One-dimensional BiPO4 nanorods and two-dimensional BiOCl lamellae: Fast low-temperature sonochemical synthesis, characterization, and growth mechanism

Article abstract of DOI:10.1021/ic050674g

Regular BiPO₄ nanorods, for the first time, and BiOCl lamellae have been successfully synthesized via a facile sonochemical method in a surfactant/ligand-free system under ambient air. The as-prepared products are characterized by XRD, TEM, SAED, FE-SEM, HRTEM, and Raman spectroscopy. The effects of pH and ultrasound irradiation on the phase and morphology of the products are studied and the sonochemical formation mechanisms of 1D and 2D structures are discussed. TEM data from samples made after different reaction times suggest an ultrasound-induced nucleation and an oriented-attachment growth mechanism.

Full text of DOI:10.1021/ic050674g

Inorg. Chem. 2005, 44, 8503−8509
One-Dimensional BiPO Nanorods and Two-Dimensional BiOCl
4
Lamellae: Fast Low-Temperature Sonochemical
Synthesis,Characterization, and Growth Mechanism
Jun Geng,†,§ Wen-Hua Hou,† Yi-Nong Lv,‡ Jun-Jie Zhu,*,† and Hong-Yuan Chen†
Department of Chemistry, Key Laboratory of Analytical Chemistry for Life Science, Nanjing
UniVersity, Nanjing 210093, P. R. China, and College of Materials Science and Engineering,
Nanjing UniVersity of Technology, Nanjing 210009, P. R. China
Received May 2, 2005
4
Regular BiPO nanorods, for the first time, and BiOCl lamellae have been successfully synthesized via a facile
sonochemical method in a surfactant/ligand-free system under ambient air. The as-prepared products are characterized
by XRD, TEM, SAED, FE-SEM, HRTEM, and Raman spectroscopy. The effects of pH and ultrasound irradiation
on the phase and morphology of the products are studied and the sonochemical formation mechanisms of 1D and
2D structures are discussed. TEM data from samples made after different reaction times suggest an ultrasound-
induced nucleation and an oriented-attachment growth mechanism.
Introduction
4
bismuth phosphates (BiPO ) were reported to be good cata-
9
,10
11
lysts, orthophosphate ion-sensors, and could be used as
Besides the conventional 1D nanorods, nanowires, and
nanotubes,1 plenty of new 2D nanomaterials, such as nano-
a separating agent for uranium, neptunium, and americium.1
2-14
-3
4
BiPO is also considerably important for improving the
4-6
disks, nanosheets, and nanofilms, have emerged recently.
electrical properties of phosphate glasses, which are of
technological interest as ionic, electronic, and fast ionic
Such accomplishments confirm the tremendous worldwide
efforts to create advanced and functional building blocks for
the development of innovative nanomaterials and smart
nanodevices. The design of well-defined and highly oriented
nanomaterials and their large-scale manufacturing at low cost,
in particular, remain crucial challenges to unfold the very
promising future of nanotechnology.
1
5,16
conductors.
However, to the best of our knowledge, there
, so
is no report on the preparation of nanostructured BiPO
far. Bi -VI -VII compounds such as BiOCl, BiSCl, and
4
III
A
A
BiSI evoked great interest because of the coexistence of
1
7,18
several physical properties.
For example, BiOCl shows
the properties of photoluminescence, thermally stimulated
conductivity, and good catalytic activity and selectivity in
the oxidative coupling of the methane (OCM) reaction.1
Bismuth and its compounds have been widely studied
recently because of their unique qualities.7,8 Among them,
9,20
*
To whom correspondence should be addressed. E-mail:
(8) Koh, Y. W.; Lai, C. S.; Du, A. Y.; Tiekink, E.; Loh, K. P. Chem.
Mater. 2003, 15, 4544.
(9) Chang, T. S.; Li, G. J.; Shin, C. H.; Lee, Y. K.; Yun, S. S. Catal.
Lett. 2000, 68, 229.
(10) Takita, Y.; Ninomiya, M.; Miyake, H.; Wakamatsu, H.; Yoshinaga,
Y. Phys. Chem. Chem. Phys. 1999, 1, 4501.
jjzhu@nju.edu.cn. Phone and Fax: +86-25-83594976.
