Electrochemical Properties of GO-rGO: Insights from Modified Hummers Synthesis and Thermal Reduction

doi:10.21203/rs.3.rs-5022101/v1

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Electrochemical Properties of GO-rGO: Insights from Modified Hummers Synthesis and Thermal Reduction

https://doi.org/10.21203/rs.3.rs-5022101/v1

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Graphene Oxide (GO) was synthesized using the Modified Hummers Method and subsequently thermally reduced to produce reduced Graphene Oxide (rGO). X-ray diffraction (XRD) analysis indicated crystalline dimensions of approximately 5 nm for GO and 7 nm for rGO, suggesting a slight increase in crystallinity after reduction. UV-vis spectroscopy revealed distinct electronic properties: GO exhibited an optical band gap of 2.42 eV, which narrowed to 1.33 eV in rGO, indicating improved conductivity in post-reduction. Raman Spectroscopy showed characteristic D and G bands, with the ID/IG intensity ratio increasing after thermal reduction, suggesting a higher degree of structural defects in rGO. Scanning Electron Microscopy (SEM) provided morphological insights, while Energy Dispersive X-ray Spectroscopy (EDAX) confirmed the reduction of oxygen-containing groups in rGO. Electrochemical analyses using Electrical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV) demonstrated enhanced charge storage and faster electron transfer kinetics in rGO compared to GO, highlighting its potential for superior supercapacitor electrodes. This study underscores the diverse properties of GO-rGO, essential for advancing electrochemical energy storage technologies.

Nanoscience

rGO

Modified Hummer’s Method

Thermal reduction method

Electrochemical studies

In the face of global energy challenges and the growing demand for efficient energy storage solutions, graphene-based materials have emerged as promising candidates for next-generation electrochemical devices. Among these, Graphene Oxide (GO) and reduced Graphene Oxide (rGO) have garnered significant attention due to their unique structural, electrical, and electrochemical properties [1]. This work provides a comprehensive exploration of the synthesis methods and resulting electrochemical properties of GO-rGO, with a particular focus on the modified Hummers method for GO synthesis and thermal reduction for rGO production [2].

The modified Hummers method, an improved version of the original Hummers method, has become the industry standard for GO synthesis due to its efficiency and scalability. This process involves the oxidation of graphite using strong oxidizing agents, resulting in a material rich in oxygen-containing functional groups [3]. These groups not only enhance GO's hydrophilicity but also serve as active sites for subsequent functionalization and electrochemical activities. The synthesis process is highly sensitive to various parameters, including the concentration of oxidizing agents, reaction time, temperature, and the nature of the starting graphite material. Each of these factors can significantly influence the oxidation degree and defect density of the resulting GO, which in turn affects its dispersibility, electrical conductivity, and overall functionality in electrochemical systems [4, 5].

The thermal reduction of GO to rGO is a critical process that significantly impacts the restoration of sp2 carbon networks. As GO is heated to temperatures ranging from 200°C to 1000°C, oxygen-containing functional groups are progressively removed, allowing carbon atoms to reorganize into sp2 hybridized bonds [6, 7]. This process gradually restores the graphene-like structure, enhancing electrical conductivity and mechanical properties. However, the restoration is often incomplete, leaving some residual defects and oxygen groups. The degree of sp2 network restoration is highly dependent on the reduction temperature, with higher temperatures generally leading to more complete reduction but potentially introducing new defects [8, 9]. During thermal treatment, GO sheets may wrinkle or fold due to rapid gas release, creating a more porous structure in rGO. The balance between sp2 restoration and retention of beneficial defects or oxygen groups is crucial for optimizing rGO's electrochemical properties, as it directly influences electron transfer capabilities and the presence of active sites for redox reactions [10].

This research is unique in its comprehensive approach to studying the relationship between synthesis processes and the resulting electrochemical properties. While previous studies have examined these materials independently, few have provided a direct comparison of their electrochemical behavior to their fabrication techniques [11, 12]. This study addresses this knowledge gap by examining how the modified Hummers method and thermal reduction process impact the structural, electrical, and electrochemical characteristics of GO and rGO [13, 14].

