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Synthesis and magnetic properties of graphene oxide-decorated cobalt, manganese and nickel ferrite nanoparticles prepared by polymerized route

Magnetic nanomaterials with emergent physical and chemical properties have paid much attention, however, their synthesis pathways are still a challenge. In this study, we described a simple method for the synthesis of graphene oxide-decorated magnetic nanoparticles GO@MFe2O4 (M = cobalt, manganese and nickel metal sites) from ferrites MFe2O4 via complex polymerization method and graphene oxide via Hummer’s method. The structure of GO@MFe2O4 were characterized by several physicochemical techniques, such as X-ray Diffraction (XRD) and Scanning electron microscope (SEM), Brunauer–Emmett–Teller (BET), point of zero charge (pHpzc) and Fourier-transform infrared spectroscopy (FTIR). The results indicated a spinel-like structure of GO@MFe2O4 with high porosity and surface area, along with strong saturation magnetization, which can become a potential candidate for various applications.


Introduction
Magnetic nanomaterials (MNPs) possess a wide range of emergent physical and chemical properties in comparison with their precursor materials [1][2][3]. In particular, magnetic ferrite nanocrystals MFe 2 O 4 (M = Co, Mn, Ni) have recently have recently paid much attention in various fields because of their remarkably sized and shape magnetic nanostructures [4][5][6].
Structurally, magnetic properties of ferrite nanocrystals MFe 2 O 4 are mainly attributable the microstructure and cation distribution [7,8]. Generally, in several normal spinels (e.g. MnFe 2 O 4 ), tetrahedral A sites are occupied by divalent cations occupy, while octahedral B sites are occupied by trivalent cations [9]. Especially, both tetrahedral A and octahedral B interstitial sites are likely to be occupied by the cations in cubic spinel MFe 2 O 4 structure [10]. These properties can be driven by a structural control of pore size and specific shape, therefore, making MNPs materials appropriate to potential applications from design of devices in biomedical sciences for protein separation, drug delivery, and cancer hyperthermia to magnetic resonance imaging (MRI) [11][12][13].  [14,15]. However, insight into a facile synthetic method for ferrite nanoparticles with well-controlled magnetic properties for various applications is still a challenge [10]. For example, Xia and Liu et al. reported that the use of CoFe 2 O 4 -graphene nanocomposites as a high-capacity anode material for lithium-ion batteries could be successfully synthesized by solvothermal method for the improvement of cycle performances [16].
Other strategies are restrained partly because of the use of toxic organic metal precursors, partly because of a prerequisite of the Ar or N 2 non-ambient atmosphere [13]. Meanwhile, complex polymerization method presents as an efficient route for the synthesis of MNPs with good chemical homogeneity [17]. This experimental procedure uses the simple, inexpensive, and rapid thermal decomposition approaches to obtain monodisperse nanoparticles. Moreover, for commercial perspective, complex polymerization method is an economic and scalable 'greener' approach to synthesis MNPs [13].
Recently, several substrates have been coated on nanoparticles MFe 2 O 4 (M=Co, Ni, Mn) to obtain the desirable magnetic properties in biomedicine and bioengineering fields [4]. However, the requirements for these coating materials are low toxicity and high biocompatibility. Fe-based nanoparticles coated with graphene oxides (GO) were reported as non-toxic in human lymphocytes [18][19][20]. The presence of functional groups of graphene oxides are to increase the oxygen-containing adsorption sites with good stability, thus making them a suitable candidate for adsorption applications [21].
Graphene is constituted by two-dimensional sp 2 -hybridized carbon single layers containing the hexagonal crystal structures. Meanwhile, graphene oxide derived from graphene via exfoliation with chemical oxidants such as H 2 O 2 , KMnO 4 , H 2 SO 4 , and K 2 S 2 O 8 . These carbonaceous compounds have exhibited several exceptional properties, playing a crucial role in dyes treatment because of its diverse kinds of nitrogen/oxygen-containing functional groups on surface, large surface area, and highly porous sites [22][23][24][25][26]. Combining between graphene oxide and MNPs aims to develop a form of nanocomposite with strong magnetism, increase the number of functional groups and decrease the agglomeration [27][28][29]. These materials might be therefore easy to perform magnetic separation and recycling in adsorption.
In this context, complex polymerization method was used to synthesize magnetic ferrite nanocrystals with cobalt, manganese and nickel oxide sites. The GO@MFe 2 O 4 (M= Co, Mn, Ni) nanocomposites could be prepared using a two-step approach. The first step was to synthesize the MFe 2 O 4 (M= Co, Mn, Ni) by complex polymerization method and graphene oxide by Hummers' method. Next, these ferrite nanocrystals was combined with dispersed graphene oxide in the presence of ethanol solution via a procedure of temperature thermal decomposition (1000 °C ) to create the GO@CoFe 2 O 4 MNPs. The samples were then characterized to show morphological, magnetic properties by several techniques such as SEM, IR, pH pzc and VSM.

