แปลบทความวิจัยRapid anodic growth o

แปลบทความวิจัยRapid anodic growth of TiO2 and WO3 nanotubes in fluoride free electrolytes

R. Hahn, J.M. Macak, P. Schmuki,
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doi:10.1016/j.elecom.2006.11.037
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Abstract
In the present work we report on the formation of bundles of high aspect ratio TiO2 nanotubes and WO3 nanopores structures with very thin tube or pore walls using anodization under “high voltage” conditions in perchlorate or chloride containing electrolytes. The bundles of TiO2 nanotubes consist of separated tubes with diameters in the range of approximately 20–40 nm and the WO3 nanopores consist of pores with diameters in the range of 30–50 nm. Growth occurs locally at specific surface locations. Both the TiO2 and the WO3 structures can be grown up to several dozens of micrometers in length within few minutes. We suggest that the growth of these high aspect structures is initiated by localized anodic breakdown event, triggered by a sufficiently high applied anodic field.

Keywords
Titanium dioxide; Tungsten trioxide; Nanotubes; Nanopores; Anodization
1. Introduction
TiO2 and WO3 are technologically very important semiconductive materials that provide a broad range of specific properties. These make the materials applicable in photocatalysis [1], solar cells [2], photolysis [3], sensing [4], and electrochromic devices [5] and [6]. As these materials become very important in nanotechnology for making very small functional devices with different purposes, it is of high scientific and technological interest to find different strategies, which allow production of nanostructures of these materials in a cheap, tunable and easily controllable manner. Classical approaches to produce for instance nanoporous or nanoparticulate TiO2 layers include typically sol–gel or hydrothermal processes using alkoxides as a starting material [7].

Recently, self-organized TiO2 nanotubes could be grown on Ti [8] using a relatively simple electrochemical approach, that is, anodization in an acidic electrolyte containing fluorides. Later, it was shown that fluoride containing electrolytes could be used to grow tubular or porous oxides also on other valve metals such as Nb, Ta, Hf, Zr, and W [9], [10], [11], [12] and [13]. In all these works, fluoride anions were used to establish conditions that mildly dissolve the anodic oxides while the anodic bias permanently provides new oxide growth. After establishing a steady state between oxide formation and dissolution, an equilibrium situation can be achieved leading to nanotubular or nanoporous oxide layers. This process usually takes up to several hours. For instance, in case of Ti, it has been shown that the growth of nanotubes with different diameters and lengths up to aspect ratios of ∼2000 can be achieved [14], [15], [16], [17], [18] and [19].

Several important applications have already been found for these structures, such as high photoelectrochemical performance under UV [20], [21] and [22] and visible light illumination [23], [24] and [25], hydrogen sensing [26], catalysis [27], [28] and [29], wettability control [30], electrochromism [31], and biological applications [32], [33] and [34]. Recently, Masuda and coworkers presented a striking alternative approach showing first experimental findings [35] that using electrolytes containing perchlorate anions and using a set of specific anodization conditions, it is possible to form bundles of high aspect ratio TiO2 nanotubes on Ti under very rapid growth conditions. These structures are intended to be used in dye-senstitized solar cells. In the present work, we investigate the anodization of Ti and W substrates in aqueous electrolytes with perchlorates or chlorides additions, to explore the general feasibility of this principle and to gain some insight into the growth mechanism of this novel growth approach.

2. Experimental part
Titanium and tungsten foils (0.1 mm, 99.6% purity, Advent Materials) were prior to electrochemical experiments degreased by sonication in acetone, isopropanol and methanol, afterwards rinsed with deionized (DI) water and finally dried in nitrogen stream. The samples were pressed together with a Cu-plate contact against an O-ring in an electrochemical cell (1 cm2 exposed to the electrolyte) and anodized at different potentials in the range of 10–100 V in aqueous electrolytes containing HClO4 and NaClO4 (0.01; 0.05; 0.1, 1 M). Anodization was carried out by stepping the potential to the desired value and holding it at the final value for a given time (typically several minutes). For the electrochemical experiments, a high-voltage potentiostat Jaissle IMP 88 and a conventional three-electrode configuration with a platinum gauze as a counter electrode and the Haber–Luggin capillary with Ag/AgCl (1 M KCl) as a reference electrode were used. All electrolytes were prepared from reagent grade chemicals. Some experiments were conducted at lower temperatures using a Lauda RM6 thermostat with a cooling coil, which was directly immersed in the electrolyte solution. A scanning electron microscope Hitachi FE-SEM S4800 and a transmission electron microscope Phillips CM 30 T/STEM were employed for the morphological and structural characterization of the formed layers. Energy dispersive X-ray analyser (EDX) fitted to the SEM chamber was used for determining the composition.

