One-pot solvothermal synthesis of Cu-modified BiOCl via a Cu-containing ionic liquid and its visible-light photocatalytic properties

Article abstract of DOI:10.1039/c3ra45670f

Novel visible-light-driven Cu-modified BiOCl uniform sphere-like materials have been successfully synthesized through a one-pot ethylene glycol (EG)-assisted solvothermal process in the presence of 1-octyl-3- methylimidazolium copper trichloride ([Omim]CuCl₃). The Cu-modified BiOCl materials were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), Raman, photoluminescence (PL) and UV-vis diffuse reflectance spectroscopy (DRS). The results of the XRD, XPS, SEM, EDS, Raman analyses indicated that metal Cu was evenly distributed on the surface of the BiOCl microspheres in the form of Cu²⁺. During the reaction process, the metal-based ionic liquid acted as the solvent, the template, the Cl source and the Cu source at the same time. It is possible to tune the morphology of the Cu-modified BiOCl materials by varying the amount of ionic liquid used. In addition, the electrochemical and photocatalytic properties of the Cu-modified BiOCl materials were investigated. After the introduction of Cu²⁺, the photocurrent of the Cu-modified BiOCl materials was higher than that of the pure BiOCl. And the Cu-modified BiOCl materials exhibited higher photocatalytic activity for the degradation of methylene blue (MB) and bisphenol A (BPA) than that of pure BiOCl. The increased photocatalytic activity of the Cu-modified BiOCl materials was attributed to its large adsorption capacity, broad light absorption band and high separation efficiency of photo-generated electrons and holes. On the basis of these findings, the Cu-modified BiOCl materials showed great promise as photocatalysts for degrading organic pollutants and other applications.

Full text of DOI:10.1039/c3ra45670f

RSC Advances
View Article Online
View Journal | View Issue
PAPER
One-pot solvothermal synthesis of Cu-modified
BiOCl via a Cu-containing ionic liquid and its
visible-light photocatalytic properties†
Cite this: RSC Adv., 2014, 4, 14281
Jun Di,a Jiexiang Xia,a Sheng Yin,a Hui Xu,b Li Xu,c Yuanguo Xu,a Minqiang Hea
a
*
and Huaming Li
Novel visible-light-driven Cu-modified BiOCl uniform sphere-like materials have been successfully
synthesized through a one-pot ethylene glycol (EG)-assisted solvothermal process in the presence of
1-octyl-3-methylimidazolium copper trichloride ([Omim]CuCl3). The Cu-modified BiOCl materials were
characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron
microscopy (SEM), energy dispersive X-ray spectrometry (EDS), Raman, photoluminescence (PL) and
UV-vis diffuse reflectance spectroscopy (DRS). The results of the XRD, XPS, SEM, EDS, Raman analyses
indicated that metal Cu was evenly distributed on the surface of the BiOCl microspheres in the form of
Cu2+. During the reaction process, the metal-based ionic liquid acted as the solvent, the template, the Cl
source and the Cu source at the same time. It is possible to tune the morphology of the Cu-modified
BiOCl materials by varying the amount of ionic liquid used. In addition, the electrochemical and
photocatalytic properties of the Cu-modified BiOCl materials were investigated. After the introduction of
Cu2+, the photocurrent of the Cu-modified BiOCl materials was higher than that of the pure BiOCl. And
the Cu-modified BiOCl materials exhibited higher photocatalytic activity for the degradation of
methylene blue (MB) and bisphenol A (BPA) than that of pure BiOCl. The increased photocatalytic activity
of the Cu-modified BiOCl materials was attributed to its large adsorption capacity, broad light absorption
band and high separation efficiency of photo-generated electrons and holes. On the basis of these
findings, the Cu-modified BiOCl materials showed great promise as photocatalysts for degrading organic
pollutants and other applications.
Received 9th October 2013
Accepted 28th January 2014
DOI: 10.1039/c3ra45670f
www.rsc.org/advances
photocatalysts to the larger visible spectral region. One of them
is the modication of TiO2, including non-metals doping,5
metals doping,6,7 precious metal deposition,8 semiconductor
1. Introduction
With the development of photocatalytic technology in terms of
environmental purication and solar energy conversion, TiO2
has so far been found to be the most promising semiconductor
photocatalyst because of its superior photocatalytic activity,
chemical stability, low cost, and environmental friendliness.1,2
However, a major drawback of TiO2,its lower utilization of solar
energy (it is only sensitive to UV light), limits its extensive
application.3,4 Therefore, the exploitation of visible-light-driven
photocatalysts is essential for the practical application of pho-
tocatalytic technology. Until now, two major approaches have
been employed to extend the light absorption of semiconductor
coupling,9 etc. The other method is to exploit novel non-TiO2-
based semiconductor photocatalysts with strong visible light
response, such as Bi2WO6,10 BiOX (X ¼ Cl, Br, I),11–13 Ag/AgX
(X ¼ Cl, Br, I),14 InNbO4,15 Ag3VO4,16 BiVO4,17 etc. However, these
methods are expected to be further developed to satisfy large-
scale applications under visible light radiation. Hence, the
potential of visible-light-driven photocatalysts with high effi-
ciency still needs to be fully investigated.
As a new type of non TiO2-based semiconductor photo-
catalyst, BiOX (X ¼ Cl, Br, I) has attracted great attention
recently due to its good photocatalytic activity under UV or
visible light illumination, unique layered structure, high
activity, and high photocorrosion stability.18 Among them,
BiOCl possesses the tetragonal matlockite structure, a layered
structure composed of [Bi2O2]2+ layers interleaved with double
Cl layers. Recently, BiOCl is reported to be an important pho-
tocatalyst for the degradation of organic compounds under
visible light illumination. For example, Zhang et al.19 prepared
BiOCl microspheres by a general one-pot solvothermal process,
aSchool of Chemistry and Chemical Engineering, Jiangsu University, 301 Xuefu Road,
Zhenjiang, 212013, P. R. China. E-mail: lhm@ujs.edu.cn; Fax: +86-511-88791108; Tel:
+86-511-88791108
bSchool of the Environment, Jiangsu University, Zhenjiang 212013, P. R. China
cSchool of Material Science and Engineering, Jiangsu University, Zhenjiang, 212013, P.
