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Langmuir 2006, 22, 398-401
Sonochemical Synthesis of Hollow PbS Nanospheres Shu Fen Wang,† Feng Gu,‡ 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, East China UniVersity of Science & Technology, Shanghai 200237, China ReceiVed July 9, 2005. In Final Form: September 24, 2005 PbS hollow nanospheres with diameters of 80-250 nm have been synthesized by a surfactant-assisted sonochemical route. The nanostructures were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), (high-resolution) transmission electron microscopy [(HR)TEM], and scanning electron microscopy (SEM) images. Structural characterization indicates that shells of the hollow spheres are composed of PbS nanoparticles with diameters of about 12 nm. The formation of the hollow nanostructure was explained by a vesicle-template mechanism, in which sonication and surfactant play important roles. Furthermore, uniform silica layers were successfully coated onto the hollow spheres via a modified Sto¨ber method to enhance their performance for promising applications.
Introduction PbS is an important semiconductor with a small band gap of 0.41 eV in bulk form and a larger exciton Bohr radius of 18 nm, which makes the quantum-confined effects more notable even for relatively larger particle sizes. Recent reports reveal that luminescent characteristics of PbS nanoparticles covering both visible and near-infrared regions are size-tunable.1 It hence can find potential applications in making electroluminescent devices such as light-emitting diodes and infrared detectors.2 Moreover, the nonlinear optical properties make it useful in optical switches.3 PbS nanocrystals with different morphologies have been prepared including wires,4 rods,5 and tubes,6 as well as specially dendritic7 and star-shaped structures,8 by different methods such as solvothermal,7,9 microwave irradiation,10 and thermal decomposition.8 Recently, hollow micro- or nanospheres have attracted much attention because of their specific structures and potential applications. Owing to their low density, large surface area, and surface permeability, hollow spheres are widely used as artificial cell, catalysts, fillers, and controlled release capsules for drugs and dyes.11 So far, hollow spheres of many inorganic compounds such as PbTe, MnO2, and TiO2 have been prepared by various methods including solvothermal,12 self-assembly13 and template-
involved methods.14 To the best of our knowledge, investigations on the preparation and characterization of PbS hollow nanospheres have never been reported. Here, we chose a surfactant-assisted sonochemical method to synthesize hollow PbS nanospheres. Sonochemical process has been proven to be a useful technique in synthesis of novel materials with unusual properties. During the sonication process, the formation, growth, and implosive collapse of bubbles in a liquid can drive many chemical reactions such as oxidation, reduction, dissolution, decomposition, and polymerization.15-17 Hollow nanospheres such as CdSe18 and MoS219 have been prepared by this method. It has been discovered that the ultrasound wave has a strong effect on the congregation and self-assembly of nanoparticles.16 Meanwhile, surfactantassisted synthesis has been proved to be effective and appealing because surfactants can act as soft templates as well as structuredirecting agents for the assembly and subsequent mineralization of inorganic precursors at the surfactant-solution interface.20 In our case, PbS hollow spheres have been synthesized via the collaborative action of surfactants and sonication. The process is very facile and easily extended to prepare other kinds of metal chalcogenide hollow spheres. Experimental Section
* To whom correspondence should be addressed: e-mail mengkailu@ icm.sdu.edu.cn. † Shandong University. ‡ East China University of Science& Technology.