3. Results and discussion
After some preliminary anodization experiments it was clear that using potential steps in perchlorate or chloride containing electrolytes, passivity breakdown conditions could be established – the latter being in line with extended work on “pitting corrosion” on Ti, see e.g., Ref. [36]. The result is that specific spots on the electrode surface become activated and very high current densities are observed. When stopping this process, after a few minutes, one can see (by eye) several white spots on the sample surface. Using a FE-SEM and zooming in on these locations, one can clearly observe nanotubular morphologies as shown in Fig. 1. Fig. 1a and b shows SEM images of bundles of closely packed TiO2 nanotubes prepared in View the MathML source and Cl− solutions. The tubes have an average diameter of 40 nm, a length of 30 μm, and a wall thickness of about 10 nm. Fig. 1c and d shows SEM images of nanoporous WO3 prepared in View the MathML source and Cl− containing electrolytes. In this case, bundles of WO3 nanopore structures show an average pore diameter of 40 nm, and a structure length of 16 μm.

SEM images of (a) TiO2 nanotubes prepared in 0.1M HClO4 at 30V for 60s in the ...
SEM images of (a) TiO2 nanotubes prepared in 0.1M HClO4 at 30V for 60s in the ...
Fig. 1.
SEM images of (a) TiO2 nanotubes prepared in 0.1 M HClO4 at 30 V for 60 s in the cross-sectional view; (b) WO3 nanopores prepared in 0.1 M HClO4 at 50 V for 60 s in the cross-sectional view; (c) TiO2 nanotubes prepared in 0.3 M NaCl solution (buffered pH 4) at 40 V for 60 s in the cross-sectional view; (d) WO3 nanopores prepared in 0.3 M NaCl (buffered pH 4) at 50 V for 60 s in the cross-sectional view.
Figure options
Based on our direct observations and as confirmed by a time sequence of experiments, the tube growth in every case starts randomly on certain points on the surface, and the amount of these tubular bundles increases with anodization time until the whole surface is covered. In order to form these nanostructured materials, sufficiently “harsh” anodization conditions must be established to cause breakdown events during the experiments. Specifically the electrolyte composition, temperature and applied potential must be such that the anodized metals undergo localized breakdown. For breakdown to occur in the case of TiO2 (or WO3) in chloride containing electrolytes, typically potentials of several 10 V must be applied [37], [38] and [39]. Further, it is very important how these potentials are applied, either by sweeping or by stepping the voltage. This fact influences the formed oxide layers in terms of its density, porosity, and defects [40]. When we applied the potential by sweeping (“mild anodization”) to relatively high potentials (up to 80 V) only the formation of compact TiO2 and WO3 layers with thicknesses proportional to the applied potentials could be observed. However, when the potential is stepped, a completely different situation occurs, i.e., breakdown events take place (as a result of the higher applied field strength) and as a side-effect, formation of the nanostructures takes place. This very different behaviour is demonstrated in Fig. 2a. In one case, it shows the current–time dependence recorded for Ti sample anodized in 0.05 M HClO4 after applying the 30 V in one step and in the other case, after sweeping the potential to 30 V with 1 V/s. It is apparent that anodization occurs under very different current flow (the currents in the case of step anodization are more than 10 × higher) considering the localized nature of the events. The local dissolution currents are anticipated to be several decades higher (as expected for “pitting” corrosion [36]).

(a) Current transients of Ti sample anodized in 0.05M HClO4 at 30V final ...
Fig. 2.
(a) Current transients of Ti sample anodized in 0.05 M HClO4 at 30 V final potential recorded after a potential step and a potential sweep (with 1 V/s); (b) current transients of W sample anodized in 0.1 M HClO4 at 50 V final potential after a potential step and a potential sweep (1 V/s); (c) current transients of Ti sample recorded after 20 V potential step in aqueous solutions containing different HClO4 concentrations. Insets are typically SEM top views of the surface acquired under these conditions.
Figure options
For Ti as a substrate, the formation of TiO2 nanotubes in View the MathML source containing solution is possible over a broad range of the experimental conditions. Bundles of nanotubes can be observed between applied potentials of 15 and 60 V, in the View the MathML source

ผลลัพธ์ (อังกฤษ) 1:

Translate research and Rapid growth of anodic WO3 TiO2 nanotubes in fluoride free electrolytes.R. Hahn, J.M. Macak, P. Schmuki, Show moredoi:10.1016/j.elecom.2006.11.037Get rights and contentAbstractIn the present work we report on the formation of bundles of high aspect ratio TiO2 nanotubes and WO3 nanopores structures with very thin tube or pore walls using anodization under "high voltage" conditions in perchlorate or chloride containing electrolytes. The bundles of TiO2 nanotubes consist of separated tubes with diameters in the range of approximately 20–40 nm and the WO3 nanopores consist of pores with diameters in the range of 30–50 nm. Growth occurs locally at specific surface locations. Both the TiO2 and the WO3 structures can be grown up to several dozens of micrometers in length within few minutes. We suggest that the growth of these high aspect structures is initiated by localized anodic breakdown event, triggered by a sufficiently high applied anodic field.KeywordsTitanium dioxide; Tungsten trioxide; Nanotubes; Nanopores; Anodization1. IntroductionTiO2 and WO3 are technologically very important semiconductive materials that provide a broad range of specific properties. These make the materials applicable in photocatalysis [1], solar cells [2], photolysis [3], sensing [4], and electrochromic devices [5] and [6]. As these materials become very important in nanotechnology for making very small functional devices with different purposes, it is of high scientific and technological interest to find different strategies, which allow production of nanostructures of these materials in a cheap, tunable and easily controllable manner. Classical approaches to produce for instance nanoporous or nanoparticulate TiO2 layers include typically sol–gel or hydrothermal processes using alkoxides as a starting material [7].Recently, self-organized TiO2 nanotubes could be grown on Ti [8] using a relatively simple electrochemical approach, that is, anodization in an acidic electrolyte containing fluorides. Later, it was shown that fluoride containing electrolytes could be used to grow tubular or porous oxides also on other valve metals such as Nb, Ta, Hf, Zr, and W [9], [10], [11], [12] and [13]. In all these works, fluoride anions were used to establish conditions that mildly dissolve the anodic oxides while the anodic bias permanently provides new oxide growth. After establishing a steady state between oxide formation and dissolution, an equilibrium situation can be achieved leading to nanotubular or nanoporous oxide layers. This process usually takes up to several hours. For instance, in case of Ti, it has been shown that the growth of nanotubes with different diameters and lengths up to aspect ratios of ∼2000 can be achieved [14], [15], [16], [17], [18] and [19].Several important applications have already been found for these structures, such as high photoelectrochemical performance under UV [20], [21] and [22] and visible light illumination [23], [24] and [25], hydrogen sensing [26], catalysis [27], [28] and [29], wettability control [30], electrochromism [31], and biological applications [32], [33] and [34]. Recently, Masuda and coworkers presented a striking alternative approach showing first experimental findings [35] that using electrolytes containing perchlorate anions and using a set of specific anodization conditions, it is possible to form bundles of high aspect ratio TiO2 nanotubes on Ti under very rapid growth conditions. These structures are intended to be used in dye-senstitized solar cells. In the present work, we investigate the anodization of Ti and W substrates in aqueous electrolytes with perchlorates or chlorides additions, to explore the general feasibility of this principle and to gain some insight into the growth mechanism of this novel growth approach.2. Experimental partTitanium and tungsten foils (0.1 mm, 99.6% purity, Advent Materials) were prior to electrochemical experiments degreased by sonication in acetone, isopropanol and methanol, afterwards rinsed with deionized (DI) water and finally dried in nitrogen stream. The samples were pressed together with a Cu-plate contact against an O-ring in an electrochemical cell (1 cm2 exposed to the electrolyte) and anodized at different potentials in the range of 10–100 V in aqueous electrolytes containing HClO4 and NaClO4 (0.01; 0.05; 0.1, 1 M). Anodization was carried out by stepping the potential to the desired value and holding it at the final value for a given time (typically several minutes). For the electrochemical experiments, a high-voltage potentiostat Jaissle IMP 88 and a conventional three-electrode configuration with a platinum gauze as a counter electrode and the Haber–Luggin capillary with Ag/AgCl (1 M KCl) as a reference electrode were used. All electrolytes were prepared from reagent grade chemicals. Some experiments were conducted at lower temperatures using a Lauda RM6 thermostat with a cooling