Facile Synthesis of Yolk–Shell Nanostructured Photocatalyst with Improved Adsorption Properties and Molecular-Sieving Properties

Article abstract of DOI:10.1002/cctc.201600505

A novel and facile method to fabricate yolk–shell nanostructured photocatalysts consisting of TiO₂ nanoparticles (NPs) as the core and spherical hollow silica as the shell was developed. In the fabrication, commercial TiO₂ NPs were directly incorporated into hollow silica spheres by utilizing oil-in-water (O/W) microemulsions as a template, followed by heat treatment to create a void space between the TiO₂ core and the silica shell region. The synthesized yolk–shell nanostructured TiO₂@SiO₂ acts as an efficient photocatalyst with both improved adsorption properties and photocatalytic efficiency, which far outperformed those of naked TiO₂ owing to the ability of the porous silica shell to adsorb/enrich small organic reactants (acetaldehyde and 2-propanol) inside the void space and efficient transportation to the adjacent active TiO₂ core. Comparative photocatalytic tests using a large organic molecule (conalbumin) revealed that the porous silica shell with an average pore diameter of 2.0 nm endowed this material with a molecular-sieving property, demonstrating its potential application in combination with organic support materials.

Full text of DOI:10.1002/cctc.201600505

DOI: 10.1002/cctc.201600505
Full Papers
Facile Synthesis of Yolk–Shell Nanostructured
Photocatalyst with Improved Adsorption Properties and
Molecular-Sieving Properties
[a, b]
[a]
[a]
[a, b]
Yasutaka Kuwahara,
Yuki Sumida, Kensei Fujiwara, and Hiromi Yamashita*
A novel and facile method to fabricate yolk–shell nanostruc-
outperformed those of naked TiO owing to the ability of the
2
tured photocatalysts consisting of TiO nanoparticles (NPs) as
porous silica shell to adsorb/enrich small organic reactants
(acetaldehyde and 2-propanol) inside the void space and effi-
2
the core and spherical hollow silica as the shell was developed.
In the fabrication, commercial TiO NPs were directly incorpo-
cient transportation to the adjacent active TiO core. Compara-
2
2
rated into hollow silica spheres by utilizing oil-in-water (O/W)
microemulsions as a template, followed by heat treatment to
tive photocatalytic tests using a large organic molecule (conal-
bumin) revealed that the porous silica shell with an average
pore diameter of 2.0 nm endowed this material with a molecu-
lar-sieving property, demonstrating its potential application in
combination with organic support materials.
create a void space between the TiO core and the silica shell
2
region. The synthesized yolk–shell nanostructured TiO @SiO2
2
acts as an efficient photocatalyst with both improved
adsorption properties and photocatalytic efficiency, which far
Introduction
[21]
TiO as one of the most promising photocatalysts has long
of TiO by reducing the number of surface active sites, thus
2
2
attracted immense interest for its widespread application in
air/water cleaning, antifouling, antibacterial protection, and
UV-light shielding owing to its stability, low cost, non-toxicity,
and suitable band gap energy for photo-induced redox reac-
limiting its practical applications.
An emerging approach that overcomes the above dilemma
is to encapsulate TiO2 particles within hollow materials. So-
called yolk–shell (or rattle-type) nanostructures consist of inner
core particles encapsulated by porous materials. Such a config-
uration allows access of reactant molecules to the core, and
the outer shell acts as a physical barrier to protect the core
from the surrounding environment and endows it with a mo-
[
1–11]
tions.
However, TiO often causes a problem regarding the
2
destruction of the TiO -applied surface under UV-light irradia-
2
tion, especially if it is supported on organic substrates (for ex-
ample, organic binders, polymer films, and organic fibers),
leading to a reduction of both the photocatalytic activity of
[22–24]
lecular-sieving property.
With modifiable properties and
[
12]
TiO and the durability of the support materials. A conven-
multi-functionalities, the yolk–shell nanostructure has been
considered as an intriguing platform material for the design
2
tional approach to inhibit the self-degradation of the TiO -ap-
2
[
25–30]
plied surface is covering the TiO surface with an inert material
and fabrication of intelligent catalysts.
Regarding yolk–
2
[
13,14]
[15,16]
[17,18]
such as carbon,
hydroxyapatite,
silica,
or porous
shell nanostructures encapsulating a TiO photocatalyst as the
2
[
19,20]
silica.
A number of studies reported to date have com-
core, some pioneering work was done by Ikeda et al., who
monly claimed that such methodology endows the TiO photo-
constructed rattle-like TiO @SiO2 composite in which TiO2
2
2
catalyst with not only size selectivity but also an increased ad-
sorption property in the photocatalytic degradation of specific
organic molecules. However, it simultaneously causes a crucial
problem with decreasing the inherent photocatalytic activity
nanoparticles (NPs) were kept inside the hollow silica by
[31–34]
a layer-by-layer fabrication technique.
