Langmuir 1999, 15, 1703-1708
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Coating Carboxylic Acids on Amorphous Iron Nanoparticles G. Kataby,† M. Cojocaru,† R. Prozorov,‡ and A. Gedanken*,† Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel 52900, and Department of Physics, Bar-Ilan University, Ramat-Gan, Israel 52900 Received August 6, 1998. In Final Form: December 9, 1998 Nanophased amorphous iron particles were coated by various carboxylic acids and their physical properties (differential scanning calorimetry, thermogravimetric analysis, temperature-programmed desorption, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and magnetism) were measured. The various properties were measured as a function of the alkyl chain length. The desorption pattern at 400 °C revealed a keto derivative.
Introduction Alkanethiols have been used extensively in coating films1-14 of metals, metal oxides, and ceramic materials by using Langmuir-Blodgett (LB) and self-assembly methods. Other amphiphiles have not attracted the same attention and have been studied less than thiols. For example, n-alkanoic acid as an amphiphile has coated various surfaces, but only a small number of reports have been documented. The reason for the interest in carboxylic acids as surfactants is related to their use as lubricants, corrosion-resistant materials, and catalysts.15-17 Different and sometimes contradicting results concerning the structure of the adsorbed film have been recorded. Allara and Nuzzo18,19 examined the formation of an n-alkanoic acid monolayer on Al. Shlotter20 et al. investigated the spontaneous adsorption of arachidic acid on Ag. They20 determined the monolayer thickness by ellipsometric measurements and found the alkyl chains to be arranged in a crystalline order and oriented approximately † ‡
Department of Physics. Department of Chemistry.
(1) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (2) Cheng, L.; Bocarsly, A. B.; Bernasek, S.; Ramanarayanan, T. A. Langmuir 1994, 10, 4542. (3) Cheng, L.; Bocarsly, A. B.; Bernasek, S.; Ramanarayanan, T. A. Chem. Mater. 1995, 7, 1807. (4) Cheng, L.; Bocarsly, A. B.; Bernasek, S.; Ramanarayanan, T. A. Langmuir 1996, 12, 392. (5) Delmarche, E.; Michel, B.; Kang, H.; Gerber, C. Langmuir 1994, 10, 4103. (6) Laibinis, P. E.; Fox, M. A.; Folkers, J. P.; Whitesides, G. M. Langmuir 1991, 7, 3167. (7) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (8) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (9) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (10) Poirier, G. E. J. Vac. Sci. Technol., B 1996, 14 (2), 1453. (11) Ulman, A.; Evans, S. D.; Shidman, Y.; Sharma, R.; Eilers, J. E.; Chang, J. C. J. Am. Chem. Soc. 1991, 113, 1499. (12) Ulman, A. An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, MA, 1991. (13) Ulman, A. Chem. Rev. 1996, 96, 1533. (14) Weisbecker, C. S.; Merrit, M. V.; Whitesides, G. M. Langmuir 1996, 12, 3763. (15) Bowden, F. P.; Tabor, D. The Friction and Lubrication of Solids; Oxford University Press: London, 1968; Vol. Part 2, Chapter 19. (16) Gaines, G. L. J. Colloid Sci. 1960, 15, 321. (17) Timmons, C. O.; Zisman, W. A. J. Phys. Chem. 1965, 69, 984. (18) Allara, G. N.; Nuzzo, R. G. Langmuir 1985, 1, 45-52. (19) Allara, D. L.; Nuzzo, G. N. Langmuir 1985, 1, 52-66. (20) Schlotter, N. E.; Porter, D.; Brigght, T. B.; Allara, D. L. Chem. Phys. Lett. 1986, 132 (1), 93.
