Structural, optical and photocatalytic properties of graphene-ZnO nanocomposites for varied compositions

Article abstract of DOI:10.1016/j.jpcs.2016.11.024

Graphene-ZnO nanocomposites (GZ) were prepared with different concentrations of ZnO by chemical method. Structural analysis by x-ray diffraction showed a hexagonal wurtzite phase of ZnO and Raman spectroscopy showed the formation of graphene. Shape of ZnO was found to be varied from sphere to rod with the increasing ZnO composition. Optical absorption at 369?nm (E_g=3.36?eV) was corresponding to the band edge emission of ZnO. Compared to pure ZnO, photoluminescence (PL) intensity of the nanocomposites had decreased drastically, due to quenching of photoemission, in the presence of graphene. The time resolved PL decay followed a triexponential behavior with a maximum lifetime of 27.97?ns. The BET surface area was in the range from 13 to 123?m²/g. Photocatalytic degradation study on methyl orange under UV light and Sun light showed a maximum degradation of 97.1% and 98.6% respectively. It was observed that the nanocomposites with optimum value of lifetime and surface area had shown the maximum degradation rate.

Full text of DOI:10.1016/j.jpcs.2016.11.024

Author’s Accepted Manuscript
Structural, Optical and Photocatalytic Properties of
Graphene-ZnO Nanocomposites for varied
compositions
Rosalin Beura, P. Thangadurai
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PII:
S0022-3697(16)30455-3
http://dx.doi.org/10.1016/j.jpcs.2016.11.024
PCS7911
DOI:
Reference:
To appear in: Journal of Physical and Chemistry of Solids
Received date: 8 August 2016
Revised date: 2 November 2016
Accepted date: 23 November 2016
Cite this article as: Rosalin Beura and P. Thangadurai, Structural, Optical and
Photocatalytic Properties of Graphene-ZnO Nanocomposites for varied
compositions, Journal of Physical and Chemistry of Solids
http://dx.doi.org/10.1016/j.jpcs.2016.11.024
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Structural, Optical and Photocatalytic Properties of Graphene-ZnO
Nanocomposites for varied compositions
Rosalin Beura and P. Thangadurai
Centre for Nanoscience and Technology, Pondicherry University, RV Nagar, Kalapet,
Pondicherry, 605014, India
thangadurai.p@gmail.com
thangaduraip.nst@pondiuni.edu.in
ABSTRACT
Graphene-ZnO nanocomposites (GZ) were prepared with different concentrations of ZnO
by chemical method. Structural analysis by x-ray diffraction showed a hexagonal wurtzite phase
of ZnO and Raman spectroscopy showed the formation of graphene. Shape of ZnO was found to
be varied from sphere to rod with the increasing ZnO composition. Optical absorption at 369 nm
(Eg = 3.36 eV) was corresponding to the band edge emission of ZnO. Compared to pure ZnO,
photoluminescence (PL) intensity of the nanocomposites had decreased drastically, due to
quenching of photoemission, in the presence of graphene. The time resolved PL decay followed
a triexponential behavior with a maximum lifetime of 27.97 ns. The BET surface area was in the
2
range from 13 to 123 m /g. Photocatalytic degradation study on methyl orange under UV light
and Sun light showed a maximum degradation of 97.1% and 98.6% respectively. It was observed
that the nanocomposites with optimum value of lifetime and surface area had shown the
maximum degradation rate.
Graphical abstract

Keywords:
Graphene-ZnO nanocomposite; Methyl orange; Photocatalytic degradation; UV-Vis
spectroscopy; Photoluminescence; Lifetime spectroscopy
1
. INTRODUCTION
Pollution has become unavoidable in the recent lifestyle due to a fast industrial and
technological development. Water pollution is the most important one. In the global scenario,
textile industry is considered as one of the largest water polluter by releasing huge amount of
dyes into the water system. The biggest challenge in this issue is that these dyes are highly stable
and remain unaffected even after chemically and biologically treated [1]. Various methods such
as adsorption on activated carbon, ultra-filtration, reverse osmosis, chlorination, advanced
oxidation processes (AOPs) etc., [2–4] have been used for the treatment of these dyes.
Heterogeneous photocatalysis, a category of AOPs, works well with the help of inorganic
photocatalyst (semiconductors) and light energy. Upon illumination by light, electron - hole pair
are generated respectively in conduction and valence bands of the photocatalyst.. These photo-

generated charge carriers react with surface molecules (such as H O, adsorbed O ) to undergo
2
2
.

