Ultralight mesoporous magnetic frameworks by interfacial assembly of prussian blue nanocubes

Article abstract of DOI:10.1002/anie.201308625

A facile approach for the synthesis of ultralight iron oxide hierarchical structures with tailorable macro- and mesoporosity is reported. This method entails the growth of porous Prussian blue (PB) single crystals on the surface of a polyurethane sponge, followed by in situ thermal conversion of PB crystals into three-dimensional mesoporous iron oxide (3DMI) architectures. Compared to previously reported ultralight materials, the 3DMI architectures possess hierarchical macro- and mesoporous frameworks with multiple advantageous features, including high surface area (ca. 117 m² g^-1) and ultralow density (6-11 mg cm^-3). Furthermore, they can be synthesized on a kilogram scale. More importantly, these 3DMI structures exhibit superparamagnetism and tunable hydrophilicity/hydrophobicity, thus allowing for efficient multiphase interfacial adsorption and fast multiphase catalysis.

Full text of DOI:10.1002/anie.201308625

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Angewandte
Communications
DOI: 10.1002/anie.201308625
Mesoporous Materials
Ultralight Mesoporous Magnetic Frameworks by Interfacial Assembly
of Prussian Blue Nanocubes**
Biao Kong, Jing Tang, Zhangxiong Wu, Jing Wei, Hao Wu, Yongcheng Wang, Gengfeng Zheng,*
and Dongyuan Zhao*
Abstract: A facile approach for the synthesis of ultralight iron
oxide hierarchical structures with tailorable macro- and
mesoporosity is reported. This method entails the growth of
porous Prussian blue (PB) single crystals on the surface of
a polyurethane sponge, followed by in situ thermal conversion
of PB crystals into three-dimensional mesoporous iron oxide
(3DMI) architectures. Compared to previously reported ultra-
light materials, the 3DMI architectures possess hierarchical
macro- and mesoporous frameworks with multiple advanta-
geous features, including high surface area (ca. 117 m2 gÀ1) and
ultralow density (6–11 mgcmÀ3). Furthermore, they can be
synthesized on a kilogram scale. More importantly, these
3DMI structures exhibit superparamagnetism and tunable
hydrophilicity/hydrophobicity, thus allowing for efficient mul-
tiphase interfacial adsorption and fast multiphase catalysis.
Very recently, an ultralight magnetic Fe2O3/carbon foam was
synthesized on a centimeter scale by pyrolyzing commercial
polyurethane (PU) sponge that had been grafted with metal
acrylate; this foam was found to be very efficient in the
absorption of oil from polluted water.[3c] Nonetheless, the
obtained ultralight materials either lack organized mesoscale
structures or are inherited from templates that are based on
complex top-down methods, which limits the development of
hierarchical structures and the design and control of meso-
structure-based host–guest interfaces and leads to an increase
in fabrication costs.
Whereas large pores and voids generally exist in ultralight
materials, mesoporous materials represent another large
category of unique structures with pore sizes between 2 and
50 nm.[4] These pores are typically produced by an organic–
inorganic assembly of precursors and template molecules,
which is followed by template removal to obtain organized
pore structures. Combinations of mesoporous materials with
large pores, such as sponges and polystyrene sphere assem-
blies,[5] have been used for preparing a variety of hierarchi-
cally porous structures, but none of these studies have
demonstrated the synthesis of ultralight materials with
densities that are comparable to those of aerogels or foams.
More critically, the transition metal oxide precursors typically
undergo fast hydrolysis or condensation reactions so that they
can only weakly interact with template molecules,[6] which
renders the synthesis of metal oxides (such as iron oxide)
mesostructures a substantial challenge. To date, a scalable
synthesis of ultralight 3D transition metal oxides with hier-
archical macro- and mesoporous structures has not been
described.
