Ga2O3 and GaN semiconductor hollow spheres

Article abstract of DOI:10.1002/anie.200353212

Carbon spheres are used as templates to prepare semiconductor hollow spheres of Ga₂O₃ (depicted) and GaN. The thickness and uniformity of the final products are predetermined by the thickness of the active layer of the carbon spheres

Full text of DOI:10.1002/anie.200353212

Angewandte
Chemie
Microspheres
Ga2O3 and GaN Semiconductor Hollow
Spheres**
Xiaoming Sun and Yadong Li*
Both monoclinic Ga2O3 and Wurtzite-structured GaN are
wide-band-gap semiconductors (energy gap, Eg = 4.9 eV and
3.39 eV, respectively) that exhibit luminescence and conduc-
tion properties, thus, they have potential applications in
optoelectronic devices,[1–8] high-temperature stable gas sen-
sors,[9] and high-temperature/high-power electronic devi-
ces.[10,11] The future of full-colored, flat panel displays, blue
lasers, and optical communication is likely to be based on
GaN.[5,12] Nanostructured Ga2O3 and GaN with varied
morphology, for instance, nanoparticles,[13] nanorods,[14] nano-
wires,[3,15] nanobelts,[4,16] and nanotubes[5–6,17–18] have been
prepared.However, monodispersed spherical structures of
the two semiconductor compounds have not been prepared
up to now.
Monodispersed micro- or nanospheres, as a kind of
competitive building blocks for future nanodevices, have
become a new focus of research.This interest has arisen
because it is now relatively easy to synthesize uniform spheres
and to control their size, and these spheres can self-assemble
into 2D or 3D colloidal arrays or photonic band gap
crystals.[19–23] The preparation of uniform micro- or nano-
spheres of semiconducting materials would enable their use as
building blocks for new photonic crystals or as model systems
for light scattering, which are of both theoretical and practical
significance.New approaches to semiconductor hollow
spheres have been made in the last five years.[24–33] Semi-
conducting oxide ceramic hollow spheres (e.g., TiO2, SnO2)
have been prepared by templating the sol–gel precursor
solutions with monodispersed latex beads.[24] By alternating
the shape and structure of the templates, the morphology of
hollow structures can also be changed.[25,26] The II–VI family
of semiconductor luminescent nanoparticles, such as CdTe,
CdS, ZnS, have been assembled on spherical templates to
form luminescent shells by using a layer-by-layer self-
assembly strategy,[27–29] sonochemical deposition,[30] or chem-
ical-bath deposition.[31–32] Very recently, our group has devel-
oped a solution-based template-free route to ZnSe semi-
conducting hollow spheres by condensing ZnSe nanoparticles
on a liquid–gas interface.[33] Herein, we report the application
[*] X. Sun, Prof. Dr. Y. Li
Department of Chemistry
The Key Laboratory of Atomic andMolecular Nanosciences
(Ministry of Education)
Tsinghua University
Beijing, 100084 (China)
Fax: (+86)10-6278-8765
E-mail: ydli@mail.tsinghua.edu.cn
[**] This work was supportedby the NSFC (20025102, 50028201,
20151001), the Foundation for the Author of National Excellent
Doctoral Dissertation of P.R. China, andthe State Key Project of
Fundamental Research for nanomaterials and nanostructures.
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DOI: 10.1002/anie.200353212
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of a facile surface-layer-absorption (SLA) templating techni-
shell of final products can be ensured in this way.Herein, we
demonstrate that gallium ions can be adsorbed into the
surface layer and then transformed into oxide and nitride
under the right conditions.
que to obtain monodispersed Ga2O3 and GaN hollow spheres.
The strategy used to obtain monodispersed hollow Ga2O3
and GaN spheres can be divided into three steps (Figure 1):
Carbon spheres were prepared according to a reported
procedure.[36] A typical image obtained by scanning electronic
microscopy (SEM) is shown in Figure 2a.Zeta potential
analysis on the spheres indicated that the surfaces of the
spheres were negatively charged in water (pH 7, z = À47 eV).
However, this value changed to À18 eV when the spheres
were ultrasonicated in a solution of GaCl3 (0.5m) for 10 min,
washed (centrifugation and redispersion in water, five cycles)
and suspension aged for 24 h before analysis.This result
implies that Ga3+ ions were adsorbed into the surface layer of
the carbon spheres.
Figure 1. Schematic mechanism for the formation of GaN hollow
spheres using carbon spheres as templates.
