J. Phys. Chem. B 2006, 110, 4193-4198
4193
Hydrothermal Synthesis of Single-Crystalline Anatase TiO2 Nanorods with Nanotubes as the Precursor Jun-Nan Nian and Hsisheng Teng* Department of Chemical Engineering and Center for Micro/Nano Technology Research, National Cheng Kung UniVersity, Tainan 70101, Taiwan ReceiVed: NoVember 21, 2005; In Final Form: January 11, 2006
Preparation of anatase TiO2 nanorods from solutions in the absence of surfactants or templates has rarely been reported. The present work has found that hydrothermal treatment of titanate nanotube suspensions under an acidic environment resulted in the formation of single-crystalline anatase nanorods with a specific crystal-elongation direction. The nanotube suspensions were prepared by treatment of TiO2 in NaOH, followed by mixing with HNO3 to different pH values. The crystal size of the anatase nanoparticles obtained from the hydrothermal treatment increased with the pH of the suspensions, and nanorods with an aspect ratio up to 6 and a long axis along the anatase [001] were obtained at a pH slightly less than 7. A mechanism for the tube-to-rod transformation has been proposed on the basis of the crystalline structures of the tubes and rods. The local shrinkage of the tube walls to form anatase crystallites and the subsequent oriented attachment of the crystallites have been suggested to be the key steps involved in the nanorod formation.
Introduction Nanosized materials have attracted growing interest as a result of their unique structure and properties.1-4 The shape and size of nanosized materials are determined by the crystallographic planes forming the samples.5 Physical, chemical, and electronic properties of atoms in different crystallographic planes are different. Hence, the shape- or size-controlled synthesis of anisotropic nanocrystals is becoming an important scope in materials chemistry.6 One-dimensional nanostructures, for example, rods, wires, belts, or tubes, exhibit a wide range of electrical and optical properties that are strongly dependent on both shape and size.7-12 To synthesize crystalline materials of these structures from a liquid phase, templates for the confinement of shape evolution13 or surfactants for binding on selective crystalline facets5,6,14 are generally employed to assist the anisotropic growth if the intrinsic surface chemistry of the desired crystals would not favor this type of shape evolution. Nanocrystalline TiO2 has been widely employed in photocatalytic or photoelectrochemical systems because of its high stability and semiconductor characteristics capable of generating charge carriers by absorbing photon energies.15 TiO2 nanoparticles can be synthesized by several methods, among which the wet-chemical synthesis is one of the most common techniques used.6,13-16 Recently, nanotubes derived from the alkali treatment of TiO2 nanoparticles under a highly basic condition have attracted growing interest because of their large surface area exposed to media as well as the unsophisticated synthesis procedure.17 While the crystalline structure is still in controversy,18-20 the nanotubes are considered a potential precursor for one-dimensional TiO2 nanostructures, for example, a nanorod, because of their specific tubular feature. To achieve this, hydrothermal treatment of these nanotubes under a less basic or even an acidic condition is conducted in an attempt to cause bond cleavage and, thus, destruction of the nanotubes. * To whom correspondence should be addressed. E-mail: hteng@ mail.ncku.edu.tw. Tel.: 886-6-2385371. Fax: 886-6-2344496.
Nucleation of the hydrated titanium ions or coalescence of the resulting fragments right from the destruction is expected to give crystalline TiO2 nanoparticles with specific elongation orientation.21-23 The present work intends to elucidate the possible role of the nanotubes as the precursor of TiO2 nanorods. Experimental Section The TiO2 source used for producing the nanotubes was a commercially available TiO2 powder (P25, Degussa AG, Germany), which consists of about 30% rutile and 70% anatase and a primary particle size of about 21 nm. In the alkali treatment for nanotube production, which was analogous to that reported,17 5 g of the P25 was mixed with 70 mL of 10 N NaOH solution, followed by treating the mixture at 130 °C in a Telfonlined autoclave for 20 h. After the treatment, the filtered sample was washed with different amounts of 0.1 N HNO3 solution to give nanotube suspensions of different pH values. The suspensions, with a volume of about 100 mL, were subjected to hydrothermal treatment in the autoclave at 175 °C for 48 h. The resulting slurries were then filtered and dried at 100 °C for 3 h to give the final products, which will be shown to be TiO2 aggregates. The phase identification of the samples at different stages of the preparation was conducted with powder X-ray diffraction (XRD) using a Rigaku RINT2000 diffractometer equipped with Cu KR radiation. The data were collected for scattering angles (2θ) ranging between 5 and 60°, with a step size of 0.01°. The microstructures were explored with a scanning electron microscope (SEM; JEOL JSM-6700F) and a high-resolution transmission electron microscope (HRTEM; Hitachi FE-2000). The pore structure of the nanotube aggregates was characterized by N2 adsorption at -196 °C using an adsorption apparatus (Micromeritics, ASAP 2010). The surface area was determined from the Brunauer-Emmett-Teller equation, and the pore volume was determined from the total amount of N2 adsorbed at relative pressures near unity (ca. 0.96).