normal to the surface. The acid headgroup was chemisorbed on the surface, to form a carboxylate species. Cave21 et al. characterized the structure of Langmuir-Blodgett films of 22-tricosenoic acid on Al using X-ray photoelectron spectroscopy. Both monolayer and bilayer were formed. A monolayer of water molecules appeared to be trapped within these interfacial layers. Tao22 investigated the selfassembled monolayer coating of carboxylic acids on the surfaces of Ag, Cu, and Al. He determined the structure of the adsorbed film, the binding geometry of the headgroup, and the packing density and found them to depend on the metal substrate and on the alkyl-chain length. The acids were found to react with all three metals to form carboxylate layers. The carboxylate species bound differently to each metal. On silver surfaces both oxygen atoms, of the carboxylate moiety, bound to the metallic surface symmetrically, and the chain extended in the trans conformation. However, on the Al and Cu surfaces, the carboxylate bound asymmetrically. For shorter alkyl-chain acids, the molecular chains were disordered and liquidlike. As the chain length increased, the adhesive interaction increased and the packing became denser. Ahn23 et al. coated a silver surface with stearic acid using selfassembled and Langmuir-Blodgett techniques. They compared the products of the two coating methods by measuring the FTIR spectra of the coated films and concluded that the two oxygens of the carboxylate bound symmetrically to the surface. Also, they indicated that the self-assembled monolayers possess more ordered crystalline structure than the LB monolayers. We have reported the self-assembled coatings of amorphous iron and amorphous iron oxide nanoparticles with SDS, OTS,24 alkane thiols,25-28 alcohols29 and a bolaamphiphile molecule30 (a bolaamphiphile is a molecule that (21) Cave, N. G.; Cayless, R. A.; Hazell, L. B.; Kinloch, A. J. Langmuir 1990, 6 (3), 529. (22) Tao, Y.-T. J. Am. Chem. Soc. 1993, 115, 4350. (23) Ahn, S. J.; Son, D. H.; Kim, K. J. Mol. Struct. 1994, 324, 223. (24) Rozenfeld, O.; Koltypin, Y.; Bamnolker, H.; Margel, S.; Gedanken, A. Langmuir 1994, 10, 627. (25) Kataby, G.; Koltypin, Y.; Cao, X.; Gedanken, A. J. Cryst. Growth 1996, 166, 760-762. (26) Kataby, G.; Prozorov, T.; Koltypin, Y.; Sucenik, C. N.; Ulman, A.; Gedanken, A. Langmuir 1997, 13, 6151. (27) Kataby, G.; Koltypin, Y.; Rothe, Y.; Hormes, J.; Felner, I.; Cao, X.; Gedanken, A. Thin Solid Films 1998, 333, 41-49. (28) Prozorov, T.; Kataby, G.; Prozorov, R.; Gedanken, A. Thin Solid Films, in press. (29) Kataby, G.; Ulman, A.; Prozorov, R.; Gedanken, A. Langmuir 1998, 14, 1512-1515.
10.1021/la981001w CCC: $18.00 © 1999 American Chemical Society Published on Web 02/03/1999
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Kataby et al. Table 1. Dependence of the Peak Position and the Width of the Bidentate Carboxylate Vibration as a Function of Number of Carbon Atoms in the Chain coated particles
Figure 1. FTIR spectra of (a) nonadecanoic acid and (b) nonadecanoic acid-coated amorphous iron.
contains two functional groups at the R and ω positions) containing a thiol and carboxylic acid at the R and ω positions. In the present study, we have extended our investigation and coated amorphous iron nanoparticles with carboxylic acids. The coated particles were examined by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), temperature-programmed desorption (TPD), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and magnetic measurements. Experimental Section The preparation of the amorphous iron nanoparticles has been described elsewhere.31 In brief, a 1 M solution of Fe(CO)5 in decane was sonicated (Sonics and Materials, VC-600, Ti horn, 20 kHz, 100 W cm-2) under argon for 3 h at -90 °C. Great care was taken to minimize oxidation of the product in further steps. The coating of the carboxylic acids was carried out from an ethanolic solution, where the amorphous iron/carboxylic acid molar ratio was 12:1. The TGA analysis was done using a Mettler TG-50, and DSC measurements were carried out on a Mettler DSC-30. Mass spectrometer measurements were carried out on a Fisons VG double focusing instrument (AutoSpec E). The mass spectra were obtained by direct insertion of the solid probe in the CI ion source. The reagent gas used was methane. The room-temperature FTIR spectra were recorded on a Nicolet (Impact 410) spectrometer. The measurements were performed using a KBr pellet. The magnetization loop measurements were conducted at room temperature using an Oxford Instrument vibrating sample magnetometer (VSM). X-ray photoelectron spectroscopy measurements were carried out using a 5600 multitechnique System (PHI). The samples were irradiated with an Al KR monochromated source (1486.6 eV), and the outcoming electrons were analyzed by a spherical capacitor analyzer employing a pass energy of 11.75 eV. The samples were analyzed at the surface and after a few minutes of sputter cleaning with a 4 kV Ar+ ion gun. The sputtering rate was 20 Å/min (on SiO2). All the samples were positively charged during measurements. This charging was compensated by using a charge neutralizer with a reference to the C1s peak of hydrocarbons (at ∼284.8 eV). The seven carboxylic acids (heptanoic, octanoic, decanoic, palmitic, heptadecanoic, stearic, and nonadecanoic) which were studied in this report were all purchased from Aldrich. They were used without any further purification.
