1.1 IntroductionNanoparticles play a vital role in high performance materials in high technologyindustries. The studies of nanoparticles started in the early 1980's and have now becomeone of the hottest worldwide research fields (Pui and Chen, 1997).There are four main processing approaches for the preparation of nanoparticles bychemical method (Riman, 1993): (1) chemistry in liquid phase including direct strike(Murata, et al., 1976), nonsolvent addition (Mulder, 1970), solvent removal (Cheng, etal., 1986), gel drying (sol-gel) (Perthuis, And Colomban, 1984) and precipitation fromhomogeneous solution (Gordon, et al., 1959); (2) chemistry between heterogeneousphase including hydrothermal synthesis (Adair, et al., 1987), molten salt synthesis(Arendt, et al., 1979), pyrolysis (Wada, et al., 1987) and spark erosion (Berkowitz, et al.,1987); (3) chemistry in a droplet including emulsions (Woodhead, et al., 1980), micelles(Gobe, et al., 1983) or microemulsions (Kandori, et al., 1988) and aerosols (Balboa, etal., 1987); (4) chemistry in the vapor phase including heating method (Mazdiyasni, et al.,1965), vapor precursors (Iwama, et al., 1982), liquid precursors (Kagawa, et al., 1983)and solid precursors (Watanabe, et al., 1986). The most attractive methods are thosewhich synthesize in the liquid medium, including methods of precipitation, reduction,dehydration, solvent evaporation, reversed micelle technology and microemulsionpolymerization, etc. In this chapter, we will focus on the nanoparticles made from bothW/O microemulsion (reversed micelles) and O/W microemulsion procedures.Hence it is necessary to introduce the definition of micelles and microemulsions beforedealing with the principles and practices of forming nanoparticles from micelles andmicroemulsions. Micelles are aggregates of surfactants in a liquid medium which areformed when the surfactant concentration exceeds the critical micelle concentration(CMC) (McBain and Salmon, 1920). It must be mentioned that this definition is only fornormal micelles; for the case of reversed micelles it is not necessary to have a CMC. In

the normal micelle the surfactant is orientated in such a way that the hydrophobichydrocarbon chains are towards the interior of the micelle, leaving the hydrophilicgroups in contact with the aqueous medium. Above the CMC, the physical state of thesurfactant molecules dissolved in water changes dramatically, and additional surfactantexists as aggregates or micelles. Thus, the bulk properties of the surfactant, such asosmotic pressure, turbidity, solubilization, surface tension, conductivity and selfdiffusion,change around the critical micelle concentration (Fig. 1.1).Figure 1.1 Changes in concentration dependence of a wide range of physicochemicalquantities around the critical micelle concentration (After Lindman,1980).2If the micelles are formed in non-aqueous medium, the aggregates are called reversedmicelles, as in this case the hydrophilic head groups are now towards the core of themicelle while leaving the hydrophobic groups outside of the micelles. The driving forcefor formation of reversed micells is the dipole-dipole interactions of the surfactant. Thenumber of aggregates is usually small and not sensitive to the surfactant concentrationand thus there is no obvious CMC (Zhao, 1991; Gutmann and Kertes, 1973; Kertes andGutmann, 1976). In both cases (micelles and reversed micelles), only a small amount ofsolubilized hydrophobic (usually oil) or hydrophilic (usually water) material exists in themicelles (Fig. 1.2). However, the solubilization can be enhanced if the concentration ofsurfactant is increased further. As the inside pool of water or oil is enlarged or swollen,the droplet size increases up to a dimension much larger than the monolayer thickness ofthe surfactants. In this case, we call them microemulsions or swollen micelles. What wenow describe as the preparation of nanoparticles from the reversed micelles may bebetter described as preparation from swollen reversed micelles or water-in-oilmicroemulsions.Figure 1.2 The structure of micelles and microemulsions (O/W and W/O) (AfterOverbeek et al., 1983).As the surfactant concentration increases further, micelles can be deformed and can