10.1021/jp0567321 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/14/2006
4194 J. Phys. Chem. B, Vol. 110, No. 9, 2006
Nian and Teng
Figure 1. pH-value dependence of the powder XRD patterns of TiO2 samples prepared from hydrothermal treatment (at 175 °C for 48 h) of nanotube suspensions with different pH values.
Results and Discussion In nanotube preparation it has been reported that treatment of TiO2 nanoparticles in NaOH resulted in the formation of lamellar sheets as a result of the rupture of Ti-O-Ti bonds.17 Washing the sheets with water and/or acid to remove the electrostatic charge led to the formation of nanotubes, of which the structure was strongly dependent on the pH value of the washing condition.18f It is thus anticipated that the structure of the products obtained from the hydrothermal treatment of the washed nanotube suspensions must be affected by the pH value of the suspensions. Figure 1 shows the XRD patterns of the products obtained from the hydrothermal treatment of the washed nanotube suspensions with different pH values at 175 °C for 48 h. For the samples obtained under an acidic condition (pH < 7), the peak positions are consistent with the standard diffraction pattern of anatase TiO2, with no other crystalline phase observed. A sharpening of the diffraction peaks with the pH value reflects the growth of crystal with decreasing acidity. By further increasing the pH value to reach a basic condition (pH ) 8.2), the sharpening of the anatase peaks was retarded, accompanied by the appearance of brookite peaks. The retarded crystal growth can be related to the surface electrostatic charge, which will be discussed. Thus, to prepare phase-pure anatase nanoparticles, an acidic environment is suggested for this hydrothermal treatment. Figure 2 shows the TEM images of the TiO2 particles obtained from the hydrothermal treatment of the nanotube suspensions under acidic conditions. A variation of the particle shape with the pH value can be seen. It has to be noted that hydrothermal treatment of the P25 TiO2 powder under acidic conditions would not lead to this structure variation. Under a highly acidic condition (pH ) 2.2) for the treatment of the nanotubes, nanoparticles with an average size of about 10 nm were obtained. By increasing the pH value to 4.0, rodlike nanoparticles with a size of about 20 × 60 nm were derived. A further increase of the pH to 5.6 resulted in the formation of uniform nanorods of about 20 × 120 nm. The foregoing XRD analysis has shown a crystal enlargement of anatase with the pH, while the TEM image has reflected the enlargement of the anatase nanoparticles to be anisotropic. In comparison with rutile TiO2, which can be obtained in a rod- or needle-shaped configuration from hydrothermal growth,22
Figure 2. TEM images of TiO2 nanoparticles prepared from hydrothermal treatment (at 175 °C for 48 h) of nanotube suspensions with pH values of (a) 2.2, (b) 4.0, and (c) 5.6.
