Dispersion of Fatty Acid Surface Modified Ceria Nanocrystals in

Dec 21, 2009 - Res. , 2010, 49 (4), pp 1947–1952 ..... by a Scientific Research Grant from the Ministry of Education, Science, Sports, and Culture o...
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Ind. Eng. Chem. Res. 2010, 49, 1947–1952

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Dispersion of Fatty Acid Surface Modified Ceria Nanocrystals in Various Organic Solvents Toshihiko Arita,† Yu Ueda,† Kimitaka Minami,‡ Takashi Naka,† and Tadafumi Adschiri*,‡ Institute of Multidisciplinary Research for AdVanced Materials and WPI Research Center: AdVanced Institute for Materials Research, Tohoku UniVersity, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

Ceria (CeO2) nanocyrstals which could be transparently dispersed in several organic solvents were synthesized by organic-ligand-assisted hydrothermal synthesis. We have studied the dispersity of the nanocrystals into typical organic solvents using dynamic light scattering (DLS) measurement. The mean diameter of the dispersant (the nanocrystals’ cluster) varied with changing the solvent species. When the solubility parameter (SP) values of the solvent and the modifier were comparable to each other, the nanocrystals tended to disperse in the solvent with the initial particle size. One of the three-dimensional SPs, i.e., the Hansen solubility parameters, brought more detailed understanding of the mechanism of the dispersion of the surface modified nanocrystals. Because of the dense modifier layer on the surface of the ceria nanocrystals, the hydrogen bonding ability of the solvent was not the dominant factor to determine dispersion of the surface modified nanocrystals. The dispersion and polar factors of the Hansen SPs could describe the ideal condition of perfect dispersion of the surface modified CeO2 nanocrystals. 1. Introduction The composites of matrix/polymer-nanoparticles are recognized to be nanostructured materials advantageous in terms of hybridization between organic and inorganic materials having quite different properties.1 So far, many researchers have tried to do dispersion of inorganic nanoparticles (NPs) into solvents by using dispersants or by surface modification of the inorganic particles.2 The affinity between the modifier and dispersant (solvent) is one of the most essential points in solubility behavior of inorganic nanocrystals in organic media.3–6 Therefore, surface modified NPs (SMNPs), which consist of a metal oxide attached to a dense organic layer, are key materials to achieving the unique structures and, thus, have been widely investigated.7–9 Generally, understanding phase behavior of this pseudobinary system, solvent/SMNPs, is quite important to establishing an aggregation-dispersion control method by tuning various thermodynamic parameters. For exploring the affinity between a solvent and SMNPs, the solubility parameters of solvent and the surface modifier have been considered.5 By use of a surface modifier having a closer solubility parameter to that of a solvent, we have succeeded in demonstrating the fully dispersed state in a pseudobinary system, surface modified metal oxide NP-solvent, at room temperature.10–14 In this paper, we studied the simple concept to explain the dispersion behavior by regarding a hybrid NP as a pseudomolecule. 2. Experimental Section 2.1. Materials. Ce(OH)4 was purchased from Wako Chemicals Ltd. and used as received. The surface modifier used was hexanoic acid, which was from Wako Chemicals Ltd. and used as received. Purified water was purchased from Daiwa Yakuhin Ltd. and filtered before using for hydrothermal synthesis. All the organic solvents used in this study were purchased from Wako Chemicals Ltd. and used as received. The CeO2 nanoparticles for UV-vis spectroscopy were purchased from C. I. * To whom correspondence should be addressed. Fax: +81-22-2175629. E-mail: [email protected]. † Institute of Multidisciplinary Research for advanced Materials. ‡ WPI Research Center: Advanced Institute for Materials Research.

