One-pot hydrothermal synthesis and selective etching method of a porous MoSe2 sand rose-like structure for electrocatalytic hydrogen evolution reaction

Article abstract of DOI:10.1039/c7ra10001a

The development of a platinum-free electrocatalyst for the hydrogen evolution reaction (HER) is highly essential to the large-scale production and application of water splitting devices. Herein we report a facile one-pot hydrothermal synthesis of composite MoSe₂@Cu₂Se. The morphology of the obtained material was characterized by scanning electron microscope (SEM) and it was found that the composite material formed a sand rose-like structure. The crystal structure and phase purity of the composite MoSe₂@Cu₂Se were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Then a selective electrochemical etching of copper from the composite was carried out and the porous MoSe₂ rose-like nanosphere was obtained. The robust 3D MoSe₂ rose-like structure exhibit remarkable activity and durability for electrocatalytic HER in acid maintaining a small onset overpotential of ～150 mV and keeping a small overpotential of 300 mV for 6 mA cm^-2 current density after 1000 cycles. Based on our data, the obtained porous sand rose-like structure material could improve the active surface area which yields higher HER catalytic activity. The present study provides a simple and effective way for the exploration of efficient Mo-based HER catalysts.

Full text of DOI:10.1039/c7ra10001a

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One-pot hydrothermal synthesis and selective
etching method of a porous MoSe2 sand rose-like
structure for electrocatalytic hydrogen evolution
reaction†
Cite this: RSC Adv., 2017, 7, 52345
*
Xuan Thai Tran, Sujittra Poorahong and Mohamed Siaj
The development of a platinum-free electrocatalyst for the hydrogen evolution reaction (HER) is highly
essential to the large-scale production and application of water splitting devices. Herein we report
a facile one-pot hydrothermal synthesis of composite MoSe2@Cu2Se. The morphology of the obtained
material was characterized by scanning electron microscope (SEM) and it was found that the composite
material formed a sand rose-like structure. The crystal structure and phase purity of the composite
MoSe2@Cu2Se were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM).
Then a selective electrochemical etching of copper from the composite was carried out and the porous
MoSe2 rose-like nanosphere was obtained. The robust 3D MoSe2 rose-like structure exhibit remarkable
activity and durability for electrocatalytic HER in acid maintaining a small onset overpotential of ꢀ150 mV
and keeping a small overpotential of 300 mV for 6 mA cmꢁ2 current density after 1000 cycles. Based on
our data, the obtained porous sand rose-like structure material could improve the active surface area
which yields higher HER catalytic activity. The present study provides a simple and effective way for the
exploration of efficient Mo-based HER catalysts.
Received 7th September 2017
Accepted 26th October 2017
DOI: 10.1039/c7ra10001a
rsc.li/rsc-advances
In recent years, many alternatives for non-precious metal-
based electrocatalysts have been developed, including transi-
Introduction
tion metal suldes, selenides, borides, carbides, nitrides,
phosphides, and a molecular catalyst family. Among all these
alternatives a type of efficient electrocatalysts based on transi-
tion metal dichalcogenide (TMD) has recently received signi-
cant attention, because of its exotic electronic structure and
these physical properties.10 TMD is a family of materials con-
sisting of more than 40 compounds having the generalized
formula MX2, wherein M is a transition metal, typically 4–7
groups and X is a chalcogen such as sulfur (S), selenium (Se)
and tellurium (Te). These transition metals have an important
catalytic behavior in the HER. The latter trend has formed with
superimposed layers of weak interactions of van der Waals
between two adjacent layers,10 this form has two large areas on
the sides (active sites) for the adsorption of ions. It has been
proven by further research that TMD slips active sites play an
important role in HER. Among them, MoSe2 is a newly
emerging catalyst owing to its low cost, high chemical stability,
and excellent electrocatalytic activity.11–15 The electrocatalytic
HER activity of MoSe2 depends strongly on its active selenium
edge sites, while its basal planes were catalytically inert.16 In
order to obtain, a high performance MoSe2 electrocatalyst
towards the HER is to rationally construct the nanostructure for
mass transfer and maximizing the number of active sites. A
