J. Phys. Chem. C 2007, 111, 5325-5330
5325
Orientated Attachment Assisted Self-Assembly of Sb2O3 Nanorods and Nanowires: End-to-End versus Side-by-Side Zhengtao Deng,†,‡ Dong Chen,† Fangqiong Tang,*,† Xianwei Meng,† Jun Ren,† and Lin Zhang† Laboratory of Controllable Preparation and Application of Nanomaterials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, and Graduate UniVersity of Chinese Academy of Sciences, China ReceiVed: December 12, 2006; In Final Form: February 16, 2007
A facile template-free aqueous solution strategy of oriented attachment assisted self-assembly of Sb2O3 nanorods and nanowires into well-defined complex nanostructures is described in this paper. It is demonstrated that at the temperature of 40 °C, the Sb2O3 nanorods with different lengths (2∼50 µm) and diameters (20∼1000 nm) would axially self-assemble into multisegmented coaxial nanowires by an oriented attachment assisted end-to-end self-assembly process. At the temperature of 80 °C, the Sb2O3 nanowires with diameter of 10∼30 nm would radially self-assemble into nanobelts with hundreds of micrometers in length, typically 400∼1000 nm in width and 20∼60 nm in thickness by an oriented attachment assisted side-by-side self-assembly process. The obtained self-assembled nanostructures were analyzed by a series of methods such as XRD, SEM, EDX, TEM, SAED, HRTEM, and photoluminescence. The formation mechanism of the obtained nanostructures was discussed. The oriented attachment assisted self-assembly strategy we presented here may open a new avenue for the controllable self-assembly of nanorods and nanowires into unique complex nanostructured materials.
Introduction A fundamental challenge in the development of nanoscience and nanotechnology is the assembly of nanoscale building blocks into functional nanostructured materials.1-5 Self-assembly is the main paradigm to achieve this goal, where previous work focused primarily on zero-dimensional (0D) building blocks, such as nanocrystals, forming ordered monolayers, superlattices, and molecular-like dimers and trimers.6-10 Very recently, Tang et al. demonstrated a template-free solution route to selfassembly of 0D CdTe nanocrystals into two-dimensional (2D) free-floating sheets.11 It is generally believed that because of the spherical symmetry of the 0D building blocks, there are no preferred orientations in such self-assembled nanostructures. For one-dimensional (1D) building blocks, such as nanorods and nanowires, self-assembly is much more appealing because it could lead to a wide range of complex nanostructures for use in electronics, photonics, and optoelectronics.12-15 A few efforts have been focused on the integration of 1D building blocks into two- and three-dimensional (2D and 3D) ordered nanostructures or complex functional architectures, which is essential for the success of bottom-up approaches toward advanced materials and devices.16-25 However, it is still a great challenge for chemists to exploit effective strategies for controllable self-assembly of 1D building blocks into various complex nanostructures. In the past few years, the normal concept for crystal growth, which is typically thought to occur via atom-by-atom addition to an existing nucleus or template, has been challenged by the particle-by-particle growth mechanism.26 As a new approach for the creation of advanced artificial materials, “oriental * To whom correspondence should be addressed. Phone: 86-1082543521. Fax: +86-10-62554670. E-mail:
[email protected]. † Technical Institute of Physics and Chemistry, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences.
