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Fabrication and magnetic properties of nickel dodecahedra Lei Zhao, Guangshu Zhang and Lijun Zhao* Here we report a one-pot route for the synthesis of nickel dodecahedra with 52.3 ± 0.1 emu g−1 of saturation magnetization. The procedure is very simple, and only three chemicals (NiCl2·6H2O, isopropanol and polyvinylpyrrolidone) are used throughout the entire synthetic process. During the reaction, it is
Received 10th January 2014, Accepted 16th January 2014
believed that the application of isopropanol and the amount of polyvinylpyrrolidone play an essential role in forming the dodecahedral morphology of the final product. Furthermore, a formation process of
DOI: 10.1039/c4dt00088a
twinning and the influence of reaction kinetic factors were proposed to explain the formation of nickel
www.rsc.org/dalton
dodecahedra.
Introduction Many scientists focus on constructing special structures for nanomaterials to tailor their physical or chemical properties and then improve their intrinsic attributes in some applied fields.1 The morphology controlled synthesis of polyhedral nanocrystals with uniform geometries has been an attractive research area for the past decade, as they have broad uses in catalysis, photonics, electronics, optical sensing and magnetic applications.2–4 Essentially, for all of these applications both the size and shape are critical parameters to control in order to maximize their suitability. For example, the reactivity and selectivity of a catalyst can be tailored by controlling the shape, as it will determine the crystallographic facets exposed on the surface of a nanocrystal and therefore the number of atoms located at the edges or corners.5 Based on these above theories, many works have been done on polyhedral nanomaterials. For instance, Huang et al. reported the synthesis of concave polyhedral Pt nanocrystals with high-index facets and Lee et al. reported the synthesis of nanocrystals with variable high-index Pd facets through the controlled heteroepitaxial growth of trisoctrahedral Au templates.6,7 These excellent works let us recognize that both the shape and the exposed crystalline faces are important and can have a marked influence on activity and selectivity. Like noble metals, the synthesis of nickel (Ni) nanostructures with well-controlled shapes has been a challenging task. Before we started to work on this subject, some efforts had been devoted to the chemical synthesis of Ni nanostructures,
Key Laboratory of Automobile Materials (Jilin University), Ministry of Education and School of Materials Science and Engineering, Jilin University, Changchun 130022, China. E-mail: [email protected]; Fax: +86-0431-85095876
This journal is © The Royal Society of Chemistry 2014
including the use of the surfactant polyvinylpyrrolidone (PVP) or the tetramethylammonium hydroxide (TMAH) assisted growth for Ni polyhedra.8,9 However, direct evidence for the preparation of Ni dodecahedra with geometrical shapes has never been reported. In this work, we report a facile solvothermal method for the preparation of Ni dodecahedra. Furthermore, both the formation mechanism and magnetic properties are investigated in detail.
Materials and methods Sample preparation In a typical preparation procedure, the samples were synthesized according to the following: 0.2 g of NiCl2·6H2O was first dissolved in 20 mL of isopropanol (IPA). The resulting mixture was sonicated until a yellow suspension was obtained. The obtained solution was magnetically stirred for about 30 min followed by the addition of 0.5 g of polyvinylpyrrolidone (PVP). The final mixture was transferred into a 30 mL Teflon-lined autoclave. The autoclave was sealed and maintained at 240 °C for 12 h and then cooled to room temperature naturally. The black product was collected and washed with distilled water and ethanol several times. The sample was then dried under vacuum at 60 °C for further characterization. Characterization The phases were identified by means of X-ray diffraction (XRD) with a Rigaku D/max 2500 pc X-ray diffractometer with Cu Kα radiation (λ) of 1.54156 (Å) at a scan rate of 0.04° s−1. The morphologies (FESEM) were characterized by a JEOL JSM-6700F field-emission scanning electron microscope. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected area electron diffraction (SAED), and energy dispersive
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X-ray spectroscopy (EDS) were carried out on a JEOL 2100F with an emission voltage of 200 kV. IR spectra of the samples were characterized using a FTIR spectrophotometer (NEXUS, 670) in KBr pellets. Room temperature hysteresis loops were collected on a VSM-7300 vibrating sample magnetometer (Lakeshore, USA).
Results and discussion Published on 20 January 2014. Downloaded by Aston University on 24/08/2014 09:56:12.
Characterization of Ni dodecahedra As shown in Fig. 1a, the Ni product mainly consists of welldefined dodecahedra with good uniformity and high symmetry. 834 edge lengths of 237 dodecahedra particles were counted by the software Nano Measurer 1.2. From the edge length distribution histograms of the Ni decahedra (inset in Fig. 1a), it can be seen that the sizes of the Ni dodecahedra distribute mainly in the range of 200–300 nm. A closer observation of two dodecahedra which is shown in Fig. 1b reveals that the surfaces are very smooth. Moreover, every dodecahedron is composed of 12 triangles. The TEM image in Fig. 1c shows the particles with representative hexagon contours if randomly oriented, as expected from their appearance in Fig. 1b. In addition, one thing to be mentioned is that the lengths of some edges in one hexagon do not look exactly alike, because they are viewed from different angles. The crystallinity and phase information can be observed from the XRD pattern shown in Fig. 1d. All diffraction peaks can be indexed to a face centered cubic phase (fcc) of Ni (JCPDS 65-2865).
