Self-Assembled ZnS Nanostructured Spheres: Controllable Crystal

Feb 22, 2007 - Katarzyna Matras-Postolek , Svitlana Sovinska , Adam Zaba , Ping Yang. Chemical Engineering and Processing: Process Intensification 201...
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J. Phys. Chem. C 2007, 111, 3893-3900

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Self-Assembled ZnS Nanostructured Spheres: Controllable Crystal Phase and Morphology Hua Tong, Ying-Jie Zhu,* Li-Xia Yang, Liang Li, Ling Zhang, Jiang Chang, Li-Qiong An, and Shi-Wei Wang State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China, and Graduate School of Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China ReceiVed: October 12, 2006; In Final Form: January 11, 2007

We report a simple biomolecule-assisted synthesis of ZnS nanostructured spheres assembled from ZnS nanocrystals with the controllable crystal phase and morphology. L-Cysteine, a biomolecule, was used as the sulfur source and played a key role in the formation of ZnS nanostructured spheres. ZnS nanostructured spheres assembled from various ZnS nanocrystal building blocks, such as nanosheets, quantum dots, nanorods, and multimorphology nanocrystals, were successfully prepared by this simple method. The crystal phase of ZnS nanostructured spheres could be controlled by introducing ethanolamine or ethanediamine as a surfacemodifying reagent in this synthetic system. The hexagonal ZnS nanocrystals were obtained in mixed solvents of water and ethanolamine at temperature as low as 95 °C. The growth mechanism of the nanostructured spheres assembled from nanorods was proposed. The optical properties of ZnS nanostructured spheres were investigated by ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) spectra.

Introduction Highly desirable physical and chemical properties are expected from nanomaterials with the complex hierarchical structures,1-3 whose syntheses bring great challenges to scientists. As an important direct band gap semiconductor, ZnS has been extensively studied and applied in various fields.4 Lowdimensional ZnS nanostructures, such as quantum dots (QDs),5 nanorods,6 nanowires,7 nanobelts,8 and nanotubes,9 were reported in the past decade. Recently, ZnS nanostructured assemblies including solid nanospheres,10 hollow nanospheres,11 nanowire/nanobelt arrays,12 nanoporous particles,13 and mesoporous aerogels14 were reported. These developments in the synthesis of ZnS nanomaterials have further extended their applications such as in diodes,15 phosphors,16 and catalysis.13 ZnS has cubic (zinc blende) and hexagonal (wurtzite) crystal phases with different properties. For example, hexagonal ZnS exhibits a higher ionization transition rate than cubic ZnS.17 At room temperature and atmospheric pressure, the bulk cubic phase of ZnS is more stable than the bulk hexagonal phase, and hexagonal ZnS is a more stable phase above 1020 °C.18 However, in the case of nanoscale size, the particle size can significantly affect the phase stability and a phase stability reversal can occur due to small particle size: cubic ZnS may transform to hexagonal ZnS at temperatures much lower than 1020 °C.19 Control of the crystal phase of ZnS nanocrystals by adjusting the reaction temperature or particle size has been reported.8,19,20 The hexagonal ZnS nanocrystals were successfully obtained at relative low temperatures, for example, at 150 °C5 and 225 °C.20 The optical properties of ZnS nanocrystals are strongly dependent on their size and morphology.21 The morphology of semiconductor nanocrystals may determine the distribution of the carrier state density and transport properties of carriers in * Corresponding author. Telephone: +86-21-52412616. Fax: +86-2152413122. E-mail: [email protected].

