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Particle-Particle Interactions and Chain Dynamics of Fluorocarbon and Hydrocarbon Functionalized ZrO2 Nanoparticles Andrew O’Donnell, Kimberly Yach, and Linda Reven* Centre for Self-Assembled Chemical Structures (CSACS), Department of Chemistry, McGill UniVersity, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6 ReceiVed August 13, 2007. In Final Form: NoVember 28, 2007 The chain conformation and dynamics of hydrocarbon and perfluorocarbon fatty acids adsorbed on 4 nm ZrO2 particles were characterized by solid-state 13C chemical shift and 19F NMR relaxation measurements, respectively, and compared to those from previous studies on lower surface area fumed metal oxide powders. The interdigitation of chains between neighboring particles, which increases with chain length, can be detected from the splitting of the 13C NMR and 19F NMR signals of the CH and CF groups, respectively. Similar to the case of alkanethiol self3 3 assembled monolayers (SAMs) on gold nanoparticles, this interdigitation allows for efficient chain packing despite the high surface curvature. The hydrocarbon chains on the ZrO2 nanoparticles are more ordered, and the reversible chain length dependent order-disorder transition temperatures are elevated relative to those of the same fatty acids adsorbed on fumed ZrO2 powder. Likewise, the 19F spin lattice relaxation times of the fluorocarbon chains approach those of the bulk acids with increasing chain length and interdigitation, indicating densely packed chains.
1. Introduction Self-assembled monolayers (SAMs) are widely employed to modify surface properties, since by definition they offer an organized and controllable array of surface functionalities.1-3 In recent years, SAMs have been increasingly used to tailor the surface chemistry of colloidal particles and other high surface area substrates. In the case of nanometer sized particles, the effect of a high surface curvature and particle-particle interactions on the molecular packing and dynamics may cause the properties of SAMs to be quite different from those deposited on flat surfaces. The properties of perfluorocarbon versus hydrocarbon monolayers on nanometer sized substrates may vary substantially due to large differences in conformation, intermolecular interactions, and chain flexibility. Perfluorocarbon SAMs offer an attractive option due to high thermal stabilities, resistance to chemical attack, and low surface energies. The origins of these properties are the strong C-F bond and large van der Waals radius of fluorine relative to that of hydrogen.4 Intermolecular interactions of perfluoroalkanes are generally considered to be weaker than those between hydrocarbon chains, which influences the final packing arrangement when these molecules are deposited as monolayers.5 The larger size of the fluorine atom also has the effect of inducing a slight twist in the perfluorocarbon chain such that a helical conformation results rather than the all-trans conformation of crystalline hydrocarbon chains.6 In addition to the chain conformation and packing, the dynamic properties of perfluorocarbon monolayers differ from their hydrocarbon counterparts. Whereas alkane SAMs undergo chain length dependent, reversible chain melting transitions,7 the large energy * To whom correspondence should be addressed. E-mail: linda.reven@ mcgill.ca. (1) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (2) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (3) Allara, D. L. Biosens. Bioelectron. 1995, 10, 771. (4) Hiyama, T. Organofluorine compounds; Springer-Verlag: Berlin-Heidelberg, 2000. (5) Lee, M. H.; Ha, T. H.; Kim, K. Langmuir 2002, 18, 2117. (6) Dixon, D. A.; Van-Catledge, F. A. Int. J. Supercomput. Appl. 1988, 2, 62. (7) Badia, A.; Lennox, R. B.; Reven, L. Acc. Chem. Res. 2000, 33, 475.
