Langmuir 1999, 15, 7111-7115
7111
Self-Assembled Monolayers of Alkanesulfonic and -phosphonic Acids on Amorphous Iron Oxide Nanoparticles C. Yee,†,| G. Kataby,† A. Ulman,*,†,| T. Prozorov,‡ H. White,§,| A. King,§ M. Rafailovich,§,| J. Sokolov,§,| and A. Gedanken*,‡ Department of Chemistry, Chemical Engineering and Material Science, Polytechnic University, Brooklyn, New York 11201, Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel 52900, Department of Materials Sciences and Engineering, State University of New York at Stony Brook, New York 11794-2275 and The NSF MRSEC for Polymers at Engineered Interfaces Received May 27, 1999. In Final Form: July 20, 1999 We have functionalized amorphous Fe2O3 nanoparticles with alkanesulfonic and octadecanephosphonic acids. TEM reveals nanoparticles 5-10 nm in diameter. FTIR spectra suggest that while in all cases the alkyl chains are packed in a solid-like arrangement, packing disorder increased with decreasing chain length. TGA of the sulfonic acid-functionalized Fe2O3 nanoparticles shows that moieties started to decompose and desorb from the iron oxide surface at about 260 °C. In the case of the octadecanephosphonic acid (OPA)-functionalized Fe2O3, moieties started to decompose and desorb at 340 °C. It is suggested that free Fe-OH groups can serve as proton donors to assist in the sulfonic acid desorption process and that because of the diprotic nature of the phosphonic acid these free surface Fe-OH groups may no longer be available. Among all, the octadecanesulfonic acid coating displays the lowest magnetization, which may be explained by the high packing and ordering of the alkyl chains on the particle surface. The saturation curve of the OPA case gives the smallest value of magnetization we have ever measured for functionalized Fe2O3 nanoparticles. It is suggested that the spin state of surface Fe3+ ions is affected by the bonded surfactant, through a mechanism of pπ-dπ P-O, and dπ-dπ Fe-P interactions and that the phosphonate empty d orbitals increase magnetic interactions between neighboring Fe3+ spins.
The discovery that the physical properties of semiconductor and magnetic particles are size-dependent opens interesting possibilities for the construction of advanced materials “from the bottom up,” provided that a technology will be in place for their precise positioning in space. One can imagine that material properties can be designed by building assemblies of nanoparticles where their position in space defines their interparticle interactions, and hence the overall properties of the assembly. This vision, which is called nanotechnology for lack of a better word, provides a connection between surfactant-coated nanoparticles, which is not a new idea, and modern materials science. To address the challenge of nanomaterials, one needs functionalized nanoparticles with molecularly engineered interfacial interactions, since hierarchical self-assembly is possible when interfacial interactions dictate the reaction path and structure.1 This can be achieved, in principle, by functionalizing the nanoparticles with surfactant molecules, like the self-assembly of surfactants on flat substrates. When these surfactant molecules have the appropriate substituent at their ω-position, interparticle interactions will provide the driving force for the self-assembly to three-dimensional structures. Recently, Tremel and co-workers showed that gold nanoparticles that are functionalized with p-mercaptophenol could serve as templates for the growth of CaCO3 crystals,2 similar to * To whom correspondence should be addressed. Telephone: (718) 260-3119. Fax: (718) 260-3125. E-mail: aulman@ duke.poly.edu. † Polytechnic University. ‡ Bar-Ilan University. § State University of New York at Stony Brook. | The NSF MRSEC for Polymers at Engineered Interfaces. (1) Kuhn, H.; Ulman, A. In Thin Films, Vol. 20, Supramolecular Assemblies: Vision and Strategy; Ulman, A., Ed.; Academic Press: Boston, 1995.