(a) Fabrication of PbS Hollow Spheres. Pb(CH3COO)2‚3H2O [Pb(Ac)2], thioacetamide (TAA), and sodium dodecylbenzenesulfonate (DBS) were used as received. Typically, 0.2 g of DBS was
(1) (a) Lim, W. P.; Low, H. Y.; Chin, W. S. J. Phys. Chem. B. 2004, 108, 13093. (b) Warner, J. H.; Thomsen, E.; Watt, A. R.; Heckenberg, N. R.; RubinszteinDunlop, H. Nanotechnology 2005, 16, 175. (2) Patel, A. A.; Wu, F.; Zhang, J. Z.; Torres-Martinez, C. L.; Mehra, R. K.; Yang, Y.; Risbud, S. H. J. Phys. Chem. B 2000, 104, 11598. (3) Kane, R. S.; Cohen, R. E.; Silbey, R. J. Phys. Chem. 1996, 100, 7928. (4) Yu, D.; Wang, D.; Meng, Z.; Lu, J.; Qian, Y. J. Mater. Chem. 2002, 12, 403. (5) Wang, S.; Yang, S. Langmuir 2000, 16, 389. (6) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446. (7) Kuang, D.; Xu, A.; Fang, Y.; Liu, H.; Frommen, C.; Fenske, D. AdV. Mater. 2003, 15, 1747. (8) Lee, S. M.; Jun, Y. W.; Cho, S. N.; Cheon, J. W. J. Am. Chem. Soc. 2002, 124, 11244. (9) Xu, L.; Zhang, W.; Ding, Y.; Yu, W.; Xing, J.; Li, F.; Qian, Y. J. Cryst. Growth. 2004, 273, 213. (10) Ni, Y.; Liu, H.; Wang, F.; Liang, Y.; Hong, J.; Ma, X.; Xu, Z. Cryst. Growth. Des. 2004, 4, 759. (11) (a) Caruso, F. Chem. Eur. J. 2000, 6, 413. (b) Huang, H.; Remsen, E. E. J. Am. Chem. Soc. 1999, 121, 3805. (12) Zou, G. F.; Liu, Z. P.; Wang, D. B.; Jiang, C. L.; Qian, Y. T. Eur. J. Inorg. Chem. 2004, 4521.
(13) (a) Wang, L. Z.; Ebina, Y.; Takada, K.; Sasaki, T. Chem. Commun. 2004, 1074. (b) Hu, J. S.; Guo, Y. G.; Liang, H. P.; Wan, L. J.; Bai, C. L.; Wang, Y. G. J. Phys. Chem. B 2004, 108, 9734. (14) (a) Zhong, Z. Y.; Yin, Y. D.; Gates, B.; Xia, Y. N. AdV. Mater. 2000, 12, 206. (b) Lei, Z. B.; Li, J. M.; Ke, Y. X.; Zhang, H. C.; Li, F. Q.; Xing, J. Y. J. Mater. Chem. 2001, 11, 2930. (15) (a) Suslick, K. S.; Choe, S. B.; Cichowlas, A. A.; Grinstaff, M. W. Nature 1991, 353, 414. (b) Suslick, K. S.; Hammerton, D. A.; Cline, R. E. J. Am. Chem. Soc. 1986, 108, 5641. (c) Suslick, K. S. Ultrasound: Its Chemical, Physical, and Biological Effects; VCH Verlagsgesellschaft: Weinheim, Germany, 1988. (16) Xu, S.; Wang, H.; Zhu, J. J.; Xin, X. Q.; Chen, H. Y. Eur. J. Inorg. Chem. 2004, 4653. (17) (a) Wang, Y.; Tang, X.; Yin, L.; Huang, W.; Hacohen, Y. R.; Gedanken, A. AdV. Mater. 2000, 12, 1183. (b) Okitsu, K.; Bandow, H.; Maeda, Y. Chem. Mater. 1996, 8, 315. (c) Pol, V. G.; Srivastava, D. N.; Palchik, O.; Palchik, V.; Slifkin, M. A.; Weiss, A. M.; Gedanken, A. Langmuir 2002, 18, 3352. (18) Zhu, J. J.; Xu, S.; Wang, H.; Zhu, J. M.; Chen, H. Y. AdV. Mater. 2003, 15, 156. (19) Dhas, N. A.; Suslick, K. S. J. Am. Chem. Soc. 2005, 127, 2368. (20) (a) Hentze, H. P.; Raghavan, S. R.; MaKelvey, C. A.; Kaler, E. W. Langmuir 2003, 19, 1069. (b) Lootens, D.; Vautrin, C.; Damme, H. V.; Zemb, T. J. Mater. Chem. 2003, 13, 2072.
