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Solution-Phase Synthesis of Spherical Zinc Sulfide Nanostructures Feng Gu,†,‡ Chun Zhong Li,‡ Shu Fen Wang,† and Meng Kai Lu¨*,† State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan 250100, China, and Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China UniVersity of Science & Technology, Shanghai 200237, China ReceiVed September 16, 2005 A facile solution-phase method has been developed to synthesize specially hollow and solid ZnS nanospheres. High-resolution TEM images on the nanospheres suggest their formation via the oriented aggregation of the primary ZnS nanocrystals. The morphology and size of the ZnS nanospheres can also be tuned easily by controlling the experimental conditions. These special spherical structures are very easily encapsulated within a uniform silica layer without any surface modification, suggesting potential applications in biochemistry and biodiagnostics.
Introduction The development of well-defined nanometer and micrometer structures may open new opportunities in catalysis, microelectronics, and photonics.1 Spherical nanostructures, including hollow spheres and solid spheres, have been intensively studied because of their unique properties, which are derived from their special morphologies. Generally, the spherically monodisperse morphology is an important factor for the low-light scattering at the surfaces, as well as the high-packing densities.2 Especially, hollow nanospheres have potential applications as photonic crystals, delivery vehicle system, fillers, and catalysts, owing to their tailored structural, optical, and surface properties.3 To date, hollow spheres of various materials such as carbons, polymers, metals, and inorganic materials have been extensively investigated in the literature.4 Of the various types of nanocrystals, semiconducting metal chalcogenide nanocrystals have been the most intensively studied because of their quantum confinement effects and size- and shape-dependent photoemission characteristics. These semiconductor nanocrystals have been applied to many different technological areas, including biological labeling and diagnostics, light-emitting diodes, photovoltaic devices, and lasers.5 Many semiconducting nanocrystals of metal sulfides with various compositions and shapes have been synthesized. ZnS is a wide band-gap semiconductor with a band-gap energy (Eg) of 3.6 eV. It has been used widely in displays, sensors, and lasers * To whom correspondence should be addressed. E-mail: mengkailu@ icm.sdu.edu.cn. † Shandong University. ‡ East China University of Science & Technology. (1) (a) Zakhidov, A. A.; Baughman, R. H.; Iqbal, Z.; Cui, C.; Khayrullin, Dantas, S. O.; Marti, J.; Ralchenko, V. G. Science 1998, 282, 897. (b) Johnson, S. A.; Ollivier, P. J.; Mallouk, T. E. Science 1999, 283, 963. (c) Holland, B. T.; Blanford, C. F.; Stein, A. Science 1998, 281, 538. (2) Kang, Y. C.; Park, S. B.; Lenggoro, I. W.; Okuyama, K. J. Phys. Chem. Solids 1999, 60, 379. (3) (a) Caruso, F. Chem. Eur. J. 2000, 6, 413. (b) Hollow and Solid Spheres and Microspheres: Science and Technology Associated with Their Fabrication and Application; Wilcox, D. L., Sr., Berg, M., Bernat, T., Kellerman, D., Cochran, J. K., Jr., Eds.; MRS Proc., Vol. 372; Materials Research Society: Pittsburgh, PA, 1994. (c) Caruso, F. AdV. Mater. 2001, 13, 11. (d) Yuan, J. K.; Laubernds, K.; Zhang, Q. H.; Suib, S. L. J. Am. Chem. Soc. 2003, 125, 4966. (4) (a) Wang, D.; Caruso, R. A.; Caruso, F. Chem. Mater. 2001, 13, 364. (b) Kulinowski, K. M.; Harsha, V.; Colvin, V. L. AdV. Mater. 2000, 12, 833. (c) Han, S.; Sohn, K.; Hyeon, Y. Chem. Mater. 2001, 13, 2337. (d) Yin, Y.; Lu, Y.; Gates, B.; Xia, Y. Chem. Mater. 2001, 13, 1146. (5) (a) Ma, Y.; Qi, L.; Ma, J.; Cheng, H.; Shen, W. Langmuir 2003, 19, 9079. (b) Huang, J.; Xie, Y.; Li, B.; Liu, Y.; Qian, Y.; Zhang, S. AdV. Mater. 2000, 12, 808. (c) Lee, S. M.; Jun, Y.; Cho, S. N.; Cheon, J. J. J. Am. Chem. Soc. 2002, 124, 11244. (d) Yu, S. H.; Yoshimura, M. AdV. Mater. 2002, 14, 296. (e) Sone, E. D.; Zubarev, E. R.; Stupp, S. I. Angew. Chem., Int. Ed. 2002, 11, 1706.
