Noble Metal Surfaces Research Articles

Synthesis and antibacterial activity of surface‐coated catechol‐conjugated polymer with silver nanoparticles on versatile substrate

In the present study, 2‐chloro‐3′,4′‐dihydroxyacetophenone (CCDP), a catechol derivative, was quaternized with poly(propylene oxide)‐g‐poly(dimethylaminoethyl methacrylate) (PPO‐g‐PDMA, PgP) to prepare surface coatings for various substrates. The surfaces of noble metals, oxides, and synthetic polymers were coated by immersion in an aqueous solution of CCDP quaternized with PgP (C‐PgP). The catechol functional groups that remained on the surface were used for deposition of Ag nanoparticles (AgNPs) on the coated surface, to provide a water‐resistant antibacterial polymer with long‐term antimicrobial activity. X‐ray photoelectron spectroscopy confirmed deposition of C‐PgP and AgNPs on the surface coated with the antibacterial polymer. Surface‐immobilized C‐PgP/AgNPs showed outstanding antibacterial activities against Staphylococcus aureus, a Gram‐positive bacterium, and Escherichia coli, a Gram‐negative bacterium. C‐PgP/AgNPs can be applied to a variety of substrates and can therefore be used as antibacterial materials in various fields. Copyright © 2016 John Wiley & Sons, Ltd.

Orbital control of Rashba spin orbit coupling in noble metal surfaces

Rashba spin orbit coupling (SOC) in noble metal surfaces is of great importance for the application of metal films in spintronic devices. By combining the density-functional theory calculations with our recently developed orbital selective external potential method, we investigate the Rashba SOC in the Shockley surface states of Au(111) and Ag(111). We find that the large Rashba SOC in the sp-character surface states of Au(111) is mainly contributed by the minor d-orbitals in the surface states. While for the sd-character surface states, although they are dominated by the d-orbitals, Rashba splitting is found to be rather small. Band structure analysis reveals that this is mainly because the sd-character surface states are well below the Fermi level and can be less influenced by the asymmetric surface potential. We demonstrate that the Rashba SOC in noble metal surfaces can be effectively manipulated by shifting the d-orbitals in the surface states, which can be physically implemented through surface decoration. Our investigation provides a deep understanding on Rashba SOC in noble metal surfaces and could be helpful to their applications in spintronic devices.

Comparing Ullmann Coupling on Noble Metal Surfaces: On‐Surface Polymerization of 1,3,6,8‐Tetrabromopyrene on Cu(111) and Au(111)

AbstractInvited for the cover of this issue is the group of Meike Stöhr at the University of Groningen. The image depicts the construction of low‐dimensional polymer networks on metal surfaces. Read the full text of the article at 10.1002/chem.201504946.

Facile fabrication of Ag dendrite-integrated anodic aluminum oxide membrane as effective three-dimensional SERS substrate

A novel surface enhanced Raman scattering (SERS)-active substrate has been successfully developed, where Ag-dendrites are assembled on the surface and embedded in the channels of anodic aluminum oxide (AAO) membrane, via electrodeposition in AgNO3/PVP aqueous system. Reaction conditions were systematically investigated to attain the best Raman enhancement. The growth mechanism of Ag dendritic nanostructures has been proposed. The Ag dendrite-integrated AAO membrane with unique hierarchical structures exhibits high SERS activity for detecting rhodamine 6G with a detection limit as low as 1×10−11M. Furthermore, the three-dimensional (3D) substrates display a good reproducibility with the average intensity variations at the major Raman peak less than 12%. Most importantly, the 3D SERS substrates without any surface modification show an outstanding SERS response for the molecules with weak affinity for noble metal surfaces. The potential application for the detection of polycyclic aromatic hydrocarbons (PAHs) was evaluated with fluoranthene as Raman target molecule and a sensitive SERS detection with a limit down to 10−8M was reached. The 3D SERS-active substrate shows promising potential for rapid detection of trace organic pollutants even weak affinity molecules in the environment.

