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Divergent mechanisms for thermal reduction of graphene oxide and their highly different ion affinities

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Abstract

The significant effects of annealing atmosphere on the thermal reduction of graphene oxide (GO) under different atmospheres, air and nitrogen, have been investigated and their distinct properties shown. Initially thermal annealing of both GO in air and nitrogen leads to elimination of unstable in-plane oxygen containing groups. For GO in air (A-RGO), as the temperature is raised to about 400 °C, oxygen starts to react with carbon atoms along the sheet and defect edges, where carbon atoms are most active, leading to the formation of oxygen functional groups which are carboxyl and epoxide as observed using the infrared absorption and X-ray photoelectron measurements. Raman results also show an increase in the amount of hybridized sp³ carbons and defects in agreement with an increase in the oxygen functional groups. In contrast, thermal reduction of GO in nitrogen (N-RGO) leads mainly to the loss of oxygen functional groups, the decrease in the amount of sp³ carbons, and the increase in the C/O ratio compared to those of A-RGO. The resulting A-RGO and N-RGO possess also very different ion affinities, for instance, A-RGO annealed at 500 °C shows much higher cation sorption capacity compared to that of the N-RGO. Divergent transformation paths between GO undergoing thermal reduction in air and N2 are proposed based on these results.

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... The strong broad band in the range 2700-3700 cm −1 is due to O-H stretching in alcohols, carboxylic acids, and water [36]. The peak at 1725 cm −1 is associated with C=O stretching of both carbonyl and carboxylic groups, whereas the sharp peak at 1582 cm −1 is characteristic of C=C skeletal vibrations of non-oxidized domains and/or of asymmetrical stretching of C=O in carboxylate salts [37][38][39]. The peak at 1399 cm −1 may be attributed to the O-H bending of tertiary alcohols as well as symmetric stretching of C=O in carboxylates salts [36,40]. ...
... The peak at 1399 cm −1 may be attributed to the O-H bending of tertiary alcohols as well as symmetric stretching of C=O in carboxylates salts [36,40]. Absorptions in the region of 900-1300 cm −1 may arise from C-O vibrations of several species (i.e., ethers, carboxylic acids, alcohols, epoxides, and ketones) [38,41]. The peak at 580 cm −1 has to be assigned to vibrations of S-O bonds of sulphate, the residue of Hummer's reaction for the production of GO [35], also detected by XPS analysis (Table 1). ...
Article
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The impact of extra-low dosage (0.01% by weight of cement) Graphene Oxide (GO) on the properties of fresh and hardened nanocomposites was assessed. The use of a minimum amount of 2-D nanofiller would minimize costs and sustainability issues, therefore encouraging the market uptake of nanoengineered cement-based materials. GO was characterized by X-ray Photoelectron Spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), Atomic Force Microscopy (AFM), X-ray Diffraction (XRD), and Raman spectroscopy. GO consisted of stacked sheets up to 600 nm × 800 nm wide and 2 nm thick, oxygen content 31 at%. The impact of GO on the fresh admixtures was evaluated by rheology, flowability, and workability measurements. GO-modified samples were characterized by density measurements, Scanning Electron Microscopy (SEM) analysis, and compression and bending tests. Permeability was investigated using the boiling-water saturation technique, salt ponding test, and Initial Surface Absorption Test (ISAT). At 28 days, GO-nanocomposite exhibited increased density (+14%), improved compressive and flexural strength (+29% and +13%, respectively), and decreased permeability compared to the control sample. The strengthening effect dominated over the adverse effects associated with the worsening of the fresh properties; reduced permeability was mainly attributed to the refining of the pore network induced by the presence of GO.
... Graphene oxide (GO) was prepared by modified hammer's method and reduced at elevated temperature to make it thermally reduced graphene oxide (TRGO) [22,23]. 3 g of graphite and 18 g of KMnO 4 were taken in a beaker. ...
... On the other hand, TRGO does not affect the crystalline portion of PANI because it is tightly packed and cannot disturbed, so the melting of PANI appear at exact 138 °C. Almost a similar trend was also found in the literature [22,25]. DSC and SEM results confirm the formation of heterogeneous phase morphology for PVC-PANI polymer blend and HNCs. ...
Article
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Polymer blends of Polyaniline (PANI) and Poly-Vinyl Chloride (PVC) were fabricated using Thermally Reduced Graphene Oxide (TRGO) as nano-level reinforcement to enhance the of electromagnetic interference (EMI) shielding effectiveness. The initial assessment of polymer blends of PVC/PANI and hybrid nanocomposites of TRGO, PANI and PVC were done with cyclic voltammetry and the results showed improved electrical conductivity of 1.0×10-15 S/cm, 1.5×10-6 S/cm and 4.2×10-5 S/cm respectively. The capacitive effect of this blend and hybrid nanocomposites were evaluated as an initial assessment in frequency range of 100Hz-5MHz. EMI-shielding effectiveness of PVC, PVC/PANI blend and PVC/PANI/TRGO hybrid nanocomposites were measured in microwave frequency region of 11GHz-20GHz. Almost 29dB shielding effectiveness was observed in case of PVC-PANI blend. By adding 5 wt% of TRGO, Shielding effectiveness was enhanced to ~56 dB. The dispersion state, interaction of filler with polymer matrix and the nature of fillers are the main reasons to the enhance EMI shielding of these hybrid nanocomposites.
... The decrease in the elemental Cs + adsorption capacity indicates that there may be a loss of some cation-binding functional groups during the sonication process, leading to a significant decrease in the sorption capacity of the sonicated GO. The adsorption of cations onto GO has been shown to be due to various functional groups on GO sheets, such as the carboxyl, hydroxyl, and epoxide groups [32][33][34][35][36] . Compton et al. 37 have also reported a decrease of carboxyl groups along the edges of GO sheets after a sonochemical treatment. ...
Article
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A sonochemical approach is widely used as a routine protocol for graphene oxide (GO) preparation prior to adsorption applications, but there is a limited understanding of its effects on the GO sorption ability. Herein, using different types of sorbates, which are cation, cationic and anionic dye, the changes in sorption capacity along with the characteristics of sonicated GO, are shown. The ultrasound treatment unexpectedly removes oxygen functional groups from GO with a concomitant decrease in GO adsorption capacity, rather than an increase in its adsorption as expected from the use of ultrasound in dispersing or exfoliating GO. The C/O ratio increases from 2.1 in pristine GO up to 2.5 in sonicated GO. The loss of oxygen functional groups is also accompanied by an increase in disorder and defects on GO sheets, along with the weakening of π-π interaction. The pathways of GO’s structural and functional groups changed by ultrasound are proposed, based on the generation of OH· radicals from ultrasonic cavitation. These radicals can react with the facile oxygenated groups of GO, such as those of epoxide, hydroxyl, and carboxyl, leading to the partial removal of these groups and the breakage of GO sheets. GO’s adsorption capacities for cation and different dyes can decrease up to 75% after sonication, which is in agreement with the above changes in GO’s characteristics. The effects are readily observed even in mild sonication such as using an ultrasonic bath, signifying their importance in many applications employing ultrasonication to disperse GO.
... As a result rGO shows less interlayer spacing in O 2 compare to N 2 . Moreover, O 2 can destroyed the carbon skeleton via ring rapturing of GO since sp 3 carbon contain in GO is higher than sp 2 carbon contain at 500 o C. In addition GO is most stable in N 2 than O 2 atmosphere that clearly observed in TGA result, GO completely combustion in 700 o C in O 2 but in N 2 20% remaining at 1000 o C. 31 The annealing reduction mainly take place in vacuum 32 or inert gas. 33,34 Becerril et al., found that during thermal annealing of GO at 1100 o C and vacuum pressure <10 -5 torr is necessary for the rGO production because the residual oxygen can be rapidly lost from the GO flim via reaction. ...
Article
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More recently, 2-D graphene oxide (GO)/reduced graphene (rGO) have altered the direction of modern science with material chemistry and physics by research as they offer different key advantages. These are (i) atomically thin 2-D nanosheets (NSs) provide a large surface area (ii) presence of maximum chemically reactive sites, and (iii) higher mechanical strength and flexibility. Considering the progresses of graphene research, we broadly and crucially discuss the formation of the growing family of 2-D GO/rGO in this review paper. Synthesis methodologies are compared, focusing to offer signs for emerging novel and adaptable synthetic methods. Their advantage use in the fields of supercapacitor are highlighted in this review.
... Thermal reduction is an effective way to produce reduced graphene oxide and it is considered a green technique because no chemicals are used compared with chemical reduction. Although several research groups have been studied the thermal reduction of GO [12][13][14][15][16][17][18][19], a detailed investigation about the structural properties of the final material using X-ray diffraction spectroscopy has not been reported so far. ...
Article
This study investigates the structure and composition of graphene produced by thermal reduction of graphene oxide (GO) at various temperatures, for short time (5 min). The influence of reduction temperature on the structural properties of graphene was studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier Transform Infrared (FTIR) spectroscopy, UV–Vis spectroscopy, thermogravimetric analysis (TGA), Raman and X-ray photoelectron spectroscopy (XPS). We demonstrated that by controlling the temperature in the annealing process, we can design materials with desired composition to meet the necessities for future or existing applications. The synthesized materials proved to have great potential in the atenolol adsorption from wastewater at room temperature (61% removal efficiency after 30 min).
... Preparation of Thermally Reduced Graphene Oxide (trGO). Improved Hammer's method was used for the preparation of graphene oxide and thermally reduced at elevated temperature [11,12]. 3g of graphite powder, 18g of KMnO4, 360ml sulfuric acid and 40ml phosphoric acid was mixed together in a 1000ml beaker at 50 0 C with continuous stirring. ...
Conference Paper
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Thermally Reduced Graphene Oxide (trGO) was successfully prepared and confirmed by XRD then dispersed in Polystyrene (PS) and Acrylonitrile-Butadiene-Styrene (ABS) polymers and evaluated for EMI shielding in microwave and infrared (IR) region. Thickness of prepared polymer/trGO composite films were 200-250 micron. It was observed that trGO has more compatibility with PS then ABS and dispersed more easily and uniformly in PS than ABS. This effect was also observed in IR shielding as ABS+15trGO have 3% transmission and PS+1% trGO have 1.5% transmission. Maximum 29dB and 25dB shielding effectiveness was measured by Vector Network Analyzer (VNA) in microwave region (9-18 GHz) of PS+2% trGO and ABS+2% trGo composite respectively. Clearly indicating that trGO is more compatible with PS than ABS and form more stable and mature interconnected network structure in PS at lower concentrations.
... The rGO was extracted from GO by thermal reduction GO has been reduced by keeping it at reduction temperature (300˚C) a sudden change in temperature causes elimination of functional groups and oxygen atoms from carbon planes and exfoliation of GO takes place to produce rGO [26]. The rGO can be considered as chemically-derived graphene, whose structure varied from one layer to multilayers [27]. Ag-doped rGO with various concentration ratios was synthesized hydrothermally, using 800 mg ...
