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Facile Synthesis of Reduced Graphene Oxide/Fe3O4 Nanocomposite Film

Background This study aims at proposing a facile method to prepare rGO/Fe3O4 composite film with adjusted magnetic properties and electronic conductivity. Methods Colloidal solution of graphene oxide (GO)/Fe3O4 nanoparticles (F-NPs) with a size in the range of 20-80 nm were prepared by a solution-blending method and heated step-by-step from room temperature to 60°C, 120°C and 160°C for 12 hours, respectively, to obtain a reduced graphene oxide (rGO)/F-NP composite film. The structure, morphology, components, magnetic properties and electrical conductivity of the composite films were characterized by X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, superconducting quantum interference devices and 4-probe instrument. Results The results indicated that the F-NPs were uniformly distributed on the graphene film, and the composite exhibited good ferromagnetic properties and conductivity, which could be adjusted easily via different loadings of F-NPs. A high content of F-NPs (200 mg) led to a strong saturation magnetization of 63.6 emu·g−1, with a coercivity of about 104.9 oersted (Oe). Whereas a high conductivity of 6.5 S·m−1was obtained at low amounts of F-NPs (40 mg). Notably, rGO/Fe3O4 composite film fabricated by this simple method is widely used in various fields including magnetoelectronics, electrochemical energy conversion and storage, and magnetic nanodevices and others. Conclusions A graphene-based film deposited by Fe3O4 nanoparticles with controllable loadings has been fabricated by a step-by-step heating treatment of GO/Fe3O4 colloidal solution.


Introduction
Because of its extraordinary 2-dimensional structure and very high specific surface area (2,630 m 2 ·g -1 ), graphene exhibits unique physicochemical properties that have attracted much attention since Geim and Novoselov of the University of Manchester obtained monolayer graphene by exfoliating graphite using a mechanical method (1)(2)(3)(4). Currently, the method for producing graphene materials such as graphene oxide (GO) flakes, graphene films and graphene nanoplatelets has been extended to exfoliating graphite using chemi-composite film. After careful measurement, the electrical conductivity of the film was found to be 0.7 S·m -1 , and the saturation magnetization value was in the range of 0.5 to 44.1 emu·g -1 when the content of Fe 3 O 4 was 5.3% to 57.9%, with the average diameter of the nanoparticles changing from 1.2 to 6.3 nm (17). Liang et al filtered rGO/Fe 3 O 4 nanoparticle dispersion that has been prepared by chemical reduction method to synthesize graphene-based films. Their graphene/F-NP hybrid films exhibited superparamagnetic properties for potential applications as magnetic-controlled switches (18). Also, Narayanan et al reported using a chemical coprecipitation technique to synthesize uniformly sized Fe 3 O 4 nanoparticles (range 10-15 nm) then mixed with GO suspension to get GO/ Fe 3 O 4 freestanding membranes by vacuum filtering. After that, the film was peeled off the filter paper and treated with 2-M hydrazine monohydrate at 90°C for 1 hour to fabricate rGO/ Fe 3 O 4 film (19). Even though graphene/Fe 3 O 4 composite film has been extensively investigated for the last few years, finding a relatively facile method and a controlled size of Fe 3 O 4 NPs on graphene still remain major challenges (17).
In this work, we used an improved thermal treatment method without adding any reducing agent, to prepare rGO/ F-NP nanocomposite film. Namely, GO/Fe 3 O 4 colloid solution prepared by the solution-blending method was heated step-by-step from room temperature to 160°C to fabricate reduced GO film instead of annealing directly up to a high temperature. The Fe 3 O 4 nanoparticles were found to be well dispersed due to the existence of graphene sheets. Magnetic properties of the nanocomposite film can be controlled readily by altering the Fe 3 O 4 nanoparticle feeding, and film thickness is dependent on the amount of GO/F-NP solution coated on a substrate. We demonstrated that rGO/F-NP films could be prepared by a low-temperature annealing treatment, which is expected to provide a simple and effective process for the preparation of graphene-based composite films. ; Xiya Reagent) were acquired. All reagents were analytical grade and used without purification. Water was purified using ultrapure water equipment.

