SiO2 nanospheres with tailorable interiors by directly controlling Zn2 and NH3·H2O species in an emulsion process

Article abstract of DOI:10.1016/j.jssc.2011.03.051

SiO₂ nanospheres with tailorable interiors were synthesized by a facile one-spot microemulsion process using TEOS as silica source, wherein cyclohexane including triton X-100 and n-octanol as oil phase and Zn² or NH₃·H₂O aqueous solution as dispersive phase, respectively. The products were characterized by Scanning Electron Microscopy, Transmission Electron Microscopy and X-ray Powder Diffraction. It was suggested that the as-synthesized silica nanospheres possessed grape-stone-like porous or single hollow interior, and also found that the ammonia dosage and aging time played key roles in controlling the size and structure of silica nanospheres. Furthermore, the comparative results confirmed that in-situ zinc species [ZnO/Zn(OH)₂] acted as the temporary templates to construct grape-stone-like interior, and a simultaneously competing etching process occurred owing to the soluble Zn(NH₃)₄² complex formation while the additional excessive ammonia was introduced. With the aging time being extended, the in-situ nanocrystals tended to grow into bigger ones by Ostwald Ripening, producing single hollow interior.

Full text of DOI:10.1016/j.jssc.2011.03.051

Journal of Solid State Chemistry 184 (2011) 1603–1607
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
2
þ
SiO nanospheres with tailorable interiors by directly controlling Zn
2
and NH ꢀ H O species in an emulsion process
Yuchao Liao , Xiaofeng Wu , Zhen Wang , Yun-Fa Chen
3
2
a,b
a
a,b
a,n
a
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
Graduate University of Chinese Academy of Sciences, Beijing 100049, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
SiO nanospheres with tailorable interiors were synthesized by a facile one-spot microemulsion process
2
Received 30 November 2010
Received in revised form
using TEOS as silica source, wherein cyclohexane including triton X-100 and n-octanol as oil phase and
2þ
Zn
or NH
3
ꢀ H
2
O aqueous solution as dispersive phase, respectively. The products were characterized
1
9 March 2011
by Scanning Electron Microscopy, Transmission Electron Microscopy and X-ray Powder Diffraction.
It was suggested that the as-synthesized silica nanospheres possessed grape-stone-like porous or single
hollow interior, and also found that the ammonia dosage and aging time played key roles in controlling
the size and structure of silica nanospheres. Furthermore, the comparative results confirmed that
Accepted 29 March 2011
Available online 30 April 2011
Keywords:
2
SiO nanospheres
in-situ zinc species [ZnO/Zn(OH)
2
] acted as the temporary templates to construct grape-stone-like
Tailorable interior
Microemulsion
2þ
interior, and a simultaneously competing etching process occurred owing to the soluble Zn(NH )
3 4
complex formation while the additional excessive ammonia was introduced. With the aging time being
extended, the in-situ nanocrystals tended to grow into bigger ones by Ostwald Ripening, producing
single hollow interior.
Temporary template
Ostwald Ripening
&
2011 Elsevier Inc. All rights reserved.
1
. Introduction
templates usually take much time, and the removal of templates by
either sintering or etching is very complicated, time-consuming
and energy-consuming. Moreover, the hollow structures obtained
via template-mediated approach are usually larger than 100 nm
because it is hard to make smaller size template particles, which
limits the application of this method. While microemulsion method
proposed first by Schulman [29] would overcome the shortcomings
above because the size of dispersed droplets is usually less than
100 nm. In addition, the advantages including mild experiment
conditions and simple procedures make microemulsion technology
a facile and efficient method for synthesizing various nano-sized
materials.
Porous materials have opened many new applications such as
catalysis, separation, thermal insulation and nanoscience due to
their respective high specific surface areas, tunable pore sizes and
high porosity. Hollow interior structure had attracted much
more attention in recent years, which was applied extensively in
catalysis [1,2], sensors [3], Li-ion batteries [4,5], microreactors [6,7],
biomedicines [8,9] and so on. Many methods such as template
method [10–13], template-free method [14,15], sacrificial template
method [16], Kirkendall effect [17–19], galvanic replacement
[20,21], have been introduced to prepare hollow interior structure.
Among them, the template method is an effective and intriguing
strategy, in which the templates are firstly prepared, then the
surface of templates is functionalized and coated with target
materials or their precursors, particles with core–shell structure
can be obtained. As the core–shell particles roasted in air or etched
with an appropriate solvent, the hollow interior structure can be
obtained. Generally, various templates such as PS spheres [22],
2
In this present work, SiO nanospheres with tailorable interiors
were obtained through just controlling the reactants dosage and
aging time of temporary templates in microemulsions. The tempor-
ary templates were firstly produced by two microemulsions, and
2
then they would be coated with SiO shells derived from TEOS and
synchronously etched due to the addition of third microemulsion.
As the hydrolysis rate of TEOS is a little faster than etching rate of
the temporary templates, nanostructured interior structures were
obtained. In contrast to conventional methods, no additional pro-
cesses such as roasting or etching were needed. And the size of the
particles could be tuned within 100 nm. Moreover, the nanostruc-
tured single or multiple interior could be achieved by just only
controlling the aging time of temporary templates. To the best of
our knowledge, this technique in which tailorable interiors were
PMMA spheres [23], C spheres [24], and SiO
2
spheres [25], droplets
[26], vesicles [27,28] are used. However, preparation procedures of
n
Correspondence to: Institute of Process Engineering, Zhongguancun Beiertiao
1
0
#, Haidian District, Beijing, China. Fax: þ86 10 62525716.
E-mail address: yfchen@home.ipe.ac.cn (Y.-F. Chen).
022-4596/$ - see front matter & 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2011.03.051

