Spinel CoFe2O4 Nanoflakes: A Path to Enhance Energy Generation and Environmental Remediation Potential of Waste-Derived rGO

Carbon nanomaterials derived from agricultural waste streams present an exciting material platform that hits multiple sustainability targets by reducing waste entering landfill, and enabling clean energy and environmental remediation technologies. In this work, the energy and photocatalytic properties of reduced graphene oxide fabricated from coconut coir using a simple reduction method using ferrocene are substantially improved by introducing metallic oxides flakes. A series of cobalt ferrite rGO/CoFe2O4 nanocomposites were assembled using a simple soft bubble self-templating assembly, and their potential for clean energy applications confirmed. The transmission electron microscopy images revealed the uniform dispersion of the metal oxide on the rGO sheets. The functional group of the as synthesized metal oxide and the rGO nanocomposites, and its individual constituents, were identified through the FTIR and XPS studies, respectively. The composite materials showed higher specific capacitance then the pure materials, with rGO spinal metal oxide nanocomposites showing maximum specific capacitance of 396 F/g at 1 A/g. Furthermore, the hybrid super capacitor exhibits the excellent cyclic stability 2000 cycles with 95.6% retention. The photocatalytic properties of the synthesized rGO nanocomposites were analyzed with the help of malachite green dye. For pure metal oxide, the degradation rate was only around 65% within 120 min, while for rGO metal oxide nanocomposites, more than 80% of MG were degraded.


Introduction
The path towards "zero carbon" or "below zero" emission reduction targets is inherently linked to finding solutions for sustainable energy generation and storage, as well as mechanisms to reduce energy consumption and waste generation. Waste from agricultural industry is increasingly attracting attention for the role it may play in environmental remediation and in reducing greenhouse gas emissions as the global communities move towards a cleaner world [1][2][3]. For material synthesis, biomass waste is a valuable source of carbon and trace minerals [4][5][6], where the micro-and nanostructure of the material may give rise to unusual material architectures without the need for complex templates or multi-step processes [7][8][9][10][11][12]. With an attractive combination of properties such as good conductivity, exhibit multiple oxidation states, which enables various redox reactions [48][49][50][51][52][53]. CoFe 2 O 4 has been previously shown to be more catalytically active when compared to NiFe 2 O 4 nanoparticles [52], whereas other studies have shown a decreased in the cell voltage of the discharging plateaus for NiFe 2 O 4 when compared to CoFe 2 O 4 , due to differences in the free energy formation of the oxides [53].
It should be noted that NiFe 2 O 4 has the inverse spinel structure, with Ni 2+ in the more active octahedral site, and CoFe 2 O 4 has a normal spinel structure, with Co 2+ and Fe 3+ occupying the tetrahedral and octahedral sites, respectively. The latter also has greater coercivity and higher remanence after magnetization compared to NiFe 2 O 4 [54]. Hence, it is worth investigating how nanocomposites comprising CoFe 2 O 4 differ from those based on NiFe 2 O 4 fabricated using a similar method. The various characterizations related to specific capacitance, supercapacitor cyclic stability and photo-catalytic properties of thus-synthesized spinel transition metal oxides/rGO nanocomposites were analyzed and reported.

Materials and Synthesis Methods
Reduced graphene oxide was synthesized from coconut coir waste in a single step process without using any hazardous chemicals. Coconut coir is a by-product of the coconut industry and is a rich source of cellulose, lignin, pectin, and hemicellulose. The coconut coir was washed with tap water to remove impurities, and then washed with deionized water. The cleaned material was dried, and once dried, the material was ground to powder in a mortar and mixed with ferrocene. The mixture was carbonized in a box furnace at 300 • C for 15 min [55] to produce black-colored reduced graphene oxide (rGO). The length and temperature have previously been optimized to produce high-quality material at the lowest possible energy budget, an important feasibility criterion for scale up and industrial translation. The results of the optimization process can be found in our previously published report [31]. Briefly, the carbonization temperature of 300 • C in the presence of air was chosen to avoid mass loss and decomposition of GO material, and the formation of graphite-like material [31]. Although strictly speaking, reduced graphene oxide is synthesized from graphite powder, and the carbonaceous products synthesized from coconut coir waste should be more accurately referred to as carbon sheets, we will use rGO for simplicity and to maintain consistency with published literature on the topic of waste-derived carbonaceous materials. The formed rGO was combined with various transition metal oxides like Co 3 O 4 , CoFe 2 O 4 , and a hydrothermal process was used to produce rGO/Co 3 O 4 , rGO/CoFe 2 O 4 composites. Briefly, for rGO/CoFe 2 O 4 material, 0.01M (0.233 g) of Co(NO 3 ) 3 ·6H 2 O, 0.02M (0.6464 g) of Fe(NO 3 ) 2 ·9H 2 O and excess amount of urea (0.1 M) were combined in a 250 mL beaker. Then, an aliquot of the ultrasonically dispersed rGO (at the concentration of 1 mg/1 mL) was added to the mixture under stirring. Cobalt (II) nitrate hexahydrate, Iron (III) nitrate nonahydrate, urea, and ammonia solution were purchased from Alfa Aesar (Lancaster, UK). Deionized water was used as solvent in all experiments. Analytical grade ethanol and acetone were used for washing purpose.
