- Email: [email protected]
Preparation and particle size characterization of Cu nanoparticles prepared by anodic arc plasma
RARE METALS Vol. 25, No. 2, Apr 2006, p. I72
Preparation and particle size characterization of Cu nanoparticles prepared by anodic arc plasma WEI Zhiqiang1’2’,XIA Tiandong2’,FENG Wangiun”, DAI Jianfeng”, W M G Qing”, LI Weixue”, and YAN ~engxun~’ 1 ) School of Sciences, Lanzhou University of Technology, Lanzhou 730050. China 2) College of Materials Science and Engineering. Lanzhou University of Technology, Lanzhou 730050,China
3) Institute of Plasma and Metal Materials, Lanzhw University, Lanzhou 73ooo0, China (Received 2004- 12-30)
Abstract Copper nanoparticles were successfully prepared in large scale by means of anodic arc discharging plasma methcd in inert atmosphere. The particle size, specific surface area,crystal structure, and morphology of the samples were characterized by X-ray diffraction (XRD), BET equation, transmission electron microscopy (TEM), and the corresponding selected area electron diffraction (SAED). The experimental results indicate that the crystal structure of the samples is fcc stn~cturethe same as that of the bulk materials. The specific surface area is 11 m2/g,the particle size distribution is 30 to 90 nm, and the average particle size is about 67 nm obtained from TEM and confirmed from XRD and BET results. The nanoparticles with uniform size, high purity, narrow size distribution and spherical shape can be prepared by this convenient
and effective method. Key words: metal materials; Cu nanoparticles; anodic arc plasma; particle size; structure [This work wasfinancially supported by the Natural Science Foundation of Gansu Province, China (No. 323042-B25-017).]
1. Introduction Metal nanoparticles exhibit novel physical and chemical properties owing to the small size effect, surface effect, quantum size effect, and quanta tunnel effect [ 1-41. In recent years, the research and development for metal nanoparticles have attracted significant interest and is still the subject of intense investigation owing to their intriguing properties and various potential applications [5-71. Because the properties depend strongly on the details of particle size, specific surface area, crystal structure, and morphology of the particles, all these aspects offer the possibility for obtaining nanoparticles with desired physical and chemical properties. Many techniques have been developed to prepare metal nanoparticles, such as gas-phase chemical reaction [8], spray pyrolysis [9], water-heating reaction [lo], laser ablation [ I l l , flame processing [12], Corresponding author: WE1 Zhiqiang
vapor deposition [131, microwave plasma synthesis [14], and sol-gel method [IS]. However, there exist some limitations in the above-mentioned methods. For example, these methods usually cannot be fulfilled promptly and generally are not suitable for large-scale production in factory, and such powders prepared by these methods cannot usually be isolated. In addition, commercial exploitation of nanoparticles is currently limited by high synthesis costs. From a practical viewpoint, it is vital to develop a way of manufacturing high quality nanoparticles in large quantities at low cost. For this reason, an efficient process has been developed in our laboratory to prepare high quality nanoparticles, which will solve all the above limitations. Some metals have been prepared and the preliminary results have shown that this technique is a promising method and has some advantages which are as follows: (1) this method is convenient, inexpensive, effective and
E-mail: zqwei741 I @ 163.corn
Wei Z.Q. et al., Preparation and particle size characterization of Cu nanoparticles prepared by
high yields; (2) the nanoparticles prepared by this method have uniform size, higher purity, narrow size distribution, and spherical shape; (3) the properties (particle size, morphology, and other characteristics) can be easily improved by varying the technological parameters; (4) this is a continuous production and suitable for bulk production in a factory. In this article, the results of preparation and particle size characterization of Cu nanoparticles prepared by anodic arc discharging plasma method with homemade experimental apparatus are investigated. The crystal structure and the average grain size are measured by X-ray diffraction (XRD), and the average crystalline size is estimated with the (111) diffraction peak using the Scherrer formula. In addition, the particle size and morphology are further characterized by transmission electron microscopy (TEM)and the corresponding selected area electron diffraction (SAED). The specific surface area is determined by the Brunauer-Emmett-Teller (BET) method with nitrogen adsorption, and the average equivalent particle size is obtained.