change their shapes to rodlike micelles, hexagonal micelles and lamellar micelles or3liquid crystals (Fig. 1.3). It is these changes that make it possible to prepare differentshapes of nanoparticles from micelle synthesis microreactors.1.2 Formation Mechanisms of Micelles and Microemulsions1.2.1 Simple Geometric FactorsThe structures of micelles can be simply determined by the geometric factors of thesurfactant at the interface, including head group area a0, the alkyl chain volume v and themaximum length lc (to which the alkyl chain can extend). According to Israelachvili(Israelachvili, et al., 1976), the packing considerations govern the geometry ofaggregation into micelles, vesicles and liposomes.Figure 1.3 A schematic phase diagram of surfactant-oil-water systems showinga variety of self-assembled structures (After Liu, J., et al., 1996).These obey the following rules:1. Spherical micelles require v/a0lc < 1/3,2. Non-spherical micelles require 1/3 < v/a0lc < 1/2,3. Vesicles or bilayers require 1/2 < v/a0lc < 1, and4. Inverted micelles require 1 < v/a0lc.In each case, the limits for the packing parameter v/a0lc can be evaluated from simplegeometry (Fig. 1.4) (Israelachvili, 1985). However, the change of environment willaffect these parameters, and thus dictate the molecular packing at the interface.4Figure 1.4 The relationship between aggregate type and geometry on thepacking requirements of surfactant head group and chains (Israelachvili, 1985).51.2.1.1 Spherical MicellesSpherical micelles are usually formed by anionic surfactants with or without cosurfactants.For an O/W micelle, this can be done by adjusting the repulsion (doublelayers) between adjacent head groups, resulting in large values for a0. In this case, themicelle radius is approximately equal to the maximum stretched out length of thesurfactant molecule and therefore the aggregates are very small. Bellare et al. (1988),using small-angle neutron scattering (SANS), have successfully visualized a sphericalmicelle of radius (3.0 ± 0.3) nm for a cryo-TEM image of a 10 mmol • dm-3

solution ofditetradecyl-dimethyl-ammonium acetate.

1.2.1.2 Cylindrical MicellesIt is a quite common phenomenon that micelles grow as the preferred surface curvaturedecreases. Any change that reduces the effective head group area will lead to the growthof micelles. There are basically three ways to form cylindrical micelles: (1) addition of aco-surfactant with a very compact head group, i.e. n-alkanol for which the–OH group issmall in comparison with a charged sulfate group, (2) changing the counterion, i.e.,changing Na+ to Mg2+ will significantly reduce the electric double layer thickness, andhence reduce the effective volume (size) of the head groups, (3) changing thehydrophilicity of non-ionic head groups by electrolyte addition or temperature change;i.e., for micelles formed by surfactants with poly(oxyethylene)(PEO) head groups, thehead groups are sensitive to changes of solvency (Tadros, 1987).1.2.2 The Critical Micelle Concentration (CMC) for SurfactantsThe CMC of a surfactant system depends on the minimum value of the interaction freeenergy per molecule μN

0. The minimum arises from the hydrophilicity of the head group,tending to increase the area per molecule, while the hydrophobicity of the alkyl tailtends to cause a decrease due to the hydrophobic bonding. From this concept, one is ableto predict how various structural features of surfactant molecules will affect their CMCvalues.Table 1.1 Typical CMC values for ionic surfactants at 25 °CSurfactant CMC/mmol • dm-3

C12H25SO4Na 8.1C12H25SO4Li 8.9C12H25SO3Na 10C12H25CO3K 12.5C12H25NH3Cl 14.76C12H25NC2H5Cl 15C12H25N(CH3)3Br 16C12H25N(CH3)3Cl 17For these ionic surfactants, there is little difference between anionic and cationic headgroups, since both have comparatively high CMC values, provided that the counterion ismonovalent. Usually, the CMC values for these systems are 1–20 mmol • dm-3

(Table

1.1). However, to change the counterion to a multivalent one tends to decrease the CMCconsiderably.For non-ionic surfactants, such as CxEy type, where x is the carbon number in the rangeof 8–18, and y is the ethylene oxide group in the range of 3–20, the CMC value isextremely low, i.e., 0.04–3 mmol • dm-3, depending on the structure of the molecules(Table 1.2).Table 1.2 Typical CMC values for non-ionic surfactants at25 °CSurfactant CMC/mmol · dm-3

