Preparation of Ultrafine Monodispersed Indium−Tin Oxide Particles in

Langmuir 1999, 15, 1599-1603

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Preparation of Ultrafine Monodispersed Indium-Tin Oxide Particles in AOT-Based Reverse Microemulsions as Nanoreactors Dae-Wook Kim,† Seong-Geun Oh,*,† and Jong-Dae Lee‡ Department of Chemical Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul, and Department of Chemical Engineering, College of Engineering, Chungbuk National University, Cheongju, Chungbuk 360-763, Korea Received November 12, 1998. In Final Form: January 7, 1999 Monodispersed ultrafine indium-tin oxide (ITO) particles were prepared in AOT-based reverse microemulsions with indium nitrate and tin chloride dissolved in water droplets. The particle size distribution, calcination rate, chemical composition, and electrical resistivity of pellets manufactured with particles were measured and compared with analogous measurements of particles formed by bulk precipitation method. ITO particles formed in microemulsions showed a narrower size distribution, faster calcination rate, and lower electrical resistivity. It was also confirmed that tin molecules are doped into the indium oxide homogeneously rather than forming a discrete tin oxide domain in indium-tin oxide particles formed in microemulsions.

Introduction The preparation of ultrafine monodispersed particles has attracted a great deal of attention because of its technological applications such as in ceramics, magnetic particles, semiconductors, superconductors, and metallic catalysts. Many inorganic particles of different chemical compositions, shape, and size distribution have been prepared through different chemical or mechanical methods.1-3 The particles prepared by these methods have irregular shapes and a wide size distribution and thus have had limited applicability in advanced technologies. To overcome this limitation, the synthesis of ultrafine inorganic particles in reverse microemulsions has been introduced and widely studied in attempts to make monodispersed ultrafine particles.4-6 Reverse microemulsion consists of nanometer-sized water droplets dispersed in a continuous oil medium and stabilized by surfactant molecules accumulated at the oilwater interface. In this case, the dispersed water droplets behave as nanoreactors for the production of inorganic particles. The reactions for the formation of particles are limited only inside dispersed water droplets. Thus the size of particles produced in microemulsions is on the order of the size of water droplets functioning as nanoreactors, which is usually less than 0.1 µm. Since monodispersed metallic particles were prepared in microemulsions by Boutonnet in 1982,7 many different types of particles have been prepared using microemulsions, such as calcium carbonate,8 titanium dioxide,9 silica,10 PbS,11 polymer latex,12 barium ferrite,13 superconductors,14 among others. * To whom all correspondence should be addressed. E-mail: [email protected]. † Hanyang University. ‡ Chungbuk National University. (1) Segal, D. Chemical Synthesis of Advanced Ceramic Materials; Cambridge University Press: Cambridge, 1989. (2) Hayashi, C. Phys. Today 1987, Dec., 44. (3) Matijevic, E. Langmuir 1994, 10, 8. (4) Sjo¨blom, J.; Lindberg, R.; Friberg, S. E. Adv. Colloid Interface Sci. 1996, 95, 125. (5) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961. (6) Bandyopadhyaya, R.; Kumar, R.; Gandhi, K. S. Langmuir 1997, 13, 3610. (7) Boutonnet, M.; Kizling, J.; Stenius, P. Colloids Surf. 1982, 5, 209. (8) Kandori, K.; Kon-no, K.; Kitahara, A. J. Colloid Interface Sci. 1987, 115, 579.