barrier to C-C bond rotations of the perfluoroalkanes renders these chains less flexible.8,9 Langmuir monolayers10-12 and SAMs13,14 of perfluoro fatty acids have been studied both experimentally and theoretically,15-17 and there are a few reports of gold and silver nanoparticles functionalized with fluorocarbon chains.18-20 In one study, gold particles of 3 nm diameter with fluorocarbon chains attached via CH2CH2SH linker groups were found to pack hexagonally.19 The interparticle spacing was determined to be slightly smaller than the bimolecular length, which was taken as evidence of interpenetration of chain ends. Similar results were found for fluoro-thiol capped silver nanoparticles.20 Solid-state NMR spectroscopy has been used to characterize surface bonding, chain conformation, and dynamics of SAMs deposited on gold nanoparticles21 and metal oxide powders.22 Metal oxide substrates provide an opportunity to study the effect of the surface curvature of the substrate particles on SAM organization, since micrometer to nanometer sized particles are available. In the case of organothiol SAMs, only 2-5 nm diameter Au nanoparticles have been studied, since larger diameter particles are difficult to prepare and the NMR peaks become broad.23 19F (8) Wunderlich, B.; Moeller, M.; Grebowicz, J.; Baur, H. AdV. Polym. Sci. 1988, 87, 44. (9) Eaton, D. F.; Smart, B. E. J. Am. Chem. Soc. 1990, 112, 2821. (10) Ha, K.; Kim, J.-M.; Rabolt, J. F. Thin Solid Films 1999, 347, 272. (11) Ha, K.; Ahn, W.; Rho, S.; Suh, S.; Synn, D.; Stelzle, M.; Rabolt, J. F. Thin Solid Films 2000, 372, 223. (12) Shibata, O.; Furuya, H.; Moroi, Y.; Saito, M.; Matuura, R. Thin Solid Films 1998, 327-329, 123. (13) Chau, L.-K.; Porter, M. D. Chem. Phys. Lett. 1990, 167, 198. (14) Wallace, R. M.; Chen, P. J.; Henck, S. A.; Webb, D. A. J. Vac. Sci. Technol., A 1995, 13, 1345. (15) Schmidt, M. E.; Shin, S.; Rice, S. A. J. Chem. Phys. 1996, 104, 2101. (16) Schmidt, M. E.; Shin, S.; Rice, S. A. J. Chem. Phys. 1996, 104, 2114. (17) Barton, S. W.; Goudot, A.; Bouloussa, O.; Rondelez, F.; Lin, B.; Novak, F.; Acero, A.; Rice, S. A. J. Chem. Phys. 1992, 96, 1343. (18) Lee, S. J.; Han, S. W.; Kim, K. Chem. Commun. 2002, 442. (19) Yonezawa, T.; Onoue, S.; Kimizuka, N. Langmuir 2001, 17, 2291. (20) Yonezawa, T.; Onoue, S.; Kimizuka, N. AdV. Mater. 2001, 13, 140. (21) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Reven, L. Langmuir 1996, 12, 1262. (22) Pawsey, S.; Yach, K.; Halla, J.; Reven, L. Langmuir 2000, 16, 3294. (23) Hostetler, M. J.; Wingate, J. E.; Zhong, C. J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17.
10.1021/la702503m CCC: $40.75 © 2008 American Chemical Society Published on Web 02/13/2008
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Figure 1. TEM images and particle size distribution of the ZrO2 nanopowder and fumed ZrO2 powder. Table 1. Coverages, 13C Chemical Shifts, C-H IR Stretching Frequencies, and the Order/Disorder Transition Temperatures Detected by NMR as a Function of Chain Length for CnH2nO2/ZrO2 13C
sample, CnH CnH2nO2/ZrO2 C13H C14H C15H C16H C19H C20H a
chemical shifts at RT (ppm)
FTIR at RT (cm-1)
type of ZrO2
% coverage
inner CH2
CH3
νs
νas
order/disorder temperature (°C)
nano nano fumeda nano fumeda nano fumeda nano fumeda nano
∼100 ∼100 64 94 71 96 75 90 65 88
31.9 31.9 30.4 32.1 33, 30 33 33 33 33 33
13 13 13 13 13 13, 15 13 13, 15 13 13, 15
2918 2921 2923 2918 2919 2919 2917 2917 2916 2918
2849 2850 2852 2850 2850 2850 2849 2849 2848 2849
n/a 5 -15 15 25 45 50 55 70 73
Data for the fumed ZrO2 powder are from ref 22.
Table 2. Surface Coverages and 19F Chemical Shifts of the CF3 Groups of the Perfluoro Fatty Acids Adsorbed on ZrO2 Nanopowdera CnF
type of ZrO2
coverage (%)
C8F
nano fumedb nano nano nano fumedb nano fumedb nano fumedb
58 50 61 62 63 66 106 73 114 83
C10F C12F C14F C16F C18F
CF3a (ppm) -83.6 (sh) -83.6 (sh) -83.6 (sh) -83.6 (37%) -83.6 (63%) -83.7 (75%)
CF3b (ppm) -84.8 -85 -85.4 -85.4 -85.2 -85 -85.0 -85 -85.2 -85
a The numbers in parenthesis for CF3a are the percent contribution to the total CF3 signal obtained from the integrated intensities. b Data for the fumed ZrO2 are taken from ref 24.
