Template-free synthesis of multiple-shell MgO/Pt hollow spheres as enhanced electrocatalysts

Article abstract of DOI:10.1039/c6ta03070j

In general, most carbon-based hollow spheres with porous structures are prepared using the so-called hard-template method. In this paper, we present a template-free strategy for the fabrication of multiple-shell MgO/Pt hollow spheres. The as-prepared MgO/Pt spheres have a highly porous and self-supporting structure with a high surface area of 16.7 m² g^-1. Moreover, our research indicates that the as-prepared multiple-shell MgO/Pt hollow spheres exhibit enhanced electrocatalytic activity and long-term stability towards the oxidation reaction of methanol.

Full text of DOI:10.1039/c6ta03070j

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Materials Chemistry A
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Template-free synthesis of multiple-shell MgO/Pt
hollow spheres as enhanced electrocatalysts†
Cite this: DOI: 10.1039/c6ta03070j
Tao Wang, Weihao Cui, Meiling Peng, Shenshen Ouyang and Sheng Wang*
Received 14th April 2016
Accepted 10th May 2016
DOI: 10.1039/c6ta03070j
www.rsc.org/MaterialsA
7
In general, most carbon-based hollow spheres with porous structures lithium-ion batteries. The Wang group developed a binary
are prepared using the so-called hard-template method. In this paper, carbonaceous sphere template method for the synthesis of WO3
8
we present a template-free strategy for the fabrication of multiple- MSHSs with an excellent photocatalytic activity. The above-
shell MgO/Pt hollow spheres. The as-prepared MgO/Pt spheres have mentioned synthetic routes, the most convenient approaches to
a highly porous and self-supporting structure with a high surface area hollow spheres, are similar in principle and based on the so-
2
À1
of 16.7 m g . Moreover, our research indicates that the as-prepared called hard-template method, in which modied carbonaceous
multiple-shell MgO/Pt hollow spheres exhibit enhanced electro- spheres with plenty of surface functional groups or carbona-
catalytic activity and long-term stability towards the oxidation reaction ceous spheres that are calcined in an inert gas atmosphere are
of methanol.
used as hard template materials. By applying high pressure
externally, large amounts of desired metal ions being adsorbed
can penetrate deeply into the template materials. The subse-
quent calcination process generates hollow spheres with
a MSHS structure. However, the synthesis conditions of the
hard-template method sometimes are complicated. A time-
saving and low-cost synthesis method is still highly desirable
and challenging. Herein, we present the fabrication of MgO/Pt
multiple-shell hollow sphere (MgO/Pt MSHS) powders using
a template-free strategy. By using a simplied two-step process,
spherical Pt–C–MgCO3 composites were rst prepared by
hydrothermal treatment and then transformed into multiple-
Hollow micro- and nano-structured spheres, especially those
with complex shell structures (multiple-shell hollow spheres,
MSHSs), can provide an enhanced surface-to-volume ratio and
reduced transport lengths for both mass and charge transport.
Their attractive properties have stimulated tremendous
research interest in recent years because of their fascinating
potential in drug delivery, medical imaging, articial cells,
1
–5
catalysis and energy storage. Over the last few years, consid-
erable efforts have been devoted to exploring various strategies
to generate MSHS structures possessing optimized physical and
chemical properties for specic applications. Thus, many types
of MSHS structures have been fabricated through various
synthesis routes. For example, Jiang and co-workers applied
calcined porous carbonaceous microspheres as templates to
ꢀ
shell hollow spheres by combustion in air at 500 C. Since Pt
nanocrystals are monodispersedly loaded onto multiple-shell
structures, the composites could provide more catalytically
active sites for electrocatalytic reactions than commercially
available Pt electrocatalysts.
2
+
absorb Zn cations, followed by a programmable heating
process to prepare ZnO with a MSHS structure, which was used
as a photoanode in DSSCs and showed high energy conversion
The hydrothermal carbonization approach is a commonly
used method for the synthesis of carbonaceous nano-
9,10
materials. Indeed, the hydrothermal carbonization approach
6
efficiency. Lou et al. reported the fabrication of metal oxide
has been utilized in various studies to combine carbonaceous
materials with other components such as inorganic nano-
particles to form composites. In addition, the surface of the
microsphere template is hydrophilic and contains abundant
hydroxyl (OH) and carbonyl (C]O) functional groups, thus
MSHSs including Co Mn O , ZnMn O , ZnCo O and
x
3Àx
4
2
4
2 4
NiCo O by a “penetration–solidication–annealing” strategy
2
4
and illustrated their high potential as negative electrodes for
1
1,12
favouring the absorption of cationic metal ions.
