Formation of monodisperse and shape-controlled MnO nanocrystals in non-injection synthesis: Self-focusing via ripening

Article abstract of DOI:10.1021/ja073023n

Formation of nearly monodiperse MnO nanocrystals by simple heating of Mn stearate in octadecene was studied systematically and quantitatively as a model for non-injection synthesis of nanocrystals. For controlling the shape of the nanocrystals, that is, rice, rods, peanuts, needles, and dots, either an activation reagent (ocadecanol) or an inhibitor (stearic acid) might be added prior to heating. The quantitative results of this typical non-injection system reveal that the formation of nearly monodisperse nanocrystals did not follow the well-known focusing of size distribution mechanism. A new growth mechanism, self-focusing enabled by inter-particle diffusion, is proposed. Different from the traditional focusing of size distribution , self-focusing not only affects the growth process of the nanocrystals, but may also play a role in controlling nucleation. Because of the simplicity of the reaction system, it was possible to also identify the chemical reactions associated with the growth and ripening of MnO nanocrystals with a variety of shapes. Through a recycling reaction path, water was identified as a decisive component in determining the kinetics for both growth and ripening in this system, although the reaction occurred at around 300°C.

Full text of DOI:10.1021/ja073023n

Published on Web 08/14/2007
Formation of Monodisperse and Shape-Controlled MnO
Nanocrystals in Non-Injection Synthesis: Self-Focusing via
Ripening
Yongfen Chen, Eric Johnson, and Xiaogang Peng*
Contribution from the Department of Chemistry & Biochemistry, UniVersity of Arkansas,
FayetteVille, Arkansas 72701
Received April 30, 2007; E-mail: xpeng@uark.edu
Abstract: Formation of nearly monodiperse MnO nanocrystals by simple heating of Mn stearate in
octadecene was studied systematically and quantitatively as a model for non-injection synthesis of
nanocrystals. For controlling the shape of the nanocrystals, that is, rice, rods, peanuts, needles, and dots,
either an activation reagent (ocadecanol) or an inhibitor (stearic acid) might be added prior to heating. The
quantitative results of this typical non-injection system reveal that the formation of nearly monodisperse
nanocrystals did not follow the well-known “focusing of size distribution” mechanism. A new growth
mechanism, self-focusing enabled by inter-particle diffusion, is proposed. Different from the traditional
“focusing of size distribution”, self-focusing not only affects the growth process of the nanocrystals, but
may also play a role in controlling nucleation. Because of the simplicity of the reaction system, it was
possible to also identify the chemical reactions associated with the growth and ripening of MnO nanocrystals
with a variety of shapes. Through a recycling reaction path, water was identified as a decisive component
in determining the kinetics for both growth and ripening in this system, although the reaction occurred at
around 300 °C.
Introduction
in this net growth stage because the growth rate of nanocrystals
increases as their size decreases.2,10 Interestingly, some recent
Formation of monodisperse nanocrystals with size and shape
control has recently become a central topic in the field of
materials chemistry.1,2 This topic not only represents a synthetic
challenge, but also is of importance in several areas of science,
such as optoelectronics,3,4 bio-medical sensing,5,6 catalysis,7
crystallization,8 and natural/bio-mineralization,9 etc. For this
topic, one established concept, “focusing of size distribution”,10
has played a key role. Hot-injection methods, initiating reaction
by the injection of cold precursors into a hot solvent, are
specifically based on this concept and have become the
mainstream approach in the synthesis of nearly monodisperse
nanocrystals.1,2 By experimental design, nucleation stops shortly
after the injection because of the subsequent decrease of
monomer concentration and reaction temperature. If the remain-
ing monomer concentration is higher than the solubility of all
nanocrystals in the system, “focusing of size distribution” occurs
reports indicate that hot injections may not be needed at all.
Nearly monodisperse nanocrystals, including oxides,11-14 II-
VI semiconductor nanocrystals,15,16 and noble metal ones,17-19
can be obtained more or less by simply heating up all premixed
reactants. Such simple non-injection-based synthesis may be
widely exploited if the mechanism on controlling the size and
size distribution of the nanocrystals can be well understood.
This along with the general importance of understanding
crystallization mentioned above was the motivation of this work.
Growth of MnO nanocrystals by simply heating Mn stearate
in a hydrocarbon solvent, sometimes with either some free
stearic acid as the inhibitor or ocadecanol as the activation
reagent, was chosen as our model system. MnO nanocrystals
are probably the most studied oxide nanocrystal system in the
recent literature, especially in terms of shape control.13,14,20-26
(11) Hyeon, T.; Lee, S. S.; Park, J.; Chung, Y.; Na, H. B. J. Am. Chem. Soc.
2001, 123, 12798-12801.
(1) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000,
30, 545-610.
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(15) Joo, J.; Na, H. B.; Yu, T.; Yu, J. H.; Kim, Y. W.; Wu, F.; Zhang, J. Z.;
Hyeon, T. J. Am. Chem. Soc. 2003, 125, 11100-11105.
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Phys. Chem. B 2006, 110, 16819-16822.
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Chem. 2004, 14, 3406-3410.
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The simplicity of our model system made it relatively easy to
follow the reactants and products along the formation of the
oxide nanocrystals as to be discussed below. Furthermore,
similar reaction systems were studied for the formation of high-
quality ZnO and In2O3 nanocrystals using hot-injection meth-
ods.27,28 However, the purpose of this work required the study
to be at a quantitative level as to be described below. In contrast,
the studies on hot-injection synthesis of ZnO and In2O3
nanocrystals were qualitative.27,28
when the size distribution was narrowed down. It should be
mentioned that a substantial decrease of particle concentration
was also observed previously for the formation of CdSe
nanocrystals in a non-injection approach,16 which is probably
the only publication reporting the particle concentration evolu-
tion for non-injection synthesis of nanocrystals. Some further
contradictions to the “focusing of size distribution” model are
also identified. For instance, the consumption of monomers was
found to be close to zero in the main duration of the
size-focusing process for the current system.
The well-studied II-VI and III-V semiconductor nanocrystal
systems can be conveniently probed using their size-dependent
optical properties with an accuracy down to a few atoms.29 MnO
nanocrystals, similar to most oxide nanocrystals, do not have
strong size-dependent optical properties. To solve this problem,
this work explored possibilities to use FTIR coupled with a
transmission electron microscope (TEM) as quantitative probes.
Because of the chosen reaction system, FTIR allowed direct
measurements of the concentrations of the precursor and the
organic side products in the reaction solution. It should be
pointed out that application of FTIR measurements in monitoring
nanocrystal synthesis has already been well demonstrated
qualitatively in the literature.30 In addition to FTIR, quantitative
analysis of TEM images of the nanocrystals offered information
on the size, shape, and size/shape distribution of the nanocrys-
tals. Combining the information from TEM and FTIR measure-
ments, one can calculate the particle concentration in the
solution. Thus, the crystallization system can be well defined.
