Langmuir 1998, 14, 4397-4401
4397
Nanosize Indium Hydroxide by Peptization of Colloidal Precipitates Luis A. Pe´rez-Maqueda,† Laifeng Wang,‡ and Egon Matijevic´* Center for Advanced Materials Processing, Clarkson University, Potsdam, New York, 13699-5814 Received February 5, 1998. In Final Form: May 1, 1998 A new procedure for the preparation of nanosized In(OH)3 particles in high concentrations without added surfactants is described. The process is based on the peptization of In(OH)3 dispersions consisting of micrometer colloids, which are obtained by the hydrolysis of InCl3 in ethylene glycol to which aqueous sodium hydroxide solutions are added. Depending on conditions, aging such systems at elevated temperatures (150-175 °C), yields colloidal particles of different morphologies (platelets, fibers), which are composed of nanometer subunits. The latter are liberated by repeated washing with water or 2-propanol, resulting in uniform spherical particles of ∼80 nm in diameter. It is suggested that an In-glycolate compound forms at high temperatures, which keeps the nanoparticles together, but it is decomposed on treating with water or 2-propanol, allowing for their release. The original micrometer-size precipitates retain their shape after calcination; hence, In2O3 particles of different morphologies can be obtained.
I. Introduction Recently, indium and indium oxide have drawn much interest, because of their semiconductor and optical properties. For many of these uses, a finely divided powder, consisting of particles uniform in size and shape, is highly desirable. The properties of materials, such as their sinterability, can be further affected, if the particle size is in the nanometer range. Several procedures have been reported for the preparation of uniform submicrometer indium hydroxide of different morphologies.1-3 In all these methods, the particles were generated by the hydrolysis of aqueous solutions of indium salts at elevated temperatures. Uniform and spherical particles were obtained only when low concentrations of the reactants in the presence of a complexing agent, such as sulfate ion1 or 2-aminobutyric acid,2 were used. It has been shown that organic solvents, e.g., ethanol, have a significant effect on the shape and other properties of particles obtained by precipitation. Thus, colloidal iron oxides of different morphologies were prepared in mixed solutions of ethanol and water or ethylene glycol and water,4-6 and zinc oxide, ceria, and zirconia were prepared in alcohols or polyol media.7,8 Uniform metallic particles have been produced in polyols, in which the latter act simultaneously as dispersants and solvents for the precursors, reducing agents, and crystal growth modifiers.9-12 The use of diols as solvents in the sol-gel
method has also been reported.13-15 Specifically, ethylene glycol offers two useful advantages, i.e., (a) a high dielectric constant, which enhances the solubility of inorganic salts (such as InCl3), and (b) a high boiling point (195 °C at atmosphere pressure), which makes it possible to carry out the preparation of inorganic compounds at relatively high temperatures. In this study, a novel process for the preparation of nanosized In(OH)3 in high concentrations without added surfactants is described. The method involves first the hydrolysis at elevated temperature of highly concentrated indium chloride solutions in ethylene glycol in the presence of aqueous sodium hydroxide, which yields micrometersize particles of different morphologies. The latter can then be peptized into uniform nanosized spheres simply by thoroughly washing such precipitates with water. The original larger particles retained their shape after calcination into In2O3. II. Experimental Section
† On leave from Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC-Universidad de Sevilla, Sevilla, Spain. ‡ On leave from Shandong Analysis and Test Center, Jinan, P. R. China. * Corresponding author.
A. Materials. Indium chloride (99.999%, Aldrich), ethylene glycol (EG, Aldrich), sodium hydroxide (Aldrich), and 2-propanol (Fisher Scientific) were used without further purification. B. Preparation of Particles in Ethylene Glycol (EG). In a typical run, 50 cm3 of EG were heated to 175 ( 1 °C in an oil bath, and then 1.1 g of solid indium chloride was introduced into the reactor. After the indium salt was dissolved, 3 cm3 of a 3 mol dm-3 aqueous sodium hydroxide solution were added dropwise into the indium chloride solution, followed by aging the system for 40 min at the same temperature under stirring. After the resulting suspension was cooled to room temperature in air, the supernatant solution was discarded, and the particles were separated from the remaining concentrated dispersion by vacuum filtration.
