Abstract
Hafnium-doped titania (Hf/Ti = 0.01; 0.03; 0.05) had been facilely synthesized via a template sol-gel method on carbon fibre. Physico-chemical properties of the as-synthesized materials were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray analysis, scanning transmission electron microscopy, X-ray photoelectron spectroscopy, thermogravimetry analysis and Brunauer-Emmett-Teller measurements. It was confirmed that Hf(4+) substitute in the Ti(4+) sites, forming Ti(1-x) Hf (x) O(2) (x = 0.01; 0.03; 0.05) solid solutions with an anatase crystal structure. The Ti(1-x) Hf (x) O(2) materials are hollow microtubes (length of 10-100 µm, outer diameter of 1-5 µm) composed of nanoparticles (average size of 15-20 nm) with a surface area of 80-90 m(2) g(-1) and pore volume of 0.294-0.372 cm(3) g(-1). The effect of Hf ion incorporation on the electrochemical behaviour of anatase TiO(2) in the Li-ion battery anode was investigated by galvanostatic charge/discharge and electrochemical impedance spectroscopy. It was established that Ti(0.95)Hf(0.05)O(2) shows significantly higher reversibility (154.2 mAh g(-1)) after 35-fold cycling at a C/10 rate in comparison with undoped titania (55.9 mAh g(-1)). The better performance offered by Hf(4+) substitution of the Ti(4+) into anatase TiO(2) mainly results from a more open crystal structure, which has been achieved via the difference in ionic radius values of Ti(4+) (0.604 Å) and Hf(4+) (0.71 Å). The obtained results are in good accord with those for anatase TiO(2) doped with Zr(4+) (0.72 Å), published earlier. Furthermore, improved electrical conductivity of Hf-doped anatase TiO(2) materials owing to charge redistribution in the lattice and enhanced interfacial lithium storage owing to increased surface area directly depending on the Hf/Ti atomic ratio have a beneficial effect on electrochemical properties.