Preprint
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Graphene oxide (GO) was obtained through modified hummers method and reduced graphene oxide (rGO) was acquired by employing heat treatment. Various concentrations (2.5, 5, 7.5 and 10 wt.%) of silver (Ag) were incorporated in GO nanosheets by adopting hydrothermal approach. Synthesized Ag decorated rGO photocatalyst Ag/rGO was characterized using X-ray diffraction (XRD) to determine phase purity and crystal structure. XRD patterns showed the formation of GO to Ag/rGO. Molecular vibration and functional groups were determined through Fourier Transform Infrared spectroscopy (FTIR). Optical properties and a decrease in bandgap with insertion of Ag were confirmed with UV-visible (Uv-vis.) spectrophotometer and Photoluminescence (PL). Electronic properties and disorders in carbon structures were investigated through Raman spectroscopy that revealed the existence of characteristic bands (D and G). Surface morphology of prepared samples was examined with Field Emission Scanning Electron Microscope (FESEM). Homogeneous distribution, size and spherical shape of Ag NPs over rGO sheets were further confirmed with the help of High-Resolution Transmission Electron Microscope (HR-TEM). Dye degradation of doped and undoped samples was examined through Uv-vis. spectra. Experimental results indicated that photocatalytic activity of Ag@rGO enhanced with increased doping ratio owing to diminished electron-hole pair recombination. Therefore, it is suggested that Ag@rGO can be used as a beneficial and superior photocatalyst to clean environment and wastewater.
... The rGO was extracted from GO by thermal reduction GO has been reduced by keeping it at reduction temperature (300°C) a sudden change in temperature causes elimination of functional groups and oxygen atoms from carbon planes and exfoliation of GO takes place to produce rGO [26]. The rGO can be considered as chemically-derived graphene, whose structure varied from one layer to multilayers [27]. Ag-doped rGO with various concentration ratios was synthesized hydrothermally, using 800 mg GO nanosheets incorporated with (25,50,75, and 100 mg) Ag in 80 ml deionized water under vigorously stirring for 20 min. ...
Preprint
Graphene oxide (GO) was obtained through modified hummers method and reduced graphene oxide (rGO) was acquired by employing heat treatment. Various concentrations (2.5, 5, 7.5 and 10 wt.%) of silver (Ag) were incorporated in GO nanosheets by adopting hydrothermal approach. Synthesized Ag decorated rGO photocatalyst Ag/rGO was characterized using X-ray diffraction (XRD) to determine phase purity and crystal structure. XRD patterns showed the formation of GO to Ag/rGO. Molecular vibration and functional groups were determined through Fourier Transform Infrared spectroscopy (FTIR). Optical properties and a decrease in bandgap with insertion of Ag were confirmed with UV-visible (Uv-vis.) spectrophotometer and Photoluminescence (PL). Electronic properties and disorders in carbon structures were investigated through Raman spectroscopy that revealed the existence of characteristic bands (D and G). Surface morphology of prepared samples was examined with Field Emission Scanning Electron Microscope (FESEM). Homogeneous distribution, size and spherical shape of Ag NPs over rGO sheets were further confirmed with the help of High-Resolution Transmission Electron Microscope (HR-TEM). Dye degradation of doped and undoped samples was examined through Uv-vis. spectra. Experimental results indicated that photocatalytic activity of Ag@rGO enhanced with increased doping ratio owing to diminished electron-hole pair recombination. Therefore, it is suggested that Ag@rGO can be used as a beneficial and superior photocatalyst to clean environment and wastewater.
... The rGO was extracted from GO by thermal reduction GO has been reduced by keeping it at reduction temperature (300°C) a sudden change in temperature causes elimination of functional groups and oxygen atoms from carbon planes and exfoliation of GO takes place to produce rGO [26]. The rGO can be considered as chemically-derived graphene, whose structure varied from one layer to multilayers [27]. Ag-doped rGO with various concentration ratios was synthesized hydrothermally, using 800 mg GO nanosheets incorporated with (25,50,75, and 100 mg) Ag in 80 ml deionized water under vigorously stirring for 20 min. ...
Article
Full-text available
Graphene oxide (GO) was obtained through modified hummers method, and reduced graphene oxide (rGO) was acquired by employing heat treatment. Various concentrations (2.5, 5, 7.5, and 10 wt. %) of silver (Ag) were incorporated in GO nanosheets by adopting hydrothermal approach. Synthesized Ag decorated rGO photocatalyst Ag/rGO was characterized using X-ray diffraction (XRD) to determine phase purity and crystal structure. XRD patterns showed the formation of GO to Ag/rGO. Molecular vibration and functional groups were determined through Fourier Transform Infrared spectroscopy (FTIR). Optical properties and a decrease in bandgap with insertion of Ag were confirmed with UV-Visible (Uv-Vis) spectrophotometer and photoluminescence (PL). Electronic properties and disorders in carbon structures were investigated through Raman spectroscopy that revealed the existence of characteristic bands (D and G). Surface morphology of prepared samples was examined with field emission scanning electron microscope (FESEM). Homogeneous distribution, size, and spherical shape of Ag NPs over rGO sheets were further confirmed with the help of high-resolution transmission electron microscope (HR-TEM). Dye degradation of doped and undoped samples was examined through Uv-Vis spectra. Experimental results indicated that photocatalytic activity of Ag@rGO enhanced with increased doping ratio owing to diminished electron-hole pair recombination. Therefore, it is suggested that Ag@rGO can be used as a beneficial and superior photocatalyst to clean environment and wastewater.
... This was undertaken by increasing the temperature rapidly, which serves to remove the oxygen atoms and attached functional groups from carbon planes leading to exfoliation of GO to produce rGO, as shown in Fig. 1 (Sengupta et al. 2018). The rGO may be considered as chemically derived GO that exhibits diversity in structure from single layer to multilayers (Le et al. 2018). ...
Article
Full-text available
A new era in the development of advanced functional materials was partly sparked by the discovery of 2D materials. In this respect, graphene is believed to have marked the origin of 2D materials. The ability to fabricate a vast majority of such advanced nanomaterials hinges upon the strength of interplanar interactions realized in their respective bulk counterparts. The present study undertakes the comparative analysis of oft-explored 2D materials such as Dirac 2D materials (GO, rGO), TMDCs (MoS 2) and 2D insulators (BN) in the context of their structural, optical, thermal and morphological parameters. Despite implementing several methodologies including a combination of physical, chemical and biological techniques, aquatic and microbial pollution remains a challenge to this day. More recently, nanomaterials have attracted considerable attention as these are believed to hold an extraordinary prospective for utilization toward environmental remediation. Among several probable candidates, 2D materials hold immense appeal due to its useful properties including high absorptivity and large surface area, which enable them to be employed for multifaceted applications. In the present study, a wide range of experimental results extracted from numerous characterization techniques (i.e., XRD, UV-Vis, FTIR, HR-TEM, XPS, DSC-TGA, and Raman) is included. For instance, optical data obtained from these materials point toward a narrow bandgap, while HR-TEM images show large surface area, which suggests that these materials hold promising prospect for use in applications that require strong catalytic activity. Further experimental results indicate that photocatalytic and sonophotocatalytic potential is significantly enhanced by 2D materials. In this respect, rGO showed 60% degradation of synthetic pollutant in 100 min and MoS 2 realized 55% degradation during the same duration. The present study suggests that rGO may be used as a superior photocatalyst in wastewater treatment and related environmental applications. Moreover, the sonophotocatalytic behavior exhibited by these materials showed consistently higher efficiency compared to the respective individual processes which is attributed to the formation of larger amounts of electron-hole pairs.
... This was undertaken by increasing the temperature rapidly, which serves to remove the oxygen atoms and attached functional groups from carbon planes leading to exfoliation of GO to produce rGO, as shown in Fig. 1 (Sengupta et al. 2018). The rGO may be considered as chemically derived GO that exhibits diversity in structure from single layer to multilayers (Le et al. 2018). ...
... Moreover, several techniques such as hydrothermal method [22], arc-discharge process [23], and chemical vapor deposition (CVD) [24] have also been employed for the doping of graphene-based materials, where stable atomic substitution with carbon are highly desirable for the tunability of optical and electronic properties of graphene [25]. Among the reduction methods, GO is mostly reduced thermally at high temperatures [26]. However, the reduction at high temperatures produces high-density defects on the edge and basal planes, which deteriorates the electrical and the optical properties [27]. ...
Article
Full-text available
Graphene and its hybrids are being employed as potential materials in light-sensing devices due to their high optical and electronic properties. However, the absence of a bandgap in graphene limits the realization of devices with high performance. In this work, a boron-doped reduced graphene oxide (B-rGO) is proposed to overcome the above problems. Boron doping enhances the conductivity of graphene oxide and creates several defect sites during the reduction process, which can play a vital role in achieving high-sensing performance of light-sensing devices. Initially, the B-rGO is synthesized using a modified microwave-assisted hydrothermal method and later analyzed using standard FESEM, FTIR, XPS, Raman, and XRD techniques. The content of boron in doped rGO was found to be 6.51 at.%. The B-rGO showed a tunable optical bandgap from 2.91 to 3.05 eV in the visible spectrum with an electrical conductivity of 0.816 S/cm. The optical constants obtained from UV-Vis absorption spectra suggested an enhanced surface plasmon resonance (SPR) response for B-rGO in the theoretical study, which was further verified by experimental investigations. The B-rGO with tunable bandgap and enhanced SPR could open up the solution for future high-performance optoelectronic and sensing applications.
... Moreover, several techniques such as hydrothermal method [22], arc-discharge process [23], and chemical vapor deposition (CVD) [24] have also been employed for the doping of graphene-based materials, where stable atomic substitution with carbon are highly desirable for the tunability of optical and electronic properties of graphene [25]. Among the reduction methods, GO is mostly reduced thermally at high temperatures [26]. However, the reduction at high temperatures produces high-density defects on the edge and basal planes, which deteriorates the electrical and the optical properties [27]. ...
... Moreover, several techniques such as hydrothermal method [22], arc-discharge process [23], and chemical vapor deposition (CVD) [24] have also been employed for the doping of graphene-based materials, where stable atomic substitution with carbon are highly desirable for the tunability of optical and electronic properties of graphene [25]. Among the reduction methods, GO is mostly reduced thermally at high temperatures [26]. However, the reduction at high temperatures produces high-density defects on the edge and basal planes, which deteriorates the electrical and the optical properties [27]. ...
... The method developed by William Hummers in the late 1950s consists of the treatment of commercial graphite powder with oxidizing agents and strong acids [81]. The resulted material is actually graphene oxide, which can be further reduced through thermal [82] or chemical [83] methods, creating the so-called reduced graphene-oxide (rGO). Probably the most effective reductant is hydrazine [84], but its toxicity to humans and the environment boosted the scientists to find greener alternatives [85]. ...
Article
Full-text available
Significant advances in graphene-based materials have facilitated the development of various composites structures in a diverse range of industry sectors. At present, the preparation of graphene-added materials is mainly developed through traditional methods. However, in recent years, additive manufacturing emerged as a promising approach that enables the printing of complex objects in a layer-by-layer fashion, without the need for moulds or machining equipment. This paper reviews the most recent reports on graphene-based photopolymerizable resins developed for stereolithography (SLA), with particular consideration for medical applications. The characteristics of the SLA technology, the most suitable raw materials and formulations and the properties of final 3D products are described. Throughout, a specific focus is placed on the mechanical properties and biocompatibility of the final 3D-printed object. Finally, remaining challenges and future directions are also discussed.
... The thermal annealing atmosphere also determines the reduction efficiency, supported by the increased C/O ratio (Fig. 8d). A considerably superior removal of oxygen containing functionalities is obtained by treated under H 2 compared to the one using argon because C and O atoms in the GO are consumed to form CO/CO 2 in Ar, whereas only oxygen is utilized to form water in H 2 during thermal reduction [45,64]. As a result, thermal annealing is usually carried out in vacuum, or an inert or reducing atmosphere. ...