boost of the catalytic activities could be achieved by reducing
Hydrogen (H2) is a promising, clean, and renewable energy
carrier to relieve our reliance on natural fossil fuels and can
reduce the growing global greenhouse effects.1–3 Growing
attention has been paid to the sustainable hydrogen evolution
reaction (HER) from the water splitting reaction, which is
a clean and environmentally benign reaction pathway.3,4 The
most effective electrocatalyst for the HER in an acidic media is
Pt group metals. However, several challenges remain in
designing and selecting an electrocatalyst material. These are
related to the expensiveness of Pt and low earth-abundance,
which hinder its practical implementation.5,6 Limited surface
active sites impede the HER from proceeding efficiently7 and
chemical and structural stability issues of electrocatalysts in
a harsh acidic environment were also another problem.8 These
limitations have motivated several researchers to dedicate
intensive efforts in designing and fabricating low-cost and
plentiful HER electrocatalysts with high catalytic activities.9
´
` ´
a Montreal,
Department of Chemistry and Biochemistry, Universite du Quebec
´
Montreal, QC, H3C 3P8, Canada. E-mail: siaj.mohamed@uqam.ca
† Electronic supplementary information (ESI) available: One-pot hydrothermal
synthesis and selective etching method of porous MoSe2 sand rose-like
structure for electrocatalytic hydrogen evolution reaction. See DOI:
10.1039/c7ra10001a
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the MoSe2 crystal size to nanoscale level and by increasing the distilled water and ethanol at least 4 times, and then dried at
exposures of its active edges to the electrolytes. In one aspect, 60 ꢂC under vacuum. To yield the nal crystalline products, the
ꢂ
a high surface area with an open structure avail the diffusion of precipitate products were dried at 500 C for 1 h in an atmo-
electrolytes, will lead smoothly to the replenishment of sphere of argon (Ar). Aer that, the copper component was
the consumed protons and lower the ohmic drop at high etched out from the composite under sonication in 0.5 M
reaction rate.17
ammonium persulfate for 30 min. Aer the etching procedure,
Herein, we demonstrate the preparation of a 3D hierarchical the porous MoSe2 sand rose-like structure was obtained
porous MoSe2 by a combination of hydrothermal and chemical (Fig. 1B). As a control experiment, the synthesis of pure MoSe2
etching methods. To the best of our knowledge, here is the rst and Cu2Se were prepared following the same method motioned
report of the MoSe2@Cu2Se synthesis by hydrothermal method. above.
Aer MoSe2@Cu2Se synthesis, the copper is selectively etched
from the alloy leading to a 3D porous MoSe2 structure formation
(sand rose-like structure). The 3D porous MoSe2 material is
Electrochemical measurement
tested as a catalyst for HER. A comparison between pure MoSe2 All electrochemical measurements were performed in a dispos-
and porous MoSe2 sand rose-like structure shows that the able three electrodes system (EP-PP, Biodevice Technology,
porous one exhibits higher catalytic activity for electrocatalytic Japan) consisted of a working electrode (carbon, 2 mm diam-
HER.
eter), a counter electrode (carbon) and a reference electrode
(Ag/AgCl). The electrochemical experiments were performed at
room temperature and were carried out using SP-300 poten-
tiostat (Bio-Logic Science Instrument, France) connected to
a personal computer and driven by EC-Lab soware. Prior to all
electrochemical measurements, 60 mg of porous MoSe2 product
and 150 mL of Naon solution (Sigma Aldrich, 5 wt%) was
dispersed in a 5 mL water-ethanol solution with a volume ratio
of 4 : 1 by at least 30 min sonication to from a homogeneous
ink. Finally, 3 mL of as-prepared slurry (containing 35 mg of
catalyst) was dropped on to the carbon-working electrode.
Finally, the as-prepared catalyst lm was dried in the air at room
temperature.
Linear sweep voltammetry (LSV) was performed in 0.5 M
H2SO4 saturated using an Ag/AgCl as the reference electrode,
and a carbon as the counter electrode. All the potentials were
calibrated to a reversible hydrogen electrode (RHE). LSV was
recorded by sweeping the potential from 0.2 to ꢁ0.8 V vs. RHE
with a scan rate of 1 mV sꢁ1 at room temperature. Cyclic vol-
tammetry (CV) was conducted for 1000 cycles between 0.1 V to
ꢁ0.8 V (vs. RHE) at 100 mV sꢁ1 for the stability test. The Nyquist
plots were measured with frequencies ranging from 200 kHz to
100 mHz at an overpotential of ꢁ250 mV. The impedance data
were then plotted to a simplied Randles circuit to extract the
series and charge-transfer resistances.