On the basis
Key Laboratory of Advanced Textile Materials and Manufacturing Technology,
Zhejiang Sci-Tech University, Hangzhou 310018, China. E-mail: wangsheng571@
hotmail.com
13
of these advantages, in this work, MgCO3 microrods and
chloroplatinic acid were used as metal precursors, and glucose
†
Electronic supplementary information (ESI) available: Experimental, was used as both a reducing agent for Pt nanocrystals and
characterization of yolk/shell MgO spheres, additional controlled experiments,
and the solubility of MgCO . See DOI: 10.1039/c6ta03070j
a carbon source. As shown in Fig. 1 (see ESI S1 for experimental
3
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Fig. 1 Schematic illustration of the preparation process of multiple-shell MgO/Pt hollow spheres.
details†), aer calcination, a MgO/Pt MSHS structure was ob-
tained, whereas only MgO with a yolk–shell structure was
formed in the absence of chloroplatinic acid.
Scanning electron microscopy (SEM) analyses were rst
performed to examine the morphology of the as-prepared
samples (the characterization of yolk–shell MgO spheres is
shown in ESI S2†). Fig. 2a and b show the SEM images of MgO/
Pt MSHSs at different magnications. The SEM images show
that the as-prepared sample is composed of a large number of
spheres with a diameter of approximately 0.8–1.0 mm. It can be
seen that the surface of each sphere is rough and uneven,
indicating that it is assembled by individual nanoparticle
subunits (Fig. 2b). The analysis of the cross-section of a broken
MgO/Pt MSHS sample ruptured by sonication reveals that the
microspheres have an interesting multiple-shell hollow struc-
ture. As further conrmed by the TEM results (Fig. 2c and d), the
spheres have a hollow structure consisting of three or four
shells in the internal space. Furthermore, high-resolution TEM
(HRTEM) analysis was employed to determine the crystallinity
of the MgO/Pt MSHS spheres. The result of HRTEM reveals
a crystalline structure of nanoparticle subunits (Fig. 2e). The
bottom-le inset is the fast Fourier transform (FFT) pattern.
Both the HRTEM image and the corresponding FFT pattern
(bright spots) conrm the single-crystalline nature of Pt nano-
particles. The observed lattice fringe corresponds to the (111)
plane of Pt according to the interplanar distance measurement
(
ꢁ0.222 nm), while nanoparticles show an interplanar spacing
of approximately 0.203 nm, which corresponds to the (400)
plane of MgO. The selected-area electron diffraction (SAED)
pattern of an individual MgO/Pt MSHS (Fig. 2f) shows diffrac-
tion signals corresponding to the characteristic diffraction
peaks of MgO.
A high-angle annular dark eld scanning TEM (HAADF- Fig. 2 (a) and (b) Low- and high-magnification SEM images of MgO/Pt
MSHSs (the inset in (b) is an enlarged SEM image of a broken shell). (c)
and (d) Low- and high-magnification TEM images of MgO/Pt MSHSs.
STEM) image and selected-area elemental analysis maps of
a MgO/Pt MSHS are shown in Fig. 3. For an individual sphere,
the uniform distribution of Mg, O and Pt could be observed
(
e) HRTEM image of an individual Pt, MgO nanoparticle and the cor-
responding FFT pattern (inset image). (f) Representative SAED pattern
throughout the entire area. The representative areas of Mg of an individual MgO/Pt MSHS.
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At rst, glucose acts as a reducing agent and reduces the Pt
precursor to the corresponding Pt nanocrystals (for the reduc-
14
tion of Pt NCs see S4†). Then, the glucose monomer can
gradually transform into well-known glucose-derived carbon-
rich hydrothermal carbon (HTC) spheres through the poly-
merization and carbonization reactions of glucose molecules
under hydrothermal conditions, which is greatly helpful in the
formation of the spherical structure. In the controlled experi-
ment, without glucose, we failed to generate the spherical
structure. Moreover, replacing glucose with other organic
substances, such as ethanol, glycol and diethylene glycol, the
products were only the aggregates of MgO microsheets (S5†).