Kasei Co., Ltd. (NanoTek). The average particle size was 14 nm, and the surface area determined BET was 50 m2 g-1. 2.2. Hydrothermal Synthesis of Surface Modified Nanoparticles. The methods to synthesize surface modified nanocrystals in this study were reported elsewhere,12,13 so here we are going to explain in brief. Pressure-resistant tube reactors (Hastelloy C) whose inner volume was 5.0 mL were used for hydrothermal synthesis with in situ surface modification. The reactor was loaded with 4.4 mL of 0.01 M of aqueous suspension of Ce(OH)4. In order to modify the surface of metal oxide nanoparticles, 0.1 mL of the surface modifier was added. The reactors were then capped tightly and put in an electric furnace whose temperature was maintained at 200 °C. The reaction was performed for 10 min including heating time (ca. 3 min) and terminated by quenching the reactor in a water bath. After quenching, the reactor was washed by distilled water and ethanol in turn to collect solid products. The obtained products were purified by three cycles of a combination of decantation and centrifugation using ethanol. Then a similar procedure was performed twice using distilled water and the particles were freeze-dried. 2.3. Characterization and Dynamic Light Scattering Measurement of Surface Modified Nanoparticles. The crystallographic identity of the solid products was evaluated by X-ray diffraction (XRD, RINT-2000 (Rigaku)) measurement comparing with the standard database, Joint Committee for Powder Diffraction Studies (JCPDS) file for CeO2 (No. 340394). The size and shape of the nanoparticles were observed by using transmission electron microscopy (TEM, JEM-1200 EXII (JEOL, Ltd.)). The chemical bonds on the surfaces of products were evaluated by Fourier transform infrared spectroscopy (FTIR, FT/IR-680Plus (JASCO)) and thermogravimetry (TG, TG-8210 (Rigaku)). A Zetasizer Nano Series ((Nano ZS), Malvern Instruments Ltd.) was used for measuring the dynamic light scattering (DLS) of dispersed particles. The DLS was measured by the following procedure. SMNPs of CeO2 were dispersed at a desired concentration in 10 mL (typical concentration was 0.1 wt % except for the concentration-dependence experiment) of various solvents: n-hexane, n-decane, cyclohexane, decalin, ethyl acetate, chloroform, dichloromethane, ben-

10.1021/ie901319c  2010 American Chemical Society Published on Web 12/21/2009

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zene, toluene, anisole, xylene m-cresol, tetrahydrofuran (THF), 1,4-dioxane, acetone, 1-propanol, 2-propanol, acetonitrile, 1-hexanol, and hexanoic acid. With these solvents, the dielectric constant and density could be varied in the range of 1.88-37.5 and 0.71-1.80 g/mL, respectively. The dispersant sizes were calculated based on a multimodal algorithm. In this study, the general purpose (GP) analysis function in the Zetasizer Nano software, a nonnegative least-squares (NNLS) algorithm, was employed to deconvolute the particle size distribution to obtain the gross average diameter of the dispersants in organic solvents. 3. Results and Discussion Long-chain carboxylic acids are widely used as stabilizers in metal oxide NP synthesis, because they can be attached onto the surface of metal oxides to form a dense layer. In this study, hexanoic acid (CH3(CH2)4COOH) was selected as a modifier for CeO2. The crystal structure of the SMNPs was examined by XRD analysis. The XRD diffraction patterns of commercially available CeO2 powders, the CeO2 SMNPs, and commercially available Ce(OH)4 powders are depicted in Figure 1d. Since Figure 1d clearly represents the spectra of commercially available CeO2, the SMNPs and JCPDS standard CeO2 showed good agreement, the SMNPs synthesized in this study must be a CeO2 nanocrystals from the crystallographic point of view. The average size (diameter) of the SMNPs were calculated to be avg. d ) 5 nm from the TEM images shown in Figure 1a. The difference between the size derived from the TEM image (TEM size) and the DLS measurement in chloroform (DLS size) was not significant, and the DLS size was larger than the TEM size. This is usual because we calculated the TEM size of the SMNPs by measuring only the core size of the SMNPs. The SMNPs actually had a dense modifier layer on the surface and the shapes of the SMNPs were not perfect spheres, so the larger size by DLS measurement was reasonable. Furthermore, it is known that the sizes determined by TEM are usually smaller than those determined by any other method. The FTIR spectra shown in Figure 2a may give us important information about how the surface modifier was immobilized on the surface of the CeO2. There was no peak due to the free carboxyl CdO stretching mode (1700 cm-1), but two strong peaks corresponding to the symmetric (1445 cm-1) and asymmetric (1535 cm-1) stretching of carboxylate anion were observed in the spectrum for the SMNPs. This indicates that CH3(CH2)4COOH conjugates with the surfaces of CeO2 by chemisorption between CH3(CH2)4COO- groups and Ce atoms15 on the surface of the CeO2. Then, the nonpolar side of modifier is free and faces the outside; thus, it changes the surface property of NPs from hydrophilic to hydrophobic. The density of the surface modifier on the CeO2 SMNPs was also calculated using the size and morphology information from TEM and the weight ratio from TG analysis, and it turned out to be 5.2 CeO2 molecules/nm2. The solvent-dependent changes of 0.1 wt % dispersion of the SMNP were measured by dynamic light scattering (DLS). The solvents examined in this study were n-hexane, n-decane, cyclohexane, decalin, ethyl acetate, chloroform, dichloromethane, benzene, toluene, anisole, xylene, m-cresol, THF, 1,4-dioxane, acetone, 1-propanol, 2-propanol, acetonitrile, 1-hexanol, and hexanoic acid. As a result of the DLS, we obtained the mean diameters of dispersants in each solvent (see Table 1). After 1 day of storage of the solution, sedimentation of the SMNPs happened in most of the solvents used, i.e., acetone, anisole, m-cresol, n-decane, 1,4-dioxane, n-hexane, 1-hexanol, 1-propanol, and acetonitrile. The rates of settling in the solvents were quite different. However, for the solvents THF, cyclohexane,

