Anodic TiO2 nanotube layers: Why does self-organized growth occur—A mini review

Abstract

The present review gives an overview of the highlights of more than 10 years of research on synthesis and applications of ordered oxide structures (nanotube layers, hexagonal pore arrangements) that are formed by self-organizing anodization of metals. In particular we address the questions after the critical factors that lead to the spectacular self-ordering during the growth of anodic oxides that finally yield morphologies such as highly ordered TiO 2 nanotube arrays and similar structures. Why are tubes and pores formed -what are the key parameters controlling these processes?Link to the published article: http://dx.doi.org/10.1016/j.elecom.2014.06.021 1 Over the past decade, the formation and application of anodic, self-organized TiO 2 nanotube arrays grown from a Ti metal (as illustrated in Fig.1) and similar oxide structures have attracted wide scientific and technological interest [1][2][3][4][5][6][7]. Self-organization during anodization has been observed already more than 70 years ago for aluminum that, when anodized in acidic electrolytes, forms hexagonally ordered porous structures [8] -such ordering can peak in a virtually perfect arrangement, as demonstrated by Masuda et al. in 1995 [9], by a meticulous optimization of the electrochemical growth conditions. These alumina structures since then have been widely used for templating (i.e., to fill the pores by a secondary material to form nanorods or nanowires, in combination with stripping the template or not), as well as for numerous other applications -excellent overviews on growth and applications of porous alumina are available; see e.g. refs. [2][3][4]10,11].In the past few years, however, research activities on ordered TiO 2 nanotube layers have in numbers of paper-output surpassed porous alumina; in the past decade, more than three thousand papers have been published dedicated to anodic TiO 2 nanotubes [1,[5][6][7]. The main reason for this enormous interest is the anticipated impact of such nanotube layers in functional applications of titanium dioxide; such as dye sensitized solar cells [12][13][14][15], photocatalysis [16,17] (including water splitting for the generation of hydrogen, pollution degradation, or the reduction of CO 2 ), biomedicine [18][19][20] (biomedical coatings of implants, drug delivery systems), ion-insertion batteries, electrochromics, etc. [21][22][23][24] These applications are, to a large extent, based on a number of almost unique features of TiO 2 [1,[25][26][27][28]: it is a semiconductor of a band-gap of 3.0 eV (rutile) -3.2 eV (anatase), with a considerably large electron diffusion length (mainly anatase), relative band-edge positions suitable to trigger a wide range of photocatalytic reactions; the material is highly biocompatible, and shows considerably good ion intercalation properties. Many of these features can be exploited in a nanotubular form even more beneficially than in powder assemblies (e.g. directional charge transport, orthogonal carrier separation, optimized and directional diffusion profile...

On the Supercapacitive Behaviour of Anodic Porous WO3-Based Negative Electrodes

“…The resulting i-t behaviour resembles other anodically grown self-organized 1D oxide structures (a most common example is TiO 2 nanotube arrays) [45,51]. Here in the first stage of the anodization the W metal substrate develops an initial compact oxide film at the metal/electrolyte interface.…”

“…1a-c show SEM images of typical anodic WO 3 nanochannel layers grown in molten o-H 3 PO 4 . The use of this electrolyte is a key factor for the growth of ordered WO 3 nanochannels as it provides an ideal equilibrium between field-assisted passivation of W metal and oxide dissolution [38,[43][44][45], thus establishing the adequate electrochemical conditions for the growth of defined nanostructures. While rapid oxide dissolution that limits the growth of the anodic structure is reported to take place in most common electrolytes [46][47][48], (nominally) pure hot o-H 3 PO 4 provides an environment with limited water content and with phosphate ions that protect anodic tungsten oxide layer from rapid dissolution [42].…”

Самоорганизация Биоактивного Наноструктурированного Оксидного Слоя На Поверхности Спеченного Порошка Губчатого Титана При Электрохимическом Анодировании

“…Особенностям роста анодных оксидных пленок (АОП) во фторсодержащих электролитах на поверхности ком-пактного титана посвящен целый ряд обзоров [7,[11][12][13][14]. Известно, что при анодировании титана во фторсодер-жащих электролитах формируются самоорганизованные нанотрубчатые оксидные пленки с диаметром трубок и толщиной, задаваемыми условиями процесса.…”

“…Принято считать [7,[11][12][13][14], что причиной формиро-вания при анодировании самоорганизованных оксидных пленок (как с пористой, так и с трубчатой мезострукту-рой) является синергия процессов роста и растворения оксидной пленки, реализующаяся под действием элек-трического поля. Согласно [7,13,14], преобразование пор в нанотрубки при анодировании титана во фторсодер-жащих электролитах происходит из-за внедрения ионов фтора в тело оксидной пленки во время роста АОП. Однако на основе выявленного факта присутствия обла-стей АОП как с пористой, так и с трубчатой мезострук-турой, можно предположить, что на формирование са-моорганизованных АОП влияет также и неоднородность рельефа анодируемой поверхности, в данном случае -частиц порошка губчатого титана.…”

“…толщины формируемого пористого/трубчатого оксидно-го слоя. Ее значение находится в диапазоне от 250 до 350 nm, что соответствует приводимым в литературе данным о толщине АОП, полученных анодированием титановой фольги в аналогичных условиях [7,[11][12][13][14]. Рентгенографирование показало, что на дифракто-граммах анодированных образцов ППМ губчатого ти-тана присутствуют лишь брэгговские пики титановой подложки, дополнительных дифракционных максимумов не выявляется [20].…”