Ligand-Controlling Synthesis and Ordered Assembly of ZnS Nanorods

Morphology Evolution of Gradient-Alloyed CdxZn1–xSeyS1–y@ZnS .... The Journal of Physical Chemistry C 0 (proofing), .... Selected-Control Hydrothe...
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J. Phys. Chem. B 2004, 108, 16002-16011

Ligand-Controlling Synthesis and Ordered Assembly of ZnS Nanorods and Nanodots Yunchao Li,† Xiaohong Li,† Chunhe Yang, and Yongfang Li* Key Laboratory of Organic Solids, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China ReceiVed: March 11, 2004; In Final Form: June 2, 2004

The shape- and phase-controlled synthesis of ZnS nanocrystals (nanorods and nanodots) was realized by the selection of ligand molecules with a simple method of thermolysing single-source precursorszinc ethylxanthate (Zn(exan)2) with octylamine (OA) or trioctylphosphine (TOP) as precursor solvent. The as-prepared nanocrystals were characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and X-ray diffraction (XRD). In hexadecylamine (HDA) + OA system, diameter- and aspect-ratio-tunable hexagonal wurtzite ZnS nanorods were attained in the temperature range of 150-250 °C, and the nanorods self-assembled into two-dimensional (2-D) aligned arrays. While in the HDA + TOP system, a shape change from rod to spherical particle and a phase transition from wurtzite to sphalerite simultaneously occurred with the increase of TOP content in the solution, and sphalerite nanodots were prepared in high TOP content or in TOP (or trioctylamine (TOA)) solution without HDA. The mechanism of the shape- and phase-controlled growth of ZnS nanocrystals by selecting ligand molecules was analyzed. The absorption and photoluminescence spectra of the wurtzite ZnS nanorods and the sphalerite ZnS nanodots were also measured and contrasted. In addition, the mechanism and the strategies of assembling 1-D ZnS nanorods on 2-D scale were discussed, and the ordered arrays of the nanorods and nanodots were obtained on a relatively large scale.

1. Introduction Semiconductor nanocrystals, especially with 1-D nanostructure, have drawn much attention in recent years due to their unique size- and shape-dependent optical and electronic properties as well as the promising applications in nanodevices.1-5 Thus, many methods have been developed for shape-controlled synthesis of semiconductor nanomaterials to attain their 1-D nanostructures, including template-assisted methods,6-8 vaporliquid-solid (VLS) process,9-10 solvothermal route,11-12 kinetic control solution growth,13-16 and self-assembly.17-20 Because ZnS is one of the most important II-VI semiconductors, various methods have been developed for the preparation of 1-D ZnS nanorods or wires in the past few years.21-27 However, it remains an interesting and difficult thing to synthesize 1-D ZnS nanostructure with controllable shape and size in quantum confinement range. Phase-controlled synthesis of nanocrystals is also very important. Because crystal structures of materials play a key role in both physical and chemical properties. For ZnS, the properties of sphalerite ZnS are different from that of wurtzite ZnS.28 Wurtzite ZnS is unstable and very difficult to be synthesized directly in solution at mild temperature.23-25 Very recently, Chen et al. has prepared wurtzite ZnS through a hydrothermal route at 180 °C.25 Simultaneously realizing shapeand phase-controlled synthesis of nanocrystals via simply changing ligand molecules is exciting and challenging in nanochemistry. Because, on one hand, it demonstrates the ability to tail-aimed material on a subtle scale, which is the foundation for practical application; on the other hand, it helps to elucidate the underlying function of ligand molecules on the controlled * Corresponding author. E-mail: [email protected]. † Also at the Graduate School, Chinese Academy of Sciences.

growth of nanocrystals. So far, there have been only a few successful examples in this shape- and phase-controlled synthesis, such as, a solvothermal route to the fabrication of wurtzite ZnS nanosheets23 or wurtzite ZnS nanorods25 and sphalerite ZnS nanospheres, a mixed surfactant system for the formation of polymorphic CdS nanoparticles and nanorods,29 sterically induced shape and phase control of GaP nanocrystals,30 and ligand selection to synthesize wurtzite and zinc blende CdTe nanocrystals.31 The development of practical strategies for the manipulation of inorganic nanoparticles and assembling them into welldefined arrays on aimed substrates is another active area in nanoscience.32-34 Although many efforts have focused on the fabrication of 2-D and 3-D arrays of nanoparticles, only a few reports have been published concerning the organization of 1-D nanorods (or nanowires) into 2-D arrays free from matrix, such as the assembly of BaCrO4 nanorods and the fabrication and alignment of Ag wires by Langmuir-Blodgett technique,35-36 the assembly of CdSe nanorods via evaporation of liquid crystals,37 and the self-assembly of Ag wires,20 Co nanodisks,38 and Ag2S nanorods.39 Herein, we report a new approach to actualize the shape- and phase-controlled synthesis of ZnS nanocrystals by thermolysing single-source precursor in different ligand solution. Wurtzite ZnS nanorods can be synthesized at 150 °C through a usual solution route (different from a hydrothermal route at high pressure) and are stable in this system. The resultant ZnS nanorods and spherical nanodots can be self-assembled into 2-D ordered arrays. To our knowledge, this is the first report on the assembly of 1-D ZnS nanorods into 2-D arrays. In addition, zinc ethylxanthate used in this work is more easily synthesized, and air-stable, even can be used under atmospheric benchtop condition, which meets the goal of environmental-friendly

