Article
pubs.acs.org/cm
Micelle-Templated Oxides and Carbonates of Zinc, Cobalt, and
Aluminum and a Generalized Strategy for Their Synthesis
Björn Eckhardt,† Erik Ortel,† Denis Bernsmeier,† Jörg Polte,† Peter Strasser,† Ulla Vainio,‡
Franziska Emmerling,§ and Ralph Kraehnert*,†
†
Technical University of Berlin, Department of Chemistry, Strasse des 17. Juni 124, D-10623 Berlin, Germany
Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, D-22607 Hamburg, Germany
§
BAM Federal Institute of Materials Research and Testing, Richard-Willstätter-Strasse 11, D-12489 Berlin, Germany
‡
S Supporting Information
*
ABSTRACT: Catalysis, energy storage, and light harvesting require functional materials with tailored porosity and
nanostructure. However, common synthesis methods that employ polymer micelles as structure-directing agents fail for zinc
oxide, for cobalt oxide, and for metal carbonates in general. We report the synthesis of the oxides and carbonates of zinc, cobalt,
and aluminum with micelle-templated structure. The synthesis relies on poly(ethylene oxide)-block-poly(butadiene)-blockpoly(ethylene oxide) triblock copolymers and a new type of precursor formed by chemical complexation of a metal nitrate with
citric acid. A general synthesis mechanism is deduced. Mechanistic insights allow for the prediction of optimal processing
conditions for different oxides and carbonates based on simple thermogravimetric analysis. Employing this synthesis, films of
ZnO and Co3O4 with micelle-controlled mesoporosity become accessible for the first time. It is the only soft-templating method
reported so far that also yields mesoporous metal carbonates. The developed synthesis is generic in nature and can be applied to
many other metal oxides and carbonates.
KEYWORDS: EISA, pore templating, metal oxide, metal carbonate, zinc oxide, cobalt oxide
■
INTRODUCTION
morphology of metal oxides on a nanometer scale is of vital
importance.
So-called “templating” is one of the most versatile methods
for controlling the nanostructure of a metal oxide during its
wet-chemical synthesis. It employs preformed nanostructures
(templates) as structure-directing agents. The templates
typically possess the inverse shape of the desired pore
morphology. Open porosity results from solidification of the
oxide framework followed by template removal. Templatebased syntheses have been reported for metal oxides with
mesopores,13,14 macropores,12,15−17 and hierarchical porosity.16−18 Depending on the nature of the employed template,
so-called hard and soft templating can be distinguished. Hard
templating is commonly employed for the synthesis of
Many applications in catalysis, energy storage, and photovoltaics rely on metal oxides that feature a specific
nanostructure. The nanostructure of a metal oxide often
determines its optical, magnetic, and catalytic properties. The
oxides of cobalt and zinc provide some of the most prominent
examples. Nanostructured cobalt-based oxides are promising
materials for electrodes in supercapacitors1,2 and in lithium ion
batteries3,4 with superior charging rates.5 Moreover, they
represent very active catalysts for the oxygen evolution reaction
in electrochemical water splitting6 and the oxygen reduction
reaction in fuel cells.7 Also, ZnO nanostructures feature unique
properties. They are used in display technologies, photovoltaics,
photocatalysis, and piezoelectric nanogenerators8 and allow the
construction of self-powered nanodevices.9−11 Moreover, the
photonic band gap of ZnO can be tailored by the introduction
of an inverse opal structure.12 Hence, control over the
© XXXX American Chemical Society
Received: February 14, 2013
Revised: June 21, 2013
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Figure 1. Synthesis scheme and deduced requirements for the synthesis of mesoporous metal oxides via metal carbonate intermediates. (1)
Formation of a soluble complex from metal nitrate and a complexing agent such as citric acid. (2) Film deposition and concurrent self-assembly of
the micelles of the polymer template with the precursor complex into an ordered mesophase. (3) Decomposition of the precursor complex into a
structurally stable metal−carbonate intermediate at low temperatures while the ordered mesophase is retained. (4) Thermal treatment in air to
remove the polymer template leading to open mesopores in the metal carbonate film. (5) Controlled decomposition of the amorphous carbonate
that forms the pore walls into the nanocrystalline metal oxide.
macroporous oxides in powder form (ZnO 12,15 and
Co3O416,17). Although hard templating can also in general
produce mesopores (ZnO13 and Co3O419,20), soft templating is
by far the most common synthesis approach for mesoporous
oxides.
Common soft-templating routines such as evaporationinduced self-assembling (EISA)21 employ micelles of amphiphilic block copolymers as the pore templates. In a typical EISA
synthesis, a solution containing an oxide precursor and the
amphiphilic block copolymers is deposited onto a substrate.
The solvent evaporates during deposition while the template
molecules are arranged into micelles. Micelles and the partially
condensed precursor assemble into an ordered mesophase. A
subsequent calcination converts this mesophase into a
mesoporous oxide film. EISA-based syntheses offer three
major advantages: (I) The pore size, pore shape, and thickness
of pore walls22 can be controlled by the structure and
concentration of the template. (II) Synthesis protocols are
simple and reproducible. (III) A wide range of metal oxides is
accessible.14 However, neither the micelle-templated synthesis
of Co3O4 films nor ZnO has been reported so far.
