Self-Organized Nanoporous Valve Metal Oxide Layers

Abstract

The chapter presents a study, which shows that formation of self-organized nanoporous oxide layers can be achieved on a whole series of valve metals directly via anodization in fluoride-containing electrolytes. In particular, nanoporous layers on IVB valve metals such as Ti, Zr and Hf are presented. A key to achieve self-organized pore formation on such metals is to tune the situation where oxide formation and local dissolution of the oxide take place at the same time. For Ti, pH strongly affect the thickness of theoxide layers, as in an acidic electrolyte the thickness is approximately 400 nm and on the other hand, in a neutral electrolyte it is approximately 2 μm. For Zr and Hf, cross-sectional morphology is strongly affected by the electrolyte. In the acid, relatively wavy tubular pores are produced, whereas in the neutral electrolyte, extremely straight tubes are formed. This can be attributed to the difference of chemical properties of electrolytes such as a buffering effect.