Fabrication of nanocomposite photocatalysts from zinc oxide

nanostructures and reduced graphene oxide

Rajendra C. Pawar, Danee Cho, Caroline S. Lee*

Department of Materials Engineering, Hanyang University, Ansan, Gyeonggi-do 426-791, Republic of Korea

a r t i c l e i n f o

Article history:

Received 5 November 2012

Received in revised form

28 December 2012

Accepted 31 December 2012

Available online 18 January 2013

Keywords:

Zinc oxide

Reduced graphene oxide

Photocatalysis

Sintering method

a b s t r a c t

Reduced graphene oxide/zinc oxide (RGO/ZnO) nanocomposites were fabricated directly using a facile

sintering method and evaluated as photocatalysts. Zinc nitrate and acetate powders were sintered in

a furnace in the presence and absence of RGO (“graphene”) sheets at 450 �C for 1 h in air. The sintered

samples were analyzed using various techniques and their photocatalytic activities toward methylene

blue (MB) degradation were evaluated. Field-emission scanning electron microscopy images revealed the

formation of pyramid- and nanorod-like structures for the cases of sintering without graphene. Sintering

with RGO powders provided a uniform distribution of ZnO nanoparticles over graphene sheets. Trans-

mission electron microscopy confirmed the good dispersion of the nanoparticles on the graphene sheets.

The nanocomposites prepared by sintering RGO with zinc acetate had superior photocatalytic activity

(40 min, k ¼ 0.0521 min�1) for MB degradation under ultraviolet (UV) irradiation. Improved photo-

catalytic activity was the result of a large specific surface area (12.83 m2 g�1) and effective photoelectron

separation and transportation by the graphene. Therefore, RGO/ZnO nanocomposites were found to be

highly promising materials for UV photocatalysis.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Water pollution has many sources, including industrial wastes,

untreated sewage, oxygen-demanding wastes, and inorganic and

organic pollutants such as acids, salts, hydrocarbons, detergents,

and toxic metals [1]. Industrial waste and city sewage discharged

into rivers are of particular concern. Industrial wastewater usually

contains specific and readily identifiable chemical compounds.

During the last 50 years, the number of industries worldwide has

grown rapidly. However, water pollution is concentrated within

a few subsectors, mainly in the form of toxic wastes and organic

pollutants. Among these, a large portion can be traced to the pro-

cessing of industrial chemicals and to the dye industry. Methods

frequently employed for water purification include adsorption,

biodegradation, electrocoagulation, nanofiltration, chlorination,

ozonation, and advanced oxidation [2].

Of these methods, the advanced oxidation process using a het-

erogeneous photocatalyst is of special interest because it can

degrade a wide range of organic pollutants [3]. In particular, ZnO is

one of the most promising catalysts because of its chemical sta-

bility, photosensitivity, fast electron transport capability, and better

light absorption compared to TiO2 [4e8]. ZnO has various impor-

tant electrical and optical properties such as high electron mobility

at room temperature (155 cm2 V�1 s�1), high exciton binding en-

ergy (60 meV), and wide bandgap energy (3.3 eV) [9,10]. However,

its catalytic activity is poor because of efficient photoelectron

recombination. Hence, attempts have been made to reduce pho-

toelectron losses by making composites from metal nanoparticles

(Ag, Pd, Au), metal oxides (TiO2, SnO2, Fe2O3), chalcogenides (ZnSe,

CdS, ZnTe), and graphene [11e22]. Among these, graphene is the

best choice to reduce photoelectron recombination and improve

the photodegradation efficiency. This is because its energy levels

are suitable for photoelectron transfer and it has a high extraction

ability for electrons from a semiconductor [23e25]. Many studies

have investigated reduced graphene oxide/zinc oxide (RGO/ZnO)

nanocomposites to reduce photoelectron recombination and

improve photocatalytic activity [26e34]. However, reported

nanocomposites have suffered from low photocatalytic efficiencies.

