Synthesis and characterization of graphene modified by iron oxide nanoparticles

Accepted Manuscript Synthesis and characterization of graphene modified by iron oxide nanoparticles

I.S. Lyubutin, A.O. Baskakov, S.S. Starchikov, Kun-Yauh Shih, Chun-Rong Lin, Yaw-Teng Tseng, Shou-Shiun Yang, Zhen-Yuan Han, Yu.L. Ogarkova, V.I. Nikolaichik, A.S. Avilov PII:

S0254-0584(18)30706-5

DOI:

10.1016/j.matchemphys.2018.08.042

Reference:

MAC 20885

To appear in:

Materials Chemistry and Physics

Received Date:

17 May 2018

Accepted Date:

18 August 2018

Please cite this article as: I.S. Lyubutin, A.O. Baskakov, S.S. Starchikov, Kun-Yauh Shih, ChunRong Lin, Yaw-Teng Tseng, Shou-Shiun Yang, Zhen-Yuan Han, Yu.L. Ogarkova, V.I. Nikolaichik, A.S. Avilov, Synthesis and characterization of graphene modified by iron oxide nanoparticles, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys.2018.08.042

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ACCEPTED MANUSCRIPT Synthesis and characterization of graphene modified by iron oxide nanoparticles I.S. Lyubutin1, A.O. Baskakov1, S.S. Starchikov1*), Kun-Yauh Shih2*), Chun-Rong Lin3, Yaw-Teng Tseng3, Shou-Shiun Yang2, Zhen-Yuan Han2, Yu.L. Ogarkova1, V.I. Nikolaichik4, and A.S. Avilov1 1Shubnikov

Institute of Crystallography of FSRC “Crystallography and Photonics” RAS, Moscow 119333, Russia 2Department of Applied Chemistry, National Pingtung University, Pingtung County 90003, Taiwan 3Department of Applied Physics, National Pingtung University, Pingtung County 90003, Taiwan 4Institute of Microelectronics Technology, RAS, Chernogolovka, Moscow District, 142432, Russia *) Corresponding authors: Prof. Kun-Yauh Shih Department of Applied Chemistry, National Pingtung University, Pingtung County 90003, Taiwan E-mail: [email protected] Dr. S.S. Starchikov Shubnikov Institute of Crystallography of FSRC “Crystallography and Photonics” RAS, Moscow 119333, Russia E-mail: [email protected] Keywords: Magnetite-graphene nanocomposites, core-shell structure, magnetic properties, chargetransfer effect, Raman and Mössbauer spectroscopy Abstract The process of interaction of graphene with iron oxide nanoparticles was investigated. First, graphene oxide (GO) modified with magnetite Fe3O4 nanoparticles was successfully synthesized. Raman and Mössbauer spectroscopy revealed that the magnetite Fe3O4 in combination with GO became non-stoichiometric, and the maghemite phase γ-Fe2O3 appears. Subsequent reduction of graphene oxide by thermal treatment leads to an increase in the fraction of maghemite content and, in addition, the hematite phase α-Fe2O3 appears in the sample annealed at above 500 oC. Meanwhile, the core-shell nanocomposites of FexOy/G appear, were FexOy consists of a mixture of the Fe3O4 , γ-Fe2O3 and α-Fe2O3 phases. The content of each phase can be varied by the annealing temperature. Magnetic, Mössbauer and Raman spectroscopy measurements indicate that graphene can interact with iron oxide. Charge-transfer from iron to graphene can occur due to delocalization of 3d electrons, which reduces the overall magnetic moment of the charge-transfer complexes. These properties can have potential applications in electronic such as supercapacitors, advanced anode materials for lithium-ion batteries, magnetically targeted drug delivery, photothermic therapy, and magnetic resonance imaging.

1. Introduction Graphene, like fullerene and carbon nanotubes are classified as a low-dimensional carbon structure. In the last decade, the study of various kinds of materials and composites based on graphene has been developing significantly for the purpose of their application in such fields as nanotechnology, nanoelectronics, materials science, chemistry and many others. So far, quite a number of possible applications of graphene have been proposed [1, 2], due to the abundance of its 1

ACCEPTED MANUSCRIPT interesting electronic and structural properties [3]. On the basis of graphene, it were encouraged to develop various kinds of nanocomposites using a modification with different nanoparticles [4–6]. Water filtration and creation of anodes for the commonly used lithium-ion batteries can be called among the possible applications of such nanostructures. Among others, nanocomposite based on graphene-modified iron oxide nanoparticles are often used [7–11]. Such composites, along with other practical use, can be applied in targeted drug delivery, supercapacitors, and for shielding electromagnetic radiation. Shortly after discovering of graphene, it was found that a large amount of information about it and other forms of carbon can be obtained by using Raman spectroscopy method [12]. In addition, this method is very sensitive to the properties of graphene and iron oxides. Meanwhile, the Mössbauer spectroscopy gives very important information in the study of composites based on iron oxides such as magnetite Fe3O4 and maghemite γ-Fe2O3, whereas the XRD method mainly shows similar diffraction patterns, which complicates distinguishing of these iron oxides. However, there are practically no publications dealing with the interaction of iron oxide and graphene nanoparticles in similar nanostructures. The features of the influence of these two composite components on each other and the effects occurring in the interface area at the junction of these two substructures have not been studied in practice. This is largely due to the difficulties in determining such subtle effects by modern techniques. In this work the "graphene - iron oxide" nanostructures were synthesized and investigated. The graphene oxide sheets, obtained by a customized Hummer’s method [13], were modified with iron oxide nanoparticles. The samples were prepared under different synthesis conditions through reduction of graphene oxide to graphene performed by heat treatment at temperatures between 500 and 900 ⁰C in air. The samples were investigated by XRD, electron microscopy (TEM and HRTEM), electron diffraction (ED) and magnetic measurements. In addition, Raman and Mössbauer spectroscopy were applied.

