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Colloids and Surfaces A: Physicochem. Eng. Aspects 320 (2008) 115–122

Interfacial coprecipitation to prepare magnetite nanoparticles:Concentration and temperature dependence

Ke Tao, Hongjing Dou, Kang Sun ∗The State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200030, China

Received 3 September 2007; received in revised form 7 January 2008; accepted 29 January 2008Available online 13 February 2008

bstract

A novel interfacial coprecipitation method to prepare magnetite nanoparticles was proposed by us recently, which possesses advantages such asacility, Gram-scale synthesis and surface functionality. In this paper, the effects of precursors concentration and reaction temperature in interfacialoprecipitation method were systematically studied to evaluate the differences between classical and interfacial coprecipitation. It was found for firstime that in both interfacial and classical coprecipitation there is a critical region of ferrous ion concentration to obtain magnetite at different amine

oncentrations, in which a phase of two-line ferrihydrite was considered to be an intermediate. Meanwhile, although the size of nanoparticles wasot obviously varied by adjusting the reaction temperature, their dispersibility and saturation magnetization can be tailored. Besides, by comparingith classical coprecipitation, the mechanism in the interfacial coprecipitation was elucidated.2008 Elsevier B.V. All rights reserved.

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eywords: Interfacial coprecipitation; Magnetite nanoparticles; Concentration;

. Introduction

Magnetic nanoparticles, especially magnetite (Fe3O4), haveeen of both technological and scientific importance nowadays1]. The potential applications in magnetic recording media [2],ensors [3], catalysts [4], and especially in biomedical fields5–8] have strongly motivated the research in the synthesis ofagnetite nanoparticles so as to match the variant requirements.Organometallic synthetic methods at elevated temperatures

n non-aqueous solvents were intensively studied in recent years9]. The typical organometallic methods in preparing iron oxideanoparticles were reported, in which Fe(C6H5N(NO)O−)310], Fe(CO)5 [11], Fe(acac)3 (acac: acetylacetonate) [12,13]ere adopted as organometallic precursors and decomposed

n hot non-aqueous/surfactant solutions. By this approach, thepeed of nucleation and growth of nanocrystals are controlledy accurately adjusting temperature and using abundant sur-

actants to produce magnetic nanoparticles with controllableize, narrow size distribution, and generally a hydrophobic sur-ace. However, this method is a nearly perfect way to prepare

∗ Corresponding author. Tel.: +86 21 34202743; fax: +86 21 34202743.E-mail address: ksun@sjtu.edu.cn (K. Sun).

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927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2008.01.051

erature; Reaction mechanism

onodispersed nanoparticles only in laboratory. The expensiverecursors and hydrophobic surface of resultant nanoparticlesimit their applications in biomedical fields.

Another important strategy to fabricate magnetite nanopar-icles is based on the coprecipitation of Fe2+ and Fe3+ ions inlkaline medium, such as ammonium or NaOH aqueous solu-ion. This coprecipitation reaction can be performed either inn aqueous system [14,15], or in an inverse microemulsionystem [16,17]. Coprecipitation in homogeneous aqueous solu-ion (denoted as classical coprecipitation in this paper) is onef the classic techniques for synthesizing magnetic nanoparti-les. After pioneer work being reported by Massart [18] andhalafalla [19], coprecipitation was widely studied in prepar-

ng magnetite nanoparticles for its extraordinary advantages,uch as gram-scale production and facility. In fact, almost allhe magnetite nanoparticles used in clinical research nowadaysre prepared by classical coprecipitation, because their surfacean be easily modified to be hydrophilic, which is suitableor biomedical applications [20,21]. However, there are stilleveral drawbacks in classical coprecipitation, such as uncon-

rollability of the size, size distribution, and the phase controlf resultant nanoparticles as well. To address these issues, sev-ral approaches were developed based on the coprecipitationeaction. For example, a great number of researches utilized

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urfactants [22], dilute acids [14] or macromolecules [23] tomprove the dispersibility of magnetite nanoparticles in water.

