Enhancement and limits of the photoelectrochemical response from anodic TiO2 nanotubes

$\mathrm{Ti}{\mathrm{O}}_2$ nanotube layers were grown on titanium by a self-organized anodic oxidation. The layers consist of arrays of individual tubes with a length of $\sim 2\mu \mathrm{m}$ , a diameter of $\sim 100\mathrm{nm}$ , and a wall thickness of $\sim 10\mathrm{nm}$ . These layers can be annealed to an anatase structure which strongly increases the photocurrent efficiency. Moreover, the nanotube layers can—under certain conditions—exhibit a drastically enhanced photocurrent compared to compact anatase layers. These strong changes in the photoresponse are attributed to the characteristics of the space charge layer within the tube wall.