Enhanced photocatalytic activity of grapheneetio2 composite under

Current Applied Physics 13 (2013) 659e663

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Current Applied Physics
journal homepage: www.elsevier.com/locate/cap

Enhanced photocatalytic activity of grapheneeTiO2 composite under
visible light irradiation
N.R. Khalid a, b, *, E. Ahmed b, Zhanglian Hong b, **, L. Sana. a, M. Ahmed a, b
a
b

Department of Physics, Bahauddin Zakariya University, Multan 60800, Pakistan
State Key Laboratory of Silicon Materials and Department of Materials Science and, Engineering, Zhejiang University, Hangzhou 310027, China

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 6 July 2012
Received in revised form
22 October 2012
Accepted 8 November 2012
Available online 21 November 2012

Novel grapheneeTiO2 (GReTiO2) composite photocatalysts were synthesized by hydrothermal method.
During the hydrothermal process, both the reduction of graphene oxide and loading of TiO2
nanoparticles on graphene were achieved. The structure, surface morphology, chemical composition
and optical properties of composites were studied using XRD, TEM, XPS, DRS and PL spectroscopy.

The absorption edge of TiO2 shifted to visible-light region with increasing amount of graphene in the
composite samples. The photocatalytic degradation of methyl orange (MO) was carried out using
grapheneeTiO2 composite catalysts in order to study the photocatalytic efficiency. The results showed
that GReTiO2 composites can efficiently photodegrade MO, showing an enhanced photocatalytic
activity over pure TiO2 under visible-light irradiation. The enhanced photocatalytic activity of the
composite catalysts might be attributed to great adsorptivity of dyes, extended light absorption range
and efficient charge separation due to giant p-conjugation system and two-dimensional planar
structure of graphene.
Ó 2012 Elsevier B.V. All rights reserved.

Keywords:
TiO2
Graphene
Composite
Extended light absorption
Photocatalytic activity

1. Introduction
Semiconductor photocatalysis is an advanced technology in air
purification, water disinfection and purification. Titanium dioxide
(TiO2) has been extensively used semiconductor in photocatalytic
and photochemical processes due to its stability, low cost and
nontoxicity [1,2]. However, narrow light response range and low
separation probability of the photoinduced electronehole pairs in
TiO2 photocatalytic system limits its technological applications.
Therefore, various studies have been directed to shift the optical
absorption of TiO2 from UV to the visible-light region and to
increase the quantum efficiency of the TiO2 photocatalyst [3e7].
Recently, graphene for improving photocatalytic properties has
emerged as a high potential material due to its high surface

area, transparency, conductivity and good interfacial contact with
adsorbents [8e10]. Furthermore, the surface properties of
graphene could be adjusted via chemical modification, which

* Corresponding author. Department of Physics, Bahauddin Zakariya University,
Multan 60800, Pakistan. Tel.: þ92 61 9210091; fax: þ92 61 9210098.
** Corresponding author. Tel./fax: þ86 571 87951234.
E-mail addresses: (N.R. Khalid), hong_zhanglian@
zju.edu.cn (Z. Hong).
1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
/>
facilitates its use in composite materials [11,12]. Thus, the combination of TiO2 and graphene is promising to simultaneously
possess excellent adsorptivity, transparency and conductivity,
which could facilitate the effective photodegradation of pollutants
during the photocatalysis. In a recent study, Williams et al. [12]
prepared TiO2egraphene nanocomposites by mixing ultrasonically TiO2 particles and graphene oxide (GO) colloids, followed by
UV-assisted photocatalytic reduction of GO. In another investigation, TiO2egraphene composite materials were prepared by
self-assembly of TiO2 nanoparticles grown on graphene sheets by
a one-step approach with the assistance of an anionic surfactant
[13]. Zhang et al. [10] synthesized high performance P25-graphene
composite photocatalyst for methyl blue degradation using
hydrothermal method. Sonophototcatalytic activity of graphene
oxide based PteTiO2 composites for DBS degradation was investigated by Neppolian et al. [14]. N. Farhangi et al. [15] investigated
Fe doped TiO2 nanowires on graphene sheets for photodegradation of 17b-estradiol (E2). Nevertheless, the understanding
of TiO2egraphene photocatalysis system is unclear.
Herein, we demonstrated a simple hydrothermal method to
prepare TiO2egraphene (GReTiO2) composites. The prepared GRe
TiO2 composites showed extended light absorption range and
higher photocatalytic activity for degradation of methyl orange
under visible-light irradiation.

