Synergistic effect for the preparation of LiMn2O4 microspheres with high electrochemical performance

Article abstract of DOI:10.1039/c3ra44571b

Under the synergistic promotion of chemical reactions, MnO₂ microspheres are successfully prepared using MnCO₃ precursor as self-template, with maximum use of the raw materials, and are then converted into spinel LiMn₂O₄ microspheres via a solid-state reaction. When applied as cathode materials for rechargeable lithium-ion batteries, the LiMn₂O₄ microspheres deliver a discharge capacity of 133.2 mA h g^-1 and 109.8 mA h g^-1 at 0.1 C and 10 C rates, respectively. More than 91.3% of the initial storage capacity is maintained after 1000 cycles at a 1 C charge and 10 C discharge rate. Furthermore, even when cycled at elevated temperature (55 °C) at a 1 C charge and 10 C discharge rate, this material showes 86.2% capacity retention after 100 cycles, which reveals a high reversible capacity, superior rate capability and excellent cycling stability under high rates both at room temperature and high temperature. The performance is comparable to the best results shown in the literature so far. This structured LiMn₂O ₄ microspheres in the present work are very promising for large-scale commercialization of high-power lithium ion batteries.

Full text of DOI:10.1039/c3ra44571b

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Synergistic effect for the preparation of LiMn O
2
4
microspheres with high electrochemical
Cite this: RSC Adv., 2014, 4, 3293
performance†
Pengli Zhu,‡*ab Xiangli Chu,‡ Fengrui Zhou, Rong Sun* and Chingping Wong*
a
a
a
bc
Under the synergistic promotion of chemical reactions, MnO
using MnCO precursor as self-template, with maximum use of the raw materials, and are then
converted into spinel LiMn microspheres via a solid-state reaction. When applied as cathode
materials for rechargeable lithium-ion batteries, the LiMn microspheres deliver a discharge capacity
2
microspheres are successfully prepared
3
2 4
O
2
O
4
ꢀ1
ꢀ1
of 133.2 mA h g and 109.8 mA h g at 0.1 C and 10 C rates, respectively. More than 91.3% of the initial
storage capacity is maintained after 1000 cycles at a 1 C charge and 10 C discharge rate. Furthermore,
ꢁ
even when cycled at elevated temperature (55 C) at a 1 C charge and 10 C discharge rate, this material
showes 86.2% capacity retention after 100 cycles, which reveals a high reversible capacity, superior rate
capability and excellent cycling stability under high rates both at room temperature and high
temperature. The performance is comparable to the best results shown in the literature so far. This
Received 22nd August 2013
Accepted 23rd October 2013
DOI: 10.1039/c3ra44571b
structured LiMn
2
O
4
microspheres in the present work are very promising for large-scale
www.rsc.org/advances
commercialization of high-power lithium ion batteries.
layered LiCoO
2
cathode materials for use in large-scale lithium
Introduction
5,8
rechargeable batteries. Despite the above mentioned advan-
Nowadays, the development of new cathode materials with tages, the observed electrochemical performances of LiMn O
2 4
better performance and safety continues to dominate the focus were found to be less impressive, with several problems, such as
of research on lithium-ion rechargeable batteries (LIB), which capacity decay and poor cycling stability especially at elevated
have bright future applications in portable electronics, temperatures, which are mainly due to the adverse conse-
stationary energy storage and next generation automobiles, quences of phase transition from the cubic to the tetragonal
such as electric vehicles (EV), hybrid electric vehicles (HEV) and phase, the dissolution of manganese in the electrolyte, elec-
1
–6
plug-in hybrid electric vehicles (PHEV). Applications in EV trolyte decomposition in the high potential regions and the
8–10
and HEV need LIB cathode materials with special properties, changes of crystallinity or particle shape.
As for LIBs, the
such as high power, high energy density, excellent rate capa- capacity of the whole battery system is usually limited by
3
,7
bility, outstanding cycle stability, good safety and low cost.