10.1021/jp0489018 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/21/2004

ZnS Nanorods and Nanodots synthesis and low cost. In nanochemistry, alkylxanthates were first introduced as capping agents.40-41 Very recently, metal salts of alkylxanthates were served as perfect and versatile singlesource precursors for synthesizing metal sulfide nanoparticles.42-44 2. Experimental Section Materials. Zinc acetate, potassium ethylxanthate, methanol, and toluene used in the present study were analytical grade reagents and purchased from Beijing Chemical Reagent Ltd. Co. of China. Trioctylphosphine (TOP, 90%, Aldrich), hexadecylamine (HDA, 90%, Aldrich), octylamine (OA, 98%, Aldrich), and trioctylamine (TOA, 98%, Acros) were used as supplied without any further purification. Zinc ethylxanthate (Zn(exan)2) was synthesized by the method similar to Nair’s.42 Synthesis. Typically, all synthesis was carried out in an inert atmosphere. ZnS nanorods were prepared with protocol A: a specified amount of zinc ethylxanthate was first dissolved in 4 mL of OA, then the solution was swiftly injected into a 4.0 g hot HDA solution through syringe under stirring, and the reaction was maintained for a specified time at specified temperature. ZnS nanodots were synthesized according to protocol B: a specified amount of zinc ethylxanthate was first dissolved in 4 mL of TOP, then the solution was swiftly injected into a 4.0 g hot HDA + TOP mixed solution with other conditions similar to protocol A; or only TOP solution (or only TOA) was used to synthesize ZnS nanodots. Afterward, the reaction solution was cooled and precipitated by acetone (or methanol), the flocculant precipitate formed was centrifuged, the upper layer liquid was decanted, and then the isolated solid was dispersed in chloroform (or toluene). The above centrifugation and isolation procedure was then repeated several times for purification of the prepared ZnS nanocrystals. Finally, the ZnS nanocrystals were re-dispersed in chloroform (or toluene) or dried under vacuum. Characterization. The nucleation process of ZnS nanocrystals in TOP and alkylamine solutions were monitored by UVvis absorption spectrometer at the initial stage. The growth rates of the ZnS nanocrystals in alkylamine solution at 150 and 250 °C were monitored via the measurement of the size change of the nanocrystals with different reaction times by transmission electron microscopy (TEM) observation. The shapes and phase structures of as-resultant ZnS nanocrystals were investigated by TEM and X-ray diffraction (XRD), etc. The coordination effects of the ligand molecules on monomer and nanocrystals were investigated by FTIR spectra and X-ray photoelectron spectroscopy (XPS), respectively. UV-vis absorption spectra were recorded on a Hitachi U-3010 spectrophotometer. Samples were dissolved in chloroform and placed in a 1 cm quartz cell, and chloroform served as reference solvent. Photoluminescence (PL) spectra were recorded on a Hitachi F-4500 spectrophotometer. The slits were set to 2.5 nm, and the excitation wavelength was set to 290 nm. TEM observation was performed with a Hitachi H-800 transmission electron microscope, accompanied by selected area electron diffraction (SAED). And, high-resolution transmission electron microscopy (HRTEM) measurements were carried out using a Hitachi H-9000NAR transmission electron microscope operated at 200 kV. The specimens for the TEM measurements were prepared by depositing a drop of a dilute solution of the sample in toluene on a carbon-coated copper grid and drying at room temperature. XRD patterns were recorded by a Rigaku D/max-2400 diffractometer operated at 40 kV voltage and 120 mA current with Cu KR radiation. Samples for XRD measurements were solid powder prepared by drying the purified product under vacuum.

J. Phys. Chem. B, Vol. 108, No. 41, 2004 16003 Infrared spectra, in the region of 500-4000 cm-1, were recorded on a PE2000 FTIR with 4 cm-1 resolution. XPS data were obtained on a VG-Scientific ESCA Lab 220i-XL spectrometer equipped with a hemisphere analyzer and an Al KR X-ray source at 1486.6 eV. Peak positions were internally referenced to the C1s peak at 284.6 eV. The samples for XPS measurements were the purified solid powder. Assembly. The assembly behavior of the nanocrystals was studied by TEM observation. The sample preparation procedure was as follows. Experiment 1: the solutions of ZnS nanorods and nanodots with a concentration of 4-6 times thicker than that for the preparation of usual TEM specimens (adjusted by toluene) were ultrasonicated for 5-10 min, and then a drop of the solutions was deposited on a carbon-coated copper grid and slowly dried at ambient condition. Experiment 2: a dilute solution of ZnS nanorods dissolved in a mixed solvent of toluene and ethanol (toluene:ethanol ) 1:1) was used for the preparation of the TEM specimen with other procedures similar to Experiment 1. 3. Results and Discussion 3.1. Shape-Controlled Synthesis of ZnS Nanocrystals. In the HDA + OA system, where OA was used as precursor solvent and HDA served as the main ligand stabilizer, 1-D ZnS nanorods were mainly formed. By changing the reaction conditions (such as reaction temperature, monomer (singlesource precursor-zinc ethylxanthate) concentration, etc.), the aspect ratio and the diameter of the nanorods can be adjusted. Figure 1 shows the TEM images of the ZnS nanocrystals prepared at different temperatures and monomer concentration conditions. At 150 °C, 0.5 g of Zn(C2H5OCS2)2 in the solution, and reaction for 10 h, the ZnS nanorods with an average 200 nm in length and 2.5 nm in width were attained, and the ZnS nanorods aggregated into bundles (Figure 1a). At 200 °C, 0.2 g of Zn(C2H5OCS2)2 in the solution, and reaction for 6 h, the resultant ZnS nanorods were an average 25 nm in length and 3.5 nm in width (Figure 1b). At 250 °C, 0.35 g of Zn(C2H5OCS2)2 in the solution, and reaction for 4 h, the formed nanorods were an average 35 nm in length and 4.5 nm in width, and a few spherical particles also appeared (Figure 1c); while at the same conditions but higher monomer concentration (0.5 g of Zn(C2H5OCS2)2), the longer nanorods with an average 50 nm in length and 4.5 nm in width were formed, and the nanorods self-assembled into 2-D ordered array (Figure 1d). Highresolution TEM (Figure 1e) of the sample corresponding to Figure 1d confirms that most rods are parallelly arranged with a distance of an average 2.4 nm (one to the other), and each rod is a single crystal with the interplanar distances of {100} of 0.338 nm which is consistent with that of wurtzite ZnS. The above results indicate that the ZnS nanorods can be attained in wide temperature and monomer concentration range in the HDA + OA system, which implies that the anisotropic growth of the ZnS nanocrystals (i.e., mainly 1-D-oriented growth along the most active facet) was easily actualized and maintained in this solution. To further support this opinion, we monitored the growth rates of the long axis and the short axis of those ZnS nanorods formed under 150 and 250 °C by TEM observation at different interval times, with the same initial monomer concentration condition (0.3 g of Zn(exan)2). As shown in Figure 2, at 150 °C, with the reaction progress (from 0 to 10 h), the long axes of the ZnS nanocrystals continue growing although the rate slows down, while their short axes nearly maintain constant. While at 250 °C, the same situation maintains for 3-4 h; then the shape transition from rod to dot turns notable for longer reaction times.

