Na0.5Ce0.5MoO4 as a new light absorption material to efficiently degrade RhB under visible light irradiation

Article abstract of DOI:10.1039/c6ra07362j

It still remains a big challenge to develop an efficient visible light responsive photocatalyst for the full utilization of solar energy. In this work, Na_0.5Ce_0.5MoO₄ has been developed as a new, high visible light absorption medium and a high electron-conducting material. It has greatly improved the visible light absorbance and the charge separation and transfer rates of the Na_0.5Ce_0.5MoO₄/MoO₃ heterojunction. Amazingly, the activity of Na_0.5Ce_0.5MoO₄ (10%)/MoO₃ is 6.4 times higher than that of MoO₃ for the photodegradation of RhB under visible light irradiation (λ ≥ 420 nm). We expect that Na_0.5Ce_0.5MoO₄ could be applied to prepare other excellent visible light heterojunction photocatalysts to fully utilize solar energy.

Full text of DOI:10.1039/c6ra07362j

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Na0.5Ce0.5MoO4 as a new light absorption material
to efficiently degrade RhB under visible light
irradiation†
Cite this: RSC Adv., 2016, 6, 52646
*
Liming Yang, Fei Teng, Juan Xu, Yang Yang, Yandong Kan and Wenhao Gu
It still remains a big challenge to develop an efficient visible light responsive photocatalyst for the full
utilization of solar energy. In this work, Na0.5Ce0.5MoO4 has been developed as a new, high visible light
absorption medium and a high electron-conducting material. It has greatly improved the visible light
absorbance and the charge separation and transfer rates of the Na0.5Ce0.5MoO4/MoO3 heterojunction.
Amazingly, the activity of Na0.5Ce0.5MoO4 (10%)/MoO3 is 6.4 times higher than that of MoO3 for the
photodegradation of RhB under visible light irradiation (l $ 420 nm). We expect that Na0.5Ce0.5MoO4
could be applied to prepare other excellent visible light heterojunction photocatalysts to fully utilize solar
energy.
Received 21st March 2016
Accepted 22nd May 2016
DOI: 10.1039/c6ra07362j
www.rsc.org/advances
used as absorbents for removal of RhB.13 Molybdenum trioxide
(MoO3), as an n-type semiconductor, is an important electro-
1. Introduction
chromic and photochromic sensitive material as gas sensors
and optical devices.14–16 Recently, the photocatalytic activity of
MoO3 has been reported for the organic pollutants degrada-
tion.17–19 As far as we know, a-MoO3 has the band gap of 2.92 eV,
which can mainly absorb UV light, but only absorb less visible
light.20 Moreover, the high recombination rate of photo-
generated charges of MoO3 has greatly hampered its practical
applications.21,22 To date, many efforts have been made to
improve its photocatalytic activity by coupling MoO3 with the
other semiconductors, such as TiO2–MoO3, Ag–MoO3, rGO/C–
MoO3 and g-C3N4–MoO3.23–26 However, these MoO3-based
composites can still not utilize the visible light effectively, or the
load of noble metal would increase the expense. Thus, it still
remains a big challenge to further improve the photocatalytic
properties of MoO3 under visible light.
Today, dyestuff wastewater pollution is a great threat to human
health and the ecosystem and has attracted public concern
widely. In recent years, semiconductor photocatalysis has been
put forward as an effective way to solve the pollution problem.1–4
TiO2, as an important photocatalyst for the decomposition of
hazardous organic pollutants, has been studied extensively.5,6
However, TiO2 can only absorb UV light, whose proportion is
only 4% of sunlight, which limits its practical applications.7–9
Therefore, it is desirable to develop visible light responsive
semiconductor photocatalysts.
In recent years, the rare earth compounds have received
extensive attention because of their technological applications
in high-performance luminescent devices, magnet, catalysts
and the other functional materials, due to the novel optoelec-
tronic and chemical properties originating from their 4f shell.10
Among various rare earth compounds, ARE(MoO4)2 (A ¼ Na, K,
RE ¼ trivalent rare earth ion) exhibit an excellent thermal and
hydrolytic stability, and can be widely used in quantum elec-
tronics and efficient phosphors.11,12 Up to now, the application
studies of ARE(MoO4)2 are still limited in the luminescence
eld. Xu et al. synthesized NaCe(MoO4)2 owers, which was
In this work, Na0.5Ce0.5MoO4 as a new material is introduced
to the eld of photocatalysis for the rst time. A new visible
light-responsive Na0.5Ce0.5MoO4/MoO3 is synthesized success-
fully via one-pot hydrothermal process and calcination treat-
ment. It not only can efficiently absorb visible light, but also has
a
high electrical conductivity. For Na0.5Ce0.5MoO4/MoO3
composites, they all clearly exhibit higher visible light absorp-
tion than MoO3 and can effectively excite more photo-generated
carriers. In addition, MoO3 and Na0.5Ce0.5MoO4 have the
matched energy bands; thus, a stable heterojunction is easy to
form between them. And Na0.5Ce0.5MoO4 presents a high
conductivity which favors for the transportation and separation
of photogenerated carries. As a result, Na0.5Ce0.5MoO4/MoO3
composites improve the photocatalytic activities under visible
light. Finally, a possible mechanism of the enhanced visible
light photocatalytic activity is put forward. Until now, few
Jiangsu Engineering and Technology Research Center of Environmental Cleaning
Materials (ECM), Jiangsu Key Laboratory of Atmospheric Environment Monitoring
and Pollution Control (AEMPC), Jiangsu Joint Laboratory of Atmospheric Pollution
Control (APC), Collaborative Innovation Center of Atmospheric Environment and
Equipment Technology (AEET), School of Environmental Science and Engineering,
Nanjing University of Information Science & Technology, 219 Ningliu Road, Nanjing
210044, China. E-mail: tfwd@163.com; Fax: +86-25-9882-1090; Tel: +86-25-9882-
1090
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra07362j
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studies have been reported on Na0.5Ce0.5MoO4/MoO3 compos- a UV-vis diffuse reectance experiment. Photoluminescence
ites. We could expect that Na0.5Ce0.5MoO4, as a new visible light (PL) spectra were measured on a uorescence spectrophotom-
absorbance and charge transfer medium, could be applied to eter (Japan, Shimadzu RF-5301PC) with the 265 nm excitation
prepare the other excellent visible light-responsive composite line of a Xe lamp as the excitation source. Nitrogen sorption
photocatalysts.
