Synthesis of ZnO decorated graphene nanocomposite for enhanced photocatalyticpropertiesS. Gayathri, P. Jayabal, M. Kottaisamy, and V. Ramakrishnan

Citation: Journal of Applied Physics 115, 173504 (2014); doi: 10.1063/1.4874877 View online: http://dx.doi.org/10.1063/1.4874877 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/17?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Electrodeposition of hierarchical ZnO/Cu2O nanorod films for highly efficient visible-light-driven photocatalyticapplications J. Appl. Phys. 115, 064301 (2014); 10.1063/1.4863468 Photocatalytic and antibacterial properties of Au-TiO2 nanocomposite on monolayer graphene: From experimentto theory J. Appl. Phys. 114, 204701 (2013); 10.1063/1.4836875 Theoretical and experimental approach on dielectric properties of ZnO nanoparticles and polyurethane/ZnOnanocomposites J. Appl. Phys. 112, 054106 (2012); 10.1063/1.4749414 Visible light photocatalytic property of Zn doped V2O5 nanoparticles AIP Conf. Proc. 1447, 351 (2012); 10.1063/1.4710024 Ag/ZnO Nanocomposites Studied by X-ray Photoelectron Spectroscopy Surf. Sci. Spectra 18, 19 (2011); 10.1116/11.20110902

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Synthesis of ZnO decorated graphene nanocomposite for enhancedphotocatalytic properties

S. Gayathri,1 P. Jayabal,1 M. Kottaisamy,2 and V. Ramakrishnan1,3,a)

1Department of Laser Studies, School of Physics, Madurai Kamaraj University, Madurai 625021, Tamilnadu, India2Department of Chemistry, Thiagarajar College of Engineering, Madurai 625014, Tamilnadu, India3Indian Institute of Science Education and Research Thiruvananthapuram, Thiruvananthapuram 695016,Kerala, India

(Received 20 February 2014; accepted 22 April 2014; published online 2 May 2014)

Zinc oxide/Graphene (GZ) composites with different concentrations of ZnO were successfully

synthesized through simple chemical precipitation method. The X-ray diffraction pattern and the

micro-Raman spectroscopic technique revealed the formation of GZ composite, and the energy

dispersive X-ray spectrometry analysis showed the purity of the prepared samples. The ZnO

nanoparticles decorated graphene sheets were clearly visible in the field emission scanning electron

micrograph. Raman mapping was employed to analyze the homogeneity of the prepared samples.

The diffuse-reflectance spectra clearly indicated that the formation of GZ composites promoted the

absorption in the visible region also. The photocatalytic activity of ZnO and GZ composites was

studied by the photodegradation of Methylene blue dye. The results revealed that the GZ

composites exhibited a higher photocatalytic activity than pristine ZnO. Hence, we proposed a

simple wet chemical method to synthesize GZ composite and its application on photocatalysis was

demonstrated. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4874877]

