Materials Chemistry and Physics 113 (2009) 412–416 Contents lists available at ScienceDirect Materials Chemistry and Physics journa l homepage: www.e lsev ier .com/ lo Synthe na Yuanzhi ong Department of Design a r t i c l Article history: Received 15 M Received in re Accepted 27 Ju Keywords: Iron–nickel Nanoparticle Chemical synt Magnetic prop hesize tonat der X partic aract surfa exte ment on m 1. Introdu Fe–Ni alloys are of great interest due to a variety of usefully magnetic and mechanical properties. For example, permalloys, which generically refers to Fe20Ni80, have a high magnetic perme- ability, low coercivity, near zero magnetostriction, and significant anisotropic magnetoresistance. Invar alloy which has a Ni concen- tration of around 35 atomic per cent exhibits an extremely low thermal exp ticulate for because of d netic mater Fe–Ni na ical and ch [2], electro Fe and Ni i inorganic s mal reduct regard to t of the ino reduction r media. If th reducing ag monly used in aqueous ∗ Correspon E-mail add 5, rat ition its the reaction temperature, and often leads to the formation of nanoparticles with an amorphous structure. Such cases are common in the reverse micelle synthesis. Nonaqueous synthetic routes which use high boiling-point organic solvents as reac- tion media provide an alternative approach to prepare metal and metallic alloy nanoparticles. The high reaction temperature pro- 0254-0584/$ – doi:10.1016/j.m ansionover awide temperature range [1]. Thenanopar- ms of Fe–Ni alloys have also received great attention iverse practical applications in the fields such as mag- ials, catalysis and medicine. noparticles have been prepared by a number of phys- emical methods, including hydrogen plasma reaction deposition in a flow cell [3], hydrogen reduction of norganic salts [4,5], hydrazine reduction of Fe and Ni alts [6,7], reverse micelle technique [8,9], hydrother- ion [10], and sonochemical decomposition [11]. In he chemical solution approaches, the co-reduction rganic salts of Fe and Ni is commonly used. The eaction can be conducted in aqueous or nonaqueous e reduction reaction is conducted in aqueous media, ents such as borohydrides and hydrazine are com- . However, the reduction of iron ions by borohydrides solution may produce various compounds, such as ding author. Tel.: +86 592 2180155; fax: +86 592 2180155. ress:
[email protected] (D.-L. Peng). vided by the solvent often leads to a highly crystalline structure, and therefore provides improved magnetic properties. One of the important issues in nonaqueous synthetic routes is the success- ful co-reduction of Fe and Ni salts in organic media. Accordingly, reducing agents which are compatible with organic media, such as hydrogen gas, superhydride (LiBEt3H), and 1,2-hexadecandiol, were usually employed in reactions. For example, Margeat et al. recently reported the synthesis of Fe–Ni nanoparticles via the decomposition of Ni[(COD)2] (COD=1,5-cyclooctadiene) and Fe[{N(SiMe3)2}2] in the presence of surfactants and hydrogen gas [13]. Tai et al. also obtained FeNi3 nanoparticles via a nonaque- ous synthetic route using 1,2-hexadecandiol as a reducing agent [14]. In this study, we report the synthesis of Fe–Ni nanoparticles via a nonaqueous organometallic route, in which nickel(II) acety- lacetonate (Ni(acac)2) and iron(III) acetylacetonate (Fe(acac)3) are decomposed inoleylamine,which servesbothas solvent and reduc- ing agent. No additional reducing agents (e.g. hydrogen gas and 1,2-hexadecandiol) are required for the reduction of the Fe and Ni precursors in the synthesis. The magnetic properties of the as- synthesized Fe–Ni nanoparticles with different Fe:Ni molar ratios are also compared. see front matter © 2008 Elsevier B.V. All rights reserved. atchemphys.2008.07.118 sis of iron–nickel nanoparticles via a no Chen, Xiaohua Luo, Guang-Hui Yue, Xuetao Luo, D Materials Science