C12H25(OCH2CH2)4OH 0.046C12H25(OCH2CH2)6OH 0.087C12H25(OCH2CH2)8OH 0.109C12H25(CH3)NO 2.11.2.3 Solubilization and Formation of Microemulsions1.2.3.1 SolubilizationThe term solubilization in this chapter refers to the dissolution of hydrophobic(hydrophilic) materials into water (or oil) to an extent greatly exceeding their normalsolubilities in water (oil). The interior of a micelle provides a hydrophobic (hydrophilic)environment in which non-polar (or polar) compounds can be accommodated. As aresult, the solubility of hydrophobic (or hydrophilic) material increases dramaticallywith increasing surfactant concentration when it reaches the CMC as shown in Fig. 1.1.The solubility behavior of surfactants is anomalous as the temperature is increased to avalue at which there is a sudden increase in solubility and the material then becomesvery highly soluble (Krafft, 1899). This is illustrated in Fig. 1.5.7Figure 1.5 Schematic representation of solubility versus temperature showinglocation of the Krafft point (After Shinoda, 1974).The process of solubilization has many applications in industrial preparations, forexample, in solubilization of insoluble drugs for intravenous injection. The process ofsolubilization by micellar systems is also important in detergency, whereby fats and oilsare removed by incorporation into the hydrocarbon core of the micelle. There are fourgeneral possible ways for the incorporation of the solubilization: (1) in the hydrocarboncore of the micelle; (2) orientation in the micelle which could be deep or shallow; (3) in

the hydrophilic portion of the surfactant (e.g., ethylene oxide of non-ionic surfactants);and (4) adsorption on the surface of the micelle (Fig. 1.6).Figure 1.6 Schematic representation of four ways of solubilization of micelles.81.2.3.2 MicroemulsionsThe microemulsion systems were first reported by Hoar and Schulman (1943), whodescribed transparent or translucent systems, formed spontaneously when oil and waterwere mixed with a relatively large amount of an ionic surfactant combined with acosurfactant, e.g., a medium size alcohol. Later, in 1959, Schulman and co-workers(Schulman, et al., 1959) introduced the concept of microemulsions as transparent ortranslucent systems with a spherical or cylindrical size range of 8–100 nm. This is theright size for preparing spherical and rod-like nanometer particles.The solubilization theories of microemulsions have been proposed by Shinoda (Shinoda,1974), who considered microemulsions as solubilized systems extended from the threecomponentphase diagrams of water-surfactant and co-surfactant (Fig. 1.7). It is clearthat in the phase diagrams there are two isotropic regions: one in the top corner, the socalled L2 phase or inverse micelles, and one in the left corner, i.e., L1 phase or normalmicelles. The L2 phase is capable of dissolving a large amount of water, thereby forminga W/O microemulsion. Similarly, the L1 phase can solubilize oil to form an O/Wmicroemulsion. Thus, O/W microemulsions can be considered as an extension of the L2

phase, whereas W/O microemulsions can be considered as an extension of the L1 phase.Figure 1.7 Schematic representation of a tree-component phase diagram forwater-surfactant and cosurfactant (After Overbeek et al., 1983).The advantages of microemulsions in many industrial processes are distinct: from theirspontaneous formation, thermodynamic stability to lack of aging. Applications are basedon the low interfacial tension (as in tertiary oil recovery), the possibility of preparingboth hydrophilic and hydrophobic nearly homogeneous nanoparticles, the small dropletsize produced and their isodisperse nature.91.3 Synthesis of Nanoparticles from W/O Microemulsions (Reversed Micelles)O/W microemulsions (reversed micelles) can be formed by ionic surfactants with double

