Self-assembly of 2D nanosheets into 3D dendrites based on the organic small molecule ANPZ and their size-dependent thermal properties

Article abstract of DOI:10.1021/cg201351j

A surfactant-assisted self-assembly technique was developed to prepare three-dimensional (3D) dendritic micro-/nanostructures of 2,6-diamino-3,5- dinitropyrazine (ANPZ). The resulting micro-/nanostructures exhibited perfect fractal features. A series of experiments indicated that the formation of the dendritic ANPZ microstructures was dominated by the process nucleation and growth → aggregation → further growth, which is similar to the growth process of inorganic dendrites. The influence of various experimental parameters on the morphology of final product was also examined, and it was found that the size and morphology of the final product can be easily controlled by adjusting experimental parameters. In addition, the ANPZ micro-/nanostructures exhibited a seemingly morphology-dependent thermal effect that was actually size-dependent and has the potential for use in energy-demanding microscale systems. This wet chemical route, which has been widely used in the synthesis of inorganic nanomaterials, will provide useful information for the design and fabrication of organic small-molecule micro-/nanostructures with novel morphologies.

Full text of DOI:10.1021/cg201351j

Article
pubs.acs.org/crystal
Self-Assembly of 2D Nanosheets into 3D Dendrites Based on the
Organic Small Molecule ANPZ and Their Size-Dependent Thermal
Properties
Bing Huang,†,‡,§ Minhua Cao,*,†,‡ Bibo Cheng,§ Jie Sun,§ Na Li,†,‡ Fude Nie,§ Haobin Zhang,§
Hui Huang,*,†,§ and Changwen Hu*,†,‡
‡
†Key Laboratory of Cluster Science, Ministry of Education of China, Department of Chemistry, and State Key Laboratory of
Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, P. R. China
§Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, P. R. China
S
* Supporting Information
ABSTRACT: A surfactant-assisted self-assembly technique
was developed to prepare three-dimensional (3D) dendritic
micro-/nanostructures of 2,6-diamino-3,5-dinitropyrazine
(ANPZ). The resulting micro-/nanostructures exhibited
perfect fractal features. A series of experiments indicated that
the formation of the dendritic ANPZ microstructures was
dominated by the process nucleation and growth →
aggregation → further growth, which is similar to the growth
process of inorganic dendrites. The influence of various
experimental parameters on the morphology of final product was also examined, and it was found that the size and morphology
of the final product can be easily controlled by adjusting experimental parameters. In addition, the ANPZ micro-/nanostructures
exhibited a seemingly morphology-dependent thermal effect that was actually size-dependent and has the potential for use in
energy-demanding microscale systems. This wet chemical route, which has been widely used in the synthesis of inorganic
nanomaterials, will provide useful information for the design and fabrication of organic small-molecule micro-/nanostructures
with novel morphologies.
advanced methods, including electrochemical deposition,19,20
1. INTRODUCTION
hard/soft template-assisted growth,21−23 and various solution
Size- and morphology-controlled organic small-molecule
techniques.24,25 Unfortunately, there are few reports so far on
nanostructures have received special attention during the past
the synthesis of dendritic nanostructures of organic small-
decade because of their unique electronic and optical
molecule materials. Therefore, it is valuable to explore an
properties, as well as the great variety and flexibility of these
materials available through molecular design.1−4 As a
effective strategy for the controllable synthesis of dendritic
nanostructures of small organic functional molecules.
However, compared with the synthesis methods of inorganic
consequence, considerable efforts have been devoted to the
controllable construction of various organic nanomaterials with
distinct shapes, such as nanotubes,5,6 nanowires,7,8 nanorods,9
nanostructures, relatively few approaches have been developed
and nanobelts.10,11 Hierarchically dendritic fractals are one type
for the fabrication of organic small-molecule nanostructures to
date, mainly including reprecipitation6,10,26 and template-
of hyperbranched complicated structures that are formed
assisted methods.5,9,27 Physical/chemical vapor deposition,7,8
generally by hierarchical self-assembly under far-from-equili-
which is the most commonly used strategy for the preparation
brium conditions, as such self-assembly is a natural and
spontaneous process.12,13 The fabrication of dendritic fractals
of inorganic nanostructures, is limited for organic small-
molecule materials because of their poor thermal stability. A
could create an opportunity to perform diverse functions such
as catalysis,14 optics,15 and molecular probe16 based on their
wet chemical route, as a “bottom-up” self-assembly approach, is
appealing because of its large-scale production, cost efficiency,
and environmental friendliness. To date, such approaches have
been widely used in the preparation of inorganic nanomaterials
and become an outstanding technology for constructing desired
materials with diverse shapes and sizes.28 Hence, the develop-
superior physical and chemical properties. Hierarchically
dendritic nanomaterials also have a variety of potential
applications in sensor technology,17 information storage,18
and so on, and they have thus attracted considerable attention
in the past several decades. It is expected that dendritic
nanostructures might play a significant role in determining their
properties because of their rich surface nanostructures.
