Titania

8180

J. Phys. Chem. C 2007, 111, 8180-8187

Fabrication of Hollow Hybrid Microspheres Coated with Silica/Titania via Sol-Gel Process and Enhanced Photocatalytic Activities Xuefeng Song and Lian Gao* State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China ReceiVed: February 10, 2007; In Final Form: April 6, 2007

In this paper, we reported a facile and effective approach for fabrication of polystyrene (PS)/silica/titania hybrid microspheres and their resultant hollow double shell hybrid spheres with a smooth and uniform shell of SiO2/TiO2. In this approach, the cationic polystyrene spheres were first synthesized by emulsifier-free emulsion polymerization using the cationic initiator. Subsequently, the PS/SiO2/TiO2 multilayer hybrid spheres were successfully formed by two-step sol-gel methods. The as-prepared hybrid spheres were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy (EDS), thermogravimetric analysis, ζ potential measurement, and N2 adsorption/desorption isotherm analysis. The effects of the titanium tetrabutoxide content, surfactant content, and the midlayer of silica on the morphology, porosity and mean pore size, specific surface area, and thermal property of the obtained hybrid spheres were systematically evaluated. The photocatalytic activity of the prepared hollow SiO2/TiO2 hybrid spheres under ultraviolet light irradiation was compared to that of commercial p-25 powder using the photocatalytic degradation of methylene blue (MB) as a model reaction. The photocatalytic activity of hybrid spheres with three layers of titania was slightly higher than that of P-25. Moreover, the photocatalytic activity of nitrogen-doped hollow SiO2/TiO2 hybrid spheres was also investigated under visible light irradiation. The photocatalytic degradation rate of nitrogen-doped hollow hybrid sphere was about two times that of nitrogen-doped P-25 powder in 5 h for the degradation of MB.

Introduction In recent years, the preparation and study of core-shell solid and hollow microspheres with well-defined structures have attracted substantial interest because of their potential applications in controlled drug delivery system, lightweight fillers, catalysis, chromatography, vessels for confined reactions, and photonic band gap material.1-6 A number of efforts to find new methods have been devoted to generating colloids with the core-shell structure, such as template-assisted sol-gel process,7-12 layer by layer (LBL) techniques,13-17 microemulsion/interfacial polymerization strategies.18-22 Among those core-shell particles, titania-coated polymer spheres are one of the model systems for the study of encapsulation and hollow sphere preparation due to the facile fabrication of polymer templet spheres and the excellent physicochemical properties of titania. Wang et al.7 obtained a core-shell structure with a core of polystyrene and shell of amorphous titania by varying the concentration of titanium tetrabutoxide (TBOT) and the volume ratio of ethanol/acetonitrile. Cheng et al.23 developed a simple route to prepare hollow titania spheres using polystyrene as a template. In this approach, the titania shells were first formed and the polystyrene cores were dissolved subsequently or synchronously. However, the above-mentioned methods that are employed for hollow microspheres of TiO2, obtain only one kind of inorganic particles used as the core or shell. There are few researches about polymer coated SiO2/TiO2 multilayer core-shell or hollow hybrid microspheres. * To whom correspondence should be addressed. E-mail: liangaoc@ online.sh.cn. Fax: +86-21-52413122. Phone: +86-21-52412718.

Methods to enhance the general titania photocatalytic activity with visible or solar light currently remain a significant issue.24-28 Nonmetal ion nitrogen-doped TiO2 have revealed high photocatalytic activity with visible light.29 So far, several methods have been reported for preparing nitrogen-doped TiO2.30-34 Pan et al.32 prepared chromium (Cr) and nitrogen (N)-doped titania (TiO2) powders by using sol-gel method and ammonia thermal treatment. Wang et al.34 have synthesized successfully a series of nitrogen-doped TiO2 nanocatalysts by a two-step hydrolysis-calcination method. However, most of the published papers deal with the nitriding or nitrogen-doping of TiO2 powders, there are scarce literatures about nitrogen-doping of hollow TiO2 microspheres and their photocatalytic behaviors. The photocatalytic behavior of nitrogen-doped hollow TiO2 microspheres is probably improved drastically because of their high specific surface area and pronounced efficiency of utilizing light. Herein, we reported a facile and efficient approach for fabrication of polystyrene (PS)/SiO2/TiO2 multilayer core-shell hybrid microspheres and their resulting hollow double shell microspheres by two-step sol-gel methods. Furthermore, we investigated photocatalytic degradation features of the nitrogendoped and hollow silica/titania microspheres for methylene blue, which was generally used as a comparative standard target in test of photocatalyst. To the best of our knowledge, this was the first report in which two kinds of inorganic particles used as shell, PS as core, for the fabrication of titania hybrid spheres by two-step sol-gel process and also in which the photocatalytic activities of N-doped TiO2/SiO2 double-shell hollow microspheres were studied.

