Dispersion Behavior of Zirconia Nanocrystals and Their Surface

Jul 31, 2007 - Jonathan De Roo , Sofie Coucke , Hannes Rijckaert , Katrien De Keukeleere , Davy Sinnaeve , Zeger Hens , José C. Martins , and Isabel ...
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Langmuir 2007, 23, 9178-9187

Dispersion Behavior of Zirconia Nanocrystals and Their Surface Functionalization with Vinyl Group-Containing Ligands Shuxue Zhou,*,† Georg Garnweitner,‡ Markus Niederberger,§ and Markus Antonietti*,† Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, Research Campus Golm, D-14424 Potsdam, Germany, Institute of Particle Technology, Technical UniVersity of Braunschweig, Volkmaroder Strasse 5, D-38104 Braunschweig, Germany, Department of Materials, Swiss Federal Institute of Technology (ETH) Zu¨rich, Wolfgang-Pauli-Strasse 10, CH-8093 Zu¨rich, Switzerland ReceiVed March 22, 2007. In Final Form: May 21, 2007 The dispersion behavior of crystalline zirconia nanoparticles with a diameter of 3.8 nm, synthesized from zirconium(IV) isopropoxide and benzyl alcohol in tetrahydrofurane (THF), methyl methacrylate (MMA), and styrene (St), was investigated using 3-(trimethoxysilyl)propyl methacrylate (MPS), ethyl 3,4-dihydroxycinnamate (EDHC), allylmalonic acid (AMA), and trimethylolpropane mono allyl ether (TMPMA) as ligating stabilizers containing polymerizable vinyl groups. Analytical ultracentrifugation (AUC) and transmission electron microscopy (TEM) analyses prove that the as-synthesized wet zirconia nanoparticles can be dispersed in THF without any agglomeration when using the appropriate ligand concentrations. Surface-adsorbed water, if intentionally introduced during the washing step, and also air humidity seriously deteriorate their dispersibility. These results suggest that the excellent dispersibility of the zirconia nanoparticles is a direct consequence of the nonaqueous synthesis approach. Fourier transform infrared spectra (FTIR) and thermogravimetric analysis (TGA) illustrate that MPS, EDHC, and AMA are chemically attached but TMPMA is physically attached to the surface of the zirconia nanoparticles. Transparent dispersions of zirconia nanoparticles can also be prepared in MMA with the help of MPS, EHDC, and AMA or in St with MPS and TMPMA, opening a promising pathway for the direct application of zirconia nanoparticles in polymer-based nanocomposites.

Introduction Because agglomerates and aggregates of nanoparticles deteriorate the mechanical properties and reduce the optical transparency of polymer-based nanocomposites,1 a perfect dispersion of nanoparticles in organic solvents or monomers, without the presence of agglomerates or aggregates, is crucial for the fabrication of high-performance polymer-based nanocomposites. Crystalline metal oxide nanoparticles are especially difficult to disperse homogeneously in organic media. The commercially available crystalline metal oxide nanoparticles are typically prepared either by flame aerosol synthesis, chemical precipitation methods, or the aqueous sol-gel process, which brings a number of specific disadvantages. These nanopowders usually consist of aggregates because of the high reaction temperature or are due to a calcination step required for inducing the crystallization of the as-synthesized metal oxide nanoparticles, or they are simply too hydrophilic to disperse into polymers. Generally, a process combining the use of surface ligands such as silane or titanate coupling agents,2-4 oleic acid,5,6 or other surfactants7 with extensive ball milling and/or sonication treatment has to be employed to enable dispersion in organic solvents, monomers, * Corresponding authors. (S.Z.) Fax: +49-331-5679502. E-mail: shuxue. [email protected], [email protected]. (M.A.) markus. [email protected]. † Max Planck Institute of Colloids and Interfaces. ‡ Technical University of Braunschweig. § Swiss Federal Institute of Technology (ETH) Zu ¨ rich. (1) Chen, G. D.; Zhou, S. X.; Gu, G. X.; Yang, H. H.; Wu, L. M. J. Colloid Interface Sci. 2005, 281, 39-350. (2) Posthumus, W.; Magusion, P. C. M. M.; Brokken-Zijp, J. C. M.; Tinnemans, A. H. A.; van der Linde, R. J. Colloid Interface Sci. 2004, 269, 109-116. (3) Chen, H.; Zhou, S. X.; Gu, G. X.; Wu, L. M. J. Dispersion Sci. Technol. 2004, 25, 837-848. (4) Cho, Y. S.; Yi, G. R.; Hong, J. J.; Jang, S. H.; Yang, S. M. Thin Solid Films 2006, 515, 1864-1871. (5) Li, Z. W.; Zhu, Y. F. Acta Chim. Sinica 2003, 61, 1484-1487. (6) Liu, J. C.; Jean, J. H.; Li, C. C. J. Am. Ceram. Soc. 2006, 89, 882-887. (7) Li, C. C.; Hang, M. H. Mater. Lett. 2004, 58, 3903-3907.

