A facile one-step route to synthesize titania hollow microspheres with incontinuous multicavities

Article abstract of DOI:10.1016/j.cclet.2013.12.002

The titania hollow microspheres with incontinuous multicavities were successfully fabricated via an oil/water (O/W) emulsion process accompanied by sol-gel reaction in the presence of polyvinylpyrrolidone (PVP). In the one-step route, the addition of PVP to the tetrabutyl titanate (TBT) 1-octanol solution as the oil phase of the O/W emulsion clearly expands the size of the cavities inside the microspheres. The n-propanol and atoleine alters the polarity of the oil phase to affect the interior structure significantly. The Span 80 is used as a stabilizer to preserve spherical shape. A preliminary mechanism based on phase-separation for the structural evolution of titania hollow microspheres with multicavities is suggested. Zirconia and alumina hollow microspheres with incontinuous multicavities can also be prepared by this one-step route successfully.

Full text of DOI:10.1016/j.cclet.2013.12.002

Chinese Chemical Letters 25 (2014) 441–446
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
A facile one-step route to synthesize titania hollow microspheres with
incontinuous multicavities
Wei-Wei Cai, Hui Yang, Xing-Zhong Guo *
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310024, China
A R T I C L E I N F O
A B S T R A C T
Article history:
The titania hollow microspheres with incontinuous multicavities were successfully fabricated via an oil/
Received 29 July 2013
water (O/W) emulsion process accompanied by sol–gel reaction in the presence of polyvinylpyrrolidone
Received in revised form 14 November 2013
Accepted 21 November 2013
Available online 7 December 2013
(
PVP). In the one-step route, the addition of PVP to the tetrabutyl titanate (TBT) 1-octanol solution as the
oil phase of the O/W emulsion clearly expands the size of the cavities inside the microspheres. The n-
propanol and atoleine alters the polarity of the oil phase to affect the interior structure significantly. The
Span 80 is used as a stabilizer to preserve spherical shape. A preliminary mechanism based on phase-
separation for the structural evolution of titania hollow microspheres with multicavities is suggested.
Zirconia and alumina hollow microspheres with incontinuous multicavities can also be prepared by this
one-step route successfully.
Keywords:
Hollow microspheres
Multicavities
Phase separation
Titania
ß 2013 Xing-Zhong Guo. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights
reserved.
1
. Introduction
Hollow microspheres have attracted much attention because of
[17]. However, the reported method to prepare transition-metal
oxide microspheres with multicavities needs heat treatment to
remove the latex template.
their special characteristics, such as low density, low thermal
conductivity, high specific surface and good flow ability [1]. So far,
various porous microspheres with unique characteristics such as
hollow spheres with a single cavity [2–4], hollow spheres with
multiple shells [5,6], and hollow sphere with multicavities [7],
have been fabricated by the templating method, emulsion
processing, high temperature smelting, layer-by-layer self-assem-
bly techniques, etc. Wherein, hollow spheres with multicavities as
an important class of hollow sphere have gained growing attention
for their potential applications such as delivery systems, adsorp-
tion and catalysis [8–10]. Although organic spheres with multi-
cavities consisting of diverse polymers have been prepared [11–
In the present work, we demonstrate a facile one-step synthesis
of cage-like porous titania microspheres via oil/water (O/W)
emulsion accompanied by
a sol–gel reaction without any
templates. The presence of polyvinylpyrrolidone (PVP) played an
important role in forming porous structure of titania hollow
microspheres with multicavities, while Span 80 had great effects
on preserving spherical structure of both macrocavities and TiO
2
particles. Moreover, zirconia and alumina hollow spheres with
multicavities were also fabricated via the same method mentioned.
It is anticipated that the method presented in this work will offer
an approach to fabricate metal oxides hollow spheres with
multicavities, including their composites.
1
3], manipulation of inorganic spherical particles with multi-
cavities is still a challenging task. So far, cage-like SiO hollow
2
spheres have been prepared by templating methods and template-
free methods in emulsion route [14–16]. However, we notice that
the preparation of metal oxide hollow spheres with multicativies
2. Experimental
2.1. Materials
2 2 2 3
including titania (TiO ), zirconia (ZrO ) and alumina (Al O ) hollow
spheres have rarely been mentioned in the literature. To the best of
our knowledge, there is only one paper concerning the transition-
metal oxide microspheres with multicavities, which were pre-
pared by using template in aerosol-assisted self-assembly method
1-Octanol, ethyl acetoacetate (EAA, AR), zirconium propoxide
(70 wt.% of popanol solution) and aluminum tri-sec-butoxide (97%)
were purchased from Aladdin Reagent Co., Ltd. Other chemicals,
i.e., tetrabutyl titanate (TBT, AR), Span 80 (CP), sodium dodecyl
sulfate (SDS, CP), alkylphenol ethoxylates (OP-10, CP) and
polyvinylpyrrolidone (PVP, K30, CP) were purchased from Sino-
pharm Chemical Reagent Co. Ltd. All the chemicals were used as
received.
*
Corresponding author.
E-mail address: msewj01@zju.edu.cn (X.-Z. Guo).
1
001-8417/$ – see front matter ß 2013 Xing-Zhong Guo. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2013.12.002

