Shell Particles with Both

Jun 10, 2010 - Kuruma Malkappa , Billa Narasimha Rao , Gandham Suresh , Ch. Venkata Ramana , Tushar Jana. Colloid and Polymer Science 2018 296 (1), ...
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Fabrication of Rattle-Type TiO2/SiO2 Core/Shell Particles with Both High Photoactivity and UV-Shielding Property Yuan Ren, Min Chen, Yang Zhang, and Limin Wu* Department of Materials Science and the Advanced Coatings Research Center of China Educational Ministry, Fudan University, Shanghai 200433, P. R. China Received February 26, 2010. Revised Manuscript Received May 26, 2010 Rattle-type TiO2@void@SiO2 particles, with commercial TiO2 particles encapsulated into hollow SiO2 shell, were fabricated by successive coating of multilayer polyelectrolytes and SiO2 shell onto TiO2 particles and then treatment by UV irradiation to remove the polyelectrolyte layers. TEM observation showed that the composite particles had a unique rattle-type structure in which there existed void space between TiO2 core and SiO2 shell. The photocatalytic degradation of Rhodamine B indicated that these composite particles with larger void space tended to have higher photoactivity. The polyurethane films doped with rattle-type TiO2@void@SiO2 composite particles had very good UV-shielding property.

Introduction In the past decades, extensive researches have been focused on nanoscale TiO2 because of its superior chemical stability, optical property, nontoxicity, photocatalytic activity, and UV absorbance property. In particular, the improvement of photocatalytic function of TiO2 nanoparticles has achieved in the fields of photocatalysts,1-6 water splittings,7,8 semiconductors in solar cells,9,10 and bacterial inhibitors.11,12 However, this excellent photoactivity may also photodegrade organic supports of TiO2 nanoparticles, such as textiles, plastics, and resins, and hence considerably limit the practical application of TiO2 nanoparticles as photocatalytic and UV absorbing agents. To work out this problem, people usually encapsulate TiO2 particles into inert inorganic substances to fabricate core/shell structure,13-16 especially TiO2/SiO2 coreshell structure since silica has low cost and mild fabrication process. Although many successful examples of such a work have been reported,17,18 this approach often faces the problem of fabricating TiO2 nanoparticles without sufficient photoactivity because the active sites of TiO2 nanoparticles would be completely shielded by the inert shell. *Corresponding author. E-mail: [email protected]. (1) Li, Y.; Zhang, H.; Guo, Z.; Han, J.; Zhao, X.; Zhao, Q.; Kim, S. J. Langmuir 2008, 24, 8351–8357. (2) Chen, Y.; Franzreb, M.; Liu, R.; Chen, L.; Chang, C.; Yu, Y.; Chiang, P. C. Ind. Eng. Chem. Res. 2009, 48, 7616–7623. (3) Li, G.; Zhang, D.; Yu, J. Environ. Sci. Technol. 2009, 43, 7079–7085. (4) Hong, X.; Wang, Z.; Cai, W.; Lu., F.; Zhang, J.; Yang, Y.; Ma, N.; Liu, Y. Chem. Mater. 2005, 17, 1548–1552. (5) Jagadale, T. C.; Takale, S. P.; Sonawane, R. S.; Joshi, H. M.; Patil, S. I.; Kale, B. B.; Ogale, S. B. J. Phys. Chem. C 2008, 112, 14595–14602. (6) Shiraishi, Y.; Saito, N.; Hirai, T. J. Am. Chem. Soc. 2005, 127, 12820–12822. (7) Osterloh, F. E. Chem. Mater. 2008, 20, 35–54. (8) Nowotny, J.; Bak, T.; Nowotny, M. K.; Sheppard, L. R. J. Phys. Chem. B 2006, 110, 18492–18495. (9) Kim, Y.; Kim, C.; Lee, Y.; Kim, K. J. Chem. Mater. 2010, 22, 207–211. (10) Jang, S. R.; Lee, C.; Choi, H.; Ko, J. J.; Lee, J.; Vittal, R.; Kim, K. J. Chem. Mater. 2006, 18, 5604–5608. (11) Kiwi, J.; Nadtochenko, V. Langmuir 2005, 21, 4631–4641. (12) Shibata, Y.; Miyazaki, T. Int. Congr. Ser. 2005, 1284, 284–289. (13) Cosa, G.; Galletero, M. S.; Fernandez, L.; Marquez, F.; Garcia, H.; Scaiano, J. C. New J. Chem. 2002, 26, 1448–1455. (14) Yoneyama, H.; Torimoto, T. Catal. Today 2000, 58, 133–140. (15) Park, O. K.; Kang, Y. S. Colloids Surf., A 2005, 257-258, 261–265. (16) Djerdjev, A. M.; Beattie, J. K.; O’Brien, R. W. J. Chem. Mater. 2005, 17, 3844–3849. (17) Wang, S.; Wang, T.; Chen., W.; Hori, T. Chem. Commun. 2008, 3756–3756. (18) Ikeda, S.; Ikoma, Y.; Kobayashi, H.; Harada, T.; Torimoto, T.; Ohtani, B.; Matsumura, M. Chem. Commun. 2007, 3753–3755.

