Prevailing Surface-Controlled Charge Storage Mechanism in Iron Oxcide with Glycine Doping for Supercapacitors

Supercapacitors as futuristic types of energy storage devices provide numerous benefits, including high power density, stability, environmentally friendliness, and fast charging and discharging speed. The primary objective of this research is to optimize the charge storage mechanism of Iron Oxcide (Fe ₃O ₄) nanomaterials with different PVP and glycine concentrations prepared by the sol-gel method. PVP and glycine doping variations the structural characteristics of materials, including modifications in lattice parameters, crystallite size. PVP doping enhances the structural stability and crystallite size of Fe ₃O ₄, resulting in bigger nanoparticles with a more continuous shape. In contrast, glycine doping dramatically alters the XRD pattern, indicating improved crystallinity, and significantly boosts electrochemical performance. SEM images show the spherical like shape of the pristine and doped Fe ₃O ₄ nanomaterials. Electrochemical characteristics demonstrate how PYP and glycine doping improve the Fe ₃O ₄ nanomaterial’s effectiveness as an electrode material for supercapacitors. When glycine is doped in Fe ₃O ₄, the specific capacitance rises to 300 Fg ^{− 1}, while undoped Fe ₃O ₄ has 94 Fg ^{− 1} at a weep rate of 5 mVs ^{− 1}. Theoretical analyses by employing Dunn’s model indicate that surface-controlled mechanisms majorly contributed to the charge storage and portion is as high as 75 % at a sweep rate of 40 mVs ^{− 1} for glycine doping.

• Iron Oxcide
• PVP and Glycine
• Sol-Gel Method
• Surface Controlled Mechanism
• Supercapacitor

1. Introduction

As the global demand for energy continues to rise, the need for efficient, cost-effective, and sustainable energy storage technologies has never been greater [1] . Supercapacitors, with their high power density, fast charge/discharge rates, excellent cycling stability, and low maintenance cost, have emerged as promising candidates for next-generation power devices, complementing traditional energy storage systems like batteries [1] . However, one of the major challenges for supercapacitors is their relatively low energy density, which is typically an order of magnitude lower than that of batteries [2] . The development of new electrode materials with enhanced energy density, without sacrificing power density or cycle life, is essential for advancing supercapacitor performance [3] [4] .

Transition metal oxides (TMOs) have gained significant attention in recent years as potential electrode materials for supercapacitors due to their pseudocapacitive behavior and the ability to store energy via faradic redox reactions [5] - [7] . Iron oxide (FeₓOᵧ) nanoparticles, in particular, are highly promising because of their environmental friendliness, low cost, and abundance, which make them attractive for large-scale applications [7] [8] . However, while iron oxide (Fe₃O₄) exhibits excellent stability and conductivity, its low capacitance and poor electron conductivity at high current densities limit its practical applications in supercapacitors [9] - [11] .

To overcome these challenges, polymer doping has been explored as an effective strategy for improving the electrochemical properties of iron oxide nanoparticles [12] [13] . For instance, Polyvinylpyrrolidone (PVP) has been used to enhance the structural integrity and stability of iron oxide nanoparticles, thereby improving the electrochemical performance [14] - [16] . Additionally, glycine doping has been investigated as another approach to enhance the electrochemical characteristics of iron oxide by modifying its surface properties and increasing the available active sites for charge storage [17] [18] . Eeu et al. [19] synthesized electrodeposited PPy/reduced graphene oxide (rGO)/iron oxide (Fe₂O₃) using a facile one-pot chronoamperometry approach. The specific capacitances are 125.7 and 102.2 Fg⁻¹ after 200 cycles at 50 mVs⁻¹ scan rate in 1.0 M of KCl electrolyte with capacitive retention at 80.3%. Devi et al. [20] investigated Fe₃O₄/rGO nanocomposite by chemical reduction method and achieved a maximum specific capacitance 416 Fg⁻¹ at 5 mVs⁻¹ with 1 M H₂SO₄ as electrolyte. Bhattacharya et al. [21] synthesized Fe₃O₄/rGO nanocomposite using a facile combination of wetchemistry and low-power sonication and exhibited a high specific capacitance of 576 Fg⁻¹ at 10 mVs⁻¹. Sagolsem et al. [22] Fe₃O₄/rGO/PVP nanocomposite films with different concentrations of Fe₃O₄ were prepared by electrospinning spray process. The minimum value of Ecorr and Icorr was 0.18 V and 4.76 l V, maximum specific capacitances were achieved at 104.12, 169.63 and 124 Fg⁻¹ at a current density of 5 Ag⁻¹.

