Electrochemical surface science: Self-assembly of Porphyrin molecules at single crystal metal electrodes

A detailed understanding of properties and processes at surfaces and interfaces requires at least two types of most basic information, chemical composition and –distribution as well as structure. While surface science in ultrahigh vacuum is blessed with a plethora of high sensitivity and highest spatial and temporal resolution due to the free accessibility of the surfaces by any kind of probe beams, investigations of solid surfaces under ambient conditions, i.e. in contact with gases or liquids, were for a long time restricted to the use of integral photon-based reflection-, absorption-, emission-, and diffraction methods. This “methodological gap” between UHV surface science and environmental interface research became immediately, at least partially, closed after the realization of the scanning tunneling microscope (STM) and following variants of proximity probes (SPM). The full applicability of this class of methods also under ambient conditions opened the door to structure information of solid-liquid interfaces of comparable resolution as in UHV at room temperature, a “quantum leap” for the understanding of e.g. interfacial electrochemistry. This, in turn, highlighted the need of reliable determination of the chemical composition and distribution at solid-liquid interfaces and pushed the development of in situ X-ray photoelectron spectroscopy (XPS).

The availability of both techniques, in situ SPM and in situ XPS closes the former methodological gap between the research in UHV and under ambient conditions. In particular, interfacial electrochemistry, being primarily interested in chemical processes at electrode/electrolyte interfaces benefits decisively of this development.

In this article, as an example, we present systematic in situ STM measurements and results on the interactions and self-assembly of porphyrins at anion modified metal/electrolyte interfaces, an important class of molecules for the functionalization of surfaces for various applications. Atomically and sub-molecularly resolved potentiostatic and potentiodynamic in situ STM images of such molecular layers are nowadays standard and wait for an in-depth theoretical analysis.