Vacancy In Graphite Research Articles

Changes of Oxygen Adsorption State of Pt Nanoparticle Catalyst on the Carbon Support By the Ion Irradiation

Pt nanoparticles (NPs) on an Ar+-irradiated glassy carbon (GC) substrate were recently found to show a higher oxygen reduction reaction (ORR) activity than those on the non-irradiated one [1]. This finding suggests that the Pt nanoparticles would be electronically modified by vacancy introduction due to the irradiation into GC. Our theoretical research revealed that vacancies in graphite would lower a d-band center of the Pt nanoparticles [2]. The downshift of d-band center of Pt 5d electrons leads to higher ORR activity because the downshift causes the downshift of the antibonding orbital level of the oxygen adsorbed states of Pt (Pt-O) closing to Fermi level and then results in weaker Pt-O bond [3]. However, it has not been revealed whether the Pt-O antibonding level of Pt NPs is downshifted by the ion irradiation into the carbon support. Therefore, we performed in situ x-ray absorption fine structure (XAFS) measurements, which can observe the unoccupied states of the Pt 5d orbital binding with the adsorbed oxygen, of Pt NPs on the irradiated GC and derived the oxygen adsorbed surface states.The GC substrates were irradiated with 380 keV Ar+ at the fluences of 7.5×1015 ions/cm2 in the Takasaki Ion Accelerators for Advanced Radiation Application (TIARA) facility of the Takasaki Advanced Radiation Research Institute. The Pt nanoparticles were then deposited by RF magnetron sputtering. The Pt deposition was also done on the non-irradiated GC substrates for comparison. The XAFS spectra were measured at BL14B1 in SPring-8. We first measured the XAFS spectra of the irradiated and non-irradiated samples reduced by 1 atm. 100 °C H2 (10%) + He (90%) gas flowing at 100 ml/min for 10min. Then the samples were exposed under mixed gas with O2 + He gas flowing at 100 ml/min at room temperature to adsorb the oxygen to Pt nanoparticles and were measured XAFS spectra. In order to observe the oxygen adsorbed surface states of Pt NPs, we derived the Dm spectra by the subtraction from the XANES spectra after the reduction from the those during the oxygen exposure.Figure 1 shows the Dμ spectra for the Pt/GC and Pt/irradiated GC together with the results of peak fitting. In the non-irradiated sample, we could fit using one peak at around 11562 eV (denoted as peak 1), whereas needed peak 1 in addition to peak 2 on the higher energy side. Since the Dμ spectra represent the change of the electronic state in Pt by oxygen adsorption, the convex peak means that new electronic states are formed at the energy position by the oxygen adsorption. The main peak (peak 1) is the antibonding level of the oxygen adsorbed Pt (Pt-O). On the other hand, the peak 2 is considered to be an oxide species that is more strongly bind to oxygen than Pt-O. It needs future study to elucidate its origin.Comparing the peak 1 of the non-irradiated and the irradiated sample, we found that the peak position for the irradiated sample is 0.7 eV lower than that for the non-irradiated one. This indicates that the Pt-O antibonding level shifts toward low energy and gets close to the Fermi level. Thus, the shift leads to easy electron filling to the antibonding level and thus to a weaker Pt-O bond. In this presentation, we will discuss the change of electronic structure of Pt nanoparticles by the ion irradiation in detail.

The Electronic State of Pt Nanoparticles on an Ion-Beam-Irradiated Carbon Support

