2434
J. Phys. Chem. C 2010, 114, 2434–2439
Fabrication of Tunable Core-Shell Structured TiO2 Mesoporous Microspheres Using Linear Polymer Polyethylene Glycol as Templates Yuming Cui, Lei Liu, Bo Li, Xingfu Zhou,* and Nanping Xu State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing UniVersity of Technology, Nanjing 210009, People’s Republic of China ReceiVed: September 6, 2009; ReVised Manuscript ReceiVed: January 4, 2010
Mesoporous core-shell structured titanium dioxide (TiO2) microspheres were successfully prepared by a facile one-step hydrothermal method using polyethylene glycol (PEG, MW 2000) as the soft template. The products were characterized in light of the morphology and chemical composition using scanning electron microscopy, transmission electron microscopy, and X-ray powder diffraction (XRD) techniques. The XRD patterns show that the direct hydrothermal synthesized product is anatase titanium dioxide. The Brunauer-Emmett-Teller surface area is 113.8 m2/g, and the average pore size is 5.78 nm. In addition, the morphology evolution of core-shell structured titanium dioxide with the different hydrothermal times was also studied. Research shows that the shell morphology and the core size of the core-shell TiO2 spheres can be easily tuned by controlling the hydrothermal time through the Ostwald ripening process. A possible growth mechanism of the mesoporous core-shell structured TiO2 hollow microspheres was also proposed in this paper. Introduction Over the past few decades, titanium dioxide (TiO2) has been widespread used in many domains, for instance, photocatalysis,1,2 electrochemical,3 high-performance hydrogen sensors,4,5 detoxification processes,6 dye-sensitized solar cells,7-9 and so on. This is because of its superior photocatalytic activity, low cost, biological and chemical inertness, easy reclamation, and nontoxicity.10 In recent years, hollow microspheres have attracted increasing scientific attention as a promising candidate because of their many attractive characteristics, such as lower density, high specific surface area, delivering ability, surface permeability, and economical use of materials. This kind of structure has been widely applied in biomedical engineering, drug carrier,11,12 chemical catalysis,13 electrode materials,14 photocatalytic,15,16 energy storage,17 and large bimolecular release.18 In addition, TiO2 hollow microspheres have also been considered as potential candidates for the above-mentioned applications. It was reported that such materials were supposed to exhibit a high photocatalytic activity and more efficient use of the light source because of their hollow core-shell structures with an appropriate inner sphere diameter that allows multiple reflections of UV light within the interior cavity.19 As a result, this promise has motivated research efforts by ever-increasing research groups seeking to develop a variety of approaches to construct hollow micro/nanostructures. Up to now, many different routes have been used to prepare these hollow micro/nanostructures, such as hydrothermal methods,20-22 self-template approaches,23 colloidal template methods,24,25 sacrificial templates,26 templatefree methods,27 and ionic liquid methods.28 Regarding TiO2 hollow nanostructures, several approaches have also been reported for the synthesis of TiO2 hollow nanostructures; for instance, Caruso et al.29 have successfully fabricated TiO2 hollow spheres by a layer-by-layer (LBL) approach, in which the composite organic-inorganic particles were first formed on the * To whom correspondence should be addressed. E-mail: zhouxf@ njut.edu.cn. Tel: +86-25-83587773. Fax: +86-25-83587773.