or polymers, analogous to the process for their dispersion in water.8 Although nanoparticle suspensions with good stability can be achieved, complete dispersions on the primary particle size level still remain a challenging task.9,10 Therefore, metal oxide nanocrystals are not the primary candidates for the preparation of transparent nanocrystal dispersions and functional nanohybrids. The synthesis of metal oxide nanocrystals by nonaqueous solgel routes at comparably low temperatures is a rather recent development.11 This synthetic approach leads directly to the formation of highly crystalline nanoparticles, without any postcalcination step necessary. Organic groups attached to the surface of the nanocrystals prevent the formation of aggregates and make their surface polarity adjustable to the medium. Completely homogeneous dispersions of nanocrystals in organic solvents have been successfully prepared from nonaqueously synthesized nanocrystals via in-situ modification for titania nanocrystals12 and post-modification for magnetite nanocrystals13 and even without any additional ligands for tin oxide nanocrystals.14 ZrO2 nanoparticles would be an ideal building block for nanocomposites because they offer several advantages such as chemical inertness, excellent thermal stability, high refractive index, and high hardness. In spite of great effort made to the surface modification and dispersion of ZrO2 nanoparticles,15-18 only very few examples regarding the successful preparation of (8) Studart, A. R.; Amstad, E.; Gauckler, L. J. Langmuir 2007, 23, 10811090. (9) Lee, C. S.; Lee, J. S.; Oh, S. T. Mater. Lett. 2003, 57, 2643-2646. (10) Liu, Y. L.; Yu, Z. F.; Zhou, S. X.; Wu, L. M. J. Dispersion Sci. Technol. 2006, 27, 983-990. (11) Niederberger, M.; Garnweitner, G. Chem.sEur. J. 2006, 12, 7282-7302. (12) Niederberger, M.; Garnweitner, G.; Krumeich, F.; Nesper, R.; Co¨lfen, H.; Antonietti, M. Chem. Mater. 2004, 16, 1202-1208. (13) Pinna, N.; Grancharov, S.; Beato, P.; Bonville, P.; Antonietti, M.; Niederberger, M. Chem. Mater. 2005, 17, 3044-3049. (14) Ba, J. H.; Polleux, J.; Antonietti, M.; Niederberger, M. AdV. Mater. 2005, 17, 2509-2512.

10.1021/la700837u CCC: $37.00 © 2007 American Chemical Society Published on Web 07/31/2007

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Scheme 1. Chemical Structures of Vinyl Group-Containing Ligands

completely homogeneous dispersions of ZrO2 nanocrystals in organic media have been reported. Charty et al.15,19 got nonaggregated tetragonal ZrO2 nanocrystals (2 nm) in n-propanol via a sol-gel process involving the use of zirconium n-propoxide, acetylacetone as the ligand, and aqueous p-toluene sulfonic acid as the catalyst. Joo et al.20 prepared homogeneous dispersions of ZrO2 nanocrystals (4 nm), synthesized from the reaction of zirconium isopropoxide with zirconium chloride in trioctylphosphine oxide (TOPO), in nonpolar organic solvents such as hexane and toluene. In these two cases, the ZrO2 dispersions are colored, which restricts their application potential. Mizuno et al.21 reported a concentrated colorless dispersion of zirconia nanocrystals (4 nm) in toluene, prepared by the hydrolysis and condensation of zirconium(IV) tert-butoxide with trimethylamine N-oxide as the catalyst, followed by annealing in dioctyl ether at 280 °C. However, this process leads to a high fraction of organic impurities, and the ZrO2 nanoparticles exhibit poor crystallinity. Recently, our group has demonstrated that the reaction of zirconium tetraisopropoxide in benzyl alcohol via nonaqueous sol-gel chemistry leads to well-stabilized, highly crystalline ZrO2 nanoparticles that can be used for the fabrication of organicinorganic nanocomposite materials for optical applications.21,22 In the present article, the dispersion behavior of such colorless ZrO2 nanocrystals (3.8 nm) in tetrahydrofuran (THF) and in various monomers is examined and discussed in more detail, where organic ligands containing a vinyl group are used. The vinyl group-containing species were chosen because they not only provide steric stabilization of the nanoparticle in organic media but also offer the ability to react them in a polymerization step, enabling their covalent attachment to the polymer matrix via their carbon-carbon double bond. This linkage provides for the stable anchoring of the particles to the matrix, diminishing the possibility of demixing of the oxidic particles from the polymer material but also enhancing the mechanical performance of the resulting nanocomposites.1 Although the as-synthesized ZrO2 nanocrystals form a white precipitate in benzyl alcohol, transparent (15) He, W.; Guo, Z. G.; Pu, Y. K. Appl. Phys. Lett. 2004, 85, 896-898. (16) Pawsey, S.; Yach, K.; Reven, L. Langmuir 2002, 18, 5202-5212. (17) Turner, M. R.; Duguet, E.; Labruge`re, C. Surf. Interface Anal. 1997, 25, 917-923. (18) Carrie`re, D.; Moreau, M.; Barboux, P.; Boilot, J. P. Langmuir 2004, 20, 3449-3455. (19) Chatry, M.; Henry, M.; Livage, J. Mater. Res. Bull. 1994, 29, 517-522. (20) Joo, J.; Yu, T.; Kim, Y. W.; Park, H. M.; Wu, F. X.; Zhang, J. Z.; Hyeon, T. J. Am. Chem. Soc. 2003, 125, 6553-6557. (21) Mizuno, M.; Sasaki, Y.; Lee, S.; Katakura, H. Langmuir 2006, 22, 71377140. (22) Garnweitner, G.; Goldenberg, L. M.; Sakhno, O. V.; Antonietti, M.; Niederberger, M.; Stumpe, J. Small, in press, 2007.

