Improved photocatalytic activity by utilizing the internal electric field of polar semiconductors: A case study of self-assembled NaNbO3 oriented nanostructures

Article abstract of DOI:10.1039/c3ra45954c

How to promote separation of photo-generated carriers is one of the key issues determining the photocatalytic activity of a semiconductor. In this paper, we report a new strategy that utilizing an internal electric field of polar semiconductors to improve photocatalytic activity and, proving it by liquid-phase photodegradation (photocatalytic RhB degradation) as well as gas-phase photodegradation (photocatalytic oxidation of CH₄) experiments upon the self-assembled NaNbO₃ oriented nanocuboids that were fabricated by a facial hydrothermal route. The formation conditions and mechanisms of the ordered nanostructures were also discussed.

Full text of DOI:10.1039/c3ra45954c

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Improved photocatalytic activity by utilizing the
internal electric field of polar semiconductors: a
case study of self-assembled NaNbO3 oriented
nanostructures†
Cite this: RSC Adv., 2014, 4, 3165
a
a
b
a
*
Linqin Jiang, Yaohong Zhang, Yu Qiu and Zhiguo Yi
How to promote separation of photo-generated carriers is one of the key issues determining the
photocatalytic activity of a semiconductor. In this paper, we report a new strategy that utilizing an
internal electric field of polar semiconductors to improve photocatalytic activity and, proving it by liquid-
phase photodegradation (photocatalytic RhB degradation) as well as gas-phase photodegradation
(photocatalytic oxidation of CH4) experiments upon the self-assembled NaNbO3 oriented nanocuboids
that were fabricated by a facial hydrothermal route. The formation conditions and mechanisms of the
ordered nanostructures were also discussed.
Received 20th October 2013
Accepted 30th October 2013
DOI: 10.1039/c3ra45954c
www.rsc.org/advances
photocatalytic activity by utilizing the internal electric eld of
polar semiconductors with a case study of self-assembled
1 Introduction
NaNbO3 oriented nanostructures.
Semiconductor photocatalysis has attracted extensive and
increasing attention due to its applications in environmental
cleansing as well as solar fuel production.1 Its basic principle
involves photogenerated electrons and holes migrating to the
surface of the semiconductor and serving as redox sources
which then react with adsorbed chemicals, leading to the redox
reactions.1d Light absorption, redox potentials and the recom-
bination of photogenerated carriers are amongst the key factors
determining the photocatalytic activities. Intensive research on
the development of high activity photocatalysts in the past
several decades has witnessed great success with strategies such
as new material exploration,2 band engineering,3 co-catalysts
loading,4 crystal facet engineering,5 photoelectrode design,6 etc.
Nevertheless, how to promote spatial separation of photo-
generated electrons and holes within a photocatalyst interior
remains a challenge.
A polar semiconductor has spontaneous electric polariza-
tion showing important applications in electronic information
technology.8 Early research on new catalyst exploration has
shown that a lot of polar semiconductors are good photo-
catalysts, for example, CdS,9 WO3,10 SrTiO3,11 Bi2WO6,12 (K, Na)
TaO3,13 etc. It is obvious that electric polarization helps the
separation of electrons and holes (Fig. 1a and b). However,
since it is relatively difficult to control the polarization states of
powder samples (Fig. 1c), there are fewer reports on conscious
application of the effect of electric polarization to powder
photocatalysis.14 Current research is based on the consideration
that spontaneous electric polarization of a polar semiconductor
can be utilized to realize the spatial separation of photogenerated
electrons and holes within the interior of a semiconductor pho-
tocatalyst if the semiconductor micro/nano-crystals can be
fabricated and their assemblage state can be well controlled. The
possible patterns satisfying this point of view are schematically
shown in Fig. 1d and e.
Fabrication of a junction structure is an effective approach
for the promotion of charge separation.7 Co-catalysts loading
has the same effect besides their function of reducing the
overpotential of chemical reactions.4 However, these only
function at the interfaces. Herein, we report improving the
2 Experimental
2.1 Preparation of photocatalysts
aKey Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of
Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, The NaNbO3 samples were prepared by hydrothermal synthesis.
