Preparation and Characterization of Netted Sphere-like CdS

Dec 7, 2007 - Synopsis. Netted spherelike CdS nanostructures consisting of the dendritic nanolines were prepared in ethylenediamine using l-cysteine a...
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CRYSTAL GROWTH & DESIGN

Preparation and Characterization of Netted Sphere-like CdS Nanostructures1 Pingtang Zhao and Kaixun Huang* Department of Chemistry, Huazhong UniVersity of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China; Accepted September 20, 2007

2008 VOL. 8, NO. 2 717–722

ABSTRACT: Netted sphere-like CdS nanostructures consisting of the dentritic nanolines were prepared in ethylenediamine using L-cysteine and cadmium nitrate tetrahydrate as precursors by a solvothermal process at 150 °C for 8 h. The reaction time, the molar ratio of precursors, solvent, and temperature can be used as the additional means to control the size and morphology. The synthesized product was characterized by X-ray powder diffraction (XRD), Fourier transform-infrared (FT-IR) spectroscopy, Field emission scanning electron microscopy (FESEM), Energy-dispersive X-ray fluorescence spectroscopy (EDXS), Transmission electron microscopy (TEM), High-resolution transmission electron microscopy (HRTEM), Electron diffraction (ED), Ultraviolet–visible (UV–vis) spectrometer, Raman spectroscopy, and Fluorescence spectrophotometer. And the possible formation mechanism of the netted spherelike CdS nanostructures is proposed. 1. Introduction Controlling the size and shape of nanomaterials is a key issue in current nanoscience research.1 Especially, semiconductor nanostructures have been attracting increasing attention because of their unique optical, mechanical, electronic, and catalytic properties, which are highly dependent on size and shape and differ from those of their bulk counterparts. They also have potential applications in optoelectronic devices.2,3 Accordingly, there has been a considerable effort directed at the preparation of semiconductor nanostructures. Among them, CdS is one of the most vital and classical II-VI group semiconductors with a direct band gap of 2.4 eV at room temperature.4,5 It is now widely used for photoelectric conversion in solar cells, in lightemitting diodes for flat-panel displays, lasers, thin film transistors, and in other optical devices based on its nonlinear properties.6–10 Recently, a thin film transistor has been fabricated on CdS nanoribbons,11 and laser properties have been demonstrated on a single CdS nanowire12 and nanoribbon.13 A number of methods have been explored to fabricate CdS crystals, such as a thermal evaporation,14–16 hydrothermal method,17 chemical vapor deposition process,18,19 templatemethod,20,21 thermal decomposition method,22 and solvothermal process.23–26 Among them, the solvothermal synthesis is an important technology for synthesizing nanostructures at low temperature. The solvothermal process utilizes a solvent under the pressure and temperature above its critical point to increase the solubility of a solid, and to speed up reactions. Under the pressure generated by the solvothermal reaction, the as-prepared nanostructures are well crystallized, and for water-sensitive reactions, solvothermal reaction can fully avoid the presence of water, so the technology was extensively applied to the preparation of nanostructures.27 Various morphologies of CdS nanostructures, such as CdS nanorods,23 tetrapods and hexagonal nanoprisms,24 nanowires with an average diameter of 25 nm and length of 20-40 µm,25 urchinlike CdS nanoflowers, branched nanowires, and fractal nanotrees,26 have been prepared by the solvothermal method. However, how to design and develop new solution-based method to prepare novel CdS * Corresponding author, Tel: +86-27-87543133; Fax: +86-27-87543632; E-mail: [email protected] (K. Huang). 1 Netted sphere-like CdS nanostructures consisting of the dentritic nanolines were prepared by a solvothermal process.

