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Synthesis, nanostructure and magnetic properties of FeCo-reduced graphene oxide composite films by one-step electrodeposition
Thin Solid Films 597 (2015) 1–6
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Synthesis, nanostructure and magnetic properties of FeCo-reduced graphene oxide composite films by one-step electrodeposition Derang Cao a, Hao Li b, Zhenkun Wang a, Jinwu Wei a, Jianbo Wang a, Qingfang Liu a,⁎ a b
Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, People's Republic of China Key Laboratory for Special Function Materials and Structural Design of the Ministry of the Education, Lanzhou University, Lanzhou 730000, People's Republic of China
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Article history: Received 27 October 2014 Received in revised form 8 November 2015 Accepted 9 November 2015 Available online 12 November 2015 Keywords: FeCo-reduced graphene oxide Composite films Magnetic properties Electrodeposition
a b s t r a c t FeCo-reduced graphene oxide (FeCo-RGO) composite film was fabricated on indium tin oxide substrate using one-step electrodeposition method. Raman spectroscopy and field emission scanning electron microscope results show that the reduced graphene oxide is coprecipitated with the FeCo film. The energy-dispersive spectrometer results demonstrate that the atomic ratio of Fe/Co in FeCo-RGO composite film is larger than that of the FeCo film under the same fabrication condition. As a result, the FeCo-RGO composite film exhibits good soft magnetic properties and high-frequency properties as well as the FeCo film. The magnetic anisotropy field and saturation magnetization of FeCo-RGO composite film are increased when compared with FeCo film. Furthermore, the ferromagnetic resonance frequency is improved from 2.15 GHz for the FeCo film to 3.9 GHz for the FeCo-RGO composite film. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Graphene, an attractive carbon material, has enjoyed significant attention in recent years due to its unique properties, such as remarkable chemical tolerance, high electrical conductivity and special optical properties [1–4] and its composite materials also show great potential applications in various fields [5–7]. However, due to its poor solubility and irreversible agglomeration in water and organic solvents as well as the difficulties in large-scale synthesis of perfect graphene [8–10], the practical applications of graphene are challenged. Compared with graphene, graphene oxide (GO) exhibits some advantages, for example, the easy availability of bulk quantities, readiness for functioning in chemical reaction, good dispersion in water and high biocompatibility [9,11,12]. Usually, the GO is reduced to graphene or becomes reduced graphene oxide (RGO) during the compositing process. The synthesis of graphene-based composite materials initially begins with RGO and metal nanoparticles [13–15]. In view of this, metal nanoparticles decorated with RGO composites have been the focus of researches in recent years, and these composites contain multifunctional areas such as catalytic, optoelectronic and magnetic materials [16–18]. Recently, electrochemical experiments of RGO have been developed, and it has been widely used in the preparation of its composite materials [19–21]. Those results provide the possibility to fabricate the RGO-metal or RGO-alloy composite film. Herein, several types of metal films such as Ag, Pt, Fe, Co, and Ni composited with graphene or ⁎ Corresponding author. E-mail address: [email protected] (Q. Liu).
http://dx.doi.org/10.1016/j.tsf.2015.11.022 0040-6090/© 2015 Elsevier B.V. All rights reserved.
RGO have been electrodeposited and showed excellent properties [16, 22–24]. Magnetic metal and alloy film, typically, FeCo-based films, as the particular soft magnetic films, are promising in high frequency applications due to its high saturation magnetization (4πMs), appropriate magnetic anisotropy (Hk) and a low coercivity (Hc) [25,26]. Fabrication and investigation of FeCo-RGO composite film are meaningful to further prove their magnetic properties or other properties. In this work, an easy, cost-effective and nature-friendly synthesis of the FeCo-RGO composite film on indium tin oxide (ITO) glass slides was achieved by one-step electrodeposition method. The resultant composite film shows a favorable nanostructure and morphology. The composite film also exhibits good soft magnetic properties and high frequency properties.
2. Experimental 2.1. Materials Graphite powders (99.85%, purity) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Sulfuric acid (H2SO4, 98%), ethanol (C2H5OH, 99.7%), phosphoric acid (H3PO4, 99.5%), potassium permanganate (KMnO4, 99.9%), hydrochloric acid (HCl, 37%), hydrogen peroxide (H2O2, 30%), ferrous sulfate heptahydrate (FeSO4·7H2O, 99.9%), cobalt sulphate heptahydrate (CoSO4·7H2O, 99.5%), boric acid (H3BO3, 99.5%), ascorbic acid (C6H8O6, 99.7%) and saccharine (C7H5O3NS, 99%) were used without further purification. Distilled water was used throughout the whole sample preparation.
