General, room-temperature method for the synthesis of isolated as well as arrays of single-crystalline ABO4-type nanorods

Article abstract of DOI:10.1021/ja046331j

Single-crystalline BaWO₄ and BaCrO₄ nanorods of reproducible shape and of varying sizes have been controllably prepared using a simple, room-temperature approach, based on the use of porous alumina template membranes. Aligned BaWO₄ and BaCrO₄ nanorod arrays can be obtained by dissolving the template. Our facile technique, which is analogous to biomineralization, offers a promising and generalized methodology to prepare other types of free-standing ABO₄ nanorods and their corresponding nanorod arrays. Extensive characterization of these samples has been performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), selected area electron diffraction (SAED), Raman spectroscopy, FT-infrared spectroscopy (FT-IR), and X-ray diffraction (XRD).

Full text of DOI:10.1021/ja046331j

Published on Web 11/02/2004
General, Room-Temperature Method for the Synthesis of
Isolated as Well as Arrays of Single-Crystalline ABO4-Type
Nanorods
Yuanbing Mao† and Stanislaus S. Wong*,†,‡
Contribution from the Department of Chemistry, State UniVersity of New York at Stony Brook,
Stony Brook, New York 11794-3400, and Materials and Chemical Sciences Department,
BrookhaVen National Laboratory, Building 480, Upton, New York 11973
Received June 21, 2004; E-mail: sswong@notes.cc.sunysb.edu; sswong@bnl.gov
Abstract: Single-crystalline BaWO4 and BaCrO4 nanorods of reproducible shape and of varying sizes have
been controllably prepared using a simple, room-temperature approach, based on the use of porous alumina
template membranes. Aligned BaWO4 and BaCrO4 nanorod arrays can be obtained by dissolving the
template. Our facile technique, which is analogous to biomineralization, offers a promising and generalized
methodology to prepare other types of free-standing ABO4 nanorods and their corresponding nanorod arrays.
Extensive characterization of these samples has been performed using scanning electron microscopy (SEM),
transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM),
energy-dispersive X-ray spectroscopy (EDS), selected area electron diffraction (SAED), Raman spectros-
copy, FT-infrared spectroscopy (FT-IR), and X-ray diffraction (XRD).
Introduction
goals have been based on principles derived from semicon-
ductor technology, solid-state chemistry, and molecular inor-
Nanoscale structures, such as nanoparticles, nanorods, nano-
wires, nanocubes, and nanotubes, have attracted extensive
synthetic attention as a result of their novel size-dependent prop-
erties.1-7 In particular, one-dimensional (1-D) materials with
their inherent anisotropy are the smallest dimension structures
that can be used for efficient transport of electrons and optical
excitations. As such, they are applicable as building blocks to
assemble the next generation of molecular electronic and com-
putational devices. Part of the challenge of developing practical
nanoscale devices for a variety of applications, including energy
storage, fuel cells, and sensing, is the ability to conveniently
synthesize well-characterizable, single-crystalline nanostructures
in order to rationally exploit their nanoscale optical, electronic,
thermal, and mechanical properties.6,8-10 Ideally, the net result
of nanoscale synthesis is the production of structures that achieve
monodispersity, stability, and crystallinity with a predictable
morphology. Many of the synthetic methods used to attain these
ganic cluster chemistry.
Strategies for the preparation of 1-D nanowires, for example,
rely on the formation from a confined alloy droplet, as described
by the vapor-liquid-solid (VLS) mechanism,3,11 the kinetic
control of growth through the use of capping reagents,12,13 the
generation through a low temperature, chimie douce solution
chemical methodology,14,15 and the use of template-inspired
methodologies.16,17 Metal oxides, in particular, represent one
of the most diverse classes of materials, with important structure-
related properties, including superconductivity, ferroelectricity,
magnetism, conductivity, and gas sensing capabilities.18,19 It is
evident that these types of 1-D oxide nanostructures will lead
to key applications in electronics and in the construction of the
next generation of electronic and computational devices.
Considerable research efforts have been undertaken to synthesize
single-crystalline nanomaterials with high purity in large quanti-
ties.1,20,21 However, there are still no generalizable guidelines
for their straightforward synthesis with specific chemical and
† Department of Chemistry, State University of New York at Stony
Brook.
‡ Materials and Chemical Sciences Department, Brookhaven National
Laboratory.
