Main applications of Fe3O4/G composites.
Preparation, properties, and applications of magnetic-graphene-based nanocomposites are reviewed. Graphene magnetic nanocomposites include those on the basis of elemental metals (Fe, Co, Ni), magnetic nanoclusters, various morphological forms of iron oxides (Fe2O3, Fe3O4), ferrites MFe2O4, 3D graphene aerogels@hierarchical Fe3O4 nanoclusters, single-molecule magnets like TbPc2 (Pc: phthalocyanine), other organometallic-containing composites (benzene-metal-graphene), as well as polycomponent nanocomposites such as Ag/Fe3O4/G (G: graphene), Fe3O4/CdS/G, or FePc/Fe3O4/GO (GO: graphene oxide), among others. Their available synthesis methods consist commonly of hydrothermal and solvothermal techniques, sol-gel autocombustion, sonoelectrochemical polymerization, thermal expansion and thermal reduction, microwave-assisted technique, and covalent bonding chemical methods. Their current and potential applications are distinct devices, in particular for colorimetric detection of glucose, construction materials, analytical, sensor and biosensor applications, environmental remediation, compounds with antibacterial properties, catalysis and photocatalysis, biological imaging, oil absorption, etc.
- environmental applications
- iron oxides
- lithium batteries
- magnetic composites
Graphene, which consists in a single layer of carbon atoms, has been intensively studied in the last decade due to its extraordinary structural and electronic properties, leading to numerous current and potential applications in electronic devices. Growing, doping, or adsorption of metal nanoparticles and other inorganic/organic species on its surface can change, improve, and manipulate graphene magnetic and electronic properties. If these particles possess magnetic properties, the whole nanocomposite can have them too, opening new opportunities and applications for the formed hybrids.
Currently, the information on magnetic graphene and graphene oxide nanocomposites has not yet been extensively reviewed despite a number of original reports. At the same time, their importance is undoubtful due to numerous current and potential applications of graphene itself, and magnetic nanoparticles in a variety of fields of chemistry, physics, medicine and biology, in the frontiers with nanotechnology. This topic is rarely presented as short sections in some reviews [1–4], dedicated to graphene, in contrast to carbon nanotubes, where a concise review was recently published . So, the main purpose of this chapter is to highlight magnetic graphene composites, and review their main synthesis methods, properties, and current and potential applications.
2. Metal-graphene composites
A certain number of magnetic metals are present in the Periodic Table. Graphene composites have been reported only with some typical metals with paramagnetic properties, such as iron, cobalt, or nickel. Among theoretical studies of metal interactions with graphene surface, quantum Monte Carlo methods, studying the consequences of placing a magnetic adatom adjacent to a vacancy in a graphene sheet [6, 7], and description of electronic and magnetic properties of the graphene-ferromagnet interfaces  are particularly relevant. In addition, the plane-wave density functional theory (DFT) was used to study the properties of nanostructures of the type C60-M-G (M: Ti, Cr, Mn, Fe, or Ni; G: graphene) . In this work, in particular, it was found that the high-spin C60-Cr-G nanostructure is more stable than its low spin analog. Structures containing distinct metals can be different. For instance, on the graphene surface, C60-Ti stands symmetrically upright, while for other metals, C60-M fragments are bent and nonequal in each composite (Figure 1). Authors suggested, based on calculations, that the C60-M fragment is flexible, respecting the bending motion. Importantly, the attachment of C60 to an M-G surface is more energetically favored than the decoration of graphene with C60-metal complexes. DFT studies were also carried out for Co-G composites (cobalt-vacancy defect  or single Co atom incorporated with divacancy in armchair graphene nanoribbon ). In this case, metal dopation was found to introduce magnetic properties into the formed composite.
Several reports were dedicated to cobalt-graphene nanocomposites. Studying single Co atoms on graphene on Pt(111) and their magnetic properties by scanning tunneling microscopy spin-excitation spectroscopy, upon hydrogen adsorption, three differently hydrogenated species were identified (Figure 2) , whose magnetic properties are very different from those of cobalt element. It was suggested that, due to the interaction of Co-G orbitals, large magnetic anisotropy stems from strong ligand field effects that take place. A series of reports studied the intercalation of cobalt between graphene and Ir(111) via distinct techniques. Thus, cobalt layers intercalated in between graphene and Ir(111) were characterized by a combination of X-ray magnetic circular dichroism and photoemission . In particular, an induced magnetic moment in the graphene layer, antiparallelly oriented to that of cobalt, was established. Magnetic ordering was found to exist beyond monolayer intercalation. In a related report , cobalt-intercalated graphene on Ir(111) was studied by DFT calculations and spin-polarized scanning tunneling microscopy. It was revealed that the strong bonding between cobalt and graphene leads to a high corrugation within the Moiré pattern, which arises due to the lattice mismatch between the graphene and the Co on Ir(111).
