Superparamagnetic high-magnetization composite microspheres with Fe3O4@SiO2 core and highly crystallized mesoporous TiO2 shell

Colloids and Surfaces A: Physicochem. Eng. Aspects 402 (2012) 60–65

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Superparamagnetic high-magnetization composite microspheres with Fe3 O4 @SiO2 core and highly crystallized mesoporous TiO2 shell Zhaogang Teng a,b , Xiaodan Su b , Guotao Chen a , Congcong Tian a , Hao Li a , Li Ai a , Guangming Lu a,∗ a

Department of Medical Imaging, Jinling Hospital, Clinical School of Medical College, Nanjing University, 305 Zhongshan East Road, Nanjing 210002, PR China Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Laboratory of Advanced Materials, Fudan University, Shanghai 200433, PR China b

a r t i c l e

i n f o

Article history: Received 23 December 2011 Received in revised form 5 March 2012 Accepted 13 March 2012 Available online 21 March 2012 Keywords: Core–shell Mesoporous Stöber Superparamagnetic Titania

a b s t r a c t We demonstrate a novel synthesis of sandwich structured superparamagnetic mesoporous microspheres with a silica-coated magnetite core and mesoporous titania shell using a block-copolymer-templating approach. The synthesis process is simple and facile, in which uniform magnetite particles were coated with silica through classical Stöber method. And then, a mesostructured P123/TiO2 composite was deposited on the silica-coated magnetite core by using Pluronic P123 block-copolymer as a template and tetrabutyl titanate (TBOT) as a precursor in ethanol aqueous solution. After calcination at 400 ◦ C, the superparamagnetic microspheres with crystallized mesoporous titania shell were obtained. The sandwich structures of the obtained composite microspheres have been confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The TEM and SEM images show that the microspheres possess a uniform diameter of ∼420 nm and a homogeneous mesoporous shell of ∼40 nm. The XRD indicates that mesoporous shell is highly crystallized anatase titania. The obtained microspheres possess tunable specific surface areas of 50–100 m2 /g, and controlled large mesopore sizes of 3.7–5.0 nm. Furthermore, the resulting superparamagnetic microspheres with a high saturation magnetization value of ∼34 emu/g could be enriched completely within 10 s under the application of a 0.2 T magnet. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Owing to its low cost, good chemical stability, high photocatalytic activity, and environmental friendliness, titania has been investigated intensively in the fields of “energy” and “environmental” [1–4]. In particular, titania is very useful in environmental remediation processes such as the purification of air and water, deodorization, self-cleaning, and sterilization [5–8]. The effectiveness of titania in practical applications is much related with its specific surface area [9,10], crystallinity [11,12], and the morphology of the material [13,14]. Crystallized titania materials with high surface area and large pores have been synthesized by using different surfactant or block copolymer as templates [15–20]. However, the titania materials are generally applied as suspensions, thus special equipments and complex processes are often required to remove the titania fine powders from large volumes of reaction solutions. This presents a major drawback for their widespread applications.

∗ Corresponding author. Tel.: +86 25 8086 0185; fax: +86 25 8480 4659. E-mail address: [email protected] (G. Lu). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2012.03.019

Magnetic materials (such as Fe3 O4 , -Fe2 O3 , or FePt) can be collected by an external magnetic field [21–23]. The magnetic motor effect is indeed attractive for the development of multi-functional composites by combining the magnetic materials with titania, which can enhance the separation and recovery of nanosized titania materials. Magnetically separable photocatalysts have been obtained by embedding submicron-sized barium ferrite in anatase titania [24]. However, the composite materials are not superparamagnetic, and thus the particles easily aggregate in practical applications due to their magnetic dipole interaction. Superparamagnetic composite materials have been prepared by combining magnetic nanoparticles with titania [25–29]. Due to the low iron oxide mass fraction contained, these composites generally show low saturation magnetization (less than 10 emu/g), which cannot be enriched easily under external magnetic field [25–27]. And the morphologies of the titania composites are often irregular in shape because of an uncontrollable sol–gel process from a too fast hydrolysis-condensation rate [28]. Moreover, the direct coating of the surface of magnetic particles with a layer of titania not only changes the properties of magnetic oxides but also deteriorates the photocatalytic activity of titania [28,29]. Silica or carbon as a passivation layer has been inserted between the magnetic particle and the titania to promote the photocatalytic activity of the titania