Experimental
Synthesis porous MoSe2
The preparation procedure of Mo@Cu selenide composite is
schematically described in Fig. 1. The Synthesis was carried out
using 0.42 g of ammonium molybdate tetrahydrate, 0.6 g cop-
per(II) sulfate pentahydrate and 1.065 g SeO2 powder in 20 mL of
distilled water under vigorous stirring for 45 minutes to form
a homogeneous solution. Aer that, 50 mL of ethylene diamine
was added with stirring continuously for 15 min at room
temperature. The nal solution has given a greenish color. The
solution was then transferred to a stainless-steel autoclave with
100 mL Teon coated and maintained at 210 ꢂC for 23 h; Fig. 1A.
Aer the reaction, the system was cooled down to room
temperature. The precipitated black obtained were collected by
centrifugation at 4000 rpm for 15 minutes, washed with
Characterization
The X-ray diffraction (XRD) was performed using a diffractom-
˚
eter (Bruker D8 Advance) with Cu-Ka radiation (l ¼ 1.54182 A)
at room temperature. X-ray photoelectron spectroscopy (XPS)
data for elemental composition and valence information was
collected with XPS PHI 5600-ci (Physical Electronics, Eden
Prairie, MN, USA). Scanning electron microscope was recorded
on JEOL (JSM7600F) with an accelerating voltage of 10 kV
and a working distance of 5 mm using secondary electron
imaging mode. Raman spectra of the lms were recorded with
alpha300 R Confocal Raman Microscope with WITec UHTS 300
spectrometer with a 532 nm laser. Transmission electron
microscopy (TEM) and Energy-dispersive X-ray spectroscopy
(EDS) was performed on a Joel JEM-2100F.
Fig. 1 (A) The procedure of the MoSe2@Cu2Se powder synthesis. (B)
The expected morphology after etching process.
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and form larger sizes, while MoSe2 retains its nanometric size
distribution in the MoSe2@Cu2Se composite. Aer etching
processes, the XRD pattern of Cu2Se disappeared, showing only
the pattern corresponding to MoSe2 (Fig. 2A-(iv)). From the XRD
result, it can be concluded that the Cu2Se was successfully
eliminated.
Results and discussion
Structure and morphology of porous MoSe2
The crystallographic structures and phase purity of as-prepared
materials were determined by XRD. Fig. 2A-(i), shows the
diffraction peaks in the two-theta range of 10–80ꢂ which implies
the crystalline nature of the obtained materials. The
asymmetric-shaped and broadening diffraction peaks, which
are the typical features of 2D nanosheets, demonstrate that the
MoSe2 rose-like structure is completely comprised of layered
akes.12 All of these diffraction peaks are in good agreement
with the characteristic planes i.e., (002), (100), (103) and (110) of
the hexagonal MoSe2 phase (JCPDS 29-0914), revealing the high
purity of the as-synthesized product. In the case of copper
selenide synthesis, Fig. 2A-(ii), the XRD patterns exhibit well-
dened peaks which were indicated with the standard pattern
of Cu2Se (JCPDS no. 01-088-2043). Fig. 2A-(iii) shows the XRD
pattern of the composite Mo@Cu selenide. It can be seen that
all diffraction peaks in the MoSe2@Cu2Se composite the X-ray
diffraction patterns can be separated from the compounds
either MoSe2 or Cu2Se, demonstrating a phase purity of the
synthesized product. In addition, the peaks compatible with
those indicated by the virgin MoSe2 are enlarged. These char-
acteristics show that the Cu2Se compound tends to aggregate
Fig. 2B-(i) presents the morphology of pure MoSe2 produced
by the solvothermal process involving (NH4)6Mo7O24$4H2O as the
Mo source and SeO2 as the Se source. The morphology of pristine
MoSe2 can be described as the rose-like microsphere that
consists of a large number of petals. While Cu2Se has an irregular
morphology in the form of dense nanoplates and/or nano-
crystals, Fig. 2B-(ii). Even though pure MoSe2 and composite
MoSe2@Cu2Se have rose-like microsphere morphology, the
petals of pure MoSe2 are thinner than that from MoSe2@Cu2Se
composite. Also, compared spaces between the petals of the
material composite are denser than MoSe2. Fig. 2B-(iv) shows
SEM images indicate that aer etching the composite of the rose-
like structures remains intact with more porous and opened
structure. Such enormous nanosheets could provide a large
number of active sites accompanied with a large specic surface
area. The dispersion homogeneity of the different components is
supported by the selected element mapping of Mo, Cu and Se
(ESI Fig. S1†). The mapping of MoSe2@Cu2Se composite sample
demonstrates clearly the existence of each element, in addition to
being well distributed over the composite material. Aer the
etching process, no copper element was detected. This mapping
conrms that the etching process has been done successfully.