In addition, nanometer-sizes films of cobalt were intercalated at the interface “graphene/Ir(111)” and were characterized by Auger electron spectroscopy and spin-polarized low-energy electron microscopy . In resulting composites, graphene top layer was found to promote perpendicular magnetic anisotropy in the Co film. It was also revealed that the magnetic anisotropy energy is significantly larger for the graphene/Co interface than for the free metallic surface. Regarding metal intercalation, contrarily to cobalt, Fe-based intercalation of GO led to the formation of reduced GO/Fe3C magnetic hybrids . Iron carbide particles were encapsulated in a graphite cage, protecting them from agglomeration.
Among other available theoretical studies on Co/G nanocomposites, the single Co layer added on graphene showed ferromagnetic ordering, with perpendicular alignment to the graphene sheet , according to the data obtained by relativistic density functional theory, at the level of generalized gradient approximation. These properties are promising for magnetism in 2D systems despite experimental difficulties in obtaining regular structures of single layer of this metal on graphene. In addition, the oxidation state of cobalt, together with possible oxidation of metal in real conditions, is also important. Thus, XPS spectral data for graphene/Co composites, obtained from CoCl2. 6H2O diluted in ethyl alcohol and highly split graphite, showed two sets of 2p3/2,1/2 lines belonging to partly oxidized and metallic Co atoms . The formation of this protective oxide layer prevents metallic cobalt from deep oxidation.
The synthesis methods for cobalt-graphene nanocomposites frequently include high-temperature decomposition of precursors. Thus, a magnetic cobalt-G nanocomposite (Figure 3) was prepared by carbonizing a self-assembly of a cobalt-based metal organic framework, ZIF-67, and GO . This composite based on cobalt and reduced graphene oxide (RGO) was used as a catalyst for the activation of peroxymonosulfate (PMS) in the process of decolorizing Acid Yellow dye in water. Resulting regeneration efficiency remained at 97.6% over 50 cycles, showing its effective and stable catalytic activity. The proposed mechanism of MCG activating PMS to generate sulfate radicals is shown in Figure 4. Another example is a combination of autocombustion and sol-gel methods, which led to Co-G nanocomposite, prepared from GO, Co(NO3)2, and citric acid as precursors . A sol-gel was first prepared and then underwent autocombustion in Ar atmosphere, at 300°C, due to the action of the produced reducing agents H2 and CH4. In the formed nanocomposite, Co nanoparticles (with a diameter of about 10 nm) are homogeneously distributed on graphene surface. The same technique can be applied for loading Ni, Cu, Ag, and Bi on graphene surface. Pyrolisis was also used, for instance, in the case of cobalt phthalocyanine (CoPc), for the purpose of fabricating organic metal/graphene composites . CoPc is capable of dispersing cobalt and its oxide onto graphene sheets due to π-interactions between them in the conditions of pyrolysis or oxidation. The process of the fabrication of Co/GC and Co3O4/GC is shown in Figure 5. The Co3O4/graphene nanocomposites showed remarkable lithium storage performance, including good rate capability, good cycle performance, and highly reversible capacity. It was suggested that CuO, Fe2O3 and other metal or metal-oxide-based graphene composites can also be prepared in this way.
The decolorization of organic dyes, such as the Acid Yellow as mentioned above, is one of the classical applications of graphene composites of cobalt and other magnetic metals. Cobalt nanoparticles (about 30 nm size), anchored on graphene sheets, were tested for heterogeneous oxidation of a dyeing pollutant, Orange II, with peroxymonosulfate (PMS) in aqueous solutions . In comparison to pure cobalt, the incorporation of Co nanoparticles into graphene sheets resulted in much higher catalytic activity for Orange II degradation. The proposed reaction mechanism is described by reactions (1)–(7). This work shows the importance of using other metal nanoparticles rather than iron for the decontamination of organic pollutants.
Co0 → Co2+ + 2e− (1)
≡Co0 + O2 + 2H2O → ≡ Co2+ + 4OH− (2)
≡Co0 + 2HSO5− → ≡ Co2+ + 2SO4−.+ 2OH− (3)
≡Co2+ + HSO5− → ≡ Co3+ + SO4−. + OH− (4)
≡Co3+ + HSO5− → ≡ Co2+ + SO5−. + H+ (5)
≡Co0 + 2 ≡ Co3+ → 3 ≡ Co2+ (6)
SO4−. + Orange II → many steps → CO2 + H2O (7)
In the case of nickel, its graphene or GO composites are not so widespread compared to cobalt or iron. Nickel itself is used as a support for graphene fabrication, for instance, by the CVD technique of CH4, leading to low-defect synthesis of bilayer graphene on evaporated polycrystalline nickel films . The CVD method also allows to obtain nickel coated with a few layers of graphene, resulting in a stable surface upon air exposure . This way, graphene-passivated (against oxidation) ferromagnetic electrodes can be fabricated, which are suitable for spin devices (namely spin-polarized oxidation-resistant electrodes as shown in Figure 6).
