Stepped Ag Research Articles

The role of single-atom Rh-dopants in the adsorption properties of OH and CO on stepped Ag(211) surfaces.

Several chemical reactions with commercial and environmental importance can benefit from the development of more active or selective heterogeneous catalysts. Particularly, those catalyzed by metallic surfaces are usually impacted by the presence of defects such as kinks and dopants. Here, we employed density functional theory calculations within van der Waals correction to investigate the effects of single-atom Rh-dopants in the adsorption properties of OH and CO on stepped Ag(211) surfaces. From our calculations and analyses, we found that the dopant is more energetically stable when replacing more coordinated (and less exposed to the vacuum) sites of the surface. However, in the presence of both molecules, this trend is inverted, and the dopant is more stable in the least coordinated site (step). While OH presents high adsorption energies on both doped and non-doped silver surfaces, CO binds weakly to the noble metal, and strongly on doped sites. The results are relevant for understanding single-atom catalysts on noble-metal surfaces, where the difference in selectivity and activity between the host metal and dopants is exploited. The charge redistribution caused by the dopant, and the appearance of a sharp peak in the density of states of the surface are used to rationalize the results and provide insights into the interactions involved in the adsorption of both molecules.

The Role of Steps on Silver Nanoparticles in Electrocatalytic Oxygen Reduction

Hydrogen fuel cell technology is an essential component of a green economy. However, it is limited in practicality and affordability by the oxygen reduction reaction (ORR). Nanoscale silver particles have been proposed as a cost-effective solution to this problem. However, previous computational studies focused on clean and flat surfaces. High-index surfaces can be used to model active steps presented in nanoparticles. Here, we used the stable stepped Ag(322) surface as a model to understand the ORR performance of steps on Ag nanoparticles. Our density functional theory (DFT) results demonstrate a small dissociation energy barrier for O2 molecules on the Ag(322) surface, which can be ascribed to the existence of low-coordination number surface atoms. Consequently, the adsorption of OOH* led to the associative pathway becoming ineffective. Alternatively, the unusual dissociative mechanism is energetically favored on Ag(322) for ORR. Our findings reveal the importance of the coordination numbers of active sites for catalytic performance, which can further guide electrocatalysts’ design.

A first-principles study of water adsorbed on flat and stepped silver surfaces.

The structural, electronic and vibrational properties of a water layer on Ag(100) and Ag(511) have been studied by first-principles calculations and ab initio molecular dynamics simulations. The most stable water structure on the Ag(100) and Ag(511) surfaces have been obtained. The AIMD results showed rather high stability of the water layer on the stepped surface at 140 K, indicating a crystal-like structure with long-range ordering. The calculated vibrational spectra at 140 K showed good agreement with the experimental results. On the Ag(100) surface, a red-shift was observed when the temperature increased from 140 K to 300 K caused by the change in the number of H-bonded (HB) hydrogen. On Ag(511), a three-fold splitting of the O-H stretch mode was observed. This can be explained by the special water structure at the stepped Ag surface: the relatively strong water-metal interaction at the step edge and weak water-terrace interaction/strong water-water interaction at the terrace, which can also explain the high stability of the water layer on the Ag(511) surface.

One Step Thermochromic Pigment Printing and Ag NPs Antibacterial Functional Finishing of Cotton and Cotton/PET Fabrics

One step thermochromic pigment printing and antibacterial functionality of cotton (100 %) and cotton/polyester blend (50/50 %) were demonstrated in this study. The improvement in antimicrobial activity against G+ve (Bacillus cereus) and G-ve (E. coli), and pigment printability were achieved by inclusion of Ag-NPs (30 g/kg) into pigment printing paste followed by printing and microwave curing at 700 W for 5 min. Modes of interactions were proposed, and surface modification was also confirmed by SEM and EDX analysis that proved the presence of Ag-NPs in cotton and cotton/ polyester blended samples. The results indicated that the colour fastness to wash and rubbing was excellent, the surface roughness reduced, and exhibited good antibacterial activity against Bacillus cereus and E. coli bacteria.

Nano-molar to milli-molar level Ag (I) determination using absorption of light by ZnS QDs without organic ligand

Despite of useful applications of Ag, it is hazardous to health and environment and hence early detection is required. Crystal defects influencing optical property is less likely utilized for Ag detection using prevalent UV–vis technique. Quantum dots (QDs) ZnS prepared by soft chemical route are exploited for the detection of Ag+ in aqueous solution for nM to mM concentrations. UV–vis and photoluminescence (PL) measurements reveal quantum confinement effect and presence of defects in ZnS nanoparticles (NPs). Controlled synthesis of ZnS NPs allows appearance of defects related peak at 400–550 nm in UV–vis spectra. A plausible mechanism is presented and elucidated by XRD and PL studies for chemical interaction between ZnS with Ag+. The interaction affects prominently the defects related absorption of ZnS NPs and allows single step Ag+ detection in aqueous medium. This absorption method is extended to other ions (Na+, K+, Mg+, Ca+, Ba+) including ‘S’ prone heavy and toxic ions like Pb, Hg, and Cd. Among them, Ag+ shows the best response with ZnS NPs. This demonstration using a portable and cost effective technique with no organic component opens up possibility for other inorganic materials.