The homogenous mixture was transferred to a stainless steel autoclave and kept for 12 h at 160 • C in a box furnace. Finally, the collected material was washed with deionized water thrice and dried overnight at 80 • C in a convection oven. Then, it was allowed to calcinate in the furnace at 300 • C for 2 h in air. Figure 1 shows the schematic representation of the synthesis procedure for the preparation of rGO/CoFe 2 O 4 . A similar protocol was used for the preparation of rGO/Co 3 O 4 composite. For the preparation of rGO/Co 3 O 4 composite, 0.01 M (0.233 g) of Co(NO 3 ) 3 ·6H 2 O excess amount of urea (0.1 M) was added, then the mixture was ultrasonically dispersed and transferred to an autoclave at 160 • C for 12 h in a box furnace. The final product was annealed at 300 • C for 2 h. From here on, the metal oxides and composites will be referred to as Co 3   Material Characterization. The X-ray diffraction (XRD) patterns of all samples were recorded with an X-ray diffractometer (Rigaku Minflux II-c X-ray diffractometer with Cu Kα radiation of wavelength 0.154 nm, Tokyo, Japan) over the 2θ range of 10-80°.The morphologies of the as-prepared samples were characterized using scanning electron microscopy (SEM) (performed on a JEOL JSM-6700F microscope, Tokyo, Japan). Fourier transform infrared (FTIR) and RAMAN spectra were collected on a Shimadzu IRPrestige-21 infrared spectrometer (Shimadzu, Japan) over the range of 4000-500cm −1 and Raman Spectroscopy (AGILTRON 1(QEB1920), Agiltron Inc., Woburn, MA, USA), respectively. UVvis adsorption spectra were recorded using a UV-vis spectrophotometer (Cary 5E, Crawley, UK) in the wavelength range of 200-1100 nm. Transmission electron microscopy (TEM) was conducted using a JEM-2100 microscope (JEOL, Tokyo, Japan) at 150 kV. Xray photoelectron spectroscopy (XPS) was conducted on XPS HSA 15000 (Hemispherical Energy Analyzer PHOIBOS 225, SPECS GmbH, Berlin, Germany). The electrochemical studies were performed using an electrochemical analyzer (Biologic VSP, Seyssinet-Pariset, France).
Electrochemical Measurements. The electrochemical test was performed using the three-electrode system. Platinum wire and Ag/AgCl were used as the counter and reference electrodes, respectively. The working electrode was prepared with 3 mg of active material, 0.3 g of polyvinylidenefluoride (PVDF) and 0.1 mL of N-methyl pyrrolidinone (NMP) solution. The paste was coated on a nickel plate and dried for 12 h at 110 °C. The exact active mass of the material was estimated as the difference in weight between the bare and paste-coated nickel plate. Cyclic voltammogram (CV) measurements were conducted over the potential window of −0.2 to 0.6 V, with 2M of KOH solution used as the electrolyte was. The galvanostatic charge-discharge (GCD) tests were conducted between −0.2 and −0.5 V. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range of 100 kHz to 0.1 Hz.
The specific capacitance Cs (Fg −1 ) can be calculated based on the GCD measurement using the following equation: where Δt and ΔV represents the discharge time (s) and voltage range (V), respectively, I is the current (A), m is the mass of the active material (g), C is the capacitance of the entire supercapacitor, which is half of the Cs. Material Characterization. The X-ray diffraction (XRD) patterns of all samples were recorded with an X-ray diffractometer (Rigaku Minflux II-c X-ray diffractometer with Cu K α radiation of wavelength 0.154 nm, Tokyo, Japan) over the 2θ range of 10-80 • .The morphologies of the as-prepared samples were characterized using scanning electron microscopy (SEM) (performed on a JEOL JSM-6700F microscope, Tokyo, Japan). Fourier transform infrared (FTIR) and RAMAN spectra were collected on a Shimadzu IRPrestige-21 infrared spectrometer (Shimadzu, Japan) over the range of 4000-500cm −1 and Raman Spectroscopy (AGILTRON 1(QEB1920), Agiltron Inc., Woburn, MA, USA), respectively. UV-vis adsorption spectra were recorded using a UV-vis spectrophotometer (Cary 5E, Crawley, UK) in the wavelength range of 200-1100 nm. Transmission electron microscopy (TEM) was conducted using a JEM-2100 microscope (JEOL, Tokyo, Japan) at 150 kV. X-ray photoelectron spectroscopy (XPS) was conducted on XPS HSA 15000 (Hemispherical Energy Analyzer PHOIBOS 225, SPECS GmbH, Berlin, Germany). The electrochemical studies were performed using an electrochemical analyzer (Biologic VSP, Seyssinet-Pariset, France).

