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Synthesis of the double-shell anatase–rutile TiO2 hollow spheres with enhanced photocatalytic activity† Shunxing Li,*ab Jie Chen,a Fengying Zheng,ab Yancai Liab and Fuying Huanga
Received 3rd August 2013 Accepted 2nd October 2013 DOI: 10.1039/c3nr04043g www.rsc.org/nanoscale
A novel double-shell TiO2 hollow sphere with an inner anatase shell and an outer rutile shell was synthesized by a simple sol–gel method and silica protected calcination process. The structure and formation mechanism was proposed based on characterization using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The double-shell spheres have a uniform diameter of 360 nm and a typical yolk–shell structure. Moreover, the double-shell TiO2 hollow spheres possess a large specific surface area (169 m2 g1). Due to the high surface area, multiple light reflection and beneficial electron conduction between the inner anatase and outer rutile shell of this special structure, the as-prepared double-shell TiO2 catalysts show remarkably enhanced photoactivity compared to the commercial P25 catalyst. In particular, rhodamine B molecules can be completely decomposed in the presence of the double-shell spheres after 60 minutes of irradiation with UV light. In addition, the high activity is retained after five cycles, indicating the stability and reusability of the double-shell catalyst.
1.
Introduction
Titanium dioxide (TiO2) is one of the most important semiconductors because of its low cost, biocompatibility and photoreactivity, and has been utilized in many applications such as photocatalysis, environmental remediation and dye-sensitized solar cells (DSSCs).1–5 Its wide band gap energy (3.0–3.2 eV) allows the absorption of UV light, generating electrons (e) and holes (h+) which can subsequently induce redox reactions. Since Honda and Fujishima rst achieved hydrogen production from water by photocatalysis with TiO2 under UV irradiation, this a
Department of Chemistry & Environmental Science, Minnan Normal University, Zhangzhou, China, 363000. E-mail: [email protected]; Fax: +86-5962591395; Tel: +86-596-2591395
b
Fujian Province Key Laboratory of Modern Analytical Science and Separation Technology, Minnan Normal University, Zhangzhou, China, 363000. E-mail: [email protected]; Fax: +86-596-2592563; Tel: +86-596-2592563
† Electronic supplementary information (ESI) available: UV-vis diffuse reection spectra and the summarisation of BET surface and average pore size. See DOI: 10.1039/c3nr04043g
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material has been widely used as a photocatalyst for water splitting and environmental remediation. TiO2 offers a number of advantageous features, including its low cost, relatively high photocatalytic activity, low toxicity, and high chemical stability. Establishing direct relationships between the structure and photocatalytic activity of TiO2 is rarely straightforward due to the intervening effects of various physicochemical properties, such as particle size, surface area, crystallinity, and morphology. However, in general, some effects are thought to be of great signicance for the photocatalytic activity of TiO2. TiO2 has three main polymorphs of crystalline phases: brookite, anatase, and rutile, of which the latter two are photocatalytically active.6,7 In particular, a mixture of anatase and rutile phases demonstrates better photocatalytic activity compared to the single anatase or rutile phase.8–10 For example, Liu et al.11 improved the photocatalytic activity for decomposing methylene blue of iodine doped TiO2 by forming a bicrystalline framework of anatase and rutile. Zhang et al.12 demonstrated an improvement in photocatalytic water splitting ability by forming an anatase–rutile TiO2 junction. The commercial P25 catalyst composed of anatase and rutile is known to show high activity, and is used not only as a benchmark but also as a model for the design of more effective TiO2 composites.13–15 The coupling of anatase and rutile phase TiO2 allows the transfer of electrons excited by ultraviolet light from anatase to rutile TiO2 as a result of the slightly lower conduction band energy of the rutile phase, and thus charge recombination can be suppressed. On the other hand, the surface area is critical to increasing sites for absorbed water and hydroxyl groups, as abundant surface adsorbed water and hydroxyl groups can potentially trap holes to form important oxidative hydroxyl radicals, _OH, and anchor dye molecules for the photodegradation of organic molecules.16–19 It is expected that the formation of TiO2 with a high surface area may be a solution to realize better photoactivity. Double or multishell nanostructures endow catalysts with higher surface areas and more reactive sites, which is benecial for the catalytic performance in sensors and lithium ion batteries.20–24 Importantly, the double or multishell
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Communication nanostructures allow multiple reections of light within the interior cavity, allowing more efficient utilization of the light source and therefore offering an improved catalytic activity for photodegradation and in dye-sensitized solar cells.25–28 Based on the above points, in order to maximize the photoactivity, it is highly desirable to combine the anatase–rutile junction and the unique double or multishell nanostructure. Herein, the double-shell anatase–rutile TiO2 hollow sphere structure was synthesized by a sol–gel and silica-protected calcination method. Unlike previous works in which the shell is a chaotic mix of anatase and rutile phases,29,30 this unique doubleshell sphere is constructed from an inner anatase hollow sphere and an outer rutile hollow sphere. The structure and formation mechanism was proposed based on characterization using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and UV-visible diffuse reection spectroscopy. Furthermore, it is found that the structures possess a high surface area, an anatase–rutile junction, and show multiple reection of UV light. Finally, the photodegradation of rhodamine B as an example clearly demonstrates the substantial role of the special double-shell structure.
2.
Experimental section
Nanoscale washed three times with ethanol, and then redispersed into 20 mL of deionized water to obtain the SiO2@TiO2 colloidal solution. The above SiO2@TiO2 solution was treated with polyvinylpyrrolidone (PVP, Mw ¼ 30 000, 0.2 g) overnight to allow for the adsorption of PVP onto the TiO2 surface, separated from solution by centrifugation, and then re-dispersed in 10 mL ethanol. The solution of SiO2@TiO2 was sequentially mixed with ethanol (13 mL), water (4.3 mL), TEOS (0.86 mL) and aqueous ammonia (28%, 0.62 mL). Aer stirring for 4 h, the resulting SiO2@TiO2@SiO2 particles were centrifuged, washed 3 times with ethanol and dried under vacuum. The second TiO2 coating procedure was the same as the method described above. In a typical synthesis procedure, the SiO2@TiO2@SiO2 samples (0.2 g) were dispersed into a mixture solution of hydroxypropyl cellulose (HPC, 0.1 g), ethanol (55 mL) and de-ionized water (0.6 mL). Aer stirring for 30 min, a mixture of 1 mL of TBOT and 5 mL of ethanol was injected into the above solution using a syringe pump at a rate of 1.5 mL min1. Aer injection, the temperature was increased to 85 C with 900 rpm stirring under reuxing conditions for 2 h. The obtained precipitate was separated by centrifugation and washed three times with ethanol.