In the research of the synthesis of magnetite nanopar-icles, it is definitely important to evaluate the influencesf reaction conditions, whatever the method was followed.specially, for coprecipitation reaction, the concentration ofrecursors and reaction temperature significantly affect theize, size distribution, phase and surface chemistry of resul-ant nanoparticles. By investigating the reaction under differentoncentrations and temperatures, it is helpful not only for obtain-ng nanoparticles with different size or properties, but alsoor elucidating the reaction mechanism. Recently we reportedfacile interfacial coprecipitation approach to fabricate mag-

etite nanoparticles [24]. In this approach, di-n-propylamine andwater/cyclohexane system acted as the alkali and medium,

espectively, so that the coprecipitation reaction was confinedo the interface between water and cyclohexane. However,lthough nanoparticles with good dispersibility in water wereroduced, the mechanism in this approach was not explicitlydentified. Consequently, influences of precursors concentra-ion and reaction temperature were studied in this paper. Theesults may be helpful not only to the phase control but also tonderstanding the mechanism in the coprecipitation reaction foragnetite nanoparticles.

. Experimental section

.1. Chemicals

FeCl2•4H2O(>98%), FeCl3•6H2O(>99%), cyclohexane>99%), di-n-propylamine (>99%), ammonia (25 wt%), andthanol (>99.5%) were purchased from Sinopharm Chemeagent Co. All these reagents were used without furtherurification. Milli-Q water was used in the preparation.

.2. Synthesis of Fe3O4 nanoparticles

Interfacial coprecipitation approach to fabricate magnetiteanoparticles reacted as the equation below, which is the sames the classical coprecipitation reaction:

e2+ + 2Fe3+ + 8OH− → Fe3O4 + 4H2O (1)

In a typical synthetic procedure, 5 mmol FeCl3•6H2O and.5 mmol FeCl2•4H2O were dissolved in 100 ml water (theolar ratio of FeCl3•6H2O to FeCl2•4H2O was fixed as 2:1 in

xperiments), the resultant solution was diluted further to vari-us concentrations. Meanwhile, 5 ml di-n-propylamine was alsoiluted to various concentrations by cyclohexane (For evaluatingemperature dependence, the concentration of aqueous solutionf ferric and ferrous ions was fixed at 50 and 25 mmol/l respec-ively, and the concentration of di-n-propylamine in cyclohexaneas fixed at 25 vol%). Under nitrogen atmosphere and vigorous

tirring (∼1200 rpm) at 25 ◦C (in evaluating the influence ofemperature, other temperatures were used, as specified in theext), 100 ml aqueous solution of iron salt was added dropwisento various volume of dipropylamine/cyclohexane solution.

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chem. Eng. Aspects 320 (2008) 115–122

he process of dropping was completed in about 15 min, andnother 15 min was kept with stirring to complete the reaction.lassical coprecipitation was also carried out as a comparison,

n which aqueous ammonia solutions at various concentrationsere used to replace dipropylamine/cyclohexane solution with

ll other factors being kept constant. To purify the product, theesultant Fe3O4 suspension was rinsed with ethanol and cen-rifuged at 11,000 rpm for 10 min. This process was repeated

times, and then the Fe3O4 nanoparticles were collected byentrifugation and freezing dried for about 24 h.

.3. Characterization

For transmission electron microscopy (TEM) studies, theispersions of nanoparticles in water were drop-cast onto aarbon coated copper grid, and TEM images were taken onJEOL JEM-2010 microscopy at an accelerating voltage of

