Dispersion and Functionalization of Nonaqueous Synthesized

Langmuir 2008, 24, 11497-11505

11497

Dispersion and Functionalization of Nonaqueous Synthesized Zirconia Nanocrystals via Attachment of Silane Coupling Agents Kaiqing Luo, Shuxue Zhou,* Limin Wu, and Guangxin Gu Department of Materials Science and AdVanced Coatings Research Center of Educational Ministry of China, Fudan UniVersity, Shanghai 200433, P. R. China ReceiVed June 20, 2008. ReVised Manuscript ReceiVed August 23, 2008 Zirconia (ZrO2) nanocrystals, synthesized from zirconium(IV) isopropoxide isopropanol complex and benzyl alcohol, were dispersed and functionalized in organic solvents using three kinds of bifunctional silane coupling agents (SCAs), 3-glycidoxypropyltrimethoxysilane (GPTMS), 3-aminopropyltriethoxysilane (APTES), and 3-isocyanatopropyltriethoxysilane (IPTES). Completely transparent ZrO2 dispersions were achieved in tetrahydrofuran (THF) with all three SCAs, in pyridine and toluene with APTES and IPTES, and in N,N-dimethylformamide with IPTES. Dynamic laser scattering (DLS) measurements and high-resolution transmission electron microscopical (HRTEM) observation indicated that the ZrO2 nanocrystals are dispersed on a primary particle size level. Fourier transform infrared spectroscopy, solid-state 13C- and 29Si NMR spectroscopy, and thermogravimetric analysis demonstrated that all three SCAs are chemically attached to the surface of the ZrO2 nanoparticles, however, in different bonding modes. Except for GPTMS/ ZrO2/THF dispersion and IPTES/ZrO2/pyridine dispersion, all other transparent dispersions have poor long-term stability. The increasing polarity, due to high amount of APTES attached and high hydrolysis and condensation degree of the bonded APTES, and the aggregation, due to interparticle coupling via the bonded triethoxysilyl group, are the causes of the poor long-term stability for the ZrO2 dispersions with APTES and IPTES, respectively. Nevertheless, the APTES-functionalized ZrO2 precipitates can be deagglomerated in water to get a stable and transparent aqueous ZrO2 dispersion via addition of a little hydrochloric acid.

Introduction Dispersion and functionalization of nanoparticles are two themes for the efficient application of nanoparticles. Homogeneous dispersion of nanoparticles, especially at the primary particle size level, is absolutely important for the formation of homogeneous nanoparticles films,1 well-ordered nanoporous films,2 or transparent nanocomposites.3 Functionalization can be employed to offer better compatibility of nanoparticles with dispersing media, to prevent nanoparticles from aggregation, as well as to render the nanoparticles with chemical reactivity. Although it has been stated that the dispersion of nanoparticles strongly depends on the chemical character, molecular weight, and grafting density of the capped ligand,4 research on the subtle relationship between dispersion and functionalization of nanoparticles in a certain dispersing medium is insufficient. Silane coupling agents (SCAs) are common ligands for functionalization of oxide nanoparticles. They are bifunctional molecules containing a trialkoxy group and an organic headgroup functionality (e.g., vinyl, glycidyl, amino, mercapto, etc.). The trialkoxy group can bind covalently to the free -OH groups at the surface of the oxide nanoparticles, while the organic headgroup determines the final chemical character of the modified surface. Methacrylopropyltrimethoxysilane (MPS), 3-glycidoxypropyltrimethoxysilane (GPTMS), and 3-aminopropyltriethoxysilane (APTES) are the most frequently adopted SCAs for various purposes. For example, MPS was used to functionalize oxide * Corresponding author. E-mail: [email protected]. (1) Isaacs, S. R.; Choo, H.; Ko, W. B.; Shon, Y. S. Chem. Mater. 2006, 18, 107. (2) Mrabet, D.; Zahedi-Niaki, M. H.; Do, T. O. J. Phys. Chem. 2008, 112(18), 7124. (3) Lee, S.; Shin, H. J. S.; Yoon, M. D.; Yi, K. J. Mater. Chem. 2008, 18, 1751. (4) Corbierre, M. K.; Cameron, N. S.; Sutton, M.; Laaziri, K.; Lennox, R. B. Langmuir 2005, 21, 6063.

nanoparticles such as silica,5-7 alumina,8,9 and zirconia10,11 nanoparticles to make the nanoparticles polymerizable in radical polymerization and thus covalently interact with a polymer matrix. GPTMS was employed to functionalize colloidal silica and titania particles for preparing photopolymerizable (or thermal-curable) nanoparticle coatings.12,13 GPTMS-capped silica nanoparticles can also react with ethylendiamine to get positively charged silica nanoparticles for gene delivery14 or enhance the anticorrosive property of epoxy/amine coatings.15 APTES can be adsorbed on magnetite or silica nanoparticles for bioapplication, e.g., immobilization of DNA,14 oligonucleotides,16 avidin protein,17 and so forth. APTES was also used to prepare aminofunctionalized γ-Fe2O3 nanoparticles that can be linked by R,ωdicarboxylic acids,18 or to fabricate amino-functionalized silica nanoparticles that can self-assemble into a nanoparticle film together with carboxylic acid-functionalized silica nanoparticles.19 (5) Chen, H.; Zhou, S. X.; Gu, G. X.; Wu, L. M. J. Dispersion Sci. Technol. 2004, 25, 837. (6) Chen, G. D.; Zhou, S. X.; Gu, G. X.; Yang, H. H.; Wu, L. M. J. Colloid Interface Sci. 2005, 281, 339. (7) Bauer, F.; Sauerland, V.; Ernst, H.; Gla¨sel, H. J.; Naumov, S.; Mehnert, R. Macromol. Chem. Phys. 2003, 204, 375. (8) Zeng, Z.; Yu, J.; Guo, Z. X. Macromol. Chem. Phys. 2005, 206, 1558. (9) Guo, Z. H.; Pereira, T.; Choi, O.; Wang, Y.; Hahn, H. T. J. Mater. Chem. 2006, 16, 2800. (10) Scholz, S; Kaskel, S. J. Colloid Interface Sci. 2008, 323, 84. (11) Zhou, S. X.; Wu, L. M. Macromol. Chem. Phys. 2008, 209, 1170. (12) Mennig, M.; Oliveira, P. W.; Frantzen, A.; Schmidt, H. Thin Solid Films 1999, 351, 225. (13) Daniels, M. W.; Francis1, L. F. J. Colloid Interface Sci. 1998, 205, 191. (14) Cso˝go¨r, Zs.; Nacken, M.; Sameti, M.; Lehr, C. M.; Schmidt, H. Mater. Sci. Eng., C 2003, 23, 93. (15) Yuan, J. J.; Zhou, S, X.; Gu, G. X.; Wu, L. M. J. Mater. Sci. 2005, 40, 3927. (16) Campoa, A. D.; Sen, T.; Lellouche, J. P.; Bruce, I. J. J. Magn. Magn. Mater. 2005, 293, 33. (17) Mohapatra, S.; Pramanik, N.; Mukherjee, S.; Ghosh, S. K.; Pramanik, P. J. Mater. Sci. 2007, 42, 7566. (18) Iida, H.; Nakanishi, T.; Osaka, T. Electrochim. Acta 2005, 51, 855. (19) An, Y. Q.; Chen, M.; Xue, Q. J.; Liu, W. M. J. Colloid Interface Sci. 2007, 311, 507.