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D. Cao et al. / Thin Solid Films 597 (2015) 1–6 Table 1 The contents of electrolyte compositions and the operating conditions of electrodeposition. Reagents/conditions
Concentration/conditions
FeSO4·7H2O CoSO4·7H2O H3BO3 C6H8O6 C7H5O3NS GO Bath temperature Operating time Deposition potential pH Anode Cathode Magnetic field
0.05 mol/L 0.05 mol/L 30 g/L 1 g/L 2 g/L 2 g/L Room temp. 800 s −1.3 V 3.5 Pt plate ITO glass 800 Oe
2.2. Preparation of graphene oxide Pure GO solution was synthesized from natural graphite powders by an improved Hummers' method [27], by which the toxic gas was not produced and the temperature is easily controlled. In the beginning, conc. H2SO4 (180 mL) and H3PO4 (20 mL) were added into a flask filled with a mixture of KMnO4 (9 g) and graphite powders (1.5 g). The above mixture was then heated up to 50 °C and stirred with a Teflon-coated magnetic stirring bar for 24 h. Whereafter, the solution was cooled to room temperature and poured onto ice (which was cooled by 200 mL distilled water) with H2O2 (3 mL). Finally, the solution was washed and centrifuged until pH = 7 (this is the obtained solution which will be used for the following codeposition). The pristine GO film was also collected on ITO substrate by dewatering and drying GO solution for verification and comparison.
2.3. Preparation of the FeCo-RGO film Electrochemical experiments were carried out in a conventional three-electrode configuration using an electrochemical workstation (CHI 860D). The reference electrode (RE) was the saturated calomel electrode (SCE). The working electrode (WE) was the indium tin oxide (ITO) conductive glass, and a platinum (Pt) strip with an area of 4 cm2 was the counter electrode (CE). All deposition potential (− 1.3 V) in our experiments referred to the SCE. The contents of
electrolyte in amounts and the operating conditions were given in Table 1. At the beginning, the pure RGO film was deposited from the GO solution. Afterwards, two sets of chemicals (each contains the same amount of FeSO4·7H2O, CoSO4·7H2O, H3BO3, C6H8O6 and C7H5O3NS) were dissolved in distilled water and the GO solution respectively. One deposition chemical that dissolved in distilled water is used for the deposition of FeCo film, and another same chemicals that dissolved in GO solution is used to deposit the composite FeCo-RGO film. The schematic diagram of the sample preparation was shown in Fig. 1(b). These two deposition solutions were then stirred for 1 h and ultrasonicated for 2 h separately before deposition. A magnetic field about 800 Oe (a horseshoe shaped permanent magnet) was applied to induce the magnetic anisotropy during deposition. The experimental schematic diagram was shown in Fig. 1(a). In more detail, prior to the deposition, the ITO glasses were cleaned by sonication in pure acetone and alcohol for 30 min respectively, and washed by distilled water finally. Pt sheet was cleaned using dilute hydrochloric acid and distilled water. The other conditions for the FeCo and FeCo-RGO films are the same except the deposition solution. All electrodes were immersed in the same electrolyte, and the deposition experiments were performed at room temperature.
2.4. Characterization Crystal structure was characterized by X-ray diffraction (XRD, D/ MAX-2400, Analytical X'Pert Pro) equipped with Cu-Kα radiation (λ = 1.5406 Å). The configuration of the XRD was using θ/θ model by the collimated light. Raman spectra were recorded using a Renishaw Raman system spectrometer (J.Y. HR800) with the 514.5 nm line of a Spectra 9000 Photometrics Ar ion laser. Atomic force microscopy (AFM) was used to image the topography of film surfaces with a MFP3D™ microscope (Asylum Research, USA) using AR mode. The morphology of the samples were imaged by a field emission scanning electron microscopy (SEM, Hitachi S-4800, Japan) equipped with an energydispersive spectrometer (EDS). The SEM operating voltage is 5 kV and the major operating voltage used for the EDS measurements is 25 kV. The thicknesses of the films were measured with a Surface Profilometer (VEECO Dektak-8). The magnetic properties of the films were measured by a vibrating sample magnetometer (VSM) (Lakeshore 7304). Permeability spectra of the samples were obtained with a vector network analyzer (VNA, PNA E8363B) ranging from 100 MHz to 10 GHz by a shorted microstrip transmission-line perturbation method.
Fig. 1. Scheme of the electrodeposition process (a) and sample preparation (b).
D. Cao et al. / Thin Solid Films 597 (2015) 1–6
Fig. 2. XRD patterns of the pristine GO, FeCo film and FeCo-RGO composite film.
