Vacancy-defected Graphene Research Articles

Critical role of vacancy defects in graphene nanosheets for enhancing asphalt binder

Clarifying the interaction between the intrinsic structure of graphene and asphalt binder is critical to achieve the practical application of graphene modified asphalt. Herein, the thermal-mechanical properties and thermal-oxidative aging resistance of asphalts modified by two types of exfoliated graphene (EG) with less vacancy and vacancy-defected graphene (VDG) were investigated via multi-scale methods, aiming at elucidating the role of vacancy defects in graphene. Rheological measurements demonstrate that VDG have stronger capability of improving the deformation resistance and aging resistance of asphalt than EG. Atomic force microscopy analysis further verifies that VDG has a strong interaction with the non-polar components of asphalt, forming dense micellar structures. In addition, molecular dynamics simulations evidence that the vacancy defects in graphene can form the electrostatic adsorption between asphalt hydrocarbon molecules (light components), increasing the concentration and densification of asphaltenes, promoting the transformation of asphalt from sol structure to gel structure, thereby significantly improving the thermal-mechanical and anti-aging properties of asphalt binder. And the interaction is also verified by infrared spectral analysis and gaussian calculation through peak displacement. The discovery of the effect of vacancy defects in graphene for reinforced asphalt deepens the understanding for developing high performance graphene modified asphalts.

A first-principles study of the adsorption of trichloroethylene on vacancy-defected graphene decorated by Cu and/or Ni dimers

The adsorption behavior of trichloroethylene (TCE) on vacancy-defected graphene (VG) decorated by Cu2/Ni2/CuNi dimers (VG-Cu2/Ni2/CuNi) was studied by using first-principles calculation. Results showed that decorating with Cu2/Ni2/CuNi could increase the adsorption performance of VG, thus forming chemical adsorption between TCE and VG-Cu2/Ni2/CuNi. Furthermore, when VG was decorated by CuNi dimer in the configuration of VG-CuNi-2, the substrate exhibited the largest adsorption energy and charge transfer amount with TCE upon its adsorption. After TCE was adsorbed on VG-CuNi-2, the electrical conductivity of the substrate was increased significantly. Therefore, VG-CuNi-2 could be used as a sensing material to detect TCE molecules.

Optimization of structures and properties of vacancy-defected graphene modified by Si atoms

本征石墨烯具有诸多优点，但因为其导电性不可控而难以直接应用。为了能够有效调节石墨烯的导电能力，使其变得可控，科学家们不断在寻找石墨烯改性的方法。高能粒子轰击方法由于能够轻易实现异质粒子在石墨烯中的掺杂，而引起了研究者的广泛关注。本文采用分子动力学方法模拟了具有一定能量的Si6团簇垂直轰击石墨烯的过程。通过可视化软件对体系的微观结构进行观察，同时记录石墨烯的结构变化。结果表明，在能量较低(0.875～55.9 eV )时，Si6并不能破坏石墨烯的结构，而能量较高(503.9 eV以上)时，Si6可以轻易破坏石墨烯结构并引入边缘规则的空位缺陷。在探明了不同能量范围下Si6轰击给石墨烯带来的影响之后，采用初始能量为0.146 eV的不同数量Si原子对轰击所得的空位缺陷石墨烯进行修补，并利用第一性原理方法对Si原子修补石墨烯所得构型进行结构优化和电子结构的计算。结果表明，Si原子的引入能够较好地饱和空位缺陷所带来的碳悬键。Si原子之间存在强烈的相互作用，且Si原子在空位缺陷处形成三维结构。Si原子的引入能成功打开石墨烯的带隙，同时会降低四空位缺陷石墨烯的导电性，该研究为石墨烯的改性提供了新思路。

Adsorption mechanism of SO2 on vacancy-defected graphene and Ti doped graphene: A DFT study

The adsorption properties of SO2 on graphene surface are explored by means of first-principle calculations using the density functional theory. The four graphene structures involved in the calculation include pristine graphene (PG), vacancy-defected graphene (VG), Ti-doped graphene (Ti-G), and Ti-doped graphene with vacancies (Ti-VG). According to the calculation results, pristine graphene is limited to forming weak physical adsorption with SO2 and vacancy defects can lead to only limited improvement of adsorption capacity. In addition, as a suitable dopant, Ti is effective in improving the adsorption energy and charge density of the adsorption system, and in promoting the formation of chemisorption. Besides, after the absorption of SO2 molecules, an impurity band appears in the band structure of Ti-doped graphene, which significantly increases the density of states near the Fermi level. In case of vacancy defects in the Ti-doped graphene, the adsorption capacity will decline. Therefore, Ti-doped graphene exhibits the highest SO2 molecules adsorption capacity among the materials calculated in this paper.

