N. Kumar, K. Shankar
University of Alberta,
Canada
Keywords: anodization, photocatalysis, hydrogen evolution, nanotube, CO2 photoreduction
Summary:
Electrochemical anodization is a simple yet highly effective technique for synthesizing vertically oriented metal oxide nanotubular and nanoporous arrays towards use in photocatalysis. Offering flexibility in geometry through voltage variation, duration and/or electrolyte composition, this method has facilitated the fabrication of a diverse range of semiconducting nanostructures, including TiO₂, Ta₂O5, and Nb₂O₅, while posing no significant challenges to commercial scalability. The majority of studies employ ethylene glycol as the non-aqueous solvent in anodization. However, our research demonstrates that varying the electrolyte composition significantly influences the physicochemical properties of the resulting nanostructures. For example, TiO₂ nanotubes grown in a formamide-based electrolyte exhibit distinct adsorbate interactions compared to those synthesized in an ethylene glycol-based electrolyte. Specifically, under CO₂ exposure, TiO₂ nanotubes formed in formamide-based electrolytes facilitate a spontaneous conversion of CO₂ to CO, attributed to their unique surface chemistry that enhances CO2 photoreduction yield. Secondly, the high viscosity of ethylene glycol promotes the formation of nanotubes with smoother walls, arranged in a closely packed honeycomb lattice. In contrast, the lower viscosity of formamide results in nanotubes with rougher walls and a more discretized morphology, which inherently provides a greater density of reactive sites beneficial for CO₂ photoreduction. Nb₂O₅ shares several properties with TiO₂ but offers distinct advantages, including a wider band gap that contributes to a higher redox potential, multiple oxide phases conducive to homojunction formation, and, in some cases, enhanced electrical conductivity compared to TiO₂. Following a similar approach, we successfully synthesized Nb₂O₅ nanotubular and nanoporous arrays using a formamide-based electrolyte instead of the conventionally reported ethylene glycol-based electrolyte. We characterized their structural and adsorptive properties and evaluated their photocatalytic performance. FTIR analysis revealed distinct adsorbate-induced spectral features, and hydrogen evolution experiments demonstrated an H₂ generation rate of 71.2 µmol g⁻¹ h⁻¹ under UV-254 nm illumination for bare Nb₂O₅ nanotubes. Given these promising results, further investigations into heterojunctions incorporating plasmonic nanostructures or additional semiconductors are warranted to enhance photocatalytic efficiency further. Combining Nb₂O₅ nanotubes with other materials could potentially improve charge separation and light absorption, leading to higher photocatalytic activity. Exploring such heterojunctions opens new avenues for developing advanced photocatalysts for environmental and energy applications. In summary, electrochemical anodization presents a versatile platform for engineering metal oxide nanostructures with tunable properties. By carefully selecting electrolyte compositions and anodization parameters, it is possible to design nanotubular and nanoporous arrays with specific characteristics suited for photocatalytic applications. The unique properties of Nb₂O₅ nanotubes, particularly when synthesized in formamide-based electrolytes, highlight the potential for developing efficient photocatalysts through controlled conditions.