C A R B O N 8 5 ( 2 0 1 5 ) 7 9 – 8 8

.sc ienced i rec t .com

Avai lab le a t www

ScienceDirect

journal homepage: www.elsevier .com/ locate /carbon

Probing the engineered sandwich network ofvertically aligned carbon nanotube–reducedgraphene oxide composites for high performanceelectromagnetic interference shielding applications

http://dx.doi.org/10.1016/j.carbon.2014.12.0650008-6223/� 2014 Elsevier Ltd. All rights reserved.

* Corresponding authors: Fax: +91 11 45609310.E-mail addresses: skdhawan@mail.nplindia.ernet.in (S.K. Dhawan), bipinbhu@yahoo.com (B.K. Gupta).

Avanish Pratap Singh a, Monika Mishra a, Daniel P. Hashim b, T.N. Narayanan c,Myung Gwan Hahm d, Pawan Kumar a, Jaya Dwivedi a, Garima Kedawat e, Ankit Gupta a,Bhanu Pratap Singh a, Amita Chandra f, Robert Vajtai b, S.K. Dhawan a,*,Pulickel M. Ajayan b, Bipin Kumar Gupta a,*

a CSIR-National Physical Laboratory, New Delhi 110 012, Indiab Department of Mechanical Engineering and Materials Science, Rice University, Houston, TX 77005, United Statesc TIFR-Centre for Interdisciplinary Sciences, Tata Institute of Fundamental Research, Hyderabad 500 075, Indiad Advanced Functional Thin Films Department, Surface Technology Division, Korea Institute of Materials Science, Gyeongnam

642831, Republic of Koreae Department of Physics, University of Rajasthan, Jaipur 302 055, Indiaf Department of Physics & Astrophysics, University of Delhi, Delhi 110 007, India

A R T I C L E I N F O

Article history:

Received 8 July 2014

Accepted 18 December 2014

Available online 24 December 2014

A B S T R A C T

Herein, we developed a strategy for fabrication of iron oxide infiltrated vertically aligned

multiwalled carbon nanotubes (MWCNT forest) sandwiched with reduced graphene oxide

(rGO) sheets network for high performance electromagnetic interference (EMI) shielding

application which offers a new avenue in this area. Such engineered sandwiched network

exhibits enhanced shielding effectiveness compared to conventional EMI shielding materi-

als. This network of exotic carbons demonstrates the shielding effectiveness value more

than 37 dB (>99.98% attenuation) in Ku-band (12.4–18 GHz), which is greater than the

recommended limit (�30 dB) for techno-commercial applications.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Electromagnetic interference (EMI) among electronic

equipments is a side-effect of their operation and it has

emerged as a serious problem. Because, it has become one

of the main causes for the degradation in the performance

of electronic system [1–3]. EMI is not only harmful for the

operation of electronic devices but may also be badly affecting

the health of human beings [4]. Picture flickering or noise dis-

tortions in television, radios and laptop screens by cellular

phone signals or wireless are some live examples of EMI in

our daily life. Moreover, EMI shielding has become more

prominent for devices operating at microwave frequency such

as telecommunication, TV picture transmission, weather

80 C A R B O N 8 5 ( 2 0 1 5 ) 7 9 – 8 8

radar, microwave electronics, microwave processing, lateral

guidance and radar surveillance systems [5]. Therefore,

microwave shielding in the form of electronic enclosures

(for computers, rooms, and aircrafts, etc.) and radiation

source enclosures (e.g., telephone receivers) are highly

desired. Owing to this, shielding materials have attracted

much attention for the proper functioning of electronics, in

all commercial and strategic sectors. Lightweight EMI shield-

ing is needed to protect the workspace and surroundings

from radiation coming from computers and telecommunica-

tion equipment as well as for protection of sensitive circuits.

The major drawback of conventional metal-based EMI shield-

ing material has higher gravimetric mass compared to light

weight carbon based composites. Therefore, nowadays, the

lightweight materials have gained much popularity and

search of suitable light weight materials with all desired

functionality for EMI shielding is the main focus areas among

global research groups [1,2].

Recently, exotic carbon materials [6] e.g., graphene [2],

graphite [7], carbon nanotubes (CNTs [1,8–10]), carbon fiber

[11] etc., have been proved as potential candidate for design-

ing microwave shielding and opens a new avenues for other

carbon structure to investigate for shielding application [10].

Graphite as a precursor for the synthesis of two-dimensional

(2D) honeycomb-structured sp2-hybridized partially reduced

graphene oxide (rGO) has gained tremendous attention due

to its superior mechanical, thermal, electrical, chemical and

optical properties. rGO sheet with residual defects and func-

tional groups, which act as polarized centers is a promising

2D material that goes beyond graphene for the development

of next generation microwave shield [3]. Therefore, high sur-

face area of rGO provides more contact in the design and pro-

cessing of high-strength, high-toughness, and low-density

composites [12].

