Self-organization is a principle found everywhere in nature, for example in the growth of organisms and phase transitions. An interesting system used to study the nonlinear governing equations giving rise to this behaviour are electrochemically anodized titania nanotubes. The physics of this system is complex, involving electrodynamics, chemical diffusion, reaction kinetics, and material stresses.
By patterning the surface of titanium with Laser Induced Periodic Surface Structures (LIPSS) prior to growth, we manipulate the subsequent electrochemical growth of the nanotubes. This is, a double-self-organized growth process, as both the nanotubes and LIPSS grow in a self-organized manner to create the LIPSS-NT structure. We, in effect, changed the ‘initial condition’ of the nonlinear growth equations, which allows us to study the titania nanotubes, and compare them to numerical predictions of the morphology.
We investigated how LIPSS structures of varying periodicity affect the nanostructure of the anodized nanotubes. The surface was patterned with a Ti:sapphire femtosecond pulsed laser (800 nm, 110 fs, 1kHz), and characterized with scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, UV-Visible spectroscopy, transmission electron microscopy, and X-ray diffraction.
Finally, we fabricated structures for photonic applications such as Surface Enhanced Raman Spectroscopy substrates, and photocatalysts in pollutant degradation. The grating-like structure of the LIPSS-NT enhances the trapping of visible light and increases the rate of photocatalytic degradation under solar irradiation.
Thomas Ward, Rakesh Arul, Junzhe Dong, Ellen Jose, Tristan Pang, Wei Gao, and M. Cather Simpson, "Pattern formation and self-organization in the growth of titanium dioxide nanotubes (Conference Presentation)," Proc. SPIE 10521, Synthesis and Photonics of Nanoscale Materials XV, 105210H (Presented at SPIE LASE: January 30, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2289337.5751467538001.
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