nanostructures and other similar semiconductors is still one of the most important tasks.26 Recently, our group has focused on fabricating the semiconductor nanostructures by a solvothermal process.28,29 In this report, the netted spherelike CdS nanostructures have been achieved by a solvothermal method using L-cysteine and cadmium nitrate tetrahydrate as precursors at 150 °C for 8 h. We use ethylenediamine as solvent, in which CdS nanorods with wurtzite structure were normally obtained.23 To the best of our knowledge, this kind of netted spherelike CdS nanostructure has not been reported previously. The possible formation mechanism of the netted spherelike CdS nanostructures is presented, and the optical properties of the CdS nanostructures were investigated. 2. Experimental Section All the chemicals were of analytical grade and purchased from Shanghai Chemical Reagent Co. In a typical procedure, equivalent molar amounts (1.4 mmol) of Cd(NO3)2 · 4H2O and L-cysteine were dissolved into 70 mL of ethylenediamine, and the solution was continuously stirred for about 10 min until a clear solution was gotten. The solution was then transferred into an 80 mL Teflon-lined stainless steel autoclave, sealed, and heated at 150 °C for 8 h in an electric oven. The autoclave was cooled to room temperature naturally. The product was collected by centrifugation at 8000 rpm for 10 min and washed with distilled water and absolute ethanol several times to remove the excess reactants and byproducts. Finally, the product was dried in a vacuum oven at 60 °C for 6 h. The yellow powders were collected for characterization. To study the crystal growth mechanism and evolution process, we also performed the experiments of different reaction time under the same other conditions. Moreover, the effects of the growth conditions, such as the temperature, solvent, and molar ratio of cadmium nitrate tetrahydrate to L-cysteine, on the sizes and morphologies of the CdS nanostructures were investigated. Scanning electron microscopy (SEM) images were obtained on a FEI Sirion 200 field emission scanning electron microscope (FESEM). The energy-dispersive X-ray fluorescence spectroscopy (EDXS) was carried out on an EDAX Inc. EAGLE III energy-dispersive X-ray fluorescence spectroscope. The X-ray diffraction (XRD) pattern of the product was recorded by employing a PANalytical B. V. (Philips) χ′ Pert PRO XRD with Cu KR radiation at a scanning rate of 0.02° s-1 in a 2θ range of 20-90°. The Fourier transform-infrared (FT-IR) spectrum was measured on a Bruker EQUINOX55 FT-IR spectrophotometer. A small amount of product was dispersed in ethanol by ultrasonic treatment for 10 min. One drop of the resulting solution was then placed onto a carbon-coated copper grid and dried at room temperature for the high-resolution transmission electron microscope

10.1021/cg070252c CCC: $40.75  2008 American Chemical Society Published on Web 12/07/2007

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Figure 1. (a) XRD patterns of the CdS nanostructures. (1) JCPDS card 41-1049; (2) XRD pattern of the as-prepared urchinlike CdS nanostructures at 150 °C for 4 h; (3) XRD pattern of the as-prepared netted spherelike CdS nanostructures at 150 °C for 8 h. (b) EDXS of the netted spherelike CdS nanostructures.

(HRTEM) visualization. The electron diffraction (ED) patterns and highresolution transmission electron microscopy (HRTEM) and transmission electron microscopy (TEM) images were carried out on a JEM-2010FEF transmission electron microscopy (TEM) at an acceleration voltage of 200 kV. Thermal gravimetric analysis (TGA) was conducted by a Setarm Setsys 16 TG/DTA/DSC integration thermal analyzer (N2 stream 50 mL/min; heating rate 10 °C/min). The ultraviolet–visible (UV–vis) absorption spectrum was recorded on a Lambda Bio 40 UV–vis spectrometer. Raman scattering spectrum was measured by a Bruker EQUINOX55 Raman microscope. The 514 nm line of an Ar ion laser was used for the excitation source. The photoluminescence (PL) spectrum was measured on a JASCO FP6500 fluorescence spectrophotometer. All the measurements were carried out at room temperature.