(1) Yura, K.; Fredrikson, K. C.; Matijevic´, E. Colloids Surf. 1990, 50, 281. (2) Hamada, S.; Kudo, Y.; Kobayashi, T. Colloids Surf A: Physicochem. Eng. Aspects. 1993, 79, 227. (3) Hamada, S.; Kudo, Y.; Minagawa, K. Bull. Chem. Soc. Jpn. 1990, 63, 102. (4) Hamada, S.; Matijevic´, E. J. Colloid Interface Sci. 1981, 84, 274. (5) Hamada, S.; Matijevic´, E. J. Chem. Soc., Faraday Trans. 1 1982, 78, 2147. (6) Matijevic´, E.; Cimasˇ, Sˇ . Colloid Polymer Sci. 1987, 265, 155. (7) Collins, I. R.; Taylor, S. E. J. Mater. Chem. 1992, 2, 1277. (8) Je´ze´quel, D.; Guenot, J.; Jouini, N.; Fie´vet, F. J. Mater. Res. 1995, 10, 77.
(9) Ducamp-Sanguesa, C.; Herreraurbina, R.; Figlarz, M. Solid State Ionics 1993, 63, 25. (10) Figlarz, M.; Ducamp-Sanguesa, C.; Fie´vet, F.; Lagier, J. P. Adv. Power Metall. Part. Mater. 1992, 1, 179. (11) Viau, G.; Fie´vet-Vincent, F.; Fie´vet, F. J. Mater. Chem. 1996, 6, 1047. (12) Fie´vet, F.; Fie´vet-Vincent, F.; Lagier, J. P.; Dumont, B.; Figlarz, M. J. Mater. Chem. 1993, 3, 627. (13) Matijevic´, E. Langmuir 1994, 10, 8. (14) Hsu, W. P.; Ro¨nnquist, L.; Matijevic´, E. Langmuir 1988, 4, 31. (15) Wilhelmy, D. M.; Matijevic´, E. J. Chem. Soc., Faraday Trans. 1 1984, 80, 563.
S0743-7463(98)00149-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/11/1998
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Table 1. Experimental Conditions for the Hydrolysis of InCl3 in Ethylene Glycol with NaOH sample no.
temp (°C)
[NaOH] (mol dm-3)
[InCl3] (mol dm-3)
[NaOH]/[InCl3]
aging time (min)
particle shapeb
mean size (µm)
Figure
1a 2 3 4 5 6 7 8 9 10 11 12 13
175 150 125 100 175 175 175 175 175 175 175 175 175
0.18 0.18 0.18 0.18 0.27 0.12 0.06 0.03 0.18 0.18 0.18 0.09 0.27
0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.05 0.15
1.8 1.8 1.8 1.8 2.7 1.2 0.6 0.3 1.8 1.8 1.8 1.8 1.8
40 40 40 40 40 40 40 40 10 5 1 40 40
P R A A A R F
∼5 ∼4
1a 1b
∼10 ∼50
2a 2b
P A A P A
∼2
3
a
2-4
Typical conditions. b Key: P, platelets; R, rods; A, aggregates; F, fibers.
