NaNoscieNce aNd NaNotechNology | NaNophysics, NaNoeNgiNeeriNg
Synthesis and characterization of thermal conductivity of nanofluids
based on Ag decorated-CNTs/graphene hybrid materials
Ngoc Anh Nguyen1, Van Trinh Pham1*, Hung Thang Bui1, Van Chuc Nguyen1, Tuan Hong Nguyen2,
Ngoc Minh Phan1,2,3, Ngoc Hong Phan1,3*
1Institute of Materials Science, Vietnam Academy of Science and Technology
2Center for High Technology Development, Vietnam Academy of Science and Technology
3Graduate University of Science and Technology, Vietnam Academy of Science and Technology
Received 16 May 2017; accepted 14 September 2017
the lower efficiency of heat transfer.
In this work, we present a new nanofluid based on silver nanoparticles
decorated on the functionalized carbon nanotubes-graphene sheet (hybrid)
materials. Briefly, carbon nanotubes and graphene sheets were first
functionalized with a hydroxyl group and carboxyl group respectively. The
hybrid material was decorated with silver nanoparticles via chemical reduction
method with the assistance of sodium hydroxyl. Finally, the obtained Ag-
Therefore, it is enormously important
to increase the heat transfer capability
of fluids. One of the most promising
methods is the addition of solid particles
with high thermal conductivity which
acts as heat carriers for fluids.
hybrid material was dispersed in ethylene glycol solution (EG) to form the
nanofluid without any surfactant. The thermal conductivity of nanofluid was
measured for different weight concentrations at different temperatures. The
results showed an increase in thermal conductivity of up to 86% for 0.045%
weight concentration at 55oC. This enhancement was due to the high thermal
conductivity of graphene, carbon nanotubes (CNTs), and Ag nanoparticles as
well as the higher surface area of Ag nanoparticles decorated on graphene and
CNTs structures. The results of Transmission Electron Microscope (TEM),
X-rays diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR)
indicated that the silver nanoparticles were formed on the surface of carbon
nanotubes and graphene sheets.
Keywords: CNTs, graphene, nanofluids, silver nanoparticles, thermal conductivity.
In 1873, Maxwell was the first
person who proposed the idea of
adding solid particles into fluids to
enhance their thermal conductivity [1].
Subsequently, researchers dispersed
microparticles into fluids to increase
the thermal conductivity of the fluids.
Nevertheless, the added microparticles
would aggregate and settle down [2]. To
address this negative aspect, a great deal
of research was carried out by dispersing
nanoparticles into fluids.
Classification numbers: 5.1, 5.5
In 1995, the term “nanofluid” was
introduced the first time by S. Choi
and J.A. Eastman at Argonne National
laboratory [3]. Generally, nanofluid is
the fluid having stable suspension of
For a decade, the development of
nanotechnology has not only minimized
the size but also improved the working
speed of electronic devices. A serious
problem in electronic devices is heat
in their performance and lifetime. In
order to solve this problem, there are
several methods for heat dissipation,
i.e. utilization of fans, thermal grease
or fluids. Most of the electronic devices
use fluids for heat dissipation such
as distilled water or ethylene glycol.
nanowires, and nanosheets, which are
typically less than 100 nm in size. There
are two phases in the system, one phase
is a liquid phase and the other one is a
solid phase. The nanoparticles used in
nanofluids are metals, oxides, carbides,
and diamond [4-7].
generation during the process of working
However, these basic fluids have poor
CNTs and graphene are the materials
at high power, leading to a decrease
owning very high thermal conductivity
*Corresponding author: Email:,
December 2017 Vol.59 Number 4
Vietnam Journal of Science,
Technology and Engineering
NaNoscieNce aNd NaNotechNology | NaNophysics, NaNoeNgiNeeriNg
(about 3000 W/m.K for CNTs and 5000
thermal conductivity was 22.22% with
synthesize graphene sheets. Potassium
W/m.K for graphene) [8-9]. It is reported
0.1wt% of Ag-graphene at 400C [15].
