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Pb electrodeposition from PbO in the urea/1-ethyl3-methylimidazolium chloride at room
temperature
Wencai He, Aimin Liu, Jinzhao Guan, Zhongning Shi,* Bingliang Gao, Xianwei Hu
and Zhaowen Wang
In this study, we dissolved PbO in a new electrolyte urea/1-ethyl-3-methylimidazolium chloride (EMIC) and
electrochemically extracted Pb. The electrochemical behavior of lead was investigated using
chronoamperometric and cyclic voltammetric techniques. The cyclic voltammograms illustrated that
lead reduction is quasi-reversible and follows a single-step, two-electron transfer process. The
chronoamperometric experiments showed that lead reduction in the PbO–urea/EMIC system follows
a 3D instantaneous nucleation and a diffusion-controlled growth at 353 K. The diffusion coefficient was
1.67 10
8
cm2 s 1. The effect of temperature on the electrochemical behavior of lead was
investigated. Results showed that electrode reaction becomes more reversible as temperature increases
from 343 to 373 K. The diffusion coefficients increase with increasing temperature and obey the
Arrhenius' law. The activation energy is estimated to be 38 kJ mol 1. Electrodeposition experiments were
Received 26th November 2016
Accepted 13th January 2017
conducted on tungsten substrates at different negative potentials and various temperatures. Scanning
electron microscopy (SEM) images of electrodeposits showed that the Pb particles became smaller and
are more densely distributed at more negative potentials and higher temperatures. The obtained
DOI: 10.1039/c6ra27383a
electrodeposits were metallic lead, as verified by X-ray diffraction (XRD) and energy dispersive
www.rsc.org/advances
spectroscopy (EDS).
1. Introduction
Lead is a useful material and is oen used in automotive leadacid batteries,1 semiconductors,2,3 and industrial X-ray shields.4
Conventionally, lead ore is processed by otation and roasting
forming PbO. Lead metal is extracted from the PbO by a hightemperature reduction in a coke-red furnace.5,6 However, this
conventional extraction method requires high temperatures.
Therefore, researchers have investigated the extraction of lead
from aqueous solutions at a relatively low temperature. These
aqueous solutions can be divided into two types: alkaline7–10 and
acid (nitrate,11,12 iodide,13 bromide,13 acetate,14 and methanesulfonate15). For example, Abrantes reported on an alkaline
solution (NaOH) that was applied to lead electrodeposition.8
Yao prepared lead nanoparticles from an acid aqueous solution
(Pb(CH3CO2)2–CH3CO2NH4–H3BO3) by adjusting the pH,
current density, deposition time and substrate materials.16
Although lead electrodeposition has been successfully conducted in alkaline and acid aqueous solutions, hydrogen
evolution on the electrode is inevitable in these systems, which
decreases current efficiency and causes massive hydrogen
embrittlement in lead products. Therefore, in this study, we
School of Metallurgy, Northeastern University, Shenyang, 110819, China. E-mail:
znshi@mail.neu.edu.cn; Fax: +86 24 83686464; Tel: +86 24 83686464
6902 | RSC Adv., 2017, 7, 6902–6910
attempted to electrodeposit lead metal using PbO as the lead
source in a new ionic liquid urea/EMIC at 353 K.
Room temperature ionic liquids (RTILs) have been
a research focus due to their hydrogen-free evolution, low
temperatures, non-ammability and negligible vapor pressure.17–19 RTILs can be considered as alternative electrolytes in
the lead electrodeposition process. Katayama et al.20 reported
on 1-butyl-1-methylpyrrolidinium bis(triuoromethylsulfonyl)
amide (BMPTFSA). Results showed that the difference of formal
potential between lead and tin was 0.11 V, indicating that it is
possible to separate lead from mixtures of lead and tin salts in
BMPTFSA. Bhatt et al.21 electrodeposited lead metal on a glassy
carbon from a DIMCARB ionic liquid (a mixture of dimethylamine and carbon dioxide). Wang et al.22 investigated the electrochemical behavior of lead in 1-butyl-3-methylimidazolium
chloride–AlCl3 system, illustrating a 3D nucleation model and
diffusion-controlled growth for lead deposition. They reported
that three phenomena (terrace expansion, island growth and
monolayer formation) and a moiré-like pattern of lead adatoms
were discovered with scanning tunneling microscopy. Similarity, Sun group23 investigated 1-ethyl-3-methylimidazolium
tetrauoroborate for its ability to electrodeposit lead metal.