Accordingly, many different types of dendrites of inorganic
and polymeric materials have been fabricated based on various
Received: October 10, 2011
Revised: May 22, 2012
Published: May 28, 2012
© 2012 American Chemical Society
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ment of a wet chemical route for the direct synthesis of organic
small-molecule nanomaterials is clearly of great importance but
also a tremendous challenge.
morphology of the ANPZ micro-/nanostructures can be
tailored by varying a series of experimental parameters. Time-
dependent experiments indicated that the fabrication process of
ANPZ dendrites mainly includes three steps: (1) nucleation
and growth, (2) aggregation, and (3) further growth. This
growth process is similar to that of inorganic dendrites.
Significantly, the ANPZ micro-/nanostructures exhibit a
seemingly morphology-dependent thermal effect that is actually
size-dependent and has potential for use in energy-demanding
microscale systems.38 To the best of our knowledge, this is the
first report of the preparation of organic small-molecule
materials with dendritic microstructures by a wet chemical
route.
In this contribution, 2,6-diamino-3,5-dinitropyrazine
(ANPZ) three-dimensional (3D) dendritic microstructures
that are self-assembled through designed intermolecular
interactions are demonstrated. With the assistance of surfactant
Pluronic P123 block copolymer [PEO20PPO70PEO20, where
PEO is poly(ethylene oxide) and PPO is poly(propylene
oxide)], a typical fractal structure can be easily constructed with
a tunable morphology. ANPZ, as a functional energetic
molecule, has potential applications in booster explosives
because of its excellent safety and high energy density.29−31
ANPZ exhibits significant intra- and intermolecular hydrogen
bonds between sheets in a layerlike crystal structure (Figure 1a)
2. EXPERIMENTAL SECTION
2.1. Materials and Reagents. 2,6-Dimethoxy-3,5-dinitro-
pyrazine (DMDP, C6H6N4O6), which was synthesized by
nitration of 2,6-dimethoxypyrazine (DMP, C6H8N2O2) accord-
ing to the literature method,29,30 was confirmed by H nuclear
1
magnetic resonance (NMR) and Fourier transform infrared
(FT-IR) spectroscopies (Figures S1 and S2, respectively, in the
Supporting Information). All other reagents were of analytical
grade and were used as received without further purification.
Urea was purchased from Sinopharm Chemical Reagent Co.,
Ltd. (Beijing, China). Absolute ethanol was purchased from
Chengdu Union Chemical Industry Reagent Research Institute
(Chengdu, China). The nonionic poly(ethylene oxide)-b-
poly(propylene oxide)-b-poly(ethylene oxide) triblock copoly-
mer (Pluronic P123, EO20PO70EO20, Mw = 5800) was
purchased from Aldrich (Milwaukee, WI). Ultrapure water
(resistivity = 18.2 MΩ·cm−1) was produced using a Milli-Q
apparatus (Millipore, Billerica, MA) and filtered using an
inorganic membrane with a pore size of 0.02 μm (Whatman
International, Ltd., Kent, U.K.) just before use.
2.2. Synthesis of Dendritic ANPZ Microstructures.
ANPZ samples with dendritic microstructures were prepared by
a facile solvothermal process in the presence of surfactant P123.
In a typical synthesis, 0.1 g of P123 and 0.48 g (8 mmol) of
urea were first dissolved in 50 mL of absolute ethanol to form a
colorless transparent solution. Afterward, 0.0345 g (0.15 mmol)
of DMDP was added to this solution under vigorous stirring at
40 °C for 30 min, and then the transparent solution was
transferred to a stainless Teflon-lined autoclave of 80 mL inner
volume, which was sealed and maintained at 120 °C for 9 h.