The void space be-
tween the core (TiO ) and the shell (SiO ) keeps the surface of
2
2
TiO uncovered and allows the diffusion of small external mole-
2
cules to the TiO2 surface, leading to a photocatalytic per-
[
35,36]
[
a] Dr. Y. Kuwahara, Y. Sumida, K. Fujiwara, Prof. Dr. H. Yamashita
Division of Materials and Manufacturing Science
Graduate School of Engineering,
formance comparable to or even better than naked TiO2.
Yang et al. developed a method to synthesize hollow silicas en-
capsulating TiO2 particles by using poly(methacrylic acid)
Osaka University
[37]
(
PMAA) as an organic template. A similar material was re-
2
-1 Yamadaoka, Suita, Osaka 565-0871 (Japan)
Fax: (+81)6-6879-7457
E-mail: yamashita@mat.eng.osaka-u.ac.jp
ported by Wu et al., who utilized two kinds of polyelectrolytes
[38]
with different electric charges as templates. In addition to
these, Demirçrs et al. fabricated a yolk–shell TiO @SiO compo-
[
b] Dr. Y. Kuwahara, Prof. Dr. H. Yamashita
Unit of Elements Strategy Initiative for Catalysts & Batteries (ESICB)
Kyoto University
2
2
site by a heat treatment method exploiting the difference in
[39]
the thermal shrinkage of SiO and TiO . These preparation
Katsura, Kyoto 615-8520 (Japan)
2
2
methods are now well established, however, most of the tech-
niques require complex multiple steps and delicate control of
Supporting information for this article can be found under http://
dx.doi.org/10.1002/cctc.201600505.
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synthetic conditions to achieve ideal TiO @SiO configurations
2
2
Results and Discussion
and the porous silica shells needed for ensuring the inherent
photocatalytic activity of TiO , which are the bottlenecks re-
2
Catalyst characterization
stricting practical implementation.
Herein, we report a new and facile method to encapsulate
TiO NPs inside hollow silica spheres (TiO @HSS) with an ideal
Field-emission (FE)-SEM images show that the synthesized ma-
terial consists of monodisperse spherical silica particles (Fig-
ure 2a). Numerous cracks are seen in the surface of the silica
spheres, suggesting the formation of a permeable silica shell.
In contrast to yolk–shell TiO @SiO analogs obtained by the
2
2
TiO @SiO configuration and porous silica structure suited for
2
2
photocatalytic applications. Fabrication of TiO @HSS was per-
2
formed by utilizing oil-in-water (O/W) microemulsions as a tem-
2
2
[
40,41]
plate.
Commercially available TiO NPs (Evonik, P25) were
conventional bottom-up approaches, TiO @HSS obtained by
2
2
directly dispersed into the O/W system with assistance by
ultrasonication, with oleic acid used as the oil phase (Figure 1,
using O/W microemulsions show a broad particle size distribu-
tion centered at approximately 180 nm (inset of Figure 2b).
Step 1). At this stage, the external surface of the TiO NPs is
From the TEM images (Figure 2b), we can see that TiO NPs
2
2
decorated with oleic acid, forming a Ti–carboxylate coordina-
are successfully encapsulated within the hollow silica spheres
[
42]
tion bond so as to have an oleophilic surface. Subsequently,
two silicon sources (tetraethyl orthosilicate (TEOS) and 3-ami-
nopropyl triethoxysilane (APTES)) were added with vigorous
stirring (Step 2), which self-assembled to form spherical organ-
ic–inorganic composites containing TiO NPs and oleic acid in
2
several minutes at room temperature owing to the neutralizing
effect between the ꢀCOOH groups of oleic acid and -NH2
groups of APTES (Step 3). To fabricate silica shells, traditional
methods require the addition of alkaline (for example, aqueous
ammonia) together with a silicon source; however, in the pre-
sented process, APTES acts as an emulsifier to form a microe-
mulsion and the -NH group of APTES act as a base to catalyze
2
the polymerization of silicon alkoxides, thereby allowing the
formation of the spherical silica shell under ambient conditions
[
40,41,43]
without the use of any external alkaline sources.
After
aging at moderate temperature under static conditions, the re-
sulting precipitate was filtered, washed, and finally calcined at
6
508C in air to obtain TiO @HSS with a yolk–shell architecture
2
(
Step 4). The resulting yolk–shell nanostructured TiO @SiO
2 2
composite provides prominent adsorption and molecular-siev-
ing properties towards organic reactants, rendering it as an ef-
fective, size-selective photocatalyst for the degradation of spe-
cific organic molecules. Furthermore, TiO NPs synthesized by
2
this method are well-confined within the hollow silica spheres,
which can insulate the TiO NPs from the surrounding environ-
2
Figure 2. (a) FE-SEM image, (b) TEM image, (c–f) STEM image and the corre-
ment and thus inhibit the self-degradation of the TiO -applied
2
sponding EDS mapping images, and (g) illustration of TiO @HSS. The inset of
2
organic surface.