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Mao and Wong
morphological composition as well as tailor-made sizes and
aspect ratios.1,20,21
been used as an oxidizing agent and as a catalyst for enhancing
vapor-phase oxidation reactions.39 Moreover, due to its excellent
photophysical and photocatalytic properties, barium chromate
is a highly efficient photocatalyst, with a particularly marked
response to visible light irradiation.40
Whereas oxide nanostructures have been synthesized by
heating and calcination of precursors,7 reversed micelle tem-
plating techniques,22,23 sol-gel processes,24 surfactant-mediated
steps,25 and hydrothermal procedures,26 in this manuscript, we
have relied on a modified template synthesis,27-29 a relatively
simple and versatile method to prepare size-controlled nano-
structures.30 This methodology involves the synthesis of the
desired material within the pores of template membranes. The
chemical and physical properties of these membranes (i.e., pore
geometry and monodisperse diameters) enable a high degree
of control over the dimensions of the resulting 1-D nanostruc-
tures. Another useful feature of this technique is that it is
extremely generalizable with respect to the types of materials
that can be synthesized. Indeed, nanotubes and nanofibrils of
conductive polymers, metals, semiconductors, carbon, and other
types of materials have been prepared within the confined
cylindrical pores of membranes.31-33 Moreover, these tubular
or fibrillar nanostructures can be assembled into a wide variety
of different architectures. For instance, if a nanostructure-con-
taining membrane is attached to a substrate and the membrane
is removed, an ordered assembly of micro- or nanostructures,
protruding out from the surface of the substrate, can be obtained,
as we have demonstrated in this work.
Though many types of templates exist for the synthesis of
1-D nanostructures, porous anodic alumina membranes are an
ideal choice for templating because of their high pore density,
parallel and straight channels, distribution of cylindrical pores
of highly uniform diameter arranged in a hexagonal array, and
size tunability of ca. 5 to 300 nm. Moreover, these templates
are thermally and mechanically stable, with readily achievable
pore densities as high as 1011 pores/cm2.31-33
In this manuscript, we have chosen to focus on the fabrication
of nanorods of BaWO4 and BaCrO4, which are compositionally
and structurally representative of the ABO4 class of metal
oxides, where A and B are two different metallic elements with
oxidation states of +2 and +6, respectively.34,35 Barium
tungstate, BaWO4 (also called Barite), for instance, is important
in the electrooptical industry due to its emission of blue
luminescence, ascribed to the influence of the Jahn-Teller effect
on the degenerated excited state of the (WO4)2- tetrahedral
structure. In addition, its interesting thermoluminescence and
stimulated Raman scattering (SRS) properties render barium
tungstate as a candidate for the design of solid-state lasers that
can emit radiation within a specific spectral region. As such,
these materials are of use for medical laser treatment applica-
tions, up-conversion fiber lasers, and analogous spectroscopic
functions.36-38 Barium chromate, BaCrO4 (also called Hashemite),
is a naturally occurring chromate analogue of Barite. It has often
Recently, remarkable progress has been achieved, regarding
the preparation of low-dimensional nanoscale metal tungstate,
chromate, and sulfate derivatives. The synthesis of ordered
microarrays of nanocrystals of both barium chromate and sulfate,
with controlled chemical composition and size distribution, was
reported using a reversed micelle templating method.41 This
methodology was subsequently extended to generate barium
tungstate nanorods; the 2-dimensional organization of these
BaWO4 and BaCrO4 nanorods at the water-air interface was
accomplished using a Langmuir-Blodgett technique.23,42 Re-
cently, high aspect-ratio, single-crystalline BaWO4 and BaCrO4
nanowires with diameters as small as 3.5 nm and lengths up to
more than 50 microns were synthesized in catanionic reverse
micelles formed by an equimolar mixture of two surfactants:
undecylic acid and decylamine.43,44 With the further use of
double-hydrophilic block copolymers as effective crystal growth
modifiers, morphological variants, namely penniform BaWO4
nanostructures, could be prepared using this technique.45 Using
an analogous idea, different BaCrO4 nanostructures have been
processed through a polymer-directed synthesis.46-48 However,
the development of facile, mild, and effective approaches for
creating size-controlled 1-D nanostructures and their associated
novel architectures remains a significant scientific challenge.