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Recently, more and more studies were concentrated on creating rattle-type TiO2@void@SiO2 particles. In this structure, the void space between TiO2 core and SiO2 shell can guarantee the high photocatalytic activity of TiO2 nanoparticles; meanwhile, the SiO2 shell can prevent the photocatalytic property of TiO2 nanoparticles from damaging any organic support when used. And the SiO2 shell derived from the sol-gel reaction of siloxane precursors under alkaline medium is always porous to facilitate the transfer of reactants.19,20 For instance, Wang et al. coated silica onto the glucose-modified TiO2 through the sol-gel method and then calcined at 773 K to remove the glucose layer.17 Ikeda et al. fabricated rattle-type TiO2@void@SiO2 particles by successive coating of commercial anatase TiO2 with a carbon layer and a SiO2 layer followed by heat treatment to remove the carbon layer.18,21,22 Demir€ors et al. reported an easy self-templated way of fabricating rattle-type particles by taking advantage of porous titania particles that shrinked after sinter.23 However, despite rattle-type TiO2@void@SiO2 showing much better photoactivity compared to core/shell particle, the composite hybrid nanoparticles severely aggregate due to the high temperature treatment, which can also depress the photocatalytic activity of TiO2 nanoparticles. In this paper, we fabricate rattle-type TiO2@void@SiO2 composite particles based on layer-by-layer assembly deposition for the first time. In this method, multiple layers of alternatively charged polyelectrolytes were coated onto the surfaces of TiO2 nanoparticles, and then a SiO2 shell derived from the sol-gel process of siloxane precursor was coated onto the outermost surfaces of the polyelectrolyte-modified TiO2 nanoparticles. When the multilayer polyelectrolytes were decomposed by photocatalytic TiO2 under UV irradiation, the void space between the TiO2 core and the SiO2 shell was formed, causing rattle-type TiO2@ void@SiO2 composite particles. Compared to the previous rattletype TiO2@void@SiO2 particles, our rattle-type structure particles (19) Tissot, I.; Reymond, J. P.; Lefebvre, F.; Bourgeat-Lami, E. Chem. Mater. 2002, 14, 1325–1331. (20) Chen, M.; Wu, L.; Zhou, S.; You, B. Adv. Mater. 2006, 18, 801–806. (21) Ikeda, S.; Kobayashi, H.; Ikoma, Y.; Harada, T.; Torimoto, T.; Ohtani, B.; Matsumura, M. Phys. Chem. Chem. Phys. 2007, 9, 6319–6326. (22) Ikeda, S.; Kobayashi, H.; Sugita, T.; Ikoma, Y.; Harada, T.; Matsumura, M. Appl. Catal., A 2009, 363, 216–220. (23) Demir€ors., A. F.; Blaaderen, A.; Imhof, A. Chem. Mater. 2009, 21, 979–984.

Published on Web 06/10/2010

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Ren et al. Scheme 1. Procedure for the Fabrication of Rattle-Type TiO2@void@SiO2 Composite Particles

have a couple of notable features: one is that the void space is obtained through the photodegradation of polyelectrolytes by TiO2 nanoparticles under UV irradiation rather than high temperature, which can obviously decrease the aggregation of TiO2@void@SiO2 composite particles; another is that the photoactivity of these rattle-type particles can be tunable by varying the void space which is easily controlled by the number of polyelectrolyte layers. Preliminary investigations showed these composite particles have excellent photocatalytic activity and UV-shielding property.