In this study, we present the synthesis of iron oxide (Fe₃O₄ nanoparticles doped with PVP and glycine using the sol-gel method. We aim to investigate how the incorporation of these polymers affects the morphology, crystallinity, and electrochemical performance of the iron oxide nanoparticles. The synthesized nanomaterials were characterized using various techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared (FTIR) spectroscopy, and cyclic voltammetry (CV). The electrochemical properties of the doped and undoped iron oxide nanoparticles were evaluated to assess their potential for supercapacitor applications.

The results show that the doping of PVP and glycine not only enhances the structural stability and surface area of iron oxide but also improves its charge storage capacity, particularly in glycine-doped iron oxide, which exhibits a significantly higher specific capacitance compared to the undoped material. These findings provide insights into the potential of polymer-doped iron oxide nanomaterials for high-performance supercapacitors and contribute to the ongoing development of advanced materials for energy storage applications [23] - [26] .

2. Materials

All of the starting raw materials, including FeCl₃∙6H₂O [FeCl₃∙6H₂O ≥ 98%, Sigma Aldrich], FeCl₂∙4H₂O [FeCl₂.4H₂O = 98%, Sigma Aldrich, PVP [powder, average M_w ~55,000 Sigma Aldrich], glycine [≥98.5%, suitable for cell culture, non-animal source, meets EP, JP, USP testing specifications], NaOH and DI-Water used for the preparation ma MNPs.

Synthesis

The iron oxide nanoparticles were prepared by using Sol gel method. The ferric chloride hexahydrate (FeCl₃∙6H₂O) 1 g and ferrous chloride tetrahydrate (FeCl₂∙4H₂O) 2.2 g were added into deionized water (100 mL). The stirring with the help of magnetic stirrer continued until the iron salt completely dissolved in deionized (DI) water. A specific amount of PVP (50 mL) and glycine (50 mL) is added to the original solution for doping. After that, the temperature was raised on a hot plate up to 75˚C by adding the sodium hydroxide (NaOH 4 g in 10 mL of DI water) solution drop wise in given solution to check the pH (7 - 9) of the solution. After maintaining the pH of samples, for 2:30 hours. The temperature is maintained at 75 ˚C, in this process solution starts changing into gel form. After passing time gel begins to burn slowly, this was an indication of formation of particles successfully. After that the sample is dried properly in Oven for 3 hrs at 90˚C. The dried sample is then converted into fine powder with help of mortar and pestle which is later annealed by furnace at 300˚C to get final sample. Final Samples are ready for characterizations (Schematic Figure 1 ).

Figure 1

Figure 1. Characterization of the final samples.

Characterization Tool

Various characterization techniques have been used to investigate the properties of synthesized nanomaterials. Structural properties were determined using X-ray diffraction (XRD) with Cu Kα radiation (λ = 0.154 nm). Functional groups were identified by Fourier Transform Infrared Spectroscopy (FTIR) using a Cary 630 spectrophotometer. The optical characteristics of prepared nanomaterials were evaluated by employing a UV–Vis spectrometer (T80). Morphological analysis was performed using scanning electron microscopy (SEM) using a FEI Nova NanoSEM 450.

Working Electrode Fabricated

A working electrode was prepared by mixing graphite, polyvinyl alcohol (binder), and synthetic materials (Fe₃O₄, PVP/Fe₃O₄, glycine/Fe₃O₄) at a ratio of 10:10:80% by weight, respectively. This mixture was then precipitated onto nickel foam. Electrochemical analysis of the fabricated working electrodes was performed in a three-electrode system using 1M KOH electrolyte solution. A platinum (Pt) counter electrode and a Hg/HgO reference electrode were used together with the working electrode, and both were connected to a CS310 workstation.