In this work, we have used a modified template synthesis
technique, originally developed for the synthesis of organic
microtubules, to successfully prepare free-standing single-
crystalline BaWO4 and BaCrO4 nanorods and their arrays by
physically placing two different precursor solutions in two halves
of a U-tube cell, respectively, separated by alumina template
membranes.49,50 In other words, we use the pores in alumina
membranes as the environment in which to control the growth
of well-defined morphologies of single crystals of our ABO4
nanostructures. The membranes used are thin, and are mounted
in a double-diffusion setup, which enables the continuous flow
of ions into the membrane pores and thus, the production of
single crystals of ABO4 nanocrystalline materials.
What is significant is that most nanostructures previously
produced by conventional templating procedures are polycrys-
talline,1 despite the variety of different deposition strategies used,
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Single-Crystalline ABO4-Type Nanorods
A R T I C L E S
including electrochemical deposition, electroless deposition,
polymerization, sol-gel deposition, and layer-by-layer deposi-
tion in nanoporous templates. The reason for the observed
polycrystallinity is that many of these prior methodologies
require additional annealing steps at high temperature.29-31,33
By contrast, as we will discuss later, the formation mechanism
of nanorods under our experimental conditions is analogous to
a biomimetic crystallization process.51 That is, the growth of
our nanorods within the confinement of alumina membranes is
analogous to the precipitation of single crystals of calcium
carbonate and calcium phosphate within the confinement offered
by gels, micelles, chitin scaffolds, and collagen matrixes.52-55
Hence, nucleation and growth of single-crystalline nanomaterials
occur essentially instantaneously through the direct chemical
interaction between ions of the two different precursor solutions
in contact with each other. The membrane acts to spatially direct
crystal growth.
Moreover, at the same time, our simplistic technique allows
for the reproducible fabrication of ordered, monodisperse 3-D
arrays of these 1-D nanomaterials. This is critical, because
assembly of nanoscale components (i.e., nanorods) is a key step
toward building functional devices,56 important for applications
including nanoscale electronics and molecular sensing. Specif-
ically, the fabrication of 3D arrays of 1-D nanomaterials, such
as nanorods, is useful for optoelectronic applications, such as
room-temperature ultraviolet lasing.57 Though a number of
preparative methods have been reported for generating these
types of nanoscale architectures,57-61 none of these techniques
appears to work for ABO4-type compound systems, with the
exception of the current work. Furthermore, although we pri-
marily focus on isolated 100 and 200 nm-sized BaWO4 and
BaCrO4 nanorods and their associated arrays, we have noted
that we can reproducibly form different sizes of these various
nanoscale architectures using alumina template membranes of
varying pore sizes.
Figure 1. Schematic of setup used to synthesize ABO4 nanostructures,
such as BaWO4 and BaCrO4 nanorods.
with deionized water. The alumina membrane was subsequently
dissolved by immersion in 1 M NaOH for about 30 min. Figure 1
schematically illustrates the experimental setup utilized.
The nanorods, which were characterized by means of X-ray
diffraction (XRD), scanning electron microscopy (SEM), transmission
electron microscopy (TEM), high resolution TEM (HRTEM), selected
area electron diffraction (SAED), energy-dispersive X-ray spectroscopy
(EDS), Raman and FT-IR analyses, were collected by centrifugation.
These were then washed with deionized water until the pH of the
product suspensions attained approximately a value of 7. To prepare
SEM samples of the corresponding BaWO4 and BaCrO4 nanorod arrays,
a previously developed procedure was utilized.29 Briefly, alumina
membranes containing BaWO4 and BaCrO4 nanorods were glued (using
Epoxy resin) to a piece of paper towel. Resulting composites were
immersed into a 1 M NaOH aqueous solution for ∼0.5 h to dissolve
the alumina. After washing steps with distilled, deionized water
followed by air-drying, samples were subsequently mounted onto SEM
stubs for imaging.
Materials Characterization. Products were characterized by a
number of different methodologies, including XRD, SEM, TEM,
HRTEM, SAED, and EDS as well as with Raman and FT-IR
spectroscopies.
X-ray Diffraction. Crystallographic and purity information on the
as-prepared barium tungstate and barium chromate nanostructure
samples were obtained using powder XRD. To generate these samples,
the as-prepared BaWO4 and BaCrO4 products, upon centrifugation, were
subsequently sonicated for about 1 min, and later air-dried upon
deposition onto glass slides. The diffraction patterns were collected
using a Scintag diffractometer, operating in the Bragg configuration
using Cu KR radiation (λ ) 1.54 Å) from 10 to 80° at a scanning rate
of 2° per minute.
