Controlling Photocatalytic Activity and Size Selectivity of TiO2

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Article

Controlling photocatalytic activity and size selectivity of TiO encapsulated in hollow silica spheres by tuning silica shell structures using sacrificial biomolecules 2

Kensei Fujiwara, Yasutaka Kuwahara, Yuki Sumida, and Hiromi Yamashita Langmuir, Just Accepted Manuscript • Publication Date (Web): 07 Jun 2017 Downloaded from http://pubs.acs.org on June 8, 2017

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Controlling photocatalytic activity and size selectivity of TiO2 encapsulated in hollow silica spheres by tuning silica shell structures using sacrificial biomolecules Kensei Fujiwara, †,‡ Yasutaka Kuwahara, †,§ Yuki Sumida, † Hiromi Yamashita*,†,§

†

Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan ‡

Paper Technology Center, Ehime Institute of Industrial Technology, 127 Mendori-cho, Shikokuchuo, Ehime 799-0113, Japan

§

Unit of Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520, Japan

ACS Paragon Plus Environment

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KEYWORDS: TiO2 photocatalyst, photocatalytic degradation, nanostructured catalyst, yolk‒ shell structure, protein

ABSTRACT:

Yolk‒shell nanostructured photocatalyst which consists of inner core photocatalytic particles and outer silica shell exhibits high photocatalytic efficiency and molecular size selectivity due to the molecular sieving property of the outer shell. Creation of extended porosity in the shell endows it with improved adsorption properties and size selectivity toward targeted reactants. In this study, yolk‒shell nanostructured photocatalyst consisting of TiO2 NPs core and porous silica shell with controllable pore size was fabricated through a facile single-step dual-templating approach utilizing oil-in-water (O/W) microemulsions and amphiphilic protein molecules. Addition of optimum amount of protein (ovalbumin) as a sacrificial template together with O/W microemulsion during the synthesis led to the expansion of average pore size from 2.0 to 3.6 nm, while retaining TiO2-encapsulated yolk‒shell nanostructures. Photocatalytic degradation tests using gaseous 2-propanol and huge proteins as model substrates clearly revealed that the obtained material (TiO2@HSS_pro) showed superior photocatalytic performances with both improved photocatalytic efficiency and molecular size selectivity due to the increased surface area and expanded pore diameter.

INTRODUCTION Photocatalysts are potent materials capable of removing the organic pollutants under ambient conditions, among which TiO2 has been regarded as one of the most attractive semiconductor photocatalysts due to its low-cost, non-toxicity, chemical stability, and high photocatalytic activity, especially effective for degradation of volatile organic compounds (VOCs).1-11 As an


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effective approach to exploit the TiO2 photocatalytic property, as well as to prohibit the selfdegradation of organic substrates under ultraviolet (UV) light irradiation, TiO2 photocatalysts are typically covered by inorganic adsorbents, such as carbon,12,13 hydroxyapatite,14,15 silica,16,17 or porous silica.18,19 Among those, porous silica materials have especially been utilized to enhance the photocatalytic activity due to its porous structures ordered in nanoscale and the associated large surface area. To date, a number of synthetic approaches for fabricating TiO2-porous silica composites with tunable nanoarchitecture, controlled composition, and multifunctionalities have extensively been explored.20-25 However, most of them cause a serious problem of decreasing the inherent photocatalytic activity of TiO2 because of covering the active sites of TiO2 surface and insufficient crystallinity of TiO2 within the confined nanospaces.26 Recently, yolk‒shell type nanostructured photocatalyst, consisting of as-crystallized TiO2 as an inner core material and hollow silica as an outer porous shell, have emerged as a new class of photocatalyst to overcome this problem, in which the encapsulated TiO2 nanoparticles (NPs) can keep their surface “free” to retain their inherent photocatalytic activity owing to the void space between the core and the shell regions.27-29 In our previous study, we developed a facile method to fabricate hollow silica spheres encapsulating as-crystallized TiO2 NPs (TiO2@HSS) by utilizing oil-in-water (O/W) microemulsion as an organic template, which showed a higher catalytic efficiency than naked TiO2 in the gas-phase photocatalytic degradation of 2-propanol/acetaldehyde under UV light irradiation owing to the ability to adsorb and condense small organic substrates inside the hollow cavity spaces.30 Furthermore, the outer shell was proven to show molecular sieving property, which allowed them to prevent the damage of TiO2-applied surface (organic fibers) even under long-term UV light irradiation.31