China. E-mail: zhiguo@irsm.ac.cn
bNew Energy Technology Center, Shanghai Institute of Microsystems and Information
All reagents were analytically pure, commercially available and
used without further purication. In a typical procedure,
0.01 mol of Nb2O5 (4 N, Sinopharm Chemical Reagent Co. Ltd)
and 1 M of NaOH (AR, 96% min, Sinopharm Chemical Reagent
Technology, Chinese Academy of Sciences, Shanghai 201807, China
† Electronic supplementary information (ESI) available: UV-vis spectral changes
of RhB, degradation kinetics, FTIR spectra, CH4 conversion, SEM images of thin
Co. Ltd) were dissolved in 70 ml deionized H2O. Aer 1 h of
lms, XRD patterns, and SEM images of NaNbO3 nanostructures, etc. See DOI:
10.1039/c3ra45954c
magnetic stirring, the precursor solution was transferred into
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of the samples were examined by scanning electron microscopy
(SEM, JSM-6700F), transmission electron microscopy (TEM),
high-resolution TEM (HRTEM) and electron diffraction (ED)
pattern were taken on JEM-2010. UV-vis diffuse reectance
spectra of the samples were recorded on a PerkinElmer Lambda
950 UV/vis/NIR spectrometer. The surface area of the samples
was measured by Brunauer–Emmett–Teller (BET, ASAP 2020C +
M). FTIR measurements were carried out by a FT-IR spectrom-
eter (Spectrum one, PerkinElmer).
2.4 Photodegradation of RhB
The photocatalytic activity experiments of the samples for
degradation of RhB were performed under ice–water bath
conditions with a 300 W Xe arc lamp as the light source. Typi-
cally, 30 mg NNB-1 powder was mixed with RhB (2 ml, with a
concentration of 40 mg Lꢁ1) in a 198 ml H2O solution. The
suspension was magnetically stirred for 1 h to reach complete
adsorption–desorption equilibrium in the dark and subse-
quently exposed to sunlight simulator irradiation with
maximum illumination time up to 240 min. The excitation
source is located at ca. 12.2 cm away from the suspension
surface. During the irradiation, the suspension was continu-
ously stirred and the reaction system was kept in an ice–water
bath. Before irradiation and at certain time intervals, about 7 ml
suspensions were sampled and centrifuged three times to
remove the residuals before characterization. The concentra-
tion of RhB was determined by measuring the absorbance in
step time using UV-vis-NR spectrophotometers (Lambda-900,
PerkinElmer).
Fig.
1 Diagram showing: (a) recombination of photo-generated
electrons and holes within a photocatalyst interior; (b) separation and
transportation of photo-generated electrons and holes driven by the
electric field of spontaneous polarization P in the interior of a polar
semiconductor; (c) powder sample with disordered distribution of P;
(d) monodisperse single crystal particles with P; (e) powder sample
with ordered arrangement of single crystal particles. For simplicity, the
case of a single crystal with multi-domain states was omitted in the
diagram.
a Teon-lined steel autoclave. The autoclave was sealed, heated
at 200 C for 24 h and then cooled down to room temperature
ꢀ
naturally. The nal product was washed with deionized water
and pure alcohol several times to remove the residues, and then
dried at 80 ꢀC overnight. The samples obtained in the presence
of 1, 2.5, 5 and 8 M of NaOH were denoted as NNB-1, NNB-2,
NNB-5 and NNB-8, respectively. Other samples were synthesized
by changing the concentration of NaOH as well as the dwelling
time.
2.5 Photooxidation of CH4
Photocatalytic oxidation of CH4 was examined upon NaNbO3 or
Pt-loaded NaNbO3 samples in a xed-bed photoreactor. The
catalyst (0.15 g) was spread on the at bottom of the reactor
(450 ml). The reaction gas, a mixture of CH4 (0.5 ml) and O2
(5 ml), was introduced into the reactor aer removing the air in
the cell by N2 carrier for 20 min. The reaction was carried out
without heating at atmospheric pressure upon photoirradiation
from a 300 W xenon lamp located at ca. 15 cm away from the
catalyst surface. At certain time intervals, the products in the
gaseous phase were sampled and analyzed by gas chromatog-
raphy (Fuli Instrument GC9720, China).
