(Invited) In Situ spectroscopy of Electrocatalytic and Photocatalytic Interfaces
Authors: Cronin Stephen B. 
Published: August 2024

We explore various aspects of electrochemistry and photoelectrochemistry using in situ spectroscopy of electrode (metal) and photoelectrode (semiconductor) interfaces in situ under electrochemical working conditions. These spectroscopies include sum frequency generation (SFG), transient reflectance/absorption spectroscopy (TAS/TRS), and surface enhanced Raman spectroscopy (SERS). Using surface enhanced Raman scattering (SERS) spectroscopy, we monitor local electric fields using Stark-shifts of nitrile-functionalized silicon photoelectrodes.6 We also report several spectroscopic methods for monitoring local electric fields (within the double layer), local pH,1 photo-induced surface potential and charge transfer2 at electrode surfaces using surface reporter molecules. The Figure below illustrates two surface reporter molecules for determining the local pH, local surface potential, and charge transfer at electrode and photoelectrode surfaces. We also measured the stacking dependence and Resonant interlayer excitation of monolayer WSe2/MoSe2 heterostructures for photocatalytic energy conversion.3 Using sum frequency generation (SFG) spectroscopy, we measure the voltage dependence of the orientation of D2O molecules at a graphene electrode surface, which is related back to the “stiffness of the ensemble”.4 Using transient absorption spectroscopy (TAS), we measure the lifetime of hot electrons photoexcited in plasmon resonant nanostructures.5 Using transient reflectance spectroscopy (TRS), we measure the photoexcited carrier dynamics in a GaP/TiO2 photoelectrode, as well as the electrostatic field dynamics at this semiconductor-liquid interfaces in situ under various electrochemical potentials.6 Here, the electrostatic fields at the surface of the semiconductor are measured via Franz−Keldysh oscillations (FKO). These spectra reveal that the nanoscale TiO2 protection layer enhances the built-in field and charge separation performance of GaP photoelectrodes.7

1 Wang, Y.Y., et al., Measuring Local pKa and pH Using Surface Enhanced Raman Spectroscopy of 4‐Mercaptobenzoic Acid. Langmuir, DOI:10.1021/acs.langmuir.3c02073 (2023).
2 Li, R., et al., SERS Detection of Charge Transfer at Electrochemical Interfaces Using Surface-Bound Ferrocene. The Journal of Physical Chemistry C, 127, 14263 (2023).
3 Chen, J., et al., Stacking Independence and Resonant Interlayer Excitation of Monolayer WSe2/MoSe2 Heterostructures for Photocatalytic Energy Conversion. ACS Applied Nano Materials, DOI:10.1021/acsanm.9b01898 (2020).
4 Montenegro, A., et al., Field-Dependent Orientation and Free Energy of D2O at an Electrode Surface Observed via SFG Spectroscopy. Journal of Physical Chemistry C, 126, 20831 (2022).
5 Wang, Y., et al., In Situ Investigation of Ultrafast Dynamics of Hot Electron-Driven Photocatalysis in Plasmon-Resonant Grating Structures. Journal of the American Chemical Society, 144, 3517 (2022).
6 Xu, Z.H., et al., Direct In Situ Measurement of Quantum Efficiencies of Charge Separation and Proton Reduction at TiO2-Protected GaP Photocathodes. Journal of the American Chemical Society, 2860-2869 (2023).
7 Wang, Y. and S.B. Cronin, Performance Enhancement of TiO2-encapsulated Photoelectrodes Based on III–V Compound Semiconductors, in Ultrathin Oxide Layers for Solar and Electrocatalytic Systems. 2022, Royal Society of Chemistry. p. 103-134.

Figure 1