crystals.22 ZnS nanostructured assemblies of low-dimensional nanocrystals with controllable crystal phase and morphology are highly desirable for exploiting novel properties and extending their applications. Herein, we report a simple biomoleculeassisted synthesis of ZnS nanostructured spheres (NSs) assembled from ZnS nanocrystals with controllable crystal phase and morphology. It should be noted that L-cysteine was used as the sulfur source and played a key role in the formation of ZnS NSs in the synthetic system reported here. Experimental Section In a typical synthetic procedure, the solution was prepared by dissolving 0.3 mmol of Zn(NO3)2‚6H2O and 0.3 mmol of L-cysteine (HSCH2CH(NH2)COOH) into 10 mL of deionized H2O and a certain amount of ethanolamine (EOA) or ethanediamine (EDA) at room temperature. For the solvothermal treatment at 200 °C, the solution was transferred into a Teflonlined stainless steel autoclave (40 mL), sealed, and heated in an oven. For heat treatment at 95 °C, the solution was transferred into a flask and heated in an oil bath. The detailed experimental parameters about the solvent, heating temperature, and time for corresponding samples are listed in Table 1. After the heat treatment, the white product was separated by centrifugation, and washed with ethanol three times. Transmission electron microscopy (TEM) was performed on a JEM-2100F field emission transmission electron microscope (JEOL, Japan). Scanning electron microscopy (SEM) was carried out with a JSM-6700F field emission scanning electron microscope (JEOL, Japan). X-ray powder diffraction (XRD) patterns were recorded on a D/MAX 2200 (Rigaku, Japan) X-ray diffractometer with Cu KR radiation (λ ) 1.54178 Å) and a graphite monochromator, operating at 40 kV and 40 mA. UVvis absorption spectra were measured on a Cary-500 spectrophotometer (Cary, USA). Photoluminescence (PL) spectra were obtained on a Fluorolog-3 spectrophotometer (Jobin Yvon, USA).

10.1021/jp066701l CCC: $37.00 © 2007 American Chemical Society Published on Web 02/22/2007

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TABLE 1: Detailed Experimental Parameters for the Synthesis of ZnS Samples and Their Morphologies and Crystal Phasesa solvent

molar ratio of Zn2+/L-Cys

temp/°C

time/h

morphology

crystal phase

A1 A2 A3

H2O H2O/EDA ) 50:1 H2O/EDA ) 50:1

1:1 1:1 1:1

200 ∼95 200

24 5 24

A4

H2O/EDA ) 10:1

1:1

200

24

spheres assembled from nanosheets spheres assembled from QDs spheres assembled from multimorphology nanocrystals dispersed multimorphology nanocrystals

A5 A6 A7 A8 A9 A10 A11b

H2O/EOA ) 50:1 H2O/EOA ) 50:1 H2O/EOA ) 50:1 H2O/EOA ) 50:1 H2O/EOA ) 10:1 H2O/EOA ) 50:1 H2O/EOA ) 50:1

1:1 1:1 1:1 1:1 1:1 1:2 2:1

200 200 200 ∼95 200 200 200

2 10 24 24 24 24 24

spheres assembled from nanorods spheres assembled from nanorods spheres assembled from nanorods aggregates of QDs dispersed nanorods dispersed nanorods spheres assembled from nanorods

cubic cubic mixture of cubic and hexagonal mixture of cubic and hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal hexagonal

sample

a The solution was prepared by dissolving 0.3 mmol of Zn(NO3)2‚6H2O and L-cysteine (HSCH2CH(NH2)COOH) in 10 mL of deionized water and a certain amount of EOA or EDA. The amount of L-cysteine used was 0.3 mmol for samples A1-A9, 0.6 mmol for sample A10, and 0.15 mmol for sample A11. L-Cys stands for L-cysteine. b A11 was a mixture of hexagonal ZnS and ZnO and partially consisted of spheres assembled from nanorods.

Figure 1. XRD patterns of ZnS samples prepared under conditions described in Table 1 and the Experimental Section. (9) (200) diffraction peak of cubic ZnS; (1) (100), (101), and (103) diffraction peaks of hexagonal ZnS.

Results and Discussion Crystal Phase of ZnS Nanostructures. ZnS nanostructured spheres (NSs) assembled from various ZnS building blocks, such as QDs, nanosheets, nanorods, and multimorphology nanocrystals, were obtained. Detailed experimental conditions for the synthesis of ZnS samples are described in Table 1 and the Experimental Section. The crystal phases of ZnS samples were determined by XRD, as shown in Figure 1. When deionized water (H2O) was used as the only solvent, the product was cubic ZnS (sample A1). When mixed solvents of H2O and EDA were used, cubic ZnS was obtained at 95 °C (A2) and mixed phases of cubic and hexagonal ZnS were obtained at 200 °C (A3 and A4 (Figure S1, Supporting Information)). When mixed solvents of H2O and EOA were used, samples (A5-A11) prepared at 95 or 200 °C consisted of hexagonal ZnS. These results indicate that the crystal phase of the prepared ZnS can be controlled by adjusting experimental parameters. The hexagonal ZnS phase was obtained in mixed solvents of H2O and EOA at temperature as low as 95 °C (A8), which is much lower than that reported previously.5,20 As mentioned above, cubic ZnS is more stable

