Improved photocatalytic efficacy of TiO2 open nanotube arrays: A view by XAS

Abstract Highly ordered TiO2 nanotube (TiNT) arrays were prepared on Ti foil. The interfacial caps between nanotube bottom and Ti foil were gradually removed by oxalic acid. The removal of caps in TiNT is known to enhance its light-harvesting capability, increase the carrier transfer, and cause effective dissociation of electron-hole pairs. The electronic and local atomic structures of TiNT at different oxalicacid immersion time were investigated by XAS. Notably, the reported enhanced light harvesting and electron collecting efficiencies in both-end open TiNT, could be strongly correlated with charge redistribution in Ti 3d orbital and structural defect. An increase in Ti 3d unoccupied states due to increase in Ti4+/Ti3+ ratio followed by a decrease in oxygen vacancy at the closed-end bottom caps after 16 h oxalicacid immersion might be one of the reasons for enhanced efficiency of both end open TiNT against one end open tube. EXAFS analysis revealed an increase in CN number and Ti–O bond length, which results in the formation of disordered TiO6 Oh symmetry that would possibly enhance the lattice phonons and eventually the efficiency of TiNT. This is one of the fundamental reasons for the improved photocatalytic properties for both-end open TiNT against closed bottom-end nanotubes.