Magneto-Optical Properties of the Magnetite-Graphene Oxide

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Functional Nanostructured Materials (including low-D carbon)

Magneto-Optical Properties of the MagnetiteGraphene Oxide Composites in Organic Solvents Alexander Solodov, Vadim Neklyudov, Julia Shayimova, Rustem Amirov, and Ayrat M Dimiev ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15129 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018

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ACS Applied Materials & Interfaces

Magneto-Optical Properties of the Magnetite-Graphene Oxide Composites in Organic Solvents Alexander Solodov, Vadim V. Neklyudov, Julia Shayimova, Rustem R. Amirov, and Ayrat M. Dimiev* Laboratory for Advanced Carbon Nanomaterials, Kazan Federal University, Kremlyovskaya Street 18, Kazan 420008, Russian Federation Corresponding author: Ayrat Dimiev, Email [email protected] Keywords: graphene oxide, iron nanoparticles, liquid crystals, magneto-optical properties, light scattering Abstract Graphene oxide aqueous solutions are known to form liquid crystals that can switch in electric fields. Magnetic fields as external stimuli are inefficient toward GO due to its diamagnetic properties, and GO is known to be insoluble in most of the organic solvents. In this study, composites of graphene oxide with oleate-protected magnetite nanoparticles were prepared as stable colloid solutions in the mixed isopropanol-chloroform solvents. The structure of the composite particles and the optical properties of their solutions can be controlled by the ratio of the mixing parent components. The as-prepared solutions are highly responsive to external magnetic field. As the consequence, the optical transmission and the direction of light scattering can be efficiently manipulated. These systems pave the way for fabricating functional materials, such as magneto-optical switches, density-gradient materials and micro-motors. Solubility in non-polar organic solvents broadens the scope of their potential applications. 1. Introduction Graphene oxide (GO) is a two-dimensional material, derived from the graphene backbone by oxidation. Due to its unique physical and chemical properties, it was successfully tested in numerous applications ranging from selective membranes and water remediation through electrode materials in energy harvesting and storage devices.1-4

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The presence of the oxygen functional groups on the surface allows this material to form stable colloidal solutions in water and some polar organic solvents.5-7 The functional groups are also responsible for the GO ability to form coordinate-covalent bonding with transition metal cations, and potentially with any Lewis acids.8-9 This property is actively employed in the design and production of composite materials with metals and metal oxides. The lion share of the studies in this field is devoted to composites with nanosized iron oxide, which exhibit magnetic properties, expanding the scope of the GO applications.10-12. For example, this composite can be used for highly processable extraction of ions from aqueous solutions, using magnetic field (magnetic separation)13-15, and as a diagnostic probe in MRI.16 Also, these composites were employed to catalyze the Fisher-Tropsch reacion,17 reduction of nitrobenzene,18 oxidation of cysteine,19 and in the heterogeneous Fenton reactions.20 Due to its 2D structure, in solutions, GO forms nematic liquid crystalline phase (LC) with the phase transition point in accordance with the Onsager theory for disc-like particles.21-25 Such LCs can be ordered into certain supramolecular structures. Due to the liquid state such crystals can change their orientation under the influence of electric field. Thus, the possibility of using GO-based liquid crystals (GO-LC) in electro-optical devices was demonstrated recently by Shen et al.26 Unlike molecular liquid crystals, GO-LCs have large anisotropy and high polarizability, which enables fabrication of electro-optical devices with macroscopic electrodes. Theoretically, the macroscopic changes in LC systems can also be influenced by magnetic field, however, the LCs exhibiting magneto-optical properties have been studied significantly less. In particular, GO is diamagnetic, and its solutions are not responsive even to the high tension magnetic fields. This obstacle can be overcomed by introducing into the system ferromagnetic particles such as iron oxide nanoparticles (IONP). In IONP, the surface iron atoms are Lewis acids and can coordinate different ligands.27 Thus, surface iron atoms ensure the binding of IONPs to functional groups of GO.8 The combination of GO ability to form LC phase, and to bind magnetic nanoparticles can be used for the synthesis of GO based composite magnetic materials. In this study, we propose a method for the preparation of a nanocomposite material consisting of GO flakes and IONPs, and demonstrate magneto-optical properties of their solutions in non-polar organic solvents. 2. Results and Discussion 2.1 Birefringence in flow


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The geometric anisotropy is an intrinsic property for all GO flakes, due to their 2D character. The shift, applied toward GO solutions, forces the GO flakes to align parallel to the shift; this can be observed as birefringence

28-29.