basis of the LBL technique, and the hybrid particles were calcined to create well-defined TiO2 hollow spheres. Cao et al.20 reported a simple method to synthesize stable TiO2 microspheres with well-defined hollow interiors by a hydrothermal precipitation of TiCl4 in the presence of urea and ammonium sulfate. Recently, Zeng’s group had reported the synthesis of Sn-doped TiO2 nanospheres via the Ostwald ripening process.30 However, it still remains a major challenge to develop facile one-step methods for the preparation of metal oxide micro/nanospheres with hollow structures. The synthesis of hollow spheres often relies on templating approaches, in which hard (e.g., inorganic, metal, and polymer particles) or soft sacrificial templates (e.g., supramolecular assemblies of surfactant and polymer) were used to create a hollow structure. However, the capability of constructing complicated structures, such as core-shell structures, is limited by the availability of a template route.19 We had proved that weak coordination among metallic species and polyethylene glycol (PEG) chain aggregates Mn+-PEG (Mn+ represents metallic species) into globules,31,32 and a series of one-dimensional metal oxide nano building blocks have been successfully assembled into hollow microspheres using these Mn+-PEG globules as soft templates.33-35 Here, in this paper, we develop these Mn+-PEG globule soft template routes further to the fabrication of complicated structures of core-shell TiO2 mesoporous microspheres; the shell morphology and the core size of the core-shell structured TiO2 microspheres can be easily tuned by controlling the hydrothermal time through the Ostwald ripening process. Experimental Section All chemical reagents were of analytical grade and used without further purification. In a typical experimental procedure, 1.7 mL of tetrabutyl titanate and 2.3 mL of HCl and 2.4 mL of glacial acetic acid were dissolved in 30 mL of ethanol to form a clarified solution. A 0.6 g portion of PEG and 0.6 g of urea (CO(NH2)2) were then added. The reaction mixture was vigor-
10.1021/jp908613u 2010 American Chemical Society Published on Web 01/25/2010
Core-Shell TiO2 Microspheres Using PEG as Templates
J. Phys. Chem. C, Vol. 114, No. 6, 2010 2435
Figure 1. (A) SEM overview of as-obtained products. (B) Typical SEM image of an individual TiO2 microsphere. (C) SEM image of a cracked TiO2 microsphere, showing the core-shell structure. (D) XRD patterns of as-synthesized TiO2 microspheres.
Figure 2. (A) TEM overview of a core-shell structured TiO2 microsphere and (B) the corresponding HRTEM image.
ously stirred for 30 min with a magnetic pulsator at room temperature until a transparent solution was obtained. The mixture was transferred into a Teflon-lined stainless steel autoclave (80 mL capacity). The autoclave was heated and held
Figure 3. TG/DSC curve of the TiO2 microspheres with a core-shell structure.
at 180 °C for 12 h and then allowed to cool to ambient temperature naturally. After the reaction, the products of the
Figure 4. Nitrogen adsorption-desorption isotherm and the corresponding Barrett-Joyner-Halenda (BJH) pore-size distribution curve (inset) of the TiO2 microspheres.
2436
J. Phys. Chem. C, Vol. 114, No. 6, 2010
Cui et al.
Figure 5. SEM images of individual perfect and cracked TiO2 microspheres synthesized with different hydrothermal reaction times: (A, B) 12 h, (C, D) 24 h, and (E, F) 48 h.
hydrothermal reaction were collected, washed with distilled water three times, and dried in the air at 80 °C. The partial products were further heated at 550 °C for 4 h. Parallel experiments were carried out to examine various synthetic parameters. X-ray powder diffraction (XRD) measurements were performed on a Bruker-D8Advance X-ray diffractometer, with graphite monochromatized high-intensity Cu KR radiation at 40 kV and 30 mA. Scanning electron microscopy (SEM) pictures were recorded on a FEI Quanta-200 instrument. The high-resolution transmission electron microscopy (HRTEM) observations and the selected area electron diffraction (SAED) were performed on a JEOL JEM-2010UHR instrument at an acceleration voltage of 200 kV. Thermal analysis (TG/DSC, STA409, Netzsch, Germany) was performed at a heating rate of10K/minunderanoxygenatmosphere.Brunauer-Emmett-Teller (BET, Omnisorp100cx, Coulter, U.S.A.) was measured to gain insight into the porous structure and distribution of the samples. Results and Discussion The morphology and structure of the product were examined by a scanning electron microscope (SEM) and a high-resolution transmission electron microscope (HRTEM). Figure 1A shows a panoramic SEM image of the as-prepared core-shell struc-
tured TiO2 microspheres. The sample mainly contains uniform spheres with diameters of about ∼1.2 µm. A high-magnification SEM image reveals that the surface of the TiO2 microspheres is rough and porous; even some particles can be found on the surface of the TiO2 microspheres (Figure 1B). The unique core-shell structure can be seen more clearly from an SEM image of a cracked TiO2 microsphere (Figure 1C); the shell thickness is about ∼50 nm, and diameter of the core is about ∼1 µm. The shell and core are rough and porous from the observation of SEM; the porous core may be greatly contributed to their high surface area measured as ∼113.8 m2/g in the following section. Figure 1D shows the X-ray diffraction (XRD) pattern of the as-synthesized titanium dioxide hollow microspheres. The XRD pattern indicates that the anatase phase of titania was obtained. The strong and sharp diffraction peaks indicate the good crystallinity of the as-synthesized products. The morphology and structure of the as-prepared core-shell structured TiO2 microsphere architectures are further characterized by HRTEM. As shown in Figure 2, TEM images of the as-synthesized products clearly show that the sample consists of TiO2 spheres in the micrometer size range (Figure 2A). The dark center and pale edge of the microspheres suggest the core-shell nature of the sample. The shell thickness of the spheres is about ∼50 nm, which is identical to the SEM
Core-Shell TiO2 Microspheres Using PEG as Templates
J. Phys. Chem. C, Vol. 114, No. 6, 2010 2437
Figure 6. Schematic representation of the formation of core-shell structured titanium dioxide hollow spheres. Below is the morphology evolution of core-shell TiO2 microspheres with the hydrothermal time.
