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Mnbius Strips of NbSe3 Morphology Design and Solid-State Chemistry.

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Highlights
Shape Control
Mbius Strips of NbSe3 : Morphology Design and
Solid-State Chemistry
Greta R. Patzke*
Keywords:
chemical vapor transport · crystal growth · niobium ·
selenium · synthesis design
The Interplay of Form and Structure in Chemistry
Chemists tend to interpret and design the “form” of a substance mainly
with respect to its internal periodic
order. This approach, however, is strictly
focussed on the concept of 230 space
groups—and it is therefore restricted to
their symmetry limitations, which neglect the important phenomenon of
complex external morphology. Consequently, rather universal “design” strategies in modern chemical synthesis are
directed towards shaping the structure
as well as the outer appearance of nanoand microscale materials. This Highlight
illustrates how solid-state chemistry can
benefit from an integral synthetic approach that has recently been utilized to
generate NbSe3 crystals with an unprecedented morphology.[1] The preparation of microcrystalline M+bius strips of
NbSe3 represents a breakthrough in
classical solid-state synthesis with respect to shape design. The remarkable
progress in the control of morphology in
nanoscale or bio-inspired inorganic
chemistry has been further developed
successfully and is outlined below.
* Thorough investigations exploring
the challenging area of “chemical
topology” at the molecular level
were driven by a desire to construct
novel molecular architectures.[2] The
well-known M+bius strip—a topological concept that fascinates both
[*] Dr. G. R. Patzke
Laboratory of Inorganic Chemistry
ETH H.nggerberg—HCI
8093 Z4rich (Switzerland)
Fax: (þ 41) 1-632-1149
E-mail: patzke@inorg.chem.ethz.ch
972
*
chemists and mathematicians—in
particular, triggered considerable
synthetic efforts until it was finally
realized as an organic compound in
1982.[3] The M+bius strip represents
a one-sided bounded surface and its
unique properties can easily be demonstrated with a simple long rectangular strip of paper. Before putting
the ends of the strip together, one
end of the paper strip is twisted
through an angle of 1808 relative to
the other so that a M+bius strip is
obtained instead of a simple cylindrical two-sided bounded surface.
Dissecting the M+bius strip twice
along its axis delivers a set of catenanes.
An elegant approach to the morphology design of nanoparticles has
recently been reported by Stupp and
co-workers: they produced CdS
nanohelices in a synthetic approach
that utilized the potential of supramolecular chemistry to build inorganic nanostructures.[4] The unique
helical shape of the CdS particles is
generated under the influence of
organic supramolecular nanoribbons
as templates.
The intriguing vision of generating
complex inorganic architectures on
the laboratory scale is currently explored in inorganic morphosynthesis.
Natural biomineralization processes
provide a continuous source of inspiration for this innovative research
area. Important developments—impressing with respect to both scientific impact and aesthetics—have
been reported, such as spongelike
vaterite spheroids, hollow shells of
mesoporous aragonite, and mesoskeletal forms of calcium phosphate
(Figure 1).[5]
Most of the above-mentioned synthetic
routes depend on the presence of a
structure-directing organic template
that controls the course of an inorganic
precipitation reaction. This pathway,
however, has its limitations since the
thermal instability of the organic component may give rise to decomposition
processes and side reactions. Therefore,
severe restrictions with respect to the
fabrication of inorganic solids at elevated temperatures are imposed on the
application of this synthetic principle in
the solid state. So which parameters
might exert efficient control over the
morphology when applied at synthesis
*
Figure 1. a) SEM images of spongelike vaterite spheroids.[12] b) SEM image of intact hollow
shells of mesoporous aragonite.[13] c) TEM image of a calcium phosphate block copolymer.[14]
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angew. Chem. Int. Ed. 2003, 42, No. 9
Angewandte
Chemie
temperatures far above the decomposition temperature of any organic additive? A convincing solution to this
crucial problem has now been presented, and it has been successfully applied
to NbSe3, a compound attracting considerable interest in both structural and
materials chemistry.
