Defective Graphene Research Articles

Self-standing graphitized hybrid Nanocarbon electrodes towards high-frequency supercapacitors

Carbon materials are considered as the ideal electrode materials for supercapacitors due to their diverse structure and nature. However, their poor frequency response is an obstacle to their application in high-frequency supercapacitors. Herein, an ultra-high temperature graphitization process at 2800 °C is proposed to fabricate carbon nanotubes/graphene hybrid films that are successfully employed as the electrode materials of high-frequency supercapacitors. By rational hybridization, the carbon nanotubes/graphene interlinked networks offer fast ion transport paths. Importantly, via a graphitization process at 2800 °C, the as-obtained hybrid films exhibit an ultrahigh in-plane conductivity of 491.81 S cm−1 and favorable out-plane conductivity of 27.98 mS cm−1. Such design brings the as-constructed high-frequency supercapacitors an unprecedented phase angle (up to −56.23°) and area capacitance (up to 230.56 μF cm−2) at 120 Hz, and their cut-off frequency can be nearly 30 times higher than that of films carbonization at 1600 °C. Such increases, further supported by density functional theory (DFT) calculations, are partly attributable to enhancements of ion response arising from the repair of edge defects of graphene. These findings will provide a new method in designing the structure of carbon electrodes for enhancing SCs frequency response performances.

Selectively Defect‐Healed Graphene Electrodes for Tungsten Diselenide Thin‐Film Transistors

Abstract2D nanomaterials such as graphene and transition metal dichalcogenides have attracted great interest as future electronic materials, especially for application in next‐generation displays, owing to their extraordinary electrical, mechanical, and optical properties. In order to achieve 2D material‐based practical devices, it is essential to heal the graphene defects which are inevitably generated during chemical vapor deposition‐based large‐area synthesis and transfer process, through doping technology. In this article, a novel approach for selective defect‐healing of graphene with electrodeposited gold nanoparticles is proposed, where the defect‐healed graphene source/drain electrodes are integrated with p‐type tungsten diselenide (WSe2) thin‐film transistors (TFTs), for the first time. This proposed device shows greatly enhanced electrical characteristics (increase of carrier mobility about ≈230.8%) by selective defect‐healing of graphene, and the performance improvement mechanism is systematically investigated in terms of conductivity and Schottky barrier engineering of graphene/WSe2 interface. Also, a long‐term stability of defect‐healed graphene electrode is achieved over a long period of a month, which is enabled by a polymer‐based passivation layer that maintains the doping effects of defect‐engineered graphene. The authors’ findings therefore provide a new strategy for developing graphene electrode‐based high‐performance TFTs, and reveal the enormous potential of graphene as an innovative conductor for prospective displays.

Successive Free-Radical C(sp 2 )–C(sp 2 ) Coupling Reactions to Form Graphene

Successive Free-Radical C(sp ² )–C(sp ² ) Coupling Reactions to Form Graphene

Fabrication of defective graphene oxide for efficient hydrogen production and enhanced 4-nitro-phenol reduction

Hydrogen has been considered as one of the most promising alternative energy source to solve the future energy demands due to its high energy capacity and emission-free character. The generation of hydrogen from non-fossil sources is necessary for the sustainable development of human life on this planet. The hydrolysis of sodium borohydride can quickly produce a large amount of hydrogen in situ and on-demand in the presence of the catalyst, which can be used as an alternative energy source. So, it is crucial to fabricate the highly efficient, robust, and economical catalyst for the production of hydrogen via hydrolysis of sodium borohydride. Herein, a facile and efficient approach for the synthesis of metal-functionalized reduced graphene oxide for the production of hydrogen at room temperature was used. Moreover, the synthesized catalyst has also been tested in the field of environmental catalysis for the reduction of toxic 4-nitrophenol to valuable 4-aminophenol in the presence of sodium borohydride. The enhanced activity of prepared metal-functionalized reduced graphene oxide is ascribed to a strong affinity between Fe–N X and reduced graphene oxide which facilitates electron transfer as well as synergistic effect. Overall, this work presents a crucial procedure for green chemistry reactions when a carbonaceous material is selected as a catalyst.

Morphological Analysis of the Rainbow Patterns Created by Point Defects of Graphene

Morphological Analysis of the Rainbow Patterns Created by Point Defects of Graphene

Scalable Graphene Defect Prediction Using Transferable Learning.

