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Formation of a Titanium Dioxide Nanotube Array Patrick Hoyer† Department of Industrial Chemistry, Tokyo Metropolitan University, Minami Ohsawa 1-1, Hachioji-Shi, 192-03 Tokyo, Japan Received September 18, 1995. In Final Form: December 11, 1995X Starting from the naturally occurring structure of porous aluminum oxide, a polymer mold suitable for the formation of titanium dioxide nanotubes was obtained. The tubular structure was formed by electrochemical deposition in the mold. After dissolution of the polymer, titanium dioxide nanotubes were obtained and characterized. The “as deposited” TiO2 tubes were amorphous, but polycrystalline anatase samples of the same structure were obtained after heat treatment. The inner diameter of the tubes decreased from about 100 to 70 nm during the crystallization. A mechanism for the electrochemical deposition is proposed. Furthermore, the preparation procedure could also be applied to other semiconducting materials.
Introduction Recently, the preparation and properties of nanotubes of different materials have received considerable attention.1-3 Depending on the chemical properties of these materials, potential applications of these specific structures might be found in catalysts as carrier materials, in pharmacy as drug delivery agents, in nanoelectronics for the isolation of ultrasmall wires, or in basic research to study host-guest chemistry in mesoscopic materials. Although titanium dioxide is a material very widely used for such applications, there are, to the best of my knowledge, no procedures described for the preparation in the form of nanotubes of either this or most other semiconducting materials. Anodically grown aluminum oxide,4 the naturally occurring nanostructure of which consists of long and narrow holes, was used as the starting material for the production of the specifically tailored TiO2. The diameter of the holes can be varied between 10 and 200 nm by altering the experimental conditons, i.e., the anodizing voltage and subsequent etching treatments. In addition, the material has a very high aspect ratio (films up to a thickness of several 100 µm can be prepared). For these reasons, porous Al2O3 has frequently been used directly as a matrix for the synthesis of macroscopic materials in the form of wires.5-7 Although polypyrrole and gold nanotubes have been reported to be formed in several porous matrices due to specific adsorption of the electroactive species onto the wall prior to deposition,3,8 there have been no reports on the production of semiconductor tubes using this process. In the present study, in order to produce the desired material, a polymer negatype structure, which serves as the mold instead of the porous alumina, was prepared.9-11 Then, titanium dioxide was electrochemically deposited onto the polymer mold whereby the surface of the polymer was covered with the deposit. After dissolution of the † Present address: Schott Research Centre, Otto-Schott-Strasse 2, 55127 Mainz, Germany. X Abstract published in Advance ACS Abstracts, March 1, 1996.
(1) Yager, P.; Schoen, P. Mol. Cryst. Liq. Cryst. 1984, 106, 371. (2) Iijima, S. Nature 1991, 354, 56. (3) Martin, C. R. Science 1994, 228, 1961. (4) O’Sullivan, J. P.; Wood, G. C. Proc. R. Soc. London, Ser. A 1970, 317, 511. (5) Moskovits, M.; Dignam, M. J. J. Chem. Soc., Faraday Trans. 2 1973, 69, 95. (6) Kawai, S.; Ueda, R. J. Electrochem. Soc. 1975, 122, 32. (7) Foss, C. A.; Hornzak, G. L.; Stockert, J. A.; Martin, C. R. J. Phys. Chem. 1994, 96, 7497. (8) Brumlik, C. J.; Martin, C. R. J. Am. Chem. Soc. 1991, 113, 3174. (9) Masuda, H.; Nishio, K.; Baba, N. Thin Solid Films 1993, 223, 1. (10) Hoyer, P.; Baba, N.; Masuda, H. Appl. Phys. Lett. 1995, 66, 2700. (11) Masuda, H.; Fukuda, K. Science 1995, 268, 1466.
Figure 1. Schematic view of the replication process. The different materials are marked.