In addition to CVD, graphene or GO hybrids with nickel have also been obtained by other methods, for example, grown on a graphene Moire on Rh(111) at 150 K . These processes of graphene decoration with nickel nanoparticles can include simultaneous reduction of GO in the process of synthesizing GO/Ni composites , in particular simultaneous reduction of GO and nickel(II) ions by the one-step far-infrared-assisted method . Also, nickel nanoparticles were loaded by a graphene nanosheet by easily scalable and reproducible direct electrochemical deposition . The graphene sheet decorated with magnetic nickel nanoparticles yielded a composite with soft magnetic and conductive properties, which efficiently promoted microwave absorbability (and was better than graphene alone).
Iron/graphene (or iron/GO) hybrids are very well studied and, together with other iron-containing nanostructures below (iron oxides and their combinations with iron, as Fe@Fe2O3), are undoubtedly the center of magnetic metal/graphene field. The hybrids can be obtained by several methods, both classic chemical and “greener” routes. Thus, focused solar radiation, as a green chemical route, was applied for the fabrication of 3D metal/metal oxide nanoparticles dispersed on 2D ultrathin graphene by a simultaneous reduction and exfoliation process . This procedure allowed the insertion of nanoparticles between ultrathin graphene layers acting as “spacers” between them. These metal/metal oxide nanoparticle dispersed graphene composites might have potential applications in environmental fields, conversion devices, energy storage sensing, and heterogeneous catalysis.
Microwave irradiation was used for the synthesis (Figure 7) of combined hierarchical 3D graphene/carbon nanotube/iron nanostructures (G-CNT-Fe) . The formed composite consists of vertically aligned CNTs, which are grown in graphene sheets along with shorter branches of CNTs stemming out from both the vertically aligned CNTs and the graphene sheets. Zero-dimensional (0D) functional iron oxide nanoparticles decorated within this 3D hierarchical nanostructure (both on 1D nanotubes and 2D graphene sheets) provide outstanding lithium storage characteristics. Iron moieties were found to be present in mixed valence states of FeOOH and Fe2O3. In addition, iron nanoparticles can be first deposited onto a graphene/Cu substrate by vacuum deposition and then the hydrogenation was carried out at 1 atm of gaseous H2 and under liquid N2 . Before the experiment, the stabilization of hydrogenated Fe nanoparticles on this support was predicted by DFT calculations, meanwhile the existence of Fe hydride is considered as nonreachable. Hydrogen was found to be released from hydrogenated Fe nanoparticles. In addition, a thermal reduction method was used for the synthesis of Fe/Fe3O4/G nanocomposite electrode material, using iron oxalate and exfoliated graphene oxide as precursors . Its application, along with nickel, for rechargeable Ni/Fe alkaline batteries (discharge and charge capacities of 280 mAh/g) in hybrid electric vehicles can be applied in a large-scale production.
Sodium borohydride has been frequently used as classic reductive agent for the reduction of iron ions and/or GO. Thus, nano zero-valent iron (nZVI) -decorated graphene sheets were prepared via sodium borohydride reduction of GO and applied for Cr(VI) removal . In a related report , nonhazardous superparamagnetic nanocomposites, consisting of iron nanoparticles (5 nm) and graphene, were synthesized from GO through intermediate formation of Fe3+/GO complexes and their further reduction with NaBH4. Methyl blue solution, a dye often present in wastewater of dyeing industry, can easily be decolorized using this nanocomposite.
Iron/G (or GO), as well as iron oxide/G (or GO) hybrids, can also have applications in the catalysis area. A novel type of oxygen reduction reaction (ORR) electrocatalyst on the basis of few-walled (2–3 walls) CNT-G complexes was reported . Abundant defects on the outer walls of the CNTs can be produced via partial unzipping of the outer CNTs walls and the formation of large quantities of graphene sheets, connected with the intact inner walls of the CNTs. These graphene sheets make easier the formation of catalytic sites for ORR on annealing in NH3. Nitrogen doping and Fe impurities are responsible for higher ORR activity in the CNT-G complexes. It was established that Fe atoms are often close to N atoms and are frequently situated along edges of defective graphene sheets. Elimination of Fe by purification leads to a considerable decrease in ORR activity. Indeed, both nitrogen and iron atomic species are important to the high ORR electrocatalytic activity. Another example is a cost-effective synthesis of nitrogen-doped graphene (NG; Figure 8) carried out by using cyanamide as a nitrogen source and graphene oxide as a precursor; iron nanoparticles were incorporated into NG using FeCl3 as precursor . This composite was used as a model for the elucidation of the influence of nonnoble metals on the electrocatalytic performance. The NG supported with iron nanoparticles (5 wt%) showed high current density (8.20 mA cm2) in an alkaline solution and an excellent methanol crossover effect, high stability in distinct media, high surface area, among other advantages, in comparison with Pt and NG-based catalysts, thus allowing platinum replacement.
