A

ttsFic©

K

1

smbdamsmmp

dt1(Fta

Ff

0d

Colloids and Surfaces A: Physicochem. Eng. Aspects 301 (2007) 113–116

Surfactant isomerization and dehydrogenation on FePt nanoparticles

Nisha Shukla ∗, Erik. B. Svedberg, J. EllSeagate Research, Pittsburgh, PA 15222, United States

Received 24 March 2006; received in revised form 10 November 2006; accepted 7 December 2006Available online 14 December 2006

bstract

Surfactant coated FePt nanoparticles were characterized using Fourier transform infrared spectroscopy (FTIR). The FTIR spectra indicate thathere is a conversion of the alkyl chain of the surfactant from the oleyl form (cis-9-octadecenyl) to the elaidyl form (trans-9-octadecenyl) duringhe synthesis of the FePt nanoparticles. This is revealed by the presence of several vibrational absorption bands in the region of the olefinic C H

tretching modes. The appearance of infrared absorption bands due to olefinic C H stretching is due to the presence of Fe in the nanoparticles. TheTIR spectrum of platinum nanoparticles does not reveal the presence of olefinic C H modes. The concentration of Fe in the FePt nanoparticles

nfluences the intensity of the olefinic C H stretching modes. The high intensity of these olefinic C H modes in nanoparticles with high Feoncentration indicates that Fe acts as a catalyst for cis to trans conversion and may lead to dehydrogentation of the alkyl chains. 2007 Elsevier B.V. All rights reserved.

oparti

cOnTecot

ibbgstpw

eywords: Nanoparticles; FTIR; Surfactant; FePt nanoparticles; Magnetic nan

. Introduction

Due to their high magnetic anisotropy, chemically synthe-ized FePt nanoparticles are potential candidates for futureagnetic media with data recording densities of 1 Tbit/in2 and

eyond. One of the key challenges in the development of high-ensity recording media is the control of the monodispersitynd the size of the magnetic grains. High-density magneticedia will require monodisperse magnetic grains with diameters

maller than 10 nm. Chemically synthesized FePt nanoparticlesay prove advantageous over conventional sputtered magneticedia because of superior control of the size and the monodis-

ersity of the magnetic grains.FePt nanoparticles can be synthesized using a method

escribed by Sun et al. [1] which involves thermal decomposi-ion of Fe(CO)5 and reduction of Pt(acac)2 in the presence of a:1 ratio of oleic acid (cis-9-octadecenoic acid) and oleylaminecis-9-octadecenoic amine) surfactants. The final product is a

ePt metal core surrounded by a layer of surfactant. The surfac-

ant coating controls the stability of the nanoparticles in solutionnd the dynamics of their self-assembly on substrate surfaces.

∗ Corresponding author at: Hamburg Hall, Carnegie Mellon University, 5000orbes Avenue, Pittsburgh, PA 15213, U.S.A. Tel.: +1 412 268 4827;ax: +1 412 268 5229.

E-mail address: nisha@andrew.cmu.edu (N. Shukla).

aocFatfF

927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2006.12.031

cles

An important challenge in the synthesis of FePt nanoparti-les is the preparation of highly monodisperse nanoparticles.nce the monodispersity of the nanoparticles is achieved theanoparticles have to be self-assembled uniformly on a surface.he uniformity of self-assembled films of nanoparticles is influ-nced by the surfactant coating on the nanoparticles and theoncentration of excess surfactant in the solution [2]. The typef the surfactant and its chain length and bonding will also effecthe self-assembly of the nanoparticles on the substrate.

Several studies have reported bonding of different acids onron oxide nanoparticles [3,4]. Turro et al. [3] have reportedonding of pyrenebutyric acid on �-Fe2O3 and indicated thatoth covalent bonding and a weak bonding of the carboxylicroup occur on the �-Fe2O3 surface. Kirwan et al. [4] havetudied bonding of poly(acrylic acid) on hematite and reportedhat the carboxylate group bonds by bidentate chelate com-lexation to the surface ferric ion. In our previous work [2],e used FTIR to study the bonding of a mixture of oleic

cid and oleylamine to the surfaces of FePt nanoparticles. Webserved that oleylamine bonds molecularly to the nanoparti-le surfaces with the NH2 group intact. Oleic acid bonds to theePt nanoparticles in both monodentate form (RC( O)–O–M)

nd bidentate form (RCO2–M). FTIR results also indicated thathere was some isomerization of the oleyl groups from the cisorm to the trans form (elaidyl groups) during synthesis of theePt nanoparticles. In that case the composition of the FePt

