Co₃O₄‑Modified TiO₂ Nanotube Arrays via Atomic Layer Deposition for Improved Visible-Light Photoelectrochemical Performance

Abstract

Composite Co₃O₄/TiO₂ nanotube arrays (NTs) were fabricated via atomic layer deposition (ALD) of Co₃O₄ thin film onto well-aligned anodized TiO₂ NTs. The microscopic morphology, composition, and interfacial plane of the composite structure were characterized by scanning electron microscopy, energy dispersion mapping, X-ray photoelectron spectra, and high-resolution transmission electron microscopy. It was shown that the ultrathin Co₃O₄ film uniformly coat onto the inner wall of the high aspect ratio (>100:1) TiO₂ NTs with film thickness precisely controlled by the number of ALD deposition cycles. The composite structure with ∼4 nm Co₃O₄ coating revealed optimal photoelectrochemical (PEC) performance in the visible-light range (λ > 420 nm). The photocurrent density reaches as high as 90.4 μA/cm², which is ∼14 times that of the pristine TiO₂ NTs and 3 times that of the impregnation method. The enhanced PEC performance could be attributed to the finely controlled Co₃O₄ coating layer that enhances the visible-light absorption, maintains large specific surface area to the electrolyte interface, and facilitates the charge transfer