A facile solution chemical route to self-assembly of CuS ball-flowers and their application as an efficient photocatalyst

Abstract

Ball-flower shaped CuS structures have been synthesized by using mixed copper chloride and thiourea in a simple hydrothermal process employing poly(vinylpyrrolidone) (PVP) as the surfactant. The morphological investigations by field emission scanning electron microscope (FE-SEM) reveal that the ball-flower shaped nanostructures are monodispersed in large quantities. The ball-flower shaped morphologies are strongly dependent on the different ratios of copper chloride to thiourea, the reaction temperature and reaction time. The possible growth mechanism of the formation of ball-flower shaped CuS products is discussed in detail. In addition, the photocatalytic activity of ball-flower shaped CuS architectures has been tested by the degradation of rhodamine B (RhB) under UV light irradiation, showing that the as-prepared ball-flower shaped CuS structures exhibit high photocatalytic activity for the degradation of RhB.

---

1. Introduction

The architectural controlled synthesis of nanostructured semiconductors has received much research attention, due to their outstanding physical and chemical properties and potential applications in numerous fields. As an important semiconductor material, copper sulfide has found many applications due to its optical properties,^1,2 chemical-sensing capability,³ ideal characteristics for solar energy absorption,⁴ thermoelectric cooling properties,⁵ fast-ion conduction at high temperature,^6,7etc. Recently, various morphologies of copper sulfide have been reported, such as plate-like,⁸ rod-like,⁹ tube-like structures,¹⁰ hollow spheres,^11,12 flower-like,¹³ snowflake-like, urchin-like patterns,¹⁴etc. Many techniques have been established to prepare copper sulfide in various forms, including hydrothermal or solvothermal methods,^15,16 microwave irradiation,¹⁷ chemical vapor deposition,¹⁸in situ template-controlled method.¹⁹ Obtaining new materials via a relative simple route and developing the morphology-controlled synthesis methodologies are a goal and evoke great interest in materials chemistry.²⁰ In solution approaches to copper sulfide, thiourea is a commonly used sulfur source, for instance, Zhu and co-workers²¹ have synthesized hollow spherical copper sulfide in water solution containing Cu(CH₃COO)₂, thiourea and 2-HP-β-CD. Qian et al. prepared CuS crystals with various morphologies using shuttle-like CuO and thiourea as raw reagents at low temperature.²² Wang et al. proposed a sonochemical approach to single crystalline CuS nanoplates.²³ However, up to now, there have been no reports on the synthesis of monodispersive ball-flower superstructures constructed with CuS nanosheets, and the controlled synthesis of CuS hierarchical nanostructures with different morphologies has not been performed.

In recent years, considerable attention has been given to the environmental problem involving organic pollutants in water. Photocatalysis is a promising technology for the treatment of contaminants, especially for the removal of organic compounds. Many investigations have been reported on utilizing metal oxide nanomaterials as photocatalysts to decompose or destroy the organic pollutants in water.²⁴ However, there is little research on the photocatalytic property of CuS superstructures with a high specific surface area, though other properties of CuS microstructure have been widely investigated.

In this work, we synthesized 3D hierarchical CuS ball-flowers by a newly designed hydrothermal self-assembly process. The simplest synthetic route to 3D nanostructures is probably a self-assembly in which ordered aggregates are formed in a spontaneous process.²⁵ Thiourea was also used as a sulfur source. However, now, an interesting but challenging question is raised, namely, what role does the thiourea play in the formation of metal sulfide nanostructures? To learn about the thiourea-based process in detail, we investigated the simplest aqueous system containing only CuCl₂ and thiourea. As expected, we found that thiourea reduces Cu(II) to Cu(I) as it combines to form a complex. The detailed shape evolution process from the intermediate complex to the final product was clearly shown, and the mechanism of formation of the hierarchical structure was studied. The photocatalytic properties of the as-obtained 3D hierarchical architectures were investigated under UV light irradiation. The results show that the synthesized CuS structures exhibit the superiority of photocatalytic performance and have exhibited much better photocatalytic properties for the photodegradation of RhB than that of other photocatalysts, such as TiO₂ nanopowders.