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catalyst by decreasing the adverse influence of the magnetic oxide core [29–31]. However, surfactant or block polymer templates have not been introduced in the titania condensation process during the preparation of magnetite-titania composites, and thus the composites generally do not have large specific surface area and uniform pore size, which is disadvantage for the improvement of catalytic activity. The Stöber approach is a facile and effective process to synthesize uniform spherical colloid silica particles in ethanol aqueous solution [32], which has also been utilized to obtain uniform mesoporous silica nanospheres (MSNs) with controllable diameter size by self-assembly process with surfactant templates [33,34]. The silica precursor hydrolyzes and condenses with controlled manner in the ethanol solution, which can further assemble with surfactant to form mesostructures. It is an expectation that the Stöber method can be used to grow the uniform mesoporous titania layers on magnetic materials, resulting in magnetic mesoporous titania composite spheres. Herein, we demonstrate a simple Stöbersolution coating method for the synthesis of superparamagnetic high-magnetization composite microspheres with a silica-coated magnetite core and highly crystallized mesoporous titania shell. By introducing magnetite particles into a classical Stöber solution, uniform amorphous silica thin layer can homogeneously be grown on the magnetite to obtain silica-Fe3 O4 composites (denoted as Fe3 O4 @SiO2 ). Then the Fe3 O4 @SiO2 was coated with P123/titania composite shell in the second Stöber solution by using Pluronic P123 block-copolymer as template and tetrabutyl titanate as precursor. The deposition of titania with Pluronic P123 on the Fe3 O4 @SiO2 surfaces are spontaneous, which represents a unique methodological advantage for tuning the coated mesostructure. After calcination at 400 ◦ C, the superparamagnetic composite microspheres with highly crystallized titania shell were obtained. The morphology and structures of the synthesized composite microspheres were investigated by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The results show that the designed sandwich structured microspheres possess a uniform diameter, tailored shell thickness, high surface area, uniform pore size, superparamagnetic property, and large magnetization.

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2.2. Synthesis of magnetite particles The spherical magnetic particles with a diameter of about 300 nm were prepared according to preciously reported solvothermal reaction [35]. Briefly, 0.65 g of FeCl3 , 0.20 g of trisodium citrate dehydrate, and 1.20 g of sodium acetate were dissolved in 20 ml of ethylene glycol under magnetic stirring. The obtained homogeneous yellow solution was transferred to a Teflon-lined stainless-steel autoclave with a capacity of 30 ml. The autoclave was heated to 200 ◦ C and maintained for 10 h, and then cooled down to room temperature. The obtained black magnetite particles were washed with water for five times, and then dried in vacuum at 60 ◦ C for 12 h. 2.3. Synthesis of Fe3 O4 @SiO2 microspheres The core–shell Fe3 O4 @SiO2 microspheres were prepared through a versatile Stöber sol–gel method. 0.08 g of the above obtained Fe3 O4 spheres were homogeneously dispersed in the mixture of ethanol (50 ml), deionized water (1 ml), and concentrated ammonia aqueous solution (1.7 ml, 25 wt%), followed by the addition of TEOS (140 ␮l). After stirring at 40 ◦ C for 6 h, the Fe3 O4 @SiO2 spheres were obtained and washed several times with water. 2.4. Synthesis of superparamagnetic mesoporous titania composite microspheres

2. Experimental

The composite microspheres were synthesized by using Pluronic P123 as template and TBOT as precursor. The above prepared Fe3 O4 @SiO2 microspheres (0.01 g) were dispersed in a mixed solution containing of Pluronic P123 (0.12 g), ethanol (50 ml), and deionized water (0.4 ml). Then 0.5 ml of TBOT was added to the dispersion. The stirring speed of the reaction solution was controlled at 300 rpm by using a D2004 electric stirrer (Shanghai Meiyingpu Co., Ltd.). After reaction at 60 ◦ C for 3 h, the products were collected with a magnet and washed with water and ethanol for three times, and then dried in an oven at 70 ◦ C. Afterward, the products were calcined in air at 400 ◦ C for 30 min to remove the template and crystallize the titania, resulting in well dispersed superparamagnetic mesoporous microspheres (denoted as Fe3 O4 @SiO2 @m-TiO2 ).