3. Results and discussion XRD patterns were obtained to investigate the phase and structure of the synthesized films. Fig. 2 shows the XRD patterns of the pristine GO powder, FeCo film and FeCo-RGO composite film. The XRD pattern of GO presents a sharp peak at 2θ = 10.7°, which is assigned to the
Fig. 3. Raman spectra of the pristine GO, RGO film, FeCo film and FeCo-RGO composite film.
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(001) peak [28]. The result reveals that many of the oxygen atoms are intercalated into the interlayer space. The weak diffraction peak located at 44.8° is assigned to FeCo (110) crystalline structure of FeCo and FeCoRGO films. As seen in the figure, the (110) peak of FeCo-RGO film broadens when compared with FeCo film, and it also shifts to the high angle, which may be due to the involved RGO that enhances the inner-stress of the film. More specifically, only FeCo phase is observed in FeCo-RGO composite film, and no diffraction peak of RGO is found. This indicates the absence of layer-stacking regularity of RGO or the relatively low content of RGO in the composite. Raman spectroscopy is a powerful nondestructive tool to distinguish the structure, crystallization and defects of carbonaceous materials [29, 30]. Fig. 3 shows the Raman spectra of the pristine GO, RGO film, FeCo film and FeCo-RGO composite film. Raman spectrum of the pristine GO displays two prominent D and G peaks at 1348 and 1585 cm− 1 and two weak 2D and D + G bands around 2690 and 2930 cm−1, respectively. As is well known, G band usually represented the E2g phonon of C sp2 atoms, and D band is a breathing mode of κ-point phonons of A1g symmetry [31], which are assigned to the local defects of disordered graphene and graphite platelets [32]. In addition, the intensities of the D and G peaks are reversed once graphene oxide is reduced to RGO. The 2D band and D + G peak, as the second order of zone-boundary phonons and the result of lattice disorders [33] respectively, refer to the combination of the G and D peaks [10,34]. As expected, no Raman scattering peaks are observed in the FeCo film [35] compared with other samples. The Raman spectrum of FeCo-RGO and RGO filmsexhibits the same characteristic peaks as GO, which show that the codeposition of FeCo and RGO on ITO substrate is well performed. Furthermore, the changed intensities of the G and D bands also indicate the formation of RGO. The as-prepared RGO and FeCo-RGO films show a blue shift of the G band when compared with that of GO [7,14], and it is also a general feature of the reduction of GO. The D/G intensity ratios of electrodeposited RGO (D/G = 1.14) and FeCo-RGO (D/G = 1.13) have increased compared with that of GO (D/G = 0.99). The results indicate that the defect regions are enhanced when the GO is reduced to RGO during deposition [36,37]. Moreover, it has been reported that the shape and position of the 2D band are related to the formation and layer number of graphene or RGO sheets [38,39]. In our present work, the intensity of the observed 2D band of FeCo-RGO increases slightly when compared with that of GO, which indicates the presence of bulk RGO in the composite, namely, the cotton-like layer GO structure is broken and then becomes discontinuous and fragmentary slice after reduction. The SEM images of the GO reveal a wrinkled texture which is associated with the presence of flexible and ultrathin GO sheets (Fig. 4(a) and (b)). The typical surface morphologies of electrodeposited
Fig. 4. Typical SEM images of the GO (a) and (b), the FeCo film (c) and (d), and the FeCo-RGO composite film (e), (f), (g) and (h). The dashed line serves as a guide to verify the partly codeposition of FeCo and RGO.
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D. Cao et al. / Thin Solid Films 597 (2015) 1–6
Table 2 The elemental composition of the films is analyzed by EDS. Samples
FeCo
FeCo-RGO
Measure times
1 2 3 Average 1 2 3 Average
Atom conc. % Fe
Co
C
O
41 41 45 42 15 28 20 21
59 59 55 58 18 31 22 24
/ / / / 45 16 36 32
/ / / / 22 25 22 23
FeCo are shown in Fig. 4(c) and (d), in which the nanoparticles with about 20 nm diameters can be easily observed. The SEM images of FeCo-RGO film are also shown in Fig. 4(e), (f), (g) and (h). The nanoparticles of FeCo and the thin blanket of RGO are seen clearly. Especially in Fig. 4(e) and (f), some of the RGO sheets grow at the boundary between the nanoparticles, while some of them deposit on the surface of the nanoparticles. This indicates that the RGO was not precipitated on the FeCo film during deposition but co-deposited with them. Moreover, from Fig. 4(e), the particle sizes of FeCo-RGO film increase, and the particle densities become more compacter when compared with pure FeCo film. According to the above results, we may deduce the codeposition process as follows. In the beginning, small GO sheets are adsorbed on cobalt and iron ions, and codeposit with them in cathode, then form a heterozygous structure. During deposition, the GO sheets are also reduced to RGO, which are grown at the grain boundary, and then form some small sheets around FeCo nanoparticles. In addition, compared with pure GO, the results shown in Fig. 4(a), (b) and (h) indicate that the cotton-like layer GO structure (Fig. 4(a) and (b)) is broken and then becomes discontinuous and fragmentary slice (Fig. 4(h)) after reduction. Those results are verified with the Raman results. EDS results of the FeCo and FeCo-RGO films are shown in Table 2. Every sample is probed three times at different regions to obtain the average atom conc. % of the FeCo film and FeCo-RGO composite film. Obviously, it is verified that RGO is codeposited with FeCo film. As shown in Table 2, the atomic ratio of Fe/Co is improved from 0.72 for the FeCo film to 0.88 for the FeCo-RGO composite film, which indicates that the deposition speed of iron and cobalt ions is changed due to the incorporation of GO solution. The reason how the GO affects the deposition speed of Fe and Co ions is under studied. It may be related to the chemical bond between the GO and Fe or Co ions. As is well known, more Fe ions are beneficial to the higher saturation in this Fe content range of FeCo-RGO composite film [40,41]. This can be corroborated in the VSM and VNA results below.
