Article
pubs.acs.org/crystal
ZnMoO4 Micro- and Nanostructures Synthesized by
Electrochemistry-Assisted Laser Ablation in Liquids and Their Optical
Properties
Y. Liang, P. Liu, H. B. Li, and G. W. Yang*
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State Key Laboratory of Optoelectronic Materials and Technologies, Institute of Optoelectronic and Functional Composite Materials,
Nanotechnology Research Center, School of Physics & Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong,
People's Republic of China
ABSTRACT: Electrochemistry-assisted laser ablation in liquids (ECLAL) is a
chemically “simple and clean” synthesis of nanoparticles. Using ECLAL, we have
synthesized zinc molybdate nanoplates and nanorods with two different phases.
These two nanostructures are characterized carefully by scanning electron
microscopy, transmission electron microscopy, X-ray diffraction analysis, Fourier
transform infrared spectroscopy, Raman scattering spectroscopy, and UV−vis
spectrophotometry. On the basis of the cathodoluminescence measurements, we
observe that the nanoplates emit no light, while the nanorods give out green light
before annealing. The optical properties of both get much better after annealing,
which indicates their potential applications in photoelectric nanodevices. The basic
physics and chemistry involved in the ECLAL fabrication and the luminescence
mechanism for products before and after annealing are discussed.
thermal methods,16,17 electrospinning calcination combinations,18 or solid state reactions19 with the above obvious
flaws. In this contribution, we synthesize mass zinc molybdate
nanoplates and nanorods with two different phases at one time
and observe different optical performance of the as-synthesized
samples before annealing. Interestingly, these nanostructures
broke down and crystallized, respectively, and exhibit much
better optical performance after annealing, which indicates their
potential applications in photoelectric nanodevices. These
investigations have further demonstrated that the ECLAL
method is a general strategy for the fabrication of simple POM
nanostructures and also an effective method for the fabrication
of rare mineral hydrates, which are hard to synthesize by
ordinary chemical methods.
1. INTRODUCTION
Polyoxometalates (POMs) represent a diverse range of microand nanoclusters based upon oxides of at least binary metal
containing Mo, W, V, or Ta and a second transition metal such
as Cu, Fe, Co, or Ni species. Such cluster building blocks can be
self-organized in complex framework which leads to functional
materials with diverse properties such as optical, electronic,
magnetic, and catalytic activity.1 To improve the performance
of POMs, several techniques for the synthesis of POM
nanostructures have been developed in recent years. However,
these techniques have many obvious flaws, for example, hightemperature or high-pressure environment, various templates
or additives, and demanding complicated synthetic procedures
causing impurities in final products, which greatly restrict their
application in industry. In this study, combining laser ablation
in liquids (LAL)2−8 and electrochemistry, we develop a facile
synthesis for simple POM nanostructures, that is, electrochemistry-assisted laser ablation in liquids (ECLAL). This
chemically “simple and clean” technique is performed in an
ambient environment without extreme temperature and
pressure and can be designed by combining interesting solid
targets, electrodes, and mother solutions.9,10 Herein, for the
fabrication of zinc molybdate nanostructures, we chose
molybdenum as a solid target for laser ablation, zinc electrodes
for the electrochemical reaction, and deionized water as a
solvent for the ECLAL synthesis.
As a typical transition metal material, zinc molybdates,
commonly applied as luminescent materials,11,12 croyogenic
phonon-scintillation detectors,13 or catalysts,14 have roused the
attention of scientists in recent years. They are mainly
synthesized by precipitation heat-treatment methods,15 hydro© 2012 American Chemical Society
2. EXPERIMENTAL PROCEDURES
2.1. Materials Preparation. The detailed the experimental setup
of ECLAL synthesis has been reported in our previous works.9,10 In
this case, a single crystalline Mo target with 99.95% purity is used as
the starting material and is initially attached to the bottom of the
rectangular quartz chamber with dimensions 6.0 × 3.0 × 6.5 cm3.
Deionized water (highly pure water) with resistivity of 16.8−17.2
MΩ·cm was then poured slowly into the chamber until the target was
covered by 2−3 mm. Meanwhile, a DC electrical field with an adjusted
voltage of 160 V, which is brought by two parallel electrodes with a
distance of 50 mm, is applied above the target. The second harmonic is
produced by a Q-switch Nd:YAG laser device with a wavelength of
532 nm, a pulse width of 10 ns, a repeating frequency of 5 Hz, and a
Received: May 15, 2012
Revised: July 29, 2012
Published: August 7, 2012
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Figure 1. SEM images of the synthesized coexisting zinc molybdates (a), micro- and nanoplates (b), and rods (c) and their corresponding images
after annealing (d, e).
pulse energy of 150 mJ/pulse. Finally, the pulsed laser was focused
onto the surface of the Mo target. During ablation, the target and
liquid environment were both maintained at room temperature. After
the whole intersection, which lasted for 30 min, the solution was
evaporated and collected for further measurements. To further
investigate the properties of zinc molybdates, the as-synthesized
samples were annealed at 500 °C in high vacuum furnace for 2 h.