Pt nanoparticle electrocatalysts on an Ar+-irradiated glassy carbon (GC) substrate were recently found to show a higher oxygen reduction reaction (ORR) activity than those on the non-irradiated one [1]. This finding suggests that the Pt nanoparticles would be electronically modified by the lattice defects in irradiated GC. Our theoretical research previously revealed that vacancies in graphite would lower a d-band center of the supported Pt nanoparticles; this is probably the reason for the higher activity [2]. We investigated here the electronic states of the interface between the Pt nanoparticles and the irradiated carbon support by X-ray absorption fine structure (XAFS) measurements in order to get an insight into the mechanism of the activity enhancement.Highly oriented pyrolytic graphite (HOPG) and glassy carbon (GC) substrates were irradiated with 380 keV Ar+. The HOPG substrate was employed so that the electronic structure could be resolved into orbital components (σ and π bonds). The Pt nanoparticles were then deposited by RF magnetron sputtering. The Pt deposition was also done on the non-irradiated HOPG and GC substrates for comparison. The XAFS spectra were measured at BL14B1 in SPring-8 and at BL-8 of the SR Center, Ritsumeikan University.Figure 1 shows the C-K XAFS spectra for the pristine HOPG, the irradiated HOPG, Pt/non-irradiated HOPG, and Pt/irradiated HOPG. This spectral measurement was made at the X-ray incident angle of 30° to obtain the X-ray absorption due to C-C π* and s* orbitals with almost the same cross-section. The pristine and irradiated HOPG substrates exhibited three peaks from a C-C π* orbital (285.3 eV), C-C σ*, σ1 orbital (291.5 and 292.5 eV), and surface carbon with adsorbates (287.5 eV). The shoulder structure around 284 eV was observed for Pt/non-irradiated HOPG, indicating the occurrence of Pt-C interaction at the interface. The intensity of this shoulder increases for Pt/irradiated HOPG as shown by the arrow. Therefore, the vacancies in the irradiated carbon support would induce the change of Pt-C interaction, leading to the different electronic structure at the interface.Our previous paper reported that the Pt nanoparticles on the irradiated carbon support were distorted [3]. According to the result of Fig. 1, the Pt-C interaction could be an origin of the observed distortion of the Pt nanoparticles, which lowered the d band center and then improved their ORR activity. In this presentation, we will additionally clarify the electronic structure of Pt nanoparticles based on the XAFS results of Pt L edge.

Magnetic states of atomic vacancies in graphite probed by scanning tunneling microscopy

Intrinsic defects in graphitic materials, like vacancies and edges, have been expected to possess magnetic states from the many-body interaction of localized electrons. However, charge screening from graphite bulk carriers significantly reduces the localization effect and hinders the observation of those magnetic states. Here, we use an ultra-low-temperature scanning tunneling microscope with a high magnetic field to observe the magnetic states of atomic vacancies in graphite generated by ion sputtering. Scanning tunneling spectroscopy reveals localized states at the vacancies, which exhibit splitting at a certain magnetic field whose separation increases with the field strength. The transition is well described by the “Anderson model,” which describes the emergence of localized magnetic states inside the metallic reservoir through electron–electron interaction. The interaction strength is estimated to be between 1 meV and 3 meV, which is supported by the density functional theory calculation. The observation provides an important foundation for application of intrinsic defects to carbon-based spintronic devices.

Mechanochemical formation of heterogeneous diamond structures during rapid uniaxial compression in graphite

We predict mechanochemical formation of heterogeneous diamond structures from rapid uniaxial compression in graphite using quantum molecular dynamics simulations. Ensembles of simulations reveal the formation of different diamondlike products starting from thermal graphite crystal configurations. We identify distinct classes of final products with characteristic probabilities of formation, stress states, and electrical properties and show through simulations of rapid quenching that these products are nominally stable and can be recovered at room temperature and pressure. Some of the diamond products exhibit significant disorder and partial closure of the energy gap between the highest-occupied and lowest-unoccupied molecular orbitals (i.e., the HOMO-LUMO gap). Seeding atomic vacancies in graphite significantly biases toward forming products with small HOMO-LUMO gap. We show that a strong correlation between the HOMO-LUMO gap and disorder in tetrahedral bonding configurations informs which kinds of structural defects are associated with gap closure. The rapid diffusionless transformation of graphite is found to lock vacancy defects into the final diamond structure, resulting in configurations that prevent $s{p}^{3}$ bonding and lead to localized HOMO and LUMO states with a small gap.