LiMn
, with a spinel structure is regarded as a prospective trochemical reversibility and stability of LiMn
candidate for these cathode materials, owing to its low cost, and elevated temperatures.
high power density, excellent structure stability, environmental
In recent years, it has been generally acknowledged that the
benignity, and the abundance of manganese resources. As such, down-sizing strategy owing to the short Li ion diffusion length
LiMn
with a 3-D lithium pathway is currently considered as can mitigate such problems, and nanostructured LiMn with
one of the most suitable substitutes to replace conventional various morphologies have been prepared. While the
nanostructured LiMn O is effective for improving the perfor-
the cathode, thus, it still very important to improve the elec-
at high rate
O
4
2 4
O
2
8,11–13
+
2
O
4
2 4
O
14–23
2
4
mance of the electrode, it adversely affects the tap density and
volumetric energy density. Generally, it is believed that the
a
Shenzhen Institutes of Advanced Technology, Chinese Academy of Science, 1068
Xueyuan Avenue, Shenzhen University Town, Shenzhen, 518055, China. E-mail: pl.
micro-size spherical particles can easily form close packed
zhu@siat.ac.cn; rong.sun@siat.ac.cn
b
8
School of Materials Science and Engineering, Georgia Institute of Technology, 771 arrays. Therefore, LiMn
2
O
4
with a micro-/nano-hybrid struc-
Ferst Drive, Atlanta, GA, 30332, USA
ture which might provide a balance between the high tap
density and short diffusion distance is highly desirable for the
design of high-performance lithium-ion batteries with high
c
Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin
NT, Hong Kong SAR, China. E-mail: cpwong@cuhk.edu.hk
†
Electronic supplementary information (ESI) available: Photographs of the
8
volumetric energy density and good rate capability. Ryu et al.
reaction; calculation of the yield of MnO
CVs results. See DOI: 10.1039/c3ra44571b
2 2 4
; BET of as-prepared LiMn O ; and
reported the synthesis of LiAl0.02Mn1.98O nanothorn spheres
4
and found that they show better electrochemical performance
‡
P. L. Zhu and X. L. Chu contributed equally to this work.
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5
than nanorods and solid spheres. Double-shelled hollow microspheres are prepared by a chemical lithiation process (eqn
ꢁ
microspheres of LiMn O reported by Ding et al. revealed a (4)) when MnO intermediates are calcined with LiOH at 750 C
2
4
2
superior rate capability and excellent cycling stability under for 12 h. Briey, the whole procedure involves four primary
10
high rates. Qiao also reported the yolk-structured LiMn O
related chemical reactions which are shown as follows:
2
4
microspheres possess remarkable stability during electro-
8
3MnCO + 2KMnO / 5MnO + K CO +2CO
(1)
(2)
(3)
chemical cycling, especially at elevated temperature. In addi-
tion, other LiMn with adjustable hollow structures have
These synthetic methods included: using
the co-precipitation MnCO as precursors; controlled oxidation
3
4
2
2
3
2
2 4
O
2
4–26
MnCO
3
+ 2HCl / MnCl
2
+ CO
2
+ H O
2
been prepared.
3
2
Mn + 2MnO
+
ꢀ
+
3
4
+ 2H
2
O / 5MnO
+ 2H
2
+ 4H
with KMnO4 combined with the Kirkendall effect or a low-
temperature heat treatment to get MnO2 or Mn O hollow
2
3
8
MnO
2
+ 4LiOH / LiMn
2
O
4
2
O + O
2
(4)
spheres directly; or MnO /MnCO complex structures in which
2
3
the residual MnCO
LiMn was obtained nally through a chemical lithiation
process, retaininh the intermediate MnO hollow, double shell
or yolk morphologies. However, these procedures either need
two-stage calcination or costly materials, as the MnCO or
3
was quickly etched by HCl and spinel
8,10,24–26
Compared with the methods used in the literature,
2 4
O
the main advantages of the approach are as follows: (1) with the
help of synergistic promotion of chemical reactions (eqn (2) and
2
(
3)), it can make full use of the raw materials, MnCO
3
and
3
KMnO , which avoid quick etching by HCl or calcination at high
4
KMnO are only take part partially in the conversion. Consid-
4
temperature to remove the inner unreacted part of MnCO ; (2)
3
ering this, we have demonstrated an economic method which
use the synergistic promotion of chemical reactions for the
the obtained LiMn O microspheres combined excellent rate
2
4
performance and cycling stability both at room temperature
large-scale synthesis of spinel LiMn
2
O
4
microspheres that
ꢁ
and high temperature (55 C), endowed by the micro-/nano-
present higher specic capacity and more stable cycle perfor-
hybrid structure.
mance at high cycle rates and elevated temperature. The
straight forward synthetic strategy for the LiMn
spheres is depicted schematically in Scheme 1.
