Mesoporous metal and metal alloy particles synthesized by aerosol-assisted confined growth of nanocrystals

Full text of DOI:10.1002/anie.201204289

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Chemie
DOI: 10.1002/anie.201204289
Nanocrystal Networks
Mesoporous Metal and Metal Alloy Particles Synthesized by Aerosol-
Assisted Confined Growth of Nanocrystals**
Qiangfeng Xiao, Hiesang Sohn, Zheng Chen, Daniel Toso, Matthew Mechlenburg,
Z. Hong Zhou, Eric Poirier, Anne Dailly, Haiqiang Wang, Zhongbiao Wu,* Mei Cai,* and
Yunfeng Lu*
Porous materials are broadly used in a diverse range of
applications; their unique properties are often influenced
significantly by the framework composition.[1–6] To date,
various porous materials were synthesized, however, the
framework compositions are mainly limited to carbon pow-
ders,[4] oxides,[3,5] metal–organic complexes,[7] and polymers.[8]
Generally, porous metals hold great promises for a broad
spectrum of applications, such as hydrogen storage and
catalysis;[1,3,4] however, the porous metals that were reported
so far are limited to noble metals, such as Pt,[9] Au,[10] Pd,[11]
and Ru.[12] For comparison, the past two decades have
witnessed remarkable progress in the synthesis of metal
nanocrystals, which were generally obtained by thermal
decomposition or reduction reaction of the precursors in the
presence of capping ligands.[13,14] Such nanocrystals could be
rapidly generated within a confined submicron-size space;
their subsequent confined growth gives three-dimensional
(3D) nanocrystal networks, the composition of which can be
defined through judicious choice of the precursors.
defined composition using an aerosol approach.[15] We started
from non-aqueous solutions (e.g., tetrahydrofuran as solvent)
of metal oleates, M(OA)x, in which M is a metal ion and OA is
oleic acid (Figure 1).[16] An atomization process with nitrogen
as the carrier gas continuously generated precursor droplets.
Solvent evaporation from the droplets enriched the precur-
Built on this hypothesis, we report herein a general
synthesis of mesoporous metal and metal alloy particles with
[*] Dr. X. F. Xiao,[+] Dr. H. Sohn,[+] Z. Chen, Prof. Y. Lu
Department of Chemical & Biomolecular Engineering
University of California
Figure 1. Formation of mesoporous particles through an aerosol-
assisted synthesis platform.
Los Angeles, CA 90095 (USA)
E-mail: luucla@ucla.edu
Dr. D. Toso, Dr. M. Mechlenburg, Prof. Z. H. Zhou
Department of Microbiology, Immunology & Molecular Genetics;
California NanoSystems Institute
Department of Physics and Astronomy, University of California
Los Angeles, CA 90095 (USA)
sors and resulted in highly concentrated M(OA)x aerosol
particles (step 1). Subsequent thermal decomposition of the
solvent molecules and the precursors (see Figure S1 in the
Supporting Information) within the heating zone created an
atmosphere that contained reducing molecules (e.g., CO and
-CHO; see gas chromatography (GC) analysis in Figure S2)[17]
and generated nanocrystals (step 2), which were sequentially
grown into nanocrystal clusters and networks (steps 3 and 4).
The subsequent sintering process further solidified the net-
works and removed the organic residuals, thus resulting in the
formation of mesoporous particles, the composition of which
was controlled through the choice of precursors. It is note-
worthy, that various metal oleates are commercially available
or can be readily synthesized, ensuring the general applic-
ability of this platform technology. It is also important to point
out that, although aerosol processes have been used to
synthesize mesoporous oxide particles (e.g., SiO2),[15] such
syntheses were often conducted by using aqueous solvents,
thus precluding their use for the synthesis of non-noble metal
particles.
Dr. E. Poirier, Dr. A. Dailly, Dr. M. Cai
General Motors Research and Development Center
30500 Mound Road, Warren, MI 48090 (USA)
E-mail: mei.cai@gm.com
Prof. H. Wang, Prof. Z. Wu
Department of Environmental Engineering
Zhejiang University
Hangzhou, 310027 (China)
E-mail: zbwu@zju.edu.cn
[+] These authors contributed equally to the work.
[**] This work was supported as part of the Molecularly Engineered
Energy Materials, an Energy Frontier Research Center funded by the
U.S. Department of Energy, Office of Science, Office of Basic Energy
Sciences under award DE-SC001342. The authors also acknowledge
the support from General Motor Inc. and IMRA America Inc.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201204289.
