Iodine-modified Au Research Articles

Porphyrin bi-layer formation induced by a surface confined reduction on an iodine-modified Au(100) electrode surface

A controlled electron transfer can be employed as a tool to drive the crafting of long-range self-assembled nanostructures of electro-active species on surfaces. In this work, we show this approach with the formation of an ordered bi-layer of tetra(N-methyl-4-pyridyl)-porphyrin molecules (TMPyP) on an iodine-modified Au(100) electrode studied by means of Cyclic Voltammetry (CV) and in situ Electrochemical Scanning Tunneling Microscopy (EC-STM). This bi-layer exists only towards negative electrode potentials between the first reduction potential of the adsorbed molecules and the desorption potential of the iodine. Starting a cathodic potential sweep at positive potentials, where the surface is covered with an adsorbed monolayer of TMPyP molecules which did not undergo any previous electrochemical reaction yet (Ref. Surfaces 2018, 1, 3–17), the formation of the ordered bi-layer coincides with the first reduction step of the adsorbed molecules; and it disappears again in a joint desorption process with the iodine. Reversal of the potential sweep leads to the opposite observations. Depending on the actual potential value as well as the scan direction up to five different competing ordered bi-layer phases can be identified. Synergistic effects between the modified substrate and the adsorbate are determined to be responsible of the corresponding structures, and at the same time surface-mediated catalytic effects shifting the first reduction step of the adsorbed molecules to a more positive potential in comparison to the reduction potential of the species from the bulk solution are observed. Moreover, based on detailed potentiodynamic STM measurements, a mechanism is also proposed to explain these observations.

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

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.

Adsorption and Electrochemical Polymerization of Pyrrole on Au(100) Electrodes as Examined by In Situ Scanning Tunneling Microscopy

Despite many efforts studying polypyrrole (PPy) with scanning probe techniques in the past several decades, its molecular structure and conformation have been elusive. This study presents the first atomic and molecular-resolution scanning tunneling microscopy (STM) images of pyrrole (Py) and PPy electrochemically produced on bare and iodine-modified Au(100) electrodes in pH 1 and 4 sulfate media. First, STM reveals a highly ordered Au(100)—(4√2 × 4√2) R45°—Py between 0.15 and 0.5 V (vs saturated calomel electrode) in 0.1 M H2SO4 containing 30 mM Py. By contrast, a disordered Py adlayer is seen in 0.1 M HClO4 medium, indicating the coadsorption of Py and an acid. This anion effect is manifested in voltammetric results obtained with Au(100), showing a pair of sharp peaks at 0.12 V in 0.1 M H2SO4 + 30 mM Py and a featureless profile seen with Au(111) or Au(100) in HClO4. Py and bisulfate (and perchloric acid) attract each other and are coadsorbed on Au(100) electrodes. The favorable adsorption of bisulfate a...

Surface−Enhanced Polymerization via Schiff-Base Coupling at the Solid–Water Interface under pH Control

On-surface polymerization realized at the solid–liquid interface represents a promising route to obtain stable and conductive organic layers with tunable properties. We present here spectroscopic evidence of π-conjugated polymer formation at the interface between an iodine-modified Au(111) and an aqueous solution. Schiff-base coupling has been used to drive the reaction by changing the pH. Scanning tunneling microscopy (STM) investigations show that the substrate acts as a template driving the formation of 1D ordered nanostructures. All the chemical states of the molecules on the surface have been identified and their evolution as a function of the pH has been monitored by synchrotron radiation X-ray photoelectron spectroscopy (XPS), demonstrating that two polymeric phases, undistinguishable by STM, exist on the surface: intermediate state and π-conjugated final product. The I/Au(111) substrate enhances the formation of π-conjugated polymers, as established comparing their production on the surface and in...

Thermodynamic self-assembly of two-dimensional pi-conjugated metal-porphyrin covalent organic frameworks by "on-site" equilibrium polymerization.