Figure 1. TEM image of CeO2 SMNPs capped with hexanoic acid (a), image of perfectly dispersed 0.1 wt % CeO2 SMNPs in CHCl3 (b), and size distribution of CeO2 SMNPs determined by TEM and DLS in CHCl3 (c). XRD spectra of CeO2 powder, SMNPs, and Ce(OH)4 powder (Aldrich) depicted together with the JCPDS standard CeO2 (34-0394) diffraction pattern (d).

chloroform, decalin, and toluene, the dispersity of the SMNPs was stable for 1 month. In the case of THF solution, the solution showed a stronger yellow color than the other solutions especially at concentrations higher than 0.1 wt %, i.e., 1 wt %. It is suggested that CeO2 was dissolved in THF to form Ce ions/complexes. It is well-known that metal ions have strong colors so that even a slight amount of Ce ions dissolved in THF could be a cause of the strong yellow color. In addition, the oxygen center of THF is widely recognized as an ether which can coordinate to Lewis acids such as alkaline metal and alkaline earth metal ions, forming adducts. It is possible to suppose that CeO2 was dissolved in THF. We examined whether CeO2 was dissolved in THF by immersing commercially available CeO2 powder (average d ) 14 nm) in the organic solvents THF and toluene. The image of the solutions (inset of Figure 3) and UV-vis spectra of the solutions supported the assumption that the CeO2 precipitates in THF showed a strong orange color, and the UV-vis spectrum of the supernatant of CeO2 in THF showed an absorption band other than the band gap of CeO2.

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Figure 3. UV-vis spectra and image (inset) of CeO2 particles in THF and toluene. The spectra were recorded with the solutions that did not contain turbidity; i.e., the UV-vis transmittance was measured with only supernatant of the solution waiting until the CeO2 nanoparticle solutions had well settled down. The image was recorded just after mixing of the CeO2 nanoparticles and the solvents.

Figure 2. (a) FTIR spectra of CeO2 SMNPs, bare CeO2 NPs, and hexanoic acid. (b) Thermogravimetry (TG) of CeO2 SMNPs. The measurement was carried out under Ar flow with temperature sweep at 10 °C/min.