Experimental Section
Materials Preparation. Alumina template membranes (Fisher
Scientific) with pore sizes of 100 and 200 nm were prepared by a two-
step aluminum anodic oxidation process.31,33 The membrane was initially
hydrated by immersion and sonication in a small volume of distilled,
deionized water for a few minutes, so as to avoid air bubble formation
within its structure or on its surface. Subsequently, the membrane was
mounted between the two halves of a U-tube cell. The half-cells were
then filled with equimolar solutions of Ba(NO3)2 and either Na2WO4
or Na2CrO4 solutions to generate barium tungstate and barium chromate,
respectively. After immersion times of up to 12 h at room temperature,
the alumina template, into which the precursors had presumably diffused
resulting in product formation, was detached and thoroughly washed
(51) Dorozhkin, S. V.; Dorozhkina, E. I.; Epple, M. Cryst. Growth Des. 2004,
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2003, 629, 2305.
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Electron Microscopy. The size and morphology of the resulting
BaWO4 and BaCrO4 products were initially characterized using a field
emission SEM (FE-SEM Leo 1550) at accelerating voltages of 15 kV.
Specifically, solid BaWO4 and BaCrO4 samples of isolated nanorods
and their associated arrays immobilized onto Epoxy resin were mounted
onto conductive carbon tapes, which were attached to the surfaces of
SEM brass stubs. These samples were then conductively coated with
gold by sputtering for 15 s to minimize charging effects under SEM
imaging.
Low magnification TEM images were taken at an accelerating
voltage of 120 kV on a Philip CM12 instrument, equipped with EDS
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Figure 2. XRD patterns arising from (A) JCPDS no. 72-0746 database
standard (BaWO4); (B) as-prepared BaWO4 nanorod sample; (C) JCPDS
no. 78-1401 database standard (BaCrO4); and (D) as-prepared BaCrO4
nanorod sample.
Figure 3. (A) Typical SEM micrograph of BaWO4 nanorods. (B) Rep-
resentative TEM image of BaWO4 nanorods. (C) Typical SEM micro-
graph of BaCrO4 nanorods. (D) Representative TEM image of BaCrO4
nanorods.
capabilities. HRTEM images and SAED patterns as well as EDS data
were obtained on a JEOL 2010F HRTEM (equipped with an Oxford
INCA EDS system and subsequent potential of performing SAED) at
an accelerating voltage of 200 kV to further characterize individual
BaWO4 and BaCrO4 nanorods. Specimens for all of these TEM studies
were prepared by depositing a drop of these aqueous suspension samples
onto a 300 mesh Cu grid, coated with a lacey carbon film. Prior to
deposition, solutions containing samples of BaWO4 and BaCrO4
nanorods, were sonicated for 2 min to ensure adequate dispersion of
the nanorods.
Optical Spectroscopy. Samples for Raman spectroscopy were
analyzed on a Kaiser micro-Raman instrument, with a 200-micron
confocal aperture, at a laser excitation of 752.5 nm using a power level
of 5 mW. Infrared spectra were obtained, using a ThermoNicolet Nexus
670 using a ZnSe single reflectance ATR accessory.
Results and Discussion
X-ray Diffraction. As-prepared BaWO4 and BaCrO4 nano-
rod samples were examined by powder XRD measurements
on a Scintag diffractometer with Cu KR radiation (Figure
2). Diffraction peaks in Figure 2B can be indexed to a
scheelite-structured, tetragonal phase BaWO4 with calculated
cell constants of a ) 5.610 Å and c ) 12.711 Å, which are
numerically close to the reported values of the bulk materials
(Figure 2A, JCPDS File No. 72-0746). By analogy, diffrac-
tion peaks in Figure 2D can be indexed to a hashemite-
structured, orthorhombic phase BaCrO4 with calculated cell
constants of a ) 9.105 Å, b ) 5.541 Å, and c ) 7.343 Å,
which are consistent with the literature data (Figure 2C, JCPDS
File No. 78-1401).