long alkyl chains alone, such as, AOT (Aerosol OT) by or a mixture of ionic andnonionic surfactants with a short oxyethylene chain dissolved in organic solvents.Reversed micelles are usually thermodynamically stable mixtures of four components:surfactant, co-surfactant, organic solvent and water. AOT, SDS (sodium dodecyl sulfate),CTAB (cetyltrimethy lammonium bromide) and Triton-X are the usual surfactants. Cosurfactantsare often aliphatic alcohols with a chain length of C6–C8. Organic solventsused for reversed micelle formation are usually alkane or cycloalkane with 6 to 8carbons.Reversed micelles can solubilize relatively large amounts of water. It is this water poolthat makes the reversed micelles particularly favorable for the synthesis of nanoparticlesbecause the water pool is in the range of nanometer size which can be controlled byadjusting the water content. Solubilization of water in the reverse micelles can beexpressed by w, the ratio of water to surfactant concentrations (w = [H2O]/[surfactant]).w is an important parameter in determining the size of the reversed micelles and thestructure of water. For a typical spherical AOT reversed micelle, there is a linearrelationship between the diameter of the water pool (D) and w. D = 0.3 w when w islarger than 15 (Pileni, et al., 1985). In addition, w is related to the structure of water. Foran AOT reverse micelle, when w increases, the structure of the water changes frombound water to free water.Due to the controllable water pool, reversed micelles are particularly favorable for thepreparation of monodisperse nanoparticles with various particle sizes. The nanoparticlescan be fabricated using the reversed micelles having the following two features: (1) thenanoparticles are harder to aggregate because the surface of the nanoparticles is coveredwith surfactants; (2) the surface of the particles can be modified further.Preparation of nanoparticles using reverse micelles can be dated back to the pioneerwork of Boutonnet et al. (Boutonnet, et al., 1982). In 1982 they first synthesizedmonodispersed Pt, Rh, Pd, Ir nanoparticles with diameters of 3–6 nm. After that, many

nanoparticles were synthesized and the method of preparing nanoparticles using reversemicelles became a world wide interest in nanoscience and nanotechnology. In thefollowing sections we will review the synthesis of various nanoparticles using thetechnique of reversed micelles.The general method to synthesizing nanoparticles using reverse micelles isschematically illustrated in Fig. 1.8. This can be divided into three cases. The first one isthe mixing of two reverse micelles. Due to the coalescence of the reverse micelles,exchange of the materials in the water droplets occurs, which causes a reaction betweenthe cores. Since the diameter of the water droplet is constant, nuclei in the differentwater cores can not exchange with each other. As a result, nanoparticles are formed inthe reversed micelles. The second case is that one reactant (A) is solubilized in thereversed micelles while another reactant (B) is dissolved in water. After mixing the two10reverse micelles containing different reactants (A and B), the reaction can take place bycoalescence or aqueous phase exchange between the two reverse micelles.Figure 1.8 Schematic illustration of various stages in the growth ofnanoparticles in microemulsions (After Leung, at al., 1988).There are essentially three procedures to form nanoparticles by reversed micelles:precipitation, reduction and hydrolysis. Precipitation is usually applied in the synthesisof metal sulfate (Qi, et al., 1996), metal oxide (Ayyub, et al., 1990; 1988), metalcarbonate (Kandori, et al., 1988; Pillai, et al., 1993) and silver halide (Dvolaitzky, et al.,1983; Hou and Shah, 1988; Chew, et al., 1990) nanoparticles. In this method two reversemicelles containing the anionic and cationic surfactants are mixed. Because everyreaction takes place in a nanometer-sized water pool, water-insoluble nanoparticles areformed.The reduction procedure is one of the most common ways to prepare metal nanoparticlesusing W/O microemulsions. By dissolving the metal salts in the reversed micelles, thesalts undergo a dissociation step inside the aqueous domain. Following a reduction step

(Men+ → Me0), a subsequent precipitation of particles can take place inside the waterpools. Strong reduction agents such as N2H4, NaBH4 and sometimes hydrogen gas canbe used.The hydrolysis procedure is usually used in the preparation of metal oxide nanoparticles.It utilizes the hydrolysis properties of metal alkoxide dissolved in oil and reacting withwater inside the droplets.1.3.1 Preparation of Nanoparticles of MetalsSince metals display surface catalytic properties, the synthesis of size-controllable andmonodisperse metal nanoparticles is of considerable importance. The reduction methodis one of the most common ways to prepare metal nanoparticles through reverse micelles.11Boutonnet et al. have prepared platinum, palladium, rhodium and iridium nanoparticlesusing reverse micelles (Boutonnet, et al., 1982; 1989). H2PtCl6 was dissolved inCTAB/water/octanol reverse micelle. Subsequent reduction with hydrazine producednanoparticles. Pd particles were formed by reducing Pd(NH2)4Cl2 or K2PdCl4

