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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


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.



Freeze-drying



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, [email protected]

| 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


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


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


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


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)


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



Freeze-drying



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, [email protected]


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.


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


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.


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%.


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


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



with the stirring rate of 5000 r min-1

and frozen by liquid nitrogen)

Near-round pores

with irregular pores

on the wall -- --




and frozen by liquid nitrogen)

Disordered irregular

pores

-- --

trGO-B-20000-LN-sugar sponge



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

three-dimensional porous graphene sponges assembled with ... · graphene sponge, hierarchical,...

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