3. Results and Discussion

XRD is a method for analyzing the structure of generated nanomaterials. The XRD analysis of both pristine and polymer doped Fe₃O₄ are presented in ( Figure 2(a) ). The XRD verifies the rhombohedral structure of iron oxide powder and the generation of single phase Fe₃O₄, which matches the JCPDS card 65 - 3107. The four diffraction peaks are observed at positions 28.94˚, 31.68˚, 45.40˚ and 56.55˚, which correspond to the (220), (311), (411), and (511) planes, respectively [27] . PVP and glycine showed a slight shift as the dopant caused the pristine, but in case of glycine, there is completely shifted at the position of 28.94˚ other ones. In comparison to naked Fe₃O₄ NPs, the polymer doping has also demonstrated impacts on the structural characteristics that differed [28] .

Figure 2

Figure 2. (a) XRD pattern of synthesized Fe 3 O 4 and PVP/glycine doped Fe 3 O 4 nanomaterials. SEM micrographs and corresponding histograms of (b) Fe 3 O 4 , (c) PVP/Fe 3 O 4 , and (d) Glycine/Fe 3 O 4 nanomaterials (h) FTIR spectra of Fe 3 O 4 , PVP/Fe 3 O 4 , and glycine/Fe 3 O 4 nanocomposite materials (i) UV-V is absorption spectrums of synthesized materials.

The average crystal size is determined using Debye-Scherrer’s technique, which adheres to Equation (1) [29] .

$D=\frac{k\lambda }{\beta \text{cosθ}}$ (1)

In this equation where D is crystallite size, k is shape crystal which value is 0.9, shows the wavelength which vale is 1.54 Å, β is FWHM and θ is Bragg’s angle. The crystal size of pristine is 12.05 nm, while polymer-doped iron oxide NPs crystallite size is enhanced shown in Table 1. So, this might be attributed to the fact that glycine has the largest crystallite size among the dopants [30] [31] .

Utilizing a scanning electron microscope, one may examine the grain size and morphology of the nanomaterials. The low and high magnification micrographs of Fe₃O₄ nanoparticles shown in Figures 2(b)-(d) show spherical grains with an average size of 53.16 ± 6.637 nm and a 0.01 is the monodisperse index (PDI) value.

Table 1

Table 1. Comparison of the crystallite size doped and undoped Iron Oxcide.

Sr.No.	Material	Average Size (nm)
1	Iron Oxcide	12.05
2	PVP/Iron Oxcide	22.89
3	Glycine/Iron Oxcide	23.68

A PDI value below 0.1 suggests that the produced NPs are monodispersed [32] . Figure 2(e) histogram demonstrates that the bare Fe₃O₄ particles are inside the nano-regime and have a great narrow distribution. When compared to pure Fe₃O₄, the shape of the nanoparticles in PVP/Fe₃O₄ composites varies very little. Spherical particles are visible in the SEM micrographs ( Figure 2(c) ). Nevertheless, the grain size has a tighter size distribution (PDI = 0.03) and is marginally on the upper side, measuring 54.39 ± 9.972 nm. The hybrid form of Fe₃O₄ with PVP is responsible for the increase in particle size because it can encourage the attachment of additional atoms to the particles during the development phase [33] . Figure 2(d) displays a mixture of glycine and Fe₃O₄, with a spherical particle shape and an average size of 48.93 ± 8.575 nm. This demonstrates that PVP plays a specific function in the NPs development process, while glycine may simply be combined with Fe₃O₄ without significantly altering the nanomaterials shape.

FTIR spectra of synthesized materials are determined in the range of 500 - 4000 cm⁻¹ as shown in ( Figure 2(h) ) highlights the comparison of bare Fe₃O₄, PVP and glycine doped Fe₃O₄ at different peaks. The pattern of PVP doped Fe₃O₄ nanoparticles is similar to the pure Fe₃O₄. But the glycine doped Fe₃O₄ nanoparticles indicated the similar behavior label at four points on the peaks [34] . Various function group existed at the different peaks as shown in Figure 2(h) . Moreover, the band of PVP doped Fe₃O₄ NPs was nearly equal to the glycine doped Fe₃O₄ NPs band and little difference absorbance level of both bands [35] . Moreover, the main peak Fe-O is observed in the band at 670 cm⁻¹ [36] , C=O attached at peak 1701 cm⁻¹, -CH₃ (asymmetric stretching) presented at peak 2967 cm⁻¹ [37] , and the band at 3745 cm⁻¹ belongs to the -OH bond of water molecules adsorbed on the surface of the particles. Many studies reported these higher frequency spectral bands. The same wave number of both spectrums was shown in graph at 1705 cm⁻¹ to 3740 cm⁻¹ [38] .

