Fabrication of Photocatalytic Paper Using TiO2 Nanoparticles

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Fabrication of photocatalytic paper using TiO nanoparticles confined in hollow silica capsules Kensei Fujiwara, Yasutaka Kuwahara, Yuki Sumida, and Hiromi Yamashita Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04003 • Publication Date (Web): 22 Dec 2016 Downloaded from http://pubs.acs.org on December 30, 2016

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Fabrication of photocatalytic paper using TiO2 nanoparticles confined in hollow silica capsules 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

KEYWORDS: TiO2 photocatalyst, photocatalytic paper, VOC removal, nanostructured catalyst, yolk-shell structure

ABSTRACT:

TiO2 nanoparticles (NPs) encapsulated in hollow silica spheres (TiO2@HSS) show a shieldingeffect that can insulate photocatalytically active TiO2 NPs from the surrounding environment, and thus prohibit the self-degradation of organic support materials under UV-light irradiation. In

ACS Paragon Plus Environment

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this study, photocatalytically-active papers were fabricated by combining TiO2@HSS and cellulose fibers, and their photocatalytic activities and durability under UV-light irradiation were examined. The yolk-shell nanostructured TiO2@HSS, which has an ample void space between inner TiO2 NPs and an outer silica shell, was synthesized by a facile single-step method utilizing an oil-in-water (O/W) microemulsion as an organic template. The thus prepared TiO2@HSS particles were deposited onto cellulose paper either by chemical adhesion process via an ionic bonding or by physical adhesion process using dual polymer system. The obtained paper containing TiO2@HSS particles with high air permeability exhibited a higher photocatalytic activity in the photocatalytic decomposition of volatile organic compounds (VOCs) than unsupported powdery TiO2@HSS particles due to the uniform dispersion on the paper with a reticular fiber network. In addition, the paper was hardly damaged under UV-light irradiation, whereas the paper containing naked TiO2 NPs showed a marked deterioration with a considerably decreased strength, owing to the ability of the silica shell to prevent the direct contact between TiO2 and organic fibers. This study can offer a promising method to fabricate photocatalytically-active papers with a photo-resistance property available for real air cleaning.

INTRODUCTION Sick building syndrome caused by indoor air pollution has recently become a serious problem. Volatile organic compounds (VOCs) are well-known indoor pollutants and are emitted from furnishing, coating, and building materials. Since the VOCs include a variety of chemicals, some of which have short- and long-term adverse effects on human bodies, development of materials/technologies capable of reducing indoor VOCs concentrations has been required. Photocatalysts are promising materials that can remove these organic pollutants because of its ability to invoke oxidation reaction by utilizing light as a sole energy source, among which TiO2


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has been regarded as one of the most attractive semiconductor photocatalysts due to its nontoxicity, low-cost, chemical stability, and high oxidation potential sufficient for VOCs degradation.1-11 For the practical use of TiO2, it is often supported on various support materials such

as

oxide

adsorbent,12,13

glass,14,15 film,16,17,18

and

paper18-20

to

materialize

photocatallytically-active composites. In particular, composite paper materials composed of a paper substrate and TiO2 photocatalysts are very useful for indoor use, since the sizes, shapes, and colors are easily tailorable according to the locations they are used, with retaining their inherent photocatalytic functionalities. Additionally, it has been reported that the paper can serve as a superior support material for gas phase catalytic reaction owing to the pore structure created by a reticular fiber network. 1 However, organic substrates are in general easily damaged under UV-light irradiation when TiO2 is directly deposited onto the substrate surface due to the strong oxidation ability of TiO2, resulting in a significant deterioration of substrate’s quality and strength. To prevent the self-degradation of TiO2-applied surface, TiO2 particles are typically covered by inert materials, such as carbon,21,22 hydroxyapatite,23,24 silica,25,26 or porous silica.27,28 Nevertheless, this method encounters a crucial problem of decreasing the inherent photocatalytic activity by covering the active sites of TiO2.29 Recently, a number of synthetic approaches to fabricate porous materials with tunable nanoarchitecture, controlled composition, and multifunctionalities have been explored.30-32 Among those, yolk-shell type nanostructured photocatalyst can be a viable option to solve the abovementioned problem. So-called “yolk-shell nanostructure” is an emerging class of materials composed of an inner core material and an outer porous shell, where a void space exists between the core and shell regions. The outer shell layer works as a molecular sieve to improve adsorption property and as a physical barrier to prevent the core material from external


