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Self‐organized anodic TiO2 nanotube (TNT) layers prepared by anodization of Ti substrates have attracted great interest within the last 20 years, due to their unique properties and low‐cost synthesis. This mini review article aims to shortly summarize the most recent developments in the TNT layer synthesis and applications. It presents the synthesis of high aspect ratio (HAR) TNT layers in a short time by accelerating the TNT layer growth rates in lactic acid containing electrolytes. Furthermore, the etching of TNT layers towards TNT bundles of homogeneous size and single tube powders is discussed, as well as the possibility to grow TNT layers on non‐planar Ti substrates, which cannot be directly connected to the potentiostat (e.g., Ti spheres or 3D Ti meshes). As a relatively new application, the employment of TNT layers in microwave resonator sensors is introduced. Last, but not least, approaches of upscaling the TNT layer size from the laboratory scale towards significantly larger scale are outlined and reviewed. [ABSTRACT FROM AUTHOR]

In this work, the anodization of Ti foils in closed bipolar cells is shown for the first time. Depending on the applied potential, i. e. direct or alternating potential, TiO2 nanotube (TNT) layers can either be prepared on one side or on both sides of a Ti foil. It is shown that due to the use of closed bipolar cells, potentials with different amplitudes can be applied within the individual two half‐cells. Additionally, different electrolytes can be used for the anodization of the two sides of a Ti substrate. Thus, the TNT layers obtained under alternating potential on both sides of a Ti foil can have completely different dimensions, i. e. TNT diameter and TNT layer thickness. Furthermore, by employing alternating potentials, black TNT layers can be synthesized on the side of the Ti foil, while being reduced during the last anodization step at negative potentials. [ABSTRACT FROM AUTHOR]

1D TiO2 nanotubes (TNTs) produced by anodization of Ti have attracted significant research and technological interest due to their excellent properties suitable for numerous applications. A huge challenge that remains is to develop reproducible protocols for the synthesis of extremely high aspect ratio (HAR) TNT layers. A new strategy for the growth of HAR TNT layers based on galvanostatic anodization of Ti in NH4F/ethylene‐glycol‐based electrolyte with the addition of lactic acid (LA) is described. The results show that the addition of LA to the electrolyte can effectively prevent the dielectric breakdown during the anodization at elevated current densities. In comparison with the LA‐free electrolyte, robust HAR TNT layers as thick as ≈44 μm can be produced within 15 min. [ABSTRACT FROM AUTHOR]

• Electrodeposited Ti and Ti foils were anodized to obtain TiO 2 nanotube (TNT) layers. • Two different thicknesses of TNT layers were prepared and extensively compared. • No morphological differences were found between layers on the two substrates. • Preferential (101) orientation of anatase revealed for layers on electrodeposited Ti. • Enhanced photo-electrochemical behavior for TNT layers on electrodeposited Ti. This study conducted a deep investigation of TiO 2 nanotube (TNT) layers of two different thicknesses anodically grown on electrodeposited Ti films. The Ti films were grown from molten salts on Ni foils. The structure of the starting metals was compared by XRD (X-Ray Diffraction) and EBSD (Electron Backscatter Diffraction analyses), which showed a strong orientation towards the titanium (101) for the electrodeposited substrate, compared to the rather polycrystalline structure of the rolled Ti. The photoelectrochemical and electrochemical properties of 1 μm and 5 μm thick TNT layers anodically grown on these substrates were examined and compared with TNT layers of the same thickness. No significant morphological and compositional differences were found using SEM (Scanning Electron Microscopy) and XPS (X-ray Photoelectron Spectroscopy) between the TNT layers grown on the two substrates. However, higher photocurrent densities and ICPE values were observed for TNT layers grown on electrodeposited Ti. An in-depth investigation using Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) analyses showed increased conductivity of the TNT layers produced on electrodeposited Ti compared to their counterparts. The carrier density (N D) for the TNT layers on electrodeposited Ti, calculated from Mott-Schottky measurements, showed a higher doping level than the TNT layers grown on Ti foils. This increase in N D results in more optimal photoelectrochemical performance of the TNT layers grown on electrodeposited Ti. All in all, the results presented herein pave the way for the use of electrodeposited Ti, with all its inherent benefits, and allow the further study of its promising properties in a wide range of applications. [ABSTRACT FROM AUTHOR]