2. Experimental

2.1. Chemicals and synthesis

All reagents were of analytical grade and were used without further purification. In a typical experiment, an appropriate amount of copper chloride was dissolved in distilled water (10 mL) and formed a blue solution of a certain concentration under constant stirring for 10 min. Then, appropriate amounts of thiourea (Tu) and PVP were dissolved in distilled water (15 mL) and formed a colorless solution. Then the above two solutions were mixed slowly, a white suspension was formed in a few minutes, which was then transferred into a 30 mL Teflon-lined autoclave and maintained at 120–180 °C for 6–24 h, then cooled to room temperature naturally. The black precipitate was filtered off, washed with distilled water and absolute ethanol several times, and then dried in vacuum at 60 °C for 4 h. For comparison, the bulk CuS powders were prepared by a liquid state reaction. Simply, appropriate amounts of CuCl₂ and Na₂S were added into the beaker under stirring for 15 min and as a result, black products were obtained.

2.2. Characterization

X-Ray powder diffraction (XRD) was carried out on an XRD-6000 (Japan) X-ray diffractometer with Cu Kα radiation (λ = 1.54060 Å) at a scanning rate of 0.05 ° s⁻¹. Scanning electron microscopy (SEM) micrographs were taken using a Hitachi S-4800 Scanning electron microscope coupled with energy dispersive X-ray spectroscopy (EDS).

2.3. Photocatalytic property

The photocatalytic activity experiments of the obtained CuS for the decomposition of RhB in air were performed at ambient temperature. A cylindrical Pyrex flask (capacity ca. 100 mL) was used as the photoreactor vessel. CuS microstructues as catalyst (10 mg) was added to an aqueous RhB solution (2.0 × 10⁻⁵ M, 50 mL) and was magnetically stirred in the dark for 30 min to ensure establishing an adsorption–desorption equilibrium. The UV light was generated from a 300 W high-pressure mercury lamp. As a comparison, the photocatalytic activity of commercial TiO₂ powders (Degussa P25, Degussa Co., the surface area ca. 45 m² g⁻¹) was also tested in the same experimental conditions. UV-vis absorption spectra were recorded at different intervals to monitor the reaction using Solid Spec-3700 UV/vis spectrophotometer.

3. Results and discussion

3.1. Structure and morphology

The typical XRD patterns as shown in Fig. 1 reveal the phase and purity of the as-obtained CuS ball-flower structures. The diffraction peaks are indexed to the data of the hexagonal phase CuS (JCPDS Card File No. 06–0464). Compared with the standard diffraction data of the hexagonal phase CuS, we can find that all peaks shift to the lower angle slightly. The reasons may be ascribed to the following reasons: on the one hand, during sample preparation, the surface of the sample was above the sample platform or the zero point of the instrument was not accurate. On the other hand, the crystallite size was a non-uniform distribution after a hydrothermal treatment, which may also lead the peaks shift to the lower angle.

Fig. 1 XRD pattern of CuS hierarchical ball-flower structures prepared at 120 °C for 24 h.

The morphology of the prepared samples was further investigated with FE-SEM, and their typical images are displayed in Fig. 2. Fig. 2a displays a representative overview of the ball-flower CuS architectures (the concentration ratio of thiourea to Cu²⁺ (4:2), 24 h, 120 °C), which shows that the as-obtained products are composed of large-scale ball-flower architectures with diameters of 1.8–2.4 µm. Fig. 2b and c show high-magnification SEM images of CuS ball-flower architectures from a different angle of view, which vividly demonstrate that the ball-flower CuS structures are built of two-dimensional nanoplates. The nanoplates are well-ordered and oriented to form ball-flower architectures. Fig. 2d shows the EDS pattern of the obtained ball-flower products, which clearly reveals that the obtained products mainly consisted of Cu and S elements, the weak O signal comes from the oxidation of air. The atomic ratio of Cu to S is 1.08:1, which closes to the stoichiometry of CuS.

Fig. 2 SEM images of (a), (b), (c) different magnifications of the as-obtained CuS ball-flower structures. (d) EDS spectrum of the obtained products.

