CdS with Various Novel Hierarchical Nanostructures by Nanobelts

Nov 4, 2009 - Many fascinating CdS hierarchical nanostructures, including nanobelt networks, flower/sphere networks by nanobelts/wires self-assembly, ...
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DOI: 10.1021/cg900780a

CdS with Various Novel Hierarchical Nanostructures by Nanobelts/Nanowires Self-Assembly: Controllable Preparation and Their Optical Properties

2009, Vol. 9 5259–5265

Shenglin Xiong,*,† Xiaogang Zhang,‡ and Yitai Qian† †

Department of Chemistry and Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Anhui 230026, China and ‡Department of Applied Chemistry, College of Material Science & Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P. R. China Received July 8, 2009; Revised Manuscript Received September 24, 2009

ABSTRACT: Many fascinating CdS hierarchical nanostructures, including nanobelt networks, flower/sphere networks by nanobelts/wires self-assembly, and nanowires, have been successfully prepared in high yields and purities by a novel dithizone (DTZ) and tetraethylenepentamine (TEPA)-synergistically directed method. A detailed study of the effect of experimental parameters on the morphology is presented. Results and analyses demonstrated that the Cd(Ac)2/dithizone (DTZ) molar ratio and the volume ratio of H2O and TEPA have an immense function in the subtle morphology control of CdS products. A possible growth process of DTZ and TEPA-synergistic-assisted gradual crystallization and subsequent self-assembling is proposed as a plausible mechanistic interpretation for the formation of those novel nanostructures. More importantly, this is the first time to synthesize wurtzite CdS ultrathin nanobelts in solution. The photoluminescence properties of various CdS hierarchical structures are also investigated.

1. Introduction CdS, an important wide-gap semiconductor with a Bohr radius of 2.4 nm1 and direct band gap of 2.40 eV,2 has been well studied in past several years because of its excellent properties in applications such as photovoltaics and lightemitting diodes for flat-panel displays.3 Also, CdS has attracted increasing interest due to its potential applications as nanoelectronic and photocatalytic materials.4 Since morphology of nanomaterials is one of the crucial factors that affects their properties, nanostructures with novel morphologies have been investigated intensively. In recent years, extensive studies have been devoted to preparing CdS nanostructures with various shapes, such as nanowires, nanobelts, and selforganized spheres in the dimension of micro/nanometers.5-7 Among them, CdS nanobelts are one of the most interesting objects. As a result, the preparation and novel property exploration of ultrathin CdS nanobelts are of significance. However, large-scale synthesis of CdS nanobelts, especially controlled synthesis of high-quality CdS nanobelts via a solution-based route at low temperature, still remains challenging for chemists. Therefore, it is of great significance to develop effective synthetic pathways to give access to semiconductor CdS nanobelts with a well-controlled shape and size for desired applications. Analogous to biomineralization in water, organic reaction routes and organic species play an elementary role in the nonaqueous or aqueous synthesis of inorganic nano/microcrystallites. In the past two decades, especially the size- and shape-tailoring effect of organic components has been used for the synthesis of semiconductor and metal oxide nanocrystals.8 Among them, amines, as a kind of organic molecules, could more strongly bond to particular surface facets of some

semiconductor nanocrystals, which can make nanostructures with novel morphologies form. In consequence, an aminebased template technique has extensive application in the preparation of diversified semiconductor micro/nanostructures.9,10 Inspired by the technique, we designed a rational tetraethylenepentamine (TEPA)-assisted method to prepare CdS nanostructures. On the basis of the TEPA-templated technique, we have successfully prepared CdS nanostructures with intriguing and complex morphologies, including nanobelt networks, flower/sphere networks by nanobelts/wires self-assembly, and nanowires. The effects of reaction parameters, such as the molar ratio of Cd(Ac)2 and dithizone (DTZ) (denoted herein as “m”), the volume ratio of water and TEPA, and solvothermal temperature, on morphological evolution have been investigated in detail. To the best of our knowledge, this is the first time for the one-step synthesis of novel CdS hierarchical nanostructures by ultrathin nanobelt self-assembly. The photoluminescence properties of the asfabricated CdS products with different shapes have been detected and they are demonstrated to display very strong photoluminescence at 670-720 nm at room temperature. 2. Experimental Section

*To whom correspondence should be addressed. E-mail: xsl8291@ustc. edu.cn;[email protected].