Generally, the particles prepared in microemulsions had ultrafine monodispersed size distribution and because of their small size showed different physical properties from those of bulk materials. After two reverse microemulsions containing different reactants were mixed, reactants are interchanged during the collision of water droplets in microemulsion as shown in Figure 1. The interchange of reactants is very fast, so that for the most commonly used microemulsions it occurs only during the mixing process.15 The reaction then takes place inside droplets (nucleation and growth) which control the final size of the particles.16 Interchange of nuclei and particles between water droplets is hindered because it would require the formation of a large hole during collisions between droplets and this in turn would require a great change in the curvature of the surfactant films around the droplets, which is not favorable energetically.17 Once the particles attain their final size, the surfactant molecules are attached to the surface of the particles, thus stabilizing and protecting them against further growth. In this study, the nanoparticles of indium-tin oxide (ITO, In2O3/SnO2) have been prepared in AOT (2-ethylhexyl sulfosuccinate)/isooctane/water microemulsions, and their particle size distribution and physical properties were compared with those of ITO particles formed by the bulk precipitation method. The unique molecular structure of AOT with its two alkyl hydrophobic chains favors an interface curved on the water core. This is the reason the AOT-based system presents a quite extended reverse microemulsion zone.18 Thus, the microemulsions formed by AOT have been used widely to form reverse micro(9) Stathatos, E.; Lianos, P.; Del Monte, F.; Levy, D.; Tsiourvas, D. Langmuir 1997, 13, 4295. (10) Esquena, J.; Tadros, Th. F.; Kostarelos, K.; Solans, C. Langmuir 1997, 13, 6400. (11) Modes, S.; Lianos, P. J. Phys. Chem. 1989, 93, 5854. (12) Antonietti, M.; Bremser, W.; Muschenborn, D.; Rosenauer, C.; Schupp, B.; Schmidt, M. Macromolecules 1991, 24, 6636. (13) Ayyub, P.; Maitra, A. N.; Shah, D. O. Physica C 1990, 168, 571. (14) Kumar, P.; Pillai, V.; Bates, S. R.; Shah, D. O. Mater. Lett. 1993, 16, 68. (15) Tondre, C.; Zana, R. J. Dispersion Sci. Technol. 1980, 1, 179. (16) Lopez-Quintela, M. A.; Rivas, J. J. Colloid Interface Sci. 1993, 158, 446. (17) Zana, R.; Lang, J. in Microemulsions: Structure and Dynamics; Friberg, S. E., Bothorel, P., Eds.; CRC Press: Boca Raton, Florida, 1987; p 153.

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Figure 1. Schematic diagram of the reaction mechanism for the formation of ultrafine particles in microemulsions.

emulsions and are well-characterized for the preparation of ultrafine particles.19 ITO has a wide applicability as an active electronic film. The conductive properties of transition-metal oxides are a consequence of electronic properties arising from the exchange of unpaired electrons between metal ions in different valence states. In ITO, tin doping at a uniform molecular level through actual substitution of indium in the oxide crystal lattice results in low electrical resistivities of transparent conductive films.20,21 ITO particles can be made by simple mixing of indium oxides and tin oxides, followed by thermal treatment at high temperature, or by a coprecipitation method of indium and tin salt solutions. However, both of these methods yield a wide range of particle size, followed by poor electrical properties of their ITO films. Usually, the particle size of ITO prepared by bulk precipitation ranges from 0.01 to 1.0 µm.22 Experimental Section Materials. AOT (Sigma, 99%) as surfactant, isooctane (Aldrich, 99.9%) as an oil phase, and deionized water (electrical resistivity, 18.2 MΩ) were used to form microemulsions. Indium nitrate pentahydrate[In(NO3)3‚5H2O, Aldrich, 99.9%] and tin chloride pentahydrate (SnCl4‚5H2O, Katayama Chemicals, Japan, 99.9%) as inorganic reactants and NH4OH (Mallinckrodt Chemicals USA, NH3 29.7%) as a precipitating agent were used as received to prepare ITO particles. Formation of Microemulsions. Indium nitrate pentahydrate (0.1 M) and 0. tin chloride pentahydrate (0.0099 M) were dissolved in 1 L of distilled water to keep a 9:1 atomic ratio of indium/tin, which has been reported to show a maximum electrical conductivity in ITO.23 This aqueous solution was added into an AOT/isooctane mixture (weight ratio of AOT/isooctane, 45:55) to prepare microemulsion I. Microemulsion II was prepared by adding 0.5 M NH4OH aqueous solution into same AOT/ isooctane mixture as in microemulsion I. A water/AOT molar ratio ()W) of 22 was maintained in both microemulsions. (18) Rouviere, J.; Couret, J. M.; Lindheimer, M.; Dejardin, J. L.; Marrony, R. J. Chim. Phys. 1979, 76, 289. (19) Koper, G. J. M.; Sager, W. F.; Smeets, J.; Bedeaux, D. J. Phys. Chem. 1995, 99, 13291. (20) Frazer, D. B. Proc. IEEE 1973, 61, 1013. (21) DuBow, J. B.; Burk, D. E. Appl. Phys. Lett. 1976, 29, 494. (22) Lehmann, H. W.; Widmer, R. Thin Solid Films 1975, 27, 359. (23) Frank, G.; Kstlin, H. Appl. Phys. A 1982, 27, 197.

