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A new experimental study on the production of clusters from copper ultrafine particles
j........ C R V S T A L t~ROWT H
Journal of Crystal Growth 128 (1993) 267-270 North-Holland
A new experimental study on the production of clusters from copper ultrafine particles C h i h i r o Kaito, T s u y o s h i W a t a n a b e , K a z u s h i O h t s u k a , F a n g y u C h e n a n d Y o s h i o Saito Department of Electronics and Information Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan Copper clusters were produced by successive processes of oxidation and reduction of copper ultrafine particles. The oxide produced was cuprous oxide (Cu20). Partly oxidized particles were converted to the original copper particles by the reduction. Completely oxidized copper particles become a spherical shell composed of microcrystallites of cuprous oxide. By the reduction of spherical shell particles, microclusters of 2-10 nm in size were produced. In the reduction process of the oxidized particles, Cu40 structure particles can be observed.
1. Introduction With increasing interest in the physical chemistry of properties at the nanometer scale, various methods [1-4] for the production of microclusters, which are clusters of very small particles, have been developed. One of the most advanced methods for producing clusters is the "gas evaporation technique" in which a material is heated in an atmosphere of inert gas. The most easily attained metallic particle size range by the gas evaporation technique is 30-100 nm. Since nucleation and growth processes take place near the evaporation source, the main particle growth mechanism is coalescence growth, as elucidated in a series of experiments by the advanced gas evaporation method [5-9]. In previous papers on the oxidation of metallic particles [9,10], it was shown that most metallic particles were oxidized by the outward movement of metallic atoms. Recently, we found that copper oxides can be reduced in vacuum by heating to about 300°C [11]. By combining these two experiments on copper metallic particles, conditions to produce the microclustering of copper atoms have been found.
2. Experimental procedure The ultrafine copper particles were produced in Ar gas pressure at 13 kPa by evaporating
copper ribbons. They were collected on carbon or SiO thin films supported on stainless steel, electron microscopic (EM) grids. The particles mounted on EM grids were oxidized in air in a furnace at 200°C for a few minutes. The reduction was done by heating the oxidized particles to 300°C in vacuum, using a Hitachi H-8101 specimen holder mounted in the Hitachi H-800 electron microscope, and in situ observations of the reduction process were carried out.
3. Results and discussion Fig. 1 shows an EM image and corresponding electron diffraction (ED) pattern before and after the reduction. As can be seen in figs. la and lb, each of particles was covered with a cuprous oxide layer. The white contrasts indicated by arrows in fig. la are due to the void clusters produced by the outward movement of copper atoms to the surface oxide layer [9]. If a copper particle of 100 nm in size is covered with an oxide layer of 10 nm, the number of copper atoms in the oxide layer is approximately 2 × 107, which corresponds to the outward movement of enough copper atoms to leave a cavity approximately 76 nm in size. Void clusters in particles can be seen as the white contrasts in EM images, as seen in fig. la. After reduction, as can be seen in fig. ld, the diffrac-
0022-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
268
C. Kaito et al. / New experimental study on production of clusters from Cu ultrafine particles
Fig. 1. EM images and ED patterns (a), (b) before and (c), (d) after the reduction of partly oxidized particles.
tion rings of oxide disappear. The void clusters seen in fig. la were filled by copper atoms by the inward movement of coppr atoms from the oxide layer. The particle size of each copper particle was returned to the original size by the reduction process. Since the oxidation of an ultrafine particle took place on its surface [9,10], and the oxide
layer became larger with time, an outward movement of metallic atoms took place. The self-diffusion of copper atoms at 200°C is estimated to be 17 n m / m i n [12], while the average oxide layer thickness produced by heating at 200°C for 1 min was 11 nm. Therefore, the self-diffusion of copper atoms may be faster than the growth of the oxide layer, and the rate determining factor in
Fig. 2. EM images (a) before and (b) after the reduction of completelyoxidized particles.
