Compositional Patterning in Carbon Implanted Titania Nanotubes

Ranging from surfaces for efficient water splitting and novel solar cells to smart biosensors: titania nanotube arrays constitute a highly functional material for various applications. Using low-energy ion implantation in the 10 keV – 100 keV range, we show a promising route for engineering the material characteristics while preserving the amorphous structure of the nanotube arrays. We report on the relationship of phenomenological effects observed upon implantation of low fluences of a few 10¹⁶ ions/cm² in the unique anisotropic and nanoporous structure: sputtering vs. re-adsorption and plastic flow, amorphization vs. crystallization and compositional patterning. Using high-resolution element mappings obtained with transmission electron microscopy, we detect a compositional patterning of oxygen and carbon. Especially, partially connected oxide and carbide domains with sizes of about 10 nm are distributed over the whole nanotube array.<br/>Our extensive theoretical treatment corroborates the experimental results. We show that the distinct nanotube environment combined with radiation enhanced diffusivity due to ion implantation and related ballistic contributions provide advantageous conditions for patterning. By applying a generalized version of the Cahn-Hilliard equation combined with driven alloys and corresponding free energy estimates for the amorphous composite, we derive characteristic length scales of pattern formation. Indeed, density functional theory calculations using amorphous TiO₂ cells show that dilute-limit mixing enthalpies are augmented for the carbon and oxygen system. Hence, the compositional patterning of carbon and oxygen is actually expected from enthalpy and entropy arguments. In contrast, a patterning of fluorine and oxygen is energetically unfavorable and not observed. In fact, the nanotubular geometry promotes the patterning due to an increase of relocation lengths compared to bulk materials. As titania nanotubes are a common basis material for advanced devices, these findings pave the way for a manifold of new optimization strategies of optical and electrical nanotube characteristics.<br/><br/><i>Funding by the Heinrich B</i><i>ö</i><i>ll foundation and German BMBF, Project EYECULTURE (FKZ 161A574C) is gratefully acknowledged. </i><br/><br/>References: A. Kupferer, A. Holm, A. Lotnyk, S. Mändl, S.G. Mayr. 2021. Adv. Funct. Mater. <i>31</i>(35), 2104250.