attachment” has attracted tremendous attention from researchers in many disciplines.27 In the past few years, a large number of oriented attachment mechanism-based growth of nanoparticles into nanostructures have been reported, such as TiO2,28 FeOOH,29 ZnO,30 CuO,31 and ZnS.32 For example, Pacholski et al. have reported the formation of high-quality single-crystalline ZnO nanorods on the basis of oriented attachment of quasi-spherical ZnO nanoparticles.30 Moreover, Yu et al. have demonstrated that 1D ZnS nanorods could be obtained from 0D ZnS nanocrystals by an oriented attachment process.32 However, previous studies were mainly focused on the oriented attachment of 0D nanoparticles, and only a few examples have been demonstrated for the formation of complex nanostructures through oriented attachment of 1D nanorods and nanowires. Among inorganic semiconducting metal oxides, antimony trioxide (Sb2O3) is particularly important because of its use for fire retardants, fillings, and catalytic agents.33-38 Recently, Sb2O3 microscale wires have been reported to be used as the effective pH electrode with characteristics of long-term stability, fast response, reproducibility, and low cost.39 Several methods for simple Sb2O3 1D building blocks have been reported in the literature such as nanorods, nanowires, nanobelts, and nanotubes.40-46 Similar to the other well-studied functional metal oxides, Sb2O3 is also expected to be an interesting semiconducting material exhibiting unique optical, electronic, and optoelectronic properties. However, less attention has been paid to the optical properties of Sb2O3, especially for their photoluminescence properties, since it is an indirect-gap semiconductor with low emission efficiency. Very recently, the room-temperature ultraviolet band-edge photoluminescence of Sb2O3 nanowires with rectangular cross sections has been reported by our group.46 Further studies are still needed for exploring the optical, electronic, and optoelectronic properties of the various Sb2O3 nanostructures to fully understand the nature of Sb2O3.
10.1021/jp068545o CCC: $37.00 © 2007 American Chemical Society Published on Web 03/22/2007
5326 J. Phys. Chem. C, Vol. 111, No. 14, 2007
Deng et al.
SCHEME 1: Illustration of End-to-End Oriented Attachment Assisted Self-Assembly of Nanorods into Multisegment Coaxial Nanowires and Side-by-Side Oriented Attachment Assisted Self-Assembly of Nanowires into Nanobelts
In this paper, we report a facile template-free strategy of oriented attachment assisted self-assembly of antimony trioxide (Sb2O3) nanorods and nanowires into regular single-crystal nanostructures, that is, multisegmented coaxial nanowires obtained by end-to-end self-assembly of nanorods and nanobelts obtained by side-by-side self-assembly of nanowires. The advantages of the present work are as follows: (1) we report a novel oriented attachment assisted template-free aqueous solution strategy to controllable self-assembly of nanorods and nanowires; the strategy is especially appealing because of the low costs and potential application in large-scale production, and it may open a new avenue for the controllable self-assembly of nanorods and nanowires into unique complexed nanostructured materials; (2) we obtained the single-crystal orthorhombic Sb2O3 multisegmented coaxial nanowires, a novel nanostructure not reported before; (3) we obtained the nanobelts by oriented attachment assisted side-by-side self-assembly of nanowires; this unique formation process of the nanobelts was not reported in the literature and may be of certain generality.
Figure 1. XRD patterns of the as-obtained Sb2O3 multisegmented coaxial nanowires (a) and nanobelts (b).
Experimental Section All of the chemical reagents used in this experiment were analytical grade. The present synthesis processes were according to our previous paper46 but altering the experimental parameters. In a typical experiment to synthesize the multisegmented coaxial nanowires, 120 mg antimony (Sb) powder (99.5%), 200 mg polyvinylpyrrolidone (PVP, MW30000), and 3 mmol ethylenediamine (EA) were added into 78 mL deionized water (DIW). Then, the mixed solution was stirred at 40 °C for 5 h. After the reaction, the resulting white solid product was filtered and washed with DIW and ethanol to remove residual ions in the product. The nanobelts were prepared following the same procedure described above for multisegmented coaxial nanowires, except the temperature was 80 °C instead of 40 °C. X-ray powder diffraction (XRD) measurement was employed with a Japan Regaku D/max γA X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.5418 Å) irradiated with a scanning rate of 0.02 deg/s. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopic (EDX) measurement were preformed using a Hitachi S-4300 scanning electron field emission microscope. A JEOL JEM-200CX microscope operating at 160 kV in the bright-field mode was used for transmission electron microscopy (TEM). Selected area electron diffraction (SAED) pattern and highresolution TEM (HRTEM) were performed on a JEOL JEM2010 electron microscope operating at 200 kV. Photoluminescence (PL) spectra were measured with a Hitachi F4500 fluorescence spectrophotometer at room temperature.