†
Department of Chemistry, Key Laboratory of Analytical Chemistry for
Life Science.
‡
College of Materials Science and Engineering.
Permanent address: Department of Chemistry, Jiangsu Institute of
§
Education, Nanjing, 210013, P. R. China.
(11) Iitaka, K.; Tani, Y.; Umezawa, Y. Anal. Chim. Acta 1997, 338, 77.
(12) Kalaiselvan, S.; Jeevanram, R. K. J. Radioanal. Nucl. Chem. 1999,
240, 277.
(13) Holgye, Z. J. Radioanal. Nucl. Chem. 1998, 227, 127.
(14) Charyulu, M. M.; Chetty, K. V.; Phal, D. G.; Sagar, V.; Naronha, D.
M.; Pawar, S. M.; Swarup, R.; Ramakrishna, V. V.; Venugopal, V. J.
Radioanal. Nucl. Chem. 2002, 251, 153.
(
1) Vayssieres, L. Encyclopedia of Nanoscience and Nanotechnology;
American Scientific Publishers: Stevenson Ranch, CA, 2004; Vol. 8,
pp 147-166.
(2) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. AdV. Funct. Mater. 2003, 13, 9.
3) Patzke, G. R.; Krumeich, F. K.; Nesper, R. Angew. Chem., Int. Ed.
(
2002, 41, 2446.
(
(
(
4) Schmidt, O. G.; Eberl, K. Nature 2001, 410, 168.
(15) Elmoudane, M.; Et-tabirou, M.; Hafid, M. Mater. Res. Bull. 2000,
35, 279.
(16) Jermoumi, T.; Hafid, M.; Et-tabirou, M.; Taibi, M.; ElQadim, H.;
Toreis, N. Mater. Sci. Eng. B 2001, 85, 28.
5) Ma, R. Z.; Bando, Y.; Sasaki, T. J. Phys. Chem. B 2004, 108, 2115.
6) Li, F.; Ding, Y.; Gao, P. X.; Xin, X. Q.; Wang, Z. L. Angew. Chem.,
Int. Ed. 2004, 43, 5238.
(
7) Molares, M.; Chtanko, N.; Cornelius, T. W.; Dobrev, D.; Enculescu,
(17) Berlincourt, D. J. Acoust. Soc. Am. 1992, 91, 3034.
(18) Whatmore, R. W. Rep. Prog. Phys. 1996, 49, 1335.
I.; Blick, R. H.; Neumann, R. Nanotechnology 2004, 15, 201.
1
0.1021/ic050674g CCC: $30.25
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 23, 2005 8503
Published on Web 10/20/2005

Geng et al.
Several efforts have been made to prepare such multiele-
mental inorganic materials,21-23 and it is still a challenge to
prepare the material by a fast and convenient method.
In recent years, sonochemical techniques have been ex-
24-26
tensively used in the synthesis of nanostructured materials.
During the acoustic cavitation process, very high tempera-
tures (>5000 K), pressures (>20 MPa), and cooling rates
(
>1010 K/s) can be achieved upon the collapse of the
2
7
bubble. Such remarkable environments provide a unique
platform for the growth of novel nanostructures. In this paper,
we present a facile procedure for the one-step selective
4
synthesis of 1D BiPO nanorods and 2D BiOCl lamellae via
a sonochemical method. We found that ultrasonic irradiation
and the pH value of the reaction system played important
roles in the formation of the final nanomaterials.
Figure 1. The XRD patterns of the as-prepared products: (a) BiPO4 and
(b) BiOCl.
Experimental Section
All the reagents used were of analytical purity and used without
graphs (HRTEM) were obtained by employing a JEOL-2010 high-
resolution transmission electron microscope with a 200 kV accel-
erating voltage. The Raman spectra were recorded on a JY HR-
further purification. Bi(NO
Chemical Reagents Factory of China. Na
HCl, and HNO were purchased from Shanghai Chemical Reagent
Limited Corporation of China. In a typical preparation of BiPO
nanorods, 20 mM Bi(NO ‚5H O and 20 mM Na PO ‚12H O were
mixed, and the pH value of the mixture was adjusted with 4 M
HNO to the range of 0.5-1. The solution was exposed to high-
3
3 2
) ‚5H O was purchased from Beijing
3
PO ‚12H O, concentrated
4
2
3
800 spectrometer provided by JY Company at room temperature
4
with an excitation wavelength of 488 nm.