To study the effects of different conditions on the properties of the final particles, different experimental parameters were investigated individually, while keeping all other experimental conditions as in the typical run, as follows: (a) reaction temperature, 100, 125, 150, and 175 °C; (b) concentration of indium chloride, 0.05, 0.1, and 0.15 mol dm-3; (c) aging time, 1, 5, 10, and 40 min. (d) molar ratio of sodium hydroxide to indium chloride adjusted over the range 0.3-2.7. C. Peptization of the Particles by Washing. To wash the particles, the remaining EG in the resulting suspension was discarded, and the solids were then dispersed in distilled water in an ultrasonic bath and separated by centrifugation at 10 000 rpm for 30 min. This procedure was repeated five times. To evaluate the influence of the washing liquid, in some experiments water was replaced by 2-propanol. D. Characterization. The size and shape of the particles were examined by scanning (SEM) and transmission (TEM) electron microscopy. The thermal behavior of the powder was studied by thermogravimetric (TGA) and differential thermal (DTA) analyses with a heating rate of 10 °C min-1 in the flow of air. The structure and the composition of the solids were evaluated by X-ray diffraction (XRD) and infrared (IR) spectroscopy. The crystallite size of the In(OH)3 was determined from the full width of the half-maximum of the (200) X-ray diffraction peak using the Scherrer equation. E. Calcination of the Particles. To evaluate the particle behavior at high temperatures, the needle- and plate-shaped particles formed in EG were calcined at 400 °C for 1 h at a heating rate of 10 °C min-1 in an open tubular furnace (static air).
III. Results A. Formation of the Particles in Ethylene Glycol (EG). The shape and size of particles obtained by the hydrolysis of indium chloride in EG/aqueous NaOH solutions depended very much on the experimental conditions, including the reaction temperature, molar ratio of sodium hydroxide to indium chloride, aging time, and the concentration of indium chloride. The conditions under which particles of different morphologies were prepared are listed in Table 1. Platelets having a mean size of 5 µm precipitated under the “standard” condition (Table 1, sample 1) are shown in Figure 1a. Lowering the reaction temperature to 150 °C yielded mostly rodlike particles ∼4 µm long (Figure 1b), while at temperatures below 125 °C, only aggregates of irregular small particles were observed. Figure 2a is the SEM of elongated (∼10 µm) particles obtained at a molar ratio of [NaOH]/[InCl3] ) 1.2, under otherwise the same conditions as in the typical run. Decreasing this molar ratio to 0.6 resulted in thinner, but longer (∼50 µm) fibers (Figure 2b), while no precipitation was observed at a ratio of 0.3. The concentration of InCl3 had a significant influence on the particle properties. Whereas dispersions displayed
Figure 1. Scanning electron micrographs (SEM) of the particles prepared by hydrolyzing 50 cm3 of a 0.1 mol dm-3 InCl3 solution in ethylene glycol (EG) in the presence of 3 cm3 of a 3 mol dm-3 aqueous NaOH solution for 40 min at (a)175 °C (sample 1 in Table 1) and (b) 150 °C (sample 2 in Table 1).
in Figure 1a were formed at 0.1 mol dm-3 InCl3, irregular aggregates precipitated at lower (0.05 mol dm-3) and higher (0.15 mol dm-3) concentrations. Short aging times resulted in smaller particles illustrated in Figure 3, which were obtained under the same conditions as those in Figure 1a, except at 10 min (sample 9, Table 1), while aggregates were observed at 1 and 5 min. B. Peptization of the Particles. Solids prepared under the standard condition (Figure 1a) and washed five times with water yielded nanosized particles of ∼80 nm shown in Figure 4. Using 2-propanol instead of water
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Figure 4. SEM of the nanosized particles obtained by washing the particles shown in Figure 1a with distilled water five times.
Figure 2. SEM of the particles obtained by hydrolyzing 50 cm3 of a 0.1 mol dm-3 InCl3 solution in EG in the presence of (a) 2 cm3 (sample 6 in Table 1) and (b) 1 cm3 of a 3 mol dm-3 aqueous NaOH solution (sample 7 in Table 1) at 175 °C for 40 min.
Figure 5. X-ray diffraction (XRD) pattern of the particles illustrated in Figure 1a (a), in Figure 4 (b), and in Figure 7 (c). Figure 3. SEM of the particles prepared by hydrolyzing a 0.1 mol dm-3 InCl3 solution in EG in the presence of 3 cm3 of 3 mol dm-3 NaOH at 175 °C for 10 min (sample 9 in Table 1).