Results and Discussion
Electrochemical Measurements. The electrochemical test was performed using the three-electrode system. Platinum wire and Ag/AgCl were used as the counter and reference electrodes, respectively. The working electrode was prepared with 3 mg of active material, 0.3 g of polyvinylidenefluoride (PVDF) and 0.1 mL of N-methyl pyrrolidinone (NMP) solution. The paste was coated on a nickel plate and dried for 12 h at 110 • C. The exact active mass of the material was estimated as the difference in weight between the bare and paste-coated nickel plate. Cyclic voltammogram (CV) measurements were conducted over the potential window of −0.2 to 0.6 V, with 2M of KOH solution used as the electrolyte was. The galvanostatic charge-discharge (GCD) tests were conducted between −0.2 and −0.5 V. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range of 100 kHz to 0.1 Hz.
The specific capacitance C s (Fg −1 ) can be calculated based on the GCD measurement using the following equation: where ∆t and ∆V represents the discharge time (s) and voltage range (V), respectively, I is the current (A), m is the mass of the active material (g), C is the capacitance of the entire supercapacitor, which is half of the C s .

Results and Discussion
Structural and chemical analysis. The XRD patterns of the obtained Co 3 O 4 , CoFe 2 O 4 , rGO/Co 3 O 4 and rGO/CoFe 2 O 4 composites are shown in Figure 2a. It is evident that the synthesized materials are crystalline in nature. All of the peaks in the spectra for Co 3  rGO/Co3O4 and rGO/CoFe2O4 composites are shown in Figure 2a. It is evident that the synthesized materials are crystalline in nature. All of the peaks in the spectra for Co3O4 can be perfectly indexed to a cubic spinel phase of Co3O4 (JCPDS no. . Discernible peaks can be seen at 2θ of 31.4, 37.2, 39.1, 45.3, 56.2, 59.9 and 65.7, corresponding to (220), (311), (400), (511) and (440), respectively. In the case of CoFe2O4, the diffraction peaks were assigned in accordance with the JCPDS no. 10-0325. The main diffraction peaks of CoFe2O4 spectra appeared at 30 Meanwhile, for both composites, the reflection peak that corresponds to (002) plain of the layered rGO has disappeared. It is speculated that the reduced graphene oxide in the rGO/Co3O4 and rGO/CoFe2O4 was fully exfoliated due to the crystal growth of Co3O4 and CoFe2O4 between the layers of the rGO sheets [31,56]. The lattice constant (a) and cell volume were calculated from XRD data, with the results shown in Table 1. The crystallite size was found to be in the range of 11.7 to 8.7nm. Briefly, the average crystallite sizes for all the synthesized nano ferrite samples were calculated using the Scherrer's equation: The lattice constant (a) and cell volume were calculated from XRD data, with the results shown in Table 1. The crystallite size was found to be in the range of 11.7 to 8.7nm. Briefly, the average crystallite sizes for all the synthesized nano ferrite samples were calculated using the Scherrer's equation: where λ is the wavelength of X-ray radiation, β is full width half maximum (FWHM) in radian and θ is the diffraction angle. Various peaks have been used from XRD pattern to understand the peak broadening with lattice strain. Equation (2) represents the Stokes and Wilson approach to calculate the strain induced broadening of the Bragg's diffraction peak: The Willamson-Hall (WH) plots were then used to calculate the values of the grain size and strain, and the results of these calculations are tabulated in Table 1. It was observed that the crystallite sizes of Co 3 O 4 and CoFe 2 O 4 decreased with the changes in the lattice strain, due to which the peaks were also shifted to lower angle. It is important to mention that the crystallite size depends on the lattice strain and plays an important role in the overall properties of the nano-structure materials.
