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Institute
We report velocity-dependent internal energy distributions of nitric oxide molecules, NO, scattered off graphene supported on gold to further explore the dynamics of the collision process between NO radicals and graphene. These experiments were performed by directing a molecular beam of NO onto graphene in a surface-velocity map imaging setup, which allowed us to record internal energy distributions of the NO radicals as a function of their velocity. We do not observe bond formation but (1) major contributions from direct inelastic scattering and (2) a smaller trapping–desorption component where some physisorbed NO molecules have residence times on the order of microseconds. This is in agreement with our classical molecular dynamics simulations which also observe a small proportion of two- and multi-bounce collisions events but likewise a small proportion of NO radicals trapped at the surface for the entire length of the molecular dynamics simulations (a few picoseconds). Despite a collision energy of 0.31 eV, which would be sufficient to populate NO(v = 1), we do not detect vibrationally excited nitric oxide.
In this study, we calculated the energetics of hydrogen atoms adsorbing on and diffusing into the first few layers of γ-Fe for the (100), (110) and (111) surfaces and for the non-magnetic (NM), ferromagnetic (FM), and antiferromagnetic single (AFM1) and double layer (AFMD) structures. These studies are relevant as they atomistically simulate the early stages of hydrogen embrittlement in steels. We employed density functional theory to establish adsorption sites and energies for each plane and the minimum energy pathways for diffusion through the first few layers with associated activation barriers. Adsorption energies for all cases vary between ∼3.7 and 4.4 eV, and the energy barriers to diffusion in the bulk region vary between ∼0.2 and 1.2 eV for the twelve cases, with the highest and lowest bulk diffusion barriers occurring in the NM(111) and the FM(100) case, respectively. We conclude that the texturing of steels in order to expose certain cleavage planes or magnetic structures can decrease the likelihood of hydrogen embrittlement.
A novel method has been implemented to prepare metal oxide nanopowders covered with known quantities of adsorbed water; we subsequently studied the γ-radiolysis of ZrO2 nanopowders covered with H2O layers. H2 yields from the adsorbed water radiolysis are of importance in multiple industrial contexts – the nuclear industry being a prime example. Measured H2 yields at water coverages of just below and above one monolayer are around 350 times greater than for neat water, but these yields decrease rapidly with increasing water loading of the ZrO2 nanoparticles, approaching the yield of bulk water at coverages of tens of water layers. The observed plateau of the yields at 0.5 to 2.0 monolayers coverage can be explained by the ease with which electronic excitations in the ZrO2 can be transferred across the interface to the first one or two adsorbed water layers. However, with increasing water loading, energy transfer to water layers further away from the interface becomes less efficient, and above ~30 water layers, most of the water is not affected by any exciton formation in the ZrO2.
The reactivity of graphene at its boundary region has been imaged using non-linear spectroscopy to address the controversy whether the terraces of graphene or its edges are more reactive. Graphene was functionalised with phenyl groups, and we subsequently scanned our vibrational sum-frequency generation setup from the functionalised graphene terraces across the edges. A greater phenyl signal is clearly observed at the edges, showing evidence of increased reactivity in the boundary region. We estimate an upper limit of 1 mm for the width of the CVD graphene boundary region.
Nitric oxide adsorption on a Au(100) single crystal has been investigated to identify the type of adsorption, the adsorption site, and the orientation and alignment of the adsorbed NO relative to the surface. This was done using a combination of 3D-surface velocity map imaging, near-ambient pressure X-ray photoelectron spectroscopy, and density functional theory. NO was observed to be molecularly adsorbed on gold at ~200 K. Very narrow angular distributions and cold rotational distributions of photodesorbed NO indicate that NO adsorbs on high-symmetry sites on the Au crystal, with the N–O bond axis close to the surface normal. Our density functional theory calculations show that NO preferentially adsorbs on the symmetric bridge (2f) site, which ensures efficient overlap of the NO π* orbital with the orbitals on the two neighbouring Au atoms, and with the N–O bond axis aligned along the surface normal, in agreement with our conclusions from the rotational state distributions. The combination of XPS, which reveals the orientation of NO on gold, with 3D-surface velocity map imaging and density functional theory thus allowed us to determine the adsorption site, orientation and alignment of nitric oxide adsorbed on Au(100).
