Edge Vacancies Research Articles

Sorption behavior of heavy metals on birnessite: Relationship with its Mn average oxidation state and implications for types of sorption sites

Abstract Sorption mechanisms of heavy metals at the mineral/water interface are largely controlled by the type and number of sorption sites on the mineral surfaces. However, in the case of layered manganese oxides, with a highly reactive interlayer (internal) region, the effects of variable substructures on their sorption sites, sorption capacities and characteristics are still obscure. Sorption experiments at pH 4.5 combined with powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were performed to investigate the sorption characteristics and mechanisms of Pb 2 + , Cu 2 + , Zn 2 + and Cd 2 + onto hexagonal birnessites with various Mn average oxidation states (AOS), together with detailed physicochemical characterizations of the birnessites. The results show that a decrease in Mn AOS of birnessites decreased considerably the sorption capacity for heavy metals, despite the fact that the specific surface area increased almost linearly. The sorption capacity for any given birnessite followed the order Pb 2 + ≫ Cu 2 + > Zn 2 + > Cd 2 + ; for Pb 2 + ranging from 1.6 to 3.9 times greater than those of the other metals, while Cu 2 + , Zn 2 + , and Cd 2 + sorbed with similar maxima among them. The large differences between maximum Pb 2 + sorption and that of the other metals were approximately of the same absolute magnitude regardless of the specific birnessites compared, as opposed to decreasing in equal relative proportions as would be expected in a homogeneous-site model. This evidence agrees well with previous work on hexagonal birnessites that proposes a general two-site structural model for metal-binding to birnessite, and the sorption maxima differences obtained in this work were used to estimate the concentration of low-energy and high-energy binding sites in the different birnessites. Relating these to previous structural information allowed us to assign these site types roughly to particle edge sites and vacancy sites at interlayers, respectively. In this manner, we found that for the birnessite containing exclusively Mn(IV), the contributions of edge sites and interlayer sites to total Pb 2 + sorption were approximately equivalent; but as Mn AOS decreased, the contribution of vacant sites at interlayers sensibly decreased, probably through the increasing presence of layer Mn(III) at the expense of vacant sites. The results presented here of highly distinctive affinity site types are valuable for a fundamental understanding of the behavior of birnessites towards heavy metal sorption, which in turn contributes both to predictions of the geochemical behavior of birnessites and to applications for remediation schemes of metal-contaminated aqueous environments.

First principle study of the thermal conductance in graphene nanoribbon with vacancy and substitutional silicon defects

The thermal conductance in graphene nanoribbon with a vacancy or silicon point defect is investigated by nonequilibrium Green’s function (NEGF) formalism combined with first-principles calculations of density-functional theory with local density approximation. The thermal conductance is very sensitive to the position of the vacancy defect, while insensitive to the position of silicon defect. A vacancy defect situated at the center of the nanoribbon generates a saddlelike surface, which greatly reduces the thermal conductance by strong scattering to all phonon modes; while an edge vacancy defect only results in a further reconstruction of the edge and slightly reduces the thermal conductance.

Room-temperature ferromagnetism in graphitic petal arrays

We report room-temperature ferromagnetism of graphitic petal arrays grown on Si substrates by microwave plasma chemical vapor deposition without catalyst. The samples have been characterized by Raman and X-ray photoelectron spectroscopy to confirm the absence of possible ferromagnetic impurities. The petals exhibit ferromagnetic hysteresis with saturation magnetization of ∼4.67 emu cm(-3) and coercivity of ∼105 Oe at 300 K, comparable to the reported behavior of few-layer graphene. Upon O2 annealing the saturation magnetization and coercivity decreased to 2.1 emu cm(-3) and ∼75 Oe respectively. The origin of ferromagnetism is believed to arise from the edge defects and vacancies in the petals.

“White Graphenes”: Boron Nitride Nanoribbons via Boron Nitride Nanotube Unwrapping

Inspired by rich physics and functionalities of graphenes, scientists have taken an intensive interest in two-dimensional (2D) crystals of h-BN (analogue of graphite, so-called "white" graphite). Recent calculations have predicted the exciting potentials of BN nanoribbons in spintronics due to tunable magnetic and electrical properties; however no experimental evidence has been provided since fabrication of such ribbons remains a challenge. Here, we show that few- and single-layered BN nanoribbons, mostly terminated with zigzag edges, can be produced under unwrapping multiwalled BN nanotubes through plasma etching. The interesting stepwise unwrapping and intermediate states were observed and analyzed. Opposed to insulating primal tubes, the nanoribbons become semiconducting due to doping-like conducting edge states and vacancy defects, as revealed by structural analyses and ab initio simulations. This study paves the way for BN nanoribbon production and usage as functional semiconductors with a wide range of applications in optoelectronics and spintronics.