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As a representative, the synthesis of mesoporous Ni
particles from Ni(OA)2 is shown in Figure 2. The transmission
electron microscopy (TEM) image of the Ni particles shows
the mesoporous structure with particle sizes averaging at
approximately 200 nm (Figure 2A-a). The high-resolution
Consistently, X-ray diffraction (XRD) analysis shows the
characteristic (111), (200), and (220) diffractions of the Ni
face-centered cubic (FCC) structure (Figure 2C).[19] Their
crystallite are estimated to be 3.4 nm in size by the Scherrer
equation, a result that is consistent with TEM observation.
The porous structure was further elucidated through 3D
tomographic reconstruction. A cross-sectional image of the
tomogram clearly exhibits a uniform spongy skeleton that is
composed of Ni nanocrystals and the surrounding C layers
(Figure 2B-a). The magnified image further reveals the
spongy structure, which is constructed from the nanocrystal
networks (Figure 2B-b). To further illustrate the structure,
total volume reconstruction was further conducted. Cross-
section images of a mesoporous particle show the variation of
electron density between the nanocrystals (yellow) and the
C layers (green; Figure 2B-c). These elemental sectional
images further corroborate homogeneous distribution of Ni
and C throughout the particle. Their convoluted image clearly
indicates that the fine C grains are uniformly passivated onto
the Ni nanocrystals. Figure 2B-d shows the total volume
reconstruction obtained by the combination of the sectional
images. Clearly, the mesoporous particles are indeed con-
structed by networks of Ni nanocrystals, which harmonize
with the TEM, STEM, and STEM-EDS results.
Consistent with the structure analysis, nitrogen adsorp-
tion–desorption studies (Figure 2D) provided a type-IV
isotherm with a uniform pore diameter averaging at 3.5 nm
(Figure S3d). A high surface area of 211 m2 gÀ1 was achieved,
which is much higher than that of the Pt–C nanocomposite
with similar metal loading ( ꢀ 19 m2 gÀ1, containing 74, 18, 7,
and 1 wt% of Pt, C, O and S, respectively).[20] It is also worth
mentioning that the surface area contributed by the micro-
pores is approximately 26 m2 gÀ1, suggesting that most of the
surface area is contributed by the mesopores within the
particles. Moreover, such a porous structure is highly stable;
for example, after sintering in nitrogen at 5508C for 6.5 h, the
particles still retained a surface area larger than 200 m2 gÀ1
(Figure S4). Similarly, mesoporous particles of Pt, Co, and Fe
with high surface area were also synthesized, proving the
general applicability of this technology for the synthesis of
various mesoporous metals (Figure S5).
Figure 2. Structure, composition, and morphology of mesoporous Ni
particles. A) a) TEM, b) HRTEM, and c) STEM images of mesoporous
Ni particles, d) chemical mapping of a representative particle dis-
persed on a SiO2-support grid. B) a) Cross-sectional image (3D tomo-
gram) of a Ni particle showing density variation between the Ni
framework and surrounding C layer, b) magnified image from (a)
further revealing the formation of Ni nanocrystal networks within the
particle. c) Cross-sectional mapping images of C (green), Ni (yellow),
and their convolution (Ni-C) obtained by the segmentation technique.
d) 3D volume reconstruction images of C (green), Ni (yellow), and
convoluted 3D networks of Ni and C showing the unique mesoporous
architecture. C) XRD patterns and D) N2 sorption isotherms of Ni
particles.
Extended from this approach, mesoporous metal alloy
particles could also be synthesized by using multiple pre-
cursors in a specified ratio. Figure 3a–c shows representative
TEM and STEM images of Ni0.5Pt0.5 particles with Ni and Pt
oleates in a molar ratio of 1:1. These particles exhibit the
mesoporous structure similar to those of the single-metal
mesoporous particles; a nitrogen sorption study showed
similar isotherms with a high surface area of approximately
143 m2 gÀ1 (Figure S6b). The TEM image also exhibits a poly-
crystalline structure, thus indicating that the alloy particles
are also composed of primary nanocrystals (Figure 3b). To
further illustrate their structure, EDS (Figure S6a) and
chemical mapping (Figure S6c) were conducted on the
particles and the results suggest a homogeneous distribution
of Ni and Pt throughout the spheres.
