Influence of a Magnetic Field on the Formation of Magnetite Particlesvia Two Precipitation Methods

Fernando Vereda,* Juan de Vicente, and Roque Hidalgo-AÄ lvarez

Grupo de Biocoloides y Fı´sica de Fluidos, Departamento de Fı´sica Aplicada, Facultad de Ciencias,UniVersidad de Granada, Granada E-18071, Spain

ReceiVed NoVember 17, 2006. In Final Form: December 21, 2006

An experimental investigation is described on the effects of the presence of a magnetic field during the fabricationof magnetite particles. We considered two well-known synthesis methods: that of Massart [IEEE Trans. Magn.1981,17, 1247-1248] for the synthesis of nanometer-sized, monodomain particles; and that of Sugimoto and Matijevic´ [J.Colloid Interface Sci.1980, 74, 227-243.] for the fabrication of micrometer-sized multidomain spherical particles.The latter method was studied with two systems of different ionic compositions that lead to two different mechanismsof growth: either growth by aggregation and recrystallization of primary particles or direct crystal growth. Whengrowth was dominated by aggregation of primary units, the magnetic field had a dramatic effect on the morphology,inducing the formation of rodlike particles. Growth dynamics of that system were studied for particles obtained inthe presence as well as in the absence of the magnetic field. Particles were also characterized by powder magnetometry,electrophoresis, X-ray diffraction, and optical absorbance techniques. Interestingly, growth dynamics of the rods crosssection were comparable to those of the diameter of the spheres. With the exception of the morphology, no othersignificant difference was found between the rodlike particles and the spheres.

1. IntroductionBecause of their potential interest for the fabrication of magnetic

fluids, either ferrofluids (FFs)1,2 or magneto-rheological fluids(MRFs),3 we decided to attempt the fabrication of magneticparticles with an elongated shape. As a first approach, we applieda magnetic field to the system during particle formation. Suchan approach has been used in the past by other authors. AcicularCo particles of approximately 30 nm of length were fabricatedby Charles and Issari4 by exposing the reactants to a field of1000 mT. Wang et al.5 fabricated magnetite nanowires by meansof a hydrothermal process in the presence of fields that rangedbetween 0 and 350 mT. They obtained particles with a crosssection of 35-100 nm and a length between 0.48 and 2.7µm.It should be mentioned that the magnetic field was not uniformwithin the volume occupied by the reactants since it was providedby permanent magnets placed under the reaction flask. Finally,Zhang et al.6 used a process based on that of Sugimoto andMatijevic7 for the synthesis of magnetite, with the particularitythat the particles were subjected during growth to fields as highas 350 mT. When that maximum field was applied, they obtained“chains of spheres” with lengths of 4-8 µm and cross sectionsof approximately 310 nm. In this paper we focus on the effectsof the presence of a magnetic field on two widely used magnetitespheres synthesis routes initially developed by Massart8 andSugimoto and Matijevic´.7

On the one hand, Massart’s method is typically used for thesynthesis of single-domain magnetite particles of ca. 10 nm ofdiameter. These are commonly employed for the fabrication of

ferrofluids.1,2The method is based on the stoichiometric mixtureof Fe2+ and Fe3+ in aqueous media, the coprecipitation of thecorresponding hydroxides [Fe(OH)2 and Fe(OH)3] upon theaddition of a strong alkali, and the relatively fast aging of thosehydroxides under vigorous stirring to form magnetite:

On the other hand, Sugimoto and Matijevic´’s phase-transforma-tion approach, reported in 1980, is used for the fabrication oflarger size (0.03-1.1 µm in diameter) magnetite particles witha relatively narrow size distribution. The process consists of theprecipitation of a ferrous hydroxide gel also in aqueous media,and the subsequent slow aging (oxidation) of that gel in thepresence of NO3- at 90°C for several hours. The nitrate ion actsas a mild oxidizing agent that reacts with the Fe(OH)2 gel as wellas with the Fe2+ in solution (see Sugimoto and Matijevic´’s work7

for specific reactions) to form magnetite. According to Matijevic´’sobservations, the oxidation of the Fe(OH)2 gel first leads to theformation of small primary particles of magnetite, which,depending on the pH of the media, can afterward aggregate andre-crystallize to form larger particles. It is found that a smallexcess of Fe2+ in solution helps to keep the pH close to theisoelectric point of magnetite, which enables the aggregationand contact-crystallization process and therefore the formationof spherical, relatively monodisperse particles. In contrast, anexcess of OH- ions brings the pH away from the isoelectricpoint and prevents the coagulation of the primary particles. Theparticles then grow by direct crystal growth (molecular addition)and show a cubic or octahedral morphology.

We decided to study these two synthesis methods because oftheir relative simplicity and wide use, and because we found noprevious reports on the effect of magnetic fields during thesynthesis of nanometric magnetite particles or on particles largerthan 0.5µm, the latter being of special interest for the fabricationof magnetorheological fluids. Therefore, in the case of Sugimotoand Matijevic’s method, we chose reactant concentrations that

* To whom correspondence should be addressed. E-mail: fvereda@ugr.es.(1) Odenbach, S.Colloid Surf., A2003, 217, 171-178.(2) Papell, S. S. U.S. Patent 3215572, 1965.(3) Ginder, J. M. InEncyclopedia of Applied Physics, Volume 16; Trigg, G.

