Semimetallic core-shell TiO2 nanotubes as a high conductivity scaffold and use in efficient 3D-RuO2 supercapacitors

Abstract

In the present work we report on TiO2 nanotube arrays (TNTAs) that were converted to a conductive scaffold established via an optimized reduction treatment in Ar/H2. These conductive TNTAs are then employed for RuO2 nanoparticle decoration. The effect of the Ar/H2 treatment is evaluated by electron energy loss spectroscopy (EELS) and electron paramagnetic resonance (EPR). The results show that, under ideal conditions, buried Ti3+ states are formed, with a higher concentration in the inner shell of the nanotube. Together with the capacitive and conductive performance, investigated by solid-state conductivity and electrochemical measurements, we find that 20 μm long TNTAs, annealed at 550°C in Ar/H2, yield an optimized and stable structure that provides a remarkably low resistivity of 13.5 KΩ/tube (vs. 70.2 MΩ for non-treated nanotubes). In cycling experiment, with a loading of only 0.048 mg.cm-2 RuO2 a specific capacitance of 1297 F.g-1 can be reached. The reason for this highly efficient use of RuO2 is that the conducting core-shell scaffold provides a unique, well dispersed RuO2 nanoparticle structure (~2.8 nm), which is responsible for the high specific capacitance, and moreover yields an excellent long-term cycling stability. Show more