Controlled Hydrothermal Synthesis of Nickel Phosphite Nanocrystals

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CRYSTAL GROWTH & DESIGN

Controlled Hydrothermal Synthesis of Nickel Phosphite Nanocrystals with Hierarchical Superstructures Gu,†,‡

Zhanjun Tianyou Jiannian Yao*,†

Zhai,†

Bifen

Gao,†

Guangjin

Zhang,†

Damei

Ke,†

Ying

Ma,*,†

and

2007 VOL. 7, NO. 4 825-830

Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Science, Beijing 100080, P. R. China, and Graduate School of the Chinese Academy of Science, Beijing 100080, P. R. China ReceiVed NoVember 2, 2006; ReVised Manuscript ReceiVed January 15, 2007

ABSTRACT: The large-scale synthesis of the hexagonal Ni11(HPO3)8(OH)6 nanocrystallites with delicate morphologies, such as nanoparticles, ellipsoidal rods, dumbbell-like and urchinlike superstructures based on nanorods as well as hollow spheres made of nanoparticles, have been prepared via a very simple hydrothermal method without any templates or catalyst. It is found that the morphology of the final products is strongly dependent on the reaction conditions, such as pH, temperature, and reaction times. A “rod-to-dumbbell-to-sphere” growth mechanism has been proposed for the formation of the urchinlike superstructures. 1. Introduction One-dimensional nanostructures, such as nanotubes, nanowires, nanorods, and nanobelts, have attracted intensive attention because of their novel physical properties and potential application in the fabrication of nanoscale devices. Recently, special attention has been focused on the fabrication of the hierarchically ordered, branched architectures based on 1D nanocrystals for the practical needs of nanoelectronics and nanophotonics.1,2 Various methods, such as the catalytic vapor-liquid-solid reaction,3 laser-assisted catalytic growth,4 template-based design techniques,5 and solution-based self-assembly routes,6 have been applied to fabricate these architectures. For example, hierarchical SiO2-Si nanostructures were synthesized via a chemical vapor deposition (CVD) process;7 penniform BaWO4 nanostructures were obtained in catanionic reverse micelles involving polymers;8 a trigonal Se nanowire network and dandelion-like architectures have been prepared by a hydrothermal method.9 However, in the most of the previous approaches, the templates or catalysts are essential for the creation of these hierarchical architectures. It is obvious that the introduction of templates or catalysts induces heterogeneous impurities, increases the production cost, and leads to difficulty for scale-up production. Thus, the development of facile, mild, and effective methods for the formation of template- and catalyst-free hierarchical architectures is still a challenge to material scientists. Compounds with open-framework structures are of great interests in material science as well as in chemistry, because of their applications as catalysts, ion-exchangers, or molecular sieves. Since Attfied and co-workers first reported the synthesis of M11(HPO3)8(OH)6 (M ) Zn, Co, and Ni) in 1993, these transition metal phosphites have attracted extensive interests because of their microporous structures and novel potential catalytic, electrical, optical, and magnetic properties.10 Meanwhile, the shape and size of inorganic materials have also been regarded as critical factors in varying their electrical, optical, and other properties. So the synthesis of transition metal phosphites with well-controlled morphology and size should be of great significance. Very recently, many efforts have been * Corresponding author. E-mail: [email protected] (J.N.Y.); yingma@ iccas.ac.cn (Y.M.). Fax: 86 10-82616517. Tel: 86 10-82616517. † Beijing National Laboratory for Molecular Sciences, Chinese Academy of Science. ‡ Graduate School of the Chinese Academy of Science.

