Synthesis of Dispersed Superfine fcc Nickel Single Crystals in Gas

Article pubs.acs.org/JPCC

Synthesis of Dispersed Superfine fcc Nickel Single Crystals in Gas Phase Wei Gao,†,‡ Shaobo Shen,*,†,‡ and Yao Cheng†,‡ †

State Key Laboratory of Advanced Metallurgy and ‡School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China ABSTRACT: A commercial raw nickel powder was used as the precursor of nickel. A powder mixture of sodium chloride and raw nickel with a molar ratio of NaCl/Ni 2.0 was chlorinated by dry chlorine at 700 °C. An anhydrous eutectic mixture with a melting point of 585 °C was formed. When the mixture was heated, some nickel complexes with a overall formula NiCl2·(NaCl)0.35, not individual NiCl2 molecules, were volatilized into gas phase, where they were reduced into metallic Ni by H2 at 900 °C. It was found that many dispersed superfine cubic (fcc) nickel single crystals with an average particle size of about 503 nm were synthesized. Their saturated magnetization was 44 emu/g at 295 K and 10 kOe. A controlled experiment without the addition of NaCl was also conducted. It was found that many irregular sphere-like nickel particles with an average particle size of ∼1435 nm were obtained in this case. It was inferred that the presence of gaseous nickel complexes or gaseous individual NaCl molecules prevented the sintering of the synthesized superfine cubic Ni crystals. This discovery may provide a novel route in synthesizing various metal crystals of regular nonspherical shapes in gas phase.

1. INTRODUCTION Various solution-phase-based methods have been proposed to synthesize nickel nanoparticles.1 Most of the synthesized products are spherical in shape.1,2 As an exception, some hexagonal and triangular Ni nanoplates were prepared by Leng et al.3 and some cubic Ni nanoparticles were synthesized by LaGrow et al.4 It was found that the saturated magnetization of cubic nickel nanoparticles was more than four times that of spherical ones of similar size.4 The magnetic behavior of the above cubic nickel nanoparticles makes them ideal for applications in bioseparation and as magnetically recyclable catalysts.4 The cubic metallic nanoparticles are quite different from spherical ones in optical and magnetic properties.2 Thus many solution-phase-based methods have been proposed to synthesize fine (nano or superfine) cubic metallic crystals with the aid of various surfactants.4−12 Some impurities were inevitably introduced into these fine metal powders during solution-synthesis due to adsorption and so on.12,13 The superfine metal powders with high purity are highly desirable for some applications. Sometimes, some expensive precursors, long reaction time, and complicated procedures are required to synthesize fine metallic powders in solution-phase-based methods.4−13 Moreover, most of the solution-phase-based methods cannot provide dispersed nickel nanoparticles.1 Highly dispersed superfine nickel particles have been widely used as Ni electrodes of multilayer ceramic capacitors (MLCCs).14 Thus superfine nickel particles with high dispersion and high purity are highly desirable. To circumvent these problems encountered in solution-phase-based methods, the initial purpose of this work was to synthesize fine-dispersed metallic nickel particles of high purity in the gas phase based on the separable © 2013 American Chemical Society

properties of various metal chlorides. In addition, process conditions, particle size, particle crystal structure, and purity were easier to control in gas-phase synthesis of superfine particles of metals.15 A commercial raw nickel powder with a purity of ∼98% and an average particle size of 24 μm was used as the precursor of nickel in the present work. A powder mixture of anhydrous sodium chloride and raw nickel with a molar ratio of NaCl/Ni 2.0 was chlorinated by dry chlorine at 700 °C. An anhydrous yellow eutectic mixture with a melting point of 585 °C, which was much lower than that (1001 °C) of pure nickel chloride, was formed. This mixture mainly consisted of both some unknown nickel complexes and sodium chloride (NaCl), which was confirmed by XRD. These studies concerning the chlorination of raw nickel powder with an extra addition of anhydrous sodium chloride will be reported elsewhere. After chlorination, the eutectic complex was heated in situ. It was found that some unknown nickel complexes, not individual NiCl2 molecules, were volatilized into the gas phase, where they were reduced into metallic Ni particles by H2 at 900 °C. The presence of gaseous unknown nickel complexes with an overall formula NiCl2·(NaCl)0.35 were confirmed by XRD, EDX, and chemical analysis. In addition to yellow nickel complexes (NiCl2·(NaCl)0.35), some white sodium chlorides NaCl were probably also volatilized into the gas phase at 900 °C because the mp of NaCl is 801 °C. It was found that some black nickel powders were formed and deposited on the inner wall of the quartz tube. When we observed the nickel particles Received: December 31, 2012 Revised: March 27, 2013 Published: April 4, 2013 9223