isotherms were performed at 77 K and <10ꢁ4 bar on a Micro-
meritics ASAP2010 gas adsorption analyzer. Each sample was
degassed at 150 C for 5 h before measurements. Surface area
was calculated by the Brunauer–Emmett–Teller (BET) method.
ꢀ
2. Experimental section
2.1 Catalyst preparation
2.3 Transient photocurrents and electrochemical
impedance spectroscopy (EIS)
All reagents were of analytical grade, purchased from Beijing
Chemical Reagents Industrial Company of China, and were
used without further purication in all our experiments. In
a typical procedure, 2 mmol of Na2MoO4$2H2O was added into
30 mL of distilled water and then 10 mmol of thiourea was
added into the above solution. Aer the Na2MoO4$2H2O and
thiourea was fully dissolved, an appropriate amount of
Ce(NO3)3$6H2O was added into the above solution and stirred
for 60 min by a magnetic stirrer at room temperature. Subse-
quently, the mixture was transferred into a Teon-lined stain-
less steel autoclave and maintained at 180 ꢀC for 48 h. Aer the
autoclave was allowed to cool to room temperature, the as-
obtained product was ltered, washed with absolute ethanol
and distilled water several times, and subsequently dried at 60
ꢀC for 6 h. Aer that, the as-obtained product was calcined in air
at 400 ꢀC for 2 h. A series of Na0.5Ce0.5MoO4/MoO3 with the
different mass ratios were prepared by adjusting the concen-
An electrochemical system (CHI-660B, China) was employed to
measure the photocurrents and electrochemical impedance
spectroscopy (EIS). Electrochemical impedance spectroscopy
(EIS) was performed from 0.1 Hz to 100 kHz at an open circuit
potential of 0.3 V and an alternating current (AC) voltage
amplitude of 5 mV. The data were analyzed by ZSimpWin so-
ware. Photocurrent measurements were carried out in
a conventional three-electrode system, in which indium-tin
oxide (ITO) glass was used as the current collector to fabricate
photo electrode, and 0.5 M Na2SO4 was used as the electrolyte
solution. MoO3/ITO, xNa0.5Ce0.5MoO4/MoO3/ITO and Na0.5
Ce0.5MoO4/ITO photo electrode were prepared by a coating
method.
-
2.4 Photocatalytic degradation reaction
2ꢁ
trations of Ce or MoO4 ions. The as-prepared samples with
The photocatalytic activities of the samples were evaluated by
the decomposition of Rhodamine B (RhB) under visible light (l
$ 420 nm), using a 500 W Xe arc lamp (CEL-HXF 300) equipped
with a visible cutoff lter as a light source. The reaction system
was placed in a sealed black box with the top opened, and was
maintained a distance of 15 cm from the light source. In
a typical photocatalytic experiment, the photocatalyst powder
(100 mg) was suspended in 200 mL of the RhB aqueous solu-
tion, with an initial concentration of 10 mg Lꢁ1. Before irradi-
ation, the suspension (200 mL) was stirred for 30 min in the
dark to ensure the establishment of an adsorption–desorption
equilibrium between the photocatalyst and RhB solution, and
then irradiated with a 500 W Xe arc lamp equipped with visible
light (l $ 420 nm). During photoreaction, 3 mL of suspension
was collected at intervals of irradiation by pipette, and subse-
quently centrifuged to remove the catalysts. UV-vis spectra were
recorded on a Spectrumlab 722 sp spectrophotometer to
determine the concentration of RhB.
different mass ratios were denoted as xNa0.5Ce0.5MoO4/MoO3,
where x refers to the mass ratios of Na0.5Ce0.5MoO4 to MoO3
(x ¼ 0%, 10%, 20%, 40%, 60%, 80%, 100%).
For comparison, phase-pure MoO3 was also prepared under
the same conditions as above but without adding Ce(NO3)3$6H2O.
