The edge- and basal-plane-specific electrochemistry of a single-layer graphene sheet

As global energy consumption accelerates at an alarming rate, the development of clean and renewable energy conversion and storage systems has become more important than ever. Although the efficiency of energy conversion and storage devices depends on a variety of factors, their overall performance strongly relies on the structure and properties of the component materials. Nanotechnology has opened up new frontiers in materials science and engineering to meet this challenge by creating new materials, particularly carbon nanomaterials, for efficient energy conversion and storage. As a building block for carbon materials of all other dimensionalities (such as 0D buckyball, 1D nanotube, 3D graphite), the two-dimensional (2D) single atomic carbon sheet of graphene has emerged as an attractive candidate for energy applications due to its unique structure and properties. Like other materials, however, a graphene-based material that possesses desirable bulk properties rarely features the surface characteristics required for certain specific applications. Therefore, surface functionalization is essential, and researchers have devised various covalent and noncovalent chemistries for making graphene materials with the bulk and surface properties needed for efficient energy conversion and storage. In this Account, I summarize some of our new ideas and strategies for the controlled functionalization of graphene for the development of efficient energy conversion and storage devices, such as solar cells, fuel cells, supercapacitors, and batteries. The dangling bonds at the edge of graphene can be used for the covalent attachment of various chemical moieties while the graphene basal plane can be modified via either covalent or noncovalent functionalization. The asymmetric functionalization of the two opposite surfaces of individual graphene sheets with different moieties can lead to the self-assembly of graphene sheets into hierarchically structured materials. Judicious application of these site-selective reactions to graphene sheets has opened up a rich field of graphene-based energy materials with enhanced performance in energy conversion and storage. These results reveal the versatility of surface functionalization for making sophisticated graphene materials for energy applications. Even though many covalent and noncovalent functionalization methods have already been reported, vast opportunities remain for developing novel graphene materials for highly efficient energy conversion and storage systems.

Superior catalytic activity and high chemical stability of inexpensive electrocatalysts for the oxygen reduction reaction (ORR) are crucial to the large-scale practical application of fuel cells. The nonprecious metal/N modified graphene electrocatalysts are regarded as one of potential candidates, and the further enhancement of their catalytic activity depends on improving active reaction sites at not only graphene edges but also its basal plane. Herein, the ORR mechanism and reaction pathways of Co-N co-doping onto the graphene basal plane have been studied by using first-principles calculations and ab initio molecular dynamics simulations. Compared to singly N-doped and Co-doped graphenes, the Co-N co-doped graphene surface exhibits superior ORR activity and the selectivity toward a four-electron reduction pathway. The result originates from catalytic sites of the graphene surface being modified by the hybridization between Co 3d states and N 2p states, resulting in the catalyst with a moderate binding ability to oxygenated intermediates. Hence, introducing the Co-N4 complex onto the graphene basal plane facilitates the activation of O2 dissociation and the desorption of H2O during the ORR, which is responsible for the electrocatalyst with a smaller ORR overpotential (∼1.0 eV) that is lower than that of Co-doped graphene by 0.93 eV. Our results suggest that the Co-N co-doped graphene is able to compete against platinum-based electrocatalysts, and the greater efficient electrocatalysts can be realized by carefully optimizing the coupling between transition metal and nonmetallic dopants in the graphene basal plane.

Ultrasensitive quantum capacitance detector at the edge of graphene

Site-selective immobilization of gold nanoparticles on graphene sheets and its electrochemical properties

Synthesis of platinum/graphene composites by a polyol method: The role of graphite oxide precursor surface chemistry

