Narrow band gap reduced TiO2-B:Cu nanowire heterostructures for efficient visible light absorption, charge separation and photocatalytic degradation

Abstract

Hydrothermally synthesized TiO2-B single-crystalline NWs were uniformly Cu-decorated via co-precipitation method to grow hybrid TiO2-B:Cu–NWs. Starting from commercial TiO2 NPs, systematic evolution of TiO2-B, R–TiO2-B and R–TiO2-B:Cu NW-structures in steps, identified a gradual narrowing of band gap from 3.23 to 2.20 eV. Generation of Ti3+ and oxygen vacancy (Ov) related defect states in TiO2 lattice facilitates enhancing absorption of visible light and also inhibit faster charge recombination via trapping of photogenerated charge carriers. Cu2O nanoparticles intimately attached with TiO2 NW surface enhance the charge carrier separation via interfacial charge transfer between Cu2O and TiO2, involving direct Z-scheme mechanism. Further, metallic copper, being present with an intermediate band gap, can promote electron tunneling across Cu-Cu2O-TiO2 ternary hetero-junctions and dissociate more efficiently the photoexcited electrons and holes from individual pairs and make those amply available for dye degradation reactions which proceed more efficiently in spite of the reduced surface area and porosity of the hetero-structured photocatalyst. Strongly reductive electrons (e−) in the CB of Cu2O produce highly reactive superoxide radicals (O2−) which dominantly contribute in dye degradation, while hydroxyl radicals (OH) produced via oxidation by holes (h+) in the VB of TiO2 also degrade the dye molecules in a relatively lesser significance. The R-TiO2-B:Cu0.2 NW catalyst degrades the MO dye very rapidly with a rate constant ~0.41 min−1 and an efficiency of dye degradation ~91%, and the MB dye even faster with a rate constant ~0.54 min−1 and an efficiency of dye degradation of ~96%, during 60 min of visible light exposure, which are significantly high as compared to the data available in the contemporary literature.