C. Kaito et aL / New experimental study on production of clusters from Cu ultrafine particles
Fig. 3. (a) Microclusters after the reduction of completely oxidized particles. H R E M images are shown in (b) and (c).
the oxidation is likely to be the diffusion rate of copper atoms in the oxide layer. By contrast, the rate determining factor in the reduction process is likely to be the decomposition rate of oxygen from C u 2 0 crystallites in vacuum. The recovery of the copper particle due to the reduction may be due to the existence of void clusters in copper particles, i.e. sink sites for copper diffusion. Figs. 2a and 2b show completely oxidized particles and the result of the reduction of the same
269
area. As elucidated in the experiments on the oxidation process of ultrafine metallic particles [9,10], the central white part and surrounding layer formation show that each particle changed to a spherical shell structure composed of oxide crystallites. Copper particles of various sizes are seen in fig. 2b. As indicated by arrows A, large copper particles appeared at the place where there is no copper particle, as seen in fig. 2a. A few nm sized clusters, as indicated by arrow B for example, were seen everywhere. As indicated by arrow C, other small clusters can be seen. Fig. 3 shows one of the typical clusters produced by the reduction. A high resolution electron microscopic ( H R E M ) image of a copper particle is shown in fig. 3b. Perfect copper microclusters were formed. In this experiment we can get microclusters of the order of lnm. The large particles indicated by arrows A in fig. 2b were not seen in fig. 2a. Formation of such a large particle is apparently due to coalescence among the microclusters produced in the spherical shell region, since the number of copper atoms was nearly maintained throughout this experiment, and each particle has a spherical shell structure. A lattice spacing with 0.39 nm distance, as shown in fig. 3c, was often observed after the reduction process at a place indicated by an ar-
()______.( Cu2
0 Cu2 0 Cu2 0 Cu2 0 Cu2 0 Cu~
1 1 "1Ij.o~ j.o-t1j.o:,j.oLJ1 4.28~
Cu2
:~0 Cu2 Cu2
1
4.28~
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1
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®
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¢~0 Cu2 Cu2 0,~ Cu2
b o~ 1.8o u.oTI
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(100)
o
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0
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Fig. 4. (a) Schematic presentations of the Cu20 structure and the derived Cu40 structure along the [100] direction. (b) (001) projection of Cu 4° structure. (c) H R E M image confirming the model in (b).
270
c. Kaito et al. / New experimental study on production of clusters from Cu ultrafine particles
row in fig. 3a. T h e distance does not correspond to the c o p p e r crystal or to cuprous oxide. Since the reduction took place from the C u 2 0 crystal, the r e d u c e d crystal model is derived from the C u 2 0 crystal structure, as shown schematically in fig. 4a. T h e atomic a r r a n g e m e n t C u 2 0 structure along the [100] direction is c o m p o s e d of layers of Cu 2, O, Cu 2, O, Cu 2, O . . . . . as shown in the upper part in fig. 4a. T h e lattice constant of C u z O is 4.28 A. Oxygen layers may be removed by the reduction, leaving a pair of adjacent Cu atom layers. The atomic a r r a n g e m e n t s of Cu and O in C u 2 0 structure are face c e n t e r e d cubic and body c e n t e r e d cubic, respectively. Therefore it is easy to change the structure to allow adjacent c o p p e r layers, as indicated in fig. 4a, with a lattice spacing of 0.39 nm. T h e composition of the r e d u c e d crystal b e c o m e s C u 4 0 . Since the crystal structure of C u 2 0 has high symmetry, the extraction of oxygen layers introduces a crystal structure as shown in fig. 4b, with the orthogonal lattice spacing of 0.39 nm. T h e enlarged H R E M image shown in fig. 4c, which shows an area such as is shown in fig. 3c, clearly shows the crossed lattice image of 0.39 nm, in a g r e e m e n t with our model. It can be concluded that the r e d u c e d C u 4 0 crystal is produced during the reduction process. In the unit cell of C u 4 0 , there are four Cu atoms with the coordinates (0.27, 0.27, 0.27), (0.73, 0.73, 0.27), (0.73, 0~27, 0.73), (0.23, 0.73, 0.73) and an O atom with the coordinate (0, 0, 0). T h e space group is assigned as P43m with lattice constant a = 0.39 nm. A similar structure has b e e n p r o p o s e d at the initial stage of oxidation of c o p p e r on the basis of H R E M image and its simulation [13]. But their assigned space group of P m m 2 could not explain
their p r o p o s e d coordinates. Details on these points may be published elsewhere in near future. In conclusion, microclusters of c o p p e r can be p r o d u c e d by using the chemical reaction process of ultrafine c o p p e r particles.