2.
Experimental
The schematic diagram of the experimental installation for obtaining metal nanoparticles is presented in Fig. 1. The experimental apparatus mainly consisted of the stainless steel vacuum chamber, the gas supply device, DC power supply current source, the plasma generator with high frequency initiator,
Fig. 1. Schematic diagram of the experimental installation.
...
173
the vacuum pump, the water-cooled collection cylinder, and the water-cooled copper crucible with a diameter of 20 cm mounted in an electrically insulated manner and connected to an arc current supply as the anode. The tungsten rod of 10 mm was mounted in an insulated and axially sliding manner and c o ~ e ~ t to e da power supply as the cathode. The temperature could be adjusted by appropriate positioning of the tungsten rod with respect to the crucible. The bulk raw material to evaporate was placed on the crucible. In the process of preparation, the vacuum chamPa and was then backfilled ber was pumped to with inert argon (purity 99.99%) to near lo3Pa. The electric arc in the argon environment was automatically ignited between the tungsten electrode and the nozzle (well cooled) with a high-frequency initiator. It was maintained by the current source at the pre-established values of the voltage and the current. Under argon pressure and electric discharge current, the ionized gases were driven through the nozzle outlet and formed the plasma jet [16]. The bulk metal Cu was heated and melted by the high temperature of the plasma, and metal atoms were detached from the metal surface when the plasma jet kinetic energy exceeded the metal superficial energy and evaporated into atom soot. The above evaporation source was a region of supersaturatedmetal vapor, where the metal atoms diffused around and collided with each other to decrease the germ-forming energy. When the metal vapor was supersaturated, a new phase. nucleates homogeneously out of the aerosol systems [17]. The droplets were rapidly cooled and combined to form primary particles by an aggregation growth mechanism [ 18-191. The free inert gas convection developed between the hot evaporation source and the cooled collection cylinder, which transported the particles out of this nucleation and growth region to the inner walls of the cylinder. The loose nanopowders can be obtained after a period of passivation and stabilization with working gas. Some metal nanopowders have been prepared and the referential experimental conditions are summarizedin Table 1.
RARE METALS, Vol. 25, No. 2, Apr 2006
I74
Table 1. Referential technology parameters for preparing metal nanopowders by anodic arc plasma
Gas pressure / kPa 0.4-1.4
Atmosphere
Arc voltage / V
Arc current / A
He. N,,Ar
20-30
60-160
The structure of the samples was characterized by a Japan Rigaku D/max-2400 X-ray powder diffraction (XRD) diffractometer using monochromatic high-intensity Cu K, radiation (A= 0.154056 nm), at a scanning speed of 2"/min from 30" to 100" (2 0 ). The particle size and morphology shape were investigated by transmission electron microscopy (TEM) and the corresponding electron diffraction (ED) with a Japan JEOL EM-1200EX microscope with an accelerating voltage of 80 kV. A small amount of the powder was dispersed in isopropanol and sonicated for 10 min, and several drops from a dmppex of the suspension samples were placed on copper grids with holey carbon coated film. The samples were placed in a vacuum oven to dry at ambient temperature before examining. The specific surface area was analyzed by multi-point full analysis of nitrogen adsorption-desorption isotherms at 77 K. The data were evaluated automatically by an America micromeritics ASAP-2010 analyzer. Approximately 0.3 to 0.5 g of powder was placed in a test tube and allowed to
Cooled condition Water
Yield rate / (gmin-')
0.5-1.3
degas for 2 h 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, the amount of liquid nitrogen adsorption or desorption from the material as a function of pressure @/PO= 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 K). As adsorption or desorption occurred, the pressure in the sample cell changed until equilibrium was established. The specific surface area of the powders was determined by the Brunauer-Emmett-Teller (BET) method based on nitrogen adsorption.