00 kV. To determine the size distribution and the mean aver-ge size, around 500 nanoparticles were measured for eachample. X-ray powder diffraction (XRD) patterns were mea-ured in reflection mode (Cu K� radiation) on a Bruker D8iffractometer. pH values of the result water layer were mea-ure by a pH meter (PHS-3TC, Tianda Instruments, Shanghai).or measuring the content of nitrogen on the surface, 580 mg2.5 mmol) nanoparticles were dispersed in 1 l water and theass of nitrogen was taken on a TOC/TN multi 3000(ChD)nalyzer (Analytik Jena, Germany). The number of ammo-ium molecules per nanoparticle (n) can be calculated based on= AMMagnetite/((2.5 × 10−3)/MNitrogenVρNA), where A (ppm)

s the value we got from TOC/TN analyzer, M is the molecu-ar weight, V (cm3) is the volume of an individual nanoparticle,(g/cm3) is the density of bulk Fe3O4, and NA is Avogadro’sumber. Zeta potentials of the samples prepared at different tem-eratures were measured on a Malvern zeta-sizer 2000 aftereing dispersed in pure water. Magnetization measurementsere performed on a vibrating sample magnetometer (VSM)

t 25 ◦C.

. Results and discussion

Although coprecipitation reaction in aqueous medium to pre-are magnetite nanoparticles has been researched for more than0 years, the difficulties in synthesizing iron oxide nanoparti-les by coprecipitation are still there, including the control ofhe particles size, size distribution, and especially the resultanthase. Many possible phases, such as ferrihydrite, akagenite�-FeOOH), Fe(OH)3, etc, may be produced at different reac-ion conditions [25]. To get pure magnetite nanoparticles, sometudies were focused on the influence of the reaction conditionsn the phase control. According to those studies, the concen-ration of reactants is one of the dominant factors in the phaseontrol, and some requirements, such as appropriate mole ratiof Fe2+ to Fe3+ and pH range, must be fulfilled to get antici-

ant phase in the coprecipitation reaction. Jolivet [26] proposedhat electron transfer between Fe2+ and Fe3+ plays a pivotalole in the formation of iron oxides, and it is the absorption ofe2+ that makes quasi amorphous ferric hydroxide transform

hysicochem. Eng. Aspects 320 (2008) 115–122 117

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K. Tao et al. / Colloids and Surfaces A: P

nto spinel magnetite. Based on theoretical analysis, Kim et.l. [27] suggested that in the pH range of 1–4, Fe3+ would beondensed as FeOOH, and Fe3O4 would be obtained in the pHange of 9–14, so it is preferred to drop or syringe iron salt solu-ion into alkali to prevent the formation of the medium FeOOHhase. Undoubtedly, the concentration of precursors is one ofhe most important factors that can influence phase controllingf products in coprecipitation reaction.

In interfacial coprecipitation, di-n-propylamine and aater/cyclohexane system acted as the alkali and medium,

espectively, so that the coprecipitation reaction was confinedo the interface between water and cyclohexane. However, it isssentially still a coprecipitation reaction. In order to explorehe different effects of concentration on the phase of productn interfacial coprecipitation and classical coprecipitation, weynthesized a series of iron oxide by these two approaches atarious concentrations of ferrous ion as well as amine. In thenterfacial coprecipitation, three kinds of typical products a, b,

respectively marked in corresponding areas A, B, C coulde obtained under different conditions, which were schemat-cally divided by two dashed lines as shown in Fig. 1. Dark,ark-red and orange solutions were produced when the concen-rations of ferrous ion and amine are located in the range A, Bnd C, respectively (as shown in the insets of Fig. 1). It wasbserved that the change of products color is a gradual process,herefore the range B between two lines should be considereds the critical concentration area between black solution andellow-orange colloidal solution. The concentration of ferrousons (CFe2+ ) and amine (Camine) have a conjunct influence onhe phase of resultant products, for example, when Camine isower than 1.4 mol/l, the critical CFe2+ decrease sharply with the

ncrease of Camine, whereas as Camine is higher than 1.4 mol/l,urther increase of Camine almost have no influence on the criti-al CFe2+ . For comparison, we carried out the similar studies inlassical coprecipitation. As shown in Fig. 2, three areas were

ig. 1. Different products prepared at variant concentration of Fe2+ and di-n-roplylamine in interfacial coprecipitation. The plot was divided into three areasy dashed lines according to their phenomena. Points like a were marked as (�)nd were classified as area A, points similar to b in area B were marked as (�),nd like c were labeled with (�) and sorted to area C. Insets shows the typicalroducts prepared at the concentrations located on point a, b and c respectively.

