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Adsorption of lactic acid on chiral Pt surfaces - A Density Functional Theory study

The adsorption of the chiral molecule lactic acid on chiral Pt surfaces is studied by Density Functional Theory calculations. First we study the adsorption of L-lactic acid on the flat Pt(111) surface. Using the oPBE-vdW functional which includes van der Waals forces on an ab initio level, it is shown that the molecule has two binding sites, a carboxyl and the hydroxyl oxygen atoms. Since real chiral surfaces are (i) known to undergo thermal roughening that alters the distribution of kinks and step edges but not the overall chirality and (ii) kink sites and edge sites are usually the energetically most favored adsorption sites, we focus on two surfaces that allow qualitative sampling of the most probable adsorption sites. We hereby consider chiral surfaces exhibiting (111) facets, in particular Pt(321) and Pt(643). The binding sites are either both on kink sites - which is the case for Pt(321) or on one kink site - as on Pt(643). The binding energy of the molecule on the chiral surfaces is much higher than on the Pt(111) surface. We show that the carboxyl group interacts more strongly than the hydroxyl group with the kink sites. The results reveal rather small chiral selectivities on the order of 20 meV for the Pt(321) and Pt(643) surfaces. L-lactic acid is more stable on Pt(321)$^S$ than D-lactic acid, while the chiral selectivity is inverted on Pt(643)$^S$. The most stable adsorption configurations of L- and D-lactic acid are similar for Pt(321) but differ for Pt(643). We explore the impact of the different adsorption geometries on the work function which is important for field ion microscopy.