Dispersibility , Shape and Magnetic Properties of Nano-Fe 3 O 4 Particles

Nano-Fe3O4 particles were prepared by a two-step microemulsion method, the influence of molar ratio of water to NP-5 (R), alkali concentration and temperature on dispersibility and shape of the nanoparticles were discussed. Magnetic studies were also carried out using VSM in this paper. It was found that the optimum preparation parameters are R = 6.0, alkali concentration = 2.5 mol·L, initial total iron concentration as 0.88 mol·L, and the temperature being 30 ̊C, the prepared nano magnetite particles have uniform size and good dispersibility with a crystal structure belonging to cubic Fe3O4 and lattice parameters of a = 8.273 Å. The results of magnetic studies show, magnetic properties of particles are influenced by dispersibility of nanoparticles which depends on size of clusters. The better dispersibility of nanoparticles leads to more ordered inner magnetic vector, and so the stronger magnetic behavior of nano-Fe3O4 particles.


Introduction
Magnetic nanoparticles have been widely studied because of their fascinating properties and wide range of potential applications in ferrofluids, information storage, pigment, medicine, biomedical and bioengineering, etc.
Many reports have not systematically studied various factors affecting dispersibility and shape of nano-Fe 3 O 4 particles, especially the relationship between dispersibility and their magnetic behavior.So in this paper, we report the preparation process of nano-Fe 3 O 4 particles us-ing two-step microemulsion method.The different factors affecting dispersibility of nano-Fe 3 O 4 particles, including NaOH concentration, reaction temperature, water-surfactant ratio, and initial total iron concentration, are discussed.The relationship between the magnetic properties and dispersion behavior of nano-Fe 3 O 4 particles is also studied.

Synthesis of Nanoparticles Fe 3 O 4
Surfactant NP-5, cosurfactant butanol, and kerosene were mixed in the ratio of 1:1:10, the muddy mixed solution was then stirred until a clear and transparent solution was obtained.The previous solution was divided into two parts of (1) and (2).
Firstly, the boiled deionized water was added to the solution of (1) with a ratio R (R is the molar ratio of water to NP-5), the microemulsion became clearer and clearer through stirring.Then certain amount of FeCl 2 •4H 2 O and FeCl 3 •6H 2 O were added into the microemulsion system with the molar ratio of Fe 2+ to Fe 3+ as 3:2 while vigorously stirring.The microemulsion labeled as (1) became brown-yellow and transparent.
Secondly, the boiled NaOH solution with the concen-tration at 2.0 mol/L, 2.5 mol/L, 3.0 mol/L, and 3.5 mol/L were added to the solution of (2) with the corresponding ratio R value previously mentioned in above step, the microemulsion labeled as (2) became clear and transparent while stirring.
After stirring microemulsion (1) and (2) for 3 h, the microemulsion (2) was dropped into the microemulsion (1) with N 2 gas passing through, the brown-yellow solution immediately turned black.The mixed system was stirred for 3.0 h and left standing for 1 h at 80˚C.After that, the upper layer was transparent and the bottom layer was black, then the upper layer was discarded and the bottom layer was washed with absolute alcohol for twice.The obtained product was dried in a vacuum oven at 80˚C for 2 h, and calcined at 350˚C for 4 h, following the dry product was calcined in tube type electric-resistance furnace at 350˚C for 4 h.Eventually, the black powders were obtained.

Characterization of Nanoparticles Fe 3 O 4
The x-ray diffraction pattern of the sample was recorded by D/max 2550 VB/PC x-ray diffractometer, using Nifiltered with Cu Kα radiation (40 mA, 40 KV, 1˚(2θ) min -1 ) at room temperature.The dispersibility and shape of samples were observed by Tecnai-12 transmission electron microscope (TEM) (120 KV).

Results and Discussions
Figure 1 shows the XRD patterns for magnetic particles prepared on the condition of R = 6.0 and C NaOH = 2.5 mol/L, the X-ray diffraction patterns of magnetic particles all display sharp peak with strong diffraction intensity, and low background, the indices (220) and (400) labeled by red color are found to be corresponding to 2θ values of 31.4 and 45.4 respectively, in a good agreement with the JCPDS card 26 -1136, which belongs to cubic system.However, the indices (220) and (400) la-  The TEM images shown in Figure 2 reveals the nanocrystallite nature of magnetite particles with a good dispersibility, the average particle size measured from the TEM images has been found to be 22 nm.