2.1. Materials and Preparation. The Cd(Ac)2 3 2H2O (99.0%, Aldrich), dithizone (99.0%, Aldrich), and TEPA used were of analytical grade. All chemicals were analytical grade, commercially available from Shanghai Chemical Reagent Co., Ltd., and used in this study without further purification. In a typical procedure, a given amount of cadmium acetate (Cd(Ac)2 3 2H2O) and a given amount of DTZ were added to a given amount of distilled water and the mixture was dispersed to form a homogeneous solution by constant strong stirring. Then, given amounts of TEPA were added to the above solution at room temperature and continually stirred for 10 min. Then, the resulting mixture was transferred into a Teflon-lined stainless autoclave (60 mL capacity). The autoclave was sealed and maintained at 140-220 °C for 20 h. The system was then cooled to ambient temperature naturally. The final product

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was collected and washed with distilled water and absolute alcohol several times, vacuum-dried, and kept for further characterization. 2.2. Characterization. The products were characterized by X-ray diffraction (XRD) recorded on a Japanese Rigaku D/max-γA rotating anode X-ray diffractometer equipped with the monochromatic high-intensity Cu KR radiation (λ = 1.5418 A˚). Scanning electron microscopy (SEM) images were taken with a field emission scanning electron microscope (FESEM, JEOL-6300F, 15 kV). Microscopy was performed with a Hitachi (Tokyo, Japan) H-800 transmission electron microscope (TEM) at an accelerating voltage of 200 kV, and a JEOL-2010 high-resolution TEM, also at 200 kV.

Figure 1. The X-ray diffraction (XRD) pattern of CdS nanobelt networks.

Xiong et al. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VGESCALAB MKII X-ray photoelectron spectrometer with an MgKa excitation source (1253.6 eV). Raman spectra were recorded on a Jobin Yvon (France) LABRAM-HR confocal laser micro-Raman spectrometer at room temperature. Photoluminescence (PL) measurements were carried out on a Perkin-Elmer LS-55 luminescence spectrometer using a pulsed Xe lamp.

3. Results and Discussion 3.1. Structure and Morphology. CdS nanobelt networks were prepared by TEPA-assisted solvothermal method with Cd(Ac)2/DTZ = 3:1 and VH2O/VTEPA = 15:30 at 180 °C for 20 h. The XRD pattern of the obtained CdS product is shown in Figure 1. All the reflection peaks can be indexed as wurtzite CdS with lattice constants a = 4.097 A˚ and c = 6.684 A˚, which is consistent with the literature values (JCPDS Card No. 41-1049). No peaks of impurities were detected, revealing the high purity of the as-synthesized products. Relatively broadened peaks display CdS crystals composed of hierarchical spheres that are small in size. According to the following results of selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM), the product is pure wurtzite CdS not the mixture of cubic phase and hexagonal phase. The second peak corresponding to the (002) plane of hexangular CdS is much narrower and stronger than other peaks in the XRD pattern. This is mainly because the stacking faults (SF) could exist in the wurtzite structure regions along the [001] direction and the periodicity of this direction is better than other directions.

Figure 2. (a) The high-magnification SEM image and (b) the TEM image of CdS nanobelt networks; (c) a TEM image of an indiviual nanobelt; (d) HRTEM image of the selected area in (c); the inset shows the corresponding SAED pattern, taken from the single nanobelt in Figure 2c.

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Figure 3. EDX analysis (a) and XPS analyses (b-d) of CdS nanobelt netowrks.

