Atomic Graphene Research Articles

Catalytic reduction of CO2 in the interface formed by monolayer graphene and metal atom (Pt, Ni, Pd, Co) doped Cu-nanoclusters: A theoretical design and investigation

In this study, pristine Cu13 nanocluster and a series of metal atoms (Pt, Ni, Pd and Co) doped Cu12X were designed and placed on the top of a monolayer graphene to form novel catalysts for CO2 reduction. Based on the designed catalysts, we studied the reaction mechanisms of five catalysts for CO2 reduction leading to three different products: CH4, CH3OH and HCOOH. By comparing the activation energies of different reaction channels of different catalysts, we obtained the three different product rate-determining steps of CO2 reduction of CH4, CH3OH and HCOOH. The selectivity order of CO2 reduction products catalyzed by graphene-supported Cu13/G and doped Cu12X/G catalysts is CH4>CH3OH>HCOOH. The order of catalyst activity is Cu12Co/G>Cu12Ni/G>Cu12Pd/G>Cu12Pt/G>Cu13/G. The doping of the four metals Pt, Ni, Pd and Co are all beneficial for improving the catalytic activity of CO2 reduction, and the catalytic activity of metal Co doping is the optimal. These results agree well with the observed phenomena. Our study reveals a correlation between the CO2 reduction catalytic activity of graphene-supported Cu13/Cu12X nanoclusters and their physical properties.

Amplifying sensing performance through gold micropatterns-induced modulation of graphene's vertical electron transfer

High-quality graphene monolayers exhibit limited vertical electron transfer properties due to their inert basal plan structure, hampering their use in electrochemical sensing devices. Overcoming this limitation while maintaining the graphene's basal plane electrical conductivity is crucial for developing sensitive and efficient electrochemical sensors. In this study, we demonstrate that incorporating a gold support beneath graphene enhances its electrochemical activity by facilitating effective hybridizations between carbon atoms in graphene and gold atoms. To achieve this, micrometer-sized gold micropatterns were fabricated on Si/SiO2 substrates to support the graphene monolayer. The presence of gold significantly improved the electrochemical activity of graphene towards redox probes in solution (ferri-/ferrocyanide) as well as for adsorbed ferrocene, indicating a synergistic effect between graphene and gold. We further investigated the adsorption effects of double-stranded DNA on pristine graphene, bare gold, and graphene with a gold underlayer, revealing that the graphene-gold combination achieved a significantly lower limit of detection for DNA hybridization events, surpassing the other configurations. These findings highlight the importance of appropriate substrate selection in the design of graphene-based electrochemical sensors, particularly in the context of DNA recognition events. The proposed approach of incorporating a gold support beneath graphene holds promise for enhancing the performance of electrochemical sensing devices without compromising the intrinsic properties of graphene.

Wear suppression and interface properties of diamond tool in micro-milling of TC4 alloy under graphene nanofluid MQL environment

The sustainable micro-milling process of TC4 alloy has garnered significant attention in aerospace engineering. However, the poor machinability of TC4 alloy results in particularly severe tool wear. Tool wear directly affects the machining quality of the parts, and traditional cutting fluids of suppressing tool wear indirectly increase production costs and pollute the environment. Therefore, to suppress the negative effects caused by tool wear, this paper proposes the application of water-based graphene nanofluid in the micro-milling of TC4 alloy in diamond tools. Firstly, micro-milling experiments and molecular dynamics simulations were conducted in different environments, including dry and water-based graphene nanofluid environments. Finally, tool surface wear morphology, milling forces, surface elements and structural phase changes were compared in detail. The results show that tool wear, including adhesive wear, abrasive wear, and edge chipping, was suppressed by graphene nanofluids compared to dry conditions, thereby increasing tool life. The graphene nanofluid improved the contact state between the tool and workpiece interface, reducing the milling forces. In addition, molecular dynamics indicates that the catalytic effect of the transition elements in the workpiece on the diamond tool was also weakened, and the carbon atoms of graphene were used as a sacrificial source to balance the catalytic effect. Graphitization and diffuse wear of the diamond were suppressed. More importantly, the water-based graphene nanofluid not only improves the wear resistance of the tool, but the LCA results show its potential for sustainability. Consequently, it holds promise for advancing the sustainable processing of challenging materials.

E-beam Patterning of Atoms in Graphene.

E-beam Patterning of Atoms in Graphene.

Top-Down Fabrication of Atomic Patterns in Twisted Bilayer Graphene.