dx.doi.org/10.1021/jp3128725 | J. Phys. Chem. C 2013, 117, 9223−9228

The Journal of Physical Chemistry C

Article

Table 1. Chemical Compositions of the Raw Nickel Powder

a

raw Ni powdera

Ni

O

MgO

Cr

Fe

Si

S

contents (% (w/w))

98.33

1.33

0.40

0.08

0.04

0.04

0.02

Average particle size was 23.4 μm.

Figure 1. Schematic diagram of the reactor used to synthesize superfine Ni powder in gas phase.

60 cm. A quartz tube 10 (Figure 1) with an outside diameter of 60 mm was inserted in the furnace. Typically, ∼0.5000 g of weighed raw nickel powder was adequately mixed with 1.0000 g of sodium chloride powder. The mixture thus obtained was placed in a quartz boat 8 (Figure 1). In another case, only 0.5000 g of weighed raw nickel powder was put in the quartz boat without the addition of sodium chloride. The quartz boat was located in the center of furnace. An argon gas with a flow rate of 100 mL/min was passed through three quartz tubes 5, 6, and 7 (Figure 1), respectively, for 1 h. Then, the furnace temperature was increased; meanwhile, three ways of argon gas with flow rates of 50, 200, and 50 mL/min were passed through the quartz tubes 5, 6, and 7, respectively. The inside diameters of the quartz tubes 5, 6, and 7 were 16, 4, and 16 mm, respectively. When the furnace temperature was increased to 700 °C and maintained at this temperature for 30 min, one way of argon gas for the tube 7 was switched to a dry chlorine with a flow rate of 50 mL/min; meanwhile, the other two ways of argon gas were maintained. After 2 h, the chlorine in tube 7 was switched to an argon gas with a flow rate of 20 mL/min; meanwhile, the furnace temperature was quickly increased to 900 °C. After reaching 900 °C, three ways of gases of H2, Ar, and Ar were passed simultaneously through the quartz tubes 5, 6, and 7, respectively, in the flow rates of 40, 200, and 200 mL/ min for 1 h. After that, the furnace was cooled to room temperature; meanwhile, only one way of argon gas was passed through the tube 10 with argon gas in a flow rate of 50 mL/ min. Most of black nickel powders produced were found to be deposited on the inner wall of the narrow section of the quartz tube 10, which possessed an average temperature of ∼50 °C during the period of hydrogen reduction. The nickel powders deposited on the narrow section of tube 10 were then flushed off the tube using anhydrous ethanol and collected in a beaker. Then, the nickel powder in the beaker was washed several times with anhydrous ethanol to remove the adsorbed impurities. Finally, the nickel powders thus obtained were immersed in anhydrous ethanol and preserved in a glass bottle with tightly screwed plastic lid. Before it was characterized using TEM and SEM, the nickel powder contained in anhydrous ethanol was pretreated using ultrasonic vibration. The nickel powder contained in anhydrous ethanol was dried in flowing argon at room temperature for 6 h before sending it to do XRD analysis. The SEM samples were prepared by dropping the solution of

synthesized with SEM, an astonishing phenomenon was found. Most of the synthesized fine nickel particles were of almost perfect cubic shapes with square facets. However, a controlled experiment performed by us clearly revealed that some agglomerated nickel particles of irregular shape were formed under very similar conditions with the exception that no sodium chloride was added in this case. A similar phenomenon was also reported by Suh et al.16 and Jang et al.17 They found that only spherical nickel or cobalt particles were formed by reducing anhydrous NiCl2 or CoCl2, respectively, with H2 in the gas phase at 900 °C in the absence of NaCl.16,17 The synthesis of cubic nickel crystal in gas phase has never been previously reported. Actually the reported gas-phase synthesis of metal particles of cubic shape was extremely rare. As only one example found so far, Harris reported a shape transformation of spherical platinum nanoparticles to cubic ones in gas phase induced by sulfur.18 Moreover, the presence of superfine cubic nickel particles in gas phase observed in the present work seems to conflict with the existing thermodynamic theories because some organic surfactants were always needed to synthesize fine cubic metal particles in solution-phase synthesis to reduce the surface energies.4−12 However, no organic surfactant was involved in the present gas-phase synthesis. So, what resulted in the formation of these almost perfect superfine cubic Ni particles?

