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.
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 = s∙ρ, where s 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, ϕv = 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.
2. Experimental
The γ-Fe2O3/Ni2O3 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 FeCl3 (40 ml, 1 M) and Ni(NO3)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 FeCl2 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 HNO3 solution (≤0.01 M), the as-prepared particles (the untreated particles) were added to boiling Fe(NO3)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.
3. 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 s vs. 1/H in high field [13] the specific saturation magnetizations ss 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. Figure 2 is a typical TEM micrograph of the particles. Statistical analysis reveals that the size of the particles fits a log-normal distribution. For the untreated particles, the median diameter
dg is 10.78 nm and the standard deviation lnsg is 0.27; for the treated particles, dg is 11.43 nm and lnsg is 0.26. The average diameter is calculated from = exp [lndg + 0.5ln 2s 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 ai 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-Fe2O3 (magnetite, PDF# 39-1346), with some weaker peaks corresponding to Ni2O3 (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-Fe2O3 and Ni2O3. However, by comparing with the pattern of untreated particles, it was found that the diffraction intensity of Ni2O3 had been weakened relative to that of γ-Fe2O3. 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(NO3)3·9H2O (PDF#01-0124).
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