3. Results and discussion Fig. 2(a) shows a TEM micrograph of Cu nanoparticles. The sample is scanned in all zones before the picture is taken. All the particles are spherical in shape, and small particles aggregate into second& particles because of their extremely small
Fig. 2. TEM micrograph (a) and SAED pattern (b) of Cu nanoparticles.
dimensions and high surface energy. Most of the particles appear to be fairly uniform in size, with smooth surface. Fig. 2(b) shows the corresponding SAED pattern. It can be indexed to the reflection of
face centered cubic (fcc) structure, and this result was also investigated by means of XRD. Tropism of the particles at random and small particles cause the widening of diffraction rings that are made up of
Wei Z.Q. et al., Preparation and particle size characterizationof Cu nanoparticlesprepared by ...
many diffraction spots, which indicates that the nanoparticles are polycrystalline structures. Electron diffraction reveals that each particle is composed of many small crystal nuclei, which proves that the particles grow by an aggregation model. From the data obtained by TEM micrographs, the particle size histograms can be drawn and the mean size of the particles can be determined. Fig. 3 shows the particle size distribution of Cu nanoparticles.The particle sizes range from 30 to 90 nm, the median diameter (taken as average particle diameter) is about 67 nm, and the particle size distribution is narrow. Fig. 4 shows the typical XRD pattern for the specimen. Due to the small size effect and incomplete inner structure of the particle, the XRD diffraction peaks are low and wide. Five broad peaks with 2 0 values of 43.44", 50.56", 74.24', 90.04', and 95.24" correspond to the (1 1l), (200), (220), (31I), and (222) planes of bulk Cu respectively, which can be assigned to Cu fcc phase. The XRD spectrum does not reveal any other phase except for the characteristic peaks of copper, indicating that the physical phases of copper nanoparticles synthesized in this work have high purity. From the full width at half maximum, the average crystalline size can be estimated with the (1 1 1 ) diffraction peak in the XRD spectra according to the Scherrer formula d = KA/(BcosO) . d is the crystallite size; K = 0.89, which is the Scherrer approximate constant related to the shape and index (hkl) of the crystals; A is the wavelength of the X-ray (Cu &,0.14954 nm); 0 is the diffraction angle; and B is the corrected half-width of the diffraction peak (in radians) given by B 2 = B i - B; , where B, is the measured half-width and B, is the half-width of a standard sample with a known crystal size greater than 100 nm. The effect of geometric (instrumental) broadening on the reflection peaks was calibrated. The average crystallite size is calculated to be around 63 nm, which corresponds well with the average particle diameter obtained from the TEM image. The BET method is based on low-temperature adsorption of nitrogen and leads to determination of
175
the specific surface area of the powder. If it is accepted that the powder consists of solid, spherical shape particles with smooth surface and the same size, the relation between surface area and average equivalent particle size is equal to D = 60004 p S,), where D is the average diameter of a spherical particle in nm, S, represents the measured surface area of the powder in m2/g, and p i s the particle density. Fig. 5 shows BET plots of Cu nanoparticles. The specific surface area of the powder is 11 m2/gcalculated with the BET equation. The corresponding average equivalent particle size is 68 nm derived from these values.
Fig. 3. Particle size distribution of Cu nanoparticles.
0 2e I Fig. 4. XRD pattern of Cu nanoparticles. (0)
0.55 I
5= 4
0.451
I
s. Y &
'4 0.35 -
0.25 0.06
2
PIP,
Fig. 5. BET plots of Cu nanoparticles.