Furthermore, the methods used for their fabrication required high

temperatures, expensive substrates, vacuum systems, and rigorous

experimental conditions and only provided the product in low

yield. Preparing nanocomposites using a single reaction step at low

cost and temperature, while maintaining good device performance,

is highly desirable. The precursor sintering method is an excellent

route to prepare oxide nanostructures in one step with high yield* Corresponding author. Tel.: þ82 31 400 4697.

E-mail address: sunyonglee@hanyang.ac.kr (C.S. Lee).

Contents lists available at SciVerse ScienceDirect

Current Applied Physics

journal homepage: www.elsevier .com/locate/cap

1567-1739/$ e see front matter � 2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.cap.2012.12.031

Current Applied Physics 13 (2013) S50eS57

[35]. The synthesis does not require complex equipment or a vac-

uum system, thereby reducing the fabrication cost. Therefore, we

followed sintering method to prepare graphene/ZnO nano-

composites for the application of photocatalyst.

In this work, we fabricated nanocomposites of RGO and ZnO

by sintering graphene nanosheets with a zinc precursor in air at

450 �C for 1 h. The catalytic efficiencies of the nanocomposites were

evaluated by monitoring the photodegradation of methylene blue

(MB) under ultraviolet (UV) irradiation. The RGO/ZnO nano-

composites exhibited superior photocatalytic performances com-

pared with those of commercial ZnO powders and ZnO

nanostructures in the absence of graphene. The improved photo-

catalytic efficiencies were attributable to effective separation,

transportation, and utilization of photoelectrons. Finally, the elec-

tronic transitions and photodegradation reaction mechanism of the

RGO/ZnO nanocomposites were investigated.

2. Experimental

2.1. Preparation of reduced graphene oxide

The Hummersmethodwas used to prepare graphene oxide (GO)

powders [36]. In brief, 2 g of powdered graphite flakes was added

with stirring to 100ml of sulfuric acid (H2SO4) that had been cooled

below 10 �C. Then, 8 g of potassium permanganate (KMnO4) was

gradually added, followed by 2 h of stirring at the same tempera-

ture. The temperature was raised to room temperature and the

mixture stirred for an additional hour. The mixture was returned to

a low-temperature bath and diluted with 100 ml of distilled water.

Hydrogen peroxide (H2O2, 30%; 20 ml) was added to the mixture to

dissolve any residual permanganate. A large amount of bubbles was

released and the color of the mixture changed to brilliant yellow

(Fig. 1a). The colored suspension was filtered and washed several

times with 1 M hydrochloric acid and distilled water. The retained

GO powders were dried in an oven at 60 �C for 12 h and stored in

a vacuum oven.

The GO powders were reduced by dispersing 100 mg in 100 ml

of distilled water with ultrasonication, adding 20 ml of hydrazine

monohydrate drop by drop and refluxing the solution at 90 �C for

2 h. The refluxed solution was filtered and the retained reduced

graphene oxide (RGO) powders were transferred to a dish. The dish

with the RGO was kept in an oven at 60 �C for 12 h and then stored

in a vacuum oven. The RGO powders were re-dispersed in distilled

water under ultrasonication for 1 h; the black color (Fig. 1b) of the

solution indicated that the GO had been reduced completely.

2.2. Preparation of RGO/ZnO nanocomposites

RGO/ZnO nanocomposites were prepared from sintered pre-

cursors in a box furnace. Initially, the zinc source (5 g; i.e., zinc

acetate (Zn(CH3COO)2$2H2O) or zinc nitrate (Zn(NO3)2$6H2O)

powder) was sintered in a furnace in air at 450 �C for 1 h. The na-

nocomposites were then fabricated by sintering the RGO powders

(20 mg) and the zinc precursor (5 g) together in the furnace at

450 �C for 1 h; the co-sintered powders were used directly for

further analyses. The samples prepared with sintered zinc nitrate,

zinc nitrate þ RGO, zinc acetate, and zinc acetate þ RGO are

denoted as ZN, ZNG, ZA, and ZAG, respectively.

