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Coexistence of Interfacial Stress and Charge Transfer in Graphene Oxide based Magnetic Nanocomposites

In this paper, we establish the existence of both compressive stress and charge transfer process in hydrothermally synthesized cobalt ferrite-graphene oxide (CoFe2O4/GO) nanocomposites. Transmission electron microscopy (TEM) results reveal the decoration of CoFe2O4 nanoparticles on GO sheets. Magnetic response of nanocomposites was confirme from superconducting quantum interference device (SQUID) magnetometer measurement. Optical properties of these nanocomposites were investigated by Raman spectroscopy. Interfacial compressive stress involved in this system is evaluated from observed blue shift of characteristic G peak of graphene oxide. Increase in full width half-maximum ( FWHM) as well as up shift in D and G peaks are clear indicator of involvement of charge transfer process between GO sheets and dispersed magnetic nanoparticles. The effect of charge transfer process is quantified in terms of shifting of Fermi level of these nanocomposites. This is evaluated from variation in contact surface potential difference (CPD) using Scanning Kelvin probe microscopy (SKPM). XRD spectra of CoFe2O4/GO confirm the polycrystalline nature of CoFe2O4 nanoparticles. Lattice strain estimated from XRD peaks are correlated to the observed Raman shift.


Introduction
Graphene oxide (GO), a derivative of graphene, is an excellent two dimensional (2D) flat sheet of hexagonally bonded sp 2 hybrid carbon atoms on which oxygen functional groups (such as hydroxyl, epoxide, carboxyl and carbonyl and hydroxyl groups) are covalently linked on their basal planes. [1][2][3][4][5][6] These functional groups attached to the negatively charged GO sheets can act as host for the positively charged ions of nanoparticles (NPs) leading to formation of nanocomposites. These nanocomposites exhibit unique properties as compared to individual components and therefore find a wide range of applications in surface-enhanced Raman scattering, sensors, catalysis and optoelectronic devices. [7][8][9][10] Nanocomposites synthesized by hydrothermal route are found to be in the form of dispersion of nucleated nanoparticles on GO sheets. It is a well known fact that, magnetic nanoparticles (MNPs) have potential applications in the field of energy storage devices, MRI and magnetic field driven drug delivery. [11][12] Particularly, Cobalt ferrite (CoFe 2 O 4 ) NPs have found lots of applications in the field of catalysis and magnetism based nano devices due to their remarkable chemical and mechanical stability, magnetic behavior, low toxicity and biocompatibility in physiological environments.

Characterization
The surface morphology of GO and CoFe 2 O 4 /GO was investigated by scanning electron microscopy (SEM) (Care-Zeiss EVO-40, working voltage 20 kV, Germany). The elemental identification of CoFe 2 O 4 /GO nanocomposite was confirmed from energy dispersive X-ray analysis (EDAX). EDAX measurement was carried out using a (Zeiss EVO ED15) microscope coupled with an (Oxford-X-MaxN) EDX detector. The magnetic properties of this nanocomposite were investigated at room temperature using a Quantum Design MPMS-7 SQUID magnetometer. From the magnetization versus applied field plot (M-H), the saturation magnetization (M s ), coercivity (H c ) and remanence magnetization (M r ) was measured. For structural analysis, transmission electron microscopy study was carried out by 200 kV TEM, JEOL 2100F, Japan. X-ray diffraction (XRD) spectra of CoFe 2 O 4 /GO nanocomposite samples were recorded using an X-ray diffractometer (Panalytical 2550-PCX-raydiffractometer). XRD data were collected using Cu-K α (λ = 0.154 nm) radiation with 2θ ranging from 10°to 70° at scanning rate 3° min -1 . Optical properties of GO and CoFe 2 O 4 /GO nanocomposite were investigated using Raman spectroscopy (HORIBA Xplora) having green laser (λ= 514 nm) excitation with a laser spot size 1 μm. The effect of CoFe 2 O 4 nanoparticles decoration on the Fermi energy level of GO sheets is monitored by scanning Kelvin probe microscopy (SKPM, KP Technology, UK).   The presence of superparamagnetic particles was examined using zero-field-cooling (ZFC) and field-cooling (FC) measurements with an applied magnetic field of 100 Oe. The magnetic susceptibility (χ) vs. temperature plot for CoFe 2 O 4 /GO nanocomposite is shown in Fig. 3 (b). For the zero-field-cooled (ZFC) case, the sample was cooled from 300 K to 2 K and then a magnetic field H = 100 Oe was turned on for magnetization (M) measurements with increasing temperature after ensuring stabilization at each temperature. Upon reaching 300 K, the data were similarly collected with decreasing temperature (FC mode) keeping the same applied field. It is clear that FC and ZFC curves show divergence at around 300 K which can be considered as the blocking temperature (T b ). Below this temperature the material shows ferromagnetic behavior above which it is superparamagnetic in nature. Above the blocking temperature, the fine nanoparticles lose their hysteresis property as evident from M-H loops. 24,25 The magnetic moments follow the direction of the applied magnetic field resulting in low remanence and low coercivity which is the characteristic feature of superparamagnetism.

TEM Studies
Microstructure analysis of these composites were carried out using TEM, for which in the first step, ethanol-based solutions of GO and CoFe 2 O 4 /GO were placed on carbon-coated copper grids followed by drying at room temperature before use. TEM image of GO and CoFe 2 O 4 /GO nanocomposite are shown in Fig. 4 (a) and Fig. 4 (b) respectively.
Where β is full width half maximum (FWHM) of diffraction peak (in radian) and θ is Bragg's diffraction angle (in degree) and Δ is lattice strain. The crystallite size was calculated from the X-ray diffraction spectra using Scherrer's formula where, the crystallite size is inversely related to Cos  Considering the fact that particle size and strain are independent of each other having a Cauchy-like form, which in combination are related to FWHM by W-H equation as follows, Where, the term Kλ/D represents the Scherrer's particle size distribution. 26,27 Fig. 5  to be 17 nm and 0.003 respectively. The crystallite size is the good agreement with the observed size of crystallites from TEM measurement and lattice strain is expected to be compressive type which will be discussed in forthcoming section.

Raman Studies
The Raman spectra of GO sheets and CoFe 2 O 4 /GO nanocomposite are shown in Fig. 6. The The D peak is shifted from 1345 cm −1 to 1354 cm −1 while the G peak is shifted from 1587 cm −1 to 1595 cm −1 (Fig. 6). This is unlike to the reported observation of red shift in case of graphene oxide based polymer nanocomposites. 30   Fermi level energy of any material is related to its work function (WF) by the equation.
Where  s is the electron affinity of the sample, and E C and E F are the conduction band energy and Fermi energy of the material respectively. 38 The estimation of Fermi level shifting is carried out in terms of work function using scanning Kelvin probe microscopy (SKPM) setup as shown in Fig. 8. The WF value of the GO sheets is measured in terms of surface potential or contact potential difference (CPD) between GO and the reference Au tip (WF=5.1 eV). Average value of CPD is given by Where Φ tip and Φ sample are the work function of tip and sample surface respectively and 'e' is the elementary charge on an electron . 39,40 The measured average CPD value of GO sheet is found to be 514 mV which corresponds to WF of 4.6 eV. In the case of CoFe 2 O 4 /GO nanocomposite, obtained average CPD is -610 mV. For simplicity in plotting, the absolute value of CPD has been taken into account as shown in Fig. 8 (b  Fig. 8 (c). This shifting may be due to electron transfer from GO to CoFe 2 O 4 which is also envisaged from changes in Raman spectra.