2.1. Chemicals

2.5. Characterization

Ferric chloride (FeCl3 ), trisodium citrate dihydrate, sodium acetate, ethylene glycol, anhydrous ethanol, concentrated ammonium aqueous solution (25 wt% NH3 ), tetrabutyl titanate (TBOT), and tetraethoxysilane (TEOS) were of analytical reagent from Sinopharm Chemical Reagent Co., Ltd. Triblock copolymers, Pluronic P123 (poly(ethylene oxide)-b-poly(propylene oxide)-bpoly(ethylene oxide), EO20 PO70 EO20 , Mw = 5800) was purchased from Aldrich. Deionized water (Millipore) with a resistivity of >10 M cm was used in all experiments.

Field emission transmission electron microscopy (FE-TEM) images were taken on a JEOL 2100F microscope (Japan) at 200 kV. Specimens for FE-TEM were prepared by dipping a drop of the colloidal solution onto carbon-coated Cu grids. Scanning electron microscopy (SEM) images were taken on a Bruker FE-SEM-4800 field emission scanning electron microscope at 15.0 kV. Nitrogen sorption isotherms were measured using a Micromeritics Tristar 3000 analyzer at −196 ◦ C. Samples were degassed at 180 ◦ C for 6 h before taking the measurements. The Brunauer–Emmett–Teller

Fig. 1. Schematic illustration of the formation process of the Fe3 O4 @SiO2 @m-TiO2 microspheres.

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Fig. 2. TEM images of (a and b) Fe3 O4 particles, (c and d) Fe3 O4 @SiO2 microspheres, and (e and f) sandwich structured Fe3 O4 @SiO2 @m-TiO2 microspheres synthesized by using Pluronic P123 as a template. Insets (b and f) are enlarged TEM images.

(BET) method was utilized to calculate the specific surface area (SBET ) using the adsorption data at p/p0 = 0.05–0.25. The pore size distribution was derived from the adsorption branch by using the Barrett–Joyner–Halenda (BJH) model. Quantum Design MPMS-XL SQUID magnetometer was used to determine the magnetic characteristics of the materials. Magnetization curves as a function of magnetic field were measured at 25 ◦ C under magnetic fields up

to 20 kOe. The thermogravimetry (TG) analysis of the samples was performed on a Shimadzu DTG-60H instrument, in air, with a temperature ramp of 10 ◦ C/min. X-ray diffraction (XRD) patterns were obtained with a Bruker model D8 focus diffractometer equipped with a copper anode producing X-ray with wavelength of 0.154 nm (40 kV, 40 mA). Data were collected in continuous scan mode with a 0.02◦ sampling interval.

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Fig. 3. TG/DTA curves of (a) bare Fe3 O4 particles and (b) Fe3 O4 @SiO2 @P123/TiO2 composite microspheres prepared by using Pluronic P123 as a template.

