1 Introduction

Recently, graphene, a single layer of carbon atoms, is one of the most crucial technological developments owing to its extraordinary physical properties. The relatively small bond length (0.0142 nm) makes graphene tougher than the steel and extremely flexible material as well. Recently its low resistivity (10− 8 Wm) and transparency makes graphene a promising material for the electronic industry, energy storage, medicine, and biosensor technology [1,2,3,4]. Furthermore it makes an effective interface between enzyme and electrode, allowing the improvement of biosensors in terms of high sensitivity and short current response time [5, 6]. So far many methods have been proposed to produce graphene among which the Hummers method is the foremost commonly used [7,8,9,10,11,12,13]. However, the graphene oxide (GO) and reduced GO flakes obtained by this method are generally in the order of micrometers. Alternatively, graphene can be directly transformed from amorphous carbides by chlorination under low temperatures and ambient pressures [14,15,16]. Among them, the most promising technique is the CVD technique to get high-quality, uniformity and large-scale (the order of cm) graphene [17, 18].

Nowadays graphene composites containing magnetic nanoparticles (MNPs) such as transition metals and their oxides are attracting great attention owing to their magnetic properties that are useful for medicine and biosensor applications [19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45]. Therefore graphene gains new functionality with the MNPs. The unique properties and production methods of these graphene-based composites are still under investigation and new and green synthesis approaches are challenging. Up to now Fe-based metal oxide nanoparticles, magnetite (Fe3O4), maghemite (γ-Fe2O3) and hematite (α-Fe2O3) have been used as contrast agents in magnetic resonance imaging, drug carriers, as well as inducers for hyperthermia of tumors [46,47,48,49]. Among them, hematite is widely studied because it is the most stable iron oxide with high resistance to corrosion, not expensive, easy to find, biocompatible, environmentally friendly and non-toxic [50,51,52,53,54]. Especially, hematite is the most promising modifying material because of the variable valence state of iron oxides that can be recovered in situ via electrochemical reducing or oxidizing during the sensing process [50].

The nanosized 3d elements (Co, Ni, Fe) and their oxides have been mostly decorated on the graphene layer because their redox states, and electrochemical stabilities [55] play an essential role to produce for glucose detection [7], and MNPs can easily connect with the glucose enzyme. Fe3O4/GO has been presented to be both glucose and H2O2 sensor [8]. Furthermore, Li et al. [9] reported Fe2O3/graphene systems show better results for the lithium storage battery. In another study, α-Fe2O3 NPs with various morphologies were synthesized and the electrocatalytic activities of dopamine (DA) and uric acid (UA) at Fe2O3/GO nanohybrids decorated glassy carbon electrodes were measured by Cai et al. [13]. In the study, the electrochemical measurements exhibited that the discal Fe2O3 NPs had the most remarkable electrochemical response toward the simultaneous detection of DA and UA.

Graphene can be decorated with nanoparticles (magnetic or non-magnetic) using different techniques such as the chemical, the electrochemical, the sputtering, etc. Selection of the decorated nanoparticle depends on the interaction between the nanoparticle and enzyme and, at the same time, degradation of the nanoparticle (cycle, photo, chemical, etc.) has a significant effect since measurement cycles cause the degradation, and removal of the nanoparticle or nanoparticles from the graphene surface decreases the sensitivity of the biosensor. Furthermore, graphene which is decorated with nanoparticle systems have been developed as well [10, 11]. In 2016, Feng et al. worked on multilayered Fe2O3/graphene where graphene oxide layers were prepared with a modified Hummers method [12]. This structure exhibits superior catalytic activity toward the oxygen evolution reaction for Li batteries. Moreover sandwich-type graphene, two slices of single-layer graphene with encapsulated nanoparticles is shown theoretically as a super-strong, conductive material enhancing desired electronic and optical properties. When binding molecules between nanoparticles and graphene are not used, nanoparticles are placed using Van der Waals forces on the SLG. Therefore, nanoparticles in biosensor applications are challenging in terms of stability. Sandwich-type graphene is proposed to overcome this issue [56]. The numbers of nanoparticles are limited by the graphene layers which significantly improved the sensitivity of the system for biosensor applications due to stability in the number of nanoparticles between layers.

In this study, Fe2O3/SLG and sandwich-type SLG/Fe2O3/SLG composites were prepared on FTO by CVD, drop-casting and sol–gel methods. Fe2O3 nanoparticles were fabricated by the sol–gel method and the CVD method was used to produce graphene. The structural characterizations were done by the X-ray diffraction (XRD), Raman spectroscopy, Energy Dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), optical microscopy and scanning electron microscopy (SEM). Magnetic properties of the nanomaterials were analyzed using a vibrating sample magnetometer (VSM) option of Quantum Design PPMS. In addition, the electrochemical analyses were carried out in a cell with three electrodes.

