Microstructural characterization of Ni nanoparticles prepared by anodic arc plasma

Materials Characterization 57 (2006) 176 – 181

Microstructural characterization of Ni nanoparticles prepared by anodic arc plasma Zhiqiang Wei a,c,⁎, Pengxun Yan b , Wangjun Feng a , Jianfeng Dai a , Qing Wang a , Tiandong Xia c a

School of Sciences, Lanzhou Univ. of Tech., Lanzhou, 730050, PR China Institute of Plasma and Metal Materials, Lanzhou University, Lanzhou, 730000, PR China State Key Lab. of Advanced New Non-ferrous Materials, Lanzhou Univ. of Tech., Lanzhou 730050, PR China b

c

Received 17 November 2005; accepted 10 January 2006

Abstract The particle size, specific surface area, crystal structure and morphology of Ni nanoparticles prepared by an anodic arc discharge plasma method were characterized via X-ray diffraction (XRD), transmission electron microscopy (TEM) and the corresponding selected area electron diffraction (SAED), X-ray energy dispersive spectrometry (XEDS), the Brunauer–Emmett– Teller (BET) equation, and the Barrett–Joyner–Halenda (BJH) method. The experimental results indicate that the crystal structure of the samples is face centered cubic (FCC) the same as that of the bulk materials, the specific surface area is 14.23 m2/g, with the particle size distribution ranging from 20 to 70 nm, the cumulative volume of the pores is 0.09cm3/g, the average pore diameter is 23 nm, and the average particle size about 47 nm. The grain size DXRD is smaller than the particle size DTEM and DBET due to agglomeration. The nanoparticles prepared by this method achieved uniform size, low impurity contamination, narrow size distribution and spherical shape. © 2006 Elsevier Inc. All rights reserved. Keywords: Ni nanoparticles; Crystal structure; Morphology; Particle size

1. Introduction Metal nanoparticles exhibit unique physical and chemical properties that differ considerably from those of the bulk solid state owing to the small size effect, surface effect, quantum size effect and quanta tunnel effect [1–4]. In recent years, metal nanoparticles have been intensively investigated due to the technological importance, theoretical interest and use in various high performance applications, such as catalysts, ferrofluids, ⁎ Corresponding author. School of Sciences, Lanzhou Univ. of Tech., Lanzhou, 730050, PR China. Fax: +86 931 2976040. E-mail address: [email protected] (Z. Wei). 1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.01.004

microwave devices, low temperature sinterable, high strength ceramics, high-density magnetic recording materials, and lubricants, etc. [5–7]. It is important to investigate the microstructure of metal nanoparticles obtained by the anodic arc discharge plasma method. This is because the properties depend strongly on the details of its microstructure, such as the chemical composition, specific surface area, phase structures, grain size and particle size, shape and agglomeration. All of these aspects offer the possibility for obtaining nanoparticles with desired physical and chemical properties. Various techniques have been developed to prepare metal nanoparticles, such as gas-phase chemical

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reaction [8], spray pyrolysis [9], water-heating reaction [10], laser ablation [11], flame processing [12], vapor deposition [13], microwave plasma synthesis [14], and sol–gel methods [15]. The arc discharge plasma method is a mature and advanced materials processing technique, which has successfully been applied for the production of metal nanoparticles in the past [4,16]. In this paper, microstructural characterizations of Ni nanoparticles prepared by an anodic arc discharge plasma method in inert atmosphere were performed. In addition, the particle size, microstructure, and morphology of the particles were characterized via Xray diffraction (XRD), transmission electron microscopy (TEM) and the corresponding selected area electron diffraction (SAED). The specific surface area and pore parameters were investigated by the Brunauer–Emmett–Teller (BET) equation and the Barrett–Joyner–Halenda (BJH) method. The chemical compositions were determined by X-ray energy dispersive spectrometry (XEDS) and elemental analysis instruments. 2. Experimental Metal nanoparticles for our experiments were prepared by an anodic arc discharging plasma technique in inert atmosphere with home-made experimental apparatus which has been described elsewhere [17]. In the process of preparation, the vacuum chamber was pumped to 10− 3 Pa and then backfilled with inert argon (purity 99.99%) to near 103 Pa. The electric arc in the inert environment was automatically ignited between the tungsten electrode and the nozzle by a high frequency initiator. The arc was then maintained by the current source at the pre-established values of the voltage and current. Under argon pressure and electric discharge current heating, the ionized gases were driven through the nozzle outlet and from the plasma jet [17]. The bulk metal was heated and melted by the high temperature, and metal atoms were detached from the metal surface when the plasma jet heating energy exceeded the metal superficial energy, evaporating into the free atom state. Above the evaporation source was a region of supersaturated metal vapor, where the metal atoms diffused and collided with each other to decrease the nucleiforming energy. When the metal vapor became supersaturated, a new phase was nucleated homogeneously out of the aerosol systems [16,18]. The droplets were rapidly cooled and combined to form primary particles by an aggregation growth mechanism [19,20]. Free inert gas convection developed

