1. บทนำOver the past 20 years, self-organizing electrochemistry has been used extensively to produce highly ordered nanoscale oxide structures such as aligned nanopores [1], [2], [3], [4], [5] and [6]/nanotubes [7] and [8] or nanochannels [9]. Such structures are usually produced by anodization of a metal under suitable electrochemical conditions. The prototype of the best ordered oxide structure is porous alumina [4], but over the past decade it has been shown that also for other metals (mainly so-called valve metals), hexagonally-aligned nanoporous and nanotubular morphologies can be grown, if an anodization treatment is carried out in dilute fluoride containing electrolytes under the “right” conditions [7] and [8]. It is interesting that some materials such as Nb [10], Ta [11] and [12], or Al [4] and [6] tend to form porous oxide morphologies while others (Zr [13], Hf [14], and Ti [7] and [8]) tend to form tubular structures. The most prominent example for the formation of a tubular morphology is titanium, where self-organized oxide nanotube layers, over the past few years, have attracted considerable interest due to high expectations for applications in biomedical [15], photocatalytic [16], or energy harvesting devices [17]. Investigations on why for some electrolytes a tubular morphology is formed while for others a porous morphology is preferred have only recently been initiated [18] and [19]. Most relevant studies were performed on TiAl [20] (i.e., an alloy consisting of the prototype of a tube forming material, Ti, and the prototype of a pore forming material, Al). For these alloys a transition from nanotubular to nanoporous structure could be observed depending on the alloy composition. More recently, a tube to pore transition could be observed for some transition metals (Zr [19], Hf [21]) if the water content in the organic fluoride electrolyte was lower than approx. 3–5%.For Ti, nevertheless, even if the water content in the electrolyte was kept below 1% always a tubular morphology was obtained, and up to now, any attempt to create ordered nanoporous structures failed [7], [8], [18] and [22]. In the present work, we however demonstrate that if anodization is carried out in a dry glove box which allows to control even very low water contents in the electrolyte and in the surrounding atmosphere, a transition from a nanotubular to an ordered nanoporous morphology structures can be obtained.2. ExperimentalFor anodization experiments we used titanium foils (0.1 mm thickness, 99.9% purity, Goodfellow). They were degreased by sonicating in acetone and methanol, were then rinsed with deionized water (DI) and dried in a nitrogen stream. All experiments were performed in a dry glove box (MBraun inert-gas glove box containing an atmosphere of purified N2). The moisture and oxygen level in the glove box were monitored and maintained less than 0.6 ppm. The used electrolytes were prepared from highly anhydrous ethylene glycol (99.8% purity, < 0.003% water content, Sigma-Aldrich) with different amounts of NH4F (0.025 M, 0.05 M, 0.1 M, and 0.2 M, Sigma-Aldrich), and different amounts of DI water additions (0, 0.3%, 0.5%, 0.7%, vol.).A two-electrode system was used in all anodization experiments with a platinum flag as the counter electrode and the Ti foil as the working electrode. Anodization was carried out with a DC power source at room temperature, by sweeping the potential from 0 to 20 V with a rate of 100 mV/s, followed by holding the potential at 20 V for 4 h. After preparation, the samples were rinsed with acetone, taken out of the glove box, and then dried in air.
The structure and morphology of the obtained films were characterized using a field-emission scanning microscope (Hitachi FE-SEM S4800). Cross-sectional measurements were carried out on mechanically cracked samples. Characterization of the samples chemical composition was carried out using EDX (EDAX Genesis, fitted to the SEM chamber) and Auger Electron Spectroscopy (AES,Physical Electronics 670).
3. Results and discussion
Extensive preliminary anodization experiments with low water content ethylene glycol based fluoride electrolytes showed, that if experiments were performed outside a dry glove box, under all conditions formation of nanotubes took place. However, if experiments were performed in the controlled atmosphere of a glove box, under optimized formation conditions, an ordered nanoporous TiO2 layer could be formed as shown in Fig. 1. This layer was obtained in an ethylene glycol electrolyte containing 0.05 M NH4F and 0.5 vol.% H2O after 4 h-anodization of Ti at 20 V. From the SEM top-view image (Fig. 1a) and the cross-sectional images (Fig. 1b and c), it is apparent that a well-aligned nanochannel structure with the pore diameter of approx. 20 nm and a layer thickness of approx. 3.5 μm, has been formed. Fig. 1d shows current density–time behavior during the formation process of the layer. The decay of the current density is in line with the characteristics typically obtained during self-organizing growth of porous Al2O3 or TiO2 nanotubular layers [7], [8] and [23].
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