Ftir Study Fept

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ED:Sabitha PAGN: ashok SCAN: Rajesh

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FTIR study of surfactant bonding to FePt nanoparticles

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Nisha Shukla*, Chao Liu, Paul M. Jones, Dieter Weller 11 13

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FePt nanoparticles coated with 2 nm thick films of surfactant have been studied as candidates for magnetic recording media. The self-assembly of these nanoparticles is influenced by the properties of their surfaces which are coated with a layer of mixed oleic acid and oleyl amine. These surfactant coated FePt nanoparticles were characterized using Fourier transform infrared spectroscopy (FTIR). The observation of both n(COO) and n(C=O) vibrational modes indicates that oleic acid bonds to the FePt nanoparticles in both monodentate and bidentate forms. The oleylamine bonds to the FePt nanoparticles through electron donation from the nitrogen atom of the NH2 group. The FTIR spectra indicate that there is a conversion of the alkyl chains from the oleyl form (cis-9-octadecenyl) to the elaidyl form (trans-9octadecenyl) during the synthesis of the FePt nanoparticles. This is revealed by the presence of several vibrational absorption bands in the region of the olefinic C–H stretching modes. The presence of elaidyl groups on the FePt surfaces is very important because the structures of the oleyl groups and the elaidyl groups are quite different and are expected to pack differently around the FePt nanoparticles. This in turn will influence the self-assembly of nanoparticles on substrates. r 2003 Published by Elsevier B.V.

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Abstract

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Received 27 November 2002; received in revised form 11 February 2003

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FePt nanoparticles have high magnetic anisotropy and have potential use as a magnetic medium for patterned data storage applications [1]. These nanoparticles are coated with surfactant films of mixed oleic acid (cis-9-octadecenoic acid) and oleylamine (cis-9-octadecenoic amine). The properties of these surfactant coatings control the stability of the particles in solution and the

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1. Introduction

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Seagate Technology, 1251 Waterfront place, Pittsburgh, PA 15222, USA

*Corresponding author. E-mail address: [email protected] (N. Shukla). 0304-8853/$ - see front matter r 2003 Published by Elsevier B.V. doi:10.1016/S0304-8853(03)00469-4

49 dynamics of their self-assembly on surfaces. Although there have been a number of studies of the self-assembly of FePt nanoparticles on substrates there are no prior studies of the surfactant coatings on the surfaces of these nanoparticles. The objective of this work is to characterize oleic acid and oleylamine surfactant coatings on FePt nanoparticles and to determine their mode of bonding to the nanoparticle surfaces. Films of unsaturated fatty acids and surfactants are widely used as surface coatings in electronics, optical, and biomedical applications [2,3]. As a result there have been a number of reports of the

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FePt nanoparticles were prepared using the synthesis described by Sun et al. [8,9]. In this synthesis monodispersed FePt nanoparticles were obtained by reaction of iron pentacarbonyl and platinum (II) acetylacetonate in a solution of dioctylether and surfactant. The surfactant used during synthesis was a 1:1 mixture of oleic acid and oleylamine. Once synthesized, the FePt nanoparticles were washed using a polar solvent (ethanol, CH3CH2OH) followed by washing with a non-polar solvent (hexane, C6H14). Following synthesis the nanoparticles were subjected to additional washing cycles to remove excess surfactant from the synthesis solution. The ‘excess surfactant’ is the fraction in solution not bonded to the FePt nanoparticles. The original mixture of surfactant coated nanoparticles and excess surfactant in dioctylether was first mixed with 10 parts of ethanol causing the particles to precipitate. The mixture was centrifuged to separate the particles from the solution. The supernatant solvent consisting of dioctylether and smaller nanoparticles was then discarded and the nanoparticles were redispersed in non-polar hexane. This sequence of precipitation in ethanol, centrifuging to separate the solvent, and redispersion in hexane was then repeated three times. Finally, the nanoparticles were dispersed in hexane. This procedure was sufficient to produce a dispersion of 3–4 nm diameter FePt nanoparticles suspended in hexane with no surfactant in the solution. The nanoparticles were characterized using FTIR. In order to obtain the FTIR spectra a drop of the nanoparticle solution was placed on the surface of a diamond attenuated total reflectance (ATR) FTIR cell and the solvent was allowed to dry. All FTIR spectra were taken using

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occurs at 286
C during synthesis and is accompanied by isomerization. Our observation of the isomerization of the oleyl chains of oleic acid and oleylamine during FePt nanoparticle synthesis provides insight into the formation of surfactant monolayers on nanoparticles.

