphous carbon from the pyrolysis of the
detergent. The nitrogen adsorption-desorption
isotherm of the materials shows typical IV
characteristics, with the hysteresis loop as H2-type
(Figure S2e). The rapid increase of nitrogen
adsorption at relatively high pressure (P/P0=0.8~1.0)
demonstrates the presence of macropores. Pore size
distribution in Figure S2f shows hierarchical pores
ranging from micropores to macropores. The average
pore width is 21 nm. The BET
(Brunauer-Emmet-Teller) specific surface area of the
trGO-B-250-LN sponge is 51 m2 g-1. TrGO-S-LN
sponge has a specific surface area of 105 m2 g-1,
almost double that of trGO-B-250-LN sponge (51 m2
g-1). While trGO sponge prepared from GO
suspension without surfactant show a BET specific
surface area of 80.1 m2 g-1. The comparison of the
specific area shows that GO sheets are better
dispersed with the assistant of the surfactant.
X-ray photoelectron spectroscopy (XPS) of the
GO-B-250-LN sponge and trGO-B-250-LN sponge
after washing by DI water was carried out (Figure S3).
The pronounced peaks include a predominant C 1s
peak at around 284.8 eV, an O 1s peak located at
around 532 eV, a weak S 2p peak at about 169.3 eV, a
Na 1s peak at around 1071.5 eV, as well as other weak
peaks of elements in low content. The relatively high
levels of O, Na, S and Si can be attributed to the
surfactant and washing assistant in the detergent.
The surfactant in the detergent is sodium dodecyl
sulfate (SDS) or sodium dodecyl benzene sulfonate
(SDBS), both of which can be removed by heat
treatment with subsequent washing process with DI
water. Elemental analysis presented in Table S1
shows the atomic ratio of some main elements,
revealing the purification trGO-B-250-LN sponge.
The removal of detergent impurities is important for
the materials. It can maintain the inherent nature and
characteristics of the basic building blocks, making
the method suitable for preparation of other porous
materials.
2.2 Tailoring Pore Morphologies
Structures of the prepared trGO-B-LN sponges can
be tuned by many factors. In this work, different
stirring rate, freezing rate and additives were
investigated to realize the control of morphologies.
The illustrations in Figure 2 show the structure
evolution with different parameters, including the
size, arrangement and wall thickness of the pores.
Figure 2b shows trGO-B-LN sponge fabricated
with the stirring rate of about 5000 r min-1
(trGO-B-5000-LN sponge), with other parameters the
same as those in Figure 2a. Near-round pores in tens
of microns large can be observed, with graphene
sheets randomly assembling on the walls. Low
magnification image was shown in Figure S4a,
displaying disordered large pores and damaged
walls. Further improving the stirring rate to 20000 r
min-1, porous graphene networks (trGO-B-20000-LN
sponge) with disordered irregular pores can be
obtained. At low magnification, the SEM image
shows fluffy graphene sheet walls and disordered
pores inherited from the detergent bubbles (Figure
S4b), which are identified with the yellow round
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research
5 Nano Res.
marks. However, at high magnification, some small
polygon-like pores can be found, preserving the
morphology of bubbles (Figure 2c). Therefore, the
tunable stirring rate can change the volumes of
bubbles made from laundry detergent, then further
change the morphologies of the final obtained
sponges.
The porous structure can be affected significantly
by the freezing rate due to the duplication of ice
formation path [40]. Except for using liquid nitrogen
as the rapid freezing medium, refrigerator was
utilized to freeze the GO bubbles relatively slowly to
get trGO-B-refrigerator sponge. The pores of
slow-frozen graphene sponge are disordered with
smooth walls, as shown in Figure 2d. The walls are
thin and near-transparent, consisting of overlapped
graphene sheets (Figure 2e).
Laundry detergent bubbles are easy to rupture
owing to the effect of gravity and surfactant [41], and
adding sugar or glycerinum in the bubble-fabrication
process can improve their stabilities. We added sugar
to laundry detergent as the mixed detergent to make
stable bubbles. The stirring rate in the fabrication
process was 20000 r min-1, the same as that in Figure
2c. SEM image of the obtained
trGO-B-20000-LN-sugar sponge is shown in Figure 2f.
