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Facile Synthesis and Formation Mechanism of Metal Chalcogenides Hollow Nanoparticles Guoxin Chen, Mutong Niu, Lifeng Cui, Feng Bao, Lihua Zhou, and Yuansheng Wang* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Graduate School of Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ReceiVed: December 3, 2008; ReVised Manuscript ReceiVed: March 11, 2009
A facile route to synthesize Cu7S4 hollow hexagonal-like nanoparticles using Cu2O nanocubes as sacrificial templates at room temperature was developed. According to the transmission electron microscopy and field emission scanning electron microscopy observations, a formation mechanism of the Cu7S4 hollow structure based on the Kirkendall effect between the Cu7S4 shell and Cu2O core at nanoscale was proposed. Interestingly, no conventional mass transport bridges between shell and core were observed, implying that a novel solutionmediated mass transportation dominates the Kirkendall process. The Cu2Se mesoboxs, with cubic morphology of the Cu2O templates preserved, were also fabricated by a similar route, which suggests that such synthesizing method might be readily extended to fabricate hollow structures of other metal chalcogenides. 1. Introduction Hollow structured materials have been drawing intense research interest on account of their unique structure-induced optical, electrical, and catalysis properties that may bring a series of opportunities for their applications as photoelectric devices, drug delivery, sensors, chemical catalysis, and so on.1–4 Various chemical methodologies have been developed to achieve this special structure, either by using removable templates such as silica and carbon spheres,5,6 emulsion droplets,7,8 micelles,9 and gas bubbles,10 or by emulsion droplets directing self-assembly of the primary particles without using external templates.11–13 Recently, the Kirkendall effect, a conventional physical process, has been introduced for the formation of hollow structured materials,14–16 where hollow structures are formed due to the generation of vacancies when one species diffuses faster than another through the interface between two components.17 As important optoelectronic,18,19 chemical sensing,20 and high temperature thermistor21 materials, copper sulfides microparticles with various morphologies of flowers, distorted rods, spheres, concaved cuboctahedrons, etc., were prepared by solvothermal reactions.22–24 Recently, several hollow structured copper sulfides nanoparticles, including the 18-facet Cu7S4 nanocages and the nonspherical CuxS mesocages, were fabricated via the Kirkendall effect.25,26 However, most of these syntheses were of great complexity in experimental procedures, and direct observations on the hollow structure formation were generally absent, probably due to the too fast Kirkendall diffusion.27 Herein, we developed a facile solvent route to synthesize the Cu7S4 hollow structured nanoparticles at room temperature by using cubic Cu2O as sacrificial templates. Transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM) observations revealed that the Cu7S4 hollow nanoparticles were formed through a Kirkendall process between the Cu7S4 shell and Cu2O core at nanoscale. Worthy of noticing, we did not find any conventional mass transport bridges between the Cu7S4 shell and Cu2O core; therefore, a novel solutionmediated mass transport mechanism should dominate the * Corresponding author. Tel.: +86-591-8370-5402. Fax: +86-591-83705402. E-mail:
[email protected].
Figure 1. XRD patterns of (a) the presynthesized Cu2O, (b) the intermediate products when Cu/S is 2:1, and (c) the final products when Cu/S is 1:2; (d) and (e) show the standard XRD patterns of Cu2O and Cu7S4.
Kirkendall process. Via a similar route, the Cu2Se mesoboxes preserving cubic morphology of the Cu2O templates were also fabricated. 2. Experimental Section As the precursors, the monodispersed Cu2O nanocubes were prepared via a modified solution route previously reported by Cao et al.25 The desired products were fabricated by adding Na2S into the Cu2O suspending solution, with a Cu/S molar ratio of 1:2 in the mixed system. In a typical experiment, 2 mL of Na2S aqueous solution (0.100 mM) was added into 20 mL of Cu2O aqueous solution (2.50 × 10-3 mM). After being stirred and reacted for 30 min at room temperature, the precipitated products were collected by centrifugation, washed several times with
10.1021/jp9012834 CCC: $40.75 2009 American Chemical Society Published on Web 04/09/2009
Synthesis of Metal Chalcogenides Hollow Nanoparticles
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Figure 2. SEM (a) and TEM (b) micrographs of the presynthesized Cu2O nanocubes; SEM (c) and TEM (d) micrographs of the Cu7S4 hollow nanoparticles; the inset of (c) shows an individual Cu7S4 particle.
