Reduced Graphene Sheets Research Articles

Effects of reductions and drying methods on electrochemically exfoliated graphene

Graphene research has advanced a lot in the last decade. Scientists are currently developing methods to make high-quality materials with distinctive attributes. Solar cells, electronic devices, energy storage and sensors are some of the potential applications involving graphene and derivatives. However, the reduction and drying steps of graphene obtained may influence the material properties and its future applications. The present work proposes a comparison of reduced graphene by sustainable methods following two different drying techniques. Freeze-drying prevents the aggregation of reduced graphene sheets, resulting in 2 ∼ 3D reduced graphene oxides, while oven-drying reduced graphene results in a flexible and self-supporting thin film. Reduced graphene exhibited low oxygen content (7 %), a low edge defect density and a high conductivity (2670 S cm−1).

Study on the Application of Nitrogen-Doped Holey Graphene in Supercapacitors with Organic Electrolyte.

We present a facile low-cost method to produce nitrogen-doped holey graphene (N-HGE) and its application to supercapacitors. A composite of N-HGE and activated carbon (AC) was used as the electrode active material in organic-electrolyte supercapacitors, and the performances were evaluated. Melamine was mixed into graphite oxide (GO) as the N source, and an ultra-rapid heating method was used to create numerous holes during the reduction process of GO. X-ray photoelectron spectra confirmed the successful doping with 2.9-4.5 at.% of nitrogen on all samples. Scanning electron micrographs and Raman spectra revealed that a higher heating rate resulted in more holes and defects on the reduced graphene sheets. An extra annealing step at 1000 °C for 1 h was carried out to further eliminate residual oxygen functional groups, which are undesirable in the organic electrolyte system. Compared to the low-heating-rate counterpart (N-GE-15), N-HGE boosted the specific capacity of the supercapacitor by 42 and 22% at current densities of 0.5 and 20 A/g, respectively. The effects of annealing time (0.5, 1, and 2 h) at 1000 °C were also studied. Longer annealing time resulted in higher capacitance values at all current densities due to the minimized oxygen content. Volumetric specific capacitances of 49 and 24 F/cm3 were achieved at current densities of 0.5 and 20 A/g, respectively. For the high-power-density operation at 31,000 W/kg (or 10,000 W/L), an energy density as high as 11 Wh/kg (or 3.5 Wh/L) was achieved. The results indicated that N-HGE not only improved the conductivity of the composite supercapacitors but also accelerated ion transport by way of shortened diffusion paths through the numerous holes all over the graphene sheets.

Effect of partially reduced fullerenol‐graphene hybrid nanofiller on photophysical and super capacitance properties of fluorescence conducting polymer nanocomposites

AbstractThe effect of partially reduced fullerenol‐graphene oxide (rFGO) nanofiller on photophysical and super capacitance properties of binary nanocomposites containing phenylene‐alt‐thiophene fluorescence conducting polymer (FCP) were investigated. Nanocomposites were prepared by solution mixing, which allows uniform nanofiller distribution into the polymer matrix. Microscopic analyses of nanocomposites display homogeneous dispersion of nanofiller into the polymer matrix. Quenching of photoluminescence reveals the photosensitized electron transfer from light‐absorbing FCP (acting as an electron donor) to fullerenol‐graphene nanofiller (acting as an electron acceptor). The presence of partially reduced graphene sheets in rFGO nanofiller facilitates the photosensitized electron transfer along the surface and also through the interlayer channels of graphene sheets due to the pillar effect of fullerenol. UV–vis and photoluminescence spectroscopic characterization data reveal excellent interaction between filler and matrix, responsible for electron transfer mechanism. Super capacitance properties of binary nanocomposites in three different electrolytes (acid, neutral and alkaline) were also investigated and the best galvanic charge discharge result of 173 F/g at 2 A/g current density was obtained at 2 wt% loading up to 500th cycle for rFGO in acid electrolyte only. The material was ineffective in the alkaline electrolyte and showed poor results in neutral electrolyte. An attempt has been made to evaluate the structure–property relationship of nanocomposites from their thermal, photophysical, and electrochemical properties.

