Arnaldo Carrasquillo

Electrode-surface coordination chemistry: ligand substitution and competitive coordination of halides at well-defined Pd(100) and Pd(111) single crystals

Abstract Ligand (adsorbate) substitution (displacement) and competitive-coordination (chemisorption) reactions between chloride, bromide and iodide anions have been studied at well-defined Pd(100) and Pd(111) single-crystal electrode surfaces in aqueous solutions. Experiments involved: (i) pretreatment of the Pd( hkl ) surfaces with a full monolayer of one halide followed by exposure to a dilute aqueous solution of another halide, and (ii) exposure of a clean Pd( hkl ) electrode to a solution that contained a binary or ternary mixture of the halides. The resulting monolayers were then characterized by low-energy electron diffraction. Auger electron spectroscopy and temperature-programmed desorption. The results were as follows: (i) the subject halides were oxidatively chemisorbed to produce well-defined halogen adlattices. (ii) In the absence of the heavier halides, chloride ions were adsorbed to form Pd(100)-(2×2)-Cl and Pd(111)-(√3×√3) R30°-Cl. (iii) These were displaced, spontaneously, irreversibly and quantitatively, by bromide ions to yield Pd(100)-(2×2)-Br and Pd(111)-(√3×√3) R30°-Br. respectively. (iv) The latter, in turn, were spontaneously, irreversibly and quantitatively displaced by iodide to produce Pd(100)-c(2×2)-I and Pd(111)-(√3×√3) R30°-I. Only Pd(100)-(2×2)-Br and Pd(111)-(√3×√3) R30°-Br were produced when the electrodes were exposed to a solution that contained a mixture of Cl − and Br − . (vi) Only Pd(100)-c(2×2)-I and Pd(111)-(√3×√3) R30°-I were produced when the electrodes were immersed in a solution that consisted of all three halides. These results, consistent with the thermal desorption data, demonstrate that the interaction of the subject halides with Pd electrode surfaces closely follows the homogeneous coordination chemistry of halo-Pd complexes: the strength of chemisorption or surface coordination decreases in the order I − > Br − > Cl − .

Electrochemical reactivity of aromatic molecules at nanometer-sized surface domains: from Pt(hkl) single crystal electrodes to preferentially oriented platinum nanoparticles.

This manuscript compares the electrochemically controlled adsorption of hydroquinone-derived adlayers and their reductive desorption from nanometer-sized Pt(111) domains present on the surface (i) of model stepped single-crystal electrodes and (ii) of preferentially oriented Pt nanoparticles. The results obtained using a stepped surface series, i.e., Pt(S)[(n - 1)(111)x(110)], suggest that in the presence of 2 mM H(2)Q((aq)) the electrochemically detected desorption-adsorption process takes place selectively from ordered Pt(111) domains present as terraces, while being precluded at other available surface sites, i.e., Pt(110) steps, where adsorption takes place irreversibly. This domain-selective electroanalytical detection scheme is employed later to selectively monitor desorption-adsorption of hydroquinone-derived adlayers from ordered, nanometer-scaled Pt(111) domains on the surface of preferentially oriented Pt nanoparticles, confirming the existence of well-ordered (111) domains on the surface of the Pt nanoparticles. A good correlation is noted between the electrochemical behavior at well-ordered Pt(hkl) surfaces and at preferentially oriented Pt nanoparticles. Key learnings and potential applications are discussed. The results demonstrate the technical feasibility of performing domain-selective decapping of nanoparticles by handle of an externally controlled parameter, i.e., the applied potential.

Electrochemistry of the I-on-Pd single-crystal interface : studies by UHV-EC and in situ STM

Abstract A single chemisorbed layer of zerovalent I atoms has been found to enhance, at ambient temperatures, the reactivity of Pd electrode surfaces. Three unique reactions (anodic dissolution in non-corrosive electrolyte, regeneration of well-ordered single-crystal surfaces, and “electrochemical digital etching”) have been investigated at Pd(111) and Pd(100) single-crystal electrodes and are described in this paper. Experimental measurements were based upon a combination of electrochemistry (EC), low-energy electron diffraction (LEED), and in-situ scanning tunneling microscopy (STM).

Domain-Selective Reactivity of Hydroquinone-Derived Adlayers at Basal Pt(hkl) Single-Crystal Electrodes

The electrochemical reactivity of hydroquinone-derived adlayers (Q((ads))) is compared at basal Pt(hkl) single-crystal surfaces, revealing that the electrochemically controlled desorption of Q((ads)) is a highly selective surface reaction. At well-ordered Pt(111) single-crystal surfaces, classical electrochemical methods are combined with in situ SNIFTIRS measurements to demonstrate that the reductive desorption of Q((ads)) and their full oxidative readsorption can be achieved, even in the presence of hydroquinone solution (H(2)Q(aq)), by controlling the potential of Pt(111) electrodes. At well-ordered Pt(111) domains, the presence of vertically adsorbed molecules within the Q((ads)) adlayer is deduced from the spectroelectrochemical SNIFTIRS measurements. The desorption mechanism, detected voltammetrically at Pt(111) electrodes, is precluded at well-ordered Pt(110) and Pt(100) single-crystal electrodes immersed in hydroquinone-containing solutions, requiring the presence of well-ordered Pt(111) surface domains in order to be detected. In clean supporting electrolyte, the partial desorption of Q((ads)) layers may take place, but predominantly from minority surface imperfections at Pt(110) and Pt(100) via a different mechanism than at Pt(111) surface domains.

