Nanostructured microspheres of MnO2 formed by room temperature solution processing

Article abstract of DOI:10.1039/b713201h

Nanostructured microspheres of manganese oxide have been prepared by the solution processing of manganese(ii) salts using 1,4,7,10-tetraazacyclododecane and utilizing a crystalline intermediate stage. The Royal Society of Chemistry.

Full text of DOI:10.1039/b713201h

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Nanostructured microspheres of MnO formed by room temperature
2
solution processing{{
ab
c
a
b
Jonathan P. Hill,* Sher Alam, Katsuhiko Ariga, Christopher E. Anson and Annie K. Powell*
b
Received (in Cambridge, UK) 29th August 2007, Accepted 19th October 2007
First published as an Advance Article on the web 1st November 2007
DOI: 10.1039/b713201h
Nanostructured microspheres of manganese oxide have been
prepared by the solution processing of manganese(II) salts using
In our investigations of biologically relevant systems we
identified the coordination complexes of manganese as being
potentially of interest. The use of such complexes of manganese
with triazacyclononane (TACN) has been demonstrated for
1,4,7,10-tetraazacyclododecane and utilizing a crystalline inter-
mediate stage.
14
industrial bleaching applications. Additionally, mixed valent
1
5
Manganese oxides from mineral and synthetic sources are of
1
significant technological importance. For instance, they are used
complexes of manganese with TACN or 1,4,8,11-tetraazacyclo-
16
tetradecane (cyclam) are well known while those of 1,4,7,10-
2
3
extensively in catalytic oxidations or as battery materials. More
recently, other oxides have also been investigated for higher
properties such as combinations of ferromagnetic and ferroelectric
tetraazacyclododecane (cyclen) have not been subjected to in depth
1
7
investigation. During work to assess the structures of dinuclear
complexes of manganese with cyclen, we discovered a method for
preparing nanostructured manganese oxide. Initially, we under-
took structural studies, which revealed a supramolecular sheet
structure involving chloride counteranions, but also found an
aggregated, nanostructured form of a manganese oxide upon
standing of the crystallization solutions.
4
effects. Now there is a significant trend towards identifying novel
properties and perhaps uses based on the nanostructuring of these
5
and other novel oxides. Nanostructuring can be achieved by a
6
variety of methods including templating, self-assembly, and
7
8
thermolysis of coordination complexes. The latter is of particular
importance since precise control of the precursor complexes can be
exercised giving products of predictable stoichiometry. One further
methodology that is gaining increasing importance involves
solution processing of inorganic materials into nanostructured
forms, which is receiving attention because of its lower environ-
mental impact and ease of implementation. Solution processing
offers other advantages of operation at large scales and
applicability to materials that are difficult to manage using
vacuum processing. Thus the development of this approach holds
significant promise as a means of accessing tailored materials. In
this communication, we report the preparation of nanostructured
manganese oxide in microspherical morphology using solution
processing of appropriate precursor materials (Scheme 1), and
which will subsequently be investigated as possible electrode
materials for new high performance batteries. Nanostructured
manganese oxides have been previously reported in several
When a methanolic solution of cyclen was mixed with
manganese(II) chloride in aqueous solution and in the presence
of sodium perchlorate a gradual colour change from pale pink to
dark green occurred consistent with formation of a dioxo-
16
dinuclear species similar to that found for cyclam. Subsequent
crystallization gave dark green prisms suitable for X-ray crystal-
lographic analysis.§ The structure of the dioxo-dinuclear unit is
given in Fig. 1 and it is consistent with a molecular formula
2
2
[Mn
(m -O) (C H N ) ]?2Cl ?ClO . The oxidation states of the
2 2 8 20 4 2 4
2
18
manganese atoms were confirmed by bond valence sum analysis
with calculated valences 3.06 and 4.03 for Mn(1) and Mn(2),
respectively (see caption of Fig. 1 for bond lengths), indicating
valence localization in the Mn O unit. Linear arrays of the
2 2
dinuclear unit formed by double hydrogen bond bridging of a
chloride ion with cyclen amine groups run parallel to the b-axis
with these linear arrays being aligned within sheets by a further
inter-array hydrogen bonding involving the remaining chloride
counterion (see ESI{). The single perchlorate counteranion per
dinuclear unit is singly hydrogen bonded at one of the remaining
cyclen amine groups and is not involved in the extended structure.
Sheets formed by amine–chloride hydrogen bonding contain
cavities, which are occupied by the perchlorate anions hydrogen
bonded to adjacent sheets.
9
10
morphologies including nanoparticles, nanosheets,
hollow
11
12
13
nanoshells, nanofibres or mesoporous materials but, to the
best of our knowledge, microspheres with a lamellar internal
nanostructure have not been reported.
a
Supermolecules Group, National Institute for Materials Science, 1-1
Namiki, Tsukuba, Ibaraki 305-0044, Japan.
E-mail: Jonathan.Hill@nims.go.jp; Tel: +81 29-860-4832
Institute for Inorganic Chemistry, University of Karlsruhe, Geb. 30.45,
b
Engesserstr. 15, Karlsruhe, Germany D-76128. E-mail: powell@aoc.uni-
karlsruhe.de; Tel: +49 721-608-2135
Fuel Cell Materials Group, National Institute for Materials Science, 1-1
c
Namiki, Tsukuba, Ibaraki 305-0044, Japan
{
Dedicated to Professor Dieter Fenske on the occasion of his 65th
birthday.
Electronic supplementary information (ESI) available: Packing dia-
Scheme 1 Illustration of microsphere synthesis. Crystallization of the
primary product is followed by its dissolution and redeposition as
nanostructured microshperes.
{
grams, EDX and XPS spectra, FTIR spectra, magnetic susceptibility data.
See DOI: 10.1039/b713201h
This journal is ß The Royal Society of Chemistry 2008
Chem. Commun., 2008, 383–385 | 383