Figure 3. TEM micrographs of the reacted products when Cu/S molar ratio in the reaction solution is (a) 4:1, (b) 2:1, and (c) 1:2. (d) HRTEM image taken from the squared region in (c); the inset shows the fast Fourier transform pattern of the squared region in (d).
deinonized water and absolute ethanol, and then dried at 40 °C in a vacuum for 4 h. Powder X-ray diffraction (XRD) patterns of the as-synthesized samples were recorded by an X-ray diffractometer
(RIGAKU-DMAX2500) with Cu KR radiation (λ ) 0.154 nm) at a scanning rate of 5°/min for 2θ ranging from 5° to 85°. The morphology and microstructure of the products were characterized by TEM (JEM-2010) equipped with an energy dispersive
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Figure 4. EDS spectra taken from the regions marked by the circles 1 and 2 in Figure 3b.
X-ray spectrometer (EDS) operated at 200 kV, and FESEM (JSM-6700F) working at 10 kV. 3. Results and Discussion XRD patterns of the prepared samples were presented in Figure 1. Pattern a is taken from the presynthesized cubic Cu2O (PDF # 050667), while pattern c from the final products can be indexed to the pure monoclinic Cu7S4 (PDF # 230958). Pattern b is from the intermediate product when the Cu/S molar ratio in the reaction solution was kept as 2:1, where both Cu2O and Cu7S4 peaks could be found. SEM and TEM micrographs of the presynthesized Cu2O, shown in Figure 2a and b, present the well-defined solid cubic morphology sized around 130 nm. The inset of Figure 2b is the selected area electron diffraction (SAED) pattern from an individual Cu2O nanocube, revealing its monocrystalline nature. Figure 2c shows the SEM micrograph of the Cu7S4 products, which are hexagonal-like particles sized around 130 nm. The sharp contrast between the dark edges and pale centers in TEM micrograph shown in Figure 2d reveals the hollow structure of the Cu7S4 particles with shell thickness of about 15 nm. To investigate the structural evolution of the Cu7S4 hollow structured nanoparticles, reactions with the mixed solution having different Cu/S molar ratios of 4:1, 2:1, and 1:2 were conducted, by modifying the amount of Na2S while keeping the amount of Cu2O fixed in the reaction system. When the Cu/S molar ratio was 4:1, the starting Cu2O solid nanocubes were converted to the cubic core/shell particles of similar size, with a narrow gap generated between the shell and core, as shown in Figure 3a. The core/shell particles with smaller cores and thicker shells were sequentially obtained when the Cu/S molar ratio was reduced to 2:1, as exhibited in Figure 3b. With further decreasing the Cu/S molar ratio to 1:2, the Cu7S4 hollow nanoparticles were finally formed, as shown in Figure 3c. Highresolution TEM (HRTEM) observation on the hollow nanoparticle revealed that the lattices of the nanocrystals in the shell are arranged in different directions, and thus the shell as a whole is polycrystalline. Figure 3d is the HRTEM image taken from
Figure 6. TEM (a) and SEM (b) micrographs of the Cu2O/Cu2Se core/ shell structures with Cu/Se molar ratio in the reaction solution of 2:1; TEM (c) and SEM (d) micrographs of the Cu2Se mesoboxes with Cu/ Se molar ratio in the reaction solution of 1:2.