Observation of birefringence in the flow of

liquid is possible if the two conditions are met: 1) the short axis of the particle is smaller than the wavelength of light, and 2) the refractive index for these particles differs from the refractive index of the solvent

30.

Thus, the phenomenon of birefringence in a flow makes it possible to

estimate the degree of the geometric anisotropy of particles, as well as to monitor in-situ the change in the shape of the particles. This effect is best observed when the birefringent material is placed between crossed polarizers (Figure 1a).

Figure 1. (a) An experimental set up for observing birefringence in a flow, (b) the birefringence in the flow for GO-IPA solution (0.2 mg/ml) with different types of agitation, (c) photographs of the solutions of parent components: GO (top line) and IONPs (bottom line) in the mixed IPA-TCM solvents acquired with crossed polarizers (shaking). The cross-shaped image is generated on the back screen. Figure 1b shows that the GO solution in isopropanol (GO-IPA) at concentration of 0.2 mg/ml is isotropic, but when shaken, light patterns caused by birefringence in the flow are observed. In 8 seconds after the agitation the system returns to the initial isotropic state. Since this effect is associated with fluid motion, the shape of the light patterns is affected by the



agitation method. Thus, stirring with a magnetic stirrer leads to the appearance of circular lines aligned along the movement of the stirrer, while shaking results in chaotic patterns of more complex shape. In addition, with shaking the observed patterns are more vivid. Therefore, the rest of the experiments described below were performed by shaking the samples. The GO-IONPs composites were synthesized in the mixture of IPA and trichloromethane (TCM) by mixing the GO-IPA solution with the IONP-TCM solution in different proportions. See Figure S1 for the photographs of parent solutions and intermediate products. The choice of the mixed solvents is explained by the fact that the parent components are soluble in solvents of different nature: GO in polar solvents, while oleate-covered IONPs in non-polar solvents. At the same time IPA and TCM are the two miscible liquids; this makes possible the mixing of the two components in different proportions. Thus, not only the GO/IONP ratio, but also the IPA/TCM ratio inevitably changes at different mixing proportions. This is why, before studying the properties of GO-IONP composites, we studied the optical properties of the parent components GO and IONP in the mixed IPA-TCM solvents of different composition. The as-prepared solutions of GO were stable in the mixed solvents in the whole range of the tested compositions for at least 2 hours. Some coagulation and precipitation was observed in 24 hours for the solutions with TCM content ≥50% (Figure S2). The IONP solutions were stable in the whole range of the tested compositions for at least one month. Expectedly, the birefringence phenomenon was observed in all the solutions of GO, and was not observed in the solutions of IONP (Figure 1c, Supplementary Video 1), which points at the geometric isotropy of the latter. Analysis of the geometry of the parent IONPs by electron microscopy (Figure S3) confirms that IONPs have the shape close to spherical, i.e. geometrically isotropic. In this control experiment (Figure 1c), we demonstrated the birefringence phenomenon on parent components with the known structure and geometry. Below, we employ this phenomenon to estimate the geometric anisotropy of the product GO-IONP nanocomposites. Mixing the two solutions GO-IPA and IONP-TCM leads to chemical interaction between GO and IONP. It is well established that in the iron oxide nanoparticles, the surface iron atoms serve as Lewis acids, and can coordinate broad range of ligands.27 This property makes them to bind oleate tails during the synthesis: the coordinate covalent bonding is formed between a surface iron atom and the carboxylic group of an oleic acid molecule. From another side, GO via its oxygen functionalities can establish coordinate covalent bonding with a broad range of


