Photoactivity of amorphous and crystalline TiO₂ nanotube arrays (TNA) films in gas phase CO₂ reduction to methane with simultaneous H₂ production

This study assessed the photoactivity of amorphous and crystalline TiO₂ nanotube arrays (TNA) films in gas phase CO₂ reduction. The TNA photocatalysts were fabricated by titanium anodization and submitted to an annealing treatment for crystallization and/or cathodic reduction to introduce Ti³⁺ and oxygen vacancies into the TiO₂ structure. The cathodic reduction demonstrated a significant effect on the generated photocurrent. The photoactivity of the four TNA catalysts in CO₂ reduction with water vapor was evaluated under UV irradiation for 3 h, where CH₄ and H₂ were detected as products. The annealed sample exhibited the best performance towards methane with a production rate of 78 μmol g_cat⁻¹ h⁻¹, followed by the amorphous film, which also exhibited an impressive formation rate of 64 μmol g_cat⁻¹ h⁻¹. The amorphous and reduced-amorphous films exhibited outstanding photoactivity regarding H₂ production (142 and 144 μmol g_cat⁻¹ h⁻¹, respectively). The annealed catalyst also revealed a good performance for H₂ production (132 μmol g_cat⁻¹ h⁻¹) and high stability up to five reaction cycles. Molecular dynamic simulations demonstrated the changes in the band structure by introducing oxygen vacancies. The topics covered in this study contribute to the Sustainable Development Goals (SDG), involving affordable and clean energy (SDG#7) and industry, innovation, and infrastructure (SDG#9).