and Engineering, College of Materials, Research Center for Materials e i n f o ay 2008 vised form 30 June 2008 ly 2008 hesis erties a b s t r a c t Fe–Ni nanoparticles have been synt mal decomposition of Ni(II) acetylace reducing agents. The analyses of pow that the as-synthesized Fe–Ni nano ture and exhibit a polydispersed ch be further controlled by introducing position also can be tuned to some Room temperature magnetic measure nanoparticles. An increased saturati tents. ction Fe65B3 In add cate /matchemphys queous organometallic route -Liang Peng ∗ & Application, Xiamen University, Xiamen 361005, PR China d via a nonaqueous solution-phase approach using ther- e and Fe(III) acetylacetonate in oleylamine without further -ray diffraction and transmission electron microscopy show les possess a face-centered cubic (fcc) crystalline struc- eristic. The particle morphology and size distribution can ctants in the reaction system, and the final chemical com- nt by varying the initial molar ratios of metal precursors. s reveal a ferromagnetic characteristic for the as-synthesized agnetization has been observed with increasing Fe con- © 2008 Elsevier B.V. All rights reserved. her than pure metal or metal alloy nanoparticles [12]. , the aqueous reaction environment practically lim- Y. Chen et al. / Materials Chemistry and Physics 113 (2009) 412–416 413 2. Experimental 2.1. Synthesis In a typical synthesis, 0.75mmol of Ni(acac)2 (96%, Alfa Aesar), 0.25mmol Fe(acac)3 (99%, Alfa Aesar), and 7mL of oleylamine (70%, Fluka) were added into a reaction flask and the mixture was stirred magnetically under a flow of high- purity argon gas. The mixture was heated to 130 ◦C and kept at this temperature for 20min, and then fast heated to 300 ◦C and maintained for 0.5h before cooling down to room temperature. The reaction products were separated by adding 30mL of ethanol followed by centrifugation, and then further washed using the mixture of hexane and ethanol. The final products were vacuum-dried and obtained as a powder. All the after-synthesis steps including centrifugation, washing and prepar- ing samples for characterization were carried out under ambient conditions. The reaction temperature, time scale, and molar ratio of metal precursors were varied in the experiments to obtain nanoparticles with a desired size and composition. In another set of experiments, small amounts of surfactants, i.e. 1,2-dodecanediol (90%, Aldrich) and trioctylphosphine (TOP, 97%, Strem) were added into the reac- tion mixtures. The remaining processing steps are the same as those in the typical synthesis. 2.2. Characterization Powder X-ray diffraction (XRD) data were collected on a Panalytical X’pert PRO X-ray diffractometer using Cu K� radiation. Transmission electron microscopy (TEM) was performed on a TECNAI F-30 and a JEM-2100HC transmission electron microscope. Bright-field (BF) images were recorded for the characterization of par- ticle morphology and size. The mean particle size was determined from measuring ∼120 separate was used to id (EDS) analyses analyses were dropping a sm vent evaporati superconducti 3. Results Amine r metallic nan nanoparticl could act as ticles with synthetic p cursors in a we prepare using oleyla Fig. 1 sh sizedusing The XRD pa Fig. 1. XRD pa cursor ratios. 50:50 (c), resp ((FeNi)3O4). Fe:Nimolar ratio of 25:75 (Fig. 1a) exhibits three distinctive diffrac- tion peaks which match well with face-centered cubic (fcc) lattice of bulk Fe–Ni alloy. The lattice constant, a, calculated from the (111) peak (a=3.523Å the incorpo an enlarged nanoparticl also shows and no oth observed (F to 50:50, th peaks cann tion peaks ( those of a s Fe–Ni nano range of Fe c in the form Fig. 2 sh which were ratio of 25: shaped mor 50–100nm een ED p line s the 5nm It is is m from es th y cry ana ompo om i f 25:7 e me 6, ra sult curre les o Ni na ly ex s tho rticl e:Ni