Firstly, as the crystallite size decreased, the grain boundary volume also increased with more volume defects associated with it. Secondly, with the reduction in the crystallite size, the pressure arising from the surface tension of crystallite interfaces induced stress fields that resulted in the lattice strain. From Table 1, it is apparent that in the case of rGO/Co 3 O 4 samples, the crystallite size is 11.6 nm, and hence, much larger than the crystallite size of rGO/CoFe 2 O 4 samples (8.7 nm). On the other hand, the surface tensioninduced pressure creates a stress field in rGO/CoFe 2 O 4 due to small crystallite size, and this in turn results in a compressive lattice strain. However, since the crystallite size is larger in rGO/Co 3 O 4 , more volume defects are associated with the grain boundaries and as a result, this ultimately exerted tensile lattice strain [57]. The presence of more defects in rGO/Co 3 O 4 over rGO/CoFe 2 O 4 boosted the faradaic redox reactions, and this clearly indicates that the atomic level strain effects played an important role in tuning the electrochemical properties of the developed composites, as it is discussed in the Electrochemical section below. Even though rGO/Co 3 Figure 2b. The peaks at 1712 and 2935 cm −1 are assigned to the bending vibration of the C=C aromatic ring of rGO and the aliphatic C-H groups, respectively. The broad peak at 3336 cm −1 is the OH stretching vibration mode of OH group. In the CO spectrum, the two distinct and sharp peaks at 552.6 and 656.6 cm −1 correspond to the stretching vibrations of the Co 2+ metal ions [58,59]. The absorption peaks at 1625 and 3349.6 cm −1 of rGO/Co 3 O 4 composite clearly indicates the Co 3 O 4 were successfully decorated on the surface of rGO sheets. The existence of the C=C peak in the spectra of all the rGO based samples suggests that the sp 2 structure of the carbon atom was retained. The absorption peak at 545. To further confirm the chemical composition of rGO/Co 3 O 4 and rGO/CoFe 2 O 4 composites, X-ray photoelectron spectroscopy (XPS) measurements were analyzed. The XPS spectra for rGO fabricated from coconut coir under similar conditions has previously been reported elsewhere [60,61]. Figure 2c shows the typical XPS survey spectra for the rGO/Co 3 O 4 and rGO/CoF 2 O 4 , which reveal the presence of Co2p, Fe2p, O1s, C1s and of Co2p, O1s, C1s in rGO/CoF 2 O 4 and rGO/Co 3 O 4 , respectively. Figure 2d-g shows the typical high-resolution spectra for Co2p, Fe2p, C1s and O1s collected for the rGO/CoFe 2 O 4 composite. The survey spectrum of rGO/Co 3 O 4 exhibited the peak at 282.7 eV corresponds to the C1s, 529.2 eV corresponds to the O 1s, and the peaks at 777.8 and 794eV corresponding to Co 2p 3/2 and Co 2p 1/2 , respectively. The core level spectrum for Co exhibited two peaks corresponding to Co 2p 3/2 and Co 2p 1/2 at 780.6 and 795.7 eV, respectively, with two satellite peaks at 786.7 and 802.9 eV, respectively, which proved the Co 2+ valance states [23]. The core level spectrum of Fe 2p shows the peaks at 711.2 and 724.5 corresponding to the Fe 2p 3/2 and 2p 1/2 , respectively. In the C 1s spectrum, the peak observed at 284.2 eV corresponds to the C-C/C=C bond and the peak at 291.0 eV is attributed to the O-C=O bond. The peak at 529.1 eV can be attributed to the metal-oxygen-carbon bond, suggesting some covalent bonding with the rGO sheets. The other peaks at 531 and 534.2 eV are associated with the metal-hydroxide (M-OH) bonds [62,63]. Hence, the XPS results confirm the formation of rGO/Co 3 O 4 and rGO/CoFe 2 O 4 nanocomposites.
Morphological analysis. The morphology of the synthesized materials was analyzed with the scanning electron microscopy. Co2p, O1s, C1s in rGO/CoF2O4 and rGO/Co3O4, respectively. Figure 2d-g shows the typical high-resolution spectra for Co2p, Fe2p, C1s and O1s collected for the rGO/CoFe2O4 composite. The survey spectrum of rGO/Co3O4 exhibited the peak at 282.7 eV corresponds to the C1s, 529.2 eV corresponds to the O 1s, and the peaks at 777.8 and 794eV corresponding to Co 2p3/2 and Co 2p1/2, respectively. The core level spectrum for Co exhibited two peaks corresponding to Co 2p3/2 and Co 2p1/2 at 780.6 and 795.7 eV, respectively, with two satellite peaks at 786.7 and 802.9 eV, respectively, which proved the Co 2+ valance states [23]. The core level spectrum of Fe2p shows the peaks at 711.2 and 724.5 corresponding to the Fe 2p3/2 and 2p1/2, respectively. In the C1s spectrum, the peak observed at 284.  Optical characterization. The optical properties of obtained nanomaterial were investigated with UV-Vis spectroscopy. Figure 4a,b shows the UV-Vis absorbance spectra of the Co3O4, CoFe2O4, rGO/Co3O4 and rGO/CoFe2O4 nanocomposite samples. The absorption broad peak around 291.1 nm confirms the formation of reduced graphene oxide from the coconut coir waste, in alignment with previously reported results for coconut coirderived rGO by our group [31]. The presence of the two UV-vis absorption bands at ~486 Optical characterization. The optical properties of obtained nanomaterial were investigated with UV-Vis spectroscopy. Figure 4a,b shows the UV-Vis absorbance spectra of the Co 3 O 4 , CoFe 2 O 4 , rGO/Co 3 O 4 and rGO/CoFe 2 O 4 nanocomposite samples. The absorption broad peak around 291.1 nm confirms the formation of reduced graphene oxide from the coconut coir waste, in alignment with previously reported results for coconut coir-derived rGO by our group [31]. The presence of the two UV-vis absorption bands at~486 nm is ascribed to Co 3 O 4 nanomaterial. The absorption peak corresponds to a charge transfer transition from O 2− to Co 2+ , i.e., band to band transition. The peak blue shift of band at 477 nm evidences that the cobalt oxide nanoflakes have been anchored onto the rGO sheets, suggesting a strong interaction between rGO and Co 3 O 4 nanocomposites. The optical absorption spectra of the CoFe 2 O 4 show the absorption peak at around 396 nm [64]. After incorporation with the rGO, the 396 nm peak shifted to the 331.6 nm, which evidences that rGO is not only a solid support, but also interacts chemically with the metal atoms [65,66]. Further, absorption spectroscopy plays an important role for the determination of the band gap value of the materials. It has been observed from the Figure S2 ( [67,68]. The spectrum for rGO flakes is shown in Figure S3 (SM).