After the autoclave had cooled naturally to room temperature,
the resulting precipitate was collected and washed several times
with ultrapure water and absolute ethanol. ANPZ micro-
structures were obtained by the natural sinking method and
dried in air atmosphere at 100 °C for 8 h.
Figure 1. (a) Layered and (b) planar structures of ANPZ.
and especially abundant intermolecular hydrogen bonds within
sheets (Figure 1b), which might be the major contributor to the
extraordinary stability of ANPZ under thermal and mechanical
shock initiation.32−35 It is well-known that morphology and size
have a significant influence on the performance of energetic
materials.36,37 Thus, the preparation of ANPZ micro-/
nanostructures is necessary. In the present work, we developed
a facile surfactant-assisted wet chemical method to synthesize
novel dendritic ANPZ microstructures in nontoxic ethanol
solvent using urea as the aminating reagent. Compared with the
reactions for the preparation of ANPZ in acetonitrile solvent, in
which traditional ammonia or NH3 gas is used as the aminating
reagent, this reaction is controllable through the adjustment of
the alcoholysis rate of urea, and as a consequence, the
nucleation and growth of ANPZ crystals can be finely tuned.
More importantly, this wet chemical route can be performed
under mild operating conditions. Moreover, the size and
2.3. Characterization. Powder X-ray diffraction (XRD)
patterns were recorded on a D8 Advance X-ray diffractometer
with Cu Kα radiation (λ = 1.5418 Å). Fourier transform
infrared (FT-IR) spectra were collected with a Spectrum One
1
IR spectrophotometer from 400 to 4000 cm−1. All H nuclear
magnetic resonance (NMR) spectra were acquired on Bruker
Advance spectrometers operating at 300 MHz, with dimethyl-d6
sulfoxide as the solvent. Field-emission scanning electron
microscopy (FE-SEM) measurements were performed with a
Hitachi S-4800 field-emission microscope operated at an
accelerating voltage of 10 kV. Transmission electron micros-
copy (TEM) and selected-area electron diffraction (SAED)
images were captured at a temperature of −174 °C on an FEI
Tecnai-20 transmission electron microscope operating at 200
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kV accelerating voltage by drop casting of the as-prepared
ANPZ dispersions on copper grids coats with Formvar and
carbon film (SPI, West Chester, PA). Thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC)
were carried out on a Simultaneous DSC-TG (SDT) Q600 unit
under N2 atmosphere at a heating rate of 10 K/min.
μm. The fractal structure was arranged along an elongated
central backbone with secondary, tertiary, and even quaternary
branches. Unlike the α-Fe2O3 fractal structure with only second
branches reported by us,24 this fractal structure is more unique
and complex. The SEM image in Figure 3c displays the detailed
morphology of the dendritic microstructures. The fundamental
building blocks of the dendrite are smooth microrods with an
average diameter of about 2 μm. This was further confirmed by
transmission electron microscopy (TEM), which was per-
formed carefully because of the poor thermal stability of the
material (Figure 3d).
3.3. Effects of Experimental Parameters on the
Morphology and Size of ANPZ Micro-/Nanostructures.
By a series of controllable experiments, it was found that the
currently used synthesis strategy provides a facile means to
tailor the morphology and assembly of ANPZ microstructures
depending on reaction parameters such as temperature, DMDP
concentration, and addition of P123.
3. RESULTS AND DISCUSSION
3.1. Sample Composition. The overall crystallinity and
purity of an ANPZ sample, prepared following the experimental
details described in section 2.2, were examined by X-ray
diffraction (XRD). As shown in Figure 2, all of the diffraction
3.3.1. Effect of Reaction Temperature. Figure 4 shows SEM
images of ANPZ microstructures obtained at different reaction
Figure 2. XRD pattern of the as-obtained sample.
peaks are in good agreement with those of the simulated XRD
pattern of ANPZ crystals and can be assigned to the monoclinic
phase of ANPZ.32 No crystalline byproducts were detected.
