Formation of Uniform Flowerlike Patterns of NiS by Macrocycle

(11) Then, in a typical synthesis, 1 mmol of NiSO4·6H2O was put in a glass bottle, and 40 mL of distilled water was added, and then 4.0 g of CT was a...
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CRYSTAL GROWTH & DESIGN

Formation of Uniform Flowerlike Patterns of NiS by Macrocycle Polyamine Assisted Solution-Phase Route Zhengcui Wu,†,‡ Cheng Pan,† Tanwei Li,† Gangjin Yang,† and Yi Xie*,† Department of Nanomaterials and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei 230026, P. R. China, and Anhui Key Laboratory of Functional Molecular Solids, College of Chemistry and Materials Science, Anhui Normal UniVersity, Wuhu 241000, P. R. China

2007 VOL. 7, NO. 12 2454–2459

ReceiVed NoVember 5, 2006; ReVised Manuscript ReceiVed July 3, 2007

ABSTRACT: A ligand family of macrocycle polyamines was first used to assist the construction of elegant inorganic microstructures. As an example, hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene (CT) was applied to the morphology control of NiS microstructures at a suitable temperature and ambient atmosphere. Highly uniform NiS flowerlike patterns with a size of 5.5–6.5 µm, which are composed of nanorods with a width range from 160 to 350 nm and a length of up to 2 µm, can be prepared on a large scale by the effect of the coordination of CT in a water system. The diffusion-limited aggregation (DLA) mechanism has been applied for production of hierarchical flowers. The experimental results demonstrated that the microstructures of the flowers strongly depended on the reaction conditions, such as the concentrations of the reactants and CT. This reaction system could be extended to the morphogenesis of other inorganic nano- and sub-micromaterials with novel morphologies and complex forms. Introduction During the past few decades, much effort has been focused on the design of rational methods for synthesizing higher ordered metal sulfide nanomaterials with specific sizes, shapes, and hierarchies because of the potential to design new materials and devices in various fields.1 Nickel monosulfide (NiS) has been the subject of considerable interest because of its properties as a metal–insulator and paramagnetic–antiferromagnetic phase changing material, and its use in hydrosulfurization catalysts and solar storage.2 A variety of elegant and efficient techniques have been carried out on the shape- and size-controlled growth ofNiScrystals,amongwhichnanorodsandtriangularnanoprisms,3,4 thin films,5 and hollow spheres6 have been successfully synthesized using solventless thermolytic decomposition or soft templates. Our group and Qian’s group have reported the availability of NiS three-dimensional (3D) microstructures with sheet-composed flowers via a polymer assistant and nanoneedles or nanobelts via dithizone as a sulfur source and ethylenediamine as a solvent, respectively.7,8 In the developed methods for generating metal sulfide nanostructures, short-chain liquid alkylamines such as ethylenediamine, n-butylamine, and diethylenetriamine have often been used as the solvent and structure-directing coordination molecular template.8,9 There are also some reports on the synthesis of metal sulfide nanostructures with long-chain solid alkylamines such as hexadecylamine, dodecylamine, etc., as the coordinate agent, surfactant, or organic template,10 but all of these structures are insoluble in water. Moreover, most of the water-soluble amine compounds in the synthesis of metal sulfides are liquid at present; the usage of abundant liquid amine will limit the extension of the method due to the indissolubility of some reactants and its irritative odor. The development of simple and effective methods for creating novel assemblies of self-supported patterns of hierarchically fractal architectures using a suitable water-soluble solid amine compound assisted * To whom correspondence should be addressed. Tel and Fax: 86-5513603987. E-mail: [email protected]. † University of Science and Technology of China. ‡ Anhui Normal University.

