On the pH sensitive optoelectronic properties of amphiphilic reduced graphene oxide via grafting of poly(dimethylaminoethyl methacrylate): A signature of p- and n-type doping

Article abstract of DOI:10.1039/c4ta03408b

Poly(N,N′-dimethylaminoethyl methacraylate) (PDMAEMA) functionalized reduced graphene oxide (rGO) is synthesized by atom transfer radical polymerization, followed by attachment to rGO via diazonium coupling. The rGO-PDMAEMA (RGP) is characterized by

Full text of DOI:10.1039/c4ta03408b

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Materials Chemistry A
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On the pH sensitive optoelectronic properties of
amphiphilic reduced graphene oxide via grafting of
poly(dimethylaminoethyl methacrylate): a
signature of p- and n-type doping†
Cite this: DOI: 10.1039/c4ta03408b
Atanu Kuila, Nabasmita Maity, Rama K. Layek and Arun K. Nandi*
0
Poly(N,N -dimethylaminoethyl methacraylate) (PDMAEMA) functionalized reduced graphene oxide (rGO) is
synthesized by atom transfer radical polymerization, followed by attachment to rGO via diazonium
1
coupling. The rGO-PDMAEMA (RGP) is characterized by H NMR, UV-Vis, FTIR and Raman spectroscopy.
TEM and AFM studies demonstrate the formation of molecular brushes of PDMAEMA chains over rGO
surface, and the TGA thermograms indicate 55 wt% grafting of PDMAEMA. RGP is dispersible in the
4 p h
widest spectrum of solvents from CCl to water [solubility parameter (d + d ), 0.6 to 58]. RGP exhibits
strongly pH dependent fluorescence properties: at pH 4, it exhibits two emission peaks, but at pH 7 and
pH 9.2, a single and broad emission peak is observed. Two emission peaks at pH 4 are attributed to
radiative decay of excitons to two kinds of holes in the rGO originating from illumination and p-type
doping, which is also characterized by a blue shift of the Raman D band. Moreover, n-type doping of
RGP at pH 9.2 is also evident due to a similar Raman shift. The dc-conductivity of RGP at pH 4 is 2
orders higher than that of pH 9.2 and the I–V characteristic curve at pH 7 exhibits a bimodal NDR
property with a rectification ratio of 5.5. The bimodal NDR property is explained with a model using
density of states and polaronic band.
Received 3rd July 2014
Accepted 31st July 2014
DOI: 10.1039/c4ta03408b
www.rsc.org/MaterialsA
13
dispersion of graphene. Therefore, due to the high cohesive
van der Waals (VDW) energy between the sheets, the solubilty of
Introduction
In the eld of material science, graphene is a unique material rGO sheets is limited to a few solvents.
due to its excellent and intriguing physical, mechanical, optical
To overcome this problem, the functionalization on rGO
To take advantage of these prop- sheets with a suitable functional group or polymer is necessary.
erties, judicious incorporation of graphene into the various The introduction of functionality to rGO sheets is mainly
1–10
and electronic properties.
14
matrixes via a uniform dispersion and ne interfacial control is divided into two approaches: (1) covalent and (2) non-cova-
11
15
necessary. Graphene oxide (GO) is dispersible in water but lent. Each type of functionalization of graphene has the ability
electrically insulating. To make it electrically conducting, GO to make it dispersed in a few specic solvents. Depending on
should be reduced to produce reduced graphene oxide (rGO), the physical properties of the modied graphene sheets, they
but the formation of a good dispersion of rGO sheets is limited may be either hydrophilic or hydrophobic. The development of
12
to only a few solvents. The solvents with lower Hildebrand modied rGO sheets that are soluble in a wide range of solvents
1
6–18
solubility parameters and lower boiling points exhibit poor is very limited.
Li et al. have prepared amphiphilic rGO by
16
graphene dispersion, whereas a solvent with a Hildebrand noncovalent functionalization of rGO with congo red and this
1
/2
solubility parameter ꢀ23 MPa and a higher boiling point dispersion is limited to DMF, DMSO, methanol, ethanol and
exhibits the best graphene dispersion. N-Methyl-pyrrolidone water. Very recently, amphiliphilic rGO was reported by the
1
/2
(
NMP) has a solubility parameter of around 23 MPa and a covalent functionalization of graphene sheets with poly(vinyl
boiling point close to 200 C, forming a high-quality stable pyrrolidone), but this procedure was unable to disperse rGO in
highly non-polar solvents like carbon tetrachloride and
ꢁ
17
19
benzene. Shen et al. have reported an amphiphilic rGO func-
tionalized with random block copolymers of hydrophilic and
hydrophobic sites by graing from the rGO surface.
Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur,
Kolkata-700 032, India. E-mail: psuakn@iacs.res.in
†
Electronic supplementary information (ESI) available: The table for Hansen
It has been reported that the amount of dispersibility of the
solubility parameters, photographs of dispersibility test for RGP with higher
molecular weight, uorescence micrographs, NMR spectra of intiatiator, PL graphene sheets is highly dependent on the molecular weight of
spectra of GO at different pH, UV-Vis spectra of RGP at different pH values. See
20
the graed polymer. However, living radical polymerization
DOI: 10.1039/c4ta03408b
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from the surface of graphene rarely shows control over the
Thus, a pH-dependent PL intensity is expected in appropri-
21
extent of graing. Because polymer-linked graphene itself ately functionalized graphenes and a detailed study of uores-
does not form a good solution and the process of degraing the cence of the RGP system at different pH values is presented
graed polymer is rarely practical or sometimes impossible, it is herein. The PDMAEMA polymer used in the study is found to
very difficult to characterise these systems by gel permeation result in different types of doping (p- and n-type) at different pH
3 2
chromatography. Thus, chemical engineering on rGO surface values due to the presence of –N(CH ) groups in the graed
with suitable polymers has become one of the critical issues for PDMAEMA from graphene moieties, and it is expected to exhibit
investigating the wide range of solubility in the development of different electronic properties when doped at different pH
graphene technologies. To overcome this challenge, herein, we values. A brief account of all the optoelectronic properties is
report on a versatile dispersibility of rGO sheets compared with discussed and an attempt is made to provide possible expla-
16,19,20
the previous reports
via the graing of poly(dimethyl nations for these interesting observations.
aminoethyl methacrylate) (PDMAEMA) (RGP) chains using
diazonium chemistry by the graing to approach.