It is well established that CNT and rGO could be the alter-

nate choice for such applications [3,8,13–16]. Because of their

unique structural properties, carbon nanotubes have been

investigated for many potential applications [17–20]. Particu-

larly, their fascinating electrical and mechanical properties

offer a new arena for the development of advanced engineer-

ing materials [21,22]. The small diameter, high aspect ratio,

high conductivity, and mechanical strength of CNT, make

them an excellent option for creating conductive composites

for high performance EMI shielding materials at low filing

[1,9]. The mechanical properties of CNT have drawn intense

interest in their potential for use as reinforcements in com-

posite materials. In addition to their high strength and elastic

constant, CNT have extremely high aspect ratios, with values

typically higher than 1000:1 and reaching as high as

2,500,000:1 [23]. As a result of these properties, CNT reinforce-

ments are expected to produce significantly stronger and

tougher composites than traditional reinforcing materials

[16].

Shielding effectiveness (SE) of any shield is governed by

dielectric attributes, magnetic permeability, thickness and

frequency of EM wave. While the absorption loss depends

on the value of er/lr, i.e., the absorption loss is maximum

when lr = er [24–26]. Since, both CNT and rGO show poor the

magnetic properties. Therefore, poor permeability of CNT

and rGO could be the fundamental limiting factor in attaining

high absorption in carbon based composites. In order to

improve matching of er and lr which is necessary for enhanc-

ing the absorption of the EM wave, ferrite particles were filled

in the gap of CNT [27–30]. Moreover, ferrite is not only envi-

ronmental friendly but also has abundant natural supply,

thus, rendering the material inexpensive. In the past decade,

a number of ferrites like Fe, Mn, Ni, Co ferrites and their

multi-component ferrites like Fe3O4, Fe2O3, BaFe12O19, CrO2,

ZnO, MnO2, Mn0.5Zn0.5Fe2O4 [14,16,20,25,31–34]were proved

to improve absorption of microwave. Among all of them

Fe3O4 is found to be most suitable for developing radar

absorbing materials because it offers high value of permeabil-

ity [33,35]. Unlike other important matrix materials, little

work has been done on the design of 2D–3D composites for

microwave shielding.

Recent advances in the fabrication of cross linking

between one-dimensional (1D) CNT and 2D rGO sheets based

composites allow us to fabricate iron oxide infiltrated verti-

cally aligned CNT forest [16,20]. In present investigation, we

attempt to design light weight 1D–2D hybrid structured, iron

oxide infiltrated vertically aligned multiwalled carbon nano-

tubes CNTs (MWCNT forest) sandwich with rGO sheets for

improving microwave shielding. The obtained results of

composite are focused on the conductivity, shielding mea-

surements, surface morphology and gross structural/micro-

structural analysis of CNT/Fe3O4/rGO three phase systems.

Moreover, the surface interaction of composite system has

been explored through Raman spectroscopy. However, to the

best of our knowledge, this type of sandwich in EMI SE

applications has not been thoroughly explored till date.

2. Experimental

2.1. Synthesis of GO and rGO

The detailed method of GO synthesis was discussed in our

earlier report [36]. In brief commercially available graphite

powder (SP-1 graphite) was purchased from Bay Carbon Cor-

poration. GO was obtained by harsh oxidation of the graphite

powder according to the customized Hummers method [36].

For thermal reduction of GO to convert it into rGO, the

obtained GO powder was transferred into a porcelain boat

and kept in a Quartz tube fitted to a high temperature fur-

nace. The powder was heated to 800 �C at 10 �C min�1 under

anoxic conditions with a gas flow of 150 cc per minute Ar–

H2 (85:15 vol%) for 2 h [37]. This resulted in a fluffy, light

weight, highly exfoliated rGO product. The filtered rGO was

heated in an air oven at 100 �C for 12 h to remove the mois-

ture. The yield of the product was more than 84%.

2.2. Preparations of the vertically aligned CNT forest

MWCNT arrays were grown on 4 inch by 1 inch silica sub-

strates by spray pyrolysis chemical vapor deposition method

at atmospheric pressure conditions using a Xylene–Ferrocene

precursor at concentrations of 20 mg/mL [38]. The substrates

were positioned in a 50 mm diameter quartz tube chamber.

The 3-zone furnace was set to temperatures of 780, 770,

780 �C at the same time argon gas flowed at a rate of 2.00 L/

min to purge the environment. A 1/16 inch diameter quartz

C A R B O N 8 5 ( 2 0 1 5 ) 7 9 – 8 8 81

tube was used as a nozzle tip to transport the precursor solu-

tion into the reaction zone and was positioned into the fur-

nace at around 300 �C. At equilibrium temperature, the

carrier gas was changed to argon/hydrogen (15% balance)

and set to a flow rate of 4.00 L/min while a separate gas flow

around the quartz tube nozzle tip was passed through a con-

centric 1/8 inch diameter steel tube set to a rate of 0.64 L/min.

Water-assisted growth was accomplished by having the noz-

zle tip gas flow through a room temperature water bubbler;

this was to promote open end caps on the CNT’s. The solution

was fed through the nozzle tip at a rate of 0.2 mL/min. The

synthesis time duration was 1 h which achieved �1 mm

carpet of MWCNT array deposited on silica substrate [44].

2.3. Fabrication of exotic carbon based sandwich network

Iron oxide infiltrated exotic carbons have been fabricated by

drop cast method. First, we cut a part of CNT forest equal to

fit the waveguide dimension and fix it in the sample holder

(Fig. 4c and d). After fixing, we slowly drop casted iron oxide

and rGO from both the side (upper and bottom face). In order

to optimize the concentration of Fe3O4 (�15 nm) infiltration,

several statistical runs have been done. For this, different

concentrations of Fe3O4 particles have been made in ethanol.