TEM image suggests that these particles are polycrystalline
and composed of primary nanocrystals with a diameter of
around 3–5 nm (Figure 2A-b). Their scanning transmission
electron microscopy (STEM) image further confirms the
mesoporous structure (Figure 2A-c). The homogeneously
distributed black and white spots throughout the particle
are identified as Ni and C, respectively, by STEM-EDS
chemical mapping analysis (EDS = energy-dispersive X-ray
spectroscopy; Figure 2A-d). The mapping analysis suggests
a homogenous distribution of C within the particles. Thermal
gravimetric analysis (TGA) indicates that these particles
contain approximately 70 and 30 wt% of Ni and C, respec-
tively. X-ray photoelectron spectrum (XPS) suggests that Ni is
present in its metallic form, evidenced from the presence of
Ni 2p signal and the absence of a NiO signal (Figure S3c).[18]
The composition of the alloyed particles can be readily
tuned by the precursor ratio. For example, Figure 3d shows
XRD patterns of a series of NixPt1Àx particles prepared from
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shows the formation of Ni nanocrystals with a diameter of
approximately 3–4 nm, which is consistent with the sizes of
the nanocrystals observed in the final porous particles. The
sample collected at 1.2 s shows the formation of nanocrystal
clusters, which sequentially grew into networks of nano-
crystals in 1.8 s. The network structure was further developed
and completed with time, evidenced from the samples
collected at 2.4 and 9.0 s. Such structure evolution confirms
extremely fast reaction kinetics; mesoporous particles were
formed continuously in a matter of seconds.
Generally, the composition of the particles is determined
by the precursors that are used. Nevertheless, for the synthesis
of mesoporous metals, the precursors that are used (metal
oleates) contain oxygen moieties, which may lead to the
formation of mesoporous oxides rather than metals. There-
fore, the composition of the particles would be determined by
reaction temperature and intrinsic parameters of the metal
moieties, such as Gibbs formation energy of metal oxides
(DG8) and electronegativity (cp).
DG8 may be generally described by a three-term equation,
DG8 = A + BT+ CTlogT, in which T is the temperature and
A, B, and C are coefficients that depend on the temperature
and the metal.[23] Figure 5 shows plots of DG8 for various
Figure 3. Structure of mesoporous alloy particles. a) TEM, b) HRTEM,
and c) STEM images of representative Ni0.5Pt0.5 alloyed particles
showing mesoporous polycrystalline structure. d) XRD of NixPt1Àx with
various compositions. e) Linear relationship between the lattice param-
eter of the alloys versus their Ni content.
mixtures of Ni(OA)2 and Pt(OA)4 with
different molar ratios. All samples
exhibit an FCC structure with character-
istic (111), (200), and (220) diffractions,
which systematically shift to the lower
2q with increasing Pt content, thus
indicating the formation of substitu-
tion-type alloys formed by replacing Ni
with Pt.[21] A linear correlation comply-
ing with the Vegardꢀs law, a = aNi x + aPt-
(1Àx), in which aNi and aPt is the unit cell
parameter of Ni or Pt and x is the molar
fraction of Ni, further confirms the
formation of alloy with tunable compo-
sition.[22] It is important to point out that
mesoporous particles of binary NixPd1Àx
alloys and ternary NixPtyPd1ÀxÀy alloys
were also synthesized with tunable
composition, further demonstrating the
general applicability of this approach
(Figure S7).
Figure 4. Structural evolution of the mesoporous particles during the continuous aerosol-
assisted formation process. TEM images of the samples collected at space time of a) 0.6 s,
b) 1.2 s, c) 1.8 s, d) 2.4 s, and e) 9.0 s, respectively, exhibiting structural evolution from nano-
crystals to nanocrystal clusters to nanocrystal networks.
The formation of such particles was achieved through such
a rapid and continuous process. The average resident time
(space time) of the aerosol particles within the heating zone
(reacting zone) is estimated to be 9 s, based on the operation
condition and the dimension of the tubular reactor (Fig-
ure S8). To dissect such a continuous process, we systemati-
cally varied the length of the heating-zone and examined the
structure of the corresponding particles that were formed. By
converting the heating-zone length into the resident time
(reaction time), structural evolution of the particles during
this continuous process could be readily probed. Figure 4a–
e shows the TEM images of the particles collected with
heating-zone lengths corresponding to a space time of 0.6, 1.2,
1.8, 2.4, and 9.0 s, respectively. The sample collected at 0.6 s
metals at 5508C (the temperature used for the synthesis), as
well as c. Considering DG8 of CO (À184.5 kJmolÀ1e) or CO2
(À395.6 kJmolÀ1e) from oxidizing the reducing agents (e.g.,
X, CO, and -CHO), the formation of mesoporous metal
particles is thermodynamically favored when DG8 is larger
than À395.6 kJmolÀ1e. In our system, the use of Pt, Pd, Cu,
and Ni oleates led to the formation of mesoporous metal
particles with DG8 greater than À149.3 kJmolÀ1. For metals
with a medium range of DG8, such as FeO (À210.5 kJmolÀ1),
the product consists of a dominant oxide phase with a small
fraction of metallic Fe. When DG8 is more negative, such as
for ZnO (À257.5 kJmolÀ1) and MnO (À331.4 kJmolÀ1), use
of the corresponding oleates leads to the formation of
mesoporous oxide particles. Since cp is a measure of the
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hydrogen at 258C, while no H2 absorption is observed for pure
MgH2 below 2008C. The saturation capacity increases with
temperature, reaching 6.2 wt% at 62 bar and 2508C; while
bulk Mg requires 4008C to reach comparable performance.