10.1021/jp071142j CCC: $37.00 © 2007 American Chemical Society Published on Web 05/16/2007

Fabrication of Hollow Hybrid Microspheres

J. Phys. Chem. C, Vol. 111, No. 23, 2007 8181

TABLE 1: Typical Experiment Recipes Information sample A B C D E F G

PS (g)

PS/SiO2 powder (g)

deionized water (g)

TBOT (g)

PVP (g)

ethanol (mL)

0.03 0.03 0.03 0.03 0.03 0.03

0.15 0.15 0.15 0.15 0.15 0.15 0.15

0.06 0.12 0.18 0.24 0.12 0.12 0.12

0 0 0 0 0.01 0.03 0.01

24 24 24 24 24 24 24

0.03

Experimental Section Materials. Titanium TBOT, styrene (St), absolute ethanol, and poly(vinyl pyrrolidone) (PVP) K30 (Mw ≈ 44 000-54 000) were purchased from Shanghai Chemical Regent Co, China. Cationic initiator 2,2-azobis-(isobutyramidine) dihydrochloride (AIBA) was bought from Wako Pure Chem, Japan. P-25 TiO2 powders was provided from Degussa Corporation, Germany. Tetraethoxysilane (TEOS), 2-propanol, and aqueous ammonia solution (∼28% by weight) were ordered from Jiangsu Yonghua Fine Chemistry Co, China. All chemicals were used as received without any purification except St, St was purified by distillation under reduced pressure. Deionized water was used for all polymerization and treatment processes. Synthesis of Cationic Polystyrene Microspheres. The cationic polystyrene cores of 262 nm diameter were fabricated by emulsifier-free emulsion polymerization using the cationic initiator AIBA, as described by Goodwin.35a Typically, 20 mL of styrene was added at room temperature to 200 mL of deionized water. Then, 0.6 g of AIBA and 2.5 g of PVP were added, and the temperature was increased gradually to 70 °C under continuous fierce stirring. The reaction solution was deoxygenated by bubbling nitrogen gas at room temperature for 1 h and stirred for 24 h at 70 °C. The resultant PS spheres were then centrifuged and washed two times with ethanol and deionized water, respectively. Asprepared PS spheres terminated in the -NH2 group were considered to be positively charged at pH value lower than 10.6. Coating PS Spheres with Silica. The coating reaction of PS beads could be directly processed at room temperature by a modified Sto¨ber procedure, a commonly used sol-gel method to get amorphous silica. A typical coating reaction was as follows: 1 mL of the PS spheres dispersion (3% by wt) was diluted with 15 mL of deionized water and 50 mL of 2-propanol. Then, 0.23 g of TEOS was added into the diluted PS dispersion under magnetic stirring when the pH value was modulated to ∼10 by dropping a certain amount of ammonia solution. The reaction process was allowed to sustain for 2 h at room temperature under continuous stirring. The as-synthesized core-shell particles were recovered by centrifugation and washed three times with 2-propanol and then dried in vacuum at 60 °C. Coating PS/SiO2 Spheres with Titania. The mixture of 0.03 g of PS/SiO2 sphere powders and 0.15 g of deionized water was sonicated in 12 mL of ethanol for 15 min. The total amount of ethanol was 24 mL, while the amount of surfactant PVP and TBOT were varied as shown in Table 1. The ultimate mixture dispersion was refluxed for 2 h at 80 °C. The particles were then centrifugated and redispersed in ethanol several times. A small part of the sample was taken for analysis. The procedure according to the recipe of sample E (as shown in Table 1) was repeated twice in order to increase coating thickness. Preparation of Hollow SiO2/TiO2 Double-Shell Hybrid Microspheres and Doping Nitrogen Procedure. The polystyrene cores were removed by calcination in a furnace. The