The UV–Vis absorption spectrum is used to study the optical properties of the synthesized Fe₃O₄, PVP/Fe₃O₄ and glycine/Fe₃O₄ nanomaterials. The bandgap and the type of electronic transitions can be determined from the absorption spectrum. When a semi-conductor absorbs photons of energy larger than the bandgap of the semiconductor, an electron is transferred from the valence band to the conduction band, there occurs an abrupt increase in the absorbance of the material to the wavelength corresponding to the bandgap energy [39] . In Figure 2(i) , the absorbance curves of Fe₃O₄, PVP/Fe₃O₄ and glycine/Fe₃O₄ show a peak at 328 nm, 339 nm, and 332 nm respectively. The band gap is determined using Tauc plot method. Figure S1 shows band gap curves for different materials. The undoped Fe₃O₄ has a band gap of 3.8 eV [18] , whereas PVP/Fe₃O₄ and glycine/Fe₃O₄ the band i.e. 2.9 eV and 3.2 eV respectively. According to the quantum size effect, the smaller the size of the NPs, the wider the energy bandgap. However, the increase in energy bandgap for doped materials with PVP and glycine indicates that several structural defects are generating allowed states between the valence and conduction band.

4. Electrochemical Analysis

Cyclic Voltammetry (CV)

The electrochemical performance of Fe₃O₄, PVP/Fe₃O₄ and glycine/Fe₃O₄ nanomaterials with different concentrations is determined by CV curves in the potential window range from 0 to 0.45 V. The sweeping speeds of 5 mVs⁻¹, 10 mVs⁻¹, 15 mVs⁻¹, 20 mVs⁻¹, 30 mVs⁻¹, and 40 mVs⁻¹ are applied with a three-electrode setup to obtain CV curves. When the scan rate rises from 5 mVs⁻¹ to 40 mVs⁻¹, the CV enclosed area expands, indicating fast electron and ion transfer on the prepared electrode surface [40] [41] . The contrast of CV curves for Fe₃O₄ and different concentrations of doped Fe₃O₄ at scan rate of 5 mVs⁻¹ is shown in Figures 3(a)-(c) . It can be observed that doping increases the peak current and area enclosed in the CV curves as compared to undoped Fe₃O₄ as a first indication of accelerated electrochemical response by PVP and glycine incorporation [42] . When comparing the response of three Fe₃O₄, PVP/Fe₃O₄ and glycine/Fe₃O₄ the oxidation peak current for PVP/Fe₃O₄ higher as compared to glycine/Fe₃O₄ contrarily, the reduction current is the highest in the case of glycine doping [39] [43] . However, for electrochemical performance parameters, the area enclosed by the CV curve is of prime focus along with peak current, and glycine/Fe₃O₄ exhibits a comparatively larger polygonal area at scan rate of 5 mVs⁻¹ than undoped and PVP doping material. The slight peak shift can be attributed to the polarization effect [44] .

The symmetrical increment in surface area for sweeping at higher scan rates indicates good rate competence, a crucial quality for the actual utilization of supercapacitors. The specific capacitance (Cs) for nanomaterials is calculated by using Equation (4) [45] [46] .

$Cs=\frac{{{\displaystyle \int }}_{vc}^{va}I\times \text{d}V}{m\times v\times \Delta V}$ (2)

where m represents the mass of active material, v is scan rate, $\Delta V$ potential window and $\int I\times \Delta V$ CV area enclosed. The specific capacitance of Fe₃O₄, PVP/Fe₃O₄ and glycine/Fe₃O₄ are 94 Fg⁻¹, 247 Fg⁻¹ and 300 Fg⁻¹ respectively at 5 mVs⁻¹.

The Randles-Sevick plot is utilized to estimate the redox contributions. A Randles-Sevick equation is used to illustrate the estimated linear relationship between the current and sweep rate. The graphs of (I_p vs. υ^1/2) and (I_p vs. υ) present faradic and non-faradic processes in the materials [47] . The linear fitting determines correlation coefficients and the R² value indicates the charge storage nature.

