Nowadays, with the rapid development of electronic devices, it is increasingly important to enhance the electrical conductivity of reduced graphene oxide (rGO). Thermal reduction (TR) temperature and time play the most crucial role as they control the electrical conductivity of rGO in terms of removal of oxygen-containing functional (OCF) groups. This work proposes a novel systematic approach for quick calibration of the OCF groups and lattice defects of GO to increase the conductivity by tuning the temperature and exposure time of the sample to the temperature. Single TR (STR) and double TR (DTR) processes were used in the current work, in which samples were exposed to temperatures of 500, 700, and 900 °C for 5 min. Further annealing took place for each sample at the same temperature with various reduction times. The results indicate that the DTR process improved the electrical conductivity of rGO samples. The highest enhancement of rGO500-5, rGO700-5, and rGO900-5 conductivities was 52.36%, 57.58%, and 231.81%, respectively. Consequently, this material can be used as a filler to get a well dispersed nanocomposite by accurate addition of rGO in a matrix, which enhances its electrical properties. Based on x-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and electrical analyses, the plausible STR and DTR mechanism of GO to rGO is effectively proposed.

Recently, the growing demand for graphene has rapidly increased due to its immense use in several applications. The wide-range industrial potential of graphene is due to its unique chemical and physical properties: high electrical and thermal conductivity, quantum Hall effect, controllable energy gap width, extraordinarily high-charge carrier mobility, and high elasticity.1,2 The high-cost synthesis of graphene, time consumption, high consumption of the oxidants, and intercalating agents are inevitable. So far, many works on improving the efficiency of the synthesis method have been reported. Therefore, extensive studies have been directed toward the synthesis of reduced graphene oxide (rGO) at a lower cost and producing properties close to graphene’s properties. rGO is a vastly used 2D material for diverse advanced applications, which can be obtained via reducing the oxygen-containing functional (OCF) groups of graphene oxide (GO).3 In turn, GO is obtained by exfoliation of graphite via several methods such as Brodie’s, Hofmann’s, Hummers’s, and Tour’s method.4–6 GO is an essential intermediate for preparing rGO by thermal reduction (TR). This material is considered an insulator due to some defects between sp2 carbon clusters and the presence of OCF groups. The existence of ether, epoxide (–O–), and hydroxyl (–OH) groups on the basal plane of the graphene sheet causes the impedance of charge carrier transfer within the graphene sheet due to defects and structural deformation. There are also inclusions of sp2-hybridized carbon-containing carboxyl and carbonyl groups concentrated mainly at the edges of a graphene sheet. Due to the unique properties of rGO, it can be used as an electrode material in supercapacitors, batteries, solar cells, and fuel cells.6,7 Amarnath and Gurunathan8 confirmed that NiO–In2O3 nanosphere-coated rGO sensing electrodes show high sensitivity toward CO2 at 50 ppm by 40% at room temperature. Ashraf et al.9 fabricated an asymmetric supercapacitor using rGO as a positive electrode, and they showed its superior electrochemical supercapacitor performance. rGO was also used as a superior photocatalyst for wastewater treatment and to clean the environment. Zhang et al.10 reported that the membrane containing polymer polyimide/rGO for water purification achieved high separation efficacy by 99.19%. Latorrata et al.11 demonstrated that rGO membranes have good capability for removing metals from several wastewaters.

Thermal reduction and chemical reduction from GO to rGO are generally used for large-scale graphene production; however, the product obtained is often of low quality. Based on literature research, more than 90% of the OCF groups from GO can be efficaciously removed by reduction processes. Despite that, restoring long-range sp2 networks and the associated electrical properties remains challenging. Thermal reduction (TR) offers a more applicable path for recovering electrical conductivity than chemical reduction. Some reducing agents, such as sodium borohydride and hydrazine, have elemental impurities, toxicity, and environmental impact when utilized in the chemical reduction process. In contrast, TR is a fast, simple, and non-toxic method and can be used in large-scale manufacturing. During the TR process, three factors strongly influence the structure and properties of rGO: temperature, time, and atmosphere. Appropriate temperature is considered a significant parameter at which OCF groups are removed efficaciously in the TR process. Sieradzka et al.12 synthesized rGO via low-temperature TR [not exceeding 300 °C, with an inert gas (nitrogen)] and investigated the influence of graphite grain size on the electrical conductivity of rGO. In the same study, at different grain sizes of graphite, <20 and <150 µm and flakes, the electrical conductivity of rGO was found to be 14, 4.5, and 11.6 S/cm, respectively. Khan and Jaafar13 studied the influence of diverse natural and TR techniques on the electrical property of rGO. They found that the highest conductivity of rGO was 0.52 S/cm using NaBH4 as a reducing agent. In contrast, by using a TR process at 180 and 220 °C under standard atmospheric conditions, the conductivities were 0.018 and 0.104 S/cm, respectively.

