Synthesis and Characterization of Large-Scale Hierarchical Dendrites

Wei Zhou , Meng Yao , Lin Guo , Yueming Li , Jinghong Li and Shihe Yang. Journal of the ... Guofeng Lin, Jianwei Zheng, and Rong Xu. The Journal of Ph...
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CRYSTAL GROWTH & DESIGN

Synthesis and Characterization of Large-Scale Hierarchical Dendrites of Single-Crystal CdS

2006 VOL. 6, NO. 8 1776-1780

Wang Qingqing, Xu Gang, and Han Gaorong* State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang UniVersity, Hangzhou 310027, China ReceiVed January 10, 2006; ReVised Manuscript ReceiVed June 8, 2006

ABSTRACT: Large-scale hierarchical CdS dendrites were synthesized by hydrothermal treatment of CdCl2 and thiourea, using poly(ethylene glycol) (PEG) as a capping agent. The products were characterized by X-ray diffraction, energy-dispersive X-ray spectrometry, transmission electron microscopy, field emission scanning electron microscopy, and selected area electronic diffraction. The individual CdS dendrite is composed of a long central trunk with secondary and tertiary branches, which preferentially grew in a parallel direction with a definite angle to the trunk. The results reveal that the dendrites are single crystalline in nature. The contrast experiments demonstrated that both thiourea and the additive PEG are necessary for building hierarchically self-assembled CdS dendrites in a water system. Moreover, the possible growth process has been discussed based on the analyses of detailed configuration and the effects of surfactants in the system. 1. Introduction Inorganic nanocrystals have exhibited many interesting novel size- and shape-dependent properties.1,2 In particular, fabrication of mesostructures with well-controlled architectures, including some shapes of higher complexity than spheres, rods, and disks, opens new pathways for more applications in future nanodevices.3-5 In the last few decades, much excellent research on the shape-controlled synthesis of inorganic crystals has been reported, such as rods/wires,1,4 saws,6 flowers,7,8 triangles,3,9 and dendrites.10-12 Among these morphologies, hierarchical dendrites have been extensively investigated in the past years owing to their special significance in understanding the growth behavior of branched fractal patterns and their potential technological applications.10,11,13 On the nanoscale, dendritic fractals are one type of hyperbranched structure, which is generally formed by hierarchical self-assembly under nonequilibrium conditions. The dendritic structures of metals have been generally considered as model systems for the study of branching and fractal growth processes for a few years.11-14 Nevertheless, several recent papers have reported the formation of inorganic material dendrites, such as oxide, hydroxide, and nanoporous silicates.7,10,15 Also, the dendritic structures of metal sulfides have been studied for their outstanding properties and potential applications in future devices.8,16,17 As one kind of important semiconductor material, cadmium sulfide has broad applications in light-emitting diodes, solar cells, or other optoelectronic devices. Recently, many methods were developed to fabricate CdS with novel morphologies.6b,9,18,19 Xie et al. have reported a kind of branch-like CdS micropatterns, using thiosemicarbazide both as a sulfur source and as a capping ligand in a methanol/water system.20 The work is important for understanding the formation of complicated CdS fractals and their potential applications in microelectronic devices. However, the shape was irregular, and the electron diffraction from the CdS architectures showed that the fractals were polycrystalline in nature. In the present paper, well-defined hierarchical CdS dendrites were synthesized in high yield by hydrothermal reaction of CdCl2 and thiourea with appropriate capping agent at suitable temperatures. The resulting * Corresponding author. Tel: 86-571-87952341. Fax: 86-571-87952341. E-mail address: [email protected].

architectures differed from those reported in previous research.17,20 Moreover, the selected area electron diffraction (SAED) patterns revealed that the trunk and branches are single crystalline in nature. To the best of our knowledge, this kind of self-assembled growth of novel hierarchical CdS dendrites by hydrothermal treatment of a Cd2+-thiourea complex has not been reported. To study the effect of thiourea and poly(ethylene glycol) (PEG) on the crystal morphology, contrast experiments were carried out, in which we substitute other sulfur resources for thiourea or employ thioglycolic acid (TGA) for PEG as the capping agent. Based on the experimental results, a possible mechanism of crystal growth was preliminarily proposed. 2. Experimental Section All analytical chemicals were used without further purification. The PEG (M ) 10 000, A.R.) was used as an additive in this system. In a typical procedure of preparing CdS dendrites, CdCl2‚2H2O (1 mmol) with 0.04 g of PEG and thiourea (3 mmol) were dissolved in two beakers containing 10 mL of distilled water. The two clear solutions were mixed together slowly to yield homogeneous Cd2+-thiourea complex solution, and then it was transferred into a 50-mL Teflonlined autoclave, which contained 20 mL of distilled water. The autoclave was maintained at 200 °C for 12 h. After the mixture cooled naturally to room temperature, the yellow precipitate was washed with distilled water and ethanol for several times, and the final product was dried in a vacuum at 60 °C for 4 h. For the contrast experiments, thiourea was substituted by Na2S‚9H2O or S powder, keeping the other conditions constant. In another respect, an appropriate dosage of TGA was used in the system as the capping agent instead of PEG. The phase purity of the as-synthesized products was measured by X-ray powder diffraction (XRD) using a Rigaku D/max-RA X-ray diffractometer with Cu KR radiation (λ ) 1.5406 Å). Field emission scanning electron microscopy (FESEM) images were taken with a JEOL 100CX scanning electron microscope. Transmission electron microscopy (TEM) images and the corresponding selected area electron diffraction (SAED) patterns were carried out on a JEM 200CX electron microscope. For TEM observation, the synthesized products were ultrasonically dispersed in ethanol, and a drop of the suspension was placed on a Cu grid coated with carbon film.