2.1. Reagents and chemicals The reagents, including anhydrous ethanol, ammonia (25–28 vol%), NaOH and polyvinylpyrrolidone (PVP) were purchased from Beijing Chemical Works and Shantou Xilong Chemical Industry Incorporated Co., Ltd. Hydroxypropyl cellulose (HPC) was purchased from Alfa Aesar. Tetraethyl orthosilicate (TEOS) was purchased from J&K Chemica, and titanium butoxide (TBOT) was obtained from Aladdin. All the reagents used were analytical grade in purity. 2.2. Synthesis of the SiO2@TiO2@SiO2@TiO2 microspheres ober SiO2 colloidal spheres were prepared by using the classic St¨ method with minor modications. In a typical synthesis operation, two solutions with equal volumes were rapidly mixed to give a total volume of 250 mL: one solution contained ethanol and TEOS, while the other contained ethanol, water, and ammonia. The contents of the water, ammonia, and TEOS solutions were adjusted to 17.0 M, 0.3 M and 0.2 M respectively. The reaction mixture generally turned turbid white as SiO2 particles were formed aer 2–30 min. The reaction was allowed to continue for 6 h at room temperature, with moderate stirring, until completion. The as-synthesized SiO2 spheres were coated with TiO2 according to the procedure reported in the literature.19,31 In a typical synthesis procedure, the SiO2 spheres (0.2 g) were dispersed into a mixture of hydroxypropyl cellulose (HPC, 0.1 g), ethanol (55 mL), and deionized water (0.6 mL). Aer stirring for 30 min, a mixture of 1 mL of TBOT and 5 mL of ethanol was injected into the above solution using a syringe pump at a rate of 1.5 mL min1. Aer injection, the temperature was increased to 85 C with 900 rpm stirring under reuxing conditions for 2 h. The obtained precipitate was separated by centrifugation,
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2.3. Synthesis of the double-shell TiO2 hollow spheres and hollow TiO2 spheres The synthesized SiO2@TiO2@SiO2@TiO2 samples were subjected to calcination crystallization and NaOH etching procedures. In a typical synthesis procedure, the SiO2@TiO2@SiO2@TiO2 samples were calcined in air at 800 C for 2 h, and then the samples were redispersed in 5 mL of NaOH aqueous solution (2.5 M) for 48 h to remove the SiO2 spheres completely. Finally, the obtained products were separated by centrifugation and washed three times with deionized water. As a comparison, hollow TiO2 spheres were synthesized using a similar method but different precursors. SiO2@TiO2 microspheres and SiO2@TiO2@SiO2 microspheres were calcined at a high temperature of 800 C, and aer the NaOH etching process, two types of hollow TiO2 spheres were obtained, labelled as outer hollow spheres and inner hollow spheres, respectively.
2.4. Characterization The size and morphology of the products were characterized by scanning electron microscopy (SEM, Hitachi S-4800), transmission electron microscopy (TEM, FEI Tecnai G2 20ST) and high resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F20) operated at 200 kV with the soware package for automated electron tomography. Powder X-ray diffraction (XRD) patterns were collected on a Rigaku D/max 2500 diffrac˚ The BET (Brutometer with Cu Ka radiation (l ¼ 1.54056 A). nauer–Emmett–Teller) surface area and pore size of the catalysts were measured using a Quadrasorb SI-MP instrument. Prior to each measurement, the sample was heated to 100 C and kept at this temperature for 3 h.
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2.5. Evaluation of photocatalytic activity for the degradation of rhodamine B Photocatalytic activity testing on the degradation of RhB was carried out in a 100 mL beaker containing 50 mL of 1 105 mol L1 RhB and 10 mg of catalyst under 500 rpm stirring at room temperature. Before the photocatalytic reaction was initiated, the solutions were rstly stirred in the dark for 1 h to ensure that rhodamine B was adsorbed to saturation on the surface of the catalysts. A 150 W high pressure mercury lamp was used as the UV light source. The concentration of rhodamine B was measured by UV-vis spectrophotometry (UV-2550) at 15 min intervals during the reaction.
3.
Results and discussion
3.1. Morphology and structure of the double-shell TiO2 spheres As illustrated in Scheme 1, the double-shell TiO2 hollow spheres can be synthesized by the following four main steps: (1) preparation of uniform silica templates; (2) deposition of an inner TiO2 layer on the silica surface to form SiO2@TiO2 spheres through the sol–gel reaction of titanium butoxide (TBOT) with hydroxypropyl cellulose (HPC) as a surfactant; (3) SiO2 coating on the SiO2@TiO2 spheres; (4) second deposition of an outer TiO2 layer on the surface of the SiO2@TiO2@SiO2 spheres; and (5) crystallization of the two TiO2 layers and etching to remove the silica template using an aqueous solution of NaOH. The morphology and structure of the intermediate and nal products were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM). From Fig. 1, the as-prepared SiO2 spheres have a uniform diameter of 148 nm (Fig. 1a). Aer TiO2 coating, the diameter of the SiO2@TiO2 microspheres increases to about 210 nm (Fig. 1b). Adding another SiO2 and TiO2 layer, the diameter of the SiO2@TiO2@SiO2 microspheres and the SiO2@TiO2@SiO2@ TiO2 microspheres further increases to about 300 nm and 375 nm, respectively (Fig. 1c and d). Moreover, the amplied TEM images (inset in Fig. 1b–d) clearly show the two, three and four layered core–shell structure of the intermediate products.
Scheme 1 Scheme of fabrication of the double-shell TiO2 hollow spheres using a sol–gel and the silica-protected calcination process.
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Fig. 1 TEM images of the products in the sol–gel process: (a) SiO2 spheres; (b) SiO2@TiO2 microspheres; (c) SiO2@TiO2@SiO2 microspheres; (d) SiO2@TiO2@SiO2@TiO2 microspheres.
