The Morphology of TiO₂ Nanotube Arrays Grown from Atomically Peened and Non-Atomically Peened Ti Films

The growth of TiO₂ nanotube arrays (TNTAs) on non-native substrates is essential for exploiting the full potential of this nanoarchitecture in applications such as gas sensing, biosensing, antifouling coatings, low-cost solar cells, drug-eluting bimedical implants and stem-cell differentiators. The direct formation of anodic TNTAs on non-native substrates requires the vacuum deposition of a thin film of titanium on the substrate followed by subsequent electrochemical anodization of the film. In this report, we studied the effect of atomic peening on the formation of Ti thin films on technologically important non-native substrates. We compared the structure and morphology of evaporated and sputtered Ti films, and correlated them to the morphology of the vertically oriented TiO₂ nanostructures that resulted subsequent to anodization of those films. We calculated a minimum value of 1.33 eV/atom for the energy of refkected neutral Ar species arriving at the substrate when a chamber pressure of 1 mTorr is used during the sputter deposition of Ti. Previous approaches relied on substrate heating to elevated temperatures during Ti thin film deposition or ion-beam assisted Ti thin film deposition as a prerequisite to form TiO₂ nanotube arrays (TNTAs). We demonstrated TNTAs on a variety of substrates at room temperature using both evaporated and sputtered Ti films without recourse to ion-beam sources. Evaporated Ti films were found to possess small grain size and high local surface roughness, which resulted in nanotubes with extremely rough sidewalls. Ti thin films formed by Ar⁺ ion sputtering at commonly used chamber pressures of 7–20 mTorr at substrate temperatures ranging from room-temperature to 250 °C possessed a highly rough surface and three-dimensional grains, which precluded the formation of ordered nanotubes upon anodization due to highly non-uniform pore nucleation processes. In contrast, Ti thin films sputtered at low chamber pressures of 0.8–2 mTorr had a low surface roughness due to the atomic peening process. Such films, even when deposited at room temperature, resulted in ordered nanotube arrays upon anodization.