SAMs on flat gold surfaces.3 The self-assembly of nanostructured materials has been reviewed recently by Fendler.4 Self-assembled monolayers (SAMs) of different adsorbates on solid surfaces have become one of the central themes in modern materials science,5,6 and molecular level engineering of stable model surfaces has been accomplished using SAMs.7 Recently, we have shown that SAMs of 4-mercaptobiphenyls are superior to those of alkanethiolate, providing stable model surfaces8 and the ability to engineer surface dipole moments.9 Such capabilities, when combined with efficient and economical preparation of functionalized nanoparticles, may contribute to the development of enabling technologies for future advanced materials. Recently, we have developed a onephase synthesis that allows facile preparation of thiolfunctionalized Pt10 as well as Au, Pd, and Ir nanoparti(2) Kuther, J.; Seshadri, R.; Nelles, G.; Assenmacher, W.; Butt, H.J.; Mader, W.; Tremel, W. Chem. Mater. 1999, 11, 1317. (3) Aizenberg, J.; Black, A. J.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 4500. (4) Fendler, J. H. Chem. Mater. 1996, 8, 1616. (5) For a review on SAMs, see: (a) Ulman, A. An Introduction to Ultrathin Organic Films: From Langinuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (b) Ulman, A. Chem. Rev. 1996, 96, 1533. (6) (a) Bascom, W. D. J. Colloid Interface Sci. 1968, 26, 89. (b) Nainyong, K. Y.; Laibinis, P. E. J. Am. Chem. Soc. 1997, 119, 2297. (c) Nuzzo, R. G.; Allara, D. L. J Am. Chem. Soc. 1983, 105, 4481. (d) Sandorff, C. J.; Garoff, S.; Leung, K. P. Chem. Phys. Lett. 1983, 96, 547. (e) Laibinis, P. E.; Whiteside, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J Am. Chem. Soc. 1991, 113, 7152. (7) Ulman, A.; Evans, S. D.; Shnidman, Y.; Sharma, R.; Eilers, J.E.; Chang, J. C. J. Am. Chem. Soc. 1991, 113, 1499. (8) Kang, J. F.; Jordan, R.; Ulman, A. Langmuir 1998, 14, 3983. (9) Kang, J. F.; Ulman, A.; Liao, S.; Jordan, R. Langmuir 1999, 15, 2095. (10) Yee, C.; Scotti, M.; Ulman, A.; White, H.; Rafailovich, M.; Sokolov, J.; King, A. Langmuir 1999, 15, 4314.
10.1021/la990663y CCC: $18.00 © 1999 American Chemical Society Published on Web 09/16/1999
7112
Langmuir, Vol. 15, No. 21, 1999
Letters
Figure 1. Sonochemical synthesis setup.
Figure 2. TEM of dodecanesulfonic acid-functionalized Fe2O3 nanoparticles.
cles.11 However, it is clear that such nanoparticles will not find wide range materials applications, due to their relatively high price, but may be utilized in, for example, sensor or bioassay applications. On the other hand, sonochemistry provides an economical route for the large scale manufacturing of metal and metal oxide nanoparticles.12 A very important application area that has attracted significant research attention is the control of magnetic storage capacity in electronic devices. There, storage volume and performance may be altered by surface fabrication of magnetic nanoparticles. We have initiated a research program to investigate the functionalization of iron oxide nanoparticles by different surfactant molecules. It is noted that while Fe2O3 nanoparticles are superparamagnetic, and thus do not have practical applications in magnetic storage media, they are excellent model systems for the development of surface functionalization. The questions we address are (a) does functionalization of magnetic nanoparticles affect their magnetic properties and (b) how is thermal stability affected by the nature of the surface molecules? These are important issues for the development of future technologies based on functionalized nanoparticles. We have prepared amorphous nanoparticles according to the process pioneered in our laboratory.13 Figure 1 shows the sonochemical setup we used. In short, a 1 M solution of Fe(CO)5 in n-decane was sonicated for 3 h under ambient atmosphere. The detailed experimental procedure has been published elsewhere.13 We have reported before on the functionalization of amorphous iron and iron oxide nanoparticles by thiols,14 alcohols,15 carboxylic acids,16 octadecyltrichlorosilane (OTS), and sodium dodecyl sulfate (SDS).17 In this paper
Figure 3. 3000-2800 cm-1 region in the IR of sulfonic acidfunctionalized Fe2O3 nanoparticles.