Furthermore, this work sheds light on the role of defects and oxygen-containing functional groups in the electrochemical performance of GO-rGO. Contrary to the conventional wisdom that emphasizes high conductivity for electrochemical applications, the findings demonstrate that controlled oxidation and defects can enhance several electrochemical properties, including pseudocapacitance and ion adsorption [15, 16]. This insight challenges the simplistic notion that reduction is always desirable and opens new avenues for optimizing GO-rGO for supercapacitor applications [17].

The study employs a range of advanced characterization techniques to provide a comprehensive understanding of charge storage processes in these materials [18]. By combining electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) with structural and morphological investigations, this work offers not only a clarification of the underlying electrochemical behavior of GO-rGO but also practical insights for enhancing their performance in supercapacitor applications [19–22].

In conclusion, this research significantly advances the understanding of GO-rGO as electrode materials for supercapacitors. By providing crucial insights into the rational design of graphene-based materials for energy storage applications, the findings have both theoretical and practical implications for the development of next-generation supercapacitors with improved performance and efficiency [23, 24]. As we continue to address global energy challenges, the insights gained from this study will be instrumental in advancing sustainable energy storage technologies and paving the way for a more energy-efficient future [25].

2.1. Preparation of GO from hummer’s method

Graphite oxide was synthesized using a modified hummer’s process. Initially, 1 g of graphite powder, 0.5 g of sodium nitrate (NaNO₃), and 23 ml of concentrated Sulfuric acid (H₂SO₄) were combined and continuously stirred for an hour. At 20°C, 3 g of potassium permanganate (KMnO₄) was progressively added into the solution, causing a color shift from black to greenish-black [26]. The reaction mixture was then kept at 35°C with continuous agitation for 12 hours. As the solution's viscosity grew, 500 ml of distilled water was added to dilute it. After 45 minutes, adding 5 ml of 30% hydrogen peroxide (H₂O₂) caused a color shift to light yellow. After a two-hour settling time, the solution was centrifuged to separate the graphite oxide precipitate. The remaining metal impurities were eliminated by treating the graphite oxide with a hydrochloric acid (HCl) solution. The resulting product was completely cleaned with deionized water to provide pure graphite oxide (Fig. 1).

2.2. Synthesis of rGO from GO by Thermal reduction method

A dry graphene oxide (GO) sample weighing 2 g was carefully measured and transferred to a clean, pre-weighed borosilicate glass beaker. The beaker containing the GO was then carefully covered in perforated aluminum foil, with the perforations strategically placed to allow for controlled gas exchange throughout the heat treatment process. The sample was reduced in a hot air oven set to 500°C (± 5°C) for 20 minutes. This high-temperature thermal treatment facilitated the in situ reduction of GO to reduced graphene oxide (Fig. 1) [27]. This thermal reduction method was chosen for its ability to produce high-quality rGO without the use of chemical reducing agents, potentially yielding a product with fewer impurities and better properties suitable for a wide range of applications in materials science and engineering [28].

3.1. Analysing structures using XRD

The X-Ray Diffraction (XRD) patterns of graphene oxide (GO) and reduced graphene oxide (rGO) reveal significant structural differences (Fig. 2). The GO pattern (red line) has a significant peak at 2θ = 10.43°, which corresponds to the (001) reflection [29]. This implies larger interlayer spacing due to the presence of oxygen-containing functional groups and intercalated water molecules. In comparison, the rGO pattern (blue line) suffers a significant change [30]. The large GO peak at 10.43° is much diminished or absent, indicating that the bulk of oxygen-containing groups were eliminated during the reduction. The peak at 2θ = 25.08° represents the (002) reflection of graphitic structures, showing partial structural restoration [31]. The broadness of this peak implies that rGO sheets are not as well-ordered as pure graphite, most likely due to remaining defects and residual functional groups. The Debye-Scherrer formula may be used to calculate the size of these materials' crystallites.