10.1021/la801943n CCC: $40.75  2008 American Chemical Society Published on Web 09/23/2008

11498 Langmuir, Vol. 24, No. 20, 2008

In addition, attachment of APTES molecules on the surface of TiO2 nanoparticles for suppressing its photocatalytic activity and enhancing its UV-shielding property was even reported.20 However, many studies focus on the characterization of the attaching mode of SCA molecules on the surface of nanoparticles and on the application of SCA-capped nanoparticles. The effects of the headgroup functionality and the grafting density of SCAs on the dispersion of nanoparticles in organic solvent (or water) are seldom addressed. Among oxide nanoparticles, ZrO2 nanoparticles would be attractive, as they offer several advantages such as chemical inertness, excellent thermal stability, high refractive index and high hardness. In spite of great efforts paid to the surface modification and dispersion of ZrO2 nanoparticles,21-24 only very few examples25-27 regarding the successful preparation of completely homogeneous dispersions of ZrO2 nanocrystals in organic media have been reported. Recently, Garnweitner et al.28 developed a novel nonaqueous route to prepare highly crystalline zirconia nanoparticles. Because of the absence of water on the nanoparticle surface, these nanoparticles were proved to be highly dispersible in tetrahydrofuran (THF), butyl acetate (BAc), and some monomers with the aid of ligands including MPS.11,29 Detailed investigation concerning on the MPS-capped ZrO2 nanocrystals11 revealed that the grafting density of MPS remarkably impacted the dispersion of ZrO2 nanocrystals in polyurethane oligomer. This article is a successive study on the surface modification and dispersion of nonaqueous synthesized ZrO2 nanocrystals, however, using GPTMS, APTES, and 3-isocyanatopropyltriethoxysilane (IPTES) as the ligands. These three SCAs were adopted because they have completely different headgroup functionalities, being propitious for inspecting the influence of the capped functional group on the dispersion of ZrO2 nanocrystals. On the other hand, the diversiform functional groups on the surface of ZrO2 nanocrystals, stemming from these three SCAs, can extend the applications of ZrO2 nanocrystals, for example, perfect building blocks for robust nanoparticle coatings (or films), and efficient nanofillers for various polymer matrices. Besides the THF that was adopted in our previous researches,11,29 other organic solvents such as toluene, N,N-dimethylformamide (DMF), pyridine, acetone, and so on, as well as even water were employed as the dispersion media of SCA-functionalized ZrO2 nanocrystals in this article. Quite different dispersion behaviors were observed and discussed. The attaching modes of the SCAs on the surface of ZrO2 nanoparticles were characterized with Fourier transform infrared spectroscopy (FTIR), solid-state 13Cand 29Si NMR spectroscopy, and thermogravimetric analysis (TGA).

Experimental Section Chemicals. Zirconium(IV) isopropoxide isopropanol complex (99.9%) and anhydrous benzyl alcohol (g99%,) were purchased from Aldrich Corp. GPTMS (g98%), APTES (g98%), and IPTES (20) Ukaji, E.; Furusawa, T.; Sato, M.; Suzuki, N. Appl. Surf. Sci. 2007, 254, 563. (21) He, W.; Guo, Z. G.; Pu, Y. K. Appl. Phys. Lett. 2004, 85, 896. (22) Pawsey, S.; Yach, K.; Reven, L. Langmuir 2002, 18, 5202. (23) Turner, M. R.; Duguet, E.; Labruge`re, C. Surf. Interface Anal. 1997, 25, 917. (24) Carrie`re, D.; Moreau, M.; Barboux, P.; Boilot, J. P. Langmuir 2004, 20, 3449. (25) Chatry, M.; Henry, M.; Livage, J. Mater. Res. Bull. 1994, 29, 517. (26) 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. (27) Mizuno, M.; Sasaki, Y.; Lee, S.; Katakura, H. Langmuir 2006, 22, 7137. (28) Garnweitner, G.; Goldenberg, L. M.; Sakhno, O. V.; Antonietti, M.; Niederberger, M.; Stumpe, J. Small 2007, 3, 1626. (29) Zhou, S. X.; Garnweitner, G.; Niederberger, M.; Antonietti, M. Langmuir 2007, 23, 9178.