I. INTRODUCTION

Currently, organic dyes and their waste products from

the industries, such as textile, paper, and plastic, contaminate

the surroundings by the discharge of the toxic, non-

biodegradable, and carcinogenic materials.1 The chemicals

found as pollutants in waste water effluents from industrial

sources must be removed before their discharge to the envi-

ronment.2 Also, such pollutants may be found in ground water

and so it is essential to remove the pollutants to achieve ac-

ceptable drinking water quality. The removal of such organic

pollutants in waste water using semiconducting materials as

photocatalysts has attracted a lot of attention in environmental

protection.3 Photocatalytic oxidation is an inexpensive pro-

cess, since it involves only with non-degradable photocata-

lytic materials and a natural or an artificial light source.4

In general, photocatalysis is based on the light absorp-

tion of semiconductor oxide photocatalyst, such as TiO2 and

ZnO, to excite the electrons from valence band to conduction

band and create electron–hole pairs.5 ZnO is a n-type, wide

band gap (Eg¼ 3.37 eV) semiconductor used in several

applications such as nano-scale electronic and optoelectronic

devices.6–9 Due to its higher photosensitivity and wide band

gap, ZnO can be considered as a promising photocatalyst

alternative to TiO2.10 However, there are still some problems

such as high recombination rate of photogenerated electron–

hole pairs that limit the use of pure ZnO for photocatalytic appli-

cations.11 Therefore, to enhance the photocatalysis efficiency, it

is essential to slow down the recombination of the charge car-

riers. Several works have been done to reduce the recombination

rate by linking the photocatalysts with other materials such as

noble metals, semiconductors, and carbon materials.12

Graphene is a two-dimensional, single atomically thin

honeycomb lattice of sp2-bonded carbon and it has outstand-

ing electrical and mechanical properties.13 These fascinating

properties of graphene would make it an exceptional

electron-transport material in the photocatalysis process than

C60 or graphite-like carbon.14–16 It has been shown that gra-

phene combined with nanoparticles, such as TiO2, ZnO,

SnO2, and CdS, show enhanced photocatalytic reduction,

higher efficiency in solar cells, and fuel cells.17 Herring

et al.18 have demonstrated the photocatalytic activity of zinc

oxide/graphene (GZ) nanocomposites prepared by micro-

wave synthesis. Ahmad et al.19 investigated photocatalytic

performance of GZ composites synthesized via solvothermal

method. Li et al.1 synthesized flower-like ZnO nanoparticles

attached on graphene oxide (GO) sheets and studied its use

for photocatalytic degradation. Lv et al.20 demonstrated pho-

tocatalytic degradation of Methylene blue (MB) by

ZnO–reduced GO (RGO)–carbon nanotube (CNT) synthe-

sized by microwave-assisted reaction. However, to the best

of our knowledge, in-situ synthesis of ZnO decorated on gra-

phene sheets (GZ composite) by one step chemical method

has not been reported so far.

In the present work, we report a simple one-step synthe-

sis of ZnO anchored on graphene sheets through chemical

reduction of GO in the presence of zinc acetate. In order to

know the structural and elemental analysis, the synthesized

GZ composites were examined by x-ray diffraction (XRD),

micro-Raman, energy dispersive x-ray (EDX), and field

emission scanning electron micrograph (FE-SEM). In addi-

tion to the SEM analysis, we have carried out Raman map-

ping of GZ composites for the first time so as to understand

the distribution of ZnO on graphene surface. The photocata-

lytic activity of the GZ composites and bare ZnO for the deg-

radation of MB was studied.a)E-mail: vr.optics1@gmail.com

0021-8979/2014/115(17)/173504/9/$30.00 VC 2014 AIP Publishing LLC115, 173504-1

JOURNAL OF APPLIED PHYSICS 115, 173504 (2014)

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II. EXPERIMENTAL

A. Materials

Graphite powder was purchased from Alfa Aesar. 37%

hydrochloric acid (HCl), 98% sulfuric acid (H2SO4), hydro-

gen peroxide (H2O2), potassium permanganate (KMnO4),

zinc acetate dihydrate (Zn(CH3COO)2�2H2O), and hydrazine

hydrate (N2H4) were purchased from Merck. All chemicals

were used as received without further purification. Deionized

water was used throughout this study.

B. Synthesis of ZnO nanoparticles

The ZnO nanoparticles were synthesized by dissolving

0.2M zinc acetate dihydrate in water. Then, the solution was

stirred for 3 h at room temperature to get transparent solu-

tion. Appropriate amount of hydrazine hydrate was added to

the above solution and stirred for another 3 h at 80 �C. The

resulting precipitate was centrifuged and washed with water

several times and dried. Finally, the powder was annealed at

450 �C for 4 h.

C. Synthesis of graphene oxide

The graphite oxide was synthesized by modified

Hummer’s method.21 Briefly, 1 g of graphite powder was

added to 80 ml of H2SO4 and stirred in an ice bath. After

30 min, 6 g of KMnO4 was slowly added to the above solu-

tion and stirred. Then, the ice bath was replaced by a water

bath (30–35 �C) and the solution was stirred overnight.