L., Ed.; VCH Publishers: New York, 1996; p 487.(4) Charles, S. W.; Issari, B.J. Magn. Magn. Mater.1986, 54-7, 743-744.(5) Wang, J.; Chen, Q. W.; Zeng, C.; Hou, B. Y.AdV. Mater.2004, 16, 137-

140.(6) Zhang, Y.; Shi, R.; Xiong, H. Q.; Zhai, Y.Int. J. Mod. Phys. B2005, 19,

2757-2762.(7) Sugimoto, T.; Matijevic, E.J. Colloid Interface Sci.1980, 74, 227-243.(8) Massart, R.IEEE Trans. Magn.1981, 17, 1247-1248.

2FeCl3 + FeCl2 + 4H2O + 8NH3 f

Fe3O4 + 8NH4+ + 8Cl- (1)

3581Langmuir2007,23, 3581-3589

10.1021/la0633583 CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 03/03/2007

would maximize the monodispersity and would yield particlesof a typical diameter of approximately 0.7µm. Furthermore, westudied Sugimoto and Matijevic´’s method with a different startingionic composition (excess OH-) that is known to lead to thedirect crystal growth of the magnetite particles. By doing so, ourstudy became of a comparative nature since we would investigatethe effect of the field on two different mechanisms of growthof particles of the same chemical compound (Fe3O4).

All synthesis procedures mentioned above were carried outin the presence and in the absence of a constant, relativelyhomogeneous magnetic field of approximately 405 mT. Becausethe preliminary results were much more promising whenSugimoto and Matijevic´’s procedure was followed in an excessof Fe2+, we dedicated more time to this system. We studied thedynamics of growth of the particles and characterized them usingpowder magnetometry, electrophoresis, X-ray diffraction, andoptical absorbance techniques.

2. Experimental Section

2.1. Synthesis of Nanometer-Sized Magnetite Particles (∼ 10nm). Ultrafine magnetite particles were fabricated following theparticular recipe given by Liu et al.9 In a screw-cap flask, 11.68 gof FeCl3‚6H2O (Scharlau, extra pure) and 4.30 g of FeCl2‚4H2O(Fluka,>99% purity) were dissolved in 200 mL of ultrapure distilledwater (Milli-Q Academic, Millipore). That water had been previouslypurged with N2and kept in a hot water bath at 85°C for approximately10 min. After the addition of the chlorides, the mixture was kept inthe bath, with N2still flowing and under vigorous mechanical stirring,for 5 min more. At this point we added 15 mL of 25% NH3‚H2O.The color of the solution turned black due to the formation ofminuscule magnetite particles. We washed the suspension 3 timeswith oxygen-free distilled water, using a magnet to acceleratesedimentation of the black solid phase and then discard thesupernatant. Finally, we let the solution dry at 50°C to obtainmagnetite powder.

We also carried out a synthesis like that described above, but ina test tube that was held inside a permanent magnet (∼405 mT) asthe one depicted in Figure 1. The volume of the final solution withthe ferric and the ferrous chlorides was 7 mL instead of 200 mL.This was achieved by placing 0.409 g of FeCl3‚6H2O and 0.150 gof FeCl2‚H2O in a test tube, and then adding ultrapure distilled wateruntil the total volume of the solution was 7 mL. The water had beenpreviously purged with N2 and heated at 85°C for approximately10 min. Although the test tube was too small for mechanical stirring,we fed N2 through a relatively thin plastic tube and kept purging the7 mL of solution until the end of the synthesis process. The test tubewas placed inside hot water at 88°C for 5 min so that the mixturereached a homogeneous temperature. At this point, we inserted thetest tube into the gap of the permanent magnet and injected 0.41 mL

of 32% NH3 with a syringe. The suspension turned black. Finally,it was washed following the procedure described in the paragraphabove.

Figure 2 shows transmission electron microscope (TEM) mi-crographs of magnetite nanoparticles synthesized both in the absenceand in the presence of the magnetic field.

2.2. Synthesis of Micrometer-Sized Magnetite Particles (∼ 1µm). The general procedure that we followed, based on that ofSugimoto and Matijevic´’s, can be divided into three steps:

1. Preparation of the Fe(OH)2 gel: It started with the preparationof a solution with the required concentrations of those chemicalsthat are not sensitive to oxidation: KOH (90%, Panreac, chemicallypure) and KNO3 (Scharlau, Spain, ultrapure). The solvent wasultrapure water (Milli-Q Academic, Millipore). The resulting solutionwas then purged in N2 for 2 h before the addition of FeSO4‚7H2O,which reacted with KOH to form Fe(OH)2. The stock solution ofFeSO4‚7H2O was also prepared with water that had been purgedwith N2 for 2 h. The mixture turned green when the FeSO4‚7H2Owas added. The green color was dark if there was an excess of Fe2+

or lighter if there was an excess of OH-.2. Curing of the gel: The mixture with the Fe(OH)2 gel was then

placed and kept for 4 h in abath of hot water or oil that had beenpreviously heated to 90°C. After that time the flask was cooleddown in iced water. A black sediment and a relatively clearsupernatant could be observed inside the reaction flask.

3. Washing of the suspension: The suspension was subjected toa washing routine that consisted of (i) decantation of the supernatant,(ii) addition of ultrapure distilled water and ultrasonic treatment toaid the re-dispersion of the sediment, and (iii) differential sedi-mentation accelerated by using a permanent magnet. The routinewas repeated approximately 7 times. This resulted in a clearsupernatant with an ionic conductivity below 2µS/cm. Once theionic conductivity was below 2µS/cm, the suspension was washedone more time with ethanol and re-dispersed again in ethanol forstorage.