Electron Microscopy. The morphology of the as-synthesized
BaWO4 and BaCrO4 nanorod samples was obtained using
FE-SEM and low magnification TEM. Typical SEM and TEM
images of BaWO4 nanorods are depicted in Figure 3A and 3B,
respectively. On the basis of these data, it can be observed that
straight, smooth, and crystalline wire-like BaWO4 nanorods,
with a uniform diameter throughout their entire length, were
produced. Diameters of BaWO4 nanorods obtained were ∼200
( 25 nm (25 different nanorods), based on the 200 nm pore
sizes of the alumina membranes used in the synthesis. The
shapes of these nanorods clearly replicated the pore structure
of the corresponding alumina templates. The lengths of the
BaWO4 nanorods were as much as several microns.
Figure 4. (A) TEM image of a single BaWO4 nanorod. (B) EDS of as-
prepared BaWO4 nanorods. The Cu peaks originate from the TEM grid.
SAED pattern (C) and HRTEM image (D) of a representative portion of a
BaWO4 nanorod, as delineated by the red square, in (A).
Figure 3, parts C and D, shows representative SEM and TEM
images of BaCrO4 nanorods, respectively. It can be observed
that smooth and crystalline wire-like BaCrO4 nanorods were
produced. Diameters of the BaCrO4 nanorods obtained were in
the range of 100 ( 15 nm (25 different nanorods), based upon
the 100 nm pore size of the alumina membranes used as the
templates. BaCrO4 nanorods synthesized reached an aspect ratio
of up to 15.
To provide further insight into the structure of the as-prepared
BaWO4 and BaCrO4 nanostructures, individual nanorods were
analyzed by HRTEM as well as using SAED and EDS. An
image of a randomly chosen single BaWO4 nanorod is shown
in Figure 4A. To confirm the chemical composition of the as-
prepared nanorods, EDS spectra (Figure 4B) were recorded at
a number of different positions along a single nanorod. The
chemical signatures obtained are identical within experimental
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A R T I C L E S
Figure 6. SEM images of as-prepared BaWO4 nanorod arrays: (A) top-
view; (B) tilt-view. Analogous images of as-prepared BaCrO4 nanorod
arrays: (C) top-view; (D) tilt-view. The less-than-optimal packing of BaCrO4
nanomaterials arises from the inhomogeneous distribution of the as-formed
individual nanorods.
Figure 5. (A) TEM image of a single BaCrO4 nanorod. (B) EDS of as-
prepared BaCrO4 nanorods. The Cu peaks originate from the TEM grid.
SAED pattern (C) and HRTEM image (D) of a representative portion of a
BaCrO4 nanorod, as defined by the red square, in (A).
and {102} planes, respectively, of an orthorhombic-phase
BaCrO4 crystal (JCPDS card no. 78-1401).
Figure 6 illustrates the top and tilt views of the SEM images
of BaWO4 and BaCrO4 nanorod arrays, respectively, grown
within the alumina templates. The SEMs show that the
nanorods are individually separated from each other. None-
theless, they form a dense, continuous network, where the
nanorods are roughly parallel to each other and vertically
oriented on the substrates to form a packed array, stretching
over micron-sized areas. This is especially true of BaWO4. The
diameters of these nanorods are about 101 ( 8 nm (50 dif-
ferent nanorods), corresponding to the diameter of channels in
the alumina template. What is important to note is that these
arrays of ABO4-type nanorods were produced very readily at
room temperature without the need for sophisticated experi-
mental setups57 (characteristic of arrays formed by vapor
transport via VLS-type methodologies), or for high-temperature
annealing (associated with sol-gel template approaches).29,30,33
Moreover, the sizes of these BaWO4 and BaCrO4 nanorods
could be readily varied by choosing membranes, possessing
different pore sizes.
To investigate the thermal stability of the as-prepared nano-
rods, samples of BaWO4 and BaCrO4 nanorods were further
heated in air for 5 h at 650 °C. Figure 7 reveals that their mor-
phologies and crystallographic orientations remain essentially
unaltered after this annealing step. It is noteworthy that the
thickness of the surface amorphous layer on the nanorods is
somewhat thinner than that of the as-prepared samples. Hence,
samples of BaWO4 and BaCrO4 nanorods should be suitable
for applications at elevated temperatures, such as high-temper-
ature catalysis.
accuracy, and essentially only Ba, W, and O elements are
observed from the nanorod structure. The Cu signal arises from
the TEM grid.