2.2 Cocatalyst loading
For cocatalyst loading, H2PtCl6$6H2O (AR, Sinopharm Chem-
ical Reagent Co. Ltd) was used as a Pt precursor. 1.0 wt% of Pt
was loaded on the NaNbO3 samples by an impregnation
method or by a photodeposition method. In the impregnation
method, the photocatalyst (0.2 g) was dispersed into an
aqueous methanol solution (30 ml) of the metal precursor for
1 h, followed by evaporation to dryness overnight. Then, the
dried powder was calcined in air atmosphere at 400 ꢀC for 2 h.
In the photodeposition method, the photocatalyst was
dispersed in methanol solution with vigorous stirring, followed
by photoirradiation from the above using a 300 W Xe lamp,
which was entirely emitted from UV to visible light, for 2 h, with
continuous stirring. The NNB-1 (or NNB-8) samples with the Pt
cocatalyst by the impregnation method and by photodeposition
were referred to as Pt–NNB-1 (or Pt–NNB-8) and Pt–NNB-1-LD
(or Pt–NNB-8-LD), respectively.
2.6 Photoelectrochemical measurement
The uorine doped tin oxide (FTO) substrates were cleaned by
ultrasonication in distilled water and absolute ethanol for
30 min sequentially. One edge of the conducting glass
substrates was covered with adhesive tape. Typically, the
suspension was prepared by grinding 0.5 g of NaNbO3 powder,
2.5 g of ethyl cellulose (10%) ethanol solution and 1.75 g
terpineol. Then the homogeneous slurries were spread on a FTO
glass substrate by the spin-coating method. The lm was dried
in air and annealed at 550 ꢀC for 15 min. The photocurrent was
measured by an electrochemical analyzer (CHI660D Instru-
2.3 Physical characterization
Phase identication of the samples was determined by X-ray ment) in a standard three-electrode system with NaNbO3 lm as
diffraction (XRD, Rigaku Miniex II). The size and morphology the working electrode, Pt wire as the counter electrode, and
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Ag/AgCl electrode as a reference electrode. A 500 W Xe arc lamp diffraction spots and streakings observed in the SAED pattern
was utilized as the light source. A 0.5 M Na2SO4 aqueous solu- are correlated with the distortion of NbO6 octahedra and atomic
tion was used as the electrolyte.
displacement in the polar NaNbO3 structure (Fig. 3d), respec-
tively.16 Further details of this kind of superstructures can be
found in ref. 15a and b.
3 Results and discussion
It is known the NaNbO3 crystal is antiferroelectric along the c
axis and ferroelectric along the b axis.16 This means there is an
internal bias electric eld along the b axis and when the single
crystal nanocuboids assemble into oriented nanostructures the
internal bias electric eld can yield the greatest returns on the
separation as well as transportation of photon-generated
carriers (as shown in Fig. 1e). Instead, when the single crystal
nanocuboids assemble into disordered nanostructures the
randomly distributed internal bias electric eld within indi-
vidual nanocuboids will make less electrons and holes move to
the surface of the samples (as shown in Fig. 1c). Therefore, the
ordered and disordered NaNbO3 nanostructures fulll the cases
described in Fig. 1e and c respectively. Two photocatalytic
experiments were then carried out to examine the activity of the
samples.
The rst one is Rhodamine B (RhB) photodegradation in
aqueous solution. The NaNbO3 samples involved include the
ordered NNB-1, disordered NNB-8, and the disordered NNB-1
aer mechanical milling. From the temporal evolution of the
spectral changes (Fig. S2†), it is found that the adsorption
effects of the samples are negligible. The decrease in absorption
intensity as well as its wavelength shi suggest the de-ethylation
reaction and cleavage of the RhB chromophore ring structure17
occurred simultaneously during the photocatalytic experi-
ments. Using the major absorption band at about 554 nm to
determine the concentration of RhB, we can compare the pho-
tocatalytic activities of the ordered and disordered NaNbO3
samples, as shown in Fig. 4a. The disordered samples show
obviously weaker activities. For instance, aer 3 h illumination
under simulated sunlight the photodegradation efficiency of
RhB upon the ordered NNB-1 and disordered NNB-8 reaches
100% and ca. 60%, respectively. Further investigation of reac-
tion kinetics indicates that the ordered NNB-1 exhibits a much
higher apparent reaction rate constant (k ¼ 0.0249 minꢁ1) than
the disordered NNB-8 (k ¼ 0.00444 minꢁ1) (Fig. S3†). FTIR
results (Fig. S4†) reveal further the destruction of RhB chro-
mophore ring structure occurred more deeply upon the ordered
NaNbO3 sample than that upon the disordered ones since the
peak attributed to the characteristic vibration of the aromatic
ring structure18 became much weaker aer illumination upon
the ordered NaNbO3 sample.