The three steps mentioned above were common to all the synthesesof micrometer-sized magnetite particles based on Sugimoto andMatijevic’s method. Specific concentrations of reactants are givenin sections 2.2.3 and 2.2.4.(9) Liu, X.; Ma, Z.; Xing, J.; Liu, H.J. Magn. Magn. Mater.2004, 270, 1-6.

Figure 1. Schematic depiction (not to scale) of the toroidal magnetused for the synthesis of magnetite particles in the presence of amagnetic field. Because of the narrow gap of the magnet, only∼7mL of the reactant mixture could be cured at a time.

Figure 2. TEM micrographs of ultrafine magnetite particlessynthesized (a) in the absence of a constant magnetic field and (b)in the presence of a magnetic field of 405 mT. This method is basedon the stoichiometric mixture of Fe2+ and Fe3+ in aqueous media,the precipitation of the corresponding hydroxides [Fe(OH)2 and Fe-(OH)3] upon the addition of a strong alkali, and the relatively fastaging of the hydroxides at approximately 85°C.

3582 Langmuir, Vol. 23, No. 7, 2007 Vereda et al.

2.2.1. Synthesis in the Presence of a Magnetic Field.A smallvolume of the Fe(OH)2 gel was cured inside the gap of the toroidalmagnet mentioned above, which was inside a water bath at 90°C.The small volume of the gap limited the amount of gel that couldbe cured at a time to 7 mL because a relatively thin test tube wasused as the reaction container (see Figure 1.) The washing of theparticles that grew exposed to the magnetic field was carried out,avoiding the ultrasonic treatment for the re-dispersion of the sedimentafter decantation. This was done to prevent the fracture of theaggregates that might have formed during the synthesis. For thesake of consistency, micrometer-sized particles synthesized in thepresence of a magnetic field are compared below with those fabricatedin the absence of field only when they originated from the sameFe(OH)2 gel.

2.2.2. Particle Growth during Aging. To observe particle growthas a function of time, small volumes (7 mL) of the same Fe(OH)2

gel were placed in different test tubes, which were then removedfrom the hot water bath at different times. To halt the curing process(i.e., avoid further oxidation of the Fe(OH)2 gel), we took the testtube where the reaction was taking place out of the hot water bath,cooled it down in iced water, and washed the suspension three timeswith oxygen-free ultrapure distilled water. We used a permanentmagnet during the washing routine to accelerate the sedimentationof the particles. Samples on TEM grids were prepared immediatelyafter the washing process to avoid further oxidation of the sample.Particle growth as a function of time was studied for Sugimoto andMatijevic’s process in an excess of [Fe2+], both in the absence andin the presence of the magnetic field.

2.2.3. Synthesis of Micrometer-Sized Particles in an Excess ofFe2+. During the preparation of the Fe(OH)2 gel we mixed, in a 250mL flask, 12.5 mL of a 1 M KOH solution, 25 mL of a 2 M KNO3

solution, and 205 mL of ultrapure water which had been purged withN2 for 10 min. After the mixture was purged with N2 for 2 h, weadded 7.5 mL of a 1 M FeSO4‚7H2O solution. The mixture turnedinto a dark green color. The relative concentrations of the reactantswere chosen to provide an excess of [Fe2+] ) 0.005 M in thesupernatant. Sugimoto and Matijevic´ observed a minimum in thewidth of the size distribution of the particles (i.e., maximummonodispersity) when their reactant mixture had a small excess of[Fe2+].

Figure 3 shows results obtained following Matijevic´’s method ina small excess of [Fe2+]. Particles shown in Figure 3a were synthesizedin the absence of a magnetic field and particles in Figure 3b weresynthesized in the presence of a magnetic field of 405 mT.

2.2.4. Synthesis of Particles in an Excess of OH-. In this case weadded 37.5 mL of 1 M KOH and 25 mL of 2 M KNO3 to 175 mLof ultrapure distilled H2O. After the mixture was purged with N2 for2 h, we added 12.5 mL of 0.5 M FeSO2‚7H2O. This solution hadalso been prepared with water that had been purged with N2 for 2h. The recipe resulted in an excess concentration of [OH-] ) 0.1M in solution. However, for consistency the concentrations of theFe(OH)2 gel and of KNO3 were kept at 0.025 and 0.2 M respectively,which are the same as when the synthesis was made in excess [Fe2+].

Figure 4 shows examples of magnetite particles that were fabricatedfollowing Sugimoto and Matijevic´’s method, but with an excess ofOH- in solution.

2.3.CharacterizationofMicrometer-SizedParticlesFabricatedin Excess Fe2+.2.3.1. Magnetic Characterization. The magnetizationof powder magnetite samples was measured as a function of theapplied external magnetic field in a MANICS DSM-8 magnetometer.The external magnetic field was swept from+4000 to-4000 kA/m,and then back to+4000 kA/m. The same procedure was followedfor both the magnetite spheres and the magnetite rods.

2.3.2. X-ray Diffraction. X-ray powder diffraction spectra wereobtained in a Bruker D8 Advance diffractometer. We used the CuKR radiation and a scanning rate of 0.15°/min.