The effect of magnetic field and disorders on the electronic transport in graphenenanoribbons

We developed a unified mesoscopic transport model for graphene nanoribbons, whichcombines the nonequilibrium Green’s function (NEGF) formalism with the real-spaceπ-orbital model. Based on this model, we probe the spatial distribution of electrons under amagnetic field, in order to obtain insights into the various signature Hall effects indisordered armchair graphene nanoribbons (AGNR). In the presence of a uniformperpendicular magnetic field (-field), a perfect AGNR shows three distinct spatial current profiles at equilibrium,depending on its width. Under nonequilibrium conditions (i.e. in the presence of an appliedbias), the net electron flow is restricted to the edges and occurs in opposite directionsdepending on whether the Fermi level lies within the valence or conduction band.For electrons at an energy level below the conduction window, the -field gives rise to local electron flux circulation, although the global flux is zero. Our studyalso reveals the suppression of electron backscattering as a result of the edge transportwhich is induced by the -field. This phenomenon can potentially mitigate the undesired effects of disorder, such asbulk and edge vacancies, on the transport properties of AGNR. Lastly, we show that theeffect of -field on electronic transport is less significant in the multimode compared to thesingle-mode electron transport.

Size Threshold in the Dibenzothiophene Adsorption on MoS2Nanoclusters

In hydrodesulfurization (HDS) of fossil fuels, the sulfur levels are reduced by sulfur extraction from hydrocarbons through a series of catalyzed reaction steps on low-coordinated sites on molybdenum disulfide (MoS(2)) nanoclusters. By means of scanning tunneling microscopy (STM), we show that the adsorption properties of MoS(2) nanoclusters toward the HDS refractory dibenzothiophene (DBT) vary dramatically with small changes in the cluster size. STM images reveal that MoS(2) nanoclusters with a size above a threshold value of 1.5 nm react with hydrogen to form so-called sulfur vacancies predominately located at edge sites, but these edge vacancies are not capable of binding DBT directly. In contrast, MoS(2) nanoclusters below the threshold perform remarkably better. Here, sulfur vacancies form predominantly at the corner sites, and these vacancies show a high affinity for DBT. The results thus indicate that very small MoS(2) nanoclusters may have unique catalytic properties for the production of clean fuels.

Pseudospin valve in bilayer graphene nanoribbons

We study the pseudospin valve effect in bilayer graphene nanoribbons using the recursive Green's function method. The pseudospin degree of freedom is associated with the electron density in two layers and can be controlled by external gate electrodes. We find that the conductance of nanoribbons shows different behavior compared to infinite systems due to the appearance of edge states and quantum confinement. Remarkably, a large on-off ratio can be achieved in nanoribbons with zigzag edges, even when the Fermi energy lies in the bulk energy gap. The influence of possible edge vacancies and interface conditions is also discussed.

Model calculations of edge dislocation defects and vacancies in α-Iron lattice

Two models of defects in perfect α-iron lattice were discussed. In the perfect bcc iron lattice 42×42×42 ao (ao = 2,87 Å) an edge dislocation was created, moving the second half of the bulk on one ao distance. This action generates a little volume in the middle of the bulk witch increases of the positron lifetime (PLT) calculated using the superimposed-atom method of Puska and Nieminen [1]. The result of 118 ps PLT in simple edge dislocation's model is in a good concurrence with earlier publications and experimental data [2]. Through the dislocation line one, two and three vacancies were localized. These models give the results for PLT of 146, 157 and 167 ps respectively. The computer simulations were performed using Finnis–Sinclair (FS) N-body potential.

The importance of defects for carbon nanoribbon based electronics

AbstractThe utilization of graphene nanoribbons for next generation nanoelectronics is commonly expected to depend on the controlled synthesis that yields a low density of defects. Edge roughness and vacancies have been shown to have a large impact on the performance of graphene nanoribbon transistors. In contrast, we show how certain defects can be used to enhance the electronic and magnetic properties of graphene nanoribbons. We explore the properties of hybrid graphene nanoribbons with armchair and zigzag features joined by an array of pentagon–heptagon structural defects. The graphene nanoribbons display an increased density of states at the Fermi level, and half metallicity in absence of external fields. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The vacancy–edge dislocation interaction in fcc metals: A comparison between atomic simulations and elasticity theory

The interaction between vacancies and edge dislocations in face centered cubic metals (Al, Au, Cu, Ni) is studied at different length scales. Using empirical potentials and static relaxation, atomic simulations give us a precise description of this interaction, mostly in the case when the separation distance between both defects is small. At larger distances, elasticity theory can be used to predict this interaction. From the comparison between both approaches we obtain the minimal separation distance where elasticity applies and we estimate the degree of refinement required in the calculation. In this purpose, isotropic and anisotropic elasticity is used assuming a perfect or a dissociated edge dislocation and considering the size effect as well as the inhomogeneity interaction.