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transition metal cations, including iron.8-9 Apparently, when the oleate-protected IONP comes into contact with GO, the latter, as a stronger chelating agent, replaces the oleate fragments from the surface iron atoms. The dense arrangement of the oxygen functional groups on GO surface affords formation of the chemical bonding simultaneously with many iron atoms of each IONP; this ensures strong bonding between GO and IONP. The HRTEM images reveal dense and uniform distribution of IONPs on GO surface (Figure S4, Figure S5). The most common size of IONPs is 4.5 ± 0.5 nm (Figure S5). Apparently, IONPs on GO surface preserve their original morphology (compare to Figure S2d). Figure S6 contains some additional characteristics of asformed composites. In particular, powder XRD analysis (Figure S6c) unambiguously confirms presence of magnetite Fe3O4, and EDAX (Figure S6d) reveals the elemental content. Both parent IONPs and product GO-IONPs exhibit strongly paramagnetic properties (Figure S6e,f; see SI section for details). The photographs of the five product solutions (12-16), obtained at different mixing ratios, as well as their content are shown on Figure 2a. The properties and stability of the as-prepared solutions are different. Thus, solutions 12 and 13 were stable for at least 4 hours but precipitated after 24 hours of standing. No birefringence was observed for solution 12; a weak effect was observed for solution 13 (Figure 2b). Solutions 14-16, were stable for at least 24 hours, and exhibited birefringence. For solutions 15 and 16, the effect is weak, and not well-visible on the photographs (Figure 2b). However, it is better pronounced on the respective videos (SI Video 2). The difference in the behavior of solutions 12-13 from that for solutions 14-16 suggests the formation of the two types of composites with different composition and structure (Figure 2c). The lack of birefringence in solution 12 suggests that the particles of as-formed nanocomposites are geometrically isotropic, whereas in solutions 14-16 they are anisotropic. Note, in solutions 12-13, the supernatants over the precipitates are colorless (Figure 2a), indicating absence of IONPs, i.e. efficient binding of all the IONPs by GO flakes.



Figure 2. (a) The photographs and the composition of GO-IONP solutions just after the preparation (top line), and after 24 hours standing (bottom line); in the table the first two lines represent the concentration of the components in the final formulations in mg/ml, the two bottom lines represent volume ratios of the mixed solvents. (b) The photographs of solutions from panel (a), obtained with crossed polarizers immediately after agitation; (c) the scheme for formation of the two types of GO-IONP composites; the oleates chains bound to IONPs are omitted for simplicity. In solution 12, GO is in a large excess relative to IONPs. This is why, in order to attain their coordination capabilities, GO flakes form folded and jammed multi-layered sandwiched structures with limited number of IONPs. This leads to the increase in the thickness of the particles, i.e. to the decrease of their aspect ratio, and as the consequence, to the loss of geometric anisotropy. We denote such type of the multi-layered particles as Composite "A" (Figure 2c). The second important factor is the aggregation, which leads to the precipitation of


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large multi-layered structures upon standing. Both these factors lead to the loss of the birefringence phenomenon in the flow. Solutions 14-16 exhibit birefringence with stirring (Figure 2b). The weak effect for solutions 15 and 16 is caused by a) lowering the GO content (only 2D GO flakes can give birth to geometrically anisotropic particles), and b) by the high content of non-bound IONPs (at high concentrations, they strongly scatter light, thus obscuring the birefringence). Nevertheless, the presence of even weak birefringence in these solutions suggests that, in contrast to solution 12, the particles retain the geometric anisotropy. Most likely, the particles forming in an excess of IONPs, consists of single-layer GO flakes densely covered by IONPs. We denote this type of the particles as Composite "B". Solution 13 demonstrates weak birefringence. It must contain particles with the morphology intermediate between type "A" and type "B". 2.2 Magneto-optical properties The presence of magnetic particles in the composites makes them susceptible to external magnetic field. Therefore, the alignment of the geometrically anisotropic particles, leading to birefringence, can be generated not only by the motion in the fluid, but also by the applied magnetic field. To estimate the influence of the magnetic field on the as-prepared solutions, their transmission was studied as the function of the analyzer rotation angle. The experimental setup used for these measurements is shown in Figure 3a. The correctness of the experimental set up operation was first verified with pure isopropanol (see Supplementary section and Figure S7 for details). Further, the light transmission for solutions 11-17 was measured as the function of the analyzer rotation angle with and without applied magnetic field.