2.3. Characterization

X-ray diffraction (XRD) of the prepared nanostructures was

carried out using a diffractometer (D/MAX-2500/PC, Cu Ka line;

Rigaku, Tokyo, Japan). Raman spectroscopic analyses of ZnO and

RGO samples were performed at room temperature using a spec-

trometer (Renishaw, Hoffman Estates, IL, USA) with 632.8 nm laser

excitation. Microstructural analysis was determined using field

emission-scanning electron microscopy (FE-SEM; JMIRA3 LM;

Tescan, Cranberry Township, PA, USA). Transmission electron mi-

croscopy (TEM; JEM-2100F; Jeol, Tokyo, Japan) was used tomeasure

the diameter of the ZnO nanorods and assess the coating unifor-

mity of the ZnO nanoparticles on the graphene nanosheets. The

specific surface areas of the ZnO and RGO powders were measured

by the BrunauereEmmetteTeller (BET) method (AS-1; Quantach-

rome Instruments, Boynton Beach, FL, USA) using nitrogen

sorption.

2.4. Photocatalytic degradation of MB

The photocatalytic activity of the synthesized nanocomposites

was measured by degrading MB (1.0 � 10�5 M, 200 ml) under ul-

traviolet (UV) light (300 W; Fig. 2). An adsorptionedesorption

equilibrium of the photocatalyst and dye molecules was obtained

by dispersing the nanocomposite powder (5 mg l�1) in the

dye solution by stirring at room temperature for 1 h in the dark.

The dispersion was irradiated with the UV lamp (incident

power ¼ 50 mW cm�2) while being stirred continuously. A 5-ml

aliquot of the dispersion was withdrawn after each 10 min of

irradiation and centrifuged. The optical absorbance of the super-

natant was measured with a spectrophotometer (V-600; Jasco,

Tokyo, Japan). The dye concentration was monitored using the

absorption band at 665 nm. The photocatalytic activities of com-

mercial ZnO powders (Junsei Chemical, Tokyo, Japan) and the MB

solutionwithout catalyst were alsomeasured for comparison under

similar conditions.

3. Results and discussion

The crystal orientation and phase composition of sintered ZN,

ZNG, ZA, and ZAG samples were analyzed by XRD (Fig. 3). Compari-

son of the observed patterns with the standard card (JCPDS No. 01-Fig. 1. Photographs of (a) graphene oxide in distilled water (0.5 mg ml�1) and (b)

reduced graphene oxide using hydrazine hydrate (0.5 mg ml�1).

R.C. Pawar et al. / Current Applied Physics 13 (2013) S50eS57 S51

089-0511) indicated the hexagonal wurtzite crystal structure of

ZnO aligned along the (101) direction. The patterns had peaks

corresponding to the (100), (002), (102), (110), (103), (200), and

(201) planes, indicating the polycrystalline nature of the ZnO

structure. The pattern for RGO is provided for comparison. The

existence of two peaks at 2q values of 20.6� and 42.6� in the RGO

XRD pattern confirmed the complete reduction of graphene oxide

by hydrazine hydrate. However, graphene peaks were not seen in

the ZNG and ZAG XRD patterns due to the small amount of graphene

sheets that had been co-sintered with the zinc precursors (a

quantity that was too small to be detected by XRD). Careful analysis

Fig. 2. Schematic illustration of the experimental setup for the photocatalytic process. The inset shows photoelectron capture by graphene from the ZnO conduction band to

degrade dye molecules.

Fig. 3. XRD patterns of ZN, ZNG, ZA, and ZAG samples.

Fig. 4. Raman spectra in the range of 300e2500 cm�1 for ZN, ZNG, ZA, and ZAG samples.

The spectrum of pure RGO is provided for reference.

R.C. Pawar et al. / Current Applied Physics 13 (2013) S50eS57S52

of the XRD patterns revealed very small changes in crystallinity

after the addition of graphene, which was evidence for the for-

mation of the RGO/ZnO nanocomposites.

