Synthesis and Characterization of Bifunctional α-Fe2O3-Ag Nanoparticles

1Centro de Fı́sisca Aplicada y Tecnoloǵıa Avanzada, Universidad Nacional Autónoma de México, Boulevard Juriquilla 3001, 76230 Santiago de Querétaro, QRO, Mexico 2Instituto de Ciencias Biomédicas, Universidad Autónoma de Ciudad Juárez, Envolvente del PRONAF y Estocolmo s/n, 32300 Ciudad Juárez, CHIH, Mexico 3Instituto de Investigaciones Metalúrgicas, UMSNH, Edificio U, Ciudad Universitaria, 58000 Morelia, MICH, Mexico


Introduction
Bimetallic nanoparticles with dumbbell-like heterostructures have recently attracted increasing attention on these nanoparticles exhibiting different physical and chemical properties and an interesting interfacial interaction between two coexisting materials. This property offers potential applications in the fields of physics, biotechnology, chemistry, materials science, biology, and catalysis [1][2][3]. In particular, the bimetallic -Fe 2 O 3 -Ag dumbbell-like nanoparticles have enhanced physical-chemical properties, since the Ag nanoparticles present an excellent catalytic activity, antibacterial effect, and a significant biocompatibility [4][5][6][7]. On the other hand, -Fe 2 O 3 possess magnetic properties. In this sense, hybrid -Fe 2 O 3 -Ag nanoparticles represent a nanostructure suitable for drug delivery, phototherapy heterogeneous catalysis, and biosensors among others [8][9][10][11]. Consequently, research of new synthesis routes for bimetallic nanoparticles is fundamental in the development of nanotechnology [12][13][14]. Under this perspective, the possibility to synthesize new nanostructures by mixing noble metals and magnetic nanoparticles offers a wide range of future applications. For this reason, the aim of this research is to present a novel and simplified synthesis route to obtain -Fe 2 O 3 -Ag dumbbell-like nanostructures. This procedure is based on the modification of the chemical reduction method [15], which is carried out at room temperature and the pH of the aqueous solution is not considered. Factors such as the quantities of metallic precursors and the amount of surfactant agent used in the synthesis process are important for determining the growth of the -Fe 2 O 3 -Ag bimetallic nanoparticles [10,16]. In order to characterize the dumbbell or heterodimers nanostructures, techniques of transmission electron microscopy (TEM), Raman spectroscopy (RMN), and cyclic voltammetry (CV) were employed.

Materials and Methods
The -Fe 2 O 3 -Ag dumbbell nanoparticles were produced using a modification of the chemical reduction method [15].  This kind of structures presents a typical configuration which consists of a nanoparticle with known composition and size and which is attached to another particle whose composition is different and its size is generally smaller. In order to probe that two different elements are present in this heterodimers nanostructures, Figure 1(b) reveals the chemical composition of the bimetallic structures obtained by EDS technique. Figure 1(c) present a -contrast image obtained by high angle angular dark field (HAADF), which illustrates that the particles with smaller size and higher contrast are associated with the Ag, due to the fact that this element has a higher atomic number ( ) in relation to the iron. Ag nanoparticles show higher contrasts region (lighter). Consequently, darker region corresponds to nanoparticle with low (Fe). In this sense, Figure 1(d) shows a bright field TEM image of an individual nanostructure where Ag nanoparticles with higher exhibit darker contrast. Also, as will be discussed later by highresolution transmission electron microscopy HRTEM technique and Raman spectroscopy, the nature of the Fe structure present in the heterodimer will be determined.

