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Direct Imaging of ALD Deposited Pt Nanoclusters inside
the Giant Pores of MIL-101
Maria Meledina, Stuart Turner,* Maria Filippousi, Karen Leus, Ivan Lobato,
Ranjith K. Ramachandran, Jolien Dendooven, Christophe Detavernier,
Pascal Van Der Voort, and Gustaaf Van Tendeloo
of MOFs have focused lots of attention
to these materials, with the main applications lying in the field of catalysis,[5–7]
gas storage and separation,[3,8,9] and
drug delivery.[10–12] The tunable pore size
can also be used as a template for confined growth of metal or metal-oxide
nanoparticles with sizes tailored to the
MOF pore diameter,[13,14] e.g., catalytic
applications.
Over the past decade, numerous
approaches have been investigated for
the embedding of nanoparticles within
MOFs.[15] Besides the typically used liquid
impregnation and solid grinding, postsynthetic metalation from the vapor-phase is
a widely used method.[15] However, this
technique provides little control over the
spatial distribution of the species within
the framework. Atomic layer deposition
(ALD) is a powerful technique that allows controlled gas phase
deposition of metal oxide monolayers on the surface of ordered
mesoporous materials.[16–21] An ALD process typically consists
of alternating precursor (A) and reactant gas (B) pulses, i.e.,
cyclic AB-type exposures that allow to deposit material with
atomic precision in a conformal way. Owing to its lower working
temperature in comparison to chemical vapor deposition, ALD
can be applied to samples with a reduced stability.[22–24]
Of pivotal importance in controlling and refining MOF material properties during synthesis is the possibility to characterize them at a local scale. Transmission electron microscopy
(TEM) is a powerful tool in order to extract information on the
atomic scale for both empty and loaded metal-organic frameworks.[5,6,14,25–29] In the past, techniques like electron diffraction[25,26] bright-field TEM,[13,15,30] high-angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM),[31]
and electron tomography[13,32] have been used to great effect to
retrieve morphological and structural information on MOFs
and nanoparticle-loaded MOFs. Recently, our group and others
have also shown the value of spectroscopic data like energy dispersive X-ray spectroscopy[6] and electron energy loss spectroscopy[14,33] in the analysis of more complex framework materials, like core-shell MOF structures.
However, the sensitive nature of MOFs makes them an obstinate candidate for electron microscopy investigations. Upon
electron beam illumination in the electron microscope, MOF
materials rapidly lose their long range order and crystallinity.[34]
MIL-101 giant-pore metal-organic framework (MOF) materials have been
loaded with Pt nanoparticles using atomic layer deposition. The final
structure has been investigated by aberration-corrected annular dark-field
scanning transmission electron microscopy under strictly controlled lowdose conditions. By combining the acquired experimental data with image
simulations, the position of the small clusters within the individual pores of
a metal-organic framework has been determined. The embedding of the Pt
nanoparticles is confirmed by electron tomography, which shows a distinct
ordering of the highly uniform Pt nanoparticles. The results show that atomic
layer deposition is particularly well-suited for the deposition of individual
nanoparticles inside MOF framework pores and that, upon proper regulation of the incident electron dose, annular dark-field scanning transmission
electron microscopy is a powerful tool for the characterization of this type of
materials at a local scale.