solution-phase route is important to technology and remains an attractive but elusive goal. Fortunately, a good candidate of macrocycle polyamine exists for the morphology construction of inorganic materials because the richness of the water-soluble macrocycle polyamine makes it possible to provide various kinds of ligands for different metals in a homogeneous water system. In this paper, we demonstrate a new example of the morphogenesis of NiS flowerlike patterns with the assistance of watersoluble solid macrocycle polyamine-hexamethyl-1,4,8,11tetraazacyclotetradeca-4,11-diene (CT). Uniform flowerlike patterns composed of individual nanorods or well-aligned nanorods can be produced on a large scale showing that CT can be used for the morphogenesis of microstructures. To our best knowledge, this is the first report of the synthesis of NiS flowerlike patterns using CT under mild reaction conditions. Experimental Section All chemicals were of analytic grade purity and used as received without further purification. The macrocycle CT was synthesized using methanol, ethylene diamine anhydrous, hydrobromic acid, and acetone as reactants according to the literature.11 Then, in a typical synthesis, 1 mmol of NiSO4 · 6H2O was put in a glass bottle, and 40 mL of distilled water was added, and then 4.0 g of CT was added into the bottle under stirring. Finally, 1 mmol of CS(NH2)2 was added, and the mixture was further stirred for about 15 min. Then, the transparent green solution was put in a 50 mL Teflon-sealed autoclave and then maintained at 180 °C for 36 h. The formed black precipitate was collected by centrifugation of the mixture and then washed with distilled water and ethanol three times. The structure of these obtained samples was characterized with the X-ray diffraction (XRD) pattern, which was recorded on a Rigaku Dmax diffraction system using a Cu KR source (λ ) 1.54187 Å). The scanning electron microscopy (SEM) images were taken with a JEOL-JSM-6700F field emission scanning electron microscope (FE-SEM, 15 kV). Transmission electron microscopy (TEM) images and high-resolution transmission electron microscopy (HRTEM) images were obtained with a Hitachi 800 system at 200 kV and a JEOL-2010 system at 200 kV, respectively. X-ray photoelectron spectroscopy (XPS) on the products was performed using an ESCALAB MK II X-ray photoelectron spectrometer and nonmonochromatized Al-Mg KR X-rays as the excitation source. The room temperature photoluminescence (PL) spectra were performed on a Jobin Yvon-Labram spectrometer with a He-Cd laser. A confocal fluorescence image was recorded with an

10.1021/cg0607856 CCC: $37.00  2007 American Chemical Society Published on Web 10/25/2007

Formation of Flowerlike Patterns of NiS

Figure 1. XRD pattern of the as-prepared flowerlike NiS product. The diffraction peaks can be indexed to hexagonal phase of NiS, indicating high crystallinity.

Figure 2. XPS survey spectra of the NiS flowerlike patterns. (a) S 2p; (b) Ni 2p3/2. inverted Olympus microscope (IX-70) by dropping aqueous dispersions of the NiS samples onto glass slides.

Results and Discussion The crystal structure and phase composition of NiS products were first characterized using XRD. Figure 1 displays a representative XRD pattern of the as-prepared NiS samples, suggesting their high crystallinity. The diffraction peaks can be readily indexed to the hexagonal phase of NiS with lattice parameters of a ) 9.609 Å and c ) 3.138 Å, which are consistent with the standard values (JCPDS Card, No. 12-41). The composition and the purity of the products were examined by XPS. The binding energy values of 852.9 eV for Ni 2p3/2 and that of 160.77 eV for S 2p are authenticated by the XPS spectra (Figure 2). The ratio of integral area for Ni 2p3/2 to S 2p is about 1.1:1, which is close to stoichiometric ratio of NiS.