The pure-phase Na0.5Ce0.5MoO4 sample was prepared with 2
mmol Na2MoO4 and 0.5 mmol Ce(NO3)3$6H2O under the
ꢀ
hydrothermal condition of 180 C for 24 h.
To evaluate the role of heterojunction on the photocatalytic
activity of the Na0.5Ce0.5MoO4/MoO3, a mechanically mixed
Na0.5Ce0.5MoO4 and MoO3 (denoted as Na0.5Ce0.5MoO4 + MoO3)
was prepared and evaluated.
2.2 Characterization
The crystal structures of the samples were determined by X-ray
powder polycrystalline diffractometer (Rigaku D/max-2550VB),
using graphite monochromatized Cu Ka radiation (l ¼ 0.154
nm), operating at 40 kV and 50 mA. The XRD patterns were
obtained in the range of 20–80ꢀ (2q) at a scanning rate of 5ꢀ
minꢁ1. The samples were characterized on a scanning electron
microscope (SEM, Hitachi SU-1510) with an acceleration voltage
of 15 keV. The samples were coated with 5 nm-thick gold layer
before observations. The ne surface structures of the samples
were determined by high-resolution transmission electron
microscopy (HRTEM, JEOL JEM-2100F) equipped with an elec-
tron diffraction (ED) attachment with an acceleration voltage of
In order to investigate reactive species, the trapping experi-
ment was performed, in which holes (h+) and hydroxyl radical
(cOH) were trapped by adding 1 mM ammonium oxalate
((NH4)2C2O4) and 10 mL dimethyl sulfoxide (DMSO) into the
RhB solution, respectively.
3. Results and discussion
3.1 Physicochemistry characterization of photocatalyst
200 kV. UV-vis diffused reectance spectra of the samples were Fig. 1 shows XRD patterns of the as-prepared samples. The
obtained using a UV-vis spectrophotometer (UV-2550, Shi- strong diffraction peaks indicate the high crystallinity of the as-
madzu, Japan). BaSO4 was used as a reectance standard in prepared products. The diffraction peaks at 2q ¼ 12.8ꢀ, 23.4ꢀ,
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Fig. 2 Fourier transform infrared (FT-IR) spectra of MoO3, Na0.5
-
Ce0.5MoO4 and 10% Na0.5Ce0.5MoO4/MoO3.
Fig. 1 X-ray diffraction (XRD) patterns of MoO3, Na0.5Ce0.5MoO4 and
xNa0.5Ce0.5MoO4/MoO3 samples, where x refers to the mass ratio of
Na0.5Ce0.5MoO4 to MoO3.
Na0.5Ce0.5MoO4 and MoO3 can be observed clearly. XRD and FT-
IR results conrm that the formation of Na0.5Ce0.5MoO4/MoO3
composite.
25.8ꢀ, 27.4ꢀ and 33.8ꢀ correspond to (020), (110), (040), (021) and
(111) crystal planes of orthorhombic MoO3 (JCPDS card no. 05-
0508), respectively. The results conrm the formation of phase-
pure MoO3. The diffraction peaks at 2q ¼ 18.3ꢀ, 28.2ꢀ, 30.6ꢀ,
33.6ꢀ and 46.2ꢀ can be perfectly indexed to the (101), (112),
The morphology of the samples was characterized by SEM.
Fig. 3 shows the SEM images of the samples. The as-prepared
MoO3 sample is composed of nanosheets, which are about
250–300 nm thick (Fig. 3a). 10% Na0.5Ce0.5MoO4/MoO3 sample
is composed of many sheets of 600–1200 nm thick and some of
small particles (Fig. 3b). Compare with pure MoO3, the sheet
thickness increases clearly. We assume that MoO3 sheets may
be coated by Na0.5Ce0.5MoO4 particles. With further increasing
the amount of Na0.5Ce0.5MoO4, most of MoO3 sheets have
broken into small irregular pieces and more nanoparticles have
(004), (200) and (204) crystal planes of tetragonal Na0.5Ce0.5
-
MoO4 with a space group of I41/a (JCPDS card no. 79-2242),13
respectively. This results conrm the formation of phase-pure
Na0.5Ce0.5MoO4. For Na0.5Ce0.5MoO4/MoO3 composites, the
XRD patterns clearly indicate the co-existence of MoO3 and
Na0.5Ce0.5MoO4 in the as-prepared nanocomposites, and no
impurity phases are observed. With increasing content of
Na0.5Ce0.5MoO4, (112) peak intensity of Na0.5Ce0.5MoO4 at 2q ¼
28.2ꢀ increases quickly, meanwhile (021) peak intensity of MoO3
at 2q ¼ 27.4ꢀ decreases gradually. Table 1 shows that the peak
intensity ratios of MoO3 to Na0.5Ce0.5MoO4 decrease clearly.