, 12, x FOR PEER REVIEW 9 of 20 nm is ascribed to Co3O4 nanomaterial. The absorption peak corresponds to a charge transfer transition from O 2-to Co 2+ , i.e., band to band transition. The peak blue shift of band at 477 nm evidences that the cobalt oxide nanoflakes have been anchored onto the rGO sheets, suggesting a strong interaction between rGO and Co3O4 nanocomposites. The optical absorption spectra of the CoFe2O4 show the absorption peak at around 396 nm [64]. After incorporation with the rGO, the 396 nm peak shifted to the 331.6 nm, which evidences that rGO is not only a solid support, but also interacts chemically with the metal atoms [65,66]. Further, absorption spectroscopy plays an important role for the determination of the band gap value of the materials. It has been observed from the Figure S2 (SM) that the band gap values of the as-synthesized materials are 1.7, 1.3, 2.41, 2.48 eV for Co3O4, rGO/Co3O4, CoFe2O4, and rGO/CoFe2O4, respectively [67,68]. The spectrum for rGO flakes is shown in Figure S3 (SM).    Figure 4c,d shows the transient photocurrent measurement to investigate the charge separation and transfer efficiency through the visible light on and off region. The plots represent the photocurrent response in samples over several ON-OFF cycles. It is worth noting that despite repeated testing of ON-OFF response, the photocurrent response was found to remain similar. From the obtained results, it is evident that the photocurrent response for pure Co 3 O 4 and CoFe 2 O 4 is very weak compared to that of their respective composites made with rGO. When rGO is integrated into the composite matrix, the improvement in the photocurrent response is attributed to an increase in the photocurrent density, illustrating that the photogenerated holes (h + ) can be effectively separated from e − in the rGO-metaloxide composite, which results in better photocatalytic performance.
Raman Analysis. The Raman spectra of Co 3 O 4 , CoFe 2 O 4 , rGO/Co 3 O 4 and rGO/CoFe 2 O 4 nanomaterials are shown in Figure 5a,b. It is known that Raman scattering is very sensitive to the microstructure of nanocrystalline materials, therefore here it is also used to further clarify the structure of the  Mechanism of CoFe2O4 growth on rGO sheets. A mechanism for the assembly of rGO/Co3O4 and rGO/CoFe2O4 composites was proposed based on the outcomes of morphological, structural and chemical characterization. The assembly on metal oxide flakes on the surfaces of reduced graphene oxide sheets was generally achieved by using a hydrothermal technique with urea as a structure-directing agent [20,[69][70][71]. Several investi- Mechanism of CoFe 2 O 4 growth on rGO sheets. A mechanism for the assembly of rGO/Co 3 O 4 and rGO/CoFe 2 O 4 composites was proposed based on the outcomes of morphological, structural and chemical characterization. The assembly on metal oxide flakes on the surfaces of reduced graphene oxide sheets was generally achieved by using a hydrothermal technique with urea as a structure-directing agent [20,[69][70][71]. Several investigations on the use of urea's structure-directing property to develop metal oxides on rGO have been published, in which metal nitrates are alkalized and then hydrolyzed with NH 3 ·H 2 O using the inductive effect provided by the hydrothermal process [20]. However, reports on the use of CO 2 soft bubble template technology for this purpose are infrequent, and reports on spinel nanostructures on rGO grown in this way are not readily available. When NH 3 ·H 2 O is added to a solution containing metal ions (Co 2+ , Fe 3+ ), the lone pair of electrons in the NH 3 ·H 2 O solution complexes with the metal ions to form [CoFe(H 2 O) 6-x (NH 3 ) x ] 2+ . This allows some configurational ions to be released back into the solution. When the temperature reaches 80 • C, the urea is hydrolyzed, releasing CO 2 and NH 3 liquid interfaces into the aqueous solution. The generated NH 3 further drives the ionization of NH 3 ·H 2 O to generate OH − . The generated CO 2 bubbles are supported on rGO, and these CO 2 bubbles on rGO act as a template for metal oxide formation. At high temperature and pressure, Fe 3+ is reduced to Fe(OH) 3 which converts into FeOOH. Now the precipitated Co(OH) 2 reacts with FeOOH to form CoFe 2 O 4 at hydrothermal conditions.
