The Structure of Modified Fe-Ni Bioxide Composite Nanoparticles Using Fe(NO3)3

Composite nanoparticles containing a γ-Fe2O3 core, Ni2O3 external shell and FeCl3·6H2O outermost layer can be synthesized by chemically induced transition in FeCl2 solution. These may be modified by treatment with Fe(NO3)3 to obtain particles for the preparation of ionic ferrofluids. Vibrating sample magnetometer (VSM) measurements and transmission electron microscopy (TEM) observations show that after Fe(NO3)3 treatment, the specific magnetization becomes weaker and the size becomes larger for treated particles compared with the untreated particles. Using energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), the structure of the particles before and after the treatment is revealed. The experimental results show that the γ-Fe2O3 core is unchanged, the Ni2O3 is dissolved partially and the FeCl3·6H2O is replaced by Fe(NO3)3·9H2O. The percentages of molar, mass and volume of these phases are deduced, and the average density of the modified particles is also estimated.


Introduction
Magnetic nanoparticles are an important class of nanoscopic magnetic systems.The preparation and characterization of magnetic nanoparticles have attracted increasing interest as particles in this size range may allow investigation of fundamental aspects of magnetic-ordering phenomena in magnetic materials with reduced dimension and could lead to new technological applications [1,2].A nanocomposite is a material composed of two or more phases in which the combination of different physical or chemical properties may lead to completely novel materials [3].In addition, magnetic nanocomposites have applications that range from ferrofluids to separation science and technology [4].
Ferrofluids (magnetic fluids) can be synthesized by dispersing nanosized subdomain magnetic particles of ~10 nm in diameter in a carrier liquid.Generally, magnetic nanoparticles in ferrofluids are coated with a surfactant to prevent aggregation [5].In the 1980s, Massart proposed a method for chemical synthesis of ferrofluids with no surfactant [6].This method has subsequently been called the Massart's method and such ferrofluids are known as ionic ferrofluids or electrical double-layered ferrofluids [7,8].In an adaptation of Massart's method for the synthesis of the ferrofluids, magnetic nanoparticles need to be treated with ferric nitrate to yield a chemically stable particle surface.
Magnetization (moment per unit volume) M is an important physical parameter used to characterize magnetic materials.In practice, the volume of a particle Vp is too difficult to measure directly, so the magnetization is obtained usually from M =•ρ, where  is the specific magnetization (moment per unit mass) and ρ is the known density of the constituent materials [9].In addition, the volume fraction of particles in ferrofluids, ϕ v = Vp/(Vp + Vc), where Vp is the volume of the particles and Vc is the volume of the carrier liquid, is also an important characteristic parameter to which the behavior of ferrofluids is related [10].Therefore, volume of the particles is an important feature for magnetic nanoparticles and is generally obtained from an accurate measure of the mass m and the known density ρ, i.e.Vp = m/ρ.However, if the nanoparticles consist of different chemical compounds, the density of the particles is no longer uniform, and their volume cannot be determined by measuring just their mass.
Recently, we have proposed a method to prepare Fe-Ni bioxide composite nanoparticles by a chemically induced transition [11].These nanoparticles might be very suitable for the synthesis of ionic ferrofluids [12].In the present work, untreated Fe-Ni bioxide composite nanoparticles and those treated with ferric nitrate have been characterized, and their chemical compositions and average density were studied.

Experimental
The γ-Fe 2 O 3 /Ni 2 O 3 composite nanoparticles were prepared by the so-called chemically induced transition method.The preparation can be divided into two steps.Firstly, an aqueous mixture of FeCl 3 (40 ml, 1 M) and Ni(NO 3 ) 3 (10 ml, 2 M; in HCl 0.05 ml) was added to NaOH solution (500 ml, 0.7 M).Then the solution was heated and boiled for 5 min, after which the black precursor gradually precipitated.Secondly, the precursor was added to FeCl 2 solution (400 ml, 0.25 M), which was then heated to boiling point for 30 min.The nanoparticles were again precipitated after the heating had stopped.After washing to pH = 7 with very dilute aqueous HNO 3 solution (≤0.01 M), the as-prepared particles (the untreated particles) were added to boiling Fe(NO 3 ) 3 solution (400 ml, 0.25 M), which was then kept boiling for 30 min.The particles were then dehydrated with acetone and allowed to dry naturally to yield the modified particles (the treated particles).
The magnetization, morphology, bulk chemical species, crystal structure and surface chemical compositions of the particles were investigated using a vibrating sample magnetometer (VSM, HH-15), transmission electron microscopy (TEM, TecnaiG220), energy dispersive Xray spectroscopy (EDX, Quanta-200), X-ray diffraction (XRD, XD-12) and X-ray photoelectron spectroscopy (XPS, SAM800).The phase structures of the particles were determined from the experimental results.

