3154
Langmuir 2000, 16, 3154-3159
Synthesis and Characterization of Nanocrystalline Yttrium Oxide Prepared with Tetraalkylammonium Hydroxides Mark D. Fokema, Eugene Chiu, and Jackie Y. Ying* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received August 26, 1999. In Final Form: December 16, 1999 Controlled chemical precipitation of yttrium hydroxynitrate has been used to synthesize nanocrystalline yttrium oxide powders with high thermal stability. The type of base employed as the precipitating agent has a significant effect on the properties of the yttrium oxide product. Precipitation with tetraalkylammonium hydroxides was shown to produce a much finer-grained product than a conventional ammonium hydroxide synthesis route. This is attributed to the higher pH that can be achieved with tetraalkylammonium hydroxides, as well as the ability of tetraalkylammonium cations to inhibit precipitate particle growth by lowering the rate of diffusion of soluble precursors to the particle surface.
Introduction In the last few decades, yttrium oxide has most commonly been used as a sintering agent for densifying ceramics1,2 and, when doped with Eu or Tb, as a phosphor material for cathode ray tubes.3 Yttrium oxide has also been examined as a bulk ceramic for high-temperature applications because of its high melting point, phase stability, and low thermal expansion.4 Because of its basic nature, yttrium oxide has more recently been applied to a wide variety of catalytic reactions, including olefin hydrogenation, olefin isomerization, aldol addition of ketones, and alcohol dehydration.5 It has also been found to be an effective catalyst for methane dimerization6 and the selective catalytic reduction of nitrogen oxides.7,8 In many of these applications, reduction of the yttrium oxide crystallite size into the nanometer regime would result in improved performance compared to coarsegrained yttrium oxide. This is particularly evident in phosphor applications, where the phosphor efficiency varies inversely as the square of the particle size due to quantum confinement effects.3 In bulk ceramic applications, nanocrystalline yttrium oxide has been found to densify much more readily than its coarse-grained counterpart.9 Improved catalytic performance can also be realized using nanocrystalline yttria, because of the ultrahigh surface areas that are associated with nanocrystalline powders. Unfortunately, few studies have been reported regarding the synthesis of thermally stable, high surface area yttrium oxide for high-temperature applications. In the past, the preparation of nanocrystalline yttrium oxide has been accomplished by a variety of physical and * To whom correspondence should be addressed. (1) Troczynski, T. B.; Nicholson, P. S. J. Am. Ceram. Soc. 1989, 72, 1488. (2) Panchula, M. L.; Ying, J. Y. To be submitted for publication in the J. Am. Ceram. Soc. (3) Goldburt, E. T.; Kulkarni, B.; Bhargava, R. N.; Taylor, J.; Libera, M. J. Lumin. 1997, 72, 190. (4) U ¨ nal, O ¨ .; Akinc, M. J. Am. Ceram. Soc. 1996, 79, 805. (5) Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. New Solid Acids and Bases; Elsevier: New York, NY, 1989; p 41. (6) Osada, Y.; Koike, S.; Fukushima, T.; Ogasawara, S.; Shikada, T.; Ikariya, T. Appl. Catal. 1990, 59, 59. (7) Fokema, M. D.; Ying, J. Y. Appl. Catal., B 1998, 18, 71. (8) Fokema, M. D.; Ying, J. Y. J. Catal., in press. (9) Kim, W.-J.; Park, J. Y.; Oh, S. J.; Kim, Y. S.; Hong, G.-W.; Kuk, I.-H. J. Mater. Sci. Lett. 1999, 18, 411.