Article
Full-text available
Thermal conductivity and thermal dissipation are of great importance for modern electronics due to the increased transistor density and operation frequency of contemporary integrated circuits. Due to its exceptionally high thermal conductivity, graphene has drawn considerable interests worldwide for heat spreading and dissipation. However, maintaining high thermal conductivity in graphene laminates (the basic technological unit) is a significant technological challenge. Aiming at highly thermal conductive graphene films (GFs), this prospective review outlines the most recent progress in the production of GFs originated from graphene oxide due to its great convenience in film processing. Additionally, we also consider such issues as film assembly, defect repair and mechanical compression during the post-treatment. We also discuss the thermal conductivity in in-plane and through-plane direction and mechanical properties of GFs. Further, the current typical applications of GFs are presented in thermal management. Finally, perspectives are given for future work on GFs for thermal management.
... After that, the hydrogen (H) will react with perchlorate radical (ClO 4 ) to form perchloric acid (HClO 4 ). NH 3 The last step is the degradation of rGO that likely occurs at higher temperatures ( > 300 °C) with the removal of the remaining oxygen functional groups and combustion of sp2 carbon framework [46] . ...
Article
This study introduces a straightforward and cost-effective route to elaborate a thermally reduced graphene oxide film (R-GOF) at 350 °C under ambient atmosphere, followed by the investigation of the kinetic triplet of the related thermal reduction mechanism by a model-free approach. The structural, the morphological, and the electrical properties of the as-prepared films are scrutinized via a series of characterization techniques. The results demonstrate the successful reduction of the graphene oxide film (GOF), leading to a prominent electrical conductivity (3275 S m−1). Moreover, a model-free approach is applied to a differential scanning calorimetry (DSC) data to evaluate the kinetic triplet of the reduction mechanism. The activation energy (Ea) is assessed according to the most accurate isoconversional methods including Friedman (FRI), Vyazovkin (VYA), convincingly suggesting that the thermal reduction process of GOF can be treated as a one step process, giving values of 88 ± 8 and 79 ± 6 kJ mol−1, respectively. The pre-exponential factor and the kinetic model of the reaction are determined by the combination of isoconversional model with a compensation effect. Conclusively, the estimated activation energy values indicate that the graphene oxide structure, morphology and reduction conditions are the main factors to be considered when modeling and predicting its thermal reduction mechanism.
... Preparation of Thermally Reduced Graphene Oxide (trGO). Improved Hammer's method was used for the preparation of graphene oxide and thermally reduced at elevated temperature [11,12]. 3g of graphite powder, 18g of KMnO4, 360ml sulfuric acid and 40ml phosphoric acid was mixed together in a 1000ml beaker at 50 0 C with continuous stirring. ...
Article
Full-text available
Thermally reduced graphene oxide (trGO) was successfully prepared and confirmed by XRD then dispersed in polystyrene (PS) and Acrylonitrile-Butadiene-Styrene (ABS) polymers and evaluated for EMI shielding in microwave and infrared (IR) region. Thickness of prepared polymer/trGO composite films were 200-250 micron. It was observed that trGO has more compatibility with PS then ABS and dispersed more easily and uniformly in PS than ABS. This effect was also observed in IR shielding as ABS+15trGO have 3% transmission and PS+1% trGO have 1.5% transmission. Maximum 29 dB and 25 dB shielding effectiveness was measured by vector network analyzer (VNA) in microwave region (9-18 GHz) of PS+2% trGO and ABS+2% trGo composite respectively. These results clearly indicating that trGO is more compatible with PS than ABS and form more stable and mature interconnected network structure in PS at lower concentrations.
... Fortunately, these detrimental functional groups may be eliminated by a variety of reduction processes that finally yields rGO, a highly conductive material akin to graphene. A few commonly used routes to achieve this transformation are through reduction via chemical means [19][20][21][22][23], thermal reduction under vacuum [17,[24][25][26] or other atmospheres [27][28][29], joule heating assisted reduction [30][31][32], reduction using microwave radiation [33][34][35][36] and by employing a few other novel methods [37][38][39][40]. A recent review J Mater Sci article [41] can be referred to by the reader for a comparative study on the various scalable routes to synthesize rGO. ...
Article
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We report on a corroborative study of the structural, morphological and electrical property alterations of free-standing graphene oxide (GO) papers subject to thermal reduction. Structural analysis performed using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and Raman techniques prove that the onset of major structural changes, characterized by removal of oxygen functionalities, occur in the 200–300 °C temperature range. The results are corroborated with related morphological changes observed using Scanning electron microscopy (SEM) and Atomic force microscopy (AFM) imaging. Elemental analysis shows the GO paper reduced at 600 °C to contain an 85 wt. % carbon content and a remnant oxygen level of 13.31 wt. %. At the highest reduction temperatures, we see evidence of vacancy-type defects impeding the overall effectiveness of the reduction process. Detailed electrical resistance measurements and current–voltage (I-V) profiling conducted using four-point probe method reveals a several orders of magnitude drop in the sample resistance once the reduction temperature exceeds 200 °C, in good agreement with the structural and morphological changes. The fundamental insights revealed through these studies will be important for future applications where the electrical and mechanical properties of free-standing GO and reduced graphene oxide (rGO) are exploited in practical devices. Graphical abstract
... To compare the photothermal laser-processing approach to conventional thermal annealing [36], we performed thermogravimetric-differential thermal analysis (TGA-DTA) combined with MS of Mod-G powder (Fig. S8b, Supporting Information) in the Ar atmosphere. Contrary to the laser treatment investigated by MS, in TGA-DTA we observe CO 2 and H 2 O release above 150 C due to eCOOH decomposition with continuous weight reduction up to 1200 C. The temperature rise above 1200 C can explain these differences in gaseous products obtained by thermal annealing and laser processing during laser treatment or by a vital contribution of photochemical reactions expected for the violet laser we used (@438 nm). ...
Article
Full-text available
Flexible electronics is a new paradigm with strong implications from healthcare to energy applications. In this context, electrically conductive polymers are the critical components. Here, we report the design, formation mechanism, and applications of a polymer nanocomposite obtained by single-step laser integration of functionalized graphene into a polymer matrix. Laser processing manipulates the physical-chemical properties of this nanocomposite in a controlled and straightforward way, tuning the electrical resistance from a dielectric (MΩ sq⁻¹) to a highly conductive material (Ω sq⁻¹). We combine experimental and computational approaches to elucidate graphene nanocomposite's nature and formation mechanism, evidencing different processes from photothermal polymer melting to shock wave mixing in a liquid phase within a millisecond time scale. We exploit these fundamental insights on the graphene/polymer nanocomposite formation in the design and fabrication of electrochemical sensing and antenna devices, showing the potential for healthcare and the Internet of Things.
... To compare the photothermal laser-processing approach to conventional thermal annealing [36], we performed thermogravimetric-differential thermal analysis (TGA-DTA) combined with MS of Mod-G powder (Fig. S8b, Supporting Information) in the Ar atmosphere. Contrary to the laser treatment investigated by MS, in TGA-DTA we observe CO 2 and H 2 O release above 150 C due to eCOOH decomposition with continuous weight reduction up to 1200 C. The temperature rise above 1200 C can explain these differences in gaseous products obtained by thermal annealing and laser processing during laser treatment or by a vital contribution of photochemical reactions expected for the violet laser we used (@438 nm). ...
... Fig. 3(a)-(c) shows the SEM surface images of pristine RGO, pristine PANI and RGO/SnO 2 /PANI composite respectively. The SEM image of RGO micrograph (Fig. 3(a)) exhibits a fluffy and wrinkled RGO morphology with many stacking layers, quite similar to the SEM images reported by Le et al (Le et al., 2018). In the SEM image shown in Fig. 3(b) PANI can be seen to be existing in cauli-flower shape morphology which is quite similar to the SEM images reported by Jin-Feng et al (Cui et al., 2014). ...
Article
A composite consisting of Reduced Graphene Oxide (RGO), Tin Oxide (SnO2) nanoparticles and Polyaniline (PANI) conducting polymer was studied as a potential counter electrode material for Dye Sensitized Solar Cells (DSSCs). This RGO/SnO2/PANI composite CE was fabricated by spray method and used as an alternative to Platinum (Pt) counter electrode (CE). In the fabrication of RGO/SnO2/PANI composite CEs, several optimizations were carried out to obtain the optimum RGO sintering temperature and the optimum amounts of RGO, SnO2 and PANI. The photovoltaic performance of the DSSCs based on the newly prepared CEs was studied by using I-V measurements. The efficiency of the DSSC with the CE with optimized RGO/SnO2/PANI composition having 9.8 μm thickness was 7.92% under the 100 mW cm⁻² (1.5AM) light illumination. After the TiCl4 treated TiO2 photoanode was used, the maximum power conversion efficiency of the DSSC based on the RGO/SnO2/PANI composite CE was increased to 8.68%, which corresponds to an impressive 94% of the efficiency of 9.22% obtained for the control DSSC made with Pt CE and TiCl4 treated photoanode measured under same light illumination. These composite CEs were characterized by X-ray diffraction, Raman spectra, FTIR spectra, SEM and TEM. Cyclic voltammetry (CV) analysis showed the excellent electro-catalytic activity of the composite CE. With high energy conversion efficiency and improved charge-transfer process of DSSCs in combination with simple preparation and relatively low-cost technique, the RGO/SnO2/PANI composite electrode demonstrates the beneficial use of this novel composite material as CEs in DSSCs.
... Thermal treatment of GO into rGO is an intricate mechanism that involves successive phases of water evaporation, removal of groups, and decomposition of carbon in the basal plane. Due to the importance of carbon to oxygen (C/O) ratio and influence of oxygen-containing groups on the resulting rGO properties, the transformation of several functional groups has also gained research interest [25]. Numerous experiments on thermal reduction of GO have been performed under different annealing environments, ranging from inert to reducing and oxidizing gases, such as argon [26,27], vacuum [28], hydrogen [29], ambient air [30][31][32], and nitrogen [33]. ...
Article
This study proposed a facile and cost-effective approach to synthesize reduced graphene oxide (rGO) thin film from graphene oxide (GO) water dispersion. The rGO thin films were deposited on glass substrates via spray pyrolysis technique (SPT) by using different numbers of spray cycles (i.e. 2 to 10). The structure, morphology, thickness, and optical properties of GO and rGO thin films were characterized using different techniques. The optical analysis showed that the energy band gap of GO and rGO thin films reduced from 4.2 to 3.27 eV as the spray cycles were increased from 2 to 10, which confirms the effect of surface coating thickness on conductivity. To test the performance of the rGO thin film as a UV photodetector device, metal-semiconductor-metal (MSM) configuration was achieved by incorporating Al metal using RF sputtering on the thin film sample prepared with 10 spray cycles. For the device, the photoresponse (R) were found to be 0.0038, 0.0084, 0.01700, and 0.0284 (A/W), while photodetector gain (G) values were 1.956, 2.05, 2.15, and 2.31 at bias voltages of 1, 2, 3, and 4 V, respectively. At the same range of bias voltages, the rise times of the photodetector were 0.08, 0.075, 0.055, and 0.055 s, while the fall time were 0.055, 0.066, 0.06 and 0.06 s, respectively, under 375 nm UV illumination (0.37 mW/cm²). The maximum sensitivity (S%) was found to be 131.84 at 4 V. The photoelectric properties and photodetection performances indicate that rGO thin film deposited on glass substrate by SPT could be a potential candidate for the development of advanced MSM optoelectronic devices.