Letters Preparation of ITO Particles. Microemulsion I and II were mixed together with a magnetic stirrer at room temperature. After 15 s, the mixed microemulsions become turbid because of the formation of particles. On the contrary, the mixing of bulk solutions of indium-tin salts and NH4OH became turbid immediately after mixing, and particles were precipitated. The microemulsion reaction proceeded for 1.5 h, enough reaction time under the vigorous agitation. The particles formed in microemulsions were not sedimented because of the adsorption of surfactant molecules into the surface of particles and the formation of the stable dispersion of particles. After the reaction, acetone was added to break the microemulsion structure and to cause sedimentation of the indium-tin hydroxide particles. The sedimented indium-tin hydroxide particles were washed with hexane and pure water successively to remove surfactant molecules and sodium ions from particle surfaces. During the washing process, centrifugal force (6500 rpm for 15 min) was applied to the samples to shorten the sedimentation time. The washed indium-tin hydroxide particles were collected and dried at 50 °C for 10 hours and crushed by marble mortar. Then, the indium-tin hydroxide particles were calcinated at 700 °C for 2 h to yield yellowish-green ITO particles. The preparation of ITO particles by bulk mixing of indiumtin salts and NH4OH aqueous solutions has been carried out to compare the characteristics of particles with those of particles formed in microemulsions. Morphology of particles. The morphology of particles formed in microemulsions and in bulk solutions was studied by Field Emission Scanning Electron Microscopy (FE-SEM, JEOL model JSM-6340F). Samples were coated with platinum by sputtering for 10 s. Photomicrographs were obtained operating at 15 kV, a working distance of 9 mm at 100 000 magnification. Crystal Oxide Formation Depending upon Temperature. Indium-tin hydroxide particles are converted to ITO by releasing water molecules from them as the temperature was increased. This water-releasing process depends on the temperature and the size of the particle. A thermogravimetric analyzer (Shimadzu, model TGA-50) and a differential thermal analyzer (Shimadzu, model DTA-50) were used to measure the water-releasing rate of indium-tin hydroxide particles at a temperature-increasing rate of 10 °C/min in the temperature range of 50∼900 °C at the ambient atmosphere. Chemical Composition of Particles. The chemical composition of particles was measured by an EDS (Energy Dispersive X-ray Spectrophotometer) attached to SEM. X-ray Diffraction Measurements. X-ray diffractometry measurements were performed on a Rigaku D/MAX RINT 2500 X-ray diffractometer. The incident wavelength was Cu KR ) 1.789 Å, and the detector moved stepwise. (∆2θ ) 0.05°) between 10° and 90° 2θ. The scan speed was 2°/min. Particle Size Measurement by Dynamic Light Scattering. Dynamic light scattering measurements were performed with a Brookhaven BI9000 AT instrument at a wavelength of 514.5 nm of Ar-Ne at 90° to measure the particle size of indium-tin hydroxide. Electrical Conductivity of ITO Pellets. To obtain pellets, 0.3 g of particles were loaded into a 12.9 mm diameter palletizer and a 600 kg/cm2 force was applied. Electrical resistivities of pellets were measured with Fluke 77 series multimeter.

Results and Discussion Regarding the formation of reverse microemulsions containing reactants in aqueous phases, ammonium hydroxide solution was easily added into the AOT/ isooctane mixture (weight ratio, 45:55) at W ) 22 to form a clear microemulsion. In the case of indium and tin salts solutions, a clear microemulsion was not readily formed upon the addition of a salt solution into AOT/isooctane mixture at the same composition. The mixture should be kept more than 20 min under the vigorous agitation to become clear. Much more energy was needed to solubilize aqueous solutions of indium and tin salts into an AOT/ isoocatne mixture than for the NH4OH solution. The amount of energy needed to solubilize indium and tin salt

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solutions in the oil phase decreased as the concentration of salts and the amount of aqueous phase decreased. Similar phenomena have been reported by Nagy et al., who also found it much more difficult to solubilize aqueous AgNO3 solution into a AOT/heptane mixture than solubilizing a KBr solution when preparing silver bromide particles in microemulsions.24 This was due to the greater decrease in the radius of the spontaneous curvature of water droplets containing AgNO3 than water droplets containing KBr. Indium nitrate and tin chloride in water droplets of microemulsion react with ammonium hydroxide to produce indium-tin hydroxide particles by the following mechanism.25

In(NO3)3 + 3NH4OH f In(OH)3V + 3NH4+ + 3NO3SnCl4 + 4NH4OH f Sn(OH)4V + 4NH4+ + 4ClBecause indium and tin salts are present in the same aqueous phase, a simultaneous doping reaction takes place between the indium and tin hydroxides proportional to their compositions, and indium-tin hydroxide particles are precipitated as they are formed.