chemical synthesis techniques. Physical techniques, such as inert gas condensation of laser-ablated yttrium oxide or inert gas condensation of sputtered or thermally evaporated yttrium followed by oxidation to yttrium oxide, have produced powders with crystallite sizes as small as 7 nm and surface areas as high as 77 m2/g at 300 °C.10,11 Chemical synthesis techniques such as chemical precipitation,12-14 sol-gel processing,15 electrospray pyrolysis,16 and combustion17 have also been used to prepare nanocrystalline yttrium oxide but have failed to produce yttrium oxide powders with surface areas in excess of 80 m2/g at temperatures above 600 °C. We have employed controlled chemical precipitation of yttrium nitrate with tetraalkylammonium hydroxides to produce ultrafine yttrium oxide powders with excellent thermal stability. Although tetraalkylammonium salts are commonly used in zeolite synthesis18 and the stabilization of metal colloids,19 until now, the use of tetraalkylammonium hydroxides (other than ammonium hydroxide) for chemical precipitation has not been reported. Experimental Section Sample Preparation. Yttrium oxide was prepared by precipitating a 0.25 M aqueous solution of Y(NO3)3‚6H2O (Alfa Aesar, 99.9%) with an aqueous organic base at a constant pH. The bases employed in this study were ammonium hydroxide (Mallinckrodt), tetraethylammonium hydroxide (TEAOH, Aldrich), and tetrabutylammonium hydroxide (TBAOH, Aldrich). The pH of precipitation was measured with an Orion 720A pH meter with a TRIODE electrode and was controlled by mixing (10) Skandan, G.; Hahn, H.; Parker, J. C. Scr. Metall. Mater. 1991, 25, 2389. (11) Betz, U.; Scipione, G.; Bonetti, E.; Hahn, H. Nanostruct. Mater. 1997, 8, 845. (12) Dogan, F.; Roosen, A.; Hausner, H. Adv. Ceram. 1987, 21, 681. (13) Tool, C. J. J.; Cordfunke, E. H. P. Solid State Ionics 1989, 32/33, 691. (14) Rasmussen, M. D.; Akinc, M.; Hunter, O., Jr. In Processing of Metal and Ceramic Powders; Metallurgical Society of AIME: Warrendale, PA, 1982; p 21. (15) Hours, T.; Bergez, P.; Charpin, J.; Larbot, A.; Guizard, C.; Cot, L. Ceram. Bull. 1992, 71, 200. (16) Rulison, A. J.; Flagan, R. C. J. Am. Ceram. Soc. 1994, 77, 3244. (17) Ekambaram, S.; Patil, K. C. J. Mater. Chem. 1995, 5, 905. (18) Burkett, S. L.; Davis, M. E. Chem. Mater. 1995, 7, 920. (19) Bo¨nnemann, H.; Braun, G.; Brijoux, W.; Brinkmann, R.; Schulze Tilling, A.; Seevogel, K.; Siepen, K. J. Organomet. Chem. 1996, 520, 143.
10.1021/la991156j CCC: $19.00 © 2000 American Chemical Society Published on Web 02/17/2000
Nanocrystalline Yttrium Oxide the two precursor solutions at a set flow rate using two liquid pumps from Fluid Metering, Inc. Following precursor addition, the precipitate was aged, centrifuged, washed three times with ethanol, dried at 120 °C, and ground in a mortar and pestle to break up any weak agglomerates. The precipitate was converted to yttrium oxide by calcination in air at 600 °C for 4 h. Sample Characterization. The surface area of yttrium oxide powders was determined using a five-point BET (BrunauerEmmett-Teller) method on a Micromeritics ASAP 2000 instrument. Phase identification was performed by powder X-ray diffraction (XRD) using a Siemens D5000 diffractometer (45 kV, 40 mA, Cu KR). Scherrer’s analysis of the broadening of the Y2O3 (222) diffraction peak was employed to obtain the volumeaveraged crystallite size. The error associated with surface area and crystallite size determinations was estimated to be ≈2%, based on measurements of duplicate samples. This unusually small degree of error can be attributed to the extremely wellcontrolled and reproducible conditions at which the materials were synthesized. The degree of agglomeration of each sample was examined by calculating the difference between the geometric surface area of each sample (based on the XRD crystallite size, density of yttrium oxide (5.01 g/cm3), and assumption of monodisperse spherical particles) and the measured BET surface area and then dividing that value by the geometric surface area. Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were performed in flowing N2 using Perkin-Elmer TGA7 and DTA7 systems, respectively. Transmission electron microscopy (TEM) was performed on a JEOL 2010 high-resolution microscope. Determination of surface charge was carried out following the procedure of Sprycha,20 which takes into account the solid solubility in the aqueous solution. A solution of nitrate salt and 0.01 M hydroxide and either 25 or 100 m2 of precipitate was titrated with 0.2 M nitric acid solution. The difference in the amount of acid required to attain a certain pH for each of the suspensions was used to calculate the surface charge. Zeta potential measurements were performed with a MATEC ESA 8000 on a 0.1 vol % suspension of precipitate with a 0.1 M potassium nitrate solution as the supporting electrolyte. The viscosity of aqueous solutions at 25 °C was determined using a Cannon-Fenske size 25 kinematic viscometer from Technical Glass Products, Inc.