Chemicals and instruments
All chemicals including iron (III) chloride, cobalt (II) chloride, manganese (II) chloride and nickel (II) chloride, citric acid (93 g) and ethylene glycol were commercially purchased from Merck KGaA. All MNPs samples were pretreated to remove moisture at 100 °C for 1 h before analyzing. The scanning electron microscope (SEM) was recorded by instrument S4800, Japan and used an accelerating voltage source of 15 kV with the magnification of 7000. An energy dispersive spectroscopy (EDS) analysis of the arrays, performed at 15 kV coupled to the field-emission scanning electron microscope. The magnetic properties were determined by Vibrating Sample Magnetometer (VSM). The FT-IR spectra were recorded by using the Nicolet 6700 spectrophotometer instrument. The N 2 adsorption/desorption isotherm was obtained using the Micromeritics 2020 volumetric adsorption analyzer system, before analysis the MPC sample was degassed in a vacuum for 6 h at 150 °C .

Preparation of graphene oxide
Graphene oxide was synthesized by Hummer's method, using the natural graphite powder exfoliated with the several oxidants [30]. In the typical experiment, 3.0 g graphite 3.0 g was added into a 500 mL flask containing 150 mL concentrated H 2 SO 4 98% placed an ice bath (3 °C -5 º C) under stirring. After that, 10.5 g KMnO 4 was slowly added for 20 min and stirred overnight at room temperature. Then, 250 mL of deionized water was poured into stirred mixture and followed by a slow addition of 5 mL H 2 O 2 30 wt% until obtained a golden yellow solution. The products were next washed with 1L HCl 3% and then 1 L deionized water for three times. The graphene oxide was suspended in distilled water and dialyzed with deionized water overnight to remove metal ions and acids.

Preparation of MFe 2 O 4 (M= Co, Mn, Ni) by polymerized method
Magnetic nanoparticles MFe 2 O 4 (M= Co, Mn, Ni) were prepared by the polymerized complex method (PC) [21]. A mixture of citric acid (93 g), 100 mL of deionized water and ethylene glycol (40 mL) were dissolved at 80 °C until obtaining a homogeneous solution. After adding another mixture of 0.36 g CoCl 2 . 6H 2 O (or 0.303 g for MnCl 2 . 6H 2 O or 0.36 g for NiCl 2 . 6H 2 O) and 0.5 g FeCl 3 . 6H 2 O, the solution was stirred for several hours at 130 °C to obtain a polymeric resin precursor. The black powder precursor could be obtained when heating the solid under air atmosphere at temperatures 1000 °C for 2h.

Preparation of GO@MFe 2 O 4 (M= Co, Mn, Ni) nanocomposites
The GO@MFe 2 O 4 (M= Co, Mn, Ni) nanocomposites were prepared by two-step procedure [26] In the typical experiment, 1.0 g MFe 2 O 4 (M= Co, Mn, Ni) material was added into a beaker containing 50 mL C 2 H 5 OH and 5 mL of GO colloidal suspension in 45 mL of water. The beaker was put in an ultrasonic bath for 1h, and after heated to 60 °C until approaching a complete vapourization. The solid was dried at 100 °C and used for the next studies.

Determination of pH pzc (pH point of zero charges)
The procedure for pH pzc determination can be performed according to a previous report [21]. The GO@MFe 2 O 4 (M= Co, Mn, Ni) nanocomposites (m o = 5.0 mg) was added a flask containing 25 mL of KCl 0.1 mol/L at the different pH 2, 4, 6, 8, 10, 12. Note that HCl and NaOH 1 mol/l were used to adjust the pH indexes. The mixture was stirred during 24 h, and then the solid was then separated. The solids were then separated from the mixture using magnetic and centrifugal methods, and their final pH 2 were measured by a pH meter. The curve was plotted via ∆pH = pH 1 -pH 2 versus pH 1 and the pH pzc was calculated at ∆pH = 0.