Fig. 1
No other crystalline impurities are detected, demonstrating that pure Ni particles were obtained. In order to obtain more information about the Ni dodecahedra, a HRTEM investigation was carried out. Through tilting the projecting angle, a quadrilateral and a hexagon can be observed from Fig. 2a and 2d, respectively. Fig. 2a shows a quadrilateral projecting from the side of the particle, yet Fig. 2d shows a hexagon projecting in front of the particle. Because of the different viewing angles, the projections of the dodecahedra are quadrilateral and hexagonal, as shown in the insets of Fig. 2a and 2d. The HRTEM images of Ni decahedra taken from different angles are shown in Fig. 2b and 2e. Moreover, Fig. 2c and 2f show magnified HRTEM images of the appointed region in Fig. 2b and 2e (signed with the same color), the same lattice features (0.203 nm) demonstrated that the surface is enclosed by (111) facets from two different projecting angle. The SAED patterns in Fig. 2g and 2h show that the Ni dodecahedra crystallized well. The SAED pattern, as shown in Fig. 2g, is taken from the dodecahedron displayed in Fig. 2a. The SAED pattern shows clear spots indexed to the ˉ12] zone axis of the fcc crystal. The Ni dodecahedra show [1 high crystallinity due to the observed highly ordered SAED spots. Furthermore, the SAED pattern obtained from the decahedron in Fig. 2d is shown in Fig. 2f, which shows clear spots indexed to the [011] zone axis of the fcc crystal. Two zone axes of Ni dodecahedra were present in the SAED, indicating that ˉ12] and [011] have a 30° rotation between the zone axes of [1 the two closed-packed lattices.
(a) Low-magnification SEM; (b) high-magnification SEM; (c) TEM and (d) XRD pattern of Ni dodecahedra.
5914 | Dalton Trans., 2014, 43, 5913–5919
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Fig. 2 (a–f ) SEM images, HRTEM images and magnified HRTEM images of two Ni dodecahedra; (g) and (h) are the corresponding SAED patterns of (a) and (d). Insets in (a) and (d) are the separately corresponding projections of the Ni dodecahedra.
The Ni crystals showed very good crystallinity because no defects were observed in the image. From the analysis of SEM, XRD and TEM, we concluded that highly crystalline, well-developed and homogeneous Ni dodecahedra were successfully synthesized by the simple solvothermal method. Formation mechanism of Ni dodecahedra To shed light on the formation mechanism of the Ni dodecahedra, a twinning mechanism was proposed. Fig. 3a shows the TEM image of a Ni dodecahedron’s top view, from which six triangle planes are clearly seen. The edge of the Ni dodecahedron along the [011] axis was detected by HRTEM, as shown in Fig. 3b. An FFT pattern was obtained in the region of the dodecahedron as labeled in Fig. 3b, which indicated the dodecahedron is a twinned crystallite. The twins are related by twinning on the (111) plane and the twins are asymmetric along the
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twin plane. Based on the above analysis, it can be confirmed that the dodecahedron is a multiply-twinned particle. Ino et al. proposed a theoretical model for the formation and development of multiply-twinned particles and following researchers further developed this theory.10–12 According to Ino’s theoretical model, a scheme of the multiply-twinned particles’ growth process was drawn and is shown in Fig. 3d. In order to make the system obtain the minimum total surface free energy, the Ni atoms stack into a tetrahedron, which is then used as a base in each step to develop the multiply-twinned particles; initially the tetrahedron twins with two others, then this grain twins for a second time with a tetrahedron on another side, and so on, until a dodecahedron is formed by a structure composed of 6 tetrahedral twinned particles. In addition, kinetic factors were also investigated to explore the possible growth mechanism of the Ni dodecahedra, we
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Fig. 3 (a) TEM image of Ni dodecahedron’s top view; (b) HRTEM image near the edge of the Ni dodecahedron along the [011] axis; (c) FFT pattern; (d) growth process schematic of multiply-twinned particles (N is the base grain, I is the first twinning, II is the secondary twinning, III is the third twinning).
