Fabrication of visible-light-responsive black TiO₂ photocatalytic hollow fiber membranes for ammonia gas removal

Improving indoor air quality is a critical public health concern, yet current photocatalytic membranes are limited in their efficiency under visible light conditions. This study presents an innovative approach by introducing nitrogen-doped black TiO₂ (BT550) onto α-Al₂O₃ hollow fiber membrane, aiming to address this limitation and enhancing photocatalytic efficiency under visible light. Nitrogen doping introduced structural modifications, including oxygen vacancies and Ti³⁺ species, which not only improved the material's visible light absorption but also enhanced charge separation efficiency. These changes enabled the nitrogen-calcined TiO₂ membrane at 550 °C (BT550M) to achieve superior ammonia removal efficiencies compared to conventional TiO₂ membranes. Compared to air-calcined membranes, the BT550M membrane demonstrated significant improvements in photocatalytic performance. Under optimized conditions, BT550M exhibited maximum ammonia removal efficiencies of 97.0 % under UV light and 82.8 % under visible light at a flow rate of 10 sccm, outperforming the air-calcined TiO₂ membrane at 550 °C (T550M). Long-term stability tests confirmed that BT550M maintained consistent performance over six days, achieving stable removal rates of 95.6–97.1 % under UV light and 81.3–83.1 % under visible light. These findings highlight the potential of the nitrogen-doped TiO₂ membranes, particularly BT550M, are highly promising for practical, continuous air purification applications, especially in removing harmful ammonia gases.