Ni ra an 10 typi aller re Fe This to th he at bccm ith a nabl ener xpos inves sis, w eren ratur rticl particles in BF TEM images. Selected area electron diffraction (SAED) entify the crystalline phases. Energy-dispersive X-ray spectroscopy were used to analyze the chemical compositions. Samples for TEM prepared by sonicating the as-synthesized powders in hexane and all volume onto a carbon-coated copper TEM grid followed by sol- on. Magnetic analyses were conducted on the dry powders using a ng quantum interference device (SQUID) magnetometer (MPMS-5). and discussion eduction is an important approach for the synthesis of oparticles, such as Cu [15], Ni [16], and Au and Ag [17] es. In our previously study, we found that alkylamines both solvent and reducing agent toproduceNinanopar- different crystalline phases [18]. To test whether the rotocol that involves the decomposition of metal pre- lkylamines works for the generation of magnetic alloys, d Fe–Ni nanoparticles with different Fe:Ni molar ratios mine as a solvent in this study. ows the XRD patterns of Fe–Ni nanoparticles synthe- differentmolar ratios ofmetal precursors in oleylamine. ttern of Fe–Ni nanoparticles obtained using an initial tterns of Fe–Ni nanoparticles synthesized using different metal pre- The molar ratios of Fe(acac)3:Ni(acac)2 are 25:75 (a), 35:65 (b), and ectively. The asterisks marked in (c) represent a spinel oxide phase have b ses. SA crystal one of of 0.20 lattice. image culated indicat primar EDS their c trum fr ratio o and th is 24:7 This re The oc molecu Fe– basical tions a nanopa initial F tial Fe: less th with a the sm cles we Fig. 4). accord when t fcc and ticles w is reaso were g upon e To synthe at diff tempe nanopa is 3.563Å, which is slightly larger than that of fcc Ni , JCPDF# 04-0850). Such a result gives an evidence of ration of Fe atoms into the fcc Ni lattice, which leads to lattice due to the larger radius of Fe atom. The Fe–Ni es obtained using an initial Fe:Ni molar ratio of 35:65 a diffraction pattern indexed to fcc lattice of Fe–Ni alloy, er diffraction peaks belonging to impurity phases are ig. 1b). However, when the initial Fe:Ni ratio increases e obtainednanoparticles exhibit several XRDdiffraction ot be indexed to fcc Fe–Ni alloy (Fig. 1c). These diffrac- marked by the asterisks in Fig. 1c) correspond well to pinel oxide ((FeNi)3O4). The above results indicate that particles with a fcc phase can be formed over a limited ontent. A high concentration of Fe precursor will result ation of oxide phases. ows the typical TEM images of Fe–Ni nanoparticles synthesized in oleylamine using an initial Fe:Ni molar 75. These nanoparticles have an angular or irregular- phology, and exhibit a wide size distribution (typically ). The crystalline characteristics of these nanoparticles confirmed by electron diffraction and HRTEM analy- attern recorded from these nanoparticles reveals a fcc tructure (Fig. 2a inset), and HRTEM image taken from nanoparticles (Fig. 2b) shows a lattice fringe spacing , corresponding well to the {111} planes of fcc Fe–Ni worthy to note that particles size observed from TEM uch larger than the mean crystallite size (12nm) cal- the XRD pattern using Debye-Scherrer equation. This at these nanoparticles are actually made up of smaller stals, and are nanocrystalline in nature. lyses were conducted on individual particles to obtain sitional information. Fig. 3 shows a typical EDS spec- ndividual nanoparticles obtained using an initial Fe:Ni 5. Both Fe and Ni peaks are observed in the spectrum, asured atomic ratio of Fe:Ni from the peak intensity ther close to the initial Fe:Ni precursor ratio (25:75). also confirms the formation of Fe–Ni nanoparticles. nce of C peak is mainly due to the absorbed organic n particle surfaces. noparticles obtainedusing an initial Fe:Ni ratio of 35:65 hibited similar particle morphologies and size