Enhanced photocatalytic reduction of aqueous Re(VII) in ambient air by amorphous TiO2/g-C3N4 photocatalysts: Implications for Tc(VII) elimination

Abstract

Although effective photocatalytic removal for aqueous ReO4− (as a chemical analogue of TcO4−) have been established, photocatalysts are not satisfactory for further applications in term of low light harvesting efficiency and fast re-dissolution of ReO2·nH2O. Herein, the amorphous TiO2/g-C3N4 composites are fabricated by a simple hydrolysis approach, producing the amorphous TiO2/30 wt% g-C3N4 (TCN-3) composite with larger specific surface area (~456 m2 g−1), narrow band gap (~2.67 eV) and excellent light-capturing ability when compared with commercial P25. Lower recombination efficiency of photo-generated electron-hole based on the Z-scheme mechanism between amorphous TiO2 and g-C3N4 was demonstrated by electron paramagnetic resonance and photoelectrochemical analysis. In addition, the as-synthesized TCN-3 exhibits an excellent photocatalytic reduction of Re(VII) in ambient air with the highest removal percentage of ~90% as compared with crystalline TiO2/g-C3N4, and can be regenerated even after eight recycles. X-ray absorption spectroscopy confirmed that the extremely slow re-dissolution of Re(IV) resulted from both shortening of ReO2·nH2O chains (decrease in coordination number of Re-Re) and association of Re(IV) with amorphous TiO2 in TCN-3 (formation of Re-Ti bond). The overall results further facilitate the photocatalytic reduction and elimination for rhenium/technetium, and open the wide-range applications for photocatalysis in the disposal of some radioactive elements.