J. Phys. Chem. C 2007, 111, 14689-14693
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Fast Synthesis of Stable Cubic Copper Nanocages in the Aqueous Phase Xiaodan Su,† Jingzhe Zhao,*,‡ Hari Bala,§ Yanchao Zhu,† Ye Gao,† Shanshan Ma,† and Zichen Wang*,† College of Chemistry, Jilin UniVersity, 2519 Jiefang Road, Changchun 130023, P.R. China, College of Chemistry and Chemical Engineering, Hunan UniVersity, Changsha 410082, P.R. China, and Institute of Materials Science and Engineering, Henan Polytechnic UniVersity, Jiaozuo 454000, P.R. China ReceiVed: June 12, 2007; In Final Form: July 20, 2007
We report here a fast approach to the synthesis of cubic Cu nanocages in higher yield through a wet chemical reductive procedure. The performing reductive reactions from Cu(II) to Cu(0) in the aqueous phase can be finished in just 3 min at the low temperature of 80 °C in the presence of sodium oleate (SOA) as a stabilizer. Both transmission electron microscopy (TEM) and field-emission scanning electron microscopy (FESEM) results indicate that the resultant cubic Cu cages are aggregates of Cu nanoparticles with 200 nm in edge lengths and the inclusive nanoparticles are about 20 nm in size. According to the inspection of the Cu nanostructure-forming mechanism, we suggest that the reduction intermediate of roughly solid Cu2O cubes formed in this strategy served as a spontaneous shape-controlled template and the cubic structure of the Cu nanostructures was evolved by morphology heredity from them. As we detected, this cubic morphology of Cu aggregates is preserved at varied reaction times, from 3 min to 2 h; it can be presumed that rapidly formed copper cubes are stable in morphology both in solution and as powders in restrained reaction time. Experiments in a longer reaction time of 24 h have also been done by us, and the results showed a trend to form Cu hollow spheres in our system at enough reaction time. Reasonably, the cubic Cu cage is a structure metastable toward hollow spherical materials and it has kinetically controlled growth.
Introduction In the past two decades, many efforts have been made in the synthesis of inorganic nanoparticles with controllable shape because of their interesting physical properties and great potential applications.1-4 Now, many kinds of materials such as nanospheres, nanocubes, nanowires, nanotubes, nanoboxes, and nanovesicles have been extensively obtained by different methods.5-9 Owing to their unique structural, optical, electrical, magnetic, thermal, and surface properties, materials with vesicles or interstices structure are competitive issues. They can be applied in areas as molecular catalysis, medicine release, chromatography separation, acoustic insulators, photonic crystals, and electronic devices.10-13 Most recently, more efforts have been dedicated to the architecture and fabrication of hollow and cage structure materials with well-defined nonspherical shapes in a variety of compositions.14,15 The promising works produced many interesting results and provided significant challenges to materials researchers. The general method for chemical synthesis of this kind of nanostructures is introduced template strategy, either a hard or soft template; in addition, the self-assembly of nanoparticles is also reachable to form hollow materials.16-21 At present, many kinds of metallic compounds with parallel nanostructures have been synthesized, such as the hollow SnO2 octahedra in Zeng’s group prepared on the basis of two-dimensional aggregation of nanocrystallites through a template-free hydrothermal route.22 Co3O4 nanoboxes * To whom correspondence should be addressed. E-mail: zhaojz@ mail.jlu.edu.cn (J.Z.);
[email protected] (Z.W.). Tel./fax: +86-7318809278 (J.Z.); +86-431-88499134 (Z.W.). † Jilin University. ‡ Hunan University. § Henan Polytechnic University.