3. Results and discussion The synthesis procedure is shown in Fig. 1. Firstly, uniform magnetite particles were introduced into a classical Stöber solution containing tetraethoxysilane, ethanol and ammonia catalyst, and thus uniform amorphous silica thin layer can homogeneously be grown on the magnetite to obtain core–shell Fe3 O4 @SiO2 microspheres. Secondly, through a block-copolymer-templating approach, a mesostructured P123/TiO2 composite was deposited on the Fe3 O4 @SiO2 microspheres in ethanol aqueous solution. Thirdly, the obtained Fe3 O4 @SiO2 @P123/TiO2 microspheres were calcined at 400 ◦ C to remove the Pluronic P123 templates and crystallize the titania, resulting in the formation of superparamagnetic Fe3 O4 @SiO2 @m-TiO2 microspheres with crystallized mesoporous TiO2 shell. The magnetite particles were prepared via a robust solvothermal reaction based on a high temperature reduction of Fe(III) salts with ethylene glycol in the presence of trisodium citrate [35]. As revealed by TEM, the obtained magnetite particles possess uniformly spherical shape and a mean diameter of ∼300 nm (Fig. 2a). High-resolution TEM (HRTEM) images clearly show that each magnetite particle is composed of plentiful nanocrystals with the size of about 4.0 nm (Fig. 2b). SEM image of the magnetite particles further confirms the uniform size of ∼300 nm and nearly spherical shape (Supporting Information Fig. S1). FT-IR spectrum of the magnetite particles show absorption bands at 1616 and 1396 cm−1 associated with carboxylate, suggesting the existence of citrate on the magnetite surface (Fig. S2). Due to numerous carboxyl groups anchored on the surface, the particles show excellent dispersibility in polar solvents, which favors the subsequent coating with silica. Through a sol–gel process by the hydrolysis and condensation of TEOS in ethanol/ammonia mixture, uniform silica layer (∼20 nm in thickness) can be formed on individual magnetite particle seed, resulting in core–shell Fe3 O4 @SiO2 microspheres (Fig. 2c). TEM and SEM images indicate that the diameter of the Fe3 O4 @SiO2 is about 340 nm (Fig. 2c, Fig. S3). Compared with the magnetite particles, the obtained Fe3 O4 @SiO2 microspheres (Fig. 2d) exhibit more regular spherical shape with smooth surface due to the deposition and growth of silica occurring on a molecular scale in the sol–gel process [32]. The subsequent deposition of titania with Pluronic P123 on the Fe3 O4 @SiO2 microspheres and calcination results in a uniform crystallized mesoporous titania shell (ca. 40 nm in thickness, Fig. 1e). The TEM and SEM images show that the microspheres are uniform in shape with a size of ∼420 nm (Fig. 1e, Fig. S4). A typical sandwich structure with a magnetite core, a nonporous silica layer in the middle layer, and a mesoporous TiO2 shell in the outer layer can be clearly observed (Fig. 1f). In order to obtain the sandwich structured composite microspheres, the coating of silica layer is very important. If directly coating the titania layer on the

magnetite particles, the magnetite would dissolve into the solution and form some small crystals, and thus the obtained microspheres show a hollow structure (Fig. S5), suggesting the silica layer can protect the magnetite in the titania coating procedures. The unique microstructure of the obtained microspheres would be very useful for many applications. First, the magnetite core provides an ability to respond to the external magnetic field. Second, the middle silica layer could protect the magnetite from etching in harsh application occasions. Third, the mesoporous titania shell offers high surface area and pore size for adsorption and catalysis of macromolecules. To confirm the Fe3 O4 phase is unchanged during the calcinations, TG/DTA curves of bare Fe3 O4 particles and the Fe3 O4 @SiO2 @P123/TiO2 composite microspheres were measured (Fig. 3). Bare Fe3 O4 underwent two exothermic reactions at 140 ◦ C and 580 ◦ C (Fig. 3a), The exotherm at 140 ◦ C is resulted from the formation of -Fe2 O3 [36]. The small weight gain (∼0.5%) in the temperature range of 120–165 ◦ C is attributed to the increase in oxygen content during the thermal conversion of Fe3 O4 to Fe2 O3 (2 Fe3 O4 → 3 -Fe2 O3 ). The exotherm at 580 ◦ C is corresponding to the phase transformation of - to ˛-Fe2 O3 . The final phase was confirmed by XRD spectrum (Fig. S6). The mass loss of 2.5% is result from the thermal decomposition of the citrate anchored on the magnetite particle surfaces. On the contrast, the two exotherms of bare Fe3 O4 do not present on the DTA curve for the sandwich structured microspheres (Fig. 3b), suggesting that the silica and titania layers protect the iron oxide phase from being transferred into - or ˛-Fe2 O3 under the calcination. The composite

Fig. 4. The wide-angle XRD patterns of (a) Fe3 O4 @SiO2 @P123/TiO2 microspheres and (b) Fe3 O4 @SiO2 @m-TiO2 microspheres. The dots and triangles indicate the typical diffraction peaks of the titania and magnetite, respectively.