In light of our previous experience on polar oxides,15 NaNbO3
semiconductor was taken as a research sample and was
fabricated via a hydrothermal route to corroborate the above
point of view. Scanning electron microscopy (SEM) images
(Fig. 2a–c) of the fabricated samples exhibit assemblages of
nanocuboids with side lengths varying from hundreds of
nanometers to several micrometers and by way of contrast,
with increasing the NaOH concentration of NaNbO3 nano-
cuboids assembled from ordered b-axis orientation (Fig. 2a
and b) to disordered random distribution (Fig. 2c). X-Ray
diffraction (XRD) patterns of the synthesized samples were all
identied to be a typical orthorhombic NaNbO3 structure type
(JCPDS 33-1270) with well developed crystallinity. No extra
peaks except for the orthorhombic NaNbO3 phase were
detected from the XRD analysis. Fig. 2d shows XRD patterns of
two typical samples (NNB-1 and NNB-8) with ordered and
disordered arrangement of NaNbO3 particles, respectively. The
fabrication conditions as well as the basic physical parameters
of the two samples are listed in Table 1.
Transmission electron microscopy (TEM) analysis of an
individual NaNbO3 particle further conrms its cuboid-shaped
structure with unequal side lengths of a–c plane (Fig. 3a). High-
resolution TEM (HRTEM) (Fig. 3b) reveals the highly crystalline
characteristics of the nanocuboid. The sharp lattice fringes with
an interplanar lattice spacing of 0.391 nm correspond to the
(101) atomic plane. The corresponding selected-area electron
diffraction (SAED) pattern (Fig. 3c and S1†) indicates a single-
crystalline feature of the nanocuboid. Note the weak satellite
In view of the fact that RhB can also harvest sunlight,
experiments with RhB photodegradation upon the ordered
NNB-1 and disordered NNB-8 under modied illumination
conditions were designed to test the possibly evolved effects
of photosensitization. From UV-vis absorption spectra
(Fig. S2†), it is clear that RhB can be excited by visible light
whereas the NaNbO3 samples can be excited only by a wave-
length shorter than ꢂ380 nm (Fig. 4b). Therefore, we used a
cut lter of 420 nm to excite the RhB but prevent the excita-
tion of the NaNbO3 samples. It was found that the RhB
degradation upon the disordered NNB-8 was governed by the
Fig. 2 (a–c) SEM images of: (a) and (b) NaNbO3 samples with ordered
nanocuboid assemblage taken from different directions; (c) the
NaNbO3 samples with disordered nanocuboid assemblage; (d) XRD
patterns of the ordered and disordered NaNbO3 samples prepared at
[NaOH] ¼ 1 M (upper one) and at [NaOH] ¼ 8 M (lower one).
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Table 1 Fabrication conditions as well as some basic physical parameters of two typical NaNbO3 samples
Band gap
(eV)
BET surface
Sample
Synthesis conditions
Lattice parameters
(m2 gꢁ1
)
˚
NNB-1 (ordered)
[NaOH] ¼ 1 M, 200 ꢀC, 24 h
a ¼ 5.473 A
3.26
1.313
˚
b ¼ 15.553 A
˚
c ¼ 5.537 A
˚
NNB-8 (disordered)
[NaOH] ¼ 8 M, 200 ꢀC, 24 h
a ¼ 5.535 A
3.26
4.488
˚
b ¼ 15.527 A
˚
c ¼ 5.508 A
Fig. 3 (a) TEM image, (b) HRTEM image, (c) ED pattern of a NaNbO3
nanocuboid (NNB-1 sample); (d) crystal structure of NaNbO3 where Na
ions are located in the cavities of NbO6 octahedra. Note the material is
ferroelectric along the b axis.16
Fig. 4 (a) Photodegradation of RhB upon the samples (along the
direction of the arrow with broken line): the ordered NNB-1 under
sunlight, the disordered NNB-1 after mechanical milling under
sunlight, the ordered NNB-1 under visible light, the disordered NNB-8
under sunlight, the disordered NNB-8 under visible light, and without
catalyst under sunlight; (b) UV-vis diffusive reflectance spectra of the
ordered NNB-1 and disordered NNB-8 samples; (c) CO2 generation
from CH4 photo-oxidation upon the samples (along the direction of
the arrow with broken line): without catalyst, Pt–NNB-1, Pt–NNB-1-
LD, NNB-8, Pt–NNB-8, Pt–NNB-8-LD, and NNB-1, under the same
sunlight illumination respectively; (d) SEM image of Pt–NNB-1 sample
prepared by the impregnation method. (e) Open circuit photovoltage
curves; (f) current–potential curves of thin films prepared by ordered
NNB-1 and disordered NNB-8 respectively.