with N2H4.Rhodium particles were formed by reducing RhCl2 with bubbling hydrogen, whereasiridium particles could be obtained by bubbling active hydrogen through 2% Pt-Al2O3 at70°C. Ag and Au colloidial nanoparticles were successfully prepared by reducing theAgNO3 and HAuCl4 in water/cyclohexane/PEGDE or PEGDE/water/n-hexane reversemicelles (Barnickel and Wokaun, 1990), where NaBH4 was used as the reductionreagent. Silver and copper salts of Aerosol OT can be used for the preparation of Ag andCu nanoparticles (Lisiecki and Pileni, 1993; Pileni, et al., 1993a; 1993b; Petit, et al.,1993; Lisiecki and Pileni, 1995). Copper nanosized particles have been synthesized inthe reverse micelles using hydrazine as a reducing reagent. The size of Cu nanoparticlescan be controlled by the water content in the reversed micelles (Lisiecki and Pileni,1995). Gold and silver nanoparticles were also produced by reducing gold chloridetetrahydrate HAuCl4 with citric acid at 80°C for half an hour (Chen, et al., 1996; Frens,

1973; Enustun and Turkevich, 1963). Nanoparticles of other metals such as Co (Chen, etal., 1994; Eastoe, et al., 1996), Ni (Lopez-Quintela and Rivas, 1993) and metal alloysFeNi (Lopez-Quintela and Rivas, 1993), Cu3Au (Sangregorio, et al., 1996) and Co-Ni(Nagy, 1989) have also been synthesized using the reversed micelles.1.3.2 Preparation of Nanoparticles of Metal SulfidesColloidal semiconductors are attracting much interest due to their applications asenhancement of photoreactivity and photocatalysis and non-linear optical properties.The key to synthetic investigation of this kind of nanoparticles must be the carefulcontrol of semiconductor size and size distribution. The precipitation method is usuallyapplied in the preparation of metal sulfide particles (Motte, et al., 1992; Hirai, et al.,1994; Ward, et al., 1993; Boalkye, et al., 1994; Modes and Lianos, 1989). CdS particleshave been synthesized in AOT and Triton reversed micelles with functional surfactantsuch as cadmium lauryl sulfate and cadmium AOT (Petit, et al., 1990; Petit and Pileni,1988). The average diameters of the particles were found to depend on the relativeamount of Cd2+ and S2-. The particles obtained from AOT were smaller and moremonodisperse than those from the Triton reverse micelle. Colloidal CdS was prepared inthe mixed sodium AOT/cadmium AOT/isooctane reverse micelle (Motte, et al., 1992).PbS nanoparticles can be prepared by mixing one polyoxyethylene dodecyl ether-nhexanereverse micelle, which supplies Pb2+ from electrolytes such as Pb(NO3)2 orPb(ClO4)2, and another reverse micelle that contains S2- from Na2S or H2S (Ward, et al.,1993). A number of nanoparticle semiconductors such as CdS (Lianos and Thomas,1987; Petit, et al., 1990; Pileni, et al., 1992; Karayigitoglu, et al., 1994), PbS (Ward, etal., 1993; Eastoe, et al., 1996), CuS (Lianos and Thomas, 1987), Cu2S (Haram, et al.,1996), Ag2S (Motte, et al., 1996), MoS3 (Boalkye, et al., 1994), CdSe (Steigerwald, et al.,1988) have also been synthesized using this method.In recent years apart from the synthesis of nanoparticles, surface modification of themetal sulfide particles has attracted much interest. The modification of the

