Fabrication of Novel CdIn2S4 Hollow Spheres via a Facile

Novel CdIn2S4 hollow spheres with small bipyramids aggregating on their surface had been successfully synthesized via a facile hydrothermal process wi...
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J. Phys. Chem. C 2008, 112, 10700–10706

Fabrication of Novel CdIn2S4 Hollow Spheres via a Facile Hydrothermal Process Lei Fan and Rong Guo* School of Chemistry and Chemical Engineering, Yangzhou UniVersity, Yangzhou 225002, People’s Republic of China ReceiVed: March 14, 2008; ReVised Manuscript ReceiVed: April 29, 2008

Novel CdIn2S4 hollow spheres with small bipyramids aggregating on their surface had been successfully synthesized via a facile hydrothermal process without any template or surfactant. The crystal structure, morphology, and optical properties of the products were characterized by X-ray diffraction, X-ray photoelectron spectra, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, UV-vis diffuse reflectance spectroscopy, and photoluminescence spectroscopy. The reaction conditions influencing the synthesis of these CdIn2S4 hollow sphere structures, such as reaction temperature, reaction time, and the kinds of solvent used, were investigated. Furthermore, on the basis of a series of observations, phenomenological elucidation of a mechanism for the growth of the CdIn2S4 hollow spheres has been presented. 1. Introduction In recent years, one of the major technological challenges in nanoscience and nanotechnology is the self-assembly of tiny nanobuilding units into larger organized conformations and geometrical architectures for device applications.1–3 An interior space for nanostructures may be further required to meet the needs of new applications. There has been increasing research in the fabrication of hollow inorganic nanomaterials owing to their potential application in optical, electronic, magnetic, catalytic, and sensoring devices ranging from photonic crystals to drug-delivery carriers and nanoreactors.4–8 At present, a variety of methods have been employed to fabricate hollow spheres of inorganic materials, including liquids droplets,9–12 latex templates,13 polymer templates,14 emulsion and microemulsion droplets,15,16 and inorganic nanoparticles.17 However, these methods require template materials, and the templates need to be removed later, so the yield of hollow spheres prepared via template route is low, or their shells are not intact, which usually leads to poor mechanical performance. Hydrothermal synthesis may be a useful tool for the synthesis of hollow spheres due to its various advantages, such as a single-step process and a low temperature. As one type of interesting materials, ternary semiconductor compounds ABmCn (A ) Cu, Ag, Zn, Cd, etc.; B ) Al, Ga, In; C d S, Se, Te) with the chalcopyrite structure have been shown to be useful due to their potential applications in solar cells, light-emitting diodes, biological labeling, and optical sensing etc.18–22 Very recently, many efforts have been focused on the preparation of these ternary chalcogenide materials with various morphologies. Xinglong Gou et al.23 have synthesized ZnIn2S4 nanomicrostructures with controlled size, and Jin-OoK Baeg et al.24 have synthesized CdIn2S4 nanotubes and “marigold-like” nanostructures. However, the development of chemical methods suitable for the synthesis of these ternary chalcogenide materials with controlled shape and size still remains a major challenge. In this work, a hydrothermal preparation of novel hollow CdIn2S4 spheres, the surface of which was composed of * Corresponding author e-mail: [email protected]; fax: 86-5147311374; phone: 86-514-7975219.