3. Results and Discussion The phase and purity of the sample were confirmed by the XRD patterns. Figure 1a (2) is the XRD pattern of assynthesized urchinlike CdS nanostructures at 150 °C for 4 h. A typical XRD pattern of as-synthesized netted spherelike CdS nanostructures at 150 °C for 8 h is shown in Figure 1a (3). All of the peaks can be indexed to the hexagonal wurtzite phase of CdS with lattice constants a ) 4.14 Å and c ) 6.73 Å, which is in good agreement with the standard data from JCPDS card 41-1049. No impurity phase can be detected. The strong and sharp diffraction peaks indicated that the as-obtained netted spherelike CdS nanostructures are well crystalline. The relative intensities of the peaks corresponding to the (002)/(100) and (002)/(101) planes varied distinctly from the standard data of the JCPDS card 41-1049, implying that the preferred growth direction of the urchinlike and netted spherelike CdS nanostructures is the [002] direction. The spectrum of EDXS from a netted spherelike CdS nanostructure is shown in Figure 1b. EDXS analysis demonstrates that the crystal consists of Cd and S element. Moreover, according to the quantitative analysis of EDXS, the Cd:S molar ratio is about 1:1.09, which is consistent with stoichiometric CdS. Figure 2 shows the SEM images of as-prepared CdS nanostructures under the typical condition using L-cysteine and cadmium nitrate tetrahydrate (Cd(NO3)2 · 4H2O) as precursors in ethylenediamine by solvothermal method at 150 °C for 8 h. A low-magnification SEM image is shown in Figure 2a, clearly exhibiting that the as-synthesized products are spherelike CdS crystals, the diameters of the spherelike CdS nanostructures are 1-3 µm. For clarifying the CdS structures, the high-magnification SEM images were shown in Figure 2b and 2c, which more clearly indicates that the products are netted spherelike CdS nanostructures, which are made of the dentritic CdS lines with the line diameter of about 10 nm. And the branches of the neighboring dendritic CdS nanolines intersect each other.

Zhao and Huang

Figure 2. SEM images of the netted spherelike CdS nanostructures fabricated by a solvothermal process at 150 °C for 8 h, with the concentrations of both L-cysteine and cadmium nitrate tetrahydrate being 0.02 mol/L. (a) Low-magnification SEM image of CdS nanostructures; (b) high-magnification SEM image of netted spherelike CdS nanostructures; (c) high-magnification SEM image of an individual netted spherelike CdS nanostructures.

The physical and chemical properties of the solvent can influence the solubility, reactivity, and diffusion behavior of the regents and the intermediate.30 Ball-like CdS crystals with an average diameter of 80 nm were obtained (Figure 3a) when water was used as solvent in place of ethylenediamine in this synthesis system. The result reveals that ethylenediamine is a key factor on forming netted spherelike CdS nanostructures. To further understand the role of ethylenediamine in this preparation, diethylenetriamine was used for this reaction. Balllike CdS crystals with diameters of about 2 µm were also formed (Figure 3b). An open sphere is shown in Figure 3c, indicating that CdS crystals are hollow spheres. It is well-known that diethylenetriamine has the property of absorbing CO2 gas in air. The CO2 in air can be absorbed during preparing the solution before being sealed into a Teflonlined stainless steel autoclave. It is not rare that the hollow spheres are formed using CO2 gas in solution as a template. Chen et al. 31 have demonstrated that the produced CO2 (better than NH3 which is more soluble) can probably form gaseous cavities under the hydrothermal growth conditions, which can act as heterogeneous nucleation centers for single bubble growth or polycrystalline aggregation. And only ball-like CdS crystals with the diameter of 0.5-1.5µm were obtained when ethanolamine was used as sulfur source under the same other conditions (Figure 3d). During the step of nanoseeds growing, ethylenediamine molecules can cap in a cis configuration onto the facets of the crystals, which is favorable to the preferred growth of the crystals. Diethylenetriamine and ethanolamine with one amino will not readily form a stable cis configuration because of the higher entropy factor.28 Ethylenediamine can also act as a structure-directing molecule that is incorporated into the inorganic framework first and then escaped from it to form nanocrystallites with desired morphologies.32 The previous studies have proven that the growth of the CdS nanorods in ethylenediamine was attributed to the highly anisotropic crystal structure and the structure-directing coordination template effect of ethylenediamine.23 To study the growth mechanism of the netted spherelike CdS nanostructures, the experiments were carried out at 150 °C for 1 h, and 4 h under the same other reaction conditionss. Spherelike crystals consisting of nanoparticles with diameters of about 10 nm were obtained after the 1 h solvothermal process and were accompanied by some particles without aggregation (Figure 4a). Figure 5 is the TGA curve of spherelike particles prepared at 150 °C for 1 h under the same other conditions. One weight loss of about 4.61% in the range of 30–150 °C is attributed to the evaporation of the adsorptive moistures and the weight loss of the decomposition pieces of the cysteine-cadmium complexes. Another weight loss of 17.93% in the range of 150-565 °C nearly consists with the stoichiometric weight loss (17.21%) of CdS(NH2CH2CH2NH2)0.5. The formation of ZnS(NH2CH2CH2NH2)0.5 have been reported by Yu et. al.33 It