The optimum infiltration between the walls have been simul-

taneously checked after drying the sample using the SEM by

cutting the MWCNT forest piece in two parts. The chopped

side shows the optimum infiltration. On the higher concen-

tration, most of Fe3O4 particles make complexation on the

upper surface of MWCNT forest and not able to infiltrate

between the walls. In the present investigation, a piece of

MWCNT forest was cut to fit the wave guide dimension

(15.9 · 7.90 mm2). Next, the MWCNT forest structure was put

into a mold and drop cast technique was utilized to infiltrate

the MWCNT forest with iron oxide. For iron oxide infiltration,

first a solution of iron oxide (1 mg iron oxide in 10 mL ethanol)

has been sonicated for 30 min. Once light brown homoge-

neous solution was achieved, 1 mL of this solution has been

drop casted in MWCNT forest followed by drying at 50 �C in

a vacuum oven. It is important to optimize the rate of drop

cast on the surface of MWCNT forest, because in case of rapid

drop cast with bigger drop size on the MWCNT surface the

MWCNT surface can collapse due to liquid surface tension.

In present case, we did several statistical runs to optimize

the amount of drop and extremely drop casting rate to avoid

any kind of collapse of MWCNT forest. During the drop cast

process, we put one drop on MWCNT forest using micro-pip-

ette after every two hours at room temperature waiting till

one drop can fully dried. According to the same procedures,

rGO layers were casted on upper and lower surface of

MWCNT forest. For sandwiching of rGO, to make the solution

for sandwiching the iron-oxide infiltrated MWCNT forest,

0.5 mg rGO in 10 mL ethanol were sonicated for 30 min.

0.1 mL of this solution has been dropped on upper and bottom

surface of infiltrated MWCNT forest till the whole upper and

lower surface is covered by rGO sheets. This sandwich net-

work has been dried in vacuum oven at 50 �C before further

characterization. The central aim of the present synthesis

method is to make highly reproducible. Keeping this aspect

in view, we employed here simple and effective method to

produce engineered sandwich structure. Schematic represen-

tation for fabrication of new exotic carbon based engineered

sandwich network is shown in Fig. 1a.

3. Results and discussion

Fig. 1b depict the X-ray diffraction pattern Fe3O4, Sample A

(pristine MWCNT forest), Sample B (iron oxide infiltrated

MWCNT forest) and Sample C (Sample B sandwiched with

rGO sheets). The main peaks for Fe3O4 have been observed

at 2h = 30.35 (d = 2.942 A), 35.66 (d = 2.515 A), 43.34

(d = 2.085 A), 53.65 (d = 1.706 A), 57.34 (d = 1.605 A), and 62.95�(d = 1.475 A) corresponding to the (220), (311), (400), (422),

(511), and (440) reflections respectively, which matches with

the standard XRD pattern of Fe3O4 (Powder Diffraction File,

JCPDS No. 88-0315). The peaks present in Fe3O4 have also been

observed in the Sample B and Sample C which confirms the

presence of Fe3O4 phase in the CNT forest. The characteristic

peak of CNT was observed at 2h = 26.660 (d = 3.3409 A) corre-

sponding to (111) reflections. The characteristic peaks of

Fe3O4 and CNT forest (Sample A) were also observed in Sam-

ple B and Sample C, which indicates the presence of Fe3O4 in

the Sample B and Sample C. Additionally, the obtained XRD

pattern of Fe3O4 were compared to Sample B and Sample C

as shown in Fig. 1b, it is clearly visualized that the phase com-

position of CNT admixed Fe3O4 exhibits composite between

CNT and Fe3O4. Furthermore, the strong interactions of com-

posite were confirmed through surface morphology and

detailed Raman spectroscopy.

Raman spectroscopy provides a very powerful tool for

investigating the proper interaction or bonding between two

components [32]. To elucidate the graphitic structure of verti-

cally aligned CNT forest and interaction among CNT, Fe3O4

and rGO, Raman spectroscopy was conducted with a

514.5 nm wavelength laser in a spectral range of 100–

3300 cm�1. Fig. 1c–f shows the Raman spectra of Fe3O4, Sam-

ple A, Sample B and Sample C respectively. The Raman spec-

trum of CNT forest (Sample A) consist of three prominent

characteristic peaks, namely the D band (disorder-induced

band), the G band (the tangential mode of graphitic structure),

and the G 0 (2D) band confirming the formation of MWCNT.

Raman spectrum of pure Fe3O4 reveals all the characteristic

bands of Fe3O4 in the low frequency region, i.e., Eg mode

(213, 274, 384, 474 cm�1), A1g mode (584 cm�1) confirming

the presence of Fe3O4. Fig. 1c–f shows the broad 2D peak of

CNT forest has been observed in all three spectra of Sample

A, Sample B and Sample C and a slight right shift in the peak

position of Sample B and Sample C from CNT and Fe3O4 has

also been observed. Slight shifting in the bands is an evidence

of interaction between these components. In addition to this,

the low frequency mode of Fe3O4 has also been seen in Sam-

ple B and Sample C due to the presence of Fe3O4 in the hybrid

material.