RARE METALS, Vol. 25, No. 2, Apr 2006
176
The average particle sizes of the as-prepared particles, calculated from the Scherrer formula, obtained based on TEM data and estimated from BET values are listed in Table 2. The particle size obtained from the BET equation and the TEM methods agree very well with the result given by X-ray line broadening. The results of TEM observations and BET methods further confirm and verify the relevant results obtained by XRD as mentioned earlier. Table 2. Average particle sizes calculated by various methods nm DBET
-
68
4ul 66
DXRD 63
4. Conclusions Copper nanoparticles were successfully prepared in large scale by means of anodic arc discharging plasma method with homemade experimental apparatus. This method is a convenient approach to prepare high-quality metal nanoparticles, and especially it can be fulfilled promptly and is suitable for large-scale production in a factory. The nanoparticles prepared by this technique have uniform size, high purity, narrow size distribution, and spherical shape. The crystal structure of the samples is fcc structure the same as that of the bulk materials. The specific surface area is 1 lrn’/g, with the particle size distribution ranging from 30 to 90 nm, and an average particle size of about 66 nm obtained from E M and confirmed by XRD and BET results.
References [ 11 Gleiter H., Nanocrystalline materials. Prog. Muter.
Sci.,1990.33(4): 223. [2] Chen Y.J., Cao M.S., and Tian Q., A novel preparation and surface decorated approach for a-Fe nanoparticles by chemical vapor-liquid reaction at low temperature, Muter. Len.,2004.58: 1481. [3] Zhang W.W. and Cao Q.Q., Structural, morphological, and magnetic study of nanocrystalline cobalt-nickel-copper particles. J. Colloid InterJiuce Sci., 2003,257: 237. [4] Cui Z. L.. Dong L.F., and Hao C.C., Microstructure and magnetic property of nano-Fe particles prepared
by hydrogen arc plasma, Mater. Sci. Eng., 2000, A
286:205. [5] Gleiter H., Materials with ultrafne microstructures: retrospectives and perspectives, Nanosttuct. Muter., 1992.1: I . [6] Chen B.J., Sun X.W., and Xu C.X., Growth and characterization of zinc oxide nano/micro-fibers by thermal chemical reactions and vapor transport deposition in air,Phys. E, 2004,21: 103. [7] Chen D.H. and Chen D.R., Hydrothermal synthesis and characterization of octahedral nickel femte particles, Powder Technol., 2003, 133 247. [8] Cao M.S. and Deng Q.G., Synthesis of nitride-iron nano meter powder by thermal chemical vapor-phase reaction method, J. Inorg. Chem., 1996,12(1): 88. [9] Karthikeyan J., Bemdt C.C., and Tikkanen J., Plasma spray synthesis of nanomaterial powders and deposits, Muter. Sci. Eng., 1997,A 238: 275. [lo] Zheng H.G.. Lang J.H., and Zeng J.H., Preparation of nickel nanopowders in ethanol-watersystem (EWS), Muter. Res. Bull., 2001,36: 947. [I I ] Gaertner G.F. and Miquel P.F., Particle generation by laser ablation from solid targets in gas flows, Nanostruct. Muter., 1993.4 (3): 559. [I21 Katz J.L. and Miquel P.F., Synthesis and applications of oxides and mixed oxides produced by a flame process, Nanostmct. Marer., 1994,4 (5): 55 1. [I31 Gunther B. and Kumpmann A., Ultrahe oxide powders prepared by inert gas evaporation, Nanostrucr. Muter., 1992,l ( I ) : 27. [I41 Vollath D. and Sickafus K.E., Synthesis of nanosized ceramic oxide powders by microwave plasma reactions, Nanosttuct. Mater., 1992, 1 (5): 427. [IS] Chen D.H. and He X.R., Synthesis of nickel femte nanoparticles by sol-gel method, Muter. Res. Bull., 2001,36: 1369. [ 161 Andre J. and Kwan W., Arc-discharge ion sources for heavy ion fusion, Nucl. Instnun. Methods Phys. Res., 2001, A 464:569. [ 171 Ioan B., Nanoparticle production by plasma, M a w . Sci. Eng., 1999,B 68:5. [ 181 Kaito C., Coalescence growth mechanism of smoke particles, Jpn. J. Appl. Phys., 1985.24: 261. [ 191 Scott J.H. and Majetich S.A., Morphology, structure, and growth of nanoparticles produced in a carbon arc, Phys. Rev., 1995, B 52: 12564.