Luo et al. (g98%) were kindly offered by Dow Corning Corp. THF (g99.5%), heptane (g98.5%), toluene (g99.5%), pyridine (g99.5%), DMF (g99.5%), isopropyl alcohol (g99.5%), n-BAc (g99.5%), acetone (g99.5%), and ethanol (g99.7%) were the products of Sinopharm Chemical Reagent Corp. Synthesis of ZrO2 Nanocrystals. ZrO2 nanocrystals were synthesized according to the procedure described elsewhere.28,30 That is, 3.33 g of zirconium(IV) isopropoxide isopropanol complex and 50 mL of benzyl alcohol were charged into a 100 mL Teflon liner. Subsequently, the Teflon liner was slid into a stainless steel autoclave and carefully sealed. The autoclave was put into an oven at a temperature of 240 °C to carry out the reaction. After 4 days, the reaction mixture was allowed to cool down, and a white turbid suspension was obtained. Dispersion and Functionalization of Zirconia Nanocrystals in Organic Solvents. The as-synthesized ZrO2 nanocrystal suspension (solid content: 33 mg/g) in benzyl alcohol was separated as a white precipitate by centrifugation or/and simple decantation to remove the benzyl alcohol. The precipitate was further washed with THF three times and directly dispersed in an organic solvent (e.g., THF, toluene, pyridine) containing a certain amount of SCA (i.e., GPTMS, APTES, or IPTES) based on a ZrO2 concentration of 0.9 wt %. The molar ratios of SCA-to-ZrO2 ranged from 0.05:1 to 0.57:1. The dispersion was sonicated at room temperature for 30 min and subsequently heated at 60 °C for 12 h or stored at room temperature. It should be noted that the organic components that inherently attached to the ZrO2 nanocrystals were counted as ZrO2 in formulating the dispersion. Dispersion of ZrO2 Nanocrystals in Water. The precipitate from the APTES-functionalized ZrO2/THF dispersion was adopted for preparation of the aqueous ZrO2 dispersion. THF was removed by centrifugation to obtain the APTES-functionalized ZrO2 precipitate. Afterward, aqueous HCl solution with a certain concentration was mixed with the precipitate. Transparent ZrO2 dispersion could be attained just by simply shaking with hands. Characterization. The transparency of the ZrO2 dispersion was determined with a UV-1800PC spectrophotometer (Shanghai MeiPu-Da Instrument Corp., Ltd., China) at a wavelength of 550 nm. The morphology and dispersion of ZrO2 nanoparticles were observed on a high-resolution transmission electron microscope (HRTEM; JEOL 2010, Japan), operated at 200 kV. The samples were deposited onto a carbon-coated copper grid by directly drying the ZrO2 dispersion that was antecedently diluted to a concentration of ∼0.05 wt %. The particle size distribution and zeta potential (z) were determined with a submicron particle size analyzer (Malvern Mastersizer Nano-ZS, U.K.) without dilution of the dispersion. Zeta potential (z) was calculated by the Henry equation: UE ) [2εzf(ka)]/ [3η], where f(ka) is the Henry function. UE represents the electrophoretic mobility of nanoparticle, and ε and η denote the dielectric constant and the viscosity of the dispersing medium, respectively. The FTIR spectra were obtained using a Nicolet Nexus 470 spectrometer (U.S.A.). The solid-state 13C NMR and 29Si NMR spectra were acquired by a Bruker DMX500 spectrometer (Germany) equipped with a 4 mm MAS probe head. TGA was carried out on a Perkin-Elmer TGA-7 (U.S.A.) at a heating rate of 10 K/min in air. The ZrO2 nanoparticles for FTIR, NMR, and TGA characterization were prepared as follows: For the cases with GPTMS and APTES ligands, the dispersions were poured with a large amount of methanol to precipitate the functionalized ZrO2 nanoparticles, and the precipitate was further washed with methanol four times, whereas, for the case with IPTES, heptane was adopted as the precipitator, and all other treatments were the same as above. The as-synthesized ZrO2 nanocrystals were treated using the same procedure, namely, washing with methanol, for the sake of comparison. The obtained SCAfunctionalized ZrO2 nanoparticles were dried under vacuum at room temperature or at 80 °C overnight. (30) Garnweitner, G.; Niederberger, M. J. Am. Ceram. Soc. 2006, 89, 1801.

Dispersion & Functionalization of ZrO2 Nanocrystals

Figure 1. Photographs of (a) an as-synthesized ZrO2 dispersion, (b) a GPTMS-functionalized ZrO2 dispersion, (c) an APTES-functionalized ZrO2 dispersion, and (d) an IPTES-functionalized ZrO2 dispersion (Treating merely with sonication at room temperature, the molar ratio of ligand-to-ZrO2 is 0.2:1; dispersing medium: THF).

Langmuir, Vol. 24, No. 20, 2008 11499

Figure 2. Particle size distribution of a ZrO2/THF dispersion using (a) GPTMS (b) APTES (c) IPTES as the ligand (preparation conditions are the same as Figure 1).

Results and Discussion Dispersion of Zirconia Nanocrystal in Organic Solvents. The dispersing of ZrO2 nanocrystals in THF using GPTMS, APTES, or IPTES as ligands was monitored by the change of the transparency of the dispersion (see Figure 1s in the Supporting Information). It was found that all ZrO2 dispersions containing SCA ligands gradually became transparent (Figure 1b,c) after sonication, while the as-synthesized ZrO2/THF dispersion remained cloudy (Figure 1a). This phenomenon on one hand indicates the assistance of SCAs for the dispersing of ZrO2 nanoparticles and, on the other hand, demonstrates the better dispersibility of nonaqueous synthesized ZrO2 nanocrystals. Additionally, we found that the sonication time to reach complete transparency is different for various ligands, ranking in the order of APTES < IPTES < GPTMS (Figure 1s). The reasons could be given as follows: Since aminated SCA can self-catalyze the hydrolysis and condensation,31 APTES naturally has the highest attaching rate via the condensation between a triethoxy group and the hydroxyl group at the surface of ZrO2 nanoparticles. In addition, the quick formation of a hydrogen bond between an amino headgroup and the hydroxyl group20 may presumably contribute to the rapid transformation of ZrO2 nanoparticles into transparent ZrO2 dispersion. Unfortunately, this kind of hydrogen bond was difficult to be probed. As for IPTES, the shorter sonication time to reach transparent dispersion should be owed to the higher reactivity of the isocyanato group relative to that of the trialkoxy group, which will be confirmed in the FTIR and NMR analyses below. The particle size distributions of transparent SCA-functionalized ZrO2 dispersions were analyzed using dynamic laser scattering (DLS), as shown in Figure 2. All dispersions have an overlapped particle size distribution with Z-average sizes of 8.4, 9.1, and 10.3 nm for GPTMS-, APTES-, and IPTES-functionalized ZrO2 dispersions, respectively. Further HRTEM observation (see Figure 2s in the Supporting Information) revealed that all transparent SCA-functionalized ZrO2 dispersions are composed of uniformally and individually dispersed ZrO2 nanoparticles with close-spherical shape and a diameter of 3-4 nm. The absolute mean size from DLS measurement is apparently higher than that from TEM observation because the former actually belongs to hydrodynamic size that includes the attached SCA shell. On the basis of DLS, TEM results, as well as the appearance of the ZrO2 dispersion, it can be concluded that GPTMS, APTES, and IPTES are efficient ligands for the dispersing of ZrO2 nanocrystals in THF at the primary particle size level. (31) Vandenberg, E. T.; Bertilsson, L.; Liedberg, B.; Uvdal, K.; Erlandsson, R.; Elwing, H.; Lundstroem, I. J. Colloid Interface Sci. 1991, 147, 103.