observation. The HRTEM (Figure 2B) observations reveal that the lattice fringes correspond to the anatase phase, and the interplanar distance calculated from the lattice fringes of the HRTEM is 0.35 nm, which corresponds to the d-spacing value of (101) crystal planes. The clear lattice image indicates the single-crystalline nature of the ∼10 nm nanoparticle building blocks of titanium oxide microspheres. Thermal properties of the as-synthesized TiO2 hollow spheres were evaluated by thermogravimetic analysis and differential scanning calorimeter (TG/DSC). Test conditions were started from 313 to 1273 K at a rate of 10 K/min in a flowing oxygen environment. The TG/DSC curves of the TiO2 microspheres are shown in Figure 3. It can be seen that the DSC plot presents an exothermic valley at 210 °C in the DSC curve, which corresponds to the thermal decomposition of PEG at 500 K.36 The weight loss occurs in a broad temperature range from 313 to 650 K due to the evaporation of absorbed water and the decomposition of PEG chains; the total weight loss is about 7%. This result shows that there are some amounts of PEG molecules existing in the core-shell structured TiO2 microspheres that cannot be completely removed by washing. PEG molecules interact with the reactants and form the composite globules,31,32 which were used as the soft templates for the formation of core-shell structured TiO2 microspheres. It is deduced that PEG molecules will be blocked in the inner of TiO2 microspheres in view of the following proposed formation mechanism. To further research into the porous structure and pore-size distribution of the as-obtained products, nitrogen adsorptiondesorption isotherms are measured to ascertain the specific surface area and pore size of the TiO2 microspheres. The corresponding results are presented in Figure 4. The nitrogen adsorption-desorption isotherm of TiO2 samples can be clas-
sified as types IV. In a low relative pressure range (below ∼0.4), the isotherm exhibits a linear absorption. However, in the high relative pressure range of 0.4-0.8 P/P0, the curve exhibits a hysteresis loop. The hysteresis observed in these isotherms is indicative of the existence of abundant mesoporous structures in the architectures, according to IUPAC classification.37 The plot of the pore-diameter distribution was determined by using the Barrett-Joyner-Halenda (BJH) method from the desorption branch of the isotherm. We find that the size of the mesopores is uniform; this uniform pore size is further confirmed by its corresponding pore-size distribution (inset in Figure 4). Figure 4 shows that TiO2 microspheres contain small mesopores (peak pore ) ca. 5.78 nm) and the BET surface area of the TiO2 microspheres is 113.8 m2/g. It is reasonable to conclude that the hydrothermal time has a great influence on the morphology of the as-synthesized titanium dioxide microspheres. Interestingly, the tunable core size and surface morphology can be realized by controlling the hydrothermal reaction time through the well-known Ostwald ripening process. Figure 5 shows a set of SEM images of TiO2 microspheres with different hydrothermal times (12, 24, and 48 h). With a short hydrothermal reaction time (12 h, Figure 5A,B), the crystallite nanoparticles of TiO2 microspheres are smaller and give a smooth surface morphology. The crystallite nanoparticles grow bigger and bring a rougher surface morphology with the increasing hydrothermal time. Moreover, the core becomes smaller while the shell gets thicker with the increasing hydrothermal time. On the basis of these images, it is speculated that the products undergo an Ostwald ripening process when the hydrothermal time is prolonged. The spherical particles are much more pronounced after 24 h of hydrothermal treatment (Figure 5C,D). With increasing the hydrothermal time to 48 h, one can obviously notice that a dramatic morphology change
