Double Carbon Vacancy Research Articles

Temperature and pressure driven functionalization of graphene with hydrogen and oxygen via ab initio phase diagrams

Functionalized graphene has numerous applications due to its remarkable and tunable physicochemical properties. However, precise control of functionalization remains challenging as direct connections between defect and functional group stability on graphene as a function of treatment conditions are missing. Here, a systematic study of graphene functionalization with hydrogen and oxygen is performed using a combination of density functional theory and ab initio phase diagrams. The interplay between carbon defects (e.g. vacancies, Stone-Wales, 555–777) and functionalization (e.g. R-H, R-H2, R-O, R-OH, and R-COOH) is examined. Generally, the process of functionalizing graphene led to significantly more stable carbon defects. By calculating Gibbs free formation energies with varying temperatures and partial pressures of H2 and H2O, ab initio phase diagrams were constructed. These diagrams revealed that the applied hydrogen and oxygen chemical potentials can be balanced during treatment to generate six distinct structures: R-H, R-O, R-OH, and R-COOH on the basal plane; and R-O and R-H2 on a double carbon vacancy. Through this systematic study, both the treatment conditions conducive to the formation of specific functional groups on graphene are pinpointed and a range of stable and metastable graphene-based structures with applications in various fields are identified.

Excellent sulfur and water resistance for CO oxidation on Pt single-atom-catalyst supported by defective graphene: The effect of vacancy type

The competitive adsorption of sulfur dioxide and water molecules will degrade the performance for CO oxidation catalyst in some processes such as automobile exhaust cleaning or Claus tail gas treatment. In order to study the effect of carbon vacancy on the sulfur and humidity resistance, the adsorption of CO, O2, SO2 and H2O molecules on Pt single-atom-catalyst supported by graphene with single carbon vacancy (Pt-SG) and double carbon vacancy (Pt-DG) have been comparatively investigated by using density functional theory calculations. It shows that the active site of Pt-SG can be blocked by the SO2 and H2O molecules due to their strong affinity towards Pt-SG. However, the carbon divacancy makes Pt atom less attractive towards SO2 and H2O molecules on Pt-DG compared with that on Pt-SG, where the pre-adsorbed SO2 and H2O molecules will be substituted by the CO molecule with larger adsorption energy on Pt-DG, which finally achieves the sulfur and water resistance for Pt-DG. In addition, an efficient Termolecular Eley-Rideal reaction path with a low energy barrier of 0.49 eV for CO oxidation is found on Pt-DG. The significant downshift of CO-5σ orbital levels plays an important role in the formation of new O-C bonds in OCOOCO intermediate on Pt-DG, where the O2-1π orbital is weakened due to its interaction with CO-5σ and CO-1π orbital, which finally facilitates the break of O-O bond and the production of CO2 molecules along the TER mechanism. Overall, the double carbon vacancy can enhance the selective adsorption for CO molecule other than the SO2 and H2O molecules on Pt atom, making the Pt-DG to be an efficient CO oxidation catalyst even in sulfur and humid environment.

Effect of boron substitution on hydrogen storage in Ca/DCV graphene: A first-principle study

By using density functional calculations, the effects of boron are investigated in the new hydrogen storage systems, which are formed by substituting different numbers of boron atoms to the first (BDDCV-F) and the second (BDDCV-S) neighbor of double carbon-vacancy (DCV). The layered host systems of boron-substituted DCV graphene are decorated with Ca metal to increase the number of adsorbed H2 molecules. Storing of H2 applications are performed by using two coordinate algorithms as CLICH (Cap-Like Initial Conditions for Hydrogens) and RICH (Rotational Initial Conditions for Hydrogens). The adsorption properties of (1–14) H2 molecules on the constructed systems are examined. The results for BDDCV-F and BDDCV-S boron-doped systems are compared with each other and those of the pure-DCV graphene. To compare the stabilities of BDDCV-systems, the formation energies are calculated. It is concluded from Mulliken charge analysis, the partial density of states and electron density differences that boron substitution process to different locations of the DCV graphene plays a crucial role on the charge transfer between Ca atom the layered host system, ionic nature and the binding properties of the systems. The herringbone-like anisotropic electron density is transformed to isotropic density with the substitution of the boron atoms. Then, the electric field, which is induced by ionic interactions and governs H2 adsorption processes, is changed and intensified along with the sheet. In this way, it can be achieved more effective H2 adsorption. It is seen from the adsorption energies of single- and double-side Ca-decorated systems that the processes of boron-substitution and Ca-decoration can considerably improve the hydrogen storage capability of pure-DCV graphene system, thus (8 and 10)H2 can be adsorbed per Ca-atom in these-type systems. The high gravimetric density of 5.80% is calculated, although larger cell and empty surface states. Moreover, the average desorption temperatures are calculated by using van't Hoff equation, and it is seen that the DCV including boron-substituted systems have closer desorption temperatures to the room temperature than pure-DCV. To check the H2 desorption of the systems, molecular dynamics simulations are performed at 200 K and 300 K temperatures.