The effect of the ligand-to-ZrO2 molar ratio on the dispersing of ZrO2 nanocrystals in THF was further investigated. It was found that the lowest ligand-to-ZrO2 molar ratios needed to obtain transparent ZrO2 dispersion (ZrO2 content 0.9 wt %) are 0.09:1, 0.05:1, and 0.06:1, for GPTMS, APTES and IPTES, respectively. Above these critical molar ratios, the transparency of the ZrO2 dispersion slightly increased as the ligand-to-ZrO2 molar ratio was increased. That is, the transmission of the ZrO2 dispersion at a wavelength of 550 nm changed from 0.81 to 0.83, 0.82 to 0.84, and 0.83 to 0.85 when the ligand-to-ZrO2 molar ratio ranges were 0.09-0.57, 0.05-0.44, and 0.06-0.45 for the cases with GPTMS, APTES and IPTES, respectively. The dispersibility of ZrO2 nanocrystals in other organic solvents (DMF, pyridine, toluene, ethane nitrile, acetone, n-BAc, and isopropyl alcohol) was also examined using GPTMS, APTES, and IPTES as ligands. The appearance of the obtained ZrO2 dispersions is summarized in Table 1. For comparison, the transparency of ZrO2 dispersions in THF is also presented in the table. Transparent ZrO2 dispersion can not be obtained for all other organic solvents involved in our experiments when GPTMS was used as the ligand. APTES worked for the preparation of transparent ZrO2 dispersion in pyridine and toluene but not in other solvents, while IPTES amazingly made the ZrO2 nanoparticles dispersible in the organic solvents with strong-, mediumor nonpolarity (i.e., DMF, pyridine, or toluene). These dispersion behaviors could be explained by the bonding modes of SCAs with ZrO2 nanoparticles. Hydrogen bond between amino group and hydroxyl group for APTES ligand and chemical bond between isocyanato group and hydroxyl group for the IPTES ligand may tend to form in toluene, consuming the polar portion of the attached SCA chains and hence enhancing the compatibility of the APTES (or IPTES)-functionalized ZrO2 nanocrytals with toluene. However, for the case with DMF as the dispersing media, the strong interaction of isocyanato group with DMF may lead the hydroxyl groups of ZrO2 nanoparticles to preferentially react with the trialkoxy group rather than the isocyanato group. The long-term stability of all transparent ZrO2 dispersions was investigated and summarized in Table 2. To eliminate the influence of moisture on the agglomeration of ZrO2 nanoparticles, the ZrO2 dispersions were stored in a well-sealed glass bottle. The GPTMS-functionalized ZrO2/THF and IPTES-functionalized ZrO2/pyridine dispersions kept their transparency even after 8 weeks of storage. But the other dispersions easily became opaque during storage, suggesting their poor storage stabilities. To further validate the poor storage stability, the SCAcontaining ZrO2/THF dispersions were stored in a 60 °C oven. Variations of the transparency and the Z-average size of the

11500 Langmuir, Vol. 24, No. 20, 2008

Luo et al.

Table 1. Summary of the Appearance of ZrO2 Dispersion in Different Organic Solventsa transparency of ZrO2 nanocrystalsb ligand

THF

DMF

pyridine

toluene

no zirconia nanoparticles and ligand 0.912 0.898 0.902 0.892 no ligand cloudyc cloudy cloudy cloudy GPTMS 0.819 cloudy cloudy cloudy APTES 0.835 cloudy 0.828 0.815 IPTES 0.852 0.835 0.846 0.827

ethane nitrile acetone

cloudy cloudy cloudy


n-BAc

isopropyl alcohol



a Preparation conditions: ZrO2 concentration ) 0.9 wt %, ligand-to-ZrO2 molar ratio ) 0.2:1, sonication time ) 30 min, and storing time ) 12 h. b The transmission at a wavelength of 550 nm. c “Cloudy” means that transparency of the dispersion was never reached during the entire process.

Table 2. Summary of the Long-Term Stability of All Transparent ZrO2 Dispersionsa ligand GPTMS APTES APTES APTES IPTES IPTES IPTES IPTES

dispersing media THF THF pyridine toluene THF DMF pyridine toluene

long-term stabilityb transparent after 8 weeks cloudy after three days cloudy after 6 h cloudy after three days cloudy after five days cloudy after one week transparent after 8 weeks cloudy after five days

a All ZrO2 dispersions were prepared by sonication for 30 min at a ligandto-ZrO2 molar ratio of 0.2 and a ZrO2 concentration of 0.9 wt %. b Stored in a sealed glass bottle at room temperature.

Figure 4. FTIR spectra of (a) as-synthesized ZrO2 nanoparticles, (b) GPTMS-functionalized ZrO2 nanoparticles (preparation conditions: GPTMS-to-ZrO2 molar ratio ) 0.2:1; sonication time ) 30 min; reaction time ) 24 h at room temperature, dried under vacuum at room temperature), and (c) GPTMS.

