Low-temperature template-free synthesis of Cu2O hollow spheres

Low-temperature template-free synthesis of Cu2O hollow spheres

ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 2285–2290 Contents lists available at ScienceDirect Journal of Crystal Growth journal homepage...

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ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 2285–2290

Contents lists available at ScienceDirect

Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro

Low-temperature template-free synthesis of Cu2O hollow spheres Yongming Sui, Yanyan Zhang, Wuyou Fu, Haibin Yang , Qiang Zhao, Peng Sun, Dong Ma, Mingxia Yuan, Yixing Li, Guangtian Zou National Laboratory of Superhard Materials, Jilin University, Changchun 130012, PR China

a r t i c l e in fo

abstract

Article history: Received 18 September 2008 Received in revised form 15 January 2009 Accepted 4 February 2009 Communicated by J.M. Redwing Available online 10 February 2009

Cuprous oxide (Cu2O) hollow spheres have been successfully prepared via a template-free hydrothermal method with the assistance of poly(vinylprrolidone) (PVP) at 90 1C. The structure and morphology of the Cu2O hollow spheres were characterized by X-ray diffraction, field-emission scanning electron microscope, and transmission electron microscope. The as-prepared hollow spheres are cubic phase Cu2O. The as-prepared products have a hollow sphere structure with outer diameters of 400–700 nm and the thickness of shells is about 40–70 nm. Poly(vinylprrolidone) is employed for the first time to improve the stabilization of crystalline phase occurred at a rate commensurate with localized Ostwald ripening and self-transformation for producing Cu2O hollow spheres. A possible PVP-assisted Ostwald ripening process is proposed for the formation of Cu2O hollow spheres on the basis of intermediate products at different growth stages. & 2009 Elsevier B.V. All rights reserved.

PACS: 78.67.Bf 81.16.Be 81.07. b 68.37.Lp Keywords: A1. Mass transfer A1. X-ray diffraction A2. Hydrothermal crystal growth B1. Oxides B2. Semiconducting materials

1. Introduction Among many inorganic materials with distinct structural and geometrical features, hollow nano/microspheres currently represent one of the fastest growing areas of materials research [1–4]. The fabrication of hollow nano/microspheres with varying sizes and shapes has attracted fascinating interest, due to their distinct low effective density, high specific surface area, and potential scale-dependent applications in photonic devices, drug delivery, lightweight fillers, active material encapsulation, ionic intercalation, acoustic insulation, surface functionalization, robust catalysts/carriers, and size-selective reactions [5–8]. According to reports of the preparation of hollow structure, there are two main categories: (i) the template-directed synthesis and (ii) the emulsion synthesis. The basis of the template-directed synthesis is adsorption of nanoparticles or polymerization on modified polymeric (e.g., polystyrene) [9–12] or inorganic (e.g., SiO2) [13–15] template surface and subsequently removal of the template by calcinations or dissolution with solvents. In the emulsion synthesis, on the other hand, the solution is emulsified Corresponding author. Tel./fax: +86 431 85168763.

E-mail address: [email protected] (H. Yang). 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.02.007

and the adsorption or reaction then takes place on the surface of sol droplets (micelles) to form the hollow spheres [16–18]. Recently, there are several special synthetic strategies that can generate spherical hollow structures. Among them, interestingly, well-known physical and chemical phenomena such as ‘‘Ostwald ripening’’ and ‘‘chemically induced self-transformation’’ have been utilized for the fabrication of hollow nano/microspheres, such as TiO2, SnO2, CdMoO4, and CaCO3, in the absence of template [19–23]. However, in general, it remains a major challenge to develop template-free, simple, mild, one-step solution route for the preparation of inorganic hollow nanostructures. Cuprous oxide (Cu2O) is a typical p-type direct band gap semiconductor with a band gap of 2.17 eV [24] and has potential applications in solar energy conversion [25], electrode materials [26], sensors [27], and catalysis [28,29]. It also has been found that high-intensity photoexcitation can give rise to the coherent propagation of Cu2O excitons through Cu2O solid owing to the large exciton binding energy of 150 meV [30]. Furthermore, its potential was demonstrated by the discovery that illuminated Cu2O could act as a stable photocatalyst for the photochemical decomposition of water into O2 and H2 under visible light irradiation [31,32]. Various Cu2O nanostructures, including nanowires [33–35], nanotubes [36], nanocubes [37], nanospheres

