The Fractal Splitting Growth of Sb2S3 and Sb2Se3 Hierarchical

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J. Phys. Chem. C 2008, 112, 672-679

The Fractal Splitting Growth of Sb2S3 and Sb2Se3 Hierarchical Nanostructures Guang-Yi Chen,† Bin Dneg,‡ Guo-Bin Cai,‡ Tie-Kai Zhang,‡ Wen-Fei Dong,† Wan-Xi Zhang,† and An-Wu Xu*,‡ College of Materials Science and Engineering, Jilin UniVersity, Changchun, 130025, China, and DiVision of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei 230026, China ReceiVed: August 28, 2007; In Final Form: NoVember 1, 2007

Complex Sb2S3 and Sb2Se3 nanostructures with a sheaf-like hierarchical morphology were prepared on a large scale at 180 °C by a simple hydrothermal method in the presence of poly(vinyl pyrrolidone) (PVP). X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy were used to characterize the products. The results indicate that threedimensional Sb2S3 and Sb2Se3 complex nanostructures were constructed by fractal splitting growth. The timedependent shape-evolution process suggests that initial stages of the growth comprise nanorod seeds. These PVP-stabilized nanorods develop in subsequent growth stages to a dumbbell structure and complete their development as a closed sphere with an equatorial notch. It has been demonstrated that PVP plays a key role in the formation of such hierarchical nanostructures. Ultraviolet-visible-near-infrared spectroscopy was further employed to estimate the band gap energy of the obtained products. The measurements of the optical properties revealed that the obtained materials have a band gap of 1.56 eV for Sb2S3 and 1.13 eV for Sb2Se3. Our work may shed some light on the design of other well-defined complex nanostructures, and the as-grown architectures may have potential applications.

Introduction There has been intensive research interest to realize rational control over the size, shape, dimensionality, and complex forms of nanocrystals due to the significance and potential to design new materials and devices in various fields such as optics, electronics, catalysis, and ceramics.1 With a deeper insight into crystal growth kinetics, nanocrystals with complex forms such as snowflakes,2 multipods,3 dendrites,4 and hyperbranched nanostructures5 were successfully synthesized in solution. Furthermore, in a strongly related field, self-assembly into highly ordered superstructures and control over the shape and size of inorganic materials are important characters of natural growth phenomena.6 Drawing inspiration from the hierarchy and assembly strategies found in nature, bioinspired morphosynthesis has focused on the fabrication of hierarchically assembled structures from nanoscale building blocks using organic templates.7 Synthetic approaches to the morphological control of inorganic minerals through polymer-controlled crystallization can create a variety of inorganic superstructures, such as helical fibers, mesocrystals, ring patterns, dumbbell structures, a sheaflike structure, and self-similar structures.8 The main group metal binary chalcogenide V2VI3 (V ) Sb, Bi, As; VI ) S, Se, and Te) compounds, as an important class of semiconductors, find applications in photovoltaics and thermoelectrics.9 Sb2S3 and Sb2Se3 are both direct band gap semiconductors and crystallize in an orthorhombic crystal structure (pbnm space group), which is isostructural with Bi2S3.10 Sb2S3 is an important semiconductor with high photosensitivity and thermoelectric power,11 and is regarded as a prospective * Corresponding author. Fax: 86-551-3600724. E-mail: anwuxu@ ustc.edu.cn. † Jilin University. ‡ University of Science and Technology of China.

Figure 1. XRD patterns of the obtained Sb2S3 (a) and Sb2Se3 (b) samples.