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environments. Furthermore, the core materials encapsulated within the shell can keep their surface “free”, thereby allowing them to retain their inherent functionalities. In our previous research, we developed a facile method to prepare the TiO2 nanoparticles encapsulated in hollow silica spheres (TiO2@HSS) by utilizing oil-in-water (O/W) microemulsion as an organic template.33-35 It has been demonstrated that the TiO2@HSS photocatalyst shows both improved adsorption properties and catalytic efficiency, which markedly outperforms those of naked TiO2 owing to the ability of the porous silica shell to adsorb/enrich small VOCs molecules inside the void space and efficient transportation to the adjacent active TiO2 core material.32 Furthermore, the porous silica shell endows this material with a molecular-sieving property, which can limit the access of huge molecules to the core TiO2, indicating the possibility for the combined use with organic support materials, such as organic binders, polymer films, and organic fibers. In addition, in contrast to the conventional approaches for the fabrication of yolk-shell nanostructured photocatalyst ever reported which require complex multiple steps and severe control of synthetic conditions,36,37 this fabrication protocol consists of minimal steps to obtain the yolk–shell nanostructure and utilizes inexpensive and abundant oleic acid as a sole organic template, which are promising for large-scale production and industrialization. In this study, yolk-shell nanostructured photocatalyst, TiO2@HSS, was combined with a cellulose fiber to fabricate composite papers with photocatalytic functionality and long-term durability even under UV-light irradiation. The fabrication of the composite papers was performed by two different processes, including chemical adhesion process via an ionic bonding (Scheme 1(a)) and physical adhesion process utilizing dual polymer system (Scheme 1(b)).38,39 The photocatalytic activity and durability under UV-light irradiation of the prepared papers were investigated. The results demonstrated that the prepared paper provides as a high photocatalytic


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activity as unsupported TiO2@HSS particles and exhibits a markedly improved durability under UV-light irradiation in comparison with the paper combined with bare TiO2.

Scheme 1. Schematic illustrations of (a) chemical adhesion process and (b) physical adhesion process to fabricate photocatalytic paper composites consisting of TiO2@HSS and cellulose fiber.

EXPERIMENTAL SECTION Materials. Tetraethoxy orthosilicate (TEOS, 95%), 3-aminopropyl triethoxysilane (APTES), oleic acid (99%), and methanol (>99.5%) were purchased from Nacalai Tesque Inc. Anionic polyacrylamide (A-PAM, V310) was purchased from Hymo, Co., Ltd. Polydiallyl dimethylammonium chloride (PDADMAC) was purchased from Aldrich, Ltd. 2,2,6,6tetramethylpiperidine-1-oxyl radical (TEMPO), sodium bromide (NaBr), 1.77 M sodium hypochlorite solution (NaClO), and toluene (>99.5%) were purchased from Wako Pure Chemicals Co., Ltd. All the chemicals were used as received without further purification. TiO2 particles with anatase and rutile phases (anatase : rutile = 7 : 3) (P25, nominal particle size = 21 nm) was purchased from Evonik Co., Ltd. A commercial softwood bleached kraft pulp (SBKP) purchased from MacKenzie Pulp Mill Co., Ltd was used as a native cellulose fiber.


