Preparation of γ-Fe2O3/ZnFe2O4 nanoparticles by enhancement of surface modification with NaOH

Background During liquid-phase synthesis of γ-Fe2O3 nanoparticles by chemically induced transition in FeCl2 solution, enhancement of surface modification by adding ZnCl2 was attempted by using NaOH. By using transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectrometry, and vibrating sample magnetometry, the dependence of the synthesis on the amount of additional NaOH was studied. Results The experimental results show that the surface of the γ-Fe2O3 nanoparticles could be modified by adding ZnCl2 to form composite nanoparticles with γ-Fe2O3/ZnFe2O4 ferrite core coated with Zn(OH)2 and adsorbed FeCl3, and that modification could be enhanced by adding NaOH. Conclusions In the experimental conditions, when the concentration of additional NaOH was below 0.70 M, the amounts of ZnFe2O4 and Zn(OH)2 phases increased slightly and that of adsorbed FeCl3 was unchanged. When the concentration of NaOH exceeded 0.70 M, the amount of FeCl3, ZnFe2O4, and Zn(OH)2 increased.


Introduction
Nanoparticles are typically defined as solids that are less than 100 nm in all three dimensions. Many physical phenomena in both organic and inorganic materials have natural length scales between 1 and 100 nm (10 2 to 10 7 atoms) [1,2]. A nanocomposite is a material composed of two or more phases with at least one phase with nanometer dimensions. Due to combination of different physical or chemical properties, composite nanoparticles may lead to completely novel materials [3]. For example, the type and geometric arrangement of surface coating on a magnetic core determine the overall size of a nanocomposite colloid and play a significant role in its biological fate in biomedical applications [4].
Magnetic nanoparticles are an important class of functional materials that have attracted increasing interest in terms of their science and their technological applications [5]. Studies on ferromagnetic (FM)-antiferromagnetic (AFM) exchange interactions in systems of fine composite particle have led to interesting applications in improving the performance of permanent magnetic materials or in exceeding the superparamagnetic limit in magnetic recording media [6]. Studies on magnetic nanoparticles have focused on the development of novel technologies for their synthesis. Liquid-phase synthesis is still one of the most commonly used methods to obtain inorganic nanoparticles [7]. Many studies have shown that surface modification, which provides additional functionality to nanoparticles, is easily accomplished after or during synthesis [1]. Recently, we proposed a new method to produce magnetic nanoparticles. This method involves chemically induced transition in which γ-Fe 2 O 3 -based nanoparticles are prepared by processing a precursor based on iron oxide hydroxide and/or metal hydroxide in FeCl 2 solution [8][9][10]. In this method, γ-Fe 2 O 3 nanoparticles can be prepared by using an amorphous coating of FeOOH and Mg(OH) 2 , as described in the following equation [10]: In another study, surface modification of the particles was undertaken during synthesis by adding ZnCl 2 to the FeCl 2 solution to prepare γ-Fe 2 O 3 /ZnFe 2 O 4 composite nanoparticles [11]. Experimental results show that when the concentration of ZnCl 2 in solution did not exceed 2 M (50 mL), γ-Fe 2 O 3 /ZnFe 2 O 4 bioxide nanoparticles coated with FeCl 3 · 6H 2 O could be prepared. Generally, alkaline solution could assist the precipitation reaction. In the present work, we attempted to enhance the surface modification by adding NaOH to the processing solution. The morphology, crystal structure, surface and bulk chemical composition, and magnetization of the as-prepared products were characterized. The structure of the particles was proposed and the role of NaOH was revealed.

Preparation
Preparation of the nanoparticles could be divided into two steps. First, the precursor based on amorphous FeOOH and Mg(OH) 2 was synthesized by coprecipitation of FeCl 3 and Mg(NO 3 ) 2 , as described in detail elsewhere [10]. In the second step, the precursor was added to 400 mL of 0.25 M FeCl 2 solution, and the resulting mixture was heated to boiling for 20 min. Afterward, a mixture of 50 mL of 1 M ZnCl 2 solution and 20 mL NaOH solution at a specific concentration was added to the boiling FeCl 2 solution, and the resulting mixture was boiled continuously for 10 min. Subsequently, the mixture was allowed to cool to room temperature, and the as-prepared particles were allowed to settle. The NaOH concentrations used for the preparation were 0.35, 0.70, 1.40, and 2.10 M, corresponding to the as-prepared samples (1), (2), (3), and (4), respectively. For comparison, modified particles were prepared without adding NaOH (sample (0)).