Preparation and particle size characterization of Cu nanoparticles prepared by anodic arc plasma WEI Zhiqiang1’2’,XIA Tiandong2’,FENG Wangiun”, DAI Jianfeng”, W M G Qing”, LI Weixue”, and YAN ~engxun~’ 1 ) School of Sciences, Lanzhou University of Technology, Lanzhou 730050. China 2) College of Materials Science and Engineering. Lanzhou University of Technology, Lanzhou 730050,China
3) Institute of Plasma and Metal Materials, Lanzhw University, Lanzhou 73ooo0, China (Received 2004- 12-30)
Abstract Copper nanoparticles were successfully prepared in large scale by means of anodic arc discharging plasma methcd in inert atmosphere. The particle size, specific surface area,crystal structure, and morphology of the samples were characterized by X-ray diffraction (XRD), BET equation, transmission electron microscopy (TEM), and the corresponding selected area electron diffraction (SAED). The experimental results indicate that the crystal structure of the samples is fcc stn~cturethe same as that of the bulk materials. The specific surface area is 11 m2/g,the particle size distribution is 30 to 90 nm, and the average particle size is about 67 nm obtained from TEM and confirmed from XRD and BET results. The nanoparticles with uniform size, high purity, narrow size distribution and spherical shape can be prepared by this convenient
and effective method. Key words: metal materials; Cu nanoparticles; anodic arc plasma; particle size; structure [This work wasfinancially supported by the Natural Science Foundation of Gansu Province, China (No. 323042-B25-017).]
1. Introduction Metal nanoparticles exhibit novel physical and chemical properties owing to the small size effect, surface effect, quantum size effect, and quanta tunnel effect [ 1-41. In recent years, the research and development for metal nanoparticles have attracted significant interest and is still the subject of intense investigation owing to their intriguing properties and various potential applications [5-71. Because the properties depend strongly on the details of particle size, specific surface area, crystal structure, and morphology of the particles, all these aspects offer the possibility for obtaining nanoparticles with desired physical and chemical properties. Many techniques have been developed to prepare metal nanoparticles, such as gas-phase chemical reaction [8], spray pyrolysis [9], water-heating reaction [lo], laser ablation [ I l l , flame processing [12], Corresponding author: WE1 Zhiqiang
vapor deposition [131, microwave plasma synthesis [14], and sol-gel method [IS]. However, there exist some limitations in the above-mentioned methods. For example, these methods usually cannot be fulfilled promptly and generally are not suitable for large-scale production in factory, and such powders prepared by these methods cannot usually be isolated. In addition, commercial exploitation of nanoparticles is currently limited by high synthesis costs. From a practical viewpoint, it is vital to develop a way of manufacturing high quality nanoparticles in large quantities at low cost. For this reason, an efficient process has been developed in our laboratory to prepare high quality nanoparticles, which will solve all the above limitations. Some metals have been prepared and the preliminary results have shown that this technique is a promising method and has some advantages which are as follows: (1) this method is convenient, inexpensive, effective and
E-mail: zqwei741 I @ 163.corn
Wei Z.Q. et al., Preparation and particle size characterization of Cu nanoparticles prepared by
high yields; (2) the nanoparticles prepared by this method have uniform size, higher purity, narrow size distribution, and spherical shape; (3) the properties (particle size, morphology, and other characteristics) can be easily improved by varying the technological parameters; (4) this is a continuous production and suitable for bulk production in a factory. In this article, the results of preparation and particle size characterization of Cu nanoparticles prepared by anodic arc discharging plasma method with homemade experimental apparatus are investigated. The crystal structure and the average grain size are measured by X-ray diffraction (XRD), and the average crystalline size is estimated with the (111) diffraction peak using the Scherrer formula. In addition, the particle size and morphology are further characterized by transmission electron microscopy (TEM)and the corresponding selected area electron diffraction (SAED). The specific surface area is determined by the Brunauer-Emmett-Teller (BET) method with nitrogen adsorption, and the average equivalent particle size is obtained.