Figure 3. The changes of the transparency (filled symbols) and Z-average size (open symbols) of ZrO2/THF dispersions as a function of storing time in an oven with a temperature of 60 °C using the ligands (a) GPTMS, (b) APTES, and (c) IPTES (ligand-to-ZrO2 molar ratio ) 0.2:1; ZrO2 concentration ) 0.9 wt %).

dispersions with storing time are plotted in Figure 3. The transparencies of APTES-containing and IPTES-containing ZrO2/ THF dispersions began to decline after 4 and 6 h of storage and became completely cloudy at 7 and 11 h of storage. Accompanying the reduction of the transparency, an increasing Z-average size was clearly observed for both APTES- and IPTES-functionalized ZrO2 dispersions, which was due to the formation of ZrO2 aggregates (or agglomerates), revealing their particle size distributions (see Figure 3s and Figure 4s in the Supporting Information). For the dispersion with APTES ligand, the increasing polarity of ZrO2 nanoparticles caused by more APTES attached may be attributed to the agglomeration of ZrO2 nanoparticles. For the dispersion with IPTES ligand, ZrO2 nanoparticles may be coupled by IPTES, resulting in the appearance of ZrO2 nanoparticle aggregates. The detailed reasons will be evidenced from the FTIR, NMR, and TGA analyses below. Figure 3 also shows that the transparency of the GPTMScontaining ZrO2 dispersion is not deteriorated after it was heated for 11 h, and the particle size distribution (Figure 5s in the Supporting Information) as well as the Z-average size does not

change with heating time, confirming the better stability of the dispersion. FTIR Analysis of SCA Attachments. To demonstrate the formation of a chemical bond between SCAs and ZrO2, and to understand the bonding mode, the SCA-functionalized ZrO2 nanoparticles obtained from depositing the transparent dispersion or from the precipitates that formed during storage were sufficiently washed with appropriate solvent (see Experimental Section) and dried for FTIR characterization. Figure 4 shows the FTIR spectra of GPTMS, as-synthesized ZrO2 nanoparticles, and GPTMS-functionalized ZrO2 nanoparticles. The as-synthesized ZrO2 nanoparticles display three strong absorption bands: 500-850, 1200-1460, and 1460-1680 cm-1. The former is due to a Zr-O bond, while the latter two bands are attributed to the inherently adsorbed organic components, as reported previously.29 Comparing with as-synthesized ZrO2 nanoparticles, two new strong absorption bands, 800-1200 and 2800-3000 cm-1, appear on the spectrum of GPTMS-functionalized ZrO2 nanoparticles, which are assigned to the stretching vibration of the Si-O-Zr bond and the C-H bond, respectively. The peaks at 910 and 1254 cm-1, respectively due to the asymmetric deformation and the symmetric axial in-phase deformation of the epoxide ring,32,33 are also visible from the spectrum. These newly occurring absorption peaks illustrate that GPTMS is chemically bonded with ZrO2 nanocrystals via its trialkoxy groups condensing with the hydroxyl groups of ZrO2 nanoparticles. Figure 5 displays the FTIR spectra of APTES-functionalized ZrO2 nanoparticles, APTES, as well as as-synthesized ZrO2 nanoparticles. Similar to GPTMS-functionalized ZrO2 nano(32) Innocenzi, P.; Brusatin, M.; Guglielmi, M.; Bertani, R. Chem. Mater. 1999, 11, 1672. (33) Innocenzi, P.; Brusatin, M. Chem. Mater. 2000, 12, 3726.


Figure 5. FTIR spectra of (a) as-synthesized ZrO2, (b) APTESfunctionalized ZrO2 nanoparticles (APTES-to-ZrO2 molar ratio ) 0.2: 1; sonication time ) 30 min; reaction time ) 24 h at room temperature, dried under vacuum at room temperature), and (c) APTES.

Langmuir, Vol. 24, No. 20, 2008 11501

Figure 7. 13C NMR spectra of (a) as-synthesized ZrO2, (b) GPTMSfunctionalized ZrO2, (c) APTES-functionalized ZrO2, and (d) IPTESfunctionalized ZrO2 (preparation conditions: ligand-to-ZrO2 ratio ) 0.2: 1; sonication time ) 30 min; reaction time ) 24 h at room temperature, dried under vacuum at 80 °C).

Figure 6. FTIR spectra of (a) as-synthesized ZrO2, (b) IPTESfunctionalized ZrO2 nanoparticles (IPTES-to-ZrO2 molar ratio ) 0.2:1; sonication time ) 30 min; reaction time ) 24 h at room temperature, dried under vacuum at room temperature), and (c) IPTES.

particles, two new absorption bands, namely, 800-1200 and 2800-3000 cm-1, are observed on the spectrum of APTESfunctionalized ZrO2 nanoparticles, suggesting APTES has been successfully attached to the ZrO2 nanoparticles. However, the asymmetric and symmetric stretching vibrations of the N-H bond, located at wavenumbers 3366 and 3296 cm-1 on the spectrum of APTES, are not seen, possibly owing to the overlapping with the stretching vibration of the hydroxyl group. Relative to the spectrum of as-synthesized ZrO2 nanoparticles, the intensity of the peak at 1580 cm-1 is enhanced on the spectrum of APTES-functionalized ZrO2 nanoparticles, which may presumably be regarded as evidence of the existence of a -NH2 group because the -NH2 group of APTES has a bending vibration at this wavenumber. Figure 6 presents the spectra of IPTES, IPTES-functionalized ZrO2 nanoparticles, and as-synthesized ZrO2nanoparticles. The strong peaks at the band of 2800-3000 cm-1 on the spectrum of IPTES-functionalized ZrO2 nanoparticles indicate the bonding of IPTES with ZrO2 nanoparticles. But no signal of an isocyanato group is observed on the spectrum. The isocyanato group must be completely consumed in the reaction with the hydroxyl group of ZrO2 nanoparticles, evidenced by the new absorption peaks at 1700 and 1538 cm-1, which are caused by the deformation of the urethane bond. The band due to the formation of the Si-O-Zr bond also appears in the range of 850-1200 cm-1. These Si-O-Zr bonds may be formed in the drying of IPTESfunctionalized nanoparticles or during the dispersing stage.