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.

Electrochemical digital etching in inert electrolyte: Reordering of ion-bombarded Pd(100) by chemisorbed-iodine-catalyzed dissolution

The anodic dissolution of an extensively disordered (ion bombarded) Pd(100) single-crystal surface in inert (halide-free) H2SO4 solution, catalyzed by a single adsorbed layer of iodine, has been found to generate a well-ordered Pd(100)-c(2 × 2)-I adlattice; reductive desorption of the adsorbed iodine yields the clean and ordered Pd(100) surface. Experimental measurements were based upon a combination of linear-sweep voltammetry, potential-step coulometry, low-energy electron diffraction, and Auger electron spectroscopy. This dissolution-reordering phenomenon is unique since the process occurs in the absence of bulk corrosive reagents, and only if a monolayer of chemisorbed iodine is present. This process may be viewed analogously to electrochemical digital etching except that bulk material is not needed to replenish the adsorbed iodine that activates the surface, and the dissolution is not immediately terminated upon regeneration of the ordered interface.

Adsorbate-catalyzed layer-by-layer metal dissolution in inert electrolyte: Pd(100)-c(2 × 2)-I

Abstract Studies on the corrosion of Pd in inert ( halide-free ) H 2 SO 4 solution, catalyzed by a single adsorbed layer of iodine, have been extended to a Pd(100) single-crystal electrode that contained an ordered c(2 × 2) adlattice of iodine. Experimental measurements were based upon a combination of linear-sweep voltammetry, potential-step coulometry, low-energy electron diffraction, and Auger electron spectroscopy. As was earlier noted with polycrystalline electrodes, Pd dissolution occurred only when iodine was present on the surface. More significantly, neither the coverage nor the ordered structure of the iodine adlattice was affected by the dissolution process. These observations strongly suggest that the iodine-catalyzed corrosion occurs one layer at a time.

Use of Model Pt(111) Single Crystal Electrodes under HMRDE Configuration To Study the Redox Mechanism for Charge Injection at Aromatic/Metal Interfaces

The electrochemical reactivity of hydroquinone-derived, catechol-derived and benzene-derived adlayers is compared at Pt(111) single-crystal surfaces (i) under stagnant hanging meniscus (HM) configuration and (ii) under hydrodynamic conditions imposed by combining the HM configuration with the rotating disk electrode (RDE) that merge in the so-called HMRDE technique. For the three cases studied, the results suggest that reductive desorption of the adlayers can be accomplished in aqueous 0.5 M H(2)SO(4) solutions within the time frame of a single cathodic scan, i.e. the first half of a single CV experiment. The results highlight the simplicity of exploiting the hydrodynamic conditions imposed by RDE as a convenient electroanalytical strategy to elucidate controversies regarding whether desorption takes place or not during electrode processes studied under the HM configuration.

Site selection in electrode reactions : quinone/hydroquinone redox at submonolayer iodine-coated electrode surfaces

We have in the past [ll investigated the influence of uncharged molecules or atoms chemisorbed in monolayer quantities at electrode surfaces on the redox kinetics of uncharged, unadsorbed electroactive species. In general, the electrontransfer kinetics of redox couples in the solution state is strongly affected by the structure and composition at the electrode-solution interface. Pronounced enhancement or retardation of the electron-transfer rates have been observed; however, the exact causes for such dramatic changes are not completely understood. The electrochemical kinetics of the benzoquinone/hydroquinone [BQ,,,,/ HQ,,,] couple in aqueous solutions is a prototypical example. Early studies [2] suggested a pH dependence of the reaction kinetics, with the minimum at pH 4. Those conclusions were disputed by later work [31 which showed that the redox kinetics were actually pH independent. Our own investigations [ll established that, in the case when species derived from HQ,,,, (or BQ,,,,) are irreversibly adsorbed on the surface, a minimum in the BQ,,,,/HQ,,,, redox kinetics occurs at pH 4. However, in the presence of a full monolayer of adsorbed iodine, which precludes chemisorption of organic intermediates, the reaction rate becomes pH independent. It was initially postulated that the role of the surface iodine was simply to prevent formation of a layer of chemisorbed HQ(,,-derived intermediates which, at pH 4, served to retard the BQ,,,,/HQ,,,, redox rate. We present here new data

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.