Isolated supramolecules on surfaces studied with scanning tunneling microscopy

To date, supramolecular chemistry is an ever growing research field owing to its crucial role in molecular catalysis, recognition, medicine, data storage and processing as well as artificial photosynthetic devices. Different isolated supramolecules were prepared by molecular self-assembly on surfaces. This review mainly focuses on supramolecular aggregations on noble metal surfaces studied by scanning tunneling microscopy, including dimers, trimers, tetramers, pentamers, wire-like assemblies and Sierpiński triangular fractals. The variety of self-assembled structures reflects the subtle balance between intermolecular and molecule–substrate interactions, which to some extent may be controlled by molecules, substrates and the molecular coverage. The comparative study of different architectures helps identifying the operative mechanisms that lead to the structural motifs. The application of these mechanisms may lead to novel assemblies with tailored physicochemical properties.

Charge-Transfer-Induced Magic Cluster Formation of Azaborine Heterocycles on Noble Metal Surfaces

We report a combined experimental and theoretical study of the adsorption and assembly of a nitrogen–boron-containing heterocycle, 1,2-dihydro-1,2-azaborine, on Au(111) and Cu(111). Despite the inherent molecular dipole moment, the self-assembly behavior is found to be highly surface dependent, with isolated molecules prevalent on Cu(111) and discrete (“magic”) clusters on Au(111). The ability to form clusters of a particular size can be understood in terms of a balance between attractive intermolecular interactions, including directional B–H···H–N dihydrogen bonding, and repulsive forces from Coulombic interactions between the charged molecules dictated by differences in the charge transfer and Pauli repulsion between the adsorbate and the surface. This work highlights the importance of metal–molecule charge transfer in the adsorption and assembly of dipolar molecules on surfaces and demonstrates that their surface-bound properties cannot be predicted a priori from gas-phase dipole moments alone.

Comparing Ullmann Coupling on Noble Metal Surfaces: On-Surface Polymerization of 1,3,6,8-Tetrabromopyrene on Cu(111) and Au(111).

The on-surface polymerization of 1,3,6,8-tetrabromopyrene (Br4 Py) on Cu(111) and Au(111) surfaces under ultrahigh vacuum conditions was investigated by a combination of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. Deposition of Br4 Py on Cu(111) held at 300 K resulted in a spontaneous debromination reaction, generating the formation of a branched coordination polymer network stabilized by C-Cu-C bonds. After annealing at 473 K, the C-Cu-C bonds were converted to covalent C-C bonds, leading to the formation of a covalently linked molecular network of short oligomers. In contrast, highly ordered self-assembled two-dimensional (2D) patterns stabilized by both Br-Br halogen and Br-H hydrogen bonds were observed upon deposition of Br4 Py on Au(111) held at 300 K. Subsequent annealing of the sample at 473 K led to a dissociation of the C-Br bonds and the formation of disordered metal-coordinated molecular networks. Further annealing at 573 K resulted in the formation of covalently linked disordered networks. Importantly, we found that the chosen substrate not only plays an important role as catalyst for the Ullmann reaction, but also influences the formation of different types of intermolecular bonds and thus, determines the final polymer network morphology. DFT calculations further support our experimental findings obtained by STM and XPS and add complementary information on the reaction pathway of Br4 Py on the different substrates.

Van Der Waals-Corrected Density Functional Theory Simulation of Adsorption Processes on Noble-Metal Surfaces: Xe on Ag(111), Au(111), and Cu(111)

The DFT/vdW-WF2s1 method based on the generation of localized Wannier functions, recently developed to include the van der Waals interactions in the Density Functional Theory and describe adsorption processes on metal surfaces by taking metal-screening effects into account, is applied to the case of the interaction of Xe with noble-metal surfaces, namely Ag(111), Au(111), and Cu(111). The study is also repeated by adopting the DFT/vdW-QHO-WF variant relying on the Quantum Harmonic Oscillator model which describes well many-body effects. Comparison of the computed equilibrium binding energies and distances, and the $C_3$ coefficients characterizing the adatom-surface van der Waals interactions, with available experimental and theoretical reference data shows that the methods perform well and elucidate the importance of properly including screening effects. The results are also compared with those obtained by other vdW-corrected DFT schemes, including PBE-D, vdW-DF, vdW-DF2, rVV10, and by the simpler Local Density Approximation (LDA) and semilocal (PBE) Generalized Gradient Approximation (GGA) approaches.