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2. Experimental section All reagents were analytical grade and used as raw materials without further purification. In a typical procedure, 0.24 g copper acetate monohydrate and 1.49 g triethanolamine (TEA) were dissolved in 60 mL ethanol–water (2:3 in volume) solution. The above mixture was stirred for 10–15 min with a magnetic. Then, 0.02 g sodium hydroxide and 0.8 g poly(vinylprrolidone) (molecular weight=30,000) were added into the above mixed solution, under constant stirring. After 10 min the solution became dark blue, then 2 mL of 1 M glucose solution was added dropwise into the blue solution with constant stirring. The resulting precursor suspension was transferred into a Teflon-lined stainless steel autoclave (V=80 mL). The autoclave was sealed and maintained at 90 1C for 6 h under self-generated pressure, and then cooled to room temperature naturally. The red precipitate was filtered off, washed several times with distilled water and absolute alcohol, and finally dried in vacuum at 60 1C for 8 h. X-ray powder diffraction (XRD) analysis was conducted on a Rigaku D/max-2500 X-ray diffractometer with Cu Ka radiation (l=1.5418 A˚). Field-emission scanning electron microscope (FESEM) images were performed on a JEOL JEM-6700F microscope operating at 5 kV. Transmission electron microscope (TEM) images and the selected area electron diffraction (SAED) patterns were obtained on a JEOL JEM-2000EX microscope with accelerating voltage of 200 kV. UV–vis absorption spectra were recorded using a spectrophotometer (3100 UV–vis–NIR).

3. Results and discussion The crystal structure of the product was confirmed by X-ray diffraction. As shown in Fig. 1, all the diffraction peaks are labeled and can be indexed according to cubic phase (JCPDS file no. 050667), not only in peak position, but also in their relative intensity. The peaks with 2y values of 29.601, 36.521, 42.441, 61.541, 73.691,

6h

(222)

(311)

(220)

(200) (110)

[38], and so on, have been successfully fabricated with a variety of techniques. It is noticed that hollow Cu2O nano/microspheres have attracted more and more attention, too. It is reported that such materials are expected to exhibit high light-collection efficiency and a fast motion of charge carriers because of their hollow structures, closely packed interpenetrating networks, and large internal surface area. Typically, Xu et al. reported the solutionphase synthesis of single-crystal hollow Cu2O spheres with gelatin as a soft template at 70 1C [39]. Zeng et al. reported the hydrothermal synthesis of Cu2O hollow nanospheres from a reductive conversion of aggregated CuO nanocrystallites without using templates at 140–180 1C [40]. Recently, Zhang et al. reported a template-free and one-pot hydrothermal synthesis of Cu2O hollow microspheres with multilayered and porous shells at 160 1C [41]. However, the Cu2O hollow nano/microspheres synthesized via a facile, template-free method at low growth temperature (below 100 1C) are seldom reported in these previous studies. In this work, we demonstrate a template-free hydrothermal method to synthesize Cu2O hollow spheres at 90 1C. The key points of the successful realization are that we introduce poly(vinylprrolidone) (PVP) to improve the stabilization of crystalline phase occurred at a rate commensurate with localized Ostwald ripening and self-transformation for producing Cu2O hollow spheres. Compared with the conventional methods, the present synthetic procedure has the advantages of simplicity (without any special equipments or templates), low growth temperature (90 1C), and high efficiency (the total growth time is approximately 6).

(111)

Y. Sui et al. / Journal of Crystal Growth 311 (2009) 2285–2290

Intensity (a.u.)