1
604
Y. Liao et al. / Journal of Solid State Chemistry 184 (2011) 1603–1607
prepared by directly controlling reactive species has not been
reported previously.
2.3. Characterizations
X-Ray Powder Diffraction (XRD) data were collected on the
X’Pert Pro powder diffractometer. Scanning Electron Microscopy
2
. Experimental
(
SEM) of type JSM-6700 F, JEOL, was used to characterize the
morphologies of the products. Transmission electron microscopy
TEM) images and Energy Dispersive Spectrometry (EDX) were
2.1. Materials
(
obtained on a JEM 2010 F operated at 200 kV. Fourier Transfer
infrared spectrums (FT-IR) were obtained on Tensor 37 type from
Brucker Company. The microemulsion droplets size was deter-
mined using dynamic light scattering technology (DLS) on Dyna-
Pro Nanostar from Wyatt Technology Corporation.
3 2
Ethanol, ammonia, tetraethoxysilane (TEOS) and Zn(NO ) , were
of analytic grade and purchased from Beijing Vas Chemical Reagent
Co, Ltd., China. Cyclohexane, triton X-100, n-octanol were of analy-
tical grade and provided by Beijing Yili Chemical Reagent Co, Ltd.,
China. Deionized water was used for all processes.
2.2. Synthesis of SiO
2
nanoparticles with tailorable interior
3. Results and discussion
Three cyclohexane emulsified systems including different
3.1. Effect of temporary templates on the interior structure of
products
dispersive phase were prepared before reaction. All the micro-
emulsions mentioned above had fixed amount of oil phase
including 10 mL cyclohexane, 1.5 mL triton X-100 and 1 mL
Generally, the existence and the shape of the templates are the
critical parameters affecting the interior structure of porous materi-
als. Here MA and MB without the addition of MC and TEOS produced
zinc species acting as the temporary templates. The composition and
crystal type of temporary templates were identified by X-Ray
Powder Diffraction (XRD). As shown in Fig. 1(A), the peaks locating
near 331, 581 and 691 can be indexed to ZnO phase (JCPDS Card No
21-1486), and the peaks near 271, between 311 and 341 can be
3 3
n-octanol, and were indexed as MA with Zn(NO ) solution as
dispersive phase, MB with dilute ammonia and MC with concen-
trated ammonia. Agitation (1000 rpm) or ultrasound (40 k Hz,
00 W) was applied to make microemulsions transparent/trans-
4
lucent and stable. First, MA and MB of equal volumes were
simultaneously poured into a beaker, and the aging time was
kept for 0.5–48 h. After the prescribed time, MC and a certain
amount of TEOS were sequentially added into the system. Half an
hour later, the mixture including MA, MB, MC and TEOS was
stopped agitating to age for 24 h. Then the mixture was demulsi-
fied, washed and centrifuged with ethanol by three cycles. The
final particles were obtained after drying at 80 1C for 12 h, the
atmosphere was air.
2
indexed to Zn(OH) phase (JCPDS Card No 20-1435). The component
of temporary templates was further confirmed by IR spectrum as
ꢁ
1
shown in Fig. 2(A). The peaks between 400 and 600 cm
are
attributed to the stretching vibrations of Zn–O, and the peak at
ꢁ
1
3459 cm
to the stretching vibration of O–H. As for the hydroxyl
coordination compound, the flexural vibration peak of M–OH tends
2
Fig. 1. XRD patterns of temporary templates (A) and SiO nanospheres (B) and dynamic light scattering spectra of MA, MB and mixture of them (C).
2
Fig. 2. FT-IR spectra of temporary templates (A) and SiO nanospheres (B).