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Figure 3. (a) SEM image of the ball-like CdS crystals as-obtained using water instead of ethylenediamine as the sulfur source under the same other conditions; (b) SEM image of the CdS crystals prepared using diethylenetriamine in place of ethylenediamine as the sulfur source under the same other conditions; (c) high-magnification SEM image of an open CdS hollow sphere as-prepared using diethylenetriamine in place of ethylenediamine as the sulfur source under the same other conditions; (d) SEM image of the ball-like CdS crystals as-obtained using ethanolamine instead of ethylenediamine as the sulfur source under the same other conditions.

Figure 4. SEM image of the products fabricated by a solvothermal process at 150 °C for 1 and 4 h, with the concentrations of both L-cysteine and cadmium nitrate tetrahydrate being 0.02 mol/L. (a) SEM image of the balllike particles prepared at 150 °C for 1 h; (b) SEM image of the urchinlike CdS nanostructures prepared at 150 °C for 4 h; (c) high-magnification SEM image of an individual urchinlike CdS nanostructure.

Figure 5. TGA curve of the spherelike particles prepared at 150 °C for 1 h under the same other conditions.

is speculated that CdS(NH2CH2CH2NH2)0.5 intermediately exists in our system. When the reaction time was further prolonged to 4 h, images b and c in Figure 4 show that urchinlike CdS crystals with diameters of about 0.5–1µm were obtained. The diameter of the needles of the urchinlike CdS crystals is about 10 nm. More details of the CdS nanostructures were investigated by HRTEM and ED. Figure 6a shows the TEM image of urchinlike CdS nanostructure. An inset is the ED pattern of a whole urchinlike CdS crystal in Figure 6a, which shows that the urchinlike CdS crystal is a polycrystal. Figure 6b is the HRTEM image of the nanoneedle of an urchin-like CdS crystal taken from Figure 6a. The measured space of the lattice fringes in the HRTEM image is 0.336 nm, which corresponds to the (002) plane of a wurtzite CdS crystal. It indicates that [001] direction is the preferred growth direction of the urchinlike CdS crystals. The HRTEM image of the dendritic line of the netted spherelike CdS nanostructure (Figure 6c) shows that the lattice distance of the trunk is 0.336 nm, which is almost in accordance with the (002) lattice distance of a wurtzite CdS crystal. It indicates that the preferential growth direction of the trunk is the [001] direction. Interestingly, the lattice distance of the branch is also 0.336 nm. The preferential growth direction of the branch is the same as the preferential growth direction of the trunk, which is similar to the result reported by Yao et al.26 FFT patterns (the insets in Figure 6c) also indicate that the preferential growth