1. Introduction
Metal-organic frameworks (MOFs) are hybrid organic–inorganic materials, in which metal ions or clusters form secondary
building units and act as nodes coordinating organic bridges
into a highly porous network.[1] By fine-tuning the geometry
and bonding of both the organic and inorganic parts of the
MOFs, materials with tailored pore size and morphology can
be obtained.[2–4] The huge surface area and controlled porosity
M. Meledina, Dr. S. Turner, Dr. M. Filippousi,
Dr. I. Lobato, Prof. G. Van Tendeloo
EMAT
University of Antwerp
Groenenborgerlaan 171, B-2020 Antwerp, Belgium
E-mail: [email protected]
Dr. K. Leus, Prof. P. Van Der Voort
Department of Inorganic and Physical Chemistry
Center for Ordered Materials
Organometallics and Catalysis (COMOC)
Ghent University
Krijgslaan 281-S3 9000, Ghent, Belgium
R. K. Ramachandran, Dr. J. Dendooven, Prof. C. Detavernier
Department of Solid State Sciences
CoCooN Research Group
Ghent University
Krijgslaan 281/S1 9000, Ghent, Belgium
DOI: 10.1002/ppsc.201500252
Part. Part. Syst. Charact. 2016,
DOI: 10.1002/ppsc.201500252
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In order to collect trustworthy crystallographic information
on the network, data need to be acquired using minimal total
electron dose. In short, when studying MOF materials, every
electron counts. Previous experiments have demonstrated that
liquid nitrogen cooling and imaging using lowered acceleration
voltage can also be beneficial to the stability of MOFs under the
electron beam,[25] although electron dose remains the most significant factor. Several authors have pointed out that annular
dark-field scanning transmission electron microscopy (ADFSTEM) imaging can provide a significant increase in contrast
for the same incident electron dose with respect to bright-field
TEM imaging.[5,33] In a recent contribution,[31] high-resolution
images from delicate Ti-Doped AlPO4-5 and Zn–MOF-74
frameworks were obtained through ADF-STEM imaging. So
far, however, no results are available on the simultaneous characterization of intact pores and loaded nanoparticles.
In this work, ADF-STEM imaging and electron tomography
are used to study the embedding of Pt nanoparticles, deposited
by ALD within a Cr-based cubic MIL-101 (Material of Institut
Lavoisier Nr. 101) framework. The MIL-101 framework was
selected, as the giant nanometer-sized pores of this framework
allow pore imaging at low magnifications and resulting low
electron dose. By combining the acquired images with detailed
image simulations, the embedding of the Pt nanoparticles
within the framework can be studied simultaneously with the
MIL-101 framework structure.
2. Results and Discussion
2.1. Imaging of the Empty MIL-101 Framework
Figure 1a displays an ADF-STEM overview image of the MIL101 crystals, synthesized according to the method published
by Jiang et al.[35] The image was acquired under low-dose conditions, keeping a beam current of approximately 10 pA and
acquiring the data at low magnification with a short dwell time
Figure 2. a) High-resolution ADF-STEM image of the MIL-101 structure
viewed along the [011] direction, together with an ADF-STEM image simulation as inset. b) Low-pass filtered ADF-STEM image with the ADF-STEM
image simulation as inset. c) MIL-101 model viewed along the [011] direction, with Cr polyhedra in green and C in brown (H and O are not shown
for clarity). The circles indicate two different pore types: the red one corresponds to a small cage with a diameter of 29 Å and the blue one to a
large cage with a diameter of 34 Å.
Figure 1. a) Overview ADF-STEM image of empty MIL-101 crystals with
a truncated octahedron shown as inset. b) HR ADF-STEM image of an
empty MIL-101 crystal viewed along the [011] direction. c) Fourier transform pattern of (b).
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of 5 μs. An important parameter in ADF-STEM mode is the
accelerating voltage. On the one hand, operating the microscope at lower voltages (60–120 kV) helps to avoid the immediate “knock-on” damage of the studied materials. On the other
hand, higher voltages (200 or 300 kV) can penetrate thicker
crystals and high-speed electrons interact less with the sample.
Also, imaging at higher voltages has the added benefit of an
improved spatial resolution. As for most highly beam sensitive
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Figure 3. a) Overview ADF-STEM image of an MIL-101 crystal loaded with
Pt nanoparticles by 40 ALD cycles. b) ADF-STEM image showing that the Pt
nanoparticle size is tailored to the MIL-101 pore size. The arrows point out
several examples: white arrows point to nanoparticles at small cage positions
(in projection), the red arrow points to a nanoparticle at a large cage position.
c) Fourier transform of the image shown in (b). d) HR ADF-STEM image of
a crystalline Pt nanoparticle and e) a partially crystallized Pt nanoparticle.
materials, useful data can only be extracted from the first
acquired image; using higher voltages with less electron beam–
sample interaction is a valid route to the acquisition of optimal
TEM data. Therefore, in these experiments, 200 and 300 kV
acceleration voltages were used.