Ab initio STUDY OF POINT DEFECTS IN 2D GRAPHENE LAYER

With a hexagonal (honeycomb) network of mono-layered carbon atoms, graphene has demonstrated outstanding electronic properties. This work describes the impact of deliberately introduced single, double and triple carbon vacancies in grapheme monolayer. In addition, these carbon vacancies were then substituted with gold atoms and their influence on the electronic properties of the two-dimensional (2D) graphene layers was investigated. In this regard, a first principle calculation was performed to examine electronic properties and formation energy of 2D graphene layer by applying density functional theory (DFT). Introduction of such defects appeared to increase the stability of the graphene sheet as confirmed by formation energy calculations. Moreover, decrease of formation energy was noticed to be significant with an increase in the number of defects. Band structure calculations described the shifting of localized states from valance to conduction bands which caused the transformation of semiconducting behavior into metallic one on the filling of carbon vacancies by gold atoms. Comparing this behavior with that of partial density of states (PDOS) it was noted that a lot of states existed in the valance band in the case of C-vacancies yielding charge free region around the vacancy. On the other hand, filling of C-vacancies by gold generated a large number of energy states in the conduction band illustrating the accumulation of charges near gold atom. Width of the peak across the Fermi level indicated the accumulative energy of electron to be almost 0.15[Formula: see text]eV. These calculated DOS and PDOS demonstrated metallic like behavior of the graphene monolayer with typical defect states.

The characterization of electronic defect states of single and double carbon vacancies in graphene sheets using molecular density functional theory

ABSTRACTA detailed picture of the electronic states manifolds of single- and double-vacancy defects in molecular models of graphene based on polycyclic aromatic hydrocarbons (PAHs) is presented. DFT calculations using various density functionals including long-range corrected ones have been performed for pyrene, circumpyrene and 7a,7z-periacene. It has been found for pyrene defect models that DFT results reproduced well the set of closely-spaced singlet and triplet states predicted by the CCSD(T) and previous MRCI + Q calculations, indicating the applicability of DFT for accessing the excited states manifolds also for larger graphene models. For the single-carbon vacancy defect, all structures have a triplet ground state. As expected, in the largest system, 7a,7z-periacene-1C, the lowest lying states are much closer in energy. For all double-vacancy defect structures, a significant rearrangement of the electronic states with increasing size of the sheet is observed. The closed-shell 1Ag state in the smallest systems is destabilised in the extended 7a,7z-periacene system, which has the 3B2u state as the ground state. As observed for the single-vacancy defect, the lowest lying states are closer in energy for the larger systems, since there are more π orbitals close in energy available. For all states, the formation of the bridging bonds for the double vacancy leads to distances shorter than for the single vacancy defect indicating a larger rigidity of the former structure which does not allow stronger distortions.

Co-doped graphene sheets as a novel adsorbent for hydrogen storage: DFT and DFT-D3 correction dispersion study

In this study, transition metals (TM) such as palladium (Pd) have been introduced on co-doped graphene and defect graphene sheets with nitrogen and boron ad-atoms to investigate the potentials of new adsorbents for hydrogen storage. The first principle studies using density functional theory (DFT) and DFT-D3 correction dispersion were undertaken to calculate the adsorption energy of hydrogen molecule on the graphene sheet. The results showed that applying Pd transition metal could enhance adsorption energy of hydrogen molecules towards pristine sheet. The main problem in applying transition metal on graphene sheets was concerned with clustering. However, the current defects in graphene sheets prevent clustering event. Our simulation results suggested that these defects reduced hydrogen adsorption and substitute dopants such as nitrogen and boron together on graphene sheets could improve the adsorption energy. Thus, two various forms of Pd decorated NB co-doped as hexagonal and double carbon vacancy (DCV) were introduced as new structures for hydrogen storage. A physical adsorption, which is appropriate for reversible hydrogen storage, was implemented for both novel adsorbents. In the two various forms of NB co-doped structures, DCV had the optimum adsorption behavior as adsorption energy level and density of state (DOS) phenomena. Moreover, the results of adsorption energy using DFT method were consistent with that of DFT-D3 correction dispersion and higher amounts of adsorption energy in DFT-D3 method were obtained. Finally, results introduced Pd decorated NB co-doped graphene sheets as a novel material for hydrogen storage.