Figure 6. (a) TEM image of an individual urchinlike CdS crystal. The inset is the electron diffraction (ED) pattern of a whole urchinlike CdS crystal in Figure 6a; (b) high-resolution TEM image of a nanoneedle taken from an urchinlike CdS crystal in Figure 6a; (c) high-resolution TEM image of the dendritic line of a netted spherelike CdS nanostructure. Insets 1 and 2 are FFT patterns of the high-resolution TEM images of the marked parts 1 and 2 in Figure 6C, respectively; (d) TEM image of the aggregating line tips of a netted spherelike CdS nanostructure; (e) TEM image of the intersectant lines of a netted spherelike CdS nanostructure.

directions of the trunk and branch are all in the [001] direction. The angle of branch to trunk is about 45°. Stacking faults within the knot between the branch and trunk marked with an arrow were observed in Figure 6c. Figure 6d shows that some tips of the dentritic nanolines can aggregate together (marked part 1 in Figure 6d). Some nanolines can also intersect each other (Figure 6e). There are several functional groups in the L-cysteine molecule, such as NH2, COOH, and SH, which have a strong tendency to coordinate with inorganic cations. Some metal sulfide nanostructures have been prepared in the presence of cysteine, where cysteine can act not only as a complexing agent but also as a sulfur source and structure-directing molecule.34–36 Cysteine has also been used to fabricate biostabilized CdS nanoparticles, where the band gap energies could be adjusted by changing the pH value and the corresponding concentration.37 Thus it is rational to conclude that cadmium ions can coordinate with cysteine to form initial precursor complexes in our approach. The FT-IR spectra of L-cysteine and the as-prepared products of different reaction times are shown in Figure 7. In the FT-IR spectrum of the as-prepared product of 1 h, the peaks at 3451, 2925, and 1122 cm-1 are the vibration modes of N-H, C-H, and C-N bonds, respectively. The presence of a carboxylate group, Vas(COO-)(asym. stretching) at 1619 cm-1 and Vs(COO-)(sym. stretching) at 1407 cm-1,38 is observed. The result indicates that as-obtained product of 1 h contains some organic compound, which is consistent with the TGA result. One sharp peak is present at 620 cm-1 and the intensity of the peak distinctly decreases after the reaction of 4 h. However, the other peaks still exist. It is speculated that the peak at 620 cm-1 is probably attributed to the vibration mode of an organic metal bond. The characteristic signal of the SH group at about

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Zhao and Huang

Figure 8. Formation process of the netted spherelike CdS nanostructures. Figure 7. (a) FT-IR spectrum of the pure L-cysteine; (b) FT-IR spectra of the as-prepared the products of different reaction time.