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ig. 2. Different products prepared at variant concentrations of Fe2+ and ammo-ia in classical coprecipitation, which was divided into three areas as that haveeen done in Fig. 1.

lso distinguished according to different appearances of resul-ant products like that in interfacial coprecipitation, except thathe CFe2+ of critical area (range B) is slightly lower than that ofnterfacial coprecipitation.

The phases of the products in three areas were characterizedy X-ray diffraction (XRD). The patterns of samples corre-ponding to the point a, b and c in Fig. 1 are shown in Fig. 3.he pattern of the sample a confirms its major composition ofe3O4 nanocrystal because the position and relative intensity ofain peaks match well to those from the JCPDS card (19–0629)

or Fe3O4. However, the interfacial coprecipitation as the con-entration of precursors located in area C (points like c) wouldeads to a quasi-amorphous product, of which two broad peaksan be found on XRD pattern at about 31◦ and 62◦, respectively.ased on the similar XRD pattern reported in literatures [28,29],

t should be concluded as the so-called “2-line ferrihydrite”FepOr(OH)s), which is an intermediate in the transformationrom ferric ions to hematite or goethite. Meanwhile, the samples

ig. 3. XRD patterns of products (a, b, c) prepared at the concentrations locatedn point a, b and c in Fig. 1, respectively.

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repared at the interim concentrations of precursors located inrea B (points like b) show quasi-amorphous background withnconspicuous peaks, indicating the mixture of the Fe3O4 and-line ferrihydrite. The samples prepared by classical copre-ipitation method exhibit similar colour and XRD patterns ashe samples prepared by interfacial coprecipitation. Obviously,ither in classical coprecipitation or interfacial coprecipitation,hen the concentration of iron salt is lower than a critical value

nd no matter what concentration and mass of amine is used,e3O4 nanocrystal would not be produced. Hence, to produceagnetite nanoparticles, there is a minimum limit of Fe2+ con-

entration which is irrelative to the concentration of amine, tour knowledge it has not been mentioned in previous references.

Based on the mechanism of classical coprecipitation ofagnetite nanoparticles proposed by some researchers [27,30],eOOH should be precipitated at first, and then would be trans-ormed to Fe3O4, that is, the reaction is proceed in two steps asollow:

e3+ + 3OH− → FeOOH + H2O (2)

FeOOH + Fe2+ + 2OH− → Fe3O4 + 2H2O (3)

This two-step mechanism can also be adapted to the interfa-ial coprecipitation because of the intrinsic nature of the reactionnd the similar phenomena. However, according to our result,he intermediated FeOOH in this two-step reaction may beonsidered as 2-line ferrihydrite (FepOr(OH)s). Obviously, thequation (2) is a reaction that prefers to proceed forward becausef the low solubility of 2-line ferrihydrite, thus the Equation (3),s a kinetically slow one, should be the critical step in coprecip-tation. Hence, it is clear that CFe2+ · (COH− )2 must be higherhan a certain value, so that the reaction would proceed spon-aneously in the forward direction of Eq. (3) to get magnetite.

ith the theory of chemical equilibrium, the value should beKsp(Fe3O4)

K⊕a ·Ksp(FepOr(OH)s)

, where Ksp is the solubility product constant,⊕a is the equilibrium constant of Eq. (3). The importance ofOH

− (pH value) in this reaction has been demonstrated byany groups. The most direct example is to add the precursor

n a reverse order, i.e., dropwise adding amine into the solu-ion of iron salt in both classical coprecipitaion and interfacialoprecipitation. By reverse addition, the orange, red, or darked mediate can be observed gradually with the addition ofmine, as mentioned by Kim [27]. It can be considered as that-line ferrihydrite is precipitated when the amine is insufficient,nd then transforms to Fe3O4 with the concentration of OH−ncrease. Similarly, as happened in the range C in our case, ifhe concentration of Fe2+ is not high enough, equation (3) wouldot be carried out, thus 2-line ferrihydrite and OH− cannot beoprecipitated so that the resultant solutions exhibit the colorf 2-line ferrihydrite. Additionally, the curves of CFe2+ versusamine exhibit a hyperbolic shape for both interfacial coprecip-

tation and classical coprecipitation, indicating that the critical

alue of CFe2+ · (COH− )2 should be a constant because COH−s directly proportional to Camine. It can be concluded that the

inimum limit of CFe2+ is certainly reasonable at correspond-ng Camine, and the two-step mechanism of the coprecipitation