This approach, presumably working for different types of
nanocrystals, is less accurate in terms of determining the size
of the nanocrystals in comparison to the existing approaches
developed for studying formation of semiconductor nanocrystals
using their size-dependent optical properties. However, it may
offer significantly more information about the molecular
mechanisms for a carefully designed system that could be fully
followed by FTIR spectroscopy. As will be described later,
this advantage was further utilized to directly verify a long
standing hypothesis that the reactivity of monomers can be
tuned by the addition of free ligands, which was repeatedly
implied by the growth kinetics of different types of semiconduc-
tor nanocrystals.31-33
A new mechanism, self-focusing, will be proposed. This
mechanism is based on inter-particle diffusion observed re-
cently,34 which allows a rapid and complete dissolution of the
small particles in the system. This means that, while the size
distribution is narrowed down in this transient stage of Oswald
ripening, the particle concentration shall decrease. Different from
the traditional “focusing of size distribution”, self-focusing
requires a high particle concentration in the solution but does
not depend on a high monomer concentration. Unlike the
traditional “focusing of size distribution”, self-focusing might
also affect the nucleation process in a system.
In addition to the growth mechanism of the nanocrystals, this
work also studied the chemical reactions involved in the
formation of nearly monodipserse MnO nanocrystals with a
variety of shapes. The identified reaction mechanism was found
to be different from the molecular mechanisms of either the
ZnO nanocrystal system or In2O3 reported previously.27,28 The
main interesting finding on this aspect is that water was
confirmed to play a key role in both growth and ripening of
the MnO nanocrystals in the current system. Although the
emphasis of this Article is on the formation of nearly mono-
disperse dot-shaped nanocrystals, some aspects related to shape
control of high-quality nanocrystals will also be briefly
described.
Results
The model reaction system chosen for studying non-
injection synthesis of high-quality nanocrystals was the forma-
tion of MnO nanocrystals in non-coordinating solvents under
elevated temperatures,14 with Mn stearate (MnSt2) as the sole
precursor and ocadecene (ODE) as the non-coordinating solvent.
If needed, free stearic acid (HSt) was used as inhibitor and
octadecanol was applied as activation reagents. Different from
the two oxide nanocrystal systems studied previously, ZnO27
and In2O3,28 no hot-injection of any chemicals was applied in
the current system, and, as pointed out above, the current system
would be quantitatively defined, instead of being at a qualitative
level for the two existing examples based on hot injections.
FTIR spectra of the aliquots taken at different reaction time
intervals were recorded, and the vibration peaks at 1556, 1740,
and 1700-1720 cm-1 are assigned as the carbonyl vibration
bands of carboxylate salt, ester, and carboxylic acid, respec-
tively.35 The -CH3 peak at 1375 cm-1 was found to be a good
reference for quantitative calculations for reactions lasting for
several hours.
The results to be discussed below suggested that formation
of monodispserse MnO nanocrystals in this non-injection-based
system was unlikely a traditional “focusing of size distribution”
process. Different from one typical feature of the “focusing of
size distribution” process described above, the particle concen-
tration in the current system was found to decrease significantly
(22) Zitoun, D.; Pinna, N.; Frolet, N.; Belin, C. J. Am. Chem. Soc. 2005, 127,
15034-15035.
(23) Ghosh, M.; Biswas, K.; Sundaresan, A.; Rao, C. N. R. J. Mater. Chem.
2006, 16, 106-111.
(24) Ould-Ely, T.; Prieto-Centurion, D.; Kumar, A.; Guo, W.; Knowles, W. V.;
Asokan, S.; Wong, M. S.; Rusakova, I.; Luettge, A.; Whitmire, K. H. Chem.
Mater. 2006, 18, 1821-1829.
(25) Shanmugam, S.; Gedanken, A. J. Phys. Chem. B 2006, 110, 24486-24491.
(26) Zhong, X.; Xie, R.; Sun, L.; Lieberwirth, I.; Knoll, W. J. Phys. Chem. B
2006, 110, 2-4.
(27) Chen, Y.; Kim, M.; Lian, G.; Johnson, M. B.; Peng, X. J. Am. Chem. Soc.
2005, 127, 13331-13337.
Dot-shaped MnO nanocrystals were formed as the sole
product in all duration of a growth reaction when ocadecanol
(28) Narayanaswamy, A.; Xu, H.; Pradhan, N.; Kim, M.; Peng, X. J. Am. Chem.
Soc. 2006, 128, 10310-10319.
(29) Brus, L. E. J. Chem. Phys. 1984, 80, 4403-9.
(30) For example: Zhang, Z.; Zhong, X.; Liu, S.; Li, D.; Han, M. Angew. Chem.,
Int. Ed. 2005, 44, 3466-3470.
(34) Thessing, J.; Qian, J.; Chen, H.; Pradhan, N.; Peng, X. J. Am. Chem. Soc.
2007, 129, 2736-2737.
(35) Bellamy, L. J. The Infra-Red Spectra of Complex Molecules, 3rd ed.;
Halsted: New York, 1975; 425 pp.
(31) Yu, W. W.; Peng, X. Angew. Chem., Int. Ed. 2002, 41, 2368-2371.
(32) Battaglia, D.; Peng, X. Nano Lett. 2002, 2, 1027-1030.
(33) Yu, W. W.; Wang, Y. A.; Peng, X. Chem. Mater. 2003, 15, 4300-4308.
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Formation of Monodisperse MnO Nanocrystals
A R T I C L E S
Figure 1. Temporal evolution of size and size distribution of MnO nanocrystals with alcohol (octadecanol) added as the activation reagents. The corresponding
FTIR spectrum for each TEM image is provided on the left side with the vibrational bands for different groups marked.
Figure 2. Temporal evolution of size and size distribution of MnO nanocrystals with free stearic acid (HSt) added as the inhibitors. The corresponding
FTIR spectrum for each TEM image is provided on the left side with the vibrational bands for different groups marked.
was added as an activation reagent. The temporal evolution of
TEM images and FTIR spectra for a typical reaction is shown
in Figure 1. When the temperature was below about 250 °C,
no particles were observed by TEM. At about 265 °C (Figure
1a), small MnO nanocrystals started to appear, and the sizes of
the dots increased rapidly (Figure 1). After the temperature
reached 310 °C for about 3 min, nearly monodisperse MnO
nanocrystals were obtained (Figure 1d). By simply looking at
the TEM images in Figure 1, one might have concluded that
this was a typical process of “focusing of size distribution”.
FTIR spectra (Figure 1, left), however, reveal a nearly
complete depletion of the MnSt2 precursor when the temperature
reached 305 °C (Figure 1b). This means that, although narrowing
of size distribution of the nanocrystals evidently occurred from
the time corresponding to Figure 1b to the time corresponding
to Figure 1d, the precursor was almost completely consumed
prior to this focusing process. Along this focusing process, the
average size showed a significant increase even without looking
at the quantitative data to be described below. These features,
growth and focusing without consuming a significant amount
of monomers, imply that this process was different from the
traditional “focusing of size distribution”. As pointed out above,
the traditional “focusing of size distribution” requires all particles
grow simultaneously, and the simultaneous growth of all
particles must consume a certain amount of monomers. Because
the monomer concentration was close to zero from Figure 1b
to Figure 1d, the monomers needed for the growth of the final
monodisperse dots were likely from the nanocrystals themselves,
which indicates a ripening process. The quantitative data to be
described below will provide further evidence to support this
conclusion.