Electrochemical studies of two-atom thick graphene sheet have attracted significant attention from the research community due to the attractive properties of its basal and edge surfaces. The graphene edge surface contains dangling bonds, unsaturated valencies, and can favorably attach to a plethora of functional groups. It has been demonstrated that the graphene edge surface has several orders of magnitude higher specific capacitance, rapid electron transfer rates and much stronger electrocatalytic activity than those of graphene basal surfaces. The graphene basal plane has been known to possess a high conductivity. Coupling these properties of the graphene basal and edge surfaces presents graphene or high quality single crystal graphite as potential electronic material. But the utilization of this versatile material in aquatic conditions is still limited due to unknown key properties of the basal and edge surfaces such as interaction of these surfaces with water and the isoelectric point. In this paper, we have tried to throw some light on the wettability characteristics of graphite basal and edge surfaces using carbon nanoparticles (fullerene and fullerol) and other characterization techniques. Highly oriented pyrolytic graphite (HOPG) is used as a model surface for fundamental studies of the wetting characteristics of graphite surfaces. HOPG is a highly anisotropic material, exhibiting different properties on its face and edge surfaces. The essential aim of this work was to provide surface chemistry information for HOPG surfaces. Fundamental wetting studies were accomplished through contact angle measurements of the as-received HOPG single crystal, which was characterized by its distinct face and edge surfaces. To study the effect of functional groups, contact angle measurements were repeated on the HOPG face and edge surfaces oxidized with different concentrations of hydrogen peroxide. Further, Raman spectroscopic studies were performed on the as-received and oxidized HOPG basal and edge surfaces. Atomic force microscopy (AFM) was also used a tool to image the HOPG surfaces and for surface force measurements to determine the iso-electric point (IEP) of the face and edge surface of HOPG. Zeta potential measurements of graphite powder in solutions of varying pH were also done to determine its iso-electric point and compare to the values reported in literature for graphitic carbon materials. Further AFM was used to study the interaction of the fullerene and fullerol nanoparticles on the graphite face and edge surfaces. The zeta potential measurements of the above mentioned carbon nanoparticles were also carried out to confirm that the interaction was due to hydrophobic or hydrophilic forces and not due to electrostatic interactions. In addition molecular dynamics simulations were also carried out to verify the interactions of the carbon nanoparticles with the graphite surfaces. Results indicate interesting charge properties of the graphite basal and edge surfaces and that the basal surface is highly hydrophobic whereas the edge is less hydrophobic (not completely hydrophilic), which makes it a promising electronic material for aquatic use.

Electro-redox-induced heterogeneous fluorescence of an individual single-layer graphene sheet was observed in real time by a total internal reflection fluorescence microscope. It was found that the fluorescence intensity of an individual sheet can be tuned reversibly by applying periodic voltages to control the redox degree of graphene sheets. Accordingly, the oxidation and reduction kinetics of an individual single-layer graphene sheet was studied at different voltages. The electro-redox-induced reversible variation of fluorescence intensity of individual sheets indicates a reversible band gap tuning strategy. Furthermore, correlation analysis of redox rate constants on individual graphene sheets revealed a redox-induced spatiotemporal heterogeneity or dynamics of graphene sheets. The observed controllable redox kinetics can rationally guide the precise band gap tuning of individual graphene sheets and then help their extensive applications in optoelectronics and devices for renewable energy.

There is enormous interest in the investigation of electron transfer rates at the edges of graphene due to possible energy storage and sensing applications. While electrochemistry at the edges and the basal plane of graphene has been studied in the past, the new frontier is the electrochemistry of folded graphene edges. Here we describe the electrochemistry of folded graphene edges and compare it to that of open graphene edges. The materials were characterized in detail by high-resolution transmission electron microscopy, Raman spectroscopy, high-resolution X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy and cyclic voltammetry. We found that the heterogeneous electron transfer rate is significantly lower on folded graphene edges compared to open edge sites for ferro/ferricyanide, and that electrochemical properties of open edges offer lower potential detection of biomarkers than the folded ones. It is apparent, therefore, that for sensing and biosensing applications the folded edges are less active than open edges, which should then be preferred for such applications. As folded edges are the product of thermal treatment of multilayer graphene, such thermal procedures should be avoided when fabricating graphene for electrochemical applications.