2
secondary reactions to produce the radical species (OH, O ) which further react with the organic
compounds and reduce them to harmless products such as H O and CO [5,6]. For better
2
2
performance, the photocatalysts should possess: (i) higher absorption affinity towards the
pollutant molecule to ease the electron exchange, (ii) lower recombination rate of the excited
electrons to provide sufficient time for the degradation of organic molecule, and (iii) proper
substrate or scaffold to immobilize the photocatalyst [7]. In addition, their performance also
depends on effective surface area [8] and concentration of surface defects. Larger surface area
would provide more active sites on the surface. Defects such as oxygen vacancies in
semiconducting oxides can also produce trapped states, helping to suppress electron-hole pair
recombination by capturing electrons [9]..
Photocatalytic activity of these semiconductor materials can further be improved by
incorporating carbon based materials. Carbon materials are excellent electron conductors and can
assist charge transfer between metal oxide and pollutant molecules. In particular, graphene is a
2
significant material because of its large surface area (2600 m /g) [10,11], high electronic
conductivity, high surface to volume ratio, easy functionalization etc. Graphene has the ability to
accept electrons in order to prevent recombination and provide a favorable surface area for
absorption of dye through π-π conjugation between dye and aromatic region of reduced graphene
oxide (RGO). The trapped electrons on graphene react with the dissolved oxygen and water to
form reactive oxygen and hydroxyl radicals which further oxidizes the dye.
The synergistic effect of metal oxides such as ZnO, TiO , SnO [9,12], V O [13], Cu O
2
2
2
5
2
[
14,15], Bi O [16] and graphene had shown improved photocatalytic performance. Out of them,
2 3
graphene-ZnO nanocomposites have greater stability towards photocatalyst degradation of

industrial dyes. Therefore, it become necessary to understand the fundamental factors that
influence the photocatalytic activity of a material to achieve maximum efficiency. This paper
explains the synthesis of graphene-ZnO with varying compositions by facile chemical synthesis
and their structural and optical properties. A detailed photocatalytic study was made with
optimized conditions of the sample so that high degradation rate for an industrial dye methyl
orange was achieved.
2
. EXPERIMENTAL PROCEDURE
Commercially available graphite flakes, sulfuric acid (H SO ) and zinc nitrate
2
4
hexahydrate (Zn(NO ) 6H O) from Himedia Chemicals-India, sodium nitrate (NaNO ) from
3
2
2
3
Fischer Scientific, potassium permanganate (KMnO ) and hydrogen peroxide (H O ) from NICE
4
2
2
Chemicals, and sodium hydroxide (NaOH) and ammonia solution (NH OH) from Merck
4
Chemicals were used for the synthesis without any further purification. Graphene oxide (GO)
was prepared by a modified Hummers method [17].
A reported procedure [18] was followed to synthesize graphene-ZnO nanocomposite with
a few modifications. Graphene oxide (0.45 g) was added to 50 ml of distilled water and sonicated
for 0.5 h. Then, sodium hydroxide was added to increase the pH from 2 to 4.0. Zinc nitrate
hexahydrate (Zn(NO ) 6H O) was added with four different mass ratios to GO in order to have
3
2
2
the GO:Zn(NO ) ratios as 1:1, 1:5, 1:10, 1:20 and they were coded as GZ1, GZ5, GZ10, GZ20
3
2
respectively. The pH of the solution was increased to 11 by adding ammonium hydroxide.
Increaseing pH value to 11 has scaled-up the Zn(OH) conversion rate. The solution was stirred
2
for another 0.5 h, after which it was transferred to a reagent bottle and kept in oven for 10 h at
9
0˚C. Finally the solution was centrifuged in water and ethanol, dried in vacuum at 70˚C and