2. EXPERIMENTAL SECTION 2.1. Materials. A commercial nickel powder with a purity of ∼98% was used as a nickel precursor. The chemical composition of the nickel powder was listed in Table 1. The chemical composition of the powder was analyzed with XRF (XRF-1800, X-ray fluorescence spectroscope, Shimadzu). The average particle size of the nickel powder was 24.6 μm based on one measurement with a laser size distribution analyzer (LMS30, Japan). The gases of argon, chlorine, and hydrogen all having a purity of over 99.99% v/v were used in this work. The powder of sodium chloride of analytical grade was used in this work, and its particle size ranged from 106 to 149 μm. Before use, it was dried at 500 °C for 2 h and then preserved in a tightly closed desiccator filled with anhydrous calcium chloride. 2.2. Synthesis and Characterization of Ni Powders. The experiments were carried out in a horizontal tubular furnace (Figure 1). The length of the heating zone of furnace is 9224



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Figure 2. (A) Low- and (B) high-magnification SEM images of superfine Ni cubic crystals with square facets synthesized by reducing anhydrous NiCl2 with H2 at 900 °C in the presence of gaseous NaCl. (C) EDX of one of cubic Ni crystals showed in panel A. (D) XRD pattern of the same batch of sample. The insert shows the facet positions of one cubic Ni crystal.

Figure 3. (A,B) SEM images of Ni particles synthesized by reducing anhydrous NiCl2 with H2 at 900 °C in the absence of gaseous NaCl. (C) XRD pattern of the same batch of sample A. (D) XRD characterization of the Ni particles of samples A−C using software MDI Jade 5.0.

anhydrous ethanol containing nickel powders on double-sided carbon conductive tapes. The latter was supported on a transparent conductive silica (SiO2) disk. TEM samples were

prepared by dropping the solution of anhydrous ethanolcontaining nickel powders on a carbon-coated copper grid. SEM observation was performed with Zeiss Auriga FIB-SEM. 9225



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Figure 4. (A) Low- and (B) high-magnification TEM images of superfine Ni cubic crystals with square facets synthesized by reducing anhydrous NiCl2 with H2 at 900 °C in the presence of gaseous NaCl. (C) Electron diffraction image recorded by aligning the electron beam perpendicular to one of square facets of the crystal shown in panel B. (D) EDX of the crystal showed in panel B.

The TEM images of nickel particles synthesized in the presence of NaCl were presented in Figure 4A,B. Different from the case using sticking carbon conductive tape in SEM observation, the nickel particles were not stuck together on the surface of smooth carbon-coated copper grid. Some highly dispersed nickel particles on the grid were thus observed with TEM (Figure 4A). Some of the nickel particles were of almost perfect cubic shape with sharp right angular corners (Figure 4A,B). The electronic diffraction image (Figure 4C) matched the reflection of {200} facets of the Ni single crystal shown in Figure 3B. No impurities were found in the cubic Ni single crystal (Figure 4B) based on the EDX analysis of TEM (Figure 4D), and thus the synthesized nickel single crystal was of high purity. Brillo and Egry19 proposed the following eq 1 to calculate the surface tension (σ(T)) of metallic nickel at different temperature T in N/m.

TEM investigation was carried out with FEI TECNAI F20. Xray diffraction (XRD) patterns of the samples powder were recorded with a Siemens D5000 X-ray diffractometer equipped with a Cu Kα radiation source (λ = 0.15405 nm). The diffraction patterns of XRD were analyzed using software of MDI Jade5.0 with the aid of JCPDF database. For the XRD analysis, the XRD pattern background and Kα 2 line contribution were first stripped; then, the pattern was smoothed and simulated sequentially. In this way, the average crystallite sizes and the lattice parameters for the samples were thus obtained. Magnetic measurement of the nickel powder was performed using the instrument of Physics Property Measurement System (PPMS-9, Quantum Design).