The panoramic FESEM images of the product in Figure S1, Supporting Information shows that network-like CdS crystals are formed in a large scale. The more structure information is supported by the high-magnification image in Figure 2a, clearly revealing these networks are constructed from the assembly of fibriform 1D nanostructures. Detailed composition and microstructural analyses of CdS networks were carried out by using TEM. Figure 2b represents a typical TEM image of CdS networks by nanobelts self-assembly (also see Figure S2, Supporting Information). Since these nanobelts assembling into the hierarchical structure are ultrathin with a thickness in the range of 2-4 nm, they curve and have excellent flexibility (showed by the arrow in Figure 2c). HRTEM (Figure 2d) and SAED (the inset in Figure 2c,d) pattern, taken from a single nanobelt with a typical width of about 30 nm in Figure 2c, show that the belts are structurally uniform and single crystal in nature with the [001] growth direction. The marked interplanar d-spacings of ca. 0.335 nm correspond to the (002) lattice planes of wurtzite CdS. All the above-mentioned experimental results clearly demonstrate that single-crystalline CdS nanobelts have successfully been fabricated. It should be pointed out that the SF exist in the wurtzite sturcture of CdS nanobelts as shown (see arrow shown in Figure 2d). A series of characterizations other than XRD measurements were conducted for composition and elemental analysis of typical CdS nanobelt networks. The EDS spectrum (Figure 3a) recorded from the product shows intense peaks of Cd and S. The copper signals come from the supporting TEM grid. EDS quantitative analysis indicates the atomic ratio of Cd and S is 50.85:49.15, close to the stoichiometry of CdS. The components of the products are further characterized by

the X-ray photoelectron spectroscopy (XPS) technique, as indicated in Figure 3b-d. Figure 3b-d shows the full XPS spectrum and high-resolution spectra of Cd 3d and S 2p, respectively. From Figure 3c,d, the binding energies of Cd 3d5/2 and S 2p3/2 are identified at 404.22 and 161.36 eV, respectively, which is consistent with the reported values in the literature and further confirms that the products are pure CdS. 3.2. Time-Dependent Experiments of the Formation of CdS Nanobelt Networks. It should be pointed out that no CdS nanobelts networks could be obtained just replacing DTZ with thioacetamide and TEPA with ethylenediamine, respectively, under similar conditions (data not shown here), which demonstrates that the formation of CdS nanobelts may be relevant to the special structure of DTZ and TEPA. Furthermore, the molar ratio between Cd(OAc)2 and DTZ and the reaction temperature also has a significant effect on the product’s final morphology and structures, with the other conditions remaining the same (see Figures 4 and 5). An appropriate ratio of Cd2þ/DTZ is crucial to the construction of novel hierarchical structures. In the present study, ultrathin CdS nanobelts can be synthesized only using DTZ as the sulfur source in a binary solution with a suitable volume ratio of water/TEPA. In order to disclose the growth process of the CdS nanobelt networks, a series of experiments were conducted, as discussed below. We followed the nucleation and growth steps by studying the samples obtained at different reaction stages using the TEM and XRD techniques. Figure 4a-d presents TEM images of samples obtained after the reaction was carried out for 30 min, 50 min, 1.5 h, and 2 h, respectively. These images clearly exhibit the morphological transformation process from the

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Figure 4. TEM images of CdS nanobelt networks obtained at 180 °C for (a) 30 min, (b) 50 min, (c) 1.5 h, (d) 2 h, and (e) XRD patterns of CdS nanobelt networks prepared at 180 °C for 30 min, 50 min, 1.5 h, and 2 h, respectively.

netlike particles to the coexistence of nanoparticles and networks by nanobelt self-assembly and finally to networks by flexible nanobelt self-assembly. XRD patterns of products prepared at 30 min, 50 min, 1.5 h, and 2 h are shown in Figure 6e, which demonstrate that amorphous CdS particles produced during the initial growth process and subsequently gradually crystallized with the reaction proceeding. On the basis of the above results and analyses, the DTZ and TEPA-synergistic-assisted gradual crystallization and subsequent self-assembling process was proposed to interpret the formation of CdS hierarchical nanostructures by nanobelt array self-assembly. At the initial stage of the solvothermal reaction, the amorphous CdS nanoclusters were generated first. In the following steps, the larger CdS crystals could serve as the seeds for nanobelt growth at the expense of the small nanoparticles through Ostwald ripening process. Although the shapes of inorganic crystals often pertain to the intrinsic crystal structure, the same material can show diverse modalities. This behavior is mainly owing to different crystal-facet energies, and moreover the growth rate of a crystal face is always a direct outcome of its surface energy in the overwhelming majority of cases. Herein, the morphology of the products was subtly determined by the facet-selective adsorption characteristic of TEPA, which could serve as a structure-directing coordination template. It is reasonable to conclude that TEPA maybe adsorb onto the (010) and (100) of the incipient CdS nuclei, which possibly originate from the match between the special CdS