The failure of EISA-based syntheses to produce templated
zinc oxide or cobalt oxide films originates from the properties
of commonly employed metal precursors. Typically, oxides
were synthesized from (I) alkoxides or partially alkoxylated
metal chlorides (e.g., SiO2,23,24 TiO2,25,26 Al2O3,27 and
ZrO228,29), (II) preformed colloidal nanocrystals [e.g., TiO2,30
Mn3O4,31 MnFe2O4, and InxSnyOz (ITO)31,32], and (III)
thermally decomposable metal compounds (e.g., IrO233). The
reasons for failure are intrinsically tied to the mechanisms of
mesophase formation and template removal. Route I based on
hydrolysis and condensation of alkoxy groups fails for
precursors with high hydrolysis and condensation rates, because
rapid condensation results in undesired precipitation prior to
mesophase assembly. Route II requires high-quality building
blocks such as a redispersible nanocrystalline colloid with a
small particle diameter (d < ∼5 nm) and a narrow size
distribution. However, for many metal oxides, such nanoparticle
syntheses remain challenging. Approach III is limited to metal
precursors that do not show excessive crystallization upon
drying; otherwise, the ordered mesostructure cannot be
formed. Moreover, oxide formation must occur at temperatures
significantly below the typical temperatures of template
combustion (∼300 °C); otherwise, pores collapse because of
premature template removal. Other constraining factors are the
limited solubility of many precursor compounds, melting
during calcination, and rapid crystallite growth of the metal
oxide before template removal. Additionally, a general
limitation of all methods described here is that they do not
provide access to the soft-templated synthesis of mesoporous
metal carbonates.
The synthesis of fine-grained metal oxides without templated
pore structure can be achieved, e.g., via the Pechini method or
the citrate method. The so-called Pechini method was originally
patented for the preparation of (untemplated) nanocrystalline
metal titanates and niobates.34 It relies on the initial formation
of chelate complexes of metal ions (originally titan, niobium,
and zirconium) with α-hydroxycarboxylic acids (for example,
lactic, citric, or glycolic acid). Subsequent heating in the
presence of a polyhydroxy alcohol (e.g., ethylene glycol)
induces polyesterification of the chelate complex, yielding an
amorphous gel. Calcination at moderate temperatures typically
converts this gel into the corresponding crystalline metal oxide
[e.g., LiMn2O4,35 YVO4:Eu,36 LaPO4:Ce,Tb,37 Eu2(WO4)3,38
and CaIn2O4:Eu39].
Alternatively, nanocrystalline metal oxides can be obtained
also by heating of the corresponding metal carbonates to
induce thermal decomposition into the respective metal oxide.
The synthesis has been used to prepare nanocrystalline MgO,40
ZnO,41 Co3O4,42 and Al2O3.43
We recently reported the first synthesis of micelle-templated
magnesium oxide.44 The preparation borrows from three
different approaches, i.e., the initial steps of the Pechini method
(complexing the metal ion), the carbonate decomposition
strategy, and pore templating with polymer micelles. The MgO
synthesis44 relied on the initial preparation of a chemical
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Table 1. Synthesis Conditions Employed for the Preparation of Micelle-Templated Mesoporous Carbonates and Oxides of Zn,
Al, Co, and Mga
precursor system
Zn(NO3)2·6H2O (444 mg) and citric acid
(144 mg)
Al(NO3)3·9H2O (563 mg) and citric acid
(144 mg)
Co(NO3)2·6H2O (437 mg) and citric acid
(144 mg)
Mg(NO3)2·6H2O (385 mg) and citric acid
(144 mg)
template
PEO213−PB184−PEO213
(70 mg)
PEO213−PB184−PEO213
(70 mg)
PEO213−PB184−PEO213
(70 mg)
PEO213−PB184−PEO213
(70 mg)
solvent
H2O and
each)
H2O and
each)
H2O and
each)
H2O and
each)
calcination (i),
carbonate
calcination (ii), oxide
ethanol (1.5 mL
60 min at 250 °C
25 min at 400 °C
ethanol (1.5 mL
60 min at 300 °C
30 min at 900 °C
ethanol (1.5 mL
60 min at 200 °C
20 min at 300 °C
ethanol (1.5
60 min at 400 °C
120 min at 400 °C and 60 min at
600 °C
a
Columns 1−3 detail the compositions of the dip-coating solution. Column 4 lists the calcination procedure (i) that yields the carbonate and column
5 the respective calcination (ii) that transforms the carbonate into the corresponding oxide.
thermal treatment procedures for (3) carbonate formation, (4)
template removal, and (5) oxide formation were established
based on thermogravimetric (TG) analysis of precursor
complexes and templates. Additional characterization revealed
the structural evolution of the pore morphology [scanning
electron microscopy (SEM) and transmission electron
microscopy (TEM)] and surface area (Kr sorption) as well as
the phase composition [Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD)] and crystallinity of the
pore walls [selected area electron diffraction (SAED) and
XRD]. The derived mechanistic picture explains the formation
of mesoporous carbonates and oxides. It reveals also why the
synthesis of mesoporous CoCO3 necessarily fails.
Mesoporous oxides and carbonates were synthesized as
summarized in Table 1. Briefly, for the synthesis of, e.g., ZnO, a
solution containing template polymer (PEO213−PB184−
PEO213), metal precursor [Zn(NO3)2·6H2O], complexing
agent (citric acid), water, and ethanol was dip-coated onto Si
substrates at a controlled temperature (25 °C) and a relative
humidity of 40%. Deposited films were calcined with procedure
(i) to obtain mesoporous carbonate (ZnCO3, 1 h at 250 °C)
and thereafter with procedure (ii) to obtain mesoporous oxide
(ZnO, 25 min at 400 °C).
precursor complex consisting of magnesium nitrate bonded to
citric acid and on thermally induced formation of a mesoporous
MgCO3 intermediate. In this synthesis, water/ethanol solutions
containing the complex and micelles of poly(ethylene oxide)block-poly(butadiene)-block-poly(ethylene oxide) (PEO−PB−
PEO) were dip-coated onto a substrate. The deposited films
were converted into mesoporous MgO in two subsequent
calcination steps performed at 400 and 600 °C.