Aer the crystallization and etching process, the doubleshell TiO2 spheres were obtained. From the SEM image, the spheres have a uniform diameter of 360 nm (Fig. 2a). The TEM image (Fig. 2b) clearly shows that the double-shell spheres have a typical yolk–shell structure and that the inner shell can randomly move in the cavity. The diameter of the inner spheres is about 210 nm. The thicknesses of the outer shell and the inner shell are about 30 and 35 nm, respectively. Importantly, the inner shell and the outer shell in the magnied TEM images are separate from each other. The inner shell consists of smaller sized TiO2 nanoparticles with 8 nm diameter, and the outer shell consists of larger sized TiO2 particles with 30 nm diameter. HRTEM images further indicate that the outer shell is highly
Fig. 2 SEM and TEM images of the double-shell TiO2 hollow spheres: (a) SEM image, (b) TEM image, and (c) magnified TEM image of a single double-shell hollow sphere; (d) magnified TEM image of the inner shell from the crushed double-shell hollow sphere, (e) HRTEM image of the outer shell in the white frame shown in (c), and (f) HRTEM image of the inner shell in the white frame shown in (d).
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Communication crystalline, with an interplanar spacing of 0.324 nm corresponding to the (110) planes of rutile TiO2, and the inner shell is highly crystalline with the interplanar spacing of 0.351 nm corresponding to the (101) planes of anatase TiO2. Furthermore, the acentric inner shell can come into contact with the outer shell and form an anatase–rutile junction, and thus electron and hole transfer may occur at the anatase–rutile junction. In brief, the double-shell structure with the anatase inner shell connected to the rutile outer shell would be a highly desirable structure for high-performance photocatalysts.32 Subsequently, X-ray diffraction (XRD) analysis was employed to investigate the crystal phase of the double-shell TiO2 spheres and other TiO2 samples. The typical XRD pattern in Fig. 3 reveals that the crystalline phase of the as-synthesized doubleshell TiO2 spheres is a mixture of the anatase phase (JCPDS no. 89-4921) and the rutile phase (JCPDS no. 21-1276). All the diffraction peaks of the inner hollow spheres and the outer hollow spheres can be indexed as single anatase phase and single rutile phase. It is obvious that the crystal phases of the inner and outer layer in the double-shell hollow sphere are the single anatase and rutile phases aer the high temperature treatment, respectively. The commercial TiO2 nanoparticles and commercial P25 catalyst are presented as a reference to conrm the phase constituents of the TiO2 hollow spheres and the double-shell TiO2 spheres. Moreover, the ratio of rutile phase in the double-shell hollow spheres is larger than that in the commercial P25 catalyst. How can the inner shell and the outer shell form two distinct crystalline phases with high temperature treatment? The possible answer is the adopted “silica-protected calcination” method and the additional silica layer on the inner TiO2 surface during the process.31,32 During heat treatment anatase formation occurs rst, and then the anatase particles start to transform to the rutile phase via the crystal growth mechanism. Due to the protection of the silica layer (the thermal expansion coefficients of anatase, rutile and silica are 10.2, 7.14 and 0.5 106 K1),32 the phase transformations of the inner TiO2 shell from amorphous to anatase and from anatase to rutile are both inhibited, and at the same temperature the outer TiO2 shell without the protection of another silica layer is transformed into the rutile phase. The different conditions the inner shell
Fig. 3 XRD patterns of the double-shell TiO2 hollow spheres and other TiO2 samples (commercial P25, commercial anatase TiO2 and the hollow TiO2 spheres).
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Nanoscale and the outer shell are under result in dissimilar crystalline processes and the different crystal phases. 3.2. BET and UV characterization of the double-shell TiO2 spheres As mentioned above, surface area is a critical factor in photodegradation. A high surface area provides more reaction sites for the hydroxyl groups and the dye molecules. Inspired by these ideas, the nitrogen absorption isotherm was measured to obtain information about the BET surface area and the pore sizes of the double-shell structured TiO2 spheres, as shown in Fig. 4. The surface area is evaluated to be 169 m2 g1 from the data points in the nitrogen adsorption–desorption isotherms. The pore size distribution (inset in Fig. 4) was determined by using the Barrett–Joyner–Halenda method from the desorption branch of the isotherm. The distribution reveals two sharp peaks at around 3.9 nm and 8.5 nm, which are due to the inner shell and the outer shell, respectively. The pores in the two shells ensure efficient access of dye molecules during the photocatalytic process. Table S1† (in the ESI) shows the BET surface areas of the double-shell TiO2 spheres and other TiO2 materials. The BET surface area of the double-shell spheres is much greater than that of commercial anatase TiO2, P25 and the outer hollow spheres, but is lower than that of the inner hollow TiO2 spheres. This result can be attributed to the size effect, as smaller particles possess a larger surface area. Firstly, the inner and outer TiO2 shells consist of 8 nm and 30 nm TiO2 nanoparticles, respectively. The smaller inner nanoparticles should have a larger surface area than the outer particles. Secondly, for the same reason, the hollow sphere with smaller diameter will also have a larger surface area than the bigger hollow spheres. 3.3. Evaluation of photocatalytic activity UV-vis absorption spectra and the photocatalytic performance of the double-shell TiO2 spheres and other samples are shown in the Fig. 5. As indicated in Fig. 5a, the double-shell spheres, the hollow spheres, the commercial anatase TiO2 and the P25 sample demonstrate similar absorption in the range 200–500 nm. The absorption of anatase TiO2 and the inner hollow
Fig. 4 Nitrogen adsorption isotherm of the double-shell TiO2 hollow spheres (inset: the pore size distribution of the inner and outer shells in the double-shell structure), showing a considerably higher surface area and the different pore diameters of the inner shell and the outer shell.