Acknowledgement This work was partly supported by a Grant-inAid for Scientific R e s e a r c h on Priority A r e a s "Crystal G r o w t h Mechanism in A t o m i c Scale" No. 04227101 from the Ministry of Education, Science and Culture, Japan.
References [1] E.R. Andrew, A. Bradbury and R.G. Eades, Arch. Sci. 11 (1958) 223. [2] I. Katakuse, T. Ichihara, Y.Fujita, T. Matsuo, T. Sakurai and H. Matsuda, Intern. J. Mass Spectrom. Ion Processes 74 (1981) 33. [3] P. Joyes and J. Van de Walle, J. Phys. B 16 (1985) 3805. [4] K. Kimoto, Y. Kamiya, M. Nonoyama and R. Uyeda, Japan. J. Appl. Phys. 17 (1963) 702. [5] C. Kaito, Japan. J. Appl. Phys. 17 (1978) 601. [6] C. Kaito, J. Crystal Growth 55 (1981) 273. [7] C. Kaito, Japan. J. Appl. Phys. 23 (1984) 525. [8] C. Kaito, Japan. J. Appl. Phys. 24 (1985) 261. [9] C. Kaito, K. Fujita and H. Hashimoto, Japan. J. Appl. Phys. 12 (1973) 489. [10] C. Kaito, K. Fujita and H. Hashimoto, Japan. J. Appl. Phys. 13 (1974) 1058. [11] C. Kaito, Y. Nakata, Y. Saito, T. Naiki and K. Fujita, J. Crystal Growth 74 (1986) 469. [12] CRC Handbook of Chemistry and Physics, 65th ed., Ed. R.C. Weast (Chemical Rubber Co., 1984) p. F46. [13] R. Guan, H. Hashimoto and T. Yoshida. Acta Cryst. B 40 (1984) 109.
Journal of Crystal Growth 128 (1993) 267-270 North-Holland
A new experimental study on the production of clusters from copper ultrafine particles C h i h i r o Kaito, T s u y o s h i W a t a n a b e , K a z u s h i O h t s u k a , F a n g y u C h e n a n d Y o s h i o Saito Department of Electronics and Information Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan Copper clusters were produced by successive processes of oxidation and reduction of copper ultrafine particles. The oxide produced was cuprous oxide (Cu20). Partly oxidized particles were converted to the original copper particles by the reduction. Completely oxidized copper particles become a spherical shell composed of microcrystallites of cuprous oxide. By the reduction of spherical shell particles, microclusters of 2-10 nm in size were produced. In the reduction process of the oxidized particles, Cu40 structure particles can be observed.
1. Introduction With increasing interest in the physical chemistry of properties at the nanometer scale, various methods [1-4] for the production of microclusters, which are clusters of very small particles, have been developed. One of the most advanced methods for producing clusters is the "gas evaporation technique" in which a material is heated in an atmosphere of inert gas. The most easily attained metallic particle size range by the gas evaporation technique is 30-100 nm. Since nucleation and growth processes take place near the evaporation source, the main particle growth mechanism is coalescence growth, as elucidated in a series of experiments by the advanced gas evaporation method [5-9]. In previous papers on the oxidation of metallic particles [9,10], it was shown that most metallic particles were oxidized by the outward movement of metallic atoms. Recently, we found that copper oxides can be reduced in vacuum by heating to about 300°C [11]. By combining these two experiments on copper metallic particles, conditions to produce the microclustering of copper atoms have been found.
2. Experimental procedure The ultrafine copper particles were produced in Ar gas pressure at 13 kPa by evaporating
copper ribbons. They were collected on carbon or SiO thin films supported on stainless steel, electron microscopic (EM) grids. The particles mounted on EM grids were oxidized in air in a furnace at 200°C for a few minutes. The reduction was done by heating the oxidized particles to 300°C in vacuum, using a Hitachi H-8101 specimen holder mounted in the Hitachi H-800 electron microscope, and in situ observations of the reduction process were carried out.