Effect of Different NaOH Concentration on Magnetite Morphology
In the preparation, the alkali concentration and watercore of microemulsion are two chief factors that affect dispersibility of Fe 3 O 4 particles.In order to discuss the effects of alkali concentration on the dispersibility and shape of Fe 3 O 4 particles, we performed five experiments under alkaline condition at concentration of 1.5, 2.0, 2.5, 3.0 and 3.5 mol•L -1 respectively.The dispersibility of prepared magnetite was investigated by transmission electron microscope, five TEM images of magnetite are shown in Figure 3.
It is found when the concentration of NaOH is lower than 2.0 mol•L -1 the agglomerated granules appear in the prepared sample, and the spindle-like particles are formed.The image recorded in Figure 3(a) exhibits rod-like form with spindle-like form crossing over.It is because the precipitate of Fe 3+ occurs at pH in the range of 3 -4, which is calculated by K θ sp, Fe(OH)3 , but precipitate of Fe 2+ occurs at pH in the range of 8 -9 calculated by K θ sp, Fe(OH)2 , indicating the amount of NaOH for precipitate of Fe 3+ is much less than that for precipitate of Fe 2+ .So when the shortfall amount of NaOH is intro-  duced into the mix solution containing Fe 3+ and Fe 2+ ions, the pH is measured about 7.5, the Fe 3+ ions preferentially form precipitates by itself instead of co-precipitating with Fe 2+ ions, the results are that more orange red FeOOH phase is formed first, but less Fe 3 O 4 phase is formed.When the concentration of NaOH reaches 2.0 mol•L -1 with the pH measuring about 8.5, the spherical particles and local agglomeration of granules are formed, as shown in Figure 3(b) The reason is that completely coprecipitation of Fe 3+ with Fe 2+ ions generally occur at pH above 9.2 as reported by reference [19].So at the pH about 8.5, the Fe 3+ ions begin to co-precipitate with the Fe 2+ ions, and form the Fe 3 O 4 phase.But this time, the residual Fe 3+ ions hydrolyzes into tiny FeOOH particles, due to lower pH value than that of coprecipitation of Fe 3+ with Fe 2+ ions, as a result, the tiny FeOOH particles produced by hydrolysis easily form local agglomeration in the sample.
In contrast to (pH ≥ 9.2) Generally speaking, under the perfect condition, coprecipitation occurs when the mole ratio of Fe 3+ to Fe 2+ is 2:1.But actually, the Fe 2+ is easily oxidized when exposed in air, so in this paper, the mole ratio of Fe 3+ to Fe 2+ is kept constant at 2:3.The whole coprecipitation equation of Fe 3+ with Fe 2+ can be written in the following: But when the concentration of NaOH becomes 3.0 mol•L -1 , slightly excessive amount of the NaOH makes particles form partial aggregates in the sample, and induces the particles increase in size.Its average size is measured larger than that of the image recorded in Figure 3(c), as can be seen in the Figure 3(d).
It is noted that when the pH of Fe 3+ and Fe 2+ solution exceeds pH of coprecipitation, the Fe 3+ and Fe 2+ ion all hydrolyze much fast and have strong tendency to precipitate.And pH increases with the concentration of NaOH increases, the excessive alkali leads to rapidly Copyright © 2011 SciRes.MSA 3 4 growing in size of clusters with different shapes, and finally formation of agglomerated sphere particles in different sizes and shapes.The higher concentration of NaOH, the more heavily agglomeration, the results can be seen in the Figure 3(e).