2.3.3. Electrokinetic Characterization. Electrophoretic mobility(µe) was measured with a ZetaPALS (Brookhaven). Every data pointpresented in the graphs shown in Figures 13 and 14 is the averageof the four measurements taken for the same sample in the courseof a “run”. The error bars are given by the standard deviation

of those four measurements. Because of the high density of magnetite(∼5.1 g/mL) and therefore its tendency to settle fast, the sampleswere sonicated for∼30 s and the measuring cell was turned overa couple of times before every run. As a preliminary step, thedependence of the measured mobility on the solid-phase concentrationof our suspensions was first investigated. At low solid concentration,mobility increased with particle concentration and then remainedpractically constant for concentrations over 0.04 mg/mL. We workedwith suspensions with either 0.06 or 0.08 mg/mL of magnetite content.

2.3.4. Optical Absorbance Characterization.Because of the fastsedimentation velocity of micrometer-sized magnetite particles inaqueous media, we used an indirect method to obtain informationabout the stability of our suspensions. This method was based onmeasurements of the optical absorbance after sedimentation andsubsequent gentle, manual re-dispersion of the sediment. By bare-eye observation of suspensions at pH) 10 and 7 with the samesolid-phase concentration, it was noticeable that after manual re-dispersion of the sediment the suspension at pH 7 presented biggeraggregates than that at pH) 10. When the absorbance through bothsuspensions was measured, we found that it was lower for thesuspension with the visible aggregates. Therefore, we decided to trythis method to obtain qualitative information about the stability ofthe suspensions. We used a Beckman DU 7400 spectrophotometerfor the absorbance measurements, which were made at a wavelengthof 540 nm. Polystyrene cells in the shape of a rectangular prism anda cross section of 1 cm2 were used as sample containers. One ofthose cells filled with water was employed as the reference.

3. Results and Discussion

3.1. Nanometer-Sized Particles. As observed, particles shownin Figure 2 are relatively polydisperse and have a characteristicdiameter of approximately 10 nm. No significant differenceswere observed between the morphology of the particles fabricatedin the absence versus in the presence of the magnetic field.Therefore, neither oriented growth of the magnetite crystals (asthat observed by Wang et al.5) nor permanent aggregation of thenanoparticles occurred. The reason for the latter could be a non-negligible Brownian motion for the particles due to their smallsize if compared to magnetostatic interparticle interactions. Arough estimation gives a diameter of 6.5 nm for the magnetostatic/thermal motion coupling parameter10 being unity. Larger sizesare needed for magnetostatic interactions to be dominant andhence for chain structures to be observed. Another possibility isthat even if particles aggregate to form chain structures due tothe action of the magnetic field, they break once the field isremoved because the electrostatic repulsion between particlesprevents the formation of permanent chains. It should bementioned that because of the concentrations used during thesynthesis, once the iron chlorides had reacted with the ammoniain aqueous media [reaction (1) above], there was still an excessof [OH-] of 0.12 M (pH∼13). Such a pH would result in highlycharged magnetite particles, as can be inferred from theelectrophoretic mobility data shown below (Figure 13).

3.2. Micrometer-Sized Particles in an Excess of Fe2+.Figure3 shows the particles resulting from carrying out Sugimoto andMatijevic’s process with a small excess of Fe2+. Particles inFigure 3a were synthesized in the absence of a magnetic fieldand particles in Figure 3b were synthesized in the presence ofa magnetic field of 405 mT. In this case a dramatic differencein the morphology of the particles can be perceived. Whereasthose synthesized in the absence of a magnetic field are spherical,those synthesized in the presence of the magnetic field are straight

(10) Bossis, G.; Volkova, O.; Lacis, S.; Meunier, A.Lect. Notes Phys.2002,594, 186-215.

Magnetic Field on Formation of Magnetite Particles Langmuir, Vol. 23, No. 7, 20073583

rodlike particles with an average aspect ratio close to 10. Themagnetic field lines up small ferromagnetic particles that jointogether and lead to rodlike micrometer-sized particles assubsequent growth proceeds.

Figure 5 shows SEM micrographs of magnetite rodlikeparticles. Light areas are shown enlarged at the bottom of thefigure. The presence of subunits in the segments of the rodlikeparticles is clear, which confirms the model of particle growthby aggregation and recrystallization of primary particles proposedby Sugimoto and Matijevic´.7 It should be noted that, in ourparticles, those subunits are not of uniform size: they are smallerin segment C and much larger in segment A. The presence ofsegments with a polyhedral shape (B and D) and a smooth surfaceis also of interest. Such morphology is not necessarily an indicationthat the segment is a single crystal11 and could be related to thegeometry of the Fe(OH)2 platelets, which could be acting as atemplate. However, the coexistence of direct crystalline growthcannot be ruled out, even if it is marginal and due toinhomogeneities in the reactant mixture or if it only occurs inthe final stages of the synthesis process. As support to thisconjecture, we have observed the presence of a small amount ofsmaller (∼200-300 nm) cubic particles among the full grown

ones (∼600-700 nm), regardless of the presence or the absenceof the magnetic field. This crystalline growth could also lead tothe formation of the polyhedral smooth segments. We hope toemploy in the near future electron diffraction techniques todetermine whether or not those segments are single crystals.