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chem. Eng. Aspects 320 (2008) 115–122

eaction can be verified except that 2-line ferrihydrite rather thaneOOH as an intermediate was produced under our experimentalonditions.

To further understand the conjunct influence of the concen-rations of Fe2+ and OH−, NaOH was chosen as the alkali in thelassical coprecipitation. It was found that the plot of its con-entration versus the concentration of Fe2+ have a similar shapeith that as weak alkali being selected, except that the critical

ange is corresponding to lower concentration of Fe2+, whichs obviously ascribed to almost complete ionization of NaOH,.e., higher concentration of OH− in NaOH solution than that inmmonium solutions with same concentration. Accordingly, theigher basicity of dipropylamine owing to the electron donatingffect of alkyl group would leads to a lower critical range thany ammonia, however, as shown in Fig. 1, the critical Caminehat can produce Fe3O4 in interfacial coprecipitaion is slightlyigher than that by ammonia in classical coprecipitation. As haseen confirmed, the interfacial coprecipitation possess similarechanism with classical coprecipitation, we suggest that it is

ecause the reaction is carried out on the interface between waternd oil instead of homogeneous medium in interfacial coprecip-tation. Although the intrinsic conditions of the coprecipitationeaction would not change in the micro-area of the interfacialayer, the concentration of OH− in the interfacial area woulde lower than that in homogenous aqueous ammonium solution31], so that the Fe3O4 can only be produced with higher crit-cal Camine in interfacial coprecipitation than that in classicaloprecipitation.

According to classical nucleation and growth theory, the con-entration is one of the predominant factors determining growthate in diffusion controlled classical coprecipitation reaction,ecause the nucleus have to grow via a long-distance mass trans-er [32]. As Massart and Cabuil reported [33], the concentrationf the iron species in classical coprecipitation has a significantffect on the overall nanoparticle size. The increase of the con-entration of iron salts will result into larger magnetite particlesith wider size distribution. Whereas in interfacial coprecipita-

ion, concentration has almost no effect on the size and sizeistribution of nanoparticles (as shown in Fig. 4). This phe-omenon should be caused by the mechanism of the reactionappened on the interface. The nucleus would be formed on thenterface between water and cyclohexane, then move towardshe water phase because of their hydrophilic surface. After theyompletely immersed in the water, they would not grow anyore because of the absence of superfluous OH− around them.

n this process, nucleation process is similar to that in clas-ical coprecipitation which was affected by the concentrationf precursors via the degree of supersaturation, S, given by= aFe2+aFe3+aOH−/Ksp(Fe3O4), where a is the activity of ions

nd Ksp is the solubility product constant. However, as almostnsoluble magnetite is, the degree of supersaturation in the copre-ipitation reaction is high enough so that the increase of theoncentration of iron salt can be ignored. What different in the

nterfacial coprecipitation from classical coprecipitation is thathere is only a relatively short time for nuleus to grow up. As aesult, the concentrations of precursors have almost no influencen the growth process of nucleus and therefore the size and size

K. Tao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 320 (2008) 115–122 119

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istribution of nanoparticles in interfacial coprecipitation. It islso means that the increasing size of nanoparticle prepared bylassical coprecipitation was originated from the growth stage.

Temperature is another important factor according to eitherhe theory or the experimental results that can work on the size,ispersibility and crystallinity in classical coprecipitation. Thelevation of temperature results into bigger and more uniformanoparticles with relatively round shape [16]. For example, inhe coprecipitation to prepare CoFe2O4 reported by Chinnasamyt al. [34], the increasing of the temperature from 70 to 98 ◦Cncreased the average particle size from 14 to 18 nm.