studied the samples (obtained with different dosages of PVP) using SEM, as shown in Fig. 4. In the synthesis of inorganic nanostructures, many organic additives have been used to modify certain crystallographic surfaces.13–15 The absorption of the organic additives on those surfaces will eliminate part of the dangling bond resulting in the reduction of surface energy, and therefore suppressing growth along these surfaces obtaining the final required morphology. Therefore, the addition of PVP is not only for reducing the surface energy but also to form a polyhedral-like structure. When no PVP was introduced into the reaction system, we observe agglomerated cubes with convexities (Fig. 4a). On the surfaces of cubes, which have higher surface energies, the existence of defects (such as kinks and steps) leads to dislocation and subsequent spiral growth to form convexities. Furthermore, particle aggregation can also reduce surface energy by eliminating the high energy facets leading to Ni in the shape of agglomerated cubes with convexities. With the introduction of PVP, we found that Ni dodecahedra gradually formed, and the uniformity and dispersion of the Ni dodecahedra began to improve (Fig. 4b). When the dosage of PVP reached 0.3, the Ni dodecahedra were taking shape, but their crystalline planes were not very clear (Fig. 4c). However, when the dosage of PVP was increased to 1.0 g, bread-like Ni particles with concaves in the center of the surfaces were observed in Fig. 4d. From the above SEM observation, it can be concluded that the nonionic surfactant PVP plays an important role in the formation of Ni dodecahedra. When the reactions were carried out in the presence of other surfactants, such as the cationic surfactant cetyltrimethylammonium bromide (CTAB) or the anionic surfactant sodium dodecyl sulfate (SDS), disordered and irregular morphologies were observed.
5916 | Dalton Trans., 2014, 43, 5913–5919
As the reaction proceeds, the polyhedral particles grow along different directions with different growth rates due to their different surface energies. In the present system, the surfactant PVP acts as not only a stabilizer to prevent the aggregation of the products but also a shape-controller to assist the formation of dodecahedral Ni crystals. As a well-known capping or stabilizing agent, PVP molecules with long chains can be adsorbed to the Ni particle surfaces via physical and chemical bonding. When we add PVP into the reaction system, it is believed that PVP tends to suppress the growth rate of the {111} planes more than that of the {100} planes, since it interacts more strongly with the {111} facets than with the n P {100} facets.16,17 Based on the Curie–Wullf theory ( Ai δi = i¼1
minimum total surface free energy), the growth rate of a crystal plane is proportional to its specific surface energy. Ai and δi are the separate area and specific surface free energy of the ‘i’ crystal plane. Under the synthesis system of IPA and PVP, n = 12 may make the total surface free energy minimum. The viscosity of the solvent is also a key factor in influencing the final morphologies of crystals. Without using PVP, the viscosity of IPA is relatively low. The solutes gradually adhere to the crystal surfaces. The light solution rises, then the heavy solution gradually sinks. Due to the effect of gravity, a vortex is generated, which makes the supply of solutes inhomogeneous and directional (Scheme 1a). Hence, crystal surfaces of the cubes are obtained with convexities. Furthermore, the cubes are hexahedrons which are occupied by six {100} planes. In order to decrease the energies of the crystal planes, agglomeration occurs. With increasing dosage of PVP, the viscosity of the solvent gradually improves. A gradual increase in viscosity will
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Fig. 4 SEM images of Ni particles obtained with different dosages of PVP: (a) 0 g, (b) 0.1 g, (c) 0.3 g and (d) 1 g; (e) SEM image of Ni polyhedrons synthesized at 260 °C with 0.5 g of PVP.
Scheme 1
Scheme of the function of PVP during the process of Ni formation.
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Fig. 5 (a) STEM image, (b) EDS spectrum, and (c) nickel element mapping image of Ni dodecahedra. (d) FT-IR spectrum and (e) room-temperature hysteresis loop of Ni decahedrons.
hinder the presence of the vortex, and then the supply of solute can diffuse (Scheme 1b). Consequently, the crystal grows in this manner with a more complete structure and uniform morphology, and then decahedrons are obtained. However, with a further increase in viscosity, diffusion of the solute becomes harder. The crystal forms under conditions of a lack of solute supply (Scheme 1c). Because the edges and corners of the crystal easily accept the solute and grow quickly, the clear crystal planes of the dodecahedra gradually disappear and bread-like Ni particles form. Apart from the above kinetic factors, temperature also has a strong influence on the final morphology. One thing to be mentioned is that the reaction temperature is vital for the formation of Ni dodecahedra. If the reaction temperature is lower than 220 °C, no Ni products can be obtained. On the contrary, if the reaction temperature is higher than 220 °C, for example 240 °C, the dodecahedra will disappear, replaced by large amounts of polyhedral-like particles with a size of about 500 nm, as shown in Fig. 4e. If we observe the SEM image carefully, it can be seen that there are some dodecahedra and icosahedra. Thus, as dictated by thermodynamics, we can speculate that the shapes of the products evolve to a sphericallike structure to reduce the surface energy with increasing temperature.18 On the basis of the above experimental observations, we found that the entire reaction process is very simple with a high repeatability because of the few reactants. Furthermore, IPA is indispensable for the formation of the dodecahedral