Figure 4C presents the SAED pattern taken from a portion
of a single BaWO4 nanorod shown in Figure 4A. We note that
the electron diffraction patterns obtained from different positions
along individual nanorods are essentially identical within
experimental accuracy (Figure S1). Whereas template methods
typically yield polycrystalline nanostructures,1 the presence of
the sharp diffraction spots rather than an amorphous ring is
suggestive of the predicted formation of single-crystalline
BaWO4. The associated electron diffraction pattern is consistent
with pure BaWO4 crystals of a tetragonal scheelite structure,
indexed as shown in Figure 4C. In Figure 4D, a HRTEM image
of a portion of an individual BaWO4 nanorod, shown in Figure
4A, is displayed. While a thin amorphous layer likely coats the
outer BaWO4 nanorod surface, the HRTEM image further
confirms that the observed BaWO4 nanorods are single crystal-
line with no defects or dislocations. The 2-D lattice fringes
reveal that the single-crystalline BaWO4 nanorods possess
interplanar spacings of about 5.09 and 3.33 Å, corresponding
to the (101) and (112) planes, respectively.
Figure 5A presents an individual BaCrO4 nanorod. EDS
elemental analysis data (Figure 5B) confirm the presence of
Ba, Cr, and O elemental signatures associated with the product
in the expected stoichiometric proportions. The SAED pattern
taken from a single BaCrO4 nanorod (Figure 5C and Figure
S2) yields sharp diffraction spots rather than rings, which are
suggestive of the presence of single-crystalline BaCrO4. The
related electron diffraction pattern is consistent with pure
BaCrO4 crystals of an orthorhombic hashemite structure, indexed
as shown in Figure 5C. Figure 5D presents a HRTEM image
of a portion of an individual BaCrO4 nanorod, initially described
in Figure 5A. This image reveals that the BaCrO4 nanorod is
single-crystalline with lattice fringe spacings of ∼4.58 and 3.36
Å, respectively. These two planes can be indexed as the {200}
Optical Spectroscopy. Optical properties of our synthesized
nanorods were also investigated. Figure 8 shows Raman spectra
of the as-prepared products and the corresponding bulk com-
mercially available samples (Aldrich) of BaWO4 and BaCrO4,
corroborating the chemical identity of our nanostructures.
Spectra of the as-prepared nanorods (Figure 8B and 8D) show
typical Raman bands, which can be attributed to crystalline
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Figure 9. FT-IR spectra of (A) bulk commercial, crystalline BaWO4
powder; (B) as-prepared single-crystalline BaWO4 nanorods; (C) bulk
commercial, crystalline BaCrO4 powder; and (D) as-prepared single-
crystalline BaCrO4 nanorods.
over, bands at 306 and 72 cm-1 can be assigned to the lattice
vibrations of BaCrO4 nanorods.
In the FT-IR spectra (Figure 9), the strong band at 802 cm-1
and its shoulder (Figure 9B) appear to result from the Eu and
Au components, respectively, of the υ3 band of BaWO4 nanorod
sample. In addition, one υ1 band at 852 cm-1 and three
additional υ3 bands at 868, 885, and 943 cm-1, respectively,
have been observed, associated with the BaCrO4 nanorod sample
(Figure 9D).62,63
Figure 7. BaWO4 nanorods upon annealing in air for 5 h at 650 °C. (A)
TEM image of a single BaWO4 nanorod. (B) EDS of the BaWO4 nano-
rod, shown in (A). Cu peaks originate from the TEM grid. SAED pat-
tern (C) and HRTEM image (D) from a portion of a BaWO4 nanorod, as
delineated by the red square in (A).
Considering the imperfect correlation between the IR spec-
tra of the bulk solid (Figure 9A, C) vs nanorod samples (Fig-
ure 9B, D), it turns out that this type of phenomenon has
been observed previously. First of all, in tungstate and molyb-
date systems,62 for some of the IR-active vibrations, the
frequency difference between the longitudinal optic and the
transversal optic modes is rather large, and the location of the
‘absorption’ maximum of these samples often depends on the
granular nature of the sample. Indeed, for the high-frequency
band at around 820 cm-1, the situation is further compli-
cated by the overlapping in the same spectral region of 2
IR-active components of υ3. Second, there is a larger degree
of surface strain, associated with the smaller nanocrystal; this
effect may be related to changes in coupling to lattice vibra-
tional modes. Third, the blue shift in the infrared absorption
peaks of our chromate and tungstate nanocrystals as compared
with their bulk counterparts may be attributed to quantum
confinement effects. Similar behavioral trends have been
observed for a number of other systems including CaCO3, InSb,
and VO2.64-67
Figure 8. Raman spectra of (A) bulk commercial, crystalline BaWO4
powder; (B) as-prepared single-crystalline BaWO4 nanorods; (C) bulk
commercial, crystalline BaCrO4 powder; and (D) as-prepared single-
crystalline BaCrO4 nanorods.