NMR is a convenient method for studying the dynamics of fluorinated chains. The 100% natural abundance and high sensitivity of 19F (83% that of 1H) coupled with a large chemical shift range are advantages to be exploited in the study of surface-
bound species. A previous 19F NMR study of perfluorinated acids adsorbed on fumed metal oxide powders found that partial and densely packed monolayers formed on TiO2 and ZrO2 powders, respectively.24 A distinctive chemical shift of the CF3 group at the monolayer/air interface was identified, and an increase in the surface coverage with increasing perfluorocarbon chain length was observed. 19F spin lattice relaxation measurements showed an enhanced mobility of the monolayers relative to the bulk acids in terms of both the frequency and amplitude of the chain motion.24 In the present study, a series of perfluorinated and hydrocarbon fatty acid monolayers on ZrO2 nanoparticles are investigated by solid-state NMR spectroscopy. The ZrO2 nanopowder consists of separated single crystals with an average diameter of 4 nm, representing a large difference in particle size compared to the previously studied fumed metal oxide powders consisting of agglomerates of 30 nm primary particles.22,24 For the hydrocarbon SAMs, the 13C chemical shift provides information about the (24) Pawsey, S.; Reven, L. Langmuir 2006, 22, 1055. (25) Deacon, G. B.; Phillips, R. J. Coord. Chem. ReV. 1980, 33, 227.
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Figure 3. 13C CP/MAS NMR spectrum of C20H, CH3(CH2)18CO2, adsorbed on the ZrO2 nanopowder with the assigned carbon signals. A contact time of 3 ms, a MAS spinning frequency of 5 kHz, and 4000 acquisitions were used. The spinning sidebands are indicated by a star.
Figure 2. PAS-IR spectra of the (a) bulk and (b) adsorbed perfluoro acids.
chain conformation whereas the chain packing of the fluorocarbon monolayers is assessed by 19F spin lattice relaxation measurements. In both systems, the presence of interdigitation of the chains is detected via the splitting of the 13C and 19F signals for the CH3 and CF3 groups, respectively. Such a splitting has not been reported previously, since the earlier NMR studies of the alkane SAMs used either the fumed metal oxide substrates where no interdigitation occurs or the 2-3 nm gold nanoparticles where the chains are completely interdigitated. The direct detection of such particle-particle interactions is important for comparisons of the properties of SAMs deposited on flat versus colloidal substrates as well as for controlling the self-assembly of nanoparticles through surface functionalization. 2. Experimental Section 2.1. Materials. Separated equiaxial cubic ZrO2 nanocrystals with an average diameter of 4 nm and a reported Brunauer-EmmettTeller (BET) surface area of 140 m2/g were purchased from Advanced Nanotechnology Ltd. (formerly Advanced Powder Technology Ltd), Welshpool, Western Australia. This substrate will be referred to as ZrO2 nanopowder, whereas the previously used substrate, VP Zirconia (Degussa Ltd., Germany), will be denoted as fumed ZrO2 powder. The reported average primary particle size of the fumed ZrO2 powder is 30 nm, and the BET surface area is 40 m2/g. The hydrocarbon fatty acids CH3(CH2)nCOOH (n ) 11, 12, 13, 14, 17, 18) were purchased from Aldrich. The analogous perfluoro fatty acids CF3(CF2)nCOOH (n ) 6, 8, 10) were purchased also from Aldrich, while CF3(CF2)nCOOH (n ) 12, 14, 16) were supplied free of charge by Exfluor Research Corporation. The hydrocarbon fatty acids will be referred to as CnH, and the perfluorocarbon fatty acids as CnF,
where n is the total number of carbons. All materials were used as received without further purification. 2.2. Sample Preparation. A total of 0.5 g of ZrO2 nanopowder was sonicated for 20 min in 150 mL of a 9:1 hexanes/ethanol solvent mixture. The perfluorinated acid (1.16 mmol, three times in excess of that necessary to fully coat the nanoparticles) was dissolved in 200 mL of the same solvent mixture. The ZrO2 nanopowder suspension was then added to this, and the mixture was washed with another 50 mL of solvent to ensure that all the ZrO2 was transferred. The reaction mixture was refluxed for 2 days with stirring. The samples were left to anneal by lowering the temperature to below the boiling point and then leaving the mixture to stir for an additional 2 days. The perfluoroacid-coated nanoparticles were then recovered by filtration and washed five times in 100 mL of the solvent mixture. The complete removal of unbound acid was confirmed by IR spectroscopy. Acetone or ethyl ether was used as the solvent for the hydrocarbon fatty acids. The ZrO2 was dispersed by sonication for approximately 15-30 min in the