CRYSTAL GROWTH & DESIGN
Fabrication of CdS Micropatterns: Effects of Intermolecular Hydrogen Bonding and Decreasing Capping Ligand
2004 VOL. 4, NO. 2 355-359
Xuanjun Zhang,† Qingrui Zhao,† Yupeng Tian,‡ and Yi Xie*,† Structure Research Laboratory and Department of Chemistry, University of Science and Technology of China, Hefei 230026, People’s Republic of China, and Department of Chemistry, Anhui University, Hefei 230039, People’s Republic of China Received August 13, 2003;
Revised Manuscript Received November 21, 2003
ABSTRACT: Novel branch-like CdS micropatterns were synthesized using thiosemicarbazide (NH2NHCSNH2) both as a sulfur source and as a capping ligand in a methanol/water system. The thiosemicarbazide/CdCl2 ratios were found to effectively play crucial roles in the morphologies of CdS crystals. With the ratios decreasing from eight to one, the CdS crystals exhibited different morphologies, from spheres to spindle-like morphologies mediated by cauliflower- and branch-like micropatterns. Control experiments using different sulfur sources showed that the NH2 groups, which could form hydrogen bonds, were necessary for the formation of the novel branch-like patterns. On the basis of the experimental results, a formation mechanism was proposed, which was supported by time-resolved experiments and IR analysis. Introduction The synthesis of inorganic materials of specific size and morphology is a key aspect in fields as diverse as modern materials, catalysis, medicine, electronics, ceramics, pigments, and cosmetics.1-4 Compared with the size control, the morphology control or morphogenesis is more demanding to achieve by means of classical chemical approaches. In the past few years, several excellent studies were carried out on shape-controlled growth of inorganic crystals, among which rods,5 triangles,6 disks,7 flowers, and dendrites8,9 have been successfully synthesized using appropriate surfactants or soft templates. Although many surfactants and/or additives were used to control the nucleation, growth, and alignment of inorganic crystals, there has been limited work about the elucidation of the relationship between the chemical nature of the materials and the final crystal morphologies. Nevertheless, several recent papers address this issue.10-13 It has been shown that the presence of a cationic charge or hydrogen-bonding site (amine) is indispensable for successful transcription of the organogel template into a silica structure.14 Shi et al. also found that hydrogen bonds are indispensable for the formation of cobalt oxide nanotubes.10 In this work, we show that not only hydrogen bonding but also the decreasing number of the capping ligand (thiosemicarbazide) can have a profound influence on the shapes of the resultant CdS crystals. Cadmium chalcogenides are important semiconductor materials and have been extensively studied15 owing to their desired applications.16 Much effort has been devoted to the synthesis of CdS rods,5,17 wires,18 and tubes.19 Peng and co-workers20 and others21 reported multi-armed CdS and/or CdSe crystals. We show here that branch-like CdS micropatterns can be obtained only * Corresponding author. E-mail:
[email protected]. Tel: 86-5513603987. Fax: 86-551-3603987. † University of Science and Technology of China. ‡ Anhui University.
using thiosemicarbazide both as a sulfur source and as a capping ligand. To the best of our knowledge, this is the first self-supported growth of CdS micropatterns via a template-free approach. To study the growth history, we examined reactions between thiosemicarbazide and CdCl2 with different reaction time, which makes it possible to arrest the CdS crystals at different growth stages. By varying different sulfur sources, we found that hydrogen bonding was necessary for the formation of the novel branch-like patterns. Experimental Procedures A total of 1 mmol of CdCl2 and 2 mmol of thiosemicarbazide was put into a Teflon-lined autoclave of 30 mL capacity. Methanol/water (v/v ) 1:1) was added to the autoclave up to 80% of the total volume. The autoclave was maintained at 80 °C for 1 h to make sure the complete dissolution of CdCl2 and thiosemicarbazide and then at 160 °C for 6 h. After cooling to room temperature, the yellow precipitate was washed with water and ethanol for several times, and the final product was dried in a vacuum at 60 °C for 2 h. It is noted that the posttreatment of the products after the reaction were carried out in fume hood to keep from excess H2S (generated in the solvothermal process). X-ray power diffraction (XRD) measurement of the asprepared sample was performed on a Rigaku D/max rA X-ray diffractometer with graphite-monochromatized CuKR radiation (λ ) 1.54178 Å) using a scanning rate of 0.06 °s-1 in the 2θ range from 20 to 60°. SEM images were carried out on an X-650 scanning electronic microanalyzer and JEOL JSM-6700F scanning electronic microanalyzer, respectively. Result and Discussion All of the obtained products were revealed to be wellcrystallized CdS under XRD measurement. The peaks of the XRD pattern (Figure 1) of the as-prepared sample
10.1021/cg0341555 CCC: $27.50 © 2004 American Chemical Society Published on Web 12/18/2003
356
Crystal Growth & Design, Vol. 4, No. 2, 2004
Zhang et al.