rod-shaped anatase from direct hydrothermal synthesis (i.e., in the absence of surfactants or templates) was rarely reported. In the progress of crystal growth, anatase TiO2 has been shown to nucleate as truncated octahedral bipyramid seeds, composed of eight equivalent {101} faces and two equivalent {001} faces as the exposed surfaces.6,23 Under equilibrium conditions, the surface possessing the largest free energy exhibits the fastest growth rate. Thus, the exposed area of the high-energy surface would be eliminated first in an ordinary crystal growth, and the crystal elongation normal to the high-energy surface would be limited.5c The {001} faces of anatase have higher surface free energy than the {101}, and this in most cases leads to the formation of bullet- or diamond-shaped nanoparticles with an axis along the [001] directions.6 This specific feature in surface energy difference prohibits the elongation of rod-shaped anatase directing along [001] under circumstances without the presence of binding-selective surfactants or directing templates. A theoretical study on TiO2 morphology (or shape) prediction using a thermodynamic model based on surface energies and tensions also obtained similar results.16d However, the rodshaped anatase with a high aspect ratio was observed in the present system, especially for preparation under a mildly acidic condition (Figure 2c, with a pH of 5.6). Further analysis of the crystalline structure of the anatase nanorods with HRTEM is depicted in Figure 3. The visible lattice fringes, projected along anatase [010], reflect that the nanorod has long and short axes along the [001] and [100] directions, respectively, with the ends faceted with two {101} faces. The inset in Figure 3 shows the
TiO2 Anatase Nanorods from Nanotubes
Figure 3. HRTEM image of a single TiO2 nanorod obtained from hydrothermal treatment (at 175 °C for 48 h) of a nanotube suspension with pH ) 5.6 and the corresponding selected-area electron diffraction pattern (inset).
Figure 4. Variation of the average stacking lengths of (004) and (200) faces and length ratios of (004)/(200) with the pH value for TiO2 nanoparticles prepared from hydrothermal treatment (at 175 °C for 48 h) of nanotube suspensions with different pH values.
selected-area electron diffraction pattern along a [010] zone axis from the nanorod. The pattern demonstrates that the anatase nanorods are single-crystalline. In fact, according to the HRTEM analysis, all the nanoparticles synthesized under an acidic condition appeared to be single-crystalline anatase, with a long axis directing along [001] for those with a rod shape. XRD sizing of the principal faces, {001} and {100}, was performed by using the Debye-Scherrer equation to give a qualitative description on the aspect ratio variation with pH during the nanorod synthesis.24 The (004) and (200) peaks were respectively used to estimate the lengths of the crystal-stacking domain along the long and short axes of the anisotropic particles. In this sizing task, products were obtained from the hydrothermal treatment lasting for 12-48 h. The crystal size was generally stabilized after 24 h of hydrothermal treatment (see Supporting Information). Figure 4 shows the influence of pH value on the crystalline sizes along the [001] and [100] directions after 48 h of hydrothermal treatment. It should be kept in mind that the XRD analysis gives the average thickness of the crystallineface stack, rather than the true lengths of the long and short axes that can be determined from the TEM images. The
J. Phys. Chem. B, Vol. 110, No. 9, 2006 4195
Figure 5. pH-value dependence of the powder XRD pattern of nanotube aggregates prepared from NaOH treatment on P25 TiO2 followed by washing with HNO3 to different pH values. The pattern at the bottom is that of H2Ti2O5‚H2O from the JCPD standards.26
crystalline sizes along both directions are small at a highly acidic condition (pH ) 2.2). They increase with the pH value to reach a maximum at a neutral condition (pH ) 6.9), which is then followed by a drastic decrease with the pH. In the acidic regime (pH < 7) of Figure 4, the crystal enlargement with the pH value is anisotropic; that is, the enlargement in the [001] direction is preferential, and the size ratio of (004)/(200) increases with the pH value to reach a maxium at pH ) 5.6. The process for the formation of anatase nanoparticles in the present work would be somewhat analogous to that for TiO2sol peptization involving the dissolution of amorphous hydrous titania, followed by crystallization.21 The peptization has been shown to be significantly enhanced by the addition of acid or heat. Because the hydrothermal treatment was conducted at a temperature as high as 175 °C in the present work, a rapid nucleation and crystallization of TiO2 would be expected after the nanotube destruction. Under this circumstance, the crystal growth may be governed by kinetics, rather than thermodynamics, thus leading to the formation of the metastable anatase.21 Anatase has been shown to have an isoelectric point (IEP) at pH ) 4.7-6.7.21 At a pH far away from the IEP, there would be strong electrostatic forces between anatase nuclei, thus retarding the aggregation and further crystal growth. This has been observed in the present work for the hydrothermal treatment conducted at pH ) 2.2 or 8.2. On the other hand, at a pH near the IEP, anatase nuclei would tend to aggregate as a result of the attractive van der Waals forces, leading to an enhanced crystal growth.21,25 The results in Figures 1 and 2 support this by showing that larger crystals were obtained from the hydrothermal synthesis with a pH near the IEP. However, the IEP factor did not explain why the enhanced crystal growth had a preferential direction along the [001] of anatase. This preference should relate to certain constraints imposed by the configuration and crystalline structure of the nanotube precursors. Thus, the correlation between the crystalline structures of nanotubes and nanorods could be looked to as offering a clue to the key mechanistic steps in the transformation of nanotubes to anatase TiO2 nanorods. Figure 5 shows the XRD patterns of the nanotubes obtained from hydrothermal treatment in NaOH, followed by washing at different pH values. The crystalline structure of the nanotubes has been determined by some studies to be anatase TiO2,18 which
4196 J. Phys. Chem. B, Vol. 110, No. 9, 2006
Figure 6. SEM images of nanotube aggregates prepared from NaOH treatment on P25 TiO2, followed by washing with HNO3 to pH values of (a) 2.2 and (b) 5.6.