Hetero-diffusion of Au epitaxy on stepped Ag(110) surface: Study of the jump rate and diffusion coefficient

We have used molecular dynamics simulations combined with an interatomic potential derived from the embedded atom method, to investigate the hetero-diffusion of Au adatom near a stepped Ag(110) surface with the height of one monoatomic layer. The activation energies for different diffusion processes, which occur on the terrace and near the step edge, are calculated both by molecular statics and molecular dynamics simulations. Static energies are found by the drag method, whereas the dynamic barriers are computed at high temperature from the Arrhenius plots. Our numerical results reveal that the jump process requires very high activation energy compared to the exchange process either on the terrace or near the step edge. In this work, other processes, such as upward and downward diffusion at step edges, have also been discussed.

Colloidal lithography with electrochemical nickel deposition as a unique method for improved silver decorated nanocavities in SERS applications

Two types of metallised nanocavities (single and hybrid) were fabricated by colloid lithography followed by electrochemical deposition of Ni and subsequently Ag layers. Introductory Ni deposition step iniciates more homogenous decoration of nanocavities with Ag nanoparticles. Silver nanocavity decoration has been so performed with lower nucleation rate and with Ag nanoparticles homogeinity increase. By this, two step Ni and Ag deposition trough polystyrene nanospheres (100, 300, 500, 700, 900nm), the various Ag surfaces were obtained. Ni layer formation in the first step of deposition enabled more precise controlling of Ag film deposition and thus final Ag surface morphology. Prepared substrates were tested as active surfaces in SERS application. The best SERS signal enhancement was observed at 500nm Ag nanocavities with normalised thickness Ni layer ∼0.5. Enhancement factor has been established at value 1.078×1010; time stability was determined within 13 weeks; charge distribution at nanocavity Ag surfaces as well as reflection spectra were calculated by FDTD method. Newly prepared nanocavity surface can be applied in SERS analysis, predominantly.

Numerical investigation of the submonolayer growth and relaxation of stepped Ag (110) and Au (110) surfaces

The terrace width distribution (TWD) as well as the adatom and island densities for some stepped fcc (110) surfaces with parallel and equidistant steps of equal stiffness are studied by means of kinetic Monte Carlo simulations. Reliable energy barriers for surface diffusion are used. They have been calculated by means of many-body potentials derived within the second moment approximation to the tight-binding model. The effects of the temperature, atom deposition flux and surface diffusion on quantities of interest in the early stage of the surface evolution process have been singled out. In most cases, linear, logarithmic and power-law behaviors are recovered for the evolution of the step width and terrace defects. The TWD is well described by the Gaussian distribution (GD) in the domain of temperature investigated but deviates from the Generalized Wigner Distribution (GWD).

Long-range surface faceting induced by chemisorption of PTCDA on stepped Ag(111) surfaces

The organic molecule PTCDA preferentially adsorbs on steps of vicinal Ag(111) surfaces and bunches them to well defined facet planes. These depend on coverage and annealing temperature and are independent of the nominal step direction and angle of inclination of the unreconstructed initial surface. We study the development of the facets and present a map of all 16 types of facets in a stereographic triangle of 35° off the [111]-direction. The faceting mechanism is interpreted as orientational phase separation originating from different bonding strengths of PTCDA on various facets. The faceting drives the system to the minimum of its surface free energy.

Metallic thin films on stepped surfaces: lateral scattering of quantum well states

Quantum well states of Ag films grown on stepped Au(111) surfaces are shown to undergo lateral scattering, in analogy with surface states of vicinal Ag(111). Applying angle resolved photoemission spectroscopy we observe quantum well bands with zone-folding and gap openings driven by surface/interface step lattice scattering. Experiments performed on a curved Au(111) substrate allow us to determine a subtle terrace-size effect, i.e., a fine step-density-dependent upward shift of quantum well bands. This energy shift is explained as mainly due to the periodically stepped crystal potential offset at the interface side of the film. Finally, the surface state of the stepped Ag film is analyzed with both photoemission and scanning tunneling microscopy. We observe that the stepped film interface also affects the surface state energy, which exhibits a larger terrace-size effect compared to surface states of bulk vicinal Ag(111) crystals.