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Nanoscale spheres is around 380 nm, and the addition of rutile TiO2 in the double-shell TiO2 spheres and P25 catalyst causes a blue shi of the absorption edge due to the increase of the amount rutile phase, which has a smaller band gap, 3.0 eV, than that of the anatase phase (3.2 eV). What's more, the diffuse reection spectrum (Fig. S1 in the ESI†) in the UV range of the doubleshell hollow spheres is much lower than that of the inner hollow spheres and nanoparticles, and apparently increases aer crushing treatment (high pressure at 100 MPa), indicating that the larger absorption of the UV light in the double-shell hollow structures is derived from the multiple light reection and scattering between the two spherical shells. Finally, the photocatalytic performance of the double-shell anatase–rutile TiO2 catalyst was evaluated for the photodegradation of rhodamine B dye under UV light. Fig. 5b shows typical adsorption spectra of an aqueous rhodamine B solution aer UV irradiation for various time periods in the presence of the double-shell TiO2 hollow spheres as a catalyst. The strong adsorption peak attributed to rhodamine B at 553 nm gradually decreased and completely disappeared aer 60 min, indicating the complete photodegradation. The performances of the various photocatalysts for rhodamine B degradation, measured from the intensity of the 553 nm peak versus time, are summarized in Fig. 5c. Before UV illumination, the mixtures of the catalysts and rhodamine B were stirred in the dark for 1 h to ensure that the dye was adsorbed to saturation on the inner and outer surfaces. The double-shell spheres and inner hollow spheres have better absorption capacity due to their high surface area. In the blank experiment (without catalyst), only 9% rhodamine B was decomposed aer UV irradiation for 75 min, but the degradation rates were notably enhanced in the presence of the photocatalysts aer a similar amount of time. The relative photocatalytic activities of the catalysts for rhodamine B degradation was in the order: double-shell TiO2 hollow
Communication spheres > inner hollow spheres > P25 > anatase TiO2 nanoparticles. In particular, the rhodamine B molecules could be completely decomposed in the presence of the double-shell spheres aer only 60 min of irradiation with UV light. Consistent with the BET and UV-vis diffuse reection studies, the double-shell spheres with an inner anatase shell and an outer rutile shell demonstrate enhanced photoactivity. In addition, the double-shell structure shows a higher catalytic activity than the hollow spheres, supporting that the crucial effect is the promotion of the rutile outer shell and the multiple reection effect rather than the high surface area. Furthermore, a reutilization experiment was conducted to evaluate the stability of the catalyst. When the photodegradation was complete, the residue was centrifuged and dried at 60 C in the air. Then, the residue was dispersed in a new rhodamine B solution and the photodegradation experiment was performed again. The double-shell hollow catalyst can retain its high activity aer ve cycles (Fig. 5d), while the activity of the commercial P25 catalyst decreased aer each cycle because the lighter P25 catalyst was difficult to completely recycle by the centrifuge process, indicating the favourable stability of the double-shell structure.
4.
Conclusions
In conclusion, a novel double-shell TiO2 structure with an inner anatase shell and an outer rutile shell was synthesized and the structural effect was investigated in detail when it used as the photocatalyst for rhodamine B degradation. The different crystal phase of the inner shell is caused by the silica layer protection, and an anatase–rutile junction may form at the tangent surface. The double-shell spheres with high surface area and the ability to reect and scatter light show enhanced photocatalytic activity for the degradation of rhodamine B. This work may provide new concepts for fabricating high-performance photocatalysts based on structural design, and the development of new synthesis methods.
Acknowledgements This work was supported nancially by the National Natural Science Foundation of China (20977074, S.X.L. and 21175115, S.X.L.), the Program for New Century Excellent Talents in University (NCET-11 0904, S.X.L), the Outstanding Youth Science Foundation of Fujian Province, China (2010J06005, S.X.L.), the Science & Technology Committee of Fujian Province, China (2012Y0065, F.Y.Z.), and the Natural Science Foundation of Fujian Province in China (2012J05031 Y. C. L.).
Fig. 5 (a) UV-vis absorption spectra of the double-shell TiO2 hollow spheres and other TiO2 samples. (b) Absorption spectra of a solution of rhodamine B in the presence of the double-shell TiO2 hollow spheres under exposure to UV light. (c) Evolution of rhodamine B concentration versus UV irradiation time (t) in the presence of the double-shell TiO2 hollow spheres and other TiO2 catalysts. (d) The reusability of the double-shelled TiO2 hollow spheres as a catalyst for rhodamine B photodegradation under the reaction conditions in (c).
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Notes and references 1 M. Murdoch, G. I. N. Waterhouse, M. A. Nadeem, J. B. Metson, M. A. Keane, R. F. Howe, J. Llorca and H. Idriss, Nat. Chem., 2011, 3, 489. 2 S. U. M. Khan, M. Al-Shahry and W. B. Ingler Jr, Science, 2002, 297, 2243.
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Communication 3 Z. Lai, F. Peng, Y. Wang, H. Wang, H. Yu, P. Liu and H. Zhao, J. Mater. Chem., 2012, 22, 23906. 4 B. Liu and E. S. Aydil, J. Am. Chem. Soc., 2009, 131, 3985. 5 D. Chen, F. Huang, Y.-B. Cheng and R. A. Caruso, Adv. Mater., 2009, 21, 2206. 6 X. Chen and S. S. Mao, Chem. Rev., 2007, 107, 2891. 7 A. L. Linsebigler, G. Lu and J. T. Yates, Chem. Rev., 1995, 95, 735. 8 D. Tsukamoto, Y. Shiraishi, Y. Sugano, S. Ichikawa, S. Tanaka and T. Hirai, J. Am. Chem. Soc., 2012, 134, 6309. 9 Y. Zhang, J. Chen and X. Li, Catal. Lett., 2010, 139, 129. 10 Z. Liu, X. Zhang, S. Nishimoto, M. Jin, D. A. Tryk, T. Murakami and A. Fujishima, Langmuir, 2007, 23, 10916. 11 G. Liu, Z. G. Chen, C. L. Dong, Y. N. Zhao, F. Li, G. Q. Lu and H.-M. Cheng, J. Phys. Chem. B, 2006, 110, 20823. 12 J. Zhang, Q. Xu, Z. Feng, M. Li and C. Li, Angew. Chem., Int. Ed., 2008, 47, 1766. 13 X. Wang, S. O. Pehkonen, J. R¨ am¨ o, M. V¨ a¨ an¨ anen, J. G. Higheld and K. Laasonen, Catal. Sci. Technol., 2012, 2, 784. 14 B. Ohtani, O. O. Prieto-Mahaney, D. Li and R. Abe, J. Photochem. Photobiol., A, 2010, 216, 179. 15 C. Deiana, E. Fios, S. Coluccia and G. Martra, J. Phys. Chem. C, 2010, 114, 21531. 16 G. Tian, H. Fu, L. Jing, B. Xin and K. Pan, J. Phys. Chem. C, 2008, 112, 3083. 17 S.-J. Kim, E. G. Lee, S. D. Park, C. J. Jeon, Y. H. Cho, C. K. Rhee and W. W. Kim, J. Sol-Gel Sci. Technol., 2001, 22, 63.