distribu- se of Fe25Ni75 nanoparticles. EDS analyses on individual es showed that the atomic ratio of Fe:Niwas close to the ratio. However, for nanoparticles obtained using an ini- tio of 50:50, a portion of smaller particles with sizes nm were observed in addition to those large particles cal size larger than 50nm. EDS analyses revealed that particles were Fe-rich oxides, whereas the larger parti- –Ni nanoparticles with a Fe:Ni ratio close to 35:65 (see result along with the XRD analysis of this sample is in e equilibrium phase diagram of Fe1−xNix diagram [19], omic percent of Ni decreases below0.67 (about 300 ◦C), ixed phaseswill occur. Generally, the Fe-rich nanopar- bcc phase are more easily to be oxidized. Therefore, it e to assume that if any redundant Fe-rich nanoparticles ated during the reaction, they could be easily oxidized ure to air, forming oxides. tigate the influences of reaction temperature on the e prepared Fe–Ni nanoparticles (initial Fe:Ni =25:75) t reaction temperatures (240–300 ◦C). The reaction e does not change the crystalline phase (fcc) of the es, however, it influences their latticevolumesandcom- 414 Y. Chen et al. / Materials Chemistry and Physics 113 (2009) 412–416 Fig. 2. Low-m Fe–Ni nanopar lamine withou indexed to fcc positions. B decrease w the lattice is 3.531Å, w 3.563Å. The temperatur is closely re Since the ra substitution This effect i is favorable reaction tem on the othe agnification TEM image (a) and high-resolution TEM image (b) of ticles synthesized using an initial Fe:Ni molar ratio of 25:75 in oley- t additional surfactants. The SAED pattern shown in the inset of (a) is lattice of Fe–Ni alloy. asically, the lattice constant of fcc Fe–Ni nanoparticles ith decreasing the reaction temperature. For example, constant of Fe–Ni nanoparticles synthesized at 240 ◦C hereas that of the particles synthesized at 300 ◦C is lattice constant of nanoparticles synthesized at a lower e is closer to that of fcc Ni. The lattice volume change lated to the incorporation of Fe atoms into fcc lattice. dius of Fe atom is slightly larger than that of Ni, the of Fe for Ni atoms will increase the fcc lattice volume. s more distinct at a higher reaction temperature which for the diffusion of Fe atoms into the lattice. A lower perature will result in particles with a smaller size but r hand inhibit the formation of Fe–Ni nanoparticles. As Fig. 3. EDS spectrumof Fe–Ni nanoparticles synthesized using an initial Fe:Nimolar ratio of 25:75. Cu signals are from TEM grids. a result, the atomic percent of Fe in Fe–Ni nanoparticles becomes lower for the samples synthesized at a lower temperature, which was verified by EDS analyses. The redundant Fe-containing compo- nents will eventually lead to the formation of Fe-rich (may contain a small por exposing to In regar Fe–Ni nano involving a ously reduc of the possi at first, and ticles. The N aFe-rich sh mutual diff observed ex is that Ni na precursor in employed r whereas Fe This indicat cles are less is that Fe–N which have oleylamine anisms for Fig. 4. EDS sp ratio of 50:50. Cu signals are tion of Ni) oxides, either during the reaction or after air. d to the formation mechanism of the as-synthesized particles, we believe that it is different from those co-reduction process wherein Fe and Ni are simultane- ed by strong reducing agents to form alloy clusters. One ble formationmechanisms is thatNi clusters are formed then serve as seeds for further growthof Fe–Ninanopar- i seeds may promote the reduction of Fe(acac)3 to form ell,which canbe easily converted into auniformalloyby usion at an elevated temperature. There are at least two perimental facts to support such a presumption. One noparticles can be formed by the decomposition of Ni oleylamine without additional reducing agents at the eaction temperature range of 200–300 ◦C (see Ref. [18]), nanoparticles cannot be obtained in such