Advances in Colloid and Interface Science, 55 (1995) 241-269 241 Elsevier Science B.V.

00227 A

P R E P A R A T I O N OF N A N O P A R T I C L E S OF SILVER HALIDES, S U P E R C O N D U C T O R S A N D MAGNETIC MATERIALS U S I N G WATER-IN-OIL M I C R O E M U L S I O N S AS N A N O - R E A C T O R S

V. P I L L A I , P. K U M A R , M.J . H O U , P. A Y Y U B a n d D.O. S H A H

Center for Surface Science & Engineering, Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA

CONTENTS

Abstract I. II. III. IV. V.

VI.

VII. VIII. IX. X.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242 Reactions in Microemulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 Part icle Synthesis in Microemulsions . . . . . . . . . . . . . . . . . . . . . . 246 Synthesis of Silver Halides . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 Synthesis of Magnetic Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . 249 A. Synthesis of Bar ium Ferr i te . . . . . . . . . . . . . . . . . . . . . . . . . 249 B. Synthesis ofT-Fe203 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 Synthesis of Superconductors . . . . . . . . . . . . . . . . . . . . . . . . . . 256 A. Synthesis of Y-Ba-Cu-O (123) Superconductors . . . . . . . . . . . . . . 256 B. Synthesis of Bi-Pb-Sr-Ca-Cu-O (2223) Superconductors . . . . . . . . . 261 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 Fu ture Directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265

ABSTRACT

Water-in-oil microemuls ions have been used for the synthesis of a var ie ty of nano-

part icles since the technique was first introduced in 1982. In this paper we have reviewed

several art icles per ta in ing to the synthesis of nanopart icles in microemulsions and described in some detail our research efforts in the past decade in the field of synthesis

of nanopart ic les of si lver halides, superconductors and magnet ic mater ia ls using water-

in-oil microemulsions as nano-reactors.

0001-8686/95/$29.00 © 1995 - - Elsevier Science B.V. All rights reserved. SSDI 0001-8686(94)00227-4

242

I. INTRODUCTION

A microemulsion may be defined as a thermodynamically stable isotropic dispersion of two immiscible liquids consisting of nano-size domains of one or both liquids in the other, stabilized by an interfacial film of surface active molecules [1]. Microemulsions may be classified as water-in-oil (w/o) or oil-in-water (o/w) depending on the dispersed and continuous phases. In both cases, the dispersed phase consists of mono- dispersed droplets in the size range of 100-1000 A [2]. It has now been shown that when microemulsions contain comparable amounts ofoil and water, some of these systems may show a bicontinuous structure [3,4].

Microemulsions were first introduced by Schulman and his co-workers in 1943 [5]. They explained that microemulsions were spontaneously formed with the uptake of water or oil due to the negative transient interfacial tension which allows the free energy to decrease as the total oil-water interfacial area increases [5,6]. At equilibrium, the oil/water interfacial tensions become zero or a very small positive number of the order of 10-2-10 .3 mN/m.

During the past fifteen years, studies on both theoretical and experi- mental aspects of microemulsions have provided a better understanding of the formation, properties, and phase behavior of microemulsions [7-13]. Due to their importance in various technological applications such as enhanced oil recovery, pharmaceutics, cosmetics, and nanoparticle synthesis, the structure and structural transitions of microemulsions have been extensively studied using X-ray and light scattering, ultracen- trifugation, spin label probes, dielectric relaxation, fluorescence probes, and neutron scattering [14-27].

As many applications depend on the dynamics of microemulsions, the characterization of the size and shape of microemulsion droplets and their inter-droplet interaction has been vigorously pursued recently. Many techniques such as light scattering, small angle neutron scattering, fluorescence quenching, stopped-flow, and electrical conductivity have been used to study the dynamics of microemulsions [28-34].

Microemulsions are industrially relevant due to their applications in a number of areas [35]. The interfacial and rheological properties of microemulsions are important to exploit surfactant-enhanced tertiary oil recovery methods. There are also obvious applications in the food and cosmetics industries, particularly using phospholipid surfactants. The ability to precisely control the size and stability of the microstructure

243

domains of microemulsions makes them useful for applications in liquid membrane technology [35,36]. Microemulsions also provide compartmen- talized liquid structures having high surface area and can therefore be used as novel and versatile media for chemical synthesis. This is signifi- cant for potential applications in enzymic reactions in microemulsions [36]. Another potential use of the fluid but structured nano-size domains of microemulsions is in the preparation of sub-micron or nano-particles of a desired size or shape, reflecting the structure and environment of the aqueous droplets [35].

Sub-micron or nanometer sized particles are of increasing scientific and technological interest [37]. There is clear evidence that small atomic clusters (10-1000 atoms) exhibit novel and hybrid properties between the molecular and bulk solid-state limits. These nanophase materials, which may contain crystalline, quasicrystalline, or amorphous phases, can be metals, ceramics, or composites with rather unique and improved me- chanical, electronic, magnetic and optical properties, than normal, coarse-grained polycrystalline materials. Because of their ultrafine sizes, and high surface area, these particles can easily overcome conventional restrictions of phase equilibria and kinetics, leading to lowering of sin- tering and solid-state reaction temperatures, and increase in sintering rate. The large fraction of atoms residing at the surfaces and grain boundaries of these materials also leads to materials with novel proper- ties. Such nanoparticles are not only of basic physical interest, but have also resulted in important technological applications such as: (a) cataly- sis, (b) high-performance ceramic materials, (e) micro-electronic devices, and (f) high density magnetic recording [37-39].