Under prolonged hydrothermal conditions, the produced bubbles act as a soft template on which the Co(OH) 2 and FeOOH aggregate to form CoFe 2 O 4 . Due to the orientation attachment, the crystal developed along a certain orientation during calcination at 350 • C for 2 h, resulting in nanoflakes. The formation of the metal oxide nanoflake is schematically illustrated in Figure 6.
Electrochemical analysis. CV was used to characterize the electrochemical behavior of the synthesized materials. The materials show pseudo-capacitance behavior because of the cobalt and iron ions of Co 3 O 4 and CoFe 2 O 4 , which is confirmed by the nonlinear plot of CV curves shown in Figure 7a,c. The effect of scan rate on the specific and interfacial capacitance has been studied at the rates of 10, 25, 50, 100 and 300 mV/s. The voltametric currents are increased with the scan rate, which indicates the pseudocapacitive nature of the as synthesized materials. The specific capacitance is obtained by dividing the mass of electrode dipped, whereas interfacial by dividing area of the electrode dipped in the electrolyte. The redox peak around at 0.31 V indicate the reversible redox reaction of the Co 2+ /Co 3+ and Fe 3+ /Fe 4+ in alkaline electrolyte. In the case of Co 3 O 4 , the redox peak is narrow than the bimetallic materials. The specific capacitance decreased with increasing scan rate because of the of inner active sites at the electrode surface not being able to maintain redox transitions as the scan rate increases. The redox reaction can be expressed by the following reactions: Under prolonged hydrothermal conditions, the produced bubbles act as a soft template on which the Co(OH)2 and FeOOH aggregate to form CoFe2O4. Due to the orientation attachment, the crystal developed along a certain orientation during calcination at 350 °C for 2 h, resulting in nanoflakes. The formation of the metal oxide nanoflake is schematically illustrated in Figure 6. Figure 6. Growth mechanism of rGO/CoFe2O4 composite is enabled by the presence of urea, which decomposes upon mild heating to produce CO2 bubbles that act as templates for metal oxide assembly on the surfaces of rGO sheets. Electrochemical analysis. CV was used to characterize the electrochemical behavior of the synthesized materials. The materials show pseudo-capacitance behavior because of the cobalt and iron ions of Co3O4 and CoFe2O4, which is confirmed by the nonlinear plot Figure 6. Growth mechanism of rGO/CoFe 2 O 4 composite is enabled by the presence of urea, which decomposes upon mild heating to produce CO 2 bubbles that act as templates for metal oxide assembly on the surfaces of rGO sheets.
The GCD curves are demonstrated in the Figures 7b,d and S4, and the resulting nonlinear graphs indicate pseudocapacitive behavior for both Co 3 O 4 and CoFe 2 O 4 . In contrast, in the case of composites, rGO/Co 3 O 4 and rGO/CoFe 2 O 4 display both pseudocapacitive and EDLC capacitive behavior. Here, the discharging time is higher than that for pure metal oxide materials, with a negligible IR drop due to the hybrid storage mechanism. The interactions between the electrolyte and electrode materials are also high because of the sheet-like morphology of both rGO and metal oxides. The specific capacitance of the as-synthesized materials was estimated to be 71.8, 220, 166 and 396 F/g for Co 3 O 4 , CoFe 2 O 4 , rGO/Co 3 O 4 , and rGO/CoFe 2 O 4 , respectively, as shown in Figure 8. The specific capacitance of the rGO/CoFe 2 O 4 is higher than rGO/Co 3 O 4 due to the number of redox reactions involved in the rGO/CoFe 2 O 4 because of its binary metal oxide nature and rGO also offer large surface area. Furthermore, this composite is highly porous in nature, as can be seen from the microscopy images. The GCD curves are demonstrated in the Figures 7b,d and S4, and the resulting no linear graphs indicate pseudocapacitive behavior for both Co3O4 and CoFe2O4. In contra in the case of composites, rGO/Co3O4 and rGO/CoFe2O4 display both pseudocapacit and EDLC capacitive behavior. Here, the discharging time is higher than that for pu metal oxide materials, with a negligible IR drop due to the hybrid storage mechanis  The capacitance properties of the materials reduce very slightly at the high number of cycles, with excellent retention of 95.6%, which confirms excellent cyclic stability of the prepared materials. Furthermore, the enhanced electrochemical properties of the materials were analyzed with the Electrochemical Impedance Spectroscopy (EIS). Nyquist plots ( Figure S5) consist of two parts, a semicircle in the high-frequency region and a nearly straight line in the low-frequency (Warburg impedance region). The radius of the semicircle represents the charge transfer resistance, while the slope of the line indicates the diffusive resistance. The small semicircle and high slope of the line confirm efficient electron transport. Among the four sample types, rGO/CoFe 2 O 4 electrodes exhibited small charge transfer resistance, as seen in Figure 8, suggesting high electrical conductivity. These results can be due to the hybrid structure of rGO and CoFe 2 O 4 nanoparticle integration, which could promote rapid electron and electrolyte transportation at the electrode surface. An intercept at high-frequency region with the real part (Z − ) is attributable to a combinational resistance. The combinational resistance includes the ionic resistance of the electrolyte, the intrinsic resistance of the active material, and the contact resistance of the active material and the current collector. The combinational resistance found to be 7.6 Ω, 6. as-synthesized materials was estimated to be 71.8, 220, 166 and 39 rGO/Co3O4, and rGO/CoFe2O4, respectively, as shown in Figure 8. of the rGO/CoFe2O4 is higher than rGO/Co3O4 due to the numb volved in the rGO/CoFe2O4 because of its binary metal oxide na large surface area. Furthermore, this composite is highly porous from the microscopy images.  