Small molecules control the formation of Pt nanocrystals: A key role of carbon monoxide in the synthesis of Pt nanocubes

Article abstract of DOI:10.1039/c0cc03671d

In many previous studies, nonaqueous synthesis of Pt nanocubes with tunable size has been achieved by the use of metal carbonyls (e.g., Fe(CO)₅, Co₂(CO)₈, W(CO)₆). The presence of zero-valent metals in the carbonyls was demonstrated as the key factor to the nanocube formation but the role of CO was entirely ignored. By using CO alone, we have now demonstrated that the favorable growth of Pt nanocubes in the presence of CO is mainly owing to the effect that the Pt (100) surface is stabilized by the co-adsorption of CO and amine.

Full text of DOI:10.1039/c0cc03671d

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Small molecules control the formation of Pt nanocrystals: a key role
of carbon monoxide in the synthesis of Pt nanocubesw
Binghui Wu, Nanfeng Zheng* and Gang Fu*
Received 5th September 2010, Accepted 21st October 2010
DOI: 10.1039/c0cc03671d
In many previous studies, nonaqueous synthesis of Pt nanocubes
with tunable size has been achieved by the use of metal carbonyls
(e.g., Fe(CO)5, Co2(CO)8, W(CO)6). The presence of zero-
valent metals in the carbonyls was demonstrated as the key
factor to the nanocube formation but the role of CO was entirely
ignored. By using CO alone, we have now demonstrated that the
favorable growth of Pt nanocubes in the presence of CO is
mainly owing to the effect that the Pt (100) surface is
stabilized by the co-adsorption of CO and amine.
to Pt,21 we however believe that CO should play an important
role in the formation of Pt nanocubes where metal carbonyls
were used.
In order to demonstrate that CO alone was able to determine
the formation of Pt nanocubes, we excluded the use of any
metal carbonyl in our studies. The nonaqueous syntheses of Pt
nanoparticles were carried out by reducing Pt(acac)2 in
mixtures containing oleylamine (OAm) and oleic acid (OLA)
in various ratios (see ESIw for details). We first chose 4 : 1
(volume ratio) OAm/OLA mixture as the solvent for the
studies. In the absence of any metal carbonyl or CO, as
illustrated in Fig. 1a, the thermal decomposition of Pt(acac)2
at 180 1C in the 4 : 1 OAm/OLA mixed solvent led to the
formation of Pt nanodendrites, consistent with previous
Colloidal nanoparticles of platinum have been extensively
explored for catalytic applications in automobile exhaust
purification, petrochemical industry, fuel-cell technology, and
fine chemical industry.1 As a surface-dependent property, the
catalysis by metal nanocrystals is often determined by their
surface structure.2–7 For instance, {100} terminated Pt nano-
cubes have been reported to exhibit significantly enhanced
electrocatalytic activity in oxygen reduction reaction.7,8 There-
fore, the development of effective synthetic methods aimed to
tailor the size and shape of Pt nanocrystals is important for
their catalysis applications.3,6,7 However, as compared to Au
or Ag, the surface energy difference among various crystal-
lographic planes of Pt is relatively large, resulting in a challenge
in controlling the shape of Pt nanoparticles.2
studies.19,20 Surprisingly, when a CO gas flow (B30 mL minÀ1
)
was simply applied on the surface of the same reaction, the
formation of Pt nanocrystals in cubic shape was observed
(Fig. 1b, Fig. S1, ESIw). The average edge length of the as-
prepared Pt nanocubes was 10.3 nm. As revealed by high-
resolution TEM (HRTEM), the well-resolved lattice fringes
indicate the single-crystallinity of the Pt nanocubes (Fig. 1c).
The lattice fringes running parallel to the edges of the cube
Recently, there have been an increasing number of reports
on the synthesis of Pt nanocrystals with high-energy facets
using electrochemistry,9–12 aqueous,6,13–15 and nonaqueous
synthetic routes.7,16–20 Several effective nonaqueous synthetic
methods have been developed to prepare monodispersed Pt
nanocubes. The nonaqueous syntheses of Pt nanocubes were
mainly achieved by reducing Pt precursors in an H2
atmosphere16,17 or in the presence of metal carbonyls, such
as Fe(CO)5,7,18 Co2(CO)8,19 and W(CO)6.20 The distinct
advantage of the carbonyl route over the hydrogen reduction
method is that monodispersed well-defined Pt nanocubes can
be synthesized in comparatively high concentrations. In all
previous studies of the carbonyl routes, the zero-valent metals
(i.e., Fe, Co, W) in the metal carbonyls were considered to play
an essential role for the shape control of Pt nanocubes. The
possible role of CO decomposed from the carbonyls was
entirely ignored. Considering that the strong binding of CO
State Key Laboratory for Physical Chemistry of Solid Surfaces and
Department of Chemistry, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen 361005, China.
E-mail: nfzheng@xmu.edu.cn, gfu@xmu.edu.cn;
Fig. 1 TEM images of (a) Pt nanodendrites synthesized without CO
and (b) 10.3 nm Pt nanocubes prepared under CO flow at 180 1C, both
using 4 : 1 OAm/OLA solvent. (c) HRTEM and fast Fourier
transform (inset) of an individual 10.3 nm Pt nanocube shown in (b).
(d) XRD patterns of randomly oriented and (100) textured Pt
nanocube assemblies.
Fax: +86 592 2183047; Tel: +86 592 2186821
w Electronic supplementary information (ESI) available: TEM images
of Pt nanoparticles made in mixed OAm and OLA, FT-IR, XRD, CV
of 10.3 nm Pt nanoparticles, and computational details. See DOI:
10.1039/c0cc03671d
c
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Chem. Commun., 2011, 47, 1039–1041 1039