R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra45670f
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 14281–14290 | 14281

View Article Online
RSC Advances
Paper
and the band gap of the as-prepared BiOCl powders was esti-
In this study, novel visible-light-driven Cu-modied BiOCl
mated to about 3.22 eV. The photocatalytic activity of BiOCl on materials were solvothermally synthesized by a Cu-containing
the degradation of MO is better than that of TiO2 (Degussa, P25) ionic liquid. A series of characterization methods were con-
under UV-vis light irradiation and that of C-doped TiO2 under ducted to characterize the synthesized Cu-modied BiOCl
visible light (l > 420 nm) irradiation. However, its photocatalytic materials. The Cu-containing ionic liquid acted not only as the
activity has also been limited by its wide band gap and the high solvent and the template, but also as the Cl source and the Cu
recombination of the photogenerated electron–hole pairs. Some source. The photocatalytic activity of the Cu-modied BiOCl
strategies have been exploited to expand the visible light materials was evaluated using methyl blue (MB) and bisphenol
absorption range and improve the quantum efficiency of BiOCl, A (BPA) as target pollutants in H2O2 system. The relationship
in order to enhance its photocatalytic activity. For instance, between the photocatalytic activity and the structural properties
Chang et al.20 used a narrow band gap material NaBiO3 coupled of the Cu-modied BiOCl materials was discussed. The reasons
with BiOCl to enlarge its visible-light-absorption ability. Gao for the enhanced photocatalytic activity of Cu/BiOCl were also
et al.21 dispersed BiOCl nanoparticles on the both surfaces of carefully illustrated.
graphene sheets to form a sandwich-like structure via a facile
chemical-bath method. The unique 2D carbon nanostructure
and the C–Bi chemical coupling in the graphene/BiOCl nano-
composite increased the separation efficiency of the electron–
2. Experimental section
2.1. Preparation of the photocatalysts
hole, and then enhanced its ability to photodegrade methyl-
2.1.1. Material and sample preparation. All chemicals were
benzene under UV irradiation, compared with bare BiOCl. It has analytical grade and used as received without purication.
been proved that the metal modication can enhance the Commercially available 30 wt% H2O2 was purchased from Sino-
structure and morphology of photocatalysts, leading to the pharm Chemical Reagent Co., Ltd. The ionic liquid 1-octyl-3-
reduction of electron–hole recombination and an improvement methylimidazolium chloride ([Omim]Cl) (99%) was purchased
of the photocatalytic activity.22–24 To the best of our knowledge, from Shanghai Chengjie Chemical Co. Ltd.
the relevant information regarding the application of metal
modied BiOCl in visible light photocatalytic degradation of imidazolium copper trichloride ([Omim]CuCl3). 0.01 mol of
organic pollutants has seldom been reported.
[Omim]Cl and 0.01 mol of CuCl2 were mixed and added into a
2.1.2. Preparation of the ionic liquid 1-octyl-3-methyl-
Ionic liquids (ILs) have received a great deal of attention as three-necked ask with stirring. The mixture was stirred at
new types of environmentally-friendly reaction media, due to room temperature for 24 h, and then the product [Omim]CuCl3
their promising properties such as negligible vapor pressure, was collected and dried under a vacuum.
wide electrochemical window, wide temperature range, good
2.1.3. Preparation of BiOCl. BiOCl powder was synthesized
dissolving ability, high thermal stability, high ionic conduc- by a one-step solvothermal method according to ref. 13. In a
tivity, non-ammability, etc.25,26 The application of ILs in typical procedure, 1 mmol of Bi(NO3)3$5H2O was dissolved into
inorganic nanomaterial preparation has received increasing an EG solution (20 mL) containing 1 mmol of ionic liquid
attention in recent years. Many inorganic nanostructures [Omim]Cl, and the mixture was stirred for 30 min, and then
have been fabricated through various IL-involved processes, transferred into 25 mL Teon-lined autoclave up to 80% of the
including high-quality TiO2 nanocrystals,27 rod-like, star-like total volume. The autoclave was then heated at 140 ꢀC for 24 h
and ower-like ZnO nano structures,28 Bi2S3, Sb2S3 nano- and cooled down to room temperature. The nal product was
rods,29 and ultrathin SmVO4 nanosheets.30 Among them, separated by centrifugation, washed with distilled water and
Lewis acidic ILs based on 1-alkyl-3-methylimidazolium metal absolute ethanol for four times, and dried under vacuum at
ꢀ
chloride ([Cnmim]Cl/MCl2) (M ¼ Zn, Fe, Cu, Mg, Ni, Co, etc.) 50 C for 24 h before further characterization.
have also been used in the synthesis of inorganic nano-
2.1.4. Preparation of Cu-modied BiOCl. Cu-modied
materials. These Lewis acidic metal-based ILs not only have BiOCl materials were prepared in a method similar to the
the general advantages of the conventional ionic liquids, but previously used solvothermal method. In a typical procedure,
also possess some unique characteristics, such as strong 1 mmol of Bi(NO3)3$5H2O was dissolved in an EG solution
magnetic eld response, moisture-stable, intermediate Lewis (20 mL) containing stoichiometric amounts of ionic liquid
acidity, etc.31–35 For example, Taubert36 introduced a novel [Omim]CuCl3, and the mixture was stirred for 30 min and then
protocol for the controlled synthesis of CuCl nanoplatelets transferred into 25 mL Teon-lined autoclave up to 80% of the
with a well-developed crystal structure and a tunable particle total volume. The autoclave was then heated at 140 ꢀC for 24 h
size and synthesized them from the Cu-containing ionic and cooled down to room temperature. The nal product was
liquid. It was possible to tune the particle size, thickness, and separated by centrifugation, washed with distilled water and
ꢀ
connectivity by varying the reaction temperature. The absolute ethanol four times, and dried under vacuum at 50 C
approach provided a simple and green method to tune the for 24 h before further characterization. Cu-modied BiOCl
properties of the CuCl nanocrystals and assemble them for materials with different Cu modication ratios were synthe-
applications in catalysis. However, few studies have been sized by adding 1/3 mmol, 1 mmol, 5 mmol and 10 mmol of
reported to synthesize inorganic nanomaterials via these [Omim]CuCl3. The nal products were marked as Cu/BiOCl
metal-based ILs. The use of metal-based ILs in inorganic (1 : 3), Cu/BiOCl (1 : 1), Cu/BiOCl (5 : 1) and Cu/BiOCl (10 : 1)
synthesis still needs to be fully exploited.
according to the different Cu/Bi molar ratios.