Characterization of graphene edge functionalization by grating enhanced Raman spectroscopy

We demonstrate a large enhancement of edge related Raman features, associated with armchair and zigzag hydrogen-terminated graphene edges. The graphene edges act as good scatterers to excite LSPP on a noble metal surface.

Topological states on the gold surface

Gold surfaces host special electronic states that have been understood as a prototype of Shockley surface states. These surface states are commonly employed to benchmark the capability of angle-resolved photoemission spectroscopy (ARPES) and scanning tunnelling spectroscopy. Here we show that these Shockley surface states can be reinterpreted as topologically derived surface states (TDSSs) of a topological insulator (TI), a recently discovered quantum state. Based on band structure calculations, the Z2-type invariants of gold can be well-defined to characterize a TI. Further, our ARPES measurement validates TDSSs by detecting the dispersion of unoccupied surface states. The same TDSSs are also recognized on surfaces of other well-known noble metals (for example, silver, copper, platinum and palladium), which shines a new light on these long-known surface states.

PH-Triggered Molecular Alignment for Reproducible SERS Detection via an AuNP/Nanocellulose Platform.

The low affinity of neutral and hydrophobic molecules towards noble metal surfaces hinders their detection by surface-enhanced Raman spectroscopy (SERS). Herein, we present a method to enhance gold nanoparticle (AuNP) surface affinity by lowering the suspension pH below the analyte pKa. We developed an AuNP/bacterial cellulose (BC) nanocomposite platform and applied it to two common pollutants, carbamazepine (CBZ) and atrazine (ATZ) with pKa values of 2.3 and 1.7, respectively. Simple mixing of the analytes with AuNP/BC at pH < pKa resulted in consistent electrostatic alignment of the CBZ and ATZ molecules across the nanocomposite and highly reproducible SERS spectra. Limits of detection of 3 nM and 11 nM for CBZ and ATZ, respectively, were attained. Tests with additional analytes (melamine, 2,4-dichloroaniline, 4-chloroaniline, 3-bromoaniline, and 3-nitroaniline) further illustrate that the AuNP/BC platform provides reproducible analyte detection and quantification while avoiding the uncontrolled aggregation and flocculation of AuNPs that often hinder low pH detection.

SERS of the anti‐inflammatory drug piroxicam adsorbed on the surface of silver or gold colloids as nanocarrier model

Piroxicam is a non‐steroidal anti‐inflammatory drug actually used with limitations because of serious side effects. It is poorly soluble in pure water and exhibit a minimum of four conformers according to the pH conditions of the solution, but not all of them are medically active. In this study, we present, firstly, the Raman characterization of piroxicam in different organic solvents (dimethyl sulfoxide, 1,4‐dioxane, ethanol, 1‐propanol). These results have permitted us to analyze surface‐enhance Raman scattering spectra of piroxicam adsorbed on gold or silver nanoparticles surface at several pHs (1, 2, 4, and 7), imitating the environment of the drug in the body, either in the gastrointestinal tract or in healthy and disease tissues. Results indicate that, below pH = 7, piroxicam is mainly in the zwitterionic conformer, and molecules are oriented parallel to the noble metal surface; however at pH ≥ 7, the main specie detected is the anionic one, differently oriented with respect to the nanoparticle surface. The metal‐piroxicam systems here characterized by surface‐enhance Raman scattering spectroscopy could constitute nanocarriers in future projects of transporting and releasing of the drug in the body. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Direct observation of many-body charge density oscillations in a two-dimensional electron gas