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Figure 1. TEM images of the ZnS nanorods prepared in the solution of HDA + OA with different reaction conditions: (a) at 150 °C, 0.5 g of Zn(C2H5OCS2)2 in the solution, and for 10 h; (b) at 200 °C, 0.2 g of Zn(C2H5OCS2)2 in the solution, and for 6 h; (c) at 250 °C, 0.35 g of Zn(C2H5OCS2)2 in the solution, and for 4 h; (d) at 250 °C, 0.5 g of Zn(C2H5OCS2)2 in the solution, and for 4 h; (e) large area HRTEM images for d (the inset (left side) is a magnified HRTEM image).

Figure 2. Variation of the long axis and the short axis of the ZnS nanocrystals with the reaction time at 150 and 250 °C, respectively. The asterisk (*) means that the values of ZnS long axes are very dispersive.

The mechanism for the anisotropic growth and the shape evolution of nanocrystals in solution have been profoundly elucidated recently by Adam and Peng15,16,45 and Lee et al.,46,47 respectively. They pointed out that the formation of elongated nanocrystals was a highly kinetics driven reaction, and a shape evolution from long rod to shorter rod and even to spherical particle was the result of the reaction system shifting from kinetics driven control to thermodynamic driven control due to the situation change of nanocrystals and their reaction environments (high monomer concentration supplies the reaction system with high kinetic drive, while high reaction temperature provides it with high thermal energy (kT) and accelerates reaction rate). Obviously, in HDA + OA system, the shape control of the ZnS

nanocrystals is the result of kinetics adjustment via variation of the reaction conditions. Under conditions similar to those mentioned above (at 200 °C, 0.3 g of Zn(C2H5OCS2)2 in the solution, and reaction for 3 h), when TOP instead of OA was used as the precursor solvent, no matter whether HDA or TOP or the mixed solution was used as the bulk ligand solution, it led to a shape change of the resultant ZnS nanocrystals. Figure 3 shows the TEM images of the ZnS nanoparticles prepared by protocol B (refer to the Experimental Section). When only TOP was served as the stabilizer, dotlike particles (nanodots) were formed and aggregated together (Figure 3a). A high-resolution TEM image (the inset) confirms that the particle is single crystal with clear interplanar distances of 0.312 nm, which may correspond to that of the {002} plane of the wurtzite ZnS or the {111} plane of the sphalerite ZnS. When a mixture of TOP and HDA (weight ratio ) 2:1) was used as the stabilizer, uniform spherical nanoparticles (90%) were mainly formed, and the nanoparticles assembled into a 2-D superlattice array (Figure 3b). Highresolution TEM (Figure 3d and the inset) corresponding to Figure 3b reveals that those particles are hexagonal arrangement on a 2-D scale and are single-crystalline with 0.312 nm for the interplanar distances of {111} and 0.274 nm for the interplanar distance of {200}, respectively, which is consistent with those of the sphalerite ZnS. When the ratio of TOP to HDA was lowered to 1:1 in the ligand solution, the content of nanorods increased in the products (Figure 3c). In addition, when only TOA was used as the stabilizer, i.e., in neat TOA solution, the formed particles also showed a dot shape and crystal nature (see Figure 3e and the inset). Moreover, the particles prepared in neat TOP or neat TOA solutions are obviously small and difficult to be isolated by standard solvent/nonsolvent precipitation technique. The difficulties may arise from the higher solubility of the small particles and the short chain length of the capping moleculessTOP or TOA.

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Figure 3. TEM images of the ZnS nanoparticles prepared at 200 °C, 0.3 g of Zn(C2H5OCS2)2, and reaction for 3 h in different ligand solutions of (a) TOP only (the inset (left above) is a magnified HRTEM image), (b) TOP:HDA ) 2:1, (c) TOP:HDA ) 1:1, (d) TOA only (the inset (left above) is a magnified HRTEM image), and (e) large-area HRTEM images for b (the inset (left above) is a magnified HRTEM image).

