A Template-free Route to Sb2S3 Crystals with Hollow Olivary Architectures Qiaofeng Han,* Juan Lu, Xujie Yang, Lude Lu, and Xin Wang* Materials Chemistry Laboratory, Nanjing UniVersity of Science and Technology, Nanjing 210094, China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 2 395–398
ReceiVed October 30, 2007; ReVised Manuscript ReceiVed December 2, 2007
ABSTRACT: Olivary Sb2S3 microcrystallines have been first synthesized via a hydrothermal process at 180 °C for 24 h using hydrochloric acid, SbCl3, and Na2S as starting materials. The powder X-ray diffraction (XRD) pattern shows the product corresponds to the pure orthorhombic phase of Sb2S3, the phase purity of which is further confirmed by energy dispersive X-ray (EDX). Transmission electron microscopic (TEM) and scanning electron microscopy (SEM) studies reveal that the irregular shaped nanobricks self-assemble into hollow olivary Sb2S3 microstructures. The possible mechanistic pathways in the formation of the structures are discussed. The design and synthesis of large-scale self-assembly of meso-, micro- and nanostructured building components have received considerable attention in nanoscience and nanotechnology due to their unique size and shaped-dependent properties.1–8 A variety of self-assembling patterns with one-dimensional (1D) structure-based materials as building units such as flower-like, dendritic-like, urchinlike or tubular-like superstructures,9–16 sheaf-like structure,17 peanut-like structures,18,19 and bullet-head-like structures20 have been produced. Most of these studies were focused on the preparationofthermodynamicallyfavorablesphericalsuperstructures.9–16 Lately, many efforts have been devoted to preparing hollow nanostructures with nonspherical morphologies such as Au or Co3O4 nanoboxes,21,22 Ag or Cu2O cages,23,24 or hematite spindle.25 Templates or surfactants are usually required to control oriented growth of building units. However, the introduction of the templates and surfactants increases the production cost. So the development of simple and template-free methods for creating novel microstructure patterns is very important to the potential studies of physical and chemical properties of materials and is still a challenge to materials scientists. Antimony trisulfide (Sb2S3), an important member of the chalcogenides, has received significant attention for potential applications in solar energy conversion,26 thermoelectric cooling technologies, and optoelectronics in the IR region.27 Several morphologies of Sb2S3 superstructures have been fabricated, including microspheres, microtubes,28 dendritic-like, or featherlike microstructures.11 Here, in this study, Sb2S3 microcrystallines with three-dimensional (3D) olivary superstructures were prepared by the hydrothermal method using SbCl3 and Na2S as raw materials without any surfactants. Analogous to our previous work on the preparation of Bi2S3 nanorods,29 the reaction processes were simple, and organic species were avoided. To our knowledge, little work has been conducted of olivary Sb2S3 microstructures using nanostructure-based bricks as building units. Experimental Procedures. All the reactants and solvents were of analytical grade and used without further purification. 1.37 g of SbCl3 was added to 8 mL of distilled water with stirring, and then about 2 mL of concentrate hydrochloric acid (HCl, 36%) was added dropwise until the white precipitate was dissolved completely. 2.40 g of sodium sulfide (Na2S · 9H2O) was dissolved in 6 mL of deionized water. Finally, these two solutions were mixed together in a Teflon-liner autoclave of 20 mL capacity and were maintained constantly at 180 °C for 24 h. When the reactions were completed, the products were filtered, washed with water and absolute alcohol several times, and then dried under vacuum before characterization. * To whom correspondence should be addressed.
Figure 1. XRD patterns of the as-prepared Sb2S3 products at (a) 120 °C, (b) 150 °C, and (c) 180 °C; the standard stick pattern for orthorhombic bulk Sb2S3 (lowest, JCPDS 06-0474).
Figure 2. Raman spectrum of the Sb2S3 crystals.
The X-ray diffraction patterns (XRD) were recorded on a Bruker D8 advanced X-ray diffractometer using Cu KR radiation (λ ) 0.1542 nm) with the range of the diffraction angle of 2θ ) 15–75°. Transmission electron microscopic (TEM), high-resolution transmission electron microscopic (HRTEM), and field emission scanning electron microscopic (FE-SEM) images were obtained on a JEM-2100 microscope (JEOL) and LEO-1530VP scanning electron microscope, respectively. Raman spectrum of Sb2S3 crystals was obtained using a Renishaw Invia spectrometer. Results and Discussion. Figure 1 shows the XRD patterns of the Sb2S3 microcrystallines prepared at different temperatures. All the reflections of Sb2S3 crystals obtained at 180 °C (Figure 1c) can be indexed to an orthorhombic phase of Sb2S3 (JCPDS Files, No.
10.1021/cg7010716 CCC: $40.75 2008 American Chemical Society Published on Web 12/28/2007
396 Crystal Growth & Design, Vol. 8, No. 2, 2008
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Figure 3. Panels (a), (b) and the lower-right inset in (b) SEM images of the olivary Sb2S3 microcrystallines; (c) and (d) EDX spectrum and an individual olive used in the EDX experiment; (e) TEM image; (f) HRTEM image of the edge of an olive.