1 hysic

nr

[oIowna

cOmsrtFco

2

up1n((s

asstictvtpcawditsT

opFtci

TAttuaods

hhwi

3

iicptoan1baaaamdue to the ν( C H) mode of the C H bond adjacent to the C Cbond, and the small peak at 1648 cm−1 is due to the ν(C C)stretch mode (6,8). In addition, the spectrum of the oleic acidshows several modes characteristic of the carboxylic acid group:

14 N. Shukla et al. / Colloids and Surfaces A: P

anoparticles was held constant such that the particles were ironich.

In this paper, we have extended our previously reported work2] to understand the effect of the FePt nanoparticle compositionn the bonding of the surfactant to the surfaces of nanoparticles.n previous work, we reported that isomerization of surfactantccurred on the nanoparticle surface during synthesis. In thisork we report a comparison of the surfactant bonding to FePtanoparticles coated with iron oxides, FePt nanoparticles with50:50 Fe to Pt composition and pure Pt nanoparticles.

Our work indicates that the composition of the nanoparti-le surface is very important to the bonding of the surfactant.ur results also indicate that Fe oxides act as catalysts for iso-erization of the surfactant. We observed that isomerization of

urfactant occurred only on FePt nanoparticles which have Feich surfaces. No isomerization was evident on FePt nanopar-icles with 50:50 composition and pure Pt nanoparticles. TheTIR of the surfactants can be used to distinguish the surfaceomposition of the FePt nanoparticles and to probe the bondingf surfactants to the nanoparticle surfaces.

. Experimental

FePt nanoparticles and Pt nanoparticles were synthesizedsing a method described by Sun et al. [1] and by theolyol process which involves reduction of Pt(acac)2 using,2-hexadecanediol. The FePt (50:50, Fe/Pt composition)anoparticles, Pt nanoparticles and iron oxide coated FePt50:50, Fe/Pt) nanoparticles were washed using a polar solventethanol, CH3CH2OH) followed by washing with a non-polarolvent (hexane, C6H14).

Following synthesis, the nanoparticles were subjected todditional washing cycles to remove excess surfactant from theynthesis solution. The ‘excess surfactant’ is the fraction ofurfactant in solution that is not bonded to the FePt nanopar-icles. The original mixture of surfactant coated nanoparticlesn dioctylether solvent was first mixed with 10 parts of ethanolausing the particles to precipitate. The mixture was centrifugedo separate the particles from the solution. The supernatant sol-ent consisting of dioctylether and smaller nanoparticles washen discarded and the nanoparticles were redispersed in non-olar solvent, hexane. This sequence of precipitation in ethanol,entrifuging to separate the solvent, and redispersion in hex-ne was then repeated three times. Finally, the nanoparticlesere dispersed in hexane. This procedure was sufficient to pro-uce a dispersion of 3–4 nm diameter nanoparticles suspendedn hexane with no free surfactant in the solution. After washing,he nanoparticles solution consists of nanoparticles with onlytrongly bonded surfactants and there is no excess surfactant.he nanoparticle concentration in the solution is 2 mg/ml.

The nanoparticles were characterized using FTIR. In order tobtain the FTIR spectra a drop of the nanoparticle solution waslaced on the surface of an attenuated total reflectance (ATR)

TIR cell having a diamond tip and the solvent was allowed

o dry. The concentration of nanoparticles in solution was keptonstant in all experiments. The amount of nanoparticle solutionn each drop was controlled by using a precision micropipette.

Faor

ochem. Eng. Aspects 301 (2007) 113–116

he nanoparticles solution was slowly dried by covering theTR surface for a good film formation. All FTIR spectra were

aken using a Nicolet Nexus 670 spectrometer with a deuteratedriglyceride sulphate (DTGS) detector. Spectra were obtainedsing 4 cm−1 resolution and 32 scans were taken for statisticalveraging. FTIR spectra of the pure samples of oleic acid, andleyl amine were obtained by putting a drop of the surfactantissolved in hexane onto the diamond ATR cell and letting theolvent evaporate before taking the spectrum.