than hexagonal ZnS at low temperatures. However, the change in surface energy of ZnS nanocrystals due to their small size may decrease the phase transformation temperature.19 On the basis of molecular dynamics simulations and thermodynamic analysis, the temperature for the transformation from cubic to hexagonal ZnS is 25 °C when the average particle size is ∼7 nm, dramatically lower than that observed in the bulk material (∼1020 °C).23 However, in the aqueous solution, hexagonal ZnS nanocrystals were actually obtained at a much higher temperature because of the effect of chemisorbed water.23 It was reported that the crystal structure of prepared ZnS nanocrystals was sensitive to some organic molecules which acted as a surface-modifying reagent.24 The surface-modifying reagent adsorbed on the surface of ZnS nanocrystals may efficiently decrease the phase transformation temperature from cubic to hexagonal ZnS. For example, hexagonal ZnS nanocrystals were prepared by a hydrothermal route at 180 °C in the presence of EDA.25 In the current synthetic system, both EDA and EOA play the important role of surface-modifying reagent, but they have different effects on the crystal phase of ZnS. Comparing the structures of EDA and EOA molecules, one can see that the EOA molecule has two different functional groups of -OH and -NH2, and that the EDA molecule has two identical functional groups of -NH2. The -OH group is more hydrophilic and has more ability to form hydrogen bonds than the -NH2 group does. On the other hand, the -OH group more easily bonds with zinc atoms on the surface of ZnS nanocrystals than the -NH2 group does. Therefore, it is more efficient for EOA molecules than for EDA molecules to adsorb on the surface of ZnS nanocrystals. The formation of chemical bonds between the surface zinc atoms of ZnS nanocrystals and the -OH groups of EOA molecules or -NH2 groups of EDA molecules may lead to a change of the surface energy of ZnS nanocrystals, which may be responsible for the formation of the hexagonal ZnS nanocrystals in H2O/EOA at a relatively low temperature, because the difference in the internal energy of ZnS between the hexagonal and cubic phases is quite small (13.4 kJ‚mol-1).24 Morphologies of ZnS. The morphology and microstructure of ZnS samples were investigated by TEM and SEM. The morphology of ZnS nanocrystals in the NSs is strongly dependent on the synthetic parameters. Although the ZnS NSs had a similar spherical morphology, they were formed by the assembly of ZnS building blocks with different crystal phases and shapes. The average size of the ZnS NSs was significantly different for different samples. Figure 2 shows the TEM and

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Figure 2. TEM and SEM micrographs of ZnS NSs (A1): (a) TEM micrograph of ZnS NSs at low magnification; (b) TEM micrograph of a single sphere; (c) FE-SEM micrograph on the surface of a sphere; (d) TEM micrograph on the edge of a sphere at higher magnification; (e) highresolution TEM (HRTEM) image on a nanosheet; (f) fast Fourier transform (FFT) image of (e).

Figure 3. TEM micrographs of ZnS NSs. (a-c) A2: (a) ZnS NSs at low magnification; (b) a single sphere; (c) HRTEM image on the edge of a sphere. (d-f) A3: (d) ZnS NSs at low magnification; (e) a single sphere; (f) edge of a sphere at higher magnification. (g-j) A4: (g) dispersed multimorphological ZnS nanocrystals at low magnification; (h) HRTEM image of a single spherical nanocrystal of cubic ZnS; (i) HRTEM image of a single ZnS nanocrystal in which the phase transformation from cubic to hexagonal is not complete; (j) HRTEM image of a single nanorod of hexagonal ZnS.

