Titanium Oxide Nanowires Originating from Anodically Grown Nanotubes: The Bamboo‐Splitting Model

Abstract

Motivated by the successful fabrication of porous alumina templates that are widely used for the preparation of nanowires and nanotubes [1][2][3][4] the anodization of metals such as tungsten, [5][6] niobium, [7][8] tantalum, [9] zirconium, [10] and iron [11] has been intensively studied to mimic the self-ordering phenomena displayed in anodic porous alumina. Among various anodic porous metal oxides, titanium oxide has attracted great attention due to its versatile applications, for example, dye solar cells, [12] highly efficient photocatalysts, [13] ultrasensitive biosensors, [14] and biocompatible dental/bone implants. [15] The preparation of highly ordered TiO 2 nanotubes by anodic oxidation of titanium was initially based on aqueous HF. [16][17] However, the maximum thickness obtainable is limited to approximately 500 nm due to fast chemical dissolution of the formed oxide in acidic aqueous HF. A few years later it was revealed that the chemical dissolution can be greatly prohibited by using buffer electrolyte consisting of a mixture of KF and NaF [18] or a mixture of NaF and Na 2 SO 4.[19] In this case, TiO 2 nanotubes with a thickness of several micrometers can be prepared; this is due to the fact that the buffer electrolyte does not rapidly dissolve the formed structures at the mouth of nanotubes but allows nanotubes to grow at the bottom by local acidification. Very recently, Grimes and co-workers have successfully demonstrated that highly ordered TiO 2 nanotube arrays with a thickness of more than 100 mm can be prepared by using NH 4 F in nonaqueous organic polar electrolyte such as ethylene glycol (HOCH 2 CH 2 OH). [20][21] The principle for the preparation of the extremely long TiO 2 nanotubes is based on minimizing water content in the electrolyte since hydrogen ions, derived by the electrolysis of water, participate in the reaction for the chemical dissolution of the formed oxide:In this Communication we demonstrate for the first time that TiO 2 nanowires with a diameter of a few tens of nanometers and a length up to a few tens of micrometers can be synthesized by simple anodization in ethylene glycol (EG) containing NH 4 F (hereafter, referred to as EG + NH 4 F solution). The anodization allows for the preparation of TiO 2 nanotubes, which are vertically split off by electric-field-directed (electro)chemical etching. This leads to the formation of TiO 2 nanowires on the entire TiO 2 nanotube arrays. The mechanism of the nanowire formation is described based on the bamboo-splitting model.Unlike the formation of highly ordered TiO 2 nanotubes reported in the literature, [20] Figure 1 shows that TiO 2 nanowires can be prepared on the entire surface when titanium is anodized under well-controlled conditions. The nanowires are more than 10 mm in length (Figure 1 a) and around 20 nm in diameter (Figure 1 c). We find that TiO 2 nanotubes with a diameter of around 100 nm exist underneath the nanowires (Figure 1 b). We assume that the nanowires originate from the vertical break of TiO 2 nanotubes; this is co...

One-Dimensional Titanium Dioxide Nanomaterials: Nanotubes

“…This so-called "nanograss" (Figure 6.a) on the top can cover part of the nanotube openings 40 (some authors refer to this etched nanotube region also as nanowires or nanobelts) [218][219][220] .…”

Transition from Anodic Titania Nanotubes to Nanowires: Arising from Nanotube Growth to Application in Dye‐Sensitized Solar Cells

“…Recently, a few researchers reported anodic formation of titania nanowires on top of the nanotubes in an organic electrolyte. [17][18][19][20] Several methods have been employed to suppress formation of the nanowires, such as growing a compact rutile layer, [21] producing a photoresist coating, [22] or polishing titanium foils prior to anodization, [23] and to eliminate the nanowires with an ultrasonication process after anodization. [24] The formation of the nanowires has been interpreted using a bamboosplitting model [17] based on alumina nanowires and nanotubes produced by etching alumina templates.…”

Research Trends in Doping Methods on TiO₂ Nanotube Arrays Prepared by Electrochemical Anodization

“…Figure 4 demonstrates that the thickness of the walls of nanotubes increase up to around twice when RuO2 is incorporated into the wall. Thus, the typical nanopore closing problem and bamboo-splitting mode is suppressed [24,25,49]. Because of the minus net charge of RuO4 -in the electrolyte, RuO4 -can be attracted to anode, where is positively biased.…”

“…It leads to destroy the nanotubular morphology on the top (so-called bamboo-splitting mode). In the bamboo-splitting mode, several parameters such as a high voltage, long anodization time, and small amount of water, enhance the dissolution of the top surface of TiO2 nanotubes, leading to split the nanotubes to nanowires [24,25]. Figure 3 shows the bamboo-splitting mode and normal TiO2 nanotubes, respectively.…”