focused on the preparation of these transition metal phosphate/ phosphite with 1D nanostructures.11 However, to the best of our knowledge, there is no report on the synthesis of Ni11(HPO3)8(OH)6 with a hierarchical structure made of onedimensional nanoscale building blocks. Here, we report a simple hydrothermal route to fabricating a novel hierarchical structure of Ni11(HPO3)8(OH)6. Urchin-like architectures based on 1D nanoscale building blocks have been successfully prepared in a large scale. 2. Experimental Section Materials. All the chemicals were of analytical grade and used without further purification. NiCl2‚6H2O, KH2PO4, HCl, KOH, KNO3, NaCl, KCl, urea, and CTAB were purchased from Beijing Chemical Reagent company. Synthesis. In a typical synthesis, NiCl2 (0.474 g, 2 mmol) and KH2PO4 (0.273 g, 2 mmol) were added to 20 mL of distilled water under stirring. The solution mixture was put in a Teflon-lined stainless steel autoclave and heated at 180 °C for 1-24 h. After reaction, the green precipitate was filtered, washed with water to remove ions possibly remaining in the final products, and dried at 60 °C. Following the above procedures, we obtained a high purity of Ni11(HPO3)8(OH)6 with hierarchical structure on a large scale. The typical yield obtained in our experiments was about 70% (based on Ni atoms). To study the formation process of the products, the experimental parameters have been varied during the synthesis. The influences of the additives on the morphology of the final products were also investigated. Various additives, including urea (0.06-2 g), CTAB (4-80 mg), NaCl (0.2-4 g), KCl (0.2-4 g), and KNO3 (0.2-6 g), were tested in this work under typical conditions for the urchinlike sample formation. Characterization. XRD analysis was performed using a Japan Rigaku D/max-2500 diffractometer with CukR radiation (λ ) 1.5418 Å). The sizes and shapes of the nanowires were observed on field emission scanning electron microscope (SEM, Hitachi, S-4300) and high-resolution transmission electron microscope (HRTEM, JEOL JEM2010 operated at 200 kV). XPS measurements were carried out with an ESCALab220i-XL spectrometer by using a twin-anode Al KR (1486.6 eV) X-ray source. All the spectra were calibrated to the binding energy of the adventitious C1s peak at 284.6 eV. The base pressure was about 3 × 10-7 Pa. For diffuse reflectance, magnetic measurements were performed on powdered samples at temperatures in the range 5.0300 K, using a Quantum Design MPMS-7 SQUID magnetometer. The magnetic field was approximately 1000 Oe, a value in the range of linear dependence of magnetization vs magnetic field, even at 5.0 K.

3. Results and Discussion Hydrothermal treatment of NiCl2 solution in the presence of KH2PO4 at 180 °C for 24 h leads to the formation of

10.1021/cg060774c CCC: $37.00 © 2007 American Chemical Society Published on Web 03/09/2007

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Figure 1. (a) XRD patterns of the as-prepared products. (b-d) XPS spectra of the as-obtained products: (b) Ni 2p, c) P 2p, (d) wide XPS spectrum of the as-prepared Ni11(HPO3)8(OH)6 products.

Ni11(HPO3)8(OH)6 with hierarchical superstructures. The XRD pattern of the as-prepared sample shown in Figure 1a could be indexed to the hexagonal phase of Ni11(HPO3)8(OH)6 with the unit cell parameters a ) 1.270 nm and c ) 0.494 nm and space group P63mc. (JCPDS card 81-1065) No peaks of other phosphites or phosphates were detected from this pattern. The peaks are strong and narrow, which indicates the good crystallinity of the as-prepared sample. X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical state of the atoms of Ni and P in the samples. The peaks at 857.2 eV displayed in Figure 1b can be attributed to Ni 2p3/2. In addition to this major peak, a strong satellite peak around 863.4 eV was also presented, which is ascribed to a multielectron excitation (shakeup peak).12a-c The peak at 875.2 eV corresponds to Ni 2p1/2, and its shakeup peak was also observed on the higher binding energy (around 881.5 eV). Similar satellite peaks for the Ni2+ have been reported by previous researchers.12d,e The peak at 133.5 eV in Figure 1c can be assigned to P 2p, which shows that all P atoms are in the +3 oxidization state.12f No peaks of elements other than Ni, P, and O are observed in the wide XPS spectrum, which further confirms the high purity of the Ni11(HPO3)8(OH)6 products (Figure 1d). The SEM images of as-prepared samples are shown in Figure 2. The overall morphology of the samples, as shown in Figure 2a, indicates that there exists a great deal of microspheres with diameters ranging from several to tens of micrometers. Close observation (Figure 2b) reveals that these spherical structures are composed of numerous nanorods and take on an urchinlike appearance. A few nanorod bundles in Figure 2b can be clearly seen near the spheres, as indicated with an arrow, which