14282 | RSC Adv., 2014, 4, 14281–14290
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
with an electrochemical analyzer (CHI660B, Chen Hua Instru-
ments, Shanghai, China) in a standard three-electrode system,
which employed a platinum wire as the counter electrode, a
saturated Ag/AgCl electrode as the reference electrode, and a
indium tin oxide (ITO) glass as the working electrode, respec-
tively. The ITO was cut into 3 cm ꢂ 1 cm slices and successively
bathed in 1 M NaOH solution for 10 min and acetone for
30 min, and then it was washed with water and dried prior to
use. 10 mg of the sample was ultrasonically dispersed in 1 mL of
absolute ethanol and 1 mL EG, and 20 mL of the resulting
colloidal dispersion (5 mg mLꢁ1) was then dip-coated onto a
0.5 ꢂ 1 cm2 ITO glass electrode. The photocurrent measure-
ments were performed at a constant potential of 0 V. A phos-
phate buffer solution (PBS, 0.1 M, pH 7.4) was prepared by
mixing stock standard solutions of Na2HPO4 and NaH2PO4, and
adjusting the pH with 0.1 M NaOH or H3PO4, which was used as
the supporting electrolyte throughout the photoelectrochemical
measurements. A 500 W Xe arc lamp was utilized as the light
source for the photoelectrochemical measurement. All the
electrochemical tests were carried out at room temperature
under an air atmosphere.
2.2. Characterization
X-ray powder diffraction (XRD) analysis was carried out on a
Bruker D8 diffractometer with high-intensity Cu-Ka (l ¼ 1.54 A).
˚
X-ray photoelectron spectroscopy (XPS) was measured on a
PHI5300 with a monochromatic Mg Ka source to explore the
elements on the surface. The eld-emission scanning electron
microscopy (FE-SEM) measurements were carried out with a
eld-mission scanning electron microscope (JEOL JSM-7001F)
equipped with an energy-dispersive X-ray spectroscope (EDS)
which was operated at an acceleration voltage of 10 kV. Raman
spectra were recorded at room temperature using a micro-
Raman spectrometer (Renishaw Invia) in a backscattering
geometry with a 532 nm laser as an excitation source. The
photoluminescence (PL) spectra of the samples were obtained
by a QuantaMaster & TimeMaster Spectrouorometer with an
excitation wavelength at 360 nm. UV-vis diffuse reectance
spectroscopy (DRS) was recorded on an UV-2450 spectropho-
tometer (Shimadzu Corporation, Japan).
2.3. Photocatalytic activity
Photocatalytic activities of the Cu-modied BiOCl powders were
evaluated by photocatalytic degradation of MB and BPA under
visible light irradiation. Experiments were carried out in a Pyrex
photocatalytic reactor under 300 W Xe lamp with a 400 nm
cutoff lter. Aeration was performed by using an air pump to
ensure a constant supply of oxygen and to complete mixing of
the solution and the photocatalyst during photoreactions. In a
typical photocatalytic experiment, 10 mg and 50 mg of Cu-
modied BiOCl powders were dispersed into 100 mL of MB
solution (10 mg Lꢁ1) and BPA solution (10 mg Lꢁ1), respectively.
The suspension was strongly and magnetically stirred in the
dark for 30 min to reach the adsorption–desorption equilibrium
of pollutant on the catalyst surface. Then, about 3 mL of the
suspension was taken from the reaction cell. Visible light illu-
mination was conducted aer 3 mL of H2O2 (30%) was added
into the suspension. During irradiation, about 3 mL of the
suspension was continually taken from the reaction cell at xed
intervals. The photocatalyst powders and the MB or BPA solu-
tion were separated by centrifuge. The MB concentration was
analyzed through a UV-vis spectrophotometer (UV-2450, Shi-
madzu) by checking the absorbance at 663 nm. For the
concentration of BPA, high performance liquid chromatog-
raphy (HPLC) was used to analyze the remnant amounts.
3. Results and discussion
3.1. Structure and morphology characterization
The XRD pattern of the as-prepared Cu/BiOCl (5 : 1) material is
shown in Fig. 1. All the peaks for the samples were readily
indexed to the tetragonal phase of BiOCl (JCPDS card no. 06-
˚
˚
0249), with lattice constants of a ¼ 3.891 A, c ¼ 7.369 A. The
narrow sharp peaks suggested that the Cu/BiOCl products were
highly crystalline. Interestingly, the intensity ratio of the (110)
peak to the (012) peak in this result of Cu-modied BiOCl
microspheres structures was larger than the data of the BiOCl
JCPDS card. The result indicated that the structure of Cu/BiOCl
(5 : 1) crystal had special anisotropic growth along the (110)
plane. However, no typical patterns of copper species were
observed in Cu-modied BiOCl, which was probably due to the
ions of Cu being modied on the surface of the BiOCl.
2.4. HPLC Analysis
The HPLC setup was equipped with two Varian ProStar210
pumps, an Agilent TC-C (18) column, and a Varian ProStar325
UV-Vis Detector at 230 nm. A solution of methanol and H2O in
the ratio 75 : 25 (v/v) was used as the mobile phase at 1 mL
minꢁ1, and 20 mL of the sample solution was injected.
2.5. Photoelectrochemical measurement
To investigate the photoelectrochemical properties of Cu-
modied BiOCl powders, the modied electrodes were
prepared. The photocurrent measurements were carried out Fig. 1 The XRD pattern of the as-prepared Cu/BiOCl (5 : 1) crystals.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 14281–14290 | 14283

View Article Online
RSC Advances
Paper
Therefore, in the XRD analysis, no impurity peaks were 1s, which is the characteristic of O2ꢁ in the Cu-modied BiOCl
observed, which conrmed the high purity of the Cu-modied materials. The above results are in good agreement with the
BiOCl products.
literature.37 Meanwhile, Fig. 2e showed the high-resolution Cu
The surface chemical composition of the Cu/BiOCl (5 : 1) 2p XPS spectra of the samples. The Cu 2p XPS peaks could be
nanostructures is analysed by XPS spectroscopy (Fig. 2). The resolved into two typical peaks, 933.75 eV and 954.28 eV, and
binding energies obtained in the XPS analysis is corrected for they were ascribed to binding energies of Cu 2p3/2 and Cu 2p1/2
,
specimen charging by internally referring to the adventitious respectively, which was due to there being Cu2+ in Cu-modied
carbon at a binding energy of 284.6 eV. The survey XPS spec- BiOCl material.38 Based on the XRD and XPS analyses, the Cu
trum (Fig. 2a) shows that the Cu-modied BiOCl microspheres element existed in the form of Cu2+, and the Cu2+ was distrib-
are composed of elements of Bi, O, Cl and Cu. The carbon peak uted on the surface of BiOCl in Cu-modied BiOCl materials.