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have a spacing of 0.19 nm which corresponds to the (200)
interplanar distance of face centered cubic (fcc) platinum.
The highly crystalline nature of the as-prepared Pt
nanocubes was also confirmed by X-ray diffraction (XRD)
analysis. As shown in Fig. 1d, when the analysis was run on the
random aggregates of the Pt nanocubes deposited on a rough
glass holder, all characteristic diffraction peaks of fcc Pt were
observed. In comparison, slow evaporation of the Pt nanocube
dispersion on a surface-polished Si(100) wafer resulted in an
oriented assembly of the Pt nanocubes. In the resultant XRD
pattern, the (200) peak intensity was significantly enhanced
while the (111) disappeared. This observation indicates that the
Pt nanocubes in the oriented assembly align perfectly flat on
the substrates with their {100} facets, further confirming that
the as-prepared Pt nanocubes have a very narrow distribution
of shape. The average crystalline size estimated from Scherrer’s
formula using the (200) peak was B10.2 nm, consistent with
the average edge length that was measured in the TEM images.
While no Pt nanocubes were produced in the absence of CO,
high-quality Pt nanocubes were readily prepared by
introducing surface CO gas flow. This result suggested the
importance of CO in the formation of Pt nanocubes, which
could originate from the strong binding of CO on Pt. We have
therefore used FT-IR to check the presence of CO adsorption
on the as-made 10.3 nm Pt nanocubes. While no CO
adsorption was observed on Pt nanodendrites prepared in
absence of CO (Fig. S2, ESIw), the freshly prepared Pt
nanocubes displayed an obvious CO band at 2082 cmÀ1 and
a weak band at 1802 cmÀ1 (Fig. 2), attributed to linear and
bridging CO respectively. The CO signal slowly decayed over
time but was still strongly present in the Pt nanocubes after
120 h exposure in air (Fig. S3, ESIw). The IR studies (Fig. S2,
ESIw) also revealed that only OAm was present in the as-
prepared Pt nanocubes although both OAm and OLA were
used, consistent with the previous report.20 In order to explain
the origin of shape control using CO, we calculated the surface
energies (j) of the (100) and (111) facets of Pt before and after
adsorption of amine (RNH2) and/or CO (see ESIw for details).
The periodic DFT calculations (Table S1, ESIw) revealed that
for clean Pt surface, j100 is higher than j111 by 0.26 eV per
atom. When RNH2 adsorbs on the surfaces at 0.25 monolayer
(ML), the difference (Dj) between j100 and j111 decreases to
0.20 eV per atom. After co-adsorption of RNH2 and CO
(0.25 ML/0.25 ML), Dj(100)À(111) is calculated to be À0.02 eV
per atom, indicating that (100) facet becomes more stable than
(111) facet. Thus, experiments and theoretical calculations reach
an agreement that CO adsorption plays a crucial role in the
formation of Pt nanocubes, which is different from the
mechanism proposed in the syntheses of Pt or Pt3M
nanocubes where metal carbonyls were used. In those systems,
the presence of zero-valent metals (i.e., Fe, Co, W) in the
carbonyls has been regarded as the key to the nanocube
formation. It is worth noting that CO binding was also
observed on the Pt nanocubes synthesized by using W(CO)6
(Fig. S4, ESIw).20
Besides the use of metal carbonyl, the OAm/OLA ratio was
also claimed as an important factor to the formation of Pt
nanocubes in many previous studies.20 However, based on our