2286

2h 0.5 h 30

40

50 2θ (deg.)

60

70

80

Fig. 1. A typical XRD pattern of the as-prepared Cu2O hollow spheres.

and 77.611 correspond to the crystal planes of 110, 111, 200, 220, 311, and 222 of crystalline Cu2O, respectively. No impurity is detected in this pattern. In particular, the crystallinity of the products is indeed increased gradually with the reaction time prolonged (e.g., from 0.5 to 6 h; Fig. 1), which indicates that Ostwald ripening (crystallites grow at the expense of the smaller ones [42], discussion detailed later) is an underlying mechanism operative in this hollowing process. Fig. 2 shows the representative FESEM and TEM images of the as-prepared Cu2O hollow spheres. The low-magnification FESEM image (Fig. 2a) indicates that the products consist of a wealth of spheres with diameters ranging from 400 to 700 nm. As shown by the black arrows in Fig. 2b, many of the spheres are ‘‘open mouth’’ or broken hemispheres that indicate the spheres are of hollow structures. High-magnification images of an individual sphere clearly show that bowl-like shape spheres are hollow, and the shell wall consists of nanoparticles with diameters of ca. 30 nm. The thickness of shell walls is approximately 40–70 nm and inner surfaces are extensively roughened, whereas the external surfaces are smooth. Although the walls of spheres are too thick for electron beam to transmit easily, the contrast of TEM images, as shown in Fig. 2d and e, also confirmed the hollow sphere structure of the products. The corresponding TEM image (Fig. 2d) further confirms that the products consist of a mixture of hollow spheres and bowl-like larger hollow spheres. In order to obtain more detailed information of the crystal structure of the spheres, a broken Cu2O hollow sphere with hemispherical shape as shown in Fig. 2c was used as a candidate for further investigation of selected area electron diffraction. The annular SAED pattern indicates that the Cu2O hollow spheres are polycrystalline. The intermediate products in different reaction durations were studied in detail by FESEM and TEM to further investigate the formation process of Cu2O spheres with hollow interior. Fig. 3 shows a series of typical intermediate morphologies corresponding to the growth process evolution of Cu2O hollow spheres. Fig. 3a shows the FESEM image of Cu2O spheres produced for 0.5 h at 90 1C. The obtained products are composed of large-scale, perfect spheres with diameters of 300–600 nm. The TEM image (Fig. 3b) exhibits that the spheres are very round and perfectly smooth. When the reaction time reaches 2 h, the FESEM observation reveals that the peripheral surface of the sphere becomes rough. Based on the corresponding TEM image (Fig. 3c) results, apparently, the outer crystallites were loosely packed and/ or with smaller crystallite sizes. When the reaction time increases

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Fig. 2. (a) Low-magnification FESEM image of the large-scale Cu2O hollow spheres. (b) High-magnification FESEM image of the Cu2O hollow spheres. Some broken spheres, the ones with open holes on the shells, as indicated by black arrows in panel b, show the hollow structure of the spheres. (c) Typical high-magnification FESEM image of the individual bowl-like Cu2O hollow spheres showing rough and smooth inner and outer surfaces, respectively. (d) TEM image of Cu2O hollow spheres. (e) High-magnification TEM image of an individual hollow sphere. (f) The corresponding SAED pattern taken from the shell of an individual hollow sphere.