ZrO2 nanocrystal dispersions in THF or monomers were readily achieved after post-functionalization treatment with the vinyl group-containing ligands. The excellent dispersibility of the ZrO2 nanocrystals was confirmed by both analytical ultracentrifugation (AUC) measurements and transmission electron microscopy (TEM). In addition, the long-term stability of the dispersions, the redispersion behavior of the ligand-functionalized ZrO2 nanoparticles, and the interactions between vinyl group-containing ligands and ZrO2 nanocrystals are discussed in detail. Experiments Chemicals. Zirconium(IV) isopropoxide (99.9%, Aldrich) and anhydrous benzyl alcohol (g99%, Aldrich) were used for the synthesis of ZrO2 nanoparticles. 3-(Trimethoxysilyl)propyl methacrylate (MPS, g98%, Sigma), ethyl 3,4-dihydroxycinnamate (EDHC, 97%, Alfa Aesar), allylmalonic acid (AMA, g98%, Fluka), and trimethylolpropane mono allyl ether (TMPMA, 98%, Aldrich) were used as the vinyl group-containing ligands. (Their structures are shown in Scheme 1.) Tetrahydrofuran (THF, g99.5%, Fluka) with monomers methyl methacrylate (MMA, g99.5%, Fluka) and styrene (St, 99%, Aldrich) were employed as the organic media for the dispersion of ZrO2 nanoparticles, and absolute ethanol (g99.9%, J. T. Baker) and methanol (g99.9%, Merck) were used for washing. All chemicals were used as received. Synthesis of ZrO2 Nanocrystals. The ZrO2 nanoparticle synthesis was described in detail elsewhere.23,22 Zirconium(IV) isopropoxide (33.3 g) was weighed into a 1 L Teflon cup and 500 mL of benzyl alcohol was added. The cup was inserted into a 1 L benchtop reactor (T316 stainless steel stirred moveable reactor no. 4525, Parr Instruments), which was sealed and heated to 210 °C under continuous stirring. After 3 days, the reaction mixture was allowed to cool down, and a white turbid liquid dispersion was obtained. Dispersion of ZrO2 Nanocrystals. A small portion (1.0 g) of the as-synthesized ZrO2 nanoparticle suspension was centrifuged to remove benzyl alcohol. The white precipitate was further washed by two repeated cycles of sonication and centrifugation with absolute ethanol (2 × 1 mL) to remove excess solvent. Without drying, the wet ZrO2 nanoparticles were mixed either with the ligand-THF solution or with the ligand-monomer solution that was previously prepared on the basis of the desired ligand-to-ZrO2 molar ratio and the targeted ZrO2 concentration in the final systems. For example, if the as-synthesized ZrO2 nanocrystal suspension had a solid content of 13.6 mg/g (0.11 mmol/g), then 2.74 mg of MPS (0.011 mmol) was first dissolved in 1.237 g (0.0172 mol) of THF or in the same amount of monomer to meet the desired MPS-to-ZrO2 molar ratio of 0.1:1 and a ZrO2 concentration of 1.1 wt %. The whole mixture consisting of the wet ZrO2 nanoparticles and the ligand-THF or (23) Garnweitner, G.; Niederberger, M. J. Am. Ceram. Soc. 2006, 89, 18011808.

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Figure 1. XRD powder pattern of the as-synthesized zirconia nanoparticles. ligand-monomer solution was further sonicated at room temperature for 10 min. After 3 days (to complete the reactions between ligands and zirconia nanoparticles), the dispersions were ready for characterization. The residual ethanol from the washing step was about 1.9 wt % in the dispersion with a ZrO2 concentration of 1.1 wt %. Unless otherwise noted, all ZrO2 concentrations and ligandto-zirconia molar ratios given in this article were calculated on the basis of the total weight of the as-synthesized ZrO2 nanocrystal hybrids (i.e., the inherently surface-bound organic species were also considered to be ZrO2 in the calculation). The influence of water on the dispersibility properties of the assynthesized ZrO2 nanocrystals was investigated by controlled introduction of water during the second washing step (i.e., instead of 1 mL of absolute ethanol, a mixture of 1 mL of ethanol and 2 drops of water was used). All other experimental steps were the same as described above. Characterization. The X-ray powder diffraction (XRD) patterns were measured in reflection mode (Cu KR radiation) on a Bruker D8 diffractometer equipped with a scintillation counter. Fourier transform infrared spectra (FTIR) were obtained with a rapid FTIR spectrometer (FTS 6000). Before each measurement, the background spectrum was taken and subtracted from the FTIR spectrum of the sample. Transmission electron microscopy (TEM) was performed on an Omega 912 (Carl Zeiss) microscope, operated at 100 kV. The samples were deposited onto a carbon-coated copper grid by directly drying the dispersion of ZrO2 nanoparticles. Analytical ultracentrifugation (AUC) was performed with a Beckman Optima XL-I centrifuge (Beckman/Coulter, Palo Alto, CA) with an on-line Rayleigh interferometer at 20 000, 25 000, or 30 000 rpm. The particle size was evaluated from a sedimentation velocity experiment assuming spherical particles with the density of ZrO2 (FZrO2 ) 5.6 g/mL).24 Thermogravimetric analysis (TGA) was performed on Netzsch TG 209 F1 at a scanning rate of 20 K/min under a nitrogen atmosphere. The samples for FTIR and TGA were treated as follows: ZrO2 nanoparticle dispersions containing a certain amount of ligand were first dried at ambient temperature (ambient drying can avoid the possible thermoinitiated polymerization of the ligand during drying) and then washed with methanol by four repeated cycles of sonication and centrifugation. For comparison, the assynthesized ZrO2 nanoparticles were additionally washed with methanol four times after the two washing steps with ethanol. All of the washed samples were further dried at 60 °C overnight.