Figure 3. Magneto-optical properties of the GO-IONP solutions. (a) The sketch of the experimental setup for measuring polarized light transmittance. (b) The dependence of the light transmission on the analyzer rotation angle for eight solutions; obtained in presence of the external magnetic field, created by the two square-shaped permanent magnets. The transmission values for each solution at open polarizers are taken as 100%. The curves for solutions 11, 12, 17 and for neat IPA overlap. (c) The photographs of solution 14 in the magnetic field created by electromagnet in "On" and "Off" positions. Without the magnetic field, all the experimental curves coincided with the curve obtained for neat isopropanol, indicating that all the solutions are isotropic (Figure S8). However, the situation changes when the solutions are placed in a constant magnetic field (Figure 3b). Expectedly, the magnetic field had no effect on solutions 11, 12, and 17; the curves overlap with those acquired in the absence of the magnetic field. Solution 11 (GO-IPA) contains


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geometrically anisotropic GO flakes, but they cannot be oriented under the magnetic field; the solution is optically anisotropic. Solution 17, on the contrary, contains magnetic particles. However, the geometric isotropy of the particles also does not make it possible to form an optically anisotropic solution. Solution 12, does not reveal optical anisotropy, consistent with the observations in the fluid motion (Figure 2b), most likely due to the loss of geometric anisotropy with formation of thick multi-layered structures of Composite "A". For solutions 14, 15, 16, the magnetic field not only enhances the transmission, but also shifts the transmission curve to the left (Figure 3b). The observed effect weakens with the decrease in the GO/IONP ratio; the decrease in GO content lowers the concentration of the anisotropic Composite "B" particles in the sequence 14-15-16. Visually, this magneto-optical effect is demonstrated for solution 14 by the experiment shown in Figure 3c (see also SI Video 3). The specific light interference picture appears immediately after turning the electromagnet "On" (the polarizers are crossed). This picture in the shape of the four-petal-flower, or a darkcross is a typical conoscopic image for uniaxial crystals. This observation additionally confirms the fact and suggests the type of the flakes' orientation in the solution under external magnetic field. The optical effect fades away within 5 seconds after turning the magnet "Off". While the optical effect shown on Figure 3b was obtained in the field of 290 mT, the effect demonstrated on Figure 3c was achieved in the field of only 76 mT. In a separate experiment, we tested how the optical effect depends on the strength of the applied magnetic field. As evident from Figure S9 and SI Video 4, the field of 10-13 mT already generates some distinguishable light patterns. The patterns have been becoming brighter with the strength of the applied field in the whole tested range, permitted by the electromagnet. This experiment demonstrates that the optical effect is proportional to the applied magnetic field. Figure S10 and SI Video 5 demonstrate how two different solutions 11 and 14 respond to the magnetic field: the light transmission of solution 14 increases, while solution 11 remains nonresponsive, confirming the difference in the structure of the composites' particles. Atypical behavior was registered for solution 13. Here, the registered increase in transmission at crossed polarizers is associated with the partial removal of the nanocomposite particles from the bulk of the solution at prolonged exposure to the magnetic field (Figure S11). Such behavior was not observed for the rest of the tested solutions. The solution decolorizes, indicating that all GO flakes participate in formation of the GO-IONPs composite, and there are



neither free GO flakes nor free IOPNs in the solution. As a result, the light beam passes through the pure solvent rather than through the colloidal solution. The proportion of the scattered light becomes smaller, and the transmitted light is larger, which leads to the increase in transmission. The separated system easily returns to its original state by shaking the solution (SI Figure S11). Thus, by controlling the GO/IONP ratio one can achieve not only orientation, but also transport of the particles through solution. This phenomenon can be used in the preparation of the density gradient materials. Since the strongest magneto-optical effect was observed with solution 14, the rest of the investigations were conducted with the GO/IONPs ratio corresponding to this solution. As the next step, we investigated if the IPA-TCM solvent composition affects the magnetooptical behavior of solutions at the fixed GO/IONPs ratio. Solution 14 was separated by centrifugation, and the still wet precipitate was redispersed in pure IPA, TCM, and in their mixtures of various compositions. The dispersions of Composite "B" in pure IPA and TCM did not exhibit magneto-optical effects, whereas in the mixed IPA-TCM solvents the effect was observed (Figure 4a). We explain this phenomenon by the partially amphiphilic character of the GO-IONP composites; apparently, they form more stable and more orderly arranged solutions in the mixed solvents rather than in pure polar or non-polar solvents.

Figure 4. (a) Photographs of composite “B” in IPA, TCM and their mixtures with crossed polarizers under the action of the magnetic field. (b) The dependence of the light transmission on the angle of the analyzer for the composite “B” solution (IPA/TCM = 50/50 vol.%) at the two different magnet positions.