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Nanoscale 18 J. Sun, L. Gao and Q. Zhang, J. Am. Ceram. Soc., 2003, 86, 1677. 19 J.-B. Joo, Q. Zhang, M. Dahl, I. Lee, J. Goebl, F. Zaera and Y. Yin, Energy Environ. Sci., 2012, 5, 6321. 20 G. Zhang, L. Yu, H. B. Wu, H. E. Hoster and X. W. Lou, Adv. Mater., 2012, 24, 4609. 21 J. Liu, H. Xia, D. Xue and L. Lu, J. Am. Chem. Soc., 2009, 131, 12086. 22 L. Zhou, D. Y. Zhao and X. W. Lou, Adv. Mater., 2012, 24, 745. 23 X. Lai, J. Li, B. A. Korgel, Z. Dong, Z. Li, F. Su, J. Du and D. Wang, Angew. Chem., Int. Ed., 2011, 50, 2738. 24 L. Han, R. Liu, C. Li, H. Li, C. Li, G. Zhang and J. Yao, J. Mater. Chem., 2012, 22, 17079. 25 X. Wu, G. Q. Lu and L. Wang, Energy Environ. Sci., 2011, 4, 3565. 26 H. Li, Z. Bian, J. Zhu, D. Zhang, G. Li, Y. Huo, H. Li and Y. Lu, J. Am. Chem. Soc., 2007, 129, 8406. 27 J. Qian, P. Liu, Y. Xiao, Y. Jiang, Y. Cao, X. Ai and H. Yang, Adv. Mater., 2009, 21, 3663. 28 Z. Dong, X. Lai, J. E. Halpert, N. Yang, L. Yi, J. Zhai, D. Wang, Z. Tang and L. Jiang, Adv. Mater., 2012, 24, 1046. 29 G. Liu, X. Yan, Z. Chen, X. Wang, L. Wang, G. Q. Lu and H.-M. Cheng, J. Mater. Chem., 2009, 19, 6590. 30 S. Sreekantan, R. Hazan and Z. Lockman, Thin Solid Films, 2009, 518, 16. 31 J. B. Joo, Q. Zhang, I. Lee, M. Dahl, F. Zarea and Y. Yin, Adv. Funct. Mater., 2012, 22, 166. 32 Y.-M. Sung, J.-K. Lee and W.-S. Chae, Cryst. Growth Des., 2006, 6, 805.
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Synthesis of the double-shell anatase–rutile TiO2 hollow spheres with enhanced photocatalytic activity† Shunxing Li,*ab Jie Chen,a Fengying Zheng,ab Yancai Liab and Fuying Huanga
Received 3rd August 2013 Accepted 2nd October 2013 DOI: 10.1039/c3nr04043g www.rsc.org/nanoscale
A novel double-shell TiO2 hollow sphere with an inner anatase shell and an outer rutile shell was synthesized by a simple sol–gel method and silica protected calcination process. The structure and formation mechanism was proposed based on characterization using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The double-shell spheres have a uniform diameter of 360 nm and a typical yolk–shell structure. Moreover, the double-shell TiO2 hollow spheres possess a large specific surface area (169 m2 g1). Due to the high surface area, multiple light reflection and beneficial electron conduction between the inner anatase and outer rutile shell of this special structure, the as-prepared double-shell TiO2 catalysts show remarkably enhanced photoactivity compared to the commercial P25 catalyst. In particular, rhodamine B molecules can be completely decomposed in the presence of the double-shell spheres after 60 minutes of irradiation with UV light. In addition, the high activity is retained after five cycles, indicating the stability and reusability of the double-shell catalyst.
1.
Introduction
Titanium dioxide (TiO2) is one of the most important semiconductors because of its low cost, biocompatibility and photoreactivity, and has been utilized in many applications such as photocatalysis, environmental remediation and dye-sensitized solar cells (DSSCs).1–5 Its wide band gap energy (3.0–3.2 eV) allows the absorption of UV light, generating electrons (e) and holes (h+) which can subsequently induce redox reactions. Since Honda and Fujishima rst achieved hydrogen production from water by photocatalysis with TiO2 under UV irradiation, this a
Department of Chemistry & Environmental Science, Minnan Normal University, Zhangzhou, China, 363000. E-mail: [email protected]; Fax: +86-5962591395; Tel: +86-596-2591395
b
Fujian Province Key Laboratory of Modern Analytical Science and Separation Technology, Minnan Normal University, Zhangzhou, China, 363000. E-mail: [email protected]; Fax: +86-596-2592563; Tel: +86-596-2592563
† Electronic supplementary information (ESI) available: UV-vis diffuse reection spectra and the summarisation of BET surface and average pore size. See DOI: 10.1039/c3nr04043g
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material has been widely used as a photocatalyst for water splitting and environmental remediation. TiO2 offers a number of advantageous features, including its low cost, relatively high photocatalytic activity, low toxicity, and high chemical stability. Establishing direct relationships between the structure and photocatalytic activity of TiO2 is rarely straightforward due to the intervening effects of various physicochemical properties, such as particle size, surface area, crystallinity, and morphology. However, in general, some effects are thought to be of great signicance for the photocatalytic activity of TiO2. TiO2 has three main polymorphs of crystalline phases: brookite, anatase, and rutile, of which the latter two are photocatalytically active.6,7 In particular, a mixture of anatase and rutile phases demonstrates better photocatalytic activity compared to the single anatase or rutile phase.8–10 For example, Liu et al.11 improved the photocatalytic activity for decomposing methylene blue of iodine doped TiO2 by forming a bicrystalline framework of anatase and rutile. Zhang et al.12 demonstrated an improvement in photocatalytic water splitting ability by forming an anatase–rutile TiO2 junction. The commercial P25 catalyst composed of anatase and rutile is known to show high activity, and is used not only as a benchmark but also as a model for the design of more effective TiO2 composites.13–15 The coupling of anatase and rutile phase TiO2 allows the transfer of electrons excited by ultraviolet light from anatase to rutile TiO2 as a result of the slightly lower conduction band energy of the rutile phase, and thus charge recombination can be suppressed. On the other hand, the surface area is critical to increasing sites for absorbed water and hydroxyl groups, as abundant surface adsorbed water and hydroxyl groups can potentially trap holes to form important oxidative hydroxyl radicals, _OH, and anchor dye molecules for the photodegradation of organic molecules.16–19 It is expected that the formation of TiO2 with a high surface area may be a solution to realize better photoactivity. Double or multishell nanostructures endow catalysts with higher surface areas and more reactive sites, which is benecial for the catalytic performance in sensors and lithium ion batteries.20–24 Importantly, the double or multishell
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Communication nanostructures allow multiple reections of light within the interior cavity, allowing more efficient utilization of the light source and therefore offering an improved catalytic activity for photodegradation and in dye-sensitized solar cells.25–28 Based on the above points, in order to maximize the photoactivity, it is highly desirable to combine the anatase–rutile junction and the unique double or multishell nanostructure. Herein, the double-shell anatase–rutile TiO2 hollow sphere structure was synthesized by a sol–gel and silica-protected calcination method. Unlike previous works in which the shell is a chaotic mix of anatase and rutile phases,29,30 this unique doubleshell sphere is constructed from an inner anatase hollow sphere and an outer rutile hollow sphere. The structure and formation mechanism was proposed based on characterization using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and UV-visible diffuse reection spectroscopy. Furthermore, it is found that the structures possess a high surface area, an anatase–rutile junction, and show multiple reection of UV light. Finally, the photodegradation of rhodamine B as an example clearly demonstrates the substantial role of the special double-shell structure.