dried PS/SiO2/TiO2 powders were placed in a crucible to calcine in a furnace at different temperatures of 550, 600, and 650 °C for 3 h, respectively. Then, the final hollow SiO2/TiO2 double shell hybrid microspheres were obtained. For the photocatalytic experiment under visible light irradiation, the resulting hollow double-shell hybrid microspheres were treated in NH3 flow at 550 °C for 3 h to dope nitrogen into the TiO2 shell of the hybrid spheres. Commercial Degussa P-25 TiO2 powder was also annealed under the same condition for 3 h for the sake of comparison. Characterization. X-ray Powder Diffraction (XRD). XRD was carried out in a Japan Rigaku D/max 2550V X-ray diffractometer using Cu KR (λ ) 0.15406 nm) radiation at 40 kV and 60 mA. The crystal size of TiO2 was estimated by applying the Schrerrer equation (φ ) kλ/β cos θ), where φ is crystal size, λ is the wavelength of the X-ray irradiation, k is usually taken as 0.89, and β is the peak width at half-maximum height after subtracting the instrumental line broadening. Percent composition calculations were made using the equation rutile % ) 100 × (0.8(IA/IR) + 1)-1, where IA is the peak intensity of anatase (101) and IR is the peak intensity of rutile (110)35b. Morphology ObserVation of the Obtained Spheres. Transmission electron microscopy (TEM) images were collected using a JEOL-2100F electron microscope with an accelerating voltage of 200 kV. The dispersions were diluted with ethanol and ultrasonicated at 25 °C for 15 min and then dried onto carboncoated copper grids before examination. Scanning electron microscopy (SEM) images were collected on a JSM 6700F fieldemission scanning electron microscope. Energy-DispersiVe X-ray Spectroscopy (EDS) Analysis. EDS was recorded on an OXFORD ISIS spectroscope to examine the composition of the shell and core of hybrid microspheres, which was attached to the JEOL-2100F electron microscope. ThermograVimetric Analysis (TGA). The obtained powders were heated in air flow from room temperature to 900 °C on a STA429C analyzer at the rate of 10 °C/min (Netzsch, Selb, Germany). N2 Adsorption/Desorption Isotherm Analysis. The nitrogen adsorption/desorption isotherms were obtained at 77 K with a Micromeritics ASAP 2010 micropore analysis system (Micromeritics, Norcross, GA). Surface areas were determined using the Barrett-Emmett-Teller (BET) method, and average pore diameters were calculated using the Barrett-Joyner-Halenda (BJH) method. Particle Size and ζ Potential Measurements. The average particle sizes of the various as-prepared microspheres and the electrophoretic mobility (EPM) measurements were carried out using a Brookhaven ZetaPlus 1DA-033 potential analyzer. Photocatalytic Reaction. Photocatalytic experiments were carried out in a Pyrex photoreactor containing required quantity of hollow SiO2/TiO2 particles and methylene blue aqueous solution. The suspension were irradiated with a 300 W mediumpressure mercury lamp with a 340-nm cutoff filter for ultraviolet light irradiation experiments, while a 400-nm cutoff filter was utilized for visible light irradiation experiments. The catalysts were suspended in a 400-mL aqueous solution of 15 mg/L methylene blue (MB) and sonicated in the dark for about 15 min. The concentration of photocatalyst was 0.3 g/L. Oxygen was bubbled into reactive suspensions at a flow rate of 100 ml/min during the whole experiments. The concentration of methylene blue in the solution was determined using the Lambda 950 UV-vis spectrophotometer by collecting the absorbance of MB at 650 nm.

8182 J. Phys. Chem. C, Vol. 111, No. 23, 2007

Song and Gao

Figure 1. TEM images of the various obtained microspheres. (a) Pure PS spheres; (b) PS/TiO2 hybrid spheres without midlayer SiO2 shell; (c) PS/SiO2 hybrid spheres; (d) PS/SiO2/TiO2 multilayer hybrid microspheres.