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Synthesis of TiO2@HSS. The synthesis of TiO2@HSS was performed according to a previously reported method.35 Typically, 20 ml of MeOH including 2 mmol of oleic acid (0.571 g) and 0.232 g of commercial TiO2 powder (Evonik P25) was ultrasonicated for 5 min and stirred at 80 °C for 30 min to modify TiO2 surface with oleic acid, thus homogeneous dispersion of TiO2 NPs in oleic acid was achieved. 57.6 ml of DI water was added after MeOH was completely evaporated, and ultrasonication was applied for 5 min to obtain uniform O/W microemulsion. Then the mixture of 13.4 mmol of TEOS (2.94 g) and 2 mmol of APTES (0.447 g) was added dropwise, followed by stirring for approximately 5 min at room temperature with vigorous stirring, left to age for 2 h at the same temperature without perturbation and aged for another 24 h at 80 °C to form the silica shell around the oil droplets. The obtained solid was washed with deionized water and EtOH several times and calcined for 6 h at 650 °C in air to remove the organic components. The molar composition of the initial gel solution was 1 OA / 1 APTES / 6.7 TEOS / 1600 H2O, and the TiO2 content in the final solid was adjusted to be ca. 20 wt%. Amine modification of TiO2@HSS. Post-synthetic modification of amine groups on the outer surface of TiO2@HSS particles was conducted as a following procedure; to 100 mL of toluene containing 2 mmol of APTES (0.447 g), 1.0 g of dried TiO2@HSS particles were added and refluxed at 120 °C for 20 h.40,41 The resulting product was filtered, washed with EtOH several times, and dried at 100 °C to give amino-functionalized TiO2@HSS (NH2-TiO2@HSS). TEMPO-mediated oxidation of cellulose. TEMPO-mediated oxidation of cellulose fiber was carried out according to a previously reported method.42, 43 3 mmol of TEMPO (0.45 g) and 30 mmol of NaBr (3.0 g) were dissolved into 3 L of deionized water. 30 g of SBKP was dispersed into the solution with stirring at room temperature, and then 150 mmol of NaClO (75 mL) was


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added dropwise to the slurry. During the reaction, 5 mM NaOH aqueous solution was added continuously to maintain pH of 9.8. After 3 h of reaction when NaOH was no longer consumed, a small amount of EtOH was added to react with any residual NaClO in the slurry. The slurry was washed thoroughly with deionized water and the obtained fibrous product was washed with 1 M HCl aqueous solution to convert sodium carboxylates to free carboxyl groups, thus TEMPO-oxidized cellulose fiber (TOCF) was obtained. Preparation of photocatalytic paper. Preparation of photocatalytic paper composites were performed by two different processes, i) chemical adhesion process via an ionic bonding between –COOH groups of TEMPO-oxidized cellulose fibers and –NH2 groups tethered on NH2TiO2@HSS particles and ii) physical adhesion process using dual polymer system. In the former process, a predetermined amount of NH2-TiO2@HSS was added to an aqueous suspension containing 0.5 wt% of TOCF, followed by stirring for 30 s at room temperature. A photocatalytic paper was made from the suspension by casting using standard sheet-forming machine (KRK Co., Ltd., No.2558-II) equipped with 150 mesh wire, followed by drying in an oven at 105 °C overnight. The thus fabricated paper was named as TiO2@HSS/paper(c). In the latter process, a predetermined amount of TiO2@HSS or P25 (bare TiO2) was added to an aqueous suspension containing 0.5 wt% of raw cellulose fiber, SBKP. After stirring for 5 min, two different kinds of polymers, cationic PDADMAC and anionic A-PAM (both 0.2 wt% per total solids), were successively added to the suspension as flocculants, followed by stirring for 5 min. The corresponding photocatalytic papers were made from the suspension by the same procedure as mentioned above. The thus fabricated papers were named as TiO2@HSS/paper(p) and TiO2/paper(p), respectively. The pulp grammage of these papers were determined to be 100 g/m2. The TiO2 content in the papers was adjusted to be 5 wt% in all cases by controlling the dosage of


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TiO2@HSS particles, unless otherwise noted. For comparison, a pure cellulose fiber paper (CF paper) was prepared by a similar procedure except for the addition of TiO2@HSS and the flocculants. 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. Nitrogen adsorption–desorption isotherms were measured at –196 °C using BELSORP-max system (MicrotracBEL Corp.). Samples were degassed at 110 °C for 6 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. Thermogravimetric (TG) analysis was carried out on a BRUKER TG-DTA2010SA system from room temperature to 600 °C at a heating rate of 10 °C/min in air flow. Fourier transform infrared spectroscopy (FTIR) were performed on a Thermo Fisher Scientific Nicolet6700 with the spectral range of 4000–400 cm-1. X-ray diffraction (XRD) patterns were recorded using a Rigaku Ultima IV. Photocatalytic activity test. Photocatalytic experiments were carried out in quartz reactor (160 mL of total volume). The photocatalytic papers (containing 5 mg of TiO2) were placed on a circular glass dish (3.6 cm in diameter) and were then mounted inside the reactor. A predetermined volume of 2-propanol gas with the saturated vapor pressure (corresponding to 100 µmol) was introduced to the reactor containing the sample. Prior to UV-light irradiation, the adsorption of 2-propanol was equilibrated by keeping for 2 h in the dark. UV-light (Intensity = 20 mW/cm2, λ > 300 nm) was emitted from a 200 W Hg-Xe lamp (SANEI Electric Co., Ltd., SUPERCURE-203S) from the upper part of the reactor. 200 μL of the reaction gas was