To date, rGO has been synthesized by the TR process of GO at different ranges of temperatures exceeding 1100 °C in either an Air,14–20 Ar,21–24 N2,25–28 and H229–32 atmosphere. Nevertheless, little has been reported on the systematical studies of the effect of time and temperature at standard atmospheric pressure and under ambient conditions on the electrical properties of rGO.

In the current work, a facile robust synthesis and rapid approach was performed via the double thermal reduction (DTR) process to further enhance the electrical conductivity of rGO by manipulating the thermal annealing time and temperature.

Graphite fine powder (purity 99.99%) was obtained from Central Drug House (CDH). The other chemicals used were potassium permanganate (KMnO4), sulfuric acid (H2SO4, 98 wt. %), hydrogen peroxide (H2O2, 30 wt. %), 10% HCl, distilled water, and ethanol. All chemicals were used as received without further purification.

In a typical procedure, GO was synthesized according to the Hummers method, and the details could be found in our previous work.33 Each step of the synthesis process of GO nanosheets is described and illustrated in Fig. 1.

FIG. 1.

Consecutive steps to illustrate the process of GO synthesis.

FIG. 1.

Consecutive steps to illustrate the process of GO synthesis.

Close modal

The rGO samples were synthesized by two-step thermal reduction [single thermal reduction (STR) and double thermal reduction (DTR)].

To obtain rGO from GO, a certain amount of GO powder was taken in a crucible and placed in a muffle furnace at different temperatures (500, 700, and 900 °C) with a constant time (5 min) at standard atmospheric pressure and under ambient conditions, and this process defined as the STR process. In this process, instead of using the gradual increase in temperature (heating rate) in the muffle furnace, as mentioned in previous studies,23,26,34 the heat shock was used for the samples, in which the prepared samples were suddenly subjected to thermal treatment at the desired temperature in the furnace. The synthesized samples were denoted as rGO500-5, rGO700-5, and rGO900-5, respectively.

The DTR process was developed to enhance the electrical properties of rGO. rGO500-5, rGO700-5, and rGO900-5 were cooled at RT for 10 min. Then, thermal treatment was imposed on rGO500-5 at 500 °C with different reduction times (3, 5, and 7 min), which were indicated by names rGO500-5-3, rGO500-5-5, and rGO500-5-7, respectively. The DTR of rGO700-5, and rGO900-5 was carried out at 700 and 900 °C, respectively, with different reduction times (3, 5, and 7 min). The obtained products are labeled as rGO700-5-3, rGO700-5-5, rGO700-5-7, rGO900-5-3, rGO900-5-5, and rGO900-5-7, respectively.

STR and DTR of GO caused volumetric changes in all rGO samples [Fig. 2(a)]. In the STR process, volumetric expansions were observed in rGO samples. rGO900-5 nanosheets were more severely exfoliated and expanded [Fig. 2(b)]. In contrast to the DTR process, rGO samples showed weight loss and yielded less rGO, and drastic weight loss occurred in rGO900-5-7 [Fig. 2(c)]. The volume expansion of the rGO samples led to the eviction of some carbon atoms at high temperatures.16,35

FIG. 2.

Top view photograph of (a) GO, (b) rGO900-5, and (c) rGO900-5-7, rGO900-5-5, and rGO900-5-3.

FIG. 2.

Top view photograph of (a) GO, (b) rGO900-5, and (c) rGO900-5-7, rGO900-5-5, and rGO900-5-3.

