Probing the passivity of iron by electrochemical tunneling microscopy and spectroscopy

Abstract

[eng] This PhD project has been fully developed at the department of physical-chemistry of the University of Barcelona. The work involves a fundamental study of passivity on iron electrodes by using advanced applications of Scanning Probe Microscopy techniques. The main concepts of the Semiconductor Electrochemistry have been introduced in a simple way to make them accessible for an electrochemist. The final conclusions deals with a quantitative diagram of the energy levels distribution at the oxide electrolyte interface. The new methodologies developed in this PhD work are extensive to the quantitative study of the electron transfer through any electrode liquid junction. After an initial electrochemical characterization of the system, we succeed in coupling our three-electrode configuration into an STM microscope. Special efforts were put on the cell design which finally resulted in a robust ECSTM system that allowed the first in situ topographic and electric measurements of the Fe oxide borate buffer interface under a strict electrochemical control. Additional ex situ microscopy and spectroscopy techniques were also used in combination to elaborate the first oxide growth mechanism and the quantitative electronic band diagram picture of the interface. Convinced that the different transitions of the electronic properties on the iron oxide surface governed its passivity and redox behavior, we went further in its electrical characterization and were embarked in the design of a new methodology to perform real electrochemical tunneling spectroscopy (ECTS). Although its experimental realization had been previously demonstrated on other ideal semiconducting electrodes, several technical difficulties had braked its wide fan of possibilities. Within the last two years of this Ph.D, we have invested efforts to solve the main technical problems and presented by the first time a novel procedure to record reproducible in situ electronic spectra of an electrode electrolyte interface as a function of the oxidation state of the substrate. Their benefits go far beyond this study as it is shown in the Appendix A of this Ph.D. We found especially advantageous the combination of ECTS with the capacitance data of the same interface on determining its corresponding band diagram in a quantitative energy scale. Finally, the last block of the Ph.D. work have been devoted to the analysis and interpretation of the in situ tunneling spectroscopy data which have been employed firstly, to elaborate a complete mechanism of the formation and dissolution of the iron passive layer, and secondly, to revisit the concept of passivity from an electronic point of view. All these background was finally used on the quantitative electronic characterization of the passivity breakdown process of an iron electrode in the presence of chloride. The picture deduced in this last study is probably the highest impact feature of the present work since it directly addresses to the real capabilities that the technique shows on a dynamic process of remarkable technical impact.