Noble Metal Surfaces Research Articles

In Situ Monitoring of Electrooxidation Processes at Gold Single Crystal Surfaces Using Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy.

Identifying the intermediate species in an electrocatalytic reaction can provide a great opportunity to understand the reaction mechanism and fabricate a better catalyst. However, the direct observation of intermediate species at a single crystal surface is a daunting challenge for spectroscopic techniques. In this work, electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (EC-SHINERS) is utilized to in situ monitor the electrooxidation processes at atomically flat Au(hkl) single crystal electrode surfaces. We systematically explored the effects of crystallographic orientation, pH value, and anion on electrochemical behavior of intermediate (AuOH/AuO) species. The experimental results are well correlated with our periodic density functional theory calculations and corroborate the long-standing speculation based on theoretical calculations in previous electrochemical studies. The presented in situ electrochemical SHINERS technique offers a unique way for a real-time investigation of an electrocatalytic reaction pathway at various well-defined noble metal surfaces.

ChemInform Abstract: NMR Techniques for Noble Metal Nanoparticles

AbstractReview: 148 refs.

Ab Initio Study Of Li2O2 On Noble Metal (Pt, Au, Pd), Pt3M (M=Fe, Co, Ni, Cu) Alloy And Pd3M (M=Fe, Co, Ni, Cu) Alloy Surfaces

Li- O2 cells has shown great potential to be one of the energy storage systems with energetic capabilities beyond the lithium-ion cell. The early ab-initio theoretical picture of the Oxygen Reduction Reaction (ORR) at the cathode envisioned a Li2O2 covered surface after the initial cycle and further ORR as occurring on the deposited Li2O2[2]. Density Functional Theory (DFT) studies invoking such mechanisms have predicted enlarged overpotentials in agreement with experimentally observed high overpotentials. But the issue of electron transfer across the poorly conducting peroxide layer in spite of low conductivity has had to be explained using mechanisms such as d-band bending [2] and poloron hopping [3]. This picture of an electrode surface that is covered and does not influence the ORR or influences it by an indirect at-distance mechanism is at odds with experimental evidence that the surface plays a crucial role in the ORR. ORR on Noble metals [1], metal oxide [4], metal alloys [5], and carbon nanotubes [6] has been reported to occur with reduced overpotential and longer cycle life. Also studies of the cathode surface have shown that the Li2O2 nucleates in various 3-D morphologies [6] and even after multiple cycles a significant portion of the surface is bare [6]. The present study addresses the twin challenges of understanding the binding of Li2O2 on active sites and identifying possible candidate materials which would facilitate easy removal of deposited Li2O2 during the charge process. The Li2O2 binding energy (B.ELi2O2) was calculated using DFT on periodic models of noble metal (Pt, Au, Pd), Pt3M (M=Fe, Co, Ni, Cu) alloy and Pd3M (M=Fe, Co, Ni, Cu) alloy surfaces after geometric optimization and by bonding Li2O2 to the surfaces at various orientations and sites. The effect of the various sites on the binding energy and the variation of the binding energy at each site with the change in material were studied. Top and Bridge sites were found to have the lowest B.ELi2O2 in case of the noble metals. Systematic permutation of the first two atomic layers of FCC (111) Pt3M and Pd3Mskin alloys slabs was carried out to find the most stable geometric surface configuration and the influence of the dopant modification of the lattice constant on the B.ELi2O2. Possible guidelines for future alloy design to achieve desired B.ELi2O2 values were formulated and B.ELi2O2 was shown to be a key parameter for screening future Li-O2ORR catalyst candidates Acknowledgement: Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Energy-dependent photoemission delays from noble metal surfaces by attosecond interferometry

How quanta of energy and charge are transported on both atomic spatial and ultrafast time scales is at the heart of modern technology. Recent progress in ultrafast spectroscopy has allowed us to directly study the dynamical response of an electronic system to interaction with an electromagnetic field. Here, we present energy-dependent photoemission delays from the noble metal surfaces Ag(111) and Au(111). An interferometric technique based on attosecond pulse trains is applied simultaneously in a gas phase and a solid state target to derive surface-specific photoemission delays. Experimental delays on the order of 100 as are in the same time range as those obtained from simulations. The strong variation of measured delays with excitation energy in Ag(111), which cannot be consistently explained invoking solely electron transport or initial state localization as supposed in previous work, indicates that final state effects play a key role in photoemission from solids.

Hierarchical effect behind the supramolecular chirality of silver(I)-cysteine coordination polymers.