To portrait the surface morphology of composite system,

we performed the scanning electron microscopy (SEM).

Fig. 2a–c contains the schematic diagram and their corre-

sponding SEM images of vertically aligned CNT forest, iron

particles (15–25 nm range) filled CNT forest and 2D rGO sand-

wiched iron particles filled CNT forest. The top and side view

of CNT forest (Fig. 2a1 and a2) clearly shows the high aspect

Fig. 1 – (a) Schematic representation exhibits the MWCNT forest (Sample A), Fe3O4 particles infiltrated MWCNT forest (Sample

B) and sandwiching the Sample B in rGO sheets (Sample C), (b) XRD patterns of Fe3O4 particles and different samples of

MWCNT forest, Fe3O4 particles infiltrated MWCNT forest and sandwiching the sample Fe3O4 particles infiltrated MWCNT

forest in rGO sheets, (c) Raman spectrum of vertically aligned CNT forest, (d) Raman spectrum of Fe3O4 particles, (e) Raman

spectrum recorded from Fe3O4 particles decorated CNT forest and (f) Raman spectra recorded from Sample C and Inset image

shows the zoom view of Sample C in 100–1000 cm�1. (A color version of this figure can be viewed online.)

82 C A R B O N 8 5 ( 2 0 1 5 ) 7 9 – 8 8

ratio of CNTs. This dense CNT forest with very high porosity,

high specific surface area have a well-organized, aligned

structure with vertical alignment as shown in Fig. 2a2. To

confirm the presence of iron oxide particles in the gap of

CNTs, SEM micrograph has been recorded by first filling a

rectangular piece of CNT s forest and broken vertically in

Fig. 2 – Schematic presentation with their counterparts of SEM images from CNT forest (Sample A) to its sandwiching

between rGO sheets: (a) schematic drawing of CNT forest; (a1) SEM images of the top surface of CNT forest and (a2) SEM image

of vertically aligned CNT forest, (b) schematic drawing of CNT forest filled with Fe3O4 particles; (b1) a top view of SEM image of

Sample B, (b2) a inside view of MWCNT forest decorated with Fe3O4 particles and (c) schematic diagram of Sample B

sandwiched between rGO sheets; (c1) a top view SEM image of Fe3O4 particles decorated in CNT forest sandwiched between

rGO sheets, (c2) TEM image of Sample C. (A color version of this figure can be viewed online.)

C A R B O N 8 5 ( 2 0 1 5 ) 7 9 – 8 8 83

two parts. Fig. 2b1 exhibits the homogenous distribution of

iron oxide particles on the upper surface of CNT forest

Fig. 2b2 ensures the existence of iron oxide particles on the

broken part (external surface) of CNT forest (Fig. 2b2). While

drying the Sample B at room temperature in the ambient

atmosphere, some cracks appeared as shown in Fig. 2b1,

while Fig. 2c1 shows the wrinkled rGO sheets on the upper

surface of CNT forest. We also investigated the TEM micros-

copy of Sample C to explore the presence of MWCNT and

rGO Fig. 2c2. We have taken small piece of hybrid structure

and dispersed in ethanol followed by sonication at 25 kHz fre-

quency for 10 min to prove the presence of rGO in hybrid

structure. The TEM result of Sample C is also supported by

its EDAX (energy dispersive X-ray analysis) study, which is

shown in Fig. S1 (see supporting information). Furthermore,

the presence of iron oxide particles in the gap of CNT forest

was investigated by the vibration sample magnetometer.

The field dependence of magnetization for the CNT forest

containing Fe3O4 particles, sandwiched between rGO sheets

have been studied by using the M–H curve at room tempera-

ture as shown in Fig. 3. The saturation magnetization (Ms)

value of the Fe3O4 synthesized by co-precipitation method

has been found to be 50 emu g�1 at an external field of

5 kOe having a small value of coercivity and negligible reten-

tivity with no hysteresis loop, indicating a superparamagnetic

nature as shown in inset of Fig. 3a. When these ferrite parti-

cles are filled in the gap of CNT forest, the Ms value has been

increased from 0.04 emu g�1 (pristine CNT) to 4.10 emu g�1.

Furthermore, from rGO sandwiching of Sample B, Ms value

a little decreases (Fig. 3b). For enhancing and controlling the

magnetic losses the sufficient magnetization value is required

for EMI shielding applications.

The S parameters Sij {S11 (S22), S12 (S21)} of the MPC compos-

ites were measured by vector network analyzer (VNA E8263B

Fig. 3 – (a and b) VSM curves of Fe3O4 and CNT forest hybrid

structure and inset of image (a) shows the VSM plot of Fe3O4

particles. (A color version of this figure can be viewed

online.)

84 C A R B O N 8 5 ( 2 0 1 5 ) 7 9 – 8 8

Agilent Technologies) in the frequency range of 12.4–18 GHz

(Ku band) using two port measurement techniques (Fig. 4).

The power coefficients, transmission coefficient (T) and

reflection coefficient (R) were calculated by the equations:

Fig. 4 – (a) Image showing the vector network analyzer for mea

the samples while (b) image showing the sample holder fitted b

for measuring the shielding performance (c) without CNT fores

color version of this figure can be viewed online.)