SEM micrographs of sample A1 prepared at 200 °C when H2O was used as the only solvent. One can see that cubic ZnS NSs with sizes of several micrometers were obtained (Figure 2a,b). These ZnS NSs were constructed with nanosheet building blocks (Figure 2c,d). Each individual nanosheet was single-crystalline in structure (Figure 2e). Figure 3 shows TEM micrographs of ZnS (A2-A4) prepared in mixed solvents of H2O and EDA. When the volume ratio of EDA/H2O was 1:50, cubic ZnS NSs (A2) assembled from quantum dots (QDs) were obtained at 95

°C (Figure 3a-c) and ZnS NSs (a mixture of the cubic and hexagonal ZnS phases) assembled from multimorphology nanocrystals (QDs, nanosheets, nanorods) were obtained at 200 °C (A3, Figure 3d-f). When the volume ratio of EDA/H2O was increased to 1:10, dispersed multimorphology nanocrystals of ZnS (a mixture of the cubic and hexagonal ZnS phases) were synthesized at 200 °C (A4, Figure 3g-j). Figure 4 shows the TEM and SEM micrographs of ZnS samples (A5-A10) prepared in mixed solvents of H2O and EOA. When mixed solvents of

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Figure 4. TEM and SEM micrographs of ZnS NSs (A5-A10). (a, b) A5: (a) TEM micrograph of ZnS NSs at low magnification; (b) TEM micrograph of a single sphere. (c-e) A6: (c) TEM micrograph of ZnS NSs at low magnification; (d) TEM micrograph of a single sphere; (e) TEM micrograph of a fragment from a ZnS sphere. (f-i) A7: (f) TEM micrograph of ZnS NSs at low magnification; (g) TEM micrograph of a single sphere; (h) FE-SEM micrograph on the surface of a sphere; (i) HRTEM image on a nanorod. (j, k) A8: (j) TEM micrograph of ZnS aggregates at low magnification; (k) HRTEM image of ZnS nanocrystals in an aggregate. (l) A9: TEM micrograph of dispersed ZnS nanorods. (m) A10: TEM micrograph of dispersed ZnS nanorods.

EOA/H2O (1:50) were used, the NSs of hexagonal ZnS assembled from nanorods were prepared at 200 °C (A5-A7, Figure 4a-i) and the aggregates of hexagonal ZnS (A8) assembled from QDs (Figure 4j,k) were obtained at 95 °C. When the volume ratio of EOA/H2O was increased to 1:10 or the molar ratio of Zn(NO3)2 to L-cysteine was decreased to 1:2, dispersed individual ZnS nanorods with a hexagonal structure were formed at 200 °C (A9 and A10, Figure 4l,m). These results show that the concentration of L-cysteine has a significant influence on the morphology of the prepared ZnS nanocrystals. ZnS NSs assembled from nanorods were prepared at a Zn(NO3)2/Lcysteine molar ratio of 1:1 (A5-A7); in contrast, dispersed ZnS nanorods (A10) were obtained at a Zn(NO3)2/L-cysteine molar ratio of 1:2. When the molar ratio of Zn(NO3)2 to L-cysteine was 2:1, the product was a mixture of ZnS and ZnO, as shown by XRD (A11, Figure 5a), and partially consisted of ZnS NSs assembled from nanorods (Figure 5b-d). In this case, L-cysteine was not enough for the formation of ZnS, leading to the formation of ZnO. Proposed Formation Mechanism of ZnS NSs. In the present synthetic system, L-cysteine was used as a sulfur source and a

capping agent for the formation of various ZnS NSs. In contrast, ZnS NSs could not be obtained when another sulfur source such as sodium sulfide (Na2S) and thiourea ((NH2)2CS) was used instead of L-cysteine. This indicates that L-cysteine played a key role in the formation of ZnS NSs. On the other hand, L-cysteine can form a hydrophilic capping on ZnS nanocrystals,26 which may be a determinative factor for the formation of ZnS NSs by the assembly of ZnS nanocrystals. The delicate balance between the kinetic growth and thermodynamic growth determines the final morphology of the nanocrystals. Under nonequilibrium kinetic growth conditions at a high concentration of the monomer, selective anisotropic growth of different crystallographic planes is preferred.27 In the present synthetic system, when the synthesis was performed at a higher temperature (200 °C), the concentration of the formed monomer was high and excessive relative to the consumption for the growth of ZnS nanocrystals, the kinetic growth occurred, and the anisotropic morphology of ZnS nanocrystals formed. Generally, the crystal plane with a higher surface energy has a faster growth rate along its normal direction. According to ZnS crystal surface energies obtained from molecular