Residual near-round pores (round mark in Figure S4c)
and trifurcate edges (yellow lineal mark in Figure S4c)
can be observed from the disordered assembly of
graphene sheets in the obtained microstructure. High
magnification image shows the filamentous graphene
assemblies (yellow arc lines in Figure S4d) and
granular structures on the walls (inset), due to the
precipitation of sugar at low temperature. The
precipitated sugar assembled around the walls of
bubbles and acted as the skeleton of graphene sheets
self-assembly.
2.3 Formation Mechanism
Figure 3a illustrates the formation mechanism of
trGO-B-LN sponge. The bubbles have fluid-filled
membranes (Figure 3b). After adding GO suspension
and stirring, GO sheets were uniformly distributed in
the walls of the bubbles (Figure 3c). Adding GO
dispersion into the detergent bubbles almost has no
obvious effect on the structure, shown as the yellow
intact pentagon. Subsequent fast freezing resulted in
long crystallization of ice on the walls, in which
process GO sheets adhere to the ice and assemble in
particular directions. TrGO-B-LN sponges were
obtained after subsequent freeze-drying and thermal
annealing at last. SEM image of sponge pores in
Figure 3d shows their intact pentagon structures,
which match the morphologies of detergent bubbles
observed in optical image of Figure 3c.
The inset of Figure 3c displays morphologies of
bubbles made with higher stirring rate of 5000 r min-1,
with smaller volume and thinner membranes. Since
rupture will occur when a lamella becomes critically
thin [41], bubbles made with higher stirring rate are
easier to rupture in the subsequent operations, which
can explain the disordered assembly of graphene
sheets in trGO-B-5000-LN and trGO-B-20000-LN
sponges.
Detergent bubbles
GO-B sponge
Wet GO bubbles
trGO-B sponge
GO suspension Fast freezing
Solid GO bubbles
Vacuum freeze drying
Thermal annealing
200 μm
(b)
200 μm
(c)
100 μm
(d)
GO/detergent solution
Prepared via sonication
Freezing Drying Thermal-annealing
Washing
trGO-S sponge
(a)
(e)
200 μm
Figure 3. Formation mechanism of trGO sponge. (a) Illustration
of the formation mechanism of trGO-B-LN sponge. (b) Optical
image of detergent bubbles prepared with the stirring rate of 250 r
min-1, showing intact pentagon and hexagon. (c) Optical image of
wet GO bubbles before freezing process. Inset: Optical image of
small bubbles prepared with high stirring rate of 5000 r min-1. (d)
SEM image of GO sponge shows an inherited pentagon of the
raw bubbles. (e) Illustration of the formation mechanism of
trGO-S-LN sponge.
By adding sugar, the viscosity of liquid in the
membranes of bubbles is improved, resulting in the
increased stability of bubbles. In the following
freezing process, sugar precipitates out at low
temperature and graphene sheets were filamentous
assembled. Materials frozen with refrigerator have
smooth walls, since the GO bubbles have enough
time to drainage, rupture and topological
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6 Nano Res.
rearrangement under a relatively low freezing rate.
With respect to trGO-S-LN sponge, the formation
mechanism is ice templating (Figure 3e). The
existence of detergent is conducive to better
dispersion of GO sheets. In the following
thermal-annealing and washing process, amorphous
carbon particles stay in the materials due to the
pyrolysis of the detergent. The pore size directly
depend on the speed of ice crystallization formed in
the freezing process.[18]
2.4 Absorption Measurement and Mechanical
Property
Due to the abundant pores in the spongy materials,
they exhibit good absorption capacities.
TrGO-B-250-LN sponge prepared from clusters of
bubbles can absorb both water and diesel oils (Figure
4a, Movie S2, S3). Our spongy material with visible
macroscopic pores is shown in Figure S5a, facilitating
the permeation of water into the structure.