100 ml of water was slowly added and stirred for 30 min. A

mixture of H2O2 (5 ml) and water (100 ml) was slowly added

to the above solution and stirred for 10 min. The solution

gradually turns to yellow color from dark brown. The solu-

tion was then filtered to get the precipitate. The filtered yel-

low cake (graphite oxide) was initially washed (3–4 times)

using HCl and then washed several times in H2O until the

neutral pH is reached. The synthesized graphite oxide was

then dispersed in water and sonicated for 4 h to get GO.

D. Synthesis of GZ composite

In the typical synthesis, dried GO powder was dispersed

in 90 ml of water (10 mg/ml) and sonicated for 30 min to

obtain GO suspension. Subsequently, 300 mg of zinc acetate

was added to the GO suspension and stirred for 3 h. Then,

30 ll of N2H4 was added and the resulting mixture was

stirred for another 3 h at 80 �C. The solution turned to black

from brown color indicating the reduction of GO to gra-

phene. The precipitate was then centrifuged (3000 rpm) for

30 min and washed several times using water and alcohol.

Later, the obtained powder was dried at 100 �C in a water

bath for 2 h and the dried powder was annealed at 450 �Cfor 4 h. The final powder is designated as GZ-1. In order to

optimize the graphene and ZnO composition to get better

photocatalytic efficiency, the process was repeated with

varying mass of zinc acetate, viz., 600 mg and 900 mg and

the final products were designated as GZ-2 and GZ-3,

respectively.

E. Characterization

Powder XRD of the synthesized samples was recorded

by PANalytical X-ray diffractometer with CuKa radiation

(k¼ 1.54 A). The structural morphology of the samples was

observed using a field emission scanning electron micro-

scope (Nova NanoSEM NPE206). The elemental analysis

was carried out using EDX spectrometry (Brucker Quantax

200 AS). Micro-Raman and Raman mapping measurements

were performed at room temperature in 180� back scattering

geometry by using HORIBA Jobin Yvon LabRAM HR 800

equipped with a charge coupled device (CCD) detector and

an automated XY motorized stage, where 632.8 nm line from

He-Ne laser was used as the excitation source. The room

temperature photoluminescence (PL) was measured by

LabRAM HR 800 using the 325 nm excitation line of He-Cd

laser and 40� objective. The UV–Visible diffuse reflectance

spectra (DRS) were recorded on a Shimadzu UV-2450

Spectrophotometer.

F. Photocatalytic activity experiments

The photocatalytic properties of the prepared samples

were estimated by monitoring the photodegradation of MB

in a home-made apparatus with a Newport-66901 Xe lamp

(300 W, 200–800 nm) as the radiation source. In each experi-

ment, 50 mg of photocatalyst was dispersed in 100 ml of MB

aqueous solution (1� 10�5M). The suspension was then

magnetically stirred in dark for 30 min. Later, the solution

was transferred to Erlenmeyer flask and exposed to the

UV-visible light irradiation and at a regular time interval of

20 min, 5 ml of the exposed solutions was sampled, centri-

fuged, and the supernatant was collected for analysis of

the MB concentration. The photocatalytic degradation pro-

cess was monitored using a UV–Vis spectrophotometer

(Shimadzu UV-2450) to record the characteristic absorption

peak (660 nm) of MB.

III. RESULTS AND DISCUSSION

ZnO nanoparticles were decorated on graphene sheets by

a facile reaction between Zn2þ and OH� ions in the aqueous

solution. The schematic illustration of the formation mecha-

nism of GZ composites is presented in Fig. 1. When dissolv-

ing zinc acetate powder into GO solution, Zn2þ ions will be

adsorbed onto the surfaces of GO sheets due to their bonding

with the O atoms of the negatively charged oxygen-

containing functional groups via electrostatic force.1,22 Once

the hydrazine hydrate solution is added to the above suspen-

sion, ZnðOHÞ2�4 and ZnO2� may bond with the functional

groups such as carboxyl and hydroxyl groups (introduced on

graphite oxide during the process of oxidation) of GO sheets

by intermolecular hydrogen bonds.23,24 At the same time, the

GO will be reduced to graphene. ZnO anchored on the gra-

phene sheets was obtained after the annealing treatment. The

obtained products were characterized by several techniques to

ensure the formation of GZ composite.