Results and Discussion Structural Properties of the ZrO2 Nanocrystals. The reaction of zirconium(IV) isopropoxide in benzyl alcohol resulted in a white, rather unstable ZrO2 nanoparticle suspension. A typical XRD pattern (Figure 1) indicates that the ZrO2 nanoparticles are highly crystalline and phase pure. They appear to have a cubic (24) Co¨lfen, H.; Pauck, T. Colloid Polym. Sci. 1997, 275, 175-180.

Figure 2. FTIR spectra of the as-synthesized ZrO2 nanoparticles, posttreated at different temperatures.

structure (JCPDS no. 49-1642), which has been verified by Rietveld refinement.22 According to the Scherrer equation applied to the (220) reflection, the crystal size of the as-synthesized ZrO2 nanoparticles is about 3.5 nm. Taking the TEM and AUC results into account (Figures 3 and 4), it can be concluded that the as-synthesized ZrO2 nanoparticles are single-crystalline. In addition to the band at 500-850 cm-1, which is due to the Zr-O bond, the FTIR spectrum (Figure 2) of an as-prepared sample displays two strong absorption bands at wavenumbers of 1200-1460 and 1460-1680 cm-1. These two bands vanish after calcination of the particles at 600 °C for 6 h, suggesting that the nanoparticles are coated by a considerable quantity of organic components. An extraordinarily broad peak is observed in the range of 2200-3360 cm-1, which also disappears after calcination and is assigned to surface hydroxyl groups. TGA analysis reveals that the weight fraction of the chemisorbed organic component (including some hydroxyl groups) on the surface of the as-synthesized ZrO2 nanoparticles is about 5 wt %. (See the weight loss between 300 and 900 °C in Figure 9 and Table 2.) The two strong peaks at 1531 and 1418 cm-1 are very similar to the peaks of asymmetric and symmetric vibrations of carboxylate groups, respectively.25,26 The weak peak at 3021 cm-1 (C-H stretching vibration of phenyl groups) and the peak at 716.8 cm-1 (out-of-plane C-H vibration of phenyl groups) suggest that the surface-bound organic species contain phenyl groups. These observations point to the presence of surface-adsorbed benzoate molecules that must stem from the benzyl alcohol solvent. The oxidation of benzyl alcohol to benzoic acid under these reaction conditions is not unusual and has been reported before, also leading to surface-complexing benzoate species.27 Dispersion of ZrO2 Nanoparticles in THF. Although the as-synthesized ZrO2 nanoparticles have some organic groups on their surface, they are still not dispersible in THF. Ligands such as MPS, EDHC, AMA, and TMPMA have to be employed to improve the dispersibility. For preliminary tests to evaluate the applicability of selected ligands, an excess of ligands (0.45:1 ligand-to-ZrO2 molar ratio) was used. Amazingly, all four ligands (25) Pawsey, S.; Yach, K.; Halla, J.; Reven, L. Langmuir 2000, 16, 32943303. (26) Tang, F. Q.; Huang, X. X.; Wang, L. Z.; Guo, J. K. Nanostruct. Mater. 1999, 11, 861-866. (27) Pinna, N.; Garnweitner, G.; Beato, P.; Niederberger, M.; Antonietti, M. Small 2005, 1, 112-121.

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Figure 3. TEM images at different magnifications of ZrO2 nanoparticle dispersions in THF with the following ligands: (a, b) MPS, (c, d) EDHC, (e, f) AMA, and (g), h) TMPMA. The ligand-to-ZrO2 molar ratio is 0.2:1, and the ZrO2 concentration is 1.1 wt % for all of these samples.

Figure 4. Particle size distributions of ZrO2 dispersions in THF with various ligand-to-ZrO2 molar ratios of (a) MPS, (b) EDHC, (c) AMA, and (d) TMPMA, respectively (ZrO2 concentration, 1.1 wt %).

transformed the cloudy zirconia dispersions into completely transparent “solutions” after sonication for 1-3 min (optical spectra in Supporting Information). The dispersions with MPS

and AMA became completely transparent even by hand shaking for several minutes. After 1 day, only a small amount of 2.4 wt % of the initial solid precipitated. TEM images (Figure 3) of the

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Figure 5. FTIR spectra of (a) MPS, (b) as-synthesized ZrO2, and (c) MPS-modified ZrO2 nanoparticles after washing (molar MPS-to-ZrO2 ratio, 0.1:1).

Figure 6. FTIR spectra of (a) EDHC, (b) EDHC-modified ZrO2 nanoparticles before washing, and (c) EDHC-modified ZrO2 nanoparticles after washing (molar EDHC-to- ZrO2 ratio, 0.1:1).

dispersions give evidence that the ZrO2 nanoparticles are, as a matter of fact, individually dispersed, as shown for MPS (Figure 3a,b), EDHC (Figure 3c,d), and AMA (Figure 3e,f). The TEM images also prove that the ZrO2 nanoparticles are spherical and monodisperse with a diameter of 3.8 nm. Although the particles are close to each other (Figure 3a-f), because of the drying process during the preparation of the TEM grid, they are clearly separated by an organic layer that is visible as white contrast around the nanoparticles. In the case of TMPMA (Figure 3g,h), dark regions of ZrO2 assemblies suggest that the dispersibility is not as high as in the other systems. The formation of these agglomerates is presumably a consequence of the potential exchange of TMPMA ligands against water from air, as will be discussed below.