Effect of coexistence of defect and dopant on the quantum capacitance of graphene-based supercapacitors electrodes

The influence of doping (B, N, Al, Si, P, S), vacancy, and Stone-Wales defect on stability, electronic structures, quantum capacitance, and surface charge storage of graphene is explored by density functional theory calculations. The results indicate that the quantum capacitance and surface charge storage can be significantly improved by the introducing of defect and doping. Furthermore, the enhancement of doped vacancy-defected graphene is more obvious than that of doped Stone-Wales defected graphene. By analyzing the density of states, it can be obtained that the improvement of quantum capacitance is resulted from the presence of localized states around Fermi level by doping or defect. Moreover, the effect of concentration of doping and defect on the graphene is also explored, and the results suggest that the quantum capacitance increases with the increasing of concentration for most modified graphene-based substrates. The findings can provide effective approach for improving the performance of graphene-based surpercapacitors.

Theoretical investigation of the catalytic and inhibiting role of boron in graphene oxidation

Boron has been considered as an important promising candidate for oxidation protection in carbon materials. However, the experimental discovery of catalytic effect of boron in graphite makes the role of boron confusing. To better understand the role of boron in carbon materials such as graphite or graphene, we investigate the oxidation behavior of boron-doped stoichiometric and vacancy-defected graphene plane by density functional theory calculations. Boron is found to be a catalyst for oxidation in the stoichiometric surface, while it acts as an inhibitor in the vacancy-defected surface. The electronic properties are calculated to explain why boron behaves differently in different surfaces. We show that the role of boron is closely associated with the redistribution of σ electrons. For instance, the introduction of boron in stoichiometric surface facilitates the delocalized σ electrons transfer from the matrix to oxygen and the adsorption energy increases with the number of transferred σ electrons; while the decrease in reactivity with boron substitution in the vacancy-defected surface is largely attributed to the transition of electrons from delocalized states to localized states, thus suppressing the activity of dangling C atoms. This work helps to provide a theoretical foundation for further investigation of oxidation-resistant carbon materials.

Double-Vacancy Controlled Friction on Graphene: The Enhancement of Atomic Pinning.

The vacancy-enhanced contact friction of graphene is mainly attributed to the vacancy-enhanced out-of-plane deformation flexibility of the graphene and the climbing of the tip out of the vacancy trap (which actually acts as a step edge). However, this mechanism does not apply for explaining the enhanced friction caused by small-sized vacancies that are unable to accommodate the tip, such as single vacancy and double vacancies, which also commonly exist in the graphene. In the present study, by performing a set of classic molecular dynamics simulations, we demonstrated that the double-vacancy defect in graphene substantially enhanced the contact friction when the tip slides over it and the pinning effect of the reconstructed lattice of the double-vacancy defect with atoms at the bottom of the tip dominated such an influence. The underlying mechanism of such an atomic pinning effect and the influence of the normal load, sliding direction, and the sliding velocity were unveiled by analyzing the obtained friction evolution and the atomic configuration and interaction between the tip and the graphene. We believe that the findings presented in this study complete the state-of-art understanding of the nanoscale friction behaviors of vacancy-defected graphene, which is essential for the implementation of their potential control.

First-principles investigation of vacancy-defected graphene and Mn-doped graphene towards adsorption of H2S

Abstract The effect of vacancy defects and doping Mn atoms on the performance of adsorption between graphene and H2S molecules has been investigated by using first-principles study. The analysis shows that although vacancy defects slightly enhances the adsorption of H2S on graphene, there is no chemical adsorption formed between them. However, after doping graphene with Mn atoms, the adsorption energy and charge transfer between graphene and H2S increase significantly and the maximum values under various adsorption sites and configurations reach 4.3228 eV and 0.171 e, respectively. In addition, the charge density between Mn-doped graphene and H2S molecules is greater than 3.000 × 10−1 e/A3, and the orbitals of Mn and S atoms are hybridized, indicating a formation of chemical bonds. Therefore, Mn doping can significantly enhance the adsorption of H2S on graphene, improving the sensitivity of graphene to H2S. Mn-doped graphene is expected to be a potential sensing material for making high-performance H2S gas sensors.