conditions. es that the nuclei for the formation of Fe–Ni nanoparti- likely to be the Fe clusters/nanoparticles. The other fact i nanoparticles can also be obtained by using Ni seeds been prepared in advance to react with Fe(acac)3 in at an elevated temperature. Similar formation mech- the generation of alloy nanoparticles have also been ectra of Fe–Ni nanoparticles synthesized using an initial Fe:Ni molar (a) Spectrum from big particles. (b) Spectrum from small particles. from TEM grids. Y. Chen et al. / Materials Chemistry and Physics 113 (2009) 412–416 415 Fig. 5. TEM im 1,2-dodecaned discussed in and NiPt na Since ole the control the reaction ogy and siz image of Fe the presenc Thesepartic an angular much narro measured m ticles revea ratio. It shou ing effects on the formation of Fe–Ni nanoparticles,whichmayhelp to obtain nanoparticles with a relatively uniform size, although olved mechanism is not straightforward. Fig. 5b presents rphologyof Fe–Ni nanoparticles (initial Fe:Ni =25:75)which ynthesized in the presence of 0.2mmol of TOP at 280 ◦C for . It can be seen that these particles have a spherical mor- y with a narrow size distribution. The measured mean size 5nm. Since TOP has a structure that one end (P-containing can reversibly coordinatesNi surface sites, and theother end hain) extends to the solvent, it can inhibit particle growth otect nanoparticles from agglomerating. Therefore, the par- ze can be effectively controlled by employing TOP. However, alyses on individual particles reveal a Fe content much less e initial ratio, and hence make the simultaneous control of stribution and composition difficult. Further investigations influence of surfactants on the control of particle size and the inv themo were s 30min pholog is 43± group) (alkyl c and pr ticle si EDS an than th size di on the ages of Fe–Ni (initial Fe:Ni =25:75) nanoparticles synthesized using iol (a) and TOP (b) as surfactants. alloy nanoparticle systems such as CoPt [20], FePt [21] noparticles [22]. ylamine cannot provide enough stabilization effects on of particle growth, two surfactants were introduced in mixtures, and their influences on theparticlemorphol- e distribution were investigated. Fig. 5a shows the TEM –Ni nanoparticles (initial Fe:Ni =25:75) synthesized in e of 1mmol of 1,2-dodecanediol at 300 ◦C for 30min. les also exhibit a polycrystalline characteristic andhave morphology; nevertheless their size distributions are wer than those of synthesized without surfactants. The ean size is 62±7nm. EDS analyses on individual par- l a chemical composition close to the initial Fe:Ni molar ld be pointed out that 1,2-dodecanediol also has reduc- compositions are underway. The room temperature hysteresis loops of the as-synthesized Fe–Ninanoparticleswithdifferent compositionsare shown inFig. 6. For the purpose of comparison, the hysteresis loop of Ni nanoparti- cles (Fig. 6a) is also comparedwith thoseof the Fe–Ninanoparticles. The Ni nanoparticles were prepared using the similar synthetic protocol which involving the decomposition of Ni(acac)2 in oley- lamine at 215 ◦C. As shown in Fig. 6, all the magnetization curves saturate quickly under the external applied filed, and exhibit mag- netic hysteresis, which is very typical for ferromagnetic materials. The measured coercivity (Hc) values are 48, 191 and 103 Oe for Ni, Fe25Ni75 and Fe35Ni65 nanoparticles, respectively. An increased Fig. 6. Hystere The insets sho sis loops of Ni (a), Fe25Ni75 (b) and Fe35Ni65 (c) nanoparticles at 300K. w the low field detail of the cycles. 416 Y. Chen et al. / Materials Chemistry and Physics 113 (2009) 412–416 trend is observed on the saturation magnetization (Ms) of these samples with increasing Fe contents. The measured Ms values for Ni, Fe25Ni75 and Fe35Ni65 nanoparticles are 48, 69 and 89emug−1, respectively. The increased saturation magnetization is attributed to the incorporation of Fe atoms which have higher magnetic moments than Ni atoms. Such a compositional-dependent behav- ior provides an important approach to achieve tunable magnetic properties of Fe–Ni nanoparticles for their many practical applica- tions. 