T
hree-dimensional (3D) porous materials with ultralow
density are a research hotspot, owing to their high surface-
to-volume ratio, easily accessible pore structures,[1] and
tailorable surface functionalities for applications that include
storage, separation, catalysis, drug delivery, and tissue engi-
neering.[2] Silica-, carbon-, and metal-based materials, in the
form of aerogels, sponges, networks, and microlattices,[3]
contribute to almost all of the 3D porous materials for
which densities below 10 mgcmÀ3 have been reported. For
instance, an ultralight carbon aerogel with a density of
0.16 mgcmÀ3 was prepared by freeze-drying of an aqueous
solution of carbon nanotubes and graphene oxide sheets.[3b]
[*] B. Kong,[+] J. Tang,[+] H. Wu, Y. Wang, Prof. G. Zheng, Prof. D. Zhao
Department of Chemistry
Laboratory of Advanced Materials
Shanghai Key Laboratory of Molecular Catalysis and Innovative
Materials, Fudan University
Shanghai 200433 (P.R. China)
E-mail: gfzheng@fudan.edu.cn
dyzhao@fudan.edu.cn
B. Kong,[+] Z. Wu, J. Wei
Department of Chemical Engineering, Monash University
Wellington Road, Clayton, VIC 3800 (Australia)
Herein, an interface-mediated growth of 3DMI frame-
works is described that was achieved by the controlled
hydrolysis and assembly of molecular precursors on the
solution–substrate interface without structure-directing sur-
factants, followed by an interface-constrained thermal pyrol-
ysis of the porous coordination polymer network (PCPN)
nanocrystals into the 3DMI frameworks with hierarchical
macropores and mesopores (Figure 1). The compounds of
the Prussian blue family are among the most well-
[+] These authors contributed equally to this work.
[**] This work was supported by the National Key Basic Research
Program of China (2013CB934104, 2012CB224805), the NSF of
China (20890123, 21322311, and 21071033), the Shanghai Leading
Academic Discipline Project (B108), the Science and Technology
Commission of Shanghai Municipality (08DZ2270500), the Pro-
gram for New Century Excellent Talents in University (NCET-10-
0357), and the Program for Professor of Special Appointment
(Eastern Scholar) at Shanghai Institutions of Higher Learning.
known PCPNs,[7] with
a typical chemical formula of
M3[M’(CN)6]2·nH2O, and based on a cubic (a-Po) network
topology, where the M and M’ ions are connected to the
À
À
cyanide ions by linear M CN M’ linkages. PU sponges with
inverse opal surfaces and different pore sizes (600–1250 mm;
Figure 2a,d) were used as the growth substrates for the
hydrolysis of K4[Fe(CN)6] (for details, see the Supporting
Information). After the synthesis, the color of the PU sponges
changed from yellow to blue, suggesting that Prussian blue
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308625.
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flexibility of the Prussian blue polyurethane nanocubes was
much greater. Two distinctive stages were observed during
stress tests (emax = 60%), including a quasi-linear elastic
region at e < 50%, which is followed by a densification
region (Figure S4). This process is highly reversible and
reproducible for more than 30 cycles.
The as-prepared Prussian blue nanocubes were subse-
quently converted into Fe2O3 nanocubes by pyrolysis in air.
The resulting product exhibited a similar framework, but its
pore size was smaller than that of the initial PU sponge
scaffold, which indicates that a replication and contraction
process of the PU macroporous frameworks has occurred.
The macropores are approximately 250 mm large (Figure 3a
Figure 1. Growth of ultralight 3DMI architectures. Three-dimensional
PU is used as the scaffold for the interfacial growth of PB–PCPN
nanocubes. Subsequent thermal treatment removes the PU scaffold
and converts the PB nanocubes into iron oxides. The obtained 3DMI
material replicates the morphology of the initial scaffold, while forming
mesoporous structures inside the nanocubes at the same time.
Figure 3. a–c) SEM images of 3DMI architectures. d) TEM image and
corresponding SAED pattern (inset) of the 3D mesoporous super-
paramagnetic architectures. e) TEM image of a corner of the super-
paramagnetic mesoporous nanocubes. The white circles indicate the
component units of crystallized iron oxide nanoparticles after pyrolysis
in air. f) HRTEM image of a crystallized iron oxide nanoparticle in
superparamagnetic mesoporous nanocubes.
Figure 2. a,d) Photographs of the PU scaffold before (a) and after (d)
interfacial growth. b) SEM image of soft PU foam scaffolds. c) SEM
images of the PCPN nanocrystals with the nanocube morphology
grown on the PU scaffold. Inset: Magnified SEM image of a single
frame of the PU scaffolds. e) PCPN nanocrystals with high density on
the scaffolds. Inset: SEM image of a single PCPN crystal. f) TEM
image and corresponding SAED pattern of one PCPN nanocrystal.
