Facet-dependent oxysulfidation of Cu2O nanomaterials: implications for improving the efficacy of nanopesticides

Copper (hydr)oxide nanomaterials are an important class of nanomaterials with various applications, including next-generation pesticides. The efficacy of these materials is largely affected by oxysulfidation, one of the most important transformation processes in the environment. Here, we show that the extent and route of oxysulfidation are facet-dependent for these materials. Specifically, oxysulfidation of Cu2O_{100} and Cu2O_{111}-two Cu2O nanomaterials with predominantly exposed {100} and {111} facets-is fast and complete, with only a hollow shell left at the end of the experiment. In comparison, oxysulfidation of Cu2O_{110}, a nanomaterial with {110} facet, is much less complete, in that, the end-product exhibits a yolk-shell structure with a large cuprite core. The varied degrees of oxysulfidation are attributable to the facet-dependent adsorption affinities of Cu2O for both oxygen and sulfide ions, leading to the formation of different initial oxysulfidation products. Unlike the porous coatings of yarrowite on Cu2O_{111} and a mixture of yarrowite and covellite on Cu2O_{100}, the condensed layer of djurleite formed on Cu2O_{110} passivates the material by sealing the surface of Cu2O, hindering subsequent copper dissolution. Consequently, Cu ions released from Cu2O_{100} and Cu2O_{111} are 2.2 and 2.4 times higher than that from Cu2O_{110}. These findings underline the important role of exposed facets in dictating the interfacial processes of soft metal-based nanomaterials, and have important implications for improving the efficiency of nanopesticides in redox dynamic rhizospheres to minimize the environmental impacts associated with the overuse of conventional pesticides.