N-doped Z-scheme heterojunctions on cobalt foam for enhanced photocatalytic degradation of levofloxacin and reduction of hexavalent chromium: Insights into charge separation and catalytic mechanisms

Abstract

The transfer of photoelectrons across heterogeneous materials stands as a cornerstone process within the domain of photocatalysis. Elucidating the mechanisms underlying electron transfer at heterojunction interfaces provides profound insights that are crucial for the rational design of efficient catalysts. In this study, we employ a hierarchical synthesis approach to fabricate N-doped Z-scheme heterojunctions directly on the surface of cobalt foam (CoF). Both experimental observations and density functional theory (DFT) calculations reveal that N-doped CoS2 nanosheets exhibit asymmetric charge distribution, functioning as localized electron traps. Further analysis of differential charge density elucidates that positive charges are predominantly concentrated in proximity to N-CoS2, whereas negative charges accumulate near ZnO, indicating substantial charge exchange between these two phases. The integration of a potent built-in electric field upon illumination facilitates the splitting of photogenerated carriers in both transverse and longitudinal directions, thereby enabling their effective separation and migration. The CoF@N-CS@ZO composites demonstrated a remarkable capacity for simultaneous degradation and reduction, achieving a 93.6 % degradation of levofloxacin (LEV) and a 96.9 % reduction of hexavalent chromium (Cr(VI)) under visible light irradiation for 85 min. Furthermore, due to the active species being anchored on the surface of the CoF, which serves as a recyclable substrate, the composites could be directly recycled post-use. Notably, CoF@N-CS@ZO maintained an efficiency of over 80 % for both LEV degradation and Cr(VI) reduction even after undergoing more than ten cycles. This research elucidates the intricate interplay of the synergistic N-doped built-in electric field on the charge transfer process, offering profound insights into the targeted design of charge-modulated materials for enhanced photocatalytic performance.