Fig. 2 shows FT-IR spectra of Na0.5Ce0.5MoO4, MoO3 and 10%
Na0.5Ce0.5MoO4/MoO3. For orthorhombic a-MoO3, the sharp
band at about 996 cmꢁ1 is assigned as the stretching vibration
of Mo]O; the band at about 867 cmꢁ1 and 595 cmꢁ1 are
associated with the vibration of bridging Mo–O–Mo bonds and
Mo–O bands, respectively.27–29 For pure Na0.5Ce0.5MoO4, one
abroad peak in 950–700 cmꢁ1 is resultant from the Mo–O
appeared (Fig. 3b and c). Obviously, the coexistence of Na0.5
-
Ce0.5MoO4 and MoO3 did signicantly affect their morphol-
ogies. Moreover, as the mass ratio of Na0.5Ce0.5MoO4 is varied
from 20% to 80%, the number of nanoparticles coated on MoO3
increases dramatically (Fig. 3d–f). These indicate that Na0.5
-
Ce0.5MoO4 nanoparticles have formed. The morphology of
Na0.5Ce0.5MoO4 is presented in the ESI, Fig. S1.† Furthermore,
the Brunauer–Emmett–Teller (BET) areas of the samples are
presented in Table 2. Among them, 10% Na0.5Ce0.5MoO4/MoO3
has a larger surface area (8.5 m2 gꢁ1) than Na0.5Ce0.5MoO4 (6.1
m2 gꢁ1), while the others are smaller than pure Na0.5Ce0.5MoO4.
However, the surface areas of Na0.5Ce0.5MoO4/MoO3 composites
are all higher than that (2.4 m2 gꢁ1) of MoO3. This can be
attributed to the generation of nanoparticles.
2ꢁ
30
stretching vibration in MoO4
.
In the case of 10% Na0.5-
Ce0.5MoO4/MoO3 sample, the characteristic bands of both
Table 1 Peak intensity ratios of xNa0.5Ce0.5MoO4/MoO3 samples with different weight ratiosa
(021) peak intensity
(112) peak intensity of
Peak intensity ratio
of (021) MoO3/(112) Na0.5Ce0.5MoO4
Sample
of MoO3 at 2q ¼ 27.36ꢀ
Na0.5Ce0.5MoO4 at 2q ¼ 28.26ꢀ
10%
20%
40%
60%
80%
560
495
236
187
60
390
545
812
937
782
1.436
0.908
0.291
0.200
0.077
a
x: the mass ratio of Na0.5Ce0.5MoO4 to MoO3.
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Fig. 3 Scanning electron microscopy (SEM) images of MoO3 and xNa0.5Ce0.5MoO4/MoO3 samples: (a) MoO3; (b) x ¼ 10%; (c) x ¼ 20%; (d) x ¼
40%; (e) x ¼ 60%; (f) x ¼ 80%.
Table 2 Specific surface areas, band gap (Eg) values and apparent first-
the diffraction spots to the center are 2.7 nm and 3.8 nm, cor-
order rate constant of the samples for the degradation of RhB under
visible light (l $ 420 nm)
responding to the inter-plane spacings of (101) and (110) of
MoO3, respectively. Likewise, in Fig. 4f, the clear diffraction
BET areaa
Band gap
(Eg, eV)
kapp
b
spots can be ascribed to the (112) and (004) Bragg reections of
Na0.5Ce0.5MoO4, and the (040) and (021) Bragg reections of
MoO3, respectively. The diffraction peaks of these planes also
appear at 28.3ꢀ and 30.6ꢀ (Na0.5Ce0.5MoO4) and 25.8ꢀ and 27.4ꢀ
for MoO3 in the XRD patterns. Hence, these observations rmly
demonstrate that the formation of intimate heterojunction
between Na0.5Ce0.5MoO4 and MoO3. This intimate hetero-
junction structure would favor for the charge separation, thus
effectively improving the photocatalytic activity.
Sample
(m2 gꢁ1
)
(minꢁ1
)
MoO3
Na0.5Ce0.5MoO4
10%
20%
40%
2.442
6.033
8.513
4.883
2.983
3.155
4.134
2.92
2.58
2.74
2.67
2.64
2.54
2.52
0.003
9.93 ꢂ 10ꢁ4
0.0221
0.0206
0.0177
0.0129
0.0044
60%
80%
a
Calculated by the Brunauer–Emmett–Teller (BET) method. b kapp is the
rate constants of pseudo rst order (minꢁ1).
