Single Vacancy Graphene Research Articles

CO2 hydrogenation to formic acid over platinum cluster doped defective graphene: A DFT study

The hydrogenation of CO2 to formic acid is an important reaction in environmental catalysis, which can both alleviate the greenhouse effect and produce useful chemicals. Platinum element catalysts need to be investigated to realize large-scale development due to the expensive and scarce features. In this work, CO2 hydrogenation to formic acid over Pt4 cluster doped single-vacancy graphene was investigated using density functional theory. Catalyst configuration was optimized to perform corresponding gas adsorption and reduction reaction. Four reaction pathways were explored according to the reaction mechanism of Langmuir-Hinshelwood (L-H), Eley-Rideal (E-R) and termolecular Eley-Rideal (TER). Kinetic analysis was used to evaluate the reaction rate of different processes under the given temperature range, and the potential effect of CO molecule was also considered to better understand the feasibility. Results showed that Pt4/SV was a stable and high activity catalyst. The minimum activation energy among different pathways was 0.56 eV and TER could be the dominant reaction mechanism of CO2 hydrogenation. This work not only provides a promising catalyst but also gives a more deep understanding of CO2 reduction technology and the future applicability of platinum metal catalysts.

The adsorption and activation of oxygen molecule on nickel clusters doped graphene-based support by DFT

The catalytic activation of O2 on nickel nanoclusters doped upon diff ;erent functionalized graphene substrates (monovacancy as well as one, two and three pyridinic nitrogen decorated) was investigated through the density functional theory calculation. We can observed the adsorption characteristics and the catalytic activation of O2 on catalysts is related to the support effect of diff ;erent functionalized graphene supports. The charge of Ni aotms has a good correlation with the adsorption energy of O2 on Nin clusters with different graphene-based support. The electron mainly transferred from the Ni clusters to the O2, and the graphene-based substrates plays a minor role in the electron transfer process. In addition, the Transition State Scaling are suitable for O2 dissociation reaction, and Transition State Scaling verify the accuracy of the O2 dissociation reaction calculation. Furthermore, in accordance with the energy barrier and the thermodynamic analysis of O2 activation reaction, the Ni3 clusters catalyst anchored single vacancy graphene decorated with three nitrogen atoms may be the most promising catalyst for O2 activation reaction. At the same time, this research will cast insight into activation reaction mechanism of O2 on metal nanoclusters catalysts and lay the foundation for the investigation of metal nanoclusters catalysts for the activation reaction of O2.

Methane activation by a single iron atom supported on graphene: Impact of substrates

A density functional theory study was conducted to investigate methane activation by a single iron atom anchored on eleven different graphene substrates. It was concluded that the structure of the graphene substrate influences the catalytic activity of single iron atom for methane activation. The adsorption energy of methane on single iron atom supported on single vacancy graphene is higher than that on double vacancy graphene. Moreover, single iron atom supported on single vacancy graphene has lower energy barrier than that on double vacancy graphene for the cleavage of the first CH bond of methane. It is noted that FeC and FeH bonds were formed for the transition states of the CH bond breakage by single iron atom decorated on single vacancy graphene, whereas the transition states of single iron atom embedded on double vacancy graphene catalytic activation of methane does not involve the formation of FeH bonds. With increase of nitrogen doping, Fe⋯C interactions are weaker whereas C⋯H or N⋯H interactions are stronger. The energy barrier for the breakage of the first CH bond of methane by a single iron atom supported on single vacancy graphene doped with three nitrogen atoms is lowest among all the catalysts investigated.

Adsorption characteristics of Co-anchored different graphene substrates toward O2 and NO molecules

The adsorption characteristics of O2 and NO on Co-anchored different graphene-based substrates (single vacancy, double vacancy and N atoms doped) have been investigated using density functional theory. The geometric stability of the single atom catalysts, adsorption configurations of gas molecules, adsorption energies, electronic structure and thermodynamic analysis have been performed. Co/vacancy-graphene shows high thermodynamic stability through calculating and comparing the binding energy of Co-anchored single atom catalysts and the cohesive energy of Co bulk. For O2 adsorption, it prefers to form two chemical bonds with the Co atom, and electron transfer dominates the formation of the strong chemical ionic bonds. While on Co single and double vacancy graphene substrates, N atom in NO invariably bonds to the Co atom, with electron transfer and orbital hybridization dominating the process of bonding formation respectively, afterwards ionic and covalent bonds formed between gas molecule and the metal atom. Additionally, electro-negativity and partial d-band centre are good descriptors of adsorption energies and can well reveal the relationship of adsorption energy with adsorption activity and the electronic structure. Co/single vacancy-graphene substrate with three pyridine nitrogen atoms (Co/SV-N123) is a promising catalyst in catalytic oxidation of NO. The results can provide reference for the further study of the NO oxidation mechanism on the Co/GN surface as well as the new non-noble-metal catalysts design.