This paper demonstrates that the self-assembly of triblock
copolymers with citric acid-based metal complexes provides
access to micelle-templated oxides and carbonates of zinc,
cobalt, and aluminum. Thus, mesoporous films of ZnO, Co3O4,
ZnCO3, and Al2(CO3)3 with micelle-controlled pore structure
become accessible for the first time. On the basis of a
mechanistic understanding, general criteria for the successful
synthesis of metal oxides and metal carbonates with controlled
porosity are deduced. Factors that are crucial for the synthesis
as well as remaining limitations are critically discussed.
■
SYNTHESIS STRATEGY
From the recently reported synthesis of mesoporous MgO,44
five criteria that a synthesis strategy with general applicability
would have to fulfill can be deduced. The deduced criteria are
illustrated in Figure 1 along with the proposed synthesis
strategy. (1) The metal salt and ligands with carboxylic acid
functionality must form a chemical complex. Chelating ligands
are preferred because of the high stability of the complexes. (2)
Polymer micelles and the metal complex must undergo selfassembly during deposition and drying to form an ordered
mesostructure. (3) The chemical complex should decompose
into a structurally stable metal carbonate at low temperatures
while the templating micelles stabilize the formed mesostructure. (4) Subsequent template removal should yield the
mesoporous metal carbonate; hence, decomposition of the
template polymer should occur at a temperature where the
carbonate remains thermally stable. (5) The final thermal
treatment should transform the carbonate into a nanocrystalline
metal oxide while retaining the templated pores.
Guided by these requirements, we analyzed for metals Zn, Al,
Co, and Mg the physical and chemical processes that would
constitute the synthesis of the respective mesoporous carbonate
and oxide. Formation of (1) a stable metal complex was studied
by electrospray ionization mass spectroscopy (ESI-MS) of
precursor solutions. Employing highly amphiphilic surfactants
PEO−PB−PEO that form stable spherical micelles already
prior to solution deposition22 assured (2) robust reproducible
mesophase formation. Ordering of micelles and pore structures
was assessed by small-angle X-ray scattering (SAXS). Adequate
■
MESOPOROUS ZINC CARBONATE AND ZINC
OXIDE
Figure 2 presents for the Zn-based material (i) calcined at 250
°C the analysis by SEM (panel a), FTIR (panel b), and XRD
(panel c). Moreover, Figure 2 shows properties of the
corresponding material (ii) calcined in addition at 400 °C
studied by SEM (panel d), FTIR (panel e), XRD (panel f), and
TEM (panels g−i). SEM analysis of sample (i) indicates that
calcination at 250 °C yields a homogeneous film. Cross-section
SEM images (Figure 2a) reveal that the formed film is ∼1100
nm thick and completely penetrated by mesopores. The pores
show elliptical shapes ∼21 nm × ∼19 nm in size. The
appearance of the pore walls is smooth and unstructured; no
crystallite shapes can be distinguished (Figure 2a). FTIR
spectra recorded on corresponding powder samples feature
intense symmetric (1384 cm−1) and asymmetric (1583 cm−1)
vibrations (Figure 2b). These bands can be assigned to zinc
carbonate,45 whereas only negligible contributions at wavenumbers indicative of ZnO (e.g., 577 cm−1) are observed.
Moreover, XRD analysis of the sample (Figure 2c) shows only
reflections that can be attributed to the substrate (silicon wafer)
and no indications of a crystalline Zn-containing phase. Kr
physisorption indicates a surface area of 86.1 m2/g. This value
is slightly smaller than the surface area typically observed for
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Figure 2. Analysis of (i) mesoporous ZnCO3 calcined for 1 h at 250 °C and (ii) mesoporous ZnO calcined for 1 h at 250 °C followed by 25 min at
400 °C by (a and d) SEM, (b and e) FTIR, (c and f) XRD, and (g−i) TEM. (a) Cross-section SEM of a mesoporous ZnCO3 film with the inset at a
higher magnification. (b) Infrared spectrum of the precursor complex calcined at 250 °C recorded in transmission mode. (c) Grazing incident XRD
analysis of amorphous ZnCO3 (i). (d) SEM top-view image of ZnO (ii). (e) FTIR spectrum of the dried precursor complex calcined by procedures
(i) and (ii). (f) XRD analysis of ZnO (ii) with reflection positions corresponding to ZnO in the hexagonal zincite structure (PDF-No. 00-036-1451).
(g−i) Electron microscopy analysis of (ii) ZnO by bright-field TEM, high-resolution TEM, and selected-area electron diffraction SAED, respectively
(indexing: hexagonal zincite structure, PDF-No. 00-036-1451).
micelle-templated oxides (∼100−250 m2/g).14 Hence, combined analytical data indicate the successful synthesis of micelletemplated zinc carbonate comprising amorphous walls and
interconnected accessible mesopores.
Further calcination (ii) at 400 °C transforms the carbonate
film (i) into ZnO while preserving the mesopore structure.