1) the adsorption of gallium ions from solution into surface
layer; 2) calcination of the composite spheres in air to remove
the carbon core, which results in oxide hollow spheres;
3) in situ conversion of the as-prepared oxide hollow spheres
into nitride hollow spheres in an ammonia atmosphere at 700–
9008C.There are some essential differences between the
current strategy and traditional strategies.[21–32,34,35] In tradi-
tional templating processes, which use latex or silica as
templates, a coating is deposited on the sacrificial core by
controlled surface precipitation/absorption of the inorganic
precursors from a solution/suspension.These processes
usually require alkoxides as a starting material (e.g., in
sol–gel methods), or use nanoparticles as “building units” and
polyelectrolytes as “glue”.[34] The core is then subsequently
removed by thermal or chemical means, thus leaving behind
hollow spheres.[35] During the process, surface modification is
usually required for efficient adsorption and the experimental
parameters must be precisely controlled to avoid heteroge-
neous agglomeration.In most cases, the growth is outward
from the template surface,[21–25,34,35] and the void formed is
approximately equal to the original size of the template.
However, in our method, the templates used were carbon
spheres prepared by dehydrating glucose or sucrose under
hydrothermal conditions.[36] The surface of the spheres is
=
hydrophilic and has a distribution of -OH and -C O groups
similar to that of polysaccharide in,[36] which makes surface
modification unnecessary.When the carbon spheres are
dispersed in solutions of metal salts, the cations are adsorbed
into the surface.Agglomeration is naturally avoided because
cations are absorbed into the surface layer to form a
composite shell rather than form a heterogeneous coating.
In the following calcination process, the metal atoms in the
shell become denser, the spheres contract and cross-link to
form metal-oxide hollow spheres, which are a smaller replica
of the carbon spheres (ꢀ 40%).The salts (eg. ., chlorides) can
thus be used as the starting material instead of alkoxides,
which reduces the preparation cost.The use of transition-
metal salts should lead to hollow spheres of different
compositions.We believe that this is a general, simple, and
cheap approach to hollow spheres.Moreover, since the
thickness of the functional “surface layer” is predetermined
by hydrothermal synthesis, the integrity and uniformity of the
Figure 2. a) Typical SEM image of carbon spheres; b) typical TEM
image of carbon spheres; c,d) typical TEM image of Ga2O3 hollow
spheres; e) TGA plots of A) templating carbon spheres andB) Ga-
adsorbed carbon spheres (inset: their differential curves); f) SEM
image of intact Ga2O3 hollow spheres 100 nm in diameter (inset: SEM
image of large hollow spheres 1500 nm in diameter, which were delib-
erately broken by crushing in a mortar to show the hollow interior);
g,h) typical TEM images of Ga2O3 hollow spheres preparedby using
templating carbon spheres from sucrose rather than glucose solutions.
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Images of the spheres obtained by transmission electron
microscopy (TEM) indicate that the gallium ions were
adsorbed into the surface layer rather than grafted onto the
surface (Figure 2b).No amorphous layer or aggregations of
gallium compounds, such as hydroxides, were observed,
though energy dispersive X-ray (EDX) analysis on the
spheres clearly revealed the existence of elemental Ga and
an atomic ratio of Ga to Cl greater than 10:1.This result
indicates that cationic Ga3+ ions were selectively absorbed in
the surface layer and anionic ClÀ ions were excluded since the
ratio of Ga3+ to ClÀ in bulk solution was 1:3, while that of
adsorbed ions was greater than 10:1 after washing cycles.We
propose that the polysaccharide-like layer at the surface of
carbon spheres serves as an adsorbent, which can chelate
Ga3+ ions and protect them from heterogeneous coagulation
or being washed off.It is noteworthy that the size of carbon
spheres can be manipulated in the range from 200 nm to
4000 nm, or even larger, by adjusting experimental parame-
ters.Smaller spheres (about 400 nm in diameter) were chosen
to show the surface morphology more clearly.
The gallium-adsorbed carbon spheres were calcined in air
to remove the carbon component, resulting in Ga2O3 hollow
spheres.TEM micrographs reveal the hollow structure of the
calcined samples (Figure 2c).A selected-area electron-dif-
fraction pattern shows that the spheres are polycrystalline.
XRD characterization indicate that the as-prepared sample is
Ga2O3 (JCPDS 060509, shown in Figure 3a).The average
crystalline size was about 4 nm, as calculated from the
broaden peaks by using the Scherrer equation.The hollow
spheres prepared from carbon spheres of 1000 Æ 50 nm in
diameter (shown in Figure 2a) had diameters of 370 Æ 20 nm,
about 60% smaller than the starting spheres and had a shell
thickness of about 16 nm as measured by using a TEM image
that had been further magnified (Figure 2d).The large
shrinkage ratio is essentially different to that obtained by
using traditional sacrificial-core methods.In these methods,
the prepared hollow spheres were nearly the same size or
slightly smaller than the template.[21–26,34,35] We believe that
the large shrinkage is caused by further dehydration of the
loosely cross-linked structure of the carbon spheres, which
leads to densification of Ga3+ ions in surface layer.Con-
sequently, cross-linking and crystallization of gallium oxide
take places when the carbon is removed in air during the
calcination process, thus resulting in metal-oxide hollow spheres.
