CRYSTAL GROWTH & DESIGN
Hierarchical Growth and Shape Evolution of HgS Dendrites Xiangying Chen, Xiong Wang, Zhenghua Wang, Xiaogang Yang, and Yitai Qian* Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China Received April 16, 2004;
2005 VOL. 5, NO. 1 347-350
Revised Manuscript Received October 6, 2004
ABSTRACT: Novel hierarchical HgS dendrites were synthesized in high yield by hydrothermal treatment of a mercury (II)-thiourea complex. The individual HgS dendrite consists of a long central trunk with secondary and tertiary sharp branches, which preferentially grow parallel to the (001) plane. The possible growth process is proposed by arresting the growth at a series of intermediate morphology stages during the shape evolution of HgS dendrites based on SEM observations. Introduction Fabrication of nano- to microscopic-scale inorganic materials with special size and morphology are of great interest for materials chemistry due to their importance in basic scientific research and potential technological applications.1,2 In the past few decades, there has been an increasing number of reports on the synthesis of inorganic crystals, such as rods,3 wires,4 plates,5 tubules,6 dendrites,7 and cubes.8 Of these shapes, hierarchical dendrites have attracted much attention stimulated by the practical importance related to some fractal growth phenomena.9 Studies on the shape control of inorganic crystals will to some extent give insights into the crystallization behavior in a nano- or micro-sized scale owing to the traditional lack of understanding of the growth history and shape evolution process. Sulfides of metals have been extensively studied for their outstanding properties and potential use in future devices.10,11 Among the metal sulfides, mercury (II) sulfide (HgS) is widely used in many fields such as image sensors and electrostatic image materials,12 ultrasonic transducers,13 and photoelectric conversion devices.12,13 HgS usually exists in two forms: the cubic phase (β-HgS, metacinnabar) and the hexagonal phase (R-HgS, cinnabar). Reports on the syntheses of mercury chalcogenides are scarce owing to the toxicity problem of mercuride. Recently, many methods were developed to fabricate HgS with different morphologies, including nanoparticles,14 nanorods,15 nanowires,16 and nanotubules.17 Herein, we present a simple route to synthesize hierarchical HgS dendrites in high yield with side branches emerging at 60 angles with respect to the central trunk, preferentially growing parallel to the (001) plane. To the best of our knowledge, it is the first report of the growth of hierarchical HgS dendrites in high yield via a solution approach. To study the growth process, reactions between HgCl2 and thiourea are carried out at different reaction times to arrest the HgS morphologies at different growth stages. Experimental Procedures All analytical chemicals were purchased from Shanghai Chemical Reagents Co. and used without further purification. * Corresponding author. Tel.: +86-551-3606647; fax: +86-5513607402; E-mail:
[email protected].
Figure 1. XRD patterns of the ground products (a) and the unground products (b) obtained at 140 °C for 4 h. In a typical procedure for preparing hierarchical HgS dendrites, anhydrous HgCl2 (2 mmol) and thiourea (3 mmol) were dissolved in two beakers containing 20 mL of distilled water, respectively. The two clear solutions were mixed together to yield white mercury (II)-thiourea complex solution, which was then transferred into a 50-mL Teflon-lined autoclave and maintained at 140 °C for 4 h. After the autoclave was cooled by quenching with cold water, the red products were filtered off, washed with distilled water and absolute ethanol several times, and dried in a vacuum at 50 °C for 4 h. XRD patterns were recorded by using a Philips X’Pert PRO SUPER X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.541874 Å). Field emission scanning electron microscopy (FESEM) images were taken with a JEOL JSM6700F scanning electron microscope. TEM images and SAED patterns were carried out on a Hitachi model H-800 transmission electron microscope. Scanning electron microscopy (SEM) images were obtained on a Hitachi (X-650) scanning electron microscope.