for many years.6 Recently doped ZnS nanomaterials have attracted considerable attention because of their superior luminescence characteristics as compared to those of their bulk counterparts.7 Several different synthetic routes have been investigated for the production of ZnS nanostructures.8 However, to our knowledge, synthesis of nanometer-sized hollow ZnS spheres has been limited by using silica and polystyrene spheres as sacrificial templates, in aqueous solutions of triblock copolymer P123.9 These preparations often suffer the limits of special equipment conditions and tedious procedures.10 When compared with the significant progress in the preparation of hollow nanospheres of other systems, the formation of well-defined ZnS hollow nanospheres has been laid behind. Therefore, it is desirable to explore diverse routes for the synthesis of spherical ZnS nanostructures with promising novel properties; especially, such methods that may be easily controllable, well-repeatable, mild, and feasible are needed urgently. In this paper, we demonstrate a facile solution chemical route for one-step, large-scale synthesis of hollow and solid ZnS nanospheres with tunable size by varying the experimental parameters. The formation mechanism of these spherical structures has been studied systematically. On the other hand, incorporation of luminescent semiconductor nanomaterials into nanospheres has been explored as a way to prepare bright biological labels and functional composite luminescent materials.11 For example, Rogach et al. encapsulated CdSe quantum dots in 40-80 nm silica nanospheres and proposed to use them as building blocks to form 3D colloid crystal microstructures.11b Herein, without any surface modification, our as-prepared spherical ZnS nanostructures can be easily (6) (a) Falcony, C.; Garcia, C.; Ortiz, A.; Alonso, J. C. J. Appl. Phys. 1992, 72, 1525. (b) Prevenslik, T. V. J. Lumin. 2000, 87-98, 1210. (7) (a) Bulanyi, M. F.; Kovalenko, A. V.; Polezhaev, B. A. Inorg. Mater. 2000, 39, 222. (b) Yang, P.; Lu, M.; Xu, D.; Yuan, D.; Song, C.; Zhou, G. J. Phys. Chem. Solids 2001, 62, 1181. (c) Xu, S. J.; Chua, D. J.; Liu, B.; Gan, L. M.; Chew, C. H.; Xu, G. Q. Appl. Phys. Lett. 1998, 73, 478. (d) Zhu, Y. C.; Bando, Y.; Xue, D. F.; Golberg, D. AdV. Mater. 2004, 16, 831. (8) (a) Li, Q.; Wang, C. R. Appl. Phys. Lett. 2003, 83, 359. (b) Jiang, Y.; Meng, X. M.; Liu, J.; Hong, Z. R.; Lee, C. S.; Lee, S. T. AdV. Mater. 2003, 15, 1195. (c) Zhao, Q.; Hou, L. S.; Huang, R. A. Inorg. Chem. Commun. 2003, 6, 971. (d) Xia, Y. N.; Gates, B.; Yin, Y. D.; Lu, Y. AdV. Mater. 2000, 12, 693. (e) Deng, Z. X.; Wang, C.; Sun, X. M.; Li, Y. D. Inorg. Chem. 2002, 41, 869. (f) Miyoshi, H.; Mori, H.; Yoneyama, H. Langmuir 1991, 7, 503. (9) (a) Ma, Y.; Qi, L.; Ma, J.; Cheng, H. Langmuir 2003, 19, 4040. (b) Velikov, K. P.; van Blaaderen, A. Langmuir 2001, 17, 4779. (c) Breen, M. L.; Donsmore, A. D.; Pink, R. H.; Qadri, S. Q.; Ratna, B. R. Langmuir 2001, 17, 903. (10) (a) Lee, J.; Sohn, K.; Hyeon, T. J. Am Chem. Soc. 2001, 123, 5146. (b) Liu, T.; Wan, Q.; Xie, Y.; Burger, C.; Liu, L.; Chu, B. J. Am. Chem. Soc. 2001, 123, 10966. (c) Cho, W.; Hanada, E.; Kondo, Y.; Takayanagi, K. Appl. Phys. Lett. 1996, 69, 278. (d) Zhang, Y.; Li, Y. D. J. Phys. Chem. B 2004, 108, 17805. (11) Chen, Y.; Ji, T.; Rosenzweig, Z. Nano Lett. 2003, 3, 581.