2.2. Characterization Techniques. Scanning electron microscopy (SEM) images of the as-synthesized samples were obtained by a
JSM-7600F field emission scanning electron microscope operated at
15 kV. Before observation, the samples were covered by a gold layer to
decrease the charge effect during the analysis. X-ray diffraction (XRD)
was performed with a Rigaku D/Max-IIIA X-ray diffractometer with
Cu Kα radiation (λ = 1.54056 Å, 40 kV, 20 mA) at a scanning rate of
1° s−1, and transmission electron microscopy (TEM) was carried out
with a JEOL JEM-2010HR instrument at an accelerating voltage of
200 kV, equipped with an energy-dispersive X-ray spectrometer
(EDS). These techniques are used to identify the crystal structure and
morphology of the products. Inductively coupled plasma-atomic (ICP)
emission spectrometry using a ThermoFisher iCAP6500Duo has been
employed to analyze the chemical content of starting materials, with an
incident power of 1150 W, a plasma gas flow of 14 L/min, and an
atomization gas flow of 0.6 L/min. To gain molecular vibration
information, Raman spectra (RS) and Fourier transformation infrared
(FTIR) spectra of samples deposited on Si substrate were recorded on
a Renishaw inVia Plus laser micro-Raman spectrometer detected with
Ar ion laser irradiation of λ = 514.5 nm and Bruker EQUINOX55
spectrometer coupled with infrared microscope, respectively. UV−vis
spectrum of the obtained liquid sample was also obtained by a UV
3150 spectrophotometer (Shimadzu, Japan). Moreover, cathodoluminescence (CL) spectroscopy with a Gatan MonoCL3 system attached
to a field emission scanning electron microscope was employed to
characterize the luminescence of the synthesized nanostructures at
room temperature.
3. RESULTS AND DISCUSSION
3.1. Morphology and Structure Characterization.
Figure 1 shows the morphology of the as-synthesized sample.
The products consist of homogeneous micro- and nanoplates
and rods under the same experiment conditions. The
corresponding high-magnification image demonstrates that
the nanoplates are actually structured as a flower-like shape
with a perfect three-dimensional geometry, about 2−3 μm in
diameter, 100 nm in thickness, and nanoscaled surface
smoothness, as shown in Figure 1b. Meanwhile, the high4488
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are more likely to be easily hydrated Zn5Mo2O11·5H2O and
nanorods are likely to be unhydrous ZnMoO4 since they
behave differently in heating process. In other words, the
Zn5Mo2O11·5H2O may decompose during the thermal treatment while ZnMoO4 remain the same at 500 °C.
Figure 3a,e shows the TEM bright-field images of a single
nanoplate and rod gently transferred onto a copper grid by
ultrasonication. HRTEM examination indicates that the
nanoplate is of single crystal structure without visible defects,
as shown in Figure 3b. This HRTEM image clearly shows the
interplanar spacing, measured as 0.272 and 0.268 nm, in good
agreement with the d values of the (22̅1) and (202)
crystallographic planes of the rhombohedral Zn5Mo2O11·5H2O.
Figure 3c gives the corresponding selected area electron
diffraction (SAED) pattern, which further proves the singlecrystal nature of nanoplates. By calculation, the diffraction point
can be indexed to (22̅1), (202), and (021) facet of
Zn5Mo2O11·5H2O. Similarly, HRTEM examination of a single
rod shows that the nanorod is of single-crystal structure. The
fringe pattern is not clear-edged and regularly arranged as highcrystallinity crystals are, which indicates that the nanorods are
not well crystallized. The planar spacing, measured as about
0.667 and 0.631 nm in Figure 3e, is very close to (1̅10) and
(01̅1) facets of triclinic ZnMoO4. Its corresponding Fourier
transform SAED analysis in Figure 3f further proves the poor
crystallinity of nanorods since the diffraction point is broadened
and is identified as (1̅10) (01̅1) and (1̅01) facets of ZnMoO4.