Consequently, the four ligands investigated are suitable for the dispersion of ZrO2 nanoparticles in THF. These results clearly point out that the dispersion behavior of nanopowders based on nonaqueous sol-gel chemistry remarkably differs from that obtained by high-temperature procedures.28 The quality of the ZrO2 dispersions as a function of ligand concentration was examined in more detail. Theoretically, the required amount to build a monolayer of ligands on the metal oxide surface is in the range of 3.0-6.9 µmol/m2 for MPS, depending on its orientation (parallel or perpendicular with respect to the nanoparticle surface).2,29 For the ZrO2 nanoparticles with (28) Ring, T. A. Fundamentals of Ceramic Powder Processing and Synthesis; Academic Press: San Diego, CA, 1996. (29) Miller, J. D.; Ishida, H. Surf. Sci. 1984, 148, 601-622.

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Figure 7. FTIR spectra of (a) AMA, (b) AMA-modified ZrO2 nanoparticles before washing, and (c) AMA-modified ZrO2 nanoparticles after washing (molar AMA-to-ZrO2 ratio, 0.1:1).

Figure 8. FTIR spectra of (a) TMPMA, (b) TMPMA-modified ZrO2 nanoparticles before washing, (c) TMPMA-modified ZrO2 nanoparticles after washing, and (d) as-synthesized ZrO2 nanoparticles (molar TMPMA-to-ZrO2 ratio, 0.1:1).

an average diameter of 3.8 nm, the theoretical molar ratio of ligand to ZrO2 should be in the range of 0.052-0.12 to give a monolayer of surface coverage. In fact, because of the existence of surface-adsorbed organic groups stemming from the nonaqueous synthesis procedure and also the incomplete attachment of ligands, an accurate prediction of the optimum ligand concentration might be more complicated. To shed some light on this problem, several experimental series were carried out. Table 1 summarizes the experimental results as a function of broad ligand concentration, and Figure 4 plots the particle size distributions determined by AUC. The results clearly indicate that transparent ZrO2 dispersions can be achieved at ligand concentrations below the theoretical limit of 0.052 in the case of samples M-5 (MPS) and T-6 (TMPMA). However, E-4 (EDHC) and A-4 (AMA) require molar ratios of at least 0.06 and 0.089, respectively, which is consistent with the

monolayer expectation. Flocculation of those transparent dispersions with low ligand concentrations occurred to our surprise in nonsealed containers after several days (M-4, T-5, and T-6) or even after one night (M-5). The destabilization may be caused by the slow adsorption of water on ZrO2 nanoparticles from air and will be discussed in detail below. AUC results in Figure 4 show that small aggregates actually exist in the lower ligand samples (M-4, T-5, and E-4); however, these aggregates are in the sub-10-nm range and therefore do not affect the transparency. Complete dispersion of ZrO2 nanoparticles is achieved at and above critical ligand-to-ZrO2 molar ratios of 0.11, 0.11, 0.089, and 0.10 for the different ligands (MPS, EDHC, AMA, and TMPMA, respectively). This is astonishingly similar and indicates a perpendicular 1:1 bound ligand monolayer. In addition, the AUC results of the dispersions with ligand content above the critical concentration have very equal particle size

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Figure 9. Thermograms of the as-synthesized and ligand-modified ZrO2 nanoparticles. Table 1. Summary of the ZrO2 Nanoparticle Dispersions in THFa sample nameb

molar ratio of ligand-to-ZrO2

M-1 M-2 M-3 M-4 M-5

0.45 0.29 0.11 0.053 0.028

colorless transparent colorless transparent colorless transparent colorless transparent colorless transparent with a little bit of precipitate

E-1 E-2 E-3 E-4

0.45 0.21 0.11 0.06

E-5

0.03

yellowish transparent yellowish transparent yellowish transparent yellowish transparent with a little bit of precipitate cloudy

A-1 A-2 A-3 A-4 A-5 T-1

0.45 0.42 0.18 0.089 0.044 0.45

colorless transparent cloudy colorless transparent

T-2 T-3 T-4 T-5 T-6 T-7

0.32 0.20 0.10 0.074 0.036 0.019

colorless transparent colorless transparent colorless transparent colorless transparent colorless transparent cloudy

appearancec

colorless transparent colorless transparent

long-term stabilityd transparent after 1 month cloudy after 1 week flocculated after 1 night

transparent after 1 month transparent with a small bit of precipitate precipitated quickly transparent after 1 month precipitated quickly flocculated after 1 week but stable if they are kept in a sealed container

flocculated after 5 days flocculated after 2 days precipitated quickly

a ZrO2 concentration: 1.10 ( 0.05 wt %. b M, E, A, and T represent MPS, EDHC, AMA, and TMPMA, respectively. c Observed directly after the sonication process. d Stored in a sealed centrifugal tube at room temperature.