2 Experimental

2.1 Graphene growth

Single-layer graphene (SLG) was produced by using CVD technique. 0.1 mm thick, 2 × 2 cm2 Cu foils of 99.8% purity was used as a metallic catalyzer. Copper foils were pre-cleaned by acetone, isopropanol, and deionized water for 10 min in an ultrasonic cleaner. Then, it was placed in a furnace. The furnace was evacuated to 10− 6 Torr and pre-heated to 1000 °C with flowing H2 at 100 sccm for 30 min. This pre-heating and annealing process for Cu is targeted to create graphene seeds for growth. After annealing, CH4 gas was let into the chamber at 30 sccm flow for 30 min. The chamber pressure was kept at 9 × 102 Torr while holding the Cu substrate at 1000 °C during the growth of the graphene film. Finally, the Cu foil covered with graphene was quickly cooled to room temperature.

2.2 Transfer procedure of graphene to FTO substrates

The large-area SLG graphene prepared on the Cu foils was transferred onto FTO substrates. To be able to carry out the transfer, graphene on Cu foil was covered homogeneously with PMMA (Polymethyl methacrylate) solution with a spin-coater rotating at 4000 rpm for 40 s. Then, the sample was floated onto Fe(NO3)3*9H2O for a day in order to have the Cu substrate etched. The remaining graphene with PMMA was transferred onto the FTO substrate by the fishing process. Then the PMMA solution was removed by acetone.

2.3 Synthesis of Fe2O3 nanoparticles

Fe2O3 was prepared by the sol–gel technique. Appropriate amount of Fe(NO3)3*9H2O was dissolved in water in order to obtain desired stoichiometry. Afterward, citric acid and ethylene glycol were added to the mixture. A viscous residue was formed by slowly boiling this solution at 200 °C. The obtained residue was dried at 300 °C until a dry-gel was formed. Finally, the residual precursor was burned in the air at 600 °C for 12 h in order to remove the organic materials produced in the chemical reactions, and the remaining powder material was grounded in a mortar to obtain homogenous materials.

2.4 Production of Fe2O3/SLG and sandwich-type SLG/Fe2O3/SLG electrode

Nanoparticles produced by the sol–gel method were dissolved in ethanol for 10 min through an ultrasonic cleaner and then the solution was left for 30 min for large particles to collapse. The nanoparticle solution was taken from near the surface by pipet and the solution was dropped onto SLG/FTO by drop-casting and dried under room conditions to produce Fe2O3/SLG composite. Also, the production of sandwich-type SLG/ Fe2O3/SLG had followed the same procedure with Fe2O3 on graphene, and then, another single-layer graphene sheet was transferred on Fe2O3/SLG.

2.5 Characterizations

X-ray diffraction (XRD) was used for the structural analysis of Fe2O3 nanoparticles. XRD patterns were collected at room temperature using Cu-Kα radiation. FullProf software was used for data processing [6]. Raman spectroscopy was used for analyses of graphene properties. The morphology analyses were performed by SEM (Ziess/Supra 55 FE-SEM) and optical microscopy (Optica B-1000MET). EDX and XPS analyses were performed for elemental analysis (Thermo Fisher Scientific model). The temperature dependence of the magnetization M(T) with Zero Field Cooled (ZFC), Field Cooled (FC) and Field Warmed (FW) protocols under 1 T magnetic field and the field dependence of the magnetization M(H) at room temperature (300 K) were measured using a Quantum Design PPMS with VSM option. Electrochemical measurements were carried on Iviumstat potentiostat/galvanostat in a conventional working cell containing SLG on FTO, and Fe2O3/SLG on FTO as a working electrode, a Pt wire as counter electrode, and Ag/AgCl as the reference electrode. The electrodes were inserted into a working cell containing 5 mM K4Fe(CN)6/0.1 M KCl between − 0.8 V and 1.2 V with 100 mV/s of the scan rate.

3 Results and discussion

Figure 1 shows the refined XRD pattern of the as-synthesized Fe2O3 nanoparticles. In the figure, the red circle symbol and the thick line represents the observed and calculated patterns, respectively. The thin black line indicates the difference between these patterns. The Bragg positions are also presented. The refinement result shows that there is a good agreement between the observed and the calculated patterns. The structure of Fe2O3 was found to be a rhombohedral crystal structure with hexagonal axes (space group; R-3cH). The calculated lattice constants are determined as a = b = 5.032 Å and c = 13.739 Å (c/a = 2.730). The result shows that the nanoparticles have α-Fe2O3 phase. The crystallite size of the sample was calculated for the strongest Bragg reflection by using Scherrer Formula [57] and found to be 44 nm.