coil, which was directly immersed in the electrolyte solution. A scanning electron microscope Hitachi FE-SEM S4800 and a transmission electron microscope Phillips CM 30 T/STEM were employed for the morphological and structural characterization of the formed layers. Energy dispersive X-ray analyser (EDX) fitted to the SEM chamber was used for determining the composition.3. Results and discussionAfter some preliminary anodization experiments it was clear that using potential steps in perchlorate or chloride containing electrolytes, passivity breakdown conditions could be established – the latter being in line with extended work on "pitting corrosion" on Ti, see e.g., Ref. [36]. The result is that specific spots on the electrode surface become activated and very high current densities are observed. When stopping this process, after a few minutes, one can see (by eye) several white spots on the sample surface. Using a FE-SEM and zooming in on these locations, one can clearly observe nanotubular morphologies as shown in Fig. 1. Fig. 1a and b shows SEM images of bundles of closely packed TiO2 nanotubes prepared in View the MathML source and Cl− solutions. The tubes have an average diameter of 40 nm, a length of 30 μm, and a wall thickness of about 10 nm. Fig. 1c and d shows SEM images of nanoporous WO3 prepared in View the MathML source and Cl− containing electrolytes. In this case, bundles of WO3 nanopore structures show an average pore diameter of 40 nm, and a structure length of 16 μm.SEM images of (a) TiO2 nanotubes prepared in 0.1M HClO4 at 30V for 60s in the ...SEM images of (a) TiO2 nanotubes prepared in 0.1M HClO4 at 30V for 60s in the ...Fig. 1. SEM images of (a) TiO2 nanotubes prepared in 0.1 M HClO4 at 30 V for 60 s in the cross-sectional view; (b) WO3 nanopores prepared in 0.1 M HClO4 at 50 V for 60 s in the cross-sectional view; (c) TiO2 nanotubes prepared in 0.3 M NaCl solution (buffered pH 4) at 40 V for 60 s in the cross-sectional view; (d) WO3 nanopores prepared in 0.3 M NaCl (buffered pH 4) at 50 V for 60 s in the cross-sectional view.Figure optionsBased on our direct observations and as confirmed by a time sequence of experiments, the tube growth in every case starts randomly on certain points on the surface, and the amount of these tubular bundles increases with anodization time until the whole surface is covered. In order to form these nanostructured materials, sufficiently "harsh" anodization conditions must be established to cause breakdown events during the experiments. Specifically the electrolyte composition, temperature and applied potential must be such that the anodized metals undergo localized breakdown. For breakdown to occur in the case of TiO2 (or WO3) in chloride containing electrolytes, typically potentials of several 10 V must be applied [37], [38] and [39]. Further, it is very important how these potentials are applied, either by sweeping or by stepping the voltage. This fact influences the formed oxide layers in terms of its density, porosity, and defects [40]. When we applied the potential by sweeping ("mild anodization") to relatively high potentials (up to 80 V) only the formation of compact TiO2 and WO3 layers with thicknesses proportional to the applied potentials could be observed. However, when the potential is stepped, a completely different situation occurs, i.e., breakdown events take place (as a result of the higher applied field strength) and as a side-effect, formation of the nanostructures takes place. This very different behaviour is demonstrated in Fig. 2a. In one case, it shows the current–time dependence recorded for Ti sample anodized in 0.05 M HClO4 after applying the 30 V in one step and in the other case, after sweeping the potential to 30 V with 1 V/s. It is apparent that anodization occurs under very different current flow (the currents in the case of step anodization are more than 10 × higher) considering the localized nature of the events. The local dissolution currents are anticipated to be several decades higher (as expected for "pitting" corrosion [36]).(a) Current transients of Ti sample anodized in 0.05M HClO4 at 30V final ...Fig. 2. (a) Current transients of Ti sample anodized in 0.05 M HClO4 at 30 V final potential recorded after a potential step and a potential sweep (with 1 V/s); (b) current transients of W sample anodized in 0.1 M HClO4 at 50 V final potential after a potential step and a potential sweep (1 V/s); (c) current transients of Ti sample recorded after 20 V potential step in aqueous solutions containing different HClO4 concentrations. Insets are typically SEM top views of the surface acquired under these conditions.Figure optionsFor Ti as a substrate, the formation of TiO2 nanotubes in View the MathML source containing solution is possible over a broad range of the experimental conditions. Bundles of nanotubes can be observed between applied potentials of 15 and 60 V, in the View the MathML source

