Ti Anodization in Alkaline Electrolyte: The Relationship between Transport of Defects, Film Hydration and Composition

Abstract

The hydration state and structural transformations of Ti films potentiostatically grown in 0.1 M NaOH are characterized utilizing EIS, Mott-Schottky and XPS techniques. Variations presented in the density of donors, the flatband potential and the fraction of species measured by XPS, in the region of characterization of these films are associated with changes in the ratio of different Ti sub-oxides within the passive film. Oxygen and hydroxyl vacancies and their respective formation rate constants are estimated from EIS spectra fits with a modified Point Defect Model. A consistent behavior is observed for the hydration state and structural transformations of the films as a function of potential. Films anodized at less positive potentials than 1.17 V vs SCE present higher hydroxyl vacancy diffusivities (e.g higher film hydration) and a heterogeneous mixture of titanium sub-oxides, whereas the opposite behavior is observed for films formed at more positive potentials than 1.17 V vs SCE, where the hydroxyl vacancy diffusivities decrease by one order of magnitude and the oxygen vacancy diffusivities increase three times. Thus, films anodized at more positive potentials present a higher dehydration, are more homogeneous and mainly composed of TiO 2 .Titanium anodization consists of the formation of an oxide layer on a Ti substrate by electrolytic polarization. The oxides formed through this methodology have become attractive due to the paramount performance exhibited by semiconductor TiO 2 films in photocatalytic and photoelectrochemical applications, biomaterials and corrosion resistance. 1-4 Indeed, the features and performance of these films associated with the aforementioned applications can be directly related to the electronic properties and crystal structure of these materials. For instance, the surface coloration, electrocatalytic activity, corrosion resistance, light refraction and absorption of the films rely upon the properties and crystal structure of the material. [5][6][7][8] This anodization could entail the transition through multiple intermediary oxidation states (TiO, Ti 2 O 3 , Ti 3 O 5 and/or Ti 4 O 7 ). 9-31 The proportion of these oxides within the anodic films varies depending on the conditions utilized to anodize, e.g. applied potential, time, electrolyte composition. This transition also affects the crystallinity and photoelectrochemical performance of TiO 2 photoanodes, as well as their corrosion resistance. 32,33 Molar fraction of chemical species vs E diagrams for the Ti-H 2 O system reveals that the formation of Ti sub-oxides could be connected to the hydration state of the oxide film, since the stability of the TiO 2 is displaced to more positive potentials when hydrated oxides are involved in the thermodynamic calculations. 34 Different characterization techniques have been used to study the formation of these intermediary oxides in different electrolytes. [10][11][12]15,18,19,21,[23][24][25]30,32,34 However, to date few studies have correlated the formation of these sub-oxides with their...

Thin-Film Materials for the Protection of Semiconducting Photoelectrodes in Solar-Fuel Generators

“…and certain pH conditions, by being biased into the "passivation" region ( Figure 2). 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 19 passage of current, and may thus allow for eventual permeation of electrolyte to the underlying semiconductor surface. 61, 68…”

Electrochemical Dechlorination of 2-Chlorophenol on Pd/Ti, Ni/Ti and Pd-Ni Alloy/Ti Electrodes

“…The foregoing was done because carbon is neither the main component nor it forms continuous and homogeneous layers over the electrode surfaces. 19,20 Pd3d doublet spectra were fitted using a Gaussian-Lorentzian mix function and Shirley background sub traction.…”

High-Field Growth of Semiconducting Anodic Oxide Films on Metal Surfaces for Photocatalytic Application

“…The oxide lattice may not withstand the large ion or electron fluxes arising at very high fields, or else, high reaction rates of corrosion due to the presence of aggressive chemical species (like chloride or fluoride ions) may locally increase the conductivity, leading to breakdown of the passive film. This may be indicated by several effects such as irregular current peaks, visible sparks, potential fluctuations, increasing electrical noise or even audible noise, depending on the oxide bandgap, and the nature and concentration of ions in solution [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48].…”

“…In general, characterization of the anodic thin films is needed, and diverse experimental techniques have been developed for these purposes: optical, electronic and/or atomic microscopy [50], electrochemical measures controlling voltage, current densities or charges [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48], electrochemical impedance spectroscopy [53,54], gravimetric measurements with the electrochemical quartz microbalance [55,56], infrared absorption, Raman, laser, UV-vis, UV-vis reflectance, luminescence, acoustic, X-rays, resonance, ellipsometry, and neutron-based spectroscopies [15, 44-47, 50, 57-62], and dynamic characterization based on photoelectrochemical methods [23,63,64].…”