)
3 3
2
3
4
2
Results and Discussion
3
Characterizations of BiPO
4
and BiOCl. The XRD
intensity ultrasonic irradiation under ambient air. Ultrasonic ir-
radiation was accomplished with a high-intensity ultrasonic probe
pattern of the as-prepared BiPO (Figure 1a) could be indexed
4
(
6
Xinzhi Co., China, JY92-2D, 0.6 cm diameter; Ti-horn, 20 kHz,
0W/cm ) immersed directly in the reaction solution. The total
to the hexagonal primitive cell (JCPDS Cards 45-1370) with
lattice constants of a ) 6.978 Å and c ) 6.474 Å. The (200)
peak appeared to have a quite strong intensity, indicating
2
reaction time was 30 min. The sonication was conducted without
cooling so that a temperature of about 333 K was reached at the
end of the reaction. A white precipitate was centrifuged, after the
reactant was cooled to room temperature; then it was washed with
distilled water and absolute ethanol in sequence and dried in air.
In the selective preparation of layered BiOCl nanocrystals, all the
procedures remained the same as above except that the pH value
of the solution was adjusted with 6 M HCl instead of 4 M HNO .
3
The final products were characterized by XRD, SEM, TEM, SAED,
HRTEM, and Raman spectroscopy.
4
that the crystalline BiPO was oriented in a particular
crystallographic direction. The XRD pattern of single-crystal
BiOCl (Figure 1b) could be indexed in terms of tetragonal
symmetry with a space group of P4/nmm and lattice constants
of a ) 3.888 Å and c ) 7.357 Å. In contrast to that of the
bulk BiOCl (JCPDS Cards 82-0485), the five peaks (001),
(002), (003), (004), and (005) at 2θ of 12.04°, 24.20°, 36.64°,
4
9.54°, and 63.16°, respectively, showed extraordinarily
The XRD (X-ray diffraction) analysis was performed by a Philips
X-pert X-ray diffractometer at a scanning rate of 4° min in the
strong intensity, which indicated single crystals in the form
of platelets with the c axis perpendicular to the platelets.
-
1
2θ range from 10° to 80°, with graphite monochromatized Cu KR
A typical SEM image of BiPO , shown in Figure 2a,
4
radiation (λ ) 0.15418 nm). Scanning electron micrographs (SEM)
and energy-dispersive X-ray analysis (EDAX) patterns were taken
on a LEO-1530VP field-emission scanning electron microscope.
Transmission electron micrographs (TEM) and selected area
electron diffraction (SAED) patterns were recorded on a JEOLJEM
reveals that this sample is composed of nanorods with
diameters of 40-60 nm and lengths of 2-5µm, which shows
a large aspect ratio of 50-80. The high-magnification SEM
image exhibits the faceted morphology of BiPO
Figure 2b) and the six-faceted trunk can be identified. The
SAED pattern (inset of Figure 2c) recorded on an individual
BiPO nanorod reveals the single-crystalline nature of the
4
nanorods
(
200CX transmission electron microscope, using an accelerating
voltage of 200 kV. High-resolution transmission electron micro-
4
(
(
19) Shtilikha, M. V.; Chepur, D. V. SoV. Phys.-Semicond. 1972, 16, 962.
20) Kijima, N.; Matano, K.; Saito, M.; Oikawa, T.; Konishi, T.; Yasuda,
H.; Sato, T.; Yoshimura, Y. Appl. Catal. A 2001, 206, 237.
sample with a preferential growth oriented along the (001)
crystalline plane. The HRTEM image (Figure 2d) shows that
the sample is structurally uniform with an interplanar spacing
of about 0.35 nm, which corresponds to the (110) lattice
(21) Ganesha, R.; Arivuoli, D.; Ramasamy, P. J. Cryst. Growth 1993, 128,
1081.