Effect of Different Water/Surfactant Molar Ratio (R)
The size of water-core is dependent upon water/surfactant ratio (R) [20], when R keeps constant, the size of water-core keeps unchanged; but when R increases, the water-core grows in size; this will lead to many topological constructions.To study the effect of R on the dispersibility of nano-Fe 3 O 4 particles, mole value of Fe 3+ was fixed at 0.7 mmol, mole value of Fe 2+ was fixed at 1.05 mmol, the concentration of NaOH fixed at 2.5mol/L, and R was controlled at 3, 5, 6 and 7.
As seen from Figure 4, the four prepared samples are heavily agglomerated shaped, local agglomerated shaped, well uniform spherical shaped and spheroidally aggregated shaped respectively.The results indicate that the distribution of the water-core in oil phase is chiefly determined by the water/surfactant molar ratio (R), the nucleation and growth of hydrolyzed core is likely to be a diffusion-controlled process through interaction between micelles.When R = 3.0, the image of nano-Fe 3 O 4 particles exhibit heavily agglomerated shaped with average particle size measuring about 10 nm, because the low water/surfactant molar ratio leads to crowded dispersion of NP-5 in the microemulsion system, and so strong interaction between micelles of NP-5.This induces water-core contracting in the oil phase, the crystal core can only grow in the crowded medium, and form heavily agglomeration at last.
When R = 5.0, the increase of water/surfactant molar ratio leads to broader dispersion of NP-5 in the microemulsion system, and the water-core can be incompletely dispersed in the oil phase.As a result, the particle size increase, with the average size measuring about 20 nm, and a local agglomeration is formed.When R = 6.0, water-core is dispersed uniformly in the oil phase, and the crystal core grows regularly, the average particle size is measured about 22 nm, because the microemulsion system may form well dispersive micelle.
When R = 7.0, the high mole ratio of water/surfactant leads to a great increase in water-core size, which decreases the distance between neighbouring water-core, thus leading to an increase in contractive interactions between neighbouring water-core.The crystal core grows fast but irregularly in the uncrowded medium, and form

Effect of Temperature on Magnetite Morphology
The dispersibility of magnetite is also depended upon the temperature for preparation.When the nano-Fe 3 O 4 particles were prepared at 30˚C, the TEM image recorded in Throughout crystallite size variations of the nanosized nano-Fe 3 O 4 particles, one can find that the particle sizes of synthesized products are increased with the synthesis temperature increasing, the particle sizes of synthesized products are 15 nm, 27.5 nm, 31 nm, and 36.5 nm, corresponding to the synthesis temperature being 30˚C, 45˚C, 60˚C, and 70˚C respectively.

Effect of Initial Total Iron Concentration on Magnetite Morphology
To study the effect of initial total iron concentration on the dispersibility of nano-Fe 3 O 4 in the microemulsion system, R was maintained at 6.0, the concentration of NaOH maintained at 2.5 mol•L -1 , and the temperature was kept at 30˚C.The sample was synthesized by controlling initial total iron concentration at 0.63, 0.88, and 1.25 mol•L -1 (in water) respectively, while the mole ratio of Fe 3+ to Fe 2+ was kept constant at 2:3.As seen from   [20].As we know, the Fe 3+ ions is hydrolyzed much faster than the Fe 2+ ions in alkaline solution according to K θ sp, Fe(OH)3 and K θ sp, Fe(OH)2 , when the initial total iron concentration is low, the mole ratio of alkaline concentration [OH -] to initial total iron concentration {[Fe 3+ ] + [Fe 2+ ]} becomes high, this induces Fe 3+ ions hydrolyzing much fast, and forming β-FeOOH particles before coprecipitation with Fe 2+ ions.
The dispersibility of nano-Fe 3 O 4 in th stem is also found to be dependent on the initial total iron concentration and size of water-core.When water/ surfactant ratio (R) is constant, the size of water-core cannot change, so dispersibility of nano-Fe 3 O 4 are mainly depended on the dispersion of Fe 3+ and Fe 2+ ions in water-core.As seen from Figure 6(b), the TEM image shows well uniform spherical shaped with lengths of 22 nm, probably because the Fe 3+ and Fe 2+ ions are dispersed uniformly in water-core when the initial total iron concentration was controlled at 0.88 mol•L -1 .
When the initial total iron concentration w lled at 1.25 mol•L -1 , excessive Fe 3+ and Fe 2+ ions may not be dispersed uniformly in water-core.It makes the new-formed tiny crystal core mutually collide to become massive agglomeration before completely growing, thus hinders the growth of the tiny core, and so results in larger sized clusters of tiny particles formation, as can be seen in Figure 6(c).By using the Scherrer formula and X-ray diffraction method, the particle diameter is measured to be 7 nm.