Results and Discussion (a) FTIR Results. The FTIR spectra of nonadecanoic acid and nonadecanoic acid-coated amorphous iron are depicted in Figure 1. The region which indicates clearly that chemical bonds were formed between the carboxylic (30) Kataby, G.; Prozorov, R.; Ulman, A.; Cojocaru, M.; Gedanken, A. Submitted for publication in J. Mater. Chem. (31) Suslick, K. S.; Choe, S.-B.; Cichowlas, A. A.; Grinstaff, M. W. Nature 1991, 353, 414.
no. of carbons
absorption, cm-1
absorption width, cm-1
neat acid absorption, cm-1
19 18 17 16 10
2917 2918 2918 2918 2819
25 25 25 25 50
2918 2918 2917 2917 2924
acid and the iron substrate is that of the CdO stretching mode. This band is observed for nonadecanoic acid at 1706 cm-1, while after the formation of the chemical bond it is observed at 1539 and 1440 cm-1. In the IR spectra of alkanoic acid coated amorphous iron, the absence of the original 1710 cm-1 peak is common to all acids. In all cases we have also observed the symmetric as well as the asymmetric stretching vibrational modes. We interpret these results in light of the previous explanations by indicating that the bonding pattern of the carboxylic acids on the surface of the amorphous iron is a combination of molecules bonded symmetrically and molecules that are bonded at an angle to the surface.22 Results of previous experimentation dealing with the deposition of carboxylic acids on metals differ in IR results in the CdO absorption region. The main difference is whether the symmetric and asymmetric CdO stretching modes are both observed in the spectrum. Ahn23 and coworkers detected only the symmetric stretching mode at 1404 cm-1 when stearic acid was deposited on silver. On the other hand, Tao22 has detected both the symmetric CdO stretching (1441 cm-1 for various carboxylic acids deposited on Cu) and the asymmetric stretching (1557 cm-1 for the carboxylic acids deposited on Cu). The presence of both the symmetric and asymmetric stretching modes is an indication that part of the carboxylate headgroups are bonded to the surface at an angle (see Figure 10 in Tao’s manuscript22). If, on the other hand, only the symmetric stretching mode is observed in the spectrum, the bonding pattern is such that both carboxylic oxygens are symmetrically bonded to the surface. The IR bands in the region of 1200-1350 cm-1 are called the progressional bands.23 The appearance of the zigzag IR band structure is indicative of a fully extended alkyl chain having a trans configurational structure which is coupled strongly with the carboxylic headgroup. It is evident from Figure 1 that the progressional bands for the alkanoic acid coated amorphous iron are hardly detected. Tao22 has explained the absence of the progressional IR absorptions in alkanoic acids coated on Cu and Al as due to the uncoupling between the carboxylic headgroup and the methylene groups. Figure 10 in his paper depicts the isolation of the carboxylic group from the polymethylene chain. The IR bands in the region of 2850-2960 cm-1 are associated with the CH3 and CH2 symmetric and asymmetric stretchings, respectively. These CH2 symmetric and asymmetric stretchings are usually analyzed by studying peak position and width as a function of the methylene chain length. If the energies of the CH2 stretchings of longer alkanoic acids appear at lower energies than the corresponding short acids, it indicates a highly ordered and a crystallike structure for the longer chain acids. The dependence of the peak position and the width of the CH2 peak as a function of the number of the methylene groups are presented in Table 1. This table indicates clearly that the peak position and the width of
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Figure 2. O1s XPS of (a) heptadecanoic acid and (b) heptadecanoic acid-coated amorphous iron. Figure 4. (a) TGA spectrum of nonadecanoic acid-coated amorphous iron nanoparticle. (b) DSC spectrum of nonadecanoic acid-coated amorphous iron nanoparticle.
