Polymer-directed synthesis of penniform BaWO4 nanostructures in reverse micelles

Article abstract of DOI:10.1021/ja029958f

Novel penniform superstructures of BaWO₄ nanowires have been successfully synthesized in catanionic reverse micelles by using a double-hydrophilic block copolymer as the directing agent. This synthetic method is very simple, mild, and controllable, and it provides a novel method for direct solution-growth of hierarchical nanostructures based on inorganic nanowires. Copyright

Full text of DOI:10.1021/ja029958f

Published on Web 03/05/2003
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Polymer-Directed Synthesis of Penniform BaWO Nanostructures in Reverse
Micelles
Hongtao Shi, Limin Qi,* Jiming Ma, and Humin Cheng
State Key Laboratory for Structural Chemistry of Stable and Unstable Species, College of Chemistry,
Peking UniVersity, Beijing, 100871, P. R. China
Received December 30, 2002; E-mail: liminqi@chem.pku.edu.cn
One-dimensional nanoscale building blocks, such as nanotubes,
nanowires, and nanorods, have attracted intensive interest due to
their importance in fundamental research and potential wide-ranging
applications.1 Many recent efforts have been focused on the
integration of nanorod/nanowire building blocks into two- and three-
dimensional ordered superstructures or complex functional archi-
tectures, which is essential for the success of bottom-up approaches
toward future nanodevices and would offer opportunities to explore
their novel collective optical, magnetic, and electronic properties.4-7
Ordered nanorod arrays have been obtained by the self-assembly
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of preformed uniform nanorods through hydrophobic interaction,
8
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DNA hybridization, and a Langmuir-Blodgett technique. For the
direct growth of ordered nanowire arrays, solid templates, such as
porous alumina10 and patterned catalysts, are usually employed
to control the directional growth. There are also attempts to
synthesize complex architectures based on nanorods. For example,
multiarmed and radially aligned semiconductor nanorods were
fabricated by solution-growth methods. Recently, a variety of novel
hierarchical ZnO nanostructures have been grown by a vapor
transport and condensation technique.13 However, the development
of facile, mild, and effective methods for creating novel architectures
based on nanowires remains a key scientific challenge.
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BaWO with a scheelite structure is an important material in the
electrooptical industry due to its emission of blue luminescence.
Because of its interesting stimulated Raman scattering (SRS)
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properties, BaWO is also a potential material for designing all-
solid-state lasers emitting radiation in a specific spectral region.1
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Figure 1. TEM (a-d) and HRTEM (e) images of penniform BaWO4
1
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nanostructures obtained in the presence of 0.5 g L PEG-b-PMAA after
aging for 8 h. Insets show the corresponding electron diffraction patterns.
Scale bars: (a) 5 µm, (b) 2 µm, (c) 200 nm, (d) 100 nm, and (e) 5 nm.
Recently, we reported a novel synthesis of high aspect-ratio single-
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crystal BaWO nanowires by using catanionic reverse micelles
15
formed by a cationic-anionic surfactant mixture. By employing
a double-hydrophilic block copolymer, poly (ethylene glycol)-block-
poly(methacrylic acid) (PEG-b-PMAA), in this synthesis system,
we have synthesized unique penniform architectures based on
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decane under mild heating. Next, 50 µL of 0.1 mol L Na WO
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solution and 100 µL of 1 g L aqueous PEG-b-PMAA solution
were added with shaking, followed by the addition of 50 µL of 0.1
mol L BaCl and vigorous shaking, giving a polymer concentra-
2
tion of 0.5 g L with respect to the aqueous phase of the reverse
micelles. Finally, the resultant mixture was incubated for 8 h at
50 °C, resulting in the formation of white precipitates.
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BaWO
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nanowires. In recent years, double-hydrophilic block
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1
copolymers have been used as effective crystal growth modifiers
for the controlled crystallization of inorganic particles in aqueous
solutions.16 When the polymer PEG-b-PMAA was combined with
aqueous micelle solutions, unusual inorganic hollow spheres were
easily produced.17 Therefore, it would be interesting to explore the
effect of the polymer on the nanowire synthesis in reverse micelles.
Herein, we report on the facile synthesis of penniform superstruc-
tures of BaWO nanowires directed by the block copolymer in
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reverse micelles, which provides a novel method for direct solution-
growth of hierarchical nanostructures based on inorganic nanowires.
Figure 1 presents the typical transmission electron microscopy
(TEM) images of the obtained penniform BaWO nanostructures
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at different magnifications. The low magnification image in Figure
1a shows that the product exhibits a featherlike appearance with
numerous, nearly parallel barbs grown perpendicular on both sides
of a shaft. In general, the “feathers” are up to 50 µm in length and
ranging from 2.5 to 4.5 µm in width, and the diameters of the shafts
lie in the range of 200-400 nm. Figure 1b shows an enlarged image
of a single feather, which has a shaft ∼230 nm in diameter and
two feathery tufts about 2 µm in length. A high magnification image
of the central part of the feather (Figure 1c) suggests that the slender,
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Penniform BaWO nanostructures were prepared in catanionic
reverse micelles by using the reported procedures15 except for the
addition of the polymer PEG-b-PMAA (PEG ) 3000 g/mol,
PMAA ) 700 g/mol, Th. Goldschmidt AG).16 In a typical synthesis,
0.782 g of a catanionic surfactant mixture formed by equimolar
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parallel barbs are actually BaWO nanowires grown perpendicular
undecyl acid and decylamine was first dissolved in 2.5 mL of
on the shaft. A high magnification image of a feathery tuft of the
3450
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J. AM. CHEM. SOC. 2003, 125, 3450-3451
10.1021/ja029958f CCC: $25.00 © 2003 American Chemical Society

