Synthesis of Single-Crystalline Cu3B2O6/CuB2O4 and Their Photocatalytic Degradation of Methylene Blue under Visible-Light Irradiation

Single-crystalline Cu3B2O6/CuB2O4 was successfully prepared by a sol-gel method from cupric nitrate/cupric acetate and boric acid, using citric acid as a foaming agent. The obtained materials were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and thermogravimetry-differential thermal analysis (TG-DTA). The photodegradation of methylene blue (MB) solution was used to evaluate the photocatalytic activity of Cu3B2O6/CuB2O4 under visible-light irradiation (400 nm<lambda<1100 nm). The results indicated that both Cu3B2O6 and CuB2O4 displayed good photocatalytic activity. Under visible-light irradiation for 6 h, the photocatalytic activities of CuB2O4 and Cu3B2O6 reached 63.36% and 99.52%, respectively, in MB aqueous solution (50 mg.L-1) containing 1g.L-1 catalyst. Ultraviolet-visible analysis showed that the width of the midgap state for Cu3B2O6 is 1.78 eV, which is much narrower than that of CuB2O4 (1.95 eV), and the band gap of Cu3B2O6 is narrow (E-g=2.34 eV). These results indicated that electron transitions can occur in both the nnidgap state and forbidden band for Cu3B2O6; this is why Cu3B2O6 has higher visible-light photocatalytic activity than CuB2O4.