Thesis
Membrane separation technology plays an important role in various fields including water treatment, chemicals and gas separation for numerous industrial fields, and food processing. There has been a renewed focus on two-dimensional(2D) materials for membrane application since their atomic thicknessand confined interlayer spacing could theoretically lead to enhanced separative performances. Either the single nanosheets themselves, or the stackingof multiple sheets can form selective membranes. The multilayer assembly of single nanosheets – forming nanolaminate membranes – creates 2D capillaries(or nanochannels) that can efficiently sieve chemical species depending ontheir size.Recent examples have been reported in the literature demonstrating the potential of 2D materials as multi- or single-layer membranes for molecular sieving(222; 260; 466; 204), gas separation (219; 246; 190), energy harvesting (467)and water desalination (198; 194).Among the different building blocks of nanolaminate membranes made of two-dimensional materials (2D), graphene oxide (GO) has been studied as a candidate for molecular sieving via size-limited diffusion in the 2D capillaries (222). Unfortunately the high hydrophilicity of GO nanosheets makes GO membranes unstable in water, while the poor control of the capillary width between the nanosheets limits the water permeance of the membranes. Other 2D materials such as exfoliated nanosheets of transition metal dichalcogenides (TMDs)constitute attractive platforms for the realization of nanolaminate membranes.Recent works carried out on nanolaminate membranes made of molybdenum disulfide (MoS2) have demonstrated improved stability (3). Within this thesis we have studied the performance of a novel type of MoS2 nanolaminate membranes with well-controlled surface chemistry of the nanosheets (14). Inorder to assess the role of surface chemistry, we explored the impact of covalent functionalization on molecular sieving toward water purification (i.e. desalination and micropollutant removal) (14). Our results open novel directions to finely tune the sieving behavior of membranes based on 2D materials.
Article
Diuron is a toxic herbicide that has contaminated underground and surface water and caused serious problems in many countries. Diuron residues in water can be treated using electrochemical advanced oxidation processes (EAOPs), and microreactor EAOPs can be developed into portable water treatment units for use in contaminated areas. Up to 91% of diuron in 10 ppm solution could be degraded within 100 s of residence time in the microreactor. Nitrates and sulfates, which are anions commonly found in natural water, retard EAOP degradation of diuron by scavenging hydroxyl radicals and suppressing hydroxyl radical generation at the anode by adsorbing to the anode surface. The apparent first-order rate constant for diuron degradation is decreased from 2.69 × 10⁻² s⁻¹ in deionized water to 1.81 × 10⁻² and 1.64 × 10⁻² s⁻¹, in water containing 50 mM nitrate and 50 mM sulfate, respectively. Sulfate does not alter the diuron degradation pathway, but nitrate forms reactive species that interact with diuron and its degradation intermediates. The concentrations of intermediates increase when the EAOP is made less effective by the presence of nitrate or sulfate, but the intermediates are less toxic than diuron. At a long residence time (e.g., 100 s), EAOP treatment decreases diuron toxicity more when the water is contaminated with nitrate or sulfate than when the water is pure, under the same conditions.
Article
Graphene enticed the scientific community for its interesting properties since its discovery. Among different synthesis routes of graphene, reduction of graphene oxide (GO) is mostly preferred because of scalability and advantage of modulation of properties of the end product. Thermal reduction of GO is considered to be the simplest and economic among different reduction techniques. The current work reports an experimental analysis of the structural evolution of GO to reduced graphene oxide (rGO) during thermal treatment. GO has been thermally annealed at an optimized temperature of 350 °C in ambient. Thermal reduction is observed after 7 min of annealing and confirmed by shifting of the first major peak from 12° to 23° in X-ray diffraction pattern. Significant carbon content enrichment and exfoliation are two aspects of the thermal reduction of GO. Carbon content suddenly enriches from 38 wt% in GO to 77 wt%. Exfoliation is confirmed by morphological alterations and decrease in carbon layers from eleven to three.
Article
Hydrothermal synthesis using graphene oxide (GO) as a precursor has been used to produce luminescent graphene quantum dots (GQDs). However, such a method usually requires many reagents and multistep pretreatments, while can give rise to GQDs with low quantum yield (QY). Here, we investigated the concentration, the temperature of synthesis, and the pH of the GO solution used in the hydrothermal method through factorial design experiments aiming to optimize the QY of GQDs to reach a better control of their luminescent properties. The best synthesis condition (2 mg/mL, 175 °C, and pH = 8.0) yielded GQDs with a relatively high QY (8.9%) without the need of using laborious steps or dopants. GQDs synthesized under different conditions were characterized to understand the role of each synthesis parameter in the materials' structure and luminescence properties. It was found that the control of the synthesis parameters enables the tailoring of the amount of specific oxygen functionalities onto the surface of the GQDs. By changing the synthesis' conditions, it was possible to prioritize the production of GQDs with more hydroxyl or carboxyl groups, which influence their luminescent properties. The as-developed GQDs with tailored composition were used as luminescent probes to detect Fe³⁺. The lowest limit of detection (0.136 μM) was achieved using GQDs with higher amounts of carboxylic groups, while wider linear range was obtained by GQDs with superior QY. Thus, our findings contribute to rationally produce GQDs with tailored properties for varied applications by simply adjusting the synthesis conditions and suggest a pathway to understand the mechanism of detection of GQDs-based optical sensors.
Article
In today’s nano scale regime, a smart electronic device is attractive and has a primary role for majority of the world’s research community, particularly scientific and engineering community. Quasi one-dimensional carbon materials are an ideal material for flexible and wearable electronic applications. Significant progress has been made in developing electronics using carbon-based polymer composites. The incorporation of micro-materials and carbon nanomaterials in polymer has been attempted since the 1990 s and has shown a number of improved properties. In this review, the performance of the polymer composite with nanophase carbon materials is explored and their applications are discussed. In recent years, a wide range of carbon nanomaterials are used to transmit electrical signals for potential applications such as electronics, chemical sensors, mechanical sensors/actuators, and smart materials. Moreover, we have also discussed carbon-based materials, especially multi-walled carbon nanotubes, that are applied on a substrate using some printing technology for flexible electronics, and the progress of CNT-based RF antenna, textile, electromagnetic and interference shielding, and sensor applications has been reported.
Article
Thin foils of graphene oxide (GO) are irradiated by a fs titanium sapphire laser at an intensity of about 10¹⁹ W/cm² in high vacuum. The produced plasma in the forward direction accelerates ions in a regime of target-normal-sheath-acceleration, thanks to the relativistic electron emission from the target surface and to their emission from the rear foil surface, generating a high electric field pulse with the positive target. The ion acceleration is measured mainly using SiC detectors in the time-of-flight configuration. Adding gold as nanoparticles or as a thin coverage film, the ion acceleration is enhanced as a result of a higher plasma electron density. The optimal acceleration is reached by varying the GO thickness, the Au nanoparticle concentration, the thin Au film thickness, and the irradiation conditions. Particularly important is the laser focal position with respect to the target surface, which is responsible for different acceleration values. In the used experimental conditions, a maximum proton energy of 2.6 MeV was obtained and the best modality to add Au atoms to the target is discussed.
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Ammonium perchlorate (AP) is the primary oxygen source for solid propellants; its thermal decomposition behavior is determinant for the overall performance of the ensuing propellant. In this perspective, a novel Intermittent, Spray Coating, Draying, and Mixing (ISCDM) method is introduced herein to prepare [email protected], [email protected], and [email protected]@GO core-shell composites. The as-synthesized materials morphological information, crystalline structure, and elemental composition are probed by powder X-ray diffraction, Raman spectroscopy, energy-dispersive X-ray spectroscopy, and scanning electron microscopy. The as-prepared CuO nanospheres were effectively embedded into the AP microparticles surface. [email protected] microparticles were continuously coated with graphene oxide (GO) nanosheets. Particle size analyzer was used to probe the particle size distribution of core-shell composites. Results show that the application of ISCDM method results in a typical narrow gaussian size distribution with an average diameter of hundred microns ranking from 158.56 μm to 166.22 μm. The thermal decomposition and the catalytic performances were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). [email protected]@GO core-shell composite exhibits an excellent thermal decomposition behavior: a low thermal decomposition temperature (LTD) of 337 ˚C, reduced values of the High thermal decomposition temperature HTD to be overlapped with the LTD step, and a significant energy release of 1543 J.g⁻¹ are registered.
Article
This work describes the chemical formation of composites based on different carbon nanostructures, such as single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT), single-layer graphene oxide (SLGO) and p-type conducting polyazulene (PAZ). The composite materials were synthesized in ethanol solution containing an appropriate amount of carbon nanostructures, azulene and ferric chloride as an oxidizing agent. The main attention was given to the electrochemical properties of these materials and their capacitance performance. The type of nanostructures influenced the morphology of the synthesized polyazulene. Thus, the relationship between the type of nanostructures present in the composite and its morphology and the electrochemical and stability properties were studied. The highest specific capacitance of 649 F g-1 was obtained for the SWCNT/PAZ composite. This value is nine times higher than the specific capacitance of pristine polyazulene synthesized under the same conditions. The SLGO/PAZ composite exhibited the lowest specific capacitance of 53 F g-1. However, this value was improved by approximately 77% by thermal treatment of the composite material at high temperature, resulting in an increase in the BET surface area as well as an increase in conductivity after heat treatment. © 2021 The Electrochemical Society ("ECS"). Published on behalf of ECS by IOP Publishing Limited.
Article
The carbon nanomaterials and congeners, e.g., graphene or graphene oxide (GO), dispose of numerous unique properties, which are not necessarily intrinsic but might be related to a content of impurities. The oxidation step of GO synthesis introduces a considerable amount of metallic species. Therefore, large-scale purification is an actual scientific challenge. Here we describe new purification technique (salt‑washing), which is based on three consecutive steps: (a) aggregation of GO sheets with NaCl (b) washing of the aggregates and (c) removing of the salt to afford purified GO (swGO). The considerably improved purity of swGO was demonstrated by ICP and EPR spectroscopy. The microscopic methods (TEM with SEAD, AFM) proved that the salt-washing does not affect the morphology or concentration of defects, showing the aggregation of GO with NaCl is fully reversible. The eligibility of swGO for biomedical applications was tested using fibroblastic cell cultures. The determined IC50 values clearly show a strong correlation between the purity of samples and cytotoxicity. Although the purification decreases cytotoxicity of GO, the IC50 values are still low proving that cytotoxic effect is not only impurities-related but also an intrinsic property. These findings may represent a serious limitation for usage of GO in biomedical applications.
Article
Graphene oxide (GO) is one of the most frequently-used graphene-family materials, but it must often be reduced in order to restore electrical conductivity for the target applications. We have demonstrated the use of non-contact fringing field RF applicators to rapidly heat and reduce GO, both in its neat form and inside a polymer matrix such as polyvinyl alcohol (PVA). For this study, GO and GO-PVA films were prepared by the vacuum filtration method. The results demonstrate quick non-contact heating of GO and GO-PVA composite films by application of RF fields. Heating rates as high as 10.9 °C/s and 1.5 °C/s have been observed for GO and GO-PVA, respectively. RF-reduced GO and GO-PVA samples have shown conductivities of 10² S/m and 10⁻¹ S/m, respectively. In addition, C/O ratio has increased from 2.44 to 5.22 when GO is exposed to RF waves which confirm that GO samples are reduced by the RF treatment. Unlike time-consuming or hazardous conventional reduction methods, RF resistively heats GO with electric fields in seconds to form reduced GO.