The UV-vis spectra of CeO2 in THF and toluene are shown in Figure 3. The new absorption bands in the spectrum of CeO2 in THF had peaks at 340 and 460 nm. The images of CeO2 solutions after hours of the mixing are depicted in the Supporting Information. It was confirmed that the dispersity of CeO2 particles was far better in THF than in toluene. In THF, CeO2 could disperse stably more than 1 day and have a yellow color whereas they could not disperse even just after dissolution in toluene and the color was white. It was suggested that the THF molecules coordinated to the Ce atoms on the surface and formed adducts; then the adducts could be regarded as a kind of pseudo surface modified nanoparticle or as a stable complex

of Ce ion. Both the pseudo surface modified CeO2 and the Ce ion complex could disperse well in THF because of the pseudo modified layer on the surface and the ligand conjugated to the dissolved Ce ion. Figure 4 depicts the size distributions of modified CeO2 NPs in several organic solvents measured by DLS. Dispersity of the SMNPs in solvents depended predominantly on the feature of the organic molecule on the surface of the crystal which interacted with solvent molecules. It is well-known that when the surface of an inorganic particle is covered densely and uniformly by organic molecules, the effect will be significant. The surface modifier density of the CeO2 SMNPs used in this work was calculated to be 5.2 molecules/nm2. The dense surface molecules occupy more than 70% in the effective volume of the organic shell of the SMNP and modify the interaction between the SMNPs. Figure 5 shows the DLS size of the aggregates/clusters as a function of solubility parameter, δ, and dielectric constant, ε, of the solvents. All the DLS sizes measured and the parameters for the solvents used in this study are listed in Table 1. ε, one of the most common parameters to classify solvent properties,

Table 1. Mean Diameter of the Dispersant Determined by DLS and Dielectric Constants, Solubility Parameters of Solvents, and Hansen Solubility Parameters of Solvents Used in This Study Hansen solubility parameters (MPa1/2) solvent

ε

δ (MPa1/2)

δt

δd

δp

δh

DLS mean diam (nm)

acetone acetonitrile anisole benzene chloroform cyclohexane m-cresol decalin n-decane dichloromethane 1,4-dioxane ethyl acetate n-hexane 1-hexanol 1-hexanoic acid 1-propanol 2-propanol THF toluene xylene

20.7 37.5 4.33 2.28 4.9 2.05 11.8 2.18 1.99 9.1 2.21 6.02 1.89 13.3 2.63 22.2 18.3 7.58 2.24 2.27

20.3 24.3 19.4 18.8 19 16.8 20.9 18 13.5 20.3 20.5 18.6 14.9 21.9 18.2 24.3 23.5 18.6 18.2 18.1

20.1 24.6 19.4 18.6 19 16.8 22.7 18.8 15.8 20.3 20.5 18.2 14.9 21.7 18.2 24.6 23.5 19.4 18.2 18

15.5 15.3 17.8 18.4 17.8 16.8 18 18.8 15.8 18.2 19 15.8 14.9 16 15 16 15.8 16.8 18 17.8

10.4 18 4.1 0 3.1 0 5.1 0 0 6.3 1.8 5.3 0 4.5 3.7 6.8 6.1 5.7 1.4 1

7 6.1 6.8 2 5.7 0.2 12.9 0 0 6.1 7.4 7.2 0 6.8 9.4 17.4 16.4 8 2 3.1

702 482 510 130 6.1 16.8 278 32.3 595 86.5 346 305 381 437 126 414 208 4.2 8.48 45

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modified layer on the surfaces. The surface modified layer could shield electrostatic interactions of the metal oxide surfaces so that the SMNPs would have stronger affinity with nonpolar organic solvents. The molecular interactions between a modifier and a solvent can also be estimated by δ of the solvents.16 The solubility parameter δi of a pure liquid i is defined by δi ) (cii)1/2 )

Figure 4. Size distributions of CeO2 SMNPs in various organic solvents measured by DLS.