Fluorescent nano-materials are very important as these
materials have a wide range of applications in optoelectronic
Experimental section
Materials
2
2–24
25
26
devices,
materials such as gold nanodots, semiconductor quantum isobutyryl bromide, 1,1,4,7,10,10-hexamethyltrietylenetetr-
dots,
sensing and biological labeling. Different nano- Graphite, p-nitrobenzylalcohol, sodium borohydride, 2-bromo-
27
2
8–30
31,32
and silicon nanoparticles,
as well as carbon-based amine (HMTETA), CuCl (Aldrich, USA), HCl, hexane, ethyl
nanomaterials such as nanodiamond, carbon dots, and carbon acetate, methanol, petroleum ether, N H , sodium nitrite,
2
4
33–40
nanotubes,
exhibit good uorescence properties. Graphene is NaHCO , NaCl, Na SO , silica (Merck, Mumbai, synthetic grade)
3 2 4
not uorescent due to its continous electronic band structure, but were used as received. Tetrahydrofuran, triethylamine,
graphene oxide (GO) with its nite electronic band gap exhibits dichloromethane, DMF (Rankem, New Delhi, analytical grade)
very weak uorescence, which arises from the radiative recom- were distilled before use and 2-(dimethylamino)ethyl methac-
bination of electron–hole (e–h) pairs in the localized electronic rylate (DMAEMA) (Aldrich, USA) was puried by column
41
states of GO. The poor emissive properties of GO originate from chromatography.
the carboxyl, hydroxyl and epoxy groups, which induce non-
Synthesis of 4-nitro-2-bromoisobutyryl bromide (NBI) from
radiative recombinations by the transfer of their non-bonding p-nitrobenzylalcohol. In a round-bottomed ask, p-nitro-
2
34,37
electrons to the holes in the sp clusters.
The partial reduction benzylalcohol (20 mmol) and triethylamine (22 mmol) were
of GO causes an enhancement in the PL intensity due to the dissolved under an inert atmosphere in anhydrous DCM (40 ml)
2
34
ꢁ
followed by stirring at 0 C, and to this solution, 2-bromoiso-
creation of more localized sp clusters and structural defects.
The formation of water soluble and blue uorescent GO is butyryl bromide (22 mmol, dissolved in 10 ml of anhydrous
reported by amide formation and epoxide ring opening by alkyl DCM) was added dropwise by a pressure-equalizer. Aer the
amines destroying the non-radiative electron–hole (e–h) recom- addition was complete, the reaction mixture was permitted to
39
ꢁ
bination centers of GO. Moreover, highly uorescent GO was stir at room temperature (30 C) for 24 h. Then, the triethyl-
produced by the non-covalent functionalization with a polymer amine hydrochloride salt was removed by ltering and the
(
methyl cellulose), and this uorescent hybrid was used as an ltrate was consecutively washed with 1% HCl, saturated
3
42
efficient nitroaromatic sensor. Recently, Ajayan et al. proposed NaHCO , brine solution and distilled water. The organic layer
3
that the PL properties of GO arise from the quasi molecular u- was collected and was dried over active Na SO . Silica column
2
4
orophores like the polycyclic aromatic compounds produced chromatography was performed using a solvent mixture of
from the electronic coupling of oxygen atoms of –COOH groups to hexane–ethyl acetate (95/5) for further purication. The nal
the adjacent carbon atom of graphene. They observed a strong product was obtained as a brownish liquid aer the evaporation
pH-dependent uorescence of GO and attributed the PL proper- of the organic solvent.
ties of GO above pH 8 to the electronically excited carboxylate ion
of GO. Kochmann et al. have explained the pH dependent PL dimethyl aminoethyl methacraylate) (NO -Ar-PDMAEMA) via
Synthesis of 4-nitro benzene functionalized poly(N,N-
43
2
ꢃ2
properties of GO as a result of the protonation and deprotonation atom transfer radical polymerization (ATRP). CuCl (26.5 ꢂ 10
of –COOH and –OH groups of GO. The non-covalent interaction mmol) was added to a nitrogen-purged vessel containing a 1 ml
ꢃ2
of poly(vinyl alcohol) (PVA) with the –OH, epoxy or carboxyl tetrahydrofuran (THF) solution of NBI (26.5 ꢂ 10 mmol).
groups (–COOH) present on the GO surface prevent the restacking Then, nitrogen-purged DMAEMA (0.7 ml) was introduced into
44
of GO sheets, thereby causing an enhancement of PL intensity.
the vessel and the vessel was closed with a rubber septum, and
Hence, the photoluminescence (PL) properties of GO originate then 70 ml of HMTETA was injected. The reaction mixture was
ꢁ
from the isolated polyaromatic structures and/or passivated then allowed to stir for 24 h at 30 C. The solution was then
34,39
surface defects prepared under the very extreme conditions.
diluted with THF and poured into excess petroleum ether. The
With a change in pH, the nature of the carboxylic, hydroxyl and synthesized polymer was separated by precipitation in the non-
epoxy group of GO also changes, causing a different degree of solvent petroleum ether. Then, the polymer was isolated, re-
passivation of the GO surface. For pH # 4, the GO is fully dissolved in THF, and then it was precipitated into excess
protonated, and at pH > 4, the anchored –COOH groups become petroleum ether. To remove any trace amount of trapped
44
deprotonated such that the PL intensity decreases signicantly.
monomer, this process was repeated three times. The sample
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was dissolved in THF and the solution was passed through a DMF–water) by stirring and ultrasonication for 1 h. Next, 1 ml of
4
7
silica column followed by solvent evaporation to obtain copper N H was added to the mixture. The solution was heated in an
2
4
ꢁ
free polymer.