(a)
(c)
0 10 20 30 40 500
5
10
15
20
(
Pa
)
(%)
30%
50%
0.7 0.8 0.9 1.0 1.1 1.2
100
200
300
400
500
trGO-250 sponge
trGO-ultrasound sponge
Q (
wt/w
t)
Density (g cm-3)
0 10 20 30 40 50
0
10
20
30
40
50
(
Pa)
(%)
Cycle 1
Cycle 10
Cycle 50
Cycle 100
0 10 20 30 40 50 60 70
0
20
40
60
80
(%)
30%
50%
70%
(P
a)
0 2 4 6 8 10
0
40
80
120
160
Q (
wt/w
t)
Cycle Number
(b)
(d)
(e) (f)
0
30
60
90
120
150
Q (
wt/w
t)
Different Materials
1
5
4
3
21
2 3
4
5
6
0 10 20 30 40 50
0
300
600
900
1200
1500
1800
σ (
Pa)
ε (%)
30%
50%
0 10 20 30 40 50
0
1000
2000
3000
4000
σ (
Pa)
ε (%)
30%
50%(g) (h)
Figure 4. Absorption and compression mechanical properties of
trGO sponge. (a) trGO-B-250-LN sponge paper can absorb water
on the leaves of plant and diesel oil (dyed with oil Blue) on the
plate. (b) trGO-S sponge shows excellent absoprtion of vegetable
oil film (dyed with oil Blue) spreading on water surface. (c)
Absorption capacity (Q) of trGO-B-250-LN and trGO-S-LN
sponges for a range of oils and organic solvents. The numbers
(1-6) represent acetone, ethanol, methanol, diesel oil, vegetable
oil and ethylene glycol, respectively. (d) Comparison of Q of
different materials. The numbers represent graphene foams [25]
(1), graphene sponge prepared by hydrothermal method [39] (2),
graphene sponge prepared by hydrothermal method with the
assistance of thiourea [43] (3), carbon nanotube sponge [44] (4)
and our trGO-S-LN sponge (5). The inset shows absorption
recyclability of trGO-S-LN sponge (apparent density=2 mg cm-3)
for hexane, which can be removed under 85oC. Triangles: the
restore weight of sponges after removing of hexane, squares: the
weight gain after absorption of hexane during different cycles. (e)
Loading and unloading compressive stress-strain curves of
trGO-B-250-refrigerator sponge (8 mg cm-3) at different set
strains of 30%, 50% and 70%, respectively, indicating complete
recovery. Inset: Magnified part at compressive strain of 30%
and 50%. (f) Cyclic stress-strain curves of the same sponge in (e)
at a maximum strain of 50%.(g,h) Loading and unloading
compressive stress-strain curves of trGO-S-LN sponges (2 and
7.9 mg cm-3 , respectively) at different set strains of 30%, 50%
Although thermal reduced graphene sheets are
hydrophobic, bulk water absorption of reduced
sponge graphene was reported by the group of Sun
et al. [42], due to the high cavity content and
appropriate pore size of the material. In order to
explore the mechanism of water absorption reason of
our material, trGO-B-250-LN sponge prepared with
pure surfactant of SDS instead of detergent were
prepared, exhibiting hydrophobicity (Figure S5b).
Thus, the water absorption ability of trGO-B-LN
sponges mainly originated from residual hydrophilic
groups of the commercial detergent. trGO-S-LN
sponge prepared with GO/SDS solution can actively
absorb oils spreading on water surface (Figure 4b).
The digital photos of a water drop on the surface
(Figure S5c) and the rapid absorption of oil drop
(Figure S5d) show excellent water resistance and
lipophilicity.
The absorption capacity is defined as Q, which
means the ratio of the final weight to the initial
weight after full absorption. Figure 4c demonstrates
the absorption for a wide range of organic solvents
and oils, with Q of trGO-B-250-LN sponge as 80-149
g g-1 with the increase of liquid density. Q of the
trGO-S-LN sponge is 260-450 g g-1, owing to the low
density and high porosity. In the case of hexane, the
adsorption capacity of trGO-S-LN sponge (Q=125) is
higher than a variety of spongy materials, including
graphene foam (36×) [25], graphene sponge prepared
by hydrothermal method (44×) [39], graphene sponge
prepared by hydrothermal treatment with the
assistance of thiourea (75×) [43] and carbon nanotube
sponge prepared by CVD method (90×) [44], as
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7 Nano Res.
shown in Figure 4d. The absorbed hexane can be
removed under heat treatment (85℃), making the
sponge reusable (inset of Figure 4d). The sponges still
keep their high adsorption capacity after 10 cycles,
acting as ideal candidates for practical applications in
the removal of organics.