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Controlling pore size and arrangement is another important aspect of porous materials, since it provides drastic impacts on their adsorption properties and molecular selectivity.32-34 A number of approaches to control the pore size of yolk‒shell nanostructured materials have extensively been explored. For example, Fang et al. fabricated yolk‒shell nanostructures having radiallyaligned porous shells by employing layer-by-layer technique, in which the core was first protected by a sacrificing intermediate silica layer, followed by alkaline etching process in alkaline media in the presence of cetyltrimethylammonium bromide (CTAB) as a pore-directing agent.35,36 Ikeda et al. fabricated yolk-shell type TiO2@SiO2 photocatalyst particles by coating of TiO2 crystals with an intermediate carbon layer and an organosilica layer, followed by heating treatment to create hollow cavity spaces and meso-size pores in the shell region.37 Apart from these, various materials have been utilized as a sacrificing intermediate layer, such as polymer38 and carbon.39 As demonstrated in these studies, well-ordered porous shell can be formed by utilizing typical alkyl ammonium salts (ex. CTAB) or organosilanes bearing long alkyl chains (ex. octadecyltrimethoxysilane) as pore-directing agents; however, these methods require complicated multiple steps and precise control of synthetic conditions to fabricate the pore sizecontrolled yolk‒shell nanostructures, and expansion of pore diameter larger than 2 nm is quite difficult because of the limited diameter of CTAB micelles. In this study, we developed a facile method to fabricate yolk‒shell type nanostructured photocatalyst, TiO2 encapsulated in hollow silica spheres (TiO2@HSS), with expanded pores in the shell region by employing dual-templating approach utilizing O/W microemulsion and amphiphilic protein molecule as organic templates (Scheme 1). It has previously been found that proteins (less than 10 nm) added in O/W microemulsion system are self-assembled at the oilwater interface and are incorporated within the silica matrix of the outer silica shell, not in the oil


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phase, during the formation of silica network owing to their amphiphilic nature.40,41 Subsequent removal of organic components (the inner oil phase and the incorporated proteins) by calcination in air results in the formation of hollow cavity space and expanded porosity in the silica shell, respectively, thus enabling the fabrication of hierarchical porous silica nanostructure by a single step. The use proteins as sacrificial templates can create meso-size pores in the shell which are larger than those derived from the conventional alkyl ammonium surfactants. From the viewpoint of catalysis, the extended porosity can allow efficient diffusion of reactant molecules and can allow the access of larger reactant molecules to the inner active sites, leading to an efficient catalytic reaction; therefore, it is expected that the present synthetic protocol enables the fabrication of yolk‒shell nanostructured TiO2 photocatalyst with improved photocatalytic performances and controllable size selectivity toward targeted reactants. Effects of protein addition during the synthesis on structures were investigated by means of FE-SEM, TEM, and N2 physisorption measurements. The photocatalytic activity and molecular size selectivity of the synthesized yolk‒shell nanostructured photocatalysts were investigated in the photocatalytic degradation of gaseous 2-propanol and huge protein molecules (conalbumin/lysozyme) under UV light irradiation. The results demonstrated that the addition of optimum amount of protein (ovalbumin) during the O/W microemulsion-templated synthesis could increase the pore diameter, while retaining TiO2-encapsulated yolk‒shell nanostructures, which led to superior photocatalytic performances with both improved photocatalytic efficiency and molecular size selectivity.


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Scheme 1. Schematic representation of the synthetic procedure of TiO2@HSS and TiO2@HSS_pro.