2521 cm-1 disappeared after 1 h compared with the IR spectrum of pure L-cysteine (Figure 7a), indicating that the S component of the product originates from the SH group of L-cysteine. N-H and C-H bonds and other peaks are not observed in the FT-IR spectrum of as-prepared netted spherelike CdS nanostructures for 8 h (Figure 7b), showing that the final product has no organic impurities. On the basis of the above analyses and the experiment results, the growth mechanism of the netted spherelike CdS nanostructures is proposed. At fist, L-cysteine and Cd2+ can form the cadmium-cysteine complexes. Upon being exposed to the high temperature and pressure in the solvothermal process, the S-C bond is broken. It results in the formation of CdS nuclei. Nanoparticles were further obtained with the continuous supply of the building blocks. It is well-known that the surface area of a sphere is the smallest under the same volume. The moving nanoparticles in the solution can aggregate each other to form spherelike crystals, driven by minimization of surface energy and H-bond interaction, which is similar to the result reported by Zhang et al.36 The growth rate is generally faster along the caxis [001] direction for the highly anisotropic wurtzite CdS structure. So an oriented growth along the [001] direction took place on the surface of the spherelike particles using ethylenediamine as a structure-directing molecule. It results in the formation of urchinlike CdS crystals. Then some tubers can emerge on the CdS nanoline side surface and initially grow along the [110] direction for the hexagonal structure, which is the same as the result reported by Wang et al.17 However, the CdS crystal prefers to grow along the [001] direction rather than the [110] direction because of its high energy, which makes the growth direction of the branch of a dendritic nanoline change from the [110] to [001] direction.17 And stacking faults at the connection spots are formed because of the short tuber rotating from the onset growth [110] direction to the preferential [001] direction. The tips of the branches of the dendritic CdS nanolines are the growing energetically favored sites. Once the tips come across each other under the high temperature and pressure in the solvothermal process, some tips can aggregate together to lower the energy; some nanolines can also intersect each other. These all contribute to form the netted spherelike CdS nanostructures. The formation process of the netted spherelike CdS nanostructure is displayed in Figure 8. To study the influence of the reaction temperature on the formation of the CdS nanostructures, we carried out several experiments at 120, 180, and 220 °C under the same other conditions. Images a and b in Figure 9 show that another kind of netted spherelike CdS crystal was obtained when the temperature was decreased to 120 °C. The diameter of the netted spherelike CdS crystals is about 1-6 µm. Figure 9c is the highmagnification SEM image of the surface of a netted spherelike CdS crystal in Figure 9b, further indicating that the netted balllike CdS nanostructure is made of some smaller netted sphere-

like CdS crystals and strawlike CdS crystals. When the temperature was increased to 180 °C, the netted spherelike CdS nanostructures with the diameter of 1-3 µm were obtained (images d and e in Figure 9), and the dendritic lines of the netted spherelike CdS clearly show fractal condition. When the temperature was further increased to 220 °C, the spherelike CdS nanostructures consisting of the dendritic CdS crystals were obtained (Figure 9f and 9g). The diameter of the spherelike CdS nanostructure is 1-2 µm. Figure 9h reveals that every dendritic CdS crystal of the spherelike CdS nanostructure has one trunk and several branches. The length of the longest branch is up to 200 nm. The CdS nanocrystals have symmetric branches (Figure 9h). The diameter of the trunk and branch of dentritic CdS nanocrystals is about 50 nm, which is larger than that at low temperature. The diffusion of smaller CdS nanostructures at low temperature is slower than that at high temperature, which is favorable to the aggregation of some smaller CdS nanostructures. So the netted spherelike CdS nanostructures with larger diameter can be obtained. At a too-high temperature, however, the adsorption of the capping ligands (ethylenediamine) on the lateral surfaces of the branch and trunk of the dentritic CdS crystal can be weakened, and thus the trunk and branch have a relatively faster lateral growth rate, which results in the formation of thicker trunk and branches. All results indicate that the reaction temperature plays a key role in the formation of the netted spherelike CdS nanostructures. To investigate the influence of the molar ratio of Cd(NO3)2 · 4H2O and L-cysteine on the morphology of CdS crystals, a series of experiments were carried out by changing the molar ratio from 2:1 to 1:1.5 and 1:2 at 150 °C under the same other conditions. SEM images of the as-prepared CdS nanostructures under different molar ratio are shown in Figure 10. When the Cd(NO3)2 · 4H2O (0.02 M) to L-cysteine (0.01 M) molar ratio is 2:1, images a and b in Figure 10 show that all the products are prickly ball-like CdS nanostructures with the smaller diameter of about 200 nm. Prickly ball-like CdS crystals with diameters of about 1-2 µm (images c and d in Figure 10) are also obtained when Cd(NO3)2 · 4H2O (0.02 M) to L-cysteine (0.03 M) molar ratio is 1:1.5. The Cd(NO3)2 · 4H2O (0.02 M) to L-cysteine molar ratio is further changed to 1:2. The product is still the prickly ball-like CdS nanostructures with diameters of about 1-3 µm consisting of nanorods (images e and f in Figure 10). It is well known that the process of a crystal growth can be divided into two stages: an initial nucleating stage and a subsequent crystal growth stage. With the continuous supply of the building blocks, the nuclei can serve as seeds for further growth. The formation of a perfect crystal requires a reversible pathway between the building blocks on the solid surface. These conditions allow the building blocks to easily adopt correct positions in developing the long-range-ordered crystalline lattice. In addition, the building blocks need to be supplied at a wellcontrolled rate in order to obtain crystals having a homogeneous composition and uniform morphology.39 The initiative concen-

