Shell Structure

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 4 805-808

Communications Controlled Crystallization of Nanoporous and Core/Shell Structure Titania Photocatalyst Particles Yun-Mo Sung,*,† Jin-Kyung Lee,‡ and Won-Seok Chae# Department of Materials Science & Engineering, Korea UniVersity, Anam-dong, Seoul 136-713, South Korea, Department of Materials Science & Engineering, Daejin UniVersity, Pochun-si, Kyunggi-do 487-711, South Korea, and Department of Chemistry, Daejin UniVersity, Pochun-si, Kyunggi-do 487-711, South Korea ReceiVed July 18, 2005; ReVised Manuscript ReceiVed March 8, 2006

ABSTRACT: Poly(ethylene oxide) (PEO)-titania (TiO2) organic-inorganic hybrid particles were synthesized using sol-gel chemistry and crystallized at 600, 800, and 900 °C. Anatase/rutile core/shell-structured titania particles were obtained through partial phase transformation (∼70%) of anatase to rutile at 800 °C. The mechanism of core/shell structure formation was proposed as the shrinkage during anataseto-rutile phase transformation, the difference in the thermal expansion coefficient (TEC) of two crystals, and further decomposition of remaining organic components trapped in the core. The core/shell particles showed enhanced photodecomposition rates compared to spherical shape particles and other commercially available particles most probably due to the increased surface area by the nanoporous and nanocrystalline structure of the anatase core. Titania (TiO2) has been well-known for its versatility in optical, electrical, and photochemical properties, and thus it has been applied to high-refractive optics, oxide semiconductors, oxygen sensors, photovoltaics, photocatalysts, etc.1-10 Although these unique properties of titania come from its own energy band gap characteristics, the physical and chemical properties of final titania products are also highly dependent upon phase composition, microstructure, crystallinity, and chemical composition, which can be modified by preparation techniques, thermal treatments, and metal ion doping.11,12 Three different crystalline forms, brookite (orthorhombic, C21), anatase (tetragonal, C5), and rutile (tetragonal, C4), have been reported in titania, among which anatase shows the highest photocatalytic performances. Anatase powders especially have been spotlighted due to their high efficiency in photocatalytic decomposition of harmful organic components. Also, it has been known that anatase powders with some fraction of the rutile phase such as Degusa P-25 show enhanced photocatalytic properties compared to pure anatase powders due to the electron and hole transfer between the two phases. However, anatase crystals not only are apt to transform to the rutile phase, but also easily agglomerate to a bulk form at an elevated temperature.13-18 Hence, special care must be taken to precisely control crystallinity, phase composition, and morphological features of titania to obtain desired properties of titania crystals. The sol-gel process, offering unique advantages for the preparation of homogeneous metal oxides, would be a method suitable to obtain titania in powder, bulk, and thin film forms.19-23 It is also the most effective way to synthesize homogenious organicinorganic hybrid materials using a soft chemistry. Titania is almost * Corresponding author. Tel: +82-2-3290-3286. Fax: +82-2-928-3584. E-mail: [email protected]. † Korea University. ‡ Department of Materials Science & Engineering, Daejin University. # Department of Chemistry, Daejin University.

amorphous as prepared from a sol-gel process and needs to be thermally treated for crystallization, which determines both the final morphology and the crystal structure of titania. In this study, core/shell structured titania particles were synthesized by controlled heat treatment of PEO-TiO2 hybrid particles. The mechanism of core/shell structure formation was discussed based upon volume shrinkage, thermal expansion coefficient (TEC) differences, and further decomposition of organic components. The photocatalytic properties of spherical titania and core/shell titania particles were compared with those of commercially available titania particles. Poly(ethylene oxide) (PEO, Mn∼100 000, Aldrich Chemical, Milwaukee, WI), Titanium(IV) isopropoxide (Aldrich Chemical, WI), 2,4-pentanedione (AcAc, Aldrich Chemical, WI), and ethanol absolute solution (Merck KGaA, Darmstadt, Germany) were used as precursors for PEO-TiO2 powder synthesis, and Ti-isopropoxide and AcAc were handled in a glovebox under dried N2 atmosphere. PEO was dissolved in absolute ethanol solution by stirring and refluxing at 60 °C for 10 h under N2 gas flow. The mixture of Ti-isopropoxide and AcAc dissolved in ethanol was added into the PEO-ethanol solution followed by stirring and refluxing at 60 °C for 10 h in N2 atmosphere. Hydrochloric acid of 1.5 mol/L, used as a catalyst for hydrolysis and polycondensation of Ti-isopropoxide, was added dropwise into the PEO-Ti-isopropoxide solution under the same atmosphere, and the final solution was vigorously stirred and refluxed at 60 °C for 6 h. The solution was aged at 60 °C for 6 h in N2 atmosphere without stirring. After aging of the sample, the yellowish and transparent solution was poured into Teflon dishes and dried in a drying oven at 60 °C for 2 days. The obtained yellowish powder was thermally treated at 600, 800, and 900 °C for 30 min or 1 h in air atmosphere. Scanning electron microscopy (SEM: Philips XL-30, Eindhoven, Netherland) and X-ray diffraction (XRD: Rigaku Ultima+ D/MAX2200, Tokyo, Japan) were performed to investigate morphology

