Non-covalent functionalization of graphene with poly(diallyl dimethylammonium) chloride: Effect of a non-ionic surfactant

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Non-covalent functionalization of graphene withpoly(diallyl dimethylammonium) chloride: Effect ofa non-ionic surfactant

Prabhsharan Kaur a, Mun-Sik Shin b, Neha Sharma a, Namarta Kaur a,Anjali Joshi a, So-Ryong Chae c, Jin-Soo Park b,*, Moon-Sung Kang b,Satpal Singh Sekhon a,d,**

a Department of Physics, Guru Nanak Dev University, Amritsar 143005, Indiab Department of Environmental Engineering, College of Engineering, Sangmyung University, 31 Sangmyungdae-gil,

Dongnam-gu, Cheonan, Chungnam Province 330-720, Republic of Koreac Department of Biomedical, Chemical, and Environmental Engineering, 701 Engineering Research Center,

University of Cincinnati, Cincinnati, OH 45221-0012, USAd Department of Physics, The University of the West Indies, St. Augustine, Trinidad and Tobago

a r t i c l e i n f o

Article history:

Received 9 May 2014

Received in revised form

9 November 2014

Accepted 12 November 2014

Available online xxx

Keywords:

Graphene

Nitrogen doping

Functionalization

Polyelectrolyte

Fuel cell

Non-ionic surfactant

* Corresponding author. Tel.: þ82 41 550 531** Corresponding author. Department of Phys2002x82591; fax: þ1 (868) 662 9904.

E-mail addresses: [email protected] (J.-S

Please cite this article in press as: Kaulammonium) chloride: Effect of a non-i10.1016/j.ijhydene.2014.11.068

http://dx.doi.org/10.1016/j.ijhydene.2014.11.00360-3199/Copyright © 2014, Hydrogen Ener

a b s t r a c t

Carbon based nanomaterials (carbon nanotubes, graphene etc) containing various hetero

atoms are promising metal free catalysts for oxygen reduction reaction in fuel cells. We

report the non-covalent functionalization of graphene with poly(diallyl dimethylammo-

nium) chloride (PDDA), a polyelectrolyte containing nitrogen, using a very simple method.

The addition of a non-ionic surfactant (Triton X-100) during functionalization has been

observed to improve the interactions between graphene and PDDA. An up-shift in the po-

sition ofG-peak in theRaman spectra, down-shift in the binding energy (B.E.) ofN1s peak and

an up-shift in the B.E. of C1s peak in XPS spectra have been observed due to an inter-

molecular charge-transfer from carbon in graphene to nitrogen in PDDA, which get

enhanced in the presence of Triton X-100. Graphene functionalized with PDDA also show

good thermal stability. The addition of a non-ionic surfactant enhances the non-covalent

functionalization of graphenewith PDDA,which is desirable fromapplications point of view.

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

Introduction

The applications of graphene in diverse fields have recently

generated much interest and are mainly due to its unique

properties such as large surface area, high intrinsic mobility,

5; fax: þ82 41 550 5313.ics, The University of the

. Park), sekhon_apd@yaho

r P, et al., Non-covalenonic surfactant, Interna

68gy Publications, LLC. Publ

high thermal conductivity etc [1,2]. However, in pristine form

graphene being chemically inert as well as insoluble in many

organic and inorganic solvents, cannot be used in many ap-

plications. In addition, it is a zero band gap material. To

render it suitable from application point of view, there is a

West Indies, St. Augustine, Trinidad and Tobago. Tel.: þ1 (868) 662

o.com (S.S. Sekhon).

t functionalization of graphene with poly(diallyl dimethy-tional Journal of Hydrogen Energy (2014), http://dx.doi.org/

ished by Elsevier Ltd. All rights reserved.

mailto:[email protected]

mailto:[email protected]

www.sciencedirect.com/science/journal/03603199

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http://dx.doi.org/10.1016/j.ijhydene.2014.11.068



i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y x x x ( 2 0 1 4 ) 1e72

need for the modification of graphene either by functionali-

zation (post-synthesis) or by doping (during in-situ synthe-

sis). Out of the two main approaches used for

functionalization (covalent and non-covalent), non-covalent

functionalization is generally preferred as it can add various

chemical moieties through van der Waals interactions

without damaging or changing the atomic structure [3].