Figure 1. XRD pattern of CdS crystals obtained by the reaction of thiosemicarbazide and CdCl2 (with a molar ratio of 2:1) at 160 °C for 6 h.
Figure 3. SEM images of CdS crystals obtained by the reaction of CdCl2 and thiosemicarbazide in a molar ratio of 2 at different reaction times: (a) 1 h; (b) 2 h; (c) 4 h; and (d) 6 h.
Figure 2. SEM (a and b), TEM images (c), and ED (d) of the CdS micropatterns by the reaction of thiosemicarbazide and CdCl2 (molar ratio of 2:1) at 160 °C for 6 h.
can be indexed to the hexagonal CdS (JCPDS 411049).22 It’s noted that the (101) reflection was comparatively strong, which is most probably related to the orientation of the CdS crystals. The relatively broad peaks probably resulted from the small secondary branches that may have nanometer scales. SEM results showed that CdS micropatterns (Figure 2) exhibit a dendritic growth tendency with mean scales of 10-20 µm. The branch-like architectures here have many small secondary branches on the main branches. The rough surface of the CdS architectures are different from those common dendritic crystals reported in the literature such as BaSO4 flowerlike crystals9,23 and sulfide patterns24 because all of the secondary branches of them have regular faces. ED diffraction on one small branch shows diffraction rings together with some sharp spots, like polycrystalline nature, which may have resulted from the numerous CdS secondary branches with very small size. It is noted that the crystal is stable and can stand the sonicating process, which indicates that the patterns were formed by crystal growth but not a simple aggregation of small nanoparticles. Time-resolved experiments (the reactions were quenched by cooling the Teflon-lined autoclave using
cold water) were carried out to investigate the formation process of the branch-like CdS micropatterns, and the corresponding SEM images of the as-prepared samples are illustrated in Figure 3. One hour’s reaction between the two sources produced some rough-surfaced spherical CdS particles (Figure 3a) with a mean diameter range from 600 nm to 3 µm; the yield is ca. 28% based on CdCl2. Figure 3b shows the CdS crystals obtained after 2 hours’ reaction, which have some short petals growing out of the original spherical core extending toward several directions. The petals extend with the prolonged reaction time. As shown in Figure 3d, interesting micropatterns formed after 6 hours’ reaction. On the basis of the experimental results, a possible mechanism for the formation of micropatterns is proposed and can be simply described in Scheme 1. Upon heating, thiosemicarbazide is attacked by the strong nucleophilic O atoms of H2O molecules leading to the weakening of the CdS double bonds. Heated at an elevated temperature, the CdS bond will be broken, and the S2- anion will slowly generate, which then reacts with Cd2+ to form CdS particles. During the growth course of the CdS crystals, thiosemicarbazide could be considered as a double functional ligand in which S atoms can strongly interact with CdS surfaces, whereas the NH2 and NH-NH2 groups do not exhibit strong interactions with CdS but mainly promote solubility in methanol/water solutions and provide interparticle interactions by hydrogen bonds. At the beginning of the reaction, the number of the CdS particles is few, and thiosemicarbazide can effectively cap most of the surface of the newly formed CdS particles. These particles then interact by H-bonds to form larger aggregates. Diffusion-control growth5,25 then occurs in the spherical aggregates as the particles are in close contact, and the surface area is reduced by particle fusion and structure rearrangement. At this stage, some spheres with a rough surface are obtained.
Fabrication of CdS Micropatterns Scheme 1.