10.1021/cg050342m CCC: $33.50 © 2006 American Chemical Society Published on Web 03/21/2006

806 Crystal Growth & Design, Vol. 6, No. 4, 2006

Figure 1. Scanning electron microscopy (SEM) images of PEO-TiO2 hybrid powder aged at 60 °C for 6 h.

and crystallinity of each powder. Transmission electron microscopy (TEM: JEOL JEM-2010, Tokyo, Japan) was performed on the core/ shell structured titania powder to identify each crystalline phase. Photocatalytic characteristics of each powder were analyzed using the decomposition of rhodamine-B in a deionized water solution. A mixed ultraviolet (UV) irradiation coming from Xenon and Mercury lamps was applied to the rhodamine-B solution containing each different photocatalytic powder for different time periods. The intensity of red color of rhodamine-B was changed with the photocatalyst powder. The rhodamine-B solution with photocatalytic powders exposed to UV light was brought to a UV-Vis spectrometer and the absorbance of 545 nm was monitored. The absorbance decrease rates corresponding to the decomposition rates of rhodamine-B were obtained and compared for each powder. PEO-TiO2 nanohybrid powders, obtained through sol-gel processing, were found to consist of particles with a broad size range of ∼1-15 µm, and an SEM image of the as-precipitated PEO-TiO2 hybrid particles is shown in Figure 1. The powder shows microspherical features and rough surface, indicating the formation of PEO-TiO2 organic-inorganic hybrids. It has been known that spherical colloidal particles can be formed by sol-gel processing of Ti(OPr)4 with acetylacetone (AcAc).24,25 The most probable mechanism of spherical-shape powder formation would be that as the titanium alkoxide is hydrolyzed and condensed, many Ti-O-Ti bonds form in three dimensions and also titanium alkoxides with ligands, that are repulsive to each other, move to the outside of the particles, resulting in spherical particles with TiO-Ti bonds inside and AcAc ligands outside. Figure 2 shows XRD patterns of PEO-TiO2 hybrid powders heat-treated at different temperatures. The hybrid powder showed apparent anatase formation after heat treatment at 600 °C for 30 min, and also showed anatase-to-rutile phase transformation after heat treatment at 800 °C for 30 min. After heat treatment at 900 °C for 30 min, the hybrids showed almost complete transformation of anatase to rutile. Figure 3 shows the SEM images of PEO-TiO2 hybrid particles heat treated at (a) 600, (b) 800, and (c) 900 °C. The samples heattreated at different temperatures could result in this apparent microstructural difference due to the difference in phase compositions. It seems that thermal treatment at 600 °C was not enough to completely remove the polymer sources because the color of the powder was still light gray and its surface shown in Figure 3a was slightly rough, although its surface roughness was reduced compared to the as-dried powder. This is highly probable since the color of pure anatase powder is known to be white. On the other hand, the powder heated at 800 and 900 °C showed a white color and the surface was rather smooth as shown in Figure 3b,c, indicating that the organic component was almost removed. This result is in good agreement with our thermal gravimetric analysis (TGA) results

Communications

Figure 2. X-ray diffraction (XRD) patterns of titania powder heated at different temperatures.