by surfactant template fabrication in absolute ethanol were reported by Chen et al.,23 and platinum-funcitonalized octahedral silica nanocages were also synthesized by Archer with intermediate (NH4)2PtCl6 as a quasitemplate.24 At the same time, noble metal particles were also frequently chosen as candidates to prepare materials with cage or hollow structures. For example, single-crystal Pd nanoboxes and nanocages were developed from sacrificial single-crystal Pd nanocubes on the basis of the corrosion technique by Xia and his co-workers.25 Qi and coworkers reported a work on rhombododecahedral silver cages by self-assembly coupled with precursor crystal templating.26 Also, Wang et al. reported a synthesis of PbTe naoboxes using a solvothermal technique,27 Chen et al.28 have synthesized uniform cubic Au nanocages with the size of about 30 nm, and similar structures of Au particles were reported by Halder and Ravishankar.29 Here, we describe a simple and fast approach to obtain uniform cubic Cu nanocages in aqueous solution through reductive reactions, on the basis of the morphology of its selfstanding intermediate Cu2O cubes, as is known that cupreous compounds (such as Cu2O, CuxS) are frequently chosen as model cases to generate polyhedral hollow nanostructures instead of normally made spherical hollow aggregates in most recent developments.30-33 The whole reaction of our strategy was finished in 3 min, and the cubic morphology and cage structure of the Cu products are stable for a reaction time as long as 2 h. These kinds of Cu nanostructures may be used as carriers applied to material release and molecular catalysis and also have potential uses as building skeletons to construct alloys or complex materials with multifarious functions, as their cage voids make it easier to uniformly disperse heterogeneous particles into their interior. Extended experiments of prolonged
10.1021/jp074550w CCC: $37.00 © 2007 American Chemical Society Published on Web 09/18/2007
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reaction times up to 24 h have also been done by us to inspect the nanostructure growth mechanism, and the additional results revealed a trend in this system to form hollow spherical materials finally after enough reaction time. The cubic cage structure is a metastable stage of stable hollow spheres, and the growth is kinetically controlled. Experimental Section In our typical experiment, 0.02 mol of CuSO4‚5H2O was dissolved in 20 mL of distilled water, and 10 mL of a 0.04 M aqueous solution of sodium oleate (SOA) was added into this CuSO4 solution. Here, SOA was used as surfactant and stabilizing reagent to prevent the Cu nanoparticles from coarseness and being oxidized. Also, it was used as a shape-controller. Then, 10 mL of a 4 M aqueous solution of NaOH was added into the mixture, and 30 mL of 50% hydrazine hydrate was quickly poured into the mixture. The whole reaction was carried out under dynamoelectric stirring in a water bath of 80 °C. A series of time-dependent experiments were done by tuning the reaction time from 3 min to 24 h. After reaction, the resulted solution was filtrated. The Cu nanostructures were obtained by washing the as-prepared precipitates with distilled water and pure ethanol several times and subsequently drying them in a vacuum system at room temperature. X-ray diffraction (XRD) detection on Cu nanocages and intermediates was performed on a Shimadzu model XRD-6000 using Cu KR radiation. The morphology and particle size of powders were characterized by a field-emission scanning electron microscope (FESEM, JEOL JSM-6700F) and a transmission electron microscope (TEM, Hitachi H-8100). The samples for TEM and FESEM measurements were prepared by dispersing the dried powders into ethanol and keeping the dispersions under ultrasonication for 5-10 min to make a light suspension; then a drop was placed on the copper grids or glass flake, which was allowed to dry at room temperature. Highresolution transmission electron microscopy (HRTEM) measurement was made on a JEOL JEM-3010 microscope to observe the crystal lattice structure of Cu nanocages. The contact angle of the prepared powders was taken at 25 °C on a FTA200 (First Ten Angstroms, Portsmouth, Virginia, U.S.) contact angle instrument. Results and Discussion We improved a fast method to generate metal Cu nanocages with well-controllable cubic shape through a wet chemical reductive procedure. In this simple procedure, hydrazine hydrate was employed as a reducing reagent and the reaction was proceeding with several changes of color in the aqueous solution system, from blue at the initial stage to dark purple at the end. Therefore, we can judge the primary stages of the reaction by combining their distinctive colors with the corresponding XRD results. On the basis of the XRD detection of the dried powders derived from different stages, we suggest that the whole reaction process from Cu(II) to Cu(0) can be described as follows: First, after NaOH solution was poured into the CuSO4 solution at 80 °C, the formed blue Cu(OH)2 floccules are quickly dehydrated and turned to black precipitates in a few seconds. The XRD result proved that the black precipitates were CuO (see Figure S1 in the Supporting Information). Second, when the reducing agent hydrazine hydrate was poured into the above mixture, the black precipitates of CuO rapidly transformed through brick-red Cu2O to Cu nanocages with a color change to dark purple in several minutes. Owing to the fast reductive reaction by this method, the appearance of brick-red Cu2O
Figure 1. TEM and FESEM micrographs of cubic Cu nanocages prepared after 3 min: (A) TEM image of the sample; (B) FESEM image of this sample in large scale.