Results and Discussion
To verify the Fe nanostructure present in the HRTEM analysis that was carried out, Figure 2 (1 11), (111), and (020) of Ag, respectively (JCPDF #04-0783). Additionally, an interplanar distance of 0.189 nm associated with the -Fe 2 O 3 (JCPDF #33-0664) was observed. This result suggests that the Fe nanoparticles were oxidized. Some reports indicate that the iron nanoparticles can be oxidized by many factors as pH of aqueous solution and oxidative etching among others [10,16]. In particular, in this synthesis process, the pH of the aqueous solution was constant; therefore, it can be deduced that oxidative species such as oxygen, Fe(III) in solution, and corrosive ions as chlorides present during the chemical synthesis favor the oxidation process.
Under this perspective, it is important to establish the structure and crystallinity of the nanoparticles. Ag and -Fe 2 O 3 -Ag nanoparticles have a cubic FCC structures. In this sense, Figure 3(a) describes a simulation model of the dumbbell-like nanostructure. The Ag nanoparticle modelling (Inodecahedron) has an average size of 7 nm according to the experimental Ag nanoparticle observed by TEM and high angle annular dark field (HAADF) images, while -Fe 2 O 3 -Ag structures show an average diameter of 20 nm. In order to verify the structures observed by HRTEM images, a simulated TEM image was constructed from the proposed model (Figure 3(b)). The method to obtain the theoretical images (HRTEM) is based on the multislice approach of electron diffraction dynamical theory [17]. This simulated image presents a close similarity to the experimental HRTEM image. Hence, this model describes accurately the -Fe 2 O 3 -Ag heterodimers formation. The axis zone used to the TEM image simulation was [201].  [17][18][19]. For this reason, it is important to accurately set the kind of iron oxide observed. Some reports indicate that by XRD technique it is possible to establish only the structural differences between two iron oxides [20][21][22]. In this regard, Raman spectroscopy can provide clear assignment of the iron oxide polymorphs through the observation of the phonon modes. The iron oxides species exhibit distinct Raman signatures; therefore, it is possible to take advantage  of the so-called phonon confinement effects to confirm the oxide present in the sample [23]. In order to demonstrate accurately the type of oxide present in the system, a Raman spectrum of dumbbell nanoparticles is presented in Figure 4. In these Raman spectra, typical intensities from the hematite structure located at 221 cm −1 (A1g), 241 cm −1 (Eg), 295 cm −1 (Eg), and 504 (A1g) are observed. However, two bands situated at 365 and 551 cm −1 are attributed to maghemite and magnetite, respectively [23]. These bands account for only minority phases, with hematite being the predominant phase, thus confirming the results obtained by HRTEM. Complementarily, the presence of the Ag nanoparticles is demonstrated by the bands situated at 1342 and 1645 cm −1 [20,21].
The formation of this kind of nanostructures can be discussed from the point of view of the solubility, since in general terms these elements are immiscible in the solid state, promoting the dumbbell configuration. In addition, the nucleation rate takes preponderant role in the formation of the Fe 2 O 3 -Ag system because Fe nanoparticles, previously formed, offer nucleation sites for forming Ag nanoparticles through a heterogeneous nucleation. Thermodynamically, Ag solution atoms nucleate in regions where the energy barrier is lower, in this case in the -Fe 2 O 3 nanoparticles. To expand the discussion regarding the factors that involved the growing of these nanostructures, the surface charges associated with the active sites of the nanoparticles should be considered [18]. Based on the fact that the particle size and structure type determine the compensation charge in the nanoparticles surface, it can be deduced that mainly the size of decahedral Ag nanoparticles would influence the charge compensation and hence the formation of Fe 2 O 3 heterostructures [24], due the fact that the decahedral nanoparticles exhibit greater reactivity. In this regard, some studies have reported these structures as a function of the -Fe 2 O 3 size/size Ag seed ratio [22,23]. It is important to highlight the synthesis methodology presented in this research since it offers a simple route for obtaining controlled bimetallic nanostructures. This synthesis is carried out at room temperature while some other methodologies involve temperature (∼400 ∘ C) [22,[24][25][26].
In order to establish a reference parameter regarding the electrochemical behavior of the bimetallic system ( -Fe 2 O 3 -Ag) and Ag nanoparticles, Figure 5 shows the cyclic voltammogram corresponding to the bimetallic and monometallic nanoparticles. It can be seen that in the case of bimetallic nanoparticles there is an increase in the current density of the redox processes involved in comparison with the monometallic nanoparticles. Additionally, -Fe 2 O 3 -Ag heterodimers nanostructures present a displacement in the values associated with the potential formation and reduction of metal oxides. Based on this behavior, it can be deduced that the two systems have different electrochemical characteristics. It is important to note that the presence of -Fe 2 O 3 increases the region where monolayers of oxides are formed. It has been reported that the position of hydrogen adsorption/desorption peak and the oxygen reduction peak is proportional to the nanoparticle size [27]. Nevertheless, the synthesis of monometallic (Ag) and bimetallic ( -Fe 2 O 3 -Ag) nanoparticles was carried out analogously under the same conditions, such that very similar particle sizes were obtained. In this  sense, the electrochemical comparison of both systems is considerably accurate. Hence, through this technique, it is possible to determine the electrochemical behavior of the monometallic (Ag) nanoparticles and the influence of hematite in the processes of oxidation and reduction.

Conclusions
The synthesis of -Fe 2 O 3 -Ag dumbbell-like nanostructures or heterodimers was performed using a novel and simplified route, in which factors as temperature and pH of the aqueous solution were constant. It was observed that the formation of these nanostructures is favored by factors such as compensation charges generated by Ag and -Fe 2 O 3 particles, which are derived from the structure type and the active sites of these entities. Moreover, structural characterization of the bimetallic system was carried out mainly by TEM techniques, and corroborated by modeling and simulation of HRTEM images. However, the different species of iron oxides have a large number of similarities as lattice parameters,spacings, and chemical composition. Thus, an analysis by Raman spectroscopy was conducted, in order to specify the type of iron oxide present in the bimetallic system. This study confirms that the majority phase of this system is hematite. Nevertheless, some traces of maghemite and magnetite were identified. On the other hand, the electrochemical characterization reveals that this kind of bimetallic nanostructure offers potential catalytic applications, given their particular behavior observed in the studies of cyclic voltammetry. It should be noted that the oxidation and reduction potentials are very well defined and hence a significant catalytic activity could be expected.