Figure 5. Size variation of modified CeO2 SMNPs as a function of solubility parameter, δ (left), and dielectric constant, ε (right), of various solvents. Vertical lines drawn in both plots represent the values for hexanoic acid. The solubility parameter could help to estimate the dispersity of SMNPs in solvents better than the dielectric constant.

clearly represents that the SMNPs used in this study can be more dispersed in the solvents which have ε values lower than 7.5. This is reasonable because the SMNPs have a dense surface

( ) ∆Evi VLi

1/2

(1)

where cii is the cohesive energy density of the pure liquid i, ∆Evi is the energy required to isothermally evaporate liquid i from the saturated liquid to the ideal gas, and VLi is the liquid molar volume of pure liquid i. There are some restrictions in using the solubility parameter theory. Only energetic contributions to the mixing process are involved, and entropic effects are not considered. Moreover, the heat of mixing is estimated from properties of the pure substances. Despite these shortcomings, the solubility parameter approach is convenient to use and helpful as a mean of estimating the expected compatibility between the membrane polymer phase and the species to be separated. Simply stated, the definition of δ is the square root of the cohesive energy density, so δ could standardize the cohesive force of solvent molecules.17 If the solubility parameter difference between the SMNP and the solvent, ∆δ, is small, it is expected the SMNP can be dissolved in solvent with good dispersity. In this case, DLS size will be close to the individual SMNP size. The left-hand side of Figure 5 shows that the DLS sizes of the CeO2 SMNPs exhibit minima of ∼4 nm at δmin ) 17-19. By means of the strong chemisorption of the modifier to the surface of the CeO2 SMNPs, many CH3(CH2)4COO groups exist on the surface of nanocrystals of CeO2. Vertical lines drawn in Figure 5 indicate the solubility parameter, δhex, and dielectric constant, εhex, of hexanoic acid. Both lines locate roughly at the minimum δmin and εmin; however, δ expressed that the ideal condition of the solvents for perfect dispersion of the CeO2 SMNPs looks better than that of ε expressed to a degree. This suggests that the solubility parameter could be a better parameter to standardize views on the solvent-dependent dispersity of the SMNPs. It is also suggested that the similarity of molecular structures between a solvent and a modifier would not be an effective factor to obtain better dispersion. n-Decane and n-hexane, with the molecular structures closer to that of

Figure 6. (a) Three-dimensional plot of HSPs for the solvents used in this study and (b) HSPs for the solvents used in this study as a function of δd and δp. Filled circles represent the solvents in which the CeO2 SMNPs showed good dispersity (DLS size < 100 nm). Filled triangles represent the solvents in which the CeO2 SMNPs showed the second-best dispersity (100 nm < DLS size < 200 nm). The large circle with the star center in (b) represents the possible optimum area for the perfect dispersion of CeO2 SMNPs. For details, see the text.

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hexanoic acid than the other solvents, did not show better dispersity and smaller DLS size. This indicates that because of the far larger size of SMNPs than the solvent molecules, we need to apply viewpoints other than similarity of the molecular structures in dispersion of SMNPs in solvent, like the cases of polymer solutions. Hansen’s three-dimensional solubility parameters (HSPs) δd (dispersion component value), δp (polarity component value), and δh (hydrogen bonding component value) can also be used for assessment of the relative affinity of an SMNP and solvent.18 Hansen assumed that the cohesive energy is responsible for the contributions from nonpolar interactions, polar interactions, and hydrogen bonding interactions. This approach divides the contributions to the overall solubility parameter into three components representing nonpolar, polar, and hydrogen bonding contributions to the cohesive energy density. Thus, the Hansen solubility parameters (δd, δp, δh) have been combined as follows: δt2

) δd + δp + δh 2

2

2

(2)

