Controlled Morphology and Crystalline Phase of Poly (ethylene oxide

May 5, 2004 - Yun-Mo Sung* and Jin-Kyung Lee. Functional Nanostructured Materials Laboratory (FNML), Department Material Science &. Engineering ...
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CRYSTAL GROWTH & DESIGN

Controlled Morphology and Crystalline Phase of Poly(ethylene oxide)-TiO2 Nanohybrids Yun-Mo Sung* and Jin-Kyung Lee Functional Nanostructured Materials Laboratory (FNML), Department Material Science & Engineering, Daejin University, Pochun-si, Kyunggi-do 487-711, South Korea Received February 23, 2004;

2004 VOL. 4, NO. 4 737-742

Revised Manuscript Received March 29, 2004

ABSTRACT: Poly(ethylene oxide) (PEO)-titania (TiO2) organic-inorganic hybrid powder having different microstructures was synthesized through a sol-gel process and crystallized by subsequent heat treatments at elevated temperatures. As-precipitated hybrid powder showed two distinct morphologies, one of which is spherical and the other is platelike depending upon aging time. It is presumed that the structural variation was highly related with the formation of hydrogen bonds between a chelating agent (2,4-pentanedione: AcAc) and a polymer component (poly(ethylene oxide): PEO). Also, microspherical particles with nanocrystalline anatase core/rutile shell structure were prepared through heat treatment at 800 °C. The mechanism of core/shell structure formation was elucidated by the shrinkage during anatase-to-rutile phase transformation, the difference in thermal expansion coefficient (TEC) of two crystals, and further decomposition of remaining organic components trapped in the core. In addition, the powder with platelike structure showed retardation in anatase-rutile phase transformation kinetics compared to that with spherical structure. Introduction Titania (TiO2), one of the most famous electronic ceramic materials, has been attracting a great deal of attention because of its versatility in optical, electrical, and photochemical properties, and thus it has found wide applications in high-refractive optics, oxide semiconductors, oxygen sensors, photovoltaics, photocatalysts, etc.1-8 Although the aforementioned unique properties of titania are coming 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.9,10 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. Especially, anatase powders have been highlighted due to their high efficiency in photocatalytic decomposition of harmful organic components. However, anatase crystals are not only apt to transform to the rutile phase, but are also easy to agglomerate to a bulk form at an elevated temperature.11-16 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.17-21 It is also the most effective way to obtain homogeneous organic-inorganic hybrid materials using a soft chemistry. Su et al.22 prepared hollow titania microspheres with anatase phase through solgel processing and they used decaoxyethylene cetyl * Corresponding author. Tel: +82-31-539-1985; fax: +82-31-5391980; e-mail: [email protected].

ether (C16(EO)10) surfactant as a template in the synthesis. Their hollow titania microspheres showed a high surface area and high pore volume with a pore size of 2.6 nm. Titania is almost 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, the morphological features and crystallization characteristics of sol-gel PEO-TiO2 hybrids were successfully controlled by using different aging and heat-treatment conditions. Also, the mechanism of core/ shell structure formation was discussed based upon volume shrinkage, thermal expansion coefficient (TEC) difference, and further decomposition of organic components. The morphological variation in titania was explained using a molecular structure model. The structure-property relationship of PEO-TiO2 hybrid was also discussed in terms of anatase-to-rutile phase transformation characteristics. Experimental Procedures Poly(ethylene oxide) (PEO, Mv 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 ethanol absolute 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. The content of PEO in the hybrids was 25 wt %. 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 or 12 h in N2 atmosphere without stirring. After aging, the yellowish and transparent solution was poured into Teflon

10.1021/cg049926z CCC: $27.50 © 2004 American Chemical Society Published on Web 05/05/2004

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roughness was reduced compared to 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, respectively, showed a white color and the surface was smooth, indicating that the organic component was almost removed. In the case of powder with a spherical shape, the aggregation did not occur during thermal treatment. It has been known that spherical colloidal particles can be formed by sol-gel processing of Ti(OPr)4 with acetylacetone (AcAc).23,24 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 particles, resulting in spherical particles with Ti-O-Ti bonds inside and AcAc ligands outside. The exchange reaction between Ti-isopropoxide and AcAc is presented in eq 1.