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characterization of oleic acid films adsorbed on the surfaces of materials such as soda lime-silicate, silica glasses, soda-silicate [4,5], apatite, dolomite [6], and titania [7]. The Fourier transform infrared spectroscopy (FTIR) studies of oleic acid adsorption on soda lime-silicate indicate that oleic acid is adsorbed on the soda lime-silicate as both metaloleate and oleic acid hydrogen bonded to Si–OH sites [4,5]. In contrast, it has been reported that oleic acid is adsorbed on fused silica only as hydrogen bonded oleic acid. On the soda-silicate glass it is reported that oleic acid completely deprotonates and bonds to Na+ ions as oleate. Ince et al. [6] have reported that adsorption of oleic acid on apatite and dolomite occurs predominantly as oleate at pH=10, whereas at pH=4 some fraction is present as oleic acid. Thistlethwaite et al. [7] have reported that on the rutile form of TiO2, oleic acid adsorption at pH=3 and 9 is dependent on the nature of the particular oxide sample. Oleic acid monomers are adsorbed to Ti4+ Lewis acid sites by coordination as s-bonded ligands through the carbonyl group oxygen. The oleic acid dimers were reported to bond to two adjacent Lewis sites via their hydroxyl oxygen atoms. At pH=3 it was observed that the CH=CH group interacted with surface OH+ 2 groups. In summary, there are a variety of modes of oleic acid bonding to surfaces that might be observed on FePt nanoparticles. In this paper we have used FTIR to study a mixture of oleic acid and oleylamine bonded to the surfaces of FePt nanoparticles. We have observed that oleylamine bonds molecularly to the nanoparticle surfaces with the NH2 group intact. Oleic acid bonds to the FePt nanoparticles in both monodentate form (RC(=O)–O–M) and bidentate form (RCO2–M). FTIR results also indicate that there is some isomerization of the oleyl groups from the cis form to the trans form (elaidyl groups) during synthesis of the FePt nanoparticles. Although several studies of oleic acid adsorbed on surfaces at room temperature have been reported in the literature, none of these studies have reported isomerization of oleic acid. While the oleic acid films observed in prior studies were adsorbed at room temperature, adsorption of oleic acid and oleylamine on the FePt nanoparticles

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Prior to characterization of the FePt nanoparticles the post-synthesis solution was analyzed using FTIR. The post-synthesis solution consists of FePt nanoparticles, excess oleic acid, and excess oleylamine dispersed in dioctylether solvent. A spectrum of the post-synthesis solution was obtained by placing a drop of the solution on the diamond window of the ATR cell. The postsynthesis solution is predominantly dioctylether. As a result the FTIR spectrum of the postsynthesis nanoparticle dispersion is identical to the FTIR spectrum of pure dioctylether, which is used as the solvent during the synthesis of the FePt nanoparticles. Clearly, in order to obtain a spectrum of the nanoparticles they must be isolated by redispersion into a solvent that will evaporate. No attempt was made to analyze or assign the modes observed in spectra of the postsynthesis solution, as it was not the focus of this work. The standard process for preparing a monodispersed sample of the FePt nanoparticles involves washing in ethanol and separating the large particles from solution by centrifugation. The heavy particles are then redispersed in hexane and surfactant. Figs. 1–3 show FTIR spectra of pure oleic acid, pure oleylamine, and FePt nanoparticles washed in hexane and ethanol (as described in Section 2) with excess oleic acid and oleylamine (excess means surfactant in the solution

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a Nicolet Nexus 670 spectrometer with a deuterated triglyceride sulfate (DTGS) detector. Spectra were obtained using 4 cm1 resolution and 32 scans were taken for statistical averaging. FTIR spectra of the pure samples of oleic acid, elaidic acid, and oleyl amine were obtained by putting a drop of the surfactant dissolved in hexane onto the diamond ATR cell and letting the solvent evaporate before taking the spectrum. The oleic acid, oleylamine, elaidic acid, ethanol, and hexane (anhydrous) were bought from Aldrich Chemical Co. and were used without further purification. The ethanol was obtained in sure seal bottles with very low water content (o0.002%).

