Anodic nanoporous niobium oxide layers grown in pure molten ortho-phosphoric acid

Abstract

The anodic oxidation of niobium is investigated in pure molten ortho-phosphoric acid (o-H3PO4). At applied potentials in the 2.5-60 V range and for electrolyte temperatures in the 60-110°C range, porous niobium oxide layers are formed. Pore ordering and morphology depend on the anodization voltage, time and temperature, i.e. the anodic oxide develops different morphologies depending on low-or high-field anodizing conditions. At 100°C and low voltages, e.g. ≤ 5 V, vertically oriented, amorphous oxide nanopores with cylindrical shape ("nanochannels") grow with an average diameter of 5-8 nm. Higher voltages (10-20 V) lead to a less ordered pore morphology, resembling a "fish bone" structure. At 40-60 V, and after a sufficiently long anodization time (≥ 30 min), an orthorhombic Nb2O5 hierarchical structure forms that shows a bimodal pore size distribution with some 100 nm wide main pores that branch out into ~ 10 nm sized nanochannels. A partial crystallization of the anodic oxide is observed at high voltages (≥ 40 V). We propose that this is due to a high field-induced crystallite nucleation in the barrier oxide at the oxide/metal interface. With time, the crystalline oxide is incorporated in the porous layer. The main pores, partially crystalline and hence more stable in the anodizing electrolyte, result from the gradual dissolution of the amorphous side nanochannels.

Advanced Nanoporous Anodic Alumina-Based Optical Sensors for Biomedical Applications

“…In the mid-1990s, AAO was applied for bottomup templated nanofabrication (Masuda and Fukuda, 1995;Masuda et al, 2001). Oxide layers generated during anodization can also be produced on materials such as niobium (Nb) (Lu et al, 2005;Sieber et al, 2005;Stróz et al, 2019;Altomare et al, 2020), silicon (Si) (Xia et al, 2000;Rodriguez et al, 2005;Formentín et al, 2015;Elia et al, 2016), tantalum (Ta) (Sieber et al, 2006;Barton et al, 2009;Wei et al, 2009;Yu et al, 2013;Wang et al, 2015;Fialho et al, 2020), titanium (Ti) (Kim et al, 2014;Zheng et al, 2014;Nguyen et al, 2017;Anitha et al, 2018), etc.…”

Optical Properties of Porous Alumina Assisted Niobia Nanostructured Films–Designing 2-D Photonic Crystals Based on Hexagonally Arranged Nanocolumns

“…Features of electrochemical behavior of valve metals (Al, Nb, Ta, Ti, etc. ), micro and nanodispersed, nanostructured oxides, including porous and tubular anodic oxides and composite materials based on them, are the objects of numerous studies [ 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 ] and review publications [ 11 , 12 , 18 , 52 , 53 , 54 , 55 , 56 , 57 , 58 ]. Particular interest in such materials is due to the fact that they have a quasi-regular structure, which is formed in the process of production as a result of self-organization processes [ 59 ].…”

Peculiar Porous Aluminum Oxide Films Produced via Electrochemical Anodizing in Malonic Acid Solution with Arsenazo-I Additive

“…The ratio of simultaneously occurring self-organized processes of metal oxidation with the formation of oxide and dissolution plays an important role in the formation of porous and tubular anodic oxides of valve metals. Different kinds of metal dissolution during anodic oxidation of aluminum and other valve metals lead to the formation of oxide films with different morphologies: barrier layers, quasi-regular porous layers, high degree self-ordering porous layers as well as tubular and nanocomposite structures [ 6 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 ]. The anodizing electrolyte is, aside from other process parameters, the main factor that determines this morphology.…”