Fig. 1
figure 1

Rietveld refinement results. Observed (red circles) and calculated (black lines) intensities of XRD patterns of Fe2O3 (Color figure online)

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The morphology of the Fe2O3 was measured by SEM and presented in Fig. 2. The images show that nanocrystals are agglomerated and the shapes of particles are nearly spherical. The size distribution of particles is also depicted in the inset. The distribution analysis of NPs shows that the mean particle diameter is 128 nm.

Fig. 2
figure 2

SEM images of the Fe2O3 nanoparticles (histograms of crystallite size diameters obtained from SEM images of the Fe2O3 nanoparticles shown in the inset)

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SLG, SLG/Fe2O3, and the sandwich-type SLG/Fe2O3/SLG on FTO have been investigated in bright and dark field mode by optical microscopy (Fig. 3a–f). The dark field microscopy process is useful to see the detail of the materials. Figure 3a–d shows that SLG is located on FTO. After Fe2O3 nanoparticles are added on SLG/FTO, they can be easily seen in the images (Fig. 3b–e). In addition, after a second graphene sheet transferred on Fe2O3/SLG, two different graphene structures can be distinguished from the edges (Fig. 3c–f).

Fig. 3
figure 3

Microscopic views of a SLG on FTO, b Fe2O3/SLG on FTO, c SLG/Fe2O3/SLG and the dark field microscope images of d SLG on FTO, e Fe2O3/SLG on FTO, f SLG/Fe2O3/SLG

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Raman spectroscopy is widely used to characterize layer number and quality of graphene. The quality of the graphene is determined by the ratio of peak intensities, ID/IG. The number of layers of graphene can be derived from the ratio of peak intensities, I2D/IG. The Raman spectrum as a single layer and the sandwich-type of the graphene obtained in this study are shown in Fig. 4a, b, respectively. The Raman spectrum in Fig. 4a shows the characteristic D band peak at 1368 cm− 1, the G peak at 1594 cm− 1, the 2D peak at 2712 cm− 1. The ratios of peak intensities were calculated as ID/IG= 0.62 and I2D/IG=1.5. General opinion on layer number of graphene is I2D/IG ratio of > 2, ∼1, and < 1 is speculated as a single-layer, bilayer, and few-layer graphene, respectively [58, 59]. Consequently, the obtained value indicates single-layer graphene synthesized on FTO. The Raman spectrum in Fig. 4b shows the characteristic D band peak at 1370 cm− 1, a G peak at 1589 cm− 1, the 2D peak at 2713 cm− 1. The ratio of 2D and G peak’s intensity is 0.65 proving that sandwich-type (few-layer) graphene is successfully produced in the study. Furthermore, optical microscopy and SEM analysis also confirm the results of the Raman analysis.

Fig. 4
figure 4

Raman spectroscopy of a Fe2O3/SLG on FTO and b sandwich-type SLG/ Fe2O3/SLG on FTO

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Figure 5 also shows SEM images of FTO (Fig. 5a), SLG (Fig. 5b), Fe2O3/SLG (Fig. 5c) and the sandwich-type SLG/Fe2O3/SLG (Fig. 5d, e) on FTO. It is possible to distinguish from the images as coated and uncoated (Fig. 5a) with SLG. Furthermore, these images show that the SLG is mostly covered with the Fe2O3 nanoparticles. In addition to seeing the differences between bare SLG/FTO surface and Fe2O3/SLG/FTO surface. The nanoparticles also appear to be trapped between the graphene layers and are covered by a graphene layer (Fig. 5d, e). Furthermore, in the images, a typically wrinkled, sheet structure of graphene and the presence of very small defects in this graphene layer can be seen.

Fig. 5
figure 5

SEM images of a FTO, b SLG on FTO, c Fe2O3/SLG on FTO d, e sandwich-type SLG/Fe2O3/SLG on FTO

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EDX spectra of the sandwich-type SLG/Fe2O3/SLG on FTO are depicted in Fig. 6. EDX is a standard method for identifying and quantifying elemental compositions. The EDX spectrum confirms the existence of C peaks of graphene, Fe, and O peaks of Fe2O3 nanoparticles. In addition, F and Sn peaks of the FTO substrate are also observed. The Si peak arises from the FTO coated glass surface.