same solvent and then added dropwise to 3 mM acid solutions. Amounts to provide an excess of 3 times the surfactant required for complete coverage were used. The mixture was refluxed with stirring for 2 days and then annealed for 1 additional day. The resultant solid was removed by centrifuging (if necessary) and filtering. The solid sample was washed three times with the solvent to remove any unbound material. After the final washing, the sample was filtered and dried under vacuum overnight. Elemental analysis provided a figure for the percentage carbon content of each sample. The surface coverage was then estimated using the measured BET value of 157 m2/g for the surface area of the nanoparticles, 40 m2/g for the fumed ZrO2, and a cross-sectional area of 30 nm2 for the chains. The % coverage for each adsorbed CnF or CnH sample was calculated from the measured wt C and the calculated wt C for monolayer coverage of 1 g ZrO2: ZrO2 surface area (m2/g) area per molecule (m2) × NA (mol-1)
× n‚12 (g/mol) ) wt C (g)
2.3. Solid-State NMR and IR Spectroscopy. 13C and 19F magicangle spinning (MAS) NMR spectra were acquired on a Chemagnetics CMX-270 MHz spectrometer with a 4 mm high-speed MAS Chemagnetics probe. A contact time of 3 ms, a recycle delay of 2 s, a MAS frequency of 5 kHz, and 4000 acquisitions were used to collect the 1H-13C CP/MAS spectra. Four scans were acquired for each 19F spectrum with a pulse delay of 2 s and MAS frequency of 15 kHz. Variable temperature 19F T1 relaxation times were measured using a saturation-recovery sequence. The temperature was controlled
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Figure 4. Variable temperature 13C MAS NMR spectra focusing on the methyl signal of a long chain fatty acid, CH3(CH2)17CO2H adsorbed on the ZrO2 nanopowder, where the chain disordering transition temperature is at ∼55 °C. The spectra were acquired by a 13C single pulse experiment rather than 1H-13C cross polarization. to within (0.2° by a Chemagnetics temperature controller. 19F relaxation times were also measured on a 600 MHz Bruker spectrometer (ν0F ) 564.5 MHz) with a 4 mm MAS probe spinning at 15 kHz. An inversion-recovery pulse sequence with Hahn echo was employed, and the temperature was controlled to within (0.1°. Deconvolution of the peaks was carried out using a Spinsight NMR software routine. Photoacoustic IR spectra were obtained on a Bio-Rad FTS6000 spectrometer operating at 4 cm-1 resolution. Thirty-two scans were collected for each spectrum to give sufficient signal-to-noise. 2.4. TEM and XRD. Samples for transmission electron microscopy (TEM) were prepared by putting a drop of solution on continuous carbon-coated copper (400 mesh) grids from Electron Microscopy Sciences (Fort Washington, PA) and allowing them to dry completely. The phase contrast images of the particles were obtained using a top-entry JEOL JEM-2000 FX electron microscope operated at an accelerating voltage of 80 kV. Micrographs were obtained at magnifications ranging from 100 000× to 340 000×. Size distributions of the particles were determined from diameters of at least 100 particles located on various locations on the grid after micrographs were enlarged 200% to 300%, using scanning software (Sigma Scan version 4.1). The powder X-ray diffraction (XRD) spectra were recorded using an auto-sampling X-ray diffractometer with Cu KR radiation at a wavelength of 1.7890 Å, an acquisition time of 2.0 s, and a slit width of 0.020°.
3. Results and Discussion 3.1. Coverages. The ZrO2 nanopowder is highly crystalline (cubic ZrO2), while the fumed ZrO2 powder is partially crystalline, with amorphous along with monoclinic and tetragonal crystalline domains. The ZrO2 nanopowder displays a sharp powder X-ray diffraction spectrum (not shown) in which the Scherrer method of particle size determination gave an average crystallite size of 3.9 ( 0.2 nm in agreement with the TEM analysis shown in Figure 1. Estimates of the surface coverages based on elemental analyses show that hydrocarbon fatty acids form complete monolayers on the ZrO2 nanopowder (Table 1). Lower coverages ranging between 65% and 75% were observed in the previous studies using fumed ZrO2 powder presumably because not all the BET measured surface area is accessible to the surfactants.22 The coverages for the perfluoro acids are also higher on the nanopowder, but complete monolayers are only obtained for the two longest chain lengths (Table 2). Whereas the surface coverage increases with chain length on the fumed ZrO2 powder, the coverage on the nanopowder stays constant with chain length and then jumps to a higher value for C16F and C18F. The higher
Figure 5. 19F MAS NMR spectra of (a) bulk and (b) adsorbed perfluoro acids.
