Mn3O4 catalyzed growth of polycrystalline Pt nanoparticles and single crystalline Pt nanorods with high index facets

Article abstract of DOI:10.1039/c0cc03656k

Rarely-observed polycrystalline twinned Pt nanoparticles (NPs) and length-controlled Pt nanorods (NRs) with exposed high index facets were prepared through an unexpectedly catalytic growth approach from low-cost Mn ₃O₄ NPs. The as-pr

Full text of DOI:10.1039/c0cc03656k

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Mn3O4 catalyzed growth of polycrystalline Pt nanoparticles and single
crystalline Pt nanorods with high index facetsw
Xuezhong Gong, Yun Yang* and Shaoming Huang*
Received 4th September 2010, Accepted 28th October 2010
DOI: 10.1039/c0cc03656k
Rarely-observed polycrystalline twinned Pt nanoparticles (NPs)
and length-controlled Pt nanorods (NRs) with exposed high
index facets were prepared through an unexpectedly catalytic
growth approach from low-cost Mn3O4 NPs. The as-prepared Pt
NRs exhibit higher catalytic activity and better stability for the
oxidization of methanol than commercial Pt/C catalysts,
indicating the promise of this catalyst in direct methanol
fuel cells.
commonly-observed Pt NPs are single crystalline. Hence,
scientists know little about their formation mechanism.2a (ii)
Highly active catalysts for direct methanol fuel cells (DMFCs).
Nanostructured Pt with a high-index facet contains a high
density of low-coordinate atomic steps and kinks which are
highly active in electro-oxidization of methanol.8 However, the
crystal growth habit always only allows the formation of NPs
bound by low-index facets. For this reason, the fabrication of
nanostructured Pt with a high index facet is considerably
difficult.7c,8 (iii) The low-cost mass production of Pt NRs.
Our method is facile and highly repeatable, besides, the used
Mn3O4 NPs are low-cost and can be prepared easily, which are
very crucial to mass production of Pt NRs.9 Furthermore, the
developed method may be also applied to synthesize other
nanostructured noble metals and inspire researchers to study
the catalytic effect of other metal oxide NPs on the growth of
Pt NPs.
The synthesis of nanostructured platinum (Pt) has been an
interesting field due to its wide applications, particularly in
catalysis, in the past few decades.1–7 Among developed synthetic
approaches, seeding growth has been widely used.1b,d,2a,3d,4,7a–c
During the seeding growth, generally, one pre-prepared NP act
as a seed (nucleating centre and active site provider) and
selective-growth of newly formed atoms occurs on the seed
surface, producing core–shell or dumbbelled NPs.1d,f,7a–c For
example, Yang’s group prepared Pt@Fe core–shell NPs by using
Pt NPs as seeds.1d In Yang’s case, the selective-growth occurred
centring the whole surface of the seed. When the selective-growth
only happens on one part of seed surface, heterostructured NPs
(dumbbell or dendrite) will form.7a–c
The formation of Pt nanostructures occurred in a sealed
heating system and detailed preparations can be seen in the
ESIw. Fig. 1 shows the high resolution transmission electron
microscopy (HRTEM) images of Mn3O4 NPs used to induce
Pt growth and the as-prepared twinned Pt NPs at different
magnifications. Before growth, the Mn3O4 NP is cubic
(Fig. 1A) as reported previously.8 After the Pt precursor was
added and the mixture was heated at 180 1C for 1 h,
interestingly, unusual polycrystalline Pt NPs formed under
the induced-growth of single crystalline Mn3O4 as shown in
Fig. 1B–D. Due to lattice mismatch, a noble metal and metal
oxide could combine into heterostructured NPs, such as
dumbbell Pt–Fe3O4.7a However, in our case, the as-prepared
polycrystalline Pt NPs have no such structure. From energy
dispersive spectrometry (EDS) and high angle annular dark
In this communication, through investigating the growth of
Pt NPs in the presence of Mn3O4 NPs, we found a new growth
approach which gives very interesting results. In our
experiments, the pre-prepared Mn3O4 NPs were added to a
mixture of oleyl amine and a Pt precursor, Pt(acac)2. During
heating at 180 1C, Pt(acac)2 can be reduced by oleyl amine. In
general, Mn3O4 NPs acted as seeds and reduced Pt atoms grew
on the Mn3O4 NPs surface to form core–shell or dumbbell
NPs. However, unexpectedly, the selective-growth was not
observed and newly formed Pt atoms nucleated separately.