is probably separated from the microspheres. The diameters and lengths of these uniform nanorods are about 50-200 nm and 5-10 um, respectively, which are varied by the reaction time. More careful observation of a typical urchin-like structure, as shown in Figure 2c, indicates that numerous nanorods with very high density grow pointing toward the center of the sphere. More details of the morphological and structural features are studied using TEM and HRTEM. It is worth noting that the urchinlike superstructures can be decomposed into the nanorod bundles and individual nanorods by sonication. The typical TEM image (Figure 3a) of these nanorods indicates these rods take on a shuttlelike appearance. The spacing of the lattice fringes was found to be about 1.10 and 0.492 nm, respectively, as shown in Figure 3b. These two planes could be well-indexed as (100) and (001) planes of the hexagonal Ni11(HPO3)8(OH)6 crystal, respectively. The FTT pattern shown in the inset of Figure 3b could be indexed perfectly, indicating that the nanorod grows along the [001] direction. The energy-dispersive spectrum (EDS) of the nanorods shows that only Ni, P, and O are contained in the samples, in which the Cu, Cr, and C peaks were generated from the supporting carbon-coated copper meshes. These results are in good agreement with the XPS results (Figure 3c). To understand the formation mechanism of the urchinlike structures, we carried out time-dependent experiments. An examination of the intermediate products collected after 1 h showed that there were a large number of rods with diameters of 200-500 nm. With an increase in the reaction time, these nanorods started to branch at their ends and further developed into dumbbell-like superstructures. By further prolonging the reaction time to 8 h, we can obtain the closed spheres based on

Nickel Phosphite Nanocrystals with Hierarchical Superstructures

Figure 2. Representative SEM images for the urchinlike assemblies of Ni11(HPO3)8(OH)6. (a) Overall morphology of the products. (b, c) High-magnification SEM image of the samples.

the nanorods (Figure 4). Interestingly, some hemispheres and unsymmetrical spheres were also found in images e and f of Figure 4. These intermediates suggest that the final stage of spheres results from a complex growth mechanism, which was called the “rod-to-dumbbell-to-sphere” transformation mechanism. In this mechanism, rodlike particles are formed first, which can grow at their ends resulting in dumbbell-like particles. These dumbbells grow further into closed spheres when the reaction is performed for a long enough time.13 Co¨elfen and co-workers have also reported on the synthesis of calcium carbonate

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aggregates with similar morphology evolution from spheres to rods to dumbbells, and spheres in the presence of the doublehydrophilic block copolymers (DHBCs).14 They believed that the specific interaction between the DHBCs and the crystal surfaces of calcium carbonate may have played an important role in controlling the morphology of the products. But, in our work, we found that the special morphology of the products is due to the crystallization conditions (mainly pH and reaction time) and does not require any additional templates or catalysts. Therefore, it is reasonable to presume that the inherent unusual growth habits of the Ni11(HPO3)8(OH)6 crystals play important roles in the formation of the final morphologies. In addition, the “rod-to-dumbbell-to-sphere” transition seems to be a general crystal growth phenomenon and was observed for several carbonate systems (CaCO3, BaCO3, MnCO3, and CdCO3) as well as a fluoroapatite system (Ca5(PO4)3OH).14-16 The synthesis parameters, such as the pH value and temperature, play important roles in controlling the morphology and composition of the final products. Controlled experiments have been carried out to investigate the influence of pH on the reaction. The urchin-like microspheres obtained in a large scale could be prepared only when the reaction system was adjusted by KOH and HCl to the pH range of 3-4.5 (Figure 5a). With an increase in pH value to 5, the branched rods were generated (Figure 5b). By further increasing the pH value to 6, we observed the rods with smooth instead of branched ends (Figure 5c). At pH 7, the morphology of the sample changed from nanorods to nanoparticles with a diameter of 50-100 nm (Figure 5d). Above pH 8, only irregular particles with undefined crystal structures are generated. This indicates that the branching of the nanorods is inhibited at high pH values. On the other hand, when the pH value was lower than 2, no products were obtained because the precipitate could dissolve in such an acidic solution. Although the variation in pH drastically changed the morphology of the as-prepared products, the crystal form of all the products obtained was not changed when the pH value was below 8. The optimal temperature for the rapid production of highly crystalline urchinlike products is 160-180 °C. If the temperature is lower than this, the microplate will be obtained. (see the Supporting Information) It is believed that the presence of additives can strongly influence the morphology of growing crystals. Therefore, we carried out the crystallization under the typical conditions for urchinlike sample formation in the presence of various additives, including urea, CTAB, NaCl, KCl, and KNO3. When the experiment was conducted in the presence of 0.06 g of urea, only the branched rods of Ni11(HPO3)8(OH)6 were obtained from