Doping of graphene with different hetero atoms in-situ dur-

ing the synthesis is another route available for modifying its

atomic and electronic structure. However, the post-synthesis

functionalization is much easier than doping during in-situ

synthesis. Nitrogen, which is next to carbon in the periodic

table, is the most appropriate dopant in graphene due to their

atomic size similarity. Recently, many studies have been re-

ported on nitrogen doped graphene, which has potential

applications in many devices like biosensors, fuel cells, and

lithium ion batteries [4e6]. Graphene functionalized with a

polyelectrolyte containing nitrogen, poly(diallyl dimethy-

lammonium) chloride (PDDA), has been reported to have

excellent activity as metal-free electrocatalyst for the oxygen

reduction reaction (ORR) in fuel cells [7]. As PDDA contains

positively charged nitrogen (Nþ) in its chemical structure, so

it has strong electron-withdrawing ability which has been

exploited for ORR. The adsorption of PDDA on graphene

sheets results in charge-transfer from C (in graphene) to Nþ

(in PDDA) and as a result it makes graphene electron defi-

cient. The positively charged carbon atoms in graphene can

then readily attract electrons from the anode of fuel cell and

in turn enhances the ORR. Theoretical studies on interactions

between graphene and a surfactant have been also reported

[9e11].

During functionalization, many surfactants could be used

as dispersing agents for carbon nanotubes (CNTs) and gra-

phene [8]. In our earlier study on PDDA functionalized multi-

wall carbon nanotubes [12], we have found that among the

various surfactants used for dispersion of carbon nanotubes,

Triton X-100 is not only the best dispersant but it also en-

hances the inter-molecular charge transfer from carbon in

CNTs to nitrogen in PDDA. As the surfactant could also be

used to disperse graphene, so it is reasonable to expect similar

behavior in PDDA-functionalized graphene. This altogether

set up the motivation for the present work.

In order to have efficient wrapping of PDDA on graphene,

we have functionalized graphene with PDDA in the presence

of a non-ionic surfactant (Triton X-100) by using a very simple

method. The effect of addition of a non-ionic surfactant on the

properties of graphene functionalized with PDDA has been

studied by Fourier transform infrared (FTIR) spectroscopy,

Raman spectroscopy, transmission electron microscopy

(TEM), X-ray photoelectron spectroscopy (XPS), and ther-

mogravimetric analysis (TGA).

Table 1 e The composition and codes of differentsamples.

Sample composition Code

Pristine graphene G1

Graphene oxide (GO) G2

Reduced graphene oxide þ PDDA G3

Reduced graphene oxide þ PDDA þ Triton X-100 G4

Experimental

Materials

The graphene nanopowder (~8 nm in thickness; ~550 nm in

average particle size) was supplied by Graphene Supermarket,

Graphene Laboratories Inc., NY. All other chemicals used in

Please cite this article in press as: Kaur P, et al., Non-covalenlammonium) chloride: Effect of a non-ionic surfactant, Interna10.1016/j.ijhydene.2014.11.068

the present study are of analytical grade, and were purchased

either from Sigma Aldrich or Merck.

Non-covalent functionalization of graphene with PDDA

The composition and codes of different samples studied in the

present work are listed in Table 1. In the following text, these

samples will be referred with their code names.

Preparation of G1The sample G1 is pristine graphene and it has been used as

received.

Preparation of G2The sample G2 is graphene oxide (GO) and it was prepared by

following themodified Hummer's method [13]. To prepare GO,

100 mg graphene nanopowder and 15 mL H2SO4 were taken in

a round bottom flask and stirred over an ice-water bath for

180min. After this, 600mg KMnO4 was added and themixture

was stirred for another 60 min while the temperature was

increased from 0 to 30 �C. The sample was then diluted by

adding 25 mL of de-ionized (DI) water, and refluxed in a hot

oil-bath until its temperature reaches 90 �C. The solution was

then allowed to cool to room temperature. After this, 60 mL DI

water and 1.8 mL H2O2 were added to it slowly. A change in

color of the sample from black to yellowish brown was

observed, which indicates the oxidation of graphene.

Preparation of G3The sample G3 is reduced graphene oxide functionalized with

PDDA. This sample was prepared by adding 1.4 mL PDDA and

100 mL DI water to GO (G2) prepared above [14]. The solution

was stirred for 7e8 h at room temperature, followed by ultra-

sonication for 120min. Further, 100mgNaBH4was then added

for the reduction of GO and it was again stirred for 30 min at

room temperature. An oil-bath heat treatment was given to

this sample at 130 �C for 180 min to ensure efficient reduction

of GO.