(11) Yee, C.; Jordan, R.; Ulman, A.; White, H.; King, A.; Rafailovich, M.; Sokolov, J. Langmuir 1999, 15, 3486. (12) Suslick, K. S.; Choe, S. B.; Cichowlas, A. A.; Grinstaff, M. W. Nature 1991, 353, 414. (13) Cao, X.; Prozorov, R.; Koltypin, Y.; Kataby, G.; Gedanken, A. J. Mater. Res. 1997, 12, 402. (14) (a) Kataby, G.; Koltypin, Y.; Cao, X.; Gedanken, A. J. Cryst. Growth 1996, 166, 760. (b) Kataby, G.; Prozorov, T.; Koltypin, Y.; Cohen, H.; Sukenik, C. H.; Ulman, A.; Gedanken, A. Langmuir 1997, 13, 6151. (c) Kataby, G.; Koltypin, Y.; Rothe, J.; Honnes, J.; Felner, I.; Cao, X.; Gedanken, A. Thin Solid Films 1998, 333, 41. (15) Kataby, G.; Ulman A.; Prozorov, R.; Gedanken, A. Langmuir 1998, 14, 1512. (16) Kataby, G.; Cojocaru, M.; Prozorov, R.; Gedankeri, A. Langmuir 1999, 15, 1703. (17) Rozenfeld, O.; Koltypin, Y,; Bamnolker, H.; Margel, S.; Gedanken, A. Langmuir 1994, 10, 627.
we report the assembly of a series of alkanesulfonic acids (ASAs, CH3(CH2)nSO3H) of various chain lengths (n ) 11, 13, 15, 17), as well as of octadecanephosphonic acids (OPA, CH3(CH2)17PO3H2) at the surface of amorphous iron nanoparticles. Our motivation for the choice of the phosphonic acid came from friction studies we have carried out using its SAMs on flat Al2O3 surfaces.18 There, we have observed that the Al3+-phosphonate surface interactions are extremely strong and that the SAM can sustain prolonged friction, even at elevated temperatures and high relative humidity, without any apparent deterioration. Therefore, we expected similar stability for the Fe3+phosphonate interactions in the functionalized Fe2O3 nanoparticles. The comparison between strong acids that can (OPA) or cannot (ASA) bridge Fe3+ sites at the nanoparticle surface and how this property affects the magnetic properties and thermal stability were the motivation for selecting the ASAs. The nanoparticles were functionalized with the different acids by sonicating a 1:1 Fe2O3/acid molar ratio in 20 mL of ethanol. The functionalized nanoparticles where washed and dried as was described elsewhere.15 Figure 2 shows a TEM image of dodecanesulfonic acid-functionalized Fe2O3 nanoparticles. The size of the particles is 5-10 nm in diameter; TEM images of the rest of the other acidfunctionalized particles are similar to that shown in Figure 2. (18) Berman, A.; Steinberg, S.; Campbell, S.; Ulman, A.; Israelachvili, J. J. Tribol. 1998, 43.
Letters
Langmuir, Vol. 15, No. 21, 1999 7113
Figure 4. 1600-1000 cm-1 region in the FTIR spectra of sulfonic acid-functionalized Fe2O3 nanoparticles.
The transmission infrared spectra of all four alkanesulfonic acid-functionalized iron oxides as compared to that of the bulk OSA are presented in Figure 3 for the 3000-2800 cm-1 region and in Figure 4 for the 16001000 cm-1 region of the spectra. For asymmetric and symmetric C-H stretching vibrations at 3000-2800 cm-1, no shifts were found: only broadening as the alkyl chain length was getting shorter. Hostefler and co-workers have carried out detailed infrared spectroscopy studies of alkanethiolate-functionalized gold nanoparticles.19 For the C16H33S-functionalized gold nanoparticles they reported 2954 cm-1 (r-), 2918 cm-1 (d-), 2873 cm-1 (r+), and 2848 cm-1 (d+). They noted that a crystalline microenvironment is seen for alkyl chains C6 or greater, as is evident by the above CH2 stretch (d′ and d). Figure 3 suggests that, for the C12-C18 sulfonic acid-functionalized Fe2O3 nanoparticles, the alkyl chains are packed in a solid-like arrangement. The line broadening indicates a possible increase in packing disorder with decreasing chain length. The CH2 scissoring motion (δ) at 1466 cm-1 (Figure 4) is typical of an all-trans methylene chain (∼1467 cm-1). The fact that the scissor is only a single band suggests that a structure with two chains in a unit cell should be rejected. However, since a completely disordered phase would also have this characteristic, one cannot offer any conclusion on the structure of the chain assembly. The RSO3- symmetric and asymmetric stretching vibrations at 1207-1177 and l064 cm-1, respectively, confirm the bonding of the sulfonic acids to the Fe2O3 nanoparticle surfaces via ionic bonds. The transmission infrared spectra of the bulk OPA (a) and of the OPA-functionalized iron oxide nanoparticles (b) are presented in Figure 5. For the asymmetric and symmetric C-H stretching vibration region at 2800-2950 cm-1, the values show that the chains are in a microcrystalline environment. The slight broadening of the bands in (b) as compared with (a) may be attributed to the alkyl chains that are less well packed in the OPAfunctionalized Fe2O3 nanoparticles as compared with the crystalline OPA in the bulk and is similar to the observations for ASA-functionalized Fe2O3 nanoparticles. Another indication of the formation of strong chemical bonds between the substrates and the alkanesulfonic and -phosphonic acid moieties is obtained from thermal gravimetric analysis (TGA) measurements. In Figure 6, we present TGA of the sulfonic acid-functionalized Fe2O3 nanoparticles. All the moieties started to decompose and (19) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604.