numerous small bipyramids, was developed using cadmium chloride, indium chloride, and thioacetamide as sources. To study the growth mechanism of CdIn2S4 hollow spheres, we systematically surveyed the growth process of CdIn2S4 by analyzing the samples at different growth stages, which made it possible to arrest the CdIn2S4 crystals in different stages of their growth. The effects of temperature and solvent were also our concern. To the best of our knowledge, such a novel hollow sphere-like architecture has not been reported. 2. Experimental Section 2.1. Chemicals and Synthesis. CdCl2 · 2.5H2O, InCl3 · 4H2O, and thioacetamide (TAA), purchased from Shanghai Chemical Reagent Corp., were used without further purification. In a typical synthesis, stoichiometric amounts of CdCl2 · 2.5H2O (0.056 g), InCl3 · 4 H2O (0.146 g), and a double excess of TAA (0.150 g) were added to a Teflon-lined stainless-steel autoclave of 30 mL capacity. The autoclave was filled with double-distilled water to 70% of its volume and then maintained at 160 °C for 16 h. Then the autoclave was air-cooled to room temperature unaided. An orange precipitate was filtered, washed with ethanol and distilled water several times, and dried in vacuum at 60 °C for 4 h. 2.2. Characterization. X-ray powder diffraction (XRD) patterns of the products were recorded with a Bruker (German) AXS D8 ADVANCE X-ray diffractometer with Cu KR radiation (λ ) 1.5418 Å) at a scanning speed of 0.4 °/min from 10° to 80°. The surface morphology and particle size were examined by scanning electron microscopy (SEM, XL-30E Philips Co., Holland, 20 kV) and transmission electron microscopy (TEM, Tecnai-12 Philips Apparatus Co. Holland, 120 kV). Selected area electron diffraction (SAED) patterns and high-resolution TEM (HRTEM) images were taken by a JEOL-2010 transmission electron microscope. X-ray photoelectron spectra (XPS) were recorded on a Thermo ESCALAB 250 spectrometer using a non-monochromatized Al KR X-ray as the excitation source and choosing C1s as the reference line. UV-visible diffuse reflectance spectra were recorded at room temperature on a Lambda-850 UV-vis spectrophotometer (Perkin-Elmer, USA) fitted with an integrating sphere diffuse reflectance accessory.

10.1021/jp8022259 CCC: $40.75  2008 American Chemical Society Published on Web 06/26/2008

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Figure 1. XRD pattern of CdIn2S4 prepared in water at 160 °C for 16 h.

Photoluminescence (PL) spectra were recorded with a Hitachi F-4500 spectrofluorometer at room temperature. 3. Results and Discussions Figure 1 shows the XRD pattern of CdIn2S4 prepared in aqueous solutions. All the peaks in the pattern reveal that CdIn2S4 product is the face-centered cubic structure, with no evidence of binary sulfides, oxides, or organic compound related to reactants. The broad XRD peaks at 2θ ) 27° (311) and 47.5° (440) are the main characteristics of the cubic spinel phase of CdIn2S4. All reflection parameters and the lattice parameter a ) 10.845 Å match well with the reported data (JCPDS card No.27-0060). The strong and sharp peaks indicate the high purity and good crystallinity of the final product. The morphology and microstructure of the products are examined by SEM and TEM. Figure 2 shows the images of products prepared in aqueous solution at 160 °C for 16 h. The low-magnification SEM image (Figure 2a) demonstrates that the products consist of relatively uniform sphere-like CdIn2S4 architectures with an average diameter of 5 µm. The microspheres are dispersed with good monodispersity. An interesting feature shown in Figure 2b is that each sphere comprises numerous three-dimensional bipyramids aggregating on its surface. The length of the bipyramids on the sphere surface is in the range of 0.4-1 µm. It is intriguing to note that CdIn2S4 bipyramids can self-aggregate into the sphere-like structure in the absence of any specific ionic additives, biological macromolecules, or synthetic organic templates. The inset pattern of Figure 2b shows the presence of typical opening hollow microspheres. Such a fascinating morphology as that observed for the first time in CdIn2S4 has not been reported hitherto. To study the surface nature of as-prepared CdIn2S4 nanostructures, the sample is also characterized by XPS (as shown in Figure 3). The single S 2p peak (Figure 3d) at 161.32 eV is indicative of sulfur present as the sulfur ion. Both the Cd 3d and In 3d spectra (Figure 3, panels b and c, respectively) show two peaks (405.36 and 412.10 eV for the Cd 3d level, 444.84 and 452.40 eV for the In 3d level), corresponding to the 3d5/2 and 3d3/2 spin-orbit spin components, respectively. As can be seen from the survey spectrum (Figure 3a), the oxygen peak at a binding energy of 533.20 eV is due to the presence of H2O absorbed on the sample surface, and no obvious impurities, for

Figure 2. SEM images of CdIn2S4 prepared in water at 160 °C for 16 h: (a) an overall morphology, (b) detailed views on average-sized spheres. The inset is a high magnification of a hollow sphere.