2. Materials and methods Materials used: Natural graphite high purity powder, -200 mesh, 99.9999% (metals basis) from Alfa Aesar, Johnson Matthey Co.Ltd. Ferrous sulfate heptahydrate (granular) from J.T. Baker Co.Ltd. The H2O2 from Showa Co.Ltd. All the other chemicals were analytical grade without further purification. All aqueous solutions were prepared with deionized water. The graphene oxide (GO) was synthesized from graphite powder by the modified Hummer's method. In this process, concentrated H2SO4 was stirred in ice bath at 5 oC. Graphite (1.5 g) and NaNO3 (1.5 g) were added with stirring for 30 min, and then KMnO4 (9 g) was added and the mixture was stirred for 2 h. Deionized water of 220 ml was added to the pasty mixture solution under stirring and the color of the solution turned to yellowish brown. H2O2 (30%, 10 mL) was added and the color turned deep green immediately. The solution was filtered and then dispersed in DI water. The mixture was washed until pH of the solution became neutral. The resultant material was dried in oven at 80 oC overnight and used for the experiment. Preparation of the reference sample of Fe3O4 (without graphene): Sodium citrate dihydrate (2 g) was added in 50 ml DI water. Iron(II) sulfate heptahydrate (0.5 g) were dissolved in 2.5 mL DI water. The iron solution and sodium hydroxide solution (2M 7.5 mL) were added to the sodium citrate dihydrate solution and sonicated for 0.5 h. The mixture was heated under reflux for 3 h. After that, the solution was filtered and then redispersed in propanone to remove impurities from the sample. The resulting material was dried in an oven at 80 ° C to remove excess propanone. Preparation of Fe3O4 /GO: 0.05 g GO was exfoliated in 50 ml DI water by ultrasonication for 30 min and form the GO suspension. Iron(II) sulfate heptahydrate (0.5 g) were dissolved in 2.5 mL DI water. The iron solution and sodium hydroxide solution (2M 7.5 mL) were added to the GO suspension and sonicated for 0.5 h. The mixture was heated under reflux for 3 h. After that, the Fe3O4/GO composite solution was filtered and then redispersed in propanone. The resultant material 2

ACCEPTED MANUSCRIPT was dried in oven at 80 oC overnight. Preparation of the reduced GO with magnetite (Fe3O4 /G): 12 g sodium citrate dihydrate and 0.3 g GO were exfoliated in 300 mL DI water by ultrasonication for 30 min thus forming a suspension of GO. Iron(II) sulfate heptahydrate (3 g) were dissolved in 15 mL DI water. The Iron(II) sulfate heptahydrate solution and sodium hydroxide solution (2M 45 mL) were added slowly into the GO suspension under stirring and then sonicated for 0.5 h. The mixture was heated under reflux for 3 h. After that, the iron oxide/GO composite solution was filtered and then redispersed in propanone. The resultant material was dried in oven at 80 oC overnight. Finally, several samples were sintered in furnace at temperatures between 500 and 900 oC for 1 h in air. A schematic diagram of the synthesis procedure is shown in Fig. 1. It was supposed that under annealing, the graphene oxide (GO) will be reduced to graphene (G) and thus the Fe3O4/G composite can be created.

Fig. 1. Schematic illustration of the synthesis procedure for FexOy –RGO nanocomposite.

3. Instrumentation (Methods of characterization) The crystal structure and phase purity of the samples were examined by X-ray powder diffraction (XRD, Mutiflex MF2100, Rigaku Co. Ltd) with the CuKα radiation ( = 1.5418 Å). The morphology and microstructure of the particles were characterized by transmission electron microscopy (TEM, JEOL JEM-2000FX equipped with INCA system of Oxford Instruments) with an accelerating voltage up to 150 kV. Raman spectra were obtained with a Princeton Instruments Acton SP2500 monochromator /spectrograph equipped with Spec-10 system with nitrogen cooled CCD detector. Spectra-Physics Beamlock 2080 Krypton laser with 647.1 nm line was used as an excitation source for a Raman signal. The Mössbauer absorption spectra of 57Fe nuclei were recorded at room and 90 K temperatures with a standard MS-1104Em spectrometer operating in the constant accelerations regime. The gamma-ray source 57Co(Rh) was at room temperature, and a 3

ACCEPTED MANUSCRIPT metal α-Fe standard absorber was used for calibration. Magnetic properties were analyzed at room temperature using a Vibrating sample magnetometer in an applied magnetic field up to 15 kOe.