Photocatalytic degradation of antibiotic drug and dye pollutants under visible-light irradiation by reduced graphene oxide decorated MoO3/TiO2 nanocomposite

This study explored the effect of reduced graphene oxide (RGO) amount (1, 3, and 5 wt%) loading on the MoO3/TiO2 nanocomposite (Gr1/MoO3/TiO2 NCs, Gr3/MoO3/TiO2 NCs, and Gr5/MoO3/TiO2 NCs) for their photocatalytic activity against the Tetracycline (TC) antibiotic drug and Rhodamine B (RhB) dye under visible light illumination and compared it with the MoO3/TiO2 nanocomposite. Crystallite sizes of MoO3/TiO2 NCs, Gr1/MoO3/TiO2 NCs, Gr3/MoO3/TiO2 NCs, and Gr5/MoO3/TiO2 NCs are 17.69 nm, 16.24 nm, 12.64 nm, and 11.74 nm, respectively, with energy band gaps of 2.72 eV, 2.59 eV, 2.49 eV, and 2.38 eV correspondingly. The photocatalytic performance of the nanocomposites was revealed to be highly affected by the weight ratio of RGO on MoO3–TiO2. The results showed that about 98% of RhB and 94% of TC drug were photodegraded under visible light irradiation during the 60 min and 80 min respectively. RhB and TC drug photocatalysis degradation showed pseudo-first-order reaction kinetics, with order rate constants of 0.06435 min−1 and 0.0351 min−1, respectively. By electrostatic interactions, the stacked MoO3 being bound to the surface of TiO2 nanoparticles and concurrently anchored onto graphene nanosheets through π–π stacking. In addition, the inclusion of reduced graphene oxide nanosheets was shown to enhance photocatalytic activity. The mechanism was also portrayed: the synergetic (interfacial) interaction of the graphene sheets and the MoO3/TiO2 was responsible for the excellent photo-degradation impact of the RhB dye and the TC drug pollutants through the Gr/MoO3/TiO2. Furthermore, the MoO3/TiO2 has been used as a migration vehicle for the visible light carrier, while a high surface area and the number of active sites of the reduced graphene sheet enhanced the photocatalytic activity.

Single-step synthesis of AgNPs@rGO composite by e-beam from DC-plasma for wound-healing band-aids

Currently, there is a huge demand for medical biomaterials aimed to improve antibacterial effect and promote wound healing. In this work, a facile and single-step approach of synthesizing silver nanoparticles (AgNPs) and reduced graphene oxide (rGO) composite material (namely, AgNPs@rGO) using high-voltage direct-current (DC) non-thermal plasma was proposed. Electron-beam (e-beam) from the DC-plasma discharge zone was implanted into the solution of AgNO3 and graphene oxide (GO), leading to the reduction of both GO and silver ions. As a result, AgNPs were formed and distributed homogeneously on the reduced graphene sheets. This composite material exhibits excellent antibacterial performance, being able to kill both Gram-positive bacteria Staphylococcus aureus (S. aureus) and Gram-negative bacteria Escherichia coli (E. coli) effectively. Furthermore, this composite material was utilized to prepare wound-healing band-aids. The in vitro assay on mice confirmed the satisfactory curing effect. The cytotoxicity for bio-compatibility and the mechanism for sterilization were also examined and clarified. As such, this work puts forward a facile and new route of preparing antibacterial biomaterial and demonstrates its promising prospect in wound-healing application.

Anchoring of graphitic carbon nitride on reduced graphene sheets by UV pulsed laser irradiation for augmented photoelectrochemical water splitting

Recently graphitic carbon nitride (GCN) has gained a great significance as it is one of the most suitable materials for large-scale photocatalytic water splitting because, it has a narrow band gap energy, photoelectrochemical stability, non-toxicity, and low-cost production. However, bare GCN is not reliable in terms of its relatively poor intrinsic photoelectrochemical performance as it possesses a low surface area, a high recombination rate of photo-induced charge carriers, and poor adhesion to the current collector. Here, we report a green, and low-cost approach to anchor GCN on reduced graphene oxide (rGO) sheets, forming rGO/GCN colloidal nanocomposite by simply irradiating a GO/GCN dispersion in methanol aqueous solution using a UV laser without further treatment. In contrast with the bare GCN, the synthesized rGO/GCN nanocomposite exhibited a higher tendency of visible light absorption (400-700 nm), a lower recombination rate of photo-induced charge carriers, excellent adhesion on the current collector, and thus higher photoelectrochemical efficiency for water splitting under visible light illumination. The photocurrent density of the rGO/GCN photoanode was enhanced by nine folds (~90 mA cm−2) in alkaline media over that one produced by the bare GCN under the same operating conditions. This study exhibits the feasibility and effectiveness of UV laser irradiation in the synthesis of high-purity rGO-based photocatalytic nanocomposites at room temperature for large-scale photoelectrochemical water splitting.

