The formation of a porous TiO 2 layer by immersing Ti in 5 M NaOH at 303 K was investigated using electrochemical measurements, a scanning electron microscope, and grazing incident X-ray diffraction. The porous layer was readily obtained in the case of the presence of titanium hydride (TiH 2 ) on the surface before immersion. The crystalline structure of the porous layer is composed of a mixture of the rutile and anatase type TiO 2 . The porous layer is hard to produce without the presence of TiH 2 . The TiH 2 is directly changed to TiO 2 by a dissolution reaction in alkaline solution. The presence of TiH 2 on the surface is an important factor for the preparation of the porous TiO 2 layer.

The influence of hydrogen on the formation of porous TiO2 in alkaline solution was investigated using an electrochemical measurement, a scanning electron microscope, and a grazing incident X-ray diffraction technique. The porous TiO2 was not obtained on the surface of titanium in 5 M NaOH solution at 303 K for 3 h. When the titanium was charged with hydrogen in 1 M H2SO4 at 303 K for 1 h before immersion, the porous TiO2 was readily produced on the surface under the same experimental conditions. The anodic polarization measurement indicated that the hydrogen-charged titanium was more readily dissolved in alkaline solution than the titanium. The cathodic reaction was the hydrogen evolution reaction on both the titanium and the hydrogen-charged titanium. For titanium, although the hydrogen penetrated into the inside of the material due to the side reaction of hydrogen evolution, the hydrogen content in the inside was so small that porous TiO2 was not produced under this condition. © 2002 The Electrochemical Society. All rights reserved.

The catalytic activity for the hydrogen evolution reaction was investigated in 1 M NaOH at 303 K. The hydrogen overpotentials of Raney-Ni electrodes obtained from NiAl 3 and Ni 2 Al 3 were lower than those from nickel rich alloys (NiAl and Ni 3 Al). Especially, NiAl 3 yielded the most active Raney-Ni cathode. This is because the fast aluminum leaching from NiAl 3 phase gives large surface area of the electrode, the formation of small micropores, and the appearance of the Ni phase.

The influence of the temperature and concentration of NaOH on the formation of porous TiO 2 was investigated. The porous TiO 2 was obtained only after a 24-h immersion in 5 M NaOH at 353 K. The X-ray diffraction pattern indicated that the porous layer consisted of a mixture of anatase and rutile types of TiO 2 . During immersion in the NaOH solution, the anodic and cathodic reactions proceed at the same time on the titanium. From the cathodic polarization measurements, the cathodic reaction proceeded mainly the hydrogen evolution reaction. Therefore, a part of the adsorbed hydrogen penetrated into the titanium. The penetration of hydrogen confirmed with a glow discharge optical emission spectroscopy. The hydrogen evolution reaction continues during immersion in the 5 M NaOH solution at 353 K. However, the hydrogen evolution did not proceed in the low concentration of NaOH solution or at below 283 K. In addition, although the hydrogen evolution continues during immersion in 5 M NaOH solution at 303 or 333 K. As a result, the content of hydrogen in the titanium is insufficient to form the porous TiO 2 layer so that the porous layer is not produced on the titanium. Titanium dioxide ͑TiO 2 ͒ has favorable optical properties and a good stability in corrosive environments. Since O'Regan and Grätzel 1 reported the high-efficiency solar cell based on dyesensitized porous TiO 2 films in 1991, porous TiO 2 has become one of the most interesting materials in the field of solar cells and photoelectrochemistry. Since then, many researchers have vigorously investigated the formation of porous TiO 2 in order to use it as a material to convert photoenergy into electric energy. 2-6 Porous TiO 2 is also an attractive material in the field of biomaterials. This is because the porous TiO 2 can enhance the adhesion between an artificial bone, which is based on a titanium alloy, and calcium phosphate. [7][8][9][10] Although these properties, such as energy conversion efficiency and adhesion resistance, may be influenced by the structure of the porous TiO 2 , such as porosity, film thickness, and so on, the factor controlling these parameters has not been defined in detail. This is because the mechanism of the preparation for porous TiO 2 in alkaline solution has not yet been clarified. Generally, the porous TiO 2 was prepared by immersing in alkaline solution at high temperature for several hours. 8-12 However, we reported that the formation of porous TiO 2 was strongly dependent on the initial surface conditions of the titanium and the porous layer was only obtained in the presence of TiH 2 or a hydrogen-absorbed layer in the titanium before immersion in NaOH solution. 13,14 Therefore, we have proposed that the surface condition of titanium before immersion is an important factor for the formation of porous TiO 2 .In this study, we prepared the porous TiO 2 on the surface of titanium by immersing it in an alkaline solution in order to investigate the influence of the solution temperature and the concentration of NaOH on the formation...

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