and Figure S5), and the struts of the 3D framework consist of
layers of packed nanocubes (Figure 3b and Figure S6). The
layer thickness is approximately 1.2 mm, which corresponds to
approximately three nanocube layers. These nanocubes have
a mesoporous structure over the entire cube with a polycrys-
talline nature (Figure 3c,d). HRTEM images reveal that the
nanocubes are assembled by fully crystallized iron oxide
nanoparticles with uniform sizes of < 10 nm (Figure 3e,f).[10]
The N2 sorption isotherms of the 3DMI frameworks that
were obtained after calcination at 350–4508C show type IV
curves with surface areas of approximately 93, 105, and
117 m2 gÀ1 (Figure 4a), and mean pore sizes of approximately
12.7, 14.1, and 18.3 nm, respectively (Figure 4b). The wide-
angle XRD pattern reveals that the PCPN-derived 3DMI
nanocubes are mainly composed of a g-Fe2O3 phase (Fig-
ure S7a).[11] The diffraction peaks of g-Fe2O3 gradually
increase in intensity (from 350 to 4508C) without phase
changes (Figure S8). X-ray photoelectron spectroscopy (XPS)
of 3DMI nanocubes revealed two main peaks at approx-
imately 725 and 710 eV, which may be associated with the Fe
2p1/2 and 2p2/3 of g-Fe2O3 (Figure S7b,c), respectively.[12] The
obtained 3DMI framework exhibits attractive superparamag-
netic properties as it benefits from the controlled crystalline
phases and grain sizes. The magnetization curves of 3DMI
frameworks show almost no hysteresis loops and only very
nanocubes had grown (Figure 2b and Figure S1). Scanning
electron microscopy (SEM) revealed that the PU surfaces are
covered by nanocubes with sharp edges and a uniform size of
approximately 300 nm (Figure 2c,e). High-resolution trans-
mission electron microscopy (HRTEM) images show that all
Prussian blue nanocubes are solid and of a uniform size
(Figure 2 f and Figure S2). The selected area electron diffrac-
tion (SAED) pattern of a single nanocube along the < 001 >
zone axis exhibits ordered diffraction spots, which indicates
that the Prussian blue nanocubes are of a single-crystalline
nature (Figure 2 f).[8] The crystallographic structure of the
Prussian blue nanocubes was further confirmed by powder X-
ray diffraction (XRD); all of the diffraction peaks could be
assigned to the face-centered-cubic Fe4[Fe(CN)6]3 structure
(Figure S3),[9] which suggests that the material is of high
purity. Furthermore, the PU foams that are loaded with
Prussian blue nanocubes could preserve their high flexibility.
The Prussian blue nanocubes are hardly flexible owing to
their face-centered-cubic Fe4[Fe(CN)6]3 crystal structure.
After they had been grown on the polyurethane surface, the
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may be used for the synthesis
of other 3DMI structures for
various applications.
Possible applications of
the 3DMI framework arise
from its strong magnetic prop-
erties (Figure 5 h), hierarchi-
cal porosity, high surface area,
and tunable surface function-
alities. The large pore sizes
can expedite molecule trans-
port,[15,16] whereas the meso-
Figure 4. a) Nitrogen sorption isotherms. b) Pore size distribution. c) Magnetization curve for the 3D
mesoporous structure.
small remnant magnetization (Mr) and coercive force (Hc) at
room temperature, suggesting a superparamagnetic or quasi-
superparamagnetic behavior (Figure 4i).[13]
pores provide large adsorption sites for target molecules. The
outstanding magnetic and adsorption properties of the 3DMI
structures were first demonstrated for the removal of heavy
metal ions, such as AsIII, from contaminated water. The AsIII
ions can be selectively adsorbed onto the iron(III) oxide
particles by ligand exchange in the coordination spheres of
the structurally defining Fe atoms (Table S1).[17] The 3DMI
materials could rapidly remove 95 wt% of the AsIII ions (d =
5 ppm in an aqueous solution) within 20 min (Figure 5i) and
are thus much more efficient than magnetic Fe2O3 nano-
particles with diameters of 8 or 170 nm. The removal
percentage of the AsIII ions was improved by 168% and
The surface wettability of the 3DMI frameworks can be
easily tuned from highly hydrophilic to highly hydrophobic by
surface functionalization.[14] Water droplets that were depos-
ited on the as-grown 3DMI frameworks quickly spread out on
the surface with a low contact angle (< 18; Figure S7d),
indicating the hydrophilic nature of the pristine 3DMI
structures. After coating with phenolic resins (resols) and
subsequent thermal treatment (for details, see the Supporting
Information), the water contact angles ranged from 83.68 to
117.58 with an increase in the resol deposition time, suggesting
a hydrophobic surface (Figure S7e,f). Furthermore, the
3DMI frameworks exhibited high thermal stability even
when they were exposed to an alcohol flame (400–6008C,
depending on the part of the flame), as both the morphology
and the nanostructure were well retained (Figures S7g, S9,
S10).