ผลลัพธ์ (อังกฤษ) 2:

Microsoft Research Rapid growth of anodic TiO2 and WO3 nanotubes in free fluoride electrolytes R. Hahn, JM Macak, P. Schmuki, Show more Doi: 10.1016 / J.elecom.2006.11.037 Get rights and content Abstract In the present we Work Report. on the formation of bundles of high aspect ratio TiO2 nanotubes and WO3 nanopores structures with very thin tube or pore walls using anodization under "high voltage" conditions in perchlorate or chloride containing electrolytes. The bundles of TiO2 nanotubes consist of separated tubes with diameters in the. range of approximately 20-40 nm and the WO3 nanopores consist of pores with diameters in the range of 30-50 nm. Growth occurs locally at specific surface locations. Both the TiO2 and the WO3 structures can be grown up to several dozens of micrometers in. Length Within few minutes. We suggest that the growth of these Structures High Aspect is initiated by anodic breakdown localized event, triggered by a High Sufficiently Applied anodic field. Keywords Titanium dioxide; Tungsten trioxide; Nanotubes; Nanopores; anodization 1. Introduction TiO2 and WO3. are technologically very important semiconductive materials that provide a broad range of specific properties. These make the materials applicable in photocatalysis [1], solar cells [2], photolysis [3], sensing [4], and electrochromic devices [5] and [. 6]. As these materials become very important in nanotechnology for making very small functional devices with different purposes, it is of high scientific and technological interest to find different strategies, which allow production of nanostructures of these materials in a cheap, tunable and easily controllable. Manner. Classical approaches to Produce for instance nanoporous or nanoparticulate TiO2 layers include typically Sol-gel or hydrothermal processes using alkoxides as a Starting Material [7]. Recently, self-Organized TiO2 nanotubes could be Grown on Ti [8] using a relatively Simple. electrochemical approach, that is, anodization in an acidic electrolyte containing fluorides. Later, it was shown that fluoride containing electrolytes could be used to grow tubular or porous oxides also on other valve metals such as Nb, Ta, Hf, Zr, and W [. 9], [10], [11], [12] and [13]. In all these works, fluoride anions were used to establish conditions that mildly dissolve the anodic oxides while the anodic bias permanently provides new oxide growth. After establishing a. steady state between oxide formation and dissolution, an equilibrium situation can be achieved leading to nanotubular or nanoporous oxide layers. This process usually takes up to several hours. For instance, in case of Ti, it has been shown that the growth of nanotubes with different. Aspect ratios of diameters and lengths up to ~2000 Can be achieved [14], [15], [16], [17], [18] and [19]. Several have already been important Applications Found for these Structures, such as. high photoelectrochemical performance under UV [20], [21] and [22] and visible light illumination [23], [24] and [25], hydrogen sensing [26], catalysis [27], [28] and [29]. , wettability control [30], electrochromism [31], and biological applications [32], [33] and [34]. Recently, Masuda and coworkers presented a striking alternative approach showing first experimental findings [35] that using electrolytes containing perchlorate anions. and using a set of specific anodization conditions, it is possible to form bundles of high aspect ratio TiO2 nanotubes on Ti under very rapid growth conditions. These structures are intended to be used in dye-senstitized solar cells. In the present work, we investigate. the anodization of Ti and W substrates in aqueous electrolytes with perchlorates or chlorides Additions, to explore the feasibility of this general principle and to gain Some Insight Into the growth mechanism of this novel growth approach. 2. Experimental Part Titanium and Tungsten foils (0.1 mm. , 99.6% purity, Advent Materials) were prior to electrochemical experiments degreased by sonication in acetone, isopropanol and methanol, afterwards rinsed with deionized (DI) water and finally dried in nitrogen stream. The samples were pressed together with a Cu-plate contact against. an O-ring in an electrochemical cell (1 cm2 exposed to the electrolyte) and anodized at different potentials in the range of 10-100 V in aqueous electrolytes containing HClO4 and NaClO4 (0.01; 0.05; 0.1, 1 M). Anodization was carried. out by stepping the potential to the desired value and holding it at the final value for a given time (typically several minutes). For the electrochemical experiments, a high-voltage potentiostat Jaissle IMP 88 and a conventional three-electrode configuration with a platinum gauze. as a counter electrode and the Haber-Luggin capillary with Ag / AgCl (1 M KCl) as a reference electrode were used. All electrolytes were prepared from reagent grade chemicals. Some experiments were conducted at lower temperatures using a Lauda RM6 thermostat with a cooling. coil, which was directly immersed in the electrolyte solution. A scanning electron microscope Hitachi FE-SEM S4800 and a transmission electron microscope Phillips CM 30 T / STEM were employed for the morphological and structural characterization of the formed layers. Energy dispersive X-ray analyser. (EDX) fitted to the Chamber SEM was used for determining the composition. 