5918 | Dalton Trans., 2014, 43, 5913–5919
structure during the solvothermal process. Before we reported this work, a lot of experiments were done on the application of solvents including alkaline solutions, deionized water, ethanol, dimethylformamide (DMF), and so on. However, it was found that these solvents can only influence the preparation of noble metal polyhedra (Au, Ag, Pd, Pt) or some kinds of metal oxide,19–23 but they have no effect on the synthesis of Ni polyhedra. Based on these reported works, we explored and applied a new volatile solvent (IPA). During the reaction, IPA plays two roles: solvent and reducing agent. The reduction process of Ni is accompanied by the dehydration and oxidation of IPA at high temperature.24 To depict the nature of the prepared Ni dodecahedra, elemental mapping analysis of the Ni dodecahedra was carried out. Fig. 5b shows the EDS spectra of the Ni dodecahedra. Combined with the STEM images (Fig. 5a) and the nickel element mappings (Fig. 5c) it was found that no O signals were detected therefore the Ni dodecahedra were composed of pure nickel. The surface state of the Ni dodecahedra was further checked by FT-IR spectroscopy. No obvious Ni–O bond is seen from Fig. 5d, which proves that the surface of Ni is not oxidized. Magnetic properties Nickel is a typical ferromagnetic material. The magnetic properties of the Ni dodecahedra are very important for its future potential application. Fig. 5e presents the hysteresis loop of the Ni dodecahedra measured at room temperature. The
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saturation magnetization (Ms) of the dodecahedra (52.3 ± 0.1 emu g−1) is very close to bulk Ni (55 emu g−1). The symmetrical structure and uniform shape of the dodecahedra may be the reason for the high Ms value. The atomic magnetic moment of iron group elements is mainly due to the contribution of electron spin. However, the contribution of electron spin orbit is small or even zero. Hence, the magnetic moment can generally be calculated by the formula μs = 2√(S(S + 1)) (S is spin quantum number).25 The crystal symmetry is mainly composed of equivalent crystal faces, and the crystal edges and corners are repeated regularly. This is because it has a regular grid structure, which is a reflection of its cyclical repetition in three-dimensional space. Ni dodecahedra have high crystal symmetry; hence, the orbital angular momentum may just be partly frozen. Furthermore, for the 3dn of Ni, n = 8, the ground state in the dodecahedron has dual degeneracy.25 The orbital angular momentum is partly frozen. Therefore, the magnetic moment of Ni can be calculated using the formulae μJ = gJ√( J ( J + 1))μB ( J = L + S, J is the total quantum number; L is the orbital quantum number). When n is larger than 5, gJ is larger than 2.25 Based on the above analysis, we have explained why the Ni dodecahedra have a high Ms value. Moreover, the Ni decahedra exhibit greatly enhanced coercivity (155.4 ± 1 Oe) compared with bulk Ni (100 Oe).26 It is known that the coercivity of magnetic materials depends strongly on various types of anisotropy (crystal anisotropy, shape anisotropy, stress anisotropy, externally induced anisotropy, and exchange anisotropy), among which shape anisotropy is predicted to produce the largest coercive forces.27 In addition, with a decrease in particle size, the formation of domain walls becomes energetically unfavorable and the coherent rotation of spins is required instead of domain wall motion for the changes in the magnetization, resulting in larger coercivity. Therefore, a higher coercivity compared with bulk nickel was observed. Moreover, similar values of coercive fields are reported by many previous researchers.28–31
Paper
Acknowledgements The financial support from the Major Science and Technology Projects Research Plan of Changchun City (13KG75) is appreciated.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Conclusions In summary, Ni dodecahedra have been fabricated by a simple solvothermal method. The presence of a twinning structure makes for the formation of Ni dodecahedra. In addition, the dosage of PVP has a direct influence on the viscosity of the IPA system. An appropriate viscosity can facilitate the formation of Ni dodecahedra. Due to the symmetric structure and uniform morphology of Ni dodecahedra, they have a high Ms value which is comparable to bulk Ni.
