A general single-source route for the preparation of hollow nanoporous metal oxide structures

Full text of DOI:10.1002/anie.200900539

Communications
DOI: 10.1002/anie.200900539
Porous Hollow Structures
A General Single-Source Route for the Preparation of Hollow
Nanoporous Metal Oxide Structures**
Lianzhou Wang,* Fengqiu Tang, Kiyoshi Ozawa, Zhi-Gang Chen, Aniruddh Mukherj,
Yingchun Zhu, Jin Zou, Hui-Ming Cheng, and G. Q. (Max) Lu*
Inorganic micro- and nanosized hollow structures have
attracted increasing attention in recent years because of
their well-defined interior voids, low density, large surface
area, stability, and surface permeability. Such materials have
potential applications in a number of areas including efficient
catalysts, optoelectronic sensors, drug-delivery carriers, pho-
tonic crystals, energy-storage devices, and chemical nano-
reactors.[1–5] Currently, a common synthetic strategy for the
fabrication of hollow structures is the utilization of various
removable templates, including soft ones such as surfactants,
emulsion droplets, micelles, vesicles, ionic solvents, and gas
bubbles, and hard ones, such as polymers, silica, carbon, metal
oxides, and metallic cores.[1–7] Recently, a few wet-chemistry
template-free methods including mutual diffusion processes
(known as the Kirkendall effect), galvanic replacement, and
the intermediate-crystal method, have been developed by
carefully tailoring the solution-based formation process of
hollow microstructures.[8–16] Despite these successes, a gen-
eral, economical, and scalable route to rationally fabricate
hollow structures with designed internal voids, controllable
shapes, and porous sizes is still a challenge and highly
desirable.
chemistry based controlled decomposition–dissolution
(CDD) process of single metal-salt sources. The preparation
of manganese oxide microboxes will be first demonstrated as
an example, followed by more metal oxide examples with
different compositions and nanoporous morphologies. It is
demonstrated that this new strategy provides a simple
template-free approach for cost-effective and scalable prep-
aration of a wide variety of hollow structures with different
compositions, variable morphologies, and tunable nanopo-
rous structures.
Our idea was inspired by examining the thermogravimet-
ric analysis (TGA) curve (Figure 1) of manganese carbonate
cubelike microcrystals (see Experimental Section, and Sup-
Herein, we present a facile and general strategy to
fabricate porous hollow microstructures by a solid-state-
[*] Dr. L. Z. Wang, Dr. F. Tang, A. Mukherj, Prof. G. Q. Lu
ARC Centre of Excellence for Functional Nanomaterials
School of Chemical Engineering and AIBN
The University of Queensland
Figure 1. Thermogravimetric analysis curve of MnCO3 microcubes. 1
and 2 indicate the two steep weight-loss steps; a and b indicate the
two plateaus during thermal decomposition process. See text for
details.
St Lucia, Qld. 4072 (Australia)
E-mail: l.wang@uq.edu.au
maxlu@uq.edu.au
Dr. K. Ozawa
National Institute for Materials Science
1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 (Japan)
porting Information Figure S1). Thermal decomposition in air
leads to the conversion of MnCO3 into manganese oxide
MnO2, Mn2O3 or Mn3O4, depending upon the calcination
temperature. The first steep weight loss (Figure 1, Step 1,
Dr. Z.-G. Chen, Prof. H.-M. Cheng
Shenyang National Laboratory for Materials Science
Institute of Metal Research, Chinese Academy of Sciences
72 Wenhua RD, Shenyang 110016 (China)
1
ðStep1Þ
MnCO3 þ = O2 ! MnO2 þ CO2
Prof. Y. C. Zhu
2
The key Laboratory of Inorganic Coating Materials
Shanghai Institute of Ceramics, Chinese Academy of Sciences
1295 Dingxi RD, Shanghai 250100 (China)
< 4008C) is associated with the reaction of MnCO3 and
oxygen in air to form the MnO2 phase and CO2 as the
byproduct, whereas the second steep weight loss (Step 2,
Prof. J. Zou
School of Engineering and
Centre for Microscopy and Microanalysis
The University of Queensland, St Lucia, QLD, 4072 (Australia)
1
1
ðStep2Þ
MnO2 ! = Mn2O3 þ = O2
2
4
[**] The financial support from Australian Research Council (through its
Centre’s grant and DPs) is gratefully acknowledged.