Compression tests were conducted on
trGO-B-250-refrigerator sponges. The compression
curves show nearly complete recovery after 30%-70%
strain (Figure 4g). The stress at the strain of 70% is
just over 70 Pa, showing the softness and
ultra-flexibility of the materials. The compression
stress-strain curve of the 100th cycle does not change
obviously compared with the first cycle (Figure 4h),
demonstrating the super-elasticity of materials. For
comparison, trGO-B-250-LN sponges can sustain
higher compression stress than those prepared with
refrigerator (Figure S6a), due to the irreversible
collapse of walls which make the materials compact.
This phenomenon can be explained according to the
microstructures of the two materials, as shown in
Figures 2a and 2d, respectively. Slot pores on the thin
walls and the regular arrangement of graphene
sheets in sponges prepared with liquid nitrogen as
freezing medium (Figure 2a) cause the fragility of the
materials, while the disordered graphene strips in
slowly frozen materials (Figure 2d) could move
easily to rearrange, benefiting the flexibility of the
materials. The comparison of trGO-B-250-LN
sponges with different densities in Figure S6b show
the improvement of mechanical properties by the
increase of materials densities.
For compression test of trGO-S-LN sponges, the
compressive stress is higher than trGO-B-LN
materials at the same strain (Figure 4g, h), due to the
closer arrangement of graphene sheets. The
improvement of stress according to the increase of
density can also be observed with the comparison.
3. Conclusion
In conclusion, we have developed a simple
strategy to prepare two kinds of sponges by the
synergistic effect of surfactant and freeze-drying. The
comparison of all the samples is concluded in Table
S2. TrGO-B-LN sponges have hierarchical pores with
the scales ranging from several nanometers to several
hundred microns. The structures can be easily tuned
by changing the freezing rate, stirring rate and
adding additives. The formation mechanism was
proposed as the self-assembly of GO sheets, copying
the morphologies of original liquid-filled detergent
bubbles. TrGO-S-LN sponge has directional aligned
graphene sheets, and the existence of surfactant in
the fabrication process benefit the dispersion of GO
sheets. The two kinds of materials both exhibited
excellent adsorption ability and compression
performance, showing great potential in cleaning
areas. The surfactant can be removed by heat
treatment and subsequent washing process, making
the approach a promising method for the structural
design and the synthesis of hierarchical porous
materials. The porous, flexibility, light weight of the
materials enable many other applications, like
electrodes, absorbents, storage media, carriers, etc.
4. Experimental Section
Preparation of trGO-B-LN sponge: GO was
purchased from XFNANO. Detergent bubbles were
prepared by adding 2g of laundry detergent (Tide
powders from P&G, Guangzhou, China) into 100 mL
of DI water with subsequent stirring rate of 250 r
min-1. Then 2 mL of GO dispersion (10 mg mL-1) was
added to 300 mL of laundry detergent bubbles. GO
sheets were uniformly distributed on the wall of
bubbles by evenly stirring, followed by
rapid-freezing with liquid nitrogen. The as-prepared
GO frozen network was vacuum freeze drying for 24
h. Finally, the trGO-B-250-LN sponge was obtained
by thermal annealing at 300℃ in Ar. For comparison,
slower freezing with refrigerator
(trGO-B-250-refrigerator sponge), higher stirring
rates of 5000 and 20000 r min-1 (trGO-B-5000-LN and
trGO-B-20000-LN sponges), higher volume ratio of
GO dispersion and detergent bubbles were also
investigated, respectively.