10.1021/la0518647 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/19/2005
Sonochemical Synthesis of Hollow PbS Nanospheres
Langmuir, Vol. 22, No. 1, 2006 399 diffractometer with graphite monochromatized Cu KR irradiation (λ ) 1.5418 Å)] and Fourier transform infrared (FT-IR) spectra (American Nicolet FTIR 20SX spectrometer).
Results and Discussion
Figure 1. XRD pattern of the as-obtained PbS hollow spheres. dissolved in 100 mL of distilled water to form a homogeneous solution. Then 1.5 g of Pb(Ac)2 and 1.0 g of TAA were added into the above solution. The mixture was then transferred to a commercial ultrasonic cleaning bath (Shanghai, S-1200H, 49 Hz, 50 W) to sonicate for 4 h. As the reaction proceeded, the solution underwent gradual changes in color from gray to black. Black precipitation was collected by filtration (equipped with a vacuum pump), washed with water and ethanol several times, and then vacuum-dried at 60 °C for 3 h. The sonication was conducted without cooling so that the temperature was raised to about 45 °C at the end of the reaction. For comparison, parallel experiments were also carried out with different surfactants or lead sources. (b) Silica Coating Process. Silica-coated PbS hollow spheres were prepared via a modified Sto¨ber method by the aid of sonication. Typically, 0.1 g of as-obtained PbS nanospheres was added to 50 mL of ethanol to sonicate for 15 min. Then 5 mL of NH3‚H2O (26%) and a given amount of tetraethyl orthosilicate (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. (c) Characterization. The morphologies of the products were characterized by transmission electron microscopy (TEM) (Japan JEM-100CXII transition electron microscope), scanning electron microscopy (SEM) (JEOL JSM-6700F field-emission microscope), and high-resolution (HR) TEM images (Philips Tecnai 20U-TWIN). The phase composition and phase structure were characterized by X-ray diffraction (XRD) patterns [Japan Rigaku D/max-γA X-ray
The XRD pattern of the product is shown in Figure 1. All diffraction peaks can be indexed well to the cubic rock salt structure of PbS (space group Fm3m, JCPDS 05-0592). The average size calculated from XRD reflections by application of Scherrer’s equation is 12 nm, implying that the hollow spheres are composed of smaller nanocrystals. The morphology and microstructure of PbS hollow nanospheres are clearly demonstrated by TEM and SEM images. As shown in Figure 2a, the strong contrast between the dark edge and the relatively bright center is evidence for their hollow nature. The diameter of the hollow sphere is in the range of 80∼200 nm and the wall thickness is roughly estimated to be around 20 nm from the enlarged TEM image of a single hollow sphere (Figure 2b). From the SEM image shown in Figure 2e, spherical nanostructures with a smooth exterior have been observed clearly. The diameter of the sphere is about 80∼250 nm, which is comparable to the TEM results. A broken sphere with apparent cavity is shown in the inset of Figure 2e, further demonstrating the hollow nature of the products. In addition, a few fragments can be found during the SEM observation, indicating that the spheres are not very compact and some of them may be destroyed by intensive postsonication. High-resolution TEM (HRTEM) provided further insight into the microstructure of the PbS nanospheres. As shown in Figure 2c, PbS nanocrystals with diameters of about 12 nm were observed, indicating that the walls of the PbS nanospheres are really constructed by small particles. Figure 2d shows a typical HRTEM image of an individual PbS nanocrystal. The distance between two lattice fringes was 0.29 nm, which is consistent with the separation of cubic PbS (200) planes. A fundamental understanding of the formation mechanism of PbS hollow nanospheres is critical for achieving control over their nanoparticle properties. To better understand the formation mechanism of PbS hollow spheres, a series of parallel experiments were carried out. Under the same experimental conditions, when no surfactant is present in the solution, only irregular PbS rods were observed (Figure 2f), which can be attributed to the oriented growth characteristics of PbS crystals.9 When the surfactant used was varied from DBS to cetyltrimethyl ammonium bromide (CTAB), well-crystalline PbS rods of 0.3-0.4 µm in width and
Figure 2. (a-e) TEM, HRTEM and SEM images of the PbS hollow spheres. For comparison, TEM images of the samples prepared under different experimental conditions are also shown: (f) in the absence of DBS, (g) CTAB used as surfactant, and (h) Pb(NO3)2 used as lead source.