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Figure 9. SEM images of the as-prepared CdS nanostructures at different reaction temperatures under the same other conditions. (a) Low-magnification SEM image of the netted spherelike CdS nanostructures prepared at 120 °C; (b) SEM image of a netted spherelike CdS nanostructure prepared at 120 °C; (c) high-magnification SEM image of the surface of a netted spherelike CdS nanostructure prepared at 120 °C; (d) SEM image of the CdS nanostructures prepared at 180 °C; (e) high-magnification SEM image of a CdS nanostructure prepared at 180 °C; (f) low-magnification SEM image of the as-prepared CdS nanostructures consisting of dentritic CdS nanocrystals at 220 °C; (g) high-magnification SEM of an individual CdS nanostructure consisting of dentritic CdS nanocrystals at 220 °C; (h) high-magnification SEM of dentritic crystals of an individual CdS nanostructure in Figure 9g.

Figure 10. SEM images of as-prepared CdS crystals with different Cd(NO3)2 · 4H2O and L-cysteine molar ratios under the same other conditions. (a) Low-magnification SEM images of the as-prepared prickly ball-like CdS crystals with a Cd(NO3)2 · 4H2O (0.02 mol/L) to L-cysteine (0.01 mol/L) molar ratio of 2:1; (b) high-magnification SEM images of the as-prepared CdS crystals with a Cd(NO3)2 · 4H2O to L-cysteine molar ratio of 2:1; (c) SEM images of the as-prepared CdS crystals with a Cd(NO3)2 · 4H2O (0.02 mol/L) to L-cysteine (0.03 mol/ L) molar ratio of 1:1.5; (d) high-magnification SEM images of the surface of one prickly ball-like CdS crystal in Figure 10c; (e) SEM images of the as-prepared CdS crystals with a Cd(NO3)2 · 4H2O (0.02 mol/L) to L-cysteine (0.04 mol/L) molar ratio of 1:2; (f) highmagnification SEM images of the surface of one prickly ball-like CdS crystal in Figure 10e.

tration of free Cd2+ in the solution increases when the Cd(NO3)2 · 4H2O to L-cysteine molar ratio is increased. As a result, the nucleation speed of a CdS crystal becomes faster, which leads to fast depletion of the precursors. So the growth speed of the crystal sharply decreases because of the fast decreasing of the precursor concentration, which is favorable for the nuclei to grow into some smaller prickly ball-like CdS crystals. It is rationally speculated that the low growth speed is unfavorable to the formation of the tubes along the [110] direction on the surface of CdS nanolines. So no netted spherelike CdS nanostructures are obtained. The initiative concentration of free Cd2+ in the solution decreases when the Cd(NO3)2 · 4H2O to L-cysteine molar ratio is decreased, which results in slowing the nucleation and growth speed. It is also unfavorable to the formation of the netted spherelike CdS nanostructures. All the results indicate that the Cd(NO3)2 · 4H2O to L-cysteine molar ratio can affect the size and morphology of the CdS crystals.

Figure 11. (a) UV–vis spectrum of the netted spherelike CdS nanostructures prepared at 150 °C for 8 h; (b) PL spectrum of the netted spherelike CdS nanostructures prepared at 150 °C for 8 h; an inset in Figure 11b is the excitation spectrum of the emission peak of the netted spherelike CdS nanostructures.