3. RESULTS AND DISCUSSION The SEM images of the nickel powder synthesized in the presence of NaCl were presented in Figure 2A,B. Most of the nickel particles were of almost perfect cubic shapes with sharp right angular corners and agglomerated loosely on sticking carbon conductive tape. Except for a very small peak of oxygen, which most likely resulted from substrate silica (SiO2), no other impurity was found in the nickel crystal according to the EDX measurement of SEM (Figure 2C). However, for the Ni particles synthesized in the absence of NaCl, most of the nickel particles were of irregularly spherical shape (Figure 3A,B). Furthermore, most of nickel particles aggregated closely to form bigger blocks (Figure 3A,B).

σ = 1.77 − 3.3 × 10−4(T − 1454)

(1)

The external pressure (ΔP) applied on the surface of nickel crystal can thus be estimated based on the following Young− Laplace eq 2:

ΔP =

2σ r

(2)

where T is temperature in Celsius and σ and r are the surface tension in newtons per meter and the radii of curvature of Ni 9226



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Figure 5. (A) Variation of distribution percentage of particle number with particle size for local particles shown in Figure 2A. (B) Magnetic measurement of the sample B (Figure 3D) carried out at 10 kOe and 295 K.

formed only in the presence of high concentration of surfactant poly(vinyl pyrrolidone) (PVP), whereas in the presence of low concentration of surfactant PVP, only octahydronal Ag nanocrystals were formed in solution-phase synthesis. LaGrow et al.4 found that regular nanocubes of Ni were only formed with the aid of a higher concentration of surfactant trioctylphosphine (TOP) in solution-phase synthesis. It usually takes several hours for the formation of fine cubic metal crystals in solution-phase synthesis, and one or several surfactants must be added in the solution synthesis to reduce the surface energies of metals4−12−. These surfactants also included halide anions in addition to common organic molecules.12 Xiong et al. found that Pd nanobars or nanorods were only formed with the assistance of bromide anion, whereas the usual surfactant PVP only favored the formation of Pd nanocrystals with cuboctahedral shape.12 The presence of bromide adsorbed on the Pd nanobars or nanorods was further confirmed by EDX analysis.12 The adsorption of halide anions (Cl−, Br−, and I−) on cubic crystals of Pt and Pd in solution-phase synthesis has been reported.21,22 A similar phenomenon, some gaseous NaCl molecules adsorbing on gaseous cubic Ni crystals, might occur for the present work. The lattice parameter (3.5163 Å) of the sample A was smaller than that (3.5237 Å) of standard sample D (Figure 3D). However, the lattice parameters of samples B and C were 3.5225 and 3.5342 Å, respectively, which were close to or larger than that of standard sample D (Figure 3D). It implied that only Ni crystals synthesized in the absence of gaseous NaCl (sample A) were compressed compared with standard sample D probably due to the existence of larger surface tension of Ni metal in this case. All of the above experiments supported the fact that the surface tensions of the fine cubic Ni crystals from sample B or C were reduced greatly during the gas-phase synthesis probably due to the presence of gaseous complexes (NiCl2·(NaCl)0.35) or gaseous individual NaCl molecules. This discovery may provide a novel route in synthesizing various metallic crystals of regular nonspherical shapes in the gas phase. A size distribution of local cubic nickel particles of sample B (Figure 3D) was obtained by approximately measuring the sizes of 204 nickel particles shown in Figure 2A with a ruler. The result is shown in Figure 5A. The measured local average particle size was 367 nm (Figure 5A), which was close to overall average particle size (503 nm) measured by XRD (Figure 3D). The minimum and maximum particle sizes measured (Figure 5A) were 158 and 1684 nm, respectively. A magnetic measurement was performed using the superfine cubic nickel