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periodically extracted using a gas-tight syringe and the amount of 2-propanol and products was determined by a gas chromatograph (Shimazu GC-14A) with both a flame ionization detector (FID) equipped with a Porapak Q column. Apparent adsorption property was determined from the adsorbed amounts of 2-propanol during 2 h under dark condition, and apparent photocatalytic activity was determined from the amounts of 2-propanol decomposed after first 1 h of UV-light irradiation Photo-resistance test of photocatalytic paper. The photocatalytic papers were irradiated with UV-light (Intensity = 4 mW/cm2, λ > 300 nm) using weather meter (Suga Test Instruments Co., Ltd., N-25). After UV-light irradiation for 10 h, 30 h, and 50 h, the tensile strength of papers were measured by horizontal tensile tester (KRK Co., Ltd., No.2000-C) to assess the degree of durability. Furthermore, the papers before and after UV-light irradiation for 50 h were observed by FE-SEM to visually check the degree of degradation of the papers.

RESULTS AND DISCUSSION Catalyst characterization. The changes of the organic functionalities of TiO2@HSS and CF paper were monitored by FT-IR (Figure 1). The amino-functionalized TiO2@HSS (NH2TiO2@HSS) shows the IR band at 1558 cm-1 which is hardly observed in the IR spectrum of the TiO2@HSS. This IR band is ascribed to the bending vibration of –NH2 groups, indicating the presence of amine groups on the surface of the NH2-TiO2@HSS (cf. Figure 1(a) and (b)). The IR band seen at 1724 cm-1 on TOCF paper is ascribed to carboxyl groups, while the IR spectrum of the original CF paper does not show such a band in this region, confirming the generation of carboxyl groups upon the TEMPO-mediated oxidation (cf. Figure 1(c) and (d)). TiO2@HSS/paper(c), a composite paper comprised of NH2-TiO2@HSS and TOCF paper, show a


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drastic shift of the band of the amine groups to 1593 cm-1, indicating the formation of an ionic bond between the carboxyl groups of TOCF and the amine groups tethered on TiO2@HSS particles (Figure 1(e)).44 Owing to such a rigid ionic bond, the TiO2@HSS particles can tightly be immobilized on the CF paper without any flocculants.

Figure 1. FT-IR spectra of (a) TiO2@HSS, (b) NH2-TiO2@HSS, (c) CF paper, (d) TOCF paper, and (e) TiO2@HSS/paper(c).

FE-SEM image of TiO2@HSS particles shows monodispersed spherical silica particles with a particle size of about 100-200 nm (Figure 2(a)). TEM image shows that the TiO2@HSS consists of TiO2 particles with a particle size of ca. 20 nm which are successfully encapsulated within the hollow silica shell with the thickness of about 20 nm (Figure 2(b)). This was more clearly evidenced by scanning transmission electron microscope (STEM) images and energy dispersive spectroscopy (EDS) mapping images (Figure S1). An ample void space was observed between the TiO2 particles and the silica shell region. The TiO2 particles showed an uneven distribution throughout the hollow silica particles; however, TiO2 particles existing outside the silica shell were scarcely observed. Figure 2(c) shows the FE-SEM image of the original CF paper in which


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sheet-like cellulose fibers were thickly entangled with each other to form a reticular fiber network. Figure 2(d) and (e) show FE-SEM images of TiO2@HSS/paper(c) and TiO2@HSS/paper(p), respectively, visualizing that TiO2@HSS particles were successfully fixed onto the fiber network. The prepared TiO2@HSS/paper(c & p) were both sheet-like shaped (Figure 2(f)) and a detachment of TiO2@HSS particles from the paper by shaking, blowing or even by peeling-off using a Scitch tape was negligible, confirming a rigid fixation of the TiO2@HSS particles onto the paper.

