Template-free synthesis of functional metal oxide nanotubes via nonclassical nucleation in a continuous injection method

One-dimensional nanotubes have attracted considerable interest owing to their remarkable surface-to-volume ratios and exceptional mass and charge transport properties, making them a pivotal class of materials. In contrast to naturally layered materials, the exploration of free-standing metal oxide nanotubes has been relatively limited due to synthetic challenges. This study presents a simple colloidal method for synthesizing well-defined metal oxide nanotubes, with a particular focus on Ga₂O₃ nanotubes, featuring adjustable dimensions and crystal structures. By varying the injection rate of the gallium precursor solution, we achieved substantial control over the length, diameter, and crystal phase of the resulting Ga₂O₃ nanotubes. The growth mechanism was elucidated through the investigation of reaction intermediates, revealing a nonclassical nucleation pathway involving the aggregation of pre-nucleation clusters. This process is promoted by a low monomer supersaturation, resulting from a slow injection rate, and a low cluster dissolution rate, associated with high metal-oxygen bond dissociation energies. As growth continues, these clusters form elongated one-dimensional nanostructures that eventually evolve into nanotubes. Additionally, the versatility of this approach was demonstrated by the successful synthesis of magnetic Fe₃O₄ and plasmonic WO_3-x nanotubes with comparable morphological features. The scalable and highly tunable method presented in this work opens new avenues for designing one-dimensional multifunctional nanomaterials with promising applications in optoelectronics, energy storage, and sensing technologies.