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.

Microporous layers on titanium (Ti) are formed by chemical treatment in highly concentrated alkaline media, and their properties and growth mechanism are examined using electrochemical techniques, in situ resistometry, scanning electron microscopy (SEM), grazing-incident X-ray diffraction (GIXRD), and glow discharge optical emission spectroscopy (GD-OES). Chemical treatment in a 5 M aqueous KOH solution yields results superior to those from the same treatment in a 5 M aqueous NaOH solution, while a 3 M aqueous LiOH solution does not produce porous layers. The cation constituting the solution plays a vital role in the process. An SEM analysis reveals that the KOH solution is the most effective in forming microporosity and that the longer the treatment time, the more porous the near-surface layer. The results of GIXRD analysis show the presence of Na(2)Ti(5)O(11) and K(2)Ti(6)O(13) in the layers formed in the NaOH and KOH solutions, respectively; in the case of the LiOH solution, TiO(2) is formed. Chemical treatment in the NaOH and KOH solutions resembles a general corrosion process with the existence of local cathodic and anodic sites. The reduction reaction produces H(2), some of which becomes absorbed in the near-surface region of Ti, while the oxidation reaction produces the above-mentioned compounds and/or an oxide layer. The presence of hydrogen (H) within the solid is detected using GD-OES. The H-containing near-surface layer partially dissolves, yielding a microporous structure. The development and dissolution of the H-containing near-surface layer of Ti upon chemical treatment in the NaOH and KOH solutions are confirmed by resistometry measurements. They point to the formation of a compact passive layer on Ti upon exposure to the LiOH solution.

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...

The formation of microporous oxide layers on titanium (Ti) by anodization in sulfuric acid (H2SO4) solution and the influence of prior hydrogen charging on their properties are examined using electrochemical techniques, scanning electron microscopy, grazing incident X-ray diffraction, and X-ray photoelectron spectroscopy. When Ti is anodized in 1 M aqueous H2SO4 solution at a high direct current (DC) potential (>150 V) for 1 min, a porous surface layer develops, and the process takes place with spark-discharge. Under these conditions, oxygen evolution at the Ti electrode proceeds vigorously and concurrently with the formation of anodic oxide. The oxygen gas layer adjacent to the Ti surface acts as an insulator and triggers spark-discharge; the latter stimulates the development of pores. In the absence of spark-discharge, the oxide layer has extended surface roughness but low porosity. A porous oxide layer can be prepared by applying a lower DC voltage (130 V) and without spark-discharge, but Ti requires prior hydrogen charging by cathodic polarization in 1 M aqueous H2SO4 solution. Mott-Schottky measurements indicate that the oxide layers are n-type semiconductors and that the charge carrier density in the anodic oxide layer on the hydrogen-charged Ti is lower than in the case of untreated Ti. The hydrogen charging also affects the flat band potential of the anodic oxide layers on Ti by increasing its value. The reduced charge carrier density brought about by hydrogen charging decreases the oxide layer conductivity and creates favorable conditions for its electrical breakdown that stimulates the development of pores. The porous layer on the hydrogen-charged Ti consists of anatase and rutile phases of TiO2; it has the same chemical composition as the porous layer obtained on untreated Ti. X-ray photoelectron spectroscopy measurements show that prior hydrogen charging does not affect the thickness of anodic oxides on Ti. The porous oxide layer on Ti enables the growth of hydroxyapatite, thus revealing good bioactivity in simulated body fluids.

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The degradation behavior of pigmented long oil alkyd resin (LAR-P) painted on steel in an aqueous 3% NaC1 solution was investigated by nanoindentation and scanning acoustic microscope (SAM) . Young's modulus (E) and hardness (H) , obtained by nanoindentation, of the LAR-P paint film on steel increased at short immersion time and then decreased with increasing immersion time. Furthermore, it wasclarified that the SAM image shows a heterogeneous E distribution. The lower E areas in paint film were observed at short immersion time, while the blisters were observed in the lower E area in the SAM images. The spread of lower E area and blister growth with increasing immersion time, which are difficult to observe by optical method, were clarified by evaluation of the SAM images.