Crystalline nanoflowers with different chemical compositions and physical properties grown by limited ligand protection

Article abstract of DOI:10.1002/anie.200601553

(Figure Presented) A varied bouquet: Crystalline nanoflowers (see picture) of compounds with different chemical and physical properties, for example, In₂O₃, ZnO, CoO, MnO, and ZnSe, are grown by a new approach, limited ligand protection (LLP). LLP destabilizes the primary nanoparticles and promotes their three-dimensionally oriented attachment into complex nanostructures.

Full text of DOI:10.1002/anie.200601553

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Nanostructures
carbon, such as 1-octadecene (ODE), was used as solvent.
Myristic acid (MA) or stearic acid (SA) was optionally used as
a free ligand. An alcohol, such as 1-octadecyl alcohol (OA) or
decyl alcohol (DA), was often added to either activate the
stable precursors or to increase the yield and stability[22] of the
oxide nanoflowers. For the ZnSe nanoflowers, an amine, such
as 1-octadecyl amine (ODA), was used as an activation
reagent and a ligand, and selenium powder dissolved in
tributylphosphine was used as a reagent.
DOI: 10.1002/anie.200601553
Crystalline Nanoflowers with Different Chemical
Compositions and Physical Properties Grown by
Limited Ligand Protection**
Arun Narayanaswamy, Huifang Xu, Narayan Pradhan,
and Xiaogang Peng*
The results for the model system of In2O3 are briefly
summarized in Figure 1, which shows representative trans-
Colloidal nanocrystals with controlled size and shape play a
key role in nanotechnology.[1–3] Organometallic[4–6] and
related alternative (or so-called greener)[7–10] synthetic meth-
ods at elevated temperatures in non-aqueous solvents are the
current mainstream strategies for producing high-quality
nanocrystals. The ability to control the size and size distribu-
tion of nanodots using these strategies is reasonably well
developed. Also, advances in the formation of 1D nano-
crystals have recently been reported.[5,11–14] These 1D struc-
tures offer a unique technical potential that is inaccessible
with the corresponding 0D nanocrystals. Reports on the
synthesis of complex 3D nanostructures, however, remain
uncommon.[15–19] Most of these structures are grown on
substrates, are large in size, and/or are polycrystalline.
Herein, we demonstrate that, by simply reducing the degree
of ligand protection to the domain of limited ligand protec-
tion (LLP), the existing mainstream synthetic chemistry for
0D and 1D nanocrystals—more specifically, the greener
approaches—can also produce high-quality complex 3D
nanostructures, such as crystalline nanoflowers. Unlike the
formation of tetrapods of II–VI semiconductors with specific
crystal structures,[15,20] we reveal that LLP coupled with 3D
oriented attachment can be applied in the preparation of
nanocrystals with different compositions, crystal structures,
and physical (magnetic and electronic) properties.
Figure 1. Formation of In2O3 nanodots by SLP and nanoflowers by LLP,
and representative TEM images of the nanostructures (the size and
shape depend on the reaction conditions).
mission electron microscope (TEM) images of the nano-
structures formed. When In(Ac)3 is used as the precursor, the
extremely short Ac ligands require that a high concentration
(2 equiv) of MA be used to stabilize dot-shaped nanocrystals
in the domain of sufficient ligand protection (SLP; Figure 1,
center panel, top left). If only 1 equivalent of the long-chain
fatty acid is present, the system is in the LLP domain. The
individual nanodots become unstable and aggregate into 3D
nanoflowers (Figure 1, center panel, top right). LLP can also
be achieved with indium salts of fatty acids with long
hydrocarbon chains, such as In(Mt)3, without addition of
free MA (Figure 1, center panel, bottom right). In compar-
ison, the addition of 1 equiv of free MA yields nearly
monodisperse nanodots (Figure 1, center panel, bottom
left). All four reactions were carried out under the same
conditions, with the exception of the types and concentrations
of the indium precursors and ligands.
Figure 2 illustrates that a variety of reaction systems can
be similarly tuned to either the regular SLP domain or the
LLP domain, through the use of metal carboxylates as
precursors and the optional addition of a free ligand in a
low concentration. This simple approach allows the growth of
both nearly monodisperse nanoparticles and nanoflowers
without any size sorting.
The size and shape of the 3D nanostructures can be varied
in several different ways (Figure 3). The number of primary
particles in each 3D ZnO aggregate is similar when MA
(Figure 3a) or SA (Figure 3b) is used as the free ligand, but
the sizes of the primary particles are different. When a lower
concentration of the free fatty acid (MA) is used (Figure 3c),
the number of primary particles in each ZnO nanoflower
increases dramatically. The primary particles in the CoO
nanoflowers formed in the early stages of the reaction
(Figure 3e) are pointier and less regular in shape than those
found in later stages (Figure 3 f).