3. Results and Discussion The dendrites were prepared by hydrothermal treatment of Cd2+-thiourea complex at 200 °C for 12 h on a large scale and in high purity. Figure 1 shows typical FESEM and TEM

10.1021/cg060017e CCC: $33.50 © 2006 American Chemical Society Published on Web 07/08/2006

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Figure 1. FESEM and TEM images of the as-prepared products: (a) a FESEM image of hierarchical CdS dendrites; (b) the corresponding EDS spectrum; (c) TEM image of a hierarchical CdS dendrite; (d) SAED pattern taken from the crystal shown in panel c.

images of hierarchical CdS dendrites obtained with 0.04 g of PEG as the surfactant. The low-magnification FESEM image in Figure 1a shows that the product consists almost entirely of such dendritic structures with a mean length of 5-8 µm along the trunk, and this indicates the high yield and good uniformity achieved with this approach. The corresponding EDS spectrum (Figure 1b) indicates that the dendrites consist of Cd and S with a ratio of 1:1. The observation based on the TEM image of a representative hierarchical dendrite (Figure 1c) further reveals the regularity of crystal morphology. This reveals a clear and well-defined dendritic fractal structure with a pronounced trunk and highly ordered branches. The SAED patterns taken from the trunk tip, the branch tip, or the connection point of the central trunk and secondary branch in CdS dendrite are identical. The SAED pattern (Figure 1d) taken from the entire dendrite clearly shows a single-crystalline structure, and the diffraction pattern can be indexed to hexagonal CdS. Figure 2 shows the XRD pattern of the as-prepared sample. The strong and sharp diffraction peaks indicate that the product was well crystallized. All the diffraction peaks in Figure 2 can be indexed to pure hexagonal structure CdS with lattice constants of a ) 4.147 Å and c ) 6.747 Å, which are consistent with the data in the standard card (JCPDS 75-1545), and no byproduct peaks were found. Compared with the standard reflection, however, the intensity of the (002) diffraction peak becomes extremely weak, and those of (100), (110), and (200) peaks become strong. Associated with the special crystalline shape, the central trunk of the CdS dendrite mostly prefers an orientation parallel to the plane of the sample for XRD measurement. Thus the changes in the intensity of the (002), (100), (110), and (200) diffraction peaks imply the orientational growth of the hierarchical CdS dendrite along the [00h] direction. The high-magnification FESEM and TEM images in Figure 3a,b exhibit the detailed configuration of the novel dendrites.

Figure 2. XRD pattern of the products obtained by hydrothermal reaction of thiourea and CdCl2 with 0.04 g of PEG at 200 °C for 12 h.

The individual CdS dendrite is composed of a long central trunk and three rows of secondary branches. It is interesting that the three rows of secondary branches arrange with a 3-fold symmetry, separated by 120°, and the branches in the same row are parallel to each other emerging at about 30°-45° with respect to the central trunk. The onsets of tertiary branches can be obviously observed in a high-magnification TEM image of part branches (Figure 3b). The tubers, as well as the regular faces, of secondary branches indicate that the whole architecture resulted from the self-growth of the CdS nucleus instead of accumulation of various crystals. Moreover, from Figure 3b, it is noted that the spot indicated by a dot arrow shows the inchoate stage of branch growth, and the spots indicated by line arrows show the subsequent growth stage. An obvious rotation of the growth direction can be observed, which implies an adjusting

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Figure 3. SEM and TEM images of the CdS dendrites prepared with various capping agents: (a, b) high-magnification SEM and TEM images of the CdS dendrites prepared with 0.04 g of PEG; (c) high-magnification SEM image of the CdS branched fractals with 20 µL of TGA; (d) TEM image of the CdS irradiated flower-like dendrites without PEG. In panel b, the spot indicated by a dot arrow shows the inchoate stage of branch growth, and the spots indicated by line arrows show the subsequent growth stage. In panel c, the plane pointed by white-line arrow show the sectional plane of the secondary branches along the c-axis direction of wurtzite CdS.