Results and Discussion Figure 1a shows the XRD pattern of the as-prepared products (ground) obtained at 140 °C for 4 h. All the diffraction peaks in Figure 1a can be indexed to hexagonal HgS (cinnabar) with lattice parameters of a ) 4.151 Å, and c ) 9.494 Å, which are consistent with the reported values (JCPDS file No. 06-0256, a ) 4.149 Å,
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Figure 2. FESEM and TEM images of the as-prepared products: (a, b) FESEM images of hierarchical HgS dendrites; (c) TEM image of a representative hierarchical HgS dendrite; (d) SAED pattern taken from panel c.
and c ) 9.495 Å). The XRD pattern of the unground products is shown in Figure 1b. It is noticeable that the (003) and (006) reflection planes are extremely strong compared with the standard reflections, which is probably related to the preferential growth of hexagonal HgS dendrites. The products are proven to be well crystallized and pure by XRD patterns. Figure 2 a,b shows the FESEM images of hierarchical HgS dendrites with different magnifications. The panoramic view with low magnification in Figure 2a demonstrates clearly that a large quantity of dendritic architectures exists with a length of 10-30 µm along the trunk. Further observation based on the high-magnification image (Figure 2b) reveals that the individual hexagonal HgS dendrite is composed of a long central trunk with secondary and tertiary sharp branches. It is interesting that the secondary branches are parallel to each other and emerge at 60 angles with respect to the central trunk. Similar phenomena are also found toward the tertiary branches. A representative TEM image of an individual hierarchical dendrite is shown in Figure 2c. The SAED pattern (Figure 2d) taken from the trunk tip or branch tip is shown to be identical and can be attributed to the [001] zone axis diffraction, which suggests that the hierarchical dendrite grows parallel to the (001) plane. This implies that the individual trunk or branch is single crystallite rather than polycrytallite. To investigate the intermediate transition from the white mercury (II)-thiourea complex precursor to R-HgS dendrites and the growth mechanism, time-dependent experiments were carried out by quenching the Teflonlined autoclave using cold water at different reaction
stages. A series of SEM images in Figure 3 show the morphology at different reaction stages corresponding to the reaction time of 0, 1, 2, 4 h, respectively. The white precursor obtained by reacting anhydrous mercury (II) chloride with thiourea in water is of irregular shape in Figure 3a. When prolonging the reaction time to 1 h, it is observed from Figure 3b that rodlike HgS crystals first form, and some short secondary branches in low yield grow out of the rodlike trunk. When increasing the reaction time to 2 h, as shown in Figure 3c, more secondary branches emerge, the secondary branches grow longer, and tertiary branches also grow out of the secondary branches. Well-defined hierarchical HgS dendrites with high yield of 95% (Figure 3d) are obtained after the initial white precursor is hydrothermally treated at 140 °C for 4 h. On the basis of the experimental results, a possible growth process demonstrating the synthesis of HgS dendrites can be simply described in Figure 4a. Upon heating, thiourea is attacked by the strong nucleophilic O atoms of H2O molecules leading to the weakening of the CdS double bonds, which will be broken to release S2- anions slowly.18 The newly formed S2- then reacts with Hg2+ to produce HgS nuclei, which grow preferentially to form the rodlike HgS crystals. Subsequently, the secondary branches branch off, parallel to each other and emerging at 60 angles with respect to the central trunk. With further growth, the tertiary family of branches grows along the directions parallel to the trunk, emerging at 60 angles with respect to the secondary branches. Zhong et al.19 have reported that the dendritic crystals are connected by the coordination polyhedron growth unit along the fastest growth direc-
Growth and Shape Evolution of HgS Dendrites
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Figure 5. TEM image of the cubic HgS nanocrystals obtained using Na2S‚9H2O as the sulfur source.
Figure 3. SEM images of hierarchical HgS dendrites obtained at different reaction times: (a) 0 h; (b) 1 h; (c) 2 h; (d) 4 h.