Preparation of trGO-S-LN sponges: 2 mL of GO
suspension (10mg mL-1) was added into 8 mL of the
lower detergent solution. Mixed solution was
obtained by ultrasound instead of stirring. The
mixture was then frozen by liquid nitrogen, followed
by vacuum freeze drying and thermal reduction at
300℃. Sponges prepared with pure GO suspension
and high GO concerntration were also prepared for
comparison.
Materials Characterizations: The morphologies of
materials were characterized with SEM (LEO 1530)
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8 Nano Res.
and TEM (JEOL 2010). Compression mechanical
properties were evaluated with INSREON 5943. Pore
size distribution was measured with
Aurosorb-iQ2-MP. The BJH model was employed to
calculate the pore size distribution. XPS was carried
out with ESCALAB 250Xi.
Solvents and oils absorption by graphene sponge:
The absorption capacity of sponges was measured
for a variety of organic solvents and oils with
different densities, including ethanol, acetone,
methanol, diesel oil, vegetable oil and ethylene glycol.
The weight before and after absorption was recorded.
In the absorption cycles, organic solvents of hexane
was used, which can be easily removed under heat
treatment (85 oC).
Acknowledgements
This work is supported by Beijing Natural Science
Foundation (2122027), National Program on Key
Basic Research Project (2011CB013000), National
Science Foundation of China (51372133), Tsinghua
University Initiative Scientific Research Program
(2012Z02102).
Electronic Supplementary Material: SEM, TEM
images, Raman spectrum, N2 adsorption-desorption
isotherms and pore size distribution of
trGO-B-250-LN sponge, XPS spectrum and elements
analysis of GO-B-250-LN sponge after heat treatment
and washing, SEM images of graphene sponge with
different treatment, hydrophilicity and
hydrophobicity of sponges, compression stress-strain
curves of trGO sponge prepared with different
parameters. Comparison of all the samples in the
manuscript. The Supplementary Material is available
in the online version of this article at
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Nano Res.
Electronic Supplementary Material
Three-Dimensional Porous Graphene Sponges
Assembled with the Combination of Surfactant and
Freeze-drying
Rujing Zhang1, Yachang Cao1, Peixu Li2, Xiaobei Zang1, Pengzhan Sun1, Kunlin Wang1, Minlin Zhong1,
Jinquan Wei1, Dehai Wu2, Feiyu Kang1,3, Hongwei Zhu1,3,4 ()
Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)
20 μm
(d)
100 μm
(c)
200 μm
(a)
30 μm
(b)
Figure S1. SEM images of trGO-B-250-LN sponge with hierarchical pores. (apparant density~8 mg cm-3) (a) GO
sponge before thermal annealing holds ordered, hundreds of micron-large, quasi-round pores with small slot pores
on the walls. (b) Enlarged view of the slot pores in a. (c) trGO-B-250-LN sponge. The yellow lines and circles on
the image indicate the slot pores and round pores on the walls respectively. (d) High magnification image of
near-round pores on the connection joint of large pores.
Address correspondence to H. W. Zhu, hongweizhu@tsinghua.edu.cn
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Nano Res.
(a)
200 nm
(c)
50 nm
(b)
200 nm
(e) (f)
1000 1500 2000 2500 3000 3500
2D-band
G-band
Inte
nsity (
a.u
.)
Raman shift (cm-1)
D-band
(d)
1 10 1000.000
0.005
0.010
0.015
0.020
dV
/dD
(cm
3 g
-1 n
m-1
)
Pore diameter (nm)
0.0 0.2 0.4 0.6 0.8 1.00
40
80
120
160
200
Quantity
Adsorb
ed (
cm
3g
-1 S
TP
)
Relative Pressure (P/P0)
Figure S2. (a-c) TEM images of trGO-B-250-LN sponge. (a) Clean graphene sheets for some parts. (b-c) Pores in
scale of tens of nanometers (b) and several nanometers (c) can be observed on the materials due to residual
impurities produced by the thermal annealing of laundry detergent. (d) Raman spectrum of the prepared
trGO-B-250-LN sponge. (e,f) N2 adsorption-desorption isotherms (e) and pore size distributions (f) of the
materials.
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Nano Res.