The quality of the RGO/ZnO nanocomposites was studied by

Raman spectroscopy (Fig. 4). The Raman spectrum for RGO was

used to clarify the Raman shifts of the bands observed for the

nanocomposites. The two strong peaks observed for RGO at about

1334 cm�1 (D band) and 1602 cm�1 (G band) are attributable to

disordered sp2-bonded carbon and to breathing vibrations of six-

membered sp2 carbon rings (E2g and A1g modes) in reduced gra-

phene [37]. In case of the ZN and ZA samples, a single peak at about

442 cm�1 is the optical phonon E2 (transverse optical) in the

Brillouin zone of the ZnO nanostructure [38]. However, the ZNGand ZAG XRD patterns contained peaks for RGO and ZnO at values

shifted toward a lower wave number (439 cm�1), indicating

a strong interaction between the RGO nanosheets and ZnO

nanostructures [39].

The surface morphology and growth of the ZnO nanostructures

with and without RGO were studied using FE-SEM. A well-faceted,

micrometer-scale pyramid-like structure was formed using zinc

nitrate sintering (Fig. 5a), with faceted growth uniformly dis-

tributed throughout the sample. However, in the presence of RGO,

the surface morphology changed from a faceted structure to one of

smooth crystals in which graphene sheets were mixed with ZnO

crystals (Fig. 5b). The low-magnification image shows an uneven

distribution of ZnO crystals and RGO and poor sample homogene-

ity. The formation of the pyramid-like structures with nitrate sin-

tering is attributable to nitrate ions from the zinc nitrate. These ions

adsorb on the ð1011Þ plane, which retards growth and results in

pyramid-like structures [40e44]. In the case of zinc acetate

Fig. 5. (a) FE-SEM image of micrometer-sized pyramid-like structures of sintered zinc nitrate powders (ZN sample) at 0.5 and 5000� magnification, (b) FE-SEM image of graphene

and zinc nitrate sintered powders for ZNG sample at 50 and 150,000� magnification, (c) FE-SEM image at 50 and 150,000� magnification of ZA sample showing nanorods formed

after zinc acetate sintering, and (d) FE-SEM image of graphene and zinc acetate powders at 50 and 150,000� magnification for ZAG sample.

R.C. Pawar et al. / Current Applied Physics 13 (2013) S50eS57 S53

sintering, nanorods formed with an average diameter of 60 nm

(Fig. 5c). The low-magnification image shows a uniform distribu-

tion of the nanorods. For RGO and zinc acetate sintering, the

nanoparticles, with an average diameter of 70 nm, were well dis-

tributed on the graphene sheet surfaces. Uniform coverage of

nanoparticles over an entire graphene sheet is evident in the low-

magnification image (Fig. 5d). The RGO/ZnO nanocomposite was

also studied by high-resolution transmission electron microscopy

(HR-TEM) (Fig. 6). The TEM image reveals a uniform distribution of

the ZnO particles over the graphene sheet. The two parallel planes

are oriented along the (101) direction and are separated by about

0.24 nm (inset of Fig. 6). The same orientation was observed in the

XRD pattern. The nanoparticles are well distributed on the gra-

phene surfaces because of strong electrostatic and electron transfer

interactions between ZnO and graphene.

The specific surface area (N2 adsorption) was measured for the

samples sintered with and without RGO (Table 1). The highest

specific surface area (12.83 m2 g�1) was found for the ZAG sample

(Fig. 7). Incorporation of graphene served to increase the surface

area, since ZA had a lower surface area (11.50 m2 g�1). The ZNsample had the lowest surface area (2.09 m2 g�1), which increased

with graphene addition (i.e., to 4.79 m2 g�1 for the ZNG sample).

Therefore, addition of graphene in both cases increased the specific

surface area. The reported specific surface areas of the RGO/ZnO

nanocomposites are higher than those reported elsewhere [35].