Article
This work focused on the kinetic analysis of the thermal reduction reaction of graphene oxide (GO) both with and without carbon suboxide (C3O2) by using Borchardt-Daniels, Kissinger, and Ozawa models. The effect of C3O2 on the thermal decomposition of GO was investigated by thermogravimetry (TG), derivative thermogravimetry (DTG), and differential scanning colorimetry (DSC) analysis. It was noticed that C3O2 tends to increase the chemical reactivity of GO due to the decreasing of the reaction activation energy Ea. The apparent reaction order is determined to be 0.7 for the thermal decomposition of both studied samples. This value evidences the complex mechanism of the thermal decomposition of GO that includes consecutive zero-order and first-order processes. Finally, by comparing the enthalpy values of GO thermal decomposition, it was determined that GO sample with the additive of C3O2 releases a lower amount of energy comparing to that of pure GO during the thermal reduction.
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The urea oxidation reaction (UOR) and nitrophenol reduction are safe and key limiting reactions for sustainable energy conversion and storage. Urea and nitrophenol are abundant in industrial and agricultural wastes, human wastewater, and in the environment. Catalytic oxidative and reductive removal is the most effective process to remove urea and 4-nitrophenol from the environment, necessary to protect human health. 2D carbon-supported, cobalt nanoparticle-based materials are emerging catalysts for nitrophenol reduction and as an anode material for the UOR. In this work, cobalt modified on a porous organic polymer (CoPOP) was synthesized and carbonized at 400 and 600 °C. The formation of CoPOP was confirmed by FT-IR spectroscopy, the 2D graphitic layer and amorphous carbon with cobalt metal by TEM, SEM, and PXRD, and the elemental composition by TEM mapping, EDX, and XPS. The catalytic activity for the 4-nitrophenol reduction was studied and the related electrocatalytic UOR was scientifically evaluated. The catalytic activity toward the reduction of 4-NP to 4-AP was tested with the addition of NaBH4; CoPOP-3 exhibited enhanced activity at a rate of 0.069 min⁻¹. Furthermore, LSV investigated the catalytic activity of materials toward UOR, producing hydrogen gas, the products of which were analyzed via gas chromatography. Among the electrocatalysts studied, CoPOP-2 exhibited a lower onset potential, and the Tafel slope was 1.34 V and 80 mV dec⁻¹. This study demonstrates that cobalt metal-doped porous organic polymers can be used as efficient catalysts to remove urea and nitrophenol from wastewater.
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Urban mining of graphite for higher value addition is considered to be a leading requirement for both the economic and environmental benefits. Herein, we adopted a scalable and novel synthetic approach using modified Hummers method to produce reduced graphene oxide (rGO) directly and graphene oxide (GO) using spent graphite material obtained from scraped Lithium-ion battery (LIB) for the first time. During the synthesis of graphene derivatives, all the metal impurities have been completely terminated through higher temperature oxidation thereby excluding the curing step of graphite. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) study clearly manifests that the reduction in lattice d-spacing, absence of impurity phase and removal of oxygen groups confirming the preparation of high-quality rGO framework. The synthesized and thermally reduced GO is used as electrode materials for non-Faradaic super capacitor application in different aqueous electrolytes. In particular, directly synthesized rGO material exhibits the superior capacitance of 156 Fg⁻¹ in neutral system, which is greater than the thermally reduced GO (90 Fg⁻¹) and commercial rGO samples (35 Fg⁻¹). The findings demonstrate that the as-proposed one-pot methodology to produce rGO endows with economic and environmental viability and also exhibits high purity, suitable for several commercial applications at the same time invigorating the worth of graphite conversion from fatigued LIBs.
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Radioactive Cs is a major by-product of nuclear power plants, with high radioactivity and long half-life. It is highly soluble in water and is difficult to remove. In this study, pristine graphene oxide (GO) synthesized via a Hummer's method has been demonstrated as a very efficient Cs sorbent with the maximum adsorption capacity of GO found to be 180, 465, 528 mg Cs/g sorbent at pH of 3, 7, and 12, respectively. The results from Fourier-transform infrared (FTIR) spectroscopy of GO before and after Cs sorption at various pH values reveal the mechanism of Cs sorption by GO. Several functional groups which are carboxyls, phenols, and hole defects containing multi-ether groups, are shown to play an important role in Cs capture. GO's affinity for other major cations found in seawater, namely, Na, K, and Mg was also evaluated, and the effect of these cations in competing with Cs for adsorption on GO was also studied. This reveals GO's exceptional ability in capturing Cs even in the presence of high concentrations of competitive cations and its high potential for use in Cs decontamination, as well as other heavy metal removal applications.
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Density functional theory was used to study the effects of different types of oxygen-containing functional groups on the adsorption of oxygen molecules and single active oxygen atoms on carbonaceous materials. During gasification or combustion reactions of carbonaceous materials, oxygen-containing functional groups such as hydroxyl(-OH), carbonyl(-CO), quinone(-O), and carboxyl(-COOH) are often present on the edge of graphite and can affect graphite’s chemical properties. When oxygen-containing functional groups appear on a graphite surface, the oxygen molecules are strongly adsorbed onto the surface to form a four-member ring structure. At the same time, the O-O bond is greatly weakened and easily broken. The adsorption energy value indicates that the adsorption of oxygen molecules changes from physisorption to chemisorption for oxygen-containing functional groups on the edge of a graphite surface. In addition, our results indicate that the adsorption energy depends on the type of oxygen-containing functional group. When a single active oxygen atom is adsorbed on the bridge site of graphite, it gives rise to a stable epoxy structure. Epoxy can cause deformation of the graphite lattice due to the transition of graphite from sp² to sp³ after the addition of an oxygen atom. For quinone group on the edge of graphite, oxygen atoms react with carbon atoms to form the precursor of CO2. Similarly, the single active oxygen atoms of carbonyl groups can interact with edge carbon atoms to form the precursor of CO2. The results show that oxygen-containing functional groups on graphite surfaces enhance the activity of graphite, which promotes adsorption on the graphite surface.
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Recently, studies on graphene-based lubrication additives have been widely researched, but few refer to their preparation by thermal reduction which shows potential in not only significantly lowering the mass-production cost, but also the simple, nonchemical process. In this study, mild thermal reduction of graphene oxide (MRGO) has been achieved by high temperature (700 °C) treatment and the product used as a lubrication additive. It shows a relatively ordered lamellar structure and a certain level of oxygen by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis, and exhibits excellent tribological properties as a lubrication additive. The friction coefficient can be reduced by as much as 30% and the rubbing surfaces display few scratches at a lower additive concentration (0.5 wt%) compared with that of base oil (Poly Alpha Olefins Type 6: PAO 6) without MRGO additive under the same friction conditions. Based on the advantages of green, low-cost and simple synthesis operation, the MRGO offers significant potential application as a lubrication additive.
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We investigate the temperature dependence of the phonon frequencies of the G and 2D modes in the Raman spectra of monolayer graphene grown on copper foil by chemical vapor deposition. The Raman spectroscopy is carried out under a 532.16 nm laser excitation over the temperature range from 150 to 390 K. Both the G and 2D modes exhibit significant red shift as temperature increases, and the extracted values of temperature coefficients of G and 2D modes are −0.101 and −0.180 cm−1 K−1, respectively, different from that of graphene on SiO2 substrate. The obtained results shed light on the anharmonic property of graphene, the complex interfacial interactions between graphene and the underlying copper foil substrate as temperature changes, and also proposes a new routine to estimate the thermal expansion coefficient of graphene on copper substrate rather than on SiO2 and SiN substrates. Furthermore, our work is instructive to study the similar temperature dependent mechanical properties, and the interfacial interactions between the other emerging two dimensional materials and their underlying substrates by temperature dependent Raman scatterings.
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Graphene oxide (GO), the functionalized graphene with oxygen-containing chemical groups, has recently attracted resurgent interests because of its superior properties such as large surface area, mechanical stability, tunable electrical and optical properties. Moreover, the surface functional groups of hydroxyl, epoxy and carboxyl make GO an excellent candidate in coordinating with other materials or molecules. Owing to the expanded structural diversity and improved overall properties, GO and its composites hold great promise for versatile applications of energy storage/conversion and environment protection, including hydrogen storage materials, photocatalyst for water splitting, removal of air pollutants and water purification, as well as electrode materials for various lithium batteries and supercapacitors. In this review, we present an overview on the current successes, as well as the challenges, of the GO-based materials for energy and environmental applications.
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A graphene nanosheet reinforced ZrB2-SiC ceramic composite (GNs/ZrB2-SiC) using graphene oxide (GO) was hot pressed at 1950°C and 30 MPa for 1 h. Raman and XPS analysis showed multilayer GNs structures were successfully introduced into the composite by in situ thermal reduction of GO during the hot pressing process. The homogeneous dispersion of GO guaranteed the uniform distribution of GNs structures in the composite. Mechanical properties such as bending strength, fracture toughness and hardness were studied for the ZrB2-SiC composite with different volumes fraction of GO. The addition of approximately 5 vol% graphene nanosheets improved the fracture toughness of ZrB2-SiC up to 7.32 MPa m0.5, and the strength was also raised to 1055 MPa. The toughening mechanisms were graphene crack bridging and pulling out induced crack deflection in the GNs reinforced composite.
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5 Rapid thermal exfoliation/reduction of graphite oxide is a fast and easy method among the oxidation-reduction approaches for graphene synthesis. In this research, we firstly demonstrated that graphene can be obtained with a surface area of 550 to 700 m 2 /g and a yield of approximately 50% through one-step rapid thermal treatment in air from 450 to 550 °C, without vacuum or protective gas through a self-protection process. Then, we further demonstrated the effective two-step thermal annealing to 10 significantly improve the C/O ratio from c. 7.3 to 25.9 in air at the relatively low temperature of 600 °C. The smart self-protecting and enhanced-oxygen-removal mechanisms were discussed.
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We report the increasingly improvement of the available specific surface area of the widespread used Vulcan XC 72R and Ketjenblack EC-600JD carbons by simple thermal pre-treatment. The treated Vulcan and Ketjenblack substrates show a specific surface area of 322 and 1631 m2 g-1 respectively instead of 262 and 1102 m2 g-1 for the as-received materials, meaning 23 and 48 % of improvement. Subsequently, when used as platinum nanoparticles (3 nm) supports, the electrochemical active surface area is enhanced by a factor of 2.2 and 1.2 for treated Vulcan and Ketjenblack carbons, respectively. Furthermore, the electrochemical investigations have highlighted a surprisingly better catalytic activity for the pre-treated Vulcan XC 72R and Ketjenblack EC-600JD supported Pt nanoparticles. In fact, the synthesized nanostructures from the so-called "Bromide Anion Exchange" method exhibit good catalytic activity toward glucose electrooxidation both in alkaline medium and phosphate buffered solution at pH 7.4. More importantly, the present catalysts are four times more active than those in the literature prepared under similar conditions for glucose dehydrogenation at low potential (0.27 V vs. Reversible Hydrogen Electrode). Consequently, these remarkable trends uncovered herein provide ample new strategic routes for the pre-treatment of Vulcan 72R and Ketjenblack carbons for widespread utilizations.