Close modal

The structure of as-synthesized GO, rGO500-5, rGO500-5-7, rGO700-5, rGO700-5-7, rGO900-5, and rGO900-5-7 was analyzed by x-ray diffraction (XRD). XRD patterns were measured on a Rigaku-D x-ray diffractometer with Cu–K radiation (λ = 1.54 Å) and with 2θ between 5° and 70°. The resulting materials’ morphology was examined with an emission scanning electron microscope (SEM, JSM 6510LV, JEOL, Japan). Fourier transform infrared spectroscopy (FTIR, Perkin Elmer spectrophotometer) was carried out to analyze the surface functional groups of the samples. Raman spectra of GO and rGO were recorded from 1000 to 2000 cm−1 on a Raman spectroscope (InVia Reflex, Renishaw, UK) using Ar ion CW Laser (514.5 nm). A four-point probe system (Scientific Equipment, Roorkee, India) was used to test the resistivity and conductivity of rGO. The rGO powder was pelleted by using a hydraulic press machine. The electrical conductivity of the prepared rGO was measured at different temperatures (from 25 to 95 °C) by the four-probe method. Multiple measurements were carried out, and the average values were obtained.

The XRD analysis of synthesized samples has been carried out to characterize the pristine crystal structure and interlayer distances of GO and rGO samples. The results are summarized in Fig. 3. The XRD pattern of rGO samples is indexed with hexagonal C (graphite) (JCPDS card no. 75-1621) phases. As it clearly appears, the lattice planes (002) and (100) matched against JCPDS card no. 75-1621 and confirmed the hexagonal phase of the rGO samples (see Fig. 3).36,37 For the GO sample, the observed diffraction peak located at (001) corresponds to reflections of the GO phase at 2θ = 9.2°, indicating an interlayer spacing (d-spacing) of 0.96 nm. Das et al.38 pointed out that the d-spacing of graphite was around 0.34 nm for the intense peak at 26.4°, which corresponds to the (002) plane. The increment in the d-spacing of the GO is the result of the OCF groups, which is intercalation of the graphite layers and means that proper oxidation has taken place. Some OCF groups such as carboxyl, epoxy, hydroxyl, and carbonyl groups were grafted on the edge and within the graphite, causing fractures in the π-bonding of graphite and dragging the layers far apart, therefore increasing the interlayer distance.

FIG. 3.

XRD patterns of GO and rGO samples.

FIG. 3.

XRD patterns of GO and rGO samples.

Close modal

rGO samples have a broad (002) peak around 2θ = 24.5°–25.8°. The broad peaks at the (002) plane of all the rGOs were slightly shifted from lower to higher angles because of increasing TR temperature, as evident in Fig. 3. Herein, the further reduction influence on the rGO samples (STR: rGO500-5, rGO700-5, and rGO900-5) was assessed, whereas DTR was applied on every sample at the same time and temperature by tuning the exposure time (3, 5, and 7 min) to thermal treatment. It was noticed that the peaks of rGO samples were remarkably flattened after applying double reduction, and their interlayer distances were also slightly decreased (Fig. 3). Both GO and rGO samples display small peaks between 43° and 44° corresponding to the (100) reference plane due to a turbostratic disorder of carbon materials.38,39 This characteristic peak was flattened with increasing TR temperature and then observed to be semi-absent at 900 °C.

The interlayer spacing (dhkl) of synthesized samples was determined from the Bragg peaks using the Bragg law,

nλ=2dhklsinθ,
(1)

where n is an integer, λ is the wavelength of Cu–Kα radiation (1.5406 Å), dhkl is the interlayer spacing, hkl are Miller indices, and θ is the Bragg angle for reflection from the (002) planes.

The interlayer spacing decreases in the following order: dGO > drGO500−5 > drGO500−5−7 > drGO700−5 > drGO700−5−7 > drGO900−5 > drGO900−5−7. This result suggests that the elimination of OCF groups is increased when DTR is applied.

The morphology of the synthesized GO and rGO nanosheets can be observed in Fig. 4. As seen from the image in Fig. 4(a), the GO visibly displayed a flat and smooth surface morphology. Thermal treatment of GO resulted in GO being reduced and exfoliated, resulting in fluffy and wrinkled rGO, as elucidated in SEM images [Figs. 4(b) and 4(g)]. The oxidation and exfoliation of graphite severely alter the surface morphology of GO with the formation of new grain boundaries. The loss of OCF groups and certain carbon atoms during thermal annealing causes the deformation of the rGO structure, as can be seen in SEM pictures. Significant structural and morphological changes occurred when the GO samples were reduced at 500, 700, and 900 °C under ambient atmospheric conditions. STR and DTR processes caused significant physical changes in the morphology of rGO samples in terms of wrinkles and pores. Thus, the morphology of the synthesized GO samples gets strongly modified by manipulating the annealing temperature. After GO underwent STR, the graphene layers of GO were exfoliated and separated. Due to oxidative treatment, rGO900-5 and rGO900-5-7 layers had a thin, undulating, and wrinkled texture, tightly stacked with sharp edges, indicating that the reduction process has occurred [Figs. 4(f) and 4(g)]. In Figs. 4(d) and 4(e), it was clearly observed that the graphene sheets possess a wavy frizzy morphology consisting of a thin wrinkled sheet-like structure, and the edges of the sheets appeared foggy. The appearance of a wavy frizzy shape in all rGO samples resulted from separated nanosheet layers. At 500 °C, the SEM image visibly shows the morphology of wrinkled rGO sheets with an increment in the distance between the stacks [Figs. 4(b) and 4(c)], referring to rapid volumetric expansion with an increase in the specific surface area of rGO.35 The increase in the distance between the stacked sheets in rGO500-5 and rGO500-5-7 was greater than in other rGO samples. In this work, further techniques will be used to investigate the rGO structure.