FTIR spectra recorded for sample (ii) (Figure 2e) show a
strong signal at 577 cm−1 indicative of ZnO, whereas only small
bands assigned to carbonate are retained.45 X-ray diffraction
data of the sample (Figure 2f) feature broad reflections at
positions of 2θ = 31.7° (100), 34.5° (002), 36.0° (101), 62.9°
(103), and 67.8° (112). These reflections can be assigned to
ZnO in the hexagonal zincite structure (PDF-No. 00-036-1451)
with crystallite sizes of ∼7 nm (Scherrer equation). Hence,
calcination at 400 °C transforms the carbonate film almost
completely into nanocrystalline ZnO. Corresponding top-view
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Figure 3. 2D SAXS pattern of films deposited from solutions with the complex of zinc nitrate and citric acid as well as micelles of the PEO213−
PB184−PEO213 polymer template after different thermal treatments. From left to right: (a and b) as deposited, (c and d) zinc carbonate (i) calcined
for 1 h at 250 °C, and (e, f) zinc oxide (ii) calcined for 1 h at 250 °C followed by 25 min at 400 °C. Samples were analyzed by SAXS with two
different incident angles of the X-ray beam of 90° (top) and 6° (bottom) relative to the substrate surface (linear intensity scale).
confirms that film shrinkage is anisotropic and restricted to
the direction perpendicular to the substrate. Moreover, the
observed ellipsoidal 2D SAXS patterns (Figure 3b,d,f) indicate
also a certain degree of lattice distortion in the cubic
mesostructure.22 SAXS analysis thus confirms that the
deposited micelles and precursor complex form a locally
ordered mesophase, and that corresponding pore ordering is
preserved also during carbonate formation, template removal,
and transition into a mesoporous zinc oxide.
SEM images (Figure 2d) indicate that films remain
homogeneous and macroscopically crack-free. The films are
completely penetrated by mesopores ∼22 nm in diameter. The
pores are locally ordered and open toward the outer film
surface. TEM analysis (Figure 2g) confirms the presence of
templated mesopores throughout the sample volume. Highresolution TEM (Figure 2h) indicates crystallites and lattice
fringes, which confirms that the pore walls are crystalline.
Furthermore, SAED analysis (Figure 2i) shows isotropic
diffraction rings with ring positions that match the reflections
of hexagonal zincite structure (PDF-No. 00-036-1451). The
homogeneous diffraction rings indicate that the pore walls
consist of randomly oriented crystallites. Hence, calcination (ii)
at 400 °C transforms the amorphous carbonate into nanocrystalline ZnO with a templated mesopore structure. However,
the surface area of the mesoporous ZnO amounts to 250 m2/g
(Kr physisorption), which is 3 times higher than that for the
corresponding ZnCO3. This increase in surface area can be
attributed to additional microporosity observed in the
crystalline pore walls of ZnO, whereas the pore walls of
amorphous ZnCO3 appeared to be rather dense.
Mesoscale ordering in Zn-containing films was analyzed after
different thermal treatments of deposited films. Figure 3 details
the evolution of order from deposited micelles (a and b) to (i)
porous ZnCO3 (c and d) and (ii) ZnO (e and f) as indicated by
two-dimensional 2D SAXS recorded in transmission at beam
incident angles of 90° (a, c, and e) and of 6° (b, d, and f)
relative to the substrate. The 2D SAXS pattern (a) recorded at
90° for the as-deposited film features an isotropic ring
corresponding to a d spacing of 38 nm. Both the d spacing
and the isotropic ring are preserved upon heat treatment at 250
°C (Figure 3c) and during further calcination at 400 °C (Figure
3e). In contrast, all 2D SAXS patterns recorded at a low
incident angle of 6° (Figure 3b,d,f) show diffraction rings with
an elliptical shape. Such diffraction patterns have been reported
also for conventional EISA-based syntheses, where the pore axis
that is oriented perpendicular to the substrate progressively
shrinks during thermal treatments.46,47 This deformation is
caused by a loss of film volume resulting in homogeneous
anisotropic film shrinkage upon drying as well as calcination.
Hence, the d spacing perpendicular to the substrate decreases
in the studied Zn-containing films from 25 nm (as deposited)
to 21 nm [carbonate (i)] and 19 nm [oxide (ii)] (Figure
3b,d,f). However, the d spacing parallel to the substrate of 38
nm remains unchanged (Figure 3a,c,e). This observation
■
FORMATION OF THE PRECURSOR COMPLEX
The proposed synthesis strategy requires the initial formation
of a stable metal−precursor complex in solution (Figure 1,
condition 1). The ability of citric acid to form complexes with
the nitrate compounds of Zn, Al, and Co was therefore assessed
by electrospray ionization mass spectroscopy (ESI-MS) of the
complex solutions. Mass spectra recorded in anion mode are
provided in the Supporting Information (Figure S2 for Zn,
Figure S4 for Al, and Figure S9 for Co).
The mass spectrum for zinc nitrate hexahydrate and citric
acid in ethanol (Figure S2 of the Supporting Information)
shows characteristic mass fragments along with the corresponding isotope pattern. All observed masses can be assigned to zinc
ions bonded to citric acid with nitrate as the counterion [i.e.,
m/z 315.93 (C6H6NO10Zn)−, m/z 378.92 (C6H7N2O13Zn)−,
m/z 507.95 (C6H5N2O13Zn2)−, m/z 507.97 (C12H14NO17Zn)−,
and m/z 571.87 (C12H12NO17Zn2)−]. Hence, zinc (metal M)
and citric acid (ligand L) form complexes with ML, M2L, ML2,
and M2L2 stoichiometries. Moreover, the ESI-MS spectra of
complexed aluminum nitrate (Figure S4 of the Supporting
Information) and cobalt nitrate (Figure S9 of the Supporting
Information) also show a similar composition. Masses
corresponding to ML and M2L2 complex stoichiometries are
observed for both metals. Moreover, Co-based solutions also
contained M2L and ML2 stoichiometries. Hence, citric acid
forms stable complexes with all studied metal ions, underlining
the generic nature of this initial synthesis step.