(b) is the particle size distribution diagram.
2
Figure 1. Schematic representation of the synthetic procedure of TiO @HSS.
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with a shell thickness of approximately 20 nm, and a void
space exists between the TiO NPs and the silica shell. The TiO2
2
NPs showed an uneven distribution throughout the silica parti-
cles, however, hardly any TiO NPs outside the silica particles
2
were observed. STEM images and energy dispersive spectros-
copy (EDS) mapping images (Figure 2c–f) clearly visualize that
multiple TiO NPs are confined in the void space of a single
2
hollow silica sphere as illustrated in Figure 2g. During the heat
treatment process, the organic components were removed by
calcination at 6508C in air (for thermogravimetric (TG) and FTIR
data, see Figures S1 and S2 in the Supporting Information,
respectively).
In general, high-temperature annealing of TiO promotes the
2
phase transition from anatase to rutile, which sometimes leads
Figure 3. N adsorption–desorption isotherms of TiO @HSS (circles) and bare
2
2
2
TiO (squares). The inset shows the corresponding BJH pore size distribution
curves.
to a reduction of photocatalytic activity of TiO . XRD patterns
2
confirmed that the TiO NPs retained their initial crystallinity
2
(anatase/rutile=7:3) even after calcination at 6508C in air (Fig-
ure S3 in the Supporting Information), and the observed size
a peak centered at 2.0 nm (inset of Figure 3), proving the exis-
tence of mesoporosity in the silica shell, which was also con-
firmed by low-angle XRD patterns (Figure S3). These results
verify that the silica shell has multimodal porosities, including
micropores, mesopores, and hollow cavities, which are impor-
tant for the adsorption and mass transfer of reactant molecules
and the subsequent photocatalytic reactions. As summarized
of the TiO NPs inside the hollow silica spheres was almost the
2
same as that of the bare TiO NPs (nominal primary particle
2
size of 21 nm), verifying that the heat treatment at this tem-
perature did not induce the phase transition and aggregation
of TiO NPs.
2
Figure 3 displays the N adsorption–desorption isotherms of
2
TiO @HSS, which show a steep increase in the amount of ad-
in Table 1, TiO @HSS shows a Brunauer–Emmett–Teller (BET)
2
2
2
ꢀ1
sorbed N in the low pressure region (p/p <0.01) and a large
surface area (SBET) of 344 m g and an intra(inter)-particle void
2
0
3
ꢀ1
hysteresis loop closing at p/p =0.5, whereas the results for the
volume (Vvoid) of 0.66 cm g , which approximately corre-
0
bare TiO NPs show a type III isotherm without such features.
sponds to the interior space volume, whereas the bare TiO
2
2
2
ꢀ1
A substantial increase in the low pressure region (p/p >1.0ꢁ
NPs have limited surface area (SBET =49 m g ) and no observa-
0
ꢀ
5
[44]
1
0 ) is a typical feature of microporous materials. Analysis
ble void volume. The textural properties of TiO @HSS are appa-
2
2
ꢀ1
of the adsorption isotherm by the Saito–Foley (SF) method,
which is often used to determine micropore size distribution
by assuming cylindrical pore geometry, indicated the presence
of micropores with a pore diameter of approximately 0.7 nm
rently lower than those of vacant HSS (S =364 m g and
BET
3
ꢀ1
Vvoid =0.69 cm g ). The reduced intra(inter)-particle void
volume is arguably a result of the occupation of part of the
interior space with TiO NPs.
2
(
for micropore analysis data, see Figure S4 in the Supporting
Information). The large hysteresis observed in the range
.5<p/p <1.0 is a phenomenon typically observed in samples
Photocatalytic tests
0
0
possessing hollow cavities surrounded by micropore/mesopore
systems, which is ascribed to the forced evaporation caused by
the tensile strength effect and phase transitions of the ad-
The photocatalytic performance of the thus-synthesized
TiO @HSS was examined in the gas-phase photocatalytic deg-
2
radation of 2-propanol under UV-light irradiation. Figure 4
compares the reaction kinetics of the photocatalytic degrada-
[
45]
sorbed phase.
In addition, the corresponding Barrett–
Joyner–Halenda (BJH) pore size distribution curve showed
tion of 2-propanol in air over TiO @HSS (containing 10 wt% of
2
Table 1. Textural properties and the rate constants for the photocatalytic reactions of TiO
Sample physisorption
2
@HSS, pure TiO
2
, and a mixture of TiO
2
and HSS.