3.2. Influencing factors

3.2.1. Concentration ratio of Cu²⁺ to thiourea. In this work, we found that CuS ball-flower structures could be selectively obtained by adjusting the concentration ratio of CuCl₂·2H₂O to thiourea and hydrothermal temperature. To determine the effect of the concentration ratio of CuCl₂·2H₂O to thiourea on the formation of the products with different shapes, the images of the products obtained from the solutions with a concentration ratio of CuCl₂·2H₂O to thiourea of 1/1.5, 1/2.0, 2/3.0, 2/4.0 and hydrothermal heat treatment at 120 °C for 24 h are shown in Fig. 3. It is clearly seen that the morphology changed greatly with the change of the concentration ratio of CuCl₂·2H₂O to thiourea. Fig. 3a and 3b display the images of the product obtained at a concentration ratio of CuCl₂·2H₂O to thiourea of 1/1.5. It can be seen that trepang-like hierarchical architectures were achieved. With further increasing the thiourea concentration to 2.0 M, a large number of dandelion-like nanostructures composed of lots of ultrathin nanosheets were observed (Fig. 3c). When the concentration ratio of CuCl₂·2H₂O to thiourea was 2.0/3.0, microsphere-like architectures were obtained, as seen from Fig. 3e. The as-synthesized product entirely takes on the ball-flower morphology when the thiourea concentration is 4.0 M (Fig. 3g).

Fig. 3 FE-SEM images of the CuS nanostructures obtained by a hydrothermal treatment for 24 h with thiourea concentration of (a), (b) 1.5 M; (c), (d) 2.0 M; (e), (f) 3.0 M; (g), (h) 4.0 M at 120 °C.

From the above results, it can be deduced that both thiourea and copper ion concentrations influence the CuS morphologies critically. When thiourea was added to the copper(II) salt solution, the color of the solution changed from typically blue to green, indicating the thiourea–copper(II) complex is formed. Further addition of thiourea or just leaving the green solution standing for a longer time will cause the solution to become colorless, indicative of conversion into thiourea–copper (I) complexes.²⁶ During the experiment, when the concentration of Tu was 1.5 or 2.0, the green color of the thiourea–copper(II) reaction solution was persistently kept until the solution was transferred to an autoclave. However when the concentration of Tu was further increased to 3.0 and 4.0, the reaction solution rapidly became colorless, implying that excess thiourea significantly speeds up the redox process. On the basis of the above analysis, formation of different coordination precursors is a crucial factor determining the crystal growth process of the copper sulfide crystals.

3.2.2. Reaction temperature. To investigate the effect of the reaction temperature on the formation of CuS ball-flower structures, a series of comparative experiments was carried out through similar processes. It is found that the reaction temperature has a significant influence on the morphology of the as-synthesized CuS products. When the reaction temperature is below 120 °C, not only the yield of the products is low, but also the morphology of the sample is irregular. When the reaction temperature is elevated to 120 °C, the product is composed of ball-flower structures, and no other structures are observed any more (sample S4, Fig. 4a). At a higher reaction temperature of 140 °C, (Fig. 4b), the CuS ball-flower structures (sample S5, Fig. 4b) became thick and rigid. No obvious changes were observed in the morphology of CuS synthesized from the higher reaction temperatures 160 (Fig. 4c), however, the surface of the CuS hierarchical ball-flower structures became compact. When the reaction temperature increased to 180 °C, the sheets assembled in the ball-flower grew thicker than those appearing at 160 °C and the CuS ball-flower structures assembled by the compactly packed flakes were observed (Fig. 4d). Therefore, we can find that the exterior morphology of the final products did not change obviously with the increase of the reaction temperature; but the sheets composed ball-flowers thickened with the reaction temperature. It might be related to the reaction rate increased at high temperature but could not alter the growth process in the present reaction system. More in-depth studies are necessary to further understand their growth process.

Fig. 4 FE-SEM images of the CuS nanostructures obtained by a hydrothermal treatment for 24 h at (a) 120 °C; (b) 140 °C; (c) 160 °C; (d) 180 °C with a copper ion concentration of 0.1 M.

The detailed experimental parameters together with the morphological properties of the corresponding products (denoted as S1–S7) are listed in Table 1. Typical SEM images of the samples are shown in Fig. 2a, 2c, 2e and 2g.

Table 1 Morphologies and structures of CuS samples

Sample	T/°C	T/h	The molar ratio of CuCl2·2H2O to thiourea	Morphologhy
S1	120	24	1/1.5	Treapang
S2	120	24	1/2.0	Dandelion
S3	120	24	2/3.0	Microsphere
S4	120	24	2/4.0	Ball-flower
S5	140	24	2/4.0	Ball-flower
S6	160	24	2/4.0	Ball-flower
S7	180	24	2/4.0	Ball-flower

---

3.3. Formation of the CuS structures

In order to understand the evolution process of the ball-flower structures of CuS material, we carried out time-dependent experiments during which samples were collected at different time intervals from the reaction mixture.

