Substrate Purity Effect on the Defect Formation and Properties of Amorphous Anodic Barrier Al₂O₃

Abstract

A comprehensive study concerning the effect of different Al metal substrate purities (i.e. 99.5 versus 99.99%) on the properties of amorphous anodic barrier Al 2 O 3 is presented. The experimental findings demonstrate that only tiny variations in the purity of the employed Al materials lead to different oxide growth rate, surface charge, structural defect and impurities content. Below the ionic recombination potential characterized by Scanning Kelvin Probe Force Microscopy, an increase of the anodizing voltage leads to an improvement of the oxide barrier properties. The larger growth rate exhibited by the higher purity Al substrate however indicates the formation of highly disordered and inhomogeneous barrier oxides. A combination of photoelectrochemical and photoluminescence spectroscopies was used to characterize structural defect concentration and confirmed the presence of significantly higher concentrations in the oxide grown on the purer Al substrates. FT-IR and RBS/ERDA results indicate that H and C species are incorporated from the electrolyte solution in the barrier oxides with higher H amounts detected in the oxide grown on the purer Al substrate.

Structural Engineering of the Barrier Oxide Layer of Nanoporous Anodic Alumina for Iontronic Sensing

“…Under high electric field, the counter migration of oppositely charged ions occurs via hopping from vacancies to vacancies, which in turn redistributes the ionic defect species in the BOL. These vacancies or point defects are made up of aluminum and oxygen vacancies, which are generated during anodization and represent fixed space charges, and their distribution in the BOL forms a gradient along the direction from the metal–oxide to oxide–electrolyte interfaces . Since all NAA membranes assessed in our study were produced under the mild anodization regime, it is reasonable to infer that the space charge density gradient in NAA membranes will be influenced by the thicknesses of their BOLs (Figure e).…”

“…The chemical composition of NAA’s nanopores consists of dielectric anodic oxide with an onion-like distribution of ionic impurities and vacancies from the outer to the inner side of the pore wall and the BOL (i.e., oxide–electrolyte and metal–oxide interfaces, respectively) . This chemical structure is characterized by two types of ionic defects: (i) acid anion impurities incorporated into the anodic oxide from the electrolyte, the concentration of which decreases from the outer (contaminated Al 2 O 3 ) to the inner (pure Al 2 O 3 ) side of the pore wall and the BOL, and (ii) negative (O 2– ) and positive (Al 3+ ) defect charge vacancies distributed across the volume of the amorphous BOL, where the number of O 2– vacancies is higher than that of the stoichiometric ratio for Al 2 O 3 (i.e., 1.5) at the oxide–electrolyte interface. − Ionic impurities and charge vacancies result from the flow of electrolytic species (i.e., O 2– , HO – , Al 3+ , COO – , SO 4 2– , and PO 4 3– ) across the BOL during the electric field-assisted growth of anodic oxide under volume expansion and compressive stress . The unique structure of NAA with well-defined nanopores provides an ideal material to develop membranes with precisely engineered properties for nanofluidic and separation applications. , Of all these, NAA structures have recently been devised as model platforms for nanopore-based iontronicsgeneration and transmission of electric signals associated with the flow of ions along bioinspired, synthetic nanochannels .…”

Ionic Current Rectification of Porous Anodic Aluminum Oxide (AAO) with a Barrier Oxide Layer

“…The opposite physical pictures for BL reported in the literature might possibly be associated with the detailed migration mechanisms of mobile ions, which depend on anodizing conditions (e.g., electrolyte, potential, substrate purity, etc.). 51 The reported electronic structures (i.e., p-or n-type) of the inner and outer oxide are actually the consequence of the space charge density formed by the local sum of negative and positive fixed charge centers, that is, e N N ( 3 2 ) − + − + . We assume that the fixed space charge density (ρ) in the barrier oxide of porous AAO is not uniform, but forms a gradient along the direction from the m/o-interface to the o/eone, and the presence of this gradient ( ρ ∇ ) across the barrier layer is responsible for ICR (the larger the gradient, the higher the rectification efficiency).…”

Anodizing of Self-Passivating WxTi1–x Precursors for WxTi1–xOn Oxide Alloys with Tailored Stability

“…However, as soon as the final anodizing voltage is reached, the electric field across the thickening oxide decreases approximately linearly with increasing oxide-layer thickness layer, resulting in an exponential decay of the anodizing current (Figure A): that is, a limiting oxide layer thickness is reached. A residual current is observed, which can be related to either a redistribution of ionic and defect species under the influence of the maintained electrical field or electrolysis processes. These contributions typically vary for different oxides, depending on resistivity, defect chemistry, and catalytic performance.…”

“…a limiting oxide layer thickness is reached. A residual current is observed, which can be related to either a redistribution of ionic and defect species under the influence of the maintained electrical field, 35 or electrolysis processes. These contributions typically vary for different oxides, depending on resistivity, defect chemistry and catalytic performance.…”