10.1021/la052539m CCC: $33.50 © 2006 American Chemical Society Published on Web 12/30/2005
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encapsulated within a uniform silica layer using a versatile modified Sto¨ber process, and novel optical properties have been exhibited, which make them appealing for practical applications in biochemistry and biodiagnostics. Experimental Section Sample Preparation. All of the reactants are analytical-grade and used without any further purification. The stock aqueous solution was prepared by dissolving 3.00 g of zinc nitrate (Zn(NO3)2‚6H2O) and 1.00-4.00 g of thioacetamide (TAA) in a 250-mL roundbottomed flask and by filling with 100-200 mL of distilled water. Afterward, the optically transparent stock solution was heated to the boiling point for refluxing (∼103 °C). Numerous bubbles can form in the flask during the refluxing period. After the mixture was refluxed for 30 min, a white precipitate was obtained, which was filtered and then washed with absolute ethanol and distilled water several times to remove possible residual impurities. After being dried in air at room temperature for several days, the products were collected for characterization. Coating of the ZnS nanospheres with silica has been achieved easily using a versatile modified Sto¨ber process by the aid of the sonication. Typically, 0.10 g of as-obtained ZnS was added to 50 mL of ethanol and the mixture sonicated for 15 min. Then 5 mL of NH3‚H2O (26%) and a certain amount of TEOS were added subsequently. The reaction lasted for 1.5 h under sonication without any cooling. The products were obtained by centrifugation, washed several times, and then vacuum-dried. Characterization. The X-ray diffraction (XRD) patterns of the samples were measured by using a Japan RigaKu D/MAX 2200PC diffractometer with Cu KR radiation (λ ) 0.15418 nm) and a graphite monochromator. Scanning electron micrograph (SEM) images were taken with a JEOL JSM-6700F scanning electron microscopy. Transmission electron micrograph (TEM) images were taken with a JEM-100CXII transmission electron microscope. High-resolution transmission electron micrograph (HRTEM) images were taken with a Philips Tecnai 20U-TWIN high-resolution TEM microscopy. Samples for the (HR)TEM were prepared by ultrasonically dispersing the product in ethanol, and then droplets were placed on carboncoated Cu grids. The Fourier transform infrared (FT-IR) spectra of the samples were collected using a NEXUS FTIR 670 infrared spectrometer. The excitation and photoluminescence (PL) spectra of the sample were measured with an Edinburgh FL920 fluorescence spectrometer.
Figure 1. XRD images of the spherical ZnS nanostructures. The inset shows the EDS spectrum of the hollow sample.