Graphene sheets have envisioned extensive applications in super-computer, novel materials, nano-electronics and sensors etc., due to their excellent two dimensional mechanical, structural, thermal and electrical properties. The graphene sheets are usually prepared by chemical reduction of graphite oxide platelets during which the graphene sheets become more and more hydrophobic, leading to irreversible aggregation via van der Waals interactions. Li et al. have successfully synthesized stable aqueous dispersions of graphene nanosheets via electrostatic stabilization. Graphene sheets can also bemodified with polymers via several covalent linkages such as amides or esters. Ye and coworkers prepared amphiphilic graphene nanoplatelets using in situ free radical polymerization method. However, these covalent modification methods will destroy the conjugated structure of graphene, leading to significant compromization of its electrical conductivity. p-p Stacking interactions exhibit incomparable advantages over other methods for graphene modification, particularly in the protection of graphene conductivity. Several p-orbital rich aromatic molecules such as 1-pyrenebutyrate, have been attached onto graphene basal planes via p-p stacking for various applications.

Graphenes are emerging electrode materials used in many technologies such as electronics, sensors, as well as energy conversion and storage. The pristine graphenes, due to the elimination of density of states (DOS) at/near the Dirac point (Fermi level), have limited activities for charge transfer and storage. Fortunately, introduction of structural defects and disorders in graphenes can increase the DOS near the Fermi level by forming mid-gap and/or defect states, similar to that in silicon and other semiconducting materials. This improvement significantly promoted the application of pristine graphenes in electrochemical sensing and energy devices. In addition, defects related electronic structure variations in graphenes provides novel opportunity in studying the electrochemical structure-property relationship [1]. However, the delicate control over the defect-related electronic structures and precise correlation between the electrochemical activity and defect density remain challenging. Researchers led by Prof. Dongping Zhan and Prof. Bin Ren in Xiamen University have made significant advance in this field. In their recent work published in J. Am. Chem. Soc. [2], the authors demonstrated that the density of vacancy defects in a graphene sheet can be precisely controlled through Ar irradiation. They prepared patterns of different defect densities on a single-layer graphene sheet, and simultaneously mapped the defect density and the heterogeneous electron transfer (ET) rate of hydroxymethylferrocene (FcMeOH) oxidation over the same patterned graphene sheet (Figure 1), using Raman spectroscopy and the so-called scanning electrochemical microscopy. This new approach allowed them to quantitatively correlate the defect density in graphenes with the electrochemical activity for the heterogeneous ET reactions. The experimental results showed that the ET activity of graphene can be improved by increasing the density of vacancy defects. In addition, they obtained an optimal ET rate of FcMeOH oxidation over a graphene sheet with a moderate defect density. At such an optimal state, a balance between the increase of Fermi DOS and the decrease of sheet conductivity due to increasing the defect density was achieved, so that the whole graphene sheet not only becomes electrochemically activated, but also maintains structural integrity. According to their density functional theory calculations, the vacancy defect can induce mid-gap states in graphene. Because multiple electronic levels and states are involved in the heterogeneous ET between a solid electrode and redox molecules, the corresponding ET rates become proportional to the electronic overlap integral between the electrode and the redox molecules [1]. The introduction of mid-gap states should be able to enlarge the overlap between the DOS distribution of graphene and the redox molecules. The work by Zhan and coworkers represents an elegant example in tailoring the structure and property of graphene through defect density engineering.

The site-dependent and spontaneous functionalization of 4-bromobenzene diazonium tetrafluoroborate (4-BBDT) and its doping effect on a mechanically exfoliated graphene (MEG) were investigated. The spatially resolved Raman spectra obtained from both edge and basal region of MEG revealed that 4-BBDT molecules were noncovalently functionalized on the basal region of MEG, while they were covalently bonded to the edge of MEG. The chemical doping effect induced by noncovalently functionalized 4-BBDT molecules on a basal plane region of MEG was successfully explicated by Raman spectroscopy. The position of Fermi level of MEG and the type of doping charge carrier induced by the noncovalently adsorbed 4-BBDT molecules were determined from systematic G band and 2D band changes. The successful spectroscopic elucidation of the different bonding characters of 4-BBDT depending on the site of graphene is beneficial for the fundamental studies about the charge transfer phenomena of graphene as well as for the potential applications, such as electronic devices, hybridized composite structures, etc.

