An analogous solution-liquid-solid (ASLS) growth route to InP hollow spheres and a honeycomb-like macroporous framework

Article abstract of DOI:10.1002/ejic.200500937

Hollow spheres and a honeycomb-like macroporous framework structure of InP have been successfully fabricated by employing Au/In core/shell composite colloid droplets formed in situ as the template and catalyst for a simple analogous solution-liquid-solid (ASLS) growth route. SEM images show that the morphology of the obtained InP is mostly micrometer-scale hollow spheres (>80 %) accreted with honeycomb-like structures (<5%). These hollow spheres have well-defined voids and homogeneous walls, and the surface of the obtained InP hollow spheres is porous, which indicates that the building blocks are InP nanoparticles formed by solidification on the surface of Au/In core/shell droplets. A possible growth mechanism for the micrometer hollow spheres and honeycomb-like macroporous framework is proposed. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200500937

FULL PAPER
DOI: 10.1002/ejic.200500937
An Analogous Solution-Liquid-Solid (ASLS) Growth Route to InP Hollow
Spheres and a Honeycomb-Like Macroporous Framework
Xiuwen Zheng,[a,b] Changzeng Liu,[a] and Yi Xie*[b]
Keywords: Materials science / Mesoporous materials / Semiconductors / Synthesis design
Hollow spheres and a honeycomb-like macroporous frame-
work structure of InP have been successfully fabricated by
employing Au/In core/shell composite colloid droplets
formed in situ as the template and catalyst for a simple analo-
gous solution-liquid-solid (ASLS) growth route. SEM images
show that the morphology of the obtained InP is mostly mi-
crometer-scale hollow spheres (Ͼ80%) accreted with honey-
comb-like structures (Ͻ5%). These hollow spheres have
well-defined voids and homogeneous walls, and the surface
of the obtained InP hollow spheres is porous, which indicates
that the building blocks are InP nanoparticles formed by so-
lidification on the surface of Au/In core/shell droplets. A pos-
sible growth mechanism for the micrometer hollow spheres
and honeycomb-like macroporous framework is proposed.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
materials. The templates used include polystyrene latex
spheres,[4a,6] liquid crystals,[7] surfactant vesicles,[8] macro-
porous active carbon,[9] emulsions,[4b] etc. So far, various
hollow spheres and macroporous materials have been fabri-
cated by this universally applied template-directed synthe-
sis, including oxides,[4a,6] graphite and diamond,[10] semi-
conductor Si,[11] CdS,[1a] and metals.[1f]
Introduction
Over the past decade, hollow microspheres and highly
porous inorganic materials with well-defined pore sizes have
become an important area of research.[1] The main thrust
of these intensive investigations stems from their wide appli-
cation,[2] such as small containers for microencapsulation,
catalysis, controlled delivery, artificial cells, light fillers,
acoustic insulation, photonic crystals, optical devices, etc.[3]
Porous spherical beads can also be used to improve the
properties of materials by increasing the accessible surface.