shows no XRD peak at 2θ of about 9-10°. The anatase phase was observed in our previous study for washing, conducted at a pH lower than 2.18f The pattern of the pH 2.2 specimen in Figure 5 is, as a matter of fact, similar to that of anatase TiO2. The other patterns in Figure 5 are different from that of anatase and have been assigned to titanates such as A2Ti3O7,19 lepidocrocite-type species,20b or A2Ti2O5‚H2O,20a,c where A represents Na and/or H. The principal frameworks of the last two were similar, as a matter of fact. By comparing the diffraction peak positions and relative intensities of the present samples with those of H2Ti2O5‚H2O from the Joint Committee on Powder Diffraction (JCPD) Standards (bottom of Figure 5),26 it was determined that layered A2Ti2O5‚H2O titanate should be the structure assigned for nanotube samples obtained from the alkali treatment. The ratio of H/Na in the nanotubes has been suggested to increase with the acidity of the post-treatment washing as a result of the replacement of Na+ in Na2Ti2O5‚H2O by H+.20a The replacement would result in the change of intensity ratio of diffraction peak 110 to peak 310 (indicated by the dash lines in Figure 5). In the present work, auxiliary experiments were conducted to calcine the nanotube samples to 700 °C, and it showed that Na2Ti6O13 is the only Na-containing species detected by XRD for the samples with the calcination (see Supporting Information). The appearance of Na2Ti6O13, which could be derived from the dehydration of NaxTi2-xTi2O5‚H2O, was found to be more abundant for nanotubes washed and suspended under a more basic condition. The Na/Ti atomic ratio of the nanotubes estimated from the energy-dispersive X-ray spectrum also showed consistent results (see Supporting Information). In addition to the Na+/H+ replacement, the arrangement of the nanotubes in their aggregates was dependent on the pH value as well. The SEM images in Figure 6 show that the nanotubes are loosely and randomly tangled under a high acidity (pH ) 2.2), while the tubes are tightly attached in sheaves under a mildly acidic condition (pH ) 5.6). The extent of coagulation of these tubes can be quantitatively analyzed using N2 adsorption (see Supporting Information). The surface area and pore volume of the nanotube aggregates were as high as 360 m2/g and 1.2 cm3/g for the low pH (i.e., 2.2) sample, and the values decreased
Nian and Teng
Figure 7. Structure models: (a) the protonic titanate, H2Ti2O5‚H2O, projected along its [001] direction (with hidden interlayer H+ and OH-); (b) the anatase TiO2 projected along its [101] direction.