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The micrometer-sized flowerlike patterns were successfully synthesized on a large scale, as revealed in Figure 3a where a panoramic field emission scanning electron microscopy (FESEM) image of the product is displayed. Figure 3a not only shows that the product consists almost entirely of such flowerlike architectures but also gives the information that flowers of high yield and good uniformity can be easily achieved with this simple and easily controlled approach. The sizes of the flowers are mostly in the range of 5.5–6.5 µm. Figure 3b presents a high magnification FESEM image of a single flower, revealing that this kind of elegant architecture consists of nanorods with diameters of 160–350 nm and lengths of up to 2 µm. Careful observation also finds that most of the nanorods have a tendency to split, indicating that the flowerlike pattern obtained in our CT system is obviously different from familiar urchinlike morphologies.8 The morphology and structure of the flowerlike NiS product were further detected by TEM, selected area electron diffraction (SAED), and HRTEM. Figure 3c shows a representative TEM image of an individual NiS flowerlike pattern. The high-magnification TEM image of the nanorods (Figure 3d) suggests that the tip of the nanorod in these patterns contains many small nanorods with a parallel-arranged assembly, confirming the above FESEM observation. Figure 3e shows the TEM image of a single nanorod obtained by violent ultrasonic treatment and its corresponding SAED pattern. The obtained j zone axis could be SAED spots projected along the [111] indexed as millerite NiS (110) and (011) planes, indicating that the nanorod is a single crystal grown in a direction parallel to the (110) lattice plane. The conclusion is also supported by the HRTEM observations, in which the clearly observed lattice fringes shown in Figure 3f,g taken from the regions labeled f and g in Figure 3e reveal that the NiS nanorods in the flowerlike patterns are highly crystalline. The crystal planes have lattice spacings of about 0.48 and 0.29 nm corresponding to (110) and (011) planes, respectively. This result further verifies that the preferential growth occurred parallel to the (110) lattice plane, which is in good agreement with SAED observations. Moreover, the split nanorods retain the same orientation as exhibited in Figure 3g. Through extensive investigations on individual nanorods from the NiS products with SAED, it is found that this orientation was maintained in all of the products. Although there was thiourea in the solution, the coordination ability of thiourea is much smaller than that of the macrocycle polyamine molecule, CT plays a more important role in coordination with Ni ions since CT has very strong coordination ability with it. First, a suitable amount of CT can effectively coordinate with Ni ions in the initial reaction stage and kinetically control the growth rate of flowerlike NiS microstructures. Being a kind of cycle ligands which can completely enclose metal ions, it is well known that CT could much easily coordinate with Ni ions, and the coordination of tetradentate macrocyclic ligands in a square planar fashion, which significantly affects the growth of NiS flowerlike patterns. It is easily supposed that macrocycle polyamine metal complexes prevent the production of large free metal ions; at the same time, thiourea is also a kind of mild sulfur and releases sulfur ions slowly, which thus are favorable for the oriented growth of the final product. To understand the growth mechanism of the NiS microstructures accurately, it is necessary to investigate the morphology of the intermediates involved in the formation. Herein, three samples were collected at different stages for observation, and their FESEM images are shown in Figure 4. When the hydrothermal reaction was processed at 180 °C for 1 h, the

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Figure 3. Typical FESEM and TEM images of the obtained NiS flowerlike patterns. (a) FESEM image of NiS flowerlike patterns at a low magnification, indicating that the flowerlike NiS can be fabricated on a large scale. (b) FESEM image of NiS flowerlike pattern at a high magnification, revealing the morphology of a single flower of NiS. (c) TEM image of the same NiS flower. (d) High-magnification TEM image of nanorods, suggesting that the tip of the rod in these patterns contains many small nanorods with a parallel-arranged assembly. (e) A single nanorod and its corresponding SAED pattern, exhibiting the single-crystalline nature of the nanorod. (f, g) High-resolution TEM images recorded in different regions of the nanorod in (e) showing the well-defined single-crystalline nature of the nanorod. The fringes of 0.48 and 0.29 nm were in accordance with the separation between the neighboring lattices of the (110) and (011) planes, respectively. The HRTEM image in (g) also reveals the same orientation of the split nanorod.