coverages in the case of the longest chain fluoro acids may be due to their lower solubilities and/or an increase in the intermolecular chain interactions, though these are weak compared to hydrocarbon chains. Although the estimated coverages for the two longest chains are slightly over 100%, no evidence of free acid is seen in the IR spectrum. Indeed, when residual free acid was present in one C18F sample, it was clearly visible in the IR, and it was removed by washing with C6F6. As discussed below, the 19F NMR signals of the CF3 groups of the surface-bound acids are split, but for unwashed samples a third signal whose chemical shift matches that of the bulk acid is detected and removed by washing with C6F6 (see the Supporting Information). Obtaining values higher than 100% suggests either that the surface area per gram of the nanopowder is greater than our estimate or that the chains are packing more tightly than we assume. Another source of error in the calculation of the surface coverage may be in the estimate of the molecular cross section that is based on the diameter of the perfluoroalkane helix. The high surface curvature provides a larger cone of volume per chain, and the limiting factor may be the footprint of the carboxylate headgroup (20 Å2) rather than the fluorocarbon chain (32 Å2). 3.2. FTIR Spectroscopy: Surface Bonding and Chain Conformation. The FTIR spectra (not shown) of the carboxylate stretching region of the hydrocarbon fatty acids adsorbed on the ZrO2 nanopowder are very similar to those in the previous study using fumed ZrO2. The separation of the asymmetric (νa(COO-)) and the symmetric (νs(COO-)) stretches (∆ν) indicates a bidentate chelating Zr-carboxylate surface bond, as reported previously.22 The widths of the carboxylate stretching bands are broad compared to those of bulk metal carboxylates, reflecting the heterogeneity of the surface adsorption sites. The methylene C-H stretching region provides information on the chain conformation, with stretch frequencies of ∼2918 cm-1 and 2850 cm-1 indicative of ordered all-trans hydrocarbon chains. The frequencies of the asymmetric and symmetric stretches of the adsorbed acids, listed
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Table 3. Comparison of the Room-Temperature 19F Spin Lattice Relaxation Times of the Perfluoro Fatty Acids in the Bulk State and Adsorbed on the Fumed and Nano ZrO2a
sample C8F C10F C12F C14F C16F C18F
T1(s) CF3 bulk acid (-83 ppm)
T1(s) CF3 fumed ZrO2 (-85 ppm)b
1.6 ( 0.1
0.51 ( 0.09
1.9 ( 0.5 1.6 ( 0.4 1.8 ( 0.2
0.73 ( 0.02 0.68 ( 0.03 0.75 ( 0.06
T1(s) CF3a nanoZrO2 (-83.6 ppm)
T1(s) CF3b nanoZrO2 (-85 ppm)
1.0 ( 0.2 1.2 ( 0.3 (1.7 ( 0.2)
0.36 ( 0.05 0.34 ( 0.05 (0.4 ( 0.05) 0.4 ( 0.1 0.40 ( 0.05 0.65 ( 0.1 0.73 ( 0.2 (1.0 ( 0.1)
T1(s) CF2 bulk acid
T1(s) CF2 fumed ZrO2b
T1(s) CF2 nanoZrO2
2.8 ( 1.3
0.50 ( 0.04
3.3 ( 0.6 3.1 ( 0.8 1.9 ( 0.1
0.57 ( 0.04 0.51 ( 0.04 0.34 ( 0.04
0.3 ( 0.1 0.30 ( 0.05 (0.39 ( 0.05) 0.3 ( 0.1 0.34 ( 0.05 0.77 ( 0.1 1.0 ( 0.3 (1.49 ( 0.1)
a The numbers in parenthesis were measured on a 600 MHz NMR instrument where ν0F ) 564.5 MHz. b Data for the fumed ZrO2 powder are from ref 24.