4. Structural properties from XRD, TEM and ED methods XRD analysis of all samples (Fig. 2) revealed reflections that can readily be indexed into a cubic phase with a spinel-type structure (space group Fd3m). These reflections correspond to magnetite Fe3O4 or/and to maghemite γ-Fe2O3 phases, and in the case of nanoparticles it is hardly to distinguish these two compounds only from XRD. Meanwhile, the Mössbauer spectroscopy is sensitive to iron valence state, and it can be applied for identification of these phases. An average particle size, estimated by Scherrer formula, is about 20 nm for the FexOy particles in graphene, and it is about 30 nm for the reference FexOy sample without graphene. In addition, unexpected peaks were observed in FexOy/G samples at about 24°, 33°, 41°, 49°, 54° and 64°, which can be well assign to the rhombohedral hematite α-Fe2O3 phase [ICSD #33643]. It was observed that these peaks became sharper under thermal treatment revealing the highest crystallinity of hematite nanoparticles in the sample annealed at 900oC (RF-900). Typically, the reduced graphene oxide has a broad XRD peak at about 26° (see the bottom pattern in Fig. 2) [14]. In the FexOy/GO sample, we found a much narrow peak at 26° and an additional peak at 41°. These peaks correspond to the (002) and (100) planes of graphene oxide nanoparticles, respectively. As shown below, the second peak was also observed in the electron diffraction pattern.

Fig. 2. XRD patterns of nanocomposites composed of the graphene oxide (GO) and reduced graphene oxide (RF) modified by iron oxide FexOy. The patterns of the reference Fe3O4 sample without graphene (top pattern) and the reduced graphene oxide sample without iron oxide (bottom paten) are also shown. 4

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Transmission electron microscopy (TEM) images of the graphene oxide (FexOy/GO) and the graphene oxide reduced under heat treatment at 900oС (FexOy/G) modified by FexOy nanoparticles are shown in Fig. 3(a,b) and Fig. 3(d,e), respectively. In both samples, the TEM images display two-dimensional plates of iron oxide with a characteristic size of about 30-50 nm in plane and about 3-5 nm in thickness. A large plate of graphene oxide is clearly seen as a light-grey color particle in Fig. 3a. As seen in Fig. 3e, after the reduction process, the iron oxide nanoparticles (dark color) became covered by a graphene layer (light-gray color), which thickness is about 3-5 nm. Thus, the core-shell FexOy /G composites create under annealing.

Fig. 3. TEM images of the FexOy/GO sample (a,b) composed of the graphene oxide (GO) (lightgray plate) modified with iron oxide FexOy nanoparticles (dark color particles) and the FexOy/G (RF-900) sample obtained by thermal treatment at 900 oC (d,e). The electron diffraction patterns of the FexOy/GO sample (c) and FexOy/G (RF-900) sample (f) are also shown. From the electron diffraction (ED) patterns shown in Fig. 3(c,f), the experimental values of interplanar distances dhkl were calculated for the FexOy/GO and FexOy /G samples (Table 1). Their comparison with the values known from literature for the space group Fd3m indicates that both magnetite Fe3O4 and maghemite γ-Fe2O3 can be present in these samples. In addition, a ring corresponding to dhkl = 2.232 Å, which is close to that of the (100) planes in graphene oxide was observed. Furthermore, the electron diffraction data of the reduced sample FexOy/G (RF-900) revealed the presence of reflections corresponding to the rhombohedral hematite phase α-Fe2O3, whereas, there are no hematite reflections in the FexOy/GO sample. This is in accordance with the XRD and Mössbauer data. More detailed information on the phase composition of iron oxides was obtained from Mössbauer measurements given below. 5

ACCEPTED MANUSCRIPT Table 1. Electron diffraction reflexes of the FexOy/GO and FexOy/G (RF-900) samples. FexOy/GO Ring №

Diameter, nm-1

dhkl , Å (exp.)

Intensity*)

Phase

hkl

dhkl , Å (tabulated)

1

18.14

2.977

m

Fe3O4

220

2.968

2

21.16

2.552

vs

Fe3O4

311

2.531

3

25.65

2.105

w

Fe3O4

400

2.099

4

31.52

1.713

w

Fe3O4

422

1.713

5

33.27

1.623

w

Fe3O4

511

1.615

6

36.34

1.486

s

Fe3O4

440

1.484

111

4.846

220

2.968

311

2.531

400

2.099

FexOy/G (RF-900) 1

11.13

4.852

vw

Fe3O4

2

14.86

3.634

w

α- Fe2O3

3

18.45

2.927

m

Fe3O4

4

20.02

2.697

m

α- Fe2O3

5

21.51

2.51

vs

Fe3O4

6

24.49

2.205

w

α- Fe2O3

7

26.06

2.072

w

Fe3O4

8

29.44

1.834

w

α- Fe2O3

9

31.88

1.694

m

Fe3O4

422

1.713

10

33.82

1.597

m

Fe3O4

511

1.615

11

36.51

1.479

s

Fe3O4

440

1.484

12

37.45

1.442

s

α- Fe2O3

*) Intensity: s is strong, vs is very strong, m is medium, w is weak, vw is very weak

