Synthesis, characterization and photoluminescence properties of graphene oxide functionalized with azo molecules

Article abstract of DOI:10.1007/s12039-013-0536-1

Two different azo molecules functionalized graphene oxide (GO) through an ester linkage have been synthesized for the first time. Chemical structure of the azo-GO hybrids was confirmed by Fourier transform infrared spectroscopy and UV-visible spectroscopy

Full text of DOI:10.1007/s12039-013-0536-1

c
J. Chem. Sci. Vol. 126, No. 1, January 2014, pp. 75–83. ꢀ Indian Academy of Sciences.
Synthesis, characterization and photoluminescence properties
of graphene oxide functionalized with azo molecules
R DEVIa, G PRABHAVATHIc, R YAMUNAa,∗, S RAMAKRISHNANb
and NIKHIL K KOTHURKARb
aDepartment of Sciences, bDepartment of Chemical Engineering and Material Science, Chemistry
and Materials Research Laboratory, Amrita Vishwa Vidhyapeetham, Coimbatore 641 112, India
cResearch and Development Centre, Bharathiyar University, Coimbatore 641 112, India
e-mail: r_yamuna@cb.amrita.edu
MS received 1 May 2013; revised 19 September 2013; accepted 19 September 2013
Abstract. Two different azo molecules functionalized graphene oxide (GO) through an ester linkage
have been synthesized for the first time. Chemical structure of the azo-GO hybrids was confirmed by
Fourier transform infrared spectroscopy and UV-visible spectroscopy. The GO functionalized with 5-((4-
methoxyphenyl)azo)-salicylaldehyde was further characterized by scanning electron microscopy (SEM), trans-
mission electron microscopy (TEM), and atomic force microscopy (AFM). The SEM studies demonstrated that
the morphology of the azo-GO hybrid was found to be similar to the GO sheets but slightly more wrinkled.
Further, TEM image of azo-GO indicates some dark spots on the GO sheets due to azo functionalization. AFM
results also reveal that the azo functionalization increases the thickness of GO sheet to 4–5 nm from 1.2–1.8 nm.
Both the azo-hybrids show absorption band around 379 nm due to the π–π* transition of the trans azo units.
Photoluminescence spectra of azo-GO hybrids show a strong quenching compared with azo molecules due to
the photoinduced electron or energy transfer from the azo chromophore to the GO sheets. It also reveals strong
electronic interaction between azo and GO sheets.
Keywords. Azo benzene functionalized graphene oxide; photo switching; azo/GO hybrid;
covalent functionalization; photochromic molecules.
1. Introduction
and reversibility. These photochromic molecules un-
dergo cis-trans isomerization with high quantum effi-
ciencies. Materials containing even small amount of
azo molecules have been shown to effect a substantial
change in optical properties and morphologies.9,10
Graphene11 as a unique 2D nanocarbon material, has
attracted significant interest owing to its distinctive struc-
ture, outstanding electronic, thermal and mechanical
properties.12–14 Graphene is one of the best candidates
for numerous applications such as nanoelectronics,
gas sensors, organic photovoltaics, field-effect transis-
tors and other nanocomposites.15–19 Though there are
numerous reports on the many exceptional properties,
and applications of both single and multi-walled car-
bon nanotubes (CNT)20 and fullerenes,21 such inten-
sive research is expected for graphene as a building
block for new nanomaterials. However, the solubility
and processability of this graphene in variety of sol-
vents are the main issues for many diverse applications
of graphene-based materials. This solubility problem
can be minimized by using graphene oxide (GO),10,22–27
which is the oxygenated form of monolayer graphene
platelets. This allows chemical functionalization of GO
Photochromic compounds, which can easily undergo
large conformational changes when exposed to light of
appropriate wavelength, are fascinating as molecular
switching devices.1 Azobenzenes are one among the
photo switching molecules that fulfill this condition.2
Due to its clean photochemistry, and substantial change
in material properties during light irradiation, these
molecules have been employed in a variety of appli-
cations such as optical data storage,3 photo switching
technologies,4 liquid crystals,5 chemo sensors6 and as
sensitizer in dye sensitized solar cells (DSSC).7 They
1
are readily synthesized and easily characterized by H
NMR, UV visible spectroscopy (UV-Vis) and Fourier
transform infrared (FT-IR) spectroscopy. When irradi-
ated with an appropriate wavelength of light,8 azo ben-
zene molecules undergo cis-trans isomerization which
enables them to act as photo switchable core because
of their thermal stability, detectability, high sensitivity
∗For correspondence
75