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Fig. 1 X-Ray crystal structure of the initial product from the reaction
between cyclen and manganese(II) chloride tetrahydrate in water–
methanol solution in the presence of sodium perchlorate. Important bond
2 2
lengths and angles about the Mn O unit: Mn(1)–O(1) 1.814(4), Mn(1)–
Fig. 3 X-Ray diffraction patterns of the products from the reaction
O(2) 1.868(5), Mn(2)–O(1) 1.765(5), Mn(2)–O(2) 1.786(4) s, Mn(1)–O(1)–
Mn(2) 97.7(2), Mn(1)–O(2)–Mn(2) 95.0(2)u.
between cyclen and manganese(II) oxalate in water–methanol. (a) Initial
microcrystalline product. (b) During crystal decomposition. (c) After
completion (y3 months). (d) Purified nanostructured microspheres. (e)
Wide angle region of the XRD spectrum of nanostructured microspheres.
Solid triangles denote peaks due to unknown compound.
Cyclen reacts in a similar fashion with other manganese salts
including manganese(II) acetate and manganese(II) oxalate. In the
case of oxalate, allowing the crystallization solutions to stand for
periods longer than required for the collection of crystals resulted
in formation of a dark brown precipitate concurrent with
dissolution of the crystalline or microcrystalline primary products.
Observation by optical microscopy revealed the particulate nature
of these precipitates (Fig. 2(a)). Scanning electron microscopy
lamellar structure with a layer thickness of y1.2 nm. Further weak
reflections with d-spacing values of 2.46 and 1.45 s are at angles
2
expected for known examples of layered MnO , respectively being
the 10 and 11 reflections due to its two dimensional hexagonal
2
0
architecture (Fig. 3(e)). The layer thickness deduced from the
XRD data (1.2 nm) indicates that there is a species intercalated in
the structure and infrared spectroscopy (see ESI{) suggests that
this is cyclen or its decomposition product(s).
(SEM, Fig. 2(b)–(d)) was used to probe further the structure of
these particles and revealed that they possess a secondary structure
of lamellar appearance.
To probe the elemental composition of the microparticles and
their precursors, electron X-ray dispersive analysis (EDX) was
performed. EDX spectra revealed an increasing proportion of
manganese going from crystals to the final nanostructured
microspheres. The final composition according to EDX is Mn
X-Ray diffraction (XRD) patterns for the manganese(II) oxalate
system are shown in Fig. 3. Fig. 3(a) is the XRD pattern of the
19
dark green microcrystalline primary product. Upon standing of
this product under the reaction liquors (i.e. the product was not
filtered) a broad peak appears in the diffraction patterns of the
materials filtered later (see Fig. 3(b)). Peaks due to the primary
product gradually decrease in intensity with a concurrent increase
in intensity of the broad peaks. Additionally, further sharp peaks
appear, which are due to a yet-to-be-identified colourless crystal-
line byproduct (Fig. 3(c)). This impurity can be removed manually
yielding samples having the XRD pattern in Fig. 3(d). The
d-spacings of the broad peaks are of a ratio 1 : 1/2 : 1/3 indicating a
(
51.2%), O (24.7%), C (16.6%), N (7.6%) suggesting a composition
containing MnO accompanied by 1/8 equivalents of cyclen, that
is, MnO (C 0.125, a formula consistent with the lamellar
2
2
8 20 4
H N )
structure and with the continued presence of an organic species.
An XPS survey scan (see ESI{) confirmed the presence of
manganese, oxygen and carbon although the peak due to nitrogen
appears broad. These data are consistent with the formula
obtained by EDX. The presence of an organic species, probably
cyclen, is confirmed by FTIR spectroscopy (see ESI{) and this is in
line with the requirement of a cationic species for the electrostatic
maintenance of the lamellar form so that the cyclen must be at
least partially protonated in order to balance the charge of the
2
1
oxide layers as in other similar materials and leading to a
formulation MnO22d(C H20N4)0.125.
8
Flocculation of aqueous solutions of manganese salts by
addition of basic reagents is a well known method for the
preparation of manganese oxo-hydroxide aggregates, which are
often gelatinous in nature and correspondingly difficult to handle.
In this case, our primary sequestration of the manganese cations as
the dinuclear cyclen complex prevents a rapid precipitation of the
expected oxo-hydroxides. Rather, a preliminary crystallization
process is followed by gradual deposition of the nanostructured
product. This process is somewhat similar to the ‘chelation-
Fig. 2 (a) Optical microscopy image of the microspherical product of the