4. Raman spectroscopy In the measurements of Raman spectra, the laser power on the sample was about 0.5 mW. Fig. 4 shows the Raman spectra of the samples obtained in the whole range of 200-3000 cm-1. The spectrum of the reference Fe3O4 sample (shown in inset of Fig. 4) revealed a high intensive mode at about 663 cm-1 and small broadened bands at about 310 and 520 cm-1, which are typical of the Fe3O4 compound [15, 16]. In addition, we observed a peak at 370 cm-1 belonging to maghemite γFe2O3, which typical peaks are also present at 377, 510, 670, 715 cm-1 [16]. The FexOy/GO sample demonstrates the similar spectrum where a mixture of magnetite Fe3O4 and maghemite γ-Fe2O3 is present. The appearance of D and G peaks in the spectrum of the FexOy/GO sample is typical for carbon structures, thus confirming the presence of graphene oxide. The strong D peak at about 1325 cm-1 is due to the breathing mode of carbon atoms in the defected C6 rings, and the G-peak at around 1580 cm-1 is related to bond stretching of sp2 carbon pairs in both rings and chains [13]. Usually, the ratio value ID/IG is widely used for characterization of carbon structures disorder. Intensive 2D peak occurred in RF-900 sample at 2660 cm-1 corresponds to the overtone of the D peak. 6

ACCEPTED MANUSCRIPT As suggested in [17,18], the crystallite sizes La of the nanographite particles can be calculated by Eq. (1) from the ratio between the integrated intensities of the D and G bands (ID/IG) taking into account the excitation laser energy in the visible range: 𝐿𝑎 = (2.4 ∙ 10

‒ 10

() 𝐼

)𝜆4𝑙 𝐼𝐷𝐺

‒1

.

(1)

‒ 10

Here λl is the laser wavelength and (2.4 ∙ 10 ) is the empirical coefficient of proportionality obtained in [17]. The calculated values given in Table 2 for all our samples very well correlate with electron microscopy data. For example, according to Fig. 3e, the diameter of the graphite shell in the RF900 sample is about 45 - 50 nm, whereas the Raman scattering data give 46 nm. Table 2. Characteristic lateral size of graphene particles in the FexOy/GO and FexOy/G samples calculated from Eq.(1). The numbers in the sample designation indicate the annealing temperature.

Sample GO-Fe3O4 RF500 RF600 RF700 RF800 RF900

ID/IG 3.37 3.36 3.61 3.37 0.68 0.92

Size (La), nm 12 13 12 12 62 46

The absence of typical peaks of iron oxides in the spectrum of RF-900 sample (Fig. 4) indicates that the carbon shell shields iron oxide nanoparticles from laser radiation. This is an additional indication that iron oxide nanoparticles are totally covered by the carbon layers. We found that the Raman spectra of RF-800 and RF-900 samples are significantly different from the spectra of the other samples. As seen in Fig. 5, the intensity ratio ID/IG for these samples approaches the value typical of graphite. As shown above, TEM images (Fig. 3e) revealed that the graphene shell covering iron oxide nanoparticles is about 3-5 nm in thickness, which corresponds to about 10-15 graphene layers. This implies that in these samples graphitization begins under thermal treatment at 800 – 900 °C. This temperature is much lower than the value of 1600°C observed in [19] for the reduced graphene oxide heated in nitrogen. Furthermore, we found that the G-band in the FexOy/GO sample at frequency 1588 cm-1 shifts to 1579 cm-1 in the FexOy/G (RF-900) sample under reduction of graphene oxide to graphene. This may be due to the presence of peak D', which also originates from defects and is located near G peak. Thus, the position of the peak G may depend on the intensity of the peak D ', which can vary with heat treatment. On the other hand, as shown in [20, 21], a decrease in the Raman G-band frequency occurs when graphene interacts with the electron-donating compound and the frequency increases when the electron-accepting compounds interact with graphene. This implies that in our samples magnetite nanoparticles act as electron donors to graphene, and charge-transfer interaction between graphene and iron oxide takes place. This effect is supported by the transformation of magnetite Fe3O4 to maghemite γ-Fe2O3 determined by our Mössbauer measurements given below in Section 5.

7

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Fig. 4. Room temperature Raman spectra for the FexOy particles combined with graphene oxide (GO) and with graphene oxide reduced under annealing at 500 oC (RF-500) and at 900oC (RF-900). Inset shows the Raman spectra in the near-frequency region for the reference iron oxide FexOy nanoparticles.

Fig. 5. The intensity ratio ID/IG of the Raman peaks for FexOy/G samples depending on the annealing temperature. Dashed straight lines are the values for our FexOy/GO sample and for bulk graphite. Solid line is a guide to the eye.

5. Mössbauer spectroscopy Mössbauer spectroscopy is a very effective method of identifying the phase composition and features of the magnetic properties of nanomaterials based on iron oxides. In particular, magnetite Fe3O4 and maghemite γ-Fe2O3 have a similar crystal structure of spinel and their identification by structural methods is very difficult, especially in the case of a mixture of different phases in the sample. Fig. 6 shows Mössbauer spectra of the reference sample of iron oxide nanoparticles 8