Adsorption behavior of O2 on vacancy-defected graphene with transition-metal dopants: A theoretical study

The influence of vacancy and dopants on the adsorption of O2 molecule on graphene was explored by using the density functional theory (DFT) method. The results indicated that the presence of vacancy-defect improved the sensitivity of graphene toward the O2 molecule. Furthermore, the two O atoms of O2 molecule separately formed chemical bonds with C atoms at the defect sites. After introducing the transition-metal (TM) dopants, the O–O bond length of O2 molecule was enlarged by the adsorption on the surface of graphene. Furthermore, the Mn-doped adsorption complexes became magnetic, which was mainly contributed by the O-2p and Mn-3d orbitals.

Buckling Analysis of Vacancy-Defected Graphene Sheets by the Stochastic Finite Element Method.

Vacancy defects are unavoidable in graphene sheets, and the random distribution of vacancy defects has a significant influence on the mechanical properties of graphene. This leads to a crucial issue in the research on nanomaterials. Previous methods, including the molecular dynamics theory and the continuous medium mechanics, have limitations in solving this problem. In this study, the Monte Carlo-based finite element method, one of the stochastic finite element methods, is proposed and simulated to analyze the buckling behavior of vacancy-defected graphene. The critical buckling stress of vacancy-defected graphene sheets deviated within a certain range. The histogram and regression graphs of the probability density distribution are also presented. Strengthening effects on the mechanical properties by vacancy defects were detected. For high-order buckling modes, the regularity and geometrical symmetry in the displacement of graphene were damaged because of a large amount of randomly dispersed vacancy defects.

Vibration Analysis of Vacancy Defected Graphene Sheets by Monte Carlo Based Finite Element Method.

The stochastic distributed placement of vacancy defects has evident effects on graphene mechanical property, which is a crucial and challenged issue in the field of nanomaterial. Different from the molecular dynamic theory and continuum mechanics theory, the Monte Carlo based finite element method (MC-FEM) was proposed and performed to simulate vibration behavior of vacancy defected graphene. Based on the Monte Carlo simulation, the difficulties in random distributed location of vacancy defects were well overcome. The beam element was chosen to represent the exact atomic lattice of the graphene. The results of MC-FEM have a satisfied agreement with that in the reported references. The natural frequencies in the certain vibration mode were captured to observe the mechanical property of vacancy defected graphene sheets. The discussion about the parameters corresponding with geometry and material property was accomplished by probability theory and mathematical statistics.

Adsorption sensitivity of defected graphene towards NO molecule: a DFT study

Based on density functional theory, we studied the adsorption of a nitrogen monoxide (NO) molecule on the surface of perfect graphene (PG) and vacancy-defected graphene (VG), with the aim of searching the potential of graphene as an NO gas sensor. Different possible configurations have been considered for adsorption on vacancy-defected graphene split. The results indicated that the adsorption of the NO molecule on VG exhibited larger adsorption energy, higher charge transfer, smaller band length than that of perfect graphene. Meanwhile, the VG structure transformed a semiconductor into a conductor by the adsorption of the NO molecule. Furthermore, the partial electronic density of states (PDOS) results showed that hybridizations between the NO molecule and VG were mainly contributed by N-2p, O-2p, and C-2p orbitals. These results could provide useful information for the design of gas sensors based on graphene.

DFT study of the adsorption of 2, 3, 7, 8-tetrachlorodibenzofuran (TCDF) on vacancy-defected graphene doped with Mn and Fe

Dioxins are highly toxic to humans and environment, and developing the effective methods to control and detect the organic pollutant is particular important. Here we performed a density functional theory (DFT) study on the adsorption of 2, 3, 7, 8-tetrachlorodibenzofuran (TCDF) molecules on the modified graphene substrates. The results indicated that the introducing of vacancy-defect and dopants (Mn and Fe) significantly improves the sensitivity toward TCDF molecules. The impurity played a crucial role for interacting with TCDF molecules. Furthermore, the adsorption of TCDF induced band-gap open in defected graphene substrates, which could be seen as electric signal to detect TCDF pollutant. The present study is expected to be useful to explore effective materials to detect and remove dioxin pollutants based on graphene.