Two-dimensional pi-conjugated metal-porphyrin covalent organic frameworks were produced in aqueous solution on an iodine-modified Au(111) surface by "on-site" azomethine coupling of Fe(III)-5,10,15,20-tetrakis(4-aminophenyl)porphyrin (FeTAPP) with terephthal dicarboxaldehyde and investigated in detail using in-situ scanning tunneling microscopy. Mixed covalent organic porphyrin frameworks consisting of FeTAPP and metal-free TAPP (H2TAPP) were prepared through simultaneous adsorption in a mixed solution as well as partial replacement of FeTAPP by H2TAPP in an as-prepared metal-porphyrin framework. In the mixed framework, the relative distribution of FeTAPP to H2TAPP was not random and revealed a preference for homo-connection rather than heteroconnection. The construction of substrate-supported, pi-conjugated covalent frameworks from multiple building blocks, including metal centers, will be of significant utility in the design of functional molecular nanoarchitectures.

Positional selectivity of reversible azomethine condensation reactions at solid/liquid interfaces leading to supramolecule formation

We used in situ scanning tunneling microscopy to investigate the formation of two-dimensional supramolecules by means of reversible azomethine condensation reactions between aqueous 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (TAPP) and terephthaldicarboxaldehyde (TPA) or benzaldehyde (BA) at the solid/liquid interface of an iodine-modified Au(111) surface. A nanomesh and a close-packed array were formed by the reaction of TAPP with the dicarboxaldehyde. Formation of these structures was driven by Schiff base (azomethine) bonding and simultaneous self-assembly controlled by adsorption and condensation equilibria. Surface cross coupling between TAPP and the monocarboxaldehyde (BA) formed highly ordered adlayers consisting solely of TAPP symmetrically disubstituted with two BA molecules attached at diagonally opposite corners. The position selectivity was achieved through simultaneous coupling reaction equilibria and the thermodynamic self-assembly.

In Situ STM Investigation of Aromatic Poly(azomethine) Arrays Constructed by “On-Site” Equilibrium Polymerization

Two-dimensional (2D) arrays of π-conjugated aromatic polymers produced by surface-selective Schiff base coupling reactions between an aromatic diamine and an aromatic dialdehyde were investigated in detail using in situ scanning tunneling microscopy. Surface-selective coupling was achieved for almost all diamine/dialdehyde combinations attempted, although several combinations did not proceed even in homogeneous aqueous alkaline solution. Most of the combinations of an aromatic diamine and a dialdehyde, except the combinations of 4,4'-azodianiline with mono/bithiophenedicarboxaldehyde, formed highly ordered π-conjugated polymer arrays on an iodine-modified Au(111) surface in aqueous solution at a suitable pH. The simplest polymer of the various combinations tested, obtained from the combination of 1,4-diaminobenzene with terephthaldicarboxaldehyde, gave a 2D array consisting of linearly connected benzene units. Poly(azomethine) adlayers caused a positive shift in the electrochemical potential of the butterfly shaped oxidative adsorption and reductive desorption of iodine. The acceleration of the reductive desorption of iodine suggests the existence of a weak interaction between the polymer layer and iodine. Not only the first polymer adlayers but also partially adsorbed secondary adlayers with "on-top" epitaxial behavior were frequently observed for all polymer systems. The alignment of the polymer chains in the adlayers possessed a certain regularity in terms of a regular interval between polymer chains because of repulsive interpolymer interactions.