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between the hot evaporation source and the cooled collection cylinder and transported the particles out of this nucleation and growth region to the inner walls of the cylinder. The loose nanoparticles could be obtained after a period of passivation and stabilization with the working gas. The crystal structure and grain sizes were analyzed by a Japan Rigaku D/max-2400 X-ray diffraction diffractometer using monochromatic high-intensity CuKa radiation (λ = 1.54056 Å), at a scanning speed of 2°–2θ h/min from 30 to 100° (2θ). The average grain size of the particles was estimated from X-ray line broadening measurements according to the Scherrer formula. A small amount of the powder was dispersed in ethanol, ultrasonically stimulated, and deposited on copper grids with a holey carbon-coated film. The samples were placed in a vacuum oven to dry at ambient temperature before examination. The particle size and morphology shape were investigated by transmission electron microscopy (TEM) and selected area electron diffraction (SAED) in a Japan JEOL JEM-1200EX microscope with an accelerating voltage of 80 kV. The main constituent elements and their relative content were determined by X-ray energy dispersive spectrometry (XEDS) in a scanning electron microscope (SEM, JEOL Ltd., Tokyo, Japan). Impurities, such as nitrogen, hydrogen, oxygen and carbon were determined by using an elemental analysis instrument (Elemental Vario EL, Germany). The specific surface area and pore parameters were measured by nitrogen adsorption–desorption isotherms at 77.35 K. The data were evaluated automatically by an America micromeritics ASAP-2010 analyzer. Approximately 0.5g of the powders were placed in a test tube and allowed to degas for 2h at 175 °C in flowing nitrogen. This removed contaminants such as water vapor and adsorbed gases from the samples. The static physisorption isotherms were obtained with liquid nitrogen, by measuring the amount of liquid nitrogen adsorption or desorption from the material as a function of pressure (p / p0 = 0.025–0.999). Data were obtained by admitting or removing a known quantity of adsorbing gas into or out of a sample cell containing the solid adsorbent maintained at a constant temperature (77.35 K), which is below the critical temperature of the adsorbate. As adsorption or desorption occurs, the pressure in the sample cell changes until equilibrium is established. The quantity of adsorbed or desorbed gas at the equilibrium pressure is equal to the difference between the amount of gas admitted or removed and the amount required to fill the space around the adsorbent.

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Fig. 1. X-ray energy dispersive spectrometry of Ni nanoparticles.

The specific surface area of the powders was determined by the Brunauer–Emmett–Teller (BET) method, and the average pore diameter and cumulative pore volume was estimated by the Barrett–Joyner–Halenda (BJH) method. 3. Results and discussion 3.1. Chemical compositions and impurities Chemical compositions of the products were analyzed using three methods. The main constituent elements and their relative concentrations were determined by XEDS. Impurities, such as nitrogen, hydrogen, oxygen and carbon were determined by elemental analysis, respectively. The analysis of the physical phases was carried out by XRD. Four elements (nickel, nitrogen, silicon and oxygen) were found in the powder by the XEDS analysis, Fig. 1. It is obvious that the silicon peak is caused by the glass substrate on which the Ni nanoparticles were mounted for XEDS analysis. Table 1 shows XEDS quantitative microanalysis that indicates a predominance of nickel (98.30 wt.%), as the main nanoparticle constituent. Table 2 lists the impurities of the sample. It shows that carbon, hydrogen, nitrogen and oxygen content for the sample were only 0.41, 0.34, 0.82 and 0.19 wt.%, Table 1 X-ray energy dispersive spectrometry quantitative microanalysis of Ni nanoparticles Element

Line

Weight (%)

K-ratio

Cnts/s

Atomic (%)

