NH2-MIL-88B (Fe α In1- α ) mixed-MOFs designed for enhancing photocatalytic Cr(vi) reduction and tetracycline elimination

Abstract

Aiming at solving the issue of wastewater purification, this work synthesized NH₂-MIL-88B (Fe _α In_1-α ) photocatalysts by a simple one-pot method, which was employed for photocatalytic reduction of Cr(vi) and oxidation of TC-HCl. Compared with traditional NH₂-MIL-88B (Fe) photocatalysts, NH₂-MIL-88B (Fe_0.6In_0.4) displayed excellent photocatalytic performance; the photocatalytic redox rate for Cr(vi) and TC-HCl reached 86.83% and 72.05%, respectively. The good photocatalytic performance might be attributed to the metal-to-metal charge transition (MMCT) between Fe-O clusters and In-O clusters formed by doping In(iii) into NH₂-MIL-88B (Fe), which provides effective active sites for the photocatalytic reduction and oxidation routes. Besides, the synergistic effect of the ligand-to-metal charge transition (LMCT) and MMCT expands the separation and transfer of photogenerated carriers and inhibits the recombination of electron-hole pairs, thus effectively improving the photocatalytic performance. Therefore, this work could provide a new method for the construction of mixed metal MOFs for the photocatalytic degradation of pollutants.

Scheme 1. Synthesis diagram of the synthetic processes of NH₂-MIL-88B (Fe_αIn_1−α).

Fig. 1. (a) XRD patterns and (b) FT-IR of NH₂-MIL-88B (Fe) and NH₂-MIL-88B (Fe_αIn_1−α) samples.

Fig. 2. SEM images of (a) NH₂-MIL-88B (Fe), (b–e) NH₂-MIL-88B (Fe_αIn_1−α) samples (α = 0.8, 0.6, 0.4, 0.2, respectively), (f) the compared SEM images of NH₂-MIL-88B (Fe_αIn_1−α), (g) TEM image of NH₂-MIL-88B (Fe_0.6In_0.4) and (h) EDX image of NH₂-MIL-88B (Fe_αIn_1−α).

Fig. 3. XPS spectra of NH₂-MIL-88B (Fe_0.6In_0.4) composite (a). Survey; (b) C 1s; (c) N 1s; (d) O 1s; (e) Fe 2p; (f) In 3d, respectively.

Fig. 4. (a) UV-vis DRS spectrum and (b) plots of (αhν)²versus photon energy (hν) of the NH₂-MIL-88B (Fe) and the NH₂-MIL-88B (Fe_αIn_1−α) samples.

Fig. 5. Photocatalytic reduction of Cr(vi) (a) blank group comparison, (b) NH₂-MIL-88B (Fe_αIn_1−α), (c) Cr(vi) on different concentrations reduction performance, and (d) the fitting of the pseudo-first-order linear line for the photocatalytic reaction. (Reaction conditions: 0.5 g L⁻¹ photocatalyst, 3 mL ethanol, RT, pH = 2).

Fig. 6. Photocatalytic degradation of TC-HCl (a) blank group comparison, (b) NH₂-MIL-88B (Fe_αIn_1−α), (c) TC-HCl on different concentrations degradation performance and (d) the fitting of the pseudo-first-order linear line for the photocatalytic reaction. (Reaction conditions: 0.1 g L⁻¹ photocatalyst, RT).

Fig. 7. ESR spectra of DMPO-˙O₂⁻ and DMPO-˙OH adducts generated NH₂-MIL-88B (Fe_αIn_1−α) after 60 seconds under visible light irradiation (λ > 400 nm) during the photocatalytic process.

Scheme 2. Possible mechanism of photocatalytic degradation of Cr(vi) and TC-HCl by NH₂-MIL-88B (Fe_αIn_1−α).