in Table 1, indicate extended chains with few gauche defects for all chain lengths examined. The infrared spectra of the bulk and surface adsorbed perfluoro fatty acids are presented in Figure 2. The most obvious change upon adsorption to the surface is the disappearance of the free acid peak at 1756 cm-1 and its replacement by two carboxylate stretching peaks. The symmetric and asymmetric carboxylate frequencies range from 1406 to 1426 cm -1 and 1658 to 1684 cm -1, respectively, and the difference between these two bands, ∆, varies between 252 and 263 cm-1. The ∆ values measured here are somewhat larger than those found upon adsorption onto the larger ZrO2 particles and closer to those found for the same acids on fumed TiO2 powder.24 In that study, it was concluded that the carboxylate stretching frequencies were consistent with the formation of a bidentate surface bond on the ZrO2 surface, similar to the case of normal fatty acids. This conclusion was based on the noted correlations between trifluoroacetato (TFA) salt carboxylate binding configurations and frequency separations for the symmetric and asymmetric stretches and the fact that other perfluoro carboxylate complexes have shown the same type of coordination as their hydrocarbon counterparts. Using the same reasoning, it was suggested that the same acids bound in a monodentate fashion to the TiO2 surface. The frequency separations seen here are almost in the range reported as being typical of monodentate binding (∆ > 260 cm-1).25 Other factors such as chain packing have been suggested to have an effect,26 so this could perhaps shift the frequency differences relative to those of the TFA salts. In any case, it is difficult to offer a definitive statement regarding the surface bond based on this data. A number of other changes are seen in the IR spectra upon adsorption. The broad peaks at 1250 and 1180 cm-1 are wellknown as characteristic features in spectra of perfluoroalkanes and poly(tetrafluoroethylene) (PTFE). A substantial narrowing of these peaks and the frequency shift are observed upon adsorption to the ZrO2 nanoparticle surface. This was also seen in the case of the fumed ZrO2 powder, and it is most likely due to reduced dipolar interactions between chains, a consequence of looser packing of the adsorbed chains relative to the bulk crystalline state.27 3.3. Solid-State NMR: 13C and 19F Chemical Shifts. The 13C CP/MAS NMR spectrum of a long chain fatty acid, C20H, adsorbed on the ZrO2 nanopowder is presented in Figure 3 along with the peak assignments. As observed in the previous study on the fumed zirconia, the carboxylate signal is almost broadened into the baseline and the methylene carbon attached to it, C2, (26) Gericke, A.; Huhnerfuss, H. Thin Solid Films 1994, 245, 74. (27) Hsu, S. L.; Reynolds, N.; Bohan, S. P.; Strauss, H. L.; Snyder, R. G. Macromolecules 1990, 23, 4565.
is also broadened and shifted to a higher frequency relative to the unbound acid.22 A single peak at 33 ppm is observed for the inner methylene carbons, indicating an all-trans conformation. The 13C chemical shifts of the inner methylene carbons of the other chain lengths are listed in Table 1. There are several important differences between the 13C NMR spectra of these acids adsorbed on the nanopowder versus the fumed zirconia powder. On the nanopowder, only one peak for the inner methylene carbons is observed for all chain lengths and the shift only decreases slightly from 33 to 32 ppm for acids with less than 14 methylene groups (C13H-C15H). On the fumed ZrO2 powder, a single component at 33 ppm is obtained for the longest chain acid (C20H) and shorter chains display two components at 33 and 30 ppm, assigned, respectively, to domains of extended all-trans chains and disordered chains with a liquidlike population of gauche conformers. The lower population of gauche defects on the nanopowder arises in all probability from the fact that all the surface area is accessible to the surfactants, giving rise to higher coverages and a denser chain packing. At the same time, as the average particle diameter of the nanopowder is 1 order of magnitude smaller than that of the fumed zirconia (4 nm versus 30 nm), the large increase in the surface curvature decreases the packing efficiency toward the chain ends. To maintain efficient chain packing, the increase of the volume of the cone of space per chain with increasing surface curvature may be compensated for by interdigitation of chains between neighboring nanoparticles. The presence of two methyl carbon signals at 13 and 15 ppm for the longest chains adsorbed on the nanopowder (C16H-C20H) provides strong evidence for these particle-particle interactions (Table 1). For smaller particles, that is, 2 nm alkanethiol capped Au nanoparticles where the average interparticle distances are found to be less than twice the extended chain lengths, only one methyl peak at 15 ppm appears for all chain lengths. Likewise, fatty acids on the 30 nm fumed zirconia powder display a single methyl peak at 12-13 ppm. The anomalously low methyl shift of 13 ppm was previously assigned to methyl groups situated at the monolayer/ air interface,22 and the 15 ppm peak corresponds to the shift observed in the bulk state, that is, methyl groups located in a hydrocarbon environment. The increase in the chain packing efficiency with interdigitation is manifested by the increased chain order, in that the appearance of the 15 ppm methyl peak for interdigitated chains coincides with an inner methylene chemical shift of 33 ppm as compared to the shorter chains that only have the 13 ppm methyl signal and a slightly smaller shift of 32 ppm, indicative of more gauche defects (Table 1). However, the two methyl signals observed for the fatty acids adsorbed on the ZrO2 nanopowder are not a direct consequence of an ordered versus disordered conformation, since
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Figure 6. Comparison of CF3 and CF2 19F spin lattice relaxation time trends of the C18 acids in bulk and on the nano and fumed ZrO2 powders.
