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Nanometer metallic copper particle synthesis in reverse micelles
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 80 (1993) 63-68 0927-7757/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved.
Nanometer micelles
63
metallic copper particle synthesis in reverse
M.P. Pilenia*b3*,I. Lisiecki” “Universitd P. et M. Curie, Laboratoire S.R.S.I. btitiment F 4 Place Jussieu, 75005 Paris, France bC.E.N. Saclay, DRECAM.-S.C.M, 91191 Gif sur Yvette, France
(Received 3 1 July 1992; accepted 27 April 1993) Abstract We describe the synthesis in situ of copper nanoparticles either surrounded or not surrounded by an oxide layer. Key words: Copper
particles;
Nanoparticles;
in reverse micelles. It is possible
particles
Reverse micelles; Synthesis
Introduction
tion of the surface of the particle changes with the experimental conditions.
The use of reverse micelles to synthesize microparticles in situ has made considerable progress in the last few years [l-9]. Sodium bis(Zethylhexy1) sulfosuccinate (usually called AOT) is most commonly used to form reverse micelles. The ternary system alkane/AOT/water presents enormous advantages (Petit [lo]): because the aggregates are spheroidal with a linear variation of the size of the droplet (Pileni [ll]) with the amount of water solubilized in the system. They have the ability to exchange the content of their water pools by collision, This makes possible the chemical reduction of metallic ions between compounds solubilized in different droplets. The water content w of the water pool is defined as the ratio of water to surfactant concentration (w = [H,O]/[AOT]). The size of the droplets is then defined by r(A)= 1.5~ (Pileni [ 111). In this paper we describe the formation of metallic copper particles. It is shown that the composi*Corresponding
to form metallic
author.
Experimental
Colloidal particles were prepared by mixing two micellar solutions with the same water content w(w = [H,O]/[AOT]), one containing the reducing agent such as either sodium tetrahydridoborate NaBH, or hydrazine and the other containing copper ions. The copper ion is associated with two surfactant molecules to form copper bis(2-ethylhexyl) sulfosuccinate. The synthesis of functionalized surfactant and the purity have been previously described (Pileni [ 123). The overall bis(2-ethylhexyl) sulfosuccinate concentration is kept constant. The absorption spectra were obtained with a Perkin-Elmer Lambda 5 and a Hewlett-Packard HP8452A spectrophotometer. A drop of this solution was evaporated under vacuum and the electron micrograph was obtained with a Jeol electron microscope (model Jem. lOOCX.2).
M.P. Pileni and I. Lisiecki/Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 63-68
64
570 nm but they also observed
Results and discussion In aqueous
solution
the addition
of a reducing
agent such as hydrazine or sodium tetrahydridoborate to an ionic copper solution induces precipitation of copper metal or bulk phase oxide, whereas in reverse micelle solution, under various experimental
conditions,
clusters
of particles
From these it can be concluded, observed with CdS semiconductors
are formed. as has been (Pileni [ 12]),
that AOT reverse micelles prevent precipitation. Colloidal dispersions cf metals exhibit absorption bands or broad regions of absorption in the UV-visible range. These are due to the excitation of plasma resonances or interband transitions and are a characteristic property of the metallic nature of the particles. The absorption spectra of colloidal copper particles have been described, including the effect on the absorption spectra of varying the size and the shape of the particle. Indeed, if the particle dimensions (Wokaun and Gordon [ 131, Meier and Wokaun [14]) are smaller than the mean free path of the conduction electrons, collisions of these electrons with the particle surface are noticed. This lowers the effective mean free path. By evaluating absorption measurements of spherical copper particles (between 10 and 100 A in diameter), Mie’s theory (Mie [15]) can be used in the expanded versions discussed by various authors (Fragstein and co-workers [ 16- 181). Briefly, the absorption spectra of fairly dilute dispersions of particles of colloidal dimensions can be calculated from the wavelength dependence of optical constants such as the refractive index and the optical frequency relative to the permittivity of the particles and the surrounding medium. These optical constants for the bulk metal are taken from the tables of Weaver et al. [ 193. For 10 nm diameter copper particles, the absorption spectrum is characterized by a broad absorption band with a 570 nm peak attributed to the plasmon band (Creighton and Eadon [20]). Yanase and Komiyama [21] demonstrated that metallic particles surrounded by an oxide monolayer are characterized by a peak centered at
a residual
absorp-
tion at 800 nm due to an oxide monolayer. From Mie’s theory, the absorption spectra
have
been simulated (Fig. 1) for particles ranging in size from 1 to 10 nm and the resulting simulations compared to the absorption spectrum in the bulk phase.