Figure 2. (a) Low-magnification and (b-f) high-magnification SEM image of the multisegmented coaxial nanowires obtained at 40 °C for 5 h; (g) EDX pattern of the typical segment in Sb2O3 multisegmented coaxial nanowires shown in c.
Results and Discussion As shown in Scheme 1, single-crystal Sb2O3 multisegmented coaxial nanowires were obtained by direct air oxidation of bulk metal antimony (Sb) in solution at 40 °C for 5 h. The XRD
Self-Assembly of Sb2O3 Nanorods and Nanowires
J. Phys. Chem. C, Vol. 111, No. 14, 2007 5327
Figure 3. (a-c) Low-magnification TEM images of different segments of the as-synthesized Sb2O3 multisegmented coaxial nanowires; (d) HRTEM image of the marked part of “1” shown in c; (e) HRTEM image of the marked part of “2” shown in c; Insets of d and e are the corresponding SAED patterns.
pattern of the as-synthesized product is shown in Figure 1a, and all of the peaks could be indexed as the orthorhombic phase valentinite cell constants a ) 4.911 Å, b ) 12.464 Å, c ) 5.412 Å (JCPDS Card No. 11-0689). No peaks of metal Sb or any other phases were detected, indicating that the multisegmented coaxial nanowires were very high-purity, single-phase. The intense and sharp diffraction peaks suggested the as-synthesized product was well crystallized. As shown in Figure 2, the morphology of Sb2O3 multisegmented coaxial nanowires was characterized by scanning electron microscopy (SEM). Figure 2a showed that the product contained a large number of multisegmented coaxial nanowires (over 60% in total) with a small number of byproduct of simple nanorods. The lengths of the multisegmented coaxial nanowires were up to hundreds of micrometers. As seen from Figure 2b2f, a series of novel multisegmented coaxial nanowires and their segments were clearly observed. The segments in the multisegmented coaxial nanowires were simple nanorods with varied lengths and the diameters; the nanorods were typically 2∼50 micrometers in length and 20∼1000 nm in diameter. Figure 2g was an EDX pattern obtained from a single segment shown in the inset of Figure 2c. Only Sb and O peaks were observed in this pattern (silicon signal from the silicon substrate), suggesting that the nanowires were composed of Sb and O. Quantitative EDX analysis showed that the atom ratio of Sb/O was 39:61, close to 2:3, indicating the composition of the as-synthesized product was Sb2O3. The multisegmented coaxial nanowires were further characterized by transmission electron microscopy (TEM) and highresolution TEM (HRTEM). Figure 3a-3c and Supporting Information Figure S1 clearly showed the formation of the multisegmented coaxial nanowires, which was consistent with the SEM observations. The diameters of the two adjacent segments in Figure 3a were 1000 and 50 nm, respectively. The selected area electron diffraction (SAED) patterns of different segments and different “sticky” points (the areas like the marked “1” in Figure 3c) were almost the same, indicating that the multisegmented coaxial nanowires were single crystal, and grew along the [001] direction. Figure 3d and 3e showed the HRTEM
Figure 4. (a, b) SEM images of the as-synthesized Sb2O3 nanobelts at 80 °C for 5 h; (c, d) low-magnification TEM images of the as-synthesized Sb2O3 nanobelts; (e) HRTEM image of the Sb2O3 nanobelts in d; (f) typical SAED pattern of the as-synthesized Sb2O3 nanobelts.
images of the marked areas in Figure 3c. The typical fringe spacing was determined to be 0.271 and 1.246 nm, which is close to (002) and (010) lattice spacing of bulk orthorhombic phase valentinite and is also consistent with the result obtained from XRD. The unique structure of the multisegmented coaxial nanowires with large diameter ratio of adjacent segments is likely to be a model materials family for a systematic experimental and theoretical understanding in the fundamental electri-
5328 J. Phys. Chem. C, Vol. 111, No. 14, 2007
Deng et al.