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Scheme 1. Illustration of the Spheres’ Formation and Coating Process
3.5-7 µm in length were observed (Figure 2g). As CTAB can form CH3-CH3-CH3-N structure and induce the sphere-rod transition of micelles in aqueous solution,21 the formation of rods is closely related to the addition of CTAB. All observations indicate that the surfactant used is the prerequisite for the formation of hollow PbS nanostructures. It is known that, during an aqueous sonochemical process, the temperature and pressure inside collapsing bubbles are elevated,15-17 which favors the reaction between TAA and water to produce H2S. The formation process of PbS nanoparticles can be summarized as follows:
their assembly. With the reaction proceeding, the formed PbS nanoparticles undergo mineralization to form a relatively compact PbS layer on the surface of the vesicles. The vesicle template can be successfully extracted by the washing process without destroying the spherical structures. However, when Pb(NO3)2 was chosen as lead source instead of Pb(Ac)2, solid PbS nanospheres were obtained (Figure 2h). In this case, formation of the solid sphere has been found to originate from the change of S2- release rate and the binding force between S2- and Pb2+ affected by the anions. As the surface
H2O )))) H• + OH• 2H• + RS f H2S + R• (RS ) CH3CSNH2) S2- + Pb2+ f PbS nPbS f (PbS)n As the Gibbs free energy of the surface is usually very high due to the large surface/volume ratio and the existence of dangling bonds, freshly formed PbS nanoparticles have a tendency to aggregate until they become stable. On the basis of the above observation, the formation of PbS hollow nanospheres can be explained by a vesicle-template mechanism. The vesicle-template mechanism has been considered as an effective interpretation for the formation of CdSe, polymer, and silica hollow spheres.22,20b Zheng et al.22a have demonstrated that CdSe hollow spheres can be formed by templating from anionic surfactant sodium dodecyl sulfate (SDS) vesicles induced by the ultrasonic irradiation. In the present case, it is known that DBS has the tendency to form vesicles under proper experimental conditions. And the as-formed vesicle is metastable and characterized by active ligands on the surface. It is therefore rational for these kinds of vesicles to act as temporary templates during the formation of hollow structure. The ultrasound wave urges the initial self-aggregation of DBS molecules to form vesicle structures with different sizes, which directly determine the diameters of the spheres. As DBS is an anionic surfactant, Pb2+ ions in the solution are easily attracted on the vesicle surfaces. Thus, the Pb2+ ion-covered vesicles can be formed, which provide nucleation domains for the subsequent reaction between Pb2+ and H2S to form PbS nanoparticles. The sonochemical process produces H2S gradually, which avoids the rapid reaction and guarantees that PbS nanoparticles grow on the surface before (21) Liu, Y.; Hou, D.; Wang, G. Chem. Phys. Lett. 2003, 379, 67. (22) (a) Zheng, X. W.; Xie, Y.; Zhu, L. Y.; Jiang, X. C.; Yan, A. H. Ultrason. Sonochem. 2002, 9, 311. (b) Mckelvey, C. A.; Kaler, E. W.; Zasadzinski, J. A.; Coldren, B.; Jung, H. T. Langmuir 2000, 16, 8285. (c) Hubert, D. H. W.; Jung, M.; Frederik, P. M.; Bomans, P. H. H.; Meuldijk, J.; German, A. L. AdV. Mater. 2000, 12, 1286.
