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Colloids and SurfacesA: Physicochem. Eng.Aspects 445 (2014) 48–53

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

 journal homepage: www.elsevier .com/ locate /colsur fa

Thermal and electrical properties of graphene/carbon nanotube

aerogels

Zeng Fana, Daniel Zhi Yong Tnga, Clarisse Xue Ting Lima, Peng Liua, Son Truong Nguyena,Pengfei Xiaoa, Amy Marconnetb, Christina Y.H. Lima, Hai M. Duonga,∗

a Department of Mechanical Engineering, National University of Singapore, Singaporeb School of Mechanical Engineering, PurdueUniversity, USA

h i g h l i g h t s

•  Studying effects of  reducing agents

and CNTs on morphologies and prop-

erties of GAs.•  Annealing and CNTs enhance electri-

cal conductivitiesof theGAsuptofive

times.•   The thermal conductivity of  the GAs

is measured to be∼0.10W/(mK).•   The thermal stabilities of  annealed

GAs are higher than those of  non-

annealed GAs.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

 Article history:

Received 5 August 2013

Accepted31 December2013

Available online 18 January 2014

Keywords:

Grapheneaerogel

Carbon nanotubes

Reduction method

Thermal annealing

Electrical property

Thermal conductivity

a b s t r a c t

Grapheneandcarbon nanotube(CNT) aerogelsprovide combinationsof mechanical,thermal, andelectri-

cal properties that are interesting for a variety of applications. In this work, the impact of three different

reducing agents (l-ascorbic acid (LAA), HI and NaHSO3) and carbon nanotubes on the morphologies and

properties of  the graphene aerogels (GAs) are studied systematically. Additional, the impact of  ther-

mal annealing at 450 ◦C for 5h under Ar environment is also investigated. Annealing treatment and the

addition CNTs enhance the electrical conductivities of  the GAs up to a factor of  5. Thermal annealing

impacts the surface area of the GAs. Specifically, the surface areas of those reduced by HI and NaHSO3

decreased by 30%, while those reduced by LAA or dispersed with CNTs increased by 15%. The thermal

conductivity of  the highly porous GAs is measured using an improved infrared microscopy technique

to be ∼0.10W/(mK). The optimization of the nanostructures and properties of the GAs is important for

various applications, such as energy storage devices and nanocomposites.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

With the looming effects of global warming, the world is look-

ing at new and safe methods of energy storage. The extraction of 

fossil fuels has had significant implications on our environment.

Currently, fossil fuels together with nuclear energy account for

93% of theworld’s energy needs.However, as energyconsumption

increases, researchers estimate that fossil fuels will be depleted in

∗ Corresponding author. Tel.: +65 97699600.

E-mail address: mpedhm@nus.edu.sg(H.M. Duong).

the next 50 years. Environmentally friendly and renewable energystorage methods are urgently needed.

Graphene aerogels (GAs) have unique 3D mesoporous struc-

tures andoutstanding properties such as lightweight, large surface

area and good electrical conductivity. Thus, GAs have recently

received much attention as promising materials for sensors [1,2],

energy storage devices [3–6] and catalysis [7,8]. Several methods

have been reported on the fabrication of the 3D GAs [9–15]. Wors-

ley et al. [9,10] used Resorcinol (R) and Formaldehyde (F) sol–gel

chemistry method to fabricate graphene-based aerogels with RF

as the binders. A cost-effective hydrothermal fabrication method,

[11,12] which does not require binders, lead to GAs having good

0927-7757/$ – see front matter © 2014 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.colsurfa.2013.12.083

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 Z. Fan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 445 (2014) 48–53 49

electricalconductivity anda relatively largesurface area. However,

this hydrothermal method required high reaction temperatures

and pressures and was unable to be scaled up for mass produc-

tion. Zhang et al. [13,14] improved the hydrothermal method and

successfully developed GAs by a chemical reduction method at

low temperatures without using any binder. The GAs exhibited

a large surface area, up to 512m2/g, and good electrical conduc-

tivity – approximately 102 S/m. Later Chen and Yan [15] reported

the mechanical and electrical properties of the GAs prepared by

a chemical reduction method of graphene oxide (GO) with vari-

ous reducing agents. After a thermal annealing treatmentunder an

inert environment, the electrical conductivity of the annealed GAs

was 5.1 times higher than the non-annealed GAs [15]. However,

the impact of different reducing agents on the GA nanostructure

has not been investigated.