Morphology Design and SolidState Chemistry
NbSe3 has long been famous for its
almost classical representation of a lowdimensional solid. The anisotropic properties arise from the high degree of
anisotropy in the crystal structure. The
latter consists essentially of three different types of [NbSe3] chains comprised of
face-sharing trigonal prisms that run
parallel to the b-axis of the monoclinic
unit cell. Consequently, NbSe3 exhibits a
distinct tendency to form crystals with a
fibrous, ribbonlike morphology. Thus, it
is a good candidate for synthetic attempts that focus on shape control.
Indeed, remarkable variations in crystal
growth that result in the formation of
large circular microcrystals have already
been reported for NbSe3, but the origin
and underlying growth mechanism of
these NbSe3 objects remains to be
elucidated.[6] Tanda et al. have now
established a synthetic breakthrough
that provides controlled access to M+bius strips of NbSe3 on the microscale.[1]
Moreover, annular and figure-of-eightshaped crystals of NbSe3 have also been
produced. The initial results with the
tantalum-based analogues (TaSe3, TaS3)
are also quite promising. The experimental approach chosen is of striking
simplicity: a direct reaction of Se and
Nb with no further additives involved.
This leads to the question of how such
an apparently simple reaction is capable
of generating a geometry as rare and
complex as the M+bius strip.
The reactants involved are indeed
elementary, but they participate in an
elaborate variation of the chemical
transport reaction. A mixture of Se
and Nb powder is treated in an evacuated quartz ampoule at 740 8C under
special growth conditions and the reaction performed is in a furnace with a
considerably extended temperature gradient. In this way, a nonequilibrium state
is established that facilitates the coexistence and of selenium in the vapor,
mist, and liquid (droplet) forms inside
the quartz ampoule. The formation of
microscale selenium droplets is essential, because they act as in situ templates
during the next step of the reaction:
ribbon-shaped NbSe3 crystals are subsequently formed along the equator of
the droplets and their interaction with
the surface tension of the viscous selenium droplet determines the shape of
the emerging crystals. There are several
cyclization pathways for the whiskershaped NbSe3 crystals on the droplet
surface which give rise to three different
morphological variations (Figure 2):
) Ring structures result when the ends
of the crystal simply meet without
any further twist (0 p).
) M+bius strips arise from twisting and
subsequent connection of the ends
(1 p).
) Figure-of-eight crystals are accessible either by double encircling or as
the product of a double twisting
(2 p).
The design strategy presented here
benefits from the bifunctional facilities
of selenium which serves both as a
reactant and as a templating agent. This
means that template-directed inorganic
syntheses can now be performed as
“one-pot-procedures” without any extra
or even thermally unstable additives
involved.
No other previous process combines
these advantages with such a broad
applicability. Moreover, chemical transport reactions provide the unique combination of an effective synthetic approach with a particular rich fund of
experimental experience that has continuously been systemized by theoretical
considerations:[7]
) Whole classes of inorganic substances (oxides, silicides, intermetallics,
etc.) are easily accessible in singlecrystalline form by means of chemical transport reactions and their
morphology could potentially be
adjusted by gas-phase deposition.
) The systematic variation and optimization of the reaction parameters
is possible, with the latter being
monitored by tailor-made computational methods.
The preparation of NbSe3 M+bius strips
is not only of aesthetic interest, but
offers a starting point for important
applications in both theoretical and
physical chemistry.
Mbius Strips of NbSe3—Unique
Topochemical Crystals
Figure 2. SEM images of the NbSe3 crystal topology.[1] The scale bars correspond to 10 mm.
a) Formation of a ring structure by direct linking of the ends (0 p). b) M.bius strip arising from
a simple twist (1 p). c) Figure-of-eight crystal formed after a double twist (2 p). Reprinted with
permission from Nature (ref. [1], copyright 2002, Macmillan Publishers Ltd).
Angew. Chem. Int. Ed. 2003, 42, 972 – 974
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
The outstanding low-dimensional
properties of the chain structure of
NbSe3 give rise to a multitude of applications for the newly detected microcrystals:
* The chemical reactivity of low-dimensional solids can be considered
to result from them lying at the
interface between solid-state and
molecular chemistry. They exhibit a
rich chemical reactivity that encompasses redox, intercalation, and spe-
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973
Highlights
*
974
cial acid–base reactions. Their structural chemistry can also be studied:
the influence on both the chain
structure of the host material and
the resulting morphology of the
ternary products can be elucidated
by partial substitution of other
atoms for niobium. This effect has
been reported, for example, in
FeNb3Se10, where a segregation of
the characteristic trigonal-prismatic
chains by iron-containing octahedral
chains was found.[8] The variation in
the physical properties reflects the
degree of anisotropy in the chemical
bonding, hence mixed selenides
should exhibit modified characteristics.