Simons et al.24 reported lead electrodeposition from 1-ethyl-3methylimidazolium bis(triuoromethanesulfonyl)imide. The
results revealed that the morphology of deposits was affected by
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the substrate material. These RTILs used lead salts (Pb(NO3)2,
PbCl2, Pb(NTf2)2) as lead sources. However, the lead salts are
extremely toxic to humans.25 Therefore, as an alternative, we
attempt to use PbO as a lead source during electrolysis.
However, most metal oxides are not soluble in molecular
solvents except aqueous acids or alkalis. Thus, some taskspecic solvents have been investigated to dissolve metal
oxides. Hua group37 have prepared various morphologies of Pb
powders in PbO–choline chloride/urea (ChCl/urea) system. As
increasing of PbO concentration from 10 to 60 mM, different
morphologies, including corals, rods, wires, needles and ferns,
and dendritic forms, were discovered respectively. Reddy38
electrodeposited Pb metal from PbO–ChCl/urea system and
made fundamental studies on electrochemical behavior of lead
in this system, including diffusion coefficient, activation
energy, nucleation/growth model, and morphology characteristics. Sun group39 reported that water-insoluble Pb compounds
(PbSO4, PbO2, and PbO) have an acceptable solubility in ChCl/
urea. Therefore, Sun group used ChCl/urea as an electrolyte to
recycle Pb metal from Pb compounds. This may provide
a method to dissolve the electrodes used in lead-acid batteries.
Hua group40 have prepared sub-micrometer lead wires in PbO–
ChCl/urea system at 343 K. The reduction of Pb(II) in this system
is a diffusion controlled quasi-reversible process at 323 to 343 K.
However, to date, the electroreduction and nucleation mechanisms of lead in PbO–urea/EMIC system have not been elucidated. In this study, urea/EMIC electrolyte was able to
selectively dissolve lead oxides to support lead electrodeposition. Recently, EMIC has been used as an electrolyte in batteries
due to its cost-effectiveness, high capacity and safety.26 Therefore, in this research, we attempt to electrodeposit lead metal
from the PbO–urea/EMIC system, which may provide a potential
application for lead storage batteries.
In this study, PbO was dissolved in urea/EMIC as a lead
source to support the lead electrochemical experiments. The
electrochemical behavior of lead in this electrolyte was investigated using cyclic voltammetry (CV) and chronoamperometry.
The effect of temperature on the electrochemical behavior of
lead was investigated. The electrodeposits were characterized by
XRD, EDS and SEM.
2.
2.1
2.2
Electrodeposition of Pb
Electrochemical experiments were performed in a threeelectrode cell using a potentiostat and galvanostat (AUTOLAB,
Metrohm PGSTAT 30, Switzerland) controlled by GPES soware.29 For voltammetric and amperometric experiments,
a tungsten wire (0.05 cm in diameter) was used as the working
electrode, a platinum wire (0.05 cm in diameter) and a silver
wire (0.05 cm in diameter) were used as the counter electrode
and the quasi-reference electrode, respectively. All the electrodes in the electrochemical cell were polished with emery
paper, degreased with an anhydrous alcohol solution in an
ultrasonic bath, and cleaned with doubly deionized water
before use. For the electrodeposition experiments of Pb, tungsten foils (0.55 cm2) were used as the working electrode. The
counter electrode and the quasi-reference electrode are identical to that used in voltammetric and amperometric experiments. The electrodeposition experiments were conducted via
potentiostatic electrolysis in the glove box. Aer electrodeposition, electrodeposits were obtained on the tungsten substrates.
The electrolyte adhered to the surface of the electrodeposits was
cleaned with anhydrous alcohol. The phase constitution and
morphology of the electrodeposits were detected using XRD
(PANalytical MPDDY 2094, the Netherlands) and an SEM
instrument (ZEISS ULTRA-43-13, Germany) equipped with an
EDS (X-Max 50, Oxford).