D = Kλ / (β cosθ),

Where D is the mean crystallite size, K is the form factor (usually 0.9), λ is the X-ray wavelength, β is the peak's FWHM in radians, and θ is the Bragg angle. The Debye Scherer formula was used to calculate the crystallite size 15nm and 27nm for GO and rGO respectively [32]. The greater backdrop of the rGO pattern indicates a more disordered structure than GO. These XRD patterns, together with the Debye-Scherrer analysis, clearly show the structural development from GO to rGO, emphasizing the removal of oxygen groups, partial recovery of graphitic structure, and variations in crystallite size during the reduction process [33].

3.2. Morphological Study

The SEM and EDAX images provide complementary information on the structural and chemical characteristics of Graphene Oxide (GO) and reduced graphene oxide (Fig. 3). SEM micrographs reveal distinct morphological differences: GO (picture a) exhibits a highly wrinkled and crumpled sheet-like shape that is typical of its oxidized condition, but rGO (image b) appears more compact with reduced folding, indicating that the graphene structure has been partially restored following reduction. A particles size of GO is 29nm and rGO is 32nm calculated by Image J software. [34, 35].

The EDAX spectra presented below support these compositional observations. Spectrum 1 (Fig. 4a), which most likely corresponds to GO, has distinct peaks for both carbon and oxygen, with a notably strong oxygen signal suggesting GO's high oxygen content. Spectrum 2 (Fig. 4b), which likely represents rGO, shows a substantially smaller oxygen peak than carbon, which is consistent with the removal of oxygen-containing functional groups during the reduction process. These combined studies successfully demonstrate the structural and chemical changes that occur when GO is converted to rGO, highlighting the need of integrating several analytical methods in material characterization [36]. There are many elements present in EDAX because the modified Hummers' method involves the use of reagents like potassium permanganate (KMnO₄), sulfuric acid (H₂SO₄), and others that may leave behind traces of elements like sulfur (S), manganese (Mn), potassium (K), and sodium (Na).

3.3. Vibrational Spectral Analysis

Figure 5 shows the FTIR spectra for GO-rGO. The vibrational spectra of the materials were analysed using FTIR to determine the presence of functional groups. This study is based on the vibrational excitation of molecular bonds generated by the absorption of infrared light energy at wavelengths ranging from 4000 to 500 cm^− 1. Following oxidation, Graphite Oxide (GO) is expected to include more oxygen-containing functional groups [38]. However, after reduction, rGO is expected to have a minimal number of oxygen-containing functional groups. In contrast, for GO, the strong and broad signal at 3432cm^− 1 demonstrated the presence of an O-H bond (hydroxyl group). Furthermore, -C = O stretching (-COOH group) was seen at 1642 cm-1, whilst C-O-C stretching (epoxy group) may be seen between 1388 and 1111 cm-1[38]. The presence of all of these carboxylic, hydroxyl, epoxide, and carbonyl groups resulted in oxygen molecules (O) being highly occupied at the edge and basal planes of GO, indicating that GO was properly synthesized [39]. The rGO signal at 3425 cm-1 grew narrower than GO's, indicating that the hydroxyl group had been substantially eliminated. Other peaks at 1642, 1384, and 1112 cm-1 became less prominent than those in GO FTIR spectra, indicating that oxygen was eliminated during the reduction with hydrazine hydrate. As a consequence, oxygen-containing functional groups were partially removed, leaving only trace amounts of functional group residue at the rGO's edge and basal plane [40].

3.4. Optical Properties of GO-rGO

The UV-Visible absorption spectra in this image show a comparative analysis of Graphene Oxide (GO) and reduced Graphene Oxide (rGO), revealing substantial structural and electrical differences. GO exhibits two distinct spectral features: a prominent peak at 232 nm owing to π→π* transitions of aromatic C = C bonds, and a pronounced shoulder at 300 nm due to n→π* transitions of C = O bonds [41]. In contrast, rGO's spectrum changes dramatically, suggesting a reduced state. The red shift of the C = C bond peak to 260–270 nm suggests that the sp2 carbon network has been re-established (Fig. 6). Simultaneously, the C = O shoulder has dropped dramatically, suggesting that oxygen-containing groups were effectively eliminated during the reduction process. Perhaps, rGO shows higher absorbance throughout the whole visible spectrum (300–800 nm), suggesting better electronic conjugation than its GO precursor. These spectrum characteristics, when combined, provide strong evidence of the structural change from GO to rGO, as well as showing the reduced material's superior electrical capabilities [42].