Environmental problems of contamination with heavy metals and other pollutants (As, Cr) can be partially solved using iron/graphene composites, in particular those containing iron in both metallic and oxidized form. A one-pot thermal decomposition method was used for the preparation of graphene nanoplatelet composites decorated with core-shell Fe-Fe2O3 nanoparticles . These nanocomposites could be separated from the liquid-phase mixture with the aid of a permanent magnet. Efficient and effective adsorption of arsenic(III) from the polluted water was observed for this material (nearly complete As(III) removal within 1 ppb) and attributed to the increased adsorption sites existing in the presence of magnetic nanoparticles. Magnetic graphene nanocomposites (MGNCs), consisting of a core@double-shell structure of the nanoparticles with crystalline Fe as the core, iron oxide as the inner shell, and amorphous Si-S-O compound as the outer shell, were prepared by a thermodecomposition process (Figure 9) . These composites were highly stable even in 1 M HCl aqueous acid and showed a fast and highly efficient removal of Cr(VI) from wastewater after 5 min (Figure 10), in contrast to several other materials (like carbon or waste biomass), the use of which require several hours or days and are not able to achieve 100% removal of Cr(VI).
Another application is related to polymers: graphene nanosheets, consisting of iron core and iron oxide shell nanoparticles, G/Fe@Fe2O3, were used as nanofillers for the fabrication of magnetic epoxy resin polymer nanocomposites . The G/Fe@Fe2O3 was found to favor char formation from the epoxy resin; in addition, the tensile strength of polymer nanocomposite with 1.0 wt% Gr/Fe@Fe2O3 was found to be 58% higher than that of the pure epoxy and it was attributed to the high stiffness of graphene. It was suggested that the porous char layer with Gr/Fe@Fe2O3 may indicate the existence of Fe@Fe2O3, which helps the formation of gas, during the decomposition process of epoxy resin.
3. Fe2O3-graphene hybrids
Iron can be present in graphene composites in the elemental form (as shown above) or as Fe core/oxide shell nanoparticles, iron oxides, among others. The ratio of iron nanoforms in different oxidation states depends, in particular, on O-containing groups present in the graphene surface, use of reductants, and other conditions. Magnetic iron-containing nanoparticles were loaded on the GO sheets due to the abundant oxygen-containing functionalities present in these carbon materials (hydroxyl, epoxy, and carboxyl functional groups), and their growth mechanism was studied . Most of these functional groups were eliminated and the magnetic nanoparticles were partially converted to iron during thermal treatment under reducing conditions. Metal nanoparticles changed the GO lattice structure and intrinsic functionalities; this effect depended on the amount of iron precursor.
The effect of pH is also very important and it was studied for several noncovalent magnetic GO-based materials, prepared using Fe2O3 microparticles, nanoparticles, and magnetic surfactants . pH adjustment was used to effectively charge repulsion or attraction between Fe2O3 particles and the GO sheets (Figure 11). Each material caused coflocculation of GO at acidic pH, leading to materials that could be captured using an external magnetic field. The adsorption of GO at low pH was explained by attractive electrical double-layer forces between the GO and Fe2O3 or surfactants. On the contrary, at higher values of pH, the dispersions are stable due to alike-charge repulsions. An intriguing effect was found with Fe2O3 nanoparticles: low concentrations resulted in the flocculation of GO and higher concentrations caused restabilization, which are explained by an effective overcharging of the GO surfaces. These systems were found to remove a model nanomaterial, gold nanoparticles, from water.
The main synthesis methods for iron(III) oxide/graphene nanohybrids are, in general, similar to those used for metals described above or Fe3O4 that are explained below. Thus, iron oxide nanoparticles encased by permeable carbon layers of few-layer graphene were synthesized by high-pressure pyrolysis of ferrocene with pristine graphene . The ferrocene precursor provides both carbon and iron, leading to the carbon-coated iron oxide, while the graphene works as a high-surface-area anchor, to obtain small iron oxide nanoparticles. This material was used to improve the electrochemical performance of iron-oxide-based electrodes on Li-ion batteries. Similarly, an iron-oleate precursor (Figure 12) was used for the preparation of an iron-oxide/graphene nanocomposite via a solventless thermal decomposition method . Highly monodisperse γ-Fe2O3 nanoparticles were found to be in close contact with graphene. This nanomaterial can serve as a potentially valuable candidate anode material for high-rate Li-ion batteries.
In addition to high-temperature destructive methods, sol-gel and coprecipitation techniques are also common. Thus, reduced graphene oxide (rGO) tethered with maghemite (γ-Fe2O3) was prepared by a sol-gel process without a reducing agent in which sodium dodecylbenzenesulfonate (NaDDBS) was added into the suspension for the prevention of undesirable formation of an iron oxide 3D network . These composites were applied as anodes for half lithium-ion cells, exhibiting improved cycle life, reversible capacity, and good rate capability. Two-step synthesis (Figure 13), consisting of homogeneous precipitation and subsequent microwave-assisted reduction of the GO with hydrazine, led to reduced graphene oxide (rGO) platelets decorated with Fe2O3 nanoparticles, uniformly distributed on the surface of platelets . The total specific capacity of rGO/Fe2O3 was determined to be higher than the sum of pure rGO and nanoparticular Fe2O3. In addition, polypyrrole (PPy) was reinforced with rGO and Fe2O3 to reach electrochemical stability and enhancement . This ternary nanocomposite film was fabricated using a one-pot chronoamperometry approach.
