Chemical Functionalization of ZnS: A Perspective from the Ligand–ZnS Bond Character

Chemical functionalization of metal sulfides plays a critical role in many fields such as materials science and froth flotation. The commonly used thiol-bearing functionalized ligands are generally considered to bind with metal sulfides covalently, and the computational binding energy is widely used to evaluate the functionality of the ligands toward metal sulfides. Herein, we studied the surface chemistry of the model ZnS and its binding with typical S- and O-terminated ligands using density functional theory calculations with an emphasis on the resulting bond character. Surprisingly, it was found that the ligand–ZnS(110) bond is essentially ionic with limited covalency. This very fundamental finding was further extended to the hydrophobization of ZnS in the context of froth flotation and rationalized the previously unresolved phenomenon that the higher the ligand–ZnS(110) binding strength, the lower the hydrophobic functionality of the ligand toward ZnS. Meanwhile, instead of the binding energy, the electronegativity of the ligand was identified as an effective computational descriptor that can accurately predict the relative hydrophobic functionality of the ligand toward ZnS. This work, therefore, further advanced our understanding of the intrinsic ligand–metal sulfide binding mechanism and highlighted the importance of computational parameters, beyond the binding energy, in guiding the first principles design of ligands with enhanced functionalities or optimizing relevant industrial processes.