T ¼ ET

EI

����

����

2

¼ jS21j2 ¼ jS12j2 ð1Þ

R ¼ ER

EI

����

����

2

¼ jS11j2 ¼ jS22j2 ð2Þ

and absorption coefficient was calculated from the relation of

(A) = 1 � R � T [1,2].

Here, it is noted that the absorption coefficient is given with

respect to the power of the incident electromagnetic wave. If

the effect of multiple reflections between both interfaces of

the material is negligible, then the relative intensity of the

effectively incident electromagnetic wave inside the material

after reflection is based on the quantity (1 � R). Therefore,

the effective absorbance (Aeff) can be described as Aeff = A/

(1 � R) with respect to the power of the effective incident elec-

tromagnetic wave inside the shielding material. Absorption

efficiency (AE) was obtained using the relation of AE = A/

(1 � R) · 100%. The plotted curves of absorption (A), reflection

(R) transmission (T) coefficients and absorption efficiency of

the composites aim to show the excellent EM wave attenua-

tion performances of the CNT forest composites with changing

frequency (see supporting material document Fig. S2).

The EM attributes, i.e., relative complex permittivity

(e* = e0 � ie00) and relative complex permeability (l* = l0 � il00) have

been calculated from experimental scattering parameters

(S11 and S21) by standard Nicholson-Ross and Weir theoretical

calculations [39] for the detailed analysis of microwave

absorption properties of vertically aligned CNT forest hybrid

structure which are shown in Fig. 5a and b. In brief, real part

of the EM parameters (e 0, l 0) is a measure of the amount of

polarization taking place in the material and represents the

storage ability of the electric and magnetic energy, while the

imaginary part (e00, l00) is signifies the dissipated electric and

magnetic energy. The values of the real and imaginary part

of permittivity are averaged over 201 data points from 12.4

to 18 GHz. In brief, From Fig. 5a, the values of e 0 increases from

Sample A to Sample C from 10.31 to 15.47, similarly, the value

of e00 are also increases from 7.98 to 9.66 for Sample A to Sam-

ple C. From these obtained results, we conclude that the fill-

ing of iron oxide particles increases the dipole polarization

and consequently e 0 increases. Furthermore, the rGO sheets

suring the scattering parameters and dielectric attributes of

etween the flanges of the waveguide. Copper sample holder

t (d) a piece of MWCNT forest fitted in the sample holder. (A

Fig. 5 – Frequency dependence of the (a) real parts and imaginary parts of the complex permittivity, (b) real parts and

imaginary parts of permeability, (c) dependence of shielding effectiveness (SEA and SER) in the frequency range 12.4–18 GHz

(d) behavior of skin depth of CNT forest hybrid structures as a function of frequency, (d) schematic presentation of possible

microwave absorbing mechanisms in the CNT forest composites. (A color version of this figure can be viewed online.)

C A R B O N 8 5 ( 2 0 1 5 ) 7 9 – 8 8 85

sandwiched Sample B provides enhanced conductivity, elec-

tric polarization as well as interfaces which improve the inter-

facial polarization, as a result the higher value e 0 was

achieved. In the same manner, e00 also increases because of

the same reasons in the presence of microwave. According

to, the EM theory, dielectric losses are the result of complex

phenomena like natural resonance, dipole relaxation, elec-

tronic polarization and its relaxation and certainly the unique

structure of the shield. When the frequency of the applied

field is increased, the ballistic electrons present in the system

cannot reorient themselves fast enough to respond to applied

electric field, and increase the dielectric constant. Iron oxide

particles presented inside and on the top surface of vertically

aligned CNT forest sandwiched rGO sheet, act as polarized

center and improves polarization which results in more

microwave absorption. High aspect ratio of the CNT filled

with Fe3O4 particles having high conductivity also enhances

the absorption properties. Ferromagnetic particles act as tiny

dipoles which get polarized in the presence of EM field and

result in better microwave absorption.

As shown in Fig. 5b the values of l 0 for Sample A, Sample B

and Sample C are 0.46, 1.03 and 1.48 respectively while the

values of l00 are 0.34, 0.48 and 0.88 respectively in the Ku-band

frequency range. The complex permeability (l* = l 0 � il00)

curve exhibits broad multi peaks which implies the presence

of a eddy current effects due to ferrite particles and natural

resonance caused by the enhanced surface anisotropy of the

small size of Fe3O4 particles. Anisotropy energy of the small

size materials, especially in the nanoscale, would be higher

due to surface anisotropic field owning to its small size effect

[40]. The higher anisotropy energy also contributes in the

enhancement of the microwave absorption. Interfacial polar-

ization occurs in heterogeneous media due to accumulation

of charges at the interfaces, formation of dipoles. Interfaces

among CNT, Iron particles and rGO sheets further contribute

to dielectric losses.

EMI SE of any material is the sum of the contributions of

the absorption (SEA), reflection (SER) and multiple reflections

(SEM) of the EM energy [14,32–34,41–43]:

SEðdBÞ ¼ SER þ SEA þ SEM ¼ �10 log ðPT=PIÞ

where PI and PT are the power of incident and transmitted EM

waves, respectively. According to Schelkunoff’s theory, SEM

can be ignored in all practical application where the shield

is thicker than the skin depth (d). For a material, the skin

depth (d) is the distance up to which the intensity of the EM

Fig. 6 – Schematic presentation of the interaction of EM wave with sandwich network of vertically aligned carbon nanotube–

reduced graphene oxide composites. (A color version of this figure can be viewed online.)

