Design Strategies for Coupling CO2 Reduction Molecular Electrocatalysts to Silicon Photocathodes

We explore strategies for enhancing the electronic interaction between silicon nanocrystals (Si NCs) and surface-tethered molecular Re electrocatalysts ([Re]) as models for CO2-reducing photocathodes. Using density functional theory (DFT) combined with electrochemical, spectroscopic, and photocatalytic measurements, we determine that the intrinsic Si (iSi) NC conduction band energy in iSi-[Re] assemblies is below the [Re] lowest unoccupied molecular orbital (LUMO) and singly occupied molecular orbital energies even for strongly quantum-confined 3.0-3.9 nm diameter hydrogen- and methyl-terminated iSi NCs, respectively. We computationally analyze design strategies to align the semiconductor conduction band edge and electrocatalyst frontier molecular orbitals by varying the iSi NC size, introducing boron as a dopant in the Si NC, and modifying the attachment chemistry to the [Re] complex aryl ligand framework. Our DFT analysis identifies a target hybrid structure featuring B-doped silicon (B:Si) NCs and a direct bond between a surface atom and an sp2-hybridized carbon of the electrocatalyst bipyridine aryl ring ligand (B:Si-CAr[Re]). We synthesize the B:Si-CAr[Re] NC assembly and find evidence of direct hybridization between the B:Si NC and the surface [Re] electrocatalyst LUMO using electrochemical measurements and transient absorption spectroscopy. This work provides a blueprint for the design of new Si photocathode-molecular electrocatalyst hybrids for CO2 reduction and related fuel-forming photocatalytic conversions.