3. Results and discussion Fig. 1 shows an EM image and corresponding electron diffraction (ED) pattern before and after the reduction. As can be seen in figs. la and lb, each of particles was covered with a cuprous oxide layer. The white contrasts indicated by arrows in fig. la are due to the void clusters produced by the outward movement of copper atoms to the surface oxide layer [9]. If a copper particle of 100 nm in size is covered with an oxide layer of 10 nm, the number of copper atoms in the oxide layer is approximately 2 × 107, which corresponds to the outward movement of enough copper atoms to leave a cavity approximately 76 nm in size. Void clusters in particles can be seen as the white contrasts in EM images, as seen in fig. la. After reduction, as can be seen in fig. ld, the diffrac-
0022-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
268
C. Kaito et al. / New experimental study on production of clusters from Cu ultrafine particles
Fig. 1. EM images and ED patterns (a), (b) before and (c), (d) after the reduction of partly oxidized particles.
tion rings of oxide disappear. The void clusters seen in fig. la were filled by copper atoms by the inward movement of coppr atoms from the oxide layer. The particle size of each copper particle was returned to the original size by the reduction process. Since the oxidation of an ultrafine particle took place on its surface [9,10], and the oxide
layer became larger with time, an outward movement of metallic atoms took place. The self-diffusion of copper atoms at 200°C is estimated to be 17 n m / m i n [12], while the average oxide layer thickness produced by heating at 200°C for 1 min was 11 nm. Therefore, the self-diffusion of copper atoms may be faster than the growth of the oxide layer, and the rate determining factor in
Fig. 2. EM images (a) before and (b) after the reduction of completelyoxidized particles.
C. Kaito et aL / New experimental study on production of clusters from Cu ultrafine particles
Fig. 3. (a) Microclusters after the reduction of completely oxidized particles. H R E M images are shown in (b) and (c).
the oxidation is likely to be the diffusion rate of copper atoms in the oxide layer. By contrast, the rate determining factor in the reduction process is likely to be the decomposition rate of oxygen from C u 2 0 crystallites in vacuum. The recovery of the copper particle due to the reduction may be due to the existence of void clusters in copper particles, i.e. sink sites for copper diffusion. Figs. 2a and 2b show completely oxidized particles and the result of the reduction of the same
269
area. As elucidated in the experiments on the oxidation process of ultrafine metallic particles [9,10], the central white part and surrounding layer formation show that each particle changed to a spherical shell structure composed of oxide crystallites. Copper particles of various sizes are seen in fig. 2b. As indicated by arrows A, large copper particles appeared at the place where there is no copper particle, as seen in fig. 2a. A few nm sized clusters, as indicated by arrow B for example, were seen everywhere. As indicated by arrow C, other small clusters can be seen. Fig. 3 shows one of the typical clusters produced by the reduction. A high resolution electron microscopic ( H R E M ) image of a copper particle is shown in fig. 3b. Perfect copper microclusters were formed. In this experiment we can get microclusters of the order of lnm. The large particles indicated by arrows A in fig. 2b were not seen in fig. 2a. Formation of such a large particle is apparently due to coalescence among the microclusters produced in the spherical shell region, since the number of copper atoms was nearly maintained throughout this experiment, and each particle has a spherical shell structure. A lattice spacing with 0.39 nm distance, as shown in fig. 3c, was often observed after the reduction process at a place indicated by an ar-
()______.( Cu2
0 Cu2 0 Cu2 0 Cu2 0 Cu2 0 Cu~
1 1 "1Ij.o~ j.o-t1j.o:,j.oLJ1 4.28~
Cu2
:~0 Cu2 Cu2
1
4.28~
•
1
®
®
Q
® ®
/
¢~0 Cu2 Cu2 0,~ Cu2
b o~ 1.8o u.oTI
®
0 i
® 394A
(100)
o
b
0
Cu
©o
Fig. 4. (a) Schematic presentations of the Cu20 structure and the derived Cu40 structure along the [100] direction. (b) (001) projection of Cu 4° structure. (c) H R E M image confirming the model in (b).