Quantum interference is a striking manifestation of one of the basic concepts of quantum mechanics: the particle-wave duality. A spectacular visualization of this effect is the standing wave pattern produced by elastic scattering of surface electrons around defects, which corresponds to a modulation of the electronic local density of states and can be imaged using a scanning tunnelling microscope. To date, quantum-interference measurements were mainly interpreted in terms of interfering electrons or holes of the underlying band-structure description. Here, by imaging energy-dependent standing-wave patterns at noble metal surfaces, we reveal, in addition to the conventional surface-state band, the existence of an 'anomalous' energy band with a well-defined dispersion. Its origin is explained by the presence of a satellite in the structure of the many-body spectral function, which is related to the acoustic surface plasmon. Visualizing the corresponding charge oscillations provides thus direct access to many-body interactions at the atomic scale.

Nanoporous titanium obtained from a spinodally decomposed Ti alloy

Porous noble metal surfaces were produced by gold-smiths in ancient times by dealloying. These processes lead to the formation of gold or other noble metal rich layers. Such dealloying occurs by corrosion as the most electro-chemically active elements dissolve and the more noble elements aggregate into clusters by phase separation (spinodal decomposition). Here we report results from an alternative path to nanoporosity where the structure patterns and length scales are first fixed in the bulk by high-temperature spinodal decomposition followed by chemical dissolution of one of the component products of the phase-separation. The process may be used to form nanoporous titanium for biochemical applications.

Probing the electrooxidation of weakly adsorbed formic acid on Pt(1 0 0)

Understanding the electrooxidation of formic acid on noble metal surfaces is both important for the insight it promises into a formic acid fuel cell and as a model for the electrooxidation of hydrocarbons more generally. As a result, this reaction has been studied for decades. Taken together this body of previous work suggests that on most noble metals the oxidation precedes both through an indirect, via adsorbed CO, and direct pathway via an intermediate that is controversial. In an attempt to rationalize large differences in electrooxidative activity as a function of metal, crystal surface, pH and the presence of other anions, a variety of intermediates in the direct pathway have been proposed. While differing in detail virtually all of the proposed mechanisms invoke a species that only weakly interacts with the metal surface and cannot be detected via conventional spectroscopic techniques. Here we overcome this challenge by characterizing the electrooxidation of formic acid on the Pt(100) surface using the surface selective nonlinear optical technique, vibrationally resonant sum frequency spectroscopy. We detect a weakly adsorbed form of formic acid and find by correlating its potential dependent spectral amplitude and oxidation peaks apparent in a simultaneously collected in situ cyclic voltammogram, that it is a reaction precursor in the direct oxidation of formic acid on this surface.

Trapping and Characterization of a Single Hydrogen Molecule in a Continuously Tunable Nanocavity.

Using inelastic electron tunneling spectroscopy with the scanning tunneling microscope (STM-IETS) and density functional theory calculations (DFT), we investigated properties of a single H2 molecule trapped in nanocavities with controlled shape and separation between the STM tip and the Au (110) surface. The STM tip not only serves for the purpose of characterization, but also is directly involved in modification of chemical environment of molecule. The bond length of H2 expands in the atop cavity, with a tendency of dissociation when the gap closes, whereas it remains unchanged in the trough cavity. The availability of two substantially different cavities in the same setup allows understanding of H2 adsorption on noble metal surfaces and sets a path for manipulating a single chemical bond by design.