Figure 5. Transmission infrared spectra of OPA in bulk (a) and OPA-functionalized iron oxide nanoparticles (b).
Figure 6. TGA of sulfonic acid-functionalized Fe2O3 nanoparticles. The heating rate was 10 °C/min. (a) 395 °C; (b) 359 °C; (c) 394 °C; (d) 416 °C.
desorb from the iron oxide surface at about 260 °C. The inflection points of these desorption temperatures are at 395, 359, 394, and 416 °C for C12, C14, C16, and C18 sulfonic acid surfactants, respectively. On the other hand, a twostep desorption pattern appeared in the case of OPAfunctionalized Fe2O3 nanoparticles. The inflection points of these desorption temperatures are at 340 and 479 °C, respectively. The bonding of the sulfonic acid surfactants to the iron oxide surfaces is the result of an acid-base reaction between the Fe-OH and S-OH groups. However, because of the size of the sulfonate group, one can assume that there exist free Fe-OH groups that could serve as the proton donors to assist in the sulfonic acid desorption
7114
Langmuir, Vol. 15, No. 21, 1999
Figure 7. Fe-OH-assisted desorption of sulfonic acid from its functionalized Fe2O3 nanoparticles.
Letters
Figure 9. Hysteresis loops of alkanesulfonic acid-functioned iron oxide nanoparticles.
Figure 8. Two possible binding schemes for phosphonate ions to surface Fe3+ ions.
process. Figure 7 presents a cartoon of a proposed FeOH-assisted mechanism, whereby the OH proton is transferred to the sulfonate group and the corresponding oxygen becomes a µ-oxo bridge. A second desorption mechanism may involve the breaking of either the S-O (P-O), or C-S (C-P) bonds. Since the latter are relatively weaker bonds, it is more likely that the majority of chains would be taken off the surface upon C-S (C-P) bond cleavage. However, this mechanism does not explain the difference in thermal stability between the sulfonic and phosphonic acids, since the C-S and C-P bonds are similar in strength, 65 and 63 kcal/mol, respectively. Therefore, the Fe-OH-assisted mechanism probably contributes to the observed thermal stability behavior. In the case of the diprotic phosphonic acid, two different bonding schemes between the Fe2O3 and the phosphonate ion can exist, as is shown in Figure 8. In the first, the phosphonate is symmetrically bonded through its two oxygen atoms, resulting in a stronger bridging geometry and a higher desorption temperature. In the second, only one oxygen atom participates in the bonding, resulting in weaker bonding and a lower desorption temperature. Furthermore, in the latter bonding scheme, there may be some free Fe-OH groups that participate in the desorption process as described above for the sulfonic acid case, and result in a lower desorption temperature. However, the inflection point in the case of OPA-functionalized Fe2O3 nanoparticles was at about 496 °C, and the OPA moiety started to decompose and desorb from the iron surface at 340 °C, significantly higher than its corresponding sulfonic acid-functionalized Fe2O3 nanoparticles. This suggests that the dominating binding in the case of OPA is that of phosphonate groups bridging two Fe3+ ions and that there are few, if any, surface Fe-OH groups to catalyze desorption. This is in agreement with the original suggestion, based on infrared spectra, that the alkyl chains in the OPA-functionalized nanoparticles are less well packed than those in the OSA counterparts. The hysteresis loops of all four alkyl sulfonic acidfunctionalized iron oxide particles are depicted in Figure
Figure 10. Effect of the surfactant chain length on the magnetic properties of sulfonic acid-functionalized Fe2O3 nanoparticles. The blue diamond represents the magnetization value for OPAfunctionalized Fe2O3 nanoparticles.