Figure 3. Fe2p XPS of (a) amorphous iron and (b) heptadecanoic acid-coated amorphous iron.
the acid-coated amorphous iron are independent of the chain length. This behavior is due to the amorphous nature of the substrate which hinders the organization of the methylene chains. This hindrance does not occur on flat surfaces. (b) XPS Data. The O1s photoelectron spectra of the heptadecanoic acid and heptadecanoic acid-coated amorphous iron particles are presented in Figure 2. A 1.3 eV shift is observed when the XPS spectrum of the acid-coated iron is compared with the XPS of the free acid. This shift is additional evidence for the formation of chemical bonds between the iron and the carboxylic acid and evidence that the bonding is occurring around the bidentate oxygen atoms of the carboxylate ion. In Figure 3 we present the Fe2p XPS spectra of an amorphous iron sample as well as C17 acid capped on the iron. Each sample has undergone a treatment by Ar ion beams. The free amorphous iron was treated for 6 min while the acid-coated iron was treated for only 3 min. In both spectra the zerovalent iron peak at 706.9 eV is observed. However, the intensity of the 706.9 eV bond in acid-coated iron is greater than that for the bare iron peak, although the latter was bombarded for a longer time and more oxidized molecules from its surface were removed. The more intense peak observed for the acid-coated iron is indicative of its effective corrosion resistance. The weak iron peak of the noncoated iron is due to an oxidation process that continues even when a monolayer of oxide is coating the surface. The oxidation reaction continues during the time between the synthesis of the iron and the XPS measurement and occurs even when the iron is kept under vacuum. The Fe2p peak at 709.65 eV (Figure 3a) is attributed to the oxidized form of the amorphous iron, whereas the peak at 708.9 eV (Figure 3b) is attributed to the carboxylate-Fe bond. The XPS results substantiate the IR data, indicating the formation of chemical bonds between the iron substrate and the oxygen atoms of the carboxylic acid. (c) TGA Results. The TGA data also indirectly provide a measure of the strength of the chemical bonding between
Figure 5. Inflection points in the weight-loss curve as a function of number of carbon atoms in the chain.
the amorphous iron substrate and the anchored acids. In Figure 4a we present the TGA spectrum of the nonadecanoic acid-coated iron particles. The spectrum shows a major weight loss at 366 °C (25% weight loss) and a minor loss at 520 °C (5% weight loss). These temperatures are the inflection points in the weight-loss curve and will serve along this discussion for comparing various systems. We assign the 366 °C drop to the dissociation of the carboxylate-iron bonds. If this temperature is a measure of the strength of the surfactant-substrate chemical bond, our conclusion is that the carboxylate-iron bond is stronger than the bond formed between the iron and the thiol or the alcohol moiety. This conclusion is based on the comparison of the inflection temperatures observed for thiol- and alcohol-coated amorphous iron. The sharp weight loss is also dependent on the length of the alkyl chain. In Figure 5 we present the values of the inflection points in the weight-loss curve as a function of the alkyl chain. The distinct feature observed is that the longer the alkyl chain, the higher the dissociation temperature. This is related to the interchain van der Waals interactions. It is worth mentioning that for short-chain-acid-coated amorphous iron, the weight loss observed at 300 ( 20 °C is much smaller, 4 ( 1%, indicating that the coverage is less than a monolayer. We will therefore concentrate mostly on the long-chain carboxylic acid-coated amorphous iron. (d) DSC Results. A typical and expected DSC spectrum is detected for the nonadecanoic acid-coated iron particles and is depicted in Figure 4b. A sharp endothermic peak is detected at 122 °C. This endothermic peak is not associated with any weight loss as can be deduced from the TGA spectrum. This temperature is assigned as the “melting temperature” of the long carbon chains. We have recently32 reported on a similar phenomenon observed for thiol-coated Fe2O3. The melting point of nonadecanoic acid
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Figure 6. Melting point temperatures of (a) pure acids and (b) acid-coated amorphous iron.