Notably known for its extraordinary thermal and mechanical properties, graphene is a favorable building block in various cutting-edge technologies such as flexible electronics and supercapacitors. However, the almost inevitable existence of defects severely compromises the properties of graphene, and defect prediction is a difficult, yet important, task. Emerging machine learning approaches offer opportunities to predict target properties such as defect distribution by exploiting readily available data, without incurring much experimental cost. Most previous machine learning techniques require the size of training data and predicted material systems of interest to be identical. This limits their broader application, because in practice a newly encountered material system may have a different size compared with the previously observed ones. In this paper, we develop a transferable learning approach for graphene defect prediction, which can be used on graphene with various sizes or shapes not seen in the training data. The proposed approach employs logistic regression and utilizes data on local vibrational energy distributions of small graphene from molecular dynamics simulations, in the hopes that vibrational energy distributions can reflect local structural anomalies. The results show that our machine learning model, trained only with data on smaller graphene, can achieve up to 80% prediction accuracy of defects in larger graphene under different practical metrics. The present research sheds light on scalable graphene defect prediction and opens doors for data-driven defect detection for a broad range of two-dimensional materials.

Bottom-up synthesis of graphene films hosting atom-thick molecular-sieving apertures

Incorporation of a high density of molecular-sieving nanopores in the graphene lattice by the bottom-up synthesis is highly attractive for high-performance membranes. Herein, we achieve this by a controlled synthesis of nanocrystalline graphene where incomplete growth of a few nanometer-sized, misoriented grains generates molecular-sized pores in the lattice. The density of pores is comparable to that obtained by the state-of-the-art postsynthetic etching (1012 cm-2) and is up to two orders of magnitude higher than that of molecular-sieving intrinsic vacancy defects in single-layer graphene (SLG) prepared by chemical vapor deposition. The porous nanocrystalline graphene (PNG) films are synthesized by precipitation of C dissolved in the Ni matrix where the C concentration is regulated by controlled pyrolysis of precursors (polymers and/or sugar). The PNG film is made of few-layered graphene except near the grain edge where the grains taper down to a single layer and eventually terminate into vacancy defects at a node where three or more grains meet. This unique nanostructure is highly attractive for the membranes because the layered domains improve the mechanical robustness of the film while the atom-thick molecular-sized apertures allow the realization of large gas transport. The combination of gas permeance and gas pair selectivity is comparable to that from the nanoporous SLG membranes prepared by state-of-the-art postsynthetic lattice etching. Overall, the method reported here improves the scale-up potential of graphene membranes by cutting down the processing steps.

Defective graphene nanosheets for drinking water purification: Adsorption mechanism, performance, and recovery

Defect-rich graphene oxide (dGO) was used as sorbent for organic contaminants of emerging concern in tap water, including drugs and dyes, and the performance compared to those of lower-defects graphene types. The role of holes and carbonyl- carboxylic groups on graphene nanosheets surface on the adsorption mechanism and efficiency was investigated. dGO showed enhanced adsorption capacity toward two fluoroquinolone antibiotics (ofloxacin, OFLOX, and ciprofloxacin, CIPRO) in tap water with a maximum capacity of 650 mg/g, compared to 204 mg/g for Hummers derived commercial GO (hGO) and 125 mg/g for less defected Brodie derived GO (bGO) for OFLOX. The role of defects on the selective adsorption of OFLOX was also modelled by MD simulations, highlighting a mechanism mainly driven by the shape complementarity between the graphene holes and the molecules. Adsorption isotherms revealed different adsorption model for dGO, with a Langmuir fitting for dGO and BET fitting for all the other investigated samples. The maximum adsorption capacity of dGO for OFLOX was about six times higher than that of Granular Activated Carbon (95 mg/g), the industrial adsorption standard technology. Finally, it was also demonstrated that dGO can be recovered from treated water by ultrafiltration, this preventing secondary contamination risks and enabling safe use of graphene nanosheets for water purification.

Impact of a Graphene Oxide Reducing Agent on a Semi-Permeable Graphene/Reduced Graphene Oxide Forward Osmosis Membrane Filtration Efficiency.

Graphene has been considered as a material that may overcome the limitations of polymer semi-permeable membranes in water treatment technology. However, monolayer graphene still suffers from defects that cause leakage. Here, we report a method of sealing defects in graphene transferred onto porous polymer substrate via reduced graphene oxide (rGO). The influence of various reducing agents (e.g., vitamin C, hydrazine) on the properties of rGO was investigated by SEM, Raman, FTIR, and XRD. Subsequently, membranes based on graphene/reduced graphene oxide were tested in a forward osmosis system using sodium chloride (NaCl). The effect of the effectiveness of the reduction of graphene oxide, the type and number of attached groups, the change in the distance between the rGO flakes, and the structure of this material were examined in terms of filtration efficiency. As a result, semi-permeable centimetre-scale membranes with ion blocking efficiency of up to 90% and water flux of 20 mL h−1 m−2 bar−1 were proposed.