In addition, graphene sheetsandcarbonnanotubes(CNTs)have

also sparked a very high interest among researchers. There are

many potential applications in many fields because of their excel-

lent electrical, thermal andmechanicalproperties. A one-of-a-kind

symbiotic relationship exists between one-dimensional CNTs and

two-dimensional graphene sheets. The fabrication of ultralight

graphene–CNT hybrid aerogels is promising for the development

of ideal materials for efficient energy storage devices such as

lithium ion batteries, transparent conductors and water purifica-

tion [16–18]. Integrating CNTs into the GAs could enhance their

electrical and mechanical properties [19].

Due to the extraordinary thermal conductivity of graphene,

graphene-reinforced polymer nanocomposites have been stud-

ied for various advanced applications including electronic thermal

management devices [20], thermal pastes [21] and smart polymers

[22,23]. Compared to graphene, the highly porous network struc-

ture of the GAs make GA nanocomposites lighter but very strong

[24]. However, up to now, there are very limited reports on the

thermal conductivityof theGAs. Specifically,Zhonget al. [24] mea-

sured only a GA sample having an extremely low surface area of 

43.15 m2/g by the laser flash technique.

In this paper, we systematically investigate the effects of vari-

ous reducing agents (l-ascorbic acid, HI and NaHSO3), annealingprocesses, and CNT concentrations (0.5–2.0mg/ml) on the mor-

phologies, electrical properties and thermal stabilities of the GAs.

Samples are annealed at 450 ◦C for 5h under an Ar environment

to enhance the electrical conductivities. X-ray diffraction (XRD) is

used to explain the nanostructure change of the GAs before and

after theannealingprocesses. Also, the thermal conductivityof the

GAs is measured by a comparative infrared microscopy technique

for the first time. The experimental results are useful to optimize

the nanostructures of GAs for the best performance electrodes of 

energy storage devices such as supercapacitors and lithium bat-

teries as well as nanocomposites, which benefit from the large

surface area and high electrical and thermal conductivity of the

GAs.

2. Experiments

 2.1. Materials

Graphite powder, sodium nitrate (NaNO3), potassium perman-

ganate (KMnO4), concentrated sulfuric acid (H2SO4), hydrogen

peroxide (30% H2O2), hydrochloric acid (HCl), l-ascorbic acid,

sodium bisulfate solution (NaHSO3) and hydroiodic acid (HI,

containingno stabilizer)werepurchasedfromSigma–AldrichCom-

pany Ltd. Ethanol waspurchasedfrom Fluka.CNTs were purchased

from Timesnano, China. All the chemicals were used as received

without furtherpurification. Grapheneoxidewas synthesized from

the graphite powder for the GA synthesis. For comparison of the

morphology and properties, GAs were also synthesized from GO

purchased from Timesnano, China.

 2.2. Synthesis of graphene aerogels (GAs)

Initially graphene oxide was prepared by oxidation of graphite

powder using H2SO4, NaNO3, according to a modified Hum-

mers’ method [25–27]. After the excessive oxidizing agents were

removed by H2O

2, graphite oxide was then separated from the

mixture by centrifugation and washed with HCl and deionized

(DI) water for purification. To convert graphite oxide into GO, the

as-prepared graphite oxide was dispersed into DI water by ultra-

sonication to form a 0.1–0.5wt% aqueous suspension. GO powder

wasfinallycollected bycentrifugationanddriedat 60◦C forseveral

days.

