Novel shape evolution of BaMoO4 microcrystals

Article abstract of DOI:10.1021/jp0634205

Dendritic BaMoO₄ microcrystals with lengths of about 5-15 μm were synthesized simply under ambient conditions by a microemulsion-mediated method within an ultrashort time. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and field-emission scanning electron microscopy (FESEM), which showed that the products were in pure tetragonal BaMoO₄ structure and that an individual dendrite had a long central stem with four array shrunken branches. Detailed studies revealed that the formation of these 3-D structures was strongly dependent on the composition of the microemulsion. At prolonged aging time, the dendrites evolved into rods and further into particles, driven by the lattice distortion energy required to evolve the crystal from a metastable to a stable state. This novel crystal shape evolution provides insight into crystallization behavior given that the growth history and shape evolution process have traditionally been poorly understood.

Full text of DOI:10.1021/jp0634205

J. Phys. Chem. B 2006, 110, 19295-19299
19295
Novel Shape Evolution of BaMoO4 Microcrystals
†
,†
†
†
†
Qiang Gong, Xuefeng Qian,* Hongliang Cao, Weimin Du, Xiaodong Ma, and
Maosong Mo‡
School of Chemistry and Chemical Technology, Shanghai Jiao Tong UniVersity, Shanghai 200240, P. R. China,
and Centre for AdVanced Materials Technology (CAMT), School of Aerospace, Mechanical and Mechatronic
Engineering J07, UniVersity of Sydney, Sydney, New South Wales 2006, Australia
ReceiVed: June 2, 2006; In Final Form: August 3, 2006
Dendritic BaMoO microcrystals with lengths of about 5-15 µm were synthesized simply under ambient
4
conditions by a microemulsion-mediated method within an ultrashort time. The products were characterized
by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and field-emission scanning
4
electron microscopy (FESEM), which showed that the products were in pure tetragonal BaMoO structure
and that an individual dendrite had a long central stem with four array shrunken branches. Detailed studies
revealed that the formation of these 3-D structures was strongly dependent on the composition of the
microemulsion. At prolonged aging time, the dendrites evolved into rods and further into particles, driven by
the lattice distortion energy required to evolve the crystal from a metastable to a stable state. This novel
crystal shape evolution provides insight into crystallization behavior given that the growth history and shape
evolution process have traditionally been poorly understood.
Introduction
In their synthesis, high temperatures or long reaction times are
necessary for the formation of 3-D hierarchical structures.
The fabrication of nano- to microscale inorganic materials
with special size and morphology is of great interest for the
study of material chemistry because of the importance in basic
scientific research and potential technological applications of
Presently, our group has prepared CaMoO4 and SrMoO4
mesocrystals via a microemulsion-mediated route.17 In this
article, we describe the successful preparation of hierarchical
dendrites of BaMoO4 by a similar method at room temperature
in an ultrashort reaction time, as well as the observation of shape
evolution from dendrites to rods and further to particles with
different aging times. To the best of our knowledge, these
peculiar shape evolutions of crystals from 3-D to 1-D and 0-D
structures have not been reported by previous researchers.
Therefore, studies on the shape control and evolution of
BaMoO4 crystals will, to some extent, provide insight into
crystallization behavior given that the growth history and shape
evolution process have traditionally been poorly understood.
1
-3
such materials.
In the past few years, considerable efforts
have been focused on the synthesis of inorganic crystals with
various morphologies, such as 0-D (particles and polyhedra),
1
3
-D (rods, wires, belts, and tubes), 2-D (disks and plates), and
-D (multipods, snowflakes, hyperbranches, and dendrites)
structures. Of these abundant shapes, 3-D hierarchical dendrites
have attracted much attention because of the practical impor-
tance related to some fractal growth phenomena. Previous
studies on dendritic crystals have mostly been focused on
specific materials and crystal shapes, limiting the general
understanding of the nanocrystal shape evolution process and
the possibility of exploiting their growth mechanisms for other
materials and geometries. In most cases, the formation of 3-D
hierarchical dendrites resulted from the evolution of 0-D or 1-D
primary crystals via an oriented attachment process or a self-
assembly process induced by the use of selective surfactants,
which lead to the production of anisotropic shapes arising from
the blocking or retarding of growth along one or more directions
through selective adhesion.4
Experimental Section
Analytical-grade Na MoO ‚2H O, BaCl ‚2H O, n-octane,
2
4
2
2
2
n-butanol, and cetyltrimethylammonium bromide (CTAB) were
purchased from Shanghai Chemical Industrial Company and
were used without further purification. The reaction was carried
out in a 50-mL glass jar (reaction temperatures lower than 100
°C) or in a 50-mL Teflon-lined stainless steel autoclave (reaction
temperatures above 100 °C).