Electron energy loss spectroscopy of the vibration modes of water on Ag(100) and Ag(115) surfaces and comparison to Au(100), Au(111) and Au(115)

Motivated by rather similar behavior of the Helmholtz capacitances of stepped Au(11n) and Ag(11n) electrodes we have extended a previous study on the vibration spectrum of water adsorbed at low temperatures on stepped gold surfaces to Ag(100), Ag(115) and Au(111) surfaces. On Ag(100) surfaces, the spectra show the presence of the typical H-bonded network of water molecules. The rather weak intensity, the absence of non-hydrogen bonded hydrogen atoms, the similarity to the infrared spectrum of ice crystallites, and the increase in the angular spread of the elastic peak are indicative of adsorption in form of three-dimensional clusters. This is stark contrast to Au(100) and Au(111) where the spectra match to a model involving stacks of water bilayers. The low coverage spectra on Ag(115) resemble the results on Au(115): A considerable fraction of the H-atoms remains in the non-H-bonded state and spectral features of water adsorbed at step-sites are identified. The first layer of water on Ag(115) surfaces should therefore have a similar structure as recently proposed in a theoretical study concerning water on Au(115). For larger doses, the experimental results on Ag(115) suggests the formation of three-dimensional clusters. This is contrary to Au(115) where the layered structure with a constant fraction of non-hydrogen-bonded H-atoms persists at higher doses.

The role of surface defects in large organic molecule adsorption: substrate configuration effects

The role of the configuration of metal surface atoms in the interaction between individual large, planar organic molecules and a metal substrate was investigated by low-temperature scanning tunneling microscopy and density functional theory calculations, including a semi-empirical correction scheme to account for dispersion effects. As test case, we used the adsorption of the oligopyridine derivative 2-phenyl-4,6-bis(6-(pyridine-2-yl)-4-(pyridine-4-yl)pyridine-2-yl)pyrimidine (2,4'-BTP) on a stepped Ag(100) surface. Both experiment, via statistical evaluation of the adsorption site and orientation of 2,4'-BTP admolecules, and theory indicate distinct structural effects. The results are compared with the adsorption behavior of pyridine derivatives and benzene on metal surfaces. Consequences on the understanding of the interaction between heteroatoms or functional groups in large organic adsorbates and metal atoms in typical nano-scaled surface defects and hence of the interaction with more realistic metal surfaces are discussed.

Lateral and vertical manipulations of single atoms on the Ag(1 1 1) surface with the copper single-atom and trimer-apex tips

We study the lateral and vertical manipulations of single Ag and Cu atoms on the Ag(111) surface with the Cu single-atom and trimer-apex tips using molecular statics simulations. The reliability of the lateral manipulation with the Cu single-atom tip is investigated, and compared with that for the Ag tips. We find that overall the manipulation reliability (MR) increases with the decreasing tip height, and in a wide tip-height range the MR is better than those for both the Ag single-atom and trimer-apex tips. This is due to the stronger attractive force of the Cu tip and its better stability against the interactions with the Ag surface. With the Cu trimer-apex tip, the single Ag and Cu adatoms can be picked up from the flat Ag(111) surface, and moreover a reversible vertical manipulation of single Ag atoms on the stepped Ag(111) surface is possible, suggesting a method to modify two-dimensional Ag nanostructures on the Ag(111) surface with the Cu trimer-apex tip.

Supramolecular Nanowires Self-Assembly on Stepped Ag(110) Surface

The self-assembly of the extended supramolecular nanowires on the stepped Ag(110) surfaces has been simulated by the kinetic Monte Carlo (KMC) method. The KMC simulations predict an optimal temperature and an optimal deposition flux for the formation of the longest supramolecular nanowire at the mesoscale. The kinetic instabilities of supramolecular nanowires self-assembly were elucidated, as were the physical and chemical mechanisms of formation of the extended supramolecular nanowires forming on the stepped Ag(110) surfaces.

First-Principles Study of Carbon Monoxide Oxidation on Ag(111) in Presence of Subsurface Oxygen and Stepped Ag(221)

The role of subsurface oxygen in Ag(111) and the step edge of vicinal Ag(221) on CO oxidation were studied by using density functional theory calculations. For high oxygen coverage, the formation of subsurface oxygen is not only energetically favorable but also kinetically likely. CO oxidation with on-surface atomic oxygen is facile with a barrier of about 0.16 eV, whereas the reaction with subsurface oxygen is hindered because of the significant barrier for diffusion of the subsurface oxygen to the surface from the subsurface region. It is found that the adsorption of molecules (CO, atomic O and O2) is stabilized in the presence of subsurface oxygen, and surface reactivity is enhanced. An energetically favorable reaction pathway for CO oxidation with O2 is identified, with a reaction barrier of 0.23 eV via a unique four-center Ο2···CΟ intermediate. A catalytic cycle for CO oxidation on Ag(111) in the presence of subsurface oxygen is proposed. On Ag(221) surfaces, our calculations show that both the adsor...