86 C A R B O N 8 5 ( 2 0 1 5 ) 7 9 – 8 8

wave decreases to 1/e of its original strength. The d is related

to angular frequency, relative permeability and total conduc-

tivity rT = (rdc + rac).

Furthermore SER and SEA can be defined as:

SER = �10 log (1 � R). SEA = �10 log (1 � Aeff) = �10 log (T/1 � R)

According to EM theory, for electrically thick samples (t > d),

frequency (x) dependence of far field losses can be expressed

in the terms of total conductivity (rT) real permeability (l0),

skin depth (d) and thickness (t) of the shield material [44] as:

SEA becomes more dominant as compared to the SER in the

microwave range. This may be caused by the shallow skin

depth and high conductivity (rac) values at such high frequen-

cies [44,45]. Fig. 5c shows the variation of the SE with frequency

in the 12.4–18 GHz range. From the experimental measure-

ment, the average SE due to absorption (SEA) of Sample A, Sam-

ple B and Sample C has been found 32.46, 33.69 and 36.01 dB

respectively i.e., SEA increases by presence of Fe3O4 particles

and it further improves by rGO sandwiching, On the other

hand, the SER decreases continuously from 3.33 to 1.57 dB.

Therefore, the total average SE achieved for Sample A, Sample

B and Sample C 35.79, 35.68 and 37.58 respectively at a critical

thickness of 2 mm. It is very interesting to note here that infil-

tration of magnetic particles and rGO sandwiching helps in

minimizing SE due to reflection. Therefore SE is mainly domi-

nated by absorption and exhibit better microwave absorption

properties in comparison with pristine CNT, pure rGO, c-

Fe2O3 particles, CNT-iron oxide composite [1], CNT/silica com-

posites [46], rGO/iron oxide composite reported earlier [14].

The skin depth of the samples has been calculated using

the relation, d ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2=rxl0

p, ðrac ¼ xe0e00Þ for determining the

critical thickness of the shield and its variation with fre-

quency has been shown in Fig. 5d. It can be noticed that the

skin depth decreases from Sample A (�2200 lm) to C

(�1200 lm), It means that the hybrid structure shield should

have a thickness of P1.2 mm. Thus, microwave absorption

properties of hybrid structure are improved by the dielectric

and magnetic losses.

The excellent microwave absorbing performance of CNT

forest hybrid structure is mainly attributed to two factors:

impedance matching and EM wave attenuation. The ideal

condition for the perfect absorber is er = lr or er/lr = 1, the

presence of iron nanoparticles in the gap of vertically aligned

CNT forest matrix has decreased the ratio of er/lr which helps

the level of impedance matching [5,25]. Furthermore the exis-

tence of residual defects/groups in rGO sheet [17,41]within

the shield enhances the microwave absorption ability of the

composites. Moreover, a visual demonstration of the micro-

wave absorption mechanism as discussed schematic is

shown in Fig. 6. From all the above, the results of CNT forest

hybrid structure illustrates that this sandwich network could

be potentially used as microwave absorbing material. The

milestone of the as-synthesized engineered sandwiched

structure is to tailor the EMI shielding effectiveness by addi-

tion of effective surface area of CNT and rGO which cannot

be achieved by isolated CNT and rGO.

4. Conclusions

We have successfully demonstrated the high performance

Fe3O4 infiltrated rGO–MWCNT forest sandwich network for

suppressing electromagnetic pollution. Ingenious engineered

sandwich network of exotic carbons consisting of iron oxide

particles infiltrated in vertically aligned highly porous CNTs

which is sandwiched along tubular axis by rGO sheets. This

composite network enhances the dielectric loss which can

be attributed to natural resonance, dipole relaxation, electron

polarization related relaxation, interfacial polarization and

the effective anisotropy energy of composite. Additionally,

the associated magnetic losses of the composites are mainly

caused by the natural resonance and the eddy currents. This

ingenious light weight composite with outstanding shielding

properties pushes its promising applications in next genera-

tion building block material for EMI shielding and stealth

technology.

C A R B O N 8 5 ( 2 0 1 5 ) 7 9 – 8 8 87

Acknowledgements

The authors wish to thank Prof. R.C. Budhani, Director, N.P.L.,

for his keen interest in the work. The authors are thankful to

Prof. O.N. Srivastava (Banaras Hindu University, Varanasi) for

his encouragement. The authors also thank Dr. N. Vijayan and

K.N. Sood for recording XRD pattern and SEM micrograph,

respectively. Authors thank financial support from Indo-US

Science and Technology Forum (IUSSTF), New Delhi, India.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at http://dx.doi.org/10.1016/j.carbon.

2014.12.065.

R E F E R E N C E S

[1] Che RC, Peng LM, Duan XF, Chen Q, Liang XL. Microwaveabsorption enhancement and complex permittivity andpermeability of Fe encapsulated within carbon nanotubes.Adv Mater 2004;16(5):401–5.

[2] Chen Z, Xu C, Ma C, Ren W, Cheng H-M. Lightweight andflexible graphene foam composites for high-performanceelectromagnetic interference shielding. Adv Mater2013;25(9):1296–300.