660

N.R. Khalid et al. / Current Applied Physics 13 (2013) 659e663

2. Experimental

(101)

2.1. Preparation of GReTiO2 composites

2.2. Sample characterization
The crystal structure of composites was identified by powder
X-ray diffraction (XRD) and patterns were collected from 10 to 80
in 2q with 0.02 steps/s using a Rigaku D/max-3B X-ray diffractometer with Cu Ka as radiation source (l ¼ 0.15406 nm) at 40 kV
and 36 mA. Chemical compositions of composites were analyzed
using X-ray photoelectron spectroscopy (XPS, Thermo-VG Scientific, ESCALAB250, monochromatic Al Ka X-ray source). All binding
energies were calibrated by using the containment carbon
(C1s ¼ 284.6 eV). Transmission electron microscopy (TEM) was
carried out on a JEOL JEM-1200EX electron microscope instrument
operated at 200 kV. TEM samples were prepared by dispersing the
final powder in ethanol, one drop of the dispersion was then
dropped on the carbonecopper grid. UVevis diffuse reflectance
spectra (DRS) were measured in the range of 300e800 nm using
a (HITACHI U-4100 UVeVis spectrometer) with an integrating
sphere accessory. The powders were pressed into pellets, and
BaSO4 was used as a reference standard for correction of instrumental background. Ther reflectance was converted to absorbance
by KubelkaeMunk equation: F(R) f K/S ¼ (1 À R)2/2R, where K is
the molar absorption coefficient, S is the scattering coefficient, and

R is the diffuse reflectance. The photoluminescence (PL) emission
spectra were obtained using (HITACHI F-4500 Fluorescence spectrophotometer). The samples excitation was made at 380 nm at
room temperature, and the emission spectra were collected
between 400 and 700 nm.

(200) (211)

(204) (220) (215)

05GR-TiO2
Intensity (a.u.)

GO was synthesized from graphite powder (99.99% Alfa Aesar) by
a modified Hummers method [16] and TiO2 nanoparticles were
preparedbysolegelmethodaccordingtoourpreviousstudy[17].GRe
TiO2 compositeswerepreparedbysimplehydrothermalmethodbased
onZhang’swork[10].Briefly,20mgofGOwasdissolvedinasolutionof
H2O(80mL)andethanol(40mL)mixtureusingultrasonictreatmentfor
2h,andthen200mgof(TiO2)wasaddedtotheobtainedGOsolutionand
stirred for another 2 h to get a homogeneous suspension. The suspension was then placed in a 200 mL Teflon-sealed autoclave and
maintainedat120  Cfor3htosimultaneouslyattainthereductionofGO
and the deposition of TiO2 on graphene sheets. Finally, the resulting
composite was recovered by filtration, rinsed by deionized water
several times, and dried at 70  C for 12 h. Finally, the weight ratio of
graphene to TiO2 was (1e10 wt.%) and the composites obtained were
labeled as 01GReTiO2, 02GReTiO2, 05GReTiO2 and 10GReTiO2
respectively.

(004)

10GR-TiO2

02GR-TiO2

01GR-TiO2

TiO2

10

20

30

40

50

60

70

2θ (angle)
Fig. 1. XRD patterns of TiO2, GReTiO2 composites with various graphene contents.

supernatant was taken out for absorption measurement. The
intensity of the main absorption peak (464 nm) of the methyl
orange dye was considered as a measure of the residual MO dye
concentration (C) and the initial concentration of dye was referred
as (C0).

2.3. Photocatalytic activity measurement
The photocatalytic activities of different composites were estimated by monitoring the degradation of methyl orange (MO) in
a home-made apparatus with a halogenetungsten lamp (400 W) as
the radiation source. The visible-light (l ! 420 nm) used in the
present study was obtained by the filter with cut-off wavelength of
420 nm. Typically, for the photocatalytic experiment, 100 mg
photocatalysts were suspended in 100 mL MO aqueous solution
with a concentration of 10 mg LÀ1 in a beaker. The suspension was
magnetically stirred for 0.5 h to reach the adsorption/desorption
equilibration without visible-light exposure. Following this, the
photocatalytic reaction was started by the exposure to the visible
light. The temperature of the suspension was kept at about 20  C by
an external cooling jacket with recycled water. After a setup
exposure time, 5 ml suspension was sampled, centrifuged, and the

80

Fig. 2. TEM images of (a) TiO2, (b) 10GReTiO2 composite.