Figure 5. SEM images of the time-dependent evolution of Sb2O3 multisegmented coaxial nanowires at 40 °C. (a) Nanorods for 30 min; (b, c) bisegmented coaxial nanowires for 120 min; (d, e) multisegmented coaxial nanowires for 5 h.
cal, optical, and thermal transport processes in 1D complex nanostructures. This work is the first example for the templatefree aqueous solution self-assembly of multisegmented coaxial nanowires. As shown in Scheme 1, single-crystal Sb2O3 nanobelts were obtained by direct air oxidation of bulk metal Sb in solution at 80 °C for 5 h. The XRD and the EDX pattern of the nanobelts were similar to those of the multisegmented coaxial nanowires. Figure 4a-4d clearly showed the formation of nanobelts. The as-synthesized Sb2O3 nanobelts were usually hundreds of micrometers in length, typically 400∼1000 nm in width and 20∼60 nm in thickness. Figure 4b showed a typical nanobelt with a thickness of 30 nm. As seen from Figure 4e, the fringe spacings were determined to be 0.271 and 1.246 nm, which were close to (002) and (010) lattice spacing of bulk orthorhombic phase valentinite. The SAED patterns of the different parts of the nanobelts were almost the same, indicating the nanobelts were single crystal in nature, and grew along the [001] direction (see Figure 4f). The formation mechanism of the particular nanostructures was studied, which indicated that the oriented attachment assisted self-assembly of Sb2O3 nanorods and nanowires in our systems was a thermodynamically driven process. At the temperature of 40 °C, the Sb2O3 nanorods with different diameters would axially self-assemble into multisegmented coaxial nanowires by an end-to-end self-assembly process. While at the temperature of 80 °C, the Sb2O3 nanowires would radially self-assemble into nanobelts by a side-by-side selfassembly process. As shown in Figure 5, the SEM images of the time-dependent evolution of Sb2O3 nanostructures were obtained at 40 °C, which indicated that the multisegmented coaxial nanowires were self-assembled from simple nanorods. Orthorhombic Sb2O3 crystallizes in the space group D2h10 and the growth rates of different faces are (001) > (010) > (100),46 thus, at a low temperature (40 °C), high-energy crystal face of (001) on the tips of the nanorods could act as the sticky points and could induce the nanorods to join in the ends to form multisegmented coaxial nanowires through oriented attachment. As a result, the individual nanorods would align themselves in the growth direction of [001], would start to recrystallize, and would form domains of multisegmented single-crystalline nanowires. It is assumed that the Sb2O3 monomers in solution might insert themselves at the “sticky” points of the coalescing nanorods to form the single-crystal Sb2O3 multisegmented
coaxial nanowires. Similar 1D nanostructure growth mechanisms have been observed in the transformation of Ag nanocrystals into Ag nanowires47 and transformation of Ge nanorods into Ge nanowires.48 Figure 6 shows the SEM images of the time-dependent evolution of the nanobelts obtained at 80 °C, which indicates that the simple nanowires with diameter of 10∼30 nm would align side-by-side to form nanowire bundles, and would further crystallize to form single-crystal nanobelts. As discussed above, the growth rates of different faces of orthorhombic Sb2O3 are (001) > (010) > (100). However, at a high temperature (80 °C), the energy of the crystal face of (010) of the nanowires is adequate for the nanowires to align side-by-side and to form nanowire bundles. Furthermore, as time continued, the nanowire bundles would further crystallize to form single-crystal nanobelts through oriented attachment (Supporting Information Figure S2). The nanobelts of semiconducting oxides were first synthesized by Pan et al. by evaporating the commercial metal oxide powders