to 4 h (Fig. 3e), the peripheral surface becomes quite rough, interestingly a few spheres have nanoholes (Fig. 3e, inset). As shown in Fig. 3f, a strong contrast between the dark edges and the pale center can be seen, which indicates that the hollow interior structure appears during this process. However, the interior is not completely hollow, the void space between core and shell is observed from the image inset of Fig. 3f, which is similar to the result reported by Liu et al. [43]. According to the TEM images, when the reaction times were 0.5 and 2 h, the particles were solid spheres. However, when the reaction time was 4 h, the TEM images revealed the existence of both core–shell structure and solid spheres. A longer reaction time brought an obvious increase in particle size. When the reaction time was 6 h, the samples retained hollow spherical morphologies with diameters of 400–700 nm. As we know, template synthesis is a general approach to obtain hollow spheres, and the template serves as a scaffold against which other materials are assembled with the morphology similar to that of the template. However, in this work, Cu2O hollow spheres were synthesized in ethanol–water solution without using any template; thus, the Ostwald ripening mechanism is proposed for the formation of Cu2O hollow spheres. The above mechanism is likely to be highly sensitive to relative rates of dissolution of the amorphous solid particles and nucleation of the crystalline phase. When the former is comparatively slow then the amorphous particles will transform in situ to solid crystalline spheres, whereas when the latter is relatively slow the amorphous phase will completely dissolve prior to crystallization, which then takes place in free solution. Only when the rates are similar will the phase transformation process initiate specifically on the surface of the amorphous particles and remain localized as the particle core is depleted [23]. To substantially understand the effect of PVP and TEA on the growth of Cu2O hollow spheres, a contrasting experiment was carried out through the similar process. First, in the absence of PVP, when there was no or low concentration of TEA (CTEAo0.02 M) in the synthetic system, the resultant products were nanoparticles and their aggregates as revealed by the SEM

image (Fig. 4a). Comparatively, at high concentrations (CTEA4 0.1 M), the products were sphere shaped with diameter of about 600 nm (Fig. 4b). The solid nature of the nanospheres was further demonstrated by TEM image, as shown as an inset in Fig. 4b. Even when the reaction time was prolonged to 12 h, the solid spheres were still preserved. Interestingly, some nanospheres were closely linked and fused into each other, which is similar to the result reported by Ni et al. [44]. On the other hand, when 0.8 g of PVP was introduced into the reaction system, even if the concentration of TEA was reduced to 0.05 M, many hollow spheres with broad-size distribution occurred in the final products together with some nanoparticles (Fig. 4c); but if the concentration of TEA was reduced to 0.01 M, similar products in Fig. 4a were obtained (not shown here). From these experimental results, it is indicated that the presence of TEA is beneficial to form the sphere-shaped particles; and the PVP plays a crucial role in the growth of hollow structure. Recently, PVP has been applied as an important surfactant for the synthesis of nanomaterials, and various nanostructures have been successfully fabricated with the assistance of it [45–47]. In the present case, most probably, addition of PVP is of primary importance not only in promoting the product of dispersive Cu2O spheres, but also in the partial stabilization of crystalline phase occurred at a rate commensurate with localized Ostwald ripening and self-transformation for producing Cu2O hollow spheres. It seems reasonable to assume that the high carbonyl-group density along the PVP backbone would provide high surface binding and stabilization of primary clusters of amorphous cuprous oxide initially nucleated from the supersaturated solution. Furthermore, reduction in the surface charge of these primary clusters by PVP adsorption would facilitate secondary aggregation. As in many surfactant-assisted or ligand-mediated syntheses of shape-controlled materials [48–51], TEA, may serve as surface modifier, plays a role of ‘‘structure-directing agent’’ that directed the aggregation of the building blocks [44]. Presumably, the hydroxyl groups of the TEA rims absorbed on Cu2O nanoparticles can interact with each other, which results in a linkage or bridging of the neighboring Cu2O nanoparticles. And then, the clusters of Cu2O nanoparticles aggregate together to a spherical aggregation.

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Fig. 3. FESEM and TEM of the Cu2O hollow spheres obtained at 90 1C for different reaction times: 0.5 h (a, b), 2 h (c, d), and 4 h (e, f). Inset: the void space between core and shell of hollow spheres.