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that is not bonded to nanoparticles). The spectrum of the FePt nanoparticles was obtained after allowing the hexane solvent to evaporate. The spectra of the oleic acid and the oleylamine were obtained by dissolving a sample of each into hexane and then applying the solution to the diamond window of the ATR cell and allowing the solvent to evaporate. The spectra in Figs. 1 and 2 show a number of peaks characteristic of the pure surfactants. The spectrum of oleic acid reveals modes characteristic of the oleyl group: the peaks at 2854 and 2922 cm1 are due to the symmetric and asym-

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Fig. 3. FTIR spectra of FePt nanoparticles washed in excess surfactant (oleic acid and oleylamine) in 1000–3500 cm1 region.

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Fig. 4. FTIR spectra of pure surfactant mixture (1:1, oleic acid and oleylamine) in 1000–3500 cm1 region.

COOH group. Clearly the spectrum of the FePt nanoparticles in excess surfactant does not seem to resemble a combination of the pure surfactant spectra. In order to understand the origin of the differences between the spectrum of the FePt nanoparticles in excess surfactant and the spectra of the pure surfactants we have obtained a FTIR spectrum (Fig. 4) of a 1:1 mixture of oleic acid and oleylamine, as used in the nanoparticle synthesis and washing process. Comparison of the FTIR spectra in Figs. 3 and 4 suggests that most of the features observed in the spectrum of the FePt nanoparticles can be attributed to the excess surfactant. The interesting thing is that the spectrum of the surfactant mixture is not a simple combination of the spectra of the pure component surfactants. The 1709 cm1 peak which is characteristic of the COOH group is absent as is the 3300 cm1 mode of the NH2 group. The most obvious explanation for the absence of these peak is that the mixture of oleic acid and oleylamine consists of an acid–base complex of COO and NH+ ions. Gasgnier [4] has reported FTIR 3 spectra of potassium carboxylates and observed modes associated with COO in the range 1430– 1565 cm1. In the FTIR spectra of potassium caprylate and potassium stearate the na (COO) and ns (COO) were observed at 1562 and 1435 cm1, and 1560 and 1426 cm1 respectively.

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metric CH2 stretching modes, the peak at 3006 cm1 is due to the n(C–H) mode of the C– H bond adjacent to the C=C bond, and the small peak at 1648 cm1 is due to the n(C=C) stretch mode. In addition, the spectrum of the oleic acid shows several modes characteristic of the carboxylic acid group: the n(C=O) mode is observed at 1709 cm1 and the weak mode at 2670 cm1 is due to the n(O–H) stretch of the dimerized acid. This spectrum is of the solid form of oleic acid in which the carboxylic acid groups are dimerized through hydrogen bonding. The spectrum of oleylamine (Fig. 2) shows peaks characteristic of the oleyl group in the 2850–3000 cm1 region and the n(C=C) stretch mode at 1647 cm1. In addition there are modes characteristic of the amine group: the peak at 1593 cm1 due to the NH2 scissoring mode and the peak at 3300 cm1 due to the n(N–H) stretching mode. The peaks in the low-frequency region of the spectra below 1500 cm1 arise from complex combinations of the n(C–C) stretch, n(C–O) stretches, CH2 deformations and other motions that are too complex to assign. The spectrum of the washed FePt nanoparticles (Fig. 3) with excess surfactant shows peaks characteristic of the oleyl group in the 2850– 3000 cm1 region but no peak at 3300 cm1 due to the n(N–H) stretching of the NH2 group and no peak at 1709 cm1 due to the n(C=O) mode of the

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n(NH) Trans n(CH=) Trans n(CH=) Trans n(CH=) Cis n(CH=) na (CH2) ns (CH2) n(OH) n(C=O) n(C=C) n(NH2) n(COO) Trans n(CH=)

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[19] [13] [13] [13] [6,13,16] [6,13,16] [6,13,16] [5,6] [5,6,13] [5,6] [19] [5,6,8,11] [13,16]

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Table 1 Infrared vibrational assignments

