Highly ordered anodic TiO 2 nanotube arrays with a tube length of 9 mm are shown to provide areal capacities of 0.24 mA h cm À2 (i.e. 96 mA h g À1 ) at a charge/discharge current density of 2.5 mA cm À2 (corresponding to a rate of 5 C) and 0.46 mA h cm À2 (i.e. 184 mA h g À1 ) at 0.05 mA cm À2 , when used as 3D free-standing anodes in Li-ion microbatteries. The present nanotube electrodes, which could be cycled for 500 cycles with only 6% loss of capacity, exhibited significantly higher energy and power densities, as well as an excellent cycling stability compared to previously reported TiO 2 -based Li-ion microbattery electrodes. The influence of parameters such as ordering, geometry and crystallinity of the nanotubes on the microbattery performance was investigated. A two-step anodization process followed by annealing of the nanotubes was found to yield the best microbattery performance. It is also demonstrated that the rate capability of the electrode depends mainly on the rate of the structural rearrangements associated with the lithiation/delithiation reaction and that the areal capacity at various charge/discharge rates can be increased by increasing the tube wall thickness or the length of the nanotubes, up to 0.6 mA h cm À2 for 100 cycles.

No abstract

No abstract

In the present work, we demonstrate the feasibility to form an aligned Nb(2)O(5) nanotube layer by self-organizing anodization of Nb in a NH(4)F-glycerol electrolyte. In order to achieve a nanotubular rather than a nanoporous layer, careful optimization of the anodization electrolyte is required. We show that only in a narrow window of electrolyte parameters highly aligned nanotubes of 50 nm inner diameter and several micrometres in length can be formed.

No abstract

In this work we report the anodic formation of highly ordered nanoporous Ta2normalO5 layers that exhibit a self-ordered alignment of pores uniformly over the anodized surface. These layers are grown by anodization of tantalum in a nonaqueous electrolyte consisting of an optimized glycerol/ethylene glycol mixture with the addition of NH4F . To reach an optimized nanoporous structure, several factors [fluoride concentration and content of supporting electrolyte (NH4)2SO4 ] need to be controlled. Once optimized, the nanopore diameter can be adjusted using the applied voltage in the range of 7–18 nm, and layers with a thickness of more than 10μm can be grown. As a result, a pore aspect ratio of 1000 can be reached.

Channel hopping: the formation of WO(3) layers with an aligned nanochannel morphology by using self-organizing anodization of a tungsten metal substrate is demonstrated. The nanochannel layers with diameters of approximately 9 nm can be grown to about 10 μm thickness. Layers optimized for length and structure are promising for visible-light photocatalytic applications, such as water-splitting photoanodes.

In this work, we report on the formation of highly aligned nanoporous or nanotubular oxide structures by anodization of Ti-Ta alloys in NH 4 F containing ethylene glycol electrolytes. We show that depending on the anodization conditions, either a hexagonally self-organized nanoporous layer or a nanotube array can be formed. Critical factors that decide on tube or pore formation are the water content of the electrolyte and the applied voltage. The present approach provides hexagonally packed oxide structures with an order comparable to that of the gold standard, anodized alumina. Layers of thicknesses Ͼ10 m and individual pore diameter in the range of 27-50 nm can easily be produced. © 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3490424͔ All rights reserved.Manuscript submitted August 13, 2010; revised manuscript received August 25, 2010. Published September 30, 2010 In the past few years, the electrochemical formation of selforganized nanoporous or nanotubular structures has attracted considerable attention from both the scientific and the technological community. Honeycomb-packed anodic Al 2 O 3 nanoporous films [1][2][3] represent one of the earliest and best-known examples for an almost ideal self-organizing electrochemical process. 1 More recently, by anodization of titanium in fluoride-containing electrolytes, it is possible to form a similar morphology, that is, self-aligned nanotube arrays. Some elements, upon anodization in aqueous fluoride electrolyte, tend to form nanotubular structures such as Ti, Zr, and Hf, whereas others, e.g., Ta, Nb, and W, tend to form nanoporous structures. Only very recently, first investigations were reported on the transition of the morphology from self-organized nanopore to nanotube structures depending on the specific anodization conditions. 12,17,23 In the present work, we investigate self-organizing anodization with an alloy that contains a typical tube formation element Ti and a typical pore formation element Ta. While in aqueous electrolytes, such an alloy forms only tubular structures, 22 we demonstrate in the present work that an organic electrolyte can induce transition to a porous structure with a virtually unprecedented degree of short-and longrange orders.Except for scientific interest in the feasibility of ordered oxide formation on these materials, Ti-Ta oxides are also of high technological significance. The oxide layers provide a very high corrosion resistance to surfaces, as Ta 2 O 5 is significantly more resistant than TiO 2 in almost any environment but particularly under cathodic reductive conditions. 24,25 Moreover, the alloy provides excellent biocompatibility but with a considerably lower average weight than pure Ta. Therefore, various Ti-Ta alloys are explored as candidates for biomedical implant materials. 26,27 In this context, the decoration of Ti implants with TiO 2 nanotubes has already shown a beneficial nature in view of cell response, 28,29 hydroxyapatite formation, 30 and for applications in drug delivery systems. 31,32 It is hence expe...