Morphological instability leading to formation of porous anodic oxide films.

Electrochemical oxidation of metals, in solutions where the oxide is somewhat soluble, produces anodic oxides with highly regular arrangements of pores. Although porous aluminium and titanium oxides have found extensive use in functional nanostructures, pore initiation and self-ordering are not yet understood. Here we present an analysis that examines the roles of oxide dissolution and ionic conduction in the morphological stability of anodic films. We show that patterns of pores with a minimum spacing are possible only within a narrow range of the oxide formation efficiency (the fraction of oxidized metal atoms retained in the film), which should exist when the metal ion charge exceeds two. Experimentally measured efficiencies, over diverse anodizing conditions on both aluminium and titanium, lie within the different ranges predicted for each metal. On the basis of these results, the relationship between dissolution chemistry and the conditions for pore initiation can now be understood in quantitative terms.

Anodic TiO2 nanotubes: double walled vs. single walled.

Electrochemical formation of self-organized TiO2 nanotube layers has been a highly active research field for more than 10 years. In the present manuscript we investigate the formation of two distinctly different anodic TiO2 nanotube morphologies, 'single walled' and 'double walled' tubes, which are formed mainly depending on the nature of the anodization electrolyte. While the widest used electrolytes are ethylene glycol (EG) based, forming double walled structures, tubes formed in dimethyl sulfoxide (DMSO) based electrolytes show a single tube walled morphology. Here we provide reasons for the formation of double walled tubes, characterize tubes for their composition, structure and certain properties, and give measures to suppress or minimize double wall formation. Except for the fact that in DMSO single walled tubes are formed, we also show that they grow sufficiently slowly to allow partial crystallization of the tubes during growth--this, in turn drastically influences their electronic properties. Finally we discuss the effects and potential consequences of double or single wall growth for TiO2 nanotube applications.

TiO2 nanotubes in dye-sensitized solar cells: critical factors for the conversion efficiency.

Particle vs tube: The present paper systematically investigates a range of fundamental geometrical and structural features of TiO(2) nanotube layers and their effect on the dye-sensitized solar cell conversion efficiency, to deduce the most promising strategies for improvement. It is found that the performance of the cells strongly depends on the morphology and crystalline structure of the nanotubes.

Self-organized, free-standing TiO2 nanotube membrane for flow-through photocatalytic applications.

In the present work we show a simple and robust fabrication process of a dense and free-standing membrane consisting of vertically oriented, both-side-open TiO2 nanotubes. This membrane structure allows direct, size-selective, flow-through photocatalytic reactions with a very high efficiency.