< 5508C) is due to the transformation of MnO2 to Mn2O3 and
release of O2,[17] as depicted in the Equations describing
Step 1 and Step 2.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200900539.
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Commonly, to obtain the pure phase of the
decomposition product as guided by the TGA curve,
thermal decomposition should be conducted at a
temperature of either plateau (a) (indicated by the
horizontal dotted line) or (b) in Figure 1. However, if
the thermal decomposition process was simply stop-
ped at halfway, as indicated by the vertical dotted line
in Figure 1, what would happen to the compound?
In this case, MnCO3 can only be partially decom-
posed or under-calcined at this intermediate temper-
ature, which should lead to an oxidation layer (MnO2)
on the surface of the particles, whereas the inner cores
should remain as the MnCO3 salt, thus forming a
“core–shell” structure. The formation of this type of
core–shell structure offers an excellent opportunity to
further tailor the microstructure into a hollow one.
Because the solubility of MnCO3 (Ksp ꢀ 10ꢁ11) is
relatively large it can be easily dissolved in diluted
acid, whereas MnO2 materials are more difficult to be
dissolved under the same acidic conditions. A simple
Figure 2. A) XRD patterns and B) FTIR spectra of a) as-prepared MnCO3, b) the
sample in (a) calcined at 3508C for 2 h, c) the sample in (b) acid washed for
~
4 h, and d) the sample in (c) further calcined at 5008C for 2 h. Symbols and
*
in XRD patterns and FT-IR spectra represent the characteristic peaks (or
vibration bands) of MnCO3 and a-Mn2O3, respectively.
acid-washing of this metal oxide/metal salt core–shell struc-
ture could easily leach out the residual MnCO3 core, giving a
hollow structure. Scheme 1 Steps a and b describe the pro-
(Figure 2Aa, JCPDS Card No. 83-1763), the intensity of X-
ray Diffraction (XRD) peaks is decreased considerably upon
the partial decomposition of MnCO3 at 3508C for 2 h with the
strongest (104) peak (2q = 31.328) still noticeable (Fig-
ure 2Ab). This process was accompanied by a color change
of the material from brown to dark black. The subsequent
acid washing in diluted acid (0.1m HCl) efficiently removed
the characteristic peak of MnCO3. The black sample after acid
washing was nearly amorphous (Figure 2Ac), which could be
assigned to an amorphous MnO2 phase, as guided by the TGA
curve at this temperature range. Further calcination of the
acid-washed sample at higher temperatures (5008C in this
case) resulted in the formation of Mn2O3 phase (Figure 2Ad,
JCPDS Card No. 89-4836). Likewise, Fourier transform
infrared spectroscopy confirmed the structure evolution
during this CDD process. Two sharp vibration bands centered
at 725 cmꢁ1 and 860 cmꢁ1, together with a broad absorbance
band centered at 1410 cmꢁ1, are the characteristic FT-IR
bands of MnCO3 (Figure 2Ba). Upon calcination at 3508C,
all the three vibration bands remained, whereas their
intensities decreased (Figure 2Bb), indicating that the
MnCO3 phase still exists in the sample treated at 3508C. A
noticeable change in this sample was the newly developed
515 cmꢁ1 band, which can be assigned to the formation of the
manganese oxide phase. Interestingly, upon acid washing of
the sample treated at 3508C, the FT-IR spectrum (Fig-
ure 2Bc) showed a drastic change, with the 725 cmꢁ1 and
860 cmꢁ1 bands almost vanishing and significant suppression
of 1410 cmꢁ1 vibration. This result suggests the removal of
MnCO3 by the acid-washing process, and is in excellent
agreement with the XRD results.