procedure from the onset orientation of branch to the normal growth direction. We speculate that the impetus of the rotation comes from the initial orientation growth of wurtzite CdS and the modulation of PEG on CdS. When appropriate TGA is substituted for PEG as the agent, the FESEM image of the corresponding products is shown in Figure 3c. From that, the evident hexagonal section demonstrates that the secondary branches are oriented growth along the c-axis direction of wurtzite CdS. And the angle of secondary branches to the trunk reaches 75°-90°. Figure 3d shows the TEM image of the products prepared without PEG, keeping the other parameters constant. The products were irradiated flower-like dendrites with shorter trunks and more secondary and tertiary branches in many directions. From the contrast experiments, it is found that PEG plays a great role in the formation of novel structures. As is known, PEG with two OH- groups is an excellent surfactant in synthesis of shape-controlled nanoparticles, since it can selectively adsorb to certain faces to decrease their surface energy and further the growth rate of these faces.2 At the same time, the adsorption of PEG on the side surface makes it possible to form well-assembled secondary and tertiary branches on a longer trunk due to its function of spatial impediment. TGA plays a similar role as PEG in the system. However, as an acidic agent with stronger polarity, TGA resulted in stronger rotation and larger angles of 75°-90° for the secondary branches to the central trunk. The reason for that should be the distinct selective adhesions of the two agents and their different interaction with Cd2+, which provides distinct growth rates in the system. In addition, the well-defined hierarchical dendrites could not only be ascribed to the organic agents. In principle, fractal and dendritic growth are diffusion-controlled growth, and nonequilibrium growth and molecular anisotropy are the prerequisites

for the formation of dendritic structures.5,12,15 Herein, anisotropy comes from the intrinsical anisotropy of the hexagonal structure CdS.2,3 Besides, thiourea may act as not only the sulfur source but also a bidentate ligand to form relatively stable Cd-thiourea complexes as the following:

The complex ions of thiourea with Cd2+ lead to a high remaining monomer concentration after the nucleation stage. Thus a nonequilibrium growth for the elongated crystals is facilitated.4,21 The contrast experiments were carried out to demonstrate the great influence of thiourea on the dendritic morphology. Figure 4a,b show the TEM images of the products prepared by substituting Na2S‚9H2O or S powder for thiourea, respectively, without changing other experiment parameters. From the images, we can see that the products were spherical CdS nanoparticles instead of elongated CdS crystals. This is because when Na2S‚9H2O or S powder is used as the precursor, an equilibrium surroundings for crystal growth would be satisfied, and the crystal morphology was closer to the equilibrium conditions. On the basis of the experimental results, a possible growth process of hierarchical CdS dendrites can be simply described as follows. There is a Cd-thiourea complex in the initial solution. Upon heating, the chelation of Cd2+-thiourea will be weakened, and Cd2+ will be released gradually. On the other hand, thiourea is attacked by the strong nucleophilic O atoms

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Figure 5. (a) The anisotropy of the hexagonal CdS; (b) the cross section of hexagonal CdS along the c-axis direction; (c) schematic view of growth process of the CdS dendrites. :: organic molecules of PEG or TGA.

strated in previous research.5 With further growth, the tertiary branches symmetrically branch off at the same definite angle with the secondary branch as secondary branches with the central trunk. The schematic view of the growth process of the CdS dendrites can be simply described in Figure 5. 4. Conclusions

Figure 4. TEM images of the CdS nanocrystals obtained using other sulfur sources: (a) Na2S‚9H2O; (b) element S powder.

of H2O molecules leading to the weakening of the CdS double bonds, which will be broken to release S2- anions slowly. Then the active S2- reacts with Cd2+ to generate CdS nuclei. Owing to the slow release of reaction ions, the elongated growth along the [001] direction of rodlike crystals is favored. Qian et al.17 have reported their growth model of hierarchical HgS dendrite. They figured out that there is a common plane of the central trunk and secondary and tertiary branches of HgS dendrite, which is parallel to the (001) plane, and that each array branch grows along the [110] direction, keeping the angle of the trunk and branches 60°. This model demonstrates that the hexagonal structure of HgS can bring about symmetrically three direction orientational growth vertical to the [001] direction. In the present work, CdS dendrite grows along [001] direction, and the cross section of each hierarchical trunk is a hexagonal (001) plane owing to the preferential growth of wurtzite CdS. Subsequently, some tubers emerge on the side surface, which are symmetrically separate with 120° between each other and initially grow along [110] direction like the formation of hierarchical HgS dendrite for their hexagonal structure. However, the CdS crystal prefers to grow along [001] direction rather than theee [110] direction due to its high surface energy. This initial impetus makes the branch growth direction of the CdS dendrite adjust from onset growth direction [110] to normal growth direction [001]. During the adjustment procedure, the introduction of organic molecules that selectively adhere to a particular crystal facet determines the conversion pace of branches. The different selectively adherence of PEG and TGA on CdS and their distinct degrees of interaction with Cd2+ make various paces of the adjustment procedures and further form diverse angles of branches to the central trunk. From Figure 3b, we can speculate that there exist some stacking faults at the connection spots resulted from the short tubers rotating from the onset growth direction to the preferential c-axis direction gradually, which has been demon-

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