Carbon dioxide and solar energy are abundant and accessible resources capable of generating<br/>liquid fuels such as ethanol and other carbon-rich compounds. Silicon is an ideal substrate for converting photons into electrons that molecular catalysts can use to generate energy-rich fuels, as its bandgap is readily tuned with doping to absorb across the visible and near-UV spectrum. The coupling of molecular catalysts to a semiconductor surface results in a subset of chemically modified electrodes known as hybrid photoelectrodes. The molecules that tether catalysts to the surfaces of these hybrid photoelectrodes are an integral part of device performance. The covalent attachment of molecules to the surface of silicon also tunes its electronic properties through dipole-assisted band bending. Selective tuning of the molecule-electrode interface is imperative to impart stability and a desired electrochemical behavior. To overcome the current limitations of hybrid photoelectrodes, we focus on the role of the silicon-anchoring group bond and the role of lateral interactions of the appended molecules. Here we highlight two ongoing projects. First is comparing the stability and electrochemical behavior of silicon-nitrogen and silicon-carbon bond-forming reactions. Using a nitrocyclocondensation (NCC) reaction, we successfully attached methyl, benzene, and pyrene groups to H-terminated Si electrodes. XPS shows that the attachment is successful and proceeds primarily via a Si−O−N−Si bridge structure. We compare these linkages to the silicon-carbon bonds that arise from hydrosilylation and Grignard reactions. Second, we highlight the benefits and limitations of silatranes, which offer a facile attachment method to SiOx/Si electrodes through the formation of siloxane bonds. We observe the reversible electrochemical behavior of Fc-silatrane on SiOx/Si electrodes and use this system to probe the effects of spacer group charge and (radius) to focus on the lateral interactions of the silatrane and SiOx surface in organic and aqueous environments, under illumination, and after continuous voltammetric cycling.

Structure-function relationships to improve the performance and stability of hybrid photoelectrodes on silicon surfaces

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ACS Spring 2023

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