Figure 8. 29Si NMR spectra IPTES-functionalized ZrO2 from (a) supernatant and (b) precipitate (preparation conditions: ligand-toZrO2 ratio ) 0.2:1; sonication time ) 30 min; reaction time ) 5 days at room temperature, dried under vacuum at room temperature).

Comparing with APTES-functionalized nanoparticles, the intensities of asymmetric stretching vibration of Si-O-C at 1081 cm-1, symmetric vibration of C-O at 1167 cm-1 as well as the rocking of -CH3 of ethoxyl group at 957 cm-1, are stronger, indicating that the ethoxyl group of IPTES was less reacted with ZrO2 nanoparticles relative to APTES. This fact indirectly demonstrates that the attachment of IPTES via reaction of the isocyanato group with the hydroxyl group is superior to that via the condensation of the triethoxy group with the hydroxyl group. 13C and 29Si NMR Analysis of SCA Attachment. Figure 7 shows 13C NMR spectra of as-synthesized ZrO2 nanoparticles and SCA-functionalized ZrO2 nanoparticles. The signals corresponding to special C-atoms were assigned and marked in the figure. The spectrum of as-synthesized ZrO2 nanoparticles (spectrum a) reveals that the inherently surface-adsorbed organic species include benzylmethoxyl, benzoate, and isopropoxyl. Some new signals at 72.6 ppm (peak 7), 24.3 ppm (peak 11), and 10.4 ppm (peak 12), attributed to the methylene group of GPTMS, appear on the spectrum of GPTMS-functionalized ZrO2 nanoparticles (spectrum b). Similarly, the signals of the methylene group are observed at 43.6 ppm (peak 13), 25.8 ppm (peak 14), and 11.2 ppm (peak 15) for APTES-functionalized ZrO2 nanoparticles and 11.8 ppm (peak 16), 22.9 ppm (peak 17), and

11502 Langmuir, Vol. 24, No. 20, 2008

Figure 9. Schematic diagram of the bonding mode of SCA on the surface of ZrO2 nanoparticles.

42.7 ppm (peak 18) for IPTES-functionalized ZrO2 nanoparticles. These signals demonstrate that all SCAs have been successfully attached to ZrO2 nanoparticles, in accord with FTIR results. The signals due to benzylmethoxyl and benzoate species still exist, but those due to isopropxyl species disappear in the spectra of SCA-functionalized ZrO2 nanoparticles. This fact suggests that the adsorbed benzylmethoxyl and benzoate species are stable and can not be replaced by SCA, while the isopropoxyl species is easily desorbed. In addition, two signals (peaks 9 and 10) at 50.8 ppm and 43.7 ppm due to the two C-atoms of the epoxide group are clearly seen in the spectrum of GPTMS-functionalized ZrO2 nanoparticles, indicating that ZrO2 nanoparticles bearing an epoxide group (Figure 9a) are successfully obtained. The signal (peak 19) at 158.6 ppm due to the C-atom of urethane bond proves the reaction of the isocyanato group with the hydroxyl group. The C signals of methoxyl or ethoxyl group are not observed for all SCA-functionalized ZrO2 nanoparticles, pointing to the reaction of most of these groups after drying at 80 °C. For the case with IPTES, the bonding mechanism is rather complex because of its bifunctional reactivity with the ZrO2 surface. To further understand the reaction mechanism between IPTES and ZrO2 nanoparticles as well as the reason for the occurrence of precipitate after heating treatment, solid-state 29Si NMR analysis of the IPTES-functionalized ZrO nano2 particles from precipitates and from supernatant was carried out. The results are given in Figure 8. These Si signals at -45.8 ppm, -49.5 ppm, and -54.2 ppm belong to T0, T1 and T2 structures. The existence of a T0 structure, namely, ZrO2 nanoparticles bearing a triethoxysilyl group (Figure 9c), indirectly confirms that IPTES was chemically bonded to the ZrO2 nanoparticles via the reaction of the isocyanato group with the hydroxyl group. However, T1 and T2 structures mean that some ethoxyl groups also reacted with ZrO2 nanoparticles. Comparing the two spectra in Figure 8, we found that the condensation degree of the adsorbed IPTES was higher for the IPTES-

Luo et al.

Figure 10. (a) Thermogravimetric curves of as-synthesized ZrO2 and the GPTMS-functionalized ZrO2 nanocrystals with different GPTMSto-ZrO2 molar ratios (0.11:1, 0.23:1, 0.39:1, and 0.57:1), and (b) their corresponding differential curves.

functionalized ZrO2 nanoparticles in the precipitate than those in the supernatant. Therefore, it is the further reaction between IPTES-functionalized nanoparticles via condensation of ethoxysilyl groups that is responsible for the formation of precipitate (see the schematic diagram in Figure 9g,h). The two peaks at -81.0 ppm and -84.2 ppm seem to be atttibuted to Q structures, but, the formation mechanism of these structures is unclear now. Similar peaks were also found in the modification of MPS with ZrO2 powder reported by Turner et al.23 They thought that these peaks possibly stemmed from ZrSiO4 powders because their values are very close to the experimentally observed peak for ZrSiO4 powders. TGA of SCA Attachment. The quantitative amount of the chemisorbed ligands was determined by TGA. Figure 10 shows the typical TGA and DTG curves of as-synthesized ZrO2 nanocrystals as well as the ZrO2 nanocrystals functionalized at different GPTMS-to-ZrO2 molar ratios. From the DTG curves, it clearly shows that both as-synthesized ZrO2 nanoparticles and GPTMS-functionalized ZrO2 nanoparticles have three distinctive stages of weight loss. The weight loss stage below 170 °C belongs to the loss of physically adsorbed water and organic solvent, while the other two weight loss stages above 170 °C are attributed to the chemisorbed organic groups (including some hydroxyl groups) on the surface of ZrO2 nanocrystal. As for GPTMSfunctionalized ZrO2, the increased weight loss above 170 °C should result from the organic components in the attached GPTMS. Since GPTMS attaches to the ZrO2 surface on the basis of T1, T2, and T3 structures, the number of GPTMS molecules attached is estimated on the basis of the assumption that GPTMS molecules are bonded to the ZrO2 surface with pure T1, T2, or T3 structure. The results are given in Table 3. The weight loss increases with increasing GPTMS-to-ZrO2 molar ratio and reaches equilibrium at a GPTMS-to-ZrO2 molar ratio greater than 0.39,