distributions that also agree nicely with the TEM results. This further confirms the high homogeneity of the ZrO2 nanocrystal dispersion obtained from the nonaqueous synthesis procedure. Consequently, a molar ratio of 0.1 was selected for the preparation of ZrO2 nanoparticle dispersions in this following study. To understand the sensitivity of the particle dispersions against uncontrolled storage, small amounts of water were introduced during the washing step. It was found that the ZrO2 nanoparticles were not dispersible anymore with EDHC, AMA, and TMPMA

ligands. Using MPS, the time to reach transparent ZrO2 dispersions extended remarkably, although previously the existence of a small amount of water was reported to be a positive factor to promote the attachment of a silane coupling agent onto the metal oxide surface.30 Similarly, water was also added to the ZrO2 dispersions to check whether the poor storage stability of samples (30) Yoshida, W.; Castro, R. P.; Jou, J. D.; Cohen, Y. Langmuir 2001, 17, 5882-5888.

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Table 2. Quantitative Results of the Chemical Attachment of Ligands on ZrO2 Nanoparticles from TGA Experimentsa

ligand

weight loss below 300 °C,%

weight loss between 300-900 °C,%b

estimated weight percentage of ligand attached,%c

number of CdC groups per nanoparticled

as-synthesized MPS EDHC AMA TMPMA

4.1 5.5 5.4 5.3 5.4

4.8 (5.3) 9.3 (10.9) 8.4 (9.7) 5.1 (5.7) 6.5 (7.4)

5.6 4.4 0.4 2.1

43.8 20.7 2.7 11.7

a Molar ligand-to-ZrO2 ratio ) 0.1:1. b Data in brackets are obtained on the basis of the remaining weight at 900 °C. c Data are calculated by the weight loss of 300-900 °C (the values in brackets) of ligand-modified ZrO2 minus that of as-synthesized ZrO2 nanoparticles. d Assuming that ZrO2 nanoparticles are monodisperse with a crystallite size of 3.8 nm.

M-4 and M-5 and the whole T-series was caused by the adsorption of water. The initially transparent ZrO2 dispersions became immediately cloudy. These observations illustrate that the water molecules readily adsorb onto the as-synthesized ZrO2 nanoparticles. The watercoated nanoparticles easily form hydrogen bonds with each other and thus produce hard aggregates, similar to the ones observed for ZrO2 nanoparticles prepared by an aqueous chemical precipitation method.31 However, the water layer also hinders direct contact of the ligands with the metal oxide surface, thus suppressing dispersibility by the ligand. Therefore, the absence of water is a key feature for the excellent dispersibility of our zirconia nanoparticles in the presence of appropriate ligands. It is important to point out that, besides water, other strongly interacting organic solvents such as dichloromethane and MMA were also found to deteriorate the dispersibility of the ZrO2 nanoparticles. Interaction between ZrO2 Nanoparticles and Vinyl GroupContaining Ligands. To elucidate the bonding fashion of the ligands to the nanoparticle surface, THF was first evaporated from the respective ZrO2 at room temperature, and the resulting solid films were then washed with methanol. Monitoring by FTIR spectroscopy demonstrated that after washing with methanol four times the spectra of all ligand-functionalized ZrO2 nanoparticles do not change with further washing. The FTIR spectra of ligand-functionalized ZrO2 nanoparticles before (except for the case with MPS) and after washing as well as the pure ligands are presented in Figures 5-8 for MPS, EDHC, AMA, and TMPMA, respectively, and are compared to the spectrum of as-prepared ZrO2 nanoparticles. The spectrum of MPS-modified ZrO2 in Figure 5c shows that new strong bands in the range of 790-1100 cm-1, due to SiO-Zr, appear instead of the peak at 1075.6 cm-1 in pure MPS, attributed to the stretching vibration of Si-O-C (Figure 5a). This observation indicates the formation of a chemical bond between ZrO2 and the MPS ligands. Compared with the FTIR spectrum of the as-synthesized ZrO2 nanoparticles (Figure 5b), the peaks at 1531 and 1418 cm-1 also remain after the modification with MPS, suggesting that the inherent organic groups are chemically stable and do not exchange. Generally, MPS molecules form a chemical bond to the metal oxide surface, which is thought to progress via hydrolysis of one or more of the alkoxy groups, followed by condensation with the surface hydroxyl groups.32 However, because there is no water in the reaction system the attachment of MPS molecules presumably involves a direct condensation between the ortho-ester and surface oxygens of the nanoparticles. (31) Wang, S. Y.; Li, X. A.; Zhai, Y. C.; Wang, K. M. Powder Technol. 2006, 168, 53-58. (32) Blitz, J. P.; Murthy, R. S. S.; Leyden, D. E. J. Am. Chem. Soc. 1987, 109, 7141-7145.