Crystal Growth & Design, Vol. 4, No. 2, 2004 357 Proposed Growth Process of the CdS Micropatterns
It is well-known that the fundamental growth mechanism is the addition and/or removal of molecules or very small particles on the crystal’s surface. Quite different from those systems, in which the number of the surfactants or ligands does not change,5,17-21 the number of thiosemicarbazides here is decreasing upon hydrolysis or decomposition during the growth course. With the formation of more and more CdS particles and the gradual decomposition of thiosemicarbazide, the ligand cannot cap most of the surface of CdS crystals but only selectively adsorb to some faces. As proposed in Scheme 1, the latter formed small particles only selectively interacting with hydrogen bonds with some faces of the former crystals that have been capped with ligands. Thus, some petals grow out of the original spherical core and extend toward several directions. Additional branches grow out of each petal leading to the formation of patterns. This mechanism is similar, but not identical, to the Oriented Attachment, which was observed in a system where the small particles are coated with small molecules that allow them to get close to each other and even facilitate attachment.25b,26 The mechanism by which thiosemicarbazide influences the morphology of the growing CdS crystals may involve its selective adsorption on certain crystal faces. If this mechanism was operating, one would have a spectroscopic consequence. It is to be expected that IR adsorptions associating with a vibrational transition of the CdS bond would be significantly affected by adsorption on the CdS surface. We therefore measured the IR spectra of free thiosemicarbazide and the CdS intermediate (washed with ethanol). The CdS absorption band of free thiosemicarbazide is located at 1645 cm-1. Because pure CdS has no apparent absorption at wavelengths between 700 and 2000 cm-1, the band at 1739 cm-1 of the CdS intermediate can be assigned to the absorption of the weakened CdS bond of thiosemicarbazide. The observed wavelength number increased 94 cm-1, similar to those observed for the alkylisocyanide-stabilized Pt and Pd nanoparticles (with a wavelength number increase of ca. 90 cm-1),27 indicating that the CdS groups is coordinated to the CdS surface. If the gradually decreasing ligand really influences the growing crystals, one would expect some spherical CdS crystals in a system with a large ratio of thiosemicarbazide/CdCl2. Because the ligand is apparently excessive, it can effectively cap most of the surface even
Figure 4. SEM images of CdS crystals obtained using different NH2CSNHNH2/CdCl2 ratios: (a) 8; (b) 4; (c) 2; and (d) 1.
when all CdCl2 has converted into CdS. The H-bond interactions cause the spherical aggregation and thus lead to spherical growth. With mediate ratios (hereafter abbreviated as R), the dendritic growth would occur at the middle and last reaction stages. In this case, branches could grow out of the core. In contrast, if the ratio (R) is very little, the ligand cannot cap most of the surface, and anisotropic growth would occur even at the former reaction stage. Figure 4 is the SEM images of CdS crystals obtained by the reaction of thiosemicarbazide and CdCl2 with a different R at 160 °C for 6 h. The results revealed that only some spheres with mean diameters of 1-4 µm (Figure 4a) were obtained when R is 8. When R is 4, some cauliflowerlike crystals were obtained (Figure 4b), together with some over-mature cauliflower, which is more like spheres, whereas some spindle-like rods (Figure 4d) formed when R is 1. These
358
Crystal Growth & Design, Vol. 4, No. 2, 2004
Figure 5. CdS particles obtained using thioacetamide (a) and HSCH2COONa (b) as sulfur sources and capping ligands.
results agree well with our above illustration; thus, an appropriate initial ligand number is needed for the formation of CdS micropatterns. Although the exact mechanism is still under investigation, the proposed growth process is possible. To investigate the influence of the chemical nature of the capping ligands on the morphologies of CdS crystals, we selected other sulfur containing ligands instead of thiosemicarbazide. From a series of control experiments, we found that NH2 groups play a crucial role in the morphologies of the CdS crystals, and the patterns can be formed using thiosemicarbazide, which has a relatively long NH-NH2 groups and more elements (NH or NH2 groups) for the formation of hydrogen bonds (as follows).
