In Situ/Operando Probe of Charge Transfer and Energetics at Semiconductor Photoelectrode/Electrolyte Interfaces

Photoelectrochemistry is a promising approach for converting solar energy to storable chemical energy in chemical fuels. The overall photon-to-fuel conversion process consists of multiple elementary steps, including charge separation, recombination, catalytic reactions. While the overall incident light-to-current conversion efficiency (IPCE) can be readily measured, identifying the microscopic efficiency loss processes remains difficult. Although transient absorption spectroscopy is a powerful tool for probing interfacial dynamics in high surface area materials, it is often not applicable on planar electrodes because of a lack of sensitivity for interfacial processes. Furthermore, there lack techniques for directly probing quasi-fermi level shift, electrostatic potential distribution (such as bend bending) and interfacial field that governs the charge carrier separation and recombination and chemical reaction. In this talk, we discuss our recent effort in developing <i>in situ</i> time-resolved linear and nonlinear spectroscopic tools for probing interfacial charge transfer and chemical reactions at planar (photo-) electrode/electrolyte interfaces. Depending on the time availability, the talk may cover some the following four topics. In the first topic, we discuss simultaneous <i>in-situ</i> transient photocurrent and transient reflectance spectroscopy (TRS) measurements of photocathodes for water reduction in photoelectrochemical cells. The kinetics of interfacial charge separation is probed through the built-in electric field change through the Franz-Keldysh effect. This study provides a time-resolved view of microscopic steps involved in the overall light to current conversion process and provides detailed insight on the main loss pathways of the photoelectrochemical system. In the second topic, we discuss our progress in directly probing molecule catalysts attached on electrodes and photoelectrodes using <i>in situ </i>sum-frequency generation (SFG) spectroscopy. In this technique, the kinetics and effect of interfacial charge transfer is probed by the change in adsorbate vibrational spectra. In the third topic, we discuss the <i>in situ</i> direct probe of the spatially varying electrostatic potential in the semiconductor space charge layer (i.e. the built-in potential) and the electric double layer at the electrode/electrolyte interface by electric field induced second harmonic generation (EFISH). We will discuss how this method is used to probe the change of built-in potential as a function of pH and light illumination, providing insight into rate limiting steps in the overall solar-to-fuel conversion. In the last topic, we show that Raman spectroscopy can be used to probe the light induced quasi-Fermi level shift of catalysts in semiconductor/insulator/catalysts junctions.