Characterization
The morphology of the particles in the samples was observed by transmission electron microscopy (TEM, Philips Tecnai 10), and their crystal structure was analyzed by X-ray diffraction (XRD, XD-2). The chemical species were measured using X-ray photoelectron spectroscopy (XPS, XSAM 800), and energy-dispersive X-ray spectroscopy (EDX, Genesis) equipped in scanning electron microscopy (SEM, Quanta-200). The magnetization was measured by using a vibrating sample magnetometer (VSM, HH-15).

Results and analysis
TEM images of the samples are shown in Figure 1. It can be seen that sample (0) consisted of nearly spherical nanoparticles. Statistical analysis showed that the size of the particles fit a log-normal distribution, with the median diameter d g about 9.78 nm and the standard deviation lnσ g = 0.28. Samples (1) -(4) images indicated that these samples consisted of irregular flake particles and nearly spherical particles, whose sizes are clearly larger than size of sample (0). This shows that NaOH can stimulate aggregation of the initial particles to grow into larger particles via oriented attachment [12]. XRD patterns ( Figure 2) reveal that the samples contained mainly γ-Fe 2 O 3 and traces of ZnFe 2 O 4 and Zn(OH) 2 , but no ZnCl 2 .
XPS measurements show that O, Fe, Zn, and Cl but no Mg and Na species were present in the samples, as illustrated in Figure 3. Therefore, the samples consisted of Fe 2 O 3 , ZnFe 2 O 4 , Zn(OH) 2 , and FeCl 3 . Binding energy data are listed in Table 1. As a comparison, the binding energy data of ZnCl 2 [13] are listed also in Table 1. Obviously, the data of measured deviate from data of ZnCl 2 . Quantitative analysis, whose relative error are less that 1 %, shows that relative to sample (0) this ratio of Fe to Cl keeps almost invariant for low NaOH concentrations (up to 0.70 M)samples (1) and (2), but it increase with NaOH concentrations above that levelsample (3) and (4). Alternatively, to use the same criteria used for the ratio of Fe to Zn of samples, (1) ≈ (2) < (3) < (4). Complete data are listed in Table 2.
The results of EDX measurements of all samples confirm the same chemical species as those detected by XPS. Results of quantitative analysis, whose relative error are less in 2 %, are listed in Table 3. Evidently, the Fe:Cl and Fe:Zn ratios show the same trends for the samples as those observed with XPS. Figure 4 shows the specific magnetization curves of the samples. All samples showed distinct FM behavior. Their specific saturation values σ s were deduced by plotting σ vs. 1/H in the high-field region [14]. These values were found to be 52.29, 53.63, 58.29, 55.09, and 47.15 emu/g for samples (0), (1), (2), (3), and (4), respectively. Magnetization of samples (0) and (1) were nearly the same, that of sample (2) was the strongest, and the magnetization weakened gradually from samples (2) to sample (4).