Monodisperse spherical nanocrystals from elongated
shapes can be obtained when elongated nanocrystals were
formed in the early stage. Generally speaking, when activation
reagents (alcohol) were not added, reactions yielded elongated
MnO nanocrystals, ranging from rice-shape, peanut-shape, rod-
shape, etc., in the early stage under the chosen reaction
conditions for the current system. If the system was allowed to
age for a sufficient amount of time, these elongated nanocrystals
would be converted to dot-shaped. Figure 2 illustrates one of
such reactions that converted nearly monodisperse peanuts
(Figure 2b and c) to nearly monodisperse dots (Figure 2d). This
reaction started with small sized dots (Figure 2a), and then these
dots grew to relatively large peanuts (Figure 2b) within a matter
of a few minutes (additional data in the Supporting Information,
Figure S1). The initially formed peanuts (Figure 2b) were with
sharp corners that were gradually smoothed out to form nearly
monodisperse round peanuts (Figure 2c). This smoothing process
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A R T I C L E S
Chen et al.
did not change the average volume of the nanocrystals notice-
ably. The FTIR spectra (Figure 2, left) indicate that precursors
were nearly completely consumed when this smoothing process
started. These features are consistent with an intra-particle
ripening process.36
Further annealing of these peanuts converted the elongated
nanocrystals into dots with a nearly monodisperse size distribu-
tion (Figure 2d). This process was noticed to be quite slow,
over 1 h, and the shape transition was a gradual process (see
additional data in Figure S1). Similar to the smoothing process
discussed in the above paragraph, the average volume of the
particles was estimated to be constant. Again, there were nearly
no precursors left prior to the shape evolution from peanuts to
dots. These facts imply that the formation of nearly monodis-
perse dots from the peanuts in Figures 2 is likely a continuation
of the smoothing process discussed in the above paragraph, or
intra-particle ripening.36
Elongated nanocrystals in other shapes were also observed
in this non-injection system (see more results below). Upon
annealing at the reaction temperatures, these elongated nanoc-
rystals were all unstable. They became less and less sharp at
corners, and eventually were converted to more or less dots.
One additional example is provided in Figure S2, which
illustrated the formation of nearly monodisperse rice, pearl-
shaped rods, and the smoothing of the rods.
Figure 3. Classic Ostwald ripening of the nearly monodisperse MnO dots.
See reaction in Figure 1.
The results discussed in this sub-section revealed that
elongated shapes are less stable in comparison to dots. Provided
with sufficient time, the elongated nanocrystals would be
converted to dots. This is qualitatively consistent with the
semiconductor nanocrystal model systems studied previously,
such as CdSe ones formed by hot-injection methods.36 However,
formation of monodisperse dot-shaped semiconductor nanoc-
rystals from elongated ones has not yet been reported for
those well-studied semiconductor nanocrystal systems, such as
CdSe ones. This difference between the current system
and the common semiconductor systems will be a topic in the
Discussion.
Classic Ostwald ripening of monodisperse dot-shaped
MnO nanocrystals was observed when the monodisperse dots
were subjected to further heating. For example, the nearly
monodisperse dots shown in Figure 1 were observed to undergo
a classic Ostwald ripening process (Figure 3), from a mono-
disperse sample to a polydispersed one. As expected, the average
volume of the nanocrystals evolved in a typical pattern for
classic Oswald ripening process after the reaction proceeded
for more than 30 min (Figure 3, top right).37 The data points in
the early stage, shorter than 30 min, did not follow the same
linear trend (see the extended plot for this period of time in
Figure 4d). This indicates the existence of a transition stage of
Ostwald ripening, which includes the period of the formation
of the monodisperse dots shown in Figure 1.
A quantitative analysis (Figure 4) of the reaction associated
with the data shown in Figures 1 and 3 was performed. Details
for the analysis are provided in the Experimental Section. The
general assumption is that Mn atoms in the solution existed only
significantly in the form of MnO nanocrystals and the carboxy-
late salt (see the last paragraph in the Discussion for justifica-
tion). Thus, the monomer concentration (Figure 4a) was obtained
Figure 4. Quantitative results for the reaction associated with Figures 1
and 3. Note: The solid trend lines were added to guide the eyes. A dotted
line was provided to indicate the starting point of classic Ostwald ripening.
by integrating the vibration band associated with the carboxylate
salt centered at about 1556 cm-1 (see Figures 1 and 2). The
average size (Figure 4d) and size distribution (Figure 4c) of
(36) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 1389-1395.
(37) Lifshitz, I. M.; Slyozov, V. V. J. Phys. Chem. Solids 1961, 19, 35-50.
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Formation of Monodisperse MnO Nanocrystals
A R T I C L E S
the nanocrystals were obtained by image analysis with about
400 particles for each point (see Experimental Section). The
particle concentration (Figure 4b) was calculated using the
amount of consumed monomers divided by the number of MnO
units in an average sized nanocrystal at a given moment.
Because TEM was used as the sole means for determining size
and size distribution, it was difficult to evaluate particles smaller
than about 2-3 nm. Consequently, it is impossible to study the
nucleation stage quantitatively using this method.
The average size (Figure 4d) of the particles increased
substantially in the very beginning (shorter than 1 min). The
monomers were consumed rapidly in this rapid growth period
(Figure 4d). This fast growth was followed by a slow growth
period until the reaction proceeded for about 30 min, and then
a rapid increase of the average size of the nanocrystals was
observed in the classic Ostwald ripening discussed above (also
see Figure 3, top right). Overall, the size distribution was
narrowed down significantly in the first 10 min (Figure 4c),
and then a significant broadening occurred after 30 min in the
classic Ostwald ripening stage. For clarity, a straight dotted line
was drawn in Figure 4 to indicate the beginning of the classic
Oswald ripening.
Figure 5. Size distribution histograms of MnO nanocrystals for two samples
shown in Figure 4.
the population of the relatively smaller ones reduced dramati-
cally. It is interesting to notice that both size distribution profiles
in Figure 5 have a tail on the small size side, although this tail
decreased significantly in the late stage. This distribution feature
is qualitatively consistent with that expected for a ripening
process.37 A more detailed discussion about the size distribution
evolution and narrowing of size distribution in this transient
stage of ripening will be offered in the Discussion.