First-principles calculations based on Hartree–Fock (HF) and density functional theory (DFT) levels of approximation have been carried out in order to study the stability of graphene clusters as a function of number of carbon atoms (N). The variation of calculated binding energy per carbon atom with corresponding number of carbon atoms (N) in graphene cluster almost saturates after the cluster size consisting of 96 carbon atoms, with binding energy per carbon atom 8.24 eV/atom. Adsorption of halogen atoms, ( F , Cl and Br ), on hydrogen passivated graphene ( H -graphene) was also studied systematically through first-principles DFT calculations by taking five different H -graphene clusters. The calculations showed that the most stable adsorption site for halogen adatoms on H -graphene being T site with binding energy 2.41 eV ( F ), 1.48 eV ( Cl ) and 1.19 eV ( Br ) on the H -graphene cluster consisting 96 carbon atoms. Moreover, on increasing the size of H -graphene cluster, the binding energy of halogen adatoms found to be increasing. The distances of adatom from the nearest carbon atom of H -graphene sheet were 1.47 Å ( F ), 2.71 Å ( Cl ) and 3.01 Å ( Br ), however, the adatom heights from the H -graphene basal plane were 2.22 Å ( F ), 2.90 Å ( Cl ), and 3.19 Å ( Br ). The bonding of halogen adatoms on H -graphene were through the charge transfer; 0.30 | e | ( F ), 0.37 | e | ( Cl ) and 0.19 | e | ( Br ), from H -graphene to adatoms and includes the negligible local distortion in the underlying planner H -graphene. Charge redistribution upon adsorption induces significant dipole moments 2.16 D ( F ), 4.81 D ( Cl ) and 3.08 D ( Br ) on H -graphene. The calculated HOMO–LUMO energy gap of adatom- H -graphene and H -graphene does not differ significantly up to the cluster size N = 30, however, beyond N = 30 adsorption of halogen adatoms significantly opens the HOMO–LUMO energy gap on H -graphene and the opening of HOMO–LUMO energy gap also saturates from N = 96. Correlation of computed HOMO–LUMO energy gap and corresponding binding energy of adatom- H -graphene systems have been also studied.

In-situ formation of 3D vertical graphene by carbonizing organic precursor in ammonia

Passivating defective regions on monolayer graphene with metal oxides remains an active area of research for graphene device integration. To effectively passivate these regions, a water-free atomic layer deposition (ALD) recipe was developed and yielded selective-area ALD (sa-ALD) of mixed-metal oxides onto line defects in monolayer graphene. The anisotropically deposited film targeted high-energy defect sites that were formed during synthesis or transfer of the graphene layer. The passivating layer exceeded 10 nm thickness with minimal deposition onto the basal plane of graphene. The mixed-metal oxide film was of comparable quality to films deposited using nonselective water-based ALD methods, as shown by X-ray photoelectron spectroscopy. The development of sa-ALD techniques to target defect regions on the graphene sheet, while keeping the basal plane intact, will provide a new mechanism to passivate graphene defects and modify the electronic and physical properties of graphene.

Artificial van der Waals heterostructures, obtained by stacking two-dimensional (2D) materials, represent a novel platform for investigating physicochemical phenomena and applications. Here, the electrochemistry at the one-dimensional (1D) edge of a graphene sheet, sandwiched between two hexagonal boron nitride (hBN) flakes, is reported. When such an hBN/graphene/hBN heterostructure is immersed in a solution, the basal plane of graphene is encapsulated by hBN, and the graphene edge is exclusively available in the solution. This forms an electrochemical nanoelectrode, enabling the investigation of electron transfer using several redox probes, e.g., ferrocene(di)methanol, hexaammineruthenium, methylene blue, dopamine and ferrocyanide. The low capacitance of the van der Waals edge electrode facilitates cyclic voltammetry at very high scan rates (up to 1000Vs-1), allowing voltammetric detection of redox species down to micromolar concentrations with sub-second time resolution. The nanoband nature of the edge electrode allows operation in water without added electrolyte. Finally, two adjacent edge electrodes are realized in a redox-cycling format. All the above-mentioned phenomena can be investigated at the edge, demonstrating that nanoscale electrochemistry is a new application avenue for van der Waals heterostructures. Such an edge electrode will be useful for studying electron transfer mechanisms and the detection of analyte species in ultralow sample volumes.