4. Conclusions Anonaqueous organometallic synthetic approach for the prepa- ration of Fe–Ni nanoparticles has been reported in this study. This simple synthetic method uses oleylamine both as solvent and reducing agent, and no additional reducing agents are required for the generation of Fe–Ni nanoparticles. The as-synthesized Fe–Ni nanoparticles typically have an irregular-shaped morphology with a wide size distribution. Their final chemical compositions can be tuned to some extent by varying the initial molar ratios of metal precursors. The formation mechanism of the as-synthesized Fe–Ni nanoparticles appears to be different from those involving a co- reductionprocesswherein Fe andNi are simultaneously reducedby strong reducing agents to formalloy clusters. The particlemorphol- ogy and size distribution can be further controlled by employing surfactants in the reaction mixtures. Room temperature magnetic measurements reveal that the as-synthesized Fe–Ni nanoparticles are ferromagnetic, and their saturation magnetization increases with increasing Fe contents. Acknowledgments This work was partially supported by the Natural Science Foun- dation of Fu National Natural Science Foundation of China (Grant No. 50701036 and 50671087). References [1] M. van Schilfgaarde, I.A. Abrikosov, B. Johansson, Nature 400 (1999) 46 (and references therein). [2] X.G. Li, A. Chiba, S. Takahashi, J. Magn. Magn. Mater. 170 (1997) 339. [3] S.H. Kim, H.J. Sohn, Y.C. Joo, Y.W. Kim, T.H. Yim, H.Y. Lee, T. Kang, Surf. Coat. Technol. 199 (2005) 43. [4] Y.J. Suh, H.D. Jang, H. Chang, W.B. Kim, H.C. Kim, Powder Technol. 161 (2006) 196. [5] H.Q. Wu, Y.J. Cao, P.S. Yuan, H.Y. Xu, X.W. Wei, Chem. Phys. Lett. 406 (2005) 148. [6] X. Su, H. Zheng, Z. Yang, Y. Zhu, A. Pan, J. Mater. Sci. 38 (2003) 4581. [7] X.W. Wei, G.X. Zhu, J.H. Zhou, H.Q. Sun, Mater. Chem. Phys. 100 (2006) 481. [8] I. Ban, M. Drofenik, D. Makovec, J. Magn. Magn. Mater. 307 (2006) 250. [9] B.L. Cushing, V. Golub, C.J. O’Connor, J. Phys. Chem. Solids 65 (2004) 825. [10] Q. Liao, R. Tannenbaum, Z.L. Wang, J. Phys. Chem. B 110 (2006) 14262. [11] K.V.P.M. Shafi, A. Gedanken, R.B. Goldfarb, I. Felner, J. Appl. Phys. 81 (1997) 6901. [12] A. Corrias, G. Ennas, Chem. Mater. 5 (1993) 1722. [13] O. Margeat, D. Ciuculescu, P. Lecante, M. Respaud, C. Amiens, B. Chaudret, Small 3 (2007) 451. [14] M.-F. Tai, J.-K. Hsiao, H.-M. Liu, S.-C. Lee, S.-T. Chen, Multifunctional Nanocom- posites International Conference Proceedings, Honolulu, HI, United States, September 20–22, American Society of Mechanical Engineers, New York, N.Y., 2006 (mn2006.17041/1). [15] S. UK. Son, I.K. Park, J. Park, T. Hyeon, Chem. Commun. (2004) 778. [16] J. Park, E. Kang, S.U. Son, H.M. Park, M.K. Lee, J. Kim, K.W. Kim, H.-J. Noh, J.-H. Park, C.J. Bae, J.-G. Park, T. Hyeon, Adv. Mater. 17 (2005) 429. [17] H. Hiramatsu, F.E. Osterloh, Mater. Chem. 16 (2004) 2511. [18] Y. Chen, D.-L. Peng, D. Lin, X. Luo, Nanotechnology 18 (2007) 505703. [19] T.B. Massalski (Ed.), Binary Alloy Phase Diagrams, 2nd ed., ASM International, Materials Park, Ohio, 1990. [20] E.V. Shevchenko, D.V. Talapin, H. Schnablegger, A. Kornowski, Ö. Festin, P. Svedlindh, M. Haase, H. Weller, J. Am. Chem. Soc. 125 (2003) 9090. [21] M. Chen, J.P. Liu, S. Sun, J. Am. Chem. Soc. 126 (2004) 8394. [22] K. Ahrenstorf, O. Albrecht, H. Heller, A. Kornowski, D. Görlitz, H. Weller, Small 3 (2007) 271. jian Province of China (Grant No. E0610027), and the Synthesis of iron-nickel nanoparticles via a nonaqueous organometallic route Introduction Experimental Synthesis Characterization Results and discussion Conclusions Acknowledgments References