came from the adventitious carbon and some trace of the EG in
The surface morphology of the as-prepared samples was
the sample. In the high-resolution spectrum (Fig. 2b), it can be examined by scanning electron microscopy. The pure BiOCl
seen that two strong peaks at 159.27 eV and 164.64 eV are samples have been reported in our previous work.13 The BiOCl
assigned to Bi 4f7/2 and Bi 4f5/2, which is characteristic of Bi3+ in was made up of irregular sphere-like structures, with an average
the Cu-modied BiOCl material. The XPS signal of Cl 2p diameter of 1–3 mm. There was a rugged area assembled in a
(Fig. 2c) at a binding energy of 198.56 eV shows the character- scattered manner by numerous BiOCl nanosheets on the
istic of Clꢁ anions. The high-resolution spectrum (Fig. 2d) surface of every BiOCl microsphere. Fig. 3 is the SEM images of
shows that the peak binding energy of 530.76 eV is assigned to O Cu/BiOCl (5 : 1) materials. From the SEM picture in Fig. 3a, it
can be clearly seen that uniform sphere-like Cu/BiOCl hierar-
chical structures were formed, and the average diameter of
the microspheres was 3–5 mm. Aer careful examination of the
high-magnication SEM images (Fig. 3b) of the sample, the
entire Cu/BiOCl microsphere structures were tightly assembled
by numerous Cu/BiOCl nanosheets with the diameter of several
nanometers. Moreover, the surface of Cu/BiOCl microsphere
was regular, without rugged areas, which was different from
pure BiOCl. It can be seen that metal-based ionic liquid [Omim]-
CuCl3 as a so template agent played a critical role in control-
ling the crystal structure and morphology of the Cu/BiOCl
microspheres. In our previous work,13 we proved that the ionic
liquid [C16mim]Cl acted as a so template, a capping agent and
the Cl source for the synthesis of pure BiOCl. In the synthesis of
Cu-modied BiOCl, metal-based ionic liquid [Omim]CuCl3 not
Fig. 3 SEM images of the Cu/BiOCl (5 : 1) microspheres structures (a)
Fig. 2 XPS spectra of the as-prepared Cu/BiOCl (5 : 1) material: (a) the low magnification SEM image; (b) top view SEM image; (c) EDS of
survey of the sample; (b) Bi 4f; (c) Cl 2p; (d) O 1s; (e) Cu 2p.
the Cu/BiOCl (5 : 1) microspheres.
14284 | RSC Adv., 2014, 4, 14281–14290
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
only acts as the solvent, the template and the Cl source, but also liquid could be easily removed from the surface of the material
acts as the metal Cu source. Fig. 3c is the EDS pattern of by washing with water and alcohol.
Cu/BiOCl (5 : 1) materials, and it indicates that the Cu-modied
BiOCl materials contain Bi, O, Cl and Cu elements, and the Si
element is attributable to the Si foil. This result further proved
that the as-prepared samples were Cu-modied BiOCl mate-
rials, which was consistent with the results of the XRD and
XPS analyses.
3.2. Photoelectrochemical properties
As we all know, the separation efficiency of electrons and holes
plays a key role in the photocatalytic reaction: the better the
electron and hole separation efficiency is, the higher the pho-
tocatalytic activity would be.42,43 The photocurrent is from the
photo-generated electrons in the conducting bands of a semi-
conductor photocatalyst when the semiconductor is excited by
visible light irradiation. Hence, the photocurrent test of a
semiconductor can be used to evaluate the separation efficiency
of electrons and holes. The transient photocurrent responses of
BiOCl and Cu/BiOCl material electrodes are recorded for several
on–off cycles of visible light irradiation, as is shown in Fig. 5.
Upon light illumination, the photocurrent increases sharply
reaching a steady state quickly. When the light is turned off, the
current returns quickly to its dark current state. The Cu-modi-
ed BiOCl samples have higher currents than that of the pure
BiOCl. Moreover, the photocurrent responses of Cu-modied
BiOCl samples increase gradually with the amount of Cu2+ from
1 : 3 to 5 : 1 (Cu/Bi molar ratios) and then the photocurrent
begins to decline. The above analysis revealed that the Cu-
modied BiOCl materials could effectively reduce the recom-
bination of photo-generated electrons and holes, and produce
longer living photo-generated carriers. Therefore, the Cu-
modied BiOCl materials were expected to exhibit an improved
photocatalytic activity for organic pollutant degradation.
The Raman spectra of the BiOCl and Cu/BiOCl (5 : 1) mate-
rials are shown in Fig. 4. Four Raman peaks of the BiOCl sample
were observed, and among them, the peak at 113.4 cmꢁ1 was
featured with broad full width half maximum (FWHM), and it
could not be ascribed to any ordinary Raman bands of BiOCl
and silicon reasonably.39 The peaks at 143.3 cmꢁ1, 197.3 cmꢁ1
and 396 cmꢁ1 were the characteristic peaks of BiOCl, which
could be ascribed to three different Raman modes of BiOCl.40
The band at 143.3 cmꢁ1 was attributed to A1g internal Bi–Cl
stretching mode and the band at 197.3 cmꢁ1 was assigned to E1g
internal Bi–Cl stretching mode.39 In addition, there was a broad
weak band at 396 cmꢁ1, which was attributed to the Eg and B1g
modes involving the motion of the oxygen atoms.41 For
Cu-modied BiOCl materials, the peak at 113.4 cmꢁ1 dis-
appeared, the two peaks at 199.2 cmꢁ1 and 397.9 cmꢁ1 had a
little blue-shi when compared with those of pure BiOCl, which
was attributed to the laser-induced compressive stress in the
BiOCl lm.41 In general, the disappearance of the Raman peak
(113.4 cmꢁ1) and the blue-shi indicated that the modied Cu2+
had interacted with the BiOCl crystal, and the modication of
Cu2+ had changed the properties of the BiOCl nanostructure.
