Graphite, graphene oxide, and reduced graphene oxide: comparative characterization of optical, morphological, structural and electric relaxation properties

2.1 Synthesis and characterization of the prepared samples

Graphene oxide (GO) was synthesized from natural graphite powder using a modified Hummers’ method [1], 12]. In a typical procedure, 1 g of graphite powder and 0.5 g of sodium nitrate (NaNO₃) were added to 23 mL of concentrated sulfuric acid (H₂SO₄, 98 %) under continuous stirring in an ice bath to maintain the temperature below 5 °C. Subsequently, 3 g of potassium permanganate (KMnO₄) was slowly added while ensuring that the temperature remained below 10 °C to prevent overheating and explosive reactions. The reaction mixture was then stirred at room temperature for 2 h until it became a thick paste. Following this, 46 mL of deionized water was added slowly, and the temperature was raised to ∼98 °C and maintained for 15 min. Subsequently, an additional 140 mL of deionized water followed by 10 mL of hydrogen peroxide (H₂O₂, 30 %) was added to terminate the reaction, resulting in a color change to bright yellow, indicating the formation of GO. The resulting mixture was repeatedly washed with 5 % HCl solution and deionized water via centrifugation (at 6,000 rpm for 10 min) until the pH of the supernatant reached neutral. The purified GO was then dried under vacuum at 50 °C overnight.

Reduced graphene oxide (rGO) was prepared by chemical reduction of the synthesized GO. Briefly, 100 mg of GO was dispersed in 100 mL of deionized water and sonicated for 1 h to obtain a stable brown dispersion. Then, 1 mL of hydrazine hydrate (N₂H₄·H₂O, 80 %) was added dropwise under continuous stirring, and the mixture was heated to 95 °C and refluxed for 4 h. During the reaction, the color of the solution gradually changed from brown to black, indicating the reduction of GO to rGO as in Figure 1. The resulting product was filtered, washed thoroughly with deionized water and ethanol to remove residual reducing agent and byproducts, and then dried under vacuum at 60 °C for 12 h. This research aimed to evaluate the optical, structural, and morphological characteristics of three different samples. To measure the absorption spectra and assess the optical properties of graphite, GO, and rGO, 2 mg of samples were dissolved in 4 ml of distilled water using an ultrasound mixer. The graphene oxide and graphite samples were prepared with accurately measured concentrations to ensure consistency and reproducibility in all characterization analyses. The resulting samples were subsequently dried and used in powder form for the remaining characterization studies.

Figure 1:

Synthesis of reduced graphene oxide from graphene oxide.

To characterize the three samples, the optical absorption spectra of graphite (GR), graphene oxide (GO), and reduced graphene oxide (rGO) were recorded using a UV–Vis/NIR spectrophotometer (Jasco V-770, Japan), and the corresponding energy band gap spectra were constructed. The functional groups present in the samples were analyzed using a Fourier-transform infrared spectrometer (FTIR, Perkin-Elmer Spectrum BX, USA) in the wavenumber range of 4,000–400 cm⁻¹. The surface morphology was examined using a field emission scanning electron microscope (FE-SEM, JSM-7610F, JEOL, Japan). Elemental composition and distribution were determined by energy-dispersive X-ray spectroscopy (EDS) attached to the SEM system (JEOL JSM-6400, Japan). Furthermore, the crystalline structure and interlayer spacing of the samples were analyzed using X-ray diffraction (XRD, Philips X’Pert APD equipped with a Cu Kα radiation source, The Netherlands).

2.2 Dielectric behavior assessment

In this section, the preparation and dielectric characterization of the three powdered samples in disc form are described. The powders were mixed with a small amount of polyvinyl alcohol (PVA) as a binder to improve mechanical strength and homogeneity, followed by milling in a planetary ball mill. The resulting mixture was pressed into pellets using a die mold. The dielectric measurements were performed in the frequency range of 10²–10⁶ Hz using a WAYNE KERR precision component analyzer (Model 6440 B, UK) under ambient temperature conditions. To minimize the influence of DC conductivity and better analyze the electrical relaxation behavior of the three samples, the recorded dielectric data initially expressed as the real (dielectric constant, ε′) and imaginary (dielectric loss, ε″) components of the complex permittivity were processed following the procedure described by Awad et al. [12] and subsequently converted into the electric modulus formalism (M*). The complex electric modulus, defined as the reciprocal of the complex relative permittivity, is given by the following relation:

M ^′(ω) and M ^″(ω) represent the real and imaginary components of the complex electric modulus, respectively, and are related to the complex dielectric permittivity through the following expressions:

Where (Mʹ) is real part of electric modulus and (Mʹʹ) is imaginary part of electric modulus.