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The chemistry underlying the aqueous dispersibility of graphene oxide (GO) and reduced graphene oxide (r-GO) is a key consideration in the design of solution processing techniques for the preparation of processable graphene sheets. Here, we use zeta potential measurements, pH titrations, and infrared spectroscopy to establish the chemistry underlying the aqueous dispersibility of GO and r-GO sheets at different values of pH. We show that r-GO sheets have ionizable groups with a single pK value (8.0) while GO sheets have groups that are more acidic (pK = 4.3), in addition to groups with pK values of 6.6 and 9.0. Infrared spectroscopy has been used to follow the sequence of ionization events. In both GO and r-GO sheets, it is ionization of the carboxylic groups that is primarily responsible for the build up of charge, but on GO sheets, the presence of phenolic and hydroxyl groups in close proximity to the carboxylic groups lowers the pKa value by stabilizing the carboxylate anion, resulting in superior water dispersibility .
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For incompletely reduced graphene oxides (RGOs), an effect of oxygen functional groups such as carboxyl, phenol, carbonyl, and quinone on electrochemical capacitive behavior was studied. To prepare RGO thin-film electrodes, a simple fabrication process by (i) dropping and evaporating the graphene oxide (GO) solution, (ii) irradiating pulsed light, and (iii) heat-treating at 200 ∼ 360 °C was applied. It was notable that the pulsed light irradiation was effective to prevent the disfiguring of deposited GO thin-film during the thermal reduction. From XRD analyses, interlayer distances of the RGOs were gradually decreased from 0.379 to 0.354 nm. As increasing the thermal reduction temperature from 200 to 360 °C, XPS O 1s spectra analyses showed that the atomic percentages of carboxyl and phenol of the RGOs were sustained as 5.40 ± 0.36 and 4.77 ± 0.41 at% respectively. Meanwhile, those of carbonyl and quinone of the RGOs were gradually declined from 3.10 to 1.81 and from 1.32 to 0.65 at% with different thermal reduction temperature respectively. For all RGO thin-film electrodes, the specific capacitance from the CV measurement in 6 M KOH was sustained as ca. 220 F g−1 at the scan of 5 mV s−1. However, in 1 M H2SO4, the specific capacitance was gradually decreased from 171 to 136 F g−1. After 100,000 cycles in 6 M KOH, 1 M H2SO4, and 0.5 M Na2SO4, the RGO (200 °C) electrodes showed ca. 92, 54, and 104% of the initial capacitances respectively. The atomic percentages of the oxygen functional groups involved in the pseudocapacitive Faradaic reaction were decreased after the cycle test. Especially in 1 M H2SO4, quinone group was decreased to ca. 48% of initial atomic percentage, which seems to be a main reason for the drastic reduction of capacitance. The specific pseudocapacitance per unit atomic percentage for either carboxyl or phenol group in 6 M KOH was obtained as 12.59 F g−1 at%−1. For carbonyl group in 1 M H2SO4, it was a slightly deviated value as 13.55 F g−1 at%−1. For quinone group in 1 M H2SO4, it was 27.09 F g−1at%−1.
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The Raman G and 2D bands of uniaxially strained graphene are studied within a non-orthogonal tight-binding model for parallel scattering geometry and laser photon energy of 2.5 eV. The derived strain rate of the G band, as well as its intensity as a function of the strain direction and light polarization angle, are found in very good agreement with previous reports. The simulated 2D band shows a complex peak structure with two or three resolved subbands. The dependence of the strain rate and the Raman intensity of the latter on the strain direction and light polarization angle follows simple trigonometric expressions. Noticeable deviations from these expressions are observed for the polarization angle dependence. It is also shown quantitatively that the contribution to the 2D band of the “inner” processes is about 10 times larger than that of the “outer” processes. Our predictions for the 2D band behavior of strained graphene can be used for monitoring the strain in graphene-based applications by Raman spectroscopy.
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In the present work, we have systematically investigated the effect of different exfoliation conditions on the synthesis of graphene from graphite oxide (GO). Four different conditions were used to exfoliate GO: Ar @ 1050 degrees C, vacuum @ 200 degrees C, H(2) + Ar mixture @ 200 degrees C, and H(2) @ 200 degrees C. Few layered graphenes obtained by these methods were characterized by thermogravimetry, diffractometry, spectroscopy, microscopy, surface, and elemental analysis techniques. New insights obtained upon a detailed analysis of these are presented. Although the morphology and characteristics of these graphenes are similar, differences are observed in the amount of functional groups present, resulting in considerable change in their electrical properties. These results show conclusively that the atmosphere for exfoliation of GO plays a critical role in low temperature syntheses of graphene. It is observed that exfoliation-reduction of GO in pure hydrogen atmosphere at 200 degrees C results in the highest quality of a few layered graphene sheets.
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In this work, the photoresponse performance of monodisperse PbSe nanocubes in the range of visible and near-infrared (NIR) (400–1500 nm) regions was enhanced by reduced graphene oxide (rGO). A simple cost-effective method is presented to synthesize monodisperse PbSe nanocubes (NCs) that are decorated on the rGO sheets. By the addition of PbSe/rGO nanocomposites with different rGO concentrations, pristine PbSe NCs were synthesized with the same method. Microscopy images showed that the size of NCs was smaller than the exciton Bohr radius (46 nm) of PbSe bulk. Therefore, the UV-Vis-IR spectroscopy result revealed that the PbSe/rGO samples had absorption peaks in the NIR region around 1650 nm and showed a blue shift compared to the absorption peak of the PbSe bulk. J-V measurements of the samples indicated that monodisperse PbSe/rGO nanocomposites had a higher resistance than the other samples under dark condition. On the other hand, the resistance of the monodisperse PbSe/rGO nanocomposites decreased under different light source illuminations while the resistance of the other samples was increased under illumination. Photodetector measurements indicated that the monodisperse morphology of the PbSe NCs enhanced the photoresponse speed and photocurrent intensity. In addition, responsivity (R) and detectivity (D*) of the samples were higher in the NIR region.
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A systematic study about the origin of defects emission of ZnSe structure was conducted by photoluminescence (PL) spectrometer at room temperature. It was observed that different intermediate energy levels in band-gap space of ZnSe structure were generated by different defects such as Se-, Zn-vacancies, Se-, Zn-interstitials, and surface states. Effects of these defects on the photocatalytic performance of ZnSe quantum dots (QDs) and ZnSe/graphene nanocomposites were investigated. The pristine ZnSe QDs and ZnSe/graphene nanocomposites were synthesized by a co-precipitation method. The PL spectra of the samples showed four emissions from four regions of the visible spectrum such as violet, green, orange, and red emissions. The violet emission was associated with the near-band-edge (NBE) of the ZnSe nanostructures, while, the other emissions were related to different defects of ZnSe structures. Annealing the samples in the H2 atmosphere caused to increase orange emission intensity and indicated that origin of orange emission was a donor-acceptor pair (DAPs) related to singly positively charged Se-vacancies (VSe) to singly negatively charged zinc vacancy (VZn⁻). Photocatalytic study of the samples to remove the methylene blue (MB) dye showed that the photocatalytic performance of the samples improved by graphene as an additive and increasing the orange emission intensity.
Article
The current research presents a simple, coast-effective, and one-pot refluxing method to synthesize Zn(1−x)MgxO nanostructures, which were decorated on graphene oxide (GO) sheets. In the first step, the effect of refluxing time on structure and morphology of the pristine ZnO nanostructures was investigated. X-ray diffraction (XRD) patterns indicated that the pristine ZnO nanostructures were formed after 8 h of the refluxing process. Field emission electron microscope (FESEM) images showed that stars-shape ZnO nanostructures were formed after 10 h of refluxing time. Further refluxing process for 12 h showed that morphology and structure of the ZnO nanostructures were not changed. However, after 14 h additional phases were formed. Therefore, ZnO and Zn(1−x)MgxO nanostars that were decorated on GO sheets were synthesized during 10 h. XRD patterns indicated that GO sheets were changed into reduced graphene oxides (rGO) during the refluxing process. Transmission electron microscope (TEM) images revealed that ZnO nanostars with more branches were decorated on rGO sheets. However, the TEM images showed that the morphology of ZnxMg(1−x)O/rGO nanocomposites were changed significantly with the increase of Mg concentration up to 6%. Photocatalytic performance of the products was examined under natural sunlight irradiation. The results showed that the rGO and Mg concentrations had significant roles in the photocatalytic performance of ZnO nanostars. The concentrations of Mg and rGO increased up to 4% were the optimum concentration for enhancing photocatalytic performance of Zn(1−x)MgxO/rGO nanocomposites. In addition, room temperature photoluminescence (PL) spectroscopy and photocurrent measurement results indicated that Mg and rGO with optimum concentration caused decrease of electron-hole recombination rate.
Article
This paper reviews progress that has been made in the use of Raman spectroscopy to study graphene and carbon nanotubes. These are two nanostructured forms of sp² carbon materials that are of major current interest. These nanostructured materials have attracted particular attention because of their simplicity, small physical size and the exciting new science they have introduced. This review focuses on each of these materials systems individually and comparatively as prototype examples of nanostructured materials. In particular, this paper discusses the power of Raman spectroscopy as a probe and a characterization tool for sp² carbon materials, with particular emphasis given to the field of photophysics. Some coverage is also given to the close relatives of these sp² carbon materials, namely graphite, a three-dimensional (3D) material based on the AB stacking of individual graphene layers, and carbon nanoribbons, which are one-dimensional (1D) planar structures, where the width of the ribbon is on the nanometer length scale. Carbon nanoribbons differ from carbon nanotubes is that nanoribbons have edges, whereas nanotubes have terminations only at their two ends.
Chapter
This chapter is aimed to overview the recent progress in the development and characterization of a new group of nanomaterials, in free-standing forms, such as fiber/yarns (1D), paper/sheet (2D), and bulk (3D), which were derived from graphene nanosheets prepared by using various fabrication technologies. These materials have special and unique mechanical, thermal, and electrical properties, with potential applications in various aspects, such as energy storage, environmental protection, wearable electronics, and so on. Significant progress and important applications of these new carbon nanomaterials will be highlighted. Materials (synthesis), processing strategies (spinning, filtration, casting, spark plasma sintering), morphologies, properties (mechanical, electrical, and thermal), and potential applications, as well as their interrelationships, will be presented and discussed in a more detailed way.
Article
The chemical modification of self-assembled graphene hydrogels is a topic of emerging interest to harness the excellent physicochemical properties of two-dimensional (2D) graphene for macroscopic applications. We synthesized a series of mechanically strong and lightweight nitrogen (N)-doped graphene hydrogels (NGHs), with different doping concentrations, through a simple one-pot hydrothermal reaction and systematically evaluated their performance as both adsorbents and photocatalysts for environmental remediation. Acridine orange (AO) was chosen as a model pollutant. The successful incorporation of N atoms into the carbon lattice of the macroscale 3D graphene-based materials was verified by Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Although the N content of the graphene macroassemblies varied inversely with doping density, a conspicuous increase in specific surface area was observed at all doping levels, resulting in a higher adsorption capacity and surface reactivity than the undoped hydrogel. The adsorption equilibrium was best represented by the Langmuir isotherm (with maximum monolayer coverage of 124 mg g−1 at 25 °C) while the adsorption kinetics followed both the pseudo-first and pseudo-second order rate expressions. Further, the NGHs could effectively photodegrade 20 mg L−1 AO aqueous solution by almost 70% within 5 h of visible light irradiation. The fairly good photooxidative ability of the NGHs originates from the synergistic effect of N functionalization and 3D interconnected mesoporous network structure, leading to greater uptake of AO, better absorption of visible light and rapid spatial separation of photogenerated electron–hole pairs.
