E. M. Patrito

Characterization of hafnium anodic oxide films: An AC impedance investigation

Abstract The electrochemical behaviour of hafnium oxide was investigated in the potential range 0–150V, in the following electrolytes: H 2 SO 4 , H 3 PO 4 , NaOH and HNO 3 . After film growth, AC impedance measurements were performed in the frequency range 0.1 Hz–100 KHz. The impedance spectra of the oxides showed a multilayer structure as the oxide thickness increased, due to anion incorporation from the electrolyte and the corrosion process enhanced by anodic breakdown at high final formation potentials. Anodic breakdown was also investigated. An electric field strength increase with thickness was observed, which could be associated to changes from amorphous to crystalline oxide structure. This fact could produce internal stresses during the oxide growth resulting in an anodic fracture which can be correlated, in turn, with dielectric constant changes.

Adsorption of hydrated hydroxide and hydronium ions on Ag(1 1 1). A quantum mechanical investigation

Abstract In this paper we have studied comparatively the adsorption of hydroxide and hydronium ions, extending our previous study on hydronium adsorption [J. Phys. Chem. B. 105 (2001) 7227] and emphasizing the adsorption of hydroxide. The calculations were performed on the 111 surface of silver using ab initio quantum mechanical methods (Hartree–Fock+Moller–Plesset second order perturbation theory). The adsorption was investigated for the bare and the hydrated ions (up to three water molecules). Binding energies, equilibrium structures and charge transfer processes were investigated. While the successive hydration of hydronium detaches the ion from the surface, the hydrated hydroxide anion remains specifically adsorbed. Charge transfer processes between the adsorbates and the surface were studied using electron density difference plots and effective charges obtained from Mulliken populations and from surface-dipole moment curves. The energetics of the surface reactions leading to the formation of the hydrated hydronium and hydroxide ions from the bare adsorbed ions and water molecules was also investigated. Both reactions are exothermic mainly due to the formation of strong hydrogen bonds. The effect of an external homogeneous electric field perpendicular to the surface on different adsorbate properties was investigated for the bare and hydrated hydroxide ion in order to model the environment of the electrical double layer. The electric field affects the orientation of the water molecules on the surface and the hydroxide surface distance.

Ellipsometric investigation of anodic hafnium oxide films

The galvanostatic oxidation of hafnium in different electrolytes was investigated by means of in-situ ellipsometry in the range 0–150 V corresponding to oxides thicknesses in the range 0–350 nm. The oxide surfaces were characterized by scanning electron microscopy (SEM) examination. Single and double-layer models were used to interpret the experimental results and it was observed that the nature of the electrolyte affects mainly the properties of the external thin layer close to the solution. The bulk oxide grown in H2SO4. NaOH and H3PO4, has a refractive index of 2.06–2.07 and an absorption coefficient of 0.02–0.03. The Δ- ψ profiles recorded during the oxidation in HNO3 reveal different stages of oxide corrosion. The films formed in this electrolyte have the lowest refractive indices as a consequence of their porous nature. An electric field strength increase with thickness was observed which could be associated to changes from amorphous to crystalline oxide structure. This fact could produce internal stresses during the oxide growth resulting in an anodic fracture. The ellipsometric results of the present work confirm previous ac impedance investigations.

Adsorption of carbonate species on silver. I. Nature of the surface bond

Abstract The adsorption of carbon trioxide, carbonate and bicarbonate is investigated on the 111, 110 and 100 surfaces of silver from a quantum mechanical point of view. Several aspects of the metal/ion bonding such as the adsorbate binding energy, charge transfer to/from the metal and the relaxation of the adsorbates upon adsorption are investigated. On the hollow site of Ag(110) bicoordinated carbonate has a binding energy of 226.9 kcal/mol. The highest binding energies for carbonate are obtained on Ag(110) while on Ag(111) and Ag(100) comparable binding energies are observed. The bicarbonate ion has a binding energy of 74.0 kcal/mol bicoordinated on the hollow site of Ag(100). The binding energy of bicoordinated CO 3 is 117 kcal/mol on Ag(100). There is a charge transfer towards the metal of 0.5 electrons in the case of carbonate and 0.25 electrons for bicarbonate. On the other hand, CO 3 adsorption leads to a charge transfer of 1.5 electrons from the metal to the adsorbate. Adsorbed carbonate and carbon trioxide have the same formal charge of −1.5 electrons and the same geometry, which is very close to that of carbonate compounds. For all adsorbates, the geometry distortion upon adsorption follows the same trend: the carbon-coordinated oxygen bond length increases while the bond length between the carbon atom and the non-coordinated oxygen atom decreases. The changes in bond lengths induced by the adsorption process are between 4 to 8%.

Electrochemical behaviour of passive zirconium alloys

Abstract The potentiodynamic oxidation of zirconium, zircaloy-2 (Zry-2) and zircaloy-4 (Zry-4) was studied in the 0 V ⩽ V ⩽ 8 V potential range. Side reactions take place during the oxidation of Zry-2 and Zry-4 in phosphate electrolytes. With Zry-2, oxygen evolution occurs at high anodic potentials. The oxidation of the alloys in nitric acid shows dissolution of their minor alloying elements but no oxygen evolution at high potentials. The role played by the alloying elements in connection with the appearance of side reactions is discussed. The oxide film were characterized by impedance measurements, X-ray photoelectron spectroscopy and Auger spectroscopy.

Formation, Characterization, and Stability of Methaneselenolate Monolayers on Au(111): An Electrochemical High-Resolution Photoemission Spectroscopy and DFT Study

We investigated the mechanism of formation and stability of self-assembled monolayers (SAMs) of methaneselenolate on Au(111) prepared by the immersion method in ethanolic solutions of dimethyl diselenide (DMDSe). The adsorbed species were characterized by electrochemical measurements and high-resolution photoelectron spectroscopy (HR-XPS). The importance of the headgroup on formation mechanism and the stability of the SAMs was addressed by comparatively studying methaneselenolate (MSe) and methanethiolate (MT) monolayers. Density Functional Theory (DFT) calculations were performed to identify the elementary reaction steps in the mechanisms of formation and decomposition of the monolayers. Reductive desorption and HR-XPS measurements indicated that a MSe monolayer is formed at short immersion times by the cleavage of the Se-Se bond of DMDSe. However, the monolayer decomposes at long immersion times at room temperature, as evidenced by the appearance of atomic Se on the surface. The decomposition is more pronounced for MSe than for MT monolayers. The MSe monolayer stability can be greatly improved by two modifications in the preparation method: immersion at low temperatures (-20 °C) and the addition of a reducing agent to the forming solution.

Adsorption of carbonate species on silver. II. Electric field effects

The adsorption of carbonate and bicarbonate ions on Ag(100) is investigated under high external electric fields, which simulate the environment of the electrochemical double layer. The effect of the electric field was investigated on several adsorbate properties such as binding energy, geometry, charge transfer and group vibrational frequencies. When positive electric fields are applied, the binding energy of the ions increases and the distance between the ions and the surface decreases. For carbonate, the bond length between the carbon atom and the coordinated oxygen atoms increases with the field while the C-O bond length involving the noncoordinated oxygen decreases. This weakening and strengthening of the bonds is reflected in the corresponding group vibrational frequencies. While the frequency of the CO fragment perpendicular to the surface increases with the field, the frequency of the CO 2 fragment decreases. The frequency of the CO group changes at a rate of 81.2 cm -1 /V. In the case of bicarbonate, the electric field strongly affects the orientation of the ion on the surface as a consequence of the effective positive charge around the hydrogen atom. The charge transfer towards the surface for both ions increases as positive electric fields are applied.