Fig. 6. The in-plane hysteresis loops of the FeCo film and FeCo-RGO composite film.
Table 3 The thickness, 4πMs, Hk and Hc of the FeCo film and FeCo-RGO composite film. Samples
Thickness (nm)
4πMs (T)
Hk (kA/m)
Hc (kA/m)
FeCo
764
1.27
4.37
FeCo-RGO
763
1.40
4.77
Hce = 2.14 Hch = 0.59 Hce = 2.62 Hch = 0.60
Atomic force microscopy (AFM) is currently the foremost method for the definitive identification of single-layer crystals and roughness [42]. Fig. 5 shows the plan-view of FeCo film and FeCo-RGO composite film. The results reveal that RMS roughness decreased when RGO is incorporated in FeCo thin film. That is because GO solution may supply appropriate condition to enhance the compactness of the film during deposition. In addition, some RGO sheets cover the FeCo film making the film surface smoother and more uniform, which in turn, may prevent the peeling of the nanosheets [43]. The hysteresis loops of the thin films are shown in Fig. 6. The typical magnetic parameters 4πMs, Hk and Hc are obtained from the loops, and the results are presented in Table 3, where 4πMs is the saturation magnetization, Hk is the magnetic anisotropy, and Hce and Hch are the easy axis and hard axis coercivities, respectively. The thicknesses of the films shown in Table 3 provide a parameter to confirm the 4πMs. Clearly, Hk, 4πMs and Hce of the FeCo-RGO film are larger than those of the FeCo film. The improved coercivity of FeCo-RGO may be caused by the pinning effect from RGO deposited between the FeCo particles. Furthermore, the higher Fe/Co atomic ratio in the FeCo-RGO film than that of
Fig. 5. The plan-view images of FeCo film (a) and FeCo-RGO composite film (b).
D. Cao et al. / Thin Solid Films 597 (2015) 1–6
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References
Fig. 7. The frequency dependence on complex permeability of the FeCo film and FeCo-RGO composite film.
the FeCo film may be the main reason why the 4πMs of FeCo-RGO film is larger than that of FeCo film. The permeability spectra of the films are shown in Fig. 7, where μ′ and μ″ represent the real and imaginary parts of complex permeability, respectively. The microwave field is performed along the easy axis of the film plane. It can be seen that the full width at half maximum of FeCo-RGO peaks decreases. This result is consistent with the AFM observations; improved material properties are related to smoother and flat films. Moreover, the resonance frequency fr increases from 2.15 GHz for FeCo film to 3.9 GHz for FeCo-RGO composite film. The fr of samples can be described by the Kittel equation [44] fr = (γ/2π)·(4πMs·Hk)1/2 for magnetic resonance. By this equation, it is easy to know that the resonance frequency is enhanced when Ms and Hk increase.
4. Conclusions The FeCo film and FeCo-RGO composite film were electrodeposited on ITO coated glass by a constant potential method without using any reducing agent. The Raman spectra and SEM results show that the codeposition of FeCo and RGO on ITO substrates is well performed. And EDS and SEM indicate that the deposition speed and particle size of iron and cobalt ions are changed during the codeposition due to the incorporation of RGO. The composite FeCo-RGO film deposited on ITO exhibits good soft magnetic properties and high-frequency properties. The Hk and 4πMs increase from 4.37 kA/m and 1.27 T for the FeCo film to 4.77 kA/m and 1.40 T for the FeCo-RGO composite film. Significantly, the fr increases from 2.15 for FeCo film to 3.9 GHz for the FeCo-RGO composite film.
