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.

Oxalic acid photooxidation on rutile nanowire electrodes.

A combined photoelectrochemical and IR spectroscopic approach was used to study the photooxidation of oxalic acid on thin films consisting of oriented rutile TiO(2) nanowires (diameter: approximately 2 nm). Anodic spikes appear in the photocurrent transients particularly at low oxalic acid concentrations and point to mass transport limitations due to the presence of narrow pores (< or = 1 nm) in the nanowire films. Accordingly, IR measurements reveal that the photocurrent decrease at low concentrations is accompanied by an adsorbate depletion as tracked by the decrease of IR bands at 1699 cm(-1) (adsorbate species A) and 1720 cm(-1) (adsorbate species B). The relative intensities of the two bands are found to change with illumination time. Based on the comparison of the equilibrium adsorption constants as deduced independently from photocurrent transients and from IR spectra, the photocatalytic oxidation of oxalate seems to proceed on the surface of rutile nanowires mainly via species A, which is also the one with the largest adsorption constant. The strong decrease of the IR band at 1720 cm(-1) is rationalized by a fast replenishment of photooxidized species A by species B. In a more general vein, the challenges and prospects of combining photoelectrochemical and spectroscopic measurements are discussed.

A macroscopic and molecular view of photoinduced reactions on nanostructured semiconductor thin films.

In an unprecedented approach, simultaneous photoelectrochemical and IR-spectroscopic measurements were performed to study photoinduced charge separation processes on nanostructured rutile thin films.

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.

Formate adsorption onto thin films of rutile TiO2 nanorods and nanowires.

We evaluate the applicability of silicon prisms to infrared (IR) spectroscopic investigations of the oxide/electrolyte interface in the attenuated total reflection (ATR) configuration. Using formic acid as a probe adsorbate, a comparison is done between a rutile nanowire film supported on Si and a rutile nanorod film supported on ZnSe. The nanowires were 2 nm in diameter and were deposited directly on Si by chemical bath deposition, whereas the nanorods were deposited on ZnSe by solution casting of an aqueous dispersion prepared from a commercial powder. The lower penetration depth of the evanescent wave for silicon was compensated by a higher internal surface area of the corresponding nanowire film. Advantageously, much higher solution concentrations can be used in the case of the Si prism without a significant contribution of solution species to the IR spectrum. Furthermore, the high chemical stability of Si opens up the possibility of performing experiments in highly acidic aqueous solutions. Upon formate adsorption at pH 3.5, a pair of intense IR bands was observed in the wavenumber range where antisymmetric v(as)(COO) vibrations are expected, namely at 1537 and 1592 cm(-1) on nanorod films and at 1544 and 1586 cm(-1) on nanowire films. The relative band intensities are different for nanorod and nanowire films. While the bands at 1537/1544 cm(-1) are assigned to formate adsorbed on the (110) face, forming the bridging bidentate (mu) structure, those at 1592/1586 cm(-1) are tentatively attributed to formate adsorbed at low coordination adsorption sites at the nanocrystal edges. Bands corresponding to the carboxylate symmetric stretching and the HCO deformation were also observed.

Photoelectrochemical behavior of nanostructured WO3 thin-film electrodes: The oxidation of formic acid.

Nanostructured tungsten trioxide thin-film electrodes are prepared on conducting glass substrates by either potentiostatic electrodeposition from aqueous solutions of peroxotungstic acid or direct deposition of WO3 slurries. Once treated thermally in air at 450 degrees C, the electrodes are found to be composed of monoclinic WO3 grains with a particle size around 30-40 nm. The photoelectrochemical behavior of these electrodes in 1 M HClO4 apparently reveals a low degree of electron-hole recombination. Upon addition of formic acid, the electrode showed the current multiplication phenomenon together with a shift of the photocurrent onset potential toward less positive values. Photoelectrochemical experiments devised on the basis of a kinetic model reported recently [I. Mora-Seró, T. Lana-Villarreal, J. Bisquert, A. Pitarch, R. Gómez, P. Salvador, J. Phys. Chem. B 2005, 109, 3371] showed that an interfacial mechanism of inelastic, direct hole transfer takes place in the photooxidation of formic acid. This behavior is attributed to the tendency of formic acid molecules to be specifically adsorbed on the WO3 nanoparticles, as evidenced by attenuated total reflection infrared spectroscopy.

In-situ infrared study of the adsorption and oxidation of oxalic acid at single-crystal and thin-film gold electrodes: a combined external reflection infrared and ATR-SEIRAS approach.