Experimental Section Materials. Poly(diallyldimethylammonium chloride) (PDDA, Mw < 200 000), poly(sodium 4-styrenesulfonate) (PSS, Mw = 70 000), and 3-hydroxytyramine hydrochloride (dopamine, DA) were all purchased form Aldrich. Tetraethyl orthosilicate (TEOS), sodium chloride (NaCl), absolute ethanol, isopropyl alcohol, an aqueous ammonia solution (28 wt %), and Rhodamine B were purchased from Shanghai Chemical Reagent Co. (China). TiO2 (P 25, 80% of anatase and 20% of rutile; mean size ∼20 nm) was purchased from Degussa (Germany). Polyvinylpyrrolidone (PVP, Mw =30 000) was obtained from Fluka. Water-borne polyurethane dispersion (PUD) was supplied by Behr Process Co. All these reagents were used as received. Deionized water was used throughout all the experiments. Preparation of Rattle-Type TiO2@void@SiO2 Composite Particles. The detailed preparation procedure of rattle-type TiO2@void@SiO2 composite particles is described in Scheme 1. 0.2 g of DA and 100 g of ethanol were charged into a 250 mL plastic vessel and stirred at room temperature for 10 min to obtain DA solution, followed by addition of 20 g of TiO2 powder, and then milled for 2 h on a bead miller with 0.3 mm ZrO2 beads as milling medium and a stirring rate of 3000 rpm to obtain stable DA-coated TiO2 dispersion. This suspension was centrifugated and washed with deionized water for six cycles to remove superfluous DA; the obtained solid was redispersed into deionized water. The surfaces of TiO2 became positively charged due to the adsorption of DA (which was proved by electrophoresis measurement). PSS and PDDA were subsequently and alternatively deposited onto the surfaces of the DA-coated TiO2 by adding polyelectrolyte aqueous solution (1 g/L) containing 0.5 M NaCl. After 20 min adsorption, excess polyelectrolyte was removed by six cycles of centrifugation (15 000 rpm, 15 min) and washing with deionized water; the obtained particles were then redispersed into deionized 11392 DOI: 10.1021/la1008413

water. Repeating the above steps, polyelectrolytes with different layers could form on the surfaces of TiO2 nanoparticles. The polyelectrolyte layer could provide a uniformly charged surface and facilitate subsequent substance adsorption.24-26 The outermost surface layer was always PSS, which could guarantee the coating of negatively charged SiO2. The encapsulation of polyelectrolyte-modified TiO2 with SiO2 shell was carried out in a mixed solution of isopropyl alcohol and water at room temperature using the typical process as follows: 25 mg of polyelectrolyte-modified TiO2 was dispersed in 3 mL of water, followed by addition of 4 mg of PVP dissolved in 12 g of isopropyl alcohol, and then 0.5 g of ammonia. This dispersion was subsequently dropwise added by a mixture of 85 mg of TEOS and 3 g of isopropyl alcohol at room temperature under magnetic stirring. This reaction system was kept stirring for 24 h to obtain SiO2-coated TiO2 composite particles. These SiO2-coated particles are designated as TiO2/PEj/SiO2, in which j denotes the number of polyelectrolyte layers. These TiO2/PEj/SiO2 composite particles were separated from the reaction medium by centrifugation and washing with water for three times and then exposed to the UV lamp (365 nm, 20 mW/ cm2) for 5 days so that the multilayer polyelectrolytes could be decomposed, leaving ahead void space between the TiO2 core and the SiO2 shell. The obtained rattle-type structure particles are designated as TiO2@voidj@SiO2, in which j indicates the number of polyelectrolyte layers forming the void. In this research, three kinds of samples, TiO2@void1@SiO2, TiO2@void5@SiO2, and TiO2@void9@SiO2, were prepared. Characterization. Morphology. A transmission electron microscope (TEM JEOL-2010F, JEOL Corp., Japan) was used to observe the morphologies of particles. The samples were first diluted with water, then deposited onto carbon-coated copper grids, and dried in air before examination. A scanning electron microscope (SEM Philips XL30 apparatus, Philips Corp., The Netherlands) was used to obtain SEM images. After being diluted with water, the samples were dried on a cover glass and sputtercoated with gold prior to examination. ζ Potential. Electrophoretic mobility (EPM) measurements were performed using a ZetaPlus ζ potential analyzer (Zetasizer Nano, Malvern Instruments, Britain). All the samples were dispersed into deionized water, and then ζ potential measurements were taken. (24) Caruso, F.; Lichtenfeld, H.; Giersig, M.; M€ohwald, H. J. Am. Chem. Soc. 1998, 120, 8523–8524. (25) Caruso, F.; M€ohwald, H. Langmuir 1999, 15, 8276–8281. (26) Caruso, F.; Spasova, M.; Susha, A.; Giersig, M.; Caruso, R. A. Chem. Mater. 2001, 13, 109–116.