However, when the coprecipitation happens on the interfaceetween water and oil, some features of resultant nanoparticlesrepared by the interfacial coprecipitation are completely dif-erent from those prepared by classical coprecipitation. Fig. 5hows the typical TEM images and size distributions of sam-les prepared by interfacial coprecipitation at 25, 40, 60 and0 ◦C, respectively. It can be found that the average size ofanoparticles almost does not change as temperature increasing,ven some smaller particles (about 6–8 nm) can be observed inhe TEM image of higher temperature samples, which is oppo-ite to the temperatures influence in classical coprecipitation.ig. 6 shows XRD patterns of samples prepared at various tem-eratures, which validated that they composed of almost pureagnetite nanoparticles. As is known [32], temperature has

ffects both on the nucleation and growth process in the for-ation of nanoparticles in classical coprecipitation. However,

or interfacial coprecipitation, the growth process was short as

entioned above, so the nucleation process has a dominant influ-

nce on the final particles size. As the activation energy of theucleus formation (given by �G∗ = 16πσ3

SLv2/3k2T 2ln2S) isnversely proportional to the square of temperature, the nucle-

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mol/l Fe2+ and 1.47 mol/l dipropylamine; (b) 15 mmol/l Fe2+ and 1.47 mol/l) are their size distribution corresponding to (a), (b) and (c), respectively.

tion rate (given by RN = A exp[−(�G*)/kT]), as well as theumber of nucleus would increase with temperature increasing,nd therefore a part of nanoparticles would reduce their sizelightly. Meanwhile, the size distribution may be also affectedy Ostwald ripening. With the increase of reaction temperatures,he process of ripening would accelerate, which means smalleranoparticles would decrease their size more easily and biggerarticles keep growing further, therefore leading to the wideningf the size distributions.

As a summary, the processes of forming nanoparticles inlassical coprecipitation and interfacial coprecipitation can bellustrated in Scheme 1. The differences of nanoparticles pre-ared by interfacial coprecipitation from those prepared bylassical coprecipitation were mainly caused by two reasons: Inhe first place, the nucleation in interfacial coprecipitation wasarried out on the interface between water and oil. Secondly,he nucleus would move towards water phase immediately. Oth-rwise, if the nucleuses stay on the interface, they would keepn growing and more prefer staying on the interface; and if theoving process needs a relative long time, the concentration and

emperature would have obvious effects.On the other hand, the elevation of temperature in interfa-

ial coprecipitation method would lead to better dispersibilityf as-prepared nanoparticles in pure water. According to ourxperiments, we found that increasing the temperature in prepa-ation can prolong the stabilized time of resultant nanoparticlesispersed in pure water. For example, the magnetite nanoparti-les prepared at 25 ◦C can be kept stable in pure water for about

0 h. However, if 60 ◦C was used in the preparation, the resultantanoparticles can be kept stable for more than 3 days withoutny stabilizing agents, and if the nanoparticles were synthesizedt 80 ◦C, they can be kept stable in pure water for more than one

120 K. Tao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 320 (2008) 115–122

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Fig. 5. TEM images of samples prepared at 25 ◦C (a), 40 ◦C (b), 60 ◦C (c) and 80

ig. 6. XRD patterns of samples prepared at 25 ◦C (a), 40 ◦C (b), 60 ◦C (c) and0 ◦C (d), respectively.

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eek. This time is long enough to be used in biomedical appli-ations, and is much longer than that by classical coprecipitationt the same conditions.

For elucidating the difference of dispersibility, the mass ofitrogen absorbed on the surface and the zeta potential was mea-ured. As show in Fig. 7, it is obviously that with the reactionemperature increasing, the ammonium salt absorbed on the sur-ace of nanoparticles is increased, and the absolute value of theeta potential is increasing as well, which perhaps leads to moretable products in pure water. Meanwhile, pH values of the waterayer just after reaction were determined as 10.85, 10.56, 10.19nd 9.89 for 25 ◦C, 40 ◦C, 60 ◦C and 80 ◦C preparation, respec-ively. The difference in pH values should be ascribed to themount of quaternary amine salt dissolved in water layer, whichepend on their reaction temperatures. Therefore the amountf ammonium salt molecules absorbed on nanoparticles surface

hould directly correspond to the reaction temperature.

