Highly efficient photoelectrochemical and photocatalytic anodic TiO2 nanotube layers with additional TiO2 coating

In this work, strong beneficial effects of thin and uniform TiO 2 coatings within TiO 2 nanotube layers for photocurrent generation and photocatalytical degradation of methylene blue are demonstrated for the first time. TiO 2 nanotube layers were coated by TiO 2 of various thicknesses (from 2.8 nm to 22 nm) using atomic layer deposition (ALD) and compared with TiO 2 nanotube layers decorated by TiO 2 nanoparticles (using established TiCl 4 treatment) and with blank (uncoated) layers. By means of photocurrent measurements and cyclic voltammetry, it is demonstrated that the most efficient charge carrier separation can be achieved for TiO 2 nanotube layers with an optimal ALD TiO 2 coating thickness ≈11 nm. Significant differences in flatband potentials and carrier density among all nanotube layers were revealed by Mott-Schottky measurements. Photocatalytic decomposition rates for methylene blue solutions were significantly enhanced for ALD TiO 2 coated TiO 2 nanotube layers compared to their uncoated or TiO 2 nanoparticles - decorated counterparts. A perfect agreement in trends was obtained for photocurrent and photocatalytic results.