Discriminative intracellular and extracellular ATP detection based on magnetically controlled antimicrobial peptide

Extracellular free adenosine triphosphate (ATP) is often encountered as an interferent in the detection of intracellular ATP, especially in biofilm systems. To resolve this problem, in this study, a novel impedimetric aptasensor was designed for the discriminative detection of intracellular and extracellular ATP using antimicrobial peptide-functionalized magnetic particles (M-AMPs). Successive fabrication of reduced graphene sheets (rGSs) and gold nano-clusters (Au NCs) on a glassy carbon electrode was conducted by simple electrodeposition methods to achieve improved detection sensitivity. Single-stranded aptamers were attached to the surface of Au NCs for selective ATP binding, and electrochemical impedance spectroscopy was conducted to analyze the change of surface state induced by ATP binding. For discriminative intracellular and extracellular ATP detection, magnetically controlled M-AMPs were introduced into the detection system for mild and rapid bacterial disintegration. In bacterial samples, the M-AMPs were firstly constrained by a magnetic field, and extracellular ATP was selectively bound to the prepared aptasensor, leading to increased impedance response. Thereafter, the magnetic field was shut down to liberate the M-AMPs for cell disruption, and the released intracellular ATP was determined by the further impedance increasement. The results demonstrated that the proposed aptasensor exhibited excellent sensitivity and selectivity, with a wide linear range from 1 nM to 100 nM and a detection limit of 0.65 nM. Remarkably, this electrochemical aptasensor provided a convenient and label-free approach for discriminative intracellular and extracellular ATP detection, and the detection reliability was verified in various bacterial samples and E. coli biofilms.

Tuning the Oxygen Content of Reduced Graphene Oxide and Effects on Its Properties.

The need to recover the graphene properties in terms of electrical and thermal conductivity calls for the application of reduction processes leading to the removal of oxygen atoms from the graphene oxide sheet surface. The recombination of carbon–carbon double bonds causes a partial recovery of the original graphene properties mainly limited by the presence of residual oxygen atoms and lattice defects. However, the loss of polar oxygen-based functional groups renders the material dispersibility rather complicated. In addition, oxygen-containing functional groups are reaction sites useful to further bind active molecules to engineer the reduced graphene sheets. For these reasons, a variety of chemical processes are described in the literature to reduce the graphene oxide. However, it is greatly important to select a chemical process enabling a thin modulation of the residual oxygen content thus tuning the properties of the final product. In this work, we will present a chemical-processing technique based on the hydroiodic acid to carefully control the degree of residual oxidation. Graphene oxides were reduced using hydroiodic acid with concentrations from 0.06 to 0.95 mol L–1. Their properties were characterized in detail and tested, and the results showed that their oxygen content was finely tuned from 33.6 to 10.7 atom %. This allows carefully tailoring the material properties with respect to the desired application, which is exemplified by the variation of the bulk resistance from 92 Ω to 14.8 MΩ of the film from the obtained rGO.

Synthesis of silver nanoparticles/porphyrin/reduced graphene oxide hydrogel as dye adsorbent for wastewater treatment

Facile and versatile strategies for the preparation of reduced graphene oxide hydrogel (rGH) are of great interest due to its various applications in pollution adsorption, catalysis, gas sensors and tissue engineering. In this study, the preparation of a novel self-assembled reduced graphene oxide hydrogel modified with silver nanoparticles (AgNPs) and various porphyrin complexes using the green hydrothermal approach were reported. The AgNPs were successfully decorated on the reduced graphene sheets via electrostatic interactions. On the other hand, the interaction of porphyrin complex with the Ag/rGH is via esterification and π-π interactions. Based on thermogravimetric analysis and BET analysis, the intercalation of porphyrin complex improved the thermal stability and specific surface area (SSA) of the modified hydrogels. Further characterizations of the modified hydrogel were accomplished using Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectrometer (FT-IR), scanning electron microscope-energy dispersive spectroscopy (SEM/EDS), and UV–Vis spectrophotometry. The adsorption capacity of the modified hydrogels increased with the integration of AgNPs and porphyrin complexes due to the increase of the number of adsorption sites. The Ag/TPP/rGH exhibited the highest adsorption capacity (130.37 mg/g) towards the cationic dye, methylene blue (MB) which is attributed to the additional binding sites from the available unoccupied aromatic structures of the TPP. All prepared hydrogels kinetic reaction conformed to the pseudo second-order model with good stability for removing MB with efficiency above 90%.