These potential technological applications have led to the development of several new techniques for the synthesis of various types of nanopar- ticles in recent years. These include gas-phase techniques such as gas- evaporation, laser vaporization, and laser pyrolysis [40-44]; vacuum synthesis techniques like sputtering, laser ablation, and ionized beam deposition [45-48]; and liquid phase techniques like precipitation from homogeneous solutions, sol-gel processing, freeze drying [49-53], etc. This article focuses on the use of the aqueous cores of water-in-oil microemulsions as reactors for the synthesis of nanoparticles. Precipita- tion reactions in microemulsions offers a novel and versatile technique for the synthesis of a wide variety ofnanophase materials with the ability to precisely control the size and shape of the particles formed [35,54,55]. It also offers a unique method to control the kinetics of particle formation

244

and growth by varying the physicochemical characteristics of the microe- mulsion system.

II. REACTIONS IN MICROEMULSIONS

In water-in-oil microemulsions, the aqueous droplets continuously collide, coalesce and break apart resulting in a continuous exchange of solute content [32,56]. The collision process depends upon the diffusion of the aqueous droplets in the continuous media, i.e. oil, while the exchange process depends on the attractive interactions between the surfactant tails and the rigidity of the interface, as the aqueous droplets approach close to each other [32].

For reactions in water-in-oil microemulsions involving reactant spe- cies totally confined within the dispersed water droplets, a necessary step prior to their chemical reaction is the exchange of reactants by the coalescence of two droplets. When chemical reaction is fast, the overall reaction rate is likely to be controlled by the rate of coalescence of droplets [32]. Therefore, properties of the interface such as interfacial rigidity are of major importance. A relatively rigid interface decreases the rate of coalescence, and hence leads to a low precipitation rate. On the other hand, a substantially fluid interface in the microemulsion enhances the rate of precipitation. Thus, by controlling the structure of the interface, one can change the reaction kinetics in microemulsions by an order of magnitude [57]. It has been further shown that the structure of oil, alcohol and ionic strength of the aqueous phase can significantly influ- ence the rigidity of the interface and the reaction kinetics [28,57].

Conceptually, if one takes two identical water-in-oil microemulsions and dissolves reactants A and B respectively in the aqueous phases of these two microemulsions, upon mixing, due to collision and coalescence of the droplets, the reactants A and B come in contact with each other and form AB precipitate. This precipitate is confined to the interior of the microemulsion droplets and the size and shape of the particle formed reflects the interior of the droplet. This is one of the principles utilized in producing nanoparticles using microemulsions (see Fig. la). However, nanoparticles can also be produced in microemulsions by the adding a reducing or precipitating agent, in the form of a liquid or a gas, to a microemulsion containing the primary reactant dissolved in its aqueous core. Figure lb shows the formation of metallic nanoparticles by adding reducing agents such as hydrazine or hydrogen gas in the microemulsion

245

Mix the two Microemulsions

Reactant B .7"~!::~';~. ,~)'-~q:i".'-~

~.~~ ~,

Collision and Coalescence of Droplets

(a)

Chemical Reaction Occurs

AB Precipitate Forr~

Soluble metal salt ~ ~',.~]7~ ~ - ] Add reducing agent (e.g hydra~ne or hydrogen)

(b)

Reduction Reaction Occurs

~ , ~ - ~,°~,~)

Metallic Pre~pitate Forms

Soluble salt of cation(s) ~T'~tf....~,.='~::~:~-~ ~--::,~?r 7 Bubble Gas through Microemulsion c:~

(c)

Chemical Reaction Occurs

Hydroxide or Oxide Precipit~.te Forms

Fig. 1. Schematic representation of synthesis of nanoparticles in microemulsions (a) using two microemulsions, (b) by adding a reducing agent to a microemulsion, and (c) by bubbling gas through a microemulsion.

containing the metal salt. Figure lc shows the formation of oxide, hydroxide or carbonate precipitates by bubbling gases like 02, NH3, or CO 2 through a microemulsion containing soluble salts of the cations. In the past decade or so, several researchers have used these techniques to synthesize nanoparticles in microemulsions. These articles are briefly described in the next section.

246

III. PARTICLE SYNTHESIS IN MICROEMULSIONS

Synthesis of ultrafine particles using reactions in microemulsions was first reported by Boutonnet et al. [54] when they obtained monodispersed metal particles (in the size range 3-5 nm) ofPt, Pd, Rh and Ir by reducing corresponding salts in water pools of water-in-oil microemulsions with hydrazine or hydrogen gas. Since then, there have been several reports in li terature where microemulsions have been used for the synthesis of a variety of nanoparticles.