spherelike product with many short nanorods on its surface was observed, as shown in Figure 4a. Figure 4b displays the image of the sample reacting for 3 h, revealing that the intermediate after reacting for 3 h was a claviform bundle small flower with well-aligned nanorods. The aligned rodlike flower turned into the flowerlike pattern with dispersed nanorods at 5 h, as seen from the FESEM image shown in Figure 4c, and then at 36 h, a beautiful NiS flowerlike pattern composed of nanorods was observed as shown in Figure 3a,b. The whole process clearly shows the diffusion-limited aggregation (DLA) phenomenon,12 and the DLA model is applied to explain the formation process, which is displayed in Scheme 1. At an elevated reaction temperature, the coordination between the CT molecule and Ni2+ was weakened, and simultaneously, S2- was gradually released from the decomposition of thiourea and attacked coordinated Ni ions from the upside and underside of the coordinate plane, which further weakened the coordination between the CT molecule and Ni2+; NiS seeds gradually formed at the expense of destroying the coordination between Ni2+ and CT. Then these fractal nuclei rapidly developed into aggregated

particles. In the subsequent growth step, the coordination of tetradentate macrocyclic ligands occurs in a square planar fashion, which leads to the growth of particles in a onedimensional direction, thus forming nanorods. As the growth continues, with respect to the large lattice energy of NiS, rearrangement and diffusion of NiS crystallites are limited to a certain extent; in addition, excess CT surrounds and adsorbs on NiS particles and limits the growth of the nanorods in length, and the nanorods begin to grow in parallel directions and form well-aligned nanorods. As nanorods grow further, bigger aligned nanorods split into smaller ones to satisfy the spatial requirements of the crystal growth during the “Ostwald ripening” process, and 3D flowerlike patterns of NiS gradually formed. That is to say, with the coordination and the subsequent adsorption of CT, the DLA model can be rationally used to discuss the growth and morphological control of NiS flowerlike patterns, which are similar to that of ZnO nanoparticles.13 On the basis of the above mechanism, we can reasonably assume that a suitable amount of CT is critical for the formation of NiS flowerlike patterns and the adsorption of CT with

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Figure 4. Typical FESEM images of NiS samples with different reaction times: (a) 1 h, (b) 3 h (inset is a magnified individual flower), (c) 5 h.

Scheme 1. Proposed Growth Process for the Formation of Nickel Monosulfide Flowerlike Patterns with Nanorods

different quantities will lead to a difference in the flowerlike patterns. A series of contrastive experiments were done at a given growth temperature to verify the mechanism. When the mass of CT is increased to 4.5 g while keeping the moles of NiSO4 · 6H2O and CS(NH2)2 constant at 1 mmol and the reaction temperature at 180 °C for 36 h, the flowerlike structures became denser and the nanorods became wider and shorter and formed aligned nanorods (exhibited in Figure 5a,b). When the mass of CT is further increased to 5.0 g while keeping other parameters constant, a further increase in the width and decrease in the length of the nanorods is observed (about 500 nm in width and 890 nm in length) and the buds in these patterns contain many fine nanorods with a parallel-arranged assembly (Figure 5c,d). The higher concentration of CT, the denser the flowers, which

suggests that increased concentration of CT limited the diffusion of NiS crystallites and further limited the growth of NiS in length while increased the growth opportunity in a parallel direction that led to the final dense structures, which supported the DLA mechanism. In addition, when CT was below 3.5 g with other parameters constant, NiS flowers accompanied with irregular shapes formed and many small particles attached to the flowers (Figure 5e, 5f), which may be the reason that the amount of CT was too little to adsorb and entirely cover the whole areas of the nanorods, which cannot completely limit the diffusion of NiS crystals in a certain range during the growth process, leading to a different growth manner of NiS crystals with adsorbed or unadsorbed CT on NiS seeds.

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Figure 5. FESEM images of NiS flowerlike patterns obtained under different reaction conditions. (a, b) 4.5 g of CT for 36 h. (c, d) 5.0 g of CT for 36 h. The higher magnification image was also inserted in top right corner of panel d, further indicating the rod structure of the NiS flowers. (e, f) 3.5 g of CT for 36 h. (g, h) 5.0 g of CT for 96 h. These samples were prepared from the mixture of 1 mmol of NiSO4 · 6H2O, 1 mmol of CS(NH2)2, and different masses of CT in a 40 mL water system at 180 °C for different reaction times.