[3] Wen B, Cao M, Lu M, Cao W, Shi H, Liu J, et al. Reducedgraphene oxides: light-weight and high-efficiencyelectromagnetic interference shielding at elevatedtemperatures. Adv Mater 2014;26(21):3484–9.

[4] Sambyal P, Singh AP, Verma M, Farukh M, Singh BP, DhawanSK. Tailored polyaniline/barium strontium titanate/expandedgraphite multiphase composite for efficient radar absorption.RSC Adv 2014;4(24):12614–24.

[5] Singh AP, Mishra M, Sambyal P, Gupta BK, Singh BP, ChandraA, et al. Encapsulation of c-Fe2O3 decorated reducedgraphene oxide in polyaniline core–shell tubes as anexceptional tracker for electromagnetic environmentalpollution. J Mater Chem A 2014;2(10):3581–93.

[6] Micheli D, Vricella A, Pastore R, Marchetti M. Synthesis andelectromagnetic characterization of frequency selective radarabsorbing materials using carbon nanopowders. Carbon2014;77:756–74.

[7] Mishra M, Singh AP, Dhawan SK. Expanded graphite–nanoferrite–fly ash composites for shielding ofelectromagnetic pollution. J Alloy Compd 2013;557:244–51.

[8] Narayanan T, Sunny V, Shaijumon M, Ajayan P,Anantharaman M. Enhanced microwave absorption innickel-filled multiwall carbon nanotubes in the S band.Electrochem Solid-State Lett 2009;12(4):K21–4.

[9] Singh AP, Gupta BK, Mishra M, Govind, Chandra A, MathurRB, et al. Multiwalled carbon nanotube/cement compositeswith exceptional electromagnetic interference shieldingproperties. Carbon 2013;56:86–96.

[10] Micheli D, Pastore R, Apollo C, Marchetti M, Gradoni G,Primiani VM, et al. Broadband electromagnetic absorbersusing carbon nanostructure-based composites. IEEE TransMicrow Theory Tech 2011;59(10):2633–46.

[11] Chiou J-M, Zheng Q, Chung DDL. Electromagneticinterference shielding by carbon fiber reinforced cement.Composites 1989;20(4):379–81.

[12] Wen B, Wang XX, Cao WQ, Shi HL, Lu MM, Wang G, et al.Reduced graphene oxides: the thinnest and most lightweight

materials with highly efficient microwave attenuationperformances of the carbon world. Nanoscale2014;6(11):5754–61.

[13] Kumar Srivastava R, Narayanan TN, Reena Mary AP,Anantharaman MR, Srivastava A, Vajtai R, et al. Ni filledflexible multi-walled carbon nanotube–polystyrenecomposite films as efficient microwave absorbers. Appl PhysLett 2011;99(11):113116–23.

[14] Singh AP, Garg P, Alam F, Singh K, Mathur RB, Tandon RP,et al. Phenolic resin-based composite sheets filled withmixtures of reduced graphene oxide, c-Fe2O3 and carbonfibers for excellent electromagnetic interference shielding inthe X-band. Carbon 2012;50:3868–75.

[15] Micheli D, Pastore R, Gradoni G, Marchetti M. Tunablenanostructured composite with built-in metallic wire-gridelectrode. AIP Adv 2013;3(11):112132.

[16] Cao M-S, Yang J, Song W-L, Zhang D-Q, Wen B, Jin H-B, et al.Ferroferric oxide/multiwalled carbon nanotube vspolyaniline/ferroferric oxide/multiwalled carbon nanotubemultiheterostructures for highly effective microwaveabsorption. ACS Appl Mater Interfaces 2012;4(12):6949–56.

[17] Dai H, Hafner JH, Rinzler AG, Colbert DT, Smalley RE.Nanotubes as nanoprobes in scanning probe microscopy.Nature 1996;384:147–50.

[18] Heer WAd, Chatelain A, Ugarte D. A carbon nanotube field-emission electron source. Science 1995;270:1179–80.

[19] Martel R, Schmidt T, Shea HR, Hertel T, Avouris P. Single andmulti-wall carbon nanotube field-effect transistors. ApplPhys Lett 1998;73:2447.

[20] Lu M-M, Cao W-Q, Shi H-L, Fang X-Y, Yang J, Hou Z-L, et al.Multi-wall carbon nanotubes decorated with ZnOnanocrystals: mild solution-process synthesis and highlyefficient microwave absorption properties at elevatedtemperature. J Mater Chem A 2014;2(27):10540–7.

[21] Williams OA, Whitfield MD, Jackman RB, Foord JS, Butler JE,Nebel CE. Formation of shallow acceptor states in thesurface region of thin film diamond. Appl Phys Lett2001;78:3460.

[22] Kleinsorge B, Ferrari AC, Robertson J, Milne WI. Influence ofnitrogen and temperature on the deposition of tetrahedrallybonded amorphous carbon. J Appl Phys 2000;88:1149.

[23] Makar J, Margeson J, Luh J. Carbon nanotube/cementcomposites – early results and potential applications. 3rdInternational Conference on Construction Materials:Performance, Innovations and Structural Implications,Vancouver. p. 1–10.

[24] Zhang C-S, Ni Q-Q, Fu S-Y, Kurashiki K. Electromagneticinterference shielding effect of nanocomposites with carbonnanotube and shape memory polymer. Compos Sci Technol2007;67:2973–80.