Metallic nanoparticles and metal clusters have a wide range of appli- cations, including their use in catalysis, as biological stains, and as ferro-fluids. Touroude et al. [58] synthesized bimetallic particles of plati- num and palladium by reduction of H2PtC16 and PdC12 with hydrazine in water/pentaethylene glycol dodecyl ether/hexadecane microemulsions. Similarly, platinum particles have also been synthesized in aerosol-OT (AOT) and cetyl trimethyl ammonium bromide (CTAB) based microemul- sions for applications in catalysis [59]. Colloidal gold and silver particles have potential applications as condensers for electron storage in artificial photosynthesis. This has led to studies by Kurihara et al. [60] on laser and pulse radiolytically induced colloidal gold formation in w/o microe- mulsions. Colloidal gold and silver particles have also been synthesized by reduction of corresponding metal salts in water-in-oil microemulsions using sodium borohydride [61,62].

Colloidal semiconductor particles, especially cadmium sulfide, have invoked a lot of interest due to their unique photochemical and photo- physical properties. These properties are drastically dependent on the size of the semiconductor nanoparticles. Several researchers have there- fore used water-in-oil microemulsions for the synthesis of such nanopar- ticles. Meyer et al. [63] have generated platinized colloidal cadmium sulfide in situ in AOT based microemulsions. Several studies have recently been published on the synthesis, growth and characterization of cadmium sulfide [64-70] and cadmium selenide [71] particles in micro- emulsions.

Colloidal particles of calcium carbonate, stabilized by surfactants, constitutes an important class of additives for oils used in lubrication for internal combustion engines. Water-in-oil microemulsions thus provide an ideal media for the synthesis of such particles. This has been exploited for the synthesis of calcium carbonate [72-75], barium carbonate [76], and strontium carbonate [M] particles by carbonation of their respective

247

salts or hydroxides in the aqueous cores of water-in-oil microemulsions. Microemulsions have also been used for the synthesis ofmonodisperse

particles of nickel, iron and cobalt boride particles, which have applica- tions in heterogeneous catalysis, by the reduction of metal salts using sodium borohydride [78,79]. Other nanoparticles synthesized in water- in-oil microemulsions include monodisperse silica particles [80-831, nano-size molybdenum sulfide particles [84], and magnetic particles such as magnetite [85,86] and maghemite [87].

In recent years, we have been successful in synthesizing a variety of nanoparticles using different microemulsion systems. These include in

situ synthesis of nanoparticles of AgC1 [881 and AgBr [57]; and microe- mulsion mediated synthesis of highly dense and microhomogeneous Y-Ba-Cu-O (123) and B-Pb-Sr-Ca-Cu-O (2223) high temperature ceramic superconductors [89-91], high coercivity barium ferrite for high-density perpendicular recording [92,93], superparamagnetic y-Fe203 nanoparti- cles [941 and zinc oxide based varistors [95]. The synthesis of these materials and their structure and properties are described in the follow- ing sections.

IV. SYNTHESIS OF SILVER HALIDES

Nanoparticles of silver halides are extremely important for applica- tions in photographic emulsions [96,97]. It is, however, difficult to obtain nanosize, monodispersed particles of silver halides by conventional meth- ods. We have synthesized nanoparticles of AgC1 [88[ and AgBr [57] using water/AOT (sodium di-2-ethylhexyl sulfosuccinate)/alkane microemul- sion systems. Two microemulsions with the same composition but differ- ing only in the nature of the aqueous phase were taken. Microemulsions were prepared by solubilizing the aqueous solutions into AOT/alkane mixtures under vigorous stirring. The concentration of AOT in alkane (for example n-heptane) was 0.15 M. The aqueous phase in the first microemulsion was silver nitrate (AgNO 3, 0.4 M) whereas the aqueous phase in the second microemulsion was either NaC1 or NaBr (0.4 M). The microemulsions had identical compositions and each solubilized 8 moles of aqueous phase per mole of surfactant (AOT). These two microemul- sions were then mixed under constant stirring. This led to the precipita- tion of AgC1 or AgBr particles within the aqueous droplets due to the reaction between the two reactants as the aqueous droplets collide,

248

Fig. 2. Transmission electron micrograph of silver halide nanoparticles synthesized in water/AOT/heptane microemulsions, (a) one day after start of reaction, (b) one week after start of reaction.

coalesce, and break again. The growth of the particles is restricted by the size of the aqueous droplets resulting in the formation of nano-size particles.

Transmission electron microscopy (TEM) was used to study the size and size distribution of these particles. Samples were prepared by drop- ping a small amount of particles dispersed in the microemulsion onto a TEM grid. Figures 2a and b show nanoparticles of AgC1 precipitated in

249

water/AOT/heptane microemulsions, observed 1 day and I week respec- tively after the start of the reaction. These micrographs show that the particles formed in the microemulsions are in the size range of 50-100 in diameter with very uniform size distribution. These nanoparticles appear to have a spherical shape. It can also be seen that these disper- sions are very stable with little or no aggregation of AgC1 particles even after a week. This has been explained on the basis of a low attractive interaction between the microemulsions droplets containing the AgC1 nanoparticles [57,88].