As shown in Fig. 5, the CuS self-assembled into hierarchical ball-flower structures consisting of sheets. In the initial stage (6 h), only a small amount of the product was obtained. As shown in Fig. 5a, the morphology of the as-produced samples at the early stage consisted of very thin flakes with sizes of about 1.2–1.5 µm, and a small quantity of aggregation. With further increasing the reaction time to 10 h, these sheets aligned with clearly oriented layers, pointing toward a common center, as displayed in Fig. 5b. With the increase of the hydrothermal time to 14 h, no single sheets remained. All of the sheets assembled into nest-like structures (Fig. 5c). Finally, when the reaction was further prolonged to 18 h, the cores in the center of the spheres dissolved completely, resulting in the formation of nest-like structures with hollow cores, as demonstrated in Fig. 5d. As the reaction proceeded, the size of the pores among the flakes decreased obviously. At the elongated reaction time of 24 h, the morphology of the final product is shown in Fig. 2a. It is clear to see that much more sheets were assembled into hierarchical structures, and the typical hierarchical ball-flower structures formed. From this point, the size and morphology of the product remained the same even at longer reaction times. In addition, the PVP molecules play an important role in preventing the spheres from agglomerating as many polymer molecules do in the formation of other spherical species.^27,28 On the other hand, PVP in this study acts as a directing agent to promote the preferential 3D growth of CuS ball-flower. We also found that other surfactants, such as CTAB and SDBS etc. did not reach the equivalent effect as the PVP did. So we can say that the PVP molecules may be unique for the formation of the ball-flower structures in the present reaction system.

Fig. 5 SEM images of the CuS nanostructures obtained by hydrothermal treatment at 120 °C for (a) 6 h; (b) 10 h; (c)14 h; (d) 18 h with thiourea concentration of 4.0 M.

The XRD patterns of the products prepared at different reaction times are shown in Fig. 6. As shown in Fig. 6, the products prepared at different reaction times have similar XRD patterns, except for relative peak intensity levels which were due to the random orientation. All diffraction peaks can be indexed as hexagonal CuS structure with calculated lattice constants of about a = 3.792 Å and c = 16.344 Å. These calculated lattice constant values are consistent with the reported data for CuS (JCPDS Card no. 06–0464).

Fig. 6 The XRD pattern of products at different reaction times: 6 h (a), 10 h (b), 14 h (c) and 18 h.

On the basis of the above analysis, we proposed a reasonable mechanism for the formation of CuS ball-flower structures. The whole evolution process is illustrated in Fig. 7. In this formation process, time was the most important controlling factor. Such a process is consistent with previous reports of a so-called two-stage growth process, which involves a fast nucleation of amorphous primary particles followed by a slow aggregation and crystallization of primary particles.^29–32 In our experiment, thiourea first coordinated with CuCl₂ to produce thiourea–copper (II) complexes, which precipitated to become the nuclei and quickly grew into the primary particles. In the following secondary growth stage, the primary particles aggregated into sheets which became the base of the nest-like structure. The sheets continued to grow by combining with the remaining primary particles, finally forming the ball-flower structure. Many forces cause the sheets to self-assemble into the ball-flower morphology, such as electrostatic and dipolar fields associated with the aggregate, hydrophobic interactions, hydrogen bonds, crystal-face attraction, and van der Waals forces.^33,34 For example, Zhong³⁵ described the evolution mechanism of a flower-like iron oxide precursor in the presence of CTAB, which followed Ostwald ripening kinetics. Further work is underway to investigate the details of the self-assembly growth mechanism.

Fig. 7 Formation mechanism of hierarchical ball-flower structures of CuS.

3.4. Photocatalytic properties of CuS ball-flower

To demonstrate the potential application of as-synthesized hierarchical ball-flower CuS microstructures in the degradation of organic contaminants, we have investigated their photocatalytic activities by choosing the photocatalytic degradation of RhB as a model reaction. Fig. 8a shows the optical absorption spectra of RhB aqueous solution (initial concentration: 2.0 × 10⁻⁵ M, 50 mL) with 10 mg of the as-prepared CuS powders after exposure to ultraviolet light (UV) for different durations. The main absorption peak locates at 553 nm, which corresponds to the RhB molecules, decreases rapidly with extension of the exposure time, and completely disappears after about 60 min. Further exposure leads to no absorption peak in the whole spectrum, which indicates the total decomposition of RhB. It is clear that the intense pink color of the starting solution gradually disappears with increasing exposure time to the UV light.