Ab initio phonon point defect scattering and thermal transport in graphene

We study the scattering of phonons from point defects and their effect on lattice thermal conductivity $\ensuremath{\kappa}$ using a parameter-free ab initio Green's function methodology. Specifically, we focus on the scattering of phonons by boron (B), nitrogen (N), and phosphorus substitutions as well as single- and double-carbon vacancies in graphene. We show that changes of the atomic structure and harmonic interatomic force constants locally near defects govern the strength and frequency trends of the scattering of out-of-plane acoustic (ZA) phonons, the dominant heat carriers in graphene. ZA scattering rates due to N substitutions are nearly an order of magnitude smaller than those for B defects despite having similar mass perturbations. Furthermore, ZA phonon scattering rates from N defects decrease with increasing frequency in the lower-frequency spectrum in stark contrast to expected trends from simple models. ZA phonon-vacancy scattering rates are found to have a significantly softer frequency dependence ($\ensuremath{\sim}{\ensuremath{\omega}}^{0}$) in graphene than typically employed in phenomenological models. The rigorous Green's function calculations demonstrate that typical mass-defect models do not adequately describe ZA phonon-defect scattering rates. Our ab initio calculations capture well the trend of $\ensuremath{\kappa}$ vs vacancy density from experiments, though not the magnitudes. This work elucidates important insights into phonon-defect scattering and thermal transport in graphene, and demonstrates the applicability of first-principles methods toward describing these properties in imperfect materials.

Structure and electronic states of a graphene double vacancy with an embedded Si dopant.

Silicon represents a common intrinsic impurity in graphene, bonding to either three or four carbon neighbors, respectively, in a single or double carbon vacancy. We investigate the effect of the latter defect (Si-C4) on the structural and electronic properties of graphene using density functional theory. Calculations based both on molecular models and with periodic boundary conditions have been performed. The two-carbon vacancy was constructed from pyrene (pyrene-2C) which was then expanded to circumpyrene-2C. The structural characterization of these cases revealed that the ground state is slightly non-planar, with the bonding carbons displaced from the plane by up to ±0.2 Å. This non-planar structure was confirmed by embedding the defect into a 10 × 8 supercell of graphene, resulting in 0.22 eV lower energy than the previously considered planar structure. Natural bond orbital analysis showed sp3 hybridization at the silicon atom for the non-planar structure and sp2d hybridization for the planar structure. Atomically resolved electron energy loss spectroscopy and corresponding spectrum simulations provide a mixed picture: a flat structure provides a slightly better overall spectrum match, but a small observed pre-peak is only present in the corrugated simulation. Considering the small energy barrier between the two equivalent corrugated conformations, both structures could plausibly exist as a superposition over the experimental time scale of seconds.

Hydrogen storage in Beryllium decorated graphene with double vacancy and porphyrin defect — A first principles study

Beryllium (Be)-decorated graphene with 585 double carbon vacancy defect and nitrogen-doped porphyrin defect are investigated for hydrogen storage applications using the first principle calculation based on density functional theory. It is found that the Be atom disperses well in the defective sites of graphene and prevents clustering. For the case of Be-decorated 585 double vacancy graphene, only two H2 molecules are adsorbed via Kubas interaction with the stretched H–H bond length of 0.8 Å. In Be-decorated porphyrin defect graphene system, four H2 molecules are molecularly chemisorbed with the H–H bond length of 0.77 Å. The chemisorptions are due to the hybridization between Be-p orbital and the H-[Formula: see text] orbital. The average binding energy of H2 molecule is found to be 0.43[Formula: see text]eV/H2 which lies within the required range that can permit recycling of H2 molecules under ambient conditions.

Enhanced Absorption and Diffusion Properties of Lithium on B,N,VC-decorated Graphene.

Systematic first-principles calculations were performed to investigate the adsorption and diffusion of Li on different graphene layers with B/N-doping and/or C-vacancy, so as to understand why doping heteroatoms in graphene anode could significantly improve the performance of lithium-ion batteries. We found that the formation of single or double carbon vacancies in graphene are critical for the adsorption of Li atoms. While the N-doping facilitates the formation of vacancies, it introduces over binding issue and hinders the Li diffusion. The presence of B takes the excessive electrons from Li and N and reduces the energy barrier of Li diffusion on substrates. We perceive that these clear insights are crucial for the further development of graphene based anode materials for lithium-ion batteries.