The low-resolution TEM images in Fig. 3A and B indicate the
rose-like structure formation of porous MoSe2. High-resolution
transmission electron microscopy (HRTEM) images of the
MoSe2 reveal the microscopic phase information as well as the
thickness of the MoSe2. It can be seen that each section of the
nanoowers presenting a shape of a petal is actually an indi-
vidual stack of 2D MoSe2 thin layers. A large amount of active
sites can be attributed to widely distributed petals, which would
offer much more active sites for HER. The spacing between two
adjacent monolayers is 0.277 nm, which is consistent with the
value of MoSe2 interlayer spacing of the (100) plane (Fig. 3C).
The selected area electron diffraction (SAED) results also reect
the (002) planes of 2H-MoSe2 clearly in the inset Fig. 3C. Fig. 3D
shows the other plane of MoSe2 obtained from the average
values for ve layers is 0.72 nm, in good accordance with the
thickness of the atomic layer of Se–Mo–Se unit where the c-axis
orients normal to the (002) lattice plane. Therefore, we conclude
that the MoSe2 porous microspheres are composed of MoSe2
monolayer akes in an incompact way. Furthermore, the
comparison between TEM images of pure MoSe2 (ESI Fig. S2A†)
and MoSe2 aer etching process (ESI Fig. S2B†), the pure MoSe2
looks denser than those MoSe2 aer etching. On the other word,
aer took Cu2Se out from composite material, MoSe2 becomes
hollow. To prove that the specic area increased indeed, Bru-
nauer–Emmett–Teller (BET) method was used to measure the
surface area of the pure and etched MoSe2. ESI Fig. S3A† shows
nitrogen adsorption and desorption isotherms for the porous
MoSe2 sample. It showed a hysteresis loop curve, which is the
characteristic of a mesoporous material. BET specic surface
Fig. 2 (A) The XRD pattern and (B) the SEM morphology of the as-
prepared materials grown by solvothermal method (i) MoSe2, (ii)
Cu2Se, (iii) MoSe2@Cu2Se and (iv) porous MoSe2 after etching the
Cu2Se.
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Fig. 4 (A) Raman spectra of (i) MoSe2, (ii) Cu2Se , (iii)MoSe2@Cu2Se and
(iv) porous MoSe2 after etched Cu2Se; inset the atomic vibration
manners of the A1g and E21g vibrational modes of MoSe2. Blue and yellow
balls represent Mo and Se atoms, respectively. (B) XPS survey and high-
resolution spectra showing the binding energies of Mo 3d and Se 3d of
porous MoSe2.
Fig. 3 (A and B) TEM images porous MoSe2 under different magnifi-
cations. (C) HRTEM image of an area of the surface of MoSe2; inset the
selected area electron diffraction profile. (D) Plot of the calibration for
measuring the spacing and the corresponding atomic stacking
models.
area for the porous MoSe2 sample was 33.59 m2 gꢁ1 while that
the porous MoSe2 was only 12.31 m2 gꢁ1. So, the surface area of
the composite increased by about 2.72 times compared with
pure MoSe2. Then the density functional theory (DFT) was
applied to calculate the pore size distribution from the
adsorption isotherm. As you can see in ESI Fig. S3B,† the
material possesses the micropores characteristic from the range
and MoSe2 composite. This Raman peak corresponding to the
out-of-plane Mo–Se phonon mode is preferentially excited for
the edge-terminated perpendicularly-oriented nanosheets.11
Moreover, aer etching process, the E12g peak intensity is much
higher than A1g peak where the E12g peak is preferentially excited
for terrace-terminated lm. It can be supported the porous
structure MoSe2 formation. These make a relatively weak layer–
layer interactions in the porous MoSe2 nanosheets happen and
in-plan Mo–Se phonon occurs. Photoluminescence spectros-
copy was used to investigate the optical emission properties of
porous MoSe2. As shown in ESI Fig. S4† the porous MoSe2 shows
bandgap energy of 1.54 eV.
