A New Route to Tune the Electrical Properties of Graphene Oxide: A Simultaneous, One-Step N-Doping and Reduction as a Tool for Its Structural Transformation

8.64.6 A New Route to Tune the Electrical Properties of Graphene Oxide: A Simultaneous, One-Step N-Doping and Reduction as a Tool for Its Structural Transformation Andjela Stefanović, Muhammad Yasir, Gerard Tobías-Rossell, Stefania Sandoval Rojano, Dušan Sredojević, Dejan Kepić, Duška Kleut, Warda Saeed, Miloš Milović, Danica Bajuk-Bogdanović et al. Special Issue Graphene and Graphene-Related Materials for Energy and Environment: Synthesis and Application Edited by Dr. Saverio Latorrata and Dr. Andrea Basso Peressut Article https://doi.org/10.3390/molecules30173579 Academic Editor: Chongjun Zhao Received: 10 August 2025 Revised: 27 August 2025 Accepted: 29 August 2025 Published: 1 September 2025 Citation: Stefanovi´c, A.; Yasir, M.; Tobías-Rossell, G.; Rojano, S.S.; Sredojevi´c, D.; Kepi´c, D.; Kleut, D.; Saeed, W.; Milovi´c, M.; BajukBogdanovi´c, D.; et al. A New Route to Tune the Electrical Properties of Graphene Oxide: A Simultaneous, One-Step N-Doping and Reduction as a Tool for Its Structural Transformation. Molecules 2025,30, 3579. https://doi.org/10.3390/ molecules30173579 Copyright: © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/ licenses/by/4.0/). Article A New Route to Tune the Electrical Properties of Graphene Oxide: A Simultaneous, One-Step N-Doping and Reduction as a Tool for Its Structural Transformation Andjela Stefanovi´c 1, Muhammad Yasir 2,* , Gerard Tobías-Rossell 3, Stefania Sandoval Rojano 3, Dušan Sredojevi´c 1, Dejan Kepi´c 1, Duška Kleut 1, Warda Saeed 2, Miloš Milovi´c 1, Danica Bajuk-Bogdanovi´c 4and Svetlana Jovanovi´c 1,* 1Vinˇca Institute of Nuclear Sciences-National Institute of the Republic of Serbia, University of Belgrade, P.O. Box 522, 11000 Belgrade, Serbia 2Carl von Ossietzky Universität Oldenburg, 26129 Oldenburg, Germany 3Institut de Ciencia de Materials de Barcelona (ICMAB-CSIC), Campus de la UAB, 08193 Bellaterra, Barcelona, Spain; gerar[email protected] (G.T.-R.); [email protected] (S.S.R.) 4University of Belgrade, Faculty of Physical Chemistry, Studentski trg 12-16, 11158 Belgrade, Serbia; [email protected] *Correspondence: [email protected] (M.Y.); [email protected] (S.J.) Abstract The presence of secondary electromagnetic waves (EMWs) results in EMW pollution and a large need for EMW-shielding materials. Therefore, new, lightweight, flexible, chemically resistant, and durable EMW shielding materials are demanded, while graphene and its derivatives meet the above-mentioned requirements. Among graphene derivatives, N-doped graphene exhibits promising electrical properties for shielding applications, although achieving sufficient N-incorporation in the graphene sheets remains a challenge. Herein, we produced graphene oxide using the modified Hummers’ method (GO) and the electrochemical exfoliation of highly ordered pyrolytic graphite. These two GO samples were thermally treated at 500 ◦ C and 800 ◦ C under a pure NH 3 gas for 1 h. UV-Vis, infrared, and Raman spectroscopies and X-ray diffraction, elemental, and thermogravimetric analyses were used to investigate the structural properties of modified GO. One of the highest levels of N-doping of GO was measured (11.25 ± 0.08 at%). The modification under a NH 3 atmosphere leads to simultaneous N-doping and reduction of graphene, resulting in the formation of electrically conductive and EMW shielding materials. Density functional theory (DFT) revealed the effect of heteroatoms on the energy band gap of GO. The cluster corresponding to N-doped rGO had a reduced bandgap of 0.77 eV. Keywords: graphene; graphene oxide; N-doping; electrochemical exfoliation; density functional theory; electro-magnetic shielding 1. Introduction Electromagnetic waves (EMWs) have become widespread owing to the extensive use of electronic devices, such as mobile phones, wireless communication devices, artificial satellites, and radars [1]. As a result of electromagnetic wave pollution, various electronic and measuring equipment malfunctioned, and significant concerns were raised about its effects on public health [ 2 ]. The primary strategy for controlling EMW exposure is the use of electromagnetic interference (EMI) shielding products, which efficiently prevent the propagation of EMWs. Driven by the urgent need for affordable, durable, miniature, Molecules 2025,30, 3579 https://doi.org/10.3390/molecules30173579 Molecules 2025,30, 3579 2 of 17 