example, In2O3 (O 1s for In2O3: 529.80, 530.30, and 530.50 eV) or chlorine ion, can be detected. As deduced from the intensities of the relevant XPS peaks, the surface stoichiometry of Cd/In/S is 1.00:2.06:4.15. The corresponding energydispersive X-ray spectrum (EDX) also indicates that the spherelike CdIn2S4 structure consists of Cd, In, and S with a ratio about 1:2:4 (see Supporting Information Figure S1), which is consistent with the stoichiometric composition of CdIn2S4. To analyze the specific structure of the microspheres, the typical TEM image of an individual CdIn2S4 nanostructure prepared at 160 °C for 16 h is shown in Figure 4a. After ultrasonic dispersion for 1 h, the CdIn2S4 spheres are disintegrated, and large quantities of bipyramids can be observed (Figure 4b). The SAED pattern (Figure 4c) indicates that the bipyramids are single crystalline in nature. The HRTEM image in Figure 4d, recorded at the area marked in Figure 4b, shows the internal atomic lattice for a nanocrystal. The particle exhibits a 0.31 nm d-spacing for (400) reflection, further confirming that bipyramids grow along the [100] direction of CdIn2S4. To study the formation process of the sphere-like CdIn2S4 structure, time-dependent experiments are carried out at 160 °C, and the resultant products are respectively studied by SEM and XRD measurements. Several obvious evolution stages can be observed and are shown in Figure 5. In the first stage (0.5 h), many micropompon-like spheres are formed, as shown in Figure 5a (the size is about 5 µm). It is easy to find that the spheres are loose and the surface is very rough. With increasing reaction time (4 h), the spheres hardly swell in size but become tight; the surface is composed of randomly ordered flakes (Figure 5b) that are interconnected with each other. Figure 5c

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Figure 3. XPS patterns of the CdIn2S4 sample: (a) survey spectrum and high-resolution spectra for (b) Cd3d, (c) In3d, and (d) S2p.

shows that, prior to the formation of the final sphere-like structure, the diameters of spheres remain unchanged, but the nanoparticles on surfaces are bigger than before, and some of them have grown to bipyramids. When reaction time reaches 16 h, large amount of spheres, whose surface is composed of bipyramids, are easy to find (see Figure 2). Meanwhile, there are some opening hollow spheres that come out in this stage, which are easy to be observe in the SEM picture. Therefore, we could conclude these spheres are hollow. Upon prolonging reaction time to 24 h (Figure 5d), the spheres do not change anymore compared to those at 16 h. It is clear that the sphere diameter does not change once the sphere is formed. Meanwhile, from XRD patterns of these samples corresponding to Figure S2, it can be seen that the intensity of peaks of the initial, intermediate, and final stages of samples is gradually enhanced. From the observed morphologies and XRD patterns of the samples at different stages, a possible interpretation for the formation process of the hollow sphere-like structure could be as follows: first, CdIn2S4 colloids are initially formed under the synthetic condition, and then the formed colloids aggregate together with loose spherical appearance, which indicates a nucleation aggregation mechanism. With the reaction continuing, the concentration of reactants decreases, the reaction rate slows down, and the crystal growth rate is the dominant step at the low supersaturation level. The formation of sphere-like archi-

tecture indicates that the nucleation and growth of CdIn2S4 are well controlled in our synthetic process. With increasing reaction time samples have a tendency to gradually crystallize, and the nanoparticles on the surface of spheres grow from small particles to flakes and then to bipyramids. Obviously, this process is not an oriented attachment mechanism.25 According to the process, an Ostwald ripening process may play a key role for the formation of the hollow structure, because the spheres are composed of numerous smaller particles, and those bipyramids grow at the expense of smaller nuclei initially formed.26 Compared to those at the outer surfaces, the particles located in the inner cores have high surface energies. Consequently, they are merged into particles on the outer surfaces, resulting in the formation of hollow interiors. A similar type of morphology has been reported for metal oxides.27,28 As for the shape evolution of the nanoparticles on the surface of the spheres, the shapes of crystals are determined by the surface free energy of individual crystal facets, and their final morphology will be determined by the competition of minimizing the total free energy of the system. In most cases, the product is composed of those facets whose developing rate is relatively slow.29 According to Wang’s theory,30 the surface energies of different planes hold the general sequence γ{111} < γ{100} < {110}. When the ratio (R) of the growth rate in the to that of the is about 1.73, it may turn out to be bipyramid. In

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Figure 4. TEM images of CdIn2S4 prepared at (a) 160 °C for16 h without ultrasonic and (b) with ultrasonic for 1 h; (c) SAED pattern of bipyramids and (d) the corresponding HRTEM image.

