No abstract

In 1999 Zwilling et al. reported on the electrochemical formation of self-assembled TiO 2 nanotubes (p-TiO 2 ) by the anodization of Ti, [1] and other reports followed soon thereafter. [2,3] A factor limiting the application of this first generation of nanotubes was their production in hydrofluoric acid based electrolytes. As a result of the high rate of chemical dissolution of TiO 2 in these solutions, the nanotubes could be grown only up to a length of 500 nm. Recently, we have shown [4][5][6][7] and explained [4] how a second generation of nanotubes with lengths up to several micrometers and aspect ratios up to 50 can be formed by adjusting the pH gradient within the growing nanotube. Common to all these anodic approaches is that the side walls of the tubes show strong irregularities, that is, the side-wall profiles show considerable thickness variations (ripples) as shown in Figure 1 a. In the present work we demonstrate how TiO 2 nanotubes with extremely smooth homogenous walls and aspect ratios up to 175 can be grown (as shown in Figure 1 b). This third generation of nanotubes is formed by suppressing local concentration fluctuations and pH bursts during anodization by using highly viscous glycerol electrolytes.In previous work it has been established that the length of the nanotubes is essentially the result of a steady-state situation between electrochemical formation of TiO 2 at the pore tip and the chemical dissolution of formed TiO 2 by fluorides from the electrolyte. [3,5] We showed how the pH at the pore tip is lowered by the hydrolysis reactions of the Ti 4+

Nanotubular material surfaces produced by the electrochemical formation of self-organized porous structures on materials such as aluminum [1,2] and silicon [3,4] have attracted significant interest in recent years. While scientific thrust is often directed towards the elucidation of the principles of the self-organization phenomena, technological efforts target applications based on the direct use of the high surface area, for example, for sensing [5,6] or controlled catalysis, [7] exploit the optical properties in photonic crystals, [8] waveguides, [9] or in 3D arranged Bragg-stack type of reflectors.[10]The highly organized structures may be used indirectly as templates [11] for the deposition of other materials such as metals, [12] semiconductors, [13] or polymers. [14] Over the past few years, nanoporous TiO 2 structures have also been formed by electrochemical anodization of titanium. [15][16][17] Although several applications have been proposed, [18,19] a wider impact of these structures has been hampered by the fact that the layers could only been grown to a limiting thickness of a few hundreds of nanometers.Herein we demonstrate for the first time how high-aspectratio, self-organized, TiO 2 films can be grown by tailoring the electrochemical conditions during titanium anodization. Figure 1 shows scanning electron microscope (SEM) images of self-organized porous titanium oxide formed to a thickness of approximately 2.5 mm in 1m (NH 4 ) 2 SO 4 electrolyte containing 0.5 wt. % NH 4 F. From the SEM images it is evident that the self-organized regular porous structure consists of pore arrays with a uniform pore diameter of approximately 100 nm and an average spacing of 150 nm. It is also clear that pore mouths are open on the top of the layer while on the bottom of the structure the tubes are closed by presence of an about 50-nm thick barrier layer of TiO 2 .The key to achieve high-aspect-ratio growth is to adjust the dissolution rate of TiO 2 by localized acidification at the pore bottom while a protective environment is maintained along the pore walls and at the pore mouth. In our previous work in HF and NaF solutions [15,20] it was established that the thickness of the porous layer is essentially the result of an equilibrium between electrochemical formation of TiO 2 at the pore bottom and the chemical dissolution of this TiO 2 in an F 2À , is essential for pore formation, however, it is also the reason that previous attempts to form porous layers in HF electrolytes always resulted in layer thicknesses in the range of some 100 nm.We tackled the problem by controlling the self-induced acidification of the pore bottom that is caused by the electrochemical dissolution of the metal (Figure 2 a-c). Main reason for the localized acidification is the oxidation and hydrolysis of elemental titanium [Eq. (1), in Figure 2]. The chemical dissolution rate of TiO 2 is highly dependent on the pH value (see Figure 2 d). Using a numerical simulation of the relevant ion fluxes we can construct the pH profile in the pore (suc...

Self-organized anodic titania nanotube layers were doped with nitrogen successfully using ion implantation. Photoelectrochemical measurements combined with XRD measurements show that the damage created by ion bombardment (that leads to a drastic decrease of the photoconversion efficiency) can be “annealed out” by an adequate heat treatment. This results in a N-doped crystalline anatase nanotube structure with strongly enhanced photocurrent response in both the UV and the visible range.

Ti O 2 nanotube layers were grown on titanium by a self-organized anodic oxidation. The layers consist of arrays of individual tubes with a length of ∼2μm, a diameter of ∼100nm, and a wall thickness of ∼10nm. These layers can be annealed to an anatase structure which strongly increases the photocurrent efficiency. Moreover, the nanotube layers can—under certain conditions—exhibit a drastically enhanced photocurrent compared to compact anatase layers. These strong changes in the photoresponse are attributed to the characteristics of the space charge layer within the tube wall.

No abstract

No abstract

The anodic formation of self-organized porous TiO 2 on titanium was investigated in 1 M ͑NH 4 ͒ 2 SO 4 electrolytes containing 0.5 wt % NH 4 F by potential sweeps to 20 V SCE . By a combination of electrochemical, morphological, and compositional information we show that the sweep rate has a significant influence on the initiation and growth of the porous structures. In the first phase of the anodization process, a precursor barrier type of oxide film is formed; underneath this film pores then start growing first randomly and then self-organize. High-aspect-ratio TiO 2 nanostructures can be obtained under optimized electrochemical conditions. These nanotubular oxide layers have single-pore diameter ranging from 90 to 110 nm, average spacing of 150 nm, and porosity in the order of 37-42%. The current work indicates that the nature of the initial barrier-type layer has a strong influence on establishing optimized pore growth conditions.