4086
Langmuir 2000, 16, 4086-4089
Synthesis of ZnO Particles by Ammonia-Catalyzed Hydrolysis of Zinc Dibutoxide in Nonionic Reversed Micelles Daisuke Kaneko,*,† Hideki Shouji,† Takeshi Kawai,†,‡ and Kijiro Kon-No†,‡ Department of Industrial Chemistry, Science University of Tokyo, and Institute of Colloid and Interface Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-0825, Japan Received July 13, 1999. In Final Form: January 26, 2000 Ultrafine ZnO particles were synthesized by ammonia-catalyzed hydrolysis of zinc di-n-butoxide (ZDB) in polyoxyethylene (6) nonylphenyl ether (NP-6)/cyclohexane solutions in both the reversed and swollen micelle regions of the solubilization diagram of aqueous NH3. In all micelle regions, rodlike ZnO particles were produced which were determined to be hexagonal single crystals having a wurtzite structure. The effects of changing Rw (the molar ratio of water to surfactant ) [H2O]/[NP-6]), Ra (the molar ratio of ammonia to surfactant ) [NH3]/[NP-6]), and ZDB on the length (〈PL〉) and width (〈PW〉) of the ZnO particles were examined. The values of both 〈PL〉 and 〈PW〉 increased slowly from about Rw ) 4. This increase reflected a change in the system, from reversed micelles with water bound to the polar groups of the surfactants to swollen micelles with semibound water interacting with hydrated polar groups. At a constant Rw (Rw ) 6.0), as Ra was increased, both 〈PL〉 and 〈PW〉 slowly decreased. The 〈PL〉 and 〈PW〉 profiles of the ZnO particles at various Rw or Ra values were similar to the micellar droplet size profile at various Rw or Ra values, respectively. These facts indicate that the particle size of ZnO is controlled by both the solubilized states of water and the size of the micellar droplets. However, the 〈PL〉 and 〈PW〉 of the synthesized particles were independent of the ZDB. The 〈PL〉 and 〈PW〉 values also correlated to the average crystallite sizes of the (002) and (110) faces of the particles as determined by X-ray diffraction measurements. The long side of the rodlike particle was also found to be parallel to the c-axis of the hexagonal system.
Introduction The reversed-micelle (RM) method of synthesizing colloidal particles, using well-defined nanosize water droplets in the interior of reversed micelles, is distinguished by the formation of uniform, ultrafine particles. The method also leads to stable dispersion systems. A significant amount of data has been accumulated about the factors affecting particle size, the mechanism and kinetics of particle formation, and the crystalline characteristics of particles such as metals,1-4 metal oxides,5-8 metal sulfides,9 silver halides,10 and metal carbonates.11 Despite the dependence of those data on the solubilized states of water droplets as a reaction field, little attention has been paid to the solubilized states, except in our work on the preparation of TiO2 and GeO2 particles in nonionic and ionic reversed micelles.12-14 * Author to whom correspondence should be addressed. † Science University of Tokyo. ‡ Institute of Colloid and Interface Science. (1) Kurihara, K.; Kizling, J.; Stenius, P.; Fendler, J. H. J. Am. Chem. Soc. 1983, 105, 2574. (2) Barnickel, P.; Wokaum, A.; Sager, W.; Eicke, H. F. J. Colloid Interface Sci. 1992, 148, 80. (3) Petti, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 12974. (4) Boutonnet, M.; Kizling, J.; Stenius, P.; Maire, G. Colloids Surf. 1982, 5, 209. (5) Gobe, M.; Kon-No, K.; Kitahara, A. J. Colloid Interface Sci. 1983, 93, 293. (6) Joselevich, E.; Willner, I. J. Phys. Chem. 1994, 98, 7628. (7) Chang, C.; Fogler, H. S. Langmuir 1997, 13, 3295. (8) Arriagada, F. J.; Osseo-Asare, K. J. Colloid Interface Sci. 1995, 170, 8. (9) Lianos, P.; Thomas, J. K. Chem. Phys. Lett. 1986, 125, 299. (10) Chew, C. H.; Gan, M.; Shah, D. O. J. Dispersion Sci. Technol. 1990, 11, 593. (11) Kandori, K.; Kon-No, K.; Kitahara, A. J. Colloid Interface Sci. 1988, 122, 78. (12) Kaneko, D.; Kawai, T.; Kon-No, K. J. Jpn. Soc. Colour Mater. 1998, 71, 225.