-13
BaMoO4 is one of the most important inorganic materials
from among the metal molybdate families, which have high
potential applications in various fields, such as photolumines-
cence and hosts for lanthanide-activated lasers.14 BaMoO4 has
usually been prepared by conventional methods including
repeated ball-milling and solid-state reaction. Recently, nano-
or microscale BaMoO4 crystals have been obtained by methods
The synthesis of BaMoO4 microcrystals was achieved simply,
via a microemulsion-mediated route. In a typical procedure, two
microemulsion solutions were prepared by adding 1.96 mL of
0.2 M BaCl2 and 1.96 mL of 0.2 M Na2MoO4 aqueous solutions
to separate n-octane/CTAB/n-butanol systems (molar ratio of
water to surfactant ω ) 33, molar ratio of n-butanol to CTAB
p ) 3.60, and [CTAB] ) 0.2 M), which were stirred for 30
min until they became transparent. Then, the two solutions were
mixed rapidly under vigorous stirring and aged at room
temperature (25 °C) for different times (from 1 min to 150 h).
The white products were separated by centrifugation, washed
with deionized water and absolute ethanol several times, and
15,16
using a polymerized complex or catanionic reverse micelles.
*
To whom correspondence should be addressed. E-mail: xfqian@
sjtu.edu.cn. Phone: +86-21-54743262. Fax: +86-21-54741297.
†
‡
Shanghai Jiao Tong University.
University of Sydney.
1
0.1021/jp0634205 CCC: $33.50 © 2006 American Chemical Society
Published on Web 09/02/2006

19296 J. Phys. Chem. B, Vol. 110, No. 39, 2006
Gong et al.
emulsions were mixed. When the aging time was less than 1
min, tetragonal BaMoO4 with 3-D microstructures could be
easily obtained. The FESEM image in Figure 2a clearly shows
that dendritic architectures with lengths of about 5-15 µm along
the stem were the exclusive morphology in the product. Further
observation revealed that the individual BaMoO4 dendrites were
composed of a long central stem with four shrunken branches.
The branches were perpendicular to the stem and were
composed of parallel-arrayed particles with lengths of about 50-
3
4
00 nm, as can be seen. When the aging time was increased to
h, the dendrites of BaMoO4 disappeared and evolved to
microrods, which were about 2-10 µm in length and 200-
00 nm in diameter. Interestingly, the BaMoO4 microrods were
4
not straight but rather were similar to bamboo, with concavo-
convex surfaces. Furthermore, the microrods were not the
exclusive morphology in the products, as a small number of
nanoparticles with sizes in the range of 100-300 nm were also
found (Figure 2b). If the aging time was increased to 24 h, a
large quantity of BaMoO4 microrods with lengths of less than
Figure 1. XRD pattern of a sample formed after being aged at room
temperature for 1 min ([CTAB] ) 0.2 M, ω ) 33).
then dried in a vacuum at 60 °C for 6 h. The as-prepared
products were characterized by powder X-ray diffraction (XRD,
Shimadzu XRD-6000 instrument with Cu KR radiation, λ )
3
µm were observed, and the number of nanoparticles clearly
1
.5406 Å), field-emission scanning electron microscopy
(
(
FESEM, FEI SIRION 200), transmission electron microscopy
TEM, JEM 100CX), and high-resolution transmission electron
increased (Figure 2c). Finally, all of the microrods evolved into
nanoparticles that were 100-400 nm in size after the aging time
was prolonged to 150 h at room temperature.
microscopy (HRTEM, JEOL 2010).