This study is concerned with the early stages of hydrogen embrittlement on an atomistic scale. We employed density functional theory to investigate hydrogen diffusion through the (100), (110) and (111) surfaces of γ-Fe. The preferred adsorption sites and respective energies for hydrogen adsorption were established for each plane, as well as a minimum energy pathway for diffusion. The H atoms adsorb on the (100), (110) and (111) surfaces with energies of ∼4.06 eV, ∼3.92 eV and ∼4.05 eV, respectively. The barriers for bulk-like diffusion for the (100), (110) and (111) surfaces are ∼0.6 eV, ∼0.5 eV and ∼0.7 eV, respectively. We compared these calculated barriers with previously obtained experimental data in an Arrhenius plot, which indicates good agreement between experimentally measured and theoretically predicted activation energies. Texturing austenitic steels such that the (111) surfaces of grains are preferentially exposed at the cleavage planes may be a possibility to reduce hydrogen embrittlement.
The effect of magnetism on hydrogen adsorption and subsurface diffusion through face-centred cubic (fcc) γ-Fe(0 0 1) was investigated using spin-polarised density functional theory (s-DFT). The non-magnetic (NM), ferromagnetic (FM), and antiferromagnetic single (AFM1) and double layer (AFMD) structures were considered. For each magnetic state, the hydrogen preferentially adsorbs at the fourfold site, with adsorption energies of 4.07, 4.12, 4.03 and 4.05 eV/H atom for the NM, FM, AFM1 and AFMD structures. A total barrier of 1.34, 0.90, 1.32 and 1.25 eV and a bulk-like diffusion barrier of 0.6, 0.2, 0.4 and 0.3 eV were calculated for the NM, FM, AFM1 and AFMD magnetic states. The Fe atoms nearest to the H atom exhibited a reduced magnetic moment, whereas the next-nearest neighbour Fe atoms exhibited a non-negligible local perturbation in the magnetic moment. The presence of magnetically ordered structures has a minimal influence on the minimum energy path for H diffusion through the lattice and on the adsorption of H atoms on the Fe(0 0 1) surface, but we computed a significant reduction of the bulk-like diffusion barriers with respect to the non-magnetic state of fcc γ-Fe.
We report the unambiguous detection of phenyl groups covalently attached to functionalised graphene using non-linear spectroscopy. Sum-frequency generation was employed to probe graphene on a gold surface after chemical functionalisation using a benzene diazonium salt. We observe a distinct resonance at 3064 cm1 which can clearly be assigned to an aromatic C–H stretch by comparison with a self-assembled monolayer on a gold substrate formed from benzenethiol. Not only does sum-frequency generation spectroscopy allow one to characterise functionalised graphene with higher sensitivity and much better specificity than many other spectroscopic techniques, but it also opens up the possibility to assess the coverage of graphene with functional groups, and to determine their orientation relative to the graphene surface.
The adsorption of O atoms on the Fe(1 1 0) surface has been investigated by density functional theory for increasing degrees of oxygen coverage from 0.25 to 1 monolayer, to follow the evolution of the Osingle bondFe(1 1 0) system into an FeO(1 1 1)-like monolayer. We found that the quasi-threefold site is the most stable adsorption site for all coverages, with adsorption energies of ∼2.8–4.0 eV per O atom. Oxygen adsorption results in surface geometrical changes such as interlayer relaxation and buckling, the latter of which decreases with coverage. The calculated vibrational frequencies range from 265 to 470 cm−1 for the frustrated translational modes and 480–620 cm−1 for the stretching mode, and hence are in good agreement with the experimental values reported for bulk FeO wüstite. The hybridization of the oxygen 2p and iron 3d orbitals increases with oxygen coverage, and the partial density of states for the Osingle bondFe(1 1 0) system at full coverage resembles the one reported in the literature for bulk FeO. These results at full oxygen coverage point to the incipient formation of an FeO(1 1 1)-like monolayer that would eventually lead to the bulk FeO oxide layer.
Molecular hydrogen production from amorphous solid water during low energy electron irradiation
(2017)
This work investigates the production of molecular hydrogen isotopologues (H2, HD, and D2) during low energy electron irradiation of layered and isotopically labelled thin films of amorphous solid water (ASW) in ultrahigh vacuum. Experimentally, the production of these molecules with both irradiation time and incident electron energy in the range 400 to 500 eV is reported as a function of the depth of a buried D2O layer in an H2O film. H2 is produced consistently in all measurements, reflecting the H2O component of the film, though it does exhibit a modest reduction in intensity at the time corresponding to product escape from the buried D2O layer. In contrast, HD and D2 production exhibit peaks at times corresponding to product escape from the buried D2O layer in the composite film. These features broaden the deeper the HD or D2 is formed due to diffusion. A simple random-walk model is presented that can qualitatively explain the appearance profile of these peaks as a function of the incident electron penetration.