Good dispersion of poly(δ-gluconolactone)-grafted graphene in poly(vinyl alcohol) for significantly enhanced mechanical strength

Here, we presented a straight forward and new approach for designing the interfacial compatibility between polyvinyl alcohol (PVA) and graphene sheets. The synthesis of poly(delta-gluconolactone) grafted from reduced graphene oxide (PGL-g-rGO) has been primarily developed by the ring-opening polymerization (ROP) of the green delta-gluconolactone monomer. The structure of PGL-g-rGO was fully confirmed using 1HNMR, XRD, FTIR, UV/Vis spectroscopy, TGA, and SEM. Poly(delta-gluconolactone) was expected to have a dual function of an efficacious modifier and a reducing agent for graphene oxide. Hence, the addition of PGL-g-rGO as a filler into PVA polymer was done into different mass ratios to obtain high-performance polymer nanocomposites. The dispersion and interfacial interaction of PGL-g-rGO sheets with the PVA matrix were studied. The thermal behavior, mechanical properties, and surface wettability of the final PVA nanocomposites were also evaluated. It can be observed that the char yield and the initial decomposition temperature of PVA were considerably enhanced after the addition of PGL-g-rGO to the polymer besides, the mechanical properties were totally improved. The use of PGL-g-rGO with only a mass ratio of 1.5% results in increasing the tensile strength and Young's modulus of PVA polymer by 124 and 121%, respectively. The elongation at break was uncommonly increased from 93 to 251% because of the formed strong hydrogen bonding between the filler and the polymer which can't only work on the efficacious convey of stress from PVA to reduced graphene sheets but also facilitate the polymer crystallization.

Synthesis of a Magnetic Fe3O4/RGO Composite for the Rapid Photo-Fenton Discoloration of Indigo Carmine Dye

In the last years, dye wastewater pollution became a major problem due to its toxicity and environmental restrictions. Indigo carmine is an indigoid dye that is largely used, and it is known by its toxicity and stability. In this work, we synthesized a magnetite/reduced graphene oxide (Fe3O4/RGO) composite with approximately 90%wt of Fe3O4 and used in the indigo carmine dye photo-Fenton discoloration (accompanied by UV–Vis spectrometry analysis). By using scanning electron microscopy (SEM), X-ray diffractometry (XRD), Laser Raman Spectroscopy (LRS), Magnetization Curves (VSM), and X-ray photoelectron spectroscopy (XPS) analyses it was observed that Fe3O4 particles of ~ 21 nm were successfully anchored on reduced graphene sheets. Zeta potential showed that the Fe3O4/RGO composite has a less negative electrostatic behavior in ultrapure water (− 46.9 mV) than the bare RGO (− 77.7 mV), promoting the attraction of indigo carmine molecule. Magnetic studies proved the ferromagnetic character of the composite with a saturation magnetization of 43 emu/g, thus promoting the easy separation from the reaction medium. In the first 5 min of reaction with the composite, the solution was completely discolored. The composite was recovered, and its reuse also showed total discoloration in the first 30 min of reaction. After first indigo carmine discoloration cycle, XPS results showed that Fe3+/Fe2+ atomic ratio was maintained, which can explain the total discoloration in the second cycle.