Scheme 1. The proposed controlled decomposition–dissolution
method for the fabrication of nanoporous hollow structures (cross-
sections only). Step a: partial thermal decomposition of metal salt-
s (yellow) in air at an intermediate temperature, forming metal oxide
outer layers (red) and leaving the cores unaffected (yellow); Step b:
dissolution of the residual metal salt core, leading to hollow porous
metal oxide shells; Step c: possible further calcination to form cagelike
hollow structures provided further thermo-induced structural shrinkage
occurs at higher temperatures.
posed fabrication procedure of such hollow structures. Noting
that there is a sharp weight loss in the TGA curve of MnCO3
at higher temperature (Step 2 in Figure 1), which leads to
crystalline shrinkage and formation unique macroporous
microcubes (SEM image of Mn2O3 in Supporting Informa-
tion, Figure S1), it is reasonable to hypothesize the formation
of cagelike hollow microstructure (assuming the structure
does not collapse), which arises from the shrinkage of shell
framework at elevated calcination temperatures (Scheme 1,
Step c). More importantly, the generality and simplicity of this
strategy mean that the approach can be extended to many
other material systems with different shapes including more
complex ones. Scheme 1 also depicts the possible preparation
of a faceted hollow particle.
Figure 3 presents typical scanning electron microscopy
(SEM) and transmission electron microscopy (TEM) images
of the samples prepared at different stages. The as-prepared
MnCO3 sample had solid micrometer-sized cubelike shape, as
shown in Figure 3a and b. After the CDD process, the particle
morphology remained nearly intact, but some broken par-
ticles in the sample revealed a hollow box-like nature
(Figure 3c). A high magnification image (Figure 3d) clearly
Figure 2 provides evidence of the removal of the MnCO3
core from the partially decomposed MnCO3 particles by acid
washing. Compared with the highly crystalline MnCO3
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increasing calcination temperature to 8008C did not lead to
the collapse of the box framework, whereas a well-defined
Mn2O3 micro-cage structure was formed (Figure 3i), despite
the significant weight loss and volume shrinkage upon heat-
treatment occurring at this temperature range.[18] The boxlike
shape was kept nearly intact whereas the box walls converted
into an interconnected crystalline framework, with a much
lower SBET of less than 10 m2 gꢁ1. Note that this kind of
microbox may be particularly useful for chemical loading and
delivery purpose because of their easily tunable size of the
pores in the box walls.
To confirm the generality of our CDD strategy, transition-
metal salts with different morphologies and compositions
were processed (Figure 4). As expected, the hollow structures
Figure 3. a) SEM and b) TEM images of the as-prepared MnCO3
microcubes; c,d) SEM and e) TEM images of the acid-washed MnO2
microboxes after calcination at 3508C for 2 h, f) HRTEM image
showing the nanoporous nature in the box framework; SEM images of
g) MnO2 boxes with thicker walls (calcined at 3508C for 4 h, followed
by acid washing) and h) Mn2O3 microboxes calcined at 6008C; i) SEM
image of Mn2O3 microcages obtained by calcination of MnO2 micro-
boxes at 8008C.
shows a broken boxlike particle with dimensions of around
1.5–2 mm and wall thickness of approximately 200 nm. TEM
images further support the hollow nature of the sample
(Figure 3e), whereas an HRTEM image shows the box wall
has a nanoporous structure (Figure 3 f). The direct evidence
from SEM and TEM characterizations clearly indicate that
the CDD strategy is very successful. N2 adsorption–desorp-
tion analysis revealed that the as-prepared MnCO3 materials
were non-porous with a very low Brunauer–Emmett–Teller
Figure 4. SEM images of a) as-prepared solid MnCO3 ellipsoidal
particles, b, c) acid-washed MnO2 hollow ellipsoids after calcination at
3508C for 2 h, d) Mn2O3 hollow ellipsoids calcined at 7008C, e) as-
prepared solid FeCO3 faceted microparticles, and f) acid-washed Fe2O3
hollow particles after calcination at 3008C for 2 h.