N O Si Ni Total

Ka Ka Ka Ka

0.51 0.58 0.61 98.30 100

0.0012 0.0021 0.0022 0.9796

0.46 1.49 3.20 338.86

2.06 2.07 1.22 94.65 100

respectively. The total amount was very low, not higher than 1.76 wt.%. It is conceivable that Ni nanoparticles are very active and adsorptive, such that when they were exposed in air they adsorbed N2, CO2 and O2 on the surface of the particle, and reacted intensely with oxygen in the air to form oxides, even at ambient temperatures. The XRD pattern (Fig. 2) does not reveal any phase other than the characteristic peaks of nickel. This result shows that the Ni nanoparticles produced in this work are of high purity. 3.2. Crystal structure and grain geometry Fig. 2 shows the typical X-ray diffraction pattern of the Ni nanoparticles. Due to the small size effect and incomplete inner structure of the particle, the XRD peaks are low and broad. On the other hand, the peaks with 2θ values of 44.52°, 51.88°, 76.40°, 92.96° and 98.48° correspond to the (111), (200), (220), (311), and (222) planes of the bulk FCC Ni phase, respectively. Fig. 3(a) shows a representative transmission electron microscopy (TEM) micrograph of Ni nanoparticles. It can be seen that most of the particles have a spherical shape, a fairly uniform size and a smooth surface. Few small particles have aggregated into secondary particles because of their extremely small dimensions and high surface energy. This sphere-chain shape is the result of magnetic forces and the attractive surface tension between the ultra-fine particles. Fig. 3(b) shows the corresponding selected-area electron diffraction (SAED) pattern. It can be indexed Table 2 Chemical composition analyses of Ni nanoparticles Element

C

H

N

O

Content (wt.%)

0.41

0.34

0.82

0.19

(311) (222)

(220)

40

179

Table 3 Comparison of interplaner spacings (dhkl) and the lattice parameter (a) with standard ASTM data (200)

Intensity [cps]

(111)

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60

80

100

2 Theta [deg] Fig. 2. XRD patterns of Ni nanoparticles.

to the reflection of a face centered cubic (FCC) structure; this result was further confirmed by means of X-ray diffraction. Tropism of the random small particles causes the broadening of the diffraction rings that are made up of many diffraction spots; this indicates that the polycrystalline structure of the nanoparticles. Electron diffraction reveals that each particle is composed of many small crystal nuclei, which is convincing proof that the particles grow by aggregation. According to the electron diffraction formula Rdhkl = λL and the X-ray diffraction relation λ = 2dhkl cosθ, the values of the interplaner spacings dhkl were calculated from the diameters of the diffraction rings, as well as from thep results of the XRD analysis. For FCC structure, ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dhkl = a / h2 þ k 2 þ l 2 , the lattice parameter (a) can be calculated from measured values for the spacing of the 111 plane. Table 3 presents the results of calculations of the lattice parameter from the interplaner spacings measured in the TEM-SAED and XRD analyses, and

Method

TEM (nm)

XRD (nm)

ASTM standard value (nm)

Interplaner spacings (dhkl) Lattice parameter (a)

0.2036

0.2036

0.2034

0.3529

0.3526

0.3524

compares them to standard ASTM data (a = 0.3524 nm); very good agreement is observed. 3.3. Particle size and agglomerating state From the full width at half maximum, the average grain size for the sample can be estimated from the (111) diffraction peak in the XRD spectra according to Kk Scherrer's formula: DXRD = Bcosh , where DXRD is the crystallite size; K = 0.89, which is the Scherrer constant related to the shape and index (hkl) of the crystals; λ is the wavelength of the X-rays (Cu Kα, 1.54056 Å); θ is the diffraction angle; and B is the measured half width height of the diffraction peak. By this method, the average crystallite size is calculated to be around 42nm. From the data obtained from TEM micrographs, the particle size histograms can be drawn and the mean size of the particles can be determined. Fig. 4 shows the particle size distribution of the Ni nanoparticles. It can be seen that the particle sizes range from 20 to 70nm, the median diameter (taken as average particle diameter) is about 47 nm, being deduced from the images, which show a relatively narrow size distribution.

Fig. 3. (a) TEM micrograph and (b) the selected area electron diffraction pattern of Ni nanoparticles.

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20 15 10 5 0 20

40

60

80

100

Volume Absorbed (cm3/g )

Frequency (%)

25 50 40 30

desorption

20 10

adsorption

Particle size (nm) 0 0.0

Fig. 4. Particle size distribution of Ni nanoparticles.