ACCEPTED MANUSCRIPT (without graphene), the FexOy particles combined with graphene oxide (FexOy/GO) and with graphene oxide FexOy/G reduced under annealing at 500 and 900oC (RF-500, RF-900). The spectra exhibits magnetic hyperfine splitting at room temperature (Fig. 6a) indicating that iron ions are in magnetically ordered state. The spectra lines are essentially broadened due to the overlap of several components corresponding to different nonequivalent states of iron ions. At room temperature, the lines are broadened into the inner part of spectra which is an evidence of superparamagnetic behavior. The central doublet in the spectrum of FexOy/GO sample, which was not annealed, indicates that a part of iron ions is in a paramagnetic state. Apparently, this originates from very small particles appearing due to the distribution of the particle size. This paramagnetic fraction decreases under the sample annealing and vanishes in the RF-900 sample (Fig. 6a). As is well known, the iron ions in the spinel structure of magnetite Fe3O4 are located in the tetrahedral (A) and octahedral [B] oxygen sites (Fe3+) [Fe3+ Fe2+]O4, whose population S is in the ratio SB/SA = 2.0. Both trivalent and divalent iron ions are in B-sites, however, at room temperature, a fast electronic exchange Fe3+ ⇆ Fe2+ occurs, which leads to an average valence of iron Fe2.5+ at the B-site [22]. On the other hand, maghemite γ-Fe2O3 can also have a cubic structure of spinel, and its chemical formula can be represented as a nonstoichiometric magnetite containing vacancies □ in octahedral sites (Fe3+) [Fe3+5/6 □1/6]2O4. In this case, the populations of [B] and (A) sites by iron ions should be in the ratio SB/SA = 1.66. However, all the iron ions in the maghemite are in the trivalent state. The hyperfine parameters of the Mössbauer spectra are different for Fe3+ and Fe2+ ions in [B] and (A) sites, and this makes it possible to distinguish between the magnetite and maghemite phases. The spectra processing of all FexOy/GO and FexOy/G nanoparticles was performed with a SpectrRelax program [23] that uses methods based on the recovery of hyperfine parameters distribution with the variation of fitting models, taking into account the distribution of nanoparticles in size. The distribution function of hyperfine magnetic fields P(H) was obtained for each sextet component corresponding to the [B] and (A) magnetic sublattices. At room temperature, the calculated hyperfine parameters for these components in the reference sample of Fe3O4 are: the isomer shift δA = 0.29(1) mm/s, δB = 0.61(1) mm/s, and magnetic 𝐴 𝐵 hyperfine field at iron nuclei 𝐻ℎ𝑓 = 484(2) kOe, 𝐻ℎ𝑓= 455(2) kOe. The value of δA = 0.29 mm/s corresponds to the high spin state of Fe3+ ions, while δB = 0.61 mm/s is a characteristic of iron ions with an intermediate degree of oxidation of about Fe2.5 +. This is a clear indication of the magnetite phase, in which an electronic exchange between Fe3+ and Fe2+ ions in B-sites leads to an intermediate valence state Fe3O4 = (Fe3+) [Fe3+ ⇆ Fe2+]O4 = (Fe3+) [Fe2.5+]2O4. At 90 K, the hyperfine components with the isomer shift of about δB = (0.98 - 1.3) mm/s, typical of Fe2+, appear in the Mössbauer spectrum of the reference sample Fe3O4 indicating stabilization of the ferrous iron in the B-sites due to freezing (blocking) of the electronic Fe3+ ⇆ Fe2+ exchange (Fig. 6b). This is a signature of the Verwey transition, which is expected in bulk magnetite at about TV ≈ 120 K [22]. However, only 13% of iron Fe2+ observed in the spectrum, which means that the electronic Fe3+ ⇆ Fe2+ exchange freezes only partly. Other hyperfine parameters are very close to the parameters of magnetite nanoparticles [24]. Furthermore, the obtained SB/SA ratio is about 1.9 instead of 2.0, which indicates that magnetite in the reference sample is slightly nonstoichiometric, and its formula can be given as (Fe3+)[Fe2.5+1.9 □0.1]O4, where □ is the vacancy. Mössbauer parameters of the FexOy/GO sample (Fig. 6b) indicate that all iron ions are in the ferric Fe3+ state, whereas no ferrous iron Fe2+ was found. Two magnetic components of the spectrum with the field Hhf values of 520 and 483 kOe correspond to the B- and A-sites of Fe3+ in maghemite γ-Fe2O3.The magnetic components with the similar parameters were also found in spectra of the reduced samples RF-500 and RF-900. The obtained SB/SA ratio in these samples is about 1.6 and 1.7, respectively, which is close to the theoretical value of 1.67 for bulk maghemite 9

ACCEPTED MANUSCRIPT (Fe)[Fe5/3□1/3]O4. This indicates that the synthesis of iron oxide nanoparticles in combination with graphite oxides (GO) leads to the formation of maghemite γ-Fe2O3 nanoparticles. An additional Mössbauer component, which has a lower value of the magnetic field Hhf , was found in the FexOy/GO and FexOy/G samples (Table 3). It can be attributed to iron ions at surface (and interface) layers of maghemite interacting with graphene. The reduction of the Hhf value can be associated with delocalization of d electrons transferring from iron to graphene. In the case of Fe3+ (3d5) ions d-orbitals are half filled, and the charge transfer from the outermost d-orbit of iron to graphene reduces the magnetic moment of iron. This correlates with the Raman spectroscopy data implying the charge-transfer interaction between graphene and iron oxide. Furthermore, after reduction of the graphene oxide, an additional component appears in the Mössbauer spectra of the FexOy/G samples RF-500 and RF-900, which hyperfine parameters correspond to the hematite α-Fe2O3 phases (Fig. 6b, Table 3). All iron ions are in ferric Fe3+ state in both samples, and a mixture of maghemite γ-Fe2O3 and hematite α-Fe2O3 phases is present. This observation well agrees with the data of electron diffraction and XRD. As follows from the area of the Mössbauer components, the fraction of the hematite α-Fe2O3 phase in the FexOy/G samples increases from about 5 to 11% (in iron content) as the temperature of synthesis was increased from 500oC (sample RF-500) to 900oC (sample RF-900). Perhaps, the formation of hematite nanoparticles is caused by the presence of maghemite nanoparticles not covered with graphene.