Magnetization curves of the samples prepared by interfacialoprecipitation at 25, 40, and 60 ◦C are shown in Fig. 8. Thelmost immeasurable coercivity and remanence indicate their

K. Tao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 320 (2008) 115–122 121

Scheme 1. Schematic illustration of differences between classic

Fig. 7. Zeta potentials and the numbers of ammonium molecules per 10 nm-sizednanoparticle of samples prepared at different temperatures.

Fig. 8. Magnetization curves of sample prepared at 25 ◦C (a), 40 ◦C (b), 60 ◦C (c)measured at 25 ◦C, respectively, inset shows the supersaturation magnetizationof samples prepared at different temperatures.

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al coprecipitation (A) and interfacial coprecipitation (B).

ingle domain of individual nanoparticles. The supersaturationagnetizations are 48, 54 and 63 emu/g for 25, 40, and 60 ◦C

repared samples, respectively. Obviously according to Fig. 7nd its inset, the supersaturation magnetization is enhanced withhe temperature increasing in preparation, and after the tempera-ure increased to about 60 ◦C, the supersaturation magnetizationould almost constant. The increase of supersaturation magne-

ization before 60 ◦C can obviously be ascribed to the increasef crystallinity. As Lee et al. [35] reported, the supersaturationf Fe3O4 nanoparticles would reach their maximum value whenhere are about 60% crystallized, which perhaps can be adaptedo our case when temperature is higher than 60 ◦C.

. Conclusion

In conclusion, we studied the interfacial coprecipitationethod in preparing magnetite nanoparticles with evaluating

he effects of the concentration of precursors and the tempera-ure in preparation. If the concentration of Fe2+ is lower thanbout 7.5 mmol/l, no matter what concentration of the amines, Fe3O4 would not be synthesized. The interfacial coprecipi-ation follows the mechanism of two separated steps, which isimilar to the coprecipitation happens in homogeneous aque-us medium. When the concentration of iron salt is higher thanhe critical limit, the size of the resultant nanoparticles wouldot change significantly. The effect of temperature in prepa-ation is totally different from the coprecipitation in aqueousedium, especially on the size. In the interfacial coprecipita-

ion, the size would not increase and dispersibility would bemproved with the temperature increasing, which is caused byhe mechanism of the formation of nanoparticles and their sur-

ace chemistry. In summary, our efforts presented in this paperurther confirmed the mechanism of fabricating nanoparticles bynterfacial coprecipitation method, and the reaction proceduresf the coprecipitation, which should be helpful for the phaseontrol in the preparation of magnetite nanoparticles.

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cknowledgment

This work was financially supported by Science and Tech-ology Committee of Shanghai (Project No. 05ZR14084) andoung Scholar’s foundation of Shanghai Jiao Tong Univ. Wehank Instrumental Analysis Center of SJTU for assistance withhe measurements.

eferences

[1] U. Jeong, X.W. Teng, Y. Wang, H. Yang, Y.N. Xia, Adv. Mater. 19 (2007)33.

[2] S.H. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, Science 287 (2000)1989.

[3] H. Zeng, J. Li, Z.L. Wang, J.P. Liu, S. Sun, Nature 420 (2002) 395.[4] M. Benz, A.M. Van der Kraan, R.J. Prins, J. Appl. Catal. A 172 (1998) 149.[5] U. Hafeli, W. Schutt, J. Teller, M. Zborowski, Scientific and Clinical Appli-

cations of Magnetic Carriers, Plenum Press, New York, 1997.[6] H. Gu, K. Xu, C. Xu, B. Xu, Chem. Commun. (2006) 941.[7] S. Mornet, S. Vasseur, F. Grasset, E. Duguet, J. Mater. Chem. 14 (2004)

2161.[8] J.W. Choi, C.H. Ahn, S. Bhansali, H.T. Henderson, Sens. Actuators. B

Chem. 68 (2000) 34.[9] T. Hyeon, Chem. Commun. (2003) 927.10] J. Rockenberger, E.C. Scher, A.P. Alivisatos, J. Am. Chem. Soc. 121 (1999)

11595.

