High-Yield Sol−Gel Synthesis of Well-Dispersed, Colorless ZrO2

Langmuir 2006, 22, 7137-7140

7137

High-Yield Sol-Gel Synthesis of Well-Dispersed, Colorless ZrO2 Nanocrystals Mikihisa Mizuno,* Yuichi Sasaki, Sungkil Lee, and Hitoshi Katakura Sony Corporation, Sendai Technology Center, 3-4-1 Sakuragi, Tagajo, Miyagi 985-0842, Japan ReceiVed March 22, 2006. In Final Form: May 26, 2006 A 93% high-yield synthesis of well-dispersed, colorless zirconium dioxide (ZrO2) nanocrystals is reported. In this synthesis, hydrolysis and condensation reactions of zirconium(IV) tert-butoxide in the presence of oleic acid (100 °C) formed ZrO2 precursors. The ZrO2 precursors were made of -Zr-O-Zr- networks surrounded by oleic acid molecules. Annealing (280 °C) the precursors dispersed in dioctyl ether caused crystallization of the -Zr-O-Zr- networks, thereby generating 4 nm ZrO2 nanocrystals stabilized with oleic acid. The particles were highly crystalline and tetragonal phase. A dense ZrO2 nanocrystal dispersion in toluene (280 mg/mL) showed a transmittance of about 90% (3 mm optical path length) in the whole visible region.

Introduction Colloidal nanocrystals are stabilized against aggregation in solution with a layer of surface coating.1-4 Such nanocrystals of colorless metal oxides, such as zirconium dioxide (ZrO2), can be employed as building blocks to develop a wide range of nanotechnological optical materials and devices. For instance, properly coated ZrO2 nanocrystals can be dispersed into polymers, providing new colorless nanocomposites with enhanced optical, mechanical, and thermal properties compared to bare polymers.5,6 Solution-phase synthesis provides a useful means of producing various metal oxide nanocrystals coated with organic surfactants.7-17 However, preparative routes to ZrO2 nanocrystals have not yet been well established. Currently developed methods to make ZrO2 nanoparticles employ sol-gel processing of zirconium alkoxides or salts.18-22 * To whom correspondence should be addressed. Phone: +81 22 367 2623. Fax: +81 22 367 2318. E-mail: [email protected]. (1) Schmid, G. Nanoparticles: From Theory to Application; Wiley-VCH: Weinheim, Germany, 2004. (2) Pileni, M. P. Langmuir 1997, 13, 3266. (3) Caruso, F. AdV. Mater. 2001, 13, 11. (4) Scott, R. W. J.; Wilson, O. M.; Crooks, R. M. J. Phys. Chem. B 2005, 109, 692. (5) Beecroft, L. L.; Ober, C. K. Chem. Mater. 1997, 9, 1302. (6) Kickelbick, G. Prog. Polym. Sci. 2003, 28, 83. (7) Trentler, T. J.; Denler, T. E.; Bertone, J. F.; Agrawal, A.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 1613. (8) Rockenberger, J.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 1999, 121, 11595. (9) O’Brien, S.; Brus, L.; Murray, C. B. J. Am. Chem. Soc. 2001, 123, 12085. (10) Sun, S.; Zeng, H.; Robinson, D. B.; Raoux, S.; Rice, P. M.; Wang, S. X.; Li, G. J. Am. Chem. Soc. 2004, 126, 273. (11) Tang, J.; Fabbri, J.; Robinson, R. D.; Zhu, Y.; Herman, I. P.; Steigerwald, M. L.; Brus, L. E. Chem. Mater. 2004, 16, 1336. (12) Cao, Y. C. J. Am. Chem. Soc. 2004, 126, 7456. (13) Park, J.; An, K.; Hwang, Y.; Park, J.-G.; Noh, H.-J.; Kim, J.-Y.; Park, J.-H.; Hwang, N.-M.; Hyeon, T. Nat. Mater. 2004, 3, 891. (14) Si, R.; Zhang, Y.-W.; You, L.-P.; Yan, C.-H. Angew. Chem., Int. Ed. 2005, 44, 3256. (15) Liu, Q.; Lu, W.; Ma, A.; Tang, J.; Lin, J.; Fang, J. J. Am. Chem. Soc. 2005, 127, 5276. (16) Zhang, Z.; Zhong, X.; Liu, S.; Li, D.; Han, M. Angew. Chem., Int. Ed. 2005, 44, 3466. (17) Epifani, M.; Arbiol, J.; Dı´az, R.; Pera´lvarez, M. J.; Siciliano, P.; Morante, J. R. Chem. Mater. 2005, 17, 6468. (18) Ward, D. A.; Ko, E. I. Chem. Mater. 1993, 5, 956. (19) Navı´o, J. A.; Hidalgo, M. C.; Colo´n, G.; Botta, S. G.; Litter, M. I. Langmuir 2001, 17, 202. (20) Woudenberg, F. C. M.; Sager, W. F. C.; Sibelt, N. G. M.; Verweij, H. AdV. Mater. 2001, 13, 514. (21) Liang, J.; Deng, Z.; Jiang, X.; Li, F.; Li, Y. Inorg. Chem. 2002, 41, 3602. (22) Deshpande, A. S.; Pinna, N.; Beato, P.; Antonietti, M.; Niederberger, M. Chem. Mater. 2004, 16, 2599.