Interestingly, although seeding growth does not occur, the
Mn3O4 NPs have a great impact on the growth of Pt NPs.
More interestingly, the formed Pt NPs are rarely-observed
polycrystalline. Further growth centering the twinned
polycrystalline Pt NPs can generate length-controlled one-
dimensional (1D) Pt NRs and importantly it was found that
high index facets exist at the edge of the formed NRs. The
result from the new induced-growth is potentially crucial to the
following several aspects. (i) The formation mechanism of
polycrystalline Pt NPs. As described by Xia,2a,3a unlike other
noble NPs, the polycrystalline Pt NPs rarely formed and
Nanomaterials and Chemistry Key Laboratory, Wenzhou University,
Wenzhou, Zhejiang 325027, P. R. China.
E-mail: Badbachier@gmail.com, Smhuang@wzu.edu.cn;
Fax: +86-57788373124
w Electronic supplementary information (ESI) available: Experimental
details, additional TEM, XRD and EDS results. See DOI: 10.1039/
c0cc03656k
Fig. 1 The TEM and HRTEM images of (A) Mn3O4 NPs and (B, C, D)
polycrystalline twinned Pt NPs.
c
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field (HAADF) results (Fig. S1w), pure Mn3O4 and twinned Pt
NPs were observed separately, which clearly indicates that
Mn3O4 NPs do not act as seed and no selective-growth on
Mn3O4 occurred during the formation of twinned Pt NPs. It
was found that only single-crystalline Pt with broad size
distribution can be generated without Mn3O4 NPs (Fig. S2w),
signifying that Mn3O4 is a key factor for forming twinned Pt
NPs. It has been proved that twinned polycrystalline Ag and
Pd NPs can be generated without oxidative agents.2f,g It is
believed that the high internal strain energies associated with
twinned structures and occurrence of oxidative etching (due to
O2 or other oxidative agents) did not favour the formation of
twinned polycrystalline NPs.2a In our reaction system,
although no treatments (introduction of N2 or other inert
gases) were employed to remove O2 which dissolves in
solvent and exists in the reactor,2e the polycrystalline Pt NPs
still formed, further confirming that the Mn3O4 NPs are
decisive during the formation of polycrystalline Pt NPs. It
must be noted that a small quantity of single crystalline Pt
nanocubes was also observed as shown in Fig. S3w.
It was very interesting to find that Pt NRs with different
lengths can be obtained when the reaction was run at 180 1C
for different given times. The products at different stages were
isolated and analyzed by TEM (Fig. 2). Distinctly, the twinned
Pt NPs had bud-like structures with 0.23 nm interplane
distance on their surface, which demonstrates that the buds
grew along h111i direction as labelled by the red arrows
(Fig. 2A). These buds grew into short Pt NRs after 3 h of
reaction (Fig. 2B) and the Pt NRs became longer after 5 h as
shown in Fig. 2C. During the whole growth process from buds
to NRs, the growth direction was unchanged along h111i as
also confirmed by XRD results (Fig. S5w). Thus, basically, the
length of the Pt NRs could be controlled by the reaction
period. It is worth noting that a further prolonging the
reaction time is unable to produce longer NRs (Fig. S8w).
10 h of reaction caused the fusion of NRs to form porous NPs
as shown in Fig. S8Aw. Once the reaction was run for 15 h
(Fig. S8Bw) or 20 h (Fig. S8Cw), all NRs lost the rod shape.
Energetically, NRs have higher surface free energy than
sphere-like NPs. Driven by the minimization of the system
energy and heat, the system tends to be a low-energy state, so
high-energy NRs will transform to low-energy NPs under
certain conditions.
Fig. 3A–D showed the HRTEM images of prepared Pt NRs
at different magnifications. In Fig. 3A and B, all the Pt NRs
grow from one centre and exhibit match-like morphology. The
thickness of NRs was about 5.6 nm as shown in Fig. 3C. The
HRTEM image in Fig. 3D gave the atomic packing of
obtained NRs. Clearly, atomic defects and steps existed on
the edge of the NRs, indicating the presence of high index
facets.
