Showing papers by "Klaus Wandelt published in 2018"

Porphyrin Layers at Cu/Au(111)–Electrolyte Interfaces: In Situ EC-STM Study

Abstract: The coadsorption of porphyrin molecules (TMPyP: tetra(N-methyl-4-pyridyl)-porphyrin), sulfate anions and copper on a Au(111) electrode was investigated by the use of cyclic voltammetry (CV) and in situ electrochemical scanning tunneling microscopy. With decreasing electrode potential the following sequence of surface phases was found: (I) an ordered $$\left( {\sqrt 3 \times \sqrt 7 } \right)R19.1^\circ - {\text{S}}{{\text{O}}_4}^{{2 - }}$$ structure on the unreconstructed Au(111)-(1 × 1) surface; (II) a disordered SO42−-layer on the still unreconstructed Au(111)-(1 × 1); (III) a $$\left( {\sqrt 3 \times \sqrt 3 } \right)R30^\circ$$ coadsorption structure of 2/3 ML Cu and 1/3 ML SO42−; (IV) a completed 1 ML Cu covered by a layer of mobile, i.e. not imaged, SO42− anions, moreover, a coadsorption layer of disordered porphyrin molecules and still mobile SO42− anions; (V) overpotentially deposited Cu-multilayers terminated by the well known Moire-type modulated $$\left( {\sqrt 3 \times \sqrt 7 } \right)R19.1^\circ - {\text{S}}{{\text{O}}_4}^{{2 - }}$$ structure (similar to bulk Cu(111)) and covered by a dense layer of flat lying TMPyP molecules showing a growing square as well as hexagonally ordered arrangement, and at even more negative potential values and low Cu concentrations in the solution (VI) a pseudomorphic underpotentially deposited Cu-monolayer covered by a $$\left( {\sqrt 3 \times \sqrt 7 } \right)R19.1^\circ - {\text{S}}{{\text{O}}_4}^{{2 - }}$$ layer and a dense, ordered porphyrin layer ontop. The formation of the various phases is driven by the potential dependent surface charge density and the resultant electrostatic interaction with the respective ions. A severe imbalance between the copper deposition and desorption current in the CV spectra suggests also the formation of CuTMPyP-metalloporphyrin on the surface which diffuses into the bulk solution.

Potential Driven Non-Reactive Phase Transitions of Ordered Porphyrin Molecules on Iodine-Modified Au(100): An Electrochemical Scanning Tunneling Microscopy (EC-STM) Study

Abstract: The modelling of long-range ordered nanostructures is still a major issue for the scientific community. In this work, the self-assembly of redox-active tetra(N-methyl-4-pyridyl)-porphyrin cations (H2TMPyP) on an iodine-modified Au(100) electrode surface has been studied by means of Cyclic Voltammetry (CV) and in-situ Electrochemical Scanning Tunneling Microscopy (EC-STM) with submolecular resolution. While the CV measurements enable conclusions about the charge state of the organic species, in particular, the potentio-dynamic in situ STM results provide new insights into the self-assembly phenomena at the solid-liquid interface. In this work, we concentrate on the regime of positive electrode potentials in which the adsorbed molecules are not reduced yet. In this potential regime, the spontaneous adsorption of the H2TMPyP molecules on the anion precovered surface yields the formation of up to five different potential-dependent long-range ordered porphyrin phases. Potentio-dynamic STM measurements, as a function of the applied electrode potential, show that the existing ordered phases are the result of a combination of van der Waals and electrostatic interactions.

Kinetics of solvent supported tubule formation of Lotus (Nelumbo nucifera) wax on highly oriented pyrolytic graphite (HOPG) investigated by atomic force microscopy.

Abstract: The time dependence of the formation of lotus wax tubules after recrystallization from various chloroform-based solutions on an HOPG surface at room temperature was studied by atomic force microscopy (magnetic AC mode) taking series of consecutive images of the formation process The growth of the tubules oriented in an upright fashion follows a sequential rodlet→ring→tubule behavior The influence of a number of factors, eg, different wax concentration in chloroform, the additional presence of water, or salts [(NH4)2SO4, NH4NO3] or a mixture of salt/water in the solution on the growth rate and orientation of the tubules is also investigated Different wax concentrations were found to have no effect on the growth rate or the orientation of tubules in none of the solutions The presence of water, however, considerably increased the growth rate of tubule formation, while the presence of salt was again found to have no effect on growth rate or orientation of tubules

Sulfate Adsorption on Copper: From upd to Bulk Copper on Au(111)

Abstract: This article reviews the most recent understanding of the early stages of copper electrodeposition on Au(111) from dilute CuSO 4 containing sulfuric acid solutions as gained by combined cyclic voltammetry (CV) and in situ scanning tunneling microscopy (EC-STM) studies. Five distinct structures are discernible as a function of copper coverage and electric potential: (i) at 2/3 ML copper a well-known (√ 3 × √ 3)R19.1°Cu + SO 4 coadsorption phase is found; (ii) the completed pseudomorphic first copper monolayer is first covered by a disordered SO 4 2 − anion layer, which (iii) only at rather negative potentials forms an ordered (√ 3 × √ 7)R19.1° SO 4 2 − adlayer, contrary to previous findings, but similar to the same SO 4 2 − anion arrangement on a bare Au(111) and a bulk Cu(111) surface; (iv) also the population of the second Cu layer passes through an intermediate (√ 3 × √ 3)R19.1° Cu + SO 4 coadsorption phase at a total (local) coverage of 5/3 ML Cu; (v) finally multilayer deposits (≥ 5ML Cu) grow on the completed second Cu layer which exhibit a similar (but not identical) SO 4 2 − induced (√ 3 × √ 7)R19.1° surface structure superimposed by a long-range Moire superstructure like on bulk Cu(111). The multilayers show a relatively high density of screw dislocations, most likely originating from the small gold islands at the Cu/Au(111) interface which result from a lifting of the gold reconstruction.