Acknowledgments This work is supported by the National Basic Research Program of China (2012CB933101) and the National Science Fund of China (11574121, 51371092).
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Synthesis, nanostructure and magnetic properties of FeCo-reduced graphene oxide composite films by one-step electrodeposition Derang Cao a, Hao Li b, Zhenkun Wang a, Jinwu Wei a, Jianbo Wang a, Qingfang Liu a,⁎ a b
Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, People's Republic of China Key Laboratory for Special Function Materials and Structural Design of the Ministry of the Education, Lanzhou University, Lanzhou 730000, People's Republic of China
a r t i c l e
i n f o
Article history: Received 27 October 2014 Received in revised form 8 November 2015 Accepted 9 November 2015 Available online 12 November 2015 Keywords: FeCo-reduced graphene oxide Composite films Magnetic properties Electrodeposition
a b s t r a c t FeCo-reduced graphene oxide (FeCo-RGO) composite film was fabricated on indium tin oxide substrate using one-step electrodeposition method. Raman spectroscopy and field emission scanning electron microscope results show that the reduced graphene oxide is coprecipitated with the FeCo film. The energy-dispersive spectrometer results demonstrate that the atomic ratio of Fe/Co in FeCo-RGO composite film is larger than that of the FeCo film under the same fabrication condition. As a result, the FeCo-RGO composite film exhibits good soft magnetic properties and high-frequency properties as well as the FeCo film. The magnetic anisotropy field and saturation magnetization of FeCo-RGO composite film are increased when compared with FeCo film. Furthermore, the ferromagnetic resonance frequency is improved from 2.15 GHz for the FeCo film to 3.9 GHz for the FeCo-RGO composite film. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Graphene, an attractive carbon material, has enjoyed significant attention in recent years due to its unique properties, such as remarkable chemical tolerance, high electrical conductivity and special optical properties [1–4] and its composite materials also show great potential applications in various fields [5–7]. However, due to its poor solubility and irreversible agglomeration in water and organic solvents as well as the difficulties in large-scale synthesis of perfect graphene [8–10], the practical applications of graphene are challenged. Compared with graphene, graphene oxide (GO) exhibits some advantages, for example, the easy availability of bulk quantities, readiness for functioning in chemical reaction, good dispersion in water and high biocompatibility [9,11,12]. Usually, the GO is reduced to graphene or becomes reduced graphene oxide (RGO) during the compositing process. The synthesis of graphene-based composite materials initially begins with RGO and metal nanoparticles [13–15]. In view of this, metal nanoparticles decorated with RGO composites have been the focus of researches in recent years, and these composites contain multifunctional areas such as catalytic, optoelectronic and magnetic materials [16–18]. Recently, electrochemical experiments of RGO have been developed, and it has been widely used in the preparation of its composite materials [19–21]. Those results provide the possibility to fabricate the RGO-metal or RGO-alloy composite film. Herein, several types of metal films such as Ag, Pt, Fe, Co, and Ni composited with graphene or ⁎ Corresponding author. E-mail address: [email protected] (Q. Liu).
http://dx.doi.org/10.1016/j.tsf.2015.11.022 0040-6090/© 2015 Elsevier B.V. All rights reserved.
RGO have been electrodeposited and showed excellent properties [16, 22–24]. Magnetic metal and alloy film, typically, FeCo-based films, as the particular soft magnetic films, are promising in high frequency applications due to its high saturation magnetization (4πMs), appropriate magnetic anisotropy (Hk) and a low coercivity (Hc) [25,26]. Fabrication and investigation of FeCo-RGO composite film are meaningful to further prove their magnetic properties or other properties. In this work, an easy, cost-effective and nature-friendly synthesis of the FeCo-RGO composite film on indium tin oxide (ITO) glass slides was achieved by one-step electrodeposition method. The resultant composite film shows a favorable nanostructure and morphology. The composite film also exhibits good soft magnetic properties and high frequency properties.
2. Experimental 2.1. Materials Graphite powders (99.85%, purity) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Sulfuric acid (H2SO4, 98%), ethanol (C2H5OH, 99.7%), phosphoric acid (H3PO4, 99.5%), potassium permanganate (KMnO4, 99.9%), hydrochloric acid (HCl, 37%), hydrogen peroxide (H2O2, 30%), ferrous sulfate heptahydrate (FeSO4·7H2O, 99.9%), cobalt sulphate heptahydrate (CoSO4·7H2O, 99.5%), boric acid (H3BO3, 99.5%), ascorbic acid (C6H8O6, 99.7%) and saccharine (C7H5O3NS, 99%) were used without further purification. Distilled water was used throughout the whole sample preparation.