3.2 Excellent visible light absorbing ability of
Na0.5Ce0.5MoO4
The ne surface structures of the 10% and 60% Na0.5Ce0.5
-
Fig. 5a shows the ultraviolet-visible light diffuse reectance
spectra (UV-DRS) of the samples. The pure MoO3 shows the
absorption edge at about 425 nm, which corresponds to a band
gap energy of 2.92 eV; while the absorption edge of pure
Na0.5Ce0.5MoO4 is about 504 nm. Obviously, the UV-DRS spectra
of Na0.5Ce0.5MoO4/MoO3 composites clearly show the spectrum
combination of both Na0.5Ce0.5MoO4 and MoO3, and the
absorption edge of the Na0.5Ce0.5MoO4/MoO3 composites show
a shi toward visible region gradually. Signicantly, the pres-
ence of Na0.5Ce0.5MoO4 improves the visible light absorption
ability of Na0.5Ce0.5MoO4/MoO3 composites and greatly extends
the absorption range of visible light, which is favorable for
Na0.5Ce0.5MoO4/MoO3 to generate more electron–hole pairs
under visible light irradiation. Thus it may be one of the reasons
that affect the photocatalytic performance. Furthermore, Tauc
plots of MoO3, Na0.5Ce0.5MoO4 and 10% Na0.5Ce0.5MoO4/MoO3
is shown in Fig. 5b. Based on the UV-DRS results, the Eg is
calculated by eqn (1) as follows:31
MoO4/MoO3 are further characterized by high-resolution
transmission electron microscopy (HRTEM). Fig. 4a and
d show the TEM images of 10% and 60% Na0.5Ce0.5MoO4/MoO3,
respectively. It could be clearly observed that numerous nano-
particles have deposited on the surface of MoO3 sheets,
consistent with SEM observation. In addition, Fig. 4b and e
show their lattice fringe images. For 10% Na0.5Ce0.5MoO4/MoO3
(Fig. 4b), the lattice fringe spacings of 0.27 nm and 0.292 nm
coincide with (101) plane of MoO3 and (004) plane of Na0.5
-
Ce0.5MoO4, respectively. For 60% Na0.5Ce0.5MoO4/MoO3
(Fig. 4e), the lattice fringe spacing of 0.315 nm coincides with
(112) crystal plane of Na0.5Ce0.5MoO4. Meanwhile, the lattice
fringe spacing of 0.326 nm coincides with (021) crystal plane of
MoO3. Besides, Fig. 4c and f show the selected area electron
diffraction (SAED) patterns of both samples. In Fig. 4c, two sets
of diffraction spots are clearly observed. The distances from two
diffraction spots to the center are determined to be 1.96 nm and
2.92 nm, corresponding to the inter-plane spacings of (204) and
(004) of Na0.5Ce0.5MoO4, respectively; and the distances from
ahv ¼ A(hv ꢁ Eg)n/2
(1)
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Fig. 4 High-resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns of xNa0.5
-
Ce0.5MoO4/MoO3 samples: (a–c) x ¼ 10%; (d–f) x ¼ 60%.
semiconductor, which is the geometric mean of the electro-
negativity of constituent atoms, Ec is the energy of free electrons
on the hydrogen scale (about 4.5 eV), Eg is the band gap energy
of semiconductor. According to the above equations, the
bottom of conduction band (CB) and the top of valance band
(VB) of Na0.5Ce0.5MoO4 are ꢁ0.06 eV and 2.60 eV, while those of
MoO3 are 0.44 eV and 3.36 eV, respectively, which is similar to
the reported literature.34 Therefore, MoO3 and Na0.5Ce0.5MoO4
have matched energy bands; and a stable heterojunction is easy
Fig. 5 (a) UV-visible diffuse reflectance spectra (UV-DRS) of the to form between them.
samples; (b) Tauc plots of MoO3, Na0.5Ce0.5MoO4 and 10% Na0.5
-
Ce0.5MoO4/MoO3.
3.3 High electrical conductivity of Na0.5Ce0.5MoO4
where a, h, v, Eg and A are absorption coefficient, Plank
constant, light frequency, band gap energy and a constant,
respectively. Among them, n is determined by the type of optical
transition of a semiconductor (n ¼ 1 for direct transition and n
¼ 4 for indirect transition). For MoO3, the value of n is taken as
1, and the Eg value of MoO3 is estimated to be 2.92 eV according
to a plot of (ahv)2 versus energy (hv).32 Accordingly, Eg values of
Na0.5Ce0.5MoO4 and 10% Na0.5Ce0.5MoO4/MoO3 are 2.66 eV and
2.72 eV, respectively. Moreover, it found that the band gaps of
Na0.5Ce0.5MoO4/MoO3 heterojunctions range from 2.67 eV to
The electrochemical impedance spectroscopy (EIS) is measured
to investigate the effect of Na0.5Ce0.5MoO4 on the electron
transfer properties. As we all know, the transportation and
separation efficiencies of photo-generated charges have
a signicant inuence on photocatalytic activity.35,36 The arc
radius in the EIS spectra reects the solid-state surface charge
transfer resistance, and a smaller semicircle radius in the EIS
Nyquist plot means a smaller electric resistance, which favors
for electron transportation.37 As a result, the fast electron
transfer rate favors the efficient separation of photo-generated
electrons and holes.38,39 As shown in Fig. 6, the Nyquist radius
of Na0.5Ce0.5MoO4 is obviously smaller than that of MoO3,
indicating that Na0.5Ce0.5MoO4 has a greatly higher conductivity
than MoO3, which favors for the transportation and separation
of photogenerated carries. For xNa0.5Ce0.5MoO4/MoO3 samples,
they all have a smaller Nyquist radius than MoO3, and 10%
Na0.5Ce0.5MoO4/MoO3 photocatalyst has the smallest surface
resistance among them. These results indicate that compared
with pure MoO3, the presence of Na0.5Ce0.5MoO4 can reduce the
electric resistance and promote the transfer and separation of
photogenerated carriers, thus improving the photocatalytic
activity of Na0.5Ce0.5MoO4/MoO3 samples.
2.52 eV, which gradually decreases with the increase of Na0.5
Ce0.5MoO4 content from 20% to 80% (Table 2).