■
THERMAL STABILITY OF PORE TEMPLATES AND
PRECURSOR COMPLEXES
The proposed synthesis strategy requires the decomposition of
the precursor complex into carbonate at temperatures where
the template polymer remains sufficiently stable (Figure 1, step
3). Moreover, access to the mesoporous carbonate implies that
template removal (Figure 1, step 4) occurs prior to
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Figure 4. Thermogravimetric analysis recorded for (a) the PEO213−PB184−PEO213 polymer and (b−e) dried complexes of citric acid with different
metal nitrates: (b) Mg(NO3)2, (c) Al(NO3)3, (d) Zn(NO3)2, and (e) Co(NO3)2. The colors mark different temperature ranges of thermal
modification of the template polymer and of the metal complexes. The green frame highlights the temperature range of the decomposition of the
template polymer. Colored areas indicate the evolution of the metal complexes: (blue) decomposition of the precursor complex into the
corresponding carbonate, (yellow) existence of the metal carbonate, and (red) decomposition of the carbonate into the metal oxide. Heating was
conducted in air at a rate of 5 K/min.
Information). CO2 is a typical decomposition product of
metal carbonates.
Hence, calcination procedure (i) employed for Zn-based
films [1 h at 250 °C (see Table 1)] exploits the temperature
range in which zinc carbonate remains stable and retains the
pore walls made of carbonate (Figure 2a). However, a
temperature of 250 °C is sufficient to slowly decompose the
template polymer (Figure 4a). Calcination (i) therefore yields
zinc carbonate with templated open mesoporosity (Figure 2a).
Moreover, thermal treatments at temperatures exceeding the
plateau range, i.e., >320 °C (Figure 4d), decompose the
carbonate into the oxide. Thus, the synthesis of mesoporous
ZnO requires a secondary calcination at 400 °C (Figure 2d).
Similar TGA curves are obtained also for the metal
complexes of Mg, Al, and Co (Figure 4b,c,e). The
decomposition of the complex and its conversion into
carbonate start in all three cases below 100 °C. Moreover,
FTIR analysis of the solid samples provides in the plateau
region evidence of the presence of the corresponding metal
carbonate (Figure S6 of the Supporting Information for Al,
Figure S11 of the Supporting Information for Co, and ref 44 for
Mg). Decomposition of the carbonate at temperatures beyond
the plateau region is supported by detection of CO2 as the main
decomposition product also for Al (Figure S8 of the Supporting
Information), Co (Figure S13 of the Supporting Information),
and Mg.44 Thus, for all studied metals, the decomposition of
the precursor complex into the corresponding carbonate (first
mass loss) and its transformation into the respective oxide
(second mass loss) can be assumed.
Depending on the metal, the position of the plateau shifts
(Figure 4). Hence, also the optimal calcination condition for
decomposition of the carbonate into the oxide (Figure 1, step
5). The thermal stability of the template polymer and of
different precursor complexes was therefore investigated by TG
analysis in air. The recorded TG curves are presented in Figure
4, contrasting the behavior of the template polymer (Figure 4a)
with that of the citric acid complexes of Mg (Figure 4b), Al
(Figure 4c), Zn (Figure 4d), and Co (Figure 4e).
TGA indicates that the template starts to decompose at a
temperature of ∼250 °C (Figure 4a).33 However, a rapid mass
loss related to combustion of the polymer occurs between 375
and 425 °C. The thermal stability of the polymer is therefore in
line with literature reports, where decomposition temperatures
between ∼200 °C (PEO106−PPO70−PEO106, Pluronic F127)48
and ∼400 °C (PEO79−PHB89, KLE)49 have been observed.
The TGA curves of all studied metal complexes show the
same typical shape (Figure 4b−e). An initial mass loss of
approximately 30−50% is followed by a plateau with a constant
mass and additional mass loss. Between 20 and 35% of the
initial mass is retained in the final stage. In the case of Zn, the
first significant mass loss occurs between 160 and 225 °C
(Figure 4d). The plateau of constant mass extends to 320 °C,
whereas a constant mass is reached at ∼380 °C. In combination
with XRD and IR analysis of phases (i) and (ii) (Figure 2), the
observed behavior is interpreted as decomposition of the
complex into carbonate (first mass loss), the presence of a
stable carbonate (plateau), and decomposition of the carbonate
into the oxide (second mass loss). This interpretation is further
supported by IR analysis of the gas phase during the second
mass loss, which detects CO2 as the main gas-phase
decomposition product (Figure S3 of the Supporting
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Figure 5. Electron microscopy images demonstrating the ability of the synthesis strategy to address different metals (Al, Mg, and Co). Images of the
carbonates of (a) Al, (b) Mg, and (c) (nonporous) Co. Oxides of (d) Al, (e) Mg, and (f) Co. Imaging methods: (a and b) cross-sectional SEM,
(inset in panel b) top-view SEM, and (d−f) top-view SEM. See the Supporting Information for further TEM analysis of Al (Figure S5) and Co
materials (Figure S10).
evidence the formation of aluminum oxide (Figure S6 of the
Supporting Information).51,52 XRD analysis detects two broad
reflections at 46.0° and 66.6° (Figure S7 of the Supporting
Information), which correspond well with the positions of
(400) and (440) reflections reported for γ-Al2O3 (PDF-No. 00050-0741). The crystallite size estimated via the Scherrer
equation amounts to 5 nm. Diffraction rings observed in SAED
correspond to (311), (400), (511), (440), (444), and (800)
reflections of γ-Al2 O3 (Figure S5b of the Supporting
Information) and confirm the phase assignment. SAXS analysis
(Figure S14a of the Supporting Information) evidences pore
ordering in the as-deposited film, the aluminum carbonate, and
the aluminum oxide at a d spacing of 39 nm parallel to the
substrate (Figure S14a of the Supporting Information, 90°).