Figure 1. Scanning electron microscopy images of PEO-TiO2 hybrids aged at 60 °C for (a) 6 and (b) 12 h. 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, respectively in air atmosphere. Scanning electron microscopy (SEM: Philips XL-39, Eindhoven, Netherland) and X-ray diffraction (XRD: Rigaku Ultima+ D/MAX-2200, Tokyo, Japan) were performed to investigate morphology and crystallinity of each powder. Transmission electron microscopy (TEM: JEOL JEM-2010, Tokyo, Japan) was performed to identify each crystalline phase of the core/shell titania powder.

Results and Discussion The PEO-TiO2 nanohybrid powders obtained through sol-gel processing were spherical or platelike depending upon the aging time period as shown in Figure 1. The powder aged for 6 h shows microspherical feature and rough surface, indicating the formation of PEO-TiO2 organic-inorganic composites. On the other hand, the powder aged for 12 h had bulky and platelike morphology. After heat treatment at elevated temperatures, both powders aged for 6 and 12 h, respectively, showed no remarkable change in the morphological features, which implies that the aging time is playing a major role in determining the final morphology of the powder. It seems that the thermal treatment at 600 °C was not enough to complete the removal of polymer sources because the color of the powder was still light gray and its surface was slightly rough, although its surface

The schematic diagram representing the mechanism of spherical particle formation is illustrated in Figure 2. On the other hand, we believe that during aging for 12 h AcAc located at the outer surface of Ti-O-Ti particles forms hydrogen bonds with PEO, resulting in the formation of a long chain between TiO2 and PEO and thus a platelike shape. The formation of hydrogen bonds between AcAc, forming a complex with titanium, and PEO is illustrated in Figure 3. The hydrogen bonds between AcAc (TiO2) and PEO determine the molecular structure of PEO-TiO2 hybrids. The hydrogen bonds between PEO and AcAC (TiO2) strongly affect the phase formation characteristics of TiO2 crystals as well. Figure 4 shows XRD patterns of PEO-TiO2 hybrids heattreated at different temperatures. The hybrid aged for 6 h shows apparent anatase formation after heat treatment at 600 °C for 30 min, while that aged for 12 h shows only initiation of anatase formation at the same heat treatment condition. Also, the former shows an anatase-to-rutile phase transformation after heat treatment at 800 °C for 30 min, while the latter shows only the formation of anatase. After heat treatment at 900 °C for 30 min, the former shows almost complete transformation of anatase to rutile, while the latter still shows the existence of a considerable amount of anatase. The XRD results imply that PEO-TiO2 hybrids aged for 12 h would form strong hydrogen bonds between AcAc (TiO2) and PEO, and thus their kinetics of anatase-to-rutile phase transformation as well as crystallization of anatase from amorphous hybrids are much slower than those aged for 6 h due to the interruption in the rearrangement of Ti-O-Ti bonds. Both the crystallization and phase transformation are based upon a diffusion process that includes dissociation of ionic bonds and movement of the free ions to lattice points.

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Figure 2. A proposed procedure for the molecular structure formation of spherical PEO-TiO2 hybrid.

Figure 3. A proposed molecular structure of platelike PEO-TiO2 hybrid.