Article
Synthesizing high-quality graphene by catalytic transformation from amorphous silicon carbide (a-SiC) through a rapid thermal treatment (RTT) method is reported. SiO2/Si substrates are coated by a-SiC films followed by Cu and Ni films deposited sequentially. The samples are then thermally annealed by RTT for the synthesis of high-quality graphene in only 3 minutes. The synergistic effect of Cu and Ni catalyst is observed. We conjecture that the inserted copper film acts not only as a catalyst or substrate for graphene growth but also as a barrier for carbon diffusion to facilitate the synthesis of monolayer graphene, while the nickel film acts as another catalyst and forms Cu-Ni alloy to lower the catalytic temperature. In this paper, we present a simple and time-saving way in preparation of high-quality graphene and put forward a brief theoretical model for the growth of graphene.
Article
The local atomic configuration of graphene oxide (GO) was investigated by identifying the different oxygen functionalities and following their evolution induced by thermal treatments in various environments (vacuum, nitrogen or argon flow). X-ray photoelectron spectroscopy and scanning transmission electron microscopy analyses were performed and electron energy-loss (EEL) spectra were acquired in different regions of GO and thermally reduced GO flakes. Experimental results show a series of characteristic peaks related to C and O K-edge shells and different features of GO thermally annealed at the same temperature but in different environments. In order to understand the experimental results, density functional theory calculations of core-loss EEL spectra of GO (C and O K-edges) in the presence of oxygen functional groups have been performed, for different combinations and/or concentrations. Such calculations have allowed for the association of the observed experimental peaks to the presence of specific oxygen functional groups, giving the opportunity to establish the atomic configurations that prevail in different ranges of annealing temperatures and environments.
Article
Graphene oxide has been utilized effectively for the adsorption of heavy metals. Incorporation of graphene oxide with magnetite nanoparticles through co-precipitation enhances separation of a newly designed magnetite graphene oxide by a magnetic field. Considering the instability of magnetite graphene oxide under different water chemistry conditions, we designed encapsulated magnetite graphene oxide inside a non-toxic alginate bead as a high-performance green chemical for the uptake of Cr(VI) and As(V) in water treatment applications (mGO/bead). The alginate, itself revealed low contribution for metal ions uptake, however this could potentially inhibit the aggregation of magnetite graphene oxide powder showing enhanced performance to extract metals from water. Material capacity minimally altered over a range of pH values for As(V), while Cr(VI) uptake varied with pH changes, which were explained by a local pH-controlled mechanism. Induced hydrolysis was facilitated by the release of Fe3+ from Fe3+-crosslinked mGO/beads, thus enhancing the removal of Cr(VI). For a complex mixture of heavy metals, influence of the presence of co-occurring ions in a mixed contaminant system (Cr(VI), As(V), Cu(II), Cd(II)) revealed an excellent performance (∼80-100% removal) by the composite material. The mGO/bead maintained its activity in wastewater and exhibited greater adsorption efficiency for both Cr(VI) and As(V) compared with activated carbon and carbon nanotube. The mGO/beads could be collected and reused for at least 5 cycles without the leaching of core mineral contents. The results indicate the advantages of mGO/beads over the popular adsorbents that can be developed as a versatile material for water treatment.
Article
An oxidation-resistant and elastic mesoporous carbon, graphene mesosponge (GMS), is prepared. GMS has a sponge-like mesoporous framework (mean pore size is 5.8 nm) consisting mostly of single-layer graphene walls, which realizes a high electric conductivity and a large surface area (1940 m2 g−1). Moreover, the graphene-based framework includes only a very small amount of edge sites, thereby achieving much higher stability against oxidation than conventional porous carbons such as carbon blacks and activated carbons. Thus, GMS can simultaneously possess seemingly incompatible properties; the advantages of graphitized carbon materials (high conductivity and high oxidation resistance) and porous carbons (large surface area). These unique features allow GMS to exhibit a sufficient capacitance (125 F g−1), wide potential window (4 V), and good rate capability as an electrode material for electric double-layer capacitors utilizing an organic electrolyte. Hence, GMS achieves a high energy density of 59.3 Wh kg−1 (material mass base), which is more than twice that of commercial materials. Moreover, the continuous graphene framework makes GMS mechanically tough and extremely elastic, and its mean pore size (5.8 nm) can be reversibly compressed down to 0.7 nm by simply applying mechanical force. The sponge-like elastic property enables an advanced force-induced adsorption control.
Article
Thermal decomposition of graphene oxide (GO) has been extensively investigated in the last decade, but the detailed reaction kinetics remains elusive so far. Here we employ an in-situ X-ray diffraction (XRD) analysis to clarify the kinetics of GO decomposition in different atmospheres and sample morphologies. The XRD peak (002), which is the major diffraction peak corresponding to the interlayer distance in GO samples, shifted from 11.5o to 23o along with significant decrease in intensity when samples were heated from 25 °C to 350 °C in air. The decomposition in air exhibits a higher reaction rate compared with that in pure nitrogen gases, because the O2 molecules in air facilitate the oxidation of carbon atoms, leading to the evolution of CO and CO2. Free-standing films of GO also decompose significantly faster than GO powders, owing to their slower heat dissipation into the environment and higher thermal conductivity within the well-stacked lamella. This study has provided new insights on the reaction kinetics of GO thermal decomposition, and offered a novel perspective on kinetic analysis based on our in-situ XRD technique.
Article
An efficient non-enzymatic biosensor electrode consisting of nitrogen-doped graphene (N-graphene) and platinum nanoflower (Pt NF) with different N-graphene loadings were fabricated on indium tin oxide (ITO) glass using a simple layer-by-layer electrophoretic and electrochemical sequential deposition approach. N-graphene was synthesized by annealing graphene oxide with urea at 900 °C. The structure and morphology of the as-fabricated non-enzymatic biosensor electrodes were determined using X-ray diffraction, field emission electron microscopy, transmission electron microscopy, Raman and X-ray photoelectron spectra. The as-fabricated Pt NF-N-graphene-modified ITO electrodes with different N-graphene loadings were utilized as a non-enzymatic biosensor electrode for the detection of hydrogen peroxide (H2O2). The behaviors of the hybrid electrodes towards H2O2 reduction were assessed using chronoamperometry, cyclic voltammetry and electrochemical impedance spectroscopy analysis. The Pt NF-N-graphene-modified ITO electrode with a 0.05 mg ml−1 N-graphene loading exhibited the lowest detection limit, fastest amperometric sensing, a wide linear response range, excellent stability and reproducibility for the non-enzymatic H2O2 detection, due to the synergistic effect between the electrocatalytic activity of the Pt NF and the high conductivity and large surface area of N-graphene.
Article
Low-temperature (≤ 300°C) thermal treatment of graphene oxide (GO) films in ambient air is examined. In particular, the role of low to moderate heating temperatures, to the evolution of the original functional groups anchored on the GO skeleton, is closely investigated by Fourier transform infra red (FT-IR) spectroscopy. The study shows that, contrary to vacuum or inert ambient heating, heating under ambient atmosphere triggers concomitant reduction and oxidation reactions. Hydroxyl and epoxy groups are progressively eliminated, but at the same time newly formed carbonyls appear due to oxidation. Electrical measurements indicate that despite the presence of oxygen containing groups in the restored graphene sp2 network, the conductivity enhances. The process, therefore, lends itself to the production of conductive reduced GO with increased functionalities suitable for application in gas sensor fabrication. The concept is evaluated with a humidity sensor where thermally reduced GO is prepared at different reduction temperatures. The evaluation unveils that a critical reduction temperature exists where sensor sensitivity is optimized.
Article
We developed a one-flask method for the thermal reduction of graphene oxide (GO) with triphenylphosphine dihalide (Ph3PX2) at 180oC. Our approach offers a potential to cost-effective mass-production of graphene nanosheets under mild and environmentally friendly conditions and to avoid the use of strong acids or reducing agents. Significantly, this reduced graphene oxide (rGO) by utilizing Ph3PX2 reductant has a C/O ratio higher than 15 and an electrical conductivity of 400 S/cm, which indicate that this synthetic method allows us to achieve graphene nanosheets with high quality when comparing with previous reduction methods.
Article
The ability to upscale the production of chemically modified forms of graphene has led to intense interest in the manufacture and commercialization of graphene-based materials. Free-standing film-like materials comprised of stacked and overlapped platelets of graphene oxide (G-O) or thermally and electrically conductive reduced graphene oxide (rG-O) are potentially useful in various applications including filtration membranes, mechanical seals, protective layers, heating elements and components of batteries or supercapacitors as well as in electronics and optoelectronics. The advances in these applications require efficient and low-cost protocols for fabricating certain types of layered materials and, as such, are urgently needed to utilize the reduced forms of G-O. Here we report an efficient and straightforward strategy to thermally reduce thin films of stacked G-O platelets while still maintaining their structural integrity. By rapidly heating confined G-O films on a hot plate set to 400 °C under an atmosphere of air, G-O films were readily converted to intact, electrically conductive, reduced thin films. The structure and degree of reduction of the resulting free-standing rG-O films were found to be comparable to those obtained by slow annealing at the same temperature.
Article
The effect of environmental factors (i.e., reaction time, pH, ionic strength, temperature and initial U(VI) concentration) on the adsorption of U(VI) on graphene oxide (GO) had been investigated by batch techniques. The macroscopic experiments indicated that the adsorption kinetics and adsorption isotherms of U(VI) on GO can be satisfactorily fitted by the pseudo-second-order kinetic model and Langmuir model, respectively. No change of ionic strength indicated that the inner-sphere surface complexation dominated the adsorption of U(VI) on GO over the wide pH range. Three surface complexation models (SCMs), including constant capacitance model (CCM), diffuse layer model (DLM) and triple layer model (TLM), had been given an excellent fits for the adsorption of U(VI) on GO with two inner-sphere surface complexes such as SOUO2+ and (SO)2UO2(OH)22− species.
Chapter
Quantum Modeling of the Elastic Scattering EventThe Frequency of the Defect-Induced Peaks: the Double Resonance ProcessQuantifying Disorder in Graphene and Nanographite from Raman Intensity AnalysisDefect-Induced Selection Rules: Dependence on Edge Atomic StructureSpecificities of Disorder in the Raman Spectra of Carbon NanotubesLocal Effects Revealed by Near-Field MeasurementsSummary
Article
The influence of reduction temperatures on the structure and the sorption capacity of thermally reduced graphene (TRGO) has been investigated systematically. A set of TRGO materials were prepared by thermal treatment of parent graphene oxide (GO) at five temperatures (T = 200, 300, 500, 700, and 900 °C). Investigations of these materials by X-ray diffraction, Raman spectroscopy and X-ray photoemission spectroscopy methods have shown that both the structure and the residual oxygen functional groups on the TRGO surface can be controlled by varying the temperature of the thermal treatment. The data on the sorption and desorption of 4He, H2, N2, Ne and Kr gases in the temperature interval T = 2–290 K clearly demonstrate that the sorption capacity of TRGO is closely related to the structural changes induced by the treatment temperatures. It is important that the sorption capacities of TRGOs treated at 300 °C and at 900 °C significantly increase for all the gases used. The prominent increase in the sorption capacity at 300 °C is attributed to the structural disorder and liberation of the pores caused by the removal of intercalated water and labile oxygen functional groups (oFGs) favored at this temperature. At 900 °C the sorption capacity increases due to the generation of new defects on the TRGO surface, which provide additional access to the internal space between the folds and sheets of the TRGO structure. By tailoring the structural properties we emphasize the potential of TRGO as a highly efficient sorbent.