Synthesis of 4-amino benzene functionalized poly(dimethyl (rGO). Upon sonication, the rGO was dispersed in the reaction
aminoethyl methacraylate) (NH -Ar-PDMAEMA). Raney nickel medium. This solution was centrifuged at 12 000 rpm to collect
3 mg) was added to the obtained NO -Ar-PDMAEMA solution in a black mass of rGO.
methanol (20 ml) at room temperature. Sodium borohydride
38.5 mg) was slowly and carefully added to this solution and
the reaction mixture was allowed to stir for 4 h. The solution was
ltered to remove catalyst, washed with methanol and imme-
diately immersed in distilled water to prevent spontaneous
oil bath at 100 C for 1 h to obtain the reduced graphene oxide
2
(
2
(
Diazo coupling reaction of 4-amino benzene functionalized
poly(dimethyl aminoethyl methacraylate) with rGO (RGP)

An aryl diazonium salt was prepared from the reaction between
NH
0
2
-Ar-PDMAEMA and 18 mg sodium nitrite in 10 ml water and
.5 ml 1 N HCl solution in an ice bath. To a dispersion of rGO in
an ice bath under stirring, the diazonium salt solution was
added, and the low temperature was maintained for 2 hours.
Aer the addition of the diazonium salt solution, aggregation
was observed. Then, the reaction mixture was centrifuged at
45
combustion. The ltrate was evaporated to dryness to obtain
NH -Ar-PDMAEMA (Scheme 1a).
2
Preparation of reduced graphene oxide (rGO)
First, graphene oxide was prepared according to Hummers
method. GO (75 mg) was dispersed in 75 ml of solvent (9 : 1
8
000 rpm and washed with doubly distilled water several times,
and the product was obtained by drying in vacuum at 35 C.
46
ꢁ
14
The resultant RGP was re-dispersed in 75 ml water, and the
entire process is shown in Scheme 1(a) and (b).
Characterization
Microscopy. Transmission electron microscopy (TEM, JEOL,
2010EX) was conducted by dispersing the rGO and RGP in water
and casting a drop of solution on a carbon-coated copper grid.
The samples were dried by the slow evaporation of the solvent at
ꢁ
room temperature (30 C) and nally under vacuum. Atomic
force microscopy (AFM) was conducted in the non-contact
mode at a resonance frequency of ꢀ250 kHz. rGO and RGP were
dispersed in water (0.01%, w/v) by sonication and lms were
ꢁ
cast on a mica surface. The samples were dried at 60 C on a hot
ꢁ
plate and nally under vacuum at 60 C for three days. The
morphology of the dried samples were studied using atomic
force microscope (Veeco, model AP 0100).
For the one-photon excitation experiment, a pulsed diode
laser with a repetition rate of ꢀ40 MHz was used. Briey, a
combination of a confocal microscope (Olympus IX-71) and
TCSPC setup (PicoQuant, MicroTime 200) was used in this
study. A water immersion objective (magnication 60ꢂ and
numerical aperture (NA) z 1.2) was used. The diffraction
limited spot size was 0.6l/1.2 ꢀ l/2. The excitation sources were
a pulsed picosecond diode laser (PDL 828-S “SEPIA II”, Pico-
Quant for 1PE at 405 nm). The uorescence from the samples
(RGP at pH 4, pH 7 and pH 9.2) was separated by a dichroic
mirror (Z405RDC, Chroma for 1 PE). To block the exciting laser
light, a suitable lter (430LP, Chroma, for 1 PE) was used in
front of the detectors. The uorescence was focused through a
pinhole (30 mm) and was detected by two single photon-count-
ing avalanche photodiodes (SPAD1 and SPAD2). The signals
were subsequently processed by the PicoHarp-300 time-corre-
lated single-photon-counting module (PicoQuant).
Spectroscopy. FTIR studies on GO, rGO, RGP samples were
performed using a Perkin-Elmer FT-IR instrument (spectrum
100). The ultraviolet-visible (UV-Vis) absorption spectra of the
DMF solutions of GO, rGO, RGP and aqueous solutions of RGP
at different pH values (4, 7 and 9.2) were examined using a UV-
Scheme
1
(a) Synthesis of NH
2
-Ar-PDMAEMA from nitrobenzyl
-Ar-PDMAEMA.
alcohol. (b) Synthesis of RGP by coupling of NH
2
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Vis spectrophotometer (Hewlett-Packard, Model 8453) in a of PDMAEMA are assigned in the spectra, conrming the
quartz cell with a thickness of 1 cm in the wavelength range of graing of PDMAEMA onto the surface of the rGO. The reason
ꢁ
1
90–1100 nm at 30 C. Photoluminescence (PL) studies of the for the broadening of the NMR peaks of PDMAEMA upon
solutions were performed using a quartz cell with a 1 cm path attachment to rGO is not clearly known and may be attributed to
length using a Horiba Jobin Yvon Fluoromax-3 instrument. The the decrease in solubility of RGP due to its attachment to the
1
H NMR spectra of the samples were recorded in CDCl
3
on a 500 insoluble rGO.