crystal in meters, respectively. The average particles size of the cubic Ni crystals shown in sample B (Figure 3D) in the presence of NaCl was ∼503 nm. The estimated ΔP values were (1.77 and 1.56) × 107 N/m2 at 100 and 900 °C, respectively. Under such huge pressures, a fine nickel crystal in solution normally has to take a nearly spherical shape to resist this external pressure.2,20 All of the peaks of samples A−C (Figure 3D) can be indexed to face-centered cubic (fcc) Ni (JCPDF no. 04-0850). Samples A and B were obtained by reducing gaseous NiCl2 and gaseous nickel complexes (NiCl2)·(NaCl)0.35, respectively, at 900 °C. The synthesis conditions of sample C were very similar to those of sample B with the exception that the H2-reduction temperature was 950 °C instead of 900 °C in this case. The crystallite size of sample A was ∼1435 nm, whereas the corresponding values of sample B and C were about 503 and 693 nm, respectively (Figure 4D). It implied that some cubic Ni crystals were sintered in the absence of NaCl. This was further confirmed by the SEM images (Figure 3A,B). The cubic Ni crystals were in thermodynamically unstable states and have higher surface energy than nearly spherical Ni crystals. To minimize the surface energies, several fine Ni cubic crystals had to merge to form a bigger block in the absence of NaCl (Figure 3A,B). In the presence of NaCl, the sintering degree of cubic Ni crystals was reduced greatly (Figure 2A,B). It is probably due to the fact that the NaCl molecules from the volatilized gaseous complexes (NiCl2·(NaCl)0.35) or volatilized individual gaseous NaCl molecules adsorbed on the surfaces of gaseous cubic Ni crystals and acted as one kind of surfactant to reduce substantially the surface energies of the gaseous Ni crystals and prevented them from sintering until the gaseous Ni crystals were transferred to a colder inner wall of the quartz tube by the carrier gases. The role of NaCl can be illustrated by the following eq 3.

When the formed Ni powder from the wall was flushed with anhydrous ethanol, NaCl and some other impurities such as FeCl3 were dissolved in ethanol and only pure Ni powder was left in solid. Xia et al.7 reported that cubic Ag nanocrystals were 9227



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particles of sample B shown in Figure 3D. The result is presented in Figure 5B. The magnetization curve at 295 K intercepts at the origin, with both remnant magnetization and coercivity absent, indicating that the superfine particles are superparamagnetic and have high crystallinity. The measured saturated magnetization of the superfine cubic particles (sample B) was 44 meu/g (Figure 5B). Their magnetic behavior renders them potential application in bioseparation and as magnetically recyclable catalysts. LaGrow reported a saturated magnetization of 10 emu/g at 300 K for the cubic Ni particles with a size of 11.6 nm.4 Carenco et al. reported a saturated magnetization of 25 emu/g at 300 K for the spherical Ni particles with a size of 21 nm.1 Lee et al. reported a saturated magnetization of 0.9 emu/g at 29 8K for the spherical Ni particles with a NiO shell and a particle size of 13 nm.23

