Epitaxial Electrodeposition of Chiral Metal Surfaces on Silicon(643)

Abstract

Surfaces of achiral materials exhibit two-dimensional chirality if they lack mirror symmetry. An example is the (643) surface of face-centered-cubic metals such as Au. The (643) and (6̅4̅3̅) surfaces are non-superimposable mirror images of each other. Chiral surfaces offer the possibility of serving as heterogeneous catalysts for chiral synthesis or providing a platform for chiral separation or crystallization. Here, we show the symmetry requirements for surface chirality, and we demonstrate that chiral surfaces can be produced by electrochemically depositing epitaxial films of Au onto commercially available Si(643) wafers. Au(643) is deposited onto one side of the wafer, and its enantiomer Au(6̅4̅3̅) is deposited on the other side of the wafer. In addition to the (643) orientation, the (8 14 17) orientation of Au is produced on the Si(643) wafers. The (8 14 17) orientation has a similar kinked surface to the (643) surface, but it has staggered kinks. Other metal films including Pt, Ni, Cu, and Ag that are electrodeposited onto the Au films exhibit strong in-plane and out-of-plane order. Hence, the method provides a pathway for producing chiral surfaces of a wide range of materials, and it obviates the need to work with expensive single crystals. The Ag/Au/Si(643) surface showed a preference for the electrochemical oxidation of d-glucose, whereas the Ag/Au/Si(6̅4̅3̅) surface showed preference for the oxidation of l-glucose.

Encoding Chiral Molecular Information in Metal Structures

“…In addition, other metal films such as Pt, Ni, Cu, and Au can also be electrodeposited on the Au films and result in asymmetric surface structures. This concept is very promising for producing chiral surfaces made out of a wide range of materials on the silicon wafers …”

“…In addition, several epitaxial metal films such as Pt, Ni, Cu, and Ag that have been electrodeposited on chiral Au supported on Si wafers, as stated above, also show very promising results with respect to the enantioselective oxidation of d ‐ and l ‐glucose . For example, silver electrodeposited on a Au/Si(6 4 3) surface is more active for the oxidation of d ‐glucose compared with l ‐glucose and vice versa for Ag electrodeposited on Au/Si( ) as shown in Figure …”

“…This concepti s very promising for producing chiral surfaces made out of a wide range of materials on the silicon wafers. [32] To enhance the chiral character of metal surfaces, it has been recently demonstrated that the combinationo fe ncoding molecular structures and mesoporousf eatures has distinct advantages, such as as ignificantly improved enantioselectivity owing to an extremelyh igh active surfacea rea. [12a, 24] Chirally imprinted Pt combining chiral information and mesoporosity has been successfully obtained by electrodeposition of metals in the presence of self-assembled lyotropic liquid crystal surfactants [33] and chiral molecules as shown in Figure 1.…”

“…[31a] In addition, several epitaxial metal films such as Pt, Ni, Cu, and Ag that have been electrodeposited on chiral Au supported on Si wafers, as stated above,a lso show very promisingr esults with respect to the enantioselective oxidation of d-a nd l-glucose. [32] For example, silver electrodeposited on aA u/ Si(6 43)s urface is more active for the oxidation of d-glucose compared with l-glucose and vice versa for Ag electrodeposited on Au/Si(6 4 3 )a sshown in Figure4. [32] The beneficial effect of combining chirality with the mesoporousfeatureso fmetal surfaces leads to avery significant enhancement of the recognition efficiency.T he enantioselectivity of molecular imprintedm esoporousp latinum hasb een examined by using differential pulse voltammetry (DPV).Avery differentb ehavior is observed when using mesoporousm etal films imprinted with one or the other enantiomer,e ven after removal of the chiral templates.…”

“…[32] For example, silver electrodeposited on aA u/ Si(6 43)s urface is more active for the oxidation of d-glucose compared with l-glucose and vice versa for Ag electrodeposited on Au/Si(6 4 3 )a sshown in Figure4. [32] The beneficial effect of combining chirality with the mesoporousfeatureso fmetal surfaces leads to avery significant enhancement of the recognition efficiency.T he enantioselectivity of molecular imprintedm esoporousp latinum hasb een examined by using differential pulse voltammetry (DPV).Avery differentb ehavior is observed when using mesoporousm etal films imprinted with one or the other enantiomer,e ven after removal of the chiral templates. It was found that the chiral mesoporousp latinum film imprinted with d-DOPAi sm uch more active for the electrooxidation of d-DOPAa nd vicev ersa for the one imprinted with the l-enantiomer.T oc onfirm the presenceo fi mprinted chiral information, the Pt electrode initially imprinted with l-DOPAw as scanned in the potential range À0.2 Vt o1 .25 V( vs. Ag/AgCl) in 0.5 m H 2 SO 4 ,r esulting in the complete destruction of the chiral information owing to an oxidation of the Pt surfacea toms ( Figure 5).…”