A progressive
plasmon
peak
upon
appearance increasing
of the 570 nm the
size of the
copper clusters occurs. It is clear that the copper particles with a diameter below 4 nm are characterized by a strong broadening of the plasmon resonance. Figure 2 shows absorption spectra of the colloidal particles, the electron micrographs and histograms observed 5 h after adding the reducing agent, in this case hydrazine, for reverse micelles with various w values. At low water content, a continuous absorption spectrum with a shoulder at 570 nm is seen (Fig. 2). Upon increasing the water content, the 570 nm band appears progressively. The electron micrographs show an increase in the size of the particles from 2 to 10 nm upon increasing the water content - from 1 to 10. At water contents up to 3.5 3 jg
2.5
z $
2 1.5
B 0
1
0.5
“,50
6 0
550 wavelengtt~
(nm)
Fig. 1. Simulated absorption spectra of metallic copper particles corresponding to various radii r of metallic particles: curve 1, r = 0.5 nm; curve 2, r = 1 nm; curve 3, I = 2 nm; curve 4, r=3nm;curve5,r=Snm;curve6,r~lOnm.
M.P. Pileni and I. Lisiecki/Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 63-68
Fig. 2. Absorption
spectra,
electron
micrographs and histograms of metallic copper particles water contents: (a) w= I; (b) w=3; (c) w= IO.
10, the size of the particles remains unchanged but the polydispersity increases. Electron diffractograms obtained at various water contents, compared with a simulated diffractogram of bulk metal copper, are in good agreement. This indicates a crystalline face-centered cubic structure (f.c.c.) with a lattice constant of 3.61. The similar behavior of the simulated and experimental absorption spectra in Figs. 1 and 2 confirms that the plasmon peak can only be observed with relatively large particles (up to 3 nm). The change in the particle size with water content can be explained in terms of the interfacial water structure: at low water content, copper ions associated with surfactant molecules are not totally hydrated and the effective number of ions participating in the chemical reduction is small. The increase in the water content induces an increase in the number of copper ions which react with hydrazine. This favors the growth of the average
65
prepared
in reverse micelles with various
particles. At relatively high water contents (up to lo), copper ions are totally hydrated and free water molecules are present [22]. This favors diffusion of copper ions inside the droplet. Electrostatic interactions between the head polar groups of the surfactant and copper ions compete with the hydration energy. The difference in these energies remains constant upon increasing the water content, which keeps the particle size constant. An increase in the polydispersity with the water content could be attributed to a decrease in interfacial rigidity [23-271. By using pure copper/AOT/water-in-oil aggregates, it has previously been shown that a change in the shape of the aggregates is induced by increasing the water content. Droplets are observed at w = 2, whereas cylindrical aggregates are formed at w=4 [21]. The reduction of copper reverse micelles was achieved with hydrazine as the reducing agent at
M.P. Pileni and I. Lisiecki/Colloids
66
various
surfactant
water content The shape resulting metal the CU(AOT)~ cal density particle
concentrations
with a constant
(w = 4). of the absorption spectrum of the clusters is unchanged by increasing concentration. The increase in opti-
is due to an increase
concentration
in the metallic
with the number
of reverse
Surfaces
A: Physicochem.