Sample preparation
We chose natural powder graphite to prepare a GO colloidal solution (20 mg·mL -1 ) using a modified Hummers' method (20). In brief, a blend of natural graphite powder (2 g) and NaNO 3 (1.5 g) was placed in cold (0°C) concentrated H 2 SO 4 (69 mL), and then KMnO 4 (9 g) was added gradually and slowly with stirring. The mixture was placed in ice water for 12 hours and then heated to 35°C for 1 hour. The deionized water (200 mL) was added when the mixture had cooled to room temperature, after heating to 60°C for 18 hours. H 2 O 2 was added dropwise until the solution became bright yellow, and then the solution was first washed by centrifugation with 10% hydrochloric acid solution for 3 times and then washed with deionized water to neutral to obtain the GO colloid solution. To prepare a composite film, 0 mg, 40 mg, 120 mg and 200 mg of Fe 3 O 4 nanoparticles were dispersed respectively in 40 mL citric acid solution (0.5%) and sonicated for 5 minutes. Ten milliliters of colloid solution of GO and the as-prepared Fe 3 O 4 nanoparticle dispersion were mixed and mechanically stirred for 20 minute to create the GO/F-NP colloidal solutions. Mixtures were transferred to glass dishes and placed in a vacuum oven, and then dried at 60°C, 120°C, and 160°C, for 12 hours, respectively. Finally, we obtained a flat and continuous rGO/ F-NP composite film after the prepared samples cooled down to room temperature in the vacuum oven. The rGO/F-NP film could easily be peeled off from the glass for investigation. For comparison, pure rGO film was also prepared. A schematic of the process is shown in Figure 1. The rGO/F-NP freestanding films were prepared through 2 steps of solution blending and step-by-step heating. In the thermal reduction process of graphite oxide, the loss of the interstitial H 2 O mainly occurred at 110°C (21). When the temperature was heated above 150°C, the functional groups of GO began to decompose with liberation of CO, CO 2 and H 2 O (22). If the annealing time is increased, the reduction of GO will be better (23). In the case of rapid heating, the structure of GO will be destroyed to form a slight soot (22). Therefore, it is expected that to obtain a film without wrinkles and pores, a step-by-step process with slow heating needs to be used, although this is time consuming.

Characterization
The structure of the composite film was characterized by X-ray diffraction (XRD, D8tools; Bruker), and the component and carbon-bonding types were identified by Fourier transform infrared spectroscopy (FT-IR, Perkin Elmer Spectrum One B) and X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250 spectrometer). The morphology was analyzed by scanning electron microscopy (SEM, JOEL JSM-6700F), the typical Raman spectra was measured by Raman spectroscopy (532 nm), the magnetic properties were tested by superconducting quantum interference devices (SQUIDs) and the resistance was tested by 4-probe instrument (Keithley, PCT-1B).