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Figure 5. XRD and TEM characterization of sample A11. (a) XRD pattern (# shows the peaks of ZnO); (b-d) TEM micrographs.

dynamics simulation,23 the growth of cubic ZnS crystals is confined along [110] and hexagonal ZnS has a preferential growth direction of [001] in the kinetic growth process. In the present synthetic system, the cubic ZnS crystal prefers to grow to form the nanosheet along the (110) plane and the hexagonal ZnS crystal prefers to grow to form the nanorod with the preferential growth direction along the [001] at a higher temperature and a higher monomer concentration. Parts e and f of Figure 2 show a high-resolution TEM (HRTEM) image and its corresponding fast Fourier transform (FFT) image of an individual nanosheet (A1); the confined growth direction of the nanosheet is [110]. Sample A3 was composed of the mixed phases of cubic and hexagonal ZnS, and multimorphologies of ZnS nanocrystals appeared, as shown in Figure 3f. Samples A5A7 consisted of the NSs of hexagonal ZnS assembled from nanorods, whose preferential growth direction was along [001] (Figure 4i). The SEM micrograph (A7, Figure 4h) on the surface of a sphere shows the multisided cross section of the nanorod. In contrast, QDs (A2 and A8) resulted from the thermodynamic growth independent of the crystal phase. In this case, ZnS nanocrystals grew at a slow and similar rate of each crystal plane when the concentration of the monomer released was low at a low temperature (95 °C). Among the different ZnS NSs reported in this paper, spheres assembled from nanorods are much more fascinating. Although there have been a few reports on similar morphology, for example CoPt alloys,28 the detailed formation mechanism is still not clear. In the present study, we propose a possible formation mechanism of ZnS NSs assembled from nanorods on the basis of the experimental observation (Figure 6). As described above, in the mixed solvents of H2O and EOA (50:1), the aggregates of hexagonal ZnS QDs were formed at 95 °C (A8, Figure 4j,k).

Figure 4e shows a broken fragment from a ZnS sphere assembled from ZnS nanorods (A6). This broken fragment was formed from a ZnS sphere under the action of intense ultrasonication. The broken fragment clearly reveals that the central core of the ZnS sphere was made up of quasi-spherical nanocrystals. This proposed mechanism may also be introduced to describe the formation of other ZnS NSs, such as the spheres assembled from nanosheets (A1) and multimorphology nanocrystals (A3), although the central core of the ZnS NSs (A1 and A3) was not clearly observed under TEM. The proposed formation mechanism is described as follows: (a) initially, ZnS QDs were formed and assembled into small nanospheres or aggregates with the help of L-cysteine capping shell at about 95 °C; (b) ZnS QDs on the surface of the nanospheres or aggregates kinetically grew to form anisotropic nanocrystals (nanorods or nanosheets) dependent on the crystal phase when the temperature increased to 200 °C. Here is an important question: why did all nanorods in the ZnS NSs (A5-A7) prepared in mixed solvents of H2O and EOA grow vertically to form the spheres? One possible answer is that the growing nanorods could regulate the direction when they touched other nanorods; another answer is that the nanorods grew only from those QDs whose [001] direction was vertically exposed to the spherical surface. Since QDs of hexagonal ZnS were formed in mixed solvents of H2O and EOA at 95 °C (A8), it is reasonable to infer that QDs of hexagonal ZnS were initially formed for samples A5A7 at a lower temperature and gradually grew into nanorods as the temperature increased to 200 °C. This is different from the samples prepared in mixed solvents of H2O and EDA. Since the NSs assembled from QDs of cubic ZnS (A2) were obtained in mixed solvents of H2O and EDA at 95 °C, it may infer that

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Figure 6. Schematic diagram of the proposed formation mechanism of ZnS NSs.