Eng. Aspects 80 (1993) 63-68
lated (3.6 l/,/3 = 2.09 A). High resolution microscopy
of the cylindrical
particles
electron shows
an
inter-reticular distance of 2 A. This value is in good agreement with that calculated from the lattice of the bulk phase. resolution various
Figure
image orientations
4 shows a high electronic
of these spherical of the reticular
particles
with
planes.
micelle aggregates (Fig. 3). At low Cu(AOT), concentrations, the clusters are spherical. Upon increasing the Cu(AOT), concentration, the forma-
By using sodium tetrahydridoborate as the reducing agent in the absence of oxygen, an absorption spectrum characteristic of metallic particles is
tion of cylindrical clusters is observed. The average number of cylindrical clusters remains constant (at about lo%), with 90% of the particles being spherical. However, a further increase in the surfactant concentration induces formation of longer cylinders with smaller radii. Electron diffraction shows the concentric circles characteristic of an f.c.c. phase with a lattice dimension equal to 3.61 A. The distance between the highest density reticular plane (111) can be calcu-
observed. Figure 4 shows a decrease in the size of the particles from 28 to 3 nm upon increasing the water content. At low water content (w = 3 and 5), the particles are homogeneously dispersed. Upon increasing the water content from w=4 to w =8, the particles become progressively associated. From electron diffraction patterns for particles formed in micelles with low water content (3 < w<5), the clusters are f.c.c. with a lattice dimension equal to 3.61, similar to what is obtained
in
n
(b)
Fig, 3. Absorption spectra, electron micrographs, copper concentrations: (a) [Cu(AOT),]=2
and histograms of metallic copper particles formed in reverse micelles at various x IO-’ M; (b) [Cu(AOT),]=4 x 10m2 M; (c) [Cu(AOT),] =2 x IO-’ M.
M.P.
Pileni and I. LisieckifCalloids
Surfaces A: Physicochem.
Eng. Aspects 80 (1993) 63-68
67
wavelength (nm)
wavelength (nm) Fig. 4. Absorption
spectra,
electron
micrographs and histograms of metallic copper particles water contents: (a) w = 3; (b) w = 5; (c) w = 10.
in the metallic bulk phase. At high water content (w = 9 and lo), a cubic phase with a lattice parameter equal to 4.27, characteristic of copper oxide Cu,O, is observed. The shape of the absorption spectrum of the colloidal particles changes with water content: at 3 < w < 6 the shapes of the absorption spectra are similar, with changes in intensity and a small residual absorption at 800 nm. Upon increasing the water content, an absorption around 800 nm appears. According to Yanase and Komiyama [2 l] the 800 nm absorption can be attributed to an oxide monolayer surrounding the metallic cluster. By comparing the microscopy data and the absorption spectra, it can be deduced that at low water content, metallic particles are formed. By increasing the water content, the metallic particles are surrounded by monolayers and multilayers of oxide which prevent further growth of the particles. At higher water content (above w = 8), pure copper
formed
in reverse micelles
at various
oxide particles are obtained. This is confirmed by the fact that below w= 7, electron micrographs show well-dispersed particles, whereas at w = 7 and 8 they are associated. Furthermore, Table 1 shows the ratio of the optical densities at 810 to 570 nm. An increase in the relative optical densities at 810 nm with the water content is clearly shown. This is probably due to the high affinity of the surfactant for the oxide. At w =9 and 10, the electron micrographs do not show the presence of metallic particles. These data could again be related to the water structure in the water pool. At low water contents, water molecules are bound at the
Table 1 Ratio of optical W Ratio
6 0.36
“Using
hydrazine
densities 7 0.49
at 800 and 570 nm 8 0.43
as a reducing
9 0.5 agent.
1
10 0.53
10” 0.17
M.P. Pileni and I. Lisiecki/Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 63-68
68
interface, whereas at high water contents, free water is present and plays a role in oxide formation. Compared to the data presented above with hydrazine, the size of metallic particles formed with sodium tetrahydridoborate is larger by one order of magnitude. The differences in the size and in the formation of oxide on changing the reducing agent could be due to the high redox potential of hydrazine compared to that of sodium tetrahydridoborate. The particles exposed to air are immediately oxidized.