˚
of 23–54 A. It has been expected that the relatively large surface
area of the as-prepared porous MoSe2 rose-like microspheres
not only can provide more active sites but also enhance the
conductivity, which may improve the performance for a further
application.
Then the Raman spectroscopy of all samples has been
carried out. The observation of the Raman spectra of MoSe2
(Fig. 4A-(i)) can be noted that there are two resonance peaks at
238 cmꢁ1 and 283 cmꢁ1 which can be well indexed to the out-of-
plane A1g and in-plane E21g modes of 2H-MoSe2, respectively.18 In
the Cu2Se Raman spectrum (Fig. 4A-(ii)), an active mode A1g at
257 cmꢁ1 is observed. It is corresponding to Cu–Se vibration
and is in good agreement with the literature.19 In the Raman
spectrum of MoSe2@Cu2Se composite (Fig. 4A-(iii)), it can be
noted that the two-mode vibration at 237 cmꢁ1 MoSe2 and at
256 cmꢁ1 Cu2Se appears in this spectrum. Aer the etching
process (Fig. 4A-(iv)), the sample exhibits two Raman peak at
237.1 cmꢁ1 and 283.1 cmꢁ1. In addition, as can be seen, the A1g
peak intensity is much higher than the E12g peak at pure MoSe2
Chemical compositions on the surface and valence states of
the porous MoSe2 rose-like microspheres were further investi-
gated by X-ray photoelectron spectroscopy (XPS) measurements.
Fig. 4B shows the XPS survey spectrum of MoSe2 aer etched
Cu2Se out. In which the peaks derived from Mo, Se, C and O
elements were detected; no Cu peak was observed compared to
composite materials sample (ESI Fig. S5†). Generally, a small
amount of oxygen may be due to surface adsorption of oxygen,
and the C 1s peak located at 284.6 eV mainly results from the
contamination from the used carbon conductive tabs. The core-
level XPS spectra of Mo 3d shows the binding energies at 229 eV
and 232.1 eV belong to Mo 3d5/2 and 3d3/2 spin orbit peaks of
MoSe2, conrming the elemental chemical state of Mo is mainly
the Mo4+ oxidation state in the hexagonal 2H phase of MoSe2. In
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case of Se, two tted peaks at 55.4 and 54.5 eV attributable to the same conditions. Fig. 5A shows the polarization curves of the
core levels of Se 3d3/2 and Se 3d5/2, respectively, are further current density (J) versus the potential of the pure MoSe2 and
illustrating Se2ꢁ of the MoSe2.
porous MoSe2 synthesis, bare carbon and commercial Pt
All of the above characterization results prove that the hybrid catalysts. Toward the negative potential direction, cathodic
MoSe2@CuSe2 is the individual compound and formation can waves rise due to the electrocatalytic reduction of proton to
be described as followed. Under the solvothermal condition, as H2. For porous rose-like microspheres, onset overpotential of
the temperature and pressure increase, Mo(VI) reduced to Mo(IV) ꢀ150 mV vs. RHE is recorded, corresponding to a small HER
by C2H8N2 and then reacts with Se from the decomposition of overpotential of ꢀ300 mV for 6 mA cmꢁ2 current density.
SeO2 to form MoSe2. At the same time, the Cu2Se nanoparticles However, pure MoSe2 exhibits a clear inferior electrocatalytic
also form via stacking of a redox reaction occurred between the activity compared to MoSe2 rose-like microspheres. It is worth
2ꢁ
copper and the Sex to form Cu2Se crystal.19 The generated noting that monolayer akes are signicantly worse with an
MoSe2 and Cu2Se nucleus accumulates and leads to nanosheets onset overpotential about ꢁ400 mV. We assume that the
growth. With the increasing time, the nanosheets of both severe aggregation of monolayer akes during electrocatalysis
materials tend to aggregate to form 2D layer and several layers leads to the diminished surface areas and thus the poor HER
stack under the inuence of the hydrogen bonding interaction performance. In this work the electrochemical performance of
and thermodynamic stability20 and nally formed 3D hierar- MoSe2@Cu2Se composite was not studied. This is due to
chical microsphere-like of composite MoSe2@Cu2Se. As result- Cu2Se is able to oxidize in the potential range for HER study.