In addition to their use in batteries, as referred above, a few other uses are known for Fe2O3/G hybrids. Thus, functional nanocomposite-based selective separation of microcystin-LR (toxin belonging to the family of microcystins produced by cyanobacteria and known to be the most toxic of this group) from contaminated water was achieved (Figure 14), applying cyclodextrin-functionalized magnetic composites consisting in porous silica and colloidal graphene . In this material, the magnetic component offers easier separation of microcystin-LR from water and the cyclodextrin constituent offers host-guest interaction with microcystin-LR. Among all experimented cyclodextrins, γ-cyclodextrin was found to offer the best performance. The studied functional nanomaterials can be used for the development of advanced water purification systems. Catalytic applications are also reported, for example, for nanosized Fe2O3 composites on carbon matrix, modified with nitrogen-doped graphene (Figure 15), that are excellent catalysts for the chemoselective hydrogenation of nitroarenes to anilines with good yields (reaction (8)) .
4. Fe3O4-graphene composites
4.1. Fe3O4/G aerogels
Among a large variety of mixed-valent-iron-oxide/graphene (or GO) composites, considerable attention is paid to aerogels in contrast to the above-described magnetic graphene hybrids. Graphene and other aerogels and routes for their synthesis are well known . The introduction of a magnetic component could lead to a larger variety of unusual properties and potential applications, namely, the easy removal of pollutants, such as crude oil. These nanocomposites can have a simple composition ( i.e., Fe3O4/G) or contain an additional component like a polymer. Thus, 3D N-doped graphene aerogel (N-GA)-supported Fe3O4 nanoparticles (Fe3O4/N-GAs; Figure 16) are known as efficient cathode catalysts for the oxygen reduction reaction (ORR) . These hybrids were prepared via a combined hydrothermal self-assembly, freeze-drying, and thermal treatment process (Figure 17). The products showed an excellent electrocatalytic activity for the ORR in alkaline electrolytes, including a lower ring current, higher current density, higher electron transfer number (∼4), lower H2O2 yield, and better durability.
3D graphene aerogels containing Fe3O4 nanoparticles (Fe3O4/GA (Figure 18), the lightest magnetic elastomer ever reported with density about 5.8 mg.cm3) were hydrothermally prepared by self-assembly of graphene, simultaneously decorated with Fe3O4 nanoparticles . They can be used to monitor the degree of compression/stretching of the material due to up to 52% reversible magnetic-field-induced strain and strain-dependent electrical resistance. Among more complex aerogels, hydrophobic graphene aerogel/Fe3O4/polystyrene composites (having extremely low density of 0.005 g cm−3, which corresponds to a volume porosity of 99.7%), with reticulated graphene structure, were solvothermally produced (Figure 19) . Porous Fe3O4 nanoparticles were found to appear as partial substitutes for ethylenediamine-assisted cross-linking and interconnections between graphene plates. These composites were applied for crude oil remediation, allowing intake capacity as much as 40 times its own mass, after 10 water-oil separation cycles.
4.2. Bicomponent Fe3O4/G hybrids
Regarding the nonaerogel type of graphene-Fe3O4 hybrids, the hydrothermal  and solvothermal synthesis  have also been widely used although other methods are also frequently used, such as the atomic layer deposition, applied not only for Fe3O4/graphene but also for Ni/graphene composites . Magnetic graphene foam with porous and hierarchical structures, on the basis of magnetite nanoparticles, was solvothermally obtained by gaseous reduction in a hydrothermal system and used for the adsorption of oil and organic solvents, thus serving for the cleanup of oil spills . Fe3O4 nanoparticles on graphene foam possessed different morphologies, nanosheet arrays, or cubic structures, while controlling the reduction degree of graphene oxide under mild conditions. Distinct ratios “iron oxide/graphene” are important for several applications. Thus, G/Fe3O4 nanocomposites with different ratios of Fe3O4 to GO (mFe3O4: mG = 0.1:1, 0.2:1, 0.4:1, 0.6:1, 0.8:1 and 1:1) were prepared by solvothermal method and used for the removal of methylene blue dye from aqueous solutions . The following morphologies were observed: uniform spherical homogeneously distributed Fe3O4 nanoparticles, with no agglomeration over the graphene sheets, and a uniform sheet-like shape of prepared graphene. Increasing the Fe3O4 nanoparticles on the surface of the graphene sheet was found to decrease the adsorption capacity, while the magnetization increased.