Results and Discussion Yttrium Oxide Structural Evolution. The product formed from the precipitation of yttrium nitrate with aqueous hydroxide has previously been identified as a yttrium hydroxynitrate of the general formula Y(OH)3-x(NO3)x, with x ≈ 0.5.21 TGA and DTA curves of a precipitate produced at a pH of 12.2 with TEAOH are shown in Figure 1. The overall weight loss corresponds very well to the transformation of Y(OH)2.5(NO3)0.5 to Y2O3 (30.3 wt % measured, 30.5 wt % expected). The very gradual weight loss and broad endothermic decomposition peaks observed in Figure 1 as well as the lack of X-ray diffraction peaks indicate that the precipitate is amorphous below 560 °C. The exothermic peak centered at 590 °C is associated with the formation of crystalline yttrium oxide, as confirmed by the X-ray diffraction pattern of the precipitate after calcination at 600 °C, which reveals peaks characteristic of cubic yttrium oxide (JCPDS 41-1105). The small weight loss above 600 °C in Figure 1 can be attributed to the desorption of carbonate groups that formed on the surface of the highly basic precipitate during exposure to air.22 The BET surface area and XRD crystallite size of the precipitate produced at a pH of 12.2 and calcined to different temperatures are presented in Figure 2. Trans(20) Sprycha, R. Colloid Surf. 1982, 5, 147. (21) Schildermanns, I.; Mullens, J.; Yperman, J.; Franco, D.; Van Poucke, L. C. Thermochim. Acta 1994, 231, 185. (22) Rosynek, M. P. Catal. Rev.-Sci. Eng. 1977, 16, 111.
Langmuir, Vol. 16, No. 7, 2000 3155
Figure 1. TGA and DTA profiles of precipitated yttrium hydroxynitrate. The sample was heated in a nitrogen atmosphere with a ramp rate of 10 °C/min.
Figure 2. (9) Crystallite size and (b) surface area of nanocrystalline yttrium oxide.
mission electron micrographs of the precipitate after calcination at 600 °C and 900 °C are presented in Figure 3. The product at 600 °C is composed of crystallites of ∼10 nm, which are loosely aggregated into submicrometersized secondary particles. At higher calcination temperatures, the overall nanostructure is maintained, with only minor coarsening of crystallites and concurrent loss of surface area. At both temperatures, the crystallite size observed in the TEM images is in good agreement with that obtained by XRD measurements. Effect of Tetraalkylammonium Cations. The production of highly dispersed nanometer-sized precipitates using wet chemical techniques has been investigated for a number of different oxide systems.23 In general, precipitate formation from a supersaturated solution occurs either through the nucleation of new particles or through the growth of existing particles. By promoting the nucleation of new particles and inhibiting the growth of existing particles, nanometer-sized colloidal suspensions can be obtained. By further minimizing the agglomeration of these primary particles during the precipitation, aging, and drying processes, a high surface area, nanocrystalline powder can be achieved. Table 1 presents the XRD crystallite size and BET surface area of 600 °C calcined yttrium oxide samples produced with ammonium hydroxide and two different tetraalkylammonium hydroxide precipitating agents at a pH of 10.9 and ionic strength of ∼0.18 mol/L. There is a (23) So¨hnel, O.; Garside, J. Precipitation-Basic Principles and Industrial Applications; Butterworth-Heinemann: Oxford, 1992; Appendix 4.