Many industries utilize the photoconductivity and fluorescence properties of ZnO.15,16 Nanoparticles have been the focus of many recent studies, because of their finite size and the fact that their large surface-to-volume ratio produces novel properties not exhibited by the bulk material.17-19 Such properties are intrinsically interesting and have important applications in areas such as microelectronics, catalysis, and optical communications. In this study, the solubilization diagram of aqueous ammonia in polyoxyethylene (6) nonylphenyl ether (NP-6)/cyclohexane solutions was constructed as a function of Rw, the molar ratio of water to surfactant () [H2O]/ [NP-6]), and Ra, the molar ratio of ammonia to surfactant () [NH3]/[NP-6]). The preparation of ZnO particles by ammonia-catalyzed hydrolysis of zinc di-n-butoxide (ZDB) was undertaken in the reversed and swollen micelle regions which contained different solubilized states of water. The factors controlling the size and crystalline characteristics of the obtained particles were examined. Experimental Section Materials. Polyoxyethylene (6) nonylphenyl ether (NP-6) (Lion Co, Ltd., Japan) was purified according to a method previously described,20 wherein the water-soluble materials were extracted with water from the NP-6/1-butanol solution several times and the NP-6 was dried at 80 °C under reduced pressure. (13) Kon-No, K. Fine Particle Science and Technology; Kluwer Academic Publishers: The Netherlands, 1996; p 431. (14) Kawai, T.; Usui, Y.; Kon-No, K. Colloids Surf. 1999, 149, 39. (15) Bahnemann, D. W.; Kormann, C.; Hoffmann, M. R. J. Phys. Chem. 1987, 91, 3789. (16) Koch, U.; Fojik, A.; Weller, H.; Henglein, A. Chem. Phys. Lett. 1985, 122, 507. (17) Kamat, P. V. Chem. Rev. 1993, 93, 267. (18) Henglein, A. Chem. Rev. 1989, 89, 1861. (19) Hoffman, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (20) Kitahara, A.; Kon-No, K. J. Phys. Chem. 1966, 70, 3394.
10.1021/la990931s CCC: $19.00 © 2000 American Chemical Society Published on Web 03/28/2000
Ammonia-Catalyzed Hydrolysis of Zinc Dibutoxide
Figure 1. Phase diagram of an aqueous NH3 solution in 0.1 mol kg-1 NP-6/cyclohexane at various Rw and Ra as determined by NMR and NIR techniques. Line A shows a saturated ammonia solution (29%). The four regions are: I, reversed micelle; II, swollen micelle; III, W/O microemulsion; and IV, two-phase separation. Zinc di-n-butoxide (ZDB) having a purity of 99.99% (High Purity Chemicals Co. Ltd., Japan), reagent-grade cyclohexane, and an aqueous ammonia solution consisting of 71% water and 29% ammonia were used without further purification. Measurement of Limiting Amount of Solubilized Aqueous NH3. The limiting amount of aqueous NH3 in cyclohexane solutions with 0.1 mol kg-1 NP-6 was determined by observing the point at which further additions of aqueous NH3 increased turbidity beyond that of the original surfactant solution at 25 °C. Measurement of 1H NMR, the Near-Infrared Spectra of Solubilized Water. The chemical shift (δH) of the solubilized water was measured using a Hitachi R-24B at 25 °C. TMS was used as an internal standard. The near-infrared (NIR) spectra of mixtures (90% D2O + 10% H2O) solubilized by 0.1 mol kg-1 NP-6 were measured using a Hitachi Model 330 spectrophotometer. D2O was used to prevent the coupling of OH stretches within one water molecule. A solubilized solution of D2O and aqueous ND3 was used as a reference. Measurement of Micellar Droplets. An Otuka Electronics DLS 700 spectrometer was used to measure the size of the micellar droplets at 25 °C. Using the translational diffusion coefficient of micelles obtained from the correlation function and assuming the micelles are spheres, their apparent hydrodynamic diameter (Dh) was calculated from the Einstein-Stokes equation.21 Synthesis of Particles. ZnO particles were synthesized at 25 °C by mixing ZDB powder with 0.10 mol kg-1 NP-6/cyclohexane solutions containing an amount of aqueous NH3 that provided a desirable Rw and Ra. The concentration of ZDB was in the range of 0.02-0.10 mol kg-1. Measurement of Size and Morphology, and Identification of Particles. The morphology of the particles was observed using a Hitachi H-9000 transmission electron microscope after a 1-week reaction time, because FT-IR spectra showed that all of the ZDB was consumed within 1 week. Rodlike particles were obtained, and the average particle length (〈PL〉), average particle width (〈PW〉), and standard deviations (σ) of the length and width of approximately 200 particles on the micrographs were calculated. The particles were identified using a Rigaku RAD X-ray diffractometer. Electron diffraction measurements were also made.