The results in Figure 1 and Figure 3 indicate that the nature of the solvent (ligand) molecules plays a key role in the shape control of the ZnS nanocrystals. Under the conditions with the same monomer concentration and the same reaction temperature, the growth of ZnS nanorods in the HDA + OA system with OA as precursor solvent was dominated under kinetics driven control even for a longer reaction time, while, in the reaction system with TOP or TOA as the precursor solvent, the ZnS nanocrystals grow into dotlike shape even with higher initial monomer concentration. Obviously, the shape difference of the nanoparticles synthesized in the two ligand solution systems is not the result of the difference in kinetic drive, but the result of the difference in ligand environment (i.e., ligand). How do the ligand molecules determine the shape of ZnS nanocrystals? The effect of the ligand molecules on the crystal structures may give some instructive information. 3.2. Phase Control of ZnS Nanocrystals. The ZnS crystal has two crystallographic structures in nature. One is wurtzite structure; the other is sphalerite structure. Hexagonal wurtzite ZnS is a thermodynamically metastable phase, which is usually stable at very high temperature, while cubic sphalerite ZnS is the more thermodynamically stable phase, which is stable at low temperature. Wurtzite ZnS is very difficult to synthesize at low temperature by a solution route. Interestingly, we easily attained the wurtzite ZnS nanorods at 150 °C in the HDA + OA ligand solution, as mentioned above. Figure 4 shows the XRD patterns of the ZnS nanorods prepared in the HDA + OA solution (from 150 to 250 °C). The XRD patterns all can be indexed as wurtzite-phased ZnS with the strong characteristic (110), (103), (112) peaks. In addition, SAED technique and HRTEM technique also confirmed the wurtzite crystalline structure of the ZnS nanorods with the (100), (002), (101) diffraction rings in the SAED pattern (see Figure 5a) and the 0.338 nm interplanar distances of {100} in the HRTEM image (see Figure 1d), respectively. The XRD patterns show obviously broadened diffraction peaks compared

Figure 4. Powder X-ray diffraction patterns of ZnS samples grown in HDA + OA ligand solution at different temperature: (a) 150 °C, 6 h; (b) 200 °C, 6 h; (c) 250 °C, 6 h; (d) 250 °C, 10 h, respectively.

to those of the bulk ZnS crystals, signifying the finite size of these crystallites. The stronger and narrower (002) peak in Figure 4 indicates that all the nanocrystals were elongated along the c-axis. It is interesting to find that there is no temperature effect on the crystal phase in the temperature range between 150 and 250 °C. The wurtzite structure can be formed at the relatively lower temperature of 150 °C. Moreover the wurtzite structure formed in the HDA + OA solution is stable even for long reaction time; for example, for the particles formed in the solution at 250 °C for 10 h, there is no change detected in their phase structure (see Figure 4d). For the particles formed in neat TOP or neat TOA, SAED patterns (Figure 5b,c) and XRD patterns (Figure 6a,b) of the samples all confirm their sphalerite phase structure. So we can assign that the interplanar distance of 0.312 nm shown in their

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Figure 5. Selected area electron diffraction (SAED) patterns of (a) the ZnS nanorods synthesized in HDA + OA, and ZnS nanodots formed in (b) neat TOA, (c) neat TOP, and (d) TOP + HDA (2:1), respectively.

Figure 6. Powder X-ray diffraction patterns of ZnS samples grown in different ligand solutions: (a) TOA only, (b) TOP only, (c) TOP: HDA ) 2:1, (d) TOP:HDA ) 1:1, and (e) HDA + OA.

HRTEM images (Figure 3a,e) corresponds to that of the {111} plane of the sphalerite ZnS. As for the samples synthesized in the HDA + TOP system, a phase transition from wurtzite to sphalerite occurred with increasing content of TOP in the mixed ligand solutions. For the samples formed in a ligand solution without TOP, for example, in HDA + OA solution, their phase structure is wurtzite. While for the samples synthesized in a ligand solution with a low ratio of TOP to HDA (1:1), the (102) peak and (103) peak of wurtzite were obscure in their XRD patterns. For the samples synthesized in a ligand solution with a higher ratio of TOP to HDA (2:1), diffraction information from wurtzite could hardly be detected in their XRD patterns, and the HRTEM image (Figure 3d) and the SAED patterns (Figure 5d) also confirm their sphalerite phase structure. Obviously, the ligand molecule plays a key role in determining the phase structure of ZnS nanocrystals in their growth process. 3.3. Mechanism of Shape- and Phase-Controlled Growth of ZnS Nanocrystals by the Selection of Ligand Molecules. To clarify the mechanism of the ligand-inducing shape- and phase-controlled growth of the ZnS nanocrystals, the effects of ligand molecules on the coordination action with the monomer