both signals remain upon thermal disordering. In Figure 4, the chains of C19H adsorbed on the ZrO2 nanopowder are completely disordered at 80 °C, as indicated by the shift of the inner methylene peak from 33 to 31 pm, yet the two methyl signals remain unchanged. (The signal-to-noise ratio is poor, since the spectra were acquired using direct excitation of 13C with long acquisition times rather than cross polarization which would attenuate the more mobile components.) The 13C methylene shifts can be used to detect reversible, chain length dependent order/disorder transitions of surfacebound monolayers.22 Table 1 compares the temperatures at which chain disordering occurs for the hydrocarbon fatty acid monolayers deposited on the nanopowder versus the fumed zirconia powder. In the case of the shorter chains, the transition temperatures are significantly higher by 20-30° on the nanopowder substrate, a reflection of the denser chain packing. The transition temperature difference between the two substrates is much smaller for the longer chain lengths. The perfluoro acids display similar evidence for particleparticle interactions when deposited on the nanopowder. The 19F MAS NMR spectra of the bulk and adsorbed perfluoro acids are presented in Figure 5, and the chemical shifts are listed in Table 2. As with the previous 19F NMR study on the fumed ZrO2 powder, the CF2 resonances of the perfluoro acids adsorbed on
O’Donnell et al.
Figure 7. Comparison of the 19F spin lattice relaxation trends with chain length of nanopowder-bound chains.
the nanopowder are broadened but the chemical shifts are unchanged relative to the bulk acids. In this case, however, the CF3 resonance is split into peaks at -83.6 and -85 ppm. The -85 ppm peak was previously assigned to CF3 groups at the monolayer/air interface. The -83.6 ppm component, which is not observed in the fumed ZrO2 samples, increases in intensity with chain length. The shift is close to the CF3 shift in the bulk acid (-82.7 ppm), and therefore, this -83.6 ppm peak is assigned to interdigitated chain ends. The integrated intensities (Table 2), determined by deconvolution of the two peaks, show that the fraction of chains that are interdigitated increases with chain length. The -83.6 ppm signal assigned to CF3 groups of interdigitated chains has line width of ∼400 kHz, twice as large as that of the 85 ppm peak (∼200 Hz). Since the line width does not change upon heating, a distribution of chemical shifts due to variation in the extent of interdigitation is proposed to be the dominant contribution. This variation is understandable given the particle size distribution presented in Figure 1 that shows a significant population of nanoparticles with diameters other than 4 nm. Although the line widths of the interdigitated CF3 groups did not decrease upon heating, they are less mobile than the free CF3 ends in terms of MHz frequency reorientations as shown by the spin lattice relaxation measurements in the next section. 3.4. 19F Spin Lattice Relaxation: Chain Packing. Whereas the chain packing of the hydrocarbon SAMs on the nanopowder versus the fumed ZrO2 powder can be compared via the 13C
Perfluorocarbon and Hydrocarbon ZrO2 Nanoparticles
chemical shift measurements of the trans/gauche populations, the high sensitivity of 19F NMR allows us to probe the relative fluorocarbon chain mobilities through spin lattice relaxation measurements. The previous study for the perfluoro acids on fumed zirconia and titania powders established a large increase in chain mobility of the adsorbed acids relative to the bulk state due to increases in both the frequency and angle of chain reorientations. The dominant chain motions of the stiff perfluorocarbon chains were proposed to be small angle reorientations about the long chain axis combined with more rapid rotation of the terminal CF3 group about its 3-fold axis.24 The room-temperature T1F values are listed in Table 3. As with the fumed ZrO2 powder, the relaxation times of the perfluoro acids adsorbed on the nanopowder are on average smaller than the bulk values, reflecting an enhanced chain mobility. However, there are significant differences between the two substrates. The temperature dependence of the CF2 relaxation times of C14 and C18 adsorbed on the fumed ZrO2 shows that the frequency of the chain motion is in the slow limit at ambient