The LLP strategy was first applied to the In2O3 nano-
crystal system, and it was not clear if the 3D oriented
attachment caused by LLP would be limited to certain crystal
structures and compositions. Herein, we intend to clarify the
generality of the LLP strategy. A detailed account of the
nanocrystal growth mechanism will be reported separately.[21]
A metal carboxylate (acetate (Ac), myristate (Mt), or
stearate (St)) was used as a precursor, and a generic hydro-
[*] Dr. A. Narayanaswamy,Dr. N. Pradhan,Dr. X. Peng
Department of Chemistry and Biochemistry
University of Arkansas
Fayetteville,AR 72701 (USA)
Fax: (+1)479-575-4049
E-mail: xpeng@uark.edu
Dr. H. Xu
Department of Geology and Geophysics
University of Wisconsin
Madison,WI 53706 (USA)
[**] Financial support from the National Science Foundation (X.P.) and
Wisconsin Alumni Research Foundation (H.X.) is acknowledged.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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an electric dipole moment in the direction of the unique c axis
of the wurtzite structure. The CoO and MnO nanocrystals
have a magnetic dipole. The ratio of cations to anions in In2O3
differs from that in the other compounds. Even for the same
cation to anion ratio (1:1), the coordination number of the
cation differs from case to case, being six in MnO and CoO,
and four in ZnO and ZnSe. ZnO and ZnSe are well-known
semiconductors.
Although LLP occurs in a variety of systems, there are
some noticeable differences among the resulting nanoflowers
(Figures 1–3). The primary nanocrystals in the In2O3 and
ZnSe nanoflowers do not seem to be faceted. The MnO
nanoflowers are formed by faceted primary particles, but the
primary nanocrystals do not seem to have a fixed morphology.
The ZnO nanoflowers are formed by nanopyramid primary
particles (Figure 4). The primary nanocrystals in the CoO
nanoflowers appear to be cubic. Evidently, the shapes of the
primary nanocrystals constituting a nanoflower are not
necessarily the same as those of the corresponding individual
nanocrystals grown under SLP (Figure 2).
The oriented attachment of the primary nanocrystals to
yield nanoflowers was studied by high-resolution (HR)TEM.
Oriented attachment refers to the formation of relatively
large crystalline structures by the attachment of crystalline
primary particles, typically dot-shaped nanocrystals.[23] The
1D oriented attachment of nanodots to form nanorods or
nanowires has been well documented, and there is strong
evidence that an electric dipole moment is the major driving
force.[12–14] Similarly, it has been reported that ferromagnetic
nanodots can be self-assembled into 1D nanowires by
magnetic dipole–dipole interactions between the primary
particles in solution.[24]
Kotov and co-workers clearly demonstrated that, for the
1D oriented attachment of CdTe nanocrystals, it was essential
to remove excess ligands in the reaction solution after the
formation of the primary CdTe nanodots.[13] Although some
reports have suggested that certain 3D nanostructures are
formed through 3D oriented attachment in the nanometer
regime,[25–27] the evidence is less convincing than in the 1D
case. Controlled 3D oriented attachment on the nanoscale
through the manipulation of ligand protection has not yet
been reported.
Figure 2. TEM images of the nanoparticles (left) and nano-
flowers (right) formed in the CoO,MnO,ZnO,and ZnSe systems,and
corresponding selected-area electron diffraction (SAED) patterns (cen-
ter). The metal precursors and free ligands used in each system are
indicated.
HRTEM indicates that 3D oriented attachment occurs,
either perfectly (In2O3 and CoO) or imperfectly (ZnO and
MnO), in the formation of the nanoflowers. The results for the
ZnO nanocrystals are particularly interesting. The primary
particles of the ZnO nanoflowers are faceted, mainly with a
pyramid shape (Figure 4). Stacking faults that may result in
local zinc blende-like domains occur in some of the nano-
pyramids. The imperfection of the 3D oriented attachment[23]
can be seen in this case: most of the ZnO nanocrystals attach
to form a flower with a common crystallographic orientation,
but a small misalignment is observed between some nano-
crystals (for example, the 1.58 mismatch between the (100)
lattice fringes in the left and right nanocrystals in the center
panel of Figure 4). The ZnO nanocrystals attach to each other
Figure 3. TEM images of the ZnO and CoO nanoflowers formed under different
reaction conditions: a) Zn(Ac)2:MA=1:4,b) Zn(Ac) 2:SA=1:4,
c) Zn(Ac)2:MA=1:1,d) Co(Ac) 2·4H2O:MA=1:2,e) Co(Ac) 2·4H2O:SA=1:2,5 min
after addition of DA,f) Co(Ac) 2·4H2O:SA=1:2,20 min after addition of DA.