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Fig. 5. Nitrogen sorption isotherms and their pore size distribution curves (inset) of core–shell structured microspheres synthesized with (a) 400 ␮l and (b) 360 ␮l H2 O.

microspheres present a broad exotherm between 100 and 650 ◦ C, corresponding to a mass loss of 29% which result from the thermal decomposition of the Pluronic P123 templates. No obvious mass loss between 500 and 600 ◦ C indicates that Pluronic P123 templates in the microspheres can be removed at low temperatures due to the hydrogen-bonding between Pluronic P123 and titania is relatively weak. The obvious exotherm at 400 ◦ C is attributed to the formation of crystallized titania. The phases of the sandwich structured microspheres have been further characterized by using XRD (Fig. 4). For the Fe3 O4 @SiO2 @P123/TiO2 microspheres, all diffraction peaks can be perfectly indexed to the phase of Fe3 O4 (Fig. 4a, JCPDS 19-629). The weak and broad diffraction peaks further suggest the nanocrystalline structure of the spherical magnetite particles, which is consistent with the TEM results. No characteristic peaks of other materials were detected, indicating that the coated SiO2 and TiO2 were both amorphous before calcination. On the contrast, relatively strong and sharp diffraction peaks appear in XRD pattern for Fe3 O4 @SiO2 @m-TiO2 microspheres (Fig. 4b). All of the new peaks can be indexed to the anatase phase of TiO2 (JCPDS 211272), indicating the amorphous TiO2 transformed to the anatase phase after calcination, which is consistent to the DTA results (Fig. 3b). Moreover, the peaks of Fe3 O4 are also observed for the Fe3 O4 @SiO2 @m-TiO2 microspheres, further suggesting the magnetite phase of the composite microspheres is unchanged. Nitrogen adsorption–desorption isotherms of the magnetic mesoporous titania microspheres prepared by using Pluronic P123 as template shows a characteristic type IV curve (Fig. 5a) with a distinct hysteresis loop of H2 in the p/p0 range of 0.4–0.7, indicating a uniform mesopore architecture. The surface area (SBET ) is calculated to be as high as of ∼52 m2 /g. Moreover, it shows a narrow pore size distribution (inset in Fig. 5a), indicating the uniform mesopores of the microspheres. The pore size calculated from adsorption branch is ∼5.0 nm. The thickness of the mesoporous titania shell can be well controlled by tuning water contents in the reaction solution. When the water content in the solution decreases from 400 to 360 ␮l, the thickness of the mesoporous titania increases from 40 to 60 nm (Fig. S7). Therefore, low water content is beneficial for the formation of thick mesoporous titania shell. Nitrogen sorption isotherms of the microspheres synthesized with 360 ␮l of water (Fig. 5b) also show a typical type-IV curve with a distinct hysteresis loop of H2 in the p/p0 range of 0.4–0.7, implying a uniform mesostructures of the composite microspheres. The surface area is calculated to be as high as 110 m2 /g, which are larger than that of the microspheres synthesized with 400 ␮l water. The pore size calculated from the adsorption branches is ∼3.7 nm (inset of Fig. 5b). These results clearly indicate that the mesoporous titania shell has high surface areas, large mesopores, and uniform pore sizes, which

Fig. 6. Room-temperature magnetization curves of the magnetite particles, Fe3 O4 @SiO2 spheres, and sandwich structured Fe3 O4 @SiO2 @m-TiO2 microspheres. Inset gives the photographs of the captured Fe3 O4 @SiO2 @m-TiO2 microspheres by a 0.2 T magnet and the redispersed composites by slight shaking after removal of the magnet.

would benefit for their applications in the field of adsorption and catalysis of macromolecules. Both the magnetite particles, Fe3 O4 @SiO2 , and Fe3 O4 @SiO2 @mTiO2 show no hysteresis on the room-temperature magnetization curves, indicating the superparamagnetic character of these materials (Fig. 6). These materials are superparamagnetic and not ferromagnetic is obviously due to the crystalline domains in the magnetite particles are very small. The Fe3 O4 @SiO2 @m-TiO2 has a saturation magnetization value as high as 34 emu/g, which can be enriched in 10 s upon the application of a 0.2 T permanent magnet and re-dispersed again after withdrawing the magnet (inset of Fig. 6), suggesting the microspheres can be conveniently separated and recycled. 4. Conclusions Sandwich structured superparamagnetic mesoporous Fe3 O4 @SiO2 @m-TiO2 microspheres with high-magnetization have been successfully prepared by using Pluronic P123 as a template. The magnetite particles are first coated with silica through a classical Stöber method. The silica layer could protect the magnetite particles during the titania coating and following calcination procedures. After deposition of P123/TiO2 composites and calcination at 400 ◦ C, crystallized mesoporous titania shell was coated on the Fe3 O4 @SiO2 microspheres. The obtained microspheres show