We have demonstrated above that Mn3O4 NPs are
indispensable to the formation of polycrystalline Pt NPs in
our reaction system. However, whether they play an important
role in the subsequent growth of Pt NRs is still unclear. 1D Pt
nanowires (NWs) were usually synthesized by preferential
growth along h111i in aqueous systems in the past2a–d due to
the fact that Pt NPs with {111} binding facets provide growth
substrates for their growth. Here, in a comparable experiment,
single crystalline octahedral Pt NPs with 8 {111} facets
were used to replace Mn3O4 NPs, but no Pt NRs were
obtained (Fig. S7w), implying that Mn3O4 NPs have a
significant impact on both the formation of polycrystalline Pt
NPs and the growth of Pt NRs. The synergistic catalysis effect
(Mn3O4 and Pt) is believed to make major contributions to the
formation of Pt NRs. It has been demonstrated that induced-
nucleation NPs with high autocatalytic ability could produce
nearly spherical NPs rather than 1D NWs and the formation of
NWs were favoured by those with low autocatalytic ability.2
Very likely, Mn3O4 NPs act as a catalytic-activity-modifier and
provide
a suitable environment for the formation of
polycrystalline Pt NPs and the subsequent growth of Pt
NRs.10–12 The detailed effect of Mn3O4 NPs on the growth
of polycrystalline Pt NPs and Pt NRs is not fully understood at
this stage and more studies need to be carried out.
Nanostructured Pt materials with high index facets have
been demonstrated to exhibit excellent catalytic ability
in electrocatalytic oxidization of methanol because low-
coordinated atoms can easily interact with reactant molecules
and serve as active sites for breaking chemical bonds, which is
Fig. 2 The TEM images and schematic illustration of Pt NRs
corresponding to different reaction times: (A1–A3) 1 h (the scale bar
is 2 nm); (B1–B3) 3 h; (C1–C3) 5 h.
Fig.
3 (A)–(D) The HRTEM images of Pt NRs at different
magnifications.
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believed to have significant roles in fuel cells.7c,8 We loaded the
as-prepared Pt NRs onto activated carbon as catalyst13 for the
electrocatalytic oxidization of methanol. TEM observation of
the catalyst (Fig. S9w) showed that Pt NRs were well dispersed
on the support. The catalytic activity was evaluated in an
electrochemical measurement system and compared with that
of a commercial Pt/C catalyst. Fig. S10A and B in ESIw depict
cyclic voltammograms of the Pt NR/C and commercial
catalysts in a mixture of 0.5 M methanol and 0.5 M H2SO4
at 25 1C. The current density had been normalized to the Pt
loading amount. In both positive and negative scanning
directions, the current density of Pt NRs/C was higher than
that of commercial Pt/C catalysts, indicating that Pt NRs/C
exhibited higher activity than commercial Pt/C catalysts.
Redox reaction on the catalysts surface often causes major
catalyst degradation, leading to a loss in activity.13 In order to
investigate their electrocatalytic stability, the current vs. time
curves were recorded at 0.65 V for 2000 s, which demonstrated
that the Pt NRs/C catalyst also had better catalytic stability
and lifetime than commercial Pt/C catalysts.
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In conclusion, we have demonstrated the successful
synthesis of unusual polycrystalline twinned Pt NPs and
length-controlled Pt NRs with high index facets by induced-
growth of low-cost Mn3O4 NPs. This method is straight-
forward and highly reproducible, which can allow scientists
to study these rarely observed Pt NPs with twinned structure.
Mn3O4 NPs used in this report can be obtained easily and with
low-cost, which is vital to scaled production for these poly-
crystalline twinned Pt NPs and Pt NRs. The developed method
may be also applied to synthesize other nanostructured noble
metals and possibly to discover other induced-materials for the
purpose. Furthermore, the Pt NRs with high index facets have
been demonstrated to be highly active for oxidization of
methanol and the obtained Pt NRs could be easily supported
on carbon to serve as highly active catalyst for this purpose.
Higher catalytic activity and lifetime of the as-synthesized Pt
NRs/C than commercial Pt/C catalysts for methanol oxidation
indicates that they may be a promising candidate for DMFC.
The authors would like to express their appreciation
for partial financial support from NSFC for outstanding
young scientist (51025207), Zhejiang science and technology
project (2010C31039), Lucheng science and technology project
(T100106).
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Chem. Commun., 2011, 47, 1009–1011 1011