Fig. 6. Mössbauer spectra at temperatures 295 (a) and 90 K (b) of the reference iron oxide Fe3O4 nanoparticles and of the FexOy particles combined with graphene oxide (GO) and with graphene oxide reduced under annealing at 500 oC (RF-500) and 900oC (RF-900). Solid lines are the calculated subspectra corresponding to nonequivalent iron sites. 10

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Table 3. Mössbauer hyperfine parameters of the reference iron oxide FexOy nanoparticles and the FexOy particles combined with graphene oxide (GO) and with graphene oxide reduced under annealing at 500 oC (RF-500) and 900oC (RF-900) measured at 90 K. δ is the isomer shift, Δ is the quadrupole shift, Hhf is the magnetic hyperfine field at iron nuclei, S is the area of the component. NN

δ, mm/s

Δ, mm/s

Hhf, kOe

S* rel.,%

Assignment

Fe3O4 S1 S2 S3

0.41(2) 0.02(2) 0.52(2) -0.03(2) 0.64(2) -0.13(2)

504(1) 524(1) 477(1)

34(1) 25(1) 28(1)

Fe3+ (tet) Fe3+ (oct) Fe3+ (oct)

S4 S5

0.98(2) 1.30(2)

451(1) 327(1)

9(1) 4(1)

Fe2+ (oct) Fe2+ (oct)

42(1) 34(1) 20(1)

Fe3+ (oct) Fe3+ (tet) Fe3+ surface/interface small NPs

0.43(2) 1.32(2)

FexOy/GO S1 S2 S3

0.46(2) 0.46(2) 0.52(2)

0.09(2) -0.01(2) 0.14(2)

D1

0.24(2)

0.69(2)

520(1) 483(1) 425(1)

4(1)

FexOy/G (RF-500) S1

0.46(2)

0.11(2)

535(1)

5(1)

α-Fe2O3

S2

0.48(2) -0.14(2)

522(1)

47(1)

Fe3+ (oct)

S3

0.43(2) -0.07(2)

501(1)

29(1)

Fe3+ (tet)

S4

0.46(2) -0.06(2)

467(1)

16(1)

D1

0.47(2)

Fe3+ surface/interface small NPs

0.84(2)

3(1)

G-FexOy/G (RF-900) S1

0.45(2)

0.26(2)

538(1)

11(1)

α-Fe2O3

S2

0.49(2) -0.11(2)

523(1)

47(1)

Fe3+ (oct)

S3

0.42(2)

0.01(2)

502(1)

28(1)

Fe3+ (tet)

S4

0.45(2) -0.01(2)

471(1)

14(1)

Fe3+ surface/interface

6. Magnetic measurements Field dependences of magnetization at room temperature are shown in Fig. 7 for the reference sample of magnetite nanoparticles, and for iron-oxide nanoparticles in the combination with graphene oxide FexOy/GO and with reduced graphene oxide FexOy/G (RF-500) and FexOy/G (RF-900). In all samples, magnetization is totally saturated in the applied field above 3 kOe. In the reference nanoparticles of magnetite Fe3O4 the value of saturation magnetization Ms is about 70.0 emu/g, which is lower than the theoretical value for the bulk magnetite (92 emu/g) [25]. Besides the 11

ACCEPTED MANUSCRIPT surface contribution, which is usually involved to explain the effect of lowering the magnetization in nanoparticles, the magnetite non-stoichiometry can be the main reason. Cation vacancies □, present in octahedral sites of the nonstoichiometric magnetite (Fe3+)[Fe2.5+(2-x) □x]O4, reduce the total magnetic moment of such a ferrimagnet. Magnetization of iron oxide FexOy nanoparticles modified by graphene is significantly reduced (Fig. 7), and the Ms value varies from about 21 to 25 emu/g depending on the thermal treatment. It seems that this effect cannot be explained only by the creation of an antiferromagnetic phase of hematite α-Fe2O3 under reduction of FexOy/GO. According to the Mössbauer data, the fraction of hematite in the RF-500 and RF-900 samples is only about 5 and 11%, respectively. First of all, the decrease of magnetization in the FexOy nanoparticles modified by graphene is associated with a large amount of diamagnetic carbon. The presence of γ-Fe2O3 maghemite and a small amount of α-Fe2O3 hematite also leads to a decrease in magnetization in comparison with pure magnetite Fe3O4. However, as it is indirectly indicated by the data of Raman and Mössbauer spectroscopy, it is necessary to take into account the possible delocalization of 3d electrons by the charge transfer from iron to graphene in the interface layer of the core-shell (FexOy/G) nanocomposites, which can lead to a decrease in the total magnetic moment of such a complex. Novel magnetic properties of 2D single-layer graphene modified with metal oxide NPs were investigated by B. Das et al. [26] using the first-principles density functional theory (DFT) calculations. It was shown that the metal oxide NPs can act as electron donors to graphene, and charge-transfer interaction between graphene and iron oxide can occur. In our case, this agrees with the conversion of magnetite Fe3O4 to maghemite γ-Fe2O3 under the influence of graphene as it was observed from our Mössbauer measurements. The overall magnetic moment of the composite system FexOy/graphene should decrease in comparison with the free iron oxide NPs. This is consistent with our experimental data. As discussed in [26], the magnetic and electronic properties of graphene modified by metal oxide nanoparticles are mainly regulated by charge transfer interactions. Such properties can have potential applications in electronic devices.