to 200 m2/g and 0.42 cm3/g for the sample with a pH of 5.6. This confirms the highly coagulated arrangement of the nanotubes under a mildly acidic condition. The pileup situation of the nanotubes might have eventually affected the anatase crystal enlargement during hydrothermal treatment of the nanotube suspensions. When the crystallographic structure is considered, the A2Ti2O5‚H2O titanate has been suggested to comprise of twodimensional layers in which TiO6 octahedra are combined through edge sharing,20,26b as shown in Figure 7. The lattice is orthorhombic, and the layered structure results in the large elongation of the unit cell along the [100] direction. Exchangeable A+ is situated between the layers, and the interlayer distance may vary with the Na+/H+ exchange.26b To form nanotubes, the individual layers would peel off and scroll along the [001] direction of the A2Ti2O5‚H2O structure with the tube axis directing along the [010] direction.19g,20a The outer and inner diameters of the tubes were found to be ∼10 and ∼6 nm, respectively, while the length of the tubes ranged from several tens to several hundreds of nanometers. Upon hydrothermal treatment of the nanotubes, the structure would proceed with rupture and dissolution, followed by rearrangement of the resulting fragments to form anatase crystals, as shown in the foregoing results. Similar to peptization,21 the size of the anatase crystals has been shown to be dependent on the pH during the hydrothermal treatment. The preferential elongation in the anatase [001], however, can be ascribed to the specific feature of the nanotubes. The zigzag configuration of the edge-shared TiO6 octahedra of the nanotube walls, as shown in Figure 7a with projection along the [001] of the titanate, is similar to the principal unit layer of the anatase TiO2 projected along [101] (Figure 7b). It is hypothesized that, upon hydrothermal treatment, the titanate framework would shrink locally, by reducing the interlayer distance, and transform into the anatase TiO2 structure. Our recent study20e and others19i,j have shown that local shrinkage of the titanate framework on the tube wall to form anatase crystallites is possible. The hypothetical scheme for this transformation is depicted in Figure 8, in which the structure models are obtained from the projection along the nanotube axis, that is, the [010] of the titanate. To form the anatase phase, it shows that the rearrangement of the
TiO2 Anatase Nanorods from Nanotubes
J. Phys. Chem. B, Vol. 110, No. 9, 2006 4197 Summary and Conclusions It has been shown that hydrothermal treatment of titanate nanotubes obtained from NaOH treatment on TiO2 resulted in the formation of single-crystalline anatase nanoparticles. For the hydrothermal treatment conducted under an acidic condition, the crystal size of the anatase nanoparticles increases with the pH value and shows an anisotropic crystal elongation in the [001] direction. Anatase nanorods with an aspect ratio up to 6 were obtained at a mildly acidic condition. On the basis of the comparison of the crystalline structures of the titanate and those of the anatase, the mechanism of the tube-to-rod structure transformation has been hypothesized to involve local shrinkage of the nanotubes by reducing the interlayer distance between the tube walls. The minute anatase crystallites produced from this shrinkage, with subsequent rupture of the nanotubes, would proceed with oriented attachment, leading to the formation of nanorods. Under a mildly acidic condition, the compact pileup feature of the nanotubes, as well as the situation near the IEP of anatase, was suggested to assist the formation of the anatase nanorods.
Figure 8. Hypothetical scheme for the transformation of layered titanate to anatase TiO2. The structure models presented are the projection along the nanotube axis, i.e., the [010] of the titanate. The final TiO2 nanorod product shown is the (010) face of the anatase phase.
TiO6 octahedra in each titanate layer is essential,27 in addition to the framework shrinkage. The final TiO2 product shown in Figure 8 is the (010) face of the anatase phase. With the gradual occurrence of this transformation at different spots of the tubular titanate structure, the nanotubes would eventually rupture and transform into small anatase crystallites, in combination with the dissolution of some tube fragments under this hydrothermal condition. It has been reported that adjacent anatase crystallites would coalesce in a way called “oriented attachment” on the highenergy {001} planes,6,23 thus leading to the formation of anatase crystals elongating along [001]. The arrangement of the octahedra shown in Figure 8c mimics the lattice fringes of the anatase nanorod shown in the HRTEM image of Figure 3. Figure 6b has shown that the nanotube aggregates were compactly attached under a pH of 5.6, which was near the IEP of anatase. This pileup configuration of the nanotubes would assist the oriented attachment of the anatase crystallites to form nanorods. However, to form the nanorods with tapered ends, shown in Figure 3, the coarsening of the particles through the Ostwald ripening mechanism to minimize (001) surfaces might take place as well, in addition to the oriented attachment, during the hydrothermal treatment of the nanotubes. Because of the oriented-attachment mechanism for the elongation, single-crystalline anatase nanorods with axes as long as 120 nm were obtained without the occurrence of the anataseto-rutile transition, which was generally observed when nanocrystalline anatase particles grew to a particular size.16d As to the appearance of the brookite phase for the hydrothermal treatment at high pH values (Figure 1), it has been shown that the brookite-type TiO2 can be produced in the presence of Na+ under hydrothermal conditions.28 In the present work, the Na+ ion was abundant under a high-pH condition, thus leading to brookite formation.