2.
Experimental
The schematic diagram of the experimental installation for obtaining metal nanoparticles is presented in Fig. 1. The experimental apparatus mainly consisted of the stainless steel vacuum chamber, the gas supply device, DC power supply current source, the plasma generator with high frequency initiator,
Fig. 1. Schematic diagram of the experimental installation.
...
173
the vacuum pump, the water-cooled collection cylinder, and the water-cooled copper crucible with a diameter of 20 cm mounted in an electrically insulated manner and connected to an arc current supply as the anode. The tungsten rod of 10 mm was mounted in an insulated and axially sliding manner and c o ~ e ~ t to e da power supply as the cathode. The temperature could be adjusted by appropriate positioning of the tungsten rod with respect to the crucible. The bulk raw material to evaporate was placed on the crucible. In the process of preparation, the vacuum chamPa and was then backfilled ber was pumped to with inert argon (purity 99.99%) to near lo3Pa. The electric arc in the argon environment was automatically ignited between the tungsten electrode and the nozzle (well cooled) with a high-frequency initiator. It was maintained by the current source at the pre-established values of the voltage and the current. Under argon pressure and electric discharge current, the ionized gases were driven through the nozzle outlet and formed the plasma jet [16]. The bulk metal Cu was heated and melted by the high temperature of the plasma, and metal atoms were detached from the metal surface when the plasma jet kinetic energy exceeded the metal superficial energy and evaporated into atom soot. The above evaporation source was a region of supersaturatedmetal vapor, where the metal atoms diffused around and collided with each other to decrease the germ-forming energy. When the metal vapor was supersaturated, a new phase. nucleates homogeneously out of the aerosol systems [17]. The droplets were rapidly cooled and combined to form primary particles by an aggregation growth mechanism [ 18-191. The free inert gas convection developed between the hot evaporation source and the cooled collection cylinder, which transported the particles out of this nucleation and growth region to the inner walls of the cylinder. The loose nanopowders can be obtained after a period of passivation and stabilization with working gas. Some metal nanopowders have been prepared and the referential experimental conditions are summarizedin Table 1.
RARE METALS, Vol. 25, No. 2, Apr 2006
I74
Table 1. Referential technology parameters for preparing metal nanopowders by anodic arc plasma
Gas pressure / kPa 0.4-1.4
Atmosphere
Arc voltage / V
Arc current / A
He. N,,Ar
20-30
60-160
The structure of the samples was characterized by a Japan Rigaku D/max-2400 X-ray powder diffraction (XRD) diffractometer using monochromatic high-intensity Cu K, radiation (A= 0.154056 nm), at a scanning speed of 2"/min from 30" to 100" (2 0 ). The particle size and morphology shape were investigated by transmission electron microscopy (TEM) and the corresponding electron diffraction (ED) with a Japan JEOL EM-1200EX microscope with an accelerating voltage of 80 kV. A small amount of the powder was dispersed in isopropanol and sonicated for 10 min, and several drops from a dmppex of the suspension samples were placed on copper grids with holey carbon coated film. The samples were placed in a vacuum oven to dry at ambient temperature before examining. The specific surface area was analyzed by multi-point full analysis of nitrogen adsorption-desorption isotherms at 77 K. The data were evaluated automatically by an America micromeritics ASAP-2010 analyzer. Approximately 0.3 to 0.5 g of powder was placed in a test tube and allowed to
Cooled condition Water
Yield rate / (gmin-')
0.5-1.3
degas for 2 h 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, the amount of liquid nitrogen adsorption or desorption from the material as a function of pressure @/PO= 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 K). As adsorption or desorption occurred, the pressure in the sample cell changed until equilibrium was established. The specific surface area of the powders was determined by the Brunauer-Emmett-Teller (BET) method based on nitrogen adsorption.