the squared region of the shell in (c), showing the lattice structure of a Cu7S4 nanocrystal. Figure 4 exhibits the EDS spectra taken from the regions marked by circles 1 and 2 in Figure 3b. Both O and S were detected from the core, while only S was found from the shell (the C and Au peaks were from the carbon film supported by Au grids). The O and S signals were from Cu2O and Cu7S4, respectively. We thus conclude that the shell of the intermediate product is Cu7S4, while the core is Cu2O. When detecting the EDS spectrum of the core, the electron beam had to penetrate through the Cu7S4 shell, resulting in the emergence of both S and O signals. Interestingly, TEM observation indicates that there are no any mass transport bridges that appear in the conventional Kirkendall process between shell and core.27–29 Besides, with decreasing Cu/S molar ratio, the shell outline of the products undergoes a series of changes from slightly truncated square to hexagonlike under TEM observation. On the basis of the results stated above, the structure evolution of the Cu7S4 hollow nanoparticles with the decrease of Cu/S molar ratio in the reacting solution (i.e., increase of the added Na2S) is proposed, as schematically demonstrated in Figure 5. Specifically, a thin Cu7S4 layer first deposits on the surface of the starting Cu2O nanocube, forming the Cu2O/Cu7S4 core/shell structure (step A) through the reaction of Cu2O with Na2S in the solution:26
Figure 5. Schematic illustration of the structure evolution of a Cu7S4 nanoparticle from a Cu2O nanocube with the decrease of Cu/S molar ratio.
Synthesis of Metal Chalcogenides Hollow Nanoparticles
14Cu2O(s) + 16S2-(aq) + O2 + 16H2O f 4Cu7S4(s) + 32OH-(aq) The Kirkendall process then takes place at the interface between the Cu7S4 shell and Cu2O core, where the outward diffusion of Cu ions from the core to shell is dominant due to its diameter being smaller than that of S ions, resulting in the generation of voids at the interface with high defect density and energy.29 As the diffusion of Cu ions proceeds, the continuously generated voids deform and combine together to form the gap (step B). With the increase of Na2S in the system, because Cu7S4 has much lower solubility than Cu2O in the aqueous solution (KspCu7S4 , KspCu2O),26 Cu ions of the Cu2O core continue to diffuse through the gap and Cu7S4 shell, and react with S ions in the solution to form Cu7S4, leading to thickening of the Cu7S4 shell and diminishing of the Cu2O core (step C). When the amount of Na2S in the system is appropriate, herein, keeping the Cu/S molar ratio at 1:2, the final Cu7S4 hollow structured nanoparticle is obtained (step D). Because no “mass bridges” were observed, a key question arises: how is the mass transported from the core to the shell through the gap between them? Because the reaction is proceeded in a solvent system, the gap between the Cu7S4 shell and Cu2O core is filled with the solution, which is believed to take the role of mediating the mass transportation in the gap; in other words, an unconventional solution-mediated mass transportation dominates the Kirkendall process. To verify the applicability of this facile synthetic route in fabricating other metal chalcogenides, the cubic Cu2Se phase was prepared using a similar method by simply replacing Na2S with selenide. As expected, the Cu2O/Cu2Se core/shell structures and Cu2Se mesoboxes, as shown in Figure 6, were obtained with Cu/Se molar ratios in the reaction solution of 2:1 and 1:2, respectively. This result suggests that such synthesizing strategy might be readily extended to fabricate hollow structures of other metal chalcogenides. 4. Conclusions In summary, the Cu7S4 hollow structured nanoparticles were successfully fabricated using the cubic Cu2O as sacrificial template via a facile solution strategy at room temperature. The Cu7S4 hollow structures were formed via the Kirkendall process between the Cu7S4 shell and Cu2O core at nanoscale assisted by a novel solution-mediated mass transportation. Using this strategy, the Cu2Se mesoboxes were also prepared, suggesting that it might be readily extended to fabricate hollow structures of other metal chalcogenides that have potential applications in many fields.