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D. Cao et al. / Thin Solid Films 597 (2015) 1–6 Table 1 The contents of electrolyte compositions and the operating conditions of electrodeposition. Reagents/conditions
Concentration/conditions
FeSO4·7H2O CoSO4·7H2O H3BO3 C6H8O6 C7H5O3NS GO Bath temperature Operating time Deposition potential pH Anode Cathode Magnetic field
0.05 mol/L 0.05 mol/L 30 g/L 1 g/L 2 g/L 2 g/L Room temp. 800 s −1.3 V 3.5 Pt plate ITO glass 800 Oe
2.2. Preparation of graphene oxide Pure GO solution was synthesized from natural graphite powders by an improved Hummers' method [27], by which the toxic gas was not produced and the temperature is easily controlled. In the beginning, conc. H2SO4 (180 mL) and H3PO4 (20 mL) were added into a flask filled with a mixture of KMnO4 (9 g) and graphite powders (1.5 g). The above mixture was then heated up to 50 °C and stirred with a Teflon-coated magnetic stirring bar for 24 h. Whereafter, the solution was cooled to room temperature and poured onto ice (which was cooled by 200 mL distilled water) with H2O2 (3 mL). Finally, the solution was washed and centrifuged until pH = 7 (this is the obtained solution which will be used for the following codeposition). The pristine GO film was also collected on ITO substrate by dewatering and drying GO solution for verification and comparison.
2.3. Preparation of the FeCo-RGO film Electrochemical experiments were carried out in a conventional three-electrode configuration using an electrochemical workstation (CHI 860D). The reference electrode (RE) was the saturated calomel electrode (SCE). The working electrode (WE) was the indium tin oxide (ITO) conductive glass, and a platinum (Pt) strip with an area of 4 cm2 was the counter electrode (CE). All deposition potential (− 1.3 V) in our experiments referred to the SCE. The contents of
electrolyte in amounts and the operating conditions were given in Table 1. At the beginning, the pure RGO film was deposited from the GO solution. Afterwards, two sets of chemicals (each contains the same amount of FeSO4·7H2O, CoSO4·7H2O, H3BO3, C6H8O6 and C7H5O3NS) were dissolved in distilled water and the GO solution respectively. One deposition chemical that dissolved in distilled water is used for the deposition of FeCo film, and another same chemicals that dissolved in GO solution is used to deposit the composite FeCo-RGO film. The schematic diagram of the sample preparation was shown in Fig. 1(b). These two deposition solutions were then stirred for 1 h and ultrasonicated for 2 h separately before deposition. A magnetic field about 800 Oe (a horseshoe shaped permanent magnet) was applied to induce the magnetic anisotropy during deposition. The experimental schematic diagram was shown in Fig. 1(a). In more detail, prior to the deposition, the ITO glasses were cleaned by sonication in pure acetone and alcohol for 30 min respectively, and washed by distilled water finally. Pt sheet was cleaned using dilute hydrochloric acid and distilled water. The other conditions for the FeCo and FeCo-RGO films are the same except the deposition solution. All electrodes were immersed in the same electrolyte, and the deposition experiments were performed at room temperature.
2.4. Characterization Crystal structure was characterized by X-ray diffraction (XRD, D/ MAX-2400, Analytical X'Pert Pro) equipped with Cu-Kα radiation (λ = 1.5406 Å). The configuration of the XRD was using θ/θ model by the collimated light. Raman spectra were recorded using a Renishaw Raman system spectrometer (J.Y. HR800) with the 514.5 nm line of a Spectra 9000 Photometrics Ar ion laser. Atomic force microscopy (AFM) was used to image the topography of film surfaces with a MFP3D™ microscope (Asylum Research, USA) using AR mode. The morphology of the samples were imaged by a field emission scanning electron microscopy (SEM, Hitachi S-4800, Japan) equipped with an energydispersive spectrometer (EDS). The SEM operating voltage is 5 kV and the major operating voltage used for the EDS measurements is 25 kV. The thicknesses of the films were measured with a Surface Profilometer (VEECO Dektak-8). The magnetic properties of the films were measured by a vibrating sample magnetometer (VSM) (Lakeshore 7304). Permeability spectra of the samples were obtained with a vector network analyzer (VNA, PNA E8363B) ranging from 100 MHz to 10 GHz by a shorted microstrip transmission-line perturbation method.
Fig. 1. Scheme of the electrodeposition process (a) and sample preparation (b).
D. Cao et al. / Thin Solid Films 597 (2015) 1–6
Fig. 2. XRD patterns of the pristine GO, FeCo film and FeCo-RGO composite film.