Ripening for the reactions with alcohol was a surprise to
us. All original stearate ligands from MnSt2 were converted into
inactive esters before the ripening processes started (Figures 1
and 4a). In principle, both inter-particle and intra-particle
ripening processes need the monomers to be reasonably soluble
in the solution as it was discussed previously.27 Consistent with
this hypothesis, it was reported that, when alcohol was added
and with similar reaction conditions, both ZnO and In2O3
nanocrystals did not show visible ripening.27,28 Thus, one would
not expect ripening to occur in the current system with a
sufficient amount of alcohol added because alcohol consumed
all active ligands, both stearates and stearic acid.
A noticeable decrease of the particle concentration was
observed in the entire duration of the reaction demonstrated here
(Figure 4b), which indicates the entire process shown in Figure
4 was ripening in nature. Consistent with this, monomers were
consumed almost completely in the very early period of this
reaction (Figure 4a). As pointed out above, however, the average
size of the nanocrystals kept increasing. This is also expected
for a ripening reaction, although there are some substantial
differences in size distribution evolution for the transient stage
and the classic stage. Different from the classic Ostwlad ripening
process (approximately after 30 min indicated by the dotted line
in Figure 4), a continued narrowing of size distribution was
observed in the transient stage of ripening, indicated by the
decrease of the standard deviation of size distribution (δ)
(Figures 4c and 1). Also consistent with a non-steady Ostwald
ripening, the temporal evolution of the average volume of the
nanocrystals did not follow the well-known linear relationship37
(see Figure 3, top right, and the related text).
To clarify this confusing observation, MnO nanocrystals were
isolated and purified from the reaction solution. These purified
nanocrystals were added back into a controlled environment to
study the ripening process. It should be pointed out that,
although the vibrational band corresponding to carboxylate
group at about 1556 cm-1 was barely observed in the unpurified
reaction mixture (Figures 1 and 2), the purified nanocrystals
showed a strong band in the region (Figure S3), indicating that
the MnO nanocrystals were protected with stearate ligands,
which is the same as that for the ZnO and In2O3 systems.27,28
The purified MnO nanocrystals were then subjected to
annealing under different conditions. When they were heated
in either pure ODE or ODE with alcohol, no significant change
of the size and size distribution was observed. One example
for these reactions is illustrated in Figure 6 (top). This indicates
that alcohol on its own cannot initiate ripening.
The evolution of size distribution profile of the MnO dots
in the reaction related to Figures 1, 3, and 4 is also found to be
consistent with narrowing of size distribution through a ripening
process. The fundamental principle for ripening, no matter if it
is in the transient stage or the late steady-state stage, is that the
solubility of nanocrystals increases as the size of the nanocrystals
decreases (see details in the Discussion). For the transient stage
shown in Figure 4, the particle concentration decreased sub-
stantially. This decrease, according to the fundamental principle,
should occur on the small size side, which is qualitatively
consistent with the TEM images of the nanocrystals shown in
Figure 1.
Quantitatively, this can be demonstrated through the size
distribution evolution in the transient stage (Figure 5). The
relatively broad size distribution at 0.8 min (Figure 4, bottom)
was significantly narrowed as the reaction proceeded to 10 min
(Figure 4, top). It should be pointed out that the overall size
distribution for the sample at 10 min was the narrowest for this
particular synthesis according to Figure 4c. The main change
on the large size side was the increase of the population, but
The second set of experiments was performed with water-
saturated Ar bubbling through the hot ODE solution of the
purified MnO nanocrystals, instead of pure Ar flow. The other
reaction conditions were maintained the same as those in the
first set of experiments. This was achieved by bubbling the Ar
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A R T I C L E S
Chen et al.
water indicates that there must be a significant amount of water
in the reaction system to take effect. This conclusion goes well
with the reaction mechanism for the current system as to be
discussed in the Discussion. Briefly, when alcohol was used,
two stoichiometric amounts of water (measured by MnO unit)
were yielded initially as a side product along the formation of
MnO nanocrystals. The water consumed through ripening
process was continuously regenerated in a ripening cycle.
Shape control of MnO nanocrystals has been a hot topic
in the past few years, and some of the examples are cited in the
references.13,14,20-26 The current reaction system can also
generate several different shapes with well-controlled size and
shape distribution. Three-dimensional nanoflowers were reported
separately,38 and the related mechanism seems to be reasonably
resolved, which will thus not be discussed here. The main focus
of this Article is the mechanism on growth of dot-shaped
nanocrystals and the related chemical reactions. The details of
formation of the elongated nanocrystals are under study. Here,
only some issues related to this work will be illustrated.
Figure 7 summarized a few typical examples of elongated
MnO nanocrystals along with a monodisperse dot sample. The
crystal structure of all of these shapes formed in the current
reaction system was identified to be cubic MnO. One repre-
sentative electron diffraction pattern is provided in Figure 7
(inset).
Figure 6. Ripening of purified MnO nanocrystals under different
conditions.
gas through a liquid water reservoir, prior to being introduced
into the reaction mixture. When the liquid water was at room
temperature, no sign of ripening was observed.
It was noticed that the purity of the MnSt2 precursor played
a visible role in determining the shape of the MnO nanocrystals
and the reproducibility of the synthesis. If a stoichiometric
amount of base was mixed with the fatty acid prior to the
addition of MnCl2, the resulting MnSt2 (after purification and
drying) was a white (or slightly pinkish) powder. If the reaction
was with excess base, or, the base was added in after the mixing
of the fatty acid and MnCl2, the resulting fatty acid salts would
be brown (see pictures in the Supporting Information, Figure
S4). This brownish color was most likely associated with the
formation of a small amount of Mn hydroxide or oxide. The
existence of such impurities made the reaction to preferentially
form elongated MnO nanocrystals with the same cubic structure,
but the results were generally less reproducible. In addition to
avoiding the preformation of Mn hydroxide and oxides,
However, when the liquid water in the reservoir was heated
to boiling, ripening became immediately evident. Under oth-
erwise the same conditions, the size distribution of the MnO
nanocrystals became dramatically broadened when both alcohol
and water steam were in place (Figure 6, middle). Even without
alcohol, meaning purified MnO nanocrystals placed back in pure
ODE, ripening of MnO nanocrystals was still observed as long
as the water steam carried by Ar was bulbed into the reaction
mixture (Figure 6, bottom).
The above results clearly indicate that water played a decisive
role for any ripening process to occur in the current reaction
system when alcohol was present. The substantial difference
between bulbing Ar through room-temperature water and boiling
Figure 7. TEM images of differently shaped MnO nanocrystals with a representative electron diffraction pattern (inset). Spherical MnO nanocrystals were
synthesized by a reaction of MnSt2 with excess 1-octadecanol. Rice-shaped and other elongated MnO nanocrystals were synthesized by reactions of MnSt2
with added free stearic acid. Rice-shaped MnO nanocrystals were formed at a very early stage. The indexes of the diffraction rings are as follows: (1) (111),
(2) (200), (3) (220), (4) (311), (5) (222), (6) (420), and (7) (422). Note: The image of a long needle has a different scale bar.
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Formation of Monodisperse MnO Nanocrystals
A R T I C L E S
the addition of inhibitor/activation reagents. One of the reactions
had no inhibitor/activation reagent added, pure MnSt2 in ODE.