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To further demonstrate the magneto-optical properties of the Composite "B" solutions, the dispersion with the concentration twice of that of solution 14 was prepared (IPA/TCM = 50/50, vol%), and the light transmission for this solution was measured as the function of the analyzer angle. Figure 4b shows that the relative light transmission at the crossed polarizers increased compared to solution 14 (compare to Figure 3b), and the curve shifted to the left (black line). Changing the magnets' position shifted the curve to the right (blue line). The obtained regularity indicates the specific orientation of the particles with respect to the magnetic field. To investigate the structure of the synthesized composites, we employed microscopy techniques. When compared to the SEM images of parent GO (SI Figure S12a), the size of the flakes of Composite "B" have been significantly decreased (SI Figure S12b). This is due to the binding IONPs: they sufficiently increase the mass of the flakes, and subsequently, the dynamic load under the sonication, while the strength of GO flakes remains the same. As the result, the flakes efficiently break into smaller fragments. The fuzziness of the boundaries of the Composite "B" particles observed on SI Figure S12b,c is caused by the presence of a high-boiling-point oleic acid fragments remaining on the surface of IONPs. The prolonged exposure to the electron beam during the acquisition removed the fuzziness, and the "islands" of the composite acquired clear boundaries (SI Figure S12d). The distribution of the composite particles over the Si/SiO2 substrate surface is an important feature of this image. For the parent GO, the flakes uniformly cover the entire substrate surface (SI Figure S12a). In contrast, Composite "B" does not readily cover the surface (SI Figure S12b,d), but tends to deposit on a top of the previously precipitated particles. This leads to formation of the multi-particle "islands", each consisting of separate, clearly visible flakes, with diameter ~1 μm. Such deposition is caused by the presence of the hydrophobic fragments of oleic acid on the surface of the Composite "B" particles, which impart the entire composite hydrophobic. The newly precipitating particles tend to deposit on hydrophobic particles rather than on the hydrophilic substrate (SI Figure S12e).



Figure 5. Comparison of the structures of composites "A" (a,b,c) and "B" (d,e,f). (a,d) SEM images; (b,e) AFM images; (c,f) the corresponding height profiles. To explain the difference in the magneto-optical behavior of composites "A" and "B" we estimated their aspect ratio by SEM and AFM analyses (Figure 5). On the SEM image of composite "A" (Figure 5a) one can see contours of some GO flakes due to the partial transparency of the material to electron beam. However, the flakes are highly aggregated, and most of them are folded and crumpled. This is why, on the AFM image the single flakes are not resolved (Figure 5b). The height of the lowest flakes islands is 25-31 nm (Figure 5c), which corresponds to the 4-6 GO/IONP sandwiched layers (1 nm + 4.5 nm = 5.5 nm). Apparently, in the deficit of IONPs, they cannot saturate the GO surface. This leads to the formation of the multi-layered sandwiched structures, as well as to the folding and jamming GO flakes. Such sandwiched and jammed structures have relatively low aspect ratio, and their solutions do not demonstrate optical anisotropy. For Composite "B" the single flakes are well-resolved both on the SEM and AFM images (Figure 5d,e). The flake's thickness is ~13 nm (Figure 5f), which is very close to the thickness of one GO layer, surrounded by IONPs, with their oleate tails, from both sides. Because GO surface is saturated by IONPs, the flakes are not aggregated and not folded. As the consequence, the composite structures poses high aspect ratio, and their solutions demonstrate optical anisotropy in full accordance with our observations, described above (Figure


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2,3,4). Thus, the microscopy methods confirmed our conclusions about the structure of composites "A" and "B" made above based on their optical properties.

Figure 6. (a) The photographs, showing light scattering under the influence of external magnetic field for solutions 13, 14, 15. The two static magnet positions are compared with the situation without magnetic field. The light source is on the right side. (b) The cylindrical permanent magnet is manually rotated near solution 14. The light source is behind the vial. The presence of geometric anisotropy in particles affects not only their ability to form nematic solutions, but also to scatter light. Spherical particles scatter light uniformly in all directions, whereas geometrically anisotropic particles scatter unequally. This difference can serve as an indicator for the presence of geometric anisotropy. In an optically isotropic solution of GO, the flakes are oriented randomly in all directions, and the summative light scattering is the same as for solutions of geometrically isotropic particles. Providing GO flakes magnetic properties changes the picture. Figure 6a shows photographs of light scattering by solutions 13, 14, and 15 with two different magnet orientations and in the absence of magnetic field. The change in the magnet position does not affect light scattering by solution 13. This observation let us classify solution 13 as composite "A", despite observing the weak birefringence effect in the



flow (Figure 2b). For solutions 14 and 15 the change in the magnet position dramatically changes the intensity of the scattered light.