L-serine-assisted synthesis of Fe3O4 impregnated N-doped RGO composites via a facile electrostatic self-assembly for synergistic adsorption of Rhodamine B

In this work, Fe3O4 nanoparticles impregnated N-doped RGO (Fe3O4-N-RGO) nanocomposite was synthesized through one-pot electrostatic self-assembly of magnetite nanoparticles (Fe3O4 NPs) on N-doped RGO sheet in the presence of L-serine (L-Ser) as a surface functionalizing and reducing agent. The surface property, structure, morphology, and magnetic properties of as-prepared Fe3O4-N-RGO nanocomposite were characterized by FTIR spectroscopy, X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), vibrating sample magnetometer (VSM), and electron paramagnetic resonance spectroscopy (EPRS). The XRD results were indicated the formation of high crystalline spinel type Fe3O4 NPs. The Fe3O4 nanoparticles with average sizes of 12 nm were formed uniformly on reduced graphene sheets. The Fe3O4-N-RGO nanocomposite was used as an adsorbent to remove Rhodamine B dye from aqueous solution. The nanocomposite shows excellent ability to adsorb organic pollutants and can be easily manipulated in a magnetic field for separation. The results suggest that Fe3O4-N-RGO nanocomposite has great potential for efficient removal of organic dyes pollutants from aqueous solution.

A high flexibility all-solid contact sulfide selective electrode using a graphene transducer.

In this paper, a novel high flexibility all-solid contact ion selective electrode (ASC-ISE) based on reduced graphene sheets (RGSs) as the ion-to-electron transducer was developed for rapid detection of sulfide. A graphene layer was firstly electrodeposited on a flexible silver wire by direct reduction of graphene oxide, and nanostructured Ag2S was then prepared as the selective membrane by electrodeposition. Scanning electrochemical microscopy was performed for the characterization of the morphological properties of the RGSs and Ag2S membranes. The evaluation of the analytical performances, such as the linear range, selectivity, stability, and practical application, of the proposed ASC-ISEs for the rapid detection of sulfide was performed. The results showed that, the ASC-ISEs exhibited a linear relationship between the obtained potential signal and sulfide concentration in the range of 0.50 μM to 1.0 mM, with a detection limit of 0.18 μM. Moreover, the ASC-ISEs showed good selectivity towards sulfide over other common interfering ions, and maintained a stable electrochemical response over 7 days. These results demonstrated that graphene was a promising material as the ion-to-electron transducer layer in the development of ASC-ISEs for sulfide detection, and the results of practical applications in tap water and seawater samples showed that the ASC-ISEs held significant promise in a broad range of applications.

Ultra-Range Bimetallic Pt–Pd Nanospheres Deposited on Reduced Graphene Sheet as Efficient Electrocatalyst Towards Electrooxidation of Methanol

Direct methanol fuel cells (DMFCs) offers promising possibilities in meeting future energy needs in a most sustainable way. To electro oxidize methanol at anode of DMFCs, till date Pt and Pt-based materials were widely employed. To improve the utilization of precious metals it is highly desirable to disperse them on conductive carbon structures. Here, we synthesized ultra-range bimetallic platinum–palladium (Pt–Pd) nanospheres (NSs) with uniform dispersion on reduced graphene oxide (RGO) sheets by a simple hydrothermal method and studied their efficacies towards methanol oxidation reaction (MOR). From HR-TEM analysis, it was found that the synthesized bimetallic Pt–Pd NSs possess an average particles size of about 5 to 7 nm consistent with XRD data. The MOR electro catalytic activity of bimetallic PtPd-RGO NSs was higher compared to homemade mono metallic Pt-RGO, Pd-RGO catalysts and commercial E-tekPt/C.

Role of rGO to improve the performance of BiVO4 nanostructures for efficient removal of heavy metals

The fabrication of efficient visible light-driven photocatalysts has gained much attention for the removal of heavy metals from water to resolve the water crises of the world by utilizing visible light in the context to resolve energy issues. Reduced graphene oxide/bismuth vanadate nano-photocatalysts were prepared by facile hydrothermal method. Various characterization techniques, including X-ray diffractometer (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), UV–Vis absorption spectroscopy (UV–Vis), photoluminescence emission spectroscopy (Koci et al. Appl Catal B 96(3–4):239–244, 2010), photocatalytic removal of heavy metals were utilized to explore the morphology, crystal structure, elemental composition, optical properties, and photocatalytic activity measurement. XRD patterns showed the monoclinic scheelite and tetragonal scheelite structure of prepared composite, while the average size of particles calculated by diffraction peaks were ranging from 25.3 to 36.5 nm. SEM micrographs showed the spherical, oval, cuboid morphology of prepared composite with average size of particles ranging from 26 to 35 nm with best accord to XRD results. The reduction in size may be very beneficial for the transportation of electrons on reduced graphene sheet interstitial sites and take part in photocatalytic performance and enhanced the removal of heavy metals. UV–Vis spectra showed the absorption of prepared composite resulting in reduction of bandgap from 2.41 to 2.13 eV with the increasing content of graphene oxide in bismuth vanadate. PL emission spectra display the suppressed recombination of charge carriers, when incorporating with reduced graphene oxide. These results show the increase in the separated charge carriers and resulting in enhanced photocatalytic activity. As prepared rGO/BiVO4 composite exhibits the high photocatalytic removal of heavy metals, because of the extended visible light adsorption, high surface to volume ratio, and increased separated charge carriers of prepared composite. This research provides a different vision to construct promising photocatalysts for the removal of heavy metals from water.