The oleic acid, oleylamine, ethanol, Pt(acac)2, Fe(CO)5, 1,2exadecanediol, cis-linoleic acid and cis-arachidonic acid andexane (anhydrous) were bought from Aldrich Chemical Co. andere used without further purification. The ethanol was obtained

n sure seal bottles with very low water content (<0.002%).

. Results and discussion

In this work we have used the standard process for prepar-ng a monodispersed sample of FePt nanoparticles by washingn ethanol and separating the large particles from solution byentrifugation as described in our previous work [2]. The largearticles were then redispersed in a hexane and surfactant mix-ure. Fig. 1 shows a FTIR spectrum of pure oleic acid, a mixturef oleic acid and oleylamine adsorbed on the Pt nanoparticlesnd mixture of oleic acid and oleylamine adsorbed on FePtanoparticles with 50:50 Fe to Pt composition, in the region of000–3100 cm−1. The spectrum of the oleic acid was obtainedy making a dilute solution of oleic acid in hexane and thenpplying the solution to the diamond window of the ATR cellnd allowing the solvent to evaporate. The spectrum of oleiccid reveals modes characteristic of the oleyl group: the peakst 2854 and 2951 cm−1 are due to the symmetric and antisym-etric CH2 stretching modes (5,6,7), the peak at 3006 cm−1 is

ig. 1. FTIR spectrum of pure oleic acid, surfactant (1:1 mixture of oleic acidnd oleylamine) adsorbed on Pt nanoparticles and surfactant (1:1 mixture ofleic acid and oleylamine) adsorbed on FePt nanoparticles in 1000–3100 cm−1

egion.

A: Ph

tmacitsia

tFcIotootimaei3atmsp1chcdcTit1

TI

V

ν

tttcν

ν

ν

ν

ν

ν

ν

t

ctmgrsmFtorCavao(s

ao2(etfaviTtAfPstates the intensity of olefinic C H modes which are presentdue to the isomerization of the cis C C bonds in the oleyl

N. Shukla et al. / Colloids and Surfaces

he ν(C O) mode is observed at 1709 cm−1 (5,6,8) and the weakode at 2670 cm−1 is due to the ν(O H) stretch of the dimerized

cid. This spectrum is of the solid form of oleic acid in which thearboxylic acid groups are dimerized through hydrogen bond-ng. Although, both oleic acid and oleylamine are adsorbed onhe nanoparticles we have reported the spectrum of only unad-orbed oleic acid because we are interested in the FTIR spectrumn the region of 2800–3200 cm−1 which is similar for both oleiccid and oleylamine.

In order to understand the origin of the differences betweenhe spectrum of the surfactant films on FePt nanoparticles (50:50,e/Pt composition), pure Pt nanoparticles and FePt nanoparticlesoated with iron oxide, a systematic study has been conducted.n the FTIR spectrum of pure oleic acid and oleylamine adsorbedn the Pt nanoparticles, (Fig. 1) the peak at 1709 cm−1 is due tohe ν(C O) stretch mode and indicates that some fraction of theleic acid is bonded to nanoparticles either in monodentate formr as an acid. The absence of a peak at 1648 cm−1 associated withhe ν(C C) stretching mode indicates that the C C double bonds not present in the alkyl groups of the surfactant. The assign-

ents of the vibrational modes in the region 2850–3000 cm−1

re well established in the literature. Table 1 shows the refer-nce frequencies and vibrational modes in this region. However,t is important to note that we do not observe any feature at006 cm−1 which would be due to the C H stretching modedjacent to the double bond C C. This is also consistent withhe absence of a peak at 1648 cm−1. The absence of a vibrational

ode at 1648 cm−1 suggests that the C C bond is absent; thepectrum is similar to that of stearic acid which does not have aeak at 3006 cm−1. Stearic acid is similar to oleic acid having an8 carbon atom chain but does not have C C bonds in the alkylhain. This suggests that hydrogenation of the oleyl group mayave occurred in the presence of Pt. Platinum is a well knownatalyst for hydrogenation of fatty acids. The FTIR spectrumue to oleic acid and oleylamine adsorbed on FePt nanoparti-les with a 50:50 Fe to Pt composition is also shown in Fig. 1.he weak peak at 1709 cm−1 is attributed to the ν(C O) stretch-

ng mode and indicates that some fraction of oleic acid is bondedo nanoparticles either in monodentate form or as an acid. The648 cm−1 peak is due to the ν(C C) stretching mode and indi-