10.1021/jp076883z CCC: $40.75 © 2008 American Chemical Society Published on Web 01/01/2008

Complex Sb2S3 and Sb2Se3 Nanostructures

J. Phys. Chem. C, Vol. 112, No. 3, 2008 673

Figure 2. SEM images of the as-made Sb2S3 nanostructures grown by hydrothermal treatment in the presence of PVP at 180 °C for 12 h.

material for solar energy as a result of its band gap covering the range of the solar spectrum and its good photoconductivity.12 Sb2Se3 also possesses excellent photovoltaic and thermoelectric properties.13 Moreover, Rosi et al. found the Peltier effect for Sb2Se3 and utilized this effect for thermoelectric cooling.14 Platakis and co-workers studied threshold and memory-switch-

ing phenomenon on Sb2Se3 materials.15 Owing to these functional properties, Sb2S3 and Sb2Se3 materials have been attracting much research attention. So far, simple onedimensional (1D) Sb2S3 and Sb2Se3 nanostructures, including nanorods,16 nanoribbons,17 and microtubes,18 have been synthesized by different methods. However, to the best of our

674 J. Phys. Chem. C, Vol. 112, No. 3, 2008

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Figure 3. SEM images of the as-made Sb2Se3 complex nanostructures obtained by hydrothermal treatment at 180 °C for 12 h.

knowledge, more complex forms such as a sheaf-like and dumbbell structure of Sb2S3 and Sb2Se3 have not been obtained to date. It is very important to employ special polymer structures and strong assembling functions to synthesize nanocrystals with desired shapes and to construct complicated superstructures from a single functional structure in materials science, chemistry, and biology; however, this approach is still in its infancy. Herein, we demonstrate an efficient method for the control synthesis of novel Sb2S3 and Sb2Se3 nanostructures with a sheaflike and dumbbell morphology that can be assembled with nanorods under hydrothermal treatment in the presence of poly(vinyl pyrrolidone) PVP polymer. These complex structures resemble the morphology of some minerals existing in nature, probably formed by the fractal splitting crystal growth, as do their natural counterparts, and they provide the possibility of observing such splitting events in future studies on the realtime crystal growth. In the present study of Sb2S3 and Sb2Se3 nanostructures, the time-dependent experiments were performed to explore temporal morphological evolution from a reaction, and a full range of crystal morphologies were observed, which are attributed to the fractal splitting growth mechanism in natural minerals. Experimental Section Preparation. All regents were analytical grade and used without any further purification. In a typical synthesis of Sb2S3 sample, 1 mmol of antimony potassium tartrate and 0.15 g of PVP (MW ) 40 000) were first dissolved in 10 mL of water, then 2 mmol of thioacetamide (TAA) was added to the above solution under stirring. The resulting homogeneous solution was transferred to a 20 mL Teflon-line stainless steel autoclave, then an appropriate amount of distilled water was added to 80% of the total capacity. The autoclave was sealed and maintained at

180 °C for 12 h. After the reaction, the autoclave was cooled to room temperature in air. The resulting black precipitate was filtered, washed with distilled water and absolute alcohol, and then dried at 60 °C in air. Sb2Se3 samples were synthesized by the same synthetic process, using freshly prepared NaHSe solution (2 mmol of Se power and 3 mmol of NaBH4 dissolved in 5 mL water) to replace 2 mmol of thioacetamide, while other conditions were kept constant. Characterization. The X-ray powder diffraction (XRD) patterns of the samples were performed on a Philips X’Pert Pro Super X-ray diffractometer with Cu KR radiation (λ ) 1.54178 Å), and the operation voltage and current were maintained at 40 kV and 40 mA, respectively. Field-emission scanning electron microscopic (FE-SEM) images were obtained with a JEOL JSM-6700F operated at a beam energy of 15.0 kV. Transmission electron microscopic (TEM) images, high-resolution transmission electron microscopic (HRTEM) images, and selected-area electron diffraction (SAED) patterns were obtained on a JEOL-2010 microscope with an accelerating voltage of 200 kV. An energy-dispersive X-ray spectroscope (EDS) was attached to the JEOL 2010. Sample grids were prepared by sonicating powdered samples in ethanol for 20 min and evaporating one drop of the suspension onto a carbon-coated, holey film supported on a copper grid for TEM measurements. Thermogravimetric (TG) and differential scanning calorimetric (DSC) analyses were carried out under a stream of argon, at a heating rate of 10 °C/min using a Shimadzu DTG-60H. Fourier transform infrared spectroscopy (FTIR) analysis was carried out using KBr disks in the region of 4000-400 cm-1 by using a Nicolet Magna-IR 750 under ambient conditions. An ultravioletvisible-near-infrared (UV-vis-NIR) spectrophotometer (Solid Spec-3700 series) was used to record the absorbance spectra of the samples.