particles in solution was short-lived. We can get intermediate Cu2O particles by stopping the addition of reductive agent hydrazine hydrate at a certain stage; that is, decreasing the introductory amount of hydrazine hydrate to parallel reaction system leads to the appearance of Cu2O as product rather than Cu nanocages. (The XRD result of Cu2O was shown in Supporting Information Figure S2.) Temperature-dependent experiments (20, 40, and 60 °C) were also done by us. From the experimental phenomena, we found that compared with the reaction at 80 °C, the reduction at lower temperatures needs a much longer reaction time. The reduction process had a prolonged existing period of brick-red Cu2O intermediates, and these Cu2O species were hard to transform to product Cu, as detected by XRD analysis. Figure 1A gives a TEM image of typical morphology and microstructure of the resultant Cu nanocages obtained in a fast reductive reaction of 3 min. The metallic Cu powders of 3 min extend the cubic cage structure with edge length of about 200 nm. From the contrast of dark edge and light center in the image, we can find these cubic Cu nanocages are constructed from small elementary particles of about 20 nm. Figure 1B shows a
Stable Cubic Copper Nanocages in the Aqueous Phase
Figure 2. TEM micrograph of the intermediate Cu2O solid cubes. The insert shows an SAED pattern of one isolated Cu2O cube.
FESEM image of the same sample as from TEM detection, which strongly suggests a large quantity and good uniformity of the as-prepared Cu nanocages were achieved by our simple and reproducible strategy. The surfaces of our cubic Cu nanocages are not smooth enough compared to those of the solid Cu nanocubes in Lu’s work.34 Zeng’s group also presented some results on fabricating metallic Cu hollow cubes with preformed Cu2O nanocubes as solid precursors.30 However their experiments both focused on the preparation in organic solvent of DMF and ethanol at higher reaction temperatures of 180 and 200 °C and a duration of a few hours. So we can say our method is simple and effective. To inspect the forming procedure of cubic Cu nanocages in our aqueous strategy, the morphology and structure of reductive intermediates CuO and Cu2O (XRD patterns shown in Supporting Information Figures S1 and S2) were further investigated by the TEM technique. From the images we found that CuO nanoparticles were fibrous with a length of 50 nm and width of 10 nm (see Supporting Information Figure S3). Intermediate Cu2O had the morphology of a cubic shape with comparatively solid interior and smooth surfaces, as can be seen in Figure 2, and the typical edge length is approaching 200 nm, which is concordant with the size of the final Cu nanocages. The spontaneously formed intermediate Cu2O in solution is reasonably suggested to be the template for cubic morphology of the resultant Cu nanocages inherited from that of the Cu2O cubes. However, the difference in the interior of Cu2O and Cu cubes is obvious so that intermediate Cu2O cubes seem completely solid and final Cu nanostructures exhibit a similar cubic shape with cage structure. In the inserted selected-area electron diffraction (SAED) pattern of one Cu2O cube in Figure 2, the diffraction rings of Cu2O can be seen clearly, as well as a list of lattice points. This means the cubic Cu2O sample prepared is polycrystalline along with several single crystals existing with perfect crystal structure. We conclude that the single crystals construct the surface layer of the cubes, and they obey the rules of the preponderant crystalloid growing tropism in the formation of the Cu2O cube. The polycrystalline structure of Cu2O inside makes it possible to be reduced from the inside, and as a result, the cage structure is formed through a nanoscale Kirkendall effect.26 Hydrazine hydrate and small Cu2O crystals can react
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Figure 3. HRTEM micrograph of a cubic Cu nanocage of 3 min. The lattice distance in the edge part of 0.246 nm is corresponding to d111 of Cu2O, and the distance in the inner part of 0.211 nm is relative to d111 of Cu (0.208 nm).