The controlled synthesis leads to materials with a welldefined morphology; the crystals have a typical truncated octahedral shape (schematically shown as inset) with preferential
exposure of {111} facets and {100}-type truncation. This morphology is typical for cubic crystals, like MIL-101 (FCC (facecentered cubic) structure, space group Fd-3m with a = 89 Å).
Typical FCC crystal defects, like the Σ = 3 {111}-type twin
boundaries, marked by arrows in Figure 1a, do occur. The presence of these twin defects proves that even though MOFs are
built up of complicated building units, their crystallographic
behavior is not significantly different from simple FCC crystals. Figure 1b shows a magnified ADF-STEM image of the
pores along the [011] direction, as evidenced by the Fourier
transform pattern in Figure 1c. The information transfer in
the image, acquired at magnification 160 k×, extends down
to 0.64 nm−1 (being the (04-4) reflection), corresponding to a
1.6 nm image resolution. It is clear that ADF-STEM imaging,
under low-dose conditions, allows the MIL-101 structure to
be visualized intact. Note that this image was acquired on
an instrument without Cs correction. The large cell parameter of MIL-101 (89 Å) is definitely an advantage and allows
useful data to be acquired at low magnifications. However, the
instruments in which the current in the electron probe can be
controlled independently of the probe size (e.g., instruments
equipped with an electron monochromator) are definitely beneficial for this type of work.
A high-resolution ADF-STEM image of the MIL-101 structure
is presented in Figure 2a. For clarity, a low-pass filtered image
is also presented in Figure 2b. ADF-STEM imaging generally
allows a relatively straightforward interpretation of the observed
image contrast. However, in the case of a highly complex material such as MIL-101 (consisting of a combination of high-Z elements (Cr) and low-Z elements (O, C, and H)), image simulations are useful for a proper interpretation of the image contrast.
In Figure 2c, the MIL-101 structure is displayed, viewed along
the [011] zone axis; it is built up of so-called super-tetrahedra,
one of which is indicated by a yellow triangle in the image.
Figure 4. a) Overview ADF-STEM image of an MIL-101 crystal loaded with Pt nanoparticles by 40 ALD cycles. b) Tomographic volume reconstruction
of the same Pt@MIL-101 crystal. c) Orthoslice through the reconstruction showing the Pt nanoparticles to be inside the MIL-101 crystal; arrows point
out several examples.
Part. Part. Syst. Charact. 2016,
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an inner pore diameter of 29 Å (indicated by the red circle), and
larger pores with a hexagonal pore opening and an inner pore
diameter of 34 Å (indicated by the blue circle). An excellent
match between the experimental data and the simulated images
is observed; the two types of pores are clearly visible. The simulations confirm that the contrast in the ADF-STEM images is
largely generated by the Cr atoms and to a far lesser extent by
the lighter elements such as C, O, and H.
2.2. Imaging of the ALD Pt Loaded MIL-101 Framework
Figure 5. a) Overview ADF-STEM image of MIL-101 crystals heavily loaded
with Pt nanoparticles by 120 ALD cycles. The inset Fourier transform pattern of the region indicated by the white rectangle evidences that the MIL101 material remains crystalline, even after heavy Pt loading, and is imaged
along the [011] zone axis orientation. b) HR ADF-STEM image of a heavily
Pt-loaded MIL-101 particle. The ordering of the Pt nanoparticles follows
the pore structure of MIL-101. c) HR ADF-STEM image of a heavily Ptloaded MIL-101 particle. The ordering of the Pt nanoparticles follows the
pore structure of MIL-101, imaged along the [−112] zone axis orientation.