GAs were preparedbychemical reductionof theGO nanosheets.

A 2mg/ml GO aqueous suspension was prepared. Three reducing

agents(l-ascorbicacid, HI,andNaHSO3) were respectivelyadded to

theGOsuspension andsonicateduntil completely dissolved. Then,

the reaction mixture was heated at 95◦C for 5h to form graphene

hydrogels. The obtained graphene hydrogels were immersed in DI

water for 3 days to remove excessive reducing agents and then

placed into ethanol for solvent exchange for another 3 days. The

wet gels were finally dried with supercritical CO2 to form the GAs.

In order to enhance the electrical conductivities of the GAs, the

as-prepared GAs were annealed at 450 ◦C for 5h underan Ar envi-

ronment.

 2.3. Synthesis of CNT–graphene hybrid aerogels

To form hybrid CNT–graphene aerogels, 10mg MWNTs were

added into 10ml DI water and dispersed with an ultrasonic pro-

cessor (Sonics, VCX130). This solution wascombined with 10ml of 

4 mg/ml GO suspension and sonicated for 4h to obtain a homoge-

nous CNT-GO complex. Then, 160mg LAA was added to the above

suspension and sonicated for 90s to completely dissolve LAA.

The reaction mixture was heated at 95 ◦C for 5 h to form the

CNT–graphene hybrid hydrogels. After solvent exchange in DIwater and ethanol, supercritical drying yielded the CNT–graphene

hybrid aerogels.

 2.4. Characterization

Bulk densities were determined from the GA weight and

size. The morphology of the CNT–graphene hybrid aerogels was

observed with a field emission scanning electron microscopy

(Model S-4300, Hitachi, Japan) with field-emission-gun operating

at 10kV. X-ray diffraction (XRD) was used with a diffractometer

(Model XRD-6000, Shimadzu, Japan) to investigate how the struc-

turesofGAswereaffectedbydifferent reducingagentsand thermal

annealing. Morphologies of the resulting GAs, such as specific sur-

face area, BJH pore size distribution, and total pore volume werecharacterized by nitrogen adsorption/desorption measurements

with a Nova 2200e (Quantachrome). Prior to characterization, the

samples were degassed under vacuum at 120 ◦C for 2h to remove

water and other physically adsorbed species. The bulk electrical

conductivity was measured using a Solartron 1260+ 1287 electro-

chemical system with twocopper plate electrodes attached to the

twoflat surfaces of thecoin-shaped sample. TGAwas conductedon

a DTG60H thermogravimetric analyzer from room temperature to

1000 ◦C in air with a heatingrate of5 ◦C/min.

The thermal conductivity of the GAs was measured using a

comparative infrared microscopy technique [28]. Briefly, a three-

layered stack consisting of a GA sample sandwiched between

two PDMS layers (with thermal conductivity of 0.18W/(mK)) was

attached to a heat sink at the bottom and a resistive heater on

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50   Z. Fanet al./ Colloids and Surfaces A: Physicochem. Eng. Aspects 445 (2014) 48–53

Fig. 1. Image of a CNT–graphene hybrid aerogel.

the top. The resistiveheatergeneratesa one-dimensionalheat flux

through thestack. A VarioCAM high resolution thermographic sys-

temcaptured the temperature distributionacross thewhole stack.

The thermal conductivity of the GA was then calculated from the

ratio of the temperature gradients in the PDMS compared to the

GAs according to Fourier’s law.

3. Results and discussion

The graphene hydrogels are formed by – stacking dur-

ing the reduction of hydrophilic GO to hydrophobic graphene

nanosheets [13,14]. In this letter, the choice of reducing agent

(LAA, HI, or NaHSO3) impacts the morphologies, electrical proper-

ties and thermal stability of the GAs. Additionally, the mechanism

of CNT–graphene hybrid aerogel formation may be also due to

the – interaction between graphene nanosheets as well as that

betweengrapheneandCNTs.During thereductionof GOin itsaque-

ous suspension, themajority of theCNTs were physically wrapped

inside the3D network of graphene hydrogel,while minority of the

CNTs may form

–

interaction with the reduced graphene oxidenanosheets. Fig. 1 shows an as-prepared CNT–graphene hybrid

aerogel byusingLAAhaving 10-mmdiameterand3-mmthickness.