The composition of the Cu/BiOCl materials was further exam-
ined by FT-IR spectroscopy. As shown in Fig. S1,† it could be
seen that the absorption band of pure BiOCl at 528 cmꢁ1 was
ascribed to the Bi–O stretching mode. For the Cu/BiOCl (5 : 1), it
can be observed that a little shi of the 528 cmꢁ1 characteristic
peak happened, which further conrms that there is an intense
interaction between the Cu2+ and BiOCl materials. As shown in
the FT-IR that, no characteristic absorption peaks of the ionic
liquid are found in the FT-IR spectra. This shows the ionic
3.3. Photoluminescence spectra analysis
It is well known that the semiconductor can be excited to
produce photo-generated electrons and holes when the energy
of the incident photons matches or exceeds the bandgap.1,44
Some electrons and holes recombine on the surface or inside of
the semiconductor, dissipating the input energy in the form of
heat or emitted light, and then the uorescence is generated.
So, the stronger the uorescence emission intensity is, the
Fig. 5 Transient photocurrent responses of BiOCl and Cu/BiOCl
Fig. 4 Raman spectra of BiOCl and Cu/BiOCl (5 : 1) photocatalysts.
photocatalysts.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 14281–14290 | 14285

View Article Online
RSC Advances
Paper
higher the recombination rate of the photo-generated carriers
will be.45,46 Fig. 6 shows the photoluminescence spectra of the
pure BiOCl and Cu/BiOCl (5 : 1) materials excited by 360 nm
irradiation. It could be found that the main emission peak was
centered at about 468 nm for the pure BiOCl sample. For
Cu-modied BiOCl materials, the position of the emission peak
in the PL spectrum was similar to that of the pure BiOCl, but the
emission intensity decreased, which indicated that the
Cu/BiOCl (5 : 1) materials had a much lower recombination rate
of photo-generated charge carriers. This demonstrated that the
recombination of photo-generated charge carriers was greatly
inhibited by the introduction of Cu2+, which showed the photo-
generated electrons and holes in the Cu/BiOCl (5 : 1) materials
had higher separation efficiency than those in the pure BiOCl.
This result also indicated that the Cu-modied BiOCl materials
were expected to exhibit an improved photocatalytic activity for
organic pollutant degradation.
Fig. 7 XRD patterns of Cu/BiOCl samples with different Cu contents.
of approximately 2–5 mm. The high-magnication SEM image
(Fig. 8c) showed that the surface of the Cu/BiOCl (1 : 1) micro-
spheres was smooth and there was no evidence for the presence
of nanosheets, which was different from the Cu/BiOCl (5 : 1)
material. EDS analysis shows that the sample are composed of
Bi, O, Cl and Cu elements (Fig. 8d). Fig. 9 displays the SEM
images of the as-synthesized Cu/BiOCl (10 : 1) sample. It could
be seen that the morphology of the sample was that of a cluster-
like structure assembled by scattered nanosheets, which could
not form the sphere-like architecture. The EDS analysis on the
Cu/BiOCl (10 : 1) microspheres (Fig. 9d) suggested that the
sample only contained the elements of Bi, O, Cl and Cu. From
the SEM images of different materials, it could be found that
microspheres with smooth surface were formed when a small
amount of the ionic liquid was added; when the amount of the
ionic liquid was increased, the surface of the microspheres
became a ower-like hierarchical structure; with the amount of
3.4. Inuence of the amount of the ionic liquid
In order to investigate the inuence of the amount of the ionic
liquid, the XRD and SEM analyses of the as-prepared samples
with different Cu/Bi molar ratios were conducted. Fig. 7 shows
the XRD patterns of Cu/BiOCl (1 : 3), Cu/BiOCl (1 : 1) and
Cu/BiOCl (10 : 1) materials. It could be seen that all the peaks
for the three samples were readily indexed to the tetragonal
phase of BiOCl (JCPDS card no. 06-0249), with lattice constants
˚
˚
of a ¼ 3.891 A, c ¼ 7.369 A. No impurity could be detected from
these patterns of BiOCl. This indicated that well-crystallized
BiOCl could be easily obtained under the current synthetic
conditions. The structures of the three Cu/BiOCl crystals also
had special anisotropic growth along the (110) plane like the
Cu/BiOCl (5 : 1) materials. No any diffraction peaks of copper
species were observed in the three Cu-modied BiOCl materials
because of the Cu2+ was modied on the surface of the BiOCl.
The SEM images of the Cu/BiOCl (1 : 1) material are shown
in Fig. 8. As can be seen from Fig. 8a and b, the Cu/BiOCl (1 : 1)
material had a uniform sphere-like architecture with diameters
Fig. 8 SEM images of the Cu/BiOCl (1 : 1) microspheres structures. (a)
The low magnification SEM image; (b) the high magnification SEM
image; (c) top view SEM image; (d) EDS of the Cu/BiOCl (1 : 1)
microspheres.
Fig. 6 PL spectra of the pure BiOCl and Cu/BiOCl (5 : 1) materials.
14286 | RSC Adv., 2014, 4, 14281–14290
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
in the visible light region resulted from the introduction of
Cu2+. Especially in the absorption with wavelengths higher than
about 550 nm, the optical absorption intensity showed a large
increase. This suggested that the Cu-modied BiOCl samples
should have better photoactivity than the pure BiOCl. The band
gap of the powders, calculated by means of Kubelka–Munk
theory, has been presented (Fig. S2†). As Fig. S2† shows, the Cu-
modied BiOCl samples have a smaller band gap than BiOCl.
The lower band gaps facilitate the electronic transition.
3.6. Photocatalytic activity
H2O2 is a strong oxidizing agent, and it can be decomposed into
H2O and O2, which will not cause secondary pollution when used
in water treatment. Many metal ions, such as Fe2+, Fe3+, Cu2+,
Mn2+, Zn2+, Ni2+ etc., can catalyze the decomposition of H2O2 to
produce hydroxyl radicals (cOH). Hydroxyl radicals are a kind of
non-selective strong oxidants, which can degrade a variety of
organic pollutants to generate CO2 and H2O. Therefore, to
investigate the photocatalytic activity of Cu-modied BiOCl
materials for degradation of methylene blue (MB), H2O2 was
employed in the photocatalytic experiment. Fig. 11a shows the
photocatalytic activity of the BiOCl and Cu-modied BiOCl
materials with different contents of Cu under visible light irra-
diation. Blank tests were made to eliminate interference: (a) the
direct photolysis of MB with H2O2 in the absence of the photo-
catalyst; (b) degradation of MB with the Cu/BiOCl (5 : 1) samples
and H2O2 in the absence of visible light. As can be seen from
Fig. 11, both blank tests showed low photocatalytic activity. It
indicated that light illumination was essential for photocatalytic
reaction. The degradation of MB with H2O2 was low. Aer pure
BiOCl was added, the activity was slightly improved, about 44%
of MB was degraded for 75 min. However, when the BiOCl
samples were modied by Cu2+, the photocatalytic activity was
signicantly increased. Aer 75 min of visible light irradiation,
the photocatalytic degradation efficiency of MB was about 98%,
96%, 100% and 93% for Cu/BiOCl (1 : 3), Cu/BiOCl (1 : 1), Cu/
BiOCl (5 : 1) and Cu/BiOCl (10 : 1), respectively. Among them,
the Cu/BiOCl (5 : 1) samples displayed the highest efficient
photocatalytic activity, in which all of MB was degraded under
visible light irradiation within 45 min.