Atomic-scale engineering typically involves bottom-up approaches, leveraging parameters such as temperature, partial pressures, and chemical affinity to promote spontaneous arrangement of atoms. These parameters are applied globally, resulting in atomic-scale features scattered probabilistically throughout the material. In a top-down approach, different regions of the material are exposed to different parameters, resulting in structural changes varying on the scale of the resolution. In this work, the application of global and local parameters is combined in an aberration-corrected scanning transmission electron microscope (STEM) to demonstrate atomic-scale precision patterning of atoms in twisted bilayer graphene. The focused electron beam is used to define attachment points for foreign atoms through the controlled ejection of carbon atoms from the graphene lattice. The sample environment is staged with nearby source materials such that the sample temperature can induce migration of the source atoms across the sample surface. Under these conditions, the electron-beam (top-down) enables carbon atoms in the graphene to be replaced spontaneously by diffusing adatoms (bottom-up). Using image-based feedback control, arbitrary patterns of atoms and atom clusters are attached to the twisted bilayer graphene with limited human interaction. The role of substrate temperature on adatom and vacancy diffusion is explored by first-principles simulations.

A first-principles study of structural, electronic and transport properties of aluminium and phosphorus-doped graphene

First-principles simulations are utilised for computing the structural, electronic, and transport characteristics of pure graphene and graphene doped with aluminium and phosphorus. The results demonstrate that by substituting Al and P atoms for carbon atoms in graphene, the material's band gap may be effectively opened. When Al or P atom replaces a carbon atom, a notable band gap value of 1.49 eV or 0.40 eV has been detected. Additionally, the thermal conductivity, Seebeck coefficient, carrier concentration, power factor, electrical conductivity, and dimensionless figure of merit of pure graphene, aluminium and phosphorus-doped graphene are examined. Our study presents the theoretical basis for using a doping mechanism with heteroatoms to improve graphene's electronic and transport properties. Al/P doped graphene has the potential to significantly improve thermoelectric efficiency due to its electronic properties and low thermal conductivity.

Interaction of electrons and positrons with two-dimensional artificially generated proton lattice and with carbon lattice

A many-body classical trajectory Monte Carlo (CTMC) method is applied in the study of scattering probabilities of electrons and positrons after interacting with a two-dimensional (2D) artificially generated, uniform lattice, composed of fixed protons. We used different lattice parameters between the protons for different simulations, where the projectiles have kinetic energies of 500 and 1000 eV. We found a very strong focusing of electrons and a very strong defocusing of the positrons at lower lattice parameters. Furthermore, we found, these effects get weaker with increasing lattice parameters. Interesting changes took place at lattice parameters 2 and 3 au. which are close to the lattice parameter between carbon atoms in graphene of value 2.68 au. We also performed a simulation of a defective lattice by removing some protons and noticed distinguishable changes in the spectra compared to the spectra of an ideal lattice. This comparison may open a way for the detection of lattice defects in real samples.

Effect of stirring on characteristics of electrochemically exfoliated graphene

Electrochemical exfoliation of graphite is a promising technique for the synthesis of graphene at a commercial scale. Electrochemical synthesis of graphene at high voltage (∼10 V) is widely reported. However, at high voltage, multilayer graphene platelets with high defect concentration are produced. In this study, high quality graphene nanoplatelets consisting of 1–5 layers were produced at low voltage (3–4 V) using KOH solution as electrolyte. The effect of electrolyte stirring on the characteristics of graphene was studied. The stirring of electrolyte during electrolysis showed profound effect on the characteristics of graphene. The scanning electron microscopic (SEM) analysis showed that the stirring of electrolyte increased the average size of graphene nanoplatelets to 360 nm from 253 nm (without stirring). Raman spectroscopy indicated the presence of 1–5 layers of carbon atoms and the same was confirmed by transmission electron microscopy (TEM). A decrease in the defect concentration was noted with the increase in number of layers of carbon atoms in graphene produced with stirring.

Promoting Oxygen Reduction Reaction by Inducing Out-of-plane Polarization in Metal Phthalocyanine Catalyst.

Metal phthalocyanine (MPc) materials with well-defined MN4 moiety offers a platform for catalyzing oxygen reduction reaction (ORR), while their practical performances are often limited by the insufficient O2 adsorption due to the planar MN4 nature. Here, we proposed a design (called as Gr-MG -O-MP Pc) that the metal of MPc (MP ) is axially coordinated to a single metal atom in graphene (Gr-MG ) through a bridge-bonded oxygen atom (O), introducing effective out-of-plane polarization to promote O2 adsorption on MPc. Manipulating the out-of-plane polarization charge by varying types of MP and MG (MP = Fe/Co/Ni, MG = Ti/V/Cr/Mn/Fe/Co/Ni) in the axial coordination zone of -MG -O-MP - were examined by density functional theory simulations. Among them, the catalyst of Gr-V-O-FePc stands out with the highest calculated O2 adsorption energy, which was synthesized successfully and verified by systemic X-ray absorption spectroscopy measurements. Importantly, it delivered a remarkable ORR performance with half-wave potential of 0.925V (versus RHE) and kinetic current density of 26.7mA cm-2 . This thus demonstrates a new and simple way to pursue high catalytic performance by inducing out-of-plane polarization in catalysts. This article is protected by copyright. All rights reserved.