heated at 350˚C to remove the hydroxides present. Pure ZnO nanoparticles were also synthesized
in a similar way as a reference sample.
Structural analysis was carried out in a powder X-ray diffractometer (Ultima-IV, Rigaku)
by using Cu-K radiation in a θ-2θ geometry. Raman spectroscopy was done in a Renishaw In
α1
via system, with a laser excitation wavelength of 514 nm. Scanning electron microscopy (SEM)
analysis was done in Hitachi (S-3400N) SEM microscope operated at 30 kV. Fourier transform
infrared (FTIR) spectra were recorded in transmission mode (Thermo Nicolet-6700) with KBr as
reference. UV-Vis absorption spectra were acquired by using Lambda 650 (Perkin Elmer)
spectrometer. Photoluminescence spectra were recorded in HORIBA FluroMax-4
spectroflurometer with a 280 nm excitation from Xenon lamp. Lifetime measurement was
performed in FLUROLOG-FL3-11 spectrofluorometer. Brunauer-Emmett-Teller (BET) specific
surface area analysis was done in Micrometrics Gemini VII 2390 by using N absorption at 77 K.
2
Photocatalytic degradation experiments on methyl orange (MO) by graphene-ZnO
nanocomposites were carried out at room temperature. Before turning on the UV light for
photocatalytic process, 0.02 mM of methyl orange was dissolved in 100 ml water and kept
overnight under stirring to obtain a homogeneous solution. Then 50 mg of the graphene-ZnO
nanocomposites was added and stirred for 1 h to attain equilibrium of absorption and desorption
between the nanocomposite and dye. Then, the solution was irradiated with UV light (8 W
power) for 6 h under continuous stirring. After every 30 minutes of irradiation, the solution was
collected, whose UV-Vis absorption spectra were acquired. Similar experiment was carried out
under direct Sun light irradiation instead of UV light that was used in the laboratory experiment.

3
. RESULTS AND DISCUSSION
3
.1.
X-ray diffraction analysis
Figure 1 presents the XRD patterns for pure ZnO and graphene-ZnO nanocomposites.
The indexed x-ray diffraction peaks are corresponding to the hexagonal wurtzite structure of
ZnO (JCPDS card #: 89-7102). Sharp XRD peaks indicates the crystalline nature of ZnO. The
XRD patterns for graphene-ZnO nanocomposites show two peaks at 24˚ and 43˚ that correspond
to graphene. Presence of graphene peak is more expressed in GZ1 as the graphene content in this
sample is highest among all samples. For the other samples, graphene peak is shown as an insert
in the respective XRD patterns. As the ZnO concentration increases, there is a decrease in
relative intensity of graphene to ZnO in the XRD peaks. Diffraction peak for the graphene phase
is not very well expressed in GZ20 because of less quantity of graphene. Crystallite size of ZnO
was calculated by using the Scherrer formula [19] from the XRD peak broadening. Crystallite
size was obtained to be 8.9, 23.6, 37.8, 29.9 and 47.0 nm for GZ1, GZ5, GZ10, GZ20 and pure
ZnO respectively.

Figure 1. XRD patterns for pure ZnO and graphene-ZnO nanocomposites along with the JCPDS
card #:89-7102) pattern for ZnO. Insert shows the magnified region in order to view the peak
corresponding to graphene.
(
3
.2. Raman spectroscopy analysis
Figure 2(a) shows the Raman spectrum for pure ZnO acquired with a laser excitation
-1
low
wavelength of 514 nm. The Raman modes at 99, 439 and 583 cm [20,21] are attributed to E
,
2

high
optical E2 , E (LO) scattering modes respectively and these are the characteristic Raman
1
-1
modes of wurtzite ZnO. The Raman modes at 333 and 1153 cm [22] represent the 2E (LO)
1
which are second order multiple phonon modes of ZnO. Raman spectra for the GZ
-1
nanocomposites are presented in Fig. 2(b). The D and G bands at 1358 and 1601 cm
respectively are the characteristic peaks for the carbon components of graphene. The G peak
arises from the stretching of C-C bond of graphite materials and is highly sensitive to strain
2
effects in sp system. The D peak is caused by the disordered structure of graphene. The 2D band
-1
-1
for graphene is observed at 2705 cm and the D+G band is located at 2916 cm . The 2D band is
the second order two phonon processes which appear usually due to multilayer structured
graphene. Intensity of the 2D band reflects the number of graphene layers present. A single layer
of graphene usually shows a sharp and intense 2D peak. The 2D band in Fig. 2(b) indicates the
presence of a few layers of graphene. The deconvoluted 2D band and D+G band of GZ20 is
shown as an insert in the Fig.2 (b). Integrated intensity ratio of the D to G bands, I /I for GZ1 is
D
G
calculated to be 1.034 (higher than I /I for GO i.e., 0.90) indicating the presence of more
D
G
defects in the graphene lattice and confirms the formation of RGO [23,24]. The I /I ratio varies
D
G
as 1.02, 1.01 and 1.03 for GZ5, GZ10 and GZ20 respectively. Figure 2(b) also shows the Raman
low
peaks corresponding to ZnO and they are located at 201 (2E2 ), 329 (second order or multiple
high
-1
phonon scattering), 425 (optical E2 mode), 576 cm (A (LO) and E (LO) vibrational modes
1
1
-1
corresponding to intrinsic defects like oxygen vacancies), 1137 cm (2E (LO) mode).
1