FIG. 4.

Scanning electron microscopy (SEM) images of (a) GO, (b) rGO500-5, (c) rGO500-5-7, (d) rGO700-5, (e) rGO700-5-7, (f) rGO900-5, and (g) rGO900-5-7. In addition, the yellow dotted ovals and arrows in SEM images of (b) and (c) highlight the distance between the stacks, while (d) and (e) represent the wavy wrinkled shape with foggy edges of the rGO sheets. The yellow dotted ovals and arrows in SEM images of (f) and (g) indicate thin layers with a wrinkled texture, tightly stacked with sharp edges.

FIG. 4.

Scanning electron microscopy (SEM) images of (a) GO, (b) rGO500-5, (c) rGO500-5-7, (d) rGO700-5, (e) rGO700-5-7, (f) rGO900-5, and (g) rGO900-5-7. In addition, the yellow dotted ovals and arrows in SEM images of (b) and (c) highlight the distance between the stacks, while (d) and (e) represent the wavy wrinkled shape with foggy edges of the rGO sheets. The yellow dotted ovals and arrows in SEM images of (f) and (g) indicate thin layers with a wrinkled texture, tightly stacked with sharp edges.

Close modal

FTIR analysis was used to validate the elimination of OCF groups in the GO after being thermally reduced (STR and DTR processes) at different reduction temperatures. Figure 5 shows the FTIR spectra for GO and rGO500-5, rGO500-5-7, rGO700-5, rGO700-5-7, rGO900-5, and rGO500-5-7. The FTIR spectrum of GO shows several characteristic peaks (Fig. 5), in which the broad peak in the range of 3300–3650 cm−1 represents the hydroxyl group (–OH) located at the edge and centered at 3390 cm−1,40 while the aromatic C=C bond is observed at 1583 cm−1.41 The peak centered at 1714 cm−1 is ascribed to the carbonyl group (C=O).42 The peak at 1397 cm−1 indicates the bending mode of the hydroxyl group (C–OH).43,44 The peaks localized at 1210 cm−1 referred to stretching and symmetric vibrations of (C–O–C). Moreover, the stretching vibration of (C–O) was noted at 1100 cm−1. These results concluded that the oxidation of graphite successfully occurred. Overall, the peak intensities were reduced obviously with the STR and DTR of GO. The OH group peaks in rGO samples were changed drastically compared to GO, in which the peaks were flattened after thermal treatment of the samples and became more flattened in rGO900-5 and rGO900-5-7. The defined peak at wavenumber 1628 cm−1 was attributed to C=C in rGO samples, which belongs to the sp2-bond carbon.45–47 It should be noted that the peak at 1628 cm−1 clearly appeared after STR and DTR processes, which may be attributed to the formation of non-oxidized graphite regions. The peak position of the C=C bond in rGO samples slightly shifted toward a lower wavenumber with increasing reduction temperature from 500 to 900 °C. In addition, the decrease in the intense peaks of C=O groups was due to exposure of the samples to higher temperature. As it is clear from the FT-IR spectra of rGO samples, at the wavenumber region of 1200–1000 cm−1, some peaks of OCF groups almost disappeared due to their nearly complete decomposition (Fig. 5).

FIG. 5.

FTIR spectra of GO and rGO samples.

FIG. 5.

FTIR spectra of GO and rGO samples.

