Large-Scale Synthesis of Single-Crystal Double-Fold Snowflake Cu2S Dendrites Zhengcui Wu, Cheng Pan, Zhenyu Yao, Qingrui Zhao, and Yi Xie* Department of Nanomaterials and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei 230026, P. R. China
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 7 1717-1719
ReceiVed January 27, 2006; ReVised Manuscript ReceiVed May 10, 2006
ABSTRACT: In this paper, novel self-supported micropatterns of double-fold snowflake copper sulfide (Cu2S) dendrites were successfully synthesized by a template- and surfactant-free method based on a simple reaction of CuSO4‚5H2O and dimethyl sulfoxide (DMSO), in which DMSO performed multiple roles as a solvent, a sulfur source, and a reducing agent. The influences of the concentration of CuSO4‚5H2O, the reaction time, and the temperature on the morphology of the products are discussed. Possible crystal growth processes are also proposed on the basis of the experimental results. Introduction Recently, much effort has been made in the design of rational methods for synthesizing higher ordered inorganic crystals with specific sizes, shapes, and hierarchies because of the potential to design new materials and devices in various fields. The architectural control of nanoparticles with well-defined shapes is a key for the success of “bottom-up” approaches toward future nanodevice fabrication.1 Dendrites, as a kind of fractal structure, which are generally formed by hierarchical self-assembly under nonequilibrium conditions,2,3 have received intensive interest in recent years. A variety of methods are used for preparing dendritic inorganic nanostructures, most of which are synthesized in the presence of organic additives or surfactants;4-7 selfassembled organic superstructures and templates with complex functionalization patterns can direct the growth of inorganic crystals with controlled morphologies and architectures.8 However, the use of surfactants or directing agents may introduce heterogeneous impurities. Of the many challenges facing crystal science, the development of simple, one-step, and effective methods for creating novel assemblies of self-supported patterns of hierarchically fractal architectures is important to technology and remains an attractive but elusive goal. It is well-known that Cu2S (chalcocite) is an interesting material for its semiconducting and photovoltaic capabilities because Cu2S is an indirect semiconductor with a bulk band gap of 1.21 eV, and it has been extensively investigated and widely used as a component of solar cells. A variety of elegant and efficient techniques have been carried out on shape- and size-controlled growth of Cu2S crystals, among which nanocrystal superlattices,9 nanorods,10 nanodisks,11 nanowires,12-13 nanoflakes,14 hexagon nanoplates,15 and monolayer plane flowerlike dendrites16 have been successfully synthesized using appropriate surfactants or soft templates. The availability of Cu2S nanostructures with well-defined morphologies and dimensions should enable the synthesis of new types of applications or should enhance the performance of currently existing photoelectric devices. However, to the best of our knowledge, the hierarchical self-assembly of Cu2S has rarely been reported. In this paper, we report the large-scale synthesis of hierarchical self-supported micropatterns of Cu2S by the reaction of CuSO4‚ 5H2O and dimethyl sulfoxide (DMSO) at a suitable concentration and temperature, in which DMSO performed multiple roles * To whom correspondence should be addressed. Tel and Fax: 86-5513603987. E-mail:
[email protected].
Figure 1. XRD pattern of the as-prepared double-fold snowflake Cu2S dendrites at [CuSO4‚5H2O] ) 0.0075 M and 180 °C for 6 h. The diffraction peaks can be indexed to the hexagonal phase of Cu2S, indicating the high crystallinity.