Charge density waves (CDWs) are
displayed by NbSe3 in an exemplary
way so that it serves as a model
substance for illustrating such phenomena. Thus, the impact of NbSe3
M+bius strips goes far beyond the
level of synthetic innovation and
gives rise to fundamental theoretical
studies. Although the appearance of
charge density waves has been investigated in general, the effect of
systems with closed-loop topologies
on CDW still needs to be elucidated.
The novel M+bius strips afford a
unique insight since topological effects in quantum mechanics can now
be studied in a real system. So what
kind of CDW behavior will be observed in closed loops of NbSe3 ?
Maybe self-interference will appear,
or even hitherto unknown processes
might be found. Microring crystals
of NbSe3 are now readily available
and have proven suitable for the
experimental study of interference
phenomena in the charge density
waves.[9, 10]
* A multitude of innovative practical
applications of these novel microcrystals appears to be on the horizon. NbSe3 is a promising cathode
material for rechargeable lithium
cells, since it is capable of intercalating three lithium ions per chalcogen
ide molecule.[11] Its unique combination of ribbonlike morphology and
metallic conductivity gives rise to a
whole set of technological benefits:
the need for either additional binders or conducting additives is eliminated. Moreover, the large surface
area of the fibers leads to enhanced
electrochemical behavior. Now that
a set of microcrystals with distinct
shapes is accessible, further developments in the field of microbatteries
will surely be possible.
* M+bius strips already imply the
presence of catenanes that would
result from their twofold longitudinal dissection. Thus, the NbSe3 M+bius strips are promising precursors
for the first inorganic catenane synthesis; they might enable the design
of new components for microelectronics.
The development of a remarkably
flexible gas-phase synthesis that could
be elaborated into a general access to
inorganic solids with a special morphology should provide entry to a new
generation of compounds for modern
materials chemistry.
[1] S. Tanda, T. Tsuneta, Y. Okajima, K.
Inagaki, K. Yamaya, N. Hatakenaka,
Nature 2002, 417, 397.
[2] H. L. Frisch, E. Wassermann, J. Am.
Chem. Soc. 1961, 83, 3789.
[3] D. M. Walba, R. M. Richards, R. C.
Haltiwanger, J. Am. Chem. Soc. 1982,
104, 3219.
[4] E. D. Sone, E. R. Zubarev, S. I. Stupp,
Angew. Chem. 2002, 114, 1781; Angew.
Chem. Int. Ed. 2002, 41, 1705.
[5] S. Mann, Angew. Chem. 2000, 112, 3532;
Angew. Chem. Int. Ed. 2000, 39, 3392.
[6] F. A. Trumbore, L. W. ter Haar, Chem.
Mater. 1989, 1, 490.
[7] R. Gruehn, R. Glaum, Angew. Chem.
2000, 112, 706; Angew. Chem. Int. Ed.
2000, 39, 692.
[8] J. Rouxel, Acc. Chem. Res. 1992, 25, 328.
[9] Y. Okajima, H. Kawamoto, M. Shiobara,
K. Matsuda, S. Tanda, K. Yamaya, Phys.
B 2000, 284–288, 1659.
[10] S. Tanda, H. Kawamoto, M. Shiobara, Y.
Sakai, S. Yasuzuka, Y. Okajima, K.
Yamaya, Phys. B 2000, 284–288, 1657.
[11] F. A. Trumbore, J. Power Sources 1989,
26, 65.
[12] D. Walsh, B. Lebeau, S. Mann, Adv.
Mater. 1999, 11, 324.
[13] D. Walsh, S. Mann, Nature 1995, 377,
320; D. Walsh, S. Mann, Adv. Mater.
1997, 9, 658.
[14] M. Antonietti, M. Breulmann, C. G.
G+ltner, H. C. C+lfen, K. K. W. Wong,
D. Walsh, S. Mann, Chem. Eur. J. 1998, 4,
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Angew. Chem. Int. Ed. 2003, 42, 972 – 974
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