Morphological evolution of anodic TiO2 nanotubes

Abstract

An experimental study on the development of TiO₂ nanostructures during anodization in fluoride-ion containing electrolyte has been conducted. The evolution of this nanostructure involves the inward growth of a porous network at the oxide/substrate interface that develops into nanotubes. A new explanation for the formation of these nanotubes during anodization is proposed based on the geometric changes observed. SEM examination reveals that the newer formed portions of the pore space closest to the oxide/substrate interface are larger in diameter than those above which form earlier in the process. This yields a structure in which pores are wider at their bases than at the outer oxide/electrolyte interface and appear to crowd each other in the lower portions as anodization proceeds. Further SEM evidence suggests that excessive crowding eventually causes some pores to degenerate into inter-tube regions and leave behind distinct nanotubes. We propose that this degeneration process is the main driving force for the structural transformations from the initial oxide layer to a network of nanopores and finally to fully developed nanotubes rather than a separate dehydration or dissolution process as proposed by other researchers. The differences in morphology between anodic TiO₂ films formed in aqueous and ethylene glycol-based electrolytes are explained using the same model.