3.1 UV–visible absorption spectra

UV–Vis spectroscopy was employed to elucidate the optical transitions and electronic structure modifications occurring during the oxidation of graphite to graphene oxide (GO) and the subsequent reduction to reduced graphene oxide (rGO). Graphite, GO, and rGO samples were dispersed in distilled water and allowed to rest for 24 h to ensure homogeneous suspension prior to analysis. As presented in Figure 2, the UV–Vis absorption spectra of the three materials exhibit distinct features corresponding to their degree of oxidation and restoration of the conjugated π-electron system. Graphite displayed a pronounced absorption peak near 284 nm, attributed to π–π* transitions of C=C bonds within the sp²-hybridized carbon network [13]. Upon oxidation to GO, this peak blue-shifted to approximately 241 nm, indicating disruption of the π-conjugation by the incorporation of oxygen-containing functional groups (hydroxyl, epoxy, carbonyl, and carboxyl). These groups break the delocalized electronic network, localizing electrons and thereby increasing the band gap energy [14], 15]. In contrast, the rGO spectrum exhibited a red-shifted absorption peak at around 264 nm, signifying partial restoration of the π–π* conjugated system following the removal of oxygen functionalities during thermal reduction. This shift toward longer wavelengths directly reflects enhanced electronic delocalization and improved conductivity in rGO compared to GO [16]. To quantify these optical transitions, the optical band gap (E _g ) was determined using a Tauc equation:

where α is the absorption coefficient, his Planck’s constant, υis the photon frequency, A is a proportionality constant, and n is the Tauc exponent, which depends on the nature of the electronic transition. For a direct allowed transition, n = 1 2 .

Figure 2:

UV–visible absorption spectra of graphite, GO and rGO.

The optical band gap was obtained from the Tauc plot of (αhυ)² versus hυ by extrapolating the linear region of the curve to the energy axis [2], 5]. According to the Beer–Lambert law (A = αcl), absorbance (A) is directly proportional to the concentration (c) and path length (l). The calculated optical band gaps were approximately 3.0 eV for GO, 2.1 eV for rGO, and 2.4 eV for graphite (Figure 3). The widening of the band gap in GO is consistent with the formation of localized electronic states caused by oxidation, while the narrowing in rGO indicates re-establishment of sp² domains and restoration of electrical conductivity. These findings are consistent with earlier reports demonstrating that reduction processes enhance π-electron delocalization and decrease the optical band gap of GO [17]. Collectively, the observed spectral shifts and band gap evolution confirm the successful oxidation and reduction of graphene derivatives, directly linking optical transitions to their corresponding structural and electronic modifications.

Figure 3:

Energy gap of graphite, GO and rGO.

3.2 FTIR spectra

Fourier-transform infrared (FTIR) spectroscopy was employed to investigate the incorporation and evolution of oxygen-containing functional groups in graphite, GO, and rGO, providing insights into structural and electronic modifications during oxidation and reduction. The FTIR spectra of the three materials are presented in Figure 4. In graphite, the spectrum exhibits a weak O–H stretching band at 3,441 cm⁻¹, likely due to adsorbed moisture, and a moderate band at 1,620 cm⁻¹ corresponding to C=C skeletal vibrations, confirming the predominance of sp²-hybridized carbon domains. Minor peaks near 1,141 and 1,020 cm⁻¹ suggest limited C–O stretching, consistent with slight surface oxidation. These features indicate that pristine graphite maintains a largely conjugated electronic structure with minimal disruption. Oxidation to GO induces pronounced spectral changes reflecting the incorporation of oxygen functionalities [18]. A broad and intense O–H stretching peak at 3,425 cm⁻¹ signals the introduction of hydroxyl groups, increasing polarity and hydrophilicity. The strong C=O stretching band at 1,712 cm⁻¹ corresponds to carboxyl and carbonyl groups at edges or defect sites, indicating disruption of the sp² network. Additional peaks at 1,612, 1,381, 1,301, 1,240, and 1,049 cm⁻¹, attributed to C=C, C–OH, and C–O–C (epoxy) groups, confirm extensive oxidation across both basal planes and edges. These structural modifications reduce electronic conjugation, consistent with the observed increase in optical band gap and decreased conductivity in GO. Following thermal reduction, rGO exhibits substantial attenuation of oxygen-related peaks, reflecting partial removal of functional groups [17], 19]. The O–H band at 3,433 cm⁻¹ persists but with lower intensity, indicating residual hydroxyl groups, while the C=C stretching band at 1,636 cm⁻¹ becomes more prominent, signifying partial restoration of sp² domains. The weak band at 1,103 cm⁻¹ confirms the presence of remaining epoxy groups. These spectral changes directly correlate with electronic restoration, as the reformation of conjugated carbon networks enhances conductivity and narrows the optical band gap relative to GO. The FTIR analysis confirms the successful oxidation of graphite to GO and the subsequent partial reduction to rGO, providing a molecular-level explanation for the observed changes in structural, optical, and electronic properties [19], [20], [21], [22].