The XRD patterns of graphene, pristine ZnO, and GZ

composites are shown in Fig. 2. All the diffraction peaks in

the XRD pattern of ZnO nanoparticles can be indexed to

173504-2 Gayathri et al. J. Appl. Phys. 115, 173504 (2014)

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hexagonal wurtzite ZnO and the planes were in good agree-

ment with the JCPDS Card 36-1451. In the XRD pattern of

graphene, two broad bands at �26� and �42� are assigned to

(002) and (100) planes of graphene, respectively. It is clearly

visible from the XRD pattern of GZ composites that as the

concentration of ZnO was increased, the intensity of the

peaks corresponding to ZnO phase increased as well.25 The

plane (002) which corresponds to graphene is very weak in

the XRD pattern of GZ-1.18 The (002) plane gives the most

dominant peak of graphite as seen in the XRD pattern of

graphite shown in the Fig. S1.26 A weak broad band of gra-

phene (002) reflection peak at �26� appeared on the XRD

pattern of GZ-2, which indicates that the ZnO nanoparticles

are anchored on the surface of graphene. The absence of gra-

phene peak in GZ-3 was due to the higher concentration of

the ZnO precursor and therefore, the graphene surface may

be covered by ZnO. Thus, XRD results confirmed the forma-

tion of ZnO on graphene surface. The absence of other peaks

except ZnO and graphene revealed the quality of the pre-

pared samples.

Raman spectroscopy is one of the sensitive tools to char-

acterize carbon based nanostructures.27 Raman spectra of

ZnO, GO, and GZ composites are presented in Fig. 3. The

bare ZnO nanoparticles showed the standard Raman modes

and they were observed at 331 (second-order vibration

related to the E2(high)-E2(low)), 438 (E2(high)), and

581 cm�1 (E1(LO)).8 According to the selection rules for

Raman scattering, in the backscattering geometry, ZnO com-

pounds with the hexagonal structure should exhibit phonons

of the symmetry E2(high) at 438 cm�1.28 Therefore, the

E2(high) mode observed in the Raman spectra of the pre-

pared ZnO [Fig. 3(a)] confirmed the formation of hexagonal

wurtzite ZnO and this is in good agreement with the XRD

result. The Raman spectrum of GO shown in Fig. 3(b) has

two broad peaks at �1335 (D band) and �1586 cm�1 (G

band) corresponding to the breathing mode of k-point pho-

tons of A1g symmetry and the first-order scattering of the E2g

phonon of sp2 carbon atoms, respectively.29 Although the

structural defects are greater in the graphene lattice (D/G ra-

tio of GO), these defects act as helpful nucleation sites for

ZnO. This results in less aggregated ZnO particles due to

FIG. 1. Schematic illustration of the

formation mechanism of GZ

composites.

FIG. 2. XRD pattern of graphene, pristine ZnO, and GZ composites.

173504-3 Gayathri et al. J. Appl. Phys. 115, 173504 (2014)

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FIG. 3. Micro-Raman spectra of (a) ZnO, (b) GO, (c) GZ-1 (d) GZ-2, and (e) GZ-3 composites.

FIG. 4. EDX spectra of ZnO and GZ composites.

173504-4 Gayathri et al. J. Appl. Phys. 115, 173504 (2014)

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their strong interaction with defect sites of graphene sheets.18

Figures 3(c)–3(e) show the Raman spectra of GZ-1, GZ-2,

and GZ-3 composites, respectively. The Raman spectra of

GZ composites displayed the modes of ZnO, defect peak

(D-band), and high intense G band. As the concentration of

ZnO was increased, the intensity of E2 (high) mode increased

as well. The D and G bands of GZ-1 resemble the profile of

multi-layer graphene. Although the graphene plane was not

observed in the XRD pattern of GZ-3, the micro-Raman

spectrum showed the Raman modes of graphene. Hence, the

formation of wurtzite hexagonal ZnO and GZ composites

was confirmed by the micro-Raman analysis.