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R Devi et al.
with variety of organic molecules, which will gene- are used as received. Solvents such as N, N-dimethyl
rate new materials with more interesting properties formamide (DMF), di-methyl sulphoxide (DMSO), ethyl
for further applications.28,29 One major advantage of acetate, chloroform, dry terahydrofuran (THF), and ethanol
using GO is its bulk synthesis from natural graphite were purified and distilled using standard procedure.
by chemical oxidation and subsequent exfoliation. The
hydrophilic surface functional groups such as epoxide,
hydroxyl, and carboxyl groups of GO makes it more
water soluble and enables further chemical modification
of graphene.23,30
2.2 Characterization and instrumentation
FT-IR spectra were recorded in Thermo Nicolet, iS10
FT-IR spectrometer using KBr disk method. Absorp-
tion spectra were carried out using Shimadzu, UV
CNT and GO have been functionalized with chro-
mophores in order to facilitate photo absorptivity in
case of photochemical devices.8,17 Solar thermal fuel,
composed of azobenzene functionalized with carbon
nanotubes has been reported in the literature.31 It has
been reported that the number of photoactive molecules
per unit volume (i.e., photo isomer concentration) is
significantly increased with respect to a solution of free
photo molecules, leading to an increased volumetric
energy density of 5–7 orders of magnitude due to the
highly ordered array of adsorbed photo molecules with
CNT substrate.31 Hybrids of azo/CNT8 and azo/GO32
through noncovalent interaction and an amide linkage,
respectively, were also studied for photo switching pro-
perties. Graphene azo poly electrolyte multilayer fab-
ricated by electrostatic layer by layer also has been
investigated for electrochemical capacitor electrode.33
Recently, there is a report on the preparation of graph-
ene grafted with azo polymer and its photo respon-
sive properties.34 In this study, we have functionalized
GO with two types of azo chromophoric compounds,
5-((4-methoxyphenyl)azo)-salicylaldehyde (azoGO-I)
and 5-((4-ethoxyphenyl)azo)-salicylaldehyde (azoGO-
II) through an ester linkage. We have chosen the azo
molecules in such a way that one end of this organic
molecule has electron donating group (methoxy,
ethoxy) and other end contains electron withdrawing
group (salicylaldehyde). We anticipate that this will
act as push-and-pull-like molecular system which in
turn will increase the photoinduced electron or energy
transfer from the azo chromophore to the GO sheets.
The synthesized azo-GO hybrids were characterized by
FTIR and UV-Vis. The azo-GO hybrid (I) has been fur-
ther characterized by transmission electron microscopy
(TEM), atomic force microscopy (AFM), and scan-
ning electron microscopy (SEM). The photophysical
properties of azo-GO hybrids I and II are also reported.
1
1800 spectrophotometer. H nuclear magnetic reso-
nance (NMR) spectra were measured in JEOL 300 MHz
multinuclear spectrometers in CDCl3. Chemical shift
was expressed in parts per million (ppm) relative
to TMS. SEM images were recorded using F E I
Quanta FEG 200-High Resolution Scanning Electron
Microscope (HRSEM). Elemental analysis was car-
ried out in PerkinElmer series II CHNS/O elemental
analyser 2400. Photo luminescence (PL) spectra were
recorded using Shimadzu RF-5301 PC spectro fluoro
photometer. Powder X-ray diffractograms were per-
formed at room temperature on a Bruker D8 Focus
X-ray diffractometer, using Ni-filtered Cu Kα radia-
tion (λ = 1.541 Å) with a scintillator detector (40 kV,
40 mA). The step time was 1 s at 0.04◦/step in a 2θ
range of 5–90◦. High Resolution Transmission Electron
Microscope (HRTEM) image was obtained by using
the JEOL JEM 2100 transmission electron microscope.
AFM carried out by using NTEGRA Prima-NT-MDT.
2.3 Synthesis of 5-((4-methoxyphenyl)azo)-
salicylaldehyde (1a)
4-Methoxyanline (1 g, 8.12 mmol) was added to 3.60 ml
of 6.00 M hydrochloric acid at 0–5◦C. Sodium nitrite
(0.57 g, 8.24 mmol in 3.60 ml of H2O) was added drop-
wise to the reaction mixture for 30 min under con-
stant mechanical stirring. A solution of salicylaldehyde
(0.99 g, 8.12 mmol) in 3.60 ml of 10% NaOH was added
slowly to the diazonium salt over the period of 1 h at 0–
5◦C. Dilute acetic acid was added to the formed brow-
nish orange precipitate. The precipitate was washed with
NaHCO3 and dried with anhydrous Na2SO4. Result-
ing orange colour solution was dried under reduced
pressure and purified on silica gel (100–200 mesh)
column using ethyl acetate, chloroform mixture (1:9).
Yield: 1.56 g, (75%), Rf value is 0.86. FT-IR (in cm−1):
1654 (C=O), 1454 (N=N), 2926, 2847 (Aliphatic C–H
Str.), 3180 (Aromatic C–H str.), 3426 (–OH str.). NMR
data in CDCl3, δ = 10.06 (s, aldehyde H), 3.94 (s,
–OCH3), 8.18 (d, aromatic 2H), 7.94 (d, aromatic 2H),
7.16 (d, aromatic 1H), 7.07 (d, aromatic 2H) ppm (for
NMR spectra, see supporting information figure S1).
2. Experimental
2.1 Materials
Thionyl chloride (SOCl2), 4-methoxyaniline (Aldrich),
4-ethoxyaniline (Aldrich), and salicylaldehyde (Aldrich)