The UV-Vis spectra (Fig. 2) are used to characterize the
MHz Bruker instrument. Raman spectra of the rGO and RGP
solutions in water were recorded using a LabSpec Raman formation of GO, rGO and RGP. The GO dispersion in DMF
spectroscope (JY T6400). The samples solutions were placed on shows an absorption peak at 240 nm, which is due to the p–p*
a microscopic glass slide and were excited at 514 nm with Ar ion transition of the graphitic rings and another at 305 nm, which is
17
laser with a scanning duration of 40 s.
Thermal study. The thermal stability of rGO and RGP was reduction with N H solution, the C]C bond becomes more
due to the n–p* transition of the substituted GO groups. Aer
2
4
measured using a Perkin-Elmer TGA instrument (Pyris Dia- aromatic in nature, resulting in a red shi of the p–p* transi-
mond TG/DTA) under a nitrogen atmosphere at a heating rate of tion peak to 270 nm. Similarly, the n–p* transition becomes
ꢁ
ꢃ1
51
1
0 C min
.
forbidden due to the removal of the oxygen groups.
The FTIR spectra of GO, rGO and RGP are compared in Fig. 3.
Conductivity measurement. The dc-conductivity of the
solvent cast lms of RGP prepared at different pH (4, 7 and 9.2) GO exhibits characteristic peaks due to the pC]O stretching
ꢃ1
was measured using the two-probe method. The lm sample vibration at 1720 cm , the O–H stretching vibration at 3400
ꢃ1
was placed between two indium-tin oxide (ITO) conducting cm , the C–O stretching vibration of the epoxide group at 1040
ꢃ1
ꢃ1 52
strips of 2 mm in width that were placed perpendicularly. The cm and the unoxidized graphitic domain at 1625 cm
. In
48
detailed procedure is discussed in our earlier publication. The the case of rGO samples, the peak for the epoxide group
area of the sample was 0.04 cm , and the thickness of the disappears due to the reduction by hydrazine hydrate. The FTIR
2
samples was measured by a screw gauge. The conductivity of the spectrum of rGO shows that the graphitic ring stretching peak
ꢃ1
samples was measured by an electrometer (Keithley, model at 1625 cm becomes more broadened, which indicates the
ꢁ
2
401) at 30 C using the equation:
formation of more new graphitic domains with wide poly-
dispersity. Like another methacrylate polymer, PDMAEMA
52
1
l
s ¼
ꢂ
graed from rGO exhibits two characteristic peaks at 1728 and
R
a
ꢃ1
1
660 cm (Fig. 3) due to the ester carbonyl stretching vibra-
where ‘l’ is the thickness, ‘a’ is the area and ‘R’ is the resistance.
Current–voltage (I–V) studies were performed using the same
samples by varying the voltage from ꢃ5 to +5 V and measuring
the current at each applied voltage.
53
tion. The characteristic peaks of the methyl and methylene
vibrations of PDMAEMA adjacent to the nitrogen atom are
ꢃ1 53
observed at 2770, 2823 and 2940 cm
. The peak for RGP at
ꢃ1
1152 cm
is a characteristic peak for the C–O stretching
vibration of PDMAEMA.
The Raman spectra of GO, rGO and RGP are compared in
Fig. 4, where the disorder-induced phonon mode (D band) and
the Raman allowed phonon mode (G band) of the GO vibra-
Results and discussion
Synthesis and characterization
ꢃ1
ꢃ1
The strategy for the preparation of RGP consists of two synthetic tions appear at 1350 cm and 1607 cm , respectively. The D
3
processes: (i) synthesis of 4-amino benzene functionalized band appears due to the vibrations of sp carbon atoms
2
poly(dimethyl aminoethyl methacraylate) (NH -Ar-PDMAEMA) (defects), and the G band arises for the vibrations of sp carbon
2
(
D G
Scheme 1a) and (ii) the preparation of rGO followed by its atoms of the graphene rings. The I /I ratio of GO has a value of
14
2
3
coupling with the former using a diazo-coupling reaction
(
isobutyryl bromide initiator (NBI) from nitro benzyl alcohol is and the G band appears at 1602 cm with an I
1.01, indicating the presence of nearly equal (50%) sp and sp
ꢃ1
Scheme 1b). In Scheme 1a, the synthesis of 4-nitro-2-bromo- domains. In the case of rGO, the D band appears at 1354 cm
ꢃ1
/I
ratio of 1.34.
/I ratio is different from the expected
D
G
1
shown. The H NMR spectrum (Fig. S1†) conrms the ATRP This increase in the I
initiator attachment from the nitro benzyl alcohol, and it is decrease in the I /I , which should occur due to reduction;
used for the synthesis of NO -Ar-PDMAEMA. The broader nature however, a similar increase in the I /I ratio of rGO is also
A probable reason for this
D G
D
G
2
D
G
1
51,54
of the above peaks in the H NMR spectrum (Fig. S2†) indicates reported by a few groups.
that PDMAEMA has grown from NBI. At the inset of the gure, observation is that GO consists of –OH, –COOH and epoxy
the GPC trace of the polymer in THF indicates that the average groups, which keeps the graphene nanosheets planar due to
ꢀ
molecular weight (M
n
) ꢀ 1500 with a PDI of 1.15, indicating an mechanical pressure from either side. Upon the reduction of
49
oligomer is produced. NO
2
-Ar-PDMAEMA is reduced to NH
2
-
epoxy groups and the removal of some fraction of other oxy-
ꢁ
Ar-PDMAEMA at 30 C by using specic reagents (Scheme 1a), genous groups, the sheets become somewhat tension-free,
which has been reported in the literature.