lightweight, and eco-friendly EMI-shielding barriers, various nanomaterials have been rapidly developing [3–5]. Common metal-based EMI shielding materials effectively block EMWs; however, challenges regarding their processability, corrosion, and cost limit their wider exploitation [ 6 ]. New materials, including conductive polymers, carbon nanotubes, and MXenes, have been utilized as fillers in various polymers, exhibiting excellent shielding effectiveness and other required properties, such as being lightweight, chemically stable, flexible, and processable. These materials reach very high values of total shielding effectiveness at very low thicknesses, such as 84.9 µ m thickness and total shielding effectiveness (EMI SE) of 44.56 dB in the X-band for carbon nanotube (CNT)-Fe 3 O 4 layer (FCFe) and MXene [ 7 ], Ti 3 C 2 T x MXene embedded in polyvinyl alcohol yielding 34.80 dB shielding efficiency at 5 µ m thickness [ 8 ], and Fe 3 O 4 -loaded cellulose/polyacrylonitrile nanofibers with Ti 3 C 2 T x MXene at 640 µ m thickness reached EMI SE of 33.2 dB [ 9 ]. Considering the ecological burden of plastic materials and the challenges of plastic composite recycling, scientific attention has focused on bio-based materials such as biochar and cellulose scaffolds as potential EMI-shielding materials [10–13]. Owing to their tunable electrical properties, mechanical strength, light weight, and chemical resistance, graphene and its derivatives have been studied for shielding applications [ 14 – 16 ]. When the incident EMWs collide with the graphene surface, they are absorbed, reflected, internally reflected, or transmitted [ 16 ]. Dielectric and magnetic losses achieve absorption loss, whereas conduction and polarization losses influence the dielectric loss. Conduction loss is associated with charge migration across graphene sheets and charge hopping from one layer to another, which occurs when materials contain graphene sheets in the construction that are high enough to allow the sheets to create a conductive [ 17 ]. This occurs in graphene composites when the mass ratio is above the threshold concentration [ 17 ]. This type of wave attenuation depends on the electrical conductivity of the material. In contrast, polarization loss occurs when dipoles in materials under an electrical field are polarized, leading to the attenuation or disappearance of the field [ 18 ]. When the dipoles return to their initial state, heat is generated and lost. In graphenebased materials, dipoles stem from polar functional groups, such as Oor N-containing functional groups, owing to the differences in electronegativity between the C atoms in graphene and the heteroatoms. These groups lead to the formation of defect sites where charge carriers can be trapped [ 19 , 20 ]. One strategy to increase EMI shielding effectiveness and electrical conductivity of graphene sheets is to incorporate electron-rich atoms in the graphene structure [ 21 , 22 ]. In the case of n-type doping of graphene, heteroatoms such as N, I, P, B, or S form localized regions with increasing electron density in the π -cloud of graphene sheets, enhancing charge carrier mobility and amplifying polarization at defect sites. It was reported that dipole polarization established between C-N bonds improves the polarization relaxation loss of EMWs, yielding an EMI shielding effectiveness of 45.6 dB in the X-band [22]. Herein, we explored the possibilities of using the thermal treatment of graphene oxides at 2 different temperatures, 500 ◦ C and 800 ◦ C, in a pure NH 3 atmosphere to achieve N-doping of GO and the effects of treatment on the electrical properties. For the first time, two different starting materials were treated: GO obtained using the modified Hummers’ method, and GO produced using electrochemical exfoliation of highly ordered pyrolytic graphite. Namely, one highly oxidized and disordered Hummers’ GO and the other with fewer oxygen-containing functional groups, and more domains with sp 2 regions were studied. A similar procedure was previously employed to achieve N-doping of GO [ 23 ], where at temperatures above 500 ◦ C, graphene was hydrophobic, with N incorporated in graphene sheets as pyridinic N, pyrrolic-N, and graphitic N, in atomic percentages (at%) Molecules 2025,30, 3579 3 of 17 between 7.7 and 12.5. Considering that pyridinic and pyrrolic N were identified as factors affecting the polarization relaxation loss, and graphitic N facilitates electron migration and enhances EMW absorption [ 22 ], this procedure aimed to improve electrical conductivity and EMI SE of GO. Using different GO as a matrix for N incorporation, we investigated how the precursor affects the ability of N atoms to build N into graphene sheets. 