3
In addition, considering that MgCO is a kind of water-
insoluble salt, we also evaluated several other insoluble salts
including carbonates and acetates in the controlled experi-
ments. However, MSHS structures were not formed using these
investigated salts (S6†). Different from the above-mentioned
insoluble salts, the solubility of MgCO in water has a unique
3
characteristic. When temperature increases, the solubility of
MgCO in water decreases signicantly (S7†). It is believed that
3
3
the specic solubility of MgCO is also important to the unique
MSHS structure.
We next investigated whether the same synthesis method
could be applied to different metal precursors. In a series of
controlled experiments, the Pt precursor was replaced by Au and
Fig. 3 (a) Dark-field TEM images of MgO/Pt MSHSs. (b–d) HAADF-
STEM-EDS mapping images and line scanning profiles of an individual
MgO/Pt MSHS.
and O atoms are substantially larger than those of Pt atoms, Pd precursors. It turned out that no MSHS structure was ob-
consistent with the results of their elemental analysis maps. In tained in both reaction systems. The reason was attributed to
addition, the formation of uniformly dispersed Pt nanoparticles the different reduction rate of metal precursors (S8†).
with an average diameter of 5 Æ 1 nm was observed. The atomic
3
As a typical strong base–weak acid salt, MgCO dissociates in
ratio of Mg : Pt was determined to be 97.5 : 2.5 by ICP-OES water and undergoes hydrolysis to yield a basic solution. The
analysis, which is in good agreement with the mole ratio of the addition of an appropriate amount of acid can disturb the
corresponding precursors, indicating the completeness of the hydrolysis equilibrium of MgCO
hydrothermal reaction. formation of Mg(HCO . However, high temperature will cause
to decompose to MgCO again (normally
3
in water, leading to the
3 2
)
As shown in Fig. S2a,† the XRD diffraction peaks match well the Mg(HCO
with the MgO JCPDS card no. 45-0946. These two peaks can be 100 C). To further identify the important role of chloroplatinic
assigned to the crystal planes (200) and (220). This result acid in the present synthesis of the MgO/Pt MSHS, we per-
)
3
2
3
ꢀ
suggests that pure MgO can be obtained via the thermal formed a series of parallel experiments replacing chloroplatinic
ꢀ
decomposition of the MgCO precursor at 500 C. Moreover, acid with different concentrations of HCl (S9†). As shown in
3
ꢀ
ꢀ
ꢀ
the diffraction peaks at 39.76 , 46.24 , and 67.45 correspond Fig. S7,† at a lower concentration of HCl (0.02 mmol), the
to the (111), (200) and (220) planes of face-centered-cubic (fcc) reaction resulted in a mixture of multiple-shell and yolk–shell
crystalline phase Pt (JCPD no. 04-0802), respectively. The MgO hollow spheres with non-uniform size. However, with
specic surface area and pore structures of MgO/Pt MSHSs increasing the concentration of HCl (0.05 mmol), broken hollow
were characterized by nitrogen adsorption and desorption spheres began to appear. Further increasing the concentration
isotherms at 77 K, which showed a BET surface area of of HCl (0.15 mmol) generated a mixture of broken hollow
2
À1
1
6.65 m g (Fig. S2b†). The sample exhibited a typical type IV microspheres (minor product) and microsheets (major
isotherm, characteristic of mesoporous materials, with an product). When the concentration of HCl was increased to
average Barrett–Joyner–Halenda (BJH) pore diameter of 0.50 mmol, only curved microsheets could be observed. When
1
1.2 nm. The broad pore size distribution in the range of the molar ratio of HCl : MgCO
mesopores strongly indicates that the MgO/Pt MSHSs have collected, because MgCl can dissolve completely in water. With
a porous and self-supporting structure that possesses a high addition of a small amount of H SO or HNO to replace HCl,
surface area and sufficient adsorption sites. On the other similar results were obtained (S10†). It is obvious that until
hand, the BET surface area of MgO yolk–shell spheres was MgCO completely transformed to MgCl , the mass of MgCO
continually decreased with the increase of the concentration of
3
$ 2 : 1, no product could be
2
2
4
3
3
2
3
2
À1
determined to be 46.0 m g (S3†).