N
2
Photocatalytic degradation of
Acetaldehyde in air
2
-Propanol in air
Conalbumin in water
R
CO2
[
a]
[b]
[c]
[d]
S
BET
V
void
V
total
D
ave
k
pro
k
act
R
CO2
k
ald
R
CO2
2
ꢀ1
3
ꢀ1
3
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
[
m g
]
[cm g
]
[cm g
]
[nm]
[min
]
[min
]
[mmolmin
]
[min
]
[mmolmin
]
[mmolmin ]
ꢀ
ꢀ
3
3
ꢀ3
ꢀ3
ꢀ2
ꢀ2
ꢀ3
TiO
TiO
TiO
2
2
2
@HSS
(P25)
+HSS
344
49
298
364
0.66
n.d.
0.47
0.69
0.99
0.34
0.72
1.0
2.0
n.d.
2.0
2.0
8.6ꢁ10
8.8ꢁ10
5.5ꢁ10
–
3.1ꢁ10
1.7ꢁ10
0.315
0.232
0.109
–
3.9ꢁ10
0.96ꢁ10
1.1ꢁ10
–
0.135
0.100
0.088
–
0.35ꢁ10
7.39ꢁ10
–
–
ꢀ3
[
e]
ꢀ3
ꢀ3
ꢀ2
0.98ꢁ10
–
HSS
[
a] Calculated by the BET method. [b] Determined from the a
inter-particle spaces. [c] Total pore volume reported at p/p =0.99. [d] Average pore diameter determined by the BJH method. [e] Physical mixture of TiO
P25) and HSS with a weight ratio of 1:9.
s
-plot by subtracting the micropore and mesopore volumes from Vtotal. Including intra- and
0
2
(
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concentration of acetone showed a gradual decrease, which
was accompanied by a rise in the CO concentration (Figure 4).
2
Table 1 summarizes the apparent rate constants for 2-propa-
nol degradation (kpro) and acetone degradation (k ), which
act
were determined by assuming pseudo-first-order reaction ki-
netics, and rates of CO evolution (RCO2). TiO @HSS gave
2
2
ꢀ
3
ꢀ1
kpro =8.6ꢁ10 min , which was almost similar to that of the
ꢀ
3
ꢀ1
naked TiO (8.8ꢁ10 min ), showing that the silica encapsula-
2
tion did not cause any hindrance for the access of small organ-
ic molecules during catalytic reactions and did not reduce the
number of active sites on the TiO surface. Of specific interest
2
are the trends of the acetone/CO concentrations; the forma-
2
tion of acetone was markedly suppressed for TiO @HSS
2
(
55 mmol at the maximum), whereas 80 mmol of acetone was
produced over naked TiO (cf. Figure 4a and 4b). The value of
2
ꢀ
3
ꢀ1
kact increased in the order TiO +HSS (0.98ꢁ10 min ) <TiO
2
2
ꢀ
3
ꢀ1
ꢀ3
ꢀ1
(
P25) (1.7ꢁ10 min ) <TiO @HSS (3.1ꢁ10 min ). Further-
2
more, TiO @HSS showed a more rapid evolution of CO com-
2
2
pared with naked TiO ; the CO formation rate, RCO2, increased
2
2
ꢀ
1
in the order TiO +HSS (0.109 mmolmin ) <TiO2 (P25)
2
ꢀ
1
ꢀ1
(
0.232 mmolmin ) <TiO @HSS (0.315 mmolmin ). These re-
2
sults clearly demonstrate the superior photocatalytic efficiency
of TiO @HSS for degradation of 2-propanol diluted in air. As
2
summarized in Table 1, a similar trend was also observed in the
photocatalytic degradation of acetaldehyde diluted in air (for
reaction kinetics data, see Figure S5 in the Supporting Informa-
tion), where a 1.35 times faster CO2 formation rate was ob-
ꢀ
1
tained for TiO @HSS (0.135 mmolmin ) compared with naked
2
Figure 4. Reaction kinetics for the photocatalytic degradation of 2-propanol
in air over (a) TiO @HSS, (b) TiO (P25), and (c) a physical mixture of TiO and
HSS with a weight ratio of 1:9 under UV-light irradiation (l>300 nm, intensi-
ꢀ
1
TiO (0.100 mmolmin ), quantitatively proving that the yolk–
2
2
2
2
shell architecture is a suitable configuration for achieving the
efficient gas-phase degradation of volatile organic molecules.
In gas-phase reactions under static conditions, diffusion and
adsorption processes of the gaseous reactants in principle play
critical roles for determining the overall reaction rate (Lang-
ꢀ
2
ty=20 mWcm ).
TiO ), the naked TiO (P25), and a physical mixture of TiO and
2
2
2
[46,47]
HSS (TiO +HSS: mixed with a weight ratio of 1:9). Notably, the
muir–Hinshelwood mechanism).