3.
3.1
Results and discussion
Cyclic voltammograms
To study the chemical behavior of lead in urea/EMIC, CV was
recorded using a tungsten working electrode at 353 K (Fig. 1).
The potential window of the blank electrolyte (urea/EMIC in
2 : 1 molar ratio) was up to approximately 2.7 V. No any redox
peaks were discovered during this potential window, indicating
that the urea/EMIC was electrochemically stable in this potential range. Aer PbO was added and dissolved into the blank
Experimental
Chemicals and materials
EMIC (Lanzhou Institute of Chemical Physics China, >98%) was
puried before the experiment.27 Urea (Sinopharm Chemical
Reagent Co., Ltd, China, 99%) and PbO (Sinopharm Chemical
Reagent Co., Ltd, China, 99.99%) were dried according to
a procedure in a previous literature.28 Tungsten foils (99.99%,
0.3 mm thick) were used as the substrates for electrodeposition.
Urea and EMIC were mixed in a molar ratio of 2 : 1 and dissolved in a beaker at 353 K until a homogeneous, colorless
transparent liquid was obtained. Moreover, 45 mmol PbO was
added into this liquid that was continuously stirred to ensure
complete PbO dissolution. A PbO–urea/EMIC electrolyte was
resulting product. All the above mixing steps were performed in
a glove box (H2O: 0.1 ppm, O2: 0.1 ppm).
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Fig. 1 Cyclic voltammograms of blank electrolyte (urea/EMIC in 2 : 1
molar ratio) using a tungsten working electrode at 353 K. The scan rate
was 0.05 V s 1.
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electrolyte, a reduction peak appeared at approximately
0.416 V while a single oxidation peak occurred near 0.132 V
(vs. Ag), as shown in Fig. 2. Both redox peaks appeared within
the potential window range. The reduction peak is assigned to
the deposition of lead and the oxidation peak was related to the
stripping of lead deposited in the previous negative scan.
CVs recorded at different scan rates in the PbO–urea/EMIC
system are shown in Fig. 3. Reduction peak potentials shi to
a negative direction while oxidation peak potentials positively
increase in succession. Due to the effect of different scan rates,
the difference between the cathodic peak potential and the
cathodic half-peak potential |Epc
Epc/2| in Fig. 3 widens, as
seen in Table 1. The minimum of |Epc Epc/2| is 43.6 mV, which
is larger than the value for a reversible reaction (33.5 mV). This
suggests that the electrode reaction is quasi-reversible.30
Accordingly, the reduction peak current also increases with
increasing scan rate. A good linear correlation between reduction peak current and scan rate is presented in Fig. 4. All these
Paper
Table 1 Comparisons of the difference between the cathodic peak
potential and the cathodic half-peak potential in Fig. 3 under different
scan rates
n/V s
1
|Epc
0.01
0.02
0.04
0.06
0.08
0.12
0.16
0.24
Epc/2|/mV
43.6
61.5
67.4
71.4
79.3
103.1
111.1
109.1
Fig. 4 The cathodic peak current density obtained from Fig. 3 as
a function of the scan rates (ln n).
Fig. 2 Cyclic voltammograms of the urea/EMIC (2 : 1 molar ratio)
containing 45 mmol PbO using a tungsten working electrode at 353 K.
Cyclic voltammograms of the urea/EMIC (2 : 1 molar ratio)
containing 45 mmol PbO using a tungsten working electrode under
different scan rates at 353 K.