semiconductor surface is also very important either from the point of view of enhancing12the stability of the nanoparticles or for providing unique physical and chemicalproperties. An additional profit from this treatment is that it allows the particles to beseparated from the micellar solution and redispersed in another solvent. Some surfacecappedsemiconductor nanoparticles have been synthesized with the cap agents such assodium hexamephosphlate (Meyer, et al., 1984; Petit and Pileni, 1988) of the surfacecappingagents such as thiophenol and phenyl (trimethyl) selenium (Steigerwald, et al.,1988; Herron, et al., 1990; Dance, et al., 1984).1.3.3 Preparation of Nanoparticles of Metal SaltsMany metal salts such as silver halide, metal sulfate and metal carbonate possess uniqueproperties. Precipitation methods are usually used to prepare the nanoparticles of thesematerials. Silver halide nanoparticles were synthesized by reacting AgNO3

with sodiumhalides in Aerosol OT W/O microemulsions (Dvolaitzky, et al., 1983; Hou and Shah,1988; Chew, et al., 1990).However, metal carbonate nanoparticles such as BaCO3, CaCO3 and SrCO3

wereprepared by bubbling CO2 through the reversed micelle solutions containing thecorresponding aqueous metal hydroxides. Kandori et al. (1987) used the hexaethyleneglycol dodecyl ether (HEGDE)/water/cyclohexane and calcium AOT based reversemicellar system to synthesize CaCO3 nanoparticles with diameters of 5.4 nm. Thenanoparticle diameter from the CaAOT system was 48–130 nm (Kandori, et al., 1987;1988). Nanoparticles of metal sulfate can also be synthesized by the precipitationmethod. Nanoparticles of AgCl (Bagwe and Khilar, 1997) and AgBr (Chew, et al., 1990;Monnoyer, et al., 1996) have been synthesized using reverse micelles.1.3.4 Preparation of Nanoparticles of Metal OxidesNanoparticles of metal oxides are usually produced by the hydrolysis method in whichthe metal alkoxides react with water droplets in the reverse micelles. Nanoparticles ofmetal oxides such as ZrO2 (Kawai, et al., 1996), TiO2 (Chang, et al., 1994; Joselevich

and Willner, 1994; Chhabra, et al., 1995), SiO2 (Osseo-Asare and Arriagada, 1990;Wang, et al., 1993; Arriagada and Osseo-Asare, 1995; Gan, et al., 1996; Chang andFogler, 1997; Esquena, et al., 1997), GeO2 (Kon-no, 1996), g-Fe2O3 (Lopez-Perez, et al.,1997) and F2O3 (Liz, et al., 1994) have been synthesized. GeO2 nanoparticles candirectly be obtained from AOT-cyclohexane W/O microemulsions by adding anhydrouscyclohexane solutions of Ge(OC2H4)4 into the microemulsions. And SiO2

nanoparticlescould be formed by adding Si(OC2H4)4 to the solubilized ammonia aqueous solution inAOT and polyoxyethylated nonylphenyl ether W/O microemulsions. Similarly, ZrO2

nanoparticles can be obtained by hydrolyzing Zr(OC4H9)4 with sulfuric acid inpolyoxyethylene nonylphenyl ether-cyclohexane systems and then washed withammonia aqueous solution. TiO2 nanoparticles can be prepared by adding benzenesolution of TiCl4 to cetylbenzyldimethylammonium chloride-benzene W/Omicroemulsions.131.3.5 Preparation of Nanoparticles of Other CompoundsYBa3CuO7-x particles were synthesized by co-precipitation of the oxalate salts of Y, Baand Cu nitrates in CTAB/n-butanol/n-octane reversed micelles (Ayyub, et al., 1988;1990). BaFe12O19 particles were synthesized by the calcination of barium-iron carbonateparticles made by mixing the two reverse micelles containing the (NH4)2CO3

and amixture of aqueous Ba(NO3)2 and Fe(NO3)3 (Pillai, V., et al., 1993).1.3.6 Synthesis of Nanowires Using Reversed MicellesThe nanoparticles fabricated in the reversed micelles are spherical particles in mostcases. However, since the optical, electric, and other properties of nanoparticles areaffected by the shape of nanoparticles, various shapes have been synthesized. Forexample, cubic Pt nanoparticles have been synthesized and they showed extremely goodcatalysis selectivity and activity (Ahmadi, et al., 1996a; 1996b). Addition of CdSnanowire into the porous aluminum oxide film will be of potential use in photoelectronics(Routkevitch, et al., 1996). Qi et al., using reversed micelles of TX-100/hexanol.cyclohexane/water, have successfully synthesized cubic BaSO4