able 1nfrared vibrational assignments

ibrational modes Frequency (cm−1) References

(NH) 3300 [12]rans ν( CH ) 3066 [5]rans ν( CH ) 3035 [5]rans ν( CH ) 3020 [5]is ν( CH ) 3006 [5–7]

a( CH2) 2922 [5–7]

s( CH2) 2854 [5–7](OH) 2670 [6,8]( C O) 1709 [5,6,8]( C C) 1647 [6,8](NH2) 1593 [12]( COO) 1512 [6,8–10]rans ν( CH ) 970 [5,7]

gs

Faa

ysicochem. Eng. Aspects 301 (2007) 113–116 115

ates that the C C double bond is intact in the oleyl groups onhe surfactant. The 1547 cm−1 peak is due to the NH2 scissoring

ode, which suggests that oleylamine is adsorbed with the NH2roup intact [2]. The intense absorption in the 2800–3000 cm−1

egion (Fig. 1) is due to the symmetric and antisymmetric CH2tretching modes of the oleyl groups but gives very little infor-ation about the bonding of either oleic acid or oleylamine to theePt nanoparticle surface. The most interesting part of the spec-

rum is in the region of 3000–3200 cm−1. The spectra of bothleic acid and oleylamine show a single peak at 3006 cm−1in thisegion. The presence of several peaks in this region indicates that

H bonds adjacent to C C bond exist in several environmentsnd are different from those in oleic acid and oleylamine. Aibrational peak at 3006 cm−1 is observed in Fig. 1 for oleiccid and oleylamine adsorbed on FePt nanoparticles and in pureleic acid but no peaks are observed at higher wave numbers3020–3100 cm−1). This mode at 3006 cm−1 is due to the CHtretch adjacent to the C C double bond.

Fig. 2 shows the FTIR spectrum of oleic acid and oleylaminedsorbed on FePt nanoparticles coated with Fe2O3/Fe3O4 andf pure linoleic acid and pure arachidonic acid in the region800–3200 cm−1. Unlike the spectrum of FePt nanoparticlesFig. 1) or the spectrum of pure Pt nanoparticles (Fig. 1), the pres-nce of several peaks in the region of 3006–3100 cm−1 indicateshe presence of C H bonds adjacent to C C bonds isomerizedrom the cis form to the trans form in the oleyl group. Thessignment of these peaks has already been discussed in our pre-ious work (2,6). The important point to note in this spectrums the intensity of the peaks in the region of 3000–3100 cm−1.he intensity of these peaks is very high and it is comparable

o the intensity of the peaks in the region of 2800–3000 cm−1.s observed in Fig. 1, the intensity of 3006–3100 cm−1 peaks

or surfactants adsorbed on FePt (50:50 composition) and puret nanoparticles is much lower than that shown in Fig. 2. Thisuggests that the amount of Fe in the FePt nanoparticles dic-

roups and perhaps due to the dehydrogenation of some C Cingle bonds.

ig. 2. FTIR spectrum of surfactant (1:1 mixture, oleic acid and oleylamine)dsorbed on FePt coated with Fe2O3/Fe3O4 during synthesis, pure linoleic acidnd pure arachidonic acid in 2800–3200 cm−1 region.

1 hysic

oaCiifioobcas−fb

dwmstt3oif[dtttit

4

i

sdt

A

n

R

[

[11] M.M. Mossoba, R.E. McDonald, J.A.G. Roach, D.D. Fingerhut, M.P.

16 N. Shukla et al. / Colloids and Surfaces A: P

In order to understand the origin of the high intensity of thelefinic C H modes observed in Fig. 2, FTIR spectra of linoleiccid and arachidonic acid were taken. Linoleic acid has 2 cis

C modes and arachidonic acid has four cis C C modes. Thedea was to understand whether the presence of more C C modesncreases the intensity of the cis C H modes which would there-ore indicate that there is dehydrogenation of the oleyl groupn oleic acid and oleylamine. An intense peak at 3012 cm−1 isbserved in the spectrum of linoleic acid (Fig. 2). The intensityf this peak is much higher than observed for oleic acid (Fig. 1)y itself or when adsorbed on FePt nanoparticles with lower Feoncentration (Fig. 1). No peaks were observed in the regionbove 3010 cm−1 as no trans linoleic acid is present. The FTIRpectrum of arachidonic acid shows an intense peak at 3012 cm1. The intensity of this peak is even higher than observed

or linoleic acid due to the presence of additional cis C Conds.