Electron diffraction was used to identify the crystal planes of the AgC1 nano-crystals. Four d-planes of the fcc lattice structure of AgC1, namely, (111), (200), (220) and (420) were identified based on the analysis of the diffraction pattern.

Thus, water-in-oil microemulsions can be used for in situ synthesis of stable dispersions of nanoparticles of silver halides.

V. SYNTHESIS OF MAGNETIC OXIDES

Finely divided magnetic nanoparticles are widely used in high density magnetic recording media and magnetically responsive fluids, novel seals and magnetically directed printing inks. The magnetic oxides commonly used for magnetic recording are 7-Fe203, Fe304, cobalt modified Fe203 and CrO 2 for longitudinal recording, and hexagonal ferrites like BaFe12Ol: ~ for high density perpendicular recording. The magnetic char- acteristics of these particles depend crucially on their shape and size and they should be essentially single domain [98,99]. These requirements can be satisfied if these particles are synthesized using water-in-oil micro- emulsions.

A. Synthesis of Barium Ferrite

Barium ferrite (BaFe12019) has been traditionally used in permanent magnets because of its high intrinsic coercivity and fairly large crystal anisotropy [100]. However, in the past decade, it has emerged as an important magnetic media for high density perpendicular recording [101]. Such technological applications require materials with a strict control of homogeneity, particle size and shape, and magnetic charac- teristics [10211.

250

The classical ceramic method for the preparation of barium ferrite consists of firing mixtures of iron oxide and barium carbonate at high temperatures (-1200°C) [103,104]. Furthermore, the ferrite must then be ground to reduce the particle size from multi domain to single domain. This generally yields mixtures which are non-homogeneous on a micro- scopic scale. Milling introduces lattice strains in the material, whereas high reaction temperatures induce sintering and coagulation of particles, both of which can be damaging if specific properties like uniform and reduced particle sizes, and high coercivities are required in the final product [105,106].

In order to achieve a homogeneity of ions at the atomic level, and to overcome the effects of milling, various techniques such as chemical coprecipitation [107,108], glass crystallization [101,109], organometallic precursor method [110,111], aerosol synthesis [112,113], and colloidal synthesis [114] have been developed to prepare ultrafine barium ferrite. We have used the aqueous cores of water-in-oil microemulsions as reac- tion media to produce uniform-sized, microhomogeneous nanoparticles of carbonate precursors for the synthesis of ultrafine barium ferrite [92,931.

We selected a microemulsion system with eetyl trimethyl ammonium bromide (CTAB) as the surfactant (12% by wt); n-butanol as the co-surfac- tant (10% by wt); n-octane (44% by wt) as the continuous oil phase; and a salt solution (34% by wt) as the dispersed aqueous phase. This system solubilizes a relatively large volume of aqueous phase in well defined nano-size droplets of stable, single phase water-in-oil microemulsions [89].

Microemulsions were prepared by solubilizing different salt solutions into CTAB/n-butanol/n-octane solutions. We took two microemulsions (microemulsion I and microemulsion II) with identical compositions but different aqueous phase. The aqueous phase in microemulsion I was a mixture of barium nitrate and ferric nitrate solutions. The aqueous phase in microemulsion II was the precipitating agent, ammonium carbonate. These two microemulsions are then mixed under constant stirring. Due to the frequent collisions of the aqueous cores of water-in-oil microemul- sions [32,56], the reacting species in mieroemulsions I and II come in contact with each other. This leads to the precipitation of bar ium- i ron- carbonate within the nano-size aqueous droplets of the mieroemulsion. The aqueous droplets act as constrained nano-size reactors for the pre- cipitation reaction, as the surfactant monolayer provides a barrier restricting the growth of the carbonate particles. This surfactant mono-

251

~O O~

i

I I

Fig. 3. Transmission electron micrograph of barium ferrite precursors synthesized in water/CTAB/1-butanol/octane microemulsions.

layer also h inde r s coagulat ion of particles. The b a r i u m - i r o n - c a r b o n a t e precipi ta te was sepa ra ted in a cent r i fuge

at 5000 r p m for 10 minutes . The prec ip i ta te was then washed in a 1:1 mix tu re of m e t h a n o l and chloroform, followed by pure m e t h a n o l to remove any oil and su r fac t an t from the particles. The prec ip i ta te was t h e n dried at 100°C. The dried precipi ta te was hea ted (calcined) at 950°C for 12 hour s to ensu re complete conversion of the carbona tes into the

hexafer r i te (BaFel2019). T ransmis s ion electron microscopy (TEM) was used to s tudy the size

and size d i s t r ibu t ion of the precursor part icles (carbonates) as well as the calcined part icles. Samples were p repa red by u l t rasonica l ly d i spers ing the par t ic les (precursors and calcined powder) in m e t h a n o l prior to depos i t ing it onto a carbon coated TEM grid. F igure 3 shows tha t the p recursor part icles formed wi th in the aqueous droplets of the microemul- sion were in the size r ange of 5-15 nm. On the o ther hand , the TEM of

252

Fig. 4. Transmission electron micrograph of microemulsion synthesized barium ferrite particles.

the calcined powder (Fig. 4) shows that these particles are in the size range of 50-100 nm, implying that growth of particles has taken place during calcination.

