Chemical transformation and dissociation of amino acids on metal sulfide surface: Insights from DFT into the effect of surface vacancies on alanine-sphalerite system

Abstract

The affinity of amino acid species toward mineral surfaces is a favorable phenomenon in green material science applications. To this end, we performed a DFT calculation to study adsorption of alanine on the sphalerite (ZnS) surface, focusing on the role of surface vacancy defects by removing single sulfur, single zinc and double neighboring congruent atoms from the outermost layer. Removal of single sulfur or zinc atom led to surface reconstruction by forming favorable local Zn-Zn and S-S bonds around the defect sites, respectively. The presence of double neighboring congruent vacancies and their relative positions altered the adsorption configuration to more complicated modes. The lowest-energy configuration was predicted when both sulfur and zinc atoms were removed from the upper hill of the surface. In this case, alanine was dissociatively adsorbed on the surface, and a tridentate bond was formed between alanine and two zinc atoms on the surface. The alanine affinity was also significantly more stable than the defect-free surface with an energy difference of about 55 kcal/mol. In another special case with two Zn and S vacancies, respectively, on the downward and upper hills, alanine underwent a chemical transformation from neutral to zwitterionic form according to an intramolecular dissociation reaction.