Novel rare earth metal–doped one-dimensional TiO2 nanostructures: Fundamentals and multifunctional applications

In last few years, titanium dioxide (TiO2)-based various dimensional (0D, 1D, 2D, and 3D) nanostructures (NSs) have been extensively investigated because of their outstanding physio-chemical properties and multifunctional applications in the variety of fields, including energy, environment, biomedicine, and so on. Particularly, one dimensional (1D) TiO2 NSs have gained much attention among the researchers as these nanomaterials can be explored in various morphologies, such as nanorods, nanotubes, nanofibers, nanowires, and so on. In addition to their unique 1D shape and large specific surface area, these 1D NSs show confinement in the radial direction making them more valuable than other dimensional NSs. However, owing to their large band gap, these NSs show inability of exploiting visible light and lower charge recombination rate becoming less efficient materials for the practical applications in the society. In order to achieve high efficiency of these materials and to improve their visible light activity, considerable efforts have been made by narrowing the band gap through doping or nanocomposite formation with other functional nanomaterials. This review article is mainly focused on the rare earth metal–doped 1D TiO2 NSs with the detailed mechanism of action, improved optoelectronic properties, and their multifunctional applications in the energy and environmental fields. It includes photocatalytic (photodegradation/surface-enhanced Raman scattering detection of organic pollutants, photocatalytic CO2 reduction, photocatalytic water splitting, and upconversion-based photocatalytic activities) sensing, solar cells, supercapacitors, lithium-ion batteries applications with an emphasis on their fundamental working principles. In addition, synthesis methods of rare earth metal–doped 1D TiO2 NSs, the doping effect on the microstructural and optical properties, and recent advances in these directions followed by challenges and future opportunities have been discussed.