In(OH)3/Sn(OH)4V + 7NH4+ + 3NO3- + 4ClIndium hydroxide particles doped with tin hydroxide are converted into indium-tin oxide particles by calcination at 700 °C for 2 h.

In(OH)3/Sn(OH)4 f In2O3/SnO2 The partial substitution of In3+ into indium oxide crystalline by Sn4+ yields the electron holes in the indiumtin oxide crystal. When electrical potential is applied to this material, electrons in electron holes move to the positive electrode. The electrical conductivity of indiumtin oxide depends on the number of electron holes, followed by the amount of tin doped in indium particles. Bedeaux and co-workers obtained the radii of water cores for a series of W as followings:19

rh )

3ν W+δ As

where rh is hydrodynamic radius of water droplets in nm, υ is the volume of water molecules (3.0 × 10-29 m3), As is the area/molecule of AOT at the oil/water interface, and d is the thickness of the surfactant monolayer. As depends on the characteristics of the ionic groups in the surfactant molecules and is given by the following equation for AOT.26

As(nm2) ) 0.596 - exp(-0.401xW) In this study, W was fixed as 22. Thus, the hydrodynamic diameter of the water pool would be 9.54 nm. When the particle size of indium-tin hydroxide was measured by dynamic light scattering, it ranged from 9 to 12 nm, as shown in Figure 2. This result implies that the size of particles formed in microemulsions is on the order of the water droplet size, and the size distribution (24) Monnoyer, Ph.; Fonseca, A.; Nagy, J. B. Colloids Surf. A 1995, 100, 233. (25) Heslop, R. B.; Jones, K. Inorganic Chemistry: A Guide to Advanced Study; Elsevier Scientific Publishing Company: Amsterdam, 1976. (26) Hilfiker, R.; Eicke, H. F.; Hammerich, H. Helv. Chim. Acta 1987, 70, 1531.

Figure 2. Particle size distribution of indium-tin hydroxide prepared in microemulsions

of indium-tin hydroxide was very narrow: within the 3 nm range. The size of particles formed in microemulsions is larger than that of water droplets by a maximum of 2.46 nm. A similar behavior was observed in the preparation of AgBr particles from AgNO3 and KBr in AOT-based microemulsions. In the case of the formation of monodisperse AgBr particles in microemulsions, the initial monomeric AgBr entity is stabilized by the adsorption of AOT ions. Because of the fast exchange between the water cores, the initially formed AgBr nuclei grow to reach a certain size, which corresponds to the thermodynamically best-stabilized species in the presence of the microemulsion. This size was greater than that of the water droplets.24 In most of the cases, the size of particles prepared in microemulsions is bigger than that of the inner water core. Despite the fact that many parameters (size of water core, kinds of oil and surfactant, concentration of reactants in water, etc.) influence the size of particles, deep understanding of the formation of nanosized particles in microemulsions is still missing.27 The powder morphology of the indium-tin oxide prepared by AOT-based microemulsions is shown in Figure 3 (bottom) when the atomic ratio of indium-tin is 9:1. The morphology of ITO particles is shown to consist of crystalline agglomerates about 25 nm in size. This means that indium-tin hydroxide particles of 9-12 nm grow to ITO particles of 25 nm during the calcination process at 700 °C. Particles are packed very densely in agglomerates. On the contrary, ITO particles prepared by bulk precipitation method are composed of spherical, elliptical, and cylindrical particles of 20-60 nm and are packed loosely in agglomerates having a large void space as shown in Figure 3 (top). When the chemical composition of particles was analyzed by EDS, it was found that the atomic percent of tin was approximately 9.0% in particles prepared in microemulsions as well as in particles formed by the bulk precipitation, which is the same composition as in aqueous phases (Figure 4). The Au peak was introduced because of the sample coating before measurement. Figure 5 shows the rate of weight loss of particles as a function of temperature, as measured by the thermal gravimetric analyzer. The particles formed in microemulsions showed a rapid weight loss between 50 and 450 °C, (27) Nagy, J. B.; Barette, D.; Fonseca, A.; Jeunieau, L.; Monnoyer, Ph.; Piedigrosso, P.; Ravet-Bodart, I.; Verfaillie, J. P., Wathelet, A. Nanoparticles in Microemulsions: A General Approach in Nanoparticles in Solids and Solutions; Fendler, J. H., Dekany, I., Eds.; NATO ASI Series 3/18, Kluwer: Dordrecht, 1996.