2.
Experimental section
Nanoscale washed three times with ethanol, and then redispersed into 20 mL of deionized water to obtain the SiO2@TiO2 colloidal solution. The above SiO2@TiO2 solution was treated with polyvinylpyrrolidone (PVP, Mw ¼ 30 000, 0.2 g) overnight to allow for the adsorption of PVP onto the TiO2 surface, separated from solution by centrifugation, and then re-dispersed in 10 mL ethanol. The solution of SiO2@TiO2 was sequentially mixed with ethanol (13 mL), water (4.3 mL), TEOS (0.86 mL) and aqueous ammonia (28%, 0.62 mL). Aer stirring for 4 h, the resulting SiO2@TiO2@SiO2 particles were centrifuged, washed 3 times with ethanol and dried under vacuum. The second TiO2 coating procedure was the same as the method described above. In a typical synthesis procedure, the SiO2@TiO2@SiO2 samples (0.2 g) were dispersed into a mixture solution of hydroxypropyl cellulose (HPC, 0.1 g), ethanol (55 mL) and de-ionized water (0.6 mL). Aer stirring for 30 min, a mixture of 1 mL of TBOT and 5 mL of ethanol was injected into the above solution using a syringe pump at a rate of 1.5 mL min1. Aer injection, the temperature was increased to 85 C with 900 rpm stirring under reuxing conditions for 2 h. The obtained precipitate was separated by centrifugation and washed three times with ethanol.
2.1. Reagents and chemicals The reagents, including anhydrous ethanol, ammonia (25–28 vol%), NaOH and polyvinylpyrrolidone (PVP) were purchased from Beijing Chemical Works and Shantou Xilong Chemical Industry Incorporated Co., Ltd. Hydroxypropyl cellulose (HPC) was purchased from Alfa Aesar. Tetraethyl orthosilicate (TEOS) was purchased from J&K Chemica, and titanium butoxide (TBOT) was obtained from Aladdin. All the reagents used were analytical grade in purity. 2.2. Synthesis of the SiO2@TiO2@SiO2@TiO2 microspheres ober SiO2 colloidal spheres were prepared by using the classic St¨ method with minor modications. In a typical synthesis operation, two solutions with equal volumes were rapidly mixed to give a total volume of 250 mL: one solution contained ethanol and TEOS, while the other contained ethanol, water, and ammonia. The contents of the water, ammonia, and TEOS solutions were adjusted to 17.0 M, 0.3 M and 0.2 M respectively. The reaction mixture generally turned turbid white as SiO2 particles were formed aer 2–30 min. The reaction was allowed to continue for 6 h at room temperature, with moderate stirring, until completion. The as-synthesized SiO2 spheres were coated with TiO2 according to the procedure reported in the literature.19,31 In a typical synthesis procedure, the SiO2 spheres (0.2 g) were dispersed into a mixture of hydroxypropyl cellulose (HPC, 0.1 g), ethanol (55 mL), and deionized water (0.6 mL). Aer stirring for 30 min, a mixture of 1 mL of TBOT and 5 mL of ethanol was injected into the above solution using a syringe pump at a rate of 1.5 mL min1. Aer injection, the temperature was increased to 85 C with 900 rpm stirring under reuxing conditions for 2 h. The obtained precipitate was separated by centrifugation,
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2.3. Synthesis of the double-shell TiO2 hollow spheres and hollow TiO2 spheres The synthesized SiO2@TiO2@SiO2@TiO2 samples were subjected to calcination crystallization and NaOH etching procedures. In a typical synthesis procedure, the SiO2@TiO2@SiO2@TiO2 samples were calcined in air at 800 C for 2 h, and then the samples were redispersed in 5 mL of NaOH aqueous solution (2.5 M) for 48 h to remove the SiO2 spheres completely. Finally, the obtained products were separated by centrifugation and washed three times with deionized water. As a comparison, hollow TiO2 spheres were synthesized using a similar method but different precursors. SiO2@TiO2 microspheres and SiO2@TiO2@SiO2 microspheres were calcined at a high temperature of 800 C, and aer the NaOH etching process, two types of hollow TiO2 spheres were obtained, labelled as outer hollow spheres and inner hollow spheres, respectively.
2.4. Characterization The size and morphology of the products were characterized by scanning electron microscopy (SEM, Hitachi S-4800), transmission electron microscopy (TEM, FEI Tecnai G2 20ST) and high resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F20) operated at 200 kV with the soware package for automated electron tomography. Powder X-ray diffraction (XRD) patterns were collected on a Rigaku D/max 2500 diffrac˚ The BET (Brutometer with Cu Ka radiation (l ¼ 1.54056 A). nauer–Emmett–Teller) surface area and pore size of the catalysts were measured using a Quadrasorb SI-MP instrument. Prior to each measurement, the sample was heated to 100 C and kept at this temperature for 3 h.