The adsorption and oxidation of oxalic acid at gold electrodes were studied by in-situ infrared spectroscopy. External reflection experiments carried out with gold single-crystal electrodes were combined with internal reflection (ATR-SEIRAS) experiments with gold thin-film electrodes. These gold thin films, with a typical thickness of ca. 35 nm, were deposited on silicon substrates by argon sputtering. As previously reported for evaporated gold films, the voltammetric curves obtained in sulfuric acid solutions after electrochemical annealing show typical features related to the presence of wide bidimensional (111) domains with long-range order. The in-situ infrared data collected for solutions of pH 1 confirmed the potential-dependent adsorption of either oxalate (Au(100)) or a mixture of bioxalate and oxalate (Au(111), Au(110), and gold thin films) anions in a bidentate configuration. The better signal-to-noise ratio associated with the SEIRA effect in the case of the gold thin-film electrodes allows the observation of the carbonyl band for adsorbed bioxalate that was not detected in the external reflection experiments. Besides, additional bands are observed between 2000 and 3000 cm(-)(1) that can be tentatively related to the formation of hydrogen bonds between neighboring bioxalate anions. The intensities of these bands decrease with increasing solution pH values, disappearing for pH 3 solutions in which adsorbed oxalate anions are the predominant species. The analysis of the intensities of the nu(s)(O-C-O) and nu(C-OH) + delta(C-O-H) bands for adsorbed oxalate and bioxalate, respectively, suggests that the pK(a) for the surface equilibrium between these species is significantly lower than that for the solution equilibrium.

A spectroscopic and electrochemical approach to the study of the interactions and photoinduced electron transfer between catechol and anatase nanoparticles in aqueous solution.

We have combined in situ photoelectrochemical and spectroscopic techniques (Attenuated Total Reflection Infrared, ATR-IR, and Resonance Raman Spectroscopy) for the study of the charge-transfer complex formed upon adsorption of catechol on anatase nanoparticles in contact with aqueous acidic solutions. Vibrational spectroscopies reveal the existence of at least two adsorbate configurations: catecholate in a chelate configuration and molecularly adsorbed catechol, with apparent values of -12.3 and -10.5 kJ mol(-1), respectively. These values are significantly less negative than the values reported for anatase colloidal dispersions. The adsorption of both catechol species on the nanoparticulate anatase thin films follows the Freundlich isotherm. As revealed by resonance Raman spectroscopy, only the adsorbed chelating catecholate forms the charge-transfer complex. The electron transfer from the adsorbate to the anatase nanoparticles has been evidenced by the development of a negative photopotential upon 514.5 or 632.8 nm laser illumination of an anatase nanostructured thin film electrode in contact with a catechol solution. The time evolution of the Raman spectra shows an increasing fluorescence indicating that, upon electron injection, catechol polymerization occurs on the TiO2 surfaces. This conclusion is confirmed by in situ ATR-IR measurements, which show a progressive broadening of the catecholate bands together with the appearance of new signals. This study illustrates the benefits of combining electrochemical, infrared, and Raman techniques for the elucidation of processes occurring at the semiconductor/solution interface. Finally, evidence is given on the different adsorption and reactivity behavior found for suspensions and nanoporous thin films under equivalent experimental conditions.

ATR-SEIRAS study of the adsorption of acetate anions at chemically deposited silver thin film electrodes.

The adsorption of acetate anions at silver thin film electrodes has been studied by in-situ infrared spectroscopy experiments with a Kretschmann internal reflection configuration. Stable silver thin films were chemically deposited on germanium substrates. Ex-situ STM images show mean grain sizes ranging from ca. 20 to 90 nm for deposition times between 2 and 20 min, respectively. The thickness of the silver film, measured by AFM, is typically around 10 nm for a deposition time of 10 min and increases up to 50 nm for a deposition time of 20 min. Roughness factors around 2.3 have been obtained for the silver films from the charge involved in lead underpotential deposition (UPD). A noticeable enhancement of the infrared absorption of adsorbed species (SEIRA effect) is observed when the silver films are used as electrodes under internal total reflection conditions. Maximum intensities of the adsorbate bands were observed for a deposition time of 10 min and an angle of incidence around 65 degrees . The potential-dependent infrared spectra of acetate and interfacial water are consistent with previously proposed models involving the existence of weakly hydrogen-bonded water molecules at potentials below the potential of zero charge and the reorientation of water molecules at potentials above the potential of zero charge. Results reported in this work suggest a weak interaction between acetate and water molecules adsorbed at the silver thin film electrodes.