Figure 5. SEM images of CuIn2S4 prepared at different reaction stages at 160 °C: (a) 0.5, (b) 4, (c) 10, and (d) 24 h. The inset is a high magnification of the CuIn2S4 sphere surface.

our case, the growth process of the bipyramid starting from small nanoparticles may limit the effect of the surface energy on the growth of CdIn2S4 bipyramids. It grows along the [100]

direction, which could be confirmed by HRTEM, and are bounded by eight {311} at last. The detailed mechanism is still in study.

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Figure 6. SEM images of CuIn2S4 prepared at different reaction temperature for 16 h: (a) 20, (b) 80, (C) 120, and (d) 180 °C. The inset is a high magnification of the CuIn2S4 sphere surface.

Figure 8. UV-diffuse reflectance spectrum (DRS) of the CdIn2S4 at different reaction temperatures: (a) 80, (b) 120, (c) 140, and (d) 160 °C.

Figure 7. CdIn2S4 prepared in different solutions: (a) in ethanol solution and (b) in pyridine solution.

Surprisingly, we find that temperature is another important factor for product morphology when other reaction conditions are kept similar. Irregular nanoparticles are formed at 20 °C (Figure 6a). From XRD patterns (Supporting Information Figure

S3) we find that the crystallinity is not good. When temperature increases to 80 °C, the flower-like spheres with diameter less than 1 µm can be found (Figure 6b). Under a hydrothermal condition at 120 °C for 16 h, the spheres with diameter about 5 µm can form, but the surfaces are all irregular nanoparticles (Figure 6c). When the reaction temperature increases to 180 °C, the sphere-like structure can also be found as in reaction at 160 °C (Figure 6d). The comparative experiments indicate that the appropriate temperature is important for the preparation of CdIn2S4 with novel hollow sphere structures. It is found that the solvent used in the synthetic system has an important influence on the morphology of CdIn2S4. If we use ethanol as solvent, very beautiful marigold-like spheres can be found (Figure 7a). By replacing water with pyridine, the product is composed of irregular sheets (Figure 7b). This may

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Figure 9. Photoluminescence of the CdIn2S4 microspheres prepared at 160 °C for 16 h. (a) Excitation spectrum at room temperature and (b) emission spectrum at 572 nm excitation.

be attributed to the differences of these solvents in alkalinity, coordination ability, viscosity, surface tension, or dipole moment, which are all important factors in influencing the shape/ size of the product in solvothermal synthesis. Effects of solvent on the final morphology of products still require further study, and the work is underway. It is important to determine the optical properties of the novel CdIn2S4 hollow spheres. To measure the band gap, UV-diffuse reflectance spectrum (DRS) characterization is employed. The DRS spectrum is obtained on a Perkin-Elmer Lambda 850 UV-vis spectroscopy machine at room temperature. Figure 8 shows the typical spectrum of CdIn2S4 products at different temperatures. On the basis of the DRS absorbance, estimation of the optical band gap (Eg) is from 1.99 to 2.15 eV with the increasing of temperature from 80 to 160 °C. The slight shift in the absorption edge may be due to the variation of particle size and difference in particle morphology. The steep absorption edge reveals a single phase of CdIn2S4 (Figure 8d), which is in good agreement with our XRD results. The room-temperature PL spectrum of the CdIn2S4 microspheres presents a sharp excitation peak at 572 nm (Figure 9a). This value agrees with the band gap of a CdIn2S4 microspheres as calculated from the corresponding UV-vis absorption edge. The emission spectrum of the microspheres excited at 572 nm reveals a stable and strong red emission band centered at 862 nm (Figure 9b), comparable to the reported data for CdIn2S4 single crystals.31 Recently, Nanu et al. reported the preparation of the three-dimensional (3D) solar cells composed of TiO2/CuInS2 nanocomposites with a remarkable energy conversion efficiency of 5%.32 They suggested that the improved performance of the 3D nanocomposite solar cells could be attributed to the enhanced interface recombination of electron-hole pairs in the light-absorbing nanomaterials. Therefore, our further work will focus on the application of the as-synthesized ternary chalcogenide microstructures in solar cells. 4. Conclusions In summary, the micrometer-scale novel hollow spheres of CdIn2S4 can be effectively constructed via a simple and mild hydrothermal process. The hierarchical CdIn2S4 particles are in a uniform spherical architecture with a diameter of 10-20 µm, which consists of numerous three-dimensional bipyramids