The synthesized samples were subjected to EDX analy-

sis in order to confirm the chemical composition of the prod-

uct. The EDX spectra of pristine ZnO and GZ composites

are shown in Fig. 4. It is very clear from Fig. 4 that our syn-

thesized samples are free from elemental impurities and it

consists of Zn, O, and C. Hence, pure GZ composites can be

prepared by simple chemical method.

The FE-SEM images of ZnO and GZ composites are

presented in Fig. 5. The uniformly distributed rod-like ZnO

nanoparticles are clearly visible in the SEM image and it is

shown in Figs. 5(a) and 5(b). The SEM images of GZ com-

posites at different concentrations show nearly similar mor-

phological features. By increasing the ZnO concentration,

the distribution of ZnO on graphene also increased. From

Figs. 5(e)–5(h), it is clear that the ZnO nanoparticles are

agglomerated and no individual particles are visible in the

SEM image. The graphene surface was marked on the SEM

images of GZ composites. At the highest ZnO concentration

(GZ-3), the ZnO nanoparticles stick on the surface of gra-

phene sheets and it can be clearly seen from Figs. 5(g) and

5(h). A similar morphology was observed for the sample

GZ-2 and its corresponding SEM image is shown in Figs.

5(e) and 5(f). But a different situation was observed for the

GZ-1 sample. In contrast to GZ-2 and GZ-3, at the lower

concentration of ZnO (GZ-1), the graphene sheets clasped

together and appeared like multi-layer graphene and ZnO

was not clearly visible in the SEM image (Figs. 5(c) and

5(d)). However, the presence of ZnO in GZ-1 can be clearly

seen from the XRD, micro-Raman, and EDX results. In the

case of GZ-2 and GZ-3, the anchoring of ZnO on both sides

of graphene sheets weakened the van der Waals force

between the graphene layers and so the graphene sheets did

not combine together during the process of annealing.

Conversely, in the case of GZ-1, the graphene sheets stacked

together due to annealing treatment of high temperature

(450 �C) and showed nearly graphite like morphology. These

results suggest that GZ composites can be prepared with tun-

able ZnO density.

In order to analyse the homogeneity of the synthesized

samples, Raman mapping measurements were carried out.

Rectangular mapping for the area of (95� 90) lm2 was made

for all the samples. The automated XY stage coupled with the

micro-Raman spectrometer moved (20� 20) steps covering

the selected area and as a result, 400 Raman spectra were col-

lected and their corresponding Raman intensity map was gen-

erated. The sequential collection of more number of spectra in

the selected region of the synthesized samples would provide

the homogeneity of our samples. Figure 6 shows the Raman

intensity maps of GZ composites obtained when plotting the

peak intensity of E2(high) band at 438 cm�1 as a function of

the spatial location. In the Raman map of GZ-1, the ZnO con-

centration is very low as expected. The bright yellow (high

intense) colour in the map represents the ZnO and the dark

yellow colour in the map is attributed to the graphene. It is

clear from the map that very few ZnO particles are anchored

on the surface of graphene and the Raman profile of the dark

area resembled the profile of multi-layer graphene which may

be due to annealing treatment. In the case of GZ-2, the con-

centration of ZnO seemed to be greater than graphene as the

brighter domain is larger than the darker domain. Hence, the

Raman map of GZ-2 suggests that the groups of agglomerated

ZnO nanoparticles are anchored on the graphene sheets. From

the Raman intensity map of GZ-3, it is clear that the ZnO par-

ticles are uniformly anchored on the graphene surface. The

Raman maps of GZ-2 and GZ-3 displayed quite similar

results. On the other hand, a completely different result was

obtained for the sample GZ-1 as the concentration of the ZnO

precursor is relatively lower than the other samples. The ho-

mogeneity of the synthesized GZ composites and the distribu-

tion of ZnO on graphene surface were successfully analysed

by Raman mapping technique.

