Facet-dependent adsorption and its effect on photocatalytic reactions: insights from diuron degradation on zinc oxide surfaces

Abstract

This study provides direct experimental evidence of facet- and pH-dependent interactions governing the adsorption and photocatalytic degradation of diuron, a persistent herbicide, on zinc oxide (ZnO) surfaces. Unlike conventional studies that assume uniform catalyst surfaces, this work systematically measured the interaction strength on different ZnO facets, using atomic force microscopy (AFM)-based force spectroscopy under conditions replicating the reaction environment. Interaction strengths among ZnO surfaces followed the order: O-terminated > Zn-terminated > mixed-terminated. The finding that ZnO powder, with polar Zn- and O-terminated surfaces, exhibited adsorption capacity approximately five times higher than that of ZnO nanorods, which have non-polar mixed-terminated surfaces, despite having an order of magnitude lower surface area further highlights the role of surface interactions in governing adsorption and subsequent photocatalytic performance. Stronger adsorption under acidic conditions enhanced the degradation efficiency, while alkaline conditions altered adsorption orientations and reduced the capacity. The degradation pathways were found to be highly facet- and pH-dependent, leading to distinct sets of intermediates with varying toxicity. Cytogenotoxicity assays revealed that certain degradation products formed under alkaline conditions were more harmful than diuron itself, underscoring the need for optimized photocatalyst designs. This work provides critical insights into facet-dependent interactions and pH effects, paving the way for more effective and safer photocatalytic solutions for environmental remediation.