Cysteine is a sulfur-containing amino acid that easily coordinates to soft metal ions and grafts to noble metal surfaces. Recently, chiroptical activity of Ag(+)/cysteine coordination polymers has been widely studied, while, on the other hand, the appearance of a plasmon-enhanced circular dichroic signal (PECD) at the plasmonic spectral region (λ > 400 nm) has been observed for AgNPs capped with chiral sulfur-containing amino acids. These two events are both potentially exploited for sensing applications. However, the presence of Ag(+) ions in AgNP colloidal solution deals with the competition of cysteine grafting at the metal NP surface and/or metal ion coordination. Herein we demonstrate that the chiroptical activity observed by adding cysteine to AgNP colloids prepared by pulsed laser ablation in liquids (PLAL) is mainly related to the formation of CD-active Ag(+)/cysteine supramolecular polymers. The strict correlation between supramolecular chirality and hierarchical effects, driven by different chemical environments experienced by cysteine when different titration modalities are used, is pivotal to validate cysteine as a fast and reliable probe to characterize the surface oxidation of AgNPs prepared by pulsed laser ablation in liquids by varying the laser wavelengths.

Heteromolecular metal–organic interfaces: Electronic and structural fingerprints of chemical bonding

Beside the fact that they attract highest interest in the field of organic electronics, heteromolecular structures adsorbed on metal surfaces, in particular donor–acceptor blends, became a popular field in fundamental science, possibly since some surprising and unexpected behaviors were found for such systems. One is the apparent breaking of a rather fundamental rule in chemistry, namely that stronger chemical bonds go along with shorter bond lengths, as it is, e.g., well-known for the sequence from single to triple bonds. In this review we summarize the results of heteromolecular monolayer structures adsorbed on Ag(111), which – regarding this rule – behave in a counterintuitive way. The charge acceptor moves away from the substrate while its electronic structure indicates a stronger chemical interaction, indicated by a shift of the formerly lowest unoccupied molecular orbital toward higher binding energies. The donor behaves in the opposite way, it gives away charge, hence, electronically the bonding to the surface becomes weaker, but at the same time it also approaches the surface. It looks as if the concordant link between electronic and geometric structure was broken. But both effects can be explained by a substrate-mediated charge transfer from the donor to the acceptor. The charge reorganization going along with this transfer is responsible for both, the lifting-up of the acceptor molecule and the filling of its LUMO, and also for the reversed effects at the donor molecules. In the end, both molecules mutually enhance their respective donor and acceptor characters. We argue that this effect is of general validity for π-conjugated molecules adsorbing on noble metal surfaces.

NMR Techniques for Noble Metal Nanoparticles

Solution phase noble metal nanoparticle growth reactions are comprised of deceptively simple steps. Analytical methods with high chemical, spatial, and temporal resolution are crucial to understanding these reactions and subsequent nanoparticle properties. However, approaches for the characterization of solid inorganic materials and solution phase molecular species are often disparate. One powerful technique to address this gap is nuclear magnetic resonance (NMR) spectroscopy, which can facilitate routine, direct, molecular-scale analysis of nanoparticle formation and morphology in situ, in both the solution and the solid phase. A growing body of work indicates that NMR analyses should yield an exciting complement to the existing canon of routine nanoparticle characterization methods such as electron microscopy and optical absorption spectroscopy. Here, we discuss recent developments in the application of NMR techniques to the study of noble metal nanoparticle growth, surface chemistry, and physical prope...

Surface plasmon-enhanced photochemical reactions on noble metal nanostructures

Nanoscale noble metals can exhibit excellent photochemical and photophysical properties, due to surface plasmon resonance (SPR) from specifically collective electronic excitations on these metal surfaces. The SPR effect triggers many new surface processes, including radiation and radiationless relaxations. As for the radiation process, the SPR effect causes the significant focus of light and enormous enhancement of the local surface optical electric field, as observed in surface-enhanced Raman spectroscopy (SERS) with very high detection sensitivity (to the single-molecule level). SERS is used to identify surface species and characterize molecular structures and chemical reactions. For the radiationless process, the SPR effect can generate hot carriers, such as hot electrons and hot holes, which can induce and enhance surface chemical reactions. Here, we review our recent work and related literature on surface catalytic-coupling reactions of aromatic amines and aromatic nitro compounds on nanostructured noble metal surfaces. Such reactions are a type of novel surface plasmon-enhanced chemical reaction. They could be simultaneously characterized by SERS when the SERS signals are assigned. By combining the density functional theory (DFT) calculations and SERS experimental spectra, our results indicate the possible pathways of the surface plasmon-enhanced photochemical reactions on nanostructures of noble metals. To construct a stable and sustainable system in the conversion process of the light energy to the chemical energy on nanoscale metal surfaces, it is necessary to simultaneously consider the hot electrons and the hot holes as a whole chemical reaction system.

