Tailoring the Ti surface via electropolishing nanopatterning as a route to obtain highly ordered TiO₂nanotubes

Abstract

Abstract. Highly-ordered TiO2 nanotubes (NTs) were synthesized by the electrochemical anodization of Ti foils subjected to electropolishing (EP) pre-treatment. We found that the Ti surface roughness plays an important role in the onset of pore nucleation in enhancing the local focusing effect of the electrical field. Additionally, EP induces the formation of dimple structures on the metal surface, which can work as a pre-pattern prior to the anodization. These shallow ripples lead to a preferential ordered pore nucleation, offering an organization improvement of the anodic oxide NTs. We found that, depending on the EP applied potential, the roughness and the spacial period of the ripple-like structures varies from 8 to 2 nm and from 122 to 30 nm, respectively. Such tuning allowed us to focus on the influence of the initial Ti pre-surface topography features on the NTs length, organization and hexagonal arrangement quality, NTs diameter and density. Our results show that an EP under 10 V is the most suitable to obtain a small Ti surface roughness, the largest NT length (40% enhancement) and the effective improvement of the ordered hexagonal NTs arrays over larger areas. Furthermore, the NTs dimensions (pore diameters and density) were found to also depend on the initial Ti surface topography. The use of optimized EP allows to obtain highly hexagonal self-ordered samples at a reduced time and cost. IntroductionThe electrochemical anodization of Ti to obtain highly-ordered nanotube (NT) arrays of TiO2 continues to gain an increasing importance for a large number of applications, such as photoelectrochemical cells for H2 production (water splitting) [1,2] and particularly for dye-sensitized solar cells (DSCs) [2]. TiO2 NTs have received great attention due to their one dimensional structure which provides a direct and efficient path for electrons [1].TiO2 NTs can be obtained by the electrochemical anodization of Ti in fluoride-based electrolytes. The addition of ethylene glycol was later found to lead to longer (hundreds of microns) and more ordered arrays of TiO2 NTs [3][4][5][6][7][8][9]. Differently from the well-known case of Al anodization, the Ti anodization presents a non-steady state, due to the out of equilibrium dissolution and oxidation processes on the NT bottom and to the additional chemical dissolution process at the NTs top [5,[10][11][12][13][14]. This unbalanced anodization leads to a maximum TiO2 NTs length, after a given anodizing time. Besides the length, other NT parameters are influenced by factors such as the electrolyte type (e.g. electrolyte concentration and pH), anodization temperature and applied potential [3][4][5][6][7][8][9][10][15][16][17][18].Aiming a degree of order comparable to that of anodic alumina oxide (AAO) at 10 % porosity [19], intense research has been performed on the organization and final ordering of TiO2 NTs in a closed-

Crystallographic Orientation Dependence of Nanopattern Morphology and Size in Electropolished Polycrystalline and Monocrystalline Aluminum: An EBSD and SEM Study

“…1,2 Electropolishing has also been extensively employed to create surface nanopatterns-ordered or quasi-ordered nanostructures such as hexagons and stripes-in various metals such as Zn, 3 Al, [4][5][6] Ti, 7 Ta, 8 and some alloys. 9 Nanopatterned metals have been used for fabrication of high precision diffraction gratings, 10 preparation of ordered anodized oxide films or nanotubes, [11][12][13][14][15] and fabrication of implanted parts with enhanced interfacial adhesion strength. [16][17][18] Nanopatterned aluminum (Al) has been used for preparing ordered anodized aluminum oxide films with controlled morphology; [11][12][13] there were studies demonstrating the potential of nanopatterned Al for quantum-dot based nano-electronic devices.…”

Modeling the Growth Kinetics of Anodic TiO2 Nanotubes

“…In the anodization the growth of the barrier layer thickness (δ b ) at the NTs bottom is governed by the high-field conduction mechanism. − While the highly studied AAO presents a steady-state anodization that results in a constant oxide δ b at the pore bottom (time independent), , TiO 2 NTs present a nonsteady-state anodization that leads to the progressive increase of δ b over time and limits the growth of NTs (because the ion diffusion path in the barrier extensively increases). ,,,− …”

A Simple and Rapid Method of Forming Double‐Sided TiO2 Nanotube Arrays

“…[24,25] In response, organic electrolyte additives (e. g., lactic acid, [26] EDTA [27] ) were shown to mitigate the effects of oxide burning whilst still allowing for the experimental simplicity and material robustness afforded by potentiostatic anodization, albeit at elevated temperatures and potentials (e. g., 60 °C, 120 V; see Figure 1, orange, Table S2). Notably, despite numerous differences in anodization parameters across the 80 publications referenced in Figure 1 (e. g., interelectrode distance, [28,29] counter electrode material, [30,31] substrate roughness/origin, [32][33][34][35] use of double anodization/ patterning, [5,36] electrolyte age, [37,38] and variations in ambient conditions [39] ), these additive approaches provide consistent and significant improvements in growth rates. Very recently, efforts to decouple the temperature and voltage influence demonstrated comparable growth rates at room temperature when supplying an even greater voltage (160 V) in a highly aged additive electrolyte.…”