 3.1. Effect of reducing agents and CNTs on the morphologies of 

GAs

 3.1.1. Reducing agent effects on the morphologies of GAs

The effect of reducing agents on the morphology of GAs was

investigatedwhen 2mg/mlGOsuspension was reduced by LAA, HI

andNaHSO3 under thesame synthesis conditions andtheamounts

of three reducing agents were enough for a sufficient reduction

[13,15]. The GA morphologies were characterized by nitrogen

Fig. 2. An SEM image of CNT–graphene hybrid aerogels.

adsorption/desorption measurements to show the specific surface

area and pore size, obtained by Brunauer–Emmett–Teller (BET)

and Barrett–Joyner–Halenda (BJH) methods, respectively. Table 1

summarizes the morphologies of the GAs reduced by LAA, HI and

NaHSO3 with and without thermal annealing conditions.The surface areas of the GAs reduced by LAA, HI and NaHSO3

were423,385and679m2/grespectivelyduetothe differentreduc-

tion effectiveness of the three reducing agents. The more efficient

reducing agent (e.g. HI) remove more functional groups (–OH,

–COOH, –O–, etc.) during the reduction processes and form more

– bonding and stacks between graphene nanosheets, which

decreases the surface area of the GAs.

After thermal annealing of the GAs, the surface areas of the GAs

reduced byHI and NaHSO3 decreased to 281 and 582 m2/g, respec-

tively. Theannealingprocess further removes thefunctionalgroups

of the GAs and causing them to further condense. More– stack

formation between the graphene sheets causes decreased GA sur-

face area. However, the surface area of the GAs reduced by LAA

increased to 487m2

/g after thermal annealing. There is no clearexplanation to explain the increase in surface area for the LAA-

reduced GAs, although it is possible that the thermal annealing

process may decrease the– interaction amonggraphene sheets

anddehydroascorbic acid (the oxidized form of LAA), in addition to

removing the functional groups on graphene sheets.

 3.1.2. CNTs effects on the morphologies of GAs

Differentconcentrationsof CNTs (0,0.5,1.0 and2.0mg/ml)were

added into the GAs to form CNT–graphene hybrid aerogels. Fig. 2

shows an SEM image of the interconnected 3D network within

theCNT–graphenehybridaerogel, where theCNTs were dispersed

inside the graphene sheet network. As seen in Table 1, the surface

 Table 1Effectof reducing agentand addition of CNTs on morphology, electrical conductivity, and thermal stability of theGAs.

Thermal annealing Surface area (m2/g) Pore size (nm) Density (g/cm3) Electrical conductivity (S/m) DTApeak temperature (◦C)

Reducing agents

LAA Without annealing 423 3.6 0.030 2.08±0.20 539

With annealing 487 3.6 0.031 4.12±0.53 568

HI Without annealing 385 3.6 0.018 3.29±0.44 591

With annealing 281 3.6 0.016 5.90±0.04 598

NaHSO3   Without annealing 679 3.6 0.020 1.67±0.03 540

With annealing 582 3.6 0.022 9.52±0.38 542

CNTs concentration (mg/ml) (LAA as the reducing agent)

0 – 738 3.6 0.037 0.20±0.01 516

0.5 – 844 3.6 0.031 0.33±0.03 495

1 – 788 3.6 0.036 0.56±0.09 496

2 – 636 3.6 0.041 0.47±0.06 532

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Fig.3. Effect of reducingagentsand thermalannealing on(a)electrical conductivity

and (b) thermal stability of theGAs.

