Topotactical Route to Multiwalled Cerium Oxide Nanotubes from MWCNTs

The development of new strategies for synthesizing 1D cerium oxide (CeO₂) hollow nanostructures has attracted much attention in recent years due to the importance of their superior properties and highly anisotropic geometry. This study reports an unpublished route of fabricating novel multiwalled CeO_2-δ nanotubes (CeO_2-δ NTs) in which the entire volume of functionalized multiwalled carbon nanotubes (f-MWCNTs) is converted into the CeO_2-δ pseudomorph through oxidation and dehydration topotactic reactions. The stable CeO_2-δ (111) planes are topotactically grown on the curved C (002) planes, preferentially exposed along the nanotube axis. In their initial condition, the novel nanotubes consist of Ce oxyhydroxide (CeO_2-x(OH)_x) with residual carbon. When heating the air up to 500 °C, CeO_2-x(OH)_x transforms gradually by dehydration into CeO_2-δ, while the residual carbon is oxidized. Despite compositional changes, nanotubes maintain their multiwalled structural integrity up to ≈550 °C. The CeO_2-δ NTs exhibit an unusually high presence of Ce³⁺ ions and surface O vacancies, contributing to a low direct band gap ranging from 2.67 to 2.32 eV compared to their NPs counterparts (3.2 eV).