Formation of anodic TiO2 nanotube or nanosponge morphology determined by the electrolyte hydrodynamic conditions

Abstract

The present work studies the effect of hydrodynamic conditions on the growth of anodic TiO2 nanostructures on Ti in a glycerol/water/NH4F electrolyte. Parameter screening for fluoride content, anodization voltage and rotation rate (Reynolds number) of a rotating anode showed that two distinctly different TiO2 morphologies could be obtained: the classic ordered nanotubes or a nanoscale sponge layer. We present conditions for TiO2 sponge formation and growth to several micrometers of thickness, and show that in specific cases sponge layers can be superior to tube morphologies, as illustrated for photoelectrochemical water-splitting under standard AM 1.5 conditions.