Li and Na Adsorption on Graphene and Graphene Oxide Examined by Density Functional Theory, Quantum Theory of Atoms in Molecules, and Electron Localization Function.

Adsorption of Li and Na on pristine and defective graphene and graphene oxide (GO) is studied using density functional theory (DFT) structural and electronic calculations, quantum theory of atoms in molecules (QTAIM), and electron localization function (ELF) analyses. DFT calculations show that Li and Na adsorptions on pristine graphene are not stable at all metal coverages examined here. However, the presence of defects on graphene support stabilizes both Li and Na adsorptions. Increased Li and Na coverages cause metal nucleation and weaken adsorption. Defective graphene is associated with the presence of band gaps and, thus, Li and Na adsorptions can be used to tune these gaps. Electronic calculations show that Li– and Na–graphene interactions are Coulombic: as Li and Na coverages increase, the metal valences partially hybridize with the graphene bands and weaken metal–graphene support interactions. However, for Li adsorption on single vacancy graphene, QTAIM, ELF, and overlap populations calculations show that the Li-C bond has some covalent character. The Li and Na adsorptions on GO are significantly stronger than on graphene and strengthen upon increased coverages. This is due to Li and Na forming bonds with both carbon and oxygen GO atoms. QTAIM and ELF are used to analyze the metal–C and metal–metal bonds (when metal nucleation is present). The Li and Na clusters may contain both covalent and metallic intra metal–metal bonds: This effect is related to the adsorption support selection. ELF bifurcation diagrams show individual metal–C and metal–metal interactions, as Li and Na are adsorbed on graphene and GO, at the metal coverages examined here.

Tungsten-Embedded Graphene: Theoretical Study on a Potential High-Activity Catalyst toward CO Oxidation.

The oxidation mechanism of CO on W-embedded graphene was investigated by M06-2X density functional theory. Two models of tungsten atom embedded in single and double vacancy (W-SV and W-DV) graphene sheets were considered. It was found that over W-SV-graphene and W-DV-graphene, the oxidation of CO prefers to Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanism, respectively. The two surfaces exhibit different catalytic activity during different reaction stages. The present results imply that W-embedded graphene is a promising catalyst for CO oxidation, which provides a useful reference for the design of a high-efficiency catalyst in detecting and removing of toxic gases.

The catalytic CO oxidative coupling to dimethyl oxalate on Pd clusters anchored on defected graphene: A theoretical study

The catalytic CO oxidative coupling to dimethyl oxalate (DMO) was studied by means of density functional theory (DFT) calculations, and effects of different sizes of Pdn cluster and different defective graphenes on this reaction were also discussed. A series of different sizes of Pdn (n = 1, 4 and 6) anchored on single-vacancy graphene (SVG) was firstly constructed, and the results show that the Pd-SVG catalyst shows remarkable catalytic activity for CO oxidative coupling to DMO. In addition, the single Pd atom supported on different defective graphenes were studied to reveal the effect of the vacancy type of graphene on reaction activity and selectivity. It showes that the favorable pathway is COCOOCH3 coupling path on the Pd supported on the graphene with double vacancy (DVG), which has excellent activity and selectivity. Therefore, Pd-DVG can be considered as effective catalyst to enhance the catalytic performance and greatly reduce cost of noble Pd-based catalysts.