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tunable specific surface area of 50–100 m2 /g, and large uniform mesopore size of 3.7–5.0 nm. The resulting Fe3 O4 @SiO2 @m-TiO2 microspheres have a saturation magnetization value as high as 34 emu/g and can be enriched in 10 s upon the application of a 0.2 T permanent magnet. The mesoporous shell is highly crystallized anatase titania with a controllable thickness of 40–60 nm. This method is very simple and allows for homogeneous deposition of mesoporous titania layers, providing many new opportunities for synthesizing various complex mesoporous materials. We expect that the composite materials can be applied in many exciting fields, such as catalysis, biomolecule separation, water treatment, and so on. Acknowledgments We greatly appreciate financial support from the National Science Foundation of China (30930028), the National Key Basic Research Program of the PRC (2011CB707700), the Major International (Regional) Joint Research Program of China (81120108013). Z. T. thanks the National Science Foundation for Post-doctoral Scientists of China (20100480030) and Shanghai Postdoctoral Sustentation Fund (11R21411500) for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.colsurfa.2012.03.019. References [1] C. Kormann, D.W. Bahnemann, M.R. Hoffmann, Photolysis of chloroform and other organic molecules in aqueous titanium dioxide suspensions, Environ. Sci. Technol. 25 (1991) 494–500. [2] A.L. Linsebigler, G.Q. Lu, J.T. Yates, Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chem. Rev. 95 (1995) 735–758. [3] H.X. Li, Z.F. Bian, J. Zhu, D.Q. Zhang, G.S. Li, Y.N. Huo, H. Li, Y.F. Lu, Mesoporous titania spheres with tunable chamber structure and enhanced photocatalytic activity, J. Am. Chem. Soc. 129 (2007) 8406–8407. [4] X. Chen, S.S. Mao, Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications, Chem. Rev. 107 (2007) 2891–2959. [5] J.E.G.J. Wijnhoven, W.L. Vos, Preparation of photonic crystals made of air spheres in titania, Science 281 (1998) 802–804. [6] A. Imhof, D.J. Pine, Ordered macroporous materials by emulsion templating, Nature 389 (1997) 948–951. [7] H. Fuji, M. Ohtaki, K. Eguchi, Synthesis and photocatalytic activity of lamellar titanium oxide formed by surfactant bilayer templating, J. Am. Chem. Soc. 120 (1998) 6832–6833. [8] J.H. Schattka, D.G. Shchukin, J. Jia, M. Antonietti, R.A. Caruso, Photocatalytic activities of porous titania and titania/zirconia structures formed by using a polymer gel templating technique, Chem. Mater. 14 (2002) 5103–5108. [9] B.Z. Tian, X.Y. Liu, B. Tu, C.Z. Yu, J. Fan, L.M. Wang, S.H. Xie, G.D. Stucky, D.Y. Zhao, Self-adjusted synthesis of ordered stable mesoporous minerals by acid–base pairs, Nat. Mater. 2 (2003) 159–163. [10] D. Chen, L. Cao, F. Huang, P. Imperia, Y.-B. Cheng, R.A. Caruso, Synthesis of monodisperse mesoporous titania beads with controllable diameter, high surface areas, and variable pore diameters (14–23 nm), J. Am. Chem. Soc. 132 (2010) 4438–4444. [11] J. Lee, M.C. Orilall, S.C. Warren, M. Kamperman, F.J. Disalvo, U. Wiesner, Direct access to thermally stable and highly crystalline mesoporous transition-metal oxides with uniform pores, Nat. Mater. 7 (2008) 222–228. [12] A. Testino, I.R. Bellobono, V. Buscaglia, C. Canevali, M. D’Arienzo, S. Polizzi, R. Scotti, F. Morazzoni, Optimizing the photocatalytic properties of hydrothermal TiO2 by the control of phase composition and particle morphology. A systematic approach, J. Am. Chem. Soc. 129 (2007) 3564–3575. [13] H.G. Yang, C.H. Sun, S.Z. Qiao, J. Zou, G. Liu, S.C. Smith, H.M. Cheng, G.Q. Lu, Anatase TiO2 single crystals with a large percentage of reactive facets, Nature 453 (2008) 638–641.

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