3. Results and discussion XRD patterns were obtained to investigate the phase and structure of the synthesized films. Fig. 2 shows the XRD patterns of the pristine GO powder, FeCo film and FeCo-RGO composite film. The XRD pattern of GO presents a sharp peak at 2θ = 10.7°, which is assigned to the
Fig. 3. Raman spectra of the pristine GO, RGO film, FeCo film and FeCo-RGO composite film.
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(001) peak [28]. The result reveals that many of the oxygen atoms are intercalated into the interlayer space. The weak diffraction peak located at 44.8° is assigned to FeCo (110) crystalline structure of FeCo and FeCoRGO films. As seen in the figure, the (110) peak of FeCo-RGO film broadens when compared with FeCo film, and it also shifts to the high angle, which may be due to the involved RGO that enhances the inner-stress of the film. More specifically, only FeCo phase is observed in FeCo-RGO composite film, and no diffraction peak of RGO is found. This indicates the absence of layer-stacking regularity of RGO or the relatively low content of RGO in the composite. Raman spectroscopy is a powerful nondestructive tool to distinguish the structure, crystallization and defects of carbonaceous materials [29, 30]. Fig. 3 shows the Raman spectra of the pristine GO, RGO film, FeCo film and FeCo-RGO composite film. Raman spectrum of the pristine GO displays two prominent D and G peaks at 1348 and 1585 cm− 1 and two weak 2D and D + G bands around 2690 and 2930 cm−1, respectively. As is well known, G band usually represented the E2g phonon of C sp2 atoms, and D band is a breathing mode of κ-point phonons of A1g symmetry [31], which are assigned to the local defects of disordered graphene and graphite platelets [32]. In addition, the intensities of the D and G peaks are reversed once graphene oxide is reduced to RGO. The 2D band and D + G peak, as the second order of zone-boundary phonons and the result of lattice disorders [33] respectively, refer to the combination of the G and D peaks [10,34]. As expected, no Raman scattering peaks are observed in the FeCo film [35] compared with other samples. The Raman spectrum of FeCo-RGO and RGO filmsexhibits the same characteristic peaks as GO, which show that the codeposition of FeCo and RGO on ITO substrate is well performed. Furthermore, the changed intensities of the G and D bands also indicate the formation of RGO. The as-prepared RGO and FeCo-RGO films show a blue shift of the G band when compared with that of GO [7,14], and it is also a general feature of the reduction of GO. The D/G intensity ratios of electrodeposited RGO (D/G = 1.14) and FeCo-RGO (D/G = 1.13) have increased compared with that of GO (D/G = 0.99). The results indicate that the defect regions are enhanced when the GO is reduced to RGO during deposition [36,37]. Moreover, it has been reported that the shape and position of the 2D band are related to the formation and layer number of graphene or RGO sheets [38,39]. In our present work, the intensity of the observed 2D band of FeCo-RGO increases slightly when compared with that of GO, which indicates the presence of bulk RGO in the composite, namely, the cotton-like layer GO structure is broken and then becomes discontinuous and fragmentary slice after reduction. The SEM images of the GO reveal a wrinkled texture which is associated with the presence of flexible and ultrathin GO sheets (Fig. 4(a) and (b)). The typical surface morphologies of electrodeposited
Fig. 4. Typical SEM images of the GO (a) and (b), the FeCo film (c) and (d), and the FeCo-RGO composite film (e), (f), (g) and (h). The dashed line serves as a guide to verify the partly codeposition of FeCo and RGO.
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Table 2 The elemental composition of the films is analyzed by EDS. Samples
FeCo
FeCo-RGO
Measure times
1 2 3 Average 1 2 3 Average
Atom conc. % Fe
Co
C
O
41 41 45 42 15 28 20 21
59 59 55 58 18 31 22 24
/ / / / 45 16 36 32
/ / / / 22 25 22 23
FeCo are shown in Fig. 4(c) and (d), in which the nanoparticles with about 20 nm diameters can be easily observed. The SEM images of FeCo-RGO film are also shown in Fig. 4(e), (f), (g) and (h). The nanoparticles of FeCo and the thin blanket of RGO are seen clearly. Especially in Fig. 4(e) and (f), some of the RGO sheets grow at the boundary between the nanoparticles, while some of them deposit on the surface of the nanoparticles. This indicates that the RGO was not precipitated on the FeCo film during deposition but co-deposited with them. Moreover, from Fig. 4(e), the particle sizes of FeCo-RGO film increase, and the particle densities become more compacter when compared with pure FeCo film. According to the above results, we may deduce the codeposition process as follows. In the beginning, small GO sheets are adsorbed on cobalt and iron ions, and codeposit with them in cathode, then form a heterozygous structure. During deposition, the GO sheets are also reduced to RGO, which are grown at the grain boundary, and then form some small sheets around FeCo nanoparticles. In addition, compared with pure GO, the results shown in Fig. 4(a), (b) and (h) indicate that the cotton-like layer GO structure (Fig. 4(a) and (b)) is broken and then becomes discontinuous and fragmentary slice (Fig. 4(h)) after reduction. Those results are verified with the Raman results. EDS results of the FeCo and FeCo-RGO films are shown in Table 2. Every sample is probed three times at different regions to obtain the average atom conc. % of the FeCo film and FeCo-RGO composite film. Obviously, it is verified that RGO is codeposited with FeCo film. As shown in Table 2, the atomic ratio of Fe/Co is improved from 0.72 for the FeCo film to 0.88 for the FeCo-RGO composite film, which indicates that the deposition speed of iron and cobalt ions is changed due to the incorporation of GO solution. The reason how the GO affects the deposition speed of Fe and Co ions is under studied. It may be related to the chemical bond between the GO and Fe or Co ions. As is well known, more Fe ions are beneficial to the higher saturation in this Fe content range of FeCo-RGO composite film [40,41]. This can be corroborated in the VSM and VNA results below.