Figure 2. Raman spectra for (a) pure ZnO and (b) graphene-ZnO nanocomposites. Insert is the
deconvoluted 2D and D+G bands of graphene in GZ20 nanocomposite.
3
.3. Microstructure analysis by SEM
Morphology and micro-structure were studied by scanning electron microscopy, and the
secondary electron SEM micrographs are presented in Fig. 3 for ZnO and GZ nanocomposites. It
is clear that pure ZnO possesses a rod shaped morphology (Fig. 3a) with an average diameter of

7
50 nm. In the case of GZ composites, the sheet-like structure of graphene embedded with ZnO
rods are noticed, more clearly in the case of GZ10 (see Fig. 3d). The GZ1 nanocomposite
contains only spherical shaped particles and no rods are observed. With increasing ZnO content,
size of the ZnO rod has also increased. Presence of Zn(NO ) .6H O that is added as a precursor
3
2
2
for ZnO, results in the heterogeneous nucleation of ZnO[7]. Hence, as we increased the amount
of Zn(NO ) .6H O precursor, it had resulted in an increase in the nucleation of ZnO. This is very
3
2
2
well concluded from the SEM images, where low Zn(NO ) .6H O concentration had yielded
3
2
2
only spherical particles , whereas rod shaped structures were obtained with increasing
concentration. Average diameter of ZnO rod varies as 540 nm and 1 µm for GZ10 and GZ20
respectively.
Figure 3. Secondary electron SEM micrographs for (a) ZnO, and graphene-ZnO nanocomposites
(b) GZ1, (c) GZ5, (d) GZ10 and (e) GZ20.

3
.4. FTIR Analysis
The FTIR spectra for the GZ nanocomposites are presented in Fig. 4. Three FTIR peaks
-1
-1
are observed at 1338, 883 and 642 cm for pure ZnO. The first one (1338 cm ) is originated
from O-H deformation and the latter two are attributed to the Zn-O stretching modes of ZnO. For
-1
the graphene-ZnO nanocomposites, a characteristic peak at 1573 cm corresponding to the
stretching vibrations of C=C is observed, which clearly confirms the formation of graphene
-1
structure. Further, there is no carboxyl group present at 1734 cm , indicating that the GO is
-
1
completely reduced to graphene [7,18,23]. The other peaks at 3409 cm (O-H group due to
-1
-1
moisture absorbed), 1232 cm (C-O vibration from epoxy group), 447 cm (characteristic peak
-1
-1
of E mode of hexagonal ZnO), 514 cm ( oxygen vacancies in ZnO) and 565-883 cm (Zn-O
2
stretching modes in ZnO lattice) [25,26]. Oxygen deficiency changes the strength of the Zn-O
bond and results in red shift of the IR absorption band. In the present case, pure ZnO possesses a
-1
peak at 642 cm . For the corresponding nanocomposites, a red shift is observed in the peak,
indicating changes in the amount of oxygen vacancies.

Figure 4. FTIR spectra of ZnO and graphene-ZnO nanocomposites measured at room
temperature for various composition ratios of ZnO.
3
.5. UV-Vis absorption spectroscopy analysis
The UV-Vis absorption spectra of ZnO and graphene-ZnO nanocomposites are presented
in Fig. 5. Pure ZnO gives a sharp excitonic absorption peak at 369 nm corresponding to the band
gap of 3.36 eV. This indicates a good crystalline and impurity suppressed ZnO structure, that
usually shows absorption in UV region [26,27]. Position of the 369 nm peak remains unchanged,
whereas, only the intensity decreases with the corresponding decrease in ZnO concentration in