Discussion
The experimental results and analysis above indicate that all of the samples were composed of γ-Fe 2 O 3 , ZnFe 2 O 4 , Zn(OH) 2 and FeCl 3 , and no ZnCl 2 . The experimental results show that the ratios of Fe to Cl and Fe to Zn obtained by XPS were less than those obtained by EDX spectrometry, and the ratio of Cl to Zn obtained by XPS agree with that obtained by EDX spectrometry (see Tables 2 and 3). Since the EDX spectrometry measurements are acquired at micrometer depths whereas XPS data are obtained from the surface layer of nanometer thickness [15,16], the experimental results suggest that the core of the particle is essentially γ-Fe 2 O 3 and the coating layers are Zn and Cl based.
XRD results show the presence of ZnFe 2 O 4 and Zn(OH) 2 phases in addition to the γ-Fe 2 O 3 phase, but no clear FeCl 3 crystal phase. These results therefore suggest that the layer of adsorbed FeCl 3 was very thin and amorphous. Therefore, the structure of the composite nanoparticles consisted of four parts, namely, a γ-Fe 2 O 3 core, external shells of ZnFe 2 O 4 and Zn(OH) 2 , and an outermost layer of adsorbed FeCl 3 . A schematic model of this structure is shown in Figure 5. Accordingly, the formation of the composite nanoparticles can be described as follows.
When the FeOOH/Mg(OH) 2 precursor was thermally treated with FeCl 2 solution, Mg(OH) 2 was dissolved, an amorphous FeOOH species was transformed into γ-Fe 2 O 3 crystallites, and Fe 2+ was oxidized to Fe 3+ . While adding ZnCl 2 to the solution, a precipitation reaction took place on the γ-Fe 2 O 3 crystallites, which can be described as follows: If y/x > 1/2, then Thus, ZnFe 2 O 4 grew epitaxially on the γ-Fe 2 O 3 crystallites and some Zn(OH) 2 outside of the ZnFe 2 O 4 layer was preserved. Clearly, additional NaOH enhanced the reaction so that x and y increased with increasing NaOH content. In addition, Fe 3+ and Cl − in the liquid phase were adsorbed and were subsequently converted to composite nanoparticles coated with FeCl 3 . Experimental results show that with increasing NaOH content, the amount of phases based on Zn increased. When the NaOH concentration was lower than 0.70 M, the FeCl 3 phase was nearly unchanged since the Fe: Cl ratio is almost invariant (Table 2); thus, only when the NaOH concentration exceeded 0.70 M did the amount of FeCl 3 increase clearly with NaOH concentration due to increasing amount of Cl. This means that the value of x in equation (2) increased with NaOH concentration at low NaOH concentrations, i.e., the amount of ZnFe 2 O 4 phase increased clearly with NaOH concentration when the NaOH concentration did not exceed 0.70 M, but it increased slightly with NaOH concentration when the NaOH concentration was > 0.70 M. As the results in Table 2 show a consistent increase of the Zn: Fe ratio with increasing NaOH concentration, it can be judged that Zn(OH) 2 increased slightly under low NaOH concentration (<0.70 M) and did clearly under high NaOH consentration (>0.70 M). According to the relation between the specific magnetization and NaOH content, the action of additional NaOH can be discussed further as follows.
The data are taken from Ref. [17]. The data are taken from Ref. [13].  As a consequence, when ∑ m i increased at low amounts of NaOH, Δϕ Cl was negative since m Cl was unchanged, i.e., −Δϕ Cl > 0; thus, equation (6) can be written as Equation (7) shows that increment of the mass fraction of the ZnFe 2 O 4 and Zn(OH) 2 phases lowered the value of σ, whereas a decrement in the mass fraction of the FeCl 3 phase increased it. For sample (1), x and y in the precipitation reaction described by equation (2) increased slightly compared with those for sample (0); hence, Δϕ Zn-Fe , Δϕ Zn , and Δϕ Cl were very small compared with their counterparts for sample (0). Therefore, σ of sample (1) was about the same as that of sample (0). For sample (2), x and y in the precipitation reaction increased, but the increment of x could be larger than y. Thus, it can be judged from equation (3) that the increment in molar content x/2 of the ZnFe 2 O 4 phase would be larger than that of the Zn(OH) 2 phase (y − x/2), i.e., Δϕ Zn-Fe > Δϕ Zn . Since σ γ-Fe − σ Zn-Fe < σ γ-Fe − σ Zn (≈ σ γ-Fe − σ Cl ) and |Δϕ Cl | is proportional to Δϕ γ-Fe + Δϕ Zn , |Δϕ Cl | (σ γ-Fe − σ Cl ) > Δϕ Zn-Fe (σ γ-Fe − σ Zn-Fe ) + Δϕ Zn (σ γ-Fe − σ Zn ). Consequently, the σ value of sample (2) was greater than those for samples (0) and (1).
When the concentration of additional NaOH exceeded 0.70 M, the amount of FeCl 3 phase increased with the NaOH content. Therefore, the variation of σ should be described as follows: Therefore, the σ weakened in the order of samples (2) to (4 Figure 5 Schematic diagram of the particle structure. d p is the diameter of the composite particle, d γ is the diameter of the γ-Fe 2 O 3 core, and d x is the depth detected by XPS.