Single Ru atom supported on defective graphene for water splitting: DFT and microkinetic investigation

We present a comprehensive understanding of sing Ru atom supported on defective graphene for water splitting using density functional theory calculations and microkinetic analysis. The structural and electronic properties of Ru atom supported single vacancy graphene (SVG), double vacancy graphene (DVG) and Stone-Wales graphene (SWG) are systematically investigated. We find that the Ru atom can be trapped effectively by the defects on each defective graphene surfaces. The binding strength of the single Ru atom onto defective graphene surfaces follows the order: Ru@SVG > Ru@DVG > Ru@SWG. After binding, the d-band centers in Ru@SVG, Ru@DVG and Ru@SWG are about −1.67, −1.36, −1.02 eV, respectively. We find that the reaction barrier of H2O splitting decreases with increase of Ru d-band center. The reaction activity of H2O splitting are as follows: Ru@SVG < Ru@DVG < Ru@SWG. The splitting of water molecule on Ru@SWG surface only need a small activation energy of 0.43 eV. In addition, in the temperature range of 300–600 K, the Ru@SWG presents better reaction activity for H2O splitting. The reaction rate and turnover frequency are orders of magnitude larger than that on Ru@SVG and Ru@DVG. Overall, our study provides insights on the significant role of a single metal atom on defective graphene.

A density functional theory study of the role of functionalized graphene particles as effective additives in power cable insulation.

The role of a series of functionalized graphene additives in power cable insulation in suppressing the growth of electrical treeing and preventing the degradation of the polymer matrix has been investigated by density functional theory calculations. Bader charge analysis indicates that pristine, doped or defect graphene could effectively capture hot electrons to block their attack on cross-linked polyethylene (XLPE) because of the π–π conjugated unsaturated structures. Further exploration of the electronic properties in the interfacial region between the additives and XLPE shows that N-doped single-vacancy graphene, graphene oxide and B-, N-, Si- or P-doped graphene oxide have relatively strong physical interaction with XLPE to restrict its mobility and rather weak chemical activity to prevent the cleavage of the C–H or C–C bond, suggesting that they are all potential candidates as effective additives. The understanding of the features of functionalized graphene additives in trapping electrons and interfacial interaction will assist in the screening of promising additives as voltage stabilizers in power cables.

Tuning Cd adsorption behaviours on graphene by introducing defects: a first-principles study

Adsorption of Cadmium (Cd) on pristine and defective graphenes, including Stone–Wales (SW) defect and single vacancy (SV) defect, was studied by first-principles calculations. It is found that Cd adatom is weakly adsorbed on the pristine and SW defective graphene, while SV defect shows a substantial increase in the bonding with Cd adatom. Analysis of electronic density-of-state indicates that the effect of SV defect on the adsorption process can be mainly ascribed to the hybridisation between C-2p and Cd-5s orbitals. The results of charge density changes due to the Cd adsorption show the substantial amount of electrons are transferred from Cd to graphene, and the magnetic moment of substrate is increased after the adsorption of Cd. These experimental results suggest that SV graphene could be a good sensor for the detection of heavy metal ions.

First-principles study of l-cysteine adsorption on vacancy graphene and Ag-doped graphene

The understanding of interactions between graphene and biomolecules is of fundamental relevance to the area of nanobiotechnology. Herein, taking l-cys as the probe molecule, its adsorptions on single-vacancy graphene (SV), double-vacancy graphene (DV), Ag doped single-vacancy graphene (AgSV) and Ag doped double-vacancy graphene (AgDV) were investigated using first-principles calculations. SV and AgSV exhibit exothermical chemisorptions while AgDV exhibits endothermical chemisorptions towards l-cys, regardless of the end type. DV shows exothermical chemisorption towards S-end l-cys and endothermical physisorption towards O-end and N-end l-cys. Two-step energy barrier related to initial symmetry broken and structural reorganization leads to differences in adsorption types and adsorption energies. Site-specific immobilization was also revealed. Calculations at 298.15K and 1atm reveal that l-cys adsorptions on SV, AgSV, the S-end, O-end adsorptions on DV are thermodynamically favourable. The results could provide guidance for further choice of graphene in bionanotechnological applications.

Stability of Ptn cluster on free/defective graphene: A first-principles study

With first-principles methods, we investigate the stability of isolated Ptn clusters from Sutton-Chen model and close-packed model, and their adsorption on defected graphene. The single-vacancy in graphene is found to enhance obviously the adsorption energy of Pt cluster on graphene due to the introduction of localized states near Fermi level. It is found that the close-packed model is more stable than Sutton-Chen model for the adsorption of Ptn cluster on single-vacancy graphene, except the magic number n=13. The cluster Pt13 may be the richest one for small Pt clusters on defected graphene due to the strong adsorption on single-vacancy. The larger cluster adsorbed on defected graphene is predicted with the close-packed crystal structure. The charge is found to transfer from the Pt atom/cluster to graphene with the charge accumulation at the interface and the charge polarization on Pt cluster. The strong interaction between Pt cluster and single vacancy can anchor effectively the Pt nanoparticles on graphene and is also expected that the new states introduced near Fermi level can enhance the catalytic characteristic of Pt cluster.