with each other and form a diffusion pair, and the coupled reaction/diffusion at the crystal/solution interface could lead to the quick formation of interconnected Cu particles around the external surfaces of the Cu2O crystals with help of surfactant SOA; the result of this method is cubes of Cu on a cage structure. Compared with the inner part, well-crystallized Cu2O surface layer are more perfect and steady, so during the reduction they should require more time to be reduced to metallic Cu, which also retains the cubic structure of the Cu cages well. A HRTEM image of edge area of one Cu nanocage in a sample of 3 min (Figure 3) provides forceful support for our suggestion. The lattice fringe measured at about 0.246 nm at the surface is in good agreement with d111 of Cu2O in the literature. The oriented crystallized Cu2O surface layer offered a possibility to remain as the cubic shape of product Cu, and the interspaces between Cu particles came into being after a fast reductive process. The HRTEM image in Figure 3 also reveals the cubic cage bodies are Cu from the fringe spacing 0.211 nm, which is close to the (111) interplanar distance 0.209 nm of face-centered cubic (fcc) copper. Especially, cubes with a cage structure are not very stable in theoretical perspectives as spherical particles; in our experiments, the cubic cage structure is stable for a relatively long reaction time in aqueous solution. To study the existing state and stability of structure, time-dependent experiments were performed with time span from 3 min to 2 h. Morphologies of the metallic Cu samples in prolonged reaction time comparing 3 min with 10 min, 30 min, and 2 h are shown in Figure 4 as TEM images. The majority of powders in each sample appeared as cubic cage structures. And it is worth noting that cubic Cu cages obtained at different reaction times have outer diameters similar to those of intermediate Cu2O cubes. Dispersed particles in TEM micrographs might be destroyed Cu cages resulting from the preparation of TEM samples by powerful ultrasonication treatment. Figure 4D presents a large-scale FESEM image of the sample of 2 h, which is similar to that of samples of 10 min and 30 min. Both the TEM and FESEM images imply that cubic cage morphology and size of resultant Cu powders remain unchanged within the reaction time from a few minutes up to 2 h. Thus, we can conclude this special cubic cage structure is
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Figure 6. TEM micrograph of a resultant Cu sample prepared after 24 h.
Figure 4. TEM and FESEM micrographs of Cu powders prepared at different reaction times: (A-C) TEM micrographs of cubic Cu nanocages prepared at 10 min, 30 min, and 2 h, respectively; (D) FESEM micrograph of Cu nanocages of 2 h in large scale.
Figure 5. XRD patterns of the Cu powders prepared in different reaction times. From (a) to (d), the reaction time is 3 min, 10 min, 30 min, and 2 h.
stable for a certain reaction time. The high stability of the cubic cage structure both in reactive solution and as dried powders will be favorable for further chemical modification, for systematic characterization, and even as building blocks of special nanostructures. However, a minor difference appeared (see Figure 4C). That is, the relatively solid edges of Cu cages existing in the samples of 3, 10, and 30 min cannot be seen clearly anymore in the sample of 2 h. For the sample of 2 h, the cage edges became rougher with combined particles of 20 nm, and the interspaces of surface particles became larger and clearer to be made out in TEM images. This would be in accordance with the reduction procedure of the surface layer. Figure 5 gives XRD results of samples prepared at different reaction times. As can be seen, apart from all of the obvious diffraction peaks at 43.3, 50.4, and 74.2° corresponding to fcc metallic Cu (JCPDS, File No.