showing the gradual thermal decomposition of the polymeric component from ∼530 to 880 °C. The PEO-TiO2 hybrids heated at 800 °C for 30 min shows the formation of core/shell structured microspherical particles as shown in Figure 3b. The structural features of core and shell are different; that is, the core is nanocrystalline and nanoporous, while the shell is microcrystalline and dense. The nanoporosity of the core would come from the decomposition of PEO and AcAc, and subsequent gas evolution during heat treatment at 800 °C. XRD patterns of the heat-treated hybrid suggest the formation of anatase and rutile phases. The volume fraction of rutile that is the degree of phase transformation from anatase to rutile (xR) was estimated using the following equation:13,18

XR )

1 0.8IA 1+ IR

(1)

where IA and IR denote integrated intensity values of anatase (101) and rutile (110) peaks, respectively. The volume fraction (xR) of rutile was determined as approximately 0.3 using eq 1. Figure 4 shows a bright-field TEM image of a core/shell particle and selected area electron diffraction (SAED) patterns obtained from core and shell, respectively. The SAED patterns from the shell were determined to correspond to the diffractions from (110), (101), (111), (211), (310), and (301) planes of rutile, while those from the core were determined as (101), (004), (200), (105), (204), (116), and (215) diffractions of anatase. The mechanism of anatase core/rutile shell structure formation can be considered from three perspectives. The first one would be the TEC difference between anatase core and rutile shell. During heat treatment anatase formation occurs first and then the surface of the anatase particle starts to transform to the rutile phase via surface nucleation and crystal growth mechanism. The heat treatment at 800 °C for 30 min induces partial transformation of anatase to rutile, and during subsequent air quenching the anatase core and rutile shell separate from each other due to the TEC difference. Anatase core shrinks more during cooling since TEC values of anatase and rutile are 10.2 and 7.14 × 10-6/K, respectively. However, this TEC difference cannot explain a ∼510% difference between the diameter of the shell inner surface and that of the core, which is observed in the SEM microstructure. Theoretically, the TEC difference can induce only a ∼0.3% difference in one dimension between the two diameters, considering a temperature drop from 800 °C. The second one would be the ∼8-10% volume shrinkage during anatase-to-rutile phase transformation. The shrinkage of rutile shell can induce separation from the anatase core. However, the ∼8-10% volume shrinkage is

Communications

Crystal Growth & Design, Vol. 6, No. 4, 2006 807

Figure 4. Transmission electron microscopy (TEM) image of an anatase core/rutile shell particle and corresponding selected area electron diffraction (SAED) patterns.

Figure 5. Comparison of photodecomposition properties of titania powders.

Figure 3. Scanning electron microscopy (SEM) images of PEO-TiO2 powder heated at (a) 600, (b) 800, and (c) 900 °C.

equivalent to only ∼3% in one dimension. The third one would be the decomposition of remaining organic components such as PEO and AcAc after separation of the shell from the core. The remaining organic components trapped inside the core can be removed after separation, and this would cause both further shrinkage of the core and formation of nanopores, which was confirmed by SEM and TEM analyses. Thus, a two-step mechanism can be proposed for the core/shell particle formation. The core and shell separate from each other during heating and/or cooling by volume shrinkage of rutile shell and TEC differences between the anatase core and rutile

shell. Further shrinkage of the core would come from the decomposition of remaining organic components trapped inside the core, which finally completes the 5-10% diameter difference between the shell and the core. Photocatalytic efficiency of the spherical powder and core/shell powder was examined using the photodecomposition rates of rhodamine-B in a deionized water solution, and the results were compared with those of other titania particles as shown in Figure 5. Spherical particles heated at 600 °C showed a low decomposition rate, while the nanoporous anatase/rutile core/shell particles showed a higher decomposition rate compared to commercially available titania powders. This would happen most probably since the core is not only nanocrystalline anatase and highly nanoporous but also is connected by the rutile shell, and thus electron and hole transfer may occur between the core and the shell. In brief, the nanoporous and nanocrystalline structured anatase core connected by rutile shell would be the highly desirable structure for the high-performance photocatalysts.