The second reaction had free stearic acid added as the inhibitor
in addition to MnSt2 and ODE. The third reaction was initiated
with the addition of octadecanol as the activation reagents into
the MnSt2 and ODE mixture. For all three cases, the precursor
concentration decreased as the reaction proceeded due to the
formation and growth of the MnO nanocrystals (Figure 8). The
depletion rate of the monomers, however, clearly differed from
one case to another. The solid line for each case in the plot
(Figure 8) is a fitting to the first-order of MnSt2 concentration.
The observed reaction constants are provided in the plot. Stearic
acid, as expected for inhibitors, indeed decreased the reactivity
of MnSt2, about a 50% decrease under the current conditions
judged by the reaction rate constants. Conversely, octadecanol
as an activation reagent increased the reactivity of the MnSt2
precursor about twice.
Figure 8. Temporal evolution of the monomer concentration under different
reaction conditions determined by time-resolved FTIR measurements.
Inset: Maximum particle concentrations for two reactions differed from
each other by the addition of alcohol versus stearic acid.
methanol used as the solvent for the synthesis of MnSt2 should
also be removed by vacuum-drying. If methanol residual were
left, the FTIR spectra would show the formation of ester even
if no octadecanol, the activation reagents used exclusively in
this work, were added. Again, precursors with methanol residual
would also make the reactions less reproducible. It should be
pointed out that, although the precursor purity played a role in
determining the reproducibility and shape of the nanocrystals,
the ripening patterns discussed above were found to be generally
true.
The theory of the shape-controlled growth of nanocrystals
developed from the results on semiconductor nanocrystal
systems further implied that the concentration of nuclei should
be low for the formation of elongated nanocrystals in compari-
son to that for the formation of dots.39 In the current system,
and probably in other systems as well, it was difficult to exactly
define the nucleus concentration. We suggest here to use the
maximum particle concentration for a given reaction system as
a measure of the nucleus concentration. The results shown in
Figure 8 (inset) indicate that the nucleus concentration following
this definition was about 2.5 times lower for the reaction with
free stearic acid in comparison with that for the one with alcohol
added. This further confirms the shape control model discussed
above.
Formation of elongated MnO nanocrystals followed the
general trend observed for semiconductor nanocrystals, although
no injections were involved in the current system. They were
typically formed under the conditions with free stearic acid
added. The existing theory39 based on studies of the shape
control of semiconductor nanocrystals indicates that, if the
activity of the monomers is low, with stearic acid as inhibitors
in the current case, the number of nuclei formed in the solution
is small. Subsequently, a high remaining monomer concentration
allows a small amount of nuclei to grow under an environment
with high chemical potential, which makes the elongated
nanocrystals stable and accessible in the reaction system. This
rapid growth of the elongated nanocrystals depletes the monomer
rapidly. As a result, the elongated nanocrystals are no longer
stable, and intra-particle ripening36 shall convert the elongated
nanocrystals to more round in shape. As shown above, this
model can well explain the results in the current system.
As discussed above, free ligands were frequently used for
reducing the nucleus concentration in a system presumably by
decreasing the reactivity of the monomers in the nucleation
stage.39 To our knowledge, this proposed reduction of the
monomer reactivity has not yet been observed directly. Gener-
ally, the monomer concentration for semiconductor nanocrystals
was calculated by the difference of the precursors added and
the monomers in the form of nanocrystals for semiconductor
nanocrystal systems.36 Using the FTIR measurements, the
precursor concentration in the current system can be directly
measured. It thus offers an opportunity to confirm this important
hypothesis.
Discussion
“Focusing of size distribution” is currently the main model
for explaining growth of nearly monodisperse nanocrystals with
dot-shape.10 To better understand the discrepancies between this
existing model and the experimental results in this work, a brief
discussion for the traditional “focusing of size distribution”
model will be provided as follows.
The traditional “focusing of size distribution” requires that
there should be no nucleation and all particles shall grow
simultaneously in the duration of narrowing of size distribution
in a reaction. The narrowing of size distribution occurs because
the growth rate increases as the size of the nanocrystals
decreases (Figure 9, left, and discussions below). This means
that this pure growth stage allows the small nanocrystals to
“catch up” with the large size nanocrystals.
The catching-up of the relatively small particles happens in
a different way dependent on whether the reaction is diffusion-
controlled or growth-controlled.10 For growth-controlled sys-
tems, the smaller particles have a high reactivity because of
their higher surface energy and relatively high surface to volume
ratio. However, growth of high-quality nanocrystals has been
mostly regarded as diffusion-controlled at present.10 One of the
main reasons is that the surface reaction sites of nanocrystals
should be largely blocked by the ligands around each nanoc-
rystal, which constitutes a good portion of the diffusion barrier.
The dynamic feature of the surface ligands allows the diffusion
through the ligand monomer to occur, but also spatially extends
the size of the diffusion barrier. This feature justifies a diffusion
Figure 8 illustrates the temporal evolution of monomer
concentration of three reactions under identical conditions except
(38) Narayanaswamy, A.; Xu, H.; Pradhan, N.; Peng, X. Angew. Chem., Int.
Ed. 2006, 45, 5361-5364.
(39) Peng, X. AdV. Mater. 2003, 15, 459-463.
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A R T I C L E S
Chen et al.
nanocrystals. Interestingly, it also revealed a substantial decrease
of the particle concentration in the course of narrowing of size
distribution. Unfortunately, the authors did not report the
evolution of monomer concentration and the size distribution
profile.
An alternative mechanism for the formation of monodis-
perse dots in these non-injection-based systems seems to be
needed as the existing “focusing of size distribution” model is
contradictory to the experimental results as discussed in the
above section. A recent discovery, inter-particle diffusion with
a high particle concentration, attracted our attention. This was
observed by studying the dissolution/ripening processes of CdS
and CdSe nanocrystals.34 Under high particle concentrations,
the complete dissolution of the small particles could occur rather
rapidly through inter-particle diffusion. This greatly reduces the
population of the relatively small particles in the solution, which
is similar to what is illustrated in Figure 5. For pure dissolution/
ripening processes studied for CdS nanocrystals, the size
distribution could be narrowed down to a few percent range
(about 7% for the example reported).34 This value is much below
the low limit of size distribution (about 25%) predicted by the
recent theoretical treatments of classic Ostwald ripening in
nanometer size regime.41,42
Although this inter-particle diffusion phenomenon was identi-
fied in dissolution/ripening of nanocrystals for pre-synthesized
nanocrystals and mostly at room temperature, it may actually
be more significant for a synthetic reaction under elevated
temperatures. Typically, a synthetic system could have a high
particle concentration, so inter-particle distance should be small.
