Barrier Layer Growth and Nanopore Initiation During Anodic Oxidation of Tungsten and Niobium

摘要

The discovery of carbon nanotubes with their variety of interesting properties have stimulated the quest for the synthesis of nanotubular and nanocolumnar structures of other substances and chemical compounds. Several recent studies have indicated that nanoporous oxides with such a structure, formed on valve metals such as Al,Ti, Zr, Nb, Ta and W, have improved properties compared to any other form of these oxides for application in catalysis, photocatalysis, sensing, photoelectrolysis and photovoltaics (1,2). Such nanoporous materials consisting of nanotube arrays have been produced by a variety of methods, one of the most promising of which is the method of anodic oxidation in suitable electrolyte solutions, by which the dimensions of the nanopores can be be accurately controlled. Uniform nanoporous oxides on Ti, for example, that consist of titania nanotube arrays of various pore sizes, depths, and wall thicknesses can be grown by precise tailoring of the electrochemical conditions. However, as these processes have been studied in depth for no more than a decade, the mechanism of pore initiation and growth is still a matter of debate (1,2).rnVery recently, Raja et al. (3) have presented a view to the mechanism of the oxide nanotube growth on Ti in which the regularity of the pore formation is ascribed to the perturbation of the surface strain energy which favours homogeneous F~- adsorption on the film surface. According to this approach, the process has been divided into the following stages: i) formation of a compact barrier film; ii) thickening of that film and subsequent microfissuring, normally referred as formation of 'easy paths'; iii) secondary porous oxide nucleation through these paths, subject to perturbation in the balance between the