2438
J. Phys. Chem. C, Vol. 114, No. 6, 2010
occurred with the transition from solid primary spherical particles to well-faceted TiO2 particles (Figure 5E,F). This is a sophisticated process of the Ostwald ripening mechanism;22,30 during this process, the inner titania crystallites of the core, which have a higher surface energy and a smaller diameter, would dissolve and transfer to the outer space in the oxide shells and redeposit and recrystallize on the better crystallized TiO2 nanoparticles of the shell. The shell morphology and the core size of TiO2 spheres can be easily tuned through the Ostwald ripening process by controlling the hydrothermal time. We believe, therefore, that the prepared tunable core-shell structured hollow spheres can be used for a wide range of applications for dye-sensitized solar cells, drug and gene delivery, and photocatalytic materials. Summing up the above results and discussions, a most plausible formation mechanism of the core-shell structured mesoporous TiO2 hollow spheres is proposed and schematically illustrated in Figure 6. It is well-known that the oxygen atom of PEG can interact with the hydrogen atom of urea by hydrogen bonding; acid can stabilize Ti(OBu)4 and form the precursor of Ti(OBu)nL4-n (L ) Cl,CH3COO), which controls the hydrolysis process of Ti(OBu)4.38 The weak coordination among metallic species and PEG chains had been proved to aggregate Mn+-PEG into globules.31,32 In our present study, the aggregated acid-stabilized Ti(OBu)nL4-n-PEG-urea globules are formed by the weak coordination interaction and the hydrogen bonding. The Ti(OBu)nL4-n-PEG-urea globules act as the soft templates for the formation of the core-shell structured TiO2 hollow microspheres. At the beginning of the hydrothermal reaction, the instantaneously existing temperature gradient along the outer surface to the inner of Ti(OBu)nL4-n-PEG-urea globules makes the formation of TiO2 nanoparticles layers on the exterior of the globules. Urea is used as a dual-role agent,39 not only as a slow-released pH-adjusting agent but also as a provider of CO2 gas bubbles, according to the reaction equation CO(NH2)2 + 3H2O f 2NH3 · H2O + CO2. The hydrothermal decomposition of the urea molecules brings tiny CO2 gas bubbles in the globules, and the tiny gas bubbles are confined in the globule by the surface-covered TiO2 nanoparticles; the bubble layer inside the Ti(OBu)4-PEG-urea globules could be realized through the interconnection of the tiny gas bubbles. The core-shell structured TiO2 microspheres were formed by the reaction of acid-stabilized Ti(OBu)nL4-n with NH3 · H2O released from the hydrothermal decomposition of urea in the confined interior of the microsphere during the hydrothermal process. Studies have clarified that Ostwald ripening will occur during the hydrothermal process; that is, the smaller, less crystalline or less dense particles in a colloidal aggregate will be dissolved gradually, while the larger, better crystallized or denser particles in the same aggregate are growing.30,40 The better crystallized TiO2 nanoparticles on the surface of the microspheres grow larger with the sacrificial dissolution of the less crystalline and smaller TiO2 nanoparticles of the core. The less crystalline TiO2 nanoparticles of the core dissolve and redeposit and recrystallize on the better crystallized TiO2 nanoparticles of the shell; this process is somewhat similar to the reported dissolution and redeposition mechanism.41,42 Eventually, the core becomes smaller and the shell gets thicker and rougher, and the solid primary spherical particles on the microsphere shell transfer into well-faceted TiO2 particles (inset of Figure 6) with the increase of the hydrothermal time, which is identical to the experimental results.