Acknowledgment. This research is supported by the National Science Council of Taiwan (NSC 92-2214-E-006-021), the Center for Micro/Nano Technology Research, National Cheng Kung University, under projects from the Ministry of Education and the National Science Council (NSC 93-212s M-006-006) of Taiwan, and NCU-ITRI Joint Research Project of National Central University (NCU-ITRI 940202). Supporting Information Available: Variation of the stacking lengths of (004) and (200) faces with hydrothermal treatment time for anatase TiO2 nanoparticles prepared from hydrothermal treatment on nanotubes; pH-value dependence of the XRD pattern of the samples obtained by calcining nanotube aggregates of different pH values at 700 °C for 2 h; variation of Na/Ti atomic ratio, surface area, and pore volume with the pH value for nanotube aggregates obtained by washing with HNO3 to different pH values. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Chen, C.-C.; Herhold, A. B.; Johnson, C. S.; Alivisatos, A. P. Science 1997, 276, 398. (b) Alivisatos, A. Science 2000, 289, 736. (c) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (d) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (e) Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711. (2) (a) Landin, L.; Miller, M. S.; Pistol, M.-E.; Pryor, C. E.; Samuelson, L. Science 1998, 280, 262. (b) Kim, Y.-G.; Walker, J.; Samuelson, L. A.; Kumar, J. Nano Lett. 2003, 3, 523. (c) Law, M.; Sirbuly, D. J.; Johnson, J. C.; Goldberger, J.; Saykally, R. J.; Yang, P. Science 2004, 305, 1269. (d) Hochbaum, A. I.; Fan, R.; He, R.; Yang, P. Nano Lett. 2005, 5, 457. (3) (a) Lieber, C. M. Nano Lett. 2002, 2, 81. (b) Huang, Y.; Duan, X.; Cui, Y.; Lieber, C. M. Nano Lett. 2002, 2, 101. (c) Barrelet, C. J.; Wu, Y.; Bell, D. C.; Lieber, C. M. J. Am. Chem. Soc. 2003, 125, 11498. (d) Zhong, Z.; Wang, D.; Cui, Y.; Bockrath, M. W.; Lieber, C. M. Science 2003, 302, 1377. (4) (a) O’Brien, S.; Brus, L.; Murray, C. B. J. Am. Chem. Soc. 2001, 123, 12085. (b) Zhou, Z.; Brus, L.; Friesner, R. Nano Lett. 2003, 3, 163. (c) Rabani, E.; Reichman, D. R.; Geissler, P. L.; Brus, L. E. Nature 2003, 426, 271. (d) Shabaev, A.; Efros, Al. L. Nano Lett. 2004, 4, 1821. (5) (a) Lee, S.-M.; Cho, S.-N.; Cheon, J. AdV. Mater. 2003, 15, 441. (b) Chang, Y.; Zeng, H. C. Cryst. Growth Des. 2004, 4, 273. (c) Siegfried, M. J.; Choi, K.-S. AdV. Mater. 2004, 16, 1743. (6) Jun, Y.-W.; Casula, M. F.; Sim, J.-H.; Kim, S. Y.; Cheon, J.; Alivisatos, A. P. J. Am. Chem. Soc. 2003, 125, 15981. (7) (a) Chang, S.-S.; Shih, C.-W.; Chen, C.-D.; Lai, W.-C.; Wang, C. R. C. Langmuir 1999, 15, 701. (b) Nikoobakht, B.; Wang, Z. L.; El-Sayed,
