Early stages of platinum electrodeposition on highly oriented pyrolytic graphite: scanning tunneling microscopy imaging and reaction pathway

The early stages of Pt electrodeposition (0.675-0.620 V vs RHE) on highly oriented pyrolytic graphite (HOPG) from chloroplatinic acid at 25 °C have been studied by ex-situ STM and SEM imaging complemented with electrochemical data. Nucleation and 3D growth of Pt initiate at HOPG surface defects. Large Pt agglomerates containing flat crystallites with well-defined geometries are found around HOPG steps. Pt crystallites formed by 1-2 nm size clusters become more compact as the electrodeposition potential is shifted negatively or the Pt electrodeposited charge is increased. High-resolution STM imaging reveals large uncovered HOPG areas with the nearest-neighbor C-C distance d = 0.24 ± 0.02 nm and fiat hexagonal Pt crystallites. Electrochemical data combined with STM imaging can be interpreted in terms of a diffusion-controlled Pt(IV) to Pt(II) reaction at HOPG and a surface reaction leading to Pt(0) at HOPG defects.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA)Facultad de Ciencias Exacta

Early stages of platinum electrodeposition on highly oriented pyrolytic graphite: scanning tunneling microscopy imaging and reaction pathway

The early stages of Pt electrodeposition (0.675-0.620 V vs RHE) on highly oriented pyrolytic graphite (HOPG) from chloroplatinic acid at 25 °C have been studied by ex-situ STM and SEM imaging complemented with electrochemical data. Nucleation and 3D growth of Pt initiate at HOPG surface defects. Large Pt agglomerates containing flat crystallites with well-defined geometries are found around HOPG steps. Pt crystallites formed by 1-2 nm size clusters become more compact as the electrodeposition potential is shifted negatively or the Pt electrodeposited charge is increased. High-resolution STM imaging reveals large uncovered HOPG areas with the nearest-neighbor C-C distance d = 0.24 ± 0.02 nm and fiat hexagonal Pt crystallites. Electrochemical data combined with STM imaging can be interpreted in terms of a diffusion-controlled Pt(IV) to Pt(II) reaction at HOPG and a surface reaction leading to Pt(0) at HOPG defects.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA)Facultad de Ciencias Exacta

Scanning Tunneling Microscopy Observation of Sulfur Electrodeposits on Graphite Single Crystals

The early stages of sulfur deposit growth on highly oriented pyrolytic graphite (HOPG) caused by HS- electrooxidation in a neutral buffered solution have been investigated using electrochemical techniques and ex situ scanning tunneling microscopy (STM). In this system sulfur deposition has been observed at −0.80 V vs SCE, i.e. a potential more negative than the reversible potential for the HS-/S reaction. The charge density was equivalent to an average surface coverage by sulfur atoms θ ≅ 1/3 monolayer (ML). Ex situ atomic resolution STM images of the layer electrodeposited at −0.8 V show sulfur submonolayers and large uncovered HOPG domains. Sulfur electroadsorption layers appear as a diluted (√3×√3) surface phase with S atoms atop C atoms of the graphite hexagons and the S−S interatomic distance d(S−S) = 0.42 nm. Further addition of S atoms to a diluted sulfur phase resulted in the formation of sulfur trimers with three S atoms placed atop the three C atoms constituting the graphite hexagons. In this case d(S−S) = 0.24 nm. Neighbor trimers originate a filled hexagonal lattice. Ex situ STM images of overpotential deposited sulfur also show submonolayer sulfur domains with a second hexagonal (√3×√3)R30° sulfur lattice with d(S−S) = 0.42 nm. A further increase of θ produces either a new honeycomb lattice with d(S−S) = 0.24 nm or a rectangular lattice formed by rows of S atoms with d(S−S) = 0.21 nm and row separation d(S−S) = 0.37 nm.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Decrease in the Roughness of Vapor-Deposited Gold Surfaces Induced by Surface Mobility

The evolution of the roughness exponent of vapor-deposited thin gold films (the object) on smooth glass substrate at 298 K has been observed by scanning tunneling microscopy. This evolution was enhanced by keeping the films at 323 K in contact with aqueous 0.5 M H2SO4 containing different KCl concentrations for 24 h. It implies a decay of the surface roughness for lengths larger than the grain size of the object leading to better ordered surface structures. Results are compared to those obtained during film grown by increasing the temperature of the glass substrate from 298 to 673 K. The analysis of surface roughness data indicated an enhancement of surface atom diffusion in ordering the gold surface structure, by either anion adsorption or deposition temperature.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Early stages of platinum electrodeposition on highly oriented pyrolytic graphite: scanning tunneling microscopy imaging and reaction pathway