85-1326), a faint peak at 36.5° located by an arrow in patterns is assigned to the (111) reflection of Cu2O (JCPDS, File No. 75-1531), which means a trace amount of Cu2O coexists with Cu particles in some of the samples. Moreover, for prolonged reaction time as from pattern a to d, the diffraction peak of Cu2O represented a trend from relativly evident to faint and then turned to obvious again, which showed that the content of Cu2O changed from decreasing to increasing. This measurement gave us information that Cu2O in the fast formed Cu sample of 3 min owed to incomplete reduction, and it mainly located as an oriented crystal layer at the surface of Cu cages which had been proved from HRTEM analysis. On further reduction at prolonged reaction time, the solid edge of Cu2O could disappear gradually until Cu nanoparticles emerged at the surface (as shown in Figure 4). These fresh Cu nanoparticles were so active that they inclined to be oxidized by dissolved O2 in solution supervened especially for the surface Cu particles. Cu2O in a sample of a longer reaction time of 2 h was the result of oxidation of Cu nanoparticles. The Cu2O surface layer in our fast formed cubic Cu cages is different from reported results by Zeng’s group; their hollow Cu cubes had a thin CuO layer at the cube surface.30 As the cubic structure was obtained by morphology heredity from the precursor Cu2O, we presume that along with the disappearance of the rudimentary Cu2O’s surface layer, the edges and corners of the cubic frame will began to fade away after a great long time. Figure 6 shows the TEM image of the resultant Cu sample prepared after 24 h, from which we can see the former cubic cage structure has changed to a quasisphere, going with a bit diminished unitary size. This change of morphology resulted from the reassembly of the outer nanoparticles in the absence of a well-crystallized Cu2O surface layer. Meanwhile, the inner nanoparticles would move outward and join the shell gradually through the action of Ostwald ripening. Ostwald ripening is an observed phenomenon in solid solution, where smaller particles are annihilated by larger ones during the reaction process because larger particles are more energetically favored than smaller particles. In our work, we suggest that cubic Cu nanocages transform to hollow spheres through this Ostwald ripening if the reaction time is long enough. In most of the cases, the Kirkendall effect and Ostwald ripening work simultaneously. We believe this adapts to our strategy; at the beginning of the reaction the Kirkendall effect prominently
Stable Cubic Copper Nanocages in the Aqueous Phase forms a cubic cage structure, and at the latter stage of the reaction Ostwald ripening dominates the reaction for evolving hollow spheres. Reasonably, the cubic Cu cage is a structure metastable to hollow spherical materials, and it is kinetically controlled growth. The morphology change of the Cu particles from cubes to truncated cubes and eventually to spheres was time-dependent. In a shorter reaction time, Cu particles exhibit a cubic cage structure. With increasing reaction time, cubic Cu cages appeared with truncated corners, and for enough reaction time, Cu particles appear as be hollow spheres. In our preparation strategy of Cu nanocages, SOA was introduced into the reaction system to act as a surfactant, a stabilizing reagent to keep project Cu nanoparticles from coarseness and oxidation, and also a shape-controller of intermediate Cu2O cubes and final Cu cages. If the synthesis went without addition of surfactant or with other surfactant, such as CTAB, PVP, PEG, and SDS, only larger Cu particles in irregular shape could be formed (see Supporting Information Figure S4). The stabilizing effect of surfactant SOA adsorbed on the simultaneously standing template Cu2O (100) surface in this reaction system can thus be demonstrated. Furthermore, on the basis of the result of contact angle detection, we found that surfactant SOA also imparted a hydrophobic nature to Cu nanocages, which can improve their stability and biocompatibility (see Supporting Information