Calculation shows that the (1̅ 2̅ 3 ) crystal planes are
perpendicular to the axis of the nanorods, indicating that
⟨1̅2̅3⟩ is the preferred growth direction of ZnMoO4. A
reasonable explanation is that the relatively low free energy of
growth of the (1̅2̅3) crystal plane leads to the ⟨1̅2̅3⟩ direction
being preferable. The EDS results, performed on HRTEM, are
illustrated in Figure 4. In addition to the carbon and copper
signal from the carbon-coated copper grid, the products are
composed of Zn, Mo, and O elements. Therefore, the TEM
results assuredly confirm that the synthesized nanoplates are
hydrate Zn5Mo2O11·5H2O and the rods are anhydrate
ZnMoO4, which corresponds to the SEM and XRD results.
Quantitative correlation between Zn5Mo2O11·5H2O and
ZnMoO4 has been obtained using the ICP technique. The
mass concentration of Zn and Mo elements is determined to be
16.01 and 7.02 mg/L; thus, the Moore percentage of Zn/Mo
under the same volume is about 3:2; therefore we can infer the
relative content of Zn5Mo2O11·5H2O and ZnMoO4 is about
4:1, which means the output of nanorods is far less than the
nanoplates in our experiment. And it is not too hard to
understand that the XRD results only contain diffraction peaks
of the Zn5Mo2O11·5H2O before annealing.
Now let us discuss the nucleation mechanism of zinc
molybdates nanocrystal synthesis upon ECLAL. The general
formation mechanism of nanostructures upon laser ablation in
liquid has been addressed in our groups’ previous work.8 When
laser ablates the solid Mo target, Mo ions and other species
plasma plume will be generated at the surface between the
liquid and solid under high temperature, high pressure, and
high density conditions, and H+, OH−, and O2− species results
from the deionized water at the same time. Mo species can
easily combine with O 2− species to form the initial
molybdenum oxide (MoO3 or even MoO42− ions) microand nanostructure nuclei. Meanwhile, the applied electric field
is suggested to play an important role in the formation
mechanism of nanostructures by ECLAL.9,10 An electro-
magnification image of the nanorods (Figure 1c) shows a
tendency of oriented growth, about 100 nm in diameter and
several micrometers in length. Careful observation reveals
several nanoscaled holes at the ends of the rods, which means
their crystallinity is not sufficient. These samples are annealed
at 500 °C in a high vacuum furnace for 2 h, and the
corresponding SEM images show different results for nanoplates and rods. Figure 1d reveals several micro- or nanoholes
formed on the surface and inner parts of the plates, which
indicates their decomposition. In contrast, the surface of
nanorods become smoother with clear edges and corners
shown in Figure 1e, which indicates improved crystallinity.
The corresponding XRD patterns for unannealed sample are
illustrated in Figure 2a. All peaks can be indexed to
Figure 2. The corresponding XRD pattern of the products obtained
before (a) and after (b) anealing. The colored lines at the bottom
correspond to the standard XRD patterns of different zinc molybdates.
rhombohedral zinc molybdenum oxide hydrate
Zn5Mo2O11·5H2O (JCPDS Card File No. 30-1486), except
one peak for Si substrate as shown. Only the highest peak is
assigned to be the (003) plane of Zn5Mo2O11·5H2O. The
crystalline structure of annealed samples is also performed as
shown in Figure 2b. Only one peak marked with a blue line can
be indexed to triclinic ZnMoO4 (JCPDS Card File No. 350765), and the other three peaks are assigned as Au (JCPDS
Card File No.65-2870), which results from the spraying
process. SEM and XRD results demonstrate that the nanoplates
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Figure 3. TEM bright field images of a single zinc molybdate micro- and nanoplates and rods (a, d). The corresponding SAED patterns (b, e) and
HRTEM images (c, f).
due to lowering of the nucleation free energy barrier.20 So in
our case, the nucleation mechanism of zinc molybdates is most
likely to be fast self-organized heterogeneous germination in
the tridimensional space. When the crystal nucleus forms, the
Zn5Mo2O11 ·5H2O and ZnMoO4 begin to form large particles.
Two factors may be responsible for the formation of these
nanoplates and nanorods. On the one hand, different kinds of
chemical reactions have taken place in the liquid environment,
which offers crystal seeds of Zn5Mo2O11·5H2O and ZnMoO4
nanostructures for growth. On the other hand, the applied
electric field can provide extra electrostatic energy and induce
the formation of different morphologies based on the theory of
crystalline habits by weakening of the surface energies. The
TEM result, which shows that the high index ⟨1̅2̅3⟩ is the
preferred growth direction of ZnMoO 4 , indicates the
importance of extra electrostatic energy during its growth
process.
