57 research outputs found
Dynamics of Pyridine Adsorption on Gold(111) Terraces in Acid Solution from in-Situ Scanning Tunneling Microscopy under Potentiostatic Control
The adsorption of pyridine (Py) on Au(111) terraces produced from aqueous 0.1 M HClO4 + 10-3 M Py at 298 K was studied in the potential range 0.15 V pzc, the potential of zero charge of the substrate, both ordered and disordered domains can be observed. Ordered domains correspond to vertically adsorbed Py molecules forming a (4 × 4) hexagonal lattice with the nearest neighbor distance d = 0.38 nm, and corrugation z = 0.04 nm. The ordered adsorbate structure disappears for E pzc, but it is recovered several minutes after stepping E backward to E > Epzc, as expected for a reversible potential-step-induced surface process. The surface concentration of adsorbed Py molecules resulting from the (4 × 4) lattice is close to 1 × 10-9 mol/cm2, a figure exceeding the average integral surface concentration value obtained from electroadsorption measurements. Therefore, disordered domains would be related to a mobile diluted Py adsorbate coexisting with ordered Py adsorbate domains leading to a nonhomogeneous Py adsorbate layer at the positively charged Au(111) terraces.Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasFacultad de Ciencias Exacta
Dynamics of Pyridine Adsorption on Gold(111) Terraces in Acid Solution from in-Situ Scanning Tunneling Microscopy under Potentiostatic Control
The adsorption of pyridine (Py) on Au(111) terraces produced from aqueous 0.1 M HClO4 + 10-3 M Py at 298 K was studied in the potential range 0.15 V pzc, the potential of zero charge of the substrate, both ordered and disordered domains can be observed. Ordered domains correspond to vertically adsorbed Py molecules forming a (4 × 4) hexagonal lattice with the nearest neighbor distance d = 0.38 nm, and corrugation z = 0.04 nm. The ordered adsorbate structure disappears for E pzc, but it is recovered several minutes after stepping E backward to E > Epzc, as expected for a reversible potential-step-induced surface process. The surface concentration of adsorbed Py molecules resulting from the (4 × 4) lattice is close to 1 × 10-9 mol/cm2, a figure exceeding the average integral surface concentration value obtained from electroadsorption measurements. Therefore, disordered domains would be related to a mobile diluted Py adsorbate coexisting with ordered Py adsorbate domains leading to a nonhomogeneous Py adsorbate layer at the positively charged Au(111) terraces.Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasFacultad de Ciencias Exacta
Scanning Tunneling Microscopy Studies of the Electrochemical Reactivity of Thiourea on Au(111) Electrodes
In situ scanning tunneling microscopy (STM) is proposed as an option to investigate the electrochemical reactivity of nontrivial systems. For this purpose the controversial electro-oxidation and electro-reduction of thiourea at Au(111) surfaces under potential control and constant temperature are considered. Sequential STM imaging show thiourea adsorption in striped arrays that evolve to a hexagonal close-packed (hcp) structure when the electron surface charge density is decreased. The transient hcp structure is electro-oxidized to formamidine disulfide (FDS) that slowly yields adsorbed sulfur. These results show that STM is a powerful tool to understand the reactivity of adsorbed molecules at conducting surfaces, by sequential imaging at the molecular level.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada
New aspects of the surface chemistry of sulfur on Au(111): Surface structures formed by gold-sulfur complexes
Sulfur adsorption on gold surfaces has been extensively studied because of the key role of sulfur species in heterogeneous catalysis, and, more recently, due to the interest in the synthesis of anisotropic gold nanoparticles with potential applications in medicine that involves sulfide reduction. Here we report new surface structures for sulfur on Au(111) by combining in situ scanning tunneling microscopy in aqueous sodium sulfide solutions and density functional theory calculations. Our results show two related lattices, (3√3×3√3) R30° (θ = 0.22) and (√7 × √7) R19.1° (θ = 0.57), that involve AuS3 complexes as building blocks. Gold‑sulfur complexes are formed by the lifting of gold atoms from the substrate surface as revealed by density functional theory calculations. These species, intermediate between adsorbed S in the well-known (√3 × √3)-R30○ lattice and adsorbed polysulfides in organized rectangular structures, explain the surface coverage of gold vacancy islands, a fingerprint of S adsorption on Au(111).Fil: Carro, Pilar. Universidad de La Laguna; EspañaFil: Andreasen, Gustavo Alfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Vericat, Carolina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Vela, Maria Elena. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Salvarezza, Roberto Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentin
Hydrogen-Induced Deformations of Metals Followed by in Situ Scanning Tunneling Microscopy : Palladium Electrolytic Hydrogen Charging and Discharging in Alkaline Solution
In situ scanning tunneling microscopy measurements of Pd single-crystal domains during hydrogen charging/discharging cycles in 0.25 M KOH at 298 K allowed us to follow deformations produced by the Pd ↔ S β(H-Pd) phase transition in real time. The stress produced by this transition leads to elastic deformations involving reversible volume changes and plastic deformations resulting in one- or two-atom high slip lines and slip bands. These results demonstrate the capability of nanoscopies to investigate solid deformations on the nanometer scale in different environments, discriminate different types of deformations, and distinguish possible additional steps that are involved in the dynamics of solids.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada
In situ sequential STM imaging of structural changes resulting from the electrodissolution of silver crystal surfaces in aqueous perchloric acid : The roughening kinetics