Figure 4:

Graphite, GO and rGO FTIR spectra.

3.3 SEM analysis

Samples were examined using a scanning electron microscope (SEM) to evaluate their morphology at high magnification. Graphite, GO, and rGO samples were analyzed, and the corresponding SEM images at 30,000× magnification are presented in Figure 5. Figure 5a shows the nanostructure of graphite, consisting of multiple thin, flat layers with smooth surfaces, characteristic of its well-ordered sp² carbon structure. In contrast, Figure 5b illustrates the morphology of graphene oxide (GO), where the layers appear wrinkled and stratified. This flake-like formation indicates successful oxidation of the graphite layers, introducing oxygen-containing functional groups that increase interlayer spacing and surface roughness. The oxidation process also results in thicker and more compressed structures compared to pristine graphite. Figure 5c depicts the morphology of reduced graphene oxide (rGO). The structure exhibits disordered, crumpled sheets with sharp edges, reflecting the partial restoration of the sp² carbon network during thermal or chemical reduction. The accumulation of these crumpled sheets leads to a less-ordered, porous architecture, which is consistent with improved surface area and electronic properties. The observed rGO morphology closely resembles that reported in the literature [23], despite differences in synthesis methods, highlighting the reproducibility of structural changes associated with GO reduction. The SEM analysis confirms the progressive morphological transformation from well-ordered graphite to wrinkled GO and finally to disordered rGO, correlating directly with the chemical and structural modifications induced by oxidation and reduction processes. Together, the SEM, FTIR, and UV–Vis analyses provide a coherent picture of the structural and electronic evolution from graphite to GO and rGO, demonstrating that oxidation introduces functional groups and disrupts conjugation, while reduction partially restores sp² domains and electronic properties.

Figure 5:

SEM images of (A) graphite, (B) GO and (C) rGO.

3.4 XRD analysis

The interlayer spacing and crystal structure of graphite, GO, and rGO were investigated using X-ray diffraction (XRD), as shown in Figure 6, with detailed crystallographic parameters summarized in Table 1. Pristine graphite exhibits a sharp and intense diffraction peak at 2θ ≈ 26.3°, corresponding to the (002) reflection plane. This indicates a well-ordered crystalline structure with minimal defects and a relatively small interlayer spacing of 0.338 nm. The crystallite size of graphite, GO, and rGO was calculated from XRD data by using Scherrer formula [24].

Where D is the crystallite size, β is the peak width at half maximum, K is a shape factor, λ is the X-ray wavelength, and θ is the Bragg angle. To estimate the strain ε in the samples, we used the Williamson–Hall equation.

Figure 6:

XRD spectra for graphite, GO and RGO.