The increase of TEA concentration may lead to more TEA molecules coating on the Cu2O nanoparticles and more hydroxyl groups interacting to strengthen the compactness between the Cu2O nanoparticles. Moreover, TEA could coordinate with Cu2+, forming a very stable complex, thus avoid the formation of Cu(OH)2 after sodium alkaline adding into the solution. The formation mechanism of Cu2O hollow spheres could be explained by a self-transformation process of the metastable aggregated particles accompanied by the localized Ostwald ripening. Recently, similar mechanisms have been used to prepare Fe3O4, Co, CdIn2S4, a-MnO2, rare earth phosphate, and magnetic hybrid copolymer–cobalt hollow spheres [52–57]. The possible formation mechanism of hollow sphere is shown by the schematic diagram in Fig. 5. In the beginning (Step a), Cu2+ in ethanol–water solution reacts first with TEA to form relatively stable complex. In our synthesis process, the formation of copper complex could decrease the concentration of free Cu2+ ions in the solution, in favor of the Cu2O particles growing at a relatively slow rate. A slow formation rate leads to the separation of nucleation and growth steps, which is crucial for high-quality crystal synthesis. Afterward, at a relatively high temperature and autogenerated vapor pressure, this complex reacts with OH in solution to form Cu2O nuclei by reduction with the glucose in conjunction with the release of TEA. In the following process, driven by the

minimization of the total energy of the system, the small primary Cu2O particles aggregate together to form nanospheres, which show broad diffraction rings in the electron diffraction analysis (Step b, inset) due to the presence of amorphous or poor crystallized phase. The formed metastable Cu2O particles would remain out of equilibrium with the surrounding solution because of their higher solubility, and so the interior were dissolved gradually and the new Cu2O nuclei were formed on the surface of the Cu2O spheres by a dissolution–crystallization mechanism with the assistance of PVP. The corresponding TEM image (Step c) reveals that the surface of Cu2O nanosphere becomes rough and the narrow diffraction rings of the samples (Step c, inset) indicate the enhancement of crystallization. At this status, crystallites located on the outermost surface would serve as starting points (or nucleation seeds) for the subsequent recrystallization process. The outer crystallites become larger crystallites on attracting the smaller crystallites underneath. The presence of large crystallites on the outermost surface can be attributed to a continued surface growth (at a slow rate under low supersaturation) after the formation of the majority of primary crystallites (at a fast rate under initial high supersaturation). As a result of this process, the core size is reduced gradually while the vacant volume inside the sphere is enlarged. As the mass being transported, the void space between core and shell is generated through the Ostwald

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Fig. 4. FESEM and TEM of the Cu2O hollow spheres obtained at different conditions: (a) without PVP and TEA. (b) The concentration of TEA was greater than 0.1 M (without PVP). (c) The concentration of TEA was 0.05 M (with 0.8 g of PVP).

(a)

(b)

(c)

0.5 h

2h

(d)

5h

(e)

6h

Fig. 5. Schematic diagram the formation process of Cu2O hollow spheres, together with the corresponding FESEM, TEM, and SAED images: (a) formation of Cu2O nanoparticles in solution. (b) Synthesis of solid spheres. (c) Formation of solid spheres with rough surface. (d) Formation of spheres with core–shell structure. (e) Formation of hollow spheres.

ripening (Fig. 3f, inset). This self-transformation process is associated with localized Ostwald ripening mechanism, resulting in the formation of core–shell Cu2O structure (Step d), which could be demonstrated by the observation of the morphology at the reaction period of 5 h. The inner core can be visualized as smaller spheres with higher curvature compared to the outer particles for a whole sphere. Owing to the higher surface energy, the core is easily dissolved and merged into particles on the outer surface with enough ripening time, resulting in the formation of a hollow structure (Step e). Fig. 6 shows the UV–vis absorption spectra of the as-prepared products. Fig. 6a with a broad peak at 556 nm corresponds to the obtained Cu2O solid spheres for 0.5 h, while Fig. 6b with a broad peak at 512 nm corresponds to the obtained Cu2O hollow spheres for 6 h. The band gaps of Cu2O calculated from the UV–vis spectra are 2.23 and 2.42 eV, respectively. It is known that the optical absorption would be affected considerably by the morphology and

crystallinity of Cu2O crystals, and a blueshift will be achieved by decreasing the overall crystal size and the hollowing process [58]. Therefore, the observation on a blueshift of Cu2O hollow spheres can be rationalized by considering that it is largely attributed to the hollowing of the sphere structures.