Figure 4. TEM images (a,b) and HRTEM image (c) of the Sb2S3 products obtained under hydrothermal treatment at 180 °C for 12 h. (d) The FFT pattern taken from the [100] zone axis. (e) The EDS spectrum of the obtained Sb2S3 complex structure. Cu peak came from the TEM grid.

Results and Discussion Synthesis of Sb2S3 and Sb2Se3 complex nanostructures was performed by hydrothermal treatment in the presence of PVP. XRD patterns of the as-synthesized Sb2S3 and Sb2Se3 samples are shown in Figure 1a,b, respectively. All diffraction peaks shown in Figure 1 can be indexed as a primitive orthorhombic lattice of Sb2S3 (stibnite, Figure 1a) and Sb2Se3 (Figure 1b), which are in good agreement with those reported in the literature (JCPDS 42-1393 for Sb2S3, and 15-0861 for Sb2Se3). No other impurities were found in the samples. The morphology and structure of the as-prepared Sb2S3 products were examined by scanning electron microscopy (SEM), as displayed in Figure 2. Small bundles with simple crystal splitting, a dumbbell structure, and a hierarchical sphere with an equatorial notch can be observed in Figure 2a,b,d,e, respectively. The high-magnification SEM image taken from a microparticle in Figure 2b shows that the rectangle-like cross section of each nanorod is clearly visible, indicating that the nanorods have a ribbon-like morphology (Figure 2c). Moreover, the high-magnification SEM image taken from an equatorial notch of a sphere (Figure 2e) shows that the fractal splitting growth is clearly observed, as shown in Figure 2f. It can be seen that a few of the microparticles have a tyre-like morphology (Figure 2g), the high-magnification SEM image (Figure 2h) shows that the fractal splitting growth is also discernible, and the nanoribbon-like morphology is clearly seen from Figure 2h.

This symmetric tyre-like structure was formed by the intergrowth of many sheaves from the same core. Similar complex nanostructures were also observed for Sb2Se3 prepared by the same method, as clearly shown in Figure 3. The SEM image in Figure 3a shows that the Sb2Se3 product is mainly composed of dumbbells with an average size of about 10 µm. SEM images of the two halves of a dumbbell mechanically broken perpendicular to the c-axis of the seed are clearly shown in Figure 3b,c. A radial pattern is discernible, which is typical for fractal areas in the seed region of the particle. Also, fractal splitting growth is clearly observed from the high-magnification SEM image in Figure 3c. Initial stages of the fractal growth comprise nanorod seeds. These PVPstabilized nanorods develop in subsequent growth stages to a dumbbell structure and complete their development as a closed sphere with an equatorial notch. The microstructures of the obtained samples were further examined with TEM, HRTEM, and fast Fourier transformation (FFT) pattern. Typical TEM images of Sb2S3 nanostructures are shown in Figure 4a-c. Figure 4a presents an individual microsphere with a cross circle, in accordance with the SEM observation (such as Figure 2c,d). The light contrast in the center indicates the Sb2S3 microsphere should be loose in the middle part. A typical TEM image taken from the edge of this sphere is shown in Figure 4b, displaying the radial growth of nanorods from the sphere center. Each microsphere consists of nanorods with diameters of 20-50 nm and lengths of several hundreds


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Figure 5. TEM images (a,b) and HRTEM image (c) of the Sb2Se3 products obtained under hydrothermal treatment at 180 °C for 12 h. (d) The SAED pattern taken from the [11h0] zone axis. (e) EDS spectrum of the obtained Sb2Se3 nanostructure. Cu peak arose from the TEM grid.