EXPERIMENTAL SECTION Materials. Tetraethoxy orthosilicate (TEOS, 95%), 3-aminopropyl triethoxysilane (APTES), oleic acid (OA, 99%), ovalbumin (OVA; 43kDa, 4.0 × 5.0 × 7.0 nm3), and methanol (>99.5%) were purchased from Nacalai Tesque Inc. TiO2 particles with anatase and rutile phases (anatase : rutile = 7 : 3) (P25®, nominal particle size = 21 nm) was purchased from Evonik Co., Ltd. Lysozyme (14 kDa, 1.9 × 2.5 × 4.3 nm3) and conalbumin (from chicken egg white, 75 kDa, 5.0 × 5.6 × 9.5 nm3) were purchased from Sigma‒Aldrich. All the chemicals were used as received without further purification. Deionized (DI) water was used throughout the experiment. Synthesis of TiO2@HSS with tunable pore size. The synthesis of TiO2@HSS was carried out according to previously described methods.30 The OA was dissolved in methanol at a concentration of 0.2 M. 0.100 g of commercial TiO2 powder (Evonik P25) was dispersed in 20


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mL of this solution at room temperature with ultrasonication for 5 min, followed by stirring at 80 °C until methanol was completely evaporated to homogenize TiO2 and OA. At this stage, the surface of the TiO2 NPs is decorated with OA by forming Ti–oleic acid moieties to give an oleophilic surface, allowing them to be uniformly dispersed in OA (for the photographs of reaction solutions, see Figure S1 in the Supporting Information). To this solution, 57.6 mL of DI water containing predetermined amount of ovalbumin (OVA) (0‒300 mg) was added, followed by ultrasonication for 5 min to form a uniform O/W microemulsion. Subsequently, the mixture of 13.4 mmol of TEOS and 2 mmol of APTES was added to the solution and vigorously stirred for 5 min in order to fabricate the silica shell around the O/W microemulsions, which was then aged for 2 h at room temperature under static conditions and left to age for another 24 h at 80 °C to form the rigid silica network. The obtained solid was washed with DI water and EtOH several times and calcined for 6 h at 650 °C in air to create the hollow structure and expanded mesoporosity by removing OA and protein molecules, respectively. The molar ratio of OA : APTES : TEOS : H2O was 1 : 1 : 6.7 : 1600 and the TiO2 content in the final solid was adjusted to be 10 wt%. The thus obtained samples were denoted as TiO2@HSS_pro(n), where n describes the amount of OVA added (n = 100, 200, 300 mg). Characterization. Field-emission scanning electron microscope (FE-SEM) images were obtained with a JEOL JSM-6500F. Transmission electron microscope (TEM) images were obtained with a Hitachi HF-2000 FE-TEM operated at 200 kV. The sample was suspended in EtOH using ultrasound, and then a droplet of the suspension was dried on a carbon grid. Thermogravimetric (TG) analysis was carried out on a BRUKER TG-DTA2010SA system from room temperature to 1000 °C at a heating rate of 10 °C min-1 in air flow. The particle size distribution was investigated at 25 °C utilizing LUMiSizer (LUM GmbH). Samples were


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dispersed in water at 0.1% w/w. Nitrogen adsorption–desorption isotherms were measured at – 196 °C using BELSORP-max system (MicrotracBEL Corp.). Samples were degassed at 300 °C for 3 h to vaporize the physisorbed water. Specific surface area was calculated by BET (Brunauer–Emmett–Teller) method using nitrogen adsorption data ranging from p/p0 = 0.05 to 0.35. X-ray diffraction (XRD) patterns were recorded using a Rigaku Ultima IV. Photocatalytic degradation of gaseous 2-propanol. The photocatalytic activity was evaluated by tracking the concentrations of 2-propanol and evolved CO2 under UV light irradiation in a closed batch system using a reactor vessel made of Pyrex glass with a volume of 160 mL. The photocatalysts (5 mg as TiO2) spread on the bottom of the Petri dish (2.7 cm in diameter) were placed in the reactor and UV light (Intensity = 20 mW cm-2, λ > 300 nm) was emitted from a 200 W Hg-Xe lamp (SANEI Electric Co., Ltd., SUPERCURE-203S). A total of 100 µmol of gaseous 2-propanol was injected through a septum into the reactor. After injection, the reactor was left for 2 h under dark condition to reach the adsorption equilibrium. Gaseous samples were regularly collected by using gas-tight syringe and the decrease in 2-propanol concentrations and the amount of gaseous CO2 evolved was quantified by using a gas chromatograph (Shimadzu GC-14A) with both a flame ionization detector (FID) equipped with a Porapak Q column and a thermal conductivity detector (TCD). The photocatalytic activity toward 2-propanol degradation and the formation rate of CO2 (described as kpro and RCO2, respectively) were calculated by assuming pseudo-first-order reaction kinetics. Photocatalytic degradation of conalbumin and lysozyme. The photocatalytic experiments were carried out in a cylindrical Pyrex reaction vessel with a volume at 35 mL sealed with a rubber septum at room temperature. The photocatalyst samples (5 mg as TiO2) were dispersed in 20 mL of aqueous solutions containing 1.0 mg of conalbumin or lysozyme as model reactants. In