Stability of Graphene on the Si (111) Surface: Insights from Reactive Molecular Dynamics Simulations

The remarkable characteristics of graphene render it well-suited for a diverse range of applications, particularly in the realm of electronic devices. After the synthesis process, the two-dimensional material known as graphene is then transferred onto a substrate. Silicon (Si) is considered a suitable choice for this purpose. Therefore, it has become essential to investigate the stability of graphene on silicon surfaces. This study utilized reactive molecular dynamics simulations to investigate the thermal stability of graphene on a Si (111) substrate across a temperature range of 300 to 1500 K. The results demonstrate the exceptional stability of graphene on this particular surface. This phenomenon can be explained by the restricted intermolecular interactions between the carbon atoms in graphene and the silicon atoms on the substrate surface. The study findings indicate that graphene exhibits a dome-shaped configuration on the Si (111) surface. In this configuration, only the carbon atoms located at the periphery of the graphene structure interact with the silicon atoms present on the underlying substrate.

Fluorinated graphene and its catalytic performance for AP pyrolysis: the promotion of F doping

We reported an experimental and theoretical study on the promotion role of F doping in the improved catalytic performance of fluorinated graphene(F-GO) for ammonium perchlorate (AP) pyrolysis. F-GO was prepared by using graphite oxide (GO) as carbon sources and CF3COOH as fluorinating agent. Compared with other samples, the F-GO-180 with the highest F doping amounts, more C-F semi-ionic and C-F covalent bond amounts exhibit superior catalytic performance, as 10 wt% of catalyst added in AP, the exothermic peak temperature proceeded in just one step was even lower than the HTD peak temperature for pure AP and the released apparent specific heat of the thermal decomposition of AP catalyzed by F-GO-180 is 4261.2 J/g, which is almost sevenfold of that of pure AP (603.5 J/g). The doping of F atoms in graphene increased surface defects and the content of C-F semi-ionic and C-F covalent bond amounts of F-GO, which provided more active sites during the thermal decomposition of AP. The molecular simulation calculation results further confirmed that the F doping could react with the defect structure of graphene to form more C-F semi-ionic and C-F covalent bond, which could act as catalytic active sites and facilitate the electron transfer ability of the graphene to promote the formation of free radicals in the reaction.

Global Optimization of Dinitrogen Clusters Bound to Monolayer and Bilayer Graphene: A Swarm Intelligence Approach

Locating the global minimum of a potential energy surface is an arduous task. The complexity of the potential energy surface increases as the number of degrees of freedom of the system increases. The highly rugged nature of the potential energy surface makes the minimization of the total energy of the molecular clusters a difficult optimization problem. A solution to this conundrum is the use of metaheuristic techniques that efficiently track down the global minima through a trade-off between exploration and exploitation. Herein, we use the swarm intelligence technique, particle swarm optimization to locate the global minima geometries of N2 clusters of size 2-10, in free and adsorbed states. We have investigated the structures and energetics of bare N2 clusters, followed by N2 clusters adsorbed on graphene and intercalated between the layers in bilayer graphene. The noncovalent interactions between dinitrogen molecules are modeled using the Buckingham potential as well as the electrostatic point charge model, while those of the N2 molecules with the carbon atoms of graphene are modeled using the improved Lennard-Jones potential. The interactions of the carbon atoms belonging to different layers in a bilayer are modeled using the Lennard-Jones potential. The bare cluster geometries and intermolecular interaction energies obtained using particle swarm optimization are found to be the same as reported in the literature, validating the use of particle swarm optimization for studying molecular clusters. The N2 molecules are found to adsorb as a monolayer on top of the graphene sheet and intercalate themselves right in the middle of the two sheets of bilayer graphene. Our study establishes that particle swarm optimization is a feasible global optimization technique for performing the optimization of high-dimensional molecular clusters, both in pristine and in confined forms.

Single-atom vibrational spectroscopy with chemical-bonding sensitivity.