area of theCNT–graphenehybridaerogels increasedwith CNTcon-

centrations up to 1 mg/ml. The surface area of the CNT–graphene

hybrid aerogels increased due to: (i) the additional surface area of thedispersed CNTs and (ii) separation of thegraphene nanosheets

bytheCNTs during theself-assemblyprocesses.However,whenthe

CNT concentration was further increased to 2mg/ml, the surface

areaof theCNT–graphenehybridaerogeldecreasedtolessthanthat

of the pure GAs. At high CNT concentrations, local agglomeration

and bundling of the CNTs may block the pores of the GAs.

 3.2. Effect of reducing agents and CNTs on electrical properties

 3.2.1. Reducing agent effects on electrical property of GAs

The electrical conductivity of the GAs were measured by a

two-probemethod and calculated from the linear current–voltage

curve. TheHI-reduced GAs have thehighest electrical conductivity

of3.29S/m, while the LAA and NaHSO3-reduced GAshad relativelylower electrical conductivities of 2.08 and 1.67S/m, respectively

(see Table 1 and Fig. 3). The differences in the electrical conduc-

tivities of the GAs reduced by different reducing agents can be

explained by the morphology differences. The GA with a lower

surface area has a higher electrical conductivity due to the more

densely packed nanostructure which enhances the electron trans-

port.

Fig. 4 shows the effects of the thermal annealing on the struc-

tures and properties of the GAs. The XRD spectrum for all the

GAs shows a broad peak around 23.5◦∼25◦ before annealing,

which is consistent with previous reports [14,29–31] and indicates

the formation of non-crystalline structures by the – stacking

among the graphene nanosheets. For the annealed GAs, the XRD

spectrum shifts to the right indicating the formation of a more

Fig. 4. XRD patterns of the GAs reduced by different reducing agents with and

without thermal annealing.

Fig. 5. Electrical conductivity andDTApeak temperature for CNT–graphene hybrid

aerogelswith various CNT concentrations.

condensed nanostructure after the thermal annealing treatment.

Duringthermalannealing,more functionalgroupsmaybe removed

and stronger – interactions could form among the graphene

sheets. Therefore, the annealed GAs had much higher electrical

conductivities than the non-annealed GAs.

 3.2.2. CNTs effects on electrical property of GAs

The addition of CNTs into the GAs also enhances the electri-cal conductivities, as shown in Table 1 and Fig. 5. When 0.5mg/ml

CNTs is added into the GA, the electrical conductivity of the

CNT–graphene hybrid aerogel is 0.33S/m, which is a 65% increase

compared to the pure GA. When the concentration of CNTs was

1 mg/ml, the electrical conductivity of the CNT–graphene hybrid

aerogel reaches a maximum of 0.56S/m, while 2.0mg/ml CNTs

slightly decreased theelectrical conductivityof the CNT–graphene

hybrid aerogel to 0.47 S/m.

Within the CNT–graphene hybrid aerogels, CNTs could create

more channels for efficient electron transfer between graphene

sheets and therefore enhance their electrical conductivities. How-

ever, when the CNT concentration exceeded 1mg/ml, the local

agglomeration of CNTs reduced the electrical conductivities of the

GAs.

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52   Z. Fanet al./ Colloids and Surfaces A: Physicochem. Eng. Aspects 445 (2014) 48–53

Fig. 6. TGA and DTA of the GAs reduced by LAA (a) without and (b) with thermal

annealing.

 3.3. Effect of reducing agents and CNTs on thermal stability of GAs

Sincetheelectrodesof theenergystoragedevices arerequiredto

work at elevated temperatures, the thermal stabilities of theGAs is

investigatedbyTGA and DTA.Asshown inFig. 3(b), theHI-reduced

GAshowsaDTApeakat598 ◦C,whichwashigher than theLAA-andNaHSO3-reducedGAs(approximately540 ◦C).DuringTGAtests,the

remaining functional groups on the graphene nanosheets are first

removed,beforethe GAreactwith theoxygen inanairatmosphere.