The H NMR spectrum of RGP (Fig. 1) conrms the graing smaller domains of graphitic rings. This may cause an increase
D G
of PDMAEMA from the surface of rGO. The six protons of the in the disorder and therefore an increase in the I /I ratio of
45
which causes the rGO sheets to become wrinkled and produce
1
50
dimethylamino group of RGP resonate at 2.3 ppm in the rGO relative to that of GO, although there is an increase in the
PDMAEMA graed from rGO and the peaks of all other protons number of graphitic domains.
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1
3
Fig. 1 H NMR spectrum of RGP in CDCl .
Fig. 2 UV-Vis spectra of GO, rGO and RGP from DMF solution.
Fig. 4 Raman spectra of GO, rGO and RGP.
attachment of the PDMAEMA chain from either side of gra-
phene, which increases the planarity of RGP due to mechanical
2 4
pressure. Moreover, it may also be due to a reduction by N H of
3
some more sp hybridized carbons, which become converted
2
into sp hybridized carbons, causing a decrease in the ID/IG
ratio. A blue shi in the D band position of rGO from GO, and
then the reversal at RGP is interesting. The higher frequency
band position of rGO may be attributed to the increase in the
wrinkled structure, which causes difficulties in the vibration of
3
disordered sp hybridized bonds, increasing the band position
ꢃ1
Fig. 3 FTIR spectra of GO, rGO and RGP.
by 4 cm . However, in the case of RGP, the recovery of planarity
due to the attachment of the polymer on either side leads to a
ꢃ1
shi back to that of GO (1350 cm ). The broadness of the G
In the case of RGP, the D band and the G band appear at 1350 band in rGO suggests an increase in the dispersity of the
ꢃ1
and 1650 cm , respectively, and the I
D
/I
G
ratio becomes 0.94, graphitic domains that may arise due to wrinkling, which
which is lower than that of rGO and GO. This may be due to the results in isolated graphitic domains.
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rGO. From the height prole, the average thickness of the RGP
is found to be ꢀ6.4 nm. This thickness is ve times higher than
that of the rGO sheet, which indicates the wrapping of the
graed polymer on the surface of the rGO sheets.
The TGA thermograms of rGO and RGP (Fig. 7) indicate 24%
and 79% weight loss, respectively. A subtraction of the two
quantities yields a graing percent of PDMAEMA on graphene
surface of 55 wt%. Again, from the gravimetric study during the
polymerization, the graing of PDMAEMA is also calculated to
be 51%, which supports the TGA data.
Fig. 5 TEM images of (a) rGO and (b) RGP.
Dispersibility in different solvents
Evidence in support of the above assertion may come from a The dispersibility of RGP in a series of solvents was tested by
ꢃ1
comparison of TEM images of rGO and RGP presented in Fig. 5a sonicating RGP (1.5 mg ml ) in various categories of solvents
and b. RGP, produced aer polymer functionalization of rGO, for half an hour. The photographs of the dispersions aer
exhibits new black domains that are associated with molecular standing for another half an hour are presented in Fig. 8. The
brushes of PDMAEMA chains over the surface of rGO (Fig. 5b). sample is found to form stable dispersions in (a) carbon tetra-
Thus, the TEM image of RGP shows a rGO sheet, which has chloride, (b) benzene, (c) toluene, (d) xylene, (e) chlorobenzene,
been decorated with blackish polymer brushes. These results
also support that the PDMAEMA chains are successfully graed
onto the surface of graphene via the diazonium coupling reac-
18,20
tion of NH
2
-Ar-PDMAEMA.
It is also important to note that
the rGO sheet is somewhat wrinkled (Fig. 5a) compared to that
of the RGP sheet, which is planar over a long region. This effect
may be attributed to the equal pressure exerted from both the
sides of the graphene sheet by polymer chains prohibiting the
formation of wrinkled sheets, as discussed above.
AFM topography and corresponding height prole diagram
are very helpful in characterizing the polymer brushes graed
onto rGO, and the results are presented in Fig. 6. The obtained
thickness of the rGO sheets is similar to that of a previous
17
report, i.e. 1.2 nm. The RGP sheets show a number of white
spots, suggesting that polymers are graed onto the surface of Fig.
7
TGA thermograms of rGO and RGP under nitrogen
atmosphere.
Fig. 6 AFM images and corresponding height profiles of (a) rGO and (b) RGP.
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ꢃ1
Fig. 8 Dispersibility of RGP in different solvents (1.5 mg ml ): (a) carbon tetrachloride, (b) benzene, (c) toluene, (d) xylene, (e) chlorobenzene, (f)
diethyl ether, (g) chloroform, (h) dioxane, (i) 1,2-dichlorobenzene (j) DCM, (k) ethyl acetate, (l) THF, (m) acetone, (n) NMP, (o) acetonitrile, (p) DMF,
(
q) DMSO, (r) ethanol, (s) methanol, and (t) water.
ꢃ1
(
f) diethyl ether, (g) chloroform, (h) dioxane, (i) 1,2 dichloro- water) is found to be ꢀ0.68 mg ml . This remarkable solubility
benzene, (j) dicholoromethane (DCM), (k) ethyl acetate, (l) properties of rGO in RGP may be attributed to the following
tetrahydrofuran (THF), (m) acetone, (n) N-methylpyrrolidone factors: (i) the polymer graing on the surface of rGO, which
0
(
NMP), (o) acetonitrile, (p) N,N -dimethyl formamide (DMF), (q) may cause an increase in the spacing between individual gra-
dimethyl sulfoxide (DMSO), (r) ethanol, (s) methanol and (t) phene sheets facilitating exfoliation in various solvents and (ii)
water, producing stable black colour dispersions. However, RGP an optimum hydrophobic/hydrophilic balance, leading to high
is not soluble in hexane and petroleum ether. The Hansen quality dispersion in a variety of solvents. The chain length of
solubility parameters of all solvents are listed in Table S1.† By PDMAEMA graed on rGO surface has great inuence on the
examining the dispersibility, it can be concluded that an solubility of the rGO sheets. We have tested another RGP
amphiphilic reduced graphene oxide is produced by graing sample with a molecular weight of PDMAEMA equal to ꢀ4700. It
PDMAEMA chains on the surface of rGO.