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mode, at 1709 cm1 (CQO) and 1647 cm1(– CQC) are well established in the literature. Our assignment of the peak at 1512 cm1 is based on several studies of acids on various substrates [5,8,11] and on standard the FTIR spectrum of sodium oleate. The standard sodium oleate FTIR spectrum shows the oleate na (COO) mode at 1558 cm1 which is similar to that observed by Lee et al. [4] for the na (COO) mode on sodium in soda-lime. However, Lee et al. have observed a range of 37 cm1 in the na (COO) mode on substrates including Al, Mg, Ca and Na. Thistlethwaite et al. [8] reported the observation of the na (COO) mode in the range 1509–1517 cm1 following adsorption of oleic acid on titania. Qingxia et al. [11] have observed the na (COO) mode on adsorption of a monolayer of 16mercaptihexadecanoic acid on nanosized Fe2O3 magnetic particles at 1527 cm1. The adsorption of oleic acid on FePt nanoparticles shows the na (COO) peak at 1512 cm1 which is very close the frequency observed by Thistlewaite and Qingxia [8,11]. The peak at 1593 cm1 is assigned to the NH2 scissoring mode. This was also confirmed by DFT calculations of small molecules such as propylamine. A detailed DFT analysis will be published later by Jones et al. [12]. All peak assignments for this work are shown in Table 1. The FTIR spectrum of the oleic acid and oleylamine surfactant layers on FePt nanoparticles in the range 2800–3500 cm1 is shown in Fig. 6.

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Our results also show peak at 1565 and 1435 cm1 which can be attributed to the –COO1 mode. The spectrum of the oleic acid—oleylamine complex (Fig. 4) dominates the spectrum of the FePt nanoparticles washed in excess (fraction of surfactant not bonded with nanoparticles) surfactant (Fig. 3). In order to understand the bonding of surfactant to FePt nanoparticles, the FePt nanoparticle solution was washed without surfactant in the hexane and ethanol wash solvents. Fig. 5 shows the 800–1800 cm1 region of the FTIR spectrum of FePt nanoparticles with no excess surfactant. This spectrum is due to oleic acid and oleylamine adsorbed on the FePt nanoparticles. The peak at 1709 cm–1 is due to the n(CQO) stretch mode and indicates that some fraction of the oleic acid is bonded to nanoparticles either in monodentate form or as an acid. The peak at 1512 cm1 is due to the na (COO) mode and indicates the presence of bidentate carboxylate bonding to the nanoparticles [11]. The 1618 cm1 peak is due to the n(C=C) stretching mode and indicates that the C=C double bond is intact in the oleyl groups on the surfactant. The 1593 cm1 peak is due to the NH2 scissoring mode which suggests that oleylamine is adsorbed with the NH2 group intact. The assignments of the vibration modes in the region 2850– 3000 cm1(–CH2) modes, at 3300 cm1(NH)

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The intense absorption in the 2800–3000 cm1 region is due to the symmetric and asymmetric CH2 stretching modes of the oleyl groups but gives very little information about the bonding of either oleic acid or oleylamine to the FePt nanoparticle surface. The broad peak in the 3300–3500 cm1 region is due to the n(NH) stretching mode, which is consistent with the previous suggestion based on the spectral feature at 1593 cm1 in Fig. 5 that the NH bonds are intact. The most interesting part of the spectrum is the region from 3000 to 3100 cm1 which is very different from the spectrum of the oleic acid–oleylamine mixture in the same region. The peaks in the 3000–3100 cm1 region are due to n(CH) stretching modes of the C–H bonds adjacent to the C=C bond. The spectra of both oleic acid and oleylamine show only a single peak at 3006 cm1 in this region. The presence of several peaks in this region indicates that the C–H bonds adjacent to the C=C bond exist in several different environments that are substantially different from those in solid oleic acid and oleylamine. Published spectra of the various solid phases of oleic acid reveal n(CH) peaks at frequencies as high as 3020 cm1 but not in a range extending as high as 3100 cm1. The very small peak at 3006 cm1 and larger peak at 3020 cm1 can be attributed to the n(CH) stretch modes in different phases of oleic acid and oleylamine but the peaks