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W.-W. Cai et al. / Chinese Chemical Letters 25 (2014) 441–446
2.2. Synthesis
The detailed preparation of the gel microspheres was as follows.
First, an external water phase was prepared by adding 4.0 g of SDS
and 8.0 g of OP-10 in 300 g of deionized water under strong stirring
at 1000 rpm. Second, 2.377 g of EAA and 6.217 g of TBT were added
to 10.450 g of 1-octanol with constant stirring for 1 h to prepare
the oil phase. Then, a certain amount (0.190, 0.571, 1.142 and
1
.903 g) of PVP and 0.571 g of Span 80 were added to the oil phase
under stirring for 1 h as an additive. Third, the mentioned oil phase
was added to the external water phase once under strong stirring
at 1000 rpm. After stirring for 24 h, the samples were collected by
centrifuge and dried at 40 8C for 24 h.
The fabrication of zirconia and alumina microspheres followed
the same process, except with zirconium propoxide and aluminum
tri-sec-butoxide as the precursors, respectively.
2.3. Characterization
The microstructures including average cavity sizes and size
distributions of the TiO
scanning electronic microscope (SEM, FEI Siron). Thermal analysis
for the TiO gel microspheres was carried out using a DTA analyzer
German Netzsch Co. STA449C) from room temperature to 700 8C
in air at a heating rate of 10 8C/min.
2
microspheres were performed using
2
(
3
. Results and discussion
It was found that the initial oil phase was a yellow transparent
solution. When the oil phase was mixed with water phase and
broken into small droplets by high-speed shearing, water
molecules around the oil droplets diffused through oil–water
interface and reacted with TBT to initiate the sol–gel process. The
sol–gel transition of oil droplets, which built the solid spheres, is
caused by two kinds of reaction, i.e., hydrolysis and polyconden-
sation [18]. The effects of PVP will be discussed and a preliminary
mechanism will be suggested accordingly.
2
Fig. 1 shows the SEM images of TiO microspheres fabricated
with different amounts of PVP. Spherical structures are obtained
for all five particles, and the surfaces of the microspheres
fabricated with different amounts of PVP are generally smooth.
However, different interior structures are observed for the five
samples. The microspheres fabricated without PVP are solid as
shown in Fig. 1b, while the addition of PVP led to cage-like porous
structures as shown in Fig. 1b, d, f, i, k. The average cavity
diameters calculated from the SEM images are ꢀ500 nm, ꢀ750 nm,
Fig. 1. SEM images of the titania microsphere synthesized with 0 (a and b), 0.190 (c
and d), 0.571 (e and f), 1.142 (h and i), and 1.903 g (j and k) PVP.
ꢀ
1.2
m
m, and ꢀ1.3
mm, respectively. In addition, the microspheres
obtained with 1.903 g of PVP show generally broken structures,
which can be attributed to the fragility caused by the decrease in
wall thickness of cavities. As shown in Fig. 2, the cavity size
distributions also become wider with the increase of PVP content
generally.
70
0.190 g PVP
60
50
40
0.571 g PVP
1.142 g PVP
.903 g PVP
1
To clarify the role of PVP in fabricating the cage-like porous
structure, DTA measurements were performed for the TiO
2
microspheres (Fig. 3a) and the supernatant phases obtained by
centrifuge (Fig. 3b), respectively. For the supernatant phase, two
exothermic peaks at around 430 and 500 8C are observed, which
3
2
1
0
0
0
0
2
are attributed to the decomposition of PVP. For the TiO micro-
spheres, only an exothermic peak at 270 8C is observed for all five
samples, which can be associated with the decomposition of
butoxy and residual 1-octanol [19]. As shown by the DTA
measurement, PVP is preferentially distributed into the water
phase, rather than the titania oligomers rich phase. It indicates that
the interaction between polymerizing titania oligomers and PVP is
less attractive, although the reported literature mentioned that
there is a strong interaction between PVP and Ti atom [20,21]. The
0.5
1.0
1.5
2.0
2.5
3.0
Cavity size (µm)
Fig. 2. Cavity size distribution of the microspheres fabricated with different PVP
contents.

W.-W. Cai et al. / Chinese Chemical Letters 25 (2014) 441–446
443
a)
b)
1.903 g PVP
1.903 g PVP
1.142 g PVP
1.142 g PVP
0
.571 g PVP
.190 g PVP
0.571 g PVP
0
0.190 g PVP
without PVP
200
without PVP
500
100
200
300
400
600 100
300
400
500
600
o
o
Temperature ( C)
Temperature ( C)
Fig. 3. DTA curves of titania microspheres fabricated with different PVP contents (a), and the supernatant phases obtained by centrifuge (b).
Fig. 4. Illustrative mechanism for the formation of hollow microspheres with multicavities.
Fig. 5. SEM images of the titania microspheres with 0.571 g PVP prepared with 20 wt.% replacement of 1-octanol by n-propanol (a and b) and atoleine (c and d), respectively.