-
Furthermore, the conduction band (CB) and valence band
(VB) potentials of MoO3 and Na0.5Ce0.5MoO4 are calculated by
the following equations:33
EVB ¼ c ꢁ Ec + 0.5Eg
ECB ¼ EVB ꢁ Eg
(2)
(3)
where EVB is the potential of valence band edge, ECB is the
potential of conduction band edge, c is the electronegativity of
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Fig.
8
Photoluminescence emission spectra (PL) of MoO3, 10%
Fig. 6 Nyquist plots of MoO3, Na0.5Ce0.5MoO4 and xNa0.5Ce0.5MoO4/
MoO3 samples in the frequency range of 0.1–106 Hz.
Na0.5Ce0.5MoO4/MoO3, and mechanical mixed 10% Na0.5Ce0.5MoO4
+ MoO3 at excitation wavelength of 265 nm.
Transient photocurrent response directly correlate with the
transportation and separation efficiencies of photo-generated
carriers.40 In order to investigate the photocurrent property,
the I–t curves of MoO3, Na0.5Ce0.5MoO4, 10% Na0.5Ce0.5MoO4/
MoO3, and mechanical mixed 10% Na0.5Ce0.5MoO4 + MoO3 are
recorded under visible light irradiation (l $ 420 nm) (Fig. 7).
The photocurrent values clearly return to a constant value while
the light is turned on. Compared with Na0.5Ce0.5MoO4, MoO3
and 10% Na0.5Ce0.5MoO4 + MoO3, the 10% Na0.5Ce0.5MoO4/
MoO3 exhibits a higher transient photocurrent density (about 4
mA cmꢁ2), which is about 5 times as high as that (about 0.8 mA
cmꢁ2) of MoO3. Compared with 10% Na0.5Ce0.5MoO4 + MoO3,
the enhanced photocurrent of 10% Na0.5Ce0.5MoO4/MoO3
Na0.5Ce0.5MoO4 + MoO3 at excitation wavelength of 265 nm. The
PL spectra of the other samples are shown in Fig. S2 (ESI†). It is
obvious that two main emission peaks of MoO3 at about 362 nm
and 425 nm are evidently decreased in intensity in the presence
of Na0.5Ce0.5MoO4. Compared with 10% Na0.5Ce0.5MoO4
+
MoO3, it could further conrm the existence of heterojunction
between Na0.5Ce0.5MoO4 and MoO3. Moreover, the presence of
Na0.5Ce0.5MoO4 can reduce the electric resistance and promote
the transfer of charges, thus improving the separation efficiency
of photogenerated carriers signicantly.
3.4 Photocatalytic activity
The photocatalytic activities of the samples were evaluated by
the photodegradation of RhB under visible light irradiation (l $
420 nm). Prior to light irradiation, the suspension was
magnetically stirred in the dark for about 30 min to reach the
absorption–desorption equilibrium between RhB and the pho-
indicates that there is a strong interaction between Na0.5Ce0.5
-
MoO4 and MoO3, which favors for efficient carrier separation at
the interface. On the other hand, Na0.5Ce0.5MoO4 has a high
electrical conductivity, as a result, which can greatly promote
the charges transfer and increase the separation rate of charges.
The photoluminescence emission spectrum (PL) of semi-
conductor could reect the recombination probability of
photogenerated electrons and holes.41,42 Fig. 8 shows the PL
spectra of pure MoO3, 10% Na0.5Ce0.5MoO4/MoO3 and 10%
tocatalyst. It can be seen from Fig. 9a that all the Na0.5Ce0.5
-
MoO4/MoO3 samples exhibit higher photocatalytic activities
than MoO3 and Na0.5Ce0.5MoO4. Aer 90 min of visible light
irradiation (l $ 420 nm), the RhB degradation over phase-pure
MoO3 and Na0.5Ce0.5MoO4 is only 24% and 10%, respectively.
Interestingly, aer coupling Na0.5Ce0.5MoO4 with MoO3, the
RhB degradation is clearly increased. For example, 10%, 20%
and 40% Na0.5Ce0.5MoO4/MoO3 could respectively degrade
94%, 85% and 84.8% of RhB in 90 min of visible light irradia-
tion (l $ 420 nm). The results show that the presence of
Na0.5Ce0.5MoO4 can effectively enhance the photocatalytic
activity of Na0.5Ce0.5MoO4/MoO3 composites. However, the
degradation efficiency gradually decreases with increasing
amount of Na0.5Ce0.5MoO4. When the Na0.5Ce0.5MoO4 mass
ratio is increased to 80%, only 39% of RhB could be degraded in
90 min. Under the same experimental condition, we also have
added the blank run, and the self-degradation of RhB is about
4% aer 90 min under visible light irradiation (l $ 420 nm). As
we mentioned above, pure Na0.5Ce0.5MoO4 has a poor photo-
catalytic activity under visible light irradiation, but Na0.5Ce0.5
MoO4 could efficiently absorb visible light, which is expected to
act as an efficient light harvester to generate photo-generated
-
Fig. 7 Transient photocurrent density of MoO3, Na0.5Ce0.5MoO4, 10%
Na0.5Ce0.5MoO4/MoO3 and mechanical mixed 10% Na0.5Ce0.5MoO4 +
MoO3 under visible light irradiation (l $ 420 nm).