Azo molecules functionalized GO
77
Figure 1. Structure of azo-GO hybrids (I and II).
C14H12N2O3 (256.26): calcd. C 65.62, H 4.72, N 10.93; 2.5 Synthesis of acyl chloride functionalized graphene
found C 65.73, H 4.65, N 10.62. The same proce- oxide (GOCl)
dure was followed for synthesizing (2a) by using 4-
GO (30 Mg) was refluxed with SOCl2 (20 ml) in the
ethoxyaniline instead of 4-methoxyaniline.
presence of DMF (0.5 mL) at 65–70◦C for 24 h under
argon atmosphere to form acyl chloride functionalized
GO (GOCl). Excess solvent and SOCl2 were removed
2.4 5-((4-Ethoxyphenyl) azo)-salicylaldehyde (2a)
by distillation.
Yield: 1.44 g, (73%), Rf value is 0.93. FT-IR (in cm−1):
1661 (C=O), 1472 (N=N), 2976, 2926 (Aliphatic C–H
2.6 Synthesis of 5-((4-methoxyphenyl)azo)-
salicylaldehyde functionalized GO (I)
Str.), 3068 (Aromatic C–H str.), 3414 (–OH str.). NMR
data in CDCl3, 25◦C, TMS: δ = 10.05 (s, aldehyde H),
1.51 (t, alkyl 3H), 4.16 (q, –OCH2), 8.12 (d, aromatic
2H), 7.93 (d, aromatic 2H), 7.15 (d, aromatic 1H), 7.05
(d, aromatic 2H) ppm. C15H14N2O3 (270.29): calcd. C
66.66, H 5.22, N 10.36; found C 65.73, H 5.39, N 10.21.
GOCl (100 Mg) and 0.58 mmol of 5-((4-methoxyphenyl)
azo)-salicylaldehyde (1a) were dissolved in DMSO
(20 ml) and refluxed at 50◦C for 65 h under N2 atm.
After completion of the reaction, the product was cooled
Figure 2. FT-IR spectra of (a) pure graphite, (b) graphene oxide, (c)
methoxy azo-GO hybrid (I) and (d) ethoxy azo-GO hybrid (II).

78
R Devi et al.
Figure 3. a and b. TEM images of the GO.

Azo molecules functionalized GO
79
Figure 4. a and b. TEM images of the methoxy azo-GO hybrid (I).