Macroporous materials with pore sizes Ͼ 50 nm and with
well-defined and controllable porosity have attracted par-
ticular interest because of their unique optical, catalytic,
and mechanical properties.[4] Macroporous supports can be
designed to provide optimal flow and improved efficiencies
in catalysis and large-molecule separation processes, as well
as to permit immobilization and stabilization of large guest
molecules.[5] Due to the importance of these specific mor-
phologies and assemblies, notable progress has been made
in structural, compositional, and morphological control, in
processing aspects of porous solids, and new applications
of these materials. Some important synthetic strategies have
also been reported for the control and preparation of these
peculiar hollow microspheres and ordered three-dimen-
sional macroporous framework. Among these methods,
templating techniques are the most appropriate and versa-
tile method to produce hollow spheres and macroporous
Recently, III–V semiconductor nanoparticles, which are
well-known electronic and optoelectronic materials, have re-
ceived renewed interest due to their technological impor-
tance.[12] InP is an especially intriguing target for research
because of its extensive use in the sources and detectors in
fiber optic communications and high-speed electronic appli-
cations. Several new strategies have been developed to syn-
thesize InP nanocrystals with different morphologies, such
as high-quality InP nanocrystals obtained in a noncoordi-
nating solvent[13] or aqueous solution,[14] InP nanowires or
nanotubes by a laser ablation process by a vapor-liquid-
solid (VLS) growth mechanism,[15] or a solution-phase syn-
thesis by a solution-liquid-solid (SLS) mechanism,[16] with
the goal of lowering the processing temperature and avoid-
ing complex reactions and toxic precursors. In addition, the
widespread research into porous III–V semiconductors is
fuelled by the exhibition of new properties and a large po-
tential for applications in nonlinear optical materials.[17]
The porosity of a III–V compound induces bandgap widen-
ing and further results in blue and ultraviolet lumines-
cence.[18] For example, two-dimensionally nanostructured
porous GaP membranes exhibit a strongly enhanced optical
second harmonic generation in comparison with the bulk
materials.[19] Our group has made significant progress
towards the synthesis of III–V semiconductors, such as
InP,[20] GaP,[14] and InAs,[21] with desired morphologies by
simple and effective solvothermal methods. Up to now,
[a] Department of Chemistry, Normal University of Linyi,
Linyi, Shandong 276000, P. R.China
[b] Nanomaterials and Nanochemistry, Hefei National Laboratory
for Physical Sciences at Microscale and Department of Chemis-
try, University of Science & Technology of China,
Hefei, Anhui 230026, China
E-mail: yxielab@ustc.edu.cn
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InP Hollow Spheres and a Honeycomb-Like Macroporous Framework
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most effort has been focused on the formation of one-di- collapse are probably due to decomposition of the tem-
mensional III–V semiconductors. One of the most impor- plates when washing with dilute HCl. Figure 3 displays the
tant growth mechanisms for one-dimensional nanostruc- SEM images of the obtained InP hollow spheres at high
tures is the VLS mechanism in chemical vapor deposition magnification; the porous nature of the surface can clearly
(CVD) or laser ablation using Au as the catalyst.[15] Re- be seen. Figure 3 also confirms that the building blocks of
cently, the VLS mechanism was extended in solution for the the InP hollow spheres are InP nanoparticles. Interestingly
preparation of InAs quantum rods with controlled lengths and surprisingly, some ordered honeycomb-like macropo-
and diameters by applying a nanocrystalline gold catalyst rous networks are also found to accrete with the hollow
to the reaction of InCl3 with tris(trimethylsilyl)arsane spheres, as shown in Figure 4. The pore diameter of the
[As(TMS)3] at 360 °C,[22] which can also be considered as ordered honeycomb-like macroporous network is approxi-
an improved SLS route.[16] A detailed investigation of these mately 2–5 µm. A key feature of these networks is the inter-
reported methods helped us find that there is one common connectedness of their pores; an SEM image at high magni-
factor, namely the existence of alloy liquid droplets that fication (Figure 4b, arrowed) shows that some of the voids
serve as a nucleation catalyst for the formation of one-di-
mensional nanostructures at the given elevated temperature,
such as the In–Au alloy liquid droplets in the formation of
InP and InAs nanowires.[15,22] The essence of the catalytic
functions of these liquid droplets is the existence of super-
saturation, which supports the whisker growth at the liquid/
solid interface. However, a high reaction temperature is a
prerequisite for the formation of In–Au alloy liquid droplets
in all these reports. We therefore wondered what the results
would be if the temperature is too low to form the In–Au
alloy in a given reaction system. In order to find the answer
to this question we decided to exploit an analogous SLS
(ASLS) growth route to synthesize InP spherical assemblies,
which can be considered as an extension of the SLS
method. The latter has traditionally been applied to grow
1D III–V semiconductors containing low-melting-point
metals such as indium by decomposition of indium-con-
taining organometallic complexes as intermediate and cata-
lyst for the directed growth of III–V one-dimensional struc-
tures.[16] In our approach, to obtain a solution-controllable
synthesis of InP with the desired hollow sphere morphology
we introduced gold nanoparticles (formed by reduction of
HAuCl4 in solution) into the classical SLS system[16,20,21]
and controlled the system temperature at 200 °C. The in-
situ formed gold nanoparticles act as a colloidal template
for the synthesis of spherical assemblies.
