Photoelectrodes consisting of metal–insulator–semiconductor
(MIS) junctions are a promising candidate architecture for water splitting
and for the CO2reduction reaction (CO2RR).
The photovoltage is an essential indicator of the driving force that
a photoelectrode can provide for surface catalytic reactions. However,
for MIS photoelectrodes that contain metal nanoparticles, direct photovoltage
measurements at the metal sites under operational conditions remain
challenging. Herein, we report a new in situ spectroscopic approach
to probe the quasi-Fermi level of metal catalyst sites in heterogeneous
MIS photoelectrodes via surface-enhanced Raman spectroscopy. Using
a CO2RR photocathode, nanoporous p-type Si modified with
Ag nanoparticles, as a prototype, we demonstrate a selective probe
of the photovoltage of ∼0.59 V generated at the Si/SiOx/Ag junctions. Because it can directly probe
the photovoltage of MIS heterogeneous junctions, this vibrational
Stark probing approach paves the way for the thermodynamic evaluation
of MIS photoelectrodes with varied architectural designs.