Tuning the Stoichiometry of Surface Oxide Phases by Step Morphology:Ag(511)versusAg(210)

We show by vibrational and photoemission spectroscopy and by ab initio calculations that in ultrahigh vacuum oxygen adsorption at stepped Ag surfaces causes the formation of surface oxide phases with different stoichiometry, depending on the morphology of the steps involved in the nucleation process. On Ag(511), characterized by close-packed steps and (100) terraces, subsurface O occupies tetrahedral interstitials and an Ag2O-like phase forms; on Ag(210), showing open step edges, subsurface oxygen ends up off center in octahedral sites and an AgO-like structure nucleates.

A statistical mechanics-based model for cubic and mixed-anisotropy ferromagnetic systems

A model based on classical statistical mechanics is used to calculate magnetization curves for cubic and mixed-anisotropy systems. Our results on Fe, Ni and SmFe 2 are in agreement with measurements on bulk single crystals of these materials. For example, the easy axes of magnetization at room temperature for Fe and Ni are [1 0 0] and [1 1 1], respectively. For SmFe 2, the easy magnetization directions at 77 K and room temperature are [1 1 0] and [1 1 1], respectively, in agreement with experiment. For the mixed-anisotropy case, we study two model systems in which both uniaxial and biaxial anisotropies are present. In the easy-[1 0 0] system, a magnetization jump to saturation is found if both anisotropies are of the same order of magnitude in contrast to easy-[1 1 0] systems where magnetization jumps are found even if the biaxial anisotropy is negligible. Examples of mixed-anisotropy material systems that exhibit a discontinuous jump in magnetization, demonstrated by our calculation, are ultra-thin films of Co/Cu and Fe on stepped Ag(0 0 1) substrates.

Comparative Study of Adatom Manipulation on Several Fcc Metal Surfaces

For a set of fcc metals, our total energy calculations based on many body potentials show that activation barriers for lateral manipulation of an adatom at a step edge depend on the tip/substrate composition. Of the six homogeneous systems studied, manipulation on stepped Ag(111) showed the lowest energy barrier for adatom hopping toward the tip, although the relative probability for this process was largest on Cu(111). For a representative Cu/Pt heterogeneous system, we find lateral manipulation of a Pt adatom along a step on Pt(111) by a Cu(100) tip to be energetically much less favorable than the reverse case of a Cu adatom manipulated by a Pt(100) tip. In the case of vertical manipulation, atomic relaxations of the tip and its neighboring atoms are found to be prominent and tip-induced changes in the bonding of the adatom to its low coordinated surroundings help explain the relative ease with which an adatom next to a step edge or a kink site may be pulled as compared to that on a flat surface.

Adsorption of acrolein on single-crystal surfaces of silver: Density functional studies

Silver selectively catalyzes the hydrogenation of the C O group of α, β-unsaturated aldehydes, indicating preferential adsorption-induced activation of this bond compared to the adjacent C C bond. Using a density functional method and periodic slab models, we studied the propensity of the flat Ag(1 1 0) and the stepped Ag(2 2 1) surfaces to activate the C O bond of adsorbed acrolein. Our calculations show acrolein to interact only weakly with all adsorption sites under study (by at most 35 kJ mol −1), resulting in very limited bond activation. Therefore, we conclude that other active sites on silver catalysts participate in the experimentally observed selective hydrogenation.

CO adsorption on the stepped Ag(2 2 1) surface

The adsorption and geometry of CO on the stepped Ag(2 2 1) surface, Ag(S)[4(1 1 1) × (1 1 1)], has been studied by temperature programmed desorption (TPD) and time-of-flight electron-stimulated desorption ion angular desorption (TOF-ESDIAD) methods. The adsorbed CO molecule exhibits two distinct desorption processes near 80 K (α 1) and near 90 K (α 2). At low coverage, only the α 2 desorption feature is observed while at higher coverage the α 1 feature can be seen as well. Upon electron bombardment, three desorption species can be identified by TOF-ESDIAD, O +, CO +, and CO* (electronically excited neutral CO species). The coverage-dependent CO* angular distribution pattern was studied by annealing the surface. At low coverage, a two-beam pattern aligned along the 〈 1 ¯ 1 0 〉 azimuth is obtained indicating that CO molecules tilt in both directions along the step edges. At higher coverage, an additional feature is observed, producing a three beam pattern, which contains contributions both from terrace and step sites. This suggests that CO preferentially adsorbs at the step sites of Ag(2 2 1) at low coverages, and that both terrace and step sites are occupied at higher coverages. It is also concluded that the CO molecules adsorb perpendicular to the terrace surface.