Oxidative addition of silanes R3SiH to the unsaturated cluster [Os3(μ-H){μ3-Ph2PCH2PPh(C6H4)}(CO)8]: Evidence for reversible silane formation in the dynamic behaviour of [Os3(μ-H)(SiR3)(CO)9(μ-dppm)

Oxidative addition of the silanes R(3)SiH (R(3)= Ph(3), Et(3), EtMe(2)) to the unsaturated cluster [Os(3)(micro-H)[micro(3)-Ph(2)PCH(2)PPh(C(6)H(4))](CO)(8)] leads to the saturated clusters [Os(3)(micro-H)(SiR(3))(CO)(9)(micro-dppm)](SiR(3)= SiPh(3) 1, SiEt(3) 2 and SiEtMe(2)3) and the unsaturated clusters [Os(3)(micro -H)(2)(SiR(3))[micro(3)-Ph(2)PCH(2)PPh(C(6)H(4))](CO)(7)](SiR(3)= SiPh(3) 4, SiEt(3) 5 and SiEtMe(2)6). Structures are based on spectroscopic evidence and a XRD structure of [Os(3)(micro-H)(SiPh(3))(CO)(9)(micro-dppm)] 1 in which all non-CO ligands are coordinated equatorially and the hydride and the silyl groups are mutually cis. From variable-temperature (1)H NMR spectra of the SiEt(3) compound 2, exchange of the P nuclei is clearly apparent. Simultaneous migrations of the SiEt(3) group and of the hydride from one Os-Os edge to another generate a time-averaged mirror plane in the molecule. VT (1)H NMR spectra of the somewhat less bulky compound [Os(3)(micro-H)(SiMe(2)Et)(CO)(9)(micro-dppm)] 3 have been analysed. Two isomers 3a and 3b are observed with the hydride ligand located on different Os-Os edges. Synchronous migration of the hydride and SiMe(2)Et groups is faster than the observed interconversion of isomers which occurs by hydride migration alone. The synchronous motion of H and SiR(3)only occurs when these ligands are mutually cis as in the major isomer 3a and we propose that this process requires the formation of a transient silane complex of the type [Os(3)(eta(2)-HSiR(3))(CO)(9)(micro-dppm)]. Turnstile rotation within an Os(CO)(3)(eta(2)-HSiR(3)) group leads to the observed exchange within the major isomer 3a without exchange with the minor isomer. This process is not observed for the minor isomer 3b because the hydride and the silyl group are mutually trans. Protonation to give [Os(3)(micro-H)(2)(SiR(3))(CO)(9)(micro-dppm)](+) totally suppresses the dynamic behaviour because there are no edge vacancies.

Vacancy charging on Si(1 1 1)-(7 × 7) investigated by density functional theory

The structure and energetics of charged vacancies on Si(111)-(7×7) are investigated using density functional theory calculations supplemented by estimates of ionization entropy. The calculations predict multiple possible charge states for the unfaulted edge vacancy in the adatom layer, although the −2 state is most stable on real Si(111) surfaces for which the Fermi level lies near the middle of the band gap.

Electronic structure and bonding of hydrogen in a screw dislocated bcc Fe

The bonding of HFe is analyzed using qualitative electronic calculations in the framework of the atom superposition and electron delocalization orbital cluster method (ASED-MO). The changes in the electronic structure of bcc Fe upon introduction, a screw dislocation are compared with Fe surfaces. A comparison is drawn with H adsorption at Fe ( 1 ̄ 1 2) surface and H absorption in the bulk Fe with a dislocation. The results suggest that the detrimental effect of H on the FeFe bonds in a screw dislocation is almost half-severe than in other defects, such as an edge dislocation or vacancy and it is limited to its first neighbor. An analysis of the orbital interaction reveals that FeH bonding involves mainly the Fe 4s and H 1s orbitals. When H is in the bulk, the region near screw dislocation core is much more repulsive than in a normal Fe lattice. These results allow us consider that H accumulation could be less favorable near a screw dislocation than an edge dislocation.