is clear from the photographs (Fig. S3†) that the dispersibility is
We are interested in investigating the reason for the broad not as good as that of the lower molecular weight PDMAEMA
range of solubility of RGP using Hansen solubility parameters and good dispersions are not produced in benzene, diethyl
17,55
(d
, d
d p
, d
h
),
where d
d
is the dispersion cohesion parameter, d
p
ether, ethyl acetate, THF and acetone. The rGO modied with a
is the polarity cohesion parameter; and d
h
is the hydrogen high molecular weight PDMAEMA shows very good dis-
bonding cohesion parameter. Current research on rGO disper- persibility in solvents with d
p
+ d
h
¼ 19–58.
sion indicates that d and dh factors are important for rGO
p
4
7
dispersion, and the values of d and d for different solvents
p
h
47
56
Fluorescence property
have established that functionalized rGO sheets could be In this section, we would like to compare the uorescence
dispersed in a number of organic solvents with a value of d + d properties of GO and RGP at different pH values (pH 4, 7 and
13–29 and 3.4–58.3, respectively. Nevertheless, their fun- 9.2). It is evident from the Fig. S4† that GO at pH 4 shows a
tionalized rGO sheets could not be dispersed in the solvents broad peak with the highest intensity from those at pH 7 and 9.2
with the d + d value of less than 3.4, but our RGP is not only and the emission peak appears at 445 nm. Upon attachment of
are listed in Table S1.† Very recently, Ruoff et al. and Qi et al.
p
h
¼
p
h
dispersed in toluene, chloroform, THF, DMF, DMSO, ethanol PDMAEMA to the surface of rGO, an interesting change in
methanol and water with the dp + d ¼ 13–58, but it is also uorescence occurs at pH 4 (Fig. 9). Here, two distinct peaks at
h
dispersed in cabon tetrachloride and benzene whose dp + dh 407 and 461 nm are observed, whereas at pH 7 and pH 9.2, a
values are ꢄ3.4. The PDMAEMA chain has both hydrophilic single and broad emission peak at 437 and 449 nm is observed,
2
group like –NMe and a hydrophobic long methylene chain, respectively. However, it is also interesting to note that in
which makes it soluble in a wide range of solvents.
contrast to the behavior of GO, the emission peak intensity in
The solubility limit of RGP is determined using gravimetric RGP increases with increasing pH.
5
6
ꢃ1
analysis, and it was found to be ꢀ1.5 mg ml for most
To achieve a moderate and detectable amount of uores-
solvents mentioned above. The rGO content in RGP is about 45 cence in GO, it is necessary to passivate the functional groups of
wt% from the TGA thermograms as well as from the gravimetric GO that remain partially ionized in aqueous solution (pH 7) by
data obtained during the polymerization process. Therefore, the the ionization of the –COOH group. The transfer of electrons
3,44
highest solubility limit of pure rGO in both very non-polar of the ionized carboxyl group to the graphitic holes causes a
(carbon tetrachloride, benzene) and very polar solvents (DMF, non-radiative decay of excitons; however, at pH 4, the –COOH
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The reasons for such a large increase in the uorescence
intensity may be attributed to the complete removal of all
protons from the –N(CH ) groups, thereby prohibiting most of
3
2
the non-radiative decay and causing a signicant increase in the
uorescence intensity. The red shis at pH 7 and pH 9.2 from

the main emission peak (407 nm) at pH 4 may be attributed to
the electrostatic n-type doping of rGO with the help of non-
bonding electrons of –N(CH ) group and the carboxylate ions,
3 2
which increase in number with an increase in the pH. This
electrostatic doping may shi the Fermi level of rGO and can
cause a signicant reduction in the band gap, yielding a large
57,58
red shi in the emission peak.
The uorescence micro-
graphs (Fig. S5†) of dried samples clearly indicate a variation in
the uorescence from bluish green for the RGP doped at pH 4 to
bright green for the RGP prepared at pH 9. This is in accordance
with the solution uorescence spectra presented in Fig. 9.
In the pH dependent UV-Vis spectra of RGP, a sharp absor-
bance peak (281 nm) is observed at pH 4, which is blue shied
by 11 nm at pH 7 and 9.2, where the absorption peaks are broad
in nature (Fig. S6†). The sharp peak at 281 nm is due to p–p*
transitions of the graphene rings, and the red shi at pH 7 and
Fig. 9 Fluorescence spectra of RGP in different pH values (inset: PL
spectrum of RGP at pH 4).
group becomes unionized and thus the non-radiative decay of 9.2 may be attributed to delocalization of p electrons of the
excitons becomes diminished, causing a ꢀ6 times increase in graphene ring due to proton doping. This proton doping also
2
the uorescence intensity compared to that at pH 7 and 9.2. causes a less dispersed sp energy level due to delocalization.
Here it is interesting to note from Fig. 9 that the PL intensity of
Proof for different types of doping in the RGP at different pH
RGP decreases with decreasing pH, which is completely oppo- values may be obtained from Raman spectroscopy (Fig. 10). The
site to the behavior of GO mentioned above. Moreover, RGP Raman spectrum of RGP at pH 4 is quite different from that of
exhibits two distinct emission peaks at 407 and 461 nm at pH 4 pH 7 as the spectrum exhibits multiple peaks. The D band
ꢃ1
in contrast to the single emission peak at 449 nm for pH 9.2 and exhibits a signicant blue shi of 36 cm , whereas the G band
ꢃ1
at 437 nm for pH 7. A probable reason for this unusual nding shows a red shi of 4 cm from that of pH 7. These shis in the
is that at pH 4, the RGP becomes fully protonated such that peak positions certainly indicate that graphene in RGP
+
59–61
–
N(CH
3
)
2
groups of PDMAEMA are converted to –N H (CH
3
)
2
.
undergoes p-type (hole) doping at pH 4.