Cui et al. Conclusions and Outlook In summary, a facile one-step hydrothermal synthesis method has been developed to prepare complicated structures of mesoporous core-shell anatase TiO2 microspheres by using PEG as the soft template. The Brunauer-Emmett-Teller surface area is 113.8 m2/g, and the average pore size is 5.78 nm. The shell morphology and the core size of the core-shell TiO2 spheres can be easily tuned by controlling the hydrothermal time through the Ostwald ripening process. This method is simple and might be extended to the preparation of other novel supermicro/nanostructures and their functionalized derivatives. We believe, therefore, that the prepared hollow spheres can be used for a wide range of applications, for instance, dye-sensitized solar cells, drug and gene delivery, and photocatalytic materials, because of their large specific surface areas and tunable core-shell structure. Acknowledgment. This research was financially supported by the National Basic Research Program (2009CB623406), the National Natural Science Foundation of China (No. 20636020), the Natural Science Foundation of Jiangsu Province (No. 08KJB150009), the Science & Technology Pillar Program of Jiangsu Province (No. BE2009679), and the Financial Foundation of State Key Laboratory of Materials-Oriented Chemical Engineering. References and Notes (1) Kisch, H.; Sakthivel, S.; Janczarek, M.; Mitoraj, D. J. Phys. Chem. C 2007, 111, 11445–11449. (2) He, Z. Q.; Xu, X.; Song, S.; Xie, L.; Tu, J. J.; Chen, J. M.; Yan, B. J. Phys. Chem. C 2008, 112, 16431–16437. (3) Tian, M.; Wu, G. S.; Adams, B.; Wen, J. L.; Chen, A. C. J. Phys. Chem. C 2008, 112, 825–831. (4) Varghese, O. K.; Gong, D.; Paulose, M.; Ong, K. G.; Dickey, E. C.; Grimes, C. A. AdV. Mater. 2003, 15, 624–627. (5) Varghese, O. K.; Gong, D. W.; Paulose, M.; Ong, K. G.; Grimes, C. A. Sens. Actuators, B 2003, 93, 338–344. (6) Zhang, Z. H.; Yuan, Y.; Shi, G. Y.; Fang, Y. J.; Liang, L. H.; Ding, H. C.; Jin, L. T. EnViron. Sci. Technol. 2007, 41, 6259–6263. (7) Greene, L. E.; Law, M.; Yuhas, B. D.; Yang, P. D. J. Phys. Chem. C 2007, 111, 18451–18456. (8) Kim, D.; Ghicov, A.; Albu, S. P.; Schmuki, P. J. Am. Chem. Soc. 2008, 130, 16454–16455. (9) Kang, T.-S.; Smith, A. P.; Taylor, B. E.; Durstock, M. F. Nano Lett. 2009, 9, 601–606. (10) Zhang, J.; Xu, Q.; Feng, Z. C.; Li, M. J.; Li, C. Angew. Chem., Int. Ed. 2008, 47, 1766–1769. (11) Wei, W.; Ma, G. H.; Hu, G.; Yu, D.; Mcleish, T.; Su, Z. G.; Shen, Z. Y. J. Am. Chem. Soc. 2008, 130, 15808–15810. (12) Zhu, Y. F.; Shi, J. L.; Shen, W. H.; Dong, X. P.; Feng, J. W.; Ruan, M. L.; Li, Y. S. Angew. Chem. 2005, 117, 5213–5217. (13) Ni, Y. H.; Tao, A. L.; Hu, G. Z.; Cao, X. F.; Wei, X. W.; Yang, Z. S. Nanotechnology 2006, 17, 