8
9 10 11 12 13 14 15 16 17 18 19
References M.P. Pileni (Ed.), Structure and reactivity in reverse micelles, Studies in Physical and Theoretical Chemistry, Vol. 65, Elsevier, Amsterdam, 1989. C. Petit, P. Lixon and M.P. Pileni, J. Phys. Chem., 94 (1990) 1598. M. Meyer, C. Wallberg, K. Kurihada and J.H. Fendler, J. Chem. Sot., Chem. Commun., 90 (1984) 90. (a) P. Lianos and J.K. Thomas, Chem. Phys. Lett., 125 (1986) 299. (b) P. Lianos and J.K. Thomas, J. Colloid Interface Sci., 117 (1987) 50. T. Dannhauser, M. O’Neil, K. Johansson, D. Whitten and G. McLendon, J. Phys. Chem., 90 (1986) 6074. S. Modes and P. Lianos, J. Phys. Chem., 93 (1989) 5854. R.R. Chandler and J.L. Coffer, J. Phys. Chem., 95 (1991) 4.
20 21 22 23 24 25 26 27
A.R. Korlan, R. Hull, R.L. Opila, M.G. Bawendi, M.L. Steigerwald, P.J. Carroll and L.E. Brus, J. Am. Chem. Sot., 112 (1990) 1327. L. Motte, C. Petit and M.P. Pileni, Langmuir, 8 (1992) 1049. C. Petit and M.P. Pileni, J. Phys. Chem., 92 (1988) 2282. M.P. Pileni, T. Zemb and C.J. Petit, Chem. Phys. Lett., 118 (1985) 414. M.P. Pileni, L. Motte and C. Petit, Chem. Mater., 4 (1992) 338. A. Wokaun, J.P. Gordon and P.F. Liao, Phys. Rev. Lett., 48 (1982) 957. M. Meier and A. Wokaun, Opt. Lett., 8 (1983) 581. G. Mie, Ann. Phys., 25 (1908) 377. C.V. Fragstein and H. Roemer, Z. Phys., 151 (1958) 54. H. Roemer and C.V. Fragstein, Z. Phys., 163 (1961) 27. C.V. Fragstein and F.J. Schoenes, Z. Phys., 98 (1967) 477. J.H. Weaver, C. Kratka, D.W. Lynch and E.E. Koch, Optical Properties of Metals, Vols. I and 2, Fachinformationszentrum, Karlsruhe, 1981. J.A. Creighton and D.G. Eadon, J. Chem. Sot., Faraday Trans., 87 (1991) 3881. A. Yanase and H. Komiyama, Surf. Sci., 248 (1991) 11. M.P. Pileni, C. Ferradini, J. Puchault and B. Hickel, J. Colloid Interface Sci., 92 (1983) 308. B.P. Binks, J. Meunier, 0. Abillon and D. Langevin, Langmuir, 5 (1989) 415. 0. Abillon, B.P. Binks, C. Otero, D. Langevin and R. Ober, J. Phys. Chem., 92 (1988) 4411. D. Guest and D. Langevin, J. Coiloid Interface Sci., 112 (1986) 208. D. Guest, L. Auvray and D. Langevin, J. Phys. Lett., 46 (1985) L-1055. R. Aveyard, B.P. Binks, S. Clark and J. Mead, J. Chem. Sot., Faraday Trans., 82 (1986) 125.
Nanometer micelles
63
metallic copper particle synthesis in reverse
M.P. Pilenia*b3*,I. Lisiecki” “Universitd P. et M. Curie, Laboratoire S.R.S.I. btitiment F 4 Place Jussieu, 75005 Paris, France bC.E.N. Saclay, DRECAM.-S.C.M, 91191 Gif sur Yvette, France
(Received 3 1 July 1992; accepted 27 April 1993) Abstract We describe the synthesis in situ of copper nanoparticles either surrounded or not surrounded by an oxide layer. Key words: Copper
particles;
Nanoparticles;
in reverse micelles. It is possible
particles
Reverse micelles; Synthesis
Introduction
tion of the surface of the particle changes with the experimental conditions.