at high temperatures, and the growth was governed by a vapor-solid process.2 In our cases, however, the Sb2O3 nanobelts were synthesized in solution and the formation process was via a side-by-side through oriented attachment assisted selfassembly of nanowires. To our knowledge, this unique formation process of the nanobelts has not been previously reported in the literature. As shown in Figure 7a, the Sb2O3 multisegmented coaxial nanowires showed a strong sharp UV emission at 374 nm (∼3.31 eV) and another weak broad blue emission between 390 and 500 nm. The UV emission corresponded to the band-edge emission which has been observed in the well-crystallized Sb2O3 nanowires with rectangular cross sections,46 while the broad blue emission profile between 390 and 500 nm was probably referred to as oxygen vacancies related defects emission, like many other well-studied semiconducting metal oxides such as ZnO49 and SnO2.50 However, the Sb2O3 nanobelts (see Figure 7b) only showed a strong blue emission at 433 nm (∼2.86 eV), which was also probably attributed to oxygen vacancy related defect emission. It is generally believed that the oxygen vacancy related defects would be introduced in the products during oriented attachment assisted self-assembly processes. As the wellcrystallized Sb2O3 nanorods with different diameters axially selfassembled into multisegmented coaxial nanowires, there were only a small amount of oxygen vacancies implanted because of the limited contact areas between the tips of the nanorods;
Self-Assembly of Sb2O3 Nanorods and Nanowires
J. Phys. Chem. C, Vol. 111, No. 14, 2007 5329
Figure 6. SEM images of the time-dependent evolution of nanobelts at 80 °C. (a) Nanowires for 30 min; (b, c) nanowire bundles for 120 min; (d, e) nanobelts for 5 h.
Figure 7. Photoluminescence (excitation at 325 nm) of the assynthesized self-assembled Sb2O3 nanostructures. (a) Multisegmented coaxial nanowires; (b) nanobelts.
in the cases that the Sb2O3 nanowires radially self-assembled into nanobelts, there were much more oxygen vacancy related defects implanted because of the huge contact areas between the nanowires. Therefore, the multisegmented coaxial nanowires showed much weaker blue emission than the nanobelts. Pure bulk antimony and bulk-phase Sb2O3 powder had a negligible or no luminescence signal under the same excitation conditions. As seen from the characterization results, such as the XRD, SAED, and HRTEM, the product was highly pure Sb2O3 nanostructures; thus, we expected that the photoluminescence spectra represented the true photoluminescence behavior of the self-assembled Sb2O3 nanostructures. Actually, there was a lack of investigation on luminescence in the Sb2O3 systems so that the origin and the mechanism of the emission require further investigations. It was reasonable to expect that the obtained nanorod and nanowire assembled Sb2O3 nanostructures could be a new member of blue light emitters. Conclusions In summary, we have demonstrated that the single-crystal Sb2O3 multisegmented coaxial nanowires were obtained by endto-end oriented attachment assisted self-assembly of nanorods, while single-crystal Sb2O3 nanobelts were obtained by oriented attachment assisted side-by-side self-assembly of nanowires, via a facile template-free aqueous solution route. The blue photoluminescence property of the new self-assembled Sb2O3 nanostructures was reported. The single-crystal orthorhombic Sb2O3