the nanocomposites. Hence, variation in the ZnO concentration in the nanocomposites did not
cause any change in the band gap of the nanocomposites.
Figure 5. UV–Vis absorption spectra for ZnO and graphene-ZnO nanocomposites.
3
.6. Photoluminescence Studies
The charge carrier separation of the photo generated electron hole pairs can be analyzed
by PL spectroscopy. Fig. 6 presents the PL emission spectra for ZnO and graphene-ZnO
nanocomposites at an excitation wavelength of 280 nm. The spectra show a sharp emission peak
in UV region with a maximum at 389 nm that corresponds to the exciton emission of ZnO [28].
Compared to pure ZnO, the PL emission intensity of the composites has decreased drastically
due to quenching of photoemission. The rate of quenching increases with increasing graphene
and this is because of the increased transfer of excited electrons to the graphene structure, thus
reducing the exciton recombination [7]. In addition, a blue shift in the PL peak position is also

observed in graphene-ZnO nanocomposites when compared to ZnO. Also, there is a sequential
blue shift as the ZnO content is decreased (i.e. from GZ20 to GZ1). This indicates that the
presence of graphene can affect the structure of ZnO [29]. A high intense broad peak at 530 nm
is observed in pure ZnO, which is due to the green emission related to defect states such as
oxygen vacancies present in ZnO. This defect band is also drastically reduced indicating the
decrease in defects with the formation of the composite with graphene.
Figure 6. Photoluminescence spectra for ZnO and graphene-ZnO nanocomposites acquired with
the excitation wavelength of 280 nm.
3
.7. Photoluminescence lifetime studies
Exciton lifetime spectra for ZnO and the nanocomposites are shown in Fig. 7. The
excitation and emission wavelengths used to record the lifetime spectra are 325 and 400 nm
respectively. The PL lifetime plot shows decay intensity against the lifetime of exciton in ns. The

experimental data is shown as symbols and the triexponential fit to the data are shown as
continuous lines in Fig. 7. Decay of ZnO emission is generally a multiexponential process with a
number of short and long components. In the present case, the decay curve follows a
triexponential behavior for all the samples. The decay curves were well fitted to the
triexponential model described by the equation

t /1
t /2
t /3
R(t)  A1e
 A2e
 A3e
where τ τ represent the fast decay components and τ represents slow decay components of
1
,
3
2
lifetime, A A are the relative amplitudes of fast decay and A is relative amplitude of slow
1
,
3
2
decay. The fast component is related to the free exciton states and the slow component is related
to the bound exciton states [30]. Table 1 shows the values of the relative contributions from the
fast and slow decay. The average PL lifetime (τ) of the material was calculated from
2
2
2
A  A   A 
1
1
2
2
3 3


A  A   A 
1
1
2
2
3 3
Table 1. Fast and slow decay times along with their amplitudes and average PL lifetime for ZnO
and graphene-ZnO nanocomposites.
Sample
code
τ1
A1
%
τ2
A2
%
τ2
A (%)
Average
3
%
(ns)
(ns)
(ns)
lifetime(ns)
ZnO
2.92
1.43
1.65
1.88
2.11
11.3
19.73
15.96
13.23
12.78
33.60
26.90
28.10
28.67
29.36
36.48
17.11
20.23
22.09
31.25
0.41
0.36
0.36
0.35
0.37
52.22
63.16
63.81
64.68
55.98
32.25
24.33
25.95
26.75
27.97
GZ1
GZ5
GZ10
GZ20

Pure ZnO has the highest lifetime of 32.25 ns compared to graphene-ZnO
nanocomposites. There is a significant decrease in the average lifetime for the graphene-ZnO
nanocomposites (from GZ20 to GZ1) which implies that there is a reduction in the charge
transfer time for lower ZnO content. This reduction of the lifetime is due to the decrease in
defects for the graphene-ZnO nanocomposites (as shown in PL results). With the reduction of
defect concentration, the chance of electron getting trapped at the forbidden region is reduced.
Hence the electron, instead of getting trapped, recombines back with holes to reduce the lifetime.
Also the presence of excessive graphene can act as kind of recombination center instead of
providing an electron pathway [31,32]. This can be very well concluded from the lifetime result
which shows that the composites with higher content of graphene have lower lifetime i.e.
recombination rate has increased. Average lifetime values for the nanocomposites GZ1, GZ5,
GZ10 and GZ20 are 24.33, 25.95, 26.75 and 27.97 ns respectively.