The DLA model can also be clearly observed in other Ni2+CT systems. For example, when the reaction time was extended from 36 to 96 h with the system containing 5.0 g of CT, 1 mmol of NiSO4 · 6H2O and 1 mmol of CS(NH2)2 at 180 °C, wellaligned bundles in a flowerlike pattern as shown in Figure 5c,d split into individual nanorods to make up the flower, shown in Figure 5g,h. The DLA mechanism also implies the important role of kinetic control on the morphology of the product. When the concentration of NiSO4 · 6H2O, CS(NH2)2 and CT were decreased simultaneously (with 0.5 mmol of NiSO4, 0.5 mmol of CS(NH2)2, and 3.0 g of CT in a 40 mL water system), the

flowers with a diameter of 3.3–3.9 µm composed of very thin nanorods (average width 50 nm) could be obtained, as exhibited in Figure 6a,b. Careful observation also shows that the nanorods are also composed of a parallel-arranged assembly of nanorods, suggesting that there are more kinetic factors on the shape control of NiS flowerlike patterns by altering the concentrations of the reactants and CT. It should also be mentioned that the reaction temperature has a significant effect on the yield of the products. A high yield of products could be produced over a temperature range from 180 to 200 °C. If the temperature is lower than 150 °C, the growth is too low. The reaction temperature increases the growth rate but does

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Figure 6. (a, b) FESEM images of NiS flowerlike patterns obtained with 0.5 mmol of NiSO4 · 6H2O, 0.5 mmol of CS(NH2)2, and 3.0 g of CT in a 40 mL water system at 180 °C for 36 h.

other inorganic semiconducting nano- and sub-micromaterials such as metal chalcogenide semiconductors with novel morphologies and complex forms since macrocycle polyamine is a large family and can coordinate with many metal ions. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 20621061) and the state key project of fundamental research for nanomaterials and nanostructures (2005CB623601).

References

Figure 7. Fluorescence spectrum (λex ) 274 nm) and the corresponding confocal fluorescence image (inset) of NiS flowerlike patterns obtained with 1 mmol of NiSO4 · 6H2O, 1 mmol of CS(NH2)2, and 4.0 g of CT in a 40 mL water system at 180 °C for 36 h.

not significantly affect the morphology of the flowerlike crystals. Increased temperature weakens the coordination between the CT molecule and Ni2+, which speeds the overall growth rate by increasing Ni2+ availability to the growing nanocrystals. The optical property of the product was also investigated. Figure 7 shows the fluorescence spectrum and the corresponding confocal fluorescence microscopic image of the flowerlike NiS sample. Under PL excitation at 274 nm, there was a broad emission around 400 nm, and the top of the emission peak was separated into two peaks located, respectively, at 393 and 408 nm, which may be due to electronic transitions caused by defects in the interfacial region.14,15 That is, the separation of the emitting peaks may be attributed to the existence of the crystal imperfections, such as point defects, dislocations, and grain boundaries in the sample of the flowerlike NiS crystals. The inset in Figure 7 shows a typical confocal image of the NiS flowerlike patterns with violet emissions, verifying that the fluorescence wavelength was in the violet-blue region of the visible light. Conclusion In conclusion, NiS flowerlike patterns composed of nanorods or well-aligned nanorods have been successfully prepared in high yield using a macrocycle polyamine assisted solution-phase route in a water system. The growth conditions are simple and also can easily be done for large-scale preparations. Timeresolved and contrastive experiments provide and confirm a diffusion-limited aggregation mechanism by which it is possible to control the various architectures. This approach opens a new route for the morphogenesis of NiS material, and it is highly possible to rapidly extend it as a general synthetic method for

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