multisegmented coaxial nanowires were a kind of new nanostructure reported for the first time and were likely to be a model materials family for a systematic experimental and theoretical understanding in the fundamental electrical, optical, and thermal transport processes in 1D complex nanostructures. The singlecrystal orthorhombic nanobelts were obtained by side-by-side oriented attachment assisted self-assembly of nanowires, and this unique formation process of the nanobelts was not reported before and may be of certain generality. The oriented attachment assisted self-assembly strategy we presented here is especially appealing because of its low costs and potential application in large-scale production, and it may open a new avenue for the self-assembly of nanorods and nanowires into unique complex nanostructured materials. By the suitable choice of source and synthetic parameters, it is reasonable to expect that the present study could be extended to other self-assembled nanostructures. Acknowledgment. The authors wish to express their appreciation to Prof. Zhonglin Wang, in Georgia Institute of Technology, United States, and Prof. Bingsuo Zou, in Hunan University, China for their helpful discussions. The authors appreciate the financial support from the National Natural Science Foundation of China (20571080, 60572031, 90406024) and Beijing Natural Science Foundation (4063042). Supporting Information Available: Additional TEM image, SAED pattern, and HRTEM image of another segment of the as-synthesized Sb2O3 multisegmented coaxial nanowires; additional TEM image of time-dependent evolution of the nanobelts. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Cui, Y.; Wei, Q. Q.; Park, H. K.; Lieber, C. M. Science 2001, 293, 1289. (2) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (3) Melosh, N. A.; Boukai, A.; Diana, F.; Gerardot, B.; Badolato, A.; Petroff, P. M.; Heath, J. R. Science 2003, 300, 112. (4) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. AdV. Mater. 2003, 15, 353. (5) Salem, A. K.; Searson, P. C.; Leong, K. W. Nat. Mater. 2003, 2, 668. (6) Alivisatos, A. P.; Johnson, K. P.; Peng, X.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P., Jr.; Schultz, P. G. Nature 1996, 382, 609. (7) Mirkin, C. A.; Lestinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607.
5330 J. Phys. Chem. C, Vol. 111, No. 14, 2007 (8) Salant, A.; Amitay-Sadovsky, E.; Banin, U. J. Am. Chem. Soc. 2006, 128, 10006. (9) Shevchenko, E. V.; Talapin, D. V.; Murray, C. B.; O’Brien, S. J. Am. Chem. Soc. 2006, 128, 3620. (10) Fu, A.; Micheel, C. M.; Cha, J.; Chang, H.; Yang, H.; Alivisatos, A. P. J. Am. Chem. Soc. 2004, 126, 10832. (11) Tang, Z. Y.; Zhang, Z. L.; Wang, Y.; Glotzer, S. C.; Kotov, N. A. Science 2006, 314, 274. (12) Wang, Z. L. Annu. ReV. Phys. Chem. 2004, 55, 159. (13) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (14) Connolly, S.; Nagaraja, R.; Fitzmaurice, D. J. Phys. Chem. B 2000, 104, 4765. (15) Duan, X. F.; Huang, Y.; Cui, Y.; Wang, J. F.; Lieber, C. M. Nature 2001, 409, 66. (16) Dujardin, E.; Hsin, L.; Wang, C. R. C.; Mann, S. Chem. Commun. 2001, 1264. (17) Caswell, K. K.; Wilson, J. N.; Bunz, U. H. F.; Murphy, C. J. J. Am. Chem. Soc. 2003, 125, 13914. (18) Salem, A. K.; Chen, M.; Hayden, J.; Leong, K. W.; Searson, P. C. Nano Lett. 2004, 4, 1163. (19) Hurst, S. J.; Payne, E. K.; Qin, L. D.; Mirkin, C. A. Angew. Chem., Int. Ed. 2006, 45, 2672. (20) Shibu Joseph, S. T.; Ipe, B. I.; Pramod, P.; Thomas, K. G. J. Phys. Chem. B 2006, 110, 150. (21) Thomas, K. G.; Barazzouk, S.; Ipe, B. I.; Joseph, S. T. S.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 13066. (22) Jiang, J. C.; Henry, L. L.; Gnanasekar, K. I.; Chen, C. L.; Meletis, E. I. Nano Lett. 2004, 4, 741. (23) Koh, Y. W.; Lin, M.; Tan, C. K.; Foo, Y. L.; Loh, K. P. J. Phys. Chem. B 2004, 108, 11419. (24) Chang, Y.; Zeng, H. C. Cryst. Growth Des. 2004, 4, 397. (25) Gu, Z. J.; Ma, Y.; Yang, W. S.; Zhang, G. J.; Yao, J. N. Chem. Commun. 2005, 3597. (26) Alivisatos, A. P. Science 2000, 289, 736. (27) Tang, Z. T., Kotov, N. A. AdV. Mater. 2005, 17, 951. (28) Penn, R. L.; Banfield, J. F. Science 1998, 281, 769. (29) Banfield, J. F.; Welch, S. A.; Zhang, H.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751.