BaWO4 and BaCrO4, respectively.62,63 According to the reported
literature, the vibrational modes of isolated BO4 tetrahedra can
be separated into internal, external translational, and rotational
modes. As shown in Figure 8B, bands at 927, 833, 797, 347,
and 334 cm-1 can be assigned to the υ1(Ag), υ3(Bg), υ3(Eg),
υ4(Bg), and υ2(Bg) modes, respectively, of the as-prepared
BaWO4 nanorods. Peaks located at 197 and 159 cm-1, as well
as at 106 and 78 cm-1, can also be assigned to their rotational
(Ag, Eg) and translational modes (Eg, Bg), respectively. In Figure
8D, peaks at positions corresponding to 909/906, 864, 432/406,
and 361/352 cm-1 could be attributed to the υ3, Ag, υ4, and υ2
modes, respectively, of as-prepared BaCrO4 nanorods. More-
The results of Raman and IR data further corroborate the other
data collected, confirming the identity of our as-prepared
nanomaterials as indeed BaWO4 and BaCrO4, respectively. The
generation of these materials provides the basis for a more
thorough future investigation of their novel optical, optoelec-
tronic, and catalytic properties for possible incorporation into
nanoscale devices. We believe that the lasing and photocatalytic
properties of these materials will be particularly intriguing.
(64) Zhu, K.; Shi, J.; Zhang, L. Solid State Commun. 1998, 107, 79.
(65) Tarte, P.; Liegeois-Duyckaerts, M. Spectrochim. Acta 1972, 28A, 2029.
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274.
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34, 533.
(63) Scheuermann, W.; Schutte, C. J. H. J. Raman Spectrosc. 1973, 1, 605.
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Single-Crystalline ABO4-Type Nanorods
A R T I C L E S
Plausible Formation Mechanism
As described previously, in the current study, we use the pores
of alumina membranes as the physical environment in which
to control the growth of well-defined morphologies of single
crystals of our ABO4 structures. The membranes used are thin,
and are mounted in a double-diffusion setup, which enables the
continuous flow of ions into the membrane pores and thus, the
production of single crystals of ABO4 nanocrystalline materials.
Based on our experimental results, we propose that single
crystals of nanoscale ABO4 materials derive from isolated,
disparate nucleation sites (consisting of ABO4 nuclei generated
via the reaction between A2+ and BO42-), which then grow by
extension through the porous network. Continued growth then
occurs at the particle surface at a rate limited by ion availability,
until the crystal impinges on the alumina template surface, which
ultimately limits further growth. Hence, by the spatially
constrained volume defined by the presence of the template itself
(Figure S3), nanorods instead of nanoparticles are formed due
to the growth environment conducive to these 1-dimensional
structures.
We note that we are not forming hollow nanotubes because
the interactions between the reagent molecules are likely stronger
than those between the reagent molecules and the pore walls.
In the case of polymeric microtubules formed in templates,
nascent polymer chains initially adsorb to the pore walls yielding
a thin polymer ‘skin’, that becomes thicker with time until
quenching with water.49 We do not believe this image accurately
reflects the growth mechanism of our system, which is ef-
fectively akin to biomineralization.
In the current study, the growth mechanism involves a double
diffusion crystallization setup process,68-70 comprising two
reservoirs of solutions that each contains a part of an insoluble
salt (ABO4), i.e., A2+ on one side and BO42- on the other side.
To prevent overly rapid mixing, the solutions are separated by
an alumina membrane that slows down diffusion and rate of
crystallization. However, when the two solutions do meet,
precipitation occurs, depleting the availability of ions in the local
environment of the growing crystal within the template. The
fact that single nanorods can grow is suggestive of the fact that
nucleation within the membrane itself is likely limited, an effect
promoted by the relatively inert surface of the membrane. The
nucleation rate is primarily dictated by the supersaturation of
the solution. Further growth of the nanorods is limited by
diffusion of ions in this localized region.
The shape of the alumina membranes has a crucial effect on
the morphology of the ABO4 phase that subsequently forms.