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26 27 28 29 30 31
S. Mann and G. A. Ozin, Nature, 1996, 382, 313. N. L. Rosi and C. A. Mirkin, Chem. Rev., 2005, 105, 1547. A. R. Tao, S. Habas and P. D. Yang, Small, 2008, 4, 310. Y. Xia, Y. J. Xiong and B. Lim, Angew. Chem., Int. Ed., 2009, 48, 60. R. Narayanan and M. A. El-Sayed, Nano Lett., 2004, 4, 1343. X. Q. Huang, et al., J. Am. Chem. Soc., 2011, 133, 4718. Y. Yu, Q. B. Zhang, B. Liu and J. Y. Lee, J. Am. Chem. Soc., 2010, 132, 18258. H. J. Song, et al., CrystEngComm, 2012, 14, 405. L. J. Zhao, Y. J. Wang and Q. Jiang, Mater. Lett., 2010, 64, 215. S. Ogawa and S. Ino, J. Cryst. Growth, 1972, 13, 48. B. J. M. Wiley, et al., Chem.–Eur. J., 2005, 11, 454. K. Yagi, et al., J. Cryst. Growth, 1975, 28, 117. B. Wiley, Y. G. Sun, B. Mayers and Y. N. Xia, Chem.–Eur. J., 2005, 11, 454. P. Toneguzzo, et al., Adv. Mater., 1998, 10, 1032. Y. J. Xiong, et al., Langmuir, 2006, 22, 8563. M. Tsuji, R. Matsuo, P. Jiang, N. Miyamae, D. Ueyama, et al., Cryst. Growth Des., 2008, 8, 2528. H. Cölfen and S. Mann, Angew. Chem., Int. Ed., 2003, 42, 2350. Y. J. Xiong and Y. N. Xia, Adv. Mater., 2007, 19, 3385. M. Leng, et al., J. Am. Chem. Soc., 2010, 132, 17084. S. D. Sun, et al., CrystEngComm, 2011, 13, 2217. C. K. Tsung, et al., J. Am. Chem. Soc., 2009, 131, 5816. J. Zhang, et al., J. Am. Chem. Soc., 2010, 132, 14012. X. Q. Huang, et al., J. Am. Chem. Soc., 2009, 131, 13916. R. J. Joseyphus, et al., J. Solid State Chem., 2007, 180, 3008. D. F. Wan and X. L. Ma, Magnetic Physics, University of Electronic Science and Technology Publishing House, 1994, p. 154. J. H. Hwang, et al., J. Mater. Res., 1997, 12, 1076. L. L. Diandra and D. R. Reuben, Chem. Mater., 1996, 8, 1770. D. Q. Zhang, G. S. Li and J. C. Yu, Cryst. Growth Des., 2009, 9, 2812. W. Hui, Z. Rui, L. Dandan and P. Wei, Chem. Mater., 2007, 19, 3506. X. M. Liu and S. Y. Fu, J. Cryst. Growth, 2007, 306, 428. Y. H. Leng, et al., Nanotechnology, 2006, 17, 1797.
Dalton Trans., 2014, 43, 5913–5919 | 5919
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Cite this: Dalton Trans., 2014, 43, 5913
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Fabrication and magnetic properties of nickel dodecahedra Lei Zhao, Guangshu Zhang and Lijun Zhao* Here we report a one-pot route for the synthesis of nickel dodecahedra with 52.3 ± 0.1 emu g−1 of saturation magnetization. The procedure is very simple, and only three chemicals (NiCl2·6H2O, isopropanol and polyvinylpyrrolidone) are used throughout the entire synthetic process. During the reaction, it is
Received 10th January 2014, Accepted 16th January 2014
believed that the application of isopropanol and the amount of polyvinylpyrrolidone play an essential role in forming the dodecahedral morphology of the final product. Furthermore, a formation process of
DOI: 10.1039/c4dt00088a
twinning and the influence of reaction kinetic factors were proposed to explain the formation of nickel
www.rsc.org/dalton
dodecahedra.
Introduction Many scientists focus on constructing special structures for nanomaterials to tailor their physical or chemical properties and then improve their intrinsic attributes in some applied fields.1 The morphology controlled synthesis of polyhedral nanocrystals with uniform geometries has been an attractive research area for the past decade, as they have broad uses in catalysis, photonics, electronics, optical sensing and magnetic applications.2–4 Essentially, for all of these applications both the size and shape are critical parameters to control in order to maximize their suitability. For example, the reactivity and selectivity of a catalyst can be tailored by controlling the shape, as it will determine the crystallographic facets exposed on the surface of a nanocrystal and therefore the number of atoms located at the edges or corners.5 Based on these above theories, many works have been done on polyhedral nanomaterials. For instance, Huang et al. reported the synthesis of concave polyhedral Pt nanocrystals with high-index facets and Lee et al. reported the synthesis of nanocrystals with variable high-index Pd facets through the controlled heteroepitaxial growth of trisoctrahedral Au templates.6,7 These excellent works let us recognize that both the shape and the exposed crystalline faces are important and can have a marked influence on activity and selectivity. Like noble metals, the synthesis of nickel (Ni) nanostructures with well-controlled shapes has been a challenging task. Before we started to work on this subject, some efforts had been devoted to the chemical synthesis of Ni nanostructures,
Key Laboratory of Automobile Materials (Jilin University), Ministry of Education and School of Materials Science and Engineering, Jilin University, Changchun 130022, China. E-mail: [email protected]; Fax: +86-0431-85095876
This journal is © The Royal Society of Chemistry 2014
including the use of the surfactant polyvinylpyrrolidone (PVP) or the tetramethylammonium hydroxide (TMAH) assisted growth for Ni polyhedra.8,9 However, direct evidence for the preparation of Ni dodecahedra with geometrical shapes has never been reported. In this work, we report a facile solvothermal method for the preparation of Ni dodecahedra. Furthermore, both the formation mechanism and magnetic properties are investigated in detail.