This hypothesis was partly evidenced by thermal gravi-
metric analysis (TGA) of Ga-loaded templates and templat-
ing carbon spheres.The similarity of the two curves (shown in
Figure 2e) revealed that the two microspheres experience
similar oxidation procedures: first (< 4008C), a slow loss of
weight attributed to the further dehydration and densification
of carbon spheres; second, sharp weight-loss peak at 414 or
4258C (Figure 2e, inset), which are attributed to burning of
the carbon spheres.The weight-loss peak from the Ga-
absorbed spheres shifts to a higher temperature; it is
considered that Ga absorption might inhibit the inner
oxidation.After the sharp weight loss, there is still a process
of weight loss (430–5508C), which might be caused by
oxidation of carbon found deep within the core.The final
weight of Ga-absorbed spheres was measured to be 3.3%.
Figure 3. a) XRD patterns of Ga2O3 andGaN hollow spheres prepared
at 7008C, 8008C, and900 8C; b,c) typical TEM image of GaN hollow
spheres preparedat 700 8C and900 8C, respectively. d) Raman spec-
trum of GaN hollow spheres preparedat 900 8C. I=intensity, arbitrary
units.
By alternating the size of templates from 200 nm to 4 mm,
the average size of hollow spheres can be manipulated in the
range from 100 nm to 1.5 mm with an almost constant
shrinkage ratio about 40%.Figure 2 f and the inset show
the SEM of some intact and individual broken hollow spheres
with diameters of 100 Æ 5 nm and 1500 nm, respectively.
Though the diameter of the hollow spheres may vary
considerably, the thickness of the shell changes little
(16 nm) even when the concentration of GaCl3 is reduced
from 0.5m to 0.1m or the adsorption temperature is increased
from room temperature to 908C.However, when the
templating carbon spheres (1000 Æ 50 nm) were prepared
from sucrose rather than glucose solution, the hollow Ga2O3
spheres had a rippled thinner shell (ꢀ 10 nm) and particle size
330 Æ 15 nm were fabricated (Figure 2g, h).Such effects are
attributed to the predetermined thickness of the “reactive-
surface layer” of the carbon spheres.During the hydro-
thermal synthesis, there is a “growing layer” surrounding the
carbon spheres, which moves outward as the size of the
carbon spheres increases.As the growth process is stopped by
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terminating the hydrothermal treatment, the growing layer
solidifies at the surface of carbon spheres and serves as a
reactive-surface layer for the adsorption of cations.The
thickness of the growing layer and the subsequent reactive-
surface layer should remain unchanged despite the diameters
of the carbon spheres increasing with increasing hydro-
thermal time.When sucrose rather than glucose was used for
preparation of carbon spheres, the thickness of growing layer
changed, resulting in different shell structures.This synthetic
aspect is also different from previous inorganic-coating
sacrificial-core methods, in which the relative ratios of the
reactants (e.g., alkoxide, organic solvent, and water) in the
sol–gel coating process considerably influence the quality and
thickness of the coating.[34] The structure of the spheres would
make it a promising material for catalysis and light advanced
ceramic materials.
When the Ga2O3 hollow spheres were heated in an
ammonia atmosphere at 7008C, GaN hollow spheres were
formed.The wide-angle XRD patterns shown in Figure 3
clearly indicate the presence of the wurtzite structure of GaN
(JCPDS 760703, space group P6mc).Lattice parameters
calculated from the patterns (a = 3.191(1), and c =
5.187(6) Š), agree well with the standard data (a = 3.190,
and c = 5.189 Š). The crystallinity of final products obtained
at 700, 800, and 9008C were compared through XRD
characterization (Figure 3a).The average crystalline sizes of
samples obtained at these temperatures were 4 nm (7008C),
7 nm (8008C), and 14 nm (9008C), according to the Scherrer
equation.These results indicate that the GaN nanoparticles
recrystallize and grow at a high temperature.Figures 3b, c
show the TEM images of the samples obtained at 7008C and
9008C, respectively.The shell of the former was uniform,
whereas the latter was decorated randomly with nanoparticles
of 20 nm in diameter.The room-temperature Raman spec-
trum of the hollow spheres (Figure 3d) shows three broad
symmetric peaks at 418, 567, and 724 cmÀ1.The second and
third peaks were assigned to first-order phonon frequencies of
E2(high) and A1(LO) modes, which have been measured at
570 and 730 cmÀ1 for GaN nanorods.[7] The lower-frequency
shift of the E2(high) peak implies that the spheres experience
some strain in structure since the shift is significantly
influenced by compressive or tensile strains.[37–38] The peak
positioned at 418 nmÀ1 is close to an acoustic overtone peak
found in the spectrum of a wurtzite GaN crystal.[39]
Figure 4. The photoluminescence spectrum of a) GaN hollow spheres
andb) GaN nanoparticles.