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at higher frequencies cannot be due to oleic acid and oleylamine. Sinclair et al. [12] have reviewed the FTIR spectra of a wide range of surfactants. Of those described, the only species that has vibrational modes at frequencies in the range up to 3100 cm1 is elaidic acid. Elaidic acid (trans-9octadecenoic acid) is the trans form of oleic acid (cis-9-octadecenoic acid). The presence of the high frequency peaks due to stretching of the C–H bond adjacent to the C=C double bond suggests that there is a conversion of oleic acid and/or oleylamine to elaidic acid or elaidic amine during synthesis of the nanoparticles. The presence of the peak at 970 cm1 in the washed FePt nanoparticle spectrum is also indicative of the presence of transsubstituted unsaturated alkyl groups. It is well established in the literature [14,15] that the prominent mode at 965–975 cm1 can be used to differentiate between cis and trans substituted unsaturated fatty acids and esters. Gruger et al. [16] have reported the FTIR spectra of oleic acid and elaidic acid adsorbed on CsI at 90 K and clearly show that there is peak at a 970 cm1 in the spectrum of elaidic acid which is absent in the spectrum of oleic acid. In order to justify our assignments in the region 3000–3100 cm1 a FTIR spectrum of solid elaidic acid was taken at room temperature as shown in Figs. 7 and 8. In Fig. 7 we observe a peak at

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NH2 group intact. We observe conversion of oleyl form to elaidyl form (conversion of the cis-9octadecenyl group to a trans-9-octadecenyl group) during synthesis. This isomerization must occur as a result of the synthesis of the FePt nanoparticles at high temperature since it is not observed during adsorption of oleic acid on surfaces at room temperature.

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References 1

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[1] S. Sun, D. Weller, C.B. Murray, The Physics of Ultrahigh-density Magnetic Recording, 2001. [2] A. Ulman, An introduction to ultra-thin orgain fil from Langmuir–Blodgett to self-assembly, Academic Press, Boston, MA, 1991. [3] G.L. Smay, Glass Technol. 26 (1985) 46. [4] M. Gasgnier, J. Mater. Sci. 20 (2001) 1259. [5] D.H. Lee, R.A. Condrate Sr., J. Mater. Sci. 34 (1999) 139. [6] D.H. Lee, R.A. Condrate Sr., W.C. Lacourse, J. Mater. Sci. 35 (2000) 4961. [7] D.E. Ince, C.T. Johnston, B.M. Moudgil, Langmuir 7 (1991) 1453. [8] P.J. Thistlethwaite, M.S. Hook, Langmuir 16 (2000) 4993. [9] S. Sun, C.B. Murray, D. Weller, Liesl Folks, A. Moser, Science 287 (2000) 1989. [10] C. Liu, N. Shukla, X. Wu, T.J. Klemmer, D. Weller, M. Tanase, D. Laughlin, to be published. [11] Qingxia Liu, Zhenghe Xu, Langmuir 11 (1995) 4617. [12] P.M. Jones, N. Shukla, C. Liu, Y.T. Hsia, D. Weller, in preparation. [13] R.G. Sinclair, A.F. Mckay, G.S. Myers, R.N. Jones, J. Am. Chem. Soc. 74 (1952) 2578. [14] H.W. Lemon, C.K. Cross, Can. J. Res. 27B (1949) 610. [15] R.S. Rasmussen, R.R. Brattain, J. Chem. Phys. 15 (1947) 120. [16] C. Vogel-Weill, A. Gruger, Spectrochim. Acta Part A 52 (1996) 1737. [17] M.J. Mendes, O.A.A. Santos, E. Jordao, A.M. Silva, Appl. Catal. A: General 217 (2001) 253. [18] C.M.M. Costa, E. Jordao, M.J. Mendes, O.A.A. Santos, F. Bozon-Verduraz, React. Kinet. Catal. Lett. 66 (1999) 155. [19] W. Erley, J.C. Hemminger, Surf. Sci. 316 (1994) L1025– L1030.

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970 cm which is due to trans form of the unsaturated fatty acid as observed by Gruger et al. [16]. In Fig. 8 we observe all three peaks at 3020, 3035 and 3060 cm1 associated with the n(CH) modes adjacent to C=C bond in elaidic acid. The isomerization of oleic acid to elaidic acid can occur during the synthesis of the FePt nanoparticles. This reaction has been observed during the hydrogenation of oleic acid on Ru–Sn catalysts [17] and on sol–gel supported Ru [18]. This prior observation supports our suggestions that isomerization is occurring during FePt nanoparticle synthesis and that we have observed elaidyl groups on their surfaces. The presence of elaidyl groups on the FePt surfaces is very important because the structures of the oleyl groups and the elaidyl groups are quite different and are expected to pack differently around the FePt nanoparticles. This in turn will influence the self-assembly of nanoparticles on substrates.

4. Conclusions

Oleic acid bonds to FePt nanoparticles in both monodentate and bidentate forms. The oleylamine bonds to FePt nanoparticles molecularly with the

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