Several methods, considered as “greener”, are sometimes reported for iron oxide/graphene composites. Thus, the “green” oxidation of Fe2+ cations in FeCl2 or FeSO4 by graphene oxide led to an in situ deposition of Fe3O4 nanoparticles onto the self-reduced graphene oxide (rGO) sheets (Figure 20) . Strongly supraparamagnetic with highly chemical reactivity, electrical conductivity, good solubility, and excellent processability G@Fe3O4 nanocomposites (with an average diameter of Fe3O4 nanoparticles of 1.2–6.3 nm; coverage density of Fe3O4 nanoparticles on graphene nanosheets of 5.3–57.9%) were prepared by a one-step “green” procedure (Figure 21) . In addition, an approach for the deposition of iron oxide nanoparticles with selective narrow size distribution (0.5–7 and 1–3 μm), supported on different sizes of graphene oxide by coprecipitation, using Fe2+ and Fe3+ aqueous salt solutions and NH3, is described in reaction (9) . The reduction of mitochondrial activity using these materials is size dependent, but the chemical functionalization of GO and Fe3O4 is a way to enhance the biocompatibility, making the system independent of the size distribution of GO.
(NH4)2Fe(SO4)2⋅6H2O + 2NH4Fe(SO4)2⋅12H2O + 8NH3⋅H2O → Fe3O4⋅4H2O + 6(NH4)2SO4 + 14H2O (9)
4.3. Multicomponent Fe3O4/G hybrids
More complex Fe3O4/G- and organic(inorganic)-containing systems are also common. Thus, coating a layer of mesoporous silica materials on each side of magnetic graphene, in the conditions of CTAB-assisted sol-gel process, with further calcination, led to obtaining magnetic graphene double-sided mesoporous nanocomposites (G/SiO2) with high surface area (168 cm2/g) and large pore volume (0.2 cm3/g) (Figure 22) . The formed materials were applied to size-selective and specific enrichment and identification of peptides (peptidomics analysis) in human urine samples, protein digest solutions, and standard peptide mixtures. Another example is G@mSiO2-C18 materials (with a surface area of 315 cm2.g−1 and a uniform pore size of 3.3 nm) with extended plate-like morphology, prepared by coating mesoporous silica layers onto graphene via surfactant-mediated cocondensation sol-gel process, and applied as magnetic solid-phase adsorbents to the selective enrichment of phthalates in water .
A few reports are dedicated to chitosan-containing magnetic materials with mainly biomedical applications. Thus, an iron oxide/graphene oxide/chitosan (Fe3O4/GO/CS) composite was obtained by a solution mixing-evaporation method . Among other data, with the incorporation of 0.5 wt% Fe3O4 and 1 wt% GO, the tensile strength and Young’s modulus of the composite significantly improved by about 28% and 74%, respectively, compared with chitosan. In addition, it was established from TGA that Fe3O4/GO/CS is less thermally stable than GO/CS composites, and graphite is more thermally stable than GO. In a related report , magnetic Fe3O4 nanoparticles were introduced into a water-dispersible and biocompatible chitosan-functionalized graphene, fabricated by a one-step ball milling of carboxylic chitosan and graphite. It could be an excellent catalyst for electrochemical biosensors, in particular for glucose detection, due to the presence of nitrogen (from chitosan) at the surface of graphene.
Also, superparamagnetic fluorescent Fe3O4/SiO2/G-CdTe QD s/chitosan nanocomposites (Fe3O4/SiO2/graphene-CdTe QDs/chitosan nanocomposites (FGQCs ), with a spherical diameter of 467 nm; QDs: quantum dots) were prepared and studied for improving the drug loading content . In addition, a magnetic composite bioadsorbent on the basis of magnetic chitosan and graphene oxide (MCGO) was prepared [66, 67]. Its adsorption capacity for methyl blue was found to be about 90% of the initial saturation adsorption capacity after being used four times. The adsorption of methyl blue on MCGO strongly depends on ionic strength and pH, showing an ion exchange mechanism.
In the case of polymers, integrated hybrid structures (stable for more than 1 year) consisting of exfoliated expanded graphite (EG flakes, both naked and functionalized with branched polyethylenimine (PEI)) and Fe3O4 nanocrystals were fabricated by an ex situ process by the integration of iron oxide nanoparticles, coated with meso-2,3-dimercaptosuccinic acid (DMSA) or poly(acrylic acid) (PAA), onto the exfoliated EG flakes under the support of N-hydroxysuccinnimide (NHS) and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) . Such materials can have antibacterial properties. For example, in situ growth of silver nanoparticles onto the polyethylenimine (PEI)-mediated magnetic reduced GO resulted in a bactericidal material, Ag@rGO-Fe3O4-PEI composite (Figure 23) . The material provides a very high killing rate of 99.9% for E. coli bacteria under a 0.5 min near-infrared (NIR) laser irradiation.