X-ray diffraction for the calcined powder showed all the characteristic peaks for barium ferrite marked in the diffractogram. No other phases were detected. This confirmed the complete conversion of the precursor carbonates into the hexagonal ferrite BaFe12019.

Room temperature magnetic property measurements for the calcined powder were carried out on a vibrating sample magnetometer (VSM) using an unoriented, random assembly of particles. The magnetization curve (Fig. 5) thus obtained yielded an intrinsic coercivity (H c) of 5397 Oe, a saturation magnetization (M s) of 61.2 emu/gram, and a remanent magnetization (M r) of 33.3 emu/gram for the microemulsion synthesized barium ferrite particles. This high value of coercivity indicates that these barium ferrite particles are essentially single domain. This observation is also confirmed by the fact that the particles formed by us are in the size range of 50-100 nm. Haneda et al.[ll5] have shown that using the theory of Kittel [116], the critical domain size for barium ferrite particles is about 1 ~tm. Also, direct observation of the domain structure of barium ferrite particles at room temperature by Goto et al. [117] confirms that the critical domain size is indeed in the region of 1 ~tm.

f - %

txO

70

50

30

10

-10

-30

-50

-70

-15

J

J

J " l l . . . . . . . I l l l , . . . . . I . . . . . . . . , i l l . . . . . I . . . . . . J l i l l l l l l l l ( I

-10 -5 0 5 10 15

253

HI (kOe)

Fig. 5. Room tempera ture magnet izat ion curve for microemulsion synthesized bar ium ferrite particles.

In summary, we have presented a new, microemulsion mediated process for the synthesis of ultrafine particles of barium ferrite. This process yielded high coercivity, single domain barium ferrite particles, which are phase pure as confirmed by X-ray diffraction. These phase pure, single domain particles can be doped by Co and Ti to reduce the coercivity and large anisotropy to make these particles suitable for high density perpendicular recording [101].

B. Synthesis of 7-Fe203

Magnetic pigments with particles in the nanometer size range have potential applications in information storage [118], color imaging [119], bioprocessing [120], ferrofluids [121], magnetic refrigeration [122] and magnetic resonance imaging [123]. The magnetic oxide commonly used for these applications is 7oFe203.

A microemulsion system, similar to the one used for the synthesis of barium ferrite precursors was used [94]. The first microemulsion had a 0.15 M solution of ferrous sulfate as the dispersed aqueous phase. To this microemulsion, a 2.0 M solution of tetra ethyl amine (TEA) in octane was

254

Fig. 6. Transmission electron micrograph of 7-F%O 3 particles synthesized in micro- emulsions.

added. Diffusion of TEA th rough the su r fac tan t monolayer into the aqueous drople ts resu l t s in the hydrolysis of Fe 2÷ ions. A second microe- muls ion , con ta in ing sod ium ni t r i te as the d ispersed aqueous phase , was added to th is suspension. This sys tem was m a i n t a i n e d at 45 + 5°C for one h o u r u n d e r cons t an t s t i rr ing. The hydrolyzed Fe 2÷ ions reac ted wi th sod ium ni t r i t e due to the f requen t collision and coalescence of the drop- lets. This led to the oxidat ion of the hydrolyzed product and the fo rmat ion of iron oxide wi th in the aqueous core of the microemuls ion. The bar r ie r provided by the su r f ac t an t monolayer res t r ic ted the g rowth of the part i- cles. T r a n s m i s s i o n electron microscopy of these part icles revealed t h a t t hey were spherical , wi th a na r row size d is t r ibut ion and a size range of 22-25 n m (see Fig. 6). X-ray diffraction of the dr ied powders showed charac ter i s t ic peaks cor responding to the crystal l ine ¥-Fe203 phase. No o ther impur i t i e s cor responding to the (~-Fe203 or Fe304 were de tec ted in the di f f ractogram.

255

~0

48

36

24

12

-12

-24

-36

-48

j f

-4.8 -3.6 -2.4 -1.2 0.0 1.2 2.4 3.6 4.8

H (kOe)

Fig. 7. Room temperature magnetization curve for 7-Fe.20:~ particles synthesized in microemulsions.

Room temperature magnetization curve for the dried powder, obtained on a VSM, is shown in Fig. 7. This curve shows no hysteresis and zero coercivity indicating that these particles were superparamagnetic at room temperature. These measurements also yielded a saturation mag- netization of 46.6 emu/g. As particle size decreases to the nanometer size range, the thermal energy becomes sufficiently large at room tempera- ture to overcome the magnetic anisotropy energy, and the bulk magnetic properties (coercivity and retentivity) vanish at room temperature, lead- ing to superparamagnetism (as shown for ~/-Fe20~ particles synthesized in microemulsions).

Ifmicroemulsions are used for in situ synthesis ofsuperparamagnetic, nanometer sized, gamma iron oxide magnetic particles, these particles

256

can be isolated easily in powder form for their use in magnetic imaging. These powders can also be dispersed in aqueous or non-aqueous media for potential applications as ferrofluids, or they can be incorporated in a polymer or glass matrix for their use in magnetic refrigeration.