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order to equilibrate adsorption state, the solution was stirred for 1.5 h under dark conditions and then oxygen was flowed through for 0.5 h. Thereafter, UV light (intensity = 2.0 mW cm-2, λ >300 nm) of the 500 W Xe lamp (SAN-EI Electric Co., Ltd., SUPER BRIGHT 500, XEF-501S) was irradiated to the vessel with magnetic stirring. The evolved CO2 was quantified by analyzing 200 μL of the reaction gas by a gas chromatograph (Shimadzu GC-14A) with a thermal conductivity detector (TCD).

RESULTS AND DISCUSSION Catalyst characterization. The morphological effect on the structure of TiO2@HSS by the addition of OVA (ovalbumin) was observed by FE-SEM and TEM analyses. Figure 1 shows the FE-SEM images (left panels) and TEM images (right panels) of the series of TiO2@HSS synthesized with varied amounts of OVA (0‒300 mg). FE-SEM image shows that TiO2@HSS particles are monodispersed spherical silica particles and a number of cracks are seen on the surface of silica sphere (Figure 1(a)). Figure 1(b), (c), and (d) clearly visualize that the number of cracks increased by the addition of larger amount of OVA. These cracks are attributed to the void spaces created by the removal of OVA by calcination in air. This can be corroborated by the weight losses during the calcination process determined by TG analyses, which showed that the weight loss increased as the amount of addition of OVA increased (Figure S2 in the Supporting Information). The average particle diameter (dave) was determined to be 118 nm (TiO2@HSS_pro(100)), 122 nm (TiO2@HSS_pro(200)), and 123 nm (TiO2@HSS_pro(300)) by analytical photocentrifuge, which was slightly smaller than that of TiO2@HSS (dave = 155 nm) but was scarcely changed upon the variation of OVA amounts added (Figure S3 in the Supporting Information). The morphology of TiO2@HSS_pro(100 and 200) were spherical,


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which is almost the same as that of TiO2@HSS (Figure 1(b) and (c)), whereas TiO2@HSS_pro(300) showed irregularly-shaped non-spherical particles (Figure 1(d)). This morphological deformation is due to the inhibition of rigid silica shell formation by the addition of excessive amount of protein; in the O/W microemulsion-templated synthesis, the carboxyl groups of oleic acid as an organic template and the amino groups of APTES as a silica source interact with each other to assemble the silica shell. When excess amount of OVA (over 300 mg) is added, OVA molecules are self-assembled at the O/W interface, occupy the space to interact with APTES, and thus hinder the silica shell formation, thereby leading to the formation of irregularly-shaped non-spherical particles. TEM image shows that TiO2@HSS were composed of the silica shell, TiO2 particles encapsulated within this silica shell and the ample void spaces between the silica shell and the TiO2 particles (Figure 1(e)). The TiO2 particles were not evenly distributed in the whole hollow silica particles, but TiO2 particles were hardly observed outside the silica shell. TiO2@HSS_pro(100 and 200) also showed TiO2 particles fully encapsulated inside the silica shell (Figure 1(f) and (g)), however some TiO2 particles existing outside the silica shell was observed for TiO2@HSS_pro(300) (Figure 1(h)). Furthermore, the thickness of the silica shell layer decreased along with the addition of increased amount of OVA. The shell thickness of TiO2@HSS was estimated to be 19–21 nm, whereas that of TiO2@HSS_pro(300) was estimated to be 13–16 nm. Nevertheless, TiO2 particles contained in the series of samples retained their initial crystallinity in all cases (Figure S4 in the Supporting Information).