Also, the composition and purity were further confirmed by 1H
NMR and FT-IR spectroscopies (Figures S3 and S4,
respectively, in the Supporting Information). It can be seen
that the purity of the product obtained by this method is
comparable to that of ANPZ prepared by other methods.29,30
3.2. Morphologies of the Typical Microstructures. The
morphology and dimensions of the same sample were
investigated by field-emission scanning electron microscopy
(FE-SEM), as shown in Figure 3. The low-magnification SEM
image in Figure 3a shows that the sample consisted almost
entirely of well-defined dendritic microstructures. Figure 3b
shows the morphology of a single dendrite, which reveals a
clear and typical fractal structure with overall lengths of 150
Figure 4. SEM images of the products synthesized in the presence of
P123 (0.1 g) at different temperatures: (a,b) 110, (c,d) 130, and (e,f)
140 °C.
temperatures. When the reaction temperature was decreased
from 120 to 110 °C and other reaction conditions were kept
constant, many sheet-branched instead of rod-branched
dendrites were formed (Figure 4a,b), revealing that low
temperature favors the formation of ANPZ dendrites with a
loose structure, which might be due to the slow kinetics and
low thermodynamic factors during the growth process.39 This
morphology evolution from rod- to sheet-branched dendrites
Figure 3. SEM images of the as-prepared ANPZ sample at (a,b) low
and (c) high magnification. (d) TEM images of a single microrod.
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with decreasing temperature is discussed in detail later in this
article. When the temperature was increased from 120 to 130
°C, dendritic microstructures, similar to that shown in Figure 3,
were obtained, but they had more flaws (Figure 4c,d).
However, when the reaction temperature was increased further
to 140 °C, most of the dendritic structures broke into
micrometer-scale fragments, as shown in Figure 4e,f. It is likely
that the marked increase of the saturated vapor pressure of the
reaction system with the increase of temperature reduced the
stability of the well-constructed dendritic microstructures. This
morphology evolution process indicates that an optimum
reaction temperature can effectively control the production rate
of ANPZ molecules and, thus, kinetically control the
aggregation and growth of the ANPZ dendrites.
shape can be explained by the fact that a large number of P123
molecules can form a stable polymolecular micelle config-
uration,41 which can promote the delicate assembly of ANPZ.
3.3.3. Effect of DMDP Concentration. In addition to the
experimental parameters discussed in the two preceding
subsections, the concentration of DMDP also has some
influence on the formation of ANPZ dendritic structures. To
investigate the effect of the concentration of DMDP on the
morphology of the final product, we carried out experiments at
120 °C for 9 h in the presence of P123 (0.1 g) but with
different precursor concentrations. As shown in Figure 6a, a low
3.3.2. Effect of P123 Amount. The relationship between the
morphology of ANPZ and the amount of P123 was studied. As
shown in Figure 5a, when no P123 was used and other
Figure 6. SEM images of the products synthesized by solvothermal
treatment at 120 °C for 9 h in the presence of P123 (0.1 g) but with
different concentrations of DMDP: (a) 0.001, (b) 0.002, (c) 0.008,
and (d) 0.016 mol L−1.
yield of etched ANPZ dendritic microstructures was obtained at
low concentration of DMDP (0.001 mol L−1). When the
concentration of DMDP was increased to 0.002 mol L−1,
bouquetlike ANPZ hierarchical structures aggregated with
microrods became predominant product (Figure 6b). The
formation of such ill-defined microstructures might be closely
relative to the amount of free ANPZ molecules in solution. If
the precursor concentration was too low, it could not
continuously supply material for ANPZ growth, leading to
the formation of imperfect structures with many holes and
cracks in both the backbone and branches.39 When the
concentration was increased to 0.003 mol L−1, well-defined
dendrites were obtained, as shown in Figure 3. Interestingly,
when the concentration of DMDP was increased from 0.003 to
0.008 and even 0.016 mol L−1, irregular dendrites were
observed (Figure 6c,d). It is well-known that the reactant
concentration can kinetically control the reaction rate. With the
increase of DMDP concentration, the reaction accelerated, and
the velocities of nucleation and crystal growth of ANPZ were
also enhanced. The newly produced ANPZ molecules
preferably form new crystal nuclei rather than attaching to
pre-existing ones. In other words, a reaction rate that is too high
at high concentration is disadvantageous for the oriented
growth of crystallites because it can inhibit anisotropic growth
and favor the formation of irregular aggregates.42 Accordingly,
the appropriate concentration of DMDP in ethanol solution is
vital for the formation of perfect dendrites.