of nanometers, as estimated from the TEM images. Figure 4c shows a typical HRTEM image of an individual Sb2S3 nanorod, having resolved lattice fringes of (010) planes (d010 ) 1.14 nm) and (001) planes (d001 ) 0.38 nm), and thus the growth of the nanorod is along the [001] direction. The corresponding FFT pattern (Figure 4d) taken from this nanorod can be indexed as an orthorhombic Sb2S3 single-crystal recorded from the [100] zone axis. EDS analysis measured from nanorods shows that the composition of the product is Sb2S3 (Figure 4e). Further investigation was carried out by TEM to reveal the organization of such self-assembled complex Sb2Se3 nanostructures. A spherical Sb2Se3 structure can be clearly seen in a typical TEM image (Figure 5a). Figure 5b shows the TEM image taken from the edge of this sphere, showing that nanorods are radially arranged from the center of the sphere. Figure 5c shows a typical HRTEM image taken from an individual Sb2Se3 nanorod, giving resolved lattice fringes of (110) planes (d110 ) 0.83 nm) and (001) planes (d001 ) 0.398 nm); the growth of the nanorod is along the [001] direction. An SAED pattern (Figure 5d) taken from this nanorod can be indexed as an orthorhombic Sb2Se3 single-crystal recorded from the [11h0] zone axis. Figure 5e is the EDS spectrum taken from an area consisting of many nanorods. Only Sb and Se peaks are observed in this spectrum, suggesting that the sample is composed of Sb and S. Quantitative EDS analysis shows that the atom ratio of Sb/Se is close to 2:3, giving the sample a composition of Sb2Se3.

Figure 6. The crystal structure of Sb2S3 viewed along the [010] direction. The Sb-S covalent bonds are indicated by solid lines, and the weak van der Waals bonds are shown by the black dashed lines. The cleavage trace is demarcated by the red dashed line.

The formation of Sb2S3 and Sb2Se3 nanorods accords well with the general orientation of growth in 1D nanostructures,1b,19 which is largely determined by the anisotropic nature of the building blocks. It can be explained on the basis of a typical Sb2S3 crystal structure, as shown in Figure 6. The crystal structure consists of infinite chains of (Sb4S6)n moieties running parallel to the c-axis and close to the [010] directions that contain two types of Sb and three types of S atoms.20 Among the three



Figure 7. Morphological evolution in a case of Sb2Se3 fractal splitting at different reaction stages. Aliquots of solution were taken out at 1 (a), 3 (b), 6 (c), and 12 h (d), respectively. The data shows that fractal splitting takes place at a very early stage of the reaction.

types of sulfur atoms, two are formally trivalent and one is divalent. Within the chain, the divalent sulfur and one of the trivalent sulfurs are connected to antimony by strong covalent bonds. However, the third sulfur is connected to the antimony of the second parallel running chain by weaker van der Waals bonds that are responsible for the cleavage of the crystal. Thus, the cleavage takes place parallel to the (010) planes, where only van der Waals bonds are to be ruptured. Consequently, Sb2S3 breaks easily along the c-axis, thus leading to the formation of a 1D structure. In fact, as already shown by previous literature reports, Sb2S3 tends to form 1D nanostructures, including nanorods and nanoribbons.16-18,21 Also, it has been shown that the nanorods of the complex structures indeed grow along the [001] direction based on our TEM measurements, which provides additional support of the fractal splitting growth mechanism for these complex nanostructures. The formation of a Sb2Se3 1D nanostructure can also be elucidated on the basis of its crystal structure because Sb2Se3 is isostructural with Sb2S3. The hydrothermal growth of complex Sb2S3 and Sb2Se3 nanostructures in the presence of PVP starts with elongated nanorod seeds followed by self-similar branching (fractal splitting growth) and ends up with anisotropic spherulitic aggregates. To shed light on the formation of the fractal splitting structure, we studied their temporal morphological evolution by taking TEM images on aliquots obtained at different time intervals from the reaction system of Sb2Se3. Figure 7 shows the evolution of the final spherulitic structure. At a very early stage (1 h), single units with no splitting were observed in Figure 7a. From panel b of Figure 7, we can see that highly complex shapes already occurred at an early stage (3 h). As the reaction proceeds, the number of nanofilaments increases, and, mean-