Figure 1. XRD pattern of obtained InP samples (a) after HCl treat-
ment and (b) without HCl treatment, indicating the presence of In.
Results and Discussion
The XRD patterns (Figure 1a) of the samples agree well
with the bulk InP reflection,[23] thus indicating the same
cubic zinc blende lattice structure. Judging from the XRD
pattern, the products have very high crystallinity under the
given experimental conditions. No component of the Au
catalyst can be detected in Figure 1a, thus indicating that
its content is less than the resolution limit of XRD. SEM
images, as shown in Figures 2a and b, give a general view
of the obtained InP micrometer hollow spheres under low
magnification. Many collapsed and hemispherical hollow
spheres can also be seen in Figure 2a, thus indicating the
hollow structure of the spheres. Figures 2c and d display
images of typical, collapsed hemispheres, from which it can
be seen that the obtained hollow spheres are about 3–6 µm
Figure 2. SEM images of obtained InP micrometer hollow spheres:
(a) and (b) show the obtained InP micrometer hollow spheres at
low magnification and (c) and (d) show the collapsed and broken
in diameter with a wall thickness of 1 µm. The opening and InP spheres at high magnification, indicating their hollow nature.
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quantification of the peaks gives an atomic ratio of In/P of
0.43:0.53. The excess P perhaps comes from the added P
raw materials. All of the observed binding energy values for
In 3d and P 2p coincide with the reported data within the
experimental errors.[24]
In both the reported VLS route and its extension in solu-
tion, the noble metal Au and the in-situ formed alloy spe-
cies function as catalysts for the nucleation and further di-
rect the growth of 1D nanostructures. Thus, the reaction
temperature must be higher than the eutectic temperature
of the alloy. Once formed, the In–Au alloy liquid droplets
dissolve the reactants and the subsequent supersaturation
supports one-dimensional structure growth at the liquid/so-
lid interface.[15,22] In our approach, however, the obtained
product is spherical rather than the usual one-dimensional
Figure 3. SEM images of obtained InP hollow spheres at high mag-
nification, showing the porous surface structure.
are connected to their neighbors by smaller windows. Such nanostructure. Our reaction temperature (200 °C) is lower
microstructures allow these macroporous networks to be than that of the melting point of bulk gold and higher than
filled with different liquid precursor solutions in the second that of the melting point of In and P4 (157 and 44.1 °C,
templating process. These small windows play important respectively), therefore the Au–In alloy cannot form. Banin
roles in the fabrication of the desired morphologies as they has reported that InAs nanorods do not yield one-dimen-
provide the necessary channels to allow the precursor solu- sional growth at a lower reaction temperature of 300 °C,
tion to flow through and form a shell throughout the entire which indicates that reaction temperatures of at least 360 °C
void. The arrow in Figure 4c shows some InP spheres inlaid are required for the onset of Au melting.[22] This value is
in the macroporous network, which further prove the pro- consistent with theoretical studies that predict a melting
posed formation mechanism.
temperature for Au146 (diameter ca. 1.6 nm) of ca. 350 °C,
XPS spectra were recorded to identify the surface com- increasing to ca. 480 °C for Au459 (diameter ca. 2.5 nm).[25]
position of the as-prepared products. The bonding energies The current low temperature (200 °C) may not suffice to
obtained from the XPS analysis were corrected for speci- activate the gold particles as catalysts to direct the growth
men charging by referencing the C 1s line to 287.60 eV. Fig- of the one-dimensional nanoscale material. However, the
ure 5a shows the XPS survey spectra of InP. No obvious reduced liquid In becomes coated on the newly formed Au
impurities (besides the Au catalyst and trace amounts of nanoparticles due to the physical tendency to reach the low-
adsorbed O2) can be detected in the samples, thus indicat- est energy level and forms Au/In core/shell droplets. These
ing that the products have high purity and the level of im- droplets further serve as a second template for the nucle-
purities is lower than the resolution limit of XPS (1 at%). ation of InP nanoparticles, therefore the function of the
The typical high-resolution spectra of In 3d and P 2p are Au/In core/shell droplets is as a template for a spherical
shown in Figure 5b. The binding energy values are structure[16] rather than a nucleation catalyst for the formation
443.74 eV for In 3d5/2 and 451.38 eV for In 3d3/2. The P 2p of a one-dimensional structure.[15,22] In the region of scat-
core shows two peaks, one at 128.88 eV corresponding to P tered Au/In core/shell beads, however, the P4 molecules in
from InP and the other at 132.95 eV (arrowed) correspond- solution will react with the In on the surface of Au/In beads
ing to oxidized P species due to exposure to the air. The (Path 1 in Scheme 1). Thereafter, the formed InP nanopar-
peak areas of these high-resolution scans were measured ticles solidify to form a compact InP layer on the surface
and used to calculate the In/P ratio for the product. The of the Au/In core/shell droplets, which blocks the further
Figure 4. (a)–(c) SEM images of obtained InP honeycomb-like macroporous network(Ͻ5%). The arrow in (c) shows some InP spheres
inlayed in the macroporous network.