proposal that the co-adsorption of CO and amine on Pt(100) is
the most essential reason for the nanocube formation, one
would expect that Pt nanocubes could be prepared even in pure
OAm. However, when the reaction was carried out in pure
OAm in the presence of surface CO gas flow, only well-
dispersed Pt polyhedral nanocrystals were produced
(Fig. 3a). Detailed analysis revealed that the major Pt
polyhedra were cuboctahedra with an average size of 5.0 nm,
which did not seem to follow the prediction from our
hypothesis on the role of CO. As indicated by the fact that
only OAm was present in the Pt nanocubes prepared in 4 : 1
OAm/OLA mixture, OAm bound strongly on the surface of Pt
nanoparticles. Considering that CO gas flowed only on the
surface of the reactions, the interaction of OAm on Pt could be
strong enough to compete with the CO binding. To obtain
nanocubes in pure OAm, the CO adsorption on Pt must be
enhanced. Following this line, we have succeeded in obtaining
Pt nanocubes in pure OAm by applying higher CO pressure.
Under 1 atm CO instead of surface CO gas flow, Pt
nanocubes were indeed produced from the reactions,
although some truncated nanocubes were also observed
(Fig. 3b). The average edge length of the as-prepared
nanocubes was 5.1 nm. When the CO pressure was further
increased to 2 atm, the cubic morphology of the prepared
nanoparticles was maintained. However, their average edge
length was reduced to 4.1 nm (Fig. 3c). It should be pointed out
that the thermal reduction of Pt(acac)2 in pure OAm at 180 1C
hardly took place, suggesting the additional role of CO as the
reductant in the synthesis. The faster nucleation caused by the
higher CO pressure could explain the size reduction of Pt
nanocubes under increased CO pressure. Both HRTEM and
XRD analysis (Fig. 3, Fig. S5, ESIw) suggested that Pt
Fig. 3 Representative TEM images of Pt nanoparticles in pure OAm
under (a) CO flow, (b) 1 atm and (c) 2 atm CO. The insets are the
corresponding HRTEM images.
Fig. 2 IR spectra of as-received oleylamine and the 10.3 nm Pt
nanocubes prepared in 4 : 1 OAm/OLA mixed solvent.
c
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nanocubes prepared under higher CO pressures were of high
quality. Furthermore, our preliminary results demonstrated
that the surface of the as-prepared Pt nanocubes was readily
activated by exchanging with n-butylamine to provide a clean
surface for electrocatalysis studies (Fig. S6, ESIw).22
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In conclusion, we have developed an effective method to
control the formation of Pt nanocubes by introducing CO.
While FT-IR measurements confirmed the co-adsorption of
CO and oleylamine on the as-prepared Pt nanocubes, periodic
DFT calculations revealed that Pt (100) surface can be
significantly stabilized by the co-adsorption of CO and
oleylamine. By understanding the key role of CO adsorption,
the synthesis of Pt nanocubes has now been successfully achieved
in the mixtures of OAm and OLA, and also in pure OAm.
We thank NSFC (20925103, 20871100, 20721001), the Fok
Ying Tung Education Foundation (121011), MSTC
(2011CB932403, 2009CB930703), NSF of Fujian Province
for Distinguished Young Investigator Grant (2009J06005)
and the Key Scientific Project of Fujian Province
(2009HZ0002-1).
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Chem. Commun., 2011, 47, 1039–1041 1041