At a fixed particle concentration, as the thermal energy of
particles increases, the frequency for the particles to encounter
each other shall increase substantially.43 This means that
narrowing of size distribution enabled by inter-particle diffusion
could occur in a synthetic system similar to the one studied
here. Because this new size-focusing phenomenon occurs by
simply mixing reaction components together, opposite to build-
up of a nucleation-growth balance by hot-injections of the
precursors,2 we tentatively suggest to name it as “self-focusing
of size distribution”, that is, “self-focusing” in short. There is
another important reason, to be discussed later, that promoted
us to call this phenomenon self-focusing.
Figure 9. Differences between traditional “focusing of size distribution”
(left) and self-focusing (right). The diffusion sphere for each nanocrystal
is depicted as a dashed circle. The solid arrows represent the diffusion flux.
sphere model2,40,41 as to be discussed below. For these reasons,
the discussions below will be limited in the diffusion-controlled
domain.
The growth rate difference between different size particles
for a typical diffusion-controlled process can be approximately
illustrated using the diffusion sphere model (Figure 9, left).2,40,41
In this model, each nanocrystal should have a same sized
diffusion sphere, and the size of the nanocrystal is negligible
in comparison to the size of its diffusion sphere.2,40,41 If the
monomer concentration is sufficiently high, more or less the
same amount of monomers should be diffused into the diffusion
spheres in this simple picture (Figure 9, left), which should make
the volume growth rate (V′) of each nanocrystal the same.2
V′ ) 1/2πd2(d′) ) constant
(1)
Equation 1 indicates that the size (d) growth rate (d′) of
nanocrystals, however, should dramatically increase as the
size (d) of the nanocrystals decreases, proportional to 1/d2. This
means that the relatively small nanocrystals shall rapidly
catch up while all particles in the solution are growing simul-
taneously. In a mean field consideration, all small nanocrystals
should grow rapidly and the small sizes in the initial size
distribution profile should be “cut off”, which results in a
symmetric size distribution.
With the particle concentrations in this work, roughly close
to 1 µmol/kg (Figures 4b and 8 (inset)), the average distance
between particles would be about 100 nm. Thus, if the thickness
of the diffusion sphere of each nanocrystal (roughly 20 nm in
average) was greater than 50 nm, overlapping of the diffusion
spheres of nanocrystals in the solution should become inevitable
from a mean field standpoint, even if ignoring the vigorous
thermal motion of the particles under elevated temperatures.
As often cited in literature, the size of a particle has been
regarded as negligible in comparison to the diameter of the
diffusion sphere for diffusion-controlled growth.2,40,41 It should
be noticed such a consideration was originally proposed for the
growth of nearly monodisperse colloidal particles in micron size
range,40 which should thus be quite valid for the discussions in
nanocrystal systems. On the contrary, because the monomer
concentration rapidly reached nearly zero (Figures 1 and 4),
In summary, there are several interesting signatures for the
traditional “focusing of size distribution” model. The first feature
is that the particle concentration during “focusing of size
distribution” should be a constant. The second feature requires
that the monomer concentration in the solution must be
sufficiently high and the monomers shall be significantly
consumed due to the simultaneous growth of all nanocrystals.
The third feature indicates that the size distribution profile of
the nanocrystals should not develop a tail on the small size side.
Evidently, numerous experimental results demonstrated in the
Results were against all of these signatures of the traditional
“focusing of size distribution”.
To our knowledge, there was only one publication in the
literature that reported the temporal evolution of particle
concentration for non-injection-based systems,16 which was
based on the growth of CdSe nanocrystals in ODE and
determined using the size-dependent optical properties of CdSe
(42) Talapin, D. V.; Rogach, A. L.; Haase, M.; Weller, H. J. Phys. Chem. B
2001, 105, 12278-12285.
(43) Atkins, P. W. Physical Chemistry, 6th ed.; W. H. Freeman & Company:
New York, 1998.
(40) Sugimoto, T. AdV. Colloid Interface Sci. 1987, 28, 65-108.
(41) Mantzaris, N. V. Chem. Eng. Sci. 2005, 60, 4749-4770.
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Formation of Monodisperse MnO Nanocrystals
A R T I C L E S
growth due to the solution-particle diffusion was insignificant
shortly after a reaction was initiated. Therefore, the narrowing
of size distribution of the dots should be largely due to self-
focusing, instead of the traditional “focusing of size distribution”.
Similar to any mechanism consideration for crystallization,
the Gibbs-Thompson equation44 (eq 2) is the starting point for
describing self-focusing in non-injection synthesis.
inter-particle diffusion (Figure 9, right). This means that the
nucleation process would be practically stopped. This is the other
reason why this alternative model might be called “self-
focusing”.
The second pathway is based on the exponential increase of
the solubility of the nanocrystals as their size decreases (eq 2).
This can be visualized by comparing the dissolution and growth
rates of two particles: one is smaller than the mean size by
∆d, and the other is greater than the mean size by ∆d. Because
of the exponential nature of the size dependence of the solubility
of the nanocrystals in a solution (eq 2), the dissolution rate of
the small nanocrystal should be substantially higher than the
growth rate for the large nanocrystal (see eq 3). Furthermore,
as dissolution/growth proceeds, this rate difference shall further
be enhanced rapidly. This is the key reason why small
nanocrystals, from either growth or nucleation, in a distribution
can be dissolved rapidly and completely through self-focusing.
There is a pure geometric argument that also favors narrowing
of size distribution under inter-particle diffusion. For instance,
if all of the monomers of a 15 nm particle are transferred onto
a 25 nm particle, the size of the 25 nm particle will only increase
by about 6%. This means that, under the control of inter-particle
diffusion, the particles smaller than the mean size in the solution
shall dissolve very rapidly, without much size increase of the
large particles. Even among the large particles with a positive
growth rate defined by eq 3, this geometric argument shall still
improve the size distribution because the relatively large ones
shall grow slower with the same amount of feedstock. For
instance, the monomers sufficient for a 25 nm particle to grow
by 6% will be enough for a 20 nm particle to grow for about
20%. This indicates that, among the large particles with a
positive growth rate, the large ones have a tendency to “wait
for” the smaller ones to catch up.
It should be pointed out that self-focusing might not only be
in place for non-injection-based synthesis. This is so because
the particle concentration in some hot-injection systems was
also found to decrease while the size distribution was narrowing
down.45 In addition, the nucleation process for injection-based
methods often lasted for a significant amount of time, a few
minutes,45 which is not really instantaneous.
As the inter-particle diffusion proceeded, the particle con-
centration would eventually drop to a critical level, which makes
inter-particle diffusion loss its dominating position. At this point,
the classic Ostwald ripening through the traditional solution-
particle diffusion would gradually kick in and become more
and more dominating in the system (Figure 3 and the late stage
in Figure 4). Consequently, the size distribution of nanocrystals
broadened.
Sd ) S∞exp(4σVm/dRT)
(2)
σ and Vm are the specific surface energy and molar volume of
a crystal, respectively. S∞ is the solubility of the bulk crystal,
and Sd is that for a crystal with its diameter (or size) as d. R is
the gas constant, and T is absolute temperature.