VI. SYNTHESIS OF SUPERCONDUCTORS

The discovery of a superconducting temperature (T c) near 30 K in La-Ba-Cu oxide by Bednorz and Muller [124], prompted many efforts to study perovskite-like oxide superconductors. This led to the discovery of new superconducting materials in the Y-Ba-Cu-O system with T c above the boiling point of liquid nitrogen [125,126]. Recently, even higher T c (> 110 K) superconductors were discovered in the Bi-Sr-Ca-Cu-O [127] and T1-Ba-Ca-Cu-O [128] systems.

It is now well recognized that the properties of high temperature oxide superconductors are critically dependent on the microstructure of the sample. Control of particle size, size distribution, morphology of precur- sors, and heat t reatment conditions are critical to obtaining a desired microstructure. The conventional method for the preparation of oxide superconductors by solid state reaction has many inherent problems like poor homogeneity, large particle size, lack of reproducibility, and longer heat t reatment times. In order to achieve a better level of homogeneity, wet chemical techniques such as coprecipitation [129-132], freeze drying [133], amorphous citrate process [134,135], and chelation [136] have been used. One can achieve an even better level of homogeneity and smaller particle sizes if the wet chemical reactions are carried out in constrained nanosize reactors, like the aqueous droplets of water-in-oil microemul- sions. We have used water-in-oil microemulsions for the synthesis of nanoparticles of oxalate precursors of both Y-Ba-Cu-O (123) and Bi-Pb- Sr-Ca-Cu-O (2223) superconductors.

A. Synthesis of Y-Ba-Cu-O (123) superconductors

YBa2Cu3O7_x superconductors were synthesized by the co-precipita- tion of oxalates of yttrium, barium and copper in the aqueous cores of water/cetyl trimethyl ammonium bromide/l-butanol/octane microemul- sion systems [89]. Two microemulsions (each consisting of 29 wt.% surfactant plus cosurfactant phase, 60 wt.% hydrocarbon phase, and

257

11 wt.% aqueous phase), one containing an aqueous solution of yttrium, barium and copper nitrates in the molar ratio 1:2:3, and the other containing ammonium oxalate solution as the aqueous phase, were mixed. This led to the precipitation of the oxalate precursors within the aqueous cores of the microemulsion. Since the two microemulsions are identical in composition, there is no destabilization of the microemulsion phase on mixing. The barrier provided by the surfactant monolayer restricts the growth of the particles and hinders inter-particle coagula- tion. Serial precipitation of the cations (which may occur in other wet chemical synthesis processes) is avoided in this approach due to the size of the "nanoreactors", which was about 130 A in the present case. The particles are also expected to be microhomogeneous due to mixing of the cations at the nano-scale level in the microemulsion droplets. The oxalate precipitate was separated, washed, dried and then calcined at 820°C for 2 hours for complete conversion to the oxide. This powder was then pressed into pellets and sintered at 925°C for 12 hours.

As a comparison, we also studied co-precipitation of yttrium, barium, copper oxalates in a bulk aqueous solution using ammonium oxalate. The oxalate precipitate thus obtained was calcined and then sintered under similar conditions as the microemulsion precipitate to produce the YBa2Cu307_ x superconductor.

DC magnetic susceptibility measurements as a function of tempera- ture were performed on a superconducting quantum interference device magnetometer (SQUID). This revealed that the microemulsion synthe- sized sample had a superconducting transition temperature (T c) of 93 K and showed 90% of the Meissner shielding as that expected from an ideal sample (-1/4~). This sintered disk also had 98% of theoretical single crystal density. This high density is due to the ability of the nanoparticles to pack closely with very few voids and upon sintering, these well-packed particles are able to form large grains (15-50 gm) with very low porosity. This can be seen from the scanning electron micrographs (SEM) of the sintered disk shown in Fig. 8a and b. On the other hand, the bulk co-precipitated sample showed a Meissner shielding of 14% of that expected from an ideal sample. This can be explained by the fact that this sample was more porous (lower density) than the microemulsion synthe- sized material due to the fact that the precursor particles were large and therefore did not pack well. This resulted in smaller grains (0.5-2.0 gm) in the sintered pellet with more pores (see Fig. 8c and d). A comparison of all properties of the microemulsion synthesized and bulk co-precipi-

258

,.Q

. ~ o

~g

~ g

o ~

m

m ~

2 ~

~ A -,"~ ~

~ . ~

• ~

259

TABLE 1

Comparison of selected physical properties ofYBa2Cu3OT_ x synthesized using microemul- sions and by bulk co-precipitation

Property Microemulsion Bulk aqueous synthesis synthesis

ESD of Y-Ba-Cu oxalate precursor

ESD of Y-Ba-Cu-O powder Calcination conditions

Grain size of sintered pellet Sintering conditions

Percent of theoretical density of sintered pellet

Fraction of ideal Meissner signal (-1/4~)

Superconducting T~

47.4 nm 380.6 nm

274.8 nm 626.6 nm 820°C, 2 h 860°C, 6 h

15-50 ~m 0.5-2.0 ~m 925°C, 12 h 925°C, 12 h

98% (_+3) 90% (_+2)

90.5% 14.4%

93 K 91 K

ESD - Estimated spherical diameter.

t a t e d s a m p l e s a re s h o w n in Tab le 1. B a s e d on t h e s e r e su l t s we can s ay

t h a t t he mic roemul s ion der ived 123 s u p e r c o n d u c t o r is s even t imes b e t t e r t h a n the one fo rmed by b u l k co-precipi ta t ion, on the bas i s of t he M e i s s n e r

sh ie ld ing r e su l t s for t he two samples .