The Ti-ions combined with AcAc and thus PEO are hard to dissociate to free ions, resulting in slow diffusion. It seems that there was not enough time to form strong hydrogen bonds between PEO and AcAc (TiO2) in the hybrids aged for 6 h. The detailed kinetics based on the activation energy for the crystallization and phase transformation of the hybrid thin films is being under investigation, and the results will be reported soon. The suggested molecular structure of PEO-TiO2 hybrids aged for 12 h would be supported by SEM images of those heat treated at an elevated temperature. Figure 5 shows an SEM image of PEO-TiO2 hybrids aged for 12 h and heat treated at 800 °C for 30 min. These hybrids maintain their platelike shape and also show formation of highly porous microstructures, indicating the removal of AcAc and PEO located between Ti-OTi bonds. The PEO-TiO2 hybrids aged for 6 h and heated at 800 °C for 30 min shows the formation of core/shell structured microspherical particles as shown in Figure 6. 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 show 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 calculated using the following equation:11,16

XR )

1 0.8IR 1+ IA

(2)

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 2. Transmission electron microscopy (TEM) was performed to identify crystal structures of the core/shell particles. Figure 7 shows a bright-field 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)

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Figure 6. Scanning electron microscopy image of an anatase core/rutile shell particle aged for 6 h and calcined at 800 °C for 30 min.

Figure 4. X-ray diffraction (XRD) patterns of heat-treated PEO-TiO2 hybrids aged at 60 °C for (a) 6 and (b) 12 h.

Figure 5. Scanning electron microscopy image of PEO-TiO2 hybrid aged for 12 h and calcined at 800 °C for 30 min.

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 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 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 subse-

quent 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 the ∼5-10% 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 an ∼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-torutile phase transformation. The shrinkage of the rutile shell can induce separation from the anatase core. However, the ∼8-10% volume shrinkage is equivalent to only ∼3% in one dimension. The third one would be the decomposition of the 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 the rutile shell and TEC difference between the anatase core and rutile shell. The further shrinkage of the core would come from the decomposition of the remaining organic components trapped inside the core, which finally complete the 5-10% diameter difference between the shell and the core. Since the core is not only nanocrystalline anatase and highly nanoporous, but also is separated from the rutile shell, the surface area of the particles is considerably increased and thus the highly enhanced functionality of the particles for photochemical applications is expected. In detail, high efficiency in the photocatalytic decomposition of organic components is expected, and this is under investigation. Summary PEO-TiO2 organic-inorganic hybrids were synthesized using sol-gel processing. The addition of AcAc into

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Figure 7. Transmission electron microscopy image of an anatase core/rutile shell particle and corresponding selected area electron diffraction (SAED) patterns. The top left-hand side and the bottom right-hand side SAED patterns correspond to anatase core and rutile shell, respectively.

Ti-alkoxide and aging for 6 h derived the formation of spherical shape hybrid powder due to the condensation of Ti-O-Ti bonds and repulsion between AcAc ligands combined to titanium. For 12 h aging the hybrids become a platelike shape most probably due to the formation of hydrogen bonds between AcAc, combined to titanium, and PEO. The heat treatment of hybrids at elevated temperatures brought crystallization of the anatase phase and subsequent transformation to rutile. The PEO-TiO2 hybrids aged for 12 h showed slower crystallization and phase transformation due to the (TiO-Ti)-AcAc-PEO-AcAc-(Ti-O-Ti) bonds. The PEO bonds would hinder the rearrangement of Ti-O-Ti bonds, and thus the crystallization and phase transformation occurring through a diffusion process would be delayed. The spherical hybrid powder heat-treated at 800 °C for 30 min showed formation of the anatase core/ rutile shell structure, which was confirmed using TEM analyses. The volume shrinkage during anatase-torutile phase transformation, the TEC difference between the two phases, and further decomposition of the remaining organic components trapped inside the core would have brought this separated core/shell double structure.

Acknowledgment. The authors would like to thank Professor W-S. Chae in Chemistry Department of Daejin Univeristy for his valuable discussion. Mr. J-Y. Park in Research Facilities Center at Daejin University helped perform SEM works. This study was supported by Korea Science and Engineering Foundation (KOSEF) though a basic science program (R05-2003-000-105030) in 2003. References (1) Vogel, R.; Mreredith, P.; Kartini, I.; Harvey, M.; Riches, J. D.; Bishop, A.; Heckenberg, N.; Trau, M.; RubinszteinDunlop, 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. (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.

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