Langmuir, Vol. 24, No. 20, 2008 11503

Table 3. Quantitative Results of the Chemical Attachment of Ligands on ZrO2 Nanoparticles from TGA Experimentsa amount of ligand-attached, µmol/m2b sample name

ligand-to-ZrO2 molar ratio

reaction time (at 60 °C), h

weight loss between 170 and 750 °C, %

final weight remaining, %

as-synthesized ZrO2 G-1 G-2 G-3 G-4 G-5 G-6 G-7 G-8 A-1 A-2c A-3d I-1 I-2c I-3d

0.11 0.23 0.39 0.57 0.14 0.14 0.14 0.14 0.19 0.19 0.19 0.17 0.17 0.17

12 12 12 12 1 3 5 8 0.1 5 5 0.2 8 8

8.1 12.7 14.1 14.9 15.2 11.6 11.9 12.5 13.2 8.4 14.2 17.0 8.4 25.9 31.7

88.1 81.7 81.3 79.4 79.1 84.6 84.1 83.9 83.3 86.8 81.7 79.4 87.2 69.7 64.8

T1

T2

T3

1.3 1.7 1.9 2.0 1.0 1.1 1.2 1.4 0.14 2.2 3.1

1.7 2.2 2.5 2.6 1.3 1.4 1.6 1.8 0.24 3.7 5.4

2.5 3.1 3.6 3.8 1.8 2.0 2.3 2.6 0.85 13.5 19.6

a The SCA-functionalized ZrO2 nanoparticles were dried at 80 °C overnight. b Assuming that ZrO2 nanoparticles are monodisperse with a crystallite size of 3.8 nm and a density of 5.6 g/cm3. c The SCA-functionalized ZrO2 nanoparticles from precipitate. d The SCA-functionalized ZrO2 nanoparticles from supernatant.

Figure 11. Thermogravimetric curves of (a) as-synthesized ZrO2, and of the APTES-functionalized ZrO2 nanocrystals (b) from the ZrO2 dispersion just when it became completely transparent under ultrasonication, (c) from the precipitate, and (d) from the supernatant of the dispersion just when it became turbid after a relatively long heat-treatment.

indicating the highest amount of GPTMS attached. The effect of reaction time on the chemisorbed amount of GPTMS was also investigated at a GPTMS-to-ZrO2 molar ratio of 0.14:1.0, and the quantitative results are given in Table 3. It is obvious that the amount of GPTMS attached increases with increasing reaction time. Therefore, both the molar ratio of GPTMS-to-ZrO2 and the reaction time are good ways to tailor the amount of GPTMS attached. For the cases using APTES or IPTES as ligands, three typical samples were adopted for TGA analysis. One was deposited from the transparent ZrO2/THF dispersion, which was prepared only after ultrasonication and a short time of heat-treatment. The other two samples came from the supernatant and the precipitate of the ZrO2/THF dispersion, which underwent a long heat-treatment and contained precipitate. The TGA curves are shown in Figures 11 and 12, and their exact preparation conditions as well as the quantitative data are given in Table 3. The results reveal that even a minute amount of APTES or IPTES attached was enough to get a transparent ZrO2 dispersion. When heating time was prolonged, the amount of chemisorbed APTES or IPTES markedly increased for the sample, both from the supernatant and from the precipitate. Moreover, it is quite interesting that the functionalized ZrO2 nanocrystals remaining

Figure 12. Thermogravimetric curves of (a) as-synthesized ZrO2 and (b) the IPTES-functionalized ZrO2 nanocrystals from the ZrO2 dispersion just when it became completely transparent under ultrasonication, as well as the IPTES-functionalized ZrO2 nanocrystals (c) from the precipitate and (d) from the supernatant of the dispersion just when it became turbid after a relatively long heat-treatment.

Figure 13. pH-dependence of the transparency of aqueous ZrO2 dispersions for APTES-functionalized ZrO2 nanoparticles prepared with APTES-to-ZrO2 molar ratios of (a) 0.09:1, (b) 0.27:1, and (c) 0.44:1. (ZrO2 concentration ) 0.8 wt %).

in supernatant have a higher amount of chemisorbed APTES (or IPTES) than those from precipitates. In addition, comparing the TGA data, we found that the highest amount of organic component was realized by IPTES, further demonstrating that IPTES was

11504 Langmuir, Vol. 24, No. 20, 2008

Luo et al.

Figure 14. TEM images of aqueous APTES-functionalized ZrO2 dispersion at different magnifications. (ligand-to-ZrO2 molar ratio ) 0.09:1; ZrO2 concentration ) 0.8 wt %).