Fig. 3
6904 | RSC Adv., 2017, 7, 6902–6910
characteristics suggest that the reduction of lead in the PbO–
urea/EMIC system is quasi-reversible and follows a single-step,
two-electron transfer process.30
To investigate the effect of PbO concentrations on the electrochemical behavior of lead in the PbO–urea/EMIC system,
a set of CV are recorded at 353 K, as shown in Fig. 5. The current
density in cathodic peak and anodic peak increases as PbO
concentration increases. Moreover, equilibrium potentials in
CV shi toward the positive direction. This can be attributed to
the fact that the equilibrium potentials are changed with the
PbO concentrations. Accordingly, the cathodic peak potential
and anodic peak potential both anodically shi with increasing
of PbO concentration from 25 to 65 mmol. The differences in
peak potentials |Epa Epc| in Fig. 5 are within range of 261.8 to
309.4 mV, as seen in Table 2. The minimum of |Epa Epc| is
261.8 mV, which is larger than the value for a reversible reaction. This suggests that the reduction of lead in PbO–urea/EMIC
system is quasi-reversible process at various PbO
concentrations30
3.2
Chronoamperometry
To study lead nucleation/growth process in the PbO–urea/EMIC
system, chronoamperometric experiments were carried out at
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Cyclic voltammograms of different PbO concentrations in the
urea/EMIC (2 : 1 molar ratio) using a tungsten working electrode scan
rates at 353 K. (a) 25 mmol, (b) 35 mmol, (c) 45 mmol, (d) 55 mmol, (e)
65 mmol. Scan rate: 0.06 V s 1.
Fig. 5
Comparisons of the difference between the cathodic peak
potential and the anodic peak potential in Fig. 5 under different PbO
concentrations
Table 2
CPbO (mmol)
|Epa
25
35
45
55
65
261.8
309.4
291.6
269.7
309.4
Epc|/mV
This current is the faradaic current, indicating that the lead
nuclei have begun to nucleate and grow. The faradaic current
increases to a maximum, im, at time, tm, as the nuclei become
bigger and the discrete diffusion zones overlap. When the
cathodic potentials are negatively shied from
0.40 to
0.52 V, the tm values tend to decrease. This may be related to
that the overlap of discrete diffusion zones requires a shorter
time and the number of nuclei gradually increases at a more
negative cathodic potential. Aer tm, all current transients
decay slowly due to the increase in diffusion layer thickness and
the overlap of diffusion zones. Relationships between current
transients aer tm and time (t 1/2) are shown in Fig. 7. Some
linear correlations are obtained, suggesting that the growth of
lead nuclei is a diffusion-controlled process.
Two classical three-dimensional (3D) models, the progressive and the instantaneous, reported by Scharier and Hills are
oen used to analyze the nucleation/growth process in metal
electrodeposition.31,32 In the progressive model, the number of
metal nuclei is gradually increased during electrodeposition.
Conversely, fast nucleation occurs at a small number of activated sites in the instantaneous model. To identify the appropriate lead nucleation/growth model in the PbO–urea/EMIC
system, the experimental current density–time transients were
compared with theoretical eqn (1) and (2).
Progressive:
(i/im)2 ¼ (1.2254tm/t)(1
Current transient curves of tungsten electrode in 45 mmol
PbO–urea/EMIC (2 : 1 molar ratio) at different cathodic potentials.
Fig. 6
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(1)
exp[ (1.2564t/tm)])2
(2)
Instantaneous:
(i/im)2 ¼ (1.9542tm/t)(1
various potentials. These were negatively added from an initial
potential to a value sufficient for nucleation/growth. In the
chronoamperometric process, current density as a function of
time transients at 353 K are recorded in Fig. 6. These curves all
show the characterizations of nucleation/growth. First, the
current values drop quickly in a short time due to the double
electrode layer charge. Then the current reverses and increases.
exp[ (2.3367t2/tm2)])2
The results, shown in Fig. 8, reveal that the lead nucleation
in the PbO–urea/EMIC system follows a 3D instantaneous
model and a diffusion-controlled growth. The experimental
current densities were larger than the theoretical values at each
cathodic potential in Fig. 8. These are probably related to partial
kinetic control of the growth process.33,34
3.3
Effect of temperature
Fig. 9 shows the effect of temperature on CV in the PbO–urea/
EMIC system. As temperature increases from 343 to 373 K, the
reduction peak potential anodically shis, and the difference
between the reduction peak and oxidation peak potentials
narrows. This means that electrode reaction becomes more
reversible at a higher temperature. Moreover, equilibrium
potentials in CV shi toward the positive direction, as shown in
Fig. 10. Similarly, the currents at the reduction and oxidation
peaks also increase with increasing temperature. This is likely
related to the enhancement of species diffusion at a higher
temperature.