These super-tetrahedra build up the highly porous MIL framework, consisting of two types of pores (sometimes referred to
as “cages”): small pores with a pentagonal pore opening having
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An overview of the MIL-101 material loaded with Pt nanoparticles by 40 ALD cycles is shown in Figure 3a. The MIL-101
particles remain crystalline after the loading procedure (see the
Fourier transform in Figure 3c) and they also keep their initial
truncated octahedral morphology. The bright contrast features
in the magnified image in Figure 3b are the Pt nanoparticles.
The size of the nanoparticles (examples are marked with
arrows) corresponds to the diameter of the MIL-101 cages, indicating a confined growth of the Pt nanoparticles within the MIL
framework cages. The positioning of the nanoparticles appears
to confirm an embedding within the MIL-101 cages; the particles marked by white arrows are likely incorporated into
the small cages, and the particles indicated by red arrows are
likely incorporated in the larger cages. Atomic resolution ADFSTEM imaging in Figure 3d,e indicates that the nanoparticles
can be either crystalline (Figure 3d) or partially crystallized
(Figure 3e). The main fraction of the nanoparticles is crystalline, as evidenced by the overview HAADF-STEM image in the
Supporting Information (Fig. S1). Note that under the severe
imaging conditions needed for atomic resolution imaging, the
basic MIL network is destroyed.
Obviously, the images presented in Figures 1, 2, 3 are 2D projections of a 3D reality. In order to confirm the loading of the Pt nanoparticles into the MIL-101 network material, an electron tomography experiment was performed. To this end, a single MIL-101
truncated octahedron is studied in Figure 4a. The reconstructed
volume (Figure 4b) and the orthoslices through the reconstructed
volume (Figure 4c) provide direct evidence that the Pt nanoparticles
are indeed embedded inside the MIL-101 material. Combined with
the high-resolution ADF-STEM imaging in Figure 3, the electron
tomography results prove that the ALD Pt material is embedded
inside both the smaller and the larger pores of the MIL-101 framework, without degrading the structure during loading.
By increasing the number of ALD cycles from 40 to 120, the
MIL-101 framework can be loaded with a higher number of Pt
nanoparticles. An example of heavily loaded MIL-101 crystals
is shown in Figure 5a. Even after heavy Pt loading, the MIL101 particles retain their truncated octahedral morphology. It is
fascinating to see that the embedded Pt nanoparticles remain
small, and fill up most of the MIL-101 pores. Indeed, the highcontrast Pt nanoparticles follow the MIL-101 framework structure in Figure 5a–c. In Figure 5a, the inset Fourier transform
pattern provides clear evidence of the arrangement of the Pt
nanoparticles inside the MIL framework, which is imaged
along the [011] zone axis orientation. An enlarged region of
an MIL-101 crystal is shown in Figure 5b. The arrows point to
separate Pt nanoparticles, arranged in the MIL framework in
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reconstruction in Figure 6b and an orthoslice through it in
Figure 6c. Gentle and fast imaging conditions allow taking the
series with a 10° tilting step keeping the loaded Pt nanoparticles
close to their initial position. Even though the MIL-101 framework is more than likely degraded after the acquisition of just
a few images, the periodical Pt nanoparticle ordering can still
be observed in both the reconstructed volume and in the orthoslices through the reconstructed volume (see also Figure S2,
Supporting Information). This effect is even more apparent in
the movie provided in the Supporting Information.
3. Conclusion
Both empty MIL-101 and ALD loaded Pt@MIL-101 materials
were characterized by “gentle” ADF-STEM imaging and electron tomography. Using low-dose techniques, consisting of
fast scanning and precise control over the beam current, highresolution imaging of these extremely sensitive metal-organic
framework materials was feasible. By combining the high-resolution annular dark-field STEM images with image simulations,
we were able to determine the exact position of the small clusters within the individual pores of a metal-organic framework.
Dark-field electron tomography confirms that ALD cycling is
capable of loading Pt nanoparticles, which are tailored to the
MIL-101 framework pore size, into the MIL-101 host.