The Relati Properties and Dispersion Behavior o Nano-Fe 3 O 4 Particles
tudy the effect of particle properties, five samples A, B, C, D and E were prepared under different conditions with constant temperature at prepared under different temperature conditions with constant R, alkaline concentration and total iron concentration.Listed in Table 1 are the nomenclatures we used for different products as well as the alkaline concentration, total iron concentration and the temperature in their preparation.-1  A, B, C, D 6(a)-(c).It was shown that cluster size and cluster distribution can be influenced in a wide range (some order of magnitude) by the variation of the preparation parameters.

C able 2. The agn c parameter f sample
Magnetic studies were carried out using a vibrating sample magnetometer (VSM BHS-55) with fields up to 30 k Gauss at room te e eight samples can be seen in Figure 7, all magnetic parameters are listed in Table 2. Magnetic properties such as saturation magnetization, coercivity and squareness are discussed as following.
1) Saturation magnetization Saturation magnetization represents magnetic intensity of magnetic materials.The magn e A, B and C shown in the F operties of nano-Fe 3 O 4 particles prepared with different alkaline concentration of 2.5, 3.0 and 3.5 mol•L -1 respectively.Whereas the magnetization curves of sample A, D and E shown in the Figure 7 display magnetic properties of nano-Fe 3 O 4 particles prepared with different initial total iron concentration at 0.88, 0.63 and 1.25 mol•L -1 respectively.The magnetization curves of sample A, A-2, A-3 and A-4 shown in the Figure 7 display magnetic properties of nano-Fe 3 O 4 particles prepared with different temperature at 30˚C, 45˚C, 60˚C and 70˚C respectively.By comparing magnetic parameters of samples listed in Table 2 with their dispersibility shown in Figures 3(c)-(e), we obtain that sample A shows the strongest saturation magnetization, chiefly due to its uniform size and best dispersion behavior.It is under alkaline condition of concentration at 2.5 mol•L -1 , that coprecipitation of Fe 3+ with Fe 2+ ions can occur.The saturation magnetization decreases to some extent with gradual increase of NaOH concentration from 2.5 mol•L -1 to 3.5 mol•L -1 .Because the nano-Fe 3 O 4 particles change from well-dispersed into partial aggregate, and finally heavily agglomerate, with an increase of alkaline concentration.
The better dispersibility of nanoparticles can be explained by the size of clusters with uniform shape.Excessive alka 3+ 2+ zing much fast, leads to nanoparticles rapidly changing into larger sized clusters with arbitrary smaller shaped particles.The better dispersibility of nanoparticles, the stronger magnetic behavior of them [21].This means inner magnetic vector becomes more ordered, with an increase of dispersion of nanoparticles, and thus the nanoparticles display stronger magnetism, leading to an increase of saturation magnetizationσs.
When different initial total iron concentration was adopted during preparation, the saturation magnetization of particles is found to be stronger wi n concentration.For sample D, the total iron concentration is so low that Fe 3+ ions hydrolyzed faster than Fe 2+ ions and formed β-FeOOH particles before coprecipitation with Fe 2+ ions.This would lead to decrease of its saturation magnetization.In the opposite side, when the total iron concentration was higher than 0.88 mol•L -1 , coprecipitation of Fe 3+ with Fe 2+ ions occurs, this prevents β-FeOOH particles from generating, causing an increase of saturation magnetization σs of sample E. But mutually collision between new-formed tiny crystal cores hinders the growth of the tiny core, leading to formation of larger sized clusters with different tiny shapes.So dispersibility of nanoparticles of sample E is worse than that of sample A, inner magnetic vector becomes less ordered also, as a result, the saturation magnetization σs of sample E becomes lower than that of sample A.
By comparing magnetic parameters of samples listed in Table 2 with their dispersibility shown in Figures 5  (a)-(d), we obtain that sample A shows the stro turation magnetization, also chiefly due to its uniform size and best dispersion behavior.When the nano-Fe 3 O 4 particles were prepared at 30˚C, the sample A was observed to have the best dispersibility, when the temperature increased from 30˚C to 70˚C, the dispersibility of the sample A-2, sample A-3 and ample A-4 decreased steadily, resulting in inner magnetic vector getting less ordered [21], and thus a progressive decrease in the saturation magnetization σs for sample A-2, sample A-3 and sample A-4.
The above results demonstrate that saturation magnetization of nano-Fe 3 O 4 particles is greatly influenced by dispersion rsed nano-Fe 3 O 4 particles with more uniform and dispersed shapes will have higher saturation magnetization, and thus display stronger magnetism.
2) Coercivity Coercivity of magnetic matericals mainly depends on anisotropy, saturation magnetization ructure paramet agneto-crystalline anisotropy can be expressed by the following formula according to Zhou.[22]: In this formula, K value depends only on property of material itself, but not related to particle size and shape, it is inversely proportional to saturation magnetization.As seen from Table 2, coercivity of sample A is higher than that of sample B and D, but lower than that of sample C and sample E. According to the formula, sample A should have the lowest coercivity compared with other samples because of its highest saturation magnetization.However, it is found that there is no such obvious linear relationship between coercivity and saturation magnetization.The coercivity of particles is related to their size of clusters and shapes.It is noted that coercivity decreased with reduction of particle size below 40nm [23].As shown in Figures 3(d)-(e) and Figures 6(a)-(c), sample A shows well uniformly spherical shaped with lengths of 22 nm, but both of samples C and E display large size of clusters, but consisting of smaller size particles than sample A. So both of samples C and E show larger coercivity than mple A a sa l .On the other hand, average size of sample B is rger than that of sample A, and sample D is 100 -250 nm length irregular rod particles, these all lead to sample A having a larger coercivity than the sample B and D.
As shown in Figures 5(a)-(d), with an increase of temperature from 30˚C to 70˚C, the dispersibility of the sample decreased progressively, resulting in the coerciveity of sample decreasing.A plot of coercivity versus the temperature is shown in Figure 8, the Figure 8 shows linear relationship between coercivity and temperature.It demonstrates that the temperature has significant influence on the anisotropy and dispersibility of the sample, with an increase of temperature, the anisotropy and dispersibility of the sample decreases, leading to coercivity decreasing.