4. Conclusions Cu2O hollow spheres were successfully synthesized at lower temperature (90 1C) with the help of PVP through an effective and facile method. The as-prepared Cu2O shows a perfectly hollow sphere structure with diameters of 400–700 nm and the thickness of shells is about 40–70 nm. The formation of the hollow spheres might be a PVP-assisted Ostwald ripening process. The Cu2O hollow spheres with low density and high surface area prepared using this method could have widespread uses as sensors,

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[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]

556 nm

Absorbance

(a)

512 nm

[27] [28] [29]

(b)

[30] [31]

300

400

500 600 Wavelength

700

800

Fig. 6. UV–vis absorption spectra of Cu2O solid spheres (a) and Cu2O hollow spheres (b).

catalysts, and material encapsulators or carriers. This method is very simple, mild, environmentally safe, and economical, and it may be general for metal oxide nano/microspheres with a hollow interior. References [1] [2] [3] [4] [5] [6] [7] [8] [9]

H. Zeng, J. Mater. Chem. 16 (2006) 649. D. Zhang, L. Qi, J. Ma, H. Cheng, Adv. Mater. 14 (2002) 1499. H. Hua, H. Zeng, Angew. Chem. Int. Ed. 43 (2004) 5206. H. Im, U. Jeong, Y. Xia, Nat. Mater. 4 (2005) 671. P. Jiang, J. Bertone, V. Colvin, Science 291 (2001) 453. Y. Wang, Y. Xia, Nano Lett. 4 (2004) 2047. X. Xu, S. Asher, J. Am. Chem. Soc. 126 (2004) 7940. Y. Wang, L. Cai, Y. Xia, Adv. Mater. 17 (2005) 473. F. Caruso, M. Spasova, A. Susha, M. Giersing, R. Caruso, Chem. Mater. 13 (2001) 109. [10] D. Wang, F. Caruso, Chem. Mater. 14 (2002) 1909. [11] V. Valtchev, Chem. Mater. 14 (2002) 4371. [12] Z. Liang, A. Susha, F. Caruso, Chem. Mater. 15 (2003) 3176.

[32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58]