80
R Devi et al.
to room temperature. The precipitate was ultra cen- (II) confirms the stretching mode of (–N=N–) bond.36
trifuged and excess azo impurity was removed by wash- The three peaks in the range of 1020–1162 cm−1
ing with ethanol until the solution became colourless.
(figure 2c and d) reveal the presence of –C–O– stretch-
ing mode of an ester linkage.37
Morphologies of GO and azo functionalized GO (I)
were investigated by SEM. SEM images of GO and
azo-GO (I) sheet morphology over a few micron length
scales are given in supporting information (figures S3
and S4). Figure S4 reveals that the graphene sheets in
the azo-GO hybrid somewhat are more wrinkled than
GO sheet (see figure S3) due to azo functionalization.
Thickness and morphology of the GO and azo-GO (I)
were further probed using AFM. AFM images of GO
deposited on freshly cleaved mica surface and its height
profile line are given in supporting information (see
figure S5 (a) and (b)). From the figure, thickness of the
GO layer is found to be ≈1.2–1.8 nm (one to two layers),
which is due to conversion of graphene to GO.38,39 The
AFM images of azo-GO hybrid (I) and its height profile
line are given in supporting information (see figure S6
(a) and (b)). This figure indicates that the thickness of
azo functionalized GO sheet is ≈4–5 nm (four to six
2.7 Synthesis of 5-((4-ethoxyphenyl)azo)-salicylalde-
hyde functionalized GO (II)
This azoGO-II hybrid was synthesized by follow-
ing the above procedure with 5-((4-ethoxyphenyl)azo)-
salicylaldehyde (2a) instead of 5-((4-methoxyphenyl)
azo)-salicylaldehyde.
3. Results and discussion
3.1 Synthesis and structure of the Azo-GO I and II
hybrids
Water soluble GO was prepared from graphite flakes
(<20 μm; Sigma Aldrich) using Hummers method.30
Distinctive powder X-ray diffractograms features of
graphite and the synthesized GO are given in the
supporting information (figure S2). For the graphite
sample, the characteristic peak of hexagonal graphite
corresponding to a d-spacing of 0.34 nm was found
at 26.66◦. Upon conversion of the graphite into GO,
the peak position shifted to a lower diffraction angle
of 10.52◦ with an increase in the interlayer spacing
of 0.84 nm. Disappearance of graphite diffraction peak
at 26.66◦ confirms the oxidation of graphite to GO.35
This increase in d-spacing is due to the intercalation of
various oxygen species mostly ether ring oxygen and
hydroxyl functional groups in between the graphene
layers.
0.3
0.2
The soluble 5-((4-methoxyphenyl)azo)salicylaldehyde-
GO (I) and 5-((4-ethoxyphenyl)azo)salicylaldehyde-GO
hybrids (II) were synthesized by suitable modification
of literature procedure.32 5-((4-Methoxyphenyl)azo)-
salicylaldehyde (1a) and 5-((4-ethoxyphenyl)azo)-
salicylaldehyde (2a) were covalently linked to GO
molecules through an ester bond by refluxing appropri-
ate azo compound with GO in DMSO at 50◦C for 65 h
under N2 atm. The chemical structure of the compound
is shown in figure 1. The azo/GO hybrid structures have
been confirmed by FT-IR spectra in the range of 500–
4000 cm−1 as shown in figure 2. GO contains various
functional groups including ether, epoxide, carboxylic
acid, phenolate and aldehyde groups. The characteristic
absorption peak assigned to carbonyl (–C=O) stretch-
ing mode of GO, azo-GO I and II is clearly visible at
1738, 1740 and 1742 cm−1, respectively as shown in
figure 2b–d.32 After functionalization, a new striking
peak at 1457 cm−1 in figure 2c and d of hybrids (I) and
(a)
0.1
(b)
0.0
300
400
500
600
700
Wavelength (nm)
Figure 5. Absorption spectra of (a) methoxy azo (1a)
(1.4 × 10−5 M) and (b) methoxy azo-GO hybrid (I)
(1.5 mg/100 ml DMF).

