  Molecular-level insight into interfacial water at buried electrode interfaces
is essential in elucidating many phenomena of electrochemistry, but
spectroscopic probing of the buried interfaces remains challenging. Here, using
surface-specific vibrational spectroscopy, we probe and identify the
interfacial water orientation and interfacial electric field at the calcium
fluoride (CaF2)-supported electrified graphene/water interface under applied
potentials. Our data shows that the water orientation changes drastically at
negative potentials (<-0.03 V vs. Pd/H2), from O-H group pointing down towards
bulk solution to pointing up away from the bulk solution, which arises from
charging/discharging not of the graphene but of the CaF2 substrate. The
potential-dependent spectra are nearly identical to the pH-dependent spectra,
evidencing that the applied potentials change the local pH (more than five pH
units) near the graphene electrode even at a current density below 1 microamp
per square centimeter. Our work provides molecular-level insights into the
dissociation and reorganization of interfacial water on an
electrode/electrolyte interface.
