Interfacial effects on charge separation and photocatalytic activity of ZnO photocatalysts modified with ionic liquids

Abstract

Zinc oxide (ZnO) is a prominent semiconductor for photocatalysis; however, its efficiency is often hampered by the rapid recombination of photogenerated electron-hole pairs. To mitigate this limitation, we designed a surface engineering approach based on the functionalization of hydrothermally synthesized ZnO nanoparticles with the protic ionic liquid triethylammonium propanesulfonate tetrafluoroborate ([TEA-PS][BF4]) at different loadings (2.5-10 wt%). X-ray diffraction (XRD) analysis confirmed the formation of wurtzite ZnO with nanometric crystallite sizes (26-47 nm), indicating that the ionic liquid (IL) influenced nucleation and crystal growth by acting as a capping agent during synthesis. Scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR) results confirmed successful surface modification and revealed IL-induced morphological changes. Photocatalytic activity was assessed via methylene blue (MB) degradation under ultraviolet (UV) light, with the optimization of catalyst mass and pH. The best performance was achieved using 40 mg of catalyst under alkaline conditions (pH 10). Under these conditions, pristine ZnO degraded 71.7% of MB after 120 min, while the ionic-liquid-modified samples reached 89.6%. The sample containing 5.0 wt% ionic liquid (Z-IL5) showed the highest efficiency, attributed to enhanced adsorption and increased accessible surface area. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were performed on IL-adsorbed ZnO clusters. The computed interaction energies and non-covalent interaction (NCI) analyses confirmed the strong interfacial anchoring of the IL, while excited-state and frontier molecular orbital descriptors indicated electronic effects consistent with improved charge separation. Finally, ionic liquid functionalization is an effective surface engineering strategy for improving the photocatalytic activity of ZnO, with interfacial effects likely contributing to reduced charge losses and enhanced degradation kinetics in aqueous media.