Correlation of lattice vibrational properties with local atomic configurations in materials is essential for elucidating functionalities that involve phonon transport in solids. Recent developments in vibrational spectroscopy in a scanning transmission electron microscope have enabled direct measurements of local phonon modes at defects and interfaces by combining high spatial and energy resolution. However, pushing the ultimate limit of vibrational spectroscopy in a scanning transmission electron microscope to reveal the impact of chemical bonding on local phonon modes requires extreme sensitivity of the experiment at the chemical-bond level. Here we demonstrate that, with improved instrument stability and sensitivity, the specific vibrational signals of the same substitutional impurity and the neighbouring carbon atoms in monolayer graphene with different chemical-bonding configurations are clearly resolved, complementary with density functional theory calculations. The present work opens the door to the direct observation of local phonon modes with chemical-bonding sensitivity, and provides more insights into the defect-induced physics in graphene.

Engineering Magnetic Anisotropy of Rhenium Atom in Nitrogenized Divacancy of Graphene.

The effects of charging on the magnetic anisotropy energy (MAE) of rhenium atom in nitrogenized-divacancy graphene (Re@NDV) are investigated using density functional theory (DFT) calculations. High-stability and large MAE of 71.2 meV are found in Re@NDV. The more exciting finding is that the magnitude of MAE of a system can be tuned by charge injection. Moreover, the easy magnetization direction of a system may also be controlled by charge injection. The controllable MAE of a system is attributed to the critical variation in dz2 and dyz of Re under charge injection. Our results show that Re@NDV is very promising in high-performance magnetic storage and spintronics devices.

Structures and electron states of (4,4)CNT-graphene hybrid system through DFTB investigation

Self-consistent density functional tight binding (SCC-DFTB) algorithm is performed to investigate the connection of a (4,4) carbon nanotube and graphene having topological defects. The examined seamless coupling CNT-graphene configurations are characterized by packing geometries, bond length, bonding energy, chemical potential, HOMO-LUMO energy gap, Mulliken populations on atoms, charge density differences, and molecular orbitals on HOMO and LUMO energy levels. Simulation results show that coupling models among the bottom atoms of the tube and the graphene atoms around the defect as well as defect patterns greatly affect configurations and electrical structures of coupled systems. Graphene having some defective patterns improves high possibility for strongly bonding the tubes. Apparent charge transfer occurs among the atoms in the tubes and graphene, and differences of transferred charges can be also found in the coupling modes. In addition, for the atoms at the connecting knots, their s and p orbitals present significant differences in the abilities of losing and gaining charge. Charge density differences as well as molecular orbitals on the HOMO and LUMO energy levels indicate the bonding between the tube and graphene as well as electron’s distribution in space.

High-throughput screening of B/N-doped graphene supported single-atom catalysts for nitrogen reduction reaction

Transitional metal single atom (TM1) doped graphene catalysts have been widely applied in electrochemical N2 reduction reaction (NRR). However, it remains a challenge for the rational design of highly active and selective electrocatalysts owing to limited knowledge of structure-activity correlations. Here, we adopted first-principle calculations to high-throughput screen the NRR performance of TM1 coordinated with two boron and two nitrogen atoms in graphene (TM1-B2N2/G). A “five-step” strategy was implemented by progressively considering different metrics such as stability, N2 adsorption, N2 activation, potential-determining step, and selectivity. As a result, a volcano plot of reactivity is established by using the valence electron number of TM1 as the descriptor. Among all catalysts, Cr1-B2N2/G exhibits superior performance with a limiting potential of -0.43 V with high selectivity of NRR interpreted by better spatial symmetry and excellent compatibility in terms of energy when N2 interacts with TM1. Our work reveals the general strategy of computational efforts to predict the next generation of advanced catalytic materials for NRR.

Large-flake graphene-modified biochar for the removal of bisphenol S from water: rapid oxygen escape mechanism for synthesis and improved adsorption performance

The combined effects of graphene and biochar for enhanced adsorption of organic pollutants have not been demonstrated yet. Therefore, the mechanisms of graphene-modified biochar synthesis and its application to adsorption of contaminants remain unclear. In this study, the effect of flake-size graphene on biochar modification and its bisphenol S (BPS) adsorption performance was explored for the first time. Three sizes of graphene oxide were used as the precursor to prepare graphene/biochar composites using pyrolysis. It was found that the graphene with a small flake size was interspersed in the macropores of biochar, while the biochar was completely or mostly wrapped by the large-sized graphene sheet, which effectively prevented the agglomeration and pore blockage of biochar. Large-flake graphene oxide modified biochar (LGB) showed the highest adsorption capacity towards BPS, exhibiting 2.8 times higher adsorption than pristine biochar. Density functional theory (DFT) calculation suggested that the maximum diffusion barrier of O atoms in graphene coated cellulose (most frequently used biochar representative) could be reduced significantly (∼46%) at pyrolysis temperature of 873 K. Taking the advantage of small amount of graphene and enhanced adsorption performance, LGB could be a promising adsorbent for the removal of certain organic pollutants from wastewater and is conducive for the development of high-valued biochar modification.