The Tauc plots (Fig. 6) in the right-hand graphs give important insights into the optical bandgaps of Graphene Oxide (GO) and reduced Graphene Oxide (rGO). The charts show the relationship between (αhv)² and photon energy (hν), allowing for linear extrapolation to the x-axis to determine each material's bandgap. The top figure, which most likely represents GO, shows a significant optical bandgap of 2.42 eV, which is consistent with GO's broken sp2 network and oxygen-rich structure [43]. In sharp contrast, the bottom figure for rGO indicates a significantly lower bandgap of 1.33 eV. This considerable decrease in bandgap energy demonstrates the restoration of the sp2 carbon network and the successful removal of oxygen functions during the reduction process, resulting in increased electrical conductivity. These Tauc plot investigations, when paired with the UV-Visible spectra, offer a comprehensive picture of the GO to rGO transition [44]. The observed red-shift in absorption peak, increased absorption across the visible spectrum, and significant reduction in bandgap all show the success of the reduction technique in removing oxygen groups and restoring a more graphene-like structure in rGO. The changes in electronic structure and optical properties are important for applications in electronics, sensors, and energy storage devices, where rGO's increased conductivity and altered electronic characteristics might boost performance [45].

3.5. Raman spectra

Raman spectroscopy gives information based on the inelastic (Raman) scattering of a molecule when subjected to monochromatic light, often a laser. Figure 7 depicts the Raman spectra for GO and RGO. Figure 6 shows that GO and RGO have two basic vibrations at 1100 and 1700 cm-1. The D vibration band, which is created by a breathing mode of j-point photons with A1g symmetry, may be discovered at 1348.31 and 1353.20 cm-1 for GO and RGO, respectively [46]. In contrast, the G vibration band generated by first-order scattering of E2g phonons by sp2 carbon was detected at 1594.19 cm-1 for GO and 1586.56 cm-1 for RGO. In addition, the presence of the stretching C-C bond, which is prevalent in all sp2 carbon systems, adds to the G vibration band. The Raman spectrum (Fig. 6) displays the disorder and tangential bands, denoted by the letters D and G [47].

Figure 7 shows a large and shifted to higher wavenumber 2D band for GO at 2716.77 cm-1. Because the 2D band is highly sensitive to graphene layer stacking, it may be used to identify graphene layers (monolayer, double layer, or multilayer). Thus, the location of the 2D band confirms that the produced GO was multilayer, as monolayer graphene is typically detected around 2679 cm-1 in the spectra [48]. The displaced location of the 2D band is due to the presence of oxygen-containing functional groups that prevent the graphene layer from stacking. In addition, rGO has a 2D band at 2706.20 cm-1, as seen in Fig. 7. It is because, after reducing GO to rGO, less residue of oxygen-containing functional groups remained, causing RGO to stack [49]. The location of the 2D band in this experiment is extremely similar to the work presented by Thakur and Karak, in which GO was reduced by employing phytochemicals taken from leaves, peels, or other parts of plants. Phytochemicals were employed instead of chemicals as a reducing agent [50].

The I_D/I_G ratio for GO was calculated to be 0.85. Following reduction, the I_D/I_G for rGO improved to 0.91 due to the restoration of sp2 carbon, despite the fact that the average size of sp2 domains fell. The higher intensity in the D band also indicated that RGO included more isolated graphene domains than GO, as well as the removal of oxygen constituent from GO upon reduction [51].