Results and Discussion Morphology Analysis of the Hybrid Microspheres. The Effect of Midlayer Silica Shell on the Morphology. The monodisperse PS template beads were prepared by emulsifierfree emulsion polymerization using the AIBA as the cationic initiator and PVP as the stabilizer. Figure 1 illustrated the TEM images of various obtained microspheres. The mean diameter of original PS particles was 262 nm determined by average particle size analyzer, which is almost consistent with the value determined by TEM image. To research whether the silica midlayer could facilitate the PS particles to be coated by titania or not, the PS/TiO2 spheres particles (sample G) were fabricated in the absence of SiO2 shell as shown in Figure 1b. It could be seen that a rough and inhomogeneous titania layer was obtained at pH 4.7. While, the titania layer become smooth and homogeneous in the presence of SiO2 shell in Figure 1d. A well-defined core-shell structure with a PS bead as core and SiO2/TiO2 as double shell was indeed formed by comparing the thickness of wall determined by TEM in parts c and d of Figure 1. The average size of silica shell is about 8 nm as revealed in Figure 1c; however, when the TBOT was added to the solution containing homogeneous PS/SiO2 hybrid spheres, the thickness of double shell was increased to ∼33 nm. So, it was evident that the thin SiO2 shell acting as midlayer changed the surface conditions of PS spheres, which should be a cause of higher affinity between SiO2 and TiO2 particles than that between PS and TiO2 particles. Additionally, the thickness of SiO2 shell could be conveniently tailored by the TEOS content. Figure 2 showed the variations of ζ potential with pH value for the pure silica spheres without titania coating, silica-coated

Figure 2. Plot of ζ potential as a function of pH for the pure silica spheres without titania coating, silica-coated PS spheres, PS/SiO2/TiO2 multilayer hybrid spheres, and the cationic PS spheres.

PS spheres, PS/SiO2/TiO2 multilayer hybrid spheres, and the cationic PS spheres. The curves gave an isoelectric point at 3.7 for silica-coated PS spheres and at 4.8 for PS/SiO2/TiO2 multilayer hybrid spheres. It was very clear that coating silica on the surface of PS particles had shifted dramatically the isoelectric point from that of cationic PS spheres (10.6) close to that of pure silica spheres (3.4), indicating that the uniformity of silica coating is perfect. While, when titania was wrapped on the surface of PS/SiO2 spheres, the value (4.8) of PS/SiO2/ TiO2 multilayer hybrid microspheres approached that (5.0) of pure bulk TiO2 spheres,8 which indicated titania coating is uniformly deposited on the surface of PS/SiO2 spheres.



Figure 3. TEM images of the coated spheres obtained at multifarious content of PVP: (a) 0 g, (b) 0.01 g, (c) 0.03 g.

The SEM morphology and EDS analysis were further conducted to present the formation of PS/SiO2/TiO2 multilayer hybrid spheres (Supporting Information). The increase of average diameter and the higher roughness of the hybrid spheres surface disclosed that the titania shell perfectly coated the core of PS/SiO2 spheres. The EDS spectra represented that the different composition in different area of the single hybrid sphere, four elements (C, O, Si, and Ti) were observed with distinct intensity in the EDS spectra. In shell layer and center, Ti content and corresponding peak intensity were much higher than that of other elements. The Effect of Surfactant Content on the Morphology. Figure 3 depicted the TEM images of the PS/SiO2/TiO2 multilayer hybrid spheres obtained at different PVP content. It was found that there was a benefit of increasing the PVP content to improve the dispersivity of as-synthesized microspheres. Although uniform titania shell on the surfaces of PS/SiO2 spheres was formed without PVP, the dispersion of hybrid spheres was not perfect (shown in Figure 3a). However, when the PVP content was increased to 0.01 g, the dispersivity was superior to that of the former (shown in Figure 3b). When the content of PVP continued to increase to 0.03 g, it was found that no uniform titania coating layer was formed, a rough surface and partial bumping or cluster of the TiO2 agglomerations were formed. This was likely to be a result of the higher PVP content, which strengthened the steric blocking effect that could prevent the titania nanoparticles from contacting with the surface of PS/ SiO2 spheres. When the effect of electrostatic attraction is higher than that of the steric blocking of PVP in some locations on the surface of the spheres, the surface of PS/SiO2 spheres could be partially covered by titania. Then, the subsequent titania formed agglomerates or cluster against the prior titania acted as growth seeds. On the contrary, when the effect of steric blocking was higher than that of the electrostatic attraction in some places, no titania particles could be formed on the surface of PS/SiO2 spheres. Thus, both cases resulted in the rough coating of titania on the surface of PS/SiO2 spheres. Depending on the above results, we could conclude that the surface morphology of the titania coating could be tailored by altering PVP content, too much PVP caused the formation of cluster or agglomeration of titania particles and consequent rough surface. The Effect of TBOT Content on the Morphology. The effect of the TBOT content on the morphology of the multilayer hybrid microspheres was investigated and shown in Figure 4. We found that it was the most convenient and reproducible to control the thickness of titania wall by adjusting the concentration of TBOT precursor. Apparently, as more TBOT content was added, the