0 200 400 600 800 1000 1200 1400
Inte
nsity
Binding Energy (eV)
trGO-250 sponge after washing
GO-250 sponge
C 1s
O 1s
Na 1s
S2pSi2p
Ca 2p
Figure S3. XPS spectra of GO-B-250-LN sponge before and after heat treatment and washing. The material is
prepared with stirring rate of 250 r min-1 with liquid nitrogen as the freezing medium.
Table S1. Elements analysis of GO and trGO sponges washed by DI water, keeping consistent with Figure S3b.
Element Atomic % (GO sponge)Atomic %
(trGO sponge after washing)
C 59.09 69.8
O 29.5 23.64
Na 6.6 1.46
S 3.5 0.64
Si 0.6 3
N 0.53 0.91
P 0.17 0.14
Ca 0.02 0.41
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Nano Res.
100 μm
(b)
100 μm
(c)
30 μm
(d)
10 μm
100 μm
(a)
Figure S4. SEM images of graphene sponge with different treatment. trGO-B-LN sponge with stirring rate of (a)
5000 r min-1 and (b) 20000 r min-1, with subsequently frozen by liquid nitrogen. (c) trGO-B-20000-LN-sugar
sponge (d) High magnification image of (c). The inset shows the enlarged view, revealing the granular structures
on the wall.
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Nano Res.
(c)(a)
(b) (d)
Figure S5. (a) trGO-B-250-LN sponge prepared with commercial detergent can absorb water. The pores are
visible to the naked eyes. (b) trGO-B-250-LN sponge prepared with pure SDS is hydrophobic. (e) A water drop on
the surface of the trGO-S-LN sponge prepared from GO/SDS solution, showing its hydrophobility. (f) It is easy
for a diesel oil drop to penetrate into the trGO-S-LN sponge.
0 5 10 15 20 25 300
10
20
30
40
(
Pa)
(%)
10%
30%
0 5 10 15 20 25 300
10
20
30
40
50
60
0 2 4 6 8 100
5
10
15
20
/P
a
(
Pa
)
(%)
Low density
High density
(b)(a)
Figure S6. Compression stress-strain curves of trGO-B-250-LN sponge prepared with liquid nitrogen as freezing
medium. (a) Curves at the set strain of 10% and 30%. (apparent density=8 mg cm-3) (b) Comparison of sponges
with low (density=8 mg cm-3) and high density (density=11.4 mg cm-3) under the set strain of 10% (inset) and
30%.
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Nano Res.
Table S2. Comparison of the preparation, structure, absorption ability and compression performance of all the
samples in the manuscript.
Controllable
FactorsSamples and Preparation Structure
Absorption
Capacity
(g g-1)
Compressive
Stress
(strain=30%)
Freezing rate
Stirring Rate
Additives
trGO-B-250-refrigerator sponge
(assembled from surfactant bubbles
with the stirring rate of 250 r min-1 and
frozen by refrigerator)
Disordered pores
with no pores on the
wall -- Dozens of Mpa
trGO-B-250-LN sponge
(assembled from surfactant bubbles
with the stirring rate of 250 r min-1 and
frozen by liquid nitrogen)
Ordered quasi-round
pores with slot pores
on the wall 80~149 Dozens of MPa
trGO-B-5000-LN sponge
(assembled from surfactant bubbles
with the stirring rate of 5000 r min-1
and frozen by liquid nitrogen)
Near-round pores
with irregular pores
on the wall -- --
trGO-B-20000-LN sponge
(assembled from surfactant bubbles
with the stirring rate of 20000 r min-1
and frozen by liquid nitrogen)
Disordered irregular
pores
-- --
trGO-B-20000-LN-sugar sponge
(assembled from surfactant bubbles
with the stirring rate of 20000 r min-1
and frozen by liquid nitrogen. The
surfactant is mixed with sugar)
Quasi-round pores
with filamentous
assembled graphene
sheets-- --
Lower part
trGO-S-LN sponge
(assembled from GO/surfactant
sonication solution and frozen by
liquid nitrogen)
Framework with
directional aligned
graphene sheets260~450 Thousands of
MPa
Recommended