Close modal

Raman spectroscopy is a useful technique to study the disorders and defects in GO and rGO crystal structures. The Raman spectra of GO and rGO samples are shown in Fig. 6. Two characteristic peaks of G and D are observed. The D and G band peaks were located at 1315–1337 and 1576–1597 cm−1, respectively. The G band is formed from the first-order Raman scattering, which is common in all sp2 carbon forms, as it arises from the C=C bond. The D band in GO is broader than the G band due to the reduction in the sp2 domain size due to the distortions, vacancies, and creation of defects in the GO structure during the oxidation process. In rGOs, the D band and G band were remarkably broadened as compared to both the bands in GO due to the increasing number of sp2 domains as a result of reduction processes, which resulted in removing the OCF groups in the rGO structure. The ratios of the intensity of the disorder-induced D band to the intensity of the Raman allowed G band (ID/IG) were measured to study the disorder of the crystal structure for GO and rGO samples and are summarized in Fig. 6 for comparison. Generally, ID/IG is used as an indicator of the lattice-defect density in carbon materials. The fewer the lattice defects, the lower is the obtained intensity ratio ID/IG. In a pure sp2 system such as carbon nanotubes (CNTs) or graphene, the ID/IG ratio can be used to recognize the existence of defects, with a higher ID/IG ratio showing a higher amount of defects.

FIG. 6.

Raman spectra of GO and rGO samples.

FIG. 6.

Raman spectra of GO and rGO samples.

Close modal

Nevertheless, rGO is a disordered material with a high content of sp3 bonds. Consequently, the decrease in the ID/IG ratio is due to an increase in the defect concentration.48,49 The defects in rGO samples are caused by the presence of OCF groups, as well as the disordering in the bonding of sp2 due to the vacancies, heptagon and pentagon rings, edge effect, etc. The ID/IG ratio was found to decrease at STR and DTR with increasing TR temperature, indicating the decrease in the degree of disorder and the increase in the size of sp2 clusters. It is well documented that the presence of OCF groups caused the defects in the rGO structure. At a high TR temperature, greater elimination of OCF groups was detected, as revealed in FT-IR analysis (see Fig. 5). This explains why the ID/IG ratio of rGO500-5 was more than that of rGO700-5 and rGO900-5. Anyway, the increase in the defects in the graphene structure at high temperatures may be attributed to some damage in the rGO structure during the elimination of OCF groups. The red-shift of the G peak was found to be in the range of 9–12 cm−1 for rGO500-5, rGO700-5, and rGO900-5, while the G peak band of samples exposed to the DTR was shifted toward a higher wavelength (red shift, lower frequency) (between 3 and 6 cm−1) than its samples undergoing STR. This shift in the G peak of rGO samples could be due to graphene’s negative thermal expansion coefficient.50,51

The resistivity (ρ and electrical conductivity (σ) of rGO are the most critical criteria to assess the degree of reduction. Temperature, time, and atmospheric conditions all play a part in the reduction process and have an impact on the electrical characteristics of rGO. The values of ρ and σ of rGO pellets were measured by the four-probe method. The measurement was carried out from 25 to 95 °C to deeply study the effect of temperature on the electrical properties of prepared rGOs.

The calculation results are summarized in Tables S1–S3 (see the supplementary material).

The resistivity and electrical conductivity of rGO samples were calculated by the following equations:52,53

ρ=I×π×tV×ln2  (Ω·cm)
(2)
σ=1ρ (S·cm1)
(3)

where ρ and σ are the resistivity and electrical conductivity of rGO, respectively. I, V, and t are the current applied, the measured voltage, and the thickness of the rGO sample, respectively.

The electrical conductivity was decreased dramatically as the TR temperature was increased from 500 to 900 °C at standard atmospheric pressure and under ambient conditions. rGO500-5 has achieved the highest electrical conductivity compared to rGO700-5 and rGO900-5. GO is an electrical insulator; however, during the reduction process at 500 °C, the OCF groups from the GO sheets, which act as traps to impede electron transportation, were removed partially, and conductive sp2-carbon domains were recovered remarkably. During the elimination of OCF groups, some of the neighboring carbon atoms could be removed along with the removal of OCF groups, which leaves some defects in the structure of rGO. With increasing TR temperature, more elimination of OCF groups was observed in rGO700-5 and rGO900-5 than in rGO500-5 (Fig. 5), and the defects in the graphene structure increased at high temperature. Thus, these defects can affect the electron transportation of graphene. Defects in rGO700-5 and rGO900-5 are the most prominent reasons for poor electrical properties,54–56 which was proved by Raman spectroscopy (Fig. 6). After the reduction in OCF groups, the electrical conductivity of rGO still lacks behind that of pristine graphene due to the OFC groups not totally eliminated as well as defects arising in the structure of rGO during the removal of OFC groups.