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Figure 5. TGA and DTA curves of particles.

Figure 6. X-ray diffractograms of particles prepared in microemulsions.

Figure 3. SEM pictures of particles prepared in microemulsion (bottom) and by precipitation method (top).

Figure 4. Chemical composition of ITO particles prepared in microemulsions.

followed by a slow loss above 450 °C. The highest rate of weight loss was observed at 460 °C. On the contrary, particles formed by bulk precipitation method showed a rapid decrease in weight between 50 and 390 °C and a slow decrease above 390 °C. The weight decreased 23% in particles formed in microemulsions and 19% in particles prepared by bulk precipitation method at 700 °C, suggesting that the smaller the particle size, the easier loss of OH- from particles was. The faster calcination rate in smaller particles might result from the faster heat rate of transfer of particles due to the larger heat-conducting area per unit mass of particles and shorter diffusion path of OH- than those in larger particles. The faster calcination

rate might result in more crystal formation in particles formed in microemulsions than in particles formed by precipitation during the calcination process. It seems that a smaller particle size and a larger surface area make it easier for particles to form crystallinity. It was reported that the rate of sintering is increased by a factor of 10 in changing from a 10- to a 1-micron size because of the higher interfacial energy in smaller particles.28 Figure 6 shows the X-ray diffraction spectrum of particles formed in microemulsions. The pattern of the spectrum coincides with that of indium oxide in both particles. It was found to deviate slightly from that of genuine indium oxide, and there was no diffraction in the spectrum of genuine tin oxide. Thus, it can be inferred that tin molecules are doped homogeneously in indium oxide and do not exist as a separate domain of tin oxide in the indium oxide. The strong intensity of particles prepared in microemulsions implies the formation of a crystal structure of ITO. The specific resistivity of pellet formed with ITO particles prepared in microemulsions was 0.072 Ω‚cm, whereas it was 0.24 Ω‚cm in the pellet formed with particles prepared by precipitation method. The electrical resistivity of pellets depends on the degree of densification of pellets. The free energy change that gives rise to densification is the decrease in surface area and lowering of surface free energy by the elimination of solid-gas interfaces. This usually takes place with the coincidental formation of new but lower-energy solid-solid interfaces. Thus, densification is easier for small particles which have a large interfacial energy than for large particles. Shah and co-workers reported that the sintered pellets of superconducting YBa2Cu3O7-x particles prepared in microemulsions had a density corresponding to 97.8% of single crystal density, which is remarkable for supercon(28) Kingery, W. D.; Bowen, H. K.; Uhlmann, D. R. Introduction to Ceramics; John Wiley & Sons: Toronto, 1991.

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ductors. These highly dense, large grains resulted from the ultrafine nature of the particles.29 In conclusion, ultrafine monodispersed indium-tin oxide particles which are widely used as transparent electrodes and electrical conductor can be prepared in the reverse microemulsions. The microemulsion method produced indium-tin hydroxide particles of 9-12 mm, and dense agglomerates composed of 25 nm spherical particles by calcination. The bulk precipitation method yielded loosely packed agglomerates composed of various shapes of particles 20-60 nm in size. Also, the calcination rate was found to be higher in particles formed in microemulsions because of smaller particle size and larger surface (29) Pillai, V.; Kumar, P.; Hou, M. J.; Ayyub, P.; Shah, D. O. Adv. Colloid Interface Sci. 1995, 55, 241.

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area per unit mass than in the particles formed by the precipitation method. It was confirmed that tin molecules were homogeneously doped in indium oxide the same as in particles formed by the bulk precipitation method and do not exist as a separate phase in the indium-tin oxide mixture. The ITO pellets formed with smaller particles showed less electrical resistivity than those formed with larger particles because of the compact packing of smaller particles in the pellet. Acknowledgment. D.W.K. is grateful to the Graduate School of Advanced Materials and Chemical Engineering and CPRC at the Hanyang University for a fellowship to carry out this research. LA9815906