Facile synthesis of reduced graphene oxide/Fe3O4 nanocomposite film

Introduction

Because of its extraordinary 2-dimensional structure and very high specific surface area (2,630 m²·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 chemical methods and chemical vapor deposition (CVD) on various substrates (5-6-7). It has been demonstrated that these 3 main types of graphene materials can be used as nanofillers for possible applications in paints, coatings and nanocomposites or as a support to load some kinds of metals or metal oxide/hydroxide including Fe₃O₄ nanoparticles (7).

As a magnetic material, Fe₃O₄ has been widely applied to the sensor (8), biomedicine (9), magnetic storage (10), pollution treatment (11), food detection (12) and other fields because of its low toxicity, high biocompatibility and high catalytic activity etc. Special attention has recently been paid to graphene/Fe₃O₄ composite film since the creation of a synergistic effect might be expected to have great potential in lithium ion batteries, microwave-absorbing materials, biomedicine and supercapacitors (13-14-15-16). Until now, a variety of methods have been explored to prepare graphene/Fe₃O₄ composites, including the hydrothermal/solvothermal method, microwave-assisted synthesis, the sol-gel method, the in situ self-assembly method and the off-site hybrid method. Using the solvothermal method at 220°C for 30 minutes, He and Gao (17) prepared a GO/Fe₃O₄ mixture that was reduced by NaOH/DEG solution at 220°C for 1 hour. The as-prepared rGO/Fe₃O₄ solution was then vacuum-filtered to form a 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 Fe₃O₄ 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/Fe₃O₄ 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 Fe₃O₄ nanoparticles (range 10-15 nm) then mixed with GO suspension to get GO/Fe₃O₄ 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/Fe₃O₄film (19). Even though graphene/Fe₃O₄ composite film has been extensively investigated for the last few years, finding a relatively facile method and a controlled size of Fe₃O₄ 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/Fe₃O₄ 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 Fe₃O₄ 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 Fe₃O₄ 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.

Methods

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-C-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 Fe₃O₄ powder material. The Fe₃O₄ 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 sp²C = C, sp³C-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 Fe₃O₄ 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, H₂O, 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 sp³ carbon atoms of disordered graphite, and the G band corresponds to the in hexagonal plane vibration of sp² 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 Fe₃O₄ nanoparticles, and a high value of 63.6 emu·g^-1was obtained by adding 200 mg of F-NPs. The unchanged coercivity for the composite with different amounts of Fe₃O₄ nanoparticles indicated that graphene-based film did not change F-NP anisotropy field – namely, 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 Fe₃O₄ content because the conductivity of Fe₃O₄ nanoparticles is lower than that of graphene sheets (16). When the content of Fe₃O₄ 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/Fe₃O₄ NP magnetic composite films step-by-step by heating the colloidal solution of GO/Fe₃O₄ 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.