Aside from the 3D hierarchical porosity, superparamag-
netism and tunable surface hydrophobicity, the 3DMI frame-
works offer remarkable ultralow density and convenient
tailorability, which may be attributed to abundant pores,
hierarchical stacking, and the non-rigid connection of the
mesoporous nanocube building blocks. The density was
measured to be as low as approximately 6 mgcmÀ3, which
makes this framework one of the lightest metal oxide
materials with a density that is comparable to those of
ultralight silica-, carbon-, and metal-based structures.[3a,b] For
instance, a 3DMI piece of approximately 1.46 g and 240 cm3
can be placed on top of an oleander flower (Figure 5a,b).
Furthermore, the bulk 3DMI frameworks can be conveniently
tailored into desired shapes, such as ultrathin films, cylinders,
cubes, hexagons, or triangular prisms (Figure 5c,e; see also
Figure S11) or packed as functional columns for filtration,
adsorption, and catalysis (Figure 5d and Figure S11d). The
3D structures were well preserved without structure cracking.
More importantly, this synthetic method for 3DMI frame-
works is scalable for mass production and possible practical
applications. For instance, by using multiple (ca. 50) PU
foams, each with a volume of 4500 cm3, which were incubated
with a Prussian blue precursor solution in a 5.5 L reaction
container, approximately 1.26 kg of 3DMI frameworks were
produced (Figure 5 f,g; see also Figures S12, S13). Moreover,
this method can be extended to a large variety of substrates,
including carbon fibers, silkworm, wool, and glass fibers
(Figure S14). Therefore, this approach is highly flexible and
Figure 5. a) Bulk ultralight 3DMI architecture. b) A 3DMI architecture
with a volume of 240 cm3 can be placed on top of an oleander flower.
c) 3DMI materials can be tailored into a cylindrical shape. d) Filtration
column filled with 3DMI material of cylindrical shape. e) An ultrathin
3DMI sheet on a sharp blade. f,g) Photographs of Prussian blue
coated polyurethane sponge and the 3DMI powder. h) Collection of
the 3DMI powder with a magnet. i) Time-dependent capture of AsIII
ions by the 3DMI material with an initial arsenic concentration of
d=5 ppm (c), compared to the uptake by iron oxide NPs with
diameters of 8 (c) and 170 nm (c). j–m) Uptake of an organic
liquid by the hydrophobic 3DMI architecture in a solid–water–oil
triphasic system. A layer of Sudan III-stained gasoline was completely
absorbed by a monolith of the hydrophobic 3DMI architecture in five
seconds.
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1315%, compared to their removal by the magnetic nano-
particles with diameters of 8 and 170 nm, respectively.
The excellent adsorption performance of the 3DMI
frameworks was further shown by oil–water separation. A
small piece (ca. 2.83 cm3) of resol-coated 3DMI material can
complete adsorb Sudan III-stained gasoline from water (Fig-
ure 5j–l).[18] This oil adsorption/removal has been interro-
gated for various petroleum products, including gasoline,
diesel, and pump oil. The resol-coated 3DMI materials
exhibited a high adsorption capacity that amounts to more
than 150 times their own weight, which is higher than for
other reported porous materials, such as Fe/C (ca. 10 times),
PU (ca. 20 times), cellulose aerogels (ca. 40 times), graphene
sponges (ca. 80 times), Fe2O3/C foam (ca. 100 times), and
graphene/carbon nanotube sponges (ca. 130 times), and
comparable to that of carbon nanotube sponges (ca.
180 times).[3c]
Furthermore, the resol-coated 3DMI frameworks were
used as a nanoreactor for multiphase catalysis. The formation
of benzaldehyde from natural cinnamaldehyde by a retro-
aldol condensation is used as a representative water–oil
biphasic reaction, which usually requires phase transfer
reagents and co-solvents.[19] Adding resol-coated 3DMI
catalysts led to a short reaction time (< 20 h), excellent
selectivity (ca. 99%) and high conversion (ca. 93%; Fig-
ure S15–S17, Table S2). These results are comparable to those
that were obtained with phase transfer reagents or co-solvent,
and substantially higher (ca. 447%) than for a reaction that is
run in the absence of resol-coated 3DMIs. This high yield is
attributed to the tunable hydrophilic/hydrophobic surface of
the 3DMI frameworks, which may thus accommodate both
hydrophilic Na2CO3 and hydrophobic cinnamaldehyde and
enable efficient molecule transfer across the oil–water
interface.[19b,20] The catalysts can be magnetically recovered
and were reused more than 15 times without a significant
drop in conversion.
Figure 6. Proposed mechanism of formation. a) Interface-induced
growth and self-assembly mechanism and reaction process for the
preparation of three-dimensional Prussian blue coordination polymer
networks on universal 3D frames. K4[Fe(CN)6] undergoes hydrolysis
and condensation on the interface of the three-dimensional PU foam
3+
and nucleates to form a linear Fe2+ CN Fe linkage layer by layer.