The total solubility parameter, δt, corresponds to the overall solubility parameter. According to the theory, we could obtain more detailed aspects in SMNP dispersion in solvents. Figure 6 depicts the three- and two-dimensional (3D and 2D) plots of HSPs of the solvent used for the DLS measurements. It is difficult to obtain clear insight to understand the dispersion of SMNPs in solvents from the 3D plot of HSPs shown in Figure 6a as expected from an imperfect correlation between DLS size and the one-dimensional SP of solvents shown in Figure 5; however, the δd vs δp plot shown in Figure 6b gives us important information. As mentioned above, the SMNPs used in this study have a dense surface modifier layer as the shell so that it is not necessary to include effects of hydrogen bonding between the SMNPs and solvent molecules. The δd vs δp plot eliminates the contribution from hydrogen bonding ability; then the filled circles gather together within the large circle, which has a radius of 2 MPa1/2, in Figure 6b. The filled circle outside the large circle represents THF, and the filled triangle outside the large circle represents hexanoic acid. As mentioned previously, THF can form adducts with CeO2 and enhance the solubility of CeO2 in THF. In addition, hexanoic acid is the surface modifier of CeO2 and the hexanoic acid molecules inside and outside the modified layer can exchange with enhancing the solubility of SMNPs. We eliminated these two points from the large circle as exceptions because the reasons for enhanced solubility in these two solvents would be different from those for the other solvents employed in this study. The large circle in Figure 6b may indicate the optimum solvent properties required for the perfect dispersion of CeO2 SMNPs. Figure 7 depicts the concentration-dependent changes of the size of dispersant measured by DLS in organic solvents. In THF and chloroform, the CeO2 SMNPs could disperse well until high concentration, whereas in n-hexane, the SMNPs could not disperse with the single NP size even from the lowest concentration. In toluene and dichloromethane, the size of dispersant varied with increasing concentration of the CeO2 SMNPs. The concentration-dependent dispersion as well as stability (time-dependent change) of the dispersion must be a next target of this study. 4. Conclusion In conclusion, hexanoic acid surface modified CeO2 nanocyrstals were successfully synthesized by a hydrothermal method. It turned out that the nanocrystals could be perfectly

Figure 7. Relation between CeO2 SMNP concentration and size of dispersants measured by DLS in various organic solvents.

dispersed in cyclohexane, chloroform, decalin, toluene, and THF. CeO2 as well as the surface modified CeO2 nanocyrstals might dissolve in THF and/or form an adduct with THF so that they showed very good dispersity into THF. DLS was employed to measure the diameter of dispersants in several organic solvents. According to the size of dispersant determined by DLS, the surface modified nanocrystals seemed to well disperse in the solvent which had a solubility parameter similar to that of the modifier layer. Using the Hansen solubility parameters, a more detailed description of and more insight into dispersity of the surface modified nanocrystals in solvents were obtained. Because of the dense modifier layer on the surface, the hydrogen bonding ability of the CeO2 surfaces could be minimized. The condition of perfect dispersion of the hexanoic acid modified CeO2 NPs in solvents could be depicted by the δd (dispersion component of the Hansen solubility parameters) vs δp (polarity component of the Hansen solubility parameters) plot of solvents. Acknowledgment This work was supported by a Scientific Research Grant from the Ministry of Education, Science, Sports, and Culture of Japan. This research was also partly supported by a grantin-aid for the COE project, Giant Molecules and Complex Systems, 2002, and KAKENHI (17206078, 19310063, 19750094, and 20226015). Supporting Information Available: Images of CeO2 solutions after hours of mixing. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Lai, S. L.; Carlsson, J. R. A.; Allen, L. H. Melting point depression of Al clusters generated during the early stages of film growth: Nanocalorimetry measurements. Appl. Phys. Lett. 1998, 72 (9), 1098. (2) Bourgeat-Lami, E. Organic-Inorganic Nanostructured Colloids. J. Nanosci. Nanotechnol. 2002, 2, 1–24. (3) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Synthesis and characterization of monodisperse nanocrystals and close-packed anocrystal assemblies. Annu. ReV. Mater. Sci. 2000, 30, 545–610. (4) Huang, S.; Sakaue, H.; Shingubara, S.; Takahagi, T. Self-Organization of a Two-Dimensional Array of Gold Nanodots Encapsulated by Alkanethiol. Jpn. J. Appl. Phys. 1998, 37, 7198–7201. (5) Huang, S.; Minami, K.; Sakaue, H.; Shingubara, S.; Takahagi, T. Optical spectroscopic studies of the dispersibility of gold nanoparticle solutions. J. Appl. Phys. 2002, 92 (12), 7486.