Figure 3. (a, b) TEM images of silica-coated PbS hollow spheres obtained after drying at 60 °C and heat treatment at 500 °C for 1 h. The bar is 100 nm. (c) XRD pattern of the silica-coated hollow spheres and (d) FT-IR spectra of the plain and silica-coated hollow spheres.
Sonochemical Synthesis of Hollow PbS Nanospheres
of the vesicle has an uncompact structure, Pb2+ and S2- ions in the solution can diffuse into the interior of vesicles. Compared with CH3COO-, NO3- is a weaker binding ligand to Pb2+,10 and the “microenvironment” around the vesicles in the case of Pb(NO3)2 is much different. Because of the weaker binding force between Pb2+ and NO3-, the barrier on the surface of the vesicle is weaker. Thus, Pb2+ and S2- ions can freely enter into the vesicles. And the reaction between Pb2+ and S2- is carried out with vesicles as “active reactors”, resulting in the formation of solid spheres. It is reported that S2- prefers to release in an alkaline environment: the lower the pH value, the lower the release rate.16 The pH value of the solution is about 3.0 and 5.0 in the case of Pb(NO3)2 and Pb(Ac)2, respectively, as the reaction proceeds for 2 h. So the release rate of S2- in the case of Pb(Ac)2 is higher than that of Pb(NO3)2. Due to the relatively higher S2release rate, PbS shells can easily form. With the reaction proceeding, especially after the mineralization, the relatively compact shell structure no longer allows Pb2+ and S2- into the cavity of the vesicle, resulting in the formation of a hollow structure. Furthermore, a stronger binding ligand between Pb2+ and CH3COO- also hinders Pb2+ from diffusing into the cavity of the vesicle. The whole process is demonstrated in Scheme 1. Due to the unstable properties of sulfides in nature and the toxicity of Pb2+, it is critical to employ a protective sheath of thermally and chemically stable materials around the as-obtained products to enhance their performance for promising applications. On this basis, silica-coated PbS hollow spheres were prepared, as silica is a biocompatible material with high stability. The coating process is very simple without any “surface decoration”, involving a modified Sto¨ber method. The formation of silica layers was clearly demonstrated by the TEM image and FT-IR spectra as shown in Figure 3. Figure 3a is a typical TEM image of coated samples. A uniform silica layer with thickness of about
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40 nm was formed over PbS hollow spheres. The shell thickness has been found to depend on the concentration of TEOS. By comparison of the FT-IR spectra of the plain and silica-coated hollow spheres as shown in Figure 3c, a pronounced change was noticed in the region of 1300-400 cm-1, which clearly indicates the presence of the silica coating. The peaks at 469, 794, and 1096 cm-1 correspond to the characteristic Si-O-Si bend, SiO-Si symmetric stretch, and Si-O-Si antisymmetric stretch, respectively.23 The existence of an amorphous silica sheath has made a contribution in the XRD pattern (Figure 3c) as increasing background at lower diffraction angles (2θ 21°∼ 26°). For comparison, a TEM image of the coated sample heat-treated at 500 °C for 1 h was also studied (Figure 3b). The intact structure of hollow spheres indicates the improvement of the stability after the silica shielding. In summary, PbS hollow spheres have been successfully prepared by a sonochemical method in the presence of DBS. In the hollow sphere formation process, sonication and surfactant play important roles. The effect of the ultrasound directs the formation of vesicle templates and the growth of the nanoparticles on the vesicle surface. The wall of the hollow spheres is composed of PbS nanoparticles with diameters of about 12 nm. The surfactant DBS as a soft template directly determines the structure of the products. Furthermore, the controllable silica coating process makes the applications of hollow spheres more feasible. Acknowledgment. This work was supported by the fund of the Chinese Ministry of Education for Excellence, State Key Laboratory (50323006). LA0518647 (23) Ma, D.; Li, M.; Patil, A. J.; Mann, S. AdV. Mater. 2004, 16, 1838.