Materials and methods Sample preparation In a typical preparation procedure, the samples were synthesized according to the following: 0.2 g of NiCl2·6H2O was first dissolved in 20 mL of isopropanol (IPA). The resulting mixture was sonicated until a yellow suspension was obtained. The obtained solution was magnetically stirred for about 30 min followed by the addition of 0.5 g of polyvinylpyrrolidone (PVP). The final mixture was transferred into a 30 mL Teflon-lined autoclave. The autoclave was sealed and maintained at 240 °C for 12 h and then cooled to room temperature naturally. The black product was collected and washed with distilled water and ethanol several times. The sample was then dried under vacuum at 60 °C for further characterization. Characterization The phases were identified by means of X-ray diffraction (XRD) with a Rigaku D/max 2500 pc X-ray diffractometer with Cu Kα radiation (λ) of 1.54156 (Å) at a scan rate of 0.04° s−1. The morphologies (FESEM) were characterized by a JEOL JSM-6700F field-emission scanning electron microscope. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected area electron diffraction (SAED), and energy dispersive
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X-ray spectroscopy (EDS) were carried out on a JEOL 2100F with an emission voltage of 200 kV. IR spectra of the samples were characterized using a FTIR spectrophotometer (NEXUS, 670) in KBr pellets. Room temperature hysteresis loops were collected on a VSM-7300 vibrating sample magnetometer (Lakeshore, USA).
Results and discussion Published on 20 January 2014. Downloaded by Aston University on 24/08/2014 09:56:12.
Characterization of Ni dodecahedra As shown in Fig. 1a, the Ni product mainly consists of welldefined dodecahedra with good uniformity and high symmetry. 834 edge lengths of 237 dodecahedra particles were counted by the software Nano Measurer 1.2. From the edge length distribution histograms of the Ni decahedra (inset in Fig. 1a), it can be seen that the sizes of the Ni dodecahedra distribute mainly in the range of 200–300 nm. A closer observation of two dodecahedra which is shown in Fig. 1b reveals that the surfaces are very smooth. Moreover, every dodecahedron is composed of 12 triangles. The TEM image in Fig. 1c shows the particles with representative hexagon contours if randomly oriented, as expected from their appearance in Fig. 1b. In addition, one thing to be mentioned is that the lengths of some edges in one hexagon do not look exactly alike, because they are viewed from different angles. The crystallinity and phase information can be observed from the XRD pattern shown in Fig. 1d. All diffraction peaks can be indexed to a face centered cubic phase (fcc) of Ni (JCPDS 65-2865).
Fig. 1
No other crystalline impurities are detected, demonstrating that pure Ni particles were obtained. In order to obtain more information about the Ni dodecahedra, a HRTEM investigation was carried out. Through tilting the projecting angle, a quadrilateral and a hexagon can be observed from Fig. 2a and 2d, respectively. Fig. 2a shows a quadrilateral projecting from the side of the particle, yet Fig. 2d shows a hexagon projecting in front of the particle. Because of the different viewing angles, the projections of the dodecahedra are quadrilateral and hexagonal, as shown in the insets of Fig. 2a and 2d. The HRTEM images of Ni decahedra taken from different angles are shown in Fig. 2b and 2e. Moreover, Fig. 2c and 2f show magnified HRTEM images of the appointed region in Fig. 2b and 2e (signed with the same color), the same lattice features (0.203 nm) demonstrated that the surface is enclosed by (111) facets from two different projecting angle. The SAED patterns in Fig. 2g and 2h show that the Ni dodecahedra crystallized well. The SAED pattern, as shown in Fig. 2g, is taken from the dodecahedron displayed in Fig. 2a. The SAED pattern shows clear spots indexed to the ˉ12] zone axis of the fcc crystal. The Ni dodecahedra show [1 high crystallinity due to the observed highly ordered SAED spots. Furthermore, the SAED pattern obtained from the decahedron in Fig. 2d is shown in Fig. 2f, which shows clear spots indexed to the [011] zone axis of the fcc crystal. Two zone axes of Ni dodecahedra were present in the SAED, indicating that ˉ12] and [011] have a 30° rotation between the zone axes of [1 the two closed-packed lattices.
(a) Low-magnification SEM; (b) high-magnification SEM; (c) TEM and (d) XRD pattern of Ni dodecahedra.
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Fig. 2 (a–f ) SEM images, HRTEM images and magnified HRTEM images of two Ni dodecahedra; (g) and (h) are the corresponding SAED patterns of (a) and (d). Insets in (a) and (d) are the separately corresponding projections of the Ni dodecahedra.
The Ni crystals showed very good crystallinity because no defects were observed in the image. From the analysis of SEM, XRD and TEM, we concluded that highly crystalline, well-developed and homogeneous Ni dodecahedra were successfully synthesized by the simple solvothermal method. Formation mechanism of Ni dodecahedra To shed light on the formation mechanism of the Ni dodecahedra, a twinning mechanism was proposed. Fig. 3a shows the TEM image of a Ni dodecahedron’s top view, from which six triangle planes are clearly seen. The edge of the Ni dodecahedron along the [011] axis was detected by HRTEM, as shown in Fig. 3b. An FFT pattern was obtained in the region of the dodecahedron as labeled in Fig. 3b, which indicated the dodecahedron is a twinned crystallite. The twins are related by twinning on the (111) plane and the twins are asymmetric along the
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twin plane. Based on the above analysis, it can be confirmed that the dodecahedron is a multiply-twinned particle. Ino et al. proposed a theoretical model for the formation and development of multiply-twinned particles and following researchers further developed this theory.10–12 According to Ino’s theoretical model, a scheme of the multiply-twinned particles’ growth process was drawn and is shown in Fig. 3d. In order to make the system obtain the minimum total surface free energy, the Ni atoms stack into a tetrahedron, which is then used as a base in each step to develop the multiply-twinned particles; initially the tetrahedron twins with two others, then this grain twins for a second time with a tetrahedron on another side, and so on, until a dodecahedron is formed by a structure composed of 6 tetrahedral twinned particles. In addition, kinetic factors were also investigated to explore the possible growth mechanism of the Ni dodecahedra, we
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Fig. 3 (a) TEM image of Ni dodecahedron’s top view; (b) HRTEM image near the edge of the Ni dodecahedron along the [011] axis; (c) FFT pattern; (d) growth process schematic of multiply-twinned particles (N is the base grain, I is the first twinning, II is the secondary twinning, III is the third twinning).