Fig. 8 (a) Time-dependent absorption spectrum of a solution of RhB solution (2.0 × 10⁻⁵ M, 50 mL) in the presence of architectural CuS (10 mg) under exposure to UV light. (b) The RhB normalization concentration (from the optical absorbance measurements at 550 nm) in the solution with different catalysts (10 mg) vs. the exposure time. Starting RhB concentration C₀: 2.0 × 10⁻⁵ M. Line a: without any catalyst under UV light. Line b: the architectural CuS in the dark. Line c: the bulk CuS powders under UV light; Line d: Degussa P25 TiO₂ powders under UV light; Line e: the architectural CuS under UV light.

Further experiments were carried out to compare the catalytic activity of the as-prepared CuS architectures, bulk CuS powders and commercial Degussa P25 TiO2 powders. Fig. 8b shows the curves of the concentration of residual RhB with irradiation time. Without any catalyst, only a slow decrease in the concentration of RhB was detected under UV irradiation. The catalytic activity of the CuS architectures in the dark was also performed, which showed that only a little decrease in the concentration of RhB was detected in the dark, which confirmed the concentration decrease of RhB solution is mainly due to photodegradation of the products. The addition of catalysts leads to obvious degradation of RhB, and the photocatalytic activity depends on the morphology. For the CuS with a hierarchical microarchitecture, however, the activity is much higher than that of bulk CuS powders (line c) and commercial Degussa P25 TiO₂ powders (line d). The RhB solution is decolorized completely by using the CuS ball-flowers after UV irradiation for 50–60 min (line e).

It is generally accepted that the catalytic process is mainly related to the adsorption and desorption of molecules on the surface of the catalyst. The high specific surface area can provide more reactive adsorption/desorption sites for photocatalytic reactions.³⁶ In order to evaluate the surface area of the obtained CuS architectures, full nitrogen sorption isotherms were measured (Fig. 9). The specific surface area was thus evaluated to be 78.6 m² g⁻¹ from data points in this pressure range by the BET equation. The result showed that the obtained CuS architectures possess a larger specific surface area than that of Degussa P25 powders (∼45 m² g⁻¹).

Fig. 9 BET measurement of CuS architectures.

Therefore, the photocatalytic superiority of the CuS architectures may be attributed to their special structural features. The hierarchically microstructured CuS can provide a greater surface area than the reference samples Degussa P25 powders, which is obviously beneficial for the enhancement of photocatalytic performance. In addition, good dispersing and uniformity also can provide a large active surface area.^37,38

4. Conclusions

In summary, nearly monodispersed CuS ball-flower structures have been successfully synthesized via a facile hydrothermal process at low-temperature. The morphologies of the architectures can be selectively produced by adjusting the concentration ratio of CuCl₂·2H₂O to thiourea and hydrothermal temperature. The possible growth mechanism is also proposed, and this method may be extends to the preparation of other metal chalcogenide semiconductors. The photocatalytic properties of the hierarchical CuS architectures was explored; the results revealed that the obtained products exhibited excellent photocatalytic activity for degradation of RhB under exposure to UV light irradiation.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (20671003, 20971003), the Key Project of Chinese Ministry of Education (209060), the Education Department of Anhui Province (2006KJ006TD) and the Program for Innovative Research Team in Anhui Normal University.