to the relatively low sublime point of 583.5 °C,20 which makes it very difficult to get clear HRTEM images. From a thermodynamic point of view, the crystalline dendrites, by virtue of its extended surface, have a considerably increased surface energy in contrast to the equilibrium shape of the crystal, and is thermodynamically unstable. Generally speaking, nonequilibrium growth and molecular anisotropy are the prerequisites for the formation of dendritic structure.21 In our system, random moving HgS nuclei formed in solution accumulate with each other to form kinetically roughened dendritic structures under certain supersaturation conditions, which is to some extent similar to the oriented attachment (OA) model pioneered by Banfield22 and Kotov.23 As contrast experiments, when thiourea is substituted by Na2S‚9H2O without changing other reaction parameters, the products are black cubic HgS nanocrystals (shown in Figure 5) instead of red hexagonal HgS dendrites. This is because the equilibrium concentration is readily attained, and the precipitate is quenched without later morphological evolution. The experimental results reveal that HgS with desired shape and phase can be selectively synthesized by programming the growth parameters in the initial synthetic procedure. Conclusion
Figure 4. (a) Schematic description of the growth process of hierarchical HgS dendrites. (b) Structural view of hierarchical HgS dendrite formed by the connection of rhombus growth unit.
tion of the corresponding crystals. In the present study, rhombus growth unit is put forward to construct the hierarchical HgS crystals with secondary or tertiary branches parallel to the (001) plane (Figure 4b). According to Zhong’s proposition described above, 〈110〉 is probably the growth direction of the hierarchical HgS crystals including the trunk and branches. To prove the growth direction of 〈110〉, it is necessary to obtain HRTEM observations. However, the hierarchical HgS crystals are sensitive to electron beam irradiation due
In summary, hierarchical HgS dendrites were synthesized in high yield via a simple hydrothermal route. The obtained HgS crystals grow preferentially parallel to the (001) plane. A series of intermediate morphologies during the shape evolution of HgS dendrites based on SEM observations were examined to propose the possible reaction process and mechanism. From a technological point of view, these obtained hierarchical HgS crystals may have potential applications in microelectronic devices. This simple method is expected to allow fabrication of other inorganic materials with the controllable phases and shapes. Acknowledgment. The National Natural Science Foundation of P. R. China and the 973 Program are gratefully acknowledged for their financial help. The
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author greatly thanks Prof. Fanqing Li for the FESEM observations and Mr. Shuai Gong for the TEM observations. References (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (3) (a) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (b) Yang, P. D.; Lieber, C. M. Science 1996, 273, 1836. (4) (a) Xia, Y. N.; Yang, P. D. Adv. Mater. 2003, 15, 351. (b) Liu, Z. P.; Yang, Y.; Liang, J. B.; Hu, Z. K.; Li, S.; Peng, S.; Qian, Y. T. J. Phys. Chem. B 2003, 107, 12658. (c) Wang, X.; Li, Y. D. Chem.-Eur. J. 2003, 9, 300. (d) Xiong, Y. J.; Xie, Y.; Li, Z. Q.; Li, X. X.; Gao, S. M. Chem.-Eur. J. 2004, 10, 654. (5) (a) Yu, J. C.; Xu, A. W.; Zhang, L. Z.; Song, K. Q.; Wu, L. J. Phys. Chem. B 2004, 108, 64. (b) Chen, S. H.; Carroll, D. L. Nano Lett. 2002, 2, 1003. (6) (a) Mo, M. S.; Zeng, J. H.; Liu, X. M.; Yu, W. C.; Zhang, S. Y.; Qian, Y. T. Adv. Mater. 2002, 22, 1658. (b) Liu, J. W.; Shao, M. W.; Chen, X. Y.; Yu, W. C.; Liu, X. M.; Qian, Y. T. J. Am. Chem. Soc. 2003, 125, 8088. (7) (a) Kuang, D. B.; Xu, A. W.; Fang, Y. P.; Liu, H. Q.; Frommen, C.; Fenske, D. Adv. Mater. 2003, 20, 1747. (b) Ma, Y. R.; Qi, L. M.; Ma, J. M.; Chang, H. M. Cryst. Growth Des. 2004, 4, 351. (c) Zhang, J.; Sun, L. D.; Jiang, X. C.; Liao, C. S.; Yan, C. H. Cryst. Growth Des. 2004, 4, 309. (8) (a) Wang, D. B.; Mo, M. S.; Yu, D. B.; Xu, L. Q.; Li, F. Q.; Qian, Y. T. Cryst. Growth Des. 2003, 3, 717. (b) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176.
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