Figure 3. (a) TEM image of the product. (b) HRTEM image of the tip of an individual nanorod from a microsphere. (c) EDS spectrum for the urchinlike superstructures of Ni11(HPO3)8(OH)6, in which the Cu, Cr, and C peaks were generated from the supporting carbon-coated copper meshes. Inset in part b shows the FTT pattern of the nanorod.

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Figure 4. SEM images of intermediate products: (a) 1, (b) 2, (c) 3, and (d) 8 h; (e) hemisphere broken perpendicular to the long seed axis; (f) unsymmetrical sphere.

Figure 5. SEM images for the products prepared at different pH values: (a) 4, (b) 5, (c) 6, and (d) 7.

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Figure 6. SEM images for our products prepared by adding different amounts of urea: (a) 0.06 g; (b) typical image of the branched nanorod in part a; (c) 0.12 g; (d) detailed view of a typical hollow sphere in part c; (e) 0.2 g; (f) 2 g. Inset in part d shows the typical TEM image of the hollow sphere.

the hydrothermal treatment at 180 °C for 24 h (images a and b of Figure 6). The addition of 0.12 g of urea to the reaction system leads to the formation of Ni11(HPO3)8(OH)6 hollow microspheres (Figure 6c). Close observation reveals that these hollow microspheres are made of nanoparticles, as shown in Figure 6d. With an increase of the dosage of urea from 0.2 to 2 g, the nanorods and microrods emerged (images e and f of Figure 6). Results from the XRD characterizations reveal that the composition and crystal form of these products also changed from hexagonal Ni11(HPO3)8(OH)6 to an undefined forms of nickel phosphate (see the Supporting Information). More indepth studies on the effect of urea on the formation of these structures are in progress. Other additives, including CTAB, NaCl, KCl, and KNO3, were also tested in this work. But our results show that these additives have little influence on the morphology of the final products. Variable-temperature magnetic susceptibility measurements of Ni11(HPO3)8(OH)6 have been carried out on a powdered sample in the range from 5 to 300 K. The molar magnetic susceptibility χm increases with decreasing temperature in the range studied (see the Supporting Information, Figure S4). The magnetic measurement of Ni11(HPO3)8(OH)6 showed that this sample was a paramagnetic material.

4. Conclusion In summary, various novel Ni11(HPO3)8(OH)6 nanostructures, such as nanoparticles, nanorods, dumbbell-like and urchinlike superstructures consisting of nanorods, and hollow microspheres made of nanoparticles, have been prepared via a simple and mild solution-growth method free of any templates and catalysts. The formation mechanism of the urchinlike structures has been investigated, and the rod-to-dumbbell-to-sphere growth mechanism clearly contributes to the creation of such structures. The as-created Ni11(HPO3)8(OH)6 superstructural architectures add to the known range of ordered nanocrystal-based patterns and can be used in studies considering the effects of spatial orientation and arrangement of 1D nanoscale building blocks on their collective sensing, catalytic, optical, electronic, and magnetic properties. In addition, this synthetic method has demonstrated that it is possible to prepare complex and hierarchical structures by a facile, mild solution approach without any template and catalysts, which could be extended to the creation of other inorganic crystals with complex forms. Acknowledgment. This work was supported by the National Natural Science Foundation of China (50221201, 90301010, 50502033), the Chinese Academy of Sciences, and

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the National Research Found for Fundamental Key Projects 973 (2006CB806200). We thank Prof. C. H. Chen and Dr. D. X. Liao from Institute of Physics, Chinese Academy of Sciences, for the magnetic measurements. Supporting Information Available: Additional SEM images, XRD, magnetic measurement results, etc. This material is available free of charge via the Internet at http://pubs.acs.org.

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