Careful inspection of the band at 1640-1750 cm-1 (Figure 5, inset) reveals that the absorption peak for the carbonyl group of surface-bound MPS splits into two peaks: one shoulder at 1710.9 cm-1 and one strong peak at 1693.5 cm-1, corresponding to the vibration of free and associated carbonyl groups, respectively. The association of the carbonyl group is presumably caused by surface interactions with the nanoparticles. Therefore, the MPS molecules on the surface of the ZrO2 nanoparticles presumably exhibit a perpendicular as well as a parallel orientation. Figure 6 shows that the FTIR spectra of EDHC-modified ZrO2 nanoparticles before (Figure 6b) and after washing (Figure 6c) are similar. Many peaks such as those at 1678 cm-1 (due to the stretching vibration of the ester group), 1621 cm-1 (due to the stretching vibration of CdC), 1491.8 cm-1 (due to the CdC ring stretch), and 1257.2 cm-1 (due to the vibrations of C-C(d O)-O and O-C-C), belonging to the EDHC chain appear, indicating the strong interaction of EDHC with the ZrO2 nanoparticles. EDHC contains a catechol function, which usually attaches to metal oxide nanoparticles by direct coordination to the metal center.12,33 The disappearance of the sharp peak at 3433.3 cm-1 (stemming from the phenolic group of EDHC) illustrates that a similar coordination mechanism applies for the attachment of EDHC to the ZrO2 nanoparticles. Two absorption peaks, a shoulder at 1703 cm-1 and another strong peak at 1678 cm-1, were observed for the carbonyl group of the attached EDHC, suggesting that EDHC has a similar orientation to MPS on the ZrO2 nanoparticle surface. Figure 7 presents the spectra of (a) pure AMA and of the AMA-functionalized nanoparticles (b) before and (c) after washing, indicating that the AMA stretching vibration of -COOH at 1691.6 cm-1 disappears directly during the functionalization treatment even before washing. This is strongly indicative of the immediate chemical reaction of most carboxyl groups. After modifying with AMA, the two original strong peaks for the as-synthesized ZrO2 nanoparticles become even stronger and shift to 1405.8 cm-1 (symmetric vibration of the carboxylate group) and 1541.2 cm-1 (asymmetric vibration of the carboxylate group), providing evidence of the chemical interaction between AMA and ZrO2. The difference of 135.4 cm-1 between the symmetric and asymmetric vibrations suggests an uncoordinated anion attachment of AMA to ZrO2 nanoparticle.25 Additional proof for the chemical absorption of AMA is provided by the existence of the weak shoulder at 1640.6 cm-1, which can be attributed to CdC. However, because of the short hydrocarbon chain of AMA, the stretching vibrations of the C-H bonds are not identified on the FTIR spectrum. Figure 8 shows that the bands at 2800-2980 cm-1, corresponding to methylene and methyl groups, and at 1067 cm-1, (33) Rajh, T.; Chen, L. X.; Lukas, K.; Liu, T.; Thurnauer, M. C.; Tiede, D. M. J. Phys. Chem. B 2002, 106, 10543-10552.

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Table 3. Redispersion Results of Surface-Functionalized ZrO2 Nanoparticles in THF redispersion results in THF ligand

ligand-to-ZrO2 molar ratio

before washing

after washing

MPS

0.1:1

soluble

soluble

EDHC

0.1:1 0.2:1

soluble soluble

partially soluble soluble

AMA

0.1:1 0.2:1

insoluble soluble

insoluble insoluble

TMPMA

0.1:1 0.2:1

insolublea insolublea

insoluble insoluble

a Soluble if the dispersion was vacuum dried. Soluble indicates completely transparent dispersions.

Table 4. Dispersion Results of ZrO2 Nanoparticles in Monomersa ligands dissolved in the monomer

MMA

St

as-synthesized MPS EDHC AMA TMPMA AA

insoluble soluble soluble soluble insoluble soluble

insoluble soluble soluble soluble

a Concentration of ZrO2, 1 wt %; ligand-to-ZrO2 molar ratio of 0.1:1. Soluble indicates transparent dispersions.

corresponding to the ether group, disappear after the TMPMAfunctionalized ZrO2 nanoparticles are washed with methanol. Thus, TMPMA can be regarded as the only physisorbed ligand for the ZrO2 nanoparticles. As a matter of fact, flocculation and precipitation from dispersions containing TMPMA-modified ZrO2 already occurred after the evaporation of a small amount of solvent at room temperature, which additionally demonstrates the weak interaction between TMPMA and ZrO2 nanoparticles. In addition, the flocculation also explains the presence of ZrO2 aggregates on the TEM images (Figure 3g,h). Nevertheless, TMPMA is still an efficient dispersant for the ZrO2 nanoparticles in THF under the condition that the dispersion is kept in a sealed container. The chemisorbed ligands were quantitatively determined by TGA (Figure 9). It is obvious that all samples exhibit two or three stages of weight loss, as we also have observed recently when analyzing fatty acid-modified ZrO2 nanoparticles.22 All samples investigated here showed a markedly lower weight loss. The weight loss below 300 °C is generally assigned to the release of physically adsorbed molecules,12,34 mainly water, methanol, and THF, and the weight loss above 300 °C is due to the desorption of chemically bonded organic groups and the dehydration of surface hydroxyls. Table 2 summarizes the quantitative results of the TGA analysis. MPS obviously is chemisorbed onto the ZrO2 nanoparticles to the largest extent, followed by EDHC. Although the chemical attachment of AMA on the ZrO2 nanoparticles is clearly proven by its FTIR spectrum, the TGA results indicate only the lowest amount for AMA, maybe because of carbonate formation during heating. Interestingly, the TGA result showed that a significant amount of TMPMA was bonded to the nanoparticle surface, although its identification was not possible on the basis of the FTIR spectrum. Table 2 also illustrates that each ligand-modified ZrO2 nanoparticle possesses a large number of CdC bonds, especially when using MPS and EDHC (34) Guerrero, G.; Mutin, P. H.; Vioux, A. Chem. Mater. 2001, 13, 43674373.