The Catalysis Effect of Na and Point Defect on NO Heterogeneous Adsorption on Carbon during High-Sodium Zhundong Coal Reburning: Structures, Interactions and Thermodynamic Characteristics

The reburning process in a furnace, a key way to reduce NOx emissions, is a heterogeneous reaction during coal combustion, in which the heterogeneous adsorption is dominant. Zhundong coal with a high content of alkali metal can enhance the reburning process. In this paper, the influence of sodium and a defect on NO heterogeneous adsorption was studied by the density functional theory, and the thermodynamic characteristic was also analyzed. The results indicate that the binding energy for NO adsorption on the pristine graphene surface (graphene-NO), Na-decorated pristine graphene surface (graphene-Na-NO), defect graphene surface (gsv-NO) and Na-decorated defect graphene (gsv-Na-NO) is −5.86, −137.12, −48.94 and −74.85 kJ/mol, respectively, and that the heterogeneous adsorption is an exothermic reaction. Furthermore, except for covalent bonds of C and N, C and O for gsv-NO, other interactions are a closed-shell one, based on the analysis of AIM, ELF and IGM. The area of electron localization for NO is graphene-Na-NO > gsv-Na-NO > gsv-NO > graphene-NO. The dispersion interaction is the main interaction force between NO and the pristine graphene surface. The δg index for the atom pairs about N–C and O–C on the pristine graphene surface is also the smallest. The density of spikes at graphene-Na-NO is bigger than that at gsv-Na-NO. Moreover, the thermodynamics characteristic showed that the reaction equilibrium constant of graphene-NO is less than those on the other surfaces under the same temperature. Thus, NO on the pristine graphene surface is the most difficult to adsorb, but the presence of sodium and a defect structure can promote its adsorption.

Transition state and formation process of Stone–Wales defects in graphene

Stone–Wales (SW) defects are possibly formed in graphene and other two-dimensional materials, and have multiple influence on their physical and chemical properties. In this study, the transition state of SW defects in graphene is determined with the fully discrete Peierls theory. Furthermore, the atomic formation process is investigated by means of ab-initio simulations. The atomic structure change and energetics of the SW transformation are revealed. It is found that the transition state is at the SW bond rotation of 34.5° and the activation energy barrier is about 12 eV. This work provides a new method to investigate SW transformations in graphene-like materials and to explore unknown SW-type defects in other 2D materials.

Unraveling electrochemical oxygen reduction mechanism on single‐atom catalysts by a computational investigation

The extension of low-expense and stable electrocatalysts for oxygen reduction reaction (ORR) is of importance to improve the performance of metal-air batteries and fuel cells. In this work, we unravel the ORR electrocatalytical mechanisms on a series of transition metal atom-doped on the defect graphene (TM@sv-graphene) by carrying out spin-polarized first-principles computations. The formation energy of the TM@sv-graphene, which is related to its stability, along with the binding energy of O2, OOH, 2OH, OH, and O species are evaluated. The calculations of Gibbs free energy show that two different pathways, O* mechanism and 2OH* mechanism, of ORR compete with each other based on the structures of intermediates (OOH, 2OH, OH and O). It is found that Co, Rh, Ir, Pt, and Mn@sv-graphene favor the O* mechanism while Ag, Au, Cd, Cu, Zn, Fe, Ni, Pd, Sc, Cr, Ti, Zr, and V@sv-graphene strongly promote the 2OH* mechanism. The overpotential (η) of the ORR is predicted and can be a valid descriptor that manifests the catalytic activity of different TM@sv-graphene. Among these catalysts, the Pd@sv-graphene (η = 1.20 V) exhibits the best activity for the ORR. The results provide new insight into electrochemical mechanism of ORR for novel single-atom electrocatalyst.

Quantifying Mn Diffusion through Transferred versus Directly Grown Graphene Barriers.