while, the individual nanofilaments grow, but mainly in the elongated direction. A sheaf-like structure was observed with the reaction time prolonging, as shown in Figure 7c. Finally, a spherical structure formed. The results clearly show that the Sb2Se3 nanostructures are observed in the form of individual nanorods with no splitting (Figure 7a), small bundles with simple splitting (Figure 7b), sheaf structures (Figure 7c), and, finally, spherulitic structures (Figure 7d). Morphogenesis evolution from rods to dumbbells and to spheres is in accordance with the fractal splitting mechanism. Future real-time TEM measurements will be of much interest and may enable definitive observation of the fractal splitting. The presence of dumbbells as intermediates for the final formation of spheres under the control of PVP is consistent with our previous observations for various metal carbonate systems using double hydrophilic block copolymers as crystal modifiers in solution,8d and is also similar to that reported by Kniep et al. about the growth of fluoroapatite22 or by Co¨lfen et al. for truncated triangular calcite mesocrystals formed in solution containing polystyrene sulfonate (PSS);23 therefore, the mechanism of morphogenesis evolution from rods to dumbbells and to spheres appears to be rather universal. However, it has to be noted that the exact growth mechanism is still unclear, although some explanation was given in the literature based on the role of intrinsic electric fields, which direct the growth of dipole crystals,22-24 and the splitting growth mechanism, which was used for explaining the formation of a sheaf-like Bi2S3 nanostructure.25 More recently, Kniep et al. used electron holography to successfully image electric fields around fluoroapatite nanorod seeds;26 this indicates that intrinsic electric dipole fields may represent a general principle in the formation of such complex


Chen et al.

SCHEME 1. Proposed Formation Mechanism of Hierarchical Nanostructures Made of Nanorodsa

a

(a) Nucleation of nanoparticles. (b) Growth of nanorod crystals controlled and stabilized by PVP molecules. (c,d) Branching at the ends of the primary rods and formation of dumbbell intermediates based on a mechanism suggested by Kniep et al.22 (e,f) Final sphere with an equatorial notch.

structures. The possible particle formation process in the presence of PVP is summarized in Scheme 1. Overall, once the nuclei are formed just after supersaturation, PVP macromolecules are preferentially absorbed to [010] planes of the Sb2S3 nanoparticles, leading to the subsequent growth along the [001] direction and the formation of nanorods, then nanorods aggregate into simple bundle nanorods, further resulting in the formation of a dumbbell structure due to the fractal splitting growth, and finally a sphere with an equatorial notch is formed, as shown in Scheme 1. In this mechanism, the aggregation of nanocrystals unavoidably results in the occlusion of polymer molecules in the threedimensional complex nanostructures. This was confirmed by thermal gravimetric analysis (TGA), which showed that a content of ca. 6 wt % polymer remained in the final product (data not shown). Trapping of the polymer within the microparticles is a direct result of the surface stabilization/aggregation process. The evolution from rod-shaped and splitting crystals to dumbbell-shaped bundles and spherulites occurred with the addition of PVP to the reaction system. When the reaction was performed in the absence of PVP, almost irregular Sb2S3 microrods with large sizes were observed, as clearly shown in the SEM image (Figure 8), indicating that PVP plays an important role in the formation of these novel Sb2S3 nanostructures. The previous studies demonstrated that the presence of PVP was found to be helpful for the formation of different types of 1D nanostructures, such as metal nanowires27 and In(OH)3 nanorods.28 As a structure-directing agent, it could be believed that the presence of PVP molecules prevents the aggregation of nanoparticles in the initial stage of the nanorod growth and