Fig. 7. Field dependences of magnetization for the reference nanoparticles of magnetite, and for iron-oxide nanoparticles in combination with graphene oxide FexOy/GO and with reduced graphene oxide samples FexOy/G (RF-500) and FexOy/G (RF-900) at room temperature. Inset shows attraction of FexOy/G nanoparticles by an external magnet.

7. Conclusions 12

ACCEPTED MANUSCRIPT Graphene modified by iron oxide nanoparticles FexOy/G was successfully synthesized. The graphene oxide (GO) was synthesized from graphite powder by the modified Hummer's method, and then the GO suspension was combined with the iron solution and exposed to heat treatment. It was supposed that under annealing, GO should be reduced to graphene and thus the Fe3O4/G composite can be created. Raman and Mössbauer spectroscopy revealed that magnetite Fe3O4 synthesized in combination with graphite oxides (GO) becomes non-stoichiometric and maghemite phase γ-Fe2O3 appeared in the FexOy/GO sample. Subsequent reduction of GO by thermal treatment leads to an increase in the fraction of maghemite content and, in addition, a small amount of the hematite phase α-Fe2O3 appears in the sample annealed above 500 ºC. The hematite phase α-Fe2O3 probably occurs in particles which are not covered by graphene. This indicates that the high-temperature reduction of graphene oxide is accompanied by oxidation of iron nanoparticles, and thus these processes are interrelated. TEM images clearly show the core-shell structure of γ-Fe2O3/G nanocomposites annealed at 900 °C. The thickness of the shell is about 3-5 nm and the shell mainly consists of graphite, which was proved by Raman spectroscopy. Magnetization of the magnetite nanoparticles bound to graphene decreases significantly, which is associated with delocalization of 3d electrons due to charge transfer from iron to graphene in the interface layer of the core-shell nanocomposites. The magnetic and electronic properties of graphene modified by metal oxide nanoparticles are regulated by charge transfer interactions. These properties can have potential applications in electronic such as supercapacitors, advanced anode materials for lithium-ion batteries, magnetically targeted drug delivery, photothermic therapy, and magnetic resonance imaging. Acknowledgments This work was supported by the Russian Scientific Foundation (Project #14-12-00848-P) in part of Mössbauer and Raman spectroscopy studies and by the Ministry of science and higher education within the State assignment FSRC «Crystallography and Photonics» RAS (Agreement No 007ГЗ/Ч3363/26) in part of electron diffraction analysis. We also thank the Ministry of Science and Technology of Taiwan (MOST 106-2112-M-153-001-MY3) for financial support.

References [1] Shahil KM, Balandin AA. Thermal properties of graphene and multilayer graphene: Applications in thermal interface materials. Solid State Communications 2012;152(15):1331–40. [2] Avouris P, Xia F. Graphene applications in electronics and photonics. MRS Bull. 2012;37(12):1225–34. [3] Allen MJ, Tung VC, Kaner RB. Honeycomb carbon: a review of graphene. Chemical reviews 2010;110(1):132–45. [4] Meidanchi A, Akhavan O. Superparamagnetic zinc ferrite spinel–graphene nanostructures for fast wastewater purification. Carbon 2014;69:230–8. [5] Chang J, Huang X, Zhou G, Cui S, Hallac PB, Jiang J et al. Multilayered Si nanoparticle/reduced graphene oxide hybrid as a high-performance lithium-ion battery anode. Advanced materials (Deerfield Beach, Fla.) 2014;26(5):758–64. 13