temperature and, in the case of C14, a T1 minimum was detected upon heating.24 Within experimental error, the CF2 relaxation times of the perfluoro acids deposited on the nanopowder remain the same or increase slightly with temperature. The temperature dependence of T1F for the CF2 and CF3 groups of the C18 acids deposited on the two different substrates is compared in Figure 6, and that of C10F and C18F on the nanopowder is compared in Figure 7. The trends in the relaxation behavior of the perfluoro chains on the nanopowder is complicated by the fact that the CF2 signal represents a mixture of free and interdigitated chains. The spin lattice relaxation times for the CF2 of the shorter chains are similar, T1F ∼ 0.3 s, but then they increase in magnitude to 0.77 s for n ) 16 where over half of the chains are interdigitated as estimated from the CF3 integrated intensities. Measurements at a higher field strength (ν0F ) 564.5 MHz) showed a small increase of the T1 values for C18F adsorbed on the nanopowder and a negligible change for C10F (Table 3). A field dependence indicates that the motional frequencies are close to or on the low-temperature side of the T1 minimum. The amplitude of the chain reorientation must also be considered. Both the rate and amplitude of motion can increase with temperature and have the same effect on T1 on the low-temperature side of the T1 minimum, but, at temperatures above the minimum, increases in rate and amplitude have opposite effects. The larger surface curvature and cone of volume available per chain on the nanopowder would allow for larger amplitudes of motion. However, this effect is counteracted by the chain interdigitation which increases with chain length. The data presented in Figure 6 support this trend,
Langmuir, Vol. 24, No. 6, 2008 2471
since the CF2 T1 values of the C18/nanoZrO2 are closer in magnitude to that of the bulk acid. Likewise, the CF3b T1 values of the C18/nanoZrO2, assigned to CF3 groups located at the monolayer/air interface, are very close to those measured for C18/fumed ZrO2, which does not have interdigitated chains. The values for the CF3a groups of the interdigitated chains fall between the bulk and C18/fumed ZrO2 samples. As noted in the previous study, interpretation of the 19F spin lattice relaxation times may also be complicated by spin diffusion that tends to equalize the T1F values.24 The possibility of spin diffusion along with the presence of a mixture of free and interdigitated chains does not allow us to interpret the 19F relaxation data of the perfluoro chains on the nanopowder beyond the conclusion that the overall magnitudes of T1F increase with chain length and the degree of interdigitation, approaching the bulk crystalline acid values.
4. Summary The chain conformation and dynamics of hydrocarbon and perfluorocarbon fatty acids adsorbed on zirconia nanopowder were characterized by 13C chemical shift and 19F NMR relaxation measurements, respectively, and compared to previous studies on lower surface area fumed metal oxide powders. We show that the interdigitation of chains of neighboring particles as a function of chain length and/or particle size can be followed by chemical shifts and intensities of NMR signals for the CH3 and CF3 groups. Similar to the case of alkanethiol SAMs on gold nanoparticles, this interdigitation allows for efficient chain packing despite the high surface curvature. The denser chain packing compensates for the increase in surface curvature such that the hydrocarbon chains on the 4 nm nanoparticles are more ordered and the orderdisorder transition temperatures are elevated relative to the same chains deposited on the 30 nm particles of the fumed ZrO2 powder. Also, the 19F spin lattice relaxation times of the fluorocarbon chains on the nanopowder approach those of the bulk acids with increasing chain length and interdigitation, indicating densely packed chains. Acknowledgment. Funding for this work was provided by the Natural Sciences and Engineering Council of Canada (NSERC) and Fonds Que´be´cois de la Recherche sur la Nature et les Technologies (FQRNT). Supporting Information Available: 19F MAS NMR spectra of the CF3 region of the perfluoro acids adsorbed on the ZrO2 nanopowder before and after washing with C6F6. This material is available free of charge via the Internet at http://pubs.acs.org. LA702503M
