TiO₂ nanotubes with different spacing, Fe₂O₃ decoration and their evaluation for Li-ion battery application

In the present work, we report on the use of organized TiO₂ nanotube (NT) layers with a regular intertube spacing for the growth of highly defined α-Fe₂O₃ nano-needles in the interspace. These α-Fe₂O₃ decorated TiO₂ NTs are then explored for Li-ion battery applications and compared to classic close-packed (CP) NTs that are decorated with various amounts of nanoscale α-Fe₂O₃. We show that NTs with tube-to-tube spacing allow uniform decoration of individual NTs with regular arrangements of hematite nano-needles. The tube spacing also facilitates the electrolyte penetration as well as yielding better ion diffusion. While bare CP NTs show a higher capacitance of 71 μAh cm^-2 compared to bare spaced NTs with a capacitance of 54 μAh cm^-2, the hierarchical decoration with secondary metal oxide, α-Fe₂O₃, remarkably enhances the Li-ion battery performance. Namely, spaced NTs with α-Fe₂O₃ decoration have an areal capacitance of 477 μAh cm^-2, i.e. they have nearly ∼8 times higher capacitance. However, the areal capacitance of CP NTs with α-Fe₂O₃ decoration saturates at 208 μAh cm^-2, i.e. is limited to ∼3 times increase.