Fig. 6. The in-plane hysteresis loops of the FeCo film and FeCo-RGO composite film.
Table 3 The thickness, 4πMs, Hk and Hc of the FeCo film and FeCo-RGO composite film. Samples
Thickness (nm)
4πMs (T)
Hk (kA/m)
Hc (kA/m)
FeCo
764
1.27
4.37
FeCo-RGO
763
1.40
4.77
Hce = 2.14 Hch = 0.59 Hce = 2.62 Hch = 0.60
Atomic force microscopy (AFM) is currently the foremost method for the definitive identification of single-layer crystals and roughness [42]. Fig. 5 shows the plan-view of FeCo film and FeCo-RGO composite film. The results reveal that RMS roughness decreased when RGO is incorporated in FeCo thin film. That is because GO solution may supply appropriate condition to enhance the compactness of the film during deposition. In addition, some RGO sheets cover the FeCo film making the film surface smoother and more uniform, which in turn, may prevent the peeling of the nanosheets [43]. The hysteresis loops of the thin films are shown in Fig. 6. The typical magnetic parameters 4πMs, Hk and Hc are obtained from the loops, and the results are presented in Table 3, where 4πMs is the saturation magnetization, Hk is the magnetic anisotropy, and Hce and Hch are the easy axis and hard axis coercivities, respectively. The thicknesses of the films shown in Table 3 provide a parameter to confirm the 4πMs. Clearly, Hk, 4πMs and Hce of the FeCo-RGO film are larger than those of the FeCo film. The improved coercivity of FeCo-RGO may be caused by the pinning effect from RGO deposited between the FeCo particles. Furthermore, the higher Fe/Co atomic ratio in the FeCo-RGO film than that of
Fig. 5. The plan-view images of FeCo film (a) and FeCo-RGO composite film (b).
D. Cao et al. / Thin Solid Films 597 (2015) 1–6
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References
Fig. 7. The frequency dependence on complex permeability of the FeCo film and FeCo-RGO composite film.
the FeCo film may be the main reason why the 4πMs of FeCo-RGO film is larger than that of FeCo film. The permeability spectra of the films are shown in Fig. 7, where μ′ and μ″ represent the real and imaginary parts of complex permeability, respectively. The microwave field is performed along the easy axis of the film plane. It can be seen that the full width at half maximum of FeCo-RGO peaks decreases. This result is consistent with the AFM observations; improved material properties are related to smoother and flat films. Moreover, the resonance frequency fr increases from 2.15 GHz for FeCo film to 3.9 GHz for FeCo-RGO composite film. The fr of samples can be described by the Kittel equation [44] fr = (γ/2π)·(4πMs·Hk)1/2 for magnetic resonance. By this equation, it is easy to know that the resonance frequency is enhanced when Ms and Hk increase.
4. Conclusions The FeCo film and FeCo-RGO composite film were electrodeposited on ITO coated glass by a constant potential method without using any reducing agent. The Raman spectra and SEM results show that the codeposition of FeCo and RGO on ITO substrates is well performed. And EDS and SEM indicate that the deposition speed and particle size of iron and cobalt ions are changed during the codeposition due to the incorporation of RGO. The composite FeCo-RGO film deposited on ITO exhibits good soft magnetic properties and high-frequency properties. The Hk and 4πMs increase from 4.37 kA/m and 1.27 T for the FeCo film to 4.77 kA/m and 1.40 T for the FeCo-RGO composite film. Significantly, the fr increases from 2.15 for FeCo film to 3.9 GHz for the FeCo-RGO composite film.
Acknowledgments This work is supported by the National Basic Research Program of China (2012CB933101) and the National Science Fund of China (11574121, 51371092).
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