Preparation of G4The sample G4 is reduced graphene oxide functionalized with

PDDA in the presence of Triton X-100. To prepare this sample,

1.4 mL Triton X-100 and 100 mL DI water were added to GO

(G2) prepared above. The mixture was stirred for 7e8 h, fol-

lowed by ultra-sonication for 120min at room temperature. To

this solution, 1.4 mL PDDA was added and it was again stirred

for 7e8 h, followed by ultra-sonication for 120 min at room

temperature. Then 100mg NaBH4 was added for the reduction

of GO and it was again stirred at room temperature for 30min.




i n t e rn a t i o n a l j o u rn a l o f h y d r o g e n en e r g y x x x ( 2 0 1 4 ) 1e7 3

The oil-bath heat treatment was given at 130 �C for 180 min to

ensure efficient reduction of GO.

The samples (G2-G4) were extracted by centrifugation at

7000 rpm for 10 min (3 times for each sample). The retrieved

samples were dried overnight in an oven at 60 �C and then

used for different studies.

Methods

Measurements of FTIR, Raman and XPSThe functionalization of graphene was studied by FTIR spec-

troscopy using KBr pellet method by FTIR spectrophotometer

(Thermo-Nicolet-5700) in 500e4000 cm�1 wavenumber range.

Raman spectroscopy measurements were carried out at room

temperature in the 500e4000 cm�1 wavenumber range using a

Raman spectrometer FRA 106/S (BRUKER OPTIK GMBH, Ger-

many) with a 1006 nm Nd-YAG laser having resolution of

4 cm�1. XPS measurements were performed with a VG

Microtech ESCALAB 201 using amonochromic Al X-ray source

(Mg (1253.6 eV)/Al (1486.6 eV), 0.5 eV).

Measurements of TGAThermal stability of all the samples was examined by TGA

using a TGA 2050 instrument (TA instruments). Heat scans

were carried out under nitrogen atmosphere in the 100e600 �Ctemperature range at a heating rate of 10 �C/min.

Fig. 1 e TEM micrographs o


Measurements of TEMTEMmicrographs for all the samples were recordedwith Libra

120, Carl Zeissmicroscope at an acceleration voltage of 120 kV.

The samples for TEM studies were prepared by placing a few

drops of the sample solution on a Lacey carbon grid.

Results and discussion

The surface morphology of all the samples has been studied

by TEM, and the micrographs are shown in Fig. 1. The mi-

crographs show the difference in the morphology of different

samples. The surface of the sample G1 (pristine graphene)

closely resembles to thin crumpled sheets which form a

disordered solid and is possibly due to the presence of impu-

rities [15]. The oxidation of graphene has been observed to

result in a change in surface morphology for the sample G2,

that is, some clusters appear in the micrograph which may be

related to the formation of GO. Some of these clusters disap-

pear after the reduction of GO as observed in the micrographs

of the samples G3 and G4. The wrapping of PDDA and Triton

X-100 on graphene skeleton is also visible in the micrographs

of the samples G3 and G4.

Next, FTIR spectra of different samples are shown in Fig. 2.

The appearance of a major peak at ~3440 cm�1 in all the

samples is due to the OeH stretching vibrations, which

f the samples G1eG4.




Fig. 2 e FTIR spectra of the samples G2eG4.Fig. 3 e Raman spectra of the samples G2eG4.


indicates the presence of small traces of oxygen [16]. The

formation of GO (the sample G2) is evident from the presence

of peaks at ~1722, ~1286 and ~1186 cm�1 which correspond to

the C]O stretching, epoxy, and hydroxyl groups, respectively

[17]. The unoxidized graphitic domains are characterized by

the skeletal vibrations appearing at ~1585 cm�1 [18], which

indicate that all sites on the graphene sheets are not

oxidized. A decrease in the intensity of oxygen related peaks

in G3, which contains reduced graphene oxide is due to a

decrease in the number of oxygen containing groups upon

reduction. The appearance of peaks at ~2949 cm�1 and

~1504 cm�1 in the spectra of the samples G3 and G4, which

have been functionalized with PDDA is related to the pres-

ence of some nitrogen containing groups [19]. The presence

of peaks at 1124, 950 and 830 cm�1 in the sample G4 proved

the effect of Triton X-100, which corresponds to ether link-

age, CH2 rock and C]O stretching vibration of Triton X-100,

respectively.