Equation 2 implies that the solubility of crystals decreases
substantially as their size increases, which is a result of the
decrease of the surface atom ratio.8 The driving force for inter-
particle diffusion is the solubility gradient between two nearby
nanocrystals with their diffusion spheres overlapping with each
other. Because the monomer concentration during the narrowing
of size distribution observed in the current system was close to
zero (see data in Figures 1 and 4), the solution-nanocrystal
diffusion can be ignored. With these facts, one can then
obtain eq 3.
d′ ) 5Dπq2(Save - Sd)[P]
(3)
where Save - Sd is the mean inter-particle solubility gradient, D
is the diffusion coefficient of monomers, q is the radius of the
diffusion sphere, and [P] is the particle concentration. Consider-
ing the rapid thermal motion of the particles in the solution,
each particle shall change its neighbors frequently. For this
reason, the solubility of the neighboring nanocrystals for a given
particle is represented by an ensemble averaged value, Save. The
particle concentration, [P], appears in both eq 3 and the inter-
particle diffusion term in the original equation.34 This is because
the number of neighboring particles and the average distance
between the particles are both related to particle concentration.
Equation 3 tells that, if a particle is smaller than the average
size, the growth rate shall be negative (dissolution). Conversely,
for a particle larger than the average size, the growth rate is
positive (growth). This means that, if a reaction is truly
controlled by inter-particle diffusion, the relatively small
particles shall always dissolve and their monomers will directly
feed the growth of their neighboring big ones (Figure 9, right).
This serves as the basis for narrowing the size distribution
through two different pathways.
The first pathway lies on controlling the generation of new
nuclei. If the system is truly controlled by inter-particle diffusion,
a self-separation of nucleation and growth should be expected,
which is different from the traditional “focusing of size
distribution”. By the initial nucleation of a large population of
nanocrystals and the subsequent growth of these nanocrystals
to relatively large sizes, monomer concentration should be
rapidly depleted (see Figure 4a as an example). This shall slow
down the nucleation rate. More importantly, the slowly formed
nuclei, very small in size and highly soluble (eq 2), became
extremely unstable and would be instantaneously consumed by
the large amount of big particles in the surroundings through
Formation of nearly monodisperse dots from elongated
nanocrystals was an interesting observation. FTIR spectra
indicate (Figure 2) that there was no net monomer consumption
in the process, and the average volume of the nanocrystals
judged by TEM (Figure 2) did not change significantly. These
features match intra-particle ripening observed in the case of
CdSe nanocrystals, which changed the CdSe nanorods to dots.36
However, the intra-particle ripening observed for CdSe nanoc-
rystals could only occur with a quite high monomer concentra-
tion, and the resulting dots were far from being monodisperse.
The first difference was likely due to the vastly different
(44) Mullin, J. W. Crystallization, 3rd ed.; Butterworth-Heinemann: Oxford,
1997.
(45) Qu, L.; Yu, W. W.; Peng, X. Nano Lett. 2004, 4, 465-469.
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A R T I C L E S
Chen et al.
solubility/stability of the metal complexes in each system.
Because of the very low solubility of Mn ions in the current
system, it was possible to have intra-particle ripening under a
very low monomer concentration.
system. With the experimental results shown above, the fol-
lowing reaction occurred as the initial step for a ripening
process. The free stearic acid generated by this reaction can
then react with the relatively small nanocrystals and dissolve
them following the same reaction shown by eq 5. Consequently,
the regenerated MnSt2 by eq 5 would grow back to a large
nanocrystal in the solution to complete a ripening process
through either eq 4 without alcohol (Figure 5, bottom) or eq 6
with alcohol (Figure 6, middle). Consistent with this hypothesis,
FTIR measurements (Supporting Information, Figure S5) showed
the gradual formation of esters in the ripening process with the
existence of alcohol. This conclusion also explains why the
ripening processes occurred much faster when both alcohol and
water steam were in place (comparing data in Figure 6, middle
and bottom panels).
The molecular mechanism discussed here offers a strong
backup of the assumption that the Mn atoms should exist in
the form of MnO nanocrystals or MnSt2. This assumption was
the basis for the calculation of monomer concentrations in the
reaction solution using FTIR spectra of MnSt2 (see Figure 4
and related text). Although alcohol and ODE could be potential
ligands for Mn ions and Mn ions could also be reduced to Mn0,
these species would not be stable at all. This is so because all
of these compounds would be much more reactive toward water
in comparison to MnSt2. As discussed above, the reaction
environment in the current system was sufficiently active to
hydrolyze MnSt2 (eq 4) and stearate ligands on the surface of
nanocrystals (eq 7). Therefore, although it is difficult to exclude
those complexes as possible intermediates, a significant con-
centration of those complexes in the reaction solution would
be unlikely.
The outstanding size distribution of the dots formed by intra-
particle ripening in the current system is also likely a result of
the low solubility of MnSt2 monomers. This means that the intra-
particle ripening of the elongated nanocrystals could be well
separated from the following classic Ostwald ripening that could
only start with a very low monomer concentration. The low
stability/solubility of the monomers in the solution made the
intra-particle ripening last for a long period of time (Figures 2
and S2), a few hours. Consequently, if the volume distribution
of the elongated nanocrystals were sufficiently narrow, a nearly
monodisperse sample of dot-shaped nanocrystals would be the
result (Figure 2). If the particle concentration were sufficiently
high, a self-focusing process prior to the classic Oswald ripening
may occur, which could further improve the size distribution
of the dots.
The molecular mechanism of the current system can be
reasonably identified because of its simplicity and all organic
species are IR active. When heating MnSt2 precursor in ODE,
with or without free stearic acid (HSt) added, the precursor went
for hydrolysis to yield MnO and HSt indicated by the increased
(decreased) intensity of the corresponding -COOH (-COO-)
vibration band (see FTIR spectra in Figure 2). Although zinc
stearate did not go for any hydrolysis,27 a similar process was
identified for the formation of In2O3 nanocrystals.28 The
hydrolysis reaction is as follows:
MnSt2 + H2O f (MnO)n + HSt
(4)
Here, (MnO)n represents MnO nanocrystals. HSt, either gener-
ated by the above reaction or added as inhibitors, made the
above reaction reversible.
(MnO)n + HSt f MnSt2 + H2O
(5)
The above chemical equations (eqs 4 and 5) should be the
reactions involved in the formation of MnO nanocrystals without
alcohol added (see Figure 2 as one example), with eq 4 for the
growth and eq 5 for dissolution needed for any ripening process,
that is, intra-particle ripening, self-focusing based on inter-
particle diffusion, and the classic Ostwald ripening.
The reactions with octadecanol (ROH) added as activation
reagents were quite different from the reactions discussed above.
Alcohol reacted with MnSt2 to form ester (see FTIR spectra in
Figure 1), H2O, and MnO nanocrystals.
