Size-dependent and step-modulated supramolecular electrochemical properties of catechol-derived adlayers at Pt(hkl) surfaces

The electrochemical reactivity of catechol-derived adlayers is reported at platinum (Pt) single-crystal electrodes. Pt(111) and stepped vicinal surfaces are used as model surfaces possessing well-ordered nanometer-sized Pt(111) terraces ranging from 0.4 to 12 nm. The electrochemical experiments were designed to probe how the control of monatomic step-density and of atomic-level step structure can be used to modulate molecule-molecule interactions during self-assembly of aromatic-derived organic monolayers at metallic single-crystal electrode surfaces. A hard sphere model of surfaces and a simplified band formation model are used as a theoretical framework for interpretation of experimental results. The experimental results reveal (i) that supramolecular electrochemical effects may be confined, propagated, or modulated by the choice of atomic level crystallographic features (i.e.monatomic steps), deliberately introduced at metallic substrate surfaces, suggesting (ii) that substrate-defect engineering may be used to tune the macroscopic electronic properties of aromatic molecular adlayers and of smaller molecular aggregates.

Pointed carbon fiber ultramicroelectrodes: a new probe option for electrochemical scanning tunneling microscopy

Carbon tips for in situ scanning tunneling microscopy studies in an electrochemical environment were prepared by electrochemical etching of carbon fibers and subsequent coating with electrodeposition paint and a silicone elastomer. The tips obtained were stable in acidic electrolyte and allowed high-resolution in situ imaging of the bare Au(111) electrode surface and of Au(111) covered by monolayers of the octyl-triazatriangulenium molecule.

Model system for the study of 2D phase transitions and supramolecular interactions at electrified interfaces: hydrogen-assisted reductive desorption of catechol-derived adlayers from Pt(111) single-crystal electrodes

Classical electroanalytical techniques and in situ FTIR are used to study the oxidative chemisorption of catechol (o-H(2)Q) and the hydrogen-assisted reductive desorption of catechol-derived adlayers (o-Q((ads))) at nearly defect-free Pt(111) single-crystal electrodes in 0.5 M H(2)SO(4). At near equilibrium conditions (lim(upsilon-->0)) the cyclic voltammetric response does not conform to the behavior expected from classical models of molecular adsorption at electrochemical interfaces. Instead, attractive interactions play a controlling role, i.e., hydrogen-assisted displacement of o-Q((ads)) takes place as an electrochemically reversible two-dimensional (2D) phase transition controlled by collision-nucleation-growth phenomena in the presence of 2 mM o-H(2)Q((aq)). In contrast, different desorption dynamics are observed when the reductive desorption of the adlayers is carried out in clean (0 mM o-H(2)Q((aq)) supporting electrolyte. Donor-acceptor (DA) interactions between the Pt(111)/o-Q((ads)) surface adduct and o-H(2)Q((aq)) are postulated as a possible intervening mechanism leading to the observed differences in the macroscopic electrochemical responses. The results also demonstrate that in aqueous solutions it is thermodynamically feasible to shift the formal oxidation potential of catechol-metal adducts to potentials near those of molecular hydrogen via chemically reversible, nondissociative interactions, taking place as a 2D phase transition.

Epitaxial supramolecular assembly of fullerenes formed by using a coronene template on a Au(111) surface in solution

Characteristic properties of the coronene layer formed on Au(111) for the epitaxial growth of various fullerenes are described. The electrochemical behavior of the coronene adlayer prepared by immersing a Au(111) substrate into a benzene solution containing coronene was investigated in 0.1 M HClO4. The as-prepared coronene adlayer on Au(111) revealed a well-defined (4 x 4) structure. Structural changes of the array of coronene molecules induced by potential manipulation were clearly observed by in situ scanning tunneling microscopy (STM). Supramolecularly assembled layers of fullerenes such as C60, C70, C60-C60 dumbbell dimer (C120), C60-C70 cross-dimer (C130), and C60 triangle trimer (C180) were formed on the well-defined coronene adlayer on the Au(111) surface by immersing the coronene-adsorbed Au(111) substrate into benzene solutions containing those molecules. The adlayers thus prepared were characterized by comparison with those which were directly attached to the Au(111) surface. The C60 molecules formed a honeycomb array with an internal structure in each C60 cage on the coronene adlayer, whereas C70 molecules were one-dimensionally arranged with the same orientations. The dimers, C120 and C130 molecules, formed an identical structure with c(11 x 4 radical3)rect symmetry. For the C130 cross-dimer molecule, C60 and C70 cages were clearly recognized at the molecular level. It was difficult to identify the adlayer of the C180 molecule directly attached to Au(111); however, individual C180 molecules could be recognized on the coronene-modified Au(111) surface. Thus, the adlayer structures of those fullerenes were strongly influenced by the underlying coronene adlayer, suggesting that the insertion of a coronene adlayer plays an important role in the formation of supramolecular assemblies of fullerenes.

Electrochemical control of the structure of two-dimensional supramolecular organization consisting of phthalocyanine and porphyrin on a gold single-crystal surface

Two-component adlayers consisting of cobalt(II) phthalocyanine (CoPc) and a metalloporphyrin such as 5,10,15,20-tetraphenyl-21H,23H-porphine copper(II) (CuTPP), 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine copper(II) (CuOEP), or 5,10,15,20-tetraphenyl-21H,23H-porphine cobalt(II) (CoTPP) were prepared by immersing either an Au(111) or Au(100) substrate in a benzene solution containing those molecules. The mixed adlayers thus prepared were investigated in 0.1 M HClO4 by cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM). The composition of the mixed adlayer consisting of CoPc and CuTPP molecules was found to vary with immersion time. CoPc molecules displaced CuTPP molecules during the modification process with increasing immersion time, and the CuTPP molecules were completely displaced by CoPc molecules in the mixed solution after a prolonged modification time, during which the underlying Au(100) substrate underwent phase transition from the reconstructed (hex) lattice to the unreconstructed (1 x 1) lattice. The two-component adlayer of CoPc and CuTPP was found to form a supramolecular adlayer with the constituent molecules arranged alternately on Au(100)-(hex). The striped structure was stable on Au(100)-(hex) at or near the open circuit potential (OCP), whereas the mixed adlayer was disordered on Au(100)-(1 x 1) at potentials more positive than OCP, where the phase transition of the arrangement of underlying Au atoms (i.e., the lifting of reconstruction) was induced electrochemically. A similar two-component supramolecular adlayer consisting of CoPc and CuTPP was formed on Au(111). A highly ordered, compositionally disordered adlayer of CoTPP and CuTPP was formed on Au(100)-(hex), suggesting that the adlayer structure is independent of the coordinated central metal ion for the formation of supramolecular nanostructures composed of those molecules. A supramolecular organization of CoPc and CuOEP was also found on Au(111). The surface mobility and the molecular reorganization of CoPc and CuOEP on Au(111) were tuned by modulation of the electrode potential. It is concluded that molecular assemblies of the two-component structure consisting of phthalocyanine and porphyrin were controlled not only by the crystallographic orientation of Au but also by the modulation of electrochemical potential.