The photocatalytic activity of the RGO/ZnO nanocomposites was

measured for degradation of MB under UV irradiation. Photo-

catalytic efficiency was assessed by the monitoring the change in

MB absorbance over time. The absorption spectrum of the dye so-

lution after 40min of illumination is shown in Fig. 8. The absorption

peak for the dye decreased gradually with increasing irradiation

due to degradation of MB molecules. The dye was completely

degraded within 40 min for the ZAG sample, indicating rapid deg-

radation. However, dye remained after 40 min of irradiation for the

commercial ZnO powders, ZN, ZNG, and ZA samples. The change in

absorbance of the MB solution with time in the presence of dif-

ferent catalysts under UV light irradiation is shown in Fig. 9; the

relative absorbance is A0/A*100, where A is the absorbance of the

MB solution at the irradiation time t and A0 is the absorbance of the

solution before irradiation (t ¼ 0). A slight decrease in the MB

concentrationwas found under UV irradiation in the absence of any

catalyst. However, photodegradation occurred after the addition of

commercial ZnO powders, with a catalytic efficiency of 64%. Gra-

phene incorporation with ZN to form ZNG improved the efficiency

from 66% to 78.6%. The photodegradation efficiency improved from

94.3% to 99.3% when graphene was sintered with ZA to form ZAG.

Hence, graphene addition improved the catalytic efficiency in all

cases. The inset to Fig. 9 graphs ln(A0/A) ¼ �kt, where A0/A is the

normalized absorbance of the MB solution, k is the apparent kinetic

rate constant, and t is the reaction time. This linear ln(A0/A) vs. t plot

demonstrates that the photodegradation of MB follows pseudo-

first-order reaction kinetics (inset of Fig. 9). The calculated value

of k for the ZAG sample (k ¼ 0.0521 min�1) was four times greater

than that for commercial ZnO powders (k ¼ 0.0105 min�1). The k

values of ZA (k ¼ 0.0292 min�1), ZN (k ¼ 0.0106 min�1), and ZNG(k ¼ 0.0144 min�1) samples were slightly higher than that of

commercial ZnO powder but lower than that of the ZAG sample

(Fig. 10). Hence, the nanocomposites prepared with RGO and zinc

acetate sintering, i.e., ZAG, had better photocatalytic activity than

the ZN, ZNG, and ZA samples. The observed photocatalytic activity is

much higher than that reported for other RGO/ZnO nano-

composites [26e34,45]. Basically, the photocatalytic activity de-

pends on the effective separation of electronehole pairs, specific

surface area, and crystallinity. Li and Cao used a RGO/ZnO com-

posite to remove Rhodamine B dye from water, which degraded in

60 min under UV light [46]. Yang et al. reported a 98% efficiency for

functionalized RGO/ZnO nanocomposites exposed to UV light for

100 min [47]. Thus, our synthesized nanocomposites exhibited

excellent photocatalytic activity. Their enhanced photocatalytic

activities are attributable to the high effective surface area (e.g.,

12.83 m2 g�1 for ZAG) and efficient separation of photoelectrons

because graphene acts as an electron acceptor (see the enlarged

schematic in Fig. 2). Additionally, the strong pep stacking in-

teractions between MB dye molecules and the graphene surface

increases the reactivity.

3.1. Possible photocatalysis mechanism

The enhanced photocatalytic activities of the RGO/ZnO com-

posites stem from suitable energy band positions of ZnO (�4.05 eV)

and graphene (�4.42 eV). This improves photoelectron transfer,

and the high electron mobility of graphene transfers the injected

electrons quickly from the interface to the sheets, minimizing

recombination losses. Hence, photocatalytic activity improved

significantly with RGO addition to ZnO. The following series of re-

action processes involved in the MB degradation are proposed:

ZnOþ hvð� 3:3 eVÞ/ZnO�

hþVB þ e�CB

�

(1)

ZnO�

hþVB þ e�CB

�

þ RGO/ZnO�

hþVB

�

þ RGO�

e�CB

�

(2)

hþVB þ H2O/Hþ þ OH�

(3)

Hþ þ OH�/

�OH (4)

RGO�

e�CB

�

þ O2/�O�

2 (5)

Fig. 6. TEM image for the ZAG sample showing the uniform distribution of ZnO

nanoparticles having an average diameter of 70 nm over the entire graphene sheet.