YBa2Cu3OT_ x s u p e r c o n d u c t o r s h a v e a lso b e e n s y n t h e s i z e d u s i n g wate r - in -o i l m i c r o e m u l s i o n s b a s e d on a nonionic s u r f a c t a n t Igepa l CO

430 ( n o n y l p h e n o x y p o l y (e thy leneoxy) e thano l ) w i th cyc lohexane as the

oil p h a s e [90]. In th is process , the a q u e o u s p h a s e in one m i c r o e m u l s i o n

cons i s t ed of y t t r i u m ace ta te , b a r i u m c a r b o n a t e and copper ace ta te , in a cat ionic ra t io of 1:2:3 (based on 10 m M of Y 3÷) d isso lved in a 50/50 (v/v)

acet ic ac id /wa te r mix tu re . The second mic roemul s ion h a d oxalic acid

d i s so lved in a 50/50 (v/v) acet ic ac id /wa te r mix tu re . Mixing of the two m i c r o e m u l s i o n s r e s u l t e d in the p rec ip i t a t ion of oxa la t e s of Y 3÷, Ba 2÷ and

Cu 2÷ in the ra t io 1:2:3 (as conf i rmed b y EDAX) wi th in the a q u e o u s cores

of t he mic roemuls ion . These oxa la te p r e c u r s o r s we re ca lc ined a t 800°C for 12 h o u r s in a i r for convers ion to the oxide. The ca lc ined p o w d e r w a s p r e s s e d into a pel le t and s i n t e r ed a t 930°C for 12 hou r s in f lowing oxygen

a t m o s p h e r e .

260

Fig. 9. Scanning electron micrograph of sintered YBa2Cu3OT_x pellet synthesized using water/Igepal CO-430/cyclohexane microemulsions.

Powder X-ray diffraction of the sintered sample showed only peaks for the orthorhombic superconducting phase ofYBa2Cu307_ x. All diffraction peaks were accounted for by an orthorhombic unit cell with a = 3.818 A, b = 3.881 .~ and c = 11.679 ./~. The sintered pellet had a density corre- sponding to 97.8% of single crystal density which is quite remarkable for superconductors sintered under ambient pressure. This is also observed in the morphology of the sintered pellet shown in Fig. 9. These highly dense, large grains result from the ultrafine nature of the precursors.

Superconducting transition temperature measurements on a SQUID revealed a T c of 92 K. The diamagnetization corrected value of zero-field cooled signal (Meissner shielding) is 93.2% of that expected from an ideal sample (-1/4n). This high value is explained on the basis of the high density and large grain sizes in the sintered superconductor.

261

B. Synthesis of Bi-Pb-Sr-Ca-Cu-O (2223) superconductors

Recently, we have also used water-in-oil microemulsions for the syn- thesis of Bi-Pb-Sr-Ca-Cu-O (2223) superconductors [129]. A nonionic surfactant Igepal CO-430 or nonylphenoxypoly (ethyleneoxy) ethanol was used for the formation of the microemulsion with cyclohexane as the external oil phase. The aqueous phase in the first microemulsion was a solution of salts of Bi, Pb, Sr, Ca, Cu in the molar ratio 1.84:0.34:1.91: 2.03:3.06 dissolved in a 50/50 (v/v) mixture of acetic acid and water. This aqueous solution was prepared by dissolving Bi203, Pb(CH3COO)2, SrCO3, CaCO 3 and Cu(CH3COO) 2 in the acetic acid/water mixture in the cationic ratio given above. The aqueous phase in the second microemul- sion was a solution of oxalic acid in a 50/50 (v/v) acetic acid and water mixture. Both the microemulsions consisted of 15 grams of surfactant, 50 ml of oil, and 10 ml of aqueous phase. Mixing of these two microemul- sions led to the formation of the oxalate precursor within the aqueous cores of the microemulsion. These precursors were in the size range of 2-6 nm (see Fig. 10). These oxalate particles were calcined at 800°C for 12 hours and then pressed into a pellet and sintered in air for 96 hours at 850°C.

DC magnetic susceptibility measurements as a function of tempera- ture (shown in Fig. 11) indicated that this superconductor had a T c of 112 K. The zero field cooled signal (diamagnetic shielding) corre- sponded to 93% of the Meissner shielding expected from an ideal sample (-1/4~). Phase analysis by X-ray diffraction showed charac- teristic peaks of the 110 K phase; (002) and (0010) at 20 = 4.8 ° and 23.9 ° respectively. This confirmed that the material was phase pure 2223 superconducting oxide.