Results and discussion
Usually, heating GO film at temperatures above 150°C will lead to the pyrolysis of oxygen-containing functional groups and thus the formation of rGO film. However, the annealing process will probably disturb the structure of the GO film, and obvious crumpled or cracked morphology will appear due to the emission of large amounts of gas (22,24). Figure 2A shows the macroscopic morphology of GO, rGO and rGO/F-NP (200 mg) films. The black GO and rGO films can be observed having no crumpled structures on the surface. By comparison, the GO film surface is smoother, even though after thermal treatment, the rGO and rGO/F-NP films still remain their integrity and continuous macroscopic structure, with only a small number of tiny pores on the surface of the composite film. We attributed the relatively smooth graphene-based composite film obtained, to the step-by-step thermal reduction process that may enable water molecules and oxygen-containing groups to be removed separately and slowly. Therefore the GO sheets get more of an opportunity to collide and link with each other through the hydrogen-bonding process to form a flat surface. The crystalline structure of the GO, rGO and rGO/F-NP composite film prepared was analyzed by XRD measurement (Fig. 2B). According to the standard XRD data for graphite (JCPDS card, file no. 41-1487), the characteristic diffraction peak at around 2θ = 26° has been replaced by a sharp peak at the position of 10.6° in the pattern for GO, indicating that the oxygen functional groups are inserted into the graphite layer. For the sample of rGO/ F-NPs (200 mg), the positions and intensities of characteristic peaks can be clearly seen, which correspond well to the standard JCPDS card of magnetite (file no. 19-0629). The XRD pattern of rGO displays a low and broad characteristic peak between 20° and 30°, which indicates that GO is reduced to rGO in the reaction process, even though the crystallization has not been carried out very well. As the C(002) peak can hardly be seen for the composite sample in the XRD pattern, the graphene sheets might be deduced to have disordered stacking in the composites (25).
As seen in the representative SEM images shown in Figure 3A, the surface of rGO film presents the typical morphology of graphene with obvious wrinkle structures. In the SEM images of rGO/F-NP (200 mg) composite films with different NP contents (Fig. 3B-D), we can see that the surface of rGO loads uniformly small F-NPs with a size in the range of 20-80 nm. The size distribution of NPs mainly originates from the initial Fe 3 O 4 powder material. The Fe 3 O 4 nanoparticles in the composite have excellent dispersibility due to the existence of the graphene sheets and lower content of NPs. Figure 3B shows relatively sparsely attached to the surface of graphene film sheets, while with high content levels, the NPs are remarkably denser (Fig. 3D).
XPS was used to further investigate the crystal phase in rGO/F-NP (200 mg) composite films. In Figure 4A and B, the C1s spectra of rGO can be deconvoluted into 5 different peaks. The binding energies at 284.5, 285.8, 286.9, 288.3 and 289.2 eV have been assigned to the sp 2 C = C, sp 3 C-C, C-OH, C = O and O = C-OH groups, respectively; while 287.8 and 290.3 eV belong to C-O-C functional groups and corresponding π-π* vibration satellite peak, respectively. The C1s spectrum of rGO hardly changes after F-NPs are added, indicating that in using the solution-blending method for the preparation of rGO/ F-NP composite, the attachment of NPs to rGO involves noncovalent bonding, which could be further confirmed by the FT-IR analysis that followed. The Fe 2p XPS spectrum of rGO/F-NP composite is given in Figure 4C. The weak binding energy peaks of Fe 2p 3/2 and 2p 1/2 suggest a small amount of Fe 3 O 4  nanoparticles used in the composite samples. Figure 4D shows the O1s region of the prepared rGO/F-NP sample. Four different peaks centered at 532.0 eV, 533.0 eV, 533.8 eV and 531.0 eV are observed, corresponding to the C-O-C, H 2 O, OH-O and O = C groups, respectively.
FT-IR analysis is shown in Figure 5A. The peak of GO at around 3,421 cm -1 can be attributed to the -OH stretching vibration, while 2 other peaks at 1,739 cm -1 and 1,626 cm -1 can be ascribed to the C = O stretching and C = C deformation vibration, respectively. The peaks at 1,171 cm -1 and 1,044 cm -1 belong to the C-O stretching vibration of the epoxy group and alkoxy (26). The absorption peaks of rGO and rGO/ F-NP (200 mg) composite decrease or disappear significantly, showing that oxygen-containing groups are removed from the graphite oxide after heat treatment.
We used Raman spectral measurement to test the spectral characteristics of the GO/F-NP (200 mg) composite film before and after reduction. The results are shown in Figure 5B. We can clearly see that there are 2 characteristic peaks in the spectrum, which are the D band (1,349.9 cm -1 ) and G band (1,589.2 cm -1 ). The D band is related to the vibrations of sp 3 carbon atoms of disordered graphite, and the G band corresponds to the in hexagonal plane vibration of sp 2 carbon atoms (27). The intensity ratio of the D band and G band of GO/F-NP composite film (I D /I G ) is increased to 1.57 from 1.13 after reduction, which proves that there are more numerous defects created during reduction of the GO/F-NPs (28).  The magnetic properties of pure F-NPs and rGO/F-NP composite with different contents were tested by SQUIDs (Fig. 6). The saturation magnetization and coercivity of the F-NPs was about 91.6 emu·g -1 and 126.3 oersted (Oe), respectively. As to the rGO/F-NP composites, the values of saturation magnetization were calculated to be about 63.6 emu·g -1 (3.0 mg), 37.5 emu·g -1 (1.6 mg) and 5.5 emu·g -1 (1.0 mg), while the value of coercivity remained unchanged at about 104.9 Oe. It can be seen that the saturation magnetization of the composites was largely dependent on the loading of the Fe 3 O 4 nanoparticles, and a high value of 63.6 emu·g -1 was obtained by adding 200 mg of F-NPs. The unchanged coercivity for the composite with different amounts of Fe 3 O 4 nanoparticles indicated that graphene-based film did not change F-NP anisotropy fieldnamely, the soft magnetic property was not changed much.
The conductivity of the rGO/F-NP composite films is shown in Table I, as measured by the 4-probe method. The results proved that the composite films had good conductivity. It is worth nothing that the films' conductivity gradually decreased with increasing Fe 3 O 4 content because the conductivity of Fe 3 O 4 nanoparticles is lower than that of graphene sheets (16). When the content of Fe 3 O 4 was 40, 120 or 200 mg, the corresponding conductivity was 6.5, 5.1 and 4.3 S·m -1 , respectively, which was higher than that of the film of He and Gao (0.7 S·m -1 ) which was prepared by chemical reduction (17). We attributed the higher conductivity to the direct and gentle thermal reduction of functional oxygen groups that facilitated the effective maintenance of the structural integrity of the composite films (29).

Conclusions
We prepared rGO/Fe 3 O 4 NP magnetic composite films step-by-step by heating the colloidal solution of GO/Fe 3 O 4 without using any chemical reduction agent. The slow thermal treatment proved to be effective in obtaining freestanding graphene-based composite film free from crack structures on the surface. The results showed that rGO/ F-NP (200 mg) composite film has a high saturation magnetization of 63.6 emu·g -1 , with coercivity of 104.9 Oe, and a good conductivity of 6.5 S·m -1 (40 mg). This simple, controllable and low-cost synthesis method makes it possible for graphene-based film to be used in many fields, including as supercapacitors, lithium ion batteries, environmental pollution treatments, sensors, microwave-absorbing materials and so on. Compared with rGO/F-NP film that has been prepared by directly heating a GO/F-NP solution up to 160°C, the slow thermal reduction approach turned out to be effective for a freestanding film with small amounts of tiny pores and free from crack structures on the surface.

Disclosures
Financial support: This project was sponsored by the National Natural Science Foundation of China (grant no. 51302019), the Natural Science Foundation of Jilin Province (grant no. 20150101029JC) and China Scholarship Council (CSC) for Xin Wang, a visiting scholar.