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(6) Huang, S.; Tsutsui, G.; Sakaue, H.; Shingubara, S.; Takahagi, T. Experimental conditions for a highly ordered monolayer of gold nanoparticles fabricated by the Langmuir-Blodgett method. J. Vac. Sci. Technol., B: Microelectron. Nanometer Struct. 2001, 19 (6), 2045–2049. (7) Schmidt, H. Nanoparticles by chemical synthesis, processing to materials and innovative applications. Appl. Organomet. Chem. 2001, 15, 331–343. (8) Manjula, S.; Kumar, S. M.; Madhu, M. G.; Suresh, R.; Raj, M. A. A sedimentation study to optimize the dispersion of alumina nanoparticles in water. Ceramica 2005, 51, 121–127. (9) Yin, M.; Willis, A.; Redl, F.; Turro, N. J.; O’Brien, S. P. Influence of capping groups on the synthesis of γ-Fe2O3 nanocrystals. J. Mater. Res. 2004, 19, 1208–1215. (10) Adschiri, T. Supercritical Hydrothermal Synthesis of OrganicInorganic Hybrid Nanoparticles. Chem. Lett. 2007, 36 (10), 1188–1193. (11) Mousavand, T.; Ohara, S.; Umetsu, M.; Zhang, J.; Takami, S.; Naka, T.; Adschiri, T. Hydrothermal synthesis and in situ surface modification of boehmite nanoparticles in supercritical water. J. Supercrit. Fluids 2007, 40 (3), 397–401. (12) Mousavand, T.; Takami, S.; Umetsu, M.; Ohara, S.; Adschiri, T. Supercritical hydrothermal synthesis of organic-inorganic hybrid nanoparticles. J. Mater. Sci. 2006, 41 (5), 1445–1448.

(13) Takami, S.; Sato, T.; Mousavand, T.; Ohara, S.; Umetsu, M.; Adschiri, T. Hydrothermal synthesis of surface-modified iron oxide nanoparticles. Mater. Lett. 2007, 61 (26), 4769–4772. (14) Zhang, J.; Ohara, S.; Umetsu, M.; Naka, T.; Hatakeyama, Y.; Adschiri, T. Colloidal Ceria Nanocrystals: A Tailor-Made Crystal Morphology in Supercritical Water. AdV. Mater. 2007, 19 (2), 203–206. (15) Kaneko, K.; Inoke, K.; Freitag, B.; Hungria, A. B.; Midgley, P. A.; Hansen, T. W.; Zhang, J.; Ohara, S.; Adschiri, T. Structural and Morphological Characterization of Cerium Oxide Nanocrystals Prepared by Hydrothermal Synthesis. Nano Lett. 2007, 7 (2), 421–425. (16) Hamley, I. W. Introduction of Soft MattersPolymer, Colloids, Amphiphiles and Liquid Crystals; John Wiley & Sons: New York, 2000. (17) Poling, B. E.; Prausnitz, J. M.; O’Connell, J. P. The properties of gases and liquids, 5th ed.; McGraw-Hill: New York, 2001. (18) Hansen, C. M. I. Solvents, plasticizers, polymers, and resinssthe three dimensional solubility parameterskey to paint component affinities. J. Paint Technol. 1967, 39 (505), 104–117.

ReceiVed for reView August 24, 2009 ReVised manuscript receiVed October 14, 2009 Accepted December 9, 2009 IE901319C