Adsorption behavior of SO2 on vacancy-defected graphene: A DFT study

The adsorption of an SO2 molecule on the perfect and point-defective graphene surfaces were investigated using density functional theory (DFT). The geometric structure, adsorption energy, charge transfer, and electronic properties were calculated and analyzed to characterize the effect of vacancy on the adsorption process of SO2 on the graphene. The result indicated that the presence of vacancy enhanced the adsorption stability with the larger adsorption energy and net charge transfer compared to that of perfect graphene. Moreover, the SO2 molecule on different adsorption sites exhibited dissimilar states because of the adsorption. Furthermore, the results of the electronic properties revealed that the adsorption of SO2 induced an opening of the band gap.

Influence of vacancy defects and 3d transition metal adatoms on the electronic and magnetic properties of graphene

Based on the density functional theory (DFT) method, we investigated the geometry stability, electronic and magnetic properties of vacancy-defected graphene with and without the adsorption of transition metal (TM) adatoms (V, Cr, and Mn).

A computational study of Na behavior on graphene

We present the first ab initio and molecular dynamics study of Na adsorption and diffusion on ideal graphene that considers Na–Na interaction and dispersion forces. From density functional theory (DFT) calculations using the generalized gradient approximation (GGA), the binding energy (vs. the vacuum reference state) of −0.75eV is higher than the cohesive energy of Na metal (EcohDFT=−1.07 eV). The binding energy approaches the Na metal cohesive energy (EcohDFT−D=−1.21 eV) when dispersion correction is included (DFT-D), with Eb=−1.14eV. Both DFT and DFT-D predict that the increase of Na concentration on graphene results in formation of Na complexes. This is evidenced by smaller Bader charge on Na atoms of Na dimer, 0.55e (0.48e for DFT) compared to 0.86e (for both DFT and DFT-D) for the single atom adsorption as well as by the formation of a NaNa bond identified by analysis of the electron density. These results suggest that ideal graphene is not a promising anode material for Na-ion batteries. Analysis of diffusion pathways for a Na dimer shows that the dimer remains stable during the diffusion, and computed migration barriers are significantly lower for the dimer than that for the single atom diffusion. This indicates that Na–Na interaction should be taken into account during the analysis of Na transport on graphene. Finally, we show that the typical defects (vacancy and divacancy) induce significant strengthening of the NaC interaction. In particular, the largest change to the interaction is computed for vacancy-defected graphene, where the found lowest binding energy (vs. the metal reference state) is about 1.15eV (1.21eV for DFT) lower than that for ideal graphene.

DFT study of formaldehyde adsorption on vacancy defected graphene doped with B, N, and S

The adsorption of formaldehyde (H2CO) on modified graphene sheets, combining vacancy and dopants (B, N, and S), was investigated by employing the density functional theory (DFT). It was found that the vacancy-defected graphene was more sensitive to absorb H2CO molecule compared with the pristine one. Furthermore, the H2CO molecule tended to be chemisorbed on vacancy-defected graphene with dopants, which exhibited larger adsorption energy and net charge transfer than that of one without dopants. The results of partial electronic density of states (PDOS) indicated that the defect-dopant combination effect on the adsorption process was mainly owing to the contribution of the hybridization between dopants and C atoms around the vacancy. We hope our results will be useful for the application of graphene for chemical sensors to detect formaldehyde gas.

First-principle study of the transition-metal adatoms on B-doped vacancy-defected graphene

The energetic, electronic, and magnetic properties of transition metal (TM) atoms absorbed on modified graphene, including B-dopant, vacancy, and combination of these two, were theoretically investigated using the density functional theory (DFT) method. It was found that the adsorption of TM atoms (V, Mn, Fe and Ni) was significantly enhanced by the presence of vacancy sites. Furthermore, the introduction of B-dopant affected the adsorption process of TM atoms and the band structure of graphene. The results of partial electronic density of states (PDOS) indicated that the influence by B-dopant was due to the interaction between the 2p orbital of B atoms and 3d orbital of TM atoms. Interestingly, the strong hybridization contributed by B-2p and C-2p at vacancy sites enlarged the adsorption energies of TM-adatoms. It is expected that the results could provide useful information to explore application of graphene as spintronic and electronic devices.