The use of reverse micelles to synthesize microparticles in situ has made considerable progress in the last few years [l-9]. Sodium bis(Zethylhexy1) sulfosuccinate (usually called AOT) is most commonly used to form reverse micelles. The ternary system alkane/AOT/water presents enormous advantages (Petit [lo]): because the aggregates are spheroidal with a linear variation of the size of the droplet (Pileni [ll]) with the amount of water solubilized in the system. They have the ability to exchange the content of their water pools by collision, This makes possible the chemical reduction of metallic ions between compounds solubilized in different droplets. The water content w of the water pool is defined as the ratio of water to surfactant concentration (w = [H,O]/[AOT]). The size of the droplets is then defined by r(A)= 1.5~ (Pileni [ 111). In this paper we describe the formation of metallic copper particles. It is shown that the composi*Corresponding
to form metallic
author.
Experimental
Colloidal particles were prepared by mixing two micellar solutions with the same water content w(w = [H,O]/[AOT]), one containing the reducing agent such as either sodium tetrahydridoborate NaBH, or hydrazine and the other containing copper ions. The copper ion is associated with two surfactant molecules to form copper bis(2-ethylhexyl) sulfosuccinate. The synthesis of functionalized surfactant and the purity have been previously described (Pileni [ 123). The overall bis(2-ethylhexyl) sulfosuccinate concentration is kept constant. The absorption spectra were obtained with a Perkin-Elmer Lambda 5 and a Hewlett-Packard HP8452A spectrophotometer. A drop of this solution was evaporated under vacuum and the electron micrograph was obtained with a Jeol electron microscope (model Jem. lOOCX.2).
M.P. Pileni and I. Lisiecki/Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 63-68
64
570 nm but they also observed
Results and discussion In aqueous
solution
the addition
of a reducing
agent such as hydrazine or sodium tetrahydridoborate to an ionic copper solution induces precipitation of copper metal or bulk phase oxide, whereas in reverse micelle solution, under various experimental
conditions,
clusters
of particles
From these it can be concluded, observed with CdS semiconductors
are formed. as has been (Pileni [ 12]),
that AOT reverse micelles prevent precipitation. Colloidal dispersions cf metals exhibit absorption bands or broad regions of absorption in the UV-visible range. These are due to the excitation of plasma resonances or interband transitions and are a characteristic property of the metallic nature of the particles. The absorption spectra of colloidal copper particles have been described, including the effect on the absorption spectra of varying the size and the shape of the particle. Indeed, if the particle dimensions (Wokaun and Gordon [ 131, Meier and Wokaun [14]) are smaller than the mean free path of the conduction electrons, collisions of these electrons with the particle surface are noticed. This lowers the effective mean free path. By evaluating absorption measurements of spherical copper particles (between 10 and 100 A in diameter), Mie’s theory (Mie [15]) can be used in the expanded versions discussed by various authors (Fragstein and co-workers [ 16- 181). Briefly, the absorption spectra of fairly dilute dispersions of particles of colloidal dimensions can be calculated from the wavelength dependence of optical constants such as the refractive index and the optical frequency relative to the permittivity of the particles and the surrounding medium. These optical constants for the bulk metal are taken from the tables of Weaver et al. [ 193. For 10 nm diameter copper particles, the absorption spectrum is characterized by a broad absorption band with a 570 nm peak attributed to the plasmon band (Creighton and Eadon [20]). Yanase and Komiyama [21] demonstrated that metallic particles surrounded by an oxide monolayer are characterized by a peak centered at
a residual
absorp-
tion at 800 nm due to an oxide monolayer. From Mie’s theory, the absorption spectra
have
been simulated (Fig. 1) for particles ranging in size from 1 to 10 nm and the resulting simulations compared to the absorption spectrum in the bulk phase.