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treatment was observed by XRD, as shown in Figure 1b.
However, it is reasonable to propose that in the dense and
thick regime of Au/In beads, which array in order, P4 mole-
cules in solution filter into and fill the void spaces among
the beads of the crystalline array. The reaction of P4 with
In molecules and subsequent solidification form a compact
InP layer on the surface of the Au/In core/shell beads. This
process is similar to the formation of InP hollow spheres.
However, due to the highly aggregated nature of the array
of Au/In beads, the solidification of the InP spheres con-
nects each other into a compact whole containing unre-
acted In. After the removal of residual In, an ordered three-
dimensional porous structure with an array of intercon-
nected spherical pores is formed (Path 2 in Scheme 1,
Ͻ5%). In addition, some InP spheres inlaid in the macro-
pores are also found (Figure 4c, arrowed), which further
demonstrates the template effect of the Au/In core/shell
droplets. This large surface area endows InP with many
promising applications in separation and adsorption. More-
over, the periodical pores can also serve as cavities to ac-
commodate diverse guest molecules. Scheme 1 outlines the
schematic process of the formation of the desired mor-
phology.
Figure 5. XPS spectra: (a) InP survey spectra, showing the presence
of Au catalyst; (b) In 3d; (c) P 2p, with an arrow indicating the
oxidized P species.
Scheme 1. Schematic illustration of the formation of InP microme-
ter hollow spheres and the honeycomb-like macroporous network:
(a) reaction of P4 with In on the surface of Au/In beads; (b) solidifi-
cation of InP nanoparticles on the surface of Au/In beads, which
leads to the formation of a core/shell structure. (c) removal of In
by HCl, leading to the formation of InP hollow spheres or a macro-
porous network (a triangle represents P4 molecules). The predomi-
nant Path 1 gives the hollow spheres and Path 2 leads to the honey-
reaction of P4 with In molecules in the beads. As a result,
when removing the unreacted In by treatment with dilute
HCl solution, the inner Au cores separate from the outer-
most InP shells and finally produce InP hollow spheres
(Ͼ80%). Upon removal of the support, some collapsed hol-
low spheres are formed. The presence of In before the HCl comb-like macroporous network.
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A series of comparison experiments were performed to at the appropriate temperature provide conditions that are
determine the effect of synthetic conditions on the forma- adequate to greatly enhance solubility, diffusion, and
tion of InP hollow spheres and macroporous networks. De- crystallization.
tailed investigation helped us determine that the ratio of In/
The same route has also been applied to the synthesis of
Au is important for controlling the synthesis of InP nano- GaP, using GaCl3 instead of InCl3·4H2O as the source. The
crystals with different assemblies, and that the optimal mo- obtained GaP micrometer spheres, with a diameter of ca.
lar ratio of In/Au for the formation of these InP hollow 3–4 µm, are shown in Figure 7. Some spheres with small
spheres and network is 25:1. At this molar ratio the product openings in the surface can also be observed (inset in Fig-
is mainly microporous hollow spheres (Ͼ80%) with a small ure 7), thus indicating that the obtained GaP spheres have
amount of a three-dimensional ordered macroporous net- a hollow structure. However, at the given temperature
work. We also found that it is difficult to obtain the desired (200 °C) the obtained GaP hollow spheres are very poorly
hollow spheres morphology if the molar ration of In/Au is crystalline.
too far above or below this ratio. For example, the obtained
products are mainly irregular particles with a small quan-
tity of rough spherical products at a ratio of 50:1. These
particles resemble an irregular stack that shows some mor-
phological similarity to that of Figure 2b. However, the
amount of obtained microscale spherical structures is less
than 10%. With a reduction of the ratio, for example to
10:1, the obtained products are poorly crystalline particles.