The corresponding periodic distance perpendicular to the
substrate decreases from 27 nm (as deposited) to 14 nm
(carbonate) and 5 nm (oxide) (Figure S14a of the Supporting
Information, 6°). The oxide Kr BET surface area amounts to
286 m2/g. Hence, calcination (ii) at 900 °C transforms
mesoporous aluminum carbonate into crystalline γ-alumina
with templated and ordered mesopore structure.
obtaining the micelle-templated mesoporous carbonates and
oxides should differ. To confirm the proposed mechanism,
calcination procedures (i) and (ii) were adjusted for Al and Co
(Table 1) based on TGA data. Additionally, Mg-based films
were prepared.44 SEM images of the synthesized micelletemplated oxides and carbonates are presented in Figure 5.
■
ALUMINUM CARBONATE AND OXIDE
TGA of the complexed aluminum nitrate shows a plateau
between 225 and 310 °C (Figure 4c). The template polymer
decomposes at 250 °C. Hence, calcination at (i) 300 °C was
chosen to produce mesoporous aluminum carbonate and
additional calcination (ii) at 900 °C to produce mesoporous
aluminum oxide and induce its crystallization.
The cross-section SEM image in Figure 5a shows the film
after calcination at 300 °C. The film is fully penetrated by
templated mesopores with the typical elliptical shape and a size
of ∼20 nm (width) × ∼9 nm (height). FTIR analysis (Figure
S6 of the Supporting Information, i) reveals two characteristic
bands indicative of aluminum carbonate, i.e., asymmetric (1608
cm−1) and symmetric (1469 cm−1) stretching vibrations.50 The
film is amorphous according to XRD (Figure S7 of the
Supporting Information, i) and features a surface area of 44.8
m2/g (Kr physisorption). Hence, thermal treatment (i) of the
deposited film forms amorphous mesoporous aluminum
carbonate.
SEM images of the film formed by additional calcination at
900 °C show that the film contains templated mesopores ∼22
nm in diameter (Figure 5d). TEM analysis confirms that the
film is fully mesoporous (Figure S5a,c of the Supporting
Information) and composed of small crystallites ∼6 nm in
diameter (Figure S5d of the Supporting Information). Two
vibrations observed at 538 and 732 cm−1 indicative of Al−O
bonds and the absence of carbonate-related vibrations in IR
■
COBALT CARBONATE AND OXIDE
TGA of the cobalt complex indicates less favorable behavior
(Figure 4e). Also here, a plateau can be observed and attributed
to the presence of cobalt carbonate. However, the plateau is
shifted to temperatures as low as 190−245 °C, i.e., below the
value of ∼250 °C required for thermal decomposition of the
template polymer. Hence, cobalt carbonate decomposes before
the template can be removed. Deposited films were therefore
calcined at (i) 200 °C to stabilize the carbonate and (ii) 300 °C
to produce a mesoporous oxide.
IR analysis of the film calcined at 200 °C (Figure S11 of the
Supporting Information, i) shows asymmetric (1585 cm−1) and
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symmetric (1392 cm−1) stretching vibrations that can be
assigned to carbonate.53 The formed film is amorphous
according to XRD (Figure S12 of the Supporting Information,
i). Unfortunately, electron microscopy imaging of the film failed
because of its instability in the electron beam. Moreover, the
film’s surface area was too small to be detected by Kr
physisorption analysis. Both observations suggest that the
polymer template is still present in the film. SAXS analysis
reveals a diffraction ring for the sample (Figure S14b of the
Supporting Information) with a d spacing of 37 nm parallel and
18 nm perpendicular to the substrate, both indicating the
presence of an ordered mesostructure. Hence, micelle structure
cobalt carbonate is formed during calcination at 200 °C, yet its
pore structure is blocked by the remaining template micelles.
This effect is in complete agreement with the TG analysis of
the precursor complex (Figure 4e) and template (Figure 4a)
and demonstrates a limitation of the proposed synthesis
strategy in its present form. However, cobalt carbonate with
open mesoporosity should become accessible when the PEO−
PB−PEO template is removed by alternative (nonthermal)
methods or when alternative templates with lower decomposition temperatures are employed.
Nevertheless, analysis of the film calcined in addition at (ii)
300 °C indicates that the mesoporous oxide can still be formed.
Top-view SEM images of the film show spherical pores ∼22 nm
in diameter (Figure 5f). TEM analysis confirms that the film is
fully porous (Figure S10c of the Supporting Information) with
pore walls composed of crystallites ∼6 nm in size (Figure S10d
of the Supporting Information). FTIR spectra contain strong
bands indicative of cobalt oxide formation, i.e., Co−O
vibrations at 663 and 570 cm−1,54 whereas only small signals
for carbonates remained (Figure S11 of the Supporting
Information, ii). The corresponding XRD pattern (Figure S12
of the Supporting Information, ii) reveals numerous reflections
that match with the reported phase PDF-No. 00-042-1467 of
crystalline Co3O4 in a spinel structure [31.1° (220), 36.9°
(311), 44.8° (400), 55.7° (422), 59.3° (511), and 65.3° (440)].
The crystallite size estimated via the Scherrer equation (311)
amounts to 7 nm. SAED analysis (Figure S10b of the
Supporting Information) evidences numerous diffraction rings
that confirm the crystallinity of the samples as well as the
assignment of the crystalline spinel phase. Furthermore, 2D
SAXS confirms the presence of an ordered pore structure
(Figure S14b of the Supporting Information). The sample’s
surface area amounts to ∼257 m2/g (Kr physisorption). Hence,
calcination (ii) of the micelle-structured (nonporous) cobalt
carbonate at 300 °C forms a nanocrystalline Co3O4 film with a
spinel structure and the desired open templated porosity.