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Figure 1. (left) FE-SEM images and (right) TEM images of (a,e) TiO2@HSS and TiO2@HSS_pro(n) (n = (b,f) 100, (c,g) 200, and (d,h) 300). The pore characteristics of the samples were investigated by N2 physisorption measurement. N2 adsorption–desorption isotherms of TiO2@HSS_pro(100, 200, and 300) showed gradual increases at low pressure region and large hysteresis loops closing at p/p0 = 0.5 in all cases (Figure 2(A)). These are typical features of yolk‒shell nanostructure which has hollow cavities surrounded by micropore/mesopore systems.42 Furthermore, the existence of mesoporosity in the


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silica shell was confirmed by low-angle XRD patterns (Figure S4(A) in the Supporting Information). These results prove that the TiO2@HSS_pro(100, 200, and 300) have the yolk‒ shell nanostructure similar to TiO2@HSS prepared without addition of protein. The BJH pore size distribution curve obtained from N2 isotherms clearly showed an expansion of the pore diameter by increasing of the amount of OVA added (Figure 2(B)). As summarized in Table 1, the BET surface area (SBET) and average pore diameter (Dave) of protein-added samples are apparently larger than the those of pristine TiO2@HSS (SBET = 344 m2/g, Dave = 2.0 nm). The SBET and Dave of TiO2@HSS_pro(100) (SBET = 415 m2/g, Dave = 2.6 nm) and TiO2@HSS_pro(200) (SBET = 463 m2/g, Dave = 3.0 nm) increased according to the amount of protein added. TiO2@HSS_pro(300) provided the largest pore diameter (Dave = 3.6 nm) among the prepared samples. No appreciable distribution peaks attributable to inter-particle pores/voids were observed in the pore diameter range up to 70 nm. Based on the above results, it is evident that the yolk‒shell nanostructured photocatalysts with expanded porous structure were fabricated through a single-step dual-templating approach. In this process, the oleic acid and the OVA molecule are considered to act as sacrificial templates to create the hollow spaces and meso-size void spaces in the silica shell region, respectively, without interrupting the respective functionalities each other.


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Figure 2. (A) N2 adsorption-desorption isotherms and (B) the corresponding BJH pore size distribution curves of (a) TiO2@HSS, (b) TiO2@HSS_pro(100), (c) TiO2@HSS_pro(200), and (d) TiO2@HSS_pro(300).

Table 1. Textural properties and the rate constants for the photocatalytic reactions of a series of TiO2@HSS synthesized with varied amount of ovalbumin.

Sample

N2 physisorption SBET a Vtotal b (m2/g) (cm3/g)

Dave c (nm)

Degradation of 2-propanol kpro RCO2 (h-1) (µmol h-1)

Degradation of conalbumin RCO2 (µmol h-1)

Degradation of lysozyme RCO2 (µmol h-1)

TiO2@HSS

344

0.99

2.0

0.51

18.8

0.02

0.13

TiO2@HSS_pro(100)

415

1.3

2.6

0.78

24.9

0.16

0.09

TiO2@HSS_pro(200)

463

1.1

3.0

0.80

26.8

0.26

0.15

TiO2@HSS_pro(300)

447

1.3

3.6

0.43

13.7

-

-

a

b

c

Determined by the BET method. Total pore volume measured at p/p0 = 0.99. Average pore diameter determined by the BJH method.