C O MMU N I C A T I O N S
Figure 2. TEM images of penniform BaWO4 nanostructures formed at
earlier stages. Aging time: (a) 0.5 h, (b) 1 h. Scale bars: 200 nm.
Figure 3. TEM images of BaWO4 nanostructures obtained in the presence
of PEG-b-PMAA with varied concentrations after aging for 8 h. [PEG-b-
PMAA]: (a) 0.05 and (b) 0.8 g L . Scale bars: (a) 10 µm, (b) 1 µm.
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feather (Figure 1d) clearly shows the nanowires are uniform in
diameter (∼3.5 nm). The related electron diffraction (ED) pattern
the formation of BaWO shafts and penniform BaWO nanostruc-
tures can be obtained in an appropriate polymer concentration range.
However, it remains unclear how the block copolymer induced the
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is consistent with pure BaWO crystals of a tetragonal scheelite
structure (a ) 0.561 nm, c ) 1.272 nm) and indicates that each
nanowire is a single crystal with the c axis along the length axis,
which is very similar to that for the BaWO nanowires obtained in
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the reverse micelles in the absence of the polymer.15 Both X-ray
diffraction (XRD) and X-ray photoelectron spectroscopy (XPS)
4 4
formation of the BaWO shafts and thus the final penniform BaWO
nanostructures. It is proposed that the presence of the polymer could
induce the formation of superaggregates consisting of primary
micelles and polymer molecules, which provided suitable sites for
analyses evidenced that the product is pure tetragonal BaWO
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the nucleation and growth of shaftlike BaWO
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crystals.
crystals (see Supporting Information). A typical high-resolution
TEM (HRTEM) image shown in Figure 1e confirms that the
BaWO nanowires are single crystals grown preferentially along
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their c axes. As compared to the ED pattern obtained from the edge
of a feathery tuft (Figure 1d), the ED pattern obtained from the
shaft together with a few adjacent nanowires (Figure 1c) exhibits
similar diffraction spots as well as discontinuous rings rather than
In summary, novel penniform superstructures of BaWO
4
nano-
wires have been synthesized in reverse micelles by using a block
copolymer as the directing agent. This synthetic method is very
simple, mild, and controllable, and it provides a novel method for
direct solution-growth of hierarchical nanostructures based on
inorganic nanowires.
Acknowledgment. This work was supported by NSFC
20003001, 20233010), the Special Fund of MOE, China (200020),
and the Doctoral Program Foundation of MOE, China. Hongtao
Shi thanks Prof. Buyao Zhu for helpful instruction and Prof. Nianzu
Wu for the XPS measurement.
full rings, indicating that the shaft itself could consists of BaWO
4
(
polycrystals with their crystallographic [001] axis orientated nearly
perpendicular to the shaft. Therefore, it may be concluded that the
obtained penniform BaWO
BaWO nanowires with a diameter of about 3.5 nm and a
polycrystalline BaWO shaft 200-400 nm in width.
The growth processes of penniform BaWO nanostructures in
reverse micelles were followed by examining the earlier stages of
their formation. As shown in Figure 2a, penniform BaWO
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nanostructures consist of single-crystal
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Supporting Information Available: XRD pattern and XPS spectra
of the penniform BaWO nanostructures (PDF). This material is
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available free of charge via the Internet at http://pubs.acs.org.
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nanostructures with a shaft width of about 220 nm and a barb length
of about 200 nm formed after 0.5 h of aging. At an aging time of
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4
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(
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