3.6. Electrochemical analysis

The electrochemical properties were assessed using CV and EIS. The measurements were performed using three electrodes. The working electrode is a Glassy Carbon Electrode (GCE) covered with an active substance, the counter electrode is platinum wire, and the reference electrode is a Saturated Calomel Electrode (SCE). Tetrabutylammonium perchlorate 1M (C₁₆H₃₆ClNO₄) was used as the electrolyte. The potential window CV extends from − 0.6 to 0.8 V, with an EIS frequency range of 0.1 Hz to 100 kHz.

3.6.1. Cyclic Voltammetry

CV measurements for GO-rGO were carried out in a 1 M C₁₆H₃₆ClNO₄ electrolyte at a potential range of -0.6 to 0.8 V. These experiments revealed a strong redox peak, showing that both electrode materials functioned in a pseudocapacitive manner. Figures 8 illustrate the CV curves for GO-rGO, respectively [52]. Curves for newly generated samples were obtained at nine different scan speeds ranging from 10 to 200 mV/s at room temperature. Even at the slowest scan rate of 10 mV/s, the area under the CV curve for rGO is clearly greater than that for GO. Because specific capacitance (Csp) is proportional to area under the CV curve, rGO has a greater specific capacitance. As shown in Fig. 8, the CV curves displayed discrete redox peaks indicating pseudocapacitive behavior, implying effective ion migration from the electrolyte to electrode [53].

The specific capacitance (Csp) values for the GO electrode were determined to be 219, 109, 73, 54, 43, 36, 27, 21, and 10 F/g^− 1 at scan speeds of 10, 20, 30, 40, 50, 60, 80, 100, and 200 mV/s, respectively (Fig. 8). At the same scan speeds, the rGO electrode had Csp values of 359, 179, 119, 89, 71, 59, 44, 35, and 17 F/g^− 1. The specific capacitance of rGO falls dramatically with increased scan speeds due to the limiting of ion migration by diffusion in the electrolyte [54]. Furthermore, peak currents for oxidation and reduction increased with scan rate, demonstrating a diffusion-controlled redox process for charge storage. Furthermore, when the scan rate increased, the peak potentials for oxidation and reduction changed slightly higher and lower, respectively, which can be ascribed to polarization effects as well as ion diffusion. The higher capacitive response of rGO in the 1 M aqueous C₁₆H₃₆ClNO₄ electrolyte is ascribed to its increased surface area when compared to GO, showing strong reversibility and pseudocapacitance of the synthesised materials [55].

Figures 9 plot the specific capacitance curves for GO-rGO vs scan rate. This graphic depicts how raising the scan rate lowers specific capacitance, mostly through the ion exchange mechanism. The higher specific capacitance of rGO was attributable to its bigger surface area when compared to GO. The increased number of redox sites in the synthesized materials results in pseudocapacitive behaviour, which improves rGO's capacitive activity. Thus, CV analysis shows that the rGO electrode is one of the most promising candidates for supercapacitor applications [56].

3.6.2. Electrochemical Impedance Spectroscopy

The Electrochemical Impedance Spectroscopy (EIS) graph (Fig. 10) presented compares the electrochemical behavior of Graphene Oxide (GO) and reduced Graphene Oxide (rGO) in the context of supercapacitor applications. The Nyquist plot displays the complex impedance, with the imaginary part (-Z (im)) plotted against the real part (Z (re)), revealing crucial insights into the materials' electrochemical properties [57]. The large semicircles in the Nyquist plots indicate high charge transfer resistance, which explains why the resistances are so large. GO is known to be less conductive than RGO due to oxygen-containing functional groups disrupting the sp2 carbon network. The RGO sample shows slightly lower impedance (smaller semicircle) compared to GO, which is shown due to its more conductive nature [58].

Both GO-rGO exhibit characteristic super capacitor behavior, with a semicircle in the high-frequency region followed by a sloped line in the low-frequency domain. However, rGO demonstrates markedly superior performance across several key parameters. The smaller semicircle diameter of rGO indicates lower charge transfer resistance, suggesting enhanced electrical conductivity and faster electron transfer kinetics compared to GO [59]. This translates to improved power delivery capabilities, a critical factor in supercapacitor performance. In the low-frequency region, rGO's more vertical line approaches ideal capacitor behavior more closely than GO's more sloped line. This indicates that rGO offers better capacitive characteristics and lower diffusion resistance, which are essential for efficient energy storage and release in supercapacitor applications [60]. The overall lower impedance values observed for rGO further reinforce its superior electrochemical performance.