(
22) Dellinger, T. M.; Braun, P. V. Scr. Mater. 2001, 44, 1893.
4
spacing of hexagonal BiPO . Combined with SAED observa-
(23) Zhu, L. Y.; Xie, Y.; Zheng, X. W.; Yin, X.; Tian, X. B. Inorg. Chem.
2
002, 41, 4560.
24) Mdleleni, M. M.; Hyeon, T.; Suslick, K. S. J. Am. Chem. Soc. 1998,
20, 6189.
tions, we are able to draw the conclusion that these BiPO
nanorods preferentially grow along the [001] direction.
The BiOCl crystals display a lamellar structure composed
4
(
1
(
(
25) Gedanken, A. Ultrason. Sonochem. 2004, 11, 47.
26) Okitsu, K.; Yue, A.; Tanabe, S.; Matsumoto, H.; Yobiko, Y.; Yoo,
Y. B. Chem. Soc. Jpn. 2002, 75, 2289.
of square nanosheets (Figure 3a and c) with dimensions of
2
(
1-4) × (1-4) µm . Careful observation shows that the
(
27) Suslick, K. S.; Choe, S. B.; Cichowlas, A. A.; Grinstaff, M. W. Nature
1991, 353, 414.
sheetlike crystals grow layer by layer, and the sheet thickness
8504 Inorganic Chemistry, Vol. 44, No. 23, 2005

1
4
D BiPO Nanorods and 2D BiOCl Lamellae
Figure 2. (a) Low-magnification and (b) high-magnification SEM images of BiPO4 nanorods, (c) a typical TEM image and SAED pattern recorded on a
single BiPO4 nanorod, and (d) a HRTEM image of the BiPO4 nanorod.
Figure 3. (a) Low-magnification SEM image of BiOCl lamellae, (b) a high-magnification SEM image showing the thickness of the lamella to be about
3
0 nm, (c) a typical TEM image and SAED pattern of a single BiOCl nanosheet, and (d) a HRTEM image of the BiOCl lamella.
is in the range of 20-30 nm (Figure 3b). The SAED pattern
of BiOCl (inset of Figure 3c) indicates that the layered
structure is perpendicular to the c axis. The HRTEM image
of a single BiOCl nanosheet (Figure 3d) exhibits good
crystalline and clear lattice fringes. The interplanar spacing
is 0.27 nm corresponding to the {110} planes of the
tetragonal system of BiOCl. The HRTEM observation
confirms, along with the XRD and SAED results, that these
BiOCl lamellae are perpendicular to the c axis.
dating back to the 1930s. In the Raman spectrum of BiPO
4
(Figure 4), the intense band observed at 963 cm is assigned
-1
to the ν PO symmetric stretching mode. The Raman band
1
4
-
1
at 1060 cm is ascribed to the ν PO antisymmetric
3
4
stretching mode. The Raman spectrum also displays bands
-
1
at 591, 560, and 474 cm , which correspond to the ν
4
bending modes of the PO units. Two bands are observed at
4
-1
4
46 and 398 cm and are considered to have originated from
the ν
2
bending modes of the PO
4
units. The Raman spectrum
To further characterize the single-crystalline BiPO
BiOCl, the Raman spectra are presented for BiPO in the
00-1400 cm region and BiOCl in the range of 100-
4
and
-1
displays an intense band at 201 cm , which may be assigned
to an OBiO symmetric bending mode. One of the strong
features of Raman spectroscopy is that bands below 400 cm
4
-
1
1
1
-1
-
1
000 cm . As far as we know, the Raman spectrum of
are readily determined. This means that intense bands
BiPO
frequency values of the free PO
in the literature
4
has rarely been investigated. The experimental
31
attributable to M-O vibrations can be ascertained.
3-
4
modes commonly quoted
were obtained by Raman measurements
28-30
(
(
29) Boyer, L. L.; Fleury, P. A. Phys. ReV. B 1974, 9, 2693.