indirect photosensitization and the high RhB degradation
activity upon the ordered NNB-1 (Fig. 4a) is largely determined by
the intrinsic excitation of the NaNbO3 semiconductor. From the
peak proles of XRD patterns (Fig. 2d) we know both the NNB
samples exhibit well developed crystallinity. The BET surface area
measurements indicate that BET surface area of the NNB-8 (4.488
m2 gꢁ1) is larger than that of the NNB-1 (1.313 m2 gꢁ1). From
these facts we can conclude that the state of assemblage of the
nanocuboids accounts for the differences of photocatalytic
activity between the NaNbO3 samples. The stability of the NNB-1
during photocatalytic reaction was further explored by recycling
tests (Fig. S5†). The photocatalytic activity of the ordered NaNbO3
nanocuboids decreases to some extent during the second cycle,
indicating the ordered nanostructure of the NNB-1 sample will be
destroyed aer long time stirring.
The second photocatalytic experiment is photocatalytic
oxidation of methane (CH4 + O2 / CO2 + H2O). The photo-
catalytic activities of the NaNbO3 samples were compared in
Fig. 5 Schematic illustration of the growth mechanism of the ordered
Fig. 4c (as well as shown in Fig. S6† the conversion of methane). NaNbO3 nanostructure.
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Obviously, the CO2 production rate upon the ordered NNB-1 is ordered NaNbO3 “cuboid-by-cuboid” arrays can only be
higher than that upon the disordered NNB-8. The effect of obtained under low NaOH concentration (e.g. 1 M) under xed
cocatalyst loading was investigated as well. Herein 1 wt% Pt dwelling time. Increasing the NaOH concentration (e.g. 8 M) will
was loaded by an impregnation method on NNB-1 (named as destroy the ordered nanostructures.
Pt–NNB-1) and NNB-8 (named as Pt–NNB-8), respectively. For
The inuence of dwelling time was performed by xing the
ꢀ
comparison purposes, the same amount of Pt was also loaded NaOH concentration (1 M) and reaction temperature (200 C)
by a photodeposition method on NNB-1 (named as Pt–NNB-1- (Fig. S11–S12†). The pure phase NaNbO3 was obtained aer 6 h
LD) and NNB-8 (named as Pt–NNB-8-LD), respectively. Both the reaction, as seen in Fig. S11.† The XRD patterns of the
samples prepared by the photodeposition method exhibit as-prepared sample for 0–3 h actually reect the chemical of
higher activity than their counterpart prepared by the impreg- Nb2O5. With increasing the dwelling time, the peaks attributed
nation method. In addition, regardless of the loading method to Nb2O5 disappear gradually and the new peaks attributed to
the Pt-loaded NNB-8 shows higher activity than the NNB-8. NaNbO3 emerge, which suggest that the process of dissolution–
However, it is interesting to note that both the Pt-loaded NNB-1 recrystallization happens for the preparation of orthorhombic
samples exhibit much lower activity than the NNB-1 sample in NaNbO3. The SEM images in Fig. S12† further reveal that there
gas phase photocatalysis, although the Pt–NNB-1 sample shows are two processes occurred simultaneously during the hydro-
signicantly higher activity on RhB degradation than the NNB-1 thermal processing. One is recrystallization of the dissolved
in aqueous environment (Fig. S7†). SEM observations of the Pt Nb2O5 on the surface of the undissolved Nb2O5 to form orien-
loaded NNB-1 samples indicate that the ordered nanostructure tated structures. The other is the generation of NaNbO3 nano-
of NNB-1 was destroyed during the process of Pt loading, as particles on the surface of the orientated structures of Nb2O5.