Figure 7. Light scattering under the influence of external magnetic field for solution 14. (a) No magnet; moderate light scattering is observed. (b) The magnet is behind the vial; the magnetic field lines are directed along the z-axis, from the magnet toward the observer. (c) The magnet is beneath the vial; the magnetic field lines are directed vertically, along the y-axis. The light source is on the left side for all the three panels (a-c). (e) and (f) The schematics showing the origin of the observed magneto-optical effects for the situations shown on panels (b) and (c), respectively. The grey circle represents the GO-IONPs flake. The magnetization vector M is directed along the flake's plane; this orients and fixes the flakes parallel to the magnetic field lines. To explain the observed magneto-optical behavior, we tested the light scattering at the third magnet position, as shown on Figure 7. While the magnet positions shown on Figures 7a and 7c


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correspond to those on the left and right columns of Figure 6a, respectfully, the magnet position on Figure 7b is different: the magnet is situated behind the vial with the magnetic field lines parallel to the z-axis, stretched out from the magnet through the vial toward an observer. At this magnet position, the scattering is not observed at all. On Figure 7c, the magnet is beneath the vial; magnetic field lines are directed vertically along the y-axis. At this magnet position, the light scattering is significantly increased compared to the situation without applied magnetic field (Figure 7a), where the composite flakes are randomly oriented. The schematics shown on Figure 7d,e explain the observed phenomena. The applied magnetic field (H) generates in a flake magnetization with the vector (M) laying in the plane of the flake. This orients and fixes the flake along the axis parallel to the magnetic field lines. At such conditions, all the possible orientations of the flakes in solution can be considered as the rotations along this axis. For the magnet position, shown on Figure 7e, the scattered light is always directed toward an observer for all the allowed flake's orientations; the observed scattered light is intense (Figure 7c). For the magnet position shown on Figure 7d, no scattered light is directed toward an observer at all the possible flake's orientations; subsequently, no scattered light is observed (Figure 7b). Thus, the observed light-scattering phenomena additionally confirm the orientation of the GO-IONP flakes in the external magnetic field, and suggest the type of the orientation typical for uniaxial crystals. The rotation of the magnet near solution 14 (Figure 6b, SI Video 7) leads to the change in the direction of the light scattering. Since the orientation of the composite particles is related to the direction of the magnetic field lines, the rotation of the magnet leads to the rotation of the composite particles. As can be seen from the SI Video S7, the solution responds almost momentarily to the magnet rotation. The systems that can convert chemical, thermal or magnetic energy to mechanical motion at the nano and microscale level are of increasing interest today, largely due to the possibility of creating nanoscale mashinery.31-32 At present day, such devices are often prepared by a multistage synthesis with low yields, which greatly complicates their production and leads to an increase in the cost of the designed products. In contrast, our particles can be easily prepared in high yields by simple wet chemistry methods, from inexpensive precursors. In our opinion, the particles of the GO-IONPs composite can act as a promising model for nano- and micro-scale motors driven by an external magnetic field.



In all our discussions above, we explained the difference between Composites "A" and "B" solely by their aspect ratios, or by geometrical anisotropy of the flakes. However, with the light scattering, the situation might be more complex. The light scattering is based on the re-emitting the light by the dipoles, induced in the material by the incident electromagnetic radiation. The induced dipoles are confined within the flake, i.e. limited by the flake's thickness. While in composite "B" the dipoles are confined within a single GO layer, with Composite "A" the situation is more complex. First of all, in the sandwiched structures, the dipoles from the neighboring GO layers will interfere with each other. Secondly, in the jammed and folded structures of Composite "A", there are numerous fragments, which are located perpendicularly or at certain angles to the main flake's body. In these fragments, the induced dipoles are oriented differently from the dipoles of the main flake. This will make the picture even more complex, leading to the loss of the optical effect. Thus, electronic effects might play critical role in the observed optical effects, in addition to the geometrical parameters. The physics behind the observed phenomena is rather complex, and deserves to be investigated in more details in a separate study. 3. Conclusions In this study, composites consisting of nano-sized magnetite and flakes of graphene oxide were prepared by mixing solutions of graphene oxide and oleate-protected magnetite nanoparticles in different ratios in the mixed isopropanol-chloroform solvents. Analysis of these mixtures for the presence of birefringence in the flow and in a magnetic field revealed formation of the two types of composites ("A" and "B") having different shapes and structures, which lead to a difference in their optical properties. Thus, Composite "A" could not form optically anisotropic media, and its structure was defined as multi-layered sandwich. Alternatively, solutions containing composite "B" exhibited birefringence both in the flow and in the magnetic field. The microscopy method analyses confirmed conclusions about the structure of the two types composites made based on their optical properties: the composite "A" particles are multilayered and geometrically isotropic, while the particles of composite "B" are mostly singlelayered and geometrically anisotropic. Dispersions of composite B possess unique magnetooptical properties. The light transmission and the direction of light scattering respond