Results and Analysis
The specific magnetization curves are shown in Figure 1.
Clearly, the magnetization of the treated particles is weaker than that of the untreated particles.From the plots of  vs. 1/H in high field [13] the specific saturation magnetizations  s are evaluated as 56.59 emu/g and 41.19 emu/g for the untreated and treated particles, respectively.
TEM observations show that both untreated and treated particles were spherical.dg is 10.78 nm and the standard deviation ln g is 0.27; for the treated particles, dg is 11.43 nm and ln g is 0.26.The average diameter <d> is calculated from <d> = exp [lndg + 0.5ln 2  g ] [14] and gives.11.18 nm for the untreated particles, and 11.82 nm for the treated particles.
The results of the EDX measurements indicated that both untreated and treated particles had O, Fe and Ni species, but the untreated particles also contained Cl and the treated particles contained N and Cl.The atomic percentages a i for these elements are listed in Table 1.
Figure 3 shows the XRD patterns for the two nanoparticles.For the untreated particles, the major diffraction peaks corresponded to γ-Fe 2 O 3 (magnetite, PDF# 39-1346), with some weaker peaks corresponding to Ni 2 O 3 (PDF#14-0481).No clear diffraction peaks corresponded to any phase based on the Cl species.In addition, for the treated particles, diffraction peaks were present for both γ-Fe 2 O 3 and Ni 2 O 3 .However, by comparing with the pattern of untreated particles, it was found that the diffraction intensity of Ni 2 O 3 had been weakened relative to that of γ-Fe 2 O 3 .In addition, some new diffraction peaks appeared, as indicated by arrows A, B, C and D. These are close to the d=0.2620, 0.2550, 0.1900 and 0.1700 nm peaks of Fe(NO 3 ) 3 •9H 2 O (PDF#01-0124).The XPS spectra are shown in

The Molar Percentages of the Phases in the Particles
The structural data and the quantitative elemental analysis from the EDX measurements provide a description of the molar ratios of the phases for both particles, as follows.
where a Fe , a Ni and a Cl are the atomic percentages of Fe, Ni and Cl, respectively.Likewise, the ratio between the Consequently, the molar percentages of γ-Fe 2 O 3   From the EDX data listed in Table 1, the values of y  , Ni and can be calculated and these are given in Table 3.
where a Fe , a Ni and a N are the atomic percentages of Fe, Ni and N, respectively.Likewise, the ratio between γ-Fe 2 O 3 and Fe(NO 3 ) 3 phases, N y  , can be written as Thus, the molar percentages of γ-Fe 2 O 3   The values of y  , Ni and N can be calculated from the data listed in Table 1, and are also listed in Table 3.Clearly, the y y Ni y  value of the treated particles (91.48/4.17) is larger than that of the untreated particles (91.74/5.63),which confirms the reduction of the Ni 2 O 3 phase after the treatment, since the molar concentrations of γ-Fe 2 O 3 in both untreated and treated particles can be regarded as equal.

The Mass Percentages of the Phases in the Particles
From the molar percentage of each phase, the mass percentages of the phases in both untreated and treated particles, z i , can be deduced from 100 where y i is the molar percentage and A i is molecular weight of the i phase in the particles.Accordingly, the mass percentage of each phase for the two particles can  4.
From the results in Table 4, it can be seen that the mass fractions of both γ-Fe 2 O 3 and Ni 2 O 3 in the treated particles are less than in the untreated particles.Therefore, the specific magnetization of the former is weaker than that of the latter, since γ-Fe 2 O 3 is ferrimagnetic, Ni 2 O 3 is weakly ferromagnetic [16], and both FeCl 3 and Fe(NO 3 ) 3 are paramagnetic.

The Volume Percentages of the Phases in the Particles
From the mass percentage z i , the volume percentage of each phase in the particle system, ϕ i , can be obtained from 100 where ϕ i is the density of the i phase.4, the values of ϕ i can be calculated, and are listed in Table 5.For the treated particles, the volume of the γ-Fe 2 O 3 phase should be same as for the untreated ones.The fact that the volume percentage of γ-Fe 2 O 3 phase for the treated particles is less than that for the untreated particles shows that the size of the former is larger than that of the latter.This is in agreement with the TEM results.