shown in Fig. 4d. Thus it can be concluded that keeping the The Nb2O5 assemblage at the initial stage of hydrothermal
ordered NaNbO3 nanostructure intact is important for the synthesis acts as a template inducing the growth of self-
improvement of CH4 oxidation.
assembled NaNbO3 ordered nanostructure. The growth process
The results of the above two experiments undoubtedly of ordered nanostructure is therefore schematically shown in
corroborate our point of view that the internal electric eld of a Fig. 5 and explained as follows:
polar semiconductor can be utilized to improve photocatalytic
During the hydrothermal treatment, the supercial part of
activity if the semiconductor particles and polarization direc- Nb2O5 particles rst hydroxylize and dissolve into the solution
tion are carefully controlled. Moreover, based on the descrip- with the aid of OHꢁ.19 At low NaOH concentration (e.g. 1 M), the
tion in Fig. 1 one can infer that (I) there is (photo)voltage alkalinity is too low to dissolve the whole Nb2O5 powder. Under
between the two end surfaces along the b axis of a NaNbO3 such a condition, two processes occur simultaneously. One is
nanocuboid and (II) photocurrent in the ordered NaNbO3 recrystallization of the dissolved Nb2O5 on the surface of the
nanostructure should be larger than that in the disordered undissolved Nb2O5 to form orientated structures. The other is
NaNbO3 nanostructure.
the generation of NaNbO3 nanoparticles on the surface of the
To examine this deduction we fabricated thin lm samples orientated structures of Nb2O5. The Nb2O5 orientated structure
using the ordered and disordered nanostructures respectively acts as a sacricial template in the subsequent stages. With
and measured their photoelectrochemical responses. SEM prolonging the dwelling time, the ordered NaNbO3 “cuboid-by-
observations indicate that the orientation characteristic of the cuboid” arrangement generates gradually under the process of
ordered nanostructure was partially retained (Fig. S8†). The nucleation and Ostwald ripening. Raising NaOH concentration
evidently larger open-circuit photovoltage (Fig. 4e) as well as tends to increase the solubility of Nb2O5. At high NaOH
larger photocurrent density (Fig. 4f) of the ordered NaNbO3 thin concentration (e.g. 8 M), the Nb2O5 is dissolved completely and
lm certainly support the aforementioned inference.
can not form orientated template in the following stages. Under
During the hydrothermal synthesis we did not obtain mono such a condition, what's obtained nally is NaNbO3 nano-
dispersed NaNbO3 (nano)crystals except for the assemblages of cuboids with disordered assemblage.
oriented NaNbO3 nanocuboids. The only reactants used in the
hydrothermal synthesis were pure Nb2O5 and NaOH. How the
orientated NaNbO3 nanostructures generated? To understand
4 Conclusions
this, a series of synthesis experiments were carried out where we In summary, we have suggested a strategy that utilizing internal
adjusted either NaOH concentration or dwelling time.
electric eld of polar semiconductors to improve photocatalytic
The relationship between the NaOH concentration and the activity and, the strategy is evidenced by both photocatalytic
microstructure of NaNbO3 was established by xing the reaction RhB degradation and CH4 oxidation upon the self-assembled
temperature (200 ꢀC) as well as dwelling time (24 h), as shown in NaNbO3 oriented nanocuboids that fabricated by a facial
Fig. S9 and S10.† When setting the concentration of NaOH from hydrothermal route. Given the resulting self-assembled
1 M to 8 M, all the prepared samples were identied by XRD to NaNbO3 oriented nanostructure as well as the strategy corrob-
be pure phase of NaNbO3. However, the morphologies of the orated, it is expected that the present results would trigger
samples prepared under different NaOH concentration were further interest not only in the fabrication of complex nano-
observed to be different as shown in Fig. S10.† When increasing structures with controllable crystalline morphology, orientation
the NaOH concentration to 8 M, the ordered nanostructures and surface architectures but also in the application of polar
were destroyed completely. Therefore, it is suggested that the semiconductors in solar energy utilization.
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X. Y. Xiao and L. Z. Zhang, J. Am. Chem. Soc., 2012, 134,
4473–4476.
Acknowledgements
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This work was nancially supported by the National Key Project
on Basic Research (Grant no. 2013CB933203), the Natural
Science Foundation of China (Grant no. 21373224), the Natural
Science Foundation of Fujian Province (Grant no. 2012J01242),
and the One Hundred Talent Program of the Chinese Academy
of Sciences.
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