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TGA Analysis. About 4 mg of rattle-type TiO2@void@SiO2 composite particles was dried at 80 °C overnight and was examined with a TGA instrument (Perkin-Elmer TGA-1) to determine the composition. All the powders were heated in nitrogen from room temperature to 800 °C at a scan rate of 10 °C min-1. Photocatalytic Activity. The photocatalytic performance of TiO2@void@SiO2 composite particles was carried out by the photodegradation of Rhodamine B under UV light as follows: Rhodamine B aqueous solution (10 mg/L) was added by the asobtained particles to keep the concentration of TiO2 of 1 g/L; the whole solution was stirred vigorously for 0.5 h in darkness to achieve adsorption-desorption of Rhodamine B molecules on the surface of catalyst before illumination. When this dispersion was exposed to the UV irradiation, the catalyst decomposed the dye molecules on its surface first; other dye molecules in the solution then moved onto the surface of catalyst and were decomposed. Thus, the change of dye concentration under UV irradiation could be used to compare the photocatalytic activity.27-29 The dye concentration indicated by the maximum absorbance at 553 nm of the dye solution as a function of time was measured by the UV-vis spectrophotometer (UV 1800 PC, Mapada, China) at the natural pH of the dye and 25 °C. UV-Shielding Performance. Two kinds of dispersions, designated as A and B were prepared before making films. The dispersion A was prepared by magnetic mixing 10 g of PUD with 5 g of Rhodamine B aqueous solution (0.5 g/L), and the dispersion B was prepared by mixing 10 g of PUD with 5 g of TiO2@void@SiO2 composite particles dispersion or with only 5 g of water. The dispersion A was coated onto a glass slide with a 120 μm rod to obtain a uniform film. After drying at room temperature for 3 days, the dispersion B was then coated onto the film with the same rod to obtain TiO2@void@SiO2-doped PU film and pure PU films. These films were dried at room temperature for another 3 days before irradiation by a UV lamp (365 nm, 20 mW/cm2). The TiO2 content was 2 wt % in these dry films based on inductively coupled plasma (ICP) analysis. The color changes of these films were measured by monitoring the absorbances of the samples at the wavelength of the maximum absorbance (560 nm) with a UV-vis spectrophotometer. UV-vis Spectrum and Color Change. The TiO2@void@ SiO2-doped PU films were prepared by coating the above dispersion B onto glass slides with a 100 μm rod and then dried for 3 days, followed by UV irradiation. The UV-vis spectra was recorded in UV-vis spectrophotometer (Mapada, UV-1800PC spectrophotometer, China) in the 235-850 nm range, while the color difference of the hybrid films was measured by a spectrophotometer (CM700d, Konica Minolta Sensing Inc., Japan) according to Commission International d’Eclairage (CIE) LAB color scale. The LAB system has a lightness scale L* and opponent color axes for rednessgreenness versus yellowness-blueness, designated a* and b*, respectively. Each color can be represented by a unique point in threedimensional coordinate used in the LAB system. The L* value characterizes the lightness of the color and ranges between 0 (dark) and 100 (light). The a* and b* values are the chroma coordinates and characterize the hue and chroma factors. The a* axis is red on the positive side and green on the negative side. The b* axis is yellow on the positive side and blue on the negative side. The higher the number, the stronger the color factors are. ΔL*, Δa*, and Δb* are the color difference values; the higher color difference value implies the more serious deterioration of films.