Deng et al. (30) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 1188. (31) Zhang, Z. P.; Sun, H. P.; Shao, X. Q.; Li, D. F.; Yu, H. D.; Han, M. Y. AdV. Mater. 2005, 17, 42. (32) Yu, J. H.; Joo, J.; Park, H. M.; Baik, S.-I.; Kim, Y. W.; Kim, S. C.; Hyeon, T. J. Am. Chem. Soc. 2005, 127, 5662. (33) Chand, N.; Verma, S. J. Fire Sci. 1991, 9, 251. (34) Sato, H.; Kondo, K.; Tsuge, S.; Ohtani, H.; Sato, N. Polym. Degrad. Stab. 1998, 62, 41. (35) Longerey, M.; Cuesta, J. M.; Gaudon, P.; Crespy, A. Polym. Degrad. Stab. 1999, 64, 489. (36) Spengler, J.; Anderle, F.; Bosch, E.; Grasselli, R. K.; Pillep, B.; Behrens, P.; Lapina, O. B.; Shubin, A. A.; Eberle, H. J.; Knozinger, H. J. Phys. Chem. B 2001, 105, 10772. (37) Liu, H. C.; Imoto, H.; Shido, T.; Iwasawa, Y. J. Catal. 2001, 200, 69. (38) Liu, H. H.; Iwasawa, Y. J. Phys. Chem. B 2002, 106, 2319. (39) Ha, Y.; Wang, M. Electroanalysis 2006, 18, 1121. (40) Guo, L.; Wu, Z. H.; Liu, T.; Wang, W. D.; Zhu, H. S. Chem. Phys. Lett. 2000, 318, 49. (41) Chen, X. Y.; Wang, X.; An, C. H.; Liu, J. W.; Qian, Y. T. Mater. Res. Bull. 2005, 40, 469. (42) Friedrichs, S.; Meyer, R. R.; Sloan, J.; Kirkland, A. I.; Hutchison, J. L.; Green, M. L. Chem. Commum. 2001, 929. (43) Ye, C. H.; Meng, G. W.; Zhang, L. D.; Wang, G. Z.; Wang, Y. H. Chem. Phys. Lett. 2002, 363, 34. (44) Zhang, Y. X.; Li, G. H.; Zhang, J.; Zhang, L. D. Nanotechnology 2004, 15, 762. (45) Sendor, D.; Weirich, T.; Simon, U. Chem. Commun. 2005, 5790. (46) Deng, Z. T.; Tang, F. Q.; Chen, D.; Meng, X. W.; Cao, L.; Zou, B. S. J. Phys. Chem. B 2006, 110, 18225. (47) Korgel, B. A.; Fitzmaurice, D. AdV. Mater. 1998, 10, 661. (48) Gerung, H.; Boyle, T. J.; Tribby, L. J.; Bunge, S. D.; Brinker, C. J.; Han, S. M. J. Am. Chem. Soc. 2006, 128, 5244. (49) Tong, Y. H.; Liu, Y. C.; Shao, C. L.; Liu, Y. X.; Xu, C. S.; Zhang, J. Y.; Lu, Y. M.; Shen, D. Z.; Fan, X. W. J. Phys. Chem. B 2006, 110, 14714. (50) Gu, F.; Wang, S. F.; Lu, M. K.; Zhou, G. J.; Xu, D.; Yuan, D. R. J. Phys. Chem. B 2004, 108, 8119.