In general, the formation of 3-D architectures of inorganic
crystals should require a long growth period. For example,
penniform BaWO4 crystals have been synthesized in catanionic
Results and Discussion
The products with different morphologies obtained in the
CTAB/microemulsion system have similar crystalline phases.
The XRD pattern (Figure 1) clearly shows that all of the
diffraction peaks can be indexed as the pure tetragonal structure
with cell parameters a ) 5.58 Å and c ) 12.82 Å (JCPDS card
no. 29-193), which confirms that the as-prepared products are
BaMoO4.
1
8
reverse micelles by Qi and co-workers. In their experiments,
the a-axis-oriented shafts were formed by oriented attachment
and fusion of c-axis-oriented shuttlelike BaWO4 nanocrystals
along the [100] axis, and then [001]-oriented nanowires or
nanobelts grew gradually from the pricks, leading to the
formation of a well-defined penniform BaWO4 nanostructure.
This process required a long aging time, and it represents the
typical growth process of hierarchical crystals from 0-D to 1-D
The shape evolutions of the BaMoO4 microcrystals depended
on the aging time at room temperature after the two micro-
Figure 2. FESEM images of samples obtained after being aged at room temperature for (a) 1 min, (b) 4 h, (c) 24 h, and (d) 150 h ([CTAB] ) 0.2
M, ω ) 33).

Shape Evolution of BaMoO4 Microcrystals
J. Phys. Chem. B, Vol. 110, No. 39, 2006 19297
Figure 3. (a) TEM image of an individual dendritic BaMoO
4
crystal. (b) HRTEM image and (c) ED pattern recorded from the tip of the stem. (d)
HRTEM image and (e) ED pattern recorded from tip of the branch. The sample was obtained after being aged at room temperature for 1 min
[CTAB] ) 0.2 M, ω ) 33).
(
Figure 4. TEM images of samples obtained after being aged at room temperature for (a) 1 min, (b) 1 h, (c) 4 h, (d) 24 h, and (e) 150 h ([CTAB]
0.2 M, ω ) 33, scale bar ) 500 nm).
)
and then 3-D. However, in our experiments, 3-D dendritic
BaMoO4 crystals were formed within a very short reaction time.
To carefully examine the structure of the BaMoO4 crystals,
transmission electron microscopy was used to study the dendrites
formed at the early stage. A representative TEM image, shown
in Figure 3a, further indicates that the dendrite indeed exhibits
a symmetric structure. Figure 3c and e are SAED patterns
recorded from the tip of the stem (area 1 in Figure 3a) and the
tip of a branch (area 2 in Figure 3a) , respectively, of an
individual dendrite. (More ED patterns are provided in Figure
SI1 of the Supporting Information.) The ED patterns are
identical and can be attributed to [010] zone axis diffraction,
which suggests that the hierarchical dendrite is a single crystal.
Representative HRTEM images of the stem tip (Figure 3b)
shows that the lattice fringe has a spacing of 0.320 nm belonging
to the (004) lattice plane of the tetragonal BaMoO4 phase,
meaning that the stem grows along the [001] direction. In
addition, the typical HRTEM image of the branch tip (Figure
consequence of rapid crystal growth due to the small disorienta-
tion of atomic roughness in the interface.
1
9
To evaluate the evolution process of the BaMoO4 crystals,
TEM images of the samples obtained at different aging times
were recorded. The product obtained at a very short aging time
(
1 min) has a dendritic structure consisting of a long stem with
numerous branches on two opposite sides of the stem (Figure
a). At increasing aging time, the branches grew wider along
4
the stem direction ([001] direction), as shown in Figure 4b. The
darker contrast of the branches (Figure 4b) indicates that they
are much thicker than those at the initial stage (Figure 4a). It is
known that the fast-growing faces of crystal will disappear
during crystal growth process. The root area that connects the
stem is much thinner than the middle part of the branch, which
means that the branches also grow rapidly along the [100] and
[010] directions. Unlike in the previous studies of other dendritic
7
-12,16,17
materials,
the branches did not grow further along the
direction of the stem to form a penniform morphology. In
contrast, the branches in the current study grew rapidly along
three directions: [001], [100], and [010]. The rapid growth along
three directions results in branches resembling spindles. It can
also be seen that the regions of the stem between two adjacent
branches are much thinner, indicating that the growth of the
branches occurred by consuming the stem. An interesting issue
regarding the dendrite growth is why the branches grow along
three directions. Such an unusual manner of growth of the
branches is likely driven by the lattice distortion energy resulting
3
d) reveals that the lattice spacing of 0.279 nm corresponds to
the (200) face, which implies that the protuberant branches are
growing along the [100] direction and perpendicular to the [001]
direction of the stem. On the basis of the SEM/TEM morphol-
ogies and the SAED results, it is reasonable to conclude that
the dendritic BaMoO4 crystals were constructed by a stem along
the [001] direction and four branches along the 〈100〉 directions.