The UV-Visible DRS absorption spectra of the prepared

GZ composites and ZnO are shown in Fig. 7(a). The

FIG. 5. SEM images of (a) and (b) ZnO, (c) and (d) GZ-1, (e) and (f) GZ-2,

and (g) and (h) GZ-3 (scale bar: 4 lm for (a), (c), (e), (g) and 2 lm for (b),

(d), (f), (h)).

173504-5 Gayathri et al. J. Appl. Phys. 115, 173504 (2014)

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absorption edge observed at 390 nm indicates the existence

of highly crystalline ZnO nanostructure.30 It is clear from the

spectra that the absorbance of the GZ composites has

increased in visible light region and showed red shift of

absorption edge when compared to bare ZnO. The maximum

absorption of visible light is observed for GZ-1 composite

which can be visibly noticed from the absorption spectra.

However, with the increase of ZnO content, the absorbance

in the visible light gradually decreased. The observed results

are in well agreement with the previous reports.12,31 This

increased visible light absorbance of GZ composites may be

due to the increase of surface electric charge of the oxides in

the GZ composites and the alteration of the process of elec-

tron–hole pair formation during irradiation.11 The obtained

results reveal the importance of graphene introduction on the

optical properties of ZnO and suggest that the incorporation

of graphene on metal oxides improves the absorption of

visible-light. The bandgap of ZnO and GZ composites was

estimated from the absorption edge of their respective

absorption spectra. The bandgap of ZnO, GZ-3, GZ-2, and

GZ-1 was estimated as 3.17, 2.90, 2.78, and 2.67 eV, respec-

tively. The observed red shift in the absorption edge of GZ

composites compared to bare ZnO indicates that the synthe-

sized GZ composites can be used as photocatalysts and the

composites could absorb more photons (visible light) than

bare ZnO. As a result, the red shift observed in the absorp-

tion spectrum might enhance the photocatalytic efficiency.5

The room temperature PL spectra of GZ composites and

bare ZnO are shown in Fig. 7(b). The PL spectrum of pristine

ZnO clearly shows a broad yellow-green emission band in

the region of 420–600 nm with a peak centred at 490 nm,

which is generally due to oxygen vacancies in ZnO lattice.32

Luminescence quenching of the emission of ZnO is observed

when it is anchored on the graphene surface. Also, the PL

spectra of GZ composites were blue shifted from that of pris-

tine ZnO. As ZnO is a good electron donor and graphene is a

good electron acceptor, the PL quenching is ascribed to the

electron transfer from the conduction band of ZnO to gra-

phene.33,34 As a result, the effective electron transfer may

reduce the recombination of photogenerated electron–hole

pairs35 and therefore the synthesized GZ composites are

expected to show better photocatalytic activity than bare

ZnO.

The photocatalytic activity of the prepared photocata-

lysts, i.e., GZ composites and bare ZnO was tested by using

FIG. 6. Raman intensity maps of (a) GZ-1, (b) GZ-2, and (c) GZ-3.

FIG. 7. (a) UV-Visible DRS and (b) PL spectra of GZ composites and bare

ZnO.

173504-6 Gayathri et al. J. Appl. Phys. 115, 173504 (2014)

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MB in aqueous solution as contaminant. MB is an intensely

absorbing organic dye in the visible region at 660 nm. The

time dependent UV-Visible absorption spectra of MB dye so-

lution in the presence of photocatalysts are shown in Fig. 8.

The absorbance of MB dye decreased with increasing irradia-

tion time. After the irradiation time of 240 min, the MB dye

was completely degraded in the presence of ZnO. With the

photocatalysts GZ-1, GZ-2, and GZ-3, the dye was degraded

at the irradiation times of 140, 100, and 160 min, respectively.

The photocatalytic degradation of MB in the presence of ZnO

and GZ composites under light irradiation is shown in Fig. 9.