3156
Langmuir, Vol. 16, No. 7, 2000
Fokema et al.
Figure 3. TEM images of nanocrystalline yttrium oxide calcined to (a) 600 and (b) 900 °C.
Figure 4. Crystallite size (closed symbols) and surface area (open symbols) of 600 °C calcined yttrium oxide precipitated with 2 M ammonium hydroxide in the presence of (b) TEAN and (9) TBAN of various concentrations. Table 1. 600 °C Calcined Yttrium Oxide Samples Precipitated with Different Bases
base NH4OH TEAOH TBAOH
geometric crystallite BET surface surface agglomeration size (nm) area (m2/g) area (m2/g) (%) 11 9 9
88 101 100
112 130 132
22 22 24
pronounced decrease in crystallite size and increase in specific surface area when a tetraalkylammonium hydroxide is used as the precipitating agent. The degree of product agglomeration does not differ significantly among the three samples. Figure 4 demonstrates the effect of adding small amounts of tetraalkylammonium cations, in the form of tetraethylammonium nitrate (TEAN, Aldrich) and tetrabutylammonium nitrate (TBAN, Aldrich), to precipitation mixtures that employ ammonium hydroxide as the base. Addition of approximately 0.25 M TEAN or 0.20 M TBAN results in a ∼10% decrease in crystallite size and ∼15% increase in surface area. Clearly, the presence of tetraalkylammonium cations is all that is required to effect an improvement in the precipitate properties. Addition of the tetraalkylammonium cations to an ammonium hydroxide precipitation leads to a product
equivalent to that obtained through precipitation with tetraalkylammonium hydroxides at the same pH. Tetraalkylammonium salts are commonly used to stabilize metal colloids because the cations adsorb on the surface of the metal particle.19,24,25 The cations form a stabilizing mantle that prevents the particles from coming close enough to one another that van der Waals attractive forces would cause the particles to agglomerate. In the yttrium oxide system, there is no apparent change in the degree of agglomeration as the concentration of tetraalkylammonium cation is increased from 0 to 0.25 M. Thus, steric inhibition of agglomeration by these cations is not a significant factor in the yttrium oxide system. The increase in surface area of the yttrium oxide product arises directly from a reduction in crystallite size, which can be attributed to the characteristics of the precipitation system during the particle formation process. The tetraalkylammonium cations inhibit particle growth and promote nucleation of new particles during precipitation. The interactions of tetraalkylammonium cations with silica have been extensively studied due to the role of tetraalkylammonium cations as structure-directing agents in zeolite synthesis.26 Tetraalkylammonium cations have been shown to specifically adsorb to the surface of silica under aqueous conditions.27-29 The mechanism for this adsorption process is not electrostatic, as the cations adsorb when the particle surface is negatively or positively charged (i.e., on both sides of the point of zero surface charge (pzc)). Rather, these species adsorb due to hydrophobic-hydrophilic interactions in the colloidal system. Tetraalkylammonium ions such as tetraethylammonium (TEA) and tetrabutylammonium (TBA) cations are highly hydrophobic. Surface adsorption of these cations to the hydrophobic siloxane bridges on the silica surface is (24) Reetz, M. T.; Helbig, W.; Quaiser, S. A.; Stimming, U.; Breuer, N.; Vogel, R. Science 1995, 267, 367. (25) Kolb, U.; Quaiser, S. A.; Winter, M.; Reetz, M. T. Chem. Mater. 1996, 8, 1889. (26) Burkett, S. L.; Davis, M. E. Chem. Mater. 1995, 7, 1453. (27) Rutland, M. W.; Pashley, R. M. J. Colloid Interface Sci. 1989, 130, 448. (28) de Keizer, A.; van der Ent, E. M.; Koopal, L. K. Colloid Surf., A 1998, 142, 303. (29) van der Donck, J. C. J.; Vaessen, G. E. J.; Stein, H. N. Langmuir 1993, 9, 3553.