The calculated crystallite size is 18.99 nm, and the lattice strain is low (ε = 0.00838), confirming the high crystallinity of graphite. Upon oxidation, GO displays a strong diffraction peak at 2θ ≈ 11.6°, reflecting a substantial increase in interlayer spacing to 0.765 nm due to the introduction of oxygen-containing functional groups (hydroxyl, epoxy, carboxyl) and intercalated water molecules. This expansion disrupts the π-conjugated network and facilitates hydration and exfoliation in aqueous media. The crystallite size decreases sharply to 3.47 nm, and the lattice strain increases to 0.10821, indicating increased disorder and turbostratic stacking. A broad secondary peak around 43° suggests irregular layer alignment and incomplete oxidation [23], [24], [25], [26]. These structural changes correlate with FTIR and UV–Vis results: the emergence of oxygen functional groups in FTIR and the blue-shift of the π–π* absorption peak in UV–Vis confirm electronic disruption and increased band gap in GO. After chemical reduction, rGO exhibits a broad and low-intensity peak at 2θ ≈ 23.1°, corresponding to a partially restored interlayer spacing of 0.385 nm. This shift indicates partial removal of oxygen functionalities and reformation of the π-conjugated network. However, the interlayer spacing does not fully return to graphite values due to residual defects. The crystallite size further decreases to 2.55 nm, and the lattice strain reduces slightly to 0.07087, reflecting partial structural recovery with remaining disorder. The broadening of the rGO peak and minor residual peaks highlight the presence of disordered stacking and incomplete elimination of oxygen groups, consistent with the partial restoration of electronic delocalization observed in UV–Vis spectra and the decreased intensity of oxygen-related bands in FTIR [27], [28], [29], [30]. XRD analysis quantitatively confirms the structural evolution from highly crystalline graphite to oxidized, disordered GO, followed by partial structural restoration in rGO. Table 1 provides a detailed comparison of interlayer spacing, crystallite size, lattice strain, FWHM, and 2θ values, supporting the interpretation of progressive oxidation and reduction processes. These findings agree with literature, where GO typically shows 2θ ≈ 10.8–11.8° (d ∼0.78–0.82 nm), shifting to ∼23–25° in rGO, reflecting partial recovery of graphitic domains while retaining some disorder [31].

3.5 EDS elemental analysis

Energy-dispersive X-ray spectroscopy (EDS) was employed to determine the elemental composition of graphite and as- prepared samples, focusing on the relative percentages of carbon and oxygen. As shown in Figure 7, pristine graphite consists entirely of carbon, reflecting its unmodified sp²-hybridized structure. Upon oxidation, GO exhibits a significant increase in oxygen content due to the introduction of oxygen-containing functional groups, such as hydroxyl, epoxy, and carboxyl, consistent with FTIR and XRD results [32]. Following reduction, rGO shows a decreased oxygen percentage, indicating partial removal of these functional groups during the reduction process and the partial restoration of the conjugated sp² carbon network [33], [34], [35], [36]. These EDS findings corroborate the chemical and structural transformations observed in UV–Vis, FTIR, SEM, and XRD analyses, confirming the successful oxidation of graphite to GO and subsequent partial reduction to rGO.

Figure 7:

(A–C) EDS analysis of graphite, GO, and rGO.

3.6 Dielectric behavior analysis

The dielectric modulus analysis of graphite, GO, and rGO reveals clear correlations between their structural order, charge transport, and relaxation behaviors (Figure 8 (A–F)). For graphite, the real modulus (M′) (Figure 8A) and imaginary modulus (M′′) (Figure 8B) components remain nearly constant and close to zero (∼10⁻⁸–10⁻⁷) up to 10⁴ Hz, followed by a sharp increase beyond 10⁵ Hz, reaching about 5.5 × 10⁻⁴ and 1.8 × 10⁻³, respectively, at 10⁶ Hz. This response confirms strong electronic conduction and minimal dipolar relaxation [37], consistent with its highly crystalline sp² network observed in XRD and SEM. In contrast, GO exhibits much higher M′ (Figure 8C) and M′′ (Figure 8D) values (∼10⁻⁴–10⁻² range), showing a gradual increase across the entire frequency spectrum, indicating pronounced interfacial and dipolar polarization due to abundant oxygen-containing groups and structural defects. This behavior reflects its insulating nature and heterogeneous charge distribution [38], as supported by FTIR and EDS analyses. For rGO, both M′ (Figure 8E) and M′′ (Figure 8F) are significantly reduced in magnitude (∼10⁻⁹–10⁻⁷ and 10⁻⁶–10⁻⁴, respectively) compared with GO, with a steep rise above 10⁵ Hz, signifying improved charge carrier mobility and reduced polarization. The absence of a relaxation peak in rGO suggests a more homogeneous conductive network and efficient charge delocalization, consistent with its partial restoration of the sp²-hybridized carbon framework [39]. The comparative modulus spectra confirm the progressive transition from a conductive (graphite) [40] to insulating (GO) [41] and back to semiconductive (rGO) behavior [42], governed by the degree of oxidation and reduction, which directly influences the materials’ dielectric relaxation and electronic transport characteristics.

Figure 8:

(A–F): Real (M′) and imaginary (M″) parts of the complex electric modulus as a function of frequency for graphite, GO, and rGO samples.