Results and Discussion Composition and Morphology of Composite Particles. In this study, a commercial TiO2 (P 25), which is composed of two (27) Li, X.; John, V. T.; He, G.; Zhan, J.; Tan, G.; McPherson, G.; Bose, A.; Sarkar, J. Langmuir 2009, 25, 7586–7593. (28) Lu, Y.; Lunkenbein, T.; Preussner, J.; Proch, S.; Breu, J.; Kempe, R.; Ballauff, M. Langmuir 2010, 26, 4176–4183. (29) Liu, R.; Ren, Y.; Shi, Y.; Zhang, F.; Zhang, L.; Tu, B.; Zhao, D. Chem. Mater. 2008, 20, 1140–1146.

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Figure 1. ζ potential measurements (mV) vs the oppositely charged layer: the layer number of zero for the DA-coated TiO2; the odd numbers for PSS deposition and the even numbers for PDDA deposition.

crystalline phases, anatase and rutile, is employed due to its better photocatalytic activity than other TiO2 materials.30,31 Since the surface charge of TiO2 nanoparticles is not high enough to adsorb polyelectrolytes, they were modified by DA first and then sequentially and alternatively adsorbed by PSS and PDDA polyelectrolytes. ζ potential measurement was taken after the deposition of each alternate polyelectrolyte layer. Figure 1 shows the ζ potential as a function of the number of polyelectrolyte layers. The original DA-coated TiO2 nanoparticles is positively charged with the ζ potential about þ10 mV. The presence of PSS causes a reversal in ζ potential (-40 mV). Subsequent deposition of PDDA onto the PSS-coated particles again reverses the ζ potential. Alternating ζ potentials can be observed with the further depositions of each oppositely charged polyelectrolyte, confirming that the stepwise growth of oppositely charged layers from the polyelectrolyte indeed occurred on the TiO2 nanoparticles. If the outermost layer of TiO2 nanoparticles was positively charged PDDA, the suspension quickly flocculated after the addition of ammonia. On the contrary, when the outermost layer of nanoparticles was negatively charged PSS, the whole system was stable. After addition of PVP, the strong hydrogen bonds could form between its carbonyl groups and the hydroxyl groups of silica; hence, silica layer could be easily coated onto the surface of TiO2. The concentration of TEOS was constant in this study, thus keeping the same thickness of deposited SiO2, ca. 10-15 nm. Figure 2 illustrates the SEM images of the original TiO2 and TiO2/PEj/SiO2 particles. It can be seen that all these particles encapsulated by multilayer polyelectrolytes and SiO2 shell maintain a relatively narrow size distribution. Compared with the uncoated TiO2 particles, coatings of polyelectrolyte layers and SiO2 shell obviously increase the mean particles size, and as the number of the polyelectrolyte layers increases, the mean particle size increases slightly. In order to remove the multilayer polyelectrolytes between the TiO2 core and the SiO2 shell, these TiO2/PEj/SiO2 composite particles were irradiated under UV light for 5 days. Figure 3 demonstrates the TEM images of these samples before and after UV irradiation, respectively. Compared with the unexposed TiO2/ PE1/SiO2 composite particles (Figure 3a), the void space in TiO2@ void1@SiO2 particles is hard to identify (Figure 3b), since the (30) Hurum, D. C.; Gray, K. A. J. Phys. Chem. B 2005, 109, 977–980. (31) Hurum, D. C.; Agrios, A. G.; Gray, K. A. J. Phys. Chem. B 2003, 107, 4545–4549.

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Figure 2. SEM images of original TiO2 (a), TiO2/PE1/SiO2 (b), TiO2/PE5/SiO2 (c), and TiO2/PE9/SiO2 particles (d).