N.R. Khalid et al. / Current Applied Physics 13 (2013) 659e663

which is consistent with the value of Tiþ4 in the TiO2 lattice [19].
O1s core level spectrum (Fig. 3c) shows main peak at 530.3 eV due
to the metallic oxides TieO bond, which is consistent with binding
energy of O2À in the TiO2 lattice and the peak appearing at 532.4 eV
was ascribed to adsorbed OHÀ on the surface of TiO2 [20]. In C1s
core level spectrum (Fig. 3d), the main peak was observed at
284.4 eV, which corresponds to the adventitious carbon adsorbed

on the surface of sample and the peak at 286.1 eV corresponds to
CeC bonds of carbonate species [15,20].
UVevisible DRS spectra of GReTiO2 composites with different
weight ratio of graphene are shown in Fig. 4(a). It is obvious that
a red shift to longer wavelength regions occurred for GReTiO2
composites. Band gap energy (Eg) for allowed indirect transitions
can be estimated using the relation

3. Results and discussion
XRD patterns of TiO2 and GReTiO2 composites with different
weight ratio of graphene are shown in Fig. 1. The patterns clearly
show peaks of anatase phase structure of TiO2, namely, the planes
(101), (004), (200), (211), (204), (220), and (215) at 2q values of ca.
25.38 , 37.82 , 48.18 , 54.4 , 62.92 , 69.92 , 74.9 respectively,
which indicate that all values are in good agreement with (JCPDS21-1272). In XRD patterns of GReTiO2 samples, graphene peaks
were not observed and structure of TiO2 was nearly unchanged in
all composites, only a slight change in FWHM of (101) peak of
anatase TiO2 was observed. It may be due to the fact that the
characteristic (002) peak at 25.9 of graphene [17,18] is weak and
might overlaps with the (101) peak of anatase TiO2 (25.4 ).
The loading of the TiO2 nanoparticles onto graphene sheets was
characterized by TEM, and the typical images of TiO2 and 10GRe
TiO2 composite are shown in Fig. 2. Image for composite sample
confirms that graphene is solid support for TiO2 nanoparticles.
During the formation of nanocomposite, the TiO2 nanoparticles
adheres to the functional groups on graphene oxide plane and
graphene oxide is reduced to form GReTiO2 composite during the
subsequent hydrothermal process.
Fig. 3a shows the XPS survey spectra of 10GReTiO2 composite,
which contains 52% O 1s, 27% Ti2p and 21% C1s. In core level XPS

spectrum of Ti2p (Fig. 3b), the Ti2p3/2 and Ti2p1/2 peaks are
located at binding energies of 459.1 eV and 464.9 eV respectively,

a

À
Á
ðahyÞ1=2 ¼ Bd hy À Eg
where a is absorption coefficient; hn is incident photon energy;
and Bd is the absorption constant. Plots of (ahn)1/2 versus hn from
the spectral data of Fig. 4(a) is presented in Fig. 4(b). The intercept
of the tangent to the plot gives a good approximation of band gap
energy for TiO2 [21]. The band gap energies estimated from the
above relation are 3.20 eV for TiO2, 3.16 eV for 01GReTiO2, 3.13 eV
for 02GReTiO2, 3.04 eV for 05GReTiO2, and 3.0 for 10GReTiO2
composite, respectively. The results obviously demonstrate the
significant influence of graphene on the optical properties, in

b

459.1 eV

Ti2p

Intensity (a.u.)

O1s

Intensity (a.u.)

661

Ti2p
Ti2s
C1s

464.9 eV

Ti3p
Ti3s

800

600

400

200

0

470

468

466

Binding energy (eV)

c

464

462

460

458

456

Binding energy (eV)

d

O1s

C1s

530.3 eV

Intensity (a.u.)

Intensity (a.u.)

284.4 eV

532.4 eV

536

534

532

530

Binding energy (eV)

528

526

286.1 eV

292

290

288

286

284

Binding energy (eV)

Fig. 3. XPS survey and core level spectra of Ti2p, O1s and C1s of 10GReTiO2 composite sample.

282

662

a

N.R. Khalid et al. / Current Applied Physics 13 (2013) 659e663
1.6

TiO2
01GR-TiO2

10GR-TiO2

02GR-TiO2

05GR-TiO2
02GR-TiO2
0.8

05GR-TiO2

Intensity (a.u.)

Absorbance

1.2

01GR-TiO2
TiO2

10GR-TiO2

0.4

0.0
300

400

500

600

700

800

Wavelength (nm)

b

450

3.0

600

650

Fig. 5. Photoluminescence spectra of GReTiO2 composites with various graphene
contents (excitation wavelength of 380 nm).