Li et al. and the nanocrystals and on the nucleation process of the ZnS nanocrystals were investigated. The coordination action of the ligand molecules on monomer and resultant ZnS nanocrystals were first investigated by FTIR technique (Figure 7). It has been well proven that the ligands containing N-donor or P-donor can coordinate with the central metal atom in metal xanthates to form corresponding adducts.48-52 In fact, TOP can well dissolve zinc ethylxanthate and forms damask clear solution, which indicates the formation of the adducts.48-50 The IR spectra in Figure 7A show that, relative to free Zn(exan)2, C-O asymmetric vibration (at 1184 cm-1) and C-O symmetric vibration (at 1124 cm-1) of these adducts (curve b) both shift to lower frequency, while its C-S stretching vibrations (at 1029 cm-1) shift to higher frequency, as many authors observed.48-52 These adducts can be stable for relatively long time at ambient conditions. For the purified ZnS sample synthesized in neat TOP solution, FTIR also confirmed the presence of TOP molecules on the nanocrystals surface (the band at 2800-2900 cm-1 and the band at 1300-1400 cm-1 can be assigned to the stretching vibrations and bending vibrations of C-H, respectively). However, relative to free TOP molecules, the characteristic band of -CH2 (adjusted to P atom) does not show obvious shift, which indicates the weak binding of TOP to the surface of ZnS. As for OA, Zn(exan)2 can be dissolved in it and form a yellowy clear solution. However, this clear solution soon turns white and turbid, which indicates OA promotes the decomposition of Zn(exan)2.53 The variation of characteristic bands at 1000-1220 cm-1 with time (in Figure 7B) also supports this opinion. The binding behavior of alkylamine (HDA or OA) molecules on the surface of ZnS nanocrystals was also confirmed by the fact that the NH2 bending vibration and the C-N stretching vibration both shift to lower frequency relative to the free alkylamine.54 The binding behavior of ligand molecules on the surface of ZnS nanocrystals was also investigated by XPS. It is well-known that long-chain alkylamine molecules (HDA, etc.) can act as a ligand to stabilize nanocrystals in solution.30,55-57 Because of the coordination of the NH2 functional group to the surface of nanocrystals, the N1s binding energy of bound HDA shifts to higher energy relative to that of free HDA. As shown in Figure 8a, there appear two types of N atoms; one locates at 398.7 eV corresponding to the N1s of free HDA (Figure 8b), and the other locates at 400.2 eV corresponding to that of bound HDA. For the ZnS prepared in neat TOP solution, XPS (Figure 8c) also confirms that TOP molecules bind to the surface of the nanocrystals since the P2p binding energy obviously shifted to higher energy relative to that of free TOP (129 eV).58 It is noteworthy that when TOP and HDA (2:1) coexist in the reaction solution, there is hardly detected any phosphine information on the surface of the resultant ZnS (as shown in Figure 8d,e), which indicates that the binding ability of HDA molecules to the surface of ZnS is relatively stronger than that of TOP molecules. The nucleation process is a key step to affect the shape and phase structure of the nanocrystals. As shown in Figure 9, the nucleation process of ZnS nanocrystals in alkylamine solution is obviously different from that in neat TOP solution. As for the former, at 200 °C, the nucleation rate is very fast: after the precursor injection for 1 min, the strong characteristic π-π* absorption peak (at ca. 304 nm) of xanthates48,50,59 is replaced by a new broad absorption peak (at ca. 290 nm)sthe ZnS clusters absorption peak. Moreover, size self-focusing phenomenon is also observed (see Figure 9A). While for the latter, the nucleation is complicated and relatively slow. As shown in

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Figure 7. (A) FTIR spectra of (a) Zn(exan)2 and Zn(exan)2 dissolved in TOP aged for (b) 5 min and (c) 24 h, (d) TOP, and (e) purified ZnS synthesized in neat TOP solution; (B) FTIR spectra of (a) Zn(exan)2, Zn(exan)2 dissolved in OA aged for (b) 5 min, (c) 1 h, and (d) 4 h, (e) OA, (f) HDA, and (g) purified ZnS synthesized in HDA + OA solution.

Figure 8. XPS spectra of (a) N1s of HDA bound to ZnS surface, i.e., N1s for ZnS prepared in HDA + OA solution, (b) N1s of free HDA, (c) P2p of ZnS prepared in neat TOP solution, (d) survey spectrum, and (e) P2p of ZnS prepared in HDA + TOP (1:2) solution.

Figure 9B, after injection of precursor for 1 min, the strong characteristic π-π* absorption peak of xanthates disappears, which indicates C-S bonds of xanthate changed.53 However, two new absorption peaks appear instead; one locates at ca. 320 nm and turns stronger with longer reaction time, and the other locates at ca. 360 nm and turns weaker and even disappears with longer reaction time. Obviously, the 320 nm peak attributes to ZnS absorption, while the 360 nm peak we assign to chargetransfer (CT) transitions between Zn and TOP.50,59 As mentioned

above, Zn(exan)2 reacts with TOP and forms the P-ligandcontaining adducts. Before injection, because of steric barriers, this CT transition is weak, while, after injection, because the part group in xanthate is removed, there leaves more chances for TOP molecules coordinating with central Zn atoms. Thus, shortly after injection, this peak turns stronger; however, with the reaction progress, it becomes dim due to the subdued coordination strength. Moreover, the ZnS clusters (nuclei) formed in alkylamine solution are obviously smaller than that

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Figure 9. UV-vis absorption spectra of (A): (a) precursor solutionsZn(exan)2 in OA, after injecting the precursor into HDA solution at 200 °C and reaction for (b) 1, (c) 3, and (d) 5 min. (B): (a) Zn(exan)2 in CH3Cl, (b) Zn(exan)2 in TOP, after injecting the precursor into TOP solution at 200 °C and reaction for (c) 1, (d) 3, (e) 5, (f) 10, and (g) 30 min (the inset is a survey of the whole spectra).