3.3. Particles in an Excess of OH-. Figure 4 shows SEMmicrographs of particles obtained following Sugimoto andMatijevic’s method in an excess of OH-. Particles shown inFigure 4a were not exposed to a magnetic field, whereas thoseshown in Figure 4b were exposed to a constant magnetic fieldof 405 mT during growth. As can be seen in Figure 4, the presenceof a magnetic field during particle growth did not have anyobvious effect on the morphology of the particles, although thosefabricated in the presence of the field seem to have a slightlysmaller typical size. Apparently, an excess of OH- prevents anypermanent field-induced aggregation. Furthermore, it is importantto realize the difference in morphology and size between theparticles fabricated in excess Fe2+ and those fabricated in excessOH-. Those fabricated in excess Fe2+ (Figure 3) have a morespherical appearance, whereas those fabricated in excess OH-

(Figure 4) show more edges, and many of them have an octahedralshape. As pointed out by Sugimoto and Matijevic´,7 this is dueto the fact that an excess of OH- shifts the pH of the solutionwith the reactants away from the isoelectric point of magnetite.Electrostatic repulsion then prevents particle growth by ag-gregation and re-crystallization of primary particles. Particlesgrow by direct crystal growth instead. Therefore, the fact that(11) Matijevic, E.Langmuir1994, 10, 8-16.

Figure 3. SEM micrographs of micrometer-sized magnetite particlesfabricated according to the method described by Sugimoto andMatijevic in an excess of Fe2+. The two types of particles were theresult of curing aliquots of the same reactant mixture at 90°C for4 h in the absence of a magnetic field (a) and in the presence of amagnetic field of 405 mT (b). Light area in (b) is shown enlargedin Figure 5.

Figure 4. SEM micrographs of magnetite particles synthesizedfollowing Matijevic’s method with an excess of [OH-] ) 0.1 M insolution: (a) in the absence of a magnetic field; (b) in the presenceof a magnetic field of 405 mT. Note the difference is typical sizebetween these particles and those shown in Figure 5.

3584 Langmuir, Vol. 23, No. 7, 2007 Vereda et al.

the rodlike particles did not form in an excess of OH- when themagnetic field was present is an indication that the growthmechanism based on contact crystallization of primary particlesis essential to the formation of the permanent rods.

3.4. Particle Growth.At this point it is of interest to pay someattention to the growth dynamics of particles synthesizedaccording to Matijevic´’s method in excess of Fe2+.

Figure 6 shows four TEM micrographs of samples of magnetitethat had been cured at 90°C for (a) 10 min, (b) 20 min, (c) 30min, and (d) 4 h. They all were fabricated from the same particularmixture of reactants following the procedure described in section2.2.3. The particle growth with time is readily noticeable. Thisgrowth is also shown in Figure 7, which shows the histogramsof those four samples shifting toward larger diameters with time.Furthermore, one can see, especially in micrograph A of Figure6, the presence of the Fe(OH)2 gel surrounding the biggermagnetite particles, and the presence in the gel of minuscule“primary” particles whose characteristic diameter was estimatedto be around 10-20 nm. The presence of the gel was a sign thatthe reaction was successfully detained by rinsing the solid phase

with oxygen-free distilled water and by drying some of it on aTEM grid. In micrograph C of Figure 6 one can see that verylittle or no Fe(OH)2 remained after 30 min of reaction.

Figure 8 shows three TEM micrographs of samples of magnetitethat had been cured at 90°C for (a) 5 min, (b) 10 min, and (c)20 min in the presence of a magnetic field of 405 mT. They allwere fabricated from the same particular mixture of reactantsfollowing exactly the same recipe that was used for the magnetitespheres shown in Figure 6. In both the TEM micrographs andin histograms shown in Figure 9 one can see that the mean rodcross section grew with time. It is also interesting to realize thatafter 5 min linear aggregates have already been formed, and thatthe segments that form those rods look in general more sphericalthan those of rods that have grown for 4 h, such as those shownin Figure 5, which look more like chains of oblate disks. Thisindicates that during growth the primary particles have a tendencyto fill in the gap between to adjacent rod segments, contributingthen to cementing the segments. This is expected since it is thespace surrounding the contact area between two touchingsegments where the most intense magnetic fields are encountered.

Figure 5. SEM micrographs showing the texture of magnetite particles fabricated following Matijevic´’s method in an excess of [Fe2+] )0.005 M and in the presence of a magnetic field. Light areas of the large image to the left are shown enlarged at the bottom. Image to theleft corresponds to light area in Figure 3b. Image to the right was taken at higher SEM magnification.

Figure 6. TEM micrographs of magnetite samples fabricatedfollowing Matijevic’s method in an excess of [Fe2+] ) 0.005 M.Each sample is the result of curing aliquots of 5 mL of the samereactant mixture for (a) 10 min, (b) 20 min, (c) 30 min, and (d) 4h. All four micrographs are shown with the same scale.

Figure 7. Histograms of particle diameter for four samples ofmagnetite spheres. Each sample was the result of curing aliquots of5 mL from the same reactant mixture for 5, 10, 20, and 30 min. Thepopulation (N) indicated for each sample in the legend is the numberof particles whose diameter was measured. The histograms showthe increase of mean sphere diameter with time.

Magnetic Field on Formation of Magnetite Particles Langmuir, Vol. 23, No. 7, 20073585

Figure 10 shows the time evolution of the mean magnetite rodcross section and the mean sphere diameter. The rate of growthis comparable. The sphere diameter and the rod cross section

were very close to each other after 5 and 20 min, and they arestill within their samples’ standard deviation after 10 min.