(Invited) In Situ Stress and Nanogravimetric Measurements during Underpotential Deposition

The underpotential deposition (UPD) of metals onto foreign metal substrates involves monolayer or submonolayer formation at potentials that are positive of the reversible electrodeposition of the bulk metal. UPD is particularly important to the understanding of electrodeposition where processes in the UPD region can be expected to influence the growth and subsequent properties of bulk thin films. In electrocatalysis, sub-monolayers of metals such as Bi, Pb, and Tl on noble metal surfaces have shown enhanced catalytic activity for a variety of electroreduction processes. Modified catalytic activity has also been observed for Pd and Pt overlayers consisting of a few monolayers. For this reason UPD processes are being examined by a variety of in situ techniques, including wafer curvature, in order to better understand the relationship between the structure of the UPD adlayers and the electrocatalytic activity. Changes in surface stress can be measured by monitoring the curvature of a wafer or cantilever while in solution and under potential control. Surface stresses on the order of 0.01 N/m can be resolved, sufficient to study the adsorption of molecular monolayers onto the electrode surface. Equation (1) quantifies the change in surface stress associated with epitaxial overlayer growth1, ΔF = Δfo + Δfpzc + Δh + Σcohtf (1) where Δfo is the intrinsic surface stress difference between the overlayer and the substrate, Δfpzc is an electrocapillary term that reflects the change in capacitance and potential of zero charge (pzc) associated with changing the surface from the substrate metal to the overlayer metal, Δh is the interface stress, and ΔΣcohtf is the product of the coherency stress and the film thickness. For sub-monolayers, Eqn. (1) may also contain an additional term that captures the contribution of the compressive strain due to island size effects2. Although the misfit term in Eqn. (1) is often assumed to dictate the overall change in surface stress, this is not always the case. The upd of Al and Cu onto Au are clear examples where the stress change is compressive even though the lattice misfit is positive. In some cases, pseudomorphic film growth of several monolayers allows the coherency term to be distinguished from the interfacial terms, which are independent of film thickness. Measurements of this type have been made for the electrodeposition of Pd onto (111)-textured Au3, and have resulted in estimates of Δh for the Pd-Au interface to be about -1.2 N/m. This is similar in both sign and magnitude to values reported for Ag-Cu4. This talk will examine the surface stress changes associated with underpotential deposition, paying particular attention to the interfacial contributions, in addition to that associated with lattice misfit.

Cu Underpotential Deposition on Ru(0001)

The underpotential deposition (UPD) has attracted research attention for many years due to its fundamental and practical importance[i]. Development of scanning probe methods (STM and AFM), and X-ray scattering techniques has enabled mechanistic in situ studies for many substrate-adlayer systems with great insight in details of UPD adlayer formation and structure, alloying and anion co-adsorption[ii]. However, in some cases, the noble metal surfaces with complex reactivity and interaction with water based electrolytes have rendered UPD studies a very difficult endeavor. One example is Ru surface for which a just few UPD studies have been reported[iii]-,[iv],[v],[vi],[vii]. Our talk presents a study of Cu UPD on Ru(0001). The Ru(0001) surface was prepared by annealing at 1400 C for one hour in Ar+H2 atmosphere (96:4), Figure 1. After cooling the Ru surface was protected by CO-saturated 0.1 M HClO4 solution and transferred to the electrochemical cell or STM cell for further studies. The example of the cyclic voltammetry and insitu STM results are shown in Figure 1. The potential has been gradually changed towards the region where Cu UPD on Ru(0001) occurs and high resolution/quality STM images were recorded. The morphological change of Ru(0001) surface due to Cu UPD process were correlated with CV data and characteristic cathodic and anodic peaks in CV were associated with particular features/process occurring during Cu UPD deposition and/or stripping. The Cu UPD process and adlayer structure on Ru(0001) is compared with corresponding Cu UPD process on other closed pack surfaces such as Au(111) and Pt(111). The apparent similarities and differences were identified and discussed during the talk. The authors acknowledge the support from NSF Chemistry division under the contract # 0955922. Figure 1. Cyclic voltammetry for Cu UPD on Ru(0001), scan rate 10 mV/s. Solution: 0.1 M HClO4 + 10-3 M Cu2+. Inset shows Ru crystal during inductive annealing. [i] S. Trasatti, J. Electroanal. Chem. 33, 351 (1971), [ii] E. Budevski, G. Staikov and W. J. Lorenz in Electrochemical Phase Formation and Growth, ed. R.C. Alkire et al. VCH, Berlin (1996), see Ref. herein. [iii] M. Vukmirovic, in Modern Aspects of Electrochemistry, vol. 42, ed. C. Vayenas et al. Springer, NY (2008). [iv] N. Bogolowski, S. Huxter, A. –E. Abd-El-Latif,G.A. Attard, H. Baltruschat, Journal of Electroanalytical Chemistry, 646, 68 (2010). [v] D. K. Gebregziabiher,Y-G. Kim, C. Thambidurai, V. Ivanova, P-H. Haumesser, J.L. Stickney, Journal of Crystal Growth, 312 1271 (2010). [vi] Y. Zhang, L. Huang, T. N. Arunagiri, O. Ojeda, S. Flores, O. Chyan, R. M. Wallace, Electrochemical and Solid-State Letters, 7 C107 (2004). [vii] M. A. Quiroz, L. Salgado, Y. Meas, Electrochimica Acta, 33 435 (1988). Figure 1