SEM images of GO@MFe 2 O 4 (M= Co, Mn, Ni)
Morphological patterns in Figure 1 compare the difference of morphological surface profiles of GO@MFe 2 O 4 (M= Co, Mn, Ni). The images of nanocomposite particles indicate the evident agglomeration and close arrangement with a spinel-like surface, which is attributable due the strong effect of magnetic properties derived from initial ferrites MFe 2 O 4 and the Vander Waals forces, creating the assembly among the primary particles. This observation can be consolidated by the change in agglomeration form of GO@MFe 2 O 4 as seen from Figure 1. The accumulative state of MNPs is the strongest in the form of GO@CoFe 2 O 4 , and weakest in form of GO@MnFe 2 O 4 , which are well matched with their saturation magnetization values [31]. Otherwise, blurred white spots appears nanosheet spinel surface was attributable to the existence  [32]. Otherwise, the average diameter of particles in Figure 1 also reveals a size distribution in the range from about 300 nm to 500 nm.
Although the saturation magnetization values of graphene oxide-decorated nanocomposites GO@MFe 2 O 4 are so far lower than those of their ferrites MFe 2 O 4 , the magnetic strength of graphene oxide-decorated ferrite also obeys the order of respective graphene oxide-free ferrites as several previous reports: MnFe 2 O 4 < NiFe 2 O 4 < CoFe 2 O 4 [33,34]. The presence of non-magnetic graphene oxide layer coating on the surface of magnetic ferrites results in a decrease in magnetic nanoparticle coverage, making the magnetization density and saturation magnetization values fall down.
Obtaining the magnetization values is a significant characteristic of GO@MFe 2 O 4 in several application fields, such as adsorption because the solid adsorbent can be easily separated by inducing a magnetic field (e.g., a magnet). Therefore, this feature can facilitate separation and recyclability processes.  Figure 3 shows the Fourier-transform infrared spectroscopy spectra of GO@MFe 2 O 4 (M= Co, Mn, Ni) MNPs. The very typical frequency bands of GO@MFe 2 O 4 at around 536, 530, and 590 cm -1 wavelengths are attributable to Fe (III)-O in octahedral and tetrahedral sites and the bands at around 452, 440, and 433 cm -1 wavelengths refer to Fe (II)-O vibrations in octahedral sites [35]. Several literatures indicated that these stretching vibrations at low frequency bands could be derived from the spinel ferrite phase of Fe 3 O 4 MNPs while the Fe-O bands at around 530-590 cm -1 is a typical band for γ-Fe 2 O 3 uniform [13,36].

FT-IR spectra of GO@MFe 2 O 4 (M= Co, Mn, Ni)
Moreover, all spectra reveal the presence of several stretching vibrations in common, such as hydroxyl groups O-H at 3420-3442 cm -1 , which is likely to originate from adsorbed H 2 O or carboxylic acid trace from graphene oxide exists on the surface after synthesis of GO@MFe 2 O 4 . In addition, carbonyl groups (e.g., ketones, aldehydes) C=O vibrations and C-H stretching of alcoholic/aromatic groups can be confirmed at 1626-1630 and 1364-1442 cm -1 , respectively [37].

pH pzc plots of GO@MFe 2 O 4 (M= Co, Mn, Ni)
The point of zero charge (pH pzc ) parameter of GO@MFe 2 O 4 nanomaterials can be described for the electrical charge density on a surface coverage, which is ascribed to a zero value [21]. Figure 4 shows the pH pzc values of a range of materials, as follows: MnFe 2 O 4 /GO (pH pzc = 6.5) < NiFe 2 O 4 /GO (pH pzc = 7.0) < CoFe 2 O 4 /GO (pH pzc = 7.3).
In adsorption, note that when pH of solution is lower than pH pzc , the nanocomposite surface is positively charged, leading to an electrostatic force between anions and positive-charged functional groups on surface. By contrast, when pH of solution is greater than pH pzc , the surface is negatively charged and it tends to attract cations from the solution. Especially, when pH of solution is as equal as pH pzc, the electrical density on a surface coverage reaches a zero value, making a balanced status in charge.  [11,12]. This phenomenon can be explained due to the effect of graphene oxide coated on the structure, providing more new pores such as micro/meso/macropores and thus increasing hierarchically porous structures and high surface areas.