Effect of the Surfactant on the Growth and Oxidation of Iron Nanoparticles

Fe nanoparticles and branched nanostructures of iron oxide were synthesized by chemical reduction in aqueous phase. The mechanism of formation of iron oxides as a function of the amount of surfactant employed during the synthesis process was studied. Specifically Fe, Fe 2 O 3 , and Fe 3 O 4 nanoparticles were obtained. The oxidation of Fe to Fe 3 O 4 and finally to Fe 2 O 3 was carried out by oxidative etching process, decreasing the amount of stabilizer agent. The structures obtained were characterized by high resolution (HRTEM) and scanning/transmission (STEM) electron microcopies, energy dispersive spectroscopy (EDS), and optical spectroscopy (UV-Vis and IR).


Introduction
In recent years iron oxide nanoparticles have been extensively studied, due to their wide applications in biomedical treatments, magnetic fluids, groundwater remediation, hazardous waste treatment, catalytic degradation, microwave adsorption, medical diagnostic, and magnetic recording [1][2][3][4][5]. These studies have also been induced by their superparamagnetic and catalytic properties [6,7]. Several synthesis routes to obtain Fe 3 O 4 magnetic nanoparticles have been reported including the chemical reduction, coprecipitation, hydrothermal process, and polyol and sol-gel methods [8][9][10][11]. However, the chemical reduction method offers an effective route to obtain very well-defined homogeneous nanoparticles sizes and morphologies. For technological applications, it is important to develop nanoparticle dispersions without aggregation or coalescence [12]. During colloidal synthesis, the nanoparticles need to be passivized to prevent coalescence. A lesser amount of surfactant than the optimum, where the surfactant does not perfectly cover the entire surface of the particles, can cause coalescence of the particles or uncontrolled increasing in particle size [13]. Specifically, in this study, Fe, Fe 2 O 3 , and Fe 3 O 4 nanoparticles were synthesized by chemical reduction. Fe(III) chloride hexahydrate (FeCl 3 ⋅6H 2 O) was employed as starting metallic salt and polyvinyl pyrrolidone (PVP) as surfactant agent. As well documented in the literature, stabilizer agent determines the size and shape of the iron nanostructures [6,14]. The oxidation degree and morphology obtained in this process are described and discussed in terms of the amount of the stabilizing agent present in the reaction.

Materials and Methods
The synthesis of iron nanostructures was carried out by the chloride method modified [15]. In this case, FeCl 3 ⋅6H 2 O as precursor and polyvinyl pyrrolidone (PVP) as surfactant were used. For the reduction of the metallic salt, sodium borohydride (NaBH 4 ) was employed. All reagents were provided by Sigma Aldrich with a purity of 99.98%. Three samples of nanoparticles were synthesized by the same synthesis route. In the first two samples (S1 and S2), equal concentrations of ferric chloride (0.01 M) were used, while in the third sample (S3) a concentration of 0.08 M was used.  The variable parameter in the process of synthesis was the concentration of stabilizing agent, for which concentrations of 1.0, 0.5, and 0.1 M were added to S1, S2, and S3 samples, respectively. The reaction was carried out at room temperature and a constant pH of the colloidal medium of 11. Table 1 specifies the synthesis conditions employed by the produced Fe nanostructures. This method is based on the Fe 3+ ions reduction using sodium borohydride (NaBH 4 ) as reducing agent according to the following reaction [15]: In general form, this reaction is conducted in two phases, initially by the (NaBH 4 ) decomposition generating hydrogen. Subsequently, hydrogen is responsible for the reduction of Fe 3+ to Fe 0 [15]. The structural and morphological characteristics of the dispersed metallic nanoparticles have been studied using a Philips Tecnai F20 transmission electron microscope (TEM) with a field emission gun attachment and dot to dot direct maximum resolution of 0.23 nm. TEM specimens were prepared by dispersing and subsequent drying of a drop of colloidal solution on a copper grid (3 mm in diameter) covered with an amorphous carbon film. Energy dispersive (EDS), X-ray diffraction (XRD), ultraviolet visible (UV-Vis), and infrared (IR) studies were also conducted.