Single and double carbon vacancies in pyrene as first models for graphene defects: A survey of the chemical reactivity toward hydrogen

Graphene is regarded as one of the most promising materials for nanoelectronics applications. Defects play an important role in modulating its electronic properties and also enhance its chemical reactivity. In this work the reactivity of single vacancies (SV) and double vacancies (DV) in reaction with a hydrogen atom Hr is studied. Because of the complicated open shell electronic structures of these defects due to dangling bonds, multireference configuration interaction (MRCI) methods are being used in combination with a previously developed defect model based on pyrene. Comparison of the stability of products derived from CHr bond formation with different carbon atoms of the different polyaromatic hydrocarbons is made. In the single vacancy case the most stable structure is the one where the incoming hydrogen is bound to the carbon atom carrying the dangling bond. However, stable CHr bonded structures are also observed in the five-membered ring of the single vacancy. In the double vacancy, most stable bonding of the reactant Hr atom is found in the five-membered rings. In total, CHr bonds, corresponding to local energy minimum structures, are formed with all carbon atoms in the different defect systems and the pyrene itself. Reaction profiles for the four lowest electronic states show in the case of a single vacancy a complex picture of curve crossings and avoided crossings which will give rise to a complex nonadiabatic reaction dynamics involving several electronic states.

Sensing sulfur-containing gases using titanium and tin decorated zigzag graphene nanoribbons from first-principles.

Atom implantation in graphene or graphene nanoribbons offers a rich opportunity to tune the material structure and functional properties. In this study, zigzag graphene nanoribbons with Ti or Sn adatoms stabilised on a double carbon vacancy site are theoretically studied to investigate their sensitivity to sulfur-containing gases (H2S and SO2). Due to the abundance of oxygen in the atmosphere, we also consider the sensitivity of the structures in the presence of oxygen. Density functional theory calculations are performed to determine the adsorption geometry and energetics, and nonequilibrium Green's function method is employed to compute the current-voltage characteristics of the considered systems. Our results demonstrate the sensitivity of both Ti- and Sn-doped systems to H2S, and the mild sensitivity of Ti-doped sensor systems to SO2. The Ti-doped sensor structure exhibits sensitivity to H2S with or without oxidation, while oxidation of the Sn-doped sensor structure reduces its ability to adsorb H2S and SO2 molecules. Interestingly, oxygen dissociates on the Ti-doped sensor structure, but it does not affect the sensor's response to the H2S gas species. Oxidation prevents the dissociation of the H-S bond when H2S adsorbs on the Ti-doped structure, thus enhancing its reusability for this gas species. Our study suggests the potential of Ti- and Sn-doped graphene in selective gas sensing, irrespective of the sensing performance of the bulk oxides.

The electronic states of a double carbon vacancy defect in pyrene: a model study for graphene.

The electronic states occurring in a double vacancy defect for graphene nanoribbons have been calculated in detail based on a pyrene model. Extended ab initio calculations using the MR configuration interaction (MRCI) method have been performed to describe in a balanced way the manifold of electronic states derived from the dangling bonds created by initial removal of two neighboring carbon atoms from the graphene network. In total, this study took into account the characterization of 16 electronic states (eight singlets and eight triplets) considering unrelaxed and relaxed defect structures. The ground state was found to be of (1)Ag character with around 50% closed shell character. The geometry optimization process leads to the formation of two five-membered rings in a pentagon-octagon-pentagon (5-8-5) structure. The closed shell character increases thereby to ∼70%; the analysis of unpaired density shows only small contributions confirming the chemical stability of that entity. For the unrelaxed structure the first five excited states ((3)B3g, (3)B2u, (3)B1u, (3)Au and (1)Au) are separated from the ground state by less than 2.5 eV. For comparison, unrestricted density functional theory (DFT) calculations using several types of functionals have been performed within different symmetry subspaces defined by the open shell orbitals. Comparison with the MRCI results gave good agreement in terms of finding the (1)Ag state as a ground state and in assigning the lowest excited states. Linear interpolation curves between the unrelaxed and relaxed defect structures also showed good agreement between the two classes of methods opening up the possibilities of using extended nanoflakes for multistate investigations at the DFT level.