30) Herzberg, G. Molecular Spectra and Molecular Structure II. Infra-
Red and Raman Spectra of Polyatomic Molecules; D. Van Nostrand:
Princeton, NJ, 1945.
(
28) Kravitz, L. C.; Kingsley, J. D.; Elkin, E. L. J. Chem. Phys. 1968, 49,
4600.
Inorganic Chemistry, Vol. 44, No. 23, 2005 8505

Geng et al.
bands than asymmetric vibrations, the strong band at 141
-
1
cm is assigned to the A1g internal Bi-Cl stretching mode.
-
1
The band at 197 cm can be assigned to the E
Bi-Cl stretching mode, while the E external Bi-X stretch-
ing is probably masked by the strong band at 141 cm . The
and B1g band, produced by motion of the oxygen atoms,
g
internal
g
-1
E
g
-
1
at about 396 cm is very weak and is not easy to observe.
The wavenumber here is similar to the reported values in
ref 23 but slightly smaller than that in the reference measure
of powder BiOCl, which probably derives from the stronger
orientation of lamellar single crystals than that of powders.
Effect of pH Value. The pH value played a key role in
32
the synthesis of 1D BiPO
uniform BiPO nanorods with a large aspect ratio could only
be obtained when the reaction system was adjusted by HNO
4
and 2D BiOCl nanocrystals. The
4
3
to the pH range of 0.5-1. With the increase of pH value,
the tendency to grow along a certain direction had been
weakened to some extent, and the BiPO prepared had lower
4
Figure 4. The Raman spectrum of BiPO4. The normal-mode frequencies
of the free PO4 ion are indicated on the top frequency axis by ν1, ν2, ν3,
and ν4.
3
-
aspect ratios and less-uniform morphologies (Figure 6 and
Table 1). On the other hand, when the pH value was lower
than 0.5, no products were obtained because the white
precipitate could be dissolved in such acidic solution.
3
3
According to Peng et al., the high chemical potential
generated by a high monomer concentration in the growth
solution favors the growth of 1D nanocrystals. Meanwhile,
faster ionic motion usually ensures a reversible pathway
between the fluid phase and the solid phase, which allows
atoms, ions, or molecules to adopt correct positions in
3
4
developing crystal lattices. In the present case, the main
reaction may take place as follows
3
+
(3-n)-
+
Bi + H PO
) BiPO V + nH
(1)
n
4
4
The strong pH-dependent relationship with the morphologies
of the products is the result of the sensitive influence of the
3
+
(3-n)-
pH on the solute concentrations ([Bi ] and [H
n
PO
4
]).
Figure 5. The Raman spectrum of lamellar BiOCl.
When the pH value of the solution before sonication was
Isostructural BiOCl is a tetragonal PbFCl-type structure
alkaline, nucleation occurred at an outburst speed forming a
large quantity of BiPO
4
precipitation, which led to low Bi3
+
with space group P4/nnm. For such a structure of space group
7
(3-n)-
D
4h , with two molecular formulas per unit cell, the Raman
and H PO
n 4
concentrations, low chemical potential, and
active modes are two A1g, B1g, and E
two distinguished bands and one weak band. Because
symmetric vibrations often give rise to more intense Raman
g
. Figure 5 consists of
slow ion motion. As the pH value of the solution was lowered
by HNO , the white precipitate was gradually dissolved
resulting in higher solute concentrations. When the pH value
3
Figure 6. TEM images of as-prepared products under various pH conditions: (a) adjusted by HNO3 to pH 2, (b) pH 4, and (c) pH 7 and adjusted by HCl
to (d) pH > 1 and (e) pH 0.5-1.
8506 Inorganic Chemistry, Vol. 44, No. 23, 2005

1
4
D BiPO Nanorods and 2D BiOCl Lamellae
Figure 7. TEM images of BiPO4 obtained after sonication for (a) 2, (b) 5, (c) 8, (d) 15, (e) 25, and (f) 60 min. A growth process from primary particles
to short nanorods then to longer nanorods is clearly observed.