Structural changes of silver terrace domains in aqueous perchloric acid solution at constant anodic current density (j) and room temperature were followed by in situ scanning tunneling microscopy sequential imaging. For j-2 silver electrodissolution proceeds at step edges without roughening. Conversely, for j-2 the silver surface becomes progressively rougher to attain, after a certain critical time, a steady roughness. In this case, the dynamic scaling theory applied to STM images indicates that the dissolving silver surface can be described as a self-affine fractal surface with a static roughening exponent α50.9060.06, and a dynamic roughening exponent β50.3660.08. The value of a is consistent with the relevant role played by surface diffusion in the silver electrodissolution mechanism, whereas the value of b is slightly higher than those derived from growth models incorporating surface diffusion. The difference in the value of β would reflect the influence of either the electric field or energetic barriers at step edges on the rate of roughness development.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada
Sequential <i>in situ</i> STM imaging of electrodissolving copper in different aqueous acid solutions
The dynamics of Cu surfaces immersed in either aqueous HClO4 or H2SO4 solution under galvanostatic conditions at room temperature was studied by in situ scanning tunneling microscopy (STM) sequential imaging. The mobile interface depends considerably on the apparent current density (j) applied to the specimen. At j = 0, the Cu topography turns out to be highly dynamic as mass transport among different domains takes place. Conversely, for j = 6 μA cm−2 an inhomogeneous attack of the Cu surface leading to a remarkable increase in roughness and to the formation of etched pits at certain surface domains can be observed. Etched pit domains drive the mobile interface to an unstable regime. The addition of HCl to those acid solutions to reach concentrations higher than 10−2 M leads to the formation of a Cu2Cl2 layer.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada
Dynamic scaling exponents of copper electrodeposits from scanning force microscopy imaging : Influence of a thiourea additive on the kinetics of roughening and brightening
Changes in the topography of Cu electrodeposits grown on polycrystalline Cu substrates at low constant current density from still aqueous concentrated CuSO4 + H2SO4 solutions, at 298 K, were studied by scanning force microscopy (SFM) at different scale lengths (L) from the nanometer level upward. The dynamic scaling theory applied to SFM images leads to exponents α = 0.87 ± 0.06 and β = 0.63 ± 0.08, which are consistent with an interface growing under an unstable regime. For similar conditions, the addition of 1,3-diethyl-2-thiourea reduces the average crystal size (〈ds〉) of electrodeposits leading to scaling exponents α = 0.86 ± 0.06 and β = 0.24 ± 0.05 for L s⟩ and a logarithmic dependence for the spatial and temporal evolution of the interface for L > 3 μm and t → 0. In an additive-free plating bath, the unstable growth regime appears to be originated by enhanced electrodeposition at protrusions due to curvature effects and further sustained by the electric and concentration fields built up around the growing deposit. The presence of the additive hinders the development of instabilities driving the evolution of the growing interface to that predicted by the Edwards-Wilkinson growth model on the asymptotic limit.Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasFacultad de Ciencias Exacta
The Influence of Adsorbates on the Growth Mode of Gold Islands Electrodeposited on the Basal Plane of Graphite
Gold electrodeposition on C(0001) from aqueous 0.5 mM AuCl3·HCl, in the range of apparent charge density (6 ≤ q ≤ 10 mC/cm2) and under mass transport kinetic control from the solution side, results in the formation of nanometer to micrometer size Au(111) islands. These islands consist of a small three-dimensional (3D) central core and large quasi-two-dimensional (2D) dendritic branches. Ex situ scanning tunneling microscopy (STM) imaging data show that the shape of islands is determined by anisotropic surface diffusion contributions. The addition of citric acid to the plating solution hinders branching and promotes 3D island growth. The adsorption of citric acid at step edges hinders interterrace gold atom surface diffusion, as concluded from in situ STM imaging. Conversely, the addition of an excess of sodium chloride enhances 2D island growth and dense branching formation. These results can be explained considering that the presence of adsorbates either increases or decreases the height of activation energy barriers for interterrace surface diffusion. In this way, the change of the island aspect ratio with the nature of the additive can be explained.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada
Dynamic scaling exponents of copper electrodeposits from scanning force microscopy imaging : Influence of a thiourea additive on the kinetics of roughening and brightening
Changes in the topography of Cu electrodeposits grown on polycrystalline Cu substrates at low constant current density from still aqueous concentrated CuSO4 + H2SO4 solutions, at 298 K, were studied by scanning force microscopy (SFM) at different scale lengths (L) from the nanometer level upward. The dynamic scaling theory applied to SFM images leads to exponents α = 0.87 ± 0.06 and β = 0.63 ± 0.08, which are consistent with an interface growing under an unstable regime. For similar conditions, the addition of 1,3-diethyl-2-thiourea reduces the average crystal size (〈ds〉) of electrodeposits leading to scaling exponents α = 0.86 ± 0.06 and β = 0.24 ± 0.05 for L s⟩ and a logarithmic dependence for the spatial and temporal evolution of the interface for L > 3 μm and t → 0. In an additive-free plating bath, the unstable growth regime appears to be originated by enhanced electrodeposition at protrusions due to curvature effects and further sustained by the electric and concentration fields built up around the growing deposit. The presence of the additive hinders the development of instabilities driving the evolution of the growing interface to that predicted by the Edwards-Wilkinson growth model on the asymptotic limit.Instituto de Investigaciones Fisicoquímicas Teóricas y AplicadasFacultad de Ciencias Exacta
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