A comparison of yolk–shell MgO spheres with MgO/Pt acid. These ndings suggest that acid contributes signicantly
MSHSs suggests that glucose, MgCO and metal precursors play to the formation of multiple-shell MgO hollow structures. In
3
a critical role in the formation of the multiple-shell of the comparison with the products generated by chloroplatinic acid,
spherical architecture.
the products of the latter show more uniform multiple-shell
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hollow structures, which may be attributed to the following two the reaction time to 2 h led to the formation of some triple-shell
reasons. First, the generation of HCl in a chloroplatinic acid hollow structures (Fig. 4e).
system is a slow release process, accompanied by reduction of Pt
To demonstrate a proof-of-concept demonstration for the
nanocrystals. This slow release process may moderate the potential use of MgO/Pt MSHSs, we investigated their electro-
formation process of MgO/Pt composites and reduce the mass catalytic properties toward the methanol oxidation reaction
loss of MgCO
nanocrystals may be involved in the construction of the core/shell spheres and commercially available Pt black (Fig. 5,
architecture. Table 1 in S11†). Fig. 5a shows the cyclic voltammograms (CVs)
3
. Secondly, a signicant amount of reduced Pt (MOR). The catalytic activity was benchmarked against MgO
Therefore, on the basis of these results, we assumed that the of the various catalysts in a 1.0 M KOH aqueous solution. In the
following reactions take place in the system. Firstly, the addi- process of methanol oxidation, an oxidation peak correspond-
tion of MgCO microrods into water generates a suspension at ing to the oxidation of freshly chemisorbed methanol was
3
room temperature. When chloroplatinic acid is added, an observed in the forward scan. In addition, an anodic peak was
equilibrium between MgCO
solution is achieved. When the temperature rises, Mg(HCO
decomposes to MgCO , together with precipitation of original forward scan.
dissolved MgCO
3
and Mg(HCO
3
)
2
in suspension detected in the reverse sweep, which is attributed to the removal
3 2
)
of carbonaceous species that are not completely oxidized in the
15,16
3
The electrochemically active surface area
3
. Simultaneously, with increasing reaction (ECSA) was calculated according to the oxide reduction peaks by
À2
temperature, Pt nanocrystals are reduced continuously. The adopting an assumption of 420 mC cm , which corresponds to
17,18
MgCO and reduced Pt NCs gradually accumulate and aggregate the adsorption of an oxygen monolayer.
The calculated
3
and are wrapped continually by growing HTC spheres, thus ECSAs of the MgO/Pt MSHSs, MgO yolk–shell spheres, and Pt
2
À1
resulting in the formation of spherical Pt–C–MgCO interme- black were 14.2, 1.2, and 26.6 m g , respectively. Although
3
diate composites. Finally, aer calcination, multiple-shell MgO/
Pt hollow spheres are achieved successfully.
To date, the formation of MSHS structures in most literature
studies relies on high external pressure, which can ensure that
metal ions penetrate deeply into ready-made HTC spheres from
external solution. However, in this work, Pt and precipitated
MgCO3 are naturally wrapped from the interior during the
entire hydrothermal process with the growth of HTC spheres.
Aer calcination, MgO/Pt MSHS structures are formed.
The effect of reaction time on the shell formation process
ꢀ
was studied in detail by carrying out the reactions at 500 C for
different reaction times (Fig. 4). Pt–C–MgCO
3
intermediate
ꢀ
ꢀ
composite nanospheres were heated to 500 C at a rate of 2 C
À1
min . The products were characterized aer different periods
of time in the heating process. Compared with the carbona-
ceous particle at room temperature in Fig. 4a, the inorganic
composites within HTC spheres gradually separated at 0.5 h,
and a decrease in the average diameter from 5 to 3.5 mm was
also observed (Fig. 4b). When the calcination time was Fig. 5 Comparison of the electrocatalytic properties of the samples:
increased to 1 h, the formation of a shell around a solid sphere (a) CV curves recorded in 1.0 M KOH solutions at a scan rate of 50 mV
À1
s
. (b) Chronoamperograms for the MOR at À0.2 V vs. SCE. (c) CV
can be observed (Fig. 4c). When the time reached 1.5 h, the
average diameter shrank continually but the solid inner core
sharply contracted and changed to a core–shell sphere, result-
curves of the catalysts for the MOR in 1.0 M KOH + 1.0 M methanol
solution at a sweep rate of 50 mV s . (d) Graphical comparison of the
mass activities of all catalysts in alkaline media. Mass activities are given
À1
ing in a core–double shell structure (Fig. 4d). Further increasing as kinetic current densities (j) normalised with the loading amount of Pt.