reaction between cyclen and manganese(II) oxalate in water–methanol. (b)
SEM image of the same sample. (c) Texture at the interior of a
microsphere. (d) Texture at the surface of a microsphere.
22
mediated’ control reported by Oaki and Imai although our
process always involves a crystalline intermediate. Nucleation of
the nanostructured product appears to occur in close association
3
84 | Chem. Commun., 2008, 383–385
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solution was filtered and allowed to stand in a vessel open to the air until
crystals of the complex began to appear. Then the solution was allowed to
stand for several weeks until a dark particulate matter was deposited, which
was subjected to further analysis. Other divalent salts of manganese were
treated similarly.
21
X-Ray crystallography. C64
3 2 8 6
H40Cl Mn N O , FW 656.79 g mol ,
orthorhombic, Pnma, a = 22.3850(14), b = 9.1934(5), c = 13.2424(10) s,
3
23
V = 2725.2(3) s , T = 200 K, F(000) = 1364, rcalc = 1.601 Mg m , m(Mo-
2
1
Ka) = 1.267 m . Stoe IPDS diffractometer, l = 0.71073 s, 15645 data
measured to 2hmax = 52u, of which 2709 unique (Rint = 0.1230), structure
solution by direct methods and full-matrix refinement against F (all data)
2
23
using SHELXTL, 192 parameters, final wR
data), R = 0.0480 (1397 with I . 2s(I)). CCDC 656863. For
crystallographic data in CIF or other electronic format see DOI:
0.1039/b713201h
2
= 0.0903, S = 0.967 (all
1
1
Scanning electron microscopy. A Hitachi S-4800 FE-SEM operating at
an accelerating voltage of 4 kV was used to collect images.
X-Ray powder diffraction. XRD of powdered samples was performed
using a Rigaku Ultima III diffractometer.
Fig. 4 Intermediate growth stage of the nanostructured microspheres on
the surface of crystals of the primary product. Inset: growth of
microspheres apparently at the interior of a crystal.
1
J. E. Post, Proc. Natl. Acad. Sci. U. S. A., 1999, 96, 3447; S. Roy, in
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with the crystalline primary product. This is most likely due to
the appearance of decompositional defects at the surfaces of the
crystals, which provide appropriate sites for growth of the
particles. Following this initial stage, growth of the microspheres
proceeds also on the glass walls of the reaction vessel. SEM
observations of the reaction solutions at intermediate stages of the
process show that the nanostructured particles appear to grow
both at the surface of the crystals and occasionally even at the
interior. However, growth of particles at the interior appears to
occur following the loss of integrity of the crystals, since at no time
could particles be observed growing inside pristine crystals (by
transmission optical microscopic observation). Fig. 4 shows the
growth of particles at two sites on the crystalline primary product.
In summary, we have discovered a method for preparing
nanostructured microspheres of manganese oxide MnO22d with a
lamellar internal structure. This solution process, which operates at
room temperature, avoids the precipitation of the manganese oxo-
hydroxides and permits preparation of these products with novel
structures. Thus we have found a means of producing potentially
useful nanostructured manganese oxide particles using a method
which is an improvement in terms of environmental impact. The
method should be applicable to other systems where precipitation
is a problem. In this case, we could obtain microparticles having
magnetic properties distinct from those of the common manganese
oxides (see ESI{). The form of these microparticles should endow
them with significant surface areas and thus they would possess
potential in the areas of catalysis or as electrode materials, and we
are now embarking on testing of their suitability for these
purposes.
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supported by Grant-in-Aid for Scientific Research on Priority
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1
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1
2
Notes and references
2
§
Synthesis. A solution of cyclen (0.1 g, 0.58 mmol) in methanol (10 mL)
was added dropwise with stirring to a solution of manganese(II) chloride
tetrahydrate (1 eq., 0.115 g) in water (10 mL). Then sodium perchlorate
22 Y. Oaki and H. Imai, Chem.–Eur. J., 2007, 13, 8564.
23 G. M. Sheldrick, SHELXTL 5.1, Bruker AXS Inc., 6300 Enterprise
Lane, Madison, WI 53719-1173, USA, 1997.
(0.4 g) was added and the mixture stirred until complete dissolution. The
This journal is ß The Royal Society of Chemistry 2008
Chem. Commun., 2008, 383–385 | 385