3. Results and discussion After anodization Some Preliminary experiments it was Clear that using electrolytes containing chloride or perchlorate potential steps in, passivity breakdown conditions could be established - the latter being in. line with extended work on "pitting corrosion" on Ti, see eg, Ref. [36]. The result is that specific spots on the electrode surface become activated and very high current densities are observed. When stopping this process, after a few minutes. , one can see (by eye) several white spots on the sample surface. Using a FE-SEM and zooming in on these locations, one can clearly observe nanotubular morphologies as shown in Fig. 1. Fig. 1a and b shows SEM images of. bundles of closely packed TiO2 nanotubes prepared in View the MathML source and Cl- solutions. The tubes have an average diameter of 40 nm, a length of 30 μm, and a wall thickness of about 10 nm. Fig. 1c and d shows SEM images. nanoporous of WO3 prepared in View the MathML Source and Cl- containing electrolytes. In this Case, bundles of WO3 nanopore Diameter Pore Structures Show an average of 40 NM, and a Length of 16 Chemically Linked structure. SEM images of (a) TiO2 nanotubes prepared. in 0.1M HClO4 at 30V for 60s in the ... SEM images of (a) TiO2 nanotubes prepared in 0.1M HClO4 at 30V for 60s in the ... Fig. 1. SEM images of (a) TiO2 nanotubes prepared in 0.1. M HClO4 at 30 V for 60 s in the cross-sectional view; (b) WO3 nanopores prepared in 0.1 M HClO4 at 50 V for 60 s in the cross-sectional view; (c) TiO2 nanotubes prepared in 0.3 M NaCl solution (. buffered pH 4) at 40 V for 60 s in the cross-sectional View; (D) WO3 Nanopores prepared in 0.3 M NaCl (buffered pH 4) at 50 V for 60 s in the cross-sectional View. Figure options Based on our. direct observations and as confirmed by a time sequence of experiments, the tube growth in every case starts randomly on certain points on the surface, and the amount of these tubular bundles increases with anodization time until the whole surface is covered. In order to form these. nanostructured materials, sufficiently "harsh" anodization conditions must be established to cause breakdown events during the experiments. Specifically the electrolyte composition, temperature and applied potential must be such that the anodized metals undergo localized breakdown. For breakdown to occur in the case of TiO2 (. or WO3) in chloride containing electrolytes, typically potentials of several 10 V must be applied [37], [38] and [39]. Further, it is very important how these potentials are applied, either by sweeping or by stepping the voltage. This fact influences the formed oxide layers in terms of its density, porosity, and defects [40]. When we applied the potential by sweeping ("mild anodization") to relatively high potentials (up to 80 V) only the formation of compact TiO2. and WO3 layers with thicknesses proportional to the applied potentials could be observed. However, when the potential is stepped, a completely different situation occurs, ie, breakdown events take place (as a result of the higher applied field strength) and as a side-. effect, formation of the nanostructures takes place. This very different behaviour is demonstrated in Fig. 2a. In one case, it shows the current-time dependence recorded for Ti sample anodized in 0.05 M HClO4 after applying the 30 V in one step and in. the other case, after sweeping the potential to 30 V with 1 V / s. It is apparent that anodization occurs under very different current flow (the currents in the case of step anodization are more than 10 × higher) considering the localized nature of the. events. The local Dissolution Currents are anticipated to be several decades higher (as expected for "pitting" corrosion [36]). (a) Current transients of 30V at Ti sample anodized in 0.05M HClO4 Final ... Fig. 2. (. a) Current transients of Ti sample anodized in 0.05 M HClO4 at 30 V final potential recorded after a potential step and a potential sweep (with 1 V / s); (b) current transients of W sample anodized in 0.1 M HClO4 at 50 V. final potential after a potential step and a potential sweep (1 V / s); (c) current transients of Ti sample recorded after 20 V potential step in aqueous solutions containing different HClO4 concentrations. Insets are typically SEM top views of the surface acquired under. these conditions. Figure options For Ti as a substrate, the Formation of TiO2 nanotubes in View the MathML Source containing Solution is possible over a Broad Range of the Experimental conditions. Bundles of nanotubes Can be observed between Applied potentials of 15 and 60 V, in. the View the MathML source