yielded the same product. Treating aggregates, produced at a lower reaction temperature (sample 4, Table 1), in the same manner resulted in a broad size dispersion with particle diameters ranging from 80 to 800 nm. C. Characterization. (i) Particles Formed in EG. While the XRD pattern of the original sample (Figure 1a) shows a well-defined peak at d ) 3.95 Å, typical for In-
(OH)3 (Figure 5a), the TGA (Figure 6a) of the same solid gives a weight loss of 40% between 100 and 350 °C, which is much larger than expected for the transformation of In(OH)3 to In2O3 (17%), indicating occlusion of EG in the precipitate. The exothermic peak at ∼300 °C in the DTA (Figure 6b) could be attributed to burning off the organic component. The particles prepared in EG /aqueous NaOH retained the shape even after calcination, as illustrated in Figure 7, which displays the sample shown in Figure 2b heated at 400 °C for 1 h. The XRD pattern (Figure 5c) indicated that this solid was converted into In2O3.
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Figure 6. (a) Thermogravimetric analysis (TGA) and (b) differential thermal analysis (DTA) of the particles shown in Figure 1a at a heating rate of 10 °C min-1 in air. (c) TGA of the In(OH)3 particles shown in Figure 4.
(ii) Nanosized Spherical Particles. The XRD pattern of the nanosized particles obtained by washing (illustrated in Figure 4) also presents a well-defined peak at d ) 3.94 Å (Figure 5b), while the XRD of the same sample calcined at 400 °C was characteristic of In2O3. The weight loss of 17% on heating between 100 and 350 °C (Figure 6) corresponds to the change from In(OH)3 to In2O3; hence, the washed nanosized particles consist of In(OH)3 without an organic contaminant. The crystallite size of the original and of the nanosized particles, as calculated from the full width of the halfmaximum of the (200) X-ray diffraction peak, was ∼20 nm. IV. Discussion A mechanism for the formation of the original and the nanosized particles can be proposed from the described experimental observations. It was established that the micrometer colloids consist of In(OH)3. Both platelets and fibers on washing with water peptize into uniform nanosized spheroids, the XRD of which indicates the same chemical composition. Such a behavior can only be explained, if the original larger particles are already built of such subunits. Over the past several years, ample experimental evidence has been produced documenting that a large majority of “monodispersed” colloids of different shapes, prepared by precipitation from solutions, are not internally homogeneous,13 as exemplified with spherical, rodlike, and hexagonal particles of cerium oxide.14 Furthermore, many uniform spheres so obtained showed X-ray characteristics of known minerals, e.g. zinc oxide of sphalerite15 or tin oxide of cassiterite,16 to mention only a few. Obviously, (16) Ocanˇa, M.; Matijevic´, E. J. Mater. Res. 1990, 5, 1083.
Figure 7. SEM (a) and TEM (b) of the particles illustrated in Figure 2b after calcination at 400 °C for 1 h in static air.
the employed process could not produce single crystals of spherical shape, and it was, indeed, proven by X-ray analysis that these particles consist of much smaller but uniform subunits. It was then demonstrated that the larger particulates were formed by aggregation of finely dispersed precursors.17 The fact that different shaped In(OH)3 can be readily peptized into nanosized particles can be understood, if one considers the generality of the composite structural properties of colloids. The difference in the behavior of In(OH)3 particles, prepared in the presence of ethylene glycol, is in the ease with which the original solids decompose on washing, while in most other cases no peptization occurred. The described behavior of In(OH)3 can further be explained by considering the large weight loss (∼40%) on heating the original powders, which exceeds the expected value of 17%, assuming transformation from In(OH)3 to In2O3. It would seem that EG is incorporated in the large particles in either of two possible forms: (a) as occluded molecules or (b) as an alcoholate compound with indium. It was previously reported that diols, especially EG, tended to produce glycolates with different metal cations.18-23 (17) Lee, S.-H.; Her, Y.-S.; Matijevic´, E. J. Colloid Interface Sci. 1997, 186, 193. (18) Calzada, M. L.; Sirera, R.; Carmona, F.; Jime´nez, B. J. Am. Ceram. Soc. 1995, 78, 1802. (19) Phillips, N. J.; Milne, S. J.; Ali, N. J.; Kennedy, J. D. J. Mater. Sci. Lett. 1994, 13, 1535. (20) Calzada, M. L.; Milne, S. J. J. Mater. Sci. Lett. 1992, 12, 1221. (21) Newman, A. A. Glycerol; CRC Press: Cleveland, OH, 1968; p 64. (22) Knelsch, D.; Groeneveld, W. L. Inorg. Chim. Acta 1973, 7, 81.