Assuming that the con-
2
amount of photocatalyst placed in the reaction vessel was ad-
secutive oxidation model mentioned above is the main reac-
tion pathway, the acetone intermediate produced on the
justed based on the mass of TiO . Under dark conditions, ap-
2
proximately 44 mmol of 2-propanol was immediately adsorbed
naked TiO surface easily diffuses into the gas phase, thus re-
2
[48]
on TiO @HSS within 30 min (Figure 4a), whereas only a slight
tarding the subsequent mineralization to CO2. In contrast,
2
amount of 2-propanol was adsorbed on the naked TiO (Fig-
the hollow silica shell of TiO @HSS prevents the diffusion of
2
2
ure 4b), demonstrating the superior adsorption property of
acetone by acting as a physical compartment between the
inner void space and the outer gas phase. The acetone mole-
cules enriched inside the void/pore space can gain easy access
to the adjacent TiO NPs core to be mineralized into CO and
TiO @HSS.
2
In the photocatalytic degradation of 2-propanol, acetone
molecules are known to be generated as the main intermedi-
ate, which are subsequently mineralized into CO and H O, as
2
2
H O, thus leading to an efficient catalytic efficiency.
2
2
2
given by the following equations:
A comparison of the reaction kinetics of TiO @HSS and
2
a physical mixture of TiO and HSS (TiO +HSS) reveals the im-
2
2
CH CHðOHÞCH þ 1=2 O ! CH COCH þ H O
ð1Þ
ð2Þ
portance of the configuration of these two components; al-
though the physical mixture showed the highest adsorption
3
3
2
3
3
2
CH COCH þ 4 O ! 3 CO þ 3 H O
performance for 2-propanol (Figure 4c), the CO
2
formation
rate (0.109 mmolmin ) under UV-light irradiation was appa-
3
3
2
2
2
ꢀ
1
ꢀ
1
Indeed, after UV-light irradiation, the concentration of
-propanol rapidly decreased together with a steep increase in
rently slower than those of TiO @HSS (0.315 mmolmin ) and
2
ꢀ1
2
naked TiO (0.232 mmolmin ). This is due to the discrete loca-
2
the acetone concentration and a modest increase in the CO2
concentration as well as the formation of trace amounts of
acetaldehyde, as a result of photocatalytic oxidation by TiO2.
After the 2-propanol concentration reached nearly zero, the
tion of the adsorption sites and catalytic active sites, that is,
the 2-propanol molecules once trapped inside the vacant HSS
have difficulty in diffusing to the TiO NPs located outside the
2
HSS. Based on the above results, it is evident that spatial
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proximity of TiO and the void space of HSS arising from the
2
yolk–shell architecture is essentially important for promoting
the photocatalytic degradation of small organic reactants.
Moreover, no appreciable changes in the morphology and
textural properties were observed for TiO @HSS even after the
2
photocatalytic use under prolonged UV-light irradiation (for
TEM, FE-SEM images, and XRD data, see Figure S6 in the Sup-
porting Information) and it was reusable without any loss of
activity (see Figure S7 in the Supporting Information), verifying
the catalyst stability and durability against UV-light irradiation.
Effect of amount of core material
The content of TiO as the core material in the TiO @SiO com-
2
2
2
posites often has a significant impact on the morphology and
photocatalytic catalytic activity. To determine the optimum
amount of the TiO core, TiO @HSS with different TiO contents
2
2
2
(5, 10, and 20 wt% in the final solids) were synthesized by
varying the added amount of TiO during the synthesis. The
2
addition of excess of TiO (>40 wt% in the final solid) did not
2
afford well-defined yolk–shell nanostructured materials owing
to the deformation of the microemulsion caused by an in-
creased solid(TiO )-to-liquid(oil/water) ratio. FE-SEM and TEM
2
images clearly show that TiO @HSS retains the yolk–shell mor-
2
phology in the TiO content range 5 to 20 wt% (Figure 5). No
2
noticeable difference was observed with the encapsulated TiO2
NPs in terms of crystal size, crystal phase, and distribution. As
summarized in Table 2, the void volume (Vvoid) monotonically
3
ꢀ1
decreased from 0.69 to 0.41 cm g and accordingly the BET
2
ꢀ1
surface area (SBET) decreased from 364 to 317 m g as the TiO2
2
Figure 5. (Left) FE-SEM image and (right) TEM image of TiO @HSS with
content increased from 0 to 20 wt%, reflecting the fact that
varied TiO
solids).