In these sol-gel methods, which rely on the hydrolysis and condensation reactions of the zirconium sources, a major problem is to control the reaction rates which are typically too fast, resulting in agglomerate or amorphous particles. A promising solution is to use nucleophilic chemical additives such as carboxylic acids and modify the reactivity of the zirconium alkoxides.23 An alternative non-hydrolytic sol-gel strategy has recently demonstrated that the high-temperature, alkyl halide elimination reaction between zirconium(IV) isopropoxide and zirconium(IV) chloride in trioctylphosphine oxide (TOPO) leads to monodisperse ZrO2 nanocrystals stabilized with TOPO.24 Unfortunately, the particles are deep green in powder form, which may limit their use in optical applications where colorlessness is critical. In this paper, we report a novel hydrolytic sol-gel approach to well-dispersed as well as colorless ZrO2 nanocrystals which appear white in powder form. The synthesis involves hydrolysis and condensation reactions of zirconium(IV) tert-butoxide, Zr(OBut)4, in the presence of oleic acid (100 °C), followed by annealing (280 °C) in a high-boiling-point solvent. Oleic acid is employed to slow the hydrolysis rate of Zr(OBut)4 and ensure a dispersible product, meanwhile the annealing process is essential to obtain high crystallinity of the particles. The synthesis method is a high yield process and readily scaled up for mass production. Experimental Section Ethanol and toluene were reagent grade and used as received. Water used was prepared by using a membrane purification system (GSR-500, Advantec). Zr(OBut)4 (Aldrich, 99.999%), trimethylamine N-oxide (Aldrich, 98%), and dioctyl ether (Aldrich, 99%) were used without further purification. Oleic acid (Aldrich, 90%) and heptadecane (Wako, g98%) were dried before use by heating at 120 °C for 1 h under vacuum. The present synthesis consisted of two steps (Scheme 1): first, controlled growth of -Zr-O-Zr- networks from hydrolysis and condensation reactions of Zr(OBut)4 in the presence of oleic acid (100 °C), forming dispersible ZrO2 precursors; second, crystallization of the -Zr-O-Zr- networks by annealing (280 °C) the precursors dispersed in dioctyl ether. In a typical experiment, for the first step of Scheme 1, 5 mmol of Zr(OBut)4, 6.2 mmol of dried oleic acid, and 43 mL of dried heptadecane were mixed in a 100 mL three-neck flask equipped with a magnetic stirrer, a reflux cooler, and a thermometer and (23) Cozzoli, P. D.; Kornowski, A.; Weller, H. J. Am. Chem. Soc. 2003, 125, 14539. (24) Joo, J.; Yu, T.; Kim, Y. W.; Park, H. M.; Wu, F.; Zhang, J. Z.; Hyeon, T. J. Am. Chem. Soc. 2003, 125, 6553.