Zhang et al.
process is different quite from that obtained using NH2 groups containing compounds as capping ligands. The experiments at different temperatures and reaction times were also carried out. Slightly higher reaction temperatures did not influence the dendritic growth tendency of the CdS crystals but resulted in relatively larger crystals. In the case of 160 °C, well-developed patterns can be obtained from 6 to 10 hours’ reaction. During the further prolonged reaction time, the small branches shortened, which probably arises from the split of bigger aggregates into smaller ones to satisfy the spatial requirements of the crystal growth during the Ostwald ripening process. This phenomenon always occurs at low monomer (both newly formed small CdS particles and precursors) concentrations, which are also observed in the growth of ZnO crystals28 under a prolonged reaction time. In conclusion, novel branch-like CdS micropatterns have been successfully synthesized using only thiosemicarbazide both as a sulfur source and as a capping ligand via a solvothermal process. On the basis of the results of a series of time-resolved experiments, a possible growth process was proposed, which was also supported by IR analysis. In addition, the relationship between the chemical nature of the materials and the final morphologies of CdS was elucidated by varying different sulfur sources, which may provide important information to the fields of crystal growth and design and morphology-controlled synthesis. This facile approach could be a general method and be extended to the synthesis of other important inorganic materials by selecting appropriate functional capping ligands. Acknowledgment. This work is supported by the National Natural Science Foundation of China and Chinese Ministry of Education. References
The driving forces for the growth is similar to several recent reports on H-bonding effects for the formation of linear structures derived from cyclic polypeptides,12c for the formation of cobalt oxide nanotubes from 1-D CoIII complexes,10 and for the formation of nanotubes from organic calix[4] hydroquinone.11 When thioacetamide (CH3CSNH2), which only has one NH2 group, was used both as a sulfur source and as a capping ligand (CH3CSNH2/CdCl2 ) 2:1) while other experimental conditions were kept unchanged, no dendrite was obtained. The SEM image of the CdS crystals is shown in Figure 5a, which reveals that almost all the product was spheres with a rough surface and mean diameters of about 4-6 µm. The rough surface may also indicate the selective adsorption of the ligand. To further study the relationship between the chemical nature and the final CdS morphologies, we also used sodium mercaptoacetate (HSCH2COONa), which cannot form intermolecular hydrogen bonding, both as a sulfur source and as a capping ligand (HSCH2COONa/CdCl2 ) 2:1). The experimental result revealed that the CdS crystals have spherical morphology and with a mean diameter of ca. 3 µm. It is noted that the surface of the particles here is smooth (Figure 5b). This indicates that the formation
(1) Mann, S.; Ozin, G. A. Nature 1996, 382, 313. (2) Yang, H.; Coombs, N.; Ozin, G. A. Nature 1997, 386, 692. (3) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; ElSayed, M. A. Science 1996, 272, 1924. (4) Matijevic, E. Curr. Opin. Colloid Interface Sci. 1996, 1, 176. (5) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanish, A.; Alivisato, A. P. Nature 2000, 404, 59. (6) Yang, J.; Fendler, J. H. J. Phys. Chem. 1995, 99, 5505. (7) Maillard, M.; Giorgio, S.; Pileni, M.