ACCEPTED MANUSCRIPT [6] Wang D, Li X, Yang J, Wang J, Geng D, Li R et al. Hierarchical nanostructured coreshell Sn@C nanoparticles embedded in graphene nanosheets: spectroscopic view and their application in lithium ion batteries. Physical chemistry chemical physics PCCP 2013;15(10):3535–42. [7] Singh K, Ohlan A, Pham VH, R B, Varshney S, Jang J et al. Nanostructured graphene/Fe₃O₄ incorporated polyaniline as a high performance shield against electromagnetic pollution. Nanoscale 2013;5(6):2411–20. [8] Qu Q, Yang S, Feng X. 2D sandwich-like sheets of iron oxide grown on graphene as high energy anode material for supercapacitors. Advanced materials (Deerfield Beach, Fla.) 2011;23(46):5574–80. [9] Ma X, Tao H, Yang K, Feng L, Cheng L, Shi X et al. A functionalized graphene oxideiron oxide nanocomposite for magnetically targeted drug delivery, photothermal therapy, and magnetic resonance imaging. Nano Res. 2012;5(3):199–212. [10] Yang X, Chen C, Li J, Zhao G, Ren X, Wang X. Graphene oxide-iron oxide and reduced graphene oxide-iron oxide hybrid materials for the removal of organic and inorganic pollutants. RSC Adv. 2012;2(23):8821. [11] Yang X, Zhang X, Ma Y, Huang Y, Wang Y, Chen Y. Superparamagnetic graphene oxide–Fe3O4 nanoparticles hybrid for controlled targeted drug carriers. J. Mater. Chem. 2009;19(18):2710. [12] Ferrari AC, Meyer JC, Scardaci V, Casiraghi C, Lazzeri M, Mauri F et al. Raman spectrum of graphene and graphene layers. Physical review letters 2006;97(18):187401. [13] Hummers WS, Offeman RE. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958;80(6):1339. [14] Vargas O, Caballero, Morales J. Enhanced Electrochemical Performance of Maghemite/Graphene Nanosheets Composite as Electrode in Half and Full Li: Ion Cells. Electrochimica Acta 2014;130:551–8. [15] Faria DLA de, Venâncio Silva S, Oliveira MT de. Raman microspectroscopy of some iron oxides and oxyhydroxides. J. Raman Spectrosc. 1997;28(11):873–8. [16] Nieuwoudt MK, Comins JD, Cukrowski I. The growth of the passive film on iron in 0.05 M NaOH studied in situ by Raman micro-spectroscopy and electrochemical polarisation. Part I: Near-resonance enhancement of the Raman spectra of iron oxide and oxyhydroxide compounds. J. Raman Spectrosc. 2011;42(6):1335–9. [17] L. G. Cançado,a_ K. Takai, and T. Enoki, M. Endo, Y. A. Kim, and H. Mizusaki, A. Jorio, L. N. Coelho, R. Magalhães-Paniago, and M. A. Pimenta, “General equation for the determination of the crystallite size La of nanographite by Raman spectroscopy”, Appl. Phys. Lett. 88, 163106 (2006) doi: 10.1063/1.2196057 [18] Andrew J. Pollard, Barry Brennan, Helena Stec, Bonnie J. Tyler, Martin P. Seah, Ian S. Gilmore, and Debdulal Roy, “Quantitative characterization of defect size in graphene using Raman spectroscopy” Appl. Phys. Lett. 105, 253107 (2014) [19] Mikhailov S. Physics and applications of graphene: Experiments. Rijeka: InTech; 2011. [20] Rakesh Voggu, Barun Das, Chandra Sekhar Rout and C N R Rao. Effects of charge transfer interaction of graphene with electron donor and acceptor molecules examined using Raman spectroscopy and cognate techniques. J. Phys.: Condens. Matter 2008;20(47):472204. 14

ACCEPTED MANUSCRIPT [21] Das B, Voggu R, Rout CS, Rao CNR. Changes in the electronic structure and properties of graphene induced by molecular charge-transfer. Chemical communications (Cambridge, England) 2008(41):5155–7. [22] Hargrove RS, Kündig W. Mössbauer measurements of magnetite below the Verwey transition. Solid State Communications 1970;8(5):303–8. [23] Matsnev ME, Rusakov VS. SpectrRelax: An application for Mössbauer spectra modeling and fitting. In: AIP; 2012, p. 178–185. [24] Dézsi I, Fetzer C, Gombkötő Á, Szűcs I, Gubicza J, Ungár T. Phase transition in nanomagnetite. Journal of Applied Physics 2008;103(10):104312. [25] Han DH, Wang JP, Luo HL. Crystallite size effect on saturation magnetization of fine ferrimagnetic particles. Journal of Magnetism and Magnetic Materials 1994;136(12):176–82. [26] Das B, Choudhury B, Gomathi A, Manna AK, Pati SK, Rao CNR. Interaction of inorganic nanoparticles with graphene. Chemphyschem a European journal of chemical physics and physical chemistry 2011;12(5):937–43.

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ACCEPTED MANUSCRIPT Highlights

1. Graphene modified by iron oxide nanoparticles FexOy/G was successfully synthesized (82) 2. The nanocomposites FexOy/G have a core-shell type design (57) 3. High temperature treatment leads to reduction of the GO and γ-Fe2O3 formation (77) 4. The charge-transfer interaction between graphene and iron oxide is discussed (76) ===================== Additional 5. Magnetic and electronic properties of such a complex are modified by charge-transfer from iron to graphene. (107) 6. Magnetite act as electron donors to graphene, and charge-transfer interaction between graphene and iron oxide takes place. (121) 7. Charge-transfer from iron to graphene occurs due to delocalization of 3d electrons in the interface layer of the core-shell nanocomposites (138) 8. Such properties can have potential applications in many electronic devices, (76) such as supercapacitors, advanced anode materials for lithium-ion batteries, magnetically targeted drug delivery, photothermal therapy, and magnetic resonance imaging . 9. XRD, TEM, ED, Raman and Mössbauer spectroscopy and magnetic measurements were used for characterization of the FexOy/G nanocomposites. (133)