Fig. 11 shows the current density–time transients for the lead
nucleation/growth process at various temperatures. The current
reaches the maximum more quickly as temperature increases.
We obtained the correlation between current density (j) and
time (t 1/2) from the decreasing transient areas, as shown in the
Fig. 11 inset. We found a number of linear relationships whose
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Current density (j) as a function of time (t 1/2) for the decreasing
portions obtained from Fig. 5 during the lead electrodeposition on
a tungsten electrode.
Fig. 7
6906 | RSC Adv., 2017, 7, 6902–6910
Fig. 8 Comparisons of the dimensionless experimental curves ob-
tained from Fig. 5 for the urea/EMIC (2 : 1 molar ratio) containing
45 mmol PbO on a tungsten electrode with the theoretical models of
three-dimensional nucleation at different potentials and 353 K.
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Cyclic voltammograms of the urea/EMIC (2 : 1 molar ratio)
containing 45 mmol PbO using a tungsten working electrode at
various temperatures. The scan rate was 0.05 V s 1.
Fig. 9
Fig. 11 Current density as a function of time transients in chro-
noamperometric experiments for the urea/EMIC (2 : 1 molar ratio)
containing 45 mmol PbO on a tungsten electrode at various
temperatures. Cathodic potential: 490 mV, inset: corresponding
Cottrell plots.
Comparisons of lead diffusion coefficients (D) in the urea/
EMIC (2 : 1 molar ratio) containing 45 mmol PbO at various
temperatures
Table 3
The equilibrium potentials for the urea/EMIC (2 : 1 molar ratio)
containing 45 mmol PbO using a tungsten working electrode at
various temperatures (obtained from Fig. 8).
Fig. 10
slopes increased with temperatures from 343 to 373 K, as
a higher temperature can facilitate the diffusion of lead species
in the PbO–urea/EMIC system.
The diffusion coefficient can be determined using eqn (3) as
follow:31
im2tm ¼ 0.1629DF2n2(C0)2
(3)
where im is a current maximum in A, tm is the time of the
current maximum in s, D is the diffusion coefficient in cm2 s 1,
F is the Faraday constant 96 485 C mol 1, n is the number of
exchanged electrons, and C0 is the metal ion bulk concentration
in mol cm 3. The values of im and tm can be obtained from
Fig. 11. Substituting the values for n, F and C0 into eqn (3), the
diffusion coefficient for various temperatures are derived and
compared, see Table 3. The values of the diffusion coefficients
increased from 1.01 10 8 to 3.16 10 8 cm2 s 1 when the
temperature increased from 343 to 373 K. This may be related to
activation energy. In addition, a comparison of lead diffusion
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Temperature (K)
D (cm2 s 1)
343
353
363
373
1.01 10
1.67 10
2.17 10
3.16 10
8
8
8
8
coefficients in various ionic liquids is shown in Table 4. The
small difference may be related to different ionic liquid systems
and temperature. Based on the Arrhenius' law, the following
eqn (4) shows relationship between the diffusion coefficients
and the activation energy.35,36
Ea
(4)
D ¼ D0 exp
RT
where D is the diffusion coefficient in cm2 s 1, D0 is the
frequency factor, Ea is the activation energy, R is the gas
constant in 8.314 J K 1 mol 1, and T is the absolute temperature
in K.
The values in Table 3 were normalized as ln(D) versus 1/T, as
shown in Fig. 12. A linear relationship was obtained. The
function between ln(D) and 1/T was shown in eqn (5).
Summarization for lead diffusion coefficients (D) in various
ionic liquids
Table 4
Ionic liquid
Temperature (K)
107D (cm2 s 1)
Reference
Urea/EMIC
BMPTFSA
DIMCARB
[C2mim][NTf2]
353
298
295
293
0.167
0.8
1.8
1.3
This work
20
21
24
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Paper
Fig. 12 Lead diffusion coefficients (ln D) in the urea/EMIC (2 : 1 molar
ratio) containing 45 mmol PbO as function of temperature (T 1).
ln D ¼ 5:02767
4570:4
T
(5)
Based on this function, we determine the value of activation
energy (Ea) to be 38 kJ mol 1.