Review of magnetic atomic structures and their properties on noble metal surfaces

Ordered arrays of magnetic nanostructures have rich physical properties and potential applications, attracting much theoretical and experimental interest. These structures can now be fabricated and investigated at scales down to the atomic level. Herein, we review recent advances in the study of magnetic atomic structures on the surfaces of noble metals. These structures include one-dimensional strings, two-dimensional hexagonal superlattices, and novel structures stabilized by quantum guiding. We focus on low-temperature scanning tunneling microscopy studies as well as kinetic Monte Carlo simulations and ab initio calculations, to discuss the self- assembly formation conditions. Combining the results of scanning tunneling spectroscopy and tight-binding calculations, we studied the spectra of these well-ordered structures. We also discuss how quantum confinement in nanocorrals significantly influences adatom diffusion and self-assembly, leading to quantum-guided self-assembly and self-regulated atom trapping in open nanocorrals.

Interaction of ionic liquids with noble metal surfaces: structure formation and stability of [OMIM][TFSA] and [EMIM][TFSA] on Au(111) and Ag(111).

Aiming at a comprehensive understanding of the interaction of ionic liquids (ILs) with metal surfaces we have investigated the adsorption of two closely related ILs, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][TFSA] and 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide [OMIM][TFSA], with two noble metal surfaces, Au(111) and Ag(111), under ultrahigh vacuum (UHV) conditions using scanning tunneling microscopy (STM). At room temperature, the ILs form a 2D liquid on either of the two surfaces, while at lower temperatures they condense into two-dimensional (2D) islands which exhibit ordered structures or a short-range ordered 2D glass structure. Comparison of the adlayer structures formed in the different adsorption systems and also with those determined recently for n-butyl-n-methylpyrrolidinium [TFSA](-) adlayers on Ag(111) and Au(111) (B. Uhl et al., Beilstein J. Nanotechnol., 2013, 4, 903) gains detailed insight into the adsorption geometry of the IL ions on the surface. The close similarity of the adlayer structures indicates that (i) the structure formation is dominated by the tendency to optimize the anion adsorption geometry, and that (ii) also in the present systems the cation adsorbs with the alkyl chain pointing up from the surface.

Bimetallic Au/Ag nanoparticle loading on PNIPAAm–VAA–CS8 thermoresponsive hydrogel surfaces using ss-DNA coupling, and their SERS efficiency

Thermoresponsive hydrogels can be efficiently used as templates for bimetallic noble metal surface loading for the fabrication of plasmonic surfaces with a wide range of applications.

Effect of the Transition Metal on Metal–Nitrogen–Carbon Catalysts for the Hydrogen Evolution Reaction

The effect of the nature of the transition metal on the structure and activity for hydrogen evolution of Metal-N-C catalysts synthesized via the pyrolysis of metal salts and a Zn-based metal organic framework was investigated. It is found that W, Mo, Cu and Zn lead to amorphous carbons with high specific area while Cr, Mn, Fe, Co and Ni lead to more graphitic carbons with a lower specific area. Metal salts with a high redox potential are fully reduced during pyrolysis while others are only partially reduced. Electrochemical activity toward hydrogen evolution was investigated at pH 1 and pH 13. Hydrogen evolution on these Metal-N-C catalysts is generally more facile at pH 1 than at pH 13, paralleling the trends observed for noble metal surfaces. The Co-, Ni- and Fe-N-C catalysts are the most active at pH 13 while Co-N-C and Cr-N-C are the most active at pH 1. The activity of the latter catalysts stems from metallic cobalt particles encapsulated in carbon and from a chromium carbo-nitride phase, respectively.

Interface Electronic Structures of the L-Cysteine on Noble Metal Surfaces Studied by Ultraviolet Photoelectron Spectroscopy

L-cysteine, one of amino acids, is well known in the bioelectronics field as a linker between proteins and metal electrodes. The interface electronic structures between L-cysteine and metals are therefore very crucial because they dominate the charge carrier injection characteristics into such biological systems. However the interface electronic structures of L-cysteine and metals have been not well understood. In this study, the electronic structures at the interfaces of sequentially deposited L-cysteine layers on the metal surfaces of Au(111), Ag(111), and Cu(111) were investigated by thickness-dependent ultraviolet photoelectron spectroscopy (UPS). At the contact regions, the electronic structures of L-cysteine revealed modification with respect to its bulk phase and significant variation depending on the substrate. The electronic structure at the interfaces including work function, secondary electron cutoff (SECO), highest occupied molecular orbital (HOMO) onset, position of an interface state, charge injection barrier, and ionization energy were estimated for each case. [DOI: 10.1380/ejssnt.2015.373]