Y. Liao et al. / Journal of Solid State Chemistry 184 (2011) 1603–1607
1605
ꢁ
1
ꢁ1
to appear below 1200 cm
[30], so the peak at 1070 cm
is
3.2. Identification of hollow-interior-nature of products
attributed to the flexural vibration Zn–OH. The analysis above
indicates the temporary templates of a mixed crystal of ZnO and
According to Table 1, Sample 2 was firstly produced. SEM
image (Fig. 4(B) inset) shows the particles of uniform size with
well dispersion. The corresponding TEM image (Fig. 4(B)) shows
an obvious contrast between the dark edge and the pale interior
of the individual spheres, indicative of porous grape-stone-like
interiors with ca. 10 nm in diameter and shell with ca. 15 nm in
thickness. Excitingly the interior size is very close to that of
the temporary templates mentioned earlier. In the corresponding
2
Zn(OH) . The size of the temporary templates is certain to affect that
of interior structure of final products. Hence, the droplets size of MA,
MB and mixture of them was investigated using dynamic light
scattering (DSL) technology. The results in Fig. 1(C) show that the
size (ca. 9 nm in diameter) of MA or MB scarcely changes before
mixing and after mixing. To avoid the changes in droplets size, in all
experiments MA and MB have identical oil phase and the equal
volumes of water phase. When MC was added, the translucent
mixture of MA and MB became transparent and no products were
collected. It may be attributed to the transformation of zinc species
ꢁ
1
FT-IR spectrum of Fig. 2(B), the adsorption peak at 1075 cm
is
attributed to the antisymmetric stretching vibration of Si–O–Si,
ꢁ
1
ꢁ1
and the two peaks at 796 cm
and 466 cm
correspond to the
2
4
þ
into soluble complex ZnðNH
3
Þ
. MC and TEOS were used and
flexural vibration of Si–O–Si. The HRTEM technique (Fig. 3(A))
indicates the particle shell of amorphous type without crystal
lattices, and there are two obvious strong peaks for Si and O in the
EDX spectrum (Fig. 3(B)). Here it should be noted that the weak
peaks for C and Cu are derived from carbon membrane and copper
mesh of sample holder. It is ascertained that the component of
produced dense nanospheres with ca. 35 nm in diameter in the
absence of MA and MB as shown in Fig. 4(A). The possible reason is
explained as following: after the addition of MC, the TEOS molecules
diffuse onto the interface between oil phase and aqueous droplets
2
and start to react with H O under alkaline condition. The details of
hydrolysis and condensation of TEOS were interpreted in previous
studies [31,32]. Because of no templates in the aqueous phase,
hydrolysis products of TEOS become more and more hydrophilic
and dissolve in the aqueous droplets. Afterwards, nanocrystals
precipitate from the supersaturated solution, aggregate and grow
into dense spheres. From the facts mentioned above, it is ascertained
that the in-situ generated zinc species nanocrystals act as temporary
templates to shape the interior structure of particles.
particle shell is amorphous SiO
XRD patterns in Fig. 1(B) that a single broad peak at 2
attributed to the silica amorphous phase, and no obvious char-
acteristic responses of zinc species are identified. All the analysis
above indicates that zinc species are dissolved in the etching
2
. Furthermore it is found from
¼231
y
2
process and SiO nanospheres with hollow-interior are formed.
3.3. Effects of dilute ammonia concentration and aging time of
temporary templates on the interior structure of products
Table 1
Experimental conditions.
In order to investigate the possible formation process and
mechanism of the nanostructured silica nanospheres, the para-
meters including reactant dosage and aging time were further
studied. It is to be remarked that the amount of MC and TEOS
were fixed in all of the experimental conditions. First, the
a
b
c
d
e
f
g
Sample
C
–
A
,V
A
C
B
V
B
V
C
AT
–
TEOS
Display
1
2
3
4
5
6
7
8
–
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
Fig. 4(A)
Fig. 4(B)
Fig. 4(C)
Fig. 4(D)
Fig. 4(E)
Fig. 4(F)
Fig. 4(G)
Fig. 4(H)
0.2, 1
0.2, 1
0.2, 1
0.4, 2
0.2, 1
0.2, 1
0.2, 1
0.4, 1
0.2, 1
0.6, 1
0.8, 2
0.4, 1
0.4, 1
0.4, 1
0.5
0.5
0.5
0.5
influence of NH
of nanospheres was investigated under the condition that Zn
concentration (0.2 mol/L) in MA was fixed. As 0.2 mol/L NH
ꢀ H
3
ꢀ H
2
O concentration in MB on interior structure
2
þ
12
3
2
O
24
48
in MB was used and dense particles with ca. 30 nm in diameter
2
formed as shown in Fig. 4(C). With the concentration of NH O
3
ꢀ H
a
2þ
in MB increased to 0.4 mol/L, the particles of interior grape-
stones-like cavities were obtained as shown in Fig. 4B. Up to
Concentration of Zn
solution in MA (mol/L).
b
c
d
e
f
2þ
Volume of Zn
solution in MA (mL).
Concentration of dilute ammonia solution in MB (mol/L).
Volume of dilute ammonia solution in MB (mL).
0.6 mol/L, the bigger particles with ca. 50 nm in diameter were
obtained, and their porous interiors are clearly identified with ca.
3 nm in diameter as shown in Fig. 4(D). Obviously, ammonia
concentration in MB plays an important role in tailoring interior
structure of nanospheres. Insufficient amount of ammonia in MB
Volume of concentrated ammonia solution in MC (mL).
The aging time of zinc species produced by MA and MB (h).
g
The volume of TEOS (mL). The composition of oil phase in all microemulsion
is identical.
2
Fig. 3. HRTEM image (A) and EDX spectrum (B) of SiO nanospheres shell.