polymer, nanotubes of the semiconductor, with stable walls, can be observed by electron microscopy. The tubes prepared cross the whole thickness of the sample and, because both the openings are accessible, it is possible by electrodeposition of metals into the tubes to obtain isolated nanowires which might show interesting catalytic properties. Such work has been started and will be presented in a forthcoming publication. Furthermore, it has been shown that the process is widely applicable to other semiconductors which were observed to form similar tubular structures under certain deposition conditions. Experimental Procedure The preparation process is shown schematically in Figure 1. Anodically grown aluminum oxide was used as the starting material. The cleaned and electropolished sheets of 99.99% Al were oxidized at 100 V in 0.5% oxalic acid solution at 17 °C for 5 min and then treated with 5% phosphoric acid at 30 °C for 1 h. Subsequently, the films were again oxidized under the same conditions for 2 h and etched in the phosphoric acid for 40 min whereby the diameter of the pores increases. A gold film of 100 Å was evaporated onto the sheets which is too thin of a film to block the holes of the membrane. After the sheets were cut into 1 cm2 pieces, methyl methacrylate was polymerized inside of the pores using benzoyl peroxide under UV illumination as the radical initiator. After the films were treated with 10% NaOH at 40 °C
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Figure 2. Scanning electron microscopy of the titania nanotubes: (a) after 1 h of electrochemical deposition “as prepared” at low magnification; (b) same at higher magnification; (c) after heat treatment at 450 °C for 1 h, low magnification, the inset shows the electron diffraction pattern obtained with a transmission electron microscope; (d) sample shown in (c) at high magnification. to remove the Al and the Al2O3, a poly(methyl methacrylate) (PMMA) replica of the porous Al2O3 layer with the thin sputtered gold film between the PMMA rods resulted. This gold film served as catalytic material for the electroless deposition of gold from a plating solution. Electrodes were prepared by contacting the form with silver paint (Dotite, Fujikura Kasei Co. Ltd.) and isolating the silver parts with epoxy resin. It is important that the PMMA mold does not dry during the whole process, as this leads to irreversible attachment of the polymer rods to each other. The electrodes were used to deposit amorphous titania on the PMMA rods using an electrochemical deposition process from the literature.12 Small amounts of NaHCO3 were added, under nitrogen, to 10 mL of 50 mM TiCl3 in hydrochloric acid (Kanto Chemical Co.) until the pH was around 2.5. The current during the electrodeposition is known to be independent of the applied potential in a potential range between -400 and +700 mV vs Ag/AgCl. In the present study, samples prepared at +400 mV vs Ag/AgCl were found to have the same structural properties as samples prepared at +500 mV in a two-electrode setup, the counter electrode being a Pt wire. Thus, the easier two-electrode cell has been used for the deposition. The electrochemical reaction of the deposition can be written as follows:12,13
nTiOH2+ + mH2O f [TiIVOx(OH)4-2x]n + 3nH+ + ne- (1) The oxide is obtained in the form of polymer-like hydrous titania. After the deposition, the electrode was washed in dilute HCl and dried in diethylene glycol at 80 °C for 10 min. The PMMA mold was dissolved in acetone at 40 °C, yielding the desired structure of amorphous TiO2 in the form of nanotubes. Heating to 450 °C for 1 h dehydrates the sample and a dense semiconducting anatase sample is produced. (12) Kavan, L.; O’Regan, B.; Kay, A.; Gra¨tzel, M. J. Electroanal. Chem. 1993, 346, 291. (13) Ragai, J.; Selim, S. I. J. Colloid Interface Sci. 1984, 115, 139.
Results and Discussion The nanostructure has been confirmed by electron microscopy. A low magnification SEM picture of the structure of the titania nanotube array after 1 h of electrochemical deposition and subsequent dissolution of the polymer mold is shown in Figure 2a). The length of the tubes is 8 µm and they are adsorbed onto the wall of their neighboring tubes to form domains. The mold was prevented from drying during the replication process to ensure parallel alignment of the PMMA rods, so the TiO2 deposition can take place on each rod separately. Thus, the domains are probably formed during the drying process after the dissolution of the polymer mold as the solvent (acetone), which separates the tubes, is removed. A picture taken at higher magnification (Figure 2b) shows that the tubes are separated from each other at close distances within the domains. From this picture, an inner hole diameter of between 70 and 100 nm and a wall diameter between 30 and 50 nm can be derived. The whole nanotubes have an outer diameter between 140 and 180 nm. The “as deposited” material is amorphous as confirmed by electron diffraction using a transmission electron microscope (TEM) which is in accord with data from unstructured films grown on indium tin oxide (ITO).12 Pictures of the tubes after heat treatment of 450 °C at low magnification and high magnification are shown in parts c and d of Figure 2, respectively. From Figure 2c it can be seen that the tubes are strongly adhered to each other while the tube opening is partly closed by titania particles which have broken away from the open end of the tube wall. An electron diffraction pattern taken with a TEM is also shown in the upper right part of Figure 2c which