A probable reason
These groups can easily dope the graphitic rings of RGP at for the red shi in G band may be due to better delocalization of
2
closer proximity due to the exible coil nature of the graed sp hybridized p-electrons of the graphitic ring in the hole
PDMAEMA chains. Thus, it produces an additional secondary doped state. The reason for the blue shi in the vibration of
hole-like structure whose energy level is possibly higher than disordered bonds mainly arising from the vibration of graphitic
that of normal holes of RGP created by illumination. These edges (D band) remains unknown, but a probable reason may
secondary holes on the graphene sheets of RGP give rise to a be proposed here. Due to the better delocalization of graphene
new decay path for the excitons, which quenches the uores-
cence intensity to some extent in the main emission peak of
RGP. Probably, the decay of GO excitons in this path is radiative
in nature, causing a weak emission peak at 461 nm. The emis-
sion peak at 407 nm may be attributed to the radiative decay of
rGO excitons of RGP, which became blue shied from that of
GO (445 nm) because the ground state energy of RGP is very low
2
due to an increase in the sp character from that of GO. A careful
observation of the uorescence spectrum of RGP at pH 7 indi-
cates the presence of a single peak (437 nm) with a small hump
(
463 nm) showing an increase in the PL intensity. A single
emission peak with a small hump may be attributed to the
absence of protonation of the –N(CH group to a major extent,
3 2
)
which decreases the degree of non-radiative decay of the RGP
excitons, causing an increase in the PL intensity. The hump at
4
63 nm may arise due to the partial protonation of a few
+
–
NH (CH ) groups, thereby producing a few secondary holes.
3 2
At pH 9.2, there is a ꢀ8-fold increase in the PL intensity with a
signicant red shi of the rGO emission peak from that of pH 4. Fig. 10 Raman spectra of RGP doped at pH 4, pH 7 and pH 9.2.
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ring electrons, the edge C–C bonds of graphene experience
some pressure from the electrons, which inhibits the vibration
of the edge bonds to some extent requiring a higher frequency
for the vibration, thereby causing the blue shi. A new peak
ꢃ1
appears at 1678 cm aer the G band, and it may be attributed
0
to the disorder activated D band, arising from the hole doping
that activates the Raman inactive phonon present in the gra-
60
phene. The other small peaks in the spectrum correspond to
the Raman active vibrations of different energies probably due
to the presence of some heterogeneous type of doping in the
graphene moiety of RGP. It is important to note that the
intensity ratio of the D and G bands (I /I ) remains the same
D
G
(
1.09) as that of pH 7 because p-type of doping does not affect
59
ꢃ1
it. At pH 9.2, there is also a blue shi in the D band by 16 cm
but the position of the G band remains unchanged. Moreover,
there exists a new D band at 1684 cm along with some small
peaks corresponding to the Raman active vibrations of different
energies probably due to the inhomogeneous doping of the
graphene moiety in RGP as stated earlier. At pH 9.2, the lone
0
ꢃ1
Fig. 11 Current–voltage (I–V) characteristic curves of RGP at pH 7.
pair of electrons of –N(CH ) groups of RGP, ionized –OH and pH 4, possibly due to a much lesser amount of trapped charges
3
2
–
COOH groups are the sources of excess electrons, which can present in the latter system than in the former. For RGP samples
dope the graphitic ring of RGP and may be termed as n-type doped at pH 7, some interesting behavior is observed (Fig. 11).
doping. The blue shi in the D band probably arises from the At the positive bias, at rst there is a typical increase in the
3
creation of more sp domains, rendering its vibration difficult current with increasing voltage and at 1.5 V, it reaches a
and causing an increase in the I
D
/I
G
ratio (¼1.45) compared to maximum, and then there is a decrease in the current. Aer
3
pH 7 (I /I ¼ 1.09). The increase in the sp domains may arise reaching 2.2 V, the current again begins to increase, showing a
D
G
due to the opening of the epoxy group of GO by the reaction of maximum at 3 V and nally decreases with increasing voltage.
39
–
N(CH3)2 groups and also for the new secondary bonding The decrease in the current with increasing voltage is in
produced between the graphene moiety and the ionized –COOH contrast to Ohm's law and the phenomenon is known as
62,63
and –OH groups. Nonetheless, it may be surmised that gra- negative differential resistance (NDR).
phene in RGP gets p-type doped at pH 4 and n-type doped at pH symmetric and it also exhibits the rectication property with a
.2 with a smaller fraction of p-type doping at pH 7. rectication ratio of 5.5, which may arise due to the presence of
partially protonated –NMe groups that can partially dope the
rGO moiety of RGP and thus act as p-type semiconductor. Here,
also a part of the –NMe groups remain undoped. The non-
The dc-conductivity of RGP samples aer doping at different pH bonding electrons of the undoped –NMe groups may cause n-
values are measured by the two probe method and the resulting type doping of the rGO of RGP, causing an intimately mixed p
The I–V curve is not
9
2
dc-conductivity and I–V property
2
2
ꢃ7
ꢃ7
ꢃ7
ꢃ1
conductivities were 16 ꢂ 10 , 8 ꢂ 10 and 0.02 ꢂ 10 S cm
and n junctions that are responsible for the rectication prop-
for pH values of 4, 7 and 9.2, respectively. It is evident from the erty of the system. Thus, it may be summarised that both NDR
results that the dc-conductivity value at pH 4 is ꢀ2 orders higher and rectication property in the system may arise from the
than that of pH 9.2. A probable reason may be the proton partial p and n type doping of RGP system at pH 7.
doping at pH 4, where in addition to the contribution of elec-
A schematic model of the density of states (DOS) of the two
tronic conductivity, the contribution of hole conductivity also electrodes and the RGP may be proposed to understand the
plays a signicant role.
bimodal NDR property under different bias voltages and it is
64,65
Due to the spectroscopic evidence of different types of illustrated in Fig. 12a–f.