Figure 8 also shows the cyclic stability of the rGO/Co3O4 an The capacitance properties of the materials reduce very slightly cycles, with excellent retention of 95.6%, which confirms excelle prepared materials. Furthermore, the enhanced electrochemical als were analyzed with the Electrochemical Impedance Spectrosc ( Figure S5) consist of two parts, a semicircle in the high-freque straight line in the low-frequency (Warburg impedance region). circle represents the charge transfer resistance, while the slope diffusive resistance. The small semicircle and high slope of the li tron transport. Among the four sample types, rGO/CoFe2O4 ele charge transfer resistance, as seen in Figure 8, suggesting high These results can be due to the hybrid structure of rGO and CoFe tion, which could promote rapid electron and electrolyte transp surface. An intercept at high-frequency region with the real par combinational resistance. The combinational resistance includes t electrolyte, the intrinsic resistance of the active material, and the active material and the current collector. The combinational resi 6.2Ω, 5.46Ω and 5.36Ω for Co3O4, rGO/Co3O4, CoFe2O4 and rGO Collectively, these results suggest that rGO/CoFe2O4 has good ele Photochemical analysis. The photo-degradation potential of thus-synthesized materials was analyzed by measuring the degradation of malachite green (MG) dye in aqueous solution in the visible light photo-reactor. To assess the photocatalytic degradation process, a small amount of the photocatalyst was added to an aliquot of MG dye solution. This solution was placed in the dark with stirring to attain adsorption-desorption equilibrium for about 30 min, and then placed under visible light irradiation. During the irradiation stage, small volumes of the aqueous reaction mixture were drawn at intervals of 30 min and their UV-Vis absorption spectra collected at the corresponding wavelength of λ max = 665 nm. A reduction in the magnitude of the absorption peak indicates the degradation of MG dye. The degradation percentage of the dye was calculated by using the following relation: where A t is the absorbance at the time interval t, A 0 is the absorbance at t = 0 min. The rate constant of photocatalytic degradation of MG for each material was obtained from the Equation (4), with the results summarized in Table 2. , more than 80% of MG were degraded. This is because rGO acts as an acceptor of the electrons generated in the metal oxide, which is suppressing the recombination of charges. The effect of pH on MG dye was studied at a fixed dye concentration and weight of the catalyst, with the solution pH varied from 3 to 11, as shown in Figure 9. The efficiency of photodegradation of MG dye linearly increases from pH 3 to 11. Malachite Green dye is a cationic dye, so in the case of the lower pH range (>7) repulsion will take place between the dye and the photocatalyst, with the breaking of the dye molecules becoming more difficult. In contrast, in the case of higher pH, the degradation takes place very easily due to the basic nature of the dye solution. This is clearly shown in the Figure S7.

CoFe2O4
0.00513 72.7 rGO/CoFe2O4 0.00667 80.8 Figure S6 shows the photocatalytic activity of Co3O4, CoFe2O4, rGO/Co3O4 and rGO/CoFe2O4 for the degradation of MG dye under the visible light irradiation. For Co3O4 and CoFe2O4, the degradation rate was only around 65% within 120 min, while for rGO/Co3O4 and rGO/CoFe2O4, more than 80% of MG were degraded. This is because rGO acts as an acceptor of the electrons generated in the metal oxide, which is suppressing the recombination of charges. The effect of pH on MG dye was studied at a fixed dye concentration and weight of the catalyst, with the solution pH varied from 3 to 11, as shown in Figure 9. The efficiency of photodegradation of MG dye linearly increases from pH 3 to 11. Malachite Green dye is a cationic dye, so in the case of the lower pH range (>7) repulsion will take place between the dye and the photocatalyst, with the breaking of the dye molecules becoming more difficult. In contrast, in the case of higher pH, the degradation takes place very easily due to the basic nature of the dye solution. This is clearly shown in the Figure S7. The kinetic performance of the nanoflakes for the degradation of MG dye was then analyzed using the Langmuir-Hinshelwood model (Equation (5)). Figure S7 gives the chemical kinetics for the removal of MG dye that fits the pseudo-first order kinetic model ln C/C o = kt, where C is the initial absorbance of MG dye at time t = 0, C o is the change in absorbance of MG dye at selected intervals of degradation, and k is the first-order rate constant.