ผลลัพธ์ (อังกฤษ) 3:

Translation research articles Rapid anodic growth of TiO2 and WO3 nanotubes in fluoride free electrolytes

R. Hahn J.M. Macak, P. Schmuki,,,

Show more doi: 10.1016 / j.elecom.2006.11.037
Get rights and Abstract content

.In the present work we report on the formation of bundles of high aspect ratio TiO2 nanotubes and WO3 nanopores structures. With very thin tube or pore walls using anodization under "high voltage." conditions in perchlorate or chloride containing. Electrolytes.The bundles of TiO2 nanotubes consist of separated tubes with diameters in the range of approximately 20 - 40 nm and the. WO3 nanopores consist of pores with diameters in the range of 30 - 50 nm. Growth occurs locally at specific surface, locations. Both the TiO2 and the WO3 structures can be grown up to several dozens of micrometers in length within few minutes.We suggest that the growth of these high aspect structures is initiated by localized anodic, breakdown event triggered. By a sufficiently high applied anodic field.


Titanium Keywords dioxide; Tungsten trioxide; Nanotubes; Nanopores; Anodization
1.? Introduction
TiO2 and WO3 are technologically very important semiconductive materials that provide a broad range of specific. Properties.These make the materials applicable in photocatalysis [1], solar cells [2], [], [photolysis 3 sensing 4], and electrochromic. Devices [] []. 5 and 6 As these materials become very important in nanotechnology for making very small functional devices. With different purposes it is, of high scientific and technological interest to find, different strategiesWhich allow production of nanostructures of these materials in a cheap tunable and, easily controllable manner. Classical. Approaches to produce for instance nanoporous or nanoparticulate TiO2 layers include typically sol - gel or hydrothermal processes. Using alkoxides as a starting material [7].

, RecentlySelf-organized TiO2 nanotubes could be grown on Ti [] using 8 a relatively simple electrochemical approach that is anodization,,, In an acidic electrolyte containing fluorides. Later it was, shown that fluoride containing electrolytes could be used to. Grow tubular or porous oxides also on other valve metals such as Nb Ta Hf Zr,,,, 9 and W [], [], [], 10 11 [] []. 12 and 13 In. All, these worksFluoride anions were used to establish conditions that mildly dissolve the anodic oxides while the anodic bias permanently. Provides new oxide growth. After establishing a steady state between oxide formation and dissolution an equilibrium, situation. Can be achieved leading to nanotubular or nanoporous oxide layers. This process usually takes up to several hours. For, instance. In case, of TiIt has been shown that the growth of nanotubes with different diameters and lengths up to aspect ratios of ∼ 2000 can be. 14 achieved [], [], [15 16], [], [] and 17 18 [19].

Several important applications have already been found for, these structures. Such as high Photoelectrochemical Performance under 20 UV [], [] [] and 21 and 22 visible light 23 illumination [], [], 24 and [], [25 hydrogen sensing 26],27 catalysis [], [], [] 28 and 29 wettability control [30], [], electrochromism 31 and biological 32 applications [], [], 33 and []. Recently 34, and Masuda coworkers presented a striking alternative approach showing first experimental 35 findings []. That using electrolytes containing perchlorate anions and using a set of specific, anodization conditionsIt is possible to form bundles of high aspect ratio TiO2 nanotubes on Ti under very rapid growth conditions. These structures. Are intended to be used in dye-senstitized solar cells. In the, present work we investigate the anodization of Ti and W. Substrates in aqueous electrolytes with perchlorates or, chlorides additionsTo explore the general feasibility of this principle and to gain some insight into the growth mechanism of this novel growth. Approach.

2. Experimental part
Titanium and tungsten foils (0.1 mm 99.6% purity, Advent Materials), were prior to electrochemical. Experiments degreased by sonication in acetone isopropanol and methanol,,Afterwards rinsed with deionized (DI) water and finally dried in nitrogen stream. The samples were pressed together with. A Cu-plate contact against an O-ring in an electrochemical cell (1 cm2 exposed to the electrolyte) and anodized at different. Potentials in the range of 10 - 100 V in aqueous electrolytes containing HClO4 and NaClO4 (0.01; 0.05; 0.1 1, M).Anodization was carried out by stepping the potential to the desired value and holding it at the final value for a given. Time (typically several minutes). For the, electrochemical experimentsA high-voltage potentiostat Jaissle IMP 88 and a conventional three-electrode configuration with a platinum gauze as a. Counter electrode and the Haber - Luggin capillary with Ag / AgCl (1 M KCl) as a reference electrode were used. All electrolytes. Were prepared from reagent grade chemicals. Some experiments were conducted at lower temperatures using a Lauda RM6 thermostat. With a, cooling coilWhich was directly immersed in the electrolyte solution. A scanning electron microscope Hitachi FE-SEM S4800 and a transmission. Electron microscope Phillips CM 30 T / STEM were employed for the morphological and structural characterization of the formed. Layers. Energy dispersive X-ray analyser (EDX) fitted to the SEM chamber was used for determining the composition.