2.0

1/2

550

Wavelength (nm)

2.5

( α hν )

500

10GR-TiO2

The photocatalytic activities of TiO2 and GReTiO2 composites
with different wt% of graphene were investigated by photodegradation of methyl orange (MO) under visible light (l ! 420 nm)
and results are shown in Fig. 6(a). A control experiment study of
MO degradation without catalyst under the same condition was
also conducted. The result indicates that the photolysis can be
ignored as the corresponding degradation is about 0.4% after
exposure for 3 h to the visible light. Photocatalytic degradation of
MO follows roughly the pseudo-first-order reaction kinetics at low
dye concentrations [24]:

05GR-TiO2

02GR-TiO2

1.5

01GR-TiO2
TiO2

1.0

0.5

0.0
1.5

2.0

2.5

3.0

3.5

4.0

Band gap (eV)
Fig. 4. (a) UVevisible DRS spectra and (b) plots of (ahn)1/2 versus photon energy (hn) of
pure TiO2 and GReTiO2 composites with various graphene loadings.

which increasing graphene amount increased the light absorption
of TiO2, similar to the results of TiO2eCNT and C-doped TiO2

[10,22].
The photoluminescence emission spectra is a useful investigation to study the efficiency of charge carrier trapping, immigration,
transfer and to understand the fate of electronehole pairs in the
field of photocatalysis over solid semiconductors [20,23]. It is
well-known that the PL emission is the result of the recombination
of excited electrons and holes either directly (bandeband) or
indirectly (via a band gap state), the lower PL intensity may show
the lower recombination rate of electrons and holes and higher
separation efficiency under light irradiation. Fig. 5 shows the
photoluminscence spectra of TiO2 and GReTiO2 composites. The PL
intensity of pure TiO2 is significantly higher than the other
composite samples, which is showing the higher recombination of
electrons and holes. Moreover, the emission intensity is decreased
with increasing graphene loadings and is found lowest for 10GRe
TiO2 composite catalyst. Therefore, it is concluded that presence
of graphene in the composite samples might be effective to
enhance the separation efficiency of electronehole pairs during the
photocatalysis.

lnðC0 =CÞ ¼ k  t
where k is the apparent first-order rate constant, used as the basic
kinetic parameter for different photocatalysts; C0 is the initial
concentration of MO in aqueous solution; and C is the residual
concentration of MO at time t. The apparent rate constant values
were deduced from the linear fitting of ln(C0/C) vs irradiation time
as shown in Fig. 6(b). The results show that the apparent rate
constant is remarkably enhanced by increasing the amount of
graphene in the composite samples. The apparent rate constant
(k ¼ 5.66 Â 10À3 minÀ1) for 10GReTiO2 composite catalyst is much
higher than that of pure TiO2 (k ¼ 0.9 Â 10À3 minÀ1). Therefore, we

attribute the significantly enhanced photocatalytic performance of
graphene based composites to the result of strong coupling
between TiO2 and graphene. Firstly, the graphene has been reported to be a competitive candidate as an acceptor material due to
its p-conjugation structure, thus in GReTiO2 system, the excited
electrons of TiO2 could transfer from the conduction band to graphene. Thus graphene in the composite serves as an acceptor
material of photogenerated electrons of TiO2 and inhibits the
electronehole recombination, which results in more charge
carriers to form reactive species and promotes the degradation of
dye. Secondly, graphene has unexpectedly excellent conductivity
due to its two dimensional planar structure. Therefore, both the
electron accepting and transporting properties of graphene in
the composite catalyst could contribute to the suppression of
charge recombination, and thereby a higher degradation rate in the
photocatalysis was achieved [10,15,22].


N.R. Khalid et al. / Current Applied Physics 13 (2013) 659e663

a

663

pure TiO2 under visible-light irradiation for MO degradation. The
enhanced photocatalytic activity of the composite catalyst might be
attributed to giant two-dimensional planar structure of graphene
and possibility of more pep interaction between composite and
organic compound.

1.0
0.9

0.8
0.7

Acknowledgments

C/C0

0.6

This work was supported partially by Natural Science Foundation of China (No. 51072180), China Postdoctoral Science Foundation (No. 20110491764), the Fundamental Research Funds for the
Central Universities (No. 2009QNA4005), and the State Key Laboratory of Silicon Materials (SKL2009-14) at Zhejiang University. N.
R. Khalid thanks to Higher Education Commission of Pakistan for
IRSIP scholarship.

0.5

MO (without catalyst)

0.4

TiO

0.3

01GR-TiO

0.2

02GR-TiO
05GR-TiO

0.1

10GR-TiO

0.0
0

20

40

60

80

100

120

140

160

180

200

Time (min)

b

0.0

-0.2

ln (C/C0)

-0.4

-0.6

-0.8
TiO
-1.0

(k = 0.00090)

01GR-TiO (k = 0.00184)
02GR-TiO (k = 0.00259)

-1.2

05GR-TiO (k =0.00404)
10GR-TiO (k =0.00566)

-1.4
0

30

60

90

120

150

180

-1

Time (min )
Fig. 6. (a) Photocatalytic degradation of MO in the presence of different catalysts
under visible light irradiation, and (b) kinetic study of MO degradation using pseudofirst order fit for different catalysts.

4. Conclusion
GrapheneeTiO2 composite photocatalysts synthesized by
hydrothermal method have extended light absorption in visiblelight range and showed enhanced photocatalytic activity than

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