Figure 7. Photoluminescence lifetime spectra for ZnO and graphene-ZnO nanocomposites
measured with the excitation and emission wavelengths of 325 and 400 nm respectively. The
lifetime spectra shows a triexponential behavior. The discrete symbols are the experimental data
and the continuous lines are the fit by using triexponential function to the experimental data.
3
.8. BET surface area measurements
Since the photocatalytic performance of the materials is highly dependent on the amount
of surface that they have, surface area measurements were done by BET method. Prior to the
measurement, the samples were heated at 200˚C for 1 h and then degassed in vacuum in order to
remove the contaminants such as absorbed water and other gases. After degassing, the samples
were analyzed by subjecting them to absorb nitrogen gas at a constant temperature of 77 K. As
adsorption occurs, pressure in the sample changes until it attains equilibrium. From the BET plot,
the specific surface area was calculated by using the BET equation [33] given as
1
c 1 P
( ) 
Q c P
1

………………………………………..(1)
P
0
Q c
m
0
m
Q[( ) 1]
P
where, P and P are the equilibrium and saturation pressures of adsorbate, Q is the quantity of
0
adsorbed gas and Q is the quantity of monolayer adsorbed gas and c is the BET constant. The
m
equation requires a linear relation between 1/Q[(P /P)-1] and (P/P ). The N
2
0
0
adsorption/desorption isotherm are shown in Fig.8. The isotherm corresponds to that of type IV
isotherm according to IUPAC classification [34]. The BET surface area of ZnO, GZ1, GZ5,
2
GZ10 and GZ20 was obtained as 0.6, 123.1, 58.4, 30.8 and 13.2 m /g respectively. The average

pore diameter of 30.4 and 42.5 nm are calculated for GZ5 and GZ20 respectively, using a Barret-
Joyner-Halenda (BJH) model. The BJH cumulative adsorption volume of pores with diameter
3
between 1.7 and 300 nm is 0.2315 and 0.1241 cm /g for GZ5 and GZ20 respectively. It is
observed that the surface area decreases with the increase in ZnO content. It is clear that the
presence of graphene in the composite increases the surface area which is expected.
Figure 8. N adsorption-desorption isotherms at 77 K for (a) GZ5 and (b) GZ20. The continuous
2
lines are guide to eyes.
3
.9. Photocatalytic Degradation Studies
Photocatalytic degradation of methyl orange (MO) by ZnO and GZ nanocomposites was
measured from their UV-vis absorption spectra. Degradation of dye was studied for 6 h of UV
light irradiation. The normalized absorbance C /C of MO in the photocatalytic degradation by
t
0

various samples are shown in Fig. 9(a), which is proportional to the maximum absorption and
derived from the change in the absorption intensity. The results in Fig. 9(a) show a continuous
decrease in the absorption intensity revealing the degradation of the MO dye with the increase of
irradiation time. The percentage of dye degradation was calculated by using the formula given as
C Ct
0
%
degradation =
 100 ……………………………………(2)
C0
where, C is the absorbance after time t and C is the initial absorbance by the dye before
t
0
degradation. The percentage of degradation by ZnO, GZ1, GZ5, GZ10 and GZ20 are 17.8, 37.1,
7.1, 91.5 and 76.7% respectively after 6 h of UV irradiation. The UV-Vis absorption spectra
9
representing the degradation of dye for the sample GZ5 is shown in Fig. 9(b). The photocatalytic
degradation reaction obeys a pseudo first order reaction kinetics, i.e.

kt
C  C e ………………………………………………………..(3)
t
0
where, k is the rate constant of the pseudo first order reaction. The reaction rate was calculated
by plotting the graph between –ln(C /C ) and irradiation time, t (Fig. 9(c)). The rate constant
t
0
-1
(
shown in Fig. 9d) was calculated to be 0.0007, 0.0016, 0.0082, 0.0061 and 0.0039 min for
ZnO, GZ1, GZ5, GZ10 and GZ20 respectively. The nanocomposites GZ5 and GZ10 have shown
the highest rate constant because they possess an optimum value of surface area and lifetime
which accounts for their enhanced photodegradation with of the dye with maximum efficiency.