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proves the heated samples to consist of nanocrystalline anastase particles. At higher magnification, the single tubes can still be distinguished in the SEM picture of Figure 2d, but the diameter of the tubes has become smaller compared to the unheated tubes. Thus, the outer diameter of the tubes is reduced to about 120 nm, with a wall thickness of about 25 nm and an inner diameter of between 50 and 70 nm. From other SEM pictures, it could be seen that the diameter of the tubes at the bottom of the film is only slightly larger compared to the diameter at the top of the tubes. In studies of electrochemically deposited polypyrrole or gold tubes in porous Al2O3 membranes, the molecules are assumed to adsorb on the wall of the membranes prior to deposition. In the case of polypyrrole,3 this results in a high degree of order of the base units in the polymer. In the case of TiO2 deposition, however, the highly charged electroactive species (see formula 1) are not likely to adsorb on the wall of the PMMA polymer before the electron transfer occurs and directional growth is improbable in an amorphous material. Another possible mechanism for the tubular growth will be considered here. In addition to the usual growth at the surface of the electrode, a deposition process through the solution between the solid PMMA cylinders might take place. This takes into account the finding by Kavan et al.12 who reported a decrease in the electron efficiency at low pH values, i.e., the concentration of dissolved Ti(IV) is increased in the vicinity of the reaction plane. In addition to the Ti(IV) species, protons are also formed during the reaction which decrease the pH and, thus, increase the solubility of Ti(IV) in the vicinity of the electrochemical reaction plane. Since a hydrous titania is formed by the polymerization of Ti(IV) via several preproducts, in addition to direct growth on the electrode surface, a deposition mechanism via precipitation of these compounds from the solution is probable. This means that some of the substances electrogenerated on the gold electrodes might diffuse through the solution between the PMMA rods to reach regions of higher pH where further polymerization can take place until precipitation on the nanostructured polymer occurs. When the deposit on the cylinders is thick enough to allow conduction (the resistivity through the film is low12) the electrochemical reaction should occur preferentially at the tube opening since the active species diffuse to this region from the solution. Again, soluble Ti(IV) species will be formed which will be deposited at the more elevated parts of the polymer, thus producing titania tubes filled with PMMA. Titania will not fill the volume between the tubes for two reasons. Firstly, it would be difficult for a Ti(III) ion to penetrate the electroactive wall between two tubes without oxidation since the time necessary to diffuse over a distance of approximately 40 nm by diffusion estimated by a simple relation14 is in the order of microseconds, i.e., all of the Ti(III) will be oxidized before penetration into the porous part between the titania tubes. Secondly, the pH gradient in the mold lowers the amount of deposited material near the location of the electrochemical reaction. (14) Moore, W. J. Physical Chemistry, 4th ed.; Longmans Green and Co., Ltd.: London, 1962; p 343.
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Figure 3. SEM picture of the cross section of the as prepared film of titania after 20 min of deposition with the upper part of the tubes removed.
In summary, the proposed mechanism for the formation of titania nanotubes is as follows: (I) Partly soluble titania is formed at the electrode surface and is deposited on the PMMA rods and on the electrode. (II) When the thickness of the titania deposit on the PMMA rods is thick enough to ensure good conductivity, TiOH2+ is oxidized preferentially at the more elevated tubular parts of the PMMA rods. (III) As some of the oxidized species are soluble, titania is deposited on the wall of the PMMA rods. Any Ti(IV) trying to enter the free space between the tubes will be deposited at the tube opening; i.e., only the tube grows. To check the proposed mechanism, a closer 45° view of the cross section of the lower part of the amorphous tubes is shown in Figure 3. The film was broken 1 µm above its base which enables a closer examination of the inner structure part of the film. The thickness of the compact bottom layer is 500 nm. The tubes appear in a relatively abrupt manner above this compact layer. The compact layer results from the first stage of the deposition process when the gold electrode is covered. The abrupt occurrence of the nanotubes corroborates the occurrence of the electrochemical reaction at the tube opening with subsequent tubular growth, as pointed out above. This means that two different deposition processes are present during the reaction. The structural aspects of the deposition should depend on the pH, buffer capacity, current density, free space in the PMMA mold, and diffusion of the ions through the solution between the PMMA rods. In the case of TiO2, electrochemical deposition in a PMMA mold with smaller distance between the polymer rods yields a compact porous film instead of nanotubes.15 Acknowledgment. A research scholarship provided by the German Research Foundation (DFG) is gratefully appreciated. LA9507803 (15) Hoyer, P.; Masuda, H. Submitted for publication.