3.4
Electrodeposition and characterization of lead
Fig. 13 shows the SEM micrographs of the electrodeposits
obtained on tungsten substrates in the PbO–urea/EMIC
system. Uniform particles were obtained at
0.43 V
(Fig. 13a). With an increase in cathodic potential, the particles
became smaller and were more densely distributed. This may
be related to the increased number of growth sites and electrodeposition rate.
Fig. 14 shows the effect of temperature on the morphologies
of the deposits. At 343 K, we observed large particles that are
spherical in shape and have a low number density. When the
temperature increased from 353 to 373 K, the size of the particles progressively decreased while their density increased. Thus,
at a high temperature, the particles are smaller and denser. This
result is mainly due to enhanced charge transfer and faster
growth of lead crystallites at a high temperature, which suggests
that temperature has a crucial effect on lead electrodeposition
that the nuclei size will decrease and the nuclei density will
increase with raising the temperature.
EDS analysis of the electrodeposits produced at 0.52 V is
shown in Fig. 15. All peaks correspond to Pb and W substrate.
No other peaks were discovered, suggesting that the main
chemical constituent of the electrodeposits is lead.
Fig. 16 shows XRD analysis of the electrodeposits obtained at
0.52 V on the tungsten substrate. All peaks correspond to lead
and the tungsten substrate. 2q values (31.306, 36.267, 52.230,
62.121, 65.238, 85.397, 88.199) are indexed to the main peaks of
lead crystals (reference code ¼ 00-004-0686). The results of the
EDS spectrum and the XRD pattern conrm that the electrodeposits consist of lead metal.
6908 | RSC Adv., 2017, 7, 6902–6910
SEM micrographs of electrodeposits produced from the urea/
EMIC (2 : 1 molar ratio) containing 45 mmol PbO on tungsten
substrates at different cathodic potentials and 353 K for 30 min: (a)
0.43 V, (b) 0.46 V, (c) 0.49 V, (d) 0.52 V.
Fig. 13
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Fig. 15 EDS spectrum of electrodeposits produced from the urea/
EMIC (2 : 1 molar ratio) containing 45 mmol PbO on a tungsten
substrate at 0.52 V for 120 min. Temperature: 353 K.
Fig. 16 XRD pattern of electrodeposits produced from the urea/EMIC
system (2 : 1 molar ratio) containing 45 mmol PbO on a tungsten
substrate at 0.52 V for 120 min. Temperature: 353 K.
4. Conclusions
Fig. 14 SEM micrographs of electrodeposits produced from the urea/
EMIC (2 : 1 molar ratio) containing 45 mmol PbO on tungsten
substrates at 0.49 V for 30 min. Temperatures: (a) 343 K, (b) 353 K, (c)
363 K, (d) 373 K.
This journal is © The Royal Society of Chemistry 2017
PbO was dissolved in urea/EMIC at 353 K and used as a lead
source for supporting electrochemical experiments. CV results
showed that lead reduction in this electrolyte is quasi-reversible
and follows a single-step, two-electron transfer process. Lead
reduction in the PbO–urea/EMIC system follows a 3D instantaneous nucleation and a diffusion-controlled growth at 353 K,
which were veried by chronoamperometric experiments. The
diffusion coefficient was 1.67 10 8 cm2 s 1. As temperature
increases from 343 to 373 K, electrode reaction becomes more
reversible. The diffusion coefficients obey the Arrhenius' law.
The activation energy is estimated to be 38 kJ mol 1. Lead
electrodeposition was conducted on tungsten substrates at
different negative potentials ( 0.43, 0.46, 0.49, 0.52 V) and
various temperatures (343, 353, 363, 373 K), respectively. SEM
images showed that the deposits became smaller and were
more densely distributed with an increase in cathodic
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potentials and higher temperatures. The deposits were identied as lead metal using XRD and EDS.
Acknowledgements
The authors would like to acknowledge the nancial support
from the National Natural Science Foundation of China (No.
51322406, 51474060 and 51574071); the Program for New Century
Excellent Talents in University (NCET-2013-0107), Ministry of
Education of China and the Fundamental Research Funds for the
Central Universities (N140205001&L1502014).
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