Erratum: Calculations of positron lifetimes in a jog and vacancies on an edge-dislocation line in Fe

The lifetimes of positrons in an edge dislocation, a jog, and vacancies, which are associated with the edge dislocation (associated vacancy), in bcc iron are calculated. The structures of the defects are obtained by static relaxation using the Finnis-Sinclair potential. The positron lifetimes are calculated using the superimposed-atom method, which was developed by Puska and Nieminen. The positrons are not localized strongly in the pure edge dislocation and the jog. The lifetimes in them are almost the same, and are only a few picoseconds longer than in the matrix. The vacancy is trapped on the edge-dislocation line, and is not collapsed completely by the strain in the core region of the dislocation. The positron lifetime in the associated vacancy is shorter than that in a vacancy in the matrix, and is compared to the experimental value observed in deformed iron. These results indicate that the deeper trap associated with the dislocation line is not the jog but the vacancy.

Calculations of positron lifetimes in a jog and vacancies on an edge-dislocation line in Fe.

The lifetimes of positrons in an edge dislocation, a jog, and vacancies, which are associated with the edge dislocation (associated vacancy), in bcc iron are calculated. The structures of the defects are obtained by static relaxation using the Finnis-Sinclair potential. The positron lifetimes are calculated using the superimposed-atom method, which was developed by Puska and Nieminen. The positrons are not localized strongly in the pure edge dislocation and the jog. The lifetimes in them are almost the same, and are only a few picoseconds longer than in the matrix. The vacancy is trapped on the edge-dislocation line, and is not collapsed completely by the strain in the core region of the dislocation. The positron lifetime in the associated vacancy is shorter than that in a vacancy in the matrix, and is compared to the experimental value observed in deformed iron. These results indicate that the deeper trap associated with the dislocation line is not the jog but the vacancy.

Computer Modelling of a Molybdenum Disulfide Catalyst: Density Functional Calculations on Molybdenum‐Sulfur Clusters

AbstractPreliminary calculations using the Density Functional Theory (DFT) method on saturated and unsaturated molybdenum‐sulfur clusters which model the active site of a molybdenum disulfide catalyst are reported. Creating an edge vacancy at molybdenum by removing sulfur atoms causes the energy of the HOMO to increase; the catalyst is a better electron donor. At the same time the charge density on basal sulfurs increases enhancing the ability of the basal plane to Interact with α‐orbitals of the thiophene molecule. Changes in the HOMO energies and the charge densities depend on the structure of the catalyst, In particular on the separation between unsaturated Mo atoms at the active site.

Simulation of carbon gasification kinetics using an edge recession model

AbstractAn edge recession modeling method is proposed as a new approach to the prediction of carbon gasification kinetics. Using this method, the variations in reactive surface area (RSA) and specific reactivity (R) with conversion were simulated for several polynuclear aromatic molecules adopted as models of carbon crystallites. The effects of crystallite size and shape, edge site reactivity and vacancies on the changes in RSA and R with conversion were investigated. As a result, RSA and R were found to be essentially independent of crystallite shape, but to depend on crystallite size and the presence of vacancies. Good agreement was obtained between model predictions and experimental data for several kinds of model carbon crystallites. In the case of structurally disordered carbons (such as chars), simultaneous gasification of model crystallites of two different sizes gave better predictions than that of single‐size crystallites. The edge recession model is proposed as an alternative, or at least complementary, approach to the more conventional (and, arguably, less physically meaningful) pore structure development models.

Identification of hydrogen platelets in proton-bombarded GaAs

The technique of cross-sectional transmission electron microscopy has been applied to study the nature of the radiation damage in GaAs bombarded with 300 keV protons to total doses of 1015 and 5×1015 protons cm−2. The results indicate that upon annealing at temperatures of 500 °C and above, the precipitated damage becomes visible in the form of hydrogen platelets (i.e., hydrogen-filled edge vacancy loops) on the {110} cleavage planes of GaAs.

The effects of cascade damage and self-ion injection on the swelling of irradiated materials

The effects on the void swelling of irradiated materials of cascade damage and (for self-ion bombardment) of self-ion injection are derived in a unified manner by means of rate theory. The collapse of vacancy clusters within cascades to form vacancy edge dislocation loops follows the general model investigated by Bullough, Eyre and Krishan, although in contrast with this previous development that given here is entirely analytical. The dependence upon macroscopic measurable quantities is thereby exposed and the complementary effects of cascade damage and self-ion injection are made evident.