As is well-known, laser ablation in liquid is a very fast process
and far from equilibrium, meanwhile the electric field is very
strong, which can be estimated as 2.7 × 103 V/m. Thus, the
whole process is very effective under the high-temperature,
chemical reaction happened: Zn2+ ions easily dissolve into the
liquid environment under strong applied electric field and form
Zn(OH)2 in the presence of H and O ions, which are also
electrolyzed by the applied field. The reaction equation is
Zn + 2H 2O → Zn(OH)2 + H 2
(1)
Then, a chemical reaction takes place between Zn species and
Mo species. These two highly active species can easily crash
into each other during the plasma quenches under the strong
electric field, which leads to the formation of zinc molybdate
metastable clusters.
2MoO3 + 5Zn(OH)2 → Zn5Mo2O11·5H 2O
(2)
MoO4 2 − + Zn 2 + → ZnMoO4
(3)
The first step for crystal nucleation is often facilitated by
heterogeneous centers.20,21 These foreign bodies range from
solid or liquid particles to the wall of the container and even the
clusters of metaphase of nucleating materials. The generally
accepted mechanism of heterogeneous nucleation is that it
follows the kinetic law for homogeneous nucleation but is faster
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Figure 4. FTIR spectrum (a), Raman spectrum (b), and UV−vis absorption spectrum (c) of the as-synthesized zinc molybdates. Panel d is the
typical Tauc plot derived from the UV−vis absorption.
895−950 and 810−880 cm−1.27,28 As we can see, bands at 913
and 866 cm−1 for nanoplates and bands at 950 and 860 cm−1
for nanorods can be assigned to symmetric stretching (ν1) and
asymmetric stretching (ν 3 ) vibration modes of MoO 4
tetrahedral units of zinc molybdates.28 The Raman band
observed at 815 cm−1 for nanorods may result from a stretching
vibration in Mo−O bonds because of the distorted MoO4
tetrahedral units. The Raman band at 345 cm−1 for nanorods
and 309 cm−1 for nanoplates can be assigned to the symmetric
bending vibration (ν2) or asymmetric bending vibration (ν4),
which usually lies in the region of 300−520 cm−1.28,29
Therefore, FTIR and Raman data further show that the
synthesized nanostructures are zinc molybdates.
3.3. UV−Vis Absorption. UV−vis spectroscopy is
employed for understanding the optical properties of the assynthesized zinc molybdates, and the data are shown in Figure
4c. Clearly, in the UV spectrum, there are two absorption bands
located at 235−250 and 210−220 nmy, which indicate two
different phases of the as-synthesized zinc molybdates
corresponding to the previous results. Typically, the optical
bandgap (Eg) of semiconductor materials can be estimated by
the classical Tauc approach,30,31 which presents the relationship
between the incident photoenergy (hν) and the absorption
coefficient (α) near the absorption edge, as follows
high-density, and high- pressure (HTHPHD) state and high
external electric field. In addition, suitable experiment
parameters play an important role in the formation of these
nanostructures.
3.2. FTIR and Raman Spectra. The chemical structure
information of the zinc molybdates is further identified by the
FTIR spectrum. Figure 4a shows the corresponding result in
the range of 400−4000 cm−1. Very few papers in the literature
have reported the FTIR spectrum of Zn5Mo2O11·5H2O and
ZnMoO4, and our analysis is based on molybdates. Several
absorption bands of samples are present; generally, the infrared
bands at 3324 and 1650 cm−1 correspond to the OH stretching
vibration and bending vibration of water molecules.22 Since we
have measured the blank sample for comparative analysis, the
bands at 1106 and 613 cm−1 are due to the SiO2 substrate.23
Other bands that are located in the range of 700−1000 cm−1
wavenumbers are mainly caused by [MoOy]n−.24,25 Bands at
1509, 1396, 2922, and 2850 cm−1 may be attributed to the
organic contamination of the obtained samples during the
sample preparation. Finally, the band at 473 cm−1 is most likely
to be Zn−O stretching mode in zinc molybdates.26
Raman scattering spectra of zinc molybdate nanoplates and
nanorods in the wavelength range 50−1200 cm−1 are also
displayed in Figure 4b. Usually, the symmetric stretching
modes (ν1) and asymmetric stretching modes (ν3) of MoO4
tetrahedral units are Raman active and observed in the region
αhν = A 0(hν − Eg )n
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The value of the exponent depends on the mechanism of
interband transition (for example, n = 1/2 for direct transitions
and n = 2 for indirect transitions). A0 is a constant called the
band tailing parameter, and Eg is the intercept of the
extrapolated linear when (αhν)1/n is plotted against hν. Figure
4d shows an example of a typical Tauc plot. In our case, the
band gap value of indirect transition for zinc molybdates is
estimated to be 4.48 and 4.72 eV from the above plots,
respectively, and 4.48 eV is close to the theoretical value of the
band gap of triclinic ZnMoO4 as reported.32 The band gap of
Zn5Mo2O11·5H2O nanoplates is determined to be 4.72 eV for
the first time by our experiment.
