Understanding the morphological evolution of anodic tantalum oxide nanostructures in acidic medium

Abstract

Nanostructures of tantalum oxide (Ta₂O₅) have been recognised as important and versatile materials in nanotechnology and medicinal science. However, the synthesis of shape- and size-controlled Ta₂O₅ nanostructures faces enormous challenges due to multi-step and expensive procedures. Over the past two decades, anodic oxidation or anodization of Ta has emerged as a facile top-down method for fabricating Ta₂O₅ nanostructures. Nevertheless, controlling the fabrication geometry, such as the thickness of the Ta₂O₅ oxide layer, and the conditions for regulating nanodimple/nanoporous/nanotube/nanorod structures remains difficult, particularly in acidic media. In the present work, we aim to understand the formation and growth mechanism of anodic Ta₂O₅ (ATO) nanostructures, and the conditions necessary for achieving the desired nanopatterns by adjusting various anodization parameters (i.e., anodization potential, time and HF : H₂O ratio in the electrolyte). By controlling the anodizing potential and HF : H₂O ratio, we have successfully obtained ATO layers with thicknesses ranging from ∼1.25 to ∼10.7 μm. Self-organized Ta₂O₅ nanotubes with a diameter of ∼55 nm were fabricated under the applied potential of 30 V for 2 or 4 min. The concepts and conditions for formation of Ta₂O₅ nanorods and agglomeration of nanotubes are revealed in this study. Furthermore, we have elucidated the rate limiting steps of Ta₂O₅ growth by applying various growth models. The crystallization of amorphous ATO structures and formation of Ta₄O and Ta₂O₅ phases were understood by temperature programmed X-ray diffraction (XRD) analyses. We anticipate that the results presented in this study will enhance the current understanding of the growth of other anodic oxide nanostructures as well.