Charge-state-dependent adsorption and photocatalysis of organic pollutants over ZnO quantum dots and MOF-derived ZnO: Experimental and computational insights

Engineering ZnO nanostructures with controlled electronic and surface properties is essential for efficient photocatalytic removal of persistent organic pollutants. However, understanding the interplay between adsorption and photocatalysis, particularly for molecules with different charge states, remains a critical challenge. Herein, ultrasmall ZnO quantum dots (ZQD) synthesized via a low-temperature precursor mixing route were directly compared with mesoporous ZnO nanoparticles (ZNP) derived from ZIF-8 calcination. Despite sharing the same wurtzite phase, both materials exhibit markedly different textural and electronic characteristics, where ZQD shows a higher specific surface area (44.50 m²g⁻¹), slight bandgap narrowing (2.9 eV), and improved charge separation relative to ZNP (12.35 m²g⁻¹; 3.0 eV). Adsorption and photocatalytic experiments using representative ionic (methyl orange), weakly ionizable (2-naphthol), and non-ionic (carbaryl) pollutants reveal distinct kinetic behaviors, with ionic methyl orange exhibiting higher adsorption capacity and degradation efficiency. Adsorption kinetics revealed chemisorption-dominated behavior for methyl orange (MO) and 2-naphthol, while carbaryl followed diffusion-controlled physisorption. Under UV-LED irradiation, ZQD achieved near-complete MO degradation within 60 min and consistently outperformed ZNP for all tested pollutants. To elucidate the underlying mechanism, density functional theory (DFT), charge density difference (CDD), non-covalent interaction (NCI), density of states (DOS), Fukui function, and excited-state (STEOM-DLPNO-CCSD) analyses were employed. The results demonstrate that ionic pollutants promote strong interfacial electronic coupling with ZnO, facilitating charge transfer, and orbital hybridization, whereas non-ionic molecules interact predominantly through weak van der Waals and hydrogen bonding interactions, resulting in limited photocatalytic activity. Importantly, photocatalytic performance is maximized at an optimal interaction strength, where sufficient charge transfer is achieved without excessive electron delocalization that can promote recombination. This work establishes a unified synthesis–structure–reactivity framework bridging adsorption and photocatalysis, highlighting the decisive role of charge-state-dependent electronic coupling and providing a rational design strategy for ZnO-based systems in complex environmental remediation.