For completeness, data regarding the growth of the magnetitespheres and the rods are summarized in Tables 1 and 2. Apartfrom the evolution of the mean sphere diameter and the meanrod cross section, it is interesting that the variation coefficients(standard deviation divided by the mean of a given population)tend to be smaller for the rods’ cross section than for the spherediameters, and that they tend to diminish with reaction time; i.e.,in relative terms the particle diameter and cross-section distribu-tions tend to narrow with time. Also, variation coefficients arecomparable, or even slightly smaller than those reported bySugimoto and Matijevic´ for spheres grown under the sameconditions.7

Data summarized in Table 2 indicate that the mean rod lengthalso grows with time. This suggests that as rod particles grow,the bonding between segments is increasingly stronger andparticles become more resistant to fracture during the cleaningprocess that followed the synthesis. However, it should be noticedthat the statistical sample population that we had available was

Figure 8. TEM micrographs of magnetite rod samples fabricated following Matijevic´’s method (in an excess of [Fe2+] ) 0.005 M) andcuring the reactant mixture in the presence of a magnetic field of 405 mT. Each sample is the result of curing aliquots of 5 mL from thesame reactant mixture for (a) 5 min, (b) 10 min, and (c) 20 min. All three micrographs are shown with the same scale. As time increases,so does the rods cross section.

Figure 9. Histograms of rod cross section for three samples ofmagnetite rods. Each sample was the result of curing aliquots of 5mL from the same reactant mixture for 5, 10, and 20 min. Thepopulation (N) indicated for each sample in the legend is the numberof rod segments whose width was measured. The histograms showthe increase of mean rod cross section with time.

Figure 10. Left: Evolution of mean sphere diameter and mean rodcross section as a function of time for three samples of magnetiterods and four samples of magnetite spheres. Standard deviationswere used for the error bars. Right: Time evolution of average rodlength for three samples of magnetite rods. As in Figure 9, eachsample was the result of curing aliquots of 5 mL from the samereactant mixture for 5, 10, and 20 min. Connecting lines are just aguide for the eye.

Table 1. Statistical Information Regarding Sphere Diameterand Rod Cross Section for Three Samples of Magnetite Rods

and Five Samples of Magnetite Spheresa

magnetite spheres magnetite rods

curingtime(min)

meansphere

diameter(nm)

variationcoefficient

(%)sample

population

meanrod

crosssection(nm)

variationcoefficient

(%)sample

population

5 265 24 61 283 16 31510 394 15 103 481 13 21420 592 22 205 595 12 107430 737 12 247

240 704 11 286

a Samples of magnetite spheres were the result of curing aliquots of5 mL from the same reactant mixture for 5 min, 10 min, 20 min, 30 min,and 4 h. Samples of magnetite rods were obtained by curing aliquotsof 5 mL from the same reactant mixture for 5, 10, and 20 min. Coefficientof variation is defined as the standard deviation divided by the mean.

Table 2. Statistical Information Regarding Rod Length forThree Samples of Magnetite Rodsa

curingtime(min)

meanlength(µm)

variationcoefficient

(%)sample

population

averageaspectratio

5 2.6 67 24 9.310 5.1 53 12 10.720 6.1 62 46 10.3

a These samples were obtained by curing aliquots of 5 mL from thesame reactant mixture for 5, 10, and 20 min. They are the same rodsamples as in Table 1.

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small (the reason is that we only counted rods that were visiblein full length in the micrographs), and that the length distributionwas very wide, as can be inferred from the fact that the threevariation coefficients are above 50%.

In summary, the mechanism of formation of magnetite in thepresence of a magnetic field by the described procedure is foundto have many similarities to the one proposed by Sugimoto andMatijevic in the absence of a magnetic field. Here, the phasetransformation process consists of the following stages.7,12

Divalent metal hydroxide gels change into a phase usuallycalled green rust which mainly consists of Fe(OH)2 platelets.Then, primary particles are nucleated on the surface of thoseplatelets due to oxidative aging (cf. Figures 6a and 8a) and remaintrapped temporarily. As the Fe(OH)2gel starts dissolving, primaryparticles coagulate, forming secondary particles, due to van derWaals forces since at this stage pH is close to the isoelectricpoint of magnetite. It is worth saying that at this point particlesare too small for magnetic interparticle forces being significantif compared to Brownian thermal forces. As mentioned above,simple dipole estimation gives a particle size of 6.5 nm in orderfor the magnetic force to overcomeKBT. Growth dominated bycoagulation of primary particle singlets (as compared to dimersand trimers of primary particles) is crucial in obtaining the desiredfinal monodispersity and has been regarded as necessary by someauthors13 for the narrowing of the size distribution that weobserved. This process determines to a great extent the sphericityand the size of the final particles.

Additional growth occurs by adhesion of additional primaryparticles to the already recrystallized spheres. The presence ofa magnetic field is relevant at this stage when the size of thesecondary particles exceeds a threshold over which magneticinterparticle forces are dominant and leads to the formation oflinear assemblies of secondary particles. Further growth of thechain segments must proceed largely unaffected by the magneticfield, mainly controlled by diffusion, adhesion, and contact re-crystallization of primary particles, as happens when no magneticfield is present. This is supported by the fact that the rate ofgrowth of the diameter of the spheres and the cross section ofthe rodlike particles are virtually equivalent (see Figure 10).Furthermore, the full-width at half-maximum (fwhm) of the peaksin the X-ray spectra are also equivalent for rodlike particles andspheres, which may suggest a similar size of the subunits thatform the particles in both cases. However, because of the strengthof the magnetic field around the area of contact between secondaryparticles, primary particles have a larger tendency to fill in thoseareas, and upon adhesion and crystallization, they provide apermanent bond between segments of the rodlike particles. Figure5 (segment C) shows how the gap between two segments hasbeen partially filled by primary particles.