as a solvent, a sulfur source, and a reducing agent. The products display an elegant fractal morphology resembling a double-fold snowflake flower, which provides us another opportunity for exploring the properties dependent on their morphologies. One of the characteristics of the above reaction is that any additives such as polymers and surfactants are not required to control the product’s sizes and morphologies. Experimental Section CuSO4‚5H2O and DMSO of analytic grade purity were purchased from Shanghai Chemical Co. Ltd. and were used as received. In a typical synthesis, 0.005 to 0.02 M CuSO4‚5H2O was added to 40 mL of DMSO to form a clear green solution of a certain concentration under constant stirring for 10 min, which was placed in a Teflon-sealed autoclave and then maintained at 180 °C for 6-12 h. The black product was isolated by centrifugation, repeatedly washed with distilled water and absolute ethanol, and finally dried in a vacuum at 60 °C for 10 h. The structure of these obtained samples were characterized by the X-ray diffraction (XRD) pattern, which was recorded on a Rigaku Dmax diffraction system using a Cu KR source (λ ) 1.54187 Å). The scanning electron microscopy (SEM) images were taken using a Sirion 200 field emission scanning electron microscope (FE-SEM, 20kV). Transmission electron microscopy (TEM) images and high-resolution transmission electron microscopy (HRTEM) images were obtained with a Hitachi 800 system at 200 kV and a JEOL-2010 system also at 200 kV.
Results and Discussion The crystal structure and phase composition of Cu2S products were first characterized using XRD analysis. Figure 1 displays
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Figure 2. Typical FESEM images of the obtained Cu2S dendrites at [CuSO4‚5H2O] ) 0.0075 M and 180 °C for 6 h. (a) At a low magnification, indicating that the double-fold snowflake Cu2S can be fabricated on a large scale. (b-d) At a high magnification, revealing the morphology of a single flower of Cu2S dendrites in different directions (b, frontal; c, flank; d, tilted frontal).
a representative XRD pattern of the as-prepared Cu2S samples, suggesting their high crystallinity. The diffraction peaks can be readily indexed to the hexagonal phase of Cu2S (JCPDS card, No. 84-206) with lattice parameters of a ) 3.912 Å and c ) 6.763 Å. The double-fold snowflake flowers were successfully synthesized on a large scale, as revealed in Figure 2a, a panoramic FESEM image of the product obtained with 0.0075 M CuSO4‚ 5H2O in 40 mL of DMSO at 180 °C for 6 h. Figure 2a not only shows that the product consists almost entirely of such dendritic structures, but it also gives the information that dendrites of high yield and good uniformity easily can be achieved with this simple and easily controlled approach. The high magnification images in Figure 2b-d show the morphology of a single flower viewed from frontal, flank, and tilted frontal perspectives, respectively. All the SEM images reveal a clear and well-defined dendritic fractal structure with a single flower consisting of a double-fold snowflake back-to-back. Each fold has six ordered petals, and each petal has many ordered small laminae. As seen from the flank of a flower, the rear of each petal has a threedimensional sandwich structure, and all flowers have a similar structure. The diagonal length of a flower was about 1.8 µm, and the thickness of each petal was about 180 nm (measured by the tip of the petal). The high symmetry and single crystallinity was revealed by TEM observations, and the TEM image of a flower is shown in Figure 3a, from which one can clearly see that the Cu2S dendrites show a 6-fold symmetric structure like a snowflake. Further structural characterization of the dendrite was carried out by HRTEM. Figure 3b is a HRTEM image taken from the area labeled b in Figure 3a, exhibiting the lattice structure at the tip of the dendrite, which has clear lattice fringes indicating its single crystallinity nature. The lattice spacing of 1.98 Å between the adjacent lattice planes in the image corresponds to the distance between (112h0) and (21h1h0) crystal planes. The selected-area electron diffraction (SAED) pattern (Figure 3c) confirms its single-crystalline structure. Moreover, the ED
Wu et al.
Figure 3. TEM images of the same Cu2S dendrites as shown in Figure 2. (a) TEM image of a double-fold snowflake structure. (b) Highresolution TEM image recorded in the region of the dendritic structure in (a) showing the well-defined single-crystalline nature of the dendritic structure. (c) SAED pattern recorded from the dendrite in (a), exhibiting not only the single-crystalline nature of the dendrite but also the doublefold structure.