Rashba spin splitting of Shockley surface states on semi-infinite crystals

We study the Rashba spin splitting of the $L$-gap surface state on the (111) surfaces of Au, Ag, and Cu, and that of the topologically protected surface state on Sb(111) by a first-principles calculation based on the embedded Green's function technique. By taking advantage of semi-infinite surface geometry, we address several issues that are difficult to handle with the finite slab calculation. For Au(111), we investigate the nonlinear dependence on wave vector $\mathbf{k}$ of the spin-splitting energy, and also at what wave number each dispersion curve of the two spin-split bands is merged into a surface-projected bulk band. Then, we discuss why Cu(111) exhibits a larger Rashba effect than Ag(111) despite the fact that the atomic orbitals of Ag have larger spin-orbit matrix elements than those of Cu. We reveal that, while the wave function of the $L$-gap surface state has strong ${p}_{z}$ character (the $z$ axis is surface normal), the Rashba spin splitting occurs through the spin-orbit matrix element between the ${d}_{{z}^{2}}$ and ${d}_{uz}$ orbital components hybridized with ${p}_{z}$, where ${d}_{uz}$ denotes a linear combination of ${d}_{xz}$ and ${d}_{yz}$ with its orbital lobes pointing to the $\mathbf{k}$ direction (defined as the $u$ axis). Cu(111) exhibits a larger spin splitting than Ag(111), since the amount of both the ${d}_{{z}^{2}}$ and ${d}_{uz}$ components mixed in its surface-state wave function is much larger than that of Ag(111). In contrast to the (111) surfaces of noble metals, the spin splitting of the topologically protected surface state on Sb(111) arises from the spin-orbit matrix element between the ${p}_{z}$ and ${p}_{u}$ orbital components. We also discuss under what condition the energy dispersion curve of a surface state intersects the boundary line between a projected bulk band and a projected bulk band gap at a large angle, instead of gradually approaching the boundary line with increasing $|\mathbf{k}|$. Finally, we comment on a recent theoretical work of Krasovskii [Phys. Rev. B 90, 115434 (2014)] on the microscopic origin of the Rashba spin splitting.

A comparative study in structure and reactivity of “FeO x -on-Pt” and “NiO x -on-Pt” catalysts

Oxide nanostructures grown on noble metal surfaces are often highly active in many reactions, in which the oxide/metal interfaces play an important role. In the present work, we studied the surface structures of FeO x -on-Pt and NiO x -on-Pt catalysts and their activity to CO oxidation reactions using both model catalysts and supported nanocatalysts. Although the active FeO1−x structure is stabilized on the Pt surface in a reductive reaction atmosphere, it is prone to change to an FeO2−x structure in oxidative reaction gases and becomes deactivated. In contrast, a NiO1−x surface structure supported on Pt is stable in both reductive and oxidative CO oxidation atmospheres. Consequently, CO oxidation over the NiO1−x -on-Pt catalyst is further enhanced in the CO oxidation atmosphere with an excess of O2. The present results demonstrate that the stability of the active oxide surface phases depends on the stabilization effect of the substrate surface and is also related to whether the oxide exhibits a variable oxidation state.

Surface-state enhancement of tunneling thermopower on the Ag(111) surface.

Thermoelectric effects in tunnel junctions are currently being revisited for their prospects in cooling and energy harvesting applications, and as sensitive probes of electron transport. Quantitative interpretation of these effects calls for advances in both theory and experiment, particularly with respect to the electron transmission probability across a tunnel barrier which encodes the energy dependence and the magnitude of tunneling thermopower. Using noble metal surfaces as clean model systems, we demonstrate a comparatively simple and quantitative approach where the transmission probability is directly measured experimentally. Importantly, we estimate not only thermovoltage, but also its energy and temperature dependencies. We have thus resolved surface-state enhancement of thermovoltage, which manifests as 10-fold enhancement of thermopower on terraces of the Ag(111) surface compared to single-atom step sites and surface-supported nanoparticles. To corroborate experimental analysis, the methodology was applied to the transmission probability obtained from first-principles calculations for the (111) surfaces of the three noble metals, finding good agreement between overall trends. Surface-state effects themselves point to a possibility of achieving competitive performance of all-metal tunnel junctions when compared to molecular junctions. At the same time, the approach presented here opens up possibilities to investigate the properties of nominally doped or gated thermoelectric tunnel junctions as well as temperature gradient in nanometer gaps.