studied the samples (obtained with different dosages of PVP) using SEM, as shown in Fig. 4. In the synthesis of inorganic nanostructures, many organic additives have been used to modify certain crystallographic surfaces.13–15 The absorption of the organic additives on those surfaces will eliminate part of the dangling bond resulting in the reduction of surface energy, and therefore suppressing growth along these surfaces obtaining the final required morphology. Therefore, the addition of PVP is not only for reducing the surface energy but also to form a polyhedral-like structure. When no PVP was introduced into the reaction system, we observe agglomerated cubes with convexities (Fig. 4a). On the surfaces of cubes, which have higher surface energies, the existence of defects (such as kinks and steps) leads to dislocation and subsequent spiral growth to form convexities. Furthermore, particle aggregation can also reduce surface energy by eliminating the high energy facets leading to Ni in the shape of agglomerated cubes with convexities. With the introduction of PVP, we found that Ni dodecahedra gradually formed, and the uniformity and dispersion of the Ni dodecahedra began to improve (Fig. 4b). When the dosage of PVP reached 0.3, the Ni dodecahedra were taking shape, but their crystalline planes were not very clear (Fig. 4c). However, when the dosage of PVP was increased to 1.0 g, bread-like Ni particles with concaves in the center of the surfaces were observed in Fig. 4d. From the above SEM observation, it can be concluded that the nonionic surfactant PVP plays an important role in the formation of Ni dodecahedra. When the reactions were carried out in the presence of other surfactants, such as the cationic surfactant cetyltrimethylammonium bromide (CTAB) or the anionic surfactant sodium dodecyl sulfate (SDS), disordered and irregular morphologies were observed.
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As the reaction proceeds, the polyhedral particles grow along different directions with different growth rates due to their different surface energies. In the present system, the surfactant PVP acts as not only a stabilizer to prevent the aggregation of the products but also a shape-controller to assist the formation of dodecahedral Ni crystals. As a well-known capping or stabilizing agent, PVP molecules with long chains can be adsorbed to the Ni particle surfaces via physical and chemical bonding. When we add PVP into the reaction system, it is believed that PVP tends to suppress the growth rate of the {111} planes more than that of the {100} planes, since it interacts more strongly with the {111} facets than with the n P {100} facets.16,17 Based on the Curie–Wullf theory ( Ai δi = i¼1
minimum total surface free energy), the growth rate of a crystal plane is proportional to its specific surface energy. Ai and δi are the separate area and specific surface free energy of the ‘i’ crystal plane. Under the synthesis system of IPA and PVP, n = 12 may make the total surface free energy minimum. The viscosity of the solvent is also a key factor in influencing the final morphologies of crystals. Without using PVP, the viscosity of IPA is relatively low. The solutes gradually adhere to the crystal surfaces. The light solution rises, then the heavy solution gradually sinks. Due to the effect of gravity, a vortex is generated, which makes the supply of solutes inhomogeneous and directional (Scheme 1a). Hence, crystal surfaces of the cubes are obtained with convexities. Furthermore, the cubes are hexahedrons which are occupied by six {100} planes. In order to decrease the energies of the crystal planes, agglomeration occurs. With increasing dosage of PVP, the viscosity of the solvent gradually improves. A gradual increase in viscosity will
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Fig. 4 SEM images of Ni particles obtained with different dosages of PVP: (a) 0 g, (b) 0.1 g, (c) 0.3 g and (d) 1 g; (e) SEM image of Ni polyhedrons synthesized at 260 °C with 0.5 g of PVP.
Scheme 1
Scheme of the function of PVP during the process of Ni formation.
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Fig. 5 (a) STEM image, (b) EDS spectrum, and (c) nickel element mapping image of Ni dodecahedra. (d) FT-IR spectrum and (e) room-temperature hysteresis loop of Ni decahedrons.
hinder the presence of the vortex, and then the supply of solute can diffuse (Scheme 1b). Consequently, the crystal grows in this manner with a more complete structure and uniform morphology, and then decahedrons are obtained. However, with a further increase in viscosity, diffusion of the solute becomes harder. The crystal forms under conditions of a lack of solute supply (Scheme 1c). Because the edges and corners of the crystal easily accept the solute and grow quickly, the clear crystal planes of the dodecahedra gradually disappear and bread-like Ni particles form. Apart from the above kinetic factors, temperature also has a strong influence on the final morphology. One thing to be mentioned is that the reaction temperature is vital for the formation of Ni dodecahedra. If the reaction temperature is lower than 220 °C, no Ni products can be obtained. On the contrary, if the reaction temperature is higher than 220 °C, for example 240 °C, the dodecahedra will disappear, replaced by large amounts of polyhedral-like particles with a size of about 500 nm, as shown in Fig. 4e. If we observe the SEM image carefully, it can be seen that there are some dodecahedra and icosahedra. Thus, as dictated by thermodynamics, we can speculate that the shapes of the products evolve to a sphericallike structure to reduce the surface energy with increasing temperature.18 On the basis of the above experimental observations, we found that the entire reaction process is very simple with a high repeatability because of the few reactants. Furthermore, IPA is indispensable for the formation of the dodecahedral
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structure during the solvothermal process. Before we reported this work, a lot of experiments were done on the application of solvents including alkaline solutions, deionized water, ethanol, dimethylformamide (DMF), and so on. However, it was found that these solvents can only influence the preparation of noble metal polyhedra (Au, Ag, Pd, Pt) or some kinds of metal oxide,19–23 but they have no effect on the synthesis of Ni polyhedra. Based on these reported works, we explored and applied a new volatile solvent (IPA). During the reaction, IPA plays two roles: solvent and reducing agent. The reduction process of Ni is accompanied by the dehydration and oxidation of IPA at high temperature.24 To depict the nature of the prepared Ni dodecahedra, elemental mapping analysis of the Ni dodecahedra was carried out. Fig. 5b shows the EDS spectra of the Ni dodecahedra. Combined with the STEM images (Fig. 5a) and the nickel element mappings (Fig. 5c) it was found that no O signals were detected therefore the Ni dodecahedra were composed of pure nickel. The surface state of the Ni dodecahedra was further checked by FT-IR spectroscopy. No obvious Ni–O bond is seen from Fig. 5d, which proves that the surface of Ni is not oxidized. Magnetic properties Nickel is a typical ferromagnetic material. The magnetic properties of the Ni dodecahedra are very important for its future potential application. Fig. 5e presents the hysteresis loop of the Ni dodecahedra measured at room temperature. The
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saturation magnetization (Ms) of the dodecahedra (52.3 ± 0.1 emu g−1) is very close to bulk Ni (55 emu g−1). The symmetrical structure and uniform shape of the dodecahedra may be the reason for the high Ms value. The atomic magnetic moment of iron group elements is mainly due to the contribution of electron spin. However, the contribution of electron spin orbit is small or even zero. Hence, the magnetic moment can generally be calculated by the formula μs = 2√(S(S + 1)) (S is spin quantum number).25 The crystal symmetry is mainly composed of equivalent crystal faces, and the crystal edges and corners are repeated regularly. This is because it has a regular grid structure, which is a reflection of its cyclical repetition in three-dimensional space. Ni dodecahedra have high crystal symmetry; hence, the orbital angular momentum may just be partly frozen. Furthermore, for the 3dn of Ni, n = 8, the ground state in the dodecahedron has dual degeneracy.25 The orbital angular momentum is partly frozen. Therefore, the magnetic moment of Ni can be calculated using the formulae μJ = gJ√( J ( J + 1))μB ( J = L + S, J is the total quantum number; L is the orbital quantum number). When n is larger than 5, gJ is larger than 2.25 Based on the above analysis, we have explained why the Ni dodecahedra have a high Ms value. Moreover, the Ni decahedra exhibit greatly enhanced coercivity (155.4 ± 1 Oe) compared with bulk Ni (100 Oe).26 It is known that the coercivity of magnetic materials depends strongly on various types of anisotropy (crystal anisotropy, shape anisotropy, stress anisotropy, externally induced anisotropy, and exchange anisotropy), among which shape anisotropy is predicted to produce the largest coercive forces.27 In addition, with a decrease in particle size, the formation of domain walls becomes energetically unfavorable and the coherent rotation of spins is required instead of domain wall motion for the changes in the magnetization, resulting in larger coercivity. Therefore, a higher coercivity compared with bulk nickel was observed. Moreover, similar values of coercive fields are reported by many previous researchers.28–31
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Acknowledgements The financial support from the Major Science and Technology Projects Research Plan of Changchun City (13KG75) is appreciated.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Conclusions In summary, Ni dodecahedra have been fabricated by a simple solvothermal method. The presence of a twinning structure makes for the formation of Ni dodecahedra. In addition, the dosage of PVP has a direct influence on the viscosity of the IPA system. An appropriate viscosity can facilitate the formation of Ni dodecahedra. Due to the symmetric structure and uniform morphology of Ni dodecahedra, they have a high Ms value which is comparable to bulk Ni.
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26 27 28 29 30 31
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