2 4
O micro-
Experimental
In the rst step, MnCO microspheres with high crystallinity
3
Synthesis of MnCO microspheres
3
are initially prepared by the ultrasonic assisted precipitation
method and used as precursors. Aerwards, the MnCO3
MnCO
tation method. In brief, 10 mM of MnSO
NH HCO were separately dissolved in 1000 ml of distilled
water, ethanol (100 ml) was added to the MnSO solution with
stirring and, aer its complete dispersion, the NH HCO solu-
3
microspheres were synthesized by a modied precipi-
27
4
2
$H O and 100 mM of
microspheres are converted into MnO
chiometric ratio KMnO from the outside to the inside
according to the chemical reaction (eqn (1)). However, with the
depletion of KMnO , the inner of the MnCO might not be
oxidized to MnO although the reaction time increased to 2 h.
2
by oxidation with stoi-
4
3
4
4
4
3
4
3
tion was poured into the MnSO $H O solution at room
4
2
2
temperature. The mixture was ultrasonically stirred for 30 min
and then ltered, washed with distilled water and ethanol for
Under this situation, dilute hydrochloric acid is added dropwise
into the system and penetrates through the surface pores,
reacting with the residual MnCO3 cores. At the same time,
three times to remove any residual impurities. Finally, MnCO
microspheres were dried at 80 C in a vacuum oven (ca. 0.1 MPa)
3
ꢁ
2
MnO is formed in the core or on the surface of the shell as a
for 12 h.
2
+
result of the redox reaction between the dissociative Mn and
ꢀ
MnO
4
(eqn (2) and (3)); at last, MnO
2
hierarchical microspheres
are obtained. Aer adding HCl, the mixture is kept stirring for Synthesis of MnO
hierarchical microspheres
2
another 1 h. These two chemical reactions (eqn (2) and (3)) can
1
.15 g as-prepared MnCO solid microspheres and 1.05 g
3
synergistically promote each other and are also helpful for
complete conversion of the precursor. Finally, the LiMn O
KMnO were separately dispersed in 100 ml and 66 ml distilled
water. Then the KMnO solution was poured into the MnCO3
4
2
4
4
solid microspheres solution under stirring. The mixture was
kept under stirring for 2 h at room temperature, then 200 ml of
HCl (0.1 M) was added dropwise into the reaction slowly and the
mixture was stirred for another 1 h, then ltered, washed with
distilled water and ethanol for three times. Finally, MnO
hierarchical microspheres were dried at 80 C in a vacuum oven
2
ꢁ
(ca. 0.1 MPa) for 12 h.
Synthesis of LiMn O microspheres
2
4
The above as-prepared MnO2 microspheres were mixed and
ground with LiOH$H O with the stoichiometric molar ratio
2
ꢁ
(
Li : Mn ¼ 1 : 2) and then the mixtures were calcined at 750 C
Scheme 1 Schematic illustration of the formation process of LiMn
microspheres.
2
O
4
for 12 h in air.
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Materials characterization
The crystal structures of MnCO , MnO and LiMn O powders
3
2
2 4
were investigated by X-ray diffraction (XRD). XRD patterns were
collected on a Rigaku D/max-2500Pc X-ray diffractometer with
˚
Cu Ka radiation (l ¼ 1.5418 A) at a voltage of 40 kV and a
current of 200 mA. The particle size and morphology of
samples were investigated with a Nova NanoSEM 450 eld-
emission scanning electron microscope (FESEM). Thermal-
gravimetric measurement (TG) was conducted on a TGA Q500
thermogravimetric analyzer (TA instruments, USA) in air at
ꢁ
ꢀ1
ꢁ
scan rate of 10 C min from room temperature to 800 C.
Nitrogen adsorption–desorption isotherms for surface area
were measured using a Micromeritics ASAP 2020 BET appa-
ratus with liquid nitrogen at 77 K.