Two-Photon Absorption of Metal-Assisted Chromophores

Aiming to understand the effect of a metal surface on nonlinear optical properties and the combined effects of surface and solvent environments on such properties, we present a multiscale response theory study, integrated with dynamics of the two-photon absorption of 4-nitro-4'-amino-trans-stilbene physisorbed on noble metal surfaces, considering two such surfaces, Ag(111) and Au(111), and two solvents, cyclohexane and water, as cases for demonstration. A few conclusions of general character could be drawn: While the geometrical change of the chromophore induced by the environment was found to notably alter (diminish) the two-photon absorption cross section in the polar medium, the effects of the metal surface and solvent on the electronic structure of the chromophore surpasses the geometrical effects and leads to a considerably enhanced two-photon absorption cross section in the polar solvent. This enhancement of two-photon absorption arises essentially from the metal charge image induced enlargement of the difference between the dipole moment of the excited state and the ground state. The orientation-dependence of the two-photon absorption is found to connect with the lateral rotation of the chromophore, where the two-photon absorption reaches its maximum when the polarization of the incident light coincides with the long-axis of the chromophore. Our results demonstrate a distinct enhancement of the two-photon absorption by a metal surface and a polar medium and envisage the employment of metal-chromophore composite materials for future development of nonlinear optical materials with desirable properties.

Unraveling the Nature of the Oxide–Metal Interaction in Ceria-Based Noble Metal Inverse Catalysts

Recently, it has been shown that model catalysts consisting of cerium oxide nanoparticles supported on noble metal surfaces are very convenient to clarify the reaction mechanism of important industrial reactions such as water–gas shift, CO oxidation, and methanol synthesis from CO2. Moreover, these special inverse catalysts are highly active, even more than the regular metal/oxide catalyst in some cases. However, there is a lack of fundamental knowledge about the electronic nature of the supported CeOx nanoparticles (NPs) and their interaction with the noble metal support. In this work we have carried out DFT calculations on CeOx/Cu(111), CeOx/Ag(111), and CeOx/Au(111) interfaces for both supported particles and periodic 2D film. The most stable and most reduced CeOx NPs are found when supported on Cu(111), followed by Ag(111) and Au(111). A similar behavior is observed for periodic 2D CeO2(111) films supported on the M(111) surfaces, although the reducibility is lower and the stability higher in extended...

Thermodynamic aspects of dehydrogenation reactions on noble metal surfaces.

The reaction free energy for dehydrogenation of phenol, aniline, thiophenol, benzoic acid, and 1,4-benzenediol on the close packed copper, silver, and gold surfaces has been studied by density functional theory calculations. Dehydrogenation of thiophenol is found to be favourable on all three surfaces while aniline does not dehydrogenate on any of them. For phenol, benzenediol and benzoic acid dehydrogenation is favourable on copper and silver only, following the general trend of an increasing reaction free energy when going form gold to silver to copper. This trend can be correlated with the changes in bond lengths within the molecule upon dehydrogenation. While copper is able to replace hydrogen, leaving small changes in the bond lengths of the aromatic ring, the metal-molecule bond is weaker for silver and gold, resulting in a partial loss of aromaticity. This difference in bond strength leads to pronounced differences in adsorption geometries upon multiple dehydrogenations.

Molecular electronic level alignment at weakly coupled organic film/metal interfaces.

Electronic level alignment at interfaces of molecular materials with inorganic semiconductors and metals controls many interfacial phenomena. How the intrinsic properties of the interacting systems define the electronic structure of their interface remains one of the most important problems in molecular electronics and nanotechnology that can be solved through a combination of surface science experimental techniques and theoretical modeling. In this article, we address this fundamental problem through experimental and computational studies of molecular electronic level alignment of thin films of C(6)F(6) on noble metal surfaces. The unoccupied electronic structure of C(6)F(6) is characterized with single molecule resolution using low-temperature scanning tunneling microscopy-based constant-current distance-voltage spectroscopy. The experiments are performed on several noble metal surfaces with different work functions and distinct surface-normal projected band structures. In parallel, the electronic structures of the quantum wells (QWs) formed by the lowest unoccupied molecular orbital state of the C(6)F(6) monolayer and multilayer films and their alignment with respect to the vacuum level of the metallic substrates are calculated by solving the Schrödinger equation for a semiempirical one-dimensional (1D) potential of the combined system using input from density functional theory. Our analysis shows that the level alignment for C(6)F(6) molecules bound through weak van der Waals interactions to noble metal surfaces is primarily defined by the image potential of metal, the electron affinity of the molecule, and the molecule surface distance. We expect the same factors to determine the interfacial electronic structure for a broad range of molecule/metal interfaces.