Figure 3

Figure 3. Cyclic voltammetry curves of (a) Fe 3 O 4 , (b) PVP/Fe 3 O 4 , and (c) glycine/Fe 3 O 4 , (d)Comparison of CV curves of all samples at scan rate 5 mV/s.

In each case, inset graph displays the diffusive response, while the main graphs represent the material’s capacitive response ( Figures 4(b)-(d) ). For the Fe₃O₄, PVP/Fe₃O₄ and glycine/Fe₃O₄, it can be seen that the charge storage mechanism is mostly due to the surface-controlled process as R² is 0.9813, 0.96037, and 0.97816 for respectively. It is seen that the surface-based capacitive response is dominanFe₃O₄, PVP/Fe₃O₄ and glycine/Fe₃O₄ t as compared to the diffusive response ( Figures 4(b)-(d) ).

The electrode’s charge storage can follow two ways, a surface-controlled process that happens quite quickly and a slower diffusion-controlled mechanism [47] . The theoretical aspect is also used to determine the ion storage nature of the developed electrodes. The power law (Equations (4-6) has been utilized for this purpose;

$I_p=a{\upsilon }^b$ (3)

where I_p is the peak current and ω is the scan rate, and a and b are constants. The b value is used to determine nature of the material i.e battery or supercapacitor. When b is equal to 0.5 or 1, there are two different scenarios that apply. If b value is 1, the material can be used as an electrode in supercapacitor (surface-controlled process) and b = 0.5 represents the battery-grade materials (diffusion-controlled method). The b value in curve case is 0.8, representing a surface-controlled mechanism or a predominantly capacitive nature of the material as shown in Figure [48] - [50] .

Figure 4

Figure 4. (a) Specifc capacity at different scan rates for all three samples. Capacitive/non-ferodic controlled mechanism differnitiation; I p vs. υ plot, (inset: I p vs. υ 1/2 ), (b) Fe 3 O 4 , (c) PVP/Fe 3 O 4 , and (d) glycine/Fe 3 O 4 .

Considering the best electrochemical performance of glycine/Fe₃O₄, the b value is determined at various static potentials between 0.23 V and 0.44 V ( Figure 5(a) ). As the potential rises, the b value starts from 0.28 at the lowest potential (0.23 V), increases to 1.11 at 0.35 V, and reduces subsequently ( Figure 5(b) ). This comparison shows that diffusion process converts generated current linearly into a surface-controlled process as the scan rate increases. Diffusion-controlled mechanism generated by fast intercalation produces the maximum current at peak potentials (ion-exchange process) [50] .

Equation (7) is used to elucidate the capacitive and diffusion-controlled effects from the overall charge storage of each electrode,

$I_p=k_{1}v+k_2{v}^{1/2}$ (4)

Here, k₁v and k₂v^1/2 represent capacitive and diffusion-limited contributions. We can utilize k₁ and k₂ to find the share of each current contributor ( Figure 5(c) ). The surface-controlled process is dominated at slow sweep speeds because ions have more time to respond. At faster scan rates, the ions are unable to interact extensively with the electrode surface and consequently have insufficient reaction time [51] . Figure 5(d) shows that the surface-controlled process controls the electrochemi-cal reaction at higher scan rates, while the diffusion-controlled process reduces for glycine/Fe₃O₄. At a scan rate of 40 mVs⁻¹, the Fe₃O₄ exhibits a 38% diffusive contribution and a 62% surface-controlled contribution ( Figure S2 ) and the PVP/Fe₃O₄ shows a 31% diffusive contribution and a 69% surface-controlled con-tribution ( Figure S3 ). However, the capacitive contribution rises to 75% at a scan rate of 40 mVs⁻¹ from 52% for glycine/Fe₃O₄. This exhibits the promise of glycine/Fe₃O₄ as an innovative material for future supercapattery applications.

Figure 5

Figure 5. Charge storage mechanism in glycine/Fe 3 O 4 (a) Draw a plot at various fixed potentials between the logs of current and scan rate, (b) Change in b at various fixed potentials (c) plot for determining k 1 and k 2 , (d) present contribution at various scan rates.