The difference in optical property for different morphologies of CdS nanostructures has attracted significant attention recently. We also carried out some basal optical property examinations to evaluate the quality of the product. Figure 11a shows the UV–vis spectrum of the as-prepared CdS nanostructures. The absorption peak of the netted spherelike CdS nanostructures is at 445nm, which shows blue-shifted compared with that of bulk CdS (513 nm).40 It indicates the presence of the quantum size effect. The luminescence mechanisms of CdS nanostructures have also been extensively studied. Usually, two emissions are observed from semiconductor nanostructures, excitonic and trapped luminescence, respectively.15 Gao et al. reported the PL properties of multiarmed CdS nanorods, and one emission peak centered at about 710 nm was observed, which came from a self-activated emission of CdS.41 Chaudhuri et al. also reported the PL spectra of CdS nanoribbons with one near-band-edge emission peak at 525 nm.14 Shen and Lee reported that two emission bands were observed in the PL spectrum of CdS multipod-based structures.38 In their results, the intense peak is at 505 nm because of the near-band-edge emission of CdS, whereas a very low intensity broad peak centered at 670 nm is attributed to the structure defects, ionized vacancies, or impurities. The room-temperature PL spectrum of the netted spherelike CdS nanostructures is shown in Figure 11b. The PL spectrum of netted spherelike CdS nanostructures holds a broad emission peak at about 570 nm with a weak shoulder at about 610 nm by the excitation wavelength of 370 nm. The peak at 570 is excitonic emission. The weak shoulder at 610 nm can be

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Figure 12. Raman spectrum of the netted spherelike CdS nanostructures prepared at 150 °C for 8 h.

attributed to deep levels associated with the sulfur vacancies, extrinsic defects, or impurities. Raman spectroscopy is also a powerful tool for the investigation of the doping concentration, lattice defect identification, and crystal orientation properties of the materials.14 Figure 12 shows the Raman spectrum of the netted spherelike CdS nanostructures. Because of the strong fluorescence background, the Raman intensities of the netted spherelike CdS nanostructures are very weak. From the spectrum, it can be seen that there are two Raman peaks at 301 and 595 cm-1, which correspond to the first- and second-order longitudinal optical phonon (LO) modes of CdS crystals, respectively. It is almost in agreement with the previous reports of the CdS structures.10,42 4. Conclusion Netted spherelike CdS nanostructures were successfully prepared by a solvothermal method using L-cysteine and cadmium nitrate tetrahydrate as precursors at 150 °C for 8 h. On the basis of the analysis of SEM images and HRTEM images, the growth mechanism of the netted spher-like CdS nanostructures is put forward. At first, nanoparticles can be formed by the decomposition of the cadmium-cysteine complexes. Spherelike crystals are further obtained through the nanoparticles aggregation. The surface nanoparticles of a spherelike crystal then experience an oriented growth along the [001] direction using ethylenediamine as a structuredirecting molecule, resulting in the formation of urchinlike CdS crystals. Some tubers can emerge on the CdS nanoline side surface and initially grow along the [110] direction for hexagonal wurtzite. However, the CdS crystal prefers to grow along the [001] direction rather than the [110] direction because of its high energy. The growth direction of the branches of the dendritic CdS nanolines changes from the [110] to [001] direction. In the meantime, the tips of CdS nanolines can aggregate together, and some nanolines intersect each other. These all result in the formation of the netted spherelike CdS nanostructure. The UV–vis absorption spectrum of the netted spherelike CdS nanostructures has an absorption peak centered at 445 nm. The PL spectrum of the netted spherelike CdS nanostructure displays a broad emission peak at 570 nm and one weak shoulder at 610 nm. These nanostructures maybe have potential application on photonic devices. Acknowledgment. This research was supported by MOST 973 program (Project 2006CB705606a). We thank the faculty from the Analysis and Test Center of Huazhong University of Science and Technology and Professor Dongshan Zhao from the Center for Electron of Microscopy of Wuhan University for the technical assistance on characterization.

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