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P / Po)

Table 4 Average particle size calculated by various methods

Fig. 5. N2 adsorption–desorption isotherms of Ni nanoparticles.

a larger particle size in the experimental results. In other words, one particle usually contained many grains. 3.4. Micropore size distribution Fig. 5 shows the typical nitrogen sorption isotherms of the Ni nanoparticles. It indicates that the sample presents typical IV adsorption. In the low-pressure region (P / P0 b 0.8), the adsorption–desorption isotherms are relatively flat, and adsorption and desorption are completely superpositioned because adsorption mostly occurs in the micropores. In the high relative pressure region (P / P0 N 0.8), the isotherms increase rapidly, and form a lag loop owing to capillary agglomeration phenomena. Fig. 6 shows BJH pore size distribution curves of the Ni nanoparticles. It shows that the micropores range from 10 to 60 nm, with most of the micropores having size smaller than 40 nm, and a narrow pore size distribution. The average pore diameter estimated from the peak position is about 23 nm. However, such micropores have not been observed in Fig. 4(a). Therefore, these particles 25

Pore volumes (cm3/g)

The surface area analysis was carried out on Ni nanoparticles by the BET method. Assuming the particles have a solid, spherical shape with smooth surfaces and same size, the specific surface area can be related to the average equivalent particle size by the equation: DBET = 6000 / (ρ · Sw) (in nm), where DBET is the average diameter of a spherical particle; Sw represents the measured specific surface area of the powder in m2/g; and ρ is the particle density in g/cm3. The specific surface area is 14.23 m2/g, which is calculated with the multi-point BET equation. The corresponding average equivalent particle size is 46nm. The average particle sizes, DTEM, obtained by TEM measurements are listed in Table 4 together with DBET obtained from BET value and DXRD estimated from the Scherrer formula. It shows that both the particle size and the grain size of Ni nanoparticles are all on the order of nanometers. It is worthwhile to note here that, for an identical specimen, the magnitude of DTEM is very similar to the value of DBET, whereas the average grain size DXRD obtained from XRD is slightly smaller than the DTEM and DBET particle sizes. This difference, we believe, is by no means fortuitous. It can be ascribed to the fact some agglomeration has occurred. In fact, TEM gives some evidence of the existence of few agglomerates with a size of less than 100 nm (see Fig. 3(a)). Because the new gamma (FCC) phase was formed by nucleation and growth independently through a solid– gas reaction during the process, some particles would be coupled together to form larger particle clusters by chemical bonding among particles, and these large hard agglomerates can not be separated by ultrasonic vibration before the TEM examinations. This leads to

20

15

10

5

0 0

Method

DBET

DTEM

DXRD

Average particle size (nm)

46

47

42

20

40

60

80

100

120

Pore diameter (nm) Fig. 6. BJH pore size distribution curves of Ni nanoparticles.

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are more probably grain clusters, i.e. small polycrystals. By assuming full saturation of the pores at the relative pressure of 0.95, the cumulative pore volume is approximately 0.09 cm3/g. 4. Conclusions (1) Microstructural characterizations of Ni nanoparticles prepared by anodic arc discharge plasma method in inert atmosphere were performed. The nanoparticles prepared by this method achieved uniform size, high purity, narrow size distribution and spherical shape. (2) The crystalline structure of the particles is FCC, the same as that of the bulk materials, the specific surface area is 14.23 m2/g, with a particle size distribution ranging from 20 to 70 nm. The cumulative pore volume is 0.09 cm 3 /g, the average pore diameter is 23 nm, and the average particle size is about 47 nm. The magnitude of DTEM is very similar to DBET, whereas the grain size DXRD is smaller than the particle size values, DTEM and DBET. This difference is ascribed to agglomeration. Acknowledgments We appreciate the financial support of the Natural Science Foundation of Gansu Province, PR China (No. 3ZS042-B25-017). References [1] Gleiter H. Nanocrystalline materials. Prog Mater Sci 1990;3 (4):223. [2] Chen YJ, Cao MS, Tian Q. A novel preparation and surface decorated approach for a-Fe nanoparticles by chemical vapor– liquid reaction at low temperature. Mater Lett 2004;58:1481. [3] Zhang WW, Cao QQ. Structural, morphological, and magnetic study of nanocrystalline cobalt–nickel–copper particles. J Colloid Interface Sci 2003;257:237.

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