Results and Discussion The hollow and solid ZnS nanospheres were synthesized by refluxing the aqueous solution of the reagents using thioacetamide (TAA) as a S2- source with the temperature held at 103 °C. The spherical morphology has been formed through the oriented aggregation of the formed ZnS primary nanocrystals, and the morphology and size of the samples can be tuned easily by adjusting the experimental parameters properly. The probable reaction process for the formation of ZnS nanospheres can be summarized as follows:
CH3CSNH2 + H2O f CH3CONH2 + H2S H2S f H+ + HS- f 2H+ + S2Zn2+ + S2- f ZnS(particle) ZnS(particle) f ZnS(sphere) The phase purity of the products was characterized by XRD. All of the diffraction peaks shown in Figure 1 could be readily indexed to the cubic phase (space group F4h3m (216)) of ZnS (JCPDS 05-0566) with lattice constant a ) 5.427 Å. It is evident that the intensity of diffraction peaks of the hollow spheres is much stronger than that of the solid spheres, indicating a bigger
Figure 2. (a, b) SEM images of the hollow spheres. (c, e) TEM images of the hollow and solid ZnS spheres; insets of parts c and e are the corresponding ED patterns. (d, f) HRTEM images of the hollow and solid sphere; insets of d and f are the corresponding processed FFT. The bar is 100 nm.
particle size. The average crystallite sizes roughly estimated on the basis of the Scherrer formula are about 20 and 8 nm for the hollow and solid spheres, respectively. The EDS results for the hollow spheres reveal the presence of Zn and S in the products with almost the same atomic ratio (inset of Figure 1). A relatively weak oxygen peak in the spectrum probably originates from unavoidable surface-adsorption of oxygen on to the spheres from exposure to air during sample processing. The morphology and microstructure of the samples were examined by SEM, TEM, and HRTEM. Typical SEM image for the hollow ZnS spheres shown in Figure 2a reveals that the sample consists of well-spherical structures and the average size is about 160 nm. Figure 2b shows a high-magnification SEM image for the hollow spheres; it is obvious that the surface of
Synthesis of Spherical Zinc Sulfide Nanostructures
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Table 1. Summary of Experimental Conditions and Results of the Resultant ZnS Samplesa sample
A
B
C
D
E
F
Zn(NO3)2/g TAA/g time/min morphologyb average diameter/nm shell thickness/nm
3.00 1.00 30 S 350
1.50 1.00 30 S 172
3.00 2.00 15 N 21
3.00 2.00 30 H 158 53
3.00 3.00 30 H 130 40
3.00 4.00 30 H 90 25
a In the above-mentioned experiments, the refluxing temperature was fixed at 103 °C, and the other experimental conditions are analogous. b The letters S, H, and N represent solid sphere, hollow sphere, and nanocrystal, respectively.
the spheres is not smooth and constructed by nanoparticles about 20 nm. As shown in Figure 1c, the clear contrast observed of the ZnS spherical product suggests a hollow structure.3b Individual ZnS spheres are composed of an empty core with a shell. The average diameter of the spheres is calculated to be 158 nm and the shell thickness was 53 nm. The TEM image for the solid spheres shown in Figure 2e exhibits spherical morphology with good uniformity. The average size is estimated to be 172 nm. The ED patterns (insets in Figure 2c,e), taken from randomly chosen hollow and solid spheres, show diffuse rings, indicating that ZnS hollow and solid spheres are both polycrystalline. The concentric rings could be assigned as diffraction from {111}, {200}, {220}, and {311} planes of face-center-cubic (fcc) ZnS from the centermost ring, respectively. For further investigation of the inner structure of ZnS, HRTEM images have been taken. Parts d and f of Figure 2 show HRTEM images of the hollow and solid ZnS spheres, respectively. The corresponding fast Fourier transform (FFT) patterns (inset of Figure 2d,e) clearly reveal the cubic structure. Detailed analysis on the lattice fringes gives an interplanar spacing of 0.31 nm, which matches well with the (111) plane separation of the standard bulk zinc blende ZnS. The morphology and size of the samples were also found to be strongly dependent on the experimental conditions such as the initial concentration of TAA, reaction time, and the molar ratio between TAA and Zn(NO3)2. By suitably adjusting these parameters, the evolution from solid to hollow morphology can occur, and the diameter will also change simultaneously. These parameters and their influence on the ZnS nanospheres are listed in Table 1. When the amount of TAA was 1.00 g or less and the other experimental conditions