in neat TOP solution, which consistently suggests the TOP molecule is a weak stabilizer for the ZnS nanocrystals. On the basis of the above experimental results and analyses, maybe, we can throw a light on the underlying mechanism for the shape-controlled and phase-controlled growth of inorganic nanocrystals. In general, shape control of inorganic nanocrystals is considered to be the result of the combination of the initial selection of nucleation with the following balanced growth between kinetic control and thermodynamic control.46 In fact, the adjustment of morphology by kinetic control is delicate and effective. As far as phase control is concerned, it is usually considered that the key controlling condition is the environmental temperature. However, the results reported here indicate that the ligand molecules induced the shape and phase change of the ZnS nanocrystals. Some experimental results have revealed that selective adsorption of ligand molecules onto particular crystallographic facets plays an important role in the shape control of nanocrystals.47 Moreover, very recently, Peng et al.31 have demonstrated the shape- and phase-controlled synthesis of CdTe nanocrystals by changing ligand molecules. They pointed out that any ligand added to a synthetic system is not only the ligand for the resulting nanocrystals but also the ligand for the precursor monomers, and a strong ligand for the monomers dominates the nucleation process mostly, while a strong ligand for the nanocrystals affects the growth process mostly.31 The reaction system here is similar to that used by Cheon et al. in the preparation of GaP nanocrystals.30 The thermodynamically stable sphalerite structure is a staggered conformation with 〈111〉 direction, and the kinetically stable wurtzite structure is an eclipsed conformation with 〈002〉 direction. The difference of steric effect as well as ligand ability between TOP (or TOA) and alkylamine (HDA or OA) molecules determines their different binding behavior. TOP is a relatively strong ligand for the monomer (can form stable complexes). Thus, the nucleation process is difficult and slow, which leaves enough chances for these complexes adjusting their pose to minimize the large steric hindrance between them and ZnS lattice in the growth process. Therefore, in neat TOP solution, staggered conformation is favored, and the formed ZnS is a sphalerite structure. A alternative interpretation is as follows: by the reason of energy, the initially formed “magicsized cluster” is mainly with zinc blend bonding geometry.45 There is short of an efficient kinetic drive to break this intrinsic

symmetry structure in TOP solution because of the difficult nucleation and relatively fast growth rate of nanocrystals (for TOP is a weak ligand for nanocrystals). Thus, the formed nanocrystals are a zinc blend structure. Because of the high symmetry of this structure, it leads to form dot shape even though there is a high monomer concentration in the ligand solution. Since HDA (or OA) is a weak ligand for the monomer and a relatively strong ligand for ZnS nanocrystals, in alkylamine solution, the nucleation is very fast, and formed ZnS cluster are exclusively bound by HDA (or OA) molecules. Because of the little steric barrier, these clusters can combine together with eclipsed conformation. Similarly, this phenomenon can be alternatively interpreted as follows: because of the fast nucleation and slow growth of the nanocrystals (bound by alkylamine molecules), the quick monomer injection results in the formation of a large quantity of “magic sized cluster” in the solution, which provides enough kinetic drive to break the intrinsic symmetry structure of zinc blend. Thus, the formation of kinetically stable wurtzite ZnS with the elongated rod shape is facilitated in the HDA + OA solution under the kinetic growth regime. As for the mixed ligand (HDA + TOP) solution system, since the single-source precursor was first dissolved in the TOP ligand solution, the initially formed ZnS nuclei are exclusively surrounded by TOP molecules and hence the formation of sphalerite nuclei is preferred. Subsequently, because the binding of ligand molecules to the nuclei (or nanocrystals) is dynamic, it leaves an opportunity for HDA molecules in the mixed solution to adsorb onto the ZnS nuclei (or nanocrystals) and the adsorbed HDA molecules affect the following growth process of the nanocrystals. Therefore, the weight ratio of TOP: HDA in the ligand solution affected the shape and phase structure of the products because there exists the adsorption competition between TOP and HDA molecules on the nanocrystals surface. 3.4. Optical Properties of ZnS Nanocrystals. The difference in optical properties for different shapes and phase structures of the semiconductor nanocrystals has attracted significant attention recently. Here we investigated the absorption and photoluminescence properties of the ZnS nanocrystals. Figure 10 shows the absorption spectra and luminescence spectra of four representative samples: two (samples a and b) are wurtzite ZnS nanorods with different diameters and aspect ratios, prepared in HDA + OA solution; the others (samples c and d)

ZnS Nanorods and Nanodots

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Figure 10. (A) UV-vis absorption and PL spectra (λex ) 290 nm) of (a) thin and long wurtzite nanorods formed at 150 °C (sample a), (b) thick and short wurtzite nanorods formed at 200 °C (sample b), (c) sphalerite nanodots formed in HDA + TOP (sample c), (d) sphalerite nanodots formed in neat TOP (sample d); (B) UV-vis absorption and PL spectra (λex ) 290 nm) of sample a with different reaction times.

are sphalerite ZnS nanodots synthesized in HDA + TOP (2:1) mixed solution and neat TOP solution, respectively. The absorption edges (or peaks) of the samples all display an obvious blue-shift compared to the bulk ZnS (335 nm) because of the quantum confinement. Obviously, the intensity of quantum confinement in the nanocrystals is determined by their diameters no matter whether they are nanorods or nanodots, and the quantum confinement in the nanorods is mainly along the diameter direction of the rods. The diameters of the samples calculated by the Wang equation60,61 are 2.1, 2.6, 3.5, and 3.0 nm for samples a-d, respectively, which are a little smaller in comparison with those observed by TEM. Interestingly, there are two peaks existing in the absorption spectrum of sample a: a sharp peak at 282 nm and a shoulder peak at 290 nm. Moreover, we found that, as the reaction proceeded, the longer wavelength peak turned stronger but the shorter wavelength peak lost its intensity, while the position of the two peaks did not change (as shown in Figure 10B). One possible explanation for this phenomenon is that there existed two stable statesstwo diameter sizes for the nanorods in sample a. However, because their diameter is in strong quantum confinement range, the difference of the two sizes (ca. 0.2 nm estimated by UV-vis) is so little that it is difficult to distinguish by TEM observation. In the growing process, probably, the increase of the diameter of the nanocrystals was step by step; i.e., the diameters of the thick nanorods would not increase until the thinner nanorods completely turned to the thick ones, which maintained rather narrow size distribution for the diameters of the nanorods grown in this system. This phenomenon was only observed in the system for sample a, where the nanocrystals possess high aspect ratio and their diameter is in the strong quantum confinement range. The photoluminescence of ZnS nanocrystals is intricate, because it is sensitive to synthetic conditions and crystal sizes and shapes.62-65 Usually, two emission peaks can be observed from semiconductor nanoparticlessexcitonic and trapped lu-