3. Results and discussion Fig. 2(a) shows a TEM micrograph of Cu nanoparticles. The sample is scanned in all zones before the picture is taken. All the particles are spherical in shape, and small particles aggregate into second& particles because of their extremely small
Fig. 2. TEM micrograph (a) and SAED pattern (b) of Cu nanoparticles.
dimensions and high surface energy. Most of the particles appear to be fairly uniform in size, with smooth surface. Fig. 2(b) shows the corresponding SAED pattern. It can be indexed to the reflection of
face centered cubic (fcc) structure, and this result was also investigated by means of XRD. Tropism of the particles at random and small particles cause the widening of diffraction rings that are made up of
Wei Z.Q. et al., Preparation and particle size characterizationof Cu nanoparticlesprepared by ...
many diffraction spots, which indicates that the nanoparticles are polycrystalline structures. Electron diffraction reveals that each particle is composed of many small crystal nuclei, which proves that the particles grow by an aggregation model. From the data obtained by TEM micrographs, the particle size histograms can be drawn and the mean size of the particles can be determined. Fig. 3 shows the particle size distribution of Cu nanoparticles.The particle sizes range from 30 to 90 nm, the median diameter (taken as average particle diameter) is about 67 nm, and the particle size distribution is narrow. Fig. 4 shows the typical XRD pattern for the specimen. Due to the small size effect and incomplete inner structure of the particle, the XRD diffraction peaks are low and wide. Five broad peaks with 2 0 values of 43.44", 50.56", 74.24', 90.04', and 95.24" correspond to the (1 1l), (200), (220), (31I), and (222) planes of bulk Cu respectively, which can be assigned to Cu fcc phase. The XRD spectrum does not reveal any other phase except for the characteristic peaks of copper, indicating that the physical phases of copper nanoparticles synthesized in this work have high purity. From the full width at half maximum, the average crystalline size can be estimated with the (1 1 1 ) diffraction peak in the XRD spectra according to the Scherrer formula d = KA/(BcosO) . d is the crystallite size; K = 0.89, which is the Scherrer approximate constant related to the shape and index (hkl) of the crystals; A is the wavelength of the X-ray (Cu &,0.14954 nm); 0 is the diffraction angle; and B is the corrected half-width of the diffraction peak (in radians) given by B 2 = B i - B; , where B, is the measured half-width and B, is the half-width of a standard sample with a known crystal size greater than 100 nm. The effect of geometric (instrumental) broadening on the reflection peaks was calibrated. The average crystallite size is calculated to be around 63 nm, which corresponds well with the average particle diameter obtained from the TEM image. The BET method is based on low-temperature adsorption of nitrogen and leads to determination of
175
the specific surface area of the powder. If it is accepted that the powder consists of solid, spherical shape particles with smooth surface and the same size, the relation between surface area and average equivalent particle size is equal to D = 60004 p S,), where D is the average diameter of a spherical particle in nm, S, represents the measured surface area of the powder in m2/g, and p i s the particle density. Fig. 5 shows BET plots of Cu nanoparticles. The specific surface area of the powder is 11 m2/gcalculated with the BET equation. The corresponding average equivalent particle size is 68 nm derived from these values.
Fig. 3. Particle size distribution of Cu nanoparticles.
0 2e I Fig. 4. XRD pattern of Cu nanoparticles. (0)
0.55 I
5= 4
0.451
I
s. Y &
'4 0.35 -
0.25 0.06
2
PIP,
Fig. 5. BET plots of Cu nanoparticles.