In situ scanning tunneling microscopy imaging of electropolymerized poly(3,4-ethylenedioxythiophene) on an iodine-modified Au(1 1 1) single crystal electrode

The electrochemical polymerization of 3,4-ethylenedioxythiophene (EDOT) on an iodine-modified Au(1 1 1) single crystal electrode in aqueous 0.10 M HClO 4 was investigated by cyclic voltammetry (CV) and electrochemical scanning tunneling microscopy (EC-STM). The cyclic voltammetric and EC-STM data revealed the stability of the iodine adlayer provided a suitable potential range for EDOT electropolymerization was controlled at 1.20 V (vs. the reversible hydrogen electrode). EC-STM was used to examine the formation of the PEDOT adlayer on the iodine-modified Au(1 1 1) electrode. In situ electropolymerization of EDOT was carried out by slowly increasing the electrode potential to 1.20 V. This process resulted in the formation of single molecular chains of PEDOT with a diameter of 0.90 nm and lengths of 5–7 nm. Higher resolution STM images further revealed PEDOT nanostructures with bent polymer backbones at angles of 105°, 144° and 180°. The growth of PEDOT multi-layers was observed when the potential was held for a longer time.

Self-assembly Monolayer of Poly-3-hexylthiophene on an Iodine-Modified Au(111) Surface

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Highly Ordered C60 Monolayer Self-Assembled by Using an Iodine Template on an Au(111) Surface in Solution

An iodine-modified Au(111) surface, (I/Au(111)), was used as a substrate to prepare a C 60 adlayer by self-organization in a benzene solution. A highly ordered C 60 adlayer was successfully prepared due to the moderate C 60-I/Au(111) interaction. Two lattice structures, (2 square root 3 x 2 square root 3) R30 degrees and p(2 x 2), were imaged for this C 60 adlayer. For the first structure, a featureless ball-like molecular shape was imaged, ascribed to the molecular rotation resulting from a symmetrical location between C 60 and iodine atoms. For the p(2 x 2) structure, the asymmetrical location of C 60 with respect to the iodine atoms freezes the C 60 molecules on the substrate, leading to a clear image of intramolecular structure. The intermediate iodine atoms in the C 60/I/Au(111) adlayer can be desorbed by electrochemically reduction without significantly affecting the ordering of the C 60 adlayer. However, the internal pattern of C 60 disappears in the absence of iodine.

Overpotential deposition of copper on an iodine-modified Au(1 1 1) electrode

Cyclic voltammetry, chronoamperometry and in situ electrochemical scanning tunneling microscopy were used to study the kinetics of nucleation and crystal growth during the initial stages of copper overpotential deposition (OPD) on a previously iodine-modified Au(1 1 1) electrode, from an aqueous solution 10 −3 M CuSO 4 in 0.05 M H 2SO 4. The starting potential during step experiments was chosen in the region where the gold electrode was completely free of the copper deposit. The recorded current transients for copper deposition onto the iodine-modified Au(1 1 1) electrode surface appear to be very complex, with the unusual presence of two or more current maxima. A new method was used for quantitative evaluation of current transients that involves the transition UPD–OPD, developed by our group [M. Palomar-Pardavé, I. González, N. Batina, J. Phys. Chem. B 104 (2000) 3545], was used for the quantitative interpretation. Our results show that, within a single current transient, copper adsorption and two types of nucleation process: two-dimensional (2D) and three-dimensional (3D) limited by lattice incorporation of copper adatoms and diffusion of Cu(II) ion, respectively, take place simultaneously. STM images revealed the enhanced growth of 3D copper on edge of I–Au(1 1 1) during the early stages of deposition. Moreover, our results strongly suggest that the iodine adlayer is constantly present, even after the striping Cu that was overpotential deposited.

Dispersion of Metallofullerene Y@C82 on Bare, C60-Modified, and Iodine-Modified Au(111) Surfaces Investigated with ECSTM

Two-dimensional (2D) assembling behaviors of the endohedral metallofullerene Y@C(82) on bare, C(60)-modified, and iodine-modified Au(111) surfaces have been investigated in 0.1 M HClO(4) solution employing electrochemical scanning tunneling microscopy (ECSTM). The results show that Y@C(82) molecules are mobile and aggregate to the terrace edges on bare and C(60)-modified Au(111) surfaces, but monodispersion of the Y@C(82) molecules is achieved on the iodine-modified Au(111) surface. The improvement of Y@C(82) dispersion on an iodine-modified gold surface is due to the strong Y@C(82)-substrate interactions. The modified-substrate method provides an effective strategy to disperse endohedral metallofullerenes.