Figure 7. Schematic diagram of growth of ZnS nanorods in ZnS NSs. HRTEM images of the end of a ZnS nanorod in ZnS NSs: (a) A5, (b) A6, and (c) A7.

cubic ZnS QDs were initially formed at a lower temperature in the case of sample A3. As the temperature increased to 200 °C, the cubic ZnS QDs gradually grew into bigger nanocrystals with different morphologies accompanying a partial phase transformation from the cubic to hexagonal ZnS, leading to the formation of multimorphological ZnS nanocrystals. Banfield et al.20 reported that the phase transformation of ZnS nanocrystals

in hydrothermal solution was dependent on the nanocrystal size and was only partially completed, which is in agreement with the result reported here. Sample A4 prepared in mixed solvents of EDA and H2O (1:10) at 200 °C was also composed of mixed phases of cubic and hexagonal ZnS (Figure S1, Supporting Information), but the morphologies were multiple, such as spherical nanocrystals of cubic ZnS (Figure 3h) and nanorods

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J. Phys. Chem. C, Vol. 111, No. 10, 2007 3899 24 h for A7). Equation 1 educed from the Gibbs-Thompson equation (see Supporting Information) indicates that the solubility of a specific crystal surface at a specific temperature is dependent on its surface area and surface energy.

SA ) C exp(σA/A1/2)

Figure 8. UV-vis and PL spectra of ZnS NSs together with information on the absorption onset, near band-edge emission, and defect emission peaks. PL spectra were recorded at an excitation wavelength of 270 nm.

of hexagonal ZnS (Figure 3j). Figure 3i shows a single ZnS nanocrystal with defects such as stacking faults and twins, indicating that the phase transformation was not complete and indicating the coexistence of the cubic and hexagonal ZnS phases.20 Additionally, ZnS NSs prepared in a single solvent of water at 200 °C were assembled from nanosheets with a cubic structure (A1). The nanosheets of cubic ZnS were formed from the kinetic growth of the QDs of cubic ZnS, as shown in Figure 6. However, in mixed solvents of EDA and H2O or EOA and H2O with a higher volume ratio (1:10), dispersed ZnS nanocrystals (A4 and A9) other than NSs were abtained (Figure 3g-j and Figure 4l). This implies the effect of the EOA or EDA molecules as a surface-modifying reagent adsorbed on the surface of ZnS nanocrystals. At a higher concentration of EOA or EDA, more EOA or EDA molecules are adsorbed on the surface of ZnS nanocrystals, which may prevent attractive aggregation of the ZnS nanocrystals, leading to the formation of dispersed ZnS nanocrystals. Size Evolution of ZnS Nanorods in NSs. The ZnS nanorods in NSs of different samples had different sizes. Sample A5 prepared in H2O/EOA (50:1) at 200 °C for 2 h had an average size of ∼150 nm length and ∼8 nm diameter (Figure 4a,b). When the solvothermal time was increased to 10 and 24 h at 200 °C, the obtained samples A6 and A7 had a similar size of ∼100 nm length and ∼15 nm diameter (Figure 4c-e and Figure 4f-i). This indicates that the growth of ZnS nanorods experienced the kinetic and thermodynamic growth stages as described in Figure 7. The larger aspect ratios of ZnS nanorods resulted from the kinetic growth induced by a high concentration of the monomer at a shorter hydrothermal time (2 h for A5). However, the decrease in length and increase in diameter for ZnS nanorods resulted from thermodynamic growth due to a low monomer concentration at a longer hydrothermal time (10 h for A6 and

(1)