Figure 7. ∆H (kcal/mol) as a function of number of carbon atoms in the chain.
is 69.4 °C. The increase in the “melting point” (mp) of the acid-coated substrate has been predicted and discussed by Landman.33 The endothermic peak is narrow, and its magnitude (∆Hfusion) is an indication of the close packing arrangements of the C19 chains.32 The higher the temperature difference between the mp of the acid-coated amorphous iron and the mp of the bare acid, the weaker the interchain interactions. When the difference of the “melting point” of the acid-coated amorphous iron and the pure acid was plotted against the number of carbon atoms for 16, 17, 18, and 19 carbons, a straight line of 50 ( 3 °C is observed. The results of the melting point temperatures of the pure acids and the acid-coated amorphous iron are presented in Figure 6. A weak exothermic peak is observed at 280 °C for the C19 acid-coated amorphous iron (see Figure 4b). It is assigned to the crystallization temperature of the amorphous particles, a process that occurs at almost the same temperature in pure amorphous iron, thiol-coated iron, and alcohol-coated iron. A much broader and less distinct exothermic peak is observed for the C17 acid-coated amorphous iron. The third endothermic peak is observed at 408 °C. This peak is assigned to the dissociation of the nonadecanoic acid from the iron substrate. It corresponds to the drop in weight observed in the TGA spectrum. The desorption temperature of the C16, C17, C18, and C19 was treated with the Redhead34 analysis process to gain information regarding the nature of the chemical bond between the substrate and the alkanoic acid. The only assumption made is that the desorption reaction is first order. The results obtained are presented in Figure 7. The results show that within the experimental error, 1.5 kcal/mol, the ∆H of the dissociation is almost independent of the chain length for the C16-C19 chains. This is surprising because we assume that the dissociation event takes place at the anchoring position of the acid to the iron and primarily involves the rupture of the Fe-O bond in addition to the separation of the weakly interacting chains. It is expected that each additional methylene group will enhance the rupture energy by about 1.5 kcal/mol. However, since the dissociation is finalized by the formation of a dimer, the van der Waals energies are released in the formation of the dimer. It is also possible that the odd-even22 phenomenon is also reflected in these results as revealed in this figure. The values calculated according to the Redhead analysis, 44-50 kcal/mol, do not exactly reflect the rupture of the Fe-OOC-R, which should be 93 ( 4.1 kcal/mol.35 The
calculated values are the sum of the energy of bonds breaking and formed upon the desorption process. As shown later, CO2 is probably released in this process and a ketonic dimer is formed. (e) TPD Results. The total ion current of hexadecanoic acid-coated amorphous iron as a function of temperature revealed a peak at 250 °C. The mass-analyzed signal showed molecules weighing 257.244 amu. This is interpreted as the MH+ (calculated mass 257.248 amu) removed from the surface. This MH+ peak is detected for all longchain alkanoic acids. This relatively low-temperature desorption cannot be due to physical adsorption since the dissociation energy would require 35 kcal/mol.34 We associate this desorption with carboxylated oxygens that are asymmetrically bonded to the amorphous iron. The same results were afforded for the following: heptadecanoic acid MH+, found 271.257 amu, calcd 271.263 amu; octadecanoic acid MH+, found 285.273 amu, calcd 285.279 amu; nonadecanoic acid MH+, found 299.294 amu, calcd 299.295 amu. In Figure 8a we display the HR-MS of nonadecanoic acid-coated amorphous iron which desorbed at the 350400 °C temperature range. The desorption peaks at the higher temperatures can be attributed to the rupture of the symmetrically bonded R-COO-Fe bonds. At these temperatures, higher molecular weight compounds which desorbed from the surface begin to appear. The HR-MS measurements of this compound predicts the following empiric formulas: C31H63O found, 451.488 amu, calcd 451.487 amu (in the case of C16:O); C33H67O, found 479.522 amu, calcd 479.519 amu (in the case of C17:O); C35H71O, found 507.548 amu, calcd 507.550 amu (in the case of C18:O); C37H75O, found 535.584 amu, calcd 535.581 amu (in the case of C19:O). The structure proposed for these compounds is that of a symmetrical ketone, CH3(CH2)nCO(CH2)nCH3 where n ) 14, 15, 17. Such ketones can be obtained by a ketonic decarboxylation of the carboxylic acids linked to the amorphous iron. The ketonic decarboxylation is a known reaction in organic chemistry.36,37 Carboxylic acids can be converted to symmetrical ketones by pyrolysis in the presence of thorium oxide:
(32) Prozorov, T.; Gedanken, A. Adv. Mater. 1998, 10, 532. (33) Leudtke, W. D.; Landman, U. J. Phys. Chem. 1996, 100, 13323. (34) Redhead, P. A. Vacuum 1962, 12, 203.