References

1. K. Takase, M. Koyano, T. Shimizu, K. Makihara, Y. Takahashi, Y. Takano and K. Sekizawa, Solid State Commun., 2002, 123, 531.
2. E. J. Silvester, F. Grieser, B. A. Sexton and T. W. Healy, Langmuir, 1991, 7, 2917.
3. A. Etkus, A. Galdikas, A. Mironas, I. Simkiene, I. Ancutiene, V. Janickis, S. Kaciulis, G. Mattogno and G. M. Ingo, Thin Solid Films, 2001, 391, 275.
4. S. T. Lakshmikvmar and A. C. Rastogi, Sol. Energy Mater. Sol. Cells, 1994, 32, 7.
5. E. Ramli, T. B. Rauchfuss and C. L. Stern, J. Am. Chem. Soc., 1990, 112, 4043.
6. M. T. Nair and P. K. Nair, Semicond. Sci. Technol., 1989, 4, 191.
7. J. C. Folmer and F. Jellinek, J. Less-Common Met., 1980, 76, 153.
8. J. Zhang and Z. K. Zhang, Mater. Lett., 2008, 62, 2279.
9. G. Mao, W. Dong and D. G. Kurth, Nano Lett., 2004, 4, 249.
10. J. Y. Gong, S. H. Yu, H. S. Qian, L. B. Luo and X. M. Liu, Chem. Mater., 2006, 18, 2012.
11. H. L. Zhu, X. Ji, D. R. Yang, Y. J. Ji and H. Zhang, Microporous Mesoporous Mater., 2005, 80, 153.
12. X. L. Yu, C. B. Cao, H. S. Zhu, Q. S. Li, C. L. Liu and Q. H. Gong, Adv. Funct. Mater., 2007, 17, 1397.
13. S. Gorai, D. Ganguli and S. Chaudhuri, Cryst. Growth Des., 2005, 5, 875.
14. L. Y. Zhu, Y. Xie, X. W. Zheng, X. Liu and G. E. Zhou, J. Cryst. Growth, 2004, 260, 494.
15. J. Zou, J. X. Zhang, B. H. Zhang, P. T. Zhao and K. X. Huang, Mater. Lett., 2007, 61, 5029.
16. S. Gorai, D. Ganguli and S. Chaudhuri, Mater. Res. Bull., 2007, 42, 345.
17. X. H. Liao, N. Y. Chen, S. Xu, S. B. Yang and J. J. Zhu, J. Cryst. Growth, 2003, 252, 593.
18. S. K. Haram, A. R. Mahadeshwar and S. G. Dixit, J. Phys. Chem., 1996, 100, 5868.
19. J. Lu, Y. Zhao, N. Chen and Y. Xie, Chem. Lett., 2003, 32, 30.
20. X. F. Duan and C. M. Lieber, Adv. Mater., 2000, 12, 298.
21. J. Z. Xu, S. Xu, J. Geng, G. X. Li and J. J. Zhu, Ultrason. Sonochem., 2006, 13, 451.
22. K. B. Tang, D. Chen, Y. F. Liu, G. Z. Shen, H. G. Zheng and Y. T. Qian, J. Cryst. Growth, 2004, 263, 232.
23. H. L. Xu, W. Z. Wang and W. Zhu, Mater. Lett., 2006, 60, 2203.
24. X. F. Song and L. Gao, J. Phys. Chem. C, 2008, 112, 15299.
25. G. M. Whitesides and M. Boncheva, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 4769.
26. M. Q. Ai, P. E. Yue, D. O. Huang, Q. L. Han and Y. S. Cheng, Cryst. Growth Des., 2005, 5, 855.
27. X. H. Li, D. H. Zhang and J. S. Chen, J. Am. Chem. Soc., 2006, 128, 8382.
28. U. Jeong, Y. Wang, M. Ibisate and Y. Xia, Adv. Funct. Mater., 2005, 15, 1907.
29. C. Burda, X. B. Chen, R. Narayanan and M. A. El-Sayed, Chem. Rev., 2005, 105, 1025.
30. Y. Cheng, Y. S. Wang, Y. H. Zheng and Y. Qin, J. Phys. Chem. B, 2005, 109, 11548.
31. R. L. Penn, J. Phys. Chem. B, 2004, 108, 12707.
32. J. Park, V. Privman and E. Matijevic, J. Phys. Chem. B, 2001, 105, 11630.
33. H. Colfen and M. Antonietti, Angew. Chem., Int. Ed., 2005, 44, 5576.
34. H. Colfen and S. Mann, Angew. Chem., Int. Ed., 2003, 42, 2350.
35. L. S. Zhong, J. S. Hu, H. P. Liang, A. M. Cao, W. G. Song and L. J. Wan, Adv. Mater., 2006, 18, 2426.
36. L. H. Zhang, H. Q. Yang, J. Yu, F. H. Shao, L. Li, F. H. Zhang and H. Zhao, J. Phys. Chem. C, 2009, 113, 5434.
37. C. H. Ye, Y. Bando, G. Z. Shen and D. Golberg, J. Phys. Chem. B, 2006, 110, 15146.
38. J. S. Hu, L. L. Ren, Y. G. Guo, H. P. Liang, A. M. Cao, L. J. Wan and C. L. Bai, Angew. Chem., Int. Ed., 2005, 44, 1269.

---