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W.-W. Cai et al. / Chinese Chemical Letters 25 (2014) 441–446
2
Fig. 6. SEM images of the TiO microspheres fabricated with 0.571 g PVP and without Span 80.
decreasing interaction is probably attributed to the decreased
polarity of titania oligomers and the increased steric hindrance
induced by polymerization [22].
the course of the phase separation. Thus we can observe that the
microspheres fabricated with n-propanol additive contain higher
pore density and smaller pore diameters. On the contrary, the
atoleine additive may suppress the sol–gel process by retarding the
diffusion of water molecule. Thus the lower pore density can be
attributed to the relatively low nucleation rate in the course of the
phase separation.
In addition, the Span 80 additive also plays an important role on
the microstructure of microspheres. It is expected that Span 80
molecules can locate on the interface of the separated domains to
act as a stabilizer so that the separated domains could experience
less deformation during the coarsening process. As shown in Fig. 6,
the absence of Span 80 will result in a coarse surface and irregular
inner cavities.
The mechanism for the formation of the hollow spheres with
multicavities may be explained on the basis of nucleation-growth
phase-separation model, which is schematically shown in Fig. 4. As
described by Nakanishi [18], the polymerization and gel forming
reaction are thermodynamically analogous to the continuous
cooling of a glass-forming liquid into a miscibility gap, which can
be described as ‘‘chemical cooling’’. An initial single-phase solution
containing a polymerizable component becomes less stable with
an increase in the molecular weight of the component, and results
in the separation into different phases. The phase-separated
domains go through a coarsening process induced by additional
molecules diffusing toward the nuclei. Also, the sol–gel transition
can freeze the phase separation structures to prevent further
coarsening of the separated domains. In this system in the
presence of PVP, the increasing molecular weight of monomer-
In the literature, porous structures can also be fabricated via W/
O/W emulsions [11,12,24]. In order to confirm the phase
2
separation mechanism, we prepared TiO gel film by spreading
the oil phase containing 0.571 g of PVP and 0.571 g of Span 80 on
water surface as depicted in Fig. 7a. The diffused water initiated the
2
TiO polymer via the sol–gel process could cause the system to be
immiscible and drive the phase separation. During the sol–gel
process, the PVP will precipitate inside the oil phase and become
the nucleation center. Then the ethyl acetoacetate released diffuses
toward the center to induce coarsening process. The sol–gel
transition finally freezes the phase separation structures to
prevent further coarsening. The increase of PVP may enhance
the phase-separation tendency, which can cause an early onset of
phase separation relative to the sol–gel transition [23], and results
in a relative long time for the coarsening process to build an
enlarged pore size. Moreover, the PVP was almost dissolved in the
water phase as characterized by the DTA measurement, which
might indicate the diffusion of PVP and ethyl acetoacetate into the
water phase due to their good water solubility.
sol–gel process of the oil phase, which produced a TiO
Fig. 7b and c shows the SEM images of the TiO gel film and the
cross section, respectively. It is observed that the TiO gel film
obtained without vigorous stirring also has a porous structure
consisting of incontinuous cavities, which further demonstrated
2
gel film.
2
2
that the porous structure of the TiO
attributed to W/O/W emulsion [25].
In addition, ZrO and Al have also been widely applied in the
2
microspheres should not be
2
2 3
O
fields of catalysis, adsorption, and separation [26–28]. Herein, the
one-step route presented in this work is proven to be applicable to
The sol–gel process inside the oil phase was greatly influenced
by the diffusion of water molecules. Thus the polarity of the solvent
used in the oil phase might have great effects on the porous
structure of the microspheres. In order to confirm the effect of the
polarity of the solvent used in the oil phase, 20 wt.% 1-octanol was
replaced by n-propanol and atoleine, respectively. Fig. 5 shows the
SEM images of the microspheres fabricated with 20 wt.% replace-
ment of 1-octanol. As shown, the microspheres fabricated in the
presence of n-propanol (Fig. 5a and b) have smaller pore diameters
(
(
ꢀ550 nm) than the microspheres without solvent replacement
Fig. 1e and f), while the microspheres fabricated in the presence of
atoleine (Fig. 5c and d) possess lower pore density. Herein, we may
suggest that the addition of polar solvent to the oil phase facilitates
the diffusion of water in the oil phase and accelerates the sol–gel
process. As mentioned, the polymerization and gel forming
reaction are thermodynamically analogous to the continuous
cooling of a glass-forming liquid into a miscibility gap. The
accelerated sol–gel process may result in high degree of ‘‘chemical
supercooling’’, which leads to a relatively high nucleation rate in
Fig. 7. Schematic picture and SEM images of the prepared titania gel film.

W.-W. Cai et al. / Chinese Chemical Letters 25 (2014) 441–446
445
Fig. 8. SEM images of zirconia (a and b) and alumina (c and d) microspheres with incontinuous multicavities.
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[
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