aggregating on the surface. In this one-pot synthesis, an appropriate temperature (160 °C) and an appropriate reaction time (16 h) are essential to the production of this type of structure. The results also show that solvent is an important factor for product morphology. These microstructures exhibit strong absorption in a wide wavelength range from visible to UV light and show intense excitation and emission luminescence at room temperature. The possible formation process of the hierarchical structure has been elucidated by SEM, TEM, and XRD measurements and can be ascribed to an aggregation and Ostwald ripening process. Our present hydrothermal synthetic system provides an appropriate crystal growth environment for the formation of novel hollow spheres architecture. The novel hollow spheres of CdIn2S4 may have potential applications in solar cells and optoelectronic devices. Acknowledgment. This work is supported by the National Natural Scientific Foundations of China (No. 20633010 and 20773106). Supporting Information Available: EDX spectrum for the sphere-like CdIn2S4 architecture prepared at 160 °C for 16 h, XRD pattern of CdIn2S4 samples prepared at different reactions stages, and XRD pattern of CdIn2S4 samples prepared at different reactions temperature. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Michael, D. W. Nature 2000, 405, 293. (2) Dai, Z. F.; Voigt, A.; Leporatti, S.; Donath, E.; Dahne, L.; Mohawald, H. AdV. Mater. 2001, 13, 1339. (3) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124. (4) Tamai, H.; Sumi, T.; Yasuda, H. J. Colloid Interface Sci. 1996, 177, 325. (5) Caruso, F. AdV. Mater. 2001, 13, 11. (6) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453. (7) Wang, D. Y.; Caruso, F. Chem. Mater. 2002, 14, 1909. (8) Huang, H.; Remsen, E. E. J. Am. Chem. Soc. 1999, 121, 3805. (9) Walsh, D.; Mann, S. Nature 1995, 377, 320. (10) Fowler, C. E.; Khushalani, D. D.; Mann, S. Chem. Commun. 2001, 19, 2028. (11) Huang, J.; Xie, Y.; Li, B.; Liu, Y.; Qian, Y.; Zhang, S. AdV. Mater. 2000, 12, 808. (12) Gao, X.; Zhang, J.; Zhang, L. AdV. Mater. 2002, 14, 290. (13) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111. (14) Shiho, H.; Kawahashi, N. Colloid Polym. Sci. 2000, 278, 270.

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Fan and Guo (23) Gou, X. L.; Cheng, F. Y.; Shi, Y. H.; Zhang, L.; Peng, S. J.; Chen, J.; Shen, P. W. J. Am. Chem. Soc. 2006, 128, 7222. (24) Kale, B. B.; Baeg, J. O.; Lee, S. M.; Chang, H. J.; Moon, S. J.; Lee, C. W. AdV. Funct. Mater. 2006, 16, 1349. (25) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, 4430. (26) Liu, Z.; Li, S.; Yang, Y.; Peng, S.; Hu, Z.; Qian, Y. AdV. Mater. 2003, 15, 1946. (27) Xu, J. S.; Xue, D. F. J. Phys. Chem. B 2005, 109, 17157. (28) Li, L. L.; Chu, Y.; Liu, Y.; Dong, L. H. J. Phys. Chem. C 2007, 111, 2123. (29) Kumar, S.; Nann, T. Small 2006, 2, 316. (30) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (31) Grilli, E.; Guzzim, M.; Moskalonov, A. V. Phys. Stat. Sol. A 1980, 69, 515. (32) Nanu, M.; Schoonamn, J.; Goossens, A. Nano Lett. 2005, 5, 1716.

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