bonded to ZrO2 nanoparticles via the formation of a urethane bond without loss of any ethoxyl groups. The reasons for the above phenomena are deduced as follows: For IPTES ligand, the attachment of IPTES leads to ZrO2 nanocrystals bearing an amino group (Figure 9f) and increases the polarity of the surface of ZrO2 nanoparticles. Because the highly polar ZrO2 nanoparticles are poorly compatible with THF, they precipitate from THF. The higher organic component of APTES-functionalized ZrO2 nanocrystals results from unreacted ethoxyl groups, which are a benefit for the compatibility between ZrO2 nanocrystals and THF. Namely, the attached APTES molecules have low hydrolysis and condensation degree (see the schematic diagram in Figure 9b). As for the IPTES ligand, the lesser amount of organic component in the precipitate is caused by the elimination of the ethoxyl group in the further hydrolysis and condensation among IPTES-functionalized ZrO2 nanoparticles, in agreement with the results observed in 29Si NMR analysis. The precipitate can not be dispersed in any organic solvents or water, futher proving that a chemical bond among ZrO2 nanoparticles is formed (see Figure 9g,h). Dispersion of Zirconia Nanocrystal in Water. Although APTES-functionalized ZrO2 dispersions would precipitate from THF if excessive APTES with a high condensation degree was attached, the precipitate could be redispersed in water when the dispersion was adjusted to an acidic status via addition of hydrochloric acid. This dispersion behavior, in one aspect, displays that the precipitate of APTES-functionalized ZrO2 nanoparticles is actually composed of soft agglomerates, demonstrating the formation mechanism of precipitates proposed above; on the other hand, the behavior shows that APTES-functionalized ZrO2 nanocrystals is one of the ways to obtain a transparent aqueous ZrO2 nanocrystal dispersion, which was rarely reported before. The dispersibility of APTES-functionalized ZrO2 nanoparticles could be attributed to the ionization of -NH2, namely, the formation of NH3+ on the surface of ZrO2 nanoparticles (see schematic diagram in Figure 9i), which is helpful to the deagglomeration of precipitate via static repulsive force. Further research has indicated that the transparency of aqueous ZrO2 dispersion depends strongly on the pH and on the APTESto-ZrO2 molar ratio for the preparation of APTES-functionalized ZrO2 nanoparticles, as shown in Figure 13. Although transparent ZrO2 dispersions were achieved under acidic conditions, the transition pH for the dispersion changing from opaque to transparent was different for different APTES-functionalized ZrO2 nanoparticles. At the APTES-to-ZrO2 molar ratio of 0.09:1, the transition pH is in the range of 4.5-5.0. When the APTES-toZrO2 molar ratio is increased to 0.27:1 and 0.44:1, the transition

Figure 15. Particle size distribution of aqueous APTES-functionalized ZrO2 dispersion (ligand-to-ZrO2 molar ratio ) 0.09:1; ZrO2 concentration ) 0.8 wt %; pH value ) 3.5).

pH lies in the range of 5.5-6.5. In other words, less HCl is needed for the dispersing of ZrO2 nanoparticles with higher amounts of APTES attached. This is because a higher amount of a chemically bonded amino group can offer better compatibility of ZrO2 nanoparticles with water. As a consequence, less ionized NH3+ was needed for the stabilization of ZrO2 nanoparticles in water. Zeta-potential measurement was conducted for the transparent aqueous ZrO2 dispersion. A potential of +31.3 mV was acquired for the sample that corresponded to the data of Figure 13, curve a, at a pH value of 3.5, confirming the positively charged surface of ZrO2 nanoparticles in aqueous phase. A typical aqueous transparent ZrO2 dispersion was characterized by HRTEM and a particle size analyzer. The HRTEM images at different magnifications (Figure 14) show that most of the ZrO2 nanoparticles are individually dispersed in water. The particle size distribution (Figure 15) displays two peaks and a high polydispersity index (PDI), implying the existence of ZrO2 aggregates. Peak 1 corresponds to the primary particle size distribution and has the percentage of 74.3%, indicating that most of ZrO2 particles are successfully deagglomerated in water. Peak 2 belongs to ZrO2 aggregates that may be formed in the stage of preparation of amino group-bearing ZrO2 nanoparticles. It is possible that these aggregates could be overcome by increasing the APTES-to-ZrO2 molar ratio during functionalization of ZrO2 nanoparticles in THF. Anyway, a transparent aqueous ZrO2 dispersion containing ZrO2 nanoparticles dispersed mostly on the primary particle size level is really fabricated.

Conclusions Three bifunctional SCAs;GPTMS, APTES, and IPTES;were used to disperse and functionalize nonaqueous synthesized ZrO2


nanocrystals in organic solvent. Completely transparent ZrO2 dispersions in THF were obtained in the presence of these three SCAs. However, only APTES and IPTES were efficient for the dispersion of ZrO2 nanocrystals in pyridine and toluene, and only IPTES was effiecient for the dispersion of ZrO2 nanocrystals in DMF. Except for GPTMS/ZrO2/THF dispersion and IPTES/ ZrO2/pyridine dispersion, all other transparent dispersions have poor long-term stability. FTIR, 13C NMR, and TGA results demonstrate that all three SCAs are chemically bonded with ZrO2 nanoparticles, but in different bonding modes. GPTMS and APTES are attached via the condensation of a trialkoxy group with the hydroxyl group of ZrO2 nanoparticles, whereas IPTES is attached via the reaction between an isocyanato group and the hydroxyl group of ZrO2 nanoparticles, leading to ZrO2 nanoparticles bearing epoxy, amino, and triethoxysilyl groups, respectively. The increasing polarity of APTES-functionalized ZrO2 nanocrystals caused by excessive attachment of APTES and high hydrolysis and condensation degree of the triethoxyl group explains the formation of precipitate in the ZrO2 dispersion with APTES. However, the precipitate in IPTES-functionalized ZrO2 dispersions is formed by interparticle coupling via the attached triethoxysilyl groups. Completely transparent ZrO2 nanocrystal dispersion in water was also achieved with APTES-

Langmuir, Vol. 24, No. 20, 2008 11505

functionalized ZrO2 nanoparticles by the addition of a little hydrochloric acid. HRTEM and DLS results reveal that most of the APTES-functionalized ZrO2 nanoparticles are deagglomerated on the primary particle size level. The transparency of the aqueous ZrO2 dispersion strongly depends on the amount of APTES attached and the pH of the dispersion. The lower the amount of APTES attached is, the lower the pH value needed to reach the transparency. The successful fabrication of transparent dispersion of ZrO2 nanocrystal bearing glycidyl, amino, and triethoxysilyl groups in various dispersing media is absolutely beneficial for the application of ZrO2 nanocrystals. Acknowledgment. We are grateful for the financial support from the New Century Excellent Talent Foundation of the Ministry of Education of China (NCET-07-0210), the National Nature Science Foundation (No. 50703005) of China, and the Shanghai Leading Academic Discipline Project (No. B113). Supporting Information Available: Change of transparency of a ZrO2 dispersion as a function of sonication time for all three SCAs; TEM images and particle size distributions for all three SCAfunctionalized ZrO2 dispersions. This information is available free of charge via the Internet at http: //pubs.acs.org. LA801943N

Dispersion and Functionalization of Nonaqueous Synthesized

Recommend Documents