11] T. Hyeon, S.S. Lee, J. Park, Y. Chung, H.B. Na, J. Am. Chem. Soc. 123

(2001) 12798.12] S. Sun, H. Zeng, J. Am. Chem. Soc. 124 (2002) 8204.13] S. Sun, H. Zeng, D.B. Robinson, S. Raoux, P.M. Rice, S.X. Wang, G. Li,

J. Am. Chem. Soc. 126 (2004) 273.

[[

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chem. Eng. Aspects 320 (2008) 115–122

14] Y.S. Kang, S. Risbud, J.F. Rabolt, P. Stroeve, Chem. Mater. 1996 (8) (1996)2209.

15] T. Freid, G. Shemer, G. Markovich, Adv. Mater. 13 (2001) 1158.16] P. Tartaj, C.J. Serna, Chem. Mater. 14 (2002) 4396.17] Y. Lee, J. Lee, C.J. Bae, J.G. Park, H.J. Noh, J.H. Park, T. Hyeon, Adv.

Funct. Mater. 15 (2005) 503.18] R. Massart, IEEE Trans. Magn. MAG-17 (1981) 1247.19] S.E. Khalafalla, G.W. Reimers, IEEE Trans. Magn. MAG-16 (1980) 178.20] Y. Sahoo, A. Goodarzi, M.T. Swihart, T.Y. Ohulchanskyy, N. Kaur, E.P.

Furlani, P.N. Prasad, J. Phys. Chem. B 109 (2005) 3879.21] F.Y. Cheng, C.H. Su, Y.S. Yang, C.S. Yeh, C.Y. Tsai, C.L. Wu, M.T. Wu,

D.B. Shieh, Biomaterials 26 (2005) 729.22] Y. Sahoo, H. Pizem, T. Fried, D. Golodnitsky, L. Burstein, C.N. Sukenik,

G. Markovich, Langmuir 17 (2001) 7907.23] V.S. Zaitsev, D.S. Filimonov, I.A. Presnyakov, R.J. Gambino, B. Chu, J.

Colloid Interface Sci. 212 (1999) 49.24] K. Tao, H. Dou, K. Sun, Chem. Mater. 18 (2006) 5273.25] X. Liang, X. Wang, J. Zhuang, Y. Chen, D. Wang, Y. Li, Adv. Funct. Mater.

16 (2006) 1805.26] E. Tronc, P. Belleville, J.P. Jolivet, J. Livage, Langmuir 8 (1992) 313.27] D.K. Kim, M. Mikhaylova, Y. Zhang, M. Muhammed, Chem. Mater. 15

(2003) 1617.28] J.L. Jambor, J.E. Dutrizac, Chem. Rev. 98 (1998) 2549.29] U. Schwertmann, J. Friedl, H. Stanjek, J. Colloid Interface Sci. 209 (1999)

215.30] J.P. Jolivet, C. Chaneac, E. Tronc, Chem. Commun. (2004) 481.31] T.M. Chang, L.X. Dang, Chem. Rev. 106 (2006) 1305.32] B.L. Cushing, V.L. Kolesnichenko, C.J. O’Connor, Chem. Rev. 104 (2004)

3893.33] R. Massart, V. Cabuil, J. Chim. Phys. Chim. Biol. 84 (1987) 967.34] C.N. Chinnasamy, B. Jeyadevan, O. Perales-Perez, K. Shinoda, K. Tohji,

A. Kasuya, IEEE Trans. Magn. 38 (2002) 2640.35] J. Lee, T. Isobe, M. Senna, J. Colloid Interface Sci. 177 (1996) 490.