The Raman spectra of different samples given in Fig. 3

show the presence of two prominent peaks. The G-peak at

around 1585 cm�1 corresponds to the splitting of E2g stretching

mode of graphene giving the structural intensity of carbon

atoms, and D-peak near 1290 cm�1 is assigned to the disor-

dered graphitic structure [20]. The peak positions and the ratio

of the intensity of D and G peaks (ID/IG) have been calculated

from Fig. 3. Upon functionalization of graphenewith PDDA, an

up-shift in the position of G-peak has been observed for the

samples G3 (1602 cm�1) and G4 (1612 cm�1) in comparison to

pristine graphene G1 (1594 cm�1). An up-shift in the peak

position of G-band is generally taken as an evidence of an

inter-molecular charge-transfer from carbon atoms in gra-

phene to Nþ in PDDA [21]. The up-shift in the G-band peak

position is large (18 cm�1) in the sample G4 than in G3 (8 cm�1),

which indicates an enhancement in the charge-transfer in the

presence of Triton X-100. Thus, the presence of Triton X-100

results in better functionalization of graphene with PDDA

along with an enhancement in the charge-transfer from gra-

phene to PDDA.


The intensity ratio (ID/IG), which corresponds to the pres-

ence of defects or disorders, has also been calculated from the

Raman spectra of different samples. Some defects may be

present even in pristine graphene due to the use of catalysts

during its synthesis. ID/IG ratio is themaximum for the sample

G2 (1.078) as it has been treated with strong acids, which

generally causes structural damage to the CeC bonds present

in graphene. A decrease in the intensity ratio for the samples

G3 (0.901) and G4 (1.057) is due to the wrapping of PDDA on

graphene. A small change in the intensity ratio indicates that

no major damage is caused to graphene upon

functionalization.

The chemical changes that take place in graphene during

oxidation and functionalization have been analyzed from XPS

results. As the B.E. values for C and N atoms are widely

different, so for each atom spectra have been plotted in the

narrow B.E. ranges and the plots are given in Fig. 4. The pris-

tine graphene consists of only CeC hexagonal skeleton and

upon oxidation a lower intensity for C1s peak has been

observed for the sample G2. Among the samples G3 and G4, G3

shows higher peak intensity as compared to G4. However,

peak B.E. for the sample G4 (284.57 eV) is at a slightly higher

B.E. value than the sample G3 (284.54 eV), which indicates an

enhanced inter-molecular charge-transfer in G4. In the C1s

XPS spectrum, another weak peak is also present at B.E. of

around 290 eV and the intensity of this peak decreases upon

functionalization of graphene and this peak almost disap-

pears for the samples G3 and G4. The disappearance of this

peak has earlier been ascribed to be due to the presence of

pep interactions between graphene and PDDA [22,23]. These

results also support that graphene has been functionalized

with PDDA.

The N1s peak is present only in the samples functionalized

with nitrogen containing polyelectrolyte (G3 and G4) (G2 does

not contain nitrogen containing PDDA) and the B.E. plots for

N1s peak are shown in Fig. 4 for these two samples only. In

order to compare the N1s peak B.E. values of the samples G3

and G4 with pure PDDA, the peak B.E. value for PDDA (402 eV)

has been taken from literature [7]. A down-shift in the N1s




Fig. 4 e XPS spectra in the narrow B.E. range for the C1s and N1s peaks for different samples.


peak B.E. value in G3, as compared to PDDA is due to the inter-

molecular charge-transfer from C in graphene to Nþ in PDDA.

A down-shift in the peak B.E. values for the samples G3

(401.75 eV) and G4 (401.09 eV) has been observed as compared

to pure PDDA. However, the down-shift in B.E. value show a

significantly higher negative shift for the sample G4 (0.91 eV)

than for the sample G3 (0.25 eV) in comparison to PDDA. It

indicates an enhanced inter-molecular charge-transfer in the

sample G4 which contains Triton X- 100, a non-ionic surfac-

tant than in G3.

The thermal stability of different samples has been

examined by studyingweight loss up to 900 �C by TGA, and the

thermograms are shown in Fig. 5. The sample G1 (pristine

graphene) shows a small weight loss beyond 600 �C only,

which is due to the presence of oxygen impurities. The sample

G2 show weight loss by a two-step process as observed in

Fig. 5. The first weight loss which occurs at around 100 �C is

due to desorption of physisorbed water [24] and the second

weight loss at around 200 �C is due to the decomposition of

oxygen containing groups. The small weight loss beyond

400 �C is due to the pyrolysis of the carbon structure [23]. The

sample G3 show relatively higher thermal stability as

compared to the sample G2 and it may be due to the wrapping

Fig. 5 e TGA plots for different samples G1eG4.


of graphene with PDDA upon functionalization. A slightly

lower thermal stability for the sample G4 as compared to G3

may be related to the presence of Triton X-100. However, pep

interactions between PDDA and graphene preserve the py-

rolysis of carbon skeleton to a considerable extent and that

provides thermal stability.