In fact, we note instances where the morphology of the ABO4
nanorods spatially maps out the interior nanoscopic profile and
localized contours of the internal pores of the alumina mem-
branes. That is, whereas the majority of as-prepared BaWO4
and BaCrO4 nanorods were straight and possessed smooth
surfaces, a small number of BaWO4 and BaCrO4 nanorods with
protrusions or depressions on their surfaces were also produced,
reflecting the morphology and inner surface roughness of the
pores from whence they were formed. Figures 10A and 10D
show examples of a hump-like protrusion from an ∼200 nm
BaWO4 nanorod as well as a pit-like depression from an ∼100
Figure 10. (A) TEM image of a single BaWO4 nanorod containing a
protrusion. (B and C) HRTEM images corresponding to different sections,
defined by the blue and green circles, respectively, of the single BaWO4
nanorod, shown in (A). (D) TEM image of a single BaCrO4 nanorod
containing a notched depression. (E) HRTEM view of the portion of the
single BaCrO4 nanorod, highlighted by a red circle in (D).
nm BaCrO4 nanorod, respectively, likely arising from imperfec-
tions within the alumina pores. It was evident that even in these
regions, the nanorods remained pure and single-crystalline,
clearly preserving their 2-D lattice fringes (i.e., Figure 10B, C,
and E).
Conclusions
The current work demonstrates the room-temperature prepa-
ration using a novel, modified template-assisted methodology
of single-crystalline BaWO4 nanorods and BaCrO4 nanorods
with different controllable sizes as well as the creation of arrays
of these nanorods in the pores of an alumina membrane at room
temperature. The resulting nanorods and the arrays have been
extensively characterized using a variety of microscopy and
spectroscopy results. Because of the simplicity and generaliz-
ability of the approach used, it is anticipated that this methodol-
ogy can be generalized to the synthesis of other important
tungstate, molybdate, chromate, sulfate, iodate, and selenate
systems at the nanoscale, such as SrWO4, BaMoO4, ZnWO4,
BaSO4, and BaIO4 as well as Ba1-xSrxWO4. Moreover, we can
likely prepare single-crystalline nanorods of smaller diameters
(to a certain extent) using smaller diameter templates.
In addition, due to the ease of preparation and the thermal
stability of the nanorods that are now available, our newly
established synthetic method could become a valuable starting
point for generating functional nanoscale devices. In particular,
creation of arrays of nanorods presents an interesting illustration
(68) Park, R. J.; Meldrum, F. C. AdV. Mater. 2002, 14, 1167.
(69) Park, R. J.; Meldrum, F. C. J. Mater. Chem. 2004, 14, 2291.
(70) Peters, F.; Epple, M. J. Chem. Soc., Dalton Trans. 2001, (24), 3585.
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of the idea of bottom-up assembly using nanoscale building
blocks. Future research will focus on more extensive investiga-
tions of the physical (e.g., optoelectronic and photocatalytic)
properties of individual nanorods and nanorod arrays.
ment, Boston College) and Dr. James Quinn (Materials Science
Department, SUNY at Stony Brook) for their help with HRTEM
and SEM/TEM analyses, respectively. The assistance of Dr.
Alasdair Bell and of the late Mr. Rudolph Schlott (Chemistry
Department, SUNY at Stony Brook) with Raman analyses and
U-tube glass fabrication, respectively, has been appreciated.
Sarbajit Banerjee and Tirandai Hemraj-Benny (Chemistry
Department, SUNY at Stony Brook) are acknowledged for their
help with the FT-IR analysis.
Acknowledgment. We acknowledge support of this work
through startup funds provided by the State University of New
York at Stony Brook as well as Brookhaven National Labora-
tory. In addition, research was supported in part by the U.S.
Department of Energy Office of Basic Energy Sciences under
Contract DE-AC02-98CH10886. Acknowledgment is also made
to the National Science Foundation for a CAREER award
(DMR-0348239), and the donors of the Petroleum Research
Fund administered by the American Chemical Society, for
support of this research. Y.M. recognizes Sigma Xi for a Grant-
in-Aid of Research. S.S.W. thanks 3M for a nontenured faculty
award. We also thank Professor Jianyu Huang (Physics Depart-
Supporting Information Available: Additional selected area
electron diffraction patterns are presented, associated with
Figures 4 and 5 of the main text. A SEM image of the cross-
section of a 200 nm pore-sized alumina template, containing
BaWO4 nanorods is also shown. This material is available free
of charge via the Internet at http://pubs.acs.org.
JA046331J
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