of manganese oxide with an ellipsoidal shape and iron oxide
with more complex faceted morphologies were successfully
prepared by simply adopting this strategy (Supporting
Information I for experimental details, and Figures S3 and
S4). This method could also be extended to commercially
available chemicals. Nickel carbonate hydrate(NiCO3·2Ni(OH)·
xH2O) and cobalt carbonate hydrate (CoCO3·xH2O), (both
of analytical grade from Aldrich without any purification),
were used for this purpose. Figure 5a shows the TGA curve of
nickel carbonate hydrate. When partial decomposition of the
salt was conducted at an intermediate temperature (3208C
indicated by red dotted line), the light green sample changed
to black (Figure 5b), whereas the acid solution after washing
process provided a direct evidence of the removal of the
residual cores, as demonstrated by greenish supernatant
solution (dissolved Ni2+ salt) and black residue (nickel
oxide) at the bottom of a beaker (Figure 5c). The nitrogen
adsorption–desorption isotherms of the sample after acid
washing exhibited higher porosity (SBET: 88 m2 gꢁ1; pore
volume: 0.30 cm3 gꢁ1), than in its counterpart before acid-
washing (SBET: 61 m2 gꢁ1; pore volume: 0.11 cm3 gꢁ1; Fig-
ure 5d). Similarly, the SBET of the cobalt oxides obtained
from a similar procedure was also increased noticeably
(Supporting Information Figures S5, S6, and S7). Note that
owing to the irregular shape of these commercial particles, it
(BET) specific surface area (SBET) of approximately 2 m2 gꢁ1
;
however, a porous structure with a higher SBET value of
22 m2 gꢁ1 was generated in the partially decomposed sample
as a result of the release of CO2 gas from the material
(Supporting Information, Figure S2A). The acid-washing
process further increased the nanoporosity in the material
(Supporting Information, Figure S2B), resulting in a SBET of
54 m2 gꢁ1. This significant change in SBET is attributable to the
removal of the non-porous MnCO3 core. The adsorption
isotherm can be classified as a type IV indicating disordered
mesoporous structure in the materials.
It is worth noting that the wall thickness and specific
surface area of the microboxes can be easily tailored by
simply controlling the calcination duration and temperature.
Figure 3g shows a SEM image of the acid-washed samples
after calcination at 3508C for a longer period of time (4 h). It
is clear that the wall thickness increased apparently, to around
400 nm. This effect is well understood because the further
decomposition of MnCO3 particles by prolonging the calci-
nation results in thicker oxidation layers of MnO2 on the
particle surfaces. Calcination at the higher temperature of
6008C resulted in formation of manganese oxide nanocrys-
tallites on the wall owing to the thermal aggregation of
nanoporous structures (Figure 3h). Interestingly, further
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(20 cm3) and then added to an aqueous solution (20 cm3) containing
sucrose (0.5 g). The mixture was stirred for 20 min and then trans-
ferred to an autoclave (50 cm3 in volume) and hydrothermally treated
at 1508C for 24 h. The resultant precipitate was washed with copious
distilled water and then was dried at 508C overnight to obtain the as-
prepared MnCO3 sample. Calcination of MnCO3 was conducted in a
furnace at appropriate temperatures for a certain period of time. The
acid-washing process was conducted by dispersing the partially
decomposed MnCO3 core–shell sample (0.5 g) in HCl (100 cm3;
0.1 moldmꢁ3) under stirring for 4 h. The resultant microbox material
was collected by centrifuging the dispersion at a speed of 2000 rpm
and then washed with water (3 ꢀ 100 ml) three times. The hollow
microbox materials were further calcined at higher temperature (500–
8008C) to produce the hollow structures with different phases and
porous sizes.
Received: January 29, 2009
Revised: March 23, 2009
Published online: August 24, 2009
Figure 5. a) TGA curve of Aldrich nickel carbonate hydrate
Keywords: controlled decomposition–dissolution ·
nanostructures · nanoporous materials · transition metals
.
(NiCO3·2Ni(OH)·xH2O); b) photo of A) the nickel carbonate hydrate
and B) partially decomposed sample (3208C). c) photo of solution
after acid-washing process, with dissolved greenish nickel salt in the
supernatant solution (indicated by red arrow) and the black nickel
oxide residue (indicated by white arrow) at the bottom of a beaker;
d) nitrogen adsorption–desorption isotherms of the A) partially cal-
cined sample and B) the sample in (A) after acid washing.
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Experimental Section
Manganese carbonate with a microsized cubelike morphology was
prepared by hydrothermal reaction of KMnO4 with sucrose. In a
typical experiment, KMnO4 (0.632 g) was dissolved in distilled water
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7051