2
contents; (a,d) 5 wt%, (b,e) 10 wt%, and (c,f) 20 wt% (in the final
a larger amount of TiO NPs was accommodated in the hollow
2
silica spheres without any deterioration of the yolk–shell nano-
structures. These combined analyses indicate that increasing
ꢀ
3
ꢀ1
the amount of TiO core content up to 20 wt% has little effect
TiO (10)@HSS (8.6ꢁ10 min ). However, these values cannot
2
2
on the yolk–shell morphology but leads to a substantial reduc-
tion in the interior space volume.
properly reflect the net photocatalytic activities, as the dosage
of catalyst in each experiment differs depending on each cata-
lyst and the effect of “adsorption (by silica)” cannot be exclud-
ed. Assuming the Langmuir–Hinshelwood mechanism, the for-
The series of TiO (x)@HSS photocatalysts (x represents the
2
TiO content in the final solid) were tested in the photocatalyt-
2
ic degradation of both 2-propanol and acetaldehyde in the
same manner as demonstrated above. In the photocatalytic
degradation of 2-propanol in air, a higher degradation rate of
mation rate of CO
photocatalytic activity. As listed in Table 2, TiO
talysts with 10–20 wt% TiO content provided slightly faster
CO formation rates (0.315 mmolmin ) than TiO
(0.311 mmolmin ). A similar trend was also observed in the
2
, RCO2, must be a good indicator of the net
@HSS photoca-
2
2
ꢀ
1
2
-propanol, kpro
,
was observed with TiO (20)@HSS (9.9ꢁ
2
(5)@HSS
2
2
ꢀ
3
ꢀ1
ꢀ3
ꢀ1
ꢀ1
10
min ) compared with TiO (5)@HSS (5.0ꢁ10 min ) and
2
2
Table 2. Effect of TiO content on the textural properties and rate constants for the photocatalytic reactions.
Sample
TiO
2
content
N
2
physisorption
Photocatalytic degradation of
2-Propanol in air Acetaldehyde in air
[
wt%]
[
a]
[b]
[c]
[d]
ave
S
[
BET
V
void
V
total
D
k
pro
k
act
R
CO2
R
CO2
2
ꢀ1
3
ꢀ1
3
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
m g
]
[cm g
]
[cm g
]
[nm]
[min
]
[min
]
[mmolmin
]
[mmolmin ]
HSS
0
5
10
20
364
360
344
317
0.69
0.68
0.66
0.41
1.0
1.0
0.99
0.63
2.0
2.0
2.0
2.4
–
–
–
–
ꢀ
ꢀ
ꢀ
3
3
3
ꢀ3
ꢀ3
ꢀ3
TiO
TiO
TiO
2
2
2
(5)@HSS
(10)@HSS
(20)@HSS
5.0ꢁ10
8.6ꢁ10
9.9ꢁ10
3.2ꢁ10
3.1ꢁ10
3.2ꢁ10
0.311
0.315
0.315
0.117
0.135
0.137
[a] Calculated by the BET method. [b] Determined from the a
s
-plot by subtracting the micropore and mesopore volumes from Vtotal. Including intra- and
inter-particle spaces. [c] Total pore volume reported at p/p =0.99. [d] Average pore diameter determined by the BJH method.
0
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photocatalytic degradation of acetaldehyde in air, where
Conclusions
TiO (10)@HSS and TiO (20)@HSS afforded RCO2 =0.135–
2
2
ꢀ
1
0
.137 mmolmin , which was approximately 1.2 times faster
Yolk–shell-type photocatalysts consisting of spherical hollow
ꢀ
1
than that of TiO (5)@HSS (0.117 mmolmin ). The subpar photo-
silica as the shell and TiO NPs as the core were fabricated
2
2
catalytic activity with TiO (5)@HSS is due to the decreased
through a facile method utilizing O/W microemulsions as
2
number of TiO NPs encapsulated within HSS, which results in
a template. Encapsulation of the TiO NPs inside the hollow
2
2
reduced accessibility of the adsorbate molecules to the TiO2
silica spheres afforded efficient photocatalysts with both im-
proved adsorption properties and catalytic efficiency, which
active sites. These results suggest that the optimum TiO con-
2
tent is around 10–20 wt%, but the variation in TiO content
markedly outperformed those of naked TiO owing to the abili-
2
2
has no drastic impact on the photocatalytic efficiency of TiO2.
ty of the porous silica shell to adsorb/enrich small organic re-
actants inside the void space and efficient transportation to
the adjacent active TiO core. Comparative studies demonstrat-
2
Sieving properties of the silica shell
ed that the optimum TiO content in the TiO @HSS photocata-
2
2
The porous silica shell is expected to act as a physical barrier
lyst was around 10–20 wt%, and the porous silica shell endow-
ed this material with a molecular-sieving property, ensuring its
potential application in combination with organic support ma-
terials, such as organic binders, polymer films, and organic
fibers. The advantages of the reported fabrication protocol in-
clude (i) use of inexpensive and abundant oleic acid as the sole
template, (ii) minimal steps to obtain the yolk–shell nanostruc-
ture, (iii) variation of material choices for the core. We expect
that this method would be easily generalized for the fabrica-
tion of various types of yolk–shell nanostructured catalysts en-
capsulating other active components.