The synthesized GZ composites exhibited better photocata-

lytic performance than bare ZnO. GZ-2 showed excellent

enhancement, i.e., 2.4 times higher than that of the bare ZnO.

From the PL spectra of the synthesized samples, we expected

degradation efficiency of the samples in the order of GZ-1

>GZ-2>GZ-3>ZnO. The observed PL quenching is attrib-

uted to the effective electron transfer from the conduction

band of ZnO to graphene which may reduce the recombina-

tion of photogenerated electron–hole pairs33 and therefore the

synthesized GZ composites are expected to show better pho-

tocatalytic activity than bare ZnO. But, the degradation effi-

ciency of the dye using various photocatalysts was in the

order of GZ-2>GZ-1>GZ-3>ZnO. As expected, the

sample GZ-2 showed better photocatalytic performance than

ZnO and GZ-3. Even though GZ-1 displayed good perform-

ance than ZnO and GZ-3, the performance was not as

expected. This result might be attributed to the following

FIG. 8. The time dependent UV-Visible absorption spectra of MB dye solution in the presence of (a) ZnO, (b) GZ-3, (c) GZ-2, and (d) GZ-1.

FIG. 9. The photocatalytic degradation of MB in the presence of ZnO and

GZ composites under light irradiation.

173504-7 Gayathri et al. J. Appl. Phys. 115, 173504 (2014)

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reason. As the concentration of ZnO is very low in GZ-1 (one

third of graphene), the graphene sheets clasped together and

appeared like multi-layer graphene because of the heat treat-

ment and so the sample appeared like ZnO on multi-layer gra-

phene (SEM image). So, the two dimensional planar structure

might have been lost, consequently the surface area

decreases. Hence, we suggest that for chemically derived

ZnO/graphene composite, GZ-2 would be the optimum con-

centration for photocatalysis application. The results are com-

parable with the literatures.1,18–20 According to the results

shown above, mechanism of MB degradation is proposed and

shown in Fig. 10. This remarkable enhancement of photocata-

lytic activity of GZ composites could be attributed to the

strong interaction between ZnO and defect sites of gra-

phene.18 After the irradiation of light, the electrons from the

valence band of ZnO may be excited to its conduction band

and consequently to graphene. The molecules of MB can be

transferred to the surface of the GZ composites (i.e., adsorp-

tion of dye) by means of p-p conjugation between dye and ar-

omatic regions of graphene.11 The increase in number of

holes initiates an oxidative pathway and therefore the

adsorbed dye can be oxidized. As a result, the photoactive

radicals generated during the reaction produce CO2, H2O, and

other intermediates and thereby leading to the degradation of

MB. These results revealed that the contaminants could be

removed successfully by the photocatalysts, which were syn-

thesized by a simple, inexpensive chemical method.

IV. CONCLUSION

ZnO decorated on graphene sheets was successfully syn-

thesized by a facile chemical route. XRD and micro-Raman

results confirmed the formation of hexagonal wurtzite ZnO

and GZ composites. The tube-like morphology of ZnO anch-

ored on the surface of graphene sheets was revealed by the

SEM analysis. The homogeneity of the GZ composites was

successfully analyzed by Raman mapping technique. The

DRS showed that the formation of ZnO on graphene surface

had promoted the absorption in the visible region.

Quenching of luminescence was observed for the GZ compo-

sites compared with bare ZnO. Finally, the photocatalytic ac-

tivity of the synthesized samples was evaluated by using MB

dye as contaminant. GZ composites showed markedly

enhanced photocatalytic performance than ZnO as expected,

and the concentration dependent study of ZnO on graphene

surface was made to understand photocatalytic efficiency.

ACKNOWLEDGMENTS

The authors acknowledge UGC-UPE for providing

micro-Raman facility and also DST-FIST, for the XRD

instrumentation facility. The authors are grateful to DST-

SERB for the financial support and providing the Raman

mapping facility.

1B. Li, T. Liu, Y. Wang, and Z. Wang, “ZnO/graphene-oxide nanocompo-

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