Fig. 9 The SEM images (a–c) and EDS (d) of the as-prepared Cu/BiOCl
(10 : 1) crystals.
the ionic liquid further increased, sphere-like architecture
could not be formed, and a cluster-like structure was formed. It
revealed that the amount of the ionic liquid played an impor-
tant role in the morphology of the products. Furthermore, it
could be predicted that the Cu/BiOCl (5 : 1) sample may has the
highest photocatalytic activity because the specic surface area
of the Cu/BiOCl (5 : 1) material was large, which could form
more active sites in photocatalytic reaction and then enhance
the photocatalytic activity.
3.5. UV-vis diffuse reectance spectra
The UV-vis diffuse reectance spectra (DRS) of the Cu/BiOCl
samples with different Cu contents are shown in Fig. 10. It was
apparent that the absorption edge of the pure BiOCl was about
370 nm. The Cu2+ modication effectively increased the
absorption in the visible light region. The colour of BiOCl is
white, and the colour of Cu–BiOCl is green. The light absorption
Fig. 11b shows the time-dependent absorption spectra of MB
solution in the presence of the Cu/BiOCl (5 : 1) samples. Under
visible light illumination, with the Cu/BiOCl (5 : 1) materials,
the color of MB solution changed from blue to light blue and
then disappeared during the reaction. The peak intensity of the
UV-vis absorption related to MB decreased sharply, and the
main absorption band at 663 nm almost completely dis-
appeared aer 45 min, which indicated the chromophoric
structure of the MB dye was decomposed.
In order to rule out the photosensitization of the Cu/BiOCl
photocatalyst in the photodegradation process, BPA was chosen
as a representative colourless model organic pollutant. Fig. S3†
shows the photocatalytic degradation of BPA in the presence of
BiOCl and Cu/BiOCl (5 : 1) under visible light irradiation with or
without 3 mL H2O2. When no H2O2 was added, both the BiOCl
and Cu/BiOCl (5 : 1) exhibit poor photocatalytic activity for the
Fig. 10 UV-vis diffuse absorption spectra of Cu/BiOCl samples with
different Cu contents.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 14281–14290 | 14287

View Article Online
RSC Advances
Paper
many factors, such as crystallinity, surface properties,
morphology, optical properties, and size.47 According to the XRD
and SEM analysis, the crystal structure was not changed, which
had little effect on improving the photocatalytic activity.
However, the morphology of the Cu-modied BiOCl materials
was regular, and more pollutant molecules could be adsorbed on
the surface of the Cu-modied BiOCl microspheres. The pho-
tocatalytic reaction involves two processes: the adsorption
process and photocatalytic degradation process aer adsorp-
tion. Therefore, the stronger the adsorption capacity was, the
better the subsequent photocatalytic degradation reaction would
be. The nitrogen adsorption and desorption isotherm analysis
(Fig. S5†) was carried out to characterize the specic surface
areas of Cu/BiOCl (5 : 1) and pure BiOCl.13 The BET specic
surface area of Cu/BiOCl (5 : 1) and BiOCl was calculated to be
36.6 m2 gꢁ1 and 16.1 m2 gꢁ1
,
respectively. It is known that
13
materials with larger specic surface areas can absorb more
active species and reactants on their surfaces. The results indi-
cate that the introduction of Cu2+ exaggerates the BET surface
area which is benecial to the improvement of photocatalytic
activity. The DRS analysis suggested that Cu2+ modication
effectively increased the absorption in the visible light region.
This meant that the Cu-modied BiOCl could absorb more
photons under the same light illumination conditions, and thus
the photocatalytic activity could be enhanced.
Recently, many researchers suggested that the modied Cu2+
was the capture center of photo-generated electrons, which can
reduce the recombination rate of photo-generated charge
carriers.48,49 For the Cu-modied BiOCl materials, the modied
Cu2+ can improve the inner charge transfer rate, while it can
effectively prevent the recombination of photo-generated elec-
trons and holes, and then the subsequent reaction can be
promoted. Moreover, the outer electron structure of Cu2+ is
3d94s0, which can turn into a more stable fully charged state Cu+
with a 3d104s0 structure. In this way, Cu2+ is combined with
electrons to increase the separation efficiency of photo-gener-
ated electrons and holes. Alternately, the simultaneous redox
process of trapping electrons in the Cu2+ states leads to Cu+ in
the following reaction:
Fig. 11 (a) Effects of Cu/BiOCl with different Cu contents on MB
degradation efficiency; (b) temporal UV-vis absorption spectral
changes during the photocatalytic degradation of MB in aqueous
solution.
degradation of BPA. When the H2O2 was added, the photo-
catalytic activity of photocatalysts improved. It can be seen that
the Cu/BiOCl (5 : 1) sample exhibits signicantly higher pho-
tocatalytic activity than pure BiOCl sample. The introduction of
Cu2+ is benecial to improve the photocatalytic activity.
To conrm the stability of the Cu/BiOCl sample, the XRD of
the Cu/BiOCl (5 : 1) sample before and aer the photoreaction
has been compared. The XRD patterns were performed to
determine the structural variations of the sample. The results
reveal that the crystal structures of the Cu/BiOCl sample do not
change aer the photocatalytic reaction (Fig. S4†). These results
indicate that the Cu/BiOCl sample has excellent stability for
photocatalytic degradation of BPA.
Cu2+ + eꢁ / Cu+
(1)
(2)
H2O2 + Cu+ / OHꢁ + cOH + Cu2+
During the photo-Fenton process, more and more cOH
radicals will be formed, and nally the pollutants will be directly
oxidized in the aqueous solution by the active species.50,51 The
XPS of the Cu/BiOCl (5 : 1) aer the photoreaction was shown in
Fig. S6.† The peaks at 932.4 eV and 952.3 eV indicate the pres-
ence of Cu+ as previously reported by others52–54 whereas the
existence of peaks at 934.7 eV and 954.9 eV can be attributed to
the presence of Cu2+
.