We quantify the mechanisms for manganese (Mn) diffusion through graphene in Mn/graphene/Ge (001) and Mn/graphene/GaAs (001) heterostructures for samples prepared by graphene layer transfer versus graphene growth directly on the semiconductor substrate. These heterostructures are important for applications in spintronics; however, challenges in synthesizing graphene directly on technologically important substrates such as GaAs necessitate layer transfer and annealing steps, which introduce defects into the graphene. In situ photoemission spectroscopy measurements reveal that Mn diffusion through graphene grown directly on a Ge (001) substrate is 1000 times lower than Mn diffusion into samples without graphene (Dgr,direct ∼ 4 × 10-18 cm2/s, Dno-gr ∼ 5 × 10-15 cm2/s at 500 °C). Transferred graphene on Ge suppresses the Mn in Ge diffusion by a factor of 10 compared to no graphene (Dgr,transfer ∼ 4 × 10-16 cm2/s). For both transferred and directly grown graphene, the low activation energy (Ea ∼ 0.1-0.5 eV) suggests that Mn diffusion through graphene occurs primarily at graphene defects. This is further confirmed as the diffusivity prefactor, D0, scales with the defect density of the graphene sheet. Similar diffusion barrier performance is found on GaAs substrates; however, it is not currently possible to grow graphene directly on GaAs. Our results highlight the importance of developing graphene growth directly on functional substrates to avoid the damage induced by layer transfer and annealing.

Multi-Scale Structure-Mechanical Property Relations of Graphene-Based Layer Materials.

Pristine graphene is one of the strongest materials known in the world, and may play important roles in structural and functional materials. In order to utilize the extraordinary mechanical properties in practical engineering structures, graphene should be assembled into macroscopic structures such as graphene-based papers, fibers, foams, etc. However, the mechanical properties of graphene-based materials such as Young’s modulus and strength are 1–2 orders lower than those of pristine monolayer graphene. Many efforts have been made to unveil the multi-scale structure–property relations of graphene-based materials with hierarchical structures spanning the nanoscale to macroscale, and significant achievements have been obtained to improve the mechanical performance of graphene-based materials through composition and structure optimization across multi-scale. This review aims at summarizing the currently theoretical, simulation, and experimental efforts devoted to the multi-scale structure–property relation of graphene-based layer materials including defective monolayer graphene, nacre-like and laminar nanostructures of multilayer graphene, graphene-based papers, fibers, aerogels, and graphene/polymer composites. The mechanisms of mechanical property degradation across the multi-scale are discussed, based on which some multi-scale optimization strategies are presented to further improve the mechanical properties of graphene-based layer materials. We expect that this review can provide useful insights into the continuous improvement of mechanical properties of graphene-based layer materials.

Electronic structural descriptors for hydrogen evolution and superior catalytic activity of graphene based structures

Defect and substrate introduced into catalyst are two feasible routes toward design of heterogeneous catalysts. It is vital to identify and understand the relationships among atomic, electronic structures and adsorbate binding ability of the catalytic surfaces. Herein, hydrogen evolution on different defect graphene with and without two-dimensional (2D) Mo2C substrate are selected as examples to explore the relationships. Three feasible electronic structural descriptors, including p and pz band centers of local atoms, electron transfer to local atoms, and deformation charge densities of composite structures, are exacted from electronic properties of the defect and substrate-supported graphene. It is found that those descriptors could predict the hydrogen binding energy quantitatively, and the hydrogen adsorption order qualitatively. The descriptor of local p and pz band centers originate from hybridization between the site atom and adsorbate. It is believed that the relationship of atomic structure, electronic structure and binding energy may be applied to other surfaces, and shed light on the nature origin of the structure–activity on electrochemistry. By tuning the descriptors by atomic structure of defect or substrate, suitable hydrogen binding ability and superior hydrogen evolution performance of graphene could be achieved. Several structures based on graphene own superior hydrogen evolution activity: the exchange current densities of S3NV1-G, S2NV2-G@Mo2C, N1-G@Mo2C, and N2-G@Mo2C are predicted to be 0.811, 0.963, 0.712, and 1.860 mA/cm2, respectively. Especially the last two ones, their negative interfacial binding energies, and the energy favorability of forming N1 and N2 defects, ensure their stabilities and easy syntheses in experiment, and enhance their potential applications.

Application of Molecular Vapour Deposited Al2O3 for Graphene-Based Biosensor Passivation and Improvements in Graphene Device Homogeneity.

Graphene-based point-of-care (PoC) and chemical sensors can be fabricated using photolithographic processes at wafer-scale. However, these approaches are known to leave polymer residues on the graphene surface, which are difficult to remove completely. In addition, graphene growth and transfer processes can introduce defects into the graphene layer. Both defects and resist contamination can affect the homogeneity of graphene-based PoC sensors, leading to inconsistent device performance and unreliable sensing. Sensor reliability is also affected by the harsh chemical environments used for chemical functionalisation of graphene PoC sensors, which can degrade parts of the sensor device. Therefore, a reliable, wafer-scale method of passivation, which isolates the graphene from the rest of the device, protecting the less robust device features from any aggressive chemicals, must be devised. This work covers the application of molecular vapour deposition technology to create a dielectric passivation film that protects graphene-based biosensing devices from harsh chemicals. We utilise a previously reported “healing effect” of Al2O3 on graphene to reduce photoresist residue from the graphene surface and reduce the prevalence of graphene defects to improve graphene device homogeneity. The improvement in device consistency allows for more reliable, homogeneous graphene devices, that can be fabricated at wafer-scale for sensing and biosensing applications.