DFTB investigation of strain affecting the combination of C60 and graphene having single-vacancy on electronic-scale

Density functional tight-binding calculations have been employed to study the effect of applying strain on the combination of C 60 and defective graphene with different coverage. Charge density difference indicate that in the more stable configurations, the active 6:6 bond atoms in C 60 combine with carbon atoms with dangling bonds in graphene by forming two typical C C covalent bonds for chemisorption, where more charge are transferred from the s or p orbitals of bonded C atoms in graphene to C atoms in C 60 whereas the charge transfer occurs only on the atoms near the defect in graphene for the physisorption case. For the comparison of different coverages of C 60 on graphene, strain engineering under smaller coverage was more likely to achieve a stable chemisorption configuration via the newly formed two C C bonds. The magnitudes and directions of the applied strain, and whether it is uniaxial or biaxial, significantly affect the combination of C 60 /graphene nanohybrid systems. • The active 6:6 bond atoms in C 60 combine with carbon atoms with dangling bonds in graphene by forming two typical C C covalent bonds for chemisorption. • Strain engineering under smaller coverage was more likely to achieve a stable chemisorption configuration via the newly formed C C bonds. • The magnitudes and directions of the applied strain, and whether it is uniaxial or biaxial, significantly affect the combination of C 60 /graphene nanohybrid system.

Exploring the effect of layer spacing of graphene for lithium single-atom diffusion using first principles calculations

Bilayer graphene is a representative two-dimensional carbon material and a promising anode material for lithium batteries.The layer spacing variation of bilayer graphene can affect the diffusion of lithium atoms, and thus affect the rate performance of lithium battery anode materials.To reveal the mechanism of the diffusion of the lithium atom in bilayer graphene at different layer spacing, the lowest energy configuration,formation energy, diffusion energy barrier, differential charge density and Mulliken population are calculated and investigated according to the first principles of density functional theory (DFT). The calculation results show that a single lithium atom is preferentially deposited at the hollow site of graphene, and the lithium atom embedded graphene gradually changes from A-B stacking mode (shifted graphene) to A-A stacking mode (aligned graphene) with the increase of layer spacing. For both A-A and A-B stacking mode bilayer graphene, the increase of layer spacing improves the formation energy of the system and reduces the stability of the structure. The increase of the layer spacing also improves the diffusion energy barrier of A-B stacking graphene and decreases that of A-A stacking. This means that the increase of layer spacing can make the diffusion of the lithium atom harder in A-B stacking while easier in A-A stacking. The results of differential charge density and Mulliken population verifiedthatwhen the layer spacing increases to 1.8 nm, the effect of bilayer graphene on the lithium atom is similar than that of monolayer graphene. The calculation results of OCV and the maximum capacity prove the potential of bilayer graphene as anode material. The results provide a new understanding of the lithium atom diffuse in bilayer graphene, and the diffusion properties of the lithium atom can be optimized by adjusting the layer spacing appropriately.

Spatial segregation of substitutional B atoms in graphene patterned by the moiré superlattice on Ir(111)

Fabrication of ordered structures at the nanoscale limit poses a cornerstone challenge for modern technologies. In this work we show how naturally occurring moiré patterns in Ir(111)-supported graphene template the formation of 2D ordered arrays of substitutional boron species. A complementary experimental and theoretical approach provides a comprehensive description of the boron species distribution, bonding configurations, interfacial interaction with Ir(111) and the impact on graphene's electronic structure. Atomically-resolved scanning tunnelling microscopy images and density functional theory calculations reveal that boron preferably forms small aggregates of substitutional defects in geometrically low regions of the moiré superlattice of graphene, by inducing local bending of graphene towards the underlying Ir(111). Surprisingly, calculations reveal that the incorporation of electron deficient boron does not lead to an enhanced p-type doping, as the local rippling of the graphene layer prompts electron uptake from the iridium substrate that compensates the initial electron loss due to substitutional boron. Scanning tunnelling spectroscopy and angle-resolved photoemission spectroscopy measurements corroborate that the arrays of boron species do not modify the electronic structure of graphene near the Fermi level, hence preserving the slight p-type doping induced by Ir(111).