On the basis of above studies, it has been observed that

the presence of a non-ionic surfactant along with graphene

and PDDA affects their interactions. The Raman and XPS

studies show an inter-molecular charge-transfer from gra-

phene to PDDA which enhances in the presence of Triton X-

100. Thus, graphene functionalized with PDDA in the pres-

ence of Triton X-100 could prove to be very advantageous as

metal-free electrocatalyst for ORR in fuel cells. However, the

properties also depend upon some other factors such as the

type of surfactant, its chemical structure, size of graphene

sheets and its atomic and electronic structure etc. The

surfactant has been found to be helpful in the dispersion of

graphene sheets. However, the mechanism involved may be

different than for CNTs due to the structural differences

between these two carbon nanomaterials. Recently, an

interaction mechanism has been suggested between a non-

ionic surfactant and graphene sheets on the basis of theo-

retical studies by using CG molecular dynamics simulations

[10]. Major factors controlling these interactions are re-

ported to be the surfactant concentration, size of graphene

sheets, hydrocarbon chain length in a surfactant, length of

head groups in a surfactant, nature of a surfactant (ionic/

non-ionic) and the orientation of surfactant backbone with

respect to the graphitic sheets. The presence of long head

groups in a surfactant has been reported to increase its

surface affinity with the carbon atoms on graphene sheets,

and so it covers the graphitic surface in a better way. The

atomic structure of the surfactant (Triton X-100) used in the

present study is shown in Fig. 6. It contains polyethylene

oxide chain as well as a benzene ring in its chemical

structure and on an average it contains 9.5 ethylene oxide

units. The presence of long chain increases its surface af-

finity with the carbon atoms in the graphene sheet. Being

non-ionic in nature, Triton X-100 does not provide electro-

static interactions with PDDA, so both interact very weakly.

PDDA and Triton X-100 could form a wrapped complex

around graphene sheets, in which the hydrocarbon chains




OO

H

x

Fig. 6 e Chemical structure of the non-ionic surfactant,

Triton X-100.


of Triton X-100 get associated with the backbone of PDDA.

As a result this system becomes more ordered and

extended, and hence could also lead to better functionali-

zation of graphene with PDDA.

Conclusions

The presence of a non-ionic surfactant during non-covalent

functionalization of graphene with PDDA has been observed

to affect their interactions. TEM and FTIR results show the

functionalization of graphene with PDDA. The PDDA con-

taining an electron-withdrawing group (Nþ) in its chemical

structure acts as a p-type dopant in graphene and its pres-

ence enhances the inter-molecular charge-transfer from

graphene to PDDA, which has been observed by an up-shift

in the position of G-peak in the Raman spectra and an up-

and down-shifts in the B.E. values of C1s and N1s peaks,

respectively in the XPS spectra. The functionalization of

graphene with PDDA gets enhanced in the presence of

Triton-X-100 and it is suitable for their applications as an

electrocatalyst for ORR in fuel cells. Graphene functionalized

with PDDA also have good thermal stability. The simple

method followed for the functionalization of graphene with

PDDA is commercially viable for fuel cells. However, more

theoretical and experimental studies including electro-

chemical characterization are desired to check their per-

formance in actual fuel cells.

Acknowledgment

This research was supported in part by the KIST Institutional

Program (Project No. 2E24841), in part by the Ministry of Edu-

cation, Science Technology (MEST) and National Research

Foundation of Korea (NRF) through the Human Resource

Training Project for Regional Innovation (Ref. No.

2012H1B8A2025906) and in part by the New & Renewable En-

ergy R&D Program of the Korea Institute of Energy Technology

Evaluation and Planning (KETEP) grant funded by the Korea

government Ministry of Trade, Industry& Energy (MOTIE) (No.

20123010010070). Authors acknowledge the help extended by

Professor Prabhjeet Singh and Dushyant during experimental

work. One of the authors (Prabhsharan Kaur) thanks CSIR,

New Delhi for the award of Senior Research Fellowship (Grant

No. SRF: 09/254(0225)/2010-EMR-1).


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Non-covalent functionalization of graphene with poly(diallyl dimethylammonium) chloride: Effect of a non-ionic surfactant