Careful observation of the HRTEM images of the stem tip and
branch tip also clearly indicates the existence of lattice distortion
at the specified square area (black arrow in Figure 3b,d). The
presence of distortion is generally considered to be the direct
2
0,21
from lattice distortion in the dendritic crystal.
The 3-D
BaMoO4 structure was obtained in a very short time, so lattice

19298 J. Phys. Chem. B, Vol. 110, No. 39, 2006
Gong et al.
slow-growing {011} and {111} faces caused by the disappear-
ance of the fast-growing {100} faces. The microrods further
grew along three directions by consuming the thinner area of
the stem, and the long rods gradually divided into short rods
and particles after being aged for 24 h (Figure 4d). Finally, the
rods evolved completely into particles when the aging time was
long enough (Figure 4e). The shape of the particles departing
from the rods was more like a spindle, indicating that the {001},
{100}, and {010} faces were minimized. Because the departure
of the particles from the rods was sequential, the particles
experienced different “Ostwald ripening” times, which resulted
in their blunt edges and multidispersion in size.
To accelerate the evolution of the BaMoO4 crystals from
dendrites to particles, comparative experiments involving a
hydrothermal process were carried out. A similar shape evolution
of BaMoO4 from 3-D to 1-D and 0-D structures was observed
within a shorter reaction time (as shown in Figure SI2).
Interestingly, when the reaction was performed at 160 °C for
2
4 h, polyhedral particles with a size of about 0.5-1 µm were
the exclusive products (Figure 5a). It is interesting to find that
the particles are 18-facet polyhedra, which is very similar to
the results obtained in a previous study on PbWO4.20 On the
basis of the above analysis, we can infer that the exposed facets
of the polyhedron are two {001} faces, eight {101} faces, and
eight {111} faces, as shown in Figure 5b,c. Unfortunately, we
cannot determine this directly from the experimental results
obtained by HRTEM or SAED. Judging from the differences
between particles obtained at different temperatures, we believe
that the high temperature and pressure encountered under
hydrothermal conditions accelerate dissolution/recrystallization
during the Ostwald ripening process, which results in the
Figure 5. (a,b) FESEM images and (c) scheme of particles obtained
through the hydrothermal reaction at 160 °C for 24 h ([CTAB] ) 0.2
M, ω ) 33).
2
2
increase in size and shearing on the edge.
distortions cannot be avoided during the crystallization process.
These lattice distortions are confirmed in the HRTEM images
of the stem and branch (Figure 3b,d). The dendritic crystals
containing numerous lattice distortions are in a metastable
structure, so they should evolve to a stable state to minimize
the total energy. In this case, highly crystalline particles are
more stable than dendritic crystals. Therefore, the branches of
the dendritic crystals completely departed from the stem and
evolved into nanoparticles when the reaction time was increased
to 4 h. That is why a small number of particles coexisted with
the rods (Figure 2b). After the branches departed from the stem
completely, the stems evolved into zigzag-shaped microrods,
as shown in Figure 4c. In accordance with previous studies,
we tentatively suggest that the microrods were enclosed by the
In our study, in addition to the shape evolution of BaMoO4
crystals from 3-D to 0-D, another interesting issue is how the
dendritic crystals develop within a very short time. Detailed
experiments revealed that the composition of microemulsion,
especially the value of ω (molar ratio of H2O to CTAB), played
a key role in controlling the initial shape of BaMoO4 in a very
short time. Figure 6 shows the TEM image of the as-synthesized
products obtained at a CTAB concentration of 0.2 M and an
aging time of within 1 min, but with ω ) 5, 15, and 30,
respectively. For ω ) 5, nanoparticles with sizes of 50-100
nm were observed in the products (Figure 6a). For ω ) 15,
microrods with lengths of 1-3 µm and widths of 50-80 nm
were obtained (Figure 6b). However, when the value of ω was
increased to 30, dendritic crystals with lengths of about 3-10
Figure 6. TEM images of samples obtained after being aged at room temperature for 1 min for ω ) (a) 5, (b) 15, and (c) 30 ([CTAB] ) 0.2 M).