The XPS analysis further certies the
53,55
3.7. The proposed mechanism
photo-Fenton process mentioned above. However, the photo-
In the photocatalytic activity analysis of the samples, it was catalytic activity will be reduced when the amount of modied
conrmed that the Cu-modied BiOCl could signicantly Cu2+ is excessive. This is because of the excessive Cu2+
enhance the photocatalytic degradation efficiency. Generally consumes a large amount of electrons, on the other hand,
speaking, the photocatalytic activity of catalysts depends on photo-generated electrons and holes can recombine through
14288 | RSC Adv., 2014, 4, 14281–14290
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
the modied Cu2+ on the surface of the BiOCl microspheres. At
the same time, the existing literature studies show that the
introduction of metal ion could cause oxygen vacancies in the
BiOCl crystals and the oxygen vacancies are stable and ener-
getically favorable within BiOCl.48,49,56–58 Therefore, it can be
inferred that the introduction of Cu2+ could cause oxygen
vacancies in BiOCl crystals. The occurrence of an oxygen
vacancy in the BiOCl crystal signicantly inuences the neigh-
boring Bi 6p states, forming a capture center of photo-excited
electrons in the forbidden band, which enhances the efficient
mobility of photo-generated carriers and improves the effective
separation rate of electron–hole pairs.56–58 The enhanced sepa-
ration rate of electron–hole pairs is benecial to the Cu2+
combines with electrons to form Cu+, which accelerates the
photo-Fenton process. As is shown in (Fig. S3 and S8†), the
Cu/BiOCl have a little higher photocatalytic activity than BiOCl
when no H2O2 was added, which indicates the generated oxygen
vacancy improves the separation rate of electron–hole pairs
and further enhances the photocatalytic activity. However, the
photocatalytic activity of the Cu/BiOCl had a signicant
improvement than pure BiOCl when H2O2 was added, which
indicates the synergetic effect of generated oxygen vacancy and
photo-Fenton process during the photodegradation. Based on
the above analysis, it can be concluded that the enhanced
photocatalytic activity of Cu-modied BiOCl microspheres can
be due to synergetic effects, including a large adsorption
capacity, a broad light absorption band and a high separation
efficiency of photo-generated electrons and holes.
References
1 M. R. Hoffmann, S. T. Martin, W. Choi and
D. W. Bahnemann, Chem. Rev., 1995, 95, 69.
2 R. M. Alberici and W. F. Jardim, Appl. Catal., B, 1997, 14, 55.
3 M. A. Shannon, P. W. Bohn, M. Elimelech, J. G. Georgiadis,
B. J. Marinas and A. M. Mayes, Nature, 2008, 452, 301.
4 L. F. Zhang, T. Kanki, N. Sano and A. Toyoda, Sep. Purif.
Technol., 2003, 31, 105.
5 F. Spadavecchia, G. Cappelletti, S. Ardizzone, C. L. Bianchi,
S. Cappelli, C. Oliva, P. Scardi, M. Leoni and P. Fermo,
Appl. Catal., B, 2010, 96, 314.
6 J. H. Chen, M. S. Yao and X. L. Wang, J. Nanopart. Res., 2008,
10, 163.
7 L. Pan, J. J. Zou, X. W. Zhang and L. Wang, Ind. Eng. Chem.
Res., 2010, 49, 8526.
8 D. V. Bavykin, A. A. Lapkin, P. K. Plucinski, L. Torrente-
Murciano, J. M. Friedrich and F. C. Walsh, Top. Catal.,
2006, 39, 151.
9 W. T. Sun, Y. Yu, H. Y. Pan, X. F. Gao, Q. Chen and
L. M. Peng, J. Am. Chem. Soc., 2008, 130, 1124.
10 L. W. Zhang, Y. J. Wang, H. Y. Cheng, W. Q. Yao and
Y. F. Zhu, Adv. Mater., 2009, 2, 1286.
11 J. X. Xia, S. Yin, H. M. Li, H. Xu, Y. S. Yan and Q. Zhang,
Langmuir, 2011, 27, 1200.
12 J. X. Xia, S. Yin, H. M. Li, H. Xu, L. Xu and Y. G. Xu, Dalton
Trans., 2011, 40, 5249.
13 J. X. Xia, J. Zhang, S. Yin, H. M. Li, H. Xu, L. Xu and Q. Zhang,
J. Phys. Chem. Solids, 2013, 74, 298.
14 M. S. Zhu, P. L. Chen and M. H. Liu, Prog. Chem., 2013, 25,
209.
4. Conclusions
15 J. Lv, T. Kako, Z. G. Zou and J. H. Ye, J. Mater. Res., 2010, 25, 159.
16 X. X. Hu and C. Hu, J. Solid State Chem., 2007, 180, 725.
17 S. S. Dunkle, R. J. Helmich and K. S. Suslick, J. Phys. Chem. C,
2009, 113, 11980.
18 J. Henle, P. Simon, A. Frenzel, S. Scholz and S. Kaskel, Chem.
Mater., 2007, 19, 366.
19 X. Zhang, Z. H. Ai, F. L. Jia and L. Z. Zhang, J. Phys. Chem. C,
2008, 112, 747.
20 X. F. Chang, G. Yu, J. Huang, Z. Li, S. F. Zhu, P. F. Yu,
C. Cheng, S. B. Deng and G. B. Ji, Catal. Today, 2010, 153,
193.
21 F. D. Gao, D. W. Zeng, Q. W. Huang, S. Q. Tian and C. S. Xie,
Phys. Chem. Chem. Phys., 2012, 14, 10572.
22 S. Sun, J. J. Ding, J. Bao, C. Gao, Z. M. Qi, X. Y. Yang, B. He
and C. X. Li, Appl. Surf. Sci., 2012, 258, 5031.
23 J. Choi, H. Park and M. R. Hoffmann, J. Phys. Chem. C, 2010,
114, 783.
24 X. X. Yang, C. D. Cao, L. Erickson, K. Hohn, R. Maghirang
and K. Klabunde, Appl. Catal., B, 2009, 91, 657.
25 J. A. Dahl, B. L. S. Maddux and J. E. Hutchison, Chem. Rev.,
2007, 107, 2228.