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2.5. Evaluation of photocatalytic activity for the degradation of rhodamine B Photocatalytic activity testing on the degradation of RhB was carried out in a 100 mL beaker containing 50 mL of 1 105 mol L1 RhB and 10 mg of catalyst under 500 rpm stirring at room temperature. Before the photocatalytic reaction was initiated, the solutions were rstly stirred in the dark for 1 h to ensure that rhodamine B was adsorbed to saturation on the surface of the catalysts. A 150 W high pressure mercury lamp was used as the UV light source. The concentration of rhodamine B was measured by UV-vis spectrophotometry (UV-2550) at 15 min intervals during the reaction.
3.
Results and discussion
3.1. Morphology and structure of the double-shell TiO2 spheres As illustrated in Scheme 1, the double-shell TiO2 hollow spheres can be synthesized by the following four main steps: (1) preparation of uniform silica templates; (2) deposition of an inner TiO2 layer on the silica surface to form SiO2@TiO2 spheres through the sol–gel reaction of titanium butoxide (TBOT) with hydroxypropyl cellulose (HPC) as a surfactant; (3) SiO2 coating on the SiO2@TiO2 spheres; (4) second deposition of an outer TiO2 layer on the surface of the SiO2@TiO2@SiO2 spheres; and (5) crystallization of the two TiO2 layers and etching to remove the silica template using an aqueous solution of NaOH. The morphology and structure of the intermediate and nal products were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM). From Fig. 1, the as-prepared SiO2 spheres have a uniform diameter of 148 nm (Fig. 1a). Aer TiO2 coating, the diameter of the SiO2@TiO2 microspheres increases to about 210 nm (Fig. 1b). Adding another SiO2 and TiO2 layer, the diameter of the SiO2@TiO2@SiO2 microspheres and the SiO2@TiO2@SiO2@ TiO2 microspheres further increases to about 300 nm and 375 nm, respectively (Fig. 1c and d). Moreover, the amplied TEM images (inset in Fig. 1b–d) clearly show the two, three and four layered core–shell structure of the intermediate products.
Scheme 1 Scheme of fabrication of the double-shell TiO2 hollow spheres using a sol–gel and the silica-protected calcination process.
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Fig. 1 TEM images of the products in the sol–gel process: (a) SiO2 spheres; (b) SiO2@TiO2 microspheres; (c) SiO2@TiO2@SiO2 microspheres; (d) SiO2@TiO2@SiO2@TiO2 microspheres.
Aer the crystallization and etching process, the doubleshell TiO2 spheres were obtained. From the SEM image, the spheres have a uniform diameter of 360 nm (Fig. 2a). The TEM image (Fig. 2b) clearly shows that the double-shell spheres have a typical yolk–shell structure and that the inner shell can randomly move in the cavity. The diameter of the inner spheres is about 210 nm. The thicknesses of the outer shell and the inner shell are about 30 and 35 nm, respectively. Importantly, the inner shell and the outer shell in the magnied TEM images are separate from each other. The inner shell consists of smaller sized TiO2 nanoparticles with 8 nm diameter, and the outer shell consists of larger sized TiO2 particles with 30 nm diameter. HRTEM images further indicate that the outer shell is highly
Fig. 2 SEM and TEM images of the double-shell TiO2 hollow spheres: (a) SEM image, (b) TEM image, and (c) magnified TEM image of a single double-shell hollow sphere; (d) magnified TEM image of the inner shell from the crushed double-shell hollow sphere, (e) HRTEM image of the outer shell in the white frame shown in (c), and (f) HRTEM image of the inner shell in the white frame shown in (d).
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Communication crystalline, with an interplanar spacing of 0.324 nm corresponding to the (110) planes of rutile TiO2, and the inner shell is highly crystalline with the interplanar spacing of 0.351 nm corresponding to the (101) planes of anatase TiO2. Furthermore, the acentric inner shell can come into contact with the outer shell and form an anatase–rutile junction, and thus electron and hole transfer may occur at the anatase–rutile junction. In brief, the double-shell structure with the anatase inner shell connected to the rutile outer shell would be a highly desirable structure for high-performance photocatalysts.32 Subsequently, X-ray diffraction (XRD) analysis was employed to investigate the crystal phase of the double-shell TiO2 spheres and other TiO2 samples. The typical XRD pattern in Fig. 3 reveals that the crystalline phase of the as-synthesized doubleshell TiO2 spheres is a mixture of the anatase phase (JCPDS no. 89-4921) and the rutile phase (JCPDS no. 21-1276). All the diffraction peaks of the inner hollow spheres and the outer hollow spheres can be indexed as single anatase phase and single rutile phase. It is obvious that the crystal phases of the inner and outer layer in the double-shell hollow sphere are the single anatase and rutile phases aer the high temperature treatment, respectively. The commercial TiO2 nanoparticles and commercial P25 catalyst are presented as a reference to conrm the phase constituents of the TiO2 hollow spheres and the double-shell TiO2 spheres. Moreover, the ratio of rutile phase in the double-shell hollow spheres is larger than that in the commercial P25 catalyst. How can the inner shell and the outer shell form two distinct crystalline phases with high temperature treatment? The possible answer is the adopted “silica-protected calcination” method and the additional silica layer on the inner TiO2 surface during the process.31,32 During heat treatment anatase formation occurs rst, and then the anatase particles start to transform to the rutile phase via the crystal growth mechanism. Due to the protection of the silica layer (the thermal expansion coefficients of anatase, rutile and silica are 10.2, 7.14 and 0.5 106 K1),32 the phase transformations of the inner TiO2 shell from amorphous to anatase and from anatase to rutile are both inhibited, and at the same temperature the outer TiO2 shell without the protection of another silica layer is transformed into the rutile phase. The different conditions the inner shell
Fig. 3 XRD patterns of the double-shell TiO2 hollow spheres and other TiO2 samples (commercial P25, commercial anatase TiO2 and the hollow TiO2 spheres).