Azo molecules functionalized GO
81
50
layers). Thickness of the GO sheet increases after func-
tionalization with methoxy azo due to the stacking of
the layers, which is similar to other reported results.40
TEM images of GO and azo-GO (I) are shown in
figures 3 and 4. Figure 4a depicts the TEM images of
azo-GO where some dark spots can be observed on
the GO sheets due to azo functionalization. Figures 3b
and 4b show stacked GO and azo-GO sheets with an
interlayer spacing of ≈0.60 and 0.65 nm for GO and
azo-GO, respectively. Compared to the pure graphene
interlayer distance of 0.34 nm, an interspacing of
0.60 nm indicates fully oxidized graphene.41 Azo func-
tionalized GO has slightly higher interlayer spacing
0.65 nm compared to GO.
(a)
40
30
20
10
0
(b)
400
500
600
700
800
Wavelength (nm)
3.2 Spectroscopy measurements
Figure 7. PL spectra of (a) methoxy azo (1a) (1.4 ×
10−5 M) and (b) methoxy azo/GO (I) (3 mg/100 ml DMF)
(λex = 400 nm) with the same absorbance value at the π–π*
transition (Abs = 0.24).
Figures 5 and 6 show UV-vis absorption spectra of
5-((4-methoxyphenyl) azo)-salicylaldehyde (1a), 5-((4-
ethoxyphenyl) azo)-salicylaldehyde (2a) and azo-GO
hybrids I and II in DMF solvent. UV-vis spectrum
of azo salicylaldehyde (1a) and (2a) shows an intense
band around 379 nm in DMF solvent which is due to
the π–π* transition of the trans azo units.42 We also
confirmed this π–π* transition peak by recording the
spectra for (1a) and (2a) in nonpolar DCM solvent;
we observed that this peak shifted to shorter wave-
length (around 354 nm). The azo-GO hybrids I, II and
the pristine azo molecules also exhibit a peak around
450 nm which is absent in the reported azo-GO hybrid
and 2-aminoazotoluene.32 The π–π* transition peak
of azo-GO hybrids I, II and the neat azo molecules
shows a blue shift of about 22 nm compared to that
of reported azo-GO hybrid and the pristine azo chro-
mophore.32 In figures 5b and 6b, intensity of the peaks
are reduced considerably due to covalent functionaliza-
tion of GO with azo molecules through an ester linkage.
This is in agreement with the reported azo-GO hybrid
where a similar suppression was observed between the
0.4
120
0.3
100
(a)
(a)
80
60
0.2
40
(b)
0.1
20
0
(b)
0.0
400
500
600
700
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Figure 8. PL spectra of (a) ethoxy azo (2a) (1.3 × 10−5 M)
Figure 6. Absorption spectra of (a) ethoxy azo (2a) (3.3 × and (b) ethoxy azo/GO (II) (3 mg/100 ml DMF) (λex =
10−5 M) and (b) ethoxy azo-GO hybrid (II) (1 mg/100 ml 400 nm) with the same absorbance value at the π–π* transi-
DMF).
tion (Abs = 0.24).

82
R Devi et al.
azo-GO compared to pristine azo.32 This indicates the Acknowledgements
interaction between the electronic energy level of azo
The authors RY and RD thank the Council of Sci-
molecules and GO in the azo-GO hybrids.29
entific and Industrial Research (CSIR), (Project No:
01(2256)/08/EMR-II), New Delhi, India for funding
and Amrita Vishwa Vidyapeetham for infrastructure.
Photoluminescence spectra of azo compounds (1a),
(2a) and azo-GO hybrids (I) and (II) in DMF (exci-
tation wavelength is 400 nm) are shown in figures 7
and 8. Emission spectra of these compounds show two
peaks which is similar to the reported azo and azo-
GO hybrids.32 Whereas in the present study, a blue
shift was observed in the azo-GO hybrids (I) and (II)
compared to pure azo molecules. Emission spectra of
these compounds also shows a blue shift compared
to the reported azo-GO hybrid and its pristine azo
molecule.32 Figures 7b and 8b show strong quenching
compared with pure azo molecules due to the photoin-
duced electron or energy transfer from the azo chro-
mophore to the GO sheets. This is in agreement with
the reported azo-GO hybrid where a similar suppression
was observed between the azo-GO compared to pure
azo (excitation wavelength is 480 nm).32 Photolumi-
nescence measurements demonstrate strong electronic
interaction between azo group and GO sheets.
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Thus, we have successfully synthesized two different
azo functionalized graphene oxides through an ester
linkage for the first time. Chemical structure of the azo-
GO hybrids were testified by the FT-IR spectra. A new
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