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Issue 3, Previous Article Next Article. Unveiling the emergence of functional materials with STM: metal phthalocyanine on surface architectures. This article is part of the themed collection: 2D materials: molecular design and engineering perspectives. You have access to this article. Please wait while we load your content Something went wrong. Try again? Cited by. Back to tab navigation Download options Please wait Article type: Review Article.
DOI: Especially, porphyrin thin films have been intensively investigated with respect to their activity as electrocatalysts in electrochemical reactions such as the reduction of O 2 for the development of efficient fuel cells [ 8 — 10 ]. Miyake et al. As a result, 5,10, tris - 4-octadecyloxyphenyl 4-pyridyl porphyrin molecules were found to form slightly undulating rows, while more kinky rows were observed with 5- 4-carboxyphenyl ,15, tris 4-octadecyloxyphenyl porphyrin.
Similarly, the ordered adlayers of three water soluble porphyrin molecules, i.
Molecular Self-Assembly at Metal-Electrolyte Interfaces
Since simple anions like halides do not adsorb on HOPG, the molecular structures in this case are only influenced by the charge state of the bare HOPG surface. With respect to metal surfaces He et al. More recently, Hai et al. Likewise, viologen based thin films have also attracted much attentions due to their widespread applications in electronic devices [ 22 — 25 ] as well as light harvesting operators [ 26 ].
Charge transfer reactions within adsorbed ordered layers of viologens as a function of the electrode potential are reported to result in reversible structure transitions [ 29 , 30 ].
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For instance, adsorption of 2. Later, Diao et al. As mentioned above, a detailed characterization of such charge transfer governed phase transitions under electrochemical conditions yields insight into the molecule-molecule and molecule-substrate interactions and, thereby, the possibility to govern the structure formation via the tunable surface charge density.
Moreover, competitive co-adsorption of two or more different organic molecules has also been investigated [ 31 — 35 ], because the resultant new intermolecular interactions between unlike molecules opens new degrees of freedom for the design of supramolecular nano-architectures.
For example, two-component films consisting of porphyrin and phthalocyanine derivatives have been examined by Kobayashi [ 31 ], Hipps [ 32 , 33 ] and recently Itaya [ 34 , 35 ], but systematic studies in this direction are still scarce. Characterization of the electrochemical and structural properties as well as phase transitions of such ordered adlayers on metal electrodes as a function of their redox-state, the pre-adsorbed anion species and the symmetry of the substrate is therefore of importance in understanding and controlling the properties of both single molecular and bimolecular adlayers in aqueous solution under electrochemical conditions.
In the present work a survey is given about the electrochemical properties, surface structures and phase transitions of a pure viologen and a pure porphyrin layer as well as of mixed layers of both molecular species forming on chloride modified Cu and Cu electrode surfaces in aqueous solution under electrochemical conditions.
Subsequently we also investigate the properties of binary adlayers consisting of both coadsorbed species. Besides the standard electrochemical method of cyclic voltametry [ 36 , 37 ] also state-of-the-art surface analytical methods are employed, namely in situ Electrochemical Scanning Tunneling Microscopy EC-STM as a local structure sensitive technique and ex situ X-ray Photoelectron Spectroscopy XPS as a chemical probe.
In particular, EC-STM enables a direct visualization of structural changes within the adsorbed molecular adlayers caused by either mere changes of the electrode potential or by surface reactions due to electron transfer processes. Chloride anions are known to adsorb strongly on copper electrodes forming well ordered phases. Exploiting the bias U b dependence of the STM images it is possible to distinguish quasi-spectroscopically between contributions originating from chloride and copper to the registered tunneling current, respectively, and, thereby, to prove that the chloride anions occupy hollow sites [ 42 ].
Large scale image: 9. Large scale image: Figure 2a is reproduced with permission from [ 38 ]. In contrast, on the Cu electrode, the adsorbed chloride phase is a function of the electrode potential. Figures 1c,d show representative atomic level STM images of the quasi-threefold symmetric chloride lattice on top of the Cu electrode surface as well as a model of this adsorbed phase.