Mechanical Properties of Vacancy-containing Graphene and Graphite Estimated by Molecular Dynamics Simulations

ABSTRACTUsing molecular dynamics (MD) simulation, we investigated the mechanical properties of graphene and graphite, which contain cluster-type vacancies. We found that as the vacancy size increases, the tensile strength drastically decreases to at least 56% of that of pristine graphene, whereas Young’s modulus hardly changes. In vacancy-containing graphene, we also found that slip deformation followed by fracture occurs under zigzag tension. In general, tensile strength decreases as the size of cluster-type vacancies increases. However, the tensile strength of graphene with a clustered sextuple vacancy increases as the vacancy disappears because slip deformation proceeds. Furthermore, we found that slip deformation by vacancies in graphite occurs less easily than in graphene.Our results suggest that the shape of vacancies affects the strengths of graphene and graphite.

Atomic Vacancy Engineering of Graphitic Surfaces: Controlling the Generation and Harnessing the Migration of the Single Vacancy

The selective introduction of atomic-scale defects with tunable density in graphite, graphene, and carbon nanotubes is highly desirable, as it could afford a better control of their properties, but difficult to realize in practice. Here, we present a plasma-based chemical approach for the selective generation of single vacancies on the graphite surface with densities between ∼5 × 102 and 3 × 105 μm−2 and apply it to the investigation of the migration behavior of this type of defect. Through scanning tunneling microscopy (STM) observation of vacancy-decorated graphite surfaces heat-treated to several different temperatures, the migration barrier for the single vacancy was deduced to be ∼0.9−1.0 eV but could be reduced to ∼0.7 eV via interaction with the STM tip. This constitutes the first direct experimental measurement of the migration barrier for the single vacancy in graphite that is consistent with theoretical calculations.

Structure and energetics of the vacancy in graphite

We determine properties of the vacancy in graphite from first principles calculations. The ground-state structure is associated with a formation energy of 7.4 eV and arises through a combination of symmetric relaxation and symmetry-breaking Jahn-Teller distortion to one of three degenerate, symmetry-related structures. The distortion results in a weak reconstructed bond and small out-of-plane atomic displacements. Dynamic switching between degenerate structures is activated by a barrier of 0.1 eV and we interpret scanning tunneling microscopy observations on the basis of thermal averaging between structures. The calculated migration energy of 1.7 eV is lower than that widely accepted from experiment, and we propose that the discrepancy is explained by a revised picture of trapping during vacancy transport, dependent on concentration. We discuss the significance of these findings in understanding defect behavior in irradiated graphite and related graphitic materials, in particular single-walled nanotubes.

Interpretation of scanning probe microscope image of the structure of atomic vacancy on graphite: ab initio periodic Hartree–Fock calculations

By using the CRYSTAL 95 program, ab initio periodic Hartree–Fock (PHF) calculations with the full potential and 6-21G∗ basis set are applied to the interpretation of scanning tunneling microscope (STM) and atomic force microscopy (AFM) image of the structure of atomic vacancy in graphite. Our results show that change in the total electron density when compared with the ideal structure is considerable at the missing atom site, but it is not considerable beyond a distance upon the missing atom site. However, the partial charge density, which originates from the states near the Fermi energy is enhanced at the atoms surrounding the vacancy. These results are in a good agreement with those of Hahn et al. (Phys. Rev. B 53 (1996) R1725), who take that vacancies are transparent in AFM but imaged as surface protrusions in STM. In conclusion, our results should clarify the reason why vacant sites of graphite are visible shown in STM, while they are not easy to detect in AFM.

Modeling vacancies in graphite via the Hückel method

It is known that when irradiating a graphite surface with ions the predominant source of defects are vacancies, Vacancies are also formed in a growing graphite sheer and cannot be filled. The theor ...

Rigidity of randomly intercalated layered solids.

We set up a harmonic spring model that describes both the layer rigidity and the size and stiffness of the intercalant species. In certain limiting cases, when (1) the layers are either perfectly floppy or perfectly rigid and (2) the stiffness of the two intercalant species are the same, the model can be solved exactly. We also give an effective-medium solution that reproduces all the known exact results, and agrees well with numerical simulations in other cases. These simulations are performed for both one- and two-dimensional systems. If the two intercalant species have the same spring constant, Vegard's law is recovered. We compute the probability distribution of the various interlayer distances and apply the results to two-dimensional alloys of Li and vacancies in graphite, and to K and Rb in graphite.