kinetically controls the growth rates of different crystallographic faces of Sb2S3 through selectively adsorbing on (010) faces. In this case, the roles of PVP in the formation of Sb2S3 nanorods are similar to those of PVP in the formation of Ag and Pb nanowires.27a-c In the absence of PVP, the reaction only produced many irregular particles of several micrometers, which often appeared as particle aggregates. This indicates that the initially formed nanorods had a strong tendency to aggregate as larger ones when no protective agents were added. When PVP was introduced in appropriate amounts, the initially formed nanorods were relatively small in dimension, and well dispersed in the solution. The selective adsorption of PVP on specific crystal faces promotes Sb2S3 crystals to grow along the [001] direction, thus leading to a bifurcation of the growth process and consequently enhancing the splitting of the rods. Optical absorption measurements were carried out to supply information on the band structures, which is one of the most important electronic parameters for semiconductor nanocrystals. Typical UV-vis-NIR absorption spectra recorded from Sb2S3 and Sb2Se3 nanostructures at room temperature are shown in Figure 9. Figure 9a shows the absorption spectrum measured from Sb2S3 nanostructures, giving the absorption edge λonset of the spectrum at about 795 nm. Figure 9b is the absorption spectrum recorded from Sb2Se3 nanostructures, showing that the absorption edge λonset of this material is located at about 1100 nm. Therefore, the band gap of Sb2S3 and Sb2Se3 complex nanostructures can be estimated to be 1.56 and 1.13 eV, respectively, using the following equation:29

Figure 8. SEM image of the Sb2S3 products obtained in the absence of PVP under hydrothermal treatment at 180 °C for 12 h.

Figure 9. UV-vis-NIR absorption spectra recorded from (a) Sb2S3 nanostructures and (b) Sb2Se3 nanostructures at room temperature.


REphoton ) A(Ephoton - Eg)1/2 where Ephoton is the corresponding phonon energy, R is the absorbance coefficient, and A is a constant. These are quite comparable to the values reported for nanoribbons and nanorods of comparable dimensions.17b,21 It has to be noted that the nanorods in our dimension range do not show a quantum confinement effect, a fact also established by other studies.17b, 21 This may be attributed to the lower Bohr’s radius for these materials. Nevertheless, the experimental band gap for complex Sb2S3 and Sb2Se3 nanostructures is close to the optimum value for photovoltaic conversion, suggesting that Sb2S3 and Sb2Se3 nanostructures may be very promising for applications in solar energy and photoelectronics. Conclusions In summary, we have introduced a new one-step approach to prepare complex Sb2S3 and Sb2Se3 nanostructures under hydrothermal treatment with PVP acting as a structure-directing agent. The formation of these complex nanostructures may be elucidated via the fractal splitting growth mechanism, as observed in some natural minerals. In the first stage, growth occurs radially from a common nucleation site. The second growth process begins with a single fiber or bundle that branches as growth continues, producing a dumbbell and eventually evolving into the spherule with an equatorial notch. Selective adsorption of polymer molecules on newly formed faces may play an important role in this mesoscale transformation. The obtained materials were characterized by XRD patterns, SEM and TEM measurements, and IR spectrum analysis in detail, and the optical properties of the obtained nanostructures were measured. This simple and facile process is favorable for the future bulk synthesis of other nanomaterials with complex nanostructures and the technological importance. Further study is needed to investigate the controlling factors for this unusual kind of morphogenesis, and to better understand the fractal splitting growth process. Acknowledgment. Support from the National Natural Science Foundation of China (20671096) and the special funding support from the Centurial Program of CAS is gratefully acknowledged. References and Notes (1) (a) Yin, Y. D.; Alivisatos, A. P. Nature 2005, 437, 664. (b) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. AdV. Mater. 2003, 15, 353. (c) Wang, D. L.; Lieber, C. M. Nat. Mater. 2003, 2, 355. (d) Wang, Z. L. J. Mater. Chem. 2005, 15, 1021. (e) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (2) Lu, Q. Y.; Gao, F.; Komarneni, S. J. Am. Chem. Soc. 2004, 126, 54. (3) (a) Jun, Y. W.; Lee, S. M.; Kang, N. J.; Cheon, J. W. J. Am. Chem. Soc. 2001, 123, 5150. (b) Zitoun, D.; Pinna, N.; Frolet, N.; Belin, C. J. Am. Chem. Soc. 2005, 127, 15034.

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The Fractal Splitting Growth of Sb2S3 and Sb2Se3 Hierarchical

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