to insulate the TiO NPs from the surrounding environment
2
and prevent the organic support from self-degradation under
UV-light irradiation. To verify such a function of the silica shell,
TiO @HSS was examined in the photocatalytic degradation of
2
conalbumin, a model organic compound with a large molecu-
3
[49,50]
lar size (75 kDa, 5.0ꢁ5.6ꢁ9.5 nm ),
in water. Figure 6
Experimental Section
Materials
Tetraethoxy orthosilicate (TEOS), 3-aminopropyl triethoxysilane
(APTES), oleic acid (99%), and methanol (>99.5%) were purchased
from Nacalai Tesque Inc. (Japan). All the chemicals were of analyti-
cal grade and used as received without further purification. TiO2
particles with anatase and rutile phases (anatase/rutile=7:3; P25)
were purchased from Evonik Co., Ltd. Conalbumin (from chicken
egg white, 75 kDa, 5.0ꢁ5.6ꢁ9.5 nm ) was purchased from Sigma–
Aldrich and used without any further purification. Deionized (DI)
water was used throughout the experiment.
Figure 6. Time course of CO
(
2
evolved from aqueous solutions of conalbumin
75 kDa, 5.0ꢁ5.6ꢁ9.5 nm ) over TiO @HSS (*), TiO (P25) (&), and a blank
experiment (ꢁ) under UV-light irradiation (l>300 nm, intensi-
3
3
2
2
ꢀ
2
ty=2.0 mWcm ) emitted from a 500 W Xe lamp.
shows the time course of CO evolution over TiO @HSS, naked
2
2
Synthesis of TiO @HSS
2
TiO (P25), and a blank experiment. The naked TiO evolved
2
2
a considerable amount of CO over time as a result of the pho-
A mixture containing commercial TiO2 powder (0.100 g, Evonik
P25), MeOH (20 mL), and oleic acid (2 mmol, 0.571 g) was ultrasoni-
cated for 5 min and stirred at 808C for 30 min to obtain a homoge-
neous suspension consisting of TiO2 nanoparticles uniformly dis-
persed in oleic acid. At this stage, the external surface of the TiO2
nanoparticles was decorated with oleic acid forming Ti–oleic acid
2
tocatalytic mineralization of conalbumin under UV-light irradia-
tion. On the other hand, CO evolution was hardly observed
2
over TiO @HSS during 10 h of UV-light irradiation, demonstrat-
2
ing a size-selectivity of the TiO @HSS photocatalyst toward or-
2
ganic reactants. This can be explained by the molecular-sieving
effect of the hollow silica shell, that is, the hollow silica shell
has an average pore diameter of 2.0 nm, which prevents the
permeation of huge reactant molecules into the inner space,
while allowing access of smaller reactant molecules (2-propa-
nol and acetaldehyde). This result again confirms the success-
[42]
moieties to give an oleophilic surface. After complete evapora-
tion of MeOH, DI water (57.6 mL) was added and subjected to
ultrasonication for 5 min to form a uniform O/W emulsion solution.
A mixture of TEOS (13.4 mmol, 2.94 g) and APTES (2 mmol, 0.447 g)
were added dropwise into this solution. The solution was vigorous-
ly stirred for approximately 5 min at room temperature, left to age
for 2 h at the same temperature under static conditions, and aged
for another 24 h at 808C to ensure the formation of the silica net-
work. The resulting product was washed with DI water and EtOH
several times, dried at 1008C, and then calcined for 6 h at 6508C in
ful encapsulation of TiO NPs inside the hollow silica spheres
2
and ensures the potential applications of TiO @HSS in combi-
2
nation with organic support materials.
air to give TiO @HSS as a white powder. The initial molar composi-
2
tion was 1OA/1APTES/6.7TEOS/1600H O and the TiO content was
2
2
adjusted to be 10 wt% (in the final solid) unless otherwise noted.
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Hollow silica spheres (HSS) without core material were synthesized
l>300 nm) emitted from a 200 W Hg/Xe lamp (SAN-EI Electric Co.,
Ltd., SUPERCURE-203S) from the upper part of the vessel. Quantifi-
cation of the concentrations of acetaldehyde and evolved CO2
were performed in the same manner as mentioned above. The
photocatalytic activity was evaluated by using the apparent rate
by a similar procedure except for the addition of TiO powder.
2
Characterization
constant (k ) calculated by assuming pseudo-first-order reaction
ald
Field-emission scanning electron microscope (FE-SEM) images were
obtained with a JEOL JSM-6500F. Transmission electron microscope
kinetics as well as the formation rate of CO (RCO2).