Vacancy in Graphite: Positron Srudies

The positron techniques of lifetime and Doppler-broadening were applied to vacancies in graphite. Vacancies were introduced into highly-oriented pyrolytic graphite by irradiation with 2 MeV electrons and fission reactor neutrons. The lifetime of positrons trapped at the vacancies was determined as 245±5 ps. Lifetime studies revealed two distinct vacancy annealing stages at 600–900°C and at 1300–1500°C. The Doppler-broadened lineshapes of the annihilation gamma-ray were taken in the directions perpendicular and parallel to the c -axis of the graphite lattice. Momentum distribution for an electron-positron pair at a vacancy was deduced and compared with the results of calculation based on the defect molecule model in the framework of LCAO-MO theory.

A defect molecule calculation for the vacancy in graphite

The defect-molecule calculations of Coulson et al. [2] of the electronic structure of a vacancy in graphite have been improved and extended to allow for symmetric relaxation. Various problems encountered previously [1,2] have been shown to be due to the use of the Goeppert-Mayer and Sklar approximation. The rebonding forces on the nearest-neighbour atoms to the vacancy have been calculated and a perfect-lattice model was used to describe lattice relaxation effects; the difficulties experienced in work on diamond and silicon [5,6] using an imperfect-lattice model were avoided. For the neutral vacancy, the stabilization energy, E v , was found to be 1.44eV, and the (symmetric) relaxation energy, E R , to be 3.32 eV. The neutral vacancy formation energy has been re-calculated, and using the above values for E v and E R a value of 8.75 eV was obtained. This is somewhat higher than experimental estimates, but the difference may be explained by a high value for the formation entropy.

9. A calculation of the lattice distortion produced by a vacancy in graphite

9. A calculation of the lattice distortion produced by a vacancy in graphite

THE DETECTION OF VACANCIES IN MOLYBDENITE

The etch-decoration method developed to detect lattice vacancies in graphite has been extended to the detection of lattice vacancies in molybdenite. Observations indicate that vacancies occur at sites from which a molybdenum atom is missing and that etch pits developed from these vacancies may be several layers deep; however, gold decoration can reveal step heights one layer deep (6.15 Å).

Determination of Lattice Vacancies in Graphite

Vacancies in graphite crystals can be rendered visible for electron microscopy by a technique which combines cleaving, etching, and decorating. The crystals are cleaved, and the vacant lattice sites in the exposed surface are attacked by a mixture of chlorine and oxygen, which expands these vacancies into monolayer loops of any desired diameter. These loops are decorated with gold to enhance their contrast. Numerous tests have established that the loop concentration is a quantitative measure of the vacancy concentration. The method is capable of determining vacancies at concentrations at least as low as 10—10 per carbon atom. The relative chemical reactivities of carbon atoms surrounding a vacancy, of carbon atoms at the edges of layers and of carbon atoms within layers have been measured.

Bond Energy of Vacancies and Interstitial Atoms in Graphite

Baker and Kelly1 have estimated that the energy to form a vacancy in graphite (that is, the energy to remove an atom from the lattice and place it on the free surface) is Uf = 3.3±0.9 eV. (The symbol ± refers to maximum limit of values.) They comment that this is less than the heat of sublimation, Us = 7.5 eV per atom, indicating an energy-relaxation of 4.2 ± 0.9 eV at the vacancy. Equality should occur2 if the broken lattice bonds around the vacancy remain broken. In graphite, however, all the lattice bonds are unsaturated, and we would expect that the unpaired electrons of the three atoms around the vacancy would form covalent bonds with electrons derived from unsaturated bonds of adjacent atoms (cf. ref. 3). These we will call partial bonds. The net energy associated with their formation (denoted by 3U1 per vacancy) we identify with the required energy relaxation. Thus:

VACANCIES AND DISLOCATION LOOPS IN GRAPHITE

Electron microscope studies were undertaken to determine concentrations and mobilities of vacancies in graphite. Crystals were quenched and annealed to various temperatures. Crystals under strain (some deliberately bent) before heat treatment contained larger numbers of loops while crystals treated under strain- free conditions showed no loops. Crystals quenched from 3000 deg C showed no loops even when bent or irradiated with neutrons, indicating that loops are not produced by coalescing of excess quenchedin vacancies. (D.C.W.)