2. Results To investigate the chemical composition and nature of the chemical bonds in the annealed GO, EA, SEM-EDS, TGA, Raman, FTIR, and XRD analyses were conducted. Table 1shows the results of the EA, indicating that C is dominantly present in each sample, ranging from 81.04 to 90.00 wt%. The wt% of H was similar in all samples, whereas the wt% of N was the highest for GO-500. Oxygen was detected between 5.69 and 8.58 wt%, while S was identified in GO-500 and HOPG-500 as a residue. The amount of N introduced to the GO samples was inversely proportional to the treatment temperature. In the two sets of samples, the N was incorporated at a higher wt% at 500 ◦C. Table 1. Elemental compositions of samples in wt% measured using an element microanalyzer. Sample C ±STD H ±STD N ±STD S ±STD O1 GO-500 81.04 ±0.21 0.69 ±0.03 11.25 ±0.08 0.21 ±0.01 6.81 GO-800 83.63 ±0.19 0.68 ±0.05 7.11 ±0.06 <0.1 8.58 HOPG-500 87.93 ±0.08 0.34 ±0.01 5.85 ±0.03 0.19 ±0.02 5.69 HOPG-800 90 ±0.13 0.41 ±0.01 3.46 ±0.02 <0.1 6.13 1The values of O wt% were calculated by subtracting the wt% of the measured elements (C, H, N, and S). The surface chemical composition and morphology of the GO samples were investigated using SEM-EDS analysis. In Figure 1, SEM images and associated EDS maps of the identified elements for GO-500 and HOPG-800 are presented. The SEM images showed layered morphology for both samples, while the element maps indicated a homogeneous and equal distribution of each detected element over the investigated surface. Table S1 (Supporting Information) lists the elemental compositions extracted from the EDS spectra for all the samples, in both wt% and at%. Compared to values in Table 1, EDS analysis showed similar but different wt% values for each element from the elemental analysis. EDS analysis was used to investigate the surface chemical compositions, and the depth of the analyzed specimen depends on the electron beam energy and atomic masses of the constituent elements present in the specimen [ 24 , 25 ]. EA studies the bulk sample that is combusted and analyzed, resulting in values representative of the overall specimen. Thus, similar values are in agreement, considering the difference in the principles of the two analyses and the confirmed results obtained. Figure 2presents the TGA of GO, GO-500, GO-800 (a), and HOPG, HOPG-500, and HOPG-800 (b). The TGA curve of GO shows an initial weight loss attributed to physically bonded water molecules (5.83 wt%) up to a temperature of ca. 150 ◦ C (Figure 2a, black continuous line). This indicates the hydrophilicity of the sample. At ca. 250 ◦ C, a large weight loss was observed (28.58 wt%), which was associated with eliminating oxygencontaining functional groups. The complete combustion of GO occurred at 552 ◦ C. For GO-500, only 1.82 wt% was lost at 150 ◦ C, while for GO-800, the weight loss was 0.01. These results indicate that the polarities of GO-500 and GO-800 were significantly changed, and that both were hydrophobic. At ca. 250 ◦ C, weight loss was not observed, while complete combustion occurred at 608 and 586 ◦ C, GO-500 and GO-800 (red and blue curves in Figure 2a), respectively. Molecules 2025,30, 3579 4 of 17 Figure 1. SEM images and EDS maps for GO-500 (a) and HOPG-800 (b). HOPG, HOPG-500, and HOPG-800 samples exhibited similar TGA curves as GO samples (Figure 2b), with temperatures of total combustion of 415, 479, and 481 ◦ C, respectively. The absence of a weight loss below 200 ◦ C in TG curves suggests that there is no presence of physisorbed water in the samples treated under pure NH 3 gas at 500 and 800 ◦ C, which is associated with a less hydrophilic character compared with the starting GO. Being a known reducing agent, NH 3 not only acts as a N source, but also induce the elimination of O-bearing functionalities from the samples, as confirmed by the disappearance of the weight loss associated with these species and the increase in the thermal stability of the material, The thermal stability of the samples closely depends on the concentration of Ncontaining groups within the conjugated lattice. In agreement with previous reports [ 23 , 26 ], Molecules 2025,30, 3579 5 of 17 the higher the N content in N-doped rGOs is (500 ◦ C treatments incorporated the larger concentration of N-based groups compared with treatments performed at 