Electrochemical measurements
Electrochemical performances of the samples were evaluated
with a standard CR2032 coin cell. The working electrodes were
composed of 80 wt% active material (LiMn O ), 10 wt%
2 4
conductive carbon black (acetylene black, ACET), and 10 wt%
polymer binder (polyvinylidene uoride, PVDF). The electrolyte
was a 1 M solution of LiPF6 in a 1 : 1 mixture of ethylene
carbonate (EC)/diethyl carbonate (DEC). The separator was
Celgard 2400. Metallic lithium was used as the counter elec-
trode and reference electrode. Cells were assembled in an
Ar-lled glove-box with moisture and oxygen concentrations
below 1.0 ppm. The charge/discharge tests were performed on a
LAND battery tester between 3.0 V and 4.5 V. The high
Fig. 1 XRD patterns of (a) MnCO solid microspheres; (b) MnO
4
hierarchical microspheres; (c) LiMn O microspheres.
3
2
2
temperature performances of the cells were tested by laying the MnCO microspheres are converted gradually into MnO and
3
2
ꢁ
cells in an oven at 55 C. The cycling voltammetry (CV) their spherical morphology is still retained (Fig. 2c). Upon
measurements were recorded on a Zahner ZENNIUM electro- closer observation from the high magnication image in
chemical station, the sweep potential range of CV was between Fig. 2d, the MnO hierarchical spheres are built with structur-
2
3
.0 V and 4.5 V at different scan rates.
ally numerous uniform nanosheets. Fig. 2e shows the SEM
images of the as-prepared LiMn particles evolved from
hierarchical MnO microspheres. The diameter of the prepared
LiMn O microspheres has become a little smaller than their
2 4
O
2
Results and discussion
2
4
The resultant products were initially examined by X-ray precursors (ꢂ0.9 mm). Fig. 2f shows very clearly that LiMn O
2
4
diffraction (XRD) to identify the crystallographic structure and microspheres are composed of many small irregularly shaped
crystallinity, and the results are presented in Fig. 1. As seen in and densely compact primary particles and their particle size is
Fig. 1a, the peaks of the MnCO
3
precursor prepared by the about 120 nm.
ultrasonic-assisted precipitation process are well indexed as a
In this synthesis method, with the help of the synergistic
promotion of chemical reactions, full use can be made of the
raw materials, MnCO and KMnO ; this can be demonstrated by
3 4
pure phase (JCPDS no. 44-1472). Aer prolonged KMnO
4
2
oxidation and HCl etching, the intermediate products MnO
(
Fig. 1b) show only two broad peaks with poor crystallinity one of the experimental phenomena. As seen in Fig. S1a (le)
8,25
24–27
which is in agreement with the literature reports.
Upon and b,† in routine methods,
aer the redox reaction
ꢁ
calcination at 750 C for 12 h, LiOH is successfully incorporated between MnCO and KMnO (eqn (1)), HCl was added to the
3
4
into the MnO
linity (Fig. 1c). All of the peaks are well indexed to a pure spinel MnCO
LiMn structure (JCPDS card no. 35-0782). resulting reaction solution and the ltrate are still red–purple,
The morphology and microstructure of the products and indicating that the KMnO only participates in the reactions
2
to obtain LiMn
2
O
4
samples with good crystal- mixture and maintained for less than 5 min to etch the inner
3
, quickly followed by separation. The color of the
2 4
O
4
their precursors were examined by a eld-emission scanning partly. Meanwhile in the synergistic method, the mixture was
electron microscope (FESEM). As shown in Fig. 2a and b, the kept stirring for at least 1 hour aer adding the HCl till the color
MnCO uniform microspheres here prepared via an ultrasonic- of the mixture changed to black. The ltrate is colourless and
3
assisted precipitation method, with a diameter of about 1 mm, this demonstrates that nearly all the KMnO took part in the
4
which is a little smaller than the other traditional co-precipi- reaction (Fig. S1a (right) and c†). On the other hand, the yield of
8,10,25
tation processes.
Upon reaction with KMnO
4
and HCl, the MnO
2
based on eqn (1)–(3) is nearly 99% (see the calculation in
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Fig. 3 Rate performance of lithium cell with a cathode material of
LiMn
C, 2 C, 5 C, and 10 C rate between 3.0–4.5 V; (b) cycling performance
at different rates (0.1–10 C rate) between 3.0–4.5 V.
2 4
O microspheres. (a) Charge/discharge profiles at 0.1 C, 0.5 C,
Fig. 2 SEM images of (a, b) MnCO
hierarchical microspheres; (e, f) LiMn O microspheres.