4198 J. Phys. Chem. B, Vol. 110, No. 9, 2006 M. A. J. Phys. Chem. B 2000, 104, 8635. (c) Kim, F.; Kwan, S.; Akana, J.; Yang, P. J. Am. Chem. Soc. 2001, 123, 4360. (8) (a) Chen, C.-C.; Chao, C.-Y.; Lang, Z.-H. Chem. Mater. 2000, 12, 1516. (b) Yang, C.-S.; Awschalom, D. D.; Stucky, G. D. Chem. Mater. 2002, 14, 1277. (c) Fang, X.-S.; Ye, C.-H.; Zhang, L.-D.; Wang, Y.-H.; Wu, Y.-C. AdV. Funct. Mater. 2005, 15, 63. (9) (a) Fang, X.-S.; Ye, C.-H.; Zhang, L.-D.; Xie, T. AdV. Mater. 2005, 17, 1661. (b) Li, L.-S.; Hu, J.; Yang, W.; Alivisatos, A. P. Nano Lett. 2001, 1, 349. (10) (a) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (b) Gates, B.; Wu, Y.; Yin, Y.; Yang, P.; Xia, Y. J. Am. Chem. Soc. 2001, 123, 11500. (11) (a) Urban, J. J.; Yun, W. S.; Gu, Q.; Park, H. J. Am. Chem. Soc. 2002, 124, 1186. (b) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, 4430. (c) Cao, M.; Hu, C.; Peng, G.; Qi, Y.; Wang, E. J. Am. Chem. Soc. 2003, 125, 4982. (d) Mao, Y.; Wong, S. S. J. Am. Chem. Soc. 2004, 126, 15245. (12) (a) Martin, C. R. Science 1994, 266, 196. (b) Martin, C. R. Acc. Chem. Res. 1995, 28, 61. (c) Limmer, S. J.; Seraji, S.; Wu, Y.; Chou, T. P.; Nguyen, C.; Cao, G. AdV. Funct. Mater. 2002, 12, 59. (d) Liu, S. M.; Gan, L. M.; Liu, L. H.; Zhang, W. D.; Zeng, H. C. Chem. Mater. 2002, 14, 1391. (e) Tai, Y.-L.; Teng, H. Chem. Mater. 2004, 16, 338. (13) (a) Miao, Z.; Xu, D.; Ouyang, J.; Guo, G.; Zhao, X.; Tang, Y. Nano Lett. 2002, 2, 717. (b) Limmer, S. J.; Cao, G. AdV. Mater. 2003, 15, 427. (c) Park, I.-S.; Jang, S.-R.; Hong, J. S.; Vittal, R.; Kim, K.-J. Chem. Mater. 2003, 15, 4633. (d) Yang, H. G.; Zeng, H. C. J. Phys. Chem. B 2004, 108, 819. (e) Weng, C.-C.; Hsu, K.-F.; Wei, K.-H. Chem. Mater. 2004, 16, 4080. (14) Cozzoli, P. D.; Kornowski, A.; Weller, H. J. Am. Chem. Soc. 2003, 125, 14539. (15) (a) Hoffman, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (b) Stone, V. F.; Davis, R. J. Chem. Mater. 1998, 10, 1468. (16) (a) Wang, C.-C.; Ying, J. Y. Chem. Mater. 1999, 11, 3113. (b) Wu, M.; Lin, G.; Chen, D.; Wang, G.; He, D.; Feng, S.; Xu, R. Chem. Mater. 2002, 14, 1974. (c) Lazzeri, M.; Vittadini, A.; Selloni, A. Phys. ReV. B: Condens. Matter 2001, 63, 155409. (d) Barnard, A. S.; Curtiss, L. A. Nano Lett. 2005, 7, 1261. (17) (a) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Langmuir 1998, 14, 3160. (b) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. AdV. Mater. 1999, 11, 1307. (18) (a) Seo, D.-S.; Lee, J.-K.; Kim, H. J. Cryst. Growth 2001, 229, 428. (b) Zhang, Q.; Gao, L.; Sun, J.; Zheng, S. Chem. Lett. 2002, 226. (c) Wang, Y. Q.; Hu, G. Q.; Duan, X. F.; Sun, H. L.; Xue, Q. K. Chem. Phys. Lett. 2002, 365, 427. (d) Yao, B. D.; Chan, Y. F.; Zhang, X. Y.; Zhang, W. F.; Yang, Z. Y.; Wang, N. Appl. Phys. Lett. 2003, 82, 281. (e) Wang, W.; Varghese, O. K.; Paulose, M.; Grimes, C. A.; Wang, Q.; Dickey, E. C. J.