06-0474). The shape of the diffraction peaks indicates that the products should be well crystallized. No peaks attributable to Sb2O3 or other impurity are observed. The intensity and shape of the diffraction peaks in Figure 1b reveal that the sample is not perfectly crystallized, while Figure 1a shows poorer crystallinity. The Raman spectrum of the olivary Sb2S3 microstructures is shown in Figure 2, which exhibits three resonant peaks at about 250, 366, and 445 cm-1 in the range of 200–800 cm-1. The positions of the peaks are in agreement with the typical frequencies observed from Sb2S3.30 Figure 3 shows the SEM and TEM images of Sb2S3 microcrystallines. The morphology of the products looks like olives with a length of about 10 µm and a diameter of about 5 µm. 3D olivary superstructures are in fact built from irregular shaped nanobricks with an average size of 800 × 400 × 100 nm. These bricks are aligned parallel to the olivary surface. Remarkably, some broken olive particles in the SEM image reveal their hollow interior (Figure
3b and lower-right inset), and the shell wall consists of several layers of building bricks, the thickness of which is about one-fifth of the olive diameter. An energy dispersive X-ray analysis (EDX) technique based on the SEM image was used to characterize the as-prepared sample. The only detected peaks in the EDX spectrum, shown in Figure 3c, are assigned to S and Sb. A single peak at 2.3 KeV is evidence that is assigned to S KR, while a group of four peaks located in the region of higher energy is assigned to Sb LR. The EDX spectrum was recorded from large sample areas, consequently, representing an average elemental composition of the antimony trisulfide. The result shows that the molar ratio of Sb/S obtained from the peak areas is 39.85:60.15, close to 2:3. TEM image (Figure 3e) shows that the sample is of olivary microcrystallines with a length of about 10 µm, corresponding to the SEM images. The typical HRTEM image (Figure 3f) of the edge of an individual Sb2S3 olive reveals that the building units
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Crystal Growth & Design, Vol. 8, No. 2, 2008 397
Figure 4. SEM and TEM images of the as-synthesized Sb2S3 with different reaction times: (a) 4 h; (b) 6 h; (c) 12 h; (d) 24 h; (e) and (f) two typical SEM images at 24 h.
are highly crystallized and free of dislocation. The regular fringes spacing of ca. 0.38 nm is in agreement with the d value of (001) lattice plane of orthorhombic Sb2S3. A time-dependent experiment was conducted to track the formation of the hollow olivary structures of orthorhombic Sb2S3. As shown in Figure 4, when the reaction time was 4 h, amorphous products were observed. If the reaction lasted for another 2 h, predominantly nanoparticles were obtained except for a small amount of olivary sphere. After 12 h of reaction, large numbers of
Figure 5. A schematic illustration of the formation of olivary Sb2S3.
olivary Sb2S3 particles were produced, but there were a few fragments. Large quantities of uniform Sb2S3 olives have been fully formed when the reaction time was prolonged to 24 h (Figure 4d). The pH value of solution also exerts an important influence on the growth of Sb2S3 microcrystallines. If the concentration of hydrochloric acid was high, no Sb2S3 deposition would be obtained because they were dissolved out; if the concentration was low, SbCl3 would strongly hydrolyze to produce a white precipitate [Sb(OH)2Cl]. When the experiment was carried out in the absence
398 Crystal Growth & Design, Vol. 8, No. 2, 2008 of hydrochloric acid with other conditions unchanged, not olivary but irregular rod-like Sb2S3 particles were observed. On the basis of the above discussion, the possible mechanistic pathway in the formation of hollow olivary structures is illustrated in Figure 5. Sb2S3 nanoparticles formed at the early stages and then aggregated into nanobricks. With increasing reaction time, the marginal or exterior alignment of these building bricks led to arching fragments. As shown in Figure 4c (marked), some fragments can be observed from the products after 12 h of reaction. Finally, fragments selfassembled into a hollow olivary structure via the oriented attachment mechanism.12 That the assembling of the building units on the olivary surface is more compact than those at the ends (Figure 4e,f) supports our guess on the formation mechanism of Sb2S3 microcrystallines. We speculate that morphology control of Sb2S3 is consistent with its chainlike crystalline structure except for the selected experimental parameters (reactant species, reaction time, pH value of the solution, reaction temperature and so on). Sb2S3 has an orthorhombic lattice with four molecules per unit cell. Each antimony atom combines with sulfur atoms to form infinite chains (SbxSy)n along the c-axis, which are weakly bound in the b direction.31 The unusual chaintype structure tends to form 1D or more complex structures.11,32,33 In this work, the geometrical shape of building units must have played a key role in the formation of olivary Sb2S3, since no surfactants or emulsions were used. Conclusion. In summary, 3D olivary Sb2S3 superstructures were successfully prepared by the hydrothermal method using SbCl3, Na2S, and HCl as raw materials without any surfactants. Compared with the solvothermal route, this method is greener. The timedependent studies reveal a morphological evolution and propose a self-assembly process of building units.
Acknowledgment. We thank Prof. Jiansheng Li for the assistance with the scanning electron microscopy and the suggestions of the reviewers. We acknowledge the National Natural Science Foundation of China (50572039) and High Technology Foundation of Jiangsu Province (BG2006043) for the financial support of this work.
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