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momentarily

to the applied magnetic field. This property paves the way for fabricating

functional magneto-responding materials. 4. Experimental Part 4.1 Synthesis of GO and IONP. All the commercially available chemical reagents were at least of analytical reagent grade and used without further purification. GO was synthesized from natural flake graphite by a modified Hummers method described in our previous reports.8,

33

Solutions of GO in isopropanol (IPA) were prepared by the gradual solvent

exchange from water to IPA by the method described elsewhere.34-35 The GO in IPA was obtained in the form of the GO-IPA gel with the GO content of 2.36 wt.%. The synthesis of the iron oxide nanoparticles (IONPs) was carried out by the methods described in refs. 36-37. IONPs were obtained in the form of the hexane solution with the IONP content 6 wt.%). 4.2 Preparation of IONPs solution in chloroform (IONPs-TCM). 2.5 ml solution of IONP in hexane (IONP 6 wt.%) was brought into a 100 ml volumetric flask, and chloroform was added up to the volume 100 ml. After mixing, the solution was sonicated in the ultrasonic bath for 30 minutes at 25-30 °C. The concentration of IONPs in the resulting solution was 1.30 mg/ml. 4.3 Preparation of the solution of GO in isopropanol (GO-IPA). 0.983 g of the GO-IPA paste (GO 2.36 wt.%) was dissolved in 15 ml of isopropanol, and sonicated for 3 hours at 25-30 °C in an ultrasonic bath. Next, the solution was diluted with IPA up to 100 ml in a volumetric flask. The concentration of GO in the resulting solution was 0.20 mg/ml. 4.4 Preparation of GO-IONP composites. GO-IPA solution and IONP-TCM solution were mixed in certain proportions. The resulted solutions were sonicated for 30 min in an ultrasonic bath. 4.5 Characterization. The scanning electron microscopy (SEM) images were acquired with a field-emission high-resolution scanning electron microscope Merlin from Carl Zeiss at accelerating voltage of incident electrons of 5 kV and a current probe of 300 pA. The TEM imaging was carried out in a transmission electron microscope Hitachi HT7700 Excellence at an accelerating voltage of 100 kV in the TEM mode. GO and IONPs particle sizes were measured manually by processing two SEM images for GO and two TEM images for IONPs using an image processing and analysis program -Altami



Studio for each of the SEM and TEM images; the sample size was more than 200 particles. An equivalent diameter was selected as geometric characteristic. 4.6 Magnets and the strengths of applied magnetic field. Two different types of magnets were used in this work. The constant NdFeB magnets were square- and cylindrically-shaped, manufactured by Supermagnete, Germany. The magnetic field according to the manufacturer is ~1.3 Tesla. However, the actual field measured by the magnetometer on the distance 3-5 cm from the magnet surface (the position of the vial with a sample in all the experiments) was 0.29 Tesla. The strength of the magnetic field created by electromagnet was 0.076 Tesla at the maximum of the applied electrical current. Supporting Information Photographs of prepared solutions, XRD spectra, TEM and SEM images, Optical transmission data are available free of charge on the ACS Publications website at DOI: Acknowledgments: The work is performed with support of the subsidy provided to Kazan Federal University for a performance of the Russian Federation Government task in a research area (Grant No. 4.7094.2017/8.9). The authors thank Professor Roman Yusupov for productive discussion of the observed magneto-optical phenomena, and Ildar Gilmutdinov for measuring paramagnetic properties of the solid materials. Conflict of Interests: The authors declare no competing interests.

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Magneto-Optical Properties of the Magnetite-Graphene Oxide

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