patterns corresponding to the other trunk tips as well as the branch tips and the central part are observed to be exactly the same, suggesting that the whole dendrite is a Cu2S single-crystal oriented along [0001] with six petals along ([101h0], ([11h00], and ([01h10], respectively. We can see many second diffraction spots in the ED pattern, for example, the six dim spots surround the central bright spot (see arrows pointed in Figure 3c), which confirm the double-fold structure of our products. A series of contrastive experiments were done and indicated that the reaction time and the concentration of CuSO4‚5H2O significantly affect the petal morphology of the dendrite-like product. For example, when the reaction time was increased from 3 to 12 h, while keeping the concentration of CuSO4‚ 5H2O constant at 0.0075 M and reaction temperature at 180 °C, the six ordered petals changed continuously from a doublefold hexagon at 3 h (Figure 4a) to a double-fold snowflake with many laminae at 6 h (Figure 2b), then to a double-fold snowflake with a few laminae at 9 h (Figure 4b), and finally to a doublefold snowflake with almost smooth petals at 12 h (Figure 4c). The change of the sandwich structure of the back petal was unconspicuous but had the tendency to fade away from 6 to 12 h. The evolvement in Cu2S morphology with the reaction time suggested that the whole growth process was a kinetic and thermodynamic competition process, when the reaction time prolonged the shape of Cu2S changed from a kinetically stabilized state (double-fold snowflake with many laminae at 6 h) to a thermodynamically stabilized state (double-fold snowflake with an almost smooth petal at 12 h). When the concentration of CuSO4‚5H2O was varied between 0.0075 and 0.02 M, while a constant reaction temperature at 180 °C was maintained, the morphologies of Cu2S evolved from a regular double-fold snowflake at a CuSO4‚5H2O concentration of 0.0075 M (Figure 2) to an incompact double-fold snowflake (Figure 4d) at a CuSO4‚5H2O concentration of 0.011 M and then to irregular dendrites only containing partly doublesnowflake flowers when the CuSO4‚5H2O concentration reached 0.02 M (Figure 4e). If the concentration of CuSO4‚5H2O was lower than 0.0075 M, the quantity of product is too low to be collected for further characterization. The higher the CuSO4‚ 5H2O concentration, the more incompact were the branches of
Single-Crystal Snowflake Cu2S Dendrites
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is limited in the [0001] direction and predominantly occurs along the six energetically equivalent {101h0} directions.11 As growth continues, methanethiol cannot completely cover the whole areas of the six faces due to its short chain length, and thus gaps form on the middle part of each plane; subsequently, the crystal growth starts along 2 × 6 energetically equivalent directions, and the repeated growth mode of the additional branches growing out of each petal along crystallographically equivalent directions will lead to the formation of hexagonal double-fold snowflake flowers.17 It should be mentioned that the reaction temperature has a significant effect on the yield of the products. A high yield of products could be produced over a temperature range from 150 to 180 °C. If the temperature is lower than 150 °C, the growth is too low. The reaction temperature increases the growth rate but does not significantly affect the morphology of the nanocrystals. Increased temperature accelerates DMSO to decompose CH3SH and enhances the thermolysis rate of C-S bond in methanethiol, which speeds the overall growth rate by increasing S monomer availability to the growing nanocrystals.
Figure 4. FESEM images of Cu2S dendrites obtained with different CuSO4‚5H2O concentrations at 180 °C for different times. (a-c) At [CuSO4‚5H2O] ) 0.0075 M and reaction times of 3, 9, and 12 h, respectively. (d, e) At a reaction time of 6 h and concentrations of CuSO4‚5H2O of 0.011 M and 0.02 M, respectively.