RARE METALS, Vol. 25, No. 2, Apr 2006
176
The average particle sizes of the as-prepared particles, calculated from the Scherrer formula, obtained based on TEM data and estimated from BET values are listed in Table 2. The particle size obtained from the BET equation and the TEM methods agree very well with the result given by X-ray line broadening. The results of TEM observations and BET methods further confirm and verify the relevant results obtained by XRD as mentioned earlier. Table 2. Average particle sizes calculated by various methods nm DBET
-
68
4ul 66
DXRD 63
4. Conclusions Copper nanoparticles were successfully prepared in large scale by means of anodic arc discharging plasma method with homemade experimental apparatus. This method is a convenient approach to prepare high-quality metal nanoparticles, and especially it can be fulfilled promptly and is suitable for large-scale production in a factory. The nanoparticles prepared by this technique have uniform size, high purity, narrow size distribution, and spherical shape. The crystal structure of the samples is fcc structure the same as that of the bulk materials. The specific surface area is 1 lrn’/g, with the particle size distribution ranging from 30 to 90 nm, and an average particle size of about 66 nm obtained from E M and confirmed by XRD and BET results.
References [ 11 Gleiter H., Nanocrystalline materials. Prog. Muter.
Sci.,1990.33(4): 223. [2] Chen Y.J., Cao M.S., and Tian Q., A novel preparation and surface decorated approach for a-Fe nanoparticles by chemical vapor-liquid reaction at low temperature, Muter. Len.,2004.58: 1481. [3] Zhang W.W. and Cao Q.Q., Structural, morphological, and magnetic study of nanocrystalline cobalt-nickel-copper particles. J. Colloid InterJiuce Sci., 2003,257: 237. [4] Cui Z. L.. Dong L.F., and Hao C.C., Microstructure and magnetic property of nano-Fe particles prepared
by hydrogen arc plasma, Mater. Sci. Eng., 2000, A
286:205. [5] Gleiter H., Materials with ultrafne microstructures: retrospectives and perspectives, Nanosttuct. Muter., 1992.1: I . [6] Chen B.J., Sun X.W., and Xu C.X., Growth and characterization of zinc oxide nano/micro-fibers by thermal chemical reactions and vapor transport deposition in air,Phys. E, 2004,21: 103. [7] Chen D.H. and Chen D.R., Hydrothermal synthesis and characterization of octahedral nickel femte particles, Powder Technol., 2003, 133 247. [8] Cao M.S. and Deng Q.G., Synthesis of nitride-iron nano meter powder by thermal chemical vapor-phase reaction method, J. Inorg. Chem., 1996,12(1): 88. [9] Karthikeyan J., Bemdt C.C., and Tikkanen J., Plasma spray synthesis of nanomaterial powders and deposits, Muter. Sci. Eng., 1997,A 238: 275. [lo] Zheng H.G.. Lang J.H., and Zeng J.H., Preparation of nickel nanopowders in ethanol-watersystem (EWS), Muter. Res. Bull., 2001,36: 947. [I I ] Gaertner G.F. and Miquel P.F., Particle generation by laser ablation from solid targets in gas flows, Nanostruct. Muter., 1993.4 (3): 559. [I21 Katz J.L. and Miquel P.F., Synthesis and applications of oxides and mixed oxides produced by a flame process, Nanostmct. Marer., 1994,4 (5): 55 1. [I31 Gunther B. and Kumpmann A., Ultrahe oxide powders prepared by inert gas evaporation, Nanostrucr. Muter., 1992,l ( I ) : 27. [I41 Vollath D. and Sickafus K.E., Synthesis of nanosized ceramic oxide powders by microwave plasma reactions, Nanosttuct. Mater., 1992, 1 (5): 427. [IS] Chen D.H. and He X.R., Synthesis of nickel femte nanoparticles by sol-gel method, Muter. Res. Bull., 2001,36: 1369. [ 161 Andre J. and Kwan W., Arc-discharge ion sources for heavy ion fusion, Nucl. Instnun. Methods Phys. Res., 2001, A 464:569. [ 171 Ioan B., Nanoparticle production by plasma, M a w . Sci. Eng., 1999,B 68:5. [ 181 Kaito C., Coalescence growth mechanism of smoke particles, Jpn. J. Appl. Phys., 1985.24: 261. [ 191 Scott J.H. and Majetich S.A., Morphology, structure, and growth of nanoparticles produced in a carbon arc, Phys. Rev., 1995, B 52: 12564.