4.4. Applications of Fe3O4/G hybrids
Solvothermally prepared G/Fe3O4 (Figure 24) is able to effectively remove both bacteriophage and bacteria in water . Indeed, it is capable of eliminating a wide range of pathogens including not only bacteriophages, but also various bacteria such as Shigella, Salmonella, E. Faecium, E. coli, E. faecalis, and S. aureus, with removal efficiencies up to 94%. Graphene oxide, particularly as magnetic Fe3O4/GO particles, was used as an adsorbent for wastewater treatment (Figure 25) . A variety of other Fe3O4/G (or GO) hybrid applications are shown in Table 1.
|Fe3O4/graphene aerogels||Removal of arsenic ions (up to 40.048 mg/g) from water|||
|Fe3O4/graphene||Ultrahigh electrochemical sorption capacity for both inorganic arsenic species (arsenate As(V) and arsenite As(III)).|||
|Magnetic graphene-oxide-supported-cyclodextrin||Heavy metal removal from wastewater. In particular, Cu(II) adsorption was found to be strongly pH-dependent and could be affected by background electrolytes, ionic strength, and citric acid.|||
|Polyethylenimine(PEI)-modified magnetic mesoporous silica and graphene oxide (MMSP-GO)||These hierarchical composites are suitable for synergistic adsorptive simultaneous removal of heavy metal ions (Pb2+, Cd2+) and humic acid.|||
|rGO/Fe3O4-based conductive and magnetic multifunctional films (membranes)||Can be produced and tested for its application in water desalination. Its potential uses also include catalysis, radiation shielding devices, biomedical fields, and supercapacitors.|||
|GO/Fe3O4||Super adsorbent to remove naphthalene, 1-naphthol, and 1-naphthylamine with different polarity. MWCNTs/FeO Fe2O3 were also produced in these synthesis as a byproduct, but graphene composite has the highest adsorption capacity among carbon-based nanomaterials. The adsorption capacity was shown to be naphthalene < 1-naphthol < 1-naphthylamine.|||
|Magnetic graphene composite absorbent (Fe3O4@PDDA/|
GO@DNA) based on poly cationic core-shell Fe3O4@PDDA (poly(diallyldimethyl ammonium chloride) (PDDA)) and GO@DNA
|Removal of trace levels of six types of polybrominated diphenyl ethers in water treatment. The nanomaterials can be reused at least 20 times for remediation purposes.|||
|A porous graphene composite affinity material, containing graphene scaffolds, Fe3O4 nanoparticles, and fully covered porous titania nanostructures||Selective capture and convenient magnetic separation of target phosphopeptides, showing superior activity in comparison with commercial TiO2 affinity materials.|||
|3D graphene foam-supported|
|Use in lithium battery anodes.|||
|The Fe3O4/G (graphene sheets) composite disks of μm dimensions||High-capacity lithium-ion battery anodes. The composites were prepared by electrostatic self-assembly between positively charged Fe3O4-APTMS [Fe3O4 grafted with (3-aminopropyl)trimethoxysilane (APTMS)] and negatively charged GO sheets and in an acidic solution (pH = 2) followed by in situ chemical reduction. An excellent rate capability as well as much enhanced cycling stability.|||
5. Ferrite-graphene and related composites
Simple or complex graphene or GO composites are known for a few ferrites, mainly those of cobalt and nickel, and usually used for environmental purposes or in batteries. NiFe2O4/G nanocomposites were prepared by mixing graphene and NiFe2O4 nanoparticles (obtained via a polyacrylamide gel method) into ethanol and further thermal drying at 60°C . This composite exhibited significantly enhanced photocatalytic activity for the degradation of methylene blue in the conditions of irradiation of simulated sunlight, while NiFe2O4 nanoparticles were inert. NOH radicals were suggested as being the most active species causing dye degradation. NiFe2O4/GO nanohybrids based on NiFe2O4 and reduced GO consisted in nanosized NiFe2O4 crystals (5–10 nm in size) confined by few-layered rGO sheets . This material seems a promising anode constituent for Li-ion batteries. The ternary nitrogen-doped graphene/nickel ferrite/polyaniline nanocomposite was hydrothermally prepared by a two-step approach process using urea, GO, Ni(NO3)2⋅6H2O, Fe(NO3)3⋅9H2O, and PANI as precursors, and applied in electrodes for supercapacitors . It was confirmed that the introduction of N-heteroatom greatly improved the electrode specific capacitance.
In the case of cobalt, its graphene composites have been synthesized mainly by hydro/solvothermally although an ultrasonic method was also reported . A CoFe2O4/G nanocomposite was hydrothermally prepared and was evaluated in the photocatalytic degradation of methylene blue dye under visible irradiation . Its photocatalytic activity depends on the surface area and the structural and optical properties of samples, and behaves better than pure cobalt ferrite. The formation mechanism of hydrothermally synthesized CoFe2O4-FGS (FGS: functionalized graphene sheets) is described in detail by Li et al. . In addition, porous CoFe2O4/rGO nanoclusters with different concentrations of graphene were solvothermally prepared (Figure 26)  and their electrochemical properties were evaluated using polyvinylidene fluoride and Na-alginate as binder materials to improve the anode performance of Li-ion batteries. The resulting CoFe2O4/rGO (20%) nanocomposites with sodium alginate binders exhibited promising electrochemical performance, such as excellent recycling behavior, good rate capability, and high reversible capacity.