that by using graphene and CNTs as
additives in nanofluids, the thermal
conductivity went up significantly. CNTs
and graphene are considered as bridges
Amiri et al. reported an enhancement
of thermal conductivity of 25% with
1wt% of Ag-CNTs dispersing in
nanofluids. S.S. Aravind, et al. examined
(NH4)2SO4, sulfuric acid (H2SO4,
98%), nitric acid (HNO3, 68%), thionyl
chloride (SOCl2), tetrahydrofuran
(THF), ethylene glycol (EG), and sodium
or networks for making heat transfer
nanofluids containing graphene-multiwall
faster. Rad Sadri, et al. showed that the
thermal conductivity rose up to 22.31%
for nanofluids containing 0.5wt% of
CNTs. This result was obtained after
40 minutes of ultrasonication at 450C
[10]. Zeinab Hajjar, et al. revealed that
carbon nanotubes (graphene-MWCNTs)
nanocomposite based on DI water and
EG. The results showed that the thermal
conductivity enhancement was 10.5%
and 87.9% with 0.04% volume fraction
of graphene-MWCNTs at 250C and 500C
from Shantou Xilong Chemical Factory
Guangdong, China. Silver nitrate
(AgNO3) and sodium borohydride
(NaBH4) were purchased from Shanghai
Aladdin Bio-Chem Technology Co.
LTD, China.
increasing thermal conductivity depends
on the concentration of graphene oxide
(GO) dispersing in nanofluids. For
example, the thermal conductivity
enhancement was 14.75% with
0.05wt % of GO, the thermal conductivity
increased by 47.57% with 0.25wt% of
GO at 400C [11]. Mehrauli et al. studied
graphene nanofluids with different
concentrations of graphene, specifically
0.025, 0.05, 0.075, and 0.1wt%. The
result displayed that the maximum
thermal conductivity enhanced 27.64%
with 0.1wt% of graphene dispersing
in nanofluids [12]. Several studies also
focused on using metallic or non-metallic
nanoparticles decorated on graphene and
CNTs to enhance heat transfer capability
of nanofluids. For an instant, Baby,
et al. studied nanofluids containing
copper oxide nanoparticles decorated on
graphene. The results depicted that the
enhancement in thermal conductivity was
approximately 28% with 0.05% volume
fraction of CuO-graphene dispersing
in DI water-based nanofluids at 250C
and thermal conductivity enhancement
was 23% with 0.07% volume fraction
in EG-based nanofluids at 500C [13].
This author group also studied the
in DI water, respectively. Whereas in
EG, thermal conductivity enhancement
was 13.7% and 24% at 250C and 500C,
respectively [16]. Recently, T.T. Baby, et
al. studied nanofluids containing silver
nanoparticles decorated on graphene-
MWCNTs based on EG. The results
showed that the enhancement of thermal
conductivity was 8% and 20% with
0.04% volume fraction of Ag/graphene-
MWCNTs at 250C and 500C, respectively
[17]. The presence of metallic or non-
metallic nanoparticles supposed to avoid
the stacking of graphene sheets and
CNTs [18].
In this study, we present the results
of synthesis and thermal conductivity
characterization of nanofluids based
on EG containing Ag nanoparticles
decorated on functional groups of
graphene and CNTs by chemical
reaction method with different weight
concentrations. The Ag-hybrid materials
were synthesized by a simple method
and performed the enhancement of high
thermal conductivity of nanofluids.
Experiment and methods
Nanofluid preparation
Schematic of the synthesis of Ag
nanoparticles decorated on the hybrid
material is shown in Fig. 1. Graphene
sheets synthesized by a plasma-assisted
electrochemical exfoliation process
[19] were functionalized with carboxyl
(-COOH) group by treatment in the
mixture of acid (HNO3: H2SO4, ratio
1:3 respectively) at 700C for 5 hours
under continuous magnetic stirring, then
filtered by distilled water and dispersed
in EG. CNTs were functionalized with
hydroxyl (-OH) group by treatment
with SOCl2 at 600C for 24 hours under
continuous magnetic stirring then
filtered by distilled water and washed
with tetrahydrofuran. After that, they
went through a treatment with EG at
120oC for 48 hours under continuous
magnetic stirring and dispersed in EG.
CNTs-graphene material was dispersed
in EG by ultrasonication for 10 minutes.
A specific amount of AgNO3 (0.05
M) solution was added to the above
solution, under continuous stirring. After
30 minutes, 20 ml of reducing solution
(a mixture of NaBH4 and NaOH) was
added to the above solution dropwise.
The reaction was as follows:
decoration of silver nanoparticles on
graphene and the reported thermal
CNTs were supported by Laboratory
of Carbon Nanomaterials, Institute of
AgNO3 + NaBH4Ag + H2 + B2H6 + NaNO3
After the reducing process completes,
the solution was filtered and washed with
Graphite rod (99.99%) was purchased
distilled water. A calculated amount of
nanofluids at 700C [14]. H. Yarmand
Ag-hybrid material was dispersed in EG
et al. also showed that the increase in
Company to be used as an electrode to
to generate nanofluid by ultrasonication.
Vietnam Journal of Science,
Technology and Engineering
December 2017 Vol.59 Number 4