Increasing the growth time leads to the accumulation of cubic
Prussian blue crystals on the PU surface. b,c) Formation mechanism
for 3D PCPNs and corresponding mesoporous iron oxide obtained by
thermal conversion. Interfacial growth of the three-dimensional PB
nanocrystal was achieved by a scalable solution-phase synthesis that is
based on decomposition and oxidation of the precursors on the 3D
template interface.
À
À
nanocubes and form continuous layers, which replicate the
original PU sponges and inherit the macropores. Further-
more, the 3DMI frameworks are composed of interconnected
g-Fe2O3 nanocrystals with uniform sizes of approximately
10 nm, which are slightly smaller than those calculated
according to the Scherrer Equation, thus confirming the
preferred orientation of the Fe2O3 nanocrystals.
The successful synthesis of the ultralight mesoporous
3DMI frameworks is attributed to the interface-mediated
crystal growth and pyrolysis. First, the PU sponge surface
Compared to the widely used sol–gel and evaporation-
induced self-assembly (EISA) methods for creating mesopo-
rous structures,[22] this method utilizes surface interactions for
crystal growth and subsequent mesopore formation. This
approach has not been applied to transition metal oxides in
the absence of template molecules. More importantly, this
approach can be conveniently scaled up at low cost, and the
obtained products inherit the tailorability of the three-
dimensional PU foam templates, which can be easily handled,
cut, and assembled without affecting their physical and
chemical properties. Combining the ultralight properties,
large surface area, and suitable mesopore sizes, abundant
surface chemistry allows for using the 3DMI materials for
a number of applications, including filtration, adsorption, and
multiphase catalysis. For filtration and adsorption, the large
pore sizes can expedite the transport of solution, whereas the
mesopores provide large adsorption sites for impurities, such
as heavy metal ions and oil molecules. For multiphase
catalysis, the addition of the 3DMI materials allows for
efficient molecule transfer between oil and water phases,
which leads to an increased conversion while obviating the
need for additional phase transfer reagents and co-solvents.
À
bears many carbamate groups (NH COO) on its main chains
and can efficiently adsorb the hydrolysis products of the
K4[Fe(CN)6] precursor (Figure 6a). The hydrolysis and con-
densation of K4[Fe(CN)6] rapidly proceeded on the PU–
solution interface, with each nitrogen terminus in a ferrocya-
nide complex featuring an octahedral geometry, and formed
3+
a Fe2+ CN Fe linkage layer by layer.[21] Increasing the
growth time led to the accumulation of Prussian blue
nanocubes on the PU surface. Afterwards, the single-crystal-
line Prussian blue nanocubes were decomposed by thermal
treatment (Figure 6b,c), during which gaseous products (e.g.,
CO2) were released, which created a shrinking force for the
À
À
À
nanocubes. Nonetheless, as the Fe O bonds on the surface are
tightly bound to the adjacent nanocubes and the substrate
underneath, this shrinking force does not result in morphol-
ogy deformation of the nanocube assembly. Instead, this
surface-constrained pyrolysis can well retain the shapes and
connections of the 3DMI nanocubes, while forming meso-
pores inside the nanocubes. Therefore, the obtained 3DMI
frameworks preserve the morphology of the Prussian blue
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In summary, we have developed a facile synthesis of
ultralight 3DMI frameworks by surface-induced crystal
growth of Prussian blue nanocubes and subsequent surface-
constrained mesopore formation. The 3DMI frameworks
have a high surface area (ca. 117 m2 gÀ1) and a hierarchical
macro- and mesoporous structure, with bimodal mean pore
sizes of 250 mm and 18 nm. The density of the 3DMI
framework is as low as approximately 6 mgcmÀ3, which is
comparable to that of ultralight silica-, carbon-, and metal-
based structures. The developed method allowed for the
synthesis of approximately 1.26 kg of tailorable 3DMI
material. These ultralight 3DMI frameworks exhibit charac-
teristics that include superparamagnetism, a tunable surface,
as well as hydrophilicity and hydrophobicity, which allows for
the adsorption of heavy metal ions and organic molecules and
multiphase catalysis. Further developments of this surface-
mediated crystal growth and mesopore formation can lead to
many opportunities for synthesizing a variety of ultralight
mesoporous transition metal oxides.
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Received: October 3, 2013
Revised: December 7, 2013
Published online: February 12, 2014
Keywords: adsorption · catalysis · magnetic properties ·
.
mesoporous materials · ultralight materials
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