Shape Evolution of BaMoO4 Microcrystals
J. Phys. Chem. B, Vol. 110, No. 39, 2006 19299
of the BaMoO4 crystals is strongly dependent on the ω value
of the microemulsion.
Conclusions
In summary, we report a novel shape evolution of BaMoO4
crystals from 3-D dendrites to 1-D rods and to 0-D particles
with prolonged aging time. Minimization of the lattice distortion
energy is assumed to drive the metastable dendrites to divide
into stable particles. In contrast to the traditional crystal growth
from 0-D to 1-D to 3-D, in this case, 3-D dendrites were easily
obtained within a very short reaction time. Detailed experimental
results revealed that the composition of the microemulsion,
especially the ω value, controls the initial shape of the BaMoO4
particles. This method can be extended to other materials to
purposefully prepare 0-D, 1-D, and 3-D structures. In fact, we
have observed a similar shape evolution process in BaWO4.
Figure 7. Scheme of BaMoO
ω within a short reaction time.
4
crystals prepared at different values of
Acknowledgment. This work is supported by the National
Natural Foundation of China, the Shanghai Shu Guang Project,
and the Shanghai Nano-Project.
µm were the exclusive products (Figure 6c). The TEM images
reveal that the morphologies of the as-synthesized products
changed gradually from nanoparticles to microrods and finally
to microdendrites with increasing ω value. It was found that
the sizes of the crystals obtained by this microemulsion process
were many times larger than the typical dimensions of individual
microemulsion droplets (5-100 nm) under appropriate reaction
Supporting Information Available: ED patterns recorded
from a single dendritic BaMoO4 crystal and TEM images of
samples obtained by a hydrothermal reaction. This material is
available free of charge via the Internet at http://pubs.acs.org.
2
3
conditions. Thus, it can be inferred that the molar ratio of
H2O to CTAB (or the water content) and the aggregation
coalescence of individual droplets are both responsible for the
formation of products with various morphologies. A schematic
diagram of the proposed growth mechanism is shown in Figure
References and Notes
(1) Alivisatos, A. P. Science 1996, 271, 933.
(2) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32,
2
+
7
. When two microemulsion solutions containing Ba and
435.
(3) Rao, C. N. R.; Kulkarni, G. U. Thomas, P. J.; Edwards, P. P. Chem.
Eur. J. 2002, 8, 29.
2-
MoO4 are mixed, nucleation and micelle fusion can occur
simultaneously, where BaMoO4 nucleation is well-known to be
very fast. When the ω value is low, the low water content
containing few exchangeable water molecules in the micro-
emulsions might cause the fusion rates between two spherical
droplets to be very low, which would result in a spherical droplet
as shown in Figure 7. Such a microemulsion droplet comprises
a centrally located spherical BaMoO4 nucleus. The subsequent
growth occurred at every direction of the spheroid, resulting in
the formation of BaMoO4 nanoparticles. When a moderate ω
value is used, the greater amount of exchangeable water in the
droplets accelerates the fusion between pairs of droplets. Such
fused microemulsion droplets comprise centrally located rodlike
BaMoO4 nuclei with water-enriched domains at the ends of the
droplets. The surfactant molecules at the sides of a cylindrical
droplet can adsorb onto the surface planes of the formed
BaMoO4 nucleus, so that these surfactant molecules become
fixed and immobile. In contrast, the surfactant molecules at the
two ends of the cylindrical droplet do not associate with the
BaMoO4 nucleus because of water-enriched domains in these
regions and are relatively free. Thus, a cylindrical microemulsion
droplet can dynamically and rapidly interact with other micro-
emulsion droplets at both ends of the droplet.24,25 This process
can result in the formation of 1-D BaMoO4 microrods along
the fast-growing [001] direction. When the ω value is high, the
formed droplets system might significantly increase the fusion
rate. With the water-enriched domains, the surfactant molecules
at the sides of the cylindrical droplet cannot stably adsorb on
the surfaces of the formed BaMoO4 nucleus and limit the growth
direction of the nucleus. Thus, the nucleus can also grow along
the [100] and [010] directions, which are perpendicular to the
fast-growing [001] direction. This process can result in the
formation of 3-D dendritic BaMoO4 crystals. From the model
and the experiment results, we can conclude that the initial shape
(
4) Lee, S. M.; Jun, Y. W.; Cho, S. N.; Cheon, J. J. Am. Chem. Soc.