-P. J. Phys. Chem. B 2003, 107, 2466-2470. (8) Champ, S.; Dickinson, J. A.; Fallon, P. S.; Heywood, B. R.; Mascal, M. Angew. Chem., Int. Ed. 2000, 39, 2716. (9) (a) Co¨lfen, H.; Qi, L.; Mastai, Y.; Bo¨rger, L. Cryst. Growth Des. 2002, 2, 191. (b) Qi, L.; Co¨lfen, H.; Antonietti, M. Angew. Chem., Int. Ed. 2000, 39, 604. (10) Shi, X.; Han, S.; Sanedrin, R. J.; Galvez, C.; Ho, D. G.; Hernandez, B.; Zhou, F.; Selke, M. Nano Lett. 2002, 2, 289. (11) (a) Hong, B. H.; Bae, S. C.; Lee, C.-W.; Jeong, S.; Kim, K. S. Science 2001, 294, 348. (b) Hong, B. H.; Lee, J. Y.; Lee, C. W.; Kim, J. C.; Bae, S. C.; Kim, K. S. J. Am. Chem. Soc. 2001, 123, 10, 748. (12) (a) Jung, J. H.; Shinkai, S.; Shimizu, T. Nano Lett. 2002, 2, 17. (b) Nakamura, H.; Matsui, Y. J. Am. Chem. Soc. 1995, 117, 2651. (c) Ghadiri, M. R.; Granja, J. R.; Milligan, R. A.; McRee, D. E.; Khazanovich, N. Nature 1993, 366, 324. (13) Zhang, X.; Xie, Y.; Yu, W.; Zhao, Q.; Jiang, M.; Tian, Y. Inorg. Chem. 2003, 42, 3734. (14) (a) Ono, Y.; Nakashima, K.; Sano, M.; Kanekiyo, Y.; Inoue, K.; Hojo, J.; Shinkai, S. Chem. Commun. 1998, 1477. (b) Jung, J. H.; Ono, Y.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2 1999, 1289. (c) Jung, J. H.; Ono, Y.; Shinkai, S. Angew. Chem., Int. Ed. 2000, 39, 1862. (d) Jung, J. H.; Nakashima,
Fabrication of CdS Micropatterns
(15)
(16) (17) (18) (19) (20) (21)
(22)
K.; Shinkai, S. Nano Lett. 2001, 1, 145. (e) Kobayashi, S.; Hanabusa, K.; Hamasaki, N.; Kimura, M.; Shirai, H. Chem. Mater. 2000, 12, 1523. (f) Kobayashi, S.; Hanabusa, K.; Suzuki, M.; Kimura, M.; Shirai, H. Chem. Lett. 1999, 1077. See reviews: (a) Alivizatos, A. P. J. Phys. Chem. 1996, 100, 13226; Hfeldt, A.; Gratzel, M. Chem. Rev. 1995, 95, 49. (b) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41. (c) Heath, J. R. Science 1995, 270, 1315. (d) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183. (e) Henglein, A. Chem. Rev. 1989, 89, 1861. (a) Mcclean, I. P.; Thomas, C. B. Semicond. Sci. Technol. 1992, 7, 1394. (b) Deshmukh, I. P.; Holikatti, S. G.; Hankre, P. P. J. Phys. D 1994, 27, 1786. (a) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 183. (b) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 1389. Zhang, X.; Xie, Y.; Zhao, Q.; Tian, Y. New J. Chem. 2003, 27, 827. Rao, C. N. R.; Govindaraj, A.; Deepak, F. L.; Gunari, N. A. Appl. Phys. Lett. 2001, 78, 1853. Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2002, 124, 3343. (a) Jun, Y.; Lee, S.-M.; Kang, N.-J.; Cheon, J. J. Am. Chem. Soc. 2001, 123, 5150. (b) Chen, M.; Xie, Y.; Lu, J.; Xiang, Y.; Zhang, S.; Qian, Y.; Liu, X. J. Mater. Chem. 2002, 12, 748. Powder Diffraction File Alphabetical Index Inorganic Phases; International Center for Diffraction Data: Swarthmore, PA, 1989.
Crystal Growth & Design, Vol. 4, No. 2, 2004 359 (23) Rautaray, D.; Kumar, A.; Reddy, S.; Sainker, S. R.; Sastry, M. Cryst. Growth Des. 2002, 2, 197. (24) (a) Lu, Q.; Gao, F.; Zhao, D. Angew. Chem., Int. Ed. 2002, 41, 1932. (b) Shen, G.; Chen, D.; Tang, K.; Qian, Y. Chem. Phys. Lett. 2003, 370, 334. (25) (a) Peng, X. G.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343. (b) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700, and references therein. (26) (a) Penn, R. L.; Banfield, J. F. Am. Mineral. 1998, 83, 1077. (b) Alivisatos, A. P. Science 2000, 289, 736. (c) Chemseddine, A.; Morita, T. Eur. J. Inorg. Chem. 1999, 235. (d) Park, S. J.; Kim, S.; Lee, S.; Khim, Z. G.; Char, K.; Hyeon, T. J. Am. Chem. Soc. 2000, 122, 8581. (27) (a) Horswell, S. L.; Kiely, C. J.; O’Neil, I. A.; Schiffrin, D. J. J. Am. Chem. Soc. 1999, 121, 5573. (b) Yonezawa, T.; Imamura, K.; Kimizuka, N. Langmuir 2001, 17, 4701. (28) Zhang, J.; Sun, L.; Yin, J.; Su, H.; Liao, C.; Yan, C. Chem. Mater. 2002, 14, 4172.
CG0341555