In summary, we have developed a platform technology
that enables the rapid and continuous synthesis of mesopo-
rous metal and metal alloy particles through confined growth
of nanocrystals. Considering the vast library of organometal-
lic precursors that are available, huge families of mesoporous
materials with compositions ranging from metals to alloys to
oxides may be readily prepared for a broad spectrum of
applications.
Experimental Section
Figure 5. Composition dependence of the mesoporous particles versus
the metallic moieties of the precursors. Estimated DG8 at 5508C,
plotted against Pauling electronegativity (cp) of the corresponding
metals.
Mesoporous particles of metals and alloys were synthesized from
metal oleates or their mixtures with specified ratios. The metal oleates
are commercially available or were prepared by reacting metal salts
or oxides with oleic acid. The precursors were dissolved in organic
solvent (e.g., THF and toluene) at a specified ratio and concentration
to form homogenous clear solutions. The precursor solutions were
atomized by using a commercial pneumatic atomizer (Model 3076,
TSI, Inc.) with nitrogen as a carrier/atomization gas. The aerosols
passed through a tubular reactor (heated at 5508C) and converted to
the mesoporous particles, which were subsequently collected by the
filter. The aerosol reactor is operated at a volumetric flow rate of
2.6 LminÀ1 (STP). The geometry of the tubular reactor within the
heating zone is 1’’ (ID) ꢁ 30’’ (L). Synthesized particles were after-
wards treated at 5508C under N2 atmosphere for 30 min. Details of
the chemicals that were used, precursor synthesis, and character-
izations are provided in the Supporting Information.
ability of chemically bonded atoms to attract electrons, the
underlying chemistry is more intimately related to cp.[24] It was
found that metal oleates in which the metal moieties possess
cp ꢁ 1.90 (Cu) tend to form mesoporous metals and metal
alloys, whereas those with lower cp form oxides. Such results
provide a roadmap to design porous materials with defined
composition (Figure S9–S15).
The capability to synthesize such mesoporous metal
particles offers access to unique materials for broad applica-
tions. In Figure S16A, a comparison of H2 uptake at 258C for
the mesoporous Ni particles, Vulcan XC-72 (a carbon powder
with a surface area of 230 m2 gÀ1), and Ni nanoparticles (30–
50 nm in diameter) is given. The mesoporous Ni particles
exhibit a H2 uptake of 0.40 wt% at 80 bar, while other
samples only show negligible H2 uptakes of approximately
10À3 wt%. Such results are also comparable to those of Ni-
decorated high-surface-area activated carbon powder.[25]
Note that extremely high pressure (6.5 ꢁ 103 bar) is needed
for large Ni particles to reach a similar H2 uptake.[26] A linear
increase of hydrogen storage with pressure is observed in the
high-pressure range. Such phenomena are similar to those
observed for metal-doped metal–organic frameworks
(MOFs) or carbon powder rather than for the metal–hydro-
gen systems, a result that can be attributed to the presence of
a large number of hydrogen binding and dissociation sites, as
well as the nanocrystalline network structure that promotes
the formation of nickel hydride.[27]
Received: June 2, 2012
Revised: July 26, 2012
Published online: && &&, &&&&
Keywords: hydrogen storage · mesoporous materials ·
.
nanocrystal networks · nanoparticles · alloys
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Communications
Nanocrystal Networks
X. F. Xiao, H. Sohn, Z. Chen, D. Toso,
M. Mechlenburg, Z. H. Zhou, E. Poirier,
A. Dailly, H. Wang, Z. Wu,* M. Cai,*
Y. Lu*
&&&& — &&&&
Mesoporous Metal and Metal Alloy
Particles Synthesized by Aerosol-Assisted
Confined Growth of Nanocrystals
From droplets to “spheres”: A platform
technology enables the rapid and contin-
uous synthesis of mesoporous metal and
metal alloy particles (see picture). The
confined growth of nanocrystals in aero-
sol droplets leads to the formation of
these particles with defined composition.
6
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