Nanocrystalline Yttrium Oxide
Figure 5. Surface charge of yttrium hydroxynitrate particles in the presence of (2) 0.02 M TBAN, (b) 0.1 M TBAN, ([) 0.1 M KNO3, and (9) 0.5 M KNO3.
favorable because of the reduction in hydrophobic surface area that is exposed to water upon adsorption.29 With a large concentration of surface hydroxyl groups, one would expect the surface of yttrium hydroxynitrate to be hydrophilic. Nevertheless, we examined whether specific adsorption of the tetraalkylammonium cations occurred on yttrium hydroxynitrate. Aqueous suspensions of precipitate, adjusted to a pH of 12, were titrated with nitric acid in the presence of different nitrate salts to determine the particle surface charge and point of zero charge (pzc). Figure 5 shows the surface charge of the resuspended precipitate as a function of pH for several different supporting electrolytes. In the presence of any electrolyte, the pzc was between a pH of 7.3 and 7.6, with the surface negatively charged at higher pH values. This is in reasonable agreement with the results of Sprycha et al.,30 who reported a pzc of 7.7 for yttrium hydroxycarbonate. In general, specific adsorption of cations causes the surface charge to become more positive, which leads to a shift in the pzc to a higher pH. The concentration of a specifically adsorbing electrolyte also has a significant effect on the surface charge and pzc. A greater electrolyte concentration produces a greater driving force for adsorption and thus raises the surface charge and pzc even more. The results presented in Figure 5 indicate that TBA cations do not specifically adsorb on the surface, while adsorption does occur in the presence of KNO3. These are surprising results, as potassium salts are normally used as indifferent electrolytes for surface titration or zeta potential measurements on oxide systems.20,31 However, while KOH fully dissociates in water, KNO3 is partially associated in aqueous solutions (pKd ) -0.2).32 The presence of nitrate groups on the surface of our material can thus provide a driving force for potassium cation adsorption to the surface. Because potassium is hydrophilically hydrated, it can easily approach the hydroxynitrate particle surface and coordinate to the surface nitrate groups. Dissociation constants of other tetraalkylammonium salts indicate that the tetraalkylammonium cations may also have an affinity for surface nitrate groups.33 However, the fact that the pzc does not change with TBAN concentration indicates that no adsorption occurs. This is likely due to the repulsive forces between the hydrophobic hydration sphere of the TBA cation and the hydrophilic hydroxide surface species (30) Sprycha, R.; Jablonski, J.; Matijevic, E. J. Colloid Interface Sci. 1992, 149, 561. (31) Komulski, M.; Matijevic, E. Colloid Surf. 1992, 64, 57. (32) Prue, J. E. Ionic Equilibria; Pergamon: Oxford, 1966; p 97. (33) Bower, V. E.; Robinson, R. A. Trans. Faraday Soc. 1963, 59, 1717.