J. Phys. Chem. C, Vol. 113, No. 18, 2009 7525 Acknowledgment. This work was supported by NSFC (50672098), the Science & Technology Projects of Fujian (2008F3114), and the projects of CAS (KJCX2-YW-M05), FJIRSM (SZD07004, 2006K02), and SKLSC (20080039). References and Notes (1) Caruso, F. AdV. Mater. 2001, 13, 11. (2) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz1, D. A. Science 2002, 298, 1006. (3) Wang, Z. L.; Daemen, L. L.; Zhao, Y.; Zha, C. S.; Downs, R. T.; Wang, X. D.; Wang, Z. L.; Hemley, R. J. Nat. Mater. 2005, 4, 922. (4) Xu, A. W.; Yu, Q.; Dong, W. F.; Antonietti, M.; Colfen, H. AdV. Mater. 2005, 17, 2217. (5) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (6) Sun, X. M.; Li, Y. D. Angew. Chem., Int. Ed. 2004, 43, 3827. (7) Putlitz, B. Z.; Landfester, K.; Fischer, H.; Antonietti, M. AdV. Mater. 2001, 13, 500. (8) Bao, J. C.; Liang, Y. Y.; Xu, Z.; Si, L. AdV. Mater. 2003, 15, 1832. (9) Zhang, D.; Qi, L.; Ma, J.; Cheng, H. AdV. Mater. 2002, 14, 1499. (10) Peng, Q.; Dong, Y. J.; Li, Y. D. Angew. Chem., Int. Ed. 2003, 42, 3027. (11) Bigi, A.; Boanini, E.; Walsh, D.; Mann, S. Angew. Chem. 2002, 114, 2267. (12) Yuan, J. K.; Laubernds, K.; Zhang, Q. H.; Suib, S. L. J. Am. Chem. Soc. 2003, 125, 4966. (13) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124. (14) Fan, H. J.; Scholz, R.; Kolb, F. M.; Zacharias, M.; Gosele, U. Solid State Commun. 2004, 130, 517. (15) Ye, L. N.; Wu, C. Z.; Guo, W.; Xie, Y. Chem. Commun. 2006, 42, 4738. (16) Shao, H. F.; Qian, X. F.; Zhu, Z. K. J. Solid State Chem. 2005, 178, 3522. (17) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711. (18) Nascu, C.; Pop, I.; Ionescu, V.; Bratu, I.; Indrea, E. Mater. Lett. 1997, 32, 73. (19) Takase, K.; Koyano, M.; Shimizu, T.; Makihara, K.; Takahasi, Y.; Takano, Y. Solid State Commun. 2002, 123, 531. (20) Glazov, V. M.; Shchelikov, O. D.; Burkhanov, A. S. Inorg. Mater. 1993, 59, 1019. (21) Setkus, A.; Galdikas, A.; Mironas, A.; Simkiene, I.; Ancutiene, I.; Janickis, V. Thin Solid Films 2001, 391, 275. (22) Gorai, S.; Ganguli, D.; Chaudhuri, S. Cryst. Growth Des. 2005, 5, 875. (23) Wu, C. Y.; Yu, S. H.; Antonietti, M. Chem. Mater. 2006, 18, 3599. (24) Wan, Y.; Wu, C. Y.; Min, Y. L.; Yu, S. H. Langmuir 2007, 23, 8526. (25) Cao, H. L.; Qian, X. F.; Wang, C.; Ma, X. D.; Yin, J.; Zhu, Z. K. J. Am. Chem. Soc. 2005, 127, 16024. (26) Jiao, S. H.; Xu, L. F.; Jiang, K.; Xu, D. S. AdV. Mater. 2006, 18, 1174. (27) Yin, Y. D.; Erdonmez, C. K.; Cabot, A.; Hughes, S.; Alivisatos, A. P. AdV. Funct. Mater. 2006, 16, 1389. (28) Fan, H. J.; Gosele, U.; Zacharias, M. Small 2007, 10, 1660. (29) Lou, X. W.; Arhcer, L. A.; Yang, Z. AdV. Mater. 2008, 20, 1.
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