3.5. Characterization of Micrometer-Sized MagnetiteParticles. 3.5.1. Magnetic Characterization. Figure 11 showsthe results. In both cases, the saturation magnetization wasapproximately 475 kA/m, which is very close to the bulk valuefor magnetite.14This is evidence that the black sediment obtainedin our synthesis was in fact magnetite. Furthermore, themagnetization curves also show a negligible remnant magnetiza-tion at zero field, which is also characteristic of a soft ferrimagneticmaterial such as magnetite.

3.5.2. X-ray Diffraction Analysis. The results are shown inFigure 12. The excellent agreement in the position of the peaks

as well as in their relative intensities between the observed X-raydiffraction patterns and that which was obtained from theAmerican Mineralogist Crystal Structure Database [which wascalculated15 from the crystallographic data provided by M.E.Fleet16] for magnetite is evidence that the black sediment obtainedin our synthesis is in fact magnetite.

When we compare the results obtained for the rods with thoseobtained for the spheres, we realize that both the magnetizationcurves and the X-ray spectra are virtually identical for both typesof particles. No significant differences are present in theircrystalline structure or in their magnetic properties. So far it canbe said that the only difference between synthesized rods andthe spheres is their morphology.

3.5.3. Electrokinetic Characterization. First of all, the de-pendence of the electrophoretic mobility on the pH wasinvestigated. Buffer solutions of pH) 4 and 5 were preparedwith acetic acid and NaOH. BIS-TRIS + HNO3 was used forpH ) 6, TRIS+ HNO3 for pH ) 7.2, and those correspondingto pH ) 8, 9, and 10 were prepared with H3BO3 + NaOH.Samples were prepared by adding the required amount of colloidsto aliquots of 25 mL of each buffer solution so that the resulting(12) Sugimoto, T.AdV. Colloid Interface Sci.1987, 28, 65-108.

(13) Privman, V.; Goia, D. V.; Park, J.; Matijevic, E.J. Colloid Interface Sci.1999, 213, 36-45.

(14) Cabuil, V. InEncyclopedia of Surface and Colloid Science; Hubbard, A.T., Eds.; Marcel Dekker: New York, 2002; p 4306.

(15) Downs, R. T.; Bartelmehs, K. L.; Gibbs, G. V.; Boisen, M. B.Am. Mineral.1993, 78, 1104-1107.

(16) Fleet, M. E.Acta Crystallogr., Sect. B1982, 38, 1718-1723.

Figure 11. Normalized magnetization (magnetization over saturationmagnetization) as a function of the external field for magnetite spheresand rodlike particles. The external magnetic field was swept from+4000 to -4000 kA/m and then back to+4000 kA/m. Themagnetization was calculated assuming a particle density of 5.17g/cm3.

Figure 12. X-ray diffraction spectra of magnetite spheres and rodlikeparticles. Open diamonds indicate the position and relative intensitiesfor bulk magnetite. This reference pattern was obtained from theAmerican Mineralogist Crystal Structure Database (which wascalculated15 from the crystallographic data provided by M.E. Fleet16

for magnetite). Insert shows peaks at 57.0 and 62.6° to illustrate theequivalent width at half-maximum of the two spectra.

Magnetic Field on Formation of Magnetite Particles Langmuir, Vol. 23, No. 7, 20073587

suspensions had a particle concentration of 0.06 mg/mL. Mobilityresults are shown in Figure 13. From interpolation between theexperimental data, the isoelectric point for spherical particledispersions seems to be very close to pH) 6.6. On the otherhand, the isoelectric point for rodlike particles was estimated tobe around pH) 6.8.

The curves of electrophoretic mobility vs pH that we obtainedfor both the magnetite rods and the spheres are consistent withpreviously reported values for the isoelectric point of magne-tite,17,18 which are usually between pH 6.5 and pH 7.0. Nosignificant difference between the isoelectric point of magnetiterods and magnetite spheres was observed in our experiments.

Second, electrophoretic mobility was measured as a functionof electrolyte concentration at a given pH. Figure 14 shows themobility of magnetite rods and spheres as a function of [NaNO3]at pH) 4 and pH) 10. The main feature that we can observeis a decrease of the mobility with electrolyte concentration. Thisdecrease is mainly induced by a double-layer compression sincethe larger concentration of free ions screens more effectively theparticle charge.

3.5.4. Optical Absorbance Characterization. First of all, wemeasured the absorbance of suspensions prepared at differentpH values. To do so, samples were prepared by mixing 1.05 mLof the desired buffer solution with 0.35 mL of the magnetitestock solutions, which resulted in a solid-phase concentration of1.0 mg/mL. The samples were then sonicated, shaken manually,and left to rest for 20 min. After that time the particles had settleddown and a sediment had formed. The samples were just turnedover manually three times right before measuring the absorbance.We prepared three samples at every pH. Every data point givenin Figures 15 and 16 is the average of the absorbance measuredfor each of the three samples.

The results for spheres and rodlike particles are shown inFigure 15. As we had observed visually, a lower absorbance isassociated with the presence of larger aggregates. It can be seenthat for both kinds of particles the absorbance is smaller whenthe pH is close to the isoelectric point. In contrast, the absorbanceis larger at pH values of 4 and 10. This means that the closerwe are to the isoelectric point of the particles (and therefore thesmaller the zeta potential is), the more likely they are to formstable aggregates.