Langmuir, Vol. 16, No. 7, 2000 3157
of the particles. Thus, in the case of hydroxynitrate powders, the “true” surface charge is best determined in the presence of a hydrophobic electrolyte rather than a conventional KNO3 electrolyte. The mechanism for precipitate size reduction in the presence of tetraalkylammonium cations is shown by the above study to not involve changes to the particle surface due to specific adsorption. The slightly greater surface charge that does exist in the presence of tetraalkylammonium cations may lead to increased repulsion of particles, as the magnitude of the double layer potential is increased, but this effect is not large, and particle agglomeration does not appear to be a critical process (Table 1). Rather, we believe that the use of the tetraalkylammonium salt leads to changes in the composition of the double layer and, hence, a change in the transport properties of this region. In aqueous solutions, tetraalkylammonium cations undergo the process of “hydrophobic hydration”. This refers to the restructuring of water molecules near the hydrophobic solute in order to accommodate the hydrophobic species while maintaining a fully hydrogen-bonded network of water.26 The tetraalkylammonium cations are approximately spherical with radius estimates of 3.14.0 Å for TEA+ and 3.8-5.4 Å for TBA+.34 The hydrophobic hydration sphere for TEA+ is approximated to contain 20 water molecules, while the hydration sphere for TBA+ may contain up to 60 water molecules.35 Simulations indicate that hydrogen bonding among water molecules is strengthened around large tetraalkylammonium solutes, and experimental results indicate that the mobility of water molecules within the hydration sphere is lower than that of bulk water.35,36 The increased “water structure” around these large solutes may act as a barrier and inhibit diffusion of species within the double layer region. If yttrium cations or hydroxylated clusters that form during precipitation are prevented from diffusing to the surface of existing particles, the nucleation of new particles will be favored over the growth of existing particles, and the final product will possess a finer grain size when crystallized. Measurements of aqueous diffusivities in the presence of tetraalkylammonium cations have not been reported in the literature. However, theoretical (Einstein-Stokes) and empirical (Wilke-Chang) relations of diffusivity and viscosity in dilute solutions predict that solute diffusivity is inversely proportional to solvent viscosity.37 Nikam and Sawant38 reported that the viscosities of 0.055 M solutions of tetraethylammonium bromide and tetrabutylammonium bromide were 1.9% and 6.4% greater than that of pure water at 30 °C. We found the viscosity of a 0.3 M TBAN solution at 25 °C (1.33 mPa‚s) to be 49% greater than the viscosity of water at the same temperature. Thus, one would indeed expect a greater diffusion barrier in the presence of tetraalkylammonium cations. The concentration of tetraalkylammonium cations in the double layer surrounding each colloidal particle is also significantly greater than that in the bulk solution. The counterion concentration within the double layer region (34) Shchipunov, Y. A. Adv. Colloid Interface Sci. 1988, 28, 135. (35) Fumino, K.; Yukiyasu, K.; Shimizu, A.; Taniguchi, Y. J. Mol. Liq. 1998, 75, 1. (36) Slusher, J. T.; Cummings, P. T. J. Phys. Chem. B 1997, 101, 3818. (37) Reid, R. C.; Sherwood, T. K. The Properties of Gases and Liquids, 2nd ed.; McGraw-Hill, Inc.: New York, 1966. (38) Nikam, P. S.; Sawant, A. B. Bull. Chem. Soc. Jpn. 1998, 71, 2055.
3158
Langmuir, Vol. 16, No. 7, 2000
Fokema et al.
Figure 6. Crystallite size of 600 °C calcined yttrium oxide precipitated with (b) NH4OH and (9) TBAOH at various pH values.
Figure 7. Surface area of 600 °C calcined yttrium oxide precipitated with (b) NH4OH and (9) TBAOH at various pH values.
can be approximated using the Boltzmann distribution39
the product in the precipitating liquor. An extremely low solubility of the product leads to rapid supersaturation of the solution upon addition of precursors, and the instantaneous nucleation of product before appreciable growth of existing particles can occur. Rasmussen et al. have reported a solubility product of 1.5 × 10-20 for yttrium hydroxynitrate in water and an ion product (IP ) [Y3+][OH-]2.5[NO3-]0.5) of ∼10-11 at the beginning of a reverse-strike precipitation with ammonium hydroxide.14 Our use of a constant-pH precipitation technique maintains much higher ion concentrations than a reverse-strike technique (where the inorganic precursor is added to a base solution), while the increase in precipitation pH from 11 to 13 with the use of TEAOH or TBAOH instead of NH4OH results in a further increase in the ion product by 5 orders of magnitude. Thus, the supersaturation during our precipitation process is significantly increased, and nucleation of new precipitate particles is favored over growth of existing particles. One would expect that changes in the pH of the precipitating mixture would also alter the stability of the colloidal precipitate and lead to changes in the degree of agglomeration of the product. No significant differences in the degree of precipitate agglomeration were observed for our materials (typically 20-25%). An increase in the pH affects the ionic strength of the mixture and may also change the surface charge on the colloidal particles. The DLVO theory and Debye-Hu¨ckel model of the double layer surrounding colloidal particles predict that colloid stability is enhanced by a high surface charge but reduced by a high ionic strength.39 A high ionic strength compresses the double layer and allows particles to approach closer to one another. Surface charge measurements indicate that the magnitude of the surface charge does not significantly change when the pH is raised above 10, while the ionic strength of our precipitation liquor increases only from 0.18 M at pH ) 10.8 to 0.21 M at pH ) 13. Thus, no significant change in the double layer structure is expected as the pH is increased for this system, and colloid stability remains essentially the same.