A modified nudged elastic band algorithm with adaptive spring lengths.

We present a modified version of the nudged elastic band (NEB) algorithm to find minimum energy paths connecting two known configurations. We show that replacing the harmonic band-energy term with a discretized version of the Onsager-Machlup action leads to a NEB algorithm with adaptive spring lengths that automatically increase the resolution of the minimum energy path around the saddle point of the potential energy surface. The method has the same computational cost per optimization step of the standard NEB algorithm and does not introduce additional parameters. We present applications to the isomerization of alanine dipeptide, the elimination of hydrogen from ethane, and the healing of a 5-77-5 defect in graphene.

Electrocatalysis under Cover: Enhanced Hydrogen Evolution via Defective Graphene-Covered Pt(111)

The production of hydrogen via water electrolysis using renewable electricity is a promising carbon-neutral technology. In this contribution, we report insights into the hydrogen evolution reaction (HER) in H2SO4 on Pt(111) and graphene-covered Pt(111), in addition to the electrochemical properties of graphene overlayers. As-prepared graphene overlayers are selectively permeable to H+ ions in the electrolyte, allowing H+ ions into the confined layer between graphene and Pt(111) while excluding SO42– and other anions. We demonstrate that defects in these as-prepared graphene overlayers can be generated from oxidation at high overpotentials or reduction from the production of H2 bubbles and postulate that HER occurs locally at only Pt(111) in the proximity of defects in graphene overlayers on as-prepared G/Pt(111) electrodes, and as defects in graphene increases, more of the Pt(111) surface becomes utilized for HER. Kinetically, the addition of defective graphene overlayers can increase the geometric HER rate by up to 200%, while Tafel slopes and [H+] reaction orders remain unchanged. These results shed kinetic insight into the nature of graphene overlayers and their effect on HER catalysis and also demonstrate the promise of confinement modifications in designing catalysts with properties closer to achieving optimum rates.

Coat multi-layered graphene on Cu with radiofrequency plasmas for anti-oxidization in hot air

Direct and fast synthesis of protective coating on metal surfaces is a major challenge in anti-corrosion applications of 2D graphene. In this article, we provided an approach to this challenge by using radiofrequency plasmas, where the mixture of CH4/H2 or pure CH4 gases can be decomposed under low temperature while the temperature of the Cu foil substrates can still be kept high by the induced eddy current. The high substrate temperature is essential for enhancing graphene quality, but many other plasma synthesis techniques seldom addressed this point. Graphene coatings with thicknesses of about 30 nm could be obtained in 3.5 min. Anti-corrosion tests examine protection coatings under various conditions. In this article, we mainly checked the protection performance of the graphene coatings against Cu oxidization in air. Anti-oxidization tests under 300 °C for 360 min proved that the coatings synthesized with pure CH4 performed better over those synthesized with mixture of CH4/H2. Tests under 200 °C for 900 min and 22 °C for 111 days demonstrated that the coatings synthesized with different gas compositions had equivalent anti-oxidization capabilities. Our calculations with density functional theory revealed that the defective single-layered graphene was unable to protect the Cu substrate, and it would be better to use multi-layered graphene in practice. The synthesis technique in this article could also be used in many other areas beyond anti-oxidization such as chemical reaction catalysis, electrode materials, electromagnetic interference shielding and so on.

Ferroelectric Controlled Gas Adsorption in Doped Graphene/In2Se3Heterostructure

AbstractTwo‐dimensional materials are excellent candidates for effective gas detection due to the large surface‐volume ratio, however the controllability to adsorb/desorb the gas molecules for recycling use is still a big challenge. In this study, different from previous strategies to modulate gas adsorption behavior via strain and external electric field, a novel approach to achieve gas adsorption control via ferroelectric (FE) switching is proposed. From first principle simulations, it is found that gas molecule adsorptions on Fe and Mn doped defective graphene can be well controlled when it is placed on the surface of FE In2Se3. The adsorption energies and charge transfer can be significantly modulated when the polarization is reversed, due to the polarization dependent electron redistribution and band state shifts near the Fermi level. The hypothesis of the FE controlled gas adsorption is further supported by the adsorption variations under the electric field. These findings provide feasible approaches and design principles for the next generation gas sensors.