2RCOOH f RCOR + CO2 An alternative method involves heating of the ferrous salt of the acid (35) Handbook of Chemistry and Physics, 78th ed.; CRC Press: New York, 1997-1998. (36) March, J. Advanced Organic Chemistry 3rd ed.; Wiley: New York, 1985; p 442. (37) Davis; Schultz J. Org. Chem. 1962, 27, 854.
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Figure 8. CI/CH4 Mass spectra of (a) nonadecanoic acid-coated amorphous iron and (b) nonadecanoic acid.
(RCO2)2FeII f R-CO-R + FeIICO3 This behavior translated to our studies attests to the binding of the carboxylic acids to the amorphous iron. For control purposes we have measured the mass spectrum under the same conditions as those for the noncoated carboxylic acids. It yielded quasimolecular ions and a MMH+ dimer. The MMH+ dimer observed in the spectra of the “free” carboxylic acids is the result of an ionmolecule reaction between protonated and neutral molecules under MS conditions. No peaks belonging to the symmetrical ketonees were observed in these cases (Figure 8b). Simultaneously with the desorption of the dimeric ketone we observe a monomeric peak even at high temperatures. For C16 acid-coated amorphous iron, the monomeric and dimeric ketone peak intensity as a function of temperature is presented in Figure 9. While the intensity of the dimeric peak increases in the temperature range 250-400 °C, the peak intensity of the monomer decreases in that range. Although this can be interpreted as due to lack of monomers at high temperatures, the figure shows an appreciable monomer amount even at 400 °C. On the other hand, we can explain the coupling between the dimer increase and monomer decrease as a result of a concerted desorption reaction occurring on the surface, upon the release of molecules, forming the dimeric ketone. (f) Magnetization Measurements. The magnetization loop of the carboxylic acid-coated amorphous iron is presented in Figure 10. The general behavior observed for the bare amorphous iron38 as well as the alcohol-coated amorphous iron29 is repeated in these measurements. It shows the lack of hysteresis, and it does not saturate until 15 kG. This superparamagnetic behavior is related to the small nanoparticles. The magnitude of the magnetization curve was found to be dependent on the length of the alkyl chain. (38) Grinstaff, M. W.; Salamon, M. B.; Suslick, K. S. Phys. Rev. B 1993, 48, 269.
Figure 9. Change in the amount of (a) hexadecanoic acid and (b) dimeric keto formed from the heated hexadecanoic acidcoated amorphous iron, as a function of temperature.
Figure 10. Magnetization loop of acid-coated amorphous iron.
We would like to comment also on the size of the magnetization at 15 kG and the general possibility of “tailoring” magnetic properties by changing the bonded chromophore and the length of the carbon chain. The magnetization measured at 15 kG for amorphous iron obtained from the sonication of pure Fe(CO)5, is 120 emu/g
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of iron.38 Lower values39 were measured for amorphous iron obtained from the sonication of dilute solutions of Fe(CO)5. We have observed that the magnetization is strongly dependent on the nature of the chromophore bonded to the amorphous iron. For example the values measured for thiols, alcohols, sulfonic acids, and phosphonic acids were 28, 60, 4, and 3 emu/g of iron, respectively. The reason such a dependence exists will be discussed in a separate paper. Conclusions The current paper presents data confirming the bonding of carboxylic acids to a nanophased amorphous iron substrate. Strong chemical bonds were formed as evident from the temperature at which these chemical bonds were ruptured. The molecules desorbed from the surface area react and yield a dimeric keto compound which is formed (39) Cao, X.; Koltypin, Y.; Kataby, G.; Prozorov, R.; Gedanken, A. J. Mater. Res. 1995, 10, 2952.
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by the reaction of two monomers and the elimination of CO2. The magnetization depends on the nature of the bonded chromophore and perhaps a series which is similar to the spectrochemical series can be formed. The series will explain the changes in the magnetization by low-spin and high-spin arrangements of the d electrons. Acknowledgment. This research was partially supported by Grant No. 94-00230 from the U.S.-Israel Binational Science Foundation (BSF), Jerusalem, Israel. We thank Dr. L. Burstein for XPS measurements in the Wolfson Applied Materials Research Center, Tel-Aviv University. R. Prozorov acknowledges support from the Clore Foundations. We thank Professor Y. Yeshurun for making available for this study the facilities of the National Center for Magnetic Measurements in the Departments of Physics at Bar-Ilan University. We also thank Dr. S. Hochberg for her editorial assistance. LA981001W