mean thickness of titania layer on the surface of the PS/SiO2 spheres increased. The average thickness of SiO2 shell on the surface of pure PS spheres was around 8 nm determined by TEM image in Figure 1c. When the TBOT content increased to 0.06 g, the mean thickness of double shell was up to 16 nm (as seen in Figure 4a). When the TBOT content was increased from 0.12 to 0.18 g, the average thickness of double SiO2/TiO2 shell would range correspondingly from ∼30 to 66 nm determined by parts b and c of Figure 4. The uniform coating was distinctly presented in the SEM images of typical broken hollow silica/ titania spheres (Figure 4(d), (e)), apparently denoting that the hollow structure with a ∼28 nm and ∼50 nm of wall thickness was fabricated for the 0.12 and 0.18 g of TBOT, respectively, and also indicating that the variation coefficients of the wall thickness is 6.7 and 24.2% by comparing the wall thickness before and after heat treatment. While, when the TBOT content is increased, the extent of agglomeration of hybrid spheres increased, which should be a result of production of more secondary titania nearby the original spheres (as shown in Figure 4e). Figure 5 further presented the effect of TBOT content on the wall thick of silica/titania hybrid microspheres. The mean particle size as a function of TBOT content was determined by light-scattering particle size analyzer. The mean particle size of silica-coated PS spheres was around 281 nm, as TBOT content increased, the mean particle size of silica/titania hybrid microspheres increased. When the TBOT content increased from 0.12 to 0.18 g, the mean particle size of the hybrid microspheres increased from 332 to 412 nm. However, when TBOT content further increased, the mean particle size of hybrid spheres was slightly increased, suggesting that the thickness of titania coating could be tailored using TBOT content in a certain range; too much TBOT caused the formation of second-phase titania particles.36 For the hollow silica/titania microspheres with threelayer titania, the mean particle size increased to 488 nm, however, numerous second-phase titania particles were formed. XRD Analysis. Figure 6 displayed the XRD spetra of the samples before and after heat treatment at different temperature of 550, 600, and 650 °C, respectively. It showed that the TiO2 nanoparticles were amorphous before calcination, formed anatase structure below 600 °C, and transformed gradually into rutile above 650 °C. The composition percent of rutile remained still low and only increased to 9.7% at 650 °C. The average crystallite size at different temperatures were calculated, which corresponded to 11.1, 13.7 and 17.3 nm for 550, 600, and 650 °C, respectively. The XRD results revealed that the heat treatment induced the increase of crystalline size and phase


Song and Gao

Figure 4. TEM images of hybrid microspheres prepared at various amount of TBOT content: (a) 0.06 g, (b) 0.12 g, (c) 0.18 g. The SEM images of typically broken hollow SiO2/TiO2 hybrid spheres for as-annealed sample B (d) and sample C (e).