Transmission Electron Microscopy.
Transmission electron microscopy (TEM) was used to observe the size and morphology of the nanoparticles. Figure 1(a) shows a bright field TEM image at low magnification from the Fe nanoparticles stabilized with 1.0 M PVP (sample S1). The morphology of the nanoparticles is spherical and the formation of agglomerates is not appreciated. This result can be attributed to the higher amount of stabilizing agent used in this sample and consequently a less interaction between the Fe surface area and the colloidal medium. The large amount of PVP used prevented the agglomeration and the oxidation of the particles. Figure 1(b) shows the chemical analysis by energy dispersive spectroscopy (EDS) from the sample S1. As can be observed in the spectrum, only the Fe signal was obtained (the C and Cu are located in the grid). Figure 1(c) shows a single Fe nanoparticle, the average particle size is 12 nm, and nonjoined particles are appreciable. To analyze the structure of the Fe nanoparticle, HREM images were performed. Figure 1 Decreasing the amount of PVP by 50 percent during second synthesis (sample S2), the oxidation of Fe nanoparticles is induced. It should be noted that oxidation of Fe nanoparticles is evident, due to the interaction with the environment generating Fe 3 O 4 particles. Figure 2 [16]. Also, it has been reported in the literature that the oxidation process is carried out primarily on the corners and edges of the nanocrystals giving rise to high reactivity regions. However, an isotropic growing takes place over the entire surface of nanocrystals where the more active sites have been oxidized. Indeed this situation depends on the surface chemical nanocrystals [6,16,17]. HRTEM image of a tip of the Fe 2 O 3 nanostructures is presented in Figure 3 It is important to mention that, in addition to the use of (NaBH 4 ) as reducing agent, this synthesis route involves oxidative species such as oxygen, Fe(III) in solution, Journal of Nanomaterials and corrosive ions as chlorides; this species promotes the oxidation process, specifically, the oxidative etching. This phenomenon is common in nature, which can be explained analogously to the corrosion of steel by air and/or water [6]. Accordingly it can be assumed that the synthetized iron particles experienced an oxidation of this nature. This is because, in this route of synthesis, the pH solution is constant and only the amount of PVP used in the synthesis process determines the formation of Fe nanostructures. This is due to the fact that the surfactant agent modifies the iron nanoparticles surface and increases their active surface sites. Therefore, the PVP amount employed in the synthesis procedure is essential in the nucleation and growth process of Fe nanostructures. It can be deduced that the Fe particles obtained from the reduction of Fe +3 by NaBH 4 can be considered as growth cores of the iron oxides. In this sense, Fe particles were synthesized with the largest amount of PVP and therefore they were better stabilized. This fact indicates that Fe surface is less susceptible to oxidative etching.
However it is important to highlight and confirm the differences in the iron oxide phases (Fe 3 O 4 and Fe 2 O 3 ) corresponding to the S2 and S3 samples. Due to the fact that -spacings corresponding to Fe 3 O 4 and Fe 3 O 2 are very similar, complementariness technique was conducted.  [18,19]. This result highlights the presence of both iron oxide phases observed by TEM. Figures 5(a) and 5(b) show the X-ray diffraction pattern of the S2 and S3 samples, respectively. In this sense, both patterns are very similar. This fact is attributed to the magnetite and maghemite presents a cubic structure and (1 1 1) lattice parameters are very close; for this reason, it is difficult to differentiate these structures even if both phases exhibit high crystallinity. However, some authors report that in the XRD pattern associated with the maghemite phase there exist two additional peaks located at 23.77 ∘ (210) and 26.10 ∘ (211). These intensities can be used to differentiate the magnetite phase [20][21][22]. In Figure 5(b) both intensities are clearly visible and therefore an oxidation is confirmed on the surface of the nanoparticles attributed to the decrease of the amount of stabilizing agent. XRD analysis supports and further confirms the presence of magnetite and maghemite, promoted by oxidative etching.
In order to evidence the absorption characteristics of the Fe, Fe 3 O 4 , and Fe 2 O 3 structures, which were completely identified and verified by TEM, EDS, and XRD techniques, a spectroscopic study of samples is presented below. Figure 6(a) shows the UV-Vis spectra corresponding to the synthesized samples with different amounts of PVP (1.0, 0.5, and 0.1 M). It has been reported that in the nanoparticles of iron oxides almost no absorption is beyond 700 nm [18]. Some authors indicate that as the degree of oxidation increases in the Fe particles, the diffuse reflectance is greater [23]. For this reason it is clearly observed that synthesized samples with lesser amounts of PVP (0.01 M) have less absorbance; this is due to its high degree of oxidation particles. These results confirm the presence of nanoparticles Fe, Fe 3 O 4 , and Fe 2 O 3 as a function of the amount of surfactant in each synthesis. Additionally it is observed that the Fe nanoparticles are better stabilized; therefore the degree of oxidation is not perceived and the absorbance is greater. In the case of Fe 3 O 4 and Fe 2 O 3 nanoparticles the oxidation process is evident, due to the formation of maghemite (Fe 2 O 3 ) as a result of oxidation of the magnetite (Fe 3 O 4 ) [6,24]. The decrease in the amount of PVP promoted the oxidation process of Fe nanoparticles and consequently an increase in the particle size of iron oxides nanoparticles. This result confirms the morphologies and particle sizes observed by transmission electron microscopy. Therefore, it is observed that the particles with a greater degree of oxidation have a greater particle size. This fact is attributed to the oxidation process that is carried out on the surface of the nanoparticles. In this sense, the particles with larger sizes promote the sedimentation of the samples that are in aqueous solution. Therefore the absorbance of these particles is reduced. Figure 6(b) shows the IR-spectrum of the nanoparticles of the S1, S2, and S3 samples. In the sample S1 (Fe nanoparticles), Fe-O bonds are not observed. In the S2 and S3 samples, two main absorption bands are observed. The band located at 668 cm −1 is associated with stretching vibration existing between Fe-O bonds [18][19][20][21]. The band located at 1633 cm −1 is assigned to O-H vibrations absorbed on the surface of Fe nanostructures [20,21]. S2 shows a specific intensity at 684 cm −1 , which confirms the presence of magnetite [25]. A characteristic absorption peak located at 793 cm −1 can be attributed to the maghemite structure [26]. However, in the  IR spectrum of the sample S3, this intensity is dissociated into two bands situated at 771 and 822 cm −1 [27]. This band splitting can be explained by the effect of finite size of Fe 2 O 3 nanoparticles, which generate several breaks in the atomic bonds of maghemite surface, producing an electrons rearrangement on the particle surface, and consequently a split of the energy levels of the maghemite [28]. On the other hand, a less-ordered maghemite (Fe 2 O 3 ) structure can be identified by multiple lattice absorption bands within 800 cm −1 [26].