5013–5018. (14) Gao, S. Y.; Yang, S. X.; Shu, J.; Zhang, S. X.; Li, Z. D.; Jiang, K. J. Phys. Chem. C 2008, 112, 19324–19328. (15) Xuan, S. H.; Jiang, W. Q.; Gong, X. L.; Hu, Y.; Chen, Z. Y. J. Phys. Chem. C 2009, 113, 553–558. (16) Subramanian, V.; Wolf, E. E.; Kamat, P. V. J. Am. Chem. Soc. 2004, 126, 4943–4950. (17) Huang, Z. B.; Tang, F. Q. J. Colloid Interface Sci. 2005, 281, 432– 436. (18) Li, Y. S.; Shi, J. L.; Hua, Z. L.; Chen, H. R.; Ruan, M. L.; Yan, D. S. Nano Lett. 2003, 3, 609–612. (19) Li, H. X.; Bian, Z. F.; Zhu, J.; Zhang, D. Q.; Li, G. S.; Huo, Y. N.; Li, H.; Lu, Y. F. J. Am. Chem. Soc. 2007, 129, 8406–8407. (20) Guo, C. W.; Cao, Y.; Xie, S. H.; Dai, W. L.; Fan, K. N. Chem. Commun. 2003, 700–701. (21) Wang, W. S.; Zhen, L.; Xu, C. Y.; Yang, L.; Shao, W. Z. J. Phys. Chem. C 2008, 112, 19390–19398. (22) Lou, X. W.; Yuan, C. L.; Archer, L. A. Small 2007, 3, 261–265. (23) Hu, Y. X.; Ge, J. P.; Sun, Y. G.; Zhang, T. R.; Yin, Y. D. Nano Lett. 2007, 7, 1832–1836. (24) Wu, D. Z.; Ge, X. W.; Zhang, Z. C.; Wang, M. Z.; Zhang, S. L. Langmuir 2004, 20, 5192–5195.
Core-Shell TiO2 Microspheres Using PEG as Templates (25) Li, G. Q.; Liu, C. Y.; Liu, Y. J. Am. Chem. Soc. 2007, 90, 2667– 2669. (26) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 16744–16746. (27) Raveendran, P.; Eswaramoorthy, M.; Bindu, U.; Chatterjee, M.; Hakuta, Y.; Kawanami, H.; Mizukami, F. J. Phys. Chem. C 2008, 112, 20007–20011. (28) Nakashima, T.; Kimizuka, N. J. Am. Chem. Soc. 2003, 125, 6386– 6387. (29) Caruso, R. A.; Susha, A.; Caruso, F. Chem. Mater. 2001, 13, 400– 409. (30) Yang, H. G.; Zeng, H. C. J. Phys. Chem. B 2004, 108, 3492–3495. (31) Zhou, X. F.; Chen, S. Y.; Zhang, D. Y.; Guo, X. F.; Ding, W. P.; Chen, Y. Langmuir 2006, 22, 1383–1387. (32) Zhou, X. F.; Zhang, D. Y.; Zhu, Y.; Shen, Y. Q.; Guo, X. F.; Ding, W. P. J. Phys. Chem. B 2006, 110, 25734–25739. (33) Zhou, X. F.; Hu, Z. L.; Chen, Y.; Shang, H. Y. Mater. Res. Bull. 2008, 43, 2790–2798.
J. Phys. Chem. C, Vol. 114, No. 6, 2010 2439 (34) Zhou, X. F.; Hu, Z. L.; Fan, Y. Q.; Ding, W. P.; Xu, N. P. Mater. Chem. Phys. 2008, 112, 592–595. (35) Zhou, X. F.; Guo, X. F.; Ding, W. P.; Chen, Y. Appl. Surf. Sci. 2008, 255, 3371–3374. (36) Yu, S. H.; Huang, H. T.; Zhou, L. M. Chem. Mater. 2007, 19, 2718–2720. (37) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A. Pure Appl. Chem. 1985, 57, 603–619. (38) Zhou, X. F.; Chu, D. B.; Lin, C. J.; Tian, Z. Q. Mater. Res. Bull. 2002, 37, 1851–1858. (39) Zhou, X. F.; Hu, Z. L.; Fan, Y. Q.; Chen, S.; Ding, W. P.; Xu, N. P. J. Phys. Chem. C 2008, 112, 11722–11728. (40) Li, J.; Zeng, H. C. J. Am. Chem. Soc. 2007, 129, 15839–15847. (41) Zhou, X. F.; Chen, Y.; Mei, H.; Hu, Z. L.; Fan, Y. Q. Appl. Surf. Sci. 2008, 255, 2803–2807. (42) Masuda, Y.; Kato, K. Cryst. Growth. Des. 2008, 8, 275–279.
JP908613U