[25] Belaabed B, Wojkiewicz JL, Lamouri S, El Kamchi N, Lasri T.Synthesis and characterization of hybrid conductingcomposites based on polyaniline/magnetite fillers withimproved microwave absorption properties. J Alloy Compd2012;527:137–44.

[26] Singh AP, Mishra M, Sambyal P, Gupta BK, Singh BP, ChandraA, et al. Incapsulation of c-Fe2O3 decorated reducedgraphene oxide in polyaniline core–shell tubes as anexceptional tracker for electromagnetic environmentalpollution. J Mater Chem A 2013;2:3581–93.

[27] Shen J, Hu Y, Shi M, Li N, Ma H, Ye M. One step synthesis ofgraphene oxide–magnetic nanoparticle composite. J PhysChem C 2010;114(3):1498–503.

[28] He H, Gao C. Supraparamagnetic, conductive and processablemultifunctional graphene nanosheets coated with high-density Fe3O4 nanoparticles. ACS Appl Mater Interfaces2010;2(11):3201–10.

88 C A R B O N 8 5 ( 2 0 1 5 ) 7 9 – 8 8

[29] Chandra V, Park J, Chun Y, Lee JW, Hwang I-C, Kim KS. Water-dispersible magnetite–reduced graphene oxide compositesfor arsenic removal. ACS Nano 2010;4(7):3979–86.

[30] Liang J, Xu Y, Sui D, Zhang L, Huang Y, Ma Y, et al. Flexible,magnetic and electrically conductive graphene/Fe3O4 paperand its application for magnetic-controlled switches. J PhysChem C 2010;114:17465–71.

[31] Xu H, Zhang H, Lv T, Wei H, Song F. Study on Fe3O4/polyaniline electromagnetic composite hollow spheresprepared against sulfonated polystyrene colloid template.Colloid Polym Sci 2013;291(7):1713–20.

[32] Ohlan A, Singh K, Chandra A, Dhawan SK. Conductingferromagnetic copolymer of aniline and 3,4-ethylenedioxythiophene containing nanocrystalline bariumferrite particles. J Appl Polym Sci 2008;108(4):2218–25.

[33] Singh K, Ohlan A, Saini P, Dhawan SK. Poly (3,4-ethylenedioxythiophene) c-Fe2O3 polymer composite–superparamagnetic behavior and variable range hopping 1Dconduction mechanism–synthesis and characterization.Polym Adv Technol 2008;19(3):229–36.

[34] Singh AP, Mishra M, Chandra A, Dhawan SK. Graphene oxide/ferrofluid/cement composites for electromagneticinterference shielding application. Nanotechnology2011;22:9.

[35] Sun X, He J, Li G, Tang J, Wang T, Guo Y, et al. Laminatedmagnetic graphene with enhanced electromagnetic waveabsorption properties. J Mater Chem C 2013;1(4):765–77.

[36] Gao W, Alemany LB, Ci L, Ajayan PM. New insights into thestructure and reduction of graphite oxide. Nat Chem2009;1(5):403–8.

[37] Gupta BK, Thanikaivelan P, Narayanan TN, Song L, Gao W,Hayashi T, et al. Optical bifunctionality of europium-complexed luminescent graphene nanosheets. Nano Lett2011;11(12):5227–33.

[38] Carey BJ, Patra PK, Hahm MG, Ajayan PM. Foam-like behaviorin compliant, continuously reinforced nanocomposites. AdvFunct Mater 2013;23(23):3002–7.

[39] Nicolson AM, Ross GF. Measurement of the intrinsicproperties of materials by time-domain techniques. IEEETrans Instrum Meas 1970;19:377–82.

[40] Chen Y-J, Gao P, Wang R-X, Zhu C-L, Wang L-J, Cao M-S, et al.Porous Fe3O4/SnO2 core/shell nanorods: synthesis andelectromagnetic properties. J Phys Chem C2009;113(23):10061–4.

[41] Ohlan A, Singh K, Chandra A, Singh VN, Dhawan SK.Conjugated polymer nanocomposites: synthesis, dielectric,and microwave absorption studies. J Appl Phys2009;106:044305–44311.

[42] Sachdev VK, Patel K, Bhattacharya S, Tandon RP.Electromagnetic interference shielding of graphite/acrylonitrile butadiene styrene composites. J Appl Polym Sci2011;120(2):1100–5.

[43] Ashokkumar M, Narayanan NT, Gupta BK, Reddy ALM, SinghAP, Dhawan SK, et al. Conversion of industrial bio-waste intouseful nanomaterials. ACS Sustain Chem Eng2013;1(6):619–26.

[44] Colaneri NF, Shacklette LW. EMI shielding measurements ofconductive polymer blends. IEEE Trans Instrum Meas1992;41:29.

[45] Das NC, Das D, Khastgir TK, Chakrraborthy AC.Electromagnetic interference shielding effectiveness ofcarbon black and carbon fibre filled EVA and NR basedcomposites. Composites A 2000;31:1069–81.

[46] Wen B, Cao M-S, Hou Z-L, Song W-L, Zhang L, Lu M-M, et al.Temperature dependent microwave attenuation behaviorfor carbon-nanotube/silica composites. Carbon2013;65:124–39.