3.4. Luminescence Performance of Zinc Molybdates.
We measured the cathodoluminescence of the as-synthesized
samples at room temperature, and the CL spectra are shown in
Figure 5. It was found that these zinc molybdate nanostructures
display different properties when annealed at 500 °C in high
vacuum furnace for 2 h. Specifically, for nanoplates, there are
two peaks after annealing, one broad emission band centered at
370 nm and another emission band centered at 510 nm, while
no peak shows before annealing. For nanorods, there is only
one emission band centered at 530 nm before and after heat
treatment; nevertheless, the luminous intensity of annealed
nanorods increased almost 30 times compared with unannealed
ones. The inset in Figure 5a,b shows the SEM images and
corresponding panchromatic CL images for unannealed
nanoplates and nanorods, while those in Figure 5c,d are images
for the annealed ones. By comparison of the panchromatic CL
images, both nanostructures also exhibited much better
luminescence performance after annealing.
Now we discuss the whole luminescence mechanism of
samples during the process. Luminescent properties of triclinic
ZnMoO4 small single crystals and bulk crystals have been
reported only by a few papers.32−36 It was found that zinc
molybdates belong to the class of self-activated phosphors, and
the intrinsic emission of ZnMoO4 originates from the radiative
annihilation of self-trapped excitons on MoO42− complexes,
similarly to other self-activated tungstates and molybdates of
AXO4.32,37 The emission peak at 530 nm for nanorods in our
experiment is close to the experimental result of 544 nm for
bulk ZnMoO4 crystals by X-ray excitation at room temperature.35 For annealed nanoplates, CL spectra show an obvious
peak at 370 nm, which is close to 380 nm (3.26 eV) and can be
attributed to the near band edge emission.38 Therefore, we infer
that the nanoplates broke down into other phases, such as ZnO
and ZnMoO4, while the nanorods crystallized during the
heating process, which can take place as follows:
Zn5Mo2O11·5H 2O → 2ZnMoO4 + 3ZnO + 5H 2O
(5)
The reaction equations are confirmed by XRD results of
samples annealed as shown in Figure 2. In our case, ZnO is not
detected because of its inhomogeneity and low content. Based
on above analysis, we can have a clear and general insight into
the basic mechanisms involved in the whole process: The
nonluminous Zn5Mo2O11·5H2O nanoplates broke down into
luminous ZnO (emission peak at 370 nm) and ZnMoO4
(emission peak at 510 nm) after annealing, while the greenemitting ZnMoO4 nanorods (peak at 530 nm) crystallized and
the luminous intensity improved several times. The peak shift
may result from imperfections of various origins (defects,
inadvertent impurities during the crystal growth process, or
compound decomposition processes) and the difference
between nanometer materials and bulk ones.32 Overall, the
whole luminescent process for zinc molybdate nanoplates and
rods are controllable, which indicates their potential applications in photoelectric nanodevices such as light-operated
switches.
4. CONCLUSION
In summary, the coexisting zinc molybdate micro- and
nanoplates and nanorods have been successfully synthesized
upon ECLAL. It was found that the as-synthesized zinc
molybdate nanostructures are identified as nonluminous
Zn5Mo2O11·5H2O nanoplates and green-emitting ZnMoO4
nanorods, and the luminescence properties of both nanostructures become much better after annealing. The controllability
of the fabrication indicated their potential applications in
photoelectric nanodevices. Our investigations have further
demonstrated that ECLAL is a general strategy for fabricating
simple POMs with specific properties and also an effective
method for the fabrication of rare mineral hydrates, which are
hard to synthesize by other means.
Figure 5. The room-temperature CL spectrum obtained from zinc
molybdate micro- and nanoplates and rods. The insets in each graph:
(a) SEM images of several plates or rods before annealing; (b)
corresponding panchromatic CL images; (c) SEM images of a single
plate or several rods after annealing; (d) corresponding panchromatic
CL images.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: stsygw@mail.sysu.edu.cn.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
NSFC (Grants U0734004 and 11004253) and the Ministry of
Education supported this work.
■
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