were the same as the typical synthesis, solid ZnS spheres could be obtained. When the amount of TAA was increased, for example, from 1.00 to 2.00 g, that is, the concentration of TAA doubled, the morphology of the samples varied from solid to hollow structure; meanwhile, the diameter decreased remarkably. When the amount of TAA was further increased, the samples still exhibited hollow morphology, however, the size decreased gradually. When the reaction time was reduced to 15 min, ZnS nanocrystals of 21 nm could be obtained rather than spheres, which is consist with the results from SEM and TEM observation in Figure 2. The formation mechanism for the spherical ZnS nanostructures has been proposed, which can be attributed to the oriented aggregation of the initially formed ZnS nanocrystals. It is known that oriented aggregation only requires a two-dimensional structural accord within the plane of the approaching surfaces,12 and the lattice continuity among attached crystals has been confirmed by HRTEM results (Figure 3). As no templates have been introduced into the reaction system, the origin of forming hollow and solid spheres with different morphology is proposed to the contribution (12) Penn, R. L. J. Phys. Chem. B 2004, 108, 12707.
Figure 3. HRTEM image showing the lattice continuity among attacked crystals (sample A).
of H2S gas bubbles formed during the reaction. It is not rare that the hollow spheres take gas bubbles as the aggregation centers.13 Earlier reports have indicated that N2 and CO2 gas bubbles acted as aggregation centers during the formation of ZnSe and BaCO3 hollow spheres.13a,b Rudloff et al. have demonstrated that spherical CaCO3 morphology can also be obtained which temporarily stabilized nanoparticles aggregated at the air/solution interface. In the present case, when the initial concentration of TAA in the precursor solution is high enough, individual H2S gas bubbles can be formed during the reaction. The formed primary ZnS nanocrystals, driven by the minimization of interfacial energy, may aggregate around the gas-liquid interface between H2S and water, resulting in the formation of hollow ZnS spheres. A higher starting concentration of TAA will give a thinner shell due to the formation of many more H2S gas bubbles in the reaction. It is important to note that the cavity size keeps almost constant for all the samples, which can support the fact that the H2S gas bubbles formed during the reaction act as the temporary template. It is known that surface modification is a major challenge in nanoparticle preparation and the addition of an extra functionality onto each individual particle directly leads to a multifunctional nanoparticle and offers the opportunity to study the mutual interactions between the new functionality and the core materials.14 These ZnS nanostructures would benefit from having a silica shell to impart wettability and biocompatibility.15 Siilcacoated nanostructures are very useful for biological applications, since they allow for surface conjugation with amines, thiols, and carboxyl groups, which in turn would facilitate the linking of biomolecules, such as biotin and avidin. A silica shell should also minimize fluorescence quenching by surface adsorbates or redox-active molecules.16 However, uniform capping of silica on the nanostructures is often problematic due to the lack of coating techniques and poor surface interactions under the usual experimental conditions. Suitable surface modification is often required when carrying out the encapsulation.16b Herein, without (13) (a) Peng, Q.; Dong, Y. J.; Li, Y. D.Angew. Chem., Int. Ed. 2003, 42, 3027. (b) Ocana, M.; Rodriguez-Clemente, R.; Serna, C. J. AdV. Mater. 1995, 7, 212. (c) Yu, S. H.; Colfen, H.; Xu, A. W.; Dong, W. F. Cryst. Growth Des. 2004, 4, 33. (d) Rudloff, J.; Antonietti, M.; Colfen, H.; Pretula, J.; Kaluzynski, K.; Penczek, S. Macromol. Chem. Phys. 2002, 203, 627. (14) Liang, H. P.; Wan, L. J.; Bai, C. L.; Jiang, L. J. Phys. Chem. B 2005, 109, 7795. (15) (a) Barbe, C.; Bartlett, J.; Kong, L.; Finnie, K.; Lin, H. Q.; Larkin, M.; Calleja, S.; Bush, A.; Calleja, G. AdV. Mater. 2004, 16, 1959. (b) Chen, C. C.; Liu, Y. C.; Wu, C. H.; Yeh, C. C.; Su, M. T.; Wu, Y. C. AdV. Mater. 2005, 17. 404. (c) Yi, D. K.; Selvan, S. T.; Lee, S. S.; Papaefthymiou, C.; Kundaliya, D.; Ying, J. Y. J. Am. Chem. Soc. 2005, 127, 4990. (d) Zhang, Y.; Li, Y. D. J. Phys. Chem. B 2004, 108, 17805. (16) (a) Nann, T.; Mulvaney, P. Angew. Chem., Int. Ed. 2004, 43, 5393. (b) Chan, Y.; Zimmer, J. P.; Stroh, M.; Steckel, J. S.; Jain, R. K.; Bawendi, M. G. AdV. Mater. 2004, 16, 2092.
