Abstract:

Recently, MOF-derived metal oxides have been demonstrated as excellent semiconductor materials. Their derivatives can retain the high porosity and high specific surface area of the parent MOFs, effectively improving the adsorption capacity and mass transfer rate of reactive substances. Herein, UiO-67 was selected as the substrate due to its high hydrothermal stability and high specific surface area. Through the in situ growth of TiO₂ nanoparticles, a series of double metal composite catalysts (Zr_xTi/C) were produced after calcination at 400 ℃. The X-ray diffraction (XRD) patterns showed that the UiO-67 crystal structure of collapsed after calcination, forming a Zr-O-C/TiO₂ heterojunction. Energy-dispersive spectrometry (EDS) mapping images showed that Ti, Zr, and O were evenly distributed throughout the materials without obvious aggregation. Compared to conventional inorganic semiconductor materials, Zr_xTi/C heterojunction catalysts provided much higher BET surface area (317 m²·g^-1) for the effective enrichment of contaminants on the catalyst surface. Tetracycline was selected as a representative antibiotic to study photodegradation performance under a 300 W Xenon lamp. Among the obtained catalysts, the Zr_0.3Ti/C heterojunction catalyst exhibited the best photocatalytic efficiency, achieving 98% degradation within 30 min in a 10 mg·L^-1 tetracycline solution. Fluorescence spectra, electrochemical impedance spectroscopy, and transient photocurrent responses showed that the Zr_0.3Ti/C heterojunction catalyst exhibited the fastest charge-hole separation rate and a maximum photocurrent density of 8.75 μA·cm^-2, which was 2.64 and 3.71 times those of UiO-67 and UiO-67_0.3/TiO₂, respectively. The mechanism of tetracycline photodegradation was determined by UV-visible diffuse-reflectance absorption spectroscopy, Mott-Schottky plots, and electron spin resonance technology. A direct Z-scheme charge separation path was formed by the transfer and recombination of photoexcited e^- in the conduction band (CB) of Zr-O-C with h⁺ in the valence band (VB) of TiO₂, which effectively reduced the combination rate of e^- and h⁺ in Zr-O-C and TiO₂. The photodegradation rate constant of Zr_0.3Ti/C was 16 and 3.7 times those of TiO₂ and Zr-O-C, respectively, due to its large specific surface area and excellent tetracycline adsorption performance. Furthermore, the Zr-O-C/TiO₂ heterostructure exhibiting suitable energy level matching and containing highly conductive carbon material improved the separation and migration of electron-hole pairs. Mechanistic studies revealed that the three types of radicals, superoxide radicals (O₂^•-), hydroxyl radicals (•OH), and a small amount of holes (h⁺) simultaneously promoted tetracycline photodegradation. After five recycling tests, Zr_0.3Ti/C heterojunction catalyst maintained 91.2% removal efficiency for tetracycline, indicating good cycle stability. Combining the synergistic effects of adsorption and photodegradation, using bimetallic heterojunction composites with high specific surface area is promising for the photodegradation of environmental pollutants.

Key words: Photocatalytic degradation, Porous catalyst, Bimetallic heterojunction, Metal organic framework material, Tetracycline