1
606
Y. Liao et al. / Journal of Solid State Chemistry 184 (2011) 1603–1607
Fig. 4. TEM and SEM (inset) images of Sample 1 (A), Sample 2 (B), Sample 3 (C), Sample 4 (D), Sample 5 (E), Sample 6 (F), Sample 7 (G), Sample 8 (H).
ꢁ
2þ
cannot furnish enough OH for the precipitation of Zn , so there
are no templates in microemulsion droplets. The hydrolysis
products of TEOS pass through the oil–water interface and
aggregate to a certain extent to form dense spheres in the center
of microemulsion droplets, as shown in Fig. 4(C). Moderate
diameter concomitantly with bigger grape-stone-like interior
with ca. 30 nm in diameter than that in Fig. 4(B). As the time
continued to be extended to 24 h, the morphologies of silica
nanospheres changed very little, but the porous interiors became
bigger or even formed single hollow interior (as indicated by
white arrows) as shown in Fig. 4(G). While the time was extended
to 48 h, hollow nanospheres with ca. 25 nm in shell thickness
and ca. 40 nm in interior diameter were obtained as shown in
Fig 4(H). It is concluded that with the aging time being prolonged,
the in-situ zinc species nanocrystals aggregate and grow into
larger or even single ones through Ostwald Ripening process,
which act as temporary templates to form the grape-stone-like or
single hollow interior nanostructures.
ꢁ
amount of ammonia in MB supplies enough OH to form zinc
species in the microemulsion droplets, which aggregate as the
temporary templates and etched gradually by MC, and the
interior structure of grape-stone-like appears as shown in
Fig. 4(B). While excessive ammonia in MB accelerates the pre-
cipitation of Zn , thus more and smaller zinc species particles
form [33,34], after the adding of MC and TEOS, the particles with
much more porous interior are collected as shown in Fig. 4(D).
The analysis above is further confirmed by the result of Fig. 4(E),
in which twofold Zn in MA and ammonia in MB was added. The
bigger silica nanospheres with ca.100 in diameter and more
closed-pack interiors with ca. 50 nm in diameter are identified
in contrast with the result in Fig. 4(B). Furthermore, another
critical factor aging time of temporary templates was investi-
gated. As the time was extended from 0.5 to 12 h, the size of silica
nanospheres in Fig. 4(F) became bigger with ca. 60 nm in
2
þ
2
þ
2
3.4. Formation mechanism of SiO nanospheres with tailorable
interior
Based on all the facts above, the possible formation mechan-
ism of nanostructured interior of silica nanospheres is proposed
and illustrated in Fig. 5. The primitive zinc species nanocrystals
(b) functioned as the temporary templates are produced

Y. Liao et al. / Journal of Solid State Chemistry 184 (2011) 1603–1607
1607
2
4
þ
. Such characteristic of the
well as soluble zinc complex ZnðNH
3
Þ
two reactants has been successfully applied in this study. As the
hydrolysis rate of TEOS is slightly quicker than the etching rate of
2
templates, the SiO shells form at the same time even in advance
of the complete dissolving of the templates, and the nanostruc-
tured interior forms. And just controlling of aging time of tempo-
rary templates produced by MA and MB can construct porous
or single hollow interior. Compared with other techniques, no
additional processes such as roasting or dissolving are needed.
This facile method is expected to be available in synthesizing
porous or single hollow interior of other inorganic oxides.
Acknowledgment
This research is financially supported by the Knowledge
Innovation Program of the Chinese Academy of Sciences (Grant
no. KJCX1. YW. 07).
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Fig. 5. Formation process of SiO nanospheres with porous and single hollow interior.
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. Conclusion
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þ
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2
þ
3
2
[
3
2
2