Figure S5). Conclusion In summary, we have demonstrated a fast synthesis of cubic Cu nanocages in aqueous phases. The whole reaction can be finished in just 3 min. The average size of the resultant Cu cages was about 200 nm in edge length, and the compositive nanoparticles in cubes were about 20 nm. Here, the intermediate Cu2O was the template, and the reduction took place from inside, so we prepared a special cubic cage structure of Cu from Cu2O’s morphology heredity. Further investigation of longer reaction times (10 min, 30 min, and 2 h) has also been done by us. Compared with 3 min, 2 h is rather a long time, so we can say that the Cu particles with a cubic cage structure formed instantly in the reaction are stable in their cubic morphology. Acknowledgment. This work was supported by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (SRF for ROCS, SEM), and the Natural Science Foundation of Jilin Province for Excellent Young Scholars (Grant No. 20040117). Supporting Information Available: XRD pattern of black CuO powders (Figure S1), XRD pattern of brick-red Cu2O intermediate particles (Figure S2), TEM micrograph of CuO nanoparticles (Figure S3), TEM image of a Cu sample synthe-
J. Phys. Chem. C, Vol. 111, No. 40, 2007 14693 sized without addition of surfactant SOA (Figure S4), and the contact angle of the Cu nanocages synthesized by our strategy (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (2) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (3) Shenton, W.; Pum, D.; Sleytr, U. B.; Mann, S. Nature 1998, 389, 585. (4) Firestone, M. A.; Williams, D. E.; Seifert, S.; Csencsits, R. Nano Lett. 2001, 1, 129. (5) Jin, R.; Cao, Y. W.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (6) Jana, N. R.; Gearheart, L. A.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (7) Lu, Q. L.; Gao, F.; Zhao, D. Y. Nano Lett. 2002, 2, 725. (8) Yin, M.; Wu, C. K.; Lou, Y. B.; Burda, C.; Koberstein, J. T.; Zhu, Y. M.; O’Brien, S. J. Am. Chem. Soc. 2005, 127, 9506. (9) Guo, L. F.; Murphy, C. J. Nano Lett. 2003, 3, 231. (10) Chang, Y.; Teo, J. J.; Zeng, H. C. Langmuir 2005, 21, 1074. (11) Park, S.; Lim, J.-H.; Chung, S.-W.; Mirkin, C. A. Science 2004, 303, 348. (12) Kim, S.-W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (13) Liang, Z.; Susha, A.; Caruso, F. Chem. Mater. 2003, 15, 3176. (14) Liang, H. P.; Wan, L. J.; Bai, C. L.; Jiang, L. J. Phys. Chem. B 2005, 109, 7795. (15) Liu, J. B.; Dong, W.; Zhan, P.; Wang, S. Z.; Zhang, J. H.; Wang, Z. L. Langmuir 2005, 21, 1683. (16) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111. (17) Filankembo, A.; Pileni, M. P. J. Phys. Chem. B 2000, 104, 5865. (18) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124. (19) Sun, Y. G.; Xia, Y. N. Nano Lett. 2003, 3, 1569. (20) Yang, Z.; Niu, Z.; Lu, Y.; Hu, Z.; Han, C. C. Angew. Chem., Int. Ed. 2003, 42, 1943. (21) Choi, W. S.; Park, J. H.; Koo, H. Y.; Kim, J. Y.; Cho, B. K.; Kim, D. Y. Angew. Chem., Int. Ed. 2005, 44, 1096. (22) Yang, H. G.; Zeng, H. C. Angew. Chem., Int. Ed. 2004, 43, 5930. (23) He, T.; Chen, D. R.; Jiao, X. L.; Wang, Y. L. AdV. Mater. 2006, 18, 1078. (24) Lou, X. W.; Yuan, C. L.; Zhang, Q.; Archer, L. A. Angew. Chem., Int. Ed. 2006, 45, 3825. (25) Xiong, Y. J.; Wiley, B.; Chen, J. Y.; Li, Z. Y.; Yin, Y. D.; Xia, Y. N. Angew. Chem., Int. Ed. 2005, 44, 7913. (26) Yang, J. H.; Qi, L. M.; Lu, C. H.; Ma, J. M.; Cheng, H. M. Angew. Chem., Int. Ed. 2005, 44, 598. (27) Wang, W. Z.; Poudel, B.; Wang, D. Z.; Ren, Z. F. AdV. Mater. 2005, 17, 2110. (28) Chen, J.; Saeki, F.; Wiley, B. J.; Cang, H.; Cobb, M. J.; Li, Z.-Y.; Au, L.; Zhang, H.; Kimmey, M. B.; Li, X. D.; Xia, Y. Nano Lett. 2005, 5, 473. (29) Halder, A.; Ravishankar, N. J. Phys. Chem. B 2006, 110, 6595. (30) Teo, J. J.; Chang, Y.; Zeng, H. C. Langmuir 2006, 22, 7369. (31) Lu, C.; Qi, L.; Yang, J.; Wang, X; Zhang, D.; Xie, J.; Ma, J. M. AdV. Mater. 2005, 17, 2562. (32) Cao, H. L.; Qian, X. F.; Wang, C.; Ma, X. D.; Yin, J.; Zhu, Z. K. J. Am. Chem. Soc. 2005, 127, 16024. (33) Jiao, S. H.; Xu, L. F.; Jiang, K.; Xu, D. S. AdV. Mater. 2006, 18, 1174. (34) Zhou, G.; Lu, M.; Yang, Z. Langmuir 2006, 22, 5900.