minescence. The excitonic emission is sharp and locates near the absorption edge of the particle, while the trapped emission is broad and stokes-shifted.66,67 However, much literature66-68 has reported that only the trapped luminescence is observed in the pure ZnS nanoparticles. In this work, as shown in Figure 10A, for the ZnS nanodots formed in HDA + TOP solution, there appear two emission peaks in their PL spectrumsthe sharp one is relatively strong and locates at 322 nm, which is attributed to the band edge (or excitonic) emission; the broad one is relatively weak and locates around 420 nm, which can be assigned to sulfur vacancies (or zinc vacancies) emission.27,69-70 For the nanodots formed in neat TOP, there show two broad emissionssone around 420 nm and the other around 460 nm in their PL spectrum, which can be attributed to sulfur vacancies emission and trapped surface states emission, respectively.69,70 The difference in PL spectra of the two nanodots indicates that the alkylamine molecules can more effectively passivate the surface of the nanocrystals than TOP molecules do. As for the two kinds of wurtzite ZnS nanorods, their PL spectra are very similar, and both show the same characters: the excitonic emission peak (322 nm) is very weak, and the interstitial atom emission peaks are strong (we attribute the 360 nm peak to interstitial sulfur emission and the 375 nm peak to interstitial zinc emission70). The peaks in the blue range are overlapped and relatively weak, which is usually assigned to sulfur vacancies or zinc vacancies emission. The difference in the above PL spectra implies that the point defect (deep traps) in the nanorods is more than that in the nanodots, which is the result of the high kinetic driven growth for the former. 3.5. Self-Assembly of ZnS Nanorods and Nanodots on a 2-D Scale. The ordered assembly of 1-D nanostructure onto an aimed substrate is interesting and intractable. At present, the effective manipulation strategies for the ordered assembly are very limited. We obtained 2-D arrays of the ZnS nanorods by self-assembly method as mentioned previously. But, how did the ordered arrays form and how can we adjust the assembly

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Li et al.

of 1-D nanorods by intention? To clarify these questions, we discuss the formation mechanism of the arrays first. In our experiments, the ZnS nanorods include an inorganic core surrounded by ligand molecules. Thus, the nanorod suspension can be treated within the hard-rod approximation with consideration of directional attraction between the rods.71 When we deposit a drop of the dilute solution of the nanorods onto the substrate (such as carbon-coated copper grid), with slow solvent evaporation, the solution in residual droplet will become more and more concentrated, so free volume available to each rods is smoothly reduced, and the rods tend to aggregate by aligning side by side due to directional capillary force and van der Waals attraction.35 As a result, the viscosity of the solution increases and finally it freezes the local liquidlike structure to form lyotropic crystalline phase.37,72 As long as the above orientation and aggregation process accomplishes before the completion of solvent evaporation, the ordered array of the nanorods on the substrate will be achieved on a local scale. In the above processes, the concentration of the nanorods in solution, the nature of the capping molecules, and the evaporation rate of the solvent are three main factors affecting the quality and the range of the assembly for the ZnS nanorods. Thus, we can adjust the assembly behavior of 1-D nanorods by variation of the three factors. To prove our opinion further, we performed two experiments, and the results are shown in Figure 11. The samples for Figure 11a,c were obtained by experiment 1, while the sample for Figure 11b was prepared by experiment 2, as described in the Experimental Section. As expected, we attained different 2-D ordered arrays on a relatively large region. In Figure 11a, the nanorods from a concentrated nanorods solution have an overall orientation order but with some overlap. Figure 11b (the TEM specimen was prepared by using the mixed solvents with toluene:ethanol ) 1:1) shows an interlinking structure of the nanorods which aligned side by side but varied orientation gradually. The interlinking assembly structure may result from the appearance of a ZnS-nanorods-rich region during the dewetting process (because of the stability difference of the ZnS nanorods in toluene and in ethanol, and the difference of evaporation rate between the two solvents). As far as the assembly of semiconductor nanodots is concerned, van der Waals interaction and dipolar attraction among nanodots are thought to be the main driving force for them to form ordered structure.72-73 Figure 11c is a typical TEM image of superlattice structure of the ZnS nanodots formed from a concentrated ZnS nanodots solution with slow solvent evaporation, in which most nanodots were hexagonally close-packed on a 2-D scale with some nanodots arranged on a 3-D mode. 4. Conclusion In summary, we developed a new and simple approach to synthesize ZnS nanocrystals on a large scale, i.e., thermolysing single-source precursor-zinc ethylxanthate in different ligand solutions with OA or TOP serving as precursor solvent. The shape- and phase-controlled synthesis of ZnS nanocrystals was realized by the selection of different ligand. In HDA + OA system, diameter- and aspect-ratio-tunable hexagonal wurtzite ZnS nanorods were formed by kinetic adjustment, some of which self-assembled into two-dimensional (2-D) aligned arrays. The wurtzite ZnS nanorods can be synthesized at 150 °C and are stable in the system, which is a fairly milder reaction condition than other methods for obtaining wurtzite ZnS nanorods. In the HDA + TOP system, a shape change from rod to dot and a phase transition from wurtzite to sphalerite simultaneously occurred with increasing content of TOP in the

Figure 11. TEM images of typical assembly structures of ZnS nanoparticles formed from (a) concentrated nanorods solution, (b) dilute nanorods solution with mixed solvent (toluene:ethanol ) 1:1), and (c) concentrated nanodots solution.

mixed solution, and the sphalerite nanodots were prepared in high TOP content or in neat TOP (or in neat TOA) solution. The results were consistently confirmed by HRTEM, SAED, and XRD experiments. On the basis of the investigation of the effects of ligand molecules on monomer coordination, nucleation process, and nanocrystals growth by FTIR, UV-vis spectra, and XPS, the mechanism of the shape- and phase-controlled growth of ZnS nanocrystals was tentatively proposed as the selective absorption of the ligand molecules due to their steric effect and the binding ability of the ligand affect the crystal structure of the initial “seed” and adjust their following growth.