Other graphene- (or GO)-ferrite hybrids are rare (for example, zinc-ferrite-reduced graphene oxide ), and their synthesis, properties, and applications are similar to those of nickel and cobalt. Thus, in a solvothermally fabricated uniform copper ferrite nanoparticle-attached graphene nanosheet (CuFe2O4/G), CuFe2O4 nanoparticles (with a diameter of about 100 nm) were shown to be densely and compactly deposited on graphene nanosheets . This nanomaterial exhibited properties of a high-performance supercapacitor material. A related hydrothermal route was used for the formation of CuFe2O4 nanoparticles into the GO sheets, and the obtained material, with GO content varying from 10 to 30 wt%, was used as stable and good reproducible enzyme-free glucose sensor . MnFe2O4 nanoparticle (MFNP)-decorated GO nanocomposites (MGONCs) were prepared through a miniemulsion and solvent evaporation process . The loading of magnetic nanocrystals depends on the ratio of GO/magnetic nanoparticles. In a related report , MnFe2O4 nanoparticles were deposited on GO by thermal decomposition of manganese(II) acetylacetonate and iron(III) acetylacetonate precursors in triethylene glycol. The resulting GO/MnFe2O4 nanohybrids showed imperceptible in vivo toxicity, negligible hemolytic activity, and very low cytotoxicity, being effective T2 contrast agents.
In addition, barium hexaferrite nanoparticles, fabricated  via the citrate sol-gel combustion method, in a reaction medium consisting of various forms of graphene nanosheets (such as reduced graphite oxide, expanded graphite oxide, and expanded graphite) were used to prepare nanocomposites as microwave absorbing material for radars . Finally, BiFeO3 nanoparticles that are important materials in photocatalysis  were prepared via a polyacrylamide gel route and transformed into a BiFeO3/G nanocomposite, by mixing with graphene in ethanol and further thermal drying at 60°C . Methyl orange (MO) was found to be efficiently degraded by this material under simulated sunlight irradiation. This photocatalytic performance can be mainly ascribed to the efficient transfer of photogenerated electrons from BiFeO3 to graphene (Figure 27). A similar reduced graphene rGO/BiFeO3 nanocomposite, prepared by self-assembly, was also reported .
An interesting example of a magnetic molecule and its composite with graphene is known for lanthanides. A device made of a graphene nanoconstruction, decorated with TbPc2 magnetic molecules (Pc: phthalocyananine), was able to electrically detect the magnetization reversal of the molecules in proximity with graphene (Figure 28) [99, 100].
6. Conclusions and outlook
The field of graphene and graphene oxide composites containing magnetic components can be considered as an important subject of the modern nanotechnology. Similarly to the filled or decorated carbon nanotubes, for which endohedral and exohedral functionalization with magnetic nanoparticles is well known, the graphene and graphene oxide magnetic nanocomposites can also be obtained via distinct methods and have a variety of applications. Graphene magnetic nanocomposites are currently known for those on the basis of elemental metals (Fe, Co, and Ni), magnetic nanoclusters, iron oxides (Fe2O3, Fe3O4) in distinct morphological forms, ferrites MFe2O4, 3D graphene aerogels@hierarchical Fe3O4 nanoclusters, single-molecule magnets such as TbPc2 (Pc: phthalocyanine), other organometallic-containing composites (benzene-metal-graphene), as well as polycomponent nanocomposites such as Ag/Fe3O4/G (G: graphene), Fe3O4/CdS/G, or FePc/Fe3O4/GO (GO :graphene oxide), etc.
Magnetic graphene and graphene oxide hybrids can be prepared by a variety of methods. Hydrothermal and solvothermal techniques are the most common. Other routes are also available, such as sol-gel autocombustion, sonoelectrochemical polymerization, thermal expansion and thermal reduction, microwave-assisted technique, as well as covalent bonding chemical methods. The formed composites have or might have several current and/or potential applications, such as devices (supercapacitors or material for anodes of lithium batteries), construction (graphene oxide-ferrofluid-cement nanocomposites), analytical, sensor, and biosensor applications (graphene-based magnetic solid-phase extraction for the determination of five chloroacetanilide herbicides (alachlor, acetochlor, metolachlor, butachlor, and pretilachlor), nitrite sensor, composites with peroxidase-like activity for colorimetric detection of glucose, etc.), environmental remediation (degradation and removal of water pollutants, such as chlorophenols, adsorbent for pesticide extraction, As and heavy metals (UVI)), compounds with antibacterial properties, and, of course, as magnetic nanocatalysts or nanophotocatalysts which can be simply magnetically removed from reaction media [101, 102].
As described above, magnetic graphene and graphene oxide composites can have applications, in particular, in the removal, decoloration, sequestration and adsorption of dyes , antibiotics and other organic contaminants, heavy metal separation (Hg, Cd, Cr, Cu, and As), and other environmental remediation aspects . Other areas include biological imaging and further bioapplications , magnetic separation, electromagnetic materials and coatings, conducting polymer nanocomposites, aligned substrate for nanodevices, and spintronic devices with perpendicular magnetic anisotropy. 3D graphene aerogels can have applications in electrode materials, supercapacitors, oil absorption, and gas sensors due to their electrical conductivity, mechanical strength, and high porosity.