10.1021/la060774e CCC: $33.50 © 2006 American Chemical Society Published on Web 07/15/2006

7138 Langmuir, Vol. 22, No. 17, 2006

Letters

Scheme 1. Hydrolytic Sol-Gel Synthesis of ZrO2 Nanocrystals

Figure 1. (a) Infrared and (b) XRD spectra of the ZrO2 precursors obtained from the first step of Scheme 1. stirred at 100 °C under a N2 flow, giving a pale yellow solution. Then 5 mL of a 2 M trimethylamine N-oxide aqueous solution was subsequently injected into the hot solution. The mixture was kept at 100 °C under a N2 flow and stirred under reflux for 24 h. The turbid mixture was cooled to room temperature by removing the heat source. Ethanol and toluene were added to the mixture in air, and a white material was precipitated and isolated by centrifuging (15 000 rpm, 10 min). The white material was dispersed in toluene, reprecipitated with ethanol, and centrifuged (15 000 rpm, 10 min) to remove the solvent. This dispersing and centrifuging procedure was repeated twice. The material, ZrO2 precursors, was then dispersed in 30 mL of dioctyl ether in another 100 mL three-neck flask with a magnetic stirrer, a reflux cooler, and a thermometer for the next step (the second step of Scheme 1). Under a N2 flow, the colorless solution was heated to 280 °C and held at this temperature under reflux for 20 h with stirring. The resulting colorless mixture was cooled to room temperature by removing the heat source and ethanol was added to precipitate a white product. The white precipitate was retrieved by centrifugation (15 000 rpm, 10 min) and washed twice by the dispersing and centrifuging procedure, producing ZrO2 nanocrystals which appeared white in powder form. The ZrO2 nanocrystals were capped with oleic acid and readily dispersed in nonpolar solvents such as toluene with good stability over months. According to thermogravimetric and differential thermal analyses (TG-DTA), 570 mg of the ZrO2 nanocrystals was obtained (without including the weight of the capping oleic acid molecules, 480 mg), corresponding to a high yield of 93%. By scaling up the abovementioned synthesis by a factor of 10 (Zr(OBut)4 50 mmol, oleic acid 62 mmol, heptadecane 430 mL, trimethylamine N-oxide aqueous solution 2 M 50 mL, dioctyl ether 300 mL), we obtained 10 g of dispersible ZrO2 nanocrystals (including the weight of capping oleic acid molecules), which is a very large-quantity production in the lab-scale synthesis of colloidal nanocrystals. Infrared absorption experiments were carried out with a Nicolet Magna 550. X-ray powder diffraction (XRD) spectra were collected on a Rigaku RAD-IIC under Co KR radiation (λ ) 0.178897 nm). Low-magnification transmission electron microscopy (TEM) was conducted using a JEOL JEM-200CX (accelerating voltage 200 kV). High-resolution TEM and selected area electron diffraction (SAED) were performed using a JEOL JEM-3000F operated at 300 kV. UV-vis transmission spectra were measured using a Hitachi U-3400.

Results and Discussion Figure 1a shows the infrared spectrum of the ZrO2 precursors obtained from the first step of Scheme 1. The broad absorption band below 1000 cm-1 corresponds to V(Zr-O-Zr) vibrations. This shows that -Zr-O-Zr- networks are formed. The two