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Fig. 9 (a) Photodegradation and (b) kinetic curves of RhB over MoO3, Na0.5Ce0.5MoO4 and xNa0.5Ce0.5MoO4/MoO3 samples under visible light
irradiation (l $ 420 nm).
carriers. So we assume that an excess amount of Na0.5Ce0.5MoO4 the UV-vis absorption spectra of RhB dye at different irradiation
in Na0.5Ce0.5MoO4/MoO3 composites would absorb more light times over 10% Na0.5Ce0.5MoO4/MoO3. The characteristic
and reduce the efficient absorption of MoO3 for generating absorption peak of RhB locates at 554 nm.46 It is obvious that
electrons and holes, which may lead to a reduced photocatalytic the absorption peak has not shied with reaction time, indi-
activity.43
cating that RhB may have been effectively degraded.47 Moreover,
Further, the photocatalytic degradation process of organic we also have conducted the MB photodegradation experiments
contaminant can be simplied as a pseudo-rst-order kinetic with MoO3, Na0.5Ce0.5MoO4 and 10% Na0.5Ce0.5MoO4/MoO3,
reaction as follow:44,45
and provided the results in Fig. S3 (ESI†). It can be seen from
Fig. S3† that 10% Na0.5Ce0.5MoO4/MoO3 exhibits a higher
photocatalytic activity than MoO3 and Na0.5Ce0.5MoO4 under
visible light irradiation (l $ 420 nm). The result is in consistent
with that of RhB photodegradation.
To investigate the stability of Na0.5Ce0.5MoO4/MoO3 under
visible light irradiation (l $ 420 nm), the degradation reaction
of RhB dye over the 10% Na0.5Ce0.5MoO4/MoO3 was cycled for
three times. As shown in Fig. S4a (ESI†), the degradation effi-
ciency of 10% Na0.5Ce0.5MoO4/MoO3 was only reduced by 5%
aer 3 cycles. Furthermore, we also investigated the XRD
pattern of the sample aer three cycles (Fig. S4b†). The results
indicate that the Na0.5Ce0.5MoO4/MoO3 photocatalyst has
a good stability in photocatalytic reaction.
ln(C0/C) ¼ kapp
t
(4)
where C0 is the initial concentration (mg Lꢁ1) of RhB while light-
on, C is the concentration of RhB at t-min irradiation (mg Lꢁ1),
and kapp is the apparent pseudo-rst-order rate constant
(minꢁ1). Fig. 9b gives the linear tted kinetic curves for the
degradation of RhB over photocatalysts under visible light
irradiation (l $ 420 nm). The determined kapp values for
degradation of RhB are summarized in Table 2. The kapp value
over 10% Na0.5Ce0.5MoO4/MoO3 is the largest among all
samples; it is approximately 7- and 22-times higher than those
of pure MoO3 and Na0.5Ce0.5MoO4, respectively. Fig. 10a shows
Fig. 10 (a) UV-vis absorption spectra of RhB solution over 10% Na0.5Ce0.5MoO4/MoO3 sample at different reaction times; (b) photocatalytic
activities of MoO3, Na0.5Ce0.5MoO4, 10% Na0.5Ce0.5MoO4/MoO3 and mechanical mixed 10% Na0.5Ce0.5MoO4 + MoO3 on the degradation of RhB
under visible-light irradiation (l $ 420 nm).
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Furthermore, in order to prove that the visible light photo- on its photocatalytic efficiency, while the photodegradation of
catalytic activity of Na0.5Ce0.5MoO4/MoO3 comes from the exci- RhB is inhibited obviously by the addition of ammonium oxalate
tation of the photocatalyst rather than the photosensitization, (Fig. 11a). This result demonstrates that holes (h+) are the main
the degradation of o-nitrophenol was also investigated over 10% active species for MoO3. Nevertheless, for 10% Na0.5Ce0.5MoO4/
Na0.5Ce0.5MoO4/MoO3, MoO3 and Na0.5Ce0.5MoO4 under visible MoO3, holes (h+) and hydroxyl radicals (cOH) are both the main
light irradiation (l $ 420 nm) (Fig. S5, ESI†). It is clear that the active species in the degradation of RhB. Moreover, the degra-
10% Na0.5Ce0.5MoO4/MoO3 shows a higher activity than MoO3 dation activity upon adding ammonium oxalate is higher than
and Na0.5Ce0.5MoO4 for the photodegradation of o-nitrophenol. that upon adding DMSO, suggesting that holes (h+) plays a more
The result indicates that the effect of dye sensitization can be important role than hydroxyl radical (cOH) in the degradation
avoided. Moreover, the blank run of o-nitrophenol has also been process of RhB (Fig. 11b).