Figure 5. SEM images of the ANPZ samples prepared at 120 °C for 9
h in the presence of P123 ethanol solution with different added
amounts: (a) 0, (b) 0.02, (c) 1, and (d) 3 g.
conditions were kept unchanged, it was found that the as-
prepared product consisted of ill-defined sheetlike particles with
a large size, and self-assembly was not observed. This behavior
can be ascribed to the inherent layered crystallographic
structure of ANPZ.32 The abundant intermolecular hydrogen
bonds within a layer can serve as the main driving force to
direct the growth of the microsheets without using additional
surfactant as a capping agent. This finding indicates that P123 is
crucial for the formation of dendritic fractals. When the amount
of P123 was 0.02 g, sheet-branched dendritic microstructures
were formed (Figure 5b), similar to the result for the
temperature of 110 °C from Figure 4a. It can be seen that
this microstructure was loose with an irregular shape, and it
seemed to be underdeveloped. This might be because the
surfactant P123 system with a relatively low concentration is in
a transition state from the individual unimer region to the
polymolecular micelle region;40 thus, the aggregation and
growth of ANPZ cannot be fully completed, resulting in the
formation of loose and irregular dendrites. Further increases in
the amount of P123 (1 and 3 g) led to the generation of well-
defined rod-branched dendrites, and no other morphologies
were observed (Figure 5c,d), which was almost completely the
same as for 0.1 g of P123 (Figure 3). The perfect and uniform
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3.4. Formation Mechanism of Dendritic ANPZ Micro-
structures. To better understand the formation mechanism of
as-obtained ANPZ dendritic microstructures, a detailed time-
dependent morphology evolution study was conducted.
Because the aminating process of DMDP has a high reaction
rate, a low temperature of 100 °C was used to decrease the
reaction rate so that the precise morphological evolution could
be observed. Therefore, the time-dependent experiments were
carried out at 100 °C for different reaction times of 9, 9.5, 10,
12, 21, and 30 h, and SEM images of the resulting samples are
shown in Figures 7 and 8. Figure 7a shows the SEM image of
the sample obtained after 9 h of solvothermal reaction. It can be
seen that the sample mainly consisted of ellipsoidal nanosheets
with a diameter of 25 μm and a thickness of less than 100 nm.
Moreover, some nanosheets self-assembled into 3D flowerlike
aggregates with a diameter of about 15 μm. SEM images of an
individual nanosheet and flowerlike aggregate are shown in
panels b and c, respectively, of Figure 7, providing more details
about their morphologies. The structure and morphology of the
nanosheets were further examined by TEM and selected-area
electron diffraction (SAED). The TEM image (Figure 7d)
clearly presents the ellipsoidal shape of the nanosheets, in
agreement with the results from SEM images. The SEAD
pattern (Figure 7e) recorded on the nanosheet can be indexed
to the [110] zone axis of monoclinic ANPZ, suggesting that the
nanosheet is a single crystal growing along the [001] direction.
After heating for 9.5 and 10 h, the flowerlike aggregates
gradually increased in amount, whereas the nanosheets
naturally decreased (Figure 7f,g). However, when the reaction
time was increased to 12 h, an interesting phenomenon was
found (see Figure 8a,b): The 3D flowerlike aggregates
completely changed to dendritic structures composed of
thinner sheets. After 21 and 30 h, the thinner nanosheets
(Figure 8a,b), consisting of dendrites, gradually changed to
thick ones and finally rods (Figure 8c−f). These results clearly
present the morphology evolution process from nanosheets to
dendrites by self-assembly.
It is well-known that organic surfactants are extensively used
as structure-directing agents or “soft templates” in the
preparation of nanostructured materials with peculiar morphol-
ogies. For example, with the aid of surfactants [such as bis(2-
ethylhexyl) sodium sulfosuccinate (AOT), cetyltrimethylam-
monium bromide (CTAB), and poly(vinylpyrrolidone)
(PVP)], a variety of inorganic nanostructures with novel
morphologies have been constructed.43,44 In this work, the use
of surfactant P123 was found to lead to the formation of well-
defined dendrites. Based on the time-dependent experiments
and analysis, along with the crystal structure of ANPZ, a
possible oriented aggregation mechanism is proposed for the
formation of ANPZ dendrites, as schematically illustrated in
Figure 9. This process mainly includes three steps: (1) In the
nucleation and growth step, first, urea (used in this work as an
aminating agent) is slowly alcoholyzed in absolute ethanol to
form NH3 under solvothermal conditions (eqs a and b),45,46
Figure 7. SEM images of ANPZ micro-/nanostructures formed in
absolute ethanol solution in the presence of P123 after reaction for
(a−c) 9, (f) 9.5, and (g) 10 h. (d) TEM image of nanosheets. (e)
SAED image of a single nanosheet.