T. Mandal, M. Fleming, D. Walt, Chem. Mater. 12 (2000) 3481. S. Kim, M. Kim, W. Lee, T. Hyeon, J. Am. Chem. Soc. 124 (2002) 7642. K. Kamata, Y. Lu, Y. Xia, J. Am. Chem. Soc. 125 (2003) 2384. A. Dinsmore, M. Hsu, M. Nikolaides, M. Marquez, A. Bausch, D. Weitz, Science 298 (2002) 1006. M. Yang, J. Zhu, J. Cryst. Growth 256 (2003) 134. S. Naik, A. Chiang, R. Thompson, F. Huang, Chem. Mater. 15 (2003) 787. H. Yang, H. Zeng, J. Phys. Chem. B 108 (2004) 3492. X. Lou, Y. Wang, C. Yuan, J. Lee, L. Archer, Adv. Mater. 18 (2006) 2325. W. Wang, L. Zhen, C. Xu, B. Zhang, W. Shao, J. Phys. Chem. B 101 (2006) 23154. H. Yang, J. Qian, Z. Chen, X. Ai, Y. Cao, J. Phys. Chem. C 111 (2007) 14067. J. Yu, H. Guo, S. Davis, S. Mann, Adv. Funct. Mater. 16 (2006) 2035. V. Agekyan, Phy. Status Solidi A 43 (1977) 11. A. Musa, T. Akomolafe, M. Carter, Sol. Energy Mater. Sol. Cells 51 (1998) 305. P. Poizot, S. Laruelle, S. Grugenon, L. Dupont, J. Taracon, Nature (London) 407 (2000) 496. J. Zhang, J. Liu, Q. Wang, X. Li, Chem. Mater. 18 (2006) 867. H. Zhang, X. Ren, Z. Cui, J. Cryst. Growth 304 (2007) 206. B. White, M. Yin, A. Hall, D. Le, S. Stolbov, T. Rahman, N. Turro, S. O’ Brien, Nano Lett. 6 (2006) 2095. D. Snoke, Science 273 (1996) 1351. M. Hara, T. Kondo, M. Komoda, S. Ikeda, K. Shinohara, A. Tanaka, J. Kondo, K. Domen, Chem. Commun. (1998) 357. P. Jongh, D. Vanmaekelbergh, J. Kelly, Chem. Commun. (1999) 1069. W. Wang, G. Wang, X. Wang, Y. Zhan, Y. Liu, C. Zheng, Adv. Mater. 14 (2002) 67. D. Singh, N. Neti, A. Sinha, O. Srivastava, J. Phys. Chem. C 14 (2007) 1638. Y. Tan, X. Xue, Q. Peng, H. Zhao, T. Wang, Y. Li, Nano Lett. 7 (2007) 3723. M. Cao, C. Hu, Y. Wang, Y. Guo, C. Guo, E. Wang, Chem. Commun. (2003) 1884. L. Gou, C. Murphy, Nano Lett. 3 (2003) 231. J. Zhang, J. Liu, Q. Peng, X. Wang, Y. Li, Chem. Mater. 18 (2006) 867. L. Xu, X. Chen, Y. Wu, C. Chen, W. Li, W. Pan, Y. Wang, Nanotechnology 17 (2006) 1501. Y. Chang, J. Teo, H. Zeng, Langmuir 21 (2005) 1074. H. Zhang, Q. Zhu, Y. Zhang, Y. Wang, L. Zhao, B. Yu, Adv. Funct. Mater. 17 (2007) 2766. W. Ostwald, 34 (1900) 495. B. Liu, H. Zeng, Small 1 (2005) 566. X. Ni, Y. Zhang, J. Song, H. Zheng, J. Cryst. Growth 299 (2007) 365. B. Xi, S. Xiong, H. Fan, X. Wang, Y. Qian, Cryst. Growth Des. 7 (2007) 1185. Y. Li, J. Liu, X. Huang, G. Li, Cryst. Growth Des. 7 (2007) 1350. Q. Xie, Z. Dai, W. Huang, W. Zhang, D. Ma, X. Hu, Y. Qian, Cryst. Growth Des. 6 (2006) 1514. X. Peng, Adv. Mater. 15 (2003) 459. Z. Liu, S. Li, Y. Yang, S. Peng, Z. Hu, Y. Qian, Adv. Mater. 15 (2003) 1946. Y. Qun, Y. Yin, B. Mayer, T. Herricks, Y. Xia, Chem. Mater. 14 (2002) 4736. Z. Tian, T. Voigt, T. Liu, B. Mckenzie, M. Mcdermott, M. Rodriguez, H. Konishi, H. Xu, Nat. Mater. 2 (2003) 821. B. Jia, L. Gao, J. Phys. Chem. C 112 (2008) 666. X. Wang, F. Yuan, P. Hu, L. Yu, L. Bai, J. Phys. Chem. C 112 (2008) 8773. L. Fan, R. Guo, J. Phys. Chem. C 112 (2008) 10700. M. Xu, L. Kong, W. Zhou, H. Li, J. Phys. Chem. C 111 (2007) 19141. M. Guan, F. Tao, J. Sun, Z. Xu, Langmiur 24 (2008) 8280. R. Qiao, X. Zhang, R. Qiu, J. Kim, Y. Kang, Chem. Mater. 19 (2007) 6485. C. Lu, L. Qi, J. Yang, X. Wang, D. Zhang, J. Xie, J. Ma, Adv. Mater. 17 (2005) 2562.