44 for details). Both employed calcination temperatures are in
agreement with the current TGA analysis. Hence, the validity of
the developed mechanistic hypothesis and synthesis approach
(Figure 1) is also confirmed for the case of MgCO3 and MgO.
■
CONCLUSIONS
We present a new approach for the synthesis of micelletemplated oxides and carbonates of zinc, cobalt, and aluminum.
The method employs a unique precursor to overcome
limitations of classical EISA-based syntheses, i.e., complexes
formed from citric acid and metal nitrates. The precursors
reliably self-assemble with amphiphilic block copolymers. Using
this approach, films of ZnO, Co3O4, ZnCO3, and Al2(CO3)3
were synthesized for the first time with micelle-controlled open
mesoporosity. The respective pores are locally ordered and
result in surface areas in the ranges of 44−86 m2/g
(carbonates) and 250−286 m2/g (oxides). The pore systems
show the uniaxial shrinkage that is typical for EISA-based
oxides. The pore sizes of the materials were readily adjusted by
changing the size of the template polymer (ZnO; Figure S15 of
the Supporting Information).
A general mechanism is proposed for the developed synthesis
(Figure 1). As in the Pechini method, the first step requires the
formation of a metal complex in solution. The complex
assembles with template micelles into an ordered mesophase
during film deposition and solvent evaporation. Sequential
thermal treatments convert the precursor complex first into a
micelle-structured amorphous carbonate, remove the template
yielding the porous carbonate, and finally convert the carbonate
into the mesoporous oxide. Carbonates and oxides feature a
cubic ordered mesopore structure. The calcination temperatures required for carbonate and oxide synthesis can be
derived from simple TG analysis of the corresponding
precursor complex. Moreover, a comparison between TGA
data of metal complex and template polymer predicts if thermal
template removal from the carbonate is feasible. The
amorphous character of the intermediate carbonate appears
to be of particular importance, because it facilitates template
removal without sintering of wall-forming crystallites and
therefore avoids degradation of the templated pore structure.
The synthesis is generic in nature and therefore applicable to a
wide range of metal oxides and carbonates.
The presented synthesis strategy for creating functional metal
oxides and carbonates can be easily tuned and optimized for
energy storage, electro catalysis, sunlight harvesting, or
biomedical applications. In particular, amorphous CaCO3
could be of interest for biocompatible implant coating and
defined model systems in bone cell culturing 55 and
biomineralization research.56 However, the synthesis should
be further refined on the basis of an improved fundamental
understanding. Physicochemical investigations could reveal the
type of interaction between the precursor complex and
micelles, guide the tailoring of the complex structure, and
explain how crystallites are formed. Most importantly,
extending the synthesis strategy to yield also bimetallic
carbonates and oxides with optimized pore structure such as
the Cu/ZnOx-based catalysts employed in industrial methanol
synthesis57 or indium-free transparent conductive oxides could
result for many applications in a tremendous improvement in
performance.
■
COMPARISON TO MAGNESIUM CARBONATE AND
OXIDE
TGA of the complex of magnesium nitrate with citric acid
features a broad plateau of constant mass between 275 and 425
°C (Figure 4b). According to the proposed mechanistic
hypothesis, calcination (i) at temperatures within this temperature window should yield mesoporous carbonate, whereas
subsequent calcination (ii) at temperatures exceeding 425 °C
should result in mesoporous magnesium oxide. We recently
reported the corresponding synthesis of magnesium carbonate
with templated mesopore structure employing 400 °C for
calcination (i).44 The transformation into mesoporous MgO
succeeded by calcination (ii) at 600 °C. Panels b and e of
Figure 5 show images of the corresponding materials (see ref
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mass, the oxide films were dissolved and the concentration was
measured in a Varian 715-ES ICP-OES instrument. The Al2O3 films
were dissolved in a mixture of H2SO4 (3 mL, 95 wt %) and H3PO4 (3
mL, 85 wt %) in 30 min at 200 °C and 20 bar in a microwave (200
W). The ZnO films were dissolved in an aqueous HCl solution (8 mL,
37 wt %) while being stirred for 30 h at 25 °C. The Co3O4 films were
dissolved in an aqueous HCl solution (8 mL, 37 wt %) while being
stirred for 40 h at room temperature.
2D SAXS patterns were recorded at DORIS III storage ring,
beamline B1 at DESY Hamburg with a PILATUS 1 M detector
(Dectris) at a sample−detector distance of 3589 or 1785 mm and an
X-ray energy of 16029 eV. 2D SAXS patterns were also recorded at
PETRA III storage ring, beamline P03 at DESY Hamburg with a
PILATUS 1 M detector at a sample−detector distance of 3161 mm
and an X-ray energy of 12956 eV. FTIR spectra were recorded on a
Perkin-Elmer Spectrum 100 instrument on samples pressed in KBr.
XRD was measured on a Bruker D8 Advance instrument (Cu Kα
radiation) with a grazing incident beam (1°). Reflections were assigned
using PDFMaintEx library version 9.0.133. TG FTIR was measured on
a Netzsch STA 409 connected to a Bruker Optik Equinox 55
instrument in air at a heating rate of 5 K/min. Gas-phase IR spectra
were assigned using the EPA vapor phase FTIR library. Electrospray
ionization mass spectra (ESI-MS) were measured with a Thermo
Scientific Orbitrap LTQ XL instrument operating at a source voltage
of 10 kV. The spray solutions were prepared by codissolving metal
nitrates and citric acid in a 2:1 molar ratio in ethanol and sprayed
directly into the ESI-MS instrument at a flow rate of 5 μL/min.