9. This figure shows that no hysteresis is detected and that the magnetization does not saturate even at 15 kG. Such a behavior is typical of a supraparamagnetic material20 and is typical of Fe2O3 nanoparticles. Interestingly, the saturation curve of OPA-functionalized Fe2O3 particles (not shown here) gives a value of ∼3 emu/g for the magnetization at 15 kG, significantly smaller than any value of magnetization we have measured before. The effect of the surfactant chain length on the magnetic properties is presented in Figure 10. That magnetization is a function of the sulfonic acid chain length can be seen better in the insert, where the magnetizations (per gram of Fe2O3) of the coated particles rank from C12 to C18 sulfonic acids in descending order. Among all, the octadecanesulfonic acid coating displays the lowest magnetization, which may be explained by the high packing and ordering of the alkyl chains on the particle surface and may be due to the high degree of interparticle chain intercalation. The diamond in Figure 10 represents the magnetization measured for the OPA-functionalized Fe2O3 nanoparticles. Comparing the magnetization values for the two C18 acids clearly suggests that the interactions of the phosphonate groups with the Fe2O3 nanoparticle surface should be the source of their magnetic properties. Interestingly, both the alcohol- and carboxylic acid-functionalized iron oxide particles15,16 give at least an order of magnitude larger magnetization than the OPA-functionalized iron. (20) Cullity, B. D. Introduction to Magnetic Materials; AddisonWesley: London, 1972.
Letters
It could be argued that this small magnetization is due to the dissolution of iron octadecyl phosphonate from nanoparticle surfaces, resulting in significantly smaller particles and hence smaller magnetization. However, this should have been even more significant for the sulfonic acid cases, since they are stronger than the phosphonic acid. Nevertheless, to check this scenario, we have compared the surface area of phosphonic and carboxylic acid-functionalized Fe2O3 nanoparticles by carrying out BET measurements. The results show surface areas of 71.5 and 72 m2/g, respectively, suggesting that dissolution cannot be the source of the observed decrease in magnetization. Thus, the nature of the interaction between the nanoparticle Fe3+ sites and the adsorbing group must be the defining factor. One possible explanation can be that the spin state of surface Fe3+ ions is affected by the bonded surfactant, through a mechanism of dπ-pπ P-O and dπ-dπ Fe-P interactions. It is noted that while many iron salts and complexes are colored, FePO4 is colorless, probably due to the same mechanism. In the case of carboxylic acid and alcohols, where the adsorbates have no empty d orbitals and where the oxygen atoms are less negatively charged, the iron is in a high spin state. Another possibility is driven from the fact that the magnetic behavior of nanoparticles depends on the magnetic interactions between neighboring magnetic spins and the sign of the interaction.18 Due to the fact that the phosphonate ion has empty d orbitals, magnetic interactions between neighboring spins may become more effective, resulting in ferromagnetic or antiferromagnetic interaction, thus reducing the observed magnetism. In conclusion, we have functionalized amorphous Fe2O3 nanoparticles with alkanesulfonic and octadecanephos-
Langmuir, Vol. 15, No. 21, 1999 7115
phonic acids. TEM reveals nanoparticles 5-10 mn in diameter. FTIR spectra suggest that while in all cases the alkyl chains pack in a solid-like arrangement, packing disorder increased with decreasing chain length. TGA of the sulfonic acid-functionalized Fe2O3 nanoparticles shows that moieties started to decompose and desorb from the iron oxide surface at about 260 °C. In the case of the OPAfunctionalized Fe2o3, moieties started to decompose and desorb at 340 °C. It is suggested that free Fe-OH groups can serve as the proton donors to assist in the sulfonic acid desorption process and that because of the diprotic nature of the phosphonic acid these free surface Fe-OH groups may no longer be available. Among all, the octadecanesulfonic acid coating displays the lowest magnetization, which may be explained by the high packing and ordering of the alkyl chains on the particle surface. The saturation curve of the OPA case gives the smallest value of magnetization we measured for any functionalized Fe2O3 nanoparticles. It is suggested that the spin state of surface F3+ ions is affected by the bonded surfactant, through a mechanism of dπ-pπ P-O, and dπ-dπ Fe-P interactions and that the phosphonate empty d orbitals increase the magnetic interactions between neighboring Fe3+ spins. Acknowledgment. This project was supported by Grant No. 94-00230 from the U. S.-Israel Binational Science Foundation (BSF), Jerusalem, Israel, and by the NSF through the GRT program (DGE 955452) and by the MRSEC for Polymers at Engineered Interfaces. C.Y. is grateful to the NSF. LA990663Y