Figure 5. The effect of the TBOT content on the mean particle size of PS/silica/titania hybrid microspheres.

transformation, verifying the coating of titania were indeed formed on the surface of SiO2 shell. Thermal Property. The TGA curves of the PS/SiO2 spheres, the titania-coated PS/SiO2 hybrid spheres obtained at various TBOT contents, and the hybrid spheres with three layers titania were shown in Figure 7. It could be seen that the titania content in hybrid spheres increased as the TBOT content and the coating number of titania layers increased. However, compared with sample C, the TiO2 content of hybrid spheres with three layers of titania exhibited a very slight increase (as shown in curves c and d), indicating that the content of titania on the surface of PS/SiO2 was not always directly proportional to TBOT content and too much loading caused the formation of secondary titania nanoparticles. For PS/SiO2 spheres (curve a), three weight

Figure 6. XRD patterns of non-heating samples and as-annealed multilayer hybrid microspheres at different temperature of 550, 600, and 650 °C for sample B, respectively.

loss stages: below 320, at 320-450, and at 450-580 °C, were observed. These corresponded to the evaporation of physically adsorbed water and residual solvent, the PS decomposition, and the dehydroxylation or/and the removal of residual organic matter. For the titania-coated PS/SiO2 hybrid spheres, three weight-loss stages still retained except the hybrid spheres with three-layer titania coating. By comparison with uncoated titania spheres (curve a) the onset temperature of decomposition of PS core was postponed, which was attributed to the coating of titania. Moreover, the weight loss stage of PS became gradually flat, not so steep as that of PS/SiO2 spheres, suggesting the coating of titania could bate the decomposition rate of PS core particles. The more titania was coated, the much lower the weight loss rate was, which disclosed that the titania-coated hybrid spheres with thicker wall were formed. For the samples



Figure 7. TGA curves for the (a) PS/SiO2 spheres, PS/SiO2/TiO2 hybrid spheres obtained at various amounts of TBOT (b) 0.06 g, (c) 0.18 g, (d) hybrid spheres with three-layer titania.

TABLE 2: Structural Properties of Various Obtained Hybrid Spheres sample sample A sample B sample B before calcination sample C sample E hybrid spheres with three layers titania

BET area (m2/g)

average pore size (nm)

pore volume (cm3/g)

213.4 96.7 437.1

11.7 11.3 4.8

0.36 0.13 0.53

54.2 131.3 59.9

8.1 11.5 12.1

0.09 0.15 0.13

coated with three layers of titania, the weight loss rate of PS was extremely slower than that of other samples, which is due to the thicker wall of titania. Porosity and Pore Size. The specific surface area and average pore size of the as-prepared different hybrid spheres were determined by BET and BJH method (listed in Table 2). The pore size distribution, the average pore diameter and the pore volume were calculated by using the adsorption branch of the isotherm. It could be seen that the BET surface area decreased with the TBOT content for sample A, sample B, sample C, as a result of the increased thickness of titania shell wall and the reduced amount of mesopore or average pore size on the surface of spheres. It was well known that the specific surface area was generally reduced with the increase of particle size. The more TBOT content was added, the more titania was coated on the surface of the template spheres, causing the increscent thickness of titania shell and resultant increased size of hybrid spheres. When the TBOT content was increased just for the samples with one layer of titania, the pore volume and the average pore size were decreased, suggesting some previous pores on the surface of the as-prepared spheres were completely filled by the latter titania nanoparticles, which could be further confirmed by the pore size distribution plots, as shown in Figure 8. When the TBOT content was low (0.06 g), there appeared several peaks from around 3-13 nm. While, with the increase of TBOT content loaded, these peaks disappeared gradually, evidencing that the fine titania nanoparticles fully filled the larger pores. By comparison with sample B and sample E, the BET value increased as the 0.01 g of surfactant PVP was added, suggesting the enhanced dispersivity of obtained hybrid sphere particles. When three layers of titania were coated on the hybrid spheres, there appeared a broad pore size distribution from

Figure 8. BJH pore-size distribution curves of sample A, sample B, sample C, and the hybrid spheres with three layers titania (inset), respectively. The pore-size distribution was determined from the absorption branch of the isotherms.