nanoparticles (Qi, et al., 1997). They have found that the water content in the reversedmicelles greatly affected the shape of the nanoparticles. Cubic nanoparticles of BaSO4

were obtained in the higher content of water. On the other hand, in the non-ionic reversemicelle C12E4/cyclohexane, adding 0.1 M BaCl2 and Na2CO3 aqueous solution to 0.2 MC12E4/cyclohexane solution, and mixing the two reversed micelles, BaCO3

nanowireswere obtained (Fig. 1.9). Hopwood and Mann have also synthesized BaSO4

nanowireusing reversed micelles (Hopwood and Mann, 1997).Figure 1.9 TEM micrographs and electron diffraction pattern of BaCO3

anowires synthesized in reversed micells (Qi, et al., 1997).141.3.7 Synthesis of Composite Nanoparticles Using Reversed MicellesComposite nanoparticles are composed of two kinds of nanoparticles, not only modifingthe properties of single semiconductor nanoparticles, but also producing some newelectric and optical properties. The composite semiconductor nanoparticles can bedivided into sandwich type and shell-core type. Sandwich type CdS-TiO2

(Spanhel, et al.,1987; Gopidas, et al., 1990; Lawless, et al., 1995 and CdS-SnO2 (Nasr, et al., 1997) havebeen prepared and show prospects in solar cell application. On the other hand, shell-coretype composite nanoparticles such as CdS-ZnS (Hirai, et al., 1994), CdS/PbS (Zhou, etal., 1993; 1994), CdS/HgS (Hasselbarth, et al., 1993; Mews, et al., 1994; Schooss, et al.,1994; Kamalov, et al., 1996; Mews, et al., 1996), CdS/Ag2S (Han, et al., 1998),CdS/CdSe (Tian, et al., 1996; Peng, et al., 1997), CdSe/ZnS (Kortan, et al., 1990; Hinesand Guyot-Sinnest, 1996; Dabbousi, et al., 1997), CdSe/ZnSe (Hoener, et al., 1992;Danek, et al., 1996) have been synthesized using different methods. They showedenhancement of photocatalytic efficiency and strong enhancement of emission.Reversed micelle is also an important method for synthesizing the compositenanoparticles. So far reversed micelles have been successfully used to synthesizecomposite nanoparticles such as CdS-ZnS (Hirai, et al., 1994), CdS-Ag2S (Han, et al.,1998) and CdSe-ZnS (Kortan, et al., 1990), CdSe-ZnSe (Hoener, et al., 1992).For shell-core type nanoparticles the synthesis contains two steps: the first step is the

formation of core nanoparticles in the reverse micelles and the second step is the growthof the shell particles on the core. CdS/ZnS (where CdS is the core and the ZnS is theshell) is a typical shell-core type composite nanoparticles and can be synthesized asfollows. Mixing the reverse micelles containing Cd2+ and S2- in a 1 : 2 ratio, one canobtain the core CdS reverse micelle solution. In this reversed micelle S2- is excess. Afterseveral minutes, Zn2+ containing reverse micelle was added. ZnS precipitated in the coreCdS nanoparticles, and a shell-core type CdS/ZnS composite nanoparticle was obtained.For the ZnS/CdS composite, the same method can be used, only changing the order ofsynthesis. Using this method, Ma et al. have prepared composite nanoparticles ofCdS/ZnS and ZnS/CdS. Another type of composite nanoparticle contains two metals notin the 1 : 1 ratio. They can be synthesized as follows. In the synthesis of coatedAg2S/CdS nanoparticles, after mixing the two reverse micelles containing the equalmolar Cd2+ and S2-, AgNO3 was added to the mixed reversed micelles. The reaction of2Ag+ + CdS(s) → Cd2+ + Ag2S. Coated Ag2S/CdS small particles with a diameter of ~10nm was obtained. The nanoparticles showed large nonlinear absorption.