Nanosize Indium Hydroxide
Figure 8. Infrared (IR) spectra (a) of EG and (b) of the solid obtained by hydrolyzing InCl3 in the presence of NaOH in EG at 175 °C (illustrated in Figure 1a).
Furthermore, alcohols adsorbed on the surfaces of metal oxides such as alumina or silica showed the presence of alcoholates when heated,18-26 and nickel ethylene glycolate was detected when nickel hydroxide interacted with ethylene glycol at elevated temperatures.27,28 In the present case, the evidence for an In-EG complex comes from IR spectra of the precipitated solids as compared to that of EG (Figure 8). A similar splitting of the νCH2 bands at 2951 and 2870 cm-1, as observed here, was found with Ni(OCH2-CH2O), which was attributed to two different protons (equatorial and axial) present in this compound.28 Consequently, it may be assumed that some indium glycolate is formed during the precipitation process, at least on the surface of the precursors. The fact that the crystallite size of ∼20 nm is the same in the original and nanosize peptized particles indicates that these initially formed crystallites aggregate to ∼80 nm subunits, which are stabilized by the In-EG complex. When washed with water or 2-propanol, the glycolates decompose; thus, the peptization of the colloidal particles into independent nanosized particles of ∼80 nm is facilitated. A dispersion precipitated in the absence of EG in an aqueous solution containing 0.1 mol dm-3 InCl3 and 0.18 (23) Fie´vet, F.; Lagier, J. P.; Blin, B.; Beaudoin, B.; Figlarz, M. Solid State Ionics, 1989, 32/33, 198. (24) Little, L. H. Infrared Spectra of Adsorbed Species, Academic Press: London, New York, 1966; p 176. (25) Sidorov, A. N. Zh. Fiz. Khim. 1956, 30, 995. (26) Greenler, R. G. J. Chem. Phys. 1962, 37, 2094. (27) Tekaia-Elhsissen, K.; Delahange-Vidal, A.; Nowogrocki, G.; Figlarz, M. C. R. Acad. Sci. Paris 1989, 309, 349. (28) Tekaia-Elhsissen, K.; Delahange-Vidal, A.; Nowogrocki, G.; Figlarz, M. C. R. Acad. Sci. Paris 1989, 309, 469.
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Figure 9. TEM picture of the particles shown in Figure 1a after one (a) and five (b) washing cycles with water.
mol dm-3 NaOH, aged in a closed vessel at 175 °C for 1 h (corresponding to the standard condition), yielded polydispersed well-crystallized micrometer particles of In(OH)3, which could not be broken up on washing. This result substantiates the described role of EG in the present system. Finally, the transmission electron micrograph in Figure 9a displays the partial decomposition of the platelet illustrated in Figure 1a after one washing only, whereas the TEM in Figure 9b shows particles (∼80 nm average diameter) of a completely peptized precursor. This TEM indicates that the nanosized In(OH)3 particles are composed of very thin needles, which may explain why only the (200) peak appears in the XRD pattern. During the calcination of the original particles the glycolates are burned off and the smaller subunits sinter together, while the original shape is retained, yielding In2O3 of different morphologies, such as fibers or platelets. The TEM micrograph (Figure 7b) confirms the composite nature of the fibers after calcination. This study describes a novel method for the preparation of nanosized In(OH)3 particles without the use of any surfactants by peptization of precipitates obtained in highly concentrated solutions. In principle the same procedure could be applied to the preparation of other nanosized metal (hydrous) oxide dispersions. Acknowledgment. A grant to L.A.P.-M. from the Direccio´n General de Investigacio´n Cientı´fica y Te´cnica (Ministerio de Educacio´n y Cultura from Spain) is gratefully acknowledged. LA980149C
