Enantioselective adsorption on chiral ceramics with medium entropy

Abstract

Chiral metal surfaces provide an environment for enantioselective adsorption in various processes such as asymmetric catalysis, chiral recognition, and separation. However, they often suffer from limitations such as reduced enantioselectivity caused by kink coalescence and atomic roughness. Here, we present an approach using medium-entropy ceramic (MEC), specifically (CrMoTa)Si₂ with a C40 hexagonal crystal structure, which overcomes the trade-off between thermal stability and enantioselectivity. Experimental confirmation is provided by employing quartz crystal microbalance (QCM), where the electrode is coated with MEC films using non-reactive magnetron sputtering technology. The chiral nature is verified through transmission electron microscopy and circular dichroism. Density-functional theory (DFT) calculations show that the stability of MEC films is significantly higher than that of high-index Cu surfaces. Through a combination of high-throughput DFT calculations and theoretical modeling, we demonstrate the high enantioselectivity (42% e.e.) of the chiral MEC for serine, a prototype molecule for studying enantioselective adsorption. The QCM results show that the adsorption amount of L-serine is 1.58 times higher than that of D-serine within a concentration range of 0-60 mM. These findings demonstrate the potential application of MECs in chiral recognition.

Fig. 1. Structure of CrSi₂-type ceramics and schematic diagram of enantiomeric adsorption.

a CrSi₂ with a C40 hexagonal crystal structure and the slab of MECs, (CrMoTa)Si₂(0001), with a random arrangement of metal elements. On the slab of MECs, there is no symmetry due to the random arrangement of metal elements. b Possible adsorption orientations of L-serine on CrSi₂(0001). There are three possible orientations (0°, 60°, and 120°) due to the rotation diad (C₂) of CrSi₂(0001). c Schematic diagram of the enantioselective adsorption on CrSi₂(0001). Negligible enantioselectivity is foreseen resulting from the rotation diad (C₂) along the (0001) direction. d Schematic diagram of enantioselective adsorption on (CrMoTa)Si₂(0001). The rotation diad (C₂) constituted by metallic atoms is broken by introducing the multiple elements in the helical site. The asymmetric surface configuration confines the enantiomers to locate at the distinct sites, enhancing the asymmetric effect from the helix structure.

Fig. 2. Preparation and characterization of MEC films.

a TEM image of the MEC films, cut by a focused ion beam at 30 kV. b HAADF-STEM image of MECs, with the inset showing the corresponding diffraction spot. c–f Elemental mapping of Cr, Mo, Ta, and Si in the MEC films by EDS, respectively. g XRD pattern of the composite silicide target and GIXRD patterns at different grazing incidence angles for MECs deposited via non-reactive magnetron sputtering. h, i AFM image and topography profile of the MEC films and the Cr transition layer. j Thickness of MEC films and Cr transition layer. k CD spectra of CrSi₂, MoSi₂, TaSi₂, and MECs. Source data are provided as a Source Data file.

Fig. 3. Enantioselective adsorption of serine enantiomers on the (CrMoTa)Si₂(0001) surface.

a, b Schematic diagrams illustrating the high and low energy adsorption configuration (μ₄ and μ_3’) for L- and D-serine. The subscript denotes the number of bonds between enantiomers and MECs. c, d Adsorption energy distribution of different arrangements of surface atoms. The inset in (d) illustrates the defined arrangement of surface atoms, where the red and blue triangles indicate the location of adsorbed molecules for L- and D-serine, respectively. e, f Total adsorption energy distribution function X and the corresponding adsorption uptake θ_t as a function of adsorption energy. Two distinct peaks are observed, corresponding to the μ₄ and μ_3’ adsorption configurations. Source data are provided as a Source Data file.

Fig. 4. Competitive adsorption of serine enantiomers on the (CrMoTa)Si₂(0001) surface.

a Schematic diagram of competitive adsorption. For multicomponent adsorption, different adsorbates compete for the adsorption sites characterized by maximum adsorption energies. Here, we weigh the adsorption energies of L- and D-serine on given sites to evaluate the adsorption preference of those sites. b, c Revised adsorption energy distribution considering the competitive adsorption of serine enantiomers (b) and the corresponding adsorption uptake (c). The adsorption uptake is obtained by ${\theta }_{\mathrm{t}}={\int }_{-\infty }^{{\epsilon }_{\mathrm{e}}}\left\{\theta \left(\epsilon \right)X\left(\epsilon \right)\right\}\mathrm{d}\epsilon$. Source data are provided as a Source Data file.

Fig. 5. Deformation energy and electronic structure analysis of enantioselective adsorption.

a Statistical results of deformation energy of serine enantiomers on MECs. The left column presents the deformation energies of the substrate, molecules, and systems for D-serine adsorbed on MEC films, while the right column presents these energies for L-serine. b Schematic diagram of charge distribution before and after the adsorption of the molecule on MECs. c–f Spatial distribution of charge transfer for L- and D-serine enantiomers on CrSi₂ (c, d) and MECs (e, f). The isosurface value is set to 0.005 and 0.05 e Å⁻³ for serine on CrSi₂ and MECs, respectively. g–i Projected density of states (PDOS) of (g) L- and (h) D-serine on CrSi₂ and MECs, and (i) ΔPDOS, which represents the difference in PDOS between L-enantiomers and D-enantiomers. Source data are provided as a Source Data file.

Fig. 6. Enantioselective adsorption of serine on MEC films.

a–f XPS spectra of C 1 s, N 1 s, and O 1 s for (a–c) L- and (d–f) D-serine adsorbed on MECs. g Real-time adsorption behaviors of serine enantiomers based on the frequency shift measured by QCM. The concentration of serine solution is 0.3 M. h The frequency shift changes of serine enantiomers on the MEC surface according to the concentration changes. The error bars represent the standard deviation of three independent samples. The red dots and lines represent the L-serine, while the blue ones correspond to D-serine. Source data are provided as a Source Data file.