Figure 2. (a) FE-SEM image of TiO2@HSS particles, (b) TEM image of TiO2@HSS particles, FE-SEM image of (c) original CF paper, (d) TiO2@HSS/paper(c), (e) TiO2@HSS/paper(p), and


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(f) a digital photograph of TiO2@HSS/paper(p).

The presence of TiO2@HSS particles in the composite papers was confirmed by N2 adsorption and TG measurements. The textural properties obtained from N2 adsorption measurement and TG are summarized in Table 1. The TiO2@HSS/paper(c) shows BET surface area (SBET) of 46 m2/g and pore volume (Vtotal) of 0.12 cm3/g, which are almost the same as those of TiO2@HSS/paper(p) (SBET = 47 m2/g, Vtotal = 0.11 cm3/g). These values are apparently higher than those of the original CF paper (SBET = 0.045 m2/g, Vtotal = n.d.) and the TiO2/paper (SBET = 0.48 m2/g, Vtotal = 0.024 cm3/g.). The increased surface area and pore volume are due to the extended porosity of TiO2@HSS particles (SBET = 317 m2/g, Vtotal = 0.63 cm3/g) arising from the yolk-shell nanostructure.32 The contents of TiO2 in the fabricated papers determined from the amounts of the residues obtained after TG measurements were 5.0 wt% in all cases (for TG curves, see Figure S2 in the Supporting Information (SI)). These results indicate that predetermined amount of TiO2@HSS particles were deposited onto CF paper with retaining their original yolk-shell nanostructure. Table 1. Textural properties and TiO2 content of the photocatalytic papers.

Sample

N2 physisorption a

TG b

c

SBET (m2/g)

Vtotal (cm3/g)

Residue content (wt%)

TiO2@HSS/paper(c)

46

0.12

29.85

5.01

3.18

TiO2@HSS/paper(p)

47

0.11

25.08

5.02

3.17

TiO2/paper

0.48

0.024

5.15

5.15

3.16

CF paper

0.045

n.d.

n.d.

n.d.

-

a

b

TiO2 content (wt%)

Band Gap d (eV)

c

Determined by the BET method. Pore volume measured at p/p0 = 0.99. Determined from the amounts of the residues obtained after TG measurements. d Determined from UV-vis spectra.

The existence of TiO2 in TiO2/paper and TiO2@HSS/paper composites was also proven by UV-vis spectra. As shown in Figure 3, TiO2/paper and TiO2@HSS/paper composites both


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exhibited absorptions in the range of λ < 400 nm, which are typical of bulk TiO2 as a semiconductor photocatalyst, suggesting that the composite papers retain the original photochemical property of TiO2. Since pure CF paper showed little UV absorption, the selfdegradation of CF paper under UV-light irradiation must be negligible and the photocatalytic activity can solely be attributed to the photocatalytic property of TiO2 encapsulated within hollow silica spheres. The insert in Figure 3 shows the plots of the modified Kubelka-Munk function [F(R∞)E]1/2 versus the energy of absorbed light (E). The band gap energy values of the TiO2@HSS, TiO2/paper, and TiO2@HSS/paper composites were estimated to be in the range of 3.16-3.18, which coincided well with that of the original TiO2.45 The existence of TiO2 was also confirmed by XRD measurement (Figure S3 in SI), which exhibited diffraction patterns assigned to anatase and rutile TiO2 phases arising from the encapsulated TiO2 particles (P25), as well as the amorphous nature of CF paper. (a) / a.u.

(b)

[F(R∞)E]

1/2

(c)

F(R∞) / a.u.

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(d)

3.17 eV

(b)

3.18 eV

(c)

3.16 eV

(d)

3.16 eV

2.0 2.5 3.0 3.5 4.0 4.5 5.0

(e)

250

(a)

E / eV

300

350

400

450

Wavelength / nm Figure 3. UV-vis spectra of (a) TiO2@HSS/paper(p), (b) TiO2@HSS/paper(c), (c) TiO2@HSS, (d) TiO2/paper, and (e) CF paper. The insert shows the plots of the modified Kubelka-Munk function [F(R∞)E]1/2 versus the energy of absorbed light (E).