Herein, the synthesis of BiOCl nanoplatelets of various dimensions is demonstrated. These materials were prepared by anodic oxidation of Bi ingots in diluted HCl under dielectric breakdown conditions, triggered by a sufficiently high anodic field. Additionally, it is shown that the use of several other common diluted acids (HNO3, H2SO4, lactic acid) resulted in the formation of various different nanostructures. The addition of NH4F to the acidic electrolytes accelerated the growth rate resulting in bismuth‐based nanostructures with comparably smaller dimensions and an enormous volume expansion observed during the growth. On the other hand, the addition of lactic acid to the acidic electrolytes decelerated the oxide growth rate. The resulting nanostructures were characterized using SEM, XRD and TEM. BiOCl nanoplatelets received by anodization in 1 M HCl were successfully employed for the photocatalytic decomposition of methylene blue dye and showed a superior performance compared to commercially available BiOCl powder with a similar crystalline structure, confirming its potential as a visible light photocatalyst. BiOCl nanoplatelets are produced by facile breakdown anodization of bismuth in diluted HCl electrolyte. The addition of lactic acid to the electrolyte decelerates the growth rate of the nanoplatelets while the addition of NH4F or accelerates the growth rate. The nanoplatelets are single crystalline and show a superior photocatalytic behaviour compared to commercial BiOCl powder. Similarly, Bi2O3 needles and Bi2O3 cubes are produced in H2SO4 and HNO3, respectively. [ABSTRACT FROM AUTHOR]

Here we report on structural properties and the photo-electrochemical response of annealed TiO 2 nanotube layers, grown on four different Ti substrates by an identical anodization process. TiO 2 nanotube layers were grown on Ti substrates by anodization in glycerol containing NH 4 F and water. The layers were then annealed at 400 °C for 1 h in air. Photocurrent densities of annealed nanotubes were recorded upon irradiation with ultraviolet (UV) light at a constant potential of 0.4 V, in a 0.1 M Na 2 SO 4 aqueous solution. Approximately, a 280% difference in the photocurrent densities was recorded from TiO 2 nanotubes fabricated from different Ti substrates. To elucidate the origin of this difference, the nanotube layers were scrutinized by Mott-Schottky measurements, X-ray diffraction (XRD) and Electron Energy Loss Spectroscopy (EELS). Inspection of the XRD patterns revealed preferential crystallographic orientations in various Ti substrates from which TiO 2 nanotube layers were produced. Subsequent EELS analyses of the annealed nanotube layers disclosed significant differences in the Ti:O stoichiometry, in accordance with XRD evaluation of preferential orientation differences among used Ti substrates. [ABSTRACT FROM AUTHOR]

A new Bi 2 O 3 structure resembling bundles of needles was prepared by anodic oxidation of a bismuth substrate in H 2 SO 4 electrolyte followed by annealing in air. Needle growth occurred on the timescale of minutes. The resulting porcupine-like needle bundles were characterized using SEM and XRD before and after annealing. The as-grown needles consisted of bismuth aqua sulfate hydroxide; however, upon annealing at 250 °C in air, tetragonal β-Bi 2 O 3 was produced without any morphological change. [ABSTRACT FROM AUTHOR]

Ideally hexagonally ordered TiO2 nanotube layers were produced through the optimized anodization of Ti substrates. The Ti substrates were firstly covered with a TiN protecting layer prepared through atomic layer deposition (ALD). Pre-texturing of the TiN-protected Ti substrate on an area of 20×20 μm2 was carried out by focused ion beam (FIB) milling, yielding uniform nanoholes with a hexagonal arrangement throughout the TiN layer with three different interpore distances. The subsequent anodic nanotube growth using ethylene-glycol-based electrolyte followed the pre-textured nanoholes, resulting in perfectly ordered nanotube layers (resembling honeycomb porous anodic alumina) without any point defects and with a thickness of approximately 2 μm over the whole area of the pattern. [ABSTRACT FROM AUTHOR]

Anodic self-organized TiO2 nanotube layers (with different aspect ratios) were electrochemically infilled with CuInSe2 nanocrystals with the aim to prepare heterostructures with a photoelectrochemical response in the visible light. The resulting heterostructure assembly was confirmed by field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). High incident photon-to-electron conversion efficiency values exceeding 55% were obtained in the visible-light region. The resulting heterostructures show promise as a candidate for solid-state solar cells. [ABSTRACT FROM AUTHOR]

In this work, we report for the first time on the use of melt spun glass-forming alloys - Ti 75 Zr 10 Si 15 (TZS) and Ti 60 Zr 10 Si 15 Nb 15 (TZSN) - as substrates for the growth of anodic oxide nanotube layers. Upon their anodization in ethylene glycol based electrolytes, highly ordered nanotube layers were achieved. In comparison to TiO 2 nanotube layers grown on Ti foils, under the same conditions for reference, smaller diameter nanotubes (~ 116 nm for TZS and ~ 90 nm for TZSN) and shorter nanotubes (~ 11.5 μm and ~ 6.5 μm for TZS and TZSN, respectively) were obtained for both amorphous alloys. Furthermore, TEM and STEM studies, coupled with EDX analysis, revealed a double-wall structure of the as-grown amorphous oxide nanotubes with Ti species being enriched in the inner wall, and Si species in the outer wall, whereby Zr and Nb species were homogeneously distributed. [ABSTRACT FROM AUTHOR]