Figure 8. Low- and high-magnification SEM images of ANPZ micro-/
nanostructures formed in absolute ethanol solution in the presence of
P123 after reaction for (a,b) 12, (c,d) 21, and (e,f) 30 h.
Figure 9. Schematic illustration of the possible growth process of
dendritic ANPZ microstructures and their building blocks.
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and then ANPZ nuclei are formed by the aminating reaction
between DMDP and NH3 (eq c). The correlative reactive
equations are as follows
CO(NH2)2 + CH3CH2OH
→ H2NCOOCH2CH3 + NH3
(a)
H2NCOOCH2CH3 + CH3CH2OH
→ CH3CH2OCOOCH2CH3 + NH3
(b)
Figure 10. XRD patterns of samples harvested at different reaction
times.
increasing reaction time, the (002) planes gradually weaken in
intensity, whereas the (100) planes strengthen markedly.
Furthermore, experiments on replacing P123 with different
surfactants further validated the important role of P123 in the
formation of the dendritic ANPZ microstructures (Figure 11).
According to the crystal structure of ANPZ (Figure 1b),
there are abundant intermolecular hydrogen bonds within a
sheet, and therefore, hydrogen bonding can serve as the main
driving force to direct the growth of sheetlike structures
(Figures 5a and 7a) following the proposed reaction process.
Subsequently, the nonionic polymer molecule P123 could
selectively anchor to the flat (100) surface of a nanosheet and
its edges of the ANPZ crystals. In the initial stage, P123 acts as
a selective capping reagent to inhibit faster growth along the
[100] direction, leading to the formation of ellipsoidal
nanosheets (structure I in Figure 9) rather than long irregular
microsheets (Figure 5a). Herein, nanosheets were used as the
building blocks for the assembly of subsequent dendrites. (2) In
the aggregation step, driven by the minimization of the total
energy of the system and van der Waals interactions between
polymer molecules, ANPZ nanosheets tend to aggregate in a
form of edge-to-edge attachments and edge-to-surface con-
junctions through van der Waals interactions between P123
molecules (Figure 7f,g). Afterward, these ellipsoidal aggregates
of ANPZ nanosheets became flowerlike structures (structure 2)
and even dendrites (structure 3) in this self-assembly process.
We speculate that the PEO groups in Pluronic P123, which
consists of PEO−PPO−PEO units, could form a crown-ether-
like conformation as a cavity structure in solution.47,48 The
hydrophobic PPO groups could be favorably adsorbed onto the
surface of the ANPZ crystals. Thus, the Pluronic chains
adsorbed on the ANPZ surface in our system could form
cavities and facilitate the formation of the dendrites.23 More
importantly, the dendrites from the self-assembly of flowerlike
structures also can be obtained when P123 is replaced by
Pluronic F127 block copolymer (PEO106PPO70PEO106) (Figure
S5 in the Supporting Information). This phenomenon indicates
that the PEO−PPO−PEO units provided by Pluronic
surfactants might be responsible for the formation of dendrites.
In structure 2, the building blocks are still nanosheets, whereas
in structure 3, these nanosheet building blocks have grown into
approximately tetrahedral sheets. (3) In the further growth
step, at longer times, the tetrahedral sheets on the dendrites
continue to grow in an oriented fashion along the [100]
direction through Ostwald ripening growth at the expense of
the smaller crystallites, resulting in the final formation of
polyhedral columns (structure 4). XRD pattern evolution of the
samples obtained at different reaction times clearly confirms the
capping function of P123 and the oriented growth process from
nanosheets to dendrites, as shown in Figure 10. It can be seen
that the (002) planes are dominant for nanosheets, but with
Figure 11. SEM images of ANPZ samples obtained using different
surfactants: (a) SDS, (b) AOT, (c) PEG-400, and (d) PEG-20000.