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Photocatalytic activity tests. The photocatalytic activities of bare TiO2, TiO2@HSS, and the fabricated paper composites were evaluated by the photocatalytic degradation of 2-propanol (100 μmol) in air under UV-light irradiation. The mass of the TiO2 placed in the vessel was adjusted to 5 mg in each experiment. Figure 4 compares the reaction profiles of the bare TiO2, TiO2@HSS, and their paper composites. While TiO2 and TiO2/paper adsorbed only a slight amount of 2-propanol within 2 h under dark conditions (< 8 µmol), adsorption of larger amounts of 2-propanol was observed over TiO2@HSS (14.5 µmol), TiO2@HSS/paper(p) (23.1 µmol), and TiO2@HSS/paper(c) (25.2 µmol). This improved adsorption property toward 2-propanol is primarily due to the porous silica shell of TiO2@HSS (Figure 4b). A further improved adsorption capacity of TiO2@HSS/paper composites than that of unsupported TiO2@HSS is assumably due to the uniform dispersion of TiO2@HSS particles on layered fiber network, which allows the porous silica shells more accessible by 2-propanol molecules. Under UV-light irradiation, it was found that TiO2/paper exhibited a higher photocatalytic activity than the unsupported, bare TiO2 particles. This improved photocatalytic reaction rate might be due to a uniform dispersion of TiO2 particles on the reticular fiber network of the paper, which offers micro-order void spaces between the fibers as observed by FE-SEM images and the associated high air permeability.46,47 It is clear that the TiO2@HSS/paper(p) also exhibited a higher photocatalytic activity than unsupported TiO2@HSS particles, substantiating that the paper can serve as an effective support material to improve the photocatalytic activity in gas phase reaction. On the other hand, TiO2@HSS/paper(c) showed as a similar photocatalytic activity as unsupported TiO2@HSS particles with a slightly reduced initial reaction rate. TOCF is reported to form a strong linkage between the functional groups of each fiber, resulting in a formation of some fibrous


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aggregates.48 It is assumed that the reticular structure of the TOCF paper is deformed during the sample preparation due to the strong linkage between fibers, which causes a reduction of void space and permeability, and thus results in a modest photocatalytic efficiency.

Figure 4. The photocatalytic degradation of 2-propanol in air over (a) TiO2 (P25) and TiO2/paper and (b) TiO2@HSS and TiO2@HSS/paper(p & c) under UV-light irradiation (λ > 300 nm, Intensity = 20 mW/cm2).

Figure 5. Comparison of adsorption properties and photocatalytic activities in the 2-propanol degradation under UV-light irradiation (λ > 300 nm, Intensity = 20 mW/cm2) of CF paper, TiO2, TiO2/paper, TiO2@HSS, and TiO2@HSS/paper(p).


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The results of photocatalytic degradation of 2-propanol was summarized in Figure 5. Indeed, the cellulose fiber paper (CF paper) hardly showed the adsorption property and photocatalytic activity. Among the samples examined in this study, TiO2@HSS/paper(p) showed the highest adsorption property and photocatalytic activity, which outperformed those of the unsupported TiO2@HSS as a powder form. Considering the fact that there is no difference in TiO2 loading amounts and band gap energies, this enhanced catalytic performance is the consequence of the improved dispersion of TiO2@HSS particles onto the reticular fiber network of CF paper. The above experimental results suggested that an air permeability of the paper is playing an important role for improving the photocatalytic activity of the immobilized TiO2 photocatalysts. In order to draw the relationship between the photocatalytic activity and air permeability of the paper support, similar catalytic experiments were carried out using the TiO2@HSS/paper(p) samples with different paper grammage (200 g/m2 and 500 g/m2). The density of TiO2 particles in each papers were always fixed to be 5 g/m2 (confirmed by ICP and TG analyses). The air permeability was evaluated by Gurley densometer (KRK Co., Ltd., No.2060). Figure 6 shows the comparison of photocatalytic activities and air permeances of the TiO2@HSS/paper(p) with different grammages, together with those of TiO2@HSS/paper(c) as a comparison. The air permeability of TiO2@HSS/paper(p) increased with decreasing the grammage of the paper and the TiO2@HSS/paper(c) showed very low air permeance compared with TiO2@HSS/paper(p) composites (Figure 6(a)). These results demonstrated that the physical adhesion process is a more effective method than chemical adhesion process for fabricating photocatalytic paper composites, since the papers prepared via physical adhesion process retains the inherent permeability of the paper support, whereas the chemical adhesion process tends to decrease the permeability of the paper. The photocatalytic activity was accordingly increased with the


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increase of the air permeance and a distinct linear correlation was observed between these two factors (Figure 6(b)), arguably demonstrating that the air permeability is an important factor dominating the photocatalytic activity of the composite paper.