Galvanostatic charge/discharge (GCD)

GCD investigation is used to investigate the electrochemical stored charge mechanism of the fabricated electrodes. Figures 6(a)-(c) shows the GCD curves of Fe₃O₄, PVP/Fe₃O₄, and glycine/Fe₃O₄ at current densities of 1, 3, 7, and 9 Ag⁻¹. The GCD spectra of all electrodes show a quasi-triangular pattern (87 jabir). 10a, b, c. The GCD profile exhibits a non-linear trend (indicated by inflation); of redox reactions

Discharge time decreases with increasing current density. The electrode has the highest capacitance and longest discharge time because it takes the longest time for OH ions to penetrate the electrical contacts at the lowest current density (1 Ag⁻¹). PVP/Fe₃O₄ and glycine/Fe₃O₄ materials take longer to discharge compared to pristine Fe₃O₄, which is also consistent with the CV results

To further explain the distinctive qualities of charge storage, Equation (5) was utilized to calculate the specifc capacity (Qs) from GCD measurements:

$Q_s=\frac{I\times \Delta t}{m}$ (5)

$\Delta t$ = discharging time, I = current density (A), m = mass loaded. The highest specific capacity is observed for glycine/Fe₃O₄ (301 Cg⁻¹) while the minimum specific capacity (121 Cg⁻¹) is observed for pristine Fe₃O₄ at a current density of 1 Ag⁻¹. Change specific capacity of the various materials according to current density are: Figures 6(a)-(d) . The graph shows that glycine/Fe₃O₄ has the highest specific capacity among the three materials. The trend in specific capacity for higher current densities is as follows: Electrochemical bonding between electrolytes because ions and active substances are generated in a short period of time, fast dynamics [52] .

Figure 6

Figure 6. Charge/discharge curves at diferent current densities: (a) Fe 3 O 4 , (b) PVP/Fe 3 O 4 , and (c) glycine/Fe 3 O 4 , (d) Specifc capacity trend at diferent current densities for diferent materials.

Figure 7

Figure 7. The Nyquist plots comparison of Fe 3 O 4 , PVP/Fe 3 O 4 , and glycine/Fe 3 O (inset; the high frequency spectra) (b) Equivalent circuit.

Electrochemical impedance spectroscopy (EIS)

EIS analyzes the electron-transport mechanism and conductivity of the produced electrodes in a frequency range of 0.3 Hz to 100 kHz at a voltage of 10 mV. The Nyquist plot displays the behavior of the imaginary and real impedance for ( Figure 7(a) ). The resistance created at the electrolyte-electrode interface during a chemical reaction is known as equivalent series resistance. The Rs values of 0.69 Ω, 0.65 Ω, and 0.52 Ω are obtained for (a) Fe₃O₄, PVP/Fe₃O₄, and glycine/Fe₃O₄, respectively ( Figure 7(b) ). The lowest Rs value glycine/Fe₃O₄ for indicates its enhanced response in electrochemical reactions with ease of electron transfer. The low resistance can help ions transfer into electrically charged materials [53] . The findings support the hypothesis that an electrode modified with glycine/Fe₃O₄ exhibits enhanced electron transfer efficiency. Figure 7(b) shows an equivalent circuit. The Rs, Rct, and C represent the solution resistance, charge transfer resistance, and capacitor components, respectively.

5. Conclusion

Fe₃O₄, PVP/Fe₃O₄, and glycine/Fe₃O₄ nanomaterials are efficiently synthesized by using the sol-gel method. Nanomaterials were investigated using the following techniques: XRD, FTIR, and SEM. Fe₃O₄ and PVP/glycine doped Fe₃O₄ nanomaterials illustrated an excellent capacitance effect when applied as supercapacitor electrodes. The Fe₃O₄ material for electrodes has a specific capacitance of 121 Fg⁻¹ at a current density of 1 Ag⁻¹. Doping PVP and glycine results in significantly increased electrochemical energy storage capacity. Glycine doped Fe₃O₄ electrodes demonstrated their maximum specific capacitance of 301 Fg⁻¹ at a current density of 1 Ag⁻¹, illustrating their electrochemical efficiency. The CV curves illustrate that the surface-controlled method increases with scan rate. At 40 mV/s, there’s a surface-controlled mechanism that controls 25% and a diffusion-controlled process that controls 75%. Because of this result, it becomes a special material for future supercapattery purposes.

Appendix

Figure 8

Figure S1. Band gap by UV-visible.

Figure 9

Figure S2. Contribution of Fe 3 O 4 .

Figure 10

Figure S3. Contribution PVP/Fe 3 O 4 .

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