Langmuir, Vol. 16, No. 9, 2000 4087
obtained above line A, which represents the saturated ammonia solution (29%), can be divided into three regions, I, II, and III (Figure 1). Region IV is the turbid region where the solution separated into two phases. Each division line in the diagram was determined according to the previously described procedure.22-24 NMR was used to measure the chemical shift of water in the system. Rw values corresponding to the two points where the slope changed in the chemical shift of water (δH) - Rw profiles, measured at various Rw/Ra ratios, are plotted in the phase diagram in Figure 1. The existence of points at which the slope in the δH - Rw profiles changes suggests that different types of water are present during interaction with the oxyethylene moieties of the surfactants. This conclusion is supported by the NIR spectrum of water in the region of the OH overtone frequencies (2υOH) agreeing with what has been previously reported.22-24 The intensity of the 1420-nm band, which was assigned to the “free” OH (nonhydrogen-bonded water) species, gradually increased with increasing Rw at a given Ra, whereas the intensity of the 1650-nm band, which was assigned to the hydrogenbonded species, increased above about Rw ) 4 in region II and increased markedly in region III. These spectral features suggest that three states of water are present in this system. In region I, below about Rw < 4, water binds directly to the oxyethylene moieties of the surfactants. In region II, with Rw between approximately 4 and 7, the excess water binds to the hydrated oxyethylene moieties through a hydrogen bond. In region III, above about Rw > 7, bulklike water builds up in the interior of the micelles. Such solubilized states of water have previously been detected by NIR spectroscopy in similar surfactant systems.22-24 The micelle behavior in each region was further examined by the dynamic light-scattering method. The apparent hydrodynamic diameter of the micelles (Dh) was measured at various Rw in the system with Ra ) 0.5. Keeping Ra constant (Ra ) 0.5), as Rw was increased in the range above Rw ) 4, the values of Dh increased rapidly. However, when Rw was kept constant (Rw ) 6) and Ra was increased, the values of Dh decreased (Figure 2). Comparison of Figures 2 and 1 revealed that this rapid increase in Dh at Rw ) 4 results from the change from region I to region II. This observation suggests that region I should be regarded as a system of reversed micelles (RMs) and region II as a system of swollen micelles (SMs), as has previously been proposed.22-24 Region III was previously proposed to be a W/O microemulsion (ME) system because of the existence of bulklike water. The hydrolysis of ZDB was undertaken in the different micellar states identified in the phase diagram shown in Figure 1. Factors Affecting Particle Size. When the hydrolysis of ZDB was undertaken in the swollen micelle region of Rw ) 6 without ammonia (Ra ) 0), aggregates approximately 500 nm in diameter were formed. However, monodispersed, rodlike particles were produced in the presence of ammonia (Figure 3). Monodispersed, rodlike particles similar to those produced in region II also were produced in region I. These particles are bigger than the ZnO nanoparticles produced by the sol-gel method,15 which were spherically shaped and had a diameter of 5 nm. However, the RM method is a simpler process in comparison to the sol-gel method. In addition, heat does not need to be applied in the RM method.
Results and Discussion Solubilized States of Aqueous NH3. The phase diagram of the solubilization region of aqueous ammonia (21) Nishikido, N.; Shinozaki, M.; Sugihara, G.; Tanaka, M. J. Colloid Interface Sci. 1981, 82, 352.
(22) Kawai, T.; Shindou, N.; Kon-No, K. Colloid Polym. Sci. 1995, 273, 195. (23) Kawai, T.; Hamada, K.; Shindou, N.; Kon-No, K. Bull. Chem. Soc. Jpn. 1992, 65, 2715. (24) Kawai, T.; Hamada, K.; Kon-No, K. Bull. Chem. Soc. Jpn. 1993, 66, 2804.
4088
Langmuir, Vol. 16, No. 9, 2000
Figure 2. Change in the apparent hydrodynamic diameter of micelles (Dh) with Rw and Ra in 0.1 mol kg-1 NP-6/cyclohexane systems.
Kaneko et al.
Figure 4. X-ray powder patterns of particles. (a) X-ray powder pattern of particles synthesized in the system with no ammonia (Rw ) 6.0 and Ra ) 0). (b) X-ray powder pattern of particles synthesized in the system with ammonia (Rw ) 6.0 and Ra ) 0.5). The peaks labeled with open circles were assigned to ZnO, and those labeled with closed circles were assigned to Zn(OH)2.
Figure 3. TEM photograph of ZnO particles synthesized at Rw ) 3.0, Ra ) 0.5 and ZDB ) 0.04 mol kg-1.