Figure 6. (a) Comparison of the photocatalytic activities in the photocatalytic degradation of 2propanol in air under UV-light irradiation (λ > 300 nm, Intensity = 20 mW/cm2) and air permeances of TiO2@HSS/paper(p) with different grammage (100, 200, and 500 g/m2) and TiO2@HSS/paper(c) as a comparison. (b) Correlation between photocatalytic degradation rates and air permeance values of the photocatalytic papers.

Durability test of papers. Figure 7 compares the relative tensile strength of the original CF paper and the prepared photocatalytic papers after the exposure to UV-light for the designated time. The strength of the original CF paper (without TiO2) was unchanged upon UV-light irradiation for 50 h. However, the strength of TiO2/paper containing naked TiO2 particles considerably decreased to ca. 50% of its initial strength after UV-light irradiation for 30 h due to the UV-light-induced degradation at the interface of the CF paper and the deposited naked TiO2 particles. Surprisingly, TiO2@HSS/paper(p) mostly retained initial strength even after UV-light


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irradiation for 50 h, despite containing the same amount of TiO2. Figure 8 shows FE-SEM images of TiO2/paper and TiO2@HSS/paper(p) before and after UV-light irradiation. After UVlight irradiation, the surface of TiO2/paper was obviously damaged and a number of wormholelike spots appeared which had never been observed before UV-light irradiation (Figure 8(a)). On the other hand, no apparent changes were observed on the surface of TiO2@HSS/paper(p) upon the UV-light irradiation for 50 h (Figure 8(b)). This markedly improved durability of the paper under UV-light irradiation is undoubtedly the consequence of the ability of the silica shell as a physical barrier to prevent the direct contact between TiO2 and the organic fibers.

Figure 7. Change of relative tensile strengths of CF paper, TiO2@HSS/paper(p), and bare TiO2/paper under UV-light irradiation (Intensity = 4 mW/cm2).


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Figure 8. FE-SEM images of (a) TiO2/paper and (b) TiO2@HSS/paper(p): (left) before UV-light irradiation and (right) after UV-light irradiation for 50 h.

CONCLUSION In this study, we fabricated photocatalytically-active paper which contains hollow silica spheres encapsulating TiO2 nanoparticles (TiO2@HSS). The TiO2@HSS was synthesized by single-step approach utilizing O/W microemulsion as an organic template. The composite photocatalytic paper consisting of the TiO2@HSS particles and cellulose fibers exhibited a high photocatalytic activity due to the reticular fiber network and high air permeability of the paper, demonstrating that the paper can serve as an effective support material without interrupting the adsorption and mass transfer of reactant molecules. Especially, the TiO2@HSS/paper fabricated via a physical adhesion process using dual polymer system showed a higher photocatalytic activity in the degradation of VOCs compared with sole TiO2@HSS particles since the inherent permeability of the paper can almost be retained. Additionally, the TiO2@HSS/paper was hardly degraded by TiO2 photocatalysis with retaining its tensile strength even after long-term UV-light


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irradiation, since the silica shell of the TiO2@HSS worked as a physical barrier to prevent the direct contact between cellulose fibers and TiO2 particles. The photocatalytically-active paper fabricated in this study is easy to handle and shows higher photocatalytic performances in the degradation of VOCs compared with the bare TiO2 particles and the unsupported TiO2@HSS particles, making it as a promising candidate for real air cleaning.

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 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. STEM image, EDS mapping images, TG, and XRD data of the material. AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] (H. Yamashita) Notes The authors declare no competing financial interest.

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Table of contents

The photocatalytically-active papers were fabricated by combining TiO2 nanoparticles encapsulated in hollow silica spheres (TiO2@HSS) and cellulose fibers, which exhibited both high adsorption property and improved photocatalytic efficiency with retaining its inherent durability under UV-light irradiation.


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Fabrication of Photocatalytic Paper Using TiO2 Nanoparticles

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