Two-Dimensional Supramolecular Arrangements of Enantiomers and Racemic Modification of 1,1‘-Binaphthyl-2,2‘-Dicarboxylic Acid

Formation of adlayers of the optically active compound 1,1'-binaphthyl-2, 2'-dicarboxylic acid (BINAC) on iodine-modified Au (111) surfaces in perchloric acid was investigated by in situ scanning tunneling microscopy (STM). Highly ordered arrays formed on the surfaces via simple spontaneous adsorption from a solution of enantiomers or the racemic BINAC, in spite of the fact that BINAC has a three-dimensionally complex stereochemical structure. Adlayers of both enantiomers essentially shared the same enantiomorphous structure. Observed parameters of the rectangular unit cell lattice for arrays of both enantiomers of BINAC were a = 2.3 +/- 0.2 nm and b = 0.7 +/- 0.2 nm. On the other hand, racemic modification formed an entirely different adlayer, which consisted of an alternate alignment of the two enantiomers, with an oblique unit cell lattice with parameters of a = 1.2 +/- 0.2 nm, b = 0.8 +/- 0.1 nm, and 74 +/- 3 degrees . No domain composed of a single enantiomer was observed. The stronger hetero-intermolecular interactions of enantiomer couples led to the formation of an alternate arrangement in the array prepared by racemic modification.

Fullerene adlayers on various single-crystal metal surfaces prepared by transfer from L films.

Fullerene adlayers prepared by the simple Langmuir-Blodgett (LB) method onto various well-defined single-crystal metal surfaces were investigated by in situ scanning tunneling microscopy (STM). The surface morphologies of fullerene adsorbed onto metal surfaces depended largely on the adsorbate-substrate interactions, which are governed by the types of surfaces. Too weak adsorption of C60 molecules onto iodine-modified Au(111) (I/Au(111)) allows surface migration of the molecules, and then, STM cannot visualize the C60 molecules. Stronger and appropriate adsorption onto bare Au(111) leads to highly ordered arrays relatively easily due to the limited surface migration of C60. On iodine-modified Pt(111) (I/Pt(111)) and bare Pt(111) surfaces, which have stronger adsorption, randomly adsorbed molecular adlayers were observed. Although C60 molecules on Au(111) were visualized as a featureless ball due to the maintenance of the rapid rotational motion (perturbation) of C60 on the surface at room temperature, those on I/Pt(111) revealed the intramolecular structures, thus indicating that the perturbation motion of molecules on the surface was prohibited.

In Situ STM Evidence for Adsorption of Rhodamine B in Solution

In situ scanning tunneling microscopy (STM) has been first employed to investigate the adsorption of RB (rhodamine B) on solid surface in solution. An I/Au(111) (iodine-modified Au(111)) surface was used as a substrate for molecular adsorption. In situ STM imaging revealed a highly ordered RB molecular adlayer. A unit cell with parameters of a = 1.71 nm and b = 2.25 nm was determined. Higher resolution STM images disclosed the structural details of the molecular adlayer. On the basis of STM results, the RB molecule is proposed to adsorb on the I/Au(111) surface by its two diethylamino functional groups in an edge-on orientation. The orientation was related to the molecular degradation.

Underpotential deposition of Cu on iodine-modified Au(1 1 1): an in situ scanning tunneling microscopy study