270
c. Kaito et al. / New experimental study on production of clusters from Cu ultrafine particles
row in fig. 3a. T h e distance does not correspond to the c o p p e r crystal or to cuprous oxide. Since the reduction took place from the C u 2 0 crystal, the r e d u c e d crystal model is derived from the C u 2 0 crystal structure, as shown schematically in fig. 4a. T h e atomic a r r a n g e m e n t C u 2 0 structure along the [100] direction is c o m p o s e d of layers of Cu 2, O, Cu 2, O, Cu 2, O . . . . . as shown in the upper part in fig. 4a. T h e lattice constant of C u z O is 4.28 A. Oxygen layers may be removed by the reduction, leaving a pair of adjacent Cu atom layers. The atomic a r r a n g e m e n t s of Cu and O in C u 2 0 structure are face c e n t e r e d cubic and body c e n t e r e d cubic, respectively. Therefore it is easy to change the structure to allow adjacent c o p p e r layers, as indicated in fig. 4a, with a lattice spacing of 0.39 nm. T h e composition of the r e d u c e d crystal b e c o m e s C u 4 0 . Since the crystal structure of C u 2 0 has high symmetry, the extraction of oxygen layers introduces a crystal structure as shown in fig. 4b, with the orthogonal lattice spacing of 0.39 nm. T h e enlarged H R E M image shown in fig. 4c, which shows an area such as is shown in fig. 3c, clearly shows the crossed lattice image of 0.39 nm, in a g r e e m e n t with our model. It can be concluded that the r e d u c e d C u 4 0 crystal is produced during the reduction process. In the unit cell of C u 4 0 , there are four Cu atoms with the coordinates (0.27, 0.27, 0.27), (0.73, 0.73, 0.27), (0.73, 0~27, 0.73), (0.23, 0.73, 0.73) and an O atom with the coordinate (0, 0, 0). T h e space group is assigned as P43m with lattice constant a = 0.39 nm. A similar structure has b e e n p r o p o s e d at the initial stage of oxidation of c o p p e r on the basis of H R E M image and its simulation [13]. But their assigned space group of P m m 2 could not explain
their p r o p o s e d coordinates. Details on these points may be published elsewhere in near future. In conclusion, microclusters of c o p p e r can be p r o d u c e d by using the chemical reaction process of ultrafine c o p p e r particles.
Acknowledgement This work was partly supported by a Grant-inAid for Scientific R e s e a r c h on Priority A r e a s "Crystal G r o w t h Mechanism in A t o m i c Scale" No. 04227101 from the Ministry of Education, Science and Culture, Japan.
References [1] E.R. Andrew, A. Bradbury and R.G. Eades, Arch. Sci. 11 (1958) 223. [2] I. Katakuse, T. Ichihara, Y.Fujita, T. Matsuo, T. Sakurai and H. Matsuda, Intern. J. Mass Spectrom. Ion Processes 74 (1981) 33. [3] P. Joyes and J. Van de Walle, J. Phys. B 16 (1985) 3805. [4] K. Kimoto, Y. Kamiya, M. Nonoyama and R. Uyeda, Japan. J. Appl. Phys. 17 (1963) 702. [5] C. Kaito, Japan. J. Appl. Phys. 17 (1978) 601. [6] C. Kaito, J. Crystal Growth 55 (1981) 273. [7] C. Kaito, Japan. J. Appl. Phys. 23 (1984) 525. [8] C. Kaito, Japan. J. Appl. Phys. 24 (1985) 261. [9] C. Kaito, K. Fujita and H. Hashimoto, Japan. J. Appl. Phys. 12 (1973) 489. [10] C. Kaito, K. Fujita and H. Hashimoto, Japan. J. Appl. Phys. 13 (1974) 1058. [11] C. Kaito, Y. Nakata, Y. Saito, T. Naiki and K. Fujita, J. Crystal Growth 74 (1986) 469. [12] CRC Handbook of Chemistry and Physics, 65th ed., Ed. R.C. Weast (Chemical Rubber Co., 1984) p. F46. [13] R. Guan, H. Hashimoto and T. Yoshida. Acta Cryst. B 40 (1984) 109.