UV-Vis and Infrared Spectroscopy.
In the IR spectrum corresponding to the S3 sample, this behavior can be observed. Generally, maghemite structures partially ordered are associated with the cubic system, due to the vacancies in the maghemite structure [27,28]. In this sense, the maghemite and magnetite show a spinel-like FCC structure with a network of oxygen that presents tetrahedral and octahedral coordination. However, distinctive element of maghemite is the presence of iron vacancies in the sublattice; the maghemite structure presents a vacancy of 2.67 cationic iron atoms located in octahedral sites [29]. The cation or vacancy distribution in octahedral positions can give rise to several crystal symmetries in maghemite which were identified in the XRD patterns. The magnetite oxidation to maghemite is described by the following equation [29]: where represents the tetrahedral site, is octahedral site, and ◻ is vacancy. The IR spectroscopy analysis confirms the oxidation carried out on the surface of the particles synthesized with lower amounts of stabilizer PVP. Consequently, it is clear that the surfactant acts on the surface of the particles, covering them and preventing the oxidization due to contact with the

Conclusions
Fe, Fe 3 O 4 , and Fe 2 O 3 nanostructures were synthesized by a chemical reduction method. Fe nanoparticles are highly reactive due to the large presented surface area. According to the obtained results, the amount of surfactant agent provides an alternative for controlling the oxidation of the nanoparticles. A greater amount of surfactant agent provides more stability and homogeneous particles size as in the case of Fe 3 O 4 particles. Different degrees of oxidation were observed as a function of the amount of stabilizing agent used during Journal of Nanomaterials 7 the synthesis process. Evidently, this fact is attributed to a greater interaction between the nanostructures and the environment. The growth of branched nanostructures is due to the oxidation carried out first on the corners and edges of iron crystals because these sites show the greatest reactivity. The isotropic growth of these structures is performed on the entire surface of the crystal, in this case by the interaction of oxygen from the environment. This fact was proved by the oxidative etching process. As a result, in this novel synthesis route, the pH solution is constant and only the amount of PVP used in the synthesis process is the variable that determines the formation of Fe nanostructures.