Figure 3. TEM images of composite particles before UV irradiation (a: TiO2/PE1/SiO2; c: TiO2/PE5/SiO2; e: TiO2/PE9/SiO2) and after UV irradiation (b: TiO2@void1@SiO2; d: TiO2@void5@SiO2; f: TiO2@void9@SiO2).

average thickness of each layer of polyelectrolyte is less than 2 nm.32 However, the thickness of the void space increases with 11394 DOI: 10.1021/la1008413

the number of the polyelectrolyte layers, being ca. 5 and 12 nm for TiO2@void5@SiO2 and TiO2@void9@SiO2 composite particles, Langmuir 2010, 26(13), 11391–11396

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Figure 4. TGA and DTA curves of TiO2@void9@SiO2 and TiO2/PE9/SiO2 composite particles.

Figure 6. Pictorial representation of photodegradation of Rhodamine B (a) without catalyst and (b) with TiO2@void9@SiO2 as catalyst.

Figure 5. Normalized photodegradation profile of Rhodamine B by P25 (1), P25-DA (f), TiO2@void1@SiO2 (2), TiO2@void5@ SiO2 (b), TiO2@void9@SiO2 (9), and blank without catalyst ([).

respectively. And a semitransparent middle layer and a relatively uniform SiO2 shell of ca. 20 nm thick can be observed. Therefore, the thickness of the void space can be easily controlled by the number of the polyelectrolyte layers. Figure 4 further presents the typical TGA and differential thermal analysis (DTA) curves of composite particles using the pair of TiO2@void9@SiO2 and TiO2/PE9/SiO2 as an example. For the TiO2/PE9/SiO2 particles, three weight loss stages, below 300, 300-450, and 450-800 °C, are observed, which correspond to the evaporation of physically adsorbed water and residual solvent, the decomposition of polyeletrolytes, and the decomposition of silica-bonded groups such as -OH and/or unhydrolyzed -OR, respectively. The total weight loss of TiO2/PE9/SiO2 in three stages is about 15%, while the weight loss of TiO2@ void9@SiO2 is much smaller, ca. 7% (Figure 4a). And from Figure 4 b, the two peaks of TiO2@void9@SiO2 are consistent with the first and third peaks of TiO2/PE9/SiO2; the second peak of TiO2/PE9/SiO2 disappears in TiO2@void9@SiO2, validating that the polyelectrolytes are completely decomposed by UV irradiation. Photocatalytic Activity of the TiO2@void@SiO2 Particles. Photocatalytic activity of the rattle-type TiO2@void@SiO2 composite particles was evaluated by decomposing Rhodamine B as model and using original TiO2 and TiO2/DA nanoparticles as controls. Figure 5 shows the normalized photocatalytic degradation profiles of Rhodamine B with equal amount of TiO2@voidj@ (32) Caruso, F.; Lichtenfeld, H.; Donath, E.; M€o1hwald, H. Macromolecules 1999, 32, 2317–2328.

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SiO2 or TiO2/DA and a blank test. A and A0 represent the absorbances of the dye at any time and the initial time during UV irridation, respectively; thus, A/A0 indicates the ratio of the remaining concentration to the initial concentration of the dye in the solution. As can be seen, Rhodamine B is hardly decomposed under UV irradiation in the absence of any catalyst. For the TiO2catalyzed systems, the relative degradation rate of the dyes can be ranked as follows: TiO2@void9@SiO2 > TiO2@void5@SiO2 > P25 > TiO2@void1@SiO2 > TiO2/DA. The reason why TiO2/ DA has the lowest photocatalytic activity is due to the completely surface coverage by DA, which shields the active sites of TiO2 particles. In the initial stage of UV irradiation, the dye concentration hardly changes because TiO2 photocatalyzes the DA first. The TiO2 nanoparticles aggregate gradually as the DA is decomposed, which leads to the decrease of photocatalytic activity. The TiO2@void1@SiO2 particles show very low photocatalytic activity; this is probably because the void space is too small to allow the adsorption of enough Rhodamine B molecules onto the surfaces of TiO2 particles. As the thickness of void space increases, the photocatalytic activity increases, as shown by TiO2@void5@SiO2 and TiO2@void9@SiO2 particles, since the larger void space can make active sites of TiO2 contact more dye molecules. The reason why TiO2@void5@SiO2 and TiO2@void9@SiO2 particles have higher photocatalytic activity than the original TiO2 particles should be attributed to better dispersibility of TiO2@voidj@SiO2 particles in water than the original TiO2 particles (see Figure 3d,f and Figure S1 in the Supporting Information). More intuitionistic visualization can be compared from the corresponding color changes between noncatalyzed and the TiO2@void9@SiO2-catalyzed solution in Figure 6. These results indicate that TiO2@ void@SiO2 is a promising novel photocatalyst with controllable photocatalytic performance. UV-Shielding Property. The UV-vis spectra of the photocatalytic particle-doped PU films as well as the pure PU film are demonstrated in Figure 7. Compared to the original TiO2 particles, when 2 wt % of TiO2 was doped, the UV cutoff of the hybrid DOI: 10.1021/la1008413