the dendrites, which suggested that an increased concentration of CuSO4‚5H2O expedited the growth speed of Cu2S and thus led to the final incompact structures. These results suggest that it is possible to control and tune the shape of Cu2S dendritic nanostructures by controlling the kinetic parameters, such as the reaction time and concentration. In this approach, it may be seen that the source of sulfur came from DMSO. At an elevated temperature, DMSO decomposes and releases CH3SH gradually. While copper methanethiolate forms from the reaction of Cu2+ and CH3SH, it finally decomposes to Cu2S when heated. Clearly, methanethiol provides the sulfur source for the growing nanocrystals; however, what role does methanethiol play in terms of controlling crystal growth? Being a kind of thiol with coordination ligands for transition metals,10 it is wellknown that methanethiol could much easily coordinate with Cu ions, and the coordination was preferential in the {101h0} planes, because the copper atoms in the {101h0} planes of Cu2S exhibit a square lattice with sites available for 4-fold coordination to adsorb sulfur. Meanwhile, the {0001} planes do not exhibit this 4-fold symmetry. Thus, Cu2S preferentially grows in the {101h0} directions as opposed to the [0001]; that is, the growth direction provides faces with the 4-fold surface sites that promote S adsorption and incorporation to form the next layer of Cu2S. In comparison, the {0001} planes are the most atomically dense structures with S atoms lying in 6-fold sites and are expected to kinetically inhibit S addition. So, the crystal structure growth
Conclusion In summary, self-supported micropatterns of double-fold snowflake Cu2S dendritic crystals have been successfully prepared via a template- and surfactant-free reaction route involving CuSO4‚5H2O and DMSO. The final product has a regular shape and narrow size distribution. Although the detailed mechanism is not very clear and still needs more investigation, it is no doubt a pretty simple and easy controlled route for producing elegant dendritic crystals. Given the generality of this approach, we hope to extend it to the growth of other dendritic fractal structures of sulfide semiconductor crystals with uniformity and high purity. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 20321101) and the state key project of fundamental research for nanomaterials and nanostructures. Z.C. Wu thanks Prof. Mingwang Shao at Anhui Normal University for valuable discussion. References (1) Duan, X. F.; Huang, Y.; Cui, Y.; Wang, J. F.; Lieber, C. M. Nature 2001, 409, 66. (2) Lisiecki, I.; Albouy, P. A.; Pileni, M. P. AdV. Mater. 2003, 15, 712. (3) Meakin, P. Phys. ReV. Lett. 1983, 51, 1119. (4) Tian, Z. R.; Voigt, J. A.; Liu, J.; McKenzie, B.; McDermott, M. J. J. Am. Chem. Soc. 2002, 124, 12954. (5) Kuang, D. B.; Xu, A. W.; Fang, Y. P.; Liu, H. Q.; Frommen, C.; Fenke, D. AdV. Mater. 2003, 15, 1747. (6) Ma, Y. R.; Qi, L. M.; Ma, J. M.; Cheng, H. M. Cryst. Growth Des. 2004, 2, 351. (7) Liu, B.; Yu, S. H.; Li, L. J.; Zhang, Q.; Zhang, F.; Jiang, K. Angew. Chem., Int. Ed. 2004, 43, 4745. (8) (a) Lopes, W. A.; Jaeger, H. M. Nature 2001, 414, 735. (b) Chen, X.; Chen, Z. M.; Fu, N.; Lu, G.; Yang, B. AdV. Mater. 2003, 15, 1413. (9) Liu, Z. P.; Liang, J. B.; Xu, D.; Lu. J.; Qian, Y. T. Chem. Commun. 2004, 23, 2724. (10) Larsen, T. H.; Sigman, M.; Ghezelbash, A.; Doty, R. C.; Korgel, B. A. J. Am. Chem. Soc. 2003, 125, 5638. (11) Sigman, M. B.; Jr.; Ghezelbash, A.; Hanrath, T.; Saunders, A. E.; Lee F.; Korgel, B. A. J. Am. Chem. Soc. 2003, 125, 16050. (12) Chen, L.; Chen, Y. B.; Wu, L. M. J. Am. Chem. Soc. 2004, 126, 16334. (13) Liu, Z. P.; Xu, D.; Liang, J. B.; Shen, J. M.; Zhang, S. Y.; Qian, Y. T. J. Phys. Chem. B 2005, 109, 10699. (14) Zhang, P.; Gao, L. J. Mater. Chem. 2003, 13, 2007. (15) Zhang, H. T.; Wu, G.; Chen, X. H. Langmuir 2005, 21, 4281. (16) Gorai, S.; Ganguli, D.; Chaudhuri, S. Mater. Lett. 2005, 59, 826. (17) Cao, M. H.; Liu, T. F.; Gao, S.; Sun, G. B.; Wu, X. L.; Hu, C. W.; Wang, Z. L. Angew. Chem., Int. Ed. 2005, 44, 4197.
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