The sintered disk also had large, lamellar grains as can be seen from the scanning electron micrograph (Fig. 12). This disk has very low porosity and had a density corresponding to 97% of the theoretical value. As in the case of the 123 superconductor, the ability of the nanoparticles to pack closely, with very few voids, results in high density disks with low porosity, and high Meissner shielding.

Thus, we have shown that uniform sized nanoparticles of precursors of superconductors precipitated in microemulsions, upon heat treatment, lead to the formation of microhomogeneous, high density superconduc- tors which show properties that are superior to those synthesized by most wet chemical methods.

262

Fig, 10. Transmission electron micrograph ofoxalate precursors for 2223 superconductors synthesized in water/Igepal CO-430/cyclohexane microemulsions.

A

¢ 2"

:: O" ¢ 1

¢~ -2" ; ,<

> , -4"

N

.,~ -6"

P.. -8"

-10 r . j

° ~

-12

-14

• ZFC o FC @ ¢

0 20 4 0 60 80 100 1 2 0 140

Temperature (K)

Fig. 11. DC magnetic susceptibility measurements as a function of temperature for 2223 superconductor synthesized using microemulsions.

263

Fig. 12. Scanning electron micrograph ofsintered Bi-Pb-Sr-Ca-Cu-O (2223) pellet synthe- sized using microemulsions.

VII. SUMMARY

The impor t ance of n a n o p h a s e technology has resu l ted in t r e m e n d o u s resea rch efforts towards the deve lopmen t of new techniques for the syn thes i s of nano-size particles. One such technique, developed a decade or so ago, was to use water-in-oil mic roemuls ions as a react ion med ium. The aqueous cores /water pools of these microemuls ions can be used as nano-size reactors for the precipi ta t ion of a wide var ie ty ofnano-par t ic les . For th is t echn ique to be successful, the r eac tan t s should be confined to the aqueous cores of the microemuls ions and should not in te rac t wi th the o ther componen t s of the microemuls ion (namely oil and surfactant) .

In this paper , we have p resen ted some of our recent research efforts in the field of syn thes i s of nanopar t ic les us ing water-in-oil microemul- sions. We have described the syn thes i s of silver halides, b a r i u m ferrite, g a m m a iron oxide and Y-Ba-Cu-O and Bi-Pb-Sr-Ca-Cu-O superconduc- tors us ing microemuls ions . All these mate r ia l s syn thes ized in microemul- sions exhibi t un ique proper t ies unl ike those of ma te r i a l s syn thes ized us ing convent iona l techniques .

264

VIII. FUTURE DIRECTIONS

Over the past decade or so, researchers all over the world have been successful in synthesizing a variety of inorganic and organic nanoparti- cles using microemulsions as reaction media. However, attempts have to be made to make this process commercially and economically feasible. The process, as it currently exists, is expensive since microemulsions require the use of oil, surfactant and cosurfactant in high quantities. After separation of the particles from the microemulsion, the supernatant which is left over should be separated to recover the oil, surfactant and cosurfactant so that these materials can be used again for making microemulsions. This would significantly lower the cost of the process and will make it commercially more viable than at present.

Another aspect that needs to be explored is the effect of components of the microemulsion (oil, surfactant, cosurfactant) on the kinetics of particle formation and size of nanoparticles formed in microemulsions. We have shown that the interfacial rigidity of droplets in microemulsions can be controlled by the structure of the oil, surfactant or cosurfactant [28,57]. A high interfacial rigidity leads to a low inter-droplet attraction and low rate of precipitation, while a fluid interface leads to a higher rate of precipitation. However, a detailed and systematic study to reveal the effect of microemulsion components on reaction kinetics of particle for- mation and the effect this has on the morphology and properties of the particles formed would be very useful.

Most of the reports in literature on the synthesis of nanoparticles in microemulsions involve the formation of spherical particles. It would be of great scientific and technological interest to form asymmetrical parti- cles like needle, disk, and egg-shaped particles in situ within the aqueous cores of water-in-oil microemulsions and to have the ability to precisely control the size and shape of these particles.

Finally, another area of interest would be in the synthesis ofnanopar- ticle composites where one material (50-100 A) may be coated by a thin layer (10-20 A) of another material. Technologically, these types of materials have several applications, for example, in the synthesis of cobalt surface doped iron oxide particles which are used in magnetic recording media.

These are some of the main areas which should be the focus of research, in the near future, in the field of synthesis of nanoparticles using microemulsions.

265

IX. A C K N O W L E D G E M E N T S

T h e a u t h o r s w i s h to a c k n o w l e d g e t h e f i n a n c i a l s u p p o r t p r o v i d e d b y

t h e N a t i o n a l S c i e n c e F o u n d a t i o n ( G r a n t nos . C B T 8 8 0 7 3 2 1 a n d C T S

8 9 2 2 5 7 4 ) , t h e E l e c t r i c P o w e r R e s e a r c h I n s t i t u t e ( G r a n t no. R P 800922 ) ,

a n d t h e A l c o a F o u n d a t i o n . W e w o u l d a l so l ike to t h a n k Prof . M.S . M u l t a n i

a n d Dr . L .M. G a n w i t h w h o m w e h a d s t i m u l a t i n g d i s c u s s i o n s on t h e s e

t op i c s .

X. R E F E R E N C E S

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