The dependence on temperature of the voltammetric behavior of Pt(111), Pt(100), and Pt(110) electrodes in 0.1 M HClO4 has been studied. A thermodynamic analysis of the hydrogen underpotential deposition (HUPD) has been carried out, taking into account that the adsorption process is accompanied by changes in the electrode double layer. The analysis for Pt(100) requires a previous deconvolution of the OH adsorption contribution whereas for Pt(110), the analysis is limited to high H coverages. The Pt−H bond energy values obtained for the Pt(hkl) electrodes agree with the values obtained in UHV. On Pt(111) and Pt(100), Δ (HUPD) changes linearly with coverage, with Frumkin repulsive parameters 27 and 9 kJ mol-1, respectively. The values obtained for Δ (HUPD) (−48 J mol-1 K-1 for Pt(111), − 56 J mol-1 K-1 for Pt(100), and from −55 to − 70 J mol-1 K-1 for Pt(110)) suggest immobile hydrogen adsorption. The fact that Δ (HUPD) depends significantly on the crystallographic orientation suggests that the symmetry of the platinum substrate strongly influences the degree of order in the water network directly bonded to the metal surface atoms.

The effect of temperature on the voltammetric OH adsorption on Pt(111) and Pt(100) electrodes in perchloric acid media has been studied. From a thermodynamic analysis based on a generalized adsorption isotherm, DeltaG degrees , DeltaH degrees , and DeltaS degrees values for the adsorption of OH have been determined. On Pt(111), the adsorption enthalpy ranges between -265 and -235 kJ mol(-1), becoming less exothermic as the OH coverage increases. These values are in reasonable agreement with experimental data and calculated values for the same reaction in gas phase. The adsorption entropy for OH adsorption on Pt(111) ranges from -200 J mol(-1) K(-1) (low coverage) to -110 J mol(-1) K(-1) (high coverage). On the other hand, the enthalpy and entropy of hydroxyl adsorption on Pt(100) are less sensitive to coverage variations, with values ca. DeltaH degrees = -280 kJ mol(-1) and DeltaS degrees = -180 J mol(-1) K(-1). The different dependence of DeltaS degrees with coverage on both electrode surfaces stresses the important effect of the substrate symmetry on the mobility of adsorbed OH species within the water network directly attached to the metal surface.

For monocrystalline TiO2 electrodes, capacitive currents are observed at potentials that are negative enough to induce the filling of conduction band states. Nanoparticulate electrodes exhibit, apart from these currents, an additional pair of capacitive peaks at more positive potentials, which can be attributed to charge traps in the band gap. We have taken advantage of the well-defined morphology and crystal structure of three different types of rutile electrodes to investigate the nature of these band gap states. In particular, nanostructured films composed of oriented wires, films of randomly distributed nanoparticles, and smooth single crystals have been used. The analysis of the cyclic voltammetry response reveals a strong dependence of the trap state concentration on the morphological structure of the films. On the basis of results concerning the surface modification of the electrodes, we propose a model with a location of these band gap states at grain boundaries. We report, furthermore, on a new procedure to prepare hierarchically organized nanostructures by direct deposition of nanowires onto nanoparticulate films in aqueous solutions at low temperature. From a practical point of view, this procedure allows for a systematic tuning of the inner surface area and the porosity of the original samples.

The dependence of the potential of zero charge (pzc) for Pt(111) surfaces in acidic aqueous solution having increasing densities of ordered monoatomic steps in the (111)−(110) and (111)−(100) zones is evaluated from CO “charge-displacement” measurements, with the objective of elucidating the influence of the electrochemical double layer on the large step-induced changes in surface potential known for the clean uncharged surfaces in ultrahigh vacuum (UHV). This experimental strategy, which involves evaluating the charge flowing at controlled potentials upon “quenching” the aqueous double layer with chemisorbed CO, yields pzc values referring to zero “total” electronic charge, . The values in both 0.1 M HClO4 and 0.5 M H2SO4 electrolytes decrease noticeably (by ca. 0.15 V) upon increasing the (110) step density, N, whereas smaller effects are found for (100) steps. The location of the values within the so-called “hydrogen” region, however, complicates interpretation of the −N dependences due to the presence of faradaic charge associated with potential-dependent hydrogen adsorption. Procedures are outlined by which this influence upon can be removed, yielding approximate estimates of pzc values referring instead to zero “free” electronic charge, , as a function of step density. The analysis followed involves extrapolating charge−potential data from higher potentials where hydrogen adsorption is essentially absent, or evaluating instead “potentials of constant (nonzero) free charge” in this “double-layer” region, achievable most readily with the data in 0.1 M HClO4. The resulting −N plots exhibit substantial negative slopes for dilute (110) stepped surfaces (N ≤ 107 cm-1), Δ values approaching ca. −0.7 V being obtained, although the dependence changes sign close to the densely stepped (110) limit. Significantly, the −N profile obtained for (110) steps is comparable to the corresponding work function (ΦM−N) behavior for the Pt(111)−(110) surfaces in UHV. This indicates that the remarkably (ca. 1 eV) lower “local” ΦM values for Pt steps relative to (110) terrace regions known to be present in the latter environment are not attenuated (or otherwise altered) greatly by the presence of the aqueous double layer. Interpretation is given in terms of current understanding of interfacial solvation effects on , and some electrochemical implications are pointed out.

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