Table 1. Influence of pH on the Crystal Structures and Morphologies of the Products
acid
pH
precipitation
XRD
BiPO4
BiPO4
BiPO4
BiPO4
BiOCl
BiPO4 + BiOCl
morphology
uniform nanorods
size
HNO3
0.5-1
gradual
instant
instant
instant
gradual
instant
(40-60) nm × (2-5) µm
2
4
less uniform nanorods
(20-40) nm × (150-500) nm
(30-40) nm × (100-250)nm
(20-30) nm × (50-150) nm
unhomogeneous nanorods
aggregated and irregular rodlike nanoparticles
lamellae
no adjustment
2
HCl
e1
>
(1-4) × (1-4) µm
1
nanorods + nanosheets
-
decreased to the range of 0.5-1, the precipitate was
completely dissolved to form a clear solution. So the solute
concentrations, the chemical potential, and the rate of the
ion motion arrived at the maximum value, which ensured
the 1D anisotropic growth. Considering the above factors,
hexagonal BiPO nanorods can be grown in an acid environ-
4
ment, especially in the pH range of 0.5-1.
produced during ultrasonic irradiation provides a favorable
environment for the anisotropic growth of the nanocrystals.
Without sonication, BiPO and BiOCl were hardly acquired
4
and the morphology was irregular. To investigate the details
of the sonochemical conversion from precursor to final
products, we have carried out a series of experiments by
employing different sonication times without changing other
preparation conditions.
When HCl solution was used to adjust the pH value instead
of HNO
3
, the final product was not BiPO
4
but BiOCl.
4
In the case of BiPO , after the solution was exposed to
Because the solubility product constant (Ksp) of BiOCl (1.8
sonication for 2 min, the clear solution became turbid (Figure
7a). EDAX measurements revealed that this precipitate was
-
31
×
×
10 , 298.15 K) is much smaller than that of BiPO
4
(1.3
-
23
10 , 298.15 K) and when there was a large quantity of
pure BiPO
4
. No diffraction peak was detected in the XRD
powders
-
Cl anion in the solution, the final product was lamellar
BiOCl, as the result of competitive reaction between eq 1
and eq 2.
pattern of this sample, indicating that the BiPO
4
obtained after 2 min of sonication was amorphous. It has
been known that three different regions are formed during
3
5
the aqueous sonochemical process: (1) the inner environ-
ment (gas phase) of the collapsing bubble, where elevated
temperatures (several thousands of degrees) and pressures
3
+
-
+
Bi + H O + Cl ) BiOClV + 2H
(2)
2
Therefore, BiPO
4
nanorods or BiOCl lamellae could be
(hundreds of atmospheres) are produced; (2) the interfacial
selectively synthesized simply through choosing the pH
regulator and controlling the pH value. We have also done
a comparative experiment keeping other preparation condi-
region between the cavitation bubbles and the bulk solution,
where the temperature is lower than in the gas-phase region
but still high enough to rupture chemical bonds and induce
a variety of reactions; and (3) the bulk solution, which is at
ambient temperature and where the reaction between reactant
molecules and radicals takes place. It is generally observed
that if the reaction occurs inside the collapsing bubbles, the
product obtained is amorphous because of the very high
quenching rate experienced by the products. On the other
hand, if the reaction takes place in the interfacial or bulk
3 4
tions unchanged but without the addition of Na PO . In this
case, only the irregularly shaped BiOCl was obtained, which
3-
infers that the PO
lamellar structure.
4
ion may play a role in the formation of
Effect of Ultrasonic Irradiation and Possible Formation
Mechanism. High-intensity ultrasonic irradiation also played
an important role in the formation of 1D and 2D products.
The transient high-temperature and high-pressure field
36
regions, one would expect to get crystalline products. In
(
31) Frost, R. L.; Weier, M. L.; Erickson, K. L.; Carmody, O.; Mills, S. J.
(35) (a) McNamara, W. B.; Didenko, Y. T.; Suslick, K. S. Nature 1999,
401, 772. (b) Suslick, K. S.; Hammerton, D. A.; Cline, R. E. J. Am.
Chem. Soc. 1986, 108, 5641. (c) Grinstaff, M. W.; Cichowlas, A. A.;
Choe, S. B.; Suslick, K. S. Ultrasonics 1992, 30, 168.