Highly porous self-assembly of nitrogen-doped graphene quantum dots over reduced graphene sheets for photo-electrocatalytic electrode

Nitrogen-doped graphene quantum dots (NGQDs) are a diverse organic catalyst, competitive with other metallic catalysts due to their low cost, high stability, biocompatibility, and eco-friendliness. Highly functional multi-edge surfaces of NGQDs play a key role in imparting superb photocatalytic and electrocatalytic activity. However, when coating NGQDs by conventional techniques, such surfaces are not exposed for catalysis, due to the unwanted overlap of NGQDs sheets. To avoid this issue, here we propose a facile technique to orient NGQDs in a three-dimensional (3D) self-assembled foam-like structure, over reduced graphene oxide coated woven carbon fabric. This 3D assembled structure provides highly exposed active surfaces, which are readily available for catalytic reactions: however, in the conventional uniformly coated NGQDs layer, catalytic activity was limited by complex diffusion. The superb catalytic activity of the assembled NGQDs was utilized for the degradation of organic pollutant (methylene blue dye) from water. Additionally, the proposed electrode revealed much higher electrocatalytic activity than the rare Pt catalyst, owing to the easy diffusion of electrolyte and fast quenching of charges through the porous structure. The assembled NGQDs showed 50% higher photocatalytic degradation compared to uniformly coated NGQDs, which was further accelerated (50%) by application of the biased potential of 2 V; i.e. photo-electrocatalysis. The novel photo-electrocatalytic electrode offers high conductivity, stability, and flexibility, which make this complete carbon electrode highly attractive for other catalytic applications such as fuel cells, supercapacitors, and water splitting.

Highly Dispersed Graphene Network Achieved by using a Nanoparticle‐Crosslinked Polymer to Create a Sensitive Conductive Sensor

AbstractElectrically conductive hydrogels (ECHs) have attracted significant interest in the past, owing to their potential applications in flexible electronic devices. The incorporation of reduced graphene sheets (GSs) is one effective way to endow hydrogels with electrical conductivity. However, inhomogeneous distribution of GSs in a hydrogel matrix has a side effect on the conductivity for GS‐based ECHs. In this work, cement‐released calcium hydroxide nanospherulites (CNSs) are innovatively employed to help disperse GSs in a poly‐acrylamide (PAM) hydrogel matrix. An excellent ECH with good mechanical performance is achieved by dispersing GSs homogeneously into the PAM hydrogel matrix with the support of CNSs. Raman measurements confirm the vital role of CNSs in dispersing GSs homogeneously into the hydrogel matrix. With a very low GS content of 0.2 wt %, the electrical conductivity of such ECH sample can reach four times than that of reported GS‐based conductive hydrogels dispersed by conventional surfactants. Furthermore, the homogeneous distribution of GSs in the hydrogel matrix also promotes energy dissipation in the mechanical property, leading to a high compression stress (214 MPa) and stretching stress (325 KPa) for such ECHs. Owing to its rapid response speed and high stability, this ECH is used as a strain sensor to monitor deformation. In a broader context, this ECH material may have potential applications in smart sensors and wearable devices.