Meanwhile, comparative experiments using traditional aminat-
ing reagents such as ammonia, NH3 gas, and ammonium
carbonate were made to confirm the indispensable function of
urea in the formation of the morphology of the ANPZ
structures in our synthetic route (Figure S6 in the Supporting
Information).
3.5. Thermal Analysis of ANPZ Micro-/Nanostructures.
From the time-dependent experiments reported in the
preceding subsection, it is clear that the samples obtained at
different reaction times are all pure ANPZ phase but with
different morphologies. To gain insight into the effect of the
morphology of the as-synthesized ANPZ micro-/nanostruc-
tures on their thermal properties, thermogravimetric (TG) and
differential scanning calorimetric (DSC) measurements were
carried out, as shown in Figure 12. DSC patterns show that
these exothermal peaks gradually shift toward a higher
temperature with increasing reaction time, and there is a shift
of approximate 8.5 °C toward ANPZ nanosheets obtained at 9
h, compared with dendritic ANPZ microstructures obtained at
30 h (Figure 12a). This result reveals that the morphology
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4. CONCLUSIONS
In summary, we have successfully synthesized ANPZ 3D
hierarchically self-assembled dendrites under solvothermal
conditions in the presence of P123. The resulting micro-
structures exhibit perfect fractal features. The structure and size
of ANPZ can be tuned by varying the experimental parameters.
A reasonable formation mechanism is proposed in detail. In
addition, the ANPZ micro-/nanostructures exhibit a seemingly
morphology-dependent thermal effect that is actually size-
dependent. This wet chemical route, which has been widely
used in the synthesis of inorganic nanomaterials, will provide
useful information for the design and fabrication of organic
small-molecule micro-/nanostructures with novel morpholo-
gies.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR and FT-IR spectra of DMDP and ANPZ, SEM
images of ANPZ formed in the presence of surfactant Pluronic
F127, and SEM images of ANPZ acquired with traditional
aminating agents. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Tel.: +86-10-68912631 (M.C.), +86-816-2495301 (H.H.).
Fax: +86-10-68912631 (M.C.), +86-816-2495856 (H.H.). E-
mail: caomh@bit.edu.cn (M.C.), huanghui@caep.ac.cn (H.H.),
cwhu@bit.edu.cn (C.H.).
Notes
Figure 12. (a) DSC and (c) TG curves of ANPZ with different
micro-/nanostructures obtained at given times at a heating rate of 10
°C/min. (b) Relationship between maximum decomposition temper-
ature and morphology: (A) nanosheets (9 h), (B) nanoflowers (9.5 h),
(C) dendritic nanostructures (12 h), (D) dendritic microstructures
with building blocks of microsheets (21 h), and (E) dendritic
microstructures with building blocks of microrods (30 h).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation of China (20731002, 21173021, 91022006,
20973023, and 11172276), the Joint Fund of the National
Natural Science Foundation of China and the China Academy
of Engineering Physics (10876002), the 111 Project (B07012),
the Program of Cooperation of the Beijing Education
Commission (20091739006), the Specialized Research Fund
for the Doctoral Program of Higher Education (200800070015
and 20101101110031), the Program for New Century
Excellent Talents in University, and the Beijing Municipal
Science & Technology Commission (Z09010300820902).
evolution from 2D nanosheets to 3D hierarchically self-
assembled dendrites is accompanied by a regular change in
decomposition temperature, which exhibits a unique morphol-
ogy-dependent thermal effect (Figure 12b). As can be seen in
the reported SEM images, the size changes from nanometer to
micrometer as the morphology evolves from nanosheets to
dendrites. Therefore, a possible reason for this phenomenon is
as follows: It is well-known that the ratio of surface atoms to
interior atoms significantly decreases as the particle size
increases. The ANPZ dendritic microstructures have a lower
surface energy and are more difficult to stimulate under thermal
initiation than small-sized nanostructures.37 Additionally,
microdendrites are highly crystalline (Figure 10), with fewer
crystal defects and dislocations, which might lead to an increase
in decomposition temperature. Moreover, TG curves exhibit a
distinct weight loss (Figure 12c). Significantly, the gradual
increase of the initial thermal decomposition temperature was
also observed for these samples from nanosheets to dentrites, is
in good accordance with the DSC results. Thus, these as-
synthesized materials also follow the small-size effect that most
of nanomaterials exhibit.
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