(36) Jeevanandam, P.; Koltypin, Y.; Gedanken, A.; Mastai, Y. J. Mater.
Chem. 2000, 10, 511.
J. Raman Spectrosc. 2004, 35, 1047.
(32) Davies, J. E. D. J. Inorg. Nucl. Chem. 1973, 35, 1531.
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P. C.; Buhro, W. E. Science 1995, 270, 1791.
Inorganic Chemistry, Vol. 44, No. 23, 2005 8507

Geng et al.
Figure 8. HRTEM images of aggregated particles at the early stage of crystal growth of (a) BiPO4 and (b) BiOCl. Panel a shows that the clear parallel
crystallographic orientation and the bottlenecks between the fused adjacent particles are still visible. White arrows in panel b highlight the boundaries in
which there are some dislocations between primary particles. We assume that the primary particles may have aggregated in an oriented fashion, producing
assemblies of oriented nanoparticles that subsequently underwent further growth (in the case of a) or they may have aggregated in a random manner and
undergone recrystallization to result in parallel orientation of the primary particles (in the case of b).
3
+
the present case, because of the nonvolatile nature of Bi ,
only a negligible amount of Bi3 ion is expected to be present
inside the cavity or bubble for a sonochemical reaction.
particles or anisotropic growth if there is a sufficient
difference in the surface energies of different crystallographic
faces. The BiPO prepared here possessed a hexagonal
4
+
3
9
However, amorphous BiPO
4
nanoparticles were obtained at
crystal structure, which exhibited anisotropic growth pref-
erentially along the [001] direction. It is well-known that
the hexagonal crystal structure, such as ZnO and CdSe, often
shows the anisotropic growth along the [001] direction.3
Therefore, the anisotropic growth is believed to be the
inherent driving force in the process of growing the
the early stage of the reaction, implying that the nuclei
formation of this sample probably occurs inside the collaps-
ing bubbles. This contradictory phenomenon makes it quite
difficult to explain the detailed nucleation mechanism at this
moment and further work is still under way.
3,40
4
1
After an ultrasonic treatment of 5 min, a large quantity of
white precipitate began to form. A typical TEM image
hexagonal nanorods.
This aggregation-growth mechanism provides a route for
the incorporation of defects, such as edge and screw
dislocations, in stress-free and initially defect-free nano-
(
Figure 7b) reveals that the sample was composed of uniform
nanorods with diameters of 20-30 nm and lengths of 50-
00 nm. The XRD results proved that this precipitate was a
4
2
1
crsytalline materials. In the HRTEM images (Figure 8),
the vast majority of particles appear to be composed of many
primary building-block particles ranging in size from less
than 5 nm to 10 nm. It is seen that the lattice planes of the
depicted particles are almost perfectly aligned and the lattice
planes go straight through the contact areas. The bottlenecks
between the two fused adjacent particles are still visible
(Figure 8a). Some edge dislocations at the interface between
the two primary building-block particles could be clearly
detected (Figure 8b). We assume that the primary particles
may have aggregated in an oriented fashion, producing
assemblies of oriented nanoparticles that subsequently un-
derwent further growth (in the case of Figure 8a), or they
may have aggregated in a random manner, undergone
recrystallization to result in a parallel orientation (in the case
of Figure 8b).
crystalline hexagonal structure. When the reaction time
increased to 8 min, the rods grew to diameters of 40-60
nm and lengths up to 1 µm (Figure 7c). After 15 min of
sonication, the diameters of the nanorods were invariable,
but the lengths increased to 2-5 µm (Figure 7d). During
these stages, we assume that the ultrasonic irradiation further
4
accomplished the crystallization of the amorphous BiPO and
the preferential 1D growth of nanocrystals. Cavitations and
shock waves created by ultrasound can accelerate solid
particles to high velocities leading to interparticle collisions
3
7
and inducing an effective melting at the point of collision.