where SA is the solubility of the crystal surface, A is the crystal surface area, σA is the surface energy, and C is a constant. According to eq 1, the (002) plane of hexagonal ZnS has a larger solubility than other planes. The turning point from the kinetic stage to thermodynamic stage is defined as that when the monomer concentration decreases to the solubility of the (002) plane due to consumption. In the thermodynamic stage, the growth of ZnS nanorods along [001] ceased and the (002) plane was dissolved to release the monomer for the further growth of other planes. As a result, the ZnS nanorods became shorter and wider. In addition, the main top surfaces of ZnS nanorods along the crystallographic direction of [001] were flat and indexed as the (002) crystal plane prepared for 2 h (A5, Figure 7a), and were semispherical for 10 h (A6, Figure 7b) and cone-shaped for 24 h (A7, Figure 7c). The thermodynamic growth would reach a balance if the time was long enough. For example, the ZnS nanorods prepared for 24 h (A7) had sizes similar to those prepared for 10 h (A6), and the planes with a lower solubility entirely replaced the (002) plane on the ends of ZnS nanorods (Figure 7c). Optical Properties of ZnS NSs. The optical properties of ZnS NSs assembled from various ZnS nanocrystals were investigated by UV-vis absorption and PL spectra. The UVvis absorption and PL spectra of ZnS NSs together with the information on the absorption onset, near band-edge emission, and defect emission peaks are shown in Figure 8. The absorption onset of the ZnS NSs except A2 was close to that of the bulk, because the average size of the ZnS nanocrystals was much bigger than the exciton Bohr size (5 nm) of bulk ZnS. In the PL spectra, near band-edge and trap emissions were observed using an excitation wavelength of 270 nm. The near band-edge emission peak of ZnS NSs was slightly red-shifted with respect to the absorption onset, which may be a result from the splitting of the valence band. The valence band top of ZnS nanocrystals split off to be two hole states Γ8 and Γ7.29 The high-energy hole state (Γ7) was responsible for the absorption transition, and the low-energy fine-structure state (Γ8) was for the near bandedge emission. Similar phenomena were also observed in CdSe nanocrystals.30 On the other hand, both absorption onset and near band-edge emission peaks of samples A6 and A7 were redshifted compared with those of sample A5, which is in agreement with the size difference of ZnS nanorods. As discussed above, the ZnS nanorods of sample A5 had a longer length and a smaller diameter than those of samples A6 and A7. The defect emission resulted from the recombination of conduction band electrons captured in defect traps with valence band holes. The radiative recombination on the surface of ZnS nanocrystals with sulfur vacancies was responsible for the defect emission with a maximum at about 430 nm,31 which is similar to bulk ZnS.32 Unlike other ZnS NSs, A2 had a more intense defect emission than near band-edge emission due to a higher surface defect state resulting from a larger surface atom ratio of QDs. Conclusion In summary, we have demonstrated a simple biomoleculeassisted method for the synthesis of a variety of ZnS nanostructured spheres assembled from ZnS nanocrystals with

3900 J. Phys. Chem. C, Vol. 111, No. 10, 2007 controllable crystal phases and morphologies. L-Cysteine, a biomolecule, was used as the sulfur source and played a key role in the formation of ZnS nanostructured spheres. ZnS nanostructured spheres assembled from various ZnS nanocrystal building blocks, such as nanosheets, QDs, nanorods, and multimorphology nanocrystals, were successfully prepared by this simple method. The crystal phase of ZnS nanostructured spheres could be controlled in this synthetic system. Hexagonal ZnS nanocrystals were obtained in mixed solvents of water and ethanolamine at temperature as low as 95 °C. Acknowledgment. Financial support from the National Natural Science Foundation of China (50472014) and the Chinese Academy of Sciences under the Program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program) is gratefully acknowledged. We also thank the financial support from the National Basic Science Research Program of China (973 Program) (Grant 2005CB522700). We thank Professor Mei-Ling Ruan and Professor Jing-Wei Feng for assistance in TEM measurements. Supporting Information Available: XRD patterns of samples A4-A6, A9, and A10 (Figure S1) and the eduction of eq 1. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. Science 2003, 302, 419. (2) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323. (3) Knoll, A.; Lyakhova, K. S.; Horvat, A.; Krausch, G.; Sevink, G. J. A.; Zvelindovsky, A. V.; Magerle, R. Nat. Mater. 2004, 3, 886. (4) Monroy, E.; Omnes, F.; Calle, F. Semicond. Sci. Technol. 2003, 18, 33. (5) Zhao, Y. W.; Zhang, Y.; Zhu, H.; Hadjipianayis, G. C.; Xiao, J. Q. J. Am. Chem. Soc. 2004, 126, 6874. (6) Panda, A. B.; Glaspell, G.; El-Shall, M. S. J. Am. Chem. Soc. 2006, 128, 2790.

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