ZnS Nanorods and Nanodots The absorption edge of the wurtzite ZnS nanorods shifted bluer than that of the sphalerite nanodots. In addition, there are two absorption peaks for the ZnS nanorods formed at 150 °C with high aspect ratio and diameter in strong quantum confinement range. The phenomenon was explained with the assumption of two stable statesstwo diameter sizes for the nanorods grown in the system. For the PL spectra of the ZnS nanocrystals, there is a relatively strong excitonic emission peak for the ZnS nanodots formed in the HDA + TOP solution, a relatively strong and broad trapped surface states emission for the ZnS nanodots formed in neat TOP, and a relatively strong interstitial atom emission for the ZnS nanorods. We also discussed the mechanism and the strategies of assembling the 1-D ZnS nanorods on a 2-D scale and attained the ordered nanorods and nanodots arrays on a relatively large scale. To our knowledge, this is the first report on the assembly of 1-D ZnS nanorods into 2-D arrays. Acknowledgment. This work was supported by the State Key Basic Research Project (No. 2001CB610507) and the “863” project, The Ministry of Science and Technology of China. References and Notes (1) Schlamp, M. C.; Peng, X. G.; Alivisatos, A. P. J. Appl. Phys. 1997, 82, 5837. (2) Mattoussi, H.; Radzilowski, L. H.; Dabbousi, B. O.; Thomas, E. L.; Bawendi, M. G.; Rubner, M. F. J. Appl. Phys. 1998, 83, 7965. (3) Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800. (4) Huynh, W.; Peng, X. G.; Alivisatos, A. P. AdV. Mater. 1999, 11, 923. (5) Huynh, W.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 225. (6) Han, W.; Fan, S.; Li, Q.; Hu, Y. Science 1997, 277, 1287. (7) Sha, J.; Niu, J.; Ma, X.; Xu, J.; Zhang, X.; Yang, Q.; Yang, D. AdV. Mater. 2002, 14, 1219. (8) Fu, M.; Zhu, Y.; Tan, R.; Shi, G. AdV. Mater. 2001, 13, 1874. (9) Wang, X.; Li, Y. J. Am. Chem. Soc. 2002, 124, 2880. (10) Zhue, D.; Zhu, H.; Zhang, Y. Appl. Phys. Lett. 2002, 80, 1634. (11) Yang, J.; Zheng, J.; Yu, S.; Yang, L.; Zhou, G.; Qian, Y. T. Chem. Mater. 2000, 12, 3259. (12) Gao, F.; Lu, Q. Y.; Xie, S. H.; Zhao, D. Y. AdV. Mater. 2002, 14, 1537. (13) Morals, A. M.; Lieber, C. M. Science 1998, 279, 208. (14) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700. (15) Adam, Z.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 1389. (16) Adam, Z.; Peng, X. G. J. Am. Chem. Soc. 2002, 124, 3343. (17) Remskar, M.; Mrzel, A.; Jesih, A.; Le´vy, F. AdV. Mater. 2002, 14, 680. (18) Terech, P.; Geyer, A.; Struth, B.; Talmon, Y. AdV. Mater. 2002, 14, 495. (19) Lu, Q. Y.; Gao, F.; Zhao, D. Y. Nano Lett. 2002, 2, 725. (20) Viau, G.; Piquemal, J. Y.; Esparrica, M.; Ung, D.; Chakroune, N.; Warmont, F.; Fie´vet, F. Chem. Commun. 2003, 1304. (21) Jiang, Y.; Meng, X. M.; Liu, J.; Xie, Z. Y.; Lee, C. S. Lee, S. T. AdV. Mater. 2003, 15, 323. (22) Zhu, Y. C.; Bando, Y.; Uemure, Y. Chem. Commun. 2003, 836. (23) Yu, S. H.; Yoshimura, M. AdV. Mater. 2002, 14, 296. (24) Deng, Z. X.; Wang, C.; Sun, X. M.; Li, Y. D. Inorg. Chem. 2003, 41, 869. (25) Chen, X. J.; Xu, H. F.; Xu, N. S.; Zhao, F. H.; Lin, W. J.; Lin, G.; Fu, Y. L.; Huang, Z. L.; Wang, H. Z.; Wu, M. M. Inorg. Chem. 2003, 42, 3100. (26) Wang, X. D.; Gao, P. X.; Li, J.; Summers, C. J.; Wang, Z. L. AdV. Mater. 2002, 14, 1732. (27) Jiang, X. C.; Xie, Y.; Lu, J.; Zhu, L.Y.; He, W.; Qian, Y. T. Chem. Mater. 2001, 13, 1213. (28) Bellotti, E.; Brennan, K. F.; Wang, R.; Puden, P. P. J. Appl. Phys. 1998, 83, 4765. (29) Simmons, B. A.; Li, S.; John, V. T.; Mcpherson, G. L.; Bose, A.; Zhou, W. L.; He, J. B. Nano Lett. 2002, 2, 263. (30) Kim, Y. H.; Jun, Y. W.; Jun, B. H.; Lee, S. M.; Cheon, J. W. J. Am. Chem. Soc. 2002, 124, 13656. (31) Yu, W. W.; Wang, Y. A.; Peng, X. G. Chem. Mater. 2003, 15, 4300.

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