ci/ci° ) exp(-zieψ/kT)
(1)
where ci is the counterion concentration in the double layer, ci° is the bulk counterion concentration, zi is the counterion valency, e is the elementary charge, and ψ is the potential within the double layer. Unfortunately, prediction of surface potentials from measured surface charges is not easily achieved for oxide systems with current theories.39 A general feature of oxide systems is a high surface charge density with only moderate surface and zeta potentials.39 A single-point measurement of the zeta potential of our Y(OH)2.5(NO3)0.5 precipitate at a pH of 11 did indeed produce a modest value of approximately -40 mV. This agrees moderately well with Sprycha et al., who reported a zeta potential of approximately -20 mV for Y(OH)CO3 at a lower pH of 10.30 The concentration of tetraalkylammonium counterions at the shear plane (where the potential is -40 mV) is then predicted by eq 1 to be five times greater than the bulk tetraalkylammonium concentration. For bulk tetraalkylammonium concentrations of 0.20-0.25 M, the high tetraalkylammonium concentrations in the double layer region (g1 M) will present a significant impediment to diffusion of reactive species to the yttrium hydroxynitrate particle; growth of the existing precipitate particles will therefore be substantially hindered. Effect of pH. The use of tetraalkylammonium hydroxides further improves the nanocrystallinity of the yttrium oxide product by allowing one to carry out the precipitation under extremely basic conditions. Figures 6 and 7 demonstrate that raising the pH of the precipitating liquor with TBAOH significantly reduces the crystallite size and increases the surface area of our 600 °C calcined yttrium oxide materials. A significant improvement is also realized with increasing pH when ammonium hydroxide is used as the precipitating agent. However, it should be pointed out that because ammonium hydroxide is only a moderately strong base, the concentration of ammonium hydroxide required to effect a precipitation above a pH of 10.5 is very high. The tetraalkylammonium hydroxide bases are nearly completely dissociated, so higher pH precipitation conditions can be easily achieved along with a high product yield. Nucleation and growth of particles during the precipitation process are strongly affected by the solubility of (39) Hunter, R. J. Zeta Potential in Colloid Science; Academic Press: San Diego, CA, 1981.
Conclusions We have reported on the use of tetraalkylammonium hydroxides as precipitation agents for the production of yttrium hydroxynitrate particles. The use of these bases impedes the diffusion of precipitate precursors to existing particles, resulting in a higher nucleation rate and an overall reduction in precipitate particle size. These strong bases also allow a higher pH to be achieved and maintained for the precipitation liquor than other organic bases,
Nanocrystalline Yttrium Oxide
further reducing precipitate particle size. The yttrium hydroxynitrate produced by this technique converts to a nanocrystalline yttrium oxide product with high surface areas when calcined to 600 °C. These yttrium oxide nanocrystals exhibit excellent dispersion and thermal stability, making them well suited for high-temperature catalytic applications.
Langmuir, Vol. 16, No. 7, 2000 3159
Acknowledgment. The authors thank Andrey J. Zarur for his assistance in the TEM studies and Martin L. Panchula for helpful discussions. Financial support of this project by the National Science Foundation (CTS9731396) is gratefully acknowledged. LA991156J