2 4
3
solid microspheres; (c, d) MnO
2
1
ESI†). This also further conrms the completely conversion of
MnCO and KMnO . So, both the high yield and colorless lter might be accounted for morphological and structural features.
liquor with prolonged reaction time demonstrated the full use First, due to the favorable spinel structure of the LiMn which
of KMnO in the synthesis of MnO microspheres. have three-dimensional lithium diffusion paths. In addition,
In order to examine the potential application of the obtained our LiMn microspheres are made up of some nanoparticles
3
4
2 4
O
4
2
2 4
O
LiMn O4 in high-power Li-ion batteries, the electrochemical with sizes below 100 nm, which lower the transmission path of
2
+
properties of LiMn O microspheres were rst tested in 2032- Li ions and accelerate ion-transportation in charge/discharge.
2
4
type coin half-cells at room temperature. To evaluate the rate
capability, the LiMn microspheres were cycled at various to test the capacity retention, the results are shown in Fig. 4a.
charge/discharge rates ranging from 0.1 C to 10 C over a The capacities reduce from incipient 123.6 and 110.2 mA h g
The cell was then cycled at a rate of 1 C and 10 C over 50 cycles
2 4
O
ꢀ1
ꢀ
1
potential window of 3.0–4.5 V (a rate of n C corresponds to a full to 122.1 and 109.8 mA h g at 1 C and 10 C rates aer 50 cycles,
charge/discharge of the theoretical capacity in 1/n hours and 1 C 98.8% and 99.6% of the initial capacity can be retained,
ꢀ
1
2 4
is 148 mA h g for LiMn O ). The rst charge and discharge respectively. These dates demonstrate the excellent capacity
proles at different rates are shown in Fig. 3a, the retention at both low and high charge/discharge rates. To
discharge capacity of the cathode is 133.2, 127.4, 125.2, 122.5, investigate the cycling capability further, the cell was cycled at 1
ꢀ
1
1
17.8 and 109.8 mA h g at 0.1, 0.5, 1, 2, 5 and 10 C, respec- C charge 10 C discharge over 1000 cycles; the capacity decreases
ꢀ
1
tively. It is notable that the discharge capacity reaches 133.2 mA slowly from 108.5 to 99.1 mA h g , 91.3% of its rst cycle
h g at a rate of 0.1 C, up to 90% of the theoretical capacity capacity, presenting excellent capacity retention (Fig. 4b). The
148 mA h g ). When the rate increase to 10 C, the discharge excellent cycling capability can be understood in terms of the
capacity is still 109.8 mA h g , 82.4% and 74.2% of the good crystallinity and hierarchical structure of the material. The
discharge capacity at 0.1 C (133.2 mA h g ) and theoretical good crystallinity was ascribed to the high synthesis tempera-
capacity (148 mA h g ), indicating an excellent rate capability. ture, which allowed the material to maintain an intact spinel
ꢀ
1
ꢀ1
(
ꢀ1
ꢀ
1
ꢀ1
Fig. 3b displays the cycling performances sequentially from cubic structure. In addition, nanoparticles of LiMn O4 were
2
0
(
.1 C to 10 C for every 5 cycles at each charge/discharge rate combined together densely to form hierarchically structured
0.1 C is 3 cycles), we can see that the capacity decay slowly, microspheres, reducing the surface area which is an important
indicate a good capability rate. The outstanding rate capability factor leading to capacity fading, restricting particle collapse
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2 4
Fig. 5 Cyclic voltammograms of the electrodes made with LiMn O
ꢀ1
recorded at the scan rate of 0.05, 0.10, 0.15 and 0.20 mV s
.
To achieve further insight into the electrochemical charac-
teristics, cyclic voltammetry (CV) measurements were recorded
on a Zahner ZENNIUM electrochemical station at different
ꢀ1
potential sweeping rates (0.05, 0.10, 0.15 and 0.20 mV s ). As
shown in Fig. 5, two couples of separate redox peaks around 4.0
2 4
and 4.1 V for typical spinel LiMn O can be observed, which
+
correspond to the two stage Li extraction/insertion from/into
the spinel phase and are consistent with the two plateaus in the
charge/discharge proles. As the scan rate increases to 0.2 mV
ꢀ
1
s
, the two redox pairs still can be easily distinguished and a
good linear relationship between peak current density (I ) and
the square root of potential scan rate (v ) is maintained
Fig. S4†). Assuming that the intercalation reaction is controlled
by the solid-state diffusion of lithium ions, the dependence of I
P
1
/2
(
p
1
/2
and v can be applied to determine the diffusion coefficient of
+
Li (DLi) on the basis of the following equation
5
3/2
1/2 0
Li Li
D v C
I
p
¼ 2.69 ꢃ 10 n
0
where n is the number of electrons per reaction species, CLi is
+
the bulk concentration of Li in the electrode (given as
ꢀ
3
0
.02378 mol cm from the theoretical density of the spinel).