Article
To allow for the use of graphene in various nanoelectronic applications, the methods for the large-scale production of graphene with controllable electrical properties need to be developed. Here, we report the results of a fundamental study on the remarkable conversion between n- and p-type reduced graphene oxide (rGO) with changes in the thermal annealing temperature. It was found that the charge carriers in rGO for temperatures of 300-450 °C and 800-1000 °C are electrons (n-type), whereas for temperatures of 450-800 °C, they are holes (p-type). This is because the individual oxygen functional groups present on rGO are determined by the annealing temperature. We found that the predominance of electron-withdrawing groups (i.e., carboxyl, carbonyl, and sp3-bonded hydroxyl, ether, and epoxide groups) resulted in p-type rGO, although that of electron-donating groups (sp2-bonded hydroxyl, ether and epoxide groups) lead to n-type rGO. In addition, as a proof of concept, a flexible thermoelectric device consisting of GO-700 and GO-1000 as p-type and n-type components, respectively, was fabricated. This device, which contained eight pairs of the two components, exhibited an output voltage of 4.1 mV and an output power of 41 nW for δT = 80 K. These results demonstrate that the carrier characteristics of rGO can be altered significantly by changing the functional groups present on it, thus allowing it to be used in various applications including flexible thermoelectrics.
Article
Here we report a two-step programmable reduction of graphene oxide (GO) which was synthesized by oxidation of graphite. X-Ray photoelectron spectroscopic (XPS) analysis confirmed the synthesis of exfoliated graphene oxide (GO) by introduction of oxygen as carboxylic (–COOH), epoxy (C–O–C) and hydroxyl (–OH) groups. The first step of GO reduction was achieved separately by (i) hydrazine (rGO11) and (ii) sodium borohydride (rGO21). Soda lime was used in the second-stage reduction of (a) hydrazine reduced GO (rGO12) and (b) sodium borohydride reduced GO (rGO22) to remove most of the remaining carboxylic functionalities from the rGO11 and rGO21 surface. XPS spectra of rGO21 showed a decrease (38 to 30%) in the oxygen whereas the further reduction of rGO21 with soda lime can further reduce the oxygen content. Quantitative analysis of C(]O)OX in GO shows about 43% of carbon atoms (C 1s signal) as carboxylic functionalities whereas the reduction of the GO with sodium borohydride reduced this signal to about 10%. The use of soda lime for both rGO11 and rGO21 further reduced the amount of carboxylic functionalities. An increase in the proportion of carbon atoms as sp2 and decrease in the oxygen functionalities were controlled in the two-step reduction. A good correlation in the conductivity of reduced GO with the percentage proportion of sp2 carbon was observed.
Article
Infrared spectroscopy in combination with density functional theory calculations has been widely used to characterize the structure of graphene oxide and its reduced forms. Yet, the synergistic effects of different functional groups, lattice defects, and edges on the vibrational spectra are not well understood. Here, we report first-principles calculations of the infrared spectra of graphene oxide performed on realistic, thermally equilibrated, structural models that incorporate lattice vacancies and edges along with various oxygen-containing functional groups. Models including adsorbed water are examined as well. Our results show that lattice vacancies lead to important blue and red shifts in the OH stretching and bending bands, respectively, whereas the presence of adsorbed water leaves these shifts largely unaffected. We also find unique infrared features for edge carboxyls resulting from interactions with both nearby functional groups and the graphene lattice. Comparison of the computed vibrational properties to our experiments clarifies the origin of several observed features and provides evidence that defects and edges are essential for characterizing and interpreting the infrared spectrum of graphene oxide.
Article
The preparation of graphitic oxide by methods described in the literature is time consuming and hazardous. A rapid, relatively safe method has been developed for preparing graphitic oxide from graphite in what is essentially an anhydrous mixture of sulfuric acid, sodium nitrate and potassium permanganate.
Article
In this article we review Raman studies of defects and dopants in graphene as well as the importance of both for device applications. First a brief overview of Raman spectroscopy of graphene is presented. In the following section we discuss the Raman characterization of three defect types: point defects, edges, and grain boundaries. The next section reviews the dependence of the Raman spectrum on dopants and highlights several common doping techniques. In the final section, several device applications are discussed which exploit doping and defects in graphene. Generally defects degrade the figures of merit for devices, such as carrier mobility and conductivity, whereas doping provides a means to tune the carrier concentration in graphene thereby enabling the engineering of novel material systems. Accurately measuring both the defect density and doping is critical and Raman spectroscopy provides a powerful tool to accomplish this task.
Article
Aerogel materials possess a wide variety of exceptional properties, including a quite low density, high specific surface area, high porosity, etc. Considering that both graphene aerogels and ZrO2 aerogels have advantages and disadvantages respectively, graphene/ZrO2 composite aerogels are prepared, by a facile step, to enable them to have low thermal conductivity and to enhance the electronic interaction between the ZrO2 nanoparticles and graphene sheets. The chemical composition and crystalline structure of the resulting graphene/ZrO2 composite aerogels, as well as the strong interaction between the graphene sheets and the ZrO2 nanoparticles, have been disclosed by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and X-ray powder diffraction (XRD). The morphology and hierarchically porous attributes of the resulting graphene/ZrO2 composite aerogels have been investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nitrogen adsorption–desorption tests. The mechanical properties, electrical conductivity, electrochemical properties and thermal conductivity (as well as thermal stability) of the resulting graphene/ZrO2 composite aerogels have also been revealed in this study.
Article
Significant changes in the Raman spectrum of single-layer graphene grown on a copper film were observed after the spontaneous oxidation of the underlying substrate that occurred under ambient conditions. The frequencies of the graphene G and 2D Raman modes were found to undergo red shifts, while the intensities of the two bands change by more than an order of magnitude. To understand the origin of these effects, we further characterized the samples by scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and atomic force microscopy (AFM). The oxidation of the substrate produced an appreciable corrugation in the substrate without disrupting the crystalline order of the graphene overlayer and/or changing the carrier doping level. We explain the red shifts of the Raman frequencies in terms of tensile strain induced by corrugation of the graphene layer. The changes in Raman intensity with oxidation arise from the influence of the thin cuprous oxide film on the efficiency of light coupling with the graphene layer in the Raman scattering process.
Article
The effects of different concentrations of graphene oxide (GO) on the structure and optical properties of ZnO nanoparticles (NPs) were investigated. The nanocomposites were synthesized via the sol-gel method in a gelatin medium. X-ray diffraction patterns (XRD) and Fourier transform infrared spectroscopy indicated that the GO sheets were reduced and changed to reduced GO (RGO) during the calcination of the nanocomposites at 400 °C. In addition, the XRD patterns of the NPs indicated a hexagonal (wurtzite) structure for all the products. Microscopic studies showed that the NPs were decorated and dispersed on the RGO sheets very well. However, these studies revealed that the RGO concentration had an effect on the crystal growth process for the ZnO NPs. Furthermore, these studies showed that the NPs could be grown with a single crystal quality in an optimum RGO concentration. According to the XRD results that were obtained from pure ZnO NPs, the calcinations temperature was decreased by the RGO. UV–vis and room temperature photoluminescence studies showed that the optical properties of the ZnO/RGO nanocomposite were affected by the RGO concentration. Finally, the obtained ZnO/RGO nanocomposite was used to generate a photocurrent. Observations showed that the photocurrent intensity of the nanocomposite was significantly increased by increasing the RGO, with an optimum RGO concentration.
Article
Grain boundaries consisting of dislocation cores arranged in a periodic manner have well-defined structures and peculiar properties and can be potentially applied as conducting circuits, plasmon reflectors and phase retarders. Pentagon-heptagon (5-7) pairs or pentagon-octagon-pentagon (5-8-5) carbon rings are known to exist in graphene grain boundaries. However, there are few systematic experimental studies on the formation, structure and distribution of periodic grain boundaries in graphene. Herein, scanning tunneling microscopy (STM) was applied to study periodic grain boundaries in monolayer graphene grown on a weakly interacting Cu(111) crystal. The periodic grain boundaries are formed after the thermal reconstruction of aperiodic boundaries, its structure agree well with the prediction of the coincident-site-lattice (CSL) theory. Periodic grain boundaries in quasi-freestanding graphene give sharp LDOS peaks in the tunneling spectra as opposed to the broad peaks of the aperiodic boundaries. This suggests that grain boundaries with high structural quality can introduce well-defined electronic states in graphene and modify its electronic properties.
Article
We report continuous monitoring of the heterogeneously distributed oxygenated functionalities on the entire surface of the individual graphene oxide flake during the chemical reduction process. The charge densities over the surface with mixed oxidized and graphitic domains were observed for the same flake after a step-by-step chemical reduction process with the electrostatic force microscopy. Quantitative analysis revealed heavily oxidized nanoscale domains (50-100 nm across) on the graphene oxide surface and a complex reduction mechanism involving leaching of sharp oxidized asperities from the surface followed by gradual thinning and formation of uniformly mixed oxidized and graphitic domains across the entire flake.
Article
The basal plane of graphene has been known as less reactive than edges, but some studies observed vacancies in the basal plane reacted with oxygen gas. Observation of these vacancies has typically been limited to nanometer scale resolution using microscopy techniques. This work demonstrates the introduction and observation of sub-nanometer vacancies in the basal plane of graphene by heat treatment in oxygen gas flow at low temperature such as 533 K or lower. High-resolution transmission electron microscopy was used to directly observe vacancy structures, which were compared with image simulations. These proposed structures contain C=O, C-O-C, and lactone-like functional groups.
Article
A computational chemistry study was conducted to reveal similarities and differences in the adsorption of molecular oxygen on the edge sites of a carbon nanotube (CNT) and a graphene nanoribbon. Two prototypical CNT molecules with a carbene and a carbyne active site were selected, and this in turn defined two corresponding graphene molecules obtained by CNT unzipping. Their electronic and thermochemical properties before and after O2 adsorption were compared using density functional theory at the B3LYP/3-21G∗ level, as implemented in the Gaussian03 software. The sensitivity of the results to the basis set used and the selected CNT diameter was also assessed. Despite significant curvature in a subnanometer-diameter CNT, more similarities than differences were revealed with respect to graphene, both in their charge density distributions and thermochemical properties. Contrary to intuitive expectations, the intrinsic activity of an edge site (at least in the prototypical O2 chemisorption process) is therefore not significantly modified when graphene is rolled up into a nanotube possessing a relatively large degree of pyramidalization. Greater differences exist between armchair and zigzag edges in both CNT and graphene. Both undergo a two-step mechanism of O2 adsorption, but O2 dissociates only on the armchair edge. Non-dissociative adsorption on the zigzag site has both a lower affinity and a higher activation energy than the dissociative adsorption on the armchair site.