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Nanoscale and the outer shell are under result in dissimilar crystalline processes and the different crystal phases. 3.2. BET and UV characterization of the double-shell TiO2 spheres As mentioned above, surface area is a critical factor in photodegradation. A high surface area provides more reaction sites for the hydroxyl groups and the dye molecules. Inspired by these ideas, the nitrogen absorption isotherm was measured to obtain information about the BET surface area and the pore sizes of the double-shell structured TiO2 spheres, as shown in Fig. 4. The surface area is evaluated to be 169 m2 g1 from the data points in the nitrogen adsorption–desorption isotherms. The pore size distribution (inset in Fig. 4) was determined by using the Barrett–Joyner–Halenda method from the desorption branch of the isotherm. The distribution reveals two sharp peaks at around 3.9 nm and 8.5 nm, which are due to the inner shell and the outer shell, respectively. The pores in the two shells ensure efficient access of dye molecules during the photocatalytic process. Table S1† (in the ESI) shows the BET surface areas of the double-shell TiO2 spheres and other TiO2 materials. The BET surface area of the double-shell spheres is much greater than that of commercial anatase TiO2, P25 and the outer hollow spheres, but is lower than that of the inner hollow TiO2 spheres. This result can be attributed to the size effect, as smaller particles possess a larger surface area. Firstly, the inner and outer TiO2 shells consist of 8 nm and 30 nm TiO2 nanoparticles, respectively. The smaller inner nanoparticles should have a larger surface area than the outer particles. Secondly, for the same reason, the hollow sphere with smaller diameter will also have a larger surface area than the bigger hollow spheres. 3.3. Evaluation of photocatalytic activity UV-vis absorption spectra and the photocatalytic performance of the double-shell TiO2 spheres and other samples are shown in the Fig. 5. As indicated in Fig. 5a, the double-shell spheres, the hollow spheres, the commercial anatase TiO2 and the P25 sample demonstrate similar absorption in the range 200–500 nm. The absorption of anatase TiO2 and the inner hollow
Fig. 4 Nitrogen adsorption isotherm of the double-shell TiO2 hollow spheres (inset: the pore size distribution of the inner and outer shells in the double-shell structure), showing a considerably higher surface area and the different pore diameters of the inner shell and the outer shell.
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Nanoscale spheres is around 380 nm, and the addition of rutile TiO2 in the double-shell TiO2 spheres and P25 catalyst causes a blue shi of the absorption edge due to the increase of the amount rutile phase, which has a smaller band gap, 3.0 eV, than that of the anatase phase (3.2 eV). What's more, the diffuse reection spectrum (Fig. S1 in the ESI†) in the UV range of the doubleshell hollow spheres is much lower than that of the inner hollow spheres and nanoparticles, and apparently increases aer crushing treatment (high pressure at 100 MPa), indicating that the larger absorption of the UV light in the double-shell hollow structures is derived from the multiple light reection and scattering between the two spherical shells. Finally, the photocatalytic performance of the double-shell anatase–rutile TiO2 catalyst was evaluated for the photodegradation of rhodamine B dye under UV light. Fig. 5b shows typical adsorption spectra of an aqueous rhodamine B solution aer UV irradiation for various time periods in the presence of the double-shell TiO2 hollow spheres as a catalyst. The strong adsorption peak attributed to rhodamine B at 553 nm gradually decreased and completely disappeared aer 60 min, indicating the complete photodegradation. The performances of the various photocatalysts for rhodamine B degradation, measured from the intensity of the 553 nm peak versus time, are summarized in Fig. 5c. Before UV illumination, the mixtures of the catalysts and rhodamine B were stirred in the dark for 1 h to ensure that the dye was adsorbed to saturation on the inner and outer surfaces. The double-shell spheres and inner hollow spheres have better absorption capacity due to their high surface area. In the blank experiment (without catalyst), only 9% rhodamine B was decomposed aer UV irradiation for 75 min, but the degradation rates were notably enhanced in the presence of the photocatalysts aer a similar amount of time. The relative photocatalytic activities of the catalysts for rhodamine B degradation was in the order: double-shell TiO2 hollow
Communication spheres > inner hollow spheres > P25 > anatase TiO2 nanoparticles. In particular, the rhodamine B molecules could be completely decomposed in the presence of the double-shell spheres aer only 60 min of irradiation with UV light. Consistent with the BET and UV-vis diffuse reection studies, the double-shell spheres with an inner anatase shell and an outer rutile shell demonstrate enhanced photoactivity. In addition, the double-shell structure shows a higher catalytic activity than the hollow spheres, supporting that the crucial effect is the promotion of the rutile outer shell and the multiple reection effect rather than the high surface area. Furthermore, a reutilization experiment was conducted to evaluate the stability of the catalyst. When the photodegradation was complete, the residue was centrifuged and dried at 60 C in the air. Then, the residue was dispersed in a new rhodamine B solution and the photodegradation experiment was performed again. The double-shell hollow catalyst can retain its high activity aer ve cycles (Fig. 5d), while the activity of the commercial P25 catalyst decreased aer each cycle because the lighter P25 catalyst was difficult to completely recycle by the centrifuge process, indicating the favourable stability of the double-shell structure.
4.
Conclusions
In conclusion, a novel double-shell TiO2 structure with an inner anatase shell and an outer rutile shell was synthesized and the structural effect was investigated in detail when it used as the photocatalyst for rhodamine B degradation. The different crystal phase of the inner shell is caused by the silica layer protection, and an anatase–rutile junction may form at the tangent surface. The double-shell spheres with high surface area and the ability to reect and scatter light show enhanced photocatalytic activity for the degradation of rhodamine B. This work may provide new concepts for fabricating high-performance photocatalysts based on structural design, and the development of new synthesis methods.
Acknowledgements This work was supported nancially by the National Natural Science Foundation of China (20977074, S.X.L. and 21175115, S.X.L.), the Program for New Century Excellent Talents in University (NCET-11 0904, S.X.L), the Outstanding Youth Science Foundation of Fujian Province, China (2010J06005, S.X.L.), the Science & Technology Committee of Fujian Province, China (2012Y0065, F.Y.Z.), and the Natural Science Foundation of Fujian Province in China (2012J05031 Y. C. L.).
Fig. 5 (a) UV-vis absorption spectra of the double-shell TiO2 hollow spheres and other TiO2 samples. (b) Absorption spectra of a solution of rhodamine B in the presence of the double-shell TiO2 hollow spheres under exposure to UV light. (c) Evolution of rhodamine B concentration versus UV irradiation time (t) in the presence of the double-shell TiO2 hollow spheres and other TiO2 catalysts. (d) The reusability of the double-shelled TiO2 hollow spheres as a catalyst for rhodamine B photodegradation under the reaction conditions in (c).
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