4. Experimental Section
Figure 6. a) Overview ADF-STEM image of MIL-101 crystals heavily
loaded with Pt nanoparticles by 120 ALD cycles. b) Tomographic volume
reconstruction of the same Pt@MIL-101 crystals. c) Orthoslice through
the reconstruction.
a periodical manner. In Figure 5c, a loaded MIL-101 crystal is
imaged along the [−112] zone axis orientation. Again, the separate Pt nanoparticles are arranged in a periodic manner, following the MIL-101 crystal structure.
In order to provide direct proof of the Pt loading inside the
MIL-101 pores, for the high Pt loading after 120 ALD cycles, an
electron tomography series was acquired here also. Two heavy
loaded crystals are shown in Figure 6a together with a volume
Part. Part. Syst. Charact. 2016,
DOI: 10.1002/ppsc.201500252
Scanning Transmission Electron Microscopy and Electron Tomography:
ADF-STEM imaging in Figures 1 and 5 was carried out on a FEI Tecnai
Osiris microscope, operated at 200 kV. The convergence semi-angle
used was 10 mrad; the inner ADF detection angle was 14 mrad. The
beam current was kept at ≈10 pA.
ADF-STEM imaging in Figures 2 and 3 was performed on a FEI
Titan “cubed” microscope, equipped with a CEOS probe-corrector and
monochromator operated at 300 kV. The convergence semi-angle used
was 22 mrad, the inner ADF detection angle was 25 mrad, and the
outer detection angle was ≈150 mrad. The beam current was kept below
10 pA. Image sizes were typically 1024 × 1024 pixels and a dwell time of
5–7 μs per pixel was used.
The high-resolution ADF-STEM image simulation for a 13 nm thick
MIL-101 structure in [011] zone axis orientation (shown as an inset in
Figure 2) was performed using the MULTEM program.[36] The multislice
frozen phonon calculation was performed by using the Einstein model
with 20 configurations, a slice thickness of 2.0 Å, and a Debye–Waller
factor of 0.5 Å2 for all atoms.
The tomographic HAADF-STEM data acquisition in the case of
Figure 4 was carried out on a FEI Tecnai electron microscope operated
at 200 kV. The angular step size was 5° and the particle was tilted from
−70° to +70°. The tomographic HAADF-STEM data acquisition in the
case of Figure 6 was carried out on a FEI Titan “cubed” microscope
operated at 120 kV. The angular step size was 10° and the particle was
tilted from −60° to +70°. Tomographic reconstruction was performed
in the FEI Inspect3D software, using 50 iterations of an iterative SIRT
algorithm. Visualization was performed in the Amira software package.
MIL-101 Synthesis and ALD of Pt Nanoparticles: The MIL-101 crystals
were synthesized according to Jiang et al.[35] Circa 200 mg of MIL-101
material was loaded in a molybdenum sample cup and placed on a
sample stage heated to 200 °C in the ALD reactor. The powder sample
was allowed to outgas and thermally equilibrate for at least 1 h under
vacuum. Pt ALD was performed at 200 °C using (methylcyclopentadienyl)
trimethylplatinum [MeCpPtMe3] (heated to 30 °C) as Pt source and
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O3 (175 μg mL−1) as reactant.[37] A static exposure mode was applied
during both ALD half-cycles. The pulse time of the precursor/reactant
was 10 s, after which the valves to the pumping system were kept closed
for another 20 s, resulting in a total exposure time of 30 s. During the
precursor and reactant exposures, the pressure in the chamber increased
to ≈5 × 10−1 and 1 mbar, respectively.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
S.T. and J.D. gratefully acknowledge the FWO Vlaanderen for a
postdoctoral scholarship. The Titan microscope used for this investigation
was partially funded by the Hercules foundation of the Flemish
government. This work was supported by the Belgian IAP-PAI network.
K.L. acknowledges the financial support from the Ghent University BOF
postdoctoral Grant 01P06813T and UGent GOA Grant 01G00710. C.D.
thanks the FWO Vlaanderen, BOF-UGent (GOA 01G01513), and the
Hercules Foundation (AUGE/09/014) for financial support.
Received: December 16, 2015
Revised: January 22, 2016
Published online:
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