In this contribution, various Ti thin substrates were explored and compared for the anodic growth of self-organized TiO 2 nanotube layers for the first time. In order to evaluate differences in the electrochemical anodization characteristics and the tube dimensions, five different Ti substrates from four established suppliers were anodized in the widely used ethylene glycol electrolytes containing 88 mM NH 4 F and 1,5 vol.% water. Two anodizations were carried out to elucidate an influence of the pre-anodized substrates used for the second anodization. By thorough evaluation of the nanotube dimensions, large variations between the dimensions of the nanotubes were found for the different substrates, ranging from ∼32 μm to ∼50 μm for the nanotube length and from ∼109 nm to ∼127 nm for the nanotube diameter after the second anodization. Upon AFM measurements, Goodfellow Ti substrates (99.99% purity), yielded the smoothest surface and the highest degree of ordering from all substrates. Moreover, considerably different consumption of Ti substrates via anodization was revealed by profilometric measurements between the original non-anodized part of the Ti substrates, and the anodized part after the removal of the nanotube layer. Orientation imaging microscopy revealed considerable differences in the size and orientation of the substrate grains. [ABSTRACT FROM AUTHOR]

In the present work we report on the influence of the age of ethylene glycol-based electrolytes on the synthesis of self-organized TiO 2 nanotube layers. Electrolytes of different ages, defined by the total duration for anodization, were explored in order to get an insight into how the tube structure changes with the electrolyte age. The results show a strong dependence of the electrolyte age upon the nanotube length and diameter — a phenomenon surprisingly not discussed in existing literature. When fresh electrolytes are employed, nanotube arrays with a high aspect ratio are received, while in older electrolytes (i.e. already used for anodization) the nanotube arrays exhibit low aspect ratios. This is a very important aspect for the reproducible synthesis of the nanotube layers. Moreover, the effect of the potential on the nanotube dimensions was investigated. Linear dependence of the diameter upon the potential was observed. Last, but not least, the influence of a potential change towards the end of the anodization time was studied. By sweeping the potential to 100 V, or to 5 V and keeping this for 1 h after applying a constant potential of 60 V for 4 h, nanotubes underwent interesting morphological changes. In particular, when slow sweeping from 60 V to 5 V was carried out, small nanotubes grew in the gaps between the initial nanotubes. Interestingly, these nanotube layers showed lower adhesion to the underlying substrates. [ABSTRACT FROM AUTHOR]

Keywords: anodization; bipolar electrochemistry; closed cell; nanotube layers; TiO2 EN anodization bipolar electrochemistry closed cell nanotube layers TiO2 3778 3778 1 10/29/21 20211013 NES 211013 B The Cover Feature b illustrates the set-up of a closed bipolar cell for the anodization of titanium foils towards TiO SB 2 sb nanotube layers. Anodization, bipolar electrochemistry, closed cell, nanotube layers, TiO2. [Extracted from the article]

• Anodization approach towards thick TiO 2 nanotube layers in short time is presented. • Aged NH 4 F/H 2 O/EG electrolytes with addition of lactic acid have been be used. • Prevention of dielectric breakdown at high applied voltages is the key feature. • TiO 2 nanotube layers (TNTs) with aspect ratio of ~450 were obtained in 15 min. • Double anodization was used to improve the homogeneity of TNTs. Over the past years, the growth of high aspect ratio (HAR) anodic TiO 2 nanotubes in a very short time (minute scale) has remained challenging. In the present report, TiO 2 nanotube (TNT) layers with HAR ≈ 450 were obtained during only 15 min of optimized anodization. The key feature is the use of NH 4 F/H 2 O/ethylene glycol (EG) electrolytes with the addition of lactic acid (LA) that prevents the dielectric breakdown and enables anodization at higher potentials compared to the classical NH 4 F/H 2 O/EG electrolyte without LA. The thickness and average diameter of the obtained TNT layers for 15 minutes' anodization at 160 V were approximately 80 µm and 170 nm, respectively. Furthermore, we show for the first time that high temperature anodizing, previously introduced as a driving force for the growth of HAR nanotubes in a short time, can be avoided by an accurate optimization of the anodization conditions at room temperature. The results clearly show that electrolytes containing LA can serve as a promising candidate for the ergonomic and economic production of HAR TNT layers on Ti substrates in very short anodization times, i.e. with the addition of LA, the anodization time can be significantly reduced from several hours or several days to 15–30 min. [ABSTRACT FROM AUTHOR]

The present work reports on the effect of thermal annealing on electrical, optical and structural properties of low aspect ratio self-organized TiO 2 nanotube layers formed by anodization of Ti. The average inner diameter and length of the nanotubes was 100 nm and 1 μm, respectively. From the whole range of temperatures (300–600 °C), annealing at 400 °C leads to the formation of crystalline TiO 2 nanotube layers with anatase structure offering best electrical and photo-electrochemical properties. Conducting atomic force microscopy studies have been explored for the first time to identify the annealing temperature of nanotube TiO 2 layers with best electrical properties. [ABSTRACT FROM AUTHOR]