The five types of nanocrystals formed belong to four
different crystal structures: bixbyite (cubic) for In2O3, rock
salt (cubic) for CoO and MnO, wurtzite (hexagonal) for ZnO,
and zinc blende (cubic) for ZnSe. The ZnO nanocrystals have
¯
by sharing several different faces, including their (001) faces
(Figure 4). Thus, the attachment cannot be caused by an
electric-dipole mechanism. As discussed below, the attach-
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In summary, a unique and general approach, LLP, has
been developed for the growth of nearly monodisperse
nanostructures through 3D oriented attachment. With LLP,
the primary nanocrystals are insufficiently protected, but the
resulting 3D nanostructures are stabilized. This principle
implies that LLP may be applicable to a broad spectrum of
colloidal nanocrystals, without drastically altering the syn-
thetic chemistry established for 0D and 1D nanocrystals in the
recent years. Our results indicate that oriented attachment, at
least in the 3D case, does not need to be driven by an electric
or magnetic dipole moment. A detailed analysis of this issue
for the In2O3 model system will be published shortly.[21] The
complex crystalline nanostructures described herein offer
unique nanoarchitectures for the development of high-
performance electronic, optoelectronic, and sensing devices.
The discovery of the LLP domain in the mainstream synthetic
chemistry of high-quality nanocrystals enhances this impor-
tant materials field. Although 1D oriented attachment has
been well documented, the results herein indicate that 3D
oriented attachment with LLP may occur more generally in
natural mineralization and materials synthesis, especially
under relatively vigorous conditions.
Figure 4. HRTEM image of part of a ZnO nanoflower (center),and fast
Fourier transforms (FFTs; left and right) of selected areas (dotted
squares) of the HRTEM image. See text for details.
¯
ment of the (001) face of one primary nanocrystal to the (001)
face of the next to form 1D ZnO nanowires would be
expected if the electric dipole, in the direction of the unique
c axis of the wurtzite structure, were playing a determining
role.
Weller and co-workers convincingly showed that ZnO
nanorods could be formed by the 1D oriented attachment of
spherical nanodots along their c axis in an alcohol solution.[12]
However, although ZnO nanocrystals have a dipole moment
along their c axis, 3D oriented attachment can still occur, as
shown in Figure 4. In comparison to our experimental Experimental Section
Individual nanocrystals (dots or pyramids) of In2O3, ZnO, CoO, and
conditions, Weller and co-workers used shorter ligands (Ac
as the sole ligand) and a significantly lower temperature
(608C). The very short ligands greatly decreased the distance
between primary particles and, thereby, enhanced the dipole
interaction between them. The low reaction temperature
reduced the thermal energy of the primary particles, allowing
them to align their dipole moments during the attachment
events.
Significantly more experiments are needed to clarify this
interesting system. The attachment of primary particles in
one, two, or three dimensions in a controllable fashion is of
considerable appeal. Further insight into this system may also
help us to understand natural mineralization processes
occurring under high-temperature and high-pressure condi-
tions. In these processes, 3D attachment, either perfect or
imperfect, should be preferred.
Similar to the electric dipole, the magnetic dipole does not
play a determining role in the 3D oriented attachment of the
magnetic CoO and MnO nanocrystals. Furthermore, both the
CoO and MnO nanoflowers are crystalline in nature, but the
MnO nanoflowers show some signs of imperfect oriented
attachment (a detailed structural analysis will be published
separately).
Although LLP makes individual nanodots unstable, the
resulting nanoflowers are generally stable in the reaction
solution and are also dispersible in nonpolar solvents after
purification. The stability of the nanoflowers is probably a
result of their complex surface structure. The ligands bound to
surface atoms in the gaps between incompletely fused
primary nanocrystals are kinetically “trapped” and, hence,
difficult to remove. Similarly, tetrapods and highly branched
nanocrystals of CdSe and CdTe were found to be more
durable and dispersible than the corresponding nano-
rods.[15,20,28]
MnO were produced by injecting the corresponding metal salt of a
long-chain fatty acid (MA or SA) into a mixture of OA and ODE at
2908C. In a typical synthesis of ZnO nanocrystals, Zn(St)2 (0.1 mmol)
and ODE (4 g) were loaded into a 25-mL three-necked flask,
degassed, and heated to 2808C under an argon atmosphere. OA
(0.5 mmol) dissolved in ODE (0.5 g) at 1508C was then injected into
the mixture, and the temperature was decreased to 2508C. After
incubating for 10 min at 2508C, SA (0.1 mmol) dissolved in ODE
(0.5 g) at 1208C was injected into the reaction mixture. The resulting
mixture was incubated for 1 h to yield pyramid-shaped ZnO nano-
crystals.
Nanoflowers of In2O3, ZnO, CoO, and MnO were formed from
the corresponding metal acetates in the presence of MA or SA. In a
typical synthesis of ZnO nanoflowers, anhydrous Zn(Ac)2
(0.1 mmol), SA or MA (0.1 mmol), and ODE (4.75 g) were heated
to 2808C under an argon atmosphere. DA (0.75 mmol) in ODE
(0.5 g) was then injected into the mixture to yield ZnO nanoflowers.
Details of the syntheses of individual nanocrystals and nanoflowers of
In2O3, CoO, MnO, and ZnSe are given in the Supporting Information.
Received: April 19, 2006
Published online: July 6, 2006
Keywords: crystal growth · ligand protection · nanostructures ·
.
oriented attachment · oxides
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