The Density of the Particles
From both mass percentages and densities of each phase, the average density of the particles containing many phases, <ρ>, can be obtained from 1 100 100 Using Formula (9), the average density can be estimated as 4.596 g/cm 3 and 4.102 g/cm 3 for the untreated and treated particles, respectively.For the treated particles, the density 4.102 g/cm 3 is very close to 4.125 g/cm 3 , which was deduced from density measurements of the corresponding ferrofluids [12].
From the average density, the magnetization curves of M(= i ρ i ) vs. H can be obtained from the specific magnetization, as shown in Figure 7.The values of saturation magnetization M s (= s •<ρ>) were obtained from  s as 260.09 and 168.96 kA/m for the untreated and treated particles, respectively.

Conclusion
Using  [17,18].For the composite nanoparticles with many phases, combining bulk with surface analysis using EDX, XRD and XPS reveals their structure and the molar, mass and volume percentages of each phase can be calculated.As a consequence,   the difference of both magnetization and size between the treated and untreated particles can be explained from the phase ratios.In addition, the average density of the particles can be obtained, which is very important for the synthesis of ferrofluids with accurate volume fractions of particles.The experiments indicated that after treatment with Fe(NO 3 ) 3 , although the γ-Fe 2 O 3 core is unchanged, the magnetization of the treated particles becomes much weaker than that of the untreated particles.Clearly, this is evidence showing that the surfaces of magnetic nanoparticles play an important role in determining their magnetization behavior.

Figure 2 Figure 1 .
Figure 1.The specific magnetization curves of (a) untreated particles and (b) treated particles.

Figure 2 .
Figure 2. Typical TEM images of (a) untreated particles and (b) treated particles.

Figure 4 .
These confirm the presence of the same chemical species as determined by the EDX measurements.Combining the EDX and XRD results, it was apparent that besides γ-Fe 2 O 3 and Ni 2 O 3 , there was also a phase based on Cl in the untreated particles and Fe(NO 3 ) 3 •9H 2 O was present in the treated particles.As a result, the O1s and Fe2p3/2 lines are split, as shown in Figure 5.The detailed binding energy data are listed in Table 2. To summarize, these measurements precisely identify γ-Fe 2 O 3 , Ni 2 O 3 and FeCl 3 phases in the untreated particles and γ-Fe 2 O 3 , Ni 2 O 3 and Fe(NO 3 ) 3 •9H 2 O phases in treated particles.In the untreated particles, the FeCl 3 phase could contain
For the untreated particles, based on γ-Fe 2 O 3 , Ni 2 O 3 and FeCl 3 •6H 2 O phases, the molar ratio between the γ-Fe 2 O 3 and Ni 2 O 3 phases, Ni y  , can be written as

Ni2O3Fe 3 TreatmentFigure 6 .
Figure 6.Schematic model of the structures of the particles.

yy
Cl For the treated particles based on γ-Fe 2 O 3 , Ni 2 O 3 and Fe(NO 3 ) 3 •9H 2 O phases, Ni y  , can be written as

Figure 7 .
Figure 7.The magnetization curves of (a) untreated particles and (b) treated particles.

Table 3 . The molar percentage of each phases in the un- treated and treated particles y i .
Table 3 and the molecular weights of γ-Fe 2 O 3 , Ni 2 O 3 , FeCl 3 •6H 2 O or Fe(NO 3 ) 3 •9H 2 O, which are listed in Table The densities of γ-Fe 2 O 3 , Ni 2 O 3 , FeCl 3 •6H 2 O and Fe(NO 3 ) 3 •9H 2 O are 4.90, 5.32, 1.844 and 1.684g/cm 3 , respectively.Thus, from the z i values listed in Table a chemically induced transition in FeCl 2 solution, composite nanoparticles containing a γ-Fe 2 O 3 core, Ni 2 O 3 shell and FeCl 3 •6H 2 O outermost layer, were synthesized.The Fe(NO 3 ) 3 solution treatment after synthesis dissolves FeCl 3 and some of the Ni 2 O 3 to give a new Fe(NO 3 ) 3 •9H 2 O layer on the undissolved part of the nanoparticles.This shows that during the Fe(NO 3 ) 3 treatment, it is easily absorbed on the ferrite particles to form a Fe(NO 3 ) 3 •9H 2 O surface layer, a result similar to Fe 3 O 4 when treated with Fe(NO 3 ) 3

Table 4 . The mass percentage of each phases in the un- treated and treated particles z i .
Fe 2 O 3 Ni 2 O 3 FeCl 3 •6H 2 O Fe(NO 3 ) 3 •9H 2 O i γ-