How can we describe the adsorption of quinones on activated carbon surfaces?

Quinone molecules have been widely studied as effective redox-active species for supercapacitors, but an understanding of the adsorption of quinones on activated carbon electrodes is very scarce. A hydroquinone molecule does not form a strong bond on pristine graphene, Stone−Wales defect, and double-vacancy surfaces, but it forms strong adsorption on single-vacancy surface. We demonstrate from first-principles calculations for various quinones that selecting an appropriate surface model is crucial in conducting a proper comparative study of the adsorption of quinone molecules. We suggest the single-vacancy graphene surface as a useful model for studying the adsorption of quinone molecules on an activated carbon electrode.

Unraveling the roles of iron in stabilizing the defective graphene-supported Pt[sbnd]Fe bimetallic nanoparticles

The adsorption behaviors of PtnFe(55-n) (n = 0, 11, 13, 18, 28, 37, 42, 44, 55) nanoparticles (NPs) on single vacancy graphene (SVG) were studied using density functional theory. It is found that the PtFe bimetallic NPs prefer to be adsorbed on the SVG through the Fe atoms when Pt and Fe atoms are both on the surfaces, because the interactions between Fe atoms and SVG are stronger than those between Pt atoms and SVG. Based on the density of states, molecular orbital and charge difference density analyses, the Fe atoms in the NPs can strongly interact with the sp2 dangling bonds and the π orbitals near the vacancy in the SVG at the same time. The interactions between the Pt atoms and the π orbitals are very weak. Meanwhile, due to the interactions between the Pt and Fe, the d-band centers of the Pt and Fe atoms in the NPs shift to lower energy, and the adsorption energies of PtnFe(55-n) (n = 18, 37, 42 and 44) NPs on the SVG are decreased compared with that of Pt55.

Adsorption of gas molecules and the induced magnetic properties of metal-decorated graphene

ABSTRACTBased on the first-principles of density-functional theory (DFT), the effects of gas adsorption on the change in adsorptive configurations, electronic structures and magnetic properties of graphene with magnetic metal atoms (MMA = Fe, Co, Ni and Mo) systems were investigated. Four metal atoms are strongly trapped at single vacancy graphene (MMA/SV-graphene) and can effectively regulate the adsorption of CO and O2 molecules. It is found that the positive charged of MMA are more prone to adsorb the O2 than that of CO molecule, and these gas molecules on MMA/SV-graphene induce a different type of magnetic properties of systems. Hence, the differences in electronic and magnetic properties of MMA/SV-graphene with the gas molecules are expected to design graphene-based gas sensors and spintronic device.

High-temperature quantum anomalous Hall effect in honeycomb bilayer consisting of Au atoms and single-vacancy graphene.

The quantum anomalous Hall effect (QAHE) is predicted to be realized at high temperature in a honeycomb bilayer consisting of Au atoms and single-vacancy graphene (Au2-SVG) based on the first-principles calculations. We demonstrate that the ferromagnetic state in the Au2-SVG can be maintained up to 380 K. The combination of spatial inversion symmetry and the strong SOC introduced by the Au atoms causes a topologically nontrivial band gap as large as 36 meV and a QAHE state with Chern number C = −2. The analysis of the binding energy proved that the honeycomb bilayer is stable and feasible to be fabricated in experiment. The QAHEs in Ta2-SVG and other TM2-SVGs are also discussed.

Modification of the catalytic properties of the Au4 nanocluster for the conversion of methane-to-methanol: synergistic effects of metallic adatoms and a defective graphene support.