Graphene-Based Assemblies As Electrode Materials for Supercapacitors

Reduced graphene oxide (RGO) based electrochemical double-layer capacitors (EDLC), have been extensively studied as good power capability can be achieved but they suffer from low capacitances (~100-150 F/g), as the reduced graphene sheets partially restack through π- π interactions limiting the resulting adsorption active surface area.[1] To avoid this restacking different paths have been followed in the literature: using graphene aerogels or expanded graphene structures.[2] In this latter case, an intercalate or pillar is used to space out the graphene layers and recover active surface area. Different nature of pillars or intercalates such as carbon nanotubes, carbon blacks, diamine, aromatic, metallic ions or big macrocyles have been described.[2,3] The results, that will be presented, depict the researches devoted to the development and study of graphene based expanded assemblies designed to be tested as electrode materials for supercapacitors. The material development involves the bridging of graphene oxide derived sheets using alkyl diamine as linkers (Fig. 1a).[3] The production of graphene galleries of height matching that of different alkyl chain length pillars was evidenced, notably with XRD (Fig. 1b). The purpose of this design was to enhance the specific capacitance by limiting graphene sheet restacking and optimizing the inter-sheet distance (d-spacing) or pore size with respect to the ion size, as was previously evidenced with other carbon materials.[4] These materials (named RPs) have been tested electrochemically with various tetraalkylammonium tetrafluoroborate salts in acetonitrile. This approach resulted in adsorption active surface area tuning, yielding clear evidence of the electrolytic ions entering the inter-layer galleries. Indeed only ions with diameter smaller than the d-spacing access the inter-layer galleries.[5a] Ex-situ solid-state NMR analysis were performed to further demonstrate the ionic species adsorption at the material surface. Further optimization dealing with pillar amount or nature and material density tuning showed the high interest of this re-aggregation limitation method, as gravimetric and volumetric capacitances 4 times higher than that of RGO (230 F/g and 210 F/cm3 respectively) have been achieved (Fig. 1c).[5b] Figure 1: a) Scheme differentiating reduced graphene oxide RGO and pillared graphene structures; b) XRD diffractograms obtained for pillared graphene prepared diamines with different chain lengths (5, 6 and 8 C atoms); c) Volumetric capacitance and power capability tests obtained on a pillared material compared to RGO.

Activated charcoal and reduced graphene sheets composite structure for highly electro-catalytically active counter electrode material and water treatment

In quest of finding a sustainable solution for metal-free counter electrode materials, reduced graphene oxide (rGO) is emerged as the best alternative due to its intrinsic high electrocatalytic activity. However, owing to its two-dimensional sheets like structure, there is re-stacking of rGO sheets, which reduces exposed surface area and hinders in electrolyte diffusion. To avoid these issues, activated charcoal (AC) is explored as an active spacer material between rGO sheets, for the first time. By loading an optimum concentration of AC in rGO, a high porosity, high conductivity, and high concentration of active sites were gathered in the single composite structure. Such synchronized features of the proposed composite were utilized for Pt-free counter electrode application in dye-sensitized solar cell (DSSc). The composite structure showed high electrocatalytic activity, with a low charge transfer resistance of 0.7 Ω, which is far lower than Pt and rGO (8.5 Ω and 7.5 Ω). The DSSc fabricated with optimized composite showed power conversion efficiency of 8.6%, compared to Pt-based DSSc with 7.9% efficiency. Additionally, the potential of the electrode was also tested for the electro-photocatalytic (99%) degradation of methylene blue dye from water, in 60 min. The proposed highly efficient nanocomposite structure possesses the highest efficiency as compared to other previously studied rGO based counter-electrodes.

High-efficiency layered sulfur-doped reduced graphene oxide and carbon nanotube composite counter electrode for quantum dot sensitized solar cells

A high-efficiency layered sulfur-doped reduced graphene oxide and carbon nanotube composite counter electrode for quantum dot sensitized solar cells is prepared via ultrasonic dispersion followed with simultaneous reduction and doping by hydrothermal using Na2S as a reductant and sulfur source. Scanning and Transmission electron microscopy show the layered composite with carbon nanotube-bridges between reduced graphene sheets. An increase in sp2C/sp3C ratio of composite is revealed for the first time by the analysis of X-ray photoelectron spectra. Electrochemical tests show that the charge transfer resistance is 1.76Ω. The power conversion efficiency of 5.70% is obtained for the quantum dot sensitized solar cell based the composite counter electrode, and is 18.26%, 76.47%, and 126.19% higher than those of unsulfured composite, reduced graphene, and carbon nanotube counter electrodes, respectively. This is mainly due to the improvement of morphological and property of sulfur-doped reduced graphene and carbon nanotube composite, which presents a high specific surface area, high conductivity, providing abundant catalytic active sites and fast electron transport channels for Sn2− reduction.