Figure 10. Particle size distribution of ZrO2 nanocrystals in MMA (MPS-to-ZrO2 molar ratio, 0.1:1; ZrO2 concentration, 6.8 wt %).

(i.e., the vinyl-containing ligand formally transforms every nanoparticle into a multifunctional monomer). Redispersion of Surface-Functionalized ZrO2 in THF. Redispersion experiments were conducted to show the efficiency of the ligands in preventing the nanoparticles from aggregating. The surface-functionalized ZrO2 nanopowders for redispersion experiments were obtained by the complete evaporation of THF in surface-functionalized ZrO2 dispersions at room temperature with or without four washings with methanol and subsequent drying at room temperature. The ZrO2 nanopowders before and after washing with methanol were dispersed in THF again by 10 min of sonication. The results are summarized in Table 3. MPS provides the best protection for the ZrO2 nanoparticles from aggregation at a ligand-to-ZrO2 molar ratio of 0.1:1. All MPSmodified ZrO2 nanoparticles are well dispersed in THF even after washing and drying. The EDHC-modified ZrO2 nanoparticles are also redispersible in THF. However, for the sample prepared at a molar ratio of 0.1:1, the ZrO2 nanoparticles are partially insoluble after washing with methanol. At a molar ratio of 0.1:1, AMA-modified ZrO2 nanoparticles cannot be redispersed at all, even without washing with methanol. However, as the molar ratio of AMA to ZrO2 is increased to 0.2:1 the nanoparticles become well dispersed in THF, but washing is still not possible. This observation indicates that besides chemically adsorbed AMA molecules the physically adsorbed AMA molecules also protect the nanoparticles from aggregating. But obviously they can be easily removed. TMPMA-modified ZrO2 nanoparticles can be vacuum dried and are still easily redispersed in THF. This observation indicates that TMPMA can prevent ZrO2 nanoparticles from aggregating as long as the adsorption of humidity onto the surface of the nanoparticles is avoided. Dispersion of ZrO2 Nanoparticles in Monomers. The dispersibility of ZrO2 nanoparticles in monomers (St and MMA) was examined using the same ligands, and the obtained results are listed in Table 4. Similar to the dispersion in THF, the assynthesized ZrO2 nanoparticles are not dispersible in both St and MMA. However, when additional MPS was present in the monomer, the ZrO2 dispersions both in St and MMA became transparent upon sonication. The AUC result in Figure 10 indicates that the ZrO2 nanoparticles are indeed dispersed in MMA down to the primary particle size level. EDHC and AMA act the same way for the dispersion of ZrO2 nanoparticles in MMA. These two ligands, however, are not soluble in St and thus cannot be used for the dispersion of ZrO2 in St. In the case of TMPMA, the ZrO2 dispersion behaves rather differently. ZrO2 nanoparticles can be dispersed in St by TMPMA but not in MMA. This is probably caused by the great affinity between MMA and

Dispersion BehaVior of Zirconia Nanocrystals

TMPMA, baffling the formation of hydrogen bonds between TMPMA and ZrO2 nanoparticles. The use of acrylic acid (AA) as a substitute for AMA was also investigated, and transparent dispersions were achieved. However, precipitation occurred after 1 week in St and 3 weeks in MMA as well as in THF. FTIR spectra of ZrO2 nanoparticles isolated from these dispersions show that AA molecules are attached to the ZrO2 nanoparticles. Presumably, precipitation is also caused by the uptake of water.

Conclusions Redispersable ZrO2 nanoparticles were synthesized by nonaqueous sol-gel chemistry using various vinyl-containing ligands (MPS, EDHC, AMA, and TMPMA) for surface functionalization. All ligands are able to disperse the ZrO2 nanoparticles in THF down to the level of the primary particle size, regardless of the type of interaction (chemical or physical bond) between ligands and ZrO2 nanoparticles. FTIR analysis shows that MPS, EDHC, and AMA are chemically bonded, whereas TMPMA is presumably only physically attached to the ZrO2 nanoparticle surface. Both the deterioration of ZrO2 nanoparticle dispersions as a result of the introduction of water and the destabilization of ZrO2 nanoparticle dispersions in contact with humid air strongly indicate that the excellent dispersibility of ZrO2 nanoparticles is

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a direct consequence of the inherent absence of water on the nanoparticle surface. Redispersion experiments of surfacemodified ZrO2 nanoparticles show that large ligand molecules such as MPS and EDHC effectively protect the nanoparticles from aggregation and thus result in good redispersibility. Transparent dispersions of ZrO2 nanoparticles can also be prepared in MMA with the help of MPS, EHDC, and AMA or in St with AMA and TMPMA, opening the pathway for the preparation of polymer-based nanocomposites on the basis of such nanoparticles synthesized in water-free media. Acknowledgment. The Max-Planck-Society is gratefully acknowledged for financial support. S.Z. thanks the Alexander von Humboldt Foundation for a research fellowship. We also thank Antje Vo¨lkel for AUC measurements, Rona Pitschke for TEM measurements, and Irina Shekova for TGA experiments. Supporting Information Available: Optical transmission spectra of ligand-modified and an as-synthesized zirconia dispersions in THF (zirconia concentration 1.1 wt %). This material is available free of charge via the Internet at http://pubs.acs.org. LA700837U