On both surfaces, the adsorbed chloride anions induce a preferential orientation of the substrate steps parallel to the close-packed chloride rows Figure 1a,c and [ 4 ]. Besides these mere structure and symmetry aspects, recent in situ XRD measurements in combination with theoretical calculations have provided additional information about the charge state of the chloride precovered copper surfaces [ 49 , 50 ].
In both cases the bond length between the Cu surface atoms and the adsorbed Cl anions is longer than the Cu—Cl bond length found in solid copper chloride compounds as well as for chloride adsorbed on the two surfaces in UHV [ 51 ]. By contrast, pre-adsorbed iodide layers are found to be bound more covalently, making their electrostatic interaction with co-adsorbed porphyrin cations less strong [ 52 ]. As mentioned in the previous section, the potential window of the copper electrodes in the pure electrolyte is limited by the copper dissolution reaction CDR at the anodic limit and the hydrogen evolution reaction HER at the cathodic limit dash black curves.
A drastic change in the voltammetric behavior is observed in the DBV-containing solution grey curve , namely three new pronounced pairs of peaks appear and the HER is shifted toward lower potentials. The shift of the HER is probably caused by viologen molecules blocking the most reactive surface sites for this reaction.
While the uncharged viologen species accumulate at the electrode surface due to their hydrophobic properties [ 4 ] the dication and radical cation species are soluble in aqueous solutions. The further reduction of the radical mono-cations to the fully uncharged viologen species DBV 0 labeled by P 3. As argued in the introduction, it is expected that the respective charge state of the molecular species as well as the presence or absence of the anion underlayer will influence the structure of the adsorbed DBV layer. In the following we present and discuss STM images obtained under non-reactive conditions, i.
Reproduced with permission from [ 54 — 56 ]. Since two different arrangements of the four building blocks are possible one of which is accentuate in Figure 3b , these cavitands occur in two circularly chiral enantiomers [ 55 ]. This interpretation is in agreement with Liu et al. Two lower binding energy peaks, i. Sweeping the electrode potential negatively, i. Furthermore, adjacent rows of the alternating stripe structure appear differently bright in the STM image.
Considering the uniaxial incommensuracy of the chloride structure underneath the most likely explanation for this phenomenon is that molecules in adjacent rows are situated in inequivalent adsorption sites. Obviously, the difference in the structure of the adsorbed viologen layers on the two kinds of surfaces is governed by the structural properties of the substrate via the chloride interlayer. For symmetry reasons, there are four orientations of rotational domains in total on the whole Cu surface.
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Regarding the findings presented later in section 2. Close-packed rows of the dimer-phase are also aligned along the step-edges like the chains of monocation-radicals in the alternating stripe phase. The high resolution STM image in Figure 6b reveals further details of the internal molecular structure, i. Individual spots are recognized as double-rodlets with an inner spacing of 0. Details are presented and discussed by Pham et al. In this section the electrochemical behavior and the structural features of a layer of 5,10,15,Tetrakis 4-trimethyl ammonium phenyl porphyrin tetra p-toluenesulfonate molecules abbreviated as H 2 TTMAPP , in which the four hydrogen atoms at the four meso positions of the central porphyrin core are replaced by four trimethy-ammonium-phenyl groups Figure 7 , are presented and discussed in detail.
Note, the CVs of both Cu and Cu electrodes in pure supporting chloride solution 10 mM HCl are shown in Figure 2 and have been discussed in section 2. RHE, appear, indicating the stepwise reduction of the molecules [ 62 ]. Note, that the corresponding re-oxidation peaks are out of the potential window of the copper electrodes, i. These peaks are only observed when HOPG serves as the working electrode; for details see [ 62 ]. The second reduction peak P 2 has been proposed to correspond to a four-electron transfer step [ 63 — 65 ]. The molecular rows, hence, neither run parallel to any high symmetry axes of the chloride lattice underneath nor to the close-packed atom rows of the Cu substrate.
By reason of surface symmetry, therefore, there are two co-existing mirror domains with respect to each close-packed direction of the chloride mesh underneath.
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