Decorating graphene with nano-clusters offers potential for a wide range of industrial applications. For catalysis, embedding precisely controlled mono- and bimetallic nanoclusters into graphene can greatly increase their catalytic activities, especially for oxidation reactions. The catalytic performance of a gold nanocluster can be modified dramatically by changing its electronic properties. The results of this work demonstrate by means of DFT calculations that by strategic doping and promotion from the support material the catalytic activity improvement of a gold-based catalyst for the partial oxidation reaction of methane can be drastically enhanced. The transition metal-mediated catalysis is significantly affected by the two spin-state reactivities over them. The investigated catalytic processes consist of N2O decomposition and methane hydroxylation over three subnanoclusters (Au5, Au4Pd, and Au4Pt) deposited on a single vacancy graphene support. It was found that graphene acts not only as a support but also supports the catalysis through charge transfer between the subnanocluster and graphene. Graphene-supported Au4Pd exhibits enhanced catalytic activity for both steps of methane-to-methanol conversion, whereas the supported Au5 is good for N2O decomposition but ineffective for methane hydroxylation, mainly due to the involvement of a very stable intermediate (methyl-hydroxo-grafted nanocluster). The activation energies for N2O decomposition, C-H bond activation and methanol formation over the supported Au4Pd cluster are 13.8, 15.7, and 24.9 kcal mol(-1), respectively. Without the graphene support, the catalytic trend is reversed and Au4Pd becomes an inert cluster for these reactions.

Oxidation of Pdn (n = 1–5) clusters on single vacancy graphene: A first-principles study

We performed extensive density-functional calculations to investigate the stability and electronic structure of Pdn (n=1–5) clusters supported on single vacancy graphene surface, and adsorption of an oxygen molecule on such clusters were studied in detail. Interaction of the oxygen molecule with bare Pd cluster was also calculated for comparison. Our results show that point defect acts as a strong binding trap for Pd clusters and is favorable for the dispersion of the Pd nanoparticles. Electronic properties analysis shows that there are electrons flowing from Pd clusters to graphene. In addition, the change of the DOS for Pd cluster are mostly attributed to the hybridization of Pd atoms and the directly adjacented three dangling C atoms. Comparative analysis of oxygen molecule adsorption on bare Pd clusters and graphene-supported Pd clusters, the existence of graphene support promotes more electrons to transfer to oxygen molecule and further makes the O–O bond elongation, which indicates graphene may be used as an effective support material. Besides, an energy decomposition scheme is also employed to give insight into the impact of graphene support on the oxygen molecule adsorption behavior.

Nickel Dimers Adsorbed on Graphene: First-Principles Study

The structural stabilities, electronic, and magnetic properties of Ni dimers adsorption on pure and single-vacancy graphene were studied using a first-principle method. It was found that structures of the Ni dimer adsorbed on single-vacancy graphene are more stable than those structures of the Ni dimer adsorbed on pure graphene. Total energy calculation results demonstrated that for Ni dimer adsorption on single-vacancy graphene and pure graphene, the most stable structure is PV2 where the Ni dimer is positioned perpendicular to the vacancy site and PH1 where Ni dimer standing perpendicular to hollow site. Furthermore, it indicated covalent bond was formed between Ni atoms and their nearest C atoms, suggestive of chemisorption. The structures where Ni dimer lay horizontal to the pure graphene layer (H(T1–T3), H(B1–B2), H(B1–B3)) showed strong magnetism, while the structures of dimer-adsorption structures on single-vacancy graphene do not exhibit magnetism. The electronic origin of magnetic states was located on Ni atoms, specifically the d orbit. The Ni dimer, positioned parallel to the graphene plane, offers the possibility for the formation of Ni atom chain or wire on graphene, which might have potential nanoelectronic applications.

Predicted giant magnetic anisotropy energy of highly stable Ir dimer on single-vacancy graphene

We predict that a combined system of Ir dimer situated on the single-vacancy (SV) graphene is extremely stable and has high magnetic anisotropy energy (MAE), promising to be a potential candidate for high density magnetic storage. Our theoretical results show that high energy gradient in the SV graphene makes it possible to move the Ir dimer from the perfect graphene area into the center of SV (CSV), leading to the formation of the highly stable system with the Ir-Ir bond axis exactly perpendicular to the graphene plane. The calculated MAE value is as large as 25.7 meV per Ir atom. The origin of giant MAE in such a system is attributed to the difference of local densities of electronic states magnetized along different directions caused by special symmetry of Ir${}_{2}$ at CSV. We also propose a feasible avenue to assemble the Ir dimers on the graphene with the pattern of SV defects for actual applications.