High-efficiency photocatalytic reduction of Cr(vi) by Z-scheme electron transfer in UiO-66-NH2@HDU-25 heterojunctions

Abstract

The heavy metal ion hexavalent chromium (Cr(VI)) in wastewater significantly threatens public health. Heterojunction photocatalysts typically demonstrate high reduction efficiency in water for Cr(VI) contamination. Overcoming the drawbacks of single metal organic frameworks (MOFs) or covalent organic frameworks (COFs), MOF@COF heterostructures inherit the advantages of large porosity, high surface area, and multiple active sites of single components. Herein, we report novel hybrid UiO-66-NH₂@HDU-25 heterojunctions based on a condensation reaction. The optimal ratio of UiO-66-NH₂@HDU-25 presented significantly superior photocatalytic activity to most reported MOFs and COFs due to heterojunction formation, which promoted charge separation. Under identical conditions, the optimally formulated UiO-66-NH₂@HDU-25 could remove over 99% of Cr(VI), approximately 1.8 and 2.6 times the original UiO-66-NH₂ and HDU-25 levels. Furthermore, the UiO-66-NH₂@HDU-25 (8 : 1) heterojunction demonstrated favorable reusability and stability over five reaction cycles. Various experiments and density functional theory (DFT) calculations investigated the charge carrier transport pathways and possible photocatalytic mechanisms. The Z-scheme mechanism for the photocatalytic Cr(VI) reduction by UiO-66-NH₂@HDU-25 was proposed and validated. This work aims to open new horizons for preparing novel photocatalyst heterojunctions with excellent visible light responsiveness for environmental water remediation.