around 5-25 nm (see inset), which was attributed to the pore between the agglomeration of a few hybrid spheres. The more the times of coating titania layer, the more secondary titania nearby the original spheres, indicating the second-phase titania would get more chances to connect the original spheres by physical attachment and then degrade the dispersivity of hybrid spheres. The BET area decreased from 96.7 to 59.9 m2/g as the number of TiO2 layers increased, which should be the result of the agglomeration of hybrid spheres by the secondary titania nanoparticles. After calcination at 600 °C for 3 h, the BET surface area of the product for sample B decreased drastically from 437.1 to 96.7 m2/g, accompanied with the decrease of pore volume from 0.53 to 0.13 cm3/g, indicating the formed mesostructure was destroyed during the calcination process.37 Catalytic Property. To explore the catalytic activity of this type of hollow hybrid spheres, the degradation of methylene blue in aqueous solution were tested. Figure 9a depicted the results on the photocatalytic decomposition of MB under visiblelight irradiation. Nitrogen-doped hollow SiO2/TiO2 hybrid spheres by annealing treatment under NH3 flow at 550 °C for 3 h obviously showed higher photocatalytic activity than N-doped P-25 powder with visible-light irradiation. Over 95% of the methylene blue was degraded after 5 h irradiation for nitrogen-doped hollow SiO2/TiO2 hybrid spheres, whereas only 45% for N-doped P-25 powder. The photocatalytic degradation rate of Nitrogen-doped sample B was about two times that of N-doped P-25 powder in 5 h. There were two possibilities regarding higher photocatalytic degradation rate of N-doped hollow sample B. One reason was the narrow band gap and the high specific surface area.38 The more available electron-hole pair for the redox reaction and higher specific surface area were responsible for the higher photocatalytic degradation rate of N-doped hollow sample B. The other was the structure defect derived from silica and the residue of polymer or the nitrogendoping process which made the growth of titania imperfect, such as oxygen defects/vacancies, which played an important role in enhancing the photocatalytic activity in the visible region.39 Figure 9b compared the degradation rate of hollow silica/titania hybrid spheres with that of the commercial P-25 powders under the UV-vis light irradiation. It was found that the photocatalytic activity of hybrid spheres with three layers titania was higher than that of P-25. While, there was no large difference between P-25 and hybrid spheres with three layers titania for UV-vis


Song and Gao the specific surface area degraded drastically. In the experiment of photocatalytic degradation of MB, the photocatalytic degradation rate of nitrogen-doped calcinated sample B was about two times that of N-doped P-25 powder in 5 h with visiblelight irradiation. Additionally, It was found that the photocatalytic activity of hybrid spheres with three layers of titania was slightly higher than that of P-25 powder, indicating that the hollow SiO2/ TiO2 hybrid spheres were likely to be one of promising catalysts. Further researches focusing on the effect of titania content, nitrogen element content in the shell on the photocatalytic activity and the catalytic mechanism would be carried on and elucidated in future works. Acknowledgment. This work was supported by the National Key Project of Fundamental Research (Grant No. 2005CB623605), the National Natural Science Foundation of China (No. 50572116) and Shanghai Nanotechnology Promotion Center (Nos. 0552nm045 and 0652nm022), respectively. Special thanks are given to Professor M. L. Ruan for TEM observation and Dr. Q. H. Zhang for his helpful discussion. Supporting Information Available: SEM images of the typically obtained sphere particles and EDS spectra of the hybrid sphere obtained before calcination in the core area and shell layer, respectively. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 9. The relationship plots of photodegradation ratio of methylene blue with time irradiated by (a) visible light and (b) UV-vis light.

light, which was due to the result that the surface area was almost the same between the hybrid spheres with three layers of titania (59 m2/g) and P-25 (∼55 m2/g). As to the high photocatalytic activity of as-prepared hybrid spheres presented in the photocatalytic experiment, one might doubt whether it is photocatalyst that played an important role in decomposing MB since MB itself could degrade by visible light. However, on the basis of our previous works,38,40 this doubt would be excluded because the efficiency of the decomposition without catalyst was weak under the visible light irradiation. Conclusions The new composite photocatalyst-hollow silica/titania hybrid microspheres had been successfully synthesized via a facile twostep sol-gel method. The TBOT content, surfactant content and the midlayer of silica significantly influenced the coating morphology of the multilayer hybrid spheres. The existence of midlayer silica shell facilitated the PS spheres to be coated effectively by titania nanoparticles at pH 10. The thickness of titania layer could be controlled by the TBOT content. A relatively low surfactant amount resulted in excellent dispersivity of hybrid spheres with uniform and smooth titania shell, whereas the high surfactant content caused anomalistic hybrid spheres with a rough surface. As the TBOT content increased, the BET surface area and pore volume decreased. By increasing the number of titania layers coating on the surfaces of the PS/SiO2 spheres, the average particle size remarkably increased, however,

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Titania

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