Conclusion
Quantitative results in this work indicate that a new mech-
anism for the formation of nearly monodisperse nanocrystals
seems to exist. Different from the traditional “focusing of size
distribution”, the newly proposed mechanism, self-focusing,
requires a reasonably high particle concentration but without
the need of a high monomer concentration in the solution. With
a high particle concentration, the diffusion spheres of nanoc-
rystals shall overlap, which causes direct inter-particle diffusion
from small particles to the relatively large ones. As a result,
the relatively small nanocrystals in solution dissolve very rapidly
and completely, which results in an automatic stop of nucleation
and the cutoff of the very small nanocrystals in the distribution.
The molecular mechanisms identified in this work implied that
water actually played a critical role in both growth and ripening
of the nanocrystals, although the reaction temperature was
around 300 °C in a hydrocarbon solvent. The mechanistic
information reported here shall not only assist development of
relatively simple synthetic chemistry for high-quality nanoc-
rystals with a variety of functions, but also impact other fields
related to crystallization. This is so because quantitative studies
of crystallization systems in nanometer regime are still rare in
MnSt2 + 2ROH f (MnO)n + 2St-OR + H2O (6)
Different from the reaction in eq 4, the above reaction has been
proven to be irreversible because of the high stability of the
esters formed (St-OR) (Figures 1 and 6 (top)) as demonstrated
in the ZnO and In2O3 nanocrystal systems.27,28 As a result, there
was no evidence for ripening for both ZnO and In2O3 nanoc-
rystal systems when alcohol was added into the reaction
systems.27,28 Several types of ripening processes, self-focusing
based on inter-particle diffusion and the classic Oswald ri-
pening, did occur in the current system with an excess of
alcohol. The tests summarized in Figure 6 reveal that water was
playing a decisive role in these ripening processes of the current
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Formation of Monodisperse MnO Nanocrystals
A R T I C L E S
of the solution before reaction at room temperature was also taken as
a reference for quantitative concentration calculation.
the literature, which has been considered as the main roadblock
for developing a meaningful theoretical framework for under-
standing crystallization.44,46
FTIR Analysis and Temporal Concentration Calculation. FTIR
spectra were recorded with a Bruker TGA-FTIR spectrometer. The
samples for FTIR measurement were taken out of the reaction solution
using a syringe, and, prior to any solidification of the reaction mixture,
a thin film of a sample was then prepared on a CaF2 salt plate for
recording FTIR spectrum. The temporal concentrations of Mn(St)2,
stearic acid, and ester were calculated from integrated peak area of the
corresponding peaks. The methyl group (-CH3) peak at 1375 cm-1
was chosen as the reference peak for peak area calculations. The
thickness of the thin films was controlled to avoid the saturation of
any FTIR peak. The FTIR spectra of each sample were measured three
times. The average concentration and standard deviation of these three
measurements were calculated. The error bars represent three times of
the standard deviation to achieve 95% confidence level.
Experimental Section
Materials. Manganese chloride, stearic acid (95%), 1-octadecanol,
1-octadecene (ODE), methanol, and toluene were purchased from
Aldrich and used as received without further purification. Tetramethy-
lamonium hydroxide was from Alfa Asear.
Preparation of Manganese Stearate Precursor. Two methods were
used to prepare manganese stearate. Method A: Manganese chloride
and 2 equiv of stearic acid were dissolved in methanol solution, and
the stearic acid was neutralized with equivalent tetramethylammoinum
hydroxide in methanol solution. During the neutralization process,
brownish Mn(St)2 precipitated. Mn(St)2 was washed with methanol three
times and dried in a vacuum oven at 60 °C overnight. Method B: The
stearic acid methanol solution was neutralized with tetramethyl am-
monium hydroxide solution and then mixed with manganese chloride
methanol solution. White Mn(St)2 precipitated immediately after mixing.
Mn(St)2 was washed with methanol three times and dried in a vacuum
oven at 60 °C overnight.
Synthesis of Dot-Shaped MnO Nanocrystals. Mn(St)2 (0.5 mmol,
white colored), 1-octadecanol (2 mmol), and ODE (5 g) were loaded
in a 25 mL three-necked flask. The mixture was heated to 310 °C at a
rate of 30 °C/min under Ar flow. The solution was kept at this
temperature for a few hours. Aliquots were taken for FTIR and TEM
analysis at different reaction intervals. The FTIR spectrum of the
solution before reaction at room temperature was also taken as a
reference for quantitative concentration calculation.
Synthesis of Peanut-Shaped MnO Nanocrystals and Shape
Transition. Mn(St)2 (1 mmol, brownish colored), 1 mmol of stearic
acid, and 5 g of ODE were loaded in a 25 mL three-necked flask. The
mixture was heated up to 300 °C at a rate of 30 °C/min. The solution
was kept at this temperature for a few hours. Aliquots were taken for
FTIR and TEM analysis. The FTIR spectrum of the solution before
reaction at room temperature was also taken as a reference for
quantitative concentration calculation.
First-Order Reaction Rate Determination. The Mn(St)2 concentra-
tion in ODE was kept as constant (0.5 mmol of reactant in 5 g of
solvent) for three reactions, pure Mn(St)2, Mn(St)2 plus free stearic
acid (0.2 mmol), and Mn(St)2 with 2 mmol of 1-octadecanol. The
reaction conditions of these three reactions were the same as those
described above. Five to seven aliquot solutions were taken during the
first few minutes after the reaction reached the designated temperature,
310 °C. Sampling started at 290 °C when 1-octadecanol was used as
activation reagent due to the very fast reaction rate. The FTIR spectrum
Transmission electron microscopy images and SAED were taken
on a JEOL 100 CX microscope. The MnO nanocrystals were dispersed
in toluene to form an optically clear solution. A copper grid covered
with a thin film of Formvar was dipped into the solution and dried in
air. The size and size distribution of the nanocrystals for a given sample
were initially determined using IMAGE J software. In each case, at
least 2000 of particles were included for the statistics. However, this
software was found to have quite low resolution, and the results were
only used as references in semiquantitative evaluation. For the
quantitative analysis shown in Figures 4 and 5, the results were based
on the manual measurements of about 400 particles for each sample.
MnO nanocrystal concentration at a given reaction moment was
determined by the combination of FTIR and TEM measurements. The
concentration of the precursor, Mn(St)2, at the given moment was
determined by the corresponding FTIR spectrum as described above.
The consumed concentration of Mn ions at the given moment was the
difference between the initial Mn(St)2 concentration and the Mn(St)2
concentration at the corresponding moment. The average number of
Mn ions in a single MnO nanocrystal was calculated using the average
size determined with TEM and the density of MnO bulk crystal. The
particle concentration of MnO nanocrystals in solution was then
calculated by dividing the consumed Mn ion concentration by the
number of Mn ions in a single MnO nanocrystal.
Acknowledgment. Financial support of this work was
provided by the National Science Foundation.
Supporting Information Available: Supporting results men-
tioned in the text. This material is available free of charge via
the Internet at http://pubs.acs.org.
(46) Peng, X. Chem.-Eur. J. 2002, 8, 334-339.
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