Figure 9. (a) Photocatalytic degradation of MO by ZnO and graphene-ZnO nanocomposites
under UV light, (b) UV-Vis absorption spectra for the sample GZ5 (c) reaction kinetic curves for
the photocatalytic degradation and (d) rate constant.
In a similar way, the photocatalytic experiments were carried out on pure ZnO and the
graphene-ZnO nanocomposites under direct Sun light irradiation. Except for the source of
irradiation, all the other experimental conditions were kept same as the ones for UV irradiation in
the laboratory experiment. All the Sunlight experiments were carried out on a bright sunny day
from 10 am to 4 pm in the month January at the location of 11.9139° N latitude and 79.8145° E
longitude. The photocatalytic degradation data for the sunlight irradiated samples are presented

in Figure 10. Degradation of the MO by the nanocomposites of varying composition follows the
same trend as under UV irradiation. In fact, the nanocomposite under sunlight has degraded the
MO faster than the same under UV irradiation. It took 6 h to achieve 97.1% of degradation under
UV irradiation (in the lab), whereas 98.6% degradation was achieved within 3 h under direct
sunlight. This can be due to the more intensity of sunlight. Thus these materials could be really
efficient for industrial applications.
Figure 10. (a) Photocatalytic degradation of MO by ZnO and graphene-ZnO nanocomposites
measured under direct Sun light irradiation, (b) UV-Vis absorption spectra for the sample GZ5
(c) reaction kinetic curves for the photocatalytic degradation and (d) rate constant.

Reusability of the photocatalyst is an important factor for practical application. Hence the cycle
test was done for GZ5 under UV light for five cycles. Everytime after the complete
photocatalytic experiments, the photocatalyst was recovered and re-used as fresh for the next
cycle photocatalytic experiment. The results of photodegradation percentage for the five cycles
are shown in Fig 11. It was observed that the photocatalyst is stable and has its maximum
efficiency (photodegradation rate of 91.8% and above) in all the five cycles.
Figure 11. Reusablity test for the photocatalyst GZ5 under UV light for five consecutive cycles.
Mechanism of Photocatalytic degradation
The photocatalytic degradation of MO occurs through several steps and schematic of this
mechanism is illustrated in Fig. 12. Fig. 12(a) represents the general mechanism for a
semiconductor metal oxide (eg. ZnO) and Fig.12 (b) represents the mechanism for graphene-
ZnO nanocomposite. In the first step, when the semiconductor is illuminated by the light energy,

electron in the valence band absorbs energy and moves to the conduction band. Hence the charge
carriers (holes in the valence band and electrons in the conduction band) are generated (Fig.12
a). In the next step, these generated charge carriers can either directly interact with organic
material to reduce them or move to the semiconductor surface where it undergoes secondary
-
reactions to result in a secondary product (free radicals like -OH, HO , O ) which further reacts
2
2
with organic molecules and degrade them to non-toxic products [8,35,36]. With the presence of
graphene the charge transfer is increased. According to a literature [31] the work function of
graphene and the conduction band of ZnO is -4.42 and -4.05 eV respectively (shown in Fig.12 c).
Since the energy level of graphene is slightly less than the CB of ZnO, the electron in the CB can
transfer to graphene. Graphene, being highly conducting, transfers electrons to the organic
pollutant which is then reduced. A similar mechanism for effective electron–hole pair separation
has been also described for graphitic carbon nitride [37] and 1D Ag@AgVO nanowires/
3
graphene/protonated g-C N nanosheet[38], where the authors have discussed the transfer of
3
4
electron based on the band alignment.

Figure 12. Schematic representation portraying the mechanism of photocatalytic reaction
occurred in (a) ZnO photocatalyst and (b) Graphene-ZnO photocatalyst (c) band diagram for
graphene-ZnO.
Photocatalytic degradation under sunlight can occur through two ways: 1. through
catalyst excitation and 2. through dye excitation. Solar spectrum contains irradiation in both UV

and visible range. The absorption wavelength of dye (methyl orange) is 465 nm (in visible
region) and graphene-ZnO catalyst is 365 nm (in UV region). Hence, both the mechanisms are
possible under sunlight. In the first mechanism the semiconductor is excited which follows the
steps as explained above. In the second mechanism, the dye acts as a sensitizer of visible light
and absorbs energy to form excited dye. The excited dye becomes a cationic dye radical by
transferring electron to the electron acceptor. This is followed by self-degradation of dye or
degradation by the reactive species. Thus, the dye has played a dual role, one as visible light
sensitizer and as as pollutant that has to be degraded [39,40]. Since both the two mechanisms can
simultaneously occur under solar irradiation, this might have resulted (in addition to higher
intensity of the solar radiation) in faster degradation of the dye under sunlight as compared to the
degradation under UV irradiation.
The most important factors that enhanced the photocatalytic activity are: 1. the
recombination time for the excited electrons that should be high (i.e. the lifetime of the material
should be high) and 2. Surface area of the material should be high so that more amount of
organic materials (pollutant molecules) are adsorbed to the surface which can promote the
degradation of the dye. All these postulates can be well accomplished with the introduction of
graphene/reduced graphene oxide (RGO). The schematic representation for graphene-ZnO
nanocomposite is shown in fig. 12(b), which shows that graphene, with a high surface area, acts
like a scaffold to hold both the catalyst as well as the organic molecule. As a result, it facilitates
better interaction between the catalyst and the organic molecules. In addition, high electron
conductivity of graphene helpedin faster movement of electrons through its surface and hence
the charge transfer is accelerated. It was observed from the results of photocatalytic degradation
that the graphene nanocomposite materials have possessed increased photodegradation rate in