The X-ray diffraction patterns of the synthesized particles are shown in Figure 4. Figure 4a is the diffraction pattern of aggregates produced in a system without ammonia present, and Figure 4b is the diffraction pattern of rodlike particles produced in a system with ammonia present. Figure 4 indicates that these ZnO particles are hexagonal ZnO having a wurtzite structure. However, the diffraction pattern of the particles in the system without ammonia (Figure 4a) contained peaks that were assigned to Zn(OH)2, in addition to peaks that were assigned to ZnO. These results indicate that ammonia not only is necessary for the formation of rodlike particles, but also plays a role as condenser of the Zn(OH)2 precursor. A similar effect of ammonia was observed in the formation of spherical SiO2 particles produced by the RM method.7,8 These SiO2 particles were amorphous, but the particles could not form without ammonia present. The preparation of ZnO particles was next undertaken at various Rw and Ra values and with varying ZDB amounts. Figure 5 shows the average particle length (〈PL〉) and average particle width (〈PW〉) of ZnO particles produced in systems with various values of Rw and Ra ) 0.5. Using
Figure 5. The average particle length 〈PL〉 and average particle width 〈PW〉, and the normalized standard deviation σ/〈PL〉, σ/〈PW〉, of the synthesized ZnO particles in systems with various Rw and Ra ) 0.5. The average crystallite size of the (110) and (002) faces is also plotted.
the Scherrer equation,25 volume-averaged crystallite sizes were calculated from the peaks in Figure 4, which appeared at 2θ ) 56° and 34.2° corresponding to the (110) and (002) faces. The peaks are also plotted in Figure 5. The (002) face is vertical and the (110) face is parallel to the c-axis in the hexagonal system. The calculated crystallite sizes of (002) coincided very well with the number averaged values of 〈PL〉, and those of (110) agreed with the 〈PW〉 values of the particles. This fact further (25) Rau, R. C. Norelco Rep. 1963, 10, 114.
Ammonia-Catalyzed Hydrolysis of Zinc Dibutoxide
Langmuir, Vol. 16, No. 9, 2000 4089
Figure 6. The average particle length 〈PL〉 and average particle width 〈PW〉, and the normalized standard deviation σ/〈PL〉, σ/〈PW〉, of the synthesized ZnO particles in systems with various Ra and Rw ) 6.0. The average crystallite size of the (110) and (002) faces is also plotted.
Figure 7. The average particle length 〈PL〉 and average particle width 〈PW〉, and the normalized standard deviation σ/〈PL〉, σ/〈PW〉, of the synthesized ZnO particles in systems with various ZDB at Rw ) 3, Ra ) 0.5. The average crystallite size of the (110) and (002) faces is also plotted.
supports the conclusion that the ZnO particles are single crystals. The electron diffraction results showed clear diffraction spots which indicated that the ZnO particles were also single crystals. The long side of the rodlike particle was also found to be parallel to the c-axis of the hexagonal system. Comparison of Figures 5 and 1 shows that the values of 〈PL〉 and 〈PW〉 both increased as Rw increased in the range above Rw ) 4, which indicates a change from reversed micelles containing bound water to swollen micelles which included semibound water. Comparison of Figures 5 and 2 shows that the particle size increased with increasing Dh as shown in Figure 2. This result may be explained by an increase in the number of reactants per micellar droplet. Hence, these facts may indicate that the size of the ZnO particles is controlled by both the solubilized states of water and the size of the micellar droplets. Such size control by micellar droplet size was also observed in ZnO particles produced in systems with various values of Ra at constant Rw (Figures 6 and 2). Considering the effect of the ZDB in systems of constant Rw and Ra, it was expected that the particle size would increase with increasing ZDB because the number of reactants per micellar droplet increases. Contrary to our expectations, however, the ZnO particle size was nearly constant as the ZDB increased, as shown in Figure 7. In systems with constant Ra and Rw and variable ZDB, the only difference may be in the number of final particles produced. Although the number of final ZnO particles was not determined, it would be less than the number of micellar droplets.22 Therefore, the particle size would not be affected by the ZDB.
From these results, it was assumed that ZDB reacted with water and that crystal growth occurred in the droplet. Therefore, it was concluded that the particle size was controlled by the droplet size, and a single crystal was produced. Conclusion Rodlike ZnO nanosize particles could be synthesized using the reversed micelle method in NP-6/aq NH3/ cyclohexane systems. The micellar region could be divided into three regions, i.e., reversed micelle, swollen micelle, and W/O microemulsion, depending on the solubilized state of water. The size of the particle depended on the region and it was controlled by both the state of water and the micellar droplet size. The rodlike particle was found to be a single crystal having a wurtzite structure, and the long side of the particle was parallel to the c-axis of the hexagonal system. Notation Rw, molar ratio of water to surfactant () [H2O]/[NP-6]); Ra, molar ratio of ammonia to surfactant () [NH3]/ [NP-6]); 〈PL〉, average particle length, nm; 〈PW〉, average particle width, nm; σ, standard deviation of particle length, particle width; δH, chemical shift of water, ppm; Dh, apparent hydrodynamic diameter of micelle, nm. LA990931S