Nian and Teng Mater. Res. 2004, 19, 417. (f) Tsai, C.-C.; Teng, H.; Chem. Mater. 2004, 16, 4352. (g) Saponjic, Z. V.; Dimitrijevic, N. M.; Tiede, D. M.; Goshe, A. J.; Zuo, X.; Chen, L. X.; Barnard, A. S.; Zapol, P.; Curtiss, L.; Rajh, T. AdV. Mater. 2005, 17, 965. (19) (a) Du, G. H.; Chen, Q.; Che, R. C.; Yuan, Z. Y.; Peng, L.-M. Appl. Phys. Lett. 2001, 79, 3702. (b) Chen, Q.; Du, G. H.; Zhang, S.; Peng, L. M. Acta Crystallogr., Sect. B 2002, 58, 587. (c) Chen, Q.; Zhou, W.; Du, G.; Peng, L.-M. AdV. Mater. 2002, 14, 1208. (d) Zhu, Y.; Li, H.; Koltypin, Y.; Hacohen, Y. R.; Gedanken, A. Chem. Commun. 2001, 2616. (e) Sun, X.; Li, Y. Chem.sEur. J. 2003, 9, 2229. (f) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Xu, H. J. Am. Chem. Soc. 2003, 125, 12384. (g) Zhang, S.; Peng, L.-M.; Chen, Q.; Du, G. H.; Dawson, G.; Zhou, W. Z. Phys. ReV. Lett. 2003, 91, 256103. (h) Yuan, Z.-Y.; Su, B.-L. Colloids Surf., A 2004, 241, 173. (i) Zhu, H.; Gao, X.; Lan, Y.; Song, D.; Xi, Y.; Zhao, J. J. Am. Chem. Soc. 2004, 126, 8380. (j) Zhu, H. Y.; Lan, Y.; Gao, X. P.; Ringer, S. P.; Zheng, Z. F.; Song, D. Y.; Zhao, J. C. J. Am. Chem. Soc. 2005, 127, 6730. (k) Thorne, A.; Kruth, A.; Tunstall, D.; Irvine, J. T. S.; Zhou, W. J. Phys. Chem. B 2005, 109, 5439. (L) Zhang, S.; Chen, Q.; Peng, L.-M. Phys. ReV. B: Condens. Matter 2005, 71, 014104. (20) (a) Yang, J.; Jin, Z.; Wang, X.; Li, W.; Zhang, J.; Zhang, S.; Guo, X.; Zhang, Z. Dalton Trans. 2003, 3898. (b) Ma, R.; Bando, Y.; Sasaki, T. Chem. Phys. Lett. 2003, 380, 577. (c) Zhang, M.; Jin, Z.; Zhang, X. G.; Yang, J.; Li, W.; Wang, X.; Zhang, Z. J. Mol. Catal. A: Chem. 2004, 217, 203. (d) Ma, R.; Fukuda, K.; Sasaki, T.; Osada, M.; Bando, Y. J. Phys. Chem. B 2005, 109, 6210. (e) Tsai, C.-C.; Teng, H. Chem. Mater. 2006, 18, 367. (21) Bischoff, B. L.; Anderson, M. A. Chem. Mater. 1995, 7, 1772. (22) (a) Wu, M.; Lin, G.; Chen, D.; Wang, G.; He, D.; Feng, S.; Xu, R. Chem. Mater. 2002, 14, 1974. (b) Li, Y.; Fan, Y.; Chen, Y. J. Mater. Chem. 2002, 12, 1387. (23) (a) Penn, R. L.; Banfield, J. F. Geochim. Cosmochim. Acta 1999, 63, 1549. (b) Banfield, J. F.; Welch, S. A.; Zhang, H.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751. (24) Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials; Wiley: New York, 1974. (25) Jolivet, J.; Henry, M.; Livage, J.; Bescher, E. Metal Oxide Chemistry and Synthesis; John Wiley: New York, 2000. (26) (a) Powder Diffraction Files of the Joint Committee on Powder Diffraction Standards, Card No. 47-0124, International Center for Diffraction Data. (b) Sugita, M.; Tsuji, M.; Abe, M. Bull. Chem. Soc. Jpn. 1990, 63, 1978. (27) Nishizawa, H.; Aoki, Y. J. Solid State Chem. 1985, 56, 158. (28) (a) Keesmann, I. Z. Anorg. Allg. Chem. 1966, 346, 30. (b) Danchille, F.; Simons, P. Y.; Roy, R. Am. Mineral. 1968, 53, 1929. (c) Watanabe, M. J. Solid State Chem. 1981, 36, 91. (d) Clearfield, A.; Lehto, J. J. Solid State Chem. 1988, 73, 98.