2
(
TEM) images were obtained with a Hitachi HF-2000 FE-TEM oper-
ated at 200 kV. The sample was suspended in EtOH using ultra-
sound, and then a droplet of the suspension was dried on
a carbon grid. The scanning transmission electron microscopy
Photocatalytic degradation of conalbumin in water
The experiments were conducted at room temperature by using
3
(
STEM) with elemental maps were acquired with a FEI TITAN80–
00 instrument operated at an accelerating voltage of 200 kV and
a cylindrical Pyrex reaction vessel (35 cm in capacity, 1.6 cm in di-
3
ameter) capped with a rubber septum. The photocatalyst (5 mg as
equipped with an EDAX TEM/SuperUTW energy-dispersive X-ray
detector. Powder X-ray diffraction (XRD) patterns were recorded by
using a Rigaku Ultima IV diffractometer with CuKa radiation
TiO ) and an aqueous solution containing conalbumin (20 mL,
2
ꢀ1
50 mgL ) were added to the reaction vessel. The solution was
magnetically stirred under dark conditions for 1.5 h to accomplish
adsorption equilibrium. After the adsorption, the solution was bub-
bled with oxygen for 30 min, and then UV-light irradiation (intensi-
(l=1.54056 ꢂ, 40 kV to 40 mA). Nitrogen adsorption–desorption
isotherms were measured at ꢀ1968C by using a BELSORP-max
ꢀ
2
system (MicrotracBEL Corp.). Samples were degassed at 3008C for
ty=2.0 mWcm , l>300 nm) from a 500 W Xe lamp (SAN-EI Elec-
tric Co., Ltd., SUPER BRIGHT 500, XEF-501S) was performed from
the side of the vessel. Stirring was continued during the photoca-
talytic reaction. At allotted time intervals, 200 mL of the reaction
gas was extracted by using a gas-tight syringe and the concentra-
3
h to vaporize the physisorbed water. The specific surface area
was calculated by the BET (Brunauer–Emmett–Teller) method by
using nitrogen adsorption data ranging from p/p =0.05 to 0.35.
0
The pore size distributions were obtained by the BJH (Barrett–
Joyner–Halenda) method. Thermogravimetric (TG) analysis was per-
formed on a RIGAKU Thermo plus EVO2 TG8121 system from room
tion of the evolved CO
mentioned above.
was quantified in the same manner as
2
ꢀ
1
temperature to 10008C at a heating rate of 108Cmin in an air
flow of 30 sccm by using a-Al O as a standard. Fourier transform
2
3
Acknowledgments
infrared spectroscopy (FTIR) was performed on a JASCO FT/IR-6100
ꢀ1
instrument in the spectral range 4000–400 cm under vacuum
ꢀ1
with a resolution of 4 cm .
This work was financially supported by a Grant-in-Aid from the
Frontier Research Base for Global Young Researchers and Division
of Photon Science and Technology, Osaka University. A part of
this work was also supported by a Grant-in-Aid for Scientific Re-
search (KAKENHI) from the Japan Society for the Promotion of
Science (JSPS) (no. 15K18270) and Iketani Science and Technology
Foundation. We acknowledge Dr. Eiji Taguchi at the Research
Center for Ultra-High Voltage Electron Microscopy, Osaka Univer-
sity, for his assistance with the TEM measurements.
Photocatalytic activity tests
Photocatalytic degradation of 2-propanol in air
The experiments were conducted at room temperature by using
3
a quartz photocatalytic reactor with an inner volume of 160 cm .
Approximately 100 mmol of 2-propanol was injected through
a rubber septum into the reaction vessel containing photocatalyst
(
5 mg as TiO ) placed on a Petri dish (2.7 cm in diameter). After 2 h
2
in the dark to achieve adsorption equilibrium, the reaction was ini-
Keywords: adsorption
nanostructures · photocatalysis
·
mesoporous
materials
·
tiated by irradiating the sample with UV light (intensity=
ꢀ2
2
0 mWcm , l>300 nm) emitted from a 200 W Hg/Xe lamp (SAN-
EI Electric Co., Ltd., SUPERCURE-203S) from the upper part of the
vessel. The reaction gas (200 mL) was extracted at given intervals
by using a gas-tight syringe and the concentrations of 2-propanol
and the formed products were quantified by using a gas chroma-
tograph (Shimadzu, GC-14A) with both a flame ionization detector
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Received: April 28, 2016
Revised: June 13, 2016
Published online on && &&, 0000
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FULL PAPERS
Oil in water for yolk in shell: A novel
and facile method to fabricate yolk–
shell nanostructured photocatalysts
Y. Kuwahara, Y. Sumida, K. Fujiwara,
H. Yamashita*
&& – &&
consisting of TiO nanoparticles as the
2
core and spherical hollow silica as the
shell was developed. The yolk–shell
nanostructures act as an efficient photo-
catalyst with both improved adsorption
properties and molecular-sieving prop-
erties, which far outperformed those of
naked TiO2.
Facile Synthesis of Yolk–Shell
Nanostructured Photocatalyst with
Improved Adsorption Properties and
Molecular-Sieving Properties
ChemCatChem 2016, 8, 1 – 9
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&
These are not the final page numbers! ÞÞ