The early stages of Pt electrodeposition (0.675-0.620 V vs RHE) on highly oriented pyrolytic graphite (HOPG) from chloroplatinic acid at 25 °C have been studied by ex-situ STM and SEM imaging complemented with electrochemical data. Nucleation and 3D growth of Pt initiate at HOPG surface defects. Large Pt agglomerates containing flat crystallites with well-defined geometries are found around HOPG steps. Pt crystallites formed by 1-2 nm size clusters become more compact as the electrodeposition potential is shifted negatively or the Pt electrodeposited charge is increased. High-resolution STM imaging reveals large uncovered HOPG areas with the nearest-neighbor C-C distance d = 0.24 ± 0.02 nm and fiat hexagonal Pt crystallites. Electrochemical data combined with STM imaging can be interpreted in terms of a diffusion-controlled Pt(IV) to Pt(II) reaction at HOPG and a surface reaction leading to Pt(0) at HOPG defects.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA)Facultad de Ciencias Exacta

A Quantum Chemistry Approach to Possible Sulfur Adsorbate Structures on the Basal Plane of Graphite Clusters

A quantum chemistry study for possible sulfur adsorbate structures on the basal plane of graphite clusters has beenmade using the MNDO method. The potential energy curves showed the possible formation of S atoms, sulfur dimer and trimer adsorbates on top and bridge substrate positions, whereas neither sulfur atom, dimer, nor trimer adsorption binding to hollow position could be formed. Results gave support to a recently reported interpretation for sulfur atom adsorption on the basal plane of graphite based upon scanning tunneling microscopy data.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Structural configurations of thin dried polyaniline films on gold(111) from scanning tunneling microscopy

The early stages of growth of polyaniline films electrodeposited on Au were studied by conventional electrochemical techniques, X-ray diffraction and ex situ scanning tunneling microscopy (STM). From high resolution STM imaging it appears that the first step of polyaniline formation involves the adsorption of aniline monomers on Au(111) terraces leading to domains with hexagonal arrays involving a phenyl-phenyl distance in the range 0.70 nm ⩽ d ⩽ 1.0 nm. The second step corresponds to the initiation of the polymer growth yielding typically 10 × 10 nm2 crystalline domains consisting of either 0.45 × 0.6 nm2 rectangular arrays or 0.75 × 0.45 nm2 arrays forming 120 ° angles. The crystalline domains coexist with highly disordered polymer domains. At advanced stages of growth, the formation of polymer fibers 2 nm in average width takes place leading to a full substrate coverage by a polymer deposit with an irregular surface.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Self-Assembled Monolayers on C(0001)

This chapter describes the application of tunneling and AFM to the study of inorganic and organic adsorbates on C(0001) at the submonolayer and ML level. The C(0001) surface can be taken as a model system for the study of adsorption processes because it is atomically smooth and exhibits a low chemical reactivity, allowing an easy handling in the atmosphere. The knowledge of adsorption on carbon is important in the field of electrocatalysis because carbon is widely used as a matrix for the dispersion of catalytically active metallic clusters.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

A new view of electrochemistry at highly oriented pyrolytic graphite

Major new insights on electrochemical processes at graphite electrodes are reported, following extensive investigations of two of the most studied redox couples, Fe(CN)64–/3– and Ru(NH3)63+/2+. Experiments have been carried out on five different grades of highly oriented pyrolytic graphite (HOPG) that vary in step-edge height and surface coverage. Significantly, the same electrochemical characteristic is observed on all surfaces, independent of surface quality: initial cyclic voltammetry (CV) is close to reversible on freshly cleaved surfaces (>400 measurements for Fe(CN)64–/3– and >100 for Ru(NH3)63+/2+), in marked contrast to previous studies that have found very slow electron transfer (ET) kinetics, with an interpretation that ET only occurs at step edges. Significantly, high spatial resolution electrochemical imaging with scanning electrochemical cell microscopy, on the highest quality mechanically cleaved HOPG, demonstrates definitively that the pristine basal surface supports fast ET, and that ET is not confined to step edges. However, the history of the HOPG surface strongly influences the electrochemical behavior. Thus, Fe(CN)64–/3– shows markedly diminished ET kinetics with either extended exposure of the HOPG surface to the ambient environment or repeated CV measurements. In situ atomic force microscopy (AFM) reveals that the deterioration in apparent ET kinetics is coupled with the deposition of material on the HOPG electrode, while conducting-AFM highlights that, after cleaving, the local surface conductivity of HOPG deteriorates significantly with time. These observations and new insights are not only important for graphite, but have significant implications for electrochemistry at related carbon materials such as graphene and carbon nanotubes