Electrical conductivity measurements of synthesized rGOs as a function of temperature (25–95 °C) under controlled atmosphere by the four‐probe method have been carried out [see Figs. 7(a)7(d) and Figs. 8(a)8(c)]. Obviously, the electrical conductivity of rGO samples increases with the increase in temperature. When the temperature was increased from 25 to 95 °C, the electrical conductivity of rGO500-5, rGO700-5, and rGO900-5 increased from 14.1, 2.5, and 1 to 20.7, 3.1, and 1.3 S/cm, respectively. These results are in agreement with Refs. 57–59, i.e., the effect of temperature on graphene increases its electrical conductivity. The rGO crystal received enough heat energy as a result of the temperature increase induced by four probes. Consequently, a lot of covalent bonds are broken due to the creation of more free electrons. Therefore, the electrical conductivity of rGO increases with an increasing number of free electrons.

FIG. 7.

Temperature dependence on resistivity of (a) rGO500s, (b) rGO700s, (c) rGO900s, and (d) rGO900-5-5 and rGO900-5-7 after magnification.

FIG. 7.

Temperature dependence on resistivity of (a) rGO500s, (b) rGO700s, (c) rGO900s, and (d) rGO900-5-5 and rGO900-5-7 after magnification.

Close modal
FIG. 8.

Temperature dependence on electrical conductivity of (a) rGO500s, (b) rGO700s, and (c) rGO900s.

FIG. 8.

Temperature dependence on electrical conductivity of (a) rGO500s, (b) rGO700s, and (c) rGO900s.

Close modal

In order to improve the electrical conductivity of rGO samples, DTR of rGOs was carried out at the same STR temperatures (500, 700, and 900 °C) with various reduction times (3, 5, and 7 min). It was observed that with increasing DTR time, the electrical conductivity for all rGO samples increased [Figs. 8(a)8(c)]. We proposed two reasons for this increment. One of the reasons is the increased reduction of OCF groups. The other reason is that no defects were further introduced into the structure. Sharma et al.60 reported that the electrical conductivity of rGO increased gradually as the reduction temperature increased, as well as rGO can reach conductivities of up to 68.4 S/cm at 1100 °C under a N2 atmosphere. Zhao et al.25 noticed that the electrical conductivity of rGOs increased slowly with an increase in the reduction temperature from 200 to 500 °C under a N2 atmosphere but at 700 and 900 °C, the conductivity decreased in the following order: σGNS-500 > σGNS-900 > σGNS-700. Anyway, rGO500-5-7 has achieved the highest conductivity value of 28.3 S/cm at 95 °C compared to the conductivity value of 20.7 S/cm for rGO500-5.

The enhancement of electrical conductivity by DTR of rGO as a function of temperature is shown in Figs. 9(a)9(c). The DTR process was highly efficient in improving the electrical conductivity of all rGO samples. We have evaluated the enhancement of electrical conductivity for the samples subjected to the DTR treatment relative to the conductivity of those samples that underwent the STR treatment. Herein, the electrical conductivity of rGO500-5-7 was improved, which is equivalent to a 52.36% enhancement relative to that of rGO500-5. However, the electrical conductivity of rGO700-5-7 was enhanced by 57.58% relative to rGO700-5, and the conductivity enhancement of rGO900-5-7 was about 231.81% relative to the conductivity of rGO900-5 at RT. All the values of the enhancement of electrical conductivity for the prepared samples are tabulated in Table S4 (see the supplementary material).

FIG. 9.

Enhancement of the electrical conductivity by the DTR process of (a) rGO500-5, (b) rGO700-5, and (c) rGO900-5 as a function of temperature.

FIG. 9.

Enhancement of the electrical conductivity by the DTR process of (a) rGO500-5, (b) rGO700-5, and (c) rGO900-5 as a function of temperature.

Close modal

The single thermal reduction of GO at standard atmospheric pressure and under ambient conditions is briefly summarized below:

  • i.

    The interlayer spacing decreases with increasing STR temperature from 500 to 900 °C. This result elucidated that some of the OCF groups were eliminated with increasing STR temperature.

  • i.