808 Crystal Growth & Design, Vol. 6, No. 4, 2006 Spherical shape PEO-TiO2 organic-inorganic hybrid particles were synthesized using sol-gel processing. The addition of AcAc into Ti-alkoxide and aging for 6 h derived the formation of spherical shape hybrid particles due to the condensation of Ti-O-Ti bonds and repulsion between AcAc ligands combined to titanium. The spherical hybrid particles heat-treated at 800 °C for 30 min showed formation of the anatase core/rutile shell structure, which was confirmed using SEM, XRD, and TEM analyses. The volume shrinkage during anatase-to-rutile phase transformation, TEC differences between the two phases, and further decomposition of remaining organic components trapped inside the core would have brought this separated core/shell double structure. The core/shell particles showed highly increased photodecomposition rates compared to other commercially available nanoparticles most possibly due to the increased surface area of the nanoporous and nanocrystalline anatase cores. Acknowledgment. This study was supported by Korea Science and Engineering Foundation (KOSEF) though a basic science program (R05-2003-000-10503-0) in 2003.

References (1) Vogel, R.; Mreredith, P.; Kartini, I.; Harvey, M.; Riches, J. D.; Bishop, A.; Heckenberg, N., Trau, M.; Rubinsztein-Dunlop, H. Chem. Phys. Chem. 2003, 4, 595. (2) Frach, P.; Gloss, D.; Goedicke, K.; Fahland, M.; Gnehr, W. M. Thin Solid Films 2003, 445, 251. (3) Francioso, L.; Presicce, D. S.; Taurino A. M.; Rella, R.; Siciliano, P.; Ficarella, A. Sensor Actuat. B-Chem. 2003, 95, 66. (4) Du, X. Y.; Wang, Y.; Mu, Y. Y.; Gui, L. L. Wang, P.; Tang, Y. Q. Chem. Mater. 2002, 14, 3953. (5) Hansel, H.; Zettl, H.; Krausch, G.; Kisselev, R.; Thelakkat, M.; Schmidt, H. W. AdV. Mater. 2003, 15, 2056.

Communications (6) Kron, G.; Rau, U.; Werner, J. H. J. Phys. Chem. B 2003, 107, 13258. (7) Bosc, F.; Ayral, A.; Albouy, P. A.; Guizard, C. Chem. Mater. 2003, 15, 2463. (8) Nakamura, R.; Imanishi, A.; Murkoshi, K.; Nakato, Y. J. Am. Chem. Soc. 2003, 125, 7443. (9) Sung, Y.-M.; Lee, J.-K. Cryst. Growth Des. 2004, 4, 733. (10) Shin, Y.-K.; Chae, W.-S.; Sung, Y.-M. Electrochem. Commun. 2006, 8, 465. (11) Hirano, M.; Joji, T.; Inagaki, M. J. Am. Ceram. Soc. 2004, 87, 35. (12) Fukuda, K.; Sasaki, T.; Watanabe, M.; Nakai, I.; Inaba, K.; Omote, K. Cryst. Growth Des. 2003, 3, 281. (13) Zhang, H. Z.; Banfield, J. F. Chem. Mater. 2002, 14, 4145. (14) Hu, Y.; Tsai, H. L.; Huang, C. L. Mater. Sci. Eng. A 2003, 344, 209. (15) Arbiol, J.; Cerda, J.; Dezanneau, G.; Cirera, A.; Peiro, F.; Cornet, A.; Morante, J. R. J. Appl. Phys. 2002, 92, 853. (16) Francisco, M. S. P.; Mastelaro, V. R. Chem. Mater. 2002, 14, 2514. (17) Zhang, Y. H.; Reller, A. Mater. Sci. Eng. C 2002, S19, 323. (18) Ting C. C.; Chen, S. Y. J. Mater. Res. 2001, 16, 1712. (19) Sung, Y.-M.; Anilkumar, G. M.; Hwang, S.-J. J. Mater. Res. 2003, 18, 387. (20) Sung, Y.-M. Cryst. Growth Des. 2004, 4, 325. (21) Peiro, A. M.; Peral, J.; Domingo, C.; Domenech, X.; Ayllon, J. A. Chem. Mater. 2001, 13, 2567. (22) Sizgek, E.; Bartlett, J. R.; Brungs, M. P. J. Sol-Gel Sci. Technol. 1998, 13, 1011. (23) Tanaka, Y.; Suganuma, M. J. Sol-Gel Sci. Technol. 2001, 22, 83. (24) Arroyo, R.; Cordoba, G.; Padilla, J.; Lara, V. H. Mater. Lett. 2002, 54, 397. (25) Kotani, Y.; Matsuda, A.; Tatsumisago, M.; Minami, T. J. Sol-Gel Sci. Technol. 2000, 19, 585.

CG050342M