intense bands around 1440 and 1550 cm-1 can be respectively assigned to the symmetric and antisymmetric stretching vibrations of carboxylate groups, Vs(COO-) and Vas(COO-). Their position and frequency splitting (∆V ) 110 cm-1) indicated that the carboxylate groups of oleic acid molecules were bound to zirconium atoms in a chelating bidentate configuration.25 The small contributions around 1300 cm-1 of δ(C-H) deformation vibrations of the alkyl groups of oleic acid molecules were overlapped with the stretching vibration Vs(COO-). High-energy bands located at 2820-3020 cm-1 correspond to V(C-H) stretching vibrations of the alkyl groups. The weak band centered at 3400 cm-1 should be assigned to V(O-H) stretching vibrations of Zr-OH or absorbed H2O. The ZrO2 precursors thus formed were dispersible in nonpolar solvents such as toluene, indicating that the hydrophobic alkyl groups of oleic acid molecules were pointed outward from the -Zr-O-Zr- networks. The structure of the ZrO2 precursors was amorphous as shown by XRD (Figure 1b). The broad reflexes should correspond to partially condensed hydrous zirconium oxide. TEM observations suggested that the ZrO2 precursors exhibited a fine particle-like morphology with dimension below 2 nm. When oleic acid was not used in the first step of Scheme 1, Zr(OBut)4 vigorously reacted with water, providing undispersible precipitates of amorphous ZrO2. Conversely, in the presence of oleic acid, the reactivity of Zr(OBut)4 toward water was significantly slowed. Zr(OBut)4 could react with oleic acid through a nucleophilic reaction before the water injection, entrance of carboxy groups and elimination of tert-butyl alcohol:26,27

Zr(OBut)4 + xRCOOH f Zr(OBut)4-x(RCOO)x + xButOH where R ) CH3(CH2)7CHdCH(CH2)7. The chemical modification decreased the hydrolytic susceptibility of Zr(OBut)4 and promoted decoupling between hydrolysis and condensation reactions, allowing the formation of the dispersible ZrO2 precursors.28 A similar reaction system has been recently applied to prepare crystalline TiO2 nanorods.23 In the present ZrO2 synthesis method, the condensation process is likely (25) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 3rd ed.; Wiley: New York, 1978. (26) Sanchez, C.; In, M. J. Non-Cryst. Solids 1992, 147&148, 1. (27) Chandler, C. D.; Roger, C.; Hampden-Smith, M. J. Chem. ReV. 1993, 93, 1205. (28) Sanchez, C.; Livage, J.; Henry, M.; Babonneau, F. J. Non-Cryst. Solids 1988, 100, 65.

Letters

Langmuir, Vol. 22, No. 17, 2006 7139

Figure 2. XRD spectrum of the ZrO2 nanocrystals.

governed by kinetics rather than thermodynamics,29 resulting in the amorphous -Zr-O-Zr- networks, so that it is essential to carry out a further step for perfect completion of fabricating ZrO2 nanocrystals. The ZrO2 precursors were then dispersed in dioctyl ether and annealed at 280 °C for 20 h under a N2 atmosphere (the second step of Scheme 1). The resulting ZrO2 nanocrystals were again dispersible in toluene. The infrared spectrum of the ZrO2 nanocrystals was similar to Figure 1a with the characteristic features of chelating carboxylate groups (Vs(COO-) ) 1450 cm-1, Vas(COO-) ) 1540 cm-1, and ∆V ) 90 cm-1) and those of alkyl groups (δ(C-H) ∼ 1300 cm-1 and V(C-H) ) 2820-3020 cm-1), indicating the chelation of oleic acid molecules onto the ZrO2 surfaces. Figure 2 shows the XRD spectrum of the ZrO2 nanocrystals. The drastic spectral change compared to Figure 1b suggested the formation of crystalline ZrO2 phase. The diffraction peaks were broadened due to the small particle size. A low-magnification TEM image of the ZrO2 nanocrystals was shown in Figure 3a. The particles were generally separated from each other due to the capping oleic acid molecules. Figure 3b shows a high-resolution TEM image of the ZrO2 nanocrystals. The fringes from the continuous lattice structure of a typical crystallite were observed, indicating that the particles possessed high crystallinity. The mean dimension of the particles was approximately 4 nm, while showing a size distribution of 4 ( 2 nm. The strong ring patterns from the SAED, as shown in Figure 3c, were indexed to tetragonal structure (P42/nmc, a ) 3.598 Å, c ) 5.152 Å, JCPDS No. 50-1089). The observed lattice spacing in Figure 3b (indicated by the arrows) was measured to be 0.24 nm corresponding to the (110) lattice plane of tetragonal phase. Let us consider the formation mechanism of the ZrO2 nanocrystals from the ZrO2 precursors. The TEM results suggested that the size of the ZrO2 precursors was smaller than that of the ZrO2 nanocrystals. This observation is consistent with TG-DTA data that the inorganic/organic (wt/wt) ratio of the ZrO2 precursors, 0.46, was lower than that of the ZrO2 nanocrystals, 1.2. During annealing at 280 °C for 20 h, the ZrO2 precursors were found to gradually crystallize as monitored by XRD measurements of samples at different annealing times. At a lower annealing temperature of 250 °C, the precursors crystallized less in 20 h. One possible explanation for these observations is that the ZrO2 precursors progressively grow at 280 °C through dissolution followed by growth similar to Ostwald ripening.29 This dissolution-growth process could induce thermodynamical recondensation of the -Zr-O-Zr- networks of the precursors and thus crystallization into the tetragonal ZrO2 nanocrystals. The metastable tetragonal phase of bulk ZrO2 was stabilized in the present particles, possibly due to the nanometer-scale particle size with large surface-to-volume ratio.30 The color of ZrO2 nanocrystal dispersion in toluene ranged from colorless to light pink depending on ZrO2 concentration. (29) Bischoff, B. L.; Anderson, M. A. Chem. Mater. 1995, 7, 1772. (30) Garvie, R. C. J. Phys. Chem. 1978, 82, 218.