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Photocatalytic activity tests. In the preliminary experiment, the photocatalytic activities of TiO2@HSS_pro(100, 200, and 300) were examined in the photocatalytic degradation of gas phase 2-propanol (100 μmol) in air under UV light irradiation. Figure 3(a) and (b) show the time course of 2-propanol concentration and CO2 evolution over the bare TiO2 and the series of TiO2@HSS_pro samples, respectively. The reaction vessel was kept for 2 h in the dark to achieve adsorption equilibrium prior to UV light irradiation. The adsorption of larger amounts of 2-propanol was observed for TiO2@HSS (44.9 µmol), TiO2@HSS_pro(100) (52.3 µmol), TiO2@HSS_pro(200) (42.5 µmol), and TiO2@HSS_pro(300) (65.0 µmol) compared to bare TiO2 (6.6 µmol) within 2 h under dark conditions owing to their hollow silica structures with large void volumes (Figure 3(a)). After UV light irradiation, the concentration of 2-propanol decreased with an increase in the CO2 concentration (Figure 3(a) and (b)). Figure 3(c) and (d) compare the apparent rate constants for 2-propanol degradation (kpro) and rates of CO2 evolution (RCO2), respectively, which are also summarized in Table 1. A 1.35 times faster CO2 evolution rate was observed by encapsulating TiO2 NPs within hollow silica spheres (cf. TiO2 and TiO2@HSS in Figure 3(d)), being consistent with the result we previously reported.30 TiO2@HSS_pro(100) and TiO2@HSS_pro(200) gave kpro values of 0.78 h-1 and 0.80 h-1, respectively, which were appreciably higher than that of the original TiO2@HSS (kpro = 0.51 h-1). The RCO2 showed the same trend, which increased in the order of TiO2@HSS (18.8 µmol h-1) < TiO2@HSS_pro(100) (24.9 µmol h-1) < TiO2@HSS_pro(200) (26.8 µmol h-1). This improved reaction rate is attributable to the extended porosity (surface area and pore volume) existing in the silica shell, which increases the number of reactants sorbable onto catalyst surface and improves the accessibility of reactants to the inner core TiO2 photocatalyst particles. However,


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TiO2@HSS_pro(300) showed a lower photocatalytic activity (kpro = 0.43 h-1 , RCO2 = 13.7 µmol h-1) than that of the original TiO2@HSS, which is likely due to the thinner silica shell and incomplete encapsulation of TiO2 within the hollow spaces of the silica shell. These results clearly demonstrate that the addition of optimum amount of OVA during the synthesis can improve the photocatalytic property, while the excess addition of OVA leads to the decreased photocatalytic efficiency due to the morphological deformation of yolk‒shell nanostructure. One may hypothesize that encapsulation of TiO2 NPs within hollow silica spheres would improve the light absorption property owing to the light scattering/reflection effect; however, no appreciable difference in light absorption was observed in the range below λ < 450 nm over the bulk TiO2 NPs and the series of TiO2@HSS samples (for UV-vis spectra, see Figure S5 in the Supporting Information), thereby we can disregard the effect of light absorption on photocatalytic activities.


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Figure 3. Time course of (a) 2-propanol concentration and (b) CO2 evolution in photocatalytic degradation of 2-propanol in air over bare TiO2 (cross), TiO2@HSS (open circle), TiO2@HSS_pro(100) (filled circle), TiO2@HSS_pro(200) (square), and TiO2@HSS_pro(300) (triangle) under UV light irradiation (λ > 300 nm, Intensity = 20 mW cm-2). Comparisons of (c) apparent rate constants for 2-propanol degradation (kpro) and (d) rates of CO2 evolution (RCO2).

Sieving properties of the silica shell. The porous shell of TiO2@HSS is expected to show different molecular size selectivity owing to the different pore sizes created in the silica shell. In order to verify the effect of pore size on the molecular sieving property, aqueous-phase photocatalytic degradation tests of large biomolecules (lysozyme (14 kDa, 1.9 × 2.5 × 4.3 nm3) and conalbumin (75 kDa, 5.0 × 5.6 × 9.5 nm3))43-45 as model reactants were tested over the series