A progressive
plasmon
peak
upon
appearance increasing
of the 570 nm the
size of the
copper clusters occurs. It is clear that the copper particles with a diameter below 4 nm are characterized by a strong broadening of the plasmon resonance. Figure 2 shows absorption spectra of the colloidal particles, the electron micrographs and histograms observed 5 h after adding the reducing agent, in this case hydrazine, for reverse micelles with various w values. At low water content, a continuous absorption spectrum with a shoulder at 570 nm is seen (Fig. 2). Upon increasing the water content, the 570 nm band appears progressively. The electron micrographs show an increase in the size of the particles from 2 to 10 nm upon increasing the water content - from 1 to 10. At water contents up to 3.5 3 jg
2.5
z $
2 1.5
B 0
1
0.5
“,50
6 0
550 wavelengtt~
(nm)
Fig. 1. Simulated absorption spectra of metallic copper particles corresponding to various radii r of metallic particles: curve 1, r = 0.5 nm; curve 2, r = 1 nm; curve 3, I = 2 nm; curve 4, r=3nm;curve5,r=Snm;curve6,r~lOnm.
M.P. Pileni and I. Lisiecki/Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 63-68
Fig. 2. Absorption
spectra,
electron
micrographs and histograms of metallic copper particles water contents: (a) w= I; (b) w=3; (c) w= IO.
10, the size of the particles remains unchanged but the polydispersity increases. Electron diffractograms obtained at various water contents, compared with a simulated diffractogram of bulk metal copper, are in good agreement. This indicates a crystalline face-centered cubic structure (f.c.c.) with a lattice constant of 3.61. The similar behavior of the simulated and experimental absorption spectra in Figs. 1 and 2 confirms that the plasmon peak can only be observed with relatively large particles (up to 3 nm). The change in the particle size with water content can be explained in terms of the interfacial water structure: at low water content, copper ions associated with surfactant molecules are not totally hydrated and the effective number of ions participating in the chemical reduction is small. The increase in the water content induces an increase in the number of copper ions which react with hydrazine. This favors the growth of the average
65
prepared
in reverse micelles with various
particles. At relatively high water contents (up to lo), copper ions are totally hydrated and free water molecules are present [22]. This favors diffusion of copper ions inside the droplet. Electrostatic interactions between the head polar groups of the surfactant and copper ions compete with the hydration energy. The difference in these energies remains constant upon increasing the water content, which keeps the particle size constant. An increase in the polydispersity with the water content could be attributed to a decrease in interfacial rigidity [23-271. By using pure copper/AOT/water-in-oil aggregates, it has previously been shown that a change in the shape of the aggregates is induced by increasing the water content. Droplets are observed at w = 2, whereas cylindrical aggregates are formed at w=4 [21]. The reduction of copper reverse micelles was achieved with hydrazine as the reducing agent at
M.P. Pileni and I. Lisiecki/Colloids
66
various
surfactant
water content The shape resulting metal the CU(AOT)~ cal density particle
concentrations
with a constant
(w = 4). of the absorption spectrum of the clusters is unchanged by increasing concentration. The increase in opti-
is due to an increase
concentration
in the metallic
with the number
of reverse
Surfaces
A: Physicochem.
Eng. Aspects 80 (1993) 63-68
lated (3.6 l/,/3 = 2.09 A). High resolution microscopy
of the cylindrical
particles
electron shows
an
inter-reticular distance of 2 A. This value is in good agreement with that calculated from the lattice of the bulk phase. resolution various
Figure
image orientations
4 shows a high electronic
of these spherical of the reticular
particles
with
planes.
micelle aggregates (Fig. 3). At low Cu(AOT), concentrations, the clusters are spherical. Upon increasing the Cu(AOT), concentration, the forma-
By using sodium tetrahydridoborate as the reducing agent in the absence of oxygen, an absorption spectrum characteristic of metallic particles is
tion of cylindrical clusters is observed. The average number of cylindrical clusters remains constant (at about lo%), with 90% of the particles being spherical. However, a further increase in the surfactant concentration induces formation of longer cylinders with smaller radii. Electron diffraction shows the concentric circles characteristic of an f.c.c. phase with a lattice dimension equal to 3.61 A. The distance between the highest density reticular plane (111) can be calcu-
observed. Figure 4 shows a decrease in the size of the particles from 28 to 3 nm upon increasing the water content. At low water content (w = 3 and 5), the particles are homogeneously dispersed. Upon increasing the water content from w=4 to w =8, the particles become progressively associated. From electron diffraction patterns for particles formed in micelles with low water content (3 < w<5), the clusters are f.c.c. with a lattice dimension equal to 3.61, similar to what is obtained
in
n
(b)
Fig, 3. Absorption spectra, electron micrographs, copper concentrations: (a) [Cu(AOT),]=2
and histograms of metallic copper particles formed in reverse micelles at various x IO-’ M; (b) [Cu(AOT),]=4 x 10m2 M; (c) [Cu(AOT),] =2 x IO-’ M.