ing in the elements are well distributed over the composite So it can affect HER results. In addition, the distinguished
material. Furthermore, the etching process does not inhibit the catalytic performance of 3D porous structure of MoSe2 is also
MoSe2 properties.
indicated by the exceptionally low Tafel slope of ꢀ56 mV per
decade (Fig. 5B). In comparison with the 3D porous structure,
higher Tafel slops of MoSe2 compacted spheres (pure MoSe2)
was measured as ꢀ71 mV per decade. We attribute these
merits to the 3D porous structures of MoSe2 rose-like micro-
spheres which retain excellent structure rigidity and possess
more catalytic sites during electrocatalysis. In the present
work, the Tafel slope is 56 mV per decade for the porous MoSe2
(rose-like structure), illustrating the Heyrovsky reaction plays
a dominant role in determining the HER rate of this catalyst
material. The Tafel slope in this work is indeed close to the
values of 3D MoS2/MoSe2 nanosheet-graphene networks
(61 mV per decade),16 MoS2/MoSe2 host lattice (50–60 mV per
decade)21 MoSe2/carbon ber aerogel hybrids (62 mV per
decade)22 and compositing MoSe2 on MoS2 (65 mV per
decade).23
Electrocatalytic analysis
The as-prepared porous MoSe2 rose-like microspheres
combine the advantages of the intrinsic properties of 2D
MoSe2 monolayer akes and the 3D porous structures, which
are believed to greatly benet their applications. Here, we
investigate their electrocatalytic activity for HER in acid. HER
is the key process to electrolytic or photoelectrochemical water
splitting. The search for efficient and robust HER electro-
catalysts is at the heart of clean energy research. To investigate
the HER performance of the synthesized porous MoSe2,
the electrochemical measurements were carried out in a N2-
saturated 0.5 M H2SO4 at room temperature using a three-
electrode cell. For comparison, HER catalytic measurement
using MoSe2, Pt and bare carbon have been conducted in the
Fig. 5 Electrochemical measurements of MoSe2 products. (A) Polarization curves and (B) corresponding Tafel plots obtained from the polar-
ization curve of MoSe2 porous microspheres in 0.5 M H2SO4 along with the bare carbon, pure MoSe2 and platinum electrodes.
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Fig. 6 (A) EIS Nyquist plots of pure MoSe2 and porous MoSe2. (B) Polarization curves of MoSe2 porous microsphere catalyst before and after
1000 potential cycles in 0.5 M H2SO4.
Impedance spectroscopy (Fig. 6A) revealed that the pure this facile methodology could be expanded to prepare more 3D
MoSe2 lms themselves contributed signicantly to the series porous structures comprising their monolayered units, which
resistance in addition to the substrate and solution resistances. will open new opportunities in exploring their undiscovered
Consequently, this may contribute to the observed trends in properties and applications.
turnover frequency. The high degree of resistance from the pure
MoSe2 is not surprising, given that MoSe2 possesses semi-
conducting as well as anisotropic charge transport properties.
Conflicts of interest
However, the charge transfer resistance of the porous MoSe2
was much smaller. The material showed a charge transfer
resistance of about 150 U. These results suggest that the
conductivity of the porous MoSe2 is much higher than that of
pure MoSe2, which indicates a much faster electron transfer
process during electrochemical reaction. Other than high elec-
trocatalytic activity, good durability is another important crite-
rion in the selection of electrocatalysts. Here, MoSe2 porous
microsphere catalyst was continuously cycled for 1000 times in
0.5 M H2SO4. The polarization curves before and aer 1000
cycles were compared as shown in Fig. 6B. Little HER activity
loss is discernible, indicating that the 3D porous structure of
MoSe2 rose-like microspheres is highly stable and no signicant
electrocatalytic active sites were lost during the cycles.
There are no conicts to declare.
Acknowledgements
This work was supported through funding from the Natural
Science and Engineering Research Council of Canada (NSERC),
the Canada Research Chairs program (CRC) and Canada
Foundation for Innovation. We acknowledge all the character-
´
ization centers including NanoQAM, Le Centre de Caracterisa-
2
´
tion Microscopique des Materiaux (CM ) and Laboratoire de
´
´
Caracterisation des Materiaux (LCM). We also thank Pr Daniel
Belanger (UQAM) for the discussion about the electrochemical
measurements.
References
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hydrothermal and chemical etching methods. This new
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