2
002, 124, 11244.
(5) Lu, Q.; Gao, F.; Komarneni, S. J. Am. Chem. Soc. 2004, 126, 54.
(6) Wang D. L.; Qian, F.; Yang, C.; Zhong, Z. H.; Lieber, C. M. Nano
Lett. 2004, 4, 871.
(7) Wen, J. G.; Lao, J. Y.; Wang, D. Z.; Kyaw, T. M.; Foo, Y. L.;
Ren, Z. F. Chem. Phys. Lett. 2003, 372, 717.
(8) Zhao, N. N.; Qi, L. M. AdV. Mater. 2006, 18, 359.
(9) Zhou, G.; Lu, M.; Xiu, Z.; Wang, S.; Zhang, H.; Zhou, Y.; Wang,
S. J. Phys. Chem. B 2006, 110, 6543.
10) Aizawa, M.; Cooper, A. M.; Malac, M.; Buriak, J. M. Nano Lett.
2005, 5, 815.
(
(11) Cheng, Y.; Wang, Y.; Chen, D.; Bao, F. J. Phys. Chem. B 2005,
1
09, 794.
12) Kuang, D.; Xu, A.; Fang, Y.; Liu, H.; Frommen, C.; Fenske, D.
AdV. Mater. 2003, 15, 1747.
13) Kuang, D.; Xu, A.; Fang, Y.; Ou, H.; Liu, H. J. Cryst. Growth
(
(
2002, 244, 379.
(14) Ryu, J. H.; Yoon, J. W.; Lim, C. S.; Shim, K. B. Mater. Res. Bull.
2
005, 40, 1468.
15) Li, Z. H.; Du, J. M.; Zhang, J. L.; Mu, T. C.; Gao, Y. N.; Han, B.
X.; Chen, J.; Chen, J. W. Mater. Lett. 2005, 59, 64.
16) Shi, H.; Qi, L.; Ma, J.; Wu, N. AdV. Funct. Mater. 2005, 15,
442.
(17) Gong, Q.; Qian, X. F.; Ma, X. D.; Zhu, Z. K. Cryst. Growth Des.
006, 6, 1821.
18) Shi, H.; Wang, X.; Zhao, N.; Qi, L.; Ma, J. J. Phys. Chem. B 2006,
10, 748.
19) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969.
20) Yang, W.; Xie, Z.; Miao, H.; Zhang, L.; An, L. J. Phys. Chem. B
2006, 110, 3969.
21) Geng, J.; Zhu, J. J.; Chen, H. Y. Cryst. Growth Des. 2006, 6, 321.
22) Liu, B.; Yu, S. H.; Li, L. J.; Zhang, Q.; Zhang, F.; Jiang, K. Angew.
Chem., Int. Ed. 2004, 43, 4745.
23) Moulik, S. P.; Paul, B. K. AdV. Colloid Interface Sci. 1998, 78,
(
(
2
(
1
(
(
(
(
(
99.
(24) Clark, S.; Fletcher, P. D. I.; Ye, X. L. Langmuir 1990, 6, 1301.
(25) Cao, M.; Wu, X.; He, X.; Hu, C. Langmuir 2005, 21, 6093.