Open top anodic Ta3N5 nanotubes for higher solar water splitting   efficiency
  In the present work we grow self-organized Ta2O5 nanotube layers in a H2SO4/NH4F electrolyte at various elevated temperatures. Under optimized conditions we obtain 4 {\mu}m long nanotubes that are well adherent to the substrate and can be grown very homogenous over large surface areas. Moreover, the key advantage of this approach is that an open top morphology (initiation layer free) is obtained. After a suitable conversion treatment in NH3 atmosphere Ta3N5 nanotubes are obtained, that after (Co-Pi+Co(OH)x) modification provide, under AM 1.5G illumination conditions, a photoelectrochemical current response of 4.7 mA cm-2 at 1.23 V vs RHE. 
Sabina Grigorescu, Seulgi So, Jeong Eun Yoo, Anca Mazare, Robert Hahn,   Patrik Schmuki
cond-mat.mtrl-sci