The green and blue rectangles
doping of RGP, it would be interesting to observe the effect of represent the valence and conduction bands of the two ITO
pH on electronic properties of this system. In Fig. 11, S7 and electrodes, respectively, and the two red lines between them
S8,† the I–V characteristic curves of RGP samples doped at indicate the positions of the highest occupied molecular orbital
different pH values are presented. From Fig. S7,† it is evident (HOMO) and lowest unoccupied molecular orbital (LUMO) of
that RGP at pH 4 exhibits semiconducting behavior showing a the RGP system. Moreover, due to partial doped nature of the
negative hysteresis. Probably, during doping by protons at pH 4, RGP at pH 7, a polaronic state is proposed to be present in the
RGP possesses some trapped charges, which adds to the current RGP very near to its HOMO position (marked as a dotted line).

ow in the forward bias, but in the reverse bias, this does not For simplicity, here we just assume that the HOMO and LUMO
occur because of charge annihilation at the forward bias of RGP do not change with voltage but that the polaronic band
causing a lower amount of current at the same voltage. In the changes with voltage. At the zero applied voltage (Fig. 12a), the
case of pH 9.2, typical semiconducting behaviors is also HOMO and polaronic bands are very close to each other but the
observed (Fig. S8†) and the hysteresis is much lower than that of difference between the two energy bands increases with
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Fig. 12 (a–f) Schematic illustration for the NDR mechanism. The blue and green rectangles represent the DOS of the two electrodes (left and
right). Two red lines and black dotted line between them indicate the positions of the HOMO, LUMO and polaronic band, respectively, of RGP
(
scattering region) with different bias voltages (Fermi energy is set to zero in all cases).
increasing applied voltage as the polaronic band is originated PDMAEMA chain in RGP. Moreover, it is important to note that
from closely spaced radical cation pairs (polaron). The DOS of in contrast to the behavior of GO, in RGP the emission peak
the electrodes (rectangles) shis up and down with increasing intensity increases with increasing pH showing ꢀ8 times
bias voltage both for le and right electrodes, respectively. With increase in the PL intensity at pH 9.2 with a signicant red shi
the nonzero bias voltage, the HOMO and polaronic band match of rGO emission peak from the graphitic emission peak at pH 4.
with the valence band of le electrode and LUMO of RGP The Raman spectrum of RGP demonstrates evidence of p-type
matches with the conduction band of the right electrodes doping at pH 4 and n-type doping at pH 9.2. The dc-conductivity
(Fig. 12b), causing an increase in the current with applied at pH 4 is ꢀ2 orders higher than that of pH 9.2 and the proton
voltage in accordance with Ohm's Law. With increase in the bias doping at pH 4 may contribute to the hole conductivity, in
voltage (>1.5 V), the current begin to decrease as there is no addition to the contribution of electronic conductivity of RGP
overlap between the conduction band of the right electrode and moieties. The I–V characteristic curves of RGP samples at pH 4
the LUMO, as well as the polaronic band of the scattering and 9.2 indicate semiconducting behavior showing a negative
region, showing the rst NDR (Fig. 12c). With a further increase hysteresis but that of the sample produced at pH 7 shows a
in the voltage (>2.2 V), the polaronic band matches the bimodal NDR property and rectication property with a recti-
conduction band of the right electrode and the current again cation ratio of 5.5. The bimodal NDR property has been
increases with increasing voltage showing a maximum at 3 V explained with a model produced using density of states of the
(Fig. 12d). Above 3 V, again both the LUMO and polaronic bands two ITO electrodes and RGP as a scattering region, assuming
of the scattering region could not match the conduction band of the formation of a new polaronic state due to its partial doping
the right electrode showing the second NDR region (Fig. 12e). At at pH 7 apart from its own HOMO and LUMO states. The
about 5 V, the LUMO of the RGP catches the higher (second) rectication property is attributed to the partial p- and n-type
conduction band (Fig. 12f) of the right electrode, causing a doping of RGP at pH 7, producing an intimate mixture of p–n
smaller and constant ow of current with increase in voltage. junctions. Thus, it may be concluded that we have devloped a
The absence of partial doping at pH 4 and 9.2 might be a reason new way of polymer assisted p- and n-type doping of graphene
for the absence of NDR property and rectication for the sample by simply changing the pH of the medium.
prepared at these pH values.
Acknowledgements
Conclusion
A.K, N.M and R.K.L acknowledge CSIR, New Delhi, for the
In conclusion, we have developed a method for the preparation
fellowship. We gratefully acknowledge DST Unit of Nanoscience
of amphiphilic reduced graphene oxide by graing PDMAEMA
using the graing to technique via the ATRP method. Since
PDMAEMA chain is amphiphilic in nature, RGP is soluble in a
wide range of solvents such as benzene and carbon tetrachlo-
ride and DMF or water. The uorescence property of RGP is
interesting and at pH 4 it exhibits two emission peaks, arising
from the radiative recombination of excitons to the graphitic
at IACS for instrumental support and Prof. K. Bhattacharyya and
S. Chattoraj of Physical Chemistry department for helping in

uorescence microscopy.
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