M.P.
Pileni and I. LisieckifCalloids
Surfaces A: Physicochem.
Eng. Aspects 80 (1993) 63-68
67
wavelength (nm)
wavelength (nm) Fig. 4. Absorption
spectra,
electron
micrographs and histograms of metallic copper particles water contents: (a) w = 3; (b) w = 5; (c) w = 10.
in the metallic bulk phase. At high water content (w = 9 and lo), a cubic phase with a lattice parameter equal to 4.27, characteristic of copper oxide Cu,O, is observed. The shape of the absorption spectrum of the colloidal particles changes with water content: at 3 < w < 6 the shapes of the absorption spectra are similar, with changes in intensity and a small residual absorption at 800 nm. Upon increasing the water content, an absorption around 800 nm appears. According to Yanase and Komiyama [2 l] the 800 nm absorption can be attributed to an oxide monolayer surrounding the metallic cluster. By comparing the microscopy data and the absorption spectra, it can be deduced that at low water content, metallic particles are formed. By increasing the water content, the metallic particles are surrounded by monolayers and multilayers of oxide which prevent further growth of the particles. At higher water content (above w = 8), pure copper
formed
in reverse micelles
at various
oxide particles are obtained. This is confirmed by the fact that below w= 7, electron micrographs show well-dispersed particles, whereas at w = 7 and 8 they are associated. Furthermore, Table 1 shows the ratio of the optical densities at 810 to 570 nm. An increase in the relative optical densities at 810 nm with the water content is clearly shown. This is probably due to the high affinity of the surfactant for the oxide. At w =9 and 10, the electron micrographs do not show the presence of metallic particles. These data could again be related to the water structure in the water pool. At low water contents, water molecules are bound at the
Table 1 Ratio of optical W Ratio
6 0.36
“Using
hydrazine
densities 7 0.49
at 800 and 570 nm 8 0.43
as a reducing
9 0.5 agent.
1
10 0.53
10” 0.17
M.P. Pileni and I. Lisiecki/Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 63-68
68
interface, whereas at high water contents, free water is present and plays a role in oxide formation. Compared to the data presented above with hydrazine, the size of metallic particles formed with sodium tetrahydridoborate is larger by one order of magnitude. The differences in the size and in the formation of oxide on changing the reducing agent could be due to the high redox potential of hydrazine compared to that of sodium tetrahydridoborate. The particles exposed to air are immediately oxidized.
8
9 10 11 12 13 14 15 16 17 18 19
References M.P. Pileni (Ed.), Structure and reactivity in reverse micelles, Studies in Physical and Theoretical Chemistry, Vol. 65, Elsevier, Amsterdam, 1989. C. Petit, P. Lixon and M.P. Pileni, J. Phys. Chem., 94 (1990) 1598. M. Meyer, C. Wallberg, K. Kurihada and J.H. Fendler, J. Chem. Sot., Chem. Commun., 90 (1984) 90. (a) P. Lianos and J.K. Thomas, Chem. Phys. Lett., 125 (1986) 299. (b) P. Lianos and J.K. Thomas, J. Colloid Interface Sci., 117 (1987) 50. T. Dannhauser, M. O’Neil, K. Johansson, D. Whitten and G. McLendon, J. Phys. Chem., 90 (1986) 6074. S. Modes and P. Lianos, J. Phys. Chem., 93 (1989) 5854. R.R. Chandler and J.L. Coffer, J. Phys. Chem., 95 (1991) 4.
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