Large-diameter light-scattering complex multipodal nanotubes with graded refractive index: insights into their formation mechanism and photoelectrochemical performance

Nanostructuring, morphology tuning, doping, as well as alloying have decisive roles in controlling the performance of functional materials. Herein, large-diameter multipodal (MP) Ti-Nb-Zr-O nanotubes (NTs) were synthesized using one-step potentiostatic anodization of a Ti-Nb-Zr alloy in a formamidebased electrolyte containing NH4F. Tubes with diameters up to 507 nm and lengths up to 36 mm were successfully fabricated. A mechanism for the MPNT formation is proposed and validated using FESEM imaging at various stages of formation. Unlike the conventional metal oxide nanotubes, the MP nanostructure formation is attributed to the bending and fusion of discrete NTs. A theoretical model considering NTs as a bottom-fixed cantilever is used to analyze the preconditions required for the formation of the MP nanostructure. The model findings suggest that bending only occurs when the net forces acting on the NTs overcome the NT stiffness as it passes a critical length. MPNTs proved to have a graded refractive index, which in turn enhanced their light harvesting characteristics. The photoelectrochemical properties of the MPNTs annealed in air were studied and compared to conventional compact NTs, where a 9-fold enhancement in the photocurrent was observed. This enhancement is ascribed to the MP morphology with its graded refractive index as well as easier charge transport. Mott-Schottky analysis demonstrated a positive shift in the flat band potential for the MPNTs when compared to the compact counterpart as well as double charge density. According to Mie's scattering theory, MPNTs are considered a promising candidate in light harvesting applications owing to their efficient light scattering and graded refractive index characteristics.