The increased thermal stability of the HI-reduced GA is likely due

to the more condensed nanostructure and lower surface area.

As shown in Figs. 5 and 6, the thermal stability of GAs is

affected by the CNT concentrations and the morphologies of the

CNT–graphene hybrid aerogels. When the dispersed CNT concen-

tration is increased up to 1.0mg/ml, the surface areas of GAs

increased and their thermal stabilities decreased due to the more

porous structures. When the CNTs formed agglomeration/bundles,

the surface area of the GAs reduced and the thermal stabilities are

larger dueto themore condensed nanostructures and the stronger

bondingbetweenCNTs–CNTsandgraphene–graphenenanosheets.

 3.4. Thermal conductivity of GAs

Using the comparative infrared thermography technique, the

thermal conductivity is measured for the GAs reduced by LAA

(without thermal annealing). One-dimensional temperature gra-

dients are obtained from the two-dimensional temperature maps

of the PDMS-GA-PDMS stack, as shown in Fig. 7. Temperature

gradients in the reference PDMS regions  ((dT /d x)PDMS)   and in

the sample region ((dT /d x)GA) are calculated through fitting with

the least-square method. The heat flux (q) across the GA sam-

ple can be calculated from the average temperature gradient in

the two PDMS regions and the thermal conductivity of PDMS

(kPDMS =0.18W/(mK)). Since each layer of the three-layered stackhasthesame cross-section, theheat fluxacross thestack shouldbe

constant and from Fourier’s Law,

q = −kPDMS ×

dT 

d x

PDMS

= −kGA ×

dT 

d x

GA

, (1)

andthethermal conductivityof theGA canbecalculated. The ther-

mal conductivity of the GA is ∼0.10W/(mK). It should be noted

that the temperature gradient in the cold-sided PDMS layer (near

theheat sink)wasfar smaller than that in thehot-sided PDMS layer

(near theresistiveheater), which indicateda considerableheat loss

from the three-layered stack, especially from the GA. Because the

highporosityof theGA andlowconductivity, convectiveheat losses

from the GA could be significant and more detailed studies of the

thermal properties of the GAs are merited.

Fig.7. Temperaturedistribution andlinearbestfit curvesfor thethree-layeredstack

consisting of a GA sample sandwiched between twoPDMS layers.

4. Conclusions

In summary, GAs were synthesized by a chemical reduction

method with three reducing agents (LAA, HI and NaHSO3) and a

thermal annealing treatmentwas further conductedon these sam-

ples to enhance the electrical conductivity by up to five times.

The results showed that the GAs synthesized with more effi-

cient reducing agent exhibited a higher electrical conductivity

and lower surface area, while those with low-efficiency reduc-

ing agent possessed a lower electrical conductivity and higher

surface area. Furthermore, CNT–graphene hybrid aerogels were

successfully developed by adding CNTs during the GA synthesis.

The addition of CNTs enhances the surface area up to 14% and the

electrical conductivity up to 2.8 times compared to the pure GAs.

Thermal stability of the GAs is affected by the choice of reducing

agent and CNT concentrations. The DTA peak temperature can be

optimized by reducing agent selection, thermal annealing treat-ment, or the addition of CNTs. The thermal conductivity of the GAs

was measured by a comparative infrared microscopy technique

and is∼0.1W/(mK). These results provide insight into optimizing

the nanostructures and properties of the GAs for various applica-

tions including the development of the electrodes, energy storage

devices, and nanocomposites in general.

 Acknowledgments

The authors deeplyacknowledge theStart up grant R-265-000-

361-133 andSERC2011 PublicSector ResearchFunding (PSF)Grant

R-265-000-424-305for the funding support.Wealso thankProf. Lu

Li and Asst. Prof. Chua Kian Jon for their support on electrical and

thermal conductivity measurements.

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