The plot of ln C/C o versus irradiation time gives the rate constant value from the obtained slope. Figure S8 illustrates that the photocatalytic reaction rate constants (k) for Co 3  These results reveal that the efficiency of the photocatalytic activity of rGO/CoFe 2 O 4 and rGO/Co 3 O 4 photocatalysts is increased with the incorporation of rGO sheets. The photocatalytic activity of the rGO/CoFe 2 O 4 nanoflakes with the k value of 0.00667 min −1 is higher than those for the other three materials under the visible light illumination. The enhancement in the efficiency of photodegradation was observed for rGO/CoFe 2 O 4 with the superior active surface area, which shows a greater degree of decolorizations for MG dye.
Mechanism of dye photodegradation. The diagrammatic representation of the MG dye degradation pathway is explained in Figure 10. In a typical process, when the visible light irradiates the material, the electrons (e − ) migrate from the conduction band (CB) to valence band (VB). Then, the holes (h + ) are created in the valence band. The holes react with hydroxyl (OH − ) coming from water to form •OH radicals. These •OH radicals can attack the bonds of the functional group of MG dye. In the case of pure CoFe 2 O 4 and Co 3 O 4 , there is no external charge carrier, and so the delocalized electrons are rapidly recombined with the holes produced in the conduction band prior to being captured by OH − , prohibiting the further dye degradation. On the other hand, rGO/Co 3 O 4 and rGO/CoFe 2 O 4 nanocomposites act as charge carrier due to the presence of rGO that could effectively confine the delocalized electrons and consequently prevent the recombination of electrons and holes. On the basis of above-mentioned factors, the holes in the CB have greater possibility to react with OH− to produce OH• radicals. A much higher dye degradation performance was observed for the rGO/CoFe 2 O 4 and rGO/Co 3 O 4 nanocomposites. The adsorption of MG dye is supported on the surface of rGO/Co 3 O 4 and rGO/CoFe 2 O 4 due to π-π interaction between aromatic domain of the MG dye and rGO nanosheets. The rGO nanosheet prevents the electron hole recombination in Co 3 O 4 and CoFe 2 O 4 as it acts as photoelectron acceptor and as such promotes an effective photocatalytic degradation of MG molecules. The functional group can be substituted by an •OH radical forming the corresponding functional products. The sulfoxide group of dye molecule can undergo a second attack by an •OH radical producing the sulfone and causing the definitive dissociation of the two benzenic rings. Finally, an almost complete mineralization of carbon, nitrogen and sulfur hetero-atoms occurred into CO 2 , NH 4+ , NO 3 .

Conclusions
In summary, the few layered reduced graphene oxide was synthesized from cocon

Conclusions
In summary, the few layered reduced graphene oxide was synthesized from coconut coir waste with a single step method. Then, the rGO/MO nanoflakes were obtained via a simple hydrothermal method, where CO 2 bubbles formed as a result of urea decomposition played an important role of a template during metal oxide flake synthesis in the surface of rGO. The as-prepared Co 3 O 4 and CoFe 2 O 4 yielded specific capacitances of 71.8 and 220 F/g at 1 A/g. The obtained rGO was employed as advanced support to grow CoFe 2 O 4 and Co 3 O 4 for achieving ideal capacitive performances. The optimal rGO/CoFe 2 O 4 composite delivered 396 F/g at 1 A/g, and demonstrated excellent cycling performance, with 94.5% of the capacitance retained after 2000 cycles. The few layered rGO nanosheets offer high surface area leading to high electrical conductivity and improved mechanical and cyclic stability of the composites. Therefore, the ultrahigh specific capacitance occurs due to the effective utilization of active material CoFe 2 O 4 on the conductive rGO sheets. The photocatalytic results show that rGO/CoFe 2 O 4 and rGO/Co 3 O 4 are excellent photocatalysts for the visible light photodegradation of MG dyes compared to bare Co 3 O 4 and CoFe 2 O 4 catalysts. This work is of particular interest to those working in the area of valorization of biomass waste, who look to enhance the properties of biomass-derived advanced materials and expand their potential applications in the area of green energy and environmental remediation, hence fully embracing the concept of "Waste to Treasure".
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nano12213822/s1, Figure S1: SEM image of pure rGO flakes; Figure S2: Tauc plots for (a) Co 2 O 3 and rGO/Co 2 O 3 , and (b) CoFe 2 O 4 and rGO/CoFe 2 O 4 . Absorption data obtained using UV-Vis spectroscopy. E g estimated as the intercept (y=0) between the linear fit to plot generated using the Tauc equation αhν= B (hν − E g ) n , where α is the optical absorbance, ν is the frequency of light, and n and B correspond to the type of transition and the length of localized state tails respectively; Figure S3: Spectrum for rGO flakes; Figure S4: (a, c) Representative CV curves for Co 3