Figure 3. (a) Low-magnification TEM image of the ZrO2 nanocrystals. (b) High-resolution TEM image of the ZrO2 nanocrystals; the lattice spacing of 0.24 nm indicated by the arrows corresponds to the (110) lattice plane of tetragonal structure. (c) SAED pattern acquired from the ZrO2 nanocrystals; the ring patterns for tetragonal phase are indexed. The particles were deposited from their toluene dispersion on amorphous carbon-coated copper grids and dried in air.

Figure 4. UV-vis transmission spectrum of the dense ZrO2 nanocrystal toluene dispersion (280 mg/mL) in a 3 mm path-length quartz cell.

For example, a dilute dispersion (10 mg/mL) was colorless, and a dense one (280 mg/mL) was light pink (the concentration values indicated are the weights of ZrO2 nanocrystals, without including those of capping oleic acid molecules, in 1 mL toluene dispersions). Figure 4 shows the UV-vis transmission spectrum of the dense dispersion (280 mg/mL, 3 mm optical path length). The transmittance gradually decreased from 100% (800 nm) to 87% (400 nm) and then dropped to 60% (350 nm), partially due

7140 Langmuir, Vol. 22, No. 17, 2006

to Rayleigh scattering from the particles. Colored particles and/ or large aggregates would give rise to a significant reduction of the transmittance, whereas in the dense dispersion, high transmittance above 87% was observed in the whole visible region, indicating that the ZrO2 nanocrystals were colorless and well dispersed in toluene.

Conclusions In conclusion, we have demonstrated the high-yield (93%) synthesis of colorless, tetragonal ZrO2 nanocrystals stabilized with oleic acid via the hydrolytic sol-gel reaction of Zr(OBut)4 in the presence of oleic acid followed by annealing in dioctyl ether. Further studies on size-controlled synthesis and better control of size distribution of the particles are underway. The

Letters

ZrO2 nanocrystals have great potential in the development of nanoparticle-based functional optical materials. The particles can be dispersed into polymers with different polarities by replacing the capping oleic acid molecules with proper ligands. Meanwhile, we expect that our synthesis approach, a hydrolysisannealing method, can be developed into a generalized strategy to prepare colloidal nanocrystals of different metal oxides such as Nb2O5, which are otherwise difficult to synthesize by conventional sol-gel methods. Acknowledgment. We acknowledge A.C.C. Yu for helpful discussions and Japan MEXT for partial support of the Nanotechnology Support Project. LA060774E