800 ◦ C), the higher the thermal stability against oxidation in air was observed. Figure 2. TGA curves of GO, GO-500, and GO-800 (a); HOPG, HOPG-500, and HOPG-800 (b). Figure 3shows Raman (a) and FTIR (b–d) spectra of GO samples annealed under pure NH 3 gas. All Raman spectra showed the presence of bands at 1355 cm −1 , assigned to the disordered or as-indicated D-band, at around 1600 cm −1 , the graphitic or G-band was observed, at 2700 cm −1 , the so-called 2D band, and the band indicated as D+G (2950 cm −1 ) was also identified [ 27 – 29 ]. In the case of HOPG-500 and HOPG-800, G bands are split, due to changes in the bond lengths and angles of graphene sheets as a result of strain [ 30 ]. Namely, external perturbations of the hexagonal symmetry of graphene occurred due to the introduction of N-functional groups in the graphene sheets. In all spectra, the D band’s intensity is higher than the G bands, indicating high disorder in the graphitic structure of the samples. The calculated values of the intensity ratios between D and G bands are listed in Table 2. Figure 3. Raman (a) and FTIR spectra (b) of GO-500, and GO-800; (c) HOPG-500, and HOPG-800. Table 2shows that the highest structural order was calculated for GO, while the most disordered sample is HOPG-800, according to I D /I G values. Additionally, the position of the G band is associated with the graphitic regions in graphene-based materials [ 30 ]. Using the Knight and White equation [ 31 ], the in-plane crystallite size (La) was estimated and listed in Table 2. The values were calculated using the relation La = 4.4 (I D /I G ) −1 , and Molecules 2025,30, 3579 6 of 17 indicate the lowest crystalline size for sample HOPG-800. Compared to other N-doped graphene, where La was 16.5 nm [ 32 ], our samples showed significantly lower in-plane crystallite size, while I D /I G values are similar. Furthermore, the positions of 2D (2699 cm −1 ) in HOPG-500 and HOPG-800 (2702 cm −1 ) are similar to the previously reported N-doped GO [32]. Table 2. Position of G bands, ID/IGratios, and La values. Sample G Band (cm−1) ID/IGLa GO 1596 1.11 4.35 GO-500 1603 1.15 3.82 GO-800 1602 1.20 3.70 HOPG-500 1587 1.25 3.55 HOPG-800 1589 1.33 3.34 FTIR spectra were used to investigate the binding nature of the elements identified using SEM-EDS and EA. In case of GO, vibrations associated with H-O ( ν ~3200–3400 cm −1 ) from physically absorbed water or OH groups, C=O ( ν ~1700 cm −1 ) from carboxyl, C=C ( ν ~1590 cm −1 ) from aromatic domains, O–H ( β (OH)) ( ν ~1372 cm −1 ), C=O ( ν ~1223 cm −1 ), C–O in C-O-C ( ν ~1042 cm −1 ), and C-O in epoxy ( ν ~ 952 cm −1 ) were observed [ 33 , 34 ]. In FTIR spectra of GO-500 and GO-800, the band at 3400 cm −1 significantly increased. Moreover, two new bands at 2922 and 2853 cm −1 from CH/CH 2 were detected. The band at 1700 cm −1 vanished in the GO-500 spectrum, while in the case of GO-800, a lower intensity and shifting up to 1739 cm −1, owing to the complete or partial removal of carboxyl groups, occurs. Finally, the 1590 cm −1 band shifted to 1552 cm −1 in GO-500 and GO-800 FTIR spectra. Both spectra show bands at 1160 cm −1 , which were associated with C-N vibrations [ 35 , 36 ], while bands at 1042 and 952 cm −1 almost completely vanished, indicating the removal of epoxy groups. In the case of HOPG, HOPG-500 and HOPG-800 samples showed strong bands attributed to C-N ( ν ~1161 cm −1 ), C=C ( ν ~1551 cm −1 ) from aromatic domains, C=O (ν~1737 cm−1), and CH/CH 2 (2922 and 2853 cm −1 ) vibrations. A strong band at 3400 cm −1 was detected in the HOPG-500 sample, while for the HOPG-800 sample, this band was not detected. The reason for this change is associated with an alteration in the polarity of HOPG-800 and an increase in its hydrophobicity, resulting in a lowering of the tendency of the material to absorb atmospheric water physically. All samples showed evident changes in FTIR spectra after thermal treatment under pure ammonia gas. At high temperature, ammonolysis led to significant changes in functional groups anchored to the graphene oxide. The new bands assigned to C-N bonds and detected in all thermally modified samples correspond to N incorporated into the graphitic lattice. Moreover, the simultaneous elimination of the O-containing moieties from the GO was confirmed by the disappearance of the bands corresponding to hydroxyl (1042 cm−1) and epoxy groups (952 cm−1). The crystal structure of the samples was analyzed by X-ray diffraction (Figure 4). The XRD profile of the initial GO has the (001) reflection at 2 θ = 11.4 ◦ , corresponding to an interplanar distance of approximately 7.8 Å [ 37 , 38 ]. On the other hand, pristine HOPG shows two broad peaks, at 2 θ≈ 24.4 ◦ and 2 θ = 12.4 ◦ , which correspond to the (002) and (001) reflections, respectively [ 39 ]. The deviation from the literature value for the (002) reflection of exfoliated graphene (2 θ = 26 ◦ ) and its broadness might be the consequence of the corrugated graphene’s structure and the increased interlayer spacing [ 40 ]. Thermally treated GO samples showed an emergence of a new feature (002) at 2 θ≈ 25 ◦ and a simultaneous disappearance of the (001) reflection. The (002) feature of GO and HOPG Molecules 2025,30, 3579 7 of 17 samples thermally treated at 800 ◦ C was shifted towards a higher angle than samples treated at 500 ◦ C. This implies a slight change in the interplanar distances toward lower values for the samples treated at the higher temperature, owing to the elimination of hydroxyl and epoxy groups from graphene’s surface under heating, as observed previously in FTIR spectra (Figure 3b,c). Figure 4. XR diffractograms of GO and HOPG, and thermally treated GO and HOPG samples. The UV-Vis absorption spectra of thermally treated GO and HOPG were recorded in toluene and presented in Figure 5. All spectra show an absorption peak at 224 nm, corresponding to the π - π * transitions of aromatic C-C bonds [ 41 , 42 ]. The peak was not shifted in position after the annealing at 800 ◦ C. Interestingly, the spectrum of GO showed an additional feature at 255 nm. This new band could be associated with N-functional groups [43]. Figure 5. UV-VIS spectra of GO-500, GO-800, HOPG-500, and HOPG-800. Molecules 2025,30, 3579 8 of 17 Further investigations of the optical and electrical properties of GO, HOPG, and Ndoped samples were carried out by recording reflection spectra. The Tauc equation was used to calculate the optical band gap. Figure S1 shows the corresponding Tauc plots, ( α h ν ) 2 as a function of h ν , where α is the absorption coefficient, his the Planck constant, and ν is the frequency [ 44 , 45 ]. The value of the optical energy band gap (Eg) was estimated as the x-intercept of an extrapolated Tauc plot, and the obtained values are listed in Table 3. It was observed that the Eg was decreased significantly after thermal treatment, from 4.88 eV to 1.88 eV. In the case of N-doping of GO produced by electrochemical exfoliation of HOPG, Eg was lower, and a small decrease was measured after thermal treatment at 500 ◦ C and 800 ◦ C, with the lowest value calculated for the HOPG derivatives corresponding to the sample treated at 500 ◦ C. The lowering in the values of Eg was previously reported for chemically reduced GO [ 45 , 46 ] or GO reduced by specific drying conditions [ 47 ]. A decrease in the Eg values leads to extended absorption in the visible part of the electromagnetic spectrum [ 41 ], improved semiconductor properties [ 46 , 48 ], and is associated with the restoring π -domains in graphene sheets. These results are in agreement with XRD and FTIR analyses, where removal of O-functional groups and improvement of sp 2 domains were reported. Table 3. Optical band gap of GO and HOPG powders. Sample Eg(eV) GO 4.88 GO-500 1.88 GO-800 1.85 HOPG 1.95 HOPG-500 1.82 HOPG-800 1.83 We used DFT calculations to investigate the electronic structure of N-doped rGO and compare it to that of rGO. Ab initio methods can provide an atomistic model of variously functionalized and N-doped rGO, revealing their optical and electronic properties. We employed these techniques to calculate the total and partial density of states (TDOS/PDOS) diagrams of rGO and N-doped rGO. These results provided information about the energy structure changes mainly occurring around the Fermi level when nitrogen atoms are embedded into the graphene core. Figure 6shows the optimized structures of [C 40 H 16 O 2 (OH) 2 ] and [C 35 H 14 O 2 N 6 ] clusters with the corresponding density of states (TDOS/PDOS) diagrams. The [C 40 H 16 O 2 (OH) 2 ] cluster, with two basal epoxy and hydroxyl groups, is constructed to mimic rGO, while the [C 35 H 14 O 2 N 6 ] cluster represents N-doped rGO. The [C 35 H 14 O 2 N 6 ] cluster has one graphitic, two pyrrolic, and three pyridinic nitrogen atoms embedded into the graphene lattice. The density of states diagram of the cluster representing rGO indicates that the bandgap is 2.16 eV, which categorizes it as a semiconducting material (Figure 6b). 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