    Some peaks of the OCF groups are flatted in the FT-IR spectrum, and some of them almost disappeared due to their nearly complete decomposition with the increase in pyrolysis temperature from 500 to 900 °C.

  • ii.

    The presence of OCF groups and the disordering in the bonding of sp2 due to the vacancies, heptagon and pentagon rings, edge effect, etc., caused the defects in rGO nanosheets. The ID/IG ratio of rGO900-5 was smaller than that of rGO500-5 and rGO700-5 as a result of increased removal of OCF groups.

  • iii.

    At high TR temperature, some carbon atoms were removed along with OCF groups, leading to structural defects on the basis of rGO sheets and open rings at the edges of sheets.

  • iv.

    The electrical conductivity of rGO500-5 was higher than that of rGO700-5 and rGO900-5, which might be due to fewer structure defects than other samples.

The DTR of rGO at standard atmospheric pressure and under ambient conditions is briefly summarized below:

  • After applying double thermal reduction, the peaks of rGOs samples were obviously flattened, and their interlayer decreased by some extent.

  • The ID/IG ratio was found to decrease at DTR with increasing TR temperature and time, revealing the decrease in the degree of disorder and increase in the size of sp2 clusters.

  • The resistivity (ρ and electrical conductivity (σ) of rGO are the most important criteria to evaluate the degree of rGO reduction.

  • The electrical conductivity of all rGO samples increased with increasing reduction time.

  • An increment in electrical conductivity of rGOs is attributed to the reduction in the OCF groups in rGO as a main factor, and the presence of fewer/no defects in rGO sheets during the DTR process might be considered as a probable reason for the increase in conductivity.

Based on the XRD analysis, functional group analysis, Raman spectroscopy analysis, morphology analysis, and electrical properties analysis, the possible reduction mechanism of rGO via STR and DTR has been illustrated in Figs. 10(a)10(f).

FIG. 10.

Proposed TR mechanism for rGO samples via STR and DTR: (a) graphite, (b) GO, (c) rGO500-5, (d) rGO900-5, (e) rGO500-5-7, and (f) rGO900-5-7.

FIG. 10.

Proposed TR mechanism for rGO samples via STR and DTR: (a) graphite, (b) GO, (c) rGO500-5, (d) rGO900-5, (e) rGO500-5-7, and (f) rGO900-5-7.

Close modal

In summary, we report an improved method to enhance the electrical conductivity of reduced graphene oxide (rGO) by DTR processes. During the STR process, the conductivity was decreased with an increase in the reduction temperature. rGO500-5 has achieved the highest conductivity value of 14.1 S/cm at 25 °C compared to the conductivity values of 2.5 and 1 S/cm for rGO700-5 and for rGO900-5, respectively. The DTR process was carried out to improve the electrical conductivity of rGO. It was observed that as the DTR process time was extended, the electrical conductivity values of all rGO samples increased. The steep increase in conductivity was recorded for rGO500-5-7, where its conductivity was 21.5 S/cm at 25 °C and 28.3 S/cm at 95 °C due to more elimination of OCF groups, indicating that the reduction in OCF groups had a greater impact on the restoration of the conductive nature of the graphite structure in rGO. These findings reflected the dramatic changes in the structural stability of the rGO nanosheet produced using TR treatment. The TR mechanism for rGO samples via STR and DTR has been effectively proposed. However, the TR of GO is a very complicated phenomenon due to the multistep removal processes of OCF groups, which are induced by thermal energy. Consequently, the TR of GO and the resultant rGO needs to be studied in great detail.

See the supplementary material for further experimental details presented in this paper, in particular the information about the resistivity and electrical conductivity values of rGO subjected to STR and DTR at 500, 700, and 900 °C as a function of temperature and the electrical conductivity enhancement of samples analyzed by DTR process.

The authors (S.M.S.A. and A.A.) are thankful to Mr. Mohd Imran Abbasi, Department of Applied Physics, A.M.U., Aligarh, India, for the generous help.

The authors have no conflicts to disclose.

Salah M. S. Al-Mufti: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Visualization (equal); Writing – original draft (equal). Asma Almontasser: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Visualization (equal); Writing – original draft (equal). S. J. A. Rizvi: Conceptualization (equal); Formal analysis (equal); Methodology (equal); Supervision (lead); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

1.
P. C.
Ooi
,
M. A. S. M.
Haniff
,
M. F. M. R.
Wee
,
C. F.
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Supplementary Material