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layer using a versatile modified Sto¨ber process. After SiO2 coating, uniform particles containing a single ZnS sphere can be clearly observed, and tailored SiO2 shell thickness could be derived under controlled experimental conditions. Figure 4a shows a typical TEM image for the silica-coated ZnS hollow spheres; it is obvious that the ZnS hollow spheres are encapulated by a uniform layer of silica, and the thickness is about 65 nm. FTIR results confirmed the presence of both silica and ZnS in the samples. Absorption bands at 460 (Si-O-Si band), 795 (SiO-Si symmetric strength), 961 (Si-O-Si strength), and 1093 and 1190 cm-1 (Si-O-Si antisymmetric strengths)17 were observed after coating ZnS hollow spheres with SiO2 (Figure 4b). The data strongly suggested that the ZnS spheres were really encapsulated within the silica matrix. Figure 4c shows the roomtemperature photoluminescence spectra from the synthesized hollow ZnS nanospheres before and after silica coating. One dominant emission band centered at 474 nm has been observed, corresponding to the well-known ZnS-related luminescence of zinc vacancies.18 It should be noted that the luminescence doubles in intense after coating ZnS with SiO2 layer. The peak position did not shift. This is terrific for biological applications. The enhancement of the luminescence intensity can be ascribed to the removal of electron capture centers on the surface of ZnS nanospheres or removal of nonradiative decay channels because of the silica shell around ZnS nanospheres.
Conclusion In conclusion, well-defined hollow and solid ZnS nanospheres have been synthesized via a facile solution-phase route without using templates. Such special structures represent good candidates for further applications in various fields of nanoscale science and technology. In addition, these spherical structures are easily encapsulated within a uniform SiO2 layer without surface modification, exhibiting unique optical properties.
Figure 4. (a) TEM image of silica-coated ZnS hollow nanospheres; the thickness of the silica shell is about 65 nm. (b) IR spectra of the hollow ZnS spheres with and without SiO2 coating. (c) Photoluminescence spectra of ZnS hollow nanospheres with and without SiO2 coating.
any surface modification, our as-prepared spherical ZnS nanostructures can be easily encapsulated within a uniform silica
Acknowledgment. This work was partially supported by the National Natural Science Foundation of China (20236020, 20176009), the Major Basic Research Project of Shanghai (04DZ14002), the Pre-973 Project of China (2002CCA2200), the Special Project for Key Laboratory of Shanghai (04DZ05622), and the Special Project for Nanotechnology of Shanghai. LA052539M (17) Ma, D.; Li, M.; Patil, A. J.; Mann, S. AdV. Mater. 2004, 16, 1838. (18) (a) Zhang, W. H.; Shi, J. L.; Chen, H. R.; Hua, Z. L.; Yan, D. S.Chem. Mater. 2001, 13, 648. (b) Rogach, A. L.; Nagesha, D.; Ostrander, J. W.; Giersig, M.; Kotov, N. A. Chem. Mater. 2000, 12, 2676.