The inset shows the HR-TEM image with an interplanar spacing of 0.247 nm and

orientation along the (101) direction.

Table 1

Summary of the measured specific surface areas (m2 g�1) and reaction rate con-

stants (min�1) for commercial ZnO powders, ZN, ZNG, ZA, and ZAG samples.

Sample details BET surface

area (m2 g�1)

Kinetic rate

constant (min�1)

Zinc nitrate (ZN) 2.09 0.0106

Zinc nitrate-Graphene

(ZNG)

4.79 0.0144

Zinc acetate (ZA) 11.50 0.0292

Zinc acetate-Graphene

(ZAG)

12.83 0.0521

R.C. Pawar et al. / Current Applied Physics 13 (2013) S50eS57S54

Fig. 7. Nitrogen absorption isotherm plots of (a) ZN, (b) ZNG, (c) ZA, and (d) ZAG samples.

Fig. 8. UVevisible absorbance spectra showing the photocatalytic degradation of MB under UV light using 5 mg l�1 of photocatalyst for (a) ZN, (b) ZNG, (c) ZA, and (d) ZAG samples.

�O�2 þ HO

�

2 þ Hþ/H2O2 þ O2 (6)

H2O2 þ �O�2/

�OHþ OH� þ O2 (7)

H2O2 þ e�CB/�OHþ OH� (8)

Under UV light with an energy �3.3 eV, electronehole pairs are

generated, and then photogenerated electrons are transferred to

RGO nanosheets as described in Equations (1) and (2). The holes in

ZnOðhþVBÞ generate OH radicals as shown in Equations (3) and (4).

The electrons captured on the RGO react with oxygen, which forms

transient superoxide radicals (Equation (5)). Finally, the superoxide

molecules react with electrons in the RGO (Equation (6)) and form

highly reactive OH radicals, which mineralize MB molecules as

shown in Equations (7) and (8). Thus, graphene plays a crucial role

in reducing recombination losses and thereby increases photo-

catalytic activity.

4. Conclusions

We have demonstrated facile and large-scale fabrication

method of RGO/ZnO nanocomposites using the precursor sintering

method. Photocatalytic performance after incorporation of gra-

phene was four times higher than that of commercial ZnO pow-

ders. Raman spectroscopy confirmed the successful formation of

RGO/ZnO nanocomposites. The uniform distribution of ZnO

nanoparticles over graphene nanosheets derives from strong

electrostatic interactions between ZnO and graphene. The nano-

composites showed superior catalytic efficiency toward photo-

degradation of MB dye under UV light. The photodegradation time

was less than reported values for other RGO/ZnO nanocomposites.

Therefore, an efficient fabrication method of photocatalyst was

demonstrated successfully using the cost-effective and facile pre-

cursor sintering method.

Acknowledgments

This research was supported by the Basic Science Research

Program through the National Research Foundation of Korea (NRF)

funded by the Ministry of Education, Science and Technology

(2011-0027380), by the Human Resources Development of the

Korean Institute of Energy Technology Evaluation and Planning

(KETEP) grant funded by the Ministry of Knowledge Economy, Re-

public of Korea (No. 2010-4010100620) and by Basic Science

Research Program through the National Research Foundation of

Korea(NRF) funded by the Ministry of Education, Science and

Technology (2012032560).

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Fig. 9. Relative absorbance for commercial ZnO powders, ZN, ZNG, ZA, and ZAG samples

as a function of UV irradiation time. The absorbance of MB in the absence of photo-

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Fig. 10. Comparison of the reaction rate constants (min�1) of MB photodegradation

between commercial ZnO powders, ZN, ZNG, ZA, and ZAG samples.

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