To study the aggregation as a function of [NaNO3], first, weprepared stock solutions with different [NaNO3] using the pH) 10 buffer solution as the solvent. Then we prepared the samplesmixing the required amounts of those solutions at pH) 10 witha given volume of the magnetite stock solution so that the particleconcentration was 0.5 mL/mg. The samples were then sonicated,

(17) Regazzoni, A. E.; Blesa, M. A.; Maroto, A. J. G.J. Colloid Interface Sci.1983, 91, 560-570.

(18) Kosmulski, M.J. Colloid Interface Sci.2006, 298, 730-741.

Figure 13. Electrophoretic mobility as a function of pH. Buffersolutions with an ionic strength of 2 mM were used to control thepH. Particle concentration was 0.06 mg/mL for all the samples withthe exception of those with pH) 6.3, which had a concentrationof 0.08 mg/mL. Lines are given as a guide.

Figure 14. Electrophoretic mobility of magnetite suspensions asa function of NaNO3 concentration. Particle concentration was 0.08mg/mL for both types of particles, and ionic strength was controlledby adding small amounts of a 1 M NaNO3 solution. Conductance,which gives an indication of the ionic strength, increased linearlywith [NaNO3]: (a) at pH) 3.95 and (b) at pH) 9.90.

Figure 15. Absorbance of magnetite suspensions at different pH.Particle concentration was 1.0 mg/mL. Each data point is the averageof three measurements carried out on three different samples. Theabsorbance was measured after allowing the magnetite particles tosettle down for 20 min and re-dispersing the sediment by tippingover the cell manually.

3588 Langmuir, Vol. 23, No. 7, 2007 Vereda et al.

shaken manually, and left to rest for 20 min. After that time thesediment was re-dispersed by turning over manually the cellsthree times right before measuring the absorbance. We had threesamples for every value of [NaNO3]. Every data point givenbelow is the average of the absorbance measured for each of thethree samples.

The absorbance of re-dispersed suspensions as a function of[NaNO3] is shown in Figure 16. As we increased [NaNO3], theabsorbance decreased. This means that more permanent ag-gregates were forming and that the suspensions were becomingmore unstable. This is expected since a larger ionic strengthresults in a more effective screening of the electrostatic repulsiveinteraction between the particles and therefore facilitates ag-gregation.

4. Conclusions

The effect of the presence of a relatively strong magnetic field(405 mT) during the synthesis of magnetite particles wasinvestigated. We considered two precipitation methods in aqueousmedia: that of Massart8 and that of Sugimoto and Matijevic´.7

The presence of the magnetic field during Massart’s processor during Matijevic’s synthesis in an excess of OH- did not haveany obvious effect on the resulting particles. However, rodlikemagnetite particles were obtained when Matijevic´ synthesis wascarried out in an excess of Fe2+ under a superimposed externalmagnetic field.

Regarding Matijevic´’s process, the different results observedfor the two different ionic compositions can be explained on thebasis of two different growth mechanisms.

With no magnetic field present and at very low excess ofFe(II), particle growth is mainly a consequence of adhesion ofprimary particles to already formed aggregates. As the aggregationof the grown (secondary) particles is not significant under theseconditions, monodisperse particles are eventually obtained.However, at large pH (excess of OH-) primary particles nucleatedin the gel are strongly charged. This prevents their coagulationand results in smaller and octahedral particles, which suggestsa direct crystal growth mechanism.

When a magnetic field is superimposed and there is an excessof Fe2+, we proposed a growth mechanism in which the rodlikeparticles start forming once the secondary particles grow overa critical size. At this point the interparticle magnetostaticinteraction is sufficient to align the secondary particles. Growthof the particles that form these linear aggregates continues thanksto the process of adhesion and recrystallization of primaryparticles, which, due to their small size, are largely unaffectedby the magnetic field. The growth process is still mainly controlledby diffusion and Van der Waals forces. However, due to thehigher field intensity in the gap between aligned secondaryparticles, primary particles tend to fill in those areas, eventuallybonding the segments of the rodlike particles.

Such mechanism, based on the aggregation of primary particles,seems to be ideal for directing with a magnetic field the growthand as a result the morphology of the final particles, as long asthe material itself is magnetic. Since the same growth mechanismhas been proposed for the formation of Ni and Co ferrites,19,20

the fabrication of rodlike particles of such compounds should beeasily accomplished.

Finally, magnetic, structural, and electrokinetic characterizationof the rodlike particles was carried out and the results comparedwith spheres prepared following the same recipe but in the absenceof a magnetic field. With the exception of the differentmorphology, no other significant differences were observed.DLVO theory qualitatively predicts the electrophoretic mobilityand stability results obtained.

Acknowledgment. The authors would like to thank F.Galisteo-Gonza´lez for providing the Bool2k software used forthe generation of particle-size distributions from SEM and TEMmicrographs. This work was supported by European Union projectERG-517604 and MEC MAT-2006-13646-C03-03 (Spain)project. F.V. acknowledges European Union project ERG-517604and Junta de Andalucı´a project FQM 392 (Spain) for financialsupport.

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(19) Regazzoni, A. E.; Matijevic, E.Corrosion1982, 38, 212-218.(20) Tamura, H.; Matijevic, E.J. Colloid Interface Sci.1982, 90, 100-109.

Figure 16. Absorbance of magnetite suspensions at different[NaNO3]. Particle concentration was 0.5 mg/mL. Each data pointis the average of three measurements carried out on three differentsamples. The absorbance was measured after allowing the magnetiteparticles to settle down for 20 min and re-dispersing the sedimentby tipping over the cell manually.

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