By comparison of the FTIR spectra of linoleic and arachi-onic acid with the FTIR spectrum of FePt nanoparticles coatedith Fe2O3/Fe3O4 (Fig. 2) we observed that the intensity of theode at 3012 cm−1 is comparable to that of the linoleic acid

ince in arachidonic acid the same mode has higher intensityhan the 2800–2950 cm−1 peaks. This suggests dehydrogena-ion of the surfactant. The other modes present in Fig. 2 at 3025,041 and 3068 cm−1 are due to the trans form and therefore notbserved in the spectra of linoleic and arachidonic acid. Thisndicates that on the iron oxide coated nanoparticles the sur-actant is both dehydrogenated and isomerized. Mossoba et al.11] have also observed that the intensity of peak at 3012 cm−1

ecreases as the contribution due to the peaks associated withhe trans form increases in intensity. They have also observedhe trans form only at 3035 cm−1. This may be due the fact thathey are looking at the pure trans forms of the fatty acid however;n our case we have the trans form of the fatty acid adsorbed onhe surface of the nanoparticles.

. Conclusion

The presence of excess Fe in FePt nanoparticles plays anmportant role in influencing the surfactant bonding to the

[

ochem. Eng. Aspects 301 (2007) 113–116

urface of the FePt nanoparticles. Our work indicates that dehy-rogenation and isomerization of surfactant from the cis formo the trans form takes place on Fe rich nanoparticles.

cknowledgement

Authors would like to thank Chao Liu for providing FePtanoparticles.

eferences

[1] S. Sun, C.B. Murray, D. Weller, L. Folks, A Moser, Monodisperse FePtnanoparticles and ferromagnetic FePt nanocrystal superlattices, Science287 (2000) 1989.

[2] N. Shukla, C. Liu, P.M. Jones, D. Weller, FTIR study of surfactant bondingto FePt nanoparticles, JMMM 266 (2003) 178.

[3] N.J. Turro, P.H. Lakshminarasimhan, S. Jockusch, S.P. O’Brien, S.G.Grancharov, F.X. Redl, Spectroscopic probe of the surface of iron oxidenanocrystals, Nano Lett. 2 (4) (2002) 325.

[4] L.J. Kirwan, P.D. Fawell, W. Van Bronswijk, In situ FTIR-ATR examinationof poly(acrylic acid) adsorbed onto hematite at low pH, Langmuir 19 (2003)5802.

[5] R.G. Sinclair, A.F. Mckay, G.S. Myers, R.N. Jones, The infrared absorptionspectrs of unsaturated fatty acids and esters, J. Am. Chem. Soc. 74 (1952)2578.

[6] D.H. Lee, R.A. Condrate, W.C. Lacourse, FTIR spectral characterizationof thin film coatings of oleic acid on glasses. Part II. Coatings on glass fromdifferent media such as water, alcohol, benzene and air, J. Mater. Sci. 35(19) (2000) 4961.

[7] C. Vogel-Weill, A. Gruger, Etude de la conformation des acides n-nonaoique, Z et E-9 octadecenoiques a 90 K par spectrometries infraougeet Raman II, Spectrochim. Acta Part A 52 (1996) 1737.

[8] D.H. Lee, R.A. Condrate, FTIR spectral characterization of thin film coat-ings of oleic acid on glasses. Part I. Coatings on glass from ethyl alcohol,J. Mater. Sci. 34 (1) (1999) 139.

[9] Q. Liu, Z. Xu, Self-assembled monolayer coatings on nanosized magneticparticles using mercaptohexadecanoic acid, Langmuir 11 (12) (1995) 4617.

10] P.J. Thistlethwaite, M.S. Hook, Diffuse reflectance Fourier transforminfrared study of the adsorption of oleate/oleic acid onto titania, Langmuir16 (2000) 4993.

Yurawecz, N. Sehat, Spectral confirmation of trans monounsaturated C-18fatty acid positional isomers, J. Am. Oil Chem. Soc. 74 (2) (1997) 125.

12] W. Erley, J.C. Hemminger, Spectroscopic identification of an HCNHspecies on Pt(1 1 1), Surf. Sci. 316 (1994) L1025–L1030.