Publications

Charge separation and transport through the reaction center of photosystem I (PSI) is an essential part of the photosynthetic electron transport chain. A strategy is developed to immobilize and orient PSI complexes on gold electrodes allowing to probe the complex's electron acceptor side, the chlorophyll special pair P700. Electrochemical scanning tunneling microscopy (ECSTM) imaging and current-distance spectroscopy of single protein complex shows lateral size in agreement with its known dimensions, and a PSI apparent height that depends on the probe potential revealing a gating effect in protein conductance. In current-distance spectroscopy, it is observed that the distance-decay constant of the current between PSI and the ECSTM probe depends on the sample and probe electrode potentials. The longest charge exchange distance (lowest distance-decay constant ?) is observed at sample potential 0 mV/SSC (SSC: reference electrode silver/silver chloride) and probe potential 400 mV/SSC. These potentials correspond to hole injection into an electronic state that is available in the absence of illumination. It is proposed that a pair of tryptophan residues located at the interface between P700 and the solution and known to support the hydrophobic recognition of the PSI redox partner plastocyanin, may have an additional role as hole exchange mediator in charge transport through PSI.© 2021 Wiley-VCH GmbH.

JTD Keywords: azurin, current distance decay spectroscopy, cytochrome c(6), electrochemical scanning tunneling microscopy (ecstm), electrochemistry, photosystem i, photosystem-i, plastocyanin, protein electron transfer, recognition, single metalloprotein, single molecules, structural basis, tunneling spectroscopy, 'current, Amino acids, Charge transfer, Chlorine compounds, Current distance decay spectroscopy, Decay spectroscopies, Distance decay, Electrochemical scanning tunneling microscopy, Electrochemical scanning tunneling microscopy (ecstm), Electrodes, Electron transfer, Electron transport properties, Gold compounds, Photosystem i, Photosystems, Protein electron transfer, Protein electron-transfer, Proteins, Scanning tunneling microscopy, Silver halides, Single molecule, Single molecules

Polycrystalline Cu2O layers have been selectively grown by electrochemical anodization of polycrystalline Cu electrodes in an alkaline medium (pH 12.85). Uniform layers with thicknesses around 100 nm have been obtained. Using electrochemical impedance spectroscopy, it was concluded that the Cu2O films behave as a p-type semiconductor. The Mott-Schottky plot gives a value for the flat band potential of U-FB = -255 mV vs silver/silver chloride electrode (SSC), an estimated carrier density N-A = 6.1 x 10(17) cm(-3), and the space charge layer width was calculated to be W-SCL = 9 nm at a band bending of 120 mV. The electronic structure of the Cu vertical bar Cu2O vertical bar electrolyte interface was for the first time probed by in situ electrochemical tunneling spectroscopy. The use of in situ electrochemical scanning tunneling microscopy allows us to directly observed the valence band edge and determine its position against the absolute energy scale to be E-VB = -4.9 eV. Finally, we constructed a quantitative electronic diagram of the Cu vertical bar Cu2O vertical bar electrolyte interface, where the positions of the valence and conduction band edges are depicted, as well as the edge of the previously reported electronic subband.

JTD Keywords: 0.1 m NaOH, Electrochemical tunneling spectroscopy, Cuprous-oxide films, Anodic-oxidation, Electronic-structure, Alkaline-solution, Aqueous-solution, Initial-stages, Passive film, Thin-films

We have measured in situ the electronic conductance spectra of the passive film formed on an Fe electrode immersed in a borate buffer solution using electrochemical tunneling spectroscopy (ECTS) and electrochemical impedance spectroscopy (EIS) techniques, and we have followed their changes as the electrode is electrochemically oxidized and reduced. We demonstrate that pre-passive Fe(II) oxide and the passive Fe(II)/Fe(III) film, behave as p- and n-type semiconductors, respectively and that their reversible inter-conversion is mediated by the availability of free charge carriers on the electrode surface. ECTS spectra have been also modeled to obtain the main electrochemical kinetic parameters of the electron transfer through both p-Fe(II) and n-Fe(III) oxides at different sample potentials and pHs values. We find that the electronic energy barrier in the oxide and its dependence with electrode potential and solution pH, determine the reactivity and passivity of iron.

JTD Keywords: Electrochemical tunneling spectroscopy, Fe passivity Electronic energy barriers, pH effect on passivity

Despite its tremendous scientific and economic impact, the mechanism that triggers metal passive film breakdown in the presence of aggressive ions remains under discussion. We have studied the iron passive film in chloride media using X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy and electrochemical tunneling spectroscopy (ECTS). Ex situ XPS reveal that the film consists exclusively of an Fe(III) oxide without chloride content. In situ ECTS has been used to build up conductance maps of the Fe electrode during its electrochemical oxidation in a borate buffer solution and its breakdown when the film is grown in the presence of chloride. This conductograms provide direct and in situ experimental evidence of chloride-induced surface states within the band gap of the oxide film (~3.3eV). These states enable new charge exchange pathways that allow hole capture at the surface of the n-type Fe(III) oxide. The blocking of VB processes that occurs in the iron passive film is no longer present in chloride media, and electrode corrosion can proceed through these new states. We propose a simple 3-step mechanism for the process, in which chloride anions form an oxidizing Fe(II) surface intermediate but do not participate directly in the reaction.

JTD Keywords: Electrochemical tunneling spectroscopy, Electronic band structure, Fe passive film, Aqueous chloride corrosion, Semiconductor decomposition, Interface states