The electrochemical deposition of Cu on iodine-modified Au(1 1 1) surfaces has been investigated by in situ electrochemical scanning tunneling microscopy (ECSTM) and cyclic voltammetry (CV) in sulfuric acid solutions. In situ ECSTM studies reveal different iodine adlayer structures before and during the process of copper underpotential deposition (UPD). At the beginning of the cathodic scan and for potentials higher than the onset of UPD a c( p×√3 R-30°) iodine structure is observed on wide terraces. For lower potentials this iodine structure transforms to a more compact (3×3) structure characterized by two different structural variations (symmetric and asymmetric) sometimes observed coexisting in the same terrace. Charge transfer analysis from CV measurements reveals that the amount of copper deposited at these potentials is not sufficient to account for this structure in the framework of a hard-ball structural model. During the UPD process itself other iodine structures are also observed as a function of copper deposition, together with an additional compression of the iodine adlayer associated with the formation of a CuI bilayer, in agreement with previously reported X-ray diffraction data. At the end of the UPD process a Cu(1×1) monolayer is formed with a lattice parameter equal to that of Au(1 1 1). The same course of structural changes was also observed during the anodic scan where stripping of the copper layer takes place, returning to the initial iodine c( p×√3 R-30°) structure. Our results strongly suggest that the iodine adlayer is constantly present as the top layer during the process of electrodeposition and stripping of Cu with no noticeable loss of iodine in the process. The observed structures are discussed in terms of iodine–copper interactions.

Novel “Wet Process” Technique Based on Electrochemical Replacement for the Preparation of Fullerene Epitaxial Adlayers

The electrochemical replacement method to form epitaxial adlayers of fullerene on Au(111) surfaces was proposed and demonstrated by in situ electrochemical STM. The new “wet process” method consists of the transfer of Lanmuir films of fullerene onto iodine-modified Au(111) surfaces at an air−water interface followed by the electrochemical removal and replacement of iodine adlayers with fullerene adlayers in solution. The fullerene adlayers prepared by this method showed excellent quality and uniformity, and they were essentially the same as epitaxial adlayers prepared by sublimation.

Two-Dimensional Crystals of Alkanes Formed on Au(111) Surface in Neat Liquid: Structural Investigation by Scanning Tunneling Microscopy

In situ scanning tunneling microscopy revealed the formation of the two-dimensional (2D) crystals of n-alkanes (CnH2n+2, n = 12−17) on a Au(111) surface in neat liquid at room temperature. The molecules were adsorbed on the gold surface with their molecular axis parallel to the surface plane. The molecular rows of even- and odd-numbered alkanes ran in the nearest-neighbor (NN) atomic direction and the next-nearest-neighbor atomic direction of the gold surface, respectively. The molecular axis was oriented close to the NN direction of the gold surface in both odd- and even-numbered alkanes. Although there are two NN directions with respect to the direction of the bridging row of the herringbone structure due to the reconstruction of the Au(111) surface with crossing angles of 30° and 90°, the molecular axis was preferentially oriented in the NN direction with a crossing angle of 30°. The 2D crystal of alkanes was not formed on the iodine-modified Au(111) surface, confirming that the molecule−substrate inte...

Structures and Dynamic Formation Processes of Porphyrin Adlayers on Iodine-Modified Au(111) in Solution: In Situ STM Study

The structures and dynamic formation processes of adlayers of 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)-21H,23H-porphine (TMPyP) on both bare Au(111) and iodine-modified Au(111) in perchloric acid have been investigated in detail by using in situ scanning tunneling microscopy (STM). Highly-ordered TMPyP arrays formed on iodine-modified Au(111), whereas disordered adlayers were consistently found on bare Au(111) surface. High-resolution STM images revealed the characteristic internal shape and orientation of each TMPyP molecule in ordered adlayers. Time-dependent in situ STM allowed direct observation of the dynamics of the self-ordering processes. Before the most stable adlayer was established in a relatively small domain, several structural changes were found to occur, including the formation of a one-dimensional ordered chain at an early stage and several phase transitions in the ordered adlayers. Once the most stable adlayer was formed as a nucleus in a domain, two-dimensional growth of the domain extended over the entire area of the terraces with the final packing arrangement.

Highly ordered molecular arrays formed on iodine-modified Au(111) in solution: in situ STM imaging

Highly ordered molecular arrays formed on iodine-modified Au(111) in solution: in situ STM imaging