Formic Acid Oxidation on Platinum- a Simple Mechanistic Study

The oxidation of small organic acids on noble metal surfaces under electrocatalytic conditions is important in industrial processes and of scientific interest, but the basic reaction mechanisms continue to be a matter of debate. Formic acid oxidation on platinum is one of the simplest of these reactions, yet even this model system remains poorly understood. Historically, proposed mechanisms for the oxidation of formic acid involve the acid molecule as a reactant, but recent studies suggest that the formate anion is the key reactant instead. Ab initio studies of this reaction do not address formate as a possible reactant, due to the difficulty of calculating a charged species near a solvated surface under potential control. Using the recently-developed Joint Density Functional Theory (JDFT) framework for electrochemistry, we perform ab initio calculations of this reaction on a Pt(111) surface to explore this reaction and help resolve the debate. When a formate anion approaches the platinum surface with H pointing towards the surface, it reacts to form CO2 and adsorbed H with no barrier on a clean Pt surface at relevant voltages. This mechanism explains the elusiveness of an active surface intermediate and leads to a reaction rate proportional to formate concentration and number of available platinum sites. Additionally, high coverages of adsorbates lead to large reaction barriers, and consequently, we expect availability of pristine metal sites to limit the experimentally observed reaction rate.

Study of metal specific interaction, F-LUMO and VL shift to understand interface of CuPc thin films and noble metal surfaces

The performances of organic electronic devices are significantly associated with their energy level alignment at organic semiconductor/metal–electrode interfaces. The electronic character of an organic semiconducting molecular over-layer on a metal surface can vary from semiconducting to metallic, depending on the nature of the molecular orbitals with respect to the Fermi level of the electrode. The general tendency of extrapolating established models for single crystal substrates to ‘real’ device substrates is highly misleading. Hence, the importance of metal specific interaction, former lowest unoccupied molecular orbital (F-LUMO) and vacuum level (VL) shift have been investigated as a function of thickness of the deposited films by means of photoelectron spectroscopy (XPS and UPS) to understand the interface between CuPc and Cu, Ag, Pt and Au foils sequentially. The XPS data provides the signature of affectability of pyrrole sites of CuPc molecules for partial charge transfer from Cu and Ag substrates while a negligible effect on pyrrole cites resulted for Pt and Au substrates. Furthermore, the appearance of F-LUMO, a hybridized state close to the Fermi level gives confirmatory information about partial charge transfer. Contrary to the general belief that vacuum level shift caused by charge transfer can partially or totally cancel that for push back effect, our observation indicates that the partial charge transfer does not play significant role in the shift of vacuum level. The entire thickness dependent electronic energy level alignment of CuPc films on all noble metal substrates is explained in terms of a combined effect of partial charge transfer and photoemission final state relaxation energy. A systematic variation of HOMO-FWHM was observed with CuPc thickness due to continuous change in molecular orientation.