Structural Characterization of Zn-MOF and Its Derived OVs-ZnO: Enhanced Visible-Light Photocatalytic Degradation for Tetracycline

A Zn-based metal-organic framework (Zn-MOF) with a two-dimensional structure was successfully constructed via a solvothermal method. X-ray single-crystal diffraction analysis shows that Zn-MOF crystallizes in the monoclinic system with the P 2₁/ c space group. The Zn²⁺ ions exhibit a four-coordinated tetrahedral configuration, forming a uninodal 4-connected tetragonal two-dimensional framework with the point symbol {4 ⁴ ·6 ² }. First-principles calculations reveal that Zn-MOF is an indirect band-gap compound with a band gap of 2.64 eV, which is mainly determined by the bpydb²⁻ anionic units. Thermogravimetric (TG) analysis indicates that Zn-MOF undergoes major weight loss in the range of 395–490°C. Therefore, ZnO with different concentrations of oxygen vacancies (OVs-ZnO) was prepared by mixing Zn-MOF with different masses of glucose and calcining the mixture at 500°C. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results confirm that the main component of the calcined samples is ZnO. Electron paramagnetic resonance (EPR) analysis shows the presence of oxygen vacancies (OVs) on the surface of the prepared ZnO samples. Ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) analysis indicates that the band gap widths (E g ) of Zn-MOF and OVs-ZnO are 2.68 eV, 2.55 eV, 2.29 eV, 2.07 eV and 1.76 eV, respectively. Electrochemical impedance spectroscopy (EIS) and photocurrent (PCR) measurements demonstrate that OVs-ZnO (after calcination) is more favorable for the separation of photogenerated electron-hole pairs than Zn-MOF. Photocatalytic degradation experiments show that within 60 min, the photocatalytic degradation rate of TC by OVs-ZnO is significantly higher than that by Zn-MOF; in particular, the degradation rate of TC by OVs-ZnO-4 can reach over 90%. Radical trapping experiments reveal that h⁺, ·OH and ·O₂⁻ are all involved in the photocatalytic degradation process of TC, with ·OH being the key reactive species.