Boosting visible light-driven photocatalysis with bismuth titanate (Bi₂Ti₄O₁₁/Bi₄Ti₃O₁₂) heterojunctions

Bismuth titanates have emerged as promising photocatalytic materials for environmental remediation due to their unique structural and electronic properties. While compounds such as Bi₂Ti₄O₁₁, Bi₂Ti₂O₇, Bi₄Ti₃O₁₂, Bi₁₂TiO₂₀, and Bi₂₀TiO₃₂ have been widely studied, the photocatalytic potential of Bi₂Ti₄O₁₁/Bi₄Ti₃O₁₂ (B2/B4) heterojunctions remain unexplored. In this work, B2/B4 heterojunctions with varying weight ratios of 0/100, 25/75, 50/50, 75/25, and 100/0 were synthesized by high-energy ball milling (4 h) and solid-state reaction at 800 °C. XRD and Raman spectroscopy confirmed the formation of pure-phase Bi₂Ti₄O₁₁ and Bi₄Ti₃O₁₂ in the B2/B4 heterojunctions. Photocatalytic activity of the synthesized heterojunctions was evaluated under artificial visible light irradiation from a 35 W Xe lamp (6000 K, 36,000 lux, 250 W/m²) and natural sunlight (450-550 W/m²) using Rhodamine B (RhB, 10 mg/L) as a model pollutant. The B2/B4-25/75 heterojunction showed the best performance, achieving 100 % RhB degradation within 4 h, surpassing both individual components (Bi₂Ti₄O₁₁: 40 %, Bi₄Ti₃O₁₂: 63 %). Under sunlight, similar degradation efficiency (near 100 %) was observed. EPR analyses and scavenger experiments confirmed that the photocatalytic process proceeds mainly via h⁺ and O₂^•--mediated oxidation Notably, photocatalytic activity significantly improved in the presence of NaCl (simulating real wastewater conditions) and at pH 3, reaching 99 % degradation in 60 min, (vs. 58 % in deionized water). In contrast to Degussa P25, which achieved complete degradation within 30 min but remained suspended after 24 h, the B2/B4 heterojunction fully settled within 30 min, enabling straightforward recovery and reuse over six consecutive cycles without detectable loss of activity. These results demonstrate that B2/B4 heterojunctions are highly efficient, solar light responsive, and easily recoverable photocatalysts with great potential for water treatment applications.