atomic surface structures and the linear molecular structure of TEPA, thus making (001) a higher-energy face as compared to the TEPA-covered (010) and (100). Direct evidence for the hypothesis requires further study. In fact, in most report of colloidal synthesis of metal or semiconductor 1D nanocrystals, the anisotropic growth of the 1D structure is often driven by using capping ligands that can bind selectively onto particular facets of the seed particles.11 This result implies that the existence of TEPA favors the formation of CdS nanobelts/wires and further self-assembles into an ordered manner to form regular hierarchical nanostructures, where TEPA molecules probably form interparticle bilayers inducing CdS nanobelts to glue together orderly during the self-assembly process. On the other hand, the specific crystal structure of wurtzite CdS may be another advantageous factor in the formation of a one-dimensional beltlike nanostructure. 3.3. Selective Synthesis of CdS Various Hierarchical Nanostructures: Influence of the Molar Ratio of Precursors. In our study, the morphology of products varies greatly with the change of the molar ratio of precursors, the volume ratio of reagents, and reaction temperature. All of parameters were found to be interdependent for the formation of CdS nanocrystals with various morphologies. Among these various parameters, the molar ratio of precursors (m) and the initial reaction temperature are particularly crucial for control of the morphology of product. To explore the influence of m on CdS morphology, a great deal of control experiments

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Figure 5. FESEM images of CdS various hierarchical nanostructures prepared in a binary solvent with VH2O/VTEPA = 15:30 at 180 °C for 20 h: (a, b) m = 2:1; (c, d) m = 1:1; (e, f) m = 1:2; (g, h) m = 1:3.

were performed by tuning m from 3/1 to 2/1, 1/1, 1/2, and 1/3 with VH2O/VTEPA = 15:30 at 180 °C for 20 h. Interestingly, it is found that the morphologies of products gradually transform from nanobelt networks (see Figure 2) to flower networks (Figure 5a,b), to sphere networks by nanobelt selfassembly (Figure 5c,d, and also see Figure S3, Supporting Information), to the coexistence of urchin-like nanostructures and rods (Figure 5e,f), and finally to 1D rodlike nanostructures (Figure 5g,h). Accordingly, it can be concluded that the morphology of products was predominated by the molar ratio of Cd(Ac)2 and DTZ, namely, m, in the present given condition. When m decreases, 1D nanostructures instead of 3D microcrystals are generated. That is to say, DTZ adjusts the growth of 1D nanobelts/nanorods. As was anticipated, CdS nanorods were obtained in the final products (see Figure 5g,h), when m decreased to 1:3, with the other conditions kept the same. Therefore, this method

provides an efficient way for the selectively controllable preparation of CdS various hierarchical structures by assembly from 1D nanostructures. 3.4. Temperature Effect. Besides the Cd(Ac)2/DTZ molar ratio, the temperature also had a fundamental effect on the shape of the product. In the case of m = 3/1, nanosphere networks by nanowires self-assembly were obtained at 140 °C, a typical SEM image of which is indicated in Figure 6a,b. When the temperature was increased to 220 °C, however, the product was composed of urchin-spheres by nanorods self-assembly, as displayed in Figure 6c,d. With regard to the formation of CdS spheres self-assembled by nanobelts/nanorods in this study, it is believed that the special CdS atomic surface structure provides structure match and spatial proximity to realize effective self-assembly induced by the linear molecular structure of TEPA. This self-assembly process is likely driven by energy minimization for the surfaces

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Figure 6. FESEM images of CdS with various hierarchical structures prepared with VH2O/VTEPA =15:30: (a, b) m = 3:1 at 140 °C for 20 h; (c, d) m = 3:1 at 220 °C for 20 h.

