Direct Probe of Quasi-Fermi Level of Metal Cocatalyst in Hybrid Photoelectrodes

Photovoltage is an essential indicator of the driving force that a photoelectrode can provide for surface catalytic reactions, such as water splitting, and CO2 reduction reaction (CO2RR). Various surface modification methods and innovative architecture designs of the photoelectrodes have been designed to boost the photovoltage generation capability. However, for heterogeneous, multi-junction photoelectrodes that contain metal nanoparticles, direct photovoltage measurements at the metal sites under operational conditions remain challenging. We reported, for the first time, an in-situ spectroscopic approach to probe the quasi-Fermi level of metal catalyst sites and thus the photovoltage generated in heterogeneous photocathodes via surface-enhanced Raman spectroscopy (SERS). We studied a CO2RR photocathode, which consists of the nanoporous p-type Si modified with Ag nanoparticles, as a prototype and demonstrated a selective probe of ~ 0.59 V photovoltage generated at the Si/SiOx/Ag junctions. We further apply this method to probe the Fermi level of the metal catalyst in a more complicated photocathode system where p-type silicon is modified with surface-bound viologen polymer and Au nanoparticles incorporated in the viologen polymer. Using surface-enhanced Raman spectroscopy, we were able to in situ determine not only the Fermi level of the Au catalyst sites but also the Fermi level of viologen polymer under different Si Fermi levels. With the help of the photoelectrochemical performance, the operando probe of the Fermi level of all three components in the photocathode demonstrates a comprehensive understanding of the electron transfer mechanism in this photocathode. This vibrational Stark probing approach paves the way for the thermodynamic evaluation of multi-junction photoelectrodes with varied architectural designs.