EXPERIMENTAL SECTION
Chemicals. Zinc nitrate hexahydrate (98%, extra pure) was
obtained from Acros. Aluminum nitrate nonahydrate (>99%, pro
analysis), magnesium nitrate hexahydrate (>99%, pro analysis), and
cobalt nitrate hexahydrate (>98%, for analysis) were purchased from
Merck. Water-free citric acid (>99.5%, pro analysis), ethanol (>99.9%,
absolute), and the HCl solution (37 wt %, pro analysis) were obtained
from Roth. Concentrated sulfuric acid (95 wt %, puriss) was purchased
from Th. Geyer. PEO−PB−PEO polymers were synthesized by
Polymer Service GmbH Merseburg.22 All chemicals were used without
further purification.
Film Synthesis. Prior to film deposition, substrates (Si wafers)
were cleaned with ethanol and heated in air (2 h at 600 °C). The dipcoating solution for zinc films was prepared by joining powders of
Zn(NO3)2·6H2O, citric acid, and a template in the amounts listed in
Table 1. The powders were dissolved in a mixture of Milli-Q water
(1.5 mL) and ethanol (1.5 mL) by being stirred overnight, resulting in
a colorless solution. Films were prepared by dip-coating substrates at a
withdrawal rate of 150 mm/min under a controlled atmosphere (25
°C, 40% relative humidity). Afterward, films were allowed to dry for at
least 10 min before being transferred into the preheated muffle
furnace. Mesoporous ZnCO3 films were obtained after calcination for
1 h at 250 °C. Mesoporous ZnO films required calcination for 1 h at
250 °C, natural cooling to room temperature, and a second calcination
for 25 min at 400 °C (preheated furnace). ZnO films with 8 and 15
nm pore diameters as well as bimodal films were obtained in a similar
fashion (see Figure S15 of the Supporting Information).
Films of magnesium carbonate and magnesium oxide were prepared
according to the method described previously44 employing the
composition and conditions listed in Table 1.
The dip-coating solution for the alumina films was prepared by
mixing the powders of Al(NO3)3·9H2O, citric acid, and PEO213−
PB184−PEO213 (Table 1). The powders were dissolved in a mixture of
Milli-Q water and ethanol by being stirred overnight, resulting in a
slightly yellow solution. Films were prepared by dip-coating substrates
at a withdrawal rate of 150 mm/min at 25 °C and 40% relative
humidity. Afterward, films were allowed to dry for at least 10 min
before being transferred into the preheated muffle furnace. The
mesoporous aluminum carbonate films were obtained by calcination
for 1 h at 300 °C. The mesoporous aluminum oxide films were
obtained after being calcined for 1 h at 300 °C, naturally cooled to
room temperature, and heated for 30 min to 900 °C (preheated
furnace).
The dip-coating solution for the cobalt-based films was prepared by
mixing the powders of Co(NO3)2·6H2O, citric acid, and PEO213−
PB184−PEO213 (Table 1). The powders were dissolved in a mixture of
Milli-Q water and ethanol by being stirred overnight, resulting in a
red/pink solution. Films were prepared by dip-coating substrates at a
withdrawal rate of 150 mm/min at 25 °C and 40% relative humidity.
Afterward, films were allowed to dry for at least 10 min before being
transferred into the preheated muffle furnace. The cobalt carbonate
films employed calcination for 1 h at 200 °C. The mesoporous cobalt
oxide films were obtained after being calcined for 1 h at 200 °C,
naturally cooled to room temperature, and heated for 20 min at 300
°C (preheated furnace).
All dip-coating solutions remained clear and without precipitants
even after 1 month but were used only in the first 5 days after
preparation to avoid depletion effects of the polymer template.
Characterization. TEM was conducted on a FEI Tecnai G 2 20 STWIN instrument that operated at 200 kV on films scraped off from
the substrates and transferred onto a copper grid coated with lacey
carbon. SEM imaging was performed using a JEOL 7401F instrument
at an acceleration voltage of 10 kV and a working distance of 4 mm.
Image J version 1.44o (http://rsbweb.nih.gov/ij) was employed to
determine the pore diameter and film thickness. Kr adsorption
isotherms were measured at 77 K with a Quantachrome Autosorb-1-C
instrument. The film samples were degassed in vacuum at 150 °C for 2
h prior to physisorption. The surface area was calculated using the
Brunauer−Emmett−Teller (BET) method. To determine the coating
■
ASSOCIATED CONTENT
S Supporting Information
*
Additional data. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Author Contributions
R.K. and B.E. designed the study. B.E. and D.B. conducted the
synthesis and material characterization. B.E., J.P., U.V., and F.E.
analyzed the materials with X-ray-based methods. B.E., E.O.,
R.K., and J.P. prepared the manuscript. P.S. contributed editing
of the manuscript and helpful discussion throughout the study.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
R.K., D.B., and E.O. acknowledge generous funding from
BMBF (FKZ 03EK3009). B.E. is thankful for financial support
from the German Cluster of Excellence in Catalysis (UNICAT)
funded by the German National Science Foundation (DFG)
and managed by the Technical University of Berlin (TU
Berlin). R.K. is grateful for support from Einstein-Stiftung
Berlin. Analytical support by Zentraleinrichtung Elektronenmikroskopie (ZELMI) at TU Berlin (TEM), Oliver Goerke
(TG FTIR), Gregor Koch (TG), and Maria Schlangen (ESIMS) is acknowledged. Portions of this research were conducted
on beamline B1 at light sources DORIS III and PETRA III at
DESY, a member of the Helmholtz Association (HGF).
■
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