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Figure 9. Color difference of PU films doped with TiO2@void1@ Figure 7. Typical UV-vis transmittance curves of original TiO2, TiO2@void1@SiO2, TiO2@void5@SiO2, and TiO2@void9@SiO2doped and pure PU films.

SiO2 (2), TiO2@void5@SiO2 (1), TiO2@void9@SiO2 (f), P25 (9), and pure PU films (b) under UV irradiation.

UV irradiation. The darker the yellowness of a film, the higher the Δb* value is, indicating that the polymer film deteriorates more seriously. As shown in Figure 9, when exposed to UV light, the film embedded with original TiO2 nanoparticles has considerably higher color difference than pure PU film, indicating that original TiO2 nanoparticles can accelerate the aging of PU film quickly, since TiO2 nanoparticles directly contact and photocatalyze the resin. However, the rattle-type TiO2@void@SiO2 particlesdoped PU films have almost the same color difference as the pure PU film. This result clearly indicates that the silica shell can prevent the TiO2 nanoparticles inside from deteriorating resin molecules.

Conclusions Figure 8. Normalized degradation of profile of Rhodamine B-doped films protected by TiO2@void1@SiO2 (9), TiO2@void5@ SiO2 (1), TiO2@void9@SiO2 (b), pure PU ([), and no blocking agent (2).

films is about 340 nm while the transparency of the film still maintains about 70% in the visible region, which is because of the relative smaller particles size and better dispersibility of the TiO2@void@SiO2 particles in water-borne PU dispersion. To determine the actual UV protection effect of TiO2@void@ SiO2 particles, a UV-sensitive material, Rhodamine B-doped PU film, was used as the substrates of the TiO2@void@SiO2-doped PU films and pure PU films. The photodegradation of dye-doped films as a function of irradiation time was measured by monitoring the maximum absorption intensity at 560 nm. As shown in Figure 8, after irradiation for 80 min, the unprotected and pure PU protected materials show more than 55% of loss, while the films protected by TiO2@void@SiO2-doped PU films only show 15% of loss. And three composite particles have nearly the same UV-shielding performance. Color Change. To check whether the rattle-type particles have any negative impact on the polymer itself or not, the color changes of these TiO2@void@SiO2-doped PU films as well as original TiO2-doped PU film during UV irradiation were further measured by a spectrophotometer. The extent of yellowness of films during the aging process under UV irradiation is evaluated by Δb*, which is the difference between the b* values after and before

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Based on this study, the rattle-type TiO2@void@SiO2 composite particles in which commercially available TiO2 encapsulated into hollow SiO2 shell have been successfully prepared by using the layer-by-layer method. The thickness of void space between the TiO2 core and SiO2 shell can be tuned from less than 2 to 12 nm by controlling the number of polyelectrolyte layers. As the thickness of void space increases, the photocatalytic activity of the rattle-type particles increases because reactants access to the active surface more easily. At the same time, these composite particles-doped PU films have better transparency and UVshielding property than the original TiO2-doped film. These results show that the rattle-type TiO2@void@SiO2 composite particles have high and tunable photocatalytic activity as well as UV-shielding performance without decomposing the supporting organic materials. Acknowledgment. Financial support of this research from National “863” Foundation, National Science Foundation of China (50903019), Science & Technology Foundation of Shanghai (0952 nm01000), Shanghai Rising-Star Foundation (10QA1400300), Shanghai Semiconductor Lighting Project (09DZ1141000), and the Shanghai Leading Academic Discipline Project (B113) is appreciated. Supporting Information Available: TEM images of original TiO2 (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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