The equivalent circuit model included in the graph helps interpret these results, illustrating the interplay between solution resistance (Rs), charge transfer resistance (Rct), double layer capacitance (Cdl), and Warburg impedance (W). rGO's performance in this model suggests it would excel in high-frequency applications, crucial for rapid charge/discharge cycles in supercapacitors [61]. In conclusion, this EIS analysis strongly indicates that RGO would be the preferred material for supercapacitor applications. Its lower impedance, superior charge transfer properties, and more ideal capacitive behavior promise better rate capability, higher power performance, and overall more efficient energy storage and delivery compared to GO. These characteristics make RGO a promising candidate for advancing supercapacitor technology and meeting the demands of high-performance energy storage systems [62].

This detailed work delves into the production and characterisation of graphene oxide (GO) and reduced graphene oxide (rGO), providing useful insights into their structural, electrical, and electrochemical characteristics. The study advances controlled production of these materials significantly by employing a modified Hummers technique for GO synthesis and an enhanced thermal reduction approach for rGO. SEM, EDAX, UV-Vis, XRD, and Raman spectroscopy were among the several characterisation methods used to disclose important structural and electrical features. UV-vis spectroscopy indicated the effectiveness of the reduction procedure, while XRD examination revealed increased crystallite size and decreased interlayer gap in rGO. Raman spectroscopy revealed structural evolution, while FTIR identified functional group changes during reduction. CV investigation revealed that the rGO electrode material had a capacitance of 359 Fg^− 1 at a scan rate of 10 mVs^− 1 in 1M aqueous C₁₆H₃₆ClNO₄. The rGO outperformed GO in terms of electrochemical performance due to increased oxygen vacancies and surface area. EIS revealed that both materials have outstanding energy storage and conductivity. An important innovation is the creation of an analogous circuit model for EIS data interpretation. This comprehensive approach, which combines advanced synthesis techniques with in-depth analyses, provides a comprehensive understanding of GO-rGO properties, having potential uses in advanced electronics, such as flexible screens and high-performance super capacitors. These materials are also being evaluated for usage in water purification systems and next-generation drug delivery platforms.

Acknowledgements

The corresponding author, Dr. V. Saraswathi, acknowledges the National College Instrument facility, National College (Autonomous), Tiruchirappalli, Tamilnadu, India for “LAB FACILITY”.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not for profit sectors.

Author Contributions

A. Cathirin Preethi took part in the investigation, methodology, data collection, writing-original draft, characterization analysis, and writing review & editing.

Dr.V.Hariharakrishnan: Supervision, writing-review, editing and he took part in methodology and conceptualization.

Dr. V. Saraswathi: Supervision, writing-review, and editing.

Corresponding author

Correspondence to Dr.V.Saraswathi

Email: [email protected]

Conflict of interest

The authors declare that there is no competing and conflict of interest in this research article.

Ethical approval

The authors declare that the principles of ethical and professional conduct have been followed in this research work.

Data availability

Data are available on request due to privacy or other restrictions.

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The authors declare no competing interests.

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Electrochemical Properties of GO-rGO: Insights from Modified Hummers Synthesis and Thermal Reduction

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Abstract

Figures

1. Introduction

2. Materials and Methods

2.1. Preparation of GO from hummer’s method

2.2. Synthesis of rGO from GO by Thermal reduction method

3. Results and discussion

3.1. Analysing structures using XRD

3.2. Morphological Study

3.3. Vibrational Spectral Analysis

3.4. Optical Properties of GO-rGO

3.5. Raman spectra

3.6. Electrochemical analysis

3.6.1. Cyclic Voltammetry

3.6.2. Electrochemical Impedance Spectroscopy

Conclusion

Declarations

References

Additional Declarations

Supplementary Files

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