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of TiO2@HSS samples. Figure 4 compares the rates of CO2 evolution (RCO2) over naked TiO2 (P25), TiO2@HSS, TiO2@HSS_pro(100), and TiO2@HSS_pro(200) observed under UV light irradiation. In the degradation of conalbumin (5.0 × 5.6 × 9.5 nm3) (Figure 4(A)), naked TiO2 showed a marked CO2 evolution (RCO2 = 0.44 μmol h-1) as a result of the photocatalytic degradation of conalbumin under UV-light irradiation. On the other hand, CO2 was hardly evolved over TiO2@HSS (RCO2 = 0.02 μmol h-1) owing to the molecular-sieving property of the hollow silica shell which inhibits the access of huge reactant molecule, conalbumin, of which size is larger than the pore size of the silica shell (Dave = 2.0 nm). Due to the expansion of porosity and average pore diameter, TiO2@HSS_pro(100) and TiO2@HSS_pro(200) showed higher CO2 evolution rates of 0.16 and 0.26 μmol h-1, respectively, than TiO2@HSS, clearly demonstrating that extended porous structure allows the access of reactant molecules larger than 5 nm. On the other hand, in the degradation of lysozyme (1.9 × 2.5 × 4.3 nm3) (Figure 4(B)), a substantial amount of CO2 was evolved over TiO2@HSS (RCO2 = 0.13 μmol h-1) since the molecular size of the lysozyme was smaller than the average pore diameter of the silica shell (Dave = 2.0 nm). The observed RCO2 of TiO2@HSS_pro(100) (RCO2 = 0.09 μmol h-1) and TiO2@HSS_pro(200) (RCO2 = 0.15 μmol h-1) were almost similar to that of TiO2@HSS (RCO2 = 0.13 μmol h-1), thus, the contribution of pore expansion seems limited towards the reactant molecules smaller than 5 nm. These results demonstrate that the size selectivity of yolk‒shell nanostructured photocatalyst can be controlled by the addition of optimum amount of protein during the preparation, and the expanded pore is especially effective for the access of reactant molecules larger than 5 nm (Scheme 2).46-47


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Figure 4. Comparison of CO2 evolution rates observed in photocatalytic degradation of (A) conalbumin (75 kDa, 5.0×5.6×9.5 nm3) and (B) lysozyme (14 kDa, 1.9 × 2.5 × 4.3 nm3) under UV-light irradiation (λ > 300 nm, Intensity = 2.0 mW/cm2) over bare TiO2, TiO2@HSS, TiO2@HSS_pro(100), and TiO2@HSS_pro(200).

Scheme 2. Schematic illustrations representing photodegradation of conalbumin and lysozyme molecules over TiO2@HSS (left) and TiO2@HSS_pro (right).

CONCLUSION In this study, we fabricated hollow silica spheres encapsulating TiO2 nanoparticles (TiO2@HSS) with expanded meso-size pores in the silica shell. The pore size controlled yolk‒


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shell nanostructure was synthesized by facile single-step dual-templating approach utilizing O/W microemulsion and protein (ovalbumin) as organic templates to expand the pore diameter and to increase the surface area. The pore diameter of the silica shell was expanded from 2.0 nm to 3.6 nm by the addition of protein while maintaining the yolk‒shell nanostructure, which endowed the encapsulated TiO2 NPs with improved photocatalytic activity and the different molecular size selectivity. Photocatalytic tests in 2-propanol degradation demonstrated that the addition of optimum amount of protein as a sacrificial template improves photocatalytic property of yolk‒ shell structured photocatalyst owing to the extended porosity of the silica shell, whereas addition of excessive amount of protein leads to the deformation of the yolk‒shell nanostructure and decreased photocatalytic efficiency. Photocatalytic tests in the degradation of large protein molecules demonstrated that the silica shell has an ability to discriminate the size of reactant molecules accessible to the core TiO2 photocatalyst. This study can offer a facile and effective synthetic method to fabricate the yolk‒shell nanostructured materials with controllable pore sizes suitable for targeted molecules. We expect that such pore size-controlled yolk‒shell nanostructured materials encapsulating functional cores can be used for various applications such as drug delivery, removal of pollutants and molecular sieves involving large organic molecules.

ACKNOWLEGDEMENT 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. This work was also supported by Iketani Science and Technology Foundation. YK and HY thank the Grant-in-Aid for Scientific Research (KAKENHI) from the Japan Society for the Promotion of Science (JSPS) (no. 26220911, 15K18270). YK and HY thank MEXT program “Elements


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Strategy Initiative to Form Core Research Center”. We acknowledge Dr. Eiji Taguchi at the Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, for his assistance with the TEM measurements. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/xxxxxx. Photographs of reaction solutions during the synthesis, TG, particle size distribution, XRD, and UV-vis data of the material. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (H. Yamashita) Notes The authors declare no competing financial interest.

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Table of contents

The yolk‒shell nanostructured photocatalyst with expanded shell porosity and improved photocatalyic performances were fabricated by utilizing protein as a sacrificial organic template.


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Controlling Photocatalytic Activity and Size Selectivity of TiO2

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