Adsorption and Electrochemical Activity:  An In Situ Electrochemical Scanning Tunneling Microscopy Study of Electrode Reactions and Potential-Induced Adsorption of Porphyrins

Abstract

The effect of adsorption on molecular properties and reactivity is a central topic in interfacial physical chemistry. At electrochemical interfaces, adsorbed molecules may lose their electrochemical activity. The absence of in situ probes has hindered our understanding of this phenomenon and electrode reactions in general. In this work, classical electrochemistry and electrochemical scanning tunneling microscopy (EC-STM) were combined to provide molecular level insight into electrochemical reactions and the molecular adsorption state at the electrolyte-electrode interface. The metal-free porphyrin 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (TPyP) adsorbed on Au(111) in 0.1 M H 2 SO 4 solution was chosen as a model system. TPyP is found to irreversibly adsorb on Au(111) over a wide range of potentials, from -0.25 to 0.6 V SCE . The adsorption state of TPyP has a dramatic effect on its electrochemistry. Preadsorbed, oxidized TPyP displays no well-defined cathodic peaks in cyclic voltammograms in sharp contrast to solution-phase TPyP. Our present work provides direct, molecular level evidence of the electrochemically "invisible" species. Electrochemical activity of absorbed species is recovered by allowing the oxidized molecule sufficient time (tens of minutes) to reduce. The redox state of adsorbed TPyP also affects the nature of the adsorption. Oxidized species can apparently only form monolayers. However, multilayers, stable enough to be imaged by STM, can form when the adsorbed TPyP is in the reduced state. This suggests that by controlling the electrochemistry one can either promote or suppress the formation of multilayers.

Dynamics of Porphyrin Electron‐Transfer Reactions at the Electrode–Electrolyte Interface at the Molecular Level

“…For example, the reduction of pre-adsorbed oxidized TPyP can be very slow, taking as long as tens of minutes at À0.05 V. However, the oxidation of adsorbed TPyP at 0.2 V is much faster, occurring in seconds. (The potential for the onset of oxidation of adsorbed TPyP is about 0.1 V.) [13] In the experiment (Figure 1), [14,15] the sample, an adsorbed monolayer of TPyP on Au(111) without TPyP molecules in the 0.1m H 2 SO 4 solution, was initially held at a potential (E 1 = À0.1 V) where all the TPyP molecules were reduced. [13] A short oxidation potential pulse (E 2 , for duration t) was applied to the sample during STM imaging.…”

“…Reduction of the oxidized TPyP may occur during the process of collecting STM images after the oxidation pulse; however, the rate of reduction of adsorbed oxidized TPyP molecules at this potential (À0.1 V) is very slow, so that reduction of oxidized TPyP molecules on the electrode surface can be neglected on the time scale of collecting one STM image. [13] Accordingly, we can probe the oxidation rate of TPyP at an Au(111) surface by comparing the number of oxidized TPyP molecules before and after the oxidation potential pulse.…”

“…[10] The density of dark spots was found to increase with the duration of the potential-pulse perturbation: the longer the duration of the potential pulse, the higher the density of dark spots. These dark spots can be assumed to be oxidized TPyP molecules and not vacancies for two reasons: 1) our previous studies indicated that the interaction between TPyP molecules and the Au substrate is mainly determined by the electrode potential and the redox state of the adsorbed TPyP molecules; oxidation results in a strong interaction between TPyP molecules and the Au(111) surface, [10,13] and 2) the dark spots display an internal structure that is characteristic of adsorbed TPyP (see the Supporting Information).…”

Electrochemical Study of Redox Reaction of Various Gold III Chloride Concentrations in Acidic Solution

“…Fourcade and Tzedakis [3] used a potentiostat as the electrochemical apparatus with a silver disk working electrode, a saturated calomel reference electrode and a platinum counter electrode to measure all the electrode potentials during the adsorption experiments. Tao et al [4] reported scanning tunneling microscopy (STM) and electrochemical study of the interplay between redox properties, adsorption, and self-assembly processes of porphins on Au surfaces.…”

Electrocatalytic Reduction of Oxygen on Graphitized Carbon Electrode Modified with 5,15-Bis(pyrid-4-yl)-3,7,13,17-tetramethyl- 2,8,12,18-tetraethylporphine and Its Cu(II) and Fe(III) Complexes

“…The H 2 T(4-Py)P is reduced in a single two-electron step (-0.07 V vs. Ag/AgCl and gold electrode). [31] In was shown [32][33][34][35][36] that metalloporphyrins and metallophthalocyanines activate a variety of electrochemical reactions. This is due to the nature of aromatic π-conjugated system of the macrocycle, conjugation of coordinated metal atom with this π-system and relatively ease to change the oxidation state of some of the central metal atom.…”