In order to understand the formation mechanism of InP
hollow spheres, the reaction was carried out without the
addition of P; all other experimental conditions and param-
eters were kept constant. The obtained In/Au composite be-
ads are spherical, as shown in Figures 6a and b, and gold
nanoparticles exhibiting high contrast can be discerned in
the cores of many of them (Figure 6b), which provides firm
evidence for our proposed mechanism.
Figure 7. GaP micrometer-sized hollow spheres obtained in the
same approach; the inset shows the small openings in the surface
of GaP spheres, which indicate the hollow structure of the obtained
GaP spheres.
Conclusions
In summary, we have demonstrated an effective analo-
gous solution-liquid-solid (ASLS) growth route to meso-
scale InP hollow spheres accreted with honeycomb-like
structures, in which the Au/In core/shell droplets formed in
situ serve as the template and catalyst. These hollow spheres
have well-defined voids and homogeneous walls. The sur-
face of the InP hollow spheres obtained is porous, which
further indicates that the building blocks are InP nanopar-
Figure 6. Obtained Au/In spherical composite particles: (a) SEM
ticles formed by solidification on the surface of Au/In core/
image, (b) TEM images; gold nanoparticles exhibiting high con-
shell droplets. A systematic investigation has shown that the
trast can be discerned in the cores.
correct ratio of indium/noble metal is a prerequisite for the
synthesis of InP novel structures, and that the optimal ratio
is 25:1. The same experimental process has been applied to
GaP hollow spheres and will be extended to other impor-
tant metal phosphides with desired morphologies.
As is well known, it is very important to select appropri-
ate solvents for the design of solid-state compounds by a
solvothermal route.[20,26] Based on our control experiments,
we found that ethylenediamine plays an important role in
the formation of InP particles under our conditions as it
can disperse yellow phosphorus to form a brown dispersion Experimental Section
and readily dissolve KBH4, which means that it increases
Analytical-grade reagents and solvents were purchased from
the surface area of the reactants and promotes the reaction
at given temperatures.[20] When other solvents were used,
such as ethanol, benzene, DMF, or pyridine, the reaction
either did not take place or the products could not be iden-
Shanghai Chemistry Co. Ltd and used without further treatment.
Typical procedure: 40 mL of ethylenediamine, 1.11 g of
InCl3·4H2O, 0.5 g of yellow phosphorus, 2 mL of an ethanol solu-
tion of HAuCl4 (0.0758 ) and 1.6 g of KBH4 were placed in a 50-
tified. Strongly polarizing solvents such as ethylenediamine mL Teflon-lined autoclave. The autoclave was maintained at 200 °C
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InP Hollow Spheres and a Honeycomb-Like Macroporous Framework
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[5]
for 10 h, and then cooled to room temperature naturally. The black
products were filtered off and washed with absolute alcohol and
dilute aqueous HCl (1 ). Finally, the products were dried in vacuo
at 50 °C for 1 h. The key to the fabrication of InP hollow spheres
and macroporous network is the control of the In/Au ratio. During
the formation of the desired InP hollow spheres, the influence of
this ratio was investigated in detail and control experiments were
performed with different molar ratios of In/Au, such as 120:1,
100:1, 80:1, 50:1, 25:1, and 10:1. In addition, in order to under-
stand the formation mechanism, different solvents, such as ethanol,
benzene, DMF, and pyridine were selected to investigate the opti-
mal experimental conditions.
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Received: October 23, 2005
Published Online: April 11, 2006
Eur. J. Inorg. Chem. 2006, 2364–2369
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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