Fig. 4 (a) Cycling performance at rates of 1 C and 10 C for 50 cycles
between 3.0–4.5 V; (b) cycling performance at 1 C charge 10 C
discharge for 1000 cycles between 3.0–4.5 V; (c) cycling performance
The calculated DLi values are listed in Table S2.† Most of the
ꢀ9
2
ꢀ1
values fall within the order of 10
cm
s , which was
ꢁ
comparable to the previously reported LiMn O samples with
2
4
at 55 C at 1 C charge 10 C discharge between 3.0–4.5 V.
23,29–31
excellent rate performance.
which is due to volume changing during charge/discharge
process. The value of the surface area is 4.0829 m g (Fig. S3†).
Conclusions
2
ꢀ1
Furthermore, we have investigated the high-temperature cycling In summary, LiMn O microspheres have been prepared
2
4
performance of the obtained LiMn
was cycled at 55 C at 1 C charge 10 C discharge over 100 cycles; spheres, which was synthesized by a facile synergistic-effect-
the capacity decreases from 116.8 to 100.7 mA h g , 86.2% of its promoted chemical reaction. When applied as cathode
2
O
4
microspheres. The cell through a solid state lithiation of MnO
2
hierarchical micro-
ꢁ
ꢀ1
ꢁ
rst cycle capacity (Fig. 4c). The cycling capability at 55 C is not materials for rechargeable Li-ion batteries, the obtained
as good as at room temperature, because of the dissolution of LiMn
2
O
4
hierarchical microsphere materials exhibit a high
2
+
ꢀ1
Mn into the electrolyte at high temperature resulting from the electrochemical capacity of 133.2 mA h g at 0.1 C charge/
3
+
2+
4+
ꢀ1
disproportion reaction, such as: 2Mn / Mn + Mn , which is discharge, excellent rate capability (109.8 mA h g at 10 C
accelerated in the presence of acids such as HF, at the higher charge/discharge, 82.4% of 0.1 C charge/discharge) and
temperature more HF is generated by the reaction: LiPF
6
2
+ H O
outstanding cycling performances (98.8% and 99.6% capacity
retention aer 50 cycles at 1 and 10 C rates, 91.3% capacity
28
/
2HF + LiF + POF
3
.
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retention aer 1000 cycles at 1 C charge 10 C discharge rates). 13 J. Y. Luo, L. Cheng and Y. Y. Xia, Electrochem. Commun.,
ꢁ
Cycled at 55 C at 1 C charge and 10 C discharge, it exhibits a
2007, 9, 1404.
ꢀ
1
discharge capacity of 100.7 mA h g with a capacity retention of 14 M. Okubo, Y. Mizuno, H. Yamada, J. Kim, E. Hosono,
8
6.2% aer 100 cycles. The superior electrochemical perfor-
H. S. Zhou, T. Kudo and I. Honma, ACS Nano, 2010, 4, 741.
15 K. M. Shaju and P. G. Bruce, Chem. Mater., 2008, 20, 5557.
16 J. Cabana, T. Vald ´e s-Sol ´ı s, M. R. Palac ´ı n, J. Oro-sol ´e ,
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mance suggests we will be able to use these obtained LiMn
as cathode materials for high-power battery applications.
2 4
O
2
007, 166, 492.
Acknowledgements
1
7 L. L. Xiong, Y. L. Xu, T. Tao, J. Song and J. B. Goodenough,
This work was supported nancially by the National Natural
J. Mater. Chem., 2012, 22, 24563.
Science Foundation of China (21101165), the Guangdong Inno- 18 H. W. Lee, P. Muralidharan, R. Ruffo, C. M. Mari, Y. Cui and
vative Research Team Program (no. 2011D052 and
D. K. Kim, Nano Lett., 2010, 10, 3852.
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(JC201005270372A, JSGG20120615161915279 JC201005270366A).
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