Visible-light-driven Cu-modied BiOCl materials were synthe-
sized by using a Cu-based ionic liquid as the solvent, the
template, the Cl source and the Cu source. The metal copper
was evenly distributed on the surface of the BiOCl microspheres
in the form of Cu2+. It was possible to tune the morphology of
Cu-modied BiOCl materials by varying the amount of the ionic
liquid. The high photocurrent and low PL intensity indicated
that the Cu-modied BiOCl materials could effectively reduce
the recombination of photo-generated electrons and holes. Cu2+
modication effectively increased the absorption in the visible
light region. The as-prepared Cu-modied BiOCl materials
displayed an enhancement of photocatalytic activity, which was
greatly improved in comparison with the pure BiOCl. In
summary, the high photocatalytic activity of the Cu-modied
BiOCl materials could be attributed to the large adsorption
capacity, broad light absorption band and high separation
efficiency of photo-generated electrons and holes.
Acknowledgements
This work was nancially supported by the National Nature
Science Foundation of China (no. 21206060, 21007021 and
21177050), Jiangsu Province (1102118C), and the Special
Financial Grant from the China Postdoctoral Science Founda-
tion (2013T60506).
26 Z. Ma, J. H. Yu and S. Dai, Adv. Mater., 2010, 22, 261.
27 K. L. Ding, Z. J. Miao, Z. M. Liu, Z. F. Zhang, B. X. Han,
G. M. An, S. D. Miao and Y. Xie, J. Am. Chem. Soc., 2007,
129, 6362.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 14281–14290 | 14289

View Article Online
RSC Advances
Paper
28 I. Yavari, A. R. Mahjoub, E. Kowsari and M. Movahedi, 45 J. Di, J. X. Xia, S. Yin, H. Xu, M. Q. He, H. M. Li, L. Xu and
J. Nanopart. Res., 2009, 11, 861.
Y. P. Jiang, RSC Adv., 2013, 3, 19624.
29 Y. Jiang and Y. J. Zhu, J. Phys. Chem. B, 2005, 109, 4361.
30 Y. Sun and W. J. Zheng, Dalton Trans., 2010, 39, 7098.
46 H. Xu, J. Yan, Y. G. Xu, Y. H. Song, H. M. Li, J. X. Xia,
C. J. Huang and H. L. Wan, Appl. Catal., B, 2013, 129, 182.
31 J. Y. Kim, J. T. Kim, E. A. Song, Y. K. Min and H. Hamaguchi, 47 I. S. Cho, S. Lee, J. H. Noh, G. K. Choi, H. S. Jung, D. W. Kim
Macromolecules, 2008, 41, 2886. and K. S. Hong, J. Phys. Chem. C, 2008, 112, 18393.
32 H. Wang, R. Y. Yan, Z. X. Li, X. P. Zhang and S. J. Zhang, 48 H. W. P. Carvalho, M. V. J. Rocha, P. Hammer and
Catal. Commun., 2010, 11, 763.
T. C. Ramalho, J. Mater. Sci., 2013, 48, 3904.
49 B. F. Xin, P. Wang, D. D. Ding, J. Liu, Z. Y. Ren and H. G. Fu,
Appl. Surf. Sci., 2008, 254, 2569.
50 H. W. P. Carvalho, A. P. L. Batista, P. Hammer and
T. C. Ramalho, J. Hazard. Mater., 2010, 184, 273.
51 A. N. Pham, G. W. Xing, C. J. Miller and T. D. Waite, J. Catal.,
2013, 301, 54.
33 K. Bica and P. Gaertner, Org. Lett., 2006, 8, 733.
34 D. H. Yin, C. Z. Li, L. Tao, N. Y. Yu, S. Hu and D. L. Yin, J. Mol.
Catal. A: Chem., 2006, 245, 260.
35 J. G. Li, Y. F. Hu, S. F. Sun, S. Ling and J. Z. Zhang, J. Phys.
Chem. B, 2012, 116, 6461.
36 A. Taubert, Angew. Chem., Int. Ed., 2004, 43, 5380.
37 J. Zhang, J. X. Xia, S. Yin, H. M. Li, H. Xu, M. Q. He, 52 S. W. Zou, C. W. How and J. P. Chen, Ind. Eng. Chem. Res.,
L. Y. Huang and Q. Zhang, Colloids Surf., A, 2013, 420, 89. 2007, 46, 6566.
38 W. D. Shi, J. Q. Shi, S. Yu and P. Liu, Appl. Catal., B, 2013, 53 D. Spasiano, L. D. P. Rodriguez, J. C. Olleros, S. Malato,
138, 184.
R. Marotta and R. Andreozzi, Appl. Catal., B, 2013, 136–
137, 56.
54 C. C. Chusuei, M. A. Brookshier and D. W. Goodman,
Langmuir, 1999, 15, 2806.
55 S. P. Xu, J. W. Ng, A. J. Du, J. C. Liu and D. D. Sun, Int.
J. Hydrogen Energy, 2011, 36, 6538.
56 X. C. Zhang, C. M. Fan, Y. W. Wang, Y. F. Wang, Z. H. Liang
and P. D. Han, Comput. Mater. Sci., 2013, 71, 135.
39 Y. Tian, C. F. Guo, Y. J. Guo, Q. Wang and Q. Liu, Appl. Surf.
Sci., 2012, 258, 1949.
40 W. G. Fateley, N. T. Mcdecitt and F. F. Bentley, Appl.
Spectrosc., 1971, 25, 155.
41 S. H. Cao, C. F. Guo, Y. Lv, Y. J. Guo and Q. Liu,
Nanotechnology, 2009, 20, 275702.
42 J. Di, J. X. Xia, S. Yin, H. Xu, L. Xu, Y. G. Xu, M. Q. He and
H. M. Li, J. Mater. Chem. A, 2014, DOI: 10.1039/c3ta14617k. 57 J. Jiang, L. Z. Zhang, H. Li, W. W. He and J. J. Yin, Nanoscale,
43 Q. J. Xiang, J. G. Yu and M. Jaroniec, J. Phys. Chem. C, 2011,
115, 7355.
44 M. A. Fox and M. T. Dulay, Chem. Rev., 1993, 93, 341.
2013, 5, 10573.
58 H. J. Zhang, L. Liu and Z. Zhou, Phys. Chem. Chem. Phys.,
2012, 14, 1286.
14290 | RSC Adv., 2014, 4, 14281–14290
This journal is © The Royal Society of Chemistry 2014