The energy generated during collision can induce the
crystallization of the amorphous particles. The improvement
of mass transport from turbulent mixing induced by cavita-
tions also contributed to the uniform morphologies of the
final products.38 Detailed HRTEM images of a series of
aggregated particles at the early stage of the crystal growth
indicate an oriented-attachment growth. In this process,
primary particles may aggregate in an oriented fashion to
produce a larger single crystal, or they may aggregate
randomly and reorient, recrystallize, or undergo phase
transformations to produce larger single crystals. This kind
of growth mode could lead to the formation of faceted
4
Therefore, the sonochemical formation of BiPO nanorods
can be divided into three steps in sequence: (1) ultrasound-
induced nuclei formation which leads to amorphous primary
BiPO
assembly of these primary nanoparticles to form the chain-
type structure of BiPO and generation of nanocrystalline
short rods, and (3) a crystal growth process, resulting in the
4
nanopaticles, (2) ultrasound-induced oriented self-
4
(39) Penn, R. L.; Oskam, G.; Strathmann, T. J.; Searson, P. C.; Stone, A.
T.; Veblen, D. R. J. Phys. Chem. B 2001, 105, 2177.
(
(
37) Doktycz, S. J.; Suslick, K. S. Science 1990, 247, 1067.
38) Suslick, K. S.; Price, G. J. Annu. ReV. Mater. Sci. 1999, 29, 295 and
the references therein.
(40) Vayssieres, L. AdV. Mater. 2003, 15, 464.
(41) Wang, X.; Li, Y. D. Angew. Chem., Int. Ed. 2002, 41, 4790.
(42) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969.
8508 Inorganic Chemistry, Vol. 44, No. 23, 2005

1
4
D BiPO Nanorods and 2D BiOCl Lamellae
Figure 9. TEM images of BiOCl obtained after sonication for (a) 2, (b) 5, (c) 8, (d) 15, (e) 25, and (f) 60 min. The coarse fringes of lamellae became
smooth, which indicates an aggregation and crystallization process.
fell to irregular small pieces (Figure 9f). The decrease in
size may be related to surface corrosion and fragmentation
of solids in the presence of high-intensity ultrasonic irradia-
tion. For brittle materials, especially layered inorganic
sulfides and oxides, interparticle collisions can induce
3
8
fragmentation. Therefore, we can see that the interparticle
collisions resulting from the ultrasonic irradiation are re-
sponsible for both the crystal growth and corrosion/
fragmentation of the crystalline solids.
Figure 10. Schematic illustration of the formation of BiPO4 nanorods
and BiOCl nanosheets under ultrasonic treatment.
formation of regular nanorods with higher aspect ratios. The
nanorods with different aspect ratios were formed during
different sonication times, which provides a facile method
Conclusion
In summary, BiPO nanorods with alternative aspect ratios
4
to control the dimensions of BiPO
4
nanorods simply by
have been successfully prepared, for the first time, via a
controlling the sonication time. The growth of BiOCl also
showed a similar self-assembly process from primary nano-
particles to lamellar structure under sonication (Figure 9).
The coarse fringes of the nanosheets became smooth,
indicating the oriented aggregation of nanoparticles at the
fringe of the nanosheets and a further crystallization process
sonochemical method. Simply by controlling the pH regula-
tor, we can selectively synthesize BiOCl lamellae in the same
reaction system, which has been proven to be a new,
convenient, and efficient route. The pH value and ultrasonic
irradiation played key roles in the formation of 1D and 2D
nanostructures. TEM data from samples made after different
reaction times suggest an ultrasound-induced nucleation and
an oriented-attachment growth mechanism.
(Figure 9b, c, and d). A schematic illustration of the
sonochemical formation mechanism is shown in Figure 10.
We have also carried out experiments by prolonging the
Acknowledgment. This work is supported by the
National Natural Science Foundation of China (Grant No.
sonication time. It was observed that the dimensions of BiPO
4
nanorods increased gradually with the increase in sonication
time and the morphology became less uniform, as can be
seen in Figure 7f. However, in the case of BiOCl, with the
increase in the sonication time, the initial square lamellae
2
0325516 and 90206037) and the Jiangsu Foundation of
China (BK2004210).
IC050674G
Inorganic Chemistry, Vol. 44, No. 23, 2005 8509