Laser‐Crystallization of TiO₂ Nanotubes for Photocatalysis: Influence of Laser Power and Laser Scanning Speed

Abstract

Titanium dioxide (TiO2) nanotubes have raised significant attention in the field of photocatalysis. However, achieving locally‐confined, tunable, and efficient crystallization remains challenging. Laser crystallization that enables surface‐confined, fast, and localized annealing has emerged as an alternative to calcination of TiO2 nanotubes. Nonetheless, questions regarding crystallization parameters and mechanism remain open. In this work, amorphous TiO2 nanotubes are synthesized by a two‐step anodization process and laser exposed (515 nm, 1 MHz, 250 fs, 256–569 mW, 1–200 mm s−1) to study the impact of laser parameters and the mechanism of laser crystallization. A thorough analysis via Raman spectroscopy, X‐ray diffraction, scanning electron microscopy, UV–vis spectroscopy, transmission electron microscopy, and degradation of methylene blue is performed and the influence of laser processing parameters on the crystallization are studied and discussed. As laser power increases, a gradual transition from amorphous to anatase and rutile phase takes place, consistent with a thermal crystallization mechanism. Contrary, with increasing laser scanning speeds, thus lower pulse numbers, anatase is converted to rutile and melting occurs. The degradation of methylene blue with anatase samples crystallized by laser and calcination exhibits equal kinetic degradation constants of 0.013 s−1cm−2. This study enhances the understanding of laser crystallization of TiO2 nanotubes.