positioned at 452, 470, 475, 483, and 494 nm.The band at
371 nm is related to band-edge emission, which could be
attributed to the recombination of excitons bound at neutral
donor sites.[9,42] But the latter is difficult to identify.[42–45] The
band positioned between the yellow luminescence (2.2 eV,
563 nm) caused by deeply trap states[43,18] and the donor–
acceptor pair emission (3.2 eV, 380 nm) [44] may be caused by
impurities.[44–45] When the used reagent (GaCl3) was directly
precipitated from solution with ammonia, dehydrated in air,
and converted to GaN in NH3 at 9008C, the as-formed GaN
nanoparticles exhibited only band-edge photoluminescence
(Figure 4b).So the emission around 470 nm may be morpho-
logically related.The shell of hollow spheres is composed of
GaN nanoparticles, while the structure of GaN quantum dots
at the electronic level has not yet been clearly determined,[42]
which further complicates the assignment of the observed
peaks to specific transitions at the present time.Further
detailed optical studies, such as absorbance, reflectance, and
photoluminescence, on GaN hollow spheres with different
sizes and crystallinities will be reported elsewhere.
In summary, size-tunable Ga2O3 and GaN semiconductor
hollow spheres with uniform shells were prepared by using
carbon spheres as templates.The size of the hollow spheres
could be adjusted in the range from 100 nm to 1.5 mm by
selecting different sized templating carbon spheres.This
approach provides a general, simple, and cheap method to
uniform hollow spheres.Surface modification and agglomer-
ation is naturally avoided.Hollow spheres of other metal
oxides may be prepared by using this method.
The tunable size range of GaN hollow spheres from
100 nm to 1.5 mm is especially attractive for optical applica-
tions because this range covers the band gaps in the spectral
regime from the ultraviolet (100–400 nm), visible (400–
800 nm), to near infrared (800–1500 nm), while the intrinsic
emission band of GaN is also in this range (370 nm).Former
studies indicate that the emission properties of semiconductor
nanoparticles are affected by the way in which they pack and
embed into the voids of the colloidal crystals.[40,41] Hollow
GaN spheres with average outside diameter of 370 nm (shown
in Figure 3c) were chosen for photoluminescence study since
they were expected to show specific interaction with the
emission of GaN.One can see that the GaN hollow spheres
showed two major emission bands (Figure 4a): one centered
at 371 nm and the other comprising several emissions
Experimental Section
Carbon template spheres: In a typical procedure, glucose or sucrose
(4–8 g, analytic purity, Beijing Chemical Reagent Factory) was
dissolved in water (40 mL) to form a clear solution.The solution
was then sealed in a 40 ml teflon-lined autoclave and maintained at
160–1808C for 4–20 h.The black or puce products were isolated after
centrifugation at 5000 rpm for 20 min.The products were centrifuged,
washed, and redispersed in water, and this cycle was repeated five
times.Next, the products were centrifuged, washed, and redispersed
in alcohol, and this cycle was repeated five times.The spheres were
then oven-dried at 808C for 4–8 h.[36]
GaN spheres: The as-prepared carbon spheres were washed,
redispersed in 0.5m GaCl3 solution, and ultrosonicated for 10 minutes.
The resulting suspension was aged for 12 h before being subjected to
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five cycles of centrifugation/wash/redispersion in water.The oven-
dried sample was heated to 4508C in a tube furnace in air and
maintained at the temperature for one hour.Ammonia ( > 99.5%,
30 sccm) was introduced and the temperature to 8008C at a rate of
38C per minute.Argon was flowed into the reaction vessel at a rate of
200 sccm while maintaining the temperature at 8008C for 3 h.Slightly
yellow powders were obtained as the sample was allowed to cool to
room temperature.Products were characterized with XRD (Bruker
D8 advance), TEM (Hitachi H800, Jeol 2010F, both operated at
200 kV), and SEM (Leo-1530).Fluorescent spectra were recorded
with a Hitachi F-4500 fluorescence spectrophotometer.The quantity
of Ga adsorbed onto the carbon spheres were determined by thermal
gravimmetric analysis (TA instruments, TGA2050) in air at a heating
rate of 108CminÀ1 from room temperature to 6008C.
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Keywords: gallium · microspheres · nanostructures ·
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semiconductors · template synthesis
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