tested and the o-nitrophenol self-degradation is about 9% aer
In order to understand the degradation process, the degra-
3 h visible light irradiation (l $ 420 nm), which might be dation of RhB over 10% Na0.5Ce0.5MoO4/MoO3 was analyzed by
attributed to a small amount of volatile under stirring process. total organic carbon (TOC). The TOC result is shown in Fig. S6
Given in the Table 2, we also show the Brunauer–Emmett–Teller (ESI†). It can be observed that aer 90 min, the TOC decreases by
(BET) areas of xNa0.5Ce0.5MoO4/MoO3 samples. Obviously, 10% about 40%, but it does not reach a complete conversion to CO2 +
Na0.5Ce0.5MoO4/MoO3 has the largest surface area (8.5 m2 gꢁ1
)
H2O. Two possible reasons can be considered. One may be that
among all samples, which could be ascribed to the formation of more time is needed to complete the mineralization under
Na0.5Ce0.5MoO4 nanoparticles. In the process of photo- visible light irradiation (l $ 420 nm); the other may be that the
degradation, the larger surface area will be favorable to dye has been decomposed into small molecules. Limited by our
increase the contact area with organic pollutant, the more active experimental conditions, we can only make a preliminary
sites will be provided to degrade organic pollutant. Thus it may inference. In fact, this needs further study in the future.
be one of the reasons that affect the photocatalytic
performance.
3.5 Activity enhancement mechanism of Na0.5Ce0.5MoO4/
MoO3
In addition, to further evaluate the role of heterojunction on
the photocatalytic activity of the Na0.5Ce0.5MoO4/MoO3, we
investigated the degradation activity of the mechanical mixed
Na0.5Ce0.5MoO4 and MoO3 with the same weight ratio as 10%
Na0.5Ce0.5MoO4/MoO3 (denoted as Na0.5Ce0.5MoO4 + MoO3)
(Fig. 10b). Compared with 10% Na0.5Ce0.5MoO4/MoO3, the
photocatalyst of mechanical mixed sample has poor photo-
catalytic activity. So we think that a strong interaction exists
between MoO3 and Na0.5Ce0.5MoO4, namely, heterojunction has
formed, thus remarkably improving the photocatalytic activity
of Na0.5Ce0.5MoO4/MoO3.
The trapping experiments are carried out in the photocatalytic
degradation process in order to detect active species. Ammonium
oxalate ((NH4)2C2O4) and dimethyl sulfoxide (DMSO) are chosen
as the holes and the radical scavengers, respectively. The results
are presented in Fig. 11. For pure MoO3, DMSO has a small effect
On base of the results above, the charge transfer and photo-
catalytic mechanism over xNa0.5Ce0.5MoO4/MoO3 hetero-
junctions is proposed. The transfer and separation efficiency of
interfacial charges can be effectively improved by the Na0.5
-
Ce0.5MoO4/MoO3 heterojunction due to their matched
conduction band (CB) and valence band (VB). As illustrated in
Fig. 12, the CB (ꢁ0.02 eV) of Na0.5Ce0.5MoO4 lies above the CB
(0.44 eV) of MoO3, while the VB (3.36 eV) of MoO3 lies below that
(2.56 eV) of Na0.5Ce0.5MoO4. Thus, a stable heterojunction is
easy to form between MoO3 and Na0.5Ce0.5MoO4. Consequently,
an interface electric eld forms in the Na0.5Ce0.5MoO4/MoO3
heterojunction. MoO3 has a band gap of 2.92 eV, thus it can
absorb a part of visible light. Moreover, Na0.5Ce0.5MoO4 has an
excellent visible light absorption, thus greatly improving visible
light absorption ability of Na0.5Ce0.5MoO4/MoO3. Under visible
Fig. 11 Trapping experiments for the photodegradation of RhB over (a) MoO3; (b) 10% Na0.5Ce0.5MoO4/MoO3 under visible light irradiation (l $
420 nm).
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Fig. 12 Proposed photocatalytic degradation and charge transfer mechanism over xNa0.5Ce0.5MoO4/MoO3 heterojunctions.
light irradiation (l $ 420 nm), both MoO3 and Na0.5Ce0.5MoO4 the Returned Overseas Chinese Scholars of State Education
can absorb the photons to generate electron–hole pairs. Since Ministry (20121707), Six Talent Climax Foundation of Jiangsu
the CB of Na0.5Ce0.5MoO4 is more negative than that of MoO3, (20100292), the Key Project of Environmental Protection
the photo-generated electrons at the CB of Na0.5Ce0.5MoO4 can Program of Jiangsu (2013005), Natural Science Foundation of
migrate to the CB of MoO3. Then the electrons stored in the CB Jiangsu province (BK2012464), the Project Funded by the
of MoO3 could be trapped by the O2 near the surface of MoO3 to Science and Technology Infrastructure Program of Jiangsu
form reactive cO2ꢁ. Meanwhile, the holes generated in the VB of (BM201380277), and A Project Funded by the Priority Academic
MoO3 can move to VB of Na0.5Ce0.5MoO4. Because holes (h+) and Program Development of Jiangsu Higher Education Institutions
hydroxyl radicals (cOH) are both main active species in the (PAPD) sponsored by SRF for ROCS, SEM (2013S002).
photocatalytic process, and holes (h+) plays a more important
role than hydroxyl radical (cOH) in the degradation of RhB. The
most holes in VB in Na0.5Ce0.5MoO4 can also directly oxidize the
organic compounds or react with H2O to form cOH radicals.
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