Figure 7. Room-temperature Raman spectrum of CdS nanobelt networks.

of nanobelts/nanorods. On the other hand, this binary solution reaction system is quite complicated, and the formation of certain nanostructures needs further revolution. Obviously, the synergetic effects of TEPA and water could also exert a key influence on the morphology of CdS nanostructures. 3.5. Optical Properties. Raman scattering spectroscopy was performed to study the structures of CdS nanocrystals. It is found that the as-fabricated CdS nanobelt networks, flower/sphere networks by nanobelts/wires self-assembly, and nanowires/belts have a similar Raman spectrum. Figure 7 indicates a typical room temperature Raman spectrum for the CdS nanobelt networks. Two Raman peaks centered at 301 and 602 cm-1 are observed. They can be assigned to the firstand second-order longitudinal optic (LO) phonon modes of CdS, respectively,12 which are polarized in the x-z face and strongly couple to the exciton along the c axis.13 The relatively broad, sharp, and symmetrical Raman peaks demonstrate that the CdS nanostructures are high crystalline quality and

Figure 8. Room-temperature photoluminescence spectra of CdS samples with various morphologies: (a) m = 3:1, VH2O/VTEPA = 15:30 at 180 °C for 20 h, (b) m = 2:1, VH2O/VTEPA = 15:30 at 180 °C for 20 h, (c) m = 3:1, VH2O/VTEPA = 15:30 at 220 °C for 20 h, (d) m = 1:1, VH2O/VTEPA = 15:30 at 180 °C for 20 h.

pure, which is in agreement with the above XRD and TEM results. From previous reports, the TO phonon frequency of single-crystalline CdS nanowires is 304, 607, and 908 cm-1, respectively.14 Compared to the above results, the TO phonon peaks of the CdS nanostructures all shift toward lower frequency, which is probably due to the effects of the small size and high surface area. The room-temperature photoluminescence (PL) spectra of the prepared different CdS nano/microstructures were measured using a Perkin-Elmer LS-55 luminescence spectrometer with an excitation slit width of 5 nm and an emission slit width of 5 nm, as shown Figure 8. With the excitation wavelength of 345 nm, it can be seen that all the curves exhibit a similar PL band and they all show two distinct emission bands that are a green emission band at about

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490 nm and a visible band at around 690 nm, which are close to those in previous reports for CdS nanocrystals.15 The former emission band can be attributed to near-band-edge emission, and the latter could be associated with structural defects, which may result from trap or surface states.16 4. Conclusions In summary, we have successfully prepared hierarchical CdS nanostructures with various novel shapes, including nanobelt networks, flower/sphere networks by nanobelts/ wires self-assembly, and nanowires, through a novel TEPAassisted solvothermal route in the present study. Morphology alteration of these nanostructures was achieved by adjusting the molar ratio of Cd(Ac)2 and DTZ, reaction temperature, and volume ratio of water and TEPA. The DTZ and TEPAsynergistic-assisted gradual crystallization and subsequent self-assembling process are proposed as a plausible mechanistic interpretation for the formation of CdS novel nanostructures in the present reaction system. The PL of these prepared structures indicates a strong emission band at about 690 nm and a weak green emission band at about 490 nm. The current process has the advantages of employing simple synthetic methods and inexpensive experimental set-ups that may readily be scaled-up for production of high-quality semiconductors. The ultrathin CdS nanobelts/nanowires can be used as ideal building blocks to construct more complex architectures for potential applications in various fields such as photocatalyzer, field-emission and novel optoelectronic devices. Acknowledgment. The financial support of this work, by the China Postdoctoral Science Foundation (No. 200801236), National Natural Science Foundation of China (No. 20901072), and the 973 Project of China (No. 2005CB623601), is gratefully acknowledged. Supporting Information Available: The panoramic FESEM image CdS nanobelts networks; TEM images of CdS nanobelts networks and sphere networks by nanobelt by-assembly. This material is available free of charge via the Internet at http://pubs. acs.org.

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