ꢀ
Fig. 4 TEM images of the samples recorded at different calcination times. (a) Before calcination and calcination at 500 C for (b) 0.5 h, (c) 1 h, (d)
1
.5 h and (e) 2 h.
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a MgO yolk–shell sphere was taken as a reference sample, it
Compared with Pt black (aggregate of Pt nanoparticles is
hardly showed electroactivity because MgO does not have approximately 10 nm, Johnson Matthey Co.), it is evident that
conductivity. The lower ECSA of MgO/Pt MSHSs is mainly the highly dispersed Pt nanocrystals on MgO MSHSs are highly
attributed to the multiple shell structure and large size of MgO/ benecial for application as electrocatalysts because the as-
Pt MSHSs, which hinder to some extent mass transfer from the prepared MSHSs are mono-dispersed and can supply sufficient
exterior to the interior of spheres. In addition, the calcination adsorption sites for reactants. Furthermore, it should be noted
ꢀ
process stabilizes the structure of nanocrystals, thus reducing that the sample was a calcined sample (500 C). Such a high
the active sites on the surface.
temperature will lead to signicant aggregation of Pt nano-
In general, the electrocatalytic activity of the catalyst can be particles in common catalysts, resulting in inactivation of
assessed by two important parameters, current density and catalysts. Therefore, the sample is suitable for application in
1
9,20
onset potential.
Fig. 5c shows the ECSA-normalized CVs of harsh environments, such as high-temperature environments.
In summary, a facile template-free strategy has been devel-
methanol oxidation with the catalysts in a 1.0 M KOH aqueous
solution containing 1.0 M methanol. The mass normalized oped for the synthesis of multiple-shell MgO/Pt hollow spheres.
current density of MgO/Pt in the positive direction sweep was The structure of multi-shell hollow spheres was characterized
À1
found to be 310.4 mA mg , which was approximately 1.55 and conrmed by TEM, SEM, HRTEM, XPR, HAADF-SEM, and
times higher than that of Pt black. The catalytic activity of nitrogen adsorption–desorption. The results reveal that MgO/Pt
samples for the MOR was evaluated by CVs in argon-saturated MSHSs have a porous and self-supporting structure that
1
.0 M KOH + 1.0 M CH OH solution, and the oxidation current possesses a high surface area and sufficient adsorption sites.
3
density was normalized by the Pt mass. Meanwhile, the onset Moreover, the as-synthesized multiple-shell hollow spheres
potential of MgO/Pt MSHSs was lower than that of Pt black exhibit high electrocatalytic activity and long-term durability
(about 7 mV).
towards the oxidation of methanol. Further studies are
In addition, the MOR process usually involves the adsorption currently underway to explore attractive applications of
of methanol on the surface of the catalyst. Some intermediate multiple-shell hollow spheres in different elds, such as
species, especially CO, can be adsorbed on the surface active heterogeneous catalysis.
sites of the catalyst, followed by the further oxidation of CO to
21
the nal product of CO2. The formation of the CO interme-
diate will signicantly block the access of methanol to the Pt
Acknowledgements
active sites, thus reducing the catalytic activity of Pt-based We thank the National Natural Science Foundation of China
catalysts and resulting in the poisoning of the catalyst. There- (No. 51471153 and 51372227), Natural Science Foundation of
fore, the ratio of the forward oxidation current density to the Zhejiang province (No. LY14E020011 and 2015C33008) and 521
backward current density, I
f
/I
b
, is an important indicator to Talent Project of Zhejiang Sci-Tech University for providing
describe the tolerance of the catalyst to the carbonaceous nancial support.
22
species. From Fig. 5c, the I
f
/I
b
value of MgO/Pt MSHSs was
observed to be 5.9, higher than that of Pt black (5.4). The
References
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f
b
effectively oxidized on the surface of MgO/Pt MSHSs and
generate less poisoning species compared with Pt black.
Moreover, to evaluate the long-term stability of the catalysts,
we conducted a CA test (Fig. 5b). At the initial stage, the CAs
generated high charging currents because of the high concen-
tration of methanol molecules on the catalyst surface. The
subsequent current decay mainly resulted from the inhibition
of the surface reaction active sites by the accumulation of CO
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