3. Results. And discussion
.After some preliminary anodization experiments it was clear that using potential steps in perchlorate or chloride containing. Electrolytes passivity breakdown, conditions could be established - the latter being in line with extended work on pitting. " Corrosion ", on Ti see e.g, Ref 36. [].The result is that specific spots on the electrode surface become activated and very high current densities are, observed. When stopping this process after a, few minutes one can, see (by eye) several white spots on the sample surface. Using a. FE-SEM and zooming in on these locations one can, clearly observe nanotubular morphologies as shown in Fig. 1. Fig.1A and B shows SEM images of bundles of closely packed TiO2 nanotubes prepared in View the MathML source and Cl − solutions.? The tubes have an average diameter of, 40 nm a length of 30 thermal m and a, wall thickness of about 10 nm. Fig. 1c and D shows. SEM images of nanoporous WO3 prepared in View the MathML source and Cl − containing electrolytes In, this case.Bundles of WO3 nanopore structures show an average pore diameter of, 40 nm and a structure length of 16 thermal m.

SEM images. Of (a) TiO2 nanotubes prepared in 0.1M HClO4 at 30V for 60s in the...
SEM images of (a) TiO2 nanotubes prepared in 0.1M. HClO4 at 30V for 60s in the...
Fig. 1.
SEM images of (a) TiO2 nanotubes prepared in 0.1 M HClO4 at 30 V for 60 s in the. Cross-sectional view;(b) WO3 nanopores prepared in 0.1 M HClO4 at 50 V for 60 s in the cross-sectional view; (c) TiO2 nanotubes prepared in 0.3 M. NaCl solution (buffered pH 4) at 40 V for 60 s in the cross-sectional view; (d) WO3 nanopores prepared in 0.3 M NaCl (buffered. PH 4) at 50 V for 60 s in the cross-sectional Figure options view.

.Based on our direct observations and as confirmed by a time sequence, of experiments the tube growth in every case starts. Randomly on certain points on the surface and the, amount of these tubular bundles increases with anodization time until. The whole surface is covered. In order to form these, nanostructured materialsSufficiently "harsh." anodization conditions must be established to cause breakdown events during the experiments. Specifically. The electrolyte composition temperature and, applied potential must be such that the anodized metals undergo localized, breakdown. For breakdown to occur in the case of TiO2 (or WO3) in chloride, containing electrolytesTypically potentials of several 10 V must be 37 applied [], [] []. 38 and 39 Further it is, very important how these potentials. Are applied either by, sweeping or by stepping the voltage. This fact influences the formed oxide layers in terms of its. ,, density porosity and 40 defects [].When we applied the potential by sweeping ("mild anodization.") to relatively high potentials (up to 80 V) only the formation. Of compact TiO2 and WO3 layers with thicknesses proportional to the applied potentials could be observed. However when,, The potential is stepped a completely, different situation occurs i.e,,.Breakdown events take place (as a result of the higher applied field strength) and as a side - effect formation of, the nanostructures. Takes place. This very different behaviour is demonstrated in Fig. 2A. In one case it shows, the current - time dependence. Recorded for Ti sample anodized in 0.05 M HClO4 after applying the 30 V in one step and in the, other caseAfter sweeping the potential to 30 V with 1 V / S. It is apparent that anodization occurs under very different current flow. (the currents in the case of step anodization are more than 10 × higher) considering the localized nature of the, events. The local dissolution currents are anticipated to be several decades higher (as expected for "pitting corrosion [])." 36

.(a) Current transients of Ti sample anodized in 0.05M HClO4 at 30V final...
Fig. 2.
(a) Current transients of Ti sample. Anodized in 0.05 M HClO4 at 30 V final potential recorded after a potential step and a potential sweep (with 1 V / s); (b). Current transients of W sample anodized in 0.1 M HClO4 at 50 V final potential after a potential step and a potential sweep. (1 V / s);(c) current transients of Ti sample recorded after 20 V potential step in aqueous solutions containing different HClO4. Concentrations. Insets are typically SEM top views of the surface acquired under these conditions.

Figure options For Ti. As a substrate the formation, of TiO2 nanotubes in View the MathML source containing solution is possible over a broad range. Of the experimental conditions.Bundles of nanotubes can be observed between applied potentials of 15 and 60 V in the, View the MathML source.