Hydrogen adsorption and storage on palladium-decorated graphene with boron dopants and vacancy defects: A first-principles study

The geometric stability and hydrogen capacity of Pd-decorated graphene with experimentally realizable boron dopants and various vacancy defects including single carbon vacancy (SV), “585”-type double carbon vacancy (585 DCV) and “555-777”-type double carbon vacancy (555-777 DCV) are investigated using the first-principles calculations based on density functional theory (DFT). It is found that among the four types of defective structures, Pd′s binding energies on SV and 585 DCV defect graphene sheets exceed the cohesive energy of the Pd metal bulk, thus Pd atoms are well dispersed above defective graphene sheets and effectively prevent Pd clustering. Up to three H2 molecules can bind to Pd atom on graphene with B dopants, SV and 555-777 DCV defects. For the cases of Pd-decorated graphene with B dopants and 555-777 DCV defect, a single H2 or two H2 are molecularly chemisorbed to Pd atom in the form of Pd–H2 Kubas complex, where the stretched H–H bond is relaxed but not dissociated. Out of two adsorbed H2, the third H2 binds to Pd atom by small van der Waals (vdW) forces and the nature of bonding is very weak physisorption. Different from above two cases, three H2 are all molecularly chemisorbed to Pd atom with stretched H–H bond for Pd-decorated SV defect graphene, the hybridization of the Pd-4d orbitals with the H2-σ orbitals and the electrostatic interaction between the Pd cation and the induced H2 dipole both contribute to the H2 molecules binding, and the binding energies of 0.25–0.41eV/H2 is in the range that can permit H2 molecules recycling at ambient conditions.

Li-decorated double vacancy graphene for hydrogen storage application: A first principles study

Abstract Lithium decoration is an effective strategy for improving the hydrogen adsorption binding energy and the storage capacity in carbon nanostructures. Here, it is shown that Li-decorated double carbon vacancy graphene (DVG) can be used as an efficient hydrogen storage medium by means of Density Functional Theory (DFT) based calculations. The Li binding energy in DVG is 4.04 eV, which is much higher than that of pristine graphene. A maximum of four hydrogen molecules adsorb on Li decorated on one side of DVG and this leads to a gravimetric storage capacity of 3.89 wt% with an average adsorption binding energy of 0.23 eV/H 2 . When Li is decorated on both sides of DVG, the gravimetric storage capacity reaches 7.26 wt% with a binding energy of 0.26 eV/H 2 which shows that desorption would take place at ambient conditions.

Hydrogen adsorption and storage of Ca-decorated graphene with topological defects: A first-principles study

As a candidate for hydrogen storage medium, geometric stability and hydrogen capacity of Ca-decorated graphene with topological defects are investigated using the first-principle based on density functional theory (DFT), specifically for the experimentally realizable single carbon vacancy (SV), 585 double carbon vacancy (585 DCV) and 555–777 double carbon vacancy (555–777 DCV) defects. It is found that Ca atom can be stabilized on above defective graphenes since Ca׳s binding energy on vacancy defect is much larger than its cohesive energy. Up to six H2 molecules can stably bind to a Ca atom on defective graphene with the average adsorption energies of 0.17–0.39eV/H2. The hybridization of the Ca-3d orbitals with H2-σorbitals and the electrostatic interaction between the Ca cation and the induced H2 dipole both contribute to the H2 molecules binding. Double-side Ca-decorated graphene with 585 DCV and 555–777 DCV defects can theoretically reach a gravimetric capacity of 5.2wt% hydrogen, indicating that Ca-decorated defective graphene can be used as a promising material for high density hydrogen storage.

Hydrogen adsorption capacity of adatoms on double carbon vacancies of graphene: A trend study from first principles

Structural stability and hydrogen adsorption capacity are two key quantities in evaluating the potential of metal-adatom decorated graphene for hydrogen storage and related devices. We have carried out extensive density functional theory calculations for the adsorption of hydrogen molecules on 12 different adatom (Ag, Au, Ca, Li, Mg, Pd, Pt, Sc, Sr, Ti, Y, and Zr) decorated graphene surfaces where the adatoms are found to be stabilized on double carbon vacancies, thus overcoming the ``clustering problem'' that occurs for adatoms on pristine graphene. Ca and Sr are predicted to bind the greatest number, namely six, of H${}_{2}$ molecules. We find an interesting correlation between the hydrogen capacity and the change of charge distribution with increasing H${}_{2}$ adsorption, where Ca, Li, Mg, Sc, Ti, Y, Sr, and Zr adatoms are partial electron donors and Ag, Au, Pd, and Pt are partial electron acceptors. The ``18-electron rule'' for predicting maximum hydrogen capacity is found not to be a reliable indicator for these systems.