comparison to pure ZnO. In our case, we have obtained a maximum of 5.5 times increase in the
photocatalytic efficiency for graphene-ZnO nanocomposites compared to pure ZnO.
Variation in the degradation rate of nanocomposites of varying ZnO ratios can be further
explained by taking into account of the lifetime and surface area of these materials. In order to
achieve higher degradation rate, the material should have an optimum value of surface area as
well as PL lifetime. From the results obtained, for pure ZnO, despite having a higher lifetime
-1
(
32.25 ns, as shown in Table 1) has lowest degradation rate (0.0007 min , as shown in Fig. 9d),
2
because its surface area is very less (0.6 m /g). Here the dye molecules are not embedded well
into its surface as a result there is reduced exchange of electron between the photocatalyst and
the dye molecules. Considering the surface area of the GZ nanocomposites there is a continuous
decrease in the surface area with the increase in the ZnO content. As the content of ZnO
increase,s more number of ZnO nanorods are on the graphene surface, thus decreasing the
surface area. Even though, higher surface area accounts for the larger absorption of the dye
2
molecules, still the nanocomposite GZ1 (with largest surface area 123 m /g) has a low
-1
degradation rate (0.0016 min ). This can be due to deficiency of the electrons (due to lower
concentration of ZnO) which are generated from the semiconductor material when illuminated
by light. Hence it can be concluded that for the best photocatalytic degradation, not any single
property, but, both the surface area and recombination rate should be optimum. This is achieved
in the samples GZ5 and GZ10 where the surface area and lifetime are optimum, which resulted
in the higher degradation percentage of 97.1% and 91% respectively within the UV irradiation
time (6 h). There are a very few literatures available for methyl orange degraded by graphene-
ZnO nanocomposites. Here in this work, 97.1% of degradation was achieved in 6 h of UV
irradiation and 98.6% in 3 h of sunlight irradiation. Since the conditions at which the

experiments were performed are not the same as in the literature therefore a direct comparison is
not possible with the reported values of the degradation efficiency. Important highlight of this
paper is, we are making a study on how the degradation is dependent and controlled by factors
such as lifetime and the surface area and reporting the optimum conditions necessary for
achieving maximum degradation rate.
4
. CONCLUSIONS
The present study had demonstrated the successful synthesis of graphene-ZnO
nanocomposites and their structural and optical properties have been well studied. In addition, it
compares the photocatalytic degradation rate of industrial dye (methyl orange) by pure ZnO as
well as by graphene–ZnO nanocomposites. Based on the photocatalytic studies, it is concluded
that graphene-ZnO nanocomposites have shown higher photocatalytic degradation of the dye
than pure ZnO. It is also concluded that in achieving such a high rate of degradation, both the PL
lifetime and surface area of the material have played a crucial role and both of them are expected
to be high/optimum. Higher the lifetime of the material, lower the rate of recombination of the
formed charged carriers. Due to higher surface area of the material more amount of organic
materials are adsorbed onto the surface of the composites which in turn promotes the degradation
of the dye. Based on the optimum conditions, the nanocomposites GZ5 and GZ10 have shown
the highest photocatalytic degradation of MO. Same experiment under Sun light irradiation has
shown that the nanocomposite took just half the time to achieve the same photocatalytic
degradation, due to its high intensity. Thus these graphene-ZnO nanocomposites could be a good
candidate for industrial photocatalytic applications, especially in keeping the environment clean.

Acknowledgement
Financial support from DST-SERB, India (Lr. No. SR/FTP/PS-137/2010 dt 16.04.2012) to carry
out this present work is gratefully acknowledged. Facilities availed from Central Instrumentation
Facility, Pondicherry University are acknowledged.
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