Fig. 6
figure 6

EDX analysis of the sandwich-type SLG/Fe2O3/SLG on FTO. C belongs to graphene and Fe and O belong to Fe2O3. F, Sn and Si peaks arise from the FTO coated glass surface

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Figure 7 represents the XPS of the sandwich-type SLG/Fe2O3/SLG on FTO. It shows the presence of various components such as carbon, oxygen, Fe, and FTO at room temperature (Fig. 7a). All the binding energies were corrected using the C 1s reference line at 284.6 eV as a standard. Furthermore, the peaks concerned groups in the C 1s spectra confirm the graphene structure (Fig. 7b). The energies at 286.7 and 289.3 eV can be appointed to C–O, and C=O, respectively. Furthermore, the Fe 2p peaks which are located at approximately 711.2 and 717.0 eV are assigned to Fe 2p3/2 and Fe 2p1/2, respectively, indicating that the Fe ions are present in the trivalent form, as depicted in Fig. 7c. The low-intensity peak located around 713.5 eV is a satellite. The O 1s contribution originates from the Fe2O3 and FTO (Fig. 7d). Generally, the XPS analysis revealed the presence of Fe2O3 nanoparticles and successfully coated the SLG on FTO.

Fig. 7
figure 7

XPS analysis of SLG/Fe2O3/SLG on FTO. Panel a shows the broadscan XPS spectra of SLG/ Fe2O3/SLG surfaces. Panels b, c and d display C, Fe and O XPS spectra of SLG/Fe2O3/SLG surfaces, respectively

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To investigate the magnetic behavior of Fe2O3 nanoparticles, the temperature dependence of magnetization was measured at 1 T, which is shown in Fig. 8. In general, Fe2O3 has a high Neel temperature of about TN = 955 K [60]. M(T) measurement showed an antiferromagnetic behavior at low temperatures. When the temperature is increased, there is a magnetic phase transition at T = 263 K antiferromagnetic to weak ferromagnetic, which is known as Morin temperature (TM) or spin-flop transition. This phase transition arises from the two sub-lattices from along to perpendicular to the [111] axis of a rhombohedral structure with hexagonal axes. In the case of spin reorientation, spins are oriented in a direction different from their previous alignment which strongly depends on the value of the applied magnetic field and presents anisotropy in the system. Above TM temperature, Fe2O3 has parasitic ferromagnetism because of spin-canting. Below this temperature, it is antiferromagnetic. Because an equal number of spins arrangement in the two sub-lattices indicates spontaneous antiparallel orientation along the [111] direction of the crystal [61].

Fig. 8
figure 8

Magnetization depending on the temperature of Fe2O3. Black, red and blue lines with dots indicate ZFC, FW and FC curves, respectively (Color figure online)

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Figure 9 shows the magnetic field dependence of the magnetization M(H) of Fe2O3 measured from − 2T to 2T magnetic field at room temperature (300 K). As seen in the figure, the Fe2O3 particles show no saturation of the magnetization due to the presence of the antiferromagnetic phase. By removing the antiferromagnetic contribution on the curve, the saturation value of the ferromagnetic Fe2O3 nanoparticles was obtained as 0.31 Am2/kg. Furthermore, the magnetization curve indicates a weak remanent magnetization of 0.16 Am2/kg and also coercivity of 0.203 T at room temperature. These results confirm that Fe2O3 nanoparticles exhibit rhombohedral α-phase [62].

Fig. 9
figure 9

Hysteresis curves of Fe2O3 at 2 T magnetic field at T = 300 K

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The electrochemical characterizations of SLG on FTO, and Fe2O3/SLG on FTO were investigated using cyclic voltammetry (CV) to understand oxidation-reduction processes of Fe2O3 nanoparticles (Fig. 10). When analyzing CV curves of materials, the charge density of Fe2O3/SLG/FTO depicts a larger area than SLG/FTO which can be attributed to the active surface area of Fe2O3 nanoparticles. In addition, the transferring of Fe2O3 nanoparticles on the FTO/SLG, the current density was increased.

Fig. 10
figure 10

Cyclic voltammetry analysis of SLG on FTO (red) and Fe2O3/SLG on FTO (blue) (Color figure online)

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4 Conclusion

In summary, to the best of our knowledge, large-area Fe2O3/SLG and sandwich-type SLG/Fe2O3/SLG composite structure were produced for the first time by CVD and sol–gel process and structure properties were investigated by using XRD, Raman spectroscopy, EDX, XPS, optical microscopy and SEM. α-Fe2O3 has a rhombohedral phase and the shapes of the particles which contain agglomerates of nanoparticles are spherical with about 128 nm particle diameter. It was observed that the remanent magnetization of Fe2O3 nanoparticles is 0.16 Am2/kg and samples showed almost 0.203 T magnetic coercivity. Furthermore, the structure analysis results indicated that Fe2O3/SLG and sandwich-type SLG/Fe2O3/SLG composites were successfully obtained. The increase in the charge density of Fe2O3/SLG/FTO was clearly observed when compared to SLG/FTO via electrochemical measurements. Moreover, these results demonstrate the feasibility of graphene sandwich-type electrodes to eliminate the main stabilization obstacle of magnetic nanoparticles especially for biosensor applications.