Indexing the Powder X-Ray Diffraction Pattern
The results of indexing the powder X-ray diffraction pattern are listed in Table 3. Table 3 shows that all the diffraction peaks in the pattern can be readily indexed by one set of lattice parameters.The largest relative deviation between the calculated D cal and experimental D exp is less than 0.347%, which indicates that the synthesized products are a single phase with cubic structure.The

Conclusions
In summary, the cubic nano-Fe 3 O 4 particles have been successfully synthesized by using two-step microemulsion method.The dispersibility, shape and anisotropy of nano-Fe 3 O 4 particles vary with R value, alkali concentration, and the temperature.The prepared nano-Fe 3 O 4 particles all exhibits dispersive spherical form with well uniform sizes measured about 22 nm under condition of R = 6.0, alkali concentration at 2.5 mol/L, with the temperature at 30˚C.Magnetic properties are influenced greatly by dispersibility and shape of particles.The crystal structure of the nano-Fe

Acknowledgements
We thank the National Natural Sci hou sci nce and technology project (G2010071 .

Figure 1
Figure 1.X-ray diffraction pattern of the nano magnetite.

Figure 2 .
Figure 2. TEM morphology of the nano magnetite.

Figure 5 (
a) exhibits well dispersed particles with uniform size measuring about 10 -20 nm.Upon increasing the synthesis temperature from 30 to 45˚C, the TEM image of Figure 5(b) has lower dispersion than the corresponding TEM image of Figure 5(a) of the sample prepared at 30˚C.When the synthesis temperature continued to increase from 45˚C to 60 and 70˚C, particles with spherical and irregular shapes with some agglomeration are noted (Figure 5(c) and Figute 5(d)), chiefly due to rapid Brown movement of nano-Fe 3 O 4 particles with temperature increasing.The increase in frequency of collision between the particles leads to kinetic energy of collision increaseing, this makes the nano-particles have strong tendency to overcome potential barrier between them, and ag-glomerate into large particles, as a result, a phenomenon of agglomeration takes place.

Figure 7 .
Figure 7. Magnetization hysteresis curves of nano-Fe 3 O 4 particles synthesized under different conditions.

Figure 8 .
Figure 8. Plot of coercivity versus the temperature.
3 O 4 particles belongs to c ic syst ence Foundation of China (No: 20971043), and the financial support from Wen-A lication to Bioprocesses," Colloids and Surfaces A: Physicochemical ub em with lattice parameters (a = 8.273 Å).