REFERENCES

(1) Carenco, S.; Boissière, C.; Nicole, L.; Sanchez, C.; Floch, P. L.; Mézailles, N. Chem. Mater. 2010, 22, 1340−1349. (2) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int. Ed. 2009, 48, 60−103. (3) Leng, Y.; Wang, Y.; Li, X.; Liu, T.; Takahashhi, S. Nanotechnology 2006, 17, 4834−4839. (4) LaGrow, A. P.; Ingham, B.; Cheong, S.; Williams, G. V. M; Dotzler, C.; Toney, M. F.; Jefferson, D. A.; Corbos, E. C.; Bishop, P. T.; Cookson, J.; Tilley, R. D. J. Am. Chem. Soc. 2012, 134, 855−858. (5) Sun, Y.; Xia, Y. Science 2002, 298, 2176−2179. (6) Xiong, Y.; Chen, J.; Wiley, B.; Xia, Y.; Yin, Y.; Li, Z. Nano. Lett. 2005, 5, 1237−1242. (7) Xia, X.; Zeng, J.; Oetjen, L. K.; Li, Q.; Xia, Y. J. Am. Chem. Soc. 2012, 134, 1793−1801. (8) Pang, M.; Cairns, A. J.; Liu, Y.; Belmabkhout, Y.; Zeng, H. C.; Eddaoudi, M. J. Am. Chem. Soc. 2012, 134, 13176−13179. (9) Ren, J. T.; Tilley, R. D. J. Am. Chem. Soc. 2007, 129, 3287−3291. (10) Zhu, C.; Zeng, J.; Tao, J.; Johnson, M. C.; Schmidt-Krey, I.; Blubaugh, L.; Zhu, Y.; Gu, Z.; Xia, Y. J. Am. Chem. Soc. 2012, 134, 15822−15831. (11) Zhang, Y.; Grass, M. E.; Kuhn, J. N.; Tao, F.; Habas, S. E.; Huang, W.; Yang, P.; Somorjai, G. A. J. Am. Chem. Soc. 2008, 130, 5868−5869. (12) Xiong, Y.; Cai, H.; Wiley, B. J.; Wang, J.; Kim, M. J.; Xia, Y. J. Am. Chem. Soc. 2007, 129, 3665−3675. (13) Handbook of Non-Ferrous Metal Powders: Technologies and Applications, 1st ed.; Neikov, O. D., Naboychenko, S. S., Dowson, G., Ed.; Elsevier: Amsterdam, 2009; pp 369−398. (14) Bai, L. Y.; Yuan, F. L.; Yan, S. K.; Hu, P.; Li, J. L. Electron. Compon. Mater. 2006, 25, 6−9 (in Chinese). (15) Gurav, A.; Kodas, T.; Pluym, T.; Xiong, Y. Aerosol Sci. Technol. 1993, 19, 411−452. (16) Suh, Y. J.; Jang, H. D.; Chang, H. K.; Hwang, D. W.; Kim, H. C. Mater. Res. Bull. 2005, 40, 2100−2109. (17) Jang, H. D.; Hwang, D. W.; Kim, D. P.; Kim, H. C.; Lee, B. Y.; Jeong., I. B. Mater. Res. Bull. 2004, 39, 63−70. (18) Harris, P. J. F. Nature 1986, 323, 792−794. (19) Brillo, J.; Egry, I. J. Mater. Sci. 2005, 40, 2213−2216. (20) Elechiguerra, J. L.; Reyes-Gasga, J.; Yacaman, M. J. J. Mater. Chem. 2006, 16, 3906−3919. (21) Lucas, C. A.; Marković, N. M.; Ross, P. N. Phys. Rev. B 1997, 55, 7964−7971. (22) Carrasquillo, A.; Jeng, J. J.; Bamga, R. J.; Temesghen, W. F.; Soriaga, M. P. Inorg. Chim. Acta 1997, 255, 249−254. (23) Lee, I. S.; Lee, N.; Park, J.; Kim, B. H.; Yi, Y. W.; Kim, T.; Kim, T. K.; Lee, I. H.; Paik, S. R.; Hyeon., T. J. Am. Chem. Soc. 2006, 128, 10658−10659.

4. CONCLUSIONS A commercial raw nickel powder with a purity of 98% and an average particle size of 24 μm was used as the precursor of nickel. A powder mixture of sodium chloride and raw nickel with a molar ratio of NaCl/Ni 2.0 was chlorinated by dry chlorine at 700 °C. An anhydrous eutectic mixture with a melting point of 585 °C was formed. This mixture was heated in situ, and some nickel complexes (NiCl2)·(NaCl)0.35 were volatilized into the gas phase, where they were reduced into metallic Ni by H2 at 900 °C. It was found that some almostperfect superfine cubic (fcc) nickel single crystals with an average particle size of ∼503 nm and high purity were synthesized. The magnetic measurement indicated that the cubic nickel particles exhibited superparamagnetic behavior. Their saturated magnetization was 44 meu/g. Such high saturated magnetization renders them the potential applications in bioseparation and as magnetically recyclable catalysts. They had good dispersibility, good crystallinity, and high purity, which render them potential application as Ni electrodes of MLCCs. A controlled experiment without the addition of NaCl was also conducted. It was found that some irregular sphere-like nickel particles with an average particle size of ∼1435 nm were obtained in this case. It was inferred that the presence of the gaseous nickel complexes or gaseous individual NaCl molecules prevented the sintering of the formed superfine cubic Ni crystals in the gas phase at 900 °C. This discovery may provide a novel route in synthesizing various metal crystals of regular nonspherical shapes in the gas phase.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: 86-1062332525. Author Contributions